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)phenolatonickel(II) methyl trimethylphosphine ((CH3)FI-Ni) and 2-tert-butyl-6-((2,6-(3,5-bis(trifluoromethyl)phenyl)phenylimino)methyl)phenolatonickel(II) methyl trimethylphosphine ((CF3)FI-Ni) are reported. In the presence of a Ni(COD)2 cocatalyst, these catalysts produce markedly different polyethylenes: densely branched oligomers with Mw = 1.4 × 103 g mol–1 for (CH3)FI-Ni vs lightly branched polyethylenes with Mw = 92 × 103 g mol–1 for (CF3)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)phenolatonickel(II) ethyl 2,4-lutidine ((CF3)FI-Ni-Et), indicate non-negligible Cβ–Hβ···F3C through-space dipolar interactions, and molecular modeling reveals that Cβ–Hβ···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 CF3 introduced β to the imino N produces only low-Mw 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.