Tailored Strength Neighboring Group Interactions Switch Polymerization to Dimerization Catalysis

A combined
experimental and theoretical study elucidates how Ni···O
neighboring group interactions drastically switch catalytic properties
toward ethylene. A range of salicylaldiminato complexes with aryloxy
groups in the 2,6-position of the N-phenyl group
was found to dimerize and oligomerize ethylene to butenes and branched
oligomers (C4, C6, C8, C10, ...) in pressure reactor experiments, while corresponding reference
catalysts with arylmethylene groups yield linear polyethylene with Mn of 100.000 g mol–1. While
both types of catalysts consume ethylene with similar high activities
(105 turnovers h–1), the rate of ß-hydride
elimination (BHE) is much increased for the case of aryloxy substitution.
Density functional theory (DFT) studies show that formation of the
relevant cis agostic complex from which BHE occurs by displacement
of ethylene from the cis-alkyl olefin complex is promoted by an Ni···O
interaction. This low energy pathway renders chain transfer competitive
with insertion chain growth. The resulting Ni···O intermediate
is rather stable and similar in energy to key species of catalysis
(ß-agostic and alkyl olefin complexes) but barely not yet an
energetic sink that would impede catalysis.