A comparative study on the CO2 hydrogenation catalyzed by Ru dihydride complexes: (PMe3)4RuH2 and (Me2PCH2CH2PMe2)2RuH2

Abstract

The phosphine complexes of Ru dihydride are model catalysts for CO₂ hydrogenation. Despite many theoretical studies, important questions remain unresolved regarding the underlying catalytic mechanisms. We report a comparative study by using density functional theory on two catalysts, (PMe₃)₄RuH₂ and (dmpe)₂RuH₂, with dmpe = Me₂PCH₂CH₂PMe₂, for which very different mechanisms have been suggested in previous studies. By comparing their energy profiles along all possible reaction paths side by side, we are able to clarify the similarity and difference between them, and provide a consistent account for all the experimental observations reported, including the kinetic models, the cis to trans transformation of (dmpe)₂RuH₂, and the significant enhancement of the catalytic rate in supercritical CO₂ for (PMe₃)₄RuH₂ and its lack thereof for (dmpe)₂RuH₂. The crucial difference between the two mechanisms involves the formation of an intermediate, in which a formate ion binds to Ru as a bidentate ligand. When this step results in the dissociation of a ligand, the reaction rate is enhanced under supercritical conditions, due to the increase in entropy, which should be a valid consideration for other catalytic reactions as well.