Cobalt(III) corroles as electrocatalysts for the reduction of dioxygen:: Reactivity of a monocorrole, biscorroles, and porphyrin-corrole dyads

Three series of cobalt(Ill) corroles were tested as catalysts for the electroreduction of dioxygen to water. One was a simple monocorrole represented as (Me(4)Ph(5)Cor)Co, one a face-to-face biscorrole linked by an anthracene (A), biphenylene (B), 9,9-dimethylxanthene (X), dibenzofuran (O) or dibenzothiophene (S) bridge, (6CY)CO2 (with Y = A, B, X, O or S), and one a face-to-face bismacrocyclic complex, (PCY)CO2, containing a Co(II) porphyrin and a Co(III) corrole also linked by one of the above rigid spacers (Y = A, B, X, or O). Cyclic voltammetry and rotating ring-disk electrode voltammetry were both used to examine the catalytic activity of the cobalt complexes in acid media. The mixed valent Co(II)/Co(III) complexes, (PCY)CO2, and the biscorrole complexes, (BCY)CO2, which contain two Co(III) ions in their air-stable forms, all provide a direct four-electron pathway for the reduction of O-2 to H2O in aqueous acidic electrolyte when adsorbed on a graphite electrode, with the most efficient process being observed in the case of the complexes having an anthracene spacer. A relatively small amount of hydrogen peroxide was detected at the ring electrode in the vicinity of E-1/2 which was located at 0.47 V vs SCE for (PCA)CO2 and 0.39 V vs SCE for (BCA)CO2. The cobalt(III) monocorrole (Me(4)Ph(5)Cor)Co also catalyzes the electroreduction of dioxygen at E-1/2 = 0.38 V with the final products being an approximate 50% mixture of H2O2 and H2O.