Effect of Cocatalysts and Solvent on Selective Ethylene Oligomerization

Ethylene oligomerization activities of chromium catalysts stabilized by different dipyrrole-based ancillary ligands, [(Ph 2 C(C 4 H 4 N) 2 )] ( 2 ), [Ph 2 C(C 4 H 4 N)(C 5 H 6 N)] ( 3 ), [(Et) 2 C(C 4 H 4 N) 2 ] ( 4 ), and [(C 6 H 5 )(C 5 H 4 N)C(C 4 H 4 N)(C 5 H 6 N)] ( 5 ), have been investigated using different activation methods, and the results have been compared with the commercial Chevron–Phillips ethylene trimerization system. Upon activation with triethylaluminum (TEA), chromium catalysts stabilized by dipyrrole-based ligands 2 – 5 showed a lower activity and selectivity compared to the Chevron–Phillips trimerization system based on 2,5-dimethylpyrrole ( 1 ) as the ancillary ligand. However, unprecedented increases in both activity and selectivity have been observed by carrying out the oligomerization in methylcyclohexane using depleted-methylaluminoxane (DMAO) along with triisobutylaluminum (TIBA) (1:2 ratio) as cocatalyst system under mild conditions, even for the Chevron–Phillips system itself. Well-defined chromium complexes, [(Ph 2 C(C 4 H 3 N) 2 )Cr(Cl)(THF) 3 ] ( 6 ) and {[Ph 2 C(C 4 H 3 N)(C 5 H 6 N]Cr(THF)(μ-Cl)} 2 ( 7 ), have been synthesized and fully characterized. Upon activating with MAO, catalyst 7 produced a statistical distribution of oligomers, whereas under identical oligomerization conditions catalyst 6/ MAO was found to be inactive. The use of MeAlCl 2 as cocatalyst to activate 7 resulted in the switching of the catalyst’s behavior from producing a statistical distribution of LAOs to the selective trimerization of ethylene to 1-hexene. The addition of dialkylzinc along with MAO resulted in an unprecedented activity increase.