The liquid and gaseous CrO2F2 have a tetrahedral geometry with C2vsymmetry, much like chromyl chloride.[3] 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 157pm, 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 6.[4]
History and preparation
Pure chromyl fluoride was first isolated in 1952 as reported by Alfred Engelbrecht and Aristid von 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 von Wartenberg prepare impure CrO2F2 by treating chromyl chloride with elemental fluorine.[6] 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:[5]
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[1] drew on von Wartenberg's approach[6] of direct fluoridation:
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 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.[2] 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.
where M is a metal. Chromyl fluoride also converts the oxides of boron and silicon to their fluorides.[8]
Chromyl fluoride reacts with alkali and alkaline earth metal fluorides in perfluoroheptane (solvent) to produce orange-colored tetrafluorodioxochromates(VI):[8]
1 2 von Wartenberg, H. (1941) "Über höhere Chromfluoride (CrF 4, CrF 5 und CrO 2F 2)" [About higher chromium fluorides (CrF 4, CrF 5 and CrO 2F 2)], Z. Anorg. Allg. Chem. [in German], 247(1-2), 135–146, doi:10.1002/zaac.19412470112.
↑ 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.
↑ Christe, Karl O.; Wilson, William W.; Bougon, Roland A. (1986). "Synthesis and characterization of CrF4O, KrF2.CrF4O, and NO+CrF5O-". Inorganic Chemistry. 25 (13): 2163–2169. doi:10.1021/ic00233a013.
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