Carbon dioxide hydrogenation catalyzed by a ruthenium dihydride: a DFT and high-pressure spectroscopic investigation
Reaction pathways during CO2 hydrogenation catalyzed by the Ru dihydride complex [Ru(dmpe)2H2] (dmpe=Me2PCH2CH2PMe2) have been studied by DFT calculations and by IR and NMR spectroscopy up to 120 bar in toluene at 300 K. CO2 and formic acid readily inserted into or reacted with the complex to form formates. Two formate complexes, cis-[Ru(dmpe)2(OCHO)2] and trans-[Ru(dmpe)2H(OCHO)], were formed at low CO2 pressure (<5 bar). The latter occurred exclusively when formic acid reacted with the complex. A RuH⋅⋅⋅ HOCHO dihydrogen-bonded complex of the trans form was identified at H2 partial pressure higher than about 50 bar. The trans form of the complex is suggested to play a pivotal role in the reaction pathway. Potential-energy profiles along possible reaction paths have been investigated by static DFT calculations, and lower activation-energy profiles via the trans route were confirmed. The H2 insertion has been identified as the rate-limiting step of the overall reaction. The high energy of the transition state for H2 insertion is attributed to the elongated Ru−O bond. The H2 insertion and the subsequent formation of formic acid proceed via Ru(η2-H2)-like complexes, in which apparently formate ion and Ru+ or Ru(η2-H2)+ interact. The bond properties of involved Ru complexes were characterized by natural bond orbital analysis, and the highly ionic characters of various complexes and transition states are shown. The stability of the formate ion near the Ru center likely plays a decisive role for catalytic activity. Removal of formic acid from the dihydrogen-bonded complex (RuH⋅⋅⋅HOCHO) seems to be crucial for catalytic efficiency, since formic acid can easily react with the complex to regenerate the original formate complex. Important aspects for the design of highly active catalytic systems are discussed.