Enhanced Activity of Proton-Responsive Protic-NHC-Ru(II) Bis(pincer) Complexes for Electrochemical Water Oxidation

A series of homoleptic ruthenium(II)-bis(pincer) complexes (Ru1-4) featuring unsymmetrical CNN pincer ligands are synthesized and characterized using spectroscopic and single-crystal X-ray diffraction techniques. Investigation of these complexes for catalytic electrochemical water oxidation reveals complexes Ru1 and Ru3, with proton-responsive protic-NHC ligands having β-NH functionalities, demonstrate better water oxidation performance compared to their classical-NHC counterparts, Ru2 and Ru4, respectively. Cyclic voltammetry at low temperatures (−30 °C) was conducted to gain insights into the potential active intermediates. The foot of the wave analysis (FOWA) was used to elucidate the operating reaction mechanism, revealing a water nucleophilic attack (WNA) mechanism. Catalytic Tafel plots are employed to evaluate the performance and for benchmarking the catalyst. The Ru1 complex is found to be the most efficient catalyst, among all four complexes in this study, at a modest overpotential (̴ 330 mV vs NHE) with an impressive TON value of 1.6 × 10⁵ over a period of 8 h, giving TOF of 5.53 s⁻¹. Even at a lower overpotential of 140 mV vs NHE, the TON (62560) for Ru1 outperforms most of the homogeneous Ru-systems, other than the Ru(bda) family of complexes, reported so far for the electrochemical water oxidation.