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

A series of nickel(II) bromide complexes, [NiBr2(Se<^>O<^>N)] (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 center dot THF. X-ray crystallographic analyses of Ni2a center dot THF confirm the tridentate character of the Se<^>O<^>N ligand. Upon activation with methylaluminoxane (MAO), Ni1-Ni4 complexes showed moderate to good activities for ethylene oligomerisation (TOF = 4.3-25.7 x 10(3) (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 degrees C and 20 min), the Ni-4/MAO catalytic system achieved TOF values up to 47.2 x 10(3) (mol ethylene) (mol Ni)(-1) h(-1) with good selectivity for alpha-C-4 (83.8 wt.%). (C) 2017 Elsevier B.V. All rights reserved.