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DOI: 10.1039/A900895K (Paper) Reference Section for: Chem. Commun., 1999, 887-888

The ansa-effect in permethyltantalocene chemistry: a [Me2Si] ansa bridge promotes olefin-insertion and reductive-elimination reactions for [Me2Si(C5Me4)2]Ta(η2-C2H4)H and [Me2Si(C5Me4)2]TaH3

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Jun Ho Shin and Gerard Parkin

Abstract

The ansa-effect, as it applies to tantalocene chemistry, has been investigated by a comparison of the chemistry of [Me2Si(C5Me4)2]TaH3 and [Me2Si(C5Me4)2]Ta(η2-C2H4)H with their permethyltantalocene counterparts; these studies demonstrate that incorporation of the [Me2Si] ansa bridge substantially enhances the rates of both reductive elimination of H2 and ethylene-insertion into a Ta–H bond.

References

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  9. [Me2Si(C5Me4)2](SnBun3)2 is generated via reaction of [Me2Si(C5-Me4)2]Li2 with Bun3SnCl.
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  14. As has been noted previously, exchange processes of this type do not require the participation of a 16-electron alkyl complexes, but may occur via an 18-electron agostic alkyl species.
  15. For ethylene insertion, ΔH‡= 15.49(33) kcal mol–1 and ΔS‡=–2.60(10) cal mol–1 K–1 corresponding to κins= 1.83 × 103 s–1 and ΔG‡= 16.44 kcal mol–1 at 100 °C. For inversion, ΔH‡= 17.96(24) kcal mol–1 and ΔS‡= 3.3(8) cal mol–1 K–1.
  16. Data for Cp*2Ta(η2-C2H4)H at 100 °C: κins= 2.4 s–1; ΔG‡= 21.3 kcal mol–1. See ref. 3(b).
  17. The term ‘inversion’ in this sense is being used to refer to ethylene–hydride site (or alkyl–vacancy) interchange. Since the molecule is not chiral, it is not intended to refer to enantiomer interconversion.
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  19. See supplementary data for the structures of [Me2Si(C5Me4)2]Ta(CO)H, [Me2Si(C5Me4)2]Ta(η2-C2H4)H, and Cp*2TaH3. The C2H4 and hydride ligands of [Me2Si(C5Me4)2]Ta(η2-C2H4)H are statistically disordered about a crystallographic mirror plane that lies in the Cpcent–Ta–Cpcent plane. CCDC 182/1209. See http://www.rsc.org/suppdata/cc/1999/887/ for crystallographic files in .cif format.
  20. Cpcent–Ta–Cpcent= 148.3°; Cpnorm/Cpnorm= 148.2°.
  21. Individual Ta–CCp bond lengths for [Me2Si(C5Me4)2]Ta(CO)H range from 2.32 Å to 2.45 Å. The difference between the Cpcent–Ta–Cpcent angle and the angle between the ring normals is 8.2° for [Me2Si(C5-Me4)2]Ta(CO)H, but only 0.1° for Cp*2TaH3.
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