Département de chimie moléculaire

The electronic and magnetic properties of polynuclear complexes, in particular the magnetic anisotropy (zero field splitting, ZFS), the leading term of the spin Hamiltonian (SH), are commonly analyzed in a global manner and no attempt is usually made to understand the various contributions to the anisotropy at the at. scale. This is esp. true in weakly magnetically coupled systems. The present study addresses this problem and studies the local SH parameters using a methodol. based on exptl. measurements and theor. calcns. This work focuses on the challenging mono $μ$-oxo bis $μ$-acetato dinuclear MnIII complex: [MnIII2($μ$-O)($μ$-OAc)2L2](PF6)2 (with L = tris-pyrrolidine-1,4,7-triazacyclononane) , which is particularly difficult for EPR spectroscopy because of its large magnetic anisotropy and the weak ferromagnetic interaction between the 2 MnIII ions. High field (up to 12 T) and high frequency (190-345 GHz) EPR expts. have been recorded for 1 at 5-50 K These data have been analyzed by employing Hamiltonian, which encompasses terms describing the local and inter-site interactions. D. functional theory and multi-ref. correlated ab initio calcns. have been used to est. the ZFS of the MnIII ions (DMn = +4.29 cm-1, EMn/DMn = 0.19) and the Euler angles reflecting the relative orientation of the ZFS tensor for each MnIII ($\alpha$ = -52°, $\beta$ 28., $\gamma$ 3.°). This anal. allowed the accurate detn. of the local parameters: DMn = +4.50 cm-1, EMn/DMn = 0.07, $\alpha$ = -35°, $\beta$ 23., $\gamma$ 2.°. The spin ladder approach has also been applied, but only the parameters of the ground spin state of 1 have been accurately detd. (D4 = +1.540 cm-1, E4/D4 = 0.107). This is not sufficient to allow for the detn. of the local parameters. The validity and practical performance of both approaches have been discussed. [on SciFinder(R)]