Reduction of Cobalt and Iron Corroles and Catalyzed Reduction of CO₂

Abstract

The role of cobalt and iron corroles in catalytic CO 2 reduction has been studied. Chemical, electrochemical, and photochemical reductions of the stable metal corroles Ph 3 PCo III (tpfc) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole), ClFe IV (tpfc), and ClFe IV (tdcc) (tdcc = 5,10,15-tris(2,6-dichlorophenyl)corrole) have been carried out in acetonitrile solutions. Stepwise reduction to the [M II (tpfc)] - and [M I (tpfc)] 2- states was observed in all cases. Gradual reduction with sodium amalgam permitted recording of the optical absorption spectra of the various oxidation states and showed that the M I state reacts with CO 2 . Cyclic voltammetry in Ar-saturated acetonitrile solutions permitted determination of the following half-wave potentials: for Ph 3 PCo III (tpfc), 1.11 V, 0.72 V, −0.42 V ( E pc ), −1.44 V, −2.3 V ( E pc ); for ClFe IV (tpfc), 0.44 V, −1.01 V ( E pc ), −1.60 V, −2.2 V ( E pc ); for ClFe IV (tdcc), 0.24 V, −1.18 V ( E pc ), −1.78 V vs SCE with a scan rate of 0.1 V s -1 . Cyclic voltammetry in CO 2 -saturated solutions indicated that the Co I and Fe I complexes react with CO 2 and that the reduced Fe(tdcc) complex is the most efficient electrocatalyst for CO 2 reduction, showing the largest catalytic currents among these corroles. Photochemical reduction in CO 2 -saturated acetonitrile solutions containing p -terphenyl (TP) as a sensitizer and triethylamine (TEA) as a reductant led to production of CO and H 2 . These experiments also show that Fe(tdcc) is more effective than the other corroles as a CO 2 reduction catalyst. The present finding that the M I oxidation states of the cobalt and iron corroles can react with CO 2 is in contrast with the case of the respective porphyrins and phthalocyanines, which do not react with CO 2 until they are reduced beyond the M I state.