Ethylene oligomerization activities of chromium catalysts stabilized by different dipyrrole-based ancillary ligands, [(Ph 2 C(C 4 H 4 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.
The synthesis, structural characterization, and ethylene polymerization performance of heterobimetallic aluminum-pyrrolyl complexes of group IV metals are described. The combination of MCl 4 (M = Ti, Zr, Hf), aluminum alkyls and pyrroles leads, depending on stoichiometry, to mono-and bis(aluminum-pyrrolyl) complexes that are remiscent of the corresponding mono-and bis-cyclopentadienyl systems. The bis(aluminum-pyrrolyl) complexes (η 5 -2,5-2 (7), obtained by treatment of TiCl 4 with equimolar amounts of trimethyl aluminum and the corresponding pyrrole ligands, were found to be moderately active for ethylene polymerization with MAO or [Ph 3 C] + [B(C 6 F 5 ) 4 ] − , in all cases producing UHMWPE. The NMR scale reaction of 5 with B(C 6 F 5 ) 3 showed the formation of a solvent-separated ion pair, formed by the abstraction by B(C 6 F 5 ) 3 of a methyl group from Al-CH 3 rather than from Ti-CH 3 . 1 H and 13 C NMR analysis of 6 and 7 revealed that several stable structural isomers exist in solution, with a slow interconversion on the NMR time scale. The dimeric zirconium complexes [(η 5 ,κ 1 -2,5-Me 2 C 4 H 2 NAlClMe 2 )ZrMeCl(μ-Cl)] 2 (8) and [(η 5 ,κ 1 -2,5-Me 2 C 4 H 2 NAlCl 2 Et)ZrCl 2 (μ-Cl)] 2 (9), prepared by the reaction of 2 equivalents of ZrCl 4 , 1 equivalent of 2,5-dimethylpyrrole, and 1 equivalent of aluminum alkyl, showed structures containing a bridging chloride between aluminum and zirconium, thereby giving structural evidence for the lack of catalytic behavior of these complexes. A possible explanation for the moderate or absence of catalytic activity of the aluminum-pyrrolyl complexes was proposed based on DFT calculations.
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