β-Diimine nickel complexes in BMI·AlCl4 ionic liquid: a catalytic biphasic system for propylene oligomerization

Abstract

β-Diimine nickel complexes have been synthesized using phenyl-amino-based ligands (NPh) substituted with none, two or three methyl groups. The resulting complexes, NiNPh, NiNPhMe2 and NiNPhMe3, which associated with ethylaluminum sesquichloride (EASC) as a co-catalyst, were tested for propylene oligomerization conducted under homogeneous and biphasic conditions using cyclohexane and the ionic liquid 1-methyl-3-butylimidazolium tetrachloroaluminate (BMI·AlCl4), respectively. The cocatalyst/nickel complex ratio was evaluated (Al/Ni=100 and 200) for both systems. Under homogeneous conditions, the highest activity was obtained with NiNPh and Al/Ni=100 (9s−1), and the highest selectivity in propylene dimers was attained with NiNPhMe3 (91%). Under biphasic conditions, all the catalysts showed activities similar or slightly superior to those obtained under homogeneous conditions; the highest activity was attained with NiNPh (10.8s−1), and the highest selectivity for propylene dimers was attained with NiNPhMe3 (98.3%). Under both reaction conditions, i.e., homogeneous and biphasic, 2-methyl-2-pentene was the major product. However, when reactions were conducted under biphasic conditions, less products were formed by isomerization in comparison with those formed using homogeneous systems. For the recycling reactions, no significant changes in the activity of the system were observed for each new cycle.