Nickel complexes supported by selenium-based tridentate ligands and their use as effective catalyst systems for ethylene dimerisation

Abstract

A series of nickel(II) bromide complexes, [NiBr2(SeˆOˆN)] (Ni1–Ni4), supported by selenium-based tridentate ligands were prepared in good yields by reacting arylselenyl-ether-pyrazolyl ligands (L1–L4) with NiBr2(DME) (DME = 1,2-dimethoxyethane). All complexes were characterised by infrared spectroscopy, elemental analysis and high-resolution mass spectrometry. The crystallisation of Ni2 in THF/Et2O leads to the formation of a trimeric nickel complex, Ni2a·THF. X-ray crystallographic analyses of Ni2a·THF confirm the tridentate character of the SeˆOˆN ligand. Upon activation with methylaluminoxane (MAO), Ni1–Ni4 complexes showed moderate to good activities for ethylene oligomerisation (TOF = 4.3–25.7 × 103 (mol ethylene) (mol Ni)−1 h−1), giving predominantly 1-butene (89.2–94.3 wt.%) as the main product. The ligand environment, i.e., the substituents on the pyrazolyl unit, as well as the reaction parameters, influence the catalytic activity towards the production of 1-butene. Under optimised reaction conditions (toluene, 500 equiv. of MAO, 30 °C and 20 min), the Ni4/MAO catalytic system achieved TOF values up to 47.2 × 103 (mol ethylene) (mol Ni)−1 h−1 with good selectivity for α-C4 (83.8 wt.%).