Selective Ethylene Tri-/Tetramerization by in Situ-Formed Chromium Catalysts Stabilized by N,P-Based Ancillary Ligand Systems

A series of N,¿P-¿based ancillary ligands have been synthesized, and the corresponding catalysts, formed in situ by mixing one of the N,¿P-¿ligands, Cr-¿(acac)¿3 and MAO, have been tested for ethylene oligomerization. Under std. ethylene oligomerization conditions (30 bar ethylene, 60 °C, methylcyclohexane solvent)¿, all of the in situ-¿formed complexes show catalytic activity, producing oligomers together with varying amts. of polyethylene (PE)¿. Of all these combinations, only the catalyst formed by mixing N-¿pyrrolyldiphenylphosphine with Cr-¿(acac)¿3 and MAO is capable of selectively oligomerizing ethylene, producing a mixt. of 1-¿hexene and 1-¿octene in varying ratios alongside a small amt. of PE. Further investigations on this catalyst system revealed that the presence of a low concn. of toluene favors the prodn. of 1-¿octene. However, in pure toluene as the solvent, the selectivity toward 1-¿hexene¿/1-¿octene is lost and a statistic mixt. of a-¿olefins is produced. Moreover, the choice of the cocatalyst is found to dramatically influence the compn. of the liq. products. By careful adjustment of the reaction conditions (temp., ethylene pressure, catalyst loading, and ligand¿/Cr ratio)¿, the 1-¿hexene¿/1-¿octene molar ratio can be tuned from 0.3 to 20 and a selectivity for 1-¿octene formation of up to 74¿% can be achieved.