Iron porphyrin-catalyzed reduction of CO₂. 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^IIIP to Fe^IIP to Fe^IP and beyond. In aqueous solutions, the Fe^IP 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^IP state of tetrakis(N-methyl-3-pyridyl)porphyrin and tetrakis(N-methyl-4-pyridyl)-porphyrin are short-lived at any pH. Decay of Fe^IP is accelerated by H⁺ and by CO₂, probably via reaction with the Fe⁰P state formed upon disproportionation of Fe1P. These reactions may lead to formation of H₂ and CO, respectively, and to formation of the chlorin, Fe^IIPH₂, as a side product. The Fe^IP 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)-Fe^IIP. The yield of free CO increases with time of photolysis and reaches turnover numbers of ∼70 molecules of CO per porphyrin molecule.