Cobalt porphyrin catalyzed reduction of CO₂. Radiation chemical, photochemical, and electrochemical studies

Several cobalt porphyrins (CoP) have been reduced by radiation chemical, photochemical, and electrochemical methods, in aqueous and organic solvents. In aqueous solutions, the Co^IP state is stable at high pH but is shorter lived in neutral and acidic solutions. Stable Co^IP is also observed in organic solvents and is unreactive toward CO₂. One-electron reduction of Co^IP leads to formation of a species that is observed as a transient intermediate by pulse radiolysis in aqueous solutions and as a stable product following reduction by Na in tetrahydrofuran solutions. The spectrum of this species is not the characteristic spectrum of a metalloporphyrin π-radical anion and is ascribed to Co⁰P. This species binds and reduces CO₂. Catalytic formation of CO and HCO₂^- is confirmed by photochemical experiments in acetonitrile solutions containing triethylamine as a reductive quencher. Catalytic reduction of CO₂ is also confirmed by cyclic voltammetry in acetonitrile and butyronitrile solutions and is shown to occur at the potential at which Co_IP is reduced to Co₀P. As compared with CoTPP, fluorinated derivatives are reduced, and catalyze CO₂ reduction, at less negative potentials.