In situ infrared spectroelectrochemical studies of tetraphenylporphyrin complexes containing manganese, iron and cobalt

Abstract

The difference spectra in the mid-infrared region associated with oxidation of the porphyrin ring in 5,10,15,20-tetraphenylporphyrin complexes of Mn^III, Fe^III and Co^III have been obtained by in situ Fourier-transform infrared spectroelectrochemistry, and are compared to those similarly observed for the corresponding metal(II)–metal(III) redox couples. Oxidation of the porphyrin ligand to the π-cation radical results in the appearance of three new bands near 1420, 1290 and 1230 cm^–1, as well as characteristic shifts and intensity changes of several bands present for the parent complex. In contrast, the difference spectra associated with the metal redox couple show less complex changes, the extent of which vary in the order Co < Fe < Mn. The spectral changes thus provide a means to differentiate between ligand- and metal-centred oxidation. In addition, the results provide insight into the nature of the infrared porphyrin ligand vibrations, as well as the character of the molecular orbitals associated with the oxidation processes.