One- and Two-Electron Reduction of Metalloporphyrins. Radiation Chemical, Photochemical, and Electrochemical Studies. Kinetics of the Decay of ?-Radical Anions

Article abstract of DOI:10.1021/j100402a040

One- and two-electron reduction products of several metalloporphyrins have been studied by steady-state and pulse radiolysis in aqueous and methanolic slutions and by photochemistry and electrochemistry in aqueous solutions.This study concentrates on the TMPyP (tetrakis(N-methyl-4-pyridyl)porphyrin) complexes of Ga^III, Ge^IV, and In^III, and the TPyP (tetra(3-pyridyl)porphyrin) complex of Sb^V, and compares them with the previously studied complexes of Zn^II, Al^III, and Sn^IV.All these porphyrins undergo one-electron reduction on the ligand to form ?-radical anions.The rates of formation and decay of these species have been studied under various conditions.The lifetimes of these radicals were found to vary greatly; those of Sb^V and Sn^IV are completely stable while some others decay in the millisecond range.The stability decreases in the order Sb^V, Sn^IV, In^III, Ge^IV, Ga^III, Al^III, Zn^II, which was found to correspond to the order of the porphyrin ring reduction potentials determined under similar conditions.Increased lifetime of the radical upon increase in electronegativity of the metal was found to be due to inhibition of porphyrin ring protonation, which is a necessary step in the formation of final products.The final two-electron reduction products are mainly phlorin anions.The same products were observed upon photochemical reduction.The potentials for the first and second one-electron reduction of the porphyrins were determined by cyclic voltammetry.The separation between the first and second wave gives an indication of the stability of the ?-radical anion.