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

Tetramerization of ethylene by chromium catalysts stabilized with functionalized N-aryl phosphineamine ligands C₆H₄(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 new catalyst system especially with m-functional PNP ligand (1) exhibited high 1-octene selectivity and productivity while giving minimum undesirable polyethylene and C₁₀ ⁺ olefin by-products. Using PhCl as a solvent at 75 °C led to a remarkable α-olefin (1-C₆ + 1-C₈) selectivity (>90 wt %) at a reaction rate of 2000 kg·g_Cr ^-1·h^-1. Under identical conditions, analogous PNP ligands bearing -CH₃, -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.