Chromyl fluoride

Chromyl fluoride
Names

IUPAC name Difluoro(dioxo)chromium
Other names Chromyl Fluoride, Chromium Difluoride Dioxide
Identifiers
CAS Number	7788-96-7^Y
3D model (JSmol)	Interactive image
ChemSpider	10329781^N
EC Number	232-137-9
PubChem CID	53471881
UNII	UB519P5O68^Y
InChI InChI=1S/Cr.2FH.2O/h;21H;;/q+2;;;;/p-2^N Key: FRLBLFFATGQISB-UHFFFAOYSA-L^N InChI=1/Cr.2FH.2O/h;21H;;/q+2;;;;/p-2/rCrF2O2/c2-1(3,4)5 Key: FRLBLFFATGQISB-UNDMLHRZAG
SMILES O=[Cr](=O)(F)F
Properties
Chemical formula	CrO₂F₂
Molar mass	121.991 g·mol⁻¹
Appearance	Violet-red crystals
Melting point	31.6 °C (88.9 °F; 304.8 K)
Boiling point	30 °C (86 °F; 303 K)^[1] Sublimes
Structure
Crystal structure	monoclinic
Space group	P2₁/c, No. 14
Point group	C_2v
Formula units (Z)	4
Hazards
Occupational safety and health (OHS/OSH):
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). N verify (what is ^YN ?) Infobox references

Chemical compound

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

The liquid and gaseous CrO₂F₂ have a tetrahedral geometry with C_2v symmetry, 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 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 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:

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.^[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.

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 → M₂[CrO₂F₄]

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 → (CF₃COO)₂CrO₂ + 2 CF₃COF

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

Chromium oxytetrafluoride is prepared by fluorination of chromyl fluoride with krypton difluoride:^[9]

CrO₂F₂ + KrF₂ → CrOF₄ + 0.5 O₂ + Kr

References

^ ^a ^b Brauer, Georg (1963) [1960]. "Chromyl Fluoride – CrO
₂F
₂". Handbook of Preparative Inorganic Chemistry, Volume 1 (2nd ed.). Stuttgart; New York: Ferdinand Enke Verlag; Academic Press, Inc. pp. 258–259. ISBN 978-0-32316127-5.
^ ^a ^b ^c Gard, G. L. (1986) "Chromium Difluoride Dioxide (Chromyl Fluoride)," Inorg. Synth., 24, 67-69, doi:10.1002/9780470132555.ch20.
^ 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.
^ 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.
^ ^a ^b ^c Engelbrecht, A.; von Grosse, A. (1952) "Pure Chromyl Fluoride," J. Am. Chem. Soc. 74(21), 5262–5264, doi:10.1021/ja01141a007.
^ ^a ^b 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.
^ ^a ^b ^c ^d 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.
^ 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.