Theoretical Study of Rhodium(III)-Catalyzed Hydrogenation of Carbon Dioxide into Formic Acid. Significant Differences in Reactivity among Rhodium(III), Rhodium(I), and Ruthenium(II) Complexes

Abstract

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.

Carbon Dioxide Hydrogenation Catalyzed by a Ruthenium Dihydride: A DFT and High‐Pressure Spectroscopic Investigation

“…The concerted insertion mechanism is different from the previously reported concerted mechanisms in that there is no initial interaction of an O atom with or coordination of an O atom to the metal center. [20][21][22]30] This relatively small activation energy is in good agreement with the facile CO 2 insertion observed experimentally. Ab initio metadynamics studies also confirmed very facile insertion of CO 2 into RuH 2 complexes with the same concerted insertion mecha- …”

CO2 Insertion into Metal–Carbon Bonds: A Computational Study of RhI Pincer Complexes

“…[22,23] To systematically vary the electronic structure of the rhodium complexes and explore steric influences on the reactivity of the CO 2 association and insertion, we turned to rhodium pincer complexes, which consist of one metal center surrounded by a tridentate ligand in a meridional fashion. Depending on the three donor atoms X, Y, Z, the ligands are usually denominated as XYZ pincers.…”

Hydrogenation for C  C Bond Formation

“…Oxidative coupling is promoted further by the coordinative unsaturation of cationic rhodium complexes. As supported by theoretical studies [57], Brønsted acid co-catalysts likely circumvent highly energetic four-centered transition structures A for s-bond metathesis, as required for direct hydrogenolysis of metallacyclic intermediates, as follows. Metallacycle protonolysis, which may itself occur through the six-centered transition structure B, delivers a rhodium carboxylate, which then engages in hydrogenolysis through the six-centered transition structure C (Scheme 8.6).…”