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Chromium Complexes Supported by Phenyl Ether-Pyrazolyl [N,O] Ligands as Catalysts for the Oligo- and Polymerization of Ethylene
Authors:Jorge L.S. Milani  Osvaldo L. Casagrande Jr
Affiliation:1. Laboratory of Molecular Catalysis, Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre, RS, 91501-970 Brazil

Contribution: Conceptualization, Formal analysis, Methodology;2. Laboratory of Molecular Catalysis, Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre, RS, 91501-970 Brazil

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.
Keywords:bidentate ligands  chromium complexes  ethylene oligomerization  phenyl ether-pyrazolyl ligands  α-Olefins
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