Much enhanced catalytic reactivity of cobalt chlorin derivatives on two-electron reduction of dioxygen to produce hydrogen peroxide.

Abstract

Effects of changes in the redox potential or configuration of cobalt chlorin derivatives (Co(II)(Chn) (n = 1-3)) on the catalytic mechanism and the activity of two-electron reduction of dioxygen (O2) were investigated based on the detailed kinetic study by spectroscopic and electrochemical measurements. Nonsubstituted cobalt chlorin complex (Co(II)(Ch1)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O2) by a one-electron reductant (1,1'-dimethylferrocene) to produce hydrogen peroxide (H2O2) in the presence of perchloric acid (HClO4) in benzonitrile (PhCN) at 298 K. 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)(Ch1) complex ([Co(II)(Ch1H)](+)), where one-electron reduction potential of [Co(III)(Ch1)](+) was changed from 0.37 V (vs SCE) to 0.48 V by the addition of HClO4 due to the protonation of [Co(III)(Ch1)](+). The introduction of electron-withdrawing aldehyde group (position C-3) (Co(II)(Ch3)) and both methoxycarbonyl group (position C-13(2)) and aldehyde group (position C-3) (Co(II)(Ch2)) on the chlorin ligand resulted in the positive shifts of redox potential for Co(III/II) from 0.37 V to 0.45 and 0.40 V, respectively, whereas, in the presence of HClO4, no positive shifts of those redox potentials for [Co(III)(Chn)](+)/Co(II)(Chn) (n = 2, 3) were observed due to lower acceptability of protonation. As a result, such a change in redox property resulted in the enhancement of the catalytic reactivity, where the observed rate constant (kobs) value of Co(II)(Ch3) was 36-fold larger than that of Co(II)(Ch1).