MECHANISTIC AND KINETIC-STUDIES OF COBALT MACROCYCLES IN A PHOTOCHEMICAL, CO2 REDUCTION SYSTEM

Photoreduction of CO2 using p-terphenyl as a photosensitizer and a tertiary amine as a sacrificial electron donor is efficiently catalyzed by cobalt macrocycles in methanolic acetonitrile solutions. Studies of the mechanism of CO2 reduction in the catalytic system using Co(II)HMD(2+) (HMD = 5,7,7,12, 14,14-hexamethyl-1,4,8,11-tetraazacyclot-4,11-diene) have been carried out using continuous and flash photolysis techniques. The transient spectra reveal the formation of p-terphenyl radical anion, the Co(I)HMD(+) complex, the 5-coordinate [Co(I)HMD-CO2](+) complex, and the six coordinate [S-Co(III)HMD-(CO22-)](+) complex (S = solvent) in the catalytic system. The importance of changes in the Co-II/I reduction potential and the geometry around the Co center was studied by the use of several different complexes.