Promoting Effect of Water in Ruthenium-Catalyzed Hydrogenation of Carbon Dioxide to Formic Acid

Abstract

A strong promoting effect of water in the catalytic hydrogenation of CO2 to formic acid with the solvento metal hydride species TpRu(PPh3)(CH3CN)H is observed. High-pressure NMR monitoring of the catalytic reaction shows that CO2 readily inserts into Ru−H to form the metal formate TpRu(PPh3)(CH3CN)(η1-OCHO)·H2O, in which the formate ligand is intermolecularly hydrogen-bonded to a water molecule. Theoretical calculations carried out at the B3LYP level show that reaction barrier of the CO2 insertion is significantly reduced in the presence of water. In the transition state of the process, electrophilicity of the carbon center of CO2 is enhanced by the formation of hydrogen bonds between its oxygen atoms and H2O. The metal formato species comes into equilibrium with another metal formate rapidly; the second formato species TpRu(PPh3)(H2O)(η1-OCHO) contains a coordinated H2O, which is intramolecularly hydrogen-bonded with the formate ligand. In view of the stability of these two metal formates under catalytic conditions, it is very likely that they are not within the major catalytic cycle of the reaction. A catalytic cycle, which accounts for the promoting effect of water, is proposed. The key species in the cycle is the aquo metal hydride species TpRu(PPh3)(H2O)H, which could be generated by a ligand displacement reaction of TpRu(PPh3)(CH3CN)H with H2O. It is proposed that TpRu(PPh3)(H2O)H is able to transfer a proton and a hydride simultaneously to CO2 to yield formic acid in a concerted manner, itself being converted to a transient hydroxo species, which then associates a H2 molecule. The aquo hydride complex TpRu(PPh3)(H2O)H is regenerated via σ-metathesis between the hydroxo and η2-H2 ligands. Theoretical calculations have been carried out to study the structural and energetic aspects of species involved in this catalytic cycle.