Cobalt corrin catalyzed photoreduction of CO2

Cobalt corrins (B-12) are found to act as homogeneous catalysts for photore duction of CO2 to CO and formic acid Photoreduction is carried out in aceto nitrile/methanol solutions containing p-terphenyl as a photosensitizer and triethylamine as a reductive quencher. Photolysis (lambda greater than or e qual to 300 nm) leads to production of CO and formic acid as well as H-2. T he rate of production of all three products is considerably higher with the corrins (hydroxocobalamin, cyanocobalamin, and cobinamide) than with cobal t tetra-m-tolylporphyrin. The mechanism of CO2 reduction in all cases is vi a a species formed by one-electron reduction of the Co(I) complex. Radiolyt ic studies of the Co(I) complex formed from hydroxocobalamin in aqueous sol utions show that this compound reacts very rapidly with solvated electrons and more slowly [k = (1.2 +/- 0.3) x 10(8) L mol(-1) s(-1)] with CO2.- radi cals to produce different products. The initial reduction product is sugges ted to be mainly the hydride formed by protonation of a Co(0) corrin, (HCoB 12)(-). The product of reaction with CO2.- is suggested to be the adduct (C O2CoB12)(2-) or its protonated form (HO2CCoB12)(-), identical to the adduct formed by reaction of CO2 with the photochemically reduced Co(I) corrin, w hich proceeds to produce CO. H-2 is formed by reaction of the hydride with a proton. Side reactions leading to hydrogenation of the macrocycle also ta ke place and limit the catalytic activity.