High-valent chromium-oxo complex acting as an efficient catalyst precursor for selective two-electron reduction of dioxygen by a ferrocene derivative

Efficient catalytic two-electron reduction of dioxygen (O₂) by octamethylferrocene (Me₈Fc) produced hydrogen peroxide (H ₂O₂) using a high-valent chromium(V)-oxo corrole complex, [(tpfc)Cr^V(O)] (tpfc = tris(pentafluorophenyl)corrole) as a catalyst precursor in the presence of trifluoroacetic acid (TFA) in acetonitrile (MeCN). The facile two-electron reduction of [(tpfc)Cr^V(O)] by 2 equiv of Me₈Fc in the presence of excess TFA produced the corresponding chromium(III) corrole [(tpfc)Cr^III(OH₂)] via fast electron transfer from Me₈Fc to [(tpfc)Cr^V(O)] followed by double protonation of [(tpfc)Cr^IV(O)]^- and facile second-electron transfer from Me₈Fc. The rate-determining step in the catalytic two-electron reduction of O₂ by Me₈Fc in the presence of excess TFA is inner-sphere electron transfer from [(tpfc)Cr ^III(OH₂)] to O₂ to produce the chromium(IV) superoxo species [(tpfc)Cr^IV(O₂^•-)], followed by fast proton-coupled electron transfer reduction of [(tpfc)Cr ^IV(O₂^•-)] by Me₈Fc to yield H₂O₂, accompanied by regeneration of [(tpfc)Cr ^III(OH₂)]. Thus, although the catalytic two-electron reduction of O₂ by Me₈Fc was started by [(tpfc)Cr ^V(O)], no regeneration of [(tpfc)Cr^V(O)] was observed in the presence of excess TFA, regardless of the tetragonal chromium complex being to the left of the oxo wall. In the presence of a stoichiometric amount of TFA, however, disproportionation of [(tfpc)Cr^IV(O)]^- occurred via the protonated species [(tpfc)Cr^IV(OH)] to produce [(tpfc)Cr ^III(OH₂)] and [(tpfc)Cr^V(O)].