Definitions

xenon tetra fluoride

Xenon difluoride

Xenon difluoride is a powerful fluorinating agent, with the chemical formula , is 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 (Weeks, 1966). Xenon difluoride is a linear molecule. It has a strong characteristic infra-red doublet at 550 cm−1 and 556 cm−1.

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.

The synthesis of XeF2 was first reported by Weeks, Cherwick, and Matheson of Argonne National Laboratory in 1962. They used an all-nickel system with sapphire windows. Equal parts Xe and F2 gases react at low pressure upon irradiation by an ultraviolet source to give XeF2. 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.

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.

Safety considerations

Xenon difluoride (XeF2) was first made by combining xenon and oxygen difluoride (OF2) in a nickel tube at 300 degree Celsius under pressure. The same compound can be made now 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).

Coordination chemistry

XeF2 can act as a ligand in coordination complexes when accompanied by AsF6. One such example is the reaction in HF solution:
Mg(AsF6)2 + 4 XeF2 → [Mg(XeF2)4](AsF6)2.

Crystallographic analysis shows that the magnesium 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 AsF6 ligands. A similar reaction is

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

In the crystal structure of this product the magnesium is octahedrally coordinated and the XeF2 ligands are axial while the AsF6 ligands are equatorial.

Many such reactions 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 has been synthesised where a metal is coordinated solely by XeF2 fluorines. The reaction is

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 one half of Ca ions is coordinated by fluorines from xenon difluoride while the coordination sphere of the other Ca ion bears both XeF2 and AsF6 ligands.

Fluorination reactions

Oxidative fluorination

An example of inorganic oxidative fluorination is

Ph3TeF + XeF2 → Ph3TeF3 + Xe

Reductive fluorination

Examples of reductive fluorination are:
2CrO2F2 + XeF2 → 2CrOF3 + Xe +O2

Aromatic fluorination

Alkene fluorination

.

Decarboxylation

Xenon difluoride will oxidatively decarboxylate carboxylic acids to the corresponding fluoroalkanes :

RCO2H + XeF2 → RF + CO2 + Xe + HF

Use as an etchant

Xenon difluoride is used as an etchant for silicon, particularly in the production of Microelectromechanical systems, or MEMS. From:
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 → 2Xe + SiF4
XeF2 has a relatively high etch rate and does not require ion bombardment or external energy sources in order to etch silicon.

References

  • Tius, M. A., Tetrahedron, Volume 51, Issue 24, 12 June 1995, Pages 6605-6634 , . A good review.
  • Taylor, S.; Kotoris, C.; Hum, G., (1999). "Recent Advances in Electrophilic Fluorination". Tetrahedron 55 (43): 12431–12477. A review of fluorination in general.
  • 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.
  • Weeks, J.; Matheson, M., "Xenon Difluoride", Inorganic Syntheses (8) 1966
  • Williamson, S., "Xenon Difluoride", Inorganic Syntheses (11) 1968
  • Šmalc, A.; Lutar, K., "Xenon Difluoride (Modification)", Inorganic Syntheses (29) 1992
  • Tramšek, M.; Benkič, P.; Žemva, B., Inorg. Chem., 43 (2), 699 -703, (2004) "First Compounds of Magnesium with XeF2"
  • 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"
  • Greenwood, N.; Earnshaw, A.,Chemistry of the Elements Second Edition, p. 894 1997
  • D. F. Halpem, "Xenon(II) Fluoride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York.

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