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

Effects of changes in the redox potential or configuration of cobalt chlorin derivatives (Co^II(Ch_n) (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-13²) 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(Ch_n)]⁺/Co^II(Ch_n) (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 (k_obs) value of Co^II(Ch₃) was 36-fold larger than that of Co^II(Ch₁).