Chromium Catalysts Based on Tridentate Pyrazolyl Ligands for Ethylene Oligomerization

Abstract

A set of chromium(III) complexes CrCl3(L) based on tridentate ligands (1, L = bis[2-(3,5-dimethylpyrazolyl)ethyl)]amine; 2, L = bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl]benzylamine; 3, L = bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl] ether; 4, L = bis[2-(3-phenyl-l-pyrazolyl)ethyl] ether; 5, L = bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl] sulfide)) have been prepared and characterized by elemental analysis, ESI-HRMS, and an X-ray diffraction study for 3. Upon activation with methylaluminoxane (MAO), these precatalysts show high activity in ethylene oligomerization (TOF = (3.4−131.0) × 103 (mol ethylene) (mol Cr)-1 h-1 at 80 °C), producing α-olefins in the range C4−C14+ with high selectively. The catalytic performances are substantially affected by the ligand environment, especially the bridge donor atom, and reaction conditions. Under optimized conditions ([Cr] = 10 μmol, 80 °C, 40 bar ethylene, MAO-to-Cr = 300), precatalyst 5 leads to TOF = 131.0 × 103 h-1 and selectivity for α-olefins in the range 89−97%.