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

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journal contribution
posted on 11.01.2018, 18:13 by Takuya Kurahashi
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 [MnIII(salen)­(CN)2] of low-spin S = 1 state, in contrast to the starting MnIII(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 [MnIII(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 MnIII(salen)­(OTf) to [MnIII(salen)­(CN)2], indicative of lowered CN double bond character for [MnIII(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 [MnIII(salen)­(CN)2] may contain only limited contribution from valence tautomeric [MnIV(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).