Phosphanyl Methanimine (PCN) Ligands for the Selective Trimerization/Tetramerization of Ethylene with Chromium

1-Phosphanyl methanimine ligands,
R2P–C­(Ph)NR′,
may be prepared via a modular, convergent synthesis in good overall
yield. Coordination chemistry studies with chromium confirm that they
bind in a bidentate κ2-P,N mode. In combination with
chromium and an activator, these ligands promote the selective oligomerization
of ethylene to 1-hexene and 1-octene with high activity. Notably,
an exceptionally high liquid fraction selectivity to saleable products
(1-hexene + 1-octene) is observed (>95%), along with a potent ligand
dependence, such that selectivity can be varied from pure trimerization
through to mixed trimerisation/tetramerisation with a 1-octene:1-hexene
(OTH) ratio of 1.7.