Xenon difluoride

Xenon difluoride is a powerful fluorinating agent, but it 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). It has a strong characteristic infra-red doublet at 550 cm⁻¹ and 556 cm⁻¹.

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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 glasswear. 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 [M^x(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

SUGGESTED MECHANISM:->CH2CL2+XeF2:-> XeCL2+CH2= +2F+ THIS POSITIVELY CHARGED FLUORIN ACTS AS ELECTRPHILE. -NO2+CH2-> NH2+CO2 RANA PRATAP SINGH rps_chem@rediffmail.com

Alkene fluorination

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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^[1]:

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 , Xenon Difluoride in Synthesis . A good review.

• 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. 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. doi:10.1021/ja00882a063.

• 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.

1. ^ 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.