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 ((CH3)­FI-Ni) and [2-tert-butyl-6-((2,6-(3,5-bis­(trifluoromethyl)­phenyl)­phenylimino)­methyl)­phenolato]­nickel­(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)­phenolato]­nickel­(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.