Electrochemical behavior of meso-substituted iron porphyrins in alkaline aqueous media

Abstract

The effect meso-substitution in iron porphyrin complexes has on their redox behavior in alkaline aqueous solutions is studied via cyclic voltammetry. The voltammetric features of the reduction of iron pyridylporphyrins suggest that the sites of electron transfer lie at the ligand, the metal ion, and the pyridyl moieties. The electron transfer reactions between the different forms of these compounds, including the oxygen reduction reaction they mediate, are outlined to show the sequence and potential ranges in which they occur in alkaline aqueous media. Under our experimental conditions, the iron porphyrins exist as μ-oxo dimmers whose activity for the electrocatalytic reduction of oxygen displays a considerable dependence on the nature of the substitutents and nitrogen isomerization (for pyridylporphyrins) and grows in the order (Fe(ms-Ph)₄P)₂О, (Fe[ms-(Py-3)Ph₃]P)₂O, (Fe[ms-(Py-4)₄]P)₂O, and (Fe[ms-(Py-3)₄]P)₂O.