Protic Ru(II)-NHC Pincer Catalyst for the Catalytic Hydrogenation of CO2 and Its Derivatives to Methanol

Abstract

Methanol production from CO₂ offers an attractive alternative to developing a sustainable energy economy. In this work, a phosphine-free Ru(II)-NHC pincer complex (1•PF₆) bearing a proton-responsive pyridyl(benzamide) appended on an N-heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1•PF₆ reveals the deprotonated iminolic form of the ligand. The acid–base equilibrium between the iminolic-amide tautomer of the ligand scaffold was examined by ¹H NMR and UV–vis spectra. The catalytic efficacy of 1•PF₆ for the hydrogenation of urea and carbamates as CO₂ derivatives, two of the most challenging carbonyl substrates, to methanol (yield 74–90%) was explored. Further, amine-assisted CO₂ capture followed by hydrogenation to formamide and its subsequent hydrogenation to methanol (yield up to 90%) were performed using catalyst 1•PF₆. A maximum TON of 1700 was attained by taking piperidine as a CO₂ capturing agent. The deprotonated complex exhibits superior activity in comparison to its protonated form, revealing metal–ligand cooperation in dihydrogen activation. Catalyst 1•PF₆ is air- and moisture-stable and thus offers operational simplicity. NMR experiments suggest the intermediacy of a [Ru–H/N–H]⁺ intermediate engaged in proton and hydride management in the catalytic pathway. A plausible catalytic cycle is proposed based on informative mechanistic experiments.