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Xenon difluoride

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Xenon difluoride
Xenon difluoride
Xenon difluoride
Names
IUPAC names
Xenon difluoride
Xenon(II) fluoride
Identifiers
3D model (JSmol)
ECHA InfoCard 100.033.850 Edit this at Wikidata
  • F[Xe]F
Properties
XeF2
Molar mass 169.30 g/mol
Appearance White solid
Density 4.32 g/cm3, solid
Boiling point 114 °C
Decomposes
Structure
parallel linear XeF2 units
Linear
0 D
Related compounds
Other cations
 Krypton difluoride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Xenon difluoride is a powerful fluorinating agent with the chemical formula XeF
2, and one of the most stable xenon compounds. Like most covalent inorganic fluorides it is moisture sensitive. It decomposes on contact with light or water vapour. Xenon difluoride is a dense, white crystalline solid. It has a nauseating odour but low vapor pressure.[1] Xenon difluoride is a linear molecule. It has a strong characteristic infrared doublet at 550 cm−1 and 556 cm−1.

Chemistry

Synthesis

Synthesis proceeds by the simple formula:

Xe + F2 → XeF2

The reaction requires heat, irradiation, or an electrical discharge. The product is gaseous, but can be condensed at −30 °C. It is purified by fractional distillation or selective condensation using a vacuum line.[2]

The first published report of XeF2 was in October 1962 by Chernick, et al.[3] However, though published later,[4] XeF2 was probably first created by Rudolf Hoppe at the University of Münster, Germany, in early 1962, by reacting fluorine and xenon gas mixtures in an electrical discharge.[5] Shortly after these reports, Weeks, Cherwick, and Matheson of Argonne National Laboratory reported the synthesis of XeF2 using an all-nickel system with transparent alumina windows, in which equal parts Xe and F2 gases react at low pressure upon irradiation by an ultraviolet source to give XeF2.[6] Williamson reported that the reaction works equally well at atmospheric pressure in a dry Pyrex glass bulb using sunlight as a source. It was noted that the synthesis worked even on cloudy days.[7]

In the previous syntheses the F2 reactant had been purified to remove H2. Šmalc and Lutar found that if this step is skipped the reaction rate actually proceeds at 4 times the original rate.[8]

Solubility

XeF
2 is soluble in solvents such as BrF
5, BrF
3, IF
5, anhydrous HF and CH
3CN, without reduction or oxidation. Solubility in HF is high, at 167g per 100g HF at 29.95°C.[9]

Safety considerations

Xenon difluoride (XeF2) is most easily made directly from xenon and fluorine. An evacuated glass container of fluorine and xenon is exposed to daylight. The usual precautions associated with use of F2 are required: grease-free, preferably fluorine passivated metal system or very dry glassware. Air must be excluded to preclude formation of xenon trioxide, an explosive (this is only true if the XeF2 sample contains XeF4 which hydrolyzes to xenon trioxide).

Derived xenon compounds

Other xenon compounds may be derived from xenon difluoride. The unstable organoxenon compound Xe(CF
3)
2 can be made by irradiating hexafluoroethane to generate CF·
3 radicals and passing the gas over XeF
2. The resulting waxy white solid decomposes completely within 4 hours at room temperature.[10]

The XeF+ cation is formed by combining xenon difluoride with a strong fluoride acceptor, such as an excess of liquid antimony pentafluoride (SbF
5):

XeF
2 + SbF
5 → XeF+
 + SbF−
6

Adding xenon gas to this pale yellow solution at a pressure of 2-3 atm produces a green solution containing the paramagnetic Xe+
2 ion,[11] which contains a Xe−Xe bond: ("apf" denotes solution in liquid SbF
5)

3 Xe (g) + XeF+ (apf) + SbF
5 (l) ⇌ 2 Xe+
2 (apf) + SbF−
6 (apf)

This reaction is reversible; removing xenon gas from the solution causes the Xe+
2 ion to revert back to xenon gas and XeF+
, and the color of the solution returns to a pale yellow.[12]

In the presence of liquid HF, dark green crystals can be precipitated from the green solution at −30°C:

Xe+
2 (apf) + 4 SbF−
6 (apf) → Xe+
2Sb
4F−
21 (s) + 3 F−
 (apf)

X-ray crystallography indicates that the Xe-Xe bond length in this compound is 309 pm, indicating a very weak bond.[10] The Xe+
2 ion is isoelectronic with the I−
2 ion, which is also dark green.[13]

Coordination chemistry

XeF2 can act as a ligand in coordination complexes of transition metals.[9] For example, in HF solution:

Mg(AsF6)2 + 4 XeF2 → [Mg(XeF2)4](AsF6)2

Crystallographic analysis shows that the magnesium atom is coordinated to 6 fluorine atoms. Four of the fluorines are attributed to the four xenon difluoride ligands while the other two are a pair of cis-AsF−
6 ligands.[14]

A similar reaction is:

Mg(AsF6)2 + 2 XeF2 → [Mg(XeF2)2](AsF6)2

In the crystal structure of this product the magnesium atom is octahedrally-coordinated and the XeF2 ligands are axial while the AsF−
6 ligands are equatorial.

Many such reactions with products of the form [Mx(XeF2)n](AF6)x have been observed, where M can be Ca, Sr, Ba, Pb, Ag, La, or Nd and A can be As, Sb or P.

Recently, a compound was synthesised where a metal atom was coordinated solely by XeF2 fluorine atoms:[15]

2 Ca(AsF6 )2 + 9 XeF2 → Ca2(XeF2)9(AsF6)4.

This reaction requires a large excess of xenon difluoride. The structure of the salt is such that half of the Ca2+ ions are coordinated by fluorine atoms from xenon difluoride, while the other Ca2+ ions are coordinated by both XeF2 and AsF−
6.

Applications

As a fluorinating agent

Xenon difluoride is a strong fluorinating and oxidising agent.[16][17] With fluoride ion acceptors, it forms XeF+
 and Xe
2F+
3 species which are even more powerful fluorinators.[9]

Among the fluorination reactions that xenon difluoride undergoes are:

  • Oxidative fluorination:
Ph3TeF + XeF2 → Ph3TeF3 + Xe
  • Reductive fluorination:
2 CrO2F2 + XeF2 → 2 CrOF3 + Xe +O2
  • Aromatic fluorination:
  • Alkene fluorination:

XeF
2 is selective about which atom it fluorinates, making it a useful reagent for fluorinating heteroatoms without touching other substituents in organic compounds. For example, it fluorinates the arsenic atom in trimethyl arsenic, but leaves the methyl groups untouched:[18]

(CH
3)
3As + XeF
2 → (CH
3)
3AsF
2

XeF
2 will also oxidatively decarboxylate carboxylic acids to the corresponding fluoroalkanes:[19][20]

RCOOH + XeF2 → RF + CO2 + Xe + HF

Silicon tetrafluoride has been found to act as a catalyst in fluorination by XeF
2.[21]

Use as an etchant

Xenon difluoride is also used as an etchant for silicon, particularly in the production of microelectromechanical systems, (MEMS). Brazzle, Dokmeci, et al., describe this process:[22]

The mechanism of the etch is as follows. First, the XeF2 absorbs and dissociates to xenon (Xe) and fluorine (F) on the surface of silicon. Fluorine is the main etchant in the silicon etching process. The reaction describing the silicon with XeF is

2 XeF2 + Si → 2 Xe + SiF4

XeF2 has a relatively high etch rate and does not require ion bombardment or external energy sources in order to etch silicon.

References

  1. ^ Weeks, J.; Matheson, M., "Xenon Difluoride", Inorganic Syntheses (8) 1966
  2. ^ Tius, M. A., Tetrahedron, Volume 51, Issue 24, 12 June 1995, pp. 6605-6634, Xenon Difluoride in Synthesis .
  3. ^ Chernick, CL and Claassen, HH and Fields, PR and Hyman, HH and Malm, JG and Manning, WM and Matheson, MS and Quarterman, LA and Schreiner, F. and Selig, HH; et al. (1962). "Fluorine Compounds of Xenon and Radon". Science,. 138 (3537). American Association for the Advancement of Science: 136--138. doi:10.1126/science.138.3537.136. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
  4. ^ Hoppe, R. ; Daehne, W. ; Mattauch, H. ; Roedder, K. (1962-11-01). "FLUORINATION OF XENON". Angew. Chem. Intern. Ed. Engl.; Vol: 1. 1: 599. doi:10.1002/anie.196205992.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Hoppe, R. (1964). "Die Valenzverbindungen der Edelgase". Angewandte Chemie. 11: 455. {{cite journal}}: Unknown parameter |notes= ignored (help)
  6. ^ Weeks, J.; Matheson, M.; Chernick, C., (1962). "Photochemical Preparation of Xenon Difluoride" Photochemical Preparation of Xenon Difluoride". J. Am. Chem. Soc. 84 (23): 4612–4613. doi:10.1021/ja00882a063.{{cite journal}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)
  7. ^ Williamson, S., "Xenon Difluoride", Inorganic Syntheses (11) 1968.
  8. ^ Šmalc, A.; Lutar, K., "Xenon Difluoride (Modification)", Inorganic Syntheses (29) 1992.
  9. ^ a b c Melita Tramšek; Boris Žemva (2006). "Synthesis, Properties and Chemistry of Xenon(II) Fluoride" (PDF). Acta Chim. Slov. 53 (2): 105–116. {{cite journal}}: Unknown parameter |day= ignored (help); Unknown parameter |month= ignored (help)
  10. ^ a b Harding, Charlie; Johnson, David Arthur; Janes, Rob (2002). Elements of the p block. Contributor Charlie Harding, David Arthur Johnson, Rob Janes. Royal Society of Chemistry (Great Britain), Open University. ISBN 0854046909.
  11. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1021/ic00050a023, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1021/ic00050a023 instead.
  12. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1021/ja00528a065, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1021/ja00528a065 instead.
  13. ^ Mackay, Kenneth Malcolm; Mackay, Rosemary Ann; Henderson, W. (2002). Introduction to modern inorganic chemistry (6th ed.). CRC Press. ISBN 0748764208.
  14. ^ Tramšek, M.; Benkič, P.; Žemva, B., Inorg. Chem., 43 (2), 699 -703, (2004) "First Compounds of Magnesium with XeF2".
  15. ^ Tramšek, M.; Benkič, P.; Žemva, B., Angewandte Chemie International Edition, 43, (2), 3456 (2004) "The First Compound Containing a Metal Center in a Homoleptic Environment of XeF2 Molecules".
  16. ^ D. F. Halpem, "Xenon(II) Fluoride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York.
  17. ^ Taylor, S.; Kotoris, C.; Hum, G., (1999). "Recent Advances in Electrophilic Fluorination". Tetrahedron. 55 (43): 12431–12477. doi:10.1016/S0040-4020(99)00748-6.{{cite journal}}: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link) A review of fluorination in general.
  18. ^ W. Henderson (2000). Main group chemistry. Great Britain: Royal Society of Chemistry. p. 150. ISBN 0854046178.
  19. ^ Can J Chem, 1986, 64, 138
  20. ^ J Org Chem, 1969, 34, 2446
  21. ^ Tamura Masanori; Takagi Toshiyuki; Shibakami Motonari; Quan Heng-Dao; Sekiya Akira (1998). "Fluorination of olefins with xenon difluoride-silicon tetrafluoride". Fusso Kagaku Toronkai Koen Yoshishu (in Japanese). 22. Japan: 62–63. Journal code: F0135B; accession code: 99A0711841.
  22. ^ Brazzle, J.D.; Dokmeci, M.R.; Mastrangelo, C.H.; Modeling and characterization of sacrificial polysilicon etching using vapor-phase xenon difluoride , 17th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 2004, pages 737-740.

Additional reading

  • Greenwood, Norman Neill; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 894. ISBN 0750633654.
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