Olivier Kahn

A Molecular-Based Magnet with a Fully Interlocked Three-Dimensional Structure

A compound has been synthesized with the formula (rad)2Mn2[Cu(opba)]3(DMSO)2.2H2O, where rad+ is 2-(4-N-methylpyridinium)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, opba is orthophenylenebis(oxamato), and DMSO is dimethyl sulfoxide. It consists of two nearly perpendicular graphite-like networks with edge-sharing Mn(II)6Cu(II)6 hexagons. The two networks are fully interlocked with the same topological relationship as that between adjacent rings of a necklace. The compound has three kinds of spin carriers: Mn(II) and Cu(II) ions, antiferromagnetically coupled through oxamato bridges, and rad+ radical cations, bridging the Cu(II) ions through the nitronyl nitroxide groups and forming Cu-rad chains. The temperature dependence of the magnetization reveals that below 22.5 K, the compound behaves as a magnet.

Magnetism of the heteropolymetallic systems

The field of heteropolymetallic systems with magnetic metal centers occupies a crossing point between biology and physics. For instance the Cu(II)-Fe(III) interaction in cytochrome oxidase is of the same nature as the Cu(II)-Mn(II) interaction in a novel system which could be the first molecular ferromagnet. The mechanism of the interaction is discussed, both from a phenomenological view point using a spin Hamiltonian, and from an orbital view point. An orbital model for the isotropic interaction is presented. It is based on the concept of natural magnetic orbitals. The mechanism of the anisotropic and antisymmetric interactions is more briefly treated. The role of the Zeeman perturbation is then considered in relation with the magnetic and EPR properties of the heterobimetallic complexes. Several examples are presented to emphasize that the nature, ferro- or antiferromagnetic of the isotropic interaction is controlled by the symmetry of the magnetic orbitals. The concept of overlap density is introduced. It permits an estimation of the magnitude of the ferromagnetic stabilization in the case of orthogonality of the magnetic orbitals. The Cu(II)-Fe(III) interaction, in relation to the situation encountered in cytochrome oxidase, the Cu(II)-Ni(II) interaction and a few additional selected examples are discussed. A section deals with the case where one of the interacting ions has an orbital degeneracy. Afterwards, the heterotrinuclear complexes are studied. The important concept of regular and irregular spin state structure is developped and the Mn(II)Cu(II)Mn(II) triad is presented as a spectacular example of irregular spin state structure. A section is devoted to the ordered bimetallic chains. The theory is presented, both at a qualitative and quantitative levels and the already reported compounds of this kind are discussed. One of them may be considered as one of the first molecular ferromagnets. The last but one section concerns the systems with even more subtle spin orders. In conclusion, the vast perspectives of this area are outlined.

Magnetism of Heterobimetallics: Toward Molecular-Based Magnets

Publisher Summary The field of heterobimetallic systems has developed tremendously over the past several years particularly in relation to the synthesis of molecular-based magnets. This chapter reviews the advancements in the field of bimetallic magnets. The chapter also discusses some key concepts in molecular magnetism—as spin delocalization, spin polarization—and the interaction between two spin carriers. Several compounds are presented in order of increasing nuclearity and dimensionality. The chapter focuses on the bridges that have allowed the design of molecular-based magnets. These bridges are oxamato, oxamido, oxalato, dithiooxalato, oximato, and cyano. Molecular magnetism has emerged as a novel field of research over the past several years. This field concerns the chemistry and the physics of open-shell molecules and molecular assemblies containing open-shell units. Among all molecules and molecular assemblies relevant to molecular magnetism, those containing two (or possibly more) kinds of metal ions have played a particularly important role.

Critical temperature of the LIESST effect in iron(II) spin crossover compounds

Abstract The light-induced crossover in a series of iron(II) compounds has been investigated by irradiating the sample at 10 K with a Kr + laser coupled to an optical fiber within a SQUID cavity. The temperature dependence of the molar fraction of the light-induced metastable HS state has been recorded for 22 compounds. The critical LIESST temperature, T c (LIESST), has been defined as the temperature for which the light-induced HS information was erased in the SQUID cavity. The dependence of T c (LIESST) as a function of the thermal spin crossover temperature, T 1/2 , has been discussed. The effect of cooperation has been analyzed and the influence of horizontal and vertical displacements of the LS and HS potential wells has been discussed on the basis of the non-adiabatic multiphonon theory.

Analytical Determination of the {Ln–Aminoxyl Radical} Exchange Interaction Taking into Account Both the Ligand‐Field Effect and the Spin–Orbit Coupling of the Lanthanide Ion (Ln=DyIII and HoIII)

Numerous compounds in which a paramagnetic LnIII ion is in an exchange interaction with a second spin carrier, such as a transition metal ion or an organic radical, have been described. However, except for GdIII, very little has been reported about the magnitude of the interactions. Indeed, for these ions both the ligand-field effects and the exchange interactions between the magnetic centers become relevant in the same temperature range; this makes the analysis of the magnetic behavior of such compounds more difficult. In this study, quantitative analyses of the thermal variations of the static isothermal initial magnetic susceptibility measured on powdered samples of the [Ln(NO3)3-[organic radical]2] (Ln = DyIII and HoIII) compounds were performed. The ligand-field effects on the Ln ions were taken into account, and the exchange interactions within a molecule were treated exactly within an appropriate Racah formalism. Values of the intramolecular [Ln-aminoxyl radical] exchange parameter have thus been rigorously deduced for both the Dy Kramers and Ho non-Kramers ion-based compounds. Ferromagnetic [Ln-radical] interactions are found for both the Dy and Ho derivatives with J = 8 cm(-1) and J = 4.5 cm(-1), respectively.

Molecular engineering of coupled polynuclear systems: Orbital mechanism of the interaction between metallic centers

Abstract The aim of this work was to propose a strategy allowing the synthesis of polynuclear complexes with expected magnetic properties. First, we recall the broad outlines of an orbital model for describing the exchange interaction in coupled systems. This model is grounded on the concepts of magnetic orbitals and for overlap density between magnetic orbitals. Then we focus on two problems, namely the design of ferromagnetically-coupled systems and the design of binuclear complexes exhibiting a very large antiferromagnetic interaction between metal ions far away from each other. When the overlap between the magnetic orbitals is zero, the antiferromagnetic contribution is also zero and the coupling is purely ferromagnetic. Two strategies may be used to obtain this result, either the strict orthogonality or the accidental orthogonality. The strict orthogonality is realized in CuVO(fsa)(in2)en, CH(in3)OH and [CuCr(fsa)(in2)en(H2O)2]Cl, 3H2O heterobinuclear complexes, where (fsa)2)en4− denotes the bichelating ligand derived from the Schiff base N,N′(1-hydroxy, 2-carboxxy benzilidene) ethylene diamine. The magnitude of the stabilization of the ground spin triplet with regard to the excited spin singlet in the former complex is explained in a topological way from the map of the overlap density. The accidental orthogonality may be predicted in planar di-μ-hydroxo copper(II) dimers with bridging angles around 92°, and in planar di-μ-azido copper(II) dimers with the azide ions bound end-on and bridging angles around 103°. A complex of this latter type was obtained, where the copper(II) ions are contained a inside cryptate cavity. The coupling is actually ferromagnetic. Concerning the design of bridging or binucleating ligands particularly able to propagate the electronic effects between metal ions far away from each other, two examples are presented: derivatives of the dithioxamide in which two copper(II) ions separated by more than 5.6 A may strongly interact through a C2S2N2 bridging network and a copper(II) binuclear cryptate, in which the interaction occurs through two azido groups bridged end-to-end. In both cases the mechanism of the interaction is specified.

High-spin α low-spin transition in [Fe(NCS)2(4,4′-bis-1,2,4-triazole)2](H2O). X-ray crystal structure and magnetic, mössbauer and EPR properties

Abstract The FeII ion in [Fe(NCS)2(4,4′-bis-1,2,4-triazole)2](H2O) has distorted tetragonal symmetry with two trans-oriented NCS− ligands. It shows a very abrupt high-spin α low-spin transition at 123.5 K on cooling and 144.5 K on warming. The compound is rather stable in vacuo at room temperature; however, samples which have passed the spin transition once lose their water of hydration above 240 K in vacuo. The dehydrated substance does not show a spin transition; it is high spin in the whole temperature range. Mossbauer ligand-field spectra and magnetic behaviour of both the hydrated and non-hydrated compounds are discussed. The spin transition has been followed by EPR measurements with the aid of traces of Cu2+ ions which could be substituted for Fe2+ in the tetragonal structure. In the high-spin phase the EPR signal is very broad and featureless; in the diamagnetic low-spin phase it is very sharp and resolved in hyperfine and superhyperfine structures. This unusual method to follow the spin transition was shown to be quite generally applicable. In the crystal structure of Fe(NCS)2(C4H4N6)2(H2O) the distances FENCS are 2.125(3) A, and FeN(ligand) 2.180(3) and 2.188(2) A. The water molecule is connected to the non-coordinating ligand N-atom by hydrogen bonding.

Synthesis and Magnetic Behavior of Rare-Earth Complexes with N,O-Chelating Nitronyl Nitroxide Triazole Ligands: Example of a [GdIII{Organic Radical}2] Compound with anS=9/2 Ground State

A ferromagnetically coupled gadolinium–radical compound is described. A series of three lanthanide complexes of general formula [Ln(organic radical)2(NO3)3] (Ln =Y3+, La3+, and Gd3+, shown on the right) have been synthesized. With the paramagnetic GdIII a ferromagnetic interaction with the ligands was found, which gives rise to a S = 9/2 ground-state spin.

Competing spin interactions and degenerate frustration for discrete molecular species

Abstract The concept of spin frustration, introduced by solid state physicists in relation with the spin glass behavior, at the origin was intimately related to the idea of high orbital degeneracy. More recently, the term spin frustration has also been used in molecular magnetism to define isolated systems with mere competing spin interactions. The idea of orbital degeneracy (or quasi-degeneracy) has unfortunately disappeared. The case of discrete molecular species for which competing spin interactions lead to an orbitally degenerate ground state, with at least one magnetic component, seems to us to be specifically important, and we suggest to define such a situation as being degenerate frustrated. When this is so, an instability of the spin populations with respect to a weak structural deformation may be observed. Several typical situations are investigated, and the occurrence of degenerate frustration in isolated open-shell molecules is discussed.

IRON(II)-1,2,4-TRIAZOLE SPIN TRANSITION MOLECULAR MATERIALS

The phenomenon of spin transition is probably one of the most spectacular examples of molecular bistability. Our main goal along this line concerns the design of molecular materials exhibiting abrupt spin transitions accompanied by thermochromic and large thermal hysteresis effects. The ideal situation, in terms of possible application of these compounds as active elements of memory devices, is realized when room temperature falls in the middle of the thermal hysteresis loop. In this context, we review our work concerning iron(II)-l,2,4-triazole compounds. We first introduce the idea that the cooperativity should be more pronounced in polymeric than in mononuclear compounds. Then we present some results concerning trinuclear iron(II)-1,2,4- triazole species. The heart of the paper is devoted to the polymeric compounds of formulae [Fe(Htrz)2(trz)](BF4) and [Fe(Htrz)3](BF4)2 • H2O. We report first on the magnetic, optical and calorimetric, then on the structural properties of these compounds. Afterwards, we introduce the concept of spin transition molecular alloy, and emphasize that it is possible to fine tune the spin transition regime of these alloys through their chemical composition. For some alloys, room temperature falls within the thermal hysteresis loop. In the conclusion, the mechanism of cooperativity in the spin transition polymeric compounds is briefly discussed.