Multi-Electron Oxidation Of Anthracene Derivatives By Nonheme Manganese(Iv)-Oxo Complexes

Six-electron oxidation of anthracene to anthraqui-none 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 mu-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 O-18-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.