A rational approach towards selective ethylene oligomerization via PNP-ligand design with an N-triptycene functionality

Abstract

Novel PNP ligands bearing an N-triptycene backbone were developed and evaluated for selective ethylene oligomerization. Upon activation with MMAO-3A, the pre-catalyst mixture containing Cr(acac)₃/ligand efficiently promotes ethylene tetramerization with remarkably high productivities (up to 1733 kg g_Cr⁻¹ h⁻¹) and C₈ olefin selectivities (up to 74.1 wt%). More importantly, ligands with a PNP moiety connecting at the 1- or 1,4-position of the triptycene molecule could achieve exceptionally high alpha (1-C₆ + 1-C₈) selectivities, exceeding 90 wt%, as a result of high 1-C₆ purity (>90 wt%) in the C₆ fraction. Based on comparative catalytic studies employing various PNP ligands with or without an N-triptycene backbone, we illustrate the fact that a rational design of PNP ligands with an optimum degree of steric profile around the N-center could provide C₆ cyclics controlled highly α-selective ethylene oligomerization.