Fluoroantimonate

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The fluoroantimonates are a family of polyatomic weakly coordinating anions composed of antimony and fluorine, consisting of the fluorine adducts of antimony pentafluoride, [(SbF5)nF]−. They occur in the internal chemistry of fluoroantimonic acid.

Contents

The most notable fluoroantimonates are hexafluoroantimonate[SbF6]− and undecafluorodiantimonate[Sb2F11]−. Both are used as components of ionic liquids and as weakly coordinating anions in the study of highly reactive cations.

Properties

Hexafluoroantimonate is the conjugate base of the superacid fluoroantimonic acid HF·SbF6. As fluoroantimonic acid is one of the strongest known acids (only weaker than the helium hydride ion and potentially some carborane acids), hexafluoroantimonate is one of the weakest known bases.

Higher fluoroantimonates are believed to be even less basic

Synthesis

Fluoroantimonates result from the fluorination of antimony pentafluoride. SbF5 is an extremely strong Lewis acid, especially towards fluoride sources.

In fluoroantimonic acid

In solutions of fluoroantimonic acid, the extreme Lewis acidity of SbF5 towards fluoride overcomes the very low basicity of hydrogen fluoride and strips it of its bonding electrons and fluoride. This forces HF to act as a Brønsted–Lowry base, producing the solvated protons which account for the mixture's superacidity:

2 HF + SbF5 ⇌ [H2F]+ [SbF6]−

While fluoroantimonic acid is often depicted according to the above - roughly analogous to the autooxidation of water into hydronium and hydroxide - this reaction is an oversimplification. In addition to reacting with hydrogen fluoride, excess SbF5 is also capable of forming Lewis adducts with fluoroantimonates, yielding a higher fluoroantimonate:

SbF5 + [SbF6]− ⇌ [Sb2F11]−

As fluoroantimonic acid is often mixed in a 1:1 ratio, [Sb2F11]− is the dominant anion in the solution. Further, solvated protons are not limited to [H2F]+, and can form heavier cations such as [H3F2]+ or [H4F3]+, leaving more SbF5 to react and form higher fluoroantimonate ions.

As salts

Fluoroantimonates may be crystallised from a solution of fluoroantimonic acid with some cation. The most common salts are of [SbF6]−, but salts of [Sb2F11]− and [Sb4F21]− [1] have been isolated in the laboratory.

As fluoroantimonic acid and antimony pentafluoride are highly reactive, other routes to fluoroantimonates are industrially desirable. Fluorination of antimonate [2] and of antimony trioxide (with hydrogen peroxide as an oxidant) [3] with HF as a solvent can yield fluoroantimonates from metal fluoride salts.

Applications

As weakly coordinating anions, fluoroantimonates can be used to study highly reactive cations. Examples include hydronium (crystallised from magic acid as the undecafluorodiantimonate) [4] , fluoronium (directly crystallised from fluoroantimonic acid as the undecafluorodiantimonate) [5] , noble gas-noble metal cations such as tetraxenonogold(II) (as the undecafluorodiantimonate) [6] and xenonomercury(II) (as a mixed salt of hexafluoroantimonate and undecafluoroantimonate) [7] , and derivatives or complexes of platinum group metals. [8]

Fluoroantimonate is also a component of ionic liquids used in catalysis.

Related Research Articles

In chemistry, a superacid (according to the original definition) is an acid with an acidity greater than that of 100% pure sulfuric acid (H2SO4), which has a Hammett acidity function (H0) of −12. According to the modern definition, a superacid is a medium in which the chemical potential of the proton is higher than in pure sulfuric acid. Commercially available superacids include trifluoromethanesulfonic acid (CF3SO3H), also known as triflic acid, and fluorosulfuric acid (HSO3F), both of which are about a thousand times stronger (i.e. have more negative H0 values) than sulfuric acid. Most strong superacids are prepared by the combination of a strong Lewis acid and a strong Brønsted acid. A strong superacid of this kind is fluoroantimonic acid. Another group of superacids, the carborane acid group, contains some of the strongest known acids. Finally, when treated with anhydrous acid, zeolites (microporous aluminosilicate minerals) will contain superacidic sites within their pores. These materials are used on massive scale by the petrochemical industry in the upgrading of hydrocarbons to make fuels.

<span class="mw-page-title-main">Magic acid</span> Chemical compound

Magic acid (FSO3H·SbF5) is a superacid consisting of a mixture, most commonly in a 1:1 molar ratio, of fluorosulfuric acid (HSO3F) and antimony pentafluoride (SbF5). This conjugate Brønsted–Lewis superacid system was developed in the 1960s by the George Olah lab at Case Western Reserve University, and has been used to stabilize carbocations and hypercoordinated carbonium ions in liquid media. Magic acid and other superacids are also used to catalyze isomerization of saturated hydrocarbons, and have been shown to protonate even weak bases, including methane, xenon, halogens, and molecular hydrogen.

An inorganic nonaqueous solvent is a solvent other than water, that is not an organic compound. These solvents are used in chemical research and industry for reactions that cannot occur in aqueous solutions or require a special environment. Inorganic nonaqueous solvents can be classified into two groups, protic solvents and aprotic solvents. Early studies on inorganic nonaqueous solvents evaluated ammonia, hydrogen fluoride, sulfuric acid, as well as more specialized solvents, hydrazine, and selenium oxychloride.

Antimony pentafluoride is the inorganic compound with the formula SbF5. This colourless, viscous liquid is a strong Lewis acid and a component of the superacid fluoroantimonic acid, formed upon mixing liquid HF with liquid SbF5 in 1:1 ratio. It is notable for its strong Lewis acidity and the ability to react with almost all known compounds.

<span class="mw-page-title-main">Hydrogen fluoride</span> Chemical compound

Hydrogen fluoride (fluorane) is an inorganic compound with chemical formula HF. It is a very poisonous, colorless gas or liquid that dissolves in water to yield an aqueous solution termed hydrofluoric acid. It is the principal industrial source of fluorine, often in the form of hydrofluoric acid, and is an important feedstock in the preparation of many important compounds including pharmaceuticals and polymers, e.g. polytetrafluoroethylene (PTFE). HF is also widely used in the petrochemical industry as a component of superacids. Due to strong and extensive hydrogen bonding, it boils at near room temperature, much higher than other hydrogen halides.

<span class="mw-page-title-main">Fluoroantimonic acid</span> Chemical compound

Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony penta­fluoride, containing various cations and anions. This mixture is a superacid that, in terms of corrosiveness, is trillions of times stronger than pure sulfuric acid when measured by its Hammett acidity function. It even protonates some hydro­carbons to afford pentacoordinate carbo­cations. Like its precursor hydrogen fluoride, it attacks glass, but can be stored in containers lined with PTFE (Teflon) or PFA.

<span class="mw-page-title-main">Selenium tetrafluoride</span> Chemical compound

Selenium tetrafluoride (SeF4) is an inorganic compound. It is a colourless liquid that reacts readily with water. It can be used as a fluorinating reagent in organic syntheses (fluorination of alcohols, carboxylic acids or carbonyl compounds) and has advantages over sulfur tetrafluoride in that milder conditions can be employed and it is a liquid rather than a gas.

<span class="mw-page-title-main">Tantalum(V) fluoride</span> Chemical compound

Tantalum(V) fluoride is the inorganic compound with the formula TaF5. It is one of the principal molecular compounds of tantalum. Characteristic of some other pentafluorides, the compound is volatile but exists as an oligomer in the solid state.

<span class="mw-page-title-main">Gold(V) fluoride</span> Chemical compound

Gold(V) fluoride is the inorganic compound with the formula Au2F10. This fluoride compound features gold in its highest known oxidation state. This red solid dissolves in hydrogen fluoride but these solutions decompose, liberating fluorine.

<span class="mw-page-title-main">Tetraxenonogold(II)</span> Chemical compound

Tetraxenonogold(II), gold tetraxenide(II) or AuXe2+
4 is a cationic complex with a square planar configuration of atoms. It is found in the compound AuXe2+
4(Sb
2F−
11)
2 (tetraxenonogold(II) undecafluorodiantimonate), which exists in triclinic and tetragonal crystal modifications. The AuXe2+
4 ion is stabilised by interactions with the fluoride atoms of the counterion. The Au−Xe bond length is 274 pm (2.74 Å). Tetraxenonogold(II) is unusual in that it is a coordination complex of xenon, which is weakly basic. It is also unusual in that it contains gold in the -2 oxidation state. It can be produced by reduction of AuF3 by xenon in the presence of fluoroantimonic acid. The salt crystallises at low temperature. Four xenon atoms bond with the gold(II) ion to make this complex.

<span class="mw-page-title-main">Fluoronium</span> Ion

The fluoronium ion is an inorganic cation with the chemical formula H
2F+
. It is one of the cations found in fluoroantimonic acid. The structure of the salt with the Sb
2F−
11 anion, has been determined. The fluoronium ion is isoelectronic with the water molecule and the azanide ion.

Acid strength is the tendency of an acid, symbolised by the chemical formula , to dissociate into a proton, , and an anion, . The dissociation of a strong acid in solution is effectively complete, except in its most concentrated solutions.

Chromium pentafluoride is the inorganic compound with the chemical formula CrF5. It is a red volatile solid that melts at 34 °C. It is the highest known chromium fluoride, since the hypothetical chromium hexafluoride has not yet been synthesized.

Polyhalogen ions are a group of polyatomic cations and anions containing halogens only. The ions can be classified into two classes, isopolyhalogen ions which contain one type of halogen only, and heteropolyhalogen ions with more than one type of halogen.

Fluorine forms a great variety of chemical compounds, within which it always adopts an oxidation state of −1. With other atoms, fluorine forms either polar covalent bonds or ionic bonds. Most frequently, covalent bonds involving fluorine atoms are single bonds, although at least two examples of a higher order bond exist. Fluoride may act as a bridging ligand between two metals in some complex molecules. Molecules containing fluorine may also exhibit hydrogen bonding. Fluorine's chemistry includes inorganic compounds formed with hydrogen, metals, nonmetals, and even noble gases; as well as a diverse set of organic compounds. For many elements the highest known oxidation state can be achieved in a fluoride. For some elements this is achieved exclusively in a fluoride, for others exclusively in an oxide; and for still others the highest oxidation states of oxides and fluorides are always equal.

<span class="mw-page-title-main">Carborane acid</span> Class of chemical compounds

Carborane acidsH(CXB
11Y
5Z
6) (X, Y, Z = H, Alk, F, Cl, Br, CF3) are a class of superacids, some of which are estimated to be at least one million times stronger than 100% pure sulfuric acid in terms of their Hammett acidity function values (H0 ≤ –18) and possess computed pKa values well below –20, establishing them as some of the strongest known Brønsted acids. The best-studied example is the highly chlorinated derivative H(CHB
11Cl
11). The acidity of H(CHB
11Cl
11) was found to vastly exceed that of triflic acid, CF
3SO
3H, and bistriflimide, (CF
3SO
2)
2NH, compounds previously regarded as the strongest isolable acids.

The fluorosulfates or fluorosulfonates are a set of salts of fluorosulfuric acid with an ion formula SO3F−. The fluorosulfate anion can be treated as though it were a hydrogen sulfate anion with hydroxyl substituted by fluorine. The fluorosulfate ion has a low propensity to form complexes with metal cations. Since fluorine is similar in size to oxygen, the fluorosulfate ion is roughly tetrahedral and forms salts similar to those of the perchlorate ion. It is isoelectronic with sulfate, SO4−2. When an organic group is substituted for the anions, organic fluorosulfonates are formed.

<span class="mw-page-title-main">Chlorine trifluoride oxide</span> Chemical compound

Chlorine oxide trifluoride or chlorine trifluoride oxide is a corrosive liquid molecular compound with formula ClOF3. It was developed secretly as a rocket fuel oxidiser.

Among pnictogen group Lewis acidic compounds, unusual lewis acidity of Lewis acidic antimony compounds have long been exploited as both stable conjugate acids of non-coordinating anions, and strong Lewis acid counterparts of well-known superacids. Also, Lewis-acidic antimony compounds have recently been investigated to extend the chemistry of boron because of the isolobal analogy between the vacant p orbital of borane and σ*(Sb–X) orbitals of stiborane, and the similar electronegativities of antimony (2.05) and boron (2.04).

<span class="mw-page-title-main">Hexafluoroarsenate</span> Chemical compound

The hexafluoroarsenate anion is a chemical species with formula AsF−6. Hexafluoroarsenate is relatively inert, being the conjugate base of the notional superacid hexafluoroarsenic acid.

References

  1. ↑ Drews, Thomas; Seppelt, Konrad (1997). "The Xe Ion—Preparation and Structure". Angewandte Chemie International Edition in English. 36 (3): 273–274. doi:10.1002/anie.199702731.
  2. ↑ CN 1743273A
  3. ↑ JP S62108730A
  4. ↑ Zhang, Dingliang; Rettig, Steven J.; Trotter, James; Aubke, Friedhelm (1996). "Superacid Anions: Crystal and Molecular Structures of Oxonium Undecafluorodiantimonate(V), [H3O][Sb2F11], Cesium Fluorosulfate, CsSO3F, Cesium Hydrogen Bis(fluorosulfate), Cs[H(SO3F)2], Cesium Tetrakis(fluorosulfato)aurate(III), Cs[Au(SO3F)4], Cesium Hexakis(fluorosulfato)platinate(IV), Cs2[Pt(SO3F)6], and Cesium Hexakis(fluorosulfato)antimonate(V), Cs[Sb(SO3F)6]". Inorganic Chemistry. 35 (21): 6113–6130. doi:10.1021/ic960525l.
  5. ↑ Mootz, Dietrich; Bartmann, Klemens (1988). "The Fluoronium Ions H2F⊕ and H3F⊕2: Characterization by Crystal Structure Analysis". Angewandte Chemie International Edition in English. 27 (3): 391–392. doi:10.1002/anie.198803911.
  6. ↑ Seidel, Stefan; Seppelt, Konrad (2000). "Xenon as a Complex Ligand: The Tetra Xenono Gold(II) Cation in AuXe 42+ (Sb 2 F 11− ) 2". Science. 290 (5489): 117–118. doi:10.1126/science.290.5489.117. PMID   11021792.
  7. ↑ Hwang, In-Chul; Seidel, Stefan; Seppelt, Konrad (2003). "Gold(<SCP>I</SCP>) and Mercury(<SCP>II</SCP>) Xenon Complexes". Angewandte Chemie International Edition. 42 (36): 4392–4395. doi:10.1002/anie.200351208. PMID   14502720.
  8. ↑ Willner, Helge; Bach, Christian; Wartchow, Rudolf; Wang, Changqing; Rettig, Steven J.; Trotter, James; Jonas, Volker; Thiel, Walter; Aubke, Friedhelm (2000). "Syntheses, Molecular Structures, and Vibrational Spectra of Chloropentacarbonylrhodium(III) and -iridium(III) Undecafluorodiantimonate(V), [Rh(CO)5Cl][Sb2F11]2and [Ir(CO)5Cl][Sb2F11]2: An Experimental and Density Functional Study". Inorganic Chemistry. 39 (9): 1933–1942. doi:10.1021/ic9911926. PMID   11428113.
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