Absorption and electrochemical properties of cobalt and iron phthalocyanines and their quaternized derivatives: Aggregation equilibrium and oxygen reduction electrocatalysis

The synthesis and investigation of the electronic properties of Co(II) and Fe(II)tetrakis(pyridine-3-yloxy)phthalocyanines, as well as the respective quaternized complexes, are reported here. After quaternization reaction, the compounds showed increased solubility in water, and their aggregation equilibrium was analyzed by varying the solution concentration, pH, and composition. Cyclic voltammograms of the four compounds showed both metal and ring centered redox processes, with the former being highly sensitive to methylation of the pyridyl groups. The catalytic effect of the phthalocyanines adsorbed on glassy carbon electrodes in the oxygen reduction reaction (ORR) was investigated by cyclic voltammetry (CV) and rotating disk voltammetry (RDV). The highest catalytic activities were observed for the Fe(II) complexes in alkaline media, and a 2 + 2 mechanism, which consists of a first complete O₂ to H₂O₂ process and a subsequent incomplete H ₂O₂ to H₂O process, is proposed.