Molecular Cobalt Catalysts for O 2 Reduction: Low-Overpotential Production of H 2 O 2 and Comparison with Iron-Based Catalysts

A series of mononuclear pseudomacrocyclic cobalt complexes have been investigated as catalysts for O 2 reduction. Each of these complexes, with Co III/II reduction potentials that span nearly 400 mV, mediate highly selective two-electron reduction of O 2 to H 2 O 2 (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 [O 2 ] 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 O 2 reduction to H 2 O 2 was estimated by measuring the H + /H 2 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 H 2 O 2 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 O 2 to H 2 O. 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.