Nickel(II) complexes bearing pyrazolylpyridines: synthesis, structures and ethylene oligomerization reactions George S. Nyamato, Mohd. Gulfam Alam, Stephen O. Ojwach* and Matthew P. Akerman Reactions of 2-bromo-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (L1) and 2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (L2) with NiCl 2 and NiBr 2 led to the formation of their respective metal complexes [NiCl 2 (L1)] (1), [NiBr 2 (L1)] (2) and [NiBr 2 (L2)] (3) in moderate to high yields. The complexes were characterized using elemental analyses, mass spectrometry and single- crystal X-ray diffraction for 2. The solid-state structure of 2 confirmed the bidentate coordination mode of L1 and formation of a monometallic compound. Activation of the nickel(II) pre-catalysts with methylaluminoxane afforded active catalysts in the ethylene oligomerization reaction to produce mainly butenes (84–86%). In contrast, activation of nickel(II) pre-catalyst 2 with ethylaluminium dichloride resulted in partial Friedel–Crafts alkylation of the toluene solvent by the preformed oligo- mers. Complex structure, nature of co-catalyst employed, type of solvent and reaction conditions influenced the catalytic be- haviour of these pre-catalysts. Copyright © 2015 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: nickel(II) complex; ethylene; oligomerization; co-catalyst; solvent Introduction Ethylene oligomerization towards the production of linear α-olefins has witnessed significant growth over past decades [1] due to the applications of oligomer products in the synthesis of plasticizers, lubricants and detergents, and as co-monomers in the synthesis of high-density polyethylene and linear low-density polyethylene. [2] The global consumption of co-monomer grade linear α-olefins in the range C 4 –C 8 is the largest and fastest-growing application. [1] These linear α-olefins are mainly produced in three full-range pro- cesses by Ethyl Corporation (INEOS), Gulf (Chevron Philips Chemical Company) and the nickel-based Shell higher olefin process (SHOP). [3] However, as a consequence of growing market demand for linear α-olefins in the range C 4 –C 10 rather than in the C 12 + range, [4] inten- sive efforts have been devoted towards finding more selective routes that can direct product distribution towards shorter α-olefins. [1] Since the discovery of α-diimine nickel(II) complexes as highly ef- ficient pre-catalysts for ethylene polymerization, [5] nitrogen-donor nickel(II) complexes have continued to attract much interest as eth- ylene oligomerization catalysts. Much effort has particularly been devoted to the modification of known ligands or to the design and development of new ligand systems with a view to enhancing both activity and selectivity. [6] We have previously reported the use of pyrazolyl late transition metal catalysts in ethylene oligomeriza- tion and polymerization reactions. [7–11] The results from those studies, which are also consistent with other recent reports, [12–15] indicate that catalytic properties greatly depend on reaction condi- tions, aluminium co-catalysts and ligand architecture. For example, (pyrazol-1-ylmethyl)pyridine nickel complexes I and II (Scheme 1) catalyse ethylene oligomerization to C 4 ,C 6 and C 8 alkenes upon activation with ethylaluminium dichloride (EtAlCl 2 ), followed by alkylation of the toluene solvent used by the pre-formed oligomers. [9] Modification of complexes of types I and II by introducing a chloromethyl group in the pyridine ring, to give com- plexes III, results in the production of mainly C 4 and C 6 oligomers upon activation with EtAlCl 2 . Even though these reactions also lead to Friedel–Crafts alkylation, the absence of any C 8 oligomer fractions demonstrates the pivotal role played by ligand design in the development of ethylene oligomerization catalysts. [16] Herein, we report the syntheses and characterization of some nickel(II) complexes bearing bidentate and tridentate (pyrazolyl)pyridine ligands (types IV and V) as well as their cata- lytic performance in ethylene oligomerization. The influences of catalyst structure, co-catalyst/complex ratio, time and pressure on the ethylene oligomerization reactions have been studied and are herein discussed. Experimental Materials and general considerations All synthetic manipulations were performed under a nitrogen at- mosphere using standard Schlenk techniques. All solvents were of * Correspondence to: Stephen O. Ojwach, School of Chemistry and Physics, University of KwaZulu-Natal, Scottsville, South Africa. E-mail: ojwach@ukzn.ac.za School of Chemistry and Physics, University of KwaZulu-Natal, Scottsville, South Africa Appl. Organometal. Chem. 2016, 30, 89–94 Copyright © 2015 John Wiley & Sons, Ltd. Full paper Received: 8 September 2015 Revised: 2 October 2015 Accepted: 6 October 2015 Published online in Wiley Online Library: 16 November 2015 (wileyonlinelibrary.com) DOI 10.1002/aoc.3402 89