Caesium fluoride

Caesium fluoride IUPAC name Caesium fluoride Other names Cesium fluoride Identifiers InChI InChI=1/Cs.FH/h;1H/q+1;/p-1 InChIKey XJHCXCQVJFPJIK-REWHXWOFAY Standard InChI InChI=1S/Cs.FH/h;1H/q+1;/p-1 Standard InChIKey XJHCXCQVJFPJIK-UHFFFAOYSA-M CAS number [13400-13-0] EC number 236-487-3 RTECS FK9650000 ChemSpider 24179 Properties Chemical formula CsF Molar mass 151.90 g/mol Appearance white crystalline solid Density 4.115 g/cm3 Melting point 682 °C (955 K) Boiling point 1251 °C (1524 K) Solubility in water 367 g/100 ml (18 °C) Dipole moment 7.9 D (gas) Structure Crystal structure NaCl, cF8 Space group Fm3m, No. 225 Coordination geometry Octahedral (Cs+) Octahedral (F−) Hazards Material safety data sheet (MSDS) External MSDS EU index number not listed GHS pictograms GHS signal word DANGER GHS hazard statements H314, H318 GHS precautionary statements P260, P264, P280, P301+330+331, P303+361+353, P363, P304+340, P310, P321, P305+351+338, P405, P501 Flash point non-flammable Related compounds Other anions Caesium chloride Caesium bromide Caesium iodide Other cations Lithium fluoride Sodium fluoride Potassium fluoride Rubidium fluoride Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Caesium fluoride sesquihydrate IUPAC name Caesium fluoride—water (2/3) Properties Chemical formula CsF·1½H2O Molar mass 178.93 g/mol Melting point 703 ºC Solubility in water 366.5 g/100 ml (18 ºC) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Caesium fluoride is an ionic compound usually found as a hygroscopic white solid. It is more soluble and more readily dissociated than sodium fluoride or potassium fluoride. It is available in anhydrous form, and if water has been absorbed it is easy to dry by heating at 100 °C for two hours in vacuo. It is therefore a useful, less hygroscopic alternative to tetrabutylammonium fluoride (TBAF) and TAS-fluoride (TASF) when anhydrous "naked" fluoride ion is needed. Like all soluble fluorides, it is mildly basic. Contact with acid should be avoided, as this forms highly toxic/corrosive hydrofluoric acid.

Chemical properties

Caesium fluoride reacts usually as a source of fluoride ion, F⁻. It therefore undergoes all of the usual reactions associated with soluble fluorides such as potassium fluoride, for example:

2CsF + CaCl₂ → 2CsCl + CaF₂

Being highly dissociated, it is quite reactive as a fluoride source under anhydrous conditions too, and it will react with electron-deficient aryl chlorides to form aryl fluorides (halex reaction). Due to the strength of the silicon–fluorine bond, fluoride ion is useful for desilylation reactions (removal of Si groups) in organic chemistry; caesium fluoride is an excellent source of anhydrous fluoride for such reactions (vide infra). As with other soluble fluorides, CsF is moderately basic, because hydrofluoric acid is a weak acid. The low nucleophilicity of fluoride means it can be a useful base in organic chemistry.

Crystal structure

Caesium fluoride has the inverse halite structure, because caesium ions are larger than fluoride ions; in the lithium, sodium, potassium, and rubidium halides, the cation is smaller than the anion. The caesium ions form a cubic closest packed array with fluoride ions in the octahedral holes.

Preparation

Caesium fluoride may be prepared by the action of hydrofluoric acid on caesium hydroxide or caesium carbonate, followed by removal of water.

Uses

Caesium fluoride is a useful base in organic chemistry, due the fact that fluoride ion is largely unreactive as a nucleophile. It is reported that CsF gives higher yields in Knoevenagel condensation reactions than KF or NaF.

Removal of silicon groups (desilylation) is a major application for CsF in the laboratory, as its anhydrous nature allows clean formation of water-sensitive intermediates. Caesium fluoride in THF or DMF can attack a wide variety of organosilicon compounds to produce an organosilicon fluoride and a carbanion, which can then react with electrophiles, for example:

Desilylation is also useful for the removal of silyl protecting groups.

Caesium fluoride is also a popular source of fluoride in organofluorine chemistry. For example, CsF reacts with hexafluoroacetone to form a caesium perfluoroalkoxide salt which is stable up to 60 °C, unlike the corresponding sodium or potassium salt.

Single crystals of the salt are transparent into the deep infrared. For this reason it is often used as the windows of cells used for infrared spectroscopy.

Precautions

Like other soluble fluorides, CsF is moderately toxic. Contact with acid should be avoided, as this forms highly toxic/corrosive hydrofluoric acid. The caesium ion (Cs⁺), or caesium chloride, is generally not considered toxic.

External links

• Entry for "Fluoride compounds" on the Australian National Pollutant Inventory