Organoantimony chemistry

Organoantimony chemistry is the chemistry of compounds containing a carbon to antimony 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:
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:
This property also sensitizes them to air.
If reduced instead, stibanes typically release substituents :
The cyclic compound stibole, a structural analog of pyrrole, has not been isolated, but substituted derivatives have. Antimony metallocenes are known as well:
The Cp*-Sb-Cp* angle is 154°.

Stiboranes

antimony compounds are called stiboranes. They can be synthesised from stibines and halogens :
As confirmed by X-ray crystallography, dichlorostiboranes feature pentacoordinate Sb with trans-diaxial chloride ligands.
The dichlorostiborane reacts with phenyl lithium to give pentaphenylantimony:
Like the organobismuth compounds, stiboranes form onium compounds and ate complexes. Unsymmetrical stiboranes can also be obtained through the stibonium ions:
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

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 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 :

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

The effect may extend vinylically: R2C=O + HBrCHCO2R -> R2C=CHCO2R + HBrIn contrast, unstabilized ylides form only with difficulty.
Like other metals, stibanes vicinal to a leaving group can eliminate before a proton. For example, diphenylstibine decomposes in heat or acid to styrene:
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, trisstibine catalyzes a Hunsdiecker-like decarboxylative oxidation of anhydrides to alcohols.
In ultraviolet light, distibines radicalize; the resulting radicals can displace iodide.

Lewis acids

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* orbitals of stiborane, and the similar electronegativities of antimony and boron.

Origin of acidity

σ*, where X describes substituents on antimony, contributes to the Lewis acidity of antimony compounds in two ways: donor–acceptor orbital interaction and electrostatic interaction. These two contributions to the Lewis acidity have been evaluated. Both contributions are studied by calculations, and the acidities of theses compounds are quantified by the Gutmann–Beckett method, Hammett acidity function, pK_a, and fluoride ion affinity. FIA is defined as the amount of energy released upon binding a fluoride ion in the gas phase. The FIA of two popular strong Lewis acids, BF₃ and B₃, are respectively.

Donor–acceptor orbital overlap

Since Lewis adducts are formed by dative bond between Lewis bases and Lewis acids, the orbital overlap between the Lewis base and σ* orbital is the source of the acidity. According to Gabbaï et al., NBO analysis of the Sb₃PPh₃ adduct indicates a donor-acceptor interaction between lp and σ*.
Lowering the LUMO energy increases the Lewis acidity. For example, Sb₃ has a higher LUMO energy and weaker FIA than Sb₃.

Electrostatic interaction

Partial positive charges on the surface of antimony compounds interact with partial negative charges. For example, Sb₃ has a more positively charged site than Sb₃ as shown in its electrostatic potential map, corresponding to higher Lewis acidity ₃ and Sb.

Structure of Lewis acidic antimony compounds

Lewis acidic antimony complexes with a variety of oxidation states and coordination numbers are known. Several salient examples are introduced below.
File:Lewis_acidic_antimony_compound_examples.png|center|thumb|767x767px|FIA indicates fluoride ion affinity. δ indicates the ³¹P NMR shift of OPEt₃ adducts. The chemical shift of OPEt₃ is 51.0 ppm, 51.2 ppm, and 47.6 ppm.File:SbCp* IBOs.png|thumb|IBO of Sb- bondings in ²⁺.|365x365px

3-coordinate Sb(III)

Although stibanes have a lone pair electrons, their antibonding orbitals with electron-withdrawing substituents renders them Lewis acidic. Sb₃ has three σ* orbitals and three Lewis acidic sites. However, as shown in the electrostatic potential map of Sb₃, only one site is accessible to Lewis bases due to the asymmetric arrangement of the three aryl rings.
In ²⁺, the η⁵-Cp* binding mode is confirmed using IBO analysis. In the solid state structure, the Sb-C bond distances between Sb and carbons in the Cp* ring are 2.394 to 2.424 Å, but the Sb–C bond distances with the toluene are 2.993 to 3.182 Å. This longer Sb–toluene distance implies toluene lability in solution.
Sb₂₂ had been predicted that a Lewis base would bind to two antimony centers in a bridging manner. However, it was observed that 2 binds with halide anions in various ratios. Cozzolono et al. suggested three reasons for its complex binding mode. First, rotational freedom around the bridge oxygen disrupts the Lewis base binding between two antimony centers. Second, intramolecular interactions between oxygen at catecholate and antimony competes with external Lewis base binding. Third, a high-polarity nucleophilic solvent, dimethylsulfoxide, is required to dissolve 2 due to the solubility and the solvent is also able to bind at antimony.

3-coordinate Sb(V)

²⁺ was not isolated. Instead, its Lewis adducts, ²⁺ and ⁺, were isolated. In the trigonal bipyramidal ²⁺, two OPPh₃ are located in axial positions and the Sb–O bond distance is similar to the sum of the covalent radii of Sb and O. In the distorted octahedral ⁺, the Sb–N distance with the dmap is shorter than reported N–Sb⁺ distances. This bond distance implies Lewis adduct formation. In addition, a reaction between dmap and ²⁺ forms ⁺. The experimental results indicate that ²⁺ is the Lewis acidic counterpart of these adducts.

4-coordinate Sb(V)

Tetrahedral stibonium cations also show Lewis acidity. Since ⁺ forms an adduct with triflate, the cation can be isolated as a salt. Short Sb–C bond distances of 2.095 Å and a tetrahedral space group in the crystal proves that isolated ⁺ is completely free of external electron donors. This cationic antimony Lewis acid shows strong acidity: firstly, ⁺ abstracts fluoride anion from weakly coordinating anions,, and secondly, the acidity measured by the Gutmann–Beckett method of ⁺ is comparable with that of the B₃ adduct in CH₂Cl₂.
SbPh₃⁺ was isolated as triflate salt. 6 has a tetrahedral structure like 5. In a solid state structure of a fluoride adduct, AntPh₃SbF, the incoming fluoride occupies the axial position of a trigonal bipyramidal structure, and the sterically-demanding anthryl is located at the equatorial site.

5-coordinate Sb(V)

Neutral Sb complexes are also Lewis acids. Compounds 7, 8 and 11 share the structure of spirocyclic stiborane. The LUMO of 8 mainly has its lobe at the antimony atoms and it renders 8 Lewis acidic. In detail, the LUMO can be assigned to as localized orbital on stiborafluorene moiety with larger nodes at the 9-position. Thus, Lewis bases bind towards trans to biphenylene and its fluoride adducts are asymmetric: 8·F⁻ has two enantiomers and 7·F⁻ has two diastereomers and four enantiomers.
A bisantimony complex is synthesized starting from xanthene. 9 has C₂ symmetry and the Sb–Sb distance is 4.7805 Å. Both antimony centers have distorted square pyramidal geometry with the geometry index τ₅ = 0.08. The base planes of the antimony centers meet face to face and this geometry allows 1:1 binding with F⁻, unlike 2.