Multi-Electron Oxidation of Anthracene Derivatives by Nonheme Manganese(IV)-Oxo Complexes

Abstract

Six-electron oxidation of anthracene to anthraquinone by a nonheme MnIV-oxo complex, [(Bn-TPEN)MnIV(O)]2+, proceeds through a rate-determining electron transfer from anthracene to [(Bn-TPEN)MnIV(O)]2+, followed by subsequent fast oxidation reactions to give anthraquinone. The reduced MnII complex ([(Bn-TPEN)MnII]2+) is oxidized by [(Bn-TPEN)MnIV(O)]2+ rapidly to produce the μ-oxo dimer ([(Bn-TPEN)MnIII-O-MnIII(Bn-TPEN)]4+). The oxygen atoms of the anthraquinone product were found to derive from the manganese-oxo species by the 18O-labelling experiments. In the presence of Sc3+ ion, formation of an anthracene radical cation was directly detected in the electron transfer from anthracene to a Sc3+ ion-bound MnIV(O) complex, [(Bn-TPEN)MnIV(O)-(Sc(OTf)3)2]2+, followed by subsequent further oxidation to yield anthraquinone. When anthracene was replaced by 9,10-dimethylanthracene, electron transfer from 9,10-dimethylanthracene to [(Bn-TPEN)MnIV(O)-(Sc(OTf)3)2]2+ occurred rapidly to produce stable 9,10-dimethylanthracene radical cation. The driving force dependence of the rate constants of electron transfer from the anthracene derivatives to [(Bn-TPEN)MnIV(O)]2+ and [(Bn-TPEN)MnIV(O)-(Sc(OTf)3)2]2+ was well-evaluated in light of the Marcus theory of electron transfer. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim