Proton-Coupled Electron-Transfer Reduction of Dioxygen Catalyzed by a Saddle-Distorted Cobalt Phthalocyanine

Proton-coupled electron-transfer reduction of dioxygen (O₂) to afford hydrogen peroxide (H₂O₂) was investigated by using ferrocene derivatives as reductants and saddle-distorted (α-octaphenylphthalocyaninato)­cobalt­(II) (Co^II(Ph₈Pc)) as a catalyst under acidic conditions. The selective two-electron reduction of O₂ by dimethylferrocene (Me₂Fc) and decamethylferrocene (Me₁₀Fc) occurs to yield H₂O₂ and the corresponding ferrocenium ions (Me₂Fc⁺ and Me₁₀Fc⁺, respectively). Mechanisms of the catalytic reduction of O₂ 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 O₂ in the catalytic reaction is switched from Co^II(Ph₈Pc) to protonated Co^I(Ph₈PcH), depending on the reducing ability of ferrocene derivatives employed. The protonation of Co^II(Ph₈Pc) inhibits the direct reduction of O₂; however, the proton-coupled electron transfer from Me₁₀Fc to Co^II(Ph₈Pc) and the protonated [Co^II(Ph₈PcH)]⁺ occurs to produce Co^I(Ph₈PcH) and [Co^I(Ph₈PcH₂)]⁺, respectively, which react immediately with O₂. The rate-determining step is a proton-coupled electron-transfer reduction of O₂ by Co^II(Ph₈Pc) in the Co^II(Ph₈Pc)-catalyzed cycle with Me₂Fc, whereas it is changed to the electron-transfer reduction of [Co^II(Ph₈PcH)]⁺ by Me₁₀Fc in the Co^I(Ph₈PcH)-catalyzed cycle with Me₁₀Fc. A single crystal of monoprotonated [Co^III(Ph₈Pc)]⁺, [Co^IIICl₂(Ph₈PcH)], produced by the proton-coupled electron-transfer reduction of O₂ by Co^II(Ph₈Pc) with HCl, was obtained, and the crystal structure was determined in comparison with that of Co^II(Ph₈Pc)

Proton-Coupled Electron-Transfer Reduction of Dioxygen Catalyzed by a Saddle-Distorted Cobalt Phthalocyanine

Proton-coupled electron-transfer reduction of dioxygen (O₂) to afford hydrogen peroxide (H₂O₂) was investigated by using ferrocene derivatives as reductants and saddle-distorted (α-octaphenylphthalocyaninato)­cobalt­(II) (Co^II(Ph₈Pc)) as a catalyst under acidic conditions. The selective two-electron reduction of O₂ by dimethylferrocene (Me₂Fc) and decamethylferrocene (Me₁₀Fc) occurs to yield H₂O₂ and the corresponding ferrocenium ions (Me₂Fc⁺ and Me₁₀Fc⁺, respectively). Mechanisms of the catalytic reduction of O₂ 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 O₂ in the catalytic reaction is switched from Co^II(Ph₈Pc) to protonated Co^I(Ph₈PcH), depending on the reducing ability of ferrocene derivatives employed. The protonation of Co^II(Ph₈Pc) inhibits the direct reduction of O₂; however, the proton-coupled electron transfer from Me₁₀Fc to Co^II(Ph₈Pc) and the protonated [Co^II(Ph₈PcH)]⁺ occurs to produce Co^I(Ph₈PcH) and [Co^I(Ph₈PcH₂)]⁺, respectively, which react immediately with O₂. The rate-determining step is a proton-coupled electron-transfer reduction of O₂ by Co^II(Ph₈Pc) in the Co^II(Ph₈Pc)-catalyzed cycle with Me₂Fc, whereas it is changed to the electron-transfer reduction of [Co^II(Ph₈PcH)]⁺ by Me₁₀Fc in the Co^I(Ph₈PcH)-catalyzed cycle with Me₁₀Fc. A single crystal of monoprotonated [Co^III(Ph₈Pc)]⁺, [Co^IIICl₂(Ph₈PcH)], produced by the proton-coupled electron-transfer reduction of O₂ by Co^II(Ph₈Pc) with HCl, was obtained, and the crystal structure was determined in comparison with that of Co^II(Ph₈Pc)

Theoretical Analysis of Cobalt Hangman Porphyrins: Ligand Dearomatization and Mechanistic Implications for Hydrogen Evolution

The design of molecular electrocatalysts for hydrogen evolution has been targeted as a strategy for the conversion of solar energy to chemical fuels. In cobalt hangman porphyrins, a carboxylic acid group on a xanthene backbone is positioned over a metalloporphyrin to serve as a proton relay. A key proton-coupled electron transfer (PCET) step along the hydrogen evolution pathway occurs via a sequential ET-PT mechanism in which electron transfer (ET) is followed by proton transfer (PT). Herein theoretical calculations are employed to investigate the mechanistic pathways of these hangman metalloporphyrins. The calculations confirm the ET-PT mechanism by illustrating that the calculated reduction potentials for this mechanism are consistent with experimental data. Under strong-acid conditions, the calculations indicate that this catalyst evolves H₂ by protonation of a formally Co­(II) hydride intermediate, as suggested by previous experiments. Under weak-acid conditions, however, the calculations reveal a mechanism that proceeds via a phlorin intermediate, in which the <i>meso</i> carbon of the porphyrin is protonated. In the first electrochemical reduction, the neutral Co­(II) species is reduced to a monoanionic singlet Co­(I) species. Subsequent reduction leads to a dianionic doublet, formally a Co(0) complex in which substantial mixing of Co and porphyrin orbitals indicates ligand redox noninnocence. The partial reduction of the ligand disrupts the aromaticity in the porphyrin ring. As a result of this ligand dearomatization, protonation of the dianionic species is significantly more thermodynamically favorable at the <i>meso</i> carbon than at the metal center, and the ET-PT mechanism leads to a dianionic phlorin species. According to the proposed mechanism, the carboxylate group of this dianionic phlorin species is reprotonated, the species is reduced again, and H₂ is evolved from the protonated carboxylate and the protonated carbon. This proposed mechanism is a guidepost for future experimental studies of proton relays involving noninnocent ligand platforms