Efficient Two-Electron Reduction of Dioxygen to Hydrogen Peroxide with One-Electron Reductants with a Small Overpotential Catalyzed by a Cobalt Chlorin Complex

Abstract

A cobalt chlorin complex (Co^II(Ch)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O₂) by one-electron reductants (ferrocene derivatives) to produce hydrogen peroxide (H₂O₂) in the presence of perchloric acid (HClO₄) in benzonitrile (PhCN) at 298 K. The catalytic reactivity of Co^II(Ch) was much higher than that of a cobalt porphyrin complex (Co^II(OEP), OEP^2– = octaethylporphyrin dianion), which is a typical porphyrinoid complex. The two-electron reduction of O₂ by 1,1′-dibromoferrocene (Br₂Fc) was catalyzed by Co^II(Ch), whereas virtually no reduction of O₂ occurred with Co^II(OEP). In addition, Co^II(Ch) is more stable than Co^II(OEP), where the catalytic turnover number (TON) of the two-electron reduction of O₂ catalyzed by Co^II(Ch) exceeded 30000. The detailed kinetic studies have revealed that the rate-determining step in the catalytic cycle is the proton-coupled electron transfer reduction of O₂ with the protonated Co^II(Ch) ([Co^II(ChH)]⁺) that is produced by facile electron-transfer reduction of [Co^III(ChH)]²⁺ by ferrocene derivative in the presence of HClO₄. The one-electron-reduction potential of [Co^III(Ch)]⁺ was positively shifted from 0.37 V (vs SCE) to 0.48 V by the addition of HClO₄ due to the protonation of [Co^III(Ch)]⁺. Such a positive shift of [Co^III(Ch)]⁺ by protonation resulted in enhancement of the catalytic reactivity of [Co^III(ChH)]²⁺ for the two-electron reduction of O₂ with a lower overpotential as compared with that of [Co^III(OEP)]⁺.