Effect of the Substituents on the Ligand of Iron Phthalocyanines Adsorbed on Graphite Electrodes on Their Activity for the Electrooxidation of 2-Mercaptoethanol

We have investigated the electrocatalytic activity of five iron phthalocyanines for the electrooxidation of 2-mercaptoethanol, with the complexes preadsorbed on a ordinary pyrolytic graphite electrode. The potential of Fe[II]/Fe[I] and Fe[III]/Fe[II] redox processes on the adsorbed phthalocyanines are shifted to more positive potentials by the effect of electron withdrawing groups −CO2−, −SO3− and −Cl and to more negative potentials by the electron-donating methoxy group compared to −H in unsubstituted FePc. The electrocatalytic activity, measured as current at constant potential, decreases as: Fe-octamethoxyphthalocyanine>Fe-phthalocyanine>Fe-tetracarboxyphthalocyanine>Fe-tetrasulfonatedphthalocyanine>Fe-perchlorinatedphthalocyanine. The most oxidant complex (Fe-perchlorinatedphthalocyanine) is the least active, which is contrary to what would be expected on thermodynamic grounds. This can be attributed to the fact that the reaction proceeds via an inner-sphere mechanism when the metal center is in state of oxidation II.