Effect of cocatalysts and solvent on selective ethylene oligomerization

Ethylene oligomerization activities of chromium catalysts stabilized by different dipyrrole-based ancillary ligands, [(Ph2C(C4H4N)2)] (2), [Ph2C(C4H4N)(C5H6N)] (3), [(Et)2C(C4H4N)2] (4), and [(C6H5)(C5H4N)C(C4H4N)(C5H6N)] (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, [(Ph2C(C4H3N)2)Cr(Cl)(THF)3] (6) and {[Ph2C(C4H3N)(C5H6N]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 MeAlCl2 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. (Figure Presented).