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[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

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.[6][3]: 442  A typical synthesis first displaces an SbIII 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

SbV-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] 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 

Lewis acids

Among pnictogen group Lewis acidic compounds,[10][11][12][13] unusual lewis acidity of Lewis acidic antimony compounds have long been exploited as both stable conjugate acids of non-coordinating anions (SbF−
6 and Sb
2F−
11),[14] and strong Lewis acid counterparts of well-known superacids (magic acids, fluoroantimonic acid). 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*(Sb–X) orbitals of stiborane, and the similar electronegativities of antimony (2.05) and boron (2.04).[15]

Origin of acidity

σ*(Sb–X), 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, pKa, 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, BF3 and B(C6F5)3, are 81 and 106 kcal/mol (340 and 440 kJ/mol) respectively.[14]

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*(Sb–X) orbital is the source of the acidity. According to Gabbaï et al., NBO analysis of the Sb(C6F5)3P(O)Ph3 adduct indicates a donor-acceptor interaction between lp(O) and σ*(Sb–C6F5).[16][17]

Lowering the LUMO (σ*(Sb–X)) energy increases the Lewis acidity. For example, Sb(C6H5)3 has a higher LUMO energy (−0.55 eV) and weaker FIA (59 kcal/mol) than Sb(C6F5)3 (−1.76 eV and 89 kcal/mol).[16]

Electrostatic interaction

Partial positive charges on the surface of antimony compounds interact with partial negative charges. For example, Sb(C6F5)3(o-O2C6Cl4) has a more positively charged site than Sb(C6F5)3 as shown in its electrostatic potential map, corresponding to higher Lewis acidity (the FIA of Sb(C6F5)3(o-O2C6Cl4) and Sb(C6F5)3 are 116 and 89 kcal/mol, respectively).[16]

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.

3-coordinate Sb(III)

Although stibanes have a lone pair electrons, their antibonding orbitals with electron-withdrawing substituents renders them Lewis acidic. Sb(C6F5)3 (3) has three σ*(Sb–C6F5) orbitals and three Lewis acidic sites. However, as shown in the electrostatic potential map of Sb(C6F5)3, only one site is accessible to Lewis bases due to the asymmetric arrangement of the three aryl rings.[19]

In [Sb(tol)(Cp*)]2+ (1), 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(4) to 2.424(4) Å, but the Sb–C bond distances with the toluene are 2.993(5) to 3.182(5) Å. This longer Sb–toluene distance implies toluene lability in solution.[18]

Sb2(o-catecholate)2(μ-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.[20]

3-coordinate Sb(V)

[SbPh3]2+ (4) was not isolated. Instead, its Lewis adducts, [SbPh3(OPPh3)2]2+ and [SbPh3(dmap)2(OTf)]+, were isolated. In the trigonal bipyramidal [SbPh3(OPPh3)2]2+, two OPPh3 are located in axial positions and the Sb–O bond distance (2.102(2) Å) is similar to the sum of the covalent radii of Sb and O (2.05 Å). In the distorted octahedral [SbPh3(dmap)2(OTf)]+, the Sb–N distance with the dmap (2.222(2) Å) is shorter than reported N–Sb+ distances. This bond distance implies Lewis adduct formation. In addition, a reaction between dmap and [SbPh3(OPPh3)2]2+ forms [SbPh3(dmap)2(OTf)]+. The experimental results indicate that [SbPh3]2+ is the Lewis acidic counterpart of these adducts.[21]

4-coordinate Sb(V)

Tetrahedral stibonium cations also show Lewis acidity. Since [Sb(C6F5)4]+ (5) forms an adduct with triflate, the cation can be isolated as a [Sb(C6F5)4][B(C6F5)4] salt. Short Sb–C bond distances of 2.095(2) Å and a tetrahedral space group in the crystal proves that isolated [Sb(C6F5)4]+ is completely free of external electron donors. This cationic antimony Lewis acid shows strong acidity: firstly, [Sb(C6F5)4]+ abstracts fluoride anion from weakly coordinating anions, SbF−
6, and secondly, the acidity measured by the Gutmann–Beckett method of [Sb(C6F5)4]+ (5) is comparable with that of the B(C6F5)3 adduct in CH2Cl2 (76.6 ppm).[22]

SbPh3(Ant)+ (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, AntPh3SbF, the incoming fluoride occupies the axial position of a trigonal bipyramidal structure, and the sterically-demanding anthryl is located at the equatorial site.[23]

5-coordinate Sb(V)

Neutral Sb(V) complexes are also Lewis acids.[24] 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− has two enantiomers and 7·F− has two diastereomers and four enantiomers.

A bisantimony complex (9) is synthesized starting from xanthene. 9 has C2 symmetry and the Sb–Sb distance is 4.7805(7) Å. Both antimony(V) 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.[25]

Inductive effect

Introduction of electron-withdrawing substituents on antimony results in increased acidity. For example, intramolecular donor–acceptor interactions of two stiboranes, o-C6H4(PPh2)[SbPh2(O2C6Cl4)] and o-C6H4(PPh2)[Sb(C6F5)2(O2C6Cl4)], 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 P–Sb bonding covalency.[26]

AIM analysis of the intramolecular donor-acceptor interactions of two stiboranes[26]
P-Sb bonding o-C6H4(PPh2)(Sb(C6F5)2(O2C6Cl4) o-C6H4(PPh2)(Sb(C6H5)2(O2C6Cl4)
Electron density at the bcp (e/rBohr3) 0.054 0.035
Delocalization index 0.38 0.24

Bisantimony compounds vs mono-antimony compounds

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-(μ-F)-Sb moiety in solid state structure of F− binding bisantimony compound 9.[25]

Reactions

Fluoride anion sensor

The 9-anthryltriphenylstibonium cation (5) shows weak fluorescent emission (Φ = 0.7%), but a corresponding fluoride adduct, fluorostiborane, shows a strong anthryl-based emission at 427 nm (Φ = 9.5% in CHCl3). Owing to its stability in water, 5 could in principle be used as an aqueous fluoride sensor.[23][24][27] Fluoride-selective electrodes were developed by using Lewis acidic antimony compounds as ionophores.[28]

Catalysis

Strongly acidic antimony compounds catalyze organic transformations such as the transfer hydrogenation and the Ritter reaction.[16]

Tetraarylstibonium cations catalyze cycloadditions between epoxides and CO2 or isocyanate to produce oxazolidinones.[29]

Lewis acidic antimony compounds can act as Z-type ligands. Owing to the strong σ-accepting ability of dicationic Sb ligand, a gold-antimony complex can catalyze styrene polymerization and hydroamination after being activated by AgNTf2.[30]

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. 1 2 3 4 5 6 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. ↑ Breunig, Hans Joachim; Häberle, Karl; Dräger, Martin; Severengiz, Tevfik (1985). "(C6H5Sb)6·(1,4-dioxane), the First Cyclohexastibane". Angewandte Chemie International Edition in English. 24: 72–73. doi:10.1002/anie.198500721.
  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. 1 2 3 4 5 6 7 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.
  10. ↑ Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W. (2013-09-20). "Lewis Acidity of Organofluorophosphonium Salts: Hydrodefluorination by a Saturated Acceptor". Science. 341 (6152): 1374–1377. Bibcode:2013Sci...341.1374C. doi:10.1126/science.1241764. ISSN 0036-8075. PMID 24052304. S2CID 35796905.
  11. ↑ Bayne, J. M.; Stephan, D. W. (2016). "Phosphorus Lewis acids: emerging reactivity and applications in catalysis". Chemical Society Reviews. 45 (4): 765–774. doi:10.1039/C5CS00516G. ISSN 0306-0012. PMID 26255595.
  12. ↑ Zhou, Jiliang; Liu, Liu Leo; Cao, Levy L.; Stephan, Douglas W. (2019-04-08). "The Arene-Stabilized η5-Pentamethylcyclopentadienyl Arsenic Dication [(η5-Cp*)As(toluene)] 2+". Angewandte Chemie International Edition. 58 (16): 5407–5412. doi:10.1002/anie.201902039. ISSN 1433-7851. PMID 30802353. S2CID 73465935.
  13. ↑ Ramler, Jacqueline; Hofmann, Klaus; Lichtenberg, Crispin (2019-12-31). "Neutral and Cationic Bismuth Compounds: Structure, Heteroaromaticity, and Lewis Acidity of Bismepines". Inorganic Chemistry. 59 (6): 3367–3376. doi:10.1021/acs.inorgchem.9b03189. ISSN 0020-1669. PMID 31891491. S2CID 209523568.
  14. 1 2 Krossing, Ingo; Raabe, Ines (2004-04-13). "Noncoordinating Anions—Fact or Fiction? A Survey of Likely Candidates". Angewandte Chemie International Edition. 43 (16): 2066–2090. doi:10.1002/anie.200300620. ISSN 1433-7851. PMID 5083452.
  15. ↑ Marczenko, Katherine M.; Zurakowski, Joseph A.; Bamford, Karlee L.; MacMillan, Joshua W. M.; Chitnis, Saurabh S. (2019-12-09). "Hydrostibination". Angewandte Chemie International Edition. 58 (50): 18096–18101. doi:10.1002/anie.201911842. ISSN 1433-7851. PMID 31591801. S2CID 242968313.
  16. 1 2 3 4 5 6 7 Yang, Mengxi; Tofan, Daniel; Chen, Chang-Hong; Jack, Kevin M.; Gabbaï, François P. (2018-10-15). "Digging the Sigma-Hole of Organoantimony Lewis Acids by Oxidation". Angewandte Chemie International Edition. 57 (42): 13868–13872. doi:10.1002/anie.201808551. ISSN 1433-7851. PMID 30151881. S2CID 52099143.
  17. ↑ Glendening, Eric D.; Landis, Clark R.; Weinhold, Frank (January 2012). "Natural bond orbital methods". WIREs Computational Molecular Science. 2 (1): 1–42. doi:10.1002/wcms.51. ISSN 1759-0876. S2CID 95586513.
  18. 1 2 Zhou, Jiliang; Kim, Hyehwang; Liu, Liu Leo; Cao, Levy L.; Stephan, Douglas W. (2020). "An arene-stabilized η 5 -pentamethylcyclopentadienyl antimony dication acts as a source of Sb + or Sb 3+ cations". Chemical Communications. 56 (85): 12953–12956. doi:10.1039/D0CC02710C. ISSN 1359-7345. PMID 32985631. S2CID 222166864.
  19. ↑ Benz, Sebastian; Poblador-Bahamonde, Amalia I.; Low-Ders, Nicolas; Matile, Stefan (2018-05-04). "Catalysis with Pnictogen, Chalcogen, and Halogen Bonds". Angewandte Chemie International Edition. 57 (19): 5408–5412. doi:10.1002/anie.201801452. PMC 5947745. PMID 29558562.
  20. ↑ Qiu, J.; Song, B.; Li, X.; Cozzolino, A. F. (2018). "Solution and gas phase evidence of anion binding through the secondary bonding interactions of a bidentate bis-antimony(III) anion receptor". Physical Chemistry Chemical Physics. 20 (1): 46–50. Bibcode:2018PCCP...20...46Q. doi:10.1039/C7CP05933G. ISSN 1463-9076. PMID 29226921.
  21. ↑ Robertson, Alasdair P. M.; Burford, Neil; McDonald, Robert; Ferguson, Michael J. (2014-03-24). "Coordination Complexes of Ph3Sb2+ and Ph3Bi2+ : Beyond Pnictonium Cations". Angewandte Chemie International Edition. 53 (13): 3480–3483. doi:10.1002/anie.201310613. PMID 24616180.
  22. ↑ Pan, Baofei; Gabbaï, François P. (2014-07-09). "[Sb(C6F5)4][B(C6F5)4]: An Air Stable, Lewis Acidic Stibonium Salt That Activates Strong Element—Fluorine Bonds". Journal of the American Chemical Society. 136 (27): 9564–9567. Bibcode:2014JAChS.136.9564P. doi:10.1021/ja505214m. ISSN 0002-7863. PMID 24946107.
  23. 1 2 3 Ke, Iou-Sheng; Myahkostupov, Mykhaylo; Castellano, Felix N.; Gabbaï, François P. (2012-09-19). "Stibonium Ions for the Fluorescence Turn-On Sensing of F – in Drinking Water at Parts per Million Concentrations". Journal of the American Chemical Society. 134 (37): 15309–15311. Bibcode:2012JAChS.13415309K. doi:10.1021/ja308194w. ISSN 0002-7863. PMID 22954306.
  24. 1 2 Hirai, Masato; Gabbaï, François P. (2014). "Lewis acidic stiborafluorenes for the fluorescence turn-on sensing of fluoride in drinking water at ppm concentrations". Chem. Sci. 5 (5): 1886–1893. doi:10.1039/c4sc00343h. hdl:1969.1/185530. ISSN 2041-6520.
  25. 1 2 3 Hirai, Masato; Gabbaï, François P. (2015-01-19). "Squeezing Fluoride out of Water with a Neutral Bidentate Antimony(V) Lewis Acid". Angewandte Chemie (in German). 127 (4): 1221–1225. Bibcode:2015AngCh.127.1221H. doi:10.1002/ange.201410085. PMID 25424599.
  26. 1 2 3 Tofan, Daniel; Gabbaï, François P. (2016). "Fluorinated antimony(V) derivatives: strong Lewis acidic properties and application to the complexation of formaldehyde in aqueous solutions". Chemical Science. 7 (11): 6768–6778. doi:10.1039/C6SC02558G. ISSN 2041-6520. PMC 5363782. PMID 28451122.
  27. ↑ Lee, Min Hyung; Agou, Tomohiro; Kobayashi, Junji; Kawashima, Takayuki; Gabba?, Fran?ois P. (2007). "Fluoride ion complexation by a cationic borane in aqueous solution". Chemical Communications (11): 1133–5. doi:10.1039/b616814k. ISSN 1359-7345. PMID 17347716.
  28. ↑ Li, Long; Zhang, Yihao; Li, Ying; Duan, Yinghui; Qian, Yi; Zhang, Peidong; Guo, Qingjie; Ding, Jiawang (2020-10-28). "Polymeric Membrane Fluoride-Selective Electrodes Using Lewis Acidic Organo-Antimony(V) Compounds as Ionophores". ACS Sensors. 5 (11): 3465–3473. doi:10.1021/acssensors.0c01481. PMID 33112603. S2CID 226054990.
  29. ↑ Yang, Mengxi; Pati, Nilanjana; Bélanger-Chabot, Guillaume; Hirai, Masato; Gabbaï, François P. (2018). "Influence of the catalyst structure in the cycloaddition of isocyanates to oxiranes promoted by tetraarylstibonium cations". Dalton Transactions. 47 (34): 11843–11850. doi:10.1039/C8DT00702K. ISSN 1477-9226. PMID 29697133.
  30. 1 2 Lo, Ying-Hao; Gabbaï, François P. (2019-07-22). "An Antimony(V) Dication as a Z-Type Ligand: Turning on Styrene Activation at Gold". Angewandte Chemie International Edition. 58 (30): 10194–10197. doi:10.1002/anie.201903964. ISSN 1433-7851. PMID 31111593.
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