Electrocatalysts based on MN4 macrocyclic structures (M = transition metal) have been studied for decades as alternatives to the use of noble metals such as Pt for O₂ reduction (ORR). The search and design of catalysts with high electrocatalytic activity for RRO can be assisted by computational chemistry and different methods based on quantum mechanics. A classical reactivity descriptor in electrocatalysis is the binding energy of key intermediates to the catalyst active sites [1,2]. In this work we have developed theoretical models to predict which MN4 complexes will present a greater activity for the RRO according to the binding energies, which according to the Sabatier Principle they need to be not too strong not too weak. Calculations were performed within the framework of density functional theory (DFT), evaluating the use of different functionals and base functions to determine the appropriate level of theory for the calculation of energies in these systems. Our results suggest that the binding energies in the interaction between oxygen and the metal center 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 center (perpendicular to the plane of the molecule) that are used in self-assembled systems. In addition to binding energies, other descriptors (e.g. hardness derived from conceptual DFT) were also evaluated. The behavior of the electrocatalytic activity of porphyrins and phthalocyanines has been analyzed in the context of volcano correlations. The systematic study of the effect of the different substitution patterns in these MN4 molecular systems on the behavior of volcano correlations allows us to understand the nature of the electronic effects that govern Sabatier's principle (which determines the limit of catalytic efficiency in a rather narrow range of binding energies), an issue that is key to optimizing the design of new catalysts for oxygen reduction and any electrochemical reaction that requires the presence of electrocatalysts in order to proceed at rates compatible with fuel cell or electrolyzer performances.

Acknowledgements: This work was supported by Anillo Project ACT 192175, Fondecyt, Projects 1181037, 1171408 & 1221798.

References: [1] J.K. Norskov et al., J Catal.,328 (2015) 35 [2] J.H. Zagal & M.T.M.Koper, Angew. Chem., 128 (2016) 14510.