Abstract
The synthesis, structural characterization, and ethylene polymerization performance of heterobimetallic aluminum-pyrrolyl complexes of group IV metals are described. The combination of MCl4 (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 (η5-2,5-Me2C4H2NAlClMe2)2TiMe2 (1), zirconium (η5-2,5-Me2C4H2NAlClMe2)2ZrClMe (2), (η5-2,5-Me2C4H2NAlCl2Et)2ZrCl2 (3), and hafnium (η5-2,5-Me2C4H2NAlClMe2)2HfClMe (4) were found to be inactive toward ethylene polymerization. By contrast, the electron-deficient mono(aluminum-pyrrolyl) piano stool complexes (η5-2,5-Me2C4H2NAlCl2Me)TiCl2Me (5), (η5-2,3-Me2C8H4NAlCl2Me)TiCl2Me (6), and (η5-3,4,5,6-C12H12NAlCl2Me)TiClMe2 (7), obtained by treatment of TiCl4 with equimolar amounts of trimethyl aluminum and the corresponding pyrrole ligands, were found to be moderately active for ethylene polymerization with MAO or [Ph3C]+[B(C6F5)4]−, in all cases producing UHMWPE. The NMR scale reaction of 5 with B(C6F5)3 showed the formation of a solvent-separated ion pair, formed by the abstraction by B(C6F5)3 of a methyl group from Al-CH3 rather than from Ti-CH3. 1H and 13C 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 [(η5,κ1-2,5-Me2C4H2NAlClMe2)ZrMeCl(μ-Cl)]2 (8) and [(η5,κ1-2,5-Me2C4H2NAlCl2Et)ZrCl2(μ-Cl)]2 (9), prepared by the reaction of 2 equivalents of ZrCl4, 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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