ESR Spectra of Six-coordinate Cobalt(III) Tetraphenylporphyrin Cation Radicals Generated by Electrochemical Oxidation II. Axial Ligand and Solvent Effects

Abstract

Abstract The ESR spectra of the π-cation radical generated electrochemically from 5,10,15,20-tetraphenylporphyr-inatocobalt(II), [CoII(tpp)], have been observed in the presence of various anions and in solvents such as halo-genated hydrocarbons, nitriles, ethers, ethanol, and organic acids. From the analyses of these spectra, the TT-cation radical has been found to coordinate axially the anion or the solvent according to the order of its coordinating ability; BF4−, ClO4−, PF6−, NO3−<nitriles<halogen anions<ethanol<CF3COO−, AcO−, SCN−, CN−. The singly-occupied molecular orbital has been assigned to be always a2uπ orbital regardless of the axial ligands. The spin densities on N and Co much increase due to the axial ligation. It has been further confirmed that the spin polarization mechanism mainly contributes to the appearance of the spin density on Co. These axial-ligand effects are quantitatively explained by a model calculation of the CNDO/2 MO method.