Single- and double-bridged PNP ligands in chromium-catalysed ethylene oligomerisation

Abstract

Several PNP-type diphosphine ligands have been synthesised and characterised, featuring a single or a double N-bridge between the P-donor atoms. PNP ligands 1 and 2 containing diazaphospholane donors have been prepared and reaction with [CrCl₃(thf)₃] results in coordination in a bidentate fashion to give dinuclear complexes [(1)CrCl₃]₂ and [(2)CrCl₃]₂ which have been characterised by scXRD analysis. In situ prepared catalysts using ligands 1 and 2 provide good activities and selectivities for the tri- and tetramerisation of ethylene reaching 35% 1-hexene and 61% 1-octene at 5400 g g⁻¹ per Cr per h in the case of 1, and 42% 1-hexene and 55% 1-octene at 17 000 g g⁻¹ (Cr) h⁻¹ in the case of 2, comparable to standard ⁱPrN(PPh₂)₂-type ligands under similar conditions. Chromium-catalysed ethylene oligomerisations with a doubly N-bridged cyclodiphosphazane ligand (4) result in a Schulz–Flory distribution of α-olefins with relatively low α values of 0.42 and 0.52. Computational studies using DFT on mononuclear chromium complexes of ligands 1 and 2 have shown that the binding of ethylene is favoured in these complexes compared to the benchmark PNP ligand ⁱPrN(PPh₂)₂ and that the oligomerisation mechanism involves both single and double ethylene insertions.