Xenon difluoride

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Xenon difluoride
Identifiers
CAS number13709-36-9 YesY
PubChem83674
ChemSpider75497 YesY
Jmol-3D imagesImage 1
Properties
Molecular formulaF2Xe
Molar mass169.29 g mol−1
AppearanceWhite solid
Density4.32 g/cm3, solid
Melting point

128.6 °C [1]

Solubility in waterDecomposes
Vapor pressure6.0×102 Pa[2]
Structure
Crystal structureparallel linear XeF2 units
Molecular shapeLinear
Dipole moment0 D
Thermochemistry
Std enthalpy of
formation
ΔfHo298
−108 kJ·mol−1[3]
Standard molar
entropy
So298
254 J·mol−1·K−1[3]
Hazards
MSDSPELCHEM MSDS
Main hazardsCorrosive to exposed tissues. Releases toxic compounds on contact with moisture.[4]
NFPA 704
NFPA 704.svg
0
3
1
OX
Related compounds
Other anionsXenon dichloride
Other cationsKrypton difluoride
Radon difluoride
Related compoundsXenon tetrafluoride
Xenon hexafluoride
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references
 
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Xenon difluoride
Identifiers
CAS number13709-36-9 YesY
PubChem83674
ChemSpider75497 YesY
Jmol-3D imagesImage 1
Properties
Molecular formulaF2Xe
Molar mass169.29 g mol−1
AppearanceWhite solid
Density4.32 g/cm3, solid
Melting point

128.6 °C [1]

Solubility in waterDecomposes
Vapor pressure6.0×102 Pa[2]
Structure
Crystal structureparallel linear XeF2 units
Molecular shapeLinear
Dipole moment0 D
Thermochemistry
Std enthalpy of
formation
ΔfHo298
−108 kJ·mol−1[3]
Standard molar
entropy
So298
254 J·mol−1·K−1[3]
Hazards
MSDSPELCHEM MSDS
Main hazardsCorrosive to exposed tissues. Releases toxic compounds on contact with moisture.[4]
NFPA 704
NFPA 704.svg
0
3
1
OX
Related compounds
Other anionsXenon dichloride
Other cationsKrypton difluoride
Radon difluoride
Related compoundsXenon tetrafluoride
Xenon hexafluoride
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Xenon difluoride is a powerful fluorinating agent with the chemical formula XeF2, 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 and low vapor pressure.[5]

Contents

Structure

Xenon difluoride is a linear molecule with an Xe–F bond length of 197.73±0.15 pm in the vapour stage, and 200 pm in the solid phase. The packing arrangement in solid XeF2 shows that the fluorine atoms of neighbouring molecules avoid the equatorial region of each XeF2 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.[2]

At high pressures, novel, non-molecular forms of xenon difluoride can be obtained. Under a pressure of ~50 GPa, XeF2 transforms into a semiconductor consisting of XeF4 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 XeF8 units.[6] However, a recent theoretical study has put these experimental results in doubt.[7]

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.[8]

The first published report of XeF2 was in October 1962 by Chernick, et al.[9] However, though published later,[10] 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.[11] 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.[12] 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.[13]

In the previous syntheses the F2 reactant had been purified to remove HF. Šmalc and Lutar found that if this step is skipped the reaction rate actually proceeds at four times the original rate.[14]

In 1965, it was also synthesized by reacting xenon gas with dioxygen difluoride.[15]

Solubility

XeF2 is soluble in solvents such as BrF5, BrF3, IF5, anhydrous HF, and CH3CN, without reduction or oxidation. Solubility in HF is high, at 167g per 100g HF at 29.95°C.[2]

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 XeF6 which hydrolyzes to xenon trioxide).

Derived xenon compounds

Other xenon compounds may be derived from xenon difluoride. The unstable organoxenon compound Xe(CF3)2 can be made by irradiating hexafluoroethane to generate CF·
3
radicals and passing the gas over XeF2. The resulting waxy white solid decomposes completely within 4 hours at room temperature.[16]

The XeF+ cation is formed by combining xenon difluoride with a strong fluoride acceptor, such as an excess of liquid antimony pentafluoride (SbF5):

XeF2 + SbF5XeF+ + 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,[17] which contains a Xe−Xe bond: ("apf" denotes solution in liquid SbF5)

3 Xe (g) + XeF+ (apf) + SbF5 (l) is in equilibrium with 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.[18]

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+
2
Sb4F
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.[16] The Xe+
2
ion is isoelectronic with the I
2
ion, which is also dark green.[19][20]

Coordination chemistry

XeF2 can act as a ligand in coordination complexes of metals.[2] 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.[21]

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:[22]

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.[23][24] With fluoride ion acceptors, it forms XeF+
and Xe2F+
3
species which are even more powerful fluorinators.[2]

Among the fluorination reactions that xenon difluoride undergoes are:

Ph3TeF + XeF2 → Ph3TeF3 + Xe
2 CrO2F2 + XeF2 → 2 CrOF3 + Xe +O2
Fluor1.png
Fluor2.png
Fluor3.png

XeF2 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:[25]

(CH3)3As + XeF2(CH3)3AsF2 + Xe

XeF2 will also oxidatively decarboxylate carboxylic acids to the corresponding fluoroalkanes:[26][27]

RCOOH + XeF2 → RF + CO2 + Xe + HF

Silicon tetrafluoride has been found to act as a catalyst in fluorination by XeF2.[28]

As an etchant

Xenon difluoride is also used as an isotropic gaseous etchant for silicon, particularly in the production of microelectromechanical systems, (MEMS). Brazzle, Dokmeci, et al., describe this process:[29]

The mechanism of the etch is as follows. First, the XeF2 adsorbs 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 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

  1. ^ Hindermann, D. K., Falconer, W. E. (1969). "Magnetic Shielding of 19F in XeF2". J. Chem. Phys. 50 (3): 1203. Bibcode 1969JChPh..50.1203H. doi:10.1063/1.1671178. 
  2. ^ a b c d e Melita Tramšek; Boris Žemva (2006). "Synthesis, Properties and Chemistry of Xenon(II) Fluoride". Acta Chim. Slov. 53 (2): 105–116. doi:10.1002/chin.200721209. http://acta.chem-soc.si/53/53-2-105.pdf. 
  3. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed.. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X. 
  4. ^ "MSDS: xenon difluoride". BOC Gases. http://www.vngas.com/pdf/g86.pdf. Retrieved 2010-06-01. 
  5. ^ James L. Weeks, Max S. Matheson. "Xenon Difluoride". Inorg. Synth. 8. doi:10.1002/9780470132395.ch69. 
  6. ^ Kim, M.; Debessai, M.; Yoo, C. S. (2010). "Two- and three-dimensional extended solids and metallization of compressed XeF2". Nature Chemistry 2 (9): 784–788. Bibcode 2010NatCh...2..784K. doi:10.1038/nchem.724. PMID 20729901.  edit
  7. ^ Kurzydłowski, D.; Zaleski-Ejgierd, P.; Grochala, W.; Hoffmann, R. (2011). "Freezing in Resonance Structures for Better Packing: XeF2Becomes (XeF+)(F−) at Large Compression". Inorganic Chemistry 50 (8): 3832–3840. doi:10.1021/ic200371a. PMID 21438503.  edit
  8. ^ Tius, M. A. (1995). "Xenon difluoride in synthesis". Tetrahedron 51 (24): 6605–6634. doi:10.1016/0040-4020(95)00362-C. 
  9. ^ 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 and others (1962). "Fluorine Compounds of Xenon and Radon". Science 138 (3537): 136–138. Bibcode 1962Sci...138..136C. doi:10.1126/science.138.3537.136. PMID 17818399. 
  10. ^ Hoppe, R. ; Daehne, W. ; Mattauch, H. ; Roedder, K. (1962). "Fluorination of Xenon". Angew. Chem. Intern. Ed. Engl. 1 (11): 599. doi:10.1002/anie.196205992. 
  11. ^ Hoppe, R. (1964). "Die Valenzverbindungen der Edelgase". Angewandte Chemie 11: 455. doi:10.1002/ange.19640761103. 
  12. ^ 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. 
  13. ^ Williamson, Stanley M.; Sladky, Friedrich O.; Bartlett, Neil (1968). "Xenon Difluoride". Inorg. Synth. 11: 147–151. doi:10.1002/9780470132425.ch31. 
  14. ^ Malc, Andrej; Lutar, Karel; Kinkead, Scott A. (1992). "Xenon Difluoride (Modification)". Inorg. Synth. 29: 1–4. doi:10.1002/9780470132609.ch1. 
  15. ^ Morrow, S. I.; Young, A. R. (1965). "The Reaction of Xenon with Dioxygen Difluoride. A New Method for the Synthesis of Xenon Difluoride". Inorganic Chemistry 4 (5): 759–760. doi:10.1021/ic50027a038.  edit
  16. ^ 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 0-85404-690-9. 
  17. ^ Brown, D. R.; Clegg, M. J.; Downs, A. J.; Fowler, R. C.; Minihan, A. R.; Norris, J. R.; Stein, L. . (1992). "The dixenon(1+) cation: formation in the condensed phases and characterization by ESR, UV-visible, and Raman spectroscopy". Inorganic Chemistry 31 (24): 5041–5052. doi:10.1021/ic00050a023.  edit
  18. ^ Stein, L. .; Henderson, W. W. (1980). "Production of dixenon cation by reversible oxidation of xenon". Journal of the American Chemical Society 102 (8): 2856–2857. doi:10.1021/ja00528a065.  edit
  19. ^ Mackay, Kenneth Malcolm; Mackay, Rosemary Ann; Henderson, W. (2002). Introduction to modern inorganic chemistry (6th ed.). CRC Press. ISBN 0-7487-6420-8. 
  20. ^ Egon Wiberg; Nils Wiberg; Arnold Frederick Holleman (2001). Inorganic chemistry. Academic Press. p. 422. ISBN 0-12-352651-5. 
  21. ^ Tramšek, M.; Benkič, P.; Žemva, B. (2004). "First Compounds of Magnesium with XeF2". Inorg. Chem. 43 (2): 699–703. doi:10.1021/ic034826o. 
  22. ^ Tramšek, M.; Benkič, P.; Žemva, B. (2004). "The First Compound Containing a Metal Center in a Homoleptic Environment of XeF2 Molecules". Angewandte Chemie International Edition 43 (26): 3456. doi:10.1002/anie.200453802. 
  23. ^ D. F. Halpem, "Xenon(II) Fluoride" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York.
  24. ^ 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.
  25. ^ W. Henderson (2000). Main group chemistry. Great Britain: Royal Society of Chemistry. p. 150. ISBN 0-85404-617-8. 
  26. ^ Patrick, Timothy B.; Johri, Kamalesh K.; White, David H.; Bertrand, William S.; Mokhtar, Rodziah; Kilbourn, Michael R.; Welch, Michael J. (1986). "Replacement of the carboxylic acid function with fluorine". Can. J. Chem. 64: 138. doi:10.1139/v86-024. 
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  28. ^ Tamura Masanori; Takagi Toshiyuki; Shibakami Motonari; Quan Heng-Dao; Sekiya Akira (1998). "Fluorination of olefins with xenon difluoride-silicon tetrafluoride" (in Japanese). Fusso Kagaku Toronkai Koen Yoshishu (Japan) 22: 62–63. Journal code: F0135B; accession code: 99A0711841. 
  29. ^ 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

External links