Theoretical methods (B3LYP, M06, and CCSD(T)) have been used to study the kinetics and thermodynamics of ethyl migratory insertion in a series of square-planar [(X ^Y)Ni(ethyl)(ethylene)] complexes (X ^Y = anionic bidentate ligand). The results are discussed qualitatively using general trans-influence arguments. When X Y, the reactions of the two possible isomers have been compared. The results reveal that when one of the coordinating groups exerts a strong trans influence (STI) and the other a weak trans influence (WTI), as in a STI ^WTI chelating ligand such as a phosphino-enolate (P ^O), one of the two isomers has an activation energy for ethylene insertion (i.e., ethyl migration) that is much less than that calculated for symmetrical bidentate ligands of either the WTI ^WTI or STI ^STI types. Specifically, a low activation energy is found when an ethyl group, coordinated trans to the STI group, migrates to the ethylene coordinated trans to the WTI group. The converse pathway in the STI ^WTI system, wherein ethyl migrates from a position trans to a WTI group, encounters a very high barrier. However, the kinetic barrier to isomerization (prior to migration) is sufficiently low to allow repeated insertions to proceed via the low-barrier pathway, in which an alkyl group in effect migrates from the position trans to the STI group to the position trans to the WTI group. The overall barrier (isomerization plus insertion) for an [(STI ^WTI)Ni(ethyl)(ethylene)] complex is less than that calculated for insertion in a WTI ^WTI analogue. Ethylene dissociation from [(X ^Y)Ni(ethyl)(ethylene)] leads to an intermediate exhibiting a Ni-ethyl β-agostic bond. Unexpectedly, the data reveal that increased trans influence exerted by the ligand trans to the ethyl α-carbon results in a strengthening of the β-agostic interactions. The [(STI ^STI) Ni(ethyl)] species, therefore, have a surprisingly low energy agostic resting state. As a result, ethylene binding to [(STI ^STI)Ni(ethyl)] is predicted to be endoergic; this contributes to an overall barrier to catalytic ethylene insertion which is greater than that calculated for (STI ^WTI)Ni-based species. These results may explain, at least in part, the favorable role of STI ^WTI chelating ligands in nickel-catalyzed olefin oligomerization. They likely also have bearing on factors influencing the activity of late-transition-metal catalysts for olefin oligomerization and polymerization more generally.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry