Département de chimie moléculaire

Understanding the factors that control the magnitude and symmetry of magnetic anisotropy should facilitate the rational design of mononuclear metal complexes in the quest for single-mol. magnets (SMMs), based on a single metal ion, with high blocking temps. and large energy barriers. The best strategy is to define magnetostructural correlations through the investigation of metal complexes. The main contribution to the magnetic anisotropy arises from the spin-orbit coupling (SOC) effect in metal-ion-based systems, so current studies focus particularly on the use of both ligands and metal ions possessing a large SOC. In this context, the authors report a unique series of halide CoIII complexes, [CoL(X)], with X=Cl, Br, I (CoX) and L = 2,2'-(2,2'-bipyridine-6,6'-diyl)bis(1,1-diphenylethanethiolate), which possess a rare intermediate S = 1 spin ground state. The S = 1 CoIII complexes are attractive species because they possess a remarkably large axial zero-field splitting (defined by D from the following Hamiltonian: H=DSz2), as well as the halide ligands inducing large SOC consts. The single-crystal x-ray structures reveal that the CoBr and CoI complexes are isostructural with the previously described CoCl complex. Their coordination sphere displays a distorted pentacoordinated square pyramidal geometry, with the halide located in the CoIII axial position. Large pos. D values of 35, 26, and 18 cm-1 are found for CoCl, CoBr, and CoI, resp., through anal. of the magnetic susceptibility data as a function of temp. To rationalize this trend, theor. calcns. based on both d. functional theory (DFT) and complete active space SCF (CASSCF) methods were performed successfully. Both the sign and magnitude of D are predicted remarkably well by these theor. approaches. The DFT calcns. also show that the resulting D values originate from a balance of several contributions, and that many factors, including differences in their structural properties and in the contribution of the halide, should be taken into account to explain the trend of D in this series of complexes. [on SciFinder(R)]