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.