Fluorinated Sterically Bulky Mononuclear and Binuclear 2-Iminopyridylnickel Halides for Ethylene Polymerization: Effects of Ligand Frameworks and Remote Substituents.

In the present work, four new mono(imino)pyridine ligands, 2-((2,4-bis(bis(4-R-phenyl)methyl)-6-fluorophenylimino)methyl)pyridine (R = H, L1; R = OCH3, L2; R = F, L3) and 2-((2-(bis(4-fluorophenyl)methyl)-4-((3-(bis(4-fluorophenyl)methyl)-4-amine-5-fluoro-phenyl)(phenyl)methyl)-6-fluorophenylimino)methyl)pyridine (L4), have been designed in good yields. Additionally, three novel benzhydryl-bridged bis(imino)pyridine ligands, 2-(2-(bis(4-R-phenyl)methyl)-6-fluoro-phenylimino)pyridine (R = H, L5; R = OCH3, L6; R = F, L7), were also prepared for comparison. All these organic compounds have been characterized by FT-IR analysis, 1H/13C NMR spectroscopy, and elemental analysis. The treatment of L1-L7 with nickel halides afforded the corresponding monometallic (Ni1-Ni4) and bimetallic (Ni5-Ni7) nickel complexes in moderate to good overall yields. Upon activation with methylaluminoxane (MAO), Ni4 Cl showed the highest activity up to 8.3 × 106 g of polyethylene (PE) (mol of Ni)-1 h-1 among Ni1-Ni7 for ethylene polymerization. In all cases, unsaturated PEs with low molecular weights (0.7-13.3 kg mol-1) were produced effectively. The introduction of remote para-substituents into the benzhydryl groups showed a beneficial effect on catalytic activity with the overall activities following the order of Ni-F > Ni-OCH3 > Ni-H. In addition, these para-substituents were also found to affect not only the catalytic performance of catalysts but also the branching content of the PE product. Generally, the resultant PE waxes were moderately branched and contained both terminal vinyls (-CH=CH2) and internal vinylenes (-CH=CH-) while with different ratios of vinyls to vinylenes. Notably, the polymers produced using para-methoxy-substituted Ni2/MAO and Ni6/MAO possessed the least branching content and uniquely high vinyl contributions.