Proton-coupled electron-transfer reduction of dioxygen catalyzed by a saddle-distorted cobalt phthalocyanine

Proton-coupled electron-transfer reduction of dioxygen (O ₂) to afford hydrogen peroxide (H ₂O ₂) was investigated by using ferrocene derivatives as reductants and saddle-distorted (α- octaphenylphthalocyaninato)cobalt(II) (Co ^II(Ph ₈Pc)) as a catalyst under acidic conditions. The selective two-electron reduction of O ₂ by dimethylferrocene (Me ₂Fc) and decamethylferrocene (Me ₁₀Fc) occurs to yield H ₂O ₂ and the corresponding ferrocenium ions (Me ₂Fc ⁺ and Me ₁₀Fc ⁺, respectively). Mechanisms of the catalytic reduction of O ₂ 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 O ₂ in the catalytic reaction is switched from Co ^II(Ph ₈Pc) to protonated Co ^I(Ph ₈PcH), depending on the reducing ability of ferrocene derivatives employed. The protonation of Co ^II(Ph ₈Pc) inhibits the direct reduction of O ₂; however, the proton-coupled electron transfer from Me ₁₀Fc to Co ^II(Ph ₈Pc) and the protonated [Co ^II(Ph ₈PcH)] ⁺ occurs to produce Co ^I(Ph ₈PcH) and [Co ^I(Ph ₈PcH ₂)] ⁺, respectively, which react immediately with O ₂. The rate-determining step is a proton-coupled electron-transfer reduction of O ₂ by Co ^II(Ph ₈Pc) in the Co ^II(Ph ₈Pc)-catalyzed cycle with Me ₂Fc, whereas it is changed to the electron-transfer reduction of [Co ^II(Ph ₈PcH)] ⁺ by Me ₁₀Fc in the Co ^I(Ph ₈PcH)- catalyzed cycle with Me ₁₀Fc. A single crystal of monoprotonated [Co ^III(Ph ₈Pc)] ⁺, [Co ^IIICl ₂(Ph ₈PcH)], produced by the proton-coupled electron-transfer reduction of O ₂ by Co ^II(Ph ₈Pc) with HCl, was obtained, and the crystal structure was determined in comparison with that of Co ^II(Ph ₈Pc).