Mono and Diorganoantimony compounds –A Review Abstract

Mono and Diorganoantimony compounds –A Review
Abstract: The chemistry of both mono and di-organometallic compounds of antimony has been
reviewed. Synthesis and structure of their compounds have been described.
Introduction:
Antimony complexes have been used in the field of medicine and cosmetics.1,2 For the
treatment of various parasitic diseases antimony containing compounds are commonly used. For
example, sodium antimony(V) gluconate is being used as a drug.3 Due to the fascinating
structural diversity varying from discrete monomeric molecular species to supramolecular
assemblies the chemistry of organoantimony(V) complexes has attracted significant attention in
recent times.4 Similar to that of cis-platin, organoantimony derivatives also exhibit significant
antimicrobial properties as well as antitumor activities5 which is associated with cytostatic
activity.6 The biological toxicity of Sb is less than Pt and Pd based anticancer substances. In
addition, organoantimony derivatives also show important functions as biocides, fungicides,
catalyst components and antioxidants. Antimony in the oxidation state of +5 is interesting
considering its hypervalent nature.7 In organic synthesis organoantimony(V) compounds have
been used either as reagents or as catalysts extensively.8
Biological activity:
Antimonials Since 1913, has been used in the field of therapeutic agent with the
introduction of Sb(III) potassium tartarate in the treatment of leishmaniasis. Pentavalent
antimonials have replaced trivalent antimonials due to less toxic nature of Sb(V) compounds
such as meglumine antimoniate for treating diseases. Meglumine antimoniate is recommended
by WHO as a first choice medicine for leishmaniasis therapy.9 Bayer in 1915 introduced the first
organometallic fungicides Upsulun which is an organomercurial compound. Since
organomecurial compounds are environment unfriendly, Beiter and Leebrick 1963 chose a series
of tri and pentavalent organoantimony and organobismuth compounds and examined their
activity against fungicides. It was found that organoantimony are moderately fungitoxic and
more effective than organobismuth compounds. Burrell and Corke in 1980 studied the
fungitoxicity of organo antimony compounds and found that fungal toxicity activity increased
with increasing the molecular weight of organic compound attached to group VA element.10

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Cristian Silvestru et al investigated the organoantimony(III) derivatives of
dithiophosphorus ligand which were found to shows antitumor properties in both in vitro and in
vivo studies.11 Cytotoxic activity to vascular endothelical cell was diminished when they
replaced the bismuth atom by antimony atom in 2-(N,N-dimethylaminomethyl)phenylbis(4-
methyl phenyl)bismuthane (DAPBi).12 (C6F5)2SbPh shows antifungal, antibacterial and
insecticidal activities. This compound has also been used as pesticide and insecticide for plant
diseases as reported by R. Kant et al. Later the group also synthesized the diaryl antimony(III)
amide which exhibits antitumor activity against mammary cancer cell line and human breast
adenocarcinoma cell line. In addition it also shows antibacterial activity against Pseudomonas
aeruginosa, Staphylococcus aureus, and Klebsiela pneumonia. It exhibits significant antifungal
properties against Aspergillus flavus and Aspergillus niger.13
Organoantimony(V) derivatives from Schiff bases exhibits higher antimicrobial activity
than organoantimony(III) derivatives against Aspergillus flavus and Escherichia coli.14 Potent
antimicrobial properties are revealed in three discrete organoantimony(III) containing
heteropolytungstates which was recently documented by U. Kortz et al.15 Organostibonic acids
have also been used as potential anticancer agents, which inhibits the DNA binding16a,b group of
B-ZIP proteins at micromolar concentration have been recently reported by Vinson et al.16c-e
Monoorganoantimony Compounds:
Monoorganoantimony(V) halides, due to their unstable nature, structural aspects of
mono-alkyl and arylantimony(V) chlorides are not fully understood.17 For example at room
temperature phenylantimony tetrachloride tends to disproportionate. Monoalkylantimony
tetrachlorides also undergoes decomposition ready.

Scheme 1
In order to stabilize the monoorganoantimony(V) chlorides, addition of neutral oxo donor
ligands have been used which stabilize the compound at room temperature. On the basis of
spectral analysis such as IR and 1H NMR it was suggested that the antimony atom is

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hexacoordinated. In solution state, CH3SbCl4·L (L = PyO or 4-CH3PyO) exists as monomer in
cis and trans forms. Above 70 ?C CH3SbCl4.L, it undergo decomposition giving rise to CH3Cl
and SbCl3L.18
?-diketones derivatives:
Monoorganoantimony (acetylacetonato) trichlorides have been synthesized either by the
reactions of phenylstibonic acid in HCl or monoorganoantimony(V) chloride with acetylacetone
at low temperature condition.19 In solution state, monoorganoantimony(acetylacetonato)
trichlorides are monomeric. The IR spectra reveal that these compounds show acetylacetone
bidentate ligation. On the basis of the observed doublet and singlet for acetylacetone-CH3 in the
1H NMR spectra, Okawara and co-workers have suggested an asymmetric octahedral structure
for PhSb(acac)Cl3 and a symmetric structure for MeSb(acac)Cl3 in which methyl group occupys
an axial position (Figure 1). Kawasaki et al have studied 1H NMR for PhSb(acac)Cl3 which
shows asymmetric octahedral structure were the phenyl group occupies an equatorial position.
For MeSb(acac)Cl3 the magnitude of the separation between methyl resonance increases
in aromatic solvents indicating an enhancement in the non-equivalance of two acetylacetone-CH3
groups as a result of the aromatic ring current effect on asymmetrically solvated solute molecule.
The dipole moment data also supports the asymmetric geometry for these compounds. Later the
single crystal X-ray diffraction analysis of MeSb(acac)Cl3 support the asymmetric structure with
distorted octahedral geometry.20

Figure 1: Asymmetric structure of an MeSb(acac)Cl3

The three-membered organoantimony ring cyclo-Sb3R3 R = (Me3Si)2CH21a was
prepared by Breunig and co-workers by reaction of RSbCl2 with Li3Sb at -40 °C. The alkyl
groups are occupying cis and trans positions. The cis-trans positions of the substituents are

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identified by 1H and 13C NMR spectra in solution state. The three antimony atoms present almost
resemble an equilateral triangular motif (Scheme 2).

Scheme 2
Polyoxometalates:
The first organoantimony-containing POM {PhSbOH}3(A-?-PW9O34)29-, is a sandwich-
type tungstophosphate which is dimeric in nature (Figure 2). The polyanion can be synthesized
by the direct interaction of diphenylantimony trihalide with three different lacunary
tungstophosphate precursors, Na9(A-?-PW9O34, K7PW11O39, or K10P2W20O70(H2O)2, in an
aqueous acidic medium under hydrothermal conditions. The cluster formation takes place by
removal of one of the phenyl group during its reaction with diphenylantimony trihalide.21b

Figure 2: Molecular structure of {PhSbOH}3(A-?-PW9O34)29-

Diorganoantimony Compounds:
Synthesis of diphenylantimony trichloride:

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Diorganoantimony(V) halides have been synthesized by treating antimony(III) chlorides
with diazonium salts or by halogenations of diorganoantimony(III) halides R2SbX with X2 (X =
Cl, Br) (Scheme 3).22

Scheme 3
Some of the reducing agents like stannous chloride, sulfur dioxide are used to reduce
diorganoantimony(V) halides to diorganoantimony(III) halides.23
Dimeric structure of diphenylantimony trichloride:
Michaelis and Reese first synthesized diphenylantimony trichloride where they obtained
it as a monohydrate. When heating the monohydrated compound to 100 ?C they readily obtained
the anhydrous compound. Initially there was uncertanity about the structure of Ph2SbCl3.
Bordner et al resolved the ambiguity by examining the single-crystal X-ray diffraction data of
the anhydrous Ph2SbCl3 and found that it exists as a dimer with chlorine bridges and geometry
around antimony was found to be octahedral (Figure 3).24

Figure 3: Dimeric structure of diphenylantimony trichloride.
Bone and Sowerby synthesized diarylantimony(V) tribromide (Ph2SbBr3) and the two
mixed halides Ph2SbBr2Cl and Ph2SbBrCl2.25 These compounds are monomeric in solid state and
geometry around antimony was found to be trigonal-bipyramidal with two phenyl groups and a

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bromine atom occupying the equatorial positions. Due to weak intermolecular interactions
arising between axial halogen atom and antimony, these units are linked to form infinite chains
in solid state. On the other hand when compared with above mentioned compounds Ph2SbCl3
shows a dimeric nature in solid state.
With various oxygen donor ligands such as DMSO and HMPA diorganoantimony(V)
halides forms monomeric covalent adducts (R2SbX3.L). Octahedral geometry has been proposed
on the basis of IR and NMR spectra.26
Diorganoantimony ?-diketone derivatives:
Diorganoantimony trihalide when treated with acetylacetone under reflux condition gave rise to
diorganoantimony(V) ?-diketones, Ph2Sb(CH3COCHCOCH3)Cl2 (Scheme 4) which are
monomeric in nature (Figure 4).27

Scheme 4

Figure 4: Diphenyldichloro(acetylacetonato)antimony & Dimethyldichloro(acetylacetonato)antimony.

?-diketonate ligand acts as bidentate ligand in both the complexes and the oxygen atoms
from acetylacetone bonds to antimony atom. Interpretations drawn from 1H NMR spectral
studies have been confirmed by single crystal X-ray diffraction analysis of Me2Sb(acac)Cl2 and
Ph2Sb(acac)Cl2.28,29 Me2Sb(acac)Cl2 compound possess a slightly distorted octahedral geometry
around antimony in which methyl groups occupy equatorial positions and are bent towards the
planar acetylacetonato group. In compound Ph2Sb(acac)Cl2 the geometry around antimony was
distorted octahedral in which the two chlorine are arranged trans to each other (Figure 4).

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Schiff base derivative of diorganoantimony:
Diorganoantimony(V) complexes with planar tridentate schiff base ligands (Trid) have
been prepared by the exchange reactions of diorganoantimony(V) chlorides with corresponding
schiff bases of trimethylantimony(V) or dimethyltin(IV) compounds (Scheme 5). These reactions
proceed due to greater Lewis acidity of R2Sb(V) compared to Me3Sb(V) or Me2Sn(IV).30 Planar
tridentate ligands (Trid) coordinate to antimony in ONO fashion, the chelating ONO atoms
assumed to be arranged in a meridional fashion and a linear C-Sb-C skeleton has been proposed
based on IR and 1H NMR studies. Certain compounds such as Me2Sb(Sah)Cl, Me2Sb(Bah)Cl,
Ph2Sb(Bah)Cl and Ph2Sb(Aah)Cl shows octahedral geometry with meridional arrangement as
proven by Mossbauer spectroscopy (Figure 5).31

Scheme 5

Figure 5: Meridional arrangement of tridentate Schiff base ligand around antimony.
Reaction with Silver salt of phosphinates:
Diphenylantimony trichloride when treated with two equivalents of silver salts of
phosphinates leads to the isolation of partially hydrolyzed product {SbPh2ClO2P(C6H11)2}2O
(Scheme 6).7a Single crystal X-ray diffraction studies reveals that antimony atoms are in
octahedral coordination with bridging phophinates cis to each other. The phosphinates in this
compound only acts a bridging ligand.

Scheme 6

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Kumara Swamy and co-workers reported antimony(V) phosphinates by reacting
diphenylantimony trichloride with three equivalence of silver acetate followed by one
equivalence of phosphinic acid leading to the isolation of dimeric compounds of formula
Ph2Sb(O2PR2)O2. Interestingly when the dimer was reacted with acetic acid / water gives the
tetra nuclear cage of formula Ph8Sb4O4(OH)2(O2P(C6H11)2)2 (Scheme 7).32

Scheme 7

Scheme 8
All the compounds are structurally characterized by single crystal X-ray diffraction
analysis. In the di and the tetra nuclear clusters, the antimony atoms are octahedral coordinated
with four membered Sb2O2 rings. In tetranuclear cluster two Sb2O2 rings are linked by oxo
bridges on two sides to give a Sb4O6 cage (Scheme 8).
Diorganoantimony based µ4-peroxo complex:
The first main group element µ4-peroxo complex of antimony was synthesized by treating
tetra-o-tolyldistibane in air and subsequent reaction with H2O2 and the intermediate (o-
Tol2Sb4)O6 (Scheme 9) was identified by mass spectrometry. The complex is stable in solution

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state. Single crystal X-ray diffraction studies reveal that antimony atoms are arranged as in the
vertices of a square planar arrangement (Figure 6).33

Scheme 9

Figure 6: µ4-peroxo Complex of Antimony

Synthesis of Quadruply bridged diorganoantimony compound:

Quadruply bridged diorganoantimony compounds (SbPh2)2(µ-O)2(µ-O2AsR2)2,34 where
R = Me or Ph have been synthesized by reacting (SbPh2BrO)2 with 2 moles of either
Na(O2AsMe2) or Na(O2AsPh2) in DCM and the mixture was refluxed for 24 h. The compounds
have been characterized by a various spectroscopic methods and analytical methods (Figure 7).

Figure 7: Quadruply bridged diorganoantimony compounds.

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Conclusion: This review describes the synthesis, spectroscopic and structure aspects of organ
antimony compounds.
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