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

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journal contribution
posted on 24.05.2004, 00:00 by Fredy Speiser, Pierre Braunstein, Lucien Saussine, Richard Welter
The phosphinooxazoline ligands 912 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 1924 was determined in solution by the Evans method to be trigonal bipyramidal for 1923 and tetrahedral for 24. Whereas complexes 1923 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].