Protic Ru(II)-NHC Pincer Catalyst for the Catalytic Hydrogenation of CO<sub>2</sub> and Its Derivatives to Methanol
Arindom Bhattacherya, Mandeep Kaur, Ravi Kumar, Joyanta Choudhury, Jitendra K. Bera
Abstract
Methanol production from CO 2 offers an attractive alternative to developing a sustainable energy economy. In this work, a phosphine-free Ru(II)-NHC pincer complex ( 1•PF 6 ) bearing a proton-responsive pyridyl(benzamide) appended on an N-heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1•PF 6 reveals the deprotonated iminolic form of the ligand. The acid–base equilibrium between the iminolic-amide tautomer of the ligand scaffold was examined by 1 H NMR and UV–vis spectra. The catalytic efficacy of 1•PF 6 for the hydrogenation of urea and carbamates as CO 2 derivatives, two of the most challenging carbonyl substrates, to methanol (yield 74–90%) was explored. Further, amine-assisted CO 2 capture followed by hydrogenation to formamide and its subsequent hydrogenation to methanol (yield up to 90%) were performed using catalyst 1•PF 6 . A maximum TON of 1700 was attained by taking piperidine as a CO 2 capturing agent. The deprotonated complex exhibits superior activity in comparison to its protonated form, revealing metal–ligand cooperation in dihydrogen activation. Catalyst 1•PF 6 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.