Nickel Complexes with New Bidentate P,N Phosphinitooxazoline and -Pyridine Ligands:  Application for the Catalytic Oligomerization of Ethylene^†

The phosphinitooxazoline 4,4-dimethyl-2-[1-oxy(diphenylphosphine)-1-methylethyl]-4,5-dihydrooxazole (9), the corresponding phosphinitopyridine ligands 2-ethyl-[1‘-methyl-1‘-oxy(diphenylphosphino)]pyridine (11) and 2-ethyl-6-methyl-[1‘-methyl-1‘-oxy(diphenylphosphino)]pyridine (12), which have a one-carbon spacer between the phosphinite oxygen and the heterocycle, and the homologous ligand 2-propyl-[2‘-methyl-2‘-oxy(diphenylphosphino)]pyridine (13), with a two-carbon spacer, were prepared in good yields. The corresponding mononuclear [NiCl₂(P,N)] complexes 14 (P,N = 9), 15 (P,N = 11), and 16 (P,N = 12) and the dinuclear [NiCl(μ-Cl)(P,N)]₂ 17 (P,N = 13) Ni(II) complex were evaluated in the catalytic oligomerization of ethylene. These four complexes were characterized by single-crystal X-ray diffraction in the solid state and in solution with the help of the Evans method, which indicated differences between the coordination spheres in the solution and the solid state. In the presence of methylalumoxane (MAO) or AlEt₃, only the decomposition of the Ni complexes was observed. However, complexes 14−17 provided activities up to 50 000 mol C₂H₄/(mol Ni)·h (16 and 17) in the presence of only 6 equiv of AlEtCl₂. The observed selectivities for ethylene dimers were higher than 91% (for 14 or 15 in the presence of only 1.3 equiv of AlEtCl₂). The activities for 14−17 were superior to that of [NiCl₂(PCy₃)₂], a typical dimerization catalyst taken as a reference. The selectivities of the complexes 14−17 for ethylene dimers and α-olefins were the same order of magnitude. From the study of the phosphinite 9/AlEtCl₂ system, we concluded that in our case ligand transfer from the nickel atom to the aluminum cocatalyst is unlikely to represent an activation mechanism.