Chromyl Fluoride, Chromium Difluoride Dioxide
3D model (JSmol)
|Molar mass||112 g/mol|
|Melting point||31.6 °C (88.9 °F; 304.8 K)|
|Boiling point||30 °C (86 °F; 303 K) Sublimes|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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The liquid and gaseous CrO2F2 have a tetrahedral geometry with C2V symmetry, much like chromyl chloride. 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. It dimerizes via fluoride bridges in the solid state.
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. 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. 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. 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
- 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. 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.
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. 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.
- Brauer, Georg (1963) . 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.
- Gard, G. L. "Chromium Difluoride Dioxide (Chromyl Fluoride)" Inorganic Syntheses 1986, Volume 24, p67.
- Hobbs, W. E. "Infrared Absorption Spectra of Chromyl Fluoride and Chromyl Chloride" J. Chem. Phys. 28, 1220. doi:10.1063/1.1744372
- 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
- Engelbrecht, A., Grosse, A. V. Pure Chromyl Fluoride. J. Am. Chem. Soc. 74, 5262.
- Green, P.J., Gard, G.L., Chemistry of Chromyl Fluoride. New Preparative routes to CrO2F2. Inorg. Chem. 16, 1243. doi:10.1021/ic50171a055
- 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