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

Six-electron oxidation of anthracene to anthraquinone by a nonheme Mn^IV-oxo complex, [(Bn-TPEN)Mn^IV(O)]²⁺, proceeds through a rate-determining electron transfer from anthracene to [(Bn-TPEN)Mn^IV(O)]²⁺, followed by subsequent fast oxidation reactions to give anthraquinone. The reduced Mn^II complex ([(Bn-TPEN)Mn^II]²⁺) is oxidized by [(Bn-TPEN)Mn^IV(O)]²⁺ rapidly to produce the μ-oxo dimer ([(Bn-TPEN)Mn^III-O-Mn^III(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 Mn^IV(O) complex, [(Bn-TPEN)Mn^IV(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)Mn^IV(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)Mn^IV(O)]²⁺ and [(Bn-TPEN)Mn^IV(O)-(Sc(OTf)₃)₂]²⁺ was well-evaluated in light of the Marcus theory of electron transfer.