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 [NiCl 2 (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 [NiCl 2 (PCy 3 ) 2 ] were inactive for ethylene polymerization in the presence of methylaluminoxane (MAO), the mononuclear complex 24 gave a TOF of 7900 mol of C 2 H 4 /((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 NaBH 4, but 19, 21, and 24 were active with AlEtCl 2 as a cocatalyst. Activities and selectivities were compared to those of [NiCl 2 (PCy 3 ) 2 ], a typical precatalyst used in the dimerization of α-olefins. Complex 19 yielded a turnover frequency of 36 300 mol of C 2 H 4 /((mol of Ni) h) in the presence of 14 equiv of AlEtCl 2 . 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 C 2 H 4 /((mol of Ni) h), respectively, higher than that of 27 200 mol of C 2 H 4 /((mol of Ni) h) obtained with [NiCl 2 (PCy 3 ) 2 ].