Chromyl fluoride

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Chromyl fluoride
Chromyl-fluoride-2D.svg
Names
IUPAC name
Difluoro(dioxo)chromium
Other names
Chromyl Fluoride, Chromium Difluoride Dioxide
Identifiers
3D model (JSmol)
ChemSpider
EC Number 232-137-9
Properties
CrO2F2
Molar mass 112 g/mol
Appearance violet crystals
Melting point 31.6 °C (88.9 °F; 304.8 K)
Boiling point 30 °C (86 °F; 303 K)[1] Sublimes
Hazards
Main hazards Oxidant
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Chromyl fluoride is an inorganic compound with the formula CrO2F2. It is a violet-red colored crystalline solid that melts to an orange-red liquid.[2]

Structure[edit]

The liquid and gaseous CrO2F2 have a tetrahedral geometry with C2V symmetry, much like chromyl chloride.[3] Chromyl fluoride crystallizes in the P21/c space group with Z=4. The Cr=O bond lengths are about 157 pm, and the Cr-F bond lengths are 181.7, 186.7, and 209.4pm. Chromium resides in a distorted octahedral position with a coordination number of six.[4] It dimerizes via fluoride bridges in the solid state.

Reactions[edit]

Chromyl fluoride is a strong oxidizing agent capable of converting hydrocarbons to ketones and carboxylic acids. It can also be used as a reagent in the preparation of other chromyl compounds.[2] Like some other fluoride compounds, CrO2F2 reacts with glass and quartz, so silicon-free plastics or metals must be used when handling the compound.

History and preparation[edit]

Pure chromyl fluoride was first isolated in 1952 as reported by Engelbrecht and Grosse.[5] It was first observed as red vapor in the early 19th century upon heating a mixture of fluorspar (CaF2), chromates, and sulfuric acid. These red vapors were initially thought to be CrF6, although some chemists assumed a CrO2F2 structure analogous to CrO2Cl2.[5] The first moderately successful synthesis of chromyl fluoride was reported by Fredenhagen who examined the reaction of hydrogen fluoride with alkali chromates. A later attempt saw Wartenberg prepared impure CrO2F2 by treating CrO2Cl2 with elemental fluorine. Another attempt was made by Wiechert, who treated HF with dichromate, yielding impure liquid CrO2F2 at -40 °C.

Engelbrecht and Grosse’s synthesis of CrO2F2, and most successive syntheses, involve treating chromium trioxide (CrO3) with a fluorinating agent:

CrO3 + 2 HF → CrO2F2 + H2O

The reaction is reversible, as water will readily hydrolyze CrO2F2 back to CrO3. Other methods include treatment with chlorine fluoride, carbonyl fluoride, or some metal hexafluorides:

CrO3 + 2 ClF → CrO2F2 + Cl2 + O2
CrO3 + COF2 → CrO2F2 + CO2
CrO3 + MF6 → CrO2F2 + MOF4 (M = Mo, W)

The last method involving the fluorides of tungsten and molybdenum are reported by Green and Gard to be very simple and effective routes to large quantities of pure CrO2F2.[2] They reported 100% yield when the reactions were conducted at 120 °C. As expected from the relative reactivities of MoF6 and WF6, the MoF6 reaction proceeded more readily.[6]

Reactions[edit]

In addition to being to oxidizing hydrocarbons to ketones and carboxylic acids, CrO2F2 participates in a variety of other reactions as reported by Brown, Green, and Gard.[7] Chromyl Fluoride can exchange fluorine atoms with metal oxides.

CrO2F2 + MO → MF2 + CrO3

Chromyl fluoride will also convert the oxides of boron and silicon to the fluorides.

Chromyl fluoride reacts with alkali and alkaline earth metal fluorides in a perfluoroheptane solvent to produce the orange-colored fluorochromates:

CrO2F2 + 2 MF → M2CrO2F4

Chromyl fluoride will react with Lewis acids as well:

CrO2F2 + 2(CF3CO)2O → CrO2(CF3COO)2 + 2CF3COF

Chromyl fluoride forms adducts with weak bases NO, NO2, and SO2.

References[edit]

  1. ^ Brauer, Georg (1963) [1960]. Handbuch Der Präparativen Anorganischen Chemie [Handbook of Preparative Inorganic Chemistry] (in German). 1. Stuttgart; New York, New York: Ferdinand Enke Verlag; Academic Press, Inc. p. 259. ISBN 978-0-32316127-5. Retrieved 2014-01-12. 
  2. ^ a b c Gard, G. L. "Chromium Difluoride Dioxide (Chromyl Fluoride)" Inorganic Syntheses 1986, Volume 24, p67.
  3. ^ Hobbs, W. E. "Infrared Absorption Spectra of Chromyl Fluoride and Chromyl Chloride" J. Chem. Phys. 28, 1220. doi:10.1063/1.1744372
  4. ^ Supel, J., Abram, U., Hagenbach, A., Seppelt, K., Technetium Fluoride Trioxide, TcO3F, Preparation and Properties Inorg. Chem., 2007, 46 (14), 5591–5595. doi:10.1021/ic070333y
  5. ^ a b Engelbrecht, A., Grosse, A. V. Pure Chromyl Fluoride. J. Am. Chem. Soc. 74, 5262.
  6. ^ Green, P.J., Gard, G.L., Chemistry of Chromyl Fluoride. New Preparative routes to CrO2F2. Inorg. Chem. 16, 1243. doi:10.1021/ic50171a055
  7. ^ Brown, S. D., Green, P.J., Gard, G.L., The Chemistry of Chromyl Fluoride, III: Reactions with Inorganic Systems. J. Fluorine Chem. 5, 203 doi:10.1016/S0022-1139(00)82482-3