View source for Organoantimony chemistry

'''Organoantimony chemistry''' is the [[chemistry]] of [[chemical compound|compounds]] containing a [[carbon]] to [[antimony]] (Sb) [[chemical bond]]. Relevant [[oxidation states]] are Sb<sup>V</sup> and Sb<sup>III</sup>. The toxicity of antimony<ref>{{cite journal|last=Filella|first=M.|title=Alkyl derivatives of antimony in the environment|journal=Metal Ions in Life Sciences|publisher=RSC publishing|location=Cambridge|volume=7|pages=267–301|isbn=978-1-84755-177-1|doi=10.1039/9781849730822-00267|year=2010|pmid=20877810 }}</ref> limits practical application in [[organic chemistry]].<ref>C. Elschenbroich, A. Salzer ''Organometallics : A Concise Introduction'' (2nd Ed) ('''1992''') from Wiley-VCH: Weinheim. {{ISBN|3-527-28165-7}}</ref>

== Syntheses ==

=== Stibines ===
An organoantimony synthesis typically begins with [[Coordination number|tricoordinate]] antimony compounds, called '''stibines'''.  [[Antimony trichloride]] reacts with [[organolithium compound|organolithium]] or [[Grignard reaction|Grignard]] reagents to give compounds of the form R<sub>3</sub>Sb:
:{{nowrap|SbCl<sub>3</sub> + 3 RLi (or RMgCl) → R<sub>3</sub>Sb}}

Stibines are weak [[Lewis acid]]s and do not form [[ate complex]]es.  As [[HSAB|soft]] [[Lewis base|Lewis donors]], they see wide use in [[coordination chemistry]]<ref name=PataiMain/>{{rp|348}} and typically react through [[oxidative addition]]:
:R<sub>3</sub>Sb  +  Br<sub>2</sub>  →  R<sub>3</sub>SbBr<sub>2</sub>
:R<sub>3</sub>Sb  +  O<sub>2</sub>  →  R<sub>3</sub>SbO
:R<sub>3</sub>Sb  +  B<sub>2</sub>H<sub>6</sub>  →  R<sub>3</sub>Sb·BH<sub>3</sub>
This property also [[Air sensitivity|sensitizes them to air]].

If reduced instead, stibanes typically release substituents (ligands):<ref name=PataiMain/>{{rp|443}}
:{{nowrap|R<sub>3</sub>Sb  +  Na + NH<sub>3</sub>  →  R<sub>2</sub>SbNa}}
:{{nowrap|R<sub>2</sub>SbBr + Mg → (R<sub>2</sub>Sb)<sub>2</sub> + MgBr<sub>2</sub>}}

The cyclic compound [[stibole]], a [[structural analog]] of [[pyrrole]], has not been isolated, but substituted derivatives have.  Antimony [[metallocene]]s are known as well:
:14SbI<sub>3</sub> + 3 ([[Cp*]]Al)<sub>4</sub>   →  [Cp{{su|b=2|p=&lowast;}}Sb]<sup>+</sup>[AlI<sub>4</sub>]<sup>−</sup> + 8Sb + 6 AlI<sub>3</sub>
The Cp*-Sb-Cp* angle is 154°.

=== Stiboranes ===
[[Coordination number|Pentacoordinate]] antimony compounds are called '''stiboranes'''.  They can be synthesised from stibines and halogens (Ph = {{chem2|C6H5}}):
:Ph<sub>3</sub>Sb  +  Cl<sub>2</sub>  →  Ph<sub>3</sub>SbCl<sub>2</sub> 
As confirmed by [[X-ray crystallography]], dichlorostiboranes feature pentacoordinate Sb(V) with trans-diaxial chloride ligands.<ref>{{cite journal |doi=10.1107/S0108270192011958 |title=Structures of triphenylantimony(V) dibromide and dichloride |date=1993 |last1=Begley |first1=M. J. |last2=Sowerby |first2=D. B. |journal=Acta Crystallographica Section C Crystal Structure Communications |volume=49 |issue=6 |pages=1044–1046 |bibcode=1993AcCrC..49.1044B }}</ref>
The dichlorostiborane reacts with [[phenyl lithium]] to give [[pentaphenylantimony]]:
:{{chem2|Ph3SbCl2   + 2 PhLi  →  Ph5Sb}}

Like the [[organobismuth]] compounds, stiboranes form [[onium ion|onium]] compounds and [[ate complex]]es.  Unsymmetrical stiboranes can also be obtained through the stibonium ions:
:R<sub>5</sub>Sb + X<sub>2</sub> → [R<sub>4</sub>Sb]<sup>+</sup>[X]<sup>−</sup>  +  RX
:[R<sub>4</sub>Sb]<sup>+</sup>[X]<sup>−</sup> + R'MgX → R<sub>4</sub>R'Sb

[[Pentaphenylantimony]] decomposes at 200&nbsp;°C to [[triphenylstibine]] and [[biphenyl]].

In the related [[pentamethylantimony|Me<sub>5</sub>Sb]], [[proton NMR]] spectra recorded at -100&nbsp;°C cannot resolve the two types of [[methyl]] protons.  This observation is consistent with rapid [[Berry pseudorotation]].

=== Distibines and antimony(I) compounds ===
[[File:PhSbhexamer.svg|thumb|Structure of (PhSb)<sub>6</sub><ref>{{cite journal |doi=10.1002/anie.198500721 |title=(C<sub>6</sub>H<sub>5</sub>Sb)<sub>6</sub>·(1,4-dioxane), the First Cyclohexastibane |date=1985 |last1=Breunig |first1=Hans Joachim |last2=Häberle |first2=Karl |last3=Dräger |first3=Martin |last4=Severengiz |first4=Tevfik |journal=Angewandte Chemie International Edition in English |volume=24 |pages=72–73 }}</ref>]]
'''Distibines''' are formally Sb<sup>II</sup> compounds, but feature tricoordinate Sb atoms with a single Sb-Sb bond.  They may have interest as [[thermochromic|thermochromes]]. For example, tetramethyldistibine is colorless when gas, yellow when liquid, red when solid just below the melting point of 18.5&nbsp;°C, shiny-blue when cooler, and again yellow at cryogenic temperatures.<ref>''Organoantimony compounds with element-element bonds'' H.J. Breunig, R. Rosler Coordination Chemistry Reviews 163 (1997) 33-53</ref><ref name=PataiMain>{{Cite book |title=The Chemistry of Organic Arsenic, Antimony, and Bismuth Compounds |publisher=Wiley |year=1994 |isbn=047193044X |editor-last=Patai |editor-first=Saul |editor-link=Saul Patai |series=Chemistry of Functional Groups |location=Chichester, UK |doi=10.1002/0470023473}}</ref>{{Rp|page=442}}  A typical synthesis first displaces an Sb<sup>III</sup> halide with an [[alkali metal]] and then reduces the resulting anion with [[ethylene dichloride]].<ref name=PataiMain />{{rp|pp=781–783}}

Like its lighter congener, [[arsenic]], organoantimony compounds can be reduced to cyclic oligomers that are formally antimony(I) compounds.<ref name=PataiMain />{{rp|pp=563–577}}

=== With other substituents ===
Sb<sup>V</sup>-N bonds are unstable, except where the N is also bonded to other electron-withdrawing substituents.<ref>{{harvnb|Patai|1994|p=340}}, which immediately undercuts itself by giving an example of an -SbCl<sub>3</sub>-NMe-... complex.</ref>

== Reactions ==
Stibine oxides undergo a sort of [[Pi backbonding|polarized-olefin]] [[Olefin metathesis|metathesis]].  For example, they mediate a carbonyl-imine exchange (Ar is any [[Electrophilic aromatic directing groups|activated]] [[arene]]):<ref name=FDOM>{{Cite book |last1=Freedman |first1=Leon&nbsp;D. |title=The Chemistry of the Metal&mdash;Carbon Bond |last2=Doak |first2=George&nbsp;O. |publisher=[[Interscience]] |year=1989 |isbn=0471915564 |editor-last=Hartley |editor-first=Frank&nbsp;Robinson |series=(Patai's)&nbsp;Chemistry of Functional Groups |volume=5 |location=Chichester, UK |pages=397–413 |chapter=The use of organoantimony and organobismuth compounds in organic synthesis |doi=10.1002/9780470772263.ch9}}</ref>{{Rp|page=399}}<blockquote>Ph<sub>3</sub>Sb=NSO<sub>2</sub>Ar&nbsp;+ PhC=O&nbsp;→ Ph<sub>3</sub>Sb=O&nbsp;+ PhC=NSO<sub>2</sub>Ar</blockquote>The effect may extend [[Vinylogy|vinylically]]:<ref>{{Harvnb|Freedman|Doak|1989|p=410}}, which ascribes the reaction instead to a Wittig-type reaction with an ylide.</ref> <chem display="block">R2C=O{} + HBrCHCO2R ->[{}\atop\ce{Bu3Sb}] R2C=CHCO2R{} + HBr</chem>In contrast, unstabilized [[ylide]]s (R<sub>3</sub>Sb=CR'<sub>2</sub>; R' not [[Electron-withdrawing group|electron-withdrawing]]) form only with difficulty (e.g. [[diazo]] reagents).<ref name=FDOM />{{Rp|pages=399–400}}

Like other metals, stibanes [[Vicinal (chemistry)|vicinal]] to a [[leaving group]] can [[Elimination reaction|eliminate]] before a proton.  For example, diphenyl(β-hydroxyphenethyl)stibine decomposes in heat or acid to [[styrene]]:<ref name="FDOM" />{{Rp|pages=400–402}}
:Ph<sub>2</sub>SbCH<sub>2</sub>CH(OH)Ph&nbsp;→ CH<sub>2</sub>=CHPh&nbsp;+ Ph<sub>2</sub>SbOH
As tertiary stibines also insert into haloalkyl bonds, tertiary stibines are powerful dehalogenating agents.<ref name=FDOM />{{Rp|page=403}}  However, stibanes poorly imitate [[active metal]] organometallics: only with difficulty do their ligands add to carbonyls or they power [[Palladium-catalysed cross coupling|noble-metal cross couplings]].<ref name=FDOM />{{Rp|pages=403–405}}

Stiboranes are gentle oxidants, converting [[acyloin]]s to [[1,2-diketone|diketones]] and [[thiol]]s to [[disulfide]]s.<ref name=FDOM />{{Rp|pages=406–408}}  In air, tris(thiophenyl)stibine catalyzes a [[Hunsdiecker reaction|Hunsdiecker]]-like [[Decarboxylation|decarboxylative]] oxidation of [[Organic acid anhydride|anhydrides]] to [[Alcohol (chemistry)|alcohols]].<ref name=FDOM />{{Rp|page=411}}

In [[ultraviolet light]], distibines radicalize; the resulting radicals can displace [[iodide]].<ref name=PataiMain />{{rp|766}}

==See also==
* [[Lewis acidic antimony compounds]]

==References==
{{Reflist}}

{{ChemicalBondsToCarbon}}

[[Category:Organoantimony compounds|*]]