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/-silyloxysubstituents were prepared and evaluated for chromium-catalyzed ethyleneoligomerization using MMAO-3A as an activator. The selected Cr/PNPsystem under optimized condition exhibited high 1-octene-selective(up to 70 wt %) ethylene tetramerization at a remarkable rate (over3000 kg gCr(-1) h(-1)). More importantly,the undesirable polyethylene selectivity was restricted to a minimumlevel of & SIM;1-2 wt % for pre-catalysts derived with ligands 1 and 2. Employing chlorobenzene as a reactionmedium yielded best productivity in conjunction to the total & alpha;-olefin(1-C-6 + 1-C-8) selectivity (& SIM;88 wt %). N-aryl PNP ligands (3 and 4) incorporating m-silyloxy substituents in the phenyl ring exhibited relativelypoorer tetramerization performance while yielding higher PE fractionas compared to their m-alkyloxy derivatives. A detailedmolecular 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 investigatedfor a reaction time of up to 2 h under optimized condition.