Reactive Intermediates Involved in Cobalt Corrole Catalyzed Water Oxidation (and Oxygen Reduction).

Abstract

A detailed investigation of the cobalt corrole Co(tpfc) as molecular catalyst for electrochemical water oxidation uncovered many important mechanism-of-action details that are crucial for the design of optimally performing systems. This includes the identification of the redox states that do and do not participate in catalysis and very significant axial ligand effects on the activity of the doubly oxidized complex. Specifics deduced for the electrocatalysis under homogeneous conditions include the following: the one-electron oxidation of the cobalt(III) corrole is completely unaffected by reaction conditions; catalysis coincides with the second oxidation event; two catalytic waves develop in the presence of OH-, and the one at lower overpotential is dominant under more basic conditions. Comparative spectroelectrochemical measurements performed for Co(tpfc) and Al(tpfc), the analogous corrole chelated by the nonredox active aluminum, revealed that the second oxidation process of Co(tpfc) is much more significantly metal-centered than the first one. EPR studies revealed that shift from fully corrole-centered to partially metal-centered in the singly oxidized complex [Co(tpfc)]+ is achievable with fluoride as axial ligand. The analogous experiment, but with hydroxide instead of fluoride, could not be performed because of a surprising phenomenon: formation of a cobalt-superoxide complex that is actually relevant to oxygen reduction rather than to water oxidation. Nevertheless, fluoride and hydroxide induce very similar effects in terms of the appearance of two catalytic waves, lowering of onset potentials, and increasing the catalytic activity. The main conclusions from the accumulated data are that the apparent pH effect is actually due to hydroxide binding to the cobalt center and that π-donating axial ligands play pivotal and beneficial roles regarding the main factors that are important for facilitating the oxidation of water.