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

A series of mononuclear pseudomacrocyclic cobalt complexes have been investigated as catalysts for O2 reduction. Each of these complexes, with CoIII/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 E1/2(CoIII/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 E1/2(MIII/II) than the Fe complexes. This behavior, which underlies the low overpotential, is rationalized on the basis of the catalytic rate law.