Substituted tridentate pyrazolyl ligands for chromium and nickel-catalyzed ethylene oligomerization reactions. Effect of auxiliary ligand on activity and selectivity

Abstract

Two new chromium(III) complexes [CrCl3(L)] based on tridentate ligands (1a, L = bis[2-(3-phenyl-1-pyrazolyl)ethyl)]amine; 2a, L = bis[2-(3-methyl-5-phenyl-l-pyrazolyl)ethyl]sulfide) have been prepared and characterized by elemental analysis. Upon activation with methylaluminoxane (MAO), these pre-catalysts showed high turnover frequencies for ethylene oligomerization under optimized conditions (TOFs = 22.9-36.4×10 3 mol C2H4 (mol CrIII)-1 h-1, [Cr] = 10.0 μmol, 80 °C, 20 bar ethylene, MAO:Cr = 300, oligomerization time = 20 min), producing α-olefins in the range C 4-C14+ with high selectively (67.71-73.47%). The catalytic performances are substantially affected by the ligand environment, especially the substituents at the 3- and 5-positions of the pyrazolyl rings. In parallel, the use of nickel complexes such as NiCl2{bis[2-(3,5-dimethyl-1- pyrazolyl)methyl]benzylamine} (3) and NiCl2{bis[2-(3,5-dimethyl-1- pyrazolyl)ethyl)]ether} (5) in oligomerization reactions carried out in the presence of triphenylphosphine (PPh3) afforded highly active catalytic systems with turnover frequencies (TOFs) varying from 36.4 to 154.2×103 mol C2H4 (mol Ni II)-1 h-1. The presence of this auxiliary ligand has a strong impact on the selectivity towards the production of α-olefins, decreasing substantially the amount of 1-butene with a concominat increase of the 2-butene fractions. Attempts to crystallize the nickel complex 3 afforded the tetrametallic [{(L)(μ3-Cl)NiCl} 4] (4, L = 1-anilinomethyl-3,5-dimethylpyrazole) which was characterized by X-ray diffraction analysis.