Iron Porphyrin-Catalyzed Reduction of CO ₂ . Photochemical and Radiation Chemical Studies

Several iron porphyrins have been reduced by photochemical and radiation chemical methods, in organic solvents and in aqueous solutions, from Fe III P to Fe II P to Fe I P and beyond. In aqueous solutions, the Fe I P 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 Fe I P state of tetrakis( N -methyl-3-pyridyl)porphyrin and tetrakis( N -methyl-4-pyridyl)porphyrin are short-lived at any pH. Decay of Fe I P is accelerated by H + and by CO 2, probably via reaction with the Fe 0 P state formed upon disproportionation of Fe I P. These reactions may lead to formation of H 2 and CO, respectively, and to formation of the chlorin, Fe II PH 2, as a side product. The Fe I P 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 CO 2 results in catalyzed reduction of CO 2 to CO and formation of (CO)Fe II P. The yield of free CO increases with time of photolysis and reaches turnover numbers of ∼70 molecules of CO per porphyrin molecule.