Xenon difluoride

Structure

Xenon difluoride is a linear molecule with an Xe–F bond length of in the vapor stage, and 200 pm in the solid phase. The packing arrangement in solid shows that the fluorine atoms of neighbouring molecules avoid the equatorial region of each molecule. This agrees with the prediction of VSEPR theory, which predicts that there are 3 pairs of non-bonding electrons around the equatorial region of the xenon atom.

At high pressures, novel, polymeric forms of xenon difluoride can be obtained. Under a pressure of ~50 GPa, transforms into a semiconductor consisting of units linked in a two-dimensional structure, like graphite. At even higher pressures, above 70 GPa, it becomes metallic, forming a three-dimensional structure containing units. A 2011 theoretical study cast doubt on these experimental results, suggesting that xenon difluoride remains stable up to 200 GPa, at which point it dissociates into an ionic solid.

The Xe–F bonds are weak. XeF2 has a total bond energy of 267.8 kJ/mol, with first and second bond energies of 184.1 kJ/mol and 83.68 kJ/mol, respectively. However, XeF2 is much more robust than KrF2, which has a total bond energy of only 92.05 kJ/mol.

Chemistry

Synthesis

Synthesis proceeds by the simple reaction:

:Xe + F2 → XeF2

The reaction needs heat, irradiation, or an electrical discharge. The product is a solid. It is purified by fractional distillation or selective condensation using a vacuum line.

The first published report of XeF2 was in October 1962 by Chernick, et al.{{cite journal |title=Fluorine Compounds of Xenon and Radon |display-authors=etal}} However, though published later,{{cite journal

In the previous syntheses the fluorine gas reactant had been purified to remove hydrogen fluoride. Šmalc and Lutar found that if this step is skipped the reaction rate proceeds at four times the original rate.

In 1965, it was also synthesized by reacting xenon gas with dioxygen difluoride.

Solubility

is soluble in interhalogen solvents such as , , , and others like anhydrous hydrogen fluoride, and acetonitrile, without reduction or oxidation. Solubility in hydrogen fluoride is high, at 167 g per 100 g HF at 29.95 °C.{{cite journal

Derived xenon compounds

Other xenon compounds may be derived from xenon difluoride. The unstable organoxenon compound can be made by irradiating hexafluoroethane to generate radicals and passing the gas over . The resulting waxy white solid decomposes completely within 4 hours at room temperature.{{cite book

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

: + → +

Adding xenon gas to this pale yellow solution at a pressure of 2–3 atmospheres produces a green solution containing the paramagnetic ion, which contains a Xe−Xe bond: ("apf" denotes solution in liquid )

: 3 Xe(g) + (apf) + (l) 2 (apf) + (apf)

This reaction is reversible; removing xenon gas from the solution causes the ion to revert to xenon gas and , and the color of the solution returns to a pale yellow.

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

: (apf) + 4 (apf) → (s) + 3 (apf)

X-ray crystallography indicates that the Xe–Xe bond length in this compound is 309 pm, indicating a very weak bond. The ion is isoelectronic with the ion, which is also dark green.{{cite book

Coordination chemistry

Bonding in the XeF2 molecule is adequately described by the three-center four-electron bond model.

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

:Mg(AsF6)2 + 4 XeF2 → Mg(XeF2)42

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

A similar reaction is:

:Mg(AsF6)2 + 2 XeF2 → Mg(XeF2)22

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

Many such reactions with products of the form Mx(XeF2)n**x have been observed, where M can be calcium, strontium, barium, lead, silver, lanthanum, or neodymium and A can be arsenic, antimony or phosphorus. Some of these compounds feature extraordinarily high coordination numbers at the metal center.

In 2004, results of synthesis of a solvate where part of cationic centers were coordinated solely by XeF2 fluorine atoms were published. Reaction can be written as:

: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 .

Applications

As a fluorinating agent

Xenon difluoride is a strong fluorinating and oxidizing agent. With fluoride ion acceptors, it forms and species which are even more powerful fluorinators.

Among the fluorination reactions that xenon difluoride undergoes are:

• Oxidative fluorination:

• Reductive fluorination: ::2 CrO2F2 + XeF2 → 2 CrOF3 + Xe +O2
• Aromatic fluorination: ::[[File:fluor1.png|250px]]

::[[File:fluor2.png|250px]]

• Alkene fluorination: ::[[File:fluor3.png|500px]]
• Radical fluorination in radical decarboxylative fluorination reactions, in Hunsdiecker-type reactions where xenon difluoride is used to generate the radical intermediate as well as the fluorine transfer source, and in generating aryl radicals from aryl silanes:

::[[File:Xe decarboxylation.tif|frameless]]

::[[File:Xe silanes.tif|frameless|438x438px]]

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 trimethylarsine, but leaves the methyl groups untouched:{{cite book | url-access = limited

: + → + Xe

XeF2 can similarly be used to prepare N-fluoroammonium salts, useful as fluorine transfer reagents in organic synthesis (e.g., Selectfluor), from the corresponding tertiary amine:

:[R–(CH2CH2)3N**:**][] + XeF2 + NaBF4 → [R–(CH2CH2)3–F][]2 + NaF + Xe

will also oxidatively decarboxylate carboxylic acids to the corresponding fluoroalkanes:

:RCOOH + XeF2 → RF + CO2 + Xe + HF

Silicon tetrafluoride has been found to act as a catalyst in fluorination by .{{cite journal

As an etchant

Xenon difluoride is also used as an isotropic gaseous etchant for silicon, particularly in the production of microelectromechanical systems (MEMS), as first demonstrated in 1995. Commercial systems use pulse etching with an expansion chamber Brazzle, Dokmeci, et al. describe this process:

The mechanism of the etch is as follows. First, the XeF2 adsorbs and dissociates to xenon and fluorine atoms on the surface of silicon. Fluorine is the main etchant in the silicon etching process. The reaction describing the silicon with XeF2 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

References

1. Hindermann, D. K., Falconer, W. E.. (1969). "Magnetic Shielding of 19F in XeF₂". [[J. Chem. Phys.]].
2. Zumdahl, Steven S.. (2009). "Chemical Principles 6th Ed.". Houghton Mifflin Company.
3. "MSDS: xenon difluoride". BOC Gases.
4. (1966). "Inorganic Syntheses".
5. (2010). "Two- and three-dimensional extended solids and metallization of compressed XeF₂". Nature Chemistry.
6. (2011). "Freezing in Resonance Structures for Better Packing: XeF₂ Becomes (XeF⁺)(F⁻) at Large Compression". Inorganic Chemistry.
7. (2013). "The Chemistry of the Monatomic Gases". Elsevier Science.
8. Hoppe, R.. (1964). "Die Valenzverbindungen der Edelgase". Angewandte Chemie.
9. (1968). "Inorganic Syntheses".
10. (2007). "Inorganic Syntheses".
11. (1965). "The Reaction of Xenon with Dioxygen Difluoride. A New Method for the Synthesis of Xenon Difluoride". Inorganic Chemistry.
12. (1992). "The dixenon(1+) cation: formation in the condensed phases and characterization by ESR, UV-visible, and Raman spectroscopy". Inorganic Chemistry.
13. (1980). "Production of dixenon cation by reversible oxidation of xenon". Journal of the American Chemical Society.
14. (2004). "First Compounds of Magnesium with XeF₂". [[Inorg. Chem.]].
15. Grochala, Wojciech. (Oct 2007). "Atypical compounds of gases, which have been called 'noble'". Royal Society of Chemistry.
16. (2004). "The First Compound Containing a Metal Center in a Homoleptic Environment of XeF₂ Molecules". [[Angewandte Chemie International Edition]].
17. Halpem, D. F.. (2004). "Encyclopedia of Reagents for Organic Synthesis". J. Wiley & Sons.
18. (1999). "Recent Advances in Electrophilic Fluorination". Tetrahedron.
19. Tius, M. A.. (1995). "Xenon difluoride in synthesis". Tetrahedron.
20. (1986). "Fluorination of activated aromatic systems with cesium fluoroxysulfate". J. Org. Chem..
21. (1993). "Rapid fluorodesilylation of aryltrimethylsilanes using xenon difuoride: An efficient new route to aromatic fluorides". Synlett.
22. (2013-07-22). "Enantioselective Fluoroamination: 1,4-Addition to Conjugated Dienes Using Anionic Phase-Transfer Catalysis". Angewandte Chemie International Edition.
23. (1986). "Replacement of the carboxylic acid function with fluorine". [[Can. J. Chem.]].
24. (1969). "Aqueous fluorination of carboxylic acid salts". [[J. Org. Chem.]].
25. (1995). "Microelectronic Structures and Microelectromechanical Devices for Optical Processing and Multimedia Applications".
26. (1997). "Controlled pulse-etching with xenon difluoride".
27. (2004). "Modeling and characterization of sacrificial polysilicon etching using vapor-phase xenon difluoride".