Substituent and axial ligand effects on the electrochemistry of zinc porphyrins

An absorption spectral and electrochemical study for the zinc(II) complexes of meso-tetraphenylporphyrin dianion (TPP), meso-tetramesitylporphyrin dianion (TMP) and meso-tetra(2,6-dichlorophenyl)porphyrin dianion (TDCPP) in the presence of nitrogeneous bases in CH2Cl2 solution is reported. The Soret and Q bands of the zinc porphyrins were red-shifted in the presence of imidazole bases, however, the formation constants (Kf) with imidazole or 2-methylimidazole titration were found to be of a similar magnitude. The Kf and the formal electrode potentials of zinc porphyrins are in agreement with the electron-donating–withdrawing properties of the substituents in the phenyl groups. Kf for complexation of the imidazole to the oxidized and reduced zinc porphyrins was obtained from the electrochemical study. The one-electron oxidation of zinc porphyrins showed greater affinity toward imidazole ligation than the other oxidation states of zinc porphyrins. ZnTDCPP+ was found to have the greatest affinity with Kf up to 1.35×108. Spectroelectrochemical methods were used to obtain absorption spectra of the complexation of zinc porphyrin cation radical with imidazole. The results showed a slight spectral difference between the complexed and uncomplexed zinc porphyrin cation radical.