Role of Electron-Withdrawing Remote Substituents in Neutral Nickel(II) Polymerization Catalysts

The novel neutral κ2-N,O-salicylaldiminato
Ni­(II) complex, [κ2-N,O-{2,6-(3′,5′-R2C6H3)2C6H3-NC­(H)-(3,5-I2-2-O-C6H2)}­NiCH3(pyridine)] (1a-pyr, R = NO2), with four nitro substituents
on the N-terphenyl motif is a catalyst precursor for ethylene polymerization
to yield linear higher molecular weight polyethylene (e.g., Mn 2.1 × 105 g mol–1 and only 2 methyl branches per 1000 carbon atoms). A comparison
with other known catalyst precursors at various polymerization conditions
shows that the catalytic properties in terms of linearity and molecular
weight are similar to the fluorinated catalyst precursor with R =
CF3, showing that the latter is not singular, but rather
suppression of chain transfer and branch formation by β-hydride
elimination can also be brought about by nonfluorinated electron-withdrawing
remote substituents.