Céline Pichon

Daniel N Woodruff

David Aguila

Yunnan Guo

Yoann Prado

Daniel Rosario

Rodica Ababei

Oleg Palamarciuc

Ie-Rang Jeon

Indrani Bhowmick

Steen Kasper

Evangelia Pedersen

Vivien Koumousi

Mihail Pianet

Ahmed Achard

Alain Bentaleb

Gilles Derré

Sigaud

Igor Ababei

Simona Ungureanu

Renaud Vallée

Keywords: service informatique, mission, gestion, accueil et bâtiment, cellule chimie

Palamarciuc, Octavian Blaj, and Amandine Foulet

Even if the often group and sub-group meetings are sometimes exhausting, they have their advantage in stimulating people to be hard workers and to attempt to do their best for Science. The "after" group meeting has more benefits. Therefore I would like to thank Elizabeth Hillard for the organization of group-

General Introduction

In the tremendously growing field of material science, molecular materials present a relatively recent and emerging interest, caused by a very rapid development of modern technology that requires better performance from existing materials. Molecule-based systems show synthetic advantages over traditional materials since they use well designed precursors with a controlled geometry and do not require energyintensive production methods. The rational choice of the building-blocks allows an efficient control of the final structural dimensionality of the molecule-based materials and thus their related physical properties.

Systems built from molecular units, that are assembled and designed to afford interesting and, sometimes, premeditated magnetic properties, give rise to the field of molecule-based magnetic materials.

Magnetic molecules are a class of fascinating materials. They contain a finite number of spin centers (e.g. paramagnetic ions) and thus provide ideal opportunities to study basic concepts of physics and magnetism. Scientists from various fields, in chemistry and physics, theoreticians and experimentalists have joined the research field on molecular magnetism in order to explore the unprecedented properties of these compounds. Molecular magnetic materials such as single-molecule magnets (SMMs), spin crossover (SC) or electron transfer (ET) complexes show tunable magnetic and/or optical properties as a function of different external stimuli (magnetic field, light, temperature or pressure). That is why these systems are promising for application in numerous areas such as information processing, high-density recording media, molecular switches, sensors and display devices.

Even though the magnetic properties are the main motivations in this field of research, building multi-functionality in a molecular magnetic material is a hot focus of research, as well. In this respect, molecular chemistry provides unique possibilities, as it allows us to design novel hybrid materials that combine in the same system several physical properties which are difficult or impossible to achieve with inorganic or solid state chemistry. In order to get hybrid magnetic materials with advanced properties, the functionalization of the organic part of interesting molecule-based magnetic materials by groups known to induce additional physical properties (liquid crystalline phases or increased solubility) is a strategy of choice as it may lead to a multifunctional behavior and/or synergetic effects.

One of the main research interests in the "Molecular Materials and Magnetism" (M 3 ) group at the Centre de Recherche Paul Pascal (CRPP) is the functionalization of the molecular magnetic materials towards hybrid systems possessing liquid crystalline properties and/or increased solubility. During her PhD thesis (2004-2007), Pauline Grondin developed the idea of grafting linear long alkyl chains onto triazole ligands, in order to obtain new [Fe II (R-trz) 3 ]A 2 compounds (where R-trz is a functionalized triazole ligand and A is a counter-anion) that showed, in addition to spin crossover, liquid crystalline properties and/or the ability to gelate various solvents. Following the same aims, the thesis work of Diana Siretanu (2008-2011) was dedicated to functionalization of Mn 12 -based SMMs, various Fe II -based SC systems and cyanido-bridged {Fe 2 Co 2 } electron-transfer complexes.

To continue the foregoing works, in this thesis we are focused on further functionalization of molecule-based magnetic materials such as Mn 12 -based SMMs and cyanido-bridged {Fe 2 Co 2 } molecular squares. In the first chapter of this thesis the reader can find general information about the two classes of I.1.

Molecule-based magnetic materials

Magnetism has been known for millennia, and for millennia interpretations of the nature of this elusive force have been produced. It was in the nineteenth and twentieth centuries (AD) when the nature of magnetism was finally understood. Magnetic materials became then the origin of major technological advances indispensable in our society. 1,2 The magnet-based materials are found everywhere in our lives: in generators, motors, loudspeakers, microphones, switches, sensors, data storage devices, medical devices and many more.

Until the 80s, magnetic materials composed of metals, metal alloys or metal oxides represented the most-studied systems. 2 The preparation and processing of such materials require high temperature and energy consuming metallurgical methodologies. Moreover the optimization of information storage techniques requires constant miniaturization of the size of the magnetic materials, but the traditional magnetic materials exhibit limitations towards size reduction by losing their useful magnetic properties. 2,3 Therefore, the development of new molecule-based magnetic materials has grown considerably in recent decades. In comparison to inorganic nanoparticles that are intrinsically polydispersed in size, molecular systems exhibit perfect monodispersity in size, volume, shape and charge, since molecules are all identical to each other. They are soluble in common solvents and their modular character opens ways to fine-tune their properties. Moreover, some of these molecular materials display remarkable magnetic properties, which are rare or unknown in traditional inorganic materials. For example, molecular systems with spin crossover phenomenon (SC), 4 photomagnetism, 5 single-molecule magnet (SMM) 6 or single-chain magnet (SCM) 7 behaviors have been discovered. Furthermore, these molecule-based materials may exhibit additional features including transparency, thermo-or photochromism, solubility, conductivity/superconductivity, low density, biocompatibility and/or recyclability affording convenient solutions for new technological needs. 8 An additional important benefit of these molecular materials is the possibility to finely tune their chemical and physical properties by the design of the molecular precursors through the modification of their ligands.

In recent years, considerable research efforts have been devoted to the fabrication of nanoscale systems, for example to reduce the size of magnetic units in information storage devices. Molecular magnetic materials could meet this expectation by giving the opportunity to store information at the nanometric or molecular scale using, for example, the promising SMMs. 2,[4][5][6]9 Although these physical characteristics and potential applications are often highlighted in numerous fundamental studies, to date none of these magnetic components have been exploited industrially for any commercial devices. 10 Indeed, these molecule-based compounds are generally produced as powders or crystalline solids, while technological devices require that some degree of self-organization or an easy processability is imparted to the material. Therefore one of the great challenges in the field of molecular magnetism is to process these compounds into technologically sustainable materials. One of the possible solutions is the elaboration of hybrid magnetic materials via functionalization of organic ligands with various groups to improve thermal or chemical stability, to induce liquid crystalline properties or, simply, enhance the solubility.

The main objective of this thesis is to develop hybrid molecule-based materials with improved properties via rational functionalization of well-known magnetic molecular complexes. The work along this I.8

thesis covers the following two topics: single-molecule magnets, and electron transfer systems. Therefore, the basic and essential concepts behind these two molecular systems chosen for functionalization are summarized in the first part of this chapter. The second part will provide a short bibliographical survey of the already reported functionalized magnetic molecule-based systems possessing liquid crystalline properties or improved solubility. Based on the literature review throughout the second part of this chapter and previously performed work in the "Molecular Materials and Magnetism" group from Centre de Recherche Paul Pascal (CRPP), the motivation and the objectives of this thesis work will be detailed in the end of this first chapter.

I.1. very low temperatures (i.e. T < 10 K). 6 Such molecules, called Single-Molecule Magnets (SMMs), can retain their magnetization below a certain blocking temperature (the temperature at which the time, τ, taken for the magnetization to relax is 100 s), 11 giving rise to M vs H hysteresis loops. These molecules represent the smallest units that may potentially be used in applications such as information storage, 6,9,10 quantum computation 12 or as low-temperature refrigerants via the magnetocaloric effect. 12c,13 The prerequisites for a system to act as an SMM are a high-spin ground state (S T ), a high uniaxial magnetic anisotropy (characterized by the negative zero-field splitting parameter, D, defined by the following spin Hamiltonian:

) with a magnetic easy axis and negligible magnetic interactions between the molecules. [6][7][8][9][10][11] Because of their small size, SMMs can be truly placed at the interface between the quantum and classical worlds. These objects made possible experimental observation of theoretically predicted quantum phenomena, such as quantum tunneling of the magnetization 14 and quantum phase interference. 15 A very fruitful research has been performed on the improvement of SMM characteristics and extraordinary advances have been made over the last two decades resulting in a huge number of new SMMs in scientific literature. Via the serendipitous self-assembly 11,16 or rational molecular design (building block approach) 11,17 coordination chemists were able to construct homo-or heterometallic complexes of various nuclearity (from 1 to 84 metal centers) exhibiting SMM behavior for 3d, [6][7][8][9][10][11]16,17 4d, 18 5d, 18a,19 4f, 20 and 5f 21 metals. Moreover, in recent years, organometallic chemistry has produced many interesting developments that account for notable "records" in terms of magnetic hysteresis and anisotropy barriers. 22 In the following paragraphs of Section I.1.1, basic theory of SMM physics is described to understand how these complexes are capable to show not only the classical magnet-like behavior but also the quantum properties.

I.1.1.2. Theoretical aspects of SMMs illustrated by the Mn 12 example

SMM complexes are often composed of metal ions (spin carriers) bonded together by various ligands, which serve to isolate the molecular core from its neighbors and ensure their cohesion. The intramolecular I.9 magnetic exchange between metal centers through the ligands is usually significantly larger than the intermolecular interactions. Therefore the macroscopic magnetic properties of the material basically reflect the properties of a single molecule.

The Mn 12 -OAc SMM molecule, [Mn 12 O 12 (O 2 CCH 3 ) 16 (H 2 O) 4 ], 6,23 that is used as precursor in synthesis of hybrid molecule-based materials possessing both SMM and liquid crystalline properties along this thesis work, is chosen to illustrate SMM properties in this section.

Each Mn 12 -OAc molecule contains eight Mn III (S = 2) and four Mn IV (S = 3/2) ions. The structure of Crystallographic structure of Mn 12 -OAc complex: (left) view along the crystal b-axis with illustration of the easy axis in Mn 12 -OAc complex situated along c axis of the unit cell; (right) view along the crystal c-axis with schematic representation of the spin alignments in the ground state of Mn 12 -OAc complex that gives an S T = 10 ground state. 6,23 Color scheme: Mn III dark green, Mn IV light green, O red, C grey. Hydrogens and lattice solvents are omitted for clarity.

SMM behavior arises from the association of a high-spin ground state with an easy-axis (i.e. Isingtype) magnetic anisotropy. The large spin ground state in Mn 12 -OAc is arising from a situation in which the spins of the four central Mn IV centers are all aligned antiparallel to the spins of the eight outer Mn III

I.1.1.2.1. SMM properties in the absence of external field

Assuming the above mentioned SMM is a S T = 10 macro-spin with the three directions of magnetization (xhard, yintermediate, and zeasy axis), the spin quantum number S T possesses 2S T + 1 sublevels, each characterized by a projection quantum number m S , where -S T ≤ m S ≤ S T . At low temperature, when the magnetic coupling is larger than the thermal energy, SMM behavior is defined by the following Hamiltonian:

( )

Eq. I.1

Chapter I -Generalities and Context I.10

where S T is the total spin ground state, spin operators along the three principal directions of magnetization (xhard, yintermediate, and zeasy axis), D and E are easy axis and transverse anisotropy parameters (that arise due to geometrical deviations from the ideal uniaxial symmetry for which E = 0).

Hence in the absence of a magnetic field and the presence of a strong uniaxial anisotropy (D is negative and |D|>>E) the Hamiltonian can be simplified to Eq. I.2

and the spin sublevels change their energy according to:

Eq. I.3. where S T is an integer total spin state and D is the zero-field splitting parameter originating from magnetic anisotropy of the system. For a half integer S T (when an odd number of unpaired electrons is present in a molecule) the energy barrier is thus Δ A = |D|(S T 2 -1/4).

In the equilibrium state at low temperature and without an applied magnetic field, half of the Mn 12 -OAc molecules is in the m S = + 10 level ("spin up") while the other half is in m S = -10 level ("spin down")

and the total magnetization of the sample is zero. To eventually overcome the energy barrier between the positive and negative m S states thermal population of the other m s levels is allowed at higher temperatures.

Chapter I -Generalities and Context I.11

I.1.1.2.

SMM properties in a longitudinal magnetic field

The presence of a magnetic field changes the previously reduced Hamiltonian (Eq. I.2) to describe an SMM as followed:

⃗ ⃗ ⃗⃗ Eq. I.5

where the last term is the Zeeman effect contribution which originates from the interaction of spin ⃗ with external magnetic field ⃗ ⃗⃗ . When an external longitudinal (parallel to the easy axis) magnetic field H z is applied, the spin sublevels change their energy according to the Zeeman coupling (Figure I-3 left):

Eq. I.6

According to the Eq. I.6, the spin sublevels with m S > 0 become energetically stabilized while those with m S < 0 values become energetically destabilized (Zeeman effect) (Figure I-3 right). This leads to a preferred population of the m S = +10 sublevel (in Mn 12 -OAc). Thus a favored orientation of the microscopic magnetic moments in the direction of the external magnetic field is creating a magnetization M ≠ 0 of the macroscopic sample.

Magnetization relaxation

On the removal of the external magnetic field, the system must go back to thermal equilibrium, the initial energy spectra are restored and spins recover their thermodynamic energy levels (Figure I-2), meaning that half of molecules return to m s = -10 (in Mn 12 -OAc) and magnetization relaxation is observed.

Due to the anisotropy barrier, schematized in Figure I-2, SMMs have a slow relaxation of magnetization. This relaxation process occurs through the coupling of the spin system with the environment.

If there is sufficient thermal energy (k B T), the molecule can absorb heat from the vibrational modes of the lattice (phonon). According to the selection rules of spin-phonon interaction, the only allowed paths are Δm S = ±1 and ±2. Therefore, in order to relax to the equilibrium state, the system must successively absorbs phonons until it reaches m S = 0, then it can reach m S = -10 through phonon emission. This is the origin of the thermal energy barrier to the slow relaxation of the magnetization in SMMs.

One of the ways to monitor the magnetization relaxation is by measuring the time decay of the magnetization, M(t), in a zero field. First, a large dc field is applied to the sample at a fixed temperature to I.12 saturate its magnetization in the easy direction. When the field is switched off, the magnetization is measured as a function of time. The experiment should be performed at different temperatures (Figure I-4). As the decay of the magnetization is usually an exponential function of time, the relaxation time τ is extracted at each temperature by taking τ = t when the magnetization reaches 1/e. Usually, the magnetization decreases exponentially with the time at fixed temperature according to the relation:

( ) ( ) Eq. I.7
where M 0 or M S (t = 0) is the saturation magnetization, M(t) is the magnetization at a given time t and τ is the relaxation time. The relaxation time is temperature dependent and follows the Arrhenius law:

( )

Eq. I.8
where Δ A is the energy barrier, k B is the Boltzmann constant, and τ 0 is the pre-exponential factor, a constant that can be experimentally determined and depends on the nature of the SMM and its environment. In the case of Mn 12 family, it varies in the range of 10 -8 -10 -10 s. From the Eq. I.8, we can conclude that higher temperature means smaller relaxation time (Figure I-4).

Figure I-4. Relaxation of the magnetization of Mn 12 -OAc at different temperatures, measured at zero applied field for a single crystal after saturation at 5 T field applied along the easy axis of magnetization. 24 Another experimental method (and usually most commonly used) that yields information about magnetization dynamics of SMMs is the alternating current (ac) magnetic susceptibility measurement (with a weak field, typically 1 -5 Oe, oscillating at a given frequency, ν exp ). The alternating magnetic susceptibility produces two quantities: an in-phase, or real, component ′ and an out-of-phase, or imaginary, component ″ as:

Eq. I.9

When the relaxation time τ of the compound is shorter than the characteristic experimental time τ exp (τ exp = (2πν exp ) -1 ), the magnetization vector is oscillating with the ac field, and only a static behaviour is observed (only the real component ′). However, when τ is longer, a dynamic behaviour is perceived and the magnetization vector is dephased (non-zero imaginary component ″). Experimentally, we can measure the Like classical magnets, below the blocking temperature SMMs exhibit magnetic hysteresis loops that are temperature and magnetic field sweeping rate dependent. A series of steps were discovered in the hysteresis loops of the Mn 12 -Ac (Figure I-6). The observed steps in the hysteresis loop correspond to a faster rate of magnetization relaxation occurring when the energy levels on opposite parts of the double-well potential are at the same energy. For these particular field values where a vertical magnetization step is observed (Figure I-6), tunneling of the magnetization is enabled, and an evident increase in the relaxation rate is observed. This result can be explained by the competition between two relaxation processes (thermal and quantum) that makes short-cut of the thermal barrier (thermally assisted quantum tunneling of the magnetization). Moreover, relaxation time at very low temperatures (below 1 K) does not follow the Arrhenius law and saturates, indicating the presence of quantum tunneling of the magnetization (QTM). QTM occurs between energetically matched m S levels on opposite sides of the barrier and can be observed as steps (relating to a loss or gain of magnetization) in the hysteresis loops (Figure I-6). This behavior can be described by the following spin-Hamiltonian:

( ) ⃗ ⃗ ⃗⃗
Eq. I.10

In the section I. 1.1.2.1, it was assumed that due to the strong uniaxial anisotropy the SMMs behavior in an applied magnetic field can be described by the Hamiltonian from Eq I. 5. As long as this Hamiltonian (Eq. I.5) is valid the two ±m S states are orthogonal to each other, and there is no possible quantum mixing. Upon scanning the external magnetic field, they simply cross over each other and the mechanism for tunneling does not exist (Figure I-3 left). In reality, however, there is always a transverse contribution, and therefore, the contribution of the second term in Eq. I.10 cannot be neglected. The transverse term, containing or spin operators, mixes two different m S states and creates a gap at the crossing of the m S levels. In zerofield the +m S and -m S levels are equal in energy and this corresponds to the most favorable condition to observe tunneling. When a magnetic field is applied along the z axis (Fig. Basic concepts to understand SMM characteristics and associated experimental techniques were introduced based on Mn 12 -OAc SMM in this section. In Chapter II, additional information regarding the family of Mn 12 -based complexes may be found. Another type of interesting bistable molecules is presented in the next section of this chapter.

I.1.2. Electron transfer complexes I.1.2.1. Generalities

Molecular materials that show a reversible change (or switching) of their physical properties as a function of external stimuli (light, temperature, pressure, magnetic or electric fields, etc.) offer appealing perspectives for the realization of energy efficient molecular-scale electronic devices. 26,27,28 A significant extension of the application perspectives was given by the development of molecular magnetism, which lead to the discovery that the switching of the magnetic properties of a molecule can be induced by temperature variation or photoexcitation. Among them, the most studied systems are those exhibiting spin crossover (SC) 4,5a or electron transfer (ET). 5b,c,29 The observed phenomena in both systems are associated with the change of the electronic configuration of metal centers that results in the concomitant major changes in their structural, magnetic and optical properties.

Electron transfer (ET) systems have attracted a great interest, since this process plays a key role in most of the fundamental processes in physics, chemistry and biology. 30 An electron transfer (ET) represents a redox process between two units, one being the donor (reductant) while the other onethe acceptor (oxidant). Rudolph A. Marcus developed the first generally accepted theory of outer-sphere (intermolecular)

ET in 1956 based on a transition state theory approach. 31 Later, Marcus theory was extended by Hush to the problem of inner sphere (intramolecular) electron transfer reactions. 32 Different other models have been developed to complete existing ones. Piepho, Krauz, and Schatz elaborated a vibronic coupling model (PKS model) 33 to calculate absorption profiles of the electron transfer systems.

A metal-to-metal electron transfer was firstly reported in the mixed valence Creutz-Taube ion,

[(H 3 N) 5 Ru II (pyr)Ru III (NH 3 ) 5 ] 5+ (pyr = pyrazine). 34 The phenomenon of metal-to-metal electron transfer is also observed in heterometallic compounds. However, the redox sites do not necessarily have to be metallic, purely organic systems are also known. 35,36 The discovery of ET in the 3D inorganic polymer I.16

Cyanido-bridged hetero-bimetallic molecular complexes, exhibiting switching of their magnetic and optical properties as the result of a thermally and/or photo-induced intramolecular metal-to-metal electron transfer, are another molecule-based magnetic system chosen for functionalization within this thesis work. In this regard, the following paragraphs of Section I. compounds due to their remarkable magnetic properties. The advantages to design magnetic functionalities with these compounds can be found in their facile synthesis and the predictable magnetic couplings via CN bridge. 37 Moreover, certain M/M' couples of these compounds exhibit metal-to-metal electron transfer phenomenon. During the ET process, an electron is exchanged between two metal ions via a cyanide group (Figure I-8), resulting in the change of electronic, magnetic, and/or optical properties of a whole system.

Electron transfer has been found to occur under the influence of temperature, 38 pressure, 39 visible light 38,40 and X-rays 41 in different PBAs. The first example K 0.2 Co 1.4 [Fe(CN) 6 ]•6.9H 2 O, that shows light-induced reversible magnetization changes, was briefly introduced in Section I. 1.2.1. In the visible region of its electronic absorption spectra, this compound exhibits an intense metal-to-metal charge transfer absorption at 550 nm, ascribed to the conversion of {Fe II LS (μ-CN)Co III LS } (Fe II LSt 6 2g , S = 0 and Co III LSt 6 2g , S = 0) into paramagnetic {Fe III LS (μ-CN)Co II HS } (Fe III LSt 5 2g , S = 1/2 and Co II HSt 5 2g e 2 g , S = 3/2) units via the metal-to-metal ET. As shown in Figure I-9 right, the irradiation with red light (660 nm) increases the ferrimagnetic ordering temperature (Curie temperature) from 16 to 19 K and concomitantly increases the magnetization. This effect is reversed via blue light irradiation (450 nm) or heating of the sample above 150 K.

Following this seminal work, several groups reported that the photomagnetic behavior as well as thermally induced electron transfer of Fe/Co Prussian blues can be modulated by changing the Co/Fe ratio I.17 present in the nonstoichiometric A j Co k [Fe(CN) 6 ]•nH 2 O (A = alkali metal ion) lattices. 5c,42 When j < 0.5, the Fe sites are fractionally occupied and the Co centers consequently contain one or more water molecules to satisfy their coordination spheres. The number of water molecules tunes the ligand field strength and redox potential of the Co centers. It was indeed found that an average CoN 5 O coordination environment and adjacent [Fe(CN) 6 ] n -sites are primary to result reversible thermally and photo-induced magnetization changes in the material. For example, in the case when A = Na, it was reported 42e that Na 0.07 Co the photo-induced electron transfer at low temperatures exhibit first-order phase transition with a thermal hysteresis between 260 -300, 230 -270 and 180 -220 K, respectively, associated with an electron transfer process between diamagnetic and paramagnetic states.

Similarly to Fe/Co PBAs, Fe/Mn cyanido-bridged 3D systems exhibit a temperature-dependent ET with a large thermal hysteresis of 116 K between 147 and 263 K. 43,44 The rubidium manganese hexacyanoferrate, Rb 0.88 Mn[Fe(CN) 6 ] 0.96 •xH 2 O, displays both thermally and photo-induced electron transfer. 43 This system shows a transition from the high temperature phase containing {Fe III LS (μ-CN)Mn II HS } pairs (Fe II , S = 1/2; Mn II , S = 5/2) to the low temperature phase made of {Fe II LS (μ-CN)Mn III HS } pairs (Fe II , S = 0; Mn III , S = 2). The electron transfer process is accompanied by a structural transformation from cubic to tetragonal space group due to the Jahn-Teller distortion resulting on Mn III sites (some additional information about Jahn-Teller distortion in Mn III ions may be found in Chapter II). The low temperature phase shows a 3D ferromagnetic order with a T C = 11 K due to the interactions between Mn III ions through the diamagnetic [Fe II (CN) 6 ] units. The light irradiation (532 nm) induces a ferrimagnetic phase due to a metal-to-metal charge transfer that yields reduced values of magnetization. The reverse process is also achieved via irradiation with 410 nm wavelength light source. The capability Fe/Mn PBAs to exhibit switching phenomena and their physical properties are however known to be intimately coupled to the exact system stoichiometry. 45 Besides Fe/Co or Fe/Mn PBAs, the chemistry of other 3D cyanido-bridged coordination polymers based on other d-block hexacyanidometallates was explored. Thus, bimetallic Prussian blue analogues like Fe/Cr, 46 V/Cr, 47 Mn/Cr, 48 Co/Os, 49 were reported to exhibit thermally and/or photo-induced metal-to-metal ET. Moreover, heterobimetallic networks based on octacyanidomolibdates (Cu/Mo) 50 or octacyanidotungstates (Co/W) 51 were reported to display similar phenomena.

However, the high dimensionality of bimetallic assemblies of Prussian blue analogues is limiting the systematic study of structure-property relationships, and furthermore the advance towards technological application due to the processing difficulties induced by their reduced solubility. In this respect, soluble and well-defined molecular fragments of these functional networks (with preserved original properties) appear to be more promising towards future applications. Therefore an active cyanido-metallate research trend has been directed towards the synthesis and investigation of discrete PBAs that have flexible molecular and electronic structures coupled with improved solubility. In the last two decades, several groups designed, isolated and studied molecular hetero-or homobimetallic analogues (fragments of various geometry) of I. 18Prussian blues by rational selection of capped building blocks and coordinatively unsaturated metal ions. 52,53,54,55 This approach has been extremely successful and diverse molecular architectures have been developed to display single-molecule magnet behavior, 17,53 spin crossover, 54 electron-transfer 52c,54e,55 or photo-induced magnetism. 56 Thus, several heterobimetallic systems were found to exhibit thermal and/or light-induced electron transfers, such as Cu/Mo, 56a,b Fe/Os, 55h Mn/Mo, 56c Mn/W, 56c Co/W, 56i and Fe/Co 52c,54e,55a-g,i,56d-g that are the most studied systems due to a well established mechanism, and therefore some of these Fe/Co systems were chosen for investigations within this thesis and are discussed in following paragraphs.

Despite some earlier reports on molecular complexes containing cyanido-bridged Fe and Co ions, 57

3). 55g

The tetranuclear {Fe 2 Co 2 } squares, that also mimic structurally PBA (one face of the cubic cell) and display reversible thermally and photoinduced intramolecular electron transfer in solid state and in solution, are one of the most studied molecular systems to date. 52c,55c-g,59 These compounds of general formula

I.20 {[(L 1 )Fe(CN) 3 ] 2 [Co(L 2 ) 2 ] 2 }(A)

Theoretical background

As described above, ET complexes, particularly cyanido-bridged {Fe 2 Co 2 } molecular squares, show experimentally interconversion between two different states (diamagnetic and paramagnetic), or eventually bistability. Therefore the following part is dedicated to the understanding the mechanism of their bistability based on a simple thermodynamic model: the binary solution model. [START_REF] Atkins | Physical Chemistry[END_REF] Given the fact that the ET process occurs between two different metals, we will consider an isolated pair to describe the occurrence of this process. The ET phenomena involve two states (paramagnetic and diamagnetic labeled A and B, respectively) that are energetically close and, therefore, in an equilibrium. The two different states can be easily identified by measuring the temperature dependence of the T product (as shown in Figures I- 10 -I-12), since each state is expected to be diamagnetic or paramagnetic that follows, for the latter, a Curie law. Thus, the T product corresponds to the Curie constants (C) of each state (C = 0 for a diamagnetic species) and is related to their molar fraction, x, according to the following equation:

T = x A •T) A + x B •T) B
Eq. I.11 where x A and x B represent the paramagnetic and diamagnetic fractions of a given system, respectively.

Assuming that x A = x, then Eq. I.11 becomes: Eq. I.14

T = x•T) A + (1 -x)•T) B Eq. I.12
According to the ideal solution model, we assume that A and B states are mixed without any interaction. If N is the total number of molecules, then

N A = x A •N = x•N is the number of molecules in the A state, while N B = x B •N = (1-x)
•N is the number of molecules in the B state. In statistical thermodynamics, the Gibbs energy of the system can be written:

Eq. I.15
where, I is the intermolecular interaction term, that is neglected in the first approximation, G A and G B are the molar Gibbs energy for molecules in A and B states, whereas the G mix is the energy of the mixture:

[ ( ) ( )]
Eq. I.16 hence:

I. 22
( ) ( ) ( ) [ ( ) ( )]
Eq. I.17

This relationship allows describing the Gibbs energy for each molecule, g N , as following:

[ ] [ ( ) ( )]
Eq. I.18

Introducing the degeneracy,

( ): [ ] [ ( ) ( )] ( ) Eq. I.19
The condition for equilibrium requires

( ) Eq. I.20
as G B is a constant. So we get:

( ) Eq. I.21
The equation can be rearranged in the form of ( ) Eq. I.22 and:

( )

Eq. I.23

For T = T*, x A = x B = x* = 0.5 and thus . Then, and .

The expression I.23 changes in:

( ( )) ( ( ))
Eq. I.24 or Eq. I.25

This allows plotting of the 𝑓( ) dependence (Figure I-13), where .

I.23

Whatever is the value of a, the function increases when T/T* increases. This model predicts that at low temperature (T/T* << 1), the system is in B state, so = 0. We can observe that all curves pass through a common point, T/T* = 1, and = 0.5. So we can conclude that T* is a characteristic temperature of the system. At this point, the system contains 50 % of molecules in state A and 50 % of molecules in state B. Note that in the literature, T* is often noted T 1/2 to reflect the composition of the system. Now let's consider a real case, for example a couple of Fe and Co ions, both in an octahedral environment. The paramagnetic state (A) consists of an Fe III LS ion (S = 1/2, 2 T g spectroscopic term, so the spin degeneracy is 2 and the orbital degeneracy, 2L + 1, is 3) and a Co II HS ion (S = 3/2, 4 T g , spin degeneracy is 4, and the orbital degeneracy is 3). The diamagnetic state (B) consists of Fe II LS (S = 0) and Co III LS (S = 0) ions that are both corresponding to the 1 A g spectroscopic term for which the spin degeneracy is 1 and the orbital degeneracy also 1. So and/or magnetic measurements, thus the value of a could be estimated between 6.0 and 7. 1. 55b,c,g This shows that in real complexes not only the electronic contributions to the degeneracy are present, but also the vibrational contributions for paramagnetic and diamagnetic states that are different, because of different geometry in both states.

g para = g A = (2×3)×(4×3), g dia = g B = (1×1)×(
In a solid phase, in the crystalline lattice, the ET is directly influenced by intermolecular interactions (mostly of elastic origin). They play an important role in the macroscopic properties of the systems. The solution model called Slitcher-Drickamer, 60 assumes that the interaction energy is responsible for the cooperative effects and the interaction term, neglected in Eq. I.15, is defined as:

( ) ( ) Eq. I.26
where W is the intermolecular interaction parameter (in units of energy) independent of temperature. Note that ( ) is a simple exchange term ( ) similar to a magnetic spin Hamiltonian -⃗ ⃗ . Insertion of this term in the molar Gibbs energy gives:

( ) ( ) ( ) [ ( ) ( )] ( )
Eq. I.27 then:

( ) [ ] [ ( ) ( )] ( ) Eq. I.28
and from the condition of equilibrium (Eq. I.20):

( ) Eq. I.29 or ( )
Eq. I.30 For given values of a, 𝑓( ) can be plotted (Figure I-14) and the function is defined by: ( )

Eq. I.31

As shown in Figure I-14 (left), the inter-molecular interaction (W) plays an important role to define the stable state depending on the temperature. Therefore, three cases can be discussed:

 W/(k B T*) < 2,

I.25

In the last case, a first order transition is associated with a thermal hysteresis (Figure I-14 right). The blue branch of the curve corresponds to unstable points, but in fact the system describes a hysteresis, drawn in dashed lines in Figure I-14 (right). It is noteworthy to mention that, in the general case, T* is not at the center of the thermal hysteresis. Note also that the hysteresis width does not depend on the thermodynamics but on kinetic criteria and thus on the experimental scanning rate of the temperature.

From this analysis, we can construct a phase diagram that describes the system at any temperature and In this section, the thermal ET process could be described by the binary solution model. This simple approach tells us that the interactions between the motifs responsible for the interconversion phenomena are crucial to exhibit a transition, and therefore to display bistability.

I.2. Soft hybrid molecule-based magnetic systems

Recent technological breakthroughs and the need for new functions generate an enormous demand for novel materials. Many of the well-established materials, such as metals, ceramics or plastics cannot fulfill all technological desires for the various new applications. Looking ahead, scientists must create sophisticated, miniaturized, recyclable, environmentally friendly, energy efficient, reliable, and inexpensive materials and systems. In this respect, molecular chemistry provides unique possibilities, as it allows us to design novel hybrid materials that combine in the same system several physical properties which are difficult or impossible to achieve with inorganic or solid state chemistry.

Nowadays, one of the great challenges in the molecular magnetism field is to process these compounds into technologically sustainable materials and thus to push them towards possible applications.

Among interesting molecular magnetic systems, materials suitable for surface deposition, 61,62 and nanoparticles, 63,64 have attracted particular attention with respect to possible applications. In order to design new generation of systems, hybrid soft materials are a strategy of choice as they can be easily processed and can show multiple-properties such as improved thermal/chemical stability, self-organizational properties, enhanced the solubility, as well as possible synergetic effects. One of the possible ways towards the elaboration of such hybrid magnetic materials is the functionalization of organic ligands of known molecular magnetic materials with various groups possessing or inducing mesogenic (liquid crystalline) behavior or improve the solubility.

I.2.1. Liquid crystalline magnetic molecular hybrids

Liquid crystals (LC) are examples of soft, molecular self-assemblies that exquisitely combine order and fluidity. These systems within components self-organize into a wide diversity of mobile and long-range I.26 ordered periodic structures. [START_REF]Handbook of Liquid Crystals[END_REF][START_REF] Tschierske | [END_REF] Moreover, LC assemblies can easily form thin films, which are intrinsically defect-tolerant because positioning errors are corrected automatically during the process of self-organization.

Their dynamic nature and stimuli responsiveness abilities further render them interesting for applications in modern technologies and particularly in organic electronics. 67 The formation of ordered yet fluid LC mesophases may thus be sought as a first step in the route to controlling the organization of molecule-based magnetic materials into low-dimensional supramolecular orders for their ultimate incorporation within functional macroscopic devices.

I.2.1.1. Liquid crystalline phases -generalities

Liquid crystals have been for a long time the subject of scientific interest since the discovery of this fourth state of matter in addition to the known states of solid, liquid, and gas. In the crystalline solid state In order to exhibit liquid crystallinity, the molecule must be geometrically anisotropic. Liquid crystals are anisotropic meaning that their physical properties are not identical in all directions. Therefore molecules with two antagonist parts (a rigid core and flexible ends) can form LC phases. To generate a LC a thermally induced microsegregation between the flexible chains and rigid cores is necessary.

(

I.27

Generally, there are three classes of LCs: lyotropic, thermotropic and amphitropic. [START_REF] Collings | Introduction to Liquid Crystals, Chemistry and Physics[END_REF] The thermotropic liquid crystals exhibit liquid-crystalline mesophases on melting from the crystal phase or cooling from the isotropic liquid, whereas the lyotropic materials exhibit liquid-crystalline mesophases when mixed with a particular solvent. The amphitropic LCs are compounds that exhibit both lyotropic and thermotropic behaviors.

The transition between a solid state and a LC state is called melting point, and it corresponds to the melting of the flexible part; the transition between the mesophase and the liquid state is called isotropic or clearing point, induced by the loss of cohesion between the molecules. When a liquid crystal melts from a solid, it exhibits one or more thermodynamically stable intermediate states called mesophases. The mesophase is called an enantiotropic when a material displays a mesophase both on cooling and heating.

However, when the mesophase is solely displayed on cooling from the isotropic liquid, and below the melting point of the material, the phase is called monotropic.

I.2.1.2. Magnetic molecule-based mesogens

The interest for novel materials with dual properties derived from metals (magnetism, optics, conductivity, color) and liquid crystals (fluidity, easy processability, order) in the late 70s was at the origin of the metallomesogen (metal-containing liquid crystals) research. 72 One of the objectives of this kind of combination was to obtain paramagnetic liquid crystals thanks to the presence of metal ions with unpaired electrons. It is well known that the orientation of diamagnetic, mainly organic, liquid crystalline phases can be achieved with strong magnetic or electric fields. For an eventual application in a small device, the magnetic field applied to align the mesophase should be small enough, but the threshold field for the alignment of diamagnetic liquid crystals is too high for this. Therefore, metal ions with unpaired electrons have been incorporated into liquid crystals. The design of liquid-crystalline materials containing metal ions is not a facile process. Nonetheless, over the years a wide range of mesogenic systems with improved physical properties via incorporation of different metal ions has been designed. As a result, an assortment of liquidcrystalline phases (both thermotropic and lyotropic) can be accessed with this approach. 72 On the other hand, the functionalization of molecule-based magnetic materials with mesogenic promoters presents a great interest in the coordination chemistry community. The self-assembling property of the LCs and the orientational behavior may affect significantly the magnetic and, generally, physical properties of molecular complexes. Particularly when the molecular materials are oriented and organized in a suitable way, their properties can be tuned or even enhanced via collective and/or synergetic effects.

In the metallomesogen design strategy described above, the main component was the mesogen while the metal ions were incorporated in it. In contrast, the development of multifunctional molecular magnetic complexes would require well-designed functionalized ligands that can coordinate without altering the magnetic core and induce mesogenic behaviors. This line of research is quite new and there are few examples of new coordination compounds exhibiting interesting magnetic phenomena combined with mesogenic properties induced by a rational ligand functionalization. Firstly, this approach has received much attention in the case of spin crossover (SC) compounds especially because the SC phenomenon can occur around room temperature and therefore these materials are promising candidates for molecule-based optical or magnetic data storage devices. Indeed, when SC materials are modified by mesogenic ligands that show transition to a LC phase near the room temperature, there are possibilities to combine the two properties (SC and LC) and consequently to induce synergetic effects. The first example of a molecular hybrid system wherein LC and SC behaviors coexist was reported by Galyametdinov et al. 73 In this work, the ligand of a known Fe(III) complex 74 was modified with I.31 dodecyloxybenzyloxy substituents (Figure I-23 left). The final complex exhibited a smectic phase above 388 K while the SC was detected below this temperature, so no synergy between these two properties of the same molecule was observed. A similar behavior was later reported for the first examples of mononuclear Fe(II) or Co(II)-based SC complex with LC phases. 75 Gaspar et al. 76,77 developed metallomesogenic systems in which the SC and the mesophase formation occur at similar temperatures. This result suggested that the structural change associated with crystal-toliquid crystal transition triggered the SC phenomenon (Figure I-23 right). There are also systems where both transitions coexist in the same temperature region but are not coupled, so liquid crystal phase transition is not the driving force of the spin state transition. 78 An eventual goal in the area of spin crossover metallomesogens is the switching/controling of the spin state using the sensitivity of liquid crystalline phases to external electric and/or magnetic fields.

Several research groups have used the functionalization strategy to create hybrid systems based on the andc) lamellar packing of the resulting rods with interdigitation, respectively, perpendicular and parallel to the average rod axis. 81 The M 3 group at CRPP has been interested in this type of system for more than ten years. Thus, different modifications of the triazole ligand and/or the counter-anions with or without long linear alkyl chains were studied with the aim to induce gel or LC properties. 80,81 In many cases cooperative and complete SC was observed near room temperature, while liquid-crystalline domains were unfortunately found to form at higher temperatures. Typical optical textures of lamellar phases were observed in the case of diamagnetic materials [Zn II (R i Trz) 3 ](A) 2 , which are thermally more stable than their Fe(II) analogues. In the latter systems, mesophases were present, but the appearance of oxidation/degradation processes occurring at high I.32 temperature, did not allow a detailed characterization of these systems. Recently, Hayami et al. 78b detected the presence of disordered columnar phases by functionalizing the triazole ligand with branched-alkyl chains. However, a complete investigation of thermotropic properties was not reported.

Practical applications of SMMs as molecular-scale units for high density information storage or "qubits" for quantum computation 9,82 require the imaging, probing, and eventually manipulating of individual molecules. Regarding this aspect, it is a challenging subject for coordination chemists to obtain SMMs functionalized by mesogenic groups as liquid crystal state without changing the magnetic properties of the material. The first example was described in 2008 by Terazzi et al. 83a ] complexes and chemical structure of mesogenic ligands used in previous reports. 83,84 Color Scheme Mn(IV) green, Mn(III) dark green, O red, C grey, functionalized part of the ligand is shown in pink.

With the aim to design Mn 12 -based LC systems with a reduced globular shape that preferentially leads to cubic phases, D. Siretanu, during her PhD thesis (CRPP, 2008 -2011), employed the same strategy by using a series of p-n-alkoxybenzoic acids (i.e. nonyloxy-, decyloxy-, dodecyloxy-and hexadecyloxybenzoic acids, Figure I-25). 84 Even though all exploited functionalized p-n-alkoxybenzoic acids are well-known for their liquid crystalline properties 85 the series of prepared functionalized Mn 12 -based complexes did not show any mesomorphic behaviour. These pioneering results highlight the great potential of this hybrid approach to increase the thermal stability of Mn 12 -based complexes without significantly affecting the magnetic properties (conservation of SMM behavior). However, tailoring precisely the mesomorphic behavior of these bulky complexes in order to control their organization is not an easy task and remains a considerable challenge. 86 The functionalized complex exhibits hexagonal columnar mesophase at room temperature, while at low temperature it is an SMM. Moreover, this complex showed the possibility to reversibly control its magnetic properties by simple thermal treatment of the studied sample. Thus the thermally trapped disordered phase (prepared by a very rapid cooling from 333 to 150 K) and the ordered crystalline state (slowly cooled from isotropic point) show different temperature dependences of the in-phase (χ') and out-of-phase (χ'') susceptibilities measured at several frequencies (Figure I-27 left). The observed differences consisted in I.34 different ratios between slowly and fast relaxing species in the disordered phase and in the crystalline one.

This system is one of the first examples of lanthanide double-decker complex showing two different relaxation processes and it is the only system in which the ratio between these responses has been shown to be reversibly modified by simple thermal treatments.

I.2.2. Other soft magnetic systems

Gels. An alternative way towards processability of materials is their incorporation into soft-matter phases, such as gels. Hydrogels and organogels result from the self-assembly of small molecules in water or organic liquids through specific noncovalent interactions. C 12 H 25 OC 3 H 6 . 87 The conversion from solution (at 273 K) to gel (at 298 K) is followed by a colour change from white to green, respectively. This property has been attributed to the conversion of the octahedral complex of cobalt in solution into a tetrahedral polymeric aggregate upon gelation. Iron(II) triazole-based coordination macromolecules are able also to gelate alkane solvents or toluene while maintaining their spin crossover related optical and magnetic switching properties. 80,88 The spin crossover gels are potentially useful for thermoreversible magneto-optical switches.

An attempt to form SMM containing physical gel was also reported. 89 Using a universal sol-gel approach, amorphous 3D coordination polymer gels were obtained via ligand substitution technique from Mn 12 -OAc and twelve different di-or tricarboxylic acids. Mn 12 cores were used as nodes while polycarboxylic acids as bridges in construction of the 3D coordination polymer gels. The final materials exhibited extraordinary porosity, containing ca. 95 % solvent by mass, they adopt the shape of the vessel and could be easily processed into thin-film xerogels. However, no investigations of magnetic properties were made and the goal to create processable magnetic physical gels still remains a crucial challenge.

Soluble systems. Solubility in conventional solvents should facilitate fabrication of films and coatings of magnetic materials, or even liquid phase processing.

I.35

Ligand functionalization with lipophilic chains of an Fe III SC complex, [Fe III (sal 2 -OR 2 trien)] (sal = salicylaldehyde, trien = triethylenetetramine and R = C 6 H 13 , C 8 H 17 or C 18 H 37 ), led to a self-assembly of the complexes in solution, preserving the spin crossover behavior. 90 When the alkyl chains are long enough (R = C 18 H 37 ), self-assembly of the complexes in solution forms discrete (microsize and submicrosize) aggregates.

Compared to the original spin crossover system, the functionalization allowed to enhance the cooperativity, in an originally poorly cooperating spin crossover Fe III complex, and to increase the SC characteristic temperature.

The substitution of acetate (OAc -) ligands in Mn 12 -OAc complex with various fatty acids not only improves the solubility of this magnetic system in various organic media but also leads to self-assembling into ordered vesicular aggregations. 91 The dynamic light scattering (DLS) investigations showed that the assemblies with a radius about 134 nm are present in chloroform. Whereas addition of methanol improves the stability of the assemblies, and spherical morphologies are thus obtained easily from an optimized mixed solvent (3 : 1 of chloroform : methanol in volume ratio). According to the authors of this report the formation of vesicular assemblies in various solvents is the first step on the route to organizing the Mn 12 complexes for their eventual ordered structures.

The presence or absence of solvate molecules and packing effects often restrain the clear observation of SC 92 or ET properties. 55f Investigation of these phenomena in solution (where the crystal packing effects are excluded) becomes obvious; as it highlights the molecular nature of the observed phenomena and gives information of substituent effects. 55f,g,92 The solid state properties of SMMs are relatively well understood, while it is not the case for the solution state. However, solution studies of the molecular magnetization dynamics of SMMs are crucial, 93 as the evolution of the magnetic properties of these systems from the solid state via solution to a surface have to be understood before being exploited. Frozen solution studies of the dynamic susceptibility allow the quantification of the contributions from solid state crystal packing effects to the molecular magnetic properties. 93a Nevertheless, solubility limitation of the SMM complexes and their low stability in solution hinder so far a broad exploration of this trend of research.

Therefore improving the solubilization of magnetic systems may help not only to process the material but also to study its intrinsic properties.

Throughout the sections I.1 and I.2 of this chapter, the basic concepts concerning single-molecule magnets (SMMs), electron transfer (ET) systems and soft hybrid molecule-based magnetic materials were I.36 presented. This basic information should help the reader to follow Chapters II -V that present the main results of this thesis.

I.3. Objectives of this research work

Even though the design of novel molecular magnetic materials and the study of their physical properties represent a major research interest in the field of molecular magnetism, it is also essential to devote a part of this effort to the shaping of existing molecular materials for future applications. Hybrid materials present high versatility in physico-chemical properties, as previously mentioned. Hybrids can be applied in many branches of materials science because they are simple to process and are amenable to design on the molecular scale. represent the most significant level of miniaturization for a magnet, they were later called single molecule magnets (SMM). 1 These molecular objects appear to be promising units for the development of future integrated nanodevices, such as molecular transistors, single-molecule spin-valves, quantum computers, ''nanosquid'' junctions, or magnetic random access memories. 2,3 The potential use of these molecular materials in a device, i.e. for high density information storage, would require a close control of their selforganization. The organization of SMMs has been reported on surfaces, in mesoporous materials or by coordination chemistry, 4 but another promising strategy towards their controlled organization and eventual use is the development of new functional hybrid molecular materials with self-organization abilities.

Metallomesogens (liquid crystalline materials incorporating metal ions) are very good candidates for such multifunctional materials, because they combine anisotropy related properties of liquid crystals with the original properties of metallic complexes. 5 Thus, the functionalization of organic ligands of known SMMs can help: (i) to isolate and protect the magnetic core by an organic shell in order to prevent interactions between objects; (ii) to orient, align, and organize magnetic units to keep easy axes in a quasi-same direction (see Chapter I), thanks to self-organization of the liquid crystalline phase; and (iii) to have fluidity and easy processability for shaping and further applications. Thus the formation of liquid crystalline mesophases may be considered as a first step towards the control of SMM self-organization into low-dimensional orders for their future incorporation into a functional device.

In this chapter our discussions are focused on functionalization of SMMs. The chapter starts with a short description of the Mn 12 -based SMM family chosen for our investigations, followed by previously reported attempts and the selected functionalization strategies to induce liquid crystalline organization. For a better understanding of functionalization effect over thermotropic and magnetic properties, the series of prepared Mn 12 -based complexes is divided into two groups according to the applied functionalization. The analytical characterizations as well as magnetic and thermotropic properties are then described and discussed II-2). In 1980, it was serendipitiously synthesized by Lis, while in 1993 for the first time it was discovered that Mn 12 -OAc shows slow relaxation of the magnetization at low temperature. 1 Under these conditions, if magnetized by an applied field it retains the magnetization for months and one single molecule functions as a nanoscale magnet. A deep and broad understanding of molecular magnetism became possible thanks to a systematic study of the family of Mn 12 SMMs. Besides its prominent magnetic properties, characterized by a high-spin ground state S T = 10, the presence of strong uniaxial magnetic anisotropy and a relatively high energy barrier (~74 K) 6 (beaten only by hexanuclear manganese complexes 7 and lanthanide-based SMMs 8 ), Mn 12 -OAc and its derivatives are versatile from the chemical point of view. Its synthesis is cheap, easy and, in addition, it is possible to modify easily the organic moiety that surrounds the inorganic core without significant modification of the magnetic properties.

Moreover, it is well known that chemical modification of Mn 12 -OAc allows a relative control of its solubility and redox potentials. 9 All these give a good enough motivation to work on a wise functionalization of ligands surrounding the Mn 12 core to get hybrid molecular materials, eventually with liquid crystalline phases.

II.2.2. Structure of Mn 12 -based complexes

The structure of Mn 12 -OAc (Figure II-2) and its derivatives can be described as a [Mn 12 (μ 3 -O) 12 ] roughly planar disk containing a central [Mn IV 4 O 4 ] 8+ cubane unit held within a non-planar ring of eight Mn III ions connected to the cube by eight μ 3 -O 2-ions and four bridging carboxylates perpendicular to the plane of the disk (two on each side). The peripheral eight Mn III ions are connected to each other by eight equatorial and four axial (two on each side of the disk) bridging carboxylate-based ligands. The Mn III ions of the outer ring are alternatively doubly bridged to one and singly bridged to two Mn IV ions (also called Mn III of type I II.47 12 Moreover there were isolated complexes with methanol molecules in axial positions instead of water. 13 Nevertheless, all these Mn 12 isomers preserved their SMM property.

As a consequence of the arrangement of Mn IV and Mn III ions in the oxo-metallic core, Mn 12 complexes have a high spin ground state S T = 10, which can be understood after the analysis of the magnetic core's structure. Assuming that the Mn IV (S = 3/2) of the central [Mn IV 4 O 4 ] 8+ cubane are coupled ferromagnetically and so are all of the Mn III (S = 2) from the external ring, the antiferromagnetic coupling between two subsets of Mn ions yields an S T = 10 value of spin ground state.

Most of the magnetic anisotropy of the ground state in Mn 12 -OAc comes from the magnetic anisotropy of outer eight Mn III ions. The coordination around each Mn III ion is actually not a regular octahedron, but is axially distorted, as two bonds are longer than the other four. In the case of high spin d 4 ion like Mn III (S = 2), the fourth electron can go into either one of the e g orbitals and a Jahn-Teller (JT) distortion arises, causing a tetragonal elongation of axial Mn III -O bonds (Figure II-4). 14 In Mn 12 complexes, the axial elongation of two trans bonds increases typically by 0.1-0.2 Å. 9 These JT elongation axes do not involve the Mn III -μ 3 -O 2- bonds, as they are the strongest bonds in the molecule. JT axes are all axially disposed, nearly perpendicularly to the plane of the [Mn 12 O 12 ] disk-like core (Figure II-5 left). The nearly-parallel alignment of the eight Mn III JT axes along the molecular z-axis is the origin of the significant uniaxial magnetic anisotropy that greatly influences the magnetic properties of Mn 12 -based systems.

In the structures of some Mn 12 complexes, it was observed that one Mn III JT elongation axis can be oriented equatorially rather than axially in regard to the Mn 12 planar core (Figure II-5 right). 15 This abnormal orientation is a local higher-energy situation stabilized by crystal packing effects and can be reoriented if the latter are relaxed. For example, the loss of co-crystallized solvent upon drying or the recrystallization from different solvents can convert to the isomer with normal orientation. 16 The ability of a molecule to exist in two distinct forms differing only in the relative orientation of one or more JT axes has been named Jahn-Teller isomerism. Thus, not only coordinated water molecules are responsible for the existence of different structural isomers of Mn 12 core, but also Mn III ions are. This structural distortion involving JT axes is at the origin of the low temperature (or fast relaxing) ac susceptibility signal, since the abnormal orientation of one JT axis alters the nearly-parallel alignment and, thus, influences the uniaxial magnetic anisotropy. ] complex with the PhSO 3 -ligands on the eight axial sites above and below the disk-like magnetic core, as expected for ligands originating from a stronger acid. 19 The non-carboxylate substituents have no significant effect on the electronic properties of the core as magnetic properties of the resulting compounds preserve the S T , D, and Δ eff values within the normal range for Mn 12 carboxylate complexes.

II.2.3. Chemistry of Mn 12 -based complexes

All of the above reported methodologies could be useful for introduction of different groups in order to design hybrid molecular materials, eventually with functional groups able to induce liquid crystalline properties.

II.3. Functionalization strategies towards liquid crystalline SMMs

Two different strategies can be applied on the Mn 12 -OAc complex in order to induce mesomorphic properties. These procedures consist in: (a) the functionalization of peripheral ligands with strongly lipophilic groups (long alkyl chains), or (b) the grafting of a mesomorphic promoter through a flexible aliphatic spacer. Both strategies allow improving interfaces and area compatibilities between moieties (rigid and soft) and were successfully used to obtain thermotropic mesophases with fullerenes, 20 octahedral coordination complexes, 21 polyoxometalates 22 or metallic clusters. 23 There are only a few examples of coordination compounds combining SMM behavior and mesomorphic properties induced by functionalization of peripheral ligands. 24,25 The first attempt to introduce liquid crystalline properties to a Mn 12 -based SMM was reported in 2008 by Terazzi et al. 24a Thus, by applying strategies mentioned above it was possible to modify original Mn 12 -OAc complex by the replacement of 16 acetate groups with gallic acid derivatives, which had three dodecyl or (cyanobiphenyloxy)undecyl chains (Scheme II-1). The magnetic properties were preserved, thermal stability (up to 425 K) was improved (if compared with the original Mn 12 -OAc starting material; < 330 K), and it was possible to induce smectic or cubic mesophases in the final compounds.

Scheme II-1. Chemical structures of gallic acid derivatives used by Terazzi et al. 24a The complex containing strongly lipophilic ligand (R 2 on Scheme II-1) such as tris(dodecyloxy)benzoic acid shows a 3D body-centered cubic mesophase 24a , that could be induced by the large number (48) of long alkyl chains. The large lipophilic density around the rigid magnetic core, formed functionalized molecules of almost spherical (spherulitic) shape that caused a cubic organization, since the systems of spherulitic shape tend to form cubic phases. 26 Besides the report of Terazzi et al.,24a Grafting a mesogenic promoter through a flexible aliphatic spacer to Mn 12 -based core is another strategy that was applied by Terazzi et al. 24a in functionalization of Mn 12 -based SMMs. The compound, in which all of the sixteen axial and equatorial acetates were replaced by the ligand with R 3 (Scheme II-1), displayed a supramolecular lamellar-like organization, with a short-ranged 2D intralayer hexagonal or square-like ordering of rigid magnetic cores above 313 K. 24a The main disadvantage of this system is the impossibility of reaching the clearing point without decomposing the compound, fact that may limit the further processing, like transposing the bulk ordering on surfaces. Later in 2011, the same team reported 24b

an attempt to decrease the clearing temperature by functionalization of benzoate-based ligands with twobranched dendritic structures (Scheme II-3) containing as terminal groups original and methylated Scheme II-4. Chemical structure of HL i (i = 5-6) ligands, based on modified 3,5-dihydroxybenzoic acid, for Mn 12 functionalization.

In the second part of our work focused on SMM functionalization (section II.5), we have chosen to functionalize 3,5-dihydroxybenzoic acid with (cyanobiphenyloxy)decyl and (biphenyloxy)decyl chains (Scheme II-4) and to study the corresponding Mn 12 complexes. With the aim to weaken the intermolecular interactions and, thus, to decrease the clearing point temperature the biphenyl moiety is used as the mesogenic terminal group (in the case of HL 6 ligand) and compared with cyanobiphenyl analogue (ligand HL 5 ). The ligand substitution is a reversible process. In order to shift the reaction equilibrium towards the formation of desired products, the reactions were carried out in the presence of an excess of the substituting ligands (HL i ), followed by the removal of AcOH as azeotrope mixture with toluene. In order to obtain the Mn 12 -OAc precursor, the procedure reported by Lis in 1980 was slightly modified (See Supporting information). Some of the benzoic acid derivatives (HL [1][2] ), used to replace the 16 acetate groups, were II.53

II.4. Functionalization of Mn

commercially available while others HL 3 and HL 4 were prepared by slightly modified published methods (see Supporting materials, Section II.7.1).

Several purification techniques were used to separate the substituted complexes from the reaction mixture and ligand excess. The compound with 4-methoxybenzoic (HL 1 ) acid was crystallized and purified by recrystallization. In our investigations, this compound served as a model system for this type of substitution. Since its structure was determined by single crystal X-ray diffraction, other analytical characterizations (including FT-IR and 1 H NMR spectroscopies) of this compound were very helpful in proving the ligand substitution and the purity of the synthesized hybrid compounds. Complexes substituted by long alkoxy chains were purified by size exclusion column chromatography, in order to separate the unreacted ligand or the partially substituted complex from the fully substituted Mn 12 -complexes. Bio-Beads S-X3 beads (neutral, porous styrene divinylbenzene copolymer beads) swelled with CH 2 Cl 2 were used as stationary phase. The ligand is more retained inside the column compared to the final product, but the separation was not easy to control. Thus, each small volume (~ 1-2 mL) of eluents was collected one by one, and then each of them was characterized by FT-IR to check the eventual presence of free ligand, acetic acid or unsubstituted acetate. This procedure was repeated twice. Three new compounds, with the general formula validated the chosen synthetic method for substitution of acetate groups of the original Mn 12 -OAc complex and allowed to check the bonding mode of the benzoate groups on the Mn 12 core. Crystals, suitable for X-ray diffraction experiment, were prepared by slow diethyl ether diffusion into dichloromethane solution of [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] in refrigerator at 277 K (4°C), and then re-crystallized by the same method. The compound crystallized in the triclinic P-1 space group with two Mn 12 molecules in the unit cell related by crystallographic center of inversion. Two water molecules and one disordered diethyl ether molecule are present in the unit cell. Structural parameters can be found in Table S.II-1.

[Mn 12 O 12 (L i ) 16 (H 2 O) x ] (i = 1-3, x = 3 or 4),
The structure of [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] (Figure II-6) is quite similar in many aspects to the previously reported Mn 12 -family complexes. 1,6,9,11a,18,24a There is a central [Mn IV 4 O 4 ] cube-like core surrounded by a nonplanar ring of eight outer Mn III ions, which are bridged and connected to the central cube via μ 3 -O 2-ions. The peripheral ligation is achieved by sixteen bridging 4-methoxybenzoate ligands (eight in equatorial, in-plane, and eight in axial, out-of-plane, positions) and only three terminal H 2 O molecules (rather than four usually found in Mn 12 complexes), two on Mn10 and one on Mn12, as it was previously observed in several other cases. 11,12 There are two types of Mn III ions inside the magnetic core. Those doubly bridged to one Mn IV are type I, and those singly bridged to two Mn IV are type II. The water molecules are bound to type II ions. As was mentioned in the paragraph II.2.2, the isomers of Mn 12 molecules can be differentiated depending on the II.54 way of water molecules coordination on type II Mn III ions. The present compound corresponds to the 2:0:1 form (see Figure II-3) as two of three water molecules are on the same Mn III ion and the third is on another Mn III ion, whereas the predominant isomer in precursor is the 1:1:1:1 form where one water molecule per type II ion is present. 1b,29 One of Mn III ions (Mn6) in [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] is consequently five-coordinated with square-pyramidal geometry (Figure II-7). This is certainly induced by a solid-state effect, and in solution, it is anticipated that all Mn III ions are six-coordinate as described previously. complexes. Thus, inside the packing, one molecule is hydrogen-bonded with two others and with a third one via π-staking. Such structural interactions have significant influence on magnetic properties and can lead to intermolecular antiferromagnetic interactions, especially when molecules have one or several reversed neighboring molecules. 31 As it was mentioned previously, the single crystal structure of the 4-methoxybenzoate substituted complex [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] confirmed our synthetic method to replace acetate ligands from the original

II.4.2.2. FT-IR spectroscopic analyses

FT-IR spectroscopy is a helpful tool to study the ligand exchange in Mn 12 -based complexes. It allows not only to confirm that the acetate ligand was replaced by another carboxylate-based ligand, but also to observe in the obtained compound if the substitution was complete or if it contains impurities such as free acetic acid or other carboxylate based ligand. Each of the benzoic acid based ligands shows very strong bands for C=O and C-O stretches that appear around 1670 -1687 cm -1 and 1254 -1276 cm -1 , respectively.

In the FT-IR spectra of the final compounds, the strong C=O band (1670 -1687 cm -1 ) from acidic ligand disappears, while two other bands around 1577 -1587 cm -1 and 1406 -1412 cm -1 appear (red curves in -5). These vibrations correspond to antisymmetrical and symmetrical OCO stretches of carboxylate ligands in their bidentate bridging mode. 32 This result suggests that the deprotonated ligand was covalently grafted onto the manganese core and no free ligand or acetic acid were present in the sample. It is worth noting that the precursor complex, [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ], (Figure S.II-6) shows C=O stretching band (~ 1700 cm -1 ) arising from free acetic acids existing in the channels formed by the packing of the complex. In the case of our compounds, the ligand itself is probably too bulky to stay in the inter-molecular void space. Also, the FT-IR spectra indicate that the removal of acetic acid as its toluene azeotrope was complete. Furthermore, no significant changes for C-O (≈ 1260 cm -1 ) and C-H (≈ 2900 cm -1 ) II. 56stretching bands indicate that the reacted ligand was well preserved without any major structural deformation after the reaction, except for the deprotonation.

II.4.2.3. 1 H NMR spectroscopic analyses

Due to the presence of paramagnetic centers, 1 H NMR spectroscopy may appear to be an inappropriate analysis technique because serious signal broadening and shifts may not allow proper peak attribution.

However, it is known that nonequivalency of the binding sites on original Mn 12 -OAc core causes apparition of three peaks at δ = 13.7, 40.9 and 47.8 ppm corresponding to methyl groups from coordinated acetic acid. 11a Thus, the absence of these peaks in the 1

II.4.2.4. Elemental and thermo-gravimetric analyses

The complete substitution in 16 positions by benzoate-based ligands was also demonstrated by elemental analysis which is in good agreement with the proposed formulas. The results from elemental analysis (Table II-1) were completed with the help of thermo-gravimetric analysis (TGA). From the thermo-gravimetric analyses (Figure S.II-9), it was determined that there were present cocrystallized water molecules in the inter-complex void space because of the first weight loss around 370 K.

The amount of absorbed water molecules in each complex was determined using both TG and elemental II.57

analysis. In the case of [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ], it was possible to detect the co-crystallized water molecules and not the lattice diethyl ether molecule. Thus, it is likely that the lattice diethyl ether molecule was lost easily as a function of exposure time in the air, because the crystals for TGA were kept in air outside mother liquor more time (about an hour) than the one chosen for X-ray analysis. Upon further heating above 420 K, all samples begin to decompose due to elimination of coordinated water, observed as a second weight lost.

Complex [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ], with the shortest alkoxy group on its ligand, shows good thermal stability up to 500 K compared to those with long alkoxy chains that begin to decompose above 420 K (Figure S.II-9). Above 550 K the decomposition leads ultimately to Mn 3 O 4 phase according to previous studies. 33

II.4.3. Magnetic measurements

Direct evidence that the Mn 12 core preserved its structural integrity and physical properties is given by magnetic measurements. Complexes were checked for SMM behavior by investigation of their magnetic properties using ac and dc modes.

The presence of frequency dependent out-of-phase ac signal (χ′′) is considered to be the characteristic feature of superparamagnetic-like or SMM behaviour. 1 An out-of-phase ac magnetic susceptibility signal is observed when the rate at which the magnetic moment of a molecule flips is close to the operating frequency of the ac magnetic field. The system is able to follow the ac field only when the magnetization relaxation is fast. Thus a non-zero frequency dependent χ′′ signal suggests the presence of slow relaxation of magnetization. From the ac susceptibility study, the effective energy barrier (Δ A ) for the relaxation of magnetization can be estimated. Moreover, this study can also give the information about the nature of the magnetization relaxation process. with g = 1.75 -1.93. This behavior is not surprising, since the substitution of the acetate from Mn 12 -OAc with benzoate, 1c propionate, 11a 3-chlorobenzoate 34 ligands or with some of functionalized benzoic acids, 24,27 leads to Mn 12 complexes with the ground state S T = 9, while low values of g factor were reported for Mn 12 complex with p-phenylbenzoic acid (g = 1.73) and for a mixed carboxylate complex with four acetates and twelve o-cyanobenzoates (g = 1.70). 35,36 The substitution of the ligand influences the intra-and intermolecular organizations, and thus the couplings, that lead to ground states different from parent compound.

A second plateau can be observed at lower temperatures, suggesting that there are two different relaxation processes. The origin of these two relaxations is attributed to Jahn-Teller (JT) isomerism. 16a,33 The ratio of each isomer present in the measured sample can be quantified by the difference of the two plateaus present in χ′T vs T plots. In the [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] sample, a ratio of 85% to 15 % of low temperature (LT) to high temperature (HT) JT isomers is present, while in other Mn 12 -functionalized samples reported herein the amount of HT species is predominant and varies between 60% and 80% (Table II-2). As the temperature decreases, there is a temperature (~8 K) below which the magnetic moment of the complex cannot stay in phase with the oscillating field for ac frequencies between 1 and 1500 Hz. In accordance, χ′′ signals appeared at the corresponding temperatures, indicating that the SMM behaviour of the samples was preserved. The maxima of the peaks in χ′′ signal shift to higher temperature as the operating frequency increases. In all of the four complexes, two peaks in out-of-phase susceptibility were observed in the temperature regions of 2-3 K and 4-7 K, confirming two different relaxation processes. Complex involving abnormally oriented Mn III JT axis that is almost perpendicular to the elongation axis of the other Mn III centres. This unusual JT orientation makes the molecule to have lower symmetry and magnetic anisotropy that is important for slow relaxation, and induces a significant rhombic (transverse) anisotropic term within the molecules. 16a,33 For the other complexes, for which no structure of the core is available, the presence of two strong peaks suggests the coexistence of different JT isomers within the sample, as already observed for many Mn 12 derivatives. 18,24,36 From the position of the peak maximum in the χ′′ versus T plot ( in the range 17 -20 cm 3 Kmol -1 that is in a good agreement with previously reported Mn 12 complexes. 1,2,11 The values of χT product slightly decrease between 280 and 120 K, suggesting a ferrimagnetic arrangement.

Δ eff /k B (K) τ 0 (s) Δ eff /k B (K) τ 0 (s) [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] 9
As expected, the χT products increase significantly below 50

II.60

Magnetization hysteresis loops at low temperatures are also an experimental manifestation of the SMM behavior. In all of the four samples, the field dependence of magnetization presents hysteresis loops with coercive fields varying from 0.03 to 0.83 T at 1.9 K with a field sweep rate of about 0.035 T•min -1

(Figure II-11). An interesting feature is the dramatic increase of the coercivity after functionalization with long alkyl chains (from 0.03 T for complex with HL 1 ligand to 0.53-0.83 T for complexes with functionalized HL 2 -HL 4 ligands), as a consequence of larger SR/FR species ratios (vide supra). The shape of The magnetic properties of the synthesized complexes are influenced by their molecular structure that is strongly dependent of the peripheral ligand substitution patterns. The relative proportion between SR and FR species seems to be dependent on the structure of functionalized benzoate ligand. For the series of Mn 12based SMMs substituted with (cyanobiphenyloxyundecyloxy)benzoates, it was reported that the position and the number of electron-donor alkoxy groups alter the electronic density of the ligand's aromatic ring, and consequently the Mn III ligand field, that plays a crucial role in stabilization of the JT elongation along the main symmetry axis of the Mn 12 -core. 24c Apparently, when the functionalization with electron-donor groups is done in 3-, 4-, and 5-positions the SR (HT) phase is found to be more stable, whereas the presence of cyanobiphenyloxyundecyloxy groups in 3-and 5-(meta-) positions or in 4-(para) contributes to an increase of the FR (LT) species fraction. Therefore the dependence of SR/FR ratio was reported as: (3-, 4-and 5substituted) > (3-and 5-substituted) > (4-substituted). 24c In the family of functionalized complexes, reported in this work, the SR/FR ratio follows the trend HL 3 (3-and 5-substituted) > HL 4 (3-, 4-and 5-substituted) > HL 2 (4-substituted). This dependence slightly differs from the one reported for the series of compounds with different (cyanobiphenyloxyundecyloxy)benzoates. 24c It appears that the difference between the trends in SR/FR ratios may come from the pulling and pressure effects of peripheral mesogens (in other words the packing) that distort the magnetic core and affect its properties. In order to identify which of the factors (donor-acceptor nature or the packing effect of the functional groups) has a greater influence on the SR/FR II.61

species ratio, the data obtained in this work was compared with previous reports concerning Mn 12 complexes with various benzoate based ligands. The model [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] compound with 4-methoxybenzoic acid, reported in this work, shows a high fraction of FR (LT) species ~ 85% that is in agreement with both trends reported above. Whereas the Mn 12 complex with 3,4,5-trimethoxybenzoic acid 24a shows almost 100 % of FR (LT) species, that is not at all in agreement with trends established above. Moreover, the complex of 4methylbenzoic acid, with a weaker donor group, shows a 15/85 (SR/FR) ratio when it is co-crystallized with one molecule of 4-methylbenzoic acid and a 95/5 (SR/FR) ratio when it is crystallized without the ligand molecule. 17,37 Other Mn 12 complexes do not show any influence of the functional group's donor nature as well. Thus, the complex with 4-t-butylbenzoic acid shows 100 % of FR (LT) species, 37 the ones with 4nonyloxy-and 4-hexadecyloxybenzoic acids show SR/FR ratios of 70/30 and 50/50, respectively, while the complex with 4-decyloxybenzoic acid shows 100 % of SR (HT) species. 27 The analysis of this data, indeed,

shows that the bulkiness of the functional groups influences the geometry of the core through the packing effects and intermolecular interactions inside the crystal or the mesophase. Therefore, we can state that the variation of the density of strongly lipophilic groups allows a better isolation of the magnetic cores and a certain control of magnetic properties through variation of the SR/FR ratio.

II.4.4. Thermotropic properties

The thermal and mesomorphic properties of synthesized complexes were first investigated by TGA, temperature dependent polarized optical microscopy (POM), and differential scanning calorimetry (DSC).

The nature of observed phase transitions was thereafter determined with small angle X-ray scattering (SAXS) investigations. Thermal data of phase transitions and thermodynamic parameters for functionalized shows an increase in viscosity at higher temperatures, and no birefringence was observed even after applying a shear distortion. These observations were not enough to determine the presence or formation of any mesophase in the investigated samples. The identification and clear assignment of mesophases and phase transitions was achieved by the combination of the results obtained using DSC and SAXS. Even though in microscopy we observe a kind of melting for this sample, DSC traces on first heating contain a very broad peak around 340 K, which is not present anymore on cooling and second heating ( All summarized indexation parameters at 303 K are shown in Table S.II-3. The reflections were indexed as ( 110), ( 111), ( 211), (221/300), ( 222), ( 321), ( 420), (432/520), (441/522), (532/611), (632/700), (662), and

II

(753). Examination of the general and systematic absences and the fail to meet the reflection conditions allowed to eliminate the face-centered (F) cubic (reflection conditions hkl: h + k = 2n, h + l = 2n, k + l = 2n), body-centered (I) cubic (reflection conditions hkl: h + k + l = 2n), and some of the primitive (P) cubic (reflection conditions 0kl: k + l = 2n, hhl: l = 2n, 00l: l = 2n or 4n) space groups. 38 The presence or absence of a center of symmetry cannot be detected on the basis of these experiments (Friedel's law), 39 therefore all non-centrosymmetric groups (with Laue classes 23, 432 and 43m ) are unlikely. This leaves only 3 Pm and 3 Pm m space groups. The exact nature of the space group is still undetermined, thus both of them a theoretically possible. Finally the latter 3 Pm m space group was retained due to its higher symmetry. The reciprocal spacing (q) of cubic phases is related to the lattice parameter (a) by: 110), ( 111), ( 200), ( 210), ( 211), ( 220), (300/221), ( 310), (311), and (321), which are consistent with a cubic lattice (see also Table S II-4) with a primitive network (P). The average unit cell parameter was calculated as a = 36.5 Å, from the slope of the 2π/q vs and a very weak scattering halo in the wide angle region, suggesting that the sample is close to the melting point. Also, if the enthalpy change of 205.9 kJ/mol is divided by 48 (the number of dodecyl chains) a value of 4 kJ/mol is obtained, that is in the range specific to the melting of dodecyl chains. 40 It is noticeable that the melting enthalpy of [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] is larger than that of [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ], this could be due to differences in the starting crystalline phase. diffuse scattering halo in wide angle region at 14 nm -1 , which corresponds to the lateral short range order of the molten alkyl chains. Further heating induces the organization of functionalized molecules such that 18 reflections were detected in the low angle range at 353 K (Figures II-13 right and II-14 left). Even though the diffraction pattern was very similar to the one obtained previously, 24a the reciprocal spacings for the observed reflections were not in the reported ratios and we were not able to confirm the reported cubic lattice. On cooling from 353 K to 313 K the diffraction pattern remained unchanged, at 293 K weak diffraction peaks disappeared while the intense ones broadened and at 283 K the system recovered its liquidlike weakly ordered phase (Figure II-14 center). Taking into the account that this surprising thermal behavior (the evolution of an apparent isotropic liquid into an organized phase between 333 and 353 K) was observed after first heating-cooling cycle, it was decided to check the reversibility by performing additional thermal II.66 cycles at longer detector distance, in order to better separate the reflections in the organized mesophase present at higher temperatures in low angle region. Indeed, upon second cooling from 353 K (Figure II-14 right) the analyzed sample of [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] complex showed similar thermal behavior, but with a shift of the liquid-like weakly ordered phase appearance to lower temperatures (between 283 and 274 K). This type of behavior termed "inverse melting" or "inverse freezing" has been observed in polymers, colloidal compounds, high-temperature superconductors, proteins, ultra-thin films, liquid crystals, and metallic alloys. 41 Different kinds of phase transitions are related to this phenomenon, in particular the solidification or appearance of an ordered phase from an isotropic liquid upon heating. In the field of liquid crystals, it is more precisely called the "re-entrance of the isotropic phase" and has been experimentally observed and theoretically explained. 41a Also, it is important to mention that no peaks were observed in DSC traces (Figure II-13 left) corresponding to this transition. This could simply mean that the transition is strongly second order. Therefore, it is difficult to give precisely the transition temperature because we detected it through the appearance of reflections in the X-ray diffraction profile, and this can only be done when a certain amount of material has already transited. Up to 20 reflections were identified after the detailed investigation of the low angle region in the temperature range between 303 and 393 K (Figure S. II-18). The diffuse halo in the wide angle region with a maximum at 14 nm -1 , corresponding to the molten alkyl chains, was present as well. But again it was impossible to index them with the reported cubic lattice. 24a density is considered 1 g/cm 3 ), but with an imperfect 3D arrangement of Mn 12 -based cores inside the mesophase. Similar imperfect 3D organization was reported, as well, in the case of some cone-shaped molecules and is caused by not uniformly spread aliphatic chains around the rigid core. 42 On third cooling from 393 K, the shape of the diffraction profiles started to change (Figure S.II-18 right). The re-entrance of the isotropic phase was observed again since the number of reflections decreased from 20 at 393 K to 9 at 303 K (indexation of observed reflections with an orthorhombic lattice was still possible and is summarized in Table S. II-7). A forth heating-cooling cycle was performed to probe the evolution of observed thermal behavior in [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] complex. Surprisingly, on heating a new diffraction pattern started to form between 353 and 373 K. Thus, 10 sharp reflections were well formed at 413 K (Figure S.II-19 left). The diffuse signal centered at 14 nm -1 of the molten alkyl chains was present in the wide-angle as well (Figures II-13 and II-15 both right). On cooling the system maintained the diffraction pattern until 274 K, showing no re-entrance of the isotropic phase. The reciprocal spacings were in the ratios 1 : 2 : 3 : 4 : 5 : 6 : 7 : 8 : 9 : 11 for the set of the observed 10 small angle reflections. The presence of reflection corresponding to 7 might be consistent with a 2D hexagonal arrangement, but no birefringence in POM and the presence of several other reflections as 2, 5, 6, 8, 11 allowed immediate elimination of such an organization. Thus, the considered ratio sequence must be 2 : 4 : 6 : 8 : 10 : 12 : 14 : 16 : 18 : 22 with reflections indexed as ( 110), ( 200), ( 211), ( 220), (310), ( 222), ( 321), ( 400 The presence of forbidden reflections and the exclusion of all non-centrosymmetric space groups allowed reducing the number of compatible body-centered cubic space groups to 3 Im and 3 Im m (which are with the same general conditions of extinction). Due to its higher symmetry the 3 Im m group was retained. The cubic lattice parameter is almost temperature-independent (a = 39.6 Å at 274 K and a = 40.

II.4.5. Description of the molecular packing inside the mesophase

The determination of the exact space group is very important for the possible description of molecules packing inside the mesophase. In the literature, thermotropic cubic phases are relatively rare and most of them possess 3 Pn m (bicontinuous), 3 Im m or 3 Ia d (that may be bicontinuous or tricontinuous) symmetries. 43,44 Due to the supramolecular nature of the reported cubic phases (including both lyotropic and II.68 thermotropic) the elementary cell of a cubic phase may contain up to several hundreds of molecules. The discontinuous (micellar) and multicontinuous models are generally used to describe the 3D organization inside the cubic phases with 3 Im m symmetry. The multicontinuous model's structure consists of multiple interwoven, infinite 3D networks (formed by the rigid parts of mesogenic molecules) separated from each other by lipophilic regions, formed by the molten aliphatic chains. 45 The discontinuous micellar model can be easily exemplified by dendrimers with strongly amphiphilic characters. 44h,i Such dendrimers are known to adopt conical shapes that can self-assemble into pseudospherical (spherulitic) supramolecular micelles. The structure of such self-assembled supramolecular micelles is composed of a polar (rigid) nucleus embedded in a non-polar soft aliphatic shell. Then, the resulting supramolecular micelles subsequently pack into three

dimensional lattices (Figure II-16a). 44h,i
For the cubic phases with symmetry there is no certain description of molecular arrangement inside the lattice, since there are only a few reported examples. 46 In the case of some imidazolium based Tshaped ionic liquid crystals, a supramolecular multicontinuous structure was proposed consisting of multiple This surprising difference may be explained by different functionalization positions inside the ligands used in the preparation of these two complexes. Thus, the benzoic ligand with a single chain in 4-(para-) position on the aromatic ring has a bigger average length and contributes to a little bit bigger lattice parameter than in the case with benzoic ligand with two dodecyl chains in 3-and 5-(meta-) positions. Further increase of the aliphatic density around the magnetic core, as in [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] complex, causes a very complex thermal history behavior that can be stabilized as a cubic phase with 3 Im m symmetry.

II.4.6. Summary of section II.4

A series of four functionalized Mn 12 -based complexes was synthesized and characterized. Various alkoxybenzoic acids have replaced acetate ligands in [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] by ligand substitution reaction, in order to organize single molecule magnets into a fluid liquid crystalline phase. In marked contrast with the original Mn 12 -Ac, which is unstable above 320 K, obtained compounds are thermally stable up to 420 K with the ligands bearing long chains and up to 470 K with p-methoxybenzoate ligand. From the ac magnetic measurements, the conservation of Mn 12 SMM property was confirmed. Our complexes exhibit JT isomerism as shown in ac out-of-phase susceptibility and X-ray crystal structure of the model compound.

Significant differences in magnetic relaxation were observed, resulting from the different ratio of JT isomers, which is itself dependent on the density of lipophilic chains, their positions inside ligand's aromatic ring and the electronic density donating-accepting nature of grafted groups of benzoate-based ligand. Our applied design strategy, consisting in grafting a various number of lipophilic chains around the rigid Mn 12 core, failed in obtaining products with less oriented and symmetric phases, but still resulted to be successful in the control of thermally induced 3D self-organization of hybrid molecules. Thus, by varying chain density around the rigid core, it is possible to stabilize crystalline cubic organization for [Mn 12 O 12 The organization inside the cubic mesophases has a molecular nature and is governed by the overall shape of rigid aromatic core, while aliphatic chains offer necessary fluid environment for self-organization. Similarly to magnetic properties, the organization and symmetry inside the observed cubic mesophases is dependent not only on the number of functional groups but on the substitution pattern inside benzoate ligands as well.

The specific molecular design in this case contributed to formation of hybrid molecular materials possessing several properties and allowed the tuning of parameters characteristic to the original Mn 12 -based core.

Obviously multiple possibilities exist for further development and optimization of magnetic and mesomorphic properties to design novel SMM complexes with liquid crystalline properties. The strategy based on the grafting of mesomorphic units through a flexible aliphatic spacer can be used (and is applied in the next section of this chapter), as it has been already successfully applied to obtain thermotropic mesophases in many cases including Mn 12 -based SMMs. 23,24 II.71

. Synthetic procedures and purification

In order to apply the functionalization strategy of grafting a mesogenic moiety through a flexible aliphatic chain to produce Mn 12 -based complexes with fluid mesophases at lower temperature, structurally related ligands HL 5 and HL 6 (Scheme II-4) were synthesized. The ligands were obtained in three steps (see Scheme II-5 and Supporting Materials for detailed protocols) following procedures reported for similar compounds. 23 Firstly, the precursors (1 CN and 1 H ) containing the mesogenic unit and the aliphatic spacer were prepared. Methyl esters MeL 5 and MeL 6 were obtained by Williamson etherification of precursor (1 CN or 1 H ) with methyl 3,5-dihydroxybenzoate in butanone. Finally, the methyl esters were hydrolysed into the corresponding carboxylic acids HL 5 and HL 6 .

Scheme II-5. Synthetic scheme for HL 5 and HL 6 ligands preparation.

The functionalization of the outer shell of the Mn 12 -based magnetic cores with (L 5 ) -or (L 6 ) -ligands was achieved by ligand-exchange reaction between the original Mn 12 -OAc with a large excess (28 eq.) of functionalized acid (HL 5 or HL 6 ) in CH 2 Cl 2 /toluene 1:1 mixture, similarly to the complexes modified with aliphatic chains (see sections II.2.3 and II.4.1 for details). The [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] complex is poorly soluble in toluene, thus in order to efficiently remove the resulting acetic acid by successive azeotropic distillations with toluene it is necessary, firstly, to dissolve the residue in CH 2 Cl 2 , then add toluene, stir for II.72 additional 3 hours, and finally evaporate to dryness. To displace favourably the equilibrium, the distillations must be repeated at least four times.

The crude functionalized molecules were purified by size exclusion column chromatography, using Bio-Beads S-X3 beads swelled with CH 2 Cl 2 (see section II.4.1) as stationary phase. The final products after drying in vacuum are vitreous dark brown solids at room temperature. The structural and physical properties of synthesized complexes were characterized by various techniques (vide infra).

II.5.2. Analytical characterization of the functionalized Mn 12 complexes

Similarly to the previously functionalized complexes (section II.4), the demonstration of complete substitution on the 16 coordination sites, as well as complete removal of the ligands taken in excess, relayed on the combination of several analytical techniques. Thus, the structural characterizations of functionalized complexes were performed in both solid state (FT-IR spectroscopy, elemental and thermo-gravimetric analyses) and in solution ( 1 H NMR spectroscopy).

II.5.2.1. FT-IR spectroscopic analyses

Once the substitution reaction was completed and the benzoic acid derived ligands were covalently grafted onto the Mn 12 core, the characteristic stretching vibration band of the carbonyl groups (~ 1676 cm -1 ) of the free benzoates disappeared (Figure S.II-7). The absence of this band and of another one around 1700 cm -1 indicates that no free ligand or acetate is present in the sample. Furthermore, no significant change of the C-O (≈ 1260 cm -1 ) and C-H (≈ 2900 cm -1 ) stretching bands indicate that the reacted ligand was preserved after the reaction, except for the deprotonation. In the case of [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] complex, the stretching vibration at 2224 cm -1 corresponding to CN group confirms the grafting of mesogenic cyanobiphenyl moiety (Figure S.II-7).

II.5.2.2. 1 H NMR spectroscopic analyses

The absence of the three peaks at δ = 13.7, 40.9 and 47.8 ppm (corresponding to methyl groups from coordinated acetic acid) in the 1 H NMR spectra of prepared complexes confirms complete substitution of acetate by functionalized benzoate-based ligands (Figure II-19).

II.73

The signals of the protons located close to the paramagnetic core are very broad to be observed, but the peripheral protons of the terminal mesogenic biphenyl and cyanobiphenyl groups are less affected (the signals are weakly broadened and their positions are slightly shifted). Thus, 1 H NMR spectroscopy can be used as a useful probe to determine whether the functionalized ligand is grafted or not (Figure II-20).

II.5.2.3. Elemental and thermo-gravimetric analyses

The complete substitution of 16 acetates from the parent Mn 12 -OAc complex by functionalized ligands was also demonstrated with the help of elemental and thermo-gravimetric (TG) analyses which were found to be in good agreement with proposed stoichiometries (Table II-4).

II.74

The presence of co-crystallized water molecules in the inter-complex void space was determined from the first mass loss between 360 and 400 K in TG analysis. Upon further heating above 420 K, the weight loss higher than 2 % suggests that both samples begin to decompose due to elimination of coordinated water and carboxylate-based organic ligands (Figure S.II-9). behavior by investigation of their magnetic properties that give direct evidence, that the dodecanuclear oxometallic core preserved its structural integrity.

To detect the slow relaxation of the magnetization, the temperature dependence of the in-phase (χ′)

and out-of-phase (χ′′) ac magnetic measurements were carried out in the 1.9-15 K range at several oscillation Constant values of χ′T = 37.7 cm Both peaks from χ′′ vs T plots shift towards higher temperatures when the frequency increases. The 3 a data obtained on second heating, g = glass, Sm = smectic phase, I = isotropic liquid; b the smectic nature of the observed mesophase was determined using SAXS technique; c this transition temperature was taken at first heating due to the decomposition on further heating cycles.

POM investigations of the functionalized [Mn 12 O 12 (L [5][6] ) 16 (H 2 O) 4 ] complexes revealed presence of birefringent optical textures (Figure II-24). Above room temperature the fluidity of the observed birefringent II.77 mesophases was increasing as the temperature was raised. The assignment of a certain mesophase to the textures observed on heating or on cooling from isotropic melt was not possible since no specific natural texture (schileren, fingerprint, focal-conic fan or others) could be obtained. However, it was possible to assign the transition to isotropic liquid due to the formation of fluid optically isotropic (black) areas. In the explain these high values by considering a large amount of attached mesogenic promoters. Therefore, if the enthalpy changes are divided by the number of attached mesogenic groups (32), values of 1.6 and 2.3 kJ/mol per mesogenic unit are then obtained. The values of enthalpy changes occurring at the clearing temperature are comparable with those observed for other biphenyl or cyanobiphenyl functionalized compounds. 23,24b,26,48 Indeed this result shows that the oxo-metallic core play a minor role in the clearing processes, which are governed by the organic moieties. Also, it is important to mention that functionalization with biphenyl moieties allowed to decrease the clearing temperature almost by 100 K in comparison with cyanobiphenyl analogue.

II.5.4.2. Small angle X-ray scattering (SAXS) investigations

More insights regarding the mesophase in both [Mn 12 O 12 (L [5][6] ) 16 (H 2 O) 4 ] systems were revealed with the help of temperature-dependent small and wide angle X-ray scattering investigations. The experiments were carried out on several heating-cooling cycles to allow thermal annealing of the mesophase in the studied samples. Thus, [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] with high clearing point temperature, was firstly heated from room temperature to 373 K and kept for several hours (with acquisition of several diffraction patterns in order to verify if the sample is stable at this temperature), then the sample was slowly cooled to 303 K and diffractograms were obtained on subsequent heating-cooling cycles in the 303-413 K temperature range (in order to avoid decomposition of the sample). Finally a set of diffraction images was acquired on heating the sample above the observed clearing point (445 K). Due to the stability of the [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] complex above clearing temperature, SAXS investigations were performed between 298 and 353 K on cooling and heating.

Diffraction patterns recorded for both [Mn 12 O 12 (L [5][6] ) 16 (H 2 O) 4 ] were quite similar. Each contained two sets of reflections in small angle region and a diffuse scattering halo in the wide angle region centered around 4.4 Å (or at 14.2-14.3 nm -1 showed as h ch in Figure II-26). The presence of diffuse halo corresponds to the lateral short range order of the molten chains and mesogenic (biphenyl or cyanobiphenyl) moieties, confirming the liquid crystalline nature of the mesophase.

II.79

The SAXS profiles recorded at different temperatures are all qualitatively equivalent and contain two sharp small angle reflections (and a third, very weak one, in the case of [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]) characteristic to a layered arrangement with reciprocal spacing in the 1:2 ( 1: 4 ) ratio for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and 1:2:3 Besides the reflections corresponding to the lamellar arrangement, several additional reflections of variable intensity were observed. These reflections were indexed in a hexagonal network as (hk0) = ( 110), ( 200), ( 210), (300) corresponding to the reciprocal spacings ratios 3 : 4 : 7 : 9 for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and as (hk0) = ( 200), ( 210), (300), ( 400), ( 410), ( 500 Even though, a few Mn 12 -based complexes bearing mesogenic promoters (cyanobiphenyl group) were also reported with smectic mesophases, the arrangement of magnetic cores was difficult to deduce from previous SAXS investigations. 24 Only the presence of broad diffusion along with two sharp interlayer reflections was reported in small angle region between 313 and 413 K. Thus, the square-like short-ranged 2D order of Mn 12 cores within the layer was deduced from molecular dynamic calculations. 24c In the case of the functionalized compounds reported in this work, it was possible to observe experimentally the short range hexagonal order of the magnetic units within the layers from SAXS profiles. Probably, the shortening of the length of the aliphatic spacer by one carbon allowed a certain stabilization of Mn 12 -cores arrangement within the lamellar structure induced by organic mesogens.

II.5.5. Summary of section II.5

By applying the functionalization strategy of grafting a mesogenic promoter via a flexible spacer, it was possible to obtain two new hybrid complexes possessing SMM and liquid crystalline properties. Two different mesogenic promoters were used in this work. Despite the bulkiness of the Mn 12 core, it was possible in both cases to induce thermotropic smectic phases. Moreover, grafting of biphenyl groups allowed decreasing the clearing point and, to the best of our knowledge, the reported [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] complex has the lowest clearing temperature among Mn 12 complexes functionalized with mesogenic promoters. Even though that both systems have similar structures and have the same packing inside the mesophase, the ratio of magnetic JT isomers is considerably different. This may be related to the presence of peripheral biphenyl moieties that induce a higher interdigitation comparing to cyanobiphenyl units. Thus, intermolecular interactions induced by cyanobiphenyl or biphenyl groups differently distort the magnetic core and, as result, affect differently their magnetic properties.

II.6. Conclusions and perspectives

Two different functionalization strategies were applied to obtain hybrid molecular materials. The functionalization with strongly lipophilic groups failed in obtaining products with less oriented and II.82 symmetric phases, but still resulted to be successful in the control of thermally induced 3D self-organization of the Mn 12 SMMs. Thus, by varying chain density around the rigid core, it was possible to stabilize crystalline cubic organization or thermotropic cubic mesophases of different symmetries. The organization inside the cubic mesophases has a molecular origin and is governed by the overall shape of rigid magnetic core, while aliphatic chains offer necessary fluid environment for self-oragnization. The second chosen strategy, functionalization by grafting a mesogenic promoter via a flexible spacer, showed that functionalized complexes form a hexagonal self-organization in layered smectic phase over a wide range of temperatures.

The rational choice of the mesogenic promoter allowed controlling the thermal properties of hybrid molecular materials. The specific molecular design in this case contributed to formation of hybrid molecular materials possessing several properties and allowed tuning of parameter characteristics to the original Mn 12based core. All synthesized complexes preserved their magnetic properties after the functionalization process. Significant difference in magnetic relaxation was observed, resulting from the different ratio of JT isomers, which appears to be influenced by the structure of the ligand and the intermolecular interactions it may induce.

To further develop these fundamental aspects and to optimize the magnetic and mesomorphic properties, it is necessary to design new liquid crystalline Mn 12 complexes or to adapt and extend functionalization strategies to other types of SMMs. In order to tailor more precisely the mesomorphic features functionalized Mn 12 complex should have a more anisotropic structure than a globular geometrical shape. For achieving this, it is necessary to introduce mesogenic ligands only in specific positions of the complex. Indeed, the 16 acetates of the Mn 12 complex are equitably distributed on equatorial and axial positions and several studies have shown that the acetates can be exchanged selectively depending on their position occupancies. 12 Based upon these studies, it is possible to functionalize the Mn 12 core regio-selectively with mesogenic ligands. The two functionalization modes (axial and equatorial) can be explored. By functionalizing the axial positions with mesogenic ligands, the complex can adopt a rod-like shape (calamitic) favoring the formation of smectic phases. In the second case (equatorial-mode functionalization), the molecular structure may adopt a disk-like form and lead, depending on intermolecular cohesion, to nematic or columnar mesophases. The regioselective substitution in Mn 12 derivatives is dependent on the acidity of carboxylic acids. Thus, in order to selectively modify original Mn 12 -OAc complex in its axial positions it will be necessary to obtain functionalized ligands with pK a lower than that of acetic acid as it is proposed in the scheme below:

Scheme II-6. Synthetic pathway for the preparation of mesogenic ligands with a low pKa.

II.83

To prepare the final complexes, a methodology described previously for the synthesis of heterogeneous Mn 12 must be employed. 12 The general procedure involves synthesis of homogeneous 11.09 (11.15). Selected FT-IR data (ATR, cm -1 ): 2916 (vs), 2847 (s), 1687 (vs), 1594 (s), 1420 (vs), 1301 (s), 1170 (s).

Preparation of 3,4,5-tris(dodecyloxy)benzoic acid (HL 4 ).

To a solution of methyl 3,4,5trihydroxybenzoate (4.00 g, 21.7 mmol) in 150 mL of acetone, anhydrous potassium carbonate (26.95 g, 195.1 mmol) was added. The mixture was heated to reflux and 1-bromododecane (19.20 g, 77.0 mmol) and a catalytic amount of potassium iodide were added. The reaction mixture was then refluxed for 4 days. The solvent was removed and CH 2 Cl 2 (200 mL) was added. The suspension was washed with water (3x100 mL).

The organic layer was dried over anhydrous sodium sulfate, filtrated and concentrated. The obtained white vitreous solid (MeL 4 ) was recrystallized from ethanol (500 mL), filtrated, washed with cold ethanol and dried under vacuum. Ethanol (150 mL) and potassium hydroxide (5.00 g, 89.1 mmol), dissolved previously in 20 mL of water, were added to MeL 4 . The solution was refluxed for 12 h, H 2 O (100 mL) was added and then acidified to pH = 1 with a 37 % HCl aqueous solution. Ethanol was removed and the resulting mixture was washed with CH 2 Cl 2 (3x100 mL). The organic layers were combined and dried over anhydrous sodium sulfate and concentrated. The product was dried for 24 h under vacuum and 10.45 g (15.48 mmol, 71%) of HL 4 were obtained, as a white vitreous solid. (0.10 g, 5.25•10 -2 mmol) in 20 mL of CH 2 Cl 2 /toluene 1:1, HL i (0.45 g (HL 2 ), 0.72 g (HL 3 ), or 0.98 g (HL 4 );

1.46 mmol, 28 eq.) was added. The mixture was stirred at 45°C during 24 h. The color of solution changed from brown to brownish black. The solution was filtrated and the solvent was then evaporated. Toluene (20 mL) was added to the residue, stirred for 3 hours, and then evaporated to dryness. This procedure was repeated three times more. 7.48 (7.53); N, 3.35 (3.49).

Preparation of 3,5-bis[(10-(4'-cyanobiphenyl-4-yloxy)decyl)oxy]benzoic acid (HL 5 ). To a solution of

MeL 5 (1.48 g, 1.77 mmol) in 100 mL of THF/EtOH 1:1 was added a solution of KOH (250 mg, 4.43 mmol) in water (5 mL). The mixture was stirred under reflux for 4 h, then H 2 O (250 mL) was added and acidified to pH = 1 with a 37 % HCl aqueous solution and stirred for 1 h. Ethanol and THF were removed and the resulting mixture was washed with CH 2 Cl 2 (3x100 mL). The organic layers were combined and dried over anhydrous sodium sulfate and concentrated. The crude product was purified by silica gel column chromatography (CH 2 Cl 2 /MeOH 100:0 and 98.5:2.5 at the end of the column). The final product was dried under vacuum over night to afford compound HL 5 as white solid in (0.99 g, 68%). After removal of the solvent, the mixture was purified by flash column chromatography on silica gel using pentane/dichloromethane (9:1) as eluent to yield 2.6 g (56%) of white crystals. 7.51 (7.58).

Preparation of methyl 3,5-bis((10-(biphenyl-4-yloxy)decyl)oxy)benzoate (MeL 6

I > 2σ(I), R 1 = Σ|(|F o | -|F c |)|/Σ|F o |. wR 2 = {Σ[w(F o 2 -F c 2 ) 2 ]/Σ[w(F o 2 ) 2 ]} 1/2 .
GoF (goodness of fit on

F 2 ) = {Σ[w(F o 2 -F c 2 ) 2 ]/ (n-p)} 1/2 ,
where n is the number of reflections and p is the total number of refined parameters.

Chapter II -Functionalization of Single Molecule Magnets: Towards Liquid Crystalline Phases II.89 q (obs) is the observed/measured reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, q (obs) is the observed/measured reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, II.99 q(obs) is the observed/measured reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, d(obs) and d(calc) are observed and calculated diffraction spacings: d(obs) = 2/q(obs), d(calc) = 1/√ , a, b and c are the lattice parameters of an orthorhombic cell, V is the volume of the orthorhombic cell unit (V = abc), Z is the number of complexes per orthorhombic cell, Z = (VNA)/Mw, where  is the density  1 g/cm 3 , NA is the Avogadro constant 6.0225  10 23 (note: 1 Å 3 = 1.0  10 -24 cm 3 ). q(obs) is the observed/measured reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, d(obs) and d(calc) are observed and calculated diffraction spacings: d(obs) = 2/q(obs), d(calc) = 1/√ , a, b and c are the lattice parameters of an orthorhombic cell, V is the volume of the orthorhombic cell unit (V = abc), Z is the number of complexes per orthorhombic cell, Z = (VNA)/Mw, where  is the density  1 g/cm 3 , NA is the Avogadro constant 6.0225  10 23 (note: 1 Å 3 = 1.0  10 -24 cm 3 ). q(obs) is the observed/measured reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, d(obs) and d(calc) are observed and calculated diffraction spacings: d(obs) = 2/q(obs), d(calc) = a/√ , a is the lattice parameter of the cubic phase, V is the volume of the cubic cell unit (V = a 3 ), Z is the number of complexes per cubic cell, Z = (VNA)/Mw, where  is the density  1 g/cm 3 , NA is the Avogadro constant 6.0225  10 23 (note: 1 Å 3 = 1.0  10 -24 cm 3 ).

II.98

II.100

II.102

II.103

III.1. Introduction

The chemistry of cyanido-bridged heterometallic materials with tunable optical and magnetic properties became a forefront topic after the report on the first photo-magnetic Co/Fe Prussian blue (PB) analogue by Hashimoto. 1 However, the high dimensionality of bimetallic assemblies of Prussian blue analogues and their non-stoichiometric lattice defects make difficult the systematic structure-property correlation investigations. Moreover, the reduced solubility of these inorganic 3D polymers limits their potential applications and requires elaboration of strategies for nanoparticles design, to increase their solubility and to create multi-functionalities at nano-scale. 2 Therefore an active cyanido-metallate research trend has been directed towards the synthesis and investigation of discrete PB analogues that have flexible molecular and electronic structures coupled with improved solubility. In the last two decades, several groups designed, isolated and studied molecular analogues (fragments of various geometry) of Prussian blue by rational selection of various capping ligands. 3,4,5,6 This approach has been extremely successful and diverse molecular architectures have been obtained with interesting magnetic properties such as single-molecule magnets, 4 spin crossover, 5 electron-transfer 6 and photo-induced magnetism. 7 Amongst these fragments, the most important complexes may be considered octanuclear cube 4b-d,6b,8 and tetranuclear square 3c,4f,5a,d,6c-g complexes, which mimic the properties and the structural units of the infinite Prussian blue network (their unit cell and a face of their cubic unit cell).

Tetranuclear cyanido-bridged {Fe 2 Co 2 } squares, 6c-g that display an electron transfer associated with a reversible switching of the magnetic and optical properties, are promising units for the development and Later in 2011 the same authors 6f observed a photoinduced electron transfer below 80 K for this molecular system as well. Thus, for these cyanido-bridged {Fe 2 Co 2 } squares, the nature of the capping ligands can tune magneto-optical bistability or stabilize paramagnetic or diamagnetic units that show no thermally and/or photoinduced intramolecular electron transfer process. 6c-g III.112 The study presented in this chapter focuses on the structural investigations of tetranuclear {Fe 2 Co 2 } square complexes (recently reported by our team) 6c,g,9 and on the perturbation of ET process in functionalized {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 } 2+ system in solid state through changing the counter-anion X -. While this kind of modification has been extensively studied for spin crossover complexes, 10 the effect of counter-anion exchange has not been investigated so far for {Fe 2 Co 2 }-based ET systems.

Herein, we present the synthesis and characterizations of four {Fe 2 Co 2 } square complexes: dimethylformamide. For compounds 1 and its functionalized analogue 2, chosen as model systems displaying thermally and photoinduced ET, additional single-crystal X-ray diffraction experiments under continuous white light irradiation were performed. While for complexes 2, 3 and 4, the counter-anion effect on the ET behavior is discussed on the basis of structural, optical and magnetic characterizations.

(1) -{[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }(OTf) 2 •3H 2 O•3DMF; 6c (2) -{[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 •H 2 O•2DMF; 6g,9 (

III.2. Synthesis and characterizations of molecular {Fe 2 Co 2 } squares

III.2.1. Synthetic strategy

The general synthetic approach adopted in the preparation of molecular Prussian blue analogues is to combine solvated metal cations with cyanido-metallate ions. In order to prevent polymerization, it is necessary to employ capping ligands, which will dictate the number and relative position of the available binding sites on the metal ions. A capped cyanido-metallate mononuclear building block may be added to a solution of solvated metal ions (or of a second capped metal ion precursor) resulting in the crystallization of cyanido-bridged molecular species whose topology is dependent upon the angle between the available binding sites of the cyanido-metallate ion. Considering cyanido-bridged square molecules, the capped metal ions should have at least two available cis binding sites to allow coordination with an angle of approximately 90° between the coordination vectors.

Scheme III-1. Schematic representation of the approach used to synthesize cyanido-bridged molecular {Fe 2 Co 2 } squares, charges on metal ions and counter-ions are omitted.

To generate a family of closely related complexes, a common solvent and synthetic strategy were used systematically, since it is known that synthetic variations can affect the crystal structure and the properties of the final compounds. In this work, similarly to previous reports, 6c,g the synthetic procedure consisted

III.2.2. FT-IR spectroscopic analyses

The stretching bands specific to cyanide groups ( ̅ CN ) are sensitive to their coordination mode (bridging or terminal) and the oxidation states of the coordinated metal ions. 12 Therefore FT-IR spectroscopy is an efficient technique to study the self-assembly of Fe III -based cyanide-containing building block with Co II ions during the formation of molecular cyanide-bridged {Fe 2 Co 2 } molecular squares. The FT-IR spectrum of the tricyanido-metallate precursor, (NEt 4 )[(Tp*)Fe(CN) 3 ], exhibits one strong ̅ CN stretching at 2115 cm -1 and another one of medium intensity corresponding to ̅ BH at 2549 cm -1 (Figure S.III-1). 6c,g,9 The stretching band of the cyanide groups in cyanido-bridged {Fe 2 Co 2 } molecular squares 1 -4 (Figure S.III-2) at room temperature is shifted to higher energies and splitted into two bands: one at 2146 -2151 cm -1 characteristic of bridging cyanides within {Fe III LS (μ-CN)Co II HS } units and a second one at 2116 -2128 cm -1 assigned to terminal cyanide. 6c,g,9 The energy of ̅ BH stretching band remained almost unchanged (2542 -2546 cm -1 ) suggesting that the electronic state of the Fe III ion remained the same as the tricyanido-metallate precursor. In the FT-IR spectra of the final compounds contain a strong C=O band (1663 -1672 cm -1 ) suggesting the presence of co-crystalized DMF molecules.

III.2.3. Structural investigations of molecular {Fe 2 Co 2 } squares

In order to study the photo-induced ET on single crystals and to describe the structure of photoinduced state, a set of single crystal X-ray diffraction (XRD) studies was carried out under light irradiation at low temperatures for complexes 1 and 2. Detailed studies of structural changes associated with thermally induced ET in complexes 1 and 2 were reported previously, 6c,g,9 but were repeated with the aim to compare them with the photo-induced state.

Single crystals of compounds 1 -4 were obtained using the same solvents and crystallization method. This technical aspect is important in establishing trends such as the counter-anion effect on the properties of cyanido-bridged {Fe 2 Co 2 } molecular squares 2 -4.

III.2.3.1. {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }[OTf] 2 •3H 2 O•3DMF (1)

Complex 1 is the first reported example of cyanide-bridged {Fe 2 Co 2 } molecular square that displays thermally and photo-induced ET associated reversible switching of magnetic and optical properties. 6c As the III.115 result of the intramolecular electron transfer and intercomplex elastic interactions, it exhibits a first-order phase transition with an 18 K thermal hysteresis loop (between 186 and 168 K, at a temperature sweep rate of 0.4 Kmin -1 ) between paramagnetic {Fe III 2 Co II 2 } and diamagnetic {Fe II 2 Co III 2 } states (Figure III-1 left). 6c Under white-light irradiation at 10 K, it is possible to convert the diamagnetic {Fe II 2 Co III 2 } phase into a paramagnetic {Fe III 2 Co II 2 } one. Also, it is important to mention that a metastable {Fe III 2 Co II 2 } paramagnetic phase can be also trapped by fast cooling from 298 K to 10 K (at a temperature sweep rate > 50 Kmin -1 ). The thermally quenched and photo-induced paramagnetic phases possess similar crystal structures and exhibit nearly identical magnetic data (Figure III-1 left). 6c However, the T values are consistently lower in quenched phase (Figure III-1 left). 6c The crystal structures have previously been reported for the high temperature phase at 230 K, for the low temperature phase at 120 K and for the quenched phase at 90 K (after flash cooling to 90 K from 300 K). 6c In order to describe the effects of the photo-and thermally induced ET transitions on the structure, in particular to compare photo-excited, thermally trapped and high temperature {Fe III 2 Co II 2 } structures, the structure of 1 was refined in four different states at five different temperatures (Figure III-2, Tables III-1 and S.III-1). To avoid crystal fracture that often arises from a first-order phase transition, the following thermal treatment protocol was employed: the crystal was quenched from 300 to 90 K (1 st crystal structure collected); warmed from 90 to 120 K (at 0.4 Kmin -1 ), and kept at 120 K for 24 h to allow complete and slow conversion to the thermodynamic diamagnetic phase (2 nd crystal structure collected); cooled to 84 K in the dark (3 rd crystal structure collected); irradiated with white light (P = 3 mW/cm 2 ) for 24 h at 83 K (4 th crystal structure collected); and, finally, the crystal was rapidly removed from the cryostream, kept for 1 h at room temperature and placed back under cryostream, preliminary warmed to 230 K (5 th crystal structure collected).

First experimental trials showed that the crystals of 1 loose crystallinity over time even if they are kept in oil at 90 K. Therefore a red prismatic crystal of 1 was placed into a sealed glass capillary with a small amount of III.116 mother liquor allowing to preserve the crystallinity and to perform the entire set of structural investigations on the same crystal.

Complex 1 crystallizes in the triclinic P-1 space group without any symmetry change after thermally and photo-induced ET associated transitions (Tables III-1 andS

III.117

As the result of the thermally and photo-induced ET transitions, differences were observed in unit cell parameters, in metal-to-ligand bond lengths and in coordination sphere deformation (especially around Co ions) (Tables III-1 andS III-1), that is the sum of the deviations from 90° of the 12 cis Φ angles in the coordination sphere of a metal ion (Co II in our case) to estimate the distortion. 13 The Co-N CN bonds are slightly shorter (~ parameter to 58°, originating from the change in N-Co-N angles, are consistent with the presence of a certain small amount of diamagnetic {Fe II LS (μ-CN)Co III LS } species. That may be indirectly confirmed by higher refinement factors of the crystal structure (R 1 = 7.17 %, wR 2 = 20.67 %) if compared to the structures of other phases observed on the same crystal of 1 (Table S.III-1).

The data collected at 120 K (after relaxation from quenched phase at 90 K) and at 84 K (after cooling from 120 K in the dark) ( S.III-1), respectively. The average bond lengths, the intermetallic distances and bond angles within the {Fe 2 (μ-CN) 4 Co 2 } core in this light-induced paramagnetic state show that the obtained crystal structure is virtually identical to the one obtained at 230 K (Tables III-1 andS The crystal structures have previously been reported for the high temperature phase at 250 K and for the low temperature phase at 120 K. 6g,9 To study the photo-induced ET on a single crystal of 2 and to determine the structure of the photoinduced state, detailed XRD studies were performed at various temperatures and under white light irradiation in collaboration with Prof. Eric Collet at Institut de Physique de Rennes (Université Rennes 1). Thus, crystal structures of 2 were refined in three different states and at two different temperatures (Figure III-4, Tables III-2 and S.III-2).

In the case of compound 2 it was possible to record the variation of the lattice parameters versus temperature (Figure III-3), since there the ET is not associated with a structural first order phase transition (that could fracture the crystal). S.III-2) show a distorted coordination sphere around the HS Co II ions, as judged by the Σ (N-Co-N) parameter of 58.5° (Table III-2 III-2). The N-Co-N bond angles show a slightly more distorted coordination sphere around Co (Table III-2

III.2.3.3. {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(ClO 4 ) 2 •2DMF (3)

X-ray crystal structure analysis of complex 3 was performed at 250 and 100 K (Tables III-2 andS 2), ( 3) and ( 4

III.2.3.5. Structural comparison of compounds (

III.2.4. Optical reflectivity studies

In order to check the optical properties of the obtained complexes and an eventual ET transfer induced color change, solid state reflectivity spectra were collected in the spectral range between 500 and 1000 nm, at different temperatures. As the reflectivity data for 1 and 2 are very similar and have already been reported, 6c,6g,9 only the temperature dependent reflectivity spectra of 3 and 4 in comparison with 2 will be described. properties. No thermally ET transfer was observed for 3 and 4, according to structural data. Thus, it is likely that on surface of the studied samples we can observe a thermally induced ET, but since the absorbance intensity did not increase as much as in 2 (or in 1 6c which has reflectivity spectra very similar to 2) we can state that it is likely only a surface effect.

III.2.5. Magnetic measurements

Both compounds 1 and 2 undergo thermally and photo-induced intramolecular electron transfer as the result of that the paramagnetic {Fe III LS (μ-CN)Co II HS } species can be converted reversibly into the diamagnetic {Fe II LS (μ-CN)Co III LS } species. The detailed discussions on magnetic properties of 1 and 2 were reported previously (Figure III-1). 6c,6g,9 Therefore, in this section, the magnetic properties of decreases to 3.8 cm 3 •K/mol at 1.8 K. At low temperatures (below 30 K), compound 4 has a different behaviour than 3. The χT product for 4 increases to a value of 6.9 cm 3 •K/mol at 7 K, then as the temperature is lowered it decreases to 5.7 cm 3 •K/mol at 1.8 K. The observed behavior in 4 suggests the presence of ferromagnetic interactions between Fe III and Co II ions with weak intermolecular antiferromagnetic interactions. Similar magnetic behavior corresponding to intramolecular ferromagnetic couplings between Fe III and Co II spin carriers was reported for complex 1 in its paramagnetic photo-induced phase (Figure III-1 left, green plot). The magnetic behavior observed for 3 and 4 is similar, but it seems that the intermolecular antiferromagnetic interactions are significantly stronger in 3. This is confirmed by the particularly short contacts between square complexes in 3 (See Section III.

III.3. Conclusions and perspectives

In Chapter III, four tetranuclear {Fe 2 Co 2 } molecular square compounds were investigated in terms of synthesis, structural characterizations, and magnetic properties. Previously reported 6c,6g,9 compounds 1 and 2 were synthesized and characterized with the aim to prove the photo-magnetic effect on single crystals and to study the nature of the photo-induced states. Another two new compounds, 3 and4 The photo-crystallographic investigations of compounds 1 and 2 showed that the light induced metastable paramagnetic phase is, as claimed and expected, composed of {Fe III LS (μ-CN)Co II HS } units. Moreover, in the case of complex 1 it was possible to determine that light irradiation is a more efficient way to obtain the metastable paramagnetic phase than the thermal quenching method. In the case of compound 2, it was possible to observe that the photo-induced paramagnetic phase is slightly different in structure than the thermodynamic one observed at high temperatures. perturbations influence several parameters such as the geometry of the {Fe 2 (μ-CN) 4 Co 2 } core, the intermolecular interactions, the solvation and the ET process. Therefore, we can conclude that electron transfer in cyanido-bridged molecular {Fe 2 Co 2 } squares is strongly dependent on the structural environment, which is influenced by the nature of counter-anion.

As a perspective towards a better understanding of counter-anion influence on ET process in cyanidobridged molecular {Fe 2 Co 2 } squares, it can be suggested the synthesis of new complexes with other counteranions, including diamagnetic ones like BF 2), ( 3) and ( 4) 4)

III.4.2. FT-IR spectroscopic analyses

Co(1)-N(7) 2.107(3) 2.098(3) Co(1)-N(4) 2.132(3) 2.125(3) Co(1)-N(3) 2.142(3) 2.149(3) Co(1)-N(2) 2.149(3) 2.155(3) Co(1)-N(1) 2.181(3) 2.175(3) Fe(1)-C(17)
1.914( 5) 1.917( 4) Fe(1)-C (18) 1.920( 5) 1.922( 5) Fe(1)-C (19) 1.927(4) 1.922( 4) Fe(1)-N (8) 1.997(3) 1.997(3) Fe(1)-N (11) 2.014(3) 2.001(3) Fe(1)-N (12) 2.009(3) 2.012 (3) Selected bond angles (°) 2)-Co(1)-N (12) 91.60( 19) N( 2)-Co(1)-N (13) 95.85( 19) N( 2 ) 2 ]/(np)} 1/2 , where n is the number of reflections and p is the total number of refined parameters. The main motivation of the design of hybrid magnetic materials is to obtain systems exhibiting novel synergetic effects and to preserve the original magnetic behavior (eventually improving it). But, besides the benefits, the functionalization of organic ligands of known molecular magnetic materials may significantly alter their original properties, as well. The functionalization of pyridine-based ligands used for the synthesis of {Fe 2 Co 2 } square complexes appears to be an efficient way to control the ET induced switching of magnetic and optical properties. Through the methyl functionalization of the bipyridine (bpy) ligands in 1 2,3 where diMebpy = 4,4'-dimethyl-2,2'-bipyridine, see also Chapter III) was obtained, preserving the magnetic and optical bistability driven by thermally and photoinduced intramolecular electron transfer process in solid state. This system with improved solubility showed the possibility to transfer the solid state properties of {Fe 2 Co 2 } complexes into a wide range of solvents. Moreover, a fine-tuning of the electron transfer and associated magnetic and optical properties of 2 in solution was demonstrated over a broad range of temperatures by simply adjusting the solvent composition and polarity. 2,3 Another square system with tert-butyl groups in 4-and 4'positions on bpy ligands {[(Tp*)Fe(CN) 3 ] 2 [Co(dtbbpy) 2 ] 2 }(PF 6 ) 2 •2MeOH (where dtbbpy = 4,4'-di-tert-butyl-2,2-bipyridine) exhibits electron transfer process in solid state and in butyronitrile solutions, that is induced by the temperature and/or by protonation. 4 In this chapter, two new functionalized complexes {[(Tp*)Fe(CN

III.4.4. Magnetic measurements

({[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }(OTf) 2 •2H 2 O•4DMF, 1 see also Chapter III), compound 2
({[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 •H 2 O•2DMF,
) 3 ] 2 [Co(diC6bpy) 2 ] 2 }[OTf] 2 •2DMF (5) and {[(Tp*)Fe(CN) 3 ] 2 [Co(diC13bpy) 2 ] 2 }[OTf] 2 •H 2 O•3CH 3 OH (6) (where diC6bpy = 4,4'-dihexyl-2,2- IV.144
bipyridine, and diC13bpy = 4,4'-ditridecyl-2,2-bipyridine) are investigated with the aim to probe the effect of the functionalization on intramolecular ET process in cyanido-bridged {Fe 2 Co 2 } molecular squares and to determine if their switching properties can be transferred in a general manner to different solvents. The full characterization of compounds 5 and 6 in solid state, and in solutions is described.

The preliminary work related to the synthesis of the complex 6 presented in this chapter was performed by Diana Siretanu during her PhD thesis (CRPP, 2008 -2011). 3 For a better understanding of functionalization effect in solid state and in solutions, the complete investigations of complexes 5 and 6 are presented together within this chapter.

IV.2. Synthesis and characterizations of functionalized molecular {Fe 2 Co 2 } squares in solid state

IV.2.1. Synthetic procedures and FT-IR characterizations

In order to apply the functionalization strategy (Figure IV-1), the diC6bpy and diC13bpy ligands were first synthesized. A large variety of functionalizations can be done on the bipyridine ligand. 5 The ligands with alkyl chains in 4-and 4'positions were synthesized from the 4,4'-dimethyl-2,2'-bipyridine following a reported protocol. 5a The synthesis generally proceeds in good yields when the alkylation is performed at low temperatures (193 K/-80°C) with the corresponding 1-bromo-or 1-iodoalkane in the presence of lithium diisopropylamide (Scheme IV-1). The 4,4'-dihexyl-2,2-bipyridine (diC6bpy) was purified by column chromatography, while the 4,4'-ditridecyl-2,2-bipyridine (diC13bpy) was purified by recrystallization from ethanol (see Section IV.5.1 for detailed protocols).

Scheme IV-1. Reaction scheme for the synthesis of 4,4-dialkyl-2,2-bipyridine ligands.

Functionalized complexes 5 and 6 were prepared using the same precursors (NEt 4 )[(Tp*)Fe(CN) 3 ] and Co(OTf) 2 , and the same synthetic procedure (see Sections III.2.1 and IV.5.1 for details) adapted from the previously reported syntheses of complexes 1 and 2. [1][2][3] Due to the presence of long alkyl chains, we had doubts that it was possible to obtain single-crystals of 5 and 6. Preliminary crystallization tests showed that addition of diethyl ether to the reaction mixture of 5 allows precipitation of red powder that shows IR stretching bands characteristic to cyanido-bridged {Fe 2 Co 2 } square complexes in the FT-IR spectra (Figure

IV-2 left

). [1][2][3][4] The strong stretching band at 2149 cm -1 corresponds to the cyanide groups in bridging mode 1- 3,6,7 and a weaker one at 2118 cm -1 is specific to the free terminal cyanide on the Fe sites. [1][2][3]8,9 Both signatures are observed in the FT-IR spectrum of the raw powder (Figure IV-2 left). The presence of the B-H stretching band from the Tp* ligand at 2554 cm -1 and the intense C-H bands at 2927 and 2856 cm -1 (from diC6bpy

IV.145 ligand) confirms, as well, the formation of functionalized complex 5. The red color of the raw powder of 5

and the high energies of CN and B-H bands suggest that the system is in the paramagnetic {Fe III LS (μ-CN)Co II HS } state. [1][2][3][4] Fortunately, it was possible to obtain single-crystals of 5 by layering the DMF solution of the reaction mixture with diethyl ether. The FT-IR spectrum of the dark red crystals of 5 is identical to the one recorded for the raw powder (Figure IV-2 left). Addition of diethyl ether to the reaction mixture of 6 in DMF did not allow precipitation of the raw powder of 6 and showed that the functionalized compound is very well soluble in ether. Therefore it was impossible to crystallize 6 using ether diffusion and layering techniques. Moreover, the red powder of 6 obtained after the evaporation of the reaction mixture is very well soluble at room temperature in most of the solvents. Nevertheless, crystals of 6 could be obtained by cooling down its methanol solution to 253 K (-20 °C). The crystals are well shaped green prisms, which re-dissolve at room temperature, forming a red solution, but re-crystalize again when temperature is decreased. The green crystals of 6 turn irreversibly red if they are kept outside the mother liquor. The FT-IR spectra of these two solid forms are represented in Figure IV-2 right. The spectrum of the red solid is very similar to the room temperature FT-IR spectra of compounds 1, 2 and 5. Only the stretching bands around 2900 cm -1 are stronger that is in good agreement with tridecyl chains present on the 4-and 4'-positions of the bipyridine. The FT-IR spectrum of green crystals shows bathochromic shifts of the peaks to lower energies. The B-H stretch shifts to 2505 cm -1 and CN stretches are present at 2097 and 2069 cm -1 that is characteristic to diamagnetic {Fe II LS (µ-CN)Co III LS } pairs in the green crystals of 6 (Figure IV-2 right), 1,3,[7][8][9] whereas the dried red crystals agree with the paramagnetic {Fe III LS (μ-CN)Co II HS } state of the square complex.

IV.2.2. Structural investigations of functionalized molecular {Fe 2 Co 2 } squares

In order to determine whether the functionalized complexes undergo ET process and the associated structural changes, single crystals of complexes 5 and 6 were subjected to temperature dependent X-ray diffraction analysis. Crystal structures of compound 5 were determined at 100 and 260 K. The crystals of 6 are green up to 280 K. At higher temperatures (around 310 K for a crystal in Paratone-N oil), the color turns IV.146

red, but unfortunately, the compound is not crystalline anymore. The heating causes the evaporation of the intermolecular solvent and the loss of the crystallinity. Therefore, the X-Ray analysis for red {Fe III 2 Co II 2 } state in 6 could not be performed. Green solvated crystals of 6 in {Fe II 2 Co III 2 } state can be obtained again only by recrystallization of the red complex in an appropriate solvent at low temperatures. ) 3 ] 2 [Co(diC6bpy) 2 ] 2 }[OTf] 2 •2DMF (5) X-ray crystal structure analysis of complex 5 was performed at 260 and 100 K (Tables IV-1 andS.IV- 1). No structural changes specific to thermally induced intramolecular ET were observed on cooling the dark red crystal of 5 from 260 to 100 K (Tables IV-1 andS.IV-1), suggesting that compound 5 is always in its paramagnetic {Fe III 2 Co II 2 } state. Single-crystal X-ray diffraction analysis showed that 5 crystallizes in the monoclinic P2 1 /c space group with a square-shaped tetranuclear core, in which Fe and Co ions are alternately bridged by cyanide ions, in the same way as in complexes 1 and 2. [1][2][3] The unit cell consists of a centro- Co II ion has a value of 80.4° at 100 K and is bigger than the ones measured for complexes 1 -4 (see Section III.2.3) in their paramagnetic state. V, Å 3 6991.7( 14) 7216( 4) 4785.4 (9) <Fe-C>, Å

IV.2.2.1. {[(Tp*)Fe(CN

IV.2.2.2. {[(Tp

*)Fe(CN) 3 ] 2 [Co(diC13bpy) 2 ] 2 }[OTf] 2 •H 2 O•3CH 3 OH (6)
In contrast to complex 5, compound 6 is very well soluble in diethyl ether, so it cannot be crystalized in the same manner. Surprisingly, crystals of 6 were obtained by cooling down its methanol solution to 253 K. The crystals of 6 are well shaped green prisms, which re-dissolve at room temperature to form a red solution. But they re-crystalize again when the temperature is decreased. Due to the high solubility at room temperature, the single crystal must be mounted very quickly on the X-ray diffractometer directly at 100 K, otherwise some parts of the crystal alter.

The crystal structure of {[(Tp*)Fe(CN 6) was solved in triclinic P1 space group. In contrast to the main parts of the tetranuclear core, some parts of the alkyl chains are not symmetrically equivalent through an inversion center, as it is in complexes 1 -5. Moreover, the resolution in P-1 space group gives unequivocal evidences that there is no center of inversion: in particular, some part of some disordered alkyl chains are too close to coexist with their image generated through the inversion center. Therefore, the crystal structure was solved in P1 space group using TWIN and BASF commands giving rise to a Flack parameter of 0.5 (racemic or twined crystal). IV-1), respectively, are in the agreement with the values measured for LS Fe II ions. [1][2][3][4][7][8][9] The heterometallic cyanido-bridged core has a distorted square shape, based on N(17)-Co(1)-N (13) and C(32)-Fe(1)-C (31) IV-1). The angular distortion parameter on the Co sites has a value of 38.5° that is a few degrees higher than the one measured in 2 in its diamagnetic {Fe II 2 Co III 2 } state at low temperatures. One bipyridine ligand has stretched alkyl chains pointing the c unit cell direction, while in the other one, the chains are bent following the direction of stretched chains. Therefore, all chains are arranged along the c axis of the unit cell (Figure S.IV-1). and are separated by about 5 Å. Because of the weak van-der-Waals interactions between the hydrocarbon chains, the thermal motion increases from the beginning to the end of the chains. Methanol molecules form hydrogen bonds 10 with terminal cyanide groups (Figure IV -7). Apparently, the H-bonds with methanol molecules are stabilizing the diamagnetic {Fe II 2 Co III 2 } state that may be confirmed by the irreversible color change from green to red and by the loss of crystallinity, when the solvated green crystals of 6 are dried under vacuum or heated above 290 K.

) 3 ] 2 [Co(diC13bpy) 2 ] 2 }[OTf] 2 •H 2 O•3CH 3 OH (

IV.2.3. Reflectivity studies

In order to check the optical properties of the obtained complexes, solid state reflectivity spectra were collected between 500 and 900 nm at different temperatures. For the complex 5 and the dry form of compound 6, the reflectivity spectrum at 280 K presents a strong absorption between 500 and 700 nm, and is similar to the spectra recorded for 1 and 2 in their paramagnetic state. [1][2][3] Absolute reflectivity at 550 nm is R 550 = 0.06 for 5, and R 800 = 0.03 for red solid form of 6 (Figure IV-8). Between 295 and 10 K, no significant changes are observed, denoting that the compounds do not undergo thermally-or photo-induced electron transfer.

IV.151

Solid state reflectivity measurements were also carried out for the solvated green crystals of 6. The spectra recorded between 280 and 10 K showed no significant changes. The sample remains strongly absorbent in the studied spectral range (Figure IV-9 left), similarly to complexes 1 and 2 in their diamagnetic states. [1][2][3] Moreover, no spectral changes were observed after the irradiation with white light for several hours, meaning that solvated crystals of 6 do not undergo photo-induced electron transfer.

IV.2.4. Magnetic measurements

Magnetic susceptibility measurements (Figure IV-10) confirmed that functionalized molecular squares 5 and 6 do not undergo reversible thermally induced ET and, thus, do not exhibit switching between red paramagnetic state and the green diamagnetic one. The functionalized complex 5 is paramagnetic in the 1.9 -270 K range, whereas complex 6 undergoes around 300 K an irreversible ET associated with a conversion between diamagnetic and paramagnetic states due to the loss of lattice MeOH molecules.

The temperature dependence of the χT product (where χ is the molar magnetic susceptibility per Fe III 2 Co II 2 unit) for complex 5 shows a value of 7.3 cm 3 •K/mol high temperatures (Figure IV-10 left). This value is slightly larger than the one expected for the sum of two magnetically isolated low-spin Fe III ions (S = 1/2 with g ≈ 2.6) and two high-spin Co II ions (S = 3/2, g ≈ 2.4). Upon cooling, χT slightly decreases to 7.0 cm 3 •K/mol at 130 K. After this minimum the χT increases smoothly until ca. 40 K, in line with a ferromagnetic coupling between the Co II and Fe III atoms through the cyanide bridges. At T < 40 K, χT sharply increases to reach maxima of ca. 11.3 cm 3 •K/mol at 7.0 K. Then, it further decreases with T because of zero-field splitting or/and antiferromagnetic intermolecular interactions to reach a value of 9.82 cm 3 •K/mol at 1.8 K. These features are consistent with ferromagnetic interactions inside the tetranuclear core of 5. Additional evidence for ferromagnetic exchange between Fe III and Co II ions in 5 is found in the fielddependence of magnetization plots. At 1.8 K, the magnetization value is monotonically increasing upon applying the external dc fields, without showing S-shape curves that could be indicative for the presence of antiferromagnetic interactions. Moreover the magnetization curves show an absence of magnetization saturation which is consistent with magnetic anisotropy of the Co II ions (Figure S.IV-5).

IV.3. Analytical characterizations of functionalized molecular {Fe 2 Co 2 } squares in diluted solutions

The structural, spectroscopic and magnetic studies showed that functionalized complexes 5 and 6 do not undergo reversible thermally or photo-induced ET and, thus, do not exhibit switching of magnetic and optical properties associated with an ET process in solid state. The methyl functionalized molecular square, 2, proved the possibility to transfer the ET driven switching properties of {Fe 2 Co 2 } complexes from solid state into solutions with a wide range of solvents.     insoluble at 298 K;"red" color at all temperatures suggesting the absence of electron-transfer; "redgreen" colors at 290 and 77 K suggesting an electron-transfer process in solution.

In order to determine if complexes 5 and 6 can exhibit ET properties and associated switching of their magnetic and optical properties in diluted solutions, the solubility of the functionalized complexes and the IV.153 properties of the resulting solutions have been studied. As it was expected, complexes 5 and 6 containing bipyridine ligands functionalized with long alkyl chains are soluble in a broader range of organic solvents in comparison to methyl functionalized complex 2 (Table IV-1). In contrast to their electronic stability in the solid state, both compounds undergo ET in solution, as demonstrated by the color change of solutions from red at room temperature to green when cooled down to 77 K (Figure IV-11). The observed color change occurs in almost all of the tested solvents (Table IV-2). For the solutions where a change of color was observed, it was assumed that the square configuration of {Fe 2 Co 2 } molecules remains intact in solution like for 1 and 2.

It is important to mention that complexes 2, 5 and 6 are well soluble in both DMSO and DMF, but no electron transfer associated color change was observed on cooling the red DMSO and DMF solutions. The color of these solutions changes very rapidly in time (from dark red to orange), the UV/Vis spectra are significantly altered. After a certain period of time (about several hours), crystalline materials of the starting materials are obtained, suggesting that these compounds decompose and/or disassemble in these solvents.

Moreover, there is no proof about the existence of the {Fe 2 Co 2 } molecules in solutions that stay red in the studied range of temperatures. For the solutions where a change of color was observed, it was assumed that the square configuration of {Fe 2 Co 2 } molecules remained intact.

IV.3.1. UV-Vis spectroscopic characterizations 5 and 6 in diluted solutions

The ET associated thermochromism observed in solutions of 5 and 6 has been quantitatively studied by temperature dependent UV-Vis spectroscopy in four different solvents (dichloromethane, ethylacetate, acetone and methanol). These measurements have been performed at Institute des Sciences Moléculaires (ISM, Université Bordeaux-1) in collaboration with Dr. D. M. Bassani.

The UV-Vis absorption spectra reported for 2 2,3 and those collected for 5 and 6 at room temperature (295 -300 K) in methanol are very similar (Figure IV-12). A broad absorption band is centered at 450 nm in addition to a small shoulder at ca. 540 nm (Figure IV-12 left). The intense absorption at 450 nm corresponds to a spin-and Laporte-allowed ligand-to-metal charge transfer (LMCT) transition, [2][3][4]7,11 while the shoulder at 560 nm may be attributed to a Co II →Fe III metal-to-metal charge transfer (MMCT) transition. [2][3][4]7 On cooling, the absorption band at 540 nm decreases progressively in intensity whereas a new absorption band, characteristic to the Fe II →Co III The analysis of obtained optical data suggests that only two species ({Fe III LS (μ-CN)Co II HS } and {Fe II LS (μ-CN)Co III LS }) are present in solution, due to the thermally induced ET, and thus the temperature dependence of paramagnetic or diamagnetic fractions fits well with the ideal solution model 12 for an equilibrium between two different states:

( ( ))
Eq. IV.1

where X is the {Fe(μ-CN)Co} fraction; X LT and X HT are low temperature {Fe II LS (μ-CN)Co III LS } and high temperature {Fe III LS (μ-CN)Co II HS } fractions, respectively; R is the ideal gas constant (8.314 JK -1 mol -1 ); ΔH is the enthalpy change; and ΔS is the entropy variation (ΔS = ΔH/T 1/2 ). The calculated ΔH, T 1/2 , and ΔS are summarized in Table IV-3. Table IV-3. Thermodynamic parameters obtained from the ideal solution modelling 12 of UV-Vis data for 5 and 6 in different solvents.

(5) ( 6) For both complexes 5 and 6 thermodynamic parameters change from one solvent to another, but the observed characteristic temperatures, T 1/2 , follow the same trend. The lowest T 1/2 is in dichloromethane, then in acetone, then ethyl acetate and the highest is in methanol. The characteristic temperatures for 6 are higher than those observed for 5. In the case of ethyl acetate, the values of ΔH or ΔS must be taken with caution as both complexes precipitated during the UV-Vis absorption measurements (Figure S.IV-4).

ΔH, kJ•mol -1 T 1/2 , K ΔS, J•K -1 •mol -1 ΔH, kJ•mol -1 T 1/2 , K ΔS, J•K -

IV.3.2. Magnetic properties of 5 and 6 in solutions

Additional evidences in support of intramolecular electron transfer in solutions of 5 and 6 were obtained from the magnetic susceptibility measurements (Figure IV-14).

Multiple preliminary tests allowed determination of optimal conditions for the magnetic studies in solutions. 3 In order to avoid a thermal hysteresis between cooling and heating modes due to the freezing/melting of the solvent (first order phase transition), temperature dependent measurements on cooling were performed until the freezing point. Such methodology allowed recording superposed cooling and heating curves (Figure S.IV-6). Above the melting or freezing temperature of the solvent, another thermal hysteresis related to precipitation (on cooling) or dissolution (on heating) of the solute (complex 5 or Regardless the complex functionalization and the solvent, the χT product at higher temperatures is essentially the same (around 6.6 cm 3 •K/mol) for solutions of 5 and 6 that is in agreement with the presence of paramagnetic {Fe III 2 Co II 2 } complexes. With decreasing temperature, the χT product rapidly decreases to small values (0.26 to 0.35 cm 3 •K/mol, Figure IV-14), consistent with the presence of diamagnetic {Fe II 2 Co III 2 } units as the majority species in solution. Practically the same value of the χT product at higher temperatures for both complexes may suggest that in solutions the tetranuclear cores adopt a similar structural geometry and there are no restraints induced by intermolecular interactions as in the solid state.

Table IV-4. Thermodynamic parameters obtained from the ideal solution modelling 12 of magnetic data for 5 and 6 in different solvents.

(5) ( 6) The χT vs T data (Figure IV-14) for 5 and 6 are also in good agreement with the ideal solution model 12 (see Eq. IV.1, where X is the χT product, X LT and X HT are χT values at low and at high temperatures, respectively). 12 Calculated thermodynamic parameters ΔH, T 1/2 , and ΔS are summarized in Table IV- (Table IV-3). These differences, especially between T 1/2 values determined spectrophotometrically and from magnetic measurements, are attributed to differences in the experimental set-ups and to the difficulty of precisely determining molar fractions by UV/Vis studies in cases where the experimentally available temperature range does not cover the entire temperature range of the ET. But as already observed by UV/Vis spectroscopy, the electron-transfer process is strongly influenced by the solvent nature. The characteristic temperatures (T 1/2 ) observed from magnetic studies follow the same trend as the ones determined from the absorption spectra. The lowest T 1/2 is in dichloromethane, then in acetone, and then ethyl acetate, whereas the highest were measured in methanol solutions (Table IV-4).

ΔH, kJ•mol -1 T 1/2 , K ΔS, J•K -1 •mol -1 ΔH, kJ•mol -1 T 1/2 , K ΔS, J•K -
The T 1/2 values evaluated and reported previously for complex 2 in various solvents 2 showed a strong dependence on solvent polarity (Table S.IV-3). 13 The ET characteristic temperature in solutions of 2 was increasing with the increase of the solvent polarity index. The trend observed for 5 and 6 in the studied solutions shows the same dependence except ethyl acetate solutions. The polarity index of ethyl acetate (4.3) is slightly lower than the polarity index of acetone (5.1). 13 Therefore it was expected that the T 1/2 value in acetone would have been higher than the one observed in ethyl acetate. But from both UV-Vis absorption and magnetic susceptibility measurements the T 1/2 determined for acetone solutions was lower than the ones observed in ethyl acetate solutions. On cooling solutions of 5 and 6 in ethyl acetate, besides thermally induced ET, a partial precipitation of both compounds occurs as well, so the process is more complex.

Therefore the characteristic temperature measured in ethyl acetate does not fit the polarity trend and must be taken into consideration with caution.

The increase in solvent polarity shifts T 1/2 to higher values for complexes 2, 5 and 6 in solutions. Since the observed change of optical and magnetic properties is associated with an intramolecular redox process it is possible that increasing solvent polarity shifts the reduction potentials (of Fe III ions) to more positive values and makes the reduction step more favorable due to a better solvation of tetranuclear cyanido-bridged core and stronger solute/solvent interactions (i.e. hydrogen-bonding and/or electron-pair donor/acceptor interactions), as have been reported for several organic systems exhibiting ET equilibria. 14 The comparison of magnetic susceptibility measurements of complexes 2, 2,3 5 and 6 solubilized in the same solvent (Figure IV-15) shows that the longer is the alkyl chain on the bipyridine ligand the higher is the characteristic temperature of the ET process. The alkyl chains are known as weak electron donating groups IV.158

and the chain length effect over the T 1/2 respects the following trend: methyl < hexyl < tridecyl. The magneto-optical bistability of the solubilized cyanido-bridged molecular {Fe 2 Co 2 } squares can be tuned by the solvent nature or/and by the nature of the functional groups attached to the capping ligands. 6) have been synthesized and their properties in solid state and in solutions have been described by structural, spectroscopic and magnetic methods.

IV.4. Conclusions and perspectives

In the solid state no reversible thermally or photo-induced ET was observed in the functionalized complexes 5 and 6. The functionalization of bipyridine ligands with long alkyl chains and the employed crystallization methods have a significant influence on packing and solvation inside in the crystal. Thus, the functionalized complex 5 is always paramagnetic in the solid state whereas the complex 6 is stabilized in diamagnetic state likely due to methanol molecules H-bonded to terminal cyanides on Fe sites. The presence of such H-bonds most likely induced a shift of the reduction potential on the iron centers in solid state.

Moreover, the electron density withdrawing effect is so strong that even under irradiation with white light, as confirmed by reflectivity studies, it has been impossible to convert the diamagnetic {Fe II LS (μ-CN)Co III LS } species into paramagnetic {Fe III LS (μ-CN)Co II HS } ones. On the other hand, the dried compound 6 is stable in the paramagnetic state, as could be observed from the structural, magnetic and optical reflectivity studies.

The studies performed in solutions confirmed that the functionalization with long alkyl chains improves the solubility of the molecular {Fe 2 Co 2 } squares in organic solvents. Moreover, these investigations revealed that functionalized squares undergo thermally-induced intramolecular ET in diluted solutions and, thus, exhibit switching of their magnetic and optical properties in solutions. Remarkably, a fine-tuning of the electron transfer associated magnetic and optical properties of 5 and 6 in solutions is observed over a broad range of temperature by simply adjusting the solvent polarity or the functionalized ligand of the Co site. This work demonstrates and generalizes the possibility to observe switchable magnetic and optical properties of the functionalized {Fe 2 Co 2 } complexes in solution, and opens up new possibilities to control molecular optical and magnetic properties by tuning the energy of the electron transfer process through variation of solvents and ligand functionalization.

The intramolecular ET between the cyanido-bridged Fe II/III and Co III/II centers is a redox process that occurs due to a small difference between the Fe and Co redox potentials. The spectroscopic and magnetic studies performed in solutions showed that solvent's polarity has a specific influence on the observed characteristic temperatures (T 1/2 ). Apparently, the medium formed by the solvents affects the redox potential of the metal ions, making either smaller or bigger the difference between the Fe and Co redox potentials.

Therefore studying the electrochemistry of the molecular squares and their precursors in various solvents may allow a better understanding the solvent and the bpy functionalization effect.

IV.159

The functionalization with hexyl and tridecyl aliphatic chains helped improving the solubility of the molecular squares. However, this work should not stop at this point and many strategies towards the preparation of hybrid systems exhibiting both ET and self-organization of liquid crystals can be developed.

For example, the series of functionalized bpy-based ligands showed in Scheme IV-2 may induce, indeed, liquid crystalline phases, while the functionalization with electron density donating groups stronger than alkyl chains may induce occurrence of ET around room temperature that can make this systems more attractive for future applications. 4,4'-dimethyl-2,2'-bipyridine (1.22 g, 6.64 mmol), 80 mL of diisopropylamine (1.86 mL, 13.29 mmol) solution in anhydrous THF were added. The resulting mixture was cooled to 193 K, and then 1.58 M solution of n-butyllithium in hexane (5.12 mL, 13.29 mmol) was added. After 30 min the obtained dark reddish brown solution was slowly combined with 10 mL of 1-bromoalkane (1-iodopentane 2.63 g or 1bromododecane 3. 30 g, 13.29 mmol) solution in anhydrous THF. The reaction mixture was kept at 193 K for 2 hours, and then it was allowed to warm slowly to room temperature and stirred for 2 more days. The reaction was quenched by adding methanol (100 mL) and then poured into 100 mL of water. The organic material was extracted with diethyl ether (3 x 100 mL), washed with water, dried over anhydrous sodium sulphate and concentrated under reduced pressure. The 4,4'-dihexyl-2,2'-bipyridine (diC6bpy) was purified by column chromatography on alumina with dichloromethane/hexane (1:10) mixture used as eluent, the yield was 1.33 g (62 %). 1 H NMR (400 MHz) in CDCl 3 (δ, ppm): 8.56 (d, J = 4.9 Hz, 1H), 8.24 (s, 1H), 7.12 (dd, J = 7.5, 4.0 Hz, 1H), 2.69 (t, J = 6.9 Hz, 2H), 1.76 -1.59 (m, 2H), 1.45 -1.15 (m, 7H), 0.88 (t, J = 6.9 Hz, 3H). Elemental analysis -Calc. (Found) for C 22 H 32 N 2 : C, 82.40 (82.29); H, 10.97 (11.01), N 6.63 (6.70). The 4,4'-ditridecyl-2,2'-bipyridine (diC13bpy) was recrystallized from ethanol at 253 K as a white microcrystalline product with the yield of 2.83 g (82.0 %). 1 H NMR (CD 3 Cl, 400 MHz, ppm): d = 8.54 (d, 2H, J = 5.0 Hz), 8.21 (s, 2H), 7.11 (dd, 2H, J = 6.0, 2.0 Hz), 2.67 (t, 4H, J = 7 Hz), 1.67 (m, 4H), 1.3-1.2 (m, 40H), 0.86 (t, 6H, J = 7.0 Hz). Anal. Calcd (Found) for C 36 H 60 N 2 : C, 83.01 (82.94); H, 11.61 (11.67); N, 5.38 (5.41). solution that was allowed to stir for 2 h. Subsequently, 4,4-R,R-2,2-bipyridine (for 5, 195 mg with R = C 6 H 13 ; for 6, 302 mg with R = C 13 H 27 , 0.6 mmol) was added and the mixture was stirred for 2 days and then filtered. Crystals of 5 suitable for X-ray diffraction were obtained by layering diethyl ether over the reaction mixture in DMF, the yield was 55 % (0.22 g). Elemental analysis -Calc. (Found) for ( 5) 50.70 (50.36); H, 5.00 (4.94); N, 18.00 (18.13). Selected FT-IR data (ATR, cm -1 ): 2953 (s), 2926 (s), 2856 (s), 2538 (m), 2148 (s), 2121 (w), 1679 (vs), 1613 (vs), 1263 (vs). Diethyl ether was added to the complex 6 (with R = C 13 H 27 ) and the solution was filtered to separate the insoluble impurities. The filtrate was dried under vacuum. The complex 6 crystallizes well from methanol solutions when cooled down to 253 K. The yield was 0. 4) N( 17)-Co(1)-N (20) 85.9(4) N( 13)-Co(1)- N(19) 93.2( 4) N( 19)-Co(1)-N (20) 82.5( 4) Co(1)-N( 13)-C (32) 168.7( 8) Co(1)-N( 17)-C (35) 165.6( 8) Fe(1)-C(33)-N (14) 176.5( 9) Fe( 1)-C(31)-N (15) 175.9( 9) Fe(1)-C(32)-N (13) 171.8( 9

Preparation of {[(

C 132 H 186 B 2 Co 2 F 6 Fe 2 N 28 O 8 S 2 ({[(Tp*)Fe(CN) 3 ] 2 [Co(diC6bpy) 2 ] 2 }(OTf) 2 •2DMF): C,

IV.5.4. Magnetic measurements in solid state

Introduction

The capping ligands on the Fe and Co centers in cyanido-bridged {Fe 2 Co 2 } squares have an important role in the occurrence of the intramolecular electron transfer associated with the switching of magnetic and optical properties. This phenomenon can be fulfilled only when the ancillary ligands on both metal centers match well a proper difference value between the redox potentials of the metal ions. Therefore by changing the capping ligands the redox potentials of the concerned sites are modified and, thus, it is possible to obtain systems showing thermally and/or photo-induced intramolecular ET 1,2,3,4 or to stabilize either the paramagnetic {Fe III 2 Co II 2 } 3b,5 or the diamagnetic {Fe II 2 Co III 2 } 3b,4b,6 state. In the case of the {[(Tp*)Fe(CN) 3 ] 2 [Co(4,4'-diRbpy) 2 ] 2 } 2+ systems (where Tp* = tris (3,5-dimethylpyrazol-1-yl)hydroborate, bpy = 2,2'-bipyridine and R = H, Me, t-Bu, C 6 H 13 or C 13 H 27 ) it was observed 1,3,4 (see also Chapter IV) that the functionalization of the bpy ligands on Co sites with weak electron donating alkyl groups contributes to an increase of ET characteristic temperature in solid state and/or in solutions. Based on these results and with the aim to determine if the functionalization of bpy ligands with stronger electron density donating groups The full set of structural, optical and magnetic characterizations of the prepared compounds 7, 8a and 8b are described herein in solid state and in solutions.

V.2.

Synthesis and characterizations of functionalized molecular {Fe 2 Co 2 } squares in solid state V.2.1. Synthetic procedures and FT-IR characterizations Functionalized complex 7 was synthesized and crystallized according to the procedure employed for preparation of complexes 1 -6 (see Sections III.2.1, III.4.1, IV.2.1, IV.5.1 and V.6.1), using (NEt 4 )[(Tp*)Fe(CN) 3 ] and Co(OTf) 2 as metal precursors (detailed preparation procedures can be found in Section III. 4.1). The dark red color and the high energy values of cyanide (bridging at 2154.1 cm -1 and terminal at 2121.7 cm -1 ) and B-H (at 2532.7 cm -1 ) stretching bands in the FT-IR spectrum of the obtained crystals (Figure V-1 left) suggest that at room temperature compound 7 is a cyanido-bridged square in its paramagnetic {Fe III 2 Co II 2 } state. 1,3-5c,6,7 In the case of compounds 8a and 8b, the synthetic strategy was modified with the aim to exchange the OTf -anion with PF 6 -. The choice of the hexafluorophosphate as counter-anion was not accidental. The same V.172 counter-anion was used in the preparation of the {[(Tp*)Fe(CN) 3 ] 2 [Co(di-t-Bubpy) 2 ] 2 }(PF 6 ) 2 •2MeOH (where di-t-Bubpy = 4,4'-di-tert-butyl-2,2-bipyridine) complex which exhibits both thermally and photo-induced ET in solid state, while in butyronitrile solutions it shows thermally and protonation induced ET. Moreover the results reported in Chapter III motivated us to perform also a study on the counter-anion effect of the systems functionalized with methoxy groups. of Co(OTf) 2 with 2 eq. of diMeObpy in methanol, followed by the addition of 1 eq. of (NEt 4 )[(Tp*)Fe(CN) 3 ] and 2 eq. of NBu 4 PF 6 . If the synthesis is performed in "wet" methanol, after the addition of NBu 4 PF 6 a green powder (8a) precipitates from red solution, whereas in the case of anhydrous methanol a red powder (8b) is formed. Both powders are poorly soluble in alcohols, especially methanol and ethanol.

The FT-IR analysis of the green powder (Figure V-1 right), showed that the prepared compound is likely a cyanido-bridged square in its diamagnetic {Fe II 2 Co III 2 } state, since the stretching bands corresponding to the bridging (2097 and 2076 cm -1 ) and terminal (2065 cm -1 ) cyanides are shifted to lower energies comparing to the ones observed in {Fe III 2 Co II 2 } cyanido-bridged squares. [1][2][3][4][5][6][7] The energy of the ̅ BH stretching band shifted to a lower energy (2518 cm -1 ) is confirming the presence of the Fe II ions. 7 During the FT-IR analysis a small amount of the sample turned irreversibly red after it was pressed inside the ATR accessory of the FT-IR spectrometer, therefore two stretching bands corresponding to the bridging and terminal cyanide ligands (at 2157 and 2125 cm -1 , respectively) in {Fe III 2 Co II 2 } cyanido-bridged squares were observed. The observed change of color may correspond to an irreversible ET and indirectly confirm that this green powder is a cyanido-bridged {Fe 2 Co 2 } complex. The dissolution of the green powder in acetonitrile affords a red solution. Green prismatic crystals of 8a were obtained by slow diffusion of diethyl ether into the red acetonitrile solution. The FT-IR analysis of the green crystals showed a spectrum similar to the one recorded in the case of the green powder (Figure V-1 right), but it is important to mention that the crystals of 8a are less stable to the mechanical stress and turn red after being crashed (for example with the ATR accessory of the FT-IR spectrometer). Therefore, the FT-IR spectrum of the green crystals of 8a corresponds to a mixture of cyanido-bridged {Fe III 2 Co II 2 } and {Fe II 2 Co III 2 } squares. Due to the poor stability of the 8a crystals under pressure, the ̅ CN and ̅ BH stretching absorptions bands of higher energies corresponding to {Fe III 2 Co II 2 } species are more intense than those at lower energies corresponding to the {Fe II 2 Co III 2 } units (Figure V-1 right). Moreover, it is important to mention that if a crystal of 8a is kept out of its mother liquor for about 5 -10 minutes, it turns irreversibly red.

The red prismatic crystals of 8b were obtained by ether diffusion in acetonitrile solution of the red powder obtained in anhydrous methanol. The FT-IR spectrum (Figure V-1 left) is in a good agreement with the red color suggesting the presence of the {Fe III 2 Co II 2 } units with stretching absorptions specific to bridging (2151.5 cm -1 ) and terminal (2119 cm -1 ) cyanides.

V.2.2. Structural investigations

V.2.2.1. {[(Tp*)Fe(CN) 3 ] 2 [Co(diMeObpy) 2 ] 2 }(OTf) 2 •4DMF (7)
Single crystal X-ray diffraction (XRD) studies of a dark red crystal of 7 were performed at 90 K after cooling it from 250 K with a 1 K•min -1 temperature sweep rate. Similar to complexes 1 -4 (see Chapter III), compound 7 crystalizes in the triclinic P-1 space group, but with a unit cell about twice bigger (Tables V-1 and S.V-I). The crystals of 8a are green at room temperature. If a crystal is kept outside its mother liquor or heated it turns rapidly red, and is not crystalline anymore. Apparently the evaporation of co-crystallized solvents causes a loss of crystallinity and an irreversible ET (as may be concluded from the color change). Therefore, the X-Ray analysis could not be performed for 8a at high temperatures. The re-crystallization of the dried red crystals of 8a by ether diffusion into acetonitrile solution affords red prismatic crystals which have the same structure as the crystals of 8b. Most likely the repetitive recrystallizations remove the lattice water molecules that stabilize diamagnetic state at room temperature.

X-ray structural analysis performed at 120 K in a sealed glass capillary (Tables V-1 and S.V-3)

showed that compound 8a crystallizes in the monoclinic C 2/c space group with four molecules in the unit cell. The structure of the central fragment is essentially similar to that of compound 1 1 in its diamagnetic The observed weak changes in reflectivity spectra on cooling mode, may be attributed to a thermally induced ET between Co II and Fe III ions in paramagnetic {Fe III 2 Co II 2 } molecular squares formed as defects on the surface of the green crystals of 8a during the preparation of the reflectivity sample. The presence of a small amount of paramagnetic red {Fe III 2 Co II 2 } defects can be also confirmed by the R 800 value that is slightly bigger than the one measured for R 550 , whereas for compounds 1 1 , 2 4 , and 6 (see also Chapters III and IV) in their diamagnetic green {Fe II 2 Co III 2 } state, the observed R 800 and R 550 are practically equal. On further cooling from 120 K, the surface of 8a becomes slightly more reflective in the near-infrared region. Upon heating from 10 K, the reflectivity above 700 nm continues to increase slowly until 70 K. This thermal behavior suggests that the compound might be indeed photo-active and that the diamagnetic green {Fe II 2 Co III 2 } species are partially photo-converted into paramagnetic {Fe III 2 Co II 2 } species below 70 K (even if the irradiation with white-light occurs only during the spectra acquisition). But the further increase of the temperature leads to a decrease of the reflectivity in the near-IR region and at 120 K the spectrum is similar to the one obtained at 120 K in cooling mode indicating the presence of only {Fe II LS (μ-CN)Co III LS } motifs in the square complex. The spectrum at 270 K is similar to the one obtained before the thermal cycle suggesting that the sample is stable under the experimental conditions and the defects induced during the sample preparation recover their high temperature {Fe III LS (μ-CN)Co II HS } state. An alternative way to visualize the thermal changes of the optical reflectivity spectra is presented in , R 800 is increasing up to 0.12 at 60 K, as expected if the photo-conversion is still active in this temperature domain. Above 60 K, the photo-conversion is less efficient than the thermal relaxation process and therefore the reflectivity at 800 nm decreases to recover at 120 K its 0.09 value. The heating V.181 above 120 K leads to a complete reversal of the changes observed on cooling such that at 270 K R 800 recovers its 0.14 value, which is identical in heating and cooling modes. In order to verify if compound 8a is indeed photo-active, the time evolution of the reflectivity, R 800 , during a white light irradiation was monitored after cooling the complex in the dark from 270 K to 10 K

V.186

Therefore additional single-crystal XRD investigations were performed with the aim to study the photoinduced effect on a single crystal of 8a and to determine the structure of the photo-induced state. The measurements were performed at 10 K before and after the irradiation with an 808 nm laser source ( 2 showed that over time (5 -15 min at room temperature and even after several hours at 120 K under N 2 cryoflux) the green crystals of 8a turn irreversibly red, if they are kept out of the mother liquor, likely due to the loss of the lattice solvent molecules. Cooling the sample from 250 to 2 K in the dark showed that 8a is diamagnetic in this temperature range (Figure V-18 right).

V.2.5.3. Magnetic and photo-magnetic properties of 8b

Magnetic susceptibility measurements were performed on the crystals of 8b kept in their mother liquor between 1.8 and 280 K (Figure V-19 left). The χT product below 200 K is nearly constant with a value of 0.36 cm 3 •K/mol, suggesting that 8b is in its diamagnetic electronic state, {Fe II 2 Co III 2 }. As the temperature was increased from 200 to 280 K, the χT product increased to a value of 8.5 cm 3 •K/mol, indicating the occurrence of the thermally induced ET (Figure V-19 left). The value of the χT product at 280 K is significantly higher than the expected theoretical value for two non-interacting LS Fe III (S = 1/2, g ≈ 2.6) and two HS Co II (S = 3/2) ions. The high value of the χT product may be explained by the presence of a significant orbital contribution to the magnetic moment of the HS Co II ions leading to a g factor around 2.8. 5a,8 On cooling from 280 K at 0.7 K/min, the measured χT product values traced the profile obtained in heating mode (with the same sweep rate of 0.7 K/min) with no thermal hysteresis suggesting that 8b displays a thermally reversible crossover between the paramagnetic and diamagnetic states associated with the intramolecular electron-transfer. This kind of thermal behavior is not surprising since no significant intermolecular interactions were observed in the crystal structure of 8b. (where dtbbpy = 4,4'-di-tert-butyl-2,2-bipyridine). 3 As it was mentioned previously, the crystals of 8b lose their crystallinity and their ET properties if they are kept outside their mother liquor more than 5 -10 min. In order to verify the effect of the lattice solvents over ET occurrence, the sample used for magnetic measurements, described above, was kept in vacuum overnight at room temperature and then re-measured in the magnetometer. Thus, as expected, the recorded χT vs T curve (Figure V-19 left, green dots) showed no thermally induced ET and confirmed that lattice solvent loss stabilizes the paramagnetic state. The temperature dependence of the χT product for dried crystals of 8b shows a value of 7.5 cm 3 •K/mol at high temperatures (Figure V-19 left, green dots). This value is slightly larger than the one expected for the sum of two magnetically isolated low-spin Fe III ions (S = 1/2) and two high-spin Co II ions (S = 3/2) but lower than the one observed for the solvated sample. This difference may be explained by the fact that after the solvent loss the structural geometry of the cyanidobridged {Fe 2 Co 2 } core has changed inducing a change of the intrinsic magnetic properties of the metal ions.

Upon cooling the dried sample, the χT product remained practically constant at 7.5 cm 3 •K/mol until 130 K.

Below 130 K, the χT increases smoothly down to 40 K, in line with the ferromagnetic coupling between the Co II and Fe III ions through the cyanide bridges. At T < 40 K, χT sharply increases to reach maxima of ca. 9.3 cm 3 •K/mol at 10.0 K. Then, it decreases with temperature, likely because of antiferromagnetic intermolecular interactions and/or the magnetic anisotropy of HS Co II ions, to reach a value of ca. 6.0 cm 3 •K/mol at 1.8 K. These features are consistent with significant ferromagnetic interactions inside the tetranuclear core. Additional evidence for ferromagnetic exchange between Fe III and Co II ions in the dried sample of 8b is found in the field-dependence of magnetization. At 1.8 K, the magnetization value is monotonically increasing upon applying the external dc fields, without showing S-shape curves that could be

V.3.

Analytical characterizations of the methoxy functionalized molecular {Fe 2 Co 2 } squares in diluted solutions

The characterizations of compounds 7, 8a and 8b, performed in solid state, have showed that the functionalization of the cyanido-bridged {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 } 2+ system with electron donating methoxy groups influences significantly the optical and magnetic properties of the final complexes, if combined rationally with other factors like the counter-anion and crystallization solvents. Identification of the right conditions that would contribute to the occurrence of the intramolecular ET is a difficult and time consuming task. It appears much easier to control the ET properties in molecular cyanido-bridged {Fe 2 Co 2 } squares when they are "placed" in solutions 3b,4 (see also Chapter IV). In solution the constraints induced by the crystal lattice packing are not present anymore and, thus, have no effect over the intrinsic molecular behavior.

The investigations of functionalized complexes 2, 4 5, and 6 4b (see also Chapters III and IV) in The spectra recorded at 300 K showed an intense broad absorption band at 450 nm with a shoulder around 540 nm and matched well with those reported for cyanido-bridged squares in their paramagnetic V.196 may suggest that in solutions the tetranuclear cores adopt a similar structural geometry and there are no intermolecular interactions as in the solid state. Table V-4. Thermodynamic parameters obtained from the ideal solution model 9 fit of the magnetic data for 7 and 8 in different solvents. The χT vs T data (Figure V-23) for 7 and 8 are also in good agreement with the ideal solution model (see Eq. IV.1 Chapter IV, where X is the χT product, X LT and X HT are χT values at low and at high temperatures, respectively). 9 Estimated thermodynamic parameters ΔH, T 1/2 , and ΔS are summarized in Table V-4. The thermodynamic parameters have values slightly different from those obtained from optical measurements (Table V-3). These differences, especially between T 1/2 values determined spectrophotometrically and from magnetic measurements, are attributed to differences in the experimental set-ups and to the difficulty of accurately determine the temperature during the UV/Vis studies (the temperature measurement is not directly close to the sample). But as already observed by UV-Vis absorption spectroscopy, the electron-transfer process is strongly influenced by the solvent nature. The characteristic temperatures (T 1/2 ) observed from magnetic studies follow the same solvent polarity dependence as the ones determined from the absorption spectra. Moreover the observed trend is basically the same as the one observed for compounds 2, 4 5, and 6 4b (see also Chapter IV) suggesting that thermally induced ET is an intrinsic property of the {[(Tp*)Fe(CN) 3 ] 2 [Co(diRbpy) 2 ] 2 } 2+ systems that can be easily observed in solutions due to the absence of the crystal lattice packing constraints or strong intermolecular interactions.

(7) (8) ΔH, kJ•mol -1 T 1/2 , K ΔS, J•K -1 •mol -1 ΔH, kJ•mol -1 T 1/2 , K ΔS, J•K -
Compounds 7 and 8 differ only by the counter-anions (OTf -in 7 and PF 6 -in 8); therefore it was surprising to observe that their solutions in the same solvent (i.e. dichloromethane) show a difference of ca.

14 K between the characteristic temperatures (Table V -4). On the other hand, the nature of the counter-anion has a significant influence on the solubility (7 is soluble in methanol, while 8 is not), on the dissociation V.197 equilibrium and, thus, on the interactions of the cationic core with solvent molecules. So, it is possible to preliminary conclude that counter-anions influence the occurrence of ET in cyanido-bridged molecular squares not only in solid state (see Chapter III), but also in solutions.

The functionalization of the bipyridine ligands on Co sites with strongly donating methoxy groups showed an important increase of the ET characteristic temperature. The donating effect of the methoxy groups is observed very well in dichloromethane, when the magnetic susceptibility plots are compared for 2 V.4.

Conclusions and perspectives

Three new tetranuclear cyanido-bridged {Fe 2 Co 2 } complexes functionalized with methoxy (MeO) groups in 4-and 4'positions of the bpy ligand on the Co sites were synthesized and their properties were investigated both in solid state and in solution, by structural, spectroscopic and magnetic methods.

The correlation between the structure and the properties of compounds The functionalization of these remarkable systems should be continued in many ways. The promising direction is to choose a functionalization of the bpy ligand with even stronger electron density donating groups like the dimethylamino group, -N(CH 3 ) 2 . On the other hand, it would be worth modulating also the functional groups on the capping ligand of the Fe site. It is for example possible to introduce additional donating methyl groups by exchanging the tris(3,5-dimethylpyrazol-1-yl)hydroborate (Tp*) ligand with tris (3,4,5-trimethylpyrazol-1-yl)hydroborate that can help reducing the difference between the redox potentials of the Fe and Co ions.

Finally, it is worth noting that although the {Fe 2 Co 2 } square complexes at the present time remain the simplest reported molecular analogs of the more sophisticated FeCo PBAs, they still raise many difficult questions regarding: the coupling between Fe and Co spin carriers in the photo-induced state; and the way ET occurs inside the tetranuclear core (does it occur in both pairs simultaneously or preferentially occurs through one Fe-CN-Co edge). Therefore our current efforts are being devoted to the study of the electronic states involved in the thermal and photo-switching using X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and ultra-fast time-resolved optical spectroscopy experiments (see next Section V.5 for preliminary results).

V.5.

Preliminary XAS and XMCD investigations

X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) investigations

were carried out at ID12 beamline of ESRF synchrotron (Grenoble, France) in collaboration with Marie-Anne Arrio (IMPMC), Philippe Sainctavit (IMPMC), Andrei Rogalev (ESRF) and Fabrice Wilhelm (ESRF).

XAS is an element and orbital selective spectroscopic tool for geometric and electronic structure elucidation. 12 XMCD is a difference spectrum of two x-ray absorption spectra (XAS) taken in a magnetic field, one taken with left circularly polarized x-rays, and one with right circularly polarized ones. 12 By closely analyzing the difference in the XMCD spectra, information can be obtained on the magnetic properties of the atom, such as its spin, orbital magnetic moment and relative orientations of the neighboring magnetic moments. 12,13 Recently, XAS and XMCD investigations were reported for a 1D {FeCo} n coordination polymer (XAS and XMCD at L edges) 14 and a cyanido-bridged molecular {Fe 2 Co 2 } square (XAS at K edges) 15 that exhibit thermally and light-induced ET. In both cases only the changes in electronic states associated with thermally induced ET were investigated. No XAS and XMCD investigations of the metastable light-induced paramagnetic state have been reported previously.

The X-ray absorption near-edge spectra (XANES, that is a type of X-ray absorption) and the dichroic ions. [1][2][3][4] Whereas previous reports on XMCD measurements at the Co and Fe K edges of the 3D Co/Fe PBAs have shown an antiferromagnetic coupling between Co II HS and Fe III LS ions and offered experimental evidence of the ferrimagnetic nature of the photo-induced metastable state in these 3D materials. 18 As has been mentioned in the previous paragraphs of this chapter, the loss of lattice solvent molecules influences significantly the occurrence of the ET in cyanido-bridged {Fe 2 Co 2 } squares. XAS investigations require ultra-high vacuum conditions. In order to avoid the solvent loss during XAS and XMCD studies the analyzed sample was kept in its mother liquor solution and covered with Kapton film. (a 1g ) → 4p (t 1u ) transition) and a weak pre-edge at 7712 eV (forbidden 1s (a 1g ) → 3d (t 2g , e g ) transition) at 298 K indicate the presence of Co II ions. 16 At 2 K, the Co K edge maximum is shifted by 3 eV to higher energy (7730 eV) compared to that at 298 K and fits the energy of the absorption maximum of Co III ions. 16,17 The high-energy shift of the EXAFS (extended X-ray absorption fine structure) oscillations observed above 7740 eV clearly demonstrates the presence of shorter Co III -to-ligand bonds in 8b at 2 K (as observed from temperature dependent crystallographic investigations of 8b, Table V-1, and reported for EXAFS oscillations in 3D Fe/Co PBAs). 18 The iron K edge spectra of 8b exhibit weaker changes upon thermally induced ET (Figure V-25 right). [16][17][18] The absorption maximum shifts from 7131 eV at 298 K to 7130 eV at 2 K. These weak changes are not surprising, given the similarity of the spin state (LS) and crystallographic data for the Fe coordination sphere obtained at 298 and 90 K for single crystals of 8b (Table V-1). The energy of the K edge absorption maximum (7131 eV) at 298 K indicates that the oxidation state of the iron ions is +3. 18 Moreover, the XAS analysis of the Fe III precursor, (NEt Photo-induced changes are not that obvious on Fe K edge. For a better observation, the light induced changes were analyzed as the difference between the room temperature (298 K) XANES and low temperature (2 K) XANES spectra of 8b before and during white light irradiation at both Co and Fe K edges (Figure V-28).

This data allowed estimating that approximately 70 %, of the studied sample, were converted from diamagnetic to paramagnetic state during ~ 18 h of irradiation (the conversion ratio was determined by monitoring the intensity at 7725 and 7131 eV in the Co and Fe K edges, respectively). The bulk of the V.203 sample was not fully photo-transformed probably due to the thickness of the sample and/or an insufficient penetration depth of the visible light. This problem is certainly in link with the position of the light source that was far away (60 cm) from the sample despite the use of a focalizing lens. As expected, no XMCD signal was observed for the diamagnetic state of 8b. The intense signal observed in the XMCD spectra corresponds to the allowed transition to the p-symmetry levels of the metal ions (~ 7725 eV at the Co K edge, and ~ 7130 eV at the Fe K edge). The other negative and very weak peak may correspond to forbidden transitions to the d levels of metal ions (~ 7712 eV at the Co K edge, and ~ 7112 eV at the Fe K edge). The presence of a dichroic signal at the K edges is due to the presence of spin-orbit coupling in the metallic p levels in the excited states (1s 1 3d n 4p 1 electronic configuration), and the exchange interaction of the p excited electrons with the 3d electrons. The small intensity of the signals (~ 0.7 % of the isotropic spectra at both edges corresponding to allowed 1s (a 1g ) → 4p (t 1u ) transitions) is due to the absence of spin-orbit coupling in the 1s level and the weakness of the spin-orbit coupling in the final state. 18,19 Weak analogous XMCD signals at K edges were also reported previously for 3D networks of Fe/Co PBA. 18 After the photo-excitation experiments, the reversibility of the thermally and photo-induced intramolecular ET was verified. The system recovered its initial state when it was warmed up to 300 K (Figure S.V-9). characterization of the ferromagnetic coupling between cobalt and iron ions in the photo-induced metastable state of compound 8b. To confirm these preliminary result and conclusions additional interpretations through theoretical calculations will be necessary.

Within the studies presented in this section, for the first time the photo-induced metastable state in a cyanido-bridged molecular {Fe 2 Co 2 } square was characterized using XAS and XMCD techniques.

Moreover, direct experimental evidences of the ferromagnetic nature of photo-induced metastable state were obtained.

V.6.2. Crystallographic data for 7, 8a and 8b V.211 In the field of molecule-based magnetism, functionalization of molecule-based molecular materials is one of directions that may, indeed, allow these molecular systems to be used in future applications. Our strategic approach in this thesis is to functionalize single-molecule magnets, and electron transfer systems towards more soluble systems and liquid crystal phases. We herein summarize the principal results obtained during this thesis work.

In the first chapter, general theoretical aspects of the single molecule magnets and electron-transfer systems were defined offering the readers the opportunity to understand easily the following Chapters II to V. Various examples of hybrid soft molecule-based magnetic materials have been described, in order to illustrate the interest of the coordination chemistry towards this class of multifunctional compounds that have promising properties to respond to technological demands. Regarding this, we attempted to get hybrid molecular soft systems in order to organize and shape magnetic complexes in controlled assemblies via a chemical control of their intrinsic magnetic properties via rational functionalization of the organic ligands.

In Chapter II, two different functionalization strategies were applied to obtain hybrid single molecule magnets. The functionalization with strongly lipophilic groups failed in obtaining products with less oriented and symmetric phases than the known cubic arrangements, but still resulted to be successful in the thermal control of the induced 3D self-organization of the Mn 12 SMMs. Thus, by varying chain density around the rigid core, it was possible to stabilize crystalline cubic organization or thermotropic cubic mesophases of different symmetries. The second chosen strategy oriented towards functionalization by grafting a mesogenic promoter via a flexible spacer, showed that functionalized complexes form a hexagonal self-organization in layered smectic phase over a wide range of temperatures. The rational choice of the mesogenic promoter allowed a control of the thermal properties of hybrid molecular materials. The specific molecular design in this case contributed to the formation of hybrid molecular materials possessing several properties and allowed tuning of parameter characteristics of the original Mn 12 -based core. As it was expected, all synthesized complexes preserved their magnetic properties after the functionalization process. Significant difference in magnetic relaxation was observed, resulting from the different ratio of JT isomers, which appears to be influenced by the structure of the ligand and the intermolecular interactions it may induce.

Chapters III -V were dedicated to functionalization of cyanido-bridged molecular squares that exhibit thermally or photo-induced ET, therefore these three chapters are summarized together herein. Even though it was not possible, at this point, to induce self-organizational properties within this class of molecule-based materials via functionalization of the bipyridine ligands, several other important achievements in this intriguing field were obtained. First of all, it was shown that the change of the counter-anion induces perturbations that influence several parameters such as the geometry of the {Fe 2 (μ-CN) 4 Co 2 } core, the intermolecular interactions, the solvation and, correspondingly, the occurrence of the ET process in solid state of these systems. The structural geometry of the cyanido-bridged core is strongly influenced by the employed functionalization as well. The bulkier is the structure of the functionalized ligand the more distorted is the shape of the final complex and, as a result, the occurrence of a thermally or light-induced ET becomes impossible. The nature of the functional groups allows modulation of the thermally and/or photoinduced ET in solid state and/or in solutions, and thus offers a tool to control this process. But, it is important to point out that in solid state the desired effect is achieved in combination with several other parameters (counter-anion, co-crystallized solvents, weaker or stronger intermolecular interactions) that influence the ET properties in cyanido-bridged {Fe 2 Co 2 } molecular squares. The studies performed in solutions confirmed that the functionalization with long alkyl chains improves the solubility of the molecular {Fe 2 Co 2 } squares in organic solvents. Moreover, these investigations revealed that functionalized squares (that show no evidence of ET in solid state) undergo thermally-induced intramolecular ET in diluted solutions and, thus, exhibit switching of their magnetic and optical properties in solutions. Remarkably, a fine-tuning of the electron transfer associated magnetic and optical properties in solutions is observed over a broad range of temperatures by simply adjusting the solvent polarity or the functionalized ligand of the Co site.

Furthermore, these studies suggest that thermally-induced ET is an intrinsic property of the {[(Tp*)Fe(CN) 3 ] 2 [Co(diRbpy) 2 ] 2 } 2+ systems that can be easily observed in solutions due to the absence of the crystal lattice packing effects or strong intermolecular interactions. Thus, the molecular nature of the observed process is highlighted.

As summarized in the previous paragraphs, the functionalization of SMM and ET complexes has been well achieved and offered an opportunity to observe and control the original properties within the designed materials. The work on functionalization of the remarkable systems, presented within this work, can be continued in many ways as has been detailed in conclusions part of each chapter. Liquid crystalline properties and improved solubility are not the only features that can be induced in a magnetic system. Via rational functionalization it is possible to control (tune) the physical properties via various effects of the functional groups; to induce affinity towards particular surfaces that may allow, as well, the control over the organization of the molecular materials in a device; or to induce biocompatibility that can make molecular magnetic materials promising candidates for targeted drug delivery or magnetic resonance imagining.

Therefore the foregoing investigations of hybrid (or rationally functionalized) magnetic systems would give a valuable contribution to various research fields. Dans ce domaine en pleine croissance qu'est la science des matériaux, les matériaux moléculaires connaissent un intérêt relativement récent et émergent, qui est motivé par le développement rapide de nouvelles technologies qui requièrent des matériaux toujours plus performants. 1,2 Les systèmes moléculaires présentent des avantages synthétiques par rapport aux matériaux traditionnels puisqu'ils utilisent des précurseurs bien définis avec une géométrie contrôlée et ne nécessitent pas de méthodes de production couteuses en énergie. Le choix rationnel des briques moléculaires permet un contrôle efficace de la structure finale des matériaux et ainsi de leurs propriétés physiques. 3,4 Le développement de matériaux construits à partir de motifs moléculaires, assemblés et conçus pour induire des propriétés magnétiques intéressantes et parfois préméditées, a donné naissance au domaine du magnétisme moléculaire.

A2: List of abbreviations

Les matériaux magnétiques ont été à l'origine d'avancées technologiques majeures devenues indispensables dans notre société. 5,6 Ces composants magnétiques sont omniprésents dans nos vie comme par exemple dans les générateurs, les moteurs, les haut-parleurs, les microphones, les commutateurs, les capteurs, les systèmes de stockage de l'informations, et bien d'autres. Jusque dans les années 80, les matériaux magnétiques les plus étudiés, étaient composés de métaux purs, d'alliages métalliques ou d'oxydes métalliques. 6 La préparation et traitement de tels matériaux nécessitent de hautes températures et des procédés métallurgiques très consommateurs en énergie. D'autre part l'optimisation des systèmes de stockage de l'information exige une continuelle miniaturisation de la taille des unités de stockage. Cependant, les matériaux magnétiques traditionnels présentent des limitations à cet égard puisqu'ils ne peuvent conserver leur aimantation au-dessous d'une certaine taille. 6,7 Par conséquent, le développement de nouveaux matériaux magnétiques basés sur des précurseurs moléculaires a connu un véritable essor ces dernières décennies. En comparaison avec les nanoparticules inorganiques qui sont intrinsèquement polydisperses en taille, les systèmes moléculaires sont parfaitement monodisperses en taille, volume, forme, et charge, puisque les précurseurs moléculaires sont tous identiques. Ils sont également solubles dans des solvants usuels et le caractère modulaire de ces matériaux offre l'opportunité d'affiner leurs propriétés. En outre, certains de ces matériaux moléculaires exhibent des propriétés magnétiques remarquables, qui sont rares ou inexistantes dans les matériaux inorganiques. Par exemple, des systèmes moléculaires présentant des phénomènes de conversion de spin (SC), 8 du photomagnétisme, 9 ou des comportements de molécule-aimant (SMM) 10 ou de chaîne-aimant (SCM) 11 ont été découverts. D'autre part, les matériaux moléculaires peuvent présenter des propriétés supplémentaires comme la transparence, le thermo-ou photochromisme, la solubilité, la conductivité/superconductivité, la faible densité, la biocompatibilité et/ou la recyclabilité offrant ainsi des solutions pratiques pour de nouveaux besoins technologiques. 12 Un autre avantage important de ces matériaux moléculaires est la possibilité de moduler finement les propriétés chimiques et physiques par le design des précurseurs moléculaires et en particulier par la modification des ligands.

Durant ces dernières années, des efforts de recherche considérables ont été dédiés à la synthèse de systèmes à l'échelle nanométrique, afin par exemple de réduire la taille des unités magnétiques dans les dispositifs de stockage de l'information. Les matériaux moléculaires pourraient répondre à cette attente en donnant l'opportunité de stocker l'information à l'échelle moléculaire en utilisant par exemple les prometteuses SMMs. 6,[8][9][10]13 Toutefois, si ces propriétés et applications potentielles sont souvent mises en valeur dans les études fondamentales, à ce jour, aucun de ces composants magnétiques n'a été exploité industriellement pour concevoir des dispositifs commerciaux. 14 Même si les propriétés magnétiques sont les principales motivations dans le domaine du magnétisme moléculaires, le développement de matériaux moléculaires multifonctionnels est une thématique de recherche également cruciale. En effet, la plupart des composés magnétiques sont produits sous forme de encombrants tels que des fullerènes, 15 des complexes de coordination octaédrique, 16 des polyoxométalates 17 ou des clusters métalliques. 18 Dans la première partie de notre travail, consacrée à la fonctionnalisation du [Mn 12 ] avec des groupes lipophiles, nous avons choisi les ligands de type benzoate HL i (schéma R-1) porteurs d'une, deux ou trois chaînes dodécane.

Schéma R-1. Structure chimique des ligands HL i (i=1-4) utilisés pour la fonctionnalisation du complexe Mn 12 .

Notre objectif a été de tester si la variation de la densité en groupements lipophiles permettrait de réduire l'encombrement globulaire des produits finaux, et à terme, si cela induirait des phases moins orientées et symétriques que les mésophases cubiques observées par Terazzi et coll. 19a Pour une meilleure compréhension et pour confirmer la complète substitution des groupements acétate par les ligands souhaités, nous avons également préparé un complexe modèle avec l'acide 4-methoxybenzoique comme ligand (HL 1 ).

Celui-ci a permis de valider définitivement notre approche synthétique notamment par l'élucidation de sa structure cristallographique. Le complexe [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] précédemment décrit par Terazzi et coll. 19a a été re-synthétisé afin de disposer d'une série complète de composés, nous permettant ainsi d'étudier l'influence de la densité lipophilique sur les propriétés magnétiques et mésomorphes.

Mn 12 -

 12 OAc can be described as a [Mn 12 (μ 3 -O) 12 ] roughly planar disk containing a central [Mn IV 4 O 4 ] 8+ cube surrounded by a non-planar ring of eight Mn III ions connected to the cube by eight μ 3 -O 2-ions and four bridging carboxylates perpendicular to the plane of the disk (two on each side Figure I-1 left).

Figure

  Figure I-1. Crystallographic structure of Mn 12 -OAc complex: (left) view along the crystal b-axis with illustration of the easy axis in Mn 12 -OAc complex situated along c axis of the unit cell; (right) view along the crystal c-axis with schematic representation of the spin alignments in the ground state of Mn 12 -OAc complex that gives an S T = 10 ground state. 6,23 Color scheme: Mn III dark green, Mn IV light green, O red, C grey. Hydrogens and lattice solvents are omitted for clarity.

  centers, to give S T = |(4 × 3/2) + (8 × -2)| = 10 (see Figure I-1 right). The second ingredient required for SMM behavior is an easy-axis anisotropy, which in Mn 12 -based complexes is brought by the nearly-parallel alignment of the eight Mn III JT axes along the molecular z-axis (Figure I-1 left, see also Chapter II, Section II.2).

Figure I- 2 . 2 .

 22 Figure I-2. Potential-energy diagram for Mn 12 -OAc SMM with an S T = 10 ground state experiencing axial zero-field splitting, Δ A = |D|S T 2 . At low temperature SMMs are blocked in the fundamental state m S = ±S T (±10 in case of Mn 12 -OAc). As S T = 10 for Mn 12 -OAc, the spin splits in total of 21 discrete energy levels (-S T ≤ m S ≤ S T ) with a strong uniaxial anisotropy energy barrier (Δ A ) of about 70 K (in Mn 12 -based complexes) and a doubly degenerated ground state m S = ±10 in zero field. The energy barrier (Δ A ) separates two lowest energy levels of m S = ±S T with largest |m S | values. As illustrated in Figure I-2, the spectra of energy can be modelled as a potential energy diagram, where one side corresponds to the spin pointing up and the other to the spin pointing down. There is no energy cost to reverse the direction of the total spin in m S = 0 state. The energy difference Δ A between the states of lowest energy and those of maximum energy is defined as: A | |

Figure I- 3 .

 3 Figure I-3. (left) Zeeman diagram as a function of the external field H z applied along the easy axis; (right) Double-well potential diagram in the presence of a longitudinal magnetic field (Zeeman effect)

″

  versus temperature at fixed frequency (Figure I-5 left), or/and the ″ versus frequency at fixed temperature. The temperature or frequency where ″ reaches its maximum value corresponds to the so-called I.13 blocking temperature (T B ), or blocking frequency (ν B ), respectively. The relaxation time (τ) can be deduced from the maxima of the ″(T) and/or ″(ν) curves at T B (solid symbols, Figure I-5 right) and/or ν B . For Mn 12 -OAc, the experimental thermally activated energy barrier is deduced from Arrhenius law (i.e. τ vs 1/T plot, Eq. I.8) as Δ A /k B = 62 K, with τ 0 = 2.1•10 -7 s (Figure I-5 right). To supplement these ac data and to ensure an accurate analysis over a wider range of temperatures, direct current (dc) magnetization decay data are often used.

Figure I- 5 .

 5 Figure I-5. (left) Imaginary component of the ac susceptibility of Mn 12 -OAc measured with frequency ranging from 10 Hz to 250 Hz. 25 (right) Temperature dependence of the relaxation time on a log scale of Mn 12 -OAc extracted from ac susceptibility data (solid symbols) in the frequency range 1-270 Hz and from time decay of the magnetization (empty symbols), the line is the fit to the Arrhenius law (Eq. I.8) with Δ A /k B = 62 K, with τ 0 = 2.1•10 -7 s. 11

Figure I- 6 .

 6 Figure I-6. Typical stepped hysteresis loops recorded on a single crystal of Mn 12 -OAc at 1.77, 2.10 and 2.64 K between 3 and -3 T with the field applied along the easy axis of magnetization with a sweep rate of 0.025 T/min. 14b

  1.3) the pairs of ±m S levels are no longer degenerate and the tunneling is suppressed (Figure I-7 left), except for values of H Z = nD/(gμ B ) (where n = 0, 1, 2...) when the levels are brought back into resonance again, corresponding to steps seen in hysteresis loops. When tunneling occurs between any m S levels, this provides a faster relaxation for the magnetization resulting in the effective energy barrier (Δ eff ) being smaller than the theoretical upper limit (Δ A ) calculated from S T 2 |D|.

Figure I- 7 .

 7 Figure I-7. Potential energy diagrams in the presence of different longitudinal fields. Resonant quantum tunneling, corresponding to vertical "steps" in hysteresis loops, occurs between m S levels on opposite sides of the barrier at specific applied fields (H z = nD•(gμ B ) -1 ).

K 0. 2

 2 Co 1.4 [Fe(CN) 6 ]•6.9H 2 O (Co/Fe Prussian blue analogue) 5b generated a great research interest due to the possibility to control magnetic properties at molecular level via temperature variation or light irradiation as the result of the metal-to-metal thermally and/or photo-induced ET occurrence. The 3D polymer contains {Fe(μ-CN)Co} motifs that exhibit a reversible metal-to-metal electron transfer converting = 3/2) units as a result of various external stimuli.

Figure I- 8 .

 8 Figure I-8. Schematic representation of the metal-to-metal electron transfer induced by external stimuli in the example of {Fe II LS (-CN)Co III LS } and {Fe III LS (-CN)Co II HS } pairs.

I. 1 . 2 . 2 .

 122 1.2 are dedicated to the occurrence of intramolecular metal-to-metal ET in various cyanido-bridged systems. Thermally and/or photo-induced ET in cyanido-bridged systems The Prussian blue analogues (PBA) of general formula, A j M k [M′(CN) 6 ] l •nH 2 O (A = alkali metal cation, M and M′ = transition metals, Figure I-9 left), are one of the most studied bimetallic coordination

Figure I- 9 .

 9 Figure I-9. (left) General structure of Prussian blue analogues, A j M k [M′(CN) 6 ] l •nH 2 O (A = alkali metal cation, yellow; M and M′ = transition metals, pink and green; C, black; N, blue; O, red; H, white). (right) Temperature dependence of the field-cooled magnetization at H = 5 G, before and after red light irradiation for K 0.2 Co 1.4 [Fe(CN) 6 ]•6.9H 2 O. 5b

  it was only in 2004 when was reported for the first time the occurrence of an intramolecular metal-to-metal ET in a molecular {Fe 2 Co 3 } complex, {[Co(tmphen) 2 ] 3 [Fe(CN) 6 ] 2 } (tmphen = 3,4,7,8-tetramethyl-1,10phenanthroline). 55a In this system of trigonal bipyramidal geometry a thermal metal-to-metal electron transfer occurs with the conversion behavior between two pairs of Fe/Co, resulting in one "extra" Co II LS ion not involved in this process, in the low temperature phase. Later in 2011, the photo-sensitivity of this compound was discovered and reported. 55i

Figure I- 10 .

 10 Figure I-10. (left) View of the molecular {Fe 4 Co 4 } cube at 250 K. Color scheme: Fe(III) green, Co(II) pink, C grey, O red, N light blue and B yellow. Lattice solvents, anions, and hydrogen atoms are omitted for clarity. (right) Temperature dependence of the χT product (with  defined as the molar magnetic susceptibility and equal to M/H; M = magnetization and H = external magnetic field) for the {Fe 4 Co 4 } cube before (black) and after (red) irradiation, and after thermal quenching (blue). 55b The first reported example of a molecular system that mimics not only the properties (thermal and photo-induced ET) but also the structure (the cubic cell, Figure I-9 left) of Prussian blues is the {[(pzTp)Fe III (CN) 3 ] 4 [Co II (pz 3 CCH 2 OH)] 4 }(ClO 4 ) 2 4H 2 O13DMF complex (pzTp = tetrapyrazolylborate, pz 3 CCH 2 OH = 2,2,2-tris(pyrazolyl)ethanol) (Figure I-10 left). 55b This compound is a stoichiometric molecular {Fe 4 Co 4 } cube with alternation of Co and Fe ions at each corner with cyanide linkers on each edge. The cubes are well isolated by the presence of bulky ligands around the metallic centers: tetrapyrazolylborate (pzTp) for the Fe and 2,2,2-tris(pyrazolyl)ethanol for the Co centers (Figure I-10 left).With the use of spectroscopic, magnetic, and crystallographic methods, a fully reversible intramolecular electron transfer, interconverting diamagnetic {Fe II LS (μ-CN)Co III LS } units into paramagnetic {Fe III LS (μ-CN)Co II HS } ones as a function of the temperature or light, has been demonstrated. This phenomenon is also

Figure I- 11 .

 11 Figure I-11. (left) ORTEP-type view of the cationic portion of {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 ]}[OTf] 2 •2H 2 O•4DMF at 230 K with thermal ellipsoids at 30% probability. Lattice solvent molecules, anions, and hydrogen atoms are omitted for clarity. Color scheme: C, black; Co, pink; Fe, orange; N, blue; B, gray. (right) T vs. T data for {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }[OTf] 2 •2H 2 O•4DMF (at 0.4 Kmin -1 ): in the dark (black, 0.1 T), after thermal quench (dark blue, 0.1 T), and after irradiation (green, 1 T, 3 mWcm -2 ).55c

Figure I- 12 .

 12 Figure I-12. (left) ORTEP-type view of the cationic part of {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 •H 2 O•2DMF at 120 K with thermal ellipsoids at 30% probability level. Lattice solvents, anions, and hydrogen atoms are omitted for clarity. Colour scheme: Fe, green; Co, pink; B, yellow; N, blue; C, black. (center) T vs. T data for {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 •H 2 O•2DMF: in the dark on cooling (blue, 0.5 T), and heating (red, 1 T) at 0.8 Kmin -1 ; after irradiation (green, 1 T, 2 mWcm -2 ) at 0.4 Kmin -1 . (right) Temperature dependence of the T product for {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 in methanol, acetonitirile, acetone, and dichloromethane solutions (0.4 Kmin -1 , 1 T; solid lines are fits to the Ideal solution model, 58 see section I.1.2.3). 55g

  2H 2 O•4DMF (where Tp* = tris(3,5-dimethylpyrazol-1-yl)hydroborate, bpy = 2,2-bipyridine, OTf = trifluoromethanesulfonate, and DMF = N,N-dimethylformamide, Figure I-11 left), exhibits a first-order phase transition with a thermal hysteresis of 18 K (at 0.4 Kmin -1 ) between 186 and 168 K, associated with an electron transfer process between paramagnetic {Fe. 55c It was not the case for the other square complexes that show a gradual conversion(Figure I-12 center), indicating the presence of significantly weaker elastic interactions between complexes in their crystal structures.55d-g,59a,b Solution studies of some of these tetranuclear {Fe 2 Co 2 } complexes offered an opportunity to observe their unique behaviors. Thus, through the methyl functionalization of the bpy ligands on{[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }[OTf] 2 Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }[OTf]2 (Figure I-12 left) (where diMebpy = 4,4'-methyl-2,2'-bipyridine), that proves the possibility to transfer the solid state properties of {Fe 2 Co 2 } complexes into a wide range of solvents. 55g Furthermore, electron transfer and associated magnetic and optical properties could be modulated by the solvent nature (Figure I-12 right). On the other hand, another square system, {[(Tp*)Fe(CN) 3 ] 2 [Co(dtbbpy) 2 ] 2 }(PF 6 ) 2 •2MeOH (where dtbbpy = 4,4'-di-tert-butyl-2,2-bipyridine), also maintained an electron transfer process in solution, that is induced not only thermally but also by protonation. 55e,f The effect of the protonation was explained by the modulation of redox potential on the metal sites. All above described {Fe 2 Co 2 } square complexes display a metastable {Fe III 2 Co II 2 } state by light irradiation at 10 K. Whereas for the complex of formula {[(pzTp)Fe(CN) 3 ] 2 [Co(bik) 2 ] 2 }(ClO 4 ) 2 •2H 2 O (where pzTp = tetrapyrazolylborate, bik = bis(1-methylimidazol-2-yl)ketone), there was observed for the first time a {Fe III 2 Co II 2 } ↔ {Fe III 2 Co II 2 } photo-reversibility under irradiation with light of different wavelengths at 20 K. Magnetic measurements and low-temperature single crystal X-ray diffraction experiments have shown that an intermolecular electron transfer converts the diamagnetic {Fe II 2 Co III 2 } units to the paramagnetic {Fe III 2 Co II 2 } metastable ones under 808 nm laser light irradiation, whereas the diamagnetic state is partially recovered under irradiation at 532 nm. 59a Also, in a very recent report on the {[(Tp)Fe(CN) 3 ] 2 [Co(diCOOEtbpy) 2 ] 2 }(ClO 4 ) 2 •2MeOH complex (where Tp = tri(pyrazol-1-yl)hydroborate and diCOOEtbpy = 4,4'-bis(ethoxycarbonyl)-2,2'-bipyridine) there has been shown the possibility to control the occurrence of the thermally induced ET via application of external hydrostatic pressure. 59b All the mentioned above reports give a good enough motivation to work on a wise functionalization of cyanido-bridged {Fe 2 Co 2 } molecular squares, since specific functional modification may open up a way forconstructing novel multifunctional materials with potential practical applications. Therefore these systems were chosen as targets for rational functionalization within the frame of this thesis.

For

  the paramagnetic A and diamagnetic B states, the T) A and T) B products are usually temperature independent. Therefore the investigation of the T = f(T) gives a direct access to x = f(T). Experimental observations have shown (Figures I-10 -I-12) that ET may occur as a conversion (Figure I-12) or a transition (Figure I-11). Therefore it was decided to describe the temperature dependence of x by both ideal and regular solution models, and finally to construct a phase diagram with the aim to understand the difference between a conversion and a transition. Electron transfer may be described as equilibrium between two states with a Gibbs energy change: positive below a temperature named T* and negative above it. When T = T* (), the following relationship is verified:

Figure I- 13 .

 13 Figure I-13. Temperature dependence of the paramagnetic molar fraction for different values of degeneracy, a, in ideal solution model.

  1×1) or g = 72, which gives a = ln72 = 4.28. According to the simulation plot in Figure I-13 the most similar case is for a = 4, where x min tends to 0.90 at T/T* > 2. In real systems, the limit of x is, indeed, very close to 1. The entropy change in {FeCo}-based ET systems can be experimentally measured by calorimetric methods or deduced from optical

Figure I- 14 .

 14 Figure I-14. (left) Temperature dependence of the paramagnetic molar fraction x for different values of W/(k B T*) at fixed a = 4, in a regular solution model. (right) Thermal hysteresis when a = 4 and W/(k B T*) = 3, ccooling mode, hheating mode.

  so is increasing when T/T* increases, and the corresponding curves are more and more steep for higher values of W/(k B T*), but possess the same inflection point given by T/T* = 1 and = 0.5. This phenomenon is a conversion. W/(k B T*) < 2,there is a vertical gap of at T/T* = 1, and the corresponding curve is abrupt. This gives a first order transition at T = T* = T C and the spin transition temperature is given by the relationship W/(k B T*) = T C . This phenomenon is a transition at the critical point.  W/(k B T*) > 2 the curve has an "S" shape, and for T/T* = 1, not only the = 0.5 solution is possible, but the Gibbs energy adopts two other local minimum values. A transition corresponding to a first order transition is observed then.

Figure I- 15 .

 15 Figure I-15. Phase diagram for the regular solution model with W > 0 in the plane (2k B T*/W, T/T*).

  Figure I-15) and a transition will be observed; otherwise if 2k B T*/W > 1, the system is beyond the critical point (green arrow from Figure I-15) and a conversion will be observed.

  Figure I-16 left) the arrangement of molecules is ordered, with a regularly repeating pattern in all directions of space. The molecules are held in fixed positions by intermolecular forces. In the liquid state, the motion overcomes the intermolecular forces that maintain a crystalline state, and the molecules move into random positions, without pattern in location or orientation (Figure I-16 right). The liquid crystal is an intermediate state between solid and liquid state. Molecules in a liquid-crystalline state are free to move around as in an isotropic liquid, however, they tend to orient in a preferred direction with one another, breaking the isotropy of the physical properties of the system. The arrangement of molecules in one of the liquid crystalline phases (i.e. smectic A) is represented in Figure I-16 (center). In this state, the molecules are still in layers, but within each layer they are arranged in random positions, although they remain more or less parallel to each other. Within layers, the molecules can slide around each other, and the layers can slide over one another. This molecular mobility produces the fluidity characteristic of a liquid.

Figure I- 16 .

 16 Figure I-16. (left) Schematic drawing of a crystalline, (center) liquid crystalline smectic and (right) liquid state.

Figure I- 17 .

 17 Figure I-17. Examples of calamitic (left) and discotic (right) molecules.

Figure I- 18 .

 18 Figure I-18. Schematic representation of a nematic phase formed by calamitic or rod-like molecules (left) and discotic or disc-like molecules (right).

Figure I- 19 .

 19 Figure I-19. Schematic representation of a smectic A phase (Sm A ) (left), and a smectic C (Sm C ) phase (right).

Figure I- 20 .

 20 Figure I-20. Schematic representation of (left) columnar hexagonal (Col h ), (center) columnar rectangular (Col r ) and (right) columnar oblique (Col o ) phases, a side view (top) and phase cuts (bottom) are shown.

Figure I- 21 .

 21 Figure I-21. Cubic lattices found for micellar cubic phases. 70bCubic mesophases (or smectic D, as they were also called 69a ) are three dimensionally ordered supramolecular assemblies with multi-continuous69 or a discrete micellar structures,70 depending on the molecular self-organization within the cubic cell.71 Mesogens showing cubic phases are a special type of liquid crystals as their physical properties are not anisotropic due to the cubic symmetry. The cubic phases are very viscous, their kinetics of formation is very slow. The detection of cubic phases is complicated, in particular on cooling from the isotropic liquid phase, because of its optically isotropic (black) defect texture when viewed between crossed polarizers. The method of choice for the characterization of cubic phases is the X-ray diffraction. However, for lyotropic phases, the cubic structure is well established and two distinct molecular organizations of cubic symmetry have been described. Therefore, the lyotropic case has been used as a model for thermotropic cubic phases. In their simplest form the molecules organize into micellar assemblies, mostly, with lattice symmetries with the and space groups (Figure I-21). 70b Other, less evident cubic organizations are the bicontinuous structures of , and symmetries. The latter cubic phases can be imagined as two interwoven networks as shown in Figure I-22. 69b Even tricontinuous networks have been suggested for compounds containing three incompatible parts. 69b However, thermotropic cubic phases are not so well understood, mainly due to their rare observation.

Figure I- 22 .

 22 Figure I-22. Network models of the three known bicontinuous cubic phases in soft matter: (left); (center); (right). The two interpenetrating networks are colored blue and yellow. White lines delineate the unit cell. 69b

Figure I- 23 .

 23 Figure I-23. (left) The chemical formula the molecular hybrid Fe(III)-based complex (top) that shows both LC and SC behaviors, the applied functionalization is shown in pink, the simulation of the molecular structure (bottom) is showed to emphasize the calamitic shape of the hybrid molecule. 73 (right) Schematic representation of structural changes associated with the Cr  LC transition that triggers the spin state crossover in [Fe(C n -trenR)](Cl) 2 . 76

  one-dimensional Fe(II)/triazole coordination polymers of general formula [Fe(R i Trz) 3 ](A) 2 (R i = 4mesogenic substituent-1,2,4-triazole, A = anion, Figure I-24). Fujigaya et al. 78a and Gaspar et al.79 modified the triazole ligand with mesogenic bis-alkyloxybenzylformylamino substituents and showed coupled LC/SC behaviors. Their studies suggested the existence of mesophases, including columnar phase, although the characteristic optical textures of these phases have not been reported. Unfortunately, the modification of ligands in these systems also induced a very gradual and incomplete SC.

Figure I- 24 .

 24 Figure I-24. (left) Representation of (a) 4-N-alkyl-1,2,4-triazole ligands, (b) the 1D chain formed through triple N1-N2 bridges upon coordination to Fe(II) or Zn(II) ions (shown as orange balls, nitrogen and carbon are, respectively, depicted as blue and gray), and (c) the space fill idealized conformation of one of these bridges with fully-stretched decyl substituent on the triazole ring. Schematic representation of (a) a hexacatenar moiety that would form repetitive triple 4-octadecyl-1,2,4-triazole bridges with fully-stretched alkyl arms in eclipsed conformation and (band c) lamellar packing of the resulting rods with interdigitation, respectively, perpendicular and parallel to the average rod axis.81

  ), or cyanobiphenyloxyundecyloxy (L 3 ) groups (Figure I-25). Depending on the chosen benzoate, 1D (smectic, Mn 12 -L 3 , Figure I-26 left) or 3D (body centred cubic, Mn 12 -L 2 ) organized mesophases that are stable up to 423 K were induced. The integrity of the complexes was preserved upon the exchange reaction as pointed out by the molecular structure of the model complex (Mn 12 -L 1 ). The magnetic properties are also preserved upon functionalization (Figure I-26 right).

Figure I- 25 .

 25 Figure I-25. Schematic representation of mesomorphic [Mn 12 O 12 (L i )(H 2 O) 4 ] complexes and chemical structure of mesogenic ligands used in previous reports.83,84 Color Scheme Mn(IV) green, Mn(III) dark green, O red, C grey, functionalized part of the ligand is shown in pink.

Figure I- 26 .

 26 Figure I-26. (left) Side view of the lamellar packing of cyanobiphenyl groups and Mn 12 complexes in the smectic layer. (right) Unaltered magnetic behaviour of SMM: temperature dependence of out-of-phase ac susceptibility, ″. 83a In very recent reports, Terazzi et al. 83b,c have used benzoate derivatives, mono-or di-substituted with cyanobiphenyloxyundecyl chains (L 3,5-7 , Figure I-25), as well as carboxylate-based ligands with twobranched dendritic structures (L 8 , Figure I-25) to prepare another series of LC Mn 12 -based systems. In these systems, the complexes self-organize, slightly above room temperature, into smectic-like bilayers with shortranged 2D arrangement of the magnetic units embedded in the aliphatic sublayer.

Figure I- 27 .

 27 Figure I-27. (left) The structure of the functionalized Tb double-decker phthalocyanine complex. (right) The frequency dependence of the out-of-phase (χ'') susceptibility at 25 K for the quenched disordered phase (open squares) and for the crystalline phase (open circles). 86 Besides Mn 12 -based systems the possibility to induce LC phases in a double-decker terbium phtalocyanine complex (Figure I-27 left) with chiral (S)-2-(dodecyloxy)propoxy groups was reported. 86 The

Figure I- 28 .

 28 Figure I-28. Thermal dependence of the magnetization for a 3 wt% [Fe II (C 18 trz) 3 ](ptol) 2 •2H 2 O/decane (65%)-CHCl 3 (35%) gel. Inset: pictures corresponding to low spin gel (pink) and high spin melting gel (white), schematic representation of polymeric structure of [Fe II (C 18 trz) 3 ](ptol) 2 •2H 2 O. 80b Among the macromolecular gelators, [M(R-trz) 3 ]A 2 (R-trz = 4-substituted-1,2,4-triazole) polymeric system presents a great interest. Kuroiwa et al. reported a Co II /triazole system, where R is C 16 H 33 or

  Via a subtle functionalization with methyl groups of the bpy ligands in cyanido-bridged {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }(OTf) 2 square complex a new system was obtained, {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 •H 2 O•2DMF (where diMebpy = 4,4'-dimethyl-2,2'-bipyridine), that preserved magnetic and optical switching ability driven by thermally and photo-induced intramolecular ET in solid state. The new functionalized system showed the possibility to transfer the solid state properties of {Fe 2 Co 2 } complexes into a wide range of solvents. 55g Furthermore the electron transfer and the associated changes of the magnetic and optical properties could be modulated by the solvent nature (Figure I-12 right).

Figure II- 1 .

 1 Figure II-1. Schematic representation emphasizing the benefits of ligand functionalization in SMMs: (left) functionalization with ligands possessing strongly lipophilic groups (long alkyl chains) can protect and isolate magnetic cores from contacts between them; (right) the self-organization of liquid crystalline phase can help to orient, align, and organize SMMs.

Figure II- 2 .

 2 Figure II-2. Crystallographic structure of Mn 12 -OAc complex (left) along the crystal a-axis, and (right) along the crystal c-axis. Color scheme: Mn III dark green, Mn IV light green, O red, C grey. Hydrogens are omitted for clarity.

Figure II- 3 .

 3 Figure II-3. Schematic representation of several Mn 12 cores with different number and coordination positions of axially coordinated H 2 O molecules: (a) [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ], 1:1:1:1 isomer; (b) [Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ], 2:0:2 isomer; (c) [Mn 12 O 12 (p-Me-PhCOO) 16 (H 2 O) 4 ], 1:2:1 isomer; (d) [Mn 12 O 12 (C 2 H 5 COO) 16 (H 2 O) 3 ], 1:1:1 isomer; (e) [Mn 12 O 12 (CHCl 2 COO) 8 (Bu t CH 2 COO) 8 (H 2 O) 3 ], 2:0:1. Color scheme: Mn(III) dark green, Mn(IV) green, O red, O water blue.

Figure II- 4 .

 4 Figure II-4. (top) d Orbital electronic configurations of distorted d 4 high spin octahedral complexes, emphasizing the effect of tetragonal elongation on the energies of d orbitals; (bottom) Schematic representation of regular octahedral complex (O h ), where all six ligands are identical and equidistant from the metal center, and the Jahn-Teller distortion with axial elongation, where two ligands in trans positions to each other are further than the other planar four ligands.

Figure II- 5 .

 5 Figure II-5. Structural view parallel to the molecular plane of different Mn 12 -based cores, emphasizing the orientation of the JT axes: (left) Mn 12 -OAc with JT axes axially disposed, and (right) [Mn 12 O 12 (CHCl 2 COO) 8 (Bu t -CH 2 COO) 8 (H 2 O) 3 ] with one abnormally oriented JT axis. Color scheme: normally oriented JT axes are shown in blue and the abnormally oriented JT axis is shown in yellow, Mn III dark green, Mn IV green, O red. Hydrogen and carbon atoms are omitted for clarity.

12

 12

Figure II- 6 .

 6 Figure II-6. Crystal structure at 100 K of [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] viewed in the bc plane (left) and in the ac plane (right). Lattice solvent molecules and hydrogen atoms are omitted for clarity. Color scheme: Mn IV green, Mn III dark green, O red, C grey. The Mn-O bond distances (Table S.II-2) indicate without ambiguity that the four metal ions in the cube-like core are Mn IV , while the surrounding manganese ring consists of eight Mn III ions. Indeed a comparison between Mn IV -O b (where O b is 4-methoxybenzoate oxygen; av. = 1.906 Å) and Mn III -O b (av. = 1.963 Å; oxygen from in-plane 4-methoxybenzoate) indicates that the cubane manganese ions are in a higher oxidation state. These assignments are supported by the Jahn-Teller elongation of the axial Mn-O bonds (2.075 -2.261 Å), which are on average longer than the equatorial Mn-O bonds (1.908 -1.972 Å). In contrast, the six Mn IV -O distances around each of the central cubane Mn IV ions are on average shorter and in a comparatively narrower range (1.836 -1.939 Å), due to the different O atoms involved, 4methoxybenzoate vs. bridging μ 3 -O 2-. A closer examination of the crystal structure showed that one JT axis is abnormally oriented as shown in Figure II-7 in yellow. The bond distances support this distortion. The two bonds on Mn8 connected to the out-of-plane 4-methoxybenzoate ligand (Mn8-O b ) are 1.933 and 1.950 Å, instead of usual elongated values. Similarly, the Mn8-μ 3 -O 2-(2.106 Å) and Mn8-O b(2.131 Å; oxygen from in-plane 4-methoxybenzoate) bonds are noticeably longer than those on other Mn III ions. This abnormal orientation is a local lower-energy situation stabilized by complicated crystal packing effects and can be reoriented if the latter are relaxed. 16a This structural distortion, involving JT axes, is at the origin of the low temperature (or fast relaxing) χ′′ ac susceptibility signal, since the abnormal orientation of one JT axis alters the nearly-parallel alignment and, thus, decreases the uniaxial magnetic anisotropy.30

Figure II- 7 .

 7 Figure II-7. Structural view of [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] parallel to the molecular plane. JT axes are shown in blue and the abnormally oriented JT axis is shown in yellow. Colour scheme: Mn IV green, Mn III dark green, O red. Several intermolecular interactions are present in the crystal packing. In one Mn 12 -molecule, two of the three water molecules coordinated to Mn III ions form O-H•••O hydrogen bonds (O•••O, ~ 2.71-2.75 Å) with methoxy oxygen atoms from the ligands of other two Mn 12 -molecules. The third water molecule coordinated to Mn 12 core is directly H-bonded with the lattice water molecule (O•••O, ~ 2.69 Å). Moreover, close π-π contacts (ca. 3.69 Å, Figure S.II-1) are formed between 4-methoxybenzoate ligands of two adjacent

Mn 12 -

 12 OAc complex. Other analytical characterizations of [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] such as FT-IR, 1 H-NMR, TGA and elemental analysis helped us to confirm the structure of the [Mn 12 O 12 (L 2-4 ) 16 (H 2 O) 4 ] analogues.

Figure

  Figure S.II-2 -Figure S.II-5). These vibrations correspond to antisymmetrical and symmetrical OCO

  H NMR spectra of prepared complexes indicates complete substitution of acetate by functionalized benzoate-based ligands (Figure II-8).

Figure II- 8 . 1 H

 81 Figure II-8. 1 H NMR spectra of [Mn 12 O 12 (OAc) 16 (H 2 O) 4 ] in CD 3 CN and [Mn 12 O 12 (L i=1-4 ) 16 (H 2 O) x=3 or 4 ] in CDCl 3 at 298 K.

Figure II- 9 .

 9 Figure II-9. Temperature dependence of χ′T and χ′′ for [Mn 12 O 12 (L i ) 16 (H 2 O) x ] (i = 1 -4, x = 3 -4) measured under various oscillating frequencies (1-1500 Hz) in zero dc field. The solid lines are guides for the eyes.The temperature dependence of the in-phase (χ′) and out-of-phase (χ′′) components of the ac susceptibility were measured in a zero static external field with different frequencies ranging from 1 to 1500 Hz (FigureII-9). The frequency-dependent decrease in χ′T at low temperature is attributed to a blocking process of the magnetization and was observed in all complexes. The values of χ′T at the plateau(8 -10 K)

[

  Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] has a predominant peak in the 2-3 K region (85% of the sample, Figure II-9), whereas the other [Mn 12 O 12 (L i=2-4 ) 16 (H 2 O) 4 ] complexes show higher intensity peaks in 4-7 K region (60-80% of the sample). The lower-temperature (LT) signal indicates the presence of fast-relaxing (FR) Mn 12 species

Figure II- 10 .

 10 Figure II-10. Inverse temperature dependence of the relaxation time in semi-logarithmic scale for synthesized [Mn 12 O 12 (L 1-4 ) 16 (H 2 O) 3-4 ] complexes using χ′′ vs T data for the FR and SR species from Figure II-9. Full lines are the best fits to an Arrhenius law. Direct-current (dc) magnetic susceptibility studies of functionalized Mn 12 complexes have been carried out in an applied magnetic field of 1 kOe in the 300 -1.8 K temperature range. The χT versus T data, where χ is the molar magnetic susceptibility, are shown in Figure S.II-10 left. The observed χT values at 280 K are

  magnetization vs. field curves and magnetization values at high fields ( 16 μ B ) show an absence of magnetization saturation (Figure S.II-11) which is consistent with magnetic anisotropy and high spin ground state (S > 8) in a randomly oriented polycrystalline samples. This is also confirmed by the M vs. H/T plots (Figure S.II-12) at different temperatures (from 1.9 to 10 K).

Figure II- 11 .

 11 Figure II-11. Magnetization vs. applied field at 1.9 K and at 0.035 T•min -1 sweep rate for all [Mn 12 O 12 (L 1-4 ) 16 (H 2 O) 3-4 ] complexes, the right figure is a zoom of the figure on the left side.

  Figure S.II-13), probably due to the very slow dynamics of the existing transition. SAXS measurements for [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ] were carried out at different temperatures. At 303 K, up to 13 sharp reflections (Figure S.II-15 left) were detected in the low and high angle range for which the reciprocal q spacings were in the ratios 2 : 3 : 6 : 9 : 12 : 14 : 20 : 29 : 33 : 38 : 49 : 76 : 83 suggesting a cubic lattice.

  k, l are Miller indices. Therefore, the lattice parameter can be calculated from the slope of the 2π plot(Figure S.II-16). From this plot, we can observe that the experimental data fit well with a straight line and the cubic lattice parameter a = 37.8 Å was deduced. The absence of liquid-crystalline phase in the material is confirmed by the absence of a broad scattering halo located around q = 13 nm -1 characteristic of the liquid like order of the molten aliphatic chains. Sharp peaks in wide angle region were present up to 380 K (Figure S.II-15 left). Thus, this information suggests a crystalline phase with a cubic symmetry in the thermally untreated complex below 380 K. At highest temperature, 393 K (below II.63 decomposition temperature of ca 420 K), in small angle region, two broad reflections at 1.8 and 2.2 nm -1 are still present in the X-ray diffraction profile, suggesting a remnant order or a short range ordering of the objects (Figure S.II-15 left). At wide angles a broad halo appears around 13 nm -1 , suggesting the presence of molten alkyl chains. However, as previously mentioned, no birefringent texture could be observed in POM, even if a large brown viscous area is forming on increasing temperature. As observed from DSC analyses, this thermal behavior is irreversible and is consistent with the fact that the SAXS profile remains unchanged as the temperature decreases after the first thermal cycle (Figure S.II-15 right). This irreversibility is likely caused by a very slow dynamics of the phase transition process. The DSC thermogram of [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] between 160 and 340 K shows two endothermic transitions which are thermally reversible (Figure II-12 left). The broad peak with a maximum at 242 K corresponds to a transition involving melting of alkyl chains and to the formation of a mesophase that was identified as cubic with the help of X-ray diffraction investigations (vide infra). The second peak observed in the DSC traces at 301 K is corresponding to the clearing point. On cooling the reverse sequence of transitions occurs, with a 20 K thermal hysteresis (at 5 K/min) for clearing point (280 K), while the maximum of the broad transition at lower temperature remained unchanged. The enthalpy change (ΔH) during the broad phase transition is about 94.6 kJ/mol (or 3 kJ/mol per dodecyl chain, if divided by the number of attached alkyl chains) and corresponds to a typical transition enthalpy of melting for dodecyl chains.40

Figure II- 12 .

 12 Figure II-12. (left) DSC traces of [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] complex between 160 and 330 (5 K/min). (right) Indexed SAXS diffraction profile of [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] at 283 K, in the inset diffraction profile at small angle range, intensity axis is in logarithmic scale.To gain more insights regarding the arrangement of the molecules in the mesophase, temperaturedependent SAXS measurements were performed on heating the sample from 283 K (the sample was preliminary heated to 320 K to bypass the clearing point, then cooled down to 255 K in freezer, and, finally, quickly transferred to sample chamber cooled to 283 K). The pattern collected at 283 K contained 10

  Figure II-13. (left) DSC traces of [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] complex between 220 and 360 (5 K/min). (right) SAXS diffraction profiles of [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] (intensities are in logarithmic scale): (a) at 273 K before first heating; (b) at 283 K on first heating; (c) at 353 K on first heating; (d) at 353 K on fourth heating.

  FigureII-13 right). At 273 K, the diffraction pattern showed a sharp reflection (centered around 15.3 nm -1 )

Figure II- 14 .

 14 Figure II-14. Small angle X-ray diffraction profiles of [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ]: (left) on 1 st heating from 273 K to 353 K; (center) on 1 st cooling from 353 K to 283 K; and (right) on 2 nd cooling from 353 K to 274 K and at a bigger detector distance than in the profiles showed on left and in the center.

Figure II- 15 .

 15 Figure II-15. Indexed SAXS profiles for [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ]: (left) at 333 K on 3 rd heating, corresponding to an intermediate mesophase with orthorhombic symmetry, only the small angle region is shown; and (right) at 353 K on 4 th cooling corresponding to a cubic mesophase with symmetry.

  ), (330/411) and (332) (Figure II-15 left, Tables S.II-8 and S.II-9). All reflections satisfied hkl: k + l = 2n, hhl: l = 2n, h00: h = 2n conditions compatible with a body-centered cubic lattice.

  Figure II-16. (a) Schematic representation of the "inverse micellar" type body-centered cubic liquid crystalline lattice with 3 Im m symmetry, the spheres represent the preferred locations of rigid cores, the interstitial space is filled with aliphatic chains, a is the lattice parameter of the cubic cell. (b) Schematic representation of the "inverse micellar" type primitive cubic liquid crystalline lattice with 3 Pm m symmetry. Single crystal structure of the [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] compound allowed to model the structure of the functionalized molecules using Chem3D software. The idealized structure of [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] complex molecule (Figure II-17) has a micelle-like spherulitic shape, where the rigid nucleus (of ~ 20 Å diameter, Figure II-17) is composed of the oxo-metallic core surrounded by 16 benzoate units, while the shell is formed from the dodecyl chains anchored to aromatic rings and is ~ 15 Å thick. Thus, the micellar (discontinuous) model appears to be more probable to describe the organization of functionalized Mn 12based molecules inside the cubic phases with both P and I symmetries(Figure II-16). Also, the theoretical size of the functionalized molecules and the determined values of lattice parameters show that the cubic lattice inside the mesophase is formed by self-organized molecules and not by supramolecular agregates as it

Figure II- 17 .

 17 Figure II-17. Structural model (calculated) of the [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] molecule viewed from equatorial (left) and axial (right) sides. The inner red sphere indicates the imaginary edge of the rigid core, the outer grey sphere is corresponding to soft aliphatic shell. Color scheme: Mn dark green, O red, C grey, H white.

Figure II- 18 .

 18 Figure II-18. Schematic representation of complete interdigitation of aliphatic chains.

II. 5 .

 5 Functionalization of Mn 12 -based SMMs by grafting a mesomorphic promoter through a flexible spacer II.5.1

Figure

  Figure II-19. 1 H NMR spectra of [Mn 12 O 12 (OAc) 16 (H 2 O) 4 ] in CD 3 CN and [Mn 12 O 12 (L 5-6 ) 16 (H 2 O) 4 ] in CDCl 3 at 298 K.

Figure II- 20 .

 20 Figure II-20. Aromatic region of 1 H NMR spectra in CDCl 3 at 298 K: (top) HL 5 and [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ]; (bottom) HL 6 and [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]. As the result of the ligands grafting and the proximity to paramagnetic core, the signals corresponding to protons H 1 and H 2 have totally disappeared. Also, the absence of signals corresponding to these protons corresponds to complete removal of unreacted ligand (used in a large excess in preparation) and validates the purification technique. The presence of sets of protons H 3 -H 6 for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and H 3 -H 7 for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ], although broadened, confirmed the grafting of the functionalized ligands (Figure II-20).

  frequencies from 1 to 1500 Hz(Figure II-21).

Figure II- 21 .

 21 Figure II-21. Temperature dependence of χ′T (top) and χ′′ (bottom) for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] (left) and [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] (right) measured under various oscillating frequencies (1-1500 Hz) in zero dc field. The solid lines are guides for the eyes.

  deduced relaxation time follows an Arrhenius law as shown in Figure II-22. The activation energy barriers Δ eff /k B and the pre-exponential factors τ 0 were estimated for both HT and LT relaxing species (Table II-5). Obtained values are almost the same for both [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] and are close to those observed for other Mn 12 complexes (see TableII-5).

Figure II- 22 .

 22 Figure II-22. Inverse temperature dependence of the relaxation time in semi-logarithmic scale for [Mn 12 O 12 (L 5- 6 ) 16 (H 2 O) 4 ] complexes using χ′′ vs T data for the FR (LT) and SR (HT) species from Figure II-21. Full lines are the best fits to an Arrhenius law. Variable-temperature dc magnetic susceptibility () data were collected on complexes [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] in the 1.8 -280 K range in a 0.1 T magnetic field (Figure S.II-10 right). The T versus T dependences are similar to those of previously studied [Mn 12 O 2 (L 1-4 ) 16 (H 2 O) 4 ]complexes, exhibiting a nearly temperature-independent value of 18 cm 3 •K•mol -1 in the 150-280 K range

Figure II- 23 .

 23 Figure II-23. Magnetization vs. applied field at 1.9 K and at 0.035 T•min -1 sweep rate for [Mn 12 O 12 (L 5-6 ) 16 (H 2 O) 4 ] complexes, the right figure is a zoom of the figure on the left side.Despite the facts that HL5 and HL 6 ligands have almost similar structures and final complexes are characterized by magnetic parameters (S, g, Δ eff /k B ) of almost similar values, the ratios of JT isomers is considerably different. This may confirm the hypothesis that peripheral cyanobiphenyl moieties form stronger intermolecular interactions that distort the magnetic core by pulling and pressure effects and, as result, affect its properties.

  case of [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] the clearing point was observed around 445 K, that is the same as the Mn 12based core decomposition temperature observed in TGA, while the [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] compound showed the formation of the isotropic liquid around 348 K. DSC thermograms were in agreement with POM observations. Both compounds showed a glass transition, corresponding to the melting/freezing of the mesophase, around 295-305 K. Only the glass transition was observed for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] complex (Figure II-25 left) in 280-410 K temperature range (in order to avoid decomposition) on several subsequent heating-cooling cycles. Thus, in order to determine the clearing point temperature fast (10 K/min) subsequent heating-cooling cycles in 300-450 K range were afterwards performed. Large and asymmetric peak assigned to the clearing point with POM was observed at 442 K on both heating and cooling. Further thermal cycles showed peak broadening and a significant decrease of enthalpy value, which may be explained by the decomposition of functionalized compound, confirmed by TG analysis.

Figure II- 24 .

 24 Figure II-24. Birefringent textures observed with POM: (a) for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] at 393 K on heating from room temperature; (b) for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] at 423 K on heating from room temperature; (c) for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] at 343 K on heating from room temperature; (d) for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] at 323 K on cooling from isotropic liquid. The [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] complex showed a reversible sharp peak at 347.5 K, corresponding to the transition from liquid crystal phase to isotropic liquid (Figure II-25). The enthalpy change of both transitions (50.1 kJ/mol for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and 73.3 kJ/mol for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]) have very high valueswhen compared to the normal range specific to clearing point of a mesophase.47 However, it is possible to

Figure II- 25 .

 25 Figure II-25. DSC traces of [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] on third heating-cooling cycle between 300 and 450 K at 10 K/min (left) and DSC traces of [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] recorded on first two heating-cooling cycles between 220 and 390 K at 5 K/min (right).

  ratios for [Mn 12O 12 (L 6 )16(H 2 O) 4 ]. Observed reflections were indexed with 001, 002 and 003 Miller indices (00l) and, by applying the Bragg's law, interlayer distances of 38.8 Å for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and 32.8 Å for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] were calculated.

Figure II- 26 .

 26 Figure II-26. Indexed SAXS profiles for: (left) [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] at 373 K on second heating from room temperature, in the inset is showed the small angle region emphasizing very close proximity of 002 and 200 reflections originating, respectively, from the lamellar ordering (00l) and from hexagonal (hk0) interlayer arrangement of electronrich Mn 12 cores; and (right) for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] at 323 K on cooling from isotropic liquid, in the inset weak reflections between 5 and 8 nm -1 are emphasized.

  ), (330) with reciprocal spacings ratios 4 : 7 : 9 : 16 : 21 : 25 : 27 for [Mn 12 O 12(L 6 ) 16 (H 2 O) 4 ] (see also tables S.II-10 and S.. The sharp shape and the strong intensity of some of these reflections indicate a long range hexagonal ordering of the electron-rich cores within the layer for both functionalized complexes. The average in-plane inter-core (or intra-layer) distances (a) of 43.5 Å for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and 52.7 Å for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] at 323 K were calculated. The interlayer distances (corresponding to d 001 reflections)are weakly temperature dependent and usually slightly increase with temperature decrease, while the intercore distances decreases by 1-2 Å (see also tables S.II-10 and S.II-11). The shortening of the lamellar interlayer distance when cyanobiphenyl unit is exchanged by biphenyl moiety may correspond to a higher interdigitation of the biphenyl moieties, as confirmed by an increase of in-plane inter-core distances and, thus, of molecular area (1617 Å 2 for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and 2405 Å 2 for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] at 323 K). Apparently shorter inter-planar distances are specific to biphenyl moieties, since similar result was observed and reported for a series of (nBu 4 N) 2 [Mo 6 Br 8 (L i) 6 ] functionalized clusters, 23 where L i are gallic acid based ligands modified with [((biphenyl)yl)oxy]-or [((4-cyanobiphenyl)yl]oxydecyl groups (exactly the II.80 same as those used in our work). Due to the absence of a strongly electron density withdrawing cyano (-CN) group (as it is schematically represented in Figure II-27 top right), the biphenyl moieties are much less polar than cyanobiphenyl ones (as it is schematically represented in Figure II-27 top right). Therefore electrostatic interactions between biphenyl groups are much weaker than those observed between cyanobiphenyl groups. This explains quite well the decrease of the isotropic transition temperature, a higher interdigitation between the [((biphenyl)yl]oxydecyl chains and, thus, a shorter interplanar distance (Figure II-27 left and center).

Figure II- 27 .

 27 Figure II-27. Schematic representation of the idealized intermolecular interactions leading to different inter-planar distances in the mesophases observed for [Mn 12 O 12 (L 5-6 ) 16 (H 2 O) 4 ] complexes, only the ligands in axial positions are shown.Theoretical distances calculated for the both biphenyl-based mesogens (9 -10 Å), the aliphatic spacer (~ 13 Å, idealized conformation), and the rigid Mn 12 core, including benzoate moieties from the ligands (height ~ 14 Å and diameter ~ 20 Å) allow existence of the calculated interlamellar distances, moreover these data suggests that mainly the ligands in axial positions are contributing to the formation of the lamellar structure. Whereas the in plane distance (a) corresponding to a 2D hexagonal arrangement of the magnetic cores inside the plane is influenced by the interdigitation of the mesogenic promoters (the higher is the interdigitation of the axially situated promoters the bigger is the molecular area, as can be observed from the schemes in FigureII-27) and the functional groups present in equatorial positions, that, apparently form only short contacts with other neighbors.The analysis of SAXS profiles at temperatures close to clearing points (Figures S.II-22 and S.II-23) confirms the presence of a lamellar structure and weaker in plane intermolecular interactions. Only the reflections corresponding to the layered morphology are present close to clearing temperatures, while those assigned to intra-layer ordering shift and broaden, forming in small-angle region a broad diffusion d' that corresponds to an average short-range correlation distance between oxo-metallic cores within the layer. Thus, the mesophase observed for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] complexes can be surely described by a smectic phase, consisting of alternating layers of interdigitated mesogens and Mn 12 cores arranged into a hexagonal array and separated by aliphatic sub-layers(Figure II-28).

Figure II- 28 .

 28 Figure II-28. Schematic representation of the lamellar packing of [Mn 12 O 12 (L 5-6 ) 16 (H 2 O) 4 ] complexes inside the smectic mesophase, the equatorial groups are omitted for clarity.

Figure S.II- 1 . 2 .

 12 Figure S.II-1. Packing arrangement illustrating π-π interactions present between adjacent [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] complexes II.7.2.2. FT-IR spectroscopic analyses

  Figure S.II-3. FT-IR spectra of HL 2 (black) and [Mn 12 O 12 (L 2 ) 16 (H 2 O) 3 ] (red).

Figure S.II- 4 .

 4 Figure S.II-4. FT-IR spectra of HL 3 (black) and [Mn 12 O 12 (L 3 ) 16 (H 2 O) 3 ] (red).

Figure S.II- 6 .

 6 Figure S.II-6. FT-IR spectrum of [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ].

Figure S.II- 8 . 9 .

 89 Figure S.II-8. FT-IR spectra of HL 6 (black) and [Mn 12 O 12 (L 6 ) 16 (H 2 O) 3 ] (red).

Figure S . 12 .

 S12 Figure S.II-11. M vs. H plot at 1.9 K for [Mn 12 O 12 (L 1-4 ) 16 (H 2 O) 3-4 ] complexes (left) and for [Mn 12 O 12 (L 5-6 ) 16 (H 2 O) 4 ] (right). The solid lines are guides for the eyes.

Figure S.II- 14 .

 14 Figure S.II-14. DSC traces of [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] on first two heating-cooling cycles between 270 and 410 K at 5 K/min.II.7.4.2. Small angle X-ray scattering (SAXS) investigations

  d (obs) and d (calc) are observed and calculated diffraction spacings:d (obs) = 2/q (obs) , d (calc) = a/√, a is the lattice parameter of the cubic phase,V is the volume of the cubic cell unit (V = a 3 ), Z is the number of complexes per cubic cell, Z = (VN A )/Mw, where  is the density  1 g/cm 3 , N A is the Avogadro constant 6.0225  10 23 (note: 1 Å 3 = 1.0  10 -24 cm 3 ).

Figure S.II- 16 .

 16 Figure S.II-16. The lattice spacing of the cubic phase, a = d 100 , of the [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ] complex can be calculated from the reciprocal gradient of the plot 2/q vs √ ⁄ . Solid black line is the linear fit. The lattice parameter a = 37.8 Å corresponds to the slope of the linear fit.

Figure

  Figure S.II-17. (left) Small angle X-ray diffraction profile of [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] on heating from 283 K to 343 K, emphasizing the transition to the isotropic phase; (right) 2/q vs √ ⁄ plot. Solid black line is the linear fit. The lattice parameter a = 36.5 Å corresponds to the slope of the linear fit.

  d (obs) and d (calc) are observed and calculated diffraction spacings:d (obs) = 2/q (obs) , d (calc) = a/√, a is the lattice parameter of the cubic phase, V is the volume of the cubic cell unit (V = a 3 ), Z is the number of complexes per cubic cell, Z = (VN A )/Mw, where  is the density  1 g/cm 3 , N A is the Avogadro constant 6.0225  10 23 (note: 1 Å 3 = 1.0  10 -24 cm 3 ).

Figure S.II- 18 .

 18 Figure S.II-18. Small angle X-ray diffraction profiles of [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] (left) on 3 rd heating from 303 K to 393 K and (right) on 3 rd cooling from 393 K to 303 K.

1 g/cm 3 ,

 13 q(obs) is the observed/measured reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, d(obs) and d(calc) are observed and calculated diffraction spacings:d(obs) = 2/q(obs), d(calc) = 1/√, a, b and c are the lattice parameters of an orthorhombic cell, V is the volume of the orthorhombic cell unit (V = abc), Z is the number of complexes per orthorhombic cell, Z = (VNA)/Mw, where  is the density  NA is the Avogadro constant 6.0225  10 23 (note: 1 Å 3 = 1.0  10 -24 cm 3 ).

Figure S.II- 19 .

 19 Figure S.II-19. Small angle X-ray diffraction profiles of [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] (left) on 4 th heating from 303 K to 413 K and (right) on 4 th cooling from 413 K to 274 K.

  q(obs) is the observed/measured reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, d(obs) and d(calc) are observed and calculated diffraction spacings:d(obs) = 2/q(obs), d(calc) = a/√, a is the lattice parameter of the cubic phase, V is the volume of the cubic cell unit (V = a 3 ), Z is the number of complexes per cubic cell, Z = (VNA)/Mw, where  is the density  1 g/cm 3 , NA is the Avogadro constant 6.0225  10 23 (note: 1 Å 3 = 1.0  -24 cm 3 ).

Figure S.II- 20 .

 20 Figure S.II-20. Small angle X-ray diffraction profiles of [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ]: (left) on 2 nd heating from 313 K to 413 K; (center) on 2 nd cooling from 413 K to 303 K; (right) on 3 rd heating from 393 K to 448 K.

  Figure S II-21. Small angle X-ray diffraction profiles of [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]: (left) on 2 nd heating from 298 K to 353 K; (right) on 2 nd cooling from 349 K to 298 K.

  observed and calculated diffraction spacings:d(obs) = 2/q(obs), d(calc) = √ √, a and c are lattice parameters of the average hexagonal organization in layers, also c corresponds to average lamellar periodicity between the layers formed by magnetic cores: c = ∑ . V is the volume of the average hexagonal unit cell V = a 2 •c•cos(π/6). S corresponds to the hexagonal basal (ab) plane area.

  investigation of switchable molecular materials. The first reported tetranuclear cyanido-bridged molecular square complex (by our group in 2010) 6c , {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }(OTf) 2 •2H 2 O•4DMF (where Tp* = tris(3,5-dimethylpyrazol-1-yl)hydroborate, OTf = trifluoromethanesulfonate, bpy = 2,2-bipyridine and DMF = N,N-dimethylformamide), exhibits both thermally and photoinduced intramolecular electron transfer process between paramagnetic {Fe III 2 Co II 2 } and diamagnetic {Fe II 2 Co III 2 } states. 6c Moreover, an electron transfer associated with first-order phase transition was observed for this system with a thermal hysteresis of 18 K (at a temperature sweep rate of 0.4 Kmin -1 ) between 186 and 168 K (see Figure III-1 left). The other two {Fe 2 Co 2 } molecular squares, reported later the same year, 6d,e had different capping ligands on Co or Fe sites and as a result showed different ET related behaviors. The complex of formula {[(pzTp)Fe(CN) 3 ] 2 [Co(bik) 2 ] 2 }(ClO 4 ) 2 •2H 2 O (where pzTp = tetrapyrazolylborate, bik = bis(1methylimidazol-2-yl)ketone) is a diamagnetic cyanido-bridged {Fe II 2 Co III 2 } square over the 2 -300 K temperature range and can be converted into the corresponding paramagnetic {Fe III 2 Co II 2 } species under light irradiation below 100 K. 6d On the other hand, the {[(Tp*)Fe(CN) 3 ] 2 [Co(dtbbpy) 2 ] 2 }(PF 6 ) 2 •2MeOH (where dtbbpy = 4,4'-di-tert-butyl-

Figure III- 1 .

 1 Figure III-1. (left) T vs. T data for {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }[OTf] 2 •2H 2 O•4DMF (1) (at 0.4 Kmin -1 , with  defined as the molar magnetic susceptibility and equal to M/H; M = magnetization and H = external magnetic field): in the dark (black, 0.1 T), after thermal quench (purple, 0.1 T), and after irradiation (green, 1 T, 3 mWcm -2 ); 6c (right) T vs. T data for {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 •H 2 O•2DMF (2) in the dark on cooling (blue, 0.5 T), and heating (red, 1 T) at 0.8 Kmin -1 , and after irradiation (green, 1 T, 2 mWcm -2 ) at 0.4 Kmin -1 . 6g Due to their ET associated magnetic and optical bistabilities, cyanido-bridged molecular {Fe 2 Co 2 } squares became interesting molecular objects for functionalization and design of hybrid systems. Successful attempts of rational functionalization of molecular {Fe 2 Co 2 } squares were performed by Diana Siretanu during her PhD thesis (CRPP, 2008 -2011). 9 Through the methyl functionalization of the bpy ligands of the {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 } 2+ , a new system was obtained, {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 •H 2 O•2DMF (where diMebpy = 4,4'-dimethyl-2,2'-bipyridine), that preserved magnetic and optical switching ability driven by thermally and photo-induced intramolecular electron transfer process. This result proved the possibility to transfer and tune the solid state properties of {Fe 2 Co 2 } complexes into a wide range of solvents. 6g,9 In the case of the methyl functionalized square, the electron transfer was not associated with a first-order phase transition with a thermal hysteresis. The square complex shows a gradual conversion between paramagnetic {Fe III 2 Co II 2 } state and diamagnetic {Fe II 2 Co III 2 } state (Figure III-1 right), indicating the presence of significantly weaker elastic interactions between complexes in the crystal structure. 6g,9

  3) -{[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(ClO 4 ) 2 •2DMF; (4) -{[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTs) 2 •6DMF, where bpy = 2,2-bipyridine, diMebpy = 4,4'-dimethyl-2,2'-bipyridine, Tp* = tris(3,5-dimethylpyrazol-1yl)hydroborate, OTf = trifluoromethanesulfonate, OTs = para-toluenesulfonate, and DMF = N,N-III.113

(

  Scheme III-1) in reaction of the tricyanido-metallate precursor (NEt 4 )[(Tp*)Fe(CN) 3 ] with DMF solution of corresponding Co 2+ salt (Co(OTf) 2 for 1 and 2, Co(ClO 4 ) 2 •6H 2 O for 3, and Co(OTs) 2 •6H 2 O for 4), followed by subsequent addition of bidentate bpy-based ligand (2,2-bipyridine for 1 and 4,4'-dimethyl-2,2'-bipyridine for 2-4). The crystallizations were carried out by subjecting the filtered reaction mixtures to vapor diffusion of diethyl ether. III.114 Both bpy-based ligands and Co(ClO 4 ) 2 •6H 2 O are commercially available. The Co 2+ salts such as Co(OTf) 2 and Co(OTs) 2 •6H 2 O were prepared by treatment of CoCO 3 with corresponding acid (see Section III.4 for detailed protocols). The cyanido-metallate (Et 4 N)[(Tp*)Fe(CN) 3 ] complex has been prepared via ligand metathesis reactions. The literature reported method 11 was modified using iron(III) acetylacetonate (Fe III (acac) 3 ) precursor salt instead of iron(II) acetate (Fe II (CH 3 COO) 2 ). Treatment of Fe III (acac) 3 with potassium tris(3,5-dimethylpyrazol-1-yl)-hydroborate (KTp*) in methanol rapidly affords a red precipitate. Subsequent treatment of the obtained precipitate with an excess of tetraethylammonium cyanide in acetonitrile affords a red hydrated iron(III) cyanido-metallate complex of formula (Et 4 N)[(Tp*)Fe(CN) 3 ]2H 2 O. This compound was purified by flash silica gel column chromatography, to eliminate the large excess of uncoordinated cyanide.

9 (Figure III- 2 .

 92 Figure III-2. Cationic portion of 1 in its: (a) quenched phase at 90K (quickly cooled from 300 to 90 K); (b) high temperature phase at 230 K; (c) low temperature phase collected at 84 K; and (d) photo-induced metastable paramagnetic phase at 83 K. Thermal ellipsoids are at 50% probability. All anions, lattice solvent molecules, and hydrogen atoms are omitted for clarity. Color scheme: C grey, Co pink, Fe green, N blue, B yellow.

2 .

 2 09 Å) than Co-N bpy bonds (2.13 -2.15 Å). Hydrogen-bonding interactions along the c axis are present between the N(3) atom from terminal cyanide ligand, lattice water and one DMF molecule (N(3)•••O(1)w, 2.835 Å and O(1)w•••O(5) DMF , 3.009 Å, Figure S.III-4b). Also, weak intermolecular π-π type interactions between several adjacent bpy ligands coordinated to Co II ions occur along b-axis, with centroid-centroid distance between bpy planes of 3.879 Å (Figure S.III-3b).Single-crystal X-ray studies of 1 obtained after rapid cooling (quenching) to 90 K (Figure III-2a, Tables III-1 and S.III-1) are in agreement with magnetic measurements (Figure III-1 left), suggesting that thermal quenching is not a very efficient way to obtain the metastable paramagnetic {Fe III LS (μ-CN)Co II HS } phase. The observed decrease of 0.04 Å in average Co-N distances (2.08 Å) and the decrease of the Σ(N-Co-N)

  Figure III-2c, Tables III-1 and S.III-1) show a marked decrease in unit cell volume and in the average Co-N distance (ca. 0.18 Å), which is consistent with the conversion of paramagnetic {Fe III LS (μ-CN)Co II HS } into diamagnetic {Fe II LS (μ-CN)Co III LS }. The values of N-Co-N angles in the coordination sphere of Co III LS ion depart less from orthogonality (Σ (N-Co-N) = 43.2 °) than those measured for Co II HS. Although, the Fe coordination sphere showed much smaller differences comparing to Co site, the III.118 change in the Fe charge is visible in the values of Fe-C bond lengths (ca. 1.92 Å at 230 K and ca. 1.89 Å at 84 K, Tables III-1 and S.III-1). Moreover, weaker intermolecular π-π type interactions are found between adjacent {Fe 2 Co 2 } complexes (centroid-centroid distance between bpy planes increased to 4.142 Å, Figure S.III-3c), while the H-bonds decreased between the N(3) atom from terminal cyanide ligand, lattice water and one DMF molecule (N(3)•••O(1)w, 2.681 Å and O(1)w•••O(5) DMF , 2.809 Å, Figure S.III-4c). After the irradiation with the white light (P = 3 mW/cm 2 ) at 83 K, it was possible to determine the structure of the photo-induced paramagnetic {Fe III LS (μ-CN)Co II HS } phase (Figure III-2d). The conversion was achieved in less than an hour (the unit cell parameters after 1 h of irradiation were the same as those corresponding to 24 h of irradiation), but the crystal was kept under white light at 83 K for 24 h to allow full conversion to the paramagnetic state and to check the stability of the crystal under these experimental conditions. Similarly to previous structures, significant changes in Co coordination sphere occurred as the result of photo-induced ET associated with the conversion from the thermodynamic diamagnetic {Fe II LS (μ-CN)Co III LS } phase to the metastable paramagnetic {Fe III LS (μ-CN)Co II HS } phase. Thus, the average Co-N distance increased to 2.11 Å and the values of the N-Co-N angles show a more distorted coordination sphere around Co II HS ion (Σ (N-Co-N) = 62.7 °) than the one observed in diamagnetic phase around the Co III LS ion at 84 or 120 K (Tables III-1 and S.III-1). The average Fe-C and Fe-N bond lengths are of 1.92 and 2.00 Å (Table

Complex 2

 2 .III-1) and similar to the thermally quenched phase. Moreover, it confirms that light irradiation (even at 83 K) is more efficient to obtain the metastable paramagnetic phase at low temperatures than the thermal quenching, as also observed from magnetic measurements (Figure III-1 left). is a methyl functionalized version of complex 1 that preserved thermally and photoinduced ET associated with a change of magnetic and optical properties. It exhibits thermally reversible electron-transfer crossover at T 1/2 = 174 K between paramagnetic {Fe III 2 Co II 2 } and diamagnetic {Fe II 2 Co III 2 } states (Figure III-1 right). 6g,9 Under white-light irradiation at 10 K, it is possible to convert the diamagnetic {Fe II 2 Co III 2 } phase into the paramagnetic {Fe III 2 Co II 2 } one (Figure III-1 right). 6g,9

Figure III- 3 .

 3 Figure III-3. Thermal evolution on cooling of lattice parameters for complex 2.

  Figure III-4. (left) Cationic portion of 2 in its: (a) high temperature phase at 240 K; (b) low temperature phase collected at 80 K; and (c) photo-induced metastable paramagnetic phase at 80 K. Thermal ellipsoids are at 50% probability. (right) Packing arrangement illustrating π-π interactions present between adjacent complexes of 2 along the crystallographic b-direction under different experimental conditions. All anions, lattice solvent and hydrogen atoms are eliminated for clarity.

  ). After cooling to 80 K in the dark, the structure of the green crystal (Figure III-4b) reveals shorter average Fe-C and Co-N bond distances of 1.882 and 1.926 Å, respectively (Tables III-2 and S.III-2). The shorter average Co-N bond distances by ca. 0.2 Å are characteristic of Co III LS ions formed after thermally induced ET associated with the {Fe III 2 Co II 2 } → {Fe II 2 Co III 2 } conversion. While the changes in average Fe-C and Fe-N bond distances at 80 K differ only slightly (by ~ 0.03 Å) from those observed at 240 K and correspond well to Fe II LS ions (Table III-2). As the result of metal-to-ligand bond length decrease, the interatomic Fe•••Fe (6.78 Å) and Co•••Co (7.05 Å) distances inside the {Fe 2 (μ-CN) 4 Co 2 } core shortened as well. The change in values of the N-Co-N angles shows a less distorted coordination sphere for the Co III LS ion (Σ (N-Co-N) = 35.9°) than that measured for the Co II HS site at 240 K.

Figure III- 5 .

 5 Figure III-5. Superimposed structures of the {Fe 2 (μ-CN) 4 Co 2 } cationic core in 2 emphasizing the differences observed under different experimental conditions. (left) Differences observed before and after white light irradiation at 80 K (P = 6 mW/cm 2 , t = 4 h). (right) Comparison of the structures corresponding to the thermodynamic high temperature paramagnetic phase and to the metastable light induced paramagnetic state. Many structural parameters changed after the irradiation with white light at 80 K (P = 6 mW/cm 2 , t = 4 h). Firstly, changes were observed from the unit cell determination measurements (Figure III-3). Thus, most of the lattice parameters increased in value (except the α angle that decreased) if compared with those measured at 80 K in the dark. Besides the changes in the unit cell size, the increase of the average Co-N distance from 1.926 Å (at 80 K in the dark) to 2.115 Å was observed after white light irradiation, as the result of photo-induced ET associated with the {Fe II 2 Co III 2 } → {Fe III 2 Co II 2 } conversion. A comparison of the

  ). Moreover, Fe-C-N and C-Fe-C bond angles have values that are higher or smaller than those observed at 240 K by 1-3°.Similarly to 1, square molecules form weak intermolecular π-π type interactions between several adjacent diMebpy molecules coordinated to Co II ions along the b-axis in crystal packing of 2 (Figures III-4 and III-6 top), but {Fe 2 Co 2 } complexes are more separated than in 1 (Figure S.III-3). A steric hindrance generating slightly weaker π-π interactions, i.e. longer overlap distances, is induced by the methyl groups present at the 4-and 4'-positions on the bipyridine ligands. Consequently, the bipyridine centroid-tocentroid distances in complex 1 vary from 3.88 to 4.14 Å (Figure S.III-3) while in complex 2 they are ranging between 4.26 and 4.34 Å (Figure III-4 right), suggesting the presence of significantly weaker elastic interactions between complexes in the crystal structure. 6g,9 Therefore, the square complex 2 shows a Boltzmann conversion between the paramagnetic excited {Fe III 2 Co II 2 } state and diamagnetic {Fe II 2 Co III 2 } ground state (Figure III-1 right). Besides intermolecular π-π interactions, the non-bridging cyanides are involved in H-bond with the co-crystallized water molecule, which is also H-bound to a DMF molecule. The existence of H-bonds was deduced from N•••O w and O w •••O DMF distances of 2.964 and 3.044 Å, respectively, at 240 K. In the low temperature diamagnetic phase before white light irradiation, these contacts are slightly shorter (2.930 and 3.014 Å, respectively), while in the light induced paramagnetic phase, they increase above the 240 K values to 2.971 and 3.057 Å, respectively. Complex 1 shows similar H-bonds, except that the measured values, corresponding to this kind of contacts, are about 0.2 Å shorter (Figure S.III-4).

  .III- 3). No structural changes specific to a thermally induced intramolecular ET were observed on cooling the red crystal of 3 from 250 to 100 K (TableS.III-3), suggesting that this compound is always in a paramagnetic {FeIII 2 Co II 2 } state. Complex 3 crystallizes in the triclinic P-1 space group with a square-shaped tetranuclear core, in which Fe and Co ions are alternately bridged by cyanide ions in the same way as in1 and 2. The unit cell consists of a centro-symmetric cyanido-bridged {Fe 2 Co 2 } square, two free perchlorate ions and two DMF molecules. The structure of the cationic {Fe 2 (μ-CN) 4 Co 2 } part at 100 K is showed in Figure III-6 left. The Fe ions are coordinated by tridentate Tp* ligand with a facial configuration, and the remaining coordination sites are occupied by three cyanide carbon atoms. Two of these three cyanide ions bridge to Co ions, while the remaining one coordinates only to Fe. The two bidentate diMebpy ligands chelate each Co ion, and the bridging cyanide groups occupy the cis positions of the Co sites (Figure III-6 left). The average Co-N bond lengths of about 2.13 Å at 100 K and 250 K are characteristic of Co II HS ions, while the average III.123 Fe-C (1.92 Å) and Fe-N (2.00 Å) (Table III-2) are similar to those measured for Fe III LS ions in the paramagnetic states of 1 and 2.

Figure III- 6 .

 6 Figure III-6. (left) The structure of the complex cation in 3 at 100 K. (right) The structure of the complex cation in 4 at 120 K. Thermal ellipsoids are at 50% probability. All anions, lattice solvent molecules, and hydrogen atoms are omitted for clarity. Color scheme: C grey, Co pink, Fe green, N blue, B yellow.

Figure III- 7 .

 7 Figure III-7. Perspective view of the crystal packing of 2 at 80 K in light-induced paramagnetic state (top) and 3 at 100 K (bottom) illustrating π-π interactions between molecular squares. Counter anions, hydrogen atoms and solvent molecules are omitted for clarity.

Figure III- 8 .

 8 Figure III-8. Perspective view of the crystal packing of 4 at 120 K, illustrating π-π interactions between molecular squares. Counter anions, hydrogen atoms and solvent molecules are omitted for clarity.

  Figure III-9, the cationic core in 3 has a structural geometry very similar to the one observed for 2 in both light-induced or thermodynamic paramagnetic states. Therefore the absence of the intramolecular ET in 3

Figure III- 9 .

 9 Figure III-9. Superimposed structures of the {Fe 2 (μ-CN) 4 Co 2 } cationic cores emphasizing the structural differences. (left) Comparison of the structures corresponding to the paramagnetic light-induced {Fe III 2 Co II 2 } phase in 2 at 80 K and always paramagnetic 3 at 100 K. (right) Comparison of the structures corresponding to the paramagnetic light-induced {Fe III 2 Co II 2 } phase in 2 at 80 K and always paramagnetic 4 at 120 K. Prepared compounds differ not only by the counter-anion, structure of the tetranuclear core and intermolecular short contacts, but by the solvation inside the crystal packing, as well. In the structure of 2, there are present two DMF molecules and a water molecule, while in the packing of 3 and 4 only DMF molecules are present (two DMF molecules in 3 and six DMF molecules in 4). Since the applied synthetic and crystallization methods were identical for the three compounds, it appears that the nature of the counter-

Figure III- 10 .

 10 Figure III-10. Comparison of the reflectivity spectra of 2 with 3 and 4 at 290 K (left) and at 90 K (right).

3 and 4

 4 in comparison with 2 are discussed (Figure III-11). Temperature dependent magnetic measurements were carried out between 1.8 and 280 K and are shown in Figure III-11 as the thermal dependence of the χT product. Above 250 K, the χT product remains nearly constant (~ 6.5 cm 3 •K/mol for 2 and 3, and ~ 6.8 cm 3 •K/mol for 4) that is in agreement with values expected for a 2:2 ratio of magnetically isolated Fe III LS (S = 1/2, g ≈ 2.6) and Co II HS (S = 3/2, g ≈ 2.4) centers. For 2, slowly decreasing the temperature (0.4 K/min) from 240 to 120 K affords a gradual decrease of χT value in 2, approaching 0.51 cm 3 •K/mol at 120 K then 0.08 cm 3 •K/mol at 1.8 K, therefore suggesting thermally generated diamagnetic {Fe II LS (μ-CN)Co III LS } units as a result of the intramolecular ET (black curve on Figure III-11).

Figure III- 11 .

 11 Figure III-11. Temperature dependence of the χT product (where  defined as the molar magnetic susceptibility and equal to M/H; M = magnetization and H = external magnetic field) for compounds 2 (black, 0.5 T, 0.4 Kmin -1 ), 3 (blue, 0.1 T, 0.4 Kmin -1 ) and 4 (pink, 0.1 T, 0.4 Kmin -1 ), solid lines are guides for the eyes.As expected from the temperature dependent structural investigations, compounds 3 and 4 do not exhibit any ET associated conversion between paramagnetic and diamagnetic species on cooling. The χT product for 3 and 4 remains roughly constant down to 120 K then decreases to values of 5.2 cm 3 •K/mol for 3 and 6.0 cm 3 •K/mol for 4 likely due to the spin-orbit coupling of the high spin Co II ion. On further cooling below 30 K, the χT product for 3 reaches a plateau and remains relatively constant until 10 K then it abruptly

  2.3.3, Figure III-7 bottom). Intermolecular π-π type interactions occur between adjacent diMebpy ligands and between adjacent pyrazole rings from capping Tp* ligands along the crystallographic b axis. As already mentioned, these interactions lead to a supramolecular 2D-like structure with centroid-to-centroid distances between diMebpy planes of 4.12 Å and between pyrazole planes of 3.94 Å (Figure III-7 bottom). In contrast, complexes are more isolated in 4 with no π-π type interactions observed between diMebpy ligands, while the centroid-to-centroid distances between pyrazole planes (from capping Tp* ligands) increased to 4.58 Å (Figure III-8). Additional evidence for ferromagnetic exchange between Fe III and Co II spin carriers in 3 and 4 is found in the field-dependence of magnetization plots (Figure S.III-5). At 1.8 K, the magnetization value is monotonically increasing upon applying the external dc fields, without showing S-shape curves that could be indicative for the presence of the antiferromagnetic interactions. Moreover the magnetization curves show an absence of magnetization saturation which is consistent with magnetic anisotropy of the Co II ions (Figure S.III-5).

  , were synthesized with the goal to study the perturbation of the ET process in functionalized {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 } 2+ system through changing the counter-anion, a change that has not yet been studied with respect to {Fe 2 Co 2 } molecular squares.

  The structural and magnetic studies of the family of closely related complexes, {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }X 2 (compounds 2, 3 and 4) showed that the occurrence of the ET process in solid state is strongly affected by the exchange of the counter-anion. The counter-anion induced Chapter III -Modulating the Structural and ET Properties of Molecular Squares in Solid State III.130

FT-

  IR data (ATR, cm -1 ): 3059 (w), 2924 (w), 2855 (w), 2535 (w), 2146 (s), 2118 (w), 1669 (vs), 1655 (vs), 1612 (s), 1543 (m), 1489 (m), 1446 (s), 1410 (s), 1382 (s), 1368 (s), 1195 (s), 1120 (m), 1092 (m), 1062 (s), 1035 (m), 1012 (s), 919 (m), 819 (m), 800 (m), 778 (m), 679 (vs), 658 (m), 645 (m), 568 (m), 561 (m).

Figure S.III- 1 .

 1 Figure S.III-1. FT-IR spectrum of tricyanido-metallate precursor (NEt 4 )[(Tp*)Fe(CN) 3 ] at room temperature.

Figure S.III- 2 .

 2 Figure S.III-2. FT-IR spectra of compounds 1 -4 at room temperature.

3 .

 3 Figure S.III-3. Packing arrangement illustrating π-π interactions present between adjacent complexes of 1 along the crystallographic b-direction under different experimental conditions. All anions, lattice solvent and hydrogen atoms are eliminated for clarity.

4 .

 4 Figure S.III-4. View of the intermolecular short contacts in crystal packing of 1 under different experimental conditions.

I

  > 2σ(I), R 1 = Σ|(|F o | -|F c |)|/Σ|F o |. wR 2 = {Σ[wGoF (goodness of fit on F 2 ) = {Σ[w(F o 2 -F c 2

Figure S.III- 5 .

 5 Figure S.III-5. Magnetization versus H/T plots at different temperatures for 3 (left) and 4 (right).

Figure IV- 1 .

 1 Figure IV-1. Schematic representation of the strategy used to get more soluble systems by functionalizing bipyridine ligand in 4-and 4'positions (blue arrows). The core and only one bipyridine ligand are emphasized in the shown {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 } 2+ complex. Color scheme: Fe green, Co pink, B yellow, N blue, C black.

Figure IV- 2 .

 2 Figure IV-2. FT-IR spectra collected between 3300 and 1800 cm -1 at 290 K: (left) for the raw powder and crystals of 5; (right) for solvated green crystals and dry red crystals of 6.

  symmetric cyanido-bridged {Fe 2 Co 2 } square, two free trifluoromethanesulfonate ions and two DMF molecules. The structure of the cationic {Fe 2 (μ-CN) 4 Co 2 } part at 100 K is showed in Figure IV-3.

Figure IV- 3 .

 3 Figure IV-3. Structure of the cationic part of 5 at 100 K. Lattice solvents, anions, and hydrogen atoms are omitted for clarity. Colour scheme: Fe green, Co pink, B yellow, N blue, C grey. The average Co-N bond length of 2.13 Å at 100 K and 260 K is characteristic of the HS Co II ions, while the average Fe-C (1.92 Å) and Fe-N (2.00 Å) bond distances (Table IV-1) are similar to those measured for the LS Fe III ions in the paramagnetic state of 1 and 2. The {Fe 2 (μ-CN) 4 Co 2 } core in 5 adopts a significatively distorted square geometry. The bond-angles N(1)-Co(1)-N(3) (99.3(2)°) and C(1)-Fe(1)-C(3) (83.8(2)°) are far from orthogonality. The cyanide bridges are notably bent especially on the cobalt sites with Co-N-C angles of 165.9(5) and 168.3(5)°, while the Fe-C-N angles range between 176.6(5) and 178.2(6)° (Table S.IV-1). The Fe-CN-Co edges are quasi identical since the Fe(1)•••Co(1) and Fe(1)•••Co(2) distances are 5.087 and 5.115 Å, respectively, while the Fe•••Fe and Co•••Co distances are 7.759 and 6.24 Å, respectively. Both iron and cobalt ions are six-coordinate. The FeN 3 C 3 environment is less distorted than the CoN 6 coordination sphere. The distortion parameter (Σ) for HS

  are N-Co-N bond angles The functionalization with hexyl chains contributes to a better isolation of {Fe 2 (μ-CN) 4 Co 2 } units in the crystal packing of 5. No intermolecular short contact, besides weak van-der-Waals interactions between the hydrocarbon chains, are observed between adjacent square complexes inside the crystal packing (as illustrated by Figure IV-4).

Figure IV- 4 .

 4 Figure IV-4. Perspective views of the crystal packing of 5 at 100 K. Lattice solvents, anions, and hydrogen atoms are omitted for clarity. Colour scheme: Fe green, Co pink, B yellow, N blue, C grey.

Figure IV- 5 .

 5 Figure IV-5. Structure of the cationic part of 6 at 100 K. Lattice solvents, anions, and hydrogen atoms are omitted for clarity. Color scheme: Fe green, Co pink, B yellow, N blue, C grey. The global structure of compound 6 (Figure IV-5), is consistent with that of compound 2 at 100 K in its diamagnetic {Fe II LS (µ-CN)Co III LS } state, 2,3 as confirmed by the green color of crystals and the FT-IR spectroscopy. The average Co-N bond distances of 1.928 Å suggest that Co ions are trivalent and in LS configuration, while the average Fe-N and Fe-C bond distances of 2.047 Å and 1.880 Å (TableIV-1),

  bond angles of 92.2(4)° and 87.8(5)°, respectively. The {Fe-(μ-CN)-Co} edges are bent. On the cobalt sites, Co-N-C angles are 165.6(8) and 168.7(8)°, while the Fe-C-N angles range between 171.8(9) and 176.5(9)° (Table S.IV-2). The Fe-CN-Co edges are quasi identical and shorter than in 5: the Fe(1) … Co(1) and Fe(1) … Co(2) distances are 4.884 and 4.892 Å, respectively. The Fe•••Fe and Co•••Co IV.149 distances are 7.129 and 6.688 Å, respectively (Table

Figure IV- 6 .

 6 Figure IV-6. Packing diagram of 6 in the bc plane emphasizing one unit cell. Lattice solvent molecules, anions, and hydrogen atoms are omitted for clarity. Color scheme: Fe green, Co pink, B yellow, N blue, C grey.

Figure IV- 7 .

 7 Figure IV-7. Packing diagram of 6 in the ac plane illustrating the H bonding between solvent molecules and terminal cyanide groups. Anions and hydrogen atoms are omitted for clarity. Color scheme: Fe green, Co pink, B yellow, N blue, C grey, O red. As a consequence, the packing of 6 shows a layered structure (Figures IV-6 and S.IV-1). The molecular squares are arranged face to face along the a axis in the (bc) plane, separated by 11.9217(13) Å equal to the a parameter of the unit cell. The aliphatic chains of adjacent layers are strongly interdigitated, separating the layers by 25.069(3) Å, value of the c parameter of the unit cell. The alkyl chains are packed

Figure IV- 8 .

 8 Figure IV-8. Reflectivity spectra of 5 (left) and red crystals of 6 (right) at 10 and 280 K (spectroscopic white light halogen source, P = 0.5 mW/cm 2 ).

Figure IV- 9 .

 9 Figure IV-9. Reflectivity spectra of (left) the green crystals of 6 at 10 and 280 K, and (right) the comparison between the reflectivity spectra of green and red crystals of 6 (spectroscopic white light halogen source, P = 0.5 mW/cm 2 ).

Figure IV- 10 .

 10 Figure IV-10. Temperature dependence of the χT product (with χ defined as the molar magnetic susceptibility and equal to M/H) for compound 5 at 0.1 T (left) and for compound 6 (right, 1-black on first cooling; 2-red on heating and 3-blue on second cooling at 0.4 K/min, 0.5 T).

Figure IV- 11 .

 11 Figure IV-11. Change of color observed when a methanol solution of 5 is cooled from room temperature (left) and cooled down with liquid nitrogen (right).

  MMCT, appears around 760 nm (Figure IV-12 left). Thus, the spectroscopic data clearly demonstrate that a thermal conversion of {Fe III LS (μ-CN)Co II HS } into {Fe II LS (μ-CN)Co III LS } units IV.154 occurs in solution. Furthermore, the presence of an isosbestic point at 608 nm suggests that only two species are involved in the thermal inter-conversion process. The temperature dependence of the {Fe III LS (μ-CN)Co II HS } and {Fe II LS (μ-CN)Co III LS } fractions can be deduced from the thermal variation of the 540 and 760 nm absorptions, respectively (Figure IV-12 right). As expected, the two fractions cross each other at 0.5 defining T 1/2 temperature as the temperature for which equal quantities of {Fe III LS (μ-CN)Co II HS } and {Fe II LS (μ-CN)Co III LS } species are present.

Figure IV- 12 .

 12 Figure IV-12. (left) UV-Vis spectra for 5 (top) and 6 (bottom) in CH 3 OH on cooling mode from 295 K to 210 K (at 0.2 to 0.3 K/min). (right) Temperature dependence of the {Fe III LS (μ-CN)Co II HS } and {Fe II LS (μ-CN)Co III LS } fractions estimated from the absorption intensities at the MMCT bands (≈ 540 nm and ≈ 760 nm) for 5 (top) and 6 (bottom) in CH 3 OH, solid lines are the fits to the ideal solution model. 12

Figure IV- 13 .

 13 Figure IV-13. Temperature dependence of the {Fe III LS (μ-CN)Co II HS } fraction estimated from the absorption intensities at the MMCT band (≈ 540 nm) for 5 (left) and 6 (right) in different solvents. Solid lines are the fits to the ideal solution model. 12

6 )

 6 was observed. Moreover, similar behavior was observed from the temperature dependent UV-visible IV.156 spectroscopy, when solubility of compounds 5 and 6(C ~ 5•10 -5 mol/L) in ethyl acetate decreased abruptly as the temperature was lowered(Figure S.IV-4). It was possible to avoid this extrinsic effect for solutions of 5 in methanol, acetone and dichloromethane by preparation of diluted solutions with the concentration range between 1 and 3•10 -4 mol/L. The dilution of solutions (5 in ethyl acetate and 6 in all four solvents) to the limit of measurability of the magnetometer did not allow suppressing the partial precipitation of the compounds(Figure S.IV-6). This precipitate can be easily seen by eyes when removing the sample from the SQUID magnetometer. Therefore only the magnetic measurements obtained on cooling modes for 5 and 6 are discussed in this part(Figure IV-14).

Figure IV- 14 .

 14 Figure IV-14. χT vs T data for 5 (left) and 6 (right) in methanol, ethylacetate, acetone and dichloromethane solutions (0.4 K/min, 1 T, solid lines are fits to the ideal solution model 12 ).

  4. The thermodynamic parameters have values slightly different from those obtained from optical measurements IV.157

Figure IV- 15 .

 15 Figure IV-15.Comparison between χT vs T data for complexes 2, 5 and 6 in dichloromethane (left), in acetone (center) and methanol (right), the solid line are fits to the ideal solution model.12

Scheme IV- 2 .

 2 Scheme IV-2. Representation of the possible functionalizations of the 2,2-bypiridine ligand

  [NEt 4 ][(Tp*)Fe(CN) 3 ]•H 2 O (165 mg, 0.3 mmol) was treated with Co(OTf) 2 (107 mg, 0.3 mmol) in 5 mL of wet DMF under argon to afford a red

I

  > 2σ(I), R 1 = Σ|(|F o | -|F c |)|/Σ|F o |. wR 2 = {Σ[w/ (n-p)} 1/2 ,where n is the number of reflections and p is the total number of refined parameters.

Figure S.IV- 1 .

 1 Figure S.IV-1. Packing diagram of 6 in the bc plane illustrating interdigitation of the long alkyl chains from diC13bpy ligands.

Figure S.IV- 3 .

 3 Figure S.IV-3. Left: UV-vis spectra for 6 in acetone (top), ethyl acetate (middle) and dichloromethane (bottom) in cooling mode from 300 K to solvent freezing point (at 0.2 to 0.3 K/min). Right: Temperature dependence of {Fe III LS (μ-CN)Co II HS } (red) and {Fe II LS (μ-CN)Co III LS } (green) fractions estimated from the analyses of absorption intensities at MMCT bands (540 nm and 760 nm, respectively); solid lines are fits of the experimental to an ideal solution model.

Figure

  Figure S.IV-4. UV/Vis spectra for 6 in CH 3 COOC 2 H 5 in cooling mode from from 242 K to 240 K (9•10 -5 mol/L).

Figure S.IV- 5 . 5 .

 55 Figure S.IV-5. Magnetization versus H/T plots at different temperatures for 5.

  (i.e. methoxy) can further increase the ET characteristic temperature, the work on functionalization of molecular {Fe 2 Co 2 } squares is continued in this chapter. By rational choice of the capping ligands, counter-anions and crystallization solvents, it was possible to synthesize three new cyanido-bridged {Fe 2 Co 2 } complexes functionalized with methoxy (MeO) groups in 4-and 4'positions of the bpy ligand: (7) -{[(Tp*)Fe(CN) 3 ] 2 [Co(diMeObpy) 2 ] 2 }(OTf) 2 •4DMF (where diMeObpy = 4,4'-dimethoxy-2,2'bipyridine, OTf = trifluoromethanesulfonate, DMF = N,N-dimethylformamide); (8a) -{[(Tp*)Fe(CN) 3 ] 2 [Co(diMeObpy) 2 ] 2 }(PF 6 ) 2 •2H 2 O•5MeCN (where MeCN = acetonitrile); (8b) -{[(Tp*)Fe(CN) 3 ] 2 [Co(diMeObpy) 2 ] 2 }(PF 6 ) 2 •6MeCN.

Figure V- 1 .

 1 Figure V-1. FT-IR spectra collected between 2800 and 1800 cm -1 at room temperature: (left) for the red crystals of 7 and red crystals of 8b; (right) for the green powder and green crystals of 8a. Compounds 8a and 8b are indeed different solvates of the same system, {[(Tp*)Fe(CN) 3 ] 2 [Co(diMeObpy) 2 ] 2 }(PF 6 ) 2 as shown by later structural investigations. The different solvation is achieved when the synthesis is carried out in "wet" or anhydrous solvents. The modified synthetic procedure for preparation of 8a and 8b consists in treatment of a diluted solution containing 1 eq.

Figure V- 2 .

 2 Figure V-2. The structures of the both complex cations in 7 at 90 K. Thermal ellipsoids are at 50% probability. All anions, lattice solvent molecules, and hydrogen atoms are omitted for clarity. Color scheme: C grey, Co pink, Fe green, N blue, O red, B yellow.

Figure V- 3 . 1 - 3 ,

 313 Figure V-3. Perspective views of the crystal packing of 7 at 90 K: (top) along the crystallographic c axis illustrating intermolecular short contacts between {Fe 2 Co 2 } squares; (bottom) down the crystallographic c axis. Counter anions, hydrogen atoms and solvent molecules are omitted for clarity.

  HS and Fe III LS ions in tetranuclear cores strongly suggest no intramolecular ET. The presence of two OTf -counter-anions and four co-crystallized DMF molecules per {Fe 2 Co 2 } square in the crystal packing of 7 contributes to a good separation of the {Fe 2 (μ-CN) 4 Co 2 } units (Figure V-3 bottom). Thus, no π-π type interactions are observed between pyrazole planes of the capping Tp* ligands, as it was observed in 2 -4, while adjacent diMeObpy ligands coordinated to the Co II ions form weak intermolecular π-π like interactions (about 4.5 Å) along crystallographic c axis (Figure V-3 top).Several crystal structures of 7 were collected within this work (including high temperatures), unfortunately the refinement factors could not be lowered to reasonable values mainly due to the strongly disordered OTf -counter-anions.

  {FeII 2 Co III 2 } state (see also Chapter III), with a square shaped core where Fe and Co ions are alternately bridged by cyanide ions. In the crystal packing, hexafluorophosphate counter anions, latticeMeCN and H 2 O molecules are also present. The cationic {Fe 2 (μ-CN) 4 Co 2 } core in 8a is nearly planar (Figure V-4). The tridentate Tp* ligand facially coordinates to the Fe ion, and the remaining coordination sites are occupied by V.176 three cyanide carbon atoms. Two of the three cyanide ions from the Fe sites coordinate in bridging mode to the Co ions, while the free non-bridging cyanide groups adopt as in the other analogus an anti orientation relative to the {Fe 2 (μ-CN) 4 Co 2 } plane. The coordination sphere around each Co ion is completed by two bidentate diMeObpy ligands.

Figure V- 4 .

 4 Figure V-4. (left) The structure of the complex cation in 8a at 120 K. Thermal ellipsoids are at 50% probability. (right) The side view of the complex cation in 8a, emphasizing the almost planar geometry of the tetranuclear core. All anions, lattice solvent molecules, and hydrogen atoms are omitted for clarity. Color scheme: C grey, Co pink, Fe green, N blue, O red, B yellow.

Figure V- 5 .

 5 Figure V-5. Packing arrangement illustrating H-bonds present between one co-crystallized H 2 O molecule and two adjacent complexes of 8a. All anions, lattice solvent and hydrogen atoms are eliminated for clarity. At 120 K, single-crystal XRD studies showed that the {Fe 2 (μ-CN) 4 Co 2 } core is a in diamagnetic state since it contains trivalent Co ions in low spin (LS) configuration (confirmed by average Co-N bond distance of approximately 1.93 Å, Table V-1) and Fe II ions, as results of the charge balance. The {Fe 2 (μ-CN) 4 Co 2 } core is nearly planar since the Fe-C-N angles range between 174.1(4) and 177.1(5)° while Co-N-C angles range between 173.6(4) and 175.6(4)° (Table S.V-3, Figure V-4). The separations between adjacent metal ions within the tetranuclear core through the cyanide bridges (Fe(1) … Co(1) and Fe(1) … Co(1)') are virtually equal, 4.946 and 4.942 Å, respectively. Nevertheless, the C(1)-Fe(1)-C(3) angle (87.91°), Fe•••Fe(7.187 Å)

Figure V- 8 .

 8 Figure V-8. Optical reflectivity spectra of 8a at selected temperatures between 280 K and 10 K on cooling (left) and on heating (right).

Figure V- 9 (

 9 Figure V-9 (left) in the form of the thermal dependence of the reflectivity at 800  5 nm (R 800 ). Upon cooling from 270 K (Figure V-9 left, blue curve), R 800 decreases from  0.14 down to  0.09 at 120 K as a signature of electron transfer occurrence in the {Fe III 2 Co II 2 } defects on sample's surface. At lower temperatures, the reflectivity slightly increases up to 0.11 at 10 K due a weak photo-conversion (induced by the reflectivity source light) of the {Fe II LS (μ-CN)Co III LS } into {Fe III LS (μ-CN)Co II HS } units. Upon warming from 10 K (Figure V-9 left, red curve), R 800 is increasing up to 0.12 at 60 K, as expected if the photo-conversion is still active in

Figure V- 9 .

 9 Figure V-9. (left) Thermal evolution of reflectivity signal recorded at λ = 800 ± 5 nm (R 800 ) of 8a in heating (red line) and cooling modes (blue line), and after 5 h of white light irradiation (0.4 mW/cm 2 ) at 10 K in heating mode (green line). (right) Time evolution of the absolute reflectivity R 800 under white light irradiation (0.4 mW/cm 2 ) during 5 h at 10 K; inset: reflectivity spectra at 10 K before and after 5 h of white light irradiation (0.4 mW/cm 2 ).

(

  Figure V-9 right inset) is quite similar to ones recorded for other cyanido-bridged {Fe 2 Co 2 } molecular squares in their paramagnetic state. 1,4 Thus, using optical reflectivity measurements, it was possible to reveal the high photosensitivity of 8a (at least at the samples' surface) under white light irradiation leading to conversion of the diamagnetic green {Fe II LS (μ-CN)Co III LS } pairs into paramagnetic red {Fe III LS (μ-CN)Co II HS } units. The red paramagnetic {Fe III 2 Co II 2 } state is metastable, and increasing the temperature to 150 K (at 4 K•min -1 ) results in the recovery of the R 800 value, 0.1, specific to the thermodynamically favored {Fe II 2 Co III 2 } state.

Figure V- 10 .

 10 Figure V-10. Optical reflectivity spectra of 8a at 10 K measured before (green) and after 5 min of irradiation (0.2 mW/cm 2 ) using wavelengths delivered by 505 nm (left) and 940 nm (center) LED sources. (right) Difference in absolute reflectivity (ΔAR) at 800 nm from the reflectivity spectra of 8a before and after 5 min of irradiation (0.2 mW/cm 2 ) at 13 different wavelengths delivered by LED sources at 10 K.

  h, P = 30 mW/cm 2 ) in collaboration with Prof. Eric Collet at Institut de Physique de Rennes (Université Rennes 1). Thus, crystal structures of 8a were refined in the two different states at 10 K (Figure V-15, Tables V-2 and S.V-3).

Figure V- 15 .

 15 Figure V-15. The structure of the cationic portion of 8a (left) before and (right) after the irradiation with an 808 nm laser source (2 h, P = 30 mW/cm 2 ) at 10 K. Thermal ellipsoids are at 50% probability. All anions, lattice solvent and hydrogen atoms are eliminated for clarity. Color scheme: C grey, Co pink, Fe green, N blue, O red, B yellow.

Figure V- 19 .

 19 Figure V-19. (left) Temperature dependence of the χT product for the crystals of 8b in their mother liquor (on heating and cooling, red and blue dots respectively, 0.5 T, 0.7 K/min) and for crystals of 8b dried in vacuum at room temperature for 12 h (green dots, 0.1 T, 1 K/min). (right) d(χT)/dT vs T plot of the temperature dependent susceptibility measurements showed on left of the crystals of 8b in their mother liquor at 0.5 T on cooling (blue dots) and heating (red dots) with a sweep rate of 0.7 K/min. Inset: the zoom of the χT vs T plot showed on the left between 220 and 280 K. A closer examination of the recorded χT vs T curves for the crystals of 8b (Figure V-19 right, inset) revealed that the conversion associated with the ET process between 220 and 280 K occurs in a two-step fashion via a possible intermediate state. This result is confirmed by the d(χT)/dT vs T plots (Figure V-19 right) showing the presence of two inflection points around 244 and 258 K. A possible intermediate state is observed in a very narrow temperature range around 250 K for a χT value of 4.2 cm 3 •K/mol that is about half

  indicative for the presence of antiferromagnetic interactions. Moreover the magnetization curves show an absence of magnetization saturation as expected in the presence of the Co II magnetic anisotropy (Figure S.V-5 right).

Figure V- 20 .

 20 Figure V-20. (left) Time evolution of the χT product for 8b during the white light irradiation (3 mW/cm 2 , 1 T) at 10 K. (right) Temperature dependence of the χT product for 8b on heating (green dots, 1 T, 0.3 K/min) after the white light irradiation (3 mW/cm 2 ) at 10 K for ~ 13 h and on cooling from 250 K in the dark (blue dots, 1 T, 0.3 K/min).

  solutions proved the possibility to transfer from solid state or to reveal into a wide range of solvents the ET driven switching properties of the functionalized {Fe 2 Co 2 } complexes. With the aim to determine if the functionalized complexes 7, 8a and 8b can exhibit thermally induced ET in solutions, the properties of the resulting solutions have been investigated.It is important to mention, that compounds 8a and 8b represent different solvates of the same system, {[(Tp*)Fe(CN) 3 ] 2 [Co(diMeObpy) 2 ] 2 }(PF 6 ) 2 , stabilized in different electronic states at room temperature. 8a is a green crystalline solid, whereas 8b is red at room temperature. The dissolution of the green crystals of 8a affords a red solution in acetonitrile (and other solvents), while the repeated re-crystallizations of the red V.193 acetonitrile solutions of 8a yield red crystals of 8b. Therefore in further investigations of the magnetic and optical properties in solutions, compounds 8a and 8b are considered the same compound and referred ascompound 8, {[(Tp*)Fe(CN) 3 ] 2 [Co(diMeObpy) 2 ] 2 }(PF 6 ) 2 .The qualitative tests for ET that consisted in cooling with liquid nitrogen the solutions of 7 and 8 showed a systematic change of color from red to green, pointing out the occurrence of intramolecular ET in solutions of 7 and 8. The qualitative tests were carried out in several solvents: methanol, ethanol, acetone, acetonitrile, dichloromethane and butyronitrile. Due to the presence of PF 6 -as counter-anion, both solvates of 8 are not soluble in alcohols, especially methanol and ethanol. Similarly to previously reported complexes 1, 1,4b 2, 4 5 and 6 4b (see also Chapters III and IV) it was assumed that the integrity of the {Fe 2 Co 2 } square molecules remained intact in solutions for which the color change was observed.V.3.1. UV-Vis spectroscopic characterizations of 7 and 8 in diluted solutionsThe absorption spectra of 7 (in methanol, Figure V-21 top left, and dichloromethane, Figure S.V-6 left) and 8 (in butyronitrile -PrCN, Figure V-21 bottom left) were measured at different temperatures in cooling mode from room temperature until the freezing point of the solvent.

Figure V- 21 .

 21 Figure V-21. (left) Selected UV-Vis spectra for 7 in CH 3 OH (top) and 8 in PrCN (bottom) on cooling mode from 300 K until the freezing point of the solvent (at 0.2 to 0.3 K/min). (right) Temperature dependence of the {Fe III LS (μ-CN)Co II HS } and {Fe II LS (μ-CN)Co III LS } fractions estimated from the absorption intensities at the MMCT bands (≈ 540 nm and ≈ 760 nm) for 7 in CH 3 OH (top) and 8 in PrCN (bottom), solid lines are the fits to the ideal solution model. 9

Figure V- 23 .

 23 Figure V-23. χT vs T data for the solutions of 7 in methanol and dichloromethane (left) and 8 in dichloromethane and butyronitrile (right), 0.4 K/min on cooling, 1 T, solid lines are fits to the ideal solution model. 9

(T 1

 1 /2 = 186 K), 5 (T 1/2 = 191 K), 6 (T 1/2 = 196 K) and 7 (T 1/2 = 216 K) (Figure V-24, compound 7 was chosen for comparison because it has the same counter-anion (OTf -) as 2, 5, and 6).

Figure V- 24 .

 24 Figure V-24.Comparison between χT vs T data for complexes 2, 5, 6 and 7 in dichloromethane, the solid line are fits to the ideal solution model.9 The comparison of the χT vs T plots in methanol solutions shows a slightly different behavior, the T 1/2 value for 7 (264 K) is higher than the ones measured for 2 and 5 (213 and 253 K respectively), but it is about 5 K lower than the one measured for 6 (269 K). The observed behavior can be explained by the fact that on cooling methanol solutions of 6, the ET occurs in the same time with the precipitation/crystallization of the sample (Figure S.IV-6 right top) that can have influence on the thermodynamic parameters. On the other hand, the observed difference may be explained by the fact that the nature of the ligand functionalization and the solvent's nature (or polarity) are likely not the only parameters that influence the ET properties of cyanido-bridged {Fe 2 Co 2 } squares in solution.

  photo-induced state is particularly interesting because macroscopic investigations of molecular cyanidobridged {Fe 2 Co 2 } squares suggest the presence of ferromagnetic interactions between Co II HS and Fe III LS

  XAS spectra at Co and Fe K-edges were recorded for crystals of 8b to confirm the changes in the electronic state of the metal ions upon thermally inducedET (Figure V-25). In the cobalt K edge absorption spectrum of 8b at 298 K (Figure V-25 left), the energies of the absorption maximum at 7727 eV (allowed 1s

Figure V- 25 .

 25 Figure V-25. XANES at Co (left) and Fe (right) K edges at 298 K (red) and 2 K (blue) for 8b.

  4 )[Fe III (Tp*)(CN) 3 ], showed the maximum absorption at the same energy value at 2 K (Figure S.V-8) suggesting that the maximum of absorption observed at 2 K for 8b (7130 eV) is corresponding to the expected Fe II LS ions.

Figure V- 26 .

 26 Figure V-26. Temperature dependence of the paramagnetic fractions estimated from the absorption intensities at 7725 eV in the Co K edge (green curve) and from the magnetic susceptibility measurements (blue curve) for 8b.The most prominent changes in the Co K edge were observed at the energy of 7725 eV. Therefore the electron transfer phenomenon was also followed as a function of temperature by XANES at this Co K edge energy. The temperature dependence of the paramagnetic {Fe III LS (μ-CN)Co II HS } fractions can be deduced from the thermal variation of the absorption at 7725 eV(Figure V-26). As expected, the obtained XANES

Figure V- 27 .

 27 Figure V-27. Changes in XANES at the Co (left) and Fe (right) K edges upon irradiation of 8b with white light at 2 K.

Figure V- 28 .

 28 Figure V-28. Evolution of the difference between the room temperature (298 K) XANES and low temperature (2 K) XANES spectra for 8b before and during white light irradiation at both Co (left) and Fe (right) K edges.

Figure V- 29 .

 29 Figure V-29. Isotropic (average, black) and dichroic (XMCD, red) absorption spectra at Co (left) and Fe (right) K edges for 8b.

  For a vacuum dried sample of 8b, the macroscopic magnetization data (Figure S.V-5 right) show no inflection point that may correspond to the presence of antiferromagnetic interactions between Fe III and Co II ions. Moreover, the temperature dependent magnetic susceptibility measurements for the same sample show a significant increase of the χT product at low temperatures (Figure V-19 left), suggesting a ferromagnetic coupling between the metal ions. Similar behavior in the magnetic susceptibility measurements have been observed for solvated sample of 8b under white light irradiation at low temperatures (Figure V-20 right). These macroscopic observations find a local counterpart in the dichroic signals. At both Fe and Co K edges, the dichroic signals have the same sign (positive for intense peaks corresponds to the allowed transition to the p levels, and negative for the very weak signals of the forbidden transitions to the d levels). The same signs (or directions) of the XMCD signals from cobalt and iron sites can be preliminary considered as a local V.204

Figure S.V- 1 .

 1 Figure S.V-1. Packing arrangement illustrating π-π interactions present between adjacent complexes in 8b under different experimental conditions: at 90 K (top), at 298 K (bottom). All anions, lattice solvent and hydrogen atoms are eliminated for clarity.

Figure S.V- 3 .

 3 Figure S.V-3. Optical reflectivity spectra of 8b at 10 K measured before (green) and after 5 min of irradiation (0.2 mW/cm 2 ) at different wavelengths delivered by 12 different LED sources.

Figure S.V- 4 .

 4 Figure S.V-4. Time evolution of R 800 for 8b during three successive cycles of 180 min irradiation with white light (0.4 mW/cm 2 ) by 160 min of no irradiation.

Figure S.V- 9 .

 9 Figure S.V-9. The XANES at room temperature at Fe and Co K edges emphasizing reversibility, after relaxation from photo-induced state for 8b.

  de la Matière Condensée de Bordeaux IMPMC Institut de minéralogie, de physique des matériaux et de cosmochimie ISM 7,8-tetramethyl-1,10-phenanthroline Tp tri(pyrazol-1-yl)hydroborate Tp* tris(3,5-dimethylpyrazol-1-yl)hydroborate

  poudres ou de solides cristallins, tandis que les dispositifs technologiques nécessitent un certain degré d'autoorganisation. Ainsi, un des grands défis dans le domaine du magnétisme moléculaire est de mettre en forme ces composés au sein de matériaux technologiquement viables. À cet égard, la chimie moléculaire offre des possibilités uniques, car elle permet de concevoir de nouveaux matériaux hybrides qui combinent plusieurs propriétés physiques dans un même système, ce qui est difficile, voire impossible à réaliser en chimie du solide ou en chimie inorganique. Afin d'obtenir des matériaux hybrides magnétiques avec des propriétés avancées, la modification de la partie organique des matériaux moléculaires par des substituants connus pour induire des propriétés physiques supplémentaires (phases liquide-cristallines ou augmentation de la solubilité) est une stratégie de choix car elle peut apporter des comportements multifonctionnels et/ou des effets de synergie entre propriétés. L'objectif principal de cette thèse est de développer des matériaux hybrides magnétiques avec des propriétés améliorées via une fonctionnalisation rationnelle de complexes moléculaires magnétiques bien connus. La conception d'hybrides magnétiques est une des principales thématiques de recherche du groupe Matériaux Moléculaires et Magnétisme au Centre de Recherche Paul Pascal. La stratégie mise en oeuvre consiste à modifier des complexes magnétiques connus en fonctionnalisant leurs ligands organiques avec divers substituants permettant d'induire des propriétés liquide-cristalline, ou tout simplement, permettant d'améliorer la solubilité dans des solvants usuels. Bien que cette approche puisse sembler évidente, elle est encore relativement peu développée et seuls quelques groupes de recherche étudient actuellement cette stratégie pour créer de nouveaux matériaux hybrides magnétiques. Cette approche peut être bénéfique pour: (i) isoler et protéger le coeur magnétique par une enveloppe organique limitant ainsi les interactions entre les objets; (ii) orienter, aligner et organiser les unités magnétiques, grâce à l'auto-organisation des mesophases; (iii) avoir la fluidité nécessaire pour la mise en forme de ces matériaux; (iv) améliorer la solubilité et/ou la stabilité thermique; et (v) contrôler ou améliorer les propriétés chimiques ou physiques de l'objet original. Dans ce travail, nous avons cherché à concevoir et d'étudier de nouveaux matériaux hybrides basés sur deux systèmes moléculaires connus pour posséder des propriétés magnétiques remarquables: (a) le complexe dodecanucléaire [Mn 12 ] de formule générale [Mn 12 O 12 (OAc) 16 (H 2 O) 4 ], qui fait partie de la grande famille des molécule-aimants (SMM) et (b) les carrés moléculaires cyanido-pontés {Fe 2 Co 2 } de formule {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 ]}[OTf] 2 , qui sont classés dans la catégorie des complexes à transfert d'électron. Ce manuscrit de thèse comprend cinq chapitres qui couvrent les deux systèmes moléculaires suivants: les molécule-aimants, et les systèmes à transfert d'électron. Ainsi, les concepts fondamentaux des deux systèmes moléculaires choisis pour ces travaux sont résumés dans la première partie du chapitre I. Dans la deuxième partie de ce chapitre, une étude bibliographique fait un bref état de l'art des études menées sur des hybrides magnétiques possédant des propriétés de cristal-liquides ou de systèmes solubles. Le chapitre II est consacré à la préparation d'hybrides magnétiques basés sur la molécule-aimant [Mn 12 ] en vue de lui induire des propriétés de cristal-liquide. Le chapitre commence par une brève description des complexes de type [Mn 12 ] que nous avons choisi, et présente les précédentes études et approches de fonctionnalisation utilisées pour induire des phases cristal-liquide. Deux stratégies ont ainsi été appliquées sur le complexe [Mn 12 -OAc] et ont consisté : (a) à la fonctionnalisation des ligands carboxylate périphériques avec des groupements fortement lipophiles (longues chaînes alkyle), ou (b) au greffage de promoteurs mésomorphes via des espaceurs aliphatiques. Les deux stratégies permettent d'améliorer les interfaces et les zones de compatibilités entre les différentes parties (rigides et souples) et ont précédemment été utilisées avec succès pour obtenir des mésophases thermotropiques avec des objets moléculaires

Schéma R- 2 .

 2 Structure chimique des ligands HL i (i = 5-6) utilisés pour la fonctionnalisation du complexe Mn 12 Dans la deuxième partie de ces travaux, nous avons choisi des ligands porteurs de chaînes (cyanobiphenyloxy)decyle et (biphenyloxy)decyle (Schéma R-2) et d'étudier les complexes [Mn 12 ] correspondants. Avec pour but de réduire les interactions intermoléculaires, et ainsi la température de clarification, le promoteur mésomorphe biphenyle (ligand HL 6 ) a été utilisé et comparé à son analogue cyanobiphenyle (ligand HL 5 ). Les six complexes [Mn 12 ] ont été synthétisés avec succès et entièrement caractérisés. En contraste avec le complexe original [Mn 12 -OAc] qui se décompose au-dessus de 320 K, les nouveaux composés sont thermiquement stables jusqu'à 420 K avec les ligands portant de longues chaînes et jusqu'à 470 K avec le ligand 4-méthoxybenzoate. A partir des mesures de susceptibilité magnétique ac (Figure R-1), la conservation du comportement molécule-aimant a été confirmée pour tous les composés. Les complexes présentent également un isomérisme Jahn-Teller (JT) comme le montrent les mesures de la partie imaginaire de la susceptibilité ac et la structure rayons X du complexe modèle. Des différences significatives de la relaxation magnétique résultant de la variation du ratio en isomères JT, ont été observées selon la fonctionnalisation appliquée.

Figure R- 1 .

 1 Figure R-1. χ′T et χ′′ en fonction de la température pour les complexes [Mn 12 O 12 (L i ) 16 (H 2 O) 4 ] (i = 2 -6), mesurés sous différentes fréquences d'oscillation (1-1500 Hz) à champ dc nul. Les lignes solides sont des guides pour les yeux.

Figure R- 2 . 20 Figure R- 3 .( 1 )( 2 )( 3 )( 4 )

 22031234 Figure R-2. Représentation schématique de l'organisation lamellaire des complexes au sein des mésophases smectiques. Les groupes équatoriaux sont omis pour plus de clarté.

Figure R- 4 .

 4 Figure R-4. Représentation schématique de la stratégie utilisée pour obtenir des systèmes plus solubles en modifiant les positions 4 et 4' des ligands bipyridine (flèches bleues). Couleurs du schéma Fe : vert, Co : rose, B : jaune, N : bleu, C : noir.

Figure R- 5 .

 5 Figure R-5. Produit χT en fonction de la température pour les composés 5 à 0.1 T (gauche) et 6 (droite, 1-noir : 1 er refroidissement; 2-rouge 1 er chauffage et 3-bleu 2 eme refroidissement à 0.4 K/min, 0.5 T).

Figure R- 6 .

 6 Figure R-6. Mesures χT vs T pour 5 (gauche) et 6 (droite) dans des solutions de méthanol, d'éthylacétate, d'acétone et de dichlorométhane (0.4 K/min, 1 T, les lignes pleines sont des courbes modèles pour des solutions idéales).

Figure R- 7 .

 7 Figure R-7. (Gauche) Produit χT en fonction de la température pour le composé 7 à 0.1 T. (Centre) Produit χT en fonction de la température pour le composé 8a en mode chauffage (points verts, 0.1 T, 0.3 K/min) après irradiation sous lumière blanche (3 mW/cm 2 ) à 10 K pendant ~ 17 h en mode refroidissement depuis 250 K et à l'abri de lumière (points bleus, 0.1 T, 0.3 K/min). (Droite) Produit χT en fonction de la température pour le composé 8b en mode chauffage (points verts, 0.1 T, 0.3 K/min) après irradiation sous lumière blanche (3 mW/cm 2 ) à 10 K pendant ~ 13 h et en mode refroidissement depuis 250 K et à l'abri de lumière (points bleus, 0.1 T, 0.3 K/min).

Figure R- 8 .

 8 Figure R-8. Mesures χT vs T pour 7 dans des solutions de méthanol et dichlorométhane (gauche) et 8 dans des solutions de dichlorométhane et butyronitrile (droite), 0.4 K/min en mode refroidissement, 1 T. Les lignes pleines sont des courbes modèles pour des solutions idéales.

Fonctionnalisation raisonnée de matériaux moléculaires magnétiques: vers des systèmes cristaux liquides, solubles, et aux propriétés physiques modulables

  There is one very important thing I learned after three and a half years of my doctoral studies -none would have been possible without the participation, support and encouragement from many people. The list of people, who assisted, supported and guided me over the past years and helped me to arrive where I am today, is very long and I wish to thank everybody who was involved in the development of this work. First of all, I would like to thank Philippe Richetti for welcoming me at the Centre of Research Paul Pascal and participating in my thesis defense jury. I thank Prof. Talal Mallah and Prof. Jaume Veciana for their time and consideration to review this thesis; Dr. Franck Camerel and Dr. Philippe Barois for their participation in the thesis defense jury. Thank you all for dedicating your time to assess my manuscript. I am deeply grateful to my supervisor, Rodolphe Clérac, for having introduced me into an interesting research field, molecule-based magnetic materials. I thank him for the guidance, and encouragement, which

		Sous la direction de : Rodolphe Clérac
		Soutenue le 30 Avril 2014
	Membres du jury :
	M. Talal Mallah	Professeur, Université Paris-Sud 11	Rapporteur
	M. Jaume Veciana	Professeur, Institut de Ciència	Rapporteur
		de Materials de Barcelona
	M. Rodolphe Clérac	Directeur de recherche, CNRS	Directeur de thèse
	M. Philippe Barois	Directeur de recherche, CNRS	Examinateur
	M. Franck Camerel	Chargé de Recherche, CNRS	Examinateur
	M. Philippe Richetti	Directeur de recherche, CNRS	Examinateur
			i

he provided through this work. His constructive criticism and collaboration have been great assets throughout my PhD. I benefited from his creativity and his intuition in various situations. He has provided guidance at key moments in my work while allowing me to work independently. Thank you, Rodolphe, for setting up the model of a real scientist, and for your patience during the last period of my PhD. I wish to dedicate special thanks to Diana Sîreţanu Codreanu for her help during my first year of PhD at CRPP and University of Bordeaux. She not only helped me to get acquainted with various experimental and analytical techniques at CRPP but also she was the person who guided me in the unknown world of "thousands" of documents and administrative centers that we should pass before and after becoming a PhD student. I would like to thank Corine Mathonière who is always ready for help. Her collaboration and high pedagogical skills were very precious during all my PhD. I thank Pierre Dechambenoit, who arrived in the group same time with me and became fully involved in our projects. He was the person who helped to figure out the problems with single crystal diffraction experiments. Most of the wonderful results obtained in this thesis were obtained as the result of collaborations with many other people. Therefore I would like to thank: (i) Dario Bassani, who allowed me to carry out the temperature dependent "change of colour" observations on his UV-Vis spectrometer; (ii) Eric Collet, Loïc Toupet and Wawrzyniec Kaszub for the photo-crystallography and ultrafast optical measurements; and (iii) Marie-Anne Arrio, Philippe Sainctavit, Edwige Otero, Andrei Rogalev and Fabrice Wilhelm for introducing me to the synchrotron research and XMCD measurements.

1. Single-Molecule Magnets I.1.1.1. Generalities It

  has been nearly 20 years since the discovery that single molecules of a dodecamanganese coordination complex, with formula [Mn 12 O 12 (O 2 CCH 3 ) 16 (H 2 O) 4 ] (Mn 12 -OAc), can act as nanomagnets at

  1.50 [Fe(CN) 6 ]•6.3H 2 O is paramagnetic and shows no thermally or photo-induced ET; Na 0.94 Co 1.15 [Fe(CN) 6 ]•3.0H 2 O is diamagnetic (between 2 and 350 K) and displays no photo-induced ET; while Na 0.60 Co 1.37 [Fe(CN) 6 ]•3.9H 2 O, Na 0.53 Co 1.32 [Fe(CN) 6 ]•4.4H 2 O and Na 0.37 Co 1.37 [Fe(CN) 6 ]•4.8H 2 O besides

  2

  •S, where L 1 is a derivative of trispyrazolylborate (Tp) and L 2 is a bidentate ligand, mostly derivatives of 2,2'-bipyridine (bpy), A is a monovalent counter-anion and S corresponds to lattice solvents. The first reported representative of this family of rationally built molecular systems, {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 ]}[OTf]

  2

Functionalization of Single-Molecule Magnets: Towards Liquid Crystalline Phases

	The design of hybrid molecule-based magnetic systems via rational ligand functionalization is one of
	the main interests of the Molecular Materials and Magnetism research group at the Centre de Recherche Paul
	Pascal. The employed strategy consists in selection of known complexes from different magnetic classes and Chapter II
	modification of their organic ligands with various functional groups known to induce liquid crystalline
	properties or, simply, improve the solubility. Although this approach may seem obvious, it is still relatively
	undeveloped and only a few research groups are currently exploring this strategy to create new molecule-
	based hybrid magnetic materials. The functionalization of organic ligands of known systems can help: (i) to
	isolate and protect the magnetic core by an organic shell in order to prevent interactions between objects; (ii)
	to orient, align, and organize magnetic units, thanks to self-organization of the liquid crystalline phase; (iii)
	to have fluidity and easy processability for shaping and further applications; (iv) to improve the solubility
	and thermal stability; and (v) to control or optimize the chemical or physical properties of the original object.
	In this work, we aim to design and study the properties of new hybrid materials by using two
	molecular systems known to possess remarkable magnetic properties: (a) the dodecanuclear complex
	[Mn 12 O 12 (OAc) 16 (H 2 O) 4 ] that is a part of the large family of single-molecule magnets (SMMs); and (b) the
	tetranuclear cyanido-bridged {Fe 2 Co 2 } molecular squares, {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 ]}[OTf] 2 , that are
	categorized into the class of electron transfer complexes.
	Chapter II is dedicated to functionalization of Mn 12 -based SMMs towards hybrid liquid crystalline
	systems via two different approaches. To help the reader, additional information regarding the family of
	Mn 12 -based SMMs is also presented in the beginning of the second chapter. Chapters III -V are focused on
	cyanido-bridged molecular squares that exhibit thermally or photo-induced ET. Thus, in Chapter III is
	discussed the possibility to modulate the structural and the ET properties in {Fe 2 Co 2 } molecular squares via
	light irradiation or counter-anion exchange. The functionalization with long aliphatic chains and its effect
	over the properties of {Fe 2 Co 2 } molecular squares in solid state and solutions are discussed in Chapter IV.
	While the effect of functionalization with strongly electron density donating methoxy group over the ET
	properties of {Fe 2 Co 2 } molecular squares is investigated in Chapter V.
	II.43

Table of Contents for Chapter II: II.1. Introduction An

  important research field at the interface of classical and quantum worlds was opened after the discovery that certain molecules can retain magnetization at molecular level. Thanks to the fact that they

II.2. Mn 12 -based SMMs: Versatile target for functionalization II.2.1. Why Mn 12 ?

	One of the most famous and, respectively, widely studied families of SMMs is the one derived from
	dodecanuclear manganese complex with a general formula [Mn III	8 Mn IV	4 O 12 (CH 3 COO) 16 (H 2 O) 4 ] (Mn 12 -OAc
	or Mn 12 -acetate for short, Figure
	for each group separately.
	II.46

  Mn 12 O 12 (CHCl 2 COO) 8 (Bu t CH 2 COO) 8 (H 2 O) 3 ], 2:0:1. Color scheme: Mn(III) dark green, Mn(IV) green, O red, O water blue.or II, respectively). Three or four H 2 O molecules are bound only to Mn III ions of type II. The original Mn 12 -

	3 COO) 16 (H 2 O) 4 ], 1:1:1:1 isomer; (b) [Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ], 2:0:2
	isomer; (c) [Mn 12 O 12 (p-Me-PhCOO) 16 (H 2 O) 4 ], 1:2:1 isomer; (d) [Mn 12 O 12 (C 2 H 5 COO) 16 (H 2 O) 3 ], 1:1:1 isomer; (e)
	[OAc has a 1:1:1:1 arrangement of coordinated H 2 O molecules (Figure II-3a, one H 2 O ligand on each Mn III
	ion in the set of four type II Mn III ). The four water molecules in [Mn 12 O 12 (PhCOO) 16 (H 2 O) 4 ] and in
	[Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ] are arranged in a 2:0:2 fashion (Figure II-3b, two H 2 O ligands on two Mn III
	type II ions). 9,10 Also, 1:2:1 and 1:1:2 isomers (Figure II-3c) have been found in different Mn 12 -based
	molecules. 1d,11 The exact number and the coordination position of the water molecules can vary from one
	Mn 12 complex to another. Thus, [Mn 12 O 12 (C 2 H 5 COO) 16 (H 2 O) 3 ] has only three coordinated water molecules
	in a 1:1:1 fashion (Figure II-3d), with one type II Mn III ion that is five-coordinated. While for
	[Mn 12 O 12 (CHCl 2 COO) 8 (Bu t -CH 2 COO) 8 (H 2 O) 3 ], the 2:0:1 isomer was reported (Figure II-3e).

  Mn 12 ligands can be replaced with non-carboxylate ligands. The treatment of Mn 12 -OAc with 8 eq. of benzenesulfonic acid yields [Mn 12 O 12 (CH 3 COO) 8 (PhSO 3 ) 8 (H 2 O) 4

	reaction between Mn(CH 3 COO) 2 and KMnO 4 in 60% aq. acetic acid in an appropriate ratio, according to the In a site-specific way,
	following simplified equation:
	44 Mn II + 16 Mn VII  5 (8 Mn III + 4 Mn IV )
	Similarly, [Mn 12 O 12 (PhCOO) 16 (H 2 O) 4 ] can be obtained by comproportionation of nBu 4 NMnO 4 with
	Mn(CH 3 COO) 2 in the presence of benzoic acid, 1c and [Mn 12 O 12 (p-R-PhCOO) 16 (H 2 O) 4 ] (R = Cl, CH 3 , tBu) by
	the analogous reaction of Mn(ClO 4 ) 2 and nBu 4 NMnO 4 in ethanol in the presence of p-R-C 6 H 4 COOH; 17 the
	yields are generally less than those obtained by carboxylate substitution method (see below).
	The second method of Mn 12 derivatives synthesis is based on ligand substitution reactions, 18 which are
	driven by the greater acidity of the added carboxylic acids RCO 2 H and/or the removal by distillation of an
	azeotrope of acetic acid and toluene:
	[Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] + 16 RCO 2 H	[Mn 12 O 12 (RCO 2 H) 16 (H 2 O) 4 ] + 16 CH 3 COOH
	It has to be emphasized that ligand substitution is a reversible process. In order to shift the reaction
	equilibrium towards the formation of desired products, several treatments with the new carboxylic acid are
	sometimes needed (to replace all acetate groups) or the reactions are carried out in the presence of an excess
	of the substituting ligands. The main advantage of this technique is that reaction yields are generally larger
	than those obtained in the previous approach. Many carboxylate substitutions have been successful in the
	Mn 12 chemistry using this approach. 18
	Derivatives of Mn 12 -based SMMs with ligands other than carboxylates or with site-specific
	modifications yielding mixed-carboxylate [Mn 12 O 12 (RCOO) 8 (R'COO) 8 H 2 O) 4 ] type complexes, have been
	achieved. 12 The structural JT distortion (discussed above) has also important consequences on the relative
	reactivity of the carboxylate groups. The JT elongation of a metal-ligand bond weakens that bond. For a
	donor ligand such as carboxylate, JT elongation will serve to maintain a greater electron density on their O
	atoms compared with the O atoms of Mn-O bonds which are not elongated by JT effect. This will make
	RCOO -groups possessing an O atom lying on a JT axis more sensitive to electrophilic attack at the O atom.
	This remark could form the basis for selective subtraction of these RCOO -groups. Thus, addition of 8
	equivalents of R′COOH to a pre-formed [Mn 12 O 12 (RCOO) 16 (H 2 O) 4 ] (where R′COOH is a stronger acid than
	RCOOH) allows direct access to the mixed-carboxylate product [Mn 12 O 12 (RCOO) 8 (R′COO) 8 (H 2 O) 4 ]:
	[Mn 12 O 12 (RCOO) 16 (H 2 O) 4 ] + 8 R′COOH	[Mn 12 O 12 (RCOO) 8 (R′COO) 8 (H 2 O) 4 ] + 8 RCOOH
	This is, indeed, not generally a useful method to obtain mixed-carboxylates, because a smaller pKa
	difference between the incoming and leaving acids yields a mixture containing species with different
	carboxylate compositions. A better approach used to prepare mixed-carboxylate complexes is to use an
	equimolar reaction of Mn 12 homo-carboxylate species containing 16 RCOO -ligands, and 16 R′COO -ligands, There are basically two synthetic procedures available for making new [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] complexes. The first involves the comproportionation between a Mn II source and Mn VII from MnO 4 respectively. The resulting 8:8 product has ligands originating from a stronger acid (small pKa, ex. R′ = -in the Cl 2 CH) in the axial positions and ligands originating from a weaker acid (higher pKa, ex. R = t BuCH 2 ) in the presence of the desired carboxylic acid (RCOOH). This was the original method used by Lis 1a to synthesize equatorial positions. Two examples of such mixed carboxylate complexes are [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] complex with a high yield of crystalline material. The method includes the [Mn 12 O 12 (Cl 2 CHCOO) 8 ( t BuCH 2 COO) 8 (H 2 O) 4 ] and [Mn 12 O 12 (Cl 2 CHCOO) 8 (C 2 H 5 COO) 8 (H 2 O) 4 ]. 12
	II.50	II.49

Scheme II-3. Structure

  of dendritic ligands reported to obtain Mn 12 -based complexes showing cyanobiphenyl induced lamellar phases. 24b

	cyanobiphenyl groups. Even though the methylation of cyanobiphenyl group helped decreasing the clearing
	point by 60 degrees, it still remained quite high; i.e. 411 K.
	Later in 2013, a series of Mn 12 -based complexes with benzoic acid derivatives containing a varying
	number of (cyanobiphenyloxy)undecyl chains was reported. 24c The clearing point was above decomposition
	temperature and the self-organization of the reported complexes inside the liquid crystalline phase was
	influenced by the presence of strong intermolecular interactions driven by cyanobiphenyl mesogenic group,
	and not by the number of attached mesogenic groups or ligand symmetry similarly to the compounds
	reported in 2011. 24b,c
	X = H or CH 3
	II.52

12 -based SMMs with strongly lipophilic groups II.4.1. Synthetic procedures and purification

	The Mn 12 -based hybrids were obtained from [Mn 12 O 12 (OAc) 16 (H 2 O) 4 ] by ligand exchange technique,
	taking the advantage of the ability of Mn 12 molecules to undergo carboxylate substitution (via ligand
	exchange on the pre-formed Mn 12 complex). This feature provides an extremely useful and convenient
	approach to get other carboxylate derivatives of Mn 12 . The acetate groups were substituted by corresponding
	carboxylic derivatives (HL i ):
	[Mn 12 O 12 (OAc) 16 (H 2 O) 4 ] + 16HL i	[Mn 12 O 12 (L i ) 16 (H 2 O) 4 ] + 16AcOH

Analytical characterizations of substituted Mn 12 complexes II.4.2.1. Single crystal X-ray analysis of the model compound [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ]

and the previously reported [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] complex, were synthesized and purified by the methods presented above. The structural and physical properties of synthesized complexes were characterized by different techniques.

II.4.2.

The crystal structure of the 4-methoxybenzoate-substituted compound, [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ],

Table II - 1 .

 II1 Elemental analyses of [Mn 12 O 12 (L 1-4 ) 16 (H 2 O) x ] (x = 3 or 4) complexes

	Compound	%C (found)	%H (found)	%C (calc.)	%H (calc.)
	[Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ]•H 2 O C 128 H 120 Mn 12 O 64	46.05	3.73	46.01	3.62
	[Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ]•2H 2 O C 304 H 476 Mn 12 O 66	62.30	8.19	62.28	8.21
	[Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ]•6H 2 O C 496 H 868 Mn 12 O 86	67.58	10.01	67.18	9.87
	[Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ]•H 2 O C 688 H 1246 Mn 12 O 99	69.89	10.73	70.25	10.68

Table II -

 II

2. Magnetic Properties for [Mn 12 O 12 (L i ) 16 (H 2 O) x ] (i = 1 -4, x = 3 or 4) Compound S g JT isomers ratios LT (%) / HT (%) Low temperature (LT) or fast relaxing (FR) JT isomer High temperature (HT) or slow relaxing (SR) JT isomer μ 0 H C (T)

  Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] compound.9 There are several examples reported previously which showed the same behaviour (predominant LT isomer), and this can be affected by either the change of ligand or of cocrystallized solvent.

In Figure II-7, it is showed that one of the Mn III ions in [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] has a Jahn-Teller elongation axis almost perpendicular to the other. According to the crystal structure of [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] the JT distorted isomer is predominantly present in this complex, that explains the II.59 occurrence of a significant out-of-phase signal at a lower temperature than for the parent [

Table II -

 II On increasing the temperature in the case of the complex functionalized with a ligand containing only one dodecyl chain, [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ], melting of the glassy solid around 383 K and the formation of large brown (optically isotropic) viscous areas are observed. Upon cooling, the compound becomes a solid glass again, showing no anisotropic region under crossed polarizers. The [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] complex, which has two dodecyl chains attached to its ligand, is an isotropic liquid on both heating or cooling from room temperature to 373 K. Upon heating, the [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] complex

	Compound	Transition a	T / K	ΔH / kJ•mol -1
	[Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ]	Cr -I b	380*	25.5*
	[Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ]	Cr -Cub	242	94.6
		Cub -I	295	23.9
	[Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ]	Cr -Cub	276	205.9
		Cub -dec	423 c	-

3. Thermal data for functionalized [Mn 12 O 12 (L 2-4 ) 16 (H 2 O) 4 ] complexes obtained by DSC at 5 Kmin -1 . a onset point on second heating, Cr = crystal, I = isotropic liquid, Cub = cubic phase, dec = decomposition; b this transition was observed only on first heating in DSC; c determined according to TGA; * data obtained on first heating. As expected, our model compound [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] did not show any mesomorphic properties and remains crystalline until its decomposition around 500 K, whereas other three compounds exhibit thermotropic properties strongly dependent on the density of lipophilic chains attached to the rigid oxometallic core. At room temperature, [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ] looks like a brown glassy solid, [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] is a dark-brown viscous liquid, and [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] is a very sticky dark-brown solid. No birefringent texture could be observed using polarized optical microscopy for these Mn 12 compounds in the explored II.62 temperature ranges (293 -400 K).

.4.4.1. [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ]

  Neither in DSC nor in SAXS [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ] complex showed a thermally reversible behavior.

  3 Å at 393 K), slightly bigger in value than those obtained for [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ] (a = 37.8 Å, crystalline primitive cubic) and [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] (a = 36.5 Å, cubic mesophase with primitive lattice), and about 2.5 times smaller than the one reported (a = 106.1 -108.1 Å). 24aThis complex thermal behavior exhibited by [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] is not surprising since it is known that the presence of intermediate metastable mesophases (with large lattice parameters) and strong influence of thermal cycles are characteristic to thermotropic cubic phases.43

  (L 2 ) 16 (H 2 O) 4 ] or thermotropic cubic mesophases of different symmetry for [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] and [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ].

Table II - 4 .

 II4 Elemental analyses of [Mn 12 O 12 (L 5-6 ) 16 (H 2 O) 4 ] complexes Functionalized [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] complexes were checked for SMM

	Compound	%C (found)	%H (found)	%N (found)	%C (calc.)	%H (calc.)	%N (calc.)
	[Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ]•12H 2 O C 848 H 976 Mn 12 N 32 O 124	71.39	6.93	3.32	71.42	6.90	3.14
	[Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]•9H 2 O C 816 H 1002 Mn 12 O 121	73.08	7.61	-	73.11	7.53	-
	II.5.3. Magnetic measurements

  3

  second plateau in χ′T vs T plots is suggesting that there are two different relaxation processes. These two magnetization relaxation processes in Mn 12 complexes have been elucidated by the Jahn-Teller isomerism, characteristic to Mn 12 family (see sectionsII.2.2 and II.4.3). The HT (high temperature or slowly relaxing) species contain Jahn-Teller (JT) isomers with a near parallel alignment of the eight Mn III JT axes along the molecular z-axis (normal orientation), whereas in the LT (low temperature or fast relaxing) species, one or more Mn III JT elongation axes are abnormally oriented (equatorially rather than axially). The ratio of each isomer present in the measured sample can be quantified by the difference of the two plateaus present in χ′T vs T plots, thus for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] the LT(%)/HT(%) ratio is 55/45 and for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] it is 35/65.

•K•mol -1 for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and 39.1 cm 3 •K•mol -1 for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ] are observed in the 8-12 K range, which correspond to χ′T values expected for complexes with S T = 9 (g = 1.83 for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ] and 1.86 for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]) in the ground state. The decrease of χ′T product below 8 K is accompanied by the appearance of the systematic frequency dependent out-of-phase χ′′ ac susceptibility. The presence of two peaks in the χ′′ vs T plots, as well II.75 as, the

Table II - 5 .

 II5 Magnetic Properties for [Mn 12 O 12 (L 5-6 ) 16 (H 2 O) 4 ] complexes

					Low temperature (LT) or	High temperature (HT) or
	Compound	S	g	JT isomers ratios LT (%) / HT (%)	fast relaxing (FR) JT isomer	slow relaxing (SR) JT isomer	μ 0 H C (T)
					Δ eff /k B (K)	τ 0 (s)	Δ eff /k B (K)	τ 0 (s)
	[Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ]	9	1.83	55 / 45	32.5	1.6×10 -9	60.4	6.2×10 -9	0.22
	[Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]	9	1.86	35 / 65	31.7	1.9×10 -9	60.8	1.1×10 -8	0.72

S: spin ground state; g: Lande factor of ground state; Δ A : potential energy barrier; τ 0 : pre-exponential factor; μ 0 H C : coercive field measured at T = 1.9 K and at 0.035 T•min -1 sweep rate.

Table II - 6 .

 II6 Thermal data for [Mn 12 O 12 (L 5-6 ) 16 (H 2 O) 4 ] complexes obtained by DSC at 5 Kmin -1 .

	Compound	Transition a	T / K	ΔC p / kJ•mol -1 •K -1	ΔH / kJ•mol -1
	[Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ]	g -Sm b	301	7.6	-
		Sm -I c	442.5	-	50.1
	[Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]	g -Sm	295.6	8.3	-
		Sm -I	347.5	-	73.

7. Supporting materials II.7.1. Experimental protocols

  complexes [Mn 12 O 12 (L i ) 16 (H 2 O) 4 ] and [Mn 12 O 12 (AcO) 16 (H 2 O) 4 ]. A ligand exchange reaction between the two compounds will generate heterogeneous [Mn 12 O 12 (L i ) 8On the other hand the functionalized complexes presented in this work, [Mn 12 O 12 (L[3][4][5][6] ) 16 (H 2 O) 4 ], can be used as precursors for systems of discotic geometry. The HL[3][4][5][6] ligands are weak acids, therefore the treatment of one of the [Mn 12 O 12 (L 3-6 ) 16 (H 2 O) 4 ] complexes with a complex containing a stronger carboxylic acid, i.e. [Mn 12 O 12 (Cl 2 CHCOO) 16 (H 2 O) 4 ], may lead to heterogeneous [Mn 12 O 12 (L i ) 8Since the acid with a lower pKa will go preferentially to axial positions whereas the functionalized ligands will remain in (or occupy) the equatorial positions, thus, forming a hybrid discotic molecule.The proposed strategies will be developed during the PhD thesis of Elena Darbinean (2013-2016), who will carry on the regioselective functionalization of SMMs in order to get liquid crystalline phases with columnar or nematic orders. Solvents, starting materials, 4-methoxybenzoic acid (HL 1 ) and 4-dodecyloxybenzoic acid (HL 2 ) were purchased from Acros, Alfa Aesar or Aldrich, and used without further purification unless otherwise stated. The mixture was heated to reflux and 1-bromododecane (13.00 g, 52.20 mmol) and a catalytic amount of potassium iodide were added. The reaction mixture was then refluxed for 4 days. The solvent was removed and CH 2 Cl 2 (200 mL) was added. The suspension was washed with water (3x100 mL).The organic layer was dried over anhydrous sodium sulfate, filtrated and concentrated. The obtained white vitreous solid (MeL 3 ) was recrystallized from ethanol (500 mL), filtrated, washed with cold ethanol and dried under vacuum. Ethanol (150 mL) and potassium hydroxide(5.00 g, 89.10 mmol), dissolved previously in 20 mL of water, were added to MeL 3 . The solution was refluxed for 12 h, H 2 O (100 mL) was added and then acidified to pH = 1 with a 37 % HCl aqueous solution. Ethanol was removed and the resulting mixture was washed with CH 2 Cl 2 (3x100 mL). The organic layers were combined and dried over anhydrous sodium sulfate and concentrated. The product was dried for24 h under vacuum and 8.19 g (16.22 mmol, 74%) of HL3 were obtained, as a white vitreous solid.

	ax (AcO) 8 eq (H 2 O) x ].
	Preparation of 3,5-bis(dodecyloxy)benzoic acid (HL 3 ). To a solution of methyl 3,5-
	dihydroxybenzoate (3.65 g, 21.7 mmol) in 150 mL of acetone, anhydrous potassium carbonate (26.95 g,
	195.1 mmol) was added.	eq (Cl 2 CHCOO) 8	ax (H 2 O) x ].

II.

1

H NMR (400 MHz) in CDCl 3 (δ, ppm): 7.22 (d, J = 2.3 Hz, 2H), 6.69 (t, J = 2.3 Hz, 1H), 3.98 (t, J = 6.5 Hz, 4H), 1.83 -1.73 (m, 4H), 1.50 -1.40 (m, 4H), 1.38 -1.23 (m, 32H), 0.88 (t, J = 6.8 Hz, 6H). Elemental analysis -Calc. (Found) for C 31 H 54 O 4 : C, 75.87 (75.90); H,

Preparation of [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ].

  Mn(CH 3 COO) 2 •4H 2 O (4.00 g, 16.32 mmol) was dissolved in 40 mL of 60 % CH 3 COOH at room temperature. After cooling down the solution to 0°C, KMnO 4 (1.00 g, 6.33 mmol) was slowly added. The mixture was then warmed up to room temperature while stirring during 30 minutes (until the KMnO 4 was totally dissolved). The solution was filtered and the filtrate was allowed to stand at room temperature for 1 day. The resulting black crystals were filtrated and washed

	with acetone (100 mL) and tetrahydrofuran (THF, 100 mL). The yield was 57.8 % (2.20 g). Selected FT-IR
	data (ATR, cm -1 ): 3400 (br), 2986 (w), 2934 (w), 1754 (w) 1709 (m), 1560 (s), 1413 (s), 1381 (s), 1017 (m),
	709 (m), 666 (s).
	Preparation of [

1

H NMR (400 MHz) in CDCl 3

(δ, ppm): 7.29 (s, 2H), 4.06 - 3.98 (m, 6H), 1.85 -1.69 (m, 6H), 1.51 -1.42 (m, 6H), 1.36 -1.21 (m, 48H)

, 0.86 (t, J = 6.8 Hz, 9H).

Elemental analysis -Calc. (Found) for C 43 H 78 O 5

: C, 76.50 (75.53); H, 11.65 (11.72)

. Selected FT-IR data (ATR, cm -1 ): 2954 (m), 2917 (vs), 2848 (s), 1679 (vs), 1585 (s), 1429 (s), 1225 (s), 1119 (vs). II.

85

Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ].

  To a suspension of [Mn 12 O 12 (CH 3 COO)16 (H 2 O) 4 ] (0.10 g, 5.25•10 -2 mmol) in 20 mL of CH 2 Cl 2 /toluene 1:1, 4-methoxybenzoic acid (HL 1 ) (0.22 g, 1.46 mmol, 28 eq.) was added. The mixture was stirred at 45°C during 24 h. The color of solution changed from brown to brownish black. The solution was filtrated and the solvent was evaporated. Toluene (20 mL) was added to the residue, stirred for 3 hours, and then evaporated to dryness. This procedure was repeated three more times. The residue was re-dissolved in CH 2 Cl 2(5 mL) and crystallized by slow diffusion of Et 2 O after 3 days at 4°C. The resulting crystals were collected by filtration, washed with Et 2 O. The yield was 0.13 g (72.6 %). The crystals for X-ray diffraction study were re-crystallized with the same method. Elemental analysis -Calc. (Found) for C 128 H 120 Mn 12 O 64 ([Mn 12 O 12 (L 1 ) 16 (H 2 O)

3 ]•H 2 O):

C, 46.01 (46.05); H, 3.62 (3.73)

. Selected FT-IR data (ATR, cm -1 ): 2905 (w), 2836 (w), 1602 (s), 1412 (vs), 1250 (s), 1170 (s), 1026 (m), 847 (w), 776 (m).

Preparation of [Mn 12 O 12 (L i ) 16 (H 2 O) 4 ] (i = 2-4).

  To a suspension of [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]

Preparation of methyl 3,5-bis[(10-(4'-cyanobiphenyl-4-yloxy)decyl)oxy]benzoate (MeL 5 ).

  The resulting residue was re-dissolved in CH 2 Cl 2 (1-2 mL) affording a viscous mixture, which was purified by two successive gel permeation chromatography with Biorad S-X3 beads swelled with CH 2 Cl 2 . The product was dried in vacuum at 40°C overnight. The yields were 66 % for [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ], 57 % for [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] and 62 % for [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ], based on [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]. After filtration, washing with CH 2 Cl 2 and evaporation of the solvent, the crude product was purified by silica gel column chromatography (CH 2 Cl 2 /pentane 1:9). The pale yellow solid was collected after solvent evaporation, then, filtrated and washed with pentane. The product was dried at 60°C overnight under vacuum to obtain compound 1 CN as white crystalline solid (3.87 g, 73%).1 H NMR (400 MHz) in CDCl 3 (δ, ppm): 7.66 (dd, J = 20.5, 8.1 Hz, 4H), 7.52 (d, J = 8.9 Hz, 2H), 6.99(d, J = 8.3 Hz, 2H), To a solution of methyl 3,5-dihydroxybenzoate (452 mg, 2.69 mmol) with anhydrous potassium carbonate (1.9 g, 13.45 mmol) in butanone (100 mL) was added 1 CN (2.45 g, 5.92 mmol) and KI in catalytic amounts. The mixture was stirred at reflux under argon atmosphere for 2 days. After addition of H 2 O (150 mL), the product was extracted with CH 2 Cl 2 (4x50 mL). The organic layer was washed with H 2 O (3x50 mL), dried over anhydrous Na 2 SO 4 , filtrated and evaporated. The crude product was purified by silica gel column chromatography (CH 2 Cl 2 /pentane 2:8). The final product was dried at 60°C under vacuum over night to obtain compound MeL 5 as white solid(1.52 g, 66%).

	4.00 (t, J = 6.5 Hz, 2H), 3.41 (t, J = 6.6 Hz, 2H), 1.89 -1.77 (m, 4H), 1.52 -1.40 (m, 4H), 1.30 (m, 8H).
	Elemental analysis -Calc. (Found) for C 23 H 28 BrNO: C, 66.67 (66.59); H, 6.81 (6.89); N, 3.38 (3.45).
	Elemental	analysis	-	Calc.	(Found)	for	C 304 H 476 Mn 12 O 66
	([Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ]•2H 2 O): C, 62.28 (62.30); H, 8.21 (8.19). Selected FT-IR data (ATR, cm -1 ): 2925 (m),
	2853 (m), 1607 (s), 1410 (vs), 1257 (s), 1173 (m), 849 (w), 780 (w). Elemental analysis -Calc. (Found) for
	C 496 H 868 Mn 12 O 86 ([Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ]•6H 2 O): C, 67.18 (67.58); H, 9.87 (10.01). Selected FT-IR data
	(ATR, cm -1 ): 2920 (s), 2852 (s), 1595 (s), 1412 (vs), 1164 (m), 840 (w), 779 (w). Elemental analysis -Calc.
	(Found) for C 688 H 1246 Mn 12 O 99 ([Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ]•H 2 O): C, 70.25 (69.89); H, 10.68 (10.73). Selected FT-

IR data (ATR, cm -1 ): 2920 (s), 2852 (s), 1567 (s), 1409 (vs), 1227 (s), 1112 (m), 868 (w), 777 (w).

Preparation of 4'-((10-bromodecyl)oxy)biphenyl-4-carbonitrile (1 CN

). To a solution of 4'hydroxybiphenyl-4-carbonitrile

(2.5 g, 12.81 mmol)

with anhydrous potassium carbonate

(3.54 g, 25.62 mmol)

in butanone (125 mL) was added 1,10-dibromodecane

(14.34 mL, 64.03 mmol)

. The mixture was stirred under reflux overnight.

1

H NMR (400 MHz) in CDCl 3 (δ, ppm): 7.64 (dd, J = 20.9, 8.6 Hz, 8H), 7.51

(d, J = 8.8 Hz, 4H), 7.15 (d, J = 2.3 Hz, 2H), 6.97 (d, J = 8.8 Hz, 4H), 6.62 (t, J = 2.3

Hz, 1H),

3.97 (dt, J = 15.7, 6.5 Hz, 8H), 3.88 (s, 3H), 1.84 -1.71 (m, 8H), 1.51 -1.28 (m, 24H)

. Elemental analysis -Calc. (Found) for C 54 H 62 N 2 O 6 : C, 77.67

(77.61)

; H,

  butanone was heated under reflux for 24 h. The mixture was filtered hot and washed with CH 2 Cl 2 .

	= 8.7 Hz, 4H), 6.64 (t, J = 2.1 Hz, 1H), 3.98 (t, J = 6.6 Hz, 4H), 3.95 (t, J = 6.6 Hz, 4H), 1.84 -1.70 (m, 8H),
	1.51 -1.39 (m, 8H), 1.39 -1.27 (m, 16H). Elemental analysis -Calc. (Found) for C 53 H 60 N 2 O 6 : C, 77.53
	(77.49); H, 7.37 (7.42); N, 3.41 (3.51). Selected FT-IR data (ATR, cm -1 ): 2922 (vs), 2852 (s), 2224 (s),1687
	(vs), 1601 (s), 1484 (s), 1301 (s), 1155 (s).
	Preparation of 4-((10-bromodecyl)oxy)biphenyl (1 H ). A mixture of 4-hydroxybiphenyl (2 g, 11.8
	mmol), 1,10-dibromodecane (17.62 g, 58.7 mmol), and anhydrous potassium carbonate (3.24 g, 23.5 mmol)
	in 50 mL

1

H NMR (400 MHz) in CDCl 3 (δ, ppm): 7.64 (dd, J = 20.9, 8.3 Hz, 8H), 7.50

(d, J = 8.7 Hz, 4H), 7.18 (d, J = 2.2 Hz, 2H), 6.97 (d, J

  Hz, 2H),3.98 (t, J = 6.6 Hz, 2H), 3.40 (t, J = 6.9 Hz, 2H), 1.90 -1.75 (m, 4H), 1.51 -1.38 (m, 4H), 1.39 -1.24 (m, 8H). Elemental analysis -Calc. (Found) for C 22 H 29BrO: C, 66.86 (66.79); H,

1

H NMR (400 MHz) in CDCl 3 (δ, ppm): 7.52 (dd, J = 13.8, 8.0 Hz, 4H), 7.40 (t, J = 7.6 Hz, 2H), 7.29 (t, J = 7.3 Hz, 1H),

6.96 (d, J II.87 = 8.6

Preparation of 3,5-bis((10-(biphenyl-4-yloxy)decyl)oxy)benzoic acid (HL 6

  ). A mixture ofmethyl 3,5-dihydroxybenzoate (0.33 g, 1.96 mmol), 1 H (1.57 g, 4.03 mmol) and anhydrous potassium carbonate(1.42g, 10.3 mmol) in 50 mL butanone was heated under reflux for 4 days. After cooling the mixture to room temperature, 100 mL of water was added . The resulting mixture was extracted several times with CH 2 Cl 2 . Combined organic fractions were dried over anhydrous Na 2 SO 4 . The solvent was removed under reduced pressure. The obtained solid was purified by flash column chromatography on silica gel using pentane/dichloromethane (7:3) as eluent to yield 1.47 g (62%) of white crystals.1 H NMR (400 MHz) in

	CDCl 3 (δ, ppm): 7.57 -7.47 (m, 8H), 7.39 (t, J = 7.7 Hz, 4H), 7.31 -7.25 (m, 2H), 7.15 (d, J = 2.3 Hz, 2H),
	6.95 (d, J = 8.6 Hz, 4H), 6.62 (t, J = 2.3 Hz, 1H), 4.02 -3.93 (m, 8H), 3.88 (s, 3H), 1.87 -1.72 (m, 8H), 1.51
	-1.39 (m, 8H), 1.33 (s, 16H). Elemental analysis -Calc. (Found) for C 52 H 64 O 6 : C, 79.56 (79.49); H, 8.22
	(8.31).

). To a solution of MeL

6

(1.43g, 1.82 mmol) in ethanol/THF (1:1) was added KOH (0.36 g , 6.1 mmoles) dissolved in a minimum amount of water. The mixture was stirred under reflux overnight. Then H 2 O (250 mL) was added and acidified to pH = 1 with a 37 % HCl aqueous solution and stirred for 1 h. Ethanol and THF were removed and the resulting mixture was washed with CH 2 Cl 2 (3x100 mL). The organic layers were combined and dried over anhydrous sodium sulfate and concentrated. The final product was dried under vacuum overnight to afford compound HL 6 as white solid. The yield was 1 g (71%).

1

H NMR (400 MHz) in CDCl 3 (δ, ppm): 1 H NMR (400 MHz, ) δ 7.

59 -7.46 (m, 8H)

, 7.40 (t, J = 6.7 Hz, 4H), 7.28 (t, J = 6.4 Hz, 2H), 6.96 (dd, J = 8.7, 2.1 Hz, 4H), 6.68 (d, J = 2.1 Hz, 1H), 4.03 -3.93 (m, 8H),

1.86 -1.71 (m, 8H), 1.46 (d, J = 6.1 Hz, 8H)

,

1.34 (s, 16H)

. Elemental analysis -Calc. (Found) for C 51 H 62 O 6

: C, 79.44 (79.51); H, 8.10 (8.20)

. Selected FT-IR data (ATR, cm -1 ): 2918 (vs), 2849 (s), 2224 (s), 1676 (vs), 1602 (vs), 1488 (s), 1271 (s), 1171 (s).

Preparation of [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ]. To

  a suspension of [Mn 12 O 12 (CH 3 COO)16 (H 2 O) 4 ] (55 mg, 2.67•10 -2 mmol) in 50 mL of CH 2 Cl 2 /toluene 1:1, 0.615g of HL 5 (0.75 mmol, 28 eq.) was added. The mixture was stirred at 55°C during 24 h. The color of solution changed from brown to brownish black. The solution was filtrated and the solvent was then evaporated. The final compound is poorly soluble in toluene, thus the dried reaction mixture was firstly dissolved in 20 mL of CH 2 Cl 2 and then toluene (20 mL) was added, stirred for 3 hours, and then evaporated to dryness. This procedure was repeated at least three more times to remove acetic acid. The resulting residue was purified by two successive gel permeation chromatographic columns with Biorad S-X3 beads swelled with CH 2 Cl 2 . The final product was dried in vacuum at 40°C overnight. The yield was 42% based on [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]. Elemental analysis -Calc. (Found) for C 848 H 976 Mn 12 N 32 O 124 ([Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ]•12H 2 O): C, 71.42 (71.39); H, 6.90 (6.93);N 3.14 (3.32).

Selected FT-IR data (ATR, cm -1 ): 2922 (m), 2852 (m), 2224 (m) 1601 (s), 1577 (s), 1410 (s), 1249 (vs), 1159 (vs), 820 (vs), 780 (m), 730 (s).

Preparation of [

Mn 12 O 12 (L 6 ) 16 (H 2 O) 4

  at 55°C during24 h. The color of solution changed from brown to brownish black. The solution was filtrated and the solvent was then evaporated. Toluene (20 mL) was then added, stirred for 3 hours, and then evaporated to dryness. This procedure was repeated at least three more times to remove acetic acid. The resulting residue was purified by two successive gel permeation chromatographic columns with Biorad S-X3 beads swelled with CH 2 Cl 2 . The final product was dried in vacuum at 40°C overnight. The yield was 45 % based on [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]. Elemental analysis -Calc. (Found) for C 816 H 1002 Mn 12 O 121 ([Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]•9H 2 O): C, 73.11 (73.08); H, 7.53 (7.61). Selected FT-IR data

	(ATR, cm -1 ): 2924 (m), 2852 (m), 1607 (s), 1577 (s), 1412 (vs), 1248 (s), 1165 (m), 832 (m), 762 (vs).
	II.7.2. Analytical characterizations
	II.7.2.1.

]. To a suspension of [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] (55 mg, 2.67•10 -2 mmol) in 50 mL of CH 2 Cl 2 /toluene 1:1, 0.619 g of HL 6 (0.75 mmol, 28 eq.) was added. The II.88 mixture was stirred

Crystallographic data for [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ]Table S .

 S II-1. Crystal data and structure refinement for [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] at 100K.

	Moiety formula	2(C 128 H 118 Mn 12 O 63 ),C 4 H 10 O,2(H 2 O)
	Empirical formula	C 260 H 250 Mn 24 O 129
	Formula weight	6757.16
	Crystal system	Triclinic
	Space group	P ̅
	Wavelength, Å	0.71073
	a, Å	15.5955(14)
	b, Å	17.3439(15)
	c, Å	29.249(3)
	α, °	74.666(4)
	β, °	83.808(5)
	γ, °	64.860(4)
	V, Å 3	6907.0(10)
	Z	1
	ρ calcd , g/cm 3	1.625
	μ, 1/mm	1.156
	Reflections collected	208696
	Independent reflections	25469
	R 1 a	0.0785
	wR 2 a	0.2348
	GoF a	1.006
	a

Table S .

 S II-2. Selected distances in Å for [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ]•1/2C 4 H 10 O•H 2 O

	Atom1	Atom2	Length / Å	Atom1	Atom2	Length / Å
	Mn1	Mn2	2.834(2)	Mn6	O7	1.860(6)
	Mn1	Mn3	2.917(3)	Mn6	O34	1.927(8)
	Mn1	Mn4	2.818(2)	Mn6	O6	1.872(7)
	Mn1	Mn11	2.784(3)	Mn6	O32	1.945(7)
	Mn1	O43	1.898(5)	Mn6	O50	2.075(6)
	Mn1	O1	1.937(8)	Mn7	O7	1.943(6)
	Mn1	O2	1.927(6)	Mn7	O46	2.182(7)
	Mn1	O11	1.879(8)	Mn7	O37	2.001(6)
	Mn1	O4	1.891(5)	Mn7	O61	2.142(7)
	Mn1	O12	1.867(6)	Mn7	O8	1.846(8)
	Mn2	Mn9	2.795(2)	Mn7	O35	1.952(8)
	Mn2	Mn3	2.852(2)	Mn8	O16	2.131(6)
	Mn2	Mn4	2.906(2)	Mn8	O9	1.882(7)
	Mn2	O3	1.908(7)	Mn8	O41	1.933(6)
	Mn2	O9	1.848(6)	Mn8	O62	1.950(6)
	Mn2	O52	1.908(5)	Mn8	O8	2.106(6)
	Mn2	O2	1.894(5)	Mn8	O38	1.943(8)
	Mn2	O4	1.939(5)	Mn9	O9	1.876(8)
	Mn2	O10	1.854(8)	Mn9	O40	2.261(5)
	Mn3	Mn4	2.830(2)	Mn9	O53	2.232(5)
	Mn3	Mn7	2.757(3)	Mn9	O10	1.909(6)
	Mn3	O7	1.875(8)	Mn9	O19	1.948(8)
	Mn3	O47	1.902(6)	Mn9	O17	1.934(6)
	Mn3	O1	1.929(6)	Mn10	O22	1.953(9)
	Mn3	O3	1.933(5)	Mn10	O14W	2.175(5)
	Mn3	O2	1.924(8)	Mn10	O13W	2.197(5)
	Mn3	O8	1.836(6)	Mn10	O20	1.932(6)
	Mn4	O1	1.889(5)	Mn10	O11	1.877(6)
	Mn4	O6	1.873(8)	Mn10	O10	1.889(8)
	Mn4	O5	1.878(6)	Mn11	O44	2.237(5)
	Mn4	O3	1.910(5)	Mn11	O56	2.165(5)
	Mn4	O4	1.895(7)	Mn11	O25	1.950(7)
	Mn4	O59	1.917(5)	Mn11	O11	1.904(6)
	Mn5	Mn4	2.772(2)	Mn11	O12	1.884(8)
	Mn5	O58	2.171(6)	Mn11	O23	1.960(8)
	Mn5	O6	1.893(6)	Mn12	O28	1.959(6)
	Mn5	O5	1.898(8)	Mn12	O15W	2.228(8)
	Mn5	O29	1.941(6)	Mn12	O55	2.141(6)
	Mn5	O31	1.957(9)	Mn12	O5	1.884(8)
	Mn5	O49	2.190(5)	Mn12	O26	1.972(9)
				Mn12	O12	1.888(6)

Table S .

 S II-3. Indexation of observed reflections at 303 K for [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ].

	T / K	Peak number	q (obs) / nm -1	d (obs) / Å	hkl	h 2 +k 2 +l 2	d (calc) / Å	Calculated cell parameters
		1	2.34	26.84	110	2	26.72
	303	2	2.89	21.73	111	3	21.82	a = 37.79 Å
	heating	3	4.10	15.32	211	6	15.43	V = 53967.30 Å 3
		4	4.97	12.64	221/300	9	12.60	Z  6
		5	5.75	10.92	222	12	10.91
		6	6.21	10.11	321	14	10.10
		7	7.41	8.48	420	20	8.45
		8	8.91	7.05	432/520	29	7.02
		9	9.50	6.61	441/522	33	6.58
		10	10.19	6.16	532/611	38	6.13
		11	11.60	5.41	632/700	49	5.40
		12	14.43	4.35	662	76	4.33
		13	15.08	4.16	753	83	4.15

Table S II-4.

 S Indexation of the reflections at 283 K for [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ].

	T / K	Peak number	q (obs) / nm -1	d (obs) / Å	hkl	h 2 +k 2 +l 2	d (calc) / Å	Calculated cell parameters
		1	2.43	25.82	110	2	25.85
	283	2	2.97	21.19	111	3	21.11	a = 36.51 Å
	heating	3	3.43	18.30	200	4	18.28	V = 49000.64 Å 3
		4	3.83	16.42	210	5	16.35	Z  3 -4
		5	4.21	14.94	211	6	14.93
		6	4.86	12.94	220	8	12.93
		7	5.14	12.23	221/300	9	12.19
		8	5.43	11.56	310	10	11.56
		9	5.69	11.05	311	11	11.02
		10	6.42	9.79	321	14	9.77

Table S .

 S II-5. Indexation of the reflections on 3 rd heating for [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ], part 1.

	T / K	Peak number	q (obs) / nm -1	d (obs) / Å	hkl	d (calc) / Å	Calculated cell parameters
	303 K	1	1.23	51.01	002	50.94
	heating	2	1.38	45.39	211	45.09	a = 110.80 Å
		3	1.77	35.50	212	35.78	b = 119.74 Å
		4	2.11	29.76	040	29.94	c = 101.87 Å
		5	2.28	27.53	400	27.70	V = 1.3515  10 6 Å 3
		6	2.49	25.27	004	25.47	Z  69
		7	2.75	22.85	422	22.54
		8	2.86	21.99	224	21.58
		9	3.09	20.31	234	20.02
		10	3.42	18.36	244	18.31
		11	3.64	17.25	424	17.89
		12	3.76	16.72	434	16.97
		13	3.88	16.21	444	15.89
		14	4.03	15.58	226	15.67
		15	4.22	14.90	080	14.97
		16	4.54	13.83	800	13.85
		17	4.80	13.10	822	13.04
		18	4.93	12.76	008	12.73
	333 K	1	1.21	51.78	002	51.56
	heating	2	1.37	45.77	211	45.44	a = 111.54 Å
		3	1.76	35.64	212	36.12	b = 120.70 Å
		4	2.09	30.06	040	30.18	c = 103.11 Å
		5	2.27	27.72	400	27.89	V = 1.388  10 6 Å 3
		6	2.46	25.50	004	25.78	Z  71
		7	2.73	22.98	422	22.72
		8	2.83	22.16	224	21.82
		9	3.07	20.48	234	20.23
		10	3.20	19.64	343	19.36
		11	3.42	18.35	244	18.49
		12	3.59	17.51	424	18.06
		13	3.72	16.89	434	17.13
		14	3.85	16.32	444	16.03
		15	4.01	15.68	226	15.85
		16	4.19	15.00	080	15.09
		17	4.53	13.87	800	13.94
		18	4.75	13.22	822	13.14
		19	4.90	12.83	008	12.89
		20	5.06	12.43	448	12.29

Table S .

 S II-6. Indexation of the reflections on 3 rd heating for [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ], part 2.

	T / K	Peak number	q (obs) / nm -1	d (obs) / Å	hkl	d (calc) / Å	Calculated cell parameters
	363 K	1	1.22	51.45	002	51.48
	heating	2	1.37	45.86	211	45.91	a = 113.12 Å
		3	1.73	36.33	212	36.33	b = 121.65 Å
		4	2.07	30.35	040	30.41	c = 102.95 Å
		5	2.24	28.03	400	28.28	V = 1.417  10 6 Å 3
		6	2.44	25.76	004	25.74	Z  72
		7	2.70	23.23	422	22.95
		8	2.80	22.46	224	21.86
		9	3.03	20.73	234	20.28
		10	3.39	18.55	244	18.56
		11	3.54	17.76	424	18.17
		12	3.69	17.02	434	17.23
		13	3.81	16.51	444	16.13
		14	3.96	15.86	226	15.85
		15	4.14	15.16	080	15.21
		16	4.30	14.62	426	14.26
		17	4.48	14.03	800	14.14
		18	4.70	13.36	822	13.30
		19	4.84	12.97	008	12.87
		20	5.60	11.21	428	11.50
	393 K	1	1.20	52.21	002	52.25
	heating	2	1.34	46.94	211	47.01	a = 116.46Å
		3	1.70	36.97	212	37.08	b = 123.01 Å
		4	2.03	30.95	040	30.75	c = 104.51 Å
		5	2.20	28.50	400	29.11	V = 1.497  10 6 Å 3
		6	2.40	26.16	004	26.13	Z  76 -77
		7	2.68	23.48	422	23.50
		8	2.76	22.80	224	22.23
		9	3.10	20.26	234	20.60
		10	3.31	18.98	244	18.84
		11	3.50	17.95	424	18.54
		12	3.63	17.32	434	17.56
		13	3.77	16.66	444	16.44
		14	3.89	16.15	425	16.36
		15	4.07	15.44	080	15.38
		16	4.25	14.79	800	14.56
		17	4.40	14.28	426	14.52
		18	4.61	13.62	822	13.67
		19	4.76	13.19	008	13.06
		20	5.06	12.43	228	12.48

Table S .

 S II-7. Indexation of the reflections on 3 rd cooling for [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ].

	T / K	Peak number	q (obs) / nm -1	d (obs) / Å	hkl	d (calc) / Å	Calculated cell parameters
	373 K	1	1.21	51.78	002	51.99
	cooling	2	1.35	46.64	211	46.67	a = 115.57 Å
		3	1.72	36.48	212	36.84	b = 121.98 Å
		4	2.05	30.64	040	30.50	c = 103.97 Å
		5	2.21	28.41	400	28.89	V = 1.466  10 6 Å 3
		6	2.41	26.09	004	25.99	Z  75
		7	2.66	23.65	422	23.33
		8	2.78	22.60	224	22.09
		9	3.14	20.03	234	20.48
		10	3.34	18.84	244	18.72
		11	3.62	17.37	434	17.45
		12	3.78	16.62	444	16.32
		13	3.91	16.09	425	16.27
		14	4.09	15.36	080	15.25
		15	4.40	14.27	426	14.45
		16	4.62	13.59	822	13.57
		17	4.77	13.18	008	13.00
	353 K	1	1.20	52.24	002	52.67
	cooling	2	1.34	46.89	211	46.77	a = 115.14 Å
		3	2.04	30.75	040	30.95	b = 123.8 Å
		4	2.22	28.27	400	28.79	c = 105.34 Å
		5	2.40	26.19	004	26.34	V = 1.502  10 6 Å 3
		6	2.65	23.69	422	23.39	Z  77
		7	2.78	22.57	224	22.33
		8	3.20	19.66	334	19.22
		9	3.61	17.39	434	17.58
		10	3.74	16.80	444	16.46
		11	3.90	16.13	425	16.39
		12	4.10	15.33	080	15.48
		13	4.40	14.27	426	14.39
		14	4.61	13.63	822	13.55
		15	4.77	13.18	008	13.17
	333 K	1	1.20	52.24	002	52.79
	cooling	2	2.04	30.75	040	30.77	a = 114.28 Å
		3	2.22	28.27	400	28.57	b = 123.08 Å
		4	2.40	26.19	004	26.39	c = 105.57 Å
		5	2.65	23.69	422	23.26	V = 1.485  10 6 Å 3
		6	2.78	22.57	224	22.33	Z  76
		7	3.20	19.66	334	19.18
		8	3.61	17.39	434	17.53
		9	3.90	16.13	425	16.37
		10	4.10	15.33	080	15.39
		11	4.40	14.27	426	14.29
	303 K	1	1.20	52.21	002	52.37
	cooling	2	2.07	30.29	040	30.78	a = 113.48 Å
		3	2.23	28.13	400	28.37	b = 123.12 Å
		4	2.42	25.93	004	26.18	c = 104.73 Å
		5	2.65	23.69	422	23.12	V = 1.463  10 6 Å 3
		6	3.16	19.88	334	19.06	Z  75
		7	3.68	17.07	434	17.42
		8	3.89	16.16	425	16.25
		9	4.44	14.14	426	14.19

Table S .II-8.

 S Indexation of the reflections on 4 th cooling for [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ], part 1.

	T / K	Peak number	q (obs) / nm -1	d (obs) / Å	hkl	h 2 +k 2 +l 2	d (calc) / Å	Calculated cell parameters
		1	2.21	28.46	110	2	28.53
	393	2	3.11	20.23	200	4	20.18	a = 40.35 Å
	cooling	3	3.81	16.48	211	6	16.47	V = 65701 Å 3
		4	4.40	14.27	220	8	14.27	Z  3 -4
		5	4.92	12.77	310	10	12.76
		6	5.40	11.63	222	12	11.65
		7	5.84	10.77	321	14	10.78
		8	6.22	10.11	400	16	10.09
		9	6.60	9.52	330	18	9.51
		10	7.32	8.59	332	22	8.60
		1	2.20	28.50	110	2	28.44
	373	2	3.12	20.16	200	4	20.11	a = 40.22 Å
	cooling	3	3.83	16.39	211	6	16.42	V = 65094 Å 3
		4	4.42	14.21	220	8	14.22	Z  3 -4
		5	4.94	12.73	310	10	12.72
		6	5.42	11.60	222	12	11.61
		7	5.85	10.74	321	14	10.75
		8	6.64	9.46	330	18	9.48
		1	2.21	28.41	110	2	28.37
	353	2	3.13	20.06	200	4	20.06	a = 40.12 Å
	cooling	3	3.84	16.38	211	6	16.38	V = 64602 Å 3
		4	4.43	14.19	220	8	14.19	Z  3 -4
		5	4.96	12.68	310	10	12.69
		6	5.43	11.57	222	12	11.58
		7	5.86	10.73	321	14	10.72
		8	6.66	9.44	330	18	9.46

Table S .

 S II-9. Indexation of the reflections on 4 th cooling for [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ], part 2.

	T / K	Peak number	q (obs) / nm -1	d (obs) / Å	hkl	h 2 +k 2 +l 2	d (calc) / Å	Calculated cell parameters
			2.22	28.34	110	2	28.35
	333		3.13	20.10	200	4	20.05	a = 40.09 Å
	cooling		3.84	16.37	211	6	16.37	V = 64452 Å 3
			4.44	14.17	220	8	14.18	Z  3 -4
			4.96	12.68	310	10	12.68
			5.43	11.57	222	12	11.57
			5.87	10.70	321	14	10.72
			6.64	9.46	330	18	9.45
			2.23	28.18	110	2	28.21
	303		3.15	19.93	200	4	19.95	a = 39.90 Å
	cooling		3.86	16.27	211	6	16.29	V = 63507Å 3
			4.46	14.09	220	8	14.11	Z  3 -4
			4.98	12.61	310	10	12.62
			5.45	11.52	222	12	11.52
			5.89	10.66	321	14	10.66
			2.24	28.01	110	2	28.04
	274		3.17	19.83	200	4	19.83	a = 39.65 Å
	cooling		3.88	16.21	211	6	16.19	V = 62355 Å 3
			4.49	14.01	220	8	14.02	Z  3 -4
			5.01	12.55	310	10	12.54
			5.51	11.40	222	12	11.45
			5.92	10.61	321	14	10.60

Table S .

 S II-10. Indexation of the reflections on 3 rd heating for [Mn 12 O 12 (L 5 ) 16 (H 2 O) 4 ].

	T / K	Peak number	q (obs) / nm -1	d (obs) /Å	hkl	h 2 +k 2 +hk	d (calc) /Å	Calculated cell parameters
	423 K		1.60	39.34	001	-	39.42	periodicity
	heating		2.54	24.72	d'	-		d = 39.42 Å
			3.18	19.75	002	-	19.71
	403 K		1.58	39.89	001	-	39.88	a = 47.71 Å
	heating		2.67	23.57	110	3	23.51	c = 39.88 Å
			3.15	19.93	002	-	19.94	V = 78592 Å 3
			4.07	15.45	200	4	20.36	S = 1971 Å 2
			5.40	11.63	210	7	15.39
	393 K		1.57	40.10	001	-	40.10	a = 47.75 Å
	heating		2.67	23.57	110	3	23.51	c = 40.10 Å
			3.13	20.05	002	-	20.05	V = 79164 Å 3
			4.07	15.45	200	4	20.36	S = 1974 Å 2
			5.40	11.63	210	7	15.39
			5.74	10.95	300	9	13.57
	343 K		1.62	38.83	001	-	38.78	a = 43.50 Å
	heating		2.92	21.49	110	3	21.75	c = 38.78 Å
			3.24	19.37	002	-	19.39	V = 63556 Å 3
			3.31	18.97	200	4	18.84	S = 1639 Å 2
			4.37	14.39	210	7	14.24
			5.03	12.49	300	9	12.56
	323 K		1.62	38.83	001	-	38.78	a = 43.21 Å
	heating		2.91	21.59	110	3	21.60	c = 38.78 Å
			3.24	19.37	002	-	19.39	V = 62699 Å 3
			3.35	18.76	200	4	18.71	S = 1617 Å 2
			4.44	14.15	210	7	14.14
			5.05	12.44	300	9	12.47
	313 K		1.61	39.03	001	-	39.18	a = 43.05 Å
	heating		2.92	21.52	110	3	21.53	c = 39.18 Å
			3.21	19.57	002	-	19.59	V = 62861 Å 3
			3.37	18.65	200	4	18.64	S = 1604 Å 2
			4.45	14.12	210	7	14.09
			5.07	12.39	300	9	12.43

Table S .

 S II-11. Indexation of reflections on cooling for [Mn 12 O 12 (L 6 ) 16 (H 2 O) 4 ]

	T / K	Peak number	q (obs) / nm -1	d (obs) /Å	hkl	h 2 +k 2 +hk	d (calc) /Å	Calculated cell parameters
	347 K		1.92	32.78	001	-	32.80	a = 53.80 Å
	cooling		2.35	26.77	110	3	26.90	c = 32.80 Å
			2.69	23.32	200	4	23.30	V = 82213Å 3
			3.57	17.60	210	7	17.61	S = 2507 Å 2
			3.83	16.41	002	-	16.40
			4.03	15.60	300	9	15.53
	345 K		1.92	32.78	001	-	32.80	a = 53.55 Å
	cooling		2.72	23.13	200	4	23.24	c = 32.80 Å
			3.58	17.53	210	7	17.57	V = 81470 Å 3
			3.83	16.39	002	-	16.40	S = 2483 Å 2
			4.07	15.45	300	9	15.50
			5.40	11.63	400	16	11.62
			5.74	10.95	003	-	10.93
	323 K		1.92	32.78	001	-	32.78	a = 52.70 Å
	cooling		2.75	22.85	200	4	22.82	c = 32.78 Å
			3.64	17.27	210	7	17.25	V = 78850 Å 3
			3.83	16.39	002	-	16.39	S = 2405Å 2
			4.13	15.23	300	9	15.21
			5.50	11.43	400	16	11.41
			5.75	10.93	003	-	10.93
			6.35	9.89	410	21	9.96
			6.87	9.15	500	25	9.13
		10	7.16	8.78	330	27	8.78
	308 K		1.91	32.90	001	-	32.86	a = 52.40 Å
	cooling		2.78	22.64	200	4	22.69	c = 32.86 Å
			3.68	17.10	210	7	17.15	V = 78157 Å 3
			3.85	16.32	002	-	16.43	S = 2379 Å 2
			4.16	15.10	300	9	15.13
			5.51	11.40	400	16	11.34
			5.73	10.97	003	-	10.95
			6.35	9.89	41	21	9.90
			6.91	9.09	50	25	9.08
		10	7.18	8.75	33	27	8.73
	298 K		1.90	33.07	001	-	33.04	a = 52.35 Å
	cooling		2.77	22.69	200	4	22.69	c = 33.04 Å
			3.67	17.13	210	7	17.15	V = 78429 Å 3
			3.83	16.41	002	-	16.52	S = 2373 Å 2
			4.15	15.13	300	9	15.13
			5.55	11.32	400	16	11.34
			5.67	11.08	003	-	11.01
			6.39	9.83	410	21	9.90
			6.91	9.09	500	25	9.08
		10	7.18	8.75	330	27	8.73

q(obs) is the observed/measured reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, d(obs) and d(calc) are

Table of Contents for Chapter III:

 of III.1. Introduction ................................................................................................................................ III.111 III.2. Synthesis and characterizations of molecular {Fe 2 Co 2 } squares ................................................... III.113 III.2.1. Synthetic strategy ........................................................................................................... III.113 III.2.2. FT-IR spectroscopic analyses ......................................................................................... III.114 III.2.3. Structural investigations of molecular {Fe 2 Co 2 } squares ................................................. III.114 III.2.3.1. {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }[OTf] 2 •3H 2 O•3DMF (1) ...................................... III.114 III.2.3.2. {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 •H 2 O•2DMF (2) ................................. III.118 III.2.3.3. {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(ClO 4 ) 2 •2DMF (3) ....................................... III.122 III.2.3.4. {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTs) 2 •6DMF (4) ........................................ III.124 III.2.3.5. Structural comparison of compounds 2, 3 and 4 ................................................. III.125 III.2.4. Optical reflectivity studies .............................................................................................. III.127 III.2.5. Magnetic measurements ................................................................................................. III.128 III.3. Conclusions and perspectives ...................................................................................................... III.129 III.4. Supporting materials ................................................................................................................... III.131 III.138 III.5. References .................................................................................................................................. III.139

III.4.1. Experimental protocols ................................................................................................... III.131 III.4.2. FT-IR spectroscopic analyses ......................................................................................... III.132 III.4.3. Crystallographic data for 1, 2, 3 and 4............................................................................. III.134 III.4.4. Magnetic measurements .................................................................................................

Table III - 1 .

 III1 Unit-cell parameters, average metal-to-ligand bond lengths, and deformation of the Co coordination sphere for 1 under different experimental conditions.

{[

(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }[OTf] 2 •3H 2 O•3DMF (1)

			230 K	90 K quenched	83 K light on	84 K light off
			{Fe III 2 Co II	2 }	{Fe III 2 Co II	2 }	{Fe III 2 Co II	2 }	{Fe II 2 Co III	2 }
	Crystal color	dark red	dark red	dark red	dark green
	Crystal system Space group	Triclinic P ̅	Triclinic P ̅	Triclinic P ̅	Triclinic P ̅
	a, Å		13.6685(11)	13.560(2)	13.5151(7)	13.3171(9)
	b, Å		13.8537(9)	13.7708(19)	13.7665(5)	13.4405(8)
	c, Å		14.8485(12)	14.805(2)	14.7657(7)	14.7961(10)
	α, °		99.514(3)	99.932(6)	99.894(2)	100.494(3)
	β, °		93.620(3)	94.494(7)	94.611(2)	94.330(3)
	γ, °		107.868(3)	107.213(6)	107.283(2)	106.519(3)
	V, Å 3		2619.8(3)	2576.4(7)	2558.8(2)	2473.7(3)
	<Fe-C>, Å	1.919		1.925		1.920		1.894
	<Fe-N>, Å	2.002		2.019		1.997		2.041
	<Co-N>, Å	2.122		2.083		2.111		1.935
	Fe(1) … Co(1), Å	5.134		5.102		5.131		4.940
	Fe(1) … Co(1)', Å	5.165		5.144		5.162		4.948
	Fe … Fe, Å	7.652		7.584		7.642		7.201
	Co … Co, Å	6.894		6.890		6.895		6.776
	Σ(N-Co-N) a , °	65.87		58.02		62.70		43.21
	a ∑	∑ |	| , where are N-Co-N bond angles

  .III-1).At 230 K, single-crystal XRD studies showed that {Fe 2 (μ-CN) 4 Co 2 } core is in paramagnetic state since it contains divalent Co ions in high spin (HS) configuration (confirmed by average Co-N bond distance of approximately 2.12 Å, TableIII-1) and Fe III ions, as results of the charge balance. The {Fe 2 (μ-CN) 4 Co 2 }

	core is nearly planar since the Fe-C-N angles range between 175.5(4) and 178.8(5)° while Co-N-C angles
	range between 173.8(4) and 176.6(4)° (Table S.III-1). The separations between adjacent metal ions within
	the tetranuclear core through the cyanide bridges (Fe(1) … Co(1) and Fe(1) … Co(2)) are 5.13 and 5.16 Å,
	respectively. Nevertheless, the C(1)-Fe(1)-C(2) angle (85.52°), Fe•••Fe (7.65 Å) and Co•••Co (6.89 Å)
	distances indicate that 1 adopts a distorted square structure (Figure III-2b). The two Fe III ions show a slightly
	distorted six-coordinated octahedral environment, FeC 3 N 3 , formed by three pyrazol nitrogen atoms from the

capping Tp* ligand and three carbon atoms from bridging and terminal cyanide groups. The average Fe-C and Fe-N bond lengths are of 1.92 and 2.00 Å (Table

S

.III-1), respectively. The two Co II ions exhibit a distorted octahedral environment, CoN 6 , which is formed by four bipyridine nitrogen atoms and two nitrogen atoms from bridging cyanide groups. The values of N-Co-N angles (Table

S

.III-1) in the coordination sphere of the HS Co II ion depart significantly from orthogonality, as can be observed from the Σ

(N-Co-N)

parameter of 65.87° (Table

Table III - 2 .

 III2 Unit-cell parameters, average metal-to-ligand bond lengths, metal-to-metal distances and deformation of the Co coordination sphere for 2 under different experimental conditions, for 3 at 100 K and for 4 at 120 K.

			(2)		(3)	(4)
		240 K	80 K	80 K	100 K	120 K
			light off	light on
		{Fe III 2 Co II 2 }	{Fe II 2 Co III 2 }	{Fe III 2 Co II 2 }	{Fe III 2 Co II 2 }	{Fe III 2 Co II 2 }
	Crystal system	Triclinic	Triclinic	Triclinic	Triclinic	Triclinic
	Space group	P ̅	P ̅	P ̅	P ̅	P ̅
	a, Å	13.585(10)	13.248(10)	13.405(10)	13.084(2)	13.813(4)
	b, Å	14.041(10)	13.813(10)	13.906(10)	13.958(2)	15.847(4)
	c, Å	14.921(10)	14.761(10)	14.864(10)	14.647(3)	16.115(4)
	α, °	69.503(6)	69.949(7)	69.232(7)	113.044(7)	101.448(12)
	β, °	89.664(6)	88.004(7)	89.180(7)	91.226(8)	97.680(11)
	γ, °	73.024(6)	71.569(7)	72.920(7)	105.495(8)	114.931(11)
	V, Å 3	2535.0(3)	2399.1(3)	2464.0(3)	2348.0(7)	3040.0(14)
	<Fe-C>, Å	1.926	1.882	1.920	1.920	1.925
	<Fe-N>, Å	1.990	2.035	1.989	2.003	1.997
	<Co-N>, Å	2.127	1.926	2.115	2.130	2.120
	Fe(1) … Co(1), Å	5.100	4.889	5.112	5.139	5.118
	Fe(1) … Co(1)', Å	5.114	4.895	5.104	5.060	5.127
	Fe … Fe, Å	7.148	6.782	7.088	7.073	7.489
	Co … Co, Å	7.296	7.052	7.356	7.350	6.992
		58.5	35.9	59.4	59.61	69.36

Σ(N-Co-N) a , ° a ∑ ∑ | | , where are N-Co-N bond angles

4. Supporting materials III.4.1. Experimental protocols Preparation of Co(OTf) 2

  ReF 6 ] 2-, which has a different charge. Besides synthesis of novel compounds changing of the crystallization conditions (i.e. solvents) of the previously reported compounds (1, 2, 3 and 4) can offer a better view of the effect that solvation inside the crystal packing can have on ET process. In particular, polar solvents should be able increase the ET characteristic temperature, as was observed in solutions for 1.6g,9 On the other hand, the chemical tuning (by functionalization, counter-anion exchange or variation of crystallization solvents and conditions) of ET properties in molecular {Fe 2 Co 2 } squares is time consuming. It appears much easier to control the intramolecular electron transfer in solutions, 6g,9 since they offer an environment which lacks lattice packing restraints and highlights the molecular nature of the observed behavior.Another promising perspective based on further functionalization of the bipyridine ligands on the Co site has been successfully applied in the next chapters. . 50 mL of trifluoromethanesulfonic acid(2.4 g, 16 mmol) solution in methanol were added dropwise to the CoCO 3 (0.95 g, 8 mmol) suspension in 20 mL of methanol. The mixture was stirred until CO 2 gas stopped to form. Then the solution was filtered and concentrated under reduced pressure. Co(OTf) 2 was precipitated as a pink solid by adding a large excess of diethyl ether. The The mixture was refluxed for 5 h. The resulting reddish-pink solution was filtered while hot. The filtrate was cooled to room temperature and pale orange blocks formed after 1 h. The crystals were collected by filtration and dried overnight under vacuum at room temperature. A second crop of crystals was obtained by concentration and cooling of the mother liquor. The yield was 28.5 g (78%). Elemental analysis -Calc. (Found) for C 14 H 26 CoO 12 S 2 (Co(CH 3 C 6 H 4 SO 3 ) 2 •6H Co 2 F 6 Fe 2 N 30 O 12 S

	4	-, PF 6 -, SbF 6 -, ReO 4	-, BPh 4 -or paramagnetic ones like [FeCl 4 ] -or
	[compound was dried in vacuum at 383 K. The yield was 2.3 g (82 %). Elemental analysis -Calc. (Found)
	for C 2 CoF 6 O 6 S 2 (Co(CF 3 SO 3 ) 2 ): C, 6.73 (6.40). Selected FT-IR data (ATR, cm -1 ): 1225 (vs), 1191 (vs), 1023
	(vs), 626 (m), 593 (m).
	Preparation of Co(OTs) 2 •6H 2 O. In a 250-mL round-bottomed flask equipped with a reflux condenser
	were mixed CoCO 3 (8.54 g, 71.8 mmol), p-toluenesulfonic acid monohydrate (27.4 g, 144 mmol) and 20 mL
	of water. C 26 H 46 BFeN 10 O 2 ((NEt 4 )[(Tp*)Fe(CN) 3 ]•2H 2 O): C, 52.28 (52.65); H, 7.76 (7.52); N, 23.45 (24.39). Selected
	FT-IR data (ATR, cm -1 ): 2982 (m), 2934 (m), 2549 (m), 2115 (s), 1540 (vs), 1447 (vs), 1415 (vs), 1392 (vs),
	1370 (vs), 1199 (vs), 1060 (vs), 997 (m), 785 (vs), 777 (vs), 645 (s).
	Preparation of {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }(OTf) 2 (1). [NEt 4 ][(Tp*)Fe(CN) 3 ]•2H 2 O (168 mg, 0.3
	mmol) was treated with of Co(OTf) 2 (107 mg, 0.3 mmol) in 5 mL of wet DMF under argon to afford a red
	solution that was allowed to stir for 2 h. Then 2,2-bipyridine (94 mg, 0.6 mmol) was added and the mixture
	was stirred for 10 min and filtered. Dark red crystals were obtained by slow diffusion of diethyl ether in the
	DMF solution. The yields was 0.23 g (69.9 %). Elemental analysis -Calc. (Found) for
	C 90 H 108 B 2

III.2 O):

C, 33.01 (32.56)

; H, 5.14

(5.05)

; S,

12.59 (12.32)

.

Selected FT-IR data (ATR, cm -1 ): 3065 (w), 3039 (w), 1180 (vs), 1122 (vs), 1035 (s), 1009 (s), 812 (s), 676 (s), 563 (s).

Preparation of (NEt 4 )[(Tp*)Fe(CN) 3 ]. Dropwise addition of KTp* (0.5 g, 1.48 mmol) dissolved in 2.5 mL of methanol to Fe(acac) 3 (0.5 g, 1.41 mmol) solution in 2.5 mL of methanol afforded a dark brown mixture that was stirred for ~ 2 h. A reddish precipitate was isolated by filtration and dried. Afterwards it was re-dissolved in 5 mL of acetonitrile and treated with 0.6 g of (NEt 4 )CN (3.8 mmol) in 5 mL of acetonitrile. The mixture was stirred for 2 h at 50°C. The excess of (NEt 4 )CN was removed by column chromatography (silica gel, CH 3 CN). The compound was purified by recrystallization from methanol solution layered with diethyl ether. The yield was 0.25 g (60 %). Elemental analysis -Calc. (Found) for 2 ({[(Tp*)Fe(CN

) 3 ] 2 [Co(bpy) 2 ] 2 }(OTf) 2 •2H 2 O•4DMF):

C, 48.45 (48.25); H, 4.89 (4.85)

;

N, 18.83 (18.91)

. Selected FT-IR data (ATR, cm -1 ): 3077 (w), 2928 (w), 2544 (w), 2150 (s), 2128 (m), 1664 (vs), 1596 (s), 1542 (s), 1441 (vs), 1414 (s), 1385 (s), 1367 (s), 1260 (vs), 1155 (s), 1029 (vs), 773 (s), 636 (s), 573 (m).

III.132

Preparation of {[(Tp*)

Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }[X] 2 (2-4)

  . [NEt 4 ][(Tp*)Fe(CN) 3 ]•2H 2 O (168 mg, 0.3 mmol) was treated with 0.3 mmol of Co(X) 2 (for 2 with 107 mg of Co(OTf) 2 ; for 3 with 109.7 mg of Co(ClO 4 )•6H 2 O; for 4 with 153 mg of Co(OTs) 2 •6H 2 O) in 5 mL of wet DMF under argon to afford a red solution that was allowed to stir for 2 h. Then 4,4-dimethyl-2,2-bipyridine (diMebpy, 114 mg, 0.6 mmol) was added and the mixture was stirred for 20 min and filtered. Dark red crystals were obtained by slow Elemental analysis -Calc. (Found) for (4) C 116 H 148 B 2 Co 2 Fe 2 N 32 O 12 S 2 ({[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }(OTs) 2 •6DMF): C, 55.78 (55.69); H, 5.97 (5.89);N, 17.94 (18.07). Selected

	diffusion of diethyl ether in the DMF solution. The yields were 0.23 g (69.9 %) for 2, 0.22 g (72.7 %) for 3
	and 0.27 g (71.4 %) for 4. Elemental analysis -Calc. (Found) for (2) C 92 H 108 B 2 Co 2 F 6 Fe 2 N 28 O 9 S 2
	({[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTf) 2 •H 2 O•2DMF): C, 50.70 (50.36); H, 5.00 (4.94); N, 18.00 (18.13).
	Selected FT-IR data (ATR, cm -1 ): 2926 (w), 2857 (w), 2546 (w), 2151, 2116 (w), 1664 (s), 1614 (s), 1541
	(s), 1444 (s), 1414 (s), 1385 (s), 1369 (s), 1263 (vs), 1202 (s), 1138 (s), 1062 (s), 1031 (s), 824 (s), 793 (s),
	635 (vs), 572 (s). Elemental analysis -Calc. (Found) for (3) C 90 H 106 B 2 Cl 2 Co 2 Fe 2 N 28 O 10
	({[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(ClO 4 ) 2 •2DMF): C, 52.42 (52.27); H, 5.18 (5.09); N, 19.02 (19.11).
	Selected FT-IR data (ATR, cm -1 ): 2926 (w), 2855 (w), 2545 (w), 2149 (m), 2119 (w), 1671 (vs), 1614 (vs),
	1542 (s), 1445 (s), 1414 (s), 1386 (s), 1369 (s), 1204 (m), 1086 (vs), 1061 (vs), 1016 (s), 920 (m), 824 (s),
	786 (s), 691 (m), 645 (m), 622(vs).

Table S .

 S III-1. Crystallographic data for 1 at various temperatures in its different phases.

		230 K	90 K	120 K	84 K	83 K
	Temperature	high temperature	quenched from	low temperature	low temperature	after white light
		phase	300 K	phase	phase	irradiation
	Crystal color	dark red	dark red	dark green	dark green	dark red
	Moiety formula		C 76 H 76 B 2 Co 2 Fe 2 N 26 , 2(CF 3 O 3 S), 3(C 3 H 7 NO), 3(H 2 O)
	Empirical formula			C 87 H 103 B 2 Co 2 F 6 Fe 2 N 29 O 12 S 2
	Formula weight			2176.23
	Crystal system Space group			Triclinic P ̅
	Wavelength, Å			0.71073
	a, Å	13.6685(11)	13.560(2)	13.3401(10)	13.3171(9)	13.5151(7)
	b, Å	13.8537(9)	13.7708(19)	13.4448(9)	13.4405(8)	13.7665(5)
	c, Å	14.8485(12)	14.805(2)	14.8307(12)	14.7961(10)	14.7657(7)
	α, °	99.514(3)	99.932(6)	100.515(3)	100.494(3)	99.894(2)
	β, °	93.620(3)	94.494(7)	94.288(3)	94.330(3)	94.611(2)
	γ, °	107.868(3)	107.213(6)	106.561(3)	106.519(3)	107.283(2)
	V, Å 3	2619.8(3)	2576.4(7)	2484.0(3)	2473.7(3)	2558.8(2)
	Z			1
	ρ calcd , g/cm 3	1.379	1.403	1.455	1.461	1.412
	μ, 1/mm	0.703	0.715	0.741	0.744	0.719
	Reflections collected	62198	56131	80919	59992	91328
	Independent reflections	9654	9413	9020	9033	9379
	a R 1 wR 2 a GoF a	0.0462 0.1529 1.068	0.0717 0.2067 1.047	0.0621 0.1850 1.057	0.0620 0.1818 1.053	0.0479 0.1398 1.047
			Selected bond distances (Å)
						III.135

Table S .

 S III-2. Crystallographic data for 2 at various temperatures in its different phases. Co 2 Fe 2 N 26 , 2(CF 3 O 3 S), 2(C 3 H 7 NO), (H 2 O) Empirical formula C 92 H 108 B 2 Co 2 F 6 Fe 2 N 28 O 9 S 2

	Temperature	240 K high temperature phase	80 K low temperature phase	80 K after light irradiation
	Crystal color	dark red	dark green	dark red
	Moiety formula C 84 H 92 B 2 Formula weight	2179.32
	Crystal system Space group		Triclinic P ̅
	Wavelength, Å		0.71073
	a, Å	13.5850(10)	13.2480(10)	13.4050(10)
	b, Å	14.0410(10)	13.8130(10)	13.9060(10)
	c, Å	14.9210(10)	14.7610(10)	14.8640(10)
	α, °	69.503(6)	69.949(7)	69.232(7)
	β, °	89.664(6)	88.004(7)	89.180(7)
	γ, °	73.024(6)	71.569(7)	72.920(7)
	V, Å 3	2535.0(3)	2399.1(3)	2464.0(3)
	Z		1
	ρ calcd , g/cm 3	1.428	1.508	1.469
	μ, 1/mm	0.724	0.765	0.745
	Reflections collected	19030	17635	17994
	Independent reflections	10704	10017	10232
	a R 1 wR 2 a GoF a	0.0741 0.1446 0.759	0.0660 0.1257 0.653	0.0733 0.1394 0.702
		Selected bond distances (Å)

Table S .

 S III-3. Crystallographic data for 3 at 100 and 250 K. Co 2 Fe 2 N 26 , 2(ClO 4 ), 2(C 3 H 7 NO) Empirical formula C 90 H 106 B 2 Cl 2 Co 2 Fe 2 N 28 O 10

	Temperature	250 K	100 K
	Crystal color	dark red	dark red
	Moiety formula C 84 H 92 B 2 Formula weight	2062.10
	Crystal system Space group		Triclinic P ̅
	Wavelength, Å		0.71073
	a, Å	13.191(3)	13.084(2)
	b, Å	14.097(3)	13.958(2)
	c, Å	14.739(3)	14.647(3)
	α, °	113.177(8)	113.044(7)
	β, °	90.673(10)	91.226(8)
	γ, °	105.633(8)	105.495(8)
	V, Å 3	2404.9(9)	2348.0(7)
	Z		1
	ρ calcd , g/cm 3	1.424	1.458
	μ, 1/mm	0.763	0.781
	Reflections collected	67613	72871
	Independent reflections	10179	8747
	a R 1 wR 2 a GoF a	0.0593 0.1921 1.033	0.0553 0.1629 1.059
		Selected bond distances (Å)
	Co(1)-N(5)	2.085(4)	2.075(

Table S .

 S III-4. Crystallographic data for 4 at 120 K.

	Temperature	120 K	Selected bond distances (Å)
	Crystal color	dark red	Co(1)-N(1)	2.090(5)
	Moiety formula	C 84 H 92 B 2 Co 2 Fe 2 N 26 , 2(C 7 H 7 O 3 S), 6(C 3 H 7 NO)	Co(1)-N(12)	2.105(5)
	Empirical formula	C 116 H 148 B 2 Co 2 Fe 2 N 32 O 12 S 2	Co(1)-N(15)	2.110(5)
	Formula weight	2497.96	Co(1)-N(1)	2.116(5)
	Crystal system Space group	Triclinic P ̅	Co(1)-N(13) Co(1)-N(14)	2.145(5) 2.153(5)
	Wavelength, Å	0.71073	Fe(1)-C(52)	1.908(6)
	a, Å	13.813(4)	Fe(1)-C(53)	1.930(7)
	b, Å	15.847(4)	Fe(1)-C(51)	1.937(6)
	c, Å	16.115(4)	Fe(1)-N(5)	1.974(5)
	α, °	101.448(12)	Fe(1)-N(8)	1.988(5)
	β, °	97.680(11)	Fe(1)-N(3)	2.029(5)
	γ, °	114.931(11)	Selected bond angles (°)
	V, Å 3	3040.0(14)	N(1)-Co(1)-N(14)	83.43(19)
	Z	1	N(12)-Co(1)-N(1)	95.02(19)
	ρ calcd , g/cm 3	1.364	N(12)-Co(1)-N(13)	76.26(19)
	μ, 1/mm	0.609	N(12)-Co(1)-N(14)	99.87(19)
	Reflections collected	86047	N(13)-Co(1)-N(14)	91.11(19)
	Independent reflections	11303	N(15)-Co(1)-N(1)	94.9(2)
	a R 1 wR 2 a GoF a	0.0788 0.2379 1.104	N(15)-Co(1)-N(13) N(15)-Co(1)-N(14) N(2)-Co(1)-N(1)	93.31(19) 76.71(19) 91.32(19)
			N(

Table IV - 1 .

 IV1 Unit-cell parameters, average metal-to-ligand bond lengths, metal-to-metal distances and deformation of the Co coordination sphere for 5 under different experimental conditions, for 6 at 100 K.

		(5)		(6)
		100 K	260 K	100 K
		{Fe III 2 Co II 2 }	{Fe III 2 Co II 2 }	{Fe II 2 Co III 2 }
	Crystal system	monoclinic	monoclinic	triclinic
	Space group	P2 1 /c	P2 1 /c	P1
	a, Å	12.8665(14)	12.939(4)	11.9217(13)
	b, Å	29.832(4)	30.070(8)	16.8399(18)
	c, Å	19.772(2)	20.136(6)	25.069(3)
	α, °	90.00	90.00	78.207(5)
	β, °	112.886(7)	112.923(18)	80.997(6)
	γ, °	90.00	90.00	78.043(6)

Table IV -2. Solubility of 2, 5 and 6 in

 IV various solvents and qualitative tests for electron-transfer in their solutions.

	Solvents	(2)	(5)	(6)
	Water			
	Nitromethane	red -green	red -green	red -green
	Methanol	red -green	red -green	red -green
	Acetonitrile	red -green	red -green	red -green
	Ethanol	red -green	red -green	red -green
	Acetone	red -green	red -green	red -green
	Ethyl acetate		red -green	red -green
	Isopropanol		red -green	red -green
	Tetrahydrofuran		red -green	red -green
	Chloroform	red	red -green	red -green
	n-Buthanol		red -green	red -green
	Dichloromethane	red -green	red -green	red -green
	Diethyl ether			red -green
	Toluene		red	
	Hexane

Table S .

 S IV-1. Crystallographic data for 5 at 100 and 260 K. Co 2 Fe 2 N 26 , 2(CF 3 O 3 S), 2(C 3 H 7 NO) Empirical formula C 132 H 186 B 2 Co 2 F 6 Fe 2 N 28 O 8 S 2

	IV.5.2. Crystallographic data for 5 and 6
		Temperature			100 K				260 K
	Crystal color			dark red				dark red
	Moiety formula C 124 H 172 B 2 Formula weight	2722.39
	Crystal system					Monoclinic
	Space group					P2 1 /c
	Wavelength, Å					0.71073
	a, Å			12.8665(14)			12.939(4)
	b, Å			29.832(4)				30.070(8)
	c, Å			19.772(2)				20.136(6)
	β, °			112.886(7)				112.923(18)
	V, Å 3			6991.7(14)				7216(4)
	Z						1
	ρ calcd , g/cm 3			1.293				1.253
	μ, 1/mm			0.539				0.522
	Reflections collected			76075				161415
	Independent reflections		12852				13122
	a R 1 wR 2 a GoF a			0.0833 0.2740 1.038				0.1200 0.3200 1.070
					Selected bond distances (Å)
	Co(1)-N(1)			2.085(6)				2.113(7)
	Co(1)-N(3)			2.091(5)				2.075(6)
	Co(1)-N(13)			2.116(5)				2.119(6)
	Co(1)-N(12)			2.128(5)				2.148(5)
	Co(1)-N(10)			2.142(5)				2.151(5)
	Co(1)-N(11)			2.168(5)				2.176(6)
	Fe(1)-C(3)			1.909(6)				1.920(8)
	Fe(1)-C(1)			1.912(7)				1.906(9)
	Fe(1)-C(2)			1.948(8)				1.936(9)
	Fe(1)-N(8)			1.970(5)				1.980(6)
	Fe(1)-N(4)			1.996(5)				1.978(6)
	Fe(1)-N(6)			2.023(6)				2.029(7)
	Fe(1)-Co(1)			5.087				5.108
	Fe(1)-Co(2)			5.115				5.107
	Fe(1)-Fe(2)			7.759				7.749
	Co(1)-Co(2)			6.624				6.656
					Selected bond angles (°)
									39 g (77.4
	%).	Elemental	analysis	-	Calc.	(Found)	for	(6)	C 184 H 292 B 2 Co 2 F 6 Fe 2 N 26 O 8 S 2
	({[(Tp*)Fe(CN) 3 ] 2 [Co(diC13bpy) 2 ] 2 }(OTf) 2 •2CH 3 OH): C, 64.51 (64.45); H, 8.59 (8.46); N, 10.63 (10.69);
	Selected FT-IR data (ATR, cm -1 ): (dry) 2922 (vs), 2852 (vs), 2532 (m), 2151 (s), 2119 (vw), 1613 (s), 1261
	(vs); (wet) 2920 (vs), 2851 (vs), 2506 (m), 2098 (s), 2069 (s), 1617 (s), 1259 (vs).
									IV.161

Table S .

 S IV-2. Crystallographic data for 6 at 100 K.

	Temperature	100 K	Selected bond distances (Å)
	Crystal color	dark green	Co(1)-N(21)	1.949(8)
	Moiety formula	(C 180 H 282 B 2 Co 2 Fe 2 N 26 ), 2(CF 3 O 3 S), 3(CH 4 O), H 2 O Co(1)-N(22)	1.929(9)
	Empirical formula	C 185 H 296 B 2 Co 2 F 6 Fe 2 N 26 O 10 S 2	Co(1)-N(17)	1.885(8)
	Formula weight	3473.78	Co(1)-N(13)	1.90(1)
	Crystal system	Triclinic	Co(1)-N(19)	1.943(9)
	Space group	P1	Co(1)-N(20)	1.96(1)
	Wavelength, Å	0.71073	Fe(1)-N(5)	2.040(9)
	a, Å	11.9217(13)	Fe(1)-N(3)	2.07(1)
	b, Å	16.8399(18)	Fe(1)-N(1)	2.031(8)
	c, Å	25.069(3)	Fe(1)-C(31)	1.85(1)
	α, °	78.207(5)	Fe(1)-C(33)	1.91(1)
	β, °	80.997(6)	Fe(1)-C(32)	1.88(1)
	γ, °	78.043(6)	Selected bond angles (°)
	V, Å 3	4785.4(9)	N(21)-Co(1)-N(22)	82.9(4)
	Z	1	N(21)-Co(1)-N(13)	89.9(4)
	ρ calcd , g/cm 3	1.201	N(21)-Co(1)-N(19)	93.0(4)
	μ, 1/mm	0.406	N(21)-Co(1)-N(20)	92.2(4)
	Reflections collected	38313	N(22)-Co(1)-N(17)	93.6(4)
	Independent reflections	16215	N(22)-Co(1)-N(13)	89.7(4)
	a R 1 wR 2 a GoF a	0.1832 0.3435 1.112	N(22)-Co(1)-N(20) N(17)-Co(1)-N(13) N(17)-Co(1)-N(19)	94.8(4) 92.2(4) 90.4(

Table of Contents for Chapter V:

 of

Table V - 1 .

 V1 Unit-cell parameters, average metal-to-ligand bond lengths, metal-to-metal distances and deformation of the Co coordination sphere for 7 at 90 K, for 8a at 120 K, and for 8b under different experimental conditions.The asymmetric unit inside the crystal lattice is composed of two halves of two discrete complex cations, [(Tp*)Fe(CN) 3 Co(bpy) 2 ], two strongly disordered trifluoromethanesulfonate ions and four cocrystallized DMF molecules. In both tetranuclear square-shaped cores, Fe and Co ions reside in alternate

			(7)	(8a)	(8b)
			90 K	120 K	90 K	298 K
			{Fe III 2 Co II 2 }	{Fe II 2 Co III 2 }	{Fe II 2 Co III 2 }	{Fe III 2 Co II 2 }
	Crystal system	Triclinic	Monoclinic	Monoclinic	Monoclinic
	Space group	P ̅	C 2/c	P 21/c	P 21/c
	a, Å		13.687(3)	31.980(4)	15.434(4)	15.1202(7)
	b, Å		16.247(4)	20.8190(10)	22.124(5)	23.1984(10)
	c, Å		27.611(6)	21.593(2)	21.318(4)	21.5742(7)
	α, °		87.698(11)	90	90	90
	β, °		84.376(11)	129.237(2)	132.717(11)	131.615(2)
	γ, °		65.203(10)	90	90	90
	V, Å 3		5547(2)	11135.1(18)	5348(2)	5657.6(4)
	<Fe-C>, Å	1.919	1.894	1.899	1.923
	<Fe-N>, Å	1.995	2.056	2.042	2.010
	<Co-N>, Å	2.123	1.925	1.924	2.126
	Fe(1) … Co(1), Å	5.091	4.946	4.934	5.166
	Fe(1) … Co(1)', Å	5.132	4.942	4.935	5.156
	Fe … Fe, Å	7.562	7.187	7.186	7.655
	Co … Co, Å	6.879	6.775	6.765	6.925
	Σ(N-Co-N) a , °	67.6	45.3	42.4	70.6
	a ∑	∑ |	| , where are N-Co-N bond angles

  TableV-1). The cyanide bridges are bent especially at the cobalt sites, Co-N-C angles vary between 165.0(8) and 169.2(8)° (TableS.V-2). The N-Co-N angles (TableS.V-2) reveal a strongly distorted coordination sphere around the HS Co II ion, as it can be observed from the Σ(N-Co-N) parameter of 67.6° (TableV-1). No structural changes observed upon cooling the crystal of 7 from 250 to 90 K and the presence of Co II

Table V - 2 .

 V2 Unit-cell parameters, average metal-to-ligand bond lengths, metal-to-metal distances and deformation of the Co coordination sphere for 8a before and after the irradiation with an 808 nm laser source (2 h, P = 30 mW/cm 2 ) at 10 K.

			(8a)
			10 K	10 K 808nm
			{Fe II 2 Co III 2 }	{Fe III 2 Co II 2 }
	Crystal system	Monoclinic
	Space group	C 2/c
	a, Å		31.980(4)	31.933(3)
	b, Å		20.8190(10)	21.262(2)
	c, Å		21.593(2)	21.772(2)
	β, °		129.237(2)	129.010(10)
	V, Å 3		11135.1(18)	11486.4(19)
	<Fe-C>, Å	1.887	1.925
	<Fe-N>, Å	2.053	2.010
	<Co-N>, Å	1.925	2.101
	Fe(1) … Co(1), Å	4.930	5.143
	Fe(1) … Co(1)', Å	4.930	5.135
	Fe … Fe, Å	7.144	7.577
	Co … Co, Å	6.797	6.945
	Σ(N-Co-N) a , °	43.9	64.1
	a ∑	∑ |	| , where are N-Co-N bond angles
	V.190

  3

  •K/mol that is about half of the value measured at 280 K. The occurrence of an intermediate state, composed of {Fe III in the 1:1 molar ratio inside the crystal packing, was also observed and reported previously for another cyanido-bridged square complex, {[(Tp*)Fe(CN) 3 ] 2 [Co(dtbbpy) 2 ] 2 }(PF 6 ) 2 •2MeOH

	{Fe II	2 Co III	2 } complexes
			2 Co II	2 } and
				V.191

  •mol-1

	Methanol	60.3	263.7	228.7	-	-	-
	Butyronitrile	-	-	-	59.2	218.9	270.3
	Dichloromethane	75.0	215.9	347.4	50.9	201.6	252.7

  .199 important role played by the counter-anions in the modulation of ET properties in cyanido-bridged molecular squares not only in solid state (see Chapter III), but also more surprisingly in solutions. The characteristic temperatures (T 1/2 ) observed from the magnetic and optical studies of 7 and 8 in solutions follow the same trend as the one observed for compounds 2, 4 5, and 6 4b (see also Chapter IV) suggesting that thermally induced ET is an intrinsic property of the {[(Tp*)Fe(CN) 3 ] 2 [Co(diRbpy) 2 ] 2 } 2+ systems that can be easily observed in solutions due to the absence of the crystal lattice packing constraints or the strong intermolecular interactions.Functionalization with donating methoxy groups shows the possibility to increase the ET characteristic temperature in solid state, if compared with compounds 1 and 2, and in solutions if compared with compounds 2, 5, and 6. But it is important to point out that in solid state, the desired effect was achieved in combination with several other parameters (counter-anion, co-crystallized solvents, weaker or stronger intermolecular interactions) that influence also the ET properties of the cyanido-bridged {Fe 2 Co 2 } molecular squares.

	7, 8a and 8b, observed in solid
	state, have showed that the functionalization of the cyanido-bridged {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 } 2+ system

with electron donating methoxy groups influences significantly the optical and magnetic properties of the final compound. With the ligands remaining the same across the three compounds, it was possible to stabilize the cyanido-bridged {[(Tp*)Fe(CN) 3 ] 2 [Co(diMeObpy) 2 ] 2 } 2+ core in various electronic states.

V

Table S .

 S V-1. Crystallographic data for 7 at 90 K. Co 2 Fe 2 N 26 O 8 , 2(CF 3 O 3 S), 4(C 3 H 7 NO) Empirical formula C 98 H 120 B 2 Co 2 Fe 2 N 32 O 18 S 2

	Temperature	90 K
	Crystal color	dark red
	Moiety formula C 84 H 92 B 2 Formula weight 2435.49
	Crystal system	Triclinic
	Space group	P ̅
	Wavelength, Å	0.71073
	a, Å	13.687(3)
	b, Å	16.247(4)
	c, Å	27.611(6)
	α, °	87.698(11)
	β, °	84.376(11)
	γ, °	65.203(10)
	V, Å 3	5547(2)
	Z	2
	ρ calcd , g/cm 3	1.456
	μ, 1/mm	0.676
	Reflections collected	212115
	Independent reflections	21322
	a R 1 wR 2 a GoF a	0.1445 0.3454 1.193

General Conclusions

Acknowledgements

III.136

Thank you all -Merci à tous -Mulţumesc tuturor

III.124

The {Fe 2 (μ-CN) 4 Co 2 } core in 3 adopts a distorted square geometry similar to the one observed for 2 in its paramagnetic state. Nevertheless, the bond-angles, N(5)-Co(1)-N (7) (89.27(14)°) and C (18)-Fe(1)-C (19) (91.08( 16)°), are close to orthogonality. The cyanide bridges are notably bent especially at the cobalt sites, both Co-N-C angles are 165.2(3) and 165.5(4)°, while the Fe-C-N angles range between 172.7(3) and 178.7(4)° (Table S. III-3). The two Fe-CN-Co edges are quasi identical since the Fe(1) … Co (1) and Weak intermolecular π-π type interactions are present between adjacent diMebpy ligands coordinated to Co II ions and between adjacent pyrazole rings from the capping Tp* ligand on Fe III site along the crystallographic b axis. These weak interactions lead to a supramolecular 2D-like structure with centroid-tocentroid distances between diMebpy planes of 4.12 Å and between pyrazole planes of 3.94 Å (Figure III-7 bottom). The distances corresponding to intermolecular interactions in 3 are slightly shorter than those measured for complex 2 in its light-induced paramagnetic {Fe III 2 Co II 2 } state at 80 K (Figure III-7 top). Since complex 3 at 100 K is in the {Fe III 2 Co II 2 } paramagnetic state, according to structural analysis, the lightinduced paramagnetic {Fe III 2 Co II 2 } state of 2 was chosen for comparison. The shorter intermolecular contacts between the adjacent capping ligands in the crystal packing of 3 may be explained by the presence of a smaller ClO 4 -counter-anion. As can be observed from the unit cell volumes measured for 2 (2464.0 Å 3 ) and 3 (2348.0 Å 3 ), a denser crystal packing is formed in 3 (Table III-2). Therefore, a too dense crystal packing caused by a smaller counter-anion can be considered as one of the factors that blocks the thermally induced ET.

III.2.3.4. {[(Tp*)Fe(CN) 3 ] 2 [Co(diMebpy) 2 ] 2 }(OTs) 2 •6DMF (4)

The structural analysis of a dark red crystal of 4 was performed at 120 K after cooling it from 250 K at 1 K•min -1 . Complex 4 crystalizes in the same triclinic P-1 space group, as previously reported for complexes 1, 2 and 3. The crystal structure consists of a discrete cyanide-bridged tetranuclear cationic core (Figure III-6 right), two para-toluenesulfonate anions (OTs -) and six co-crystallized DMF molecules.

The analysis of the average Co-N (2.12 Å), Fe-C (1.925 Å) and Fe-N (1.997 Å) bond distances (Tables III-2 and S. III-4) shows that the {Fe 2 (μ-CN) 4 Co 2 } core is composed of Co II HS and Fe III LS ions.

The geometry of the {Fe 2 (μ-CN) 4 Co 2 } square is significantly more distorted than in 2 or 3. Even though the two Fe-CN-Co edges are quasi identical (the Fe(1) … Co (1) and Fe(1) … Co(2) distances are 5.118 and 5.127 Å, respectively), the Fe•••Fe and Co•••Co distances within the core are significantly different at 7.489 and 6.992 Å, respectively (Table III-2). The cyanide bridges are bent especially at the cobalt sites, both Co-N-C angles are 163.7 (5) and 168.5 (5)° (Table S. III-4). The N-Co-N angles (Table S.III-4) reveal a more distorted coordination sphere around the HS Co II ion in 4 than in 2 or 3, as it can be observed from the Σ (N-Co-N) parameter of 69.36° (Table III-2).

The presence of large OTs -counter-anions and six DMF per molecular square in the crystal packing of IV.163

IV.167

Table S.IV-3. Polarity index of several commonly used solvents.

Solvents

Polarity index (P') 13 Similarly to compound 7, the presence of interstitial MeCN molecules and PF 6 -counter-anions allows a good separation between cationic cores, thus no significant short intermolecular interactions between the {Fe 2 Co 2 } squares are observed inside the crystal packing of 8a. Each of co-crystallized water molecules forms two hydrogen bonds (N (7) (Figure V-5). Hydrogen-bonds between lattice water molecules and terminal cyanide groups were also observed in 1 1 and 2 4 (see also Chapter III), which exhibit both thermally and photo-induced ET. If in complexes 1 and 2, bearing bpy or diMebpy ligands on Co sites, these H-bonds are considered as one of the factors that contribute to the occurrence of intramolecular ET. Then in the case of cyanido-bridged {Fe 2 Co 2 } complexes with diMeObpy ligands, the electron-withdrawing effect of these H-bonds over the redox potential of the Fe centers is enough to stabilize the diamagnetic {Fe II 2 Co III 2 } species in solid state. The same stabilization effect was observed with MeOH molecules H-bonded to terminal cyanide groups in complex 6 4b (see also Chapter IV) and in the recently reported {[(Tp)Fe(CN) 3 ] 2 [Co(diCOOEtbpy) 2 ] 2 }(ClO 4 ) 2 •2MeOH complex 6 (where Tp = tri(pyrazol-1-yl)hydroborate and diCOOEtbpy = 4,4'-bis(ethoxycarbonyl)-2,2'bipyridine).

V.2.2.3. {[(Tp*)Fe(CN)

The structural analysis of a dark red prismatic crystal of 8b (obtained by ether diffusion in acetonitrile solution of the red powder obtained in anhydrous methanol, see Section V.2.1), was performed at 90 and at 298 K (Tables V-1 andS S. V-4) show a distorted coordination sphere around the LS Co III ions, as judged by the Σ (N-Co-N) parameter of 42.4° (Table V-1). Similarly to the metal-to-ligand bond lengths, the short-contact distance changes as the result of the intramolecular ET from ~ 3.9 Å at 298 K to ~ 3.8 Å at 90 K (Figure S.V-1).

For the three crystallographically characterized compounds 7, 8a and 8b, structural changes associated with a thermally induced ET process were observed only in the compound 8b.

V.179

As can be deduced from the crystallographic data of 7, 8a and 8b presented in Table V-1, the cationic {Fe III 2 Co II 2 } core in 7 is significantly more distorted than the one observed in 8b at 298 K with the same electronic configuration on metal centers. The presence of trifluoromethanesulfonate anions and cocrystallized DMF molecules in the crystal packing of 7 strongly influence the molecular structure of the tetranuclear core and contribute to the stabilization of the paramagnetic {Fe III 2 Co II 2 } state. Both 8a and 8b complexes show very similar structures at low temperatures (at 120 and 90 K, respectively. See Table V-1), but only the compound 8b exhibits thermally induced ET, while 8a is stabilized in the diamagnetic {Fe II 2 Co III 2 } state, likely due to the H-bonded water molecules.

V.2.3. Optical reflectivity studies

Surface reflectivity spectra of complexes 7, 8a and 8b were collected between 500 and 1000 nm, at different temperatures, with the aim to determine the presence of any changes of the optical properties associated with an eventual thermally-and/or photo-induced ET process. The reflectivity spectrum of 7 at 280 K presents a strong absorption between 500 and 700 nm (R 550 = 0.22, while R 800 = 0.75), and is similar to the spectra recorded for other cyanido-bridged {Fe 2 Co 2 } molecular squares in their paramagnetic state. 1,4 Between 280 and 10 K, no significant changes are observed, denoting that complex 7 does not undergo thermally-induced electron transfer (Figure V-7).

V.2.3.1. Reflectivity measurements for compound 7

V.2.3.2. Reflectivity measurements for compound 8a

It is important to mention that the green crystals of 8a turn red if they are kept outside the mother liquor for more than 5 -10 min or if they are subjected to a mechanical stress like smoothing the surface during the preparation of a sample for the optical reflectivity measurements. Therefore, the optical reflectivity measurements were carried out between 270 and 10 K on freshly filtered green crystals of 8a.

At 270 K, the green crystals are strongly absorbent in the studied spectral range (R 550 = 0.05, while R 800 = 0.14, Figure V-8), similarly to 1 1 , 2 4 , and 6 (see also Chapters III and IV) in their diamagnetic {Fe II 2 Co III 2 } state. Between 270 and 120 K, the complex becomes even more strongly absorbent above 700

V.182

In order to fully characterize the metastable photo-induced {Fe III 2 Co II 2 } state, additional photoexcitation experiments were performed at selected wavelengths ranging from 940 to 365 nm (±10 nm) using LED sources. The optimal wavelength to photo-convert the diamagnetic {Fe II 2 Co III 2 } state into the paramagnetic {Fe III 2 Co II 2 } one was determined by checking the efficiency of the 13 different LEDs in similar conditions at 10 K: 5 min of irradiation at a power of 0.2 mW/cm 2 (Figures V-10 and S.V-2). Before each excitation, the sample was heated to 250 K and cooled down to 10 K in the dark to fully relax the photogenerated {Fe III 2 Co II 2 } sites. The plot of the difference between the absolute reflectivity at 800 nm before and after LED irradiation, ΔAR 800 = R 800afterLED -R 800before , as a function of the LED wavelength ( The optical reflectivity studies revealed the photo-sensitivity of 8a and thus the possibility to convert the diamagnetic green {Fe II 2 Co III 2 } species into a paramagnetic {Fe III 2 Co II 2 } metastable state. Moreover, the additional photo-excitation experiments using LED sources allowed the identification of the wavelength with the highest efficiency to induce the photo-excited state that was useful for further investigations of lightinduced ET in complex 8a.

V.2.3.3. Reflectivity measurements for compound 8b

Several preliminary tests showed that the ET transfer process (confirmed by the temperature dependent structural investigations) in crystals of 8b, which are red at room temperature, is strongly dependent on the presence of the interstitial solvent molecules inside the crystal lattice. Therefore the temperature dependent reflectivity studies were performed below room temperature between 280 and 10 K in order to avoid the loss of co-crystallized solvent molecules. At 276 K, the surface of compound 8b is strongly absorbent and shows a reflectivity spectrum weak in intensity that contains a strong and broad absorption between 500 and 700 nm (absolute reflectivity at 550 nm R 550 = 0.015) and a slightly higher reflectivity above 700 nm (R 800 = 0.06). The observed strong Chapter V: Functionalization of Molecular {Fe 2 Co 2 } Squares with Strongly Donating Methoxy Group V.183 absorption of 8b can be explained by the very dark red color of the crystals at room temperature and/or partial conversion at 276 K of the {Fe III 2 Co II 2 } species into the diamagnetic {Fe II 2 Co III 2 } species that show strong absorption between 500 and 1000 nm.

On cooling from 276 to 246 K, the complex 8b becomes strongly absorbent above 700 nm (Figure V-11 left) as the result of thermally-induced electron-transfer between Co II and Fe III ions. Upon further cooling until 100 K the reflectivity in the near-IR region continues to decrease, but not as significantly as between 276 and 246 K. Below 100 K, an increase of reflectivity is observed above 700 nm as a possible signature of the photo-induced formation of the paramagnetic {Fe III 2 Co II 2 } species under the spectroscopic white light (P = 0.4 mW/cm 2 ) as already observed for 8a. Heating the sample leads to a complete reversal of the spectral changes observed upon cooling. The spectrum at 276 K is similar to the one obtained before the thermal cycle suggesting that the sample is stable under the experimental conditions, while the further heating until 286 K shows a significant decrease of absorption in the near-IR region, suggesting the occurrence of thermally-induced ET. (left) Thermal evolution of reflectivity signal recorded at λ = 800 ± 5 nm (R 800 ) for 8b in heating (red line) and cooling modes (blue line), and after 5 h of white light irradiation (0.4 mW/cm 2 ) at 10 K in heating mode (green line). (right) Time evolution of the absolute reflectivity R 800 under white light irradiation (0.4 mW/cm 2 ) during 5 h at 10 K; inset: reflectivity spectra at 10 K before and after 5 h of white light irradiation (0.4 mW/cm 2 ).

The observed thermal evolution of the absolute reflectivity at 800 nm, R 800 , allows an easier probing of the ET process over the whole temperature range (Figure V-12 left). Upon cooling from 276 K, R 800 decreases from  0.06 at 276 K down to  0.03 at 250 K as a signature of the electron transfer process.

Between 250 and 120 K, R 800 decreases smoothly from 0.03 to 0.025 and below 120 K an increase of R 800 is observed as a possible signature of a photo-conversion of the diamagnetic {Fe II LS (μ-CN)Co III LS } into paramagnetic {Fe III LS (μ-CN)Co II HS } units by the white light source used for spectra acquisition. The presence of thermal hysteresis effect between 10 and 140 K (Figure V-12 left, red and blue curves) reinforces this hypothesis. Upon heating the sample, a complete reversal of the spectral changes is observed (R 800 recovers its value of 0.06 at 276 K) showing no thermal hysteresis above 250 K and suggesting that thermally induced ET is associated with a conversion of the diamagnetic {Fe II LS (μ-CN)Co III LS } units into the paramagnetic {Fe III LS (μ-CN)Co II HS } pairs. Whereas the increase of R 800 to 0.073 at 281 K suggests that the thermal V.184 {Fe III 2 Co II 2 } → {Fe III 2 Co II 2 } conversion is not finished at 281K and has a characteristic temperature, T 1/2 , between 250 and 298 K (on the basis of single-crystal XRD data collected at 298 K).

To follow the photo-excitation of 8b, the sample was cooled down in the dark at 10 K and then exposed to a constant white light (P = 0.4 mW/cm 2 ). At 10 K, the optical reflectivity at 800 nm (R 800 ) was Additional photo-excitation experiments using 14 LED sources of specific wavelengths ranging from 1050 to 365 nm (±10 nm) were performed on 8b (Figures V-13 and S.V-3), employing the same experimental protocol as for 8a (5 minutes of irradiation at 10 K with a LED source at a power of 0.2 mW/cm 2 , see Section V.2.3.2 for details). To summarize and visualize the changes induced by the LED irradiations, the difference of absolute reflectivity at 800 nm (ΔAR 800 ) was plotted as a function of LED Even though all the employed LED sources are capable to convert {Fe II excitation while the 505 nm LED was chosen to probe de-excitation. During the first set of successive irradiation cycles at 10 K, the power of both LED sources was set to 0.2 mW/cm 2 , with one cycle consisting of 160 min of irradiation with the 940 nm LED followed by 180 min of irradiation with the 505 nm LED. To summarize and visualize the ET photo-reversibility in 8b induced by the successive LED irradiations at 10 K, the absolute reflectivity at 800 nm (R 800 ) was monitored as a function of time. As judged by the time evolution of R 800 during the first set of successive irradiation cycles, a photo-reversible effect was observed the surface of 8b (Figure V-14), but surprisingly the recorded curves do not superimpose. The first 940 nm LED irradiation curve appears to be considerably below the following ones. Moreover each curve corresponding to the excitation with the 940 nm LED source to paramagnetic {Fe III 2 Co II 2 } state (Figure V-14 left: red dots, except the first one) looks quasi-saturated but at different R 800 values. In contrary to excitation data, the curves corresponding to the irradiation with the 505 nm LED source exhibit quite efficient recovery of the thermodynamic {Fe II 2 Co III 2 } state (Figure V-14 left: green dots), the obtained R 800 value is higher than the initial one, but is relatively constant after each irradiation cycle. Increasing the power of the LED sources and the time of exposure to LED irradiation showed similar quasi-reversible switching between {Fe II 2 Co III 2 } and {Fe III 2 Co II 2 } states at the surface of 8b (Figure V-14 left). In order to verify if the observed de-excitation caused by 505 nm LED irradiation is real and not confused with the relaxation in the dark, the latter was monitored. The R 800 time dependence was measured during three cycles consisting of 180 min exposure to the white light (P = 0.4 mW/cm 2 ) followed by 160 min of no irradiation (Figure S.V-4). No significant relaxation was observed in the dark, therefore we can conclude that at least at the surface of 8b it is possible to observe a {Fe II 2 Co III 2 } ↔ {Fe III 2 Co II 2 } photo-reversibility with different wavelengths at 10 K. To confirm the nature of the observed photo-switching, it is necessary to perform additional investigations including photo-magnetic measurements on a bulk polycrystalline sample of 8b and X-ray crystallography under irradiation.

V.2.4. Photo-crystallographic investigations of compound 8a

Optical reflectivity measurements of 8a revealed the possibility to photo-convert at 10 K the diamagnetic {Fe II 2 Co III 2 } state into a paramagnetic {Fe III 2 Co II 2 } species at the surface of the studied sample.

At 10 K before irradiation, the structure of the square motif in the green crystals of 8a (Figure V-15, Tables V-2 and S.V-3) shows no significant differences compared with the one determined at 120 K (See Section V.2.2.2, Figure V-4, Tables V-1 and S.V-3). The measured average bond lengths match well with those previously observed for Fe II and Co III ions in LS configurations.

Many structural parameters changed after the irradiation with an 808-nm laser light at 10 K (2 h, P = 30 mW/cm 2 ), even if the space group remains C2/c. An increase of the unit cell volume by 350 Å 3 was observed after the irradiation (Table V-2). Important changes in the coordination sphere of the Co ion occurred (Tables V-2 and S.V-3). The average Co-N bond length became 2.086( 7) Å, which is close to that expected for a Co II HS ion, suggesting an almost complete {Fe II 2 Co III 2 } → {Fe III 2 Co II 2 } photo-conversion. In addition, the N-Co-N angle values in the HS Co II coordination sphere departed more significantly from orthogonality than those measured for the LS Co III ion (Tables V-2 and S.V-3). Although the Fe coordination sphere showed only smaller differences (Tables V-2 andS 7)') decreased from 2.924 and 2.870 Å to 2.810 and 2.787 Å, respectively, at 10 K.

V.188

The photo-crystallographic investigations performed at 10 K for the green crystals of 8a are in agreement with the optical reflectivity study and proved the possibility to convert the diamagnetic {Fe II LS (μ-CN)Co III LS } species into paramagnetic {Fe III LS (μ-CN)Co II HS } units as the result of the intramolecular light induced ET.

V.2.5. Magnetic measurements

According to the structural and optical data, compound 7 does not exhibit an ET process, 8a is stabilized in the diamagnetic {Fe II 2 Co III 2 } state between 10 and 270 K, but exhibits an {Fe II 2 Co III 2 } → {Fe III 2 Co II 2 } conversion as the result of a photo-induced ET, while compound 8b shows both thermally and light-induced ET. To further investigate the electron transfer mechanism and the resulting changes of the electronic states, the magnetic properties of 7, 8a and 8b as a function of temperature and/or light exposure were studied.

V.2.5.1. Magnetic properties of 7

As expected, magnetic susceptibility measurements carried out for 7 between 1.

V.2.5.2. Magnetic and photo-magnetic properties of 8a

The dark green color of the crystals at room temperature suggests that 8a is in its diamagnetic {Fe II 2 Co III 2 } state. Optical reflectivity measurements and photo-crystallographic investigations at 10 K suggested the possibility to photo-induce the conversion of the diamagnetic {Fe II 2 Co III 2 } state into a metastable paramagnetic {Fe III 2 Co II 2 } state by an intramolecular electron transfer. Therefore, the magnetic and photo-magnetic properties of 8a were studied. At 10 K in the dark, the measured χT product was about 0.3 cm 3 •K/mol, confirming that the investigated sample is globally diamagnetic consistently with the presence of two low-spin Co III (S = 0) and two low-spin Fe II (S = 0) ions. Under irradiation with a white halogen lamp (3 mW/cm 2 ) at 10 K, the χT product increases with the irradiation time (Figure V-18 left). This increase of the magnetization points out the occurrence of a photo-induced electron transfer associated with the conversion of the diamagnetic {Fe II LS (μ-CN)Co III LS } species into paramagnetic {Fe III LS (μ-CN)Co II HS } ones. Apparently due to an insufficient light-penetration depth and/or relatively low efficiency of the irradiation, an incomplete conversion was observed. The χT product did not reach saturation and the expected values of ca. 6.5 -7 cm 3 •K/mol even after ~ 17 h of irradiation; χT = 2.1 cm 3 •K/mol at 10 K (Figure V-18 left). Upon heating in the dark from 2 to 45 K, the χT product smoothly decreased from 2.5 cm 3 •K/mol towards 1.9 cm 3 •K/mol, thus suggesting the presence of intramolecular ferromagnetic couplings in the photo-generated {Fe III LS (μ-CN)Co II HS } species (Figure V-18 right), as it was reported for other cyanido-bridged {Fe 2 Co 2 } squares. 1,2a Consistently with the reflectivity study, the photo-induced metastable state readily converts into the thermodynamic {Fe II LS (μ-CN)Co III LS } state above 70 K. Upon further heating the χT product recovered the initial value of 0.3 cm 3 •K/mol and remained constant between 90 and 250 K. The studied polycrystalline sample of 8a was not heated above 250 K during the magnetic measurements. Indeed preliminary tests V.194 {Fe III 2 Co II 2 } state 2a,3b,4 (see also Sections IV.3.1 and IV.5.3 in Chapter IV). The strong absorption at 450 nm was assigned to the ligand-to-metal charge transfer (LMCT) bands from the Fe and Co chromophores, whereas the shoulder band at 540 nm was assigned to Co II →Fe III metal-to-metal charge transfer (MMCT) transition. 2a,3b,4,10 As the temperature was lowered (Figures V-21 and S.V-5 left), the band at 450 nm is shifted to the higher energy region due to the decrease of the Co II →Fe III MMCT band (540 nm) in intensity.

Concomitantly, a new broad absorption peak appears, assignable to the Fe II →Co III MMCT band at 760 nm. 2a,3b,4,11 The spectral changes confirmed that solutions of 7 and 8 exhibit a thermally induced ET associated with {Fe III 2 Co II 2 } → {Fe II 2 Co III 2 } conversion upon cooling. The temperature dependence of the absorption spectra (Figures V-21 and S.V-5) showed an isosbestic point around 603 -608 nm (depending on the employed solvent) suggesting that only two species are involved in the thermal inter-conversion process.

Table V-3. Thermodynamic parameters obtained from the ideal solution model 9 fit of the UV-Vis data for 7 and 8 in different solvents. V.195

The estimated thermodynamic parameters have values close to those deduced for complexes 2, 4 5, and 6 4b (see also Chapter IV, ΔH ~ 50 -75 kJ•mol -1 and ΔS ~ 190 -350 J•K -1 •mol -1 ). Both the enthalpy and the entropy are changing with employed solvents, but no certain solvent polarity dependence could be observed, except for the characteristic temperature, T 1/2 . Both parameters are more than two times greater than the ones reported in solid state, 1,4,10a suggesting that the relatively large values might be caused by the reorganization of solvent molecules or any other solvent effect upon electron transfer.

In spite of the fact that the temperature dependence of the absorption spectra was collected only in a few solvents, the obtained results allowed to prove the occurrence of the thermally induced ET in solutions of 7 and 8. In solid state, the paramagnetic (red) state of complex 7 was stabilized without any evidence of ET. Moreover, the ET characteristic temperature is modified as a function of the employed solvent (Figure V-22), as already discussed for complexes 2, 4 5, and 6 4b (see also Chapter IV). In fact, the methanol and dichloromethane were chosen for the quantitative investigations of the optical properties in solution because previous studies of 2, 4 5, and 6 4b (see also Chapter IV) showed that the largest difference between the ET characteristic temperatures were obtained for these solvents due to a large difference between their polarities.

Another important observation was made in the case of compound 8. Its solvates 8a and 8b have a completely different behavior in solid state. The green color of the crystals of 8a at room temperature, as well as the magnetic susceptibility measurements confirm that the diamagnetic state is stabilized in solid state. On the other hand, during the solubilization, the green crystals afford a red solution that exhibits ET

V.3.2. Magnetic properties of 7 and 8 in solutions

From the magnetic susceptibility measurements, additional evidences in support of an intramolecular ET in solutions of 7 and 8 were obtained (Figures V-23 and S.V-7). The magnetic susceptibility measurements were carried in dichloromethane for both compounds, in methanol 7, and in butyronitrile for 8 at concentrations of 6.6 -10.4•10 -4 mol/L until the freezing point of the employed solvents. These solvents and concentrations were chosen to avoid any precipitation of the solid during the cooling or freezing of the solvent.

At high temperatures, both complexes showed practically the same χT product around 6.6 cm 3 •K/mol, that is in agreement with the presence of paramagnetic {Fe III 2 Co II 2 } complexes, and is similar to the χT values measured for 2, 4 5, and 6 4b (see also Chapter IV). With decreasing temperature, the χT product rapidly decreases to small values (0.2 to 0. Compound 7 that has a different counter-anion (OTf -) and was crystallized under different conditions from the ones employed for 8a and 8b, exhibits no ET and is stabilized in its paramagnetic {Fe III 2 Co II 2 } state. Compound 8a is stabilized in its diamagnetic {Fe II 2 Co III 2 } state that shows no thermally-induced ET between 2 and 300 K, as confirmed by the green color of the crystals at room temperature and the magnetic susceptibility measurements. Optical reflectivity and magnetic measurements of 8a supported by the photocrystallographic investigations demonstrated the occurrence of the photo-induced ET associated with a conversion between the diamagnetic {Fe II 2 Co III 2 } and paramagnetic {Fe III 2 Co II 2 } states at low temperatures. Compound 8b that differs from 8a only by the solvation inside the crystal lattice exhibits both thermally-and light-induced ET. Magnetic measurements of 8b showed the occurrence of thermally-induced ET in a twostep fashion via a possible intermediate state that is, most, likely composed of a 1:1 molar mixture of diamagnetic and paramagnetic complexes, as already reported for another cyanido-bridged {Fe 2 Co 2 } molecular square. 3 The compared crystallographic data for 7, 8a and 8b presented in Table V-1, showed that the cationic {Fe III 2 Co II 2 } core in 7 is significantly more distorted than the one observed in 8b at 298 K, with the same electronic configuration on metal centers. The presence of trifluoromethanesulfonate anions and cocrystallized DMF molecules in the crystal packing of 7 strongly influence the geometrical shape of the tetranuclear core and contribute to the stabilization of the paramagnetic {Fe III 2 Co II 2 } state. Both 8a and 8b complexes show very similar structures at low temperatures (at 120 and 90 K, respectively. See Table V-1), but only the compound 8b exhibits thermally induced ET, while 8a is stabilized in its diamagnetic {Fe II 2 Co III 2 } state, likely due to the H-bonded water molecules. Thus, structural perturbations inside the crystal packing such as the change of the molecular shape and weaker/stronger intermolecular interactions induced by anion exchange can either facilitate or inhibit the intramolecular ET in cyanido-bridged {Fe 2 Co 2 } molecular squares.

Additional optical reflectivity measurements employing different light wavelengths revealed that the diamagnetic {Fe II 2 Co III 2 } state in 8a and 8b can be converted into the paramagnetic {Fe III 2 Co II 2 } one by a photo-induced ET using 1050 to 365 nm irradiation wavelengths (with an optimal wavelength of 940 nm, and the least efficient one of 505 nm). Successive light irradiations with 940 nm and 505 nm LED sources proved the possibility of a photo-reversible {Fe II 2 Co III 2 } ↔ {Fe III 2 Co II 2 } conversion at the surface of 8b. To confirm the occurrence of the photo-reversible ET in this compound, it will be necessary to perform additional photo-magnetic investigations.

The studies performed in solutions revealed that functionalized squares 7 and 8 (8a and 8b are different solvates of the same system, {[(Tp*)Fe(CN) 3 ] 2 [Co(diMeObpy) 2 ] 2 }(PF 6 ) 2 , and were referred as 8 in solution studies) undergo thermally-induced intramolecular ET and, thus, exhibit switching of magnetic and optical properties in diluted solutions. In spite of the fact that the magnetic and optical characterizations were performed only in a few solvents, the obtained results revealed additional information about the occurrence of thermally induced ET in solutions of {Fe 2 Co 2 } molecular squares. It was observed that the ET characteristic temperatures in the same solvent (i.e. dichloromethane) show a difference of ca. 14 K for compounds 7 and 8 that differ only by the counter-anions (OTf -in 7 and PF 6 -in 8). This result highlights the V.205

V.6. The mixture was stirred for 1 h to give a dark red solution and green powder. The green powder was filtrated and dried in air. Dark green crystals of 8a were obtained by slow diffusion of diethyl ether into the red solution resulting from the dissolution of the green powder in CH 3 CN. The yield was 0.12 g (35 %). Selected FT-IR data (ATR, cm -1 ): 2974.5 (w), 2931.5 (w), 2869.7 (w), 2534.9 (w), 2152.9 (m), 2122. V.207 V.208

Supporting materials

Thermogravimetric analyses (TGA)

Thermogravimetric analyses were carried out using samples of 10 -20 mg, at 5 K/min in the nitrogen atmosphere using a thermogravimetric analyzer Setaram TAG 16.

Fourier transformed infrared (FT-IR) analysis

The FT-IR spectra were recorded between 4000 and 550 cm -1 on a Thermal Scientific Nicolet TM 6700 ATR (attenuated total reflectance) spectrometer equipped with a Smart iTR diamond window.

Nuclear magnetic resonance

1 H NMR spectra were collected with JEOL ECS 400 instrument at room temperature, using CDCl 3 or CD 3 CN as solvent. Chemical shifts of 1 H NMR signals were quoted to internal standard TMS (δ = 0.00 ppm)

and expressed as chemical shifts in ppm (δ), multiplicity, coupling constant (Hz), and relative intensity.

Single crystal X-ray analysis

The X-ray diffraction data for the single crystals of [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ], 1, 3-7, 8a and 8b were collected with a Bruker APEX II diffractometer, using graphite monochromated MoK α radiation (λ = 0.71073 Å). The program SAINT was used to integrate the data, which were thereafter corrected for absorption using SADABS. 1 The structure was solved by direct methods and refined by least squares on F 2 with SHELXL-97. 2 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were assigned to ideal positions using the appropriate HFIX command in SHELXL-97. Irradiation of the crystals of 1 with white light (P = 3 mW/cm 2 ) at 83 K was performed with a halogen-tungsten light source (Leica CLS 150 XD tungsten halogen source) coupled through an optical fiber that was directed towards the analyzed crystal.

Light was kept on during out of equilibrium data collections.

Structural investigations of compounds 2 (at various temperatures and under continuous white light irradiation from the sample illumination lamp of the diffractometer P = 6 mW/cm 2 ) and 8a (at 10 K before and after irradiation with a 808 nm laser source, P = 30 mW/cm 2 ) were performed on a four-circle Oxford Diffraction Xcalibur 3 diffractometer (MoK α radiation) with a 2D Sapphire 3 CCD detector (Institut de Physique de Rennes, Université Rennes 1). The single crystals were placed either in an Oxford Diffraction Helijet helium-flow cryostat, allowing reaching 10 K (8a) or in an Oxford Cryosystems nitrogen-flow cryostat, at 80 K ( 2), for the complete data collection. The unit cell parameters and the data reduction were obtained with CrysAlis software from Oxford Diffraction. 3 The structures were solved with SIR-97 4 and refined with SHELXL-97.

Small and wide angle X-ray scattering (SAXS and WAXS)

X-Ray scattering experiments at small and wide angles were collected on a home-made experimental set-up equipped with a Rigaku Nanoviewer (XRF microsource generator, MicroMax 007HF), with an 1200-W rotating anode (λ = 1.54180 Å) coupled to a confocal Max-FluxH Osmic mirror (Applied Rigaku Technologies) and a MAR345 image plate detector (Marresearch GmbH). The studied samples were mounted in glass capillaries of 1 mm diameter. Diffraction patterns were integrated with the FIT2D software. 5

Polarized optical microscopy

The thermal behavior and the optical textures of the compounds were studied with an Olympus BX51TRF polarizing microscope equipped with a Mettler Toledo FP82HT hot-stage.

Differential scanning calorimetry

The transition temperatures and enthalpies changes were measured by differential scanning calorimetry with a Q2000 TA instrument. DCS traces were recorded at different scanning rates, from 1 to 10 K/min in a nitrogen environment. Measurements were carried out using closed Tzero aluminium pans.

Calibration of temperature and melting enthalpy was performed with an indium and tin sample. Transition enthalpies changes were deduced by integration of the DSC peaks after polynomial baseline corrections.

UV/Vis analysis UV/Vis spectra were recorded between 350 and 1000 nm using a Varian Cary 5000 UV/Vis recording spectrophotometer at Institut des Sciences Moleculaires (ISM). Variable temperature UV/Vis data were obtained using an Oxford Liquid Nitrogen Cryostat Optistat ON (static) equipped with an Oxford ITC502 controller operating between 77 and 300 K.

Reflectivity analysis

Surface reflectivity measurements were performed on a home-built system at different temperatures ranging from 10 to 300 K using freshly filtered compounds. Heating and cooling rates were maintained at 4 K•min -1 during the measurements. This setup collects the light reflected by the sample (sum of direct and diffuse reflected light) that is analyzed by a high-sensitivity Hamamatsu 10083CA spectrometer between 500 and 1000 nm. The spectra are compared to a white reference obtained with a NIST traceable standard for reflectance (SphereOptics, ref SG3054). The background, collected with the light source switched off, is subtracted from all measurements. The reflectivity can be plotted as a function of temperature, time, or wavelength. Different light emitting diodes (LEDs) operating between 365 and 1050 nm and bought from Thorlabs were used for excitation measurements. As the samples are potentially photosensitive, the light exposure time was minimized during all the experiments keeping the samples in the dark except during the spectra measurements when white light is shined on the sample surface (P = 0.4 mW/cm 2 ). The temperature dependence of the reflectivity spectra was followed during a cycle of cooling-heating. For all the excitation/de-excitation experiments performed at 10 K, the sample was initially placed at this temperature keeping the sample in the dark to avoid any excitation.

Magnetic measurements

Magnetic susceptibility measurements were carried out on a Quantum Design SQUID magnetometer MPMS-XL (CRPP) operating between 1.8 and 400 K for applied dc fields ranging from -7 to 7 T.

Measurements in solid state were performed on a microcrystalline samples that were filtered from its mother liquor and dried in air less than 2 minutes prior the use. Experiments in solution were carried out in sealed plastic straws under argon. Magnetic data was corrected for the sample holder and diamagnetic contributions for all magnetic measurements. Photomagnetic measurements were performed at ICMCB using a halogentungsten light source (Leica CLS 150 XD tungsten halogen source adjustable from 0.5 mW/cm 2 to 1 W/cm 2 ) coupled through an optical fiber directed into the magnetometer cavity. Samples of ca. 3 mg were packed into a thermoformed plastic straw placed about 3.5 cm from the optical fiber (the power of the irradiation light was chosen carefully to find a good compromise between minimizing thermal heating of the sample and maximizing the photoconversion efficiency). Sample temperatures were corrected for the light-induced heating (average ±2 K at 10 K under white light irradiation), and the calibration of experimental sample temperatures was referenced to the data collected in absence of light.

XAS (XANES) and XMCD measurements

The experiments have been performed on the ID12 beamline of the ESRF synchrotron (Grenoble, France). 6 The flux of circularly polarized photons delivered by a helical undulator is monochromatized by a Si(111) two-crystal monochromator. The absorption is recorded in total fluorescence yield by a silicon diode.

More instrumental information can be found in ref 6.

The XMCD signal has been measured by reversing both the photon helicities and the direction of the magnetic induction and was obtained by the appropriate averaging of the various cross sections. For a given energy, the XMCD signal is the difference between the XAS spectra with right and left circularly polarized X-rays. The specimen magnetization is parallel to the photons wave vector, and in the electric dipole approximation, reversing the photon helicity is equivalent to reversing the magnetic induction. Spectra are recorded in a fixed magnetic field (either +17 or -17 T), and the circular polarization is flipped at each energy point.

Rational functionalization of molecular magnetic materials: towards liquid crystalline phases, improved solubility and modulation of physical properties

The work presented in this thesis was focused on the design and investigation of novel hybrid materials via ligand functionalization of the single-molecule magnets (SMMs) and electron transfer complexes. Chapter I contains general information about these two classes of the magnetic systems. In order to illustrate the motivation behind our work, a brief review on previously reported soft hybrid magnetic systems, is presented. Chapter II is dedicated to the functionalization of [Mn 12 ]-based SMMs towards hybrid liquid crystalline systems via two different approaches: (a) the functionalization of peripheral ligands with strongly lipophilic groups (long alkyl chains), or (b) the grafting of mesogenic promoters through flexible aliphatic spacers. Chapters III -V are focused on cyanido-bridged molecular {Fe 2 Co 2 } squares that exhibit thermally or photo-induced electron transfer. Thus, in Chapter III, the possibility to modulate the electron transfer properties in {Fe 2 Co 2 } molecular squares via the use of different counteranion is discussed. The functionalization with long aliphatic chains and its influence over the properties of {Fe 2 Co 2 } molecular squares in solid state and solutions are discussed in Chapter IV. Finally, the effect of the ligand functionalization with strongly electron density donating groups (methoxy) over the electron transfer properties of {Fe 2 Co 2 } molecular squares is investigated in Chapter V.