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), Ni4Cl 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.