- Source: Organoantimony chemistry
Organoantimony chemistry is the chemistry of compounds containing a carbon to antimony (Sb) chemical bond. Relevant oxidation states are SbV and SbIII. 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: 348 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):: 443
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
=Distibines are formally SbII 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.: 442 A typical synthesis first displaces an SbIII halide with an alkali metal and then reduces the resulting anion with ethylene dichloride.: 781–783
Like its lighter congener, arsenic, organoantimony compounds can be reduced to cyclic oligomers that are formally antimony(I) compounds.: 563–577
= With other substituents
=SbV-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):: 399 Ph3Sb=NSO2Ar + PhC=O → Ph3Sb=O + PhC=NSO2ArThe effect may extend vinylically:
R
2
C
=
O
+
HBrCHCO
2
R
→
Bu
3
Sb
R
2
C
=
CHCO
2
R
+
HBr
{\displaystyle {\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).: 399–400
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:: 400–402
Ph2SbCH2CH(OH)Ph → CH2=CHPh + Ph2SbOH
As tertiary stibines also insert into haloalkyl bonds, tertiary stibines are powerful dehalogenating agents.: 403 However, stibanes poorly imitate active metal organometallics: only with difficulty do their ligands add to carbonyls or they power noble-metal cross couplings.: 403–405
Stiboranes are gentle oxidants, converting acyloins to diketones and thiols to disulfides.: 406–408 In air, tris(thiophenyl)stibine catalyzes a Hunsdiecker-like decarboxylative oxidation of anhydrides to alcohols.: 411
In ultraviolet light, distibines radicalize; the resulting radicals can displace iodide.: 766
See also
Lewis acidic antimony compounds