Protic Ru(II)-NHC\nPincer Catalyst for the Catalytic\nHydrogenation of CO<sub>2</sub> and Its Derivatives to Methanol

Methanol production from CO<sub>2</sub> offers an attractive\nalternative\nto developing a sustainable energy economy. In this work, a phosphine-free\nRu(II)-NHC pincer complex (<b>1•PF</b><sub><b>6</b></sub>) bearing a proton-responsive pyridyl(benzamide) appended on\nan N-heterocyclic carbene (NHC) has been synthesized. The molecular\nstructure of <b>1•PF</b><sub><b>6</b></sub> reveals\nthe deprotonated iminolic form of the ligand. The acid–base\nequilibrium between the iminolic-amide tautomer of the ligand scaffold\nwas examined by <sup>1</sup>H NMR and UV–vis spectra. The catalytic\nefficacy of <b>1•PF</b><sub><b>6</b></sub> for\nthe hydrogenation of urea and carbamates as CO<sub>2</sub> derivatives,\ntwo of the most challenging carbonyl substrates, to methanol (yield\n74–90%) was explored. Further, amine-assisted CO<sub>2</sub> capture followed by hydrogenation to formamide and its subsequent\nhydrogenation to methanol (yield up to 90%) were performed using catalyst <b>1•PF</b><sub><b>6</b></sub>. A maximum TON of 1700\nwas attained by taking piperidine as a CO<sub>2</sub> capturing agent.\nThe deprotonated complex exhibits superior activity in comparison\nto its protonated form, revealing metal–ligand cooperation\nin dihydrogen activation. Catalyst <b>1•PF</b><sub><b>6</b></sub> is air- and moisture-stable and thus offers operational\nsimplicity. NMR experiments suggest the intermediacy of a [<b>Ru–H/N–H</b>]<sup>+</sup> intermediate engaged in proton and hydride management\nin the catalytic pathway. A plausible catalytic cycle is proposed\nbased on informative mechanistic experiments.