Oligomerization of ethylene by dinuclear chromium catalysts bearing phenyl-bridged bifunctional imine-pyrrolide/thiophene ligands

Dinuclear chromium(III) complexes [Cr(Z)-2-(HCN)-2-OCH₃C₆H₄)(THF)Cl₂]₂ (Z= C₄H₃N, L1-Cr₂) or [Cr(Z)-2-(HCN)-2-OCH₃C₆H₄)(THF)Cl₃]₂ (Z = C₄H₃S, L2-Cr₂; Z = 5-methyl-C₄H₂S, L3-Cr₂; Z = 5-phenyl-C₄H₂S, L4-Cr₂) were synthesized by reaction of the phenyl-bridged bifunctional imine-pyrrole/thiophene ligands with [CrCl₃(THF)₃]. DFT calculations for L2-Cr₂ revealed that both isomers (anti and syn) are stable presumably due to the nonrigid skeleton of this class of ligands. On activation with MAO, all chromium complexes showed catalytic activity in ethylene oligomerization, providing a non-selective distribution of α-olefins (C₄-C₁₂⁺) together with varying amounts of polymer (3.6 – 45.0 wt%). The nature of the pendant heterocyclic moieties plays a substantial influence on the catalytic performances of these binuclear Cr catalysts. L1-Cr₂ bearing pyrrolide unit yielded a significant quantity of polymer (45.0 wt%) while the binuclear chromium complexes based on thiophene unit showed higher productivities towards production of oligomers (up to 94.4 wt%). These latter Cr complexes showed higher productivities in comparison to their mononuclear analogues. DFT calculations, assuming the formation of cationic chromium (III) species after treatment with MAO in ethylene atmosphere, suggested the coordination of the thiophene group to the chromium metal center, and thus influencing the catalytic performance of L2Cr₂ – L4-Cr₂. By adjusting the reaction conditions, L3-Cr₂ showed TOF of 131,100 mol ethylene·mol Cr⁻¹·h⁻¹ producing predominantly oligomers (89.3 wt% of total products) with high selectivity for α-olefins (>91.5 wt%).