Hydrogenation of CO2to methanol by the diphosphine-ruthenium(ii) cationic complex: a DFT investigation to shed light on the decisive role of carboxylic acids as promoters

In order to provide alternative paths for the CO2 conversion processes, we present a quantum-chemical investigation of the CO2 hydrogenation to methanol catalyzed by the recently proposed diphosphine-ruthenium(ii) cationic complex, named Ru2, in presence of carboxylic acids. Our results have shown that, in absence of carboxylate anion, CO2 molecule is not activated towards the hydride nucleophilic attack and, consequently, its hydrogenation does not occur. On the other hand, the addition of carboxylic acid 1) allows the replacement of the p-cymene ligand and, therefore, the formation of the active metal-carboxylate specie proposed experimentally, and 2) activates the CO2 molecule, thus confirming the experimental suggestion that carboxylic acid acts as promoter. Moreover, the influence of the size of the carbon chain in the carboxylate acid has been also investigated and rationalized. Eventually, DFT calculations have been carried out to explore the experimentally proposed catalytic cycle for the CO2 hydrogenation to methanol.