Theoretical Study of Ruthenium-Catalyzed Hydrogenation of Carbon Dioxide into Formic Acid. Reaction Mechanism Involving a New Type of σ-Bond Metathesis

Ruthenium-catalyzed hydrogenation of CO2 into formic acid was theoretically investigated with the DFT(B3LYP) method, where cis-RuH2(PH3)4 was adopted as a catalyst model. Theoretical calculations show that (1) CO2 insertion into the Ru−H bond occurs with an activation energy (Ea) of 29.3 kcal/mol in cis-RuH2(PH3)4 and with an Ea value of 10.3 kcal/mol in cis-RuH2(PH3)3; (2) six-membered σ-bond metathesis of RuH(η1-OCOH)(PH3)3(H2) occurs with a much smaller Ea value (8.2 kcal/mol) than four-membered σ-bond metathesis (Ea = 24.8 kcal/mol) and five-membered H−OCOH reductive elimination (Ea = 25.5 kcal/mol); (3) three-membered H−OCOH reductive elimination requires a very much larger Ea value of 43.2 kcal/mol; (4) if PH3 dissociates from cis-RuH2(PH3)4, the CO2 hydrogenation takes place through the CO2 insertion into the Ru−H bond of RuH2(PH3)3 followed by the six-membered σ-bond metathesis, where the rate-determining step is the CO2 insertion; and (5) if PH3 does not dissociate from cis-RuH2(PH3)4 and cis-RuH(η1-OCOH)(PH3)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 insertion. 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 (3) the ruthenium(II) complex with three phosphine ligands is expected to be a good catalyst. The electronic processes and characteristic features of the CO2 insertion reaction and the σ-bond metathesis are discussed in detail.