Ethylene oligomerizations to low-carbon linear α-olefins by structure modulated phenoxy-imine nickel(II) complexes combined with aluminum sesquichloride

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-R2C6H3N = C-3-R′C6H3O)2Ni] (R′ = 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 × 108 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.