A strong promoting effect of water in the catalytic hydrogenation of CO 2 to formic acid with the solvento metal hydride species TpRu(PPh 3 )(CH 3 CN)H is observed. High-pressure NMR monitoring of the catalytic reaction shows that CO 2 readily inserts into Ru-H to form the metal formate TpRu(PPh 3 )(CH 3 CN)(η 1 -OCHO)‚H 2 O, 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 CO 2 insertion is significantly reduced in the presence of water. In the transition state of the process, electrophilicity of the carbon center of CO 2 is enhanced by the formation of hydrogen bonds between its oxygen atoms and H 2 O. The metal formato species comes into equilibrium with another metal formate rapidly; the second formato species TpRu(PPh 3 )(H 2 O)(η 1 -OCHO) contains a coordinated H 2 O, 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(PPh 3 )(H 2 O)H, which could be generated by a ligand displacement reaction of TpRu(PPh 3 )(CH 3 CN)H with H 2 O. It is proposed that TpRu(PPh 3 )(H 2 O)H is able to transfer a proton and a hydride simultaneously to CO 2 to yield formic acid in a concerted manner, itself being converted to a transient hydroxo species, which then associates a H 2 molecule. The aquo hydride complex TpRu(PPh 3 )(H 2 O)H is regenerated via σ-metathesis between the hydroxo and η 2 -H 2 ligands. Theoretical calculations have been carried out to study the structural and energetic aspects of species involved in this catalytic cycle.
The indenylruthenium hydride complex (eta(5)-C(9)H(7))Ru(dppm)H was found to be active in catalyzing the hydration of nitriles to amides. The chloro analogue (eta(5)-C(9)H(7))Ru(dppm)Cl was, however, found to be inactive. Density functional theory calculations at the B3LYP level provide explanations for the effectiveness of the hydride complex and the ineffectiveness of the chloro complex in the catalysis. It is learned that the presence of a Ru-H.H-OH dihydrogen-bonding interaction in the transition state lowers the reaction barrier in the case of (eta(5)-C(9)H(7))Ru(dppm)H, but in the chloro system, the corresponding transition state does not contain this type of interaction and the reaction barrier is much higher. A similar dihydrogen-bond-promoting effect is believed to be responsible for the catalytic activity of the hydrotris(pyrazolyl)borato (Tp) ruthenium complex TpRu(PPh(3))(CH(3)CN)H in CH(3)CN hydration. The chloro analogue TpRu(PPh(3))(CH(3)CN)Cl shows no catalytic activity.
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