Electrochemical Water Oxidation Mechanisms Involving Macrocyclic Copper(II) Complexes: Ligand Ring Size Effects on...

A key challenge of electrocatalytic water oxidation for H2 production remains in modulating structural and electronic features of transition metal complexes to enhance catalytic performance. Herein, inspired by previous experimental and computational studies on the macrocyclic catalyst [Cu(14-TMC)]²⁺ (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), we present a theoretical investigation based on Density Functional Theory (DFT) to examine the mechanistic impacts of its ring size reduction. To this end, we evaluated the water oxidation catalytic cycle mediated by [Cu(12-TMC)]²⁺, providing a comprehensive analysis of the electrochemical oxidation, OO bond formation, and O₂ evolution steps. Subsequently, we compare mechanistic features of [Cu(14-TMC)]²⁺ and [Cu(12-TMC)]²⁺ highlighting similarities and differences in the key reaction routes and intermediates, revealing that ligand ring size affects the electronics, steric hindrance and, consequently, the coordination numbers of these species. Notably, the rate-determining step of both catalytic cycles is the OO bond formation exhibiting significant differences in their mechanisms, especially regarding the structures of key intermediates. Despite that, both mechanisms have comparable energy barriers. For instance, the Gibbs free energy barriers are computed to be 18.96 and 19.26 kcal/mol for [Cu(12-TMC)]²⁺ and [Cu(14-TMC)]²⁺ catalysis, respectively. However, [Cu(12-TMC)]²⁺ provided more intricate mechanisms due to being more susceptible to ligand reorganization in the Cu coordination sphere.