Backbone-controlled mechanistic switch in macrocyclic Cu(II) complexes: Directing oxygen evolution reaction pathways...

Despite significant advancements in catalytic efficiency of molecular catalysts for oxygen evolution reaction (OER), the intricate relationship between molecular structure, catalytic performance, and reaction mechanisms remains insufficiently understood. Here, we demonstrate how subtle modifications in the backbone structure of macrocyclic Cu(II) complexes dictate a mechanistic switch between metal-based and ligand-based oxidation pathways during electrocatalytic water oxidation reaction. By systematically varying the carbon chain length of the ligand backbone, it reveals that rigid backbones favor metal oxidation, leading to enhanced electron density at the Cu center and lower OO bond formation energy (overpotential = 743 mV; k_cat = 37.8 s^-1; ΔG = 33.89 kcal mol^-1). In contrast, flexible backbones promote ligand oxidation, dispersing positive charge delocalization and thereby improving stability and Faradaic efficiency during prolonged electrolysis. Combined experimental and computational studies confirm that backbone rigidity controls the electron distribution, steering the reaction toward distinct active intermediates: a Cu^III-O· species for rigid systems versus a ligand-radical pathway for flexible systems. These findings establish a precise structure-mechanism correlation, highlighting how minimal backbone adjustments can selectively activate metal- or ligand-based redox processes, with broad implications for designing efficient and stable molecular OER catalysts.