Tailored strength neighboring group interactions switch polymerization to dimerization catalysis

Abstract

A combined experimental and theoretical study elucidates how Ni∙∙∙O neighboring group interactions drastically switch catalytic properties towards ethylene. A range of salicylaldiminato complexes with aryloxy groups in 2,6-position of the N-phenyl group were 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 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. 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.