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).