Proton-Coupled Electron-Transfer Reduction of Dioxygen Catalyzed by a Saddle-Distorted Cobalt Phthalocyanine

Proton-coupled electron-transfer reduction of dioxygen (O2) to afford hydrogen peroxide (H2O2) was investigated by using ferrocene derivatives as reductants and saddle-distorted (α-octaphenylphthalocyaninato)­cobalt­(II) (CoII(Ph8Pc)) as a catalyst under acidic conditions. The selective two-electron reduction of O2 by dimethylferrocene (Me2Fc) and decamethylferrocene (Me10Fc) occurs to yield H2O2 and the corresponding ferrocenium ions (Me2Fc+ and Me10Fc+, respectively). Mechanisms of the catalytic reduction of O2 are discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The active species to react with O2 in the catalytic reaction is switched from CoII(Ph8Pc) to protonated CoI(Ph8PcH), depending on the reducing ability of ferrocene derivatives employed. The protonation of CoII(Ph8Pc) inhibits the direct reduction of O2; however, the proton-coupled electron transfer from Me10Fc to CoII(Ph8Pc) and the protonated [CoII(Ph8PcH)]+ occurs to produce CoI(Ph8PcH) and [CoI(Ph8PcH2)]+, respectively, which react immediately with O2. The rate-determining step is a proton-coupled electron-transfer reduction of O2 by CoII(Ph8Pc) in the CoII(Ph8Pc)-catalyzed cycle with Me2Fc, whereas it is changed to the electron-transfer reduction of [CoII(Ph8PcH)]+ by Me10Fc in the CoI(Ph8PcH)-catalyzed cycle with Me10Fc. A single crystal of monoprotonated [CoIII(Ph8Pc)]+, [CoIIICl2(Ph8PcH)], produced by the proton-coupled electron-transfer reduction of O2 by CoII(Ph8Pc) with HCl, was obtained, and the crystal structure was determined in comparison with that of CoII(Ph8Pc).