Ethylene oligomerization promoted by chromium complexes bearing pyrrolide–imine–amine/ether tridentate ligands

Abstract

Chromium(III) complexes [CrCl₂(L)(THF)] based on monoanionic tridentate ligands [2a, L = {2-(C₄H₃N-2′-CHN)C₂H₄NHPh}; 2b, L = {5-tert-butyl-2-(C₄H₂N-2′-CHN)C₂H₃NHPh}; 2c, L = {2-(C₄H₃N-2′-CHN)C₂H₄OPh}] have been prepared. Complexes 2a and b were converted into the monomeric acetonitrile adducts [CrCl₂(L)(NCMe)] [3a, L = {2-(C₄H₃N-2′-CHN)C₂H₄NHPh}; 3b, L = {5-tert-butyl-2-(C₄H₂N-2′-CHN)C₂H₃NHPh}] by reaction with acetonitrile at room temperature. All Cr complexes were characterized by IR spectroscopy, elemental analysis, magnetochemistry for 2a–c, and by X-ray crystallography for 3a and 3b. Upon activation with methylaluminoxane (MAO), chromium precatalysts 2a and 2c showed good activity in ethylene oligomerization (TOF = 47.0–57.0 × 10³ (mol ethylene)(mol Cr)⁻¹ h⁻¹ at 80 °C), producing mostly oligomers (93.0–95.6 wt% of total products). On the other hand, under identical oligomerization conditions, 2b/MAO behaved as a polymerization catalyst generating predominantly polyethylene (73.0 wt%). However, the catalytic behavior of the precatalyst 2b can be adjusted by varying the MAO-to-Cr ratio. Thus, the use of 500 equiv. causes a dramatic shift from polymerization to ethylene oligomerization, eventually producing mainly lighter α-olefin fractions [α-C₄ (68.7 wt%) and α-C₆ (19.2 wt%)]. A further increase in the amount of MAO (1000 equiv.) leads to a more balanced distribution of oligomers, with a drastic decrease in the α-C₄ and increase in the α-C₈ fractions.