Theoretical study of ruthenium-catalyzed hydrogenation of carbon dioxide into formic acid. Reaction mechanism involving a new type of sigma-bond metathesis

Ruthenium-catalyzed hydrogenation of CO2 into formic acid was theoretically investigated with the DFT(B3LYP) method, where cis-RuH2(PH3)(4) was adopte d as a catalyst model. Theoretical calculations show that (1) CO(2)insertio n into the Ru-H bond occurs with an activation energy (E-a) of 29.3 kcal/mo l in cis-RuH2(PH3)(4) and with an E-a value of 10.3 kcal/mol in cis-RuH2(PH 3)(3); (2) six-membered sigma-bond metathesis of RuH(eta'-OCOH)(PH3)(3)(H-2 ) occurs with a much smaller E-a value (8.2 kcal/mol) than four-membered si gma-bond metathesis (E-a = 24.8 kcal/mol) and five-membered H-OCOH reductiv e elimination (E-a = 25.5 kcal/mol; (3) three-membered H-OCOH reductive eli mination requires a very much larger E-a value of 43.2 kcal/mol); (4) if Ph -3 dissociates from cis-RuH2(PH3)(4), the CO2 hydrogenation takes place thr ough the CO2 insertion into the Ru-H bond of RuH2(PH3)(3) followed by the s ix-membered sigma-bond metathesis, where the rate-determining step is the C O2 insertion; and (5) if PH3 does not dissociate from cis-RuH2(PH3)(4) and cis-RuH(eta(1)-OCOH)-(Ph-3)(4), the CO2 hydrogenation proceeds through the CO2 insertion into the Ru-H bond of Cis-RuH2(PH3)(4) followed by the H-OCOH reductive elimination, where the rate-determining step is the CO2 insertio n. From the above conclusions, one might predict that (1) excess phosphine suppresses the reaction, (2) the use of solvent that facilitates phosphine dissociation is recommended, and (13) the ruthenium(II) complex with three phosphine ligands is expected to be a good catalyst. The electronic process es and characteristic features of the CO2 insertion reaction and the a-bond metathesis are discussed in detail.