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

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 17000 g g(-1) (Cr) h(-1) in the case of 2, comparable to standard (PrN)-Pr-i(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 alpha-olefins with relatively low a 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 (PrN)-Pr-i(PPh2)(2) and that the oligomerisation mechanism involves both single and double ethylene insertions.