Elucidating the reversed proton relay mechanism: dual regulatory role of pendant carboxylates relevant to water oxidation.

To elucidate the "reversed proton relay" mechanism in mediating water oxidation under neutral conditions, we designed two Ru(ii) complexes: [RuII(tda-κ-N3O)(isoq)2] (Ru1), featuring a coordination-oversaturated pendant carboxylate, and [RuII(tpc-κ-N3O)(isoq)2][PF6] (Ru1'), lacking this structural motif. In Ru1, coordination oversaturation renders the carboxylate dynamically accessible, enabling it to function as a proton switch that extracts protons from the metal center. Density functional theory (DFT) calculations, along with comparative experimental data, confirm that the carboxylate facilitates catalytic switching: in its deprotonated state, it stabilizes key intermediates and promotes O2 evolution via reversed proton transfer. Online high-resolution mass spectrometry (HRMS) further identifies key intermediates, providing direct experimental evidence for this unconventional proton relay pathway. Collectively, these findings uncover a previously unrecognized mode of proton transfer and offer valuable mechanistic insights for the rational design of efficient water oxidation catalysts under neutral conditions.