Unveiling meta -Alkyloxy/-Silyloxy-Substituted N -Aryl PNP Ligands for Efficient Cr-Catalyzed Ethylene Tetramerization.

Novel N -aryl-functionalized PNP ligands ( 1-4 ) bearing m -alkyloxy/-silyloxy substituents were prepared and evaluated for chromium-catalyzed ethylene oligomerization using MMAO-3A as an activator. The selected Cr/PNP system under optimized condition exhibited high 1-octene-selective (up to 70 wt %) ethylene tetramerization at a remarkable rate (over 3000 kg gCr -1 h -1 ). More importantly, the undesirable polyethylene selectivity was restricted to a minimum level of ∼1-2 wt % for pre-catalysts derived with ligands 1 and 2 . Employing chlorobenzene as a reaction medium yielded best productivity in conjunction to the total α-olefin (1-C 6 + 1-C 8 ) selectivity (∼88 wt %). N -aryl PNP ligands ( 3 and 4 ) incorporating m -silyloxy substituents in the phenyl ring exhibited relatively poorer tetramerization performance while yielding higher PE fraction as compared to their m -alkyloxy derivatives. A detailed molecular structure of the best-performing pre-catalyst 1 -Cr was established by single-crystal X-ray diffraction analysis. The stability of 1 /Cr-based catalyst system was investigated for a reaction time of up to 2 h under optimized condition.