Molecular Cobalt Catalysts for O₂ Reduction: Low-Overpotential Production of H₂O₂ and Comparison with Iron-Based...

A series of mononuclear pseudomacrocyclic cobalt complexes have been investigated as catalysts for O₂ reduction. Each of these complexes, with Co^III/II reduction potentials that span nearly 400 mV, mediate highly selective two-electron reduction of O₂ to H₂O₂ (93-99%) using decamethylferrocene (Fc*) as the reductant and acetic acid as the proton source. Kinetic studies reveal that the rate exhibits a first-order dependence on [Co] and [AcOH], but no dependence on [O₂] or [Fc*]. A linear correlation is observed between log(TOF) vs E_1/2(Co^III/II) for the different cobalt complexes (TOF = turnover frequency). The thermodynamic potential for O₂ reduction to H₂O₂ was estimated by measuring the H⁺/H₂ open-circuit potential under the reaction conditions. This value provides the basis for direct assessment of the thermodynamic efficiency of the different catalysts and shows that H₂O₂ is formed with overpotentials as low as 90 mV. These results are compared with a recently reported series of Fe-porphyrin complexes, which catalyze four-electron reduction of O₂ to H₂O. The data show that the TOFs of the Co complexes exhibit a shallower dependence on E_1/2(M^III/II) than the Fe complexes. This behavior, which underlies the low overpotential, is rationalized on the basis of the catalytic rate law.