Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

Abstract

A new series of Cr (III) complexes [Cr{1‐(3‐phenoxypropyl)‐1H‐pyrazole}Cl3]2 (Cr1), [Cr{1‐(3‐phenoxypropyl)‐3,5‐dimethyl‐1H‐pyrazole}Cl3]2 (Cr2), and [Cr{1‐(3‐phenoxypropyl)‐3‐phenyl‐1H‐pyrazole}Cl3]2 (Cr3) have been synthesized and characterized by elemental analysis, high‐resolution mass spectrometry (HRMS) and IR spectroscopy. Upon activation with methylaluminoxane (MAO), chromium precatalysts Cr2 and Cr3 showed moderate activity in ethylene oligomerization [TOF = 17,900–29,200 mol (ethylene)·mol (Cr)−1·h−1 at 80 °C] with Schultz‐Flory distribution of oligomers (K = 0.54–0.66) and production of polymer varying from 2.8 to 6.7 wt.%. On the other hand, under identical oligomerization conditions, Cr1/MAO behaved as a polymerization catalyst generating predominantly polyethylene (63.7 wt%). The amount of 1‐butene is the largest component in the liquid fraction suggesting that these precatalysts operate via a Cossee‐Arlman mechanism. The catalytic activities, selectivity and product distribution are quite sensitive to the R‐group at the 3‐ and 5‐position of the pyrazolyl ring. Based on the electronic and steric effects of R‐ substituents, it is possible to stablish a trend of activity: Cr2(PzMe2) > Cr3(PzPh) > Cr1(Pz). Moreover, the effect of oligomerization parameters (cocatalyst, temperature, [Al]/[Cr] molar ratio, time) on the activity and on the product distribution were examined.