Synthesis, X-ray structural analysis, and ethylene polymerization studies of group IV metal heterobimetallic aluminum-pyrrolyl complexes

Abstract

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-Me 2C 4H 2NAlClMe 2) 2TiMe 2 (1), zirconium (η 5-2,5- Me 2C 4H 2NAlClMe 2) 2ZrClMe (2), (η 5-2,5-Me 2C 4H 2NAlCl 2Et) 2ZrCl 2 (3), and hafnium (η 5-2,5-Me 2C 4H 2NAlClMe 2) 2HfClMe (4) were found to be inactive toward ethylene polymerization. By contrast, the electron-deficient mono(aluminum-pyrrolyl) piano stool complexes (η 5-2,5-Me 2C 4H 2NAlCl 2Me)TiCl 2Me (5), (η 5-2,3- Me 2C 8H 4NAlCl 2Me)TiCl 2Me (6), and (η 5-3,4,5,6-C 12H 12NAlCl 2Me)TiClMe 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 3C] +[B(C 6F 5) 4] -, in all cases producing UHMWPE. The NMR scale reaction of 5 with B(C 6F 5) 3 showed the formation of a solvent-separated ion pair, formed by the abstraction by B(C 6F 5) 3 of a methyl group from Al-CH 3 rather than from Ti-CH 3. 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 [(η 5,κ 1-2,5-Me 2C 4H 2NAlClMe 2)ZrMeCl(μ-Cl)] 2 (8) and [(η 5,κ 1-2,5-Me 2C 4H 2NAlCl 2Et)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. © 2012 American Chemical Society.