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

Kentaro Mase, Kei Ohkubo, Shunichi Fukuzumi

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41 Scopus citations

Abstract

Effects of changes in the redox potential or configuration of cobalt chlorin derivatives (CoII(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 (CoII(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 CoII(Ch1) complex ([CoII(Ch1H)]+), where one-electron reduction potential of [CoIII(Ch1)]+ was changed from 0.37 V (vs SCE) to 0.48 V by the addition of HClO4 due to the protonation of [CoIII(Ch1)]+. The introduction of electron-withdrawing aldehyde group (position C-3) (CoII(Ch3)) and both methoxycarbonyl group (position C-132) and aldehyde group (position C-3) (CoII(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 [CoIII(Chn)]+/CoII(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 CoII(Ch3) was 36-fold larger than that of CoII(Ch1).

Original languageEnglish
Pages (from-to)1808-1815
Number of pages8
JournalInorganic Chemistry
Volume54
Issue number4
DOIs
StatePublished - 16 Feb 2015

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© 2015 American Chemical Society.

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