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

Abstract

To elucidate the "reversed proton relay" mechanism in mediating water oxidation under neutral conditions, we designed two Ru(II) complexes: [Ru II (tda-κ-N 3 O)(isoq) 2 ] (Ru1), featuring a coordination-oversaturated pendant carboxylate, and [Ru II (tpc-κ-N 3 O)(isoq) 2 ][PF 6 ] (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 O 2 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.