Abstract

Electrocatalysts based on bioinspired MN4 macrocyclic complexes (M = transition metal, N4 = phthalocyanines, porphyrins, etc.) have been studied for decades as alternatives to the use of expensive noble metals like Pt for promoting the oxygen reduction reaction (ORR). The rational design of MN4 molecules for the ORR requires knowledge of reactivity descriptors. A classical reactivity descriptor in electrocatalysis is the binding energy (coordination of O2 to the central metal) of key intermediates to the catalyst active sites. We have developed semi-empirical models to predict which MN4 complexes will present a higher activity for the ORR according to the binding energies, which, according to the Sabatier Principle need to be not too strong or weak. It has been found that the M(III)/(II) redox potential of the central metal in the complex correlates linearly with the binding energies of reaction intermediates. Therefore, this experimental parameter is a powerful reactivity descriptor. The binding energies and redox potentials in the interaction between oxygen and the metal centre of the MN4 systems are strongly affected by changes in the electronic structure caused both by the presence of substituents in the aromatic rings (in the same plane of the molecule) and also by axial ligands in the metal centre (perpendicular to the plane of the molecule) that are used in self-assembled systems. These findings agree with the reports of Lever and Alexiou that there is a clear direct linear correlation between the M(III)/(II) redox potentials and the total electronic contributions of the planar ligand and extra planar axial ligands.