Article

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

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan.
Journal of the American Chemical Society (Impact Factor: 11.44). 04/2005; 127(11):4021-32. DOI: 10.1021/ja043697n
Source: PubMed

ABSTRACT 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.

0 Followers
 · 
95 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: Carbon dioxide reacts with hydrogen, alcohols, acetals, epoxides, amines, carbon–carbon unsaturated compounds, etc. in supercritical carbon dioxide or in other solvents in the presence of metal compounds as catalysts. The products of these reactions are formic acid, formic acid esters, formamides, methanol, dimethyl carbonate, alkylene carbonates, carbamic acid esters, lactones, carboxylic acids, polycarbonate (bisphenol-based engineering polymer), aliphatic polycarbonates, etc. Especially, the productions of formic acid, formic acid methyl ester and dimethylformamide with a ruthenium catalyst; dimethyl carbonate and urethanes with a dialkyltin catalyst; 2-pyrone with a nickel-phosphine catalyst; diphenyl carbonate with a lead phenoxide catalyst; the alternating copolymerization of carbon dioxide and epoxides with a zinc catalyst has attracted attentions as the industrial utilizations of carbon dioxide. The further development of these production processes is expected.
    Catalysis Today 06/2006; 115(1-4-115):33-52. DOI:10.1016/j.cattod.2006.02.024 · 3.31 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Ru-catalyzed hydrogenation of carbon dioxide to formic acid was theoretically investigated with DFT and MP4(SDQ) methods. In the presence of water molecules, the reaction proceeds as follows: ( 1) Carbon dioxide forms the adduct cis-Ru(H)(2)(PMe3)(3)(H2O)(CO2), in which the C and O atoms of CO2 interact with the H ( hydride) ligand and the H atom of H2O, respectively. ( 2) Nucleophilic attack of the H ligand to CO2 takes place easily to afford a Ru-(eta(1)-formate) intermediate, Ru(H)(PMe3)(3)(eta(1)-OCOH)(H2O), with a much smaller activation barrier than that of the CO2 insertion into the Ru-H bond, which is the rate-determining step in the absence of water molecules. ( 3) The rate-determining step is the coordination of a dihydrogen molecule with the Ru-(eta(2)-formate) complex, Ru(H)(PMe3)(3)(eta(2)-O2CH)(H2O), the activation barrier of which is smaller than that of the CO2 insertion into the Ru-H bond. (4) The metathesis of the Ru-(eta(1)-fomate) moiety with the dihydrogen molecule easily occurs in Ru(H)(PMe3)(3)(eta(1)-OCOH)(H-2)( H2O) to afford formic acid with a moderate activation barrier. On the basis of these results, it should be concluded that the early half of the reaction mechanism changes by the presence of water molecules, which is the reason for the acceleration by water molecules. One of the most important results is that the aqua ligand accelerates the nucleophilic attack of the H ligand to CO2 because the hydrogen-bonding interaction between the aqua ligand and carbon dioxide decreases the activation barrier and increases the exothermicity. Theoretical calculations clearly show that similar acceleration is induced by amines and alcohols.
    Organometallics 07/2006; 25(14). DOI:10.1021/om060307s · 4.25 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Half-sandwich complexes with 4,4′-dihydroxy-2,2′-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT) are highly efficient catalysts for the hydrogenation of bicarbonate in an alkaline aqueous solution without an amine additive. The generation of an oxyanion by the deprotonation of the two hydroxy substituents on the catalyst ligand caused a dramatic enhancement of catalytic activity due to the strong electron-donating ability of the oxyanion. Turnover frequencies (TOF) up to 42,000 h−1 and turnover numbers (TON) up to 222,000 have been obtained by using iridium catalysts under 6 MPa at 120 °C. The production of formate (TOF = 3.5 h−1) was observed even under ambient conditions (0.1 MPa, 30 °C).
    Journal of Photochemistry and Photobiology A Chemistry 09/2006; 182(3):306-309. DOI:10.1016/j.jphotochem.2006.04.025 · 2.50 Impact Factor
Show more