Cobalt(III) Corroles as Electrocatalysts for the Reduction of Dioxygen: Reactivity of a Monocorrole, Biscorroles, and Porphyrin−Corrole Dyads

Abstract

Three series of cobalt(III) corroles were tested as catalysts for the electroreduction of dioxygen to water. One was a simple monocorrole represented as (Me_4Ph_5Cor)Co, one a face-to-face biscorrole linked by an anthracene (A), biphenylene (B), 9,9-dimethylxanthene (X), dibenzofuran (O) or dibenzothiophene (S) bridge, (BCY)Co_2 (with Y = A, B, X, O or S), and one a face-to-face bismacrocyclic complex, (PCY)Co_2, 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)Co_2, and the biscorrole complexes, (BCY)Co_2, 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 H_2O 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)Co_2 and 0.39 V vs SCE for (BCA)Co_2. The cobalt(III) monocorrole (Me_4Ph_5Cor)Co also catalyzes the electroreduction of dioxygen at E_(1/2) = 0.38 V with the final products being an approximate 50% mixture of H_2O_2 and H_2O.