Hydrogen Production\nfrom Aqueous Formic Acid through\nthe Ligand Design Strategy in Half-Sandwich Ruthenium Complexes

Herein, we synthesized water-soluble\nruthenium complexes\n[(η<sup>6</sup>-<i>p</i>-cymene)Ru(κ<sup>2</sup>-L)]<sup>+</sup> ([<b>C-1</b>]–[<b>C-5</b>]) ligated\nwith\nsubstituted bis-imidazole methane-based ligands (<b>L1–L5</b>) and the molecular structures of the representative complexes [<b>C-2</b>] and [<b>C-4</b>] were established by single-crystal\nX-ray diffraction. We screened the synthesized complexes for the catalytic\ndehydrogenation of formic acid (FA) in water, where substitution on\nthe bis-imidazole methane ligands was found to exert a significant\nimpact on the catalytic activity of the complexes. The results inferred\nthat, among the screened catalysts, [<b>C-5</b>] outperformed\nothers with an initial turnover frequency (TOF) of 1831 h<sup>–1</sup> at 90 °C. One of the most notable features of [<b>C-5</b>] was its exceptional long-term stability, as it maintained efficient\nH<sub>2</sub> production from FA for 35 catalytic runs and remained\nactive even after 60 days without any significant deactivation, reaching\na turnover number (TON) of 35,000. Furthermore, reaction kinetics\nand the influence of various reaction parameters are thoroughly examined;\ncomprehensive mass and NMR investigations under both catalytic and\ncontrol conditions are conducted, and theoretical studies are performed\nto gain more insights into the reaction pathway of FA dehydrogenation\nover the studied catalysts.