Nickel(II) complexes bearing pyrazolylimine ligands: Synthesis, characterization, and catalytic properties for ethylene oligomerization

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.

Graphical abstractThe steric bulk and electronic effect of phenyl-substituted pyrazolylimine ligands had a significant influence on the structure of their nickel complexes: ligands with more bulky and electron withdrawing substituents on N-phenyl ring produced four-coordinate nickel complexes, whereas the ligands with less bulky and electron donating substituents on N-phenyl ring formed bis-pyrazolylimine dinickel tetrahalides and six-coordinate bis-pyrazolylimine nickel dihalides. Activated by methylaluminoxane (MAO), the nickel complexes 1, 2, 5 and 6 showed moderate to high activity for ethylene oligomerization.Figure optionsDownload full-size imageDownload as PowerPoint slide