Chromyl fluoride

Structure

The liquid and gaseous CrO₂F₂ have a tetrahedral geometry with C_2V symmetry, much like chromyl chloride. 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. 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. 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

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. 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.

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. 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.