P-Terphenyl-Sensitized Photoreduction of CO2 with Cobalt and Iron Porphyrins. Interaction between CO and Reduced Metalloporphyrins

Abstract

Iron and cobalt porphyrins (FeP and CoP) are utilized as electron-transfer mediators to effect photochemical reduction of CO2 in homogeneous solutions. The species that activate and reduce CO2 are the Fe0P and Co0P formed by reduction of the starting materials. Reduction of the metalloporphyrins (MP) is achieved by photolysis in dimethylformamide or acetonitrile solutions containing triethylamine (TEA) as a reductive quencher. The photoreduction is efficient for the MIIIP → MIIP stage and probably occurs by an intramolecular electron transfer from an axially bound TEA. However, TEA does not bind to the reduced metal complexes, and the quantum efficiency is much lower for the subsequent reduction steps. Considerably higher quantum yields are obtained by adding p-terphenyl (TP) as a sensitizer. TP is very effectively photoreduced by TEA to form the radical anion, TP•-, which has a sufficiently negative reduction potential to reduce CoIP and FeIP rapidly to their M0P state. The rate constants for these react...