Unravelling the chain growth mechanism in Cr/NNN-catalysed ethylene oligomerization

Abstract

A special and controversial Cr/NNN ethylene oligomerization system delivering oligomers in a Schulz–Flory distribution, which was suggested to follow a metallacycle mechanism, has been systematically investigated by means of density functional theory (DFT) methods. Both Cossee–Arlman and metallacycle mechanisms have been considered. DFT calculation results demonstrated that Cossee-type chain propagation operates in this case while the metallacyclic chain growth is less likely to occur. A larger kinetic isotope effect (KIE) on the β-H elimination was found to hinder the transfer of deuterium atoms to the metal centre. As such, abundant even-numbered isotopomers were observed in the GC-MS spectrum while odd-numbered products only occupied a small portion. The presence of the KIE has a significant effect on the distribution of isotopomers. The Cossee–Arlman catalytic cycle with a strong KIE may lead to unnoticeable H/D scrambling and for this reason the analysis of isotopic product peaks in C₂H₄/C₂D₄ co-oligomerization should be taken with good care.