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 (O-2) by octamethylferrocene (Me(8)Fc) produced hydrogen peroxide (H2O2) 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 Me(8)Fc in the presence of excess TFA produced the corresponding chromium(III) corrole [(tpfc)Cr-III(OH2)] via fast electron transfer from Me(8)Fc to [(tpfc)Cr-V(O)] followed by double protonation of [(tpfc)Cr-IV(O)](-) and facile second-electron transfer from Me(8)Fc. The rate-determining step in the catalytic two-electron reduction of O-2 by Me(8)Fc in the presence of excess TFA is inner-sphere electron transfer from [(tpfc)Cr-III(OH2)] to O-2 to produce the chromium(IV) superoxo species [(tpfc)Cr-IV(O-2(center dot-))], followed by fast proton-coupled electron transfer reduction of [(tpfc)Cr-IV(O-2(center dot-))] by Me(8)Fc to yield H2O2, accompanied by regeneration of [(tpfc)Cr-III(OH2)]. Thus, although the catalytic two-electron reduction of O-2 by Me(8)Fc 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)CrIII(OH2)] and [(tpfc)Cr-V(O)].