Selective Dimerization/Oligomerization of α-Olefins by Cobalt Bis(imino)pyridine Catalysts Stabilized by Trifluoromethyl Substituents: Group 9 Metal Catalysts with Productivities Matching Those of Iron Systems

A series of bis(imino)pyridine ligands containing aryl substituents with o-trifluoromethyl
units, along with their cobalt and iron complexes, has been prepared and characterized.
The crystal structures of several complexes are reported. Both the cobalt and iron complexes,
when activated by methylaluminoxane cocatalysts, afford much higher olefin oligomerization/polymerization activities than their nonfluorinated relatives. Enhanced performance is seen
not only in higher peak activities but also in longer catalyst lifetimes, suggesting that the
trifluoromethyl group significantly improves catalyst stability. The most impressive activity
increase is observed for a cobalt catalyst combining an o-CF3 and an o-F unit. This system
is more active than its iron counterpart in ethylene polymerization, reaching >100 000 g
mmol-1 h-1 bar-1. Propene, 1-butene, and 1-hexene are oligomerized by this catalyst at rates
much higher than for nonfluorinated relatives. Highly linear dimers predominate in each
case, the remainder being trimeric and tetrameric products. The principal product of propene
dimerization is 1-hexene (60−73% of total), whereas for 1-butene and 1-hexene internal
olefins are obtained (E isomers predominate). No isomerization of 1-hexene is seen under
the reaction conditions. The catalysts operate by a mechanism involving (1,2)- followed by
(2,1)-insertion steps. Combinations of chain growth and step growth account for the formation
of linear trimers and tetramers of propene.