Efficient Two-Electron
Reduction of Dioxygen to Hydrogen Peroxide with One-Electron Reductants
with a Small Overpotential Catalyzed by a Cobalt Chlorin Complex
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Abstract
A cobalt chlorin complex (Co<sup>II</sup>(Ch)) efficiently
and selectively catalyzed two-electron reduction of dioxygen (O<sub>2</sub>) by one-electron reductants (ferrocene derivatives) to produce
hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) in the presence of
perchloric acid (HClO<sub>4</sub>) in benzonitrile (PhCN) at 298 K.
The catalytic reactivity of Co<sup>II</sup>(Ch) was much higher than
that of a cobalt porphyrin complex (Co<sup>II</sup>(OEP), OEP<sup>2β</sup> = octaethylporphyrin dianion), which is a typical
porphyrinoid complex. The two-electron reduction of O<sub>2</sub> by
1,1β²-dibromoferrocene (Br<sub>2</sub>Fc) was catalyzed by Co<sup>II</sup>(Ch), whereas virtually no reduction of O<sub>2</sub> occurred
with Co<sup>II</sup>(OEP). In addition, Co<sup>II</sup>(Ch) is more
stable than Co<sup>II</sup>(OEP), where the catalytic turnover number
(TON) of the two-electron reduction of O<sub>2</sub> catalyzed by
Co<sup>II</sup>(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<sub>2</sub> with
the protonated Co<sup>II</sup>(Ch) ([Co<sup>II</sup>(ChH)]<sup>+</sup>) that is produced by facile electron-transfer reduction of [Co<sup>III</sup>(ChH)]<sup>2+</sup> by ferrocene derivative in the presence
of HClO<sub>4</sub>. The one-electron-reduction potential of [Co<sup>III</sup>(Ch)]<sup>+</sup> was positively shifted from 0.37 V (vs
SCE) to 0.48 V by the addition of HClO<sub>4</sub> due to the protonation
of [Co<sup>III</sup>(Ch)]<sup>+</sup>. Such a positive shift of [Co<sup>III</sup>(Ch)]<sup>+</sup> by protonation resulted in enhancement
of the catalytic reactivity of [Co<sup>III</sup>(ChH)]<sup>2+</sup> for the two-electron reduction of O<sub>2</sub> with a lower overpotential
as compared with that of [Co<sup>III</sup>(OEP)]<sup>+</sup>