Highly Efficient Ethylene Tetramerization Using Cr CatalystsConstructed with Trifluoromethyl-SubstitutedN-Aryl PNP Ligands

Tetramerization of ethylene by chromium catalysts stabilized with functionalizedN-aryl phosphineamine ligandsC6H4(m-CF3)N(PPh2)2(1), C6H4(p-CF3)N(PPh2)2(2), C6H4(o-CF3)N???PPh2-PPh2(3), and C6H3(3,5-bis(CF3))N(PPh2)2(4)was evaluated. The parameter optimization includes temperature, co-catalyst, and solvent. Upon activation with MMAO-3A, the newcatalyst system especially withm-functional PNP ligand (1) exhibited high 1-octene selectivity and productivity while givingminimum undesirable polyethylene and C10+olefin by-products. Using PhCl as a solvent at 75 degrees C led to a remarkable alpha-olefin (1-C6+ 1-C8) selectivity (>90 wt %) at a reaction rate of 2000 kgmiddotgCr-1middoth-1. Under identical conditions, analogous PNP ligands bearing-CH3,-Et, and-Cl functional moieties at the meta position of theN-phenyl ring displayed significantly lower reactivity. Thecatalyst withp-functional ligand (2) exhibited lower activity and comparable selectivities, while the Cr/PPN (with ligand3) systemgave no noticeable reactivity. The molecular structure of the precatalyst (1-Cr), exhibiting a monomeric structural feature, waselucidated with the aid of single-crystal X-ray diffraction study.