Organoantimony chemistry

Organoantimony chemistry is the chemistry of compounds containing a carbon to antimony (Sb) chemical bond. Relevant oxidation states are Sb^V and Sb^III. The toxicity of antimony limits practical application in organic chemistry.

Syntheses

Stibines

An organoantimony synthesis typically begins with tricoordinate antimony compounds, called stibines. Antimony trichloride reacts with organolithium or Grignard reagents to give compounds of the form R₃Sb:

SbCl₃ + 3 RLi (or RMgCl) → R₃Sb

Stibines are weak Lewis acids and do not form ate complexes. As soft Lewis donors, they see wide use in coordination chemistry and typically react through oxidative addition:

R₃Sb + Br₂ → R₃SbBr₂

R₃Sb + O₂ → R₃SbO

R₃Sb + B₂H₆ → R₃Sb·BH₃

This property also sensitizes them to air.

If reduced instead, stibanes typically release substituents (ligands):

R₃Sb + Na + NH₃ → R₂SbNa

R₂SbBr + Mg → (R₂Sb)₂ + MgBr₂

The cyclic compound stibole, a structural analog of pyrrole, has not been isolated, but substituted derivatives have. Antimony metallocenes are known as well:

14SbI₃ + 3 (Cp*Al)₄ → [Cp^∗
₂Sb]⁺[AlI₄]⁻ + 8Sb + 6 AlI₃

The Cp*-Sb-Cp* angle is 154°.

Stiboranes

Pentacoordinate antimony compounds are called stiboranes, and can be synthesised from stibines and halogens:

Ph₃Sb + Cl₂ → Ph₃SbCl₂

Ph₃SbCl₂ + 2 PhLi → Ph₅Sb

Like their heavier congeners, the organobismuth compounds, stiboranes form onium compounds and ate complexes. Asymmetric compounds can also be obtained through the stibonium ion:

R₅Sb + X₂ → [R₄Sb]⁺[X]

[R₄Sb]⁺[X] + R'MgX → R₄R'Sb

Stibonium halides (R₄SbX) tend to dimerize.

Trigonal-bipyramidal molecule pentaphenylantimony decomposes at 200 °C to triphenylstibine and biphenyl. In the related Me₅Sb, proton NMR at -100 °C cannot resolve different methyl protons.

Distibines and antimony(I) compounds

Structure of (PhSb)₆

Distibines are formally Sb^II compounds, but feature tricoordinate Sb atoms with a single Sb-Sb bond. They may have interest as thermochromes. For example, tetramethyldistibine is colorless when gas, yellow when liquid, red when solid just below the melting point of 18.5 °C, shiny-blue when cooler, and again yellow at cryogenic temperatures. A typical synthesis first displaces an Sb^III halide with an alkali metal and then reduces the resulting anion with ethylene dichloride.

Like its lighter congener, arsenic, organoantimony compounds can be reduced to cyclic oligomers that are formally antimony(I) compounds.

With other substituents

Sb^V-N bonds are unstable, except where the N is also bonded to other electron-withdrawing substituents.

Reactions

Stibine oxides undergo a sort of polarized-olefin metathesis. For example, they mediate a carbonyl-imine exchange (Ar is any activated arene):

Ph₃Sb=NSO₂Ar + PhC=O → Ph₃Sb=O + PhC=NSO₂Ar

The effect may extend vinylically: $${\ce {R2C=O{}+HBrCHCO2R->[{} \atop {\ce {Bu3Sb}}]R2C=CHCO2R{}+HBr}}$$In contrast, unstabilized ylides (R₃Sb=CR'₂; R' not electron-withdrawing) form only with difficulty (e.g. diazo reagents).

Like other metals, stibanes vicinal to a leaving group can eliminate before a proton. For example, diphenyl(β-hydroxyphenethyl)stibine decomposes in heat or acid to styrene:

Ph₂SbCH₂CH(OH)Ph → CH₂=CHPh + Ph₂SbOH

As tertiary stibines also insert into haloalkyl bonds, tertiary stibines are powerful dehalogenating agents. However, stibanes poorly imitate active metal organometallics: only with difficulty do their ligands add to carbonyls or they power noble-metal cross couplings.

Stiboranes are gentle oxidants, converting acyloins to diketones and thiols to disulfides. In air, tris(thiophenyl)stibine catalyzes a Hunsdiecker-like decarboxylative oxidation of anhydrides to alcohols.

In ultraviolet light, distibines radicalize; the resulting radicals can displace iodide.

See also

• Lewis acidic antimony compounds