Elimination and activation of methane and larger hydrocarbons

This paper describes a study of hydrocarbon activation and elimination. Important chemical conclusions include the following. There is little evidence for significant agostic interactions between Zr and C-H (or N-alpha-H-alpha) bonds. Replacement of small model ligands with bulkier, more realistic ligands causes the Zr coordination to pyramidalize in the putative CH activating species, which should facilitate coordination of hydrocarbon substrate. For substrates that have a pi-system (ethylene, benzene, and acetylene), only acetylene showed evidence for interaction between Zr and the pi-system on the leaving group in the elimination transition state, suggesting activation and elimination will be relatively insensitive to substituent effects for arenes or olefins. Following the intrinsic reaction coordinates for alkane elimination shows the substrate to remain in the coordination sphere of the d(0) imido. Calculated energies for alkane binding to the d(0) Zr-imido become more negative as the alkane becomes larger. In all cases, elimination of a hydrocarbon containing a pi-system results in a pi-complex in preference to a sigma-complex. The correlation between calculated and experimental hydrocarbon elimination barriers is respectible given the approximations made in the computational models.