Proton-Coupled Electron-Transfer
Reduction of Dioxygen
Catalyzed by a Saddle-Distorted Cobalt Phthalocyanine
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Abstract
Proton-coupled electron-transfer reduction of dioxygen
(O<sub>2</sub>) to afford hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) was investigated
by using ferrocene derivatives as reductants and saddle-distorted
(α-octaphenylphthalocyaninato)cobalt(II) (Co<sup>II</sup>(Ph<sub>8</sub>Pc)) as a catalyst under acidic conditions.
The selective two-electron reduction of O<sub>2</sub> by dimethylferrocene
(Me<sub>2</sub>Fc) and decamethylferrocene (Me<sub>10</sub>Fc) occurs
to yield H<sub>2</sub>O<sub>2</sub> and the corresponding ferrocenium
ions (Me<sub>2</sub>Fc<sup>+</sup> and Me<sub>10</sub>Fc<sup>+</sup>, respectively). Mechanisms of the catalytic reduction of O<sub>2</sub> are discussed on the basis of detailed kinetics studies on the overall
catalytic reactions as well as on each redox reaction in the catalytic
cycle. The active species to react with O<sub>2</sub> in the catalytic
reaction is switched from Co<sup>II</sup>(Ph<sub>8</sub>Pc) to protonated Co<sup>I</sup>(Ph<sub>8</sub>PcH), depending on the
reducing ability of ferrocene derivatives employed. The protonation
of Co<sup>II</sup>(Ph<sub>8</sub>Pc) inhibits the
direct reduction of O<sub>2</sub>; however, the proton-coupled electron
transfer from Me<sub>10</sub>Fc to Co<sup>II</sup>(Ph<sub>8</sub>Pc) and the protonated [Co<sup>II</sup>(Ph<sub>8</sub>PcH)]<sup>+</sup> occurs to produce Co<sup>I</sup>(Ph<sub>8</sub>PcH) and [Co<sup>I</sup>(Ph<sub>8</sub>PcH<sub>2</sub>)]<sup>+</sup>, respectively,
which react immediately with O<sub>2</sub>. The rate-determining step
is a proton-coupled electron-transfer reduction of O<sub>2</sub> by
Co<sup>II</sup>(Ph<sub>8</sub>Pc) in the Co<sup>II</sup>(Ph<sub>8</sub>Pc)-catalyzed cycle with Me<sub>2</sub>Fc, whereas it is changed to the electron-transfer reduction of [Co<sup>II</sup>(Ph<sub>8</sub>PcH)]<sup>+</sup> by Me<sub>10</sub>Fc in
the Co<sup>I</sup>(Ph<sub>8</sub>PcH)-catalyzed cycle with Me<sub>10</sub>Fc. A single crystal of monoprotonated [Co<sup>III</sup>(Ph<sub>8</sub>Pc)]<sup>+</sup>, [Co<sup>III</sup>Cl<sub>2</sub>(Ph<sub>8</sub>PcH)], produced by the proton-coupled electron-transfer
reduction of O<sub>2</sub> by Co<sup>II</sup>(Ph<sub>8</sub>Pc) with HCl, was obtained, and the crystal structure was
determined in comparison with that of Co<sup>II</sup>(Ph<sub>8</sub>Pc)