Département de chimie moléculaire

The aim of this work was to det. and understand the origin of the electronic properties of MnIV complexes, esp. the zero-field splitting (ZFS), through a combined exptl. and theor. investigation on five well-characterized mononuclear octahedral MnIV compds., with various coordination spheres (N6, N3O3, N2O4 in both trans (trans-N2O4) and cis configurations (cis-N2O4) and O4S2). High-frequency and -field EPR (HFEPR) spectroscopy has been applied to det. the ZFS parameters of two of these compds., MnLtrans-N2O4 and MnLO4S2. While at X-band EPR, the axial-component of the ZFS tensor, D, was estd. to be +0.47 cm-1 for MnLO4S2, and a D-value of +2.289(5) cm-1 was detd. by HFEPR, which is the largest D-magnitude ever measured for a MnIV complex. A moderate D value of -0.997(6) cm-1 has been found for MnLtrans-N2O4. Quantum chem. calcns. based on two theor. frameworks (the D. Functional Theory based on a coupled perturbed approach (CP-DFT) and the hybrid Ligand-Field DFT (LF-DFT)) have been performed to define appropriate methodologies to calc. the ZFS tensor for MnIV centers, to predict the orientation of the magnetic axes with respect to the mol. ones, and to define and quantify the phys. origin of the different contributions to the ZFS. Except in the case of MnLtrans-N2O4, the exptl. and calcd. D values are in good agreement, and the sign of D is well predicted, LF-DFT being more satisfactory than CP-DFT. The calcns. performed on MnLcis-N2O4 are consistent with the orientation of the principal anisotropic axis detd. by single-crystal EPR, validating the calcd. ZFS tensor orientation. The different contributions to D were analyzed demonstrating that the d-d transitions mainly govern D in MnIV ion. However, a deep anal. evidences that many factors enter into the game, explaining why no obvious magnetostructural correlations can be drawn in this series of MnIV complexes. [on SciFinder(R)]