Highly Efficient Ethylene Tetramerization Using Cr Catalysts Constructed with Trifluoromethyl-Substituted N -Aryl PNP Ligands.

Tetramerization of ethylene by chromium catalysts stabilized with functionalized N -aryl phosphineamine ligands C 6 H 4 ( m -CF 3 )N(PPh 2 ) 2 ( 1 ), C 6 H 4 ( p -CF 3 )N(PPh 2 ) 2 ( 2 ), C 6 H 4 ( o -CF 3 )N=PPh 2 -PPh 2 ( 3 ), and C 6 H 3 (3,5-bis(CF 3 ))N(PPh 2 ) 2 ( 4 ) was evaluated. The parameter optimization includes temperature, co-catalyst, and solvent. Upon activation with MMAO-3A, the new catalyst system especially with m -functional PNP ligand ( 1 ) exhibited high 1-octene selectivity and productivity while giving minimum undesirable polyethylene and C 10 + olefin by-products. Using PhCl as a solvent at 75 °C led to a remarkable α-olefin (1-C 6 + 1-C 8 ) selectivity (>90 wt %) at a reaction rate of 2000 kg·g Cr -1 ·h -1 . Under identical conditions, analogous PNP ligands bearing -CH 3 , -Et, and -Cl functional moieties at the meta position of the N -phenyl ring displayed significantly lower reactivity. The catalyst with p -functional ligand ( 2 ) exhibited lower activity and comparable selectivities, while the Cr/PPN (with ligand 3 ) system gave no noticeable reactivity. The molecular structure of the precatalyst ( 1 -Cr), exhibiting a monomeric structural feature, was elucidated with the aid of single-crystal X-ray diffraction study.