Highly-efficient ethylene tetramerization chromium(III) catalysts based on N-substituted diphosphinoamine (PNP) ligands

Abstract

The Cr(III) precatalysts based on novel N-substituted bis(diphenylphosphine)amine ligands (PNP-type L1-L8) have been confirmed to be successfully synthesized and used to test ethylene tri-/tetramerization properties. Precatalyst 1 with a sec-butyl moiety exhibites remarkable activity of 2257 kg∙g⁻¹∙h⁻¹ with 61.3 % of 1-C₈⁼. Compared with precatalyst 1, precatalyst 2, with a larger steric hindrance of a sec-butylphenyl moiety at a N-atom, leading to a significant decrease in catalytic activity to 339 kg∙g⁻¹∙h⁻¹. Nevertheless, the selectivity for 1-C₈⁼ increases to 67.9 %. It seems that increasing the spatial volume of the N-alkyl substituents make it more selective for 1-C₈⁼ and lower catalytic activity. Single-crystal analysis shows that complexes 9 and 10 make from ligands 1 and 2 both adopt bidentate (k²-P, P) coordination mode. Simultaneously, the theoretical calculations also suggest that, for precatalyst 1, Path 1 toward tetramerization is thermodynamically more favorable than the other two pathways.