The title reaction was theoretically investigated, where cis-[RhH(2)(PH(3))(3)](+) and cis-[RhH(2)(PH(3))(2)(H(2)O)](+) were adopted as models of the catalyst. The first step of the catalytic cycle is the CO(2) insertion into the Rh(III)-H bond, of which the activation barrier (E(a)) is 47.2 and 28.4 kcal/mol in cis-[RhH(2)(PH(3))(3)](+) and cis-[RhH(2)(PH(3))(2)(H(2)O)](+), respectively, where DFT(B3LYP)-calculated E(a) values (kcal/mol unit) are given hereafter. These results indicate that an active species is not cis-[RhH(2)(PH(3))(3)](+) but cis-[RhH(2)(PH(3))(2)(H(2)O)](+). After the CO(2) insertion, two reaction courses are possible. In one course, the reaction proceeds through isomerization (E(a) = 2.8) of [RhH(eta(1)- OCOH)(PH(3))(2)(H(2)O)(2)](+), five-centered H-OCOH reductive elimination (E(a) = 2.7), and oxidative addition of H(2) to [Rh(PH(3))(2)(H(2)O)(2)](+) (E(a) = 5.8). In the other one, the reaction proceeds through isomerization of [RhH(eta(1)-OCOH)(PH(3))(2)(H(2)O)(H(2))](+) (E(a) = 5.9) and six-centered sigma-bond metathesis of [RhH(eta(1)-OCOH)(PH(3))(2)(H(2)O)](+) with H(2) (no barrier). RhH(PH(3))(2)-catalyzed hydrogenation of CO(2) proceeds through CO(2) insertion (E(a) = 1.6) and either the isomerization of Rh(eta(1)-OCOH)(PH(3))(2)(H(2)) (E(a) = 6.1) followed by the six-centered sigma-bond metathesis (E(a) = 0.3) or H(2) oxidative addition to Rh(eta(1)-OCOH)(PH(3))(2) (E(a) = 7.3) followed by isomerization of RhH(2)(eta(1)-OCOH)(PH(3))(2) (E(a) = 6.2) and the five-centered H-OCOH reductive elimination (E(a) = 1.9). From these results and our previous results of RuH(2)(PH(3))(4)-catalyzed hydrogenation of CO(2) (J. Am. Chem. Soc. 2000, 122, 3867), detailed discussion is presented concerning differences among Rh(III), Rh(I), and Ru(II) complexes.

No abstract

Ruthenium-catalyzed hydrogenation of CO 2 into formic acid was theoretically investigated with the DFT(B3LYP) method, where cis-RuH 2 (PH 3 ) 4 was adopted as a catalyst model. Theoretical calculations show that (1) CO 2 insertion into the Ru-H bond occurs with an activation energy (E a ) of 29.3 kcal/mol in cis-RuH 2 (PH 3 ) 4 and with an E a value of 10.3 kcal/mol in cis-RuH 2 (PH 3 ) 3 ; (2) six-membered σ-bond metathesis of RuH(η 1 -OCOH)(PH 3 ) 3 (H 2 ) occurs with a much smaller E a value (8.2 kcal/mol) than four-membered σ-bond metathesis (E a ) 24.8 kcal/mol) and five-membered H-OCOH reductive elimination (E a ) 25.5 kcal/mol);(3) three-membered H-OCOH reductive elimination requires a very much larger E a value of 43.2 kcal/mol; (4) if PH 3 dissociates from cis-RuH 2 (PH 3 ) 4 , the CO 2 hydrogenation takes place through the CO 2 insertion into the Ru-H bond of RuH 2 (PH 3 ) 3 followed by the six-membered σ-bond metathesis, where the rate-determining step is the CO 2 insertion; and (5) if PH 3 does not dissociate from cis-RuH 2 (PH 3 ) 4 and cis-RuH(η 1 -OCOH)-(PH 3 ) 4 , the CO 2 hydrogenation proceeds through the CO 2 insertion into the Ru-H bond of cis-RuH 2 (PH 3 ) 4 followed by the H-OCOH reductive elimination, where the rate-determining step is the CO 2 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 CO 2 insertion reaction and the σ-bond metathesis are discussed in detail.

A theoretical study of oxidative additions of H−CH3, CH3−CH3, H−SiR3, and SiR3−CH3 (RH, Cl, or Me) to Pt(PH3)2 was carried out with ab initio MO/MP2-MP4SDQ, CCD, and CCSD methods. The oxidative addition reactions of C−H and Si−H σ-bonds occur through a planar transition state (TS) structure, in accordance with the expectation from an orbital interaction diagram. However, the oxidative addition reactions of CH3−CH3 and SiH3−CH3 take place through a nonplanar TS structure, unexpectedly; the dihedral angle δ between PtP2 and PtXC planes (X = C or Si) is about 70° for X = Si and about 80° for X = C. Intrinsic reaction coordinate calculation of the SiH3−CH3 oxidative addition clearly indicated that this nonplanar TS is smoothly connected to the planar product on the singlet surface. The dihedral angle δ at the TS is larger in the SiMe3−CH3 and SiCl3−CH3 oxidative additions than that in the SiH3−CH3 oxidative addition. Electron distribution in the TS and effects of bulky substituent on the dihedral angle suggest that not an electronic factor but a steric factor is responsible for the nonplanar TS structure of the C−C and Si−C oxidative addition reactions.