a series of complexes of the type MLnX where M=Cu(II), Ni(II), and Cr(III), L=β-diketonate (n=1 for Cu (II) and Ni(II), n=2 for Cr(III)) and X=bridging anion was synthesized in order to study the effect of the bridging group on the magnetic exchange interaction parameter, J.

High symmetry and low coordinated single-ion lanthanides have been a successful recipe to design high performance single molecule magnets (SMMs). However, enhancement of the magnetic properties of polymetallic SMMs is an important challenge. Therefore, this thesis describes the use of redox non-innocent tetrazine-based ligands and d- and f- elements in order to fine-tune the electronic structure of the resulting compounds to promote strong exchange interactions between the spin carriers. As reported in the literature, radical-bridged ligands represent a potential strategy to improve the magnetic properties of polymetallic SMMs. Thus, chapter one introduces principal concepts that govern the physical properties of metal complexes containing radical-bridged ligands. Chapter two describes the magnetic properties of a unique air-stable tetratopic radical- bridged bpymtz•− (3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine) templating four Ni(II) metal ions. The dc magnetic studies along with DFT calculations reveal strong ferromagnetic exchange coupling between the Ni− bpymtz•− of J = 98 cm-1 with a spin ground state of S = 9/2. Chapter three describes the application of another tetrazine-based ligand, bpytz (3,6- bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine) to probe magnetic exchange interactions in a {CoII4} supramolecular square. The modelling of dc susceptibility data shows significant Co(II) - bpytz•− magnetic coupling of J = - 118 cm-1 for a spin ground state ST = 4. While the non-reduced analog displays weak Co(II)-Co(II) exchange of J = - 0.64 cm-1 (S = 0 ground state). Additionally, the radical-radical magnetic exchange contribution was probed with an analogous {ZnII4} square, where a J = -15.9 cm-1 was found. Chapter four extends the application of reduced tetrazine ligands to lanthanide systems. Here we demonstrate that the systematic reduction of the ligand with cobaltacene (CoCp2) led to the formation of a strongly coupled bpytz•−−bpytz•− bridging ligand. Magnetic measurements combined with ab initio calculations confirm unprecedented intramolecular pi-dimerization preventing strong magnetic Dy(III)−bpytz•− communication. Chapter five describes the synthesis and characterization of {LnIII4} (Ln = Dy, Gd and Lu) where the Ln(III) ions are bridged by peripheral bpytz•−. The oxophilicity and high coordination numbers preferred by lanthanides ions lead to the formation of a cubane core made up of metal ions bridged by hydroxy ligands (M3-OH−). Experimental and computational studies were applied to verify the nature and strength of the magnetic interactions between the spin carriers.

An intriguing magnetodielectric behavior is observed in triangular or tetrahedral frustrated and low-dimensional system. Therefore, the spin-tetrahedral and low-dimensional compound copper-tellurides (Cu2Te2O5Br2) is suggested that has magnetodielectric behavior. Tetragonal Cu2Te2O5Br2 contains clusters of four Cu2+ (S = 1/2) in a planar coordination. These tetrahedral form weakly coupled sheets within the crystallographic a-b plane. Therefore, this system is ideal to study the interplay between the spin frustration on a tetrahedron with localized low-energy excitations and collective magnetism induced by inter-tetrahedral couplings. In this material a strongly reduced magnetic transition temperature To = 11.5 K in comparison with a dominant magnetic exchange of 30 K is found. Low-dimensional systems with triangular geometries are considered as prominent candidates for applications using novel magnetoelectric materials. At the highest applied magnetic field 90 kOe, the temperature dependent dielectric behavior with almost frequency independent well defined maxima at Tm ~ 30 K and To ~ 11.5 K are enhanced compared with that at zero field. We suggest that the observed magnetodielectric coupling can arise from exchange striction involving frustrated tetramer clusters and inter-cluster exchange bridges with polarizable lone-pair electrons on Te4+ ions.

No description available.

Ce travail de thèse portait sur la synthèse et l'étude de complexes magnétiques redox-actifs comme prototypes pour la conception d'aimants moléculaires à haute température. L'activité redox est assurée par le ligand pontant, qui peut moduler et parfois améliorer significativement les propriétés magnétiques. Après un chapitre d'introduction présentant les derniers développements dans le domaine des matériaux magnétiques moléculaires, un accent particulier est mis sur l'importance d'avoir un fort couplage d'échange magnétique J entre les porteurs de spin. Une étude bibliographique présentant deux approches émergentes pour augmenter J dans les composés polynucléaires est également présentée et discutée. Le chapitre 2 présente les synthèses et caractérisations de complexes dinucléaires [M2(tphz)(tpy)2](PF6)n (M = Co(II) ou Ni(II); n = 4, 3, 2, tphz = tétrapyridophénazine, tpy = terpyridine) construits à partir de ligands pontant (tphz) et bloquant (tpy) fortement coordinants et redox-actifs. Les études approfondies de ces composés montrent que le ligand pontant redox-actif peut être utilisé comme un outil de choix pour promouvoir une délocalisation des spins, de forts couplages magnétiques, ainsi que de la commutabilité. L’analyse des résultats obtenus permet également de mieux comprendre les paramètres clés pour l’élaboration de systèmes fortement couplés magnétiquement. Dans le prolongement de ce travail visant à sélectionner les meilleurs composants pour la conception rationnelle d'aimants moléculaires à haute température, le chapitre 3 décrit une nouvelle série de complexes mononucléaires [Cr(III)(tphz)(tpy)](CF3SO3)n (n = 3, 2, 1). Les complexes mono- et doublement réduits présentent des interactions magnétiques remarquablement fortes entre les ions métalliques et les ligands radicalaires, et pourraient servir d'unités magnétiques intéressantes pour la conception d'aimants de plus hautes nucléarités. / The thesis work aims at the synthesis and study of redox-active magnetic molecules as prototypes towards the design of molecule-based magnets with high operating temperature, a prerequisite for technological applications. The redox activity is provided by the bridging ligand, which could tune and sometimes enhance significantly the magnetic properties of the resulting molecular architectures. After an introduction chapter presenting the latest developments in the field of molecule-based magnetic materials, special emphasis is given on the importance of having large magnetic exchange coupling J between the spin carriers to reach high operating temperature. This is supported by a bibliographic study concerning two emerging approach to enhance J values in polynuclear compounds. Chapter 2 presents the syntheses and characterizations of dinuclear M(II) complexes [M2(tphz)(tpy)2](PF6)n (M = Co or Ni; n = 4, 3, 2, tphz = tetrapyridophenazine) built by using strongly complexing, redox-active bridging ligand (tphz), and terpyridine (tpy) as capping ligands. The extensive studies on these compounds show that the redox-active bridging ligand can be used as a tool to promote spin delocalization, high spin complexes and magnetic multi-switchability. Importantly the work reveals the key parameters towards building strongly magnetically coupled systems. As a continuation research of finding the best magnetic components for the rational design of high temperature molecule-based magnets, Chapter 3 describes a new series of [Cr(III)(tphz)(tpy)](CF3SO3)n (n = 3, 2, 1) mononuclear complexes. Both the mono and doubly-reduced complexes show remarkable magnetic interactions between metal center and radical ligands, which could further act as interesting magnetic units for the design of higher nuclearities magnets.

In this thesis we have developed electronic and spin model Hamiltonians to understand magnetism in molecule based magnets like photomagnets, high-nuclearity transition metal complexes and single molecule magnets. In chapter 1, we provide an overview of molecular magnets. Here, we present a survey on the literature available on molecule based magnets. The chapter throws light on various phenomena found in molecular magnetic systems that range in dimensions from 3D down to molecular dimension. This is followed by a brief introduction to high-nuclearity transition metal complexes and single molecule magnets (SMMs). In the last two sections of this chapter, we discuss Light Induced Excited Spin State Trapping (LIESST) and photomagnetism in some molecular systems. Chapter 2 discusses various theoretical models that have been developed for magnetism. We begin with an introduction to the spin Hamiltonian and the origin of direct and kinetic exchange in simple systems and extend it to larger systems. Then we introduce the concept of superexchange proposed by Goodenough and Kanamori, followed by introduction to anisotropic Dzyalashinskii-Moria (DM) exchange and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. We also discuss molecular magnetic anisotropy, long-range magnetic interactions and higher order exchange interactions. These are effective model Hamiltonians that do not provide microscopic origin of magnetism, hence electronic model Hamiltonians need to be invoked. We introduce electronic model Hamiltonians like Huckel, Hubbard and Pariser-Parr-Popple (PPP) models and then present numerical techniques like valencebond (VB) and constant MS techniques that are used to exactly solve these model Hamiltonians. We present a many-body electronic model involving the active orbitals on the transition metal ions for photomagnetism in MoCu6 cluster, in chapter 3. The model is exactly solved using a valence bond approach. The ground state solution of the model is highly degenerate and is spanned by five S=0 states, nine S=1 states, five S=2 states and one S=3 state. The orbital occupancies in all these states correspond to six Cu(II) ions and one diamagnetic Mo(IV ) ion. The optically excited chargetransfer (CT) state in each spin sector occurs at nearly the same excitation energy of 2.993 eV for physically reasonable parameter values. We find that the excitation cross sections in different spin manifolds are similar in magnitude. The lifetime of the S=3 excited states is expected to be the largest as the number of states below that energy is very sparse in this spin sector compared to other spin sectors. This shows that photomagnetism is not due to preferential excitation to the S = 3 state. The inputs from the electronic model allows us to develop a kinetic model. In this model, photomagnetism is attributed to a long lived S=3 charge transfer excited state for which there appears to be sufficient experimental evidence. Based on this postulate, we model photomagnetism by including internal conversions and intersystem crossings. The key feature of the model is the assumption of existence of two kinds of S=3 states; one of which has no direct pathway for internal conversion and the other characterized by slow kinetics for internal conversion to the lowenergy states. The trapped S=3 state can decay via a thermally activated barrier to the other S = 3 state. The experimental XMT vs. T variation for two different irradiation times are fitted using Arrhenius dependence of the rate constants in the model. Conventional superexchange rules predict ferromagnetic exchange interaction between Ni(II) and M (M = MoV ,WV , NbIV ). Recent experiments show that in some systems this superexchange is antiferromagnetic. To understand this feature, in chapter 4 we develop a microscopic model for Ni(II) - M systems and solve it exactly using a valence bond approach. We identify direct exchange coupling, splitting of the magnetic orbitals and interorbital electron repulsions, on the M site as the parameters which control the ground state spin of various clusters of the Ni(II) - M system. We present quantum phase diagrams which delineate the high-spin and low-spin ground states in the parameter space. We fit the spin gap to a spin Hamiltonian and extract the effective exchange constant within the experimentally observed range, for reasonable parameter values. We also find a region in the parameter space where an intermediate spin state is the ground state. These results indicate that the spin spectrum of the microscopic model cannot be reproduced by a simple Heisenberg exchange Hamiltonian. The electronic model for A − B systems has been employed to reproduce the experimental magnetic data of the { NiW }2 system. In chapter 5, we present a theoretical approach to calculate the molecular magnetic anisotropy parameters, DM and EM for single molecule magnets in any eigenstate of the exchange Hamiltonian, treating the anisotropy Hamiltonian as a perturbation. Neglecting inter-site dipolar interactions, we calculate molecular magnetic anisotropy in a given total spin state from the known single-ion anisotropies of the transition metal centers. The method is applied to Mn12Ac and Fe8 in their ground and first few excited eigenstates, as an illustration. We have also studied the effect of orientation of local anisotropies on the molecular anisotropy in various eigenstates of the exchange Hamiltonian. We find that, in case of Mn12Ac, the molecular anisotropy depends strongly on the orientation of the local anisotropies and the spin of the state. The DM value of Mn12Ac is almost independent of the orientation of the local anisotropy of the core Mn(IV ) ions. In the case of Fe8, the dependence of molecular anisotropy on the spin of the state in question is weaker. We have also calculated the anisotropy constants for several sets of exchange parameters and find that in Mn12Ac the anisotropy increases with spin excitation gap while in Fe8, the anisotropy is almost independent of the gap. We have modeled the magnetic property of Nb6Ni12 cluster using a spin Hamiltonian in chapter 6. From Goodenough-Kanamori rules we should expect a ferromagnetic exchange between Nb and Ni ions. However, the magnetic studies indicate that the interaction is antiferromagnetic. We give reasons for the anomaly and fit the XMT data using an antiferromagnetic Heisenberg model. The observed XMT value at 2 K however does not correspond to ferrimagnetic ground state of Stot=9 and we invoke intermolecular interaction to explain this feature.

The diethyl ester of o-phenylenebis(oxamic acid) (opbaH2Et2) was treated with an excess of RNH2 in MeOH to cause the exclusive formation of the respective o-phenylenebis(N(R)-oxamides) (opboH4R2, R = Me 1, Et 2, nPr 3) in good yields. Treatment of 1–3 with half an equivalent of [Cu2(AcO)4(H2O)2] or one equivalent of [Ni(AcO)2(H2O)4] followed by the addition of four equivalents of [nBu4N]OH resulted in the formation of mononuclear bis(oxamidato) type complexes [nBu4N]2[M(opboR2)] (M = Ni, R = Me 4, Et 5, nPr 6; M = Cu, R = Me 7, Et 8, nPr 9). By addition of two equivalents of [Cu(pmdta)(NO3)2] to MeCN solutions of 7–9, novel trinuclear complexes [Cu3(opboR2)(L)2](NO3)2 (L = pmdta, R = Me 10, Et 11, nPr 12) could be obtained. Compounds 4–12 have been characterized by elemental analysis and NMR/IR spectroscopy. Furthermore, the solid state structures of 4–10 and 12 have been determined by single-crystal X-ray diffraction studies. By controlled cocrystallization, diamagnetically diluted 8 and 9 (1%) in the host lattice of 5 and 6 (99%) (8@5 and 9@6), respectively, in the form of single crystals have been made available, allowing single crystal ESR studies to extract all components of the g-factor and the tensors of onsite CuA and transferred NA hyperfine (HF) interaction. From these studies, the spin density distribution of the [Cu(opboEt2)]2− and [Cu(opbonPr2)]2− complex fragments of 8 and 9, respectively, could be determined. Additionally, as a single crystal ENDOR measurement of 8@5 revealed the individual HF tensors of the N donor atoms to be unequal, individual estimates of the spin densities on each N donor atom were made. The magnetic properties of 10–12 were studied by susceptibility measurements versus temperature to give J values varying from −96 cm−1 (10) over −104 cm−1 (11) to −132 cm−1 (12). These three trinuclear CuII-containing bis(oxamidato) type complexes exhibit J values which are comparable to and slightly larger in magnitude than those of related bis(oxamato) type complexes. In a summarizing discussion involving experimentally obtained ESR results (spin density distribution) of 8 and 9, the geometries of the terminal [Cu(pmdta)]2+ fragments of 12 determined by crystallographic studies, together with accompanying quantum chemical calculations, an approach is derived to explain these phenomena and to conclude if the spin density distribution of mononuclear bis(oxamato)/bis(oxamidato) type complexes could be a measure of the J couplings of corresponding trinuclear complexes. / Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

Les ions lanthanide sont employés avec succès pour la synthèse de molécules-aimants caractérisées par une relaxation lente du moment magnétique d’origine purement moléculaire. Ces propriétés sont principalement liées à la forte anisotropie intrinsèque de ces ions, ainsi qu’à leur moment magnétique intense. Dans le cas de complexes contenant plusieurs porteurs de spin (métaux 3d, 4f ou radicaux organiques), la proximité de ces centres peut induire entre ces derniers une interaction de couplage aussi appelée échange magnétique. La rationalisation de telles propriétés est notamment rendue possible par l’approche ab initio. / Lanthanide ions have been used successfully in the synthesis of single molecule magnets for more than a decade. This particular class of molecules shows slow relaxation of their magnetization from purely molecular origin. This property come mainly from the strong single ion anisotropy of these ions, and from their high magnetic moment. In the case of complexes with more than one spin carrier (3d, 4f metal or organic radical), coupling interactions can arise. These are called magnetic exchange. In this framework, ab initio calculations are a useful tool for magneto-structural correlations.

Les travaux de recherche présentés dans cette thèse portent sur la méthode de décomposition de l'interaction d'échange ($J$) entre les centres magnétiques d'une molécule portant des électrons célibataires par l'approche à brisure de symétrie. Le but de cette méthode est d'extraire à l'aide des calculs non-restreints les différentes contributions physiques au couplage magnétique : l'échange direct ($J_0$) entre les orbitales magnétiques, l'échange cinétique ($\Delta J_{KE}$) permettant la délocalisation des orbitales magnétiques et la polarisation des orbitales de cœur ($\Delta J_{CP}$). La première partie de ce travail est consacrée à la théorie de la méthode de décomposition dans le cas le plus simple de deux électrons dans deux centres magnétiques d'un système centro-symétrique. La physique des contributions est expliquée en lien avec la présentation des outils méthodologiques et théoriques utilisés dans cette approche. La deuxième partie de cette thèse concerne l'implémentation de cette méthode dans le logiciel Orca et son application à des systèmes non-centrosymétriques. La dernière partie de ces travaux porte sur les développements méthodologiques que j'ai pu mener au cours de ces trois années de thèse. Une nouvelle approche permettant d'extraire la contribution de polarisation en spin est présentée. De plus, une proposition de généralisation de la décomposition de l'interaction d'échange magnétique basée sur la théorie des hamiltoniens effectifs est discutée. Un effort particulier a été porté sur l'explication et la signification physique de l'approche à brisure de symétrie dans le formalisme Hartree-Fock et la théorie de la fonctionnelle de la densité / This work is focused on the decomposition of the magnetic exchange coupling ($J$) between magnetic centers in the broken-symmetry approach. The purpose of this method is to extract from unrestricted calculations the different contributions to the magnetic coupling: the direct exchange ($J_0$) between the magnetic orbitals, the kinetic exchange interaction ($\Delta J_{KE}$) allowing the delocalization of the magnetic orbitals and the core polarization ($\Delta J_{CP}$) of non-magnetic electrons. The first part of this thesis is centered on the theory of the decomposition method in the simplest case of a centro-symmetric system with two electrons in two magnetic centers. The physical meaning is explained in relation with methodological and theoretical tools used in this approach. The second part presents the implementation of the method in the Orca package and its application to non centro-symmetric systems. In particular, this application highlights the interest of a such automatic method in standard quantum package. The last part of this work is focused on the methodological developments carried out during these three years. An innovative method avoiding the spin contamination problem is presented to extract the spin polarization effects. Furthermore, a generalization of the decomposition of $J$ to more complicated systems with more than two electrons in two magnetic centers is discussed. Special attention was given to the explanation and physical meaning of broken symmetry approach in Hartree-Fock and Density Functional Theory

Dans une première partie, nous avons étudié quelques propriétés de molécules magnétiques impliquant des radicaux organiques (seuls ou conjointement avec des terres rares). Nous avons ainsi pu interpréter l'évolution de la susceptibilité magnétique et de l'aimantation en fonction de la température en évaluant par des approches ab initio fonctions d'onde les constantes d'échange ou le tenseur g au sein de ces matériaux. De plus, nous avons chercher à définir les conditions pour que des matériaux à base de radicaux organiques présentent simultanément des propriétés magnétiques et conductrices. Nous avons ainsi examiné différentes familles de composés et l'influence de la structure géométrique et chimique des radicaux organiques utilisés. Pour cette partie, nous avons extrait les intégrales physiques pertinentes par la méthode des Hamiltoniens effectifs.Dans une deuxième partie, nous avons utilisés ces quantités physiques (intégrale de saut, répulsion sur site, échange) pour décrire le phénomène de transport dans des jonctions pour lesquelles les effets de la corrélation électronique ne peuvent être écartés. Munis de ces paramètres ab initio, nous avons développé un modèle phénoménologique permettant de décrire la conduction moléculaire à l'aide d'un jeu d'équations maîtresses. Nous avons ainsi cherché à mettre en évidence l'intérêt des approches post Hartree-Fock empruntant une fonction d'onde corrélée et de spin adapté dans la description du transport électronique. Que ce soit dans le cas de transport polarisé en spin ou non, l'approche utilisée (mono ou multi-déterminentale) conditionne qualitativement et quantitativement la caractéristique courant/tension. / In a first part, we studied the magnetic properties of organic radicals (coupled with rare earth or between each other). We calculated the magnetic exchange and the g-tensor of these compounds to understand their magnetic susceptibility and thei magnetization curves via ab initio calculations based on the wave-function. We studied how the chemistry and the crystal stacking affect meaningful parameters linked to magnetism and conduction. Those parameters were extracted with the thory of effective Hamiltonians fo various families of organic radicals. From the observed trends for the different parameters, we predicted some ways to obtain multifunctional compounds. In a second part, we used the same parameters (hoping integral, coulombic repulsion, magnetic exchange) to describe transport properties through highly correlated molecular junctions. From the ab initio parameters, we developed a phenomenological model based on master equations to describe the electronic transport. We stressed the importance of a multiconfigurational description to reproduce properly the transport properties for spin unpolarized and spin polarized situations. In both cases, the mono- or multi-configurational description affects qualitatively and quantitatively the predicted conductance curve.