Ruthenium(II)-catalyzed hydrogenation of carbon dioxide to formic acid. Theoretical study of real catalyst, ligand effects, and solvation effects

Ruthenium-catalyzed hydrogenation of carbon dioxide to formic acid was theoretically investigated with DFT and MP4(SDQ) methods, where a real catalyst, cis-Ru(H)2(PMe3)3, was employed in calculations and compared with a model catalyst, cis-Ru(H)2(PH3)3. Significant differences between the real and model systems are observed in CO2 insertion into the Ru(II)-H bond, isomerization of a ruthenium(II) eta1-formate intermediate, and metathesis of the eta1-formate intermediate with a dihydrogen molecule. All these reactions more easily occur in the real system than in the model system. The differences are interpreted in terms that PMe3 is more donating than PH3 and the trans-influence of PMe3 is stronger than that of PH3. The rate-determining step is the CO2 insertion into the Ru(II)-H bond. Its deltaG(o++) value is 16.8 (6.8) kcal/mol, where the value without parentheses is calculated with the MP4(SDQ) method and that in parentheses is calculated with the DFT method. Because this insertion is considerably endothermic, the coordination of the dihydrogen molecule with the ruthenium(II)-eta1-formate intermediate must necessarily occur to suppress the deinsertion. This means that the reaction rate increases with increase in the pressure of dihydrogen molecule, which is consistent with the experimental results. Solvent effects were investigated with the DPCM method. The activation barrier and reaction energy of the CO2 insertion reaction moderately decrease in the order gas phase > n-heptane > THF, while the activation barrier of the metathesis considerably increases in the order gas phase < n-heptane < THF. Thus, a polar solvent should be used because the insertion reaction is the rate-determining step.