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

Abstract

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(salen)(CN)] of low-spin S = 1 state, in contrast to the starting Mn(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(salen)(X)] 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 upon conversion from Mn(salen)(OTf) to [Mn(salen)(CN)], indicative of lowered C═N double bond character for [Mn(salen)(CN)]. 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(salen)(CN)] may contain only limited contribution from valence tautomeric [Mn(salen)(CN)], 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).

Variation of the Emission Efficiency and Wavelength from Fluorescent Zinc Salen Complexes upon Systematic Structural Modifications

“…The present study shows a clear correlation between E ox – E red and λ em , as shown in Figure . The E ox of salen complexes comes from the one-electron oxidation of the phenolate moiety, as indicated from the detailed studies on phenoxyl radicals from salen complexes by us and others. − The E red of salen complexes bearing redox-inactive metal is assigned as arising from one-electron reduction of the azomethine moiety, which is indeed not redox-innocent. , Then, the most probable photoexcitation pathway is a transition from the phenolate moiety as an electron donor to the azomethine moiety as an electron acceptor, as shown in Figure .…”

“… 41 − 44 The E red of salen complexes bearing redox-inactive metal is assigned as arising from one-electron reduction of the azomethine moiety, which is indeed not redox-innocent. 45 , 46 Then, the most probable photoexcitation pathway is a transition from the phenolate moiety as an electron donor to the azomethine moiety as an electron acceptor, as shown in Figure 13 .…”