Iron porphyrin-catalyzed reduction of CO2. Photochemical and radiation chemical studies

Several iron porphyrins have been reduced by photochemical anal radiation chemical methods, in organic solvents and in aqueous solutions, from (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 acidic solutions. The (FeP)-P-I state of tetrakis(N-methyl-3-pyridyl)porphyrin and tetrakis(N-methyl-4-pyridyl)porphyrin are short-lived at any pH. Decay of (FeP)-P-I is accelerated by H+ and by CO2, probably via reaction with the (FeP)-P-0 state formed upon disproportionation of (FeP)-P-I. These reactions may lend to formation of H-2 and CO, respectively, 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 organic solvents. This is observed by photolysis of iron tetraphenylporphyrin and several of its derivatives (e.g., trimethyl-, dichloro- and pentafluorophenyl), mainly in dimethylformamide and acetonitrile solutions, using triethylamine as a reductive quencher, Further photoreduction 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 molecules of CO per porphyrin molecule.