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

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.