DFT Study of the Products, Potential Energy Surface, and Substituent Effects for Methyl Radical Addition to [Rh(PMe3)2(CO)X] (X = Halogen or CN)

Abstract

Methyl radical addition to [trans-Rh(PMe3)2(CO)Cl] (1) has been studied using density functional theory utilizing ECPs on the heavy elements. At the B3LYP level, Δ H 298 ∘ for the transformation from separate reactants to the five-coordinate 17-e Rh−alkyl product (2) is −8.6 kcal/mol. On the other hand, Δ H 298 ∘ for formation of the four-coordinate 15-e Rh−acyl radical (3) resulting from methyl addition to the carbonyl of 1 is −11.5 kcal/mol. The latter result implies that coordination of CO in 1 reduces the exothermicity of its reaction with CH3 by 5.2 kcal/mol. A 3D potential energy surface of the given reaction reveals that the lowest energy path from separate reactants to 2 does not have any electronic barrier. Similarly, there is no distinct transition state for direct methyl attack on the carbonyl of 1. Instead, the lowest energy trajectory connecting the separate reactants to the acyl product passes through a CO insertion transition state (TS2,3) corresponding to methyl migration between the rhodium and carbon atoms of the Rh−CO bond, with Δ H 298 ∘ of TS2,3 being only 3.1 kcal/mol above the separate reactants. To explore the extent to which the methyl affinity of the coordinated carbonyl in the given system may change when the substituents on the metal are varied, additional calculations have been carried out on the F, Br, I, and CN analogues of 1. Attempts have been made to account for the calculated methyl affinity trends using the ionization and Rh−CO bond dissociation energies of the square-planar reactants.