Chemistry: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
UNII
Properties
CrF2O2
Molar mass 121.991 g·mol−1
Appearance violet crystals
Melting point 31.6 °C (88.9 °F; 304.8 K)
Boiling point 30 °C (86 °F; 303 K) Sublimes
Structure
monoclinic
P21/c, No. 14
C2v
4
Hazards
Main hazards Oxidant
Related compounds
Related compounds
chromyl chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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.[1]

Structure

The liquid and gaseous CrO2F2 have a tetrahedral geometry with C2v symmetry, much like chromyl chloride.[2] Chromyl fluoride dimerizes via fluoride bridges (as O2Cr(μ-F)4CrO2) in the solid state, crystallizing 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 209.4 pm. Chromium resides in a distorted octahedral position with a coordination number of six.[3]

History and preparation

Pure chromyl fluoride was first isolated in 1952 as reported by Alfred Engelbrecht and Aristid von Grosse.[4] 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.[4] 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 von Wartenberg prepare impure CrO2F2 by treating chromyl chloride with elemental fluorine.[5] Another attempt was made by Wiechert, who treated HF with dichromate, yielding impure liquid CrO2F2 at −40 °C.

Engelbrecht and von Grosse's synthesis of CrO2F2, and most successive syntheses, involve treating chromium trioxide with a fluorinating agent:[4]

CrO3   +   2 HF   →   CrO2F2   +   H2O

The reaction is reversible, as water will readily hydrolyze CrO2F2 back to CrO3.

The approach published by Georg Brauer in the Handbook of Preparative Inorganic Chemistry[6] drew on von Wartenberg's approach[5] of direct fluoridation:

CrO2Cl2   +   F2   →   CrO2F2   +   Cl2

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.[1] They reported 100% yield when the reactions were conducted at 120 °C. As expected from the relative reactivities of MoF6 and WF6, the molybdenum reaction proceeded more readily than did the tungsten.[7]

Reactions

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.[1] Like some other fluoride compounds, CrO2F2 reacts with glass and quartz, so silicon-free plastics or metal containers are required for handling the compound. Its oxidizing power in inorganic systems has also been explored.[8] 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.[8]

Chromyl fluoride reacts with alkali and alkaline earth metal fluorides in perfluoroheptane (solvent) to produce orange-colored fluorochromates:[8]

CrO2F2   +   2 MF   →   M2CrO2F4

Chromyl fluoride also reacts with Lewis acids, drawing carboxylate ligands from organic acid anhydrides and producing an acyl fluoride byproduct:[8]

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

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

References

  1. 1.0 1.1 1.2 Gard, G. L. (1986) "Chromium Difluoride Dioxide (Chromyl Fluoride)," Inorg. Synth., 24, 67-69, doi:10.1002/9780470132555.ch20.
  2. Hobbs, W. E. (1958) "Infrared Absorption Spectra of Chromyl Fluoride and Chromyl Chloride," J. Chem. Phys. 28(6), 1220-1222, doi:10.1063/1.1744372.
  3. Supeł, J.; Abram, U.; Hagenbach, A.; Seppelt, K. (2007) "Technetium Fluoride Trioxide, TcO3F, Preparation and Properties." Inorg. Chem., 46(14), 5591–5595, doi:10.1021/ic070333y.
  4. 4.0 4.1 4.2 Engelbrecht, A.; von Grosse, A. (1952) "Pure Chromyl Fluoride," J. Am. Chem. Soc. 74(21), 5262–5264, doi:10.1021/ja01141a007.
  5. 5.0 5.1 von Wartenberg, H. (1941) "Über höhere Chromfluoride (CrF4, CrF5 und CrO2F2)" [About higher chromium fluorides (CrF4, CrF5 and CrO2F2)], Z. Anorg. Allg. Chem. [in German], 247(1-2), 135–146, doi:10.1002/zaac.19412470112.
  6. Cite error: Invalid <ref> tag; no text was provided for refs named Brauer
  7. Green, P. J.; Gard, G. L. (1977) "Chemistry of Chromyl Fluoride. 5. New Preparative routes to CrO2F2," Inorg. Chem. 16(5), 1243–1245, doi:10.1021/ic50171a055.
  8. 8.0 8.1 8.2 8.3 Brown, S. D.; Green, P.J.; Gard, G.L. (1975) "The Chemistry of Chromyl Fluoride III: Reactions with Inorganic Systems," J. Fluorine Chem. 5(3), 203-219, doi:10.1016/S0022-1139(00)82482-3.