Nickel Complexes with Oxazoline-Based P,N-Chelate Ligands:  Synthesis, Structures, and Catalytic Ethylene Oligomerization Behavior^†

The phosphinooxazoline ligands 9−12 have been prepared and used to examine and compare the catalytic properties of their corresponding Ni(II) complexes in ethylene oligomerization. The molecular structure of rac-2-[1‘-(diphenylphosphanyl)ethyl]-4,4-dimethyl-4,5-dihydrooxazole (10) has been determined by X-ray diffraction. The paramagnetic Ni(II) complexes [NiX(μ-X)(P,N)]₂ (19, X = Br, P,N = 2-[(diphenylphosphanyl)methyl]-4,4-dimethyl-4,5-dihydrooxazole (9)); 20, X = Cl, P,N = 9; 21, X = Cl, P,N = 10); 22, X = Cl; P,N = 2-[(diphenylphosphanyl)methyl]-4-(R)-phenyl-4,5-dihydrooxazole (12)); 23, X = Br, P,N = 12) and [NiCl₂(P,N)] (24; P,N = 2-[1‘-(diphenylphosphanyl)-1‘-methylethyl]-4,4-dimethyl-4,5-dihydrooxazole (11)) were prepared in yields of 79−90%, and 19 and 24 have been characterized by X-ray diffraction. The former is a dinuclear complex with distorted-trigonal-bipyramidal geometry around the Ni(II) centers, whereas 24 is a mononuclear, tetrahedral complex, which underlines the strong influence of a substituent on the carbon atom α to P. The metal coordination sphere in complexes 19−24 was determined in solution by the Evans method to be trigonal bipyramidal for 19−23 and tetrahedral for 24. Whereas complexes 19−23 and [NiCl₂(PCy₃)₂] were inactive for ethylene polymerization in the presence of methylaluminoxane (MAO), the mononuclear complex 24 gave a TOF of 7900 mol of C₂H₄/((mol of Ni) h) with a selectivity for 1-butene of 38%. Complexes 19, 20, and 22 were inactive for the oligomerization of ethylene in the presence of NaBH₄, but 19, 21, and 24 were active with AlEtCl₂ as a cocatalyst. Activities and selectivities were compared to those of [NiCl₂(PCy₃)₂], a typical precatalyst used in the dimerization of α-olefins. Complex 19 yielded a turnover frequency of 36 300 mol of C₂H₄/((mol of Ni) h) in the presence of 14 equiv of AlEtCl₂. In the presence of only 6 equiv of cocatalyst, the Ni complexes 21 and 24 showed TOF values of 38 100 and 45 900 mol of C₂H₄/((mol of Ni) h), respectively, higher than that of 27 200 mol of C₂H₄/((mol of Ni) h) obtained with [NiCl₂(PCy₃)₂].