Nickel
and palladium complexes bearing “sandwich”
diimine ligands with perfluorinated aryl caps have been synthesized,
characterized, and explored in ethylene polymerization reactions.
The X-ray crystallographic analysis of the precatalysts 16 and 6b shows differences from their nonfluorinated
analogues 17 and 19, with the perfluorinated
aryl caps centered precisely over the nickel and palladium centers,
which results in higher buried volumes of the metal centers relative
to the nonfluorinated analogues. The sandwich diimine-palladium complexes 5a and 5b containing perfluorinated aryl caps
polymerize ethylene in a controlled fashion with activities that are
substantially increased compared with their nonfluorinated analogues.
Migratory insertion rates in relevant methyl ethylene complexes agree
with the activities exhibited in bulk polymerization experiments.
DFT studies suggest that facility of ethylene rotation from its preferred
orientation perpendicular to the Pd-alkyl bond into a parallel in-plane
conformation contributes to the higher polymerization activity for 5b relative to 18a. For these palladium systems,
polymer molecular weights can be controlled via hydrogen addition
(hydrogenolysis), which is unusual for late-transition-metal-catalyzed
olefin polymerizations with no catalyst deactivation occurring. Sandwich
diimine-nickel complexes 6a and 6b with
perfluorinated aryl caps show ethylene polymerization activities that
are about half of those of classical tetraisopropyl-substituted catalyst 2 but again are more active than the analogous nonfluorinated
sandwich complexes. Ethylene polymerizations exhibit living behavior,
and branched ultrahigh-molecular-weight polyethylenes (UHMWPEs) with
very low-molecular-weight distributions (less than 1.1) are obtained.
The activated nickel catalysts are stable in the absence of monomer
and show good long-term stability at 25 °C.