Nickel(II) Heterocyclic Carbene Complexes as Catalysts for Olefin Dimerization in an Imidazolium Chloroaluminate Ionic Liquid

Nickel(II) heterocyclic carbene complexes of the formula NiI2(carbene)2 (carbene =
3-methyl-1-propylimidazolin-2-ylidene (1), 1-butyl-3-methylimidazolin-2-ylidene (2), 1-isopropyl-3-methylimidazolin-2-ylidene (3), 1,3-diisopropylimidazolin-2-ylidene (4)) have been
prepared and fully characterized by both spectroscopic methods and single-crystal X-ray
structure analysis. All complexes have “square-planar” nickel environments, with the donor
atoms pairwise trans, all obligate, except for one of the two independent molecular types
found in 2, in which, with different substituents at the nitrogen atoms, the two rotamers
are observed as independent molecules in the one crystal. Complexes 1−4 were tested as
catalysts for propene and 1-butene dimerization in both toluene and an imidazolium−chloroaluminate ionic liquid. While the complexes are inactive to sparingly active in toluene,
highly active catalysts are produced in the ionic liquid. In all cases the complexes are more
active toward 1-butene dimerization than is NiCl2(PCy3)2 in the ionic liquid. The results are
interpreted in terms of catalyst stabilization by the imidazolium-based ionic liquid. In toluene
the complexes decompose rapidly via reductive elimination of imidazolium cations, while in
the ionic liquid no decomposition is observed. While reductive elimination of imidazolium is
possible in the ionic liquid, a rapid reoxidation via addition of the solvent imidazolium cation
seems possible and may prevent the formation of Ni0 deposits associated with catalyst
deactivation.