Efficient two-electron reduction of dioxygen to hydrogen peroxide with one-electron reductants with a small overpotential catalyzed by a cobalt chlorin complex.

A cobalt chlorin complex (Co(II)(Ch)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O(2)) by one-electron reductants (ferrocene derivatives) to produce hydrogen peroxide (H(2)O(2)) in the presence of perchloric acid (HClO(4)) 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 O(2) by 1,1'-dibromoferrocene (Br(2)Fc) was catalyzed by Co(II)(Ch), whereas virtually no reduction of O(2) 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 O(2) 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 O(2) 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 HClO(4). 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 HClO(4) 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 O(2) with a lower overpotential as compared with that of [Co(III)(OEP)](+).