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 [CrCl 3 (thf) 3 ] results in coordination in a bidentate fashion to give dinuclear complexes [( 1 )CrCl 3 ] 2 and [( 2 )CrCl 3 ] 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 i PrN(PPh 2 ) 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 i PrN(PPh 2 ) 2 and that the oligomerisation mechanism involves both single and double ethylene insertions. Chromium catalysts with diazaphospholane ligands have shown good activities and selectivities for ethylene tri- and tetramerisation. Oligomerisations with a doubly N-bridged cyclodiphosphazane result in a Schulz-Flory distribution of α-olefins.