Synthesis and Characterization of a Dinuclear Manganese(III,III) Complex with Three Phenolate Ligands

A dinuclear manganese complex ([Mn2L(m-OAc)2]PF6) has been synthesized, where L is the trianion of 2,6-bis{[(2-hydroxy-3,5-di-tert-butylbenzyl)(2-pyridylmethyl)amino]methyl}-4-methylphenol, a ligand with three phenolate groups. The two pseudo-octahedrally coordinated Mn ions are bridged via the two bidentate acetate ligands and the 4-methylphenolate group of the ligand. We have characterized the complex with electrochemistry, spectroelectrochemistry and EPR spectroscopy. Electrochemically the Mn2III,III complex undergoes two metal-centered quasi-reversible one-electron reduction steps (E1/2 = 0.04 and -0.32 V vs. SCE). Reduction to the Mn2II,III state results in transformation into a modified complex with slightly different redox properties. One-electron oxidation (E1/2 = 0.96 V vs. SCE) affords the Mn2III,IV state while further one-electron oxidation (E1/2 = 1.13 V vs. SCE) presumably involves ligand oxidation. High valent Mn complexes involving MnIV or MnV centers are of particular interest as intermediates in catalytic water oxidation. The redox potentials of [Mn2L(m-OAc)2]+ show the expected stabilization of higher manganese oxidation states compared with the related complex, [Mn2(bpmp)(m-OAc)2]+. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002