Drastic Redox Shift and Electronic Structural Changes of a Manganese(III)-Salen Oxidation Catalyst upon Reaction with...

Flexible redox properties of a metal complex are important for redox catalysis. The present study shows that the reaction of a manganese(III) salen complex, which is a well-known oxidation catalyst, with hydroxide ion gives a transient manganese(III) species with drastically lowered redox potential, where the redox difference is -1.21 V. The reaction with cyanide ion gives a stable manganese(III) species with almost the same spectroscopic and redox properties, which was characterized as an anionic [Mn^III(salen)(CN)₂]^- of low-spin S = 1 state, in contrast to the starting Mn^III(salen)(OTf) having usual high-spin S = 2 manganese(III). The present study has thus clarified that the drastic redox shift comes from an anionic six-coordinate [Mn^III(salen)(X)₂]^- species where X is either OH^- or CN^-. Resonance Raman measurements show that the stretching band of the imino group shifts from 1620 to 1597 cm^-1 upon conversion from Mn^III(salen)(OTf) to [Mn^III(salen)(CN)₂]^-, indicative of lowered C═N double bond character for [Mn^III(salen)(CN)₂]^-. The observed deformation of a salen ligand is a clear indication of an increased electron population on the imino π*-orbital upon formation of low-spin manganese(III). It was proposed that the electronic structure of [Mn^III(salen)(CN)₂]^- may contain only limited contribution from valence tautomeric [Mn^IV(salen^- •)(CN)₂]^-, in which the imino group of a salen ligand is reduced by one-electron via intramolecular electron transfer from low-spin manganese(III). The present study has clarified an unexpected new finding that a salen ligand works as a reservoir for negative charge to stabilize low-spin manganese(III).