SITE OF ONE-ELECTRON REDUCTION OF NI(II) PORPHYRINS - FORMATION OF NI(I) PORPHYRIN OR NI(II) PORPHYRIN PI-RADICAL ANION

The products of one-electron reduction of a series of nickel(II) porphyrins (NiIIP) have been examined by radiolytic reduction in protic media to determine whether NiIP or the π-radical anion NiIIP·- is formed. Kinetic spectrophotometric detection was utilized to record absorption spectra of the product immediately after one-electron reduction, and γ-radiolysis was used for recording spectra of the stable reduction products. In the submillisecond time scale, certain NiII-porphyrins were found to be reduced to NiIP, whereas others gave NiIIP·-. The initial reduction product was dependent on the nature of substituents on the porphyrin but not on the redox potential; a meso-4-pyridyl group seems to direct the reduction toward the porphyrin ring, whereas in most other cases reduction occurred on the metal. On longer time scales (minutes) all porphyrins produced the two-electron reduced products, chlorin or phlorin anion. This suggests that both NiIP and NiIIP·- disproportionate to give NiIIP and NiIIP2-, such that for NiIP an intramolecular electron transfer takes place from NiI to its porphyrin ligand. © 1990 American Chemical Society.