Ethylene Oligomerization with Cr−NHC Catalysts: Further Insights into the Extended Metallacycle Mechanism of Chain Growth
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Abstract
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 α-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−C4 (±Cr−C6)). 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 α-olefins into the metallacycle mechanism. Changing the ligand C^N^C 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.
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