Chromium catalysts based on PNP(NR₂)₂ ligands for selective ethylene oligomerization

High active and selective 1–3 chromium catalysts based on the corresponding Ph₂PN(cyclopentyl)P (NR₂)₂-type L¹–L³ ligands (L¹: R = methyl, L²: R = ethyl, L³: R = isopropyl) have been explored for selective ethylene tri-/tetramerization. We found that the ligand substituents and the experimental conditions considerably influenced the catalytic performance of 1–3 based catalysts. Combining a higher ethylene pressure can elevate the selectivity of 1-octene (67.98%) of precatalyst 1 with high catalytic activity (9.3 × 10⁶ g/[molCr h]). Furthermore, precatalysts 2–3 efficiently offer selective ethylene trimerization and afforded 97.76% 1-hexene selectivity with 5.8 × 10⁶ g/(molCr h) catalytic activity (in case precatalyst 3). A relatively low catalyst mass, high Al/Cr molar ratio, and high ethylene pressure display excellent catalytic activity and selectivity for ethylene oligomerization.