Cyclometalated Ru(II) Complexes Derived from Dibasic CNO Pincer Ligands as Molecular Catalysts for Electrocatalytic Hydrogen Evolution in Alkaline Medium

Electrocatalytic hydrogen (H₂) production through water splitting is a key strategy for sustainable energy conversion. However, the application of molecular electrocatalysts in the hydrogen evolution reaction (HER) is often limited by poor electrical conductivity and inefficient charge transfer. Ruthenium complexes have recently attracted considerable attention as HER electrocatalysts due to their favorable redox properties and structural tunability. In this work, a series of cyclometalated ruthenium complexes, [Ru(CO)(PPh₃)₂L] (C1-C5), incorporating CNO pincer-type hydrazone ligands (L1-L5), were synthesized and comprehensively characterized by spectroscopic and analytical techniques. Single-crystal X-ray diffraction analysis of complex C4 revealed an octahedral coordination geometry at the ruthenium center and confirmed cyclometalation via C-H bond activation. The electrocatalytic HER activity of the complexes was evaluated under alkaline conditions using linear sweep voltammetry on glassy carbon and carbon cloth electrodes. The catalysts exhibit overpotentials of -400 to -439 mV vs RHE on glassy carbon and -197 to -544 mV vs RHE on carbon cloth at a current density of 10 mA cm⁻². Density functional theory calculations indicate that alkaline HER proceeds predominantly through a Volmer-Heyrovsky mechanism. These results demonstrate the potential of cyclometalated ruthenium hydrazone complexes as effective molecular electrocatalysts for alkaline hydrogen evolution.