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 ^[1] limits practical application in organic chemistry. ^[2]

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 ^{[3] : 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): ^{[3] : 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. They can be synthesised from stibines and halogens (Ph = C₆H₅):

Ph₃Sb + Cl₂ → Ph₃SbCl₂

As confirmed by X-ray crystallography, dichlorostiboranes feature pentacoordinate Sb(V) with trans-diaxial chloride ligands. ^[4] The dichlorostiborane reacts with phenyl lithium to give pentaphenylantimony:

Ph₃SbCl₂ + 2 PhLi → Ph₅Sb

Like the organobismuth compounds, stiboranes form onium compounds and ate complexes. Unsymmetrical stiboranes can also be obtained through the stibonium ions:

R₅Sb + X₂ → [R₄Sb]⁺[X]⁻ + RX

[R₄Sb]⁺[X]⁻ + R'MgX → R₄R'Sb

Pentaphenylantimony decomposes at 200 °C to triphenylstibine and biphenyl.

In the related Me₅Sb, proton NMR spectra recorded at -100 °C cannot resolve the two types of methyl protons. This observation is consistent with rapid Berry pseudorotation.

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. ^{[6] [3] : 442} A typical synthesis first displaces an Sb^III halide with an alkali metal and then reduces the resulting anion with ethylene dichloride. ^{[3] : 781–783}

Like its lighter congener, arsenic, organoantimony compounds can be reduced to cyclic oligomers that are formally antimony(I) compounds. ^{[3] : 563–577}

With other substituents

Sb^V-N bonds are unstable, except where the N is also bonded to other electron-withdrawing substituents. ^[7]

Reactions

Stibine oxides undergo a sort of polarized-olefin metathesis. For example, they mediate a carbonyl-imine exchange (Ar is any activated arene): ^{[8] : 399}

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

The effect may extend vinylically: ^[9] $${\displaystyle {\ce {R2C=O{}+HBrCHCO2R->[{} \atop {\ce {Bu3Sb}}]R2C=CHCO2R{}+HBr}}}$$In contrast, unstabilized ylides (R₃Sb=CR'₂; R' not electron-withdrawing) form only with difficulty (e.g. diazo reagents). ^{[8] : 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: ^{[8] : 400–402}

Ph₂SbCH₂CH(OH)Ph → CH₂=CHPh + Ph₂SbOH

As tertiary stibines also insert into haloalkyl bonds, tertiary stibines are powerful dehalogenating agents. ^{[8] : 403} However, stibanes poorly imitate active metal organometallics: only with difficulty do their ligands add to carbonyls or they power noble-metal cross couplings. ^{[8] : 403–405}

Stiboranes are gentle oxidants, converting acyloins to diketones and thiols to disulfides. ^{[8] : 406–408} In air, tris(thiophenyl)stibine catalyzes a Hunsdiecker-like decarboxylative oxidation of anhydrides to alcohols. ^{[8] : 411}

In ultraviolet light, distibines radicalize; the resulting radicals can displace iodide. ^{[3] : 766}

See also

• Lewis acidic antimony compounds

References

1. Filella, M. (2010). "Alkyl derivatives of antimony in the environment". Metal Ions in Life Sciences. 7. Cambridge: RSC publishing: 267–301. doi:10.1039/9781849730822-00267. ISBN   978-1-84755-177-1. PMID   20877810.
2. C. Elschenbroich, A. Salzer Organometallics : A Concise Introduction (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN   3-527-28165-7
3. Patai, Saul, ed. (1994). The Chemistry of Organic Arsenic, Antimony, and Bismuth Compounds. Chemistry of Functional Groups. Chichester, UK: Wiley. doi:10.1002/0470023473. ISBN   047193044X.
4. Begley, M. J.; Sowerby, D. B. (1993). "Structures of triphenylantimony(V) dibromide and dichloride". Acta Crystallographica Section C Crystal Structure Communications. 49 (6): 1044–1046. Bibcode:1993AcCrC..49.1044B. doi:10.1107/S0108270192011958.
5. . doi:10.1002/anie.198500721.{{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
6. Organoantimony compounds with element-element bonds H.J. Breunig, R. Rosler Coordination Chemistry Reviews 163 (1997) 33-53
7. Patai 1994 , p. 340, which immediately undercuts itself by giving an example of an -SbCl₃-NMe-... complex.
8. Freedman, Leon D.; Doak, George O. (1989). "The use of organoantimony and organobismuth compounds in organic synthesis". In Hartley, Frank Robinson (ed.). The Chemistry of the Metal—Carbon Bond. (Patai's) Chemistry of Functional Groups. Vol. 5. Chichester, UK: Interscience. pp. 397–413. doi:10.1002/9780470772263.ch9. ISBN   0471915564.
9. Freedman & Doak 1989 , p. 410, which ascribes the reaction instead to a Wittig-type reaction with an ylide.