Rational Functionalization of Single-Molecule Magnets: Towards Liquid Crystalline (LC) Phases Chapter II -Rational Functionalization of Single-Molecule Magnets: Towards LC Phases II.50 Chapter II -Rational Functionalization of Single-Molecule Magnets: Towards LC Phases

Keywords: molecular magnetism, liquid crystal, single-molecule magnet, spin crossover, electron transfer

Chapter IV

These three years of PhD taught me that scientific work cannot be accomplished without the help and support from many people. Herein, I wish to thank everybody who guided and encouraged me, and who was involved in the evolution of this work. Thank you for you constructive criticism and good advices in difficult situations. His meticulousness showed me a model of a great scientist. Thank you for patience and understanding in the last moments of my PhD.

I wish to dedicate special thanks to Daniel Rosario-Amorin for help during this thesis. Besides the experimental work, scientific advices and research analyses, I thank you for your contribution, guidance and patience during the writing of this manuscript. Thank you for the time you took to explain and advise me during these three years, especially during the last months. I wish also dedicate special thanks to Dmitri Mitcov, who helped me in the first months of the PhD and introduced me to Mn 12 and {Co 2 Fe 2 } chemistry and, always, found time for discussion and help even after his departure from the lab. I would like to express my gratitude to many people, who actively contributed to this work: Yunnan Guo, who initiated the project on spin crossover during his post-doctoral stay in our team, Vincent Montigaud, who helped me to reproduce and characterise some of the spin crossover and electron transfer systems during the last months of the thesis when I was busy with the preparation of the manuscript, Abhishake Mondal, who helped me for the synthesis of Fe(III) precursors and to solve, with David Aguilà, some crystallographic problems as well as Maria Asensio for providing some of organic precursors used for the synthesis of ligands. I would like to strongly acknowledge Ahmed Bentaleb, who was very often solicited for SAXS measurements and was always ready to schedule these experiments as soon as possible, even during the week-ends or vacations. Thank you for your positive energy and the time you dedicate to this work. I would like to thank Pierre Dechambenoit for teaching me the crystallography. Thank you for your help with single crystal X-ray diffraction experiments and to explain me how to solve the crystal structures. I appreciated your advices and fruitful discussions. I thank Mathieu Rouzières for setting up the magnetic measurements and help on the treatment of the data. I would like also to thanks our dear neighbours from the Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB) for their participation in our studies. Especially, I wish to thank Corine Mathonière, who has been collaborating with our team for many years on electron transfer and photomagnetic systems. I thank you for the time you spent on photomagnetic measurements and for your precious help on optical studies. I would like to thank Alain Wattiaux, who kindly proposed to characterise some of our compounds by Mössbauer spectroscopy, Patrick Rosa, for giving us access to VSM magnetometer, needed for hightemperature measurements and Laeticia Etienne for elemental analysis.

General Introduction

The humanity knows about magnetism more than millennia. Nowadays, the interest to development of new magnetic materials and their interesting properties increased significantly. The first uses of magnetism are dating from 1060 in China, when Shen Kua found a process to magnetize the iron needles and in this respect he invented the compass. 1 This invention gave a rise to a number of devices that are based on magnetic compounds. The theoretical basis was drafted by Curie, Weiss, and Langevin. By 80s, the majority of the magnetic systems were studied as inorganic materials that are surrounded by metals or metal oxides.

Later, the magnetism science developed with molecules that brought out novel properties. All these gave an opportunity to a new field that was called molecular magnetism. Molecular magnetism is the main discipline discussed in this thesis work. Our research is focused on functionalization of new magnetic materials and study of their physical and chemical properties.

The main motivation in this area of research is the magnetic properties that give the opportunity to multi-functionality of the magnetic material. Thus, we tried to develop hybrid magnetic material in order to obtain soft systems that can be easy-processable. Regarding this, we functionalize interesting molecule-based magnetic material by known groups that can induce liquid crystalline behavior or have a better solubility.

Not at least, the original magnetic properties of the functionalized materials must be preserved.

Therefore, the main research objective in the "Molecular Materials and Magnetism" (M3) group at the Centre de Recherche Paul Pascal (CRPP) is focused on the functionalization of 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 spin crossover (SC) systems and cyanido-bridged {Fe 2 Co 2 } electron-transfer complexes. Continuing in this direction, the thesis work of Dmitri Mitcov (2010-2014) was focused in further functionalization of Mn 12 -based SMMs and cyanido-bridged {Fe 2 Co 2 } electron-transfer complexes via anion exchange.

Taking into account the foregoing works, in this thesis we are focused on rational functionalization of molecule-based magnetic materials such as Mn 12 -based SMMs, Fe II -based SC systems and cyanidobridged {Fe 2 M 2 } (M = Co 2+ , Ni 2+ ) molecular squares. In Chapter I of this thesis, the reader can find general information about the three classes of magnetic complexes (single-molecule magnets, spin crossover complexes and electron transfer systems) chosen for functionalization, followed by several examples of already reported hybrid magnetic materials. In Chapter II, rational ligand functionalization of singlemolecule magnets towards liquid crystalline phases is discussed. The third Chapter is focused on two strategies of functionalization of Fe II -based SC systems. Chapter IV describes the cyanido-bridged {Fe 2 M 2 } (M = Co 2+ , Ni 2+ ) molecular squares and the influence of various functionalizations and solvation on the occurrence of the thermally, photo-induced electron transfer in solutions or liquid crystalline behaviour. All chapters present a conclusion with a summarized overview of the obtained results. Finally, general

I.1. Introduction

Molecular magnetism is a relatively young research field, which has nonetheless grown fairly quickly with considerable advance, thanks to the possible interdisciplinary research between chemistry, physics and materials science. Materials in this field are built through the bottom-up approach using well-defined building blocks (metals and ligands, or the coordination complexes). This approach not only allows to predict and control the structural dimensionalities and architectures of the materials, but also sometimes their related magnetic properties. One of the most important advantages of molecular materials is that molecules are all identical, therefore allowing relatively easy experiments on large assemblies of them, while still being able to probe, for example, quantum effects. This means that we can relate macroscopic and microscopic properties through studying the simple quantum mechanics and thermodynamics. These remarkable physical properties are often rare or unknown in traditional inorganic materials. For example, molecular systems displaying spin crossover phenomenon (SCO), 1 photomagnetism, 2 single-molecule magnet (SMM) 3 or single-chain magnet (SCM) 4 behaviors and metal-to-metal electron transfer (ET) 2,5 process have been discovered. Independently of their academic interest, these molecular systems could potentially overcome some of the limitations of inorganic magnetic materials and find a place of choice in various technological areas. In this respect, a great challenge concerns their processing, which need to be easy and efficient without altering their intrinsic properties, and one of the possible options would be their integration in soft matter.

In the first part of this chapter, we will provide general information and basic theoretical concepts on three classes of molecule-based magnetic materials, namely SMM, SCO and ET systems that were investigated along this PhD work. The second part will provide a short bibliographic survey on moleculebased magnetic soft materials, which will help the reader to understand the motivations of our work.

I.2. Molecule-based magnetic materials I.2.1. Single-Molecule Magnets (SMMs)

A single-molecule magnet (SMM) is a molecule that is able to retain magnetization at low temperature, giving rise to a magnetization versus field hysteresis loop. Therefore, a SMM behaves as bulk magnet but the bistability displayed is purely originating from the molecular level. The phenomenon was first observed in the early 90's on the now very well-known dodecamanganese complex [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] (Mn 12 -OAc for short), 6 which synthesis and preliminary magnetic characterization were reported by Lis a decade before. 7 The prerequisites for a molecule to act as an SMM are a high value of the spin ground state (S T ), a high uniaxial magnetic anisotropy with a magnetic easy axis and negligible magnetic interactions between the molecules. 3,6,8 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 9 and quantum phase interference. 10 Furthermore, these molecules could be potentially used in Chapter I -Generalities and context I.9 applications such as information storage, 6,8b quantum computation 11 or as low-temperature refrigerants via the magnetocaloric effect. 11c,12 Since the discovery of SMM behavior in Mn 12 -OAc, a very fruitful research has been performed on the improvement of SMM characteristics and extraordinary advances have been made resulting in the development of large number of SMMs. Thus, coordination chemists were able to construct homo-or heterometallic complexes of various nuclearity exhibiting SMM behavior for 3d, [6][7][8]13 4d, 14 5d, 14a,15 4f, 16 and 5f. 17 Moreover, in recent years, organometallic chemistry has produced many interesting developments that account for notable "records" in terms of magnetic hysteresis and anisotropy barriers. 18 In the following paragraphs of Section I.2.1, 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.2.1.1. Theoretical aspects illustrated by Mn 12 -OAc complex

Single-molecule magnet compounds are composed of the metal ion that are bonded together by various ligands that help to isolate and protect the molecular core from its neighbors and ensure their cohesion. The intramolecular magnetic interactions between metal centers are significantly larger than the intermolecular interactions. Thus, the macroscopic magnetic properties of the material reflect the properties of a single molecule. 6,7 The Mn 12 -OAc molecule, 6,7 which is used along Chapter II as precursor for the synthesis of metallomesogenic materials, is chosen to illustrate the theoretical aspects of the SMM behavior. The structure of Mn 12 -OAc can be described as a [Mn 12 (μ- O) 12 ] roughly planar disk containing a central [Mn IV 4 O 4 ] 8+ cube-like core surrounded by a non-planar ring of eight Mn III ions that are connected to the central core via eight μ 3 -O 2-ions and four bridging carboxylates perpendicular to the plane of the disk (Figure I-1). The SMM property of this molecule is due to the association of a large spin ground state value and strong uniaxial magnetic anisotropy. The spin ground state S T value in Mn 12 -OAc arises from a situation in which the spins of the four central Mn IV centers are all aligned anti-parallel to the spins of the eight outer Mn III centers, to give S T = |(4 × 3/2) + (8 + (-2))| = 10. The strong uniaxial anisotropy can be explained by the nearly parallel alignment of the eight Mn III Jahn-Teller (JT) axes along the molecular z-axis.

Chapter I -Generalities and context

I.10

I.2.1.2. Magnetic properties in the absence of external field

Taking into account the ground spin state, S T = 10, and considering the three directions of magnetization are described as xhard, yintermediate, zeasy axis, the spin quantum number S T is described by 2S T + 1 sublevels, each of them 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, the SMM behavior is defined by the following Hamiltonian (eq. 1):

𝐻 = 𝐷𝑆 𝑇𝑧 2 + 𝐸 (𝑆 𝑇𝑥 2 -𝑆 𝑇𝑦 2 ) (eq.1)
where S T is the total spin ground state: 𝑆 𝑇𝑥 2 , 𝑆 𝑇𝑦 2 , 𝑆 𝑇𝑧 2 spin operators along the three principal directions of magnetization (xhard, yintermediate, 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 magnetic field and in the presence of a strong uniaxial anisotropy (D is negative and |D|>>E) the Hamiltonian can be described as:

𝐻 = 𝐷𝑆 𝑇𝑧 2 (eq.2)
and the spin sublevels change their energy according to:

𝐸 𝑚 𝑠 =

I.2.1.3. Magnetic properties in longitudinal magnetic field

In the presence of an applied magnetic field the previous Hamiltonian (eq. 1) is described as followed:

𝐻 = 𝐷𝑆 𝑇𝑧 2 -𝑔𝜇 𝐵 𝑆 𝑇 ⃗⃗⃗⃗ • 𝐻 ⃗ ⃗ (eq. 4) where the last term is the Zeeman contribution resulting from the interaction of spin 𝑆 𝑇 ⃗⃗⃗⃗ with external magnetic field 𝐻 ⃗ ⃗ . When an external longitudinal magnetic field H z is applied (along to easy axis), the spin sublevels change their energy according to the Zeeman coupling (Figure I-3):

𝐸 = 𝐷𝑚 𝑧 2 -𝑔𝜇 𝐵 𝑚 𝑠 • 𝐻 𝑧 (eq. 5) According to eq.5, the spin sublevels with positive m s value are energetically stabilized, while the sublevels with negative m s value become energetically destabilized (Zeeman effect). Consequently, the population of the m s = +10 sublevel is preferred. Thus, the favored orientation of the magnetic moments in the same direction as the applied field, gives rise to the magnetization M ≠ 0 of the macroscopic sample.

I.2.1.4. Relaxation of the magnetization

Upon removal of the external magnetic field, the initial energy spectrum is restored and spins recovered their thermodynamic energy levels with a certain dynamics and thus a magnetization relaxation is observed. In a classical paramagnetic system, the relaxation into the unmagnetized equilibrium initial state is often too fast to be observed (<< 1 s, even at low temperature). 19 In contrast, SMMs display a slow relaxation of magnetization due to the strong anisotropy barrier shown earlier (Figure I-2). The relaxation process is taking place through the coupling of the spin system with the environment. The molecule can absorb heat from the vibrational modes of the lattice (phonon) in the case where there is sufficient thermal energy (k B T).

Because of the selection rules of spin-phonon interaction, ∆m s = ±1 and ±2 are the only paths possible. In order to relax into the equilibrium state, the system must absorb phonon until it reaches m s = 0, where it can thereafter reach m s = -10 through phonon emission.

There are two methods used to monitor the relaxation process. First method consists to measure the time decay of the magnetization (M(t)) in a zero field. For that, a large dc field is first applied to the sample at a fixed temperature in order to saturate the magnetization in the easy direction. After removal of the magnetic field, the magnetization is then measured as a function of time (Figure I-4). As the decay of I.12 magnetization is an exponential function of time, the relaxation time τ can be determined for every temperature by considering τ = t when the magnetization reaches 1/e. Relaxation of the magnetization for 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. 20 In the simplest cases, the magnetization decreases exponentially with the time at fixed temperature according to the relation:

𝑀(𝑡) = 𝑀 0 • exp (-𝑡/𝜏) (eq. 6) where M 0 or M s (t = 0) is the saturated magnetization, M (t) is the magnetization at given time t and τ is the relaxation time. The relaxation time is temperature dependent and it follows the Arrhenius law:

𝜏 = 𝜏 0 • 𝑒𝑥𝑝 ( ∆ 𝐴
𝑘 𝐵 𝑇 ) (eq. 7) where ∆ 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 crystal packing. For Mn 12 analogues, the relaxation time (𝜏 0 ) varies in the range of 10 -8 -10 -10 s. From the eq.7, we can conclude that higher temperature means faster relaxation time.

The second method used to determine the relaxation time and give information about magnetization dynamics of a SMM is the alternating current (ac) magnetic susceptibility measurements (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 χ' and an out-of-phase or imaginary, component χ" as:

χ ac = χ' + iχ" (eq.8)
The relaxation time (τ) depends on the characteristic experimental time τ exp (τ exp = (2πν exp ) -1 ). Consequently, when τ of the compound is shorter than τ exp , the magnetization vector is oscillating with the ac field, and only a static behavior is observed (only the real component χ'). In the other case, when the τ is longer, the magnetization vector is dephased and non-zero χ" signal can be measured. The χ" versus temperature at a given frequency χ"(T) and/or χ" versus frequency at fixed temperature χ"(ν) can be measured. The temperature or frequency at which the χ" signal reaches it maximum value correspond to the blocking temperature (T B ) or blocking frequency (ν B ) (Figure I-5, left). The relaxation time (τ) can be deduced from the maxima of the χ"(T) and / or χ"(ν) curves at T B and / or ν B . In the case of Mn 12 -OAc, the experimental thermally activated energy barrier is fitted by the Arrhenius law (τ vs 1/T plot, eq.9) as Δ A /k B = 62 K, with τ 0 Chapter I -Generalities and context I.14

I.2.2. Spin Crossover Complexes (SCOs) I.2.2.1. Generalities

The term spin crossover (SCO) design the phenomenon or complexes that exhibit a change of electronic configuration of its/their metal center(s) as function of an external physical or eventually chemical stimuli. 1 Spin-crossover phenomenon involves drastic change in magnetic properties, color, vibrational and structural properties, dielectric constant and electrical resistance. 1,22 For instance, in Fe(II) complexes, the SCO is associated with paramagnetic-to-diamagnetic switching. The ability of these materials to switch their physical properties may have interest for numerous practical applications such as sensors, switches, data storage and display devices. 23 The field was initiated more than eighty years ago by the pioneering work of Cambi and Szegö, who reported anomalous magnetic susceptibilities on a series of Fe III dithiocarbamate [Fe(R 2 NCS 2 ) 3 ] complexes. 24 However, it is only much later that the comprehension of the phenomenon began. Thus, the notion of spin equilibrium has been mentioned for the first time by Griffith in 1956. 25 Stoufer and co-workers. 26 demonstrated shortly later for the first time that the temperature can induce spin conversion phenomenon on a cobalt (II) compound, namely [Co(L) 2 ]I 2 (with L = bis- (2,6-pyridinedialdihydrazone ligand). This was followed by the discovery of the first iron(II) [Fe(phen) 2 (NCS) 2 ] (where phen = 1,10-phenanthroline) spincrossover compound by Madeja and König. 27 At the same period, the concept of switching between two different states, namely the high-spin state (HS) and the low-spin state (LS), has been introduced by A. H.

Ewald and co-workers, 28 who reinvestigated more in detail the magnetic properties of Fe III dithiocarbamate complexes reported earlier by Cambi and Szegö. Since then, SCO phenomenon has been observed in many complexes of Fe II , 1 Fe III , 29 Co II , 30 Co III , 31 Cr II , 32 Mn II , 33 Mn III , 34 and Mo II . 35 Examples of dinuclear, 36 trinuclear, 37 tetranuclear 38 or polymer 39 SCO systems have been also described. In most common situation, Chapter I -Generalities and context I.15 the SCO is triggered by the change of temperature 1 or irradiation with light 40,41 but other stimuli such as pressure, 42 magnetic 43 or electric 44 fields can be used.

I.2.2.2. Ligand field theory

In octahedral symmetry, the five d orbitals of a transition metal ion are split into two sets of orbitals, namely t 2g and e g orbitals. The set of lower energy corresponds to t 2g and is composed by the three 𝑑 𝑦𝑧 , 𝑑 𝑦𝑧 and 𝑑 𝑧𝑥 orbitals, while e g level of higher energy corresponds to the two 𝑑 𝑧 2 and 𝑑 𝑥 2 -𝑦 2 orbitals (Figure I-8).

Therefore, metal ions with d 4 to d 7 configuration may exist in either high-spin (HS) or low-spin (LS) states, depending of the energy separating the t 2g and e g levels. For instance, octahedral Fe II metal ion (3d 6 ), which are the most represented in SCO systems, can be found in either t 2g 6 LS state or in the t 2g 4 e g 2 HS state. The crystal field spitting, ∆, between the t 2g and e g levels, corresponds to the ligand field spitting (Dq) and is related to the ligand field strength as: ∆ = 10D q (eq. 9) where D q ∝ 𝜇 𝑟 𝑛 , n = 5-6, μ is the dipole moment and r is the metal to ligand distance. The energy of crystal field depends on the nature of ligands or more generally on the environment of the metal center. 45 The magnitude of the crystal field spitting along with the pairing energy (Π) of the complex determine whether the LS or HS configuration are stabilized.

 when Π > ∆, the energy required to populate the e g orbitals is inferior to the one required for pairing the spins and the electrons will populate both levels accordingly to the Hund's rule, leading therefore to the high spin (HS) situation. In Fe II metal ion, the spin value is S = 2 and the ground state is a paramagnetic HS state with a fundamental spectroscopic term 5 T 2g .

 when Π < ∆, the energy to overcome the gap is too high and the electrons will fill only the orbitals t 2g according to Pauli's rule, leading to the low spin (LS) situation. For Fe II , all the electrons are paired, giving rise to the diamagnetic (S = 0) ground state with a fundamental spectroscopic term 1 A 1g .

 when Π ~ ∆, the two spin configurations are possible. The stabilization of either LS or HS states is then very sensitive to external perturbations such as temperature and spin crossover may occur.

Chapter I -Generalities and context

I. 16 The pairing energy Π does not change significantly from one complex to another and it is worth about 15000 cm -1 for Fe II complexes. 46 In contrast, the crystal field is strongly dependent of the environment.

Therefore, to predict the spin configuration of a complex and determine whether spin crossover is possible, it is necessary to know the crystal field. Such information can be assessed by optical measurements and explained using Tanabe-Sugano diagram (Figure I-9). 46,47 For example, the [Fe(H 2 O) 6 ] 2+ complex has a ∆ of about 10400 cm -1 , which is much lower than the pairing energy and the complex is HS whatever the temperature. The presence of cyanide ligands, is known to lead to high ligand field strength and for example [Fe(CN) 6 ] 4-complex is characterized by ∆ = 33000 cm -1 and is diamagnetic. The compound [Fe(ptz showing the energy of the excited ligand-field states in units of the Racah parameter of electron-electronic repulsion B relative to the respective ground state, versus the ligand-field strength 10Dq also in units of B. The calculation was performed using the electrostatic matrices in the strong-field. 46,47 Beside the changes of the magnetic and optical properties that can be respectively monitored by magnetic susceptibility measurements and vis-UV or reflectivity spectroscopies, other critical changes of the physical features are occurring during SCO. Especially, the change of the spin configuration is accompanied by a change of the vibrational modes giving the possibility to probe the SCO phenomenon by infrared or Raman spectroscopies. These methods are particularly relevant when complexes contains cyanides or thiocyanate ligands since their characteristic stretching vibrations (~ 2100-2150 cm -1 ) are usually well isolated in the spectra. The SCO also affects the nuclear spin density of the metal center, giving access to [START_REF] Prassides | Mixed Valency Systems Applications in Chemistry[END_REF] Fe Mössbauer spectroscopy, which can differentiate the LS and HS configurations though their differing isomer shifts and quadrupole splitting, as well as NMR characterization methods. Crystallography and more particularly single-crystal X-ray diffraction analysis is also a very useful tool to probe SCO. Indeed, the conversion from LS into HS configuration involves an expansion of the coordination sphere resulting from the elongation of the coordination bond distance of about 0.25 Å. 46,48 This phenomenon can be explained by the population of the anti-bonding e g orbitals in the HS state, which reduce the σ character of the bond and limit overlapping between the t 2g orbitals from metal ion and vacant π* orbitals from the ligand (retro-

Chapter I -Generalities and context

I.17 donating π interactions). 49 For Fe II metal ion, the typical bond length values are 1.8 -2.0 Å for the LS phase and 2.0 -2.2 Å for the HS state. Beside the local changes of the coordination sphere, the SCO process might affect significantly the overall crystal packing leading sometime to a crystallographic phase transition. 50 The single-crystal X-ray diffraction analysis is therefore essential to correlate the properties originating from the molecular level to the macroscopic behavior observed in solid-state.

I.2.2.3. Conversion versus transition: thermodynamic aspects

As described above, SCO complexes show an interconversion between two different states. In this section we describe the mechanism of spin crossover based on simple thermodynamic models for binary systems. [START_REF] Atkins | Physical Chemistry[END_REF] As SCO complexes are essentially isolated paramagnetic species that exhibit Curie paramagnetism, the measurement of the molar magnetic susceptibility multiplied with the temperature as a function of temperature (χT vs T) is the principal technique used to characterize spin crossover phenomenon.

The χT product corresponds to the Curie constants of each state and is related to their molar fraction x, according to the equation: χT = x HS • (χT) HS + x LS • (χT) LS (eq. 10) Considering x HS = x, then eq.10 becomes:

χT = x • (χT) HS + (1-x) • (χT) LS (eq.11)
For the HS and LS states, (χT) HS and (χT) LS products are in most of cases temperature independent (Curie law). Therefore χT = f(T) gives access to x = f(T). In the following, the model for the temperature dependence of x is described for both ideal and regular solution models. Spin crossover may be described as equilibrium between two states:

𝐿𝑆 ⇄ HS

with Gibbs energy change: ΔG = ΔH -TΔS (eq. 12) This energy is positive below a temperature named T* and negative above it. When T = T*, the following relationship is verified: ΔH = T*ΔS (eq. 13) Considering the ideal solution model, it is assumed that the HS and LS states are mixed without any interaction. If N is the total number of molecules, then N HS = x HS •N = x•N is the number of molecules in the HS state and N LS = x LS •N = (1-x)•N is the number of molecule in the LS state. In statistical thermodynamics, the Gibbs energy of the system can be described as: G = N HS G HS + N LS G LS + G mix (eq. 14) where, G HS and G LS are the molar Gibbs energy for HS and LS molecules, and the G mix is the energy of the mixture:

G mix = -TS mix = Nk B T[xlnx + (1-x)ln(1-x)]
(eq. 15) hence:

G = xN(H HS -k B Tlng HS ) + (1 -x)N(H LS -k B Tlng LS ) +Nk B T [xlnx + (1-x)ln(1-x)]
(eq. 16) This relationship allows describing the Gibbs energy per molecule, g N , as following:

Chapter I -Generalities and context I.18

g 𝑁 = 𝐺 𝑁 = 𝑥 [∆𝐻 -𝑘 B 𝑇ln g 𝐻𝑆 g 𝐿𝑆 ] + 𝑘 B 𝑇[𝑥ln𝑥 + (1 -𝑥)ln(1 -𝑥)] + 𝐺 𝐿𝑆 (eq.17)
Introducing the logarithmic degeneracy ratio, 𝑎 = ln ( g 𝐻𝑆 g 𝐿𝑆

):

g 𝑁 = 𝐺 𝑁 = 𝑥[∆𝐻 -𝑘 B 𝑇𝑎] + 𝑘 B 𝑇[𝑥ln𝑥 + (1 -𝑥)ln(1 -𝑥)] + 𝐺 𝐿𝑆 = g 𝑁 ′ (𝑥) + 𝐺 𝐿𝑆 (eq.18)
The condition for equilibrium requires:

( ∂g 𝑁 ′ 𝜕𝑥 )
𝑇,𝑁 = 0 (eq. 19) As G LS is a constant, the relation becomes:

( ∂g 𝑁 ′ 𝜕𝑥 ) 𝑇,𝑁 = ∆𝐻 -𝑘 B 𝑇𝑎 + 𝑘 B 𝑇ln 𝑥 𝑚𝑖𝑛 1-𝑥 𝑚𝑖𝑛 = 0
(eq.20)

The equation can be rearranged in the form of: 21) or: 22) For T = T*, x HS = x LS = x* = 0. The expression eq.22 changes in:

1-𝑥 𝑚𝑖𝑛 𝑥 𝑚𝑖𝑛 = 𝑒𝑥𝑝 ( ∆𝐻 𝑘 B 𝑇 -𝑎) (eq.
𝑥 𝑚𝑖𝑛 = 1 1+𝑒𝑥𝑝( ∆𝐻 𝑘 B 𝑇 -𝑎) (eq.
𝑥 𝑚𝑖𝑛 = 1 1+𝑒𝑥𝑝( ∆H 𝑘 B ( 1 𝑇 - 1 𝑇 * )) = 1 1+𝑒𝑥𝑝(𝑎( 𝑇 * 𝑇 -1))
(eq. 23) or

𝑇 𝑇 * = 𝑎 𝑎+ln 1-𝑥 𝑚𝑖𝑛 𝑥 𝑚𝑖𝑛
(eq. 24) This allows the plotting of the . Whatever is the value of a, the function 𝑥 𝑚𝑖𝑛 increases when T/T* increases. This model predicts that at low temperature (T/T* << 1), the system is in diamagnetic (LS) state, so 𝑥 𝑚𝑖𝑛 = 0. We can observe that all curves pass through a common point, T/T* = 1, and 𝑥 𝑚𝑖𝑛 = 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 HS and 50 % of molecules in state LS.

Note that in the literature, T* is often noted T 1/2 to reflect the composition of the system.

Let's consider now a real example of Fe II octahedral system. The paramagnetic state (HS) corresponds to the 5 T 2g spectroscopic term, giving the spin degeneracy of 5 and the orbital degeneracy (2L HS + 1) of 3.

The diamagnetic state (LS) corresponds to the 1 A 1g spectroscopic term for which spin degeneracy is 1, and the orbital degeneracy is 1. Therefore, g HS = (5×3), g LS = (1×1) or g = 15, which gives a = ln15 = 2.71.

According to the simulation plot provided in Figure I-9, the most similar case is for a = 3, where x min tends to 0.95 at T/T* > 2. But in real systems, the limit of x is very close to 1. The entropy change in Fe II system can be experimentally measured by calorimetric methods or deduced from optical and/or magnetic measurements, thus the value of a could be experimentally estimated between 4.2 and 9.6. [START_REF] Boca | Theoretical Foundations of Molecular Magnetism in Current Methods in Inorganic Chemistry[END_REF][START_REF] Köng | Nature and dynamics of the spin-state interconversion in metal complexes[END_REF] This shows that in real complexes not only the electronic contributions to the degeneracy are present, but also the I.19 vibrational contributions for paramagnetic and diamagnetic states, which are different due to the different geometry in both states. In a solid state the spin crossover is directly influenced by intermolecular interactions (mostly of elastic origin), which play an important role in the macroscopic properties of the systems. Therefore we can add the interaction energy, which is responsible for the cooperative effects, in the regular solution model.

Hence, the Gibbs energy of the system can be written as: 25) where, the interaction energy term 𝐼(𝑥) is defined as: 26) where W is the intermolecular interaction parameter (in units of energy) that is independent of temperature.

G = N HS G HS + N LS G LS + G mix + I (eq.
𝐼(𝑥) = 𝑊𝑥(1 -𝑥) (eq.
Note that 𝐼(𝑥) is a simple exchange term in the mean-filed approximation: 𝑊𝑥(1 -𝑥) = 𝑊𝑥 𝐻𝑆 𝑥 𝐿𝑆 . Thus, the insertion of this term in the molar Gibbs energy gives:

𝐺 = 𝑥𝑁(𝐻 𝐻𝑆 -𝑘 B 𝑇lng 𝐿𝑆 ) + (1 -𝑥)𝑁(𝐻 𝐿𝑆 -𝑘 B 𝑇lng 𝐿𝑆 ) + 𝑁𝑘 B 𝑇[𝑥ln𝑥 + (1 -𝑥)ln(1 -𝑥)] + 𝑊𝑥(1 -𝑥)
(eq. 27) then: 28) and from the condition of equilibrium (eq. 19): 29) or

g 𝑁 ′ (𝑥) = 𝑥[∆𝐻 -𝑘 B 𝑇𝑎] + 𝑘 B 𝑇[𝑥ln𝑥 + (1 -𝑥)ln(1 -𝑥)] + 𝑊𝑥(1 -𝑥) (eq.
∆𝐻 -𝑘 B 𝑇𝑎 + 𝑘 B 𝑇ln 𝑥 𝑚𝑖𝑛 1-𝑥 𝑚𝑖𝑛 + 𝑊(1 -2𝑥 𝑚𝑖𝑛 ) = 0 (eq.
𝑘 B 𝑇 = ∆𝐻+𝑊(1-2𝑥 𝑚𝑖𝑛 ) 𝑎+ln 1-𝑥 𝑚𝑖𝑛 𝑥 𝑚𝑖𝑛
(eq. 30) The dependence Three cases are possible:

 W/k B T* < 2,

I.2.2.4. Light-induced spin crossover

The discovery of a light-induced spin crossover by Gütlich and co-workers in 1984, 40 had an enormous impact in the field of spin switchable materials. In this seminal work, the authors reported that [Fe(ptz) 6 ](BF 4 ) 2 (ptz = 1-propyltetrazole) complex can be converted at 10 K from the stable LS ( 1 A 1g ) state to the metastable HS ( 5 T 2g ) state by irradiating with green light. This phenomenon, known as Light-Induced

Excited Spin-State Trapping (LIESST), can be followed by optical and magnetic measurements. . The first step is a spin-allowed excitation via green light (≈ 530 nm) or red light (≈ 820 nm), followed by two intersystem crossing processes ending at the HS or LS, respectively.

The proposed mechanism for light-induced spin crossover is represented by Jablonski-type energy diagram (Figure I-13). The excitation with green light (≈ 530 nm) at low temperature induces a transition from the 1 A 1 ground state into a 1 T 1 excited state, which relaxes rapidly descending over two successive steps, 1 T 1 → 3 T 1 → 5 T 2 , populating the 5 T 2 metastable state. When the temperature is low enough, the system remains trapped in same state since the energy barrier associated to the transition into the 1 A 1 state is greater than the thermal energy of the system. At cryogenic temperature, the relaxation process can be very slow and a life time of about eleven days was for example found when the [Fe(ptz) 6 ](BF 4 ) 2 complex is maintained at 20 K. 54 When the temperature is increasing, the system can overcome the energy barrier and thus relaxes into the thermodynamic diamagnetic phase. In 1986, Hauser reported a reversed photomagnetic effect, referred as reverse-LIESST, [START_REF] Köng | Nature and dynamics of the spin-state interconversion in metal complexes[END_REF] During the past half-century, the interest in electron transfer (ET) systems has undergone a remarkable expansion. 2,5,56 Indeed, electron transfer phenomenon is one the most fundamental processes in physics, chemistry and biology. [START_REF] Prassides | Mixed Valency Systems Applications in Chemistry[END_REF] The ET is a redox process between two units, one being the reductant (donor) and the second one being the oxidant (acceptor). In 1956, Rudolph A. Marcus developed the first generally accepted theory of outer-sphere (intermolecular) ET based on a transition state theory approach. [START_REF] Marcus | [END_REF] Then, Hush extended the theory to the problem of inner sphere (intramolecular) ET reactions. 59 Since then, several other models have been proposed to complete existing ones. 60 The first reported synthetic molecular system showing metal-to-metal electron transfer is the wellknown mixed valence Creutz-Taube ion of general formula [(H 3 N) 5 Ru II (pyr)Ru III (NH 3 ) 5 ] 5+ (pyr = pyrazine). 61 Such ET processes have been also observed in heterometallic compounds 62 as well as in purely organic systems. Chapter I -Generalities and context

I.23
Cyanido-bridged hetero-bimetallic molecular complexes, exhibiting switching of their magnetic and optical properties via ET process, are another class of molecules proposed during this thesis work to develop hybrid materials. In this regard, the following Section I.2.3.2 is dedicated to a more detailed description of these interesting systems.

I.2.3.2. Examples of molecular Fe III /M II cyanide-based complexes

The discovery of Prussian blue and their analogues has raised much interest in many fields, including magnetism. [START_REF] Verdaguer | Magnetism -Molecules to Materials[END_REF] Prussian blue (PB) and their analogues have the general formula

A x M y [M'(CN) 6 ] i •nH 2 O (A =
alkaline ion, M and M' = transition metals). Certain M/M' couples of these materials can exhibit metal-tometal electron transfer through the cyanide bridges. Moreover, ET process has been found to occur under the influence of temperature, [START_REF] Ohkoshi | [END_REF] pressure, 66 visible light [START_REF] Ohkoshi | [END_REF]67 and X-rays 68 in different PBAs.

As mentioned above, the first example that shows light-induced reversible magnetization changes was left). 56a In the visible region of its electronic absorption spectra, this material exhibits an intense metal-to-metal charge transfer absorption at 550 nm, ascribed to the conversion The Fe/Mn PBAs were also reported to exhibit photo-and/or thermally-induced ET with a large thermal hysteresis of 116 K between 147 and 263 K. 70,71 Rb 0.88 Mn[Fe(CN) 6 ] 0.96 •nH 2 O system exhibits both thermally and photo-induced ET between {Fe III LS (μ-CN)Mn II HS } pair (Fe II , S = 1/2; Mn II , S = 5/2) and {Fe II LS (μ-CN)Mn III HS } pair (Fe II , S = 0; Mn III , S = 2). A structural transformation from cubic to tetragonal is also accompanying the ET due to Jahn-Teller distortion on the Mn III sites. A ferromagnetic order was shown at low temperatures (T C = 11 K) due to interactions between Mn III ions through the diamagnetic [Fe II (CN) 6 ] units. A ferrimagnetic phase can be induced after irradiation at 532 nm as a result of metal-to-metal charge transfer, therefore showing a decrease of magnetization. By irradiating at 410 nm the initial state can be recovered. The ability Fe/Mn PBAs to exhibit ET phenomenon is known to be intimately coupled to the exact system stoichiometry. 72 Beside this systems, other PB analogues like V/Cr, 73 Mn/Cr, 74 Co/Os, 75 as well as many polycyanometalates-based compounds, 76 i.e. Co/W, 77 Cu/Mo, 78 featuring ET properties were reported.

K 0.2 Co 1.4 [Fe(CN) 6 ]•6.9H 2
As shown above in the Co/Fe PBAs, the variation of the amount of [Fe(CN) 6 ] 3-vacancies greatly influences the cobalt ion coordination sphere, which is completed by water molecules, and consequently the structural, magnetic and optical features of materials. Although the control of vacancy amount is quite well mastered through the nature and concentration of alkaline ions (Na + , K + , Rb + , Cs + ), their repartitions in the material is not homogeneous, limiting a systematic study of structure-property relationships. Furthermore, the low solubility of these 3D polymers limits their processability, even though some interesting strategies involving nanoparticles synthesis 79 or dipping techniques 80 have been explored. In this context, the development of molecular systems with lower structural dimensionality such as 2D or 1D polymers 81 or discrete molecules emerged as an appealing strategy to overcome these limitations. 5c,82-92 The first nonpolymeric cyanide-bridged Fe/Co system exhibiting thermally-induced ET, was reported by Dunbar, Achim and co-workers. 83 A non-stoichiometric pentanuclear {[Co(tmphen 3,4,7,8-tetramethyl-1,10-phenanthroline) was obtained by the self-assembly of [Fe(CN) 6 ] Tetranuclear {Fe 2 Co 2 } squares are currently the most widely molecular ET systems investigated. [85][86][87][88][89][90] Among these interesting studies, we should cite Prof. Oshio's work that revealed for the first time that the ET is not only occurring in solid-state but also in solution. 88b They revealed that the ET process of (dtbbpy = 4,4'-ditertbutyl-2,2'-bipyridine) in butyronitrile can be triggered by the change of pH. Mathonière, Clérac, Holmes and co-workers showed that the ET process of {Fe 2 Co 2 } squares in solution is strongly dependent of the nature of the solvent and the T 1/2 increases with the polarity index. 85b,86,87 Therefore, they showed the possibility to finely tune the ET temperature by using mixture of solvents.

) 2 ] 3 [Fe(CN) 6 ] 2 } complex (tmphen =
{[(Tp*)Fe(CN) 3 ] 2 [Co(dtbbpy) 2 ] 2 ]}(PF 6 ) 2
Strong research efforts have been also devoted to develop dinuclear analogues of the Fe/Co PBA materials. During her thesis, Jeon showed that [(bbp)Fe(CN) 3 Co(PY5Me 2 )](OTf) •2.5MeOH complex (bbp = 2,6-bis(benzimidazol-2-yl)pyridine; PY5Me 2 = 2,6-bis (1,1-bis(2-pyridyl)ethyl)pyridine) displays intramolecular ET in solution induced by the change of pH, which was explained by the protonation of the bbp ligand that affects the redox potential of the iron ion site. 91 In solid state, no ET was displayed but only a SCO from Co II HS into Co II LS was observed. The absence of ET in solid state was explained by the large difference between the redox potentials of the two building blocks. By a close examination of previous fails in obtaining ET in dinuclear complexes, it was predicted that the difference between the redox potential of the two building blocks should be close to 1 V to expect ET in the resulting pair. In this respect, it was proposed to replace the [(bbp)Fe(CN) 3 ] -building block of the previous pair by [(Tp)Fe(CN) 3 ] -precursor, which afford a Δ Fe/Co of ca. 0.96 V. The resulting [(Tp)Fe(CN) 3 Co(PY5Me 2 )](OTf) pair was the first example of a dinuclear molecule featuring thermally and photo-induced ET in solid state. 92 Despite this rational and successful strategy, the design and prediction ET process in molecular systems is far to be trivial, due to the strong and unpredictable crystal packing effects that might affect the redox potential of both Fe and Co sites and annihilate the metal-to-metal redox process.

I.3. Soft magnetic hybrid materials

As seen in the previous sections molecule-based materials may exhibit fascinating physical properties including magnetic, optical and other like (super)conductivity originating from the molecular level. Beside their academic interest, which allowed to understand some of the most fundamental concepts of magnetism (including both classical or quantum physics), these materials may be considered for numerous practical applications covering various fields ranging from biomedicine 93 to spintronics. 94 In particular, the almost limitless demand for increasing data storage capacities and at the same time for miniaturization of the devices push progressively the magnetic recording technologies toward the superparamagnetic frontier. With conventional materials, this limit could be reached for a storage capacity of about 400 Gbit/in 2 , 94 while a capacity of about 200 Tbit/in 2 could be theoretically achieved using single-molecule magnets. 94,95 However, there are still huge breakthroughs to achieve, before such materials can be utilized in applications. This will require improvements and/or innovations of both molecular materials and technologies. Among them, one of the great challenges will be to develop efficient ways to process these molecules and control their organization, especially on surfaces. 96,97 In this scope, intensive research has been dedicated to the deposition of these compounds and multiple approaches based on self-assembled monolayers (SAM), 98 Langmuir-Blodgett films, 99 thermal sublimation, 100 electrospray 101 or lithographic 102 methods have been explored. An emerging approach to facilitate their deposition is to implement the molecules within soft phases such as gels or liquid-crystals, or to develop materials with low-melting point and enhanced solubility. 95,103 The following sections are mainly dedicated to liquid-crystal systems, which constitute the main approach developed in the thesis. Before the discussion on few representative examples of SCO and SMM complexes with liquid-crystal properties (Section I.3.2), the reader can find general information about liquidcrystals and metallomesogens. In the last section, we will describe several other soft-matter-based systems.

I.3.1. Generalities on liquid-crystals and metallomesogens

I.3.1.1. Liquid-crystals

Since their first observation in the late 19 th century, 104 liquid-crystals (also known as mesogens) have been subject of important scientific curiosity and extensive industrial applications. 105 A liquid crystalline phase (or mesophase) is an intermediate state between a crystalline solid and a liquid. In contrast to a crystalline phase which is characterized by an order in the three directions and absence of mobility of their constituents (atoms, ions, molecules) and with a liquid phase in which, there is no order but motion of their constituents, a liquid crystalline phase combines both of them. Indeed, the molecules are free to move around as in a liquid but tend to be oriented and organized as in a crystal. As illustrated in Figure I-18, the molecule tend to orients themselves in the same direction and form, for example, layers (smectic phase), while no positional order is present within and between the layers. To behave as a liquid-crystal, the molecules have to be geometrically anisotropic in the sense that their physical properties are not identical in all the directions. Thus, the molecules must be composed by two antagonist parts: one rigid core or head and one or multiple flexible arms. Also, to generate a mesophase a microsegregation between the flexible and rigid parts is required.

There are three types of liquid-crystals: thermotropic, lyotropic and amphitropic. 105 A thermotropic liquid-crystal is a material that exhibits liquid crystalline properties as a function of the temperature. The temperature at which the material transits from a solid into a mesophase is referred as melting point and corresponds to the melting of the flexible parts, while the transition from a mesophase into the isotropic liquid phase is designed as clearing point. In the other hand, lyotropic materials are able to form mesophases by addition of a solvent, and their physical features are therefore strongly dependent of the formulation (nature of solvent, concentration, etc.). Amphitropic liquid-crystals are compounds which display characteristics of both thermotropic and lyotropic materials.

Thermotropic liquid-crystals can be divided into three principal types of mesophases, namely nematic, smectic and columnar phases. The organization in either type of mesophases is dictated by the geometry of the molecules and interactions between them. Therefore, it is not rare to have successively different types of phases by increasing the temperature. The molecules can be classed in two categories according to their shape: rod-like, also called calamitic molecules or disk-like, also known as discotic molecules (Figure I-19).

It should be noticed that molecular systems without such geometry (globular) may also display liquid

I.29

The nematic (abbreviated by N) phase has the simplest structure of all of the mesophases and is also the least ordered. The nematic phase is characterized by molecules that have a tendency to align parallel to each other with an organization following a preferred direction vector 𝑛 ⃗ , parallel to the longitudinal axis (Figure I-20). The nematic phase can be found for both calamitic and discotic liquid-crystals. The properties of a liquid-crystal are generally characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and X-ray scattering in small and wide angle (SAXS and WAXS). The DSC is used to determine the temperature of different phase transitions and establishes the phase diagrams. The different transitions can be identified based upon the POM observations and X-ray scattering investigations. The POM is used to detect the presence of birefringence textures, which can be characteristic of a specific type of mesophase (N, Sm, Col). These characteristic textures are generally observed after the first heating into the isotropic liquid phase. The exact nature of the organization is usually accessible thanks to X-ray scattering.

I.3.1.2. Metallomesogens

Metallomesogens are metal complexes that exhibit liquid crystalline properties. 109 The field emerged in the late 70s. At the origin, the main interest was to introduce paramagnetic metal centers in order to facilitate the orientation of liquid crystal with electrical or magnetic fields. 109,110 Indeed the orientation of traditional (organic) liquid crystals requires strong magnetic or electric fields, which become a major issue for miniaturization purpose. The strategy was therefore to functionalize existing mesogens with coordinating functions in order to bind metal ions. 109,110 The philosophy of metallomesogen field has been rapidly extended; Metallomesogenic systems were developed not only to facilitate liquid-crystal alignment but to facilitate the processing and eventually improve the physical properties of existing complexes, leading to multifunctional materials of interest for various fields of material science, including molecular magnetism. 95,103 Examples of metallomesogens based on molecular magnetic materials are discussed later in section I-3.2. The strategy to design metallomesogens consists to functionalize or eventually the counter ions with functions able to induce liquid crystalline properties on the resulting materials. The design of liquid crystalline materials containing metal ions is not trivial. The stability of the complexes needs to be considered since the formation of liquid crystalline phases requires temperature and/or addition of solvents. The thermal properties and organization depend on the overall structure and shape of the complexes and not only on the mesogenic ligands/counter ions. Thus a functionalized complex may exhibit liquid crystalline properties while the ligands do not, and vice versa. The organization of mesophases is not always easy to predict especially for octahedral metal ions (which is the most common environment for spin crossover compounds) or polynuclear systems and can be even more complicated by the presence of counter ions, which might play a role on the self-organization through intermolecular interactions. However in some cases a prediction can be achieved considering the shape of ligand(s), the preferred coordination sphere symmetry of the metal center or the global architecture of the rigid core of the parent complex. 109 For example, numerous studies have been reported on metalloporphyrines 111 and metallophthalocyanines (Figure I-24). 112 These macrocycle ligands have discotic shape forming columns thanks to π-π stacking and it is well-known that these general features are maintained upon coordination of a metal center. Also, considering the trend of certain metal to adopt a square planar symmetry, the geometry of the complex can be adapted through a rational design of the ligands. 113 For instance, the ligand Pzpy illustrated in

I.3.2. Metallomesogens based on molecular magnetic materials

Researchers from both molecular magnetism and liquid-crystal communities progressively realize that the association of the interesting physical properties of their respective molecular systems in a same material might provide numerous advantages for these two fields of material science. For instance, peoples from liquid-crystal community will find in the development of SCO-based metallomesogens, the opportunity to introduce photo-or thermochromic properties and then, generate optically active liquid crystals or to align liquid crystals thanks to the paramagnetic ions. In the other hand, researchers from molecular magnetism field will find in liquid-crystals a way to shape and process their materials by forming self-organized thin films and concomitantly the opportunity to study the effect of soft matter in the magnetic behavior of their complexes. However, this pluridisciplinar research field is relatively young and the number of examples remains limited.

I.3.2.1. Metallomesogenic materials based on spin crossover complexes

Among molecular materials with interesting magnetic properties, SCO compounds were the first and are still the most widely studied metallomesogenic systems. 103 Indeed, beside the reasons cited just above, they received much consideration because the SCO phenomenon can be tuned to occur around room temperature. Thus, through a judicious design of the ligands, there is a possibility to synchronize in temperature the SCO and crystal-to-mesophase transitions and eventually generate an interplay between the two properties. 103 An eventual goal is to control of the spin switching using the sensitivity of liquid crystalline phases to external electric and/or magnetic fields.

The first complex combining liquid crystal and SCO properties was reported by In this study, a Schiff base ligand functionalized with a dodecyl tail was used to generate a Fe III complex [Fe(L) 2 ](PF 6 ) (with L = N-dodecyloxysalicylidenyl-N'-ethyl- N-ethylenediamine; Figure I-25).

The complex has a calamitic geometry, therefore inducing smectic mesophase (Sm A ) above 388 K. Due to Chapter I -Generalities and context I.33 the presence of paramagnetic ions, the material exhibits improved magnetic anisotropy and the authors demonstrated the possibility to align the liquid-crystal by applying magnetic field. The thermally-induced SCO was found to be gradual and appears at a much lower temperature than crystal-to-mesophase transition in such a way that no synergy between the two phenomena can be expected. The same conclusion was found for the first mononuclear Fe II complex . 116 ] + emphasizing the calamitic shape of the metallomesogen. 115 Gaspar and co-workers, have shown that a synchronization of the SCO and liquid-crystal transitions could be achieved by using complexes known for exhibiting SCO in the range of temperature where the solid-to-mesophase transition is usually observed. 117 Thus, they reported that the thermally-induced SCO in [Fe(C n -tren)](Cl) 2 starts exactly after the onset of the Cr ↔ Sm A phase transition, suggesting that the structural changes associated with the Cr ↔ Sm A transition trigger the spin crossover (Figure I-26). There are also systems where both transitions coexist in the same temperature region but are not coupled, so liquidcrystal phase transition is not the driving force of the spin state conversion. 118 86,[119][120][121][122] Firstly, because these molecular systems are very well-known and have been already investigated for the preparation of hybrid systems 103b including nanoparticles, Langmuir-Blodgett films, and gels, and secondly, because they have an appropriate geometry to induce liquid-crystalline features. Indeed, the scaffold formed by the triazole ligands can be seen as a succession of paddle wheels Chapter I -Generalities and context I.34 (discotic) interconnected to each other by the Fe II metal ions, and the formation of columnar mesophases can be therefore predicted. ) and showed in some cases interplays between the liquid-crystal and SCO behaviors. Unfortunately, the modification of ligands in these systems also induced a gradual and incomplete SCO, while the clearing temperatures are near or above decomposition temperatures. Our team has been also interested in this molecule-based system. Thus, modifications of the triazole ligand with long linear alkyl chains were proposed ([Fe(R-Trz) 3 ](A) 2 with R = C n H 2n+1 (n = 10 to 18) and A = OTs -, Cl). 86,121 In many cases, cooperative and complete SCO were 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(R-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 temperature, did not allow a detailed characterization of these systems. 121

I.3.2.2. Metallomesogenic materials based on single-molecule magnets

Single-molecule magnets (SMMs) displaying mesomorphism are extremely rare in the literature and only two different molecular systems, namely [Mn 12 O 12 (RCOO) 16 (H 2 O) 4 ] (Mn 12 for short) and terbium(III) double decker phtalocyanine (TbPc 2 ) complexes, have been reported so far. 86,87,123,124 Similarly to spin crossover system, the implementation of SMMs within liquid crystalline materials is an alternative strategy to control their organization and potentially a facile route toward surface deposition. Beside this aspect, the formation of metallomesogenic systems may provide additional advantages. As mentioned earlier, one of the prerequisite for a complex to behave as a SMM is the absence of intermolecular magnetic interaction. Thus the decoration of these complexes with a bulky organic shell (alkyl chains) may afford the insulation required to prevent interactions between magnetic cores or surface upon deposition. Additionally, taking into account the fluidity in liquid-crystals and the paramagnetic nature of SMMs, this may give the opportunity to orient at will these molecular magnets along their easy axis by applying a magnetic field. This latter aspect remains only conceptual since no attempt to align SMM-based liquid crystals has been reported to date.

I.35

The first examples of mesomophic SMMs were described by Mesomorphic Mn 12 complexes were obtained by substitution of the sixteen acetate ligands by benzoate moieties bearing three dodecyloxy or one to three cyanobiphenyloxyundecyloxy mesogenic substituents. 123a,c When strongly lipophilic 3,4,5-tris-(dodecyloxy)benzoate ligand was employed, the Mn 12 complex adopts a globular shape and the resulting mesophase organization was found cubic (body centred). In contrast, complexes containing pending calamitic smectogen units (cyanobiphenyl) were found to self-organise into bilayer lamellar-like mesophases with a short-ranged 2D arrangement, whose exact nature (hexagonal or square) could not be determined unambiguously (Figure I-28). The SMM properties were preserved upon the ligand exchange reaction and the authors reported a significant enhancement of their thermal stability in comparison to the original Mn 12 -OAc. Unfortunately, the clearing point of these systems is close or above decomposition temperature, compromising their use for a proper surface deposition. During their PhD theses in our team, Siretanu 86 and Mitcov 87 confirmed the formation of crystalline or mesomorphic cubic phases when benzoate ligands are functionalized by linear alkyl chains, whatever the number or length (nonyl to hexadecyl). They revealed that the replacement of cyanobiphenyl smectogen unit by biphenyl analogue was a successful approach to decrease significantly the clearing temperature of the resulting liquid-crystal material and therefore the isotropic liquid phase can be reached without any degradation of the Mn 12 core. In addition, the existence of a smectic phase with a 2D hexagonal order was clearly evidenced by X-ray scattering measurements.

Besides Mn 12 -based systems, the possibility to induce liquid-crystal phases in a double-decker terbium phtalocyanine complex with chiral (S)-2-(dodecyloxy)propoxy groups was reported (Figure I-29, left). 124 The functionalized complex exhibits hexagonal columnar mesophase at room temperature, while at low temperature, it behaves as a SMM. Moreover, this complex showed the possibility to reversibly control its magnetic properties by simple thermal treatment. 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-29, right). This system is one of the first examples of lanthanide double-I.36 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.3.3. Other soft magnetic systems

As seen in the previous section, one of the favorite methods to promote mesomorphic complexes consists to decorate the ligands with flexible alkyl chains. Independently, this approach may afford additional interesting features for processing purposes such as the increase of solubility, the lowering of melting temperature, etc., and also in term of magnetic properties or for the study of the magnetism itself. 95,103 With few examples, we describe in the following sections some of the benefits provided by the functionalization with alkyl chains.

I.3.3.1. Magnetic gels

The grafting of alkyl chains onto the ligand might give to the resulting complex the ability to gelate solvents. This characteristic is particularly interesting with SCO systems to produce eventual thermochromic gels or to help their processing thought the formation of xerogel film (dried gel). 103 The first physical SCObased physical gels were developed by our team on a series of polymeric compound [Fe(R-Trz) 3 ](OTs) 2 (with R = C n H 2n+1 , n = 13, 16, 18; see (b) in Figure I-27). 121a,125 These complexes, which exhibit thermotropic liquid-crystal properties, are able to efficiently gelate toluene, various alkanes and mixtures with chloroform even at low concentration (< 1 wt%). The magnetic and optical properties of the complexes were found likely unchanged in the gel phase whatever the composition. By studying their phase diagrams, it has been possible to tune the formulation of the gel and to synchronize the SCO temperature (T 1/2 ) with gelation/melting transition (Figure I-30).

Kuroiwa et co-workers reported a gel-like phase of Co II /R-Trz (R = C 12 H 25 OC 3 H 6 ) coordination polymer in chloroform. 126 The blue gel-like phase turns into a pink solution by cooling at 0 °C. This molecular system is not properly speaking a SCO. Indeed, a reversible dissociation/assembly of a polymeric aggregate of Co II in tetrahedral symmetry (T d ) into mononuclear complexes with symmetry octahedral (O h ) is observed at 25 °C. At lower temperature 0 °C, these complexes self-assemble into oligomeric or polymeric species.

I.3.3.2. Soluble systems

Ligand functionalization with lipophilic chains can help to improve the solubility of the resulting complexes in various organic solvents, giving access to low or non-polar solvents like alkanes. Systems with enhanced solubility are quite important for nanostructuration processes based on wet-methods.

For example, Langmuir-Blodgett (LB) is a very commonly used technique to shape molecular objects in well-organized multilayer films. 99 This method does not need necessarily the functionalization of the complexes and was for example used as the first approach to deposit Mn 12 -OAc on surface. 99b However, for non-modified complexes, addition of surfactants is required, while this role can be played in a more controllable way by the functionalization of the ligand(s) with lipophilic chains. The first example of SCO in LB film was reported by Kahn and co-workers on the [Fe II (OP 3 ) 2 (NCS) 2 ] complex where OP 3 is a 1,10phenanthroline onto which three long aliphatic chains were grafted. 99a The authors confirmed by infrared spectroscopy that the occurrence SCO in the 2D system in the same fashion as the bulk sample. Delhaès and co-workers reported the first example of SCO LB film characterized by means of magnetometry, namely [Fe II (R-bpy) 2 (NCS) 2 ] (with R,R-bpy = 4,4-bipyridine derivative; R = (CH 2 ) 12 (CF 2 ) 5 CF 3 )). 127 By comparing the results with the one obtained for the bulk sample, they revealed a strong impact of the lamellar organization in LB film on the SCO phenomenon. Nonetheless, after several annealing at high temperature, the magnetic behavior was found barely identical to the one in bulk.

Another use of alkyl-chain-functionalized ligands is the formation of emulsions. Wu and co-workers reported the dispersion of stearic acid modified Mn 12 complex (Mn 12 -C 18 ) in water through an emulsionassisted self-assembly (Figure I-31). 128 For that, they prepared an oil-in-water emulsion by ultrasonic treatment of a solution complex in hexane and a water solution containing an alkyl/poly(ethylene oxide) surfactant (C 18 EO 10 ). The complexes were isolated from water by C 18 EO 10 surfactant, forming stable waterdispersed Mn 12 -C 18 /C 18 EO 10 aggregates. The molecule-based assemblies were found compatible with physiological media and were proposed as potential MRI contrast agent. The same group showed the possibility to form vesicular assemblies of Mn 12 -C 14 (C 14 = pentadecanoate) complex in organic media. 129 For more academic purposes, the enhancement of the solubility of magnetic complexes gives also the opportunity to study their magnetic/optical properties of complexes in solution.

The ligand functionalization by a lipophilic chain of a known Fe III SCO complex, [Fe III (sal 2 -OR 2 trien)] (sal = salicylaldehyde, trien = triethylenetetramine, R = C 6 H 13 , C 8 H 17 , C 18 H 37 ), led to a self-assembly of the complex in solution and the spin crossover behavior was found to be preserved (Figure I-32). 130 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 functionalized one presents an optimization in crossover temperature.

Extensive efforts have been devoted to explore the influence of anions and/or salts on the SCO properties, especially in the solid state. 131 The presence or absence of solvate molecules and packing effects often restrain the clear observation of host-anion interactions and the effects on SCO properties. bpy ligand was functionalized with extended alkyl chains (hexyl or tridecyl), which allowed investigations in a larger range of solvents including non-polar media like hexane or toluene. 86,87 These two new compounds did not revealed any ET ability in solid-state, while ET was almost systematically found in solution, highlighting the role of crystal packing in the annihilation of the ET phenomenon. The comparison with previous complexes in the same media allowed to reveal the influence of the substituents (CH 3 vs. C 6 H 13 vs.

C 13 H 27 ) on the ET temperature and an increase of the T 1/2 was observed when the electro-donating character increase.

I.3.3.3. Unconventional magnetic behaviors

As illustrated for metallomesogenic systems the implementation of molecule-based magnetic systems in soft matter may promote unusual situations. This is particularly prevalent for spin switchable compounds, whose behavior is strongly dependent of solid-state effects. Especially, it was shown that the occurrence of a phase transition from a solid phase into a mesophase, and the resulting change of the molecule arrangements might affect the metal ion coordination sphere and provoke spin switching. 103 Independent of metallomesogens, such phase transitions are quite common in systems containing flexible alkyl chains. This includes phase transitions resulting from change of the alkyl chain conformations or phase transition (melting) due to reducing cohesion this material. When the material is cooled from 300 or 400 K, a decrease of the χT products is observed down to ca. 190 K (n = 14) or 220 K (n = 16) as expected for a gradual SCO (Figure I-32, center and right). However, on further cooling, an increase of χT products is observed showing a non-natural transition into the HS phase. In heating the reverse process is observed at higher temperature, revealing wide thermal hysteresis. In the case of [Co(C 14 -terpy) 2 ] 2+ complex, a second SCO is observed at low temperature from the HS1 phase into the LS phase without showing any hysteretic behavior. The reverse-SCO phenomena were attributed to crystal-tocrystal phase transitions involving strong changes of configuration of the alkyl chains (Figure I-33, left). Chapter I -Generalities and context I. 40 Real and co-workers recently reported a series of neutral Fe II complexes based on pyridylbenzohydrazone ligands functionalized with long alkyl chains (Figure I-34). 136 Interestingly, all the materials melt at low temperature (below 400 K) and this transition is accompanied by a change of the spin state, suggesting therefore for the first time an interplay between SCO and a crystal-to-liquid transition. In addition, due to the natural hysteresis between the melting/crystallization transitions, the SCO itself was found hysteretic. The authors explained that the SCO temperature can be modulated by the melting point which is directly related to the alkyl chain length. This study that appeared during this thesis, utilizes the same SCO compounds that we had initially targeted for investigation in metallomesogen systems and we will show some additional interesting effects of the functionalization with alkyl chains in Chapter III.

I.4. Conclusions and motivation of the thesis

Throughout this chapter, we have shown that molecular materials, including single-molecule magnets (SMMs), spin crossover (SCO) compounds and electron transfer (ET) systems present fascinating physical properties with interest for both academic research and potential applications. Even though the design of novel molecular magnetic materials and the study of their physical features represent a major activity in the field of molecular magnetism, it is also essential to devote a part of the efforts to shape and process the existing molecular materials. As shown in the second part of the chapter, soft materials present high versatility in physico-chemical properties. The implementation of these molecules in soft matter might afford interesting advantages for their processing, which include (i) self-organization abilities in liquid crystalline or gel phases, (ii) the increase of the solubility in conventional solvents facilitating their deposition on surface using wet-methods and (iii) the lowering of the melting temperature giving the opportunity to form thin films using spin-coating technique. In addition, this approach may afford some interesting features regarding their magnetic/optical properties. For example, the functionalization of SMMs with strongly lipophilic ligand may help to isolate and protect the magnetic cores and therefore prevent intermolecular Chapter I -Generalities and context I.41 magnetic interactions between the molecules, while the alignment of their easy magnetic axes with a magnetic field may be potentially assessed in a fluid phase. It was also shown that interplays between structural phase transitions inherent to the soft matter and the SCO process can be eventually promoted, while the increase of the solubility gives access to fundamental studies of the spin switching phenomenon excluding crystal packing effects and conversely check the influence of the solvatosphere.

All these reasons have motivated our team along the years to design molecule-based magnetic soft materials via a rational ligand functionalization. The employed strategy consists in selection of known complexes from different magnetic classes and modification of their organic ligands with various functional groups known to induce liquid crystalline properties or, simply, improve the solubility. This PhD work aimed to contribute to these efforts by continuing the work achieved during the theses of Siretanu and

II.1. Introduction

The discovery of the first single-molecule magnet (SMM), 1 in the early 90's has opened an important research field at the interface of classical and quantum worlds. Single-molecule magnets are complexes which are able to retain magnetization at low temperature, acting therefore as a "nano" magnet. 2 These molecular objects appear as promising units for potential applications in molecular memories, quantum computing and spintronic devices. 3,4 One of the important steps towards possible applications is to develop efficient methods to organize and process these nanomagnets. The organization of these systems has been reported on surfaces, in mesoporous materials or in coordination networks. 5 Another promising strategy towards their controlled organization is the development of the new hybrid materials with liquid crystal (LC) properties. 6 Indeed, metallomesogens (metal complexes which exhibit liquid crystalline properties) are very convenient materials in term of processability: they self-organize and are able to form suitable thin films for surface deposition.

Moreover, taking into account the fluidity of liquid crystals, they can be eventually oriented with magnetic fields and this could be useful in order to position the easy axes of SMMs in the same direction (Figure II-1).

Additionally, the organic shell provided by the mesogenic ligands may play an important role to isolate and protect the magnetic core and thus to prevent from interactions between objects (Figure II-1). A part of this PhD work aimed to develop and study new metallomesogens based on SMMs and more precisely based on the well-known dodecanuclear manganese Mn 12 -OAc. This chapter starts by short description of general aspects on Mn 12 -based complexes and previous studies of these complexes in metallomesogens. Then the general functionalization strategy used in this work to promote liquid crystalline properties will be presented. The synthesis, analytical characterizations, magnetic and thermotropic properties of the complexes prepared during this work will be then discussed and compared with previous examples described in the literature.

II.2. Selected SMM: Mn 12 -based complexes II.2.1. Why Mn 12 ?

The dodecamanganese complex with general formula [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] (Mn 12 -OAc) was the first SMM discovered. 1 It was synthesized in 1980 by Lis et al,1a while the evidences of slow relaxation of magnetization at low temperature were reported in 1993 by Sessoli et al. 1b,d,f,g Despite the discovery of thousands new SMMs, Mn 12 -OAc remains one of the most popular and widely studied system. Indeed, it displays a large energy barrier against thermal relaxation (ca. 60 K) 2 allowing to retain magnetization for months in absence of a magnetic field. Moreover, its synthesis is straightforward and inexpensive, and it is possible to replace easily the acetate ligands by other bidentate ligands to generate new Mn 12 derivatives. Such chemical modifications allow for a relative control of the solubility, redox potentials and magnetic properties of the Mn 12 core. 7

II.2.2. Structure of Mn 12 -based complexes

The Mn 12 -OAc molecule and its derivatives are mixte-valence polyoxymetallate complexes of Mn III and Mn IV ions. The general structure is composed by a cubane core Mn IV The crown consists in an alternation of two types of Mn III ions which are referred as type I or type II according to their connectivity with Mn IV ions: Mn III ions of type I are bridged to only one Mn IV via two μ-O 2-ions while Mn III ions of type II are connected to two different Mn IV ion via μ 3 -O right). Also, the structures contain usually four water ligands, which are always coordinated on type II Mn III

Axial carboxylates

Equatorial carboxylates

sites. In the original Mn 12 -OAc, these water ligands are equivalently distributed over the four Mn III sites of type II and the complex is then referred as 1:1:1:1 isomer (Figure III-3(a)). The exact number and disposition of the water molecules can vary from one Mn 12 complex to another. For example, the four water molecules in [Mn 12 O 12 (CF 3 COO) 16 Mn 12 -based complexes have a high spin ground state S T = 10, which can be understood by analysis of magnetic core's structure. Indeed, the Mn IV ions from cubane core (S = 3/2) are ferromagnetically coupled to each other and are antiferromagnetically coupled with the outer Mn III ions (S = 2) giving an S T = 10.

The magnetic anisotropy of the ground state in Mn 12 -OAc is mainly due to the anisotropy of the eight outer Mn III ions. The coordination sphere around each Mn III ion is not perfect octahedron, but is axially distorted, since two Mn III -O bonds are longer than the other ones. This distortion can be explained by the Jahn-Teller (JT) effect. The JT theory states that "any non-linear molecule with a spatially degenerate electronic ground state will undergo a geometrical distortion that removes that degeneracy, because the distortion lowers the overall energy of the species".

II.55

In the case of Mn III ion (d 4 ) in high spin configuration (S = 2), the fourth electron occupy either one of the e g oribitals causing a JT distortion associated with a tetragonal elongation of trans Mn III -O bonds (Figure II-4). 11 These distortions do not involve the Mn III -μ 3 -O 2-bonds, as they are the strongest bonds in the molecule. The JT elongation axes are nearly perpendicular to the plane of the [Mn 12 O 12 ] disk-like core (Figure II-5 (a)). Their nearly-parallel alignment along the molecular z-axis is the origin of the uniaxial magnetic anisotropy that greatly influences the magnetic properties of Mn 12 -based systems.

In some Mn 12 -based structures, one of the JT elongation axes can be oriented equatorially rather than axially in regard to the Mn 12 core ( O) 3 ] with one abnormally oriented JT axis. 12 Color scheme: normally oriented JT axes are shown in blue and the abnormally oriented JT axis is shown in light green; Mn III green, Mn IV green, O red.

It is common to found samples in which both forms coexist. The ability of Mn 12 -based molecules to exist in two different forms is known as Jahn-Teller isomerism. 10 The existence of two JT isomers is the origin of two distinct slow relaxation modes of the magnetization. Indeed, the abnormal orientation on one JT axis affect the uniaxial magnetic anisotropy and, thus, the slow relaxation of magnetization (around 10 -3 -1 s) occurs at lower temperature (ca, 2 -3 K) in respect with the isomer with normal JT orientation (4 -7 K).

II.2.3. Chemistry of Mn 12 -based complexes

One of the advantages of the Mn 12 -based molecules is their ability to undergo carboxylate substitution via ligand exchange reactions. 7,13 This characteristic provides a convenient and efficient approach to get new Mn 12 -based derivatives with improved properties, in term of solubility, stability in solution, thermal stability or magnetism. The ligand substitution is driven by the greater acidity of added carboxylic acids and/or the ability to remove the acetate ligands. It was emphasized that the ligand substitution is actually a reversible process (eq. 1), and in order to shift the reaction equilibrium towards the formation of desired products, the synthesis requires the use of an excess of the substituting ligands as well as the removal of the produced acetic acid by an azeotropic distillation with toluene.

[Mn 12 O 12 (CH 3 COO) This method is nonetheless not very efficient to prepare mixed-carboxylate complexes and yields to mixtures of complexes. In contrast, this method was found to be the only possible way to produce the mixed- O) 4 ] in which the benzene sulfonate ligands occupy exclusively the axial sites due to its very low basicity. 14a The second approach is more successful to obtain the desired products. The synthesis involves an equimolar reaction of two homoleptic Mn 12 complexes containing carboxylate ligands with significantly different pKa (eq. 3).

ligands complex [Mn 12 O 12 (OAc) 8 eq (PhSO 3 ) 8 ax (H 2
[ All these synthetic methods could be useful in order to design hybrid molecular materials, eventually with functional groups able to induce liquid crystalline properties.

II.2.4. Mn 12 -based complexes with liquid crystal properties

There are only few examples of coordination compounds combining SMM and mesomorphic properties. 15,16 The first SMM-containing metallomesogens were reported in 2008 on Mn 12 -based systems. 15a

In this study, Terazzi and co-workers obtained two hybrids complexes by replacing the sixteen acetate ligands of the Mn 12 -OAc by gallate ligands bearing three dodecyl or (cyanobiphenyloxy)undecyl chains. The SMM behavior was preserved in both cases, and mesomorphic properties were found to be significantly different according to the ligand functionalization strategy employed. The complex containing strongly lipophilic ligands, ie. tris(dodecyloxy)benzoate, self-organizes into a body-centered cubic mesophase. 15a This unusual liquid crystal organization can be explained by the globular shape (spherulitic) of the complex, induced by the large number ( 48) of long alkyl chains, which favors such cubic organizations. 16,17 On the other hand, the complex containing cyanobiphenyl units forms bilayer lamellar-like mesophases in which the magnetic cores display an intralayer short-ranged 2D organization. More recently they extended this strategy by varying the number and position of (cyanobiphenyloxy)undecyl substituents attached on the benzoate moieties and confirmed the ability of these complexes to generate smectic-like organizations. In these materials, the lamellar organization mainly results from the intermolecular π-π interactions between the cyanobiphenyl units, which are well-known calamitic smectogens, while the number of attached mesogenic groups or ligand symmetry have only a minor influence on the mesophase organization (variations only on periodicity and the compactness). The 2D organization displayed by the magnetic cores was found to be in all cases short-ranged, while its exact nature (rectangular or hexagonal)

could not be unambiguously determined experimentally. One of the main drawbacks of these systems is their high clearing point temperatures, which are near or above the decomposition temperature, limiting their potential use for surface deposition. Indeed, the formation of thin film would require first the melting into the isotropic liquid phase and then the homogenization of the surface deposition, eventually by spin-coating methods.

Scheme II-1. Chemical structure of benzoate derivatives used by Terazzi et al. 15a,c Our team in CRPP has been interested on the design of Mn 12 -based metallomesogens since 2008. 16,18 Similarly to Terazzi and co-workers' studies, our first strategy was to develop Mn 12 complexes containing strongly lipophilic carboxylate ligands. Thus, a series of complexes functionalized with 16 benzoate ligands bearing one, two or three alkyl chains (L [1][2][3][4] , Scheme II-2) was prepared by Diana Siretanu and Dmitri Mitcov during their PhD thesis. 16,18 The study aimed to determine how the number and length size of lipophilic substituents influence the mesomorphic properties of final complexes. More precisely, we sought to determine whether a reduction of the number of lipophilic groups (one or two chains) would afford a less II.58 globular bulkiness than the previously reported complexes and ultimately would provide mesophases with lower symmetry (nematic or smectic). This study emphasized that, despite the reduction of the number of alkyl chains, the overall geometry of the complexes remains pseudo-spherical leading therefore to cubic or pseudo-cubic organizations. Additionally, when benzoate ligands containing only one alkyl chain were used, the ratio between the rigid and flexible counterparts does not favor the formation of mesophases and only crystalline or amorphous phases were detected. Nonetheless, it have been shown that the variation of the lipophilic density around the Mn 12 core has a significant influence on the thermal (strong difference on the melting and clearing temperatures) and cubic self-organization (different symmetries) of resulting complexes.

As previously reported, 15 our team also investigated a second strategy consisting in the introduction of pending calamitic smectogens on the Mn 12 molecule. In addition to the cyanobiphenyl units proposed by

Terazzi and co-workers, a complex containing biphenyl groups was also prepared (L 7 , Scheme II.2). 16 This latter was found particularly interesting since it allows for a significant decrease of the clearing temperature (ca. 350 K) that is about 70 K below the decomposition temperature, making of this compound a more suitable system for surface deposition purpose. Additionally, small modifications of the ligand were found very efficient to control the liquid crystal organization. Indeed, small-angles X-ray scattering investigations revealed for the first time the formation bilayer lamellar mesophases in which the intralayer Mn 12 cores form a long-range 2D order with hexagonal symmetry.

Scheme II-2. Chemical structure of benzoate derivatives used in our team for the functionalization of the Mn 12 complex. 16,18

II.2.5. Functionalization strategy used in the thesis

In continuation with these promising investigations, we attempted a new functionalization strategy to induce liquid crystalline features on the Mn 12 complex. In contrast with previous studies in which the sixteen acetate groups were replaced by mesogenic ligands, we alternatively introduced these ligands only on certain positions of the Mn 12 core using the regioselective functionalization methodology (see section II.2.3). Here, the goal is to induce more structural anisotropy on the Mn 12 core in order to facilitate the self-organization and eventually promote unprecedented mesomorphs. Two approaches can be envisaged: the For this work, we have been mainly focused on the first approach. In order to achieve the desired siteselective functionalization, non-mesogenic ligands with low pK a are required to occupy the axial positions of Mn 12 molecule (see section II.2.3). Several ligands, including benzene sulfonate, dichloroacetate, trichloroor trifluoroacetate have been tested in our preliminary studies, and the latter one was found to be the most convenient and efficient for the preparation of hybrids. For the equatorial positions, we selected ligands based on benzoate platforms since the pK a of such ligands (pK a ~ 4.2 -4. 4) is significantly higher than the one of the trifluoroacetate (pK a = 0.3) (Scheme II-3). Additionally, the functionalization of this platform by one or multiple mesogenic substituents is easily accessible from a synthetic point of view. 15,16 In order to highlight the effect of our functionalization strategy on magnetic and self-organization properties, we selected the ligands used in the thesis of D. Mitcov. 16 Indeed, this will allow for a direct comparison with the homo-substituted analogues [Mn 12 O 12 (L i ) 16 (H 2 O) 4 ] previously reported. Also, for a better understanding of the site-selective substitution and in order to validate our synthetic strategy, we also prepared a complex containing non-mesogenic ligands [Mn 12 O 12 (CF 3 COO) O) 4 ] have been previously reported in D. Mitcov's thesis 16 or elsewhere 8,9,12,13 and their synthesis and characterization are concisely recalled in sections II.3.1 and II.3.2.

III.3.1. Synthesis of carboxylate ligands

The compounds HL 3 and HL 4 are not commercially available and were prepared, in two and three steps synthesis, respectively. 15c, 16 In the case HL 3 , the first step is the synthesis of the precursor methyl tris(dodecyloxy)benzoate 1 by a Williamson reaction (Scheme II-4).

Scheme II-4. Synthetic pathway for HL i (i=3-4)

II.61

The etherification coupling between the 1-bromododecane and the methyl 3,4,5-trihydroxybenzoate requires potassium carbonate K 2 CO 3 to deprotonate the phenolic positions and a catalytic amount of potassium iodide was used to promote the reaction by halide exchanges. A saponification of the compound is thereafter performed using KOH to produce HL 3 . For HL 4 , the first step involve the preparation of 4-((10bromodecyl)oxy)-1,1'-biphenyl 2 by a Williamson reaction between 1,10-dibromodecane and 4hydroxybiphenyl (Scheme II-4). A large excess of 1,10-dibromodecane is necessary to insure a monocoupling of the aliphatic spacer, and it is removed at the end by column chromatography. The ligand HL 4 is then obtained following the same reactional sequence used for HL 3 by ether coupling reaction between 2 and methyl 3,5-dihydroxybenzoate followed by a saponification of the precursor 3 (Scheme II-4).

II.3.2. Synthesis and characterization of homo-substituted Mn 12 complexes

As mentioned in Section II.2. 4). This latter was obtained accordingly to the procedure described by Lis et al. 1a by reaction of KMnO 4 with Mn(CH 3 COO) 2 in acetic acid solution. The complex was isolated as black crystals, for which the unit cell was consistent with the

reported structure of [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]•2CH 3 COOH•4H 2 O. 1
The replacement of the acetate ligands was carried out using an excess (~28 equiv) of HL i in order to shift the reaction equilibrium toward the formation of desired complexes. Also, to prevent from reaction reversal, the out coming acetic acid was removed by several successive distillations as an azeotropic mixture with toluene.

[Mn 12 O 12 (CH 3 COO) O) 4 ], which are not suitable for crystallization due to the presence of flexible and "greasy" moieties, were purified by size exclusion column chromatography, using Bio-Beads S-X3 beads (neutral, porous styrene divinylbenzene copolymer beads) as stationary phase. The removal of ligand excess or partially substituted Mn 12 complexes from the fully substituted one was monitored by FTIR (vide infra).

The separation is nonetheless not very easy to control and at least two successive purifications by column chromatography are required to isolate the desired products.

The formation of the expected complexes is established by FTIR and 1 H NMR measurements, which were found essentially identical to those previously reported. 15,16,18 The IR spectra display strong absorptions in the 1577 -1604 cm -1 and 1370 -1412 cm -1 ranges (Figures II-7 does not appear anymore, confirming the replacement of all acetate ligands by the desired ligands.

Furthermore, the C=O stretches at ~ 1700 cm -1 and ~ 1680 cm -1 ascribed to molecules of acetic acid and benzoic acid derivatives, respectively, were not observed, in agreement with a complete removal of acetic acid by azeotropic distillation and ligand excess by recrystallization or column chromatography. The removal of the acetate ligands is also supported by the disappearance, in 1 H NMR, of the three singlet resonances found at δ 13.7, δ 40.9 and δ 47.8, in the parent complex Mn 12 -OAc and attributed to acetate However, a proper assignment of these signals remains difficult. 13b

II.3.3. Synthesis and characterization of mixed-carboxylate Mn 12 complexes

The mixed-carboxylate compounds [Mn 12 O 12 (CF 3 COO) 8 (L i ) 8 (H 2 O) 4 ] were prepared by reacting an equimolar amount of the two types of homo-substituted Mn 12 complexes in CH 2 Cl 2 (eq. 5). After a filtration to remove minor decomposition products, the complexes were isolated as brownish powder or glassy solids.

[Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ] + [Mn 12 O 12 (L i ) 16 (H 2 O) 4 ] 2[Mn 12 O 12 (CF 3 COO) 8 (L i ) 8 (H 2 O) 4 ] (eq.5)

The formation of mixed-carboxylate complexes by NMR and IR is not trivial to confirm and proving the expected regioselectivity with these techniques is likely impossible. In order to facilitate interpretations and validate the synthetic strategy, an important effort was first made to isolate the model compound Single crystals suitable for X-ray diffraction experiments were obtained by slow evaporation of a solution of the complex in THF:DCM (1:1). The X-ray diffraction analysis reveals that the polyoxometallic Mn 12 core is preserved after reaction and its structure is similar to previously reported Mn 12 -based complexes. [8][9][10][11][12] Views of the complex are shown in O 12 (L 1 ) 16 (H 2 O) 3 ]•0.5Et 2 O•H 2 O are arranged in 1:1:1:1 and2:0:1 fashion, 8,16 respectively, meaning that a reorganization of water ligands is also occurring during the reaction or crystallization. O) 3 ] displays one abnormally oriented JT axis, almost perpendicular to the other ones. 16 Thus, considering the strong impact of these JT axes on the magnetic anisotropy, we can predict that the magnetic behavior of [Mn 12 O 12 (CF 3 COO) The methoxybenzoate ligands bridging the Mn3 and Mn4 centers were found strongly distorted. In order to improve the quality of the structure, many different crystallization conditions were tested.

Recrystallization using diethyl ether vapor diffusion into a dichloromethane solution of complex at 4 o C gave after few days black crystal of [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ], which co-crystallizes with one molecule of diethyl ether. Although the quality of the crystal remained poor due to rapid loss of solvent, the data collection confirmed the expected composition of the complex and minor changes on the structure were found in respect to the previous one. In this case, the compound crystallizes in a tetragonal I4 1 /a space group in which the unit cell contains four Mn 12 molecules. Views of the complex are shown in Figure II-10 and Xray data parameters are summarized in Table S.II-1. By symmetry, the Mn 12 complex is composed of a crystallographic unique Mn IV (Mn3) center and only two Mn III metal ions (one Mn III of type I, Mn1, and one

Mn2 O6 Mn4 Mn3 Mn2 Mn11 Mn1 Mn3 Mn4 II.65
Mn III of type II, Mn2). Consequently, the water ligands are equivalently distributed over the four Mn III sites of type II, giving rise to the 1:1:1:1 isomer instead of the 2:0:2 isomer described earlier. This illustrates, as previously described in the literature, 8 the versatility of these molecular systems to undergo isomerization in solution and highlights the importance of the reaction/crystallization conditions. Nonetheless, the magnetic behavior of these two compounds is expected to be very similar since no major, changes were found between the structures.

II.3.3.2. FTIR spectroscopy

Similarly to the homo-substituted complexes, the FTIR measurement performed on a single crystal of

II.3.3.4. Elemental and thermogravimetric analyses

The composition of the complexes was confirmed by the complementary elemental and thermogravimetric analysis (TGA). The results for the elemental analysis are gathered in the Table II-

II.4. Magnetic measurements

In general, the replacement of the peripheral acetates of the Mn 12 -OAc complex by other carboxylate ligands, and eventually by non-carboxylate ligands, allows for the preservation of SMM behavior. However, the distortions and deformations caused to Mn 12 core by these chemical modifications may change some features including the spin ground state, blocking temperature or energy barrier. The magnetic properties of the four new complexes [Mn 12 O 12 (CF 3 COO) 8 (L i ) 8 (H 2 O) 4 ] (i = 1 -4), were fully characterized by magnetic measurements in both dc and ac modes and the results were compared with their precursors.

II.4.1 Magnetic measurements in ac mode

The presence of frequency dependent out-of-phase ac signal (χ′′) is considered to be the characteristic feature of superparamagnetic-like or SMM behavior. In these molecular systems, a non-zero frequency dependent χ′′ signal can be measured when the rate at which the magnetic moment of a molecule flips and the operating frequency of the ac magnetic field are very close. The system is not able to follow the ac field due to the presence of slow relaxation of the magnetization.

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 frequencies ranging from 1 to 1500 Hz. For all the complexes, an abrupt decrease of the χ'T product is systematically observed on lowering temperature below 8 K, which is indicative of a blocking process of magnetization (Figure II-12 and II-13). Accordingly, χ′′ signals appear at the corresponding temperatures, confirming that all the samples display a SMM behavior.

For [Mn 12 O 12 (CF 3 COO) O) 3 ]. 16 In this case, the occurrence of SMM behavior at lower temperature can be explained by the lower symmetry and magnetic anisotropy which involve a significant rhombic term within the structure. On the other hand, the SR mode is related to the second JT isomer involving nearly uniaxial JT elongations along the main molecular axis as shown in the structures of [Mn 12 O 12 (CF 3 COO) O) 3 ] as predicted by the analysis of their crystallographic structures. O) 4 ]. It is difficult to rationalize the origin of this behavior as no crystallographic structure can be reasonably obtained. However, Terazzi and co-workers suggested that the ratio FR/SR in mesomorphic Mn 12 systems is directly related to the compactness of the supramolecular organization: The fast relaxation phase increase when the Mn 12 cores are closer. 15 The relaxation times (τ) at different temperatures can be determined from the position of peak maxima in χ" vs T plots (Figure II-12-13) as τ = (2πν max ) -1 (where ν max is the frequency of the oscillating magnetic field). In all the samples, the thermally activated relaxation process of the magnetization follows the Arrhenius law (Orbach process) as typically observed in Mn 12 derivatives. By fitting the τ vs. 1/T plots, the energy barrier Δ eff /k B can be calculated using the relation τ = τ 0 •exp(Δ eff /k B T), where, k B is the Boltzmann constant, and τ 0 a pre-exponential factor (Figure II-14, Table II-2). The Δ eff /k B for the SR species are in the range 64 -70.1 K, as typically found in Mn 12 -OAc and its derivatives (Table II-2

II.4.2 Magnetic measurements in dc mode

Magnetic measurements performed in dc mode were also characteristic of Mn , which is in a good agreement with the homo-substituted precursors and other Mn 12 relatives. 1,11,15a,c,16,19,20 The χT product decrease progressively in lowering the temperature between 280 and 120 K, as consequence of the intramolecular antiferromagnetic coupling between the Mn IV and Mn III ions. As expected, the χT products increase significantly below 120 K, reaching maximum values The opening of a hysteresis magnetization loop at low temperature is a characteristic feature of SMM behaviors. For all the samples, the field dependence of magnetization performed at 1.9 K with dc field (Table II-2). The significant decrease of the coercivity can be explained by the presence of the FR species. The shape of magnetization versus field curves and magnetization values (ca. 17

II.5. Thermal and self-organization properties

One of the main motivations of this work was to determine how the Mn 12 functionalization might influence the thermal and self-organization properties in respect to the previous strategies. In particular we

Chapter II -Rational Functionalization of Single-Molecule Magnets: Towards LC Phases II.72 sought to determine whether the regioselective functionalization might change the overall geometry of the molecule toward a discotic shape, allowing therefore the formation of columnar or nematic mesophases.

Also, an important aspect, toward possible deposition on surface concerns the clearing temperature, which needs to be accessible at moderate temperature in order to avoid decomposition. These particular features were investigated by temperature-dependent measurements including themogravimetry (TGA), polarized optical microscopy (POM), differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS).

As expected the complexes containing "short" ligands ([Mn 12 O 12 (CF 3 COO) O) 4 ] precursor (385 K), which suggests stronger cohesion between the molecules in the material due probably to the decrease of aliphatic functions. It is worthwhile to note that the clearing temperature occurs above the temperature at which the compound starts to decompose. However, considering that the loss of the water ligands is very progressive and should not alter significantly the overall organization, the non-reversibility is more likely due to a very-slow dynamic of the thermal behavior than a decomposition process.

SAXS investigations were performed between 298 and 473 K. During the first heating between room temperature and 430 K, the SAXS profiles were essentially the same and display numerous reflections in both small and wide-angle regions (Figure II-18), which indicates that the compound is crystalline. The absence of liquid crystalline behaviors is also supported by the absence of diffuse halo in wide-angle region which is typically observed in liquid crystalline materials and is attributed to the short-range ordering of the melted lateral alkyl chains. Hence, this confirms that the ratio between the rigid and flexible moieties is not favorable to promote mesomorphism as previously found for the [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ] precursor. 16 The SAXS profiles show also the presence of a small and broad signal in low-angle region (ca. 2.2 nm -1 ),

suggesting that the crystalline phase coexist with a glass phase. The intensity of this signal increases with the temperature, indicating that the material converges progressively toward an amorphous solid. The coexistence of phases is not unusual in macromolecular self-assemblies especially when samples have a virgin thermal history. At 458 K and above, the sharp reflections disappear and only a weak and broad signal II.74 in low-angle region (ca. 2.1 nm - 1) is observed in agreement with the conversion into the isotropic liquid phase. 111), ( 211), ( 220), (221/300), ( 222), ( 400), (241) (using Pm m space groups. The exact nature of the space group is still undetermined, but both of them a theoretically possible. However, 3 Pm m space group seems the most probable due to its higher symmetry.

√ℎ 2 +𝑘 2 + 𝑙 2 ).
The lattice parameter a is related to the reciprocal spacing q by a = 2𝜋√ℎ 2 + 𝑘 2 + 𝑙 2 / 𝑞. Extracted from the slope of 2π/q vs 1 / √ℎ 2 + 𝑘 2 + 𝑙 2 plots (Figure S.II.10), the lattice parameter was found to be almost independent of the temperature, ca 35 Å. This value is comparable to the one found for its homo-substituted precursor (ca. 37.8 Å), 16 suggesting that the Mn 12 molecules inside the cubic mesophase are arranged in both case is very similar fashion. This study shows that the overall geometry of the complex remains pseudo-spherical even though the mesogenic ligands are only on the equatorial sites of the molecules, presuming that the flexible chains are randomly spread all over the core. The size of the primitive cubic cell also indicates that the aliphatic chains are strongly interdigitated. Indeed, the complex can be idealized as spherulitic micelle in which the rigid nucleus, composed of the oxo-metallic core has a diameter of ca. 20 Å (based on the X-ray crystal structure), while the shell formed by the dodecyl chains has a thickness of ca. 15 Å. Therefore, after taking into account a maximal interdigitating of the aliphatic chains, the minimal lattice parameter a min required for a primitive cubic cell can be estimated to 35 Å (using the relation a min = 2r c + l ch where r c is the radius of the rigid core, and l ch is the maximal thickness of the soft aliphatic shell, see Figure II-19).

II.5.2. [Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ] (three C 12 chains ligand)

Similarly to the previous complex, POM analyses for [Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ] did not reveals any birefringent texture over all the range of temperature tested, indicating that the material is either cubic or amorphous. The DSC thermogram shows only one reversible transition at 258 K, which corresponds to the melting of the aliphatic shell and transition into a mesophase (Figure II-20). Indeed, the enthalpy (ΔH) of phase transition is about 134.6 kJ/mol, i.e. 5.3 kJ/mol per dodecyl function, which is in the typical range for the melting of such substitutent. 22 Similarly to its precursor [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ], the clearing point is above the thermal stability of the complex ( > 410 K) and no peak was detected in DSC up to 473 K.

The exact nature of phase below 258 K could not be determined by means of SAXS investigations since this temperature cannot be reached with our experimental setup. The first thermal cycles performed up to 358 K (Figure S.II-11 and S.II-12, left), confirmed the presence of liquid crystalline features by the presence of a diffuse scattering halo around 14 nm -1 . However, the material is only weakly organized since only one broad and intense peak at 2.10 nm -1 was observed. This latter corresponds to a short-range organization between Mn 12 molecules with a separating distance of 29.9 Å. When the sample is further heated above 363 K, the material becomes more fluid allowing an efficient reorganization of the molecules. The exact temperature of the clearing point was not determined accurately since no signal was detected in this range of temperature by DSC, but it is located between 423 and 448 K according to the SAXS experiments. Also, we should emphasized, that the organization is not fully recovered on cooling and only the first sharp peak, d 110, was observed while a broad signal in low-angle region is also present. This might be the result of a slow-dynamic thermal behavior as found for [Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ] or due to a partial decomposition of the molecular structure.

Chapter II -Rational Functionalization of Single-Molecule Magnets: Towards LC Phases

II.77

The reflections displayed in SAXS profiles between 298 and 423 K, are in the ratio of √2 ∶ √4 ∶ √6 : √8 ∶ √9 ∶ √10 ∶ √14 and were thereafter indexed as ( 110), ( 200), ( 211), ( 220), (221/300), ( 310 S. II-4). Therefore, the cubic mesophase can be viewed by the micellar model described above (Figure II-19), in which the lipophilic shell of Mn 12 molecule is strongly interdigitated.

II.5.3. [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ] (two chains containing biphenyl moieties)

In contrast with previous compounds, the POM analyses of [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ] revealed the presence of fluid birefringent textures after the first thermal annealing at 348 K, suggesting a liquid crystalline behaviors (Figure II-22). The textures are homogeneous and stable from room temperature up to ~ 348 K, where the material become an isotropic liquid (black isotropic phase). No significant hysteretic behavior between the disappearance of textures on heating and their appearance on cooling was detected as expected for LC behavior. The exact nature of the mesophase is nonetheless difficult to determine since the textures are only composed of very small domains as for many macromolecular and/or metallomesogenic systems.

DSC thermograms are in agreement with POM observations. The DSC thermograms show a reversible broad peak around 345 K, which corresponds to the conversion from a mesophase into the liquid isotropic phase (Figure II-23). Hence, the clearing point is much lower than the other complexes functionalized with lipophilic substituents and occurs well below the decomposition temperature, making of this compound the best candidate for a potential surface deposition so far. It is worth to note that the clearing point is almost identical to the one reported for the homoleptic [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] precursor (ca. 347 K). Nonetheless, this is not very surprising since it has been previously described that the clearing temperature is related to the decorrelation on interactions between the pending pro-mesomorphic units rather than the interaction between the Mn 12 cores. 15c In other words, it depends essentially of the nature and not on the number of these units. II.78

Hence these results confirms that the use of biphenyl pending units is a more judicious choice than cyanobiphenyl groups to obtain low-clearing materials. 16 indicates that the material self-organizes into a lamellar mesophase similarly to its homo-substituted precursor and other relatives. 15a,c,16 The interlamellar distance d is temperature-independent and calculated as 34.2 Å (using d =

1 𝑛 ∑ 𝑙 • 𝑑 00𝑙 𝑛 1
, where n is the number of reflections and d 00l = 2/q). The value is comparable II.79

to the one found for [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] (ca. d = 32.8 Å), 16 and lower than those found for analogue complexes containing cyanobiphenyl smectogens (ca. 40 Å), 15c,16 in agreement with the lower cohesion between biphenyl units and thus stronger interpenetration between layers. 16 Even though lamellar organizations are not incompatible with columnar phases (columnar lamellar Col L ), the formation of such mesophases is quite improbable in our case. Indeed, this would require a stacking of the cores along the main molecular axis while the flexible substituents in the molecular plane form the lamellar organization. Thus, considering that the eight equatorial ligands are equivalently distributed all over the Mn(III) crown, there is no reason to privilege one direction to form a layers and columnar with hexagonal or square symmetry would be more likely expected. Also, considering the diameter of the crown and the length of the chains and biphenyl units, a higher value for the interlayer distance should be expected. Finally, no evidence for a shortdistance stacking between the cores was found in the wide-angle region. The mesomorphic organization can For the [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] precursor, it have been previously shown that Mn 12 cores in the sublayers display a long-range 2D order with an hexagonal symmetry. 16 In our case, no evidence of hexagonal pattern can be detected. In fact, the SAXS profiles display a very broad scattering which is overlapped with the intense reflection 002 and centered at 3.80 nm -1 (Figure II-24). This correspond to a distance d' = 17 Å, which is comparable to those found in the series of complexes reported by Terazzi et al. 15a,c and might be attributed to a randomly and short-range 2D order of Mn 12 cores. The exact nature of this organization remains uncertain as no computational modeling was performed; however, considering the structural similitude with the previously reported complexes, the distance is more compatible with a square symmetry. It is worthwhile to note that the broad scattering is slightly shifted to longer distance (20 Å) upon the first thermal annealing, presuming some substructure changes of the 2D arrangement. As expected, all the complexes preserve SMM properties, confirming the versatility of these molecular systems to undergo ligand substitutions without altering significantly the structure and properties of the magnetic core. However, substantial changes on some magnetic features were displayed in respect to their respective homoleptic precursors [Mn 12 O 12 (L 1-4 ) 16 (H 2 O) x ] (x = 3, 4 As mentioned early, the main motivation of this work was to determine how our functionalization strategy might influence the thermal and self-organization feature of the final materials in comparison to the homo-substitution method used before.

II.6. Conclusions and perspectives

Similarly to their respective precursors, the two complexes containing dodecyl chains present hightemperature clearing points, in such way that the integrity of the complexes is not completely assured after annealing into the isotropic phase. Also, in contrast with our expectations, no columnar or nematic phase was obtained. Indeed, complexes self-organize either into crystal (with L 1 ) or liquid crystalline (with L 2 ) cubic phases, indicating that no improvement on the overall structural anisotropy was achieved. This result may reflect the tendency of the flexible moieties to reorganize themselves all around the Mn 12 core leading therefore to a pseudo-spherical geometry of the complex.

Once again, no columnar mesophase was obtained for [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ], and the material self-organizes in the same fashion as its precursor (bilayer lamellar phase). Indeed, this can be explained by the presence of the pending biphenyl units, which tend to form layers. Considering the architecture of the molecule, the best way to achieve this result involves a tilt of the flexible spacers on either side of the Mn 12 core (perpendicular to the molecular plane). This trend to form layers could have been overcome if Mn 12 cores had a strong ability to stack and form columns; however, the presence of trifluoroacetate ligands in the axial positions is not an ideal situation to promote such strong stacking. Even though, our ligand design strategy did not allow us to promote new mesomorphic features, this material could be an interesting candidate for surface deposition. Indeed, similarly to its parent complex, the clearing point was found at a quite low temperature (345 K) and well below the decomposition temperature making them suitable for the preparation of homogeneous thin film or eventually monolayers. Additionally, the

Chapter II -Rational Functionalization of Single-Molecule Magnets: Towards LC Phases

II.81

presence of fluorine in axial positions might promote specific interactions with surfaces (eg. silicon surface)

and thus help to control the orientation of molecules in one specific direction (easy axis perpendicular to the surface).

Many modifications on the structure of the ligands might be considered to improve the selforganization of the molecular systems and eventually promote new mesomorphs. In order to prevent from the reorganization of the flexible chains around the magnetic core as in our two first complexes, we should increase the separating distance between these two antagonist moieties. One idea would be to add a rigid spacer between the anchoring part of the ligand (carboxylate) and the mesogenic one. For instance, we could use for the equatorial positions, ligands containing diphenyl acetylene or terphenyl platforms instead of the benzoate one used so far (Scheme II-5).

To induce columnar phases, the ligands in the axial positions should be designed to promote efficient stacking via supramolecular interactions (eg. H-bonds or π-π stacking). For instance, we could add a phenyl ring while conserving fluoride groups in α-position of the carboxylate for keeping low pK a .

Scheme II-5. Possible targets for the functionalization of equatorial and axial positions of the Mn 12 core.

As mentioned before, we should also consider a second strategy consisting in the introduction of mesogenic ligands only in the axial positions of the core, which will promote the formation of lamellar phase. Thus, by playing on the design of the non-mesogenic ligands (in equatorial positions), we could promote specific interactions between molecules and improve the in-plane 2D organization.

II.7. Supporting materials II.7.1. Experimental protocols

All the solvents and starting materials were purchased from Acros, Alfa Aesar or Aldrich, and used without further purification unless otherwise stated. 7.51 (7.61).

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

Preparation of 4-((10-bromodecyl)oxy)-1,1'-biphenyl (2

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

A mixture of methyl (8.31). added. The reaction mixture was stirred at 45°C for 24 hours during which the solution color turned from brown to brownish black. After cooling to room temperature, the mixture was filtered and the solvent was removed. Toluene (40 mL) was added to the residue, stirred for 3 hours, and then evaporated to dryness. This procedure aiming to remove the produced acetic acid as an azeotropic mixture with toluene was repeated three more times. The brownish-black residue was dissolved in CH 2 Cl 2 (8 mL) ) and 0.98 g (HL 3 ), 1.13 g (HL 4 ) 1.46 mmol, 28 eq.) was added. The mixture was stirred at 45°C (or 55°C for HL 4 ) for 24 hours during which the solution color changed from brown to brownish black. The solution was filtered and the solvent was then evaporated. Toluene (20 mL) was added to the residue, stirred for 3 hours, and then evaporated to dryness.

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

This procedure was repeated three times more.

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. Mn( 1)-O( 1) 1.917( 9) Mn(3)-O( 4) 1.957( 7) Mn(1)-O(1)'

1.922( 8) Mn(3)-O( 4)' 1.872( 10) Mn( 1)-O(1)''

1.914( 7) Mn( 3)-O( 4)'' 1.941( 10)

Mn IV -O (μ3-O) (eq)
Mn( 1)-O( 2) 1.881( 9) Mn( 3)-O( 1) 1.857( 10) Mn( 1)-O( 4)

1.832( 7) Mn( 3)-O(3) 1.867( 10) Mn IV -O (TFA) (ax)

Mn( 1)-O(3) 1.914( 10) Mn( 3)-O( 12) 1.948( 10) Mn III -O (μ3-O) (eq) Mn( 2)-O( 4) 1.947( 8) Mn( 1)-O( 1) 1.880( 10) Mn( 3)-O( 2)

1.882( 7) Mn( 1)-O( 3) 1.878( 10) Mn( 3)-O( 4)

1.878( 9) Mn( 2)-O( 1) 1.909( 10) Mn( 4)-O( 2)

1.926( 8) Mn( 2 q (obs) is the observed reciprocal spacing, q = 4sin/,where  is the wavelength (1.5418 Å), 2 is the diffraction angle, d (obs) and d (calc) are observed and calculated diffraction spacing distances: d (obs) = 2/q (obs) , d (calc) = a/√ℎ 2 + 𝑘 2 + 𝑙 2 , 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 ). = 2/q (obs) , d (calc) = a/√ℎ + 𝑘 2 + 𝑙 2 , 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 ).

III.1. Introduction

During the recent decades, an important research activity has been dedicated on the development of molecular materials that display critical changes of their physical properties as function of an external stimuli such as temperature, pressure, light irradiation and magnetic or electric fields. In this scope, spin-crossover (SCO) compounds are among the most studied. 1 The SCO phenomenon is encountered in certain transition metal complexes and involves a change of the spin configuration of the metal ion center. 1 It is accompanied by important changes in magnetic and optical properties as well as structural features. For instance, in Fe(II)

complexes, the SCO is associated with paramagnetic-to-diamagnetic switching. The particular features displayed in these compounds may have interest for numerous practical applications such as sensors, molecular switches, data storage and display devices. Although these potential applications are often highlighted in numerous fundamental studies, to date none of these magnetic components have been utilized

industrially for any commercial devices. One of the main issues involves the fabrication of technologically sustainable materials. Indeed, these compounds are generally produced as powders or crystalline solids which are not the best candidates for technological devices. Thus, important research efforts have been devoted to the processing of SCO compounds by depositing them on surfaces, 2 inserting them into polymer matrices, 3 forming nanoparticles, 4 or gels. 5 More recently, another interesting approach has been proposed and consists in the preparation of spin crossover hybrids displaying liquid crystalline (LC) properties. 6 This approach received much consideration because the SCO phenomenon can be tuned around room temperature and this could be used for the development of advanced multifunctional materials with advantages in both molecular magnetism (processing/shaping, tuning of magnetic and optical features) and LC (introduction of paramagnetic, photo-or thermochromic properties) fields. The first complex combining LC and SCO properties was reported in 2001 by Galyametdinov and co-workers on a Fe(III) complex. 7 In this study, a

Schiff base ligand (salicylidenyl-N'-ethyl-N-ethylenediamine) was functionalized with a dodecyl tail, providing a calamitic geometry to the resulting complex. The material self-organizes into a lamellar mesophase (smectic A) above 388 K and displays a gradual SCO with a T 1/2 = 125 K. The first metallomesogen system based on a Fe(II) SCO complex, namely [Fe(L) 2 (NCS) 2 ] (with L = 3,4,5tris(hexadecyloxy)-N-(pyridin-2-ylmethylene)aniline), was reported shortly later by Hayami and coworkers. 8 Once again, the compound exhibits LC properties above room temperature, while the SCO was found at much lower temperature (T 1/2 = 217 K). Subsequently, several groups proposed to develop molecular systems that display SCO at higher temperature in order to promote a synergetic effect between SCO and LC properties. these promising examples, the number of SCO systems within metallomesogens remains very limited and most of the time materials display gradual and/or incomplete SCO. [6][7][8][9][10] Additionally, the behavior is often irreversible after clearing due to decomposition and/or oxidation and/or lattice solvent loss processes.

Chapter III -Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials III.101 During this PhD work, we proposed to develop new metallomesogens based on a mononuclear Fe(II) complex containing Schiff base ligands belonging to the family of pyridylaroylhydrazones (Scheme III. 1).

Scheme III-1. General structure of functionalized [Fe II (L) 2 ] complexes based on pyridylaroylhydrazone ligands.

These monoacid ligands exhibit keno-enol tautomerism, giving rise to versatile coordination chemistry where dicationic, monocationic and neutral forms of divalent metal ion complexes can be isolated. [11][12][13][14][15][16] The coordination chemistry of these ligands with various 3d-5d [11][12][13][14][15] and 4f 16 divalent and trivalent ions have been widely studied since more than 30 years. Among them, much attention has been devoted to Fe(II) systems.

Indeed, these ligands are able to bind efficiently and selectively Fe(II), forming robust complexes and have been intensively examined as potential chelators for clinical treatments of iron overload diseases and tumor growth inhibitors. 12 In other hand, the interest for their magnetism is very recent and only few studies have been reported so far. [13][14][15] To the best of our knowledge, only one example of Fe(II) dicationic complex have been magnetically characterized and found to exhibit a Curie-Weiss paramagnetic behavior, 14 while no data is available for the monocationic form. More interestingly, few examples of neutral complexes were found to display photo-and/or thermo-induced SCO according to the functionalization. [13][14][15] Figure Although no examples with LC properties have been reported, the architecture and features of these complexes make them prime candidates for the development of metallomesogens systems. Indeed, they have a bent geometry which might favor, after an appropriate functionalization, the formation of lamellar or eventually banana-like (BN) mesophases. Additionally, they are quite stable in temperature and against oxidation, which is a sine qua non condition for eventual surface deposition (upon reaching the isotropic liquid phase). The absence of counter-anions is also a clear advantage since their role on the selforganization (and magnetic behavior) remains somewhat unclear. Finally, the spin crossover can be tuned above room temperature, giving the opportunity to promote an interplay between the LC and SCO properties. Indeed, Real and co-workers reported very recently a series of long chain-functionalized analogue

Chapter III -Mononuclear Fe II spin-crossover complexes: from crystalline toward soft materials

III.102
complexes displaying spin transition with large hysteresis induced by the melting/crystallization transitions (Figure III-1), 15 and this could be potentially transposed in metallomesogenic systems.

In the first part of the chapter, we discuss on the synthesis and characterization of model complexes containing non-mesogenic ligands. Indeed, only few examples of SCO systems based on aroylhydrazone ligands have been described. In the second part of the chapter, we describe our attempts to promote liquid crystalline features on this molecular system by using strongly lipophilic ligands. Several sites are available for the grafting of mesogenic substituents and we mainly focused on the functionalization of the aryl scaffold (Scheme III-1). Although these attempts were unsuccessful, the interesting effects of these functional groups on thermo-structural and thermo-magnetic properties on the resulting materials are discussed in detail.

III.2. SCO complexes with non-mesogenic ligands

In this section, we describe the synthesis, structural and magnetic characterizations of a serie of Fe II complexes containing non-mesogenic ligands (Scheme III-2). The initial goal was to test whether the introduction of methoxy functions on the aryl ring is appropriate for the preservation of SCO features, before going further on the functionalization with alkoxy mesogenic substituents. Indeed, no example of complexes functionalized with such functional groups was available during the maturation of the project (a series of complexes containing alkyloxy chains have been described since then 15 ). Thus, three new ligands containing one to three methoxy groups and corresponding complexes were synthetized and characterized.

Scheme III-2. General pathway for the synthesis of N'-(pyridin-2-ylmethylene)benzohydrazide ligands.

All the ligands were prepared in good yields by a two-step synthesis according to general procedures described in the literature (Scheme III-2). 12,13 In short, benzohydrazide precursors 2a-c were obtained after treatment of the methyl benzoate derivatives with excess of hydrazine. Then, the HL 1-3 ligands were synthetized by an equimolar condensation of 2a-c with 2-pyridinecarboxaldehyde. ] can be prepared according to the literature, [12][13][14] by reacting a stoichiometric amount of HL 1 and Fe(ClO 4 ) 2 •6H 2 O (ratio 2:1) in presence of excess of triethylamine (Scheme III-3). This latter is required to deprotonate the ligand and promote the formation of desired neutral complex (enolate form Interestingly, [Fe(L 1 ) 2 ] can be isolated as different polymorphs and solvatomorphs according to the reaction conditions. When the reagents are combined in methanol and the reaction mixture is stirred at RT, a dark green precipitate, whose composition was found as [Fe(L 1 ) 2 ]•MeOH and thereafter referred as α-phase, appears quickly in less than one hour. However, if the reaction mixture is warmed to 50 °C, the dark green microcrystalline powder turns rapidly and irreversibly to olive green. This latter has the exact same chemical composition (not a solvatomorph), and is then referred as β-polymorph. The exact mechanism for the transformation from the α to β-phase remains not completely elucidated and understood since the existence of polymorphism in this system was fortuitously discovered only very recently. However, this result strongly III-1 (detailed crystal data, refinement parameters, bond lengths and angles can be found in Table S.III-1 in supporting information section). , where 𝜑 𝑖 are cis X-Fe-X bond angles with X = O or N

III.2.1 Mononuclear Fe(II) complexes based on HL

suggests that α-{[Fe(L 1 ) 2 ]•MeOH} complex
In both cases, the FeN 4 O 2 coordination sphere departs strongly from an ideal octahedral symmetry as shown from the Σ (X-Fe-X) parameter (with X = O or N), which sums the deviations from 90° of the 12 cis angles in the coordination sphere of Fe(II) ion. 17 A Σ (X-Fe-X) parameter of 81.1° was found for α-complex, while the distortion in β-{[Fe(L 1 ) 2 ]•MeOH} structure (157.6°) is much more pronounced (Tables III-1 and S.III-1). These values are comparable to those reported for closely related analogues in LS and HS configurations, respectively. 13 The average Fe-N (py) , Fe-N (im) and Fe-O bond lengths in α-{[Fe(L 1 It is reasonable to assume that this latter will affect more efficiently the ligand field strength of the complex since O3 is directly involved in the Fe(II) coordination sphere and it should be noticed that the complex, which is paramagnetic at 120 K, become diamagnetic after removal of the methanol molecule (vide infra). However, the real role of the methanol molecule in the stabilization of the paramagnetic phase remains uncertain. Indeed, no III.108 crystallographic structure could be successfully obtained for the desolvated complex and it is thus difficult to estimate how the removal of methanol molecule might affect the overall crystal packing. Doubts about its contribution were also raised after the isolation of another [Fe(L 3 ) 2 ]•MeOH relater complex featuring the opposite behavior. This complex, which will be discussed more in detail in section III.2.3, displays a similar H-bond with an oxygen donor and was found diamagnetic until it loses the MeOH above 400 K. The complex is then stabilized in the HS configuration for a large range of temperature on cooling (gradual SCO with T 1/2 of ca. 340 K). The stabilization of [Fe(L 1 In both cases, diffractograms patterns fits well with the simulated ones even though some variations can be detected due to the difference in temperature between the experimental and simulated diffraction

Chapter III -Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials

III.109
patterns and the resolution of the used technique. In each sample, no peak associated to the second polymorph could be found within the detection limit, approving our synthetic procedures for isolating pure fractions of each phase.

III.2.1.2.2. Desolvation process

As mentioned before, both [Fe(L 1 , where 𝜑 𝑖 are X-Fe-X bond angles with X = O or N Measurements were performed at higher temperature to detect eventual structural changes that are often indicative of SCO behavior. The unit cell parameters of [Fe(L 1 ) 2 ] remain unchanged up to 350 K, while important modifications were found at 370 K, which corresponds well to the temperature at which thermochromism (dark green to olive green) is featured in the powder sample. A contraction and increase of a and b parameters, respectively, and an expansion of about 5% of the unit cell are observed, while no crystallographic space group change is featured, remaining the monoclinic P2 1 /c space group (Tables III-2 and S.III-2). Similarly to solvent loss situation, a modification of the conformation of one of the methoxy groups is occurring upon heating to 370 K (Figure III-7). These structural changes can be unambiguously ascribed to spin switching phenomenon by the examination of the Fe(II) coordination sphere. Indeed, the volume of the FeN 4 O 2 speudo-octahedron increase from 9.3 to 11.5 Å 3 consequently to the elongation of the Fe-N and Fe-O bond lengths. The octahedral symmetry is also more distorted than in the previous structure at 120 K (with a Σ (X-Fe-X) parameter of 144.0 o ), as expected for structures in HS configuration. It is worthwhile to mention that the average bond length values, Fe-N (py) 2.203 Å, Fe-N (im) 2.060 Å, and Fe-O 2.079 Å, are slightly shorter than expected in fully paramagnetic analogues (if compared for example with β-{[Fe(L 1 ) 2 ]•MeOH}), suggesting the presence of a residual diamagnetic species at 370 K. Attempts to measure crystals at higher temperature to reveal the pure paramagnetic phase were unfortunately unsuccessful due to degradation of the single crystal. In both LS and HS phases, the structural changes appear mainly in the b-and/or a-direction (c-parameter is almost unchanged for all the structures) and this can be explained by the conservation of the supramolecular π-chains along the c-axis. The distance C2•••C13 remains the same upon desolvation and SCO processes (Table III-2). Once again, the measured crystal was representative of the bulk sample as verified by the powder X-ray diffractogram, which is virtually identical to the simulation provided by the structure at 120 K (Figure III-8, left).

Complex β-[Fe(L 1 ) 2 ] remains crystalline upon methanol loss. However, the desolvation process create multiple microfractures in such manner that crystals fall apart into tiny crystallites (microcrystalline powder and needles) that were unfortunately not suitable for single-crystal X-ray analysis. Additional attempts to remove the methanol in a soften fashion, using exfoliation techniques (after soaking the crystals in Et measurements were performed on SAXS setup that is not appropriate for high-resolution determination when used for wide windows of angles.

III.2.1.3. Spin crossover properties

III.2.1.3.1. Thermally-induced spin crossover properties

Temperature-dependent magnetic and DSC measurements were performed to further probe SCO features in these complexes. As mentioned above, α-{[Fe(L 1 ) 2 ]•MeOH} is not suitable for such experiments since it is diamagnetic at room temperature and does not survive upon heating (desolvation). The thermal dependence of the χT product (where χ is the molar magnetic susceptibility) for the solvent-free α-[Fe(L 1 On heating up to 320 K, the χT value is constant and worth ca. 0.09 cm 3 •K•mol -1 , indicating that the sample is almost purely diamagnetic (Fe(II) LS (S = 0)), in agreement with X-ray diffraction data. When heated further, the χT product rises slightly and gradually up to 0.43 cm 3 •K•mol -1 at 360 K, indicating a statistic conversion of about 10 % of species into the HS state. Above this temperature, the χT product sharply increases, suggesting a first-order spin transition. This indicates that above 360 K, the system is not anymore able to support local structural changes (expansion of the coordination sphere) and a deeper and propagative transformation of the material, involving notably the change of the conformation of one methoxy group, is occurring. At 400 K, the material is not fully paramagnetic as shown by χT value of 2.92 cm 3 •K•mol -1 , which is lower than expected for Fe II HS (S = 2 with g ~ 2). Nevertheless, the shape of the curve suggests that the pure HS phase could be reached at higher temperature. The interconversion between the paramagnetic and diamagnetic phases is perfectly reversible. However, the transition appears at much lower temperature (T 1/2↓ = 332 K) on cooling mode, revealing a thermal hysteresis of about 38 K (for a sweeping rate of 0.3 K/min). The presence of this hysteresis clearly confirms the first-order spin transition and indicates a strong cooperativity within the crystal packing. The width of the hysteresis loop is almost independent of the sweeping rate (measured between 15 and 0.14 K/min), presuming that the metastable phases have a long life-time. 1,18 The SCO was confirmed by DSC experiments that revealed only a single transition in both heating and cooling modes ( Above 40 K, the χT product is ca. 3.4 cm 3 •K•mol -1 , which is typical for Fe(II) in HS configuration with g ~ 2. At low temperature, the χT value decrease slightly, owing to weak antiferromagnetic interactions or magnetic anisotropy on the Fe(II) site. Accordingly, the DSC of the solvated compound did not revealed any transition upon the first cooling from 290 K to 200 K, while a broad signal around 350 K, corresponding to the methanol loss, is observed during the first heating (Figure S.III-5). On subsequent cooling/heating cycles, the material displays a reversible transition around 230 and 260 K, on cooling and heating modes, respectively. The temperature-dependent magnetic measurements performed on the desolvated sample (after DSC) confirmed unambiguously that the transitions observed in DSC reflects a hysteretic spin transition behavior (Figure III-10, right). Similarly to α-[Fe(L 1 ) 2 ], the χT versus T plots reveal only one reversible transition, confirming the absence of the second polymorph in the bulk sample. The χT values at low and high-temperature plateaus are 0.04 and 3.4 cm 3 K mol -1 , which is consistent with a complete transition from the LS to HS configuration. The T 1/2↑ and T 1/2↓ can be estimated to 262 and 235 K, respectively, giving rise to a hysteresis loop of about 27 K for a sweeping rate of 0.3 K/min (Table III-3). These values are about 100 K lower than those obtained for α-[Fe(L 1 ) 2 ], illustrating the huge impact of crystal packing on the critical temperature of the SCO phenomenon. Once again, the hysteresis width is almost independent of the scan rate as shown by the magnetic measurements performed at 15 to 0.14 K/min and DSC experiments (see Figure

S.III-5).

It is worthwhile to mention that the transitions in β-[Fe(L 1 ) 2 ] are sharper than the previous case, which might indicate a stronger cooperativity in the system, even though this is not reflected by a wider hysteresis loop. The enthalpy ΔH and entropy ΔS parameters, determined by DSC, were comparable for both complexes and are in the range of values reported for mononuclear Fe(II) SCO complexes (Table III-3). 19

III.2.1.3.2. Photo-induced spin crossover properties

Preliminary temperature-dependent reflectivity measurements were also performed in order to confirm the thermally-induced SCO behavior and determine whether this latter can triggered by means of light.

The spectra of both samples are barely identical and display a maximal absorption around 700 nm (Figure III-11, top). The optical features of the thermally-induced paramagnetic phase of α-[Fe(L 1 ) 2 ] could not be characterized since this latter is well above the temperature window of our experimental setup. Thus, no significant change of the spectrum was detected between 285 and 10 K ( polymorph, an increase of the reflectivity is also observed in this region, however, it is not possible to estimate the photo-conversion since no spectrum is available for the thermally-induced paramagnetic phase.

Then, the temperature was raised in order to probe the thermal relaxation of the photo-excited phase (Figure III-11, bottom). In both samples, the reflectivity intensity is stable up to ca. 40 K, while it drops rapidly above, reaching the thermodynamic diamagnetic phase around 90 K (α-phase) and 110 K (β-phase). It should be noticed that the relaxation temperatures are probably over estimated since, in this range of temperature, the thermal relaxation process competes with the photo-conversion induced by the spectroscopic light source used during the spectra acquisition. Additional experiments using different light sources of specific III.116 wavelengths (ranging from 1050 to 365 nm) were performed to determine the optimal conditions for the photo-excitation. Using the same experimental protocol, it was found that the red light (630 nm) is the most efficient for the photo-conversion of both polymorphs and was selected for studies of the photomagnetism. Photomagnetic measurements were only performed for β-[Fe(L 1 ) 2 ] complex to date. The sample was cooled down to 10 K, irradiated with red light and the photo-conversion with time was monitored by measuring the magnetic susceptibility. Despite very long exposure (ca. 13 h), the χT value did not worth much than 0.56 cm 3 K mol -1 , meaning that less than 15 % of species were converted into the metastable paramagnetic phase (Figure III-12). This result might be explained by the low efficiency of the light to penetrate the dark green sample, in such a way that only the material in surface can be activated. After irradiation, the sample was cooled down to 2 K in the dark and the evolution of the photo-induced paramagnetic phase with temperature was followed. Between 2 and 10 K, the χT product increases from 0.09 to 0.57 cm 3 •K•mol -1 , indicating the presence antiferromagnetic interactions within the packing. Then, the χT value decrease progressively to reach the thermodynamic diamagnetic phase at 65 K. Hence, these studies confirmed that both polymorphs display thermally and photo-induced SCO properties. However, the light irradiation was not found to be very efficient to promote spin switching, probably due to the darkness of the sample, and only a very partial photo-conversion could be detected as in all other Fe(II) analogues reported so far in the litterature. 13

III.2.1.4. Synthesis and properties of cationic complexes

As mentioned earlier, pyridyl benzohydrazone ligands exhibit keto-enol tautomerism allowing the isolation of cationic and neutral forms of complexes according to the experimental conditions (Scheme III-5). 11,12,14 With Fe(II) ion, numerous examples of neutral and dicationic pyridylbenzohydrazone-based complexes have been described, [11][12][13][14][15] while no example of mono-charged complex has been reported. During this work, we were able to isolate both dicationic [Fe(HL 1 ) 2 ] 2+ and mono-cationic [Fe(HL 1 )(L 1 )] + complexes.

Scheme III-5. Keto-enol tautomerism in pyridylbenzohydrazone-based complexes

The dicationic form can be obtained according to literature, 14 by reacting HL 1 with an excess of Fe(ClO 4 ) 2 (ligand:metal ratio 1:1) in MeOH at room temperature. The complex was isolated as red-purple single-crystals after slow evaporation of the reaction mixture. Interestingly, when the reaction is performed in stoichiometric conditions (ligand:metal ratio 2:1), both mono-cationic and dicationic complexes are concomitantly formed. Indeed, upon addition of the iron salt into a MeOH solution of ligand, the reaction mixture turns instantly to red-purple as expected for the formation of [Fe(HL 1 ) 2 ] 2+ complex, but a shiny gray precipitate corresponding to [Fe(HL 1 )(L 1 )] + complex appears after a while. Thus, by using slow diffusion technique, it was possible to isolate single-crystalline dark grey needles of this material, while the dicationic form, much soluble, remains in solution. We should notice that the slow evaporation of the remaining filtrate allowed us to confirm the presence of [Fe(HL 1 ) 2 ] 2+ complex but this synthetic method was not suitable to isolate the dicationic form as pure (mixture with mono-cationic complex and free ligand).

XRD analysis revealed that the dicationic complex crystallizes in the monoclinic P2 1 /c space group. A view of the structure at 120 K is shown in Figure III-13 (left) and selected crystallographic data are gathered in Table III-3 (detailed crystal data, refinement parameters, bond lengths and angles are summarized in Table S3). , where 𝜑 𝑖 are cis X-Fe-X bond angles with X = O or N

In the other hand, the monocationic complex was found to crystallize into the triclinic P-1 space group (Figure III-13 and Tables III-3 and S.III-3). The unit cell consists of a heteroleptic complex, a disordered perchlorate ion and a methanol molecule, which interacts with the complex through O9-H9 or dicationic complexes (Table III-3 S.III-3).

As suspected by the X-ray diffraction analyses at 120 K, magnetic susceptibility measurements for both dicationic and monocationic complexes did not show any sign of thermally-induced SCO (Figure III-14). Above 30 K, the χT products remain nearly constant and worth 3.3 to 3.5 cm ). The χT products decrease slightly at low temperature as a result of weak intermolecular antiferromagnetic interactions or magnetic anisotropy on Fe(II) centrer. These results suggest that the presence of one or two ligands in the ketone form strongly weakened the ligand field splitting, stabilizing systematically the paramagnetic phase. Potentially we might take advantage of this feature and use protonation/deprotonation as chemical stimuli to promote spin switching between the diamagnetic neutral complex and the paramagnetic cationic forms.

III.2.2. Synthesis and properties of [Fe(L 2 ) 2 ] complexes III.2.2.1. Synthetic approach and general comments

Similarly to [Fe(L 1 ) 2 ] complex, [Fe(L 2 ) 2 ] was found to exhibit polymorphism. When HL 2 and Fe(ClO 4 ) 2 •6H 2 O (ratio 2:1) are combined in MeOH in presence of excess of triethylamine, and the resulting dark-green solution is allowed to stand at room temperature, dark green needles corresponding to solventfree [Fe(L 2 ) 2 ] complex are obtained in good yield within one hour. The complex, which will be thereafter referred as α-phase, crystallizes in the monoclinic C2/c space group. The isolated sample was analytically pure as shown by the powder X-ray diffractogram that is identical to the simulation provided by the crystal structure at 120 K (Figure III-15, left). When the rate of the reaction is artificially increased by applying stirring, a second polymorph, corresponding to the thermodynamic product and called thereafter βpolymorph, is progressively formed as shown by the appearance of additional diffraction peaks on the powder diffractogram (Figure III-15, center). This second polymorph was isolated in good yield as large and irregular prismatic crystals by diffusion of methanol into a dichloromethane solution of [Fe(L 2 ) 2 ] complex and found to crystallize in the triclinic P-1 space group. It is worth noting that a very small fraction of α-[Fe(L 2 ) 2 ] (needles) was systematically obtained after the recrystallization and was separated mechanically from β-[Fe(L 2 ) 2 ] crystals. Following this workup, an analytically pure sample of the β-phase was isolated as , where 𝜑 𝑖 are cis X-Fe-X bond angles with X = O or N

As mentioned above, [Fe(L 2 ) 2 ] polymorphs crystallize in two distinct space groups (C2/c and P-1 for α-and β-phase, respectively). In the same fashion as previous [Fe(L 1 ) 2 ] system, both polymorphs reside in a hexa-coordinated Fe(II) complex surrounding by two anionic ligands in mer configuration. Beside this general feature, strong differences between the two polymorphs are displayed at both molecular and packing scale. In the α-polymorph, the two chelating ligands are crystallographically equivalents and related by a 2fold symmetry axis, which crosses the Fe(II) site along b-axis (asymmetric unit composed of a half molecule), while no symmetry is found in the β-form. We should also notice the differences in the rotamer conformations featured by the methoxy groups, which adopt a syn/anti configuration (in respect with the position 4 of the 3,5 dimethoxyaryl moiety) in the case of the α-[Fe(L 2 , where 𝜑 𝑖 are X-Fe-X bond angles with X = O or N. The measurements performed up to 450 K, did not reveal any change of crystallographic space group in the material, which remains in the triclinic P-1 space group. Similarly, almost no evolution of the a, b and c parameters was detected with temperature, while a moderate increase of the lattice volume (less than 5 %)

was found upon heating to 450 K. However, clear changes of the lattice distortion were observed and a substantial increase of α angle was found above 400 K, while the β and γ angle decrease significantly. The distortion of the lattice is unambiguously associated to a SCO phenomenon as shown by the important increase of the Fe-X bond lengths and Σ (X-Fe-X) parameter. At 450 K, the coordination bond distances are nonetheless shorter than expected in purely paramagnetic materials especially if compared with α-polymorph (vide supra). The fully paramagnetic phase could not be reached for this compound since the decomposition was estimated to occur around 480 K by TGA. Similarly to α-[Fe(L 1 ) 2 ] complex, one methoxy group in one of the two ligands changes its conformation during the SCO process from the antiperiplanar configuration to the synperiplanar one (Figure III-20). We should also notice that due to the increase of the coordination sphere and distortion of the lattice, less supramolecular interactions were found in the crystal packing at 450 K. Especially, the distance between C4 and C10 atoms increases

III.2.2.3. Magnetic properties

Temperature-dependent magnetic susceptibility measurements were performed to confirm the behavior of the two polymorphs (Figure III-21). In agreement with X-ray crystal structure, no SCO feature was detected for α-[Fe(L 2 ) 2 ] complex, which exhibits a typical Curie-Weiss paramagnetic behavior. The χT product is ca. 3.15 cm 3 K mol -1 down to about 30 K, where it drop to 1.0 cm 3 K mol -1 due to antiferromagnetic interactions within the crystal packing or magnetic anisotropy on the Fe(II) site. It is worthwhile to mention that no transition was detected by magnetic and calorimetric measurements at higher temperatures (up to 400 K for squid measurements and 473 for DSC measurements), indicating that the complex is thermodynamically stable (no conversion toward the β-form is possible). The sample of β-[Fe(L 2 ) 2 ] polymorph was found to be purely diamagnetic up to about 300 K as shown by the χT product of ca. 0.02 cm 3 • K•mol -1 (Figure III-22, left). This value confirms that the α-polymorph formed as an impurity during the recrystallization in CH 2 Cl 2 /MeOH, was efficiently removed (vide supra). Above 300 K, the χT product increases gradually to reach 1.1 cm 3 •K•mol -1 , revealing the onset of SCO behavior in agreement with XRD analysis at 400 K. Preliminary investigations were performed at higher temperatures using vibrating sample magnetometer (VSM). The measurements revealed that the thermally-induced SCO behavior is actually more complex than we expected and remains not clearly understood. On the first heating from 300 K to 450 K, an incomplete two-step SCO is observed as shown by the presence of two peaks at 397 and 421 K on the plot of χT product derivative (Figure III-22, inset). The high-temperature plateau, reached at ca. 433 K, has a value of ca. 2.62 cm 3 •K•mol -1 , which is quite far from expected value for a purely paramagnetic phase, confirming the relatively short coordination bond distances (for an Fe(II) HS site) found at 450 K. In fact, assuming that the χT product and bond lengths for pure HS phase should be close to those recorded in the α-form and taking this as reference, a good correlation between the magnetic and crystallographic data can be found in β-[Fe(L 2 ) 2 ]. The correlation between the temperature-dependent χT and <Fe-N (py) > is shown as an example in Figure III-22 (right). Therefore, we can estimate that about 20 % of the species remain in the LS configuration at 450 K. Upon the first heating, the behavior is not reversible. On cooling, a sharp transition is observed at 396 K, giving rise to a thermal hysteresis of about 24 K. However, the material does not return into the diamagnetic phase and reaches instead a plateau at ca. 390 K, which corresponds to a new phase containing both HS and LS species in approximatively equal proportion (χT = 1.48 cm 3 K mol -1 ). Below ca. 240K, a gradual and stepwise conversion into an almost pure diamagnetic phase is observed. On second thermal cycle, the behavior is found perfectly reversible, confirming a three-step SCO process, which includes one hysteretic spin transition in high-temperature range. Nonetheless, this result should be taken with care.

Indeed, one attempt to probe the thermal behavior by Mössbauer spectroscopy was performed and did not reflect perfectly the previous magnetic measurements. The Mössbauer spectrum performed at 298 K after a thermal treatment at 450 K (for 30 min), confirmed that the material does not return in the diamagnetic phase, but the HS/LS ratio was 4/1 instead of 1/1 as expected (Figure III-23). Further investigations including temperature dependent single-crystal and powder X-ray diffraction measurements need to be performed on the complex to confirm and better understand the behavior.

III.2.3.1. Synthesis and structural description

Complex [Fe(L 3 ) 2 ] can be obtained in the similar fashion as previous samples described in the chapter, using a slightly modified crystallization procedure. Indeed, no single-crystal could be isolated by direct crystallization in methanol, and the material was recovered as a microcrystalline powder. Thus, a recrystallization was performed by methanol diffusion into a solution of complex in dichloromethane and the compound was obtained in good yield as dark green plate-like crystals. The complex was found to crystallize in the monoclinic P2 1 /n space group, in which the assymetric unit is composed of one hexa-coordinated [Fe(L 3 ) 2 ] complex co-crystallized with one molecule of methanol. A view of the structure is given in Figure III-24 (left) and selected crystallographic data are summarized in Table III-6 (detailed data can be found in Table S.III-6 as supporting material). Temperature, K 120 120

Crystal system Monoclinic Triclinic

Space group , where 𝜑 𝑖 are cis X-Fe-X bond angles with X = O or N and Σ (X-Fe-X) distortion parameter, 81.0°, which are comparable to previous analogues in same spin configuration. The unit cell was checked at 400 K, and was exactly the same as at low temperature, showing that no SCO process is occurring and also that the methanol molecule is preserved in the structure.

P2 1 /n P-1 a, Å 9
Unfortunately, no measurement could be achieved at higher temperature, due to the desolvation process which affect dramatically the quality of the crystal. Additional attempts to remove the methanol molecule in a soften fashion, were performed using lower temperature with reduced pressure conditions but in all cases no crystal of enough quality could be obtained. A solvent-free version of the complex was obtained by recrystallization, using diethyl ether diffusion into a dichloromethane solution of complex. The compound was isolated in low-yield as tiny dark green plates and found to crystallize in the triclinic P-1 space group

III.2.3.2. Thermal and magnetic behavior

As seen earlier in the structure of [Fe(L 3 The magnetism of the solvent-free complex obtained directly by crystallization could not been studied so far, as we did not obtained a sufficient amount of the material to perform all the measurements and its preparation was found not very easy to reproduce (most of the time only precipitate was formed). However, taking into account the DSC performed up to 450 K, the single-crystal X-ray diffraction checked up to 400 K and the absence of color change, we are quite confident that the material does not display any SCO feature up to decomposition.

III.2.4. Summary and concluding remarks

In the first part of the chapter, we described the synthesis, structural and magnetic characterizations of a series of mononuclear Fe(II) complexes based on pyridylbenzohydrazone ligands functionalized with one to three methoxy groups. Our initial concern was to verify if the addition of these donating groups would fulfill the electronic requirements for SCO, before going further on the functionalization with long alkyloxy chains (assuming that the donating character of these functionalized ligands is about the same as a methoxy).

From our study, we can conclude that the presence of methoxy functions on the benzohydrazide scaffold is not incompatible with SCO, since such features were displayed for complexes based on the three ligands.

Chapter III -Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials

III.131
However, the SCO characteristics are strongly dependent on the way the molecules are organized in solidstate and on the solvation and therefore SCO phenomenon was not systematically observed.

The main interesting achievement of this work has been the discovery of polymorphism in these molecular systems. Indeed, polymorphism in SCO materials has received much interest over the recent years since they give the opportunity to better understand (or at least to emphasize) the role of crystal packing and supramolecular interactions on SCO phenomenon, which includes effects on the SCO temperature and cooperativity. The first example of polymorphic SCO was reported by Köning and co-workers on the very well-known [Fe(phen) 2 (NCS) 2 ] complex (phen = phenanthroline). 20 Despite thousands of new SCO materials developed since then, only relatively few of them have shown the ability to exist in two or more crystalline forms. 21 Also, most of the time, polymorphs are obtained as mixtures, in such a way that it is not always possible to separate them efficiently and then to perform a full investigation of their respective properties. 22 Except for the raw sample of β-[Fe(L 2 ) 2 ] complex that contained small fraction of the β-form and needed mechanical separation, we developed synthetic procedures to isolated all the other polymorphs as pure samples. We should emphasize that the isolation of pure fractions of each [Fe(L 1 ) 2 ]•MeOH polymorph by applying different synthetic procedures, was essential since the crystals of the two polymorphic forms have the same morphology and color (at least by eyes) and it would be not possible to separate them in case of a mixture. For this complex, we have seen that a transformation from the kinetic to the thermodynamic phase can occur in the mother liquor upon a thermal activation. To the best of our knowledge, the observation of such transformation has been reported only in one other system, namely [Fe(tpa)(NCS) 2 ] (with tpa = tris-(pyridylmethyl)amine), which is converted successively into two polymorphs when soaked in DMF/water mixture for a long time (one month). 23 An evolution of the α-phase into β-phase with time was evidenced in [Fe(L 2 ) 2 ], but we found that the change of crystallization solvents was a better option to isolate the two different polymorphs. [Fe(L 3 ) 2 ] polymorphs were obtained fortuitously using a different approach involving in one hand the formation of a the solvent-free complex by crystallization and in the other hand by desolvation of [Fe(L 3 ) 2 ]. This strategy have been already successfully employed to produce polymorphs of different SCO systems such as recently reported by Aromí and co-workers on [Fe (1,3-bpp) 2 ](ClO 4 ) 2 complex

(1,3-bpp = 2-(pyrazol-1-yl)-6-(pyrazol-3-yl)-pyridine). 24 It should be mentioned that one other example of

pyridylbenzohydrazide-based [Fe(L) 2 ] complex (L = (E)-2-hydroxy-N'-(1-(pyridin-2- yl)propylidene)benzohydrazide
) is known. 25,26 Unfortunately, no synthetic, crystallographic or magnetic data for these materials are available (published on a PhD thesis 25 ) and only computational calculations predicting the critical temperatures of the two polymorphs were published. 26 As in many other systems, 27 only one of the polymorphs exhibits SCO feature in the case of [Fe(L 2 N,N'-bis[(1-methyl-1,2,3-triazol-4-yl)methylene]propane-1,3-diamine), was reported to exhibit hysteretic spin transition for both isolated polymorphs and the hysteresis width did not

Chapter III -Mononuclear Fe II spin-crossover complexes: from crystalline toward soft materials III.132 exceed 7 K for high sweeping rates. 28 When possible, we have shown the critical differences that are displayed on the crystals packing and we have attempted to identify the main factors responsive to their magnetic behaviors. Remarkably the three structures that feature the highest number of π-π contacts (β-

{[Fe(L 1 ) 2 ]•MeOH}, α-[Fe(L 2 ) 2
] and solvent-free [Fe(L 2 ) 2 ] complexes) do not exhibit SCO feature suggesting that the proliferation of such interactions in the packing might create a certain stress to the molecules that need to expand or contract during the SCO process. However, taking into account the multiple and complex intermolecular interactions existing in these molecular materials, the discussion remain only qualitative and should be supported by deeper investigations including computational calculations.

III.3. Functionalization of FeN 4 O 2 complexes with alkyl chains

As described in the first part of the chapter, the strong impact of the crystal packing on SCO phenomenon reinforced our conviction that these molecular systems are interesting candidates for the development of metallomesogens or at least soft systems with low-melting temperature. Indeed, we can expect that a crystal-to-mesophase phase transition and resulting changes on the self-organization might affect significantly Fe(II) coordination sphere and eventually provoke spin switching. Interplays between the SCO phenomenon and a matter phase transition has been actually shown very recently by Real and coworkers on a series of pyridylbenzohydrazide-based complexes functionalized with strongly lipophilic groups. 15 Although non-mesogenic, these materials melt at low temperature and a change of the spin configuration was systematically observed during the transition into the liquid phase. The natural thermal hysteresis existing between the melting and crystallization (first order phase transition) was reflected on the spin crossover phenomenon, which was then revealed as a hysteretic process, which is strongly dependent of the scan rate. It is worth to recall that interplays between the SCO process and crystal-to-mesophase transitions have been previously described, notably on [Fe(RTrz) 3 ](X) 2 systems (RTrz = 4-substituent-1,2,4trizole; X = nonovalent anions). 9,10 Furthermore, the introduction of flexible alkyl chains might also give rise to unconventional magnetic behaviors such as thermally-induced reverse-SCO (ie. conversion from HS into LS by increasing the temperature) as shown by Hayami and co-workers on [Co(Cn-terpy) 2 ](BF 4 ) 2 (Cn-terpy = 4'-alkyloxy-2,2':6',2''-terpyridine, n = 14, 16). 29 During this work, our attempts to produce metallomesogens mainly relied on the functionalization of the benzohydrazyde moiety by one to three long alkyl chains. This strategy was unfortunately unsuccessful to promote the desired liquid crystalline features but the interesting effects of the functionalization on thermo-structural and thermo-magnetic behaviors are discussed in this part of the chapter.

III.3.1. Synthesis of ligands and [Fe(L 4-8 ) 2 ] complexes

Five ligands containing one to three dodecyl or octadecyl chains were targeted during this work and were prepared by a three-step synthesis (Scheme III-6). The non-commercial 1d-g precursors were obtained accordingly to procedures described in Chapter II, by a Williamson coupling with the corresponding alkyl bromide. Then, the same sequential steps and reaction conditions used for the preparation of analogues with

Chapter III -Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials III.133 methoxy functions were employed for the synthesis of the five HL 4-8 ligands. Their respective complexes were obtained in same fashion as previously described with only slight modifications of the synthetic procedure. Indeed, when reactions were performed at room temperature, the raw samples systematically contained unreacted free-ligand, which could not be separated efficiently from desired compounds.

Reactions were thereafter carried out in methanol at 60 °C for 2 hours and products were recovered as green powders after cooling to room temperature. The purity of the samples was verified by elemental analysis and FTIR, which show the characteristic signatures for chelating enolate ligands around 1600-1580 cm -1 (ν (C=N-N=C) ) and 1360 cm -1 (ν (C-O) ) and did not reveal any sign of free-ligand (ν (C=O) expected around 1640 cm -1 ). As seen in Chapter II, the presence of dodecyl chains can be also clearly identified by the strong double band around 2900 cm -1 attributed to C-H stretching vibrations. Confirmation of the structure by single-crystal X-ray diffraction analysis was also possible in the cases of [Fe(L 4 ) 2 ] and [Fe(L 5 ) 2 ] complexes and their structural descriptions are discussed later in the chapter.

III.3.2. Properties of [Fe(L 4 ) 2 ] complex III.3.2.1. Thermo-structural features

As we first thought that it would be not possible to isolate crystals of [Fe(L 4 It is important to mention that the irreversible behavior is not consecutive to a decomposition process as often observed in mesogenic SCO systems. 6 Indeed, the material remains homogeneous in aspect and color upon heating up to 485 K, while TGA measurements did not revealed significant weight loss below 520 K (Figure S.III-6). In addition, the FTIR spectrum after the thermal treatment displays the same characteristic signatures for chelating ligands, even though minor differences could be detected in the overall spectrum due to important changes in the packing organization. Finally, DSC performed on additional cycles did not revealed any evolution in term of temperature and enthalpy values for the reversible interconversion between the two crystalline phases (Cr3 and Cr4) and melting/crystallization transitions (Figure S.III-7). In fact, these results suggests that the Cr1 and Cr2 phases are related to a kinetic product inherent to the synthetic conditions employed, which is then converted at 432 K into a thermodynamic one, as found before in other SCO systems. 30 Assuming that, we

III.136

SAXS measurements confirmed that the transition at ca. 320 K observed during the cooling corresponds to the return into Cr1 phase (Figure III-31, center and right). It is worth to note that the organization in Cr1

phase is slightly temperature-dependent, showing notably an increase of interlayer distance, while no change with temperature could be perceived within Cr2 phase. No significant evolution of the thermograms could be detected over cycling (same enthalpy values) confirming the stability of the material and reversibility of its thermal behavior in the range of temperature tested. We should nonetheless mention that a slight shift of the transition temperature is observed between the 1 st and 2 nd cycles, which can be explained by minor structural rearrangement as classically found during first annealing.

From these prior investigations, it was not possible to determine unambiguously the nature of the In order to understand the origin of the Cr1→Cr2 transition, further investigations including, magnetic measurements and X-ray diffraction studies on single-crystals were performed.

III.3.2.2. Magnetic measurements and single-crystal X-ray diffraction analysis

Direct current magnetic susceptibility measurements were first performed up to 400 K, in order to prevent from the irreversible transformation occurring around 430 K. The temperature dependence of the χT product is shown in Figure III-33 (top). At low temperature, the χT value is 0.05 cm 3 •K•mol -1 and remains nearly constant up to 335 K, confirming the diamagnetic state predicted by the dark green color of the material. Above this temperature, a first-order spin transition is displayed around 340 K, which corresponds well to the first transition, Cr1 to Cr2, recorded in DSC. In cooling, the reverse transition appears at ca. 321 K, giving rise to a thermal hysteresis of about 19 K (for a sweeping rate of 0.6 K/min). The SCO is completely reversible, even though a slight shift in temperature is observed between the first and second

Chapter III -Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials III.137 heating as seen before by DSC and SAXS investigations. Remarkably, the χT product in the plateau is 1.65 cm 3 •K•mol -1 , which corresponds to about half of expected value for a full conversion into the HS state. This result suggests therefore the formation of a mixed {LS•HS} phase, which also explain the absence of significant change of color.

When the temperature is further raised, a second sharp transition generating a fully paramagnetic phase , where 𝜑 𝑖 are cis X-Fe-X bond angles with X = O or N

The compound crystallizes in the monoclinic P2 1 /c space group as shown by the structure collected at Thus, the only reasonable ways to explain the magnetic behaviors involve a CSB during the SCO process. This was, indeed, confirmed at 360K, at which a transformation from monoclinic P2 1 /c space group into orthorhombic P2 1 2 1 2 was observed. Therefore, the centrosymmetry is lost, giving rise to two Chapter III -Mononuclear Fe II spin-crossover complexes: from crystalline toward soft materials III.140 crystallographically unique complexes (Figure III-35, right). Each of them is located on a 2-fold rotation axis that crosses the metal center along a-axis and each complex can be then described by two equivalent halves of molecule.

As mentioned earlier in the chapter, pyridylaroylhydrazone-based [Fe(L) 2 ] complexes are chiral molecules due to the asymmetry of the Schiff base ligand. However, in all previous cases (including [Fe(L 4 ) 2 ] at 250 K) the Δ and Λ enantiomers were related by a symmetry inversion center. In the present case, no symmetry operation can rely the two enantiomers and the asymmetric unit is then composed of the racemic pair. The analysis of the coordination sphere confirmed that the spin configuration is not the same in the two distinct sites; The Δ-form having a Fe(II) in HS state, while Λ-form display the LS configuration (Table III-7). Actually, the reverse situation can be find in the crystal, which has been refined as an inverse- 37). The cohesion between layers is mainly assured through van der Waals interactions between the dodecyl chains, which were found almost perfectly interdigitated. As seen before, the interlayer distance increases substantially between the low and hightemperature phases (from 20.40 to 23.13 Å) and this can be explained by an increase of the bent angle formed by the almost linear chains and the metal-complex core (see Figure III-36, right). At 250 K, this III.141 angle can be estimated to ca. 87°, which is close to the expected value for octahedral symmetry with mer configuration, while the value increase at 360 K to about 105° and 110°for the LS and HS species, respectively. This changes are probably driven by the increase of the distortion of octahedral symmetry (Σ (X- Fe-X) = 158.8°) in the HS specie and provokes deep changes of the aliphatic organization in both species. Especially, this includes a change of the rotamer conformation of alkyloxy functions in order to minimize torsions and maintain an optimal interdigitating (see inset Figure III-36).

Beside the change of the bent geometry impacting the interlayer distance along b-direction, important modifications of the metal center organization within the layers are also observed. As mentioned earlier, the metal center layers are nearly planar (especially true at 360 K, see Figure III-36), but important displacements of the molecules and modest change of their orientation is occurring within the ac plan (Figure III-37). More particularly, the important contraction of the c lattice parameter, from 14.23 Å at 250 K to 10.95 Å at 360 K, is accompanied by the loss of the metal center alignment in this direction. A modest contraction of a lattice parameter is also found, giving rise to a general contraction of the ac surface area from 117.3 Å 2 to 109.9 Å 2 . Taking into account this contraction and considering that the SCO process requires an expansion of the coordination sphere, this might explain that only half of the Fe(II) species are converted into the HS configuration. Relatively few supramolecular interactions insure the cohesion within the metal center layers. Indeed, the orientation of the molecules is not favorable for establishing π-π contacts

III.142

In the above section, we described the dramatic structural changes arising from the CSB that accompanies the SCO process and after this analysis we should attempt to answer to the question: what is the driving force behind this behavior?

Obviously, the occurrence of CSB rely on the strong cooperativity resulting from elastic interactions in the packing and the manifestation of that have been experimentally evidenced by the magnetic measurements that revealed an abrupt and hysteretic spin transition. Although the origin of CSB cannot be determined with certitude, some intriguing points make us think that the presence of alkyl chains is not innocent and possibly be the main driving force behind this particular behavior. The first evidence, is the suspicious contraction of the metal center layers (ac plan), while an expansion is more likely expected considering the natural expansion of the coordination sphere after conversion into the HS state. Of course, a contraction of one of the lattice faces is not unusual but should be motivated by certain criteria, which include: the displacement of certain molecules in the third direction and/or a reorientation of the molecules in such a way that their specific area in the considered plane is minimized and/or the creation of favorable interactions. As seen before, the metal centers remain perfectly in plane in the high-temperature phase, while the moderate reorientation of the molecules is mainly resulting from the change of the β angle from 128 to 90° and does not contribute to minimize their specific surface in the ac plane. Moreover, the relatively few number and nature of the supramolecular interactions within the layers are not very convicting to describe a strongly cooperative system. In fact, we suggest that the contraction of the ac plane, which prevent from full conversion of the Fe(II) sites, is dictated by the cooperative effects between alkyl chains, which maintain strong interdigitating whatever the phase. Indeed, taking into account the natural increase of the bent geometry induced the increase of the coordination sphere distortion, the conservation of an almost perfect interdigitating requires mechanically a contraction of the ac plane.

III.3.3. Extension to other complexes with mono-alkylated ligands

In order to reinforce our hypothesis about the key role played by the alkyl chains on crystallographic symmetry breaking and check whether it is possible to improve the behavior by obtaining this time a reversible two-step SCO, we proposed to design new closely related complexes such as [Fe(L 5 Our first strategy was to slightly modify [Fe(L 4 ) 2 ] complex by adding a chloride function onto the pyridyl ring, assuming that this minor modification would not affect significantly the overall organization and would therefore allow CSB in the resulting [Fe(L 5 ) 2 ] material. Furthermore, the idea behind the introduction of a chloride function was to promote SCO at lower temperature in such a way that the second SCO step could eventually occurs before the irreversible transformation found previously. However, this ligand design strategy did not fulfill our expectations, and the tiny change of the ligand completely changed both structural and magnetic features.

V / Å 3 4940.2 Z 4 <Fe-N py >, Å 1.967 <Fe-N im >, Å 1.867 <Fe-O>, Å 2.003 Σ(X-Fe-X), ° 83.6
The compound was isolated as sheet-like single-crystals by slow diffusion of methanol into a solution of the complex in dichloromethane. Single-crystal X-ray diffraction analysis performed at 120 K revealed that the complex crystallizes in the monoclinic P2 1 /c space group in which the asymmetric unit is composed of one hexa-coordinated complex. A view of the complex is shown in Figure III-38 and selected crystallographic parameter are summarized in Table III-8 (see also Table S.III-8). The LS configuration was confirmed by the analysis of the coordination sphere, for which the averages Fe-X (1.867 -2.003 Å) and Σ (X- Fe-X) parameter (83.6°) are in the same range of values found for analogues in LS configuration (Table III-8 Attempts to determine the structure at higher temperature were unsuccessful. The unit cell was checked at 320 K, and revealed important changes of the lattice, suggesting the occurrence of SCO in this range of temperature. The Bravais lattice was not anymore compatible with monoclinic P but more likely with triclinic P-1 (α = 79.93°, β = 87.94°, γ = 87.72°), suggesting once again the occurrence of CSB with temperature. The lattice distances were found slightly longer while the cell volume increased of about 7%.

Unfortunately, the crystal did not survive very long time at this temperature, and the data set collection was not sufficient to attain an acceptable refinement of the structure. It should be nonetheless mentioned that the preliminary solution suggested that at least two crystallographically independent Fe(II) sites are present.

Magnetic susceptibility measurements were performed to confirm the occurrence of SCO. The temperature dependence of the χT product is displayed in Figure III-40. The first runs were performed between 1.8 and 300 K, and revealed a gradual and incomplete SCO that is reversible on cooling. The χT product tends to reach a plateau with a value of about 1.1 cm 3 •K•mol -1 , which indicates that about one third of Fe(II) species were converted into the HS state. The incomplete SCO occurs in a stepwise fashion as suggested by the plots of derivative of the χT product that show a maximum at 175 K and a weak and broad deformation centered at 240 K. When the temperature is further raised, the product χT product increases sharply around 350 K, reaching thereafter a plateau of 3.1 cm 3 •K•mol -1 corresponding to a full conversion into the paramagnetic phase. During the cooling, the reverse process is not observed, indicating that the material was irreversibly converted into another crystalline phase. Thereafter, the material exhibits a SCO between 340 and 220 K (with T 1/2 = 280 K), which is found perfectly reversible on additional heating and cooling cycles. The χT value in the low-temperature plateau is ca. 1.6 cm 3 •K•mol -1 indicating that the SCO occurs between a mixed {LS•HS} phase and a fully paramagnetic phase. possible to isolate single-crystals of enough good quality for X-ray diffraction analysis (only tiny pellets could be obtained). Thus, the compound was mainly characterized by DSC, SAXS and magnetic susceptibility measurements on microcrystalline powder samples. Attempts to characterize these transformations by SAXS investigations were unsuccessful since the material does not survive very long time in the liquid phase.

Susceptibility magnetic measurements were performed to confirms the presence of SCO in [Fe(L 6 ) 2 ].

The temperature dependence of product χT is displayed in Figure III-45. At low temperature, the complex is diamagnetic, which is consistent with dark green color of the microcrystalline sample. The χT value is 0.11 cm 3 •K•mol -1 and remains constant up to ca. 200 K at which a slight and very gradual increase of the χT value up to 0.49 cm 3 •K•mol -1 at 360 K is detected. Above this temperature, a sharp transition centered at 380 K is displayed in agreement with DSC experiments. The χT product in the high-temperature plateau is 1.66

cm 3 •K•mol -1
, which is the half of expected value for Fe II HS , showing once again the coexistence of both LS and HS species in this crystalline phase. On cooling the reverse transition is observed at 357 K, revealing a thermal hysteresis of 23 K. No magnetic measurement were performed so far above 400 K, nevertheless the DSC and SAXS experiments strongly suggest that the compound will behave in the same manner as [Fe(L 4 ) 2 ], leading irreversibly to the fully paramagnetic phase after the thermal treatment above 410 K.

III.3.4. Functionalization with multiple dodecyl chains

As seen in the previous section, the functionalization of pyridylbenzohydrazone ligands lead to interesting magnetic properties on resulting complexes, systematically showing the formation of an intermediate {LS•HS} phase. However, no liquid crystalline features could be observed in these materials.

Although their melting is accessible at relatively low temperature, giving the possibility to form homogeneous thin films, the resulting materials lose irremediably their SCO ability upon such thermal treatment. With the aim to promote reversible behavior with liquid crystalline features, we applied a classical strategy, which consist to introduce additional greasy substituents onto the ligand scaffold. Therefore two new ligands bearing two (L 7 ) and three (L 8 ) dodecyloxy chains were considered for the preparation of Fe(II) SCO complexes (Scheme III-8). L 7 ) 2 ] and[Fe(L 8 ) 2 ], respectively. By cooling from the isotropic liquid phase down to room temperature (with a sweep rate of ca. 5 K/min), no birefringence was detected and the material solidify as thin homogeneous film that appears black under polarized light, excluding the hypothesis of liquid crystalline features in these materials. The DSC of [Fe(L 7 ) 2 ] complex revealed a monotropic thermal behavior (Figure III-46, left). Indeed, in addition to the melting transition found around 380 K, the material displays a weak endothermic peak at 337 K followed by an exothermic signal around 344 K, while only one exothermic peak, corresponding to the solidification of material is observed in cooling mode. The presence of this endothermic/exothermic process on heating mode was attributed to the formation of a metastable liquid-like phase around 334 K and reentrance into solid state at 344 K as illustrated by the optical micrographs recorded during the first heating that show the onset of melting at 333 K and solidification at 343 K (Figure III-46, right). It should be noticed that no birefringent texture was observed for this fluid phase which discards the possibility of metastable liquid-crystalline phase.

The phenomenon of reentrance or "inverse melting" is not rare especially in polymer, colloid and liquid-crystal materials, 32 and has been notably observed on the [Mn 12 O 12 (L) 16 (H 2 O) 4 ] (with L = 3,4,5tris(dodecyloxybenzoate)) metallomesogen used as precursor in Chapter II. 33 This behavior is often explained by the existence of a low-ordered phase which tends to reorganize itself with increasing temperature thanks to the increase of fluidity (or decrease of viscosity) and it involves and equilibrium between a fluid and solid phases. The presence of such low-ordered phase was confirmed by the SAXS experiments (Figure III-47). At room temperature, the SAXS profile exhibits only two weak and broad equidistant reflections at 2.16 and 4.33 nm -1 that correspond to random lamellar ordering with separating distance of ca. 29.1 Å as well as a broad and diffuse halo in the wide-angle region (centered at 14 nm -1 ) attributed to short-range lateral order between dodecyl chains. When the temperature is increased, a reorganization occurs, giving rise to a poorly crystalline phase resembling to those observed for mono-alkylated analogues at high temperature.

The crystalline phase remains predominantly lamellar as shown by the intense peak at 1.86 nm -1 for which a periodicity up to the fifth order was found at 343 K. This value corresponds to an interlamellar distance of about 33.7 Å, which is comparable to the one found for [Fe(L 4 ) 2 ] in the high-temperature phase (Cr3 : 34.5 Å). It should be noticed that the crystallization is a relatively slow-dynamic process and its observation was only possible due to the slow sweeping rate (1 K/min) and pre-acquisition isothermal equilibrium (typically 30 min) applied. The time evolution of the crystallization from the metastable liquid-like phase measured at 343 K is provided in Figure III-48 and reveals that at least 35 minutes are required for a complete solidification of the material.

Additional SAXS were performed to probe the behavior up to the melting temperature and revealed a reorganization of the material around 373 K into another poorly crystalline phase, while the conversion into the isotropic liquid phase was confirmed at 393 K (Figure III-49, left). Even though the experimental conditions were the same, almost no crystallization was detected during the cooling down to room Chapter III -Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials III.151 temperature and the material remains likely as a glass, confirming the monotrope thermal behavior predicted by DSC. This can be shown by the low intensity of reflection peaks in comparison to those recorded during the heating (see for example diffractogram at 343 K, Figure III-49, right). This was also confirmed by POM analysis, which revealed a lower kinetic for the formation of birefringent textures (several hours) during the cooling from the liquid phase than the first heating from the metastable liquid-like phase (see inset Figure III-49). It is worthwhile to mention that solidification/crystallization was detected at higher temperature during SAXS and POM than DSC analyses, and this can be explained by the slow dynamic of the thermal behaviors and differences of sweeping rates between the experiments. -50, left). By analogy with previous examples, the intense peak at 1.59 nm -1 was attributed to the lamellar organization and the interlamellar distance was calculated as 39.5 Å, which is favorably comparable to the distance, 42.1 Å, reported by Real and co-workers on a very similar analogue (which differs from L 8 ligand only by an additional methyl group on the Schiff base function). 15 Upon heating to 338 K, the crystalline organization is lost a the material become amorphous.

Chapter III -Mononuclear Fe II spin-crossover complexes: from crystalline toward soft materials III.152 This transition into a glass phase was observed in DSC, which shows an intense endothermic signal at 326 K (Figure III-50, right). The shape of the signal and enthalpy value (56.6 kJ/mol), is quite unsual for a glass transition and suggests that the transformation is associated or provoked by a SCO. This was confirmed thereafter by the magnetic measurements. On cooling from the liquid phase, the material does not crystallize again and solidify as a glass even when very slow sweeping rates are applied (no birefringent texture was observed).

III.3.4.2. Magnetic measurements

Preliminary magnetic susceptibility measurements on [Fe(L 7) 2 ] and [Fe(L 8 ) 2 ] were performed between 1.8 and 400 K with an applied field of 1 T. The χT versus temperature plot for [Fe(L 7 ) 2 ] is displayed in This particular behavior was confirmed on a second heating run, even though the stepwise and reverse-SCO effects are less pronounced, confirming the slow dynamic of the crystallization after the first thermal annealing (vide supra). Looking more closely these data, the crossover in the liquid and glass phases follow barely the same pathway, while the "bump" in χT is essentially associated to the partial crystallization occurring above 343 K, which suggests therefore that the crystalline phase is paramagnetic. Assuming that, we might eventually promote a fully paramagnetic phase by playing on the dynamic of the experiment.

Chapter III -Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials III.153 The reverse SCO effect was also visualized by the change of color as illustrated in the optical micrographs shown in top of Figure III-52. For that, the material was heat up to the metastable liquid-like phase (338 K) in order to form a homogeneous thin film and allowed to stand at 343 K for 40 minutes during which the material crystallizes progressively (the evolution of crystallization was checked under polarized light). As the temperature increase up to 383 K, the green material turns progressively toward olive green as a result of the gradual SCO, while a darkening of the color can be perceived around 390 K (at which the compound reach the liquid phase). Then, a gradual lightening of the color is again observed by increasing the temperature. On cooling, only a gradual change from olive green into dark green is shown, confirming the monotrope character of the material (Figure III-52 bottom).

The temperature dependence of χT product for [Fe(L 8 ) 2 ] is shown in Figure III-53. As the temperature increase an abrupt increase of the χT value is observed between 315 K and 343 K at which the value was 2.52 cm 3 •K•mol -1 , indicating and incomplete SCO transition. This latter correspond well to the transition from the poorly crystalline into glass phase deduced from DSC and SAXS experiment. Interestingly, such crystal-to-glass transition was not observed in the purely diamagnetic [Zn(L 8 ) 2 ] analogue, which remains crystalline up to the melting around 360 K, suggesting therefore that the loss of crystallinity is induced by the SCO transition (see Figure S.III-8). Similarly to [Fe(L 7 ) 2 ], a steep drop of the χT value to 1.92 cm 3 •K•mol -1 , is detected when the material starts to melt above 343 K, while the onset of a crossover is exhibited in the liquid phase (above 354 K). On cooling down to 343 K, the curve follows almost the same pathway as in heating and the χT value jumps again to 1.92 cm 3 •K•mol -1 as the result of the transition into glass phase. Below this temperature, the material does not return into the diamagnetic phase but exhibits a very gradual and incomplete crossover to reach a value of about 1.55 cm 3 •K•mol -1 at 50 K. This behavior was found perfectly reversible over cycling and found independent of the sweeping rate or applied magnetic field (up to 7 T), showing no orientation with field as previously described for SCO-based soft materials. 7 Although the introduction of additional dodecyl chains onto the ligand scaffold was unsuccessful to promote liquid crystalline properties, this strategy allowed us to reveal interesting effects of the functionalization. The two compounds melt at quite low temperature and can then be used to form homogeneous thin films. This is particularly interesting for [Fe(L 7 ) 2 ] that shows a reversible spin crossover behavior. Although incomplete, we revealed the existence of reverse-SCO phenomenon, which is very unusual and has been only observed on few examples of Co(II) SCO. 29 This latter is induced by a phase transition, which is to the best of our knowledge unprecedented.

III. 4. Conclusions and perspectives

In this chapter, we have been focused on the synthesis and study of pyridylbenzohydrazone-based SCO with ambition to induce liquid crystalline properties to these materials. The discovery of SCO features in this family of FeN 4 O 2 complexes is relatively recent and only a few examples are reported. [13][14][15] Therefore, we dedicated an important part of the project on the study of model complexes in order to learn more on the coordination chemistry and properties of these molecular systems. The main achievements of this work have been already summarized in section III.2.4 and we recall here the main conclusions to emphasize the motivations of our current and future work. Through the isolation of polymorphic forms of [Fe(L 1-3 ) 2 ] complexes, we have highlighted the strong impact of crystal packing effects on the magnetic properties. We have seen that no SCO is featured when the crystal packing involves a multitude of π-π interactions.

However, it remains adventurous to conclude on the basis of these few examples considering the complexity and diversity of supramolecular interactions found in these materials. New analogues, which include derivatives of HL 1 functionalized by various substituents (electro-donating or withdrawing functions and extended π-systems) in various positions of the ligand scaffold, are currently under investigation to have a better overview on the correlation structure-properties of these interesting molecular systems. Active researches are still in progress on the three molecular systems described in the manuscript, especially for [Fe(L 1 ) 2 ] that exhibits hysteretic spin transition processes in both polymorphic forms and for which a Chapter III -Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials III.155 transformation from the α-polymorph into the β-one have been observed for the solvated forms in the mother liquor (in methanol). One of our great motivation is to learn more about the mechanism of this transformation. In our recent work, we have demonstrated that a thermal activation is necessary since no conversion is occurring at room temperature even after a long stay in the mother liquor (2 months).

Experiments to probe the kinetic of the transformation at different temperatures are currently in progress by DSC and powder X-ray diffraction investigations. From our preliminary results, no evidence for the formation of intermediate polymorphic forms was revealed (in contrast with [Fe(tpa)(NCS) 2 ] previously reported showing the generation of two successive polymorphs 23 ), suggesting that the transformation involves a direct pathway. However, the chemistry of this complex is more surprising and rich than we thought. Indeed, an evolution with time from the solvent-free α-[Fe(L 1 ) 2 ] into a new γ-[Fe(L 1 ) 2 ] polymorphic form have been very recently found when the reaction is performed in acetonitrile instead of methanol (at room temperature). This latter have been structurally and magnetically characterized and found to exhibit an abrupt spin transition at 326 K without significant thermal hysteresis (selected crystallographic, calorimetric and magnetic data are shown for information in section III.5.5). Moreover, a transformation from α-[Fe(L 1 ) 2 ] toward another polymorphic form that have been yet clearly identified is occurring in acetonitrile upon thermal activation at 50 °C. In all the cases, polymorphs were obtained in a serendipitous fashion but this promising results should motivate us to systematically dedicate more energy on synthetic aspects for this family of complexes and more generally for SCO systems.

In the second part of this work, we were focused on the functionalization of the complexes with strongly lipophilic substituents with objective to promote mesomorphic features on the resulting materials.

Although, the strategy employed was inefficient to reach our initial goal, interesting features originating from the functionalization were highlighted during the work. Especially, we reported the formation of intermediate phase in which both LS and HS species coexist when a mono-alkylation (with dodecyl or octadecyl chains) is applied. This behavior observed in the three [Fe(L 4-6 ) 2 ] complexes, was attributed to a crystallographic symmetry breaking (CSB) occurring during the SCO process and have been unambiguously evidenced in one case, [Fe(L 4 ) 2 ], by single-crystal X-ray diffraction. Three additional examples, including one with shorter substituent (hexyl) and two analogues of [Fe(L 4 ) 2 ] and [Fe(L 6 ) 2 ] with additional substituents in the Schiff base function, have been recently developed in the group and the preliminary results seems to confirm similar behaviors supporting the key-role played by the alkyl chains on CSB process that accompanies the SCO process. In all the cases, irreversible transformations into new polymorphic phases were found upon thermal treatment at more or less high temperature. In the case of [Fe(L 4 ) 2 ] (and probably also [Fe(L 6 ) 2 ]), the material is converted into an poorly crystalline phase that has no SCO ability (paramagnetic), while [Fe(L 5 ) 2 ] undergoes transformation into a thermodynamic product, which displays a reversible SCO from a pure HS phase into mixed {LS•HS} phase. In the other side, we have seen that the introduction of multiple dodecyl chains gives access to soft materials with low-melting points. The transition into the liquid phase has a direct impact on the Fe(II) coordination sphere and a partial SCO is occurring as previously described by Real and co-workers on a series of analogue complexes. 15 However, our situation was different and the melting was accompanied by an unexpected reverse-SCO, which is to the best of our Chapter III -Mononuclear Fe II spin-crossover complexes: from crystalline toward soft materials III.156 knowledge unprecedented. Numerous studies might be realized in continuation to this work. Especially, one of our future directions could be the preparation of solid solutions of [Fe x Zn (1-x) (L) 2 ]. For complexes with mono-alkylated ligands, it would be indeed interesting to study the impact of these SCO-inactive spectator ions on the CSB process. In the case of [Fe(L 8 ) 2 ], which loses crystallinity upon SCO, the dilution in Zn(II) complex might help in this case to maintain it (since the [Zn(L 8 ) 2 ] was found crystalline up to melting) and thus obtain reversible SCO.

Of course, the elaboration of metallomesogens remains one of our main priority and several strategies to reach this goal are considered. First, we can introduce calamitic smectogen units (biphenyl or cyanobiphenyl), which have been successful to promote bilayer lamellar mesophase in the case of Mn 12 SMM (see Chapter II). 33 One other approach is to induce more geometrical constrains to the complex, especially by elaborating [2 x 2] grids in which the mesogenic substituents would be oriented perpendicular to the [Fe 4 L 4 ] core (Figure III-54). Although several grids based on pyrylaroylbenzohydrazide ditopic ligands have been recently reported with different 3d and 4f metal ions, 34 no example are known with Fe(II) ion. We have recently prepared one model Fe(II)-complex in collaboration with Kruger's group (University Canterbury, New Zealand) and studies are in progress to determine if this latter displays SCO features. The non-commercial 1d-g precursors were prepared according to procedures described in Chapter II.

All the ligands were prepared in the same fashion and the general procedure is given for HL 1 . To a solution of methyl 4-methoxybenzoate (10.0 g, 60.2 mmol, 1 eq) in hot ethanol (25 mL) was added hydrazine monhydrate (14.8 mL, 300.9 mmol, 5 eq) and the reaction mixture was refluxed for 16 h. After cooling down to room temperature, the resulting precipitate was collected by filtration, washed several times with water (3 x 25 mL), cold ethanol (3 x 10 mL) and dried several hours under vacuum. The precursor 2a was obtained as white powder (Yield 7.2 g, 72 %). To a sample of 2a (5 g, 30.0 mmol, 1 eq) in methanol (20 mL) was added 2-pyridinecarboxaldehyde (3.42 mL, 36.0 mmol, 1.2 eq) and the mixture was stirred at RT for 12 h. The white precipitate formed was collected by filtration. After washing with cold MeOH (3 x 5 mL) and Et 2 O ( 2x 10 mL), HL 1 was obtained as white powder O). 13 C{ 1 H} NMR (100 MHz, d 4 -MeOH): δ 166. 7, 164.6, 154.6, 150.1, 148.7, 138.6, 130.9, 125.9, 125.7, 122.2, 115.0, 56.0. Selected FT-IR data (ATR, cm -1 ): 1643 (vs, C=O), 1603, 1586, 1550, 1509, 1467, 1434, 1343, 1283, 1248, 1180, 1148, 947, 920, 841, 776, 744, 667, 633, 616. Elemental analysis -Calc. (Found) for C 14 H 13 N 3 O 2 : C, 65.87 (66.02); N, 16.46 (16.43); H, 5.13 (5.09 O), 1.45-1.23 (m, 36 H, CH 2 ), 0.88 (t, J = 5.9 Hz, 6H, CH 3 ). Selected FT-IR data (ATR, cm -1 : 2920 (ν C-H ), 2851 (ν C-H ), 1645 (ν C=O ), 1590, 1554, 1464, 1349, 1301, 1269, 1246, 1150, 1071, 1096, 776. Elemental analysis -Calc. (Found) for C 37 H 59 N 3 O 3 : C, 74.83 (74.42); N, 7.08 (7.11); H, 10.01 (10.89).

(E)-4-methoxy-N'-(pyridin-2-ylmethylene)benzohydrazide (HL

(E)-3,4,5-tris(dodecyloxy)-N'-(pyridin-2-ylmethylene)benzohydrazide (HL 8 ). Yield 95 %. 1 O), 1.5-1.0 (m, 54 H, CH 2 ), 0.88 (m, 9H, CH 3 ). Selected FT-IR data (ATR, cm -1 ): 2920 (ν C-H ), 2851 (ν C-H ), 1639 (ν C=O ), 1580, 1552, 1465, 1329, 1232, 1145, 992, 776, 756, 617. Elemental analysis -Calc. (Found) for C 49 H 83 N 3 O 4 : C, 75.63 (76.47); N, 5.40 (5.47); H, 10.75 (10.72).

III.5.1.2. Synthesis and characterization of Fe(II) complexes

Preparation of α-[Fe(L 1 ) 2 ] polymorph.

To a solution of HL 1 (100 mg, 3.92•10 -4 mol, 1 eq.) and trimethylamine (100 μL, 7.88•10 -4 mol, 2 eq.) in methanol (4 mL), was added a solution of Fe(ClO 4 ) 2 •6H 2 O (71.1 mg, 1.96•10 -4 mol, 0.5 eq.). The dark green solution was homogenized and then allowed to stand at room temperature for 24 hours. The resulting dark green needles of α-[Fe(L 1 ) 2 ]•MeOH were collected by filtration. The α-[Fe(L 1 ) 2 ] complex is obtained by natural desolvation of the complex in air : Yield 75 mg, 68 %. Elemental analysis -Calc. (Found) for C 28 H 24 FeN 6 O 4 : C, 59.59 (59.32); N, 14.89 (14.66); H, 4.29 (4.40).

Selected FT-IR data (ATR, cm -1 ): 1602 (ν C=N-N=C ), 1583 (ν C=N-N=C ), 1516, 1466, 1450, 1437, 1404, 1350 (ν C- O ), 1287, 1246, 1162, 1141, 1102, 1057, 1028, 917, 879, 838, 809, 755, 740, 694, 677, 625. 1 ) 2 ] polymorph. Fe(ClO 4 ) 2 •6H 2 O (71.1 mg, 1.96•10 -4 mol, 0.5 eq.) was added into a solution of HL 1 (100 mg, 3.92•10 -4 mol, 1 eq.) and trimethylamine (100 μL, 7.88•10 -4 mol, 2 eq.) in methanol (8 mL) C, 59.59 (58.89); N, 14.89 (14.70); H, 4.29 (4.57). Selected FT-IR data (ATR, cm -1 ): 1597 (ν C=N-N=C ), 1583 (ν C=N-N=C ), 1511, 1444, 1355 (ν C- O ), 1296, 1248, 1162, 1143, 1105, 1057, 1023, 919, 902, 850, 838, 763, 698, 674, 638, 615. Preparation of [Fe(HL 1 ) 2 ](ClO 4 ) 2 . HL 1 (100 mg, 3.92•10 -4 mol, 1 eq.) and Fe(ClO 4 ) 2 •6H 2 O (143 mg, 3.94•10 -4 mol, 1 eq.) were combined in methanol (5 mL) and the mixture was stirred at room temperature for 2 hours. The clear red-purple solution is then allowed to stand for slow evaporation. Red-purple plate-like crystal were collected after about 3 weeks: Yield 67 mg, 45 %. Elemental analysis -Calc. (Found) for C 28 H 26 Cl 2 FeN 6 O 12 : C, 43.94 (43.36); N, 10.98 (11.00); H, 3.42 (3.71).Selected FT-IR data (ATR, cm -1 ): 1588, 1562, 1503, 1468, 1357, 1314, 1303, 1262, 1175, 1096 (ClO 4 ), 1041, 1018, 909, 846, 773, 761, 742, 617.

Preparation of β-[Fe(L

II.5.2. Selected FTIR spectra

Selected bond distances

Fe(1)-N( 1) 1.955( 5) 1.965( 7) 2.194( 6)

Fe( 1)-N( 2) 1.858( 6) 1.868( 7) 2.061( 8)

Fe( 1)-N( 4) 1.965( 4) 1.951( 7) 2.213( 8)

Fe( 1)-N( 5) 1.869( 5) 1.855( 8) 2.059( 8)

Fe( 1)-O(1) 1.959( 5) 1.981( 5) 2.077( 5)

Selected bond distances

Fe(1)-N( 1) 1.936( 5) 1.956( 6)

Fe( 1)-N( 2) 1.855( 5) 1.872( 5)

Fe( 1)-N(4) 1.934( 4) 1.954( 5)

Fe(1)-N( 5) 1.849( 5) 1.860( 5)

Fe( 1

Selected bond distances

Fe(1)-N( 1) 1.947( 3)

Fe( 1)-N( 2) 1.863( 3)

Fe( 1)-N( 4) 1.871( 4)

Fe( 1)-N( 5) 1.986( 4) , where 𝜑 𝑖 are cis X-Fe-X bond angles with X = O or N.

IV.1. Introduction

The interest in cyanide-based single-molecule magnets (SMMs) and electron transfer (ET) systems is in direct line with the famous Prussian blue (PB) and its analogues. These systems have been intensively studied, in particular by the groups of Girolami, 1 Hashimoto,2 Verdaguer 3 and Miller. 4 An experimental and detailed theoretical understanding of the magnetic interaction through bridging cyanide ligands has been achieved in the relation to the involved metal ions and structural/geometrical parameters. However, the reduced solubility of these inorganic compounds limit their potential applications and requires elaboration of strategies for nanoparticles design, to increase their solubility and to create multi-functionalities at nanoscale. 5 Since then, a new cyanide-metallate research started towards the synthesis and investigation of discrete PB analogues that have flexible molecular and electronic structures coupled with improved solubility. Several groups designed and studied molecular analogues of PB by rational choice of various capping ligands. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] Many of these systems exhibited unusual properties such as single-molecule magnets, [7][8][9] spin crossover 10 and electron transfer. . These molecular squares are well-known to behave as single-molecule magnets 8,9 or display thermally and photo-induced electron transfer [15][16][17][18][19] in the case of Ni II and Co II analogues, respectively, and have been intensively studied by our team. 9,15,19 The aim of our study is to play on the ligand functionalization in order to promote liquid-crystal properties in the resulting materials or improve their solubility in usual organic solvents without altering significantly their intrinsic magnetic properties. This approach have been initiated during the PhD theses of D. Siretanu and D. Mitcov, during which bipyridine ligands were functionalized with alkyl chains (hexyl or tridecyl) and used for the preparation of {Fe 2 Co 2 } squares. 19 This strategy brought interesting properties and appeared to be an efficient way to control thermally-induced ET properties in solution. However, the functionalization was not appropriate to induce IV.188

The packing shows weak intermolecular π-π interactions between benzoate moieties of two adjacent complexes (3.37 Å, Figure IV-6, left)

IV.2.4. Thermotropic properties

In order to investigate thermotropic properties on {Fe 2 Ni 2 } squares, polarized optical microscopy (POM), differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) were performed.

Preliminary investigations on complexes with ligand L 3 and L 4 , (3 and 4, respectively) did not reveal any liquid crystalline behaviour. The two compounds melt into the liquid phase around 400 K based upon POM analysis, while the transition associated to the melting could not be clearly observed by DSC. On cooling, no birefringence texture could be detected down to room temperature, suggesting a poorly crystalline or amorphous behaviour. The SAXS profiles during the first heating displayed an intense reflection at 1.98 and 1.70 nm -1 for 3 and 4, respectively, as well as very weak peaks in low and wide angle region, confirming a poorly crystalline behaviour (Figure S.IV-2). The sharp reflection in the low-angle region was attributed to the interlamellar distance imposed by the dodecyl chains and the values are 31.7 and 37.2 Å for 3 and 4, respectively, indicating strong differences in the organization between the two compounds. On cooling, the intensity of reflections decreased significantly, indicating a slow-dynamic process for crystallization of the materials that solidify mainly as a glass as seen before in Chapter III. In contrast, the two squares containing When heating further, a transition occurs between 338 and 343K and the two reflections disappear, indicating that the material reaches the isotropic liquid phase. The behaviour is perfectly reversible and the liquid crystalline organization is recovered at 328 K during the cooling. The two sharp reflections observed at 1.61 and 3.18 nm -1 are in ratio 1:2, showing that the molecules form a lamellar arrangement. The distance between the d 001 and d 002 layers is almost independent of the temperature and was calculated as 39.1 Å (Table IV -4). This value is in fair agreement to the expected distance separating the biphenyl groups from each side of the square (Figure IV-9). This result suggests that the material exhibits a bilayer lamellar organization as found before in Mn 12 SMMs functionalized with biphenyl or cyanobiphenyl smectogen units (see in Chapter IV-4). This value is higher than the one found in 5, as depicted in other molecular systems previously described, and attributed to lower electrostatic interaction between biphenyl units. 19b,26 the observed/measured reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, d (obs) is observed and calculated diffraction spacings:

d (obs) = 2/q (obs) , d is interlamellar distance d = 1 𝑛 ∑ 𝑖 • 𝑑 00𝑖 𝑛 𝑖=1
IV.199

The packing of 7 shows a layered structure (Figure IV-16). The molecular squares are arranged face to face in pairs along the a-axis in the (ab) plane. The alkyl chains of adjacent layers are strongly interdigitated, separating the layers by 23.020 Å, corresponding to c parameter of the unit cell. The aliphatic chains are packed and separated by approximately 5 Å. Because of the van der Waals interactions between the hydrocarbon chains, the thermal motion is increasing on entirely chain. Because of the strong agitation of carbon atoms corresponding to the long alkyl chain the R 1 and wR 2 values are significantly high (Table IV-6). Therefore, the structure is emphasized in ball and sticks (Figure IV-15). Furthermore, the water molecules form hydrogen bonds with terminal cyanide groups (Figure IV-17).

IV.3.4. Magnetic measurements in solid state

Magnetic susceptibility measurements confirmed that the 7 does not undergo reversible thermally induced ET (Figure IV-18). The χT value from 1.8 to 305 K is nearly constant with the value of 0.11 cm 3 •K/mol confirming that 7 is in a diamagnetic {Fe II 2 Co III 2 } configuration as shown by XRD analysis and by its green color at room temperature. As the temperature increase from 306 to 336 K, the χT product increases abruptly up to 5.02 cm 3 •K/mol, indicating the occurrence of thermally induced ET (Figure IV-18, red line). The χT product is lower than the expected value ~ 7 cm 3 K/mol for two non-interacting Fe III LS (S = 1/2) and two Co II HS (S = 3/2) ions. The low value of the χT product may be explained by the presence of diamagnetic impurities such as free ligand. On cooling the reverse process is not observed and the material remains paramagnetic. This irreversible ET behavior is probably induced by the desolvation of the compound (MeOH), which is thereafter stabilized in the paramagnetic phase, as shown before in the analogue complex containing tridecyl substituents. 19 From 350 down to 240 K, the T product slightly Similarly, the temperature dependence of the χT product for complex 10 shows a value of 5.8 -5.9

IV.3.5. Characterization of {Fe 2 Co 2 } molecular squares in solution

The structural and magnetic studies showed that functionalized complexes 7-10 do not undergo reversible thermally (Figure S.IV-3) or photo-induced ET (according to reflectivity measurements) in solid state. By analogy, the previous analogues with 4,4-dimethoxy2,2'bipyridine ligands exhibited or not ET properties in solid-state depending on the nature of the counter anion and solvation, suggesting a strong influence of the packing on the redox phenomenon. 19b To determine whether ET process, associated with switching of the magnetic and optical properties, is favorable for our complexes, we performed measurements in diluted solutions.

As expected, complex 7 containing bipyridine ligands functionalized with long alkyl chains is soluble in a broad range of organic solvents (Table IV- 7). In most of the case, a change of color from red to green is observed upon cooling to 77 K (Figure IV-20), which is a clear indication for a conversion from the paramagnetic into the diamagnetic phase as the result of a thermally-induced ET process. Complex 9 was found poorly soluble in polar solvents and thus ET process occurs only in a limited number of solvents. For (eq. 3) for the two redox pairs, X is the {Fe(μ-CN)Co} fraction; X LT and X HT are low-temperature {Fe(μ-CN)Co} and high-temperature {Fe(μ-CN)Co} fractions, respectively; R is the ideal gas constant (8.314

J•K -1 •mol -1 )
and ΔH is the enthalpy change. The calculated ΔH, T 1/2 , and ΔS extracted for the fits are summarized in For both compounds thermodynamic parameters are in the same ranges indicating that two distinct substituents (dodexyloxyl vs. 4-(decyloxy)-1,1'-biphenyl) have almost the same electronic effects on the Cocoordination sphere and thus only a minor influence of the redox process in solution. More interestingly, the T 1/2 is strongly solvent-dependent and is shifter to higher temperature with increasing polarity index, as shown previously with other square analogues 19 In the same respective solvents, the T 1/2 values were found slightly higher than those found for the analogue with methoxy substituents and significantly higher compared to the analogue with methyl substituents. 15b,19 This results can be explained by the higher electrodonating of the alkyoxy substituents which stabilize the diamagnetic species at higher temperature.

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

IV.4. Conclusions and Perspectives

During this work, we were interested in the functionalization of magnetic complexes {Fe III M II } 2 (M = Co, Ni) molecular squares in order to promote liquid crystalline properties in these materials or improve their solubility in usual organic solvents. In the first part of the work, our strategy consisted to attach multiple lipophilic substituents onto the bipyridine platform or to graft smectogen units (biphenyl and cyanobiphenyl)

through an aliphatic spacer. These second approach was found successful to obtain the desired liquid-crystal properties, since all the complexes exhibited such features. The {Ni 2 Fe 2 } complexes 5 and 6 form bilayer lamellar mesophase in a large range of temperature as found before in one Mn 12 complex described in Chapter II. Upon functionalization with mesogenic substituents, the magnetic behaviour of {Fe III Ni II } squares seems slightly affected as shown by our initial magnetic investigations on complex 5. Nonetheless, the study revealed the preservation of SMM behavior and the magnetic parameters were in the same ranges to those obtained for the model complexes 1 and 2. The ac magnetic susceptibilities of these three compounds revealed ferromagnetic coupling within the complex, giving a ground state S = 3. The complexes showed temperature dependence of ac susceptibility when a static dc field of 1000-1500 Oe is applied and effective barriers Δ eff /k B ranging from 12 to 23 K were deduced. Unfortunately, such functionalization was found inappropriate to promote ET features in the {Fe 2 Co 2 } molecular squares and the complexes remains paramagnetic (red) in both solid-state and in solution. Thus, in the second part of this work, we attempt a new design strategy involving the grafting of mesogenic substituents onto the bipyridine ligand through an ether coupling. Four new tetranuclear new cyanido-bridged complexes, 7-10, have been synthesized and their properties in solid state and in solutions have been described by structural, spectroscopic and magnetic methods. No evidence of reversible thermally-induced ET in solid-state could be deduced in these materials.

In one case, complex 7, an irreversible and incomplete ET was found probably induced by the loss of solvent.

Optical studies performed in solutions revealed that complexes undergo thermally-induced intramolecular ET. It was found that the ET characteristic temperatures T 1/2 are strongly dependent of the nature of the solvent, confirming the results obtained in the team for other {Fe 2 Co 2 } analogues. 15b,19 Even though no mesomorphic feature was deduced, the second approach seems more appropriate for {Fe 2 Co 2 } squares since ET process was at least found in solution. These promising results should encourage us to tune the packing through the change of the solvation or counter anions in order to eventually promote ET in both solid and solution phase. We should also continue on the derivatization of this type of ligands by applying the same strategy as before (ie. grafting of additional greasy substituents and calamitic smectogens units) to promote mesomorphic features on the resulting materials.

Chapter

. , 10.45 (9.97). , 10.81 (11.11). 6.73 (7.11); N, 8.11 (8.60).

Preparation of 4,4'-bis((10-([1,1'-biphenyl]-4-yloxy)decyl)oxy)-2,2'-bipyridine (L 8

Preparation of {[(

{[(

IV.5.2. FTIR spectroscopic analyses

III.222

certain stress to the molecule. In the second part of the chapter, we described our attempts to promote liquid crystalline features on this molecular system by using strongly lipophilic ligands. Five complexes were synthesized and characterized magnetically, thermotropic and structurally. Thus, by increasing the chain length we could not induce liquid crystalline behavior, while was observed the presence of a liquid phase. Also, by multiplying the dodecyl chains, two compounds revealed same features as for the compound that contained one long alkyl chain. It was reported the formation of intermediate phase in which both LS and HS species coexist when a mono-alkylation (with dodecyl or octadecyl chains) is applied. This behavior observed in the three [Fe(L 4-6 ) 2 ] complexes and was explained by a crystallographic symmetry breaking (CSB) that is occurring during the SCO process and has been unambiguously evidenced in one case. From the other side, we have seen that the introduction of multiple dodecyl chains gives access to soft materials with low-melting points.

As our functionalization was accompanied by unexpected reverse-SCO behavior, numerous studies might be realized in continuation to this work. One of the perspective is the preparation of solid solutions of [Fe x Zn (1-x) (L) 2 ]. Also, the goal is still to obtain metallomesogens and several strategies to reach this are considered. Firstly, we can add the calamitic smectogen units (i.e. biphenyl, cyanobiphenyl), that have been applied successfully and lead to a bilayer lamellar mesophase for Mn 12 SMM (see Chapter II). The other goal is to induce more geometrical constrains to the complex, especially by elaborating [2 x 2] grids in which the mesogenic substituents would be oriented perpendicular to the [Fe 4 L 4 ] core.

Chapter IV is reported the functionalization of cyanido-bridged molecular squares with general Many attempts was done to induce ET in solid-state in {Fe 2 Co 2 } squares, unfortunately, all failed. These results confirmed the strong impact of crystal packing effects on the phenomenon. Even though, we should continue our investigations on the derivatization of this type of ligands by applying the same strategy as before to promote mesomorphic features on the resulting materials. 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. Magnetic data was corrected for the sample holder and diamagnetic contributions for all magnetic measurements. Photomagnetic measurements were performed at ICMCB using a halogen-tungsten 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.

 Mössbauer spectroscopy

The Mössbauer measurements were carried out on Mössbauer spectrometer HALDER type (ICMCB) constant acceleration using as a radioactive source at room temperature. The spectra were performed at 298 K using a cryostat. The refinement of the spectrum was carried out taking into account the distribution of hyperfine parameters.

A2: List of abbreviations

Fonctionnalisation rationnelle de matériaux moléculaires: vers des liquides et des cristaux-liquides magnétiques

Développer des méthodes efficaces pour mettre en forme les matériaux moléculaires magnétiques demeure un enjeu majeur et représente une étape essentielle en vue de possible applications. A cet égard, l'élaboration d'hybrides magnétiques présentant des propriétés de cristaux-liquides ou des systèmes solubles apparait comme une approche prometteuse. Ce travail de thèse a été axé sur la conception, la synthèse et la caractérisation de nouveaux hybrides basé sur des molécule-aimants (SMMs), des complexes à conversion de spin (SCO) et systèmes à transfert d'électrons (ET). Le chapitre I contient des informations générales et concepts théoriques sur ces trois classes de complexes magnétiques (SMM, SCO et ET) ainsi qu'un aperçu bibliographique sur les hybrides magnétiques connus. Le chapitre II est axé sur nos travaux de fonctionnalisation de molécules-aimants basés sur le complexe Mn 12 , en vue d'obtenir des phases cristaux-liquides. Dans le chapitre III, l'étude d'une série de complexes à conversion de spin de type Fe(II)pyridylbenzohydrazone au sein de phases cristallines ou de phases molles est décrite. Le chapitre IV est dédié à l'étude de complexes tetranucléaire a pont cyanure de type {Fe 2 M 2 } (M = Co 2+ , Ni 2+ ), qui sont connus pour présenter des propriétés de transfert de charge ou SMM avec l'ion Co(II) et Ni(II), respectivement. Dans ces trois chapitres expérimentaux, l'influence de la fonctionnalisation des ligands sur l'auto-organisation et les propriétés thermiques et magnétiques des matériaux résultants est discutée en détail.

Mot clés: magnétisme moléculaire ; cristal liquide ; molécule-aimant ; conversion de spin ; transfert d'électron ;

Rational functionalization of molecular materials: towards magnetic liquids and liquid crystals

Developing efficient methods to process molecular magnetic materials remains a considerable challenge and constitutes one of the critical steps toward possible applications. In this scope, the development magnetic hybrids featuring liquid crystal properties or improved solubility appears as a promising approach. This thesis work aimed to design, synthetize and characterize new hybrid materials based on the single-molecule magnets (SMMs), spin crossover (SCO) and electron transfer (ET) complexes. Chapter I contains general information and theoretical concepts on these three classes of magnetic complexes (SMMs, SCO and ET complexes), followed by a bibliographic survey on hybrid magnetic materials. Chapter II, rational is focused on the functionalization of Mn 12 -based SMM towards liquid crystalline phases. In Chapter III, a series of pyridylbenzohydrazone-based Fe(II) SCO complexes is investigated in both crystalline and softmatter phase. Chapter IV is dedicated to the study of cyanido-bridged {Fe 2 M 2 } molecular squares (M = Co(II), Ni(II)), which are known to exhibit SMM and thermally-or photo induced ET, respectively with Co(II) and Ni(II). In these three experimental chapters, the influence of ligand functionalization on self-organization, thermal and magnetic properties of the resulting materials is discussed in detail.

Firstly

  , I would like to thank Philippe Richetti, Cécile Zakri for welcoming me at the Centre de Recherche Paul Pascal. I would like to thank Prof. Michael Shatruk and Prof. Dominique Luneau for their time and consideration to review this thesis work; Prof. Cécile Zakri for her participation in my thesis defense jury. Thank you all for your time to evaluate this manuscript. I am deeply grateful to my supervisor, Rodolphe Clérac, for showing me an interesting research field, molecular magnetic materials. Thank you for the guidance, encouragement and support during this work.

I

  would like to mention many people who helped with scientific expertise and technical support: I thank Harald Bock and Fabien Durola for the discussions in organic chemistry and liquid-crystals; Elizabeth Hillard for advices in coordination chemistry and crystallization and Claude Coulon for making magnetism more easy to understand. I would like to thanks François Dole, Stéphanie Exiga, Xavier Brilland and Alain Derré for the training on DSC, NMR, mass spectroscopy, FTIR and Vis-UV instruments and their availability when there were technical problems. Finally I would like to thank Anne Facq, Jaon-Luc Laborde, Natalie Touzé, Corine Amengual, Elisabeth Hortolland, Béatrice Dupin, Fréderic Louerat, Mbolotiana Rajaoarivelo, Stéphane Bechemin and Lionel Chevalier for the administration, informatics, chemical and security support.

I

  would like to address many thanks to Dmitri Mitcov, Mihail Secu and Evangelia Koumousi for their help during my installation in France and precious guidance to deal with French administration. Thank you for your and friendly support and encouragement. Thank you to all my colleagues, who contributed to the development of a warm atmosphere at work: Kasper Steen Pedersen, Matias Urdampilleta, Daniel N. Woodruff, Vivien Pianet, Yoann Prado, Jun-Liang Liu, Anandi Srinivasan, Vianney Grau, Miguel Cortijo, Xiaozhou Ma, Marli Ferreira, Edivandro Girotto, Deise Santos, Gabi Cabral, Guillame Naulet and Antoine Robert. We had the opportunity to learn from each other not only science, but the traditions and interesting things about the countries from where we are coming. Thank you to my dear friends and collegues since bachelor Vladimir Bulicanu, Petru Apostol and Dumitru Samohvalov for being avaible to help and encourage me in difficult moments. I would like to thank Erasmus Mundus MID program for financial support. Finally, my hearty thanks are express to my family. I thank my loving, encouraging and patient husband, Gheorghe. Even if we were far away from each other during my PhD, he always understood and found time to listen to me. Thank to my darling parents: my father Razmic and my mother Vasilisa. Thank you my brother David and his wife Irina, my lovely niece Bianca and my mother-in-law Maria.

Figure I- 1 .

 1 Figure I-1. Crystallographic structure of the Mn 12 -OAc molecule (left) along the crystal a-axis and (right) along the crystal c-axis. Mn(IV) light green, Mn(III) dark green, O red, C grey. Hydrogens has been omitted for clarity.6,7

  the S T = 10 of Mn 12 -OAc, the spin splits into 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 (Figure I-2).

Figure I- 2 . 2 .

 22 Figure I-2. Potential-energy diagram for Mn 12 -OAc 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 . The spin corresponds to different projections (Figure I-2) of the total spin along the easy axis of the molecule (in the case of Mn 12 -OAc, it corresponds to the c-axis of the crystal Figure I-1, left). The highest sublevel m S corresponds to the lowest energy level. The energy barrier ∆ A , representing the difference between highest and lowest energy levels of the S T ground state, is equal to ∆ A = |D|•𝑆 𝑇 𝑧 2 for integer spins and ∆ A = |D|•𝑆 𝑇 𝑧 2 -1/4 for non-integer spins. The pathway to reverse the magnetization over the top of the energy barrier can only be reached by thermal energy.

Figure I- 3 .

 3 Figure I-3. Potential energy-diagram in the presence of a longitudinal magnetic field (Zeeman effect).

Figure

  Figure I-4. Relaxation of the magnetization for 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.20

= 2 .

 2 1•10 -7 s (Figure I-5, right). 3a

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. 21a (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.9) with Δ A /k B = 62 K, with τ 0 = 2.1•10 -7 s. 3a Below the blocking temperature, the magnetization versus field curves show hysteresis which depends on the sweeping rate of the magnetic field and becomes larger with temperature decreasing. A series of steps are also detected in the hysteresis loops of the Mn 12 -OAc (Figure I-6, left). 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 (Figure I-6, right). For these particular field values, tunneling of the magnetization is enabled, and an evident increase in the relaxation rate is observed.

Figure I- 6 .

 6 Figure I-6. (left) Magnetization vs. field plots for Mn 12 -OAc emphasizing the stepwise hysteresis loop at low temperature. (right) Double-well potential diagram in the presence of a longitudinal field. 9b Resonant magnetization corresponding to vertical "steps" occurs when the energy levels are aligned between of two halves of the diagram.

Figure I- 7 .

 7 Figure I-7. (left) (a) Structure of the trans-[Re IV Cl 4 (CN) 2 ] 2-complex. (b) Energy level diagram: The blue shading is schematic representation of the acoustic phonon density with the cut-off at the Debye temperature. The arrows indicate direct (blue), Raman (grey) and Orbach (green) relaxation pathways from state |KD 1 -› to |KD 1 + ›. (right) Frequency dependence of the in-phase (χ', top) and out-of-phase (χ", bottom) components of the ac magnetic susceptibility at different temperatures of a polycrystalline sample (Bu 4 N) 2 [Re IV Cl 4 (CN) 2 ]•2DMA under a 2000 Oe dc field. The solid line are the best fits to the generalized Debye model. 21b

Figure I- 8 .

 8 Figure I-8. Splitting of d orbitals in an octahedral field created by six ligands for Fe II (d 6 ) ion with the spectroscopic terms associated with each configuration.

  ) 6 ] 2+ (ptz = 1-propyltetrazole) is HS at room temperature and becomes LS at low temperature. The study of its optical spectra at 295 K and 10 K gives ∆ HS /k B = 16990 K and ∆ LS /k B = 27940 K.46

Figure I- 9 .

 9 Figure I-9. Tanabe-Sugano diagram for a transition metal ion with six d electrons, showing the energy of the excited ligand-field states in units of the Racah parameter of electron-electronic repulsion B relative to the respective ground state, versus the ligand-field strength 10Dq also in units of B.The calculation was performed using the electrostatic matrices in the strong-field.46,47

  5 and thus ln 𝑥 * 1-𝑥 * = 0. Then, ∆𝐻 -𝑘 B 𝑇 * • 𝑎 = 0 and 𝑎 = ∆𝐻 𝑘 𝐵 𝑇 * .

𝑇𝑇*

  = f(x min ) dependence (Figure I-10), where 𝑇 * = ∆𝐻 𝑘 B •𝑎

Figure I- 10 .

 10 Figure I-10. Temperature dependence of the paramagnetic molar fraction 𝑥 𝑚𝑖𝑛 for different values of degeneracy, a (ideal solution model).

Figure I- 11 .

 11 Figure I-11. (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 x min 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 andx min = 0.5. This phenomenon is a spin conversion. W/k B T* = 2,there is an abrupt curve corresponding to x min at T/T* = 1. This gives a first-order transition at T/T* = T c and the spin transition temperature is given by the expression W/k B T* = T c This phenomenon is a spin transition at the critical point.  W/k B T* > 2, the curve has an "S" shape, and for T/T* = 1, not only the x min = 0.5 solution is possible, but the Gibbs energy adopts two other local minimum values. A spin transition corresponding to a first transition is observed. In this case, a first order transition is associated with thermal hysteresis (Figure I-11, right).

Figure I- 12 .

 12 Figure I-12. Phase diagram for the regular solution mode with W > 0 in the plane ( 𝑇 𝑇 * , 2𝑘 𝐵 𝑇 * 𝑊 )

Figure I- 13 .

 13 Figure I-13. Jablonski-type diagram representing the mechanism for LIESST, 40 and reverse-LIESST 54 of d 6 octahedral transition metal complex (e.g. [Fe(ptz) 6 ](BF 4 ) 2 ). The first step is a spin-allowed excitation via green light (≈ 530 nm) or red light (≈ 820 nm), followed by two intersystem crossing processes ending at the HS or LS, respectively.

Figure I- 14 .

 14 Figure I-14. Interconversion between the paramagnetic (Fe III LS -CN-Co II HS ) and diamagnetic (Fe II LS -CN-Co III LS ) electronic configurations due to thermally and photo-induced metal-to-metal electron transfer processes.

  Scheme I-15. (left) General structure of Prussian blue analogues, A x M y [M′(CN) 6 ] l •nH 2 O (A = alkaline ion, yellow; M and M′ = transition metals, M green and M' pink ; 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. 56a

  Scheme I-16. (left) View of the molecular {[(pzTp)Fe III (CN) 3 ] 4 [Co II (pz 3 CCH 2 OH)] 4 }(ClO 4 ) 2 4H 2 O13DMF cube at 250 K. Color scheme: Fe(III) orange, Co(II) blue, C grey, O red, N light blue and B pink. Lattice solvents, anions, and hydrogen atoms are omitted for clarity. (right) Temperature dependence of the χT product for the {Fe 4 Co 4 } cube before (black) and after (red) irradiation, and after thermal quenching (blue). 84 Shortly latter, the same research teams consortium reported the first tetranuclear square, namely {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }(OTf) 2 •2H 2 O•4DMF (Tp* = tris(3,5-dimethylpyrazol-1-yl)hydroborate, bpy = 2,2-bipyridine, OTf = trifluoromethanesulfonate, Figure I-17), that represents the elementary face of the cubic unit in PBAs and displays reversible thermally and photo-induced ET. 85a The material exhibits a firstorder phase transition with a thermal hysteresis of 18 K (at 0.4 K) between 186 and 168 K, associated with an electron transfer process between paramagnetic {Fe III 2 Co II 2 } and diamagnetic {Fe II 2 Co III 2 } states (Figure I-17, right).

Figure I- 17 .

 17 Figure I-17. (left) View of {[(Tp*)Fe(CN) 3 ] 2 [Co(bpy) 2 ] 2 }(OTf) 2 •2H 2 O•4DMF square at 230 K. Color scheme: C, grey; Co, blue; Fe, orange; N, light blue; B, pink. Lattice solvent molecules, anions, and hydrogen atoms are omitted for clarity. (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 ). 85a

Figure I- 18 .

 18 Figure I-18. (left) Schematic representation of a crystalline, (center) liquid crystalline phase and (right) isotropic liquid.

  Figure I-19. Examples of calamitic (N-(4-alkoxybenzylidene)-4-alkylaniline) (left) and discotic (triphenylene derivatives) (right) molecules.

Figure I- 20 .

 20 Figure I-20. Schematic representation of nematic phase for calamitic molecules (left) and discotic molecules (right).

Figure I- 21 .

 21 Figure I-21. (left) Schematic representation of a smectic A phase (Sm A ) and (right) a smectic C (Sm C ) mesophase. The columnar mesophases are formed only for discotic molecules and results from the stacking of rigid cores in columns. The arrangement of columns forms a bidimensional organization. The symmetry of this 2D organization can be hexagonal (Col h ) (Figure 4c), rectangular (Col r ) and oblique (Col o ) (Figure I-22). In some cases, columns can arrange themselves into smectic layers and are referred as lamellar columnar Col L or form nematic phases (N col ).

Figure I- 22 .

 22 Figure I-22. (left) Schematic representation of columnar hexagonal (Col h ) phase, (center) columnar rectangular (Col r ) phase and (right) columnar oblique (Col o ) phase. Thermotropic cubic phases (also known as smectic D) are 3D ordered supramolecular systems with a multi-continuous, 106 or a discrete micellar structure, 106,107 that depends on the molecular self-organization in the cubic cell (Figure I-23). 108 Cubic phases are a special type of mesophases as their physical properties are not anisotropic due to the cubic symmetry. Therefore, the detection of cubic phases is complicated because of their optically isotropic (black) texture when observed under polarized light. Nonetheless, cubic mesophases can be evidenced by small-angle X-ray scattering measurements. Cubic mesophases of lyotropic liquid-crystals are more common and two distinct organization symmetries have been described. In the simplest case, molecules organize themselves into micellar assemblies, leading to symmetries with the and space groups (Figure I-23, top). 107b Bicontinuous organization with , and symmetries can be also observed. 106b These can be described as two interwoven networks as shown in Figure I-23 (bottom). However, thermotropic cubic phases are not so well understood since their observation is not so usual.

Figure I- 23 .

 23 Figure I-23. (top) Schematic representation of (left) Primitive cubic and (right) Body centred cubic phase. 107b (bottom) Network models of the three known biscontinuous cubic phases in soft matter: (left) 𝐼𝑎3 ̅ 𝑑; (center) 𝑃𝑛3 ̅ 𝑚; (right) 𝐼𝑚3 ̅ 𝑚. The two interpenetrating networks are coloured blue and yellow. White lines delineate the unit cell. 106b

  Figure I-24 has a calamitic geometry but it was rationally designed to Chapter I -Generalities and context I.32 form discotic complex with Pt II metal ion (square planar). In the last example of Figure I-24 (right), tetra(alkanoato)dimetal derivatives are known to have a paddle wheel architecture, and the functionalization of carboxylate ligands will preserve the discotic geometry, leading therefore preferentially to columnar phases. 114

Figure I- 24 .

 24 Figure I-24. Chemical structure of metallomesogens with predictable columnar organizations: (left) metallophthalocyanine, 112 (center) [Pt(Pzpy) 2 ](Cl) 2 (with Pzpy = 3-(3,5-bis(alkyloxy)phenyl)-(5-pyridin-2yl)pyrazole) 113 and (right) tetra(alkanoato)-dicopper(II).114

Figure I- 25 .

 25 Figure I-25. (left) Chemical structure for [Fe(L) 2 ](PF 6 ) and (right) Computational simulation of [Fe(L) 2 ] + emphasizing the calamitic shape of the metallomesogen. 115

Figure I- 26 .

 26 Figure I-26. (left) Schematic representation of structural changes associated with the CrLC transition triggering the spin crossover in [Fe(C n -tren)](Cl) 2 (with C n -tren = tris[3-aza-4-((alkoxy)-2-pyridyl))but-3-enyl]amine). (right) χT vs. T for n = 16, 18, 20., the black areas indicate the hysteresis loops, blue domain indicates crystalline phase and yellow domain indicates liquid crystal phase. 117

Figure I- 27 .

 27 Figure I-27. (left) Chemical structure of [Fe(R-Trz) 3 ] 2+ . (right) Schematic representation of the column formed by triazole ligands. The red disks represent the rigid core, blue disks represent the flexible shell and pink balls represent the Fe(II) sites.

-

  Fujigaya's group 119 and Gaspar's group 120 reported the formation of hexagonal Col h and lamellar columnar Col L mesophases on [Fe(R-Trz) 3 ](A) 2 (with RTrz = 5-bis(alkoxyloxy)-N-(4H-1,2,4-triazol-4yl)benzamide A = OTs -, OTf -, ClO 4

Figure I. 28 .

 28 Figure I.28. (left) Computational simulation emphasizing the bilayer lamellar-like mesomorphic organization of Mn12 complexes containing calamitic smectogen units (right) Unaltered magnetic behavior of SMM: temperature dependence of out-of-phase ac susceptibility, ″.123a

Figure I- 29 .

 29 Figure I-29. (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). 124

Figure I- 30 .

 30 Figure I-30. (left) Temperature-composition pseudo-phase diagram for binary [Fe(R-Trz) 3 ](OTs) 2 /toluene (R = C 18 H 37 ). red circle: melting observed in heating; blue square: gelation observed on cooling; violet line: T 1/2 ; (right) Thermal dependence of the magnetization for [Fe II (C 18 trz) 3 ](OTs) 2 /toluene (3 wt%). For this composition condition, the T 1/2 and melting temperature are synchronized. Inset: pictures corresponding to LS gel (pink) and HS liquid (colorless). 125

Figure I- 31 .

 31 Figure I-31. Formation of Mn 12 -C 18 /C 18 EO 10 aggregates through emulsion-assisted self-assembly method. 128

  Investigation of SCO in solution becomes obvious, as it could give information of substituent effects since in solution, crystal packing effects are excluded. 132 Same considerations have led to the exploration of electron transfer phenomenon in solution.88b

Figure I- 32 .

 32 Figure I-32. Schematic representation of [Fe III HS (sal 2 -OR 2 trien)] complex in solid state, and self-assembly in solution that induce temperature dependence spin crossover in solution. 130 As mentioned earlier, {[(Tp*)Fe(CN) 3 ] 2 [Co(R,R-bpy) 2 ] 2 } 2+ molecular squares (Tp* = tris(3,5dimethylpyrazol-1-yl)hydroborate, R,R-bpy = 4,4'-disubstituent-2,2-bipyridine with R = H or Me) have been studied in solution and the strong influence of the solvent on the T 1/2 was shown. 85b More recently, the

  Hayami and co-workers reported examples of [Co(C n -terpy) 2 ](BF 4 ) 2 complexes (terpy = 2,2':6,2"terpyridine) functionalized with long alkyl chains (n = 14 and 16).133 While the parent [Co(terpy) 2 known to exhibit gradual SCO, 134 they reported in these complexes reverse-SCO.

  Figure I-33. Structures of the [Co(C 14 -terpy) 2 ](BF 4 ) 2 •MeOH in the HS2 phase at 190 K (a), the HS1 phase at 190 K (b), and the LS phase at 10 K (c). Lattice solvent and counter-anion are omitted for clarity. (center) χT vs. T plots for [Co(C 14 -terpy) 2 ](BF 4 ) 2 •MeOH. (right) χT vs. T plots for [Co(C 16 -terpy) 2 ](BF 4 ) 2 . 133

Figure I- 34 .

 34 Figure I-34. (left) Chemical structure of [Fe(L) 2 ] (L = pyridylmethylbenzohydrazone functionalized by alkyl chains (R = C n H 2n+1 with n = 12, 14, 16; R' = C n H 2n+1 with n = 1, 2, 4, 5) (right) χT vs. T plots for various [Fe(L) 2 ] complexes showing SCO/melting synchronism. 136

Figure II- 1 .

 1 Figure II-1. Schematic representation emphasizing the advantages of SMM-based hybrids: (left) the formation of fluid phases that can help to organize, align and orient SMMs and (right) the isolation/protection of magnetic cores by an organic shell.

4 O 4

 44 connected to a non-planar crown of eight Mn III ions through eight μ 3 -O 2-ions and four bridging carboxylates in axial positions (two in each side of the Mn III crown; see the structure of Mn 12 -OAc in Figure II-2). The eight Mn III ions from crown are interconnected by twelve bridging carboxylate ligands (eight in equatorial positions and four in axial positions).

Figure II- 2 .

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

  (H 2 O) 4 ] are arranged in a 2:0:2 fashion (Figure III-3(b)), 8 while the compound [Mn 12 O 12 (CHCl 2 COO) 8 (tBu-CH 2 COO) 8 (H 2 O) 3 ] contains only three water molecules with a 2:0:1 arrangement (Figure III-3(c)).9

Figure II- 3 .

 3 Figure II-3. Schematic representation of Mn 12 cores with different numbers and positions of H 2 O ligands: (a) [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ], 1:1:1:1 isomer, 1a (b) [Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ], 2:0:2 isomer, 8 (c)

Figure II- 5 .

 5 Figure II-5. Views of Mn 12 cores parallel to the molecular plane in: (a) Mn 12 -OAc with all JT axes nearly perpendicular to the molecular plane; 1c (b) [Mn 12 O 12 (CHCl 2 COO) 8 (Bu t -CH 2 COO) 16 (H 2O) 3 ] with one abnormally oriented JT axis.12 Color scheme: normally oriented JT axes are shown in blue and the abnormally oriented JT axis is shown in light green; Mn III green, Mn IV green, O red.

  ligands on the equatorial or on the axial positions of the Mn 12 . Using a selective functionalization of the equatorial positions with mesogenic ligand; we can expect that the complexes will adopt a discotic geometry, which will lead to nematic or columnar mesophases according to the intermolecular cohesion,(Figure II-6, left). In the second case, we can expect that the complexes which should have a rod-like shape (calamitic) will favor the formation of smectic or eventually nematic phases at high temperature(Figure II-6, right).

Figure II- 6 .

 6 Figure II-6. Schematic representation and correlation between the molecular geometry and expected supramolecular organization

  ), which are assigned respectively to ν OCO(as) and ν OCO(s) stretching vibrations of coordinated benzoate in bridging mode. The ν OCO(s) stretching band associated to the acetate groups at 1381 cm -1 in the [Mn12 O 12 (CH 3COO) 16 (H 2 O) 4 ] spectrum (Figure S.II-1), II.62

  groups on non-equivalent binding sites (Figure S.II-5, top left). 13a The 1 H NMR spectra show also broad signals corresponding to the aliphatic shell and pending biphenyl units, while the proton signals associated to the benzoate ring are completely extinguished (from the expected range ca. 6-8 ppm) due to their close proximity with the paramagnetic core (Figure S.II-5, bottom). This assignment confirms that the complexes remain intact in solution and no free ligand is present after the purification.

Figure II- 7 .

 7 Figure II-7. FTIR spectra at room temperature: (left) of HL 1 and [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ]; (right) of HL 2 and [Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ].

  Figure II-8 and X-ray data parameters are summarized in Table S.II-1.

Figure II- 8 . 64 (

 864 Figure II-8. Crystal structure of [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ] at 120 K viewed in the bc plane (left) and in the ac plane (right). Hydrogen atoms are omitted for clarity. Color scheme: Mn IV green, Mn III dark green, O red, C grey, F yellow.

Figure II- 9 .

 9 Figure II-9. Views of the Mn 12 core in [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ] illustrating: (left) the arrangement of the four H 2 O ligands (2:0:2 isomer), (right) the orientation of JT axes (in blue). Color scheme: Mn IV green, Mn III dark green, O red.

Figure II. 10 .

 10 Figure II.10. Crystal structure of [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ]•Et 2 O at 120 K viewed in the ac plane (left) and in the ab 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, F yellow.

[

  Mn12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2O) 4 ] shows the characteristic absorption bands of bridging carboxylate ligands at 1603 (ν OCO(as) ) and 1393 cm -1 (ν OCO(s) ) (FigureII-11, left).However, some absorption bands (1673 and 1426 cm -1 of the [Mn 12O 12 (CF 3 COO) 8 (H 2 O) 4 ] precursor)found in the spectra of the homo-substituted precursors (FigureII-11, left), are not present anymore suggesting the removal of benzoate and trifluoroacetate ligands from the axial and equatorial positions, respectively, and confirming the absence of these precursors in the sample. The same features were also observed for the three mixed-carboxylate complexes containing mesogenic ligands(Figures II-11, right and II.66

Figure II- 11 .

 11 Figure II-11. (left) FTIR spectra for [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ] and its precursors; (right) FT-IR spectra for [Mn 12 O 12 (CF 3 COO) 8 (L i ) 8 (H 2 O) 4 ] (i = 1-4).

  1 and the TGA thermograms are given in the Figure S.II-6. The elemental analyses, performed on powder for all the samples, are in fair agreement with the proposed formula [Mn 12 O 12 (CF 3 COO) 8 (L i ) 8 (H 2 O) 4 ], although, II.67best agreements require addition of water molecules (for L 1x = 5; L 2x = 5.5; L 3x = 2; L 4x = 5, as co-crystallized solvent. The presence of co-crystallized water molecules was confirmed by TGA, which show the corresponding weight losses in the range 350-400 K. It should be nonetheless mentioned that the exact number of water molecule cannot be determined accurately since the weight of water molecules correspond to less than 1% of the overall sample weight.

Figure II- 12 .

 12 Figure II-12. Temperature dependence of χ'T and χ" for [Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ] (left), [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ] (center) and [Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ] (right) measured under various oscillating frequencies (1-1500 Hz) with H dc = 0 Oe.

Figure II- 13 .

 13 Figure II-13. Temperature dependence of χ'T and χ" for [Mn 12 O 12 (CF 3 COO) 8 (L 1-4 ) 8 (H 2 O) 4 ] measured under various oscillating frequencies (1-1500 Hz) with H dc -0Oe. For [Mn 12 O 12 (CF 3 COO) 8 (L 2-4 ) 8 (H 2 O) 4 ] complexes, the χ'T values in the plateau 8-12 K range from 35 to 37 cm 3 •mol -1 (Figure II-13), which is consistent with S T = 9 with g factors of 1.72 -1.86, as previously reported for their respective homo-substituted precursors [Mn 12 O 12 (L 2-4 ) 16 (H 2 O) 4 ], 16 and relatives

Figure II- 14 .

 14 Figure II-14. Inverse temperature dependence of the relaxation time in semi-logarithmic scale for [Mn 12 O 12 (CF 3 COO) 8 (L 1-4 ) 8 (H 2 O) 4 ] complexes. Full lines are the best fits to an Arrhenius law.

  12 derivatives and relevant of SMM behaviour. The χT versus T data are shown in the Figure II-15. The χT values at 280 K are in the range 17 -19 m 3 Kmol -1

of 35 -

 35 Figure II-15. χT versus T plots at 0.1 T for [Mn 12 O 12 (CF 3 COO) 8 (L i ) 8 (H 2 O) 4 ] (i = 1 -4) complexes.

  varying between -7 to 7 T, reveals the presence of hysteresis loops(Figure II-16). The coercive field μ 0 H C (ie, the magnetic field required to inverse the magnetization) is around 1.5 T for [Mn 12O 12 (CF 3 COO) 8 (L i ) 8 (H 2O) 4 ] (i = 1, 2, 4) while it is found much lower for the mixed-carboxylate complex containing L 3 and for the homo-substituted complexes [Mn 12 O 12 (L i ) 16 (H 2O) x ] (x = 3-4; i = 1-4)

Figure II- 16 . 4 ) 8 (

 1648 Figure II-16. Magnetization vs. applied at 1.9 K and at 0.035 T•min -1 sweep rate for all [Mn 12 O 12 (F 3 CCOO) 8 (L 1- 4 ) 8 (H 2 O) 4 ] complexes, the right figure is a zoom of the figure on the left side.

  μ B ) at high fields show an absence of saturation (Figure S.II-8) which is consistent with magnetic anisotropy and high spin ground state (S T > 8) in a randomly oriented polycrystalline samples. This is also confirmed by the M vs. HT - 1 plots (Figures S.II-7) at different temperatures (from 1.9 to 10 K).

Figure II- 17 .

 17 Figure II-17. DSC thermograms for [Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ] (10 K/min).

Figure II- 18 .

 18 Figure II-18. (left) Small-angle X-ray scattering profiles for [Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ] during the 1 st heating from 313 K to 473 K; (right) Indexed SAXS diffraction profile at 383 K.

  The examination of general and systematic absences and fails 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) space groups. The presence or absence of center of symmetry cannot be detected on the basis of these experiments (Friedel's law), 21 therefore all non-centrosymmetric groups (with Laue classes 23, 432 and 43m ) are unlikely. This leaves only 3 Pm and 3

Figure II- 19 .

 19 Figure II-19. Schematic representation of the primitive cubic crystalline lattice 𝑃𝑚3 ̅ 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 update the caption.

  Chapter II -Rational Functionalization of Single-Molecule Magnets: Towards LC Phases II.76 Thus, up to seven sharp reflections appears in the low-angle region (Figure II-21, left). This liquid crystalline organization, is then maintained on cooling in all the range of temperature tested (Figure II.21, right).

Figure II- 20 .

 20 Figure II-20. DSC thermogram of [Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ] complex on 3 rd cycle of heating and cooling between 200 and 420 K (10 K/min).

Figure II- 21 .

 21 Figure II-21. Small angle X-Ray diffraction profiles in logarithmic scale for [Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ]: (left) on 3 rd heating between 263 and 363 K in high angle region; (center) on 3 rd cooling between 363 and 298 K in high angle region; (right) indexed diffraction profile of [Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ] on 3 rd cooling cycle at 363 K, only the small angle region is shown.

  ) and (321), which is once again well compatible with a primitive cubic lattice with 𝑃𝑚3 ̅ 𝑚 symmetry (Figure II-21 Table S.II-4). The unit cell parameter a is almost the same in all the range of temperature (298 -423 K) and about 39 Å. (Figure S.II-13 and Table

Figure II- 22 .

 22 Figure II-22. Birefringent textures observed by POM for [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ]: (left) at 338 K after cooling from isotropic liquid phase and (right) at 298 K.

Figure II- 23 .

 23 Figure II-23. DSC thermograms For [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ] complex between 280 and 420 (10 K/min).

Figure II- 24 .

 24 Figure II-24. SAXS diffraction profiles for [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ]: (left) on 1 st heating between 303 and 353 K; (center) on 2 nd cooling between 343 and 303 K; (right) indexed SAXS diffraction profile on 1 st heating cycle at 303 K.

  be more likely described by the previously reported bilayer model in which the mesogenic substituents are tilted in both side of the Mn 12 core (along the main molecular axis) to form the lamellar organization (FigureII-25).

Figure II- 25 .

 25 Figure II-25. Schematic representation of the lamellar organization of [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] complex inside the smectic mesophase.

2 .

 2 Figure S.II-2. FTIR spectrum of [Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ] and its precursors.

Figure S.II- 3 .

 3 Figure S.II-3. FTIR spectrum of [Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ] and its precursors.

Figure S.II- 4 . 7 .

 47 Figure S.II-4. FTIR spectrum of [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ] and its precursors.

Figure S.II- 8 .

 8 Figure S.II-8. M vs H/T plots at 1.9 K for [Mn 12 O 12 (CF 3 COO) 8 (L 1-4 ) 8 (H 2 O) 4. The solid lines guides for the eyes.

Figure S .

 S Figure S.II-10. d (obs) vs 1 √ℎ 2 + 𝑘 2 + 𝑙 2 ⁄ plot for [Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ] at 383 K. The black line corresponds to the best linear fit. The lattice parameter a = 35.21 Å corresponds to the slope of the linear fit.

Figure S.II- 11 .

 11 Figure S.II-11. SAXS diffractograms for [Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ]: (left) on first heating between 303 and 358 K; (right) on first cooling between 358 and 298 K

Figure S .

 S Figure S.II-13. d (obs) vs 1 √ℎ 2 + 𝑘 2 + 𝑙 2 ⁄ plot for [Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ] at 343 K. The black line corresponds to the best linear fit. The lattice parameter a = 38.95 Å corresponds to the slope of the linear fit.

Figure S.II- 14 .

 14 Figure S.II-14. SAXS diffractograms for [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ]: (left) on 3 rd heating from 343 to 368 K and (right) on 3 rd cooling from 363 to 343 K.

9 , 10

 910 Such realization was demonstrated for the first time in 2002 by Fujigaya and coworkers, which reported a series of [Fe(RTrz) 3 ](ClO 4 ) 2 complexes (with RTrz = 5-bis(alkoxyloxy)-N-(4H-1,2,4-triazol-4-yl)benzamide) in which the SCO is triggered by a crystal-to-mesophase transition. 9 Despite

III- 1 .

 1 Figure III-1. Spin transition induced by a crystal-to-liquid phase transition. Reprinted and adapted from ref. 15.

  is actually a kinetic product that undergo transformation into the thermodynamic one, β-{[Fe(L 1 ) 2 ]•MeOH}, upon thermal activation. Single-crystals of each polymorphic form were isolated as dark green thin needles, which are undistinguishable from each other by eyes.Typically, α-{[Fe(L 1 ) 2 ]•MeOH} single crystals were obtained after mixing the different reagents in methanol and allowing the solution to stand at RT for one day. Same result can be obtained using slow diffusion technique, by layering two methanol solutions containing in one side HL1 and triethylamine and in the other side Fe(ClO 4 ) 2 •6H 2 O (crystals are obtained within 24 hours). For the β-form, crystals were obtained using the first procedure at 50 °C. By analogy with our previous observations in powder samples, when a solution containing α-{[Fe(L 1 ) 2 ]•MeOH} crystals is warmed to 50 °C, a complete conversion into the β-phase is observed within 24 hours. From this experiment, it is nonetheless not possible to determine if this phase conversion is due to a crystal-to-crystal transition or if it is due to a progressive dissolution of α-{[Fe(L 1 ) 2 ]•MeOH} complex in the hot solution, which recrystallizes thereafter directly in the β-phase.However, when the reagents are directly dissolved in hot methanol(50 °C), the analysis of the first crystals formed revealed essentially the α-phase, supporting the hypothesis of a crystal-to-crystal transformation from kinetic into thermodynamic phase. When separated from the mother liquor, α-{[Fe(L 1 ) 2 ]•MeOH} complex does not undergo transformation into β-phase. Instead, the complex loses rapidly its methanol molecule, leading to the solvent-free α-[Fe(L 1 ) 2 ] complex. Although the process is much slower, a complete desolvation of β-{[Fe(L 1 ) 2 ]•MeOH} is also occurring by natural evaporation, giving rise to β-[Fe(L 1 ) 2 ] form.Both [Fe(L 1 ) 2 ] polymorphs are thermodynamically stable since no interconversion between the two phases was detected over time or thermal treatments (from 1.8 K to 470 K).Chapter III -Mononuclear Fe II spin-crossover complexes: from crystalline toward soft materialsIII.104 Due to loss of solvent, a complete characterization of the two [Fe(L 1 ) 2 ]•MeOH products was not possible but their polymorphism was nonetheless evidenced by single-crystal X-ray diffraction analysis (vide infra). The chemical and phase purity of the two samples were mainly deduced from the characterization on the dried compounds (including EA, mass spectroscopy (ESI-MS), FTIR, single-crystal or powder X-ray diffraction and DSC) combined with the analysis of the desolvation process by TGA. FTIR was found to be a very convenient technique to routinely identify the nature of different phases in bulk samples, which was systematically confirmed by X-ray diffraction, calorimetric and/or magnetic measurements (vide infra). The FTIR spectra of desolvated complexes and the ligand are compared in FigureIII-2.

Figure III- 2 .

 2 Figure III-2. (left) FTIR spectra for HL 1 (black), α-[Fe(L 1 ) 2 ] (dark green) and β-[Fe(L 1 ) 2 ] (olive green). (right) zoom of the region 1000 -550 cm -1 emphasizing the difference between the polymorphs.The strong absorption band at 1643 cm -1 ascribed to ν (C=O) stretching vibration in the free ligand is missing in FTIR spectra of isolated complexes confirming its absence in the bulk samples(Figure III-2). The spectra display intense double bands in the region 1602 -1583 cm -1 and strong peaks at 1350 cm -1 (α-phase) and 1355 cm -1 (β-phase), which are assigned to ν (C=N-N=C) and ν (C-O) , respectively, confirming the presence of chelating ligands in the enolate form.13a,b,14 The formation of the expected neutral complexes is also supported by the absence of the strong band around 1050 -1100 cm -1 expected for perchlorate-containing samples. The two spectra are not completely superimposable and strong differences are especially found in the far-infrared region. This reflects the difference in the bending vibration modes of aromatic C-H as consequence of distinct crystal packing in αand β-phase.Powder samples of the dried complexes have the same color as their respective solvated forms, ie. dark green and olive green for αand β-phase, respectively. Based upon previous descriptions on closely related complexes,15 these colors suggest that α-[Fe(L 1 ) 2 ] and β-[Fe(L 1 ) 2 ] are in the low-spin (LS) and high-spin (HS) configuration, respectively. The two dried compounds exhibit thermochromism (dark green ↔ olive green) upon heating at 380 K (α-phase) or cooling in liquid nitrogen (β-phase), suggesting that both

Figure III- 3 .

 3 Figure III-3. ORTEP views of α-{[Fe(L 1 ) 2 ]•MeOH} (left) and of β-{[Fe(L 1 ) 2 ]•MeOH} (right) at 120 K. Thermal ellipsoids are depicted at 50 % probability. The blue dashed lines correspond to hydrogen bonds. Color Figure: Fe orange, O red, C grey, N blue, H light pink.

  ) 2 ]•MeOH} are 1.953, 1.865 and 1.976 Å, respectively, confirming unambiguously that the compound is in LS state at 120 K. The coordination bond lengths in the β-polymorph are much longer, Δ Fe-X of ca. 0.2 Å, and the III.107 volume of the FeN 4 O 2 pseudo-octahedron is about 27 % bigger than the previous one, pointing clearly the HS configuration (Tables III-1 and S.III-1).These results nicely illustrate how the coordination sphere of a complex having the same chemical composition can be affected and stabilized in different spin configuration depending on effects arising purely from the crystal packing. From the multiples weak interactions that characterize the two distinct polymorphs, we should highlight the different manner that have the complexes for interacting with the methanol molecules through hydrogen bonds and with neighboring complexes through π-π contacts(Figure III-4).

Figure III- 4 .

 4 Figure III-4. Crystal packing showing the π-π interactions (red dashed lines) and hydrogen bonds (blue dashed lines) in α-{[Fe(L 1 ) 2 ]•MeOH} (top) and β-{[Fe(L 1 ) 2 ]•MeOH} (bottom). Other weak interactions are omitted for clarity. Color Figure: Fe II LS orange, Fe II HS green,O red, C grey, N blue, H light pink. In α-phase, the methanol molecule is hydrogen bonded with one hydrazone ligand via a nitrogen atom (N6•••O5, ca. 2.85 Å), while the second ligand establishes π-π interactions (C2•••C13, ca. 3.32 Å) with two neighboring complexes, creating a 1D supramolecular chain along c-axis (see top of Figure III-4). The presence of these π-π interactions might be responsible for the slight dissymmetry on the coordination bond lengths that were found systematically longer when provided by the ligand featuring this type contact. In βpolymorph, a hydrogen bond is also formed between the methanol molecule and the complex (see Figures III-3 and III-4 bottom), but this time through an oxygen donor (O3•••O5, ca. 2.87 Å) instead of a nitrogen, causing a dissymmetry on the Fe-O bonds (Fe1-O3 is ca. 0.03 Å shorter than Fe1-O1). It is reasonable to

) 2 ]

 2 •MeOH in the paramagnetic β-phase was tentatively attributed to the multiple π-π interactions. Each complex interacts with two neighboring complexes through a double π-π contact (C3•••C8, ca. 3.31 Å, and C5•••C12, ca. 3.36 Å), generating a supramolecular chain along a-axis (Figure III-4, bottom). The π-chains dimerize through an additional π-π contact involving C4 atoms from an adjacent chain (C4•••C4, ca. 3.37 Å). π-π contacts play a critical role on the cooperativity of SCO through elastic interactions, however, it is reasonable to assume that the proliferation of such interactions in β-{[Fe(L 1 ) 2 ]•MeOH} packing might create a certain frustration on the Fe(II) coordination sphere, which needs to expand or contract to undergo spin configuration change and ultimately prevent from SCO. This hypothesis would need nonetheless to be confirmed by the analysis of the crystal packing of the desolvated complex that exhibit spin transition at low temperature and important efforts are in progress in order to obtain the crystal structure of this material. The stabilization of the HS configuration might also arises from other supramolecular effects that did not appear clear in our eyes, which include multiple weak C-H•••X interactions (with X = N, O, π). Powder X-ray diffraction measurements were performed in order to determine whether the collected single-crystals were representative of bulk samples. The measurements were carried at RT using wet ground crystals (β-phase) or directly in the mother liquor (α-phase). The diffractograms are shown in Figure III-5 and compared with simulations provided by the crystal structure determinations at 120 K.

Figure III- 5 .

 5 Figure III-5. Experimental (298 K) and simulated (120 K) powder diffraction patterns for α-{[Fe(L 1 ) 2 ]•MeOH} (left) and β-{[Fe(L 1 ) 2 ]•MeOH} (right). Note: for convenience reasons, measurements were performed on a SAXS setup that is not appropriate for high resolution determination when used for wide windows of angles.

) 2 ]

 2 •MeOH products are subject to complete desolvation, leading therefore to two distinct solvent-free polymorphs. This was verified by TGA measurements on freshly filtered samples that show a weight loss of 5.4 -5.5 %, corresponding perfectly to the expected value, 5.37 %, for the loss of one methanol molecule (Figure III-6). For α-{[Fe(L 1 ) 2 ]•MeOH}, the desolvation process is very fast, even at room temperature. The TGA experiment showed that the methanol loss is actually completed during the initial isothermal equilibrium at 304 K (see the inset on left Figure III-6). From the weight loss vs. time plot, a half-time decay t 1/2 of about 6 minutes can be estimated while only traces of methanol remain after 1 hour.

Figure III- 6 .

 6 Figure III-6. (top left) TGA thermogram for α-{[Fe(L 1 ) 2 ]•MeOH} (inset) time dependence of the weight loss at 304 K. (top right) TGA thermogram for β-{[Fe(L 1 ) 2 ]•MeOH}.(inset) zoom highlighting the MeOH loss. (bottom) packing emphasizing the environment of MeOH molecules in α phase (left) and β phase (right). MeOH is shown in blue, O-H•••X (X = N or O) in dashed blue lines and C-H•••X (X = O, π) in dashed pink lines.

Figure III- 7 .

 7 Figure III-7. Projection of the crystal packing along c-axis and structure for α-{[Fe(L 1 ) 2 ]•MeOH} at 120 K (left) and α-[Fe(L 1 ) 2 ] at 120 K (middle) and 370 K (right). The FeN 4 O 2 coordination spheres are represented in orange and green pseudo-octahedrons for the LS and HS configurations, respectively.

  2 O or Chapter III -Mononuclear Fe II spin-crossover complexes: from crystalline toward soft materials III.112 hexane) were performed, but it remains unsuccessful so far. Nonetheless, powder X-ray diffraction experiments confirmed that the compound is structurally different from α-[Fe(L 1 ) 2 ] complex, reflecting their polymorphism (Figure III-8, right).

Figure III- 8 .

 8 Figure III-8. (left) Experimental (298 K) and simulated (120 K) powder diffraction patterns for α-[Fe(L 1 ) 2 ]. (right) Comparison between powder diffraction patterns of α-[Fe(L 1 ) 2 ] and β-[Fe(L 1 ) 2 ] (298 K). Note: For convenience reasons,

  ) 2 ] analogue, is shown in Figure III-9 and selected magnetic and thermodynamic parameters are summarized inTable III-3.

Figure III- 9 .

 9 Figure III-9. (left) Temperature dependence of the χT product for α-[Fe(L 1 ) 2 ] with an applied dc field of 1 T (scan rate: 0.14 to 15 K/min). (inset) dχT/dT vs. T plots at 0.3 to 15 K/min. (right) DSC data for α-[Fe(L 1 ) 2 ] between 220 and 390 K.

  Figure III-9, right), while the hysteresis loop width is also found independent of the scan rate (See Figure S.III-5).

Figure III- 10 .

 10 Figure III-10. (left) Temperature dependence of the χT product for β{-[Fe(L 1 ) 2 ]•MeOH} with an applied dc field of 1000 Oe, (inset) 1/χ vs. T plot. Solid red lines are the best fits to Curie-Weiss law. (Right) Temperature dependence of the χT product for β-[Fe(L 1 ) 2 ] with an applied dc field of 10000 Oe. (inset) dχT/dT vs. T plots at 0.3 K/min.

  Figure III-11, top left). For β-[Fe(L 1 ) 2 ] polymorph, the differences between high and low-temperature spectra are more relevant, especially in the region 530-720 nm, for which an increase of the absorption is observed upon cooling (Figure III-11, top center), in agreement with a spin transition from the paramagnetic into diamagnetic phase. The temperature dependence of the absolute reflectivity at 600 nm (noted R 600 ) in cooling and heating modes is shown in Figure III-11 (bottom right) and reflects quite well the hysteretic spin transition behavior revealed by magnetic measurements, even though slight shifts on the temperature transitions are observed (probably associated with the reflectivity versus magnetism techniques, which are respectively probing the surface and III.115 buck of the compound). It is worth noting that a slight increase of the R 600 value is observed below 80 K, due to a weak photo-conversion induced by the light required during the reflectivity acquisition.

Figure III- 11 .

 11 Figure III-11. (top) Optical reflectivity spectra at 285 K, 10 K and 10 K after irradiation with white light for α-[Fe(L 1 ) 2 ] (left) and β-[Fe(L 1 ) 2 ] (middle).(top right) Time dependence of the reflectivity during the photo-excitation. (bottom) temperature dependence of the R 625 and R 600 for α-[Fe(L 1 ) 2 ] (left) and β-[Fe(L 1 ) 2 ] (right). All temperaturedependent measurements were performed at 4 K/min.

Figure III- 12 .

 12 Figure III-12. (left) Time dependence of χT product during the photo-excitation for β-[Fe(L 1 ) 2 ]. (right) temperature dependence of the χT for β-[Fe(L 1 ) 2 ]

Figure III- 13 .

 13 Figure III-13. ORTEP views of [Fe(HL 1 ) 2 ](ClO 4 ) 2 (left) and [Fe(HL 1 )(L 1 )](ClO 4 )•MeOH (center) at 120 K. Thermal ellipsoids are depicted at 50 % probability. (right) Supramolecular dimer formed by [Fe(HL 1 )(L 1 )] + complexes. The dashed blue lines correspond to hydrogen bonds. Color Figure: Fe orange, O red, C grey, N blue, H light pink. The asymmetric unit is composed of one hexa-coordinated Fe(II) complex and two perchlorate counter-anions, which are hydrogen bonded with the complex via the NH functions (N•••O ~ 2.86 Å). In addition to the two perchlorate counter-anions that confirm the dicationic nature of the complex, the presence of ligands in the ketone form is also supported by the C-O bond lengths, ca.1.25 Å, which are shorter than those found in the neutral [Fe(L 1 ) 2 ] polymorphs, ca. 1.29 Å, while the C-N(H) distances, ca. 1.37 Å, are longer in comparison to the corresponding enolate form, ca 1.34 Å. The Fe-O and Fe-N bond distances,

  ). In contrast with the previous example, no hydrogen bond is established between the perchlorate anion and the NH function. Instead, this latter establishes a hydrogen bond (N3-H3A•••O3, 2.79 Å) with a neighboring complex, forming a supramolecular dimer (FigureIII-13, right). Similarly to the dicationic complex, the Fe-N and Fe-O bond distances and distortion of the coordination, clearly point the HS configuration at 120 K. Attempts were also made to determine whether the spin configuration can change after removal of the methanol molecule. However, no desolvation occurs even after a thermal treatment at 420 K for 24 hours under vacuum. Solvent-free crystals were obtained in low yield (~ 5 %), by diffusion of diethyl ether vapor into a solution of complex in dichloromethane. Although the quality of the structure was poor, the analysis of the coordination sphere was once again in fair agreement with the paramagnetic state down to 120 K (Figure S.III-4 and Table

Figure III- 14 .

 14 Figure III-14. Temperature dependence of the χT product for [Fe(HL 1 ) 2 ](ClO 4 ) 2 (left) and [Fe(HL 1 )(L 1 )](ClO 4 )•MeOH (right) with an applied dc field of 1000 Oe. Sweep rate: 1.3 K/min.

  Figure III-17). Thus, for each molecule eight carbon atoms are involved in π-π overlapping with four neighboring analogues. The FeN 4 O 2 coordination sphere is also directly implicated in the supramolecular assembly through weak C-H•••O bonds. Each O1 donor atom interacts with H4 and H6 from a neighboring molecule (C4•••O1: 3.28 Å; C6•••O1: 3.35 Å), participating to the extension of the network along c-axis (dashed green lines in Figure III-17).

Figure III- 17 .

 17 Figure III-17. Crystal packing of α-[Fe(L 2 ) 2 ] emphasizing the 3D supramolecular network formed by π-π and C-H•••O(Fe) contacts. C10•••C12 contacts are shown in dashed pink lines, C5•••C6 contacts in dashed blue lines and C-H•••O in dashed green lines. Other weaker interactions are omitted for clarity. Color Figure: Fe orange, O red, C grey, N blue. H, light pink. In the second polymorph (β), the crystal packing implicates less π-π contacts. Indeed, only the carbon atoms C4 and C10 from each molecule are involved in short contacts with an adjacent complex (C4•••C10: 3.22 Å), forming a supramolecular dimer (Figure III-18, left). Each oxygen donor from the coordination sphere forms only one weak C-H•••O contact (C21•••O1: 3,26 Å; C30•••O2: 3.32 Å; Figure III-18, right). The stronger contribution in both π-π and C-H•••O(Fe) interactions found in α-polymorph in comparison to

Figure III- 18 .

 18 Figure III-18. Crystal packing of β-[Fe(L 2 ) 2 ] emphasizing the supramolecular dimer formed by the π-π contacts (left) and 3D supramolecular network formed by the C-H•••O(Fe) interactions (right). Other weaker interactions are omitted for clarity. Color Figure: Fe orange, O red, C grey, N blue.Single-crystal X-ray diffraction analyses were performed at higher temperature in order to reveal eventual SCO features in β-[Fe(L 2 ) 2 ] polymorph. Selected crystallographic data are gathered in TableIII-5 and S.III-5 and their evolution with temperature are plotted in Figure III-19.

Figure III- 19 .

 19 Figure III-19. Evolution of selected crystallographic parameters and coordination bond distances with temperature for β-[Fe(L 2 ) 2 ] complex.

Figure III- 20 .

 20 Figure III-20. Crystal packing along the a-axis and molecular structure for β-[Fe(L 2 ) 2 ] emphasizing the distortion of the lattice and change of rotamer conformation with temperature: 300 K (left), 400 K (middle) and 450 K (right).

  from 3.22 to 3.41 Å. Similarly, C30-H30•••O2 interaction was found weaker (C30•••O2: 3.41 Å), while C21-H21•••O1 contact found previously cannot be deduced in the present case.

Figure III- 21 .

 21 Figure III-21. Temperature dependence of the χT product for α-[Fe(L 2 ) 2 ] (left) and β-[Fe(L 2 ) 2 ] (right). Sweep rate: 1.3 K/min.

Figure III- 22 .

 22 Figure III-22. (left). Temperature dependence of the χT product for β-[Fe(L 2 ) 2 ] at 10000 Oe (sweep rate: 10 K/min). inset: derivative of χT product as function of the temperature. (right) Temperature dependence of χT product and average Fe-N (Py) bond distances for α-[Fe(L 2 ) 2 ] and β-[Fe(L 2 ) 2 ] polymorphs.

Figure III- 23 .

 23 Figure III-23. Mössbauer spectra for β-[Fe(L 2 ) 2 ] at 298 K (left) and 298 K after thermal treatment at 450 K.

Figure III- 24 .

 24 Figure III-24. Crystal structure of [Fe(L 3 ) 2 ]•MeOH complex (left) and solvent-free [Fe(L 3 ) 2 ] complex (right) at 120 K. Color Figure: Fe orange, O red, C grey, N blue, H light pink. Table III-6. Crystallographic data for [Fe(L 3 ) 2 ]•MeOH and [Fe(L 3 ) 2 ] complexes at 120 K. Compound [Fe(L 3 ) 2 ]•MeOH [Fe(L 3 ) 2 ]

Figure III- 25 .

 25 Figure III-25. (top) Projection of the crystal packing viewed along a-axis for [Fe(L 3 ) 2 ]•MeOH complex (left) and solvent-free [Fe(L 3 ) 2 ] complex (right). (Bottom) Projection of crystal packing of [Fe(L 3 ) 2 ] emphasizing the π-π interactions along b-axis. The methanol molecule establishes a hydrogen bond with the complex via one of oxygen atom from the coordination sphere (O1•••O9 = 2.83 Å), as found previously in β-{[Fe(L 1 ) 2 ]•MeOH} (Figure III-3). At 120 K, the compound is diamagnetic as shown by the Fe-X bond distances ranging from 1.860 to 1.979 Å,

(

  Figure III-24 right, Tables III-6 and SIII-6). The molecules inside the crystal packing are arranged in a complete different manner comparing to the solvated form as shown in Figure III-25. Especially, while no ππ contact is present in the [Fe(L 3 ) 2 ]•MeOH, a strong π-π stacking is displayed in the solvent-free structure. Thus, each molecule interacts with two adjacent complexes by C•••C and C•••N contacts of 3.22 to 3.35 Å,giving rise to a supramolecular chain along b-axis. Nevertheless, despite these strong differences of the packing, no significant change is observed in the coordination sphere, which is once again consistent with LS III.129 configuration at 120 K and no evolution of the unit was detected up to 400 K, suggesting no SCO feature in this material.

  ) 2 ]•MeOH, the methanol molecule interacts strongly with the lattice via the hydrogen bond and additional weak C-H•••X (O, π) short contacts, in such a way that no desolvation was observed by X-ray diffraction analysis up to 400 K. Thermogravimetric measurements were performed to confirm the stability of the compound and determine at which temperature the loss of methanol molecule occurs. The thermogram shown in Figure III-26 (left), reveals only minor weight loss up to ca. 420 K, while an abrupt decrease corresponding to 5.5 % is observed between 420 and 430 K, in good agreement with the removal of one methanol molecule. The desolvated complex remains stable at this range of temperature (decomposition occurring at about 500 K), giving then the opportunity to probe eventual SCO features in this desolvated material.

Figure III- 26 .

 26 Figure III-26. (left) TGA data for [Fe(L 3 ) 2 ]•MeOH complex. (center) DSC thermogram for [Fe(L 3 ) 2 ]•MeOH complex. (right) DSC thermogram for solvent-free [Fe(L 3 ) 2 ] complex obtained by crystallization (Et 2 O diffusion).DSC measurement performed on [Fe(L 3 ) 2 ]•MeOH complex revealed an highly energetic endothermic peak (ΔH = 36 kJ/mol) at 433 K, which is compatible with a phase transition induced by the loss of the methanol molecule based upon TGA analysis(Figure III-26, center). After desolvation, the material displayed a reversible broad signal around 340 K, suggesting the occurrence of a spin crossover. This is supported by the ΔS value of ca. 20.7 J/mol•K, which is in the range of values found in the compounds reported earlier in the chapter. Interestingly, this reversible process could not be deduced in the DSC of the solvent-free complex obtained by recrystallization (using diethyl ether diffusion, vide supra), which remains silent up to 400 K (FigureIII-26, right), confirming the absence of SCO feature predicted by XRD. This result indicates that the samples produced by the two different methods (crystallization vs. desolvation), are structurally distinct and therefore polymorphs from each other. This might explain the strong differences found between the X-ray structures of solvated and solvent-free complexes. Some minor differences between the two solvent-free samples were revealed by FTIR (see Figure S.III-2), although additional measurements including powder X-ray diffraction analysis must be performed to confirm unambiguously polymorphism in these materials.

Figure III- 27 .

 27 Figure III-27. (left) Temperature dependence of the χT product for [Fe(L 3 ) 2 ]•MeOH with an applied dc field of 10000 Oe. (right) dχT/dT vs. T plots. Full symbols: data measured on VSM magnetometer at 5 K/min. Empty symbols: data measured on SQUID magnetometer at 1.3 K/min.

  (L 3 ) 2 ], highlighting the strong impact of the supramolecular organization (in solid-state) on the SCO phenomenon. Most spectacular properties were found in the desolvated [Fe(L 1 ) 2 ] polymorphs that both showed spin transition at very different temperatures (ΔT ~ 100 K) and with wide hysteresis of 38 K (αform) and 27 K (β-form) at 0.3 K/min. To the best of our knowledge only one other example, namely [Fe(L Me )(NCS) 2 ] (L Me =

) 2 ]

 2 complex due to flexible dodecyl chains, we mainly focused our investigations on microcrystalline powder samples. The thermal behavior was thereafter confirmed by X-ray diffraction studies on single-crystals. The compound was first checked for liquid crystalline properties by POM, DSC and SAXS measurements. The absence of liquid crystalline feature in this material was rapidly evidenced by POM analysis. The melting was detected around 475 K, as illustrated by the formation of black isotropic phase observed in crossed polarizers mode(Figure III-28).

Figure III- 28 .

 28 Figure III-28. Optical micrographs for [Fe(L 4 ) 2 ] (in crossed polarizers mode), showing the growth of crystalline birefringent textures during the cooling from isotropic liquid phase.During the cooling, the reappearance of birefringence was detected around 450 K. The growth from limited focal points (defects) and aspect of birefringent textures was clearly indicative of crystalline materials. This is also supported by the large thermal hysteresis observed between the melting and crystallization. Upon cooling down to room temperature, no significant change of the texture could be perceived and the material remains as a homogeneous thin film.DSC measurements were performed in order detect an eventual transition associated to a SCO process.Unexpectedly, in addition to the melting point found at 474 K, two phase transitions at 337 and 432 K were observed during the first heating, indicating that the material exists successively in three crystalline forms(noted Cr1, Cr2 and Cr3 in Figures III-29).

Figure III- 29 .

 29 Figure III-29. DSC data of [Fe(L 4 ) 2 ] complex for the 1 st heating/cooling cycle.

Figure III- 30 .

 30 Figure III-30. SAXS profiles for [Fe(L 4 ) 2 ] on 1 st heating (left) and 1 st cooling (right).The SAXS profiles corresponding to the different phases are shown in FigureIII-30 (left). The first two phases were found to be highly crystalline. A lamellar order corresponding to the separation distance

  suggested that the thermal behavior might be reversible if the temperature is maintained below the second transition. To verify this hypothesis a new set of DSC and SAXS measurements were performed up to 375 K (FigureIII-31). In these conditions, DSC revealed only one endothermic transition at ca. 340 K in heating and the corresponding exothermic transition around 320 K on cooling mode(Figure III-31, left).

Figure III- 31 .

 31 Figure III-31. (left) DSC thermogram for [Fe(L 4 ) 2 ] between 290 and 365 K. (center) SAXS profile for [Fe(L 4 ) 2 ] on heating from 295 to 375 K (2 nd heating). (right) SAXS profile for [Fe(L 4 ) 2 ] on cooling from 375 to 295 K (2 nd cooling).

  Cr1→Cr2 transition. It seems legit to attribute this feature to a SCO induced phase transition since a similar process could not be detected in this range of temperature in the DSC of purely diamagnetic [Zn(L 4 ) 2 ] analogue, which remarkably display a transition around 430 K corresponding to the irreversible transformation in [Fe(L 4 ) 2 ] complex(Figure S.III-8). However, some doubts remained since no significant change of color could be perceived upon the transition into Cr2 phase and the material appears as a dark green powder, which is the usual color for the LS configuration(Figure III-32). A change of color to olive green is actually observed, but upon the transition into Cr3 phase, suggesting a conversion into the paramagnetic phase. It is worth to note that the material remains thereafter olive green upon cooling from the liquid phase down to room temperature, suggesting the absence of SCO during the Cr3→Cr4 transition.

Figure III- 32 .

 32 Figure III-32. Optical micrographs for [Fe(L 4 ) 2 ] recorded during the first heating (under parallel polarizers). From left to right: Cr1, Cr2, Cr3 and liquid phases.

(

  χT =3.25 cm3 •K•mol -1 ) occurs around 424 K in agreement with the change of color observed above the second transition(Figure III-33, bottom). As mentioned above, the existence of a structural phase transition at about the same temperature in the diamagnetic [Zn(L 4 ) 2 ] analogue, makes us think that the SCO is not the driving force behind this latter transition but is most likely a fortuitous consequence of the reorganization of the packing that stabilizes the HS configuration.

Figure III- 33 .

 33 Figure III-33. (top left) Temperature dependence of the χT product for [Fe(L 4 ) 2 ] between 200 and 400 K. (top right) dχT/dT vs. T plots. (bottom left) Temperature dependence of the χT product for [Fe(L 4 ) 2 ] between 300 and 440 K. (bottom right) dχT/dT vs. T plots. In other words, the SCO is induced by the formation of a new polymorphic form and not the reverse as classically observed. It should be recalled that the analogic situation was found for [Fe(L 1 ) 2 ]•MeOH during the transformation in mother liquor from the kinetic (diamagnetic) α-polymorph into the thermodynamic

  250 K (Figure III-35 left, Tables III-7 and S.III-7). The asymmetric unit is composed of only one [Fe(L 4 ) 2 ]molecule, discarding therefore the hypothesis of multiple non-equivalent sites in the low-temperature phase.The analysis of the Fe(II) coordination sphere confirmed the LS configuration and the average Fe-X bond lengths, ranging from 1.867 to 1.966 Å, and Σ (X-Fe-X) parameter, 85.7°, are favorably comparable to other examples with LS configuration described earlier.

Figure III- 35 .

 35 Figure III-35. (Top) Views of the two [Fe(L 4 ) 2 ] enantiomers at 250 K (left) and 360 K (right). The blue star represent the inversion center of symmetry relating the two crystallographically equivalent enantiomers in the P2 1 /c space group. The blue and red vectors represent the 2-fold symmetry axes crossing the two non-equivalent isomers in the P2 1 2 1 2 space group. (bottom) projection of the packing at 360 K, emphasizing the distribution of LS (orange) and HS (green) species in the space. H-atoms are omitted for clarity.

  twin and characterized by a flack parameter of 0.49. A view emphasizing the repartition of the LS and HS species in the packing is provided in Figure III-35 (bottom) and shows the formation of HS and LS channels along a-axis, which are alternated in b-and c-directions.

Figure III- 36 .

 36 Figure III-36. Projection of the packing viewed along a and c directions for [Fe(L 4 ) 2 ] at 250 K (top) and 360 K (bottom). (inset) Fragment of [Fe(L 4 ) 2 ] structures emphasizing the change of the conformation of alkoxy functions. For clarity, only the ether of the dodexyloxy function is displayed. FeN 4 O 2 coordination sphere are shown in orange and green octahedrons for LS and HS species, respectively. The packing can be described by nearly planar Fe(II) layers parallel to the ac plan and separated from each other by a distance d = b/2 (Figures III-35 to III-37). The cohesion between layers is mainly assured

  Figure III-37. In the low-temperature phase, interactions mainly arises from C-H•••π contacts between pyridyl and benzohydrazide rings, while only one interaction involving directly the coordination sphere (C-H•••O = 2.30 Å), is present. At high temperature, all the oxygen donors are involved in C-H•••O contacts, although these interactions are found weaker (2.41 and 2.70 Å) than at 250 K.

Figure III- 37 .

 37 Figure III-37. Projection of the packing viewed along b-axis for [Fe(L 4 ) 2 ] at 250 K (left) and 360 K (right) emphasizing the weak C-H•••X interactions (X = O, N, π). Weak interactions involving the coordination sphere are displayed in red dashed lines and the other short contacts are shown in violet dashed lines. Dodecyl chains are omitted for clarity.

7 .

 7 Scheme III-7. Chemical structure of [Fe(L 5 ) 2 ] and [Fe(L 6 ) 2 ] complexes

Figure III- 38 .

 38 Figure III-38. ORTEP views of [Fe(L 5 ) 2 ] at 120 K. Thermal ellipsoids are depicted at 50 % probability.

  ). The architecture of [Fe(L5 ) 2 ] and crystal packing is significantly different from the non-chlorinated [Fe(L4 ) 2 ] analogue. The metal centers inside the packing arrange themselves in nearly planar layers parallel to bc plan and separated from each other through the interdigitated dodecyl chains(Figure III-39). In contrast with [Fe(L 4 ) 2 ] complex, the bent geometry cannot be observed, and the two alkyl chains are oriented in antiparallel fashion(Figures III-38 and III-39, left). These are nearly orthogonal to b-axis and tilted from the caxis with an angle of about 45°. The cohesion between the layers is mainly maintained through electrostatic van der Waals inter-chain interactions and weak C-H•••Cl (2.86 Å) contacts involving the terminal methyl position of one chain and one chlorine atom from the neighboring layer. The unexpected arrangement of the alkyl chains can be explained by the very different organization of the molecules in the bc plane. Indeed, in contrast with the previous example, numerous weak interactions are established between the pyridylbenzohydrazone ligands to generate supramolecular layers. Especially, this includes multiple C•••C Chapter III -Mononuclear Fe II spin-crossover complexes: from crystalline toward soft materials III.144 and C•••N π-π contacts with distances ranging from 3.18 to 3.39 Å (blue dashed lines in Figure III-39, right), as well as Cl•••π contacts of 3.21 Å (green dashed lines).

Figure III- 39 .

 39 Figure III-39. Projection of the packing for [Fe(L 5 ) 2 ] at 120 K viewed along b-axis (left) and a-axis (right). The π-π and Cl•••π interactions are shown as blue and green dashed lines, respectively. In the right figure, dodecyl chains, hydrogen atoms and other C•••X interactions (X = O, N, π) are omitted for clarity.

Figure III- 40 .

 40 Figure III-40. Temperature dependence of the χT product (left) and derivative of χT product as function of the temperature for single-crystal of [Fe(L 5 ) 2 ]. Applied dc field of 10000 Oe; Sweeping rate: 1.4 K/min.Magnetic measurements were also performed on the microcrystalline sample obtained before the recrystallization and revealed a quite different magnetic behavior(Figure III-41). Indeed, the thermally virgin sample is found to be already in a {LS•HS} phase as shown by the χT value of 1.67 cm 3 •K•mol -1 in the low-temperature plateau. As the temperature increase, a conversion from the {LS•HS} into the HS phase (3.26 cm 3 •K•mol -1 ) appears at slightly higher temperature (T 1/2 = 295 K) and steeper fashion than observed for the previous sample. On cooling the reverse conversion take places in a stepwise fashion, giving rise to a weak thermal hysteresis. These results clearly indicate that the two samples are not in the same polymorphic form. Evidences of their polymorphism were deduced from the FTIR spectra that show slight differences in the overall spectral window, although additional measurements (powder X-ray diffraction) need to be performed to unambiguously confirm that.

Figure III- 41 .

 41 Figure III-41. Temperature dependence of the χT product (left) and derivative of χT product as function of the temperature for microcrystalline powder of [Fe(L 5 ) 2 ] (sample before recrystallization). Applied dc field of 10000 Oe; Sweeping rate: 1.4 K/min.

Figure III- 42 .

 42 Figure III-42. (left) DSC thermograms for [Fe(L 6 ) 2 ] between 300 and 400 K at 10 K/min. (center) SAXS profiles of [Fe(L 4 ) 2 ] during the 2 nd heating. (right) SAXS profiles of [Fe(L 4 ) 2 ] during the 2 nd cooling. The DSC and SAXS measurements revealed that [Fe(L 6 ) 2 ] complex has a complicate thermal behavior, which is nonetheless relatively similar to its analogue with dodexyloxy functions. The DSC between 300 and 400 K (Figure III-42, left), displays a reversible transition around 380 K (heating) and 355 K (cooling), that was thereafter ascribed to a spin transition process based upon magnetic measurements. Both phases were found to be well crystalline as shown by the SAXS diffractograms (Figure III-42, center and right). By analogy with [Fe(L 4 ) 2 ] complex, the first intense reflection found at 2.40 and 2.16 nm -1 at 303 K and 393 K, respectively, was attributed to the separating distance between two adjacent metal center layers. The interlamellar distances, 26.2 Å at 303 K and 29.0 Å at 393 K, are larger than those found for [Fe(L 4 ) 2 ] in their respective crystalline phases as a direct result of the increase of alkyl chain length.

Figure III- 43 .

 43 Figure III-43. DSC thermograms for [Fe(L 6 ) 2 ] on 3 rd and 4 th heating/cooling cycle between 300 and 420 K (left), on 5 rd and 6 th heating/cooling cycle between 300 and 470 K (center) and for [Zn(L 6 ) 2 ] on 1 st heating between 300 and 420 K (right). Sweeping rate: 10K/min

Figure III- 45 .

 45 Figure III-45. Temperature dependence of the χT product (left) and derivative of χT product as function of the temperature for [Fe(L 6 ) 2 ]. Applied dc field of 10000 Oe; Sweeping rate: 1.3 K/min. Additional analogue ligands functionalized with one alkyl chain were recently prepared in the team, and the preliminary investigations of resulting Fe(II) complexes revealed systematically the coexistence of LS and HS species in an intermediate phase confirming the important role of alkyl chains in the CSB process.

Scheme III- 8 .

 8 Scheme III-8. Chemical structure of [Fe(L 7 ) 2 ] and [Fe(L 8 ) 2 ] complexes

Figure III- 46 .

 46 Figure III-46. (left) DSC thermograms for [Fe(L 7 ) 2 ] between 280 and 400 K at 10 K/min. (right) Optical micrographs of [Fe(L 7 ) 2 ] recorded during the 1 st heating emphasizing the formation of metastable liquid-like phase (starting at 333 K) and beginning of solidification at 343 K (parallel polarizers mode).

Figure III- 47 .

 47 Figure III-47. SAXS profiles for [Fe(L 7 ) 2 ] recorded between 293 and 343 K on heating (left) and cooling (right) modes.

Figure III- 48 .

 48 Figure III-48. Optical micrographs for [Fe(L 7 ) 2 ] at 343 K in parallel polarizers mode (top) and crossed polarizers mode (bottom) emphasizing the crystal growth from the metastable liquid phase. Performed during the first heating.

Figure III- 49 .

 49 Figure III-49. (left) SAXS profiles for [Fe(L 7 ) 2 ] recorded between 293 and 393 K during the 2 nd heating. (right) Comparison of SAXS profiles at 343 K on heating from 293 K and cooling from 393 K. (Inset) Polarized optical micrograph after slow cooling (1K/min) from the isotropic liquid phase down to 343 K, isothermal at 343 K for 12 hours, and cooling down to room temperature.

Figure III- 50 .

 50 Figure III-50. (left) SAXS profiles for [Fe(L 8 ) 2 ] recorded during the 1 st heating from 298 up to 368 K. (center) SAXS profiles for [Fe(L 8 ) 2 ] recorded during the cooling from between 368 down to 298 K.(right) DSC thermogram of [Fe(L 8 ) 2 ] for the 1 st and 2 nd heating/cooling cycles between 260 and 400 K. DSC sweeping rate: 10 K/min DSC and SAXS measurements for [Fe(L 8 ) 2 ] revealed a quite different and simpler thermal behavior than previous example. At room temperature, the SAXS profile exhibits one sharp and intense reflection at 1.59 nm -1 as well as weak and broad signals in the low and wide-angle regions, suggesting a poorly crystalline or plastic crystal behavior (Figure III-50, left). By analogy with previous examples, the intense

Figure III- 51 .

 51 FigureIII-51. Below 200 K, the χT product is ca. 0.23 cm3 •K•mol-1 , in agreement with an almost pure diamagnetic phase. As the temperature increases from 200 to 283 K, a progressive increase of the χT product up to 2.94 cm 3 •K•mol -1 is observed, confirming the occurrence of a gradual SCO. The derivative of the χT plot indicates that the conversion occurs in a two-step fashion, which can be explained by the slow crystallization of the material after the formation of the metastable liquid-like phase around 340 K (vide supra). At 283 K, the material starts to melt into the thermodynamic liquid phase and the χT value drops steeply to 2.42 cm 3 •K•mol -1 , while the onset of a gradual SCO happening in the liquid phase can be deduced above 395 K.

Figure III- 51 .

 51 Figure III-51. Temperature dependence of the χT product (left) and derivative of χT product as function of the temperature for [Fe(L 7 ) 2 ]. Applied dc field of 10000 Oe; Sweeping rate: 1.4 K/min.

Figure III- 52 .

 52 Figure III-52. (Top) Optical micrographs of [Fe(L 7 ) 2 ] recorded during the first heating from 343 K up to 423 K emphasizing the reverse-SCO between 383 and 390 K. (bottom) Optical micrographs of [Fe(L 7 ) 2 ] recorded during the cooling from the isotropic liquid phase (423 K) down to 303 K emphasizing the gradual conversion from the HS (olive green) liquid phase into the LS (dark green) solid phase.

Figure III- 53 .

 53 Figure III-53. Temperature dependence of the χT product (left) and derivative of χT product as function of the temperature for [Fe(L 8 ) 2 ]. Applied dc field of 10000 Oe; Sweeping rate: 1.3 K/min.

Figure III- 54 . 6 .

 546 Figure III-54. Strategy proposed to elaborate [Fe 4 L 4 ] grids with mesomorphic properties (the functionalization is represented as turquoise arrows). Note: structure of [Fe 4 L 4 ]•NEt 3 (R = H; R' = Me) at 120 K. 35 Hydrogen atoms and solvent lattice are omitted for clarity.

  preheated to 50°C. After homogenization of the dark green solution, the mixture was allowed to stand at room temperature for 20 hours. The resulting dark green needles of β-[Fe(L 1 ) 2 ]•MeOH were collected by filtration. The needle sample was dried under vacuum at 40 °C for 12 hours to obtain the solvent-free β-[Fe(L 1 ) 2 ]•complex: Yield 79 mg, 72 %. Elemental analysis -Calc. (Found) for C 28 H 24 FeN 6 O 4 :

Figure S.III- 1 .

 1 Figure S.III-1. FTIR spectra for α-[Fe(L 2 ) 2 ] (black) and β-[Fe(L 2 ) 2 ] (red)

Figure S.III- 3 .

 3 Figure S.III-3. Projection of the packing of α-{[Fe(L 1 ) 2 ]•MeOH} (left) and β-{[Fe(L 1 ) 2 ]•MeOH} (right) emphasizing the two enantiomers that are related by an inversion center of symmetry (blue star).

Figure S.III- 4 .

 4 Figure S.III-4. Structure and packing for the solvent-free [Fe(HL 1 )(L 1 )](ClO 4 ).

Figure S.III- 6 .

 6 Figure S.III-6. Thermogravimetric data for [Fe(L 4 ) 2 ]. Sweeping rate: 5 K/min

Figure S.III- 8 . 9 .

 89 Figure S.III-8. DSC thermograms for [Zn(L 4 ) 2 ] (left) and [Zn(L 8 ) 2 ] (right).

  Figure S.III-10. Projection of the crystal packing for γ-[Fe(L 1 ) 2 ] at 120 K emphasizing the intermolecular π-π interactions.

Figure

  Figure S.III-11. (left) DSC data for γ-[Fe(L 1 ) 2 ] between 220 and 400 K at 10 K/min. (right) DSC data for γ-[Fe(L 1 ) 2 ] at different sweeping rates.

Figure S.III- 12 .

 12 Figure S.III-12. Temperature dependence of the χT product for γ-[Fe(L 1 ) 2 ] at 0.1-1 T (sweep rate: 1.3 to 0.03 K/min).

IV. 4 .IV. 5 .IV

 45 ) 2 ] 2 }[OTf] 2 (7) .................................... IV.197 IV.3.4. Magnetic measurements in solid state ............................................................................. IV.200 IV.3.5. Characterization of {Fe 2 Co 2 } molecular squares in solution .......................................... IV.201 Conclusions and perspectives ....................................................................................................... IV.204 Supporting materials .................................................................................................................... IV.206 IV.5.1.Experimental protocols .................................................................................................... IV.206 IV.5.1.1. Synthesis of the organic precursors and ligands........................................................... IV.206 IV.5.1.2. Synthesis of metal ion precursors and complexes ........................................................ IV.208 IV.5.2. FTIR spectroscopic analyses ........................................................................................... IV.211 IV.5.3. Powder X-ray diffraction data ......................................................................................... IV.212 IV.5.4. Magnetic measurements .................................................................................................. IV.213 IV.5.5. UV-Vis studies in diluted solution .................................................................................. IV.215

11 - 21 Figure IV- 1 .

 11211 Figure IV-1. Schematic representation of {Fe III 2 M II 2 }. Color scheme: Fe green, M (Co or Ni) pink, B yellow, N blue, C grey. The green arrows represent the position chosen for the functionalization of the bipyridine ligand Going further toward systems with easy processability, we proposed during this thesis to develop soft materials based on {[(Tp*)Fe III (CN) 3 ] 2 [M II (R,R-bpy) 2 ] 2 }(A) 2 (where Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate; M II = Ni II or Co II ; R,R-bpy = 4,4'-disubstituent-2,2'-bipyridine derivative; A = monovalent anion) (Figure IV-1). These molecular squares are well-known to behave as single-molecule magnets 8,9 or

  . Moreover, weak intermolecular C-H•••π interactions between benzoate and pyrazole rings from Tp* are present along b-axis. H-bonds are also established between the nitrogen atom N(7) from terminal cyanide ligand, a water molecule and one DMF molecule (N(7)•••O(1)w, 2.92 Å and O(1)w•••O(12) DMF , (2.84 Å, Figure IV-6, right).

Figure IV- 6 .

 6 Figure IV-6. (left) Packing arrangements illustrating π-π interactions present between adjacent complexes of 2 along the crystallographic a-axis at 120 K. (right) View of the intermolecular short contacts in the crystal packing of 2 at 120 K. All anions, lattice solvent molecules and hydrogen atoms are omitted for clarity.

L 5

 5 and L 6 (5 and 6) were found to exhibit mesomorphism. The DSC and SAXS data for the two complexes are shown in Figure IV-7 and IV-8 and thermodynamic parameters are summarized in Table IV-3. The DSC of 5 revealed an endothermic transition at 337 K, while the corresponding opposite peak is observed during the cooling at 326 K (Figure IV-7, left). SAXS measurements were performed to confirm the nature of the phases below and above the transition. From room temperature up to 338 K, only two sharp Chapter IV -Soft materials based on cyanido-bridged {Fe III 2 low reciprocal spacing region (q) and very diffuse halo around 14 nm -1 are observed as typically found in liquid-crystal materials (Figure IV-8, top).

II ) .

 ) The DSC of 6 shows two reversible transitions(Figure IV-7, right). The first peak with the maximum of 315 K corresponds to a transition from a crystalline phase into smectic mesophase as evidenced by SAXS experiment. Upon further heating a second peak appears at 347 K and was attributed to the clearing into isotropic liquid phase based upon POM and SAXS measurements. Once again, the thermal behavior is reversible. Between the two transitions, SAXS profiles display only two sharp reflections that are in ration 1:2 as well as a broad and diffuse halo around 14 nm -1 , confirming the existence of a lamellar mesophase in the material(Figure IV-8, bottom). The interlamellar distance deduced from the two sharp reflections is almost temperature-independent and is 44.6 Å (Table

Figure IV- 7 .

 7 Figure IV-7. (left) DSC data for 5 recorded during the 2 nd heating/cooling cycle between 300 and 370 K, (right) DSC data for 6 recorded during second heating/cooling cycle between 260 and 375 K

Figure IV- 16 .

 16 Figure IV-16. Packing diagram of 7 in the bc plane emphasizing one unit cell. Lattice solvent molecules, anions, and hydrogens are omitted for clarity. Color scheme: Fe green, Co pink, B, N and C grey.

Figure IV- 17 .

 17 Figure IV-17. Packing diagram of 7 in the ab plane illustrating the H bonding between solvent molecules and terminal cyanide groups. Anions and hydrogen atoms are omitted for clarity. Color scheme: Fe orange, Co purple, B pink, N blue, C grey.

  decreases from 5.11 to 4.21 cm 3 K/mol due to the intrinsic spin orbit coupling of the cobalt centers. At lower temperatures, the T values increase to ca. 4.55 cm 3 K/mol at 7.7 K, suggesting the presence of intramolecular ferromagnetic interactions between Co II HS and Fe III LS centers. Below 5 K, the T product drops to ca. 2.40 cm 3 K/mol due to intermolecular antiferromagnetic interactions between adjacent molecular squares or zero-field splitting.

Figure IV- 18 .

 18 Figure IV-18. Temperature dependence of the χT product (with χ defined as the molar magnetic susceptibility and equal to M/H) for compound 7 collected in an applied dc field of 1 T.

Figure IV- 19 .

 19 Figure IV-19. Temperature dependence of the χT product (with χ defined as the molar magnetic susceptibility and equal to M/H) for compound 8 (left) and of 10 (right) at 0.1 T.

  cm 3 •K/mol at high temperature (Figure IV-19, right), which is slightly lower than expected for the sum of two magnetically isolated low spin Fe III ions (S = ½) and high-spin Co II (S = 3/2) ions. Upon cooling the χT product remains roughly constant down to 50 K and then increases abruptly up to 8.0 cm 3 •K/mol at 10 K. Once again, the χT value decreases down to 4.15 cm 3 •K/mol at 1.8 K due to antiferromagnetic intermolecular interactions and/or the magnetic anisotropy of Co II HS ions. For both samples, additional evidences for ferromagnetic exchanges between Fe III LS and Co II HS ions are found in the field-dependence of magnetization. At 1.8 K, the magnetization values are increasing constantly upon applying external dc fields (Figure S.IV-8), without presenting any S-shape curves that could be an indicative for the presence of antiferromagnetic interactions. For 10, the magnetization curves show an absence of saturation presuming magnetic anisotropy of the Co II site (Figure S.IV-8, right).

  a change of color was observed, it is assumed that the {Fe 2 Co 2 } molecules remain intact (no dissociation).

Figure IV- 20 .

 20 Figure IV-20. Change of color observed when a methanol solution of 7 is cooled from room temperature (left) and cooled down with liquid nitrogen (right).

  at 298 K; red -color at all temperatures suggesting the absence of electron-transfer; red-green -colors at 290 and 77 K suggesting an electron-transfer process in solution. The thermochromism observed in solutions of 7 and 9 was characterized by Vis/UV spectroscopy in three different solvents (methanol, butyronitrile and dichloromethane). The temperature-dependent Vis/UV spectra in butyronitrile are shown in Figure IV-21 (left) and those recorded in methanol and dichoromethane can be found in Figures S.IV-9 and S.IV-10. At high temperature, a broad absorption band is centered at 470 nm in addition to a small shoulder at approximately 560 nm (Figure IV-21, left). The intense absorption at 470 nm corresponds to a spin and Laporte-allowed ligand-to-metal charge-transfer (LMCT) transition, whereas the shoulder at 560 nm is attributed to a Co II → Fe III metal-to-metal charge-transfer (MMCT) transition. 15b,16bChapter IV -Soft materials based on cyanido-bridged {FeIII 2

Figure IV- 21 .

 21 Figure IV-21. (left) UV-Vis spectra for 7 (top) and 9 (bottom) in butyronitrile on cooling mode from 298 to 170 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 7 (top) and 9 (bottom) in butyronitrile, solid lines are fits to the ideal solution model. As the temperature decreases the absorption band at 560 nm decreases progressively in intensity, while a new absorption band, characteristic of the a Fe II → Co III MMCT, where in the same time a band appears around 770 nm (Figure IV-21, left). Therefore, the spectroscopic data unambiguously demonstrate that the thermal conversion of {Fe III LS (μ-CN)Co II HS } into {Fe II LS (μ-CN)Co III LS } pairs occurs in solution. Furthermore, the isosbestic point at 600 nm strongly suggests that only two species are involved in the interconversion process. 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 absorptions at 540 and 760 nm, respectively, and plotted as function of the temperature (Figure IV-21, right and Figures S.IV-9 and S.IV-10). As expected, two fractions, cross each other at 0.5 defining the T 1/2 temperature at which {Fe III LS (μ-CN)Co II HS } and {Fe II LS (μ-CN)Co III LS } species are equally present. Since only two species are in equilibrium, we can fits these plots using the binary ideal solution model 29 described in Chapter I:

Figure IV- 22 .

 22 Figure IV-22. 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 7 in dichloromethane T 1/2 = 207.8 K, butyronitrile T 1/2 = 221.0 K and methanol T 1/2 = 267.8 K. Solid lines are the fits to the ideal solution model.

  treated with Co(OTf) 2 (36 mg, 0.08 mmol) in 3 ml of DMF to afford a red solution that was allowed to stir for 2 hours. Subsequently, L 8 (96 mg, 0.12 mmol) in 3 ml of DCM was added and the mixture was stirred for 2 days at room temperature and then filtered. To the clear solution, diethyl ether (25 mL) was added, and the resulting green powder was collected by filtration and washed with methanol (3 x 5 mL): Yield 40 %. Selected FT-IR data (ATR, cm -1 ): 2925 (s), 2853 (s), 2516 (b, ν BH ), 2154 (b, ν CN ), 2112 (m, ν CN ), 2093 (s, ν CN ), 2063 (s), 1668 (vs), 1609 (vs), 1558 (s), 1487 (s), 1450 (s), 1248 (s), 1251 (vs), 1223 (s), 1155 (s), 1028 (vs), 861 (m

  Yield 40 %. Selected FT-IR data (ATR, cm -1 ): 2921 (s), 2854 (s), 2521 (b, ν BH ), 2159 (s, ν CN ), 2119 (m, ν CN ), 1585 (s), 1545 (s), 1519 (s), 1450 (s), 1283 (s), 11242 (vs), 1191 (s), 1016 (s), 833 (s), 763 (vs), 696 (s), 600 (s). Elemental analysis -Calc. (Found) for C 252 H 300 B 2 Co 2 F 6 Fe 2 N 26 O 16 P 2 : C, 67.40 (67.85); H,

Figure S.IV- 1 . 2 .

 12 Figure S.IV-1. FTIR spectrum at room temperature for: (top) tricyanido-metallate precursor (EtN 4 )[(Tp*)Fe(CN) 3 ], (bottom left) nickel trifluoromethanesulfonate and (bottom right) cobalt trifluoromethanesulfonate.

Figure S.IV- 3 . 4 .

 34 Figure S.IV-3. Small angle X-ray diffraction profiles (left) for 7 on 1 st heating from 298 to 368 K and (right) for 9 on 1 st heating from 276 K to 353 K.

Figure S.IV- 5 .

 5 Figure S.IV-5. M vs H (top) and M vs H/T (bottom) data for: (left) 1, (center) 2 and (right) 5 between 1.8 and 8 K with sweep-rates of 100-200 Oe/min.

Figure S.IV- 6 .

 6 Figure S.IV-6. Plots of the in-phase (top), χ', and out-of-phase (bottom), χ'', components of the ac magnetic susceptibility versus temperature, T, under no external magnetic field at different frequencies between 10 and 10000 Hz of the compounds: (left) 1 and (right) 2.

Figure S.IV- 7 .

 7 Figure S.IV-7. Plots of the in-phase (top), χ', and out-of-phase (bottom), χ'', components of the ac magnetic susceptibility versus frequency, v, under an external magnetic field at different at the indicated temperatures: (left) 1 at 1500 Oe, (center) 2 at 1000 Oe and (right) 3 at 1500 Oe.

Figure S.IV- 8 .

 8 Figure S.IV-8. M vs H (top) and M vs H/T (bottom) data for: 8 (left) and of 10 (right) between 1.8 and 8 K with sweeprates of 100-200 Oe/min.

Figure S .

 S Figure S.IV-10. (left) UV-Vis spectra for 7 (top) and 9 (bottom) in dichloromethane on cooling mode from 218 to 180 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 7 (top) and 9 (bottom) in dichloromethane, solid lines are fits to the ideal solution model.

  formula {[(Tp*)Fe III (CN) 3 ] 2 [M II (R,R-bpy) 2 ] 2 }(A) 2 (where Tp* = hydridotris(3,5-dimethylpyrazol-1yl)borate; M II = Ni II or Co II ; R,R-bpy = 4,4'-disubstituent-2,2'-bipyridine derivative; A = monovalent anion). Three squares shaped {Fe III (μ-CN)Ni II } complexes functionalized were synthesized and their properties were investigated in solid, by structural, thermal and magnetic methods. The magnetic studies show intramolecular ferromagnetic coupling and appreciable magnetic anisotropy of the complexes. This functionalization gives the possibility to induce liquid crystalline behavior in case for two complexes {[(Tp*)Fe(CN) 3 ] 2 [Ni(L 5 ) 2 ] 2 }(OTf) 2 and {[(Tp*)Fe(CN) 3 ] 2 [Ni(L 6 ) 2 ] 2 }(OTf) 2 . These compounds showed a smectic mesophase and preserved the magnetic properties. For {Fe 2 Co 2 } we attempt a new design strategy involving the grafting of mesogenic substituents onto the bipyridine ligand through an ether coupling. Four new tetranuclear new cyanido-bridged complexes, 7-10, and have been synthesized and their properties in solid state and in solutions have been described by structural, spectroscopic and magnetic methods. By performing optical investigations was found that the ET characteristic temperatures T 1/2 are strongly dependent of the nature of the solvent, confirming the results obtained in the team for other {Fe 2 Co 2 } analogues.

  alkyloxy-2,2':6',2''-terpyridine, n = 14, ont été à l'origine d'avancées technologiques majeures devenues indispensables à une société moderne. Les composants magnétiques sont omniprésents dans notre quotidien comme dans les générateurs électriques, les moteurs, les haut-parleurs, les microphones, les commutateurs, les capteurs, les disques durs, etc. Jusque dans les années 80, la majorité des systèmes magnétiques développés et étudiés sont des matériaux inorganiques basés sur des métaux purs, des alliages ou des oxydes métalliques. Cependant, le développement de nouveaux matériaux magnétiques basés sur des précurseurs moléculaires a connu un essor considérable au cours de ces dernières décennies. Les matériaux moléculaires présentent des avantages synthétiques par rapport aux analogues inorganiques puisqu'ils utilisent des précurseurs bien définis permettant un excellent contrôle de l'architecture, de la dimensionnalité des matériaux finaux et ainsi de leurs propriétés physiques. Par ailleurs, ces systèmes moléculaires sont accessibles par des voies souvent beaucoup plus douces et économe en énergie, impliquant la chimie de coordination ou chimie organométallique. Outre l'aspect synthétique, les propriétés magnétiques de ces matériaux moléculaires sont souvent rares ou inaccessible dans les matériaux inorganiques traditionnels.Ainsi, des systèmes moléculaires présentant des phénomènes de conversion de spin, 1 du photomagnétisme 2 ou encore des comportements de molécule-aimant 3 ou chaîne-aimant 4 ont été découverts. Ces matériaux sont d'autant plus attrayants que leurs propriétés magnétiques peuvent être modulées sur mesure en modifiant les précurseurs moléculaires qui les constituent. Même si les propriétés magnétiques sont les motivations principales de ce domaine de recherche, d'autres propriétés, qui font souvent défaut dans les matériaux inorganiques comme la transparence, le thermo-photochromisme, la biocompatibilité, la recyclabilité, peuvent être présentes dans ces systèmes moléculaires, offrant ainsi des solutions pratiques pour répondre aux nouveaux besoins technologiques.Des efforts de recherche considérables ont été dédiés ces dernières décennies 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 aux propriétés uniques pourraient répondre à cette attente en donnant l'opportunité de stocker l'information à l'échelle du nanomètre voir à l'échelle de la molécule. Toutefois, il reste un immense chemin à parcourir avant que ces matériaux moléculaires puissent être utilisés dans de telles applications et ceci impliquera des innovations et/ou améliorations aussi bien des systèmes moléculaires et technologies requises. Parmi cela, un des défis sera de développer des méthodes pour organiser et mettre en forme ces composés. A cet égard, d'intense recherches ont été dédiées aux dépôts de ces matériaux sur des surfaces et de nombreuses approches basées par exemples sur les monocouches auto-organisées (SAM : self-assembled monolayer), la lithographie, la sublimation ou les films Langmuir-Blodgett ont été explorées. Une autre alternative pour faciliter leur dépôt consiste à insérer ces composés au sein de phases molles telles que des gels ou des cristaux-liquides. La conception d'hybrides magnétiques est devenue une des principales activités de recherche du groupe Matériaux Moléculaires et Magnétisme (M 3 ) au Centre de Recherche Paul Pascal (CRPP). 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 leur 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. C'est dans cette philosophie que ce travail de thèse a été réalisé. Ainsi, nous nous sommes intéressé à divers systèmes moléculaires incluent des molécule-aimants, des complexes à conversion de spin et transfert d'électron avec pour premier objectif d'induire des propriétés de cristalliquide par une fonctionnalisation rationnel de leur ligands. Diverses mesures physico-chimiques ont été réalisées afin de mieux comprendre l'effet de nos stratégies de fonctionnalisation aussi bien sur le comportement magnétique que sur l'auto-organisation des matériaux hybrides produits. Ce manuscrit de thèse comprend quatre chapitres. Dans le premier chapitre, les concepts fondamentaux sur les comportements de molécule-aimant, de conversion de spin et transfert d'électron sont y sont résumés. Dans la deuxième partie de ce chapitre, une étude bibliographique fait un bref état de l'art des études menées sur des hybrides magnétiques, notamment possédant des propriétés de cristal-liquide, de gel ou des systèmes solubles. Le chapitre II est consacré à la préparation et étude d'hybrides magnétiques basés sur le complexe dodecamanganèse [Mn 12 O 12 (OAc) 16 (H 2 O) 4 ] (Mn 12 -OAc, Figure R-1 (a)). Cette molécule est le premier exemple connus de molécule-aimant et demeure avec ses analogues celle qui est la plus étudiée à ce jour. D'autre part, c'est un des seules types de molécule-aimant ayant fait l'objet d'étude dans des systèmes cristal-liquides. 5 Dans les précédentes études, 5 des phases liquides cristallines ont pu être obtenues en remplacent les seize ligands acétate du complexe par des ligands benzoate porteurs de substituants mésogènes. Deux stratégies avaient ainsi été explorées: (a) la fonctionnalisation des ligands benzoate avec des groupements fortement lipophiles (longues chaînes alkyle), ou (b) le greffage de promoteurs smectogènes via des espaceurs aliphatiques. Dans le premier cas des phases cristallines ou mésophases cubiques ont été obtenues en raison de la forme globulaire des complexes tandis que des mésophases lamellaires ont été observées en utilisant la seconde approche. Dans la continuité de ces travaux, nous avons cherché à savoir s'il était possible de générer d'autres types d'organisation et d'améliorer les propriétés thermotropes de ces systèmes (en particulier, diminuer la température de clarification). A cet égard, nous avons proposé une nouvelle approche consistant à introduire des ligands mésogènes uniquement sur des positions spécifiques du complexe Mn 12 en se basant sur une fonctionnalisation régiosélective. En effet, en modifiant la molécule par des ligands mésogènes sur les positions équatoriales, nous espérerions que la molécule adopte une structure de forme discotique qui serait, selon la cohésion intermoléculaire, favorable à la formation de mésophases nématiques ou colonnaires (Figure R-1 (b)). Dans le cas d'une fonctionnalisation des positions axiales par des ligands mésogènes, il peut être anticipé que la structure moléculaire soit de type calamitique, ce qui conduirait à une organisation lamellaire. Au cours de ce travail de thèse, nous nous sommes essentiellement focalisée sur la première option. Une fonctionnalisation régiosélective du complexe Mn 12 est possible en raison de la différence de réactivité entre les ligands carboxylate présents sur les positions axiales et ceux occupant les positions équatoriales. Celle-ci peut s'expliquer par les élongations de Jahn Teller sur les ligands axiaux. De ce fait, les positions axiales sont stabilisées par des ligands de faible pK a alors que des ligands plus basiques sont plus favorables pour les positions équatoriales.

Figure R- 1 .

 1 Figure R-1. (a) Structure cristallographique du complexe Mn 12 -OAc. Les liaisons sujettes à une élongation de Jahn Teller sont montrées en bleu (b) Stratégies de fonctionnalisation envisagées pour obtenir des phases cristal-liquides.

Figure R- 2 .

 2 Figure R-2. Structure cristalline du complexe [Mn 12 O 12 (CF 3 COO) 8 ax (L 1 ) 8 eq (H 2 O) 4 ] vue selon le plan bc (gauche) et plan ac (droite). Les atomes d'hydrogène sont omis pour plus de clarté. Couleur du schéma: Mn IV vert clair, Mn III vert fonce, O rouge, C gris, F jaune.

Figure R- 3 .Scheme R- 2 .

 32 Figure R-3. Dépendance en température du χ'T et χ" pour les complexes [Mn 12 O 12 (CF 3 COO) 8 (L 1-4 ) 8 (H 2 O) 4 ] mesuréess pour des champs oscillants de différentes fréquences (1-1500 Hz) et sous un champ permanent H dc nul.

Figure R- 4 .

 4 Figure R-4. Dépendance en température du produit χT pour les trois formes polymorphes du complexe [Fe(L 1 ) 2 ].

Schéma R- 3 .

 3 Structure chimiques des complexes [Fe(L i ) 2 ] synthétisés en vue d'induire des propriétés cristal-liquides. Bien que cette stratégie n'a permis en aucun cas d'obtenir les propriétés cristal-liquides désirées, la fonctionnalisation du ligand par des chaines alkyle a permis de mettre en évidence des effets intéressants sur le comportement magnétique des matériaux hybrides. Les complexes contenant les ligands L 4 -L 6 (une seule chaine alkyle), montrent une transition de spin incomplète due à la formation d'une phase intermédiaire où les deux espèces Bas-Spin (BS) et Haut-Spin (HS) sont présentes. Par exemple, le complexe [Fe(L 4 ) 2 ] exhibe une transition entre la phase diamagnétique BS et la phase intermediaire BS-HS (Figure R-5, gauche). Ce phénomène a été directement attribué à la présence des chaines alkyle. En effet, la transition de spin est accompagnée par une augmentation de la distance interlamellaire (axe b) résultant du changement de la géométrie coudée du complexe (Figure R-5, droite). Par conséquent, afin de maintenir une interdigitation des chaines, ceci entraine une compression de la couche contenant les centres métalliques (plan ac),

Figure R- 4 .

 4 Figure R-4. (Gauche) Dépendance en température du produit χT pour [Fe(L 4 ) 2 ]. (Droite) Structure cristallographique du complexe [Fe(L 4 ) 2 ] dans la phase BS et phase intermédiaire BS-HS. Les sites BS et HS sont respectivement représentés en orange et vert.

Figure R- 6 .

 6 Figure R-6. Evolution des profils SAXS avec la température pour les complexes 5 (gauche) et 6 (droite).

Figure R- 7 .

 7 Figure R-7. Dépendance en température de la fraction {Fe III LS (μ-CN)Co II HS } pour le complexe 7 dans du dichlorométhane, du butyronitrile et du méthanol. La fraction a été estimé par l'absorption sur la bande de transfert de charge intermétallique a 540 nm.

  2

  63

  Hashimoto and co-workers reported that Na 0.07 Co 1.50[Fe(CN) 6 ]•6.3H 2 O and Na 0.94 Co 1.15 [Fe(CN) 6 ]•3.0H 2 O do not display ET and remain paramagnetic and diamagnetic, respectively, 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 systems display photo-induced ET as well as thermally-induced first-order transition with a thermal hysteresis of about 40 K. 69e

6 ] n -sites are primary to result reversible thermally and photo-induced magnetization changes in the material. For example,

  2 ] 2+ building blocks. The ET was found to occur between the {Co 3

			II Fe III 2 } high-temperature
	phase and {Co 2 II Co III Fe II Fe III } low-temperature phase with a statistical disorder of Co III and Fe II in two
	positions. After exposition to moisture, the new phase was generated and the authors reported the occurrence
	of ET from {Co 3	II Fe III	2 } phase into {Co II Co 2 III Fe II 2 } by cooling below 200 K. This latter was photo-active
	(generating the {Co 3 II Fe III 2 } metastable phase) in contrast to previous {Co 2 II Co III Fe II Fe III } low-temperature
	phase.

83c

After this seminal work, our team developed in collaboration with Mathonière and Holmes's groups the first molecular fragment that mimics not only the properties (thermal and photo-induced ET) but also the structure of 3D Fe/Co Prussian Blue analogue (cubic elementary unit; Figure

I-16

  Fe III (CN) 3 ] 4 [Co II (pz 3 CCH 2 OH)] 4 }(ClO 4 ) 2 4H 2 O13DMF (pzTp = tetrapyrazolylborate, pz 3 CCH 2 OH = 2,2,2-tris(pyrazolyl)ethanol) was obtained by a rational building block approach using capping ligands. The complex exhibits a reversible first-order transition between {FeIII 4 Co II 4 } high temperature and {FeII 4 Co III 4 } low-temperature phase as a result of thermally-induced intramolecular ET occurring at 255 K. The ET process is accompanied by a paramagnetic-to-diamagnetic switching as a result

).

84

This octanuclear Chapter I -Generalities and context

I.25 complex, {[(pzTp)

  2

  O 12 (CH 3COO) 16 (H 2O) 4 ], 1:1:1:1 isomer, 1a (b) [Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ], 2:0:2 isomer, 8 (c) [Mn 12 O 12 (CHCl 2 COO) 8 (tBu-CH 2 COO) 8 (H 2O) 3 ], 2:0:1 isomer.9 Color scheme: Mn(IV) light green, Mn(III) green, O red, O water blue.

  10

	Figure II-4. (top) d orbital electronic
	configurations of distorted d 4 high spin
	complexes;	(bottom)	Schematic
	representation of regular octahedral complex
	(O h ) and the influence of the Jahn-Teller
	distortion with axial elongation.

z Chapter II -Rational Functionalization of Single-Molecule Magnets: Towards LC Phases

  Due to the presence of JT elongation in the axial Mn III -O bonds, the reactivity of carboxylate ligands is different depending to their occupation in axial or equatorial positions (See FiguresII-2). Indeed, the Mn III -O bonds with JT distortion are weaker than the other ones making the axial positions more sensitive to nucleophilic substitutions. Also, the JT elongations maintain a greater electron density on the oxygen atoms and will favor therefore the presence of ligands with low basicity. Taking into account the difference of reactivity between axial and equatorial positions, it is possible to make regioselective substitutions on the Mn 12 core and few examples of mixed-ligand complexes [Mn 12 O 12 (RCOO) 8 (L) 8 (H 2O) 4 ] (with RCOO = carboxylate and L = carboxylate or sulfonate ligands) have been reported.9,14 Two synthetic approaches have been explored.The first method involves the direct addition of 8 equivalents of ligand HL (in its protonated form) to a Mn 12 O 12 (RCOO) 8 (L) 8 H 2O) 4 ] (eq.2).

	pre-formed	[Mn 12 O 12 (RCOO) 16 (H 2 O) 4 ]	complex	to	generate	a	mixed-ligand	complex
	[
		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 (eq.1)
	II.56

14

[Mn

12

O 12 (RCOO) 16 (H 2 O) 4 ] + 8 HL [Mn 12 O 12 (RCOO) 8 (L) 8 (H 2 O) 4 ] + 8 RCOOH (eq.2)

  Mn 12 O 12 (OAc) 8 (Ph 2 PO 2 ) 8 (H 2 O) 4 ], in which the eight bulky diphenyl phosphonate ligands are not exclusively on the axial positions as it is expected, but are equivalently distributed over the axial and equatorial positions. 14b However, when only four bulky (PhO) 2 PO 2 -ligands are used, the previous rule is respected and the product [Mn 12 O 12 (PhCOO) 12 ((PhO) 2 (PO 2 ) 4 (H 2 O) 4 ] contains the four non-carboxylate ligands in axial positions. 14c

	eq (H 2 O) 4 ] and
	[Mn 12 O 12 (CHCl 2 COO) 8 ax (C 2 H 5 COO) 8

Mn 12 O 12 (RCOO)

16

(H 2

O) 4

] + [Mn

12

O 12

(R'

COO)

16

(H 2

O)

4 ] 2[Mn 12 O 12 (RCOO) 8 (R′COO) 8 (H 2

O)

4 ] (eq.

3)

This results in an 8:8 product in which the ligands with lower basicity are in the axial positions while the ligands with stronger basicity occupy the equatorial positions. To the best of our knowledge, only two examples of mixed-carboxylate complexes, namely [Mn 12 O 12 (CHCl 2 COO) 8 ax ( t BuCH 2 COO) 8 eq (H 2

O) 4

] have been reported so far.

9

It is worthwhile to mention that the electronic discrimination between axial and equatorial positions can be overcome by steric effects. This is the case in the mixed-ligands complex [

Functionalization of Mn 12 -based SMMs: Synthesis and Characterizations Accordingly

  to previous studies,9 the most efficient way to obtain mixed-carboxylate complexes [Mn 12 O 12 (RCOO) 8 (R'COO) 8 (H 2 O) 4 ] involves an equimolar reaction of the corresponding homo-substituted complexes. All the carboxylate ligands and Mn 12 precursors required for the preparation of the [Mn 12 O 12 (CF 3 COO) 8

	ax (L i ) 8	eq (H 2

8 (L 1 ) 8 (H 2

O) 4

].

Chapter II -Rational Functionalization of Single-Molecule Magnets: Towards LC Phases

II.60

Scheme II-3. Chemical structure of ligands L 1-4 used for the functionalization of equatorial positions of Mn 12 complex.

II.3.

  3, homo-substituted complexes [Mn 12 O 12 (L i ) 16 (H 2 O) 4 ] were prepared by ligand exchange reactions starting from [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] (eq.

[

  Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2O) 4 ], as crystals and to determine its structure by single-crystal X-ray diffraction analysis.

II.3.3.1. Single-crystal X-

ray diffraction analyses of [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ]

Table II - 1 .

 II1 Elemental analyses of [Mn 12 O 12 (CF 3 COO) 8 (L 1-4 ) 8 (H 2 O) 4 ]•xH 2 O (x = co-crystallized water molecules)

	Compound	%C (found)	%H (found) %C (calc.) %H (calc.)
	[Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ]•5H 2 O C 80 H 74 F 24 Mn 12 O 61	30.11	2.07	30.73	2.39
	[Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ]•5.5H 2 O C 168 H 251 F 24 Mn 12 O 61.5	46.75	5.92	46.17	5.79
	[Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ]•2H 2 O C 360 H 644 F 24 Mn 12 O 74	58.62	8.94	59.46	8.93
	[Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ]•5H 2 O C 424 H 506 F 24 Mn 12 O 85	62.86	6.67	63.04	6.31

Rational Functionalization of Single-Molecule Magnets: Towards LC Phases

8 (L 1 ) 8 (H 2

O) 4

], the χ'T value measured at the plateau 8 -12 K, ca. 54 cm 3 /mol, is favorably comparable to the value of [Mn 12

O 12

(CF 3

COO)

16

(H 2 O) 4 ], ca. 58 cm 3 /mol and in good accordance with S T = 10 with Landé factor g = 1.96 as typically found for the parent Mn 12 -OAc (Figure II-Chapter II -II.68 12, Table

II-2)

. It is worth noting that these values are significantly higher than the precursor [Mn 12

O 12

(L 1 ) 16 (H 2

O)

3 ] for which a S T = 9 with g = 1.93 was found in agreement with previous investigations.

16

Table II - 2 .

 II2 Magnetic Properties for [Mn 12 O 12 (CF 3 COO) 8 (L 1-4 ) 8 (H 2O) 4 ] complexes and its precursors.

				JT isomers	Fast relaxing (FR) JT	Slow relaxing (SR) JT
	Compound	S	g	ratios SR (%) / FR (%)	Δ eff /k B (K)	isomer	τ 0 (s)	Δ eff /k B (K)	isomer τ 0 (s)	μ 0 H C (T)
	[Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ]	10 1.86	100	-		-	70.1	4.8×10 -9	1.28
	[Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ]	9 1.75	100	-		-	69.1	6.0×10 -9	1.37
	[Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ]	9 1.80	35/65	51.1	1.64×10 -11	62.4	3.8×10 -9	0.17
	[Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ]	9 1.72	100	-		-	69	6.0×10 -9	0.97
	[Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ]	10 1.96	100	-		-	71.0	5.3×10 -9	1.16
	[Mn 12 O 12 (L 1 ) 16 (H 2 O) 3 ]	9 1.93	49/51	29.6	1.28×10 -9	63.7	4.7×10 -9	0.3
		9 1.91	70/30	34.7	2.7×10 -10	59.1	9.7×10 -9	0.55
		9 1.93	60/40	37.6	4.1×10 -10	58.0	2.0×10 -8	0.53
		9 1.86	65/35	31.7	1.9×10 -9	60.8	1.1×10 -8	0.72

[Mn 12

O 12

(L 2 ) 16 (H 2

O)

4 ] * [Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] * [Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] * * from ref. 16. S: spin ground state; g: Landé factor of ground state; Δ eff : potential energy barrier; k B : Boltzmann constant; τ 0 : preexponential factor; μ 0 H C (T): coercive field.

  8 (L 1 ) 8 (H 2O) 4 ] and its precursors) do not show any mesomorphic properties and the discussion will be thereafter essentially focused on the characterization of the [Mn 12O 12 (CF 3COO) 8 (L 2-4 ) 8 (H 2 O) 4 ] complexes and the comparison with their respective homo-substituted precursors. Phase transition temperatures, structural and thermodynamic parameters for [Mn 12 O 12 (CF 3 COO) 8 (L 2-4 ) 8 (H 2 O) 4 ] and [Mn 12 O 12 (L 2-4 ) 16 (H 2 O) 4 ] complexes are summarized in Table II-3.

Table II - 3 .

 II3 Parameters describing the thermal and structural properties of [Mn 12 O 12 (CF 3 COO) 8 (L 2-4 ) 8 (H 2 O) 4 ] and their precursors

	Compound	Transition a	T / K	ΔH / kJ•mol -1
	[Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ] [Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ]	Cr -I a Cr -Cub b Cub -I c	454 258 ~ 448	243.3 134.6 --
	[Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ]	Sm -I b	345	52.7
	[Mn 12 O 12 (L 2 ) 16 (H 2 O) 4 ] *	Cr -g a g -I a	340 385	--25.5
	[Mn 12 O 12 (L 3 ) 16 (H 2 O) 4 ] *	Cr -Cub b Cub -dec d	276 423	205.9 --
	[Mn 12 O 12 (L 4 ) 16 (H 2 O) 4 ] *	g -Sm b Sm -I b	295 347	--73.3

II.5.1. [

Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ] (one C 12 chain ligand)

The POM analysis for [Mn 12

O 12

(CF 3 COO) 8 (L 2 ) 8 (H 2

O) 4

] was not conclusive to determine the presence or absence of liquid crystalline features since no birefringent texture can be detected in all the temperature Chapter II -

Rational Functionalization of Single-Molecule Magnets: Towards LC Phases

  Indeed, the absence of birefringent textures can be either due to a homotropic arrangement as typically observed in cubic phases or the absence of any organization (amorphous solid). By increasing temperature above 420 K, an increase of the fluidity is observed but it is difficult to determine accurately the temperature at which the compound reaches the isotropic liquid phase. The DSC thermogram on first heating(Figure II-17) displays only one transition at 454 K which is in agreement with the transition into the isotropic phase based on POM and confirmed by the SAXS investigations. The thermal behavior is not reversible and the DSC thermograms remain silent on cooling and further heating/cooling cycles. The transition into the isotropic liquid phase appears at much higher temperature for this complex than its [Mn 12O 12 (L 2 ) 16(H 2

	range tested.
	II.73

  Mn 12 -based SMMs containing two types of carboxylate ligands are extremely rare in the literature and the extension to mesomorphic systems was not reported before. By applying a regioselective methodology, it was possible to prepare successfully four new complexes [Mn 12O 12 (CF 3COO) 8

	ax (L 1-4 ) 8 eq (H 2 O) 4 ].

  ). More particularly, most of the complexes display only one relaxation mode of the magnetization process (SR mode) in contrast with their precursors that show systematically both SR and FR modes. This was explained by the presence of only one JT isomer in the sample (with all JT distortions nearly orthogonal to the molecular plane) as deduced from the crystallographic structure of [Mn 12O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2O) 4 ]. As direct consequence, the magnetization hysteresis loops were found significantly larger than those found in [Mn 12O 12 (L 1-4 ) 8 (H 2 O) x ]. An exception appears for the [Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2O)

4 ] complexes, which does not follow the same trend and contains both JT isomers (where the FR specie is predominant), although it is difficult to understand the origin of this behavior and how to control the presence or not of the two JT isomers.

  3

  inert atmosphere for 4 days. After removal of the solvent, CH 2 Cl 2 (200 mL) was added to the light brown solid and the organic layer was washed with H 2 O (3x100 mL). The organic fraction was dried over anhydrous Na 2 SO 4 , filtered and the solvent removed under vacuum. After recrystallization from

	). To a solution of methyl 3,4,5-trihydroxybenzoate
	(4.00 g, 21.7 mmol) in 150 mL of butanone, was subsequently added anhydrous K 2 CO 3 (26.95 g, 195.1
	mmol), 1-bromododecane (19.20 g, 77.0 mmol) and a catalytic amount of KI (5 mol%). The reaction mixture
	was reflux under

ethanol (200 mL), filtration, and drying under vacuum, 1 is obtained as white powder. The precursor 1 is dissolved in hot EtOH (150 mL) and a solution of KOH in water was added

(20 wt%, 20 mL, 89.1 mmol)

and the reaction mixture was refluxed for 12 hours. After cooling to room temperature, the solution was diluted with H 2 O (100 mL) and a HCl (37 wt%) was dropwise added until reaching pH = 1. The solution is concentrated under reduced pressure, and extracted several times with CH 2 Cl 2 (3x100 mL). The combined organic layer was dried over anhydrous Na 2 SO 4 and filtered. After removal of the solvent, HL

3

was obtained as white powder: Yield 9.80 g, 66 %.

1

H NMR (400 MHz) in CDCl 3 (δ, ppm): 7.30 (s, 2H), 4.06 -3.80 (m, 6H), 1.85 -1.70 (m, 6H), 1.51 -1.41 (m, 6H), 1.38 -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 (76.39); H, 11.65 (11.80)

. Selected FT-IR data (ATR, cm -1 ): 2952 (m), 2916 (vs), 2847 (s), 1680 (vs), 1585 (s), 1504 (m), 1466 (m), 1429 (s), 1332 (s), 1225 (s), 1119 (vs), 968 (m), 863 (m), 720 (s).

Rational Functionalization of Single-Molecule Magnets: Towards LC Phases

  O (300 mL) and extracted several times with CH 2 Cl 2 (3x100mL). The organic fraction was dried over anhydrous Na 2 SO 4 , filtered and the solvent was removed under reduced pressure. The obtained solid was purified by column chromatography on silica gel using pentane/dichloromethane (7:3) as eluent. After evaporation, 3 was obtained as white microcrystalline powder: Yield 4.15 g, 89%.1 H NMR (400 MHz) in

	with H 2 CDCl 3 (δ, ppm): 7.58 -7.45 (m, 8H), 7.40 (t, J = 7.7 Hz, 4H), 7.30 -7.26 (m, 2H), 7.15 (d, J = 2.3 Hz, 2H),
	6.96 (d, J = 8.6 Hz, 4H), 6.63 (t, J = 2.3 Hz, 1H), 4.01 -3.93 (m, 8H), 3.87 (s, 3H), 1.88 -1.70 (m, 8H), 1.51
	-1.38 (m, 8H), 1.33 (s, 16H). Elemental analysis -Calc. (Found) for C 52 H 64 O 6 : C, 79.56 (79.46); H, 8.22
	II.83

3,5- dihydroxybenzoate (0.988 g, 5.88 mmol)

, 2

(4.7 g, 12.

09 mmol) and anhydrous K 2 CO 3 (4.

27 g, 4.26 mmol)

in 150 mL butanone was refluxed for 4 days. After cooling to room temperature, the mixture was diluted Chapter II -

  4

Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4

  ). To a solution of 3 (4.00 g, 5.086 mmol) in hot ethanol/THF (1:1; 100 mL) was added KOH (0.96 g, 17.08 mmol) dissolved in a minimum amount of water. The mixture was stirred under reflux for 12 hours. After cooling to room temperature, the solution was diluted with H 2 O (100 mL) and HCl (37 wt%) was dropwise added until reaching pH = 1. The solution is concentrated under reduced pressure, and extracted several times with CH 2 Cl 2 (3x300 mL). The organic layers were combined and dried over anhydrous Na 2 SO 4 , filtered and evaporated. The product HL4 was isolated as white powder: Yield 1.85 g, 47 %.

	Preparation of [Preparation of [

1

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

56 -7.46 (m, 8H), 7.39 (t, J = 6.7 Hz, 4H), 7.27 (t, J = 6.4 Hz, 2H), 6.94 (dd, J = 8.7, 2.1 Hz, 4H), 6.67 (d, J = 2.1 Hz, 1H), 4.01 -3.92 (m, 8H), 1.84 -1.72 (m, 8H), 1.44 (d, J = 6.1 Hz, 8H), 1.33 (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). ]. 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 few days. The resulting black crystals were filtered and washed with acetone (100 mL) and tetrahydrofuran (100 mL): Yield 2.10 g, 55 %. Selected FT-IR data (ATR, cm -1 ): 2934 (w), 1752 (w) 1704 (m), 1554 (s), 1381 (s), 1021 (m), 706 (m), 666 (s).

Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ].

	A suspension of [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] (1.868 g, 1.0
	mmol) in CH 2 Cl 2 (50.5 ml) and CF 3 COOH (10 ml) was stirred for 24 hours at room temperature. The
	solution was then filtered, transferred into in thin tube and layered with hexanes. After a week, black crystals
	were filtered and washed with Et 2 O and hexane: Yield 0.66 g, 30%. Selected FT-IR data (ATR, cm -1 ): 1673

(s), 1635 (s), 1597 (s), 1426 (s), 1599 (s), 1154 (s). Elemental analysis -Calc. (Found) for C 32 H 8 F 48 Mn 12 O 48 : C, 14.07

(14.40); H, 0.30 (0.90)

.

Chapter II -

Rational Functionalization of Single-Molecule Magnets: Towards LC Phases II.84 Preparation of [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 ] (0.573 g, 0.3 mmol) in 20 mL of CH 2 Cl 2 /toluene (1:1), 4-methoxybenzoic acid (HL 1 ) (1.27 g, 8.4 mmol, 28 eq.) was

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

  and crystallized by slow diffusion of Et 2 O vapor at 4°C. After 2 days, the resulting crystals were collected by filtration and washed with Et 2 O: Yield 0.53 g, 71.6%. 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, 31.64 (30.11); H, 2.12 (2.07). Selected FT-IR data (ATR, cm -1 ): 2928 (w), 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, HL i (0.45 g (HL 2

	2837 (w), 1601 (s), 1382 (vs), 1248 (s), 1168 (s), 1024 (m), 847 (w), 776 (m).

Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ]

  . A solution of complex [Mn 12 O 12 (L 1 ) 16 (H 2 O) 4 ] (0.125 g, 0.043 mmol) in CH 2 Cl 2 (4 mL) was treated with a solution of complex [Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ] (0.153 g, 0.043 mmol) in THF (4 mL) . After 24 hours, the solution was filtered and the filtrate was allowed to stand for slow evaporation. The resulting brownish black crystals were collected by filtration, and washed with small fraction of Et 2 O and THF: Yield 68 mg, 50 %. Elemental analysis -Calc. (Found) for C 80 H 74 Mn 12 O 61 :

	Preparation of [

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 eluted with CH 2 Cl 2 . The compounds [Mn 12

O 12

(L i ) 16 (H 2

O)

4 ], were isolated as brownish black powder or glassy solids. The yields were 70 % (i = 2), 61 % (i = 3) and 42 % (i = 3). 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 (62.32); H, 8.21 (8.17). Selected FT-IR data (ATR, cm -1 ): 2922 (m), 2853 (m), 1604 (s), 1380 (vs), 1250 (s), 1173 (m), 848 (w), 779 (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 (67.30); H, 9.87 (9.72). Selected FT-IR data (ATR, cm -1 ): 2924 (s), 2852 (s), 1561(s), 1412 (vs), 1248 (m), 1114 (m), 762 (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 (69.85); H, 10.68 (10.79)

. Selected FT-IR data (ATR, cm -1 ): 2920 (s), 2852 (s), 1567 (s), 1409 (vs), 1227 (s), 1112

(m)

, 868 (w), 777 (w). C,

30.73 (30.11); H, 2.39 (2.07)

. Selected FT-IR data (ATR, cm -1 ): 2968 (w), 1660 (s),

1603 (s), 1393 (vs), 1257 (s), 1177 (vs), 1146 (vs), 847 (s), 776 (s)

.

Chapter II -

Rational Functionalization of Single-Molecule Magnets: Towards LC Phases II.85 Preparation of [Mn 12 O 12 (CF 3 COO) 8 (L i ) 8 (H 2 O) 4 ] (i = 2-4).

  To a suspension of complex [Mn12 O 12 (CF 3COO) 16 (H 2O) 4 ] (0.164 g, 0.0602 mmol) in dry CH 2 Cl 2 (2 mL) was added a solution of [Mn 12 O 12 (L i ) 16 (H 2 O) 4 ] (0.35 g (i=2), 0.704 g (i=3), 0.80 g (i=4); 0.0602 mmol) in dry CH 2 Cl 2 (2 mL) . After 24 hours, the solution was filtered and the solvent was evaporated to dryness. The product was dried in vacuum at 40°C overnight. The yields were 53 % for [Mn 12 O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ], 48% for [Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ] and 52% for [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ]. Elemental analysis -Calc. (Found) for C 80 H 64 F 24 Mn 12 O 56 ([Mn 12O 12 (CF 3 COO) 8 (L 2 ) 8 (H 2 O) 4 ]): C, 46.17(46.75); H, 5.79 (5.92). Elemental analysis -Calc. (Found) for C 360 H 624 F 24 Mn 12 O 72 ([Mn 12 O 12 (CF 3 COO) 8 (L 3 ) 8 (H 2 O) 4 ]): Calc. (Found) for C 424 H 496 F 24 Mn 12 O 80 ([Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ]): C, 63.04(62.86); H, 6.31 (6.67). Selected FT-IR data (ATR, cm -1 ):

		Selected
	FT-IR data (ATR, cm -1 ): 2924 (s), 2849 (s), 1603 (s), 1601 (s), 1493 (s), 1409 (vs), 1255 (s), 1175 (vs), 817
	(s), 775 (s). C, 59.46 (58.62); H, 8.93 (8.94). Selected FT-IR data (ATR, cm -1 ): 2920 (s), 2850 (s), 1710 (s), 1672 (s),
	1409 (vs), 1376 (vs), 1491 (s), 1409 (vs), 1201 (vs), 1146 (s), 1063 (s), 905 (m), 848 (m), 823 (s), 776 (m),
	716 (s), 696 (s).	Elemental analysis -

2922 (s), 2850 (s), 1664 (s), 1605 (s), 1518 (s), 1484 (s), 1463 (s), 1410 (vs), 1380 (s), 1246 (s), 1155 (vs), 1110 (s), 1046 (s), 829 (s), 762 (vs), 691 (s). II.7.2. Analytical characterizations II.7.2.1.

Crystallographic data for [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ]Table S .

 S II-1. Crystal data and structure refinement for [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ] at 120K.

	Empirical formula	C 80 H 56 F 24 Mn 12 O 56	C 84 H 66 Mn 12 O 57 F 24
	Formula weight	3028.52	3102.61
	Crystal system	Orthorhombic	Tetragonal
	Space group	C222	I4 1 /a
	Wavelength, Å	0.71073	0.71073
	a (Å)	17.7568(16)	34.586(4)
	b (Å)	28.5324(18)	34.586(4)
	c (Å)	32.136(2)	12.6043(11)
	α = β = γ (°)	90	90
	V (Å 3 )	16282(2)	15077(4)
	Z		2	4
	ρ calcd (g/cm 3 )	0.618	1.465
	μ (mm -1 )	0.494	1.077
	Reflections collected	105640	4949
	Independent reflections	5372	2649
	R 1	a	0.0786	0.1135
	wR 2 a	0.2220	0.4404
	GoF a	1.032	1.132
	a

Table S .

 S II-2. Selected bond length distances for [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ] and [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ]•Et 2 O Bond Type [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ] [Mn 12 O 12 (CF 3 COO) 8 (L 1 ) 8 (H 2 O) 4 ]•Et 2 O

	Mn-O	Length / Å	Mn-O	Length / Å
	Mn IV -O (cub)

Table S .

 S II-3. Indexation of observed reflections on 1 st heating for [Mn 12 O 12 (F 3 CCOO) 8 (L 2 ) 8 (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
			3.16	19.90	111	3	20.06
	313 K		4.30	14.61	211	6	14.18
			5.34	11.76	220	8	11.76
			5.57	11.28	300/221	9	11.58	a = 34.75 Å
			6.04	10.41	311	11	10.47	V = 41.9•10 3 Å
			7.33	8.57	400	16	8.68	Z ≈ 6
			849	7.40	332	22	7.41
			9.32	6.74	510/431	26	6.81
			10.27	6.12	-	31	6.24
		10	10.41	6.03	440	32	6.14
			3.13	20.06	111	3	20.21
	348 K		4.31	14.60	211	6	14.29
			5.34	11.75	220	8	12.37
			5.49	11.44	221/300	9	11.67	a = 35.01Å
			6.09	10.31	311	11	10.55	V = 42.9•10 3 Å
			7.16	8.76	400	16	8.75	Å 3 ÅÅ 3 Z ≈ 6
			8.37	7.51	241	21	7.63
			8.63	7.27	-	23	7.30
			10.07	6.23	-	31	6.28
		10	10.21	6.15	440	32	6.18
			3.11	20.18	111	3	20.27
	363 K		4.32	14.53	211	6	14.33
			5.39	11.66	220	8	11.70
			5.44	11.54	221/300	9	11.70	a = 35.12 Å
			6.11	10.28	222	12	10.13	V = 43.3•10 3 Å 3
			7.01	8.86	400	16	8.78	Z  6
			8.31	7.56	241	21	7.66
			8.68	7.23	-	23	7.32
			8.96	7.00	500	25	7.02
		10	9.98	6.29	-	31	6.30
			3.10	20.22	111	3	20.32
	383 K		4.34	14.46	211	6	14.37	a = 35.21 Å
			5.37	11.69	221/300	9	11.73	V = 43.6•10 3 Å 3
			6.14	10.23	222	12	10.16	Z  6
			6.98	9.00	400	16	9.09
			8.22	7.63	241	21	7.68
			8.81	7.13	224	24	7.18
			9.85	6.38	-	30	6.42
			10.25	6.13	-	33	6.13
		10	10.68	5.88	-	35	5.95
			3.06	20.53	110	3	20.51
			4.35	14.44	211	6	14.51
	431 K		5.31	11.81	221/300	9	11.84
			6.13 6.83 8.11	10.25 9.19 7.74	222 -241	12 15 21	10.25 9.17 7.75	a = 35.53Å V = 44.8•10 3 Å 3
			8.36	7.51	332	22	7.57	Z  6
			8.63	7.27	224	24	7.25
			9.67	6.49	-	30	6.48
		10	10.17	6.17	-	33	6.18

Table S .

 S II-4. Indexation of observed reflections on 3 rd cooling and 4 th heating for [Mn 12 O 12 (F 3 CCOO) 8 (L 3 ) 8 (H 2O) 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
			2.27	27.66	110	2	27.51
			3.22	19.50	200	4	19.45
	298 K a		3.91 4.53	16.04 13.84	211 220	6 8	15.88 13.75	a = 38.90 Å V = 58.8•10 3 Å 3
			4.75	13.20	221/300	9	12.96	Z ≈ 5
			5.05	12.43	310	10	12.30
			5.98	10.49	231	14	10.39
			2.27	27.68	110	2	27.54
			3.20	19.61	200	4	19.47
	343 K a		3.91 4.51	16.04 13.92	211 220	6 8	15.90 13.77	a = 38.95 Å V = 59.1•10 3 Å 3
			4.75	13.20	221/300	9	12.98	Z ≈ 5
			5.05	12.43	310	10	12.31
			5.97	10.51	231	14	10.41
			2.25	27.89	110	2	27.90
			3.20	19.60	200	4	19.73
	353 K a		3.91 4.52	16.03 13.89	211 220	6 8	16.11 13.95	a = 38.95 Å V = 59.1•10 3 Å 3
			4.9	13.10	221/300	9	13.15	Z ≈ 5
			5.05	12.43	310	10	12.47
			5.96	10.52	231	14	10.54
			2.25	27.91	110	2	27.94
			3.20	19.62	200	4	19.76
	363 K a		3.92 4.52	16.04 13.90	211 220	6 8	16.13 13.97	a = 39.52 Å V = 61.7•10 3 Å 3
			4.79	13.11	221/300	9	13.17	Z ≈ 5
			5.05	12.44	310	10	12.49
			5.98	10.50	231	14	10.56
			2.23	28.11	110	2	28.49
	303 K b		3.17 3.88 4.46 4.86	19.81 16.18 14.06 12.90	200 211 220 221/300	4 6 8 9	20.15 16.45 14.24 13.43	a = 40.3 Å V = 65.4•10 3 Å 3 Z ≈ 5
			5.10	12.31	310	10	12.74
			2.25	27.89	110	2	27.93
	348 K b

a = 39.51 Å V = 61.6•10 3 Å 3

  -3 rd cooling; b -4 th heating; q (obs) is the observed reciprocal spacing, q = 4sin/,where  is the wavelength (1.5418 Å), 2 is the diffraction angle, d (obs) and d (calc) are observed and calculated diffraction spacing distances: d (obs)

		3.18	19.70	200	4	19.75
		3.89	16.14	211	6	16.12
		4.48 4.77	14.00 13.14	220 221/300	8 9	13.96 13.17	Z ≈ 5
		5.08	12.35	310	10	12.49
		2.27	27.67	110	2	27.40
		3.21	19.58	200	4	19.38	a = 38.75 Å
	403 K b	3.89	16.14	211	6	15.82	V = 58.2•10 3 Å 3
		4.49	13.97	220	8	13.70	Z ≈ 5
		5.03	12.49	310	10	12.25

a

Table S

 S

.II-5. Indexation of observed reflections on first cooling for [Mn 12 O 12 (CF 3 COO) 8 (L 4 ) 8 (H 2 O) 4 ]. T / K Peak number q (obs) /

nm -1 d (obs) /Å hkl Calculated cell parameters

					Chapter III
		343 K	1	1.84	34.17	001
		cooling	2	3.68	17.08	002	d = 34.10
			3	3.82	16.44	d'
			4	5.55	11.32	003
		323 K	1	1.84	34.18	001
		cooling	2 3	3.68 3.80	17.07 16.52	002 d'	d = 34.16
			4	5.52	11.39	003
		303 K	1	1.84	34.17	001
		cooling	2	3.68	17.08	002	d = 34.18
			3	3.80	16.52	d'
			4	5.51	11.40	003
	1 𝑛	∑ 𝑖 • 𝑑 00𝑖 𝑛 𝑖=1	; d' corresponds to the distance of the short-range 2D organization
							III.97

q (obs) is the observed reciprocal spacing, q = 4sin/,where  is the wavelength, 2 is the diffraction angle, d (obs) observed diffraction spacing: d (obs) = 2/q (obs) , d is average of lamellar periodicity: d =

Mononuclear Fe II Spin-Crossover Complexes: From Crystalline Toward Soft Materials

Table of Contents for Chapter III: III.1. Introduction

 of .................................................................................................................................. III.

100 III.2. SCO complexes with non-mesogenic ligands

  ............................................................................. III.102 III.2.1. Mononuclear Fe(II) complexes based on HL 1 ligand ................................................... III.102 III.2.1.1. Synthesis of [Fe(L 1 ) 2 ] and general comments ............................................................. III.102 III.2.1.2. Analytical characterizations ........................................................................................ III.105 III.2.1.3. Spin crossover properties ............................................................................................ III.112 III.2.1.4. Synthesis and properties of cationic complexes ........................................... III.116 III.2.2. Synthesis and properties of [Fe(L 2 ) 2 ] complexes .................................................... III.119 III.2.2.1. Synthetic approach and general comments ................................................................. III.119 III.2.2.2. Structural description of [Fe(L 2 ) 2 ] polymorphs .......................................................... III.120 III.2.2.3. Magnetic properties ..................................................................................................... III.124 III.2.3. Synthesis and properties of [Fe(L 3 ) 2 ] complexes ........................................................... III.127 III.2.3.1. Synthesis and structural description ............................................................................ III.127 III.2.3.2. Thermal and magnetic behavior .................................................................................. III.129 III.2.4. Summary and concluding remarks ................................................................................. III.130

III.3. Functionalization of Fe II N 4 O 2 complexes with alkyl chains

..................................................... III.132 III.3.1. Synthesis of ligands and [Fe(L 4-8 ) 2 ] complexes ............................................................. III.132 III.3.2. Properties of [Fe(L 4 ) 2 ] complex ..................................................................................... III.133 III.3.2.1. Thermo-structural features .......................................................................................... III.133 III.3.2.2. Magnetic measurements and single-crystal X-ray diffraction analysis....................... III.136 III.3.3. Extension to other complexes with mono-alkylated ligands .......................................... III.142 III.3.3.1. Structural and magnetic properties of [Fe(L 5 ) 2 ] complex ........................................... III.143 III.3.3.2. Thermo-structural and magnetic properties of [Fe(L 6 ) 2 ] complex .............................. III.145 III.3.4. Functionalization with multiple dodecyl chains ............................................................ III.148 III.3.4.1. Thermo-structural properties ....................................................................................... III.148 III.3.4.2. Magnetic properties ..................................................................................................... III.152 III.4. Conclusions and perspectives .......................................................................................................

III.154 III.5. Supporting materials

  Synthesis and characterization of HL 1-8 ligands ......................................................... III.157 III.5.1.2. Synthesis and characterization of Fe(II) complexes ................................................... III.159 III.5.2. Selected FTIR spectra .................................................................................................... III.162 III.5.3. Crystallographic data ..................................................................................................... III.163 III.5.4. Thermal data .................................................................................................................. III.171 III.5.5. Structural, thermal and magnetic properties of γ-[Fe(L 1 ) 2 ] complex ............................. III.173 III.6. References ...................................................................................................................................... III.175

.................................................................................................................... III.157 III.5.1. Experimental protocols .................................................................................................. III.157 III.5.1.1.

1 ligand III.2.1.1. Synthesis of [Fe(L 1 ) 2 ] and general comments

[Fe(L 1 ) 2

Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials III

  .103 absence of base, the cationic forms, [Fe(HL 1 )(L 1 )] + and [Fe(HL 1 ) 2 ] 2+ , can be selectively isolated depending on the synthetic conditions. The synthesis, structural descriptions and magnetic properties of these cationic complexes are briefly discussed in section III.2.1.4. Synthetic pathway for the formation of [Fe(L 1 ) 2 ]

	Scheme III-3.

). Indeed, in Chapter III -

Table III - 1 .

 III1 Crystallographic data for [Fe(L 1 ) 2 ]•MeOH polymorphs at 120 K.

	Compound	α-{[Fe(L 1 ) 2 ]•MeOH}	β-{[Fe(L 1 ) 2 ]•MeOH}
	Temperature, K	120	120
	Crystal system	Monoclinic	Triclinic
	Space group	P2 1 /c	P-1
	a, Å	24.1677(6)	8.7893(4)
	b, Å	13.2155(4)	11.1963(5)
	c, Å	8.7284(2)	14.8884(7)
	α, °	90	111.717(2)
	β, °	99.860(1)	91.037(2)
	γ, °	90	97.507(2)
	V, Å 3	2746.6(1)	1345.96
	Z	4	2
	<Fe-N (py) >, Å	1.953	2.233
	<Fe-N (im) >, Å	1.865	2.114
	<Fe-O>, Å	1.976	2.101
	V FeN4O2 , Å 3	9.34	11.84
	Σ(X-Fe-X), a °	81.1	157.6
	a ∑ = ∑ |90 -𝜑 𝑖 | 12 𝑖=1

Table III - 2 .

 III2 Crystallographic data for α-{[Fe(L 1 ) 2 ]•MeOH} and α-[Fe(L 1 ) 2 ]

	Compound	α-{[Fe(L 1 ) 2 ]•MeOH}		α-[Fe(L 1 ) 2 ]
	Temperature, K	120	120	260	350 a	370
	Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
	Space group	P2 1 /c	P2 1 /c	P2 1 /c	P2 1 /c	P2 1 /c
	a, Å	24.1677(6)	22.7027(14)	22.806(6)	22.88	21.451(3)
	b, Å	13.2155(4)	12.9995(8)	13.1051(14)	13.22	14.565(3)
	c, Å	8.7284(2)	8.7405(6)	8.7743(9)	8.80	8.6226(8)
	β, °	99.860(1)	98.705(3)	98.307(4)	98.32	96.467(6)
	V, Å 3	2746.6(1)	2549.8(3)	2595.1(3)	2634.2	2676.9(8)
	Z	4	4	4	--	4
	<Fe-N (py) >, Å	1.953	1.960	1.958	--	2.203
	<Fe-N (im) >, Å	1.865	1.865	1.862	--	2.060
	<Fe-O>, Å	1.976	1.970	1.967	--	2.079
	Σ(X-Fe-X), b °	81.1	80.3	80.4	--	144.0
	C8•••C13, Å	3.318	3.311	3.362	--	3.341
	a Only the unit cell was checked at 350 K. b ∑ = ∑ |90 -𝜑 𝑖 | 12 𝑖=1

Table III - 3 .

 III3 Selected SCO parameters for [Fe(L 1 ) 2 ] polymorphs

	Compound	α-[Fe(L 1 ) 2 ]	β-[Fe(L 1 ) 2 ]
	T 1/2↑ , a K	370	262
	T 1/2↓ , a K	332	235
	ΔT 1/2 , a K	38	27
	ΔH↑ / ΔH↓, b kJ•mol -1	18.4 / 18.0	12.7 / 10.9
	ΔS↑ / ΔS↓, b J•mol -1 •K -1	50 / 52	48 / 46
	a determined by magnetic measurements at 0.3 K/min, b determined by DSC measurements at 10 K/min.

Table III - 3 .

 III3 Crystallographic data for [Fe(HL 1 ) 2 ] 2+ and [Fe(HL 1 )(L 1 )] + compounds at 120 K.

	Compound	[Fe(

HL 1 ) 2 ](ClO 4 ) 2 [Fe(HL 1 )(L 1 )](ClO 4 )•MeOH

	Temperature, K	120	120
	Crystal system	Monoclinic	Triclinic
	Space group	P2 1 /c	P-1
	a, Å	11.320(1)	10.876(1)
	b, Å	12.611(1)	11.557(1)
	c, Å	21.564(2)	13.444(1)
	α, °	90	72.307(2)
	β, °	90.458(2)	79.470(2)
	γ, °	90	79.920(2)
	V, Å 3	3078.4(1)	1569.9(1)
	Z	4	2
	Σ(X-Fe-X), a °	165.5	125.3
	<Fe-N (py) >, Å	2.199	2.225
	<Fe-N (im) >, Å	2.121	2.107
	<Fe-O>, Å	2.122	2.153
	C=O, Å	1.251 (av)	1.233
	C-O, Å	--	1.289
	(O)C=N, Å	--	1.336
	(O)C-N(H), Å	1.366 (av)	1.364
	a ∑ = ∑ |90 -𝜑 𝑖 | 12 𝑖=1

Mononuclear Fe II spin crossover complexes: from crystalline toward soft materials

3

•K•mol

-1

, in good Chapter III -III.119 agreement with the expected value for Fe II HS (S = 2, with g ≈ 2

Table III - 4 .

 III4 Crystallographic data for [Fe(L 2 ) 2 ] polymorphs at 120 K.

	Compound	α-[Fe(L 2 ) 2 ]	β-[Fe(L 2 ) 2 ]
	Temperature, K	120	120
	Crystal system	Monoclinic	Triclinic
	Space group	C2/c	P-1
	a, Å	23.0807(7)	9.2954(4)
	b, Å	10.8025(3)	12.2093(5)
	c, Å	11.8221(2)	13.4330(6)
	α, °	90	70.760(2)
	β, °	105.654(1)	79.229(2)
	γ, °	90	85.841(2)
	V, Å 3	2838.3	1413.8
	Z	4	2
	<Fe-N (py) >, Å	2.261	1.954
	<Fe-N (im) >, Å	2.111	1.866
	<Fe-O>, Å	2.112	1.978
	Σ(X-Fe-X), a °	166.6	83.0
	a ∑ = ∑ |90 -𝜑 𝑖 | 12 𝑖=1

Table III -

 III

	Temperature	120 K	300 K	390 K	400 K	450 K
	Crystal system	Triclinic	Triclinic	Triclinic	Triclinic	Triclinic
	Space group	P-1	P-1	P-1	P-1	P-1
	a, Å	9.2954(4)	9.3765(3)	9.402(2)	9.3875(6)	9.611(2)
	b, Å	12.2093(5)	12.2641(3)	12.415(2)	12.4485(8)	12.544(2)
	c, Å	13.4330(6)	13.5941(4)	13.709(2)	13.7215(9)	13.543(9)
	α, °	70.760(2)	70.018(1)	69.595(5)	69.609(3)	72.868(7)
	β, °	79.229(2)	78.963(1)	78.414(6)	78.233(3)	74.071(7)
	α, °	85.641(2)	84.903(1)	83.639(5)	83.296(3)	75.934(7)
	V, Å 3	1413.8	1441.5	1467.7	1469.6	1478.0
	Z	2	2	2	2	2
	<Fe

5.

Crystallographic data for β-[Fe(L 2 ) 2 ] between 120 and 450 K

-N (py) >, Å

	1.954	1.957	2.010	2.035	2.212

<Fe-N (im) >, Å

		1.866	1.867	1.916	1.938	2.090
	<Fe-O>, Å	1.978	1.981	2.001	2.005	2.067
	Σ(X-Fe-X), a °	83.0	83.9	96.8	102.5	144.9

a ∑ = ∑ |90 -𝜑 𝑖 | 12 𝑖=1

  General pathway for the synthesis of N'-(pyridin-2-ylmethylene)benzohydrazide ligands and complexes. Conditions: (i) C n H 2n+1 Br, K 2 CO 3 , butanone, reflux, 16h. (ii) N 2 H 4 •H 2 O, EtOH, reflux, 16h. (iii) 2pyridinecarboxaldehyde or 5-chloropicolinaldehyde, RT, MeOH, 16h. (iv) Fe(ClO 4 ) 2 •6H 2 O, NEt 3 , MeOH, 60°C, 2h.

Scheme III-6.

Table III - 7 .

 III7 Crystallographic data for [Fe(L4 ) 2 ] at 250 and 360 K .

	Temperature / K	250	360
	Crystal system	Monoclinic	Orthorhombic
	Space group	P2 1 /c	P2 1 2 1 2
	a / Å	10.4722(14)	10.036(2)
	b / Å	40.801(5)	46.263(10)
	c / Å	14.2308(16)	10.951(2)
	β / ˚	128.100(7)	90
	V / Å 3	4787.9(11)	5084.5(18)
	Z	4	4
	Flack parameter	--	0.49
	Bond Lengths	Site 1	Site 1 (Fe1)	Site 2 (Fe2)
	<Fe-N py >, Å	1.938	1.917	2.203
	<Fe-N im >, Å	1.867	1.854	2.092
	<Fe-O>, Å	1.966	1.972	2.073
	Σ(X-Fe-X), a °	85.7	81.8	158.8
	a ∑ = ∑ |90 -𝜑 𝑖 | 12 𝑖=1

Table III - 8 .

 III8 Crystal data for [Fe(L 5 ) 2 ]

	Temperature / K	120
	Crystal system	Monoclinic
	Space group	P2 1 /c
	a / Å	22.8322(13)
	b / Å	12.6464(8)
	c / Å	17.092(11)
	β / ˚	94.39(3)

  ).Yield 74 %. 1 H NMR (400 MHz, d 4 -MeOH): δ 8.57 (d, J = 4.8 Hz, 1H, CH py ), 8.38 (s, 1H, CH im ), 8.32 (d, J = 7.5 Hz, 1H, CH py ), 7.96 (d, J = 9.0 Hz, 2H, CH ar ), 7.90 (t, J = 7.9 Hz, 1H, CH py ), 7.43 (t, J = 6.1 Hz, 1H, CH py ), 7.06 (d, J = 9.2 Hz, 2H, CH ar ), 3.85 (s, 3H, CH 3

  ). CH py ), 7.00 (s, 2H, CH ar ), 6.54 (s, 1H, CH ar ), 3.84 (t, 4H, CH 2 O), 1.87 (m, 4H, CH 2 CH 2

	= 5.8 Hz, 1H,
	III.159

  H NMR (400 MHz, CDCl 3 ): δ 8.49 (m, 1H, CH py ), 8.39 (s, 1H, CH im ), 8.05 (m, 1H, CH py ), 7.60 (t, 1H, CH py ), 7.21-7.04 (m, 3H, CH py + CH ar ), 3.89 (m, 6H, CH 2 O), 1.94-1.87 (m, 6H, CH 2 CH 2

Table S .

 S III-1. Crystallographic data for [Fe(L 1 ) 2 ]•MeOH polymorphs

	Compound	α-{[Fe(L 1 ) 2 ]•MeOH}	β-{[Fe(L 1 ) 2 ]•MeOH}
	Temperature, K	120	260	120	290
	Crystal description	Dark green needle	Dark green needle	Green needle	Green needle
	Moiety formula	C 28 H 24 FeN 6 O 4 , CH 4 O	C 28 H 24 FeN 6 O 4 , CH 4 O	C 28 H 24 FeN 6 O 4 , CH 4 O	C 28 H 24 FeN 6 O 4 , CH 4 O
	Empirical formula	C 29 H 28 FeN 6 O 5	C 29 H 28 FeN 6 O 5	C 29 H 28 FeN 6 O 5	C 29 H 28 FeN 6 O 5
	Formula weight	596.42	596.42	596.42	596.42
	Crystal system	Monoclinic	Monoclinic	Triclinic	Triclinic
	Space group	P2 1 /c	P2 1 /c	P-1	P-1
	Wavelength, Å	0.71073	0.71073	0.71073	0.71073
	a, Å	24.1677(6)	22.808(2)	8.7893(4)	8.883(13)
	b, Å	13.2155(4)	13.1051(14)	11.1963(5)	11.170(17)
	c, Å	8.7284(2)	8.7743(9)	14.8884(7)	15.09(3)
	α, °	90	90	111.717(2)	111.71(5)
	β, °	99.860(1)	98.307(4)	91.037(2)	91.93(7)
	γ, °	90	90	97.507(2)	96.71(5)
	V, Å 3	2746.6(1)	2595.1(4)	1345.96(11)	1377(4)
	Z	4	4	2	2
	ρ calcd , g/cm 3	1.408	1.439	1.472	1.584
	μ MoKα , 1/mm	0.600	0.628	0.612	0.617
	R 1 a	0.0406	0.1174	0.0332	0.0974
	wR 2 b	0.0895	0.2881	0.0941	0.2399
	GoF c	1.055	1.115	1.101	0.993
			Selected bond distances
	Fe(1)-N(1)	1.961(3)	1.965(7)	2.2618(13)	2.246(12)

Table S .

 S III-2. Crystallographic data for α-[Fe(L 1 ) 2 ]

	Temperature, K	120	260	370
	Crystal description	Dark green needle	Dark green needle	Green needle
	Moiety formula	C 28 H 24 FeN 6 O 4	C 28 H 24 FeN 6 O 4	C 28 H 24 FeN 6 O 4
	Empirical formula	C 28 H 24 FeN 6 O 4	C 28 H 24 FeN 6 O 4	C 28 H 24 FeN 6 O 4
	Formula weight	564.38	564.38	564.38
	Crystal system	Monoclinic	Monoclinic	Monoclinic
	Space group	P2 1 /c	P2 1 /c	P2 1 /c
	Wavelength, Å	0.71073	0.71073	0.71073
	a, Å	22.7027(14)	22.808(2)	21.451(3)
	b, Å	12.9995(8)	13.1051(14)	14.565(3)
	c, Å	8.7405(6)	8.7743(9)	8.6226(15)
	β, °	98.705(3)	98.307(4)	96.467(6)
	V, Å 3	2549.8(3)	2595.1(4)	2676.9(8)
	Z	4	4	4
	ρ calcd , g/cm 3	1.470	1.439	1.400
	μ MoKα , 1/mm	0.639	0.628	0.609
	R 1 a	0.0879	0.1174	0.0815
	wR 2 b	0.1872	0.2881	0.1792
	GoF c	1.226	1.115	1.090

Table S .

 S III-3. Crystallographic data for [Fe(HL 1 ) 2 ] 2+ and [Fe(HL 1 )(L 1 )] + complexes Moiety formula C 28 H 26 FeN 6 O 4 ,2ClO 4 C 28 H 25 FeN 6 O 4 ,ClO 4 ,CH 4 O C 28 H 25 FeN 6 O 4 ,ClO 4

	Compound	[Fe(HL 1 ) 2 ](ClO 4 ) 2	[Fe(HL 1 )(L 1 )](ClO 4 )•MeOH	[Fe(HL 1 )(L 1 )](ClO 4 )
	Temperature, K	120	120	120
	Crystal description	Purple plate	Greenish gray needle	Greenish gray block
	Empirical formula	C 28 H 26 FeCl 2 N 6 O 12	C 29 H 29 FeClN 6 O 9	C 28 H 25 FeClN 6 O 8
	Formula weight	764.03	694.10	664.83
	Crystal system	Monoclinic	Triclinic	Orthorhombic
	Space group	P2 1 /c	P-1	Pnna
	Wavelength, Å	0.71073	0.71073	0.71073
	a, Å	11.3202(9)	10.8762(4)	14.9875(12)
	b, Å	12.6110(13)	11.5572(5)	25.0245(18)
	c, Å	21.564(2)	13.4441(6)	14.8207(14)
	α, °	90	72.307(2)	90
	β, °	90.458(2)	79.470(2)	90
	γ, °	90	79.920(2)	90
	V, Å 3	3078.4(5)	1569.92(12)	5558.6(8)
	Z	4	2	8
	ρ calcd , g/cm 3	1.351	1.474	1.510
	μ MoKα , 1/mm	0.623	0.628	0.686
	R 1 a	0.0462	0.0641	0.0783
	wR 2 b	0.1299	0.2002	0.2621
	GoF c	1.068	1.049	0.896
		Selected bond distances

Table S .

 S III-4. Crystallographic data for α-[Fe(L 2 ) 2 ] complex

	Temperature, K	120	300
	Crystal description	Green needle	Green needle
	Moiety formula	C 30 H 28 FeN 6 O 6	C 30 H 28 FeN 6 O 6
	Empirical formula	C 30 H 28 FeN 6 O 6	C 30 H 28 FeN 6 O 6
	Formula weight	624.42	624.42
	Crystal system	Monoclinic	Monoclinic
	Space group	C2/c	C2/c
	Wavelength, Å	0.71073	0.71073
	a, Å	23.0807(7)	23.0694(19)
	b, Å	10.8025(3)	10.8626(7)
	c, Å	11.8221(2)	11.9644(11)
	β, °	105.6540(10)	105.223(4)
	V, Å 3	2838.26(13)	2893.0(4)
	Z	4	4
	ρ calcd , g/cm 3	1.611	1.566
	μ MoKα , 1/mm	0.595	0.607
	R 1 a	0.0356	0.0762
	wR 2 b	0.1246	0.1933
	GoF c	0.974	1.246
		Selected bond distances

Table S .

 S III-5. Crystallographic data for β-[Fe(L 2 ) 2 ] complex

	Temperature, K	120	300	390	400	450
	Crystal description			Dark green block
	Moiety formula			C 30 H 28 FeN 6 O 6
	Empirical formula			C 30 H 28 FeN 6 O 6
	Formula weight			624.42
	Crystal system			Triclinic
	Space group			P-1
	Wavelength, Å			0.71073
	a, Å	9.2954(4)	9.3765(3)	9.4017(15)	9.3875(6)	9.285(3)
	b, Å	12.2093(5)	12.2641(3)	12.4153(19)	12.4485(8)	12.217(4)
	c, Å	13.4330(6)	13.5941(4)	13.709(2)	13.7215(9)	13.445(4)
	α, °	70.760(2)	70.0180(10)	69.595(5)	69.609(3)	70.591(15)
	β, °	79.229(2)	78.9630(10)	78.414(6)	78.233(3)	78.987(15)
	γ, °	85.641(2)	84.9030(10)	83.629(5)	83.296(3)	85.177(14)
	V, Å 3	1413.84(11)	1441.50(7)	1467.8(4)	1469.61(16)	1411.6(7)
	Z	2	2	2	2	2
	ρ calcd , g/cm 3	1.266	1.242	1.599	1.078	1.116
	μ MoKα , 1/mm	0.334	0.328	0.580	0.311	0.321
	R 1 a	0.0311	0.0384	0.0483	0.0452	0.0767
	wR 2 b	0.0974	0.1251	0.1584	0.1318	0.1956
	GoF c	0.540	0.985	0.968	0.952	0.989
			Selected bond distances
	Fe(1)-N(1)	1.9555(17)	1.959(2)	2.009(3)	2.034(3)	1.953(6)
	Fe(1)-N(2)	1.8653(17)	1.864(2)	1.910(2)	1.936(3)	1.844(6)
	Fe(1)-N(4)	1.9534(17)	1.956(2)	2.008(3)	2.037(3)	1.968(6)
	Fe(1)-N(5)	1.8679(17)	1.870(2)	1.923(3)	1.940(3)	1.853(6)
	Fe(1)-O(1)	1.9686(14)	1.9717(18)	1.990(2)	1.997(2)	1.975(5)
	Fe(1)-O(2)	1.9880(14)	1.9901(17)	2.013(2)	2.013(2)	1.985(5)
			Selected bond angles
	N(1)-Fe(1)-N(2)	81.49(7)	81.38(9)	79.52(10)	78.99(11)	81.5(3)

Table S .

 S III-6. Crystallographic data for [Fe(L 3 ) 2 ] and [Fe(L 3 )]•MeOH complexes

	Compound	[Fe(L 3 ) 2 ]	[Fe(L 3 ) 2 ]•MeOH
	Temperature, K	120	120
	Crystal description	Dark green plate	Dark green plate
	Moiety formula	C 32 H 32 FeN 6 O 8	C 32 H 32 FeN 6 O 8 ,CH 4 O
	Empirical formula	C 32 H 32 FeN 6 O 8	C 33 H 36 FeN 6 O 9
	Formula weight	684.48	716.52
	Crystal system	Triclinic	Monoclinic
	Space group	P-1	P2 1 /n
	Wavelength, Å	0.71073	0.71073
	a, Å	10.720(2)	9.1200(4)
	b, Å	12.981(3)	25.3030(10)
	c, Å	13.188(2)	15.1708(6)
	α, °	102.214(9)	90
	β, °	113.967(8)	102.552(2)
	γ, °	90.034(10)	90
	V, Å 3	1631.3(5)	3417.2(2)
	Z	2	4
	ρ calcd , g/cm 3	1.347	1.673
	μ MoKα , 1/mm	0.515	0.906
	R 1 a	0.0694	0.1007
	wR 2 b	0.1971	0.2294
	GoF c	0.970	1.131

Table S .

 S III-7. Crystallographic data for [Fe(L 4 ) 2 ] complex.

	Temperature / K	250		360
	Crystal description	Dark green sheet		Green sheet
	Moiety formula	C 50 H 68 FeN 6 O 4		C 50 H 68 FeN 6 O 4
	Empirical formula	C 50 H 68 FeN 6 O 4		C 50 H 68 FeN 6 O 4
	Formula weight	931.03		931.03
	Crystal system	Monoclinic		Orthorhombic
	Space group	P2 1 /c		P2 1 2 1 2
	Wavelength, Å	0.71073		0.71073
	a / Å	10.4722(14)		10.036(2)
	b / Å	40.801(5)		46.263(10)
	c / Å	14.2308(16)		10.951(2)
	β / ˚	128.100(7)		90
	V / Å 3	4787.9(11)		5084.5(18)
	Z	4		4
	Flack parameter	--		0.49
	μ, 1/mm	0.364		0.343
	a R 1 wR 2 a GoF a	0.0633 0.1872 0.952		0.0748 0.2254 0.938
		Site 1 (LS)	Site 1(LS)	Site 2 (HS)
		)-O(1)	1.964(4)	1.979(4)
	Fe(1)-O(2)	1.976(3)	1.965(4)
			Selected bond angles
	N(1)-Fe(1)-N(2)	81.8(2)	81.3(2)

Table S .

 S III-8. Crystallographic data for [Fe(L 3 ) 2 ] and [Fe(L 3 )]•MeOH complexes

	Compound	[Fe(L 3 ) 2 ]
	Temperature, K	120
	Crystal description	Dark green sheet
	Moiety formula	C 50 H 66 Cl 2 FeN 6 O 4
	Empirical formula	C 50 H 66 Cl 2 FeN 6 O 4
	Formula weight	941.84
	Crystal system	Monoclinic
	Space group	P2 1 /c
	Wavelength, Å	0.71073
	a, Å	22.8322(13)
	b, Å	12.6464(8)
	c, Å	17.0912(11)
	β, °	94.392(3)
	V, Å 3	4920.2(5)
	Z	4
	ρ calcd , g/cm 3	1.271
	μ MoKα , 1/mm	0.464
	R 1 a	0.0581
	wR 2 b	0.1793
	GoF c	0.773

Table S .

 S III-9. Crystallographic data for γ-[Fe(L 1 ) 2 ] complex Moiety formula C 28 H 24 FeN 6 O 4 C 28 H 24 FeN 6 O 4

	Temperature, K	120	340
	Crystal description	Dark green block	Dark green block
	Empirical formula	C 28 H 24 FeN 6 O 4	C 28 H 24 FeN 6 O 4
	Formula weight	564.38	564.38
	Crystal system	Triclinic	Triclinic
	Space group	P-1	P-1
	Wavelength, Å	0.71073	0.71073
	a, Å	10.6982(19)	10.6225(14)
	b, Å	11.190(2)	11.3450(15)
	c, Å	12.531(2)	12.8603(17)
	α, °	114.058	116.17
	β, °	111.426	109.99
	γ, °	92.199	90.00
	V, Å 3	1243.7(4)	1286.1(3)
	Z	2	2
	ρ calcd , g/cm 3	1.502	1.328
	μ MoKα , 1/mm	0.655	0.626
	R 1 a	0.0295	0.0380
	wR 2 b	0.1400	0.1463
	GoF c	1.274	1.274
	<Fe-N (py) >, Å	1.960	2.199
	<Fe-N (im) >, Å	1.870	2.070
	<Fe-O>, Å	1.997	2.076
	Σ(X-Fe-X), d °	86.3	142.7
	a I > 2σ(I), R 1 = Σ|(|F o | -|F c |)|/Σ|F o |. b wR 2 = {Σ[w(F o fit on F 2 ) = {Σ[w(F o 2 ) 2 ]/(n-p)} 1/2 , where n is the number of reflections and p is the total 2 -F c 2 ) 2 ]} 1/2 . c GoF (goodness of 2 ) 2 ]/Σ[w(F o 2 -F c number of refined. d ∑ = ∑ |90 -𝜑 𝑖 | 12 𝑖=1

Table of Contents for Chapter IV:

 of Synthetic approach .......................................................................................................... IV.181 IV.2.2. FTIR spectroscopic analyses ........................................................................................... IV.183 IV.2.3. Structural descriptions ..................................................................................................... IV.184 IV.2.3.1. Structure of [(Tp*)Fe(CN) 3 ] 2 [Ni(L 1 ) 2 ] 2 }[OTf] 2 (1) ...................................................... IV.184 IV.2.3.2. Structure of [(Tp*)Fe(CN) 3 ] 2 [Ni(L 2 ) 2 ] 2 }[OTf] 2 (2) ...................................................... IV.186 IV.2.4. Thermotropic investigations ............................................................................................ IV.188 IV.2.5. Magnetic properties ......................................................................................................... IV.191 IV.2.5.1. Static magnetic measurements for 1, 2 and 5 ............................................................... IV.191 IV.2.5.2. Dynamic magnetic measurements for 1, 2 and 5 ......................................................... IV.192

	IV.1. Introduction ................................................................................................................................... IV.180
	IV.2. Synthesis and characterization of {Fe 2 Ni 2 } molecular squares ................................................. IV.181
	IV.2.1.

IV.3. Synthesis and characterization of {Fe 2 Co 2 } molecular squares

  IV.3.1. Synthetic approach .......................................................................................................... IV.195 IV.3.2. FTIR spectroscopic analyses ........................................................................................... IV.196 IV.3.3. Structural description of [(Tp*)Fe(CN) 3 ] 2 [Co(L 7

................................................ IV.195

Table IV - 3 .

 IV3 Phase transition temperatures and thermodynamic parameters for 5 and 6

	Compound	Transition a	T / K	ΔH / kJ•mol -1
	5	Sm-I I-Sm	337.3 325.9	27.0 29.2
	6	Cr-Sm Sm-I	315.1 347.3	98.0 22.9
	Cr = crystalline phase; I = isotropic liquid; Sm = smectic phase.

Table IV -

 IV

				5						6
	T / K	Peak number	q (obs) / nm -1	d (obs) /Å	hkl	Interlamellar distance	T / K	Peak number	q (obs) / nm -1	d (obs) /Å	hkl Interlamellar distance
	293	1 2	1.61 3.20	39.02 19.63	001 002		298	1 2 3	1.40 2.08 2.83 22.20 002 44.87 001 11 30.2 -
	303	1 2	1.61 3.20	39.02 19.63	001 002		318	1 2	1.41 2.82	44.56 001 22.28 002
	313	1 2	1.61 3.21	39.02 19.57	001 002		323	1 2	1.41 2.82	44.56 001 22.28 002
	323	1 2	1.60 3.21	39.26 19.57	001 002	d = 39.1	333	1 2	1.41 2.82	44.56 001 22.28 002	d = 44.6
	328	1 2	1.60 3.19	39.26 19.69	001 002		343	1 2	1.41 2.83	44.56 001 22.20 002
	333	1 2	1.59 3.21	39.51 19.57	001 002
	338	1 2	1.61 3.21	39.02 19.57	001 002
	q (obs)

4. Indexation of SAXS profiles for 5 and 6

Table IV -

 IV

	Solvents	Polarity index (P')	7	9
	Water	9.0	×	×
	Nitromethane	6.8	red -green	×
	Methanol	6.6	red -green	×
	Dimethyl sulfoxide	6.5	red	red
	Dimethylformamide	6.4	red	red
	Acetonitrile	6.2	red -green	×
	Ethanol	5.2	red -green	×
	Acetone	5.1	red -green	red -green
	Butyronitrile	4.6	red -green	red -green
	Ethyl acetate	4.3	red -green	×
	Tetrahydrofuran	4.2	red -green	red -green
	Chloroform	4.1	red -green	red
	Dichloromethane	3.4	red -green	red -green
	Diethyl Ether	2.9	red -green	×
	Toluene	2.

7. Solubility of 7 and 9 in various solvents and qualitative tests for electron-transfer in their solutions.

Table IV -8. Chapter IV -Soft materials based on cyanido-bridged {Fe III 2 (μ-CN) 4 M II 2 } molecular squares

 IV

IV.204

Table IV - 8 .

 IV8 Thermodynamic parameters obtained from the ideal solution model 20 fit of the UV-Vis data for 7 and 9 in different solvents.

	7	9
	ΔH,
	kJmol -1

IV -Soft materials based on cyanido-bridged {Fe III 2 (μ-CN) 4 M II 2 } molecular squares

	IV.5
	IV.206

. Supporting materials IV.5.1. Experimental protocols IV.5.1.1. Synthesis of the organic precursors and ligands

  Solvents and starting materials were purchased from Acros, Alfa Aesar or Aldrich, and used without further purification unless otherwise stated. The non-commercial benzoic acid derivatives were prepared according to procedures described in Chapter II. , 8.1 mmol) in 200 ml methanol was added 4 ml of 87 wt% sulfuric acid and the mixture was refluxed for 24 h. Then clear solution was cooled to room temperature. Water (200 ml) was added and the excess of methanol removed under vacuum. The pH was adjusted to neutral with NaOH solution (2M) and the resulting precipitate was filtered, washed with water and dried under vacuum. The precursor A was isolated as a white powder: Yield 2.61 g (98 %). 1 H NMR (400 MHz) in CDCl 3 (δ, ppm): 8.95 (dd, J = 3.0 Hz, 2H), 8.66(d, J = 5.0 Hz, 2H), 7.90 (dt, J = 5.1 Hz, 2H), 3.97 (s, 6H).

	Preparation of 4,4'-dimethoxycarbonyl-2,2'-bipyridine A. To a suspension of 2,2'-bipyridine-4,4'-
	dicarboxylic acid (2,2 gElemental analysis -Calc. (Found) for C 12 H 12 N 2 O 2 : C, 66.65 (65.98), H, 5.59 (5.80), N, 12.96 (12.90).

Preparation of 2,2'-bipyridine-4,4'-diyldimethanol B.

Sodium borohydride

(7.23 g, 19.2 mmol)

was slowly added to a suspension of A (2.61 g, 9.6 mmol) in 180 ml of ethanol absolute. The mixture was refluxed for 3 hours and cooled to room temperature. Then, the excess of borohydride was quenched with 180 ml of saturated ammonium chloride solution. The ethanol was removed under vacuum and the precipitate was dissolved in a minimal amount of water. The resulting solution was extracted with ethyl acetate (5 x 180 ml), dried over anhydrous sodium sulfate, filtered and the solvent was removed under vacuum. The compound B was obtained as a white powder: Yield 1.26 g (75 %). 1 H NMR (400 MHz) in CDCl 3 (δ, ppm): 8.65

(d, J = 4.9 Hz, 2H), 8.36 (m, 2H), 7.36 (d, J = 4.8 Hz, 2H), 4.83 (q, J = 7.2 Hz, 4H)

.

Preparation of 4,4'-bis(bromomethyl)-2,2'-bipyridine C.

  The functionalized bipyridine B(1.26 g, 5.88 mmol) was dispersed in a mixture of 48 % HBr (25 ml) and concentrated sulfuric acid (9 ml) was add. The resulting solution was refluxed for 6 hours and then allowed to cool to room temperature. The reaction mixture is diluted with 50 ml of water and the pH was adjusted to pH = 7 with NaHCO 3 . The product was

	extracted with chloroform (3 x 50 ml), dried over anhydrous sodium sulfate, filtered and evaporated to
	dryness. The compound C was isolated as a white powder: Yield 1.23 g (62 %). 1 H NMR (400 MHz) in
	CDCl 3 (δ, ppm): 8.66 (t, J = 7.2 Hz, 2H), 8.42 (d, J = 9.9 Hz, 2H), 7.36 (dd, J = 4.9 Hz, 2H), 4.47 (d, J = 11.2
	Hz, 4H). Elemental analysis -Calc. (Found) for C 12 H 10 Br 2 N 2 : C, 42.14 (41.96); H, 2.95 (3.06); N, 8.19

(8.18)

. Selected FT-IR data (ATR, cm -1 ): 3019

(m)

, 2968 (m), 1592 (s), 1554 (s), 1456 (s), 1373 (s), 1210 (s), 1119

(m)

, 990

(m)

, 903 (m), 851 (s), 832 (s), 742

(m)

, 637 (vs), 559 (vs).

Preparation of L

[1][2][3][4][5][6]

ligands. All the ligands were prepared in the same fashion and the general procedure is described for L 1 . To a solution of C (0.349 g, 1.02 mmol) and 4-methylbenzoic acid (0.23 g,

Chapter

IV -Soft materials based on cyanido-bridged {Fe III 2 (μ-CN) 4 M II 2 } molecular squares

  The mixture was stirred 24 hours at room temperature under Argon. The solvent was removed, and the sticky residue was dispersed in diethyl ether (80 ml) with vigorous stirring. After 3 hours, the white precipitate was collected by filtration, washed with water, cold methanol and dried under vacuum, leaving L 1 as a white powder: Yield 430 mg(95 %). Selected FT-IR data (ATR, cm -1 ): 3065(m), 3036 (m), 2951 (m),

	1.68 mmol) in 20 ml of THF was added tetrabutylammonium fluoride (TBAF) (1 M in THF, 2.5 ml, 2.5
	mmol). 2923 (m), 2857 (m), 1715 (vs), 1604 (s), 1437 (s), 1347 (s), 1277 (vs), 1180 (s), 1118 (s), 1021 (m), 847 (m),
	812 (s), 701 (s), 590 (m). 1 H NMR (400 MHz) in CDCl 3 (δ, ppm): 8.68 (d, J = 10.8 Hz, 2H), 8.45 (d, J = 10.7
	Hz, 2H), 8.00 (t, J = 8.9 Hz, 4H), 7.38 (d, J = 4.9 Hz, 2H), 5.44 (s, 4H), 2.41 (s, 6H). Elemental Analysis -
	Calc. (Found) for C 28 H 24 N 2 O 4 : C, 74.32 (73.91); H, 5.35 (5.68); N, 6.19 (6.18).
	L 2 : Yield 76 %. Selected FT-IR data (ATR, cm -1 ): 3065 (m), 3035 (m), 2952 (m), 2927 (m), 2854 (m), 1721
	(vs), 1598 (s), 1455 (s), 1347 (m), 1269 (vs), 1178 (s), 1116 (s), 1020 (m), 847 (m), 812 (vs), 699 (s), 592
	(m). 1 H NMR (400 MHz) in CDCl 3 (δ, ppm) for 8.67 (d, J = 5.0 Hz, 2H), 8.46 (s, 2H), 8.00 (d, J = 8.2 Hz,
	4H), 7.39 (d, J = 5.0 Hz, 2H), 5.44 (s, 4H), 2.66 (m, 4H), 1.58 (t, J = 7.5 Hz, 6H), 1.37 (m, J = 7.4 Hz, 4H),
	0.93 (t, J = 8.4 Hz, 6H). Elemental Analysis -Calc. (Found) for C 34 H 36 N 2 O 4 : C, 76.09 (76.07); H, 6.76
	(7.05); N, 5.22 (5.21).
	74 H 116 N 2 O 8 : C, 76.51
	(76.40); H, 10.06 (10.27); N, 2.41 (2.64).
	L 4 : Yield 68 %. Selected FT-IR data (ATR, cm -1 ): 2916 (vs), 2848 (s), 1713 (vs), 1600 (s), 1587 (m), 1501
	(m), 1429 (s), 1388 (m), 1358 (m), 1332 (s), 1214 (vs), 1118 (vs), 811 (m), 759 (m), 719 (m). 1 H NMR (400
	MHz) in CDCl 3 (δ, ppm): 8.65 (d, J = 5.1 Hz, 2H), 8.48 (s, 2H), 7.35 (d, J = 3.8 Hz, 2H), 7.31 (d, J = 1.9
	Hz, 4H), 5.42 (s, 4H), 3.99 (d, J = 6.5 Hz, 4H), 1.77 (m, 12H), 1.59 (s, 8H), 1.43 (s, 8H), 1.23 (d, 96H), 0.86
	(d, J = 7.5 Hz, 18H). Elemental Analysis -Calc. (Found) for C 98 H 164 N 2 O 10 : C, 76.91 (76.45); H, 10.80
	(10.95); N, 1.82 (1.98).
	L 5 : Yield 55%. Selected FT-IR data (ATR, cm -1 ): 3032 (w), 2929(s), 2851 (s), 1719 (s), 1598 (s), 1521 (s),
	1488 (s), 1466 (s), 1449 (s), 1250 (s), 1163 (s), 1028 (s), 831 (s), 759 (s), 691 (m). 1 H NMR (400 MHz) in
	CDCl 3 (δ, ppm): 8.689 (d, 2H), 8.472 (t, 2H), 7.553 (dd, 8H), 7.516 (dd, 8H), 7.413 (dd, 8H), 7.38 (d, 2H),
	7.299 (t, 4H), 7.24 (d, 4H), 6.966 (d, 8H), 6.677 (t, 2H), 5.445 (s, 4H), 3.992 (m, 16H), 1.805 (m, 16H),
	1.415 (m, 48H).
	L 6 : Yield 90 %. Selected FT-IR data (ATR, cm -1 ): 3035 (w), 2922(s), 2851 (s), 2225 (s, ν CN ), 1717 (s), 1600
	(s), 1521 (s), 1494 (s), 1465 (s), 1444 (s), 1249 (s), 1165 (s), 1054 (s), 819 (s), 761 (s), 675 (m). 1 H NMR
	(400 MHz) in CDCl
	IV.207

L

3

: Yield 71 %. Selected FT-IR data (ATR, cm -1 ): 2915 (vs), 2848 (s), 1713 (s), 1599

(m)

, 1587

(m)

, 1501

(m), 1429 (s), 1358 (m), 1332 (s), 1215 (vs), 1118 (vs)

, 1036 (m), 841 (s), 812 (s), 760 (s), 719

(m)

. 1 H NMR (400 MHz) in CDCl 3 (δ, ppm) for 8.70 (d, J = 5.1 Hz, 2H), 8.49 (s, 2H), 7.42 (d, J = 4.6 Hz, 2H), 7.21 (d, J = 2.3 Hz, 4H), 6.65 (t, J = 2.3 Hz, 2H), 5.45 (s, 4H), 3.95 (t, J = 6.3 Hz, 8H), 1.76 (m, 8H), 1.43 (m, 8H), 1.31-1.19

(m, 64H)

, 0.87-0.83 (t, J = 6.9 Hz, 12H). Elemental Analysis -Calc. (Found) for C 3 (δ, ppm): 8.69

(d, 2H), 8.482 (t, 2H), 7.658 (dd, 16H,), 7.526 (d, 8H), 7.396 (d, 2H), 7.22 (d, 4H), 6.98 (d, 8H), 6.671 (t, 2H), 5.448 (s, 4H), 3.991 (t, 16H), 1.805 (m, 16H), 1.407 (m, 48H)

.

Preparation of 4,4'-diol-2,2'-bipyridine E.

To a solution of 4,4'-dimethoxy-2,2'-bipyridine (0.288 g, 1.3 mmol) in acetic acid (12 mL) was added hydrobromic acid

(48 wt%, 12 ml)

and mixture was refluxed Chapter

IV -Soft materials based on cyanido-bridged {Fe III 2 (μ-CN) 4 M II 2 } molecular squares

  After cooling to room temperature, the solution was diluted with water (100 mL) and neutralized with aqueous solution of NH 4 OH. This resulting white powder was filtered, washed with water (2 x 20 mL) and ethanol (2 x 20 mL) and dried under vacuum: Yield 2.06 g (78 %). 1 H NMR (400 MHz) in CD 3 OD (δ, ppm): 8.58(d, J = 5.0 Hz, 2H), 8.25 (s, 2H), 7.41 (d, J = 5.0 Hz, 2H). Selected FT-IR data (ATR, cm -1 ): 2535 The reaction mixture was then refluxed for 24 hours. The solvent was removed and CH 2 Cl 2 (50 mL) was added. The suspension was washed with water (3x50 mL). The organic layer was dried over anhydrous sodium sulfate, filtrated and concentrated. The obtained white vitreous solid was filtrated, washed with diethyl ether and dried under vacuum: Yield 340 mg(48 %). Elemental analysis -Calc. (Found) for C 34 H 56N 2 O 2 : C, 77.81 (78.02); H, 10.76 (11.54);N, 5.34 (5.49). 1 H NMR (400 MHz) in CDCl 3 (δ, ppm): 8.45(d, J = 5.7 Hz, 2H), 7.99 (d, J = 2.5 Hz, 2H), 6.84 (dd, J = 5.7 Hz, 2H), 4.14 (t, J = 6.5 Hz, 4H), 3.89 (m,

	overnight. (m), 1583 (vs), 1557 (vs), 1457 (vs), 1434 (s), 1389 (s), 1389 (s), 1289 (vs), 1029 (vs), 984 (s), 896 (vs), 813
	(vs), 660 (m), 582 (s), 573 (s).
	Preparation of 4,4'-bis(dodecyloxy)-2,2'-bipyridine (L 7 ). To a solution of E (0.25 g, 1.33 mmol) in 25
	mL of dimethylformamide, anhydrous potassium carbonate (0.979 g, 6.65 mmol) was added. The mixture
	was heated to reflux and 1-bromododecane (0.73 g, 2.92 mmol) and a catalytic amount of potassium iodide
	were added.
	IV.208

4H), 1.38 -1.14 (m, 36H)

, 0.45

(t, J = 6.8 Hz, 6H)

. Selected FT-IR data (ATR, cm -1 ): 2916 (vs), 2848 (vs), 1579 (vs), 1579 (vs), 1562 (s), 1393 (s), 1378 (s), 1296 (vs), 1243 (vs), 1021 (vs), 865

(m)

, 840 (vs), 717

Synthesis of metal ion precursors and complexes Preparation of M(OTf) 2 •6H 2 O (M = Co 2+ , Ni 2+ ).

  To a 10 mL of trifluoromethanesulfonic acid (0.5 g,

	E (0.5 g, 2.66 mmol) in 25 mL of dimethylformamide, anhydrous potassium carbonate (0.979 g, 6.65 mmol),
	4-((10-bromodecyl)oxy)-1,1'-biphenyl (2.28 g, 5.9 mmol) and a catalytic amount of potassium iodide were
	added. The reaction mixture was then refluxed for 24 hours. After evaporation of the solvent, the residue was
	dispersed in 150 mL of water and the product extracted with CH 2 Cl 2 (3 x 30 mL). The organic layer was
	dried over anhydrous sodium sulfate, filtrated and the solvent removed under vacuum. The white vitreous
	solid was filtrated, washed with diethyl ether and dried under vacuum leaving L 8 as a white solid: Yield 340
	mg (56 %). Elemental analysis -Calc. (Found) for C 54 H 64 N 2 O 4 : C, 80.56 (80.05); H, 8.01 (8.34); N, 3.48
	(2.95). 1 H NMR (400 MHz) in CDCl 3 (δ, ppm): 8.42 (m, 2H), 7.99 (s, 2H), 7.93 (t, J = 5.1 Hz, 2H), 7.50 (m,
	8H) 7.40 (t, J = 7.6 Hz, 4H), 6.96 (m, 4H), 3.96 (t, J = 6.6 Hz, 4H), 1.81 (m, 6H), 1.46 (m, 8H), 1.33 (s,
	16H). Selected FT-IR data (ATR, cm -1 ): 2919 (vs), 2851(vs), 1581 (vs), 1562 (vs), 1394 (s), 1378 (m), 1297
	(s), 1243 (vs), 1043 (s), 1003 (vs), 853 (s), 833 (s), 763 (s), 717 (m).
	IV.5.1.2.

). To a solution of 3.2 mmol) solution in water (30ml) were added slowly MCO 3 (0.2 g, 1.6 mmol). 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 and

Ni(OTf)

2 as a green solid. The compounds were dried in vacuum at 110 °C. The yield were 2.3 g (82 %) for Co(OTf) 2 and 2.84 g (85 %) Ni(OTf) 2 . Elemental analysis Chapter

IV -Soft materials based on cyanido-bridged {Fe III 2 (μ-CN) 4 M II 2 } molecular squares

  Found) for C 2 H 12 CoF 6 O 12 S 2 (Co(CF 3 SO 3 ) 2 •6H 2O): C, 5.16 (5.87), H, 2.60 (2.74). Selected FT-IR data (ATR, cm -1 ): 1648(m), 1614(m), 1230 (vs), 1188 (vs), 1033 (vs), 719 (m), 633 (s), 580 (s). Elemental analysis -Calc. (Found) for C 2 H 12 NiF 6 O 12 S 2 (Ni(CF 3 SO 3 ) 2 •6H 2 O): C, 5.17(5.86); H, 2.60 (2.74). Selected FT-IR data (ATR, cm -1 ): 1634 (s), 1219 (vs), 1190 (vs), 1030 (vs), 693(m), 625 (vs).

	-Calc. (Preparation of (NEt 4 )[(Tp*)Fe(CN) 3 ]. KTp* (0.5 g, 1.48 mmol) dissolved in 2.5 mL of methanol was
	dropwise added to Fe(acac) 3 (0.5 g, 1.41 mmol) solution in 2.5 mL of methanol affording a dark brown
	mixture that was stirred for 2 hours. 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 hours 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 0.23 g (55 %). Elemental analysis -Calc. (Found) for
	1·2H 2 O, C 26 H 46 BFeN 10 O 2 : C, 52.28 (52.70); H, 7.76 (7.58); N, 23.45 (24.25). Selected FT-IR data (ATR, cm -
	1 ): 2982 (m), 2934 (m), 2549 (m, ν BH ), 2115 (s, ν CN ), 1541 (vs), 1446 (vs), 1414 (vs), 1370 (vs), 1200 (vs),
	1059 (vs), 1028 (m), 814 (vs), 773 (vs), 691 (s), 643 (vs).
	Preparation of {[
	IV.209

(Tp*)Fe(CN) 3 ] 2 [Ni(L 1 ) 2 ] 2 }[OTf] 2

	C 154 H 148 B 2 Ni 2 F 6 Fe 2 N 28 O 23 S 2 : C, 58.06 (58.50); H, 4.68 (4.69); N, 12.30 (11.71). Selected FT-IR data (ATR,
	cm -1 ): 2926 (m), 2845 (m), 2533 (m, ν BH ), 2164 (s, ν CN ), 2117 (m, ν CN ), 1716 (vs), 1672 (s), 1611 (s), 1543
	(s), 1446 (s), 1414 (s), 1371 (s), 1264 (vs), 1178 (s), 1147 (s), 1102 (s), 1062 (s), 1030 (s), 837 (s), 752 (vs).

(1). [NEt 4 ][(Tp*)Fe(CN) 3 ]•2H 2 O (17 mg, 0.03 mmol) was treated with Ni(OTf) 2 (18 mg, 0.04 mmol) in 3 ml of DMF to afford a red solution that was allowed to stir for 2 hours. Subsequently, L 1 (30 mg, 0.06 mmol) in 2 ml of DCM was added and the mixture was stirred for 2 days at room temperature and then filtered. Red solution were evaporated affording a brownish-orange powder. Brownish-orange crystals of 1•H 2 O•2CH 3 CN were obtained by slow diffusion of diethyl ether into a solution of 1 in CH 3 CN (3 mL): Yield 52 %. Elemental analysis -Calc. (Found) for

Preparation of {[(Tp*)Fe(CN) 3 ] 2 [Ni(L 2 ) 2 ] 2 }[OTf] 2 .(2). [

  NEt 4 ][(Tp*)Fe(CN) 3 ]•2H 2 O (17 mg, 0.03 mmol) was treated with Ni(OTf) 2 (18 mg, 0.04 mmol) in 3 ml of DMF to afford a red solution that was allowed to stir for 2 hours. Subsequently L 2 (48 mg, 0.06 mmol) in 2 ml of DCM was added and the mixture was stirred for 2 days at room temperature and then filtered. Slow diffusion of diethyl ether vapor into the mother liquor at 5 °C afford brownish-orange crystal of 2•H 2 O•2DMF after few days: Yield 68 %. Elemental analysis -Calc. (Found) for C 186 H 220 B 2 Ni 2 F 6 Fe 2 N 30 O 28 S 2 : C, 59.94(60.61); H, 5.76 (5.81);N, 10.87 (10.10).

	Selected FT-IR data (ATR, cm -1 ): 2917 (s), 2926 (m), 2847 (m), 2533 (m, ν BH ), 2152 (s, ν CN ), 2119 (m, ν CN ),
	1716 (s), 1672 (s), 1607 (s), 1611 (s), 1543 (s), 1446 (s), 1414 (s), 1371(s), 1264 (vs), 1178 (s), 1102 (s),
	1030 (s), 837 (s), 752 (s), 691 (s).

Preparation of [{(Tp * )Fe(CN)

3 } 2 {Ni(L 3-6 ) 2 } 2 ][OTf] 2 complexes

. All the complexes were prepared in the same fashion and products were obtained as glassy solid with 56 to 85 % yields. The general procedure is given for [{(Tp * )Fe(CN) 3 } 2 {Ni(L 5 ) 2 } 2 ][OTf] 2 . In a vial, a solution of [PPh 4 ][(Tp*)Fe(CN) 3 ] (23 mg, 2.9•10 -5 mol) is dissolved in 5 mL MeCN, and Ni(OTf) 2 •6H 2 O (13.5 mg, 2.9•10 -5 mol) is added. The solution is Chapter

IV -Soft materials based on cyanido-bridged {Fe III 2 (μ-CN) 4 M II 2 } molecular squares IV

  .210 stirred for 10 minutes at RT and then a solution of L 5 (100 mg, 5.8•10 -5 mol) in 20 mL of DCM is to the vial.The reaction mixture is stirred at room temperature for 24 hours. The clear solution is concentrated to about 3 mL during which a sticky solid precipitate. The product is removed and the remaining solid is washed several times with MeCN and MeOH. The brownish-orange residue is purified by size extrusion column chromatography (Bio-Beads S-X3 beads) using CH 2 Cl 2 as eluent.

	[{(Tp*)Fe(CN) 3 } 2 {Ni(L 3 )} 2 ][OTf] 2 (3): Yield 56 %. Selected FT-IR data (ATR, cm -1 ): 2921 (s), 2851 (s),
	2533 (w), 2161 (m), 1723 (s), 1593 (s), 1444 (s), 1220 (s), 1159 (s), 1030 (s), 841 (m), 761 (s), 636 (s)
	[{(

Tp*)Fe(CN) 3 } 2 {Ni(L 4 )} 2 ][OTf] 2 (4):

  Yield 82 % Elemental analysis -Calc. (Found) for C 430 H 700 B 2 Ni 2 F 6 Fe 2 N 26 O 46 S 2 : C, 69.80 (69.04); H, 9.54 (9.63); N, 4.92 (4.72). Selected FT-IR data (ATR, cm -1 ): 2920 (s), 2851 (s), 2530 (w, ν BH ), 2159 (m, ν CN ), 1718 (s), 1583 (s), 1429 (s), 1332 (s), 1202 (s), 1111

	(s), 1029 (s), 760 (s), 637 (s)
	[{(Tp*)

Fe(CN) 3 } 2 {Ni(L 5 )} 2 ][OTf] 2 (5):

  Yield 75 % Elemental analysis -Calc. (Found) for C 494 H 572 B 2 Ni 2 F 6 Fe 2 N 26 O 54 S 2 : C, 72.65 (71.92); H, 7.06 (7.09); N, 4.46 (4.42). Selected FT-IR data (ATR, cm -1 ): 2917 (s), 2849 (s), 2536 (m, ν BH ), 2162 (m, ν CN ), 2120 (b, ν CN ), 1722 (s), 1607 (s), 1487 (s), 1471 (s), 1447 (s), 1222 (s), 1159 (vs), 1029(s), 1029 (vs), 831 (vs), 761 (vs), 695 (s), 636 (s).

	[{(

Tp*)Fe(CN) 3 } 2 {Ni(L 6 )} 2 ][OTf] 2 (6):

  Yield 85 %. Selected FT-IR data (ATR, cm -1 ): 3029 (w), 2917 (s), 2849 (s), 2542 (w, ν BH ), 2225 (s, ν CN ), 2155(m, ν

CN ), 1722 (s), 1601 (s), 1523 (m), 1493 (s), 1465 (s), 1446 (s), 1248 (s), 1161 (s), 1029 (s), 820 (s), 763 (m), 636(s).

Preparation of {[(Tp*)Fe(CN)

3 ] 2 [Co(L 7 ) 2 ] 2 }[OTf] 2 (7)

  .[NEt 4 ][(Tp*)Fe(CN) 3 ]•2H 2 O (35 mg, 0.06 mmol) was treated with Co(OTf) 2 (36 mg, 0.08 mmol) in 3 ml of DMF to afford a red solution that was allowed to stir for 2 hours. Subsequently, L 7 (63 mg, 0.12 mmol) in 3 ml of DCM was added, and the mixture was stirred for 2 days at room temperature and then filtered. The red solution obtained was evaporated, redissolved in 50 ml of MeOH and set for slow evaporation, after five days, resulting green needles are collected by filtration and washed with cold methanol (2 x 1 mL). Yield 30 %. Selected FT-IR data (ATR, cm -1 ): 2921 (vs), 2852 (vs), 2506 (b, ν BH ), 2160 (b, ν CN ), 2106 (m, ν CN ), 2088 (s, ν CN ), 2051(s), 1611 (vs), 1559(m), 1599 (s), 1495 (s), 1452 (vs), 1334 (s), 1251 (vs), 1223 (s), 1155 (s), 1028 (vs), 861(m). Elemental analysis -Calc. (Found) for 7•6H 2 O C 174 H 280 B 2 Co 2 F 6 Fe 2 N 26 O 20 S 2 : C, 59.96 (60.75); H, 8.10 (8.03); N

Soft materials based on cyanido-bridged {Fe III 2 (μ-CN) 4 M II 2 } molecular squares IV.211 Preparation of {[(Tp*)Fe(CN) 3 ] 2 [Co(L 7-8 ) 2 ] 2 }[PF 6 ] 2 .

  ). Elemental analysis -Calc. (Found) for C 254 H 300 B 2 Co 2 F 6 Fe 2 N 26 O 22 S 2 : C, 67.82(66.10); H, 6.72 (6.49);N, 8.10 (7.80). To a solution Co(OTf) 2 (36 mg, 0.08 mmol) in 2 ml of MeOH was added L 7 ( 63 mg, 0.12 mmol) or L 8 (96 mg, 0.12 mmol) in MeOH (7 ml) and the resulting pale yellow solution was stirred for 30 minutes at room temperature. Then, [NEt 4 ][(Tp*)Fe(CN) 3 ]•2H 2 O (35 mg, 0.06 mmol) in CH 3 OH (2 ml) and NBu 4 PF 6 (46 mg, 0.12 mmol) were subsequently added and the reaction mixture was stirred for 2 days. The resulting red precipitate was collected by filtration and washed with cold methanol.

	Chapter IV -{[(

Tp*)Fe(CN) 3 ] 2 [Co(L 7 ) 2 ] 2 }[PF 6 ] 2 (8).

  Yield 45 %. Selected FT-IR data (ATR, cm -1 ): 2922 (s), 2852 (s), 2521 (m, ν BH ), 2159 (s, ν CN ), 2125 (m, ν CN ), 1608 (s), 1562 (s), 1543 (s), 1494 (s), 1448 (s), 1324 (s), 1204 (vs), 1061 (s), 1027 (s), 831 (vs), 778 (s). Elemental analysis -Calc. (Found) for C 174 H 268 B 2 Co 2 F 6 Fe 2 N 26 O 8 P 2 : C, 61.31 (60.34); H, 8.02 (7.78); N

) 8 (H 2O) 4 ] were smaller than for the [Mn 12O 12 (CF 3COO) 8 (L 1 ) 8 (H 2 O) 4 ] or parent [Mn 12 O 12 (CF 3 COO) 16 (H 2 O) 4 ]and Mn 12 -OAc compounds supporting an S T = 9 ground state, as suggested by the ac measurements.

* From ref.16; a determined by DSC on 1 st heating (irreversible behavior); b determined by DSC on 2 nd heating; c estimated by SAXS on 4 th heating; d estimated by TGA; Cr = crystal, I = isotropic liquid; Cub = cubic phase; g = glass; Sm = smectic phase; dec = decomposition.The complexes are thermally stable up to ca. 410 K since only minor weight losses attributed to lattice solvent molecules (water) were detected by TGA. By analogy, the parent complex Mn 12 -OAc starts to decompose at 320 K, confirming as previously reported, the benefit of benzoate functionalization to enhance the thermal stability of the Mn 12 core.15,16 Between ca. 410 -480 K, a slight decrease, probably associated with the loss of water ligands, is observed while a major decomposition of the Mn 12 core occurs above 480 K. Considering the low-kinetic for the loss of water ligands, temperature-dependent measurements can be performed up to 480 K without alteration the complexes, although reaching such extreme temperature was not necessary in most of the case.

, where 𝜑 𝑖 are cis X-Fe-X bond angles with X = O or N

} molecular squaresIV.177

} molecular squaresIV.180

} molecular squares

} molecular squaresIV.212

} molecular squaresIV.213

} molecular squaresIV.215

Acknowledgements i 2 } molecular squares

IV.190 2 } molecular squares IV.216

shown by the powder X-ray diffraction, which did not reveal any traces of α-[Fe(L 2 ) 2 ] complex and was found to be in good agreement with the simulated powder pattern (Figure III-15, right). Similarly to [Fe(L 1 ) 2 ] complexes, both polymorphs can be clearly distinguished by FTIR spectroscopy, which reveals differences in the 1800 -550 cm -1 window, while the characteristic absorption bands in the region 1600 -1580 cm -1 and 1350 cm -1 ascribed to ν (C=N-N=C) and ν (C- O) , respectively, are displayed confirming the presence of chelating enolate ligands (Figure S.III-1).

Powdered samples of the polymorphs are olive green and dark green, respectively for α-[Fe(L 2 ) 2 ] and β-[Fe(L 2 ) 2 ], as expected for this type of complexes in the HS and LS configurations, respectively, suggesting once again the strong impact of polymorphism on the Fe(II) coordination sphere and resulting magnetic properties. However, in contrast with previous system, no change of color could be clearly detected upon cooling down to liquid nitrogen temperature (α-phase) or heating up to 400 K (β-phase), presuming the absence of SCO feature in this range of temperatures.

III.2.2.2. Structural description of [Fe(L 2 ) 2 ] polymorphs

Single-crystal X-ray diffraction analyses were performed in order to reveal the differences between the two polymorphs. Views of the complexes collected at 120 K are displayed in III.138 (paramagnetic) β-phase. Afterward, the behavior becomes irreversible and the material remains in the HS configuration down to 300 K, showing that the Cr3-to-Cr4 transition is not associated to SCO process. Magnetic measurements after this thermal treatment must be performed below 300 K to check whether a return into the diamagnetic phase is possible (only DSC down to 200 K were performed and did not reveal any evidence for SCO).

The formation of the intermediate mixed {LS•HS} phase around 340 K retained our attention since the process is perfectly reversible (below 400 K) and such behavior is not so usual in SCO systems. Two scenarios can be considered to explain this particular phenomenon: (i) the compound crystallize in such a way that two (or more) crystallographically unique complexes are present in the packing and due to their distinct environment, only one or half species experience SCO with temperature, or (ii) all the Fe(II) sites are equivalent at low temperature and the SCO involves a crystallographic symmetry breaking (CSB) generating two (or more) crystallographically and magnetically non-equivalent species. 31 In order to investigate the structural-properties relationship and have a better overview on this compound, important efforts were made to isolate it as single-crystals and solve the structures of both diamagnetic and mixed {LS•HS} phases. Dark green sheet-like single-crystals were obtained in moderate yield by recrystallizing of the microcrystalline powder after dissolution in dichloromethane and layering with hexane. The single-crystal X-ray diffraction measurements were performed below (250 K) and above (360 K) the transition. For the pertinence of the following structural analysis, we should first mention that the isolated single-crystals are structurally identical to the microcrystalline sample used for previous measurements. This can be shown by comparing the SAXS diffractograms to the simulated powder patterns provided by X-ray diffraction on a single-crystal (Figure III-34).

where n is the number of reflections and p is the total number of refined.

IV.181 mesomorphic features, while spin switchable properties were not maintained in solid-state. During this PhD work, we applied the same approaches as used in previous chapters, which consisted to introduce additional strongly lipophilic substituents or calamitic smectogen units such as biphenyl and cyanobiphenyl groups in order to favor mesomorphic properties. This Chapter is divided in two parts dedicated to the two types of molecular squares. In the first part, we discuss on the synthetic approach for the preparation of bipyridinebased mesogenic ligands and {Fe 2 Ni 2 } complexes, as well as the characterization of their thermotropic properties and preliminary investigations of their magnetic properties. The second part dedicated to {Fe 2 Co 2 } systems, is mainly oriented toward their investigations in solution.

IV.2. Synthesis and characterization of {Fe 2 Ni 2 } molecular squares

IV.2.1. Synthetic approach

In order to attach multiple mesogenic substituents onto bipyridine ligand, we tested several synthetic strategies. The first one consisted to deprotonate and alkylate the methyl positions of the 4,4'-dimethyl-2,2'bipyridine (Scheme IV-1), as previously described for the preparation of analogues with hexyl and tridecyl alkyl chains. 19 As test reaction, we used the precursor 3,5-bis(dodecyloxy)benzyl bromide, which has been previously prepared in the lab by reduction of methyl 3,5-bis(dodecyloxy)benzoate with lithium aluminium hydride (LiAlH 4 ), followed by a bromination of the resulting alcohol with phosphorus tribromide (PBr 3 ).

Unfortunately, this method was not very efficient. Even though the deprotonation was selective and complete, as verified by deuteration tests, the alkylation was not operating properly. Indeed, only traces of the expected compound were found in NMR spectra of the crude sample. This was explained by a major decomposition of the benzyl bromide derivative, which notably involves a breaking of the Ar-C bond.

Scheme IV-1. Unsuccessful strategy to prepare mesogenic 2,2'-bipyridine ligand analogues.

Thereafter, we followed another strategy that involves an ether coupling reaction between 3,5bis(dodecyloxy)benzyl bromide and 2,2'-bipyridine-4,4'-diyldimethanol B (Scheme IV-2). This latter was prepared by esterification of the 2,2'-bipyridine-4,4'-dicarboxylic acid, followed by a reduction of the resulting ester A using sodium borohydride (NaBH 4 ).

Scheme IV-2. Second strategy tested to introduce mesogenic substituent in 2,2-bipyridine ligand.

The ether coupling reaction was carried using sodium hydride (NaH) in slight excess (2.2 equivalents) under inert and anhydrous conditions. Even though the conversion of the reaction was acceptable (~ 40 %),

Chapter IV -Soft materials based on cyanido-bridged {Fe III 2 (μ-CN) 4 M II 2 } molecular squares

IV.182 the product could not be isolated in good yield. Indeed, the purification of the desired compound by column chromatography was efficient since it has a very similar polarity with the mono-functionalized by-product.

The third approach proposed was to graft the mesogenic part through an ester link by reaction between C, obtained by bromination of B, and benzoic acid precursors D 1-6 using tetrabutylammonium fluoride (TBAF) as mediator (Scheme IV-3). The non-commercial precursors D 1-6 were prepared according to

Chapter II. This procedure was found particularly efficient and we were able to produce four new ligands containing long dodecyl chains or smectogen units, L 3-6 , as well as two ligands containing short substituents, L 1-2 , which were used as model for our studies.

Scheme IV-3. Synthetic pathway for the preparation of L 1-6 ligands

The general synthetic approach for preparation of molecular PB analogues is the combination between solvated metal cations and cyanido-metallate ions. In order to prevent from polymerization, it is necessary to employ capping ligands, which will limit the number of available binding sites on the metal ions. formed during the reaction. Although not isolated, the evidence of this impurity was revealed by the presence of a characteristic χ" signal around 15 K in zero dc field, 22,23 during our initial ac magnetic measurements.

To prevent from its formation, attempts to optimize the reaction conditions were made, for example by using pre-formed {[(Tp*)Fe III (CN) 3 ] 2 [Ni II (DMF) 4 ] 2 }(A) 2 square (using isolated single-crystals) or non-coordinating solvent (nitromethane). Unfortunately, none of these conditions were successful to give better results. Thus, the impurity was removed by size exclusion column chromatography, using Bio-Beads S-X3 beads (neutral, porous styrene divinylbenzene copolymer beads) as stationary phase.

IV.2.2. FTIR spectroscopic analyses

The stretching bands specific to cyanide groups (𝜈̅ CN ) are sensitive to their coordination mode (bridging and terminal) and the oxidation states of the coordinated metal ions. 24 Thus, infrared spectroscopy The stretching band of the cyanide groups in cyanido-bridged {Fe 2 Ni 2 } molecular squares at room temperature is shifted to higher energies and divided into two bands: one at 2151 -2164 cm -1 that is characteristic of bridging cyanides within {Fe III LS (μ-CN)Ni II HS } units and the second one around 2120 cm -1 corresponding to terminal cyanide. 8,9 For 6 the cyanido-bridged band around 2225 cm -1 is too high due to the presence of cyanide that is coming from ligand L 6 . Furthermore, the presence of the B-H stretching band from the Tp* ligand at 2535 -2926 cm -1 and the intense C-H bands at 2930 and 2859 cm -1 from the functionalized bipyridine ligands, confirms the formation of functionalized {Fe 2 Ni 2 } molecular squares.

IV.2.3. Structural descriptions

Single crystals of complex 1 were obtained by recrystallization of the raw powder. The compound was dissolved in acetonitrile, and slow diffusion of diethyl ether vapor was applied. The single-crystal X-ray diffraction analysis of the complex was performed at 120 K. A view of the structure is shown in Figure IV-3 and selected crystallographic data are gathered in Table IV-1 IV-1) are similar to those measured for Fe III in low-spin (LS) configuration. 8,9 The {Fe 2 (μ-CN) 4 Ni 2 } core adopts a distorted square geometry. Indeed, the cyanide bridges are significant bent especially on the nickel sites for which the two Ni-N-C angles were 165.5( 4)° and 174.3(3)°, while the two Fe-C-N angles are 174.9( 5)° and 176.5( 4)° (Table IV-1 22) Fe( 1)-N( 8) 2.013( 5) c, Å

19.7033( 13) Fe( 1)-N( 6) 2.014( 4) α , ° 88.590( 3) Fe( 1)-N( 4) 1.968( 3) β, ° 86.991( 3) Fe( 1)-C( 2) 1.925( 5) γ , ° 80.915( 4) Fe( 1)-C( 1) 1.932( 6) V, Å 3 3813.2( 4)

Selected bond angles (°) ρ calcd , g/cm 3 1.387 N( 11 1) 174.9( 5)

, where n is the number of reflections and p is the total number of refined parameters.

In the crystal packing, strong intermolecular π-π type interactions between adjacent {Fe 2 Ni 2 } complexes are present (2.97 and 2.63 Å, Figure IV-4, left). An hydrogen bond is also present between the 2)

Ni( 1)-N( 6) 2.042(5) b, Å 22.319( 2) Fe( 1)-N( 9) 1.933( 7

, where n is the number of reflections and p is the total number of refined parameters.

IV.2.5. Magnetic properties

The two model complexes 1, 2 and one mesomorphic analogue, 5, were checked for SMM properties by magnetic susceptibility measurements in both dc and ac modes. The characteristic magnetic parameters extracted from these investigation are gathered in Tables IV-5 and results form dc and ac measurements are discussed in sections IV.2.5.1 and IV.2.5.2, respectively.

IV.2.5.1. Static magnetic measurements for 1, 2 and 5

The temperature dependence of the χT products of the {Fe 2 Ni 2 } squares were collected in a static dc field of 0.1 T over a range of temperatures (1.8 -300 K, Figure IV-10). For 1 and 2, the measurements were performed on freshly filtered crystals. The χT values at room temperature (3.52, 3.20 and 3.89 cm 3 mol -1 K, for 1, 2 and 5, respectively), are in agreement with the presence of magnetically isolated Fe III LS (S = ½, g = 2.6 -2.8) and Ni II (S = 1, g = 2.2 -2.3) ion in a 2:2 ratio, assuming that significant orbital contributions are present on Fe III LS ions. 8,9,27 With decreasing temperatures, the χT values gradually increase between 300 and and8.12 cm 3 mol -1 K at 5.5 K are observed, which decrease slightly to 6.4, 6.7 and 7.6 cm 3 mol -1 K at 1.8 K for 1, 2, 5, respectively.

Considering an idealized symmetrical square structure for these compounds, the magnetic data were initially modeled in the frame of an isotropic Heisenberg spin Hamiltonian:

where J is the average exchange interaction between Fe III LS and Ni II sites through the cyanido-bridge, and S i are the spin operators for each metal ions (S Ni(II) = 1 and S Fe(III) = ½). Subsequent application of the van Vleck equation to Kambe's vector coupling scheme allowed for analytical expression (eq.2) of the magnetic susceptibility in the low field approximation: 28 Taking into account only data above 5 K to avoid the effects of intermolecular interactions and/or magnetic anisotropy, the best fit parameters J/k B = 9.7, 9.6 and 10.6 K with g values of 2.14, 2.16, 2.19 for 1, 2, 5, respectively. The intra-complex magnetic interactions lead to an S T = 3 spin ground state.

𝜒 = 𝑁

The field dependence of the magnetization collected for three squares confirms that all the compounds exhibit significant magnetic anisotropy for cyanide-based SMMs (Figure S.IV-5). At 1.85 K the magnetization values found (5.5 μ B for 1 and 2, 5 μ B for 5) confirm that the S T = 3 spin ground. This conclusion is consistent with previously described magnetic data (χT versus T data). Moreover, the M versus H/T data obtained for the {Fe 2 Ni 2 } squares is confirming that significant magnetic anisotropy is present below 8 K. We note that no hysteresis is observed above 1.85 K in the M versus H data and is consistent with behavior reported for a variety of polynuclear cyanide-bridged complexes. [7][8][9]27

IV.2.5.2. Dynamic magnetic measurements for 1, 2 and 5

In order to probe the magnetization dynamics of 1, 2 and 5, ac susceptibility measurements were first performed in the absence of a static dc magnetic field. The in-phase χ′ and out-of-phase χ′′ components of the ac susceptibility were measured as function of the temperature at various ac field frequencies (Figure S.IV-6). In all cases, the onset of frequency-dependent χ′′ signal are observed below 4 K, while the maxima of the

Chapter IV -Soft materials based on cyanido-bridged {Fe III 2 (μ-CN) 4 M II 2 } molecular squares

IV.193 peak could not be observed at low temperature, suggesting that slow relaxation of magnetization is only operative below 1.9 K. Therefore, it was not possible from these data to extract accurately the characteristic relaxation times. These results indicate that in absence of an applied field, the systems are not able to retain magnetization and relax fast above 1.9 K, as found previously in {Fe III 2 Ni II 2 } analogues. 8,9 In these systems, slow relaxation processes in measurable temperature or frequency ranges can be activated by applying small static magnetic fields. 8,9 Thus to determine the optimal fields, ac susceptibility measurements were performed at 1.9 with applied dc magnetic fields ranging from 0 to 10000 Oe. (Figure IV-11). The three complexes show ac signals with relaxation modes lower than 10 kHz for dc field above 200 Oe. The maxima of the χ′′ signals decrease in frequency with increasing dc field. Strong differences of this evolution were found between the three compounds. For example with an applied field of 1000 Oe, the maxima of the χ′′ signals are found at about 80 Hz, 500 Hz and 1200 Hz for 2, 1 and 5, respectively, meaning that 2 is characterized by an higher energy barrier. From these data, the optimal dc fields were estimated to 1000 Oe From the maxima of the χ′′ signals, it was possible to extract the characteristic relaxation times τ at different temperature. In all cases, the relaxation time follows an Arrhenius law, where the ln(τ) versus 1/T plots. From the linear regression fits, the energy barriers Δ eff /k B were estimated to 15, 23 K and 12 K respectively for 1, 2 and 5 (Figure IV-13), which is in agreement with previous value found in {Fe III 2 Ni II 2 } analogues. 8,9 Hence, these investigations revealed the strong influence of the functionalization on the magnetic behavior, and confirmed the possibility to preserve SMM features on a mesomorphic complex 5. Although, no measurement was performed so far for the other complexes functionalized with mesogenic substituents, we assume that this behavior will be maintained.

IV.3. Synthesis and characterization of {Fe 2 Co 2 } molecular squares

In the second part of this project, we have been interested in the functionalization of cyanido-bridged {Fe 2 Co 2 } molecular squares with the aim to promote liquid crystal properties or at least to improve their solubility in usual organic solvents. Our initial strategy was to develop the analogues of {Fe 2 Ni 2 } squares described earlier using the same ligands. Several of these complexes were prepared and our preliminary investigations confirmed remarkably the same liquid crystalline features when L 5 and L 6 are used, while no liquid crystal properties are observed with L 3 and L 4 ligands are employed. All the complexes were isolated as red powder or glassy solids which is the characteristic color for the paramagnetic phase. Unfortunately, upon cooling to liquid nitrogen temperature, none of them revealed change of color into green as expected for ET process. The absence of ET in solid state is not unusual, 16,19 and can be either explained by crystal packing effects, ie. the intermolecular interactions, or by the functionalization of the bipyridine ligands that changes the redox potential of the metal center, making the ET unfavourable. The first hypothesis was discarded since even in solution, no change of color could be detected. Hence, this preliminary study revealed that the functionalization proposed was successful to promote liquid crystalline properties in these molecular squares but was unfortunately inappropriate to obtain ET properties and highlighted the necessity to revise the design of our systems.

IV.3.1. Synthetic approach

In order to promote ET properties in {Fe 2 Co 2 } squares, two strategies might be applied: First, by changing the ligand on the Fe III building block and second, by changing the ligands on the Co II center.

During this work we were mainly focused on the second strategy. In particular, assuming that the change of the redox potential is mainly due to the presence of benzylic carbon functions, we looked for an alternative spacer between the bipyridine platform and the mesogenic part. Previous work in our team showed that {Fe 2 Co 2 } molecular squares functionalized with 4,4' dimethoxy-2,2'-bipyridine exhibit ET in solid-state according to the nature of counter-anion and solvation. 19b Taking into account this result, we proposed to synthetize two new ligands, L 7 and L 8 , containing mesogenic groups linked to the bipyridine through an ether function (Scheme IV-5).

Scheme IV-5. Synthetic pathway for the preparation of L 7 and L 8 .

The ligands were prepared in two steps following the pathway described in Scheme IV-5. The precursor 4,4'-dimethoxy-2,2'-bipyridine was treated with a mixture of HBr/acetic acid and refluxed for The four prepared complexes will be thereafter referred as following:

IV.3.2. FTIR spectroscopic analyses

The oxidation state of the two set of complexes was confirmed by FTIR spectroscopy. In 7 and 9

complexes, the stretching vibration of cyanide ligands in bridging mode were spitted in two bands at ca. 2090 and 2057 cm -1 , while the stretching vibration corresponding to terminal cyanide is found around 2110 cm -1 . These values were similar to those previously found in the diamagnetic analogue {Fe II 2 Co III 2 } (with 4,4'-dimethoxy-2,2'-bipyridine ligand and PF 6-counter anion). 19b In both cases, a very weak band is observed around 2160 cm -1 , suggesting the presence of a small fraction of paramagnetic {Fe III 2 Co II 2 } square in the sample. This latter was previously explained by solvent loss, which induce an irreversible ET into the paramagnetic phase. 19b

In 8 and 10 complexes, the stretching vibrations for cyanide ligands in bridging mode are significantly shifted to higher energy, ca. 2160 cm -1 , as expected for the paramagnetic phase right). 11b,(15)(16)(17)(18)(19) The terminal cyanide band is almost unperturbed compared to 7 and 9, and is found around 2120 cm -1 (Figure IV-14, left). In all complexes, the formation of expected complexes is also supported by the presence of the B-H stretching band from the Tp* ligand at 2506-2921 cm -1 and the intense double band at 2940 and 2 } molecular squares IV.197

IV.3.3. Structural description of {[(Tp

The crystals of 7 are green at room temperature, which is an indication for the diamagnetic configuration. When the crystal is kept outside the mother liquor, it remains stable (no loss of solvent) and no change of color could be perceived. However, when the crystal is heated up to 320 K, the crystals turn irreversibly to red and lose crystallinity. Therefore, single-crystal X-Ray diffraction studies could not be performed at high temperatures.

The crystal was collected at 120 K and showed that the compound crystallizes in triclinic P-1 space group.

The asymmetric unit is composed of one cationic {Fe 2 (μ-CN) 4 Co 2 } molecular square, two trifluorosulfonate counter anions and six co-crystallized water molecules. The structure of the central fragment is essentially similar to those previously reported. [15][16][17][18][19] The central fragment is constituted from a square shaped core where 9) Co( 1)-N( 13) 1.926( 7) b, Å 17.2525( 13) Fe( 1)-N( 1) 2.034( 8) c, Å 23.0199( 16) Fe( 1)-N( 3) 2.037( 7) α, ° 88.523( 4) Fe( 1)-N( 5) 2.036( 7) β, ° 80.997( 6) Fe( 1)-C( 16) 1.867( 7) γ, ° 73.821( 4) Fe( 1)-C( 17) 1.892( 9) V, Å 3 4631.69 Fe(1)-C( 18) 1.893( 8)

Selected bond angles (°) ρ calcd , g/cm 3 1.245 N( 11)-Co( 1)-N( 10

, where n is the number of reflections and p is the total number of refined parameters.

Two of the three cyanide ions from the Fe sites coordinate the Co ions in bridging mode, while the free cyanide groups adopt an anti orientation relative to the {Fe 2 (μ-CN) 4 Co 2 } plane. The coordination sphere of Co ion is completed by two bidentate L 7 ligands. The {Fe 2 (μ-CN) 4 Co 2 } core is nearly planar as shown by Fe-C-N angles, which range between 174.7( 7) and 177.6( 7)°, while Co-N-C angles range between 114.2( 6) and 126.2( 6)° (Table IV -6, Figure IV-15). In addition, the C( 17)-Fe(1)-C( 16) angle ( 90 IV -6). The oxidation state of the molecular square, {Fe II 2 (μ-CN) 4 Co III 2 } was unambiguously confirmed by the examination of the coordination sphere of the Co ion site. The Co-N bond distance range from 1.899 to 1.940 Å, which is in agreement with Co III in its LS state (Table IV-6), and Fe II LS ion was therefore deduced to satisfy the charge balance. The Co III LS configuration is also supported by the relatively low distortion of the coordination sphere (Σ (N-Co-N) = 34.2°).

General Conclusions

III.221

At the end of this manuscript, we will recall the main results obtained during this thesis work.

In the first introduction chapter, we outline theoretical aspects of the single molecule magnets, spin cross-over and electron-transfer systems, to make the readers to understand the subsequent Chapters II to IV. Several examples have been described that show the richness of coordination chemistry. This literature summary emphasizes that hybrid systems are multifunctional materials. Towards this, we attempt to obtain hybrid materials in order to induce and organize magnetic materials by functionalizing with organic ligands.

Chapter II is dedicated on synthesis and characterization of new Mn 12 -based SMMs towards hybrid liquid crystalline systems by applying a rational functionalization. In contrast with previous studies in which the sixteen acetate groups were replaced by mesogenic ligands, we alternatively introduced these ligands only on certain positions of the Mn 12 core using the regioselective functionalization methodology. The four Mn 12 -based complexes were successfully synthesized and fully characterized. All the complexes preserve SMM properties, confirming the versatility of these molecular systems to undergo ligand substitution without altering significantly the structure and properties of the magnetic core. The two complexes containing dodecyl chains present high-temperature clearing points, where the integrity of the complexes is not confirmed after annealing into the isotropic phase. Indeed, complexes self-organize into liquid crystalline cubic phases, indicating that no improvement on the overall structural anisotropy was achieved. This result may reflect the tendency of the flexible moieties to reorganize themselves all around the Mn 12 core leading therefore to a pseudo-spherical geometry of the complex. For the compound containing two biphenyl moieties, no columnar mesophases was obtained. The complex self-organizes in the same fashion as its precursor (bilayer lamellar phase). Even though, the chosen strategy of functionalization did not reveal desired properties, this compound could be an interesting candidate for surface deposition.

In order to avoid of the reorganization of the flexible chains around the magnetic core as in our two first complexes, we should increase the separating distance between these moieties. One idea would be to add a rigid spacer between the anchoring part of the ligand (carboxylate) and the mesogenic one. In this respect, we can induce the columnar phases, by ordering this ligand in axial position of the Mn 12 -core.

In Chapter III, we described the synthesis and characterization of functionalized spin cross-over systems based on Fe II L 2 complexes with a N 4 O 2 coordination sphere. In the first part of the chapter, five complexes containing non-mesogenic ligands containing donating groups were synthesized and characterized. According to this functionalization, we can conclude that the presence of methoxy functions on the benzohydrazide scaffold is not incompatible with SCO, since such features were emphasized for compounds [Fe(L 1 ) 2 ], [Fe(L 2 ) 2 ] and [Fe(L 3 ) 2 ] and their polymorphs. But an interesting achievement of this work is the presence of polymorphism in these molecular systems. Moreover, the three structures that feature the highest number of π-π contacts (β-{[Fe(L 1

 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 (FTIR) analysis

The FTIR 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.

 Electrospray Ionization Mass Spectrometry (ESI-MS) analysis

ESI-MS spectra were performed using an Electro Spray Ionizationtime-of-light (TOF) mass spectrometer (LCQ Advantage; ThermoFicher) by analyzing positive ions.

 Nuclear magnetic resonance 1 H NMR and 19 F NMR spectra were collected with JEOL ECS 400 instrument at room temperature, using CDCl 3 , CD 3 CN, DCM or THF 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 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.

 X-ray Powder Diffraction (XRD)

The X-ray powder diffraction data for compounds were collected with Philips PW 1820 diffractometer with Bragg-Bentano geometry (λ = 1.5406 Å). The angular range extends from 5 to 60° in θ with a step acquisition of 1 or 10 seconds.

 Small angle X-ray scattering (SAXS) X-Ray diffraction experiment at small and wide angle were performed 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. 3

 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 PerkinElmer UV-Vis/NIR recording spectrophotometer. 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