Catalytic two-electron reduction of dioxygen by ferrocene derivatives with manganese(V) corroles

Electron transfer from octamethylferrocene (Me₈Fc) to the manganese(V) imidocorrole complex (tpfc)Mn^V(NAr) [tpfc = 5,10,15-tris(pentafluorophenyl)corrole; Ar = 2,6-Cl₂C₆H₃] proceeds efficiently to give an octamethylferrocenium ion (Me₈Fc⁺) and [(tpfc)Mn^IV(NAr)]^- in acetonitrile (MeCN) at 298 K. Upon the addition of trifluoroacetic acid (TFA), further reduction of [(tpfc)Mn^IV(NAr)]^- by Me₈Fc gives (tpfc)Mn^III and ArNH₂ in deaerated MeCN. TFA also results in hydrolysis of (tpfc)Mn^V(NAr) with residual water to produce a protonated manganese(V) oxocorrole complex ([(tpfc)Mn^V(OH)]⁺) in deaerated MeCN. [(tpfc)Mn^V(OH)]⁺ is rapidly reduced by 2 equiv of Me₈Fc in the presence of TFA to give (tpfc)Mn^III in deaerated MeCN. In the presence of dioxygen (O₂), (tpfc)Mn^III catalyzes the two-electron reduction of O₂ by Me₈Fc with TFA in MeCN to produce H₂O₂ and Me₈Fc⁺. The rate of formation of Me₈Fc⁺ in the catalytic reduction of O₂ follows zeroth-order kinetics with respect to the concentrations of Me₈Fc and TFA, whereas the rate increases linearly with increasing concentrations of (tpfc)Mn^V(NAr) and O₂. These kinetic dependencies are consistent with the rate-determining step being electron transfer from (tpfc)Mn^III to O₂, followed by further proton-coupled electron transfer from Me₈Fc to produce H₂O₂ and [(tpfc)Mn^IV]⁺. Rapid electron transfer from Me₈Fc to [(tpfc)Mn^IV]⁺ regenerates (tpfc)Mn^III, completing the catalytic cycle. Thus, catalytic two-electron reduction of O₂ by Me₈Fc with (tpfc)Mn^V(NAr) as a catalyst precursor proceeds via a Mn^III/Mn^IV redox cycle.