Organoantimony-based Lewis acids are organoantimony compounds that exhibit the property of Lewis acidity. The high Lewis acidity of antimony pentafluoride has long been known, one consequence of which are non-coordinating anions ( and ). Another consequence is the use of SbF<sub>5</sub> to produce superacids (magic acids, fluoroantimonic acid). It follows that the behavior of SbF<sub>5</sub> could be replicated with organoantimony compounds. Some related compounds, including antimony(III) derivatives, also display Lewis acidity.
In general members of pnictogen group Lewis acidic compounds, Lewis-acidic antimony compounds have been investigated to extend the isolobal analogy between the vacant p orbital of borane and ÃÂ*(SbâÂÂX) orbitals of stiborane, and the similar electronegativities of antimony (2.05) and boron (2.04).
The unoccupied ÃÂ*(SbâÂÂX) oribital contributes to the Lewis acidity of antimony compounds in two ways: donorâÂÂacceptor 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 GutmannâÂÂBeckett method, Hammett acidity function, pK<sub>a</sub>, and fluoride ion affinity (FIA). 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, BF<sub>3</sub> and B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>, are respectively.
Since Lewis adducts are formed by dative bond between Lewis bases and Lewis acids, the orbital overlap between the Lewis base and ÃÂ*(SbâÂÂX) orbital is the source of the acidity. NBO analysis of the Sb(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>P(O)Ph<sub>3</sub> adduct indicates a donor-acceptor interaction between lp(O) and ÃÂ*(SbâÂÂC<sub>6</sub>F<sub>5</sub>).
Lowering the LUMO (ÃÂ*(SbâÂÂX)) energy increases the Lewis acidity. For example, Sb(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub> has a higher LUMO energy (âÂÂ0.55 eV) and weaker FIA (59 kcal/mol) than Sb(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> (âÂÂ1.76 eV and 89 kcal/mol).
Partial positive charges on the surface of antimony compounds interact with partial negative charges. For example, Sb(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(o-O<sub>2</sub>C<sub>6</sub>Cl<sub>4</sub>) has a more positively charged site than Sb(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> as shown in its electrostatic potential map, corresponding to higher Lewis acidity (the FIA of Sb(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>(o-O<sub>2</sub>C<sub>6</sub>Cl<sub>4</sub>) and Sb(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> are 116 and 89 kcal/mol, respectively).
Lewis acidic antimony complexes with a variety of oxidation states and coordination numbers are known. Several salient examples are introduced below.
Although stibanes have a lone pair electrons, their antibonding orbitals with electron-withdrawing substituents renders them Lewis acidic. Sb(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> (3) has three ÃÂ*(SbâÂÂC<sub>6</sub>F<sub>5</sub>) orbitals and three Lewis acidic sites. However, as shown in the electrostatic potential map of Sb(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>, only one site is accessible to Lewis bases due to the asymmetric arrangement of the three aryl rings.
In [Sb(tol)(Cp*)]<sup>2+</sup> (1), the ÷<sup>5</sup>-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(4) to 2.424(4) à, but the SbâÂÂC bond distances with the toluene are 2.993(5) to 3.182(5) à. This longer SbâÂÂtoluene distance implies toluene lability in solution.
Sb<sub>2</sub>(o-catecholate)<sub>2</sub>(ü-O) (2) 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 (3:1, 2:1, 1:1, 1:2, 1:3). 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.
[SbPh<sub>3</sub>]<sup>2+</sup> (4) was not isolated. Instead, its Lewis adducts, [SbPh<sub>3</sub>(OPPh<sub>3</sub>)<sub>2</sub>]<sup>2+</sup> and [SbPh<sub>3</sub>(dmap)<sub>2</sub>(OTf)]<sup>+</sup>, were isolated. In the trigonal bipyramidal [SbPh<sub>3</sub>(OPPh<sub>3</sub>)<sub>2</sub>]<sup>2+</sup>, two OPPh<sub>3</sub> are located in axial positions and the SbâÂÂO bond distance (2.102(2) à) is similar to the sum of the covalent radii of Sb and O (2.05 à). In the distorted octahedral [SbPh<sub>3</sub>(dmap)<sub>2</sub>(OTf)]<sup>+</sup>, the SbâÂÂN distance with the dmap (2.222(2) à) is shorter than reported NâÂÂSb<sup>+</sup> distances. This bond distance implies Lewis adduct formation. In addition, a reaction between dmap and [SbPh<sub>3</sub>(OPPh<sub>3</sub>)<sub>2</sub>]<sup>2+</sup> forms [SbPh<sub>3</sub>(dmap)<sub>2</sub>(OTf)]<sup>+</sup>. The experimental results indicate that [SbPh<sub>3</sub>]<sup>2+</sup> is the Lewis acidic counterpart of these adducts.
Tetrahedral stibonium cations also show Lewis acidity. Since [Sb(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>+</sup> (5) forms an adduct with triflate, the cation can be isolated as a [Sb(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] salt. Short SbâÂÂC bond distances of 2.095(2) àand a tetrahedral space group in the crystal proves that isolated [Sb(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>+</sup> is completely free of external electron donors. This cationic antimony Lewis acid shows strong acidity: firstly, [Sb(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>+</sup> abstracts fluoride anion from weakly coordinating anions, , and secondly, the acidity measured by the GutmannâÂÂBeckett method of [Sb(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]<sup>+</sup> (5) is comparable with that of the B(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub> adduct in CH<sub>2</sub>Cl<sub>2</sub> (76.6 ppm).
SbPh<sub>3</sub>(Ant)<sup>+</sup> (6) (where Ant is 9-anthryl) was isolated as triflate salt. 6 has a tetrahedral structure like 5. In a solid state structure of a fluoride adduct, AntPh<sub>3</sub>SbF, the incoming fluoride occupies the axial position of a trigonal bipyramidal structure, and the sterically demanding anthryl is located at the equatorial site.
Neutral Sb(V) 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 (Sb). Thus, Lewis bases bind towards trans to biphenylene and its fluoride adducts are asymmetric: 8÷F<sup>âÂÂ</sup> has two enantiomers and 7÷F<sup>âÂÂ</sup> has two diastereomers and four enantiomers.
A bisantimony complex (9) is synthesized starting from xanthene. 9 has C<sub>2</sub> symmetry and the SbâÂÂSb distance is 4.7805(7) à. Both antimony(V) centers have distorted square pyramidal geometry with the geometry index ÃÂ<sub>5</sub> = 0.08. The base planes of the antimony centers meet face to face and this geometry allows 1:1 binding with F<sup>âÂÂ</sup>, unlike 2.
Introduction of electron-withdrawing substituents on antimony results in increased acidity. For example, intramolecular donorâÂÂacceptor interactions of two stiboranes, o-C<sub>6</sub>H<sub>4</sub>(PPh<sub>2</sub>)[SbPh<sub>2</sub>(O<sub>2</sub>C<sub>6</sub>Cl<sub>4</sub>)] and o-C<sub>6</sub>H<sub>4</sub>(PPh<sub>2</sub>)[Sb(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>(O<sub>2</sub>C<sub>6</sub>Cl<sub>4</sub>)], have been analyzed by AIM. AIM analysis of electron density at the bond critical point (bcp) and delocalization index indicates that electron-withdrawing substituents on Sb(V) lead to an increased PâÂÂSb bonding covalency.
A bisantimony complex (9) is a stronger Lewis acid than a monoantimony compound (8) because both Lewis acidic sites cooperatively contribute to the Lewis base binding. The electrostatic potential map of 9 shows positive charges on the Sb centers facing each other.
This cooperativity is supported by the Sb-(ü-F)-Sb moiety in solid state structure of F<sup>âÂÂ</sup> binding bisantimony compound 9.
The 9-anthryltriphenylstibonium cation shows weak fluorescent emission (æ = 0.7%), but a corresponding fluoride adduct, fluorostiborane, shows a strong anthryl-based emission at 427 nm (æ = 9.5% in CHCl<sub>3</sub>). Fluoride-selective electrodes were developed by using Lewis acidic antimony compounds as ionophores.
Strongly acidic antimony compounds catalyze transformations such as the transfer hydrogenation and the Ritter reaction.
Tetraarylstibonium cations catalyze cycloadditions between epoxides and CO<sub>2</sub> or isocyanate to produce oxazolidinones.
Lewis acidic antimony compounds can act as Z-type ligands. Owing to the strong ÃÂ-accepting ability of dicationic Sb ligand, a gold-antimony complex can catalyze styrene polymerization and hydroamination after being activated by AgNTf<sub>2</sub>.