Chromyl fluoride

Chromyl fluoride
Other names Chromyl Fluoride, Chromium Difluoride Dioxide
Identifiers
CAS number	7788-96-7
Properties
Molecular formula	CrO2F2
Molar mass	112 g/mol
Appearance	violet crystals
Melting point	31.6°C
Hazards
Main hazards	Oxidant
Y fluoride (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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.^[1]

Structure

The liquid and gaseous CrO₂F₂ have a tetrahedral geometry with C_2V symmetry, much like chromyl chloride.^[2] Chromyl fluoride crystallizes in the P2_1/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.^[3] It dimerizes via fluoride bridges in the solid state.

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 metals must be used when handling the compound.

History and preparation

Pure chromyl fluoride was first isolated in 1952 as reported by Engelbrecht and 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 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.^[1] 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.^[5]

Reactions

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.^[6] 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

1. ^ ^{a b c} Gard, G. L. "Chromium Difluoride Dioxide (Chromyl Fluoride)" Inorganic Syntheses 1986, Volume 24, p67.
2. ^ Hobbs, W. E. "Infrared Absorption Spectra of Chromyl Fluoride and Chromyl Chloride" J. Chem. Phys. 28, 1220. doi:10.1063/1.1744372
3. ^ 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
4. ^ ^{a b} Engelbrecht, A., Grosse, A. V. Pure Chromyl Fluoride. J. Am. Chem. Soc. 74, 5262.
5. ^ Green, P.J., Gard, G.L., Chemistry of Chromyl Fluoride. New Preparative routes to CrO₂F₂. Inorg. Chem. 16, 1243. doi:10.1021/ic50171a055
6. ^ 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