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A study of some novel cationic complexes of the group 4 metals and their applications in Ziegler-Natta catalysis

By Mark Leslie Butcher

Abstract

The reactions between antimony(v) chloride and the monocyclopentadienyl metal trichlorides of Group 4 (CpMCl3 for M = Ti, Sr and Hf) in acetonitrile solution have afforded new hexachloroantimonate salts. These cationic metal-containing products have been isolated and characterised using IR, 1H NMR, elemental analysis, and in two cases, X-ray crystallography. The analogous pentamethylcyclopentadienyl (Cp*) systems have also been investigated, with largely similar results.\ud \ud Depending on the stoichiometry employed in the reaction, the relevant mono-, di- and tri-cationic complexes can be formed via halide exchange. For the Cp/Cp*TiCl3 series, a 1:1 or 2:1 ratio of Sb:Ti affords [CpTiCl2(MeCN)3][SbCl6] and [CpTiCl(MeCN)4][SbCl6]2 respectively. A ratio of 1:6 is required to yield pure [CpTi(MeCN)5][SbCl6]3. The 16-electron, pseudo-octahedral geometry of the latter complex was confirmed by X-ray analysis.\ud \ud A similar situation is found in the analogous Zr and Hf cases. However, the formation of the triply-charged complexes is accompanied by an expansion in coordination number about the metal centre; X-ray crystallographic studies revealed the 18-electron [CpZr(MeCN)6][SbCl6]3 to exhibit a pseudo-pentagonal bipyramidal geometry.\ud \ud The reactivity of the CpTi3+ salt towards a variety of both neutral and anionic species has been investigated. In acetonitrile solution, the complex rects with amines to give insoluble mixtures of partially substituted products. Adduct formation with ethers and trimethylphosphine is slow and only occurs when non-coordinating solvents are employed. The complex reacts with inorganic salts to reform neutral CpTiX3 compounds by simple anion exchange.\ud \ud The Ziegler-Natta catalytic activity of the cationic series has been studied. In the presence of methylaluminoxane cocatalysts, the complexes polymerise ethene but their activity is inversely proportional to the charge on the metal

Topics: QD
OAI identifier: oai:wrap.warwick.ac.uk:2542

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