Ruthenium(II)-Catalyzed Hydrogenation of Carbon Dioxide to Formic Acid. Theoretical Study of Significant Acceleration by Water Molecules

Ru-catalyzed hydrogenation of carbon dioxide to formic acid was theoretically investigated with DFT and MP4(SDQ) methods. In the presence of water molecules, the reaction proceeds as follows: (1) Carbon dioxide forms the adduct cis -Ru(H) 2 (PMe 3 ) 3 (H 2 O)(CO 2 ), in which the C and O atoms of CO 2 interact with the H (hydride) ligand and the H atom of H 2 O, respectively. (2) Nucleophilic attack of the H ligand to CO 2 takes place easily to afford a Ru-(η 1 -formate) intermediate, Ru(H)(PMe 3 ) 3 (η 1 -OCOH)(H 2 O), with a much smaller activation barrier than that of the CO 2 insertion into the Ru−H bond, which is the rate-determining step in the absence of water molecules. (3) The rate-determining step is the coordination of a dihydrogen molecule with the Ru-(η 2 -formate) complex, Ru(H)(PMe 3 ) 3 (η 2 -O 2 CH)(H 2 O), the activation barrier of which is smaller than that of the CO 2 insertion into the Ru−H bond. (4) The metathesis of the Ru-(η 1 -fomate) moiety with the dihydrogen molecule easily occurs in Ru(H)(PMe 3 ) 3 (η 1 -OCOH)(H 2 )(H 2 O) to afford formic acid with a moderate activation barrier. On the basis of these results, it should be concluded that the early half of the reaction mechanism changes by the presence of water molecules, which is the reason for the acceleration by water molecules. One of the most important results is that the aqua ligand accelerates the nucleophilic attack of the H ligand to CO 2 because the hydrogen-bonding interaction between the aqua ligand and carbon dioxide decreases the activation barrier and increases the exothermicity. Theoretical calculations clearly show that similar acceleration is induced by amines and alcohols.