Abstract : A series of Co(III) corrole complexes were tested as catalysts for the electroreduction of dioxygen to water. Cyclic voltammetry and rotating ring–disk electrode voltammetry were both used to examine the catalytic activity of the cobalt complexes in acidic media. Altogether twenty-nine related compounds were examined.
A simple monocorrole represented as (Me4Ph5Cor)Co, a face-to-face biscorrole complex, (BCY)Co2, linked by an anthracene (A), biphenylene (B), 9,9-dimethylxanthene (X), dibenzofuran (O) or dibenzothiophene (S) bridge and a face-to-face porphyrin–corrole, (PCY)Co2, containing a Co(II) porphyrin (P) and a Co(III) corrole (C) linked by one of the above rigid spacers (with Y = A, B, X, O) provided a direct four-electron pathway for the reduction of O2 to H2O.
The catalytic reactivity of seven heterobinuclear cofacial porphyrin–corrole complexes (PCY)MClCoCl, M being either an iron(III) or manganese(III) ion were examined and compared on one hand to related dyads with a single Co(III) corrole macrocycle linked to a free-base porphyrin with the same set of linking bridges, (PCY)H2Co, and on the other hand to dicobalt porphyrin–corrole dyads having the formula (PCY)Co2. The data indicates that the E1/2 values where electrocatalysis is initiated is related to the initial site of electron transfer, which is the Co(III)/Co(II) porphyrin reduction process in the case of (PCY)Co2 and the Co(IV)/Co(III) corrole reduction in the case of (PCY)MnClCoCl, (PCY)FeClCoCl and (PCY)H2Co. The overall data also suggests that the catalytically active form of the biscobalt dyad in (PCY)Co2 contains a Co(II) porphyrin and a Co(IV) corrole.
Finally, a series of dicobalt cofacial porphyrin–corrole dyads bearing substituents at the meso positions of the corrole ring were tested for the electroreduction of dioxygen. The examined compounds are represented as (PMes2CY)Co2, where Y = 9,9-dimethylxanthene (X), dibenzofuran (O), or diphenylether (Ox). The catalytic behavior of the three investigated cofacial porphyrin–corrole dyads was compared to two cobalt corrole complexes possessing different meso-substituents, (F5PhMes2Cor)Co and (TPFCor)Co with F5PhMes2Cor = 10-(pentafluorophenyl)-5,15-bis(2,4,6-trimethylphenyl)corrole and TPFCor = 5,10,15-tris(pentafluorophenyl)corrole, respectively. The overall data shows that meso-substitution of porphyrin-corrole dyads disfavors the four-electron pathway and a Co(IV)/Co(III) corrole reduction process is involved in the reduction of dioxygen.