IRON PORPHYRIN-CATALYZED REDUCTION OF CO2 - PHOTOCHEMICAL AND RADIATION-CHEMICAL STUDIES

Several iron porphyrins have been reduced by photochemical anal radiat ion chemical methods, in organic solvents and in aqueous solutions, fr om (FeP)-P-III to Fe-II to (FeP)-P-I and beyond, In aqueous solutions, the (FeP)-P-I state is relatively stable for the tetrakis(N-methyl-2- pyridyl)porphyrin at high pH but is shorter lived in neutral and acidi c solutions. The (FeP)-P-I state of tetrakis(N-methyl-3-pyridyl)porphy rin and tetrakis(N-methyl-4-pyridyl)porphyrin are short-lived at any p H. Decay of (FeP)-P-I is accelerated by H+ and by CO2, probably via re action with the (FeP)-P-0 state formed upon disproportionation of (FeP )-P-I. These reactions may lend to formation of H-2 and CO, respective ly, and to formation of the chlorin, (FePH2)-P-II, as a side product, The (FeP)-P-I state is also observed as a stable product in several or ganic solvents. This is observed by photolysis of iron tetraphenylporp hyrin and several of its derivatives (e.g., trimethyl-, dichloro- and pentafluorophenyl), mainly in dimethylformamide and acetonitrile solut ions, using triethylamine as a reductive quencher, Further photoreduct ion in the presence of CO2 results in catalyzed reduction of CO2 to CO and formation of (CO)-(FeP)-P-II. The yield of free CO increases with time of photolysis and reaches turnover numbers of similar to 70 mole cules of CO per porphyrin molecule.