Modeling Fischer-Tropsch chemistry. The thermochemistry and insertion kinetics of ClRuH(CH2)

Abstract

The insertions of metal-bound CH/sub x/ into M-H and M-CH/sub 3/ bonds have been proposed as the chain initiation and propagation steps, respectively, in the Fischer-Tropsch reductive polymerization of CO to form alkanes. As a model for this important elementary reaction, the authors have examined the properties and migratory insertion reactivity of a prototypical coordinatively saturated complex ClRuH(CH/sub 2/) using ab initio methods (generalized valence bond + configuration interaction). The Ru==CH/sub 2/ double bond is covalent, with D/sub e/(Ru==C) = 84.7 kcalmol. The optimum geometry has the CH/sub 2/ plane perpendicular to the ClRuH plane, with a rotational barrier of greater than or equal to 13.6 kcalmol. The lowest energy conformer of the singlet state of ClRuH(CH/sub 2/) has an in-plane ..pi.. bond, which facilitates the insertion of the CH/sub 2/ ligand into the adjacent Ru-H bond. Using analytic gradient techniques combined with GVB-MCSCF wave functions to find the minimum energy pathway, they find that the insertion proceeds with a moderate barrier (11.5 kcalmol) and is exothermic by 7.1 kcalmol. From a thermodynamic cycle designed to probe basis set and electron correlation deficiencies, they estimate an actual barrier to insertion of 10.9 +- 1.7 kcalmol and an exothermicity of 10.5more » +- 1.0 kcalmol.« less