Theoretical Study of CO Migratory Insertion Reactions with Group 10 Metal−Alkyl and −Alkoxide Bonds

Abstract

The results of density functional calculations on the alternative migratory insertion reactions of CO with the M−OMe and M−Me bonds of group 10 M(Me)(OMe)(PH3)2 model systems are reported. For all three metals insertion into the M−OMe bond to form methoxycarbonyl products is thermodynamically favored over insertion into the M−Me bond to give acyls. This preference is small when M = Ni (ΔΔER = 3 kcal/mol) but increases down the triad and becomes significant for M = Pt (ΔΔER = 12 kcal/mol). Both associative five-coordinate and phosphine displacement four-coordinate mechanisms for migratory insertion were considered. For Ni associative mechanisms are more accessible and the lowest energy pathway is for reaction with the Ni−Me bond. With Pd and Pt the five- and four-coordinate pathways are close in energy, and for Pd there is a small kinetic preference for insertion into the Pd−OMe bond. For Pt however there is a clear kinetic preference for reaction with the Pt−OMe bond. During migratory insertion into M−OMe bonds the methoxide ligand rotates in the transition state to allow the participation of an oxygen lone pair in C−O bond formation while maintaining some residual M···OMe interaction. This M···OMe interaction is retained to some extent in the three-coordinate methoxycarbonyl species formed along the four-coordinate pathways. For an isostructural series of reactive species the trend in activation energy is always Ni < Pd ≪ Pt for reaction with the M−Me bond and Ni > Pd < Pt (with Pt > Ni) for reaction with the M−OMe bond. Trends in the computed thermodynamic and kinetic data of the alternative migratory insertions can be understood in terms of metal−ligand homolytic bond strengths. All M−C bonds studied show a marked increase down the group 10 triad, whereas much less variation is seen in the M−OMe bonds, which results in reaction with the M−OMe bonds being generally favored. A key additional driving force, however, is the stronger C−O bond formed in the methoxycarbonyl product compared to the C−C bond of the alternative acyl species.