Complexes of an aminebis(phenolate) [O,N,O] donor ligand and EPR studies of isoelectronic, isostructural Cr(III) and Mn(IV) complexes

Ligating properties of a tridentate ligand, methylamino-N,N- bis(2-methylene-4,6-dimethylphenol), H2L, with [O,N,O]-donor atoms are described. The ligand H2L yields complexes [Fe-2(III) L-3] (1), [(FeL2)-L-III] (1a), [(MnL2)-L-IV] (2), [(MnL2)-L- III](-) (2a), and [(CrL2)-L-III](-) (3) characterized by various physical techniques, including IR, MS, UV-Vis, electrochemical, EPR, Mossbauer and magnetic susceptibility measurements. The crystal structures of 1, 2 and 3 have also been determined. Complex 1 contains a dinuclear Fe2L3 unit with two different 5- and 6-coordinated iron(III) centers. The overall geometry around the manganese(IV) ion in 2 and the chromium(III) ion in 3 is best described as a distorted meridional octahedron with two trans-positioned nitrogens of the aminebis(phenolate) ligand. The isoelectronic [(CrL2)-L- III](-) (3) and [(MnL2)-L-IV] (2) with distinctive metal-ligand covalent binding characteristics have been used as probes for comparison of EPR spectra. Complex 2 containing Mn(IV) with its higher ionic charge and covalency has a significantly larger zero-field splitting parameter D than that of Cr(III) in 3. That the electrochemical oxidations are ligand-centered, i.e. formation of phenoxyl radicals from the coordinated phenolates, except for the manganese(III) compound 2a, where the first oxidation occurs at metal center generating Mn(IV) species 2, have been shown by voltammetric methods. (C) 2002 Published by Elsevier Science B.V