Displacement of weakly coordinating anions from zirconocene alkyl cations by trialkyl phosphine : a model for olefin coordination in homogeneous Ziegler-Natta catalysis

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1998
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Beck, Stefan
Prosenc, Marc-Heinrich
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Journal of Molecular Catalysis A: Chemical ; 128 (1998), 1-3. - pp. 41-52. - ISSN 1381-1169
Abstract
Displacement of the anion H3C–B(C6F5)3 from the zirconocene contact-ion pair [(C5H5)2Zr(CH3)+⋯(μ-H3C)–B(C6F5)3] by a series of phosphines and formation of a bisphosphine complex by uptake of a second phosphine ligand have been studied by NMR methods in C6D6 solutions. Evidence is presented that associated, outer-sphere ion pairs [(C5H5)2Zr(CH3)(PR3)]+ H3C–B(C6F5)3 predominate at zirconocene concentrations up to ca. 2 mM; higher aggregates become apparent at higher zirconocene concentrations. Equilibrium constants for the formation of mono- and bis-PMe3 complexes have been determined; the latter are found to be highly sensitive to steric perturbations. These data, together with density-functional estimates for reaction enthalpies, lead to the following qualitative conclusions with regard to olefin coordination in zirconocene-based polymerization catalysts: (i) The olefin substrate displaces the borate anion only from a small equilibrium fraction of the zirconocene alkyl cations present; (ii) the predominant fraction of the resulting zirconocene alkyl olefin cation remains in an outer-sphere association with the counteranion; (iii) mutual displacement of olefin and counteranion from the Zr center is slow compared to typical olefin insertions; (iv) uptake of a second olefin under formation of a cationic zirconocene alkyl diolefin complex is rather unlikely even at elevated olefin concentrations.
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ISO 690BECK, Stefan, Marc-Heinrich PROSENC, Hans-Herbert BRINTZINGER, 1998. Displacement of weakly coordinating anions from zirconocene alkyl cations by trialkyl phosphine : a model for olefin coordination in homogeneous Ziegler-Natta catalysis. In: Journal of Molecular Catalysis A: Chemical. 128(1-3), pp. 41-52. ISSN 1381-1169. Available under: doi: 10.1016/S1381-1169(97)00161-1
BibTex
@article{Beck1998Displ-23878,
  year={1998},
  doi={10.1016/S1381-1169(97)00161-1},
  title={Displacement of weakly coordinating anions from zirconocene alkyl cations by trialkyl phosphine : a model for olefin coordination in homogeneous Ziegler-Natta catalysis},
  number={1-3},
  volume={128},
  issn={1381-1169},
  journal={Journal of Molecular Catalysis A: Chemical},
  pages={41--52},
  author={Beck, Stefan and Prosenc, Marc-Heinrich and Brintzinger, Hans-Herbert}
}
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    <dcterms:abstract xml:lang="eng">Displacement of the anion H&lt;sub&gt;3&lt;/sub&gt;C–B(C&lt;sub&gt;6&lt;/sub&gt;F&lt;sub&gt;5&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;−&lt;/sup&gt; from the zirconocene contact-ion pair [(C&lt;sub&gt;5&lt;/sub&gt;H&lt;sub&gt;5&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;Zr(CH&lt;sub&gt;3&lt;/sub&gt;)+⋯(μ-H&lt;sub&gt;3&lt;/sub&gt;C)–B(C&lt;sub&gt;6&lt;/sub&gt;F&lt;sub&gt;5&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;−&lt;/sup&gt;] by a series of phosphines and formation of a bisphosphine complex by uptake of a second phosphine ligand have been studied by NMR methods in C&lt;sub&gt;6&lt;/sub&gt;D&lt;sub&gt;6&lt;/sub&gt; solutions. Evidence is presented that associated, outer-sphere ion pairs [(C&lt;sub&gt;5&lt;/sub&gt;H&lt;sub&gt;5&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;Zr(CH&lt;sub&gt;3&lt;/sub&gt;)(PR&lt;sub&gt;3&lt;/sub&gt;)]&lt;sup&gt;+&lt;/sup&gt; H&lt;sub&gt;3&lt;/sub&gt;C–B(C&lt;sub&gt;6&lt;/sub&gt;F&lt;sub&gt;5&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;−&lt;/sup&gt; predominate at zirconocene concentrations up to ca. 2 mM; higher aggregates become apparent at higher zirconocene concentrations. Equilibrium constants for the formation of mono- and bis-PMe3 complexes have been determined; the latter are found to be highly sensitive to steric perturbations. These data, together with density-functional estimates for reaction enthalpies, lead to the following qualitative conclusions with regard to olefin coordination in zirconocene-based polymerization catalysts: (i) The olefin substrate displaces the borate anion only from a small equilibrium fraction of the zirconocene alkyl cations present; (ii) the predominant fraction of the resulting zirconocene alkyl olefin cation remains in an outer-sphere association with the counteranion; (iii) mutual displacement of olefin and counteranion from the Zr center is slow compared to typical olefin insertions; (iv) uptake of a second olefin under formation of a cationic zirconocene alkyl diolefin complex is rather unlikely even at elevated olefin concentrations.</dcterms:abstract>
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