Ethylene oligomerizations to low-carbon linear a-olefins by structure modulated phenoxy-imine nickel(II) complexes combined with aluminum sesquichloride Il Kim * , Chang Hun Kwak, Jae Sung Kim, Chang-Sik Ha Department of Polymer Science and Engineering, Pusan National University, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Republic of Korea Received 4 October 2004; received in revised form 9 March 2005; accepted 25 March 2005 Available online 26 April 2005 Abstract A series of ethylene oligomerization catalysts based on bidentate phenoxy-imine ligands coordinated to nickel is reported. The phenoxy- imine nickel(II) complexes of the general structure, [(2,6-R 2 C 6 H 3 N = C-3-R 0 C 6 H 3 O) 2 Ni] (R 0 = H, 1; tBu = 2; R = Me, a; Et, b; iPr, c), modifying their structural characteristics by changing the nature of the ligand substituents on the imino and aromatic moieties were prepared by using one shot synthesis. The formation of active species in the reaction of 1c complexes with ethylaluminum sesquichloride (EAS) was investigated UV–vis spectroscopy. At Al/Ni molar ratio >25, the formation of a cationic nickel species, active in the oligomerization of ethylene, was supposed to occur and this result was in agreement with the actual oligomerization results. In the presence of ethylene and EAS cocatalyst, all complexes are extremely active for the ethylene oligomerization, with activities greater than 5  10 8 g oligomer/mol Ni h atm, regardless of the catalyst structure. All catalysts yielded butenes as major products. The catalytic behavior of the homogeneous systems prepared in situ from 1c and different organoaluminum co-catalysts in the activation of ethylene was also investigated. In particular, the effect of reaction parameters, such as temperature, the Al/Ni molar ratio and the type of solvent, was studied. Changing the reaction parameters it was possible to modify the chemo- and regioselectivity of the catalytic process towards the formation of target olefin products, even if they were not decisive factors to control product distribution. # 2005 Elsevier B.V. All rights reserved. Keywords: Ethylene; Ethylaluminum sesquichloride; Homogeneous catalysis; Nickel; Oligomerization; Phenoxy-imine ligands 1. Introduction The oligomerization of ethylene is one of the major industrial processes for the production of linear a-olefins [1]. These oligomers in the range of C 6 –C 20 are used as comonomers in the polymerization of ethylene to give linear low-density polyethylene (LLDPE) or for the preparation of detergents and synthetic lubricants. The neutral Ni com- plexes are the most widely used as homogeneous catalysts for industrial ethylene oligomerization due to their high activity and capability to direct the reaction toward specific target products [2]. Catalysts currently used in industry for the shell higher olefin process (SHOP) contain Ni(II) complexes bearing bidentate mono-anionic ligands [3–7] and cationic Ni(II) a-diimine complexes were reported to be effective ethylene oligomerization catalysts [8]. The selectivity of the oligomerization process is determined by the control of the carbon–carbon bond formation and of the extension of the parallel double-bond migration isomerization. The control of these reactions is extremely attractive from both academic and technological viewpoints. High-performance Ni catalysts that selectively produce oligomers have been obtained via tailoring of the surrounding ligands [9,10]. The development of new ligands in order to obtain selective Ni catalysts is of great interest. As a means of the continuation of the work to develop new nickel catalyst system, we investigated a series of phenoxy-imine nickel(II) complexes which have been widely investigated since early 1960s [11,12] because of www.elsevier.com/locate/apcata Applied Catalysis A: General 287 (2005) 98–107 * Corresponding author. Tel.: +82 51 5102466; fax: +82 51 5137720. E-mail address: ilkim@pnu.edu (I. Kim). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.03.026