Iron(III) fluoride
Encyclopedia
Iron fluoride, also known as ferric fluoride, is an inorganic compound
. It exists in an anhydrous form (formula FeF3) as well as two hydrated forms (both formula FeF3·3H2O). It is produced commercially mostly for laboratory work and is not as useful as the related compound iron(III) chloride
. Anhydrous iron(III) fluoride is white while the hydrated forms are light pink.
.
Fe(III)F6 centres interconnected by linear Fe-F-Fe linkages. In the language of crystallography
, the crystals are classified as rhombohedral with an R-3c space group
. The structural motif is similar to that seen in ReO3
. Although the solid is nonvolatile, it evaporates at high temperatures, the gas at 987 °C consists of FeF3, a planar molecule of D3h symmetry
with three equal Fe-F bonds, each of length 176.3 pm. At very high temperatures, it decomposes to give FeF2 and F2.
Two crystalline forms - or more technically, polymorph
s - of FeF3·3H2O are known, the α and β forms. These are prepared by evaporation of an HF solution containing Fe3+ at room temperature (α form) and above 50°C (β form). The space group
of the β form is P4/m, and the α form maintains a P4/m space group with a J6 substructure. The solid α form is unstable and converts to the β form within days. The two forms are distinguished by their difference in quadrupole splitting from their Mössbauer spectra.
with hydrogen fluoride:
It also forms as a passivating film upon contact between iron (and steel) and hydrogen fluoride
. The hydrates crystallize from aqueous hydrofluoric acid.
The material is a fluoride acceptor. With xenon hexafluoride
it forms [FeF4]XeF5].
Some cross coupling reaction are catalyzed by ferric fluoride-based compounds. Specifically the coupling of biaryl compounds are catalyzed by hydrated iron(II) fluoride complexes of N-heterocyclic carbene ligands. Other metal fluorides also catalyse similar reactions. Iron(III) fluoride has also been shown to catalyze chemoselective addition of cyanide to aldehydes to give the cyanohydrin
s.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
. It exists in an anhydrous form (formula FeF3) as well as two hydrated forms (both formula FeF3·3H2O). It is produced commercially mostly for laboratory work and is not as useful as the related compound iron(III) chloride
Iron(III) chloride
Iron chloride, also called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl3. The colour of iron chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red...
. Anhydrous iron(III) fluoride is white while the hydrated forms are light pink.
Chemical and physical properties
Iron(III) fluoride is a thermally robust, paramagnetic solid, consisting of high spin Fe(III) centers, which is consistent with the pale colors of all forms of this material. Both anhydrous iron(III) fluoride as well as its hydrates are hygroscopicHygroscopy
Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing material becoming physically 'changed,' somewhat, by an increase in volume, stickiness, or other...
.
Structure
The anhydrous form is adopts a simple structure with octahedralOctahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
Fe(III)F6 centres interconnected by linear Fe-F-Fe linkages. In the language of crystallography
Crystallography
Crystallography is the experimental science of the arrangement of atoms in solids. The word "crystallography" derives from the Greek words crystallon = cold drop / frozen drop, with its meaning extending to all solids with some degree of transparency, and grapho = write.Before the development of...
, the crystals are classified as rhombohedral with an R-3c space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
. The structural motif is similar to that seen in ReO3
Rhenium trioxide
Rhenium trioxide is a red solid with a metallic lustre. It is the only stable trioxide of the Group 7 elements .-Structure:...
. Although the solid is nonvolatile, it evaporates at high temperatures, the gas at 987 °C consists of FeF3, a planar molecule of D3h symmetry
Symmetry group
The symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...
with three equal Fe-F bonds, each of length 176.3 pm. At very high temperatures, it decomposes to give FeF2 and F2.
Two crystalline forms - or more technically, polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
s - of FeF3·3H2O are known, the α and β forms. These are prepared by evaporation of an HF solution containing Fe3+ at room temperature (α form) and above 50°C (β form). The space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
of the β form is P4/m, and the α form maintains a P4/m space group with a J6 substructure. The solid α form is unstable and converts to the β form within days. The two forms are distinguished by their difference in quadrupole splitting from their Mössbauer spectra.
Preparation, occurrence, reactions
Anhydrous iron(III) fluoride is prepared by treating virtually any anhydrous iron compound with fluorine. More practically and like most metal fluorides, it is prepared by treating the corresponding chlorideChloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
with hydrogen fluoride:
- FeCl3 + 3 HF → 2 FeF3 + 3 HCl
It also forms as a passivating film upon contact between iron (and steel) and hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
. The hydrates crystallize from aqueous hydrofluoric acid.
The material is a fluoride acceptor. With xenon hexafluoride
Xenon hexafluoride
Xenon hexafluoride is a noble gas compound with the formula XeF6 and the highest of the three binary fluorides of xenon, the other two being XeF2 and XeF4. All are exergonic and stable at normal temperatures. XeF6 is the strongest fluorinating agent of the series...
it forms [FeF4]XeF5].
Applications
The primary commercial use of iron(III) fluoride in the production of ceramics.Some cross coupling reaction are catalyzed by ferric fluoride-based compounds. Specifically the coupling of biaryl compounds are catalyzed by hydrated iron(II) fluoride complexes of N-heterocyclic carbene ligands. Other metal fluorides also catalyse similar reactions. Iron(III) fluoride has also been shown to catalyze chemoselective addition of cyanide to aldehydes to give the cyanohydrin
Cyanohydrin
A cyanohydrin is a functional group found in organic compounds. Cyanohydrins have the formula R2CCN, where R is H, alkyl, or aryl. Cyanohydrins are industrially important precursors to carboxylic acids and some amino acids...
s.