Chromyl fluoride

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]

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]

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

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]

CrO₂F₂ + 2 MF → (M⁺)₂[CrO₂F₄]²⁻

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 Lewis bases NO, NO₂, and SO₂.

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

2 CrO₂F₂ + 2 KrF₂ → 2 CrOF₄ + O₂ + 2 Kr