Hydrogen Production from Aqueous Formic Acid through the Ligand Design Strategy in Half-Sandwich Ruthenium Complexes

Abstract

Herein, we synthesized water-soluble ruthenium complexes [(η⁶-p-cymene)Ru(κ²-L)]⁺ ([C-1]–[C-5]) ligated with substituted bis-imidazole methane-based ligands (L1–L5) and the molecular structures of the representative complexes [C-2] and [C-4] were established by single-crystal X-ray diffraction. We screened the synthesized complexes for the catalytic dehydrogenation of formic acid (FA) in water, where substitution on the bis-imidazole methane ligands was found to exert a significant impact on the catalytic activity of the complexes. The results inferred that, among the screened catalysts, [C-5] outperformed others with an initial turnover frequency (TOF) of 1831 h^–1 at 90 °C. One of the most notable features of [C-5] was its exceptional long-term stability, as it maintained efficient H₂ production from FA for 35 catalytic runs and remained active even after 60 days without any significant deactivation, reaching a turnover number (TON) of 35,000. Furthermore, reaction kinetics and the influence of various reaction parameters are thoroughly examined; comprehensive mass and NMR investigations under both catalytic and control conditions are conducted, and theoretical studies are performed to gain more insights into the reaction pathway of FA dehydrogenation over the studied catalysts.