Ethylene oligomerization with Cr-NHC catalysts: Further insights into the extended metallacycle mechanism of chain growth

The mechanism of ethylene oligomerization with chromium(III) complexes of bis(carbene)pyridine ligands in combination with MAO has been investigated. Oligomerization with mixtures of CH2CH2/ CD2CD2, reveals that this system produces alpha-olefins via an extended metallacycle mechanism. Deviation form a Schulz-Flory distribution has been explained on the basis of less favorable product release at the early stages of metallacycle growth (Cr-C-4 (+/- Cr-C-6)). Significant amounts of branched olefins (methylidenes) and linear internal olefins are produced at the high catalyst loadings employed in this study, and these result from secondary incorporation of alpha-olefins into the metallacycle mechanism. Changing the ligand (CNC)-N-Lambda-C-Lambda coordination mode in any way resulted in a sharp loss of activity. In the case of bidentate carbene-pyridine and carbene-thiophene ligands, this is due to a change in the mechanism from metallacycle to linear chain growth. Thus, only those catalysts which support a metallacycle mechanism promote oligomerization with high activity.