Phospholane-Based Ligands for Chromium-Catalyzed Ethylene Tri-and Tetramerization

Chromium complexes with bis(phospholane) ligands were synthesized and evaluated for ethylene tetramerization in a high-throughput reactor. Three ligand parameters-the phospholane substituent, the ligand backbone, and the type of id phosphine (cyclic vs acyclic)-were investigated. The size of the phospholane substituent was found to impact the selectivity of the resulting catalysts, with smaller substituents leading to the production of larger proportions of 1-octene. Changing the ligand backbone from 1,2-phenylene to ethylene did not impact catalysis, but the use of acyclic phosphines in place of the cyclic phospholanes had a detrimental effect on catalytic activity. Selected phospholane-chromium complexes were evaluated in a 300 mL Parr reactor at 70 degrees C and 700 psi of ethylene pressure, and the ethylene oligomerization performance was consistent with that observed in the smaller, high-throughput reactor. MeDuPhos-CrCl3(THF) (MeDuPhos = 1,2-bis(2,5-dimethylphospholano)- benzene; THE = tetrahydrofuran) gave activity and selectivity for 1-octene (54.8 wt %) similar to the state-of-the-art i-PrPNP- CrCl3(THF) (64.0 wt %) (PNP = bis(diphenylphosphino)amine), while EtDuPhos-CrCl3(THF) (EtDuPhos = 1,2-bis(2,5-diethylphospholano)benzene) exhibited even higher activity, with catalyst selectivity shifted toward 1-hexene production (90 wt %). These results are surprising, given the prevalence of the aryl phosphine motif in ligands used in ethylene oligomerization catalysts and the inferior performance of previously reported catalysts with alkyl phosphine-containing ligands.