Influence of the Conjugated Structure of Redox Ligands on the PCET Process in Water Oxidation Reactions

To solve the high overpotential and low catalytic efficiency bottlenecks in the oxygen evolution reaction (OER), this work focuses on the precise regulation of redox-active ligand conjugation on proton-coupled electron transfer (PCET). We propose a strategy to optimize catalyst performance by tuning the ligand conjugation scope and degree. Using parent ligand L₁ as a scaffold, ligands L₂ and L₃ with extended conjugation were designed and synthesized, yielding ruthenium(III) hydroxylated complexes [(L₂^N5-)-Ru^III-OH]⁺ and [(L₃^N5-)-Ru^III-OH]⁺, fully characterized by ¹H NMR, ESI-HRMS, and XRD. Electrochemical measurements show that extended conjugation significantly lowers oxidation potentials. The OER overpotentials decrease to 93 and 100 mV from 183 mV of the L₁ complex, with catalytic rates reaching 7.75 and 4.95 s^-1, far higher than 1.26 s^-1. DFT and kinetic studies reveal that oxidation potential differences stem from the SOMO energies of Ru⁴⁺ intermediates. Base-assisted water nucleophilic attack is the rate-determining step, with energy barriers reduced to 17.6 and 16.7 kcal/mol for L₂/L₃ systems, much lower than the 26.4 kcal/mol for L₁. This study establishes the conjugation-PCET-activity relationship, offering a theoretical guide for designing low overpotential, high-performance OER catalysts.