Nickel(II) complexes bearing pyrazolylimine ligands: Synthesis, characterization, and catalytic properties for ethylene oligomerization |
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Authors: | Yuan-yuan Wang Hai-yang Gao Qing Wu |
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Affiliation: | Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China |
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Abstract: | Four phenyl-substituted pyrazolylimine ligands 2-(C3HN2Me2-3,5)(C(Ph) N(4-R2C6H2(R1)2-2,6)) (L1: R1 = iPr, R2 = H; L2: R1 = H, R2 = NO2; L3: R1 = R2 = H; L4: R1 = H, R2 = OCH3) were synthesized. The influences of steric bulk and electronic effect of pyrazolylimine ligands on the structures of their corresponding nickel complexes were investigated. Ligands with more bulky and electron withdrawing substituents on N-phenyl ring produced four-coordinate nickel complexes (2-(C3HN2Me2-3,5))(C(Ph) (4-R2C6H2(R1)2-2,6)NiBr2 (1, R1 = iPr, R2 = H; 2, R1 = H, R2 = NO2)), whereas the ligands with less bulky and electron donating substituents on N-phenyl ring formed bis-pyrazolylimine dinickel tetrahalides (bis-2-(C3HN2Me2-3,5))(C(Ph) N(4-R2C6H2 (R1)2-2,6)Ni2Br4 (3, R1 = R2 = H; 4, R1 = H, R2 = OCH3)) and six-coordinate nickel dihalides (bis-2-(C3HN2Me2-3,5))(C(Ph) N(4-R2C6H2(R1)2-2,6) NiBr2 (5, R1 = R2 = H;6, R1 = H, R2 = OCH3)). The solid-state structures of complexes 1, 4 and 5 have been confirmed by X-ray single-crystal analyses. Activated by methylaluminoxane (MAO), complexes 1, 2, 5 and 6 showed moderate to high activity for ethylene oligomerization, and complex 5 revealed the highest activity up to 8.96 × 105 g oligomer/(mol Ni · h). The proportions of resultant oligomers were mainly C4-C8 and a little C10-C14 determined by gas chromatography/mass spectrometry. |
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Keywords: | Pyrazolylimine nickel complexes X-ray single-crystal analyses Ethylene oligomerization Methylaluminoxane |
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