Suppression of β-hydride chain transfer in nickel(II)-catalyzed ethylene polymerization via weak fluorocarbon ligand-product interactions

The synthesis and characterization of two neutrally charged Ni(II) ethylene polymerization catalysts, [2-tert-butyl-6-((2,6-(3,5-dimethylphenyl) phenylimino)methyl)phenolato]nickel(II) methyl trimethylphosphine ((CH ₃)FI-Ni) and [2-tert-butyl-6-((2,6-(3,5-bis(trifluoromethyl)phenyl) phenylimino)methyl)phenolato]nickel(II) methyl trimethylphosphine ((CF ₃)FI-Ni) are reported. In the presence of a Ni(COD) ₂ cocatalyst, these catalysts produce markedly different polyethylenes: densely branched oligomers with M _w = 1.4 × 10 ³ g mol ^-1 for (CH ₃)FI-Ni vs lightly branched polyethylenes with M _w = 92 × 10 ³ g mol ^-1 for (CF ₃)FI-Ni and with ∼6.5× the polymerization activity and with much greater performance thermal stability. HOESY 2D ¹⁹F, ¹H NMR spectra of a model Ni-ethyl compound, [2-tert-butyl-6-((2,6- (3,5-bis(trifluoromethyl)phenyl)phenylimino)methyl)phenolato]nickel(II) ethyl 2,4-lutidine ((CF ₃)FI-Ni-Et), indicate non-negligible C _β-H _β•••F ₃C through-space dipolar interactions, and molecular modeling reveals that C _β-H _β•••F(C) distances can be as small as ∼2.61 Å during the polymerization process. Furthermore, there is no structural or spectroscopic evidence for fluorocarbon inductive effects on the structure, bonding, and reactivity of these complexes, and a catalyst with CF ₃ introduced β to the imino N produces only low-M _w oligomers with low activity. These results argue that weak (ligand)C-F•••H-C(polymer) interactions can significantly influence the chain transfer characteristics of these catalysts.