Catalytic electroreduction of molecular oxygen using iron or cobalt 4,4′,4″,4‴-tetracarboxyphthalocyanine

The electroreduction of oxygen was examined in aqueous solution containing iron or cobalt 4,4′,4″,4‴-tetracarboxyphthalocyanine (FeTCPc or CoTCPc). In the presence of FeTCPc ([FeTCPc] > ca. 3 × 10^-5 M), oxygen was reduced quantitatively to water via hydrogen peroxide (H₂O₂) and the reaction could be explained by an electrochemical (EC) catalytic regeneration mechanism. In the presence of CoTCPc ([CoTCPc] > ca. 3 × 10^-5 M), oxygen was reduced quantitatively to H₂O₂ at a potential closely coincident with that for Co^II- to Co^ITCPc reduction, and moreover the resultant H₂O₂ was partially reduced to water at the same and more negative potentials. Magnetic circular dichroism and electronic absorption spectra have suggested that FeTCPc and CoTCPc as the reaction intermediates are low-spin divalent and low-spin monovalent complexes, respectively. The results are stated in comparison with those so far reported for iron and cobalt porphyrins and phthalocyanines.