Reduction of Cobalt and Iron Corroles and Catalyzed Reduction of CO2

The role of cobalt and iron corroles in catalytic CO2 reduction has been studied. Chemical, electrochemical, and photochemical reductions of the stable metal corroles Ph3PCoIII(tpfc) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole), ClFeIV(tpfc), and ClFeIV(tdcc) (tdcc = 5,10,15-tris(2,6-dichlorophenyl)corrole) have been carried out in acetonitrile solutions. Stepwise reduction to the MII(tpfc)- and MI(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 MI state reacts with CO2. Cyclic voltammetry in Ar-saturated acetonitrile solutions permitted determination of the following half-wave potentials: for Ph3PCoIII(tpfc), 1.11 V, 0.72 V, −0.42 V (Epc), −1.44 V, −2.3 V (Epc); for ClFeIV(tpfc), 0.44 V, −1.01 V (Epc), −1.60 V, −2.2 V (Epc); for ClFeIV(tdcc), 0.24 V, −1.18 V (Epc), −1.78 V vs SCE with a scan rate of 0.1 V s-1. Cyclic voltammetry in CO2-saturated solutions indicated that the CoI and FeI complexes react with CO2 and that the reduced Fe(tdcc) complex is the most efficient electrocatalyst for CO2 reduction, showing the largest catalytic currents among these corroles. Photochemical reduction in CO2-saturated acetonitrile solutions containing p-terphenyl (TP) as a sensitizer and triethylamine (TEA) as a reductant led to production of CO and H2. These experiments also show that Fe(tdcc) is more effective than the other corroles as a CO2 reduction catalyst. The present finding that the MI oxidation states of the cobalt and iron corroles can react with CO2 is in contrast with the case of the respective porphyrins and phthalocyanines, which do not react with CO2 until they are reduced beyond the MI state.