Keywords: paramagnetic NMR, molecular magnetic materials, cyanide ligand, building blocks, self-assembly, prussian blue analogues, cyanide--bridged low dimensional compounds, spin--crossover complexes, spin density

The work presented in this Ph.D. dissertation focuses on the application of NMR spectroscopy for studying molecular magnetic materials. A particular attention is devoted to cyanide--contaning magnetic systems: cyanide building blocks, low--dimensional polymetallic complexes and extended networks (Prussian blue analogues). Basically, we try to show through selected examples that NMR can be used as a powerful structural and magnetic probe to address a variety of questions related to paramagnetic materials (chapter 2, 4, 5). Besides, we also use here NMR to study spin equilibrium in solution in a family of Fe(II) spin-crossover complexes (chapter 3). Apart from the use of various NMR techniques, different physical techniques including FT--IR and UV--vis spectroscopy (in solid--state and solution), TGA, X--ray diffraction, EPR spectroscopy, Polarized neutron diffraction (only for [Fe(Tp)(CN) 3 ] --compound), and SQUID magnetometry are used to obtain reliable structural and electronic characterization of the probed magnetic materials and to rationalize and support the results obtained from NMR spectroscopy. Theoretical DFT calculations (in collaboration with theoreticians) are also performed on some selected compounds to support the experimental observations. The first chapter of the dissertation provides a short overview of some basic concepts of NMR spectroscopy in solid state and in solution and gives very briefly some few examples of NMR studies on a variety of paramagnetic systems. The first experimental chapter of the thesis focuses on the use of 13 C and 15 N solid-state NMR spectroscopy to probe local magnetic properties and to estimate the spin density and its distribution onto the cyanide ligands. It thus addresses a fundamental question for magnetochemists: how the unpaired electron delocalized from the paramagnetic source onto the bridging ligand to give rise to the magnetic exchange interaction? In the third chapter of the thesis, the solution state paramagnetic 1 H NMR spectroscopy has been used to study Fe(II) mononuclear switchable complexes, which undergo a change of their magnetic (and optical) properties upon external stimuli. More specifically, the thermally--induced spin-crossover equilibria of the [Fe II (R--bik) 3 ] 2+ complexes has been studied by variable temperature 1 H NMR and the results are compared to those obtained by other techniques. The fourth chapter deals with the magneto--structural characterization of some polynuclear complexes based on the new cyanide--based [Co III ( Me2 Tp)(CN) 3 ] --building block. NMR spectroscopy of the quadrupolar 59 Co nucleus is used as a probe for following the magnetic behaviour of these Co(III) containing molecular systems at different temperature. It also allows to investigate the spin extension mechanism over the cyanide bridge. Finally the chapter 5 intends to explore the use of 113 Cd NMR spectroscopy as both a local structural and magnetic probe for studying Fe--Cd based molecular materials. The first part of the chapter 5 focuses on the use of 113 Cd NMR as a local structural probe. It is used here to investigate the electrochemical insertion and extraction of Li + ions into the lacunary Cd 3 [Fe(CN) 6 ] . 15H 2 O prussian blue analogue that could be used a electrode material.

The second part of this chapter focuses on the use of 113 Cd NMR as a magnetic probe. It deals with the investigation of the role of geometric or electronic parameters on the spin delocalization mechanism, from the paramagnetic source to the divalent metal ion connected via the cyanide bridge.

Introduction

Metal ions are omnipresent in chemical and biological processes. Some of them are paramagnetic due to the presence of unpaired electrons in their valence orbitals. Nuclear Magnetic Resonance (NMR) spectroscopy is one of the important techniques for structural characterization of the paramagnetic and diamagnetic species in solid state as well as in solution. However, the presence of paramagnetic centre has a dramatic influence on the NMR signal. It results in a wide spread of the NMR chemical shifts and a broadening of the signals, which sometimes leads to the fallacy that NMR is not very useful for characterizing paramagnetic compounds. The progresses in NMR equipment (improved performance of the analog to digital converters, radio--frequency pulses, etc.) have allowed to partially removing the problems associated with spectral broadening and resolution. In recent times a marge variety of paramagnetic systems have ben studied through NMR such as cathode materials for lithium--ion batteries, paramagnetic metalloproteins and porphyrins, organometallic compounds (sometimes involved in catalytic processes), inorganic complexes and various lanthanide complexes, etc. The general interests behind studying those systems through NMR mainly lie on the local structural characterization, spin dynamics investigation and to follow the electronic and chemical behaviour. In the present contribution we intend to extend this research work to molecular magnetic materials. In the first chapter of the Ph.D. a short description about the basic principles used in NMR spectroscopy and a glimpse of selected research work in the field of paramagnetic NMR is highlighted.

In the four experimental chapters we intend to illustrate how NMR spectroscopy can be used to investigate cyanide--based molecular magnetic materials based.

The design of molecular magnetic materials is an emerging domain of materials science that offers potential application in emerging field such as molecular electronics. These molecular systems represent an alternative to the use of more traditional magnetic oxide and alloys. The usual synthetic route to design this kind of materials is based on the self--assembly of the preformed building blocks. These building blocks exhibit specific electronic and structural information that allow to control the architecture and the physical properties of the resulting materials. In order to understand the behaviour of the molecular magnetic materials, the characterization of the electronic properties and the structure of the building block are of high importance. Structural information can be obtained from XRD experiments when single crystals are available and electronic information can be obtain through different techniques including NMR. How electronic characterization is related to NMR technique? The answer lies in the term called (super)hyperfine coupling. The (super)hyperfine coupling is the interaction between the spin of the unpaired electron and the nuclear spin of probed nucleus. By measuring this interaction through NMR on different nuclei, it is thus possible to reach valuable electronic information such as the distribution of unpaired electronic in a paramagnetic complex. In the second chapter of the thesis we thus explore the use of NMR spectroscopy to probe the spin density on the cyanide ligand in various molecular building blocks through solid 13 C and 15 N solid--state NMR spectroscopy. The selected building blocks exhibit various topologies but the same low--spin d 5 electronic configuration, with only one unpaired electron. Our aim is here to compare how the geometry, the nature of the ligand or the nature of the valence shell (3d or 4d) may influence spin delocalization. When possible the results are compared to theoretical calculations or other experimental techniques such as polarized neutron diffraction.

The third chapter deals with the study of switchable molecular systems, which underg o a change of their magnetic (and optical) properties upon an external stimulus. Here we investigate more specifically the thermo--induced spin--crossover phenomenon in a series of Fe(II) tris--chelate complexes. Taking advantage of the temperature dependency of the (paramagnetic) chemical shift, variable temperature 1 H solution NMR and variable temperature Evans' NMR of these compounds have been performed in order to follow the spin state change and extract the thermodynamic parameters associated to the spin equilibrium. The accuracy of the result obtained from NMR spectroscopy are compared to those obtained from magnetic susceptibility measurements, and UV--vis spectroscopy.

In the fourth chapter, we investigated polymetallic systems based on the new [Co(Tp Me2 )(CN) 3 ] --building block. These Co(III) complexes are studied through 59 Co NMR spectroscopy in solid and solution. Although, the quadrupolar nature of the Co(III) nuclei makes the analysis complicated, the 59 Co chemical shift, and quadrupolar coupling parameters were carefully investigated. A clear influence of the magnetic state of the neighbouring atom is observed on the 59 Co NMR signals. Before this work, 59 Co solid--state NMR spectroscopy has been used mainly to investigate various small molecules, biological model systems, homogeneous catalysts and octahedral basic inorganic complexes. Here, we use the 59 Co NMR technique to study molecular systems having different magnetic properties.

The fifth chapter of the thesis is a continuation of a preliminary study on FeCd PBAs in which the 113 Cd NMR spectroscopy was used to probe both structural and magnetic properties at the local scale.

The first part of this chapter is devoted to the evaluation of FeCd Prussian blue analogues as cathode materials for lithium ion batteries. The comparing the 113 Cd NMR spectra recorded at different stages of the Li + intercalation, with the spectra of the FeCd PBAs model compounds, it is possible to highlight the local structural changes that occur during the Li insertion, and to correlate them with the electrochemical properties of the material.

In the second part, we focus on the magnetic information that can be extracted from 113 Cd NMR. More specifically, our goal is to study how geometric or electronic parameters can influence the spin delocalization from the magnetic source (the iron ion) to the Cd(II) ion through the cyanide bridge. We thus investigate a series of new low dimensional {Fe--CN--Cd} compounds whose crystal structure has been precisely determined. The results are compared to each other and with those obtained in PBAs.

Introduction

Near 1902 physicist P. Zeeman first observed the splitting of spectral lines when atoms are subjected to a strong external magnetic field. For the discovery of the so--called Zeeman effect, he was awarded Nobel Prize. It was exactly 50 years later that physicists F. Bloch and E. Purcell shared a Nobel prize for putting the so--called nuclear Zeeman effect to practical use by constructing the first "Nuclear Magnetic Resonance" spectrometer. 1,2 Since this time, NMR spectrometers have considerably evolved and numerous NMR methods for application in both the solid and solution--state have been developed. 3 Nowadays Nuclear Magnetic Resonance appears as one of the major technique for elucidation of molecular structure and dynamics. The large capabilities of NMR spectroscopy makes it a very powerful technique for the investigation of wide range of materials such as polymers, organic compounds, organometallics, inorganic coordination compounds, biomolecules. 4 There are a number of experimental techniques existing for structural investigation of the materials including conventional X--ray, neutron or electron diffraction. Generally, structural investigation at atomic scale can be carried out by vibrational spectroscopies (infrared, UV--vis, Raman), X--ray absorption (EXAFS, XANES) or by various multinuclear NMR techniques. Unlike techniques such as X--ray diffraction, which conventionally require the presence of long--range atomic order, NMR spectroscopy is almost equally useful for studying the atomic environments of the most disordered materials as for the most ordered single crystals systems. 5 In the other hand, the importance of solution--state nuclear magnetic resonance (NMR) is already well established since long time. In solution--state NMR, spectra consist of sharp signals, due to averaging of anisotropic interactions by rapid molecular tumbling. In the solid state, this kind of motion is normally absent. Therefore spectra are influenced by the anisotropic interactions and they get broad. These anisotropic interactions on one hand are responsible for lowering the resolution of distinct sites, but on the other hand they provide valuable structural and dynamic information. High--resolution solid--state NMR spectra can provide the same type of information that is obtained from corresponding solution NMR spectra, but a number of special techniques and equipment are required, including magic--angle spinning, cross polarization, special pulse sequences, enhanced probe electronics, etc. 6 Nowadays the availabilities of the well--equipped NMR spectrometers enable to extract these anisotropic interactions and corresponding structural information accurately. 7--10

Basic principles of NMR

Once the nucleus is subjected to a magnetic field, its magnetic moment (µ) interacts with the external magnetic field (B 0 ) and experiences a torque given by: τ = µμ × B 0 (1) This causes the nucleus to precess along the direction of applied magnetic field at an angular frequency of ω as shown in figure 1.

Figure 1. Precession of the magnetic moment of a nucleus in the presence of a static magnetic field. 8 Now, the magnetic moment vector of a nucleus in a magnetic field also precesses with a characteristic angular frequency called the Larmor frequency (ω), which is a function of γ and B 0 :

ω = γ B 0 (2)
Where γ is the gyromagnetic ratio (Charge to mass ratio that is unique to each nucleus) and it is also the proportionality constant between the magnetic moment and nuclear spin.

γ = q/2m = 2!"/ℎ! (3)
The angular Larmor frequency (ω), in units of radians per second can be converted into the linear frequency ! (in reciprocal seconds or Hz) by division by 2π:

! precession = ω/2π = γ B 0 /2! (4)
This is the fundamental equation of NMR, which correlates the applied radio frequency with the magnetic field strength.

The precessional motion causes the tip of the magnetic moment vectors (either up or down) to trace out a circular path (Figure 2). It was already mentioned that the torque experienced by a nucleus in the presence of a magnetic field leads to precession of the nuclei about B 0 . The angular frequency (ω) can be obtained from the various 2I+1 angular momentum spin states. The energy required for a transition from the higher to the lower energy state, for the case of I=1/2, can be expressed by the difference in their energies:

E = E f -E i = ℎ! precession = (h/2π).ω = (h/2π). γ B 0 (5) Hence, from the above discussion we can have a brief overview about the behaviour of a single spin under an external magnetic field. Therefore, we can investigate that how a collection of many spins behaves. In the presence of an external magnetic field individual spins will either align with or against the field (Figure 3). Parallel orientations implies the lower energy state with m = ½ (spin up), while anti--parallel orientations points to a higher energy state with m = -½ (spin down).

According to the Boltzmann distribution always there will be slightly more amount spins in the lower energy state than that of the high--energy state (Figure 3). This gives rise to a net longitudinal magnetization that has cylindrical symmetry due to spin precession. 4

Interactions in NMR

If a nucleus is subjected to a magnetic field, it experiences many different types of interaction. The total interaction energy of the nucleus is result of all the individual Hamiltonians it experiences. The information about the atomic scale surroundings of a nucleus is contained within the interactions between the nucleus and those surroundings. Some of the interactions are much stronger than the others, but all contain valuable information about the nucleus surroundings. These interactions are summarised in the internal Hamiltonian (ℋ !"# ) mentioned below. (6) Where ℋ ! , ℋ ! , ℋ !" , ℋ ! , ℋ !"#" and ℋ ! are the hamiltonian for Zeeman, dipolar, chemical shielding, spin--spin coupling, electrical quadrupolar and paramagnetic interactions (valid only for paramagnetic compounds). In diamagnetic complexes, spin--1/2 nuclei observe different kind of interactions including dipolar interactions (through--space interactions with nearby nuclear magnetic moments), chemical shielding (modification of the magnetic field due to surrounding electrons) and indirect spin--spin coupling or J coupling (interaction of nuclear magnetic moments mediated by intermediate electron spins). In materials containing paramagnetic centres, the unpaired electrons strongly interact with the nuclei resulting in a large chemical shift (Fermi contact and Knight shift interactions) and effective broadness (dipolar interaction) on the NMR spectra. Nuclei with spin I>1/2 are also affected by the electric quadrupole interaction (H Q ), an interaction between the nuclear electric quadrupole moment and the gradient in the electric field at the nucleus. 5,11 Although this is an electrical interaction it depends on the magnetic quantum number and so affects the NMR spectrum. The main NMR interactions are summarized below (Table 1).

ℋ !"# = ℋ ! + ℋ ! + ℋ !" + ℋ ! + ℋ ! + ℋ !"#"
Table 1: Summary of NMR interactions. 5 Interaction Nature Information Amplitude (Hz)

Field dependence Zeeman Interaction with main magnetic field

where ! is the external magnetic field, ! = -!! is the Cartesian tensor of the Zeeman interaction and 1 is a unit tensor. This is the strongest interaction and as such dominates. All other interactions of the nucleus with its surroundings are several orders of the magnitude smaller and can therefore be treated as perturbations of the Zeeman Hamiltonian ℋ ! . 13

Dipolar interaction

The dipolar interaction arises from the coupling between two magnetic dipoles. More specifically, the dipolar interaction results from interaction of one nuclear spin with a magnetic field generated by another nuclear spin, and vice versa. In the figure given below, j and k are two magnetic dipoles. They are separated by a distance of r. This is a direct through space interaction, which is dependent upon the ! of each nucleus, as well as r --3 :

Figure 4. The dipolar interactions between two magnetic dipoles (j and k)

The dipolar Hamiltonian is expressed by the following equation: 5

ℋ ! = ! ! !! ! ! ! ! ℏ ! ! (! + ! + ! + ! + ! + !) (10)
The term A--F is called dipolar alphabets. Normally, only A and B term is often considered when considering dipolar coupling.

! = ! !" ! !" 3!"# ! ! -1 (11)
! = - ! ! ! !! ! !! + ! !! ! !! 3!"# ! ! -1 (12)
In the above expression of dipolar Hamiltonian the

! ! !! ! ! ! ! ℏ
! ! term is is called as the dipolar coupling constant (d) which describes how large the dipolar coupling is between two nuclei at a given distance. It is possible to express the dipolar Hamiltonian in a more specific way, separated for heteronuclear and homonuclear case.

ℋ ! !!"!#$ = ! ! !! ! ! ! ! ℏ ! ! ! 2! !" ! !" 3!"# ! ! -1 (13)
ℋ ! !!"! = ! ! !! ! ! !,! ! ! ! ! ℏ ! ! ! 2! !" ! !" - ! ! ! !! ! !! + ! !! ! !! 3!"# ! ! -1 (14)
It is observed from the above equation that for both homonuclear and heteronuclear cases the interaction consists similar angular dependence of 3!"# ! ! -1 . Here θ represents the angle between the external magnetic field (B 0 ) and the vector joining two dipoles. This actually turns out to be very useful to remove the effect of anisotropic broadening from the NMR spectra. 13 Moreover, this interaction is completely independent of B 0 . The dipole coupling is very useful for molecular solid--state structural studies, since it is only dependent on the intermolecular distance (r --3 ). By measuring this dipolar coupling one can accurately estimate the distances between nuclei and hence the geometrical form of the molecule. In liquid due to free movement of the molecules this interaction is averaged out.

Chemical shift interaction

Nuclei are surrounded by electrons. Since electrons are moving charges, they obey to the laws of electromagnetic induction. Hence, the applied magnetic field (B 0 ) induces circulation in the electron cloud surrounding the nucleus such that a secondary magnetic field opposed to B 0 is produced. 14 Thus, the local magnetic field observed by a nucleus, is smaller than the applied field. The nucleus is shielded from the external field by its surrounding electrons by an amount equal to σΒ 0 where σ is known as the shielding constant. 3,4 Therefore, the resulting field observed at the nucleus could be expressed as (15) The nucleus precesses at the Larmor frequency: (16) The absolute magnetic shielding, σ, in ppm actually refers the difference in shielding between the frequency of the bare nucleus, ! !"#$ , and the frequency of the same nucleus in the species under investigation, ! ! :

B nucl = B 0 (1--σ)
! ! = ! !! (1 -!)! !
! !!" = 10 ! ! !"#$ -! ! ! !"#$ (17)
The chemical shift, δ, is the difference in shielding between the nucleus in the species under investigation, ! ! , and the shielding of the same nucleus in a reference compound, ! !"# :

! !!" = 10 ! × (! !"# -! ! ) (1 -! !"# ) = 10 ! × (! ! -! !"# ) (! !"# ) (18
) Because ! !"# is often a small number compared to 1, frequently the latter approximation is used. In addition to that since the extent of shielding is proportional to the external magnetic field B o , we use field independent units for chemical shifts: δ values, whose units are ppm (parts per million). Moreover, it would be inconvenient and confusing to always have to convert NMR data according to the field strength of the instrument used. Therefore, the resonance frequencies are usually expressed not as absolute values in Hz, but rather as values relative to a common standard, generally the signal generated by the protons in TMS.

Chemical Shift Tensors

Depending on the local symmetry at the nuclear site, the magnitude of the chemical shift will vary as a function of the orientation of the molecule with respect to the external magnetic field. This orientation dependence of the chemical shift is referred to as chemical shift anisotropy (CSA). 15 Mathematically, the chemical shift anisotropy is described by a second--rank tensor (a 3 by 3 matrix), which in the case of the symmetric part of the chemical shift (CS) tensor consists of six independent components. Generally, one is able to express the chemical shift tensor in a coordinate frame where all off--diagonal elements vanish. In this principal axis system, the chemical shift tensor is fully described by the three diagonal elements -- the principal components --and the three Eigen vectors or Euler angles describing the orientation of the principal axes with respect to an arbitrary frame (Figure 5). In the standard convention, the principal components of the chemical shift tensor, (δ 11 , δ 22 , δ 33 ), are labeled according to the IUPAC rules. 16 They follow the high frequency--positive order. Thus, δ 11 corresponds to the direction of least shielding, with the highest frequency, while δ 33 corresponds to the direction of highest shielding, with the lowest frequency as shown in figure 6. The isotropic values, δ iso , are the average values of the principal components, and correspond to the center of gravity of the line shape.

Indirect dipole coupling or J--coupling

The J--coupling is a quantum mechanical--based indirect interaction of nuclear moments, which is mediated via the electrons in chemical bonds between the nuclei. The first nucleus perturbs the surrounding electrons, which results an additional magnetic field at the second nucleus. In contrast to dipolar coupling, which is considered as through--space interaction, scalar coupling is a through--bond interaction. Therefore, it is most significant for covalent compounds. 5 The J--coupling is exclusively intramolecular and two spins have a measurable indirect dipole coupling only if they are linked together through a small number of bonds, including hydrogen bonds. 13 The magnitude of this coupling depends on the degree of covalence of the bonds, the number of bonds between the nuclei and the gyromagnetic ratio. The Hamiltonian for the J-coupling is given by 5

ℋ ! = ℎ! !" ! ! • ! ! (19)
J is the indirect spin--spin or scalar coupling constant and it is expressed in Hz. The source of this coupling can be spin--orbital, spin--dipolar and Fermi contact interactions. 17 In this case the Fermi contact interaction is probably the most dominant one. There is no B 0 dependence of this interaction (Table 1). The J--coupling is mainly observed in liquids since it is often obscured in solids by other interactions. The spectral pattern arises due to this coupling provides important insight to the local spin network and has proved very useful in liquids for identifying the local fragment of molecules like proteins.

Quadrupolar interaction

Most nuclei in the periodic table have a spin I larger than 1/2. These spins are called quadrupole spins and they are sensitive to the electric field gradient (whose magnitude is expressed as eq) generated by their surroundings. The coupling of the nuclear electric quadrupole moment eQ (a property of the nucleus) with an electric field gradient (a property of the sample) is called the quadrupole interaction.

The Hamiltonian of the quadrupolar interaction can be expressed as: 11

ℋ ! = !" !!(!!!!) ! • ! • ! (20)
where, V represents EFG tensor.

! !"# = ! !! 0 0 0 ! !! 0 0 0 ! !!
The principal components of the electric field gradient (EFG) tensor are defined such that |V 11 | ≤ |V 22 | ≤ |V 33 |. Since the EFG tensor is traceless, isotropic tumbling in solution averages it to zero. The magnitude of the quadrupolar interaction is given by the nuclear quadrupole coupling constant:

! ! = !"! !! ! (!" !"#) (21)
The asymmetry of the quadrupolar interaction is given by the asymmetry parameter, 11 11 (22) Where, 0 ≤ ! ! ≤ 1 and ! !! = !" If ! ! = 0, the EFG tensor is axially symmetric. The quadrupolar interaction, unlike all of the other anisotropic NMR interactions, can be written as a sum of first and second order interactions:

! ! = ! !! !! !! ! !!
ℋ ! = ℋ ! ! + ℋ ! ! (23)
The first--order energy perturbation to the Zeeman transition energy, m, of magnitude: 5

! ! ! = ! ! ! ! !! !!!! ! ! 3!"# ! ! -1 + ! ! !"# ! !"#$2! (3! ! -! ! + 1 ) (24)
Indeed, the quadrupolar interaction can be large, but in most cases it is smaller in magnitude than that of than the Zeeman interaction and its effect is typically described as a perturbation to the Zeeman energy levels. The below figure shows the perturbation of the four Zeeman energy levels for a spin I = 3/2 nucleus by the quadrupolar interaction. The degeneracy of the three single--quantum (Δm = ±1) transitions, all of which have frequency ω L under the Zeeman interaction, is lifted when a quadrupolar interaction is present.

There are now two distinct types of transitions: (i) a central transition (with m = +1/2 ↔ m = -1/2), unaffected by the quadrupolar interaction to first order, at ω L ; and (ii) two satellite transitions (with m = -3/2 ↔ m = -1/2 and m = +1/2 ↔ m = +3/2), with frequencies of ω L -2ω Q and ω L + 2ω Q , respectively. Here, ! ! is the quadrupolar coupling frequency and it s expressed in MHz. Figure 7. (Left) The four energy levels for a I = 3/2 system system in a single crystal in (state a) zero Zeeman field B 0 , (b) in the presence of B 0 , (c) including the first--order quadrupole interaction, and (d) including the second--order quadrupole interaction. (Right) Their corresponding spectra are represented as arrows in (b'), (c'), and (d') respectively. In the figure ω Q is the quadrupole coupling and ω L is the Larmor frequency. 14 If the quadrupolar interaction becomes larger as the result of increasing EFGs, the quadrupolar interaction can no longer be treated as a first--order perturbation on the Zeeman Hamiltonian. It is often considered as a second--order perturbation to the Zeeman interaction. The magnitude of the second--order energy perturbation is inversely proportional to ! ! , therefore at higher magnetic field strengths the contribution from the quadrupolar interaction is reduced. 14

! ! ! !, ! -1 = (25)
where,

! ! = !! ! !!(!!!!)
In the above equation C n (I,m) are the coefficient for second--order quadrupole effects,

! ! ! ! = 1 + ! ! ! ! ,
! ! cosθ and ! ! cosθ are second and fourth order Legendre functions. ! ! β, γ, ! ! is the function of β, γ (Euler angles that describe the orientation of B 0 with respect to principle axis system) and ! ! .

The (d') portion in figure 7 represents the effect of ℋ ! ! is to shift the energy levels further (state d). As a result, an additional shift with respect to ω L , called the second--order quadrupole shift, occurs for each absorption line. However, this second--order quadrupole shift of a line is much smaller than the ±2ω Q shift due to ℋ ! ! . 14 The further detail about quadrupolar interaction will be discussed in chapter 4.

The paramagnetic interaction or hyperfine interactions will be discussed in section 7.1.

NMR in liquids

In case of liquid, rapid isotropic tumbling averages the anisotropic or secular components of the nuclear interactions to zero. The secular terms in all interaction tensors consists of an angle dependence in the form of (1 -3!"# ! ! ). If the term is integrated over all orientation angles, one can show that the anisotropic contributions to the interactions vanish and only the isotropic part remains. 18,19 The dipole and first order quadrupole interaction tensors are traceless and therefore vanish completely in isotropic liquids.

The terms remaining are only the isotropic parts of interaction Hamiltonians:

ℋ = -!ℏ! ! + !"! ! ! ! + !! ! ! ! (26)
In a typical solution--state NMR spectrum the resonance lines are narrow, therefore multiplet structure can be revealed without any problem 13 as shown in Figure 8. The presence of three sets of signals in the above NMR spectra indicatess that there are three different chemical environments that the protons can be observed: (a) three protons of terminal methyl (CH 3 ) group, (b) two protons of the methylene (CH 2 ) carbon and (c) the single proton attached the oxygen of the alcohol group (OH). Here, the CH 2 group of the ethanol appears at about 4.1 ppm with quartet spilitting pattern (with intensity 1:3:3:1). The quartet pattern arises due to coupling with three hydrogens attached to the neighbouring CH 3 group. The CH 3 group at about 1.3 ppm with a triplet pattern (with intensity 1:2:1) is undergone coupling with the two protons attached to the CH 2 group. The proton of the -OH group appears as a singlet and is not coupling with the other H atoms.

Solid--State NMR / Magic angle spinning (MAS)

In the solid--state, the molecular tumbling does not usually occurs. Therefore anisotropic effects with an orientational dependence are not averaged by motional effects. This leads to broad (sometimes featureless) lineshapes that can hinder the extraction of chemical shift information. 13 C NMR (7.04 T) spectra of glycine indicative of narrow and broad line shape and in solution and solid state respectively. The NMR spectra have been produced from data taken from the Spectral Data Base System for Organic Compounds at the National Institute of Materials and Chemical Research in Japan. The above figure (figure 9) clearly shows that the anisotropy is absent in solution, which leads to a narrow spike like signals of the carbons. In contrast, the shape of the signals in solid--state static spectrum reflects the presence of anisotropic effects in the compound. It may be correlated to the different geometry of the electronic/chemical surrounding around the two carbon atoms: rhombic nature of the C1 carbon and a roughly axial symmetry for carbon 2.

At this point, the so--called "magic angle spinning (MAS) technique" becomes useful to gain resolution. It has been observed that, the rotation of the sample at a magic angle is sufficient to eliminate the CSA for the carbons in a large extent. Thus, a narrow, well resolved signals could be obtained for this compound through MAS NMR. A brief description about the MAS NMR spectroscopy is described hereunder.

MAS NMR

An significant breakthrough in solid--state NMR came when in 1959 Andrew and Lowe discovered that some of the factors responsible for broadening in the materials could be diminished through rapid rotation of the sample at a particular angle to the magnetic field axis (the magic angle ! ! = 54.74 0 ). 6 The interactions like anisotropic chemical shift, dipolar and first order quadrupolar interaction contains orientation dependence of (1 -3!"# ! !), where ! is the angle between the tensor principal axes system direction and the external magnetic field B 0 . 6,20 If the sample is spun around the magic angle ! ! , the orientation dependence of the interaction can be expressed as: 13

3!"#

! ! -1 = 1 2 3!"# ! ! ! -1 3!"# ! ! -1 + ! 3!"# ! ! ! -1 3!"# ! ! -1 (28
) Now, if the angle ! ! is selected in such a way that (3!"# ! ! ! -1) becomes zero, then anisotropic part of the interaction vanishes and only the isotropic contribution remains. During the acquisition of a MAS NMR spectra some equispaced lines are often observed on each side of the central resonance. These lines are called the spinning side--bands. They appears due to the modulation of the interaction by the physical act of spinning the sample. As they are harmonics of the spinning speed, they can be distinguised from the isotropic (true) peaks. Their position will change with the spinning speed, moving away from the central peak at higher speeds. They are visible only when the sample is turn at a lower rate than the magnitude of the anisotropic interaction. The figure 11 represents the variation in the number of spinning sidebands with MAS spinning speed. Figure 11. 31 P MAS NMR spectra (9.4 T) of Na 5 P 3 O 10 recorded at a spinning speed of (a) 13 kHz and (b) 4.6 kHz. The istropic peaks are (marked by asterisk) not moving with spinning speed and it is located at 1.3 ppm and --7.5 ppm. Spinning sidebands are diminished in manifold upon increasing MAS spinning speed. 20 The broadening in the spectra of spin I = ½ nuclei arises mainly due to interactions between the magnetic dipoles of the adjacent nuclei which are anisotropic in powder samples. In quadrupolar nuclei where spin I > ½ the interaction between non zero nuclear--electric quadrupole moment with the electric field gradient causes the broadening associated with these nuclei. Some of these interactions consist (3cos 2 θ m -1) term in the second--order Legendre polynomial P 2 (cosθ m ), where θ m is the angle between the axis of the applied magnetic field and the principal axis of interaction. At the magic angle cos θ m = 1/√3 and the term P 2 (Cosθ m ) becomes zero (Fig 12).

Figure 12. second and fourth order Legendre polynomial as a funtion of θ (inclination angle of the rotor). 5,12 !" !"

The rate of MAS must be greater than or equal to the magnitude of the anisotropic interaction to average it to zero. MAS therefore removes the broadening of the spectra of quadrupolar nuclei arising from first-order quadrupolar interactions, but not broadening due to higher order interactions which have angular dependence other than (3cos 2 θ m -1). In complex spectra of samples containing several overlapping sites, the removal of higher order broadening may be important, and can be accomplised by spinning at more than one angle either simultaneously as in double rotation (DOR) or sequentially as in double angle spinning (DAS). 21 MQMAS is a one of the commonly used high resolution 2D NMR technique for studying quadrupolar nuclei. 21c,21d The main advantage of this technique is by manipulating spin coherences (through use of pulses), the second and fourth rank Legendre Polynomials can be averaged throughout the experiment.

Hence from the aforementioned discussion about MAS, we can conclude that it helps to remove the dipole-dipole and/or the chemical shift anisotropy (CSA) interactions, as well as the first--order quadrupolar interactions. However, the spectral resolution depends on the amplitude of each of these interactions and therefore to the rotational speed used during spectral recording. The different rotors (currently used) allow to turn from 7 kHz to 67 kHz corresponding to rotor diameters ranging from 7 mm to 1.3 mm. Bruker has recently (2015) launched a smaller diameter one (0.7 mm) and can reach a maxium speed of 111 kHz.

Pulse sequences used in this thesis

Single pulse

In single pulse NMR experiment the magnetization of the sample is turned with the rotating magnetic field B 1 . It is mentioned just before that there are more magnetic moments aligned with the field than against it. This implies when all the opposing magnetic moments have cancelled each other out, the net population difference will create a bulk magnetization vector (called M o ) aligned along the direction of external magnetic field B 0 . The magnetic behaviour of the system can be represented like this: Figure 13. Evolution of the magnetization (a) before (b) during and (c) after the application of a radiofrequency pulse. In the above figure, a pulse of radio frequency radiation is applied along the x axis. The magnetic field of this radiation is given by the symbol B 1 . The application of this radio frequency pulse spreads the bulk magnetization vector (M 0 ) from its equilibrium position and results the M 0 rotates clockwise about the x axis and is reprented by M. The amount of the rotation is gouverned by pulse duration. In most of the FT--NMR experiments, the pulse duration is selected in such a way that the magnetization vector rotates by 90°.

The detector is placed along the y--axis. Therefore, in a static frame of reference the net magnetic moment will be spinning around the y'--axis at the Larmor frequency. This motion constitutes a radio--frequency signal, which can be detected. When the radiofrequency pulse ends, the nuclei relax and return to their equilibrium positions, and the signal decays. This decaying signal contains the sum of the frequencies from all the target nuclei. The resulting signals cannot be recorded directly due to high frequency. Therefore it is mixed with a lower frequency signal to produce an interferogram of low frequency. This interferogram is digitized, and is called the Free Induction Decay, (FID). Fourier transformation of the FID yields a frequency domain spectrum. 9 In practical purpose, this 90° pulse sequence is repeated to improve signal--to--noise ratio. The amplitude of the signal after being Fourier transformed (S) will depend on spin--latice relaxation time (T 1 ) and the repetitions time (TR) of the sequence. Therefore, the signal equation can be expressed as:

S = k!(1! e !TR T 1 ) (29)
where k is a proportionality constant and ρ refers density of spins in the sample. T 1 is also called as longitudinal relaxation time. It characterizes the time taken by the magnetization vector to return to the equilibrium position along the z axis. The repeating time should be long enough (TR ≈ 5T 1 ) so that all spins totally remain in equilibrium position. This is the necessary condition to obtain a quantitative spectrum. However, there is another important process that occurs also during the NMR experiment. With the passage of time after the RF pulse puts the spins in the transverse plane, the magnetic moments interact with one another and lose their phase coherence in the xy--plane. This loss of transverse magnetization is characterized by the time constant T 2 , called the spin--spin or the transverse relaxation time. More specifically, it refers the return to equilibrium of the transverse components M x and M y i.e. M x and M y again become zero. It implies a return to an equal distribution that does not require a system energy change. This process is dispersive.

Spin echo technique

In the single pulse technique the rf field should be ended before starting detection of signal. As this shut down process of the rf field is not instantaneous, magnetization can already start spreading in the xy--plane. Therefore one might partly lose signal. This signal loss problem can be resolved by using spin echo or the ! pulse is not carefully set it can result in residual incoherent, or non--refocused, transverse magnetization, which leads to phase distortions in the observed spectrum. This problem can be circumvented by running the sequence more than once with the phase of the ! ! and ! pulses varied in each successive run (known as a phase cycle). A very specific phase cycling method, known as an EXORCYCLE, can be implemented and makes the Hahn--echo far less sensitive to errors in the setting of both the ! ! and ! pulses. 10 The pulse sequenced used in Hahn--echo technique and various state of the magnetization observed during different steps of the Hahn--echo sequence is illustrated in the figure 15.

NMR of paramagnetic materials

The investigation of paramagnetic compounds through the help of NMR was ignored during the first decade of the application of this spectroscopic method to chemical problems, because of technical limitations. Nowadays, the development of improved NMR equipments allowed to change that situation in a large extent. Paramagnetic spectra undergo two main problems. The spectra of paramagnetic metal complexes are still generally not so well resolved as those of corresponding diamagnetic compounds. The very short T 1 relaxation time in paramagnetic species causes a strong signal broadening that even makes sometimes the signal invisible (signal width at half--height is proportional to T 1 ). The half--widths of the NMR signals lie between few hertz to several thousand hertz for dissolved paramagnetic species. Another problem is the shifts of the signals that extend over a range that is several orders of magnitude greater than for the analogous diamagnetic compounds. This requires the use of large spectral windows, which sometimes leads to baseline deformations. In 1957 the detection of 1 H NMR signal of nickelocene (two unpaired spins, S=1) in toluene marked the beginning of solution--state NMR spectroscopy applied to paramagnetic complexes. 24 The real growth for studying the NMR of paramagnetic species at an extensive scale was started at 1990.

Hyperfine interactions

The NMR method is extremely sensitive to the presence of unpaired electrons in the environment of the probed nuclei through hyperfine interactions (using the nomenclature inherited from EPR). The effects can be roughly divided into two groups, signal shifts due to the Fermi--contact or the Knight shift 25a interactions and spreading of the signal due to dipolar interaction.

The Hamiltonian H SI for the hyperfine interaction could be expressed as:

H SI = A c ! Î ! Ŝ + Î ! ! A dip ! Ŝ (30)
where, A c is the coupling constant of the Fermi contact term, ! A dip the dipolar tensor and the hyperfine interaction and Î and Ŝ represent the spin operators for the nucleus and the unpaired electrons, respectively.

Fermi--contact interaction

The Fermi--contact interaction arises due to the presence of a certain density of unpaired electrons on the observed nucleus. This interaction generally leads to a large displacements of the NMR signals in both positive or negative direction which depends on the sign of the A C , itself dependent electron spin density on the probed nuclei.

Therefore, the "Fermi contact shift" is proportional to the spin density at the observed nucleus: 25b

! FC T ,iso = µ 0 a 0 3 " e 2 g av 2 (S +1) 9k B T
# is (0) (31) In the above equation β e is the Bohr magneton, S is the electron spin multiplicity, k B is the Boltzman constant, T is the absolute temperature, g av is the avearge electronic 'g' factor, μ 0 is the vaccuum permeability, a 0 is the Bohr radius and ρ is (0) is the spin density in the s orbital of the probed nuclei.

The Fermi contact term also can be expressed as a function of magnetic susceptibility. In fact it is directly related to the components of magnetic susceptibility tensor by the equation given below. 25b

! FC T = ! A C 3(" N 2# )$ % xx g xx + % yy g yy + % zz g zz " # $ $ % & ' ' (32
) Now the A C term can further be expressed as:

A H = (4 / 3)g e !" N K (33) here, K is the theoritically derived proportionality constant correlating the orbital spin density contribution to the nuclear spin density. In the equation (32) χ xx , χ yy and χ zz are the principal axis components of the susceptibility tensor. g xx , g yy and g zz are the three component of electronic g tensor. For the simple case of no zero field spilitting we can write 25b 22

! ii = g ii 2 " 2 S(S +1) 3k B T
(34) It is clear from the equation ( 31) that the Fermi contact shift is inversely proportional to the temperature. However, equation ( 32) is also reflecting the same feature if we consider the fact that magnetic susceptibility of a paramagnetic species is inversely proportional to the temperature (see equation 34). Therefore paramagnetic chemical shift has a strong dependency on temperature and thus the accurate knowledge about the recording temperature is required during the analysis of the paramagnetic NMR data. The temperature dependence of paramagnetic chemical shift is highlighted in chapter 3 and partly in chapter 4 (will be described later). The unpaired electron density or spin density can extend to the ligands of a paramagnetic metal complex, in the following ways: 25a (a) Unpaired d electrons of the metal d orbitals fill unoccupied antibonding orbitals of the ligands that have σ or π character ("back--donation"). This mechanism is particularly pronounced in the observed signal shifts in the NMR spectra of titanium(III) and vanadium(III) complexes. 25c (b) Electrons in the highest occupied orbitals of the ligand or ligands are partly transferred into unoccupied or unfilled d orbitals of the metal (donation). This transfer mechanism is possible in particular for the more highly charged ions of the 3d metals. A positive spin density also remains in the ligand orbitals. 25a (c) Partly occupied d orbitals of the metal overlap with occupied orbitals of ligand atoms having a sterically favorable arrangement, but which are not bonded directly (in the conventional sense of a chemical bond) to the metal atom or ion. This "direct interaction" is probably responsible e.g. for the strong negative shifts of the signals in the 1 H--NMR spectra of substituted and unsubstituted bis(cyclopentadienyl) complexes of vanadium(II) and chromium(II). 25c In these compounds, positive spin density is transferred directly from the transition metal ion to the protons of the cyclopentadienyl rings, with the result that the signals of these protons are displaced by up to 360 ppm toward lower fields. 25a,25d

Dipolar interaction

The dipolar interaction occurs in space between the electron and the nuclear spin and it varies with 1/r 3 (where r is the distance between paramagnetic source and the nucleus under study). This interaction causes a significant enlargement of the NMR signals due to the high gyromagnetic ratio of the electronic spin. Although, the enlargement can be partially averaged through magic angle spinning. Furthermore, in the case where the g tensor describing the electron spins is anisotropic, this dipolar interaction contains a so--called "pseudo--contact" contribution that will create an additional chemical shift. Thus, the observed chemical shift corresponds to the Fermi contact and pseudo--contact shift, the latter being generally much lower than the first. This interaction could be expressed as:

! ! !"# = ! ! !! • ! ! ! !!! !!" • (! !! ! -! ! ! ) • !(!, !) • !(!, !) (35
) with β the Bohr magneton, S the electron spin multiplicity, k the Boltzman constant, T the absolute temperature, g the electronic 'g' factor, μ 0 the magnetic constant and G(r,θ) reflects the fact that the interaction depends on the dipole orientation with r being the distance between the metal and the relevant nucleus, θ is the angle between r and magnetic axis of the molecule. The f (g,D) term is the function of the g factor and the zero--field splitting D.

Knight shift

It has been suggested in 1970 that the electronic distribution in conducting solids is not exactly same with the paramagnetic or diamagnetic solids. 25a In a conducting solid the electrons are delocalised and form a conduction electron cloud. The conduction electrons occupy a Fermi distribution within the electronic states of the material in a pairwise manner. In the absence of external magnetic field the pairing implies a contribution of zero magnetic moment of the conducting electrons. However, upon applying an external field normal α, β distribution will be there. The differences in their number will lead to a net magnetization and hence a suceptibility termed as pauli susceptibility (Pauli paramagnetism). The shift in the fieldthus produced can be obtained through the estimation of field responsible for the susceptibility. This can be expressed in terms of the hyperfine field generated by the conduction electrons, 36) where, Ω is the volume per electron.

!B = µ 0 4! 8! 3 " p " #(0) 2 F B 0 (
The Knight shift (K) has the same origin as the Fermi--Contact shift i.e., the presence of a certain magnetization on the probed nuclei. But unlike the Fermi contact shift, the Knight shift is independent of the temperature. This effect was first observed in copper by W.D. Knight. It does not specify what is the mechanism that contributes to the shift or that is alone responsible for the shift, unlike the case in the terms "Fermi--contact shift" and "pseudo--contact shift". 25a

Solid--State paramagnetic NMR

Many compounds of interest in chemistry, physics and biology contain paramagnetic centers. In the periodic table more than 1/3 of the elements shows paramagnetism. From the magnetic point of view paramagnetic compounds are fundamental because interesting magnetic behaviours originate from the presence of unpaired electrons. The effects arising from paramagnetism are associated with the electronic configuration of the metal and on the molecular structure leading to variety of information about chemical properties, electronic states of the paramagnetic center and about structural restraints in the surrounding environment. 26 For providing detailed structural information on paramagnetic complexes in solids, 26 X--ray crystallography has been most useful, but only for compounds that can be successfully crystalized. Solid-state NMR is a powerful technique for charecterization of both crystalline and noncrystalline samples. 27 Therefore solid--state NMR spectroscopy is used to invesigate challeging paramagnetic systems including cathode materials for lithium--ion batteries, paramagnetic metalloproteins and porphyrins, organometallic compounds (sometimes involved in catalytic processes), inorganic complexes and prussian blue analogues, various lanthanide complexes etc. Pioneering contribution of the scientists like McConnel, Fermi, Van--Vleck, Bertini are highly important in this context. 28 Notably, there is a difficulties to differentiate the importance of solid state NMR over other techniques of science like EPR, magnetochemistry, PND, XRD, DFT, quantum mechanics etc. All of these techniques are interrelated and very important to extract complete structural, chemical and electronic behaviour of different complexes. The significant developments in the domain of high--resolution solid--state NMR spectroscopy (through the use of iltra fast MAS, combined dipolar decoupling and MAS, cross--polarization etc.) provide detail information about the spin structure of the solid matter. Therefore very small amount of spin density can be determined in atomic scale. Such spin density measurement are manifested carefully in several reported organometallic complexes, inorganic coordination complexes, organic radicals, inorganic prussian blue analogues. 28--31 High resolution solid--state NMR plays a key role in the stuctural, chemical and dynamical characterization of a wide range of biological materials modified with spin levels. 26b, 28f--g

The main interests for studying paramagnetic systems through solid--state NMR spectroscopy are: (i) the local structural characterization, (ii) probing nuclear spin dynamics; (iii) following the electronic and chemical behaviour. The development of different solid--state NMR techniques (ultrafast MAS, cross-polrization, deccoupling etc.) related to signal enhancement makes these investigations possible. In the following section selected examples of the use of NMR spectroscopy to study challenging paramagnetic systems will be briefly described.

Cathode materials for Li ion batteries

Lithium intercalation materials or sometimes called as insertion materials have been widely investigated to develop new electrode materials for compact, high--voltage rechargeable batteries. 31 The main problems still persists in the reproducible production of these materials. In addition, structural changes might occur during the charging and discharging processes. 32 It may affect both short--term and long--term behavior of the material within the battery.

Therefore accurate knowledge about the local structure in these materials is important to the understanding of their performance as electrodes. The long--range structural information of these materials is generally available from diffraction methods, but it provides no direct information regarding the local order and environment of the cations. 33 Thus the solid state NMR is an utmost technique for characterizing local structure in these materials, even in highly disordered systems. The lithium nuclei 6 Li and 7 Li are typically used as probes because it is the lithium ions that are directly involved in the electrochemical processes.

Both of the 6 Li and 7 Li are quadrupolar in nature with nuclear spin quantum number 1 and 3/2 respectively. The natural abundance of 7 Li (92.6 %) provides good sensitivity for direct NMR measurement, but also 6 Li (7.4%) can easily be observed without enrichment. Besides, the smaller gyromagnetic ratio and quadrupolar moment of ( 6Li compared to 7 Li) make it a potential candidate for high resolution MAS NMR. 33 The main problem with 7 Li (I=3/2) NMR spectra is that it is often affected by homonuclear dipolar coupling and the second--order quadrupolar interaction, both of them can give rise to broadening. 34 In contrast, 6 Li (I=1) exhibits much weaker homonuclear dipolar coupling. 33 The recording of high--resolution NMR spectra of the lithium--transition metal oxide system is further complicated by the paramagnetism of these materials. The interactions between the nuclear and unpaired electron spins in these systems cause broadening of the spectrum and sometimes undetectable NMR resonances. The extremely fast MAS spinning rate (≥ 40kHz) is one of the commonly used method to diminishing interactions, reducing the number of sidebands and simplifying the spectra. 35 Spin--echo mapping techniques have also been used in paramagnetic systems for the detection of extremely broad resonances. 36 A large number of 6 Li and some 7 Li NMR experiments have already been performed on the LiCoO2, LiNi 1-- y CoO 2 , LiMn 2 O 4 and Li 2 MnO 3 . 37 7 Li MAS NMR of Li--Mn--O and Li--Mn--V--O compounds have also been reported. 38 The interpretation of the spectra was often complicated by the large numer of spinning sidebands and poor signal--to--noise ratios; hence, accurate attribution of the chemical shift is not observed each time. In 1991, the observed shifts were often interpreted in terms of a Knight--shift mechanism. 39 Six years later (1997) by Mustarelli et al. showed that the major source of the large shifts in these compounds is the Fermi--contact shift mechanism. 40 More recently (1998) wide--line NMR studies have also interpreted 7 Li shifts in terms of a hyperfine or contact interaction. 41 Sometimes, variable--temperature static 7 Li NMR studies is performed on the Li--Mn oxide systems to investigate possible magnetic ordering of the Mn 3+ /Mn 4+ ions below the cubic--totetragonal phase transition and the Neel temperature. 42 Moreover, in most of the cases solid state 6/7 Li NMR spectroscopy is basically used to study the local orderof the Li cations in the oxide systems and to investigate the structural and electronic properties that are responible for the large shifts observed in these systems. Some of the selected examples are highlighted below.

Li--Mn spinels

The spinels have appeard to be a cheap alternative cathode material that can be charged and discharged sufficiently rapidly to meet the power requirements for use in electric vehicles. The spinel materials, LiMn 2-- O 4 , have been the most extensively investigated among all the lithium manganates. 43 These materials can be cycled at ca. 4 V (vs Li) from LiMn 2 O 4 to MnO 2 . NMR spectroscopy has been performed on these systems to probe structural changes before and after electrochemical cycling. The 6 Li or 7 Li MAS NMR spectra of the stoichiometric spinel LiMn 2 O 4 exhibit a large hyperfine shift of more than 500 ppm (with respect to the diamagnetic reference compound of Li). 3,40,44 LiMn 2 O 4 is basically a mixed--valent compound consisting of both Mn 3+ and Mn 4+ ions; it is also a hopping semiconductor. In this system the electronic hopping occurs between the e g orbitals of the manganese ions. As the hopping time scale is fast compared to the NMR time scale (ca. 10 --5 s), the lithium spins "experience" an average manganese oxidation state of +3.5, and thus there is only one magnetically inequivalent lithium site. The NMR spectra are clearly sensitive to this hopping mechanism, and NMR spectroscopy is thus very useful to follow the partial charge-ordering process that occurs just below room temperature in the stoichiometric material. 3b Figure 17. 6 Li MAS NMR (9 kHz) spectra of the spinel LiMn 2 O 4 (structure shown on the left) as a function of synthesis temperature (adapted from ref 33a). The spectra were recored at a magnetic field of 4.3 T using quadrupolar echoes (90° -τ -90° -τ -acq).

x M' x
The manganese oxidation state of +3.5 is obtaineded in the stoichiometric material LiMn 2 O 4 synthesized at 850 °C. 45 The above figure represents that the 6 Li MAS NMR spectra of this compound vary significantly with reaction temperature (Figure 17). Although the structure of the stoichiometric spinel contains only one crystallographic site for lithium, a large number of different local environments are observed for the compounds synthesized at lower temperatures. The spectrum of the material synthesized at a temperature of 600 °C resembles to the spectrum of samples with excess lithium (Li 1.05 Mn 1.95 O 4 ), which typically show improved cycling behavior. 46 The spinning sideband pattern observed at 40 kHz resembles to that found for the octahedral site in the spinel Li 0.5 Zn 0.5 [Mn 0.5 Li 0.5 ]O 4 , confirming that this resonance is due to Li substitution (for Mn) on the octahedral site of the spinel structure. Therefore, the spectra provided clear evidence that the samples prepared at lower temperature contain excess Li, despite their stoichiometry. 3 The additional resonances in the spectra of the nominally stoichiometric material LiMn 2 O 4 were assigned to tetrahedral lithium environments nearby Mn 4+ ions. These Mn 4+ ions arise from the substitution of Li + in the octahedral site: For every Li + substituted on this site, five Mn 3.5+ ions must be oxidized to Mn 4+ .

Cr 3+ --and Ni 2+ --substituted layered Lithium Manganates

Lithium NMR spectroscopy has also been employed to investigate the local structure and cycling behaviour of a number of alternative layered manganese cathode materials. One of these kind of materials consists of several cathodes, in particular, containing ) where the charging and discharging cycles involve multiple electron transfers per transition--metal ion. 47 Chromium was shown to cycle reversibly between Cr 3+ and Cr 6+ in the mixed Cr--Mn oxide Li[Li 0.2 Mn 0.4 Cr 0.4 ]O 2 . 6 Li NMR studies proved that the material contains two types of local surroundings (a) Li 2 MnO 3 --like and (b) Mn 4+ -doped LiCrO 2 . The Li ions in the Mn 4+ --doped LiCrO 2 regions were only removed on cycling and appeared to be responsible for the capacity, whereas the Li 2 MnO 3 parts were inactive. The toxicity of Cr 6+ restricts the use of this cathode material. 47,48 The widely studied layered material LiNiO 2 functions via the reversible cycling of nickel between oxidation states of 3 and 4. But it was also proved that nickel could also be cycled between Ni 2+ and Ni 4+ at higher voltages in both layered materials and spinels. The presence of Ni 2+ in the spinels LiNi x Mn 2--x O 4 was clearly demonstrated by NMR. 49 This kind of Ni 2+ /Ni 4+ feature was also obtained in layered LiNi 0.5 Mn 0.5 O 2 and its Li-excess versions at a much lower voltage. 49

LiCoO 2 related cathode materials:

NMR spectroscopy has also been applied to follow the cycling of Li x CoO 2 . The results indicate that Co 4+ ions are formed during the initial stages of deintercalation. 50 A broad resonance was observed for x = 0.94 (where x is the lithium content in LixCoO 2 ) and the shift was ascribed to a Knight shift, arising from the interaction between the Li nuclei and the conduction electrons in the t 2g * band formed from the overlapping t 2g cobalt orbitals.

Paramagnetic metalloproteins and porphyrins

Solid--state NMR has appeared to be a reliable tool for investigating structural detail of biological systems that cannot be induced to form sufficiently diffracting crystals and is also not feasible through solution NMR due to size limitations. Membrane proteins, fibril, precipitated proteins etc. belong to this category. 52 The size of the investigated proteins has markedly increased during last several years, even if it still remains limited to systems smaller than 100 amino acids, at least in the structures reported till date. 53 Although routine generation of such structures continues to show considerable challenges. This is mainly due to a number of competing effects, which make the detection of signals close to a paramagnetic centre difficult. [START_REF] Haeberlen | High Resolution NMR in Solids[END_REF] A major step forward was represented by the complete attribution of the 13 C, 15 N resonances of human Cu 2+ , Zn--SOD in 2007, where nuclei as closer as 5 Å to the Cu 2+ ion were observed . 55 This work emphasized the fact that solid--state NMR is applicable to the detection of low--γ nuclei ( 13 C), which are less susceptible to paramagnetic broadening than 1 H. Also, efficient dipolar--based polarization transfers can be used for cross--polarization and for homonuclear correlation experiments. 54a Another important breakthrough came in the study of proteins by solid--state ultrafast magic angle spinning NMR (60 kHz) and the ability to use 1 H detection. These techniques have been used on highly paramagnetic proteins, significantly diminishing the "blind sphere" of resonances broadened beyond detection belonging to spins close to the metal center, and providing a powerful source of structural constraints for protein structure determination. 54a An example where ultrafast MAS was applied to a paramagnetic protein is shown in the figure given below, demonstrating the example of CoMMP--12 (Fig 19). The use of 22 kHz MAS NMR technique suffered from the difficulty in observing closer nuclear spins which was attributed due to increase of paramagnetic shift anisotropy effects in close proximity to the paramagnetic center. 56 The 13 C detection was limited to the observation of nuclei located further than 9 Å from the Co 2+ ion. Ultrafast MAS at 60 kHz has enabled the detection of resonances in closer proximity to the Co 2+ ion than before, up to 5.6 Å. As already observed for proteins in solution, PCS (pseudo contact shift) of 13 C nuclei can also be used as structural restraints in the solid state. PCS are directly related to the nuclear position with respect to the metal through a simple and quantitative relationship, and a very good agreement between the experimental values and those predicted on the basis of a molecular structure was obtained. 54c, 57 Although the determination of PCS will not be sufficient enough to completely define a protein structure, but yet it can be easily determined and could be helpful, together with the other diamagnetic restraints, in the structure calculation method. 58 More importantly, it is found that when PCS are used together with even a few unambiguous restraints, a low resolution but comparatively accurate structure can be already determined.

Prussian Blue and cyanometallates

In the field of molecular chemistry the paramagnetic hexacyanometallates always occupy a superior position because of their frequent presence in many magnetically valuable systems. The major prototypes are (a) bulk magnetic Prussian blue analogues (b) multiple spin clusters (c) ferromagnetic coordination polymers (d) nanomagnets and low dimensional magnets etc. 59--61 In addition these hexacyanometallates have been used recently to perpare ''Hybrid materials'' exhibiting chirality or conducting properties. 62 In all these compounds the main magnetic behaviour is based on the fact that the unpaired electron(s) is transmited from [M(CN) 6 ] 3--to the adjacent metal centre. It is well established that the magnetic interactions in the materials is mediated through the cyanide ligands via superexchange mechanism. 29,30 It should be noted that the ambidentate nature of the cyanide ligands make these hexacyanometallates potentially active member for taking part in the various self assembly processes realted to design of multiple spin system. It is well known from the reported work on these compounds that to have a clear idea about the magnetic interactions, information regarding the distribution of unpaired electrons in the starting compounds is necessary. 29,30 Particulary, the amount of unparied electron on C and N atoms is highly important because cyanide ligand is the carrier of magnetic informations in all of these aforementioned compounds. Although there are not too much choices for experimentally studying spin density distributions in building blocks. The major techniques are (a) X--ray magnetic circular dichroism (XMCD) (b) polarized neutron diffraction (PND) and (c) solid--state NMR. 63,64 XMCD is a suitable technique for maping the spin structure of the metal ions in molecule--based magnetic materials containing a macroscopic magnetic moment, but not for probing spin transfer mechanism in cyanometallates or other molecular building block. 63 PND is suitable for spin density studies. But, due its requirement of large size single crystals (which is not always possible) it is often limited to many cases. Indeed a PND study of Cs 2 K[Fe(CN) 6 ] was performed which gave good results for the spin density at iron, but it was less precise for the C and N atoms, claiming --5% and +6% of an unpaired electron, respectively. 29 Therefore for studying these paramagnetic hexacyanometallates the solid--state NMR technique has been appeared as an important alternative. Moreover, it provides the advantage of measuring powder samples and it is also a relatively inexpensive experimental setup compared to XMCD and PND. From the seminal work of Van Vleck and McGarvey it was proved that the chemical shift of a paramagnetic compound provide access to spin densities, because they are related to the Fermi contact coupling between the nucleus under study and the unpaired electrons. 13 C, 15 N NMR spectroscopy how much spin density is located on the 2s orbital of C and N atom of the cyanide ligand of hexacyanometallate compounds. 29 The 13 C and 15 N MAS NMR spectra of microcrystalline samples of isotopically enriched [Fe(CN) 6 ] 3-are given in figure 20.

It was observed that the spin density at the C atoms is negative, while it is positive at the N atoms. They were also able to draw the conclusion that the spin delocalization proceeds through π--type orbitals with They emphasized the fact that the spin distribution in different directions of the crystal lattice could also be distinguished as well from this kind of study. 29 The step forward to the afforementioned study was investigating magnetic materials derived from these cyanometallates building block. One of the most important magnetic systems based on the cyanometallate building block is Prussian blue. 59 Prussian blue and Prussian blue anlogues (PBAs) are still a hot topic of reasearch due to its diverse application in various fields including gas storage, biosensors, heterogeneous catalysis, waste recovery, molecular sieves and in magnetism. [START_REF] Banci | [END_REF] The flexibility of the PBAs has provided many interesting magnetic materials which has enriched the field of molecular magnetism.

PBAs are basically insoluble porus inorganic polymers whose simple composition and cubic structures coveres a large family of (often nonsoichiometric) compounds. 30a The structure of this PBAs generaly obtaineded through powder X--ray diffraction experiments. The powder XRD technique results a long--range averaged view of the cubic framework of the Prussian blue, but unables to provide a precise description of the local structural parameters such as the environment of the metal ions. There are some other techniques like EXAFS (extended X--ray absorption fine structure), 67,68 Mössbauer spectroscopy could provide more local information. But the EXAFS techniques need elaborate instrumental setups and Mössbauer is generally limited to iron. 69 Therefore, solid--state NMR spectroscopy is recalled for the structural elucidation. In addition to that the potential use of NMR spectroscopy to extract magnetic information makes the solid--state NMR studies of PBAs more meaningful. In 2009, Lescöuezec et al. deeply investigated some cadmium(II) containing PBAs through 113 Cd solid--state NMR spectroscopy. 30a They were able to provide a complete view of the spin transfer mechanism and the local structer of PBAs. They showed that the distribution of the vacancies in the lattice of PBAs is not random and probably quantified by local charge balance. In addition to that, the cadmium chemical shift were proven as an useful indirect magnetic probe to detect the small amount of spin density on Cd(II). Furthermore, in their study they pointed out an important feature. They observed that the presence of iron(III) centres in the vicinity of the 113 Cd induced a magnifying glass effect on the spectra, which results an enhancement of spectral resolution compared to that observed in the diamagnetic samples. The resolution of the reported spectra were good enough to reveal the presence of isomeric coordination sites in PBAs structure.

Organometallic compounds

The structural aspect of organometallic complexes in solid--state is highly important due to its versatile application in supramolecular chemistry and solid state reactions. 70 X--ray diffraction analysis is undoubtedly the most useful techinque for obtaining structural information of organometallic species, but exclusively for those compounds which can be sucessfully crystalized. Solid--state NMR is thus a useful technique for studying crystalline as well as noncrystalline powdered samples. However, only few high--resolution solid-state NMR studies of paramagnetic organometallic solids have been reported in literature whereas solution--state NMR studies on paramagnetic organometallic complexes have been carried out since long time. 71,[START_REF]Fitting of the Absorbance vs temperature plot of SCO complexes (obtained from solution UV--vis study) (a) [Fe(Me--bik)[END_REF] The major differences in solid--state from solution NMR of a paramagnetic organometallic complex arise mainly due to large bulk susceptibility anisotropy, electron spin diffusion, and unaveraged nuclear nuclear and nuclear--electron dipolar coupling. 73 It causes low resolution and difficulties in signal detection. Despite all these facts, it has been proved that a combination of MAS and high--power proton decoupling, together with variable--temperature operation produce nice spectrum and able to provide important information regarding structure and spin dynamics of the compound. Using this two combination Some recent studies demonstrated that 13 C, 1 H, and 2 D MAS spectra of 2 D--labeled paramagnetic complexes exhibit well--resolved lines without the requirement of 1 H decoupling. 71c,72 Although this method could not be easily employed because a large number of various selectively 2 D--and/ or 13 74 Before his report 13 C paramagnetic solid--state NMR spectra of organometallic complexes often suffered from line broadening caused by insufficient 1 H RF decoupling due to large paramagnetic shifts. In order to overcome this problem they used MAS spin rate (ν R ≥ 20 kHz) and broadband ramped (or adiabatic) CP sequence. The ramped CP sequence helps to increase the sensitivity through using a large 1 H polarization and a short 1 H T 1 in paramagnetic complexes. It is well known since long that the very fast MAS experiment removes a large part of 1 H-- 13 C dipolar couplings as well as 1 H--1 H dipolar couplings. 74 The use of a fast MAS (24 kHz) probe equipped with a small RF coil allowed them to apply high--power 1 H and 13 C RF fields covering large bandwidths sufficient for many paramagnetic complexes. The 13 C MAS spectra of Cu II (dl--alanine)

Paramagnetic NMR in solution

The NMR spectroscopic studies of paramagnetic complexes in solution provide valuable information on the molecular structure (chemical shift) and dynamics (structure--property relationships). Unlike in solid--state due to rapid molecular tumbling most of the anisotropic interactions are vanished in solution. Therefore only the hyperfine coupling between unpaired electrons and resonating nuclei results in characteristic shifts and signal broadening. The shape and position of the shift acts as sources of information on the electronic and spatial structures of ligands in paramagnetic complexes. The main obstacle for obtaining a nice paramagnetic NMR spectrum in solution is arise mainly due to the long electronic relaxation time of the paramagnetic metal ions and broadenings arising from metal--centred dipolar relaxation.

Table 2. Electron--relaxation times of some metal ions and nuclear line broadenings induced on a proton at 5 Å from the metal at 2.35 T. Adapted from ref (75). Metal ions (τ e ) in sec Δν in Hz Ti 3+ 10 --9 --10 --10 1000--100

V 2+
5 x 10 --10 2500 v 3+ 5 x 10 --12 5 vo 2+ 10 --8 --10 --9 10000--1000

Cr 3+ 5 x 10 --10 2500

Cr 2+ 10 --11 80

Mn 3+ 10 --10 --10 --11 800--80

Mn 2+ 10 --9 --10 --10 12000--1200

Fe 3+ (HS) 10 --10 --10 --11 1200--120

Fe 3+ (LS) 10 --11 --10 --12 10--1

Fe 2+ (HS) 10 --11 --10 --12 80--10

Co 2+( HS) 10 --11 --10 --12 50--5 co 2+ (LS) 10 --9 --10 --10 1000--100

Ni 2+ 10 --10 --10 --12 300--3 cu 2+

(1--3) x 10 --9 3000--1000

Ru 3+ 10 --11 --10 --12 10--1

Re 3+ 10 --11 30

Gd 3+ 10 --8 --10 --9 400000--60000

Dy 3+ 8 x 10 --13 100

Ho 3+ 8 x 10 --13 100

Tb 3+ 8 x 10 --13 100

Tm 3+ 8 x 10 --13 70

The table 2 is representative of electron--relaxation times of some paramagnetic metal ions and their effective broadness (observed on 1 H NMR spectrum). Thus the metal ions which consist favourable electron--relaxation time are high--spin tetrahedral nickel(II), penta--or hexacoordinate high--spin cobalt(II), low--spin iron(III), the lanthanides(III) except gadolinium(III), and high--spin iron(II). Hexacoordinate nickel(II), tetrahedral cobalt(II), and high spin iron(III) are borderline cases. However, a number of solution NMR studies relaled to those aforementioned paramagnetic metal ions are reported in the literature. In this context we would like to mention that when two or more metal ions are magnetically coupled, new electron energy levels are available which leads to a shortening of the electron relaxation times. In other words, polymetallic systems in which magnetic coupling occurs are generally suitable for NMR investigation. 75 The challenging paramagnetic sytems where paramagnetic solution NMR plays crucial roles include proteins, organometalic compounds, lanthanide compounds, supramolecular assemblies etc. Some of the selected example associated to such sytems will be highlighted here.

Iron--sulfur proteins

The Iron--sulfur proteins are one of the first non--heme proteins to be studied by NMR. They contain cysteine (Cys) residues as iron ligands. 76 These proteins could further be described as a non--heme iron proteins containing polymetallic centers in which the iron ions contains tetrahedral surroundings, generally with bridging sulfide ions and cysteinyl residues (HO 2 CCH(NH 2 )CH 2 SH) as ligands. The paramagnetic chemical shift of β--CH 2 and the α--CH protons of these Cys group arise due to hyperfine interaction with Fe(II)/Fe(III). 77 Iron-sulfur proteins transfer electrons over a wide range of reduction potentials, and their iron content can range from 1 to more than 12 Fe atoms per protein molecule. Four kinds of iron-sulfur centers, containing one, two, three, and four iron atoms, respectively, are known in electron--transfer proteins (Fig 23).

Although they differ in the number of sulfur atoms provided by cysteine thiolates versus sulfide, in all cases the iron is coordinated to four sulfur ligands in a roughly tetrahedral environment. The iron--sulphur proteins with one and two iron atoms are selected for the further discussion.

Rubredoxin (with one Fe centre)

Rubredoxin contains a mononuclear iron center tetrahedrically coordinated to four cysteines. It could be obtained in both oxidized (Fe III ) and reduced (Fe II ) form. The long electronic relaxation time (≈10 --9 s) of Fe(III) in oxidized rubredoxin results a broadening of the β--CH 2 cysteine proton signal that they remain almodt undetectable (very low intensity) in solution state 1 H NMR spectrum. 78,79 The 1 H NMR signal assignment of the reduced form of rubredoxin is given in figure 24 In the case of oxidized form of the Fe 2 S 2 Ferredoxins, the two iron(III) ions exhibits a strong antiferromagnetic coupling. 81 Although, the magnetic coupling between two iron centres is not so effective on reducing their electronic relaxation times. 82 Therefore, broad resonance (ca. 2000 Hz at 4.69 T) was observed in the 1 H NMR spectrum near 34--37 ppm (see figure 25). These signals corresponded to eight protons of Fe 2 S 2 Ferredoxins and they displayed anti--Curie dependence. [START_REF] Abragam | The Principles of Nuclear Magnetism[END_REF] Wheareas, a set of sharp signals around 45--15--ppm were obtained into the reduced form of Fe 2 S 2 Ferredoxins (Figure 25). dependence of these signals. [START_REF] Dunham | [END_REF] Dunham also predicted a Curie behavior and considerably larger shifts for the proton signals of oxidised Fe 2 S 2 Ferredoxins. The latter were not detected until 1984, when Bertini et al. reported an experiment using a larger spectral window, which allowed the observation of a group of broad and very far--shifted signals (over 100 ppm) corresponding to four protons with a Curie temperature dependence. 86 In this way, the initial predictions of Dunham et al. were confirmed, providing a useful method for identifying protons sensing iron(II) and those sensing iron(III).

Study of olefin polymerization catalyst

Chromium based catalysts play a significant role in coordination polymerization of α olefins. 87 The chromium--based heterogeneous systems have appeared to be one of the important alternative of well-established Ziegler--Natta catalyst. 88 The study of homogeneous diamagnetic Ziegler--Natta catalysts by NMR is widespread and demonstrates valuable information about the nature of the active species. In the case of chromium(III)--based catalytic systems the paramagnetic nature of the metal ion leads to large shift and severe broadening in the observed 1 H NMR signals. 87 As a result the signal assignment often suffers from lack of accuracy. It has been observed in many cases that the catalyst is activated with a cocatalyst to avoid the overlap of the NMR--signals of the catalyst and the cocatalyst. These kinds of experiments are normally performed with non--standard activating agents and/or isotopically marked systems. Hence if methylaluminoxane (MAO) is used as activating agent, at least 50 equivalents of Al per catalyst molecule are required. 87 Thus, the normal window for 1 H--NMR is dominated by the excess of cocatalyst. In this situation, paramagnetic systems could be advantageous for NMR studies. As the paramagnetic signals are often much more shifted, the overlap with the diamagnetic signals will not happen. Therefore, valuable information regarding the metal environment of trialkylaluminium and MAO--activated catalysts could be obatined. During last several years, amino--functionalized Cp--based chromium(III) complexes are reported in the literature. 88--90 The reaction of this compounds with MAO gives highly active catalysts for olefin polymerization, which are very stable at high temperatures.

In order to increase the stability of Cr(III) half sandwich compounds, especially quinolyl or N,N-dimethylanilinyl functionalised cyclopentadienyl ligands were often introduced. These ligands consist an ideal, rigid topology for the formation of half sandwich compounds. Now, it is well known that organolithium and Grignard reagents lead to alkylation at the ligand, but MAO only reacts at the metal center. As the formation of the metal--alkyl or --hydride is required for the activation of the catalysts, the synthesis of chloro--alkyl derivative of quinolyl functionalised cyclopentadienyl ligand is necessary.

The synthetic scheme of this kind of reaction is given below (Figure 26). The treatment of [CrCl 2 Me(THF) 3 ] with potassium salt of quinolyl functionalised cyclopentadienyl ligand (9K) leads to chloro and chloro-alkyl containing chromium(III) half sandwich complexes. 87,[START_REF] Enders | Co--Polymers of Ethylene with C3--C9 R--Olefins[END_REF]91 The chloro--alkyl derivative ( 8) can be used as precatalysts for olefin polymerization reaction. The 1 H NMR study of these complexes was reported by Enders et al. 87 The important outcome of their NMR study is highlighted below.

The 1 H--NMR spectrum of 1 showed six resonances outside the ''diamagnetic region''. A total of eight proton signals were expected for compound 1. The resonances appeared in the 0--10 ppm region were difficult to assign because they may be hidden by residual solvent signals or diamagnetic impurities. 87 A very broad and weak signal around --80 ppm was also observed for 1 (Fig 27). The signals at +28 and --41 ppm were ascribed to the methyl groups at the Cp--ring (attribution was confirmed from the integartion value). The paramagnetic shifts in 8 are larger as compared to the shifts in 1. The Fermi contact shift is notably stronger at H 4 , which is in a para position relative to the metal. A new signal appears at --4 ppm, which belongs to H 7 . In 1, this resonance was hidden in the diamagnetic region. 87 The invisibility of the the chromium bonded CH 3 signal was ascribed due to proximity to the paramagnetic center. Therefore they were strongly shifted and extremely broadened. The assignment of the Cp--methyl groups was more difficult. On going from 1 to 8, the mirror plane disappears, and consequently, the former two signals split into four. Three of the latter signals were obtained in the expected --50 to +50 ppm range, whereas the fourth signal appeared at a very low field (asterisks in Figure 22). Although Enders et al. were able to identify the four methyl groups of 8, they were unable to provide any definitive conclusion about specificity of the signals. The final assignments of 1H NMR signals for these compounds were done with the help of quantum mechanical calculations.

Conclusion

In this introductory chapter we have discussed about the basic principles of NMR and its application in solid and solution state. As previously mentioned the presence of paramagnetic species in a system leads to strong shifts and broadening of NMR signals. Sometimes, this leads to the misconcept that NMR spectroscopy is less useful for characterizing paramagnetic compounds. But interestingly, it has been observed in many cases that this paramagnetic nature of the compound allows to extract many fundamental aspects of the molecules including spin density delocalization, various isotropic and anisotropic interactions, structural parameters, etc. In this PhD work, we will present the NMR study of some interesting pramagnetic systems in solid and solution state.

For the solid--sate NMR study, we have used MAS technique to minimize the various solid--state interactions.

Hahn--echo pulse sequence was used to minimize baseline deformations. 13 C, 15 N and 113 Cd nuclei are probed in solid state. Lately, we extend our research to the quadrupolar 59 Co nuclei also. The 59 Co solid-state NMR studies of some of the selected compounds allow to estimate different quadrupolar parameters and CSA parameters associated to them. The observed paramagnetic chemical shift in our 13 C, 15 N and 113 Cd NMR spectra mainly arise due to the hyperfine (super) interaction between the electronic spin of paramagnetic metal ion and the nuclear spin of the probed nuclei. It has been observed that in all our paramagnetic cases ( 13 C, 15 N and 113 Cd) the Fermi contact term was the dominating contributor to the overall hyperfine interactions. Besides, taking profit of the X--ray diffraction analysis and EPR analysis, we were able to estimate dipolar contribution in those systems. Chemical shift anisotropy paramerers have also been estimated. The result obtained from NMR analysis was compared to those obtained from various other solid state techniques including XRD, IR, EPR, PND (in case of [Fe(Tp)CN 3 ] --), and SQUID.

The paramagnetic solution NMR study of some of the Fe(II) complexes have been carried out. These compounds exhibit variation in magnetic behaviour with temperature. Therefore, variable temerature 1 H NMR and Evans' NMR of these complexes have been carried out to interprete their magnetic behaviour. In chapter 4, we will also mention about the solution 59 Co NMR of some Co(III) complexes.

Therefore, by taking the profit of advancement in paramagnetic NMR techniques we investigate various interesting paramagnetic systems including mononuclear magnetic building blocks, spin crossover compounds, Prussian Blue Analogues, low dmensional chains, high nuclarirty cluster based compounds.

Chapter 2

Probing spin density in partially blocked M(III) cyanometallates (M = Fe, Ru).

Introduction

Magnetism has been a reserved subject for physicists during a long period, but since the pioneering work of Bleaney and Bowers, the development of simple theoretical models has allowed the molecular chemists to enter progressively in this field. 1 The molecular approach of magnetism is often based on chemical self-assembly process. In the self--assembly approach, the geometric and electronic properties of tailored building block complexes of "blocked topology" allows to orientate their self--assembly toward polymetallic materials of controlled architecture and physical properties. Because of the versatility of the molecular chemistry, the properties of the building blocks can be tuned almost at will. So if chemists are able to understand the links between the properties of the final desired compounds and those of their building--blocks, a vast number of new and original molecular-based materials could become available. Since the late seventies, the field of molecular magnetism has significantly grown. Originally, the study of simple molecular compounds allowed to test the model of the superexchange interaction based on orbital symmetry consideration (the Kahn's model). 2 On this basis, the field has quickly evolved toward the design of more sophisticated molecular magnetic materials exhibiting original properties (switchable magnetic materials, 3 single molecule magnets, 4 multifunctional materials, 5 etc.). Nowadays numerous research efforts deal with the insertion of these original magnetic molecules into hybrid systems aiming at developping original functionalities that could be used in future technological devices (molecular memories, molecular switches, etc.).

For designing and optimizing materials that are assembled from paramagnetic building blocks, key information is the distribution of unpaired electrons in the starting compounds. Of particular interest is the fraction of unpaired electrons (the spin density) at the bridging ligand atoms, which are aimed at linking neighbouring spin centres in the resulting magnetic material. 3 Indeed, spin density plays an important role not only in mononuclear building blocks but also in polynuclear paramagnetic materials as it governs the magnetic exchange. The role of the coligands (which allow to tune the electronic properties of the building blocks) on the overall spin distribution into the complex is for example of high interest. However, there are not many choices for experimentally studying spin density distributions in molecular materials. X--ray magnetic circular dichroism is a powerful method for determining the spin and orbital magnetic moments of the metal ions in molecule-based magnetic materials. 6 However, the spin moment transfered to ligand is often small and not always accessible by this technique. When large single crystals are available, polarized neutron diffraction (PND) is well suited for determining the atomic magnetic moments that are induced by magnetic fields. Spin density estimation can be obtained from these data but once again, the data are not always accurate enough to determine small amount of spin density that are delocalized onto the ligands. An alternative experimental method is NMR spectroscopy because the signal shifts are an indirect measure of the spin density at the nucleus under investigation. The NMR technique is readily avaible and require relatively inexpensive experimental setup in comparison to PND and XMCD techniques. Such studies have been carried out on metallocene complexes by F.H. Köhler, 7 and these works have been recently extended to the study of cyanide compounds (hexacyanometallates, Prussian blue analogues, etc). 8,9 Cyanide--based materials are good candidates for such NMR studies because: (i) the cyanide bridging ligand is known to efficently promote electronic (and magnetic exchange) interaction between the metal ions it links; (ii) the large number of cyanide building block complexes with tunable electronic and geometric properties should allow to establish useful magneto--structural correlation; (iii) the cyanide ligand is available as 13 C and 15 N enriched (although expensive, ca. 500 €/g), facilitating the NMR investigation of paramagnetic species.

In the continuation of the previous work on hexacyanometallates, we have selected in this chapter some M(III) cyanide building blocks that exhibit various blocking ligands and various topology and electronic properties.

These building blocks represent interesting starting materials which could be used for the design of functional molecular materials such as Single Chain Magnets (SCM), Single Molecule Magnet (SMM) and photomagnetic molecules. The molecular structures of these compounds were obtained by X--ray diffraction on single crystals. One of our main interest is to investigate the role of the blocking ligands (eg. their donor character), the influence of the symmetry, the role of the metal ions, etc. on the electronic and magnetic properties of the cyanide complexes. The electronic and magnetic properties of these compounds were probed by EPR spectroscopy and SQUID magnetometry. The spin density extension from metal center to the cyanide ligands was probed by NMR and compared when possible to DFT and/or PND studies.

Synthesis, structural and spectroscopic characterization of M(III) building blocks 2.1 Synthesis and Elemental Analyis

The ligands including KTp, K Me2 Tp, pcq and H 2 salen are prepared following literature procedures. 5) are literature--known compounds and they are synthesized without any significant change as compared to the literature protocols. 12,13,14 PPh The synthesis of (4) --was already reported as ammonium salt by Leung et al. 15,16 In the present case some modification have been applied in order to get a better yield (69% in our present work 48% in the published one). 2 mmol of solid KCN are added to a 30 ml methanolic solution containing 0.6 mmol of [Ru(salen)(PPh 3 )Cl]. 16 The brown mixture is stirred and refluxed for 4 hour. The resulting green solution is then cooled down to room temperature and evaporated to dryness. 20 mL of distilled water are added to the green residue and the solution is stirred and filtered. Then 0. 5) are literature--known compounds. 10,11,12 Therefore, no detailed description about their crystal structures are given here. Only selected angles and distances will be mentioned together with atom numbering of the structure. The detail description of the crystal stucture of this compound is given in chapter 4. Table 5. Selected bond distances (Å) and angles (deg) for 5 --anion.

Ru(1)--O(1)

2.0146( 12)

Ru(1)--O(2)
2.0132( 12)

Ru(1)--O(3)
2.0056( 12)

Ru(1)--O(4)
2.0101( 12)

Ru(1)--C(1)
2.0769( 18)

Ru(1)--C(2)
2.0701( 18)

N(1)--C(1)
1.146( 2)

N(2)--C(2)
1.152(2)

Ru(1)--C(1)--N (1) 178. 30(15) Ru(1)--C(2)--N (2) 178.86( 16)

O 88.96(6)

Infrared spectroscopy

Fourier Transform Infrared spectroscopy (FT--IR) is a useful analytical tool in cyanide chemistry, which justifies its widespread use as a characterization method in this work. The C ! N functional groups exhibit characteristic stretching vibrations in the 2200--2000 cm --1 region of the IR spectra which carries important structural and electronic information: (i) its bonding mode (terminal or bridging); (ii) the oxidation state of the C--coordinated metal; (iii) if the cyanide is involved in weak interactions such as H--bond. Hence, it is literature-known that ferricyanides always absorb ca. above 2110 cm --1 , that is at higher frequency than ferrocyanides (< 2130 cm --1 ) and free cyanide (~ 2080 cm --1 ), but far below organic nitriles (2260 -2222 cm --1 ). Bridging coordination mode usually shifts absorption bands to higher frequencies, while weak interactions have the same --but much smaller--effect. Solid--state FT--IR spectra were recorded for the M(III) cyanido complexes 1--5 at room temperature. In order to make the comparison easier, selected absorption frequencies are reported in the table below. When the compound is available as tetraphenylphosphonium salt, it contains the following characteristic frequencies: 525, 721, 995, 1108, 1438 and 1483 cm --1 . The aromatic C-H of the phenyl ring also absorbs in between 3000 -3100 cm --1 , however, these absorptions are very weak and not visible when the sample is wet (freshly filtered). Table 6. Selected FT--IR (ATR) frequencies of cyanido M(III) complexes (4 cm --1 resolution) XRD data indicate that the M(III)-C cyanide bond lengths are in average slightly longer than those observed in their M(II) analogues, which is exactly the opposite of that usually obserbed in coordination compounds. This is due to a greater π back--bonding character in the M(II) cyanide bonds than in their M(III) analogues. Enhanced electron density in the CN antibonding π* orbitals weakens the CN triple bonds, which reduces the stiffness of the associated harmonic oscillator describing the stretching vibration mode. This explains that in all the presented M(III) species, the cyanide stretches absorb at about 2120 cm --1 , while all M(II) species exhibit absorption at lower frequency than 2060 cm --1 , thus complying with the empirical rule mentioned above. Borohydride frequencies range from 2500 to 2540 cm --1 , without a clear trend regarding the iron/cobalt oxidation state or the donor properties of the scorpionate ligand. However, for a given scorpionate ligand, the B-H moiety of the Fe(III)/Co(III) species always absorbs at higher frequency than in its Fe(II)/Co(II) analogue. Finally, all complexes exhibit very weak but characteristic CH stretching vibrations between 3000 and 3200 cm -- 1 : the pyrazolyl C-H absorbing between 3110 and 3165 cm --1 .

The cyanide vibrations observed with 13 C and 15 N enriched cyanides compounds are all lower because 13 C and 15 N atoms are heavier than the natural abundant 12 C and 14 N isotopes (Table 7).

Table 7. Isotope effect of enriched cyanides on the cyanide stretching frequencies (expressed in cm --1 ) on some cyanido M(III) complexes (4 cm --1 resolution) in FT--IR spectroscopy.

Compound

! { = (g x + g y ) 2 g II = g z (1)
and ! !" can be calculated by considering, g av = (g x + g y + g z ) 3

(2)

In the case of PPh The spectra exhibits an axial pattern. Hyperfine interaction is also observed in the g z component but its origin is unclear (the same feature is observed in both 14 N and 15 N--enriched sample). Simulation of the spectrum considering this interactions gives best fit with the experimental results. Although the spectrum is not axial, for the needs of simplification for estimating the dipolar contribution to the chemical shift we will use ! ! = 2.379, ! ∥ = 1.693 and g av = 2.150.

Solid State NMR spectroscopy

NMR spectroscopy is one of the most widely used techniques for characterization of the materials both in solid state and in solution. In earlier days, NMR spectroscopy was exclusively used on diamagnetic compounds, whereas EPR spectroscopy was used for paramagnetic compounds. Actually these techniques are complementary as the electronic relaxation is inversely proportional to the nuclear one, so that it is difficult to get good spectra through both techniques on the same species. Although, the present study with these low-spin d 5 complexes seems to be a favourable case where both EPR and NMR spectra can be obtained which is not often the case. It should be noted that the nuclear relaxation is favoured by solid state measurements (in solids molecules are close to each other resulting in a more dipolar electronic interaction that increases electronic relaxation time and conversely descreases nuclear relaxation time). In case of EPR, cooling down the sample at low temperature leads to a slower electronic relaxation which favours spectral resolution in EPR (sensitivity also increases in EPR upon cooling as the differences in Boltzman populations increases). In contrast, with increasing temperature the electronic relaxation becomes faster and conversely nuclear relaxation becomes slower.

The short spin--lattice relaxation time of the paramagnetic species is the main obstacle for recording a NMR spectrum as it leads to broad signals, which are difficult to detect. Another problem arises from the very wide chemical shift ranges (far wider than "normal" diamagnetic range) that requires short acquisition window which can lead to truncated FIDs (if a signal is found!) and thus strongly distorted baseline. Although the first paramagnetic hexacyanidometallate solid--state NMR spectra were published in 1960, 18 the design of new generations of spectrometers with considerably improved electronics has been the key factor to obtain better spectra of paramagnetic species. Nowadays, if the electron--nucleus interaction is not too strong (which is often the case for paramagnetic 1 H NMR), acceptable data sets with can be obtained with routine tools. As mentioned in chapter 1, the Hahn--echo pulse sequence can also be useful in order to partly overcome the problems linked to fast relaxation. Besides, the rapid sample rotation about an axis inclined at the 'magic angle' of 54.74° to the Zeeman field direction (so--called MAS technique) can partially remove many sources of broadening (dipolar interaction, chemical shift anisotropy, first order quadrupolar interaction, etc.) from the NMR spectrum and enable fine features to be revealed. Nowadays solid--state NMR spectroscopy has emerged as a powerful technique to investigate a diversity of paramagnetic systems such as cathode materials for lithium--ion batteries, [20][21][22][23] metalloproteins and porphyrins, [24][25][26] organometallic or coordination compounds (sometimes involved in catalytic processes), 27,28 inorganic polymers such as prussian blue analogues, 8,9,29,30 lanthanide complexes. In this work our aim is to use NMR spectroscopy to probe the magnetic properties of cyanide paramagnetic building block complexes at the atomic scale.

It is well known that the cyanide bridge allows efficent magnetic and electronic communication between the metal centres it links. The distribution and orientation of unpaired electrons along the cyanide ligands in the mononuclear paramagnetic building blocks carries important information for designing multinuclear magnetic compounds. Solid--state NMR studies on hexacyanidometallates and Prussian Blue Analogues (PBA) clearly reveals the fact that NMR spectroscopy applied to paramagnetic species (paramagnetic NMR) provide access not only to the local structural information but also to the local magnetic information. 8,9,30 In the present contribution the following changes has been implemented compared to the previous study on hexacyanometallates: 1) the topology of the building block has been changed from hexacyano to bis-- and tris-cyanido complexes, 2) various blocking ligands have been incorporated to investigate their role on the spin delocalizatio,n and 3) two different paramagnetic sources (with the same electronic LS--d 5 configuration) have been used in this study.

Paramagnetic NMR: Background theory

The experimental chemical shift (in ppm) of a paramagnetic species can be decomposed as the sum of two main contributions:

! T exp = ! T dia +! T para (3) ! ! !"
# is the diamagnetic contribution to the chemical shift (usually independent of T) in the absence of any influence from the unpaired electron(s). It is also approximated as the chemical shift one would measure for a isostructural diamagnetic species. 31,32 ! ! !"#" is the paramagnetic contribution to the experimental chemical shift and is solely due to the unpaired electron. More specifically, ! ! !"#" term comes from the hyperfine interaction, which is the sum of a dipolar contribution and a contact term.

! T para = ! T con +! T dip (4)
! ! !"# , or the pseudocontact term, is due to the dipolar through--space interaction between two magnetic moments and it can be estimated using geometric and electronic parameters. 33 Actually the pseudocontact shift critically depends on the g--tensor anisotropy and it has a 1/! ! ! dependence, where ! ! is the distance between the paramagnetic species and nuclear under study. Its contribution to the ! ! !"#" is significant when the NMR active nucleus is in close proximity to the paramagnetic centre. 8,34 As the unpaired electrons can extend onto the atom of the ligands (through spin polarization or delocalization through molecular orbitals), the pseudocontact chemical shift can further be described as a sum of through-space metal--centered pseudocontact (! !" !"# ) term and through--space ligand--centered pseudocontact (! !" !"# ) term. 31,33,35

! T dip = ! MC dip +! LC dip (5)
The metal--centered pseudocontact shift can be obtained from the following equation:

! MC dip = µ 0 4" # e 2 9k B T 3cos 2 $ j !1 r j 3 F ' j " (S, g II , D) (6)
The formula assumes an axial symmetric g tensor, the point--dipole approximation and an anisotropy arising from zero field spilitting. 8,9,34 Depending on S the F' term takes the form:

S = ½: F 1/2 ' (S, g) = S(S +1)(g II 2 ! g " 2 )
S = 1:

F 1 ' (S, g, D) = S(S +1)(g II 2 ! g " 2 ) 1! (g II 2 + 0.5g " 2 ) 3(g II 2 ! g " 2 ) D k B T # $ % & ' ( S = 1: F 3/2 ' (S, g, D) = S(S +1)(g II 2 ! g " 2 ) 1! 4(g II 2 + 0.5g " 2 ) 5(g II 2 ! g " 2 ) D k B T # $ % & ' (
where, ! ! is the vectors joining the paramagnetic source and the nucleus under studiy, ! ! is the angle between r j and the principal magnetic axis of the molecule. In this formula, ! ! is the Boltzmann constant, T, the standard temperature, ! ∥ and ! ! , the electron g factors (parallel and perpendicular to the magnetic axis), S, the electron spin quantum number, ! ! is the vaccuum permeability, ! ! , the Bohr magneton, and all are given in SI units.

The value obtained from the equation ( 4) can be converted to ppm by the multiplication by 10 6 .

For S = ½ (low spin iron(III) ion) with strong axial symmetry, the metal--centered dipolar signal shift at temperature T can be expressed as:

! MC dip = µ 0 4" # e 2 S(S +1) 9k B T 3cos 2 $ j !1 r j 3 (g II 2 ! g " 2 ) j # = 174.29 3cos 2 $ j !1 r j 3 (g II 2 ! g " 2 ) j # (7)
The ligand--centered pseudocontact (! !" !"# ) term is generally left aside in many of the published work as it is difficult to estimate. The ! !" !"# contribution originates from the interaction between the nuclear magnetic moment and the unpaired electron delocalized onto the ligands. 33 It has been shown to be weak for the N atom and negligible for the C in the low--spin d 5 [Fe III (CN) 6 ] 3--complex. It will be left aside in the present case also.

! ! !"# is a through--bond interaction called Fermi contact term.
It is due to the interaction between the unpaired electron delocalized onto the probed atom and the nuclear magnetic moment. In other words, it is correlated to the probability of finding the unpaired electron(s) onto the nuclei, and it thus arises from the presence of unpaired electron in s orbital. 32,33 The isotropic hyperfine coupling due to the through--bond interaction is commonly obtained from the isotropic contact shift by the equation (assuming an axial anisotropy): 33 (8) where is a function of the components of the g tensor and the zero--field spilitting D. In the case of S = ½, one gets . This formula is valid when only one spin state is populated, and when the ground state can be described by a spin quantum number, S. However it is also generally used as approximation in more complicated case (when the ground term is a T term with first order spin--orbit coupling), considering that the orbital contribution could be included in the g term (described as g av ) extracted from magnetometry data.

! 298K,iso con = g e " e S(S +1) 3# N k B T A s F(g II , D)
The isotropic hyperfine coupling, , can be further linked to the spin density located in the s orbital of the i th electron shell, ! is (0) 2 , by using the following formula: 32

A s (X) = µ 0 3S g e ! e " N # is (0) 2 = µ 0 3S g e ! e " N $ is (0) (9) F(g ii , D) g ii F(g ii , D) =1 A s (X)
In the calculation, we will assume that all the spin density is located in the 2s orbital of the C and N atoms (neglecting the 1s orbitals). Therefore by combining equation ( 8) and ( 9) one can notice that the isotropic contact term is directly correlated to the spin density at the observed nucleus: 8,9,33 ! 298K,iso

con = µ 0 a 0 3 " e 2 g av 2 (S +1) 9k B T # is (0) (10)
Therefore, ! 298K,iso con = " is (0)g av 2 3 / 2 5.072 !10 "5 (10') Here , the spin density is expressed in (a.u) --3 unit. Alternatively it can be experessed in Å --3 by removing the a 0 3

factor.

The spin density experienced at the i th s orbital of the observed nucleus can then be converted to a fractional spin density as follows:

! is (0) = f is " is (0) 2 (11)
In this expression is a theoretical value corresponding to the amount of spin density at the nucleus, considering that the i th s orbital has one unpaired electron (the function describes a semioccupied orbital and its spin density Ψ !" 0 ! is given in Å -3 or in a.u. --3 ). These values are tabulated and can be found in previous work. 36 Hence the dimensionless fractional spin density in the i th s orbital is expressed as: 8

f is = 9k B T µ 0 g av 2 ! e 2 S(S +1) " is (0) 2 # 298K,iso con = 3.426 !10 "4 # 298K,iso con g av 2 S(S +1) " is (0) 2 (12)
In the equation (10') constants and the standard temperature (298 K) have been inserted, has been replaced by and the shift,

! !"#!,!"# !"# , is expressed in ppm whereas Ψ !" 0 ! is given in Å 3 .
It should be noted that depends on the selected wave function .

In the present case we assume that the spin density is located in the 2s orbital of the observed nuclei (excluding the 1s). Therefore i th s orbital refers to the 2s orbital in all the followwing calculations. 8 The spin density can thus be obtained from the contact term after the estimation of both the diamagnetic and the dipolar contributions. In this work, the diamagnetic contribution is estimated by measuring the signal shift of diamagnetic Co(III) analogues.

It is noteworthy that both Fermi contact and pseudo--contact terms are inversely proportional to the temperature, which makes paramagnetic NMR spectroscopy data useless if the actual temperature inside the probe is unknown. Since MAS--NMR spectroscopy involves very high spinning rates of the sample (up to 67 kHz, depending on the rotor size), the sample undergoes a non--negligible increase of its internal temperature

! is ! is g e g av f is ! is (0)
compared to the set point temperature. This can be monitored and adjusted by using an internal temperature standard. Indeed, the isotropic chemical shift of nickelocene [Ni(Cp) 2 ] protons in function of the temperature is tabulated in the literature: 37 12.89 (13) For each of the following measurement, some freshly ground nickelocene is inserted in the middle of the rotor in order to measure and adjust the temperature during the measurement. A proton spectrum is acquired before and after each 13 C and 15 N measurement in order to check the stability of the inner temperature during the measurement. Two spectra are acquired at two different carefully selected spinning rates but at the same inner temperature in order to identify the isotropic peak from the side band pattern.

T = ! 79477 ! exp !
3.2 13 C, 15 N MAS--NMR spectra of 1--5

Compound 1--5 were synthesized using 13 C (99%) and 15 N (98%) enriched potassium cyanide at a 1.0 mmol scale and their solid--state paramagnetic NMR spectra were recorded.

The solid--state NMR spectra were acquired using magic angle spinning technique, by rotating the sample at the magic angle (θ ≈ 54.74°). The acquired spectra showed isotropic chemical shift, δ iso , and a spinning sideband pattern, regularly spaced by the MAS spin rate, which could be used to determine the chemical shift anisotropy (CSA) of the nuclei. The superposition of two or more spectra at different (carefully chosen) spin rates but the same temperature allows to determine the isotropic signal.

In the following section, the chemical shift anisotropy tensors of all the solid state NMR spectra were determined using the module "Solids Line Shape Analysis" from Bruker's software package Topspin. The Haeberlen--Mehring--Spiess convention was used for CSA analysis. 38 Every paramagnetic contribution to the chemical shift is referenced against its own diamagnetic contribution.

Ideally, one should have one isostructural diamagnetic reference compound per paramagnetic species.

! T ,iso exp = (! T ,xx exp +! T ,yy exp +! T ,zz exp ) / 3 ! T ,xx exp ! T ,yy exp ! T ,zz exp ! T ,zz exp !! T ,iso exp " ! T ,xx exp !! T ,iso exp " ! T ,yy exp !! T ,iso exp !! T exp = ! T ,zz exp " (! T ,xx exp + ! T ,yy exp ) / 2 ! T exp = (" T ,yy exp !" T ,xx exp ) / (" T ,zz exp !" T ,iso exp )
However, the diamagnetic M(III) equivalents of each of the paramagnetic M(III) cyanometallates are not always accessible, so the diamagnetic low--spin cobalt(III) complex PPh 4 [Co III ( Me2 Tp)(CN) 3 ] will be taken as diamagnetic reference for all of the paramagnetic species mentioned in this chapter. This complex exhibits the same overall C 3v symmetry as the paramagnetic PPh 4 [Fe III (Tp)(CN) 3 ] complex, which is the one which has been more deeply investigated. The error bar introduced here is however small. The change in 13 C and 15 N chemical shifts do not vary significantly when changing the blocking ligand (δ( 13

C MAS--NMR spectroscopy

The 13 C enriched sample was prepared using about 100 mg of microcrystalline 13 C--enriched compound packed in a 4 mm zirconium oxide (ZrO 2 ) rotor. 13 C spectra of this diamagnetic reference were recorded in a 500 MHz Bruker spectrometer equipped with a BL probe head and the chemical shift scale was referenced against TMS (adamantane as secondary reference). The spectra were recorded with spinning speed of 10 and 6 kHz, with a d1 delay of 5 seconds between two successive scans. Superposition of the two spectum allowed the identification of three kinds of isotropic signals, as depicted in the figure given below. Haeberlen Analysis (HBA) of the overnight recorded 6 kHz MAS--NMR spectrum was performed (Figure 10). Best results (88.35% overlap) could be obtained by considering only two different cyanide 13 C environments, at δ iso = 138.8 and 133.2 ppm respectively, and three PPh 4 + / Me2 Tp isotropic values (δ iso = 125. 1, 120.4 and 117.4 ppm).

Experimental parameters obtained from the 13 C MAS--NMR spectrum of PPh 4 [Co III ( Me2 Tp)(CN) 3 ] are summarized in Table 8. Due to its low intensity, the Me2 Tp methyl signal was not included. At these spin rates, the broadness of the peaks does not allow the third cyanide to be resolved. The results of the Haeberlen analysis are expressed for the sake of clarity and are coherent with previously published work. 8,9 Table 8. Haeberlen Analysis results for the simulation of the 6 kHz 13 Both contributions exhibit strong axial symmetry, with η ≈ 0, which is consistent with the MAS--NMR spectra of hexacyanometallates reported in the literature. 8,9 Actually, the cyanide bridges being linear (along the z axis for instance), the local x and y axis are intervertible, so that the anisotropy tensor contributions along these two axes are expected to be identical, but quite different from the last one. Contribution (a) exhibits an almost half as great anisotropy Δδ exp ( 13 C) than contribution (b). They are also negative, indicating that the nucleus is less shielded in two directions than it is in the third one (oblate geometry).

The signals c--d--e (see inset of Figure 10) cannot be ascribed to specific carbon in the tetraphenyl phosphonium salt or in the ligand. The first two have a strong axial symmetry and a strong negative anisotropy in the same order of magnitude as the cyanides signal sets. Signal set (e), however, exhibits a strong non--axial symmetry and a very small anisotropy (46.3 ppm, to be compared with the --252.3 and --217.2 ppm for contributions (c) and ( d)).

Since it is not possible to ascribe each cyanide in the structure to its NMR signal with sufficient accuracy, the following mean set of parameters (Table 9), associated with uncertainties, will be used in this work.

Table 9. Mean values and associated uncertainties for the cyanide contributions to the 13

( Me2 Tp)(CN) 3 ] Mean value [ppm] Uncertainty [ppm] ! !,!"# !"# ( 13 C) 136.0 ± 2.8 ! !,!! !"# ( 13 C) --48.1 ± 27.5 ! !,!! !"# ( 13 C) 226.3 ± 16.3 ! !,!! !"# ( 13 C) 229.8 ± 19.6 ∆! ! !"# ( 13 C) --276.2 ± 45.4 !" ! !"# ( 13 C) 0.
019 -

N MAS--NMR spectroscopy

The 15 N enriched sample of PPh 4 [Co III (Tp Me2 )(CN) 3 ] was prepared in a similar way as the 13 C enriched sample and its 15 N NMR spectra was recorded with the same experimental setup. The 15 N chemical shift was referenced against CH 3 NO 2 (with NH 4 NO 3 as a secondary reference). The spectra were recorded using a Hahn echo pulse sequence, at 3, 6 and 10 kHz, with a d1 delay of either 60 or 120 seconds. The isotropic values were determined by superposition of the 6 and 10 kHz spectra as depicted in Figure 11. At these spinning rates, all the three cyanide contributions are resolved, but very few spinning bands define the overall shape of the CSA (Chemical Shift Anisotropy). It is also not possible to attribute precisely each isotropic value to a specific cyanide ligand. The 6 kHz spectrum was simulated and the results are reported in Table 7. A fourth nitrogen environment is clearly visible at δ iso = --62. ; this is coherent with the linearity of cyanides and the previous 13 C results displayed in this work. Their anisotropy is bigger than for 13 C and ranges from --490.4 to --523.6 ppm. In order to have a closer look at the CSA, a spectrum was recorded and simulated at a spin rate of 3 kHz (Figure 13). At 3 kHz, more likely the solid state interactions are not totally averaged so the three different cyanides are not resolved anymore, and the spectrum can be simulated using only two different nuclei. Despite 72 hours of acquisition, the signal to noise ratio was still quite low, so that (d) was not clearly visible and therefore it is neglected for the simulation. The parameters of the signals obtained at 3kHz, are roughly comparable to 6 kHz but the limited signal to noise ratio did not allow to improve them significantly.

Table 10. Haeberlen analysis for the simulation of the 6 kHz 15 N MAS--NMR spectrum of PPh 4 [Co III ( Me2 Tp CN) 3 ] using four different isotropic shifts. Chemical shifts are given in ppm.

The poor signal to noise ratio and low resolution of the 3 kHz spectrum compared to the 6 kHz makes the analysis less reliable than that of 6 kHz. The diamagnetic PPh 4 [Co III ( Me2 Tp)(CN) 3 ] reference will be therefore described by the following mean values and uncertainties (Table 11).

Table 11. Mean values and associated uncertainties for the cyanide contribution to the 15 N MAS--NMR spectrum of PPh 4 [Co III ( Me2 Tp)(CN) 3 ], based on the experimental parameter set listed in Table 10. Chemical shifts are given in ppm.

PPh

4 [Co III ( Me2 Tp)(CN) 3 ] Mean value [ppm] Uncertainty [ppm] ! !"# !"# ( 15 N) --80.3 ± 3.5 ! !! !"# ( 15 N) --418.0 ± 15.1 ! !! !"# ( 15 N) 73.8 ± 11.5 ! !! !"# ( 15

C MAS--NMR spectroscopy

The sample was prepared using about 10 mg of dried ground crystalline 13

C--enriched PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O
and was packed in a 1.3 mm ZrO 2 rotor. The 13 C spectra were acquired at magic angle spinning in a 700 MHz spectrometer equipped with a 1.3--BL probe head, and the chemical shift is referenced against TMS (adamantane as secondary reference). An attempt of recording the 13 C spectrum in 300 MHz with 4 mm rotor was made, but due to the large chemical shift anisotropy (up to several thousand ppm), the use of high spinning speed (and thus small rotor size) is necessary. Actual temperatures for each measurement are reported below. The spectra are simulated by using Haeberlen analysis. The experimental spectra (60 and 65 KHz) are depicted in Figure 1. Although the different cyanide contributions can not be attributed at a first glance, a significantly better overlap between experimental and simulated spectra was obtained when using two different 13 C nuclei. Simulation and experiment results match partly because of the spectral bump, which is artificially flatten during the baseline correction. As previously reported for the hexacyanidometallates, 3,436 the isotropic chemical shifts of the cyanide carbons are strongly shifted to lower frequency compared to their diamagnetic reference. The experimental isotropic shifts exhibit high negative values, --3726 and --3755 ppm at 334.6 K, that leads to negative values of spin density on the probed atoms. This is coherent with a dominent spin polarization mechanism from the metal to the carbon, as observed in previous study concerning hexacyanometallates. 8b,9a

PPh 4 [Co III ( Me2 Tp)(CN) 3 ] Spinning rate 6 kHz Contribution a b c d ! !,!"# !"# ( 15 N) --76.8 --80.2 --84.0 --62.4 ! !,!! !"# ( 15 N) --413.8 --407.2 --433.1 --423.6 ! !,!! !"# ( 15
(K) ! !"#$ ! !"#$%" (ppm) ! !"#$ ! !"#$% (ppm) T actual (K) ! ! !"# (ppm) 1.3--BL 60
Both signals exhibit strongly axial CSA tensor (the asymmetry parameters are close to zero, from the Haeberlen analysis). The perpendicular chemical shift is weaker (more deshielded) than the axial one, leading to a negative anisotropy parameter (Δδ = --2150 and --2033 ppm). It is worth noticing that the overall chemical shift anisotropy (CSA) of about 2660 ppm is in absolute value tenfold bigger than for the diamagnetic PPh 4 [Co III (Tp Me2 )(CN) 3 ] reference. Moreover the symmetry of the signals is modified with an prolate shape compared to the oblate for PPh 4 [Co III (Tp Me2 )(CN) 3 ] and previously reported [Fe(CN) 6 ] 3--compound.

N MAS--NMR spectroscopy

The 15 N MAS--NMR spectroscopy was recorded in a 4 mm rotor. The rotor was prepared using 100 mg of ground 15

N cyanide--enriched PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O.
The 15 N spectra were acquired on a 400 MHz spectrometer, equipped with a 4--BL probe head. The temperature inside the rotor was controlled by setting the nickelocene chemical shift to --245.9 ppm (6 kHz) and --245.4 ppm (10 kHz) which correspond to 310.3 and 311.0 K, respectively (table 13). The chemical shifts were referenced against CH 3 NO 2 .

!"#$%&'()*+,*-./* 012(3"#(4'&%* *+,*-./* Table 13. Actual temperatures for the 15 The superposition of the 6 and 10 kHz spectra allows the determination of three clearly defined cyanide sites, though grouped in one signal bush, at δ iso = 522, 502 and 474 ppm. This is about 554-602 ppm left--shifted compared to the diamagnetic reference. Even though each isotropic signal corresponds to one of the three slightly different cyanides (determined by X--Ray diffraction analysis), no specific attribution can be done. This positive paramagnetic contribution corresponds to a positive spin density perceived by the nitrogen nuclei. The result is consistent with previous paramagnetic NMR studies on hexacyanometallates. 3,29,41 Figure 16. 15 The 6 kHz spectrum was simulated using Haeberlen analysis (Figure 17). Interestingly, 15 N spectra of this compound do not exhibit the same symmetry as that of the diamagnetic reference. Indeed, while the cobalt(III) complex exhibits strongly axial CSA tensors (mean η = 0.087), the iron(III) species clearly exhibits non axial contributions, with asymmetry parameter ranging from 0.785 to 0.912. Simulation attempts with axial contributions failed at reproducing the overall shape of the signal with physically reasonable parameters.

[K] ! !"#$ ! !"# [ppm] T actual [K] ! ! !"# [ppm] 4--BL 6
The empty, antibonding cyanide molecular orbital of π* symmetry (whose principal contribution is the p orbitals of the nitrogen atom) possesses an adequate symmetry to interact with the d orbitals containing the unpaired electron through backbonding. This is expected to result in a significant, positive spin delocalisation into the p orbital of the nitrogen atoms. The axial nature of the said orbital (perpendicular to the CN triple bond) that interacts with the 2s N orbital could therefore be responsible for the sweep from an axial--symmetric signal without unpaired electron to a strong non axial one in its presence.

! !"#,!! !"#"
--5111 --5062 1896 a 73 38

! !"#,!! !"#"
--3242 --3307 345 505 438

! !"#,!! !"#" --3233 --3304 --435 a 1169 1188 (! !"#,!"# !"#" ) av --4350.2 602.6
a The anisotropy parameter of this contribution is positive, which leads to an artificial change in the xx and zz Haeberlen convention compared to the reference mean parameters. In order to get meaningful results for this contribution, the chemical shift are on this occasion calculated as:

! !! ! = ! !! ! -! !! !"# and ! !! ! = ! !! ! -! !! !"# instead of the normal ! !! ! = ! !! ! -! !! !"# and ! !! ! = ! !! ! -! !! !"#
The spectra simulation is likely showing that the unpaired electron distribution about the cyanide carbon atoms exhibits an axial symmetry for the PPh 4 [Fe(Tp)(CN) 3 ]••H 2 O complex, whereas the cyanide nitrogen atoms significantly deviate from axiality (with three different contributions). Hence, if the shift anisotropies are visually represented by spheroids, this means that they would be clearly prolate at C and scalene (with three different axes) at N. This observation contrasts with that obtained for previously reported [Fe(CN) 6 ] 3-- compounds where strongly axial symmetry were observed for both C and N sites (respectively oblate and prolate). 9 Infact, this observation might be correlated to the higher magnetic anisotropy observed in the PPh 4 [Fe(Tp)(CN) 3 ]••H 2 O as shown by PND analysis of the compound (see below).

Solid--state NMR spectroscopy of PPh 4 [Fe(pcq)(CN) 3 ] (3) 13 C MAS--NMR spectroscopy

The sample was prepared using 10 mg of crystalline sample of 13 C enriched 3 and was packed in a 1.3 mm rotor. The 13 C MAS--NMR spectra were recorded on a 700 MHz Bruker spectrometer equipped with a 1.3--BL probe head and the chemical shift scale was referenced against TMS (secondary reference being adamantane). The sample 13 C MAS--NMR spectrum was recorded at an inner temperature of 311.4 K (δ NiCp2 = --245.05 ppm) in the rotor, using a Hahn--Echo pulse sequence. The temperature data is summarised in Table 15.

Table 15. Actual temperature of 13 C MAS--NMR measurement of 3 recorded with a 1.3--BL probe head and a Bruker AV--700 spectrometer.

Probe

Spinning rate (kHz)

T set point (K) ! !"#$ ! !"#$%" (ppm) ! !"#$ ! !"#$% (ppm) T actual (K) ! ! !"# (ppm) 1.3 BL 60 245 --245.0 --245.1 311.44 --1900
Intially 13 C MAS NMR spectra of this compound were recorded on 700 MHz NMR with a spinning speed of 25 and 30 kHz (and using a 2.5 mm rotor). The recorded spectra showed poor signal to noise ratio and a large envelope around ca. --1000 to --2150 ppm. Despite long aquisition time, no correct attribution of the isotropic peak and spinning sidebands were possible. The close observation of the 30 kHz MAS spectrum revealed that, each equispaced spikes exhibit spilitting pattern which might be indicative of different 13 C sites. The large width of the signal cleary indicates large chemical shift anisotropy. In order to minimize the CSA interaction and to have estimation about the isotropic chemical shift, experiments with high MAS spinning speed were required.

Figure 18. 13 C MAS--NMR spectra of 3 at 30 kHz (black) and 25 kHz (red) at 310.4 K and 310.1 K respectively. The spectra were acquired at a Larmor frequency of 175.37 MHz.

Even at very high speed (60 kHz), the observed CSA was remarkably large and spinning sidebands were not distinguished in this spectrum. The isotropic shift can only be roughly estimated and it is located around --1900 ppm. Therefore, the simulation of the 60 kHz spectrum is highly hypothetical and largely suffers from accuracy. The broadness of the signal could be due to dipolar coupling beetween one of the cyanides' carbon and the proton of the pcq ligand that are located at distances range from ca. 2.67 Å to 2.74 Å. This distances are clearly shorter than the van der Walls radius of C--H (ca. 2.90 Å). Therefore, this 13 C--H dipolar coupling is expected to be pronounced. On the other hand, the shortest distance between the cyanides' carbon and the proton of the Tp ligand in the PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O complex is ca. 3.04 Å (higher than van der Walls radius of C--H),

-4000 -3000 -2000 -1000 0 1000

! [ppm] !"#$%&# '(#$%&#
therefore the dipolar 13 C--H coupling is anticipated to be absent. Therefore we have performed proton decoupled 13C MAS NMR spectrum, but no significant improvement was observed.

Figure 19. 13 C MAS--NMR spectra of 3 at 60 kHz (black) at T = 311.44 K acquired at a Larmor frequency of 175.37 MHz.

The paramagnetic contribution is strongly negative, and comparable with that of 1. The roughly estimated isotropic shift leads to negative values of spin density which is coherent with a dominent spin polarization mechanism from the metal to the carbon, as observed in 1. 15

N MAS--NMR spectroscopy

The 4 mm rotor was prepared using 100 mg of ground 15 N cyanide--enriched sample of compound 3. The 15 N MAS--NMR spectra were recorded on a 300 MHz spectrometer, equipped with a 4--MQ probehead. inner temperature was set to the chemical shift of nickelocene of δ = --250.16 ppm, which corresponds to an actual temperature of 304.81 K. The spectra at 8 and 12 kHz were acquired using a Hahn--echo pulse sequence, and the baselines were manually corrected. Table 16. Actual temperature of 15 N MAS--NMR measurements of 3 recorded with a 4--MQ probehead and a Bruker AV--300 spectrometer.

Probe

Spinning rate (kHz) In strong contrast with the 13 C spectra, the three cyanides exhibit well separated 15 N isotropic chemical shifts around 761.0, 694.6 and 606.9 ppm. The absence of strong residual dipolar coupling with the Fe(III) centre is probably the main factor for obtaining resolved 15 N spectra. The 8 kHz MAS spectrum exhibits more resolved spinning sidebands and therefore carries more information about the CSA. Hence, it was simulated with Haeberlen analysis and the results are summarized in table 17.

T set point (K) ! !"#$ ! !"#$%" (ppm) ! !"#$ ! !"#$% (ppm) T actual (K) ! ! !"# (ppm)
The spectrum (both experimental and simulated) shows three cyanide contributions, two of them exhibiting a similar side--band pattern, are probably due to the trans--cyanide that are images from each other through the -3000 -2000 -1000

! [ppm] !"#$%&# !"
pseudo--miror plane containing the pcq ligand. The third signal, is likely due to the cyanide in the pcq meridional plane.

Figure 20. 15 N MAS--NMR spectra of 3 at 8 kHz (red) and 10 kHz (black) at 304.9 K acquired at a Larmor frequency of 30.46 MHz. The isotropic peaks (three), which are not shifted, are marked with an asterisk. The two trans sites exhibit non axial symmetry with η = 0.98 and 0.46 respectively for the 760 ppm and 693 ppm at 304.9 K. The anisotropy parameters of the two sites are smaller in absolute value than the anisotropy parameters found for 1. They amount to Δδ = --787 and --650 ppm, but the sign, as already explained, is an artificial consequence of the Haeberlen convention. It was therefore treated consequently in the following spin

! !,!! !"# -- 294 260 236 ! !,!! !"# -- 493 532 766 ! !,!! !"# -- 1034 1287 1280 ∆! ! !"# -- --469 --650 --787 !" ! !"# -- 0.47 0.46 0.98 ! !"#,!! !"#"
--728 694 669

! !"#,!! !"#" -- 429 469 708 ! !"#,!! !"#" -- 952 1211 1204 (! !"#,!"# !"#" ) av --2127.3
784.9 The overall 15 N spectrum simulation showed that the spin distribution about the cyanides' nitrogen atoms deviate from axiality compared to the diamagnetic reference. The cyanide nitrogen atoms of diamagnetic reference compound showed a prolate like chemical shift anisotropy tensor. In contrast in the case of PPh 4 [Fe(pcq)(CN) 3 ], we observe a scalene spheroids with three different CSA tensor values. The results are similar to those observe for the PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O where a scalene spheroid symmetry is observed for the nitrogen CSA but they contrast to the strongly axial symmetry observed for [Fe(CN) 6 ] 3--. 8

Solid--state NMR spectroscopy of PPh 4 [Ru(salen)(CN) 2 ] (4) 13

C MAS--NMR spectroscopy

The 13 C MAS--NMR spectra of the compound were recorded in a 300 MHz spectrometer with a spinning speed of 12 kHz and 14 kHz. Unfortunately, we did not observe the expected 13 C signals (coming from cyanide carbon) in both of the recorded spectra. Only signals in the diamagnetic range (between 0 ppm to +200 ppm) were observed and ascribed to the carbons within the PPh 4 + counter cation. Other measurments need to be carry out at higher magnetic field and higher MAS frequency.

have almost no impact on the determination of isotropic chemical shift(s) because of the linewidth of the signals (W 1/2 ≈ 970 Hz).

Table 18. Tabulated report of 15 Although the crystal structure of compound 4 displays two slightly different cyanide bonds (one would have expected two different isotropic 15 N signals), due to a broad linewidth and low resolution in the spectra they could not be resolved.

(K) ! !"#$ ! !"#$%" (ppm) ! !"#$ ! !"#$% (ppm) T actual (K) ! ! !"# (ppm)
Figure 22. 15 N MAS--NMR spectra of 4 at 3 kHz (red) and 5 kHz (black) at 308.7 K acquired at a Larmor frequency of 30.46 MHz. The isotropic peak is marked with an asterisk.

Both spectra were simulated with Haberlen Analysis, with the best overlap at 3 kHz (90.34%). The extracted values are reported in Table 19.

At 308.8 K, the isotropic signal, δ iso = 346.4 ppm, is shifted by 426.7 ppm at higher frequency from the mean isotropic value of the diamagnetic reference. This corresponds to a positive paramagnetic contribution to the chemical shift, and a positive spin density. The Haeberlen analysis of the 15 N spectrum shows a deviation from axial symmetry (η = 0.453) with a roughly scalene shaped CSA tensor. This contrasts with the results previously found in diamagnetic reference compound but they are similar to those obtained for 1, and 3. All attempts using axial simulations led to absurd deconvolution results. The anisotropy parameters are positive and in absolute value smaller than those previously mentioned in PPh 4 [Fe III (Tp)(CN) 3 ] compound {Δδ = --502.49 ppm versus +1434, --1272 and --1282 ppm for PPh 4 [Fe III (Tp)(CN) 3 ]}, pointing out a lower anisotropy in the spin density localisation of 4 than that of 1. The 15 N spectral simulation results are given in table 19.

-1000 0 1000 Table 19. The solid state 15 N NMR datas and chemical shift of PPh 4 [Ru(salen)

! [ppm] ! "#$%&# # '#$%&#
(CN) 2 ]. PPh 4 [Ru III (salen)(CN) 2 ] 15 N ! !,!"# !"# 345.6 ! !,!! !"# 11 ! !,!! !"# 219 ! !,!! !"# 806 ∆! ! !"# --502 !" ! !"# 0.45 ! !"#,!! !"#" 444 ! !"#,!! !"#" 151 ! !"#,!! !"#" 729 ! !"#,!"# !"#"

437.6

Solid--state NMR spectroscopy PPh 4 [Ru(acac) 2 (CN) 2 ] (5) 13

C MAS--NMR spectroscopy

Microcrystalline sample of 13 C--enriched 5 (50 mg) were mixed and packed in a 2.5 mm rotor. The 13 C spectra were recorded at 700 MHz with MAS frequency of 30 and 25 kHz and using a Hahn--echo sequence to limit baseline deformations. Initial attempts with 4 mm rotor at 300 MHz were unsuccessful due to to low resolution of the spectrum. Table 20. Actual temperature of 13 C MAS--NMR measurements of 5 recorded with a 2.5--BL probehead and a Bruker AV--700 spectrometer.

Probe Spinning rate (kHz) Superposition of the 30 and 25 kHz spectra (Figure 24) allows the determination of the isotropic signals at δ iso = --1279.5 and --1334 ppm at 310.5 K. A recyle delay (d1) of 0.1 sec was used for both the experiments.

T set point (K) ! !"#$ ! !"#$%" (ppm) ! !"#$ ! !"#$% (ppm) T actual (K) ! ! !"# (ppm)
Figure 24. 13 C MAS--NMR spectra of 5 at 30 kHz (red) and 25 kHz (black) at T = 310.5 K acquired at a Larmor frequency of 175.37 MHz. The isotropic peaks are marked with an asterisk.

The isotropic chemical shifts of the cyanide carbons are strongly shifted to low frequency as compared to the diamagnetic reference, with δ = --1279.5 and --1334 ppm at 310.5 K. This is consistent with the 13 C MAS--NMR for the other paramagnetic cyanido M(III) complexes presented in this work and points to a negative spin density on the carbon atom. This negative paramagnetic contribution is coherent with a spin delocalisation mechanism.

The signals are also sharper (half width of 25 ppm at 30 KHz) compared to 3, which leads to better spectral resolution and a much better accuracy on the determination of the chemical shift and CSA tensor. Two cyanide contributions are observed in both spining rates. A closer look at the simulated 30 kHz spectra (Figure 25, table 22) indicates that the signal exhibits a lower chemical shift anisotropy than in PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O or [Fe(CN) 6 ] 3--, with three different main tensor values. The asymmetry parameters are deviate from axiality both 30 and 25 kHz. The spectra exhibits deformations of the baseline, it can be ascribed due to the larger window size of the experiment. A spectral window of ca. 5700 ppm was used for the measurement.

-4000 -3000 -2000 -1000 0 1000

!"#$%&# # '(#$%&# ! [ppm]
)# Figure 25. Simulated (top) and experimental (bottom) 13 C MAS--NMR spectrum of 5 acquired at a Larmor frequency of 175.37 MHz, δ(NiCp 2 ) = --245.7 ppm (T = 310.58 K) and a spinning rate of 30 kHz. Overlap: 83.64%.

N MAS--NMR spectroscopy

The 15 N MAS--NMR spectra were recordred using 100 mg of ground 15 N cyanide--enriched sample, on a 300 MHz spectrometer equipped with a 4--MQ probehead. The spectra at 5 and 2 kHz were acquired using a Hahn--echo pulse sequence in order to limit the baseline deformation due to the large spectral window. Two isotropic shifts are observed at 286.6 and 279.01 ppm, which can be ascribed to each of the two crystallographically indepent cyanide ligands.

Table 21. Actual temperature of the 15 N MAS--NMR measurements of 5 recorded with a 4--MQ probehead and a Bruker AV--300 spectrometer.

Probe

Spinning rate (kHz) The positive value of chemical shift implies a positive spin density on the nitrogen atoms of the cyanide. The spin density in the 2s orbital of the nitrogen atoms results from the polarisation of this orbital by the spin density delocalised from the ruthenium (III) ion into the 2p orbitals of the said nitrogen atoms. The 2 kHz 15 N MAS--NMR spectrum could be simulated with Haberlen analysis and the results are summarized in Table 17. The two 15 N sites exhibit strongly non axial symmetry with η = 0.863 and 0.918 for the 286.

T set point (K) ! !"#$ ! !"#$%" (ppm) ! !"#$ ! !"#$% (ppm) T actual (K) ! ! !"# (ppm)
! !,!! !"# --1641 --1917 --36 --28 ! !,!! !"# --1231 --1450 269 314 ! !,!! !"# --957 --1060 614 737 ∆! ! !"# --547 --662 --477 --553 !" ! !"# 0.75 0.88 0.92 0.86 ! !"#,!! !"#" --1593 --1869 396 404 ! !"#,!! !"#" --1457 --1676 202 248 ! !"#,!! !"#" --1187 --1290 529. 656 Mean ! !"#,!"# !"# --1475.9 383.4
The Haeberlen analysis of the 13 C and 15 N NMR spectra of 5 show that the spin distribution about the cyanide carbon and nitrogen atoms deviates significantly from axiality, whereas in case of the diamagnetic PPh 4 [Co( Me2 Tp)(CN) 3 ] compound a strong axiality was observed. Therefore, we can assume that the involvement of unpaired electron may lead to this change in symmetry. The 13 C and 15 N CSA tensor show scalene like spheroids shapes. This observation matches with that observed for PPh 4 [Fe III (Tp)

(CN) 3 ]••H 2 O.

Spin density calculations in 1--5

As mentioned before, due to lack of isostructural diamagnetic reference for each of the paramagnetic building block, the chemical shift value of PPh 4 [Co(Tp Me2 )(CN) 3 ] is used as diamagnetic reference, with:

! T dia ( 13 C) = +136 ppm ! T dia ( 15 N ) = !80.3ppm
These values are similar to those observed in other diamagnetic cyanido complexes and they are significantly lower than those measured in the paramagnetic compounds. Therefore, error arising due to lack of isostructural diamagnetic reference is limited.

NMR provides access to spin density through the determination of Fermi contact chemical shift. In order to extract this term one must evaluate the dipolar contribution to the paramagnetic chemical shift (! ! !"# ). The determination of the dipolar contribution of chemical shift requires: (a) the distance between the paramagnetic metal ion and the probed nuclei which is marked as r j ; (b) the angle between r j and the principal magnetic axis of the molecule which is marked as θ j ; (c) ! ! and ! ∥ values. The r j value is obtained directly from the single crystal X--ray diffraction analysis.. The ! ! and ! ∥ factors can be determined from the EPR spectra. It is however much more difficult to experimentally determine the main magnetic axis of a molecule. Magnetometry on single crystals can be used but accurate results are obtained only if the crystal symmetry does not induce averaging effect (because of the presence of miror plane or symmetry axes). Generally only estimates of the dipolar interaction are obtained. We used this approach for all the compounds expect of PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O. In the case of 1, the "susceptibility tensor method" was applied to extract the orientation of the main magnetic axis from PND measurements. This experiment, which is part of the PhD thesis of Karl Ridier, requires a strong expertise and has been carry out at the CEA of Saclay in the group of B.

Gillon. It can be carried out only if large single crystals are available and it requires long PND measurement sessions (PND measurments need to be carried out at low magnetic field and under three different orthogonal orientations of the single crystal).

In order to get a better accuracy in the estimation of dipolar contribution, we have also used for 1 the more sophisticated equation ( 14) instead of the more generally used equation (6). Indeed McGarvey et al. demonstrated that when there is accurate information about g tensor and θ j , the metal ion contribution to the pseudocontact chemical shift can be expressed through the following equation: 33

! MC dip = " 2 S(S +1) 9kTr j 3 {[g zz 2 ! 1 2 (g xx 2 + g yy 2 )](1! 3cos 2 # j ) + 3 2 (g yy 2 ! g xx 2 )sin 2 # j cos2$} ( 14
)
where ψ is the angle between the projection of r j in the xy plane and the x axis. 43 The physical meanings of θ j, r j and the other parameters of this equation were already mentioned before.

For the other paramagnetic compound (3, 4 and 5) we have followed the more simple approach to obtain estimates of the dipolar contribution by considering the two extreme cases where the nuclei is in axial position (θ j = 0°) or in equatorial position (θ j = 0°). Hence from the above discussion it is clear that the dipolar contribution or the overall spin density calculation There is a significant influence of ! ! and ! ! in the Fe-- 13 C and Fe-- 15 N dipolar contribution of

PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O.
In principle, a ½--spin located on the iron ion can induce a dipolar contribution up to +640 ppm (! ! = 90°) or --320 ppm (! ! = 0°) at the cyanide carbons (case a), 1.919 Å away. Since nitrogen lies farther from the iron(III) ion, the dipolar contributions to the paramagnetic chemical shift is smaller (case b), but it can still range from +158 ppm to --79 ppm. This can have a huge impact on the calculated spin density in regard to the overall paramagnetic chemical shifts of these nuclei. However as the geometry is accidentally favourable in the present case, with angle close to 120° and weak value of cos 2 θ, the dipolar contribution remain limited.

Using the experimental data above, accurate estimates of the dipolar contributions between the 13 C nuclei of the cyanide and Fe(III) centre are obtained, the values being --12.05 ppm, --12.05 ppm and +19.06 ppm. The metal centered dipolar contribution arising from the interaction between 15 N atom of the cyanide ligands and Fe(III) ion amount to --3.58 ppm, --3.58 ppm and +4.46 ppm.

•••••• 13 C distance [Å] 1.919 1.911(6) -1.930(6) Fe•••••• 15 N distance [Å] 3.061 3.046 -3.072 B••••••Fe-C angle [°] 126.5 124.5 -127.6 B••••••Fe••••••N angle [°] 125.5 122.6 -127.1 ! !",!"#! !"# (Fe--13 C) [ppm]
--4.41 +19.69 / --12.05

! !",!"#! !"#
(Fe-- 15 N) [ppm] --2.71 +4.46/ --3.58 13C) [ppm] --10.36 +20.9/ --15.63

! !",!"#! !"#! ( 13 C) [ppm] --0.114 +1.21 / --0.94 ! !",!"#! !"#! ( 15 N) [ppm] --0.008 +0.072 / --0.09 ! !"!#$,!"#! !"# (
! !"!#$,!"#! !"# (15N) [ppm] --2.81 +4.53 / --3.67
In order to have a better view on the dipolar contribution, we have also calculated the ligand centered dipolar term (represented as

! !",!"#! !"#! ) for PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O compound.
The ligand centered dipolar contribution is relevant for T 2 or T 1 terms, where there is an extensive mixing of pure spin and orbital states via a spin--orbit coupling mechanism. The p--orbital ligand contribution to the molecular orbitals generally represents the ligand centered dipolar contribution. Therefore, fractional electron density on the p orbital of the the probed nuclei need to be considered. The ligand contribution to the pseudocontact shift is given by the equation 33

! LC,298K dip' = ! 16" 2 f 2 r !3 p 45kT 1+ 11 6V (1! e !(3/2)V " # $ % & ' 1+ 2e !(3/2)V " # % & (15)
where, ! !! ! is the average value of r --3 for the ligand p orbital, ! = ! !", and f is the fractional electron density on the p orbital of probed nuclei. Koh et al. reported the fraction of unpaired electron into the 2p orbital of the 13 C and 15 N nuclei. 36 The total amount of the dipolar contribution

[! !"!#$,!"#! !"# ] is then obtained from the sum of ! !",!"#! !"# and ! !",!"#! !"#!
.The obtained values for the ligand centered contribution (table 19) are very low and could be reasonably neglected.

The contact term is further deduced by substracting the dipolar contribution from the paramagnetic chemical shift. Since chemical shifts of paramagnetic species are strongly dependent upon temperature and every spectrum was acquired at a different temperature, direct comparison of raw data can not be performed. Assuming the Curie law is valid for these species near room temperature one can calculate the corresponding Fermi contact contribution to the chemical shift at 298 K: 32

! !"#,!"# !"# = ! !,!"# !"# -! !"# !"# ! 298 -! !"#,!"# !"# (16)
The spin density on the carbon and nitrogen atoms are then estimated using the following equation ( 17)

The It is worth mentioning that the EPR measurements are carried out at low temperature and thus reflect the electronic state of the compound at these temperatures. In contrast, NMR experiments are performed near room temperature. Therefore the probed magnetic state of the paramagnetic species is not exactly the same. Actually, the effective g factors may change with the temperature owing to, for instance, the spin population of higher states and phase transitions. An example is K 3 [Fe(CN) 6 ] whose g factor anisotropy is much smaller at 100 K than at 20 K. 8b The spin density in 3--5 have been calculated by using the equations described in the section 3.1. As previously mentioned, the dipolar contribution, ! !"#! !"# , has been calculated considering two extreme cases: the 13 C/ 15 N nucleus lying either along the M(III) main magnetic axis (! ! = 0°) or perpendicular to this axis (! ! = 90°). The ligand centered term was neglected for sake of simplicity in the calculation. Although, it has been observed that the dipolar contribution are small compare to the paramagnetic chemical shift, they are not negligible. The ! ! and ! ∥ values were obtained from the EPR at low temperature. The Fermi contact term was calculated as mentioned above by substraction of the dipolar chemical shift from paramagnetic chemical shift value. The Landé factor (g av ) at room temperature was deduced from bulk suceptibility measurement. The dimensionless fractional spin density in the 2S orbital of the cyanide carbon and nitrogen atom of these compounds have been derived by dividing the ρ 2S term with |ψ 2S (0)|2 term (given in a.u --3 unit). The f 2S term depends on the wave function |ψ 2S (0)| 2 , which must be known for the compound under study.

! 298K,iso con = µ 0 a 0 3 " e 2 g av 2 (S +1) 9k B T # is (0) ! 298K con ! 2s f 2s ! 298K con' ! 2s ' f 2s

Spin density deduced from other techniques

NMR technique is an indirect method to estimate spin density and the calculations are based on a series of approximations. In order to check its validity we thus need to compare the obtained results to other techniques. However there are only few experimental tools that allow to meaure the unpaired electron distribution. Polarized neutron diffraction is a direct method which allows to measured local magnetic moments (that can be further converted into spin densities) from diffraction patterns. 8a Howevere this technique requires large single crystals and heavy physical instruments. Moreover it does not allow to measure small amount of magnetic moments. X--ray magnetic circular dichroism (XMCD) is another powerful method that gives access to spin and orbital magnetic moments on metal ions. However, as for PND, it requires large instruments and it does not allow to detect small magnetic moments. Theoretical DFT or ab initio calculation are thus often used to get information about the spin density in paramagnetic compounds. In the case of

! 298K con,axial ! 298K con,equitorial ! 298K
!"#$ %#$ %&$ %'$ (&$ (#$ ('$ ()$ (*$ (+$
The spin density located at the cyanide carbon atoms is negative and it ranges from --0.032 a.u --3 to --0.051 a.u --3 (average value of --0.041 a.u --3 ). The spin density obtained for the cyanide nitrogen atoms is positive, smaller than that on the carbons, and ranges from +0.0078 a.u --3 to +0.0362 a.u --3 (mean value of +0.022 a.u --3 ). Besides DFT calculation revealed that the nitrogen Tp atoms bear a small amount of negative spin density. It thus seems that in the present case a bigger portion of spin is drifted towards cyanide ligands compared to the Tp ligand.

PPh 4 [Ru III (acac) 2 (CN) 2 ] [5]

Therotical estimation of the spin density on different atoms of this building block is already reported in the literature. DFT calculations were carried out using a 3--21G basis set for the ruthenium atom.

(b) Spin density through PND experiment

PND method provides access to the spatial distribution of the local magnetic moments in magnetic systems. 45,46 More precisely, in paramagnetic species this diffraction techniques provides the magnetization density ! M (r

!

) , induced by an external magnetic field. The comparison of the diffraction patterns of "up" and "down" polarized neutrons allows to obtain a set of experimental magnetic structure factors F M (h, k, l), which permits the determination of the magnetic moment density ! M (r ! ) in the whole space. The magnetization density is the sum of two contributions, namely, the spin density reflecting the spin distribution of the unpaired electrons, and the orbital momentum density, corresponding to the magnetic momenta created by the rotation of the unpaired electrons on their orbit around the direction of the applied magnetic field. Unlike XMCD techniques, the PND method is not able to separate these two contributions. Another drawback of the technique (beside the need of large crystals and polarized neutron source) is its low sensitivity to small amount of magnetic moments.

In the current study we observe indeed that PND is able to determine the spin density on the Fe(III) atom of the [Fe III (Tp)(CN) 3 ] --complex, but it fails in measuring the small magnetic moments located in the the C and N atom of the ligands. The spin densities obtained in Bohr magnetic moment can be converted into fraction of electron and atomic unit. The experimental mean spin density located in the cyanide carbon atoms amounts to --0.028 a.u --3 , but ranges from --0.021 a.u --3 to --0.041 a.u --3 . The average spin density obtained for the cyanide nitrogen atoms amounts to +0.036 a.u --3 , but ranges from +0.027 a.u --3 to +0.051 a.u --3 .

Discussion

It is observed by NMR that all the complexes exhibit negative spin density on their cyanide carbon 2s orbitals, and a positive one at the cyanide nitrogen atoms. This observation is in agreement with the PND study on 1 and the reported DFT calculations. This points to a dominating spin polarisation mechanism from the magnetic source to the cyanide carbon atoms as previously reported for hexacyanometallates. On the contrary, the positive spin density observed on the cyanide nitrogen atom is expected to arise from a delocalization mechanism (the unpaired electron is localized in a molecular orbital that is delocalized onto the ligand atoms).

It is worth noting that the polarization from the carbon to the nitrogen would also gives rise to a positive spin density on the carbon (as the spin density sign transmitted by polarization alternates. This is consistent with the fact that (i) the these partially--blocked cyanometalates have no unpaired electrons in d σ (e g ) orbitals (ii) a singly occupied d π (t 2g ) orbital interacts with the π* ligand orbitals with a bigger contribution on the N atom. The polarization mechanism of the M--C link is also consistent with the retained axial symmetry of the 13 C MAS--NMR spectra.

The precise attribution of the signals related to the enriched cyanides through NMR spectroscopy is not possible (in contrast with PND and DFT analysis). Only average values of spin density are thus obtained. The mean electron spin density located on the 2s orbital of cyanide carbon atoms of 1 is found to be ca. --0.020 a.u -- 3 . This compares well with the values obtained from PND and DFT analysis with average values of --0.039 a.u --3

!"#$#%##&' ($"$))%*+&' ($"$)*%*#&' ($"$,*%*)&' $"$+#%#-&' $"!$#%#*&' ($"$)*%#.&' ($"$./%#,&' ($"$-.%*!&' $"$+*%#,&'
and --0.028 a.u --3 on the cyanide carbon atoms. The mean electron density located on the the 2s orbital of cyanide carbon atoms of 3 and 5 are lower, ca. --0.011 a.u --3 and --0.0085 a.u --3 , respectively. In fact, the DFT calculation on 5 shows that there is more spin in the equatorial plane of the compound, than along the M--CN axis. The average spin density on the carbon atoms of the cyanide ligand amount to --0.011 a.u --3 which agrees well with the NMR data. The lower spin density value in 3 and 5 compared to 1 indicates that the propagation of the magnetic information by polarisation of the orbitals located on carbon atoms is likely less effective. Moreover, the DFT analyses reveale that the major portion of unpaired spin is transmited into the cyanide ligands in PPh 4 [Fe III (Tp)(CN) 3 ], whereas the major portion of unpaired spin is transmited to the acac ligands in PPh 4 [Ru III (acac) 2 (CN) 2 ]. The higher spin density found by DFT compared to NMR is probably due the fact that NMR probes electron spin density located in s orbitals through the contact term whereas DFT calculation considers the overall spin density on the carbon atoms. In absolute values, the electron spin density located in the 2s orbitals of the cyanide nitrogen atoms is smaller than that found in the 2s orbitals of the carbon atoms of the cyanides. In case of 1, 3, 4 and 5, the experimental mean spin densities found by 15 N MAS--NMR amount to 0.0027 a.u --3 , 0.0041 a.u --3 , 0.0027 a.u --3 and 0.0022 a.u --3 respectively. The higher spin density value in the cyanide nitrogen of 3 (compared to the other compounds) is reflected in its chemical shift value. The spin density value in PPh 4 [Fe III (Tp)(CN) 3 ] ranges from 0.0026 to 0.0028 a.u --3 . It is worth noticing that DFT calculations lead to more spread values, from 0.0080 to 0.0295 a.u --3 , depending on the cyanide. Overall, the value of the spin density obtained from DFT is large compared to the value extracted from MAS--NMR. This is as expected, since the main propagation mechanism of the unpaired electron to the nitrogen atom should be the delocalisation of a fraction of the spin density located on the iron ion to the antibonding π* molecular orbital, whose main contribution is located on the nitrogen. This electron spin density in turn polarises the 2s orbital of the same nucleus, which is the information obtained by the analysis of the isotropic chemical shift performed in this work. This is likely related to the symmetry change observed (compared to the diamagnetic reference) in the 15 N MAS--NMR spectra. In the cases of 5 the spin density value ranges from +0.0021 a.u --3 to +0.0023 a.u --3 . Once again, DFT analysis showed a higher average spin density of +0.009 a.u --3 . The other Ru(III) building block, PPh 4 [Ru III (salen)(CN) 2 ] exhibits a slightly higher spin density than 5 (on the basis of 15 N MAS--NMR analysis). MAS NMR spectroscopy of partially blocked cyanometalates provides information on the extension mechanism of the unpaired electron from the metal center to the carbon and nitrogen atoms of the CN ligands. Similar studies were carried out on the hexacyanometallates. The observed spin density value on the carbon and nitrogen atoms for the reported [Fe III (CN) 6 ] 3--were --0.0275 a.u --3 and +0.0064 a.u --3 respectively. In the current NMR analysis the spin density on the carbon atoms amount to --0.020 a.u --3 and --0.011 a.u --3 respectively for the PPh 4 [Fe III (Tp)(CN) 3 ] and PPh 4 [Fe III (pcq)(CN) 3 ]. The spin density observed on the nitrogen atoms are ca. +0.0027 a.u --3 and +0.0041 a.u --3 respectively for the PPh 4 [Fe III (Tp)(CN) 3 ] and PPh 4 [Fe III (pcq)(CN) 3 ] compound. The lower value of these complexes compared to [Fe III (CN) 6 ] 3--indicates a more favourable orbital overlap between the cyanide fragments and the iron ion in the ferricyanide coimplexes than the partially blocked Fe(III) complexes.

Conclusion

In this chapter we have tried to investigate how does the spin density transmit from the paramagnetic source to bridging ligand. The measurement of 13 C and 15 N isotropic chemical shifts have allowed estimating the fermi contact terms, allowing to follow the spin distribution on cyanide ligands. It has been experimentally shown that the polarization mechanism is efficient on the carbon atom whereas direct delocalization mechanism transmits positive spin density on nitrogen atoms.

Besides, chemical shift anisotropy (CSA) in these complexes have been studied and is likely correlated to the anisotropy in the unpaired electron distribution. There is a lowering of the CSA symmetry from axial (either oblate or prolate) to scalene one. This is particularly marked in the PPh 4 [Fe(Tp)(CN) 3 ] complex. It has been observed that the orientation of the local magnetic anisotropy deduced from PND studies could be related to the symmetry of the CSA tensor obtained from 13 C analysis.

The results on the other complexes are too partial to lead to final conclusions and complementary theoretical and experimental studies are required. One of our future aim is for example to perform more sophisticated ab initio calculations on the PPh 4 [Fe(Tp)(CN) 3 ] and on the other complexes to better investigate their electronic state and to compare this outcome with the NMR results.

The NMR analyses through the isotropic chemical shift do not provide direct information about the spin density in p orbitals. Therefore, direct comparison of NMR result with the DFT and PND results has to be taken with care. We are currently trying to extract the spin density arising from the p orbital of carbon and nitrogen atoms via CSA analysis. It should also be noted that NMR experiments are carried out at room temperature whereas the PND data are collected at low temperature and DFT analysis probed the low lying electronic state. Some discrepencies are thus expected as we are not necessarily probing the same electronic states.

Introduction

Solution NMR of paramagnetic complexes. The investigation of paramagnetic compounds with the help of nuclear magnetic resonance has been neglected during long time, mainly because of the technical problems associated with the measurement of species exhibiting fast nuclear relaxation. Nowadays the technological improvements of the spectrometers (eg. improved analog--to--digital converters, high precision RF control etc.) have made possible to record large family of paramagnetic complexes (provided that nuclear relaxation is not too fast). In general, however, the spectra of paramagnetic systems are not so well resolved as those of the corresponding diamagnetic compounds. For example nucleus--nucleus coupling is very rarely observed in such spectra. The half--widths of the NMR signals are between ca. 1 Hz and several thousand of Herz for dissolved paramagnetic species and the chemical shifts can extend over a range that is several orders of magnitude greater than that observed for the analogous diamagnetic compounds. Indeed the chemical shifts of paramagnetic species contains an important contribution from the so--called paramagnetic shift (see general introduction). This contribution does not directly depend on the chemical surrounding of the probed nucleus but actually it is correlated to the amount of unpaired electron felt by the nuclei under study. 1 The usual chemical shift NMR tables become thus useless for assigning the NMR signals of paramagnetic species. However using peaks integration, chemical substitutions, or other tools (see below) it is still possible in some cases -- although more challenging -- to identify the number and the nature of the species in solution. In a similar way as for the study of diamagnetic samples, it is thus possible to use NMR spectroscopy for identifying the nature of the molecules in solution or for studying chemical equilibria. 2 NMR of paramagnetic species can also provide magnetic information. Roughly speaking, as the chemical shifts bear information about the (local) magnetic susceptibilities it can be in principle used as an indirect tool for measuring magnetic susceptibilities in solution. 3 The most known approach for investigating magnetic properties through NMR is the Evan's NMR method. 4 In this approach the chemical shift of a diamagnetic reference (eg. the solvent) is used to indirectly measured the magnetic susceptibility of the medium by comparing its chemical shift in the presence and in the absence of the paramagnetic species (of known concentration). In Evans NMR it is thus the influence of the bulk susceptibility created by a dissolved paramagnetic species on the chemical shift of a reference diamagnetic species which is examined. 4a Another approach could be to directly investigate the chemical shift of a nucleus which seats on the paramagnetic species itself. This approach is particularly useful if the paramagnetic contribution arises mainly from the Fermi contact chemical, and not to the dipolar contribution (see chapter 1). From the pioneering work of Van--Vleck and Mc--Garvey, it is well known that the paramagnetic chemical shift is a measure of the local magnetic susceptibilty. 3 Temperature dependent NMR measurement can thus be used to probe the changes in the magnetic properties of the system in solution. 5 In this chapter, we propose to explore both approaches to investigate molecular magnetic systems that exhibit strong change in their magnetic properties with temperature. In recent times, strong research interest have been devoted to the study of switchable molecular magnetic systems which exhibit for example spin--crossover phenomenon, electron transfer coupled spin transition (ETCST), photomagnetism, etc. 6,7 These systems grab much attention because they have appeared as potential candidates for future molecular electronic devices. 6 In this present study, we selected a molecular magnetic systems which exhibit spin--crossover (SCO) behaviour. Before entering into further discussion we should remind what is spin--crossover? Spin crossover (SCO) is a phenomenon that occurs in some metal complexes wherein the spin state of the complex changes due to external stimuli. The spin crossover (SCO) phenomenon is generally observed for octahedral 3d metal ions with configuration from d 4 to d 7 . In O h symmetry these metal complexes can adopt two different electronic ground states depending on the splitting of the d orbitals in the e g and t 2g subsets. When the energy gap between these subsets (Δ) is greater than the inter--electronic repulsion energy (P) the electron tend to occupy the lower energy orbitals (t 2g ) and the metal complex is in a low spin (LS) state. In the reverse case, when Δ is smaller than P the metal complex adopts a high--spin state. For those complexes where Δ and P have similar values, the difference in energy between the HS and LS states (ΔE 0 HL ) is of the order of magnitude of the thermal energy (k B T). So when ΔE 0 HL ≈ k B T, the application of some relatively minor external perturbation effects (pressure, temperature, light--irradiation, electric field, etc.) can induce a change in the spin state. This is known as spin--crossover phenomenon. This means that the two spin state can be interconverted in a controlled, detectable and reversible manner, by the action of an external stimulus. The ΔE 0 HL term contains both entropic and enthalpic contribution but the so--called SCO phenomenon is an entropy--driven process. In a SCO complex, the LS↔HS conversion is associated to changes in all the properties that depend on the distribution of the 3d valence electrons, especially optical, vibrational, magnetic and structural properties. These changes allow to probe the occurrence of SCO phenomenon and monitor its temperature dependency. 7a--c The Fe(II) SCO complexes are of particular interest as the spin crossover implies important changes in both magnetic and optical properties of the complex. Indeed in this case the diamagnetic low--spin (LS) state, S = 0 (t 2g 6 ) is reversibly converted into a paramagnetic high--spin (HS) state, S = 2 (t 2g 4 e g 2 ). Moreover, the spin transition is here accompanied by significant reorganisation in the coordination sphere with notable variation of metal--ligand distance, ca. d Fe-- N ≈ 0.2 Å, that can favour cooperative effect in the solid state via intermolecular interactions. If the intermolecular interactions between the spin--crossover molecules are large enough, this leads to hysteresis effect. In solution the interactions between the molecules undergoing spin change are virtually negligible and the course of the transition can be described by Boltzmann statistics with a temperature dependent distribution over HS and LS vibronic state. The spin transition is thus described as a spin equilibrium and the thermodynamic parameters that are exclusively ascribed to the spin crossover process itself can be determined using techniques that allowed to determine the relative amount of LS and HS species at variable temperatures. 8 The SCO in solution can be followed by different techniques. 1 H NMR spectroscopy in solution at variable temperature was already shown to be a useful and sensitive method for detecting changes in electronic ground state in transition metal complexes.

Selected SCO complexes. During last few years, a number of cyanide--based polynuclear magnetic complexes featuring the bis(imidazolyl)ketone ligands (R--bik) have been investigated by our group. 9 Some of them containing the {Co(bik) 2 (N--) 2 } subunit (N--is cyanide--based metalloligand such as [Fe(Tp)(CN) 3 ] --, [W(CN) 8 ] 3--) show electron--transfer coupled to spin transition (ETCST), and others containing the {Fe(bik) 2 (N--) 2 } subunit exhibit spin transition (ST). 9,10 These systems are switchable molecules whose electronic, magnetic or optical properties can be reversibly switched by external stimuli (temperature change, pressure, light irradiation, etc) they appears as good candidates for designing future molecular electronic devices. As their insertion into advanced materials often require their solubilization in solution, NMR spectroscopy is not only usefull to investigate their magnetic behaviour but also their stability in solution. In this first work, we chose to primarly focus on the study of simple mononuclear Fe(II) complexes with molecular formula [Fe(R--bik) 3 ](BF 4 ) 2 in order to investigate how slight changes in the ligand could affect the switchable properties of the complex. These [Fe(R--bik) 3 ] 2+ complexes are reminiscent of the well known [Fe(phen) 3 ] 2+ and [Fe(bipy) 3 ] 2+ complexes (bipy and phen are the well--known α--diimine ligands 1,2--bipyridine and 1,10--phenanthroline) which are always LS. 11,12 In the present case, the R--bik chelate ligand form six members rings which induce weaker ligand field so that the resulting [Fe(R--bik) 3 ] 2+ complexes show LS--HS transition.

Scheme 1. bis--imidazolyl chelate ligands used in this work

In this work, we explore the use of variable temperature solution 1 H NMR to investigate the SCO equilibrium in the three [Fe II (R--bik) 3 ] 2+ complexes. The results are compared with the better known Evans NMR method and with solution SQUID magnetometry and variable temperature UV--vis spectrometry. The structure of the complexes have also been characterized in solid--state, and their (photo)magnetic behaviour investigated. In addition to that, DFT calculations have been performed to probe the structural and electronic alterations in complexes 1--3 that lead to the observed differences in the magnetic and photomagnetic behaviour of 1--3. B3LYP/TZV calculated HS--LS energy gap rationalizes the presence of SCO behaviour in complexes 1--3.

Solid--state study 2. Synthesis, structural and basic spectroscopic characterizations

Preparation of [Fe(V--bik) 3 ](BF 4 ) 2 (3)

To a colourless solution of V--bik (321 mg, 1.5 mmol) in a mixture of acetonitrile / water (4:1) (20 mL) is added a colourless solution of Fe(BF 4 ) 2 6H 2 O (169 mg, 0.5 mmol) in the same solvent (5 mL), causing an immediate colour change to blue. The solution is stirred for 30 minutes at room temperature and filtered. 2. In the three cases, the iron(II) metal centres are in N 6 coordination sphere. They are coordinated by three N,N--chelate--bonded R--bik ligands, resulting in a slightly distorted octahedral coordination geometry. The Fe--N distances range from 1.983(2)--2.008( 2) Å ( 1), 1.988( 13)--2.004( 13) Å ( 2) and 1.977(3)--1.995(3) Å (3). These bond lengths are typical of a low--spin iron(II) metal centre (S = 0). 15 It is worth noticing that other crystal structures with the [Fe II (Me--bik) 3 ] 2+ cation have already been reported with other counter anions. 16,17,18 The crystal structure of [Fe(Me--bik) 3 ](ClO 4 ) 2 measured at 298 K 16 reveals a high--spin iron(II) compound with an average Fe--N distance of 2.14 Å, whereas in the compounds [Fe(Me--bik) 3 ]Cl 2 17 and

[Fe(Me--bik) 3 ](OTf) 2 18 both measured at 150 K, a low--spin Fe(II) ion is found with an average Fe--N distances of 1.98 Å. These distances are coherent with those encountered in this work. In the present case the average Fe--N distance are ca. 1.99 Å, indicating the occurence of a low--spin Fe(II) ion at this temperature.

In (1), the 3). In contrast the bite angle in the well known LS [Fe II (phen) 3 ] 2+ complex, which exhibits five--member rings is much lower, 82.77°, and it results a distorted octahedral geometry of the complex. 12b

!"# $%# $&# $'# $(# $)# $*# !"# !$# !%# !$&# !%&# !"&# '(# complexes (Table
= { Σ [ w(F o 2 -F c 2 ) 2 ] / Σ [ w(F o 2 ) 2 ] } 1/2 R1 = Σ | |F o | -|F c | | / Σ |F o |
The sum of the deviation of the twelve cis--N--Fe--N angles from 90°, Σ, 19 often used to evaluate the distortion from octahedral geometry, amounts to 13.42°(1), 7.25°(2) and 20.33° (3). These data are in agreement with the low--spin nature of these six--member ring complexes. 15 This is in contrast with the situation in related five--member rings, such as in the LS [Fe(phen) 3 ] 2+ compound where the distortion parameter is 47.64°. The interplanar dihedral angles angles between the imidazole rings are quite different inside a same complex: 4.05°, 9.02° and 11.55°(1), 5.17°, 10.44° and 13.03 °(2) and 2.16°, 10.79° and 21.81°(3). This shows the flexibility of these ligands as compare to the rigid phen α--diimine. All these three compounds have the same average C=O distance, which amounts to 1.22 Å. However, significant differences in Fe--C--O angles are observed for the three comounds. While the Fe--C--O angle approaches linearity for 2 (ranges from 177 to 180°), it deviates significantly in 3 (ranges from 160 to 169°) and slightly in 1 (ranges from 173 to 178°) from linearity.

The geometry of the coordination sphere have been analysed by using continuous shape measure analysis (CShM). 20 The reported values help to determine the match between an idealized polyhedron and the actual coordination sphere. The analysis reveals that all three complexes exhibit only very small distorsion from the ideal octahedral coordination environment (Table 3 ). 20,21 Finally there is no important intermolecular interaction in 1--3, neither strong hydrogen bonding or π-π interaction (in agreement with the gradual spin transition observed for the three compounds, see below).

Nonetheless short contacts (interatomic shorter than the sum of the van der Waals radii) are found between C--H groups from methyl and ethyl and the imidazol ring plane, which are oriented vertically to those substituents. Here, the crystal packing of the complexes is thus most probably influenced by these CH•••π interactions between imidazolyl ring and alkyl proton. Such a CH•••π interaction is often referred to as a point--to--face or T--shaped arrangement 22 and their energy lie in the range of 1--2 kJ/mol. 23 In 1 and 2 the average distance of CH•••C(π) contacts are 2.78 Å and 2.77 Å respectively. These distances are quite significant as they lie at the middle of the accepted distance range for such supramolecular interaction (from 2.6 to 3.1 Å). 24 When looking at such interaction, it is also possible to look at the intermolecular CH•••ring--centroid interaction. Although it is more significant in 2 (2.67 Å) than 1 (2.92 Å), they remain both cases within the accepted distance range.

electron cloud (Figure 2). Whereas, in compound 1, different Me--bik ligands from two different molecules are involved in donation and acceptance respectively. In compound 3 no significant CH

--H•••(π) contacts of the SCO complexes [Å] Compound [Fe(Me--bik) 3 ](BF 4 ) 2 0.5H 2 O (1)
[Fe(Et--bik

) 3 ](BF 4 ) 2 (2)
[Fe(V--bik The three spin--crossover complexes consists of [FeN 6 ] coordination sphere which can be also found in widely studied [Fe II (phen) 3 ] 2+ (1,10--phenanthroline) or [Fe II (bipy) 3 ] 2+ (2,2'--bipyridyl) complexes. 12 The bipy-or phen--containing trischelate Fe(II) compounds are LS in nature at any temperature whereas the R--bik containing these three compounds exhibit SCO behavior (see below). This difference may arise from the different chelate ring size. Ligands like bipy or phen form more stable 5--membered ring with 3d metal centres whereas R bik ligand forms 6--membered ring. The M--L bond may thus be weaker in the case of R--bik complexes however the measured M--L distance is very similar. Another important effect to consider is of course the aromatic system of the bipy ligand which favor pi--back donation and strong ligand field. The R-bik igand are aromatic too, but the imidazolyl ring exhibit a dihedral angle that break down the aromaticity. The distorsion of the ligand is also noticeable in the Fe--C--O angle which deviates from linearity.

) 3 ](BF 4 ) 2 (3) Shortest average intermolecular C--H•••C(π) contacts, a,b
90 ! ! 1 12
"

Solid--State FT--IR Spectroscopy

The spectra of all the compounds were recorded at room temperature in the ATR mode. They exhibit characteristic vibrations that are listed in table 5. From the IR spectra of these three SCO complexes, it is observed that upon coordination of the R--bik ligands to a Fe(II) center, no significant shift of the carbonyl stretching vibration occurs. The carbonyl stretching vibration is almost identical to that in the free ligand. Some sharp characteristic vibrations below 1170 cm --1 indicate of the presence of non--coordinated tetrafluoroborate anions. 25

Thermogravimetric analysis

The thermogravimetric analysis (TGA) experiments were carried out on 1, 2 and 3. Crystals of these compounds have been filtered just before the experiment as they rapidly lose their crystallization solvent molecules in air. The experiments were performed under nitrogen flow up to 130 °C, with heating rates of 2 °C /min. Compound 1 undergoes a continuous weight loss between 20 and 130 °C of approximately 1.3 %, which matches well with that expected for the removal of 0.5 crystallization water molecule (according to single-crystal XRD study). The compound is then stable under N 2 atmosphere up to 140° C. 2 exhibit a very small weight loss (ca. 0.5%) that could correspond solvent molecules adsorb to the crystal surface. A very small step is also observed near 80°C (and could correspond to crystallization solvent) although no traces of solvent molecules were found from the XRD analysis and elemental analysis. Compound 3 undergoes a first weight loss near room temperature due to adsorb solvent on the crystals. The second one near 50 °C (ca. 2%) and a third one between 95 and 130 °C (ca. 1%), could be due to the removal of ca. 1 and 0.5 crystallization water molecules (1 molecule of water correspond to 2% of the total sample mass) respectively. We assume that the first weight loss is due to the removal of water molecules that are weakly bonded either with the [Fe II (Vinyl--bik) 3 ] 2+ units or with the BF 4 --counter anions and the second loss originates from the water molecules that are strongly supported. Due to the limited quality of the XRD data, no H--bonded solvent molecules were observed however the elemental analysis showed the presence of 0.5 molecules of water water molecule. The observed water molecules in elemental analysis were most probably the strongly H--bonded ones. Therefore the resultant formula of the compound considering XRD, TGA and elemental analysis data is [Fe(V--bik) 3 ](BF4) 2 1.5H 2 O.

1, [Fe(Me--bik) 3 ](BF 4 ) 2 2, [Fe(Et--bik) 3 ](BF 4 ) 2 3, [Fe(V--bik) 3 ](BF 4 ) 2

Variable temperature optical study in solid state

Solid--state UV--Vis spectra were measured in the range of 200--1200 nm on a CARY 5000 double--beam spectrophotometer equipped with the Eurolabo variable--temperature cell (21525, quartz windows) and Specac temperature controller. The measurements were performed on KBr pellets (approximately 2 mg of fresh crystallites of compounds are dispersed without any grinding in ca. 99 mg of KBr, this latter being previously ground). This sample preparation aimed at minimizing the formation of crystalline defects that alter the SCO characteristics. Absorption spectra were recorded within the temperature range 100 --450 K.

The below figure represents the spectral evolution of this three compound between 100 and 450 K. The band observed around ca. 615 nm (1), 624 nm ( 2) and 630 nm (3) at low temperature can be assigned to the metal to ligand charge transfer (MLCT) band of the Fe(II) chromophores (Fig 4). This MLCT transition is responsible for the deep colour of the compound. Upon heating up to 450 K, its intensity decreases markedly and a blue shift is observed. The decrease in intensity upon heating is due to the spin crossover phenomenon, the high--spin species being dominant above ca. 350 K. Indeed, the larger metal--ligand bond length in the Fe(II) high--spin state results in a weaker overlap between metal--centred and ligand--centred orbitals as compared to the low--spin state. 15a Therefore, the intensity of the MLCT band strongly decreases upon converting the LS into HS species. Conversely there is an increase of the intensity around ca. 452 nm, 461 nm and 481 nm respectively for 1, 2 and 3, which is associated to the formation of HS complex. 26 This change in the optical properties of the compounds is coherent with a thermally--induced spin--state switching of the Fe(II) ion centered nearly around (275--350K). Finally the very intense band located below 400 nm is likely due to the intra--ligand charge transfer (ILCT). Some additional low temperature discrepancies observed in the NIR region are assigned to some variations in the baseline upon recording the spectra. 26

Study of magnetic properties of 1--3 4.1 Bulk magnetic properties (spin transition) in solid state

Magnetic susceptibility data were collected using a Quantum Design SQUID magnetometer (MPMS5S Model) calibrated against a standard palladium sample. The magnetic susceptibility values were corrected from the diamagnetism of the molecular constituents and of the sample holder. The measurements in the solid state were carried out with fresh crystals of 1, 2 and 3 in the temperature range 4--400 K. Magnetic field of 1 T was used for the study.

The χ M T vs T curves are depicted in the figure below (χ M being the molar magnetic susceptibility per Fe(II) ion). The χ M T product at low temperature (ca. 4--175 K) for fresh samples of 1, 2 and 3 are ca. 0.15, 0.06 and 0.01 cm 3 mol -1 K respectively. These values are in agreement with a S = 0 low--spin state for the Fe(II) ion. Upon warming the χ M T value increases gradually from 180 up to 400 K. reaching ca. 3.11 (1), 3.40 (2) and 2.80 (3) cm 3 mol -1 K at 400 K. These values are lower than those expected for an S = 2 high--spin state for the Fe(II) ion (with χ M T = 3.6 cm 3 mol -1 K and g = 2.

2) and could be attributed to incomplete spin transition, which implies the occurrence of some residual fraction of LS Fe(II) even at 400 K. 15 This behaviour is reproducible upon cooling (and over several cycles) for 1 and 2. In contrast for compound 3 the system reverts upon cooling to the diamagnetic state following a different path than in the first heating run. 3 exhibits thus an irreversible change probably due to the loss of crystallization molecule (evidenced from TGA analysis, see above). This assumption was further verified by measuring magnetic susceptibility on a previously dehydrated sample of compound 3. The χ M T curve is then the same as in the cooling mode and reversible upon cycling. 18 The T 1/2 of the SCO process (according to bulk magnetic susceptibility measurement) for these compounds are ca. 316 K (1), 277 K (2), 342 K (3, fresh) and 302 K (3, dehydrated). The steep nature of the χ M T curve of compound 2 is likely correlated with the stronger intermolecular interactions. XRD analysis of this compound revealed the presence of significant supramolecular CH•••π interaction. This kind of interactions between the SCO molecules cause an increase in cooperativity into the system which, in turn, is responsible for the steeper nature of the curve. 7 The χ M T vs T data were fitted by using a regular solution model (expressed by the following equation) in order to extract the thermodynamic parameters: 27 ln (1! n HS ) n HS ! f Hs ( )

" # $ % = &H + '( f HS +1! 2n HS ) [ ] RT ! &S R
where ΔH, ΔS, and Γ are the enthalpy and the entropy variations and the parameter accounting for cooperativity associated with the spin conversion, respectively. The parameter f HS accounts for the HS molar fraction at low temperature and n HS represents the HS molar fraction. 3) J K --1 mol --1 and Γ = 0 (1) 0.05, 0.21 (2) 0.0 (3) kJ mol --1 respectively to the heating and cooling cycle of these three complexes.

(χ M T) HS was kept fixed at 3.4 cm 3 K mol --1 for 1 and 2, which is consistent with the 94.4 % of HS species at 400 K. In case of 3, which shows incomplete transition, the (χ M T) HS was fixed at 2.95 cm 3 K mol --1 The thermodynamic values may suffer from errors due to the incomplete nature of the spin transition. More importantly, in 1 and 3 the solvent loss during the measurement makes the solid state fit (see annex section for curve--fitting plots) less accurate. Indeed the intermolecular interactions that play a role in cooperativity are changing upon heating. In contrast, the simulation obtained for compound 2 (which does not contain crystallization molecules) matches well with the experimental result and it seems to be reliable. The solid state fit plot of these compounds are given below.

Photomagnetic properties in solid state

Photomagnetic measurements were carried out on fresh and dehydrated samples by using a sample holder equipped with an optical fibre. In a typical experiment 0.5 mg of finely ground crystals are deposited on an adhesive tape. The sample is separated from the end of the fibre by 5.5 cm. All the irradiations are carried out at 20 K using laser diode (7 m W cm --2 ) in the visible range. The temperature was set at 20 K in order to minimize the temperature variation induced by switching the light on / off.

A light--induced excited spin state trapping (LIESST) effect was observed on fresh samples of 1, and 3. In contrast 2 does not exhibit any photo--induced magnetization upon irradiation in the visible range at 20 K. Fresh sample of compound 1 undergoes a significant photomagnetic effect with the four different laser sources covering the visible range but the increase of χ M T product is much higher when using a 635 nm laser source. The value of χ M T reaches 2.39 cm 3 mol 1 K after 50 min (Figure 6). This value is lower than that expected one for a Fe(II) high--spin complex. It suggests a partial conversion of the diamagnetic state into a photo--induced metastable paramagnetic state. For compound 3, a weaker photomagnetic effect is observed. The compound undergoes a slight increase of the magnetization upon irradiation. The biggest effect is observed at 635 nm, but the value of χ M T reaches only 0.45 cm 3 mol 1 K after 50 min of irradiation (Figure 6). It is worth noticing that the maximum LIESST effect was observed in case of 1 and 3 with the 635 nm wavelengths, which falls in the region of MLCT transition.

The photomagnetic measurement on dehydrated samples are efficient only for compound 3. In contrast with the fresh compound, the dehydrated phase of 3 exhibits a maximal LIESST effect upon irradiation with the 808 nm laser. The value of χ M T amounts to 1.48 cm 3 mol 1 K after 50 min of irradiation (Figure 7). This value clearly indicates a partial conversion of the diamagnetic state into a photo--induced metastable paramagnetic state. The thermal relaxation of the photo--induced state has been probed for fresh samples of 1 and 3. After turning off the laser, the sample is warmed up from low temperature to 85 K at 0.3 K min -1 . The slight decrease in χ M T values observed on the relaxation curves at low temperature could be due to the zero--field splitting or to intermolecular antiferromagnetic interactions.The plateau observed at higher temperature HS conversion but only in the dehydrated phase. 9c For the {Fe III 2 Fe II 2 } square, the most efficient photomagnetic effect occurs in the 700--900 nm range whereas for the {Mo v 2 Fe II 2 } square it is observed at 405 nm. Significant differences are also observed on the relaxation temperature, which are lower in the square complexes (T LIESST = 45 and 48 K, respectively). These results show how critical is the role of the N--donor atom on the photomagnetic process of the {Fe(bik) 2 (N--) 2 } subunit (eg by modifying the nature of the excited state). It thus shows a stronger dipolar relaxation which results in a broadening of its spectral signal. Indeed the major part of the relaxation arises from the dipolar contribution which is proportional to 1/r 6 (where r is the distance between the metal centre and the NMR nucleus). 1 The other signal at 21.34 ppm is thus attributed to the proton H b .

Four different signals of the imidazole ring with the relative intensity 1:1:2:3 are observed for compound 2.

The sharp signal around 2.93 ppm (d) with relative intensity 3H is assigned to the CH 3 group of the ligand. The broad signal near 3.99 ppm is assigned to H a whereas H b is observed at 20.22 ppm. Interestingly, an unusual behaviour is observed for the methylene protons of the ethyl group. The signal of the CH 2 is splitted into two peaks, (c) and (c'), that are observed at 8.69 ppm and 7.73 ppm with the relative intensity 1:1. Indeed the Fe(II) trischelate complex is chiral and these two protons are diastereotopic. 34a--b The diastereotopic methylene hydrogens appear as a doublet only below ca. 320 K (as their exhange is slow as compared to the NMR time scale). Upon increasing the temperature, coalescence is observed indicating a dynamic exchange process with a relatively low activation energy. 34c The full range variable temperature behaviour of these diastereotopic protons is further described in the section 6.2.

In the case of compound 3, five signals of the imidazole ring with same intensity are observed at 310 K. The three vinylic protons are assigned according to their coupling constant value and considering that J trans > J cis > J geminal for the vinylic system. Two doublets around 7.20 ppm (d)

Magnetic susceptibility in solution through NMR

Our goal is to explore the use of NMR spectroscopy to investigate the HS--LS spin equilibrium of our three model complexes in solution. More precisely we wish to determine the temperature dependence of the relative amount of LS (or HS) species, in order to extract the thermodynamic quantities (ΔH and ΔS) relative to the spin equilibrium in a given solvent. The measurement of the magnetic susceptibilities versus temperature is often used to determine the amount of high--spin species in solid--state. It can also be carry out on solution although it is more complicated from an experimental point of view. As the spin equilibrium is often accompanied by changes in the optical properties, it can also be studied by measuring the temperature dependence of an absorption band, which is characteristic of one of the two spin state. In the following section we compare the different techniques including VT 1 H NMR, Evans' NMR, solution state SQUID magnetometry and electronic absorption spectroscopy to investigate the spin--equilibrium of 1--3 in acetonitrile solution.

Determination of magnetic susceptibility through Evans' NMR a) Principle.

The Evans' NMR method provides an indirect access to the magnetic susceptibility of paramagnetic solutes in a diamagnetic solvent. More precisely, it is based the measurement of the frequency shift (Δδ) that the diamagnetic solvent undergoes in presence of paramagnetic species. It represents an alternative to susceptibility measurements through magnetometry methods which are usually --and more easily-- carried out on solid--state samples. Therefore one can extract γ (amount of HS mole fraction) and thermodynamic parameters related to SCO equilibrium from this measurement. This NMR method is simple and quite reliable, however the accessible temperature range is more limited than that usually explored in magnetometry (typically 200--400 K for NMR instruments equipped with a variable temperature set up). In practice, a special double walled NMR tube is used. The solution of the paramagnetic complex is placed in the inner narrow--bore tube, and the solvent is poured in the outer tube, which is a regular NMR tube (see Figure 10). The separation between the two solvent signals, Δf (measured in Hz), is proportional to the bulk magnetic susceptibility of the solution. The mass susceptibility or gram susceptibility (c) of the dissolved paramagnetic substance is given by the expression: 4 (1

)
where f is the operating radio frequency of the spectrometer, m is the concentration of the paramagnetic ion in the inner tube in g cm --3 , χ 0 is the gram susceptibility of pure CD 3 CN solvent, d 0 and d s are the density of the pure solvent and paramagnetic solution respectively. 35 For highly paramagnetic substances the last term can often be neglected without serious error. 4 In Evans' equation, 2π/3 represents the shape factor of the magnet which refers to an iron core magnet (sample axis perpendicular to the magnetic field). In our present study a cylindrical sample in a superconducting magnet (sample axis parallel to the magnetic field) was used. Therefore the shape factor becomes 4π/3. 4,36 The gram susceptibility is calculated using the following equation:

and the molar susceptibility value is calculated in the usual way:

! M = ! ! M.W. ( 3
)
finally, χ M is then corrected for the diamagnetic susceptibility calculated from the Pascal tables. 37

b) Experimental study on 1--3.

In the present study, the experiments were carried out on a Bruker Avance II 300 spectrometer from --43 to +57 °C (230 --330 K) in acetonitrile--d 3 solvent by using the CD 2 HCN residual proton signal as a reference.

Here or below. The temperature was limited to +57 °C because of the evaporation of the CD 3 CN solvent from the double walled NMR tube. 8,38 The solutions concentration of the paramagnetic samples were fixed to 10 --2 mol L --1 to obtain signals with noticeable intensity. The resulting NMR spectra exhibits two solvent peaks near 2 ppm: one is due to the CD 3 CN in the outer tube (which occurs at 581.91 Hz) and the other is due to the paramagnetically shifted CD 3 CN signal from the inner tube (Figure 11). The signal arsing from the inner tube is present on the left side (higher chemical shift) of each spectrum compare to the pure CD 3 CN (outer tube).

! = 3!f 2" fm + ! 0 + ! 0 d 0 " d S ( ) m ! = 3!f 4" fm + ! 0
The gram susceptibility of the pure actonitrile solvent (χ 0 ) is already reported in the literature and it amounts to --0.534 X 10 --6 g cm --3 . 38 The frequency of the spectrometer is (f) 3 x 10 8 Hz. At 330 K, the measured χ M T product of 1, 2 and 3 are 2.15, 2.24 and 1.64 cm 3 mol --1 K, respectively. A gradual decrease of the χ M T product was observed upon cooling. The results obtained from the variable temperature Evans' NMR indicate the occurence of a SCO process in solution, which starts at ca. 280 K and which is incomplete at the 320 K for the three compounds. The data are not accurate enough to establish reliable value of spin-equilibrium transition, T 1/2 . In the case of 2, the solution magnetic susceptibility is surprinsingly high at low temperature. This could be due to the presence of a paramagnetic side product, which can not be detected by 1H NMR because of fast nuclear relaxation. A possible oxidation of the Fe(II) complex might be responsible for the high susceptibility value measured in the present case, however we never observed sign of oxidation in this family. We thus assume that the paramagnetic impurity has another origin. Actually, the presence of undetectable paramagnetic impurities is one of the drawback of the method. 39 Another inconvenient of the Evans'method arises from the limited available temperature range. In the present case, low temperatures can not be investigated since the two probed signals overlap below 230 K (or below 250 K for the V--bik derivative). Morover, below this temperature the freezing temperature of solvent is reached. However the evaporation of the solvent above 330 K is the main limitation for studying the SCO phenomenon in 1--3. Beside these limitations, the accuracy of the results suffer from several approximations. The quality of the results first depends on the accurate knowledge of the concentrations.

Considering the small quantities of weighed sample used, very careful work is necessary. The method also requires in principle accurate determination of Δf whose measurement error is ca. 5%. 33 ! M !10 "3 (cm 3 mol "1 ) ! M corr !10 "3 (cm 3 mol "1 ) ! M corr T (cm 3 mol !1 K ) Evans'method gives access to a bulk susceptibility so that undetectable paramagnetic impurities that contribute to the measured susceptibility are source of error, as in the case of 2. An alternative method, which consists in studying the temperature dependence of a 1H signal of the SCO complex itself will be presented in the following section.

Variable temperature 1 H NMR (a) Introductive remarks

In a paramagnetic system, the observed chemical shift can be expressed as a sum of paramagnetic and diamagnetic chemical shift. The paramagnetic chemical shift ( ) in solution can further be described as a sum of the pseudocontact term (δ PC ) and the Fermi--contact term (δ FC ): 1

The term is determined by the through--space electron--nuclear interactions and referred as dipolar coupling between the nuclear magnetic moment and the magnetic moment of the unpaired electron treated as a point dipole. It can be expressed through the following equation: 1,3 (5)

The Fermi--contact term is a through--bond electron--nuclear interactions. It arises from the electron spin delocalization onto the nucleus. The Fermi contact shift is determined by the following relationship: 3,40 (6) As mentioned by McGarvey, the Fermi contact term can be expressed as a function of magnetic susceptibility. In fact it is directly proportional to the components of magnetic susceptibility tensor. 3 (7) Hence in paramagnetic complexes, the chemical shift can be treated as a local magnetic susceptibility. This is however true only if the Fermi--contact shift dominates over the pseudo--contact term (or dipolar term). Therefore, by performing variable temperature NMR study on a spin transition complex in solution and by studying the temperature dependence of the chemical shifts, one can follow the spin crossover process, i.e. the conversion of diamagnetic to paramagnetic species or vice versa. 33 This approach has been used for example by Weber and Walker who showed that it is a powerful complement to Evans' method for studying Fe(II) SCO complexes. 33,39 As the Fe(II) spin--state switching is faster than the NMR timescale, the observed proton NMR resonances represent the population--weighted average of the HS and LS states of the molecule at a given temperature. Peak shift can thus be related to amount of high--spin species and thus compared to Evans' method. Thus one can roughly write: (8) The accurate determination of the δ values for a purely HS or LS species is very helpful, but it is not always possible in a large temperature range (because of experimental limitation and/or solvent boiling or freezing point). In the present case the explored temperature range is 230 K to 350 K.

(b) Experimental Results

General Observations.

For 1--3 we observe that analyte concentrations of 10 --2 M are necessary to get acceptable intensities for the paramagnetically shifted/broadened signals. Since both diamagnetic and paramagnetic species may be simultaneously present, it is important to probe the "normal" 1 H window (standard ≈12 ppm) as well as a much larger frequency range. Signal ranges may drastically vary for spin--crossover complexes but a 100 ppm range is sufficient for the present case.

The figures 13--15 represent the variable temperature 1 H NMR spectra of 1--3 in CD 3 CN. The first important observations are the following ones:

(i) the three compounds seems to be stable through the explored temperature range;

! PC = µ 0 4" #S(S +1) 9kT 3cos 2 $ !1 r 3 (g II 2 ! g " 2 ) ! FC T = 2" A H #gS(S +1) 3$ N hk B T ! FC T = ! A H 3 " N 2# ( ) $ % xx g xx + % yy g yy + % zz g zz " # $ $ % & ' ' % HS = ! HS !! LS ( )
! LS "100

(ii) the three complexes are the only detectable paramagnetic species and they represent the major species in solution. No by--product are observed except H 2 O, THF, and some free ligand in the case of Et--bik (see details below);

(iii) the temperature dependence of the chemical shift supports the occurrence of a spin--state equilibrium as shown by the fact that the chemical shifts of the observed protons clearly increase upon heating. This is the contrary of the usual behavior observed for standard paramagnetic species whose paramagnetic chemical shift depends roughly on 1/T (Curie law). Effect of the spin equilibria on the chemical shifts.

The strong chemical shift and their signal half--width indicate clearly that paramagnetic species are present at high temperature. 20 The chemical shifts gradually move toward the diamagnetic region upon cooling and are found close to the values of the free ligand at 230 K. Heating of the sample results an increased population of the high--spin species which conversely results paramagnetic shifting of the signals. The evaporation of the solvent above 350 K prevent the study at higher temperature.

Focusing on the H b protons of all of the three complexes we can observe that their chemical shift increase following a sigmoidal pattern as the temperature increases. This particular proton exhibits the highest chemical shift value at high temperature. Here, the Fe(II)--H b distance is ca. ≥5 Å so that the pseudo--contact shift can be reasonably neglected. The observed chemical shift is thus mostly due to the Fermi--contact shift (in particular at high temperature), which itself largely depends on the spin density on the nucleus under study. It is thus quite directly related to the local magnetic susceptibility and can be conveniently used as a probe to follow the thermal population of the HS state (see below). The H b protons, which display sharp signal and exhibit the highest chemical shift variation with temperature will be thus selected for the further variable--temperature studies that allow to estimate the thermodynamic parameters associated with the spin--crossover equilibria in 1--3. distance is ≥3 Å leads to a stronger pseudo--contact shift, which is expected to show a decrease as the temperature increases. In the other hand, the Fermi--contact shift is not negligible and show the opposite trend as the ratio of HS molecule increases (with the increase in temperature, the H a proton experiences more spin density transmitted through the chemical bonds from Fe(II) centre resulting in an increase of its chemical shift). For all the other protons in these three complexes, a gradual increase of the chemical shift is observed as the temperature increases. All of them are far apart from the Fe(II) centre: the Fe II --H distance well above 5 Å results in negligible pseudo--contact interaction. increases from 230 K to 310 K as the compound becomes more paramagnetic. This phenomenon can be explained by considering the "magnifying glass effect" on the NMR spectrum. 41 Actually, the presence of an increasing amount of delocalized spin density on the ligand (as the temperature increases and the number of HS molecules increases) induces a striking signal spread such that the resolution can be notably improved compared to the diamagnetic signals. Indeed, in the low temperature range (230--260 K) the compound is diamagnetic and as a result the separation between the signals is considerably reduced. At 240 K, only one signal is observed at 300 MHz. As for standard diamagnetic species, experiment at higher magnetic field (600 MHz 1 H NMR) in the low temperature range (230--290 K) allow to improve the resolution and to distinguish the two overlapping signal. Finally, at high temperature (above 310 K) the exchange between the two chiral complexes becomes fast and the two diasterotopic protons show The observation (ii) we made above which state that only one paramagnetic species is observed in solution could appear contradictory with the measurement of a significant paramagnetic contribution at all temperature in the case of compound 2 (see Evans'method). As the amount of free ligand is non negligeable in that case (the 1 H NMR experiment is reproducible and gives always the same result), we assume that the [Fe II (Et--bik) 3 ] 2+ may show a chemical equilibrium with the paramagnetic [Fe II (bik) 2 (S) 2 ] 2+ complex (S = solvent), which is expected to be high spin. Indeed this complex (that we previously used as building block to prepare polynuclear species) is stable in acetonitrile and NMR silent. Indeed the recorded 1 H NMR spectrum of in situ prepared [Fe II (Et--bik) 2 (CH 3 CN) 2 ](BF 4 ) 2 complex, shows only the signals of the [Fe II (Et--bik) 3 ] 2+ species and traces of free Et--bik ligand. It is also worth noticing that the mass spectrometric data also show the presence of a significant amount of free Et--bik ligand in the case of 2, in contrast with 1 and 3.

A closer look at the variable temperature NMR spectra of 2 shows that the presence of free Et--bik is only clearly observed within 230--290 K. At a first glance one could imagine that the peaks vanish at high temperature (>300 K) but actually they get broader above 300 to 350 K. There is indeed a drastic change in the spectral half--width of the free Et--bik ligand signals within 10 K between 290 K and 300 K. This feature is not due to an experimental artifact as the acquisition and proceeding parameters are the same, so as the sample. The experiment has been repeated several times leading always to the same observation. The broadening of the signals in NMR spectra generally arise from ( 1) the presence of paramagnetic species, and/or (2) a chemical exchange between different species in solution whose kinetics is comparable to the NMR time scale. It is worth noticing that in the present case the spin--crossover process is centered around 290--320 K (from Evans' method and VT 1 H NMR result). Therefore the drastic change in the line width of the free Et--bik ligand which occurs above 290 K is likely correlated to the chemical equilibrium: [Fe II (Et-bik) 3 ] 2+ = [Fe II (Et--bik) 2 (CH 3 CN) 2 ] 2+ + Et--bik, and the ligand exchnage in the coordination sphere of the Fe(II).

As the [Fe II (Et--bik) 3 ] 2+ paramagnetic form becomes dominant (above T 1/2 ), the signals of the Et--bik broaden.

Comparison between the NMR methods.

VT NMR method probes the temperature dependancy of all protons in the paramagnetic species. In the other hand Evans' method monitors the thermal behaviour of the solvent. Therefore susceptibility obtained from VT NMR method is more "local" in nature. Overall, the variable temperature 1 H NMR study appears as an interesting alternative to the Evans' NMR method in order to follow the spin--crossover phenomena. Indeed the VT NMR study (δT vs T measurement) appears to be more efficient over Evans' technique as: i) a wider temperature range can be explored;

ii) The chemical shift variation of adequately chosen 1 H protons of the SCO complex are much more important than those observed on spectators molecules (the solvent in Evans method), so that the measurments of the SCO equilibrium can be more accurate; (c) the behaviour of the SCO complex can be directly probed without being perturbated by the presence of paramagnetic side--product.

iii) The quantity of paramagnetic sample used in VT NMR is higher than in Evans' method, therefore it consits of less error related to weight or impurities of the sample.

In adition to that, the variable temperature 1 H NMR study provides additional magnetic and chemical information. Probing each protons behaviour in VT NMR provide an overview on the delocalization of the unpaired electron in the molecule. On the other hand, information on possible chemical exchange (see the unusual behaviours mentioned above) can be be observed in VT NMR study.

Bulk magnetic susceptibility of solutions through SQUID magnetometry

SQUID magnetometry is another possible method to follow SCO process. It is spreadly used to study solid-state sample but it can also be used to study solution. In that case however, the diamagnatic contribution of the sample holder and the solvant is high compare to the paramagnetic contribition of the sample. Compare to Evans' method the SQUID magnetometry still offers higher sensitivity and accuracy. It allows to measure small sample quantity and more dilute solutions. However as for the Evans' method, SQUID measurements probe bulk magnetic properties which can partly arise from the presence of magnetic impurities. Sample analysis also requires the mass to be known accurately. A wider temperature range is available in the SQUID magnetometer. However aggregation problem related to low solubility (crystallization or precipitation) are possible. Recent reports illustrate the use of SQUID measurements to study frozen solutions, including a study of the effect of solvent variations on the spin--transition of one complex. 30 The magnetic susceptibility of 1, 2 and 3 were measured under a magnetic field of 1 Tesla in the temperature range 220--350 K in CD 3 CN. The study was carried out in CD 3 CN solvent to have a better correlation with the NMR study. The diamagnetic correction was estimated by measuring the magnetic susceptibility on CD 3 CN solution. The χ M T vs T curves of 1, 2 and 3 exhibit a similar shape as that fround in Evans'measurement that account for a spin--equilibrium, starting from ca. 280 K and uncomplete at 350 K. The χ M T product for 1, 2 and 3 at 330 K are 2.00, 2.64 and 1.61 cm 3 mol --1 K --1 respectively and they compare quite well with those found by Evans' method, with χ M T value of 2.15, 2.24 and 1.64 cm 3 mol --1 K --1 respectively for 1, 2 and 3 at 330 K.

Figure 19. Thermal dependence of χ M T for three complexes (1,black; 2,red; 3,blue)

Variable temperature optical study in solution

Electronic absorption spectroscopy represents another possible mean to investigate thermally--induced spin crossover equilibria. For Fe(II) SCO complexes, the HS state is usually less absorbing than the LS one. Indeed the intensity of the metal to ligand charge transfer (MLCT) in Fe(II) SCO complexes vary markedly with the spin state. UV--visible investigations of spin crossover behaviour are potentially more sensitive than Evans method as more dilute solutions can be used to track the spin state of the complex (as the MLCT absorption coeficient is high). However in the present solution UV--Vis study the explored experimental temperature range is much narrower than that of the solid state. The freezing of the solution prevents the investigation below ca. -43 °C (230 K) and boiling of the solvent does not allow to go beyond +81 °C (354 K). The spectra were recorded in the temperature range of 264 --349 K with an interval of 5 K. The concentration of the measured solutions were ca. 5 x 10 --4 mol L --1 (1), 2 x 10 --4 mol L --1 (2) and 1 x 10 --4 mol L --1 (3). The variable temperature spectra in the range 350 nm -- 800 nm are depicted in figure 20. The intense bands with maxima at ca. 594 nm ( 1), 598 nm ( 2) and 618 nm ( 3) are ascribed to metal--to--ligand charge-transfer (MLCT) band. The intensity of the MLCT bands decreases with increasing temperature. This feature is commonly explained by a less favourable orbital overlap due to longer metal-ligand bonds and it clearly points to a conversion from the low--spin to the high--spin state. 27c Isobestic points are observed (near 784 nm and 400 nm in 1, near 756 nm and 392 nm in 2 and near 785 nm and 392 nm in 3) and they varries also with temperature. The observed molar extinction coefficients, ε, can be expressed as a function of the real extinction coefficient of the HS and LS states taking into account the relative fraction of HS and LS (equation 9). ( 9)

The plot of the molar extinction coefficients ε at (594 nm ( 1), 598 nm ( 2) and 618 nm ( 3)) against temperature exhibit sigmoidal shape (see annex), as it is expected for a spin crossover equilibrium. The transition is clearly uncomplete in the explored temperature range, therefore the molar extinction coefficients ε HS and ε LS of the pure spin states have to be determined from a fit (equation 9) related to mole fraction of molecules in the HS state (γ Hs ). 27e The partial experimental data make therefore more difficult the extraction of the thermodynamic parameters associated to the SCO equilibrium (see below).

Determination of thermodynamic parameters of the SCO equilibrium in solution

From the solution study of 1--3 it is observed that the transition temperature of the three complexes are similar ( 1), slighlty higher ( 2) or slightly lower (3) than those observed in solid state. The substitution of the methyl by the ethyl group (which exhibits a slightly higher inductive donor effect) or the substitution of the methyl by the vinyl group (which may increases the aromaticity of the ligand and its π--acid character) do not apparently induce significant change in the ligand field.

Overall the nature of the spin transition curve in solution, obtained from the δT vs T approach, Evans' method, solution SQUID measurement and the variable temperature UV--vis study are similar to each other and they were all simulated by using the regular solution model in order to extract relevant thermodynamic parameters (Figure 21): 27,33 ln

(1! n HS ) / n Hs [ ] = "H RT ! "S R # $ % & ' ( (10)
ΔH, ΔS are the enthalpy, entropy variations associated with the spin crossover equilibrium. R is the universal gas constant. The HS molar fraction, n HS , term has been deduced in the various techniques through the equations given below: 40

n HS = (! M T ) T ! (! M T ) LT (! M T ) HT ! (! M T ) LT " # $ % & ' (11)
(! M T ) T is the value of the product (! M T ) at any temperature. The (! M T ) LT and (! M T ) HT correspond to the value of at low temperature and high temperature resepectively.

!(T ) = ! LS +" HS (T )(! HS !! LS ) ! M T (b) Variable temperature 1 H NMR study in solution. n HS = (!T ) T ! (!T ) LT (!T ) HT ! (!T ) LT " # $ % & ' (12)
Here, (!T ) T is the value of the product of temperature and chemical shift at any experimental temperature. The (!T ) LT and (!T ) HT corresponds to the limit values at low and high temperature respectively. (c) Variable temperature UV--vis study in solution:

n HS = (!) T ! (!) LT (!) HT ! (!) LT " # $ % & ' (13)
(!) T , (!) LT and (!) HT are the apparent molar extinction coefficient at any temperature, and the extreme values deduced obtained by simulation of the sigmoidale curve at low temperature and at high temperature.

The calculated values of the thermodynamic parameters obtained from the different method are reported in table 8 and9.

Table 8. Thermodynamic parameters of the spin transition of the three complexes obtained from susceptibility measurements in solution using the regular solution model (ca. 10 --2 mol/L). 27,33 VT NMR Evans' NMR Solution state SQUID

ΔH HL (KJ mol --1 ) ΔS HL (J mol --1 K --1
)

T 1/2 (K) ΔH HL (KJ mol --1 ) ΔS HL (J mol --1 K --1
)

T 1/2 (K) ΔH HL (KJ mol --1 ) ΔS HL (J mol --1 K --1
) 9. Thermodynamic parameters of the SCO process of the three complexes obtained from variable temperature UV--vis study in solution (ca. 10 --4 mol/L). The values of the thermodynamic parameters obtained from VT NMR and Evans' NMR are similar, and coherent with those found in litterature for Fe(II) SCO complexes. Although, they show some deviations with those obtained from the solution--state SQUID measurements. This could be due to the different experimental conditions in SQUID magnetometry as compare to NMR spectroscopy. Indeed it is worth noticing that the versus T curves obtained from the SQUID measurements do not show a clear inflexion point at T 1/2 in contrast with those obtained from NMR spectroscopy which seem more complete, with a more pronounced sigmoidal shape. The limited data obtained in SQUID are likely responsible for

T 1/2 (K
Compounds ΔH SCO (KJ mol --1 ) ΔS SCO (J mol --1 K --1 ) T 1/2 (K) [Fe(Me--bik) 3 ] (BF 4 ) 2
! M T
stronger errors in the ΔH and ΔS estimates.

UV--vis study in solution reveals that the transition temperatures T 1/2 lie almost in the same range for 1--3.

And close to those found by other techniques. However the partial data make the simulation of the spin equilibrium very approximative and the accuracy of various physical parameters obtained from this method is clearly questionable. Indeed the thermodynamic parameters obtained from the variable temperature UV--vis study differs largely from those obtained from NMR or solution SQUID method. Other reasons may explain such discrepancies. For example, chemical equilibrium such as [Fe(R--bik) 3 ] = [Fe(R--bik) 2 ] + R--bik might be more pronounced at low concentration values. Complementary investigations are needed to solve this problem (eg by recording the UV--Vis spectra in a broader temperature range, or by investigating the NMR spectra at variable concentrations).

The thermodynamic parameters obtained from the solution vary also significantly as compared to those obtained in solid--state (see above). More specifically, the thermodynamics parameters (associated to the SCO process) obtained for powder and solution are quite close for 1, but differ significantly for 2 and 3.

Entropy and enthalpy values associated with the spin crossover in the solution state are higher than that of the solid state for all the three complexes although, as in solution, strong intermolecular interactions are absent in solid--state. Similar discrepency was also observed in previously reported Fe(II) complexes. 33 1, VT NMR method 2, Evans' NMR method 3, Solution--state SQUID Overall it should be notice that in the present case the best SCO equilibrium curve (with the more pronounced sigmoidal curve is obtained from the variable temperature NMR study. The main advantage of this technique as compare to the others is the following one: the possible presence of other species does not interfere with the measurement, as the physical value measured is a selected chemical shift of the SCO complexes, which directly reflect its magnetic state.

Theoretical Studies

The following theoretical study has been carry out by Pr. Gopalan Rajaraman and his co--workers (IIT Bombay, India). DFT 42 calculations were performed on the X--ray structural coordinates using the Gaussian 09 43 and ORCA 44 2.9.1 suite programs.

In order to have a better insight into the electronic structure and magnetic and optical properties of complexes 1--3, density functional calculations using hybrid B3LYP functional were carried out on 1--3. All the three complexes have been optimized in their respective high--spin (t 2g 4 e g 2 , 5 T 2g ) and low--spin (t 2g 6 e g 0 , 1 A 1g ) ground states considering the X--ray structural coordinates (of the LS state obtained at 200 K) as the initial starting point. The computed structural parameters are consistent with the X--ray structures (see Table 9). Calculations clearly reveal that in all three cases the Fe--N distances in LS states are in the range of 2.03 to 2.05 Å while at the HS surface the bond lengths are in the range of 2.22 to 2.25 Å. Computed energetics reveal high--spin as the ground state for all the complexes, with energy differences between the HS and LS being estimated to 8.8, 6.0, 6.1 kJ/mol for complexes 1--3 respectively. The HS--LS energy gaps estimated for complexes 1--3 are thus very small and they are in the range expected for the observation of spin--crossover behaviour. 45 More precise calculations would require the use of actual X--ray data for the HS states. Unfortunatelly, these structural data could not be obtained in the present cases as the compounds lose their cristallinity upon heating. Table 10. B3LYP optimized selected bond parameters for complex 1--3.

To gain further insights into the nature of bonding, d--based orbitals of complexes were computed and they are plotted in Figure 22. The nature of the spin ground state is determined by the orbital splitting and the pairing energy. For complex 1--3, calculations yield the following electronic configuration (d 1 for the high--spin configuration, with the highest orbital splitting, ΔE(d xy --d z2 ), being around ca. 142 kJ/mol (11 800 cm --1 ). Remarkably, the presence of ketonic groups promotes delocalization of the π clouds between the two imidazole rings leading to stronger back--bonding effect and a stabilization of the LS state.

xy ) 2 (d yz ) 1 (d xz ) 1 (d x2-- y2 ) 1 (d z2 )
Besides in complexes 1--3, significant C--H ... O interactions between the alkly/vinyl hydrogen and the keto oxygens are detected. 46 The nature of this interaction however vary within the set of complexes 1--3 (C--H ... O distances are 2.55 Å, 2.30 Å and 2.26 Å for complexes 1, 2 and 3, respectively) and this interaction is also found to alter the donor abilities of the imidazole ring that can further leads to the differences in the observed magnetic properties. Computed spin density plots of complexes 1 and 3 are shown in Figure 23. The spin density at the metal centre has an octahedral shape and the spin density on the Fe atoms are found to efficiently delocalize to the ligands, as it is experimentally observed by NMR spectroscopy. The nitrogen atom coordinated to the Fe centre in complex 1 exhibit spin density values in the range of 0.021--0.023 (fraction of electrons). The oxygen atom of the keto group also bears significant amount of spin densities. Although the quoted values are small, the differences observed in complexes 1--3 clearly indicate significant structural and electronic alteration upon ligand modifications possibly leading to the variations observed in the SCO properties. Moreover in order to estimate the optical properties of the complexes 1--3, TD--DFT calculations were performed using B3LYP functionals. The computed absorption spectra are shown in Figure 24. Complexes 1--3 show an intense charge--transfer band near ~315 nm, which corresponds to LMCT transition. Markedly, complexes 1--3 also exhibit LMCT transitions at higher wavelength near 630 nm for complex 1, 641 nm for complex 2 and 650 nm in case of complex 3 in fair agreement with the experimental results. More precicely this peak corresponds to d xy (beta) to ligand--centred π* CT band. As the ligand π* orbitals are relatively low--lying in energy for complexes 1--3, this transition is found to occur at quite high wavelengths.

Concluding remarks

In this present contribution, we have performed structural, magnetic, spectroscopic, and optical studies to investigate the spin transition process in three related [Fe II (R--bik) 3 ] 2+ complexes containing similar α-keto-β--diimine ligands. These experimental results have been compared to DFT calculations.

It is well known that iron(II) complexes, in particular those with adequate N 6 coordination sphere, may exhibit SCO phenomenon, however joint solution and solid--state studies are rare and most of the investigations on SCO complexes focus mainly on solid--state properties. Here we made an attempt to study the SCO phenomenon in closely related compounds in both solid state and solution in order to observe how the switching properties of these systems are influenced by their surrounding (solvate in solution and neighboring molcules in solid--state).

We used usual magnetometry and optical methods to study the spin transition in the solid--state. Variable temperature data were used for analyzing the spin--transition process and for extracting the associated thermodynamic parameters. As only weak supramolecular interactions are observed in the structures of the compounds, these remain moderate and gradual spin transition are observed with almost no cooperative effect except for 2, which shows more efficient CH--π like interactions. Overall the condensed phase induces a shift of the transition temperature toward lower temperatures.

For analysing the spin--crossover process in solution, we used four different methods. The Evans' NMR method has been first used and its limitations have been discussed. One of them is the fact that this method allows to determine the bulk susceptibility of the solution, so if paramagnetic impurities are present (or chemical exchange occurs) the measurement can lead to erroneous results. Bulk susceptibility was also extracted from measurment in solution with the SQUID magnetometer. Although this technique is more sensitive than the Evans method (dilute solution can be used) it suffers from more or less the same inconvenients. As the SCO phenomenon induces important optical changes, variable temperature UV--vis study was used to follow the HS -- LS equilibrium. Here the simulation based on the partial spin--crossover data is highly approximative, which is reflected into the estimated thermodynamic parameters that show significant discrepencies with those found by the oher techniques. Complementary measurements are required to better understand the origin of these differences.

Overall it should be noticed that in the present case the best SCO equilibrium curve (with the more pronounced sigmoidale curve) is obtained from the variable temperature NMR study. By tracking the temperature dependence of adequately chosen 1 H chemical shifts (which bear a strong contribution from the Fermi contact term), we were able to follow the spin--crossover process of all the three compounds. The main advantage of this technique as compare to the others is the following one: the possible presence of other species do not interfer with the measurment as the physical value measured is a selected chemical shift of the SCO complex which directly reflect its magnetic state. Thermodynamic values coherent with those previously measured in other Fe(II) SCO complexes and similar to those obtained by the other techniques on the same complexes were obtained.

Beside the study of the spin equilibrium, 1 H NMR allowed to study the stability of the species over the explored temperature range and the possible occurrence of chemical equilibria, although one need to keep in mind that possible paramagnetic impurity may be NMR silent. For compound 2, we were able for example to observe an isomerization between the two stereoisomers, Δ--Λ. Whereas the two isomers can be detetced at low temperature (as the signal are diamagnetic) with high field NMR spectroscopy, at higer temperature, the presence of spin density on the probed nuclei lead to an enlargment of the chemical shift range and an improved resolution (as long as coalescence doe not occurs).

Finally, theoretical calculations based on density functional methods have been performed for all three complexes and the employed methodology describe both the structure and electronic properties of these complexes correctly. The computed energy difference between the HS and LS structures rationalize the presence of SCO properties in complexes 1--3. The computed electronic structure and the computed spin densities reveal that the presence of C=O group in complex 1--3 lead to enhancement of the π acidic nature of the ligand, therefore increasing the ligand field strength leading to the stabilization of LS state as the ground state. Actually we recently checked that the substitution of the CO by a methylene group lead to high--spin complexes.

Introduction

The investigated systems

The cyanometallates are an extremely rich family of complexes, 1a--b that have been succesfully used as building--blocks for the construction of wide range of polynuclear coordination clusters and extended networks with tunable properties and architectures. 1c,2,3 In particular, these complexes have proved successful in designing molecule--based magnetic materials. 2 Indeed, the cyanide ligand exhibits various advantage to build magnetic materials: (i) this ambidentate ligand stabilizes a variety of transition metal centers and oxidation states giving so access to a large number of heterometallic species; (ii) it favours electronic communication between the metal ions it links, and more particularly it promotes exchange magnetic interaction; (iii) as the μ--CN linkages between two metal centers are generally linear, reliable prediction of the magnetic exchange interaction can be established by using simple orbital--based model of the magnetic exchange interaction. 3--9 Morover, many cyanometallates are stable and versatile building blocks whose electronic properties and geometry can be adjusted. It is thus possible through the self-assembly of these building--blocks to prepare molecular materials whose magnetic, optical, and electronic properties can be tuned in a systematic fashion. 2,5--9 Among the various cyanometallate complexes, the fac--[M n (L)(CN) 3 ] n--3 building block (L = R 3 tacn, tmphen, triphos, Cp), which contain a facially coordinated tripodal ligand (L) and only three cyanide ligands in orthogonal positions are particularly suitable to build low dimensional compounds. 10--12 To date, only few cyanometalates (M III = Fe, Cr, V) have been reported with the use of the well--known poly(pyrazolyl)borates ligand. 13 These building--blocks appeared to be useful synthons for designing magnetic moleclar systems, such as for example Single Molecule Magnets, Single Chain Magnets and photomagnetic complexes. 13--17 The fac--[Fe III (L)(CN) 3 ] --complexes (L = tris(pyrazolyl)borate and related ligands) have been extensively used in our group and in others to design Fe--Co photomagnetic polynuclear complexes. 17 In this chapter, we report the preparation and characterization of a new tris (3,5--dimethylpyrazol--1--yl)borate (so--called Tp*) cobalt(III) tricyanido building block. This diamagnetic analogue of the fac--[Fe III (Tp)(CN) 3 ] --can be used to obtain polymetallic model compounds (where Fe(III) is substituted by Co(III)) that are usefull in order to rationalize the magnetic properties of the polymetallic systems. It can also be used itself as ligand to assemble paramagnetic systems with potentially interesting magnetic properties. Here we present the first results of its use as metalloligand: (a) toward a mononuclear [Fe II (Me--bik) 2 (S) 2 ] 2+ complex (where S = solvent) to lead a square complex exhibiting spin transition and (b) toward a dinuclear [Co II 2(μ-- OH 2 )(piv) 4 (Hpiv) 4 ] complex to lead a two--dimensional network. Actually the main objective of the present research is two--fold: --exploring the coordination chemistry of new cyanometallate complexes to build molecular magnetic materials; -- taking advantage of the diamagnetic nature of Co(III) centre to investigate if 59 Co solid state NMR can be used to probe the magnetic properties of these compounds.

Aim of this work: a magneto--structural study

In most of the cases, the cobalt ions exists in the oxidation states Co(II) or Co(III) in the coordination complexes. The Co(II) state is found for example in many molecular magnetic compounds because of its interesting magnetic properties. 18,19 The Co(III) state, which is diamagnetic in octahedral surrounding, is for example found in the well--known cobalamin and in its model compounds (here the Co atom can adopt the oxidation state I--III in the catalytic cycle). 20,21 59 Co solid--state NMR spectroscopy has been used in the past to probe various small molecules, biological model systems, homogeneous catalysts and octahedral basic inorganic complexes. 21--24 In most of these cases the cobalt exhibits a +III oxidation state (in some cases it is +I) and the complexes are diamagnetic. Actually, when the cobalt atom bears unpaired electrons, the relaxation becomes too fast and no signal can be detected by standard NMR techniques. In our present work we thus intend to probe the 59 Co NMR signals of the diamagnetic Co(III) ions belonging to the [Co III (L)(CN) 3 ] --building block in 1--3. Because the cyanide is efficient in transmitting electronic information, we expect these signals to be sensitive to the magnetic properties of the neighboring metal ions. The 59 Co NMR spectra obtained at different temperatures will thus be compared to the magnetic properties obtained by SQUID magnetometry in order to establish how sensitive is the 59 Co to the magnetic properties of the materials. We also plan to investigate if reliable structural information can be obtained from these data. It is indeed well--established that solid--state NMR is a powerful technique to provide information regarding the structure and the coordination environments of the probed metal centers. 25 It is examplified for instance by the solid state NMR study of cadmium prussian blue analogues by Lescouëzec et al. 26 59 Co solid--state NMR, has been commonly employed on mononuclear species, 22,27 however, it can also be used to study heteronuclear as well as polynuclear complexes. Such studies are potentially useful for identifying polymorphs in the bulk phase, and for building correlations between local structure and the NMR observables. 28 In this study we will thus compare the NMR results with the structural informations that we obtain from single crystal X--ray crystallography.

Some details on 59 Co NMR

The study of 59 Co NMR spectra of solid samples allows to extract information about the chemical shift (CS) and electric field gradient (EFG) tensors. 22,29 The isotropic 59 Co chemical shifts span a range of more than 18000 ppm 22 and are hence very sensitive to subtle changes in molecular structure. 21,23 However, despite favourable 59 Co nuclear magnetic properties, the large quadrupole moment of 59 Co (42 fm 2 ) often gives rise to broad NMR signals of the central transition (m S = +1/2 → m S = -1/2) for solid powder samples, precluding many solid--state 59 Co NMR studies. 30,31 This problem was circumvented in some early NMR works by obtaining 59 Co NMR spectra of single crystals. 32,33 For example, in their pioneering work, Spiess and coworkers used this technique to determine the 59 Co CS and EFG tensors for numerous octahedral complexes including that of cobaltocenium nitrate, which has a quadrupolar coupling constant of 165.6 MHz. 33 Unfortunately, such experiments are time consuming, and obtaining crystals of suitable size and quality is often difficult. An alternative is to use high magnetic field. The increasing availability of NMR spectrometers with B 0 > 17.5 T makes these experiments easier. 27,35 As the line broadening of the central transition arising from the second--order quadrupolar interaction depends inversely on B 0 , 34 recording NMR spectra of non--integer spin quadrupolar nuclei at the highest possible applied magnetic field strength leads to thiner NMR signals.

The first powder 59 Co NMR spectra of highly symmetric cobalt(III) complexes was reported by Eaton et al. 36 However the general theory for powder NMR line shape simulations for quadrupolar nuclei in low symmetry sites was not available at that time. 36 This could have impede them to get quantitative results on the interactions responsible for the observed anisotropic line shapes. Exactly three years after, methods for the analysis of powder line shapes of half--integer--spin quadrupolar nuclei (I = n/2 where n = 3, 5, 7, or 9), which have chemical shielding and quadrupolar tensors that are both non--axially symmetric and non-coincident, were indepently reported by two research group. 37,38 Since then, the chemical shift and quadrupolar tensors of various anionic cobalt complexes have been determined using the more general NMR line shape theory. 39,40 In this report, we present the 59 Co solid--state powder NMR spectra of the three cobalt(III)--containing complexes 1--3. Their CS and EFG tensors have been determined, as well as the relative orientations of the tensors in each complex. IR (ATR): ν max / cm --1 2534 (BH), 2166 (CN from Co III --CN--Fe II ), 2136 (terminal CN), 1636 (C=O), 1544, 1419, 1261, 1147, 1033, 900. The elemental analysis matches with the molecular formula { : C, 43.14; H, 4.33; N, 23.17 Found: C, 43.56; H, 4.26; N, 23.07. O 12 ). Elemental analyses calcd (%): C, 46.34; H, 5.77; N, 14.74 Found: C, 46.56; H, 4.48; N, 14.71. Compound 3 is obtained by self--assembly of 1 with preformed dinuclear complex [Co II 2(μ--OH 2 )(piv) 4 (Hpiv) 4 ] in CH 3 CN. This versatile dinuclear complex can recombine in solution to yield homo--and hetero--valent species with nuclearities ranging from three to fourteen cobalt ions. 42 This recombination is very much dependent on temperature. Therefore reaction temperature has a strong influence on the nuclearity of the final compound. In the present case, we performed the reaction at room temperature.

[Co{( Me2 Tp)}(CN) 3 ] 2 [Fe(Me-- bik) 2 ] 2 }(OTf)
{[Co II 2 (piv) 3 ] 2 [Co( Me2 Tp)(CN) 3 ] 2 ••2CH 3 CN} ∞ (3) [PPh 4 ][Co III ( Me2 Tp)(CN) 3 ]••CH 3 CN (0.

FT--IR analysis of 1, 2, and 3

The infrared spectrum (at room temperature) of 1 exhibits characteristic vibation at high energy ! BH (2531 cm --1 ) and ! CN (2131 cm --1 ) strectches that indicate the presence of Co(III) centres. The presence of only one cyanide stretching frequency around 2131 cm --1 indicates that all cyanides adopt a terminal coordination mode. The infrared spectrum of 2 exhibits a ! BH vibration around 2534 cm --1 and two cyanide stretching vibrations (2166, 2136 cm --1 ) which are characteristic of bridging [e.g. {Co III LS (μ--CN)Fe II HS }] unit and terminal cyanides respectively. The infrared spectrum of 3 also displays strong ! BH (2553 and 2535 cm --1 ) and cyanide stretching vibrations (2180, 2166 cm --1 ). The cyanide stretching vibrations of this compound clearly suggests that all cyanides are in bridging mode. The small stretching vibration around 2252 cm --1 is ascribed due to solvated CH 3 CN (! CN ) molecule.

Description of the crystal structures

Crystallographic parameters of all the compounds are given in table 1. Table 1. Crystallographic parameters of compounds 1, 2 and 3. The analysis of the Co(III) coordination sphere in these three complexes has been given in table 3. It is observed from the analysis that the Co(III) centres in all these three compounds are low spin in nature and they holds an almost ideal octahedral geometry (S ≈ 0.1).

R1 = Σ | |F o | -|F c | | / Σ |F o | wR2 = { Σ [ w(F o 2 -F c 2 ) 2 ] / Σ [ w(F o 2 ) 2 ] }
!" ### $%&' !" ## $(&' !" ## $)&'

Thermogravimetric analysis

The thermogravimetric analysis (TGA) experiments were carried out on 1, 2 and 3. Crystals of these compounds have been filtered just before the experiment as they decompose rapidly in air. The experiments were performed under nitrogen flow up to 150 °C, with heating rates of 2 °C / min. Basically, information regarding the thermal stability of the compound, molecular formula and amount of solvent loss upon heating could be obtained from this analysis. The analysis reveals that Compound 1 undergoes a significant weight loss throughout the explored temperature range. The weight lost occur in 2 stages. The first one occured from room temperature (1 loses crystallinity very quickly upon removal from the mother liquor) to 97°C and it corresponds to the loss of solvated acetonitile molecules (observed in XRD also), whereas the second one occurs above 97°C. The second decrease is probably due to the loss of solvated water molecules (observed from the elemental analysis). Therefore, the formula of the compound based on the result of TGA analysis, XRD and elemental analysis is

PPh 4 [Co( Me2 Tp)(CN) 3 ]••CH 3 CN••3H 2 O.
2 contains four acetonitrile and two water molecules (according to single crystal XRD study). This corresponds to 1.8 % x 4 and 0.8 % x 2 of the sample weight. As acetonitrile molecules are not involved in H--bonding (in contrast with the water molecules) they might be lost at room temperature. Indeed, 2 loses very quickly its crytallinity, so that an unkown amount of crystallization molecule is probably lost before starting the TGA experiment at ambient temperature. One can assume that the 1% weight loss near room temperature (from 20 to 31 °C) corresponds to one H--bonded water molecule. After this weight loss the compound is almost stable under nitrogen atmosphere up to 150 °C. The weight loss mathches with the removal of one molecule of acetonitrile molecule. Therefore, from the gravimetric analysis the formula of the compound become

{[Co{( Me2 Tp)}(CN) 3 ] 2 [Fe(Me--bik) 2 ] 2 }(OTf) 2 ••H 2 O
, which differs from EA and XRD results. Although, the result obtained from the XRD and EA for this compound are considered to be more reliable. The compound 3 undergoes almost no weight loss up to 120 °C. A slight decrease in total mass is observed above 120°C. Indeed, two molecule of acetonitrile per square unit of the 2D network is observed in single crystal X--ray analysis. But due to rapid loss of the solvent upon removal from the mother solution, we can not unambiguously ascribe the removal of specific crystallization molecules in TGA. Hence, the formula of the compound considering the result of TGA analysis, XRD and elemental analysis is

{[Co II 2 (piv) 3 ] 2 [Co( Me2 Tp)(CN) 3 ] 2 } ∞ .

Electronic properties of 1--3 3.1 Magnetic behaviour of 1, 2 and 3

Variable--temperature (2.0--400 K) magnetic measurements were carried out with a Quantum Design SQUID magnetometer MPMS XL--7 at an applied external field of 1 T. In a typical experiment, a fresh sample was packed in a polyethylene bag, which was introduced in the SQUID at 200 K under helium flow and frozen before purging under vacuum. The susceptibility measurements were carried out from 200 to 2 K, then from 2 K to 400 K, and finally from 400 to 2 K. Photomagnetic measurements were carried out by using a sample holder equipped with an optical fiber. In a typical experiment 0.6 mg of finely ground crystals were deposited on a adhesive tape. The sample was separated from the end of the fiber by 5.0 cm. All the irradiations were carried out at 20 K to minimize the temperature variation induced by the light. The magnetic susceptibility values were corrected for the diamagnetic contribution of the constituent atoms as well as from the residual diamagnetic signal of the sample holder. Bulk molar magnetic susceptibilty (χ M ) measurements were carried out on the fresh samples (for 1, 2 and 3) and previously desolvated sample (only for 2). These three compounds show very different magnetic behaviours. Whereas 1 is diamagnetic, 3 is paramagnetic throughout the explored temperature range. In contrast 2 shows a strong temperature dependence of its χ M T product (Figure 8).

Figure 8. ! M T product versus T plot of 1 (orange), 2 (green) and 3 (violet) measured at 2 K/min under H = 1 T, first upon heating (1, 2--400 K) then upon cooling (2, 400--2 K) and finally upon heating again (3, 2--400 K).

Compound 1 shows the expected behaviour for a low--spin Co III complex (S Co = 0). The magnetic behaviour of 2 significantly depends on the presence of solvant molecules and on the measurement procedure (particularly scan rate). The ! M T curve of a fresh sample of 2 first exhibits a pseudo plateau up to ca. 270 K with ! M T = 0.34 cm 3 mol --1 K and then increases abruptly to reach almost a pseudo plateau at ca. 358 K, with ! M T = 7.02 cm 3 mol --1 K. This behaviour is ascribed to a spin change of the two Fe(II) centres with T 1/2 = 292 K (Figure 8). The χ M T value agrees well with that expected for a molecular square containing two low--spin Co(III) ions (S=0) and two high--spin Fe(II) ions (S = 2, ! M T = 2

x 3.63 = 7.26 cm 3 mol --1 K with g ≈ 2.2). 46,47 This net increase of ca. 6.68 cm 3 mol --1 K can be ascribed to an almost complete spin crossover of the two Fe(II) ions (ca. 92% of LS -HS conversion). It represents a greater amount of spin--state conversion than our previously reported {Mo V 2 Fe II 2 } 2 rhombus molecule but compares well with the reported mixed--valence {Fe III 2 Fe II 2 } 2 square. 46 This behaviour is irreversible as after heating at 400 K, a gradual spin--crossover curve shifted toward lower temperatures is observed.

The ! M T curve is then reproducible upon cycling in the 2.0-400 K range (ca. T 1/2 = 213 K). The ! M T curve measured after heating exhibits an unexpected feature: a small transition between ca.115--185 K is observed whose origin is still unclear. The discrepancies observed in the ! M T curves before and after heating up to 400 K likely result from the partial loss of crystallization solvent molecules upon heating and under reduced pressure in the SQUID magnetometer. Indeed, it is well know that the removal of solvent molecules can strongly influence the spin--transition process. Actually TGA analysis showed that 2 exhibits a small but noticable weight loss near room temperature (the crystals quickly lose their crystallinity when removed from the mother liquor solution). The magnetic study of a previously desolvated sample of 2 exhibits the same behaviour, confirming this hypothesis.

The ! M T value for each {Co III 2 Co II 4 } repeating unit of the 2D network in 3 is 9.31 cm 3 mol --1 K at 300 K. This value lies within the expected range for the following set of ions: (i) two low--spin Co(III) ions (S = 0), and (ii) four high--spin Co(II) ions (S = 3/2). Two of them are tetrahedral (2.1 < χ M T < 3.1 cm 3 mol --1 K per ion at 300 K) and the two other exhibit a trigonal bipyramidal geometry (2.4 < ! M T < 3.4 cm 3 mol --1 K

! M T / cm 3 mol -1 K T / K (3) (2) (1) 1 2 3 1 2 3
per ion at 300 K). 49 Upon lowering the temperature, the ! M T value remained almost unchanged up to 45 K, then decreases, rapidly down to 2.0 K, with ! M T = 8.13 cm 3 mol --1 K. The sharp decrease at low temperature might be ascribed to weak antiferromagnetic exchange interactions. 48

Thermochromic properties of 2

Variable temperature solid--state UV--vis spectra were measured on a PerkinElmer Lambda 1050 WB spectrophotometer in the 300--1400 nm range. The measurements were performed on KBr pellets (approximately 3 mg of fresh crystallites were mixed and grinded with ca. 200 mg of KBr). The experiments were performed in two steps using two different experimental setup: (a) by heating the compound from low temperature (100 K) to room temperature using He gas flow and (b) by heating from room temperature (300 K) to high temperature (450 K) under inert an argon flow.

The variable temperature solid--state UVvis study of 2 gives further support for the occurence of the spin transition in 2 and reveals a significant thermochromism effect, which is associated to the thermally-induced spin--state change. The spectrum of 2 exhibits a strong broad band centred at 612 nm together with a shoulder at higher energy, which can be ascribed to the metal to ligand charge transfer (MLCT) band of the {Fe II (Me--bik) 2 (NC--) 2 } chromophores (Figure 9). The MLCT band was also observed in the previously reported molecular square complexes containing the same {Fe II (Me--bik) 2 (NC--) 2 } subunit. 46 The intensity of the band decreases upon heating, which is coherent with a spin transition as observed in the related {Fe III 2 Fe II 2 } 2 , {Mo V 2 Fe II 2 } 2 square complexes. 46 More specifically, the intensity decrease is due to the larger metal--ligand bond length in the high--spin state, which results in a smaller overlap between metal--centred and ligand--centred orbitals as compared to the low--spin state. According to the results obtained from the optical study, the spin transition seems to be centered nearly above room temperature. Absorbance Wavelength (nm) ---300 K ---450 K

Photomagnetic properties of 2

The magnetic state of 2 can also be switched by irradiation at low temperature. The photo--sensitivity of 2 was checked at 20 K by using various laser--diodes (λ= 900, 808, 635, 532 and 405 nm, with the respective powers: 7, 6, 12, 10 and 5 mW cm --2 , Figure 10). A fresh sample of compound 2 undergoes a significant increase of the magnetization whose maximum effect occurs with the 635 nm laser diode.

The ! M T value increases from ca. 0.21 to 5.61 cm 3 mol --1 K, and the saturation is observed after ca. 60 minutes under the present experimental conditions using a 635 nm laser light (Figure 10). This increase corresponds to ca. 74 % conversion of the diamagnetic state to the paramagnetic state. The photomagnetic effect is ascribed to a Light--Induced Excited Spin--State Trapping (LIESST) effect, the low--spin Fe(II) (S = 0) state being converted into a metastable high--spin Fe(II) (S = 2) state by the laser-light irradiation. Upon heating the sample at 0.5 K min --1 , the light--induced excited spin state persists up to T LIESST = 58 K (Figure 10 -- right). In this spin crossover compound, the T LIESST temperature is somehow higher than that measured in the related {Fe III 2 Fe II 2 } 2 square (T LIESST = 45 K) and rhombous {Mo V 2 Fe II 2 } 2 (T LIESST = 48 K) complexes, which also contain the {Fe II (Me--bik) 2 (NC-) 2 } SCO subunits. 46 405 and 700 nm laser lights were the most efficient wavelength for {Fe III 2 Fe II 2 } 2 square and rhombous {Mo V 2 Fe II 2 } 2 molecule, respectively, whereas in the present case the highest increase in magnetization occured with the 635 nm laser light. 46 The LIESST effect likely involve here a MLCT excited state as the most effecient wavelength (635 nm) matches the transition observed at ca. 612 nm, and assigned to a MLCT transition (see Figure 9). Cobalt--59 is the cobalt's main magnetically active isotope, is a nucleus for which NMR observation should in principle be easy. Actually 59 Co was one of the first nuclei to be studied using NMR spectroscopy. There is indeed a large number of stable low--spin diamagnetic Co(III) complexes that are well--suited for NMR studies. 59 Co chemical shifts were also the subject of early theoretical considerations. This stable 100% naturally abundant nucleus possesses a relatively high magnetogyric ratio, and due to the magnetic mixing of its ground and excited electronic states, it may experience a substantial paramagnetic deshieldings that allows to reveal subtle changes in chemical environments.

Actually the 59 Co exhibits the largest known chemical shift range, of about 20000 ppm.

The 59 Co measurements are however complicated by the high nuclear spin (I = 7/2) of this isotope, which is associated with a quadrupole moment that provides a very efficient relaxation mechanism. As a consequence, this leads to broad resonances in solution that may be difficult to observe. Though free from this tumbling--induced relaxation, solid NMR observations are also expected to be problematic, as the anisotropy of the quadrupole and the shielding interactions will endow nuclei in different crystallites with widely different resonance frequencies. Solid--state 59 Co NMR: Basic considerations Before moving to the interpretation of the experimental spectra, it is necessary to harp on the parameters that affect line shapes in solid--state 59 Co NMR spectrum and to summarize the strategies that were adopted for extracting these parameters. Like all other quadrupolar nuclei, 59 Co NMR signals are also defined by locally induced magnetic fields (chemical shifts) and by the interaction between the 59 Co nucleus and its surrounding electronic gradients (illustrated in figure 11). 21d For a quadrupolar nucleus, only the Zeeman, quadrupolar and chemical shielding interactions need to be considered. There are two situations that can arise depending on the relative strength of the quadrupolar coupling with respect to the Zeeman interaction: either first--order description is enough (Q 1 ) or second--order quadrupolar effects (Q 2 ) need to be taken into account. Q 1 is sufficient to describe the response of the quadrupolar nucleus when the quadrupolar interaction is moderate and can be described as a first--order perturbation of the Zeeman interaction (similar to all other NMR interactions that are only first--order perturbations). 23a When quadrupolar effects become too large to be appropriately described by the first--order term in a Zeeman--based perturbative expansion, the inclusion of a second term is required to redress this deficiency. Therefore the overall hamiltonian for a quadrupolar nuclei can be expressed by the following equation: 50

H total = H Q (1) + H CS (1) + H Q (2) (1)
Although, the dominant local interaction affecting solid--state 59 Co NMR spectra is usually the quadrupolar coupling but the central transition (CT) is still affected by the quadrupole interaction via second--order effects as well as by conventional shielding and dipolar anisotropies. 51 The line shapes of these spectra are then given by a first--order chemical shift proportional to the Larmor frequency ,

H (1) CS = ! 0 " CS iso +" CS aniso (# CS ,$ CS ) ! " # $ S z (2),
Where S z is the z component of the spin angular momentum operator.

The shape of the CT also depends on second--order quadrupole effect, which can be expressed as

H Q (2) = (e 2 qQ / h) 2 ! 0 " Q (iso,2) +" Q (aniso,2) (# Q ,$ Q ) ! " # $ S z (3)
and is proportional to the square to the quadrupole coupling and inversely proportional to the applied magnetic field B 0 . The combination of H (1) CS and H Q (2) terms result central transition spectra, which are centered at the sum of isotropic chemical and quadrupole--induced shifts (QIS). The CT is broadened by anisotropic shielding and second--order quadrupole effects as mentioned previously. 21d

The correct expressions for these contributions in terms of the quadrupolar coupling parameters (including quadrupole coupling constant and asymmetry parameter η Q ), the chemical shifts (including isotropic, anisotropic and asymmetry η CS ), and the Euler angles (which describes the orientation between the external magnetic field and the principle axis system of the shielding tensor) have already been discussed in literature. 52,53 Hence, it follows from these arguments that the powder line shape arising from a single 59 Co site will depend on several parameters ( , ,η CS , η Q , , ), which can be determined by numerically fitting the experimental patterns.

Theoretically, these various NMR parameters can be obtained more accurately from 59 Co spectra recorded on static samples as a function of the external magnetic field strength. Because the different dependences of H Q (2) and H (1) CS term (equation 2 and 3) permits to remove several ambiguities regarding these couplings that usually remain after single B 0 determinations. However, this variable--field static NMR approach becomes inadequate for determining small quadrupole coupling constants in the presence of dominating shielding anisotropies. This complication might be dealt with by using MAS spectroscopy as it was shown for example in the case of porphyrin complexes. 21d Another tempting alternative consists of collecting data while rapidly spinning the sample with respect to the external magnetic field. [START_REF] Haeberlen | High Resolution NMR in Solids[END_REF]55 It allows to scale the relatively small residual quadrupole effects and remove broadenings arising from shielding and dipolar interactions, thus breaking the broad 59 Co NMR patterns into manifolds of sharp spinning sidebands. Recording such spectra as a function of spinning speeds yields an accurate estimate of a site's isotropic centerband, whose position is given by:

! centerband = ! CS iso + (e 2 qQ / h) 2 " 0 2 ! Q (iso,2) (4)
In the other hand, when monitored as a function of ν 0 these well--defined centreband frequencies provide access for the estimation of even small quadrupole coupling contributions, as well as the values of the solid state isotropic chemical shifts. Reliable information about the shielding anisotropy of different cobalt sites can also be extracted from these data by simulating the MAS sideband patterns observed at various spinning rates and magnetic fields. 56--58 The resulting sideband line shapes actually

! 0 e 2 qQ / h ! cs iso ! cs aniso e 2 qQ / h !, ",# ! 0
depend on the three chemical shift tensor parameters, on two quadrupole parameters ( e 2 qQ / h , η Q ), and on the relative orientation between the shielding and quadrupolar tensors. Instead of implementing a full fit of these sideband patterns by numerically searching for the optimum values of these eight parameters, it is often assumed that the asymmetries of the shift and quadrupolar tensors are equal and that their orientations are coincident. 21c,21d Finally, the isotropic shift is determined from the centerband positions. Therefore, the shielding anisotropy and its asymmetry parameter are become the only unknown variables remaining in the MAS data simulations. Such approach has been used for compound 1. However, in the present case, it resulted in no significant change in the quality of the spectrum (with MAS spin rate 55 KHz and relatively long acquisition times) probably as a result of a fast decay of the signals. Consequently, we centred our attention retrieving reliable static 59 Co line shapes amenable to quantitative simulation.

NMR powder line shapes for quadrupolar system

It is of central importance to realise that this second--order quadrupole broadening has an isotropic part as well as anisotropic terms that are proportional to second--order and fourth--order Legendre polynomials. It is apparent that there is no single angle which can simultaneously satisfy that P 2 (cosθ) = 0 and P 4 (cosθ) = 0. Spinning around a single axis can completely remove either one or the other of these terms but will then only partially average the other. By far the most common approach is to spin at the conventional magic angle of 54.7° as this ensures that all the first--order effects are removed to live with the consequences of only partially averaging the P 4 (Cosθ) term. 12. Example of the line shapes, without and with MAS, of second--order quadrupolar broadened powder spectra of the central transition.

! -! ! - ! ! ! !"! ! ! ! + 1 - ! ! Figure
In the above figure ! ! is the Larmor frequency, ! ! is the frequecy of quadrupolar coupling.

The ! ! can be expressed as:

! Q = 3(e 2 qQ / h) 2I(2I !1) = 3C Q 2I(2I !1) (5)
Regarding CS and EFG tensors, it is only possible to obtain the orientations of the tensors with respect to the molecule using single--crystal NMR, a dipolar interaction, or in high crystal symmetry systems. 23a Therefore in order to describe fully the interactions responsible for the anisotropic NMR line shape of a quadrupolar nucleus in the solid--state, eight variables are needed (also described in the previous section). Now, by looking at the centreband obtained from MAS NMR, one can estimate the second--order quadrupolar shift. The consequence of the residual second--order term is that the centre of gravity of the lineshape does not coincide with the isotropic chemical shift. There is an isotropic second--order quadrupolar shift, which is given generally by the following equation: 25

! !,!"# ! !, ! = - !! ! ! !"! ! ! ! !!!! ! ! ! + 1 -9! ! -1 -3 (1 + ! ! ! ! ) (15)
The position of the isotropic chemical shift is obviously a characteristic of the peak, but must be quoted in the context of the applied magnetic field used since it is not a universal characteristic. Hence for the transition, the effect of the residual second--order isotropic shift is to produce a shift of the centre of gravity of δ cg , which means that the overall position is given by 25

! !" = ! !",!"# - ! !" !(!) ! ! ! ! ! ! 1 + ! ! ! ! (16)

Solution--state 59 Co NMR studies

One 59 Co solution NMR study was performed on 1 and 2. This study was mainly performed to confirm our attribution regarding the chemical shift values obtained in solid--state NMR. Due to the insolubility of compound 3, we could not study it by 59 Co solution NMR. Therefore for 3 solid--state 59 Co NMR was essential to estimate the quadrupolar parameters.

The spectra were recorded on 600 MHz spectrometer (B 0 = 14.09 T, ν( 59 Co) ≈ 142.39 MHz) using CD 3 CN solvent. The basic trends displayed by these shifts can be understood in terms of the classical "atom in a molecule" formalisms of 59 Co NMR shielding. According to these crystal field models, peak positions are dictated by the paramagnetic shielding contributions, which in a first approximation, are inversely proportional to the energy difference, ΔE between occupied and vacant 3d metal orbitals. 59,60 Therefore when the lignads become strong field the crystal field spilitting will increase and it will result an up--field shift for the 59 Co resonances. In the present case ligand linked with Co(III) centre is strong field in nature which causes a up--field shift for the 59 Co resonances than the reported B 12 compounds. 21 !1 / 2 " +1 / 2 In addition to isotropic shifts, solution resonances can, in principle, provide information about the electronic field gradients surrounding the metal ion by analysis of their line widths. [START_REF] Spiess | Rotation of Molecules and Nuclear Spin Relaxation[END_REF] As mentioned earlier, however, a more direct alternative to measuring quadrupole interactions consists of analyzing the 59 Co NMR line shapes observed in the solid.

Experimental 59 Co Solid--State NMR results

59 Co magic--angle spinning (MAS) and static NMR experiments were performed at B 0 = 16.4 T [ν( 1 H) ≈ 700

MHz, ν( 59 Co) ≈ 166.12 MHz] using a Bruker AVANCE III spectrometer. Fresh samples were powdered and packed just before the experiment into a 3.2 mm outer diameter zirconia rotor for static and MAS 59 Co NMR (20 kHz) experiments. For compound 1 MAS experiments were also performed 55 kHz (using 1.3 mm rotor). Solid--state NMR signals were acquired using a solid echo experiment, (solid π/2 pulse length is scaled from the solution π/2 pulse length by a factor equal to 1/(I + ½)), which takes the form π/2-τ-π, where τ represents interpulse delay. An interpulse delay of 15 μs and a recycle delay of 0.75 s were used for both static and MAS 59 Co NMR experiments. Prescan delay of 6.50 μs was used for all 59 Co NMR experiments except for the 55 kHz MAS spectrum, where prescan delay of 4.60 μs was used. The rf powers and chemical shift reference for 59 Co were determined using 1 M K 3 [Co(CN) 6 ] solution in H 2 O (δ( 59 Co) = 0.0 ppm). NMR lineshape analysis was carried out using TOP spin programme.

The cobalt chemical shifts for the three compounds range from +3545 ppm (1 260K ) to +4800 (3 260K ) ppm, and the shielding of the cobalt nuclei for the complexes 2, 3 are significantly higher than that for the parent PPh 4 [Co( Me2 Tp)(CN) 3 ]••CH 3 CN complex (+3545 ppm). In the theory proposed by Ramsey, 62 nuclear magnetic shielding is described as the sum of a diamagnetic, σ d , and a paramagnetic, σ p , component. The former arises from the ground electronic--state (associated to the electronic configuration (t 2g ) 6 ) and is largely insensitive to the chemical environment. The second one, σ p , arises from the mixing of the ground state and the excited electronic states (that have electronic configurations (t 2g ) 6-n (e g ) n where 1 < n < 4) 63 and it is inversely related to their energy difference, The σ p contribution is very sensitive to the local environment, in contrast with σ d . The large chemical shift range for cobalt nuclei in symmetric octahedral d 6 complexes has been attributed to the significant σ p contribution in the literature.

In our case, we also need to consider an additional paramagnetic contribution that arises from the delocalization of the spin density from the neighbouring paramagnetic centers through the cyanide bridges and from the dipolar contribution between the 59 Co probed nucleus and the magnetic source (eg the Co(II) or Fe(II) neighbouring ion). These contributions are the so--called Fermi contact term and dipolar term that are mentioned in the chapter 1 and 2. As the distance from the neigboring paramagnetic source is relatively large (ca. 5 Å), the dipolar contribution is expected to be moderate. Moreover, as the three compounds exhibit the same [Co III ( Me2 Tp)(CN) 3 ] --subunits where all Co(III) ions exhibit the same surrounding (see section 2.3 of this chapter), we will assume that the σ d contribution is likely similar. As a consequence, the differences in chemical shifts between the diamagnetic reference 1 and the compounds 2 and 3 are expected to be mainly due to the spin delocalization from the paramagnetic neigboring ions. The chemical shift value for the three compounds measured at different temperatures (260 K and 320 K) is given in Table 4. A sharp increase in the Co(III) chemical shift measured at 260 K is observed upon moving from 1 to 3 ( ). This is correlated to the number of paramagnetic centres linked to the Co(III) ions : from zero in 1 to three in 3. The linked paramagnetic atom clearly contributes to the overall nuclear magnetic shielding value of these complexes. The case of 2 is more problematic. It is evidenced from the magnetic study that (i) compound 2 exhibits thermo--induced spin crossover phenomenon; (ii) dehydration favors the formation of high--spin Fe(II) ions. According to the magnetic study, the Fe(II) ions are expected to be essentially diamagnetic at 260 K. However it appears that the magnetic susceptibility already smoothly increases below 260 K (Figure 8). Moreover we can not discard that the partial drying of the compound upon sample preparation (grinding and packing) favour the occurence of HS Fe(II) ions. So the presence of small amount of Fe(II) could account for the small positive shift between 1 and 2.

More strikingly, fresh sample of compound 2 shows a chemical shift value of +3710 ppm, which increases to +4204 ppm upon heating the sample at 320 K (Figure 15, left). This is undoubtedly due to the diamagnetic--paramagnetic conversion upon heating. As the local magnetic susceptibility of the

! 260 K (1) < ! 260 K, fresh (2) < ! 260 K (3)
compound increases with temperature, the chemical shift value also increases. It is worth reminding that the Fermi contact chemical shift is related to the local magnetic susceptibility through this equation: 64

! FC T = ! A s 3(" N / 2# )$ % xx g xx + % yy g yy + % zz g zz " # $ $ % & ' ' (5)
where, A s is the hyperfine coupling constant.

This phenomenon is often observed in solution NMR spectra of Fe(II) / Fe(III) spin crossover complexes. 65 In order to carefully interpret the magnetic data in terms of 59 Co NMR, we have recorded the static 59 Co NMR spectra of a previously dehydrated (in TGA) sample of 2 (inset of Figure 14). The dehydrated sample exhibits a 59 Co chemical shift of ca. +4159 ppm, which is ≈ +450 ppm higher than the fresh sample. After looking at the magnetic curve (Figure 8) one can clearly observe that the spin transition curve (s--shaped) is shifted towards low temperature and becomes gradual in nature after dehydration of the sample. Therefore, the magnetic state of 2 260K, dehy and 2 260K, fresh is not same. The 2 260K, dehy compound exhibits a higher paramagnetic character as compared to the fresh sample at same temperature. This difference in magnetic property is reflected in the position and shape of the CS tensor.

In contrast, in the case of compound 3, the chemical shift value at 260 K, +4800 ppm, decreases down to 4650 ppm upon heating the sample at 320 K (Figure 15). This observation is in well agreement with the paramagnetic nature of the compound. According to Curie law, the magnetization of a paramagnetic material is inversely proportional to the temperature. Chemical shift (more specifically, Fermi contact shift) should thus roughly follow the same trend with temperature. As described in chapter 3, chemical shift can be treated as local magnetic susceptibility and for a paramagnetic compound chemical shift is related to the temperature through the equation given below: 66 The spans of the cobalt CS tensors for the studied compounds range from +427 (1 260K ) to +1377 ppm (2 260K, dehy ). They are somewhat lower than those reported for other low--symmetry octahedral complexes. 27,29a,33 In the case of compound 2, a significant increase of the span (ca. +682 ppm) is observed upon heating from 260 K to 320 K. This is coherent with a change in the magnetic properties of the compound. The fresh sample which is almost diamagnetic (260 K) becomes more paramagnetic upon heating, leading to a spectral broadening. This observation is in line with the variable temperature SQUID magnetometry study (Figure 8). Similarly, the dehydrated sample, 2 260K, dehy , exhibits a much larger span than the fresh one, 2 260K, fresh at 260 K. Finally, compound 3 shows a span of ca. +539 to +774 ppm (respectively at 260 K and 320 K) which does not vary significantly as it remains paramagnetic throughout the explored temperature range, although its value is significantly higher than of the parent compound 1.

! FC T = g av µ B S(S +1) 3" N kT A s (6)
Figure 16. Cobalt--59 static NMR spectra of fresh samples of 1, 2 and 3 at 16.4 T. Simulated spectra (red) are displayed above experimental spectra (black). Left side of the figures represents those spectra which are obtained at 260 K, right side represents spectra at 320 K. 59 Co chemical shift values are expressed in ppm.

The NMR parameters obtained from the best--fit simulations are summarized in Table 4. Despite the difficulties in fitting the spectra acquired at 16.4 T, in particular the splitting of the horns in the powder patterns, the spectra simulations allow the determination of C Q, η Q , δ iso, and the principal components of the CS tensor. Although one should keep in mind that although a higher field (16.4 T) reduces the broadening arising from the quadrupolar interaction, it has opposite effect on the contribution arising from the anisotropy in the magnetic shielding. 29a When analyzing the spectra of quadrupolar nuclei under static conditions, contributions from CSA should be considered (in the present case CSA is masked by the quadrupolar broadening). Under favourable conditions, the relative orientation of the EFG and magnetic shielding frames can also be quantified. It should be noted that due to the lack of high symmetry at the cobalt atom, there may be other sets of angles (i.e., α, β, γ) which would produce good fits of the experimental data; this is a general caveat which is not specific to the current study. The important point is that isotropic chemical shift and quadrupolar coupling magnitude are well defined as a result of spectral fitting. 28 The positive value of chemical shift (+3545 to +4800 ppm) is in well agreement with the oxidation state +III of the cobalt atom.

The measured C Q ( 59 Co) value ranges from 22.4.6 to 38. 5 MHz, which is comparable to that observed in most of the reported octahedral Co(III) complexes. 22,28,33 Although for highly symmetric Co(III) complexes, the value (C Q ( 59 Co)) can reach to ≈3 MHz. 21d,22 It is observed that the quadrupolar tensors are non--axial in 2--3, roughly in agreement with the structure obtained by single--crystal XRD. The deviation from axaiality is probably due to the change in the second coordination sphere of the cobalt (III) centre. More specifically, in 2 and 3 cyanides of the Co(III) building block participate in bridging which is not the case for 1.

In addition to that, as observed from the XRD analysis, the distance between two most spatially proximate cobalt (III) nuclei is >7 Å for compound 2 and 3. This discards the contribution of homonuclear cobalt dipolar coupling. In order to estimate the second order quadrupolar shift in the building block and to accurately determine the isotropic chemical shift, 59 Co MAS experiment was performed (Figure 17). The centrebands produced under these conditions will also have characteristic lineshapes even though the shapes will be very different from the static ones. The greatest agreement between experimental 20 kHz MAS spectrum and the simulated one is achieved by using the following prameters: δ iso = +3583.5 ppm; δ 11 = +3756 ppm; δ 22 = +3588 ppm; δ 33 = +3403 ppm; Ω = 353 ppm. The experimental position of the centre of gravity of the centreband is located at the position (δ cg = +3545 ppm) that would be expected using C Q ( 59 Co), η Q and δ iso values mentioned in Table 5, after adjusting known second--order quadrupolar shift, δ Q , which is equal to -54.6 ppm. (i.e., the centre of gravity of the centreband, δ cg, is calculated to be located at δ iso + δ Q = 3583.5 --54.6 = +3529 ppm, slightly deviates with the observations). 25,28 Finally another MAS experiment on compound 1 with a MAS rate of 55 kHz was performed, however no significant improvement was observed (see annex chapter). Therefore we mainly focused our studies on static NMR spectra for the other compounds. Due to the lack of a well--defined second--order quadrupolar line shape in this MAS NMR spectrum (compound 1), one can only conclude that the 59 Co MAS NMR spectrum of 1 is consistent with the presence of a Co(III) nuclei, but it is not possible to further comment on the line shape broadening due to second--order quadrupolar effects.

- ! !" !(!) ! ! ! ! ! ! 1 + ! ! ! ! = -

Conclusion

A ] ∞ 2D network were structurally characterized and their optical and magnetic properties were investigated. Variable temperature magnetic measurements of these compounds in solid--state reveal that 1 is diamagnetic and 3 is paramagnetic throughout the entire temperature range, whereas 2 shows a thermally driven spin transition near room temperature, with T 1/2 = 291 K. The spin transition can also be induced by light irradiation at low temperature (LIESST effect).

The 59 Co chemical shift value have been used to probe the magnetic behaviour of these compounds. The 59 Co chemical shifts of +3545 ppm, +3710 ppm and +4800 ppm observed at 260 K for (fresh) 1, 2 and 3 respectively, are clearly correlated to their magnetic state: "the more paramagnetic, the more shifted". The positive shift indicates a positive spin density of the Co(III) ions which is transmitted along the M II --N--C--Co III link (M = Fe, Co). This contrasts with the negative spin density observed on the cadmium nuclei when studying the Fe III --C--N--Cd II link. The shape of the Co(III) signals were also shown to significantly depend on the magnetic state of the metal centres to which the [Co III ( Me2 Tp)CN 3 ] --is linked.

The quadrupolar coupling parameters for all these complexes have also been determined through the analysis of the 59 Co NMR spectra of solid samples. The quadrupolar and chemical shift tensor parameters are shown to be slightly sensitive to the magnetic behaviour of the compounds, as expected. Simulations of the 59 Co NMR spectra for these complexes indicate that the overall C Q is positive and comparable to the recently reported azide containing [NaCo III 2 ] system. 28 The homonuclear cobalt dipolar coupling is expected to be absent in the present case. The spans of the cobalt CS tensors are also quite low compared to the reported Co(III) compounds. In the case of the spin--crossover complex (2) they exhibit a remarkable variation with temperature, which is correlated to the spin--state change. To our knowledge this study represents probably the first report of solid state 59 Co NMR spectroscopic characterization of a spin--crossover compound containing quadrupolar Co(III) centre.

One of our future objectives is to obtain variable temperature 59 Co NMR spectra on single crystals of these complexes in order to reach more accurate information about the 59 Co CS and EFG tensors. Another interesting perspective will be to study in more details the temperature dependence of the CS and EFG tensor in switchable FeCo sytems (in the well--known Prussian Blue Analogues, or in FeCo photomagnetic molecules) featuring ETCST (electron transfer coupled spin transition) phenomenon. We also hope this work will encourage other researchers to use the approach described herein to obtain more detailed quadrupolar parameters when investigating quadrupolar molecular magnetic complexes. We think that these results can demonstrate at least to some extent the utility of solid--state NMR in providing insights on the structural and magnetic nature of Co(III)--based polynuclear species.

General Introduction

Prussian blue, which was fortuitously discovered in Berlin by the dye--maker Johann Jacob Diesbach in 1703, 1 is sometime considered as the oldest coordination polymer. Actually, this pigment, which is better known by the chemists as ferric ferrocyanide, is the eldest member of the so--called Prussian blue analogues (PBAs) family, whose general formula is A n M' x [M(CN) 6 ] y (H 2 O) z . Here, A is an alkali cation, M and M' are six coordinated transition metal ions. The PBAs are non--stoichiometric compounds and depending on the synthetic conditions, the amount of inserted cations and the amount of [M(CN) 6 ] n--vacancies can vary. Because of their remarkable structural and chemical versatility, that allow a fine--tuning of their chemical and physical properties, these materials are still attracting the interest of researchers interested in various fields such as (hydrogen) gas storage, 2 (bio)sensors, 3 heterogeneous catalysis, 4 waste recovering, 5 or molecular sieves. 6 In the field of molecular magnetism, the flexibility of the PBAs has allow the synthesis of numerous interesting materials, including for example molecule--based magnet exhibiting magnetic order above room temperature 7 or photomagnetic materials. 8 In all cases, the properties of the material dramatically depend on its chemical composition. This is due to the fact that structural, chemical, and electronic parameters (the stoichiometry, the nature and the amount of the alkali ions, the amount of vacancies, the bending of the M'--N--C cyanide bridge, etc.) are strongly connected and they play a key role on the physical properties. The rationalization of their properties often requires to get reliable structural information however PBAs are insoluble polymers and their structure is usually obtained through powder X--ray diffraction experiments. This technique leads to a long--range averaged view of their cubic framework, but it fails in giving a precise description of the local structural parameters such as the surrounding of the metal ions.

Recently our group explored the use of 113 Cd solid state NMR spectroscopy to probe the local structure of PBAs. 9,10 The 113 Cd was selected as this is a ½ nuclear spin which exhibits good receptivity and abundance and which can be used to prepare PBAs. Two series of stoichiometric model compounds were prepared: (i) one of general formula CsCd II [M III (CN) 6 ] containing inserted alkali ion and no vacancy ; (ii) and another one of formula Cd II 3 [M III (CN) 6 ] 2 0 containing no inserted alkali ion but one third of hexacyanometallate vacancies (0 represents a [M(CN) 6 ] 3-- vacancy, M III are paramagnetic Fe III or diamagnetic Co III ions). As mentioned previously in this work, depending on the amount of spin density delocalized on the probe nucleus, its chemical shift is more or less shifted. Here, the more paramagnetic Fe(III) ions are linked through the cyanide bridges to the 113 Cd probe (spin ½), the more shifted is the chemical shift (Figure 1). This simple approach allowed in both families of model compounds (and thanks to the use of various Fe(III)/Co(III)-doped samples) to identify and quantify the nature of the divalent sites in the materials, and consequently the nature of the vacancies. We were able for example to reveal the role of the local charge balance on the occurrence of the difference divalent sites: {Cd(--NC--Fe) n (H 2 O) 6--n }.

Besides this structural study, valuable insights into the spin--transfer mechanism (which affect the magnetic properties) were obtained by NMR. The 13 C and 15 N MAS NMR study on [M III (CN) 6 ] 3--building blocks and some partially blocked M(III) cyanometallates (described in chapter 2) has already shown that there is a large amount of negative spin density at the carbon atom of the CN ligand, whereas at the nitrogen there is a small amount of positive spin density. 11,12 In the PBAs derivatives it was observed that there is a negative spin density on the Cd ions arising from an efficient polarization mechanism. The spin density was shown to be more efficiently transmitted in the lacunary PBAs Cd 3 [Fe III (CN) 6 ] 2 than in the non lacunary one, CsCd[Fe III (CN) 6 ]. Here the bending of the cyanide bridge might be responsible for a more or less efficient spin delocalization, but structural data obtained through powder XRD on these phases are not accurate enough to check that hypothesis.

In this chapter, our goal is twofold. In the part A, we wish to use 113 Cd NMR in order to probe the local structure in insertion materials based on FeCd PBAs for energy storage application (lithium ion batteries). This work is carried out in collaboration with Prof Masashi Okubo from the University of Tokyo. In the part B, we wish to investigate the spin delocalization over the {Fe--CN--Cd} bridges in low dimensional model compounds in order to try to better understand, which structural and electronic parameters govern the spin delocalization mechanism. 6), 0.25 (7), 0.5 (8), 0.75 (9) and 1 (10). Doted lines refer to six different cadmium surroundings having a variable number of neighbouring Fe. The spectra were recorded at 305 K, under a 2.35 T magnetic field, and at a MAS frequency of 5 kHz.

!

Introduction

The research on renewable energy sources is one of the major topics in science worldwide. It is often associated with the search of efficient energy storage. Among the different energy storage technology, lithium--ion batteries (LIBs) have already proven to be successful as the power source of electric devices (laptops, cell phones, etc.) and vehicles, one of their main advantages being their high energy density. They are commonly made of an insertion cathode (usually a metal oxide) and a carbon anode that are separated by an organic electrolyte containing lithium salts. Recently researchers demonstrated that Prussian blue analogues could alternatively be used as insertion cathode. The open framework structure of PBAs can indeed offers several advantages as compared to other electrode materials such as oxides, including greater durability and faster kinetics. Indeed, monovalent cations (Li + , Na + , K + and NH 4 + ) in aqueous electrolytes can show fast intercalation into PBA--based electrodes, allowing extremely high chargedischarge rate capability. 13--14 This is a consequence of their robust cubic framework where large interstitials sites may host alkali ions and zeolitic water molecules. In lacunary PBAs (of formula M 3 [M'(CN) 6 ]) the ion channels connecting the interstitial sites are similar in size to solvated alkali ions such as potassium allowing rapid transport of the ions through the lattice. intercalated Li + per formula unit Li x Cd 1.5 [Fe(CN) 6 ] (on the assumption that the electric current is completely consumed in Li + intercalation/deintercalation).

In order to better understand the intercalation process, we carried out ex--situ powder XRD (in Tokyo) and solid--state NMR spectroscopy (in Paris) of various sample with different Li + insertion rates. Eight samples were prepared as followed for the NMR study:

1: pure Fe--Cd PBA before mixing with acetylene black; 2: Fe--Cd PBA mixed and ground with acetylene black (x = 0); 3--8, 3: 14 mAh/g discharge (x = 0.27); 4: 30 mAh/g discharge (x = 0.57); 5: Full discharge (x = 0.76); 6: 10 mAh/g charge (x = 0.57); 7: 26 mAh/g charge (x = 0.27) and 8: Full (40 mAh/g) charge (x= 0).

Powder XRD analyses

The reversible nature of the lithiation process is supported by the powder X--ray diffraction experiments. Each of the 2--8 samples are isostructural compounds. The data for 2--8 matches well with the reported Prussian blue of Fm3m space group. 5 The peaks indexed to the cubic Fe--Cd PBA gradually shift on the lower angles on Li + insertion. The unit cell parameter, a, shows almost reversible contraction/expansion upon Li intercalation/deintercalation, similarly as that observed for Na + , K + , or Mg 2+ insertion in other PBAs. 15--16

113 Cd NMR study

The Solid State NMR measurements were carried out on powder samples packed in a 2.5 mm ZrO 2 rotor. The 113 Cd NMR spectra of 1--8 were acquired by magic angle spinning (MAS) in 500 MHz and 300 MHz (for compound 8 only) spectrometers equipped with a 2.5--BL probe head. The temperature inside the rotor was

! !"# # $"% & ' (
accurately determined by recording the nickelocene (internal temperature reference) proton signal relative to external adamantane. 17 The isotropic 113 Cd signal shifts were measured relative to external {Cd(NO 3 ) 2 (H 2 O) 4 } (δ = --100 ppm).

A look--back at the previous study on Fe--Cd PBAs

Before going to the experimental results concerning the intercalation of Li + in the Cd II 3 [Fe III (CN) 6 ] PBAs, we would like to have a quick look at the reported studies on this starting material. As previously observed, the spectra exhibit three different cadmium sites, {Cd II (NC--Fe III ) 3 (OH 2 ) 3 };, {Cd II (NC--Fe III ) 4 (OH 2 ) 2 } and {Cd II (NC--Fe III ) 5 (OH 2 )}, with the isotropic chemical shift value of --995 ppm, --1300 ppm and --1500 ppm respectively at 304.4 and 305.7 K. The signal arising from the {Cd II (NC--Fe III ) 4 (OH 2 ) 2 } site shows a splitting pattern which is ascribed to cis/trans isomer surroundings as previously described. 10 v 113 Cd spectra of Fe--Cd PBA--acetylene black mixed sample (2)

The 113 Cd NMR spectra of the sample 2 were recorded and their evolution with time has been monitored. It was observed that the compound ages or may evolve with heating. Figure 5 show different spectra acquired at different spinning speeds. For those spectra acquired at higher speed rates, the temperature has been reduced to avoid heating effects due to the spinning. The same sample recorded after 7 days exhibits new features that may be due to an aging effect. In addition to the previously (Day 0) observed three signals, a set of new signals between --300 and --600 ppm is observed. This new pattern is ascribed to a unique site that exhibits a strong CSA and thus side band patterns (the new signals are separated from each other by the spinning speed, 12.5 kHz). The spectrum recorded after 15 days shows the same strong isotropic signal around --430 ppm, accompanied with some small satellite peaks, which are unambiguously attributed to the spinning side bands (separated from the central signal by the MAS spinning speed, 20 kHz). Moreover, the three initial signals at lower chemical shift completely disappeared, whereas a new signal located near 87 ppm accounts for the presence of diamagnetic species. The spectra obtained after 19 days is similar. The slight differences lie in (a) the increase of the intensity of the diamagnetic signal (87 ppm) and (b) the increased number of side bands at --430 ppm because of the slower MAS spinning speed.

Overall the disappearance of the initial strongly shifted chemical shifts and the appearance of less shifted ones point to a reduction of the Fe III ions into iron II . Actually the isotropic signal at --430 ppm and 87 ppm may be ascribed to {Cd II (NC--Fe III ) 2 (dia) 4 } and {Cd II (dia) 6 } species, respectively, where dia is either a coordinated water molecule or a reduced (NC--Fe II ) moieties.

This results show that the preparation of the electrode material, and the mixing with acetylene black, may favour the decomposition (reduction) of the PBAs. All the following experiments were carry out on freshly prepared samples. => ?? @A=5BC ??? D + @% * ED# # => ?? @A=5BC ??? D 6 @% * ED * # => ?? @A=5BC ??? D ! @% * ED ! # 0 123 #4#5-+""#778# =>@A=5BC ??? D * @% * ED 6 F# #=>@A=5BC ??? D@% * ED + # # =>@% * ED G #

113 Cd spectra of the intercalation materials (3--8)

The following figure represents the ex--situ 113 Cd MAS NMR spectra obtained at different stages of the Li + intercalation/deintercalation process. All the spectra were recorded on freshly prepared compounds.

Fig 8.

113 Cd MAS--NMR spectra of 1--8. The sample number and spinning speed is indicated on the left side of spectra. All the spectra were obtained on a 500 MHz NMR instrument except the top one, which was acquired with a 300 MHz NMR machine.

3 represent the spectrum of the 27% lithiated material. Theoretically each Li + ion should reduce one Fe(III) to Fe(II). Therefore 27% of Fe(II) should be present in 3. Hence one can expect the appearance of signals at lower chemical shift and ascribed to new sites such as {Cd(NC--Fe II )(NC--Fe III ) 3 (H 2 O) 2 }, etc.. Indeed, the experimental spectrum shows that the intensity of the signals related to the {Cd II (NC--Fe III ) 4 (dia) 2 } and {Cd II (NCFe III ) 5 (dia)} sites decrease whereas a slight increase of intensity is observed for the {Cd II (NCFe III ) 3 (dia) 3 } site. Here "dia" refers to either H 2 O or (NC--Fe II ) moieties. This is thus in agreement with the expected trends. Moreover, the appearance of a new intense signal around --430 ppm indicates the presence of a cadmium centre linked with two Fe(III) centre. (Note that we would expect a cadmium site linked to only one Fe(III) ion at a lower chemical shift). The small peaks on both sides of the signal are spinning sidebands whose position change with the spinning speed. Another important observation is the change in shape of the peak due to the {Cd II (NCFe III ) 4 (H 2 O) 2 } site. It seems that one of the isomer site (see description above) tends to be reduced more easily than the other one. We may assume that it is the cis isomer (with two water molecules located next to each other on the cadmium coordination sphere), which is located next to large voids. Therefore it could be easier for Li + ions to access these vacancies, favouring the reduction of the Fe(III) neighbouring ions. The spectrum 4 represents the 57% lithiated material. It shows significant changes as compared to 3: (i) the signals corresponding to the initial cadmium sites site (with three, four and five Fe(III) neighbouring centres) disappear; (ii) the isotropic signals located at --440 ppm strongly increases; (ii) a new signal at 92 ppm which correspond to a cadmium ion linked only to diamagnetic species. (The spectrum was recorded at 20 and 30 kHz to check again that the small peaks located at both sides of the --440 ppm signal are spinning side bands). Overall all these changes are coherent with a reduction of some Fe(III) to Fe(II) ions upon lithium intercalation. However, we surprisingly do not observe a signal that could be ascribed to a cadmium ion surrounded by only one paramagnetic Fe(III) ion.

The spectrum 5 corresponds to the sample, which contains the maximum amount of Li + . It shows the presence of an intense "diamagnetic" cadmium site and a small signal ascribed to some residual {Cd II (NCFe III ) 2 (dia) 4 } sites. This indicates that almost all the Fe(III) ions are reduced. In fact from the charge/discharge plot (figure 2) we can observe that it is not possible to introduce more Li + ions in the compound. This is coherent with a complete reduction of the Fe(III) sites, which is due on the one hand to the intercalation properties, and on the other hand to an aging effect. Spectra 6, 7 and 8 represent the spectra obtained during the delithiation process. 7 represent the spectrum of the compound that contains 27% of lithium. It exhibits a similar spectrum as 3, with the following features: (i) the purely diamagnetic signal around 92 ppm gets completely vanished; (ii) the appearance of a significant amount {Cd II (NCFe III ) 2 (dia) 4 } sites around --440 ppm; (iii) the presence of three cadmium sites with chemical shift value of --1000 ppm, --1325 ppm and --1520 ppm, that are due to {Cd II (NCFe III ) 3 (dia) 3 }, {Cd II (NCFe III ) 4 (dia) 2 } and {Cd II (NCFe III ) 5 (dia)} sites respectively. Therefore, the overall shape of 7 is very similar to that of 3. 8 show the spectrum of the compound containing 0 % of lithium. It was first recorded with 500 and NMR spectrometers on a sample, which partly aged. A more careful experiment on a fresh sample of 8 with a higher scan number leads to a spectrum similar to that of 2, (i.e. PBA mixed acetylene black) although some residual signals are observed at lower chemical shift. A careful observation of the spectrum also reveals a slight left--shift in the chemical shifts in 8 as compared to 2.

Overall the NMR study of the lithium intercalation shows that the process is reversible although some aging effects can affect the intercalation properties of the materials.

Conclusion

Ex situ 113 Cd MAS NMR studies performed on these battery materials allowed to have a better view on the intercalation phenomenon, by probing the local structure of various partially lithiated and delithiated PBA-based electrode materials. Actually, the presence of paramagnetic sources near the cadmium probe results in an enlargement of the spectral window and enables us to observe and partly identify different cadmium sites, at different stages of the intercalation process. The current study establish a clear experimental correlation between the upper limit of the Li insertion in the materials and the number of residual Fe(III) sites. It also underlines the aging effect on the PBA mixed acetylene black material.

Although this study is specific to the Fe--Cd PBA, it may be informative on the alkali intercalation process, which is carried out more generally in other PBAs.

These preliminary results have also raised some questions, which are not clearly resolved for the moment. Why is the CSA of the {Cd II (NCFe III ) 2 (dia) 4 } site so big compare to the original sites? Why is there some preferred cadmium sites in the compound? Indeed, the {Cd II (NCFe III ) 4 (dia) 2 } and {Cd II (NCFe III ) 2 (dia) 4 } sites exhibit higher intensities as compared to the others. We did not identify for example the {Cd II (NCFe III ) 1 (dia) 5 } site upon lithiation and dilithiation process.

Further experiments deals with the study of intermediates stoichiometry like 2--3, 3--4, etc. and Li+ NMR.

Introduction

The work presented in the Part B is associated to the compounds which are low in dimension compared to the Prussian blue and its analogues. Recently a paramount research have been carried out on these kind of low dimensional compounds. They are some times called as molecular model compounds of Prussian blue. The main interest to study this kind of systems are lies in the following facts (a) the compounds could be crystallized easily (b) for better understanding the physical properties of the more sophisticated 3D compounds like prussian blue (c) potential application in nano electronic devices. Lescouëzec et al. clearly demonstrated in his work on PBAs that 113 Cd nuclei could be used as a structural and magnetic probe to study PBAs. 1 It was observed that the CsFeCdPBAs exhibit lower spin density compared to CdFePBAs. 1 The authour anticipated that it could be correlated with the bending of the cyanide bridge, which make the spin transfer less efficient and therefore addressed the important question about the role of geometric parameters on spin--transfer mechanism. 1a In order to find out the solution of this question we have prepared a family molecular model compounds containing {M III --CN--Cd II } motif. The advantage of the current study is that all the compounds have been charecterized by single crystal XRD technique. Therefore accurate information about the dipolar interaction, which has a 1/r 3 dependence, has been obtained. It will lead to a better accuracy in spin density calculation. Although, the dipolar interaction is expected to be weak. Therefore, paramagnetic shift is expected to reflect the contact term and thus the spin extension. For probing the role of different paramagnetic metal ions and/or to investigate how diffused nature of the 4d orbital play crucial role in spin transfer mechanism, we have studied the solid state 113 Cd NMR study on two Ru III --CN--Cd II bridge containing network like compounds with molecular formula [{Ru(salen)(CN) 2 }{Cd(CH 3 CN)}] . H 2 O and [{Ru(acac) 2 (CN) 2 }{Cd}] ∞ . The diamagnetic reference for the present study is two zig--zag chains based on {Co III --CN--Cd II } motif. In addition to spin density calculation, chemical shift parameters of these compounds are analyzed also.

stretching vibration of the ligand, whereas those at higher energy are ascribed to OH stretching of water molecules (ν OH ). Table 1. Selected IR vibrations in 1--10 (ATR mode at 298 K)

Compound

ν CN (cm --1 ) ν C--H (cm --1 ) ν B--H (cm --1 ) ν M--CN (cm --1 ) Bridging Terminal 4)-1.894(4) Å) are in good agreement with those observed in the low--spin iron(III) and cobalt(III) building block. 2,5 It must be noted that the values of the two M III -C bonds corresponding to the two trans coordinated cyanido groups (with respect to the phenanthroline ring) in 6 and 8 are significantly shorter than those of the cis coordinated ones. The crystal structure of 3 is based on zig--zag chains that contain pending [Fe(Tp)(CN) 3 ] --units. There are thus two crystallographically independent Fe(III) centres: one (Fe2) is coordinated to two Cd(II) centre through two cyanide bridges, while the other one (Fe1) coordinates one Cd(II) centre through one cyanide bridge. The coordination sphere of each of these Fe(III) contains three Tp nitrogen atoms and three cyanide carbon atoms. The average Fe III -N(Tp) bond lengths are 1.995(3) Å and 1.994(3) Å respectively for Fe (1) and Fe(2), which is slightly higher than in 1 and 2. The average angle subtended at the iron centre by the chelating Tp ligand amounts to 90.52° and 90.51° respectively for Fe1 and Fe2 centre. These values match well with those found in 1 and 2 and in the previously reported chains based on the [Fe(Tp)(CN) 3 ] --building block. 2,4 The Cd(II) ion exhibits a distorted octahedral coordination sphere (see table 2), which is made of one oxygen from a water molecule, two nitrogen atoms from the coordinated bipyrimidine and three nitrogen atoms from cyanide bridges. The distance between the cadmium and cyanide nitrogen range from 2.30( 3

!"#$ %#$ %&$ %'$ %($ )#$ )&$ )'$ )($ )*$ )+$ %,#$ %,#-$ .#$ !"#-$ !"#$ !#$ !%$ !&$ !'$ (#$ (%$ (&$ ('$ ()$ (*$ !+#$ !+#,$ -#$ !"#,$
"#$ %#$ %&$ %'$ %($ )#$ )&$ )'$ )($ )*$ )+$ %,#$ %,#-$ .#$ !"#-$ !"#$ !#$ !%$ !&$ !'$ (#$ (%$ (&$ ('$ ()$ (*$ !+#$ !+#,$ -#$ !"#,$
"# $%# $"# $&# $'# !%# !%&# !&# !'# !(# !)# $*%# $*%+# ,%# -.%+# ,/# -./+# -.%# -./# $/# !/# !%)# !%"# !%0# !%%# !%1# !%/# !//# !/&# !/'# !"# $'# !%'# $0# !%(# $2# !%2# $%1# !0# $(# !2# $)# !/%# $%/#

[{Ru(salen)(CN) 2 }{Cd(CH 3 CN)}].H 2 O (9)

The structure of 9 is made of a two--dimensional network built up from trans--[Ru III (salen)(CN) 2 ] -anions that connect Cd(II) cations and intercalated crystallization water molecules. Both cyanide nitrogen atoms of each trans--[Ru(salen)(CN) 2 ] --units are linked to Cd(II) ion in equatorial plane and generate a square--based sheets. The absence of terminal cyanide in the structure is also reflected in the IR stretching vibration of 9. The bond distances of each [Ru(salen)(CN) 2 ] --unit in the polymeric compound are: Ru--O(salen) = 2.012 Å; Ru--N(salen) = 1.998 Å and Ru--C (CN) = 2.075 Å. Similar bond distances are observed in the reported Ru III --Mn II 2D compound based on [Ru(salen)(CN) 2 ] --. 3d--g The Ru--C--N angle amounts to 175.85° in this compound, which also matches with the reported heterometallic compound derived from [Ru(salen)(CN) 2 ] --. 3 Each Cd(II) center is penta coordinated and has a vacant octahedral environment (according to SHAPE analysis, see table 2). The coordination sphere of cadmium ion is made of four cyanide nitrogen atoms (arising from four [Ru(salen)(CN) 2 ] --units) in the equatorial positions and one nitrogen coming from coordinated acetonitrile molecule in the axial positions. The Cd--NC--Ru unit is bent towards Cd(II) centre with an angle of 164.99°. The Cd--N(CH 3 CN) distance is 2.297 Å and Cd--N(bridge) distance is 2.245 Å. The intralayer Ru------Cd distance is 5.382 Å whereas the shortest interlayer Ru------Cd distances are 9.28 and 10.65 Å, and the closest interlayer Ru------Ru separation is 8.60 Å. In this crystal structure, each Ru atom exhibits a slightly distorted octahedral coordination sphere, where as Cd(1) and Cd(2) exhibits a distorted tetrahedral geometry (distortion is more in case of Cd1, see table 2). The ideal geometry of the metal centre has been analysed by SHAPE programme and is listed in table 2. 6 The average Cd-N(CN) bond distance is 2.16 Å for both Cd(1) and Cd( 2 The highest the "shape" value is, the more distorted the coordination sphere is. Here OC--6 refers octahedral geometry, TPR--6 refers trigonal prismatic geometry, VOC--5 refers vacant octahedral geometry and T--4 refers tetrahedral geometry. All the NMR spectra were recorded on freshly prepared samples. They were obtained using a Hahn--echo (90--τ--180) sequence in order to minimize the baseline deformations, 7,8 using 4.00 μS pulse width, 0.004 s acquisition time, 6.5 μS pre--scan delay, 15 μS inter--pulse delay, and 0.5 s recycling delay for the paramagnetic samples. In the case of diamagnetic samples, 10 s recycling delay was used.

!"#$ !"%$ !%$ !#&$ '#$ '#($ )&$ )#*$ )%$ )+$ ,-#$ ,-#$ ,-#$ ,-#$
The experimental data were simulated with the Topspin software, using the spectra with the lowest MAS spinning speed to extract more accurate CAS tensor values.

113 Cd NMR spectra of diamagnetic reference (7--8)

As it was not possible to obtain isostructural diamagnetic reference for all the polymetallic Fe--Cd compounds, 7 and 8 were used as diamagnetic reference for all the complexes. The isotropic chemical shift of the zig--zag double chains are located at ca. --27.3 ppm and --28.6 ppm, which lies in the lower region of the expected diamagnetic range of 113 Cd signal (+500 to --125 ppm). These values are quite shifted as compared to those measured in the Fe--Cd PBAs. Indeed, the compounds Cd The 113 Cd MAS NMR spectra of these three compounds were recorded at a spinning speed of 5 and 8 kHz. The compound 1, 2 and 3 exhibit isotropic 113 Cd signal at --920.24 ppm (1), --439.63 ppm (2), --748.77 ppm (3) and --503.4 ppm (4) (at 298 K), which are thus found out of the diamagnetic range (+500 to -125 ppm). As there is only one cadmium surrounding in each of these samples, only one isotropic signal is observed in each case. The strong left shift of the chemical shifts are due to the hyperfine interaction between the 113 Cd nuclear spin and the unpaired electron located on Fe(III) centre. The negative sign of the chemical shift indicates a negative spin density on the cadmium that arises by spin polarization mechanism from the positive spin density located on the nitrogen (see chapter 2). As observed for PBAs, the more Fe(III) centres are linked to Cd(II) ions (four (1), two (2 and 4), and three ( 3)) the more spin density is delocalised onto the cadmium site and more the 113 Cd NMR signal is shifted. Looking at the work on the Fe--Cd PBAs by Flambard and1). In addition to that, we should remember that the paramagnetic chemical shift is determined by the subtraction of the chemical shift value obtained for a diamagnetic analogue compound to the experimentally observed chemical shift value. Here, as no isostructural diamagnetic reference have been obtained, approximations have been made using 7 and 8 as diamagnetic references.

The average Fe--Cd distances in 1--4 lie in the range of ca. 5.15--5.28 Å, which is close to the average distances observed in PBA compounds (ca. 5.30 Å). The lower paramagnetic chemical shift value observed on the 113 Cd nuclei could thus be correlated with the bending of the cyanide bridge. In case of CsCd PBA the cyanide bridge is almost linear, whereas in these compounds the bridge is bent with an average angle of ca. 138°--167° ( 1), 154°--161° (2), 153° ( 4) and 151°--167° (3). As for 1--4, the cyanide bridge is significantly bent in these compounds with average angle of ca. 135°--138° ( 5) and 157°--159° (6). The bending of the bridge is particularly prominent in the case of 5. Therefore the spin transfer is expected to be less efficient, however 5 exhibits quite surprisingly the higher paramagnetic chemical shift as compared to 1 and 6, which also exhibit similar cadmium surroundings. By looking at the structural parameters one could imagine that it might be correlated to the short intramolecular Fe--Cd distance which amounts to 5.01 Å for 5 and ca. 5.33 Å for 6. As shorter distances generally result in a greater dipolar contribution to the paramagnetic shift. However, this contribution is not expected to dominate the paramagnetic chemical shift here, as the distance between the probed nuclei and the paramagnetic source is quite large, and estimate of the dipolar contribution are small (see below).

-2000 -1500 -1000 -500 0

! [ppm]

!" #"$%&" " '"$%&" Overal, if we considere only the influence of geometrical effects (Fe--Cd distances, cyanide bending angle) on the spin transfer mechanism, it is not possible to draw clear conclusion at that stages. The electronic factors are also expected to play an important role on the spin delocalization. For example, the greater π-acid nature of the bipy and phen ligands (and the unfavourable cyanide bending) should leads to a lowering of the portion of spin transmitted to the Cd centre. However experimental results show opposite trends. In order to explore the role of different paramagnetic metal ions and/or to investigate how the diffused 4d orbitals can influence the spin transfer mechanism, we have studied the solid state 113 Cd NMR study on two (Ru--CN--Cd)--containing compounds, 9--10. We first selected two known cyanide--Ru(III) complexes for comparison reason, as these compounds exhibit the same electronic configuration as the low--spin iron(III) complexes that are described above. Our solid state 15 N NMR measurements on these Ru(III) building blocks (chapter 2), showed that there is less spin density on the cyanide nitrogen atoms in the complexes PPh 4 [Ru(acac) Compound 9 exhibits one isotropic peak around --438 ppm at 298 K that is ascribed to the unique Cd(II) site observed by XRD on single crystals. This Cd(II) ion is linked to four paramagnetic Ru(III) centre via cyanide bridges. It is clear that the negative 113 Cd chemical shift is due to the hyperfine interaction between the 113 Cd nucleus and the four paramagnetic neighbours. Therefore one Ru(III) ion would contribute roughly to ca. --109 ppm to the observed chemical shift value, whereas one Fe(III) ions in the previous cases (1--6) contributes roughly to ca. ≥ --255 ppm to the observed chemical shift value. This agrees with the expected trends, taking into accont the lower spin density delocalized on the cyanide nitrogen atoms.

In the case of compound 10, a broad 113 Cd signal is observed. The low resolution of the spectra makes difficult the attribution of the isotropic chemical shift and prevent their simulation. In addition to that single crystal X--ray analysis of this compound reveals that there are two types of cadmium sites with surrounding [Cd( 1

Chemical Shift Anisotropy (CSA) analyses

The electron cloud around a probed nucleus is generally anisotropic (because of chemical bond etc.). This anisotropy is reflected in the chemical shift, which shows orientation dependence with respect to the external magnetic field direction. Here, in order to determine chemical shift anisotropy of the studied complexes, Haeberlen analysis was carried out using the module ''Solids Line Shape Analysis" from the Bruker's software, Topspin. This enables the determination of experimental isotropic shift

, where , and are the principal component of the chemical shift tensor (ordered as ). The parameter describing the anisotropy is here expressed as and the asymmetry parameter is . The line broadening factor (LB) indicates the broadness of the siganals. 9

The obtained simulated CSA parameters are reported in table 3--4. ! T exp = (" T ,yy exp ! " T ,xx exp ) (" T ,zz exp ! " T ,iso exp ) The analysis reveals that the CSA tensors of the cadmium sites are not perfectly axial and exhibits a negative anisotropy parameters. The values obtained from the paramagnetic compounds are in line with those obtained for the diamagnetic references, except that the presence of spin density on the Cd nuclei leads to higher values. Indeed the two diamagnetic reference compound (7, 8) exhibits anisotropy parameters, ranging from --119.76 to --124.09 ppm. These close value of ∆! ! !"# shows that the change of the blocking ligands attached to Co(III) centre (bipy, phen) has no significant influence on the chemical shift anisotropy. The anisotropy in the paramagnetic {Fe--CN--Cd} bridged complexes is significantly increased, as a results of the strong hyperfine contribution. It ranges from --314.33 to --774.58 ppm for the more. The paramagnetic anisotropy in 9, which contains (Ru--CN--Cd) bridge, is lower in magnitude as compared to that in the Fe(III) complexes (1, 5 and 6) which also exhibit {Cd(NC--para) 4 (H 2 O) 2 } site, but here again it is an effect of the lower spin density. Therefore, roughly we can say that the more the spin density, more the anisotropy is. However, in case of compound 1, the spin density is higher but the anisotropy is low. It is ascribed due to the low symmetry of the cadmium site (due to presence of four crystallographically independent Fe III atoms linked to Cd II ). The decrease in the magnitude of the anisotropy parameters of 4 and 2 compared to 3 might be due to the strong deviation from ideal octahedral geometry. However, despite having an almost octahedral geometry 7 and 8 exhibit small anisotropic parameters and could be ascribed solely due to absence of hyperfine interaction.

The asymmetry parameter (!" ! !"# ) Carries more information about the local geometry of the nucleus as it significantly depends on the symmetry of the Cd(II) site. Among all these complexes, 2 exhibits the lowest symmetry in the Cd(II) sites as its lagely deviates from the ideal octahedral geometry (observed by SHAPE anlysis, table 2). Its CSA thus deviates strongly from axiality and exhibits a !" ! !"# value of 0.99. Although, the reason behind the observation of higher !" ! !"# value i.e., deviation of the CSA tenor from axiality in compound 7 is still unclear to us. The SHAPE analysis in compound 5, 6 and 8 reveals that these compounds hold almost ideal octahedral cadmium surrounding, therefore the observed !" ! !"# value in this complexes lies almost in the same range. They show moderate distortion from axial symmetry and hold an roughly rhombic pattern of the CSA tensor. As there are four different Fe(III) centre linked to cadmium atom in 1, the net symmetry into the Cd(II) site becomes lower than that in 3--6, which exhibit only one type of Fe(III) centre. The compound 1 thus deviates from axiality and shows a !" ! !"# value of 0.77.

Spin density from 113 Cd NMR

The theortical background concerning the estimation of the spin density has already been described in chapter 2 and 3 and will thus not be detailed here.

The dipolar contribution was estimated through the dipole-dipole point approximation by using Equation ( 2), in which g || and g⊥ are the electron g factors parallel and perpendicular to the magnetic axis, r is the distance between paramagnetic source and the nuclear under study, and is the angle between r and the principle magnetic axis of the molecule. 10,11 (1)

As the main magnetic axis is not known, the dipolar contribution of the chemical shift was calculated considering the two extreme cases: an axial poition (θ =0°) and the equatorial one (θ =0°). The accurate information about r (M--Cd distance) was obtained from the XRD data. As in all the cases, r is greater than 5 Å, the dipolar contribution obtained (in both axial or equatorial geometry) is much quite low as compared to the paramagnetic chemical shift. This last one can thus provide a good estimate of the spin density felt by the probed nucleus (the fremi contribution). The electronic g factors obtained from the EPR analysis of the building blocks were used for calculating the dipolar conribution to the chemical shift. In this context we would like to remind the limitations of this approach. In order to estimate the dipolar contribution, one needs accurate information about the anisotropy of the magnetic susceptibility. In principle, it should be the local magnetic susceptibility anisotropy experienced by the probed site, but for practical reasons, it is commonly expressed as a g tensor, which is generally obtained from the EPR studies. The EPR data often need however to be obained at low temperature (< 50 K) (temperature at which signals have satisfactory intensity and good spectral resolution), and thus reflect the electronic state of the compound at these temperatures. In contrast all the NMR spectra are recorded near room temperature. Therefore one should idealy use the g⊥ and g⊥ value obtained from EPR studies around room temperature which is not possible in the present case. Using values obtained at low temperature however leads to an overestimate of the dipolar contribution as the anisotropy is more important at these temperature.

Once estimates of the dipolar contribution obtained, the spin density can be deduced from contact term by the following equation 12 The g av factor can be obtained from magnetic susceptibility measurment. The results of the spin--density calculations are summarised in table 7--9.

g|| and g⊥ calculation

The g|| and g⊥ value for the building block including [Fe(Tp)(CN) 3 ] --, [Ru(acac) 2 (CN) 2 ] --and [Ru(salen)(CN) 2 ] -- were already mentioned in the chapter 2. The g|| and g⊥ value for the building block [Fe(Ttp)(CN) 3 ] --and [Fe(phen)(CN) 4 ] --were obtained by performing low temperature EPR mesurements. We made several attempt to record the EPR spectra of the [Fe(bipy)(CN) 4 ] --compound at low temperature (in X and Q band), but we were not able to successful due to low spectral resolution. Therefore, g|| and g⊥ value of [Fe(phen)(CN) 4 ] --compound will be used for the dipolar shift calculation of [Fe(bipy)(CN) 4 ] --. All the EPR studies were carried out with the help of Dr. Yanling Li (Université Pierre et Marie Curie). The experimental EPR spectra of Fe(Ttp)(CN) 3 ] --, and [Fe(phen)(CN) 4 ] --are given in the annex chapter.

gav calculation

The estimation of the Landé factor g av at room temperature was obtained from bulk suceptibility measurement (already reported in the literature) using the expression given below: 2 g eff 2 = g av 2 = 3k(! M T ) exp ! A " 2 S(S +1)

The g av value for all the paramagnetic building blocks at 300 K are tabulated below. 2--4 (a.u --3 ) --2.51x10 --3 --2.51x10 --3 --4.81x10 --3 --4.85x10 --3 --4.35x10 --3 --4.38x10 --3 (a.u --3 ) --2.28x10 --3 --2.29x10 --3 --4.83x10 --3 --4.86x10 --3 --4.36x10 --3 --4.39x10 From the above tabulated data, we can clearly see that the 113 Cd spin densities in all of these paramagnetic M--CN--Cd based compounds is negative. This confirms the previous observation on PBAs: the polarization mechanism from the nitrogen to the cadmium atom, leads to a negative 113 Cd spin densities. 1,7,8 In the present case the spin density increases with the increase of the number of paramagnetic centre attached to Cd(II) ion. This is solely due to the increase in (super) hyperfine coupling (the interaction between the unpaired electron and the observed nuclear spin). There is a huge difference in the spin density value between Ru--CN--Cd based compounds and Fe--CN--Cd based compounds. This is due to the presence of lower amount of delocalised spin on the cyanide nitrogen atoms of PPh 4 [Ru(acac) 2 (CN) 2 ] and PPh 4 [Ru(salen)(CN) 2 ] building blocks as compared to that in the iron complexes.

The differences in the observed chemical shift value, which is a measure of spin density also, between 9 and 10 arise probably due to differences in spin density distribution in these compounds. Although we do not have any strong evidence in favour of this hypothesis. The DFT calculation for 10 is reported in literature and it shows small amount of positive spin density (+ 0.009 (a.u. --3 )) into the cyanide nitrogen atom. As already mentioned in chapter 2 the DFT calculation in PPh 4 [Ru(salen)(CN) 2 ] is needed to be performed to make any definitive comment about its spin distribution on various atoms. The cadmium sites in 2 and 4 represents similar sarrounding of Cd(NC--Fe) 2 . The estimated spin density value of 2 is slightly lower than 4 inspite of the following facts: (a) cyanide bridge in 2 is slightly more bent (ca. 157.5°) than 4 (ca. 152.6°) (b) Fe--cd distance in 2 is ca. 5.18 Å, where as in 4 it is 5.23 Å. The observed spin density in 5 is higher than 6 inspite of more bending of the cyanide bridges. These observations clearly reveals that in the present case electronic factors play dominant role over geometric factors in the process of spin--density transfer from the paramagnetic sources to the cadmium probe. The previously described (chapter 2) spin density studies on the building blocks PPh present case matches well with the reported studies for PBAs. The differences in spin density value arise due to different calculation approach. More specifically, in the case of PBAs, a g av value of 2.16 was used for the calculation whereas in our present case g av value of 2.69--2.75 was used for the spin density calculation of Fe--CN--Cd containing compounds. It makes the main difference. Otherwise, the present result is in well agreement with the reported studies on PBAs.

In summary, we can say this is the continuation of the work on PBAs. In the present contribution, we intend to get more insight onto the role of geometric and electronic parameters in controlling the spin density delocalization. As a result, several low dimensional {Fe III --CN--Cd II }--based compounds and two {Ru II --CN--Cd II }-based compound have been prepared, structurally characterized and their 113 Cd NMR have been investigated. By performing this study, we have obtained the following important information of these systems:

(a) A qualitative link between number of paramagnetic neighbours linked to Cd(II) ion and the observed chemical shift could be obtained. (b) The dipolar contribution does not dominate to the overall paramagnetic shift. (c) Interestingly, it is observed that compounds with similar paramagnetic neighbouring may exhibit significant differences (see Ru compounds) in the observed chemical shift if the spin delocalization in the building block is diferent. (d) For a same family of Fe(III) compounds, the current datas are insufficient to draw any definitive cnculsion about the role of geometrical and electronic factors on spin density delocalization. (e) Thus, the preliminary results using low dimensional model compounds are thus too limited and do not allow to bring definitive conclusion. Moreover, this result should be compared with theoretical calculations and/or with other sophisticated model compounds.

Conclusion and outlook

In this work, we tried to illustrate through selected examples how useful NMR spectroscopy can be to investigate cyanide--based magnetic molecular materials. The diversity of its potential uses is illustrated through the diversity of the questions that are addressed in this work. NMR allows for example to probebetter than any other experimental technique-small amount of spin densities and reveals the mechanism of spin delocalization in paramagnetic building blocks. This technique can also provide precious structural information, sometime with a higher accuracy than in diamagnetic compounds. It has been used here for example to monitor the Li + intercalation into insertion materials that exhibit potential application in energy storage.

In chapter 2, electronic properties of several mononuclear M(III) cyanide building blocks have been probed through solid state 13 C and 15 N NMR spectroscopy. More specifically, the spin density and its distribution along the cyanide bridges is revealed by taking advantage of the fact that the chemical shift of a paramagnetic species is related to the hyperfine interaction, which itself is correlated to the spin density. It has been observed that the unpaired electron extends from the metal ion to the cyanide carbons through an efficient spin polarisation phenomenon. In contrast, a spin delocalization mechanism is responsible for the positive spin density measured on the nitrogen atoms. The observed spin density in these nitrogen atoms is indeed positive and arises from the presence of unpaired electron that are delocalized in the 2p orbitals through π--back bonding. This phenomenon is general for the cyanide building blocks as expected. Our analysis of the CSA anisotropy is only qualitative in this work, and one of our goal is to investigate more deeply the spin density in 2p orbital through CSA tensor analysis. Actually, it is known that in paramagnetic species, anisotropic components of the chemical shift originate from the presence of unpaired electron in p or d orbitals on the probed nuclei. However our deeper investigation on the electronic properties of the PPh 4 [Fe(Tp)(CN) 3 ] building block through PND and NMR and its comparison with previous measurements on [Fe(CN) 6 ] 3--let us think that the magnetic anisotropy could also be reflected in the lower symmetry of the CSA tensors. Solid--state NMR studies on single crystals undoubtedly represent an important future objective to go further in probing the anisotropy in these materials. In order to extend the influence on the magnetic anisotropy on the NMR signals we have extended our work to the study of Ru(III) complexes that exhibit higher spin--orbit coupling than the iron(III) complexes (keeping the electronic configuration the same). At the present stage, it is however very difficult to make any definitive conclusion as the geometry of the complex and the nature of the ligands have undoubtedly a major influence on both spin delocalization and CSA tensors. More complete investigations of isostructural anisotropic Fe(III), Ru(III) or Os(III) building blocks is highly necessary in order to address these questions.

In the chapter 3, we have investigated the spin--crossover behaviour in solution and in solid--state of a family of Fe(II) complexes, [Fe II (R--bik) 3 ] 2+ , based on α--ketone--β--diimine ligands . Here we have shown that the ligand modification has no major influence on the thermally--induced spin transition in solution. The DFT studies showed that ligand functionalization modulates the electronic properties of the ligand but do not significantly alter the HS--LS energy gap. The solid--state behaviour is clearly more affected by ligand modification as small changes in supramolecular interaction were shown to enhance the cooperativity effect. The photo--induced spin--state change (LIESST effect) appears to be more significantly modified by ligand functionalization and/ or intermolecular interactions but the rationalization of these differences is a challenging task that requires a number of advanced measurements. This is beyond the scope of the present work, which focuses on the use of NMR spectroscopy. Actually, a more particular attention has been here devoted to the study of the spin--crossover equilibria of the [Fe II (R--bik) 3 ] 2+ complexes in solution by variable temperature 1 H NMR. The results have been compared to those obtained by other techniques such as solution SQUID magnetometry, UV--vis experiment, Evans' method. Overall, the results of these getting more insight on the electronic and structural parameters which govern the spin--extension mechanism along the cyanide bridge by investigating Fe--Cd low dimensional model compounds. In part A, the electrochemical insertion and extraction of Li + ions into the lacunary Cd 3 [Fe(CN) 6 ] . 15H 2 O prussian blue analogue has been investigated. The 113 Cd NMR allowed to probe the structural and electronic evolutions at different stages of the alkali insertion/extraction process. In this study, the presence of paramagnetic Fe(III) sources close to the cadmium centre causes a strong enlargement of the spectral window and allows to well identify different cadmium sites. As the Li + get inserted, the Fe(III) are reduced into diamagnetic Fe(II) ions and the Cd signals undergo strong shift that can be rationalized using the data obtained on model PBAs. It should be mentioned that the powder XRD analysis of these battery materials couldn't provide such accurate local structural information. Here we clearly evidenced that the limited capacity of the PBA materials for Li + insertion is directly correlated to a partial reduction of the material during the preparation of the electrode material (PBA --acetylene black). We observed a fairly good structural reversibility upon the intercalation and deintercalation of the Li + , although an aging effect is observed on the mixture PBA --acetylene black. We are currently investigating more precisely the nature of the cadmium sites (and vacancies) that form upon Li + insertion to have a better view on the intercalation process.

The work presented in the part B of chapter 5 focuses on the magnetic information that can be obtained from 113 Cd NMR. In the previous report of 113 Cd NMR study on PBAs, Lescouëzec et coll. observed that in the CsFeCd PBA, the paramagnetic sources delocalize less spin density than in the lacunary CdFe PBA. It was assumed that a bending of the cyanide could indeed alter the spin delocalization along the {Fe--CN--Cd} path. In order to try answering the question we initiate a study on a family of lower dimensional model compounds containing {M III --CN--Cd II } motif. The advantages to study these low dimensional compounds are the following: (a) they can be obtained as single crystal (in contrast with PBAs) so that accurate structural information can be obtained through XRD; (b) the building block approach allows to tune both the structure and the electronic properties of the systems. Overall, the current results are too partial to draw any definitive conclusion. It is clear that structural, chemical and electronic factors are intricate: changing a ligand leads to changes in the distortion of the coordination sphere, in the geometry of the cyanide bridge but also in the electronic properties of the paramagnetic complex. At this stage, it seems that a broader panel of molecular models and complementary magnetic characterizations would be required to have a convenient set of experimental data. In all these figures Y--axis is indicating χ M T (cm 3 mol -1 K) and X--axis is indicating T (K). The solid state χ M T vs T plot fitting of the three SCO complexes mentioned in chapter 3 is given here.

List of abbreviations

FTIR spectra of compounds

NMR spectra of different compounds

!"# $## $"# %## %"# &## &"# ! $ % & !"# $## $"# %## %"# &## &"# ! $ % & !"# $## $"# %## %"# &## &"# ! $ % & !"# $## $"# %## %"# &## &"# ! $ % & !"# $## $"# %## %"# &## &"# ! $ % & !"# $## $"# %## %"# &## &"# ! $ %
Avance 300 and Bruker Avance 600 (300 MHz and 600 MHz) spectrometer. The chemical shifts are expressed in ppm and are referenced in respect to TMS ( 1 H). When possible, the solvent signals were used as internal secondary reference. The multiplicity of the NMR signals are given using the following abbreviations: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, and br = broad signal. High and low temperature experiments were performed on calibrated spectrometer with a 4% methanol in MeOD--d 4 sample for low temperature corrections and a 80% glycol in DMSO--d 6 sample for high temperature corrections. Particular attention was given to paramagnetic samples, so that they are given time to reach thermal equilibrium. Solid--state MAS--NMR samples were prepared from microcrystalline samples packed in zirconium oxide rotors. The size of the rotor was adapted to the spectrometer probe head, it was tilted at the magic angle (θ ≈ 54.7°) and spinned at spectrometer--dependant high frequency. Data were collected on a Bruker Avance 500 spectrometer equipped with a 4--BL MAS--NMR probe head (4 mm diameter rotor - 500 MHzmax. 14 kHz), on a Bruker Avance 300 spectrometer equipped with a 4--MQ MAS--NMR probe head (4 mm diameter rotor -300 MHz -max. 14 kHz), or on a Bruker Avance 700 equipped with a 1.3/2.5--BL probe head (1.3, 2.5 mm diameter rotor - 700 MHz - max. 35/67 kHz), depending on the nature of the nucleus and the properties of the samples. The quadrupolar 59 Co static NMR experiments were recorded on a Bruker Avance 700 spectrometer equipped with a 3.2--BL probe head.

For paramagnetic compounds, some nickelocene was added as internal temperature probe and sample temperature was tuned using a BCU Xtreme cooling unit. About 100 mg sample was needed in case of a 4 mm rotor, but only 10 mg needs for a 1.3 mm rotor.

As previously mentioned the Herzfeld--Berger Analysis was carried out using the module "Solids Line Shape Analysis" from Bruker"s software package Topspin.

Figure 2 .

 2 Figure 2. Precession of the magnetic moment in each of the two possible spin states of an I=1/2 nucleus in external magnetic field B 0 .

Figure 3 .

 3 Figure 3. The alignment of spins in the presence of a static magnetic field and the resulting net magnetization.

Figure 5 .

 5 Figure 5. Graphical representation of the principal components of chemical shift in laboratory frame.

Figure 6 .!

 6 Figure 6. Schematic representation of an axially symmetric chemical shift tensor where (a) δ 11 = δ 22 ≠ δ 33 (top) and (b) δ 11 ≠ δ 22 = δ 33 (bottom). In both cases, δ iso = (1/3)(δ 11 + δ 22 + δ 33 ).16b

Figure 8 .

 8 Figure 8. Solution state 1 H NMR (2.11 T, CDCl 3 ) spectra of ethanol indicative of narrow line shape and muliplet structure. The NMR spectra have been produced from data taken from the Spectral Data Base System for Organic Compounds at the National Institute of Materials and Chemical Research in Japan.

Figure 9 .

 9 Figure 9. Solution (left) and solid--state static (right)13 C NMR (7.04 T) spectra of glycine indicative of narrow and broad line shape and in solution and solid state respectively. The NMR spectra have been produced from data taken from the Spectral Data Base System for Organic Compounds at the National Institute of Materials and Chemical Research in Japan.

Figure 10 .

 10 Figure 10. Schemetic representation of the sample rotation at a magic angle.

T 2 * 2 *

 22 is the name given to the observed value of the decay constant. The figure mentioned below is indicative of T dependancy of free induction decay.

Figure 14 .

 14 Figure 14. Figure represents free induction decay (FID) upon applying a 90° pulse.

Figure 16 .

 16 Figure 16. Number of articles published per year in the field of paramagnetic NMR. (Figure is taken from ISI web of knowledge).

Figure 18 .

 18 Figure18.7 Li MAS NMR spectra for Li x CoO 2 (asterisk indicate spinning sidebands). The LiCoO 2 local environment and resonance due to the metallic phase are marked with asterisk (adapted from ref 50). The spectra were recorded at 30.04 T using 90° pulse sequence. The NMR results(Fig 18) suggest that the biphasic region that exists in Li x --CoO 2 from 0.75 ≤ x ≤0.94 is due to this phase transition. Additional7 Li resonances have been observed for Li--excess nonstoichiometric Li 1+x CoO 2 due to the presence of paramagnetic Co 2+ ions that were formed in conjunction with oxygen and cobalt vacancies.51 Some other interesting materials which is often studied by solid--state Li NMR includes lithium phosphates (LiFePO 4 ), lithium vanadates (LiV 3 O 8 ), lithium vanadium phosphates (Li 3 V 2 (PO 4 ) 3 and Li 3 FeV(PO 4 ) 3 ) etc.

Figure 19 .

 19 Figure19.13 C NMR spectra of microcrystalline CoMMP--12 recorded (a) at 22 kHz MAS at 20 T (10k scans, 3s interscan delay, total time = 15 h) and (b, c) at 60 kHz MAS at 21.2 T (15k scans, 2s and 0.2s interscan delay), respectively, total times = 15h and 3h). Note that sharp diamagnetic signals in c are suppressed with respect to b. All the spectra were recorded using 90° pulse sequence. Adapted from ref 56.As already observed for proteins in solution, PCS (pseudo contact shift) of13 C nuclei can also be used as structural restraints in the solid state. PCS are directly related to the nuclear position with respect to the metal through a simple and quantitative relationship, and a very good agreement between the

29 Figure 20 .

 2920 Figure 20. (Left) Spin densities at the axial CN ligands of Cs 2 K[Fe(CN) 6 ]. (Right) 13 C and 15 N MAS NMR spectra of Cs 2 K[Fe(CN) 6 ] at 326.4 K. Rotational frequencies of 15 kHz and 5 kHz for 13 C and 15 N, respectively. The axial and equatorial isotropic signals are labeled with ☐ and O, respectively. (Figure is adapted from ref 29a). The spectra were recorded at a magnetic field of 75.43 T ( 13 C) and 30.40 T ( 15 N).

30

 30

Figure 21 .

 21 Figure 21. (Left) 113 Cd MAS NMR spectra of Cd 3 [Fe x Co 1--x (CN) 6 ]••15H 2 O. Iron contents are x=0 (1), 0.25 (2), 0.5 (3), 0.75 (4), and 1 (5). The spectra were recorded at 305 K in a magnetic field of 2.35 T, at a MAS frequency of 5 kHz. (Right) Spin sign pattern of an Fe III --CN--Cd II --NC--Fe III fragment of Cd 3 [Fe(CN) 6 ] 2 ••15 H 2 O. The arrows represent positive (up) and negative (down) spin densities. (Figure is adapted from ref 30a).

Figure 22 .

 22 Figure22.13 C MAS spectra of Cu(DL--alanine) 2 ••(H 2 O) recorded at 25.14T with (a) adiabatic CP at ν R = 24 kHz, (b) onepulse excitation at ν R = 24 kHz, and (c) one--pulse excitation at ν R = 5 kHz, where cw 1 H decoupling (100 kHz) was applied only in (c). A total of 512 scans was recorded for each spectrum with recycle delays of 0.15 s in (a,b) and 0.3 s in (c). (Figure is adapted from ref 74).

Figure 23 .

 23 Figure 23. Schematic representation of the structurally characterized iron--sulfur centers in proteins:monoiron center, as present in rubredoxin, Fe 2 S 2 cluster, Fe 3 S 4 cluster, and Fe 4 S 4 cluster respectively from left to right.75

Figure 24 .

 24 Figure 24. Right figure shows the 1 H NMR spectrum (recorded at 7.04 T) of reduced D. gigas rubredoxin at 328 K: (A) 260--100 ppm region, (B) 20 to --10 ppm region. Adapted from ref (79c). Left figure is indicative of α and β protons in rubredoxin. It is expected that eight resonances will be observed for the cysteine β--CH 2 protons. The apperance of only four signals in the experimental spectrum could be explained by considering quasi--D 2d symmetry of the metal center obtained in X--ray analysis. 80a,b The broadness of the C,D signals than that of A,B was explained by considering a ligand conformation placing one β proton closer to the iron ion for each geminal pair. Each signal would then correspond to two protons of different cysteine residues symmetrical to one another. The large line widths of these signals (up to 4500 Hz) have been appeared as an obstacle to perform NOE or 2D experiments on them in order to perform a sequence specific assignment of the ligands. One recent work demonstrates the inability to detect 2D connectivities near the paramagnetic center both in the oxidized and reduced forms. 80 B. Fe 2 S 2 proteins

84

 84

Figure 25 . 1 H

 251 Figure 25. 1 H NMR spectra of Fe 2 S 2 ferredoxins: (A) oxidized ferredoxin from P. umbilicalis, recorded at 303 K and 4.69 T (B) 1 H NMR spectrum of reduced Fe 2 S 2 spinach ferredoxin at 7.046 T (signals L, M, N, O, A and E follow a Curie behavior, whereas signals B, C, D and F display an anti--Curie behavior). Adapted from ref 75.

Figure 26 .

 26 Figure 26. Synthetic scheme of paramagnetic chromium(III) olefin polymerization catalysts. Figure is adapted from ref 87.The 1 H NMR study of these complexes was reported by Enders et al.87 The important outcome of their NMR study is highlighted below.

Figure 28 .

 28 Figure 28. 1H NMR spectra in CDCl 3 (295 K, 200 MHz) of 1 (top) and reaction product of 9K with [CrCl 2 Me(THF) 3 ] (bottom). The dotted lines connect signals of 1, while the dashed lines point to corresponding signals in 8. Asterisks show the signals of the four nonequivalent CH 3 groups in 8. Figure is adapted from ref 87.The paramagnetic shifts in 8 are larger as compared to the shifts in 1. The Fermi contact shift is notably stronger at H 4 , which is in a para position relative to the metal. A new signal appears at --4 ppm, which belongs to H7 . In 1, this resonance was hidden in the diamagnetic region.87 The invisibility of the the chromium bonded CH 3 signal was ascribed due to proximity to the paramagnetic center. Therefore they were strongly shifted and extremely broadened. The assignment of the Cp--methyl groups was more difficult. On going from 1 to 8, the mirror plane disappears, and consequently, the former two signals split into four. Three of the latter signals were obtained in the expected --50 to +50 ppm range, whereas the fourth signal appeared at a very low field (asterisks in Figure22). AlthoughEnders et al. were able to identify the four methyl groups of 8, they were unable to provide any definitive conclusion about specificity of the signals. The final assignments of 1H NMR signals for these compounds were done with the help of quantum mechanical calculations.

Figure 1 .

 1 Figure 1. Molecular structure of the anion in 1. Side (left) and top (right) view (along the Fe1••••••B1 axis). Yellow-brown, orange, grey and blue spheres represent Fe, B, C and N atoms, respectively; H atoms, solvent molecules and tetraphenyl phosphonium countercation are omitted for clarity.

Figure 2 .

 2 Figure 2. Molecular structure of the anion in 2. Side (left) and top (right) view (along the Co1••••••B1 axis). Yellow, orange, black and blue spheres represent Co, B, C and N atoms, respectively; H atoms, solvent molecules and tetraphenyl phosphonium countercation are omitted for clarity.

Figure 3 .

 3 Figure 3. Molecular structure of the anion in 3. Yellow--brown, red, grey and blue spheres represent Fe, O, C and N atoms, respectively; H atoms and tetraphenyl phosphonium countercation are omitted for clarity.

Figure 4 .

 4 Figure 4. Molecular structure of the anion in 4. Green, red, grey and blue spheres represent Ru, O, C and N atoms, respectively; H atoms and tetraphenyl phosphonium countercation are omitted for clarity.

Figure 5 .

 5 Figure 5. Molecular structure of the anion in 5. Green, red, grey and blue spheres represent Ru, O, C and N atoms, respectively; H atoms and tetraphenyl phosphonium countercation are omitted for clarity (adapted from ref 14b).

PPh 4 [

 4 Fe III (Tp)(CN) 3 ]••H 2 O (1)The EPR spectrum of PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O[1] was recorded at 5 K before the beginning of this work (Figure6).

Figure 6 .

 6 Figure 6. EPR spectra of ground PPh 4 [Fe(Tp)(CN) 3 ]••H 2 O (1) in X--band (9.42 GHz) at 5 K. PPh 4 [Fe III (Tp)(CN) 3 ] exhibits a orthorhombic g--tensor, with ! ! = 1.1, ! ! = 0.57 and ! ! = 3.7. The EPR signal intensity decreases with increasing temperature and completely disappeared above 50 K. For the needs of the spin density calculations presented in the rest of this chapter, one can define a ! ! and a ! ∥ as follows:

  g

4 [

 4 Fe III (Tp)(CN) 3 ]••H 2 O [1], we obtain : ! ! = 0.8 and ! ∥ = 3.7.PPh 4 [Fe(pcq)(CN) 3 ] (3) The X--band EPR spectrum of a crysalline sample of PPh 4 [Fe(pcq)(CN) 3 ] [3] was recorded at 4 K.

Figure 7 .

 7 Figure 7. X--band EPR spectra of a polycrystalline sample of PPh 4 [Fe(pcq)(CN) 3 ] at 4 K.

Figure 8 .

 8 Figure 8. X--band EPR spectra on a crystalline sample of PPh 4 [Ru(salen)(CN) 2 ] at 4 K. PPh 4 [Ru III (acac) 2 (CN) 2 ] (5)The EPR spectrum of PPh 4[Ru II (acac) 2 (CN) 2 ] [5] was reported by L. Toma et al.14 The Q--band EPR spectrum of a polycrystalline sample of this compound at 67 K (Figure7) shows a rhombic pattern with g 1 = 2.55, g 2 = 2.21 and g 3 = 1.69.

Figure 9 .

 9 Figure 9. Q--band EPR spectra of a polycrystalline sample of trans--PPh 4 [Ru(acac) 2 (CN) 2 ] at 67 K. 14a

Scheme 1 .

 1 Scheme 1. chemical shift tensor with principal components. 38 Solid--state NMR spectroscopy of PPh 4 [Co(Tp Me2 )(CN) 3 ] (2)

  C) = 135.1 ppm and δ( 15 N) = --83 and --78 ppm in [Co III (CN) 6 ] 3-and δ( 13 C) = 136 ppm and δ( 15 N) = --80.2 ppm in [Co III ( Me2 Tp)(CN) 3 ] --) as compared to the overall large signal shift measured in this work. 8b

Figure 9 .

 9 Figure 9. 13 C MAS--NMR spectra of the diamagnetic PPh 4 [Co III ( Me2 Tp)(CN) 3 ] at 6 (red) and 10 kHz (black) recorded at a 13 C--Larmor frequency of 125.77 MHz. The three types of isotropic signals are marked with an asterisk (isotope enriched cyanides), a black square (PPh 4 + / Me2 Tp), and a circle (Me groups of Me2 Tp).

Figure 10 .

 10 Figure 10. Simulated (top) and experimental (bottom) 13 C MAS--NMR spectra of the diamagnetic PPh 4 [Co III ( Me2 Tp)(CN) 3 ] at 6 kHz and at a 13 C--Larmor frequency of 125.77 MHz. The position of the isotropic peaks is marked with an asterisk, and the enlarged area is displayed in the inset.

Figure 11 .

 11 Figure 11. 15 N MAS--NMR spectra of the diamagnetic PPh 4 [Co III ( Me2 Tp CN) 3 ] at 10 (black curve) and 6 kHz (red curve) at a Larmor frequency of 50.87 MHz. The isotropic peaks are noted with an asterisk and a circle indicates residual PPh 4 C 15 N. The inset shows the isotropic peaks of the 10 kHz spectrum.

  4 ppm and unambiguously attributed to a small portion of co--crystallized PPh 4 C 15 N. The three cyanide sites at δ iso at --76.8 (a), --80.2 (b) and --84.0 (c) ppm are close to the isotropic values reported in the literature for the hexacyanocobaltate(III) (--81.9 ppm), PPh 4 [Co III (bipy)(CN) 3 ] (--68.1 ppm and --81.1 ppm) and PPh 4 [Co III (phen)(CN) 3 ] (--66.6 ppm and --79.9 ppm). 8b Signals (a) and (b) exhibit a strongly axial symmetry (η a ≈ η b ≈ 0) while (c) departs slightly from axiality (η c ≈ 0.216)

Figure 12 .

 12 Figure 12. Simulated (top) and experimental (bottom) 15 N--MAS--NMR spectra of PPh 4 [Co III ( Me2 Tp)(CN) 3 ] at 6 kHz and at a Larmor frequency of 50.87 MHz. The three enriched cyanide isotropic peaks are marked with an asterisk, the impurity peak is marked with a circle. Best overlap for 6 kHz: 80.37%

Figure 13 .

 13 Figure 13. Simulated (top) and experimental (bottom) 15 N--MAS--NMR spectra of the diamagnetic PPh 4 [Co III ( Me2 Tp)(CN) 3 ] at 3 kHz and at a Larmor frequency of 50.87 MHz. The cyanide isotropic peaks are marked with an asterisk. Best overlap for 3 kHz: 85.04%

  rate, the temperature was set in such a way that the 1 H nickelocene signal was ca. δ = --229 ppm. The actual temperature inside the rotor was thus 334.6 K and 334.4 K respectively for 60 KHz and 65.5 kHz MAS spin rate. The inner temperature slightly decreased overnight during the 65.5 kHz measurement, so that a slight broadening of the signals width could occur. The spectra were acquired using a Hahn--echo sequence, and their baseline distortions were manually corrected. Superposition of the spectra obtained at 60 and 65.5 kHz allowed the determination of the isotropic shift (! ! !"# = --3752 ppm). Another measurement at 48 kHz (not displayed) was carried out in order to confirm this attribution.

Figure 14 .

 14 Figure 14. 13 C MAS--NMR spectra of PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O at 60 kHz (red) and 65.5 kHz (black) at T = 334 K acquired at a 13 C--Larmor frequency of 175.37 MHz. The isotropic peak, which is not shifted, is marked with an asterisk.

Figure 15 .

 15 Figure 15. Simulated (top) and experimental (bottom) 13 C spectra of PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O at 60 kHz at T = 334.6 K acquired at a Larmor frequency of 175.37 MHz.

  Figure 16. 15 N MAS--NMR spectra of PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O at 6 kHz (red) and 10 kHz (black) at T = 310 K acquired at a 15 N--Larmor frequency of 40.55 MHz. The isotropic peak, which is not shifted, is marked with an asterisk. Inset: zoom on the isotropic signal set at 10 kHz.

Figure 17 .

 17 Figure 17. Simulated (top) and experimental (bottom) 15 N MAS--NMR spectrum of PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O acquired at a Larmor frequency of 40.58 MHz, δ(NiCp 2 ) = --245.9 ppm (T = 310.3 K) and a spinning rate of 6 kHz.

Figure 21 .

 21 Figure 21. Simulated (top) and experimental (bottom) 15 N MAS--NMR spectrum of 3 acquired at a Larmor frequency of 30.46 MHz, δ(NiCp 2 ) = --250.1 ppm (T = 304.8 K) and a spinning rate of 8 kHz. Overlap: 80.64%.

  density calculations. The third signal, located aroud 607 ppm also exhibit a non axial symmetry with η = 0.47. This signal has a lower anisotropy prameter (Δδ = --469 ppm) in absolute value compared to the other two signals.

Figure 23 .

 23 Figure 23. Simulated (top) and experimental (bottom) 15 N MAS--NMR spectrum of 4 acquired at a Larmor frequency of 30.46 MHz, δ(NiCp 2 ) = --247.05 ppm (T = 308.8 K) and a spinning rate of 3 kHz. Overlap: 90.34%.

Figure 26 .

 26 Figure26.15 N MAS--NMR spectra of 5 at 2 kHz (red) and 5 kHz (black) at 308.8 K acquired at a Larmor frequency of 30.46 MHz. Isotropic peaks are marked with an asterisk.

  6 ppm and 279.01 ppm (308.8 K) signal, respectively and anisotropy parameters of Δδ = --553 and --477 ppm. The values are comparable to the above described Ru(III) complex, PPh 4 [Ru(salen)(CN) 2 ].

Figure 27 .

 27 Figure 27. Simulated (top) and experimental (bottom) 15 N MAS--NMR spectrum of 5 acquired at a Larmor frequency of 30.46 MHz, δ(NiCp 2 ) = --247.0 ppm (T = 308.8 K) and a spinning rate of 2 kHz. Overlap: 94.49%.

4. 1

 1 Spin density on the cyanide in [Fe(Tp)(CN) 3 ] --from NMR PND experiments shown that the easy axis of magnetisation (z axis) lies along the pseudo C 3 symmetry axis (B/C••••••Fe axis). Thus the ! ! ranges from 124.53° to 127.29° for 13 C and from 122.64° to 127.12° for 15 N. The ψ angles are ca. 30°, 30° and 90° for three cyanide ligands. The r j value ranges from ca. 1.91 Å to 1.93 Å for Fe III --C and from 3.04 Å to 3.07 Å for Fe III --N.

Figure 28 .

 28 Figure 28. Fe III magnetic susceptibility ellipsoid obtained from PND experiments (susceptibility tensor method) within its coordination octahedron. The local easy magnetization axis (χ 1 in blue) is nearly parallel to the C 3 pseudo--axis (in red) of the molecule. (Courtesy of B. Gillon et al., CNRS--CEA, LLB).

' 4 . 2

 42 Spin density of other low--spin d 5 complexes (3--5) from NMR

compound 1 ,

 1 we have made an attempt to study the spin structure of the paramagnetic PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O complex through PND method in the collaboration with the group of B. Gillon (CEA). We will only mentioned here below the experimental results obtained from this study. The theoretical calulation of spin density on PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O and PPh 4 [Ru III (acac) 2 (CN) 2 ] made by other groups will also be mentioned. (a) Spin density through DFT calculations PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O [1]A preliminary analysis of the electronic properties of the compound was carried out through DFT calculation by Karine Costuas (University of Rennes 1, France). DFT calculations (using [B3LYP] hybrid functionals).

Figure 29 .

 29 Figure 29. Left figure shows the spin density distribution of PPh 4 [Fe III (Tp)(CN) 3 ] (grey atoms: carbon, blue ones: nitrogen, orange one: iron, yellow one: boron) at the [B3LYP] level of theory. Positive spin density in red, negative spin density in blue (Courtesy of Dr. Karine Costuas). Right figure is showing the atom labelling of the cyanide ligands in [Fe III (Tp)(CN) 3 ] --unit.

14

 14

Figure 30 .

 30 Figure 30. Spin density map for model 5. The values of the atomic spin densities in electron units are: +0.86 (Ru), +0.04 (O), -0.02 (C cyano ), +0.01 (N cyano ). Spin density is plotted with cutoff at 0.010 e (adapted from ref 14a). The spin density expressed in electron fraction in the work of L. Toma et al. has been converted into atomic units. The average spin density on the cyanide carbon atoms of [Ru III (acac) 2 (CN)2 ] --is --0.011 a.u --3 , whereas, the mean spin density on the cyanide nitrogen atoms is +0.009 a.u --3 . Here most of the spin density arising from the paramagnetic Ru(III) ion extends into the equatorial plane of the compound. The oxygen atoms of the acac ligand bear for example much more spin density than carbon and nitrogen atom of cyanide ligand.

Figure 31 .

 31 Figure 31. Atomic spin populations of [Fe III (Tp)(CN) 3 ] --obtained from polarized neutron diffraction (PND) through multipole analysis. The numbers are given in μ B unit.

2. 1

 1 Preparation of the complexes Materials. All chemicals and solvents were used as received. Ligands including Me--bik [bis(1-methylimidazol--2--yl)ketone], Et--bik [bis(1--ethylimidazol--2--yl)ketone] and V--bik [bis(1--vinylimidazol--2-yl)ketone] are prepared according to literature procedures.

3 )Figure 1 :

 31 Figure 1: View of the [Fe II ( R bik) 3 ] 2+ cation in the crystal of 1, 2 and 3 at 200 K and atom numbering of the coordination sphere. All hydrogen atoms and tetrafluoroborate anions have been omitted for clarity. Dark blue plate--like crystals 1 and 2 suitable for X--ray diffraction were obtained in methanol whereas compound 3 crystalized in acetonitrile--water (4/1) solution. All the structures were determined at 200 K. Their crystal structures consists of [Fe II ( R bik) 3 ] 2+ cationic units, crystallization molecules (only for compound 1) and BF 4 --counter anions, which are interlinked by van der Waals interactions. A view of the cationic trischelate complexes in 1, 2 and 3 are depicted in Figure 1. Selected bond lengths and angles are given in Table2. In the three cases, the iron(II) metal centres are in N 6 coordination sphere. They are coordinated by three N,N--chelate--bonded R--bik ligands, resulting in a slightly distorted octahedral coordination geometry. The Fe--N distances range from 1.983(2)--2.008(2) Å (1), 1.988(13)--2.004(13) Å (2) and 1.977(3)--1.995(3) Å (3). These bond lengths are typical of a low--spin iron(II) metal centre (S = 0).15 It is worth noticing that other crystal structures with the [Fe II (Me--bik) 3 ] 2+ cation have already been reported with other counter anions.16,17,18 The crystal structure of [Fe(Me--bik) 3 ](ClO 4 ) 2 measured at 298 K 16 reveals a high--spin iron(II) compound with an average Fe--N distance of 2.14 Å, whereas in the compounds [Fe(Me--bik) 3 ]Cl 2 17 and

Figure 2 :

 2 Figure 2: Intermolecular C--H•••π interactions (black dotted lines) between adjacent complexes [H atoms not involved in interactions have been omitted for clarity] in 1 (left) and 2(right).Finally there is no important intermolecular interaction in 1--3, neither strong hydrogen bonding or π-π interaction (in agreement with the gradual spin transition observed for the three compounds, see below). Nonetheless short contacts (interatomic shorter than the sum of the van der Waals radii) are found between C--H groups from methyl and ethyl and the imidazol ring plane, which are oriented vertically to those substituents. Here, the crystal packing of the complexes is thus most probably influenced by these CH•••π interactions between imidazolyl ring and alkyl proton. Such a CH•••π interaction is often referred to as a point--to--face or T--shaped arrangement22 and their energy lie in the range of 1--2 kJ/mol.23 In 1 and 2 the average distance of CH•••C(π) contacts are 2.78 Å and 2.77 Å respectively. These distances are quite significant as they lie at the middle of the accepted distance range for such supramolecular interaction (from 2.6 to 3.1 Å).24 When looking at such interaction, it is also possible to look at the intermolecular CH•••ring--centroid interaction. Although it is more significant in 2 (2.67 Å) than 1 (2.92 Å), they remain both cases within the accepted distance range.

  [a] List of the intermolecular C--H•••(π) contacts [CH•••C < 2.9 Å, CH•••centroid < 3.0 Å (distances are valid for 1 and 2. 3 is enlisted only for comparison reason)] [b]The sum of the contact radii of carbon(1.70 Å ) and hydrogen (1.20 Å ) is 2.90 Å as given by: A.Bondi, J.Phys. Chem. 1964, 68, 441.

Figure 3 .

 3 Figure 3. Thermo--gravimetric analysis of 1, 2 and 3 in the 20--130 °C range at 2°C/min (under a dinitrogen flow of 100mL/min).

1 , 2 2, 2 3, 2 Figure 4 .

 12224 Figure 4. UV--Vis spectra of 1, 2 and 3 monitored as a function of temperature between 100 (LS, blue) and 450 K (HS, reddish brown).

Figure 5 .

 5 Figure 5. Plot of χ M T vs T for compound 1, 2 and 3 under an applied magnetic field of 1 T: fresh sample introduced at 200 K. (red: first measurement upon heating; grey: way back upon cooling).

Figure 6 .

 6 Figure 6. χ M T vs irradiation time of fresh samples of 1 and 3 with under laser light irradiation at 20 K.

Figure 7 .

 7 Figure 7. χ M T vs irradiation time of dehydrated sample of 3 under laser light irradiation at 20 K.

Figure 8 .

 8 Figure 8. Temperature dependence of the χ M T product for 1(a) after irradiation at 808 nm laser light (b) 3 after irradiation at 405 nm laser light. However these results can already be compared to those obtained on related SCO systems. Recently our group studied a number of switchable molecular systems containing the {Fe(bik) 2 (N--) 2 } subunit (where N--is a cyanide--based complex), among them a {Fe III 2 Fe II 2 } and a {Mo v 2 Fe II 2 } molecular squares (N--= [Fe(Tp)(CN) 3 ] -- and [Mo(CN) 8 ] 3--, respectively). 9c, 10b Both of them exhibit a much more efficient LIESST effect, with an almost 100 % LSHS conversion but only in the dehydrated phase. 9c For the {Fe III 2 Fe II 2 } square, the most efficient photomagnetic effect occurs in the 700--900 nm range whereas for the {Mo v 2 Fe II 2 } square it is observed at 405 nm. Significant differences are also observed on the relaxation temperature, which are lower in the square complexes (T LIESST = 45 and 48 K, respectively). These results show how critical is the role of the N--donor atom on the photomagnetic process of the {Fe(bik) 2 (N--) 2 } subunit (eg by modifying the nature of the excited state).

  The below figure represents the NMR spectra of the three mononuclear iron(II) complexes in acetonitrile--d 3 at 310 K.

Figure 9 . 1 H

 91 Figure 9. 1 H NMR spectra of the three mononuclear complexes [Fe(Me--bik) 3 ](BF 4 ) 2 (1), [Fe(Et--bik) 3 ](BF 4 ) 2 (2) and [Fe(Vinyl--bik) 3 ](BF 4 ) 2 (3) in CD 3 CN at 310 K.

Figure 10 .

 10 Figure 10. Schematic representation of Evans' NMR tube. Regular bore NMR tube contains diamagnetic reference solvent, paramagnetic sample solution is in the narrow capillary tube.

Figure 11 .

 11 Figure 11. Superposition of Evans' NMR spectrum of three compounds, obtained at 300 K in a double-walled NMR tube, in CD 3 CN (inner) vs pure CD 3 CN (outer). CD 3 CN signals are marked with an asterisk(*).The signal arsing from the inner tube is present on the left side (higher chemical shift) of each spectrum compare to the pure CD 3 CN (outer tube).

Figure 12 .

 12 Figure 12. Thermal dependence of χ M T for three complexes (1, black; 2, red; 3, blue respectively) obtained

Figure 13 :

 13 Figure 13: 1 H NMR spectral change upon heating a solution of [Fe(Et--bik) 3 ](BF 4 ) 2 (2) in acetonitrile--d 3

Figure 14 : 3 Figure 15 : 1 H

 143151 Figure 14: 1 H NMR spectral change upon heating a solution of [Fe(Et--bik) 3 ](BF 4 ) 2 (2) in acetonitrile--d 3

Figure 16 . 2 Figure 17 .

 16217 Figure 16. Thermal variation of δT for the H b proton of three complexes (1,black; 2,red; 3,blue respectively) obtained from variable temperature NMR study.

Figure 18 .

 18 Figure 18. CH 2 signal(*) of compound 2 obtained from NMR study at 300 MHz and 600 MHz

Figure 20 .

 20 Figure 20. UV--vis spectral changes observed for (a) 1, (b) 2 and (c) 3 in acetonitrile upon variation of temperature in the range 264 K to 349 K. Inset represents mole fraction in the HS state (γ HS ) vs temperature curve.The spectra were recorded in the temperature range of 264 --349 K with an interval of 5 K. The concentration of the measured solutions were ca. 5 x 10 --4 mol L --1 (1), 2 x 10 --4 mol L --1 (2) and 1 x 10 --4 mol L --1 (3). The variable temperature spectra in the range 350 nm -- 800 nm are depicted in figure20. The intense

Figure 21 .

 21 Figure 21. Regular solution model plot: data obtained from different method (in CD 3 CN) of the three SCO complexes.

Figure 22 .

 22 Figure 22. Left figure represents energy splitting of d--based orbital's for 1. Right one represents the molecular orbital d(x 2 --y 2 ) that shows the presence of extended π--interaction in 1.

Figure 23 .

 23 Figure 23. B3LYP computed spin density plot for complex 1.

Figure 24 .

 24 Figure 24. B3LYP computed optical properties for complex 1--3.

Figure 2 .

 2 Figure 2. Infrared spectra of 1, 2 and 3 at room temperature in the 2000--2800 cm --1 range (ATR).

Figure 3 .

 3 Figure 3. X--ray structure of the [Co III ( Me2 Tp)(CN) 3 ] --in compound 1. Yellow, orange, grey and blue spheres represent Co, B, C and N atoms, respectively; H atoms and solvent molecules are omitted for clarity.

Figure 6 .

 6 Figure 6. View of a {Co III 2 Co II 4 } repeating unit (highlighted with sea--blue colour) in the 2D network. In the {Co III 2 Co II 4 } core unit, the Co III --CN--Co II edges are quasi identical [4.962 and 5.040 Å] and the angles at the corners slightly deviates from orthogonality [Co III --Co II --Co III = 91.39° and Co II --Co III --Co II = 88.61°]. Although the cyanide bridges are bent on the Co II side [Co II --N--C value ranges from 163.3(2) to 167.1(2)°], they remain close to linearity on the Co III one [Co III --C--N value ranges from 172.4(2) to 179. 5(2)°]. In the [Co III ( Me2 Tp)(CN) 3 ] --subunit the third cyanide (Co III --C1--N1) acts as a linker between neighbouring {Co III 2 Co II 4 } units. The Co III --C cyano bond distances range from 1.871(2) to 1.889(2) Å. These values are slightly higher than those observed in the related low--spin [Co III (Tp Me2 )(CN) 3 ] --building block and coherent with a low--spin cobalt(III) ion. The angles subtended by the tripodal ligands at the cobalt(III) ions slightly deviate from orthogonality [86.95(9) to 92.9(1)°]. As already mentioned two different types of Co(II) centres are present in the complex: one cobalt atom (Co2) is in a CoN 2 O 3 surrounding while the other (Co1) exhibits a CoN 1 O 3 coordination sphere. The Co II --N NC bond lengths vary between 2.025(2) to 2.057(2) Å for Co2 and for Co1 they are 2.024(2) Å. These values match well the observed bond lengths in trigonal bipyramidal and tetrahedral HS Co(II) complexes in the case of Co2 and Co1 respectively. Overall the single crystal XRD experiment supports the presence of cobalt(III) and cobalt(II) ions at 200 K, and are also coherent with the magnetic data. The layers are quite well isolated from each other, the shortest intermolecular Co III --Co III and Co III --Co II distances between two layers being 12.00 and 10.05 Å.

Figure 7 .

 7 Figure 7. Thermo--gravimetric analysis of 1 (orange), 2 (green) and 3 (purple) in the 20--150 °C range at 2°C/min (under a dinitrogen atmosphere).

Figure 9 .

 9 Figure 9. Variable temperature solid--state UV--Vis spectra of crystallites of 2 (a) Left figure shows heating the sample from 100--300 K (dispersed in KBr) and (b) Right figure represents heating the sample from 300--450 K (dispersed in KBr).

Figure 10 .

 10 Figure 10. (a) Left figure indicates the ! M T product vs. Irradiation time of a fresh sample of 2 under laser light irradiation (b) right one represents LIESST effect after irradiating the sample with 635 nm laser diode.

Figure 11 .

 11 Figure 11. Nuclear charge distribution in a spin ½ nuclei and quadrupolar nuclei (I>1/2)

Figure 13 .

 13 Figure 13. Solution 59 Co NMR spectra of 1 and 2. The isotropic shift δ iso and full--width--at--half--height (FWHH) values are indicated on the right.

Figure 14 .

 14 Figure 14. Cobalt--59 NMR static NMR spectra of fresh samples of 1, 2 and 3 acquired at 16.4 T, 260 K. Inset represents static NMR spectra of a previously dehydrated (in TGA) sample of 2 at 260 K, 16.4 T.

Figure 15 .

 15 Figure 15. Evolution of Cobalt--59 static NMR spectra of fresh samples of 2 (Left) and 3 (right) upon variation of temperature at 16.4 T. High temperature spectra is displayed above low temperature one.

a

  The EFG parameters are C Q = eQV ZZ /h and η Q = (V XX -V YY )/V ZZ where |V ZZ | ≥ |V YY | ≥ |V XX |, e is the electron charge and h is Planck's constant.

Figure 17 .

 17 Figure 17. Cobalt--59 MAS NMR spectra of 1 at 16.4 T, ν MAS = 20 kHz. Simulated spectra are displayed above experimental spectra. The centreband is denoted with an asterisk.

Fig 1 .

 1 Fig 1. 113 Cd MAS NMR spectra of CsCd[Fe x Co 1--x (CN) 6 ].0.5H 2 O. Iron contents are x = 0 (6), 0.25 (7), 0.5 (8), 0.75 (9) and 1(10). Doted lines refer to six different cadmium surroundings having a variable number of neighbouring Fe. The spectra were recorded at 305 K, under a 2.35 T magnetic field, and at a MAS frequency of 5 kHz.

Fig 2 .

 2 Fig 2. Schematic representation of the PBA structure with [M(CN) 6 ] . vacancies and inserted alkali cation (C: grey, N: blue, O: red, alkali ion: violet).

Fig 4 .

 4 Fig 4. Ex--situ powder X--ray diffraction pattern for the Li+ intercalation materials 2--8 (compounds 2--3 and 4--5 measured by XRD represent intermediate state between the compounds 2 and 3, and 4 and 5 studied by NMR).

Fig 5 .

 5 Fig 5. 113 Cd NMR spectra of Cd 3 [Co x Fe 1--x (CN) 6 ] 2 (x = 1 (1), 0.75 (2), 0.50 (3), 0.25 (4), 0 (5)). The vertical lines indicate the position of the {Cd(--NC--Fe) n (H 2 O) 6--n } sites.

  In order to check the purity of the parent compound, the 113 Cd MAS NMR spectra of CdII 1.5 [Fe III (CN) 6 ] . 7H 2 O was recorded. The spectra were recorded at two different MAS spinning rates to identify the isotropic chemical shift (which should not move with MAS spin rate).

Fig 6 .

 6 Fig 6. Experimental 113 Cd MAS--NMR spectrum of 1 acquired at a MAS spinning speed of 20 and 30 kHz. Spectra recorded at 500 MHz NMR.

Fig 7 .

 7 Fig 7. Evolution of 113 Cd MAS--NMR spectra of 2 acquired at different interval of time. Spectra recorded at 500 MHz NMR (for those spectra aquired at high speed, the temperature has been decreased to avoid the heating effect due to the spinning).

Finally

  , in order to investigate the role of different blocking ligands on spin transfer mechanism, we have performed the solid state 113 Cd NMR study on molecular zig--zag chains of formula [{Fe(bipy)(CN) 4 } 2 {Cd(H 2 O) 2 }] and [{Fe(phen)(CN) 4 } 2 {Cd(H 2 O) 2 }] .4H 2 O. In order to investigate the influence of cadmium surrounding on spin density we have prepared various topological chains based on [Fe(Tp)(CN) 3 ] -- building block with molecular formula [{Fe(Tp)(CN) 3 } 2 {Cd(H 2 O) 2 }] . 5H 2 O, [{Fe(Tp)(CN) 3 }{Cd(nc)(NO 3 )}] and [{Fe(Tp)(CN) 3 } 2 {Cd(bpym)(H 2 O)}]. In addition to that one tri nuclear unit based on [Fe(Ttp)(CN) 3 ] --building block is also studied through 113 Cd NMR.

  The compounds 6 and 8 are isostructural. Both of them crystallize in the monoclinic P2 1 /n space group. Their structure consist of neutral centrosymmetric double zigzag chains of formula [{M III (phen)(CN) 4 } 2 {Cd II (H 2 O) 2 }].XH 2 O (M = Fe(6) and Co(8)) and water molecules of crystallization, which are linked by hydrogen bonds and van der Waals interactions. Within each chain, the [M III (phen)(CN) 4 ] --unit acts as a bismonodentate ligand toward Cd II (H 2 O) 2 units through two of its four cyanide groups in cis positions affording bimetallic double chains containing M III 2 Cd II 2 tetranuclear motifs (Figure1). Two nitrogen atoms from the diimine ligands and four cyanide carbon atoms, featuring a distorted octahedral geometry, coordinate each M(III) metal atom in 6 and 8. The average M III --phen nitrogen bond lengths (2.003(3), 1.970(1) Å in 6 and 8 respectively) and the average angles subtended by the phen at the iron atom (82.1(1)° and 83.24(4)° in 6 and 8 respectively) are close to those reported for the previously reported double zigzag chains based on the [M III (phen)(CN) 4 ] --building block.2 The Fe III -C bond lengths in 6 (1.931(3)-1.959(3) Å) and the Co III -C ones in 8(1.865(

Figure 1 .

 1 Figure 1. Perspective view of a fragment of the zig--zag chain in 6 (left) and 8 (right). The metal atoms are represented as brown (Fe), yellow (Co) and violet (Cd) spheres, and the crystallization water molecules have been omitted for clarity.

Figure 2 .Figure 3 .

 23 Figure 2. Perspective view of hydrogen bonds (marked with orange--geen dotted lines) involving the coordinated water in (left) 6 and (right) 8. The M III --C-N angles for both terminal (177.0(3)--177.9(3)° in 6 and 177.3(1)--177.4(1)° in 8) and bridging (176.8(3)--177.2(3)° in 6 and 177.4(1)--178.0(1)° in 8) cyanide groups slightly deviate from ideal linearity. The cadmium atoms are hexacoordinated: two water molecules in trans positions and four cyanide nitrogen atoms build a distorted octahedral surrounding around the metal atoms. SHAPE analysis reveals that Cd(II) sites in these two molecules holds a nearly ideal octahedral geometry (See table 2 and reference therein).The Cd-O(w) bond lengths are 2.312 Å (6) and 2.285 Å (8), whereas the Cd-N bond distances are 2.307--2.364 Å (6) and 2.283--2.357 Å (8). The values of the Cd II -N-C links range from [157.2°--159°] for 6 and [155.2°--158.7°] for 8. These values clearly indicate deviations from linearity. The C-N bond lengths for terminal cyanide groups [1.146(4)--1.153(4) Å (6) and 1.152(2)--1.153(2) Å (8)] are very close to the C-N bond distances for the bridging ones [1.154(4)--1.162(4) Å (6) and 1.148(2)--1.153(2) Å (8)]. The values of the intrachain Fe------Cd separation through bridging cyanides are 5.301--5.353 Å, and intrachain Co--Cd separation is 5.192--5.280 Å. The shortest interchain Fe------Fe and Co------Co separations are 9.945 Å and 9.424 Å for 6 and 8, respectively. The shortest intermolecular M------Cd separations are 7.613 Å (6) and 7.528 Å (8), respectively. Finally, hydrogen bonds involving the crystallization water molecules and one of the terminal cyanide ligands, and the coordinate water molecule connect the chain to each others and likely play an important role to stabilize the crystal lattices in 6 and 8 (Figure2).

!

Figure 4 .

 4 Figure 4. Perspective view of a fragment of the chain in 1. The metal atoms are represented as brown (Fe) and violet (Cd) spheres, and the crystallization water molecules have been omitted for clarity. Inside the chain, the basic rhombus building unit, {Cd 2 (µ--CN) 4 Fe 2 } are connected by the trans--Cd II (H 2 O) 2 moities. Each Cd(II) ion is coordinated by four nitrogen atoms of four individual [Fe III (Tp)(CN) 3 ] --units in an equatorial plane, and two oxygen atoms of H 2 O molecules in an axial positions. SHAPE analysis reveals that each Cd(II) site consists distoeted octahedral geometry (see table 2 and reference therein). The Cd-N distances are in the range 2.247(5)--2.364(4) Å, and the Cd-O distances are in the range 2.341(4)--2.346(5) Å. Each Fe(III) atom is coordinated by three Tp nitrogen atoms and three cyanide carbon atoms, in a distorted octahedral geometry. The average length of the Fe III -N(Tp) bonds is 1.971 Å and the average angle subtended at the Fe(III) by the chelating Tp ligand is 90.71°. These values are comparable to those found in reported double--zigzag chains based on [Fe III (Tp)(CN) 3 ] --building block. 2,4 The cyanide bridges are significantly bent to the Cd(II) side with C--N--Cd II ranges from 138.91° to 167.02°. The intramolecular Fe III ••••••Cd II separations are in the range: 5.032--5.279 Å. The shortest intermolecular Fe III ••••••Fe III , Cd II ••••••Cd II and Fe III ••••••Cd II separations are 7.764, 11.363 and 8.178 Å respectively. The hydrogen bonds involving terminal cyanide, CN------O water , and coordinated water, Cd(H 2 O) 2 ------O water, stabilize the crystal lattice of 1 (Figure 5).

!

Figure 5 .

 5 Figure 5. View of the hydrogen bonds in 1 (dotted green--orange lines). Organic ligands have been partially removed for clarity.

Figure 6 .

 6 Figure 6. Perspective view of a fragment of the zig--zag chain in 2. The metal atoms are represented as brown (Fe) and violet (Cd) spheres. H atoms are omitted for clarity.The Cd(II) ion is hexa--coordinated and exhibit a very highly distorted CdN 4 O 2 octahedral surrounding (see table2). The coordination sphere of each Cd(II) ions is completed by two nitrogen atom coming from the coordinated neocuproine molecule, two oxygen from nitrate anion and two nitrogen of the cyanide bridged [Fe(Tp)(CN) 3 ] --units. The Cd--N(CN) distances ranges from 2.220(4)--2.313(4) Å and average Cd--N(neocuproine) amounts to 1.973(4) Å. The Cd--O(NO 3 ) bond length are 2.379 and 2.512 Å. The cyanide bridge is more bent towards Cd(II) site and deviates significantly from linearity (154° and 161.47°). Overall the Cd(II) coordination sphere is significantly more distorted than in the previous compounds (double chains). The average intrachain Fe------Cd distance is 5.19 Å whereas the shortest interchain Fe------Cd distances is 8.93 Å.

Figure 7 .Figure 8 .

 78 Figure 7. perspective view of a fragment of the structure of 3. The metal atoms are represented as brown (Fe) and violet (Cd) spheres.[{Fe(Ttp)(CN) 3 } 2 {Cd(nc) 2 }] (4)The crystal structure of 4 consists of trinuclear unit, where a cadmium ion is linked by two [Fe(Ttp)(CN) 3 ] -- units in trans position. The Cd(II) ion exhibits a significantly distorted trigonal prismatic coordination sphere (according to SHAPE analysis, see table2) made of four nitrogen atoms of two neocuproine ligand and two nitrogen atoms from a cyanide ligand of two [Fe(Ttp)(CN) 3 ] --units. The [Fe(Ttp)(CN) 3 ] --units are equivalent (through a C2 axis) and coordinates the Cd(II) ion through only one cyanide bridge. The cyanide bridge is linear on the iron side [with Fe(1) -C(1) -N(1) = 178.76°] but significantly bent towards the Cd(II) centre (like in all our previous compounds) with a C--N--Cd angle of 152.59°. In this compound, the Fe(III) centre exhibits a slightly distorted octahedral surroundings and its coordination sphere is made of three Ttp nitrogen atoms and three cyanide carbon atoms. The average Fe III -N(Ttp) length is 1.984 Å and the average angle subtended at the Fe(III) by the chelating Ttp ligand is 90.71°. These values are very close to those reported in the chains based on the [Fe(Tp)(CN) 3 ] --or [Fe(Ttp)(CN) 3 ] --building block.2,4

Figure 9 .

 9 Figure 9. (Left) perspective view of the 2D network in 9. (Right) View of the asymmetric unit of 9. The metal atoms are represented as green (Ru) and violet (Cd) spheres. Crystallization water molecules are removed for clarity.

Figure 10 .

 10 Figure 10. (Left) perspective view of the two 3D network in 10. (Right) View of the asymmetric unit of complex 10. The metal atoms are represented as green (Ru) and violet (Cd) spheres.

  in[Å] and angles in [°]; N--N is an alphadiimine ligand; Shape factor for the cadmium(II) octahedral coordination sphere calculated from continuous shape analysis(Alvarez et al.).

6

 6

Figure 11 .

 11 Figure 11. Experimental (top) and simulated (bottom) 113 Cd MAS--NMR spectrum of 7 acquired at 11.75 T. Isotropic peak is marked with an asterisk (*).

Figure 12 .

 12 Figure 12. Experimental (top) and simulated (bottom) 113 Cd MAS--NMR spectrum of 8 acquired at 11.75 T. Isotropic peak is marked with an asterisk (*).

  et al. (see part A), we observe that cadmium ions in a {Cd(NC--Fe) 2 (dia) 4 }, {Cd(NC--Fe) 3 (dia) 3 } and {Cd(NC--Fe) 4 (dia) 2 } surroundings exhibit isotropic signals near --1092 , --827 and --547 ppm.1 These values are in fair agreement with those observed here and match with the cadmium coordination sphere: {Cd(NC--Fe) 4 (OH 2 ) 2 } (1) {Cd(NC--Fe) 2 (N--N)(η 2 --O 3 N)} (2), {Cd(NC--Fe) 2 (N--N) 2 } (4) and {Cd(NC--Fe) 3 (N--N) 2 (OH 2 )} (3).

Figure 13 .

 13 Figure 13. Experimental (top) and simulated (bottom) 113 Cd MAS--NMR spectrum of 1 acquired at 11.75 T. Isotropic peak is marked with an asterisk (*).

Figure 14 .

 14 Figure 14. Experimental (top) and simulated (bottom) 113 Cd MAS--NMR spectrum of 2 acquired at a Larmor frequency of 110.99 MHz. Isotropic peak is marked with an asterisk (*).

Figure 15 .

 15 Figure 15. Experimental (top) and simulated (bottom) 113 Cd MAS--NMR spectrum of 3 acquired at 11.75 T. Isotropic peak is marked with an asterisk (*).

Figure 16 .

 16 Figure 16. Experimental (top) and simulated (bottom) 113 Cd MAS--NMR spectrum of 4 acquired at a 11.75 T. Isotropic peak is marked with an asterisk (*).

1

 1

3. 3 113

 3 Cd NMR of the [Fe III (L)(CN) x ] --based compounds, L = bipy, phen (5--6)In contrast with 1--4, the isostructural diamagnetic compounds, [{Co(L)(CN) 4 } 2 {Cd(H 2 O) 2 }], have been obtained for 5 and 6. The deduced paramagnetic shifts are thus expected to be more acurate in the present case. The 113 Cd MAS NMR spectra of these compounds were recorded at spinning speeds ranging from 5--12 kHz. Their isotropic paramagnetic chemical shifts are --1083.7 ppm (5) and --978 ppm (6) at 298 K. These values compare well with those previously observed in X and in the PBAs for the {Cd(NC--Fe) 4 (dia) 2 } surroundings. The presence of only one isotropic value is in well agreement with the XRD study which indicate the presence of only one {Cd(NC--Fe) 4 (H 2 O) 2 } site.

Figure 17 .

 17 Figure 17. Experimental (top) and simulated (bottom) 113 Cd MAS--NMR spectrum of 5 acquired at a 11.75 T. Isotropic peak is marked with an asterisk (*).

Figure 18 .

 18 Figure 18. Experimental (top) and simulated (bottom) 113 Cd MAS--NMR spectrum of 6 acquired at a 11.75 T. Isotropic peak is marked with an asterisk (*).

Figure 19 .

 19 Figure 19. Experimental (top) and simulated (bottom) 113 Cd MAS--NMR spectrum of 9 acquired at 11.75 T. Isotropic peak is marked with an asterisk (*).

  ) II (NC--Ru III ) 4 ] and [Cd(2) II (NC--Ru III ) 4 ] in this compound. The isotropic paramagnetic chemical shift is roughly located in between --2.8 to --206.1 ppm at 298 K. The shape of the NMR spectrum also indicative of the presence of two cadmium(II) sites. The observed chemical shift is much lower than the one observed in 9 and almost lies in the diamagnetic range of113 Cd signals. The average Ru--Cd bond distances and C--N--Cd bridging angles in 10 are ca. 5.31 Å and 166.5°--168.5° which is close to those measured in 9 (Fe--Cd 5.38 Å; C--N--Cd 164°).

Figure 20 .

 20 Figure 20. Experimental 113 Cd MAS--NMR spectrum of 10 acquired at a Larmor frequency of 110.99 MHz. Hence the differences in the 113 Cd chemical shift value between these two compounds clearly arise electronic factors. More specifically the differences in the spin density distribution in the PPh 4 [Ru(acac) 2 (CN) 2 ] and PPh 4 [Ru(salen)(CN) 2 ] is likely responsible for these differences in the chemical shift values.

Scheme 1 .

 1 Scheme 1. chemical shift tensor with principal components.9

4 [

 4 Fe(Tp)(CN) 3 ], PPh 4[Ru(salen)(CN) 2 ] and PPh 4[Ru(acac) 2 (CN) 2 ] permits us to draw a map of spin densities on the M III --C--N--Cd II --N--C--M III backbone in 1, 2, 3 ,9 and 10. The alternation of the spin--density sign demonstrates the role of the polarisation mechanism in the spin density transfer from paramagnetic source to the cadmium ion. In our present study each iron atom transmits ca. 0.0011 (a.u --3 ) spin density to the cadmium centre, whereas the previous studies on CdII 3 [Fe III x Co III 1--X (CN) 6 ] 2 . 15H 2 O and CsCd II [Fe III x Co III 1--x (CN) 6 ] . 0.5H 2O reveal that each iron transmits 0.0027 and 0.0020 (a.u --3 ) resprctively to the cadmium centre.1 Although the chemical shift value obtained per iron atom in our

Chapter 2 Figure 1 .

 21 Figure 1. IR absorption spectra of the crystalline sample of PPh 4 [Fe III (Tp)(CN) 3 ] (1). The spectrum is recorded at room temperature in ATR mode.

Figure 2 .

 2 Figure 2. IR absorption spectra of the crystalline sample of PPh 4 [Co III (Tp Me2 )(CN) 3 ] (2). The spectrum is recorded at room temperature in ATR mode.

Figure 3 .

 3 Figure 3. IR absorption spectra of the crystalline sample of PPh 4 [Fe III (pcq)(CN) 3 ] (3). The spectrum is recorded at room temperature in ATR mode.

Figure 4 .

 4 Figure 4. IR absorption spectra of the crystalline sample of PPh 4 [Ru III (salen)(CN) 2 ] (4). The spectrum is recorded at room temperature in ATR mode.

Figure 5 . 3 Figure 6 .

 536 Figure 5. IR absorption spectra of the crystalline sample of PPh 4 [Ru III (acac) 2 (CN) 2 ] (5). The spectrum is recorded at room temperature in ATR mode.

Figure 7 .

 7 Figure 7. IR absorption spectra of the fresh crystalline sample of [Fe(Et--bik) 3 ](BF 4 ) 2 (2). The spectrum is recorded at room temperature in ATR mode.

Figure 8 .Figure 9 .

 89 Figure 8. IR absorption spectra of the fresh crystalline sample of [Fe(V--bik) 3 ](BF 4 ) 2 (3). The spectrum is recorded at room temperature in ATR mode.

Figure 10 .

 10 Figure 10. IR absorption spectra of the fresh crystalline sample of {[Co II 2 (piv) 3 ] 2 [Co III (Tp Me2 )(CN) 3 ] 2 ⋅2CH 3 CN} ∞ (3). The spectrum is recorded at room temperature in ATR mode.

Figure 11 .

 11 Figure 11. IR absorption spectra of the fresh crystalline sample of [{Fe III (Tp)(CN) 3 } 2 {Cd II (H 2 O) 2 }]⋅5H 2 O (1). The spectrum is recorded at room temperature in ATR mode.

Figure 12 .

 12 Figure 12. IR absorption spectra of the fresh crystalline sample of [{Fe III (Tp)(CN) 3 }{Cd II (nc)(NO 3 )}] (2). The spectrum is recorded at room temperature in ATR mode.

Figure 15 .

 15 Figure 15. IR absorption spectra of the fresh crystalline sample of [{Ru III (salen)(CN) 2 }{Cd II (CH 3 CN)}]⋅H 2 O (9). The spectrum is recorded at room temperature in ATR mode.

Figure 1 .

 1 Figure 1. 13 C MAS--NMR spectra of PPh 4 [Ru(salen)(CN) 2 ] at 12 kHz (red) and 14 kHz (black). The spectra are acquired at a Larmor frequency of 75.43 MHz. The signals marked with an asterisk represents the diamagnetic signal arise from the PPh 4+ counter cation asterisk. The expected signals due to the13 C of the cyanide ligand were not observed in the above spectra.

Figure 2 .Figure 3 .

 23 Figure 2. The 13 C static NMR spectrum of PPh 4 [Fe(pcq)(CN) 3 ], recorded at at a Larmor frequency of 75.43 MHz.The broad signal obtained in the range of ≈ --1000 to --3000 ppm is ascribed as a signal from13 C enriched cyanide carbon, where as the signal centred at ≈ +200 ppm is probably due to the PPh 4 + counter cation or carbon from the pcq ligand.

Figure 4 .

 4 Figure 4. Cobalt--59 MAS NMR spectra of PPh 4 [Co( Me2 Tp)(CN) 3 ] at 16.4 T, ν MAS = 55 kHz. The centreband is denoted with an asterisk. The spinning sidebands are represented by red dotted lines.

Figure 5 .Fig 6 .

 56 Figure 5. Superposition of the Cobalt--59 MAS NMR spectrum of PPh 4 [Co( Me2 Tp)(CN) 3 ], ν MAS = 20 kHz (black) and the static Cobalt--59 NMR spectrum (red) of PPh 4 [Co( Me2 Tp)(CN) 3 ]. B 0 = 16.4 T.

  Ni 2+ or Cr 3+ (Li[Li 0.2 Mn 0.4 Cr 0.4 ]O 2 and Li[Ni 0.5 Mn 0.5 ]O 2

  C--labeled samples are needed for signal assignments.Liu et al. reported line narrowing of13 C MAS spectra for a nonlabeled sample without decoupling at a moderate spinning frequency (ν R ) of 11 kHz. 72b Although signals suffered from line broadening and it needs a long--time signal accumulation to ensure sufficient sensitivity. The further improvement came fromIshii et al. in 2003.

  2

  ••(H 2 O) are given below (Fig 22).

4 [Co( Me2 Tp)(CN) 3 ] (2)

	The synthesis of this building block is described in chapter 4. The elemental analysis on the crystals of
	compound 2 matches with the hydrated sample of molecular formula PPh 4 [Co( Me2 Tp)CN 3 ]••3H 2 O
	(C 42 H 48 BPCoN 9 O 3 ). Elemental analyses calcd (%): C, 60.95; H, 5.85; N, 15.23 Found: C, 61.66; H, 5.65; N, 15.04.

PPh 4 [Ru(salen)(CN) 2 ] (4)

2.2 Structural analysis PPh

  4 [Fe III (Tp)(CN) 3 ]••H 2 O (1), PPh 4 [Fe III (pcq)(CN) 3 ] (3), and PPh 4 [Ru III (acac) 2 (CN) 2 ] (

	68 mmol of PPh 4 Cl solution (5 mL H 2 O) is dropwisely added to the green
	filtrate with constant stirring. A dark green precipitate forms immediately which is kept for sometime to be
	settled down before filtration. The precipitate is washed with water and Et 2 O and dried in air. Crystals suitable
	for X--ray crystallography are obtained by slow diffusion of diethyl ether into a solution of PPh 4 [Ru(salen)(CN) 2 ]
	in MeOH. Yield 69% Elemental analyses for C 42 H 36 PRuN 4 O 2 calcd (%): C, 66.3; H, 4.77; N, 7.36 Found: C, 66.96;
	H, 4.65; N, 7.04.

Table 1 .

 1 Selected bond distances (Å) and angles (deg) for 1.

	Fe(1)--C(1)	1.910(6)	Fe(1)--N(4)	1.970(4))
	Fe(1)--C(2)	1.929(7)	Fe(1)--N(5)	1.987(4)
	Fe(1)--C(3)	1.917(5)	Fe(1)--N(6)	1.984(4)
	C(1)--N(1)	1.136(9)	C(2)--N(2)	1.135(8)
	C(3)--N(3)	1.158(8)
	C(1)-Fe(1)-C(2)	86.7(3)	C(1)-Fe(1)-N(4)	91.7(2)
	C(1)-Fe(1)-N(6)	179.3(2)	C(1)-Fe(1)-C(3)	87.1(2)
	C(2)-Fe(1)-N(6)	92.6(2)	C(2)-Fe(1)-N(4)	178.2(2)
	C(2)-Fe(1)-N(5)	91.8(2)	C(1)-Fe(1)-N(5)	92.8(2)
	C(2)-Fe(

Table 2 .

 2 Selected bond distances (Å) and angles (deg) for 2.

	Co(1)--C(1)	1.876 (17)	Co(1)--N(4)	1.987 (12)

Table 3 .

 3 Selected bond distances (Å) and angles (deg) for 3.

	Fe(1)--C(1)	1.954(3)	Fe(1)--N(4)	1.976(3)
	Fe(1)--C(2)	1.959(3)	Fe(1)--N(5)	1.894(2)
	Fe(1)--C(3)	1.975(3)	Fe(1)--N(6)	1.971(2)
	C(1)--N(1)	1.145(4)	C(2)--N(2)	1.150(4)
	C(3)--N(3)	1.149(4)
	C(1)-Fe(1)-C(2)	86.49(13)	C(1)-Fe(1)-N(4)	95.24(12)
	C(1)-Fe(1)-N(6)	98.44(12)	C(1)-Fe(1)-C(3)	88.00(12)
	C(2)-Fe(1)-N(6)	86.46(11)	C(2)-Fe(1)-N(4)	92.84(11)
	C(2)-Fe(1)-N(5)	95.79(11)	C(1)-Fe(1)-N(5)	177.43(12)
	C(2)-Fe(1)-C(3)	173.58(12)	N(4)-Fe(1)-N(5)	83.47(11)
	N(4)-Fe(1)-N(6)	166.23(11)	N(4)-Fe(1)-C(3)	90.95(11)
	N(6)-Fe(1)-C(3)	91.09(11)	N(6)-Fe(1)-N(5)	82.92(11)
	Fe(1)--C(1)--N(1)	178.8(3)	Fe(1)--C(2)--N(2)	176.1(3)
	Fe(1)--C(3)--N(3)	174.8(3)	C(3)-Fe(1)-N(5)	89.79(11)

trans--PPh 4 [Ru III (Salen)(CN) 2 ] [4]

Table 4 .

 4 Selected bond distances (Å) and angles (deg) for 4 --anion.

	Ru(1)--O(1)	2.02(1)	Ru(1)--O(2)	2.03(1)
	Ru(1)--N(1)	2.01(1)	Ru(1)--N(2)	2.00(1)
	Ru(1)--C(17)	2.04(2)	Ru(1)--C(18)	2.09(2)
	N(3)--C(17)	1.16(2)	N(4)--C(18)	1.12(2)
	Ru(1)--C(18)--N(4)	176.01(1)	Ru(1)--C(17)--N(3)	178.02(1)
	O(1)--Ru(1)--O(2)	95.3(4)	O(1)--Ru(1)--N(1)	90.7(5)
	O(1)--Ru(1)--N(2)	173.4(6)	O(1)--Ru(1)--C(17)	90.3(6)
	O(1)--Ru(1)--C(18)	91.3(6)	O(2)--Ru(1)--N(1)	173.5(5)
	O(2)--Ru(1)--N(2)	90.9(6)	O(2)--Ru(1)--C(17)	90.1(6)
	O(2)--Ru(1)--C(18)	89.6(6)	N(1)--Ru(1)--N(2)	83.1(6)
	N(1)--Ru(1)--C(17)	87.5(6)	N(1)--Ru(1)--C(18)	92.6(7)
	N(2)--Ru(1)--C(17)	91.9(6)	N(2)--Ru(1)--C(18)	86.5(6)
	C(17)--Ru(1)--C(18)	178.3(7)

trans--PPh 4 [Ru III (acac) 2 (CN) 2 ] [5]

1--5. All

							values are
	given in cm --1 .
	No	Compound	ν CN	ν BH	ν CH	ν CH2	Ring stretch
	1	PPh 4 [Fe(Tp)(CN) 3 ]••H 2 O 2123	2502	3115, 3133,	-	1501
					3150
	2	PPh 4 [Co( Me2 Tp)(CN) 3 ]	2132	2531	3133	2966, 2930 1547
	3	PPh 4 [Fe(pcq)(CN) 3 ]	2111	-	3060	-	1630, 1597, 1570
	4	PPh 4 [Ru(salen)(CN) 2 ]	2090	-	3057	-	1599, 1522, 1434
	5	PPh 4 [Ru(acac) 2 (CN) 2 ]	2097	-	3090, 3058	2921	1544, 1518

  C MAS--NMR spectrum of PPh 4 [Co III ( Me2 Tp)(CN) 3 ] using five different isotropic shifts (in ppm). Best overlap: 88.35%. The two cyanide isotropic chemical shifts (a) and (b) at δ iso = 138.8 ppm and 133.2 ppm are coherent with the values found for other reported diamagnetic polycyanide cobalt(III) complexes, like Cs 2 K[Co III (CN) 6 ] (135.1 ppm), PPh 4 [Co III (bipy)(CN) 4 ] (133.4 ppm) and PPh 4 [Co III (phen)(CN) 4 ] (133.1 ppm) (in acetonitrile solution). 8b

	Compound			PPh 4 [Co III ( Me2 Tp)(CN) 3 ]
	Contribution	a	b	c	d	e
	!"# ( 13 C) ! !"# !"# ( 13 C) ! !! ! !! !"# ( 13 C)	138.8 --75.6 242.6	133.2 --20.6 210.0	125.1 --43.0 208.9	120.4 --24.4 191.4	117.4 148.3 116.7
	!"# ( 13 C) ! !! Δδ exp ( 13 C)	249.4 --321.6	210.2 --230.8	209.6 --252.3	194.2 --217.2	87.2 46.3
	ηδ exp ( 13 C)	0.032	0.001	0.004	0.019	0.954

state NMR spectroscopy of PPh 4 [Fe III

	N)	103.2	± 24.9
	Δδ exp ( 15 N)	--506.5	± 16.1
	ηδ exp ( 15 N)	0.087	-
	Solid--

(Tp)(CN) 3 ]••H 2 O (1)

Table 12 .

 12 Actual temperatures used for13 C MAS--NMR measurement of PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O recorded with a 1.3--BL probehead and a Bruker AV--700 spectrometer.

	N)	85.3	83.2	52.9	5.2
	! !,!! !"# ( 15 N)	98.1	83.3	128.1	231.2
	!"# ( 15 N) ∆! ! !" ! !"# ( 15 N)	--505.5 0.038	--490.4 0.0	--523.6 0.216	--541.8 0.626

  N MAS--NMR spectra of PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O at 6 and 10 kHz at a 15 N--Larmor frequency of 40.55 MHz.

	Probe	Spinning rate [kHz]	T set point

Table 14 .

 14 The solid state NMR datas and chemical shifts (in ppm) of PPh 4 [Fe(Tp)(CN) 3 ]••H 2 O

	[Fe III (Tp)(CN) 3 ] --	13 C			15 N
	! !,!"# !"#	--3726	--3755	522	502	474
	! !,!! !"#	--5159	--5110	1478	--345	--380
	! !,!! !"#	--3016	--3081	419	579	512
	! !,!! !"#	--3003	--3074	--332	1273	1292
	∆! ! !"#	--2150	--2033	1434	--1272	--1282
	!" ! !"#	0.009	0.005	0.785	0.819	0.912

Table 17 .

 17 The solid state NMR datas and chemical shifts (in ppm) of PPh 4 [Fe(pcq)(CN) 3 ].

	[Fe III (pcq)(CN) 3 ] --	13 C		15 N
	! !,!"# !"#	--1900	607	693	760

  N MAS--NMR measurements of 4 recorded with a 4--MQ probehead and a Bruker AV--300 spectrometer.

	Probe	Spinning rate (kHz)	T set point

Table 22 .

 22 The solid state NMR datas and chemical shifts (in ppm) of PPh 4[Ru(acac) 2 (CN) 2 ]

	PPh 4 [Ru(acac) 2 (CN) 2 ]	13 C		15 N
	! !,!"# !"#	--1276.3	--1475.7	282.4	340.7

Table 23 .

 23 Selected bond lengths and angles for PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O: mean values and range calculated dipolar contributions at the observed nuclei13 C and15 N.

	PPh 4 [Fe III (Tp)(CN) 3 ]	Mean value	Range
	Fe

Table 24 .

 24 Paramagnetic chemical shift and spin density value of the enriched C and N atoms in compound 1.

	PPh 4 [Fe III (Tp)(CN) 3 ]	13 C	15 N
	(in ppm) a	--4369.31	601.66
	(in (a.u) -3 ) a	--0.0198	0.0027
	a	--0.0044	0.00034
	(in ppm) b	--4369.81	601.68
	(in (a.u) -3 ) b	--0.0198	0.0027
	b	--0.0044	0.00034
	a Estimated value considering only metal centred dipolar contribution.

g av value for PPh 4 [Fe III (Tp)(CN) 3 ]••H 2 O at 300 K, g = 2.73, can be obtained from magnetometry. In the present case, spin densities are expressed in (a.u) --3 unit. It is assumed that the spin is in the valence 2s orbital. The involvement of 1s orbitals through cross terms with 2s orbitals is not excluded, but that should not affect the general conclusions. The spin density and fraction of spin density values for PPh 4 [Fe III (Tp)(CN) 3 ] are reported in the table here under. b Estimated value considering total dipolar contribution (metal centered and ligand centered).

  The g av values of these compounds are already reported in literature and they amounts to 2.56, 2.33 and 2.42 respectively for [Fe III (pcq)(CN) 3 ] --, [Ru III (salen)(CN) 2 ] --and [Ru III (acac) 2 (CN) 2 ] --.13,14a,44

Table 25 . Dipolar and contact contribution to the paramagnetic chemical shift and spin density values of the enriched

 25

	Compounds	PPh 4 [Fe III (pcq)(CN) 3 ] PPh 4 [Ru III (salen)(CN) 2 ] PPh 4 [Ru III (acac) 2 (CN) 2 ]
		13 C	15 N	13 C	15 N	13 C	15 N
	! !"#! !"# (ppm)	[--72.7;	[--18.2;	--	[+16.5;	[--109.3;	[--29.16;
		+36.3]	+9.1]		--8.2]	+54.6]	+14.55]
	(ppm)	--2055.1 +803.4	--	+425.2	--1393.7	+405.3
	(ppm) --2164.2 +776.1	--	+450.0	--1557.8	+361.60
	(a.u. --3 )	--0.0106 +0.0041	--	+0.0026	--0.0080	+0.0023
	(a.u. --3 )	--0.0111 +0.0040	--	+0.0028	--0.0090	+0.0021
	f 2S	--0.0024 +0.00051	--	+0.00034 --0.0019	+0.00027

C and N atoms in compound 3--5.

Table 26 . Atomic spin densities on different atoms of [Fe III

 26

			Atomic spin density	Atomic spin density	Atomic spin density
			in CH 3 CN (a.u --3 )	in CH 2 Cl 2 (a.u --3 )	in H 2 O (a.u --3 )
	Fe1	1.0477	1.0337	1.0358	1.0328
	C1	--0.0397	--0.0326	--0.0334	--0.0323
	N1	0.0126	0.0080	0.0085	0.0078
	C2	--0.0483	--0.0417	--0.0424	--0.0414
	N2	0.0318	0.0267	0.0271	0.0264
	C3	--0.0517	--0.0440	--0.0450	--0.0438
	N3	0.0362	0.0295	0.0304	0.0293
	N4	--0.0059	--0.0092	--0.0088	--0.0092
	N5	--0.0117	--0.0088	--0.0093	--0.0085
	N6	--0.0008	--0.00258	--0.0023	--0.0026

(Tp)(CN) 3 ] --obtained from Density functional theory (DFT) analysis (for the atom labelling see figure

29

).

Atom labels Atomic spin density in Vaccum (a.u --3 )

13,14 Preparation of [Fe(Me--bik) 3 ](BF 4 ) 2 ⋅0.25 H 2 O (1), [Fe(Et--bik) 3 ](BF 4 ) 2 (2):

  BF 4 ) 2 ⋅ 6H 2 O (169 mg, 0.5 mmol) in methanol(5 mL). The resulting deep blue solution is stirred for 30 minutes at room temperature and filtered. [Fe(Me--bik) 3 ](BF 4 ) 2 ⋅ 0.25 H 2O and [Fe(Et--bik) 3 ](BF 4 ) 2 are obtained as dark plate--like crystals after few days by slow evaporation of the solution under ambient conditions. Yield: 77 (1)% , 55 (2)%.

Similar synthetic procedures are applied for 1 and 2. To a colourless methanolic (20 mL) solution of the ligand (Me--bik: 285 mg; Et--bik: 327 mg; 1.5 mmol) is added a solution of Fe([Fe(Me--

bik) 3 ](BF 4 ) 2 ⋅ 0.25 H 2 O (1)

  Anal. Calcd for C 27 H 30.5 B 2 F 8 FeN 12O 3.25 : C, 40.31; H, 3.82; N, 20.89 Found: C, 40.71; H, 3.89; N, 20.78. ESI--MS: m/z 313.30 {[Fe II (Me--bik) 3 ] 2+ , calcd 313.10}, 523.30 {[Fe II (Me--bik) 2 (BF 4 )] + , calcd 523.10}, 713.60 {[Fe II (Me--bik) 3 (BF 4 )] + , calcd 713.40}.

	[Fe(Et--bik) 3 ](BF 4 ) 2 (2) Anal. Calcd for C 33 H 42 B 2 F 8 FeN 12 O 3 : C, 44.83; H, 4.79; N, 19.01 Found: C, 45.33; H, 4.88;
	N, 19.10. ESI--MS: m/z 219.30 {[(Et--bik)+H] + , calcd 219.21}, 241.16 {[(Et--bik)+Na] + , calcd 241.11}, 355.21
	{[Fe II (Et--bik) 3 ] 2+ , calcd 355.13}, 443.27{[Fe II (Et--bik) 3 (BF 4 ) 2 +2H] 2+ , calcd 443.16}, 459.28 {[(Et--bik) 2 +Na] + , calcd
	459.40}.

Table 1 .

 1 Crystallographic Data for 1

,2 and 3 (T = 200 K)

		Compound 1	Compound 2	Compound 3
	Formula	C 27 H 30.5 FeN 12 O 3.25 B 2 F 8	C 33 H 42 FeN 12 O 3 B 2 F 8	C 33 H 42 FeN 12 O 3 B 2 F 8
	FW (g mol -1 )	804.60	884.25	872.13
	Wavelength (Å)	1.54178	0.71073	1.54180
	Space group	pbca	C2/c	P1
	a (Å)	15.3604 (3)	21.1447 (5)	9.3631 (2)
	b (Å)	11.6415 (3)	16.0717 (4)	11.4700 (3)
	c (Å)	37.4475 (7)	12.7572 (3)	18.6414 (4)
	α (°)	90	90	99.121 (2)
	β (°)	90	114.657 (1)	93.443 (2)
	γ (°)	90	90	90.240 (10)
	V (Å 3 )	6696.3 (3)	3940.01 (17)	1972.92 (8)
	Z	8	4	2
	Density (g cm --3 )	1.596	1.491	1.468
	m (mm --1 )	4.489	0.472	3.856
	F (0,0,0)	3284	1824	888
	h, k, l max	18, 13, 44	30, 22, 18	11, 13, 22
	N par	503	271	578
	N ref	5733	6016	7152
	GOF	0.968	0.997	0.992
	θ max (°)	66.693	30.542	66
	R (reflections)	0.0457 (5168)	0.0457 (5250)	0.0820 (5853)
	wR2 (reflections)	0.1162 (5733)	0.1360 (6016)	0.2206 (7152)
	S	1.037	1.078	0.999
	wR2

Table 3 .

 3 •••C(π) / CH•••ring-centroid short interactions (less than sum of C--H van der Waals radii) are present, but for comparison reason the the CH•••C(π) / CH•••ring--centroid distances are calculated and mentioned in table 4. Comparison of coordination sphere of the SCO complexes.

		[Fe(Me--bik) 3 ](BF 4 ) 2 0.5H 2 O	[Fe(Et--bik) 3 ](BF 4 ) 2	[Fe(V--bik) 3 ](BF 4 ) 2 [Fe(phen) 3 ] (NO 3 ) 2 2H 2 O	21
	Temperature (K)	200 K	200 K	200 K	293 K
	d[Fe--N(L)] av	1.997 Å	1.997 Å	1.987 Å	1.973 Å
	bite (N--Fe--N) av angle 89.11°	89.59°	88.90°	82.77°
		13.42°	7.25°	20.33°	47.64°
	trans(N--Fe--N) av angle 178.04°	178.56°	177.55°	174.33°
	Interplanar dihedral	4.05, 11.55, 9.02	5.17, 10.44,13.03	2.16, 10.79, 21.81 0, 0, 0
	angles (°)
	Shape factor S (OC--6) 0.0419	0.019	0.063	0.423

Table 4 .

 4 Shortest intermolecular C

Table 5 .

 5 Selected IR vibrations ATR mode (298 K, ν in cm --1 )

	Complex	ν(C--H)	ν(C=O)	ν(C=C)	ν(B--F)
	Me--bik	3095, 2949	1628
	Et--bik	3110, 2977	1631
	V--bik	3140, 3117, 2998 1642	1621
	[Fe(Me--bik) 3 ](BF 4 ) 2	3142, 2973	1634		1165, 1087, 1037, 692, 655
	[Fe(Et--bik) 3 ](BF 4 ) 2	3126, 2988	1636		1163, 1090, 1029, 693, 653
	[Fe(V--bik) 3 ](BF 4 ) 2	3140, 3005	1643	1628	1160, 1085, 1033, 690, 655

  and 5.89 ppm (e) are attributed to H d and H e respectively. H c appears as doublet of doublet (dd) near 9.55 ppm. The most paramagnetically shifted H b signal is observed at 16.46 ppm and the broad H a signal is observed at 5.17 ppm.

	Table 6. Proton NMR data (δ in ppm) at 310 K in CD 3 CN
		H a	H b	H c	H c'	H d	H e
	Me--bik	7.12	7.24	3.92
	Et--bik	7.40	7.04	4.51		1.45
	V--bik	7.22	7.68	5.07		7.57	5.51
	[Fe(Me--bik) 3 ](BF 4 ) 2	4.41	17.22	7.29
	[Fe(Et--bik) 3 ](BF 4 ) 2	4.12	16.03	7.39	6.73	2.47
	[Fe(V--bik) 3 ](BF 4 ) 2	5.40	12.88	6.45		8.92	5.78

Table 7 .

 7 Magnetic susceptibility obatained from Evans' NMR measurement on 1, 2 and 3

	Temperature
	(K)

  obtained from solution state SQUID measurements in CD 3 CN.The existence of some paramagnetic impurities in compound 2, probably [Fe II (Et--bik) 2 (CH 3 CN) 2 ] 2+ , is also evidenced by this technique (the experiment was carry out several times) as a high χ M T value is observed at low temperature. Overall the SQUID magnetometry experiments on solutions reveal a gradual spin transition in solution. These results are coherent with those observed by the Evans method and the VT 1 H NMR technique.

Table

										)
	[Fe(Me--bik) 3 ] (BF 4 ) 2	36.8	116.9	315	35.8	113.8	317	38.4	120.6	318
	[Fe(Et--bik) 3 ] (BF 4 ) 2	37.7	119.2	317	37.8	119.5	313	45.9	144.1	319
	[Fe(V--bik) 3 ] (BF 4 ) 2	40.4	124.6	324	38.4	118.4	328	29.6	89.0	334

  2

  ••MeCN••2H 2 O (C 76 H 91 B 2 Co 2 F 6 Fe 2 N 35 O 12 S 2 ). Elemental analyses calcd (%)

  FormulaC 44 H 45 B 1 Co 1 N 10 P 1 C 82 H 100 B 2 Co 2 F 6 Fe 2 N 38 O 12 S 2 C 70 H 104 B 2 Co 6 N 20 O 12

		1	2	3
	Mol Wt. [g mol--1] 814.63	2239.24	1792.92
	Temperature [K]	200(2)	200(2)	200(2)
	space group	Monoclinic, P2 1 /c Triclinic, P--1,	Monoclinic, P2 1 /c
	a [Å]	16.3036(10)	13.2020(3)	11.9945(2)
	b [Å]	9.8538(7)	13.2278(3)	21.3557(4)
	c [Å]	26.4565(17)	16.2937(3)	16.5074(3)
	α[°]	90	66.4853(8)	90
	β[°]	103.452(3)	86.3463(8)	94.8110(10)
	γ[°]	90	75.1913(8)	90
	V [Å 3 ]	4133.7(5)	2520.12(5)	4213.49
	Z	4	1	4
	ρ calcd [gcm --3 ]	1.309	1.475	1.408
	μ[mm --1 ]	0.499	5.931	1.172
	Reflections	127793	37335	48479
	Collected
	Independent	13744	8852	10487
	refrections
	R1 [I≥2σ(I)]	0.0546	0.0574	0.0741
	WR2 [I≥2σ(I)]	0.1385	0.1054	0.0388
	GOF on F 2	1.0035	0.9989	1.003
		1/2

Table 3 .

 3 Comparison of selected structural data and geometry of 1, 2 and 3.

	1	2	3

Table 4 .

 4 Spin dependent factors of the central transition

	I	3/2	5/2	7/2	9/2
	F(I)	1/3	2/25	5/147	1/54

  28

Table 5 .

 5

		C Q / MHz	η Q	δ iso /ppm	Ω/ppm
	1 260K	24.2	0.16	3545	427.3
	2 260K,fresh	22.4	0.67	3710	488.5
	2 320K,fresh	38.1	0.69	4204	1170.6
	2 260K,dehy	38.5	0.02	4159	1377.5
	3 260K	36.3	0.66	4800	539.1
	3 320K	28.9	0.80	4649	774.2

  new Co(III) tricyanido buidling block of formula [Co III ( Me2 Tp)(CN) 3 ] --was synthesized and characterized. It was used as metalloligand toward mononuclear [Fe II (Me--bik) 2 (S) 2 ] 2+ and binuclear [Co

	OH 2 )(piv)	4	(Hpiv)	4	] species. The resulting [Fe II	2 Co III	2 ] square complex and [Co II	4 Co III	2

Table 2 :

 2 Geometric parameters of compound 1--10 relevant for spin density study.

	Compounds Cd	II ------N(CN) av (C--N--Cd	II ) angles intramolecular	Shortest	Cd	II	surrounding	Shape
				(M	III ------Cd	II ) av	intermolecular	Factor of Cd	II
							(M	III ------Cd	II )	site
	1	2.30	138.91, 143.55,	5.15			8.18		Cd	II (NC--Fe	III ) 4 (OH 2 ) 2	0.4195(OC--6)
			155.25, 167.02
	2	2.26	154.0, 161.47	5.19			8.93	Cd	II (NC--Fe	III ) 2 (N--N)(η	2 --O 3 N) 4.3909(OC--6)
	3	2.33	150.7, 167.5	5.28			8.25	Cd	II (NC--Fe	III ) 3 (N--N)(OH 2 )	1.6719(OC--6)
	4	2.29	152.59	5.23			9.06		Cd	II (NC--Fe	III ) 2 (N--N) 2	3.6182(TPR--6)
	5	2.34	135.56, 138.12	5.01			7.04		Cd	II (NC--Fe	III ) 4 (OH 2 ) 2	0.3645(OC--6)
	6	2.33	157.22, 159.02	5.33			7.61		Cd	II (NC--Fe	III ) 4 (OH 2 ) 2	0.0777(OC--6)
	7	2.32	135.4, 137.56	4.93			6.97		Cd	II (NC--Co	III ) 4 (OH 2 ) 2	0.3947(OC--6)
	8	2.31	155.23, 158.69	5.24			7.53		Cd	II (NC--Co	III ) 4 (OH 2 ) 2	0.0906(OC--6)
	9	2.24	164.99	5.38			9.28		Cd	II (NC--Ru	III ) 4 (NCCH 3 )	0.8784(VOC--5)
	10	2.16	166.51, 168.54	5.31			7.89		Cd(1)

II

(NC--Ru III

3. Solid--state 113 Cd NMR Cadmium

  --113 NMR spectra were obtained using Bruker Avance 500 NMR spectrometer operating at 11.75 T. The spectra were acquired with magic angle spinning (MAS). Samples were packed in 4.0 mm ZrO 2 rotors and each sample was recorded at different MAS spinning speed ranging from 3.00 to 12.5 kHz in order to determine the isotropic chemical shift. All 113 Cd NMR chemical shifts are referenced by setting the external Cd(NO 3 ) 2 (H 2 O) 4 peak to --100 ppm. Nickelocene was used as internal temperature reference (the temperature was accurately measured by recording the nickelocene proton signal relative to external adamantane). The isotropic chemical shift values (δ T iso ) of all paramagnetic compounds (except 6) were converted to paramagnetic shift at T by subtracting the113 Cd signal shift of the diamagnetic [{Co(bipy)(CN) 4 } 2 {Cd(H 2 O) 2 }] . 4H 2 O (--27.31 ppm).

		Then, (δ para	T ) was converted to	by considering the
	Curie law	. The 113 Cd spectra were fitted using Topspin software.

  3 [Co(CN) 6 ] . 15H 2 O and CsCd[Co(CN) 6 ] . 0.5H 2 O, exhibit chemical shifts at δ( 113 Cd) = +71 and +81 ppm, respectively. 1 It is thus worth noting that although the cadmium ions exhibit the same surrounding, {Cd(NC--Co) 4 (H 2 O) 2 }, in the Cd 3 [Co(CN) 6 ] . 15H 2 O PBA and in the chains 7--8, their chemical shift differ by ca. 100 ppm. This of course introduces some uncertainties on the deduced paramagnetic chemical shift value.

			! 298 para
	! T para !1 / T
		!"
	#"$%&"
	"
	'"$%&"
	500	0	-500
		! [ppm]

  2 (CN) 2 ] and PPh 4[Ru(salen)(CN) 2 ]than in the PPh 4 [Fe(Tp)(CN) 3 ] building block. It amounts to ca. +0.0028 (a.u) --3 and ca. +0.0030 (a.u) --3 respectively for PPh 4 [Ru(acac) 2 (CN) 2 ] and PPh 4 [Ru(salen)(CN) 2 ] whereas it reaches ca. +0.0063 (a.u) --3 in the PPh 4 [Fe(Tp)(CN) 3 ] building block. Therefore we can anticipate that the spin density delocalization should be weaker in the present case. We thus expect a lower 113 Cd chemical shift (in 9--10) as compared to that observed in the [Fe(Tp)(CN) 3 ]--based compounds.

	!"#$%&'()*
	+*,-.*
	(/0(1"#(2'&%*		3*
	+*,-.*
	0	-500	-1000	-1500	-2000
			! [ppm]
			!"
	#"$%&"
	'()*"$%&"
	0	-500	-1000	-1500	-2000
			! [ppm]
	#$%&'()*+"
	,"-./"
	*01*2$%*3)('"		!"
	,"-./"
	0	-500	-1000	-1500	-2000
			! [ppm]

Table 3 .

 3 CSA parameters obtained from Haeberlen analysis on the 113 Cd NMR spectra of 1--6.

	Compound	1	2	3	4	5	6
	! !,!"# --514.54	--383.37 --721.14 --647.70

Table 4 .

 4 CSA parameters obtained from Haeberlen analysis on the 113 Cd NMR spectra of 7--10.

	Compound	7	8	9	10
	! !,!"# !"# (ppm)	--27.31	--28.56	--438.87	In between --2.8 to --206.1
	! !,!! !"# (ppm)	--124.09	--119.76	--479.19	--
	! !,!! !"# (ppm)	--28.52	--59.55	--467.53	--
	! !,!! !"# (ppm)	70.68	93.63	--369.9	--
	!"# (ppm) ∆! ! !"# !" ! ! !"#,!! !"#" (ppm)	--145.17 0.84 --124.09	--136.8 0.49 --119.76	--62.12 0.17 --492.85	------
	! !"#,!! !"#" (ppm)	--28.52	--59.55	--480.94	--
	! !"#,!! !"#" (ppm)	70.68	93.63	--380.51	--

Table 5 .

 5 g|| and g⊥ factors for paramagnetic building blocks obtained from EPR measurements.

	Building blocks		g ||	g⊥
	[Fe(Tp)(CN) 3 ]	--			3.7	0.8
	[Fe(Ttp)(CN) 3 ]	--		3.72	0.83
	[Fe(phen)(CN) 4 ]	--	2.60	2.72
	[Ru(salen)(CN) 2 ]	--	1.96	2.33
	[Ru(acac) 2 (CN) 2 ]	--	1.69	2.38

Table 6 .

 6 g av factor for paramagnetic building blocks obtained from bulk magetic susceptibility measurements at 300 K.

	Building blocks	g av
	[Fe(Tp)(CN) 3 ]	--		2.73
	[Fe(Ttp)(CN) 3 ]	--	2.69
	[Fe(bipy)(CN) 4 ]	--	2.75
	[Fe(phen)(CN) 4 ]	--	2.75
	[Ru(salen)(CN) 2 ]	--	2.33
	[Ru(acac) 2 (CN) 2 ]	--	2.42

Table 7 .

 7 Chemical shift and spin density values in 1--3.

	Compound 1	Compound 2	Compound 3
	MAS
	frequency

Table 9 .

 9 Chemical shift and spin density values in 7--10.

	--3

4. Crystallographic Tables The

  crystallographic tables for the compounds mentioned in chapter 5 (Part B) are only stated here. The crystallographic tables for rest of the compounds are already described in the corresponding chapters.

		Compound 3 Compound 5 Compound 9	Compound 4 Compound 6 Compound 10
	Formula Formula Formula	C 64 H 56 Cd 2 Fe 1 N 44 O 2 C 14 H 10 Cd0.5Fe 1 N 6 O 1 Compound 7 C 18 H 22 CdN 6 O 4 Ru 2	C 58 H 48 B 2 CdFe 2 N 26 C 16 H 14 Cd 0.53 Fe 6 N 6 O 3 Compound 8 C 12 H 16 CdN 2 O 4 Ru
	Mol Wt. [g mol--1] Mol Wt. [g mol--1] Formula Mol Wt. [g mol--1]	1964.88 309.32 C 28 H 20 Co 2 CdN 12 O 2 509.88	1354.91 450.37 C 32 H 28 CdCo 2 N 12 O 6 465.74
	Temperature [K] Temperature [K] Mol Wt. [g mol--1] Temperature [K]	Compound 1 200(2) 200(2) 786.81 250(2)	Compound 2 200(2) 200(2) 906.92 298
	Formula space group space group Temperature [K] space group Mol Wt. [g mol--1] a [Å] a [Å] space group a [Å] Temperature [K] b [Å] b [Å] a [Å] b [Å] space group c [Å] c [Å] b [Å] c [Å] a [Å] α[°] α[°] c [Å] α[°] b [Å] β[°] β[°] α[°] β[°] c [Å] γ[°] γ[°] β[°] γ[°] α[°] V [Å 3 ] V [Å 3 ] γ[°] V [Å 3 ] β[°] Z Z V [Å 3 ] Z γ[°] ρ calcd [gcm --3 ] ρ calcd [gcm --3 ] Z ρ calcd [gcm --3 ] V [Å 3 ] μ[mm --1 ] μ[mm --1 ] ρ calcd [gcm --3 ] μ[mm --1 ] Z Reflections Collected Reflections Collected μ[mm --1 ] Reflections Collected ρ calcd [gcm --3 ] Independent reflections Independent reflections Reflections Collected Independent reflections μ[mm --1 ] R1 [I≥2σ(I)] R1 [I≥2σ(I)] Independent reflections R1 [I≥2σ(I)] Reflections Collected WR2 [I≥2σ(I)] WR2 [I≥2σ(I)] R1 [I≥2σ(I)] WR2 [I≥2σ(I)] Independent reflections GOF on F 2 GOF on F 2 WR2 [I≥2σ(I)] GOF on F 2	C 24 H 32 B 2 CdFe 2 N 18 O 6 Monoclinic Monoclinic 200(2) Tetragonal 914.35 15.448(3) 7.8489(6) Monoclinic 14.8392(6) 250(2) 8.8943(6) 15.0576(13) 7.7069(4) 14.8392(6) Orthorhombic 30.079(6) 12.9065(17) 14.9018(9) 16.8301(7) 13.6583(4) 90 90 12.7622(7) 90 20.2450(6) 91.752(15) 94.380 90 90 14.3947(3) 90 90 94.308 90 90 4130.9(11) 1520.9(2) 90 3706.0(3) 90 2 4 1461.56 8 90 1.58 1.70 2 1.80 3980.31(19) 16.83 1.85 14.30 4 42866 21094 149.86 2722 1.53 11742 4364 10406 2722 13.08 0.0358 0.0435 2527 0.0337 19911 0.0349 0.0573 0.0286 0.0644 8231 1.1141 0.9999 0.0680 0.9990	monoclinic Monoclinic 200(2) Monoclinic C 26 H 22 B 1 CdFe 1 N 12 O 3 23.69(2) 7.656(2) 10.679 Monoclinic 729.60 8.695(7) 15.656(3) 11.0608 7.5404(3) 200(2) 29.18(5) 15.557(5) 11.2023 15.4891(7) triclinic 90 90 119.583 15.3065(5) 7.5016(5) 96.04(9) 93.508 108.848 90 11.5697(7) 90 90 90 93.662 17.4123(12) 5980(10) 1861.2 2136.73 90 105.563(2) 4 2 4 1784.06 101.792(2) 1.51 1.61 1.47 2 91.639(3) 8.95 13.94 1.24 1.69 1419.52(10) 6712 14138 20978 15.71 2 6712 5040 4285 36082 1.71 0.0583 0.03 0.0528 6791 13.13 0.0784 0.063 0.1361 0.0203 19114 6473 0.9906 1.0000 0.0438 0.9885
	R1 [I≥2σ(I)] GOF on F 2	0.0402 0.9995	0.0401 0.9999
	WR2 [I≥2σ(I)]	0.1096	0.0652
	GOF on F 2	0.9985	1.0072

N MAS--NMR spectroscopyThe15 N MAS--NMR spectra were obtained in a 300 MHz Bruker spectrometer equipped with a 4--MQ probe head. In order to determine the isotropic chemical shift (mainly for the estimation of spin density) the compound was measured at 5 and 3 KHz. 100 mg of ground15 N cyanide--enriched sample was taken and packed in a 4 mm rotor. The temperature during both experiments slightly varied (Table15), however these changes

S,axial ! 298K 2S,equitorial

and 7 are isostructural. These two compounds are reminiscent of

and 8. Their crystal structure consist of centrosymmetric neutral rhombohedron chain of formula [{M III (bipy)(CN) 4 } 2 {Cd II (H 2 O) 2 }] where the [M III (bipy)(CN) 4 ] --unit acts as a monodentate ligand through two of its four cyanide groups toward a central trans--[Cd(H 2 O) 2 ] 2+ motif.

2S,equitorial

Annexes

Table of contents

Solution--state study

Characterization of the complexes through 1 H NMR a) Experimental details

The NMR spectra of 1--3 in deuterated acetonitrile solution were recorded on a Bruker Avance II 300 spectrometer operating at a 1 H Larmor frequency of 299.955 MHz and equipped with a BBMS variable-temperature unit. The spectra were registered over the temperature range, --43 to +77 °C (230 to 350 K), referenced by the resonance from residual solvent protons (CHD 2 CN, 1.94 ppm relative to TMS). The temperature was calibrated using the standard Wilmad methanol and ethylene glycol samples. Deuterated acetonitrile-d 3 (D, 99.8%) was purchased from Euriso--top and all the starting materials required for the preparation of the ligands were purchased from Sigma Aldrich. The three [Fe II (R--bik) 3 ](BF 4 ) 2 sample solutions were prepared immediately prior to the recording of the NMR data. NMR samples for variable temperature NMR experiments were prepared by dissolving stoichiometric amount of Fe(BF 4 ) 2

. 6H 2 O and imidazolyl--ketone ligands (iron tetrafluoroborate : ligand = 1:3 in mmol) in 3 mL of CD 3 CN. The concentrations of all three paramagnetic samples were set to 10 --2 mol L 1 . 0.45 mL of each solution was used for the NMR experiments.

b) Generalities

In a paramagnetic species, the magnetic interactions between the NMR--active nuclei and unpaired electrons lead to significant line broadening and strongly temperature--dependent chemical shifts. In fact routine structural assignment of paramagnetic spectra are not easily accessible because of : (i) the chemical shifts occur far outside the usual range of diamagnetic molecules making the signal assignment difficult, 28 (ii) the rapid nuclear relaxation of paramagnetic complexes results in line broadening of the spectral signals.

Therefore, peak integrations may become unreliable in some cases. Indeed, paramagnetic species can be detected by NMR only when the nuclear relaxation of the NMR active nuclei are not too fast (ie. the electronic relaxation is fast enough). In fact, there is a qualitative statement, derived from the Solomon equation: 29 the shorter the electronic relaxation times of the metal complex is, the sharper the NMR signals are. For example the situation is often favourable the case with high--spin cobalt(II) paramagnetic complexes that exhibit short electronic relaxation leading to NMR spectra with good resolution. 30,31 Indeed, the electronic relaxation rates in HS cobalt(II) complexes increases with the coordination number, and therefore the relaxation times of the protons are found to follow the opposite order. 30a These complexes are therefore excellent candidates to follow substitution and isomerization processes by variable temperature NMR. 30 The situation of high--spin iron(II) complexes is less favorable as the electronic relaxation is a bit slower, however, there are several reported high--spin iron(II) complexes having fast electronic spin lattice relaxation times that give rise to NMR features sharp enough for investigation. 30, 32 & ref therein This is the case in our present study. NMR spectra observed for the three samples in CD 3 CN contain moderatly broad line--shapes and strongly shifted chemical shifts as compared to the free ligand but all the proton can be observed and identified. The signal assignment and the interpretation of the paramagnetic NMR of [Fe II (R--bik) 3 ](BF 4 ) 2 complexes (with up to five proton environments) can be done based on the integration of the signals, the comparison of the three derivatives (chemical substitution) and the broadening of the signal (which is higher for proton close to the paramagnetic source). The signal assignement can be further confirmed by using DFT computations that provide the sign of the spin density on the proton (which corresponds to the sign of the paramagnetic chemical shift, see chapter 1). 32,33 Chapter 4

Molecular magnetic materials based on the new cyanide--based [Co III ( Me2 Tp)(CN) 3 ] --building block:

A combined magnetic and 59 Co NMR study.

Synthesis, structural and spectroscopic characterizations

2.1 Synthesis of 1, 2, and 3 Figure 1. Synthetic scheme of the fac--[Co III (Tp Me2 )(CN) 3 ] --building block All chemicals and solvents were used as commercially obtained without further purification. Ligands including potassium hydrotris(2,5--dimethylpyrazol--1--yl)borate (K Me2 Tp) and Me--bik [bis(1--methylimidazol--2--yl)ketone] were prepared according to literature procedures. 41 The preparation of the diamagnetic complex [Co II 2 (μ--OH 2 )(piv) 4 (Hpiv) 4 ] was already reported in the literature. 42 FTIR spectra were collected in the 200-4000 cm -1 range with a Tensor 27 Bruker FTIR spectrophotometer working in the ATR mode. Thermogravimetric analysis (TGA) were carried out with a TGA analyzer (TAI instrument, SDT Q600) at a rate of 2 °C min --1 . All TGA experiments were performed under a flow of N 2 (100 mL/min).

PPh 4 [Co III ( Me2 Tp)(CN) 3 ]••CH 3 CN (1)

CoCl 2 ••6H 2 O (4 mmol) was dissolved in 80 mL of distilled THF and the resulting blue solution was stirred at room temperature for 30 min. A colourless solution of K Me2 Tp (4 mmol) in 80 mL of distilled THF was then added dropwise via cannula to the blue solution of CoCl 2 ••6H 2 O, resulting in a violet solution. After stirring for 2 h, KCN (13.2 mmol) dissolved in 65 mL of a methanol:water mixture (1:2) was added very slowly and the resulting orange solution was allowed to stirr for another 30 min. Finally PPh 4 Cl (4 mmol) dissolved in 60 mL methanol:water mixture (1:2) was added slowly to the reaction mixture. The orange--yellow solution was stirred overnight at room temperature and then filtered. Slow evaporation of the filtrate under ambient conditions afforded yellow cubic crystals that were further recrystallized from MeCN. Yield 68% IR (ATR): ν max / cm --1 2531 (BH), 2133 (CN), 1547, 1482, 1434, 1416, 1314, 1209, 1106 (PPh 4 ) and 1063.

The elemental analysis on the crystals matches with the hydrated sample of molecular formula PPh 4 [Co III ( Me2 Tp)CN 3 ]••3H 2 O (C 42 H 48 BPCoN 9 O 3 ). Elemental analyses calcd (%): C, 60.95; H, 5.85; N, 15.23 Found: C, 61.66; H, 5.65; N, 15.04. 2) crystallizes as red plate like crystals in the triclinic (P--1) space group. Its structure consists of a {Co III 2 Fe II 2 } 2 square motifs, triflate anions (OTf --) and crystallization molecules. In this centosymmetric square complex the two [Co III ( Me2 Tp)(CN) 3 ] --building--blocks act as bis-monodentate ligands toward two {Fe II (bik) 2 } 2+ units through two cis cyanide groups. The remaining terminal cyanide ligand adopts an anti--orientation to the {Fe 2 (μ--NC) 4 Co 2 } skeleton (Figure 4). The Co III -CN-Fe II edges are almost identical (ca. 4.971(3) Å) and the angles at the corners slightly deviate from orthogonality [88. 39(13)° and 90.59 (10)° at Co1 and Fe1, respectively]. Slightly distorted octahedral Co 1 N 3 C 3 and Fe 1 N 6 surroundings results from the contribution of three nitrogen atoms of the tridentate ( Me2 Tp) --ligand and three cyanide--carbon atoms [at Co1] and four nitrogen atoms from two bidentate bik ligands plus two cyanide--nitrogen atoms [at Fe1]. The SHAPE analysis was performed for both Fe(II) and Co(III) coordination sphere, 45 the lowest value calculated by SHAPE is obtained for the octahedron in both cases (0.059 for the Fe and 0.099 for the Co) and point to slighty distorted coordination sphere. The cyanide bridges are slightly bent on the Fe II side [Fe1-N1-C1 = 176.8( 2 3) Å] and Fe II -N cyanide distances [1.949(3) and 1.951(3) Å] are coherent with the occurrence of a low--spin iron(II) in a N 6 surrounding. 44,46 Actually these distances obtained at 200 K are comparable with those observed in our previously reported cyanide--bridged {Fe III 2 Fe II 2 } and {Mo V 2 Fe II 2 } complexes. 46a,46b The spectroscopic and magnetic data also support the occurrence of a low--spin {Co III LS -CN-Fe II LS } pair below room temperature. 46c,47 The molecular squares are well isolated from each other, the shortest intersquare metal-metal bond distances being 9.779 Å. The terminal cyanide groups are connected to the (OTf) --counterions through two hydrogen--bonded water molecules along a CN( 3 303) OTf--path (Fig. 4). The N(3) CN -- and O(3) 2 (piv) 3 ] bimetallic complex. In the bimetallic cobalt complex, one of the Co(II) ions is tetra coordinated and the other is penta--coordinated. The SHAPE analysis was performed and it reveals that the ideal structure of the Co(III) centre is close to octahedron whereas penta--coordinated Co(II) centres exhibit a trigonal bipyramide (shape factor: 0.947) and tetra-coordinated Co(II) centre have a distorted tetrahedral coordination sphere (shape factor: 1.229). 45 The penta--coordinated Co(II) centre coordinates to two nitrogen atoms from the CN bridges (forming square motif) and three μ--η 1 :η 1 bridging pivalate group, whereas the tetrahedral Co(II) centre coordinate to one nitrogen atom from the [Co III ( Me2 Tp)(CN) 3 ] --and three oxygen atoms from μ--η 1 : η 1 bridging pivalate (Me 3 CCO 2 --) group. 48 Chapter 5

{[Co{(

113 Cd NMR as a local structural and magnetic probe probe for the study of:

(A) insertion cathodes based on Fe--Cd Prussian blue analogues (B) low dimensional {M--CN--Cd} containing model compounds . sites are missing. Its structure thus contains channels that connect the interstitial tetrahedral sites that can potentially be occupied by alkali ions. The compound was prepared by precipitation method (PBA are highly insoluble compounds) following the literature reported procedure. 5 Actually, the Fe--Cd PBA readily forms as a thin powder when adding hexacyanometallates in concentrated aqueous solution of cadmium chloride. The material preparation and the Li + insertion experiment were carried out by Dr. Masashi Okubo and his co--workers at the Department of Chemical System Engineering, University of Tokyo, Hongo.

Galvanostatic intermittent titration

Galvanostatic intermittent titration technique (GITT) was used to study the intercalation/deintercalation of the lithium ions. At first, the Cd--Fe PBA was mixed with a conducting material (in the present case acetylene black) and the mixture was ground and dried in vacuum. Then two electrodes were prepared from this material, one used as working electrode and the other as counter electrode. The weight and the surface of the counter electrode were sufficiently greater than those of the working one, thus neither capacity nor the kinetics of the counter electrode limited the electrochemical reaction at the working electrode. An organic electrolyte containing lithium salts was used in these experiments. :.

PART B

Low dimensional {M--CN--Cd}--containing model compounds --735.40

Cd surrounding Cd(NC--Fe) 4 Cd(NC--Fe) 2 Cd(NC--Fe) 3 studies are coherent, for instance, the transition temperature of the spin crossover equilibrium for these three compounds lies between ca. 300--315 K. It was observed that variable temperature 1 H NMR method is the most efficient (compared to other solution technique used) technique to follow the spin crossover equilibria. In 1 H NMR, the observation of spin--crossover equilibrium in solution lies on the fact that the chemical shift of a paramagnetic species is a measure of local magnetic susceptibility. It is therefore the only magnetic state of the complex, which is tracked. In contrast, in the other techniques, the physical quantities that are obtained can be contaminated by the presence of paramagnetic impurities, Moreover 1 H NMR allow parallely to check the stability of the species in solution over the explored temperature range and the possible occurrence of chemical equilibrium. In the case of [Fe(Et--bik) 3 ](BF 4 ) 2 , thanks to the presence of diastereotopic protons, we have been able to observe the presence of the isomerization between the two optical isomers Δ--Λ, of the trischelate complex. Interestingly, whereas the two closely spaced isomeric signals could be distinguished at low temperature only if high field NMR spectroscopy is used, at higher temperature the presence of spin density on the probed nuclei results in an enhancement of the chemical shift range and an improved resolution that allow us to observe the isomeric signals even at low field. The thermodynamic parameters associated to the SCO equilibria have been obtained from the different techniques including variable temperature 1 H NMR, Evans' NMR, solution SQUID measurement and variable temperature UV--vis analysis. As the explored temperature is limited, these results suffer from error, once again those obtained from NMR seems to be the more reliable ones. One of our goal is to explore the SCO in other solvent to explore broader temperature range and more importantly, we wish to study the variable temperature solution NMR of more sophisticated switchable systems, such as Fe--Co photomagnetic complexes which exhibit electron transfer coupled to a spin transition (ETCST). In the chapter 4, the magneto--structural properties of some polynuclear complexes based on the new cyanide--based [Co III (Tp Me2 )(CN) 3 ] --building block have been investigated. We synthesized and studied here a cyanide--bridged {Fe II 2 Co III 2 } square complex and a {Co II 4 Co III 2 } ∞ two--dimensional network that have been obtained from the self--assembly of [Co III (Tp Me2 )(CN) 3 ] --building block with a mononuclear [Fe II (Me-bik) 2 (S) 2 ] 2+ complex and a binuclear [Co II 2(μ--OH 2 )(piv) 4 (Hpiv) 4 ] complex. The compounds have been structurally characterized by XRD experiment and their magnetic properties investigated. The variable temperature bulk magnetic susceptibility measurements on these compounds reveal that the [Co III (Tp Me2 )(CN) 3 ] --precursor is diamagnetic, the {Fe II 2 Co III 2 } square complex shows a thermally--induced spin-transition and the {Co II 4 Co III 2 } ∞ 2D network is paramagnetic. Parallely, we have taken advantage of the diamagnetic nature of the Co(III) ion, to use the quadrupolar 59 Co nucleus as a probe for following the magnetic behaviour of these related systems at different temperature. We observed that the 59 Co chemical shift of these complexes can also be used to probe the magnetic behaviour of these compounds. Actually the shape and position of 59 Co signals are sensitive to the magnetic behaviour of the compounds. In particular there is a significant change on the 59 Co signal with temperature as the iron(II) in the {Fe II 2 Co III 2 } square complex undergo a spin transition. The quadrupolar coupling parameters in these complexes are comparable to the reported Co(III) octahedral complexes. It would be now interesting to study 59 Co NMR spectra on single crystals to extract more accurate information about the 59 Co CS and EFG tensors. One of our future aim is to investigate the temperature dependency of the CS and EFG tensor in Co(III) containing switchable Fe--Co systems showing ETCST behaviour. Chapter 5 is a continuation of a study on Fe--Cd Prussian blue analogues, in which it has been shown that accurate local magnetic and structural information could be obtained by 113 Cd NMR measurements. Here the aim was twofold: (A) taking profit of the local structural information that can be obtained by 113 Cd NMR to rationalize the Li + intercalation process in an insertion material with potential use for Li batteries; (B) Wavenumber / cm -1

Crystal Structure Data Collection and Refinement

A single crystal of each compound was selected, mounted onto a Hamilton cryoloop using Paratone--N oil and placed in the cold flow produced with an Oxford Cryocooling device. Due to rapid solvent loss, even in oil, all the crystals were fully recovered with glue and the exposure time for frames was minimized for data collection at high temperature. Intensity data were collected with a Bruker Kappa--APEXII with graphite--monochromated Mo--Kα radiation (λ = 0.71073 Å). Data collection was performed with APEX2 suite. Unit--cell parameters refinement, integration and data reduction were carried out with SAINT program. SADABS was used for multi--scan absorption corrections. In the WinGX suite of programs, the structure were solved by direct methods with SHELXS--97 and refined by full--matrix least--squares methods using SHELXL--97. Almost all non--hydrogen atoms were refined anisotropically; only atoms of solvent molecules or disordered parts were refined isotropically. Hydrogen atoms were placed at calculated positions and refined with a "riding model". The following quality factors were used:

" wF 0 2 "

EPR spectroscopy

EPR measurements were performed using a Bruker ESP300 E spectrometer, at the 9.42 GHz (X band) and a 33 GHz (Q band) frequency. Calibrated silica tubes (suprasil grade) were filled with dry powdered sample was used. The EPR spectra were recorded at low temperature (using a cooled helium flow device).

UV--vis spectroscopic measurement in solution

UV--visible spectra were recorded on a JASCO V--670 equipped with a ETC--717 Peltier module. The spectra were recorded in pure acetonitrile solutions at room temperature. Samples were placed in 1 cm path length quartz cuvette and generally data were collected in the range 300--1200 nm.

TGA Analysis

TGA measurements were performed on a Perkin Elmer STA--Q600 simultaneous thermal analyzer. Usually the samples were heated at 2 °C/min from 20 °C to 140 °C under N 2 flow.

NMR spectroscopy