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

Abstract

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 ratedetermining 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.

The Carbon Dioxide Molecule and the Effects of Its Interaction with Electrophiles and Nucleophiles

“…In general they are promoted by a previous coordination of CO 2 Recently, in a theoretical study [151] of the hydrogenation of CO 2 to formic acid using Ru II catalysts in the presence of water, no coordination of CO 2 to the metal center was identified, but low-energy assemblages with the C and O atoms of CO 2 interacting with H À bound to the metal or the H + of H 2 O, respectively. In the absence of water, CO 2 directly coordinates to the Ru center to afford Ru(H) 2 …”

Selective Formic Acid Decomposition for High‐Pressure Hydrogen Generation: A Mechanistic Study

“…Taking these experimental observations into account, it is likely that the monohydride complex 6 can also react with HCOO À and enter a second catalytic cycle (lower cycle in Scheme 1), involving a dihydride species, commonly observed in CO 2 hydrogenation mechanisms. [24,35,36] The b-elimination step in this cycle leads to a CO 2 moiety trans to a hydride ligand, which is therefore more readily released than in 3. Hence, the step that was rate determining in the first catalytic cycle is accelerated in the second cycle due to the trans effect of the hydride ligand.…”

Carbon Dioxide Coordination Chemistry and Reactivity of Coordinated CO2

“…Another recent example of the question of the formation of intermediate metal -CO 2 complexes in these reactions was the theoretical study by Ohnishi et al [84] of the hydrogenation of CO 2 to formic acid by Ru II catalysts. In the presence of water, there was no direct metal coordination of CO 2 , but formation of adducts in which the C and O atoms of CO 2 interacted with the H (hydride) ligand and the H atom of H 2 O: cis -Ru(H) 2 (PMe 3 ) 3 (H 2 O)(CO 2 ).…”