Synthesis and properties of triphenylantimony(V) dithiolate derivative

Article abstract of DOI:10.1134/S1070363213100174

Interaction of substituted benzene-1,2-dithiols I-III with triphenylantimony(V) dichloride was studied. As a result of the exchange reaction of 3,6-dichlorobenzene-1,2-dithiol with Ph₃SbCl₂ in the presence of triethylamine triphenylantimony(V) 3,6-dichlorobenzene-1,2- dithiolate IV was obtained. Electrochemical properties of the free dithiols and the complex were explored. According to the cyclic voltammetry, the oxidation of the complex is irreversible and corresponds to one electron transfer with follow-up chemical stage. The antioxidant activity of compounds I-IV was examined in the process of oleic acid autoxidation. A more effective inhibition of the oleic acid oxidation was found at adding dithiol and better protective effect was observed in the presence of Ph₃SbCl₂ compared with the complex IV.

Full text of DOI:10.1134/S1070363213100174

ISSN 1070-3632, Russian Journal of General Chemistry, 2013, Vol. 83, No. 10, pp. 1900–1905. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © S.A. Smolyaninova, A.I. Poddel’sky, I.V. Smolyaninov, N.T. Berberova, 2013, published in Zhurnal Obshchei Khimii, 2013, Vol. 83,
No. 10, pp. 1693–1699.
Synthesis and Properties of Triphenylantimony(V) Dithiolate
Derivative
S. A. Smolyaninovaa, A. I. Poddel’skyb, I. V. Smolyaninovc, and N. T. Berberovaa
a Astrakhan State Technical University, ul. Tatishcheva 16, Astrakhan, 414025 Russia
e-mail: thiophen@mail.ru
b Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, Nizhni Novgorod, Russia
c Southern Scientific Center of the Russian Academy of Sciences, Astrakhan, Russia
Received October 18, 2012
Abstract—Interaction of substituted benzene-1,2-dithiols I–III with triphenylantimony(V) dichloride was
studied. As a result of the exchange reaction of 3,6-dichlorobenzene-1,2-dithiol with Ph3SbCl2 in the presence
of triethylamine triphenylantimony(V) 3,6-dichlorobenzene-1,2-dithiolate IV was obtained. Electrochemical
properties of the free dithiols and the complex were explored. According to the cyclic voltammetry, the
oxidation of the complex is irreversible and corresponds to one electron transfer with follow-up chemical stage.
The antioxidant activity of compounds I–IV was examined in the process of oleic acid autoxidation. A more
effective inhibition of the oleic acid oxidation was found at adding dithiol and better protective effect was
observed in the presence of Ph3SbCl2 compared with the complex IV.
DOI: 10.1134/S1070363213100174
We have previously shown that complexes of
triphenylantimony(V) with catecholate and o-amido-
phenolate ligands exhibit an antiradical activity [1].
The common feature of these compounds is the ability
to reversibly bind molecular oxygen [2–4]. The
presence of electron-withdrawing substituents in the
ligand favors the formation of the catecholates stable
in air, while the antimony(V) complex containing
donor groups can fix molecular oxygen [5]. The
variation the nature of substances in the redox-active
ligand affects the antioxidant capacity of these
compounds in the process of free radical oxidation of
unsaturated fatty acids and lipids. o-Amidophenolate
and catecholate complexes of triphenylantimony(V)
with donor groups prevent oxidation and promote the
decomposition of hydroperoxides. In contrast to the
above-mentioned compounds, the catecholate triphenyl-
antimony(V) derivatives with acceptor substituents
have only a short-term inhibitory effect.
antimony(V) compounds containing a redox-active
ligands the oxidation potential may be one of the
defining characteristics of the antioxidant activity [6].
The structure of the previously studied complexes
includes a five-membered metallocycle formed by the
antimony(V) ion and a catecholate (o-amidophenolate)
ligand capable of existing in several redox levels [7].
Replacing the heteroatoms in the ring for sulfur should
affect significantly the properties of these compounds.
Currently, the literature provides some examples of the
antimony(III/V) dithiolate complexes [8, 9], but data
on organometallic compounds of antimony(V) with
sulfur-containing bidentate ligands have not been found.
In order to find new antimony compounds pos-
sessing unusual properties we carried out a reaction
between triphenylantimony(V) chloride with benzene-
1,2-dithiol (I), 4-methylbenzene-1,2-dithiol (II) and
3,6-dichlorobenzene-1,2-dithiol (III). We studied the
electrochemical properties, antioxidant activity of free
dithiols and the resulting complex IV in the inhibition
of the oxidation of oleic (cis-9-octadeca-enoic) acid.
Study of electrochemical properties of the com-
plexes showed that a correlation existed between the
inhibition efficiency toward the oleic acid free radical
oxidation and the compound oxidation potential: the
easier it releases an electron, the higher its inhibition
efficiency. Consequently, in the series of the triphenyl-
The exchange reaction of dithiols (I–III) with
Ph3SbCl2 occurs in the presence of triethylamine.
Under the anaerobic conditions the reaction with
1900

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SYNTHESIS AND PROPERTIES OF TRIPHENYLANTIMONY(V) DITHIOLATE DERIVATIVE
Oxidation potentials of dithiols I–III and complex IV
measured by CV metod on glassy carbon electrode, CH2Cl2,
V = 0.2 V s–1, 0.1 M NBu4ClO4, c = 3×10–3 mol L–1, Ar, vs.
Ag/AgCl/KCl(sat.)a
dithiols I and II results in the formation of the solution
of unstable dithiolate complexes which decomposes
during the isolation. In contrast to the substituted
o-amidophenolates and the triphenylantimony(V) cate-
cholates that form spiroendoperoxides, the dithiolate
antimony derivatives are oxidized by atmospheric
oxygen to the triphenylantimony(V) oxide and the cor-
responding disulfides. The introduction of the acceptor
atoms of chlorine in 3 and 6 positions of the aromatic
ring of benzene-1,2-dithiol stabilizes dianionic S,S'-
coordinated ligand, leading to the formation of
complex IV which is stable to atmospheric oxygen.
Compound
Е1pа, V
n
Е*ра, V
1.43
1.0
1.52
I
1.41
1.58
1.0
1.5
1.47
1.60
II
III
1.46
1.23
1.0
2.0
–
–
IV
(CatCl)SbPh3
b
a
*
E1pa is value of the peak potential of the first anodic process, Epa
is value of the peak potential of the first oxidation process
obtained at the use of a platinum anode; n is the number of
electrons transferred during the first anode stage versus
ferrocene as a reference . b [5].
Cl
Cl
SH
S
2 Et3N
+
Ph3SbCl2
SbPh3
_2 [Et3NH]Br
SH
S
Cl
Cl
by two irreversible one-electron redox transition shifted
to the anode area (Fig. 1). In the case of the oxygen-
containing analog of compound IV, the triphenylanti-
mony(V) 3,4,5,6-tetrahlorcatecholate (CatCl)SbPh3, the
introduction of the acceptor chlorine atoms in the
catecholate ligand led to an increase in the number of
electrons involved in the electrode reaction to two, and
was accompanied by the decoordination of o-benzo-
quinone [7]. The oxidation of complex IV in the first
step leads to the formation of an unstable intermediate
radical cation, which can be represented as two
resonance forms A and B.
IV
Complex IV was isolated from toluene in a
preparative yield. The compound was characterized by
IR, 1H, 13C NMR, mass spectroscopy, its structure was
fully consistent with the assumed formula.
The mechanism of the action of many antioxidants
in the inhibition of free radical processes includes the
hydrogen transfer or sequential deprotonation and loss
of an electron, which leads to the formation of stable
radicals of the antioxidant. The dissociation energy of
the bond H–X (X is O, N, or S) determines the ease of
the reaction with active radical. There are various
experimental and computational methods for the
determination of this indicator. One of experimental
methods of assessment of the dissociation energies of
H–X bond is a method that takes into account the
values of the redox potential and the acid function
[10]. The oxidation potential characterizes the
reducing ability of the compound and is one of the
factors determining the antioxidant activity. We
investigated the electrochemical properties of complex
IV and free dithiols I–III by the method of cyclic
voltammetry (CV) (see the table).
The irreversible oxidation indicates a low degree of
delocalization of the charge and spin in the aromatic
ring and points to the predominant contribution of the
localized form B, which is consistent with the known
calculated data and the results of ESR studies of
phenylthiyl radical [11]. Phenoxyl and phenylaminyl
radicals have delocalized structure [12], and in the case
of the phenylthiyl radical, on the contrary, the localiza-
tion of spin density on the sulfur atom is observed.
Unsubstituted phenylthiyl radicals and monoanionic
forms of the transition metal complexes with o-imino-
thiobenzosemiquinone radicals react to form dimeric
disulfide adducts [11, 13]. The cation-radical inter-
mediate B dissociates to form uncoordinated thiyl
radical C, which dimerizes to form binuclear dication.
The formation of a disulfide bond is reflected in the
reverse scan of CV by a broad reduction peak at
–0.2 V (Fig. 1a), presumably corresponding to the
reduction of the disulfide complex. Similar changes in
In complex IV the central antimony atom is in the
highest oxidation state, therefore, the observed
electrochemical activity can be regarded as a change in
the redox state of the coordinated ligand. In contrast to
the previously studied triphenylantimony(V) cate-
cholates [7], whose first anodic process is quasi-
reversible, the dithiolate complex IV is characterized
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SMOLYANINOVA et al.
Cl
Cl
Cl
Cl
Cl
S
S
S
S
S
SbPh3
S
SbPh3
SbPh3
Cl
A
B
Cl
SbPh3
Cl
Cl
Cl
S
S
S
SbPh3
S
S
S
Cl
Ph3Sb
C
Cl
CV were observed earlier for a series of transition
metal complexes with coordinated thiophenolate anion
[14]. The second anodic process registered in the
voltammogram (Fig. 1b) at the potential Epa = 1.89 V
may equally correspond to further oxidation of the
intermediate B accompanied by decoordination of the
ligand, or to the oxidation of disulfide complex.
occurs at a more cathodic potential (0.05–0.1 V) than
the corresponding oxygen analogs [14, 15]. On the
contrary, the oxidation of compound IV occurs at a
potential by 0.23 V higher than that of catecholate
complex (CatCl)SbPh3, which is not consistent with the
results of electrochemical studies of the free ligands.
As expected, the oxidation potential of benzene-1,2-
dithiol is shifted to the cathode region (see the table)
compared to pyrocatechol (1.48 V). This fact may
indicate a more covalent character of the antimony–
sulfur bond in the complex IV than the antimony–
It is known that the oxidation of sulfur compounds
I, mA
SH
SH
SH
SH
_
_e
R
R
S
_H+
R
SH
oxygen bond in the catecholate derivative [14].
The electrochemical oxidation of dithiols on glassy
carbon electrode is of irreversible one-electron type,
and it leads to the formation of low stable radical
cations.
Е, V
After the electrode process the chemical stage of
deprotonation takes place. The use of platinum anode
allowed to observe on the voltammogram reverse
branch the deprotonation step in all three dithiols
Fig. 1. CV of oxidation of complex IV; (1) in the range of
potential sweep from –0.5 to 1.8 V, (2) in the range of
potential sweep from –0.5 to 2.1) (CH2Cl2, glassy carbon
anode, Ag/AgCl/KCl, 0.1 M NBu4ClO4, c = 3×10–3 M, argon).
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SYNTHESIS AND PROPERTIES OF TRIPHENYLANTIMONY(V) DITHIOLATE DERIVATIVE
(–0.14 to –0.22 V). It should be noted that the values
of oxidation potentials of dithiols I–III on the
platinum anode are shifted to the anodic area,
compared to the results for the carbon glass electrode
(see table). The resulting thiyl radical is cabable to
enter into further dimerization reaction. The value of
the oxidation potentials of dithiols I–III is affected by
the nature of substituents on the phenyl ring: the
electron-donor methyl group slightly shifts the oxida-
tion potential to the cathode region, while the electron-
withdrawing chlorine atoms impede the oxidation.
сROOH, mM
The reactivity and thermodynamic properties of
free thiols determine a possibility of their application
as electron carriers, antidotes for heavy metal
poisoning, in the functioning of antioxidant protection
systems of the body [16]. The interaction of thiols with
free radicals through the hydrogen atom abstraction, as
well as the reactivity of disulfides generated at the
oxidation of thiols toward hydroperoxides allow the
application of these compounds as inhibitors of free-
radical processes. Based on the electrochemical data it
is possible to predict that the greatest antioxidant
activity should exhibit compounds II, I and IV.
τ×10–3, s
Fig. 2. Kinetic curves of accumulation of ROOH during the
oxidation of oleic acid at 60°C in the presence of additives
(1 mM): (1) no additives, Ph3SbCl2 (2), 3,6-dichloro-1,2-
dithiobenzene (3), 1,2-ditiobenzene (4), 3-methyl-1,2-
dithiobenzene (5), complex IV (6), 3,6-dichloro-1,2-dithio-
benzene with Ph3SbCl2 (7).
be more effective inhibitor. The antimony(V)
derivative Ph3SbCl2 (Fig. 2, curve 2) has a promoter
effect increasing the content of hydroperoxides, which
is consistent with previous results [19]. It is seen from
Fig. 2, curve 7, that the promoting effect of Ph3SbCl2
may be obscured by the dithiol III additives which can
act as a chelator. Obviously, the combined sulfur-
containing ligand and Ph3SbCl2 has a more pro-
nounced inhibitory effect on the concentration of
hydroperoxides than synthesized complex IV.
Comparative data on the antioxidant activity of the
compounds in the autoxidation of oleic acid at 60°C
within 5 h are shown in Fig. 3.
The study of the oxidation of unsaturated fatty
acids is a model reaction of lipid oxidation by
peroxides in cell membranes. The oxidation of oleic
acid (RH) with oxygen leads to the formation of
substituted allyl radicals that interact with O2
producing peroxyl radicals ROO•. The corresponding
cis- and trans-isomeric hydroperoxides ROOH are the
main products in the initial stage of the oxidation
process [17, 18]. In this paper we continue our study of
antioxidant activity of the triphenylantimony(V)
complexes with redox-active ligands and free ligands
on a model example of oxidation of oleic [(Z)-octadec-
9-enoic] acid at 60°C. In addition to studying the
antioxidant activity of thiols I–III, we consider their
chelating properties by an example of interaction with
Ph3SbCl2.
The analysis of the results showed that the
maximum activity among the studied dithiols have the
substituted dithiobenzenes II and III. The inhibitory
activity of dithiols is due to the homolytic decom-
position of S–H bond at the action of active ROO•
radicals. The dimerization of thiyl radicals leads to the
formation of disulfides, which are capable of further
oxidation at the SH group and can destruct
hydroperoxides forming sulfoxides [16]. Complex IV
is a one-electron donor forming at the oxidation a di-
cationic compound inactive toward hydroperoxy
radicals, unlike disulfides formed at the oxidation of
the ligands.
The results of investigation of the kinetics of
accumulation of hydroperoxides are presented in Fig. 2.
Adding a dithiol I–III or complex IV leads to a
decrease in the concentration of hydroperoxide in the re-
action medium. The explored dithiols show more pro-
nounced inhibitory effect on the process of free radical
oxidation of oleic acid during the time of the experi-
ment as compared with complex IV. Based on the electro-
chemical data, the highest inhibitory activity would be
expected in compound II, but dithiol III turned out to
Thus, in this work we obtained a new dithiolate
complex of triphenylantimony(V) IV stable stable
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SMOLYANINOVA et al.
1
without further purification. The H and 13C NMR
spectra were recorded on a Bruker AVANCE DPX-
200 spectrometer, internal reference TMS, solvent
CDCl3. IR spectra were recorded on a FT-IR
spectrometer FSM 1201 from samples in mineral oil.
The mass spectra of compounds were recorded on a
mass spectrometer Polaris Q/Trace GC Ultra (Ion Trap
analyzer), 70 eV, ion source temperature 250°C, the
sample temperature 50–450°C. Experiments on the
synthesis and studying the properties of the complex
were carried out in evacuated ampules in the absence
of oxygen. Used solvents were purified and dried freed
by usual technique [20].
Fig. 3. Relative content of ROOH in oleic acid in the
Complex IV. To a solution of triphenylantimony
dichloride (0.256 g, 0.5 mmol) in 40 mL of toluene was
added with stirring a solution of 3,6-dichlorobenzene-
1,2-dithiol (0.106 g, 0.5 mmol) in 30 mL of toluene and
then a solution of triethylamine (0.14 mL, 1.0 mmol) in
10 mL of toluene. The reaction mixture was stirred at
room temperature for 3 h, the solution was con-
centrated to about 1/4 of the volume, 20 mL of hexane
was added, and the solution was left overnight at –18°C.
The solution was filtered from the formed triethyl-
ammonium chloride precipitate, and the solvent was
replaced by hexane (~30 mL). At standing this solution
for two days at –18°C a powdery precipitate of the
complex formed, which was filtered off, washed twice
with hexane, and dried in a vacuum. Yield 0.23 g
(81.8%). IR spectrum (vaseline oil, cm–1): 1572 m,
1528 m, 1476 m, 1432 s, 1396 s, 1365 w, 1326, 1303
w, 1262 w, 1239 m, 1187 w, 1179 w, 1158 s, 1116 w,
1075 s, 1067 s, 1061 s, 1019 w, 997 s, 913 w, 820 m,
805 m, 799 m, 773 w, 731 s, 727 s, 695 s, 688 s, 600
course of 5 h autoxidation at 60°C; additive concentration
(1 mM): (1) Ph3SbCl2; (2) benzene-1,2-ditiol, (3) 4-methyl-
1,2-ditiobenzene, (4) 3,6-dichloro-1,2-ditiobenzene, (5) com-
plex IV, (6) 3,6-dichloro-1,2-ditiobenzene + Ph3SbCl2.
ROOH content in oleic acid without additives is taken as
100%.
under aerobic conditions, studied the redox properties
of dithiols I–III and complex IV, and estimated their
activity by the example of autoxidation of oleic acid.
According to the cyclic voltammetry the oxidation of
dithiols I–III and complex IV is irreversible and
proceeds as a one-electron transfer with follow-up
chemical stage. Unlike triphenylantimony(V) cate-
cholates, the oxidation of complex IV results in an
unstable monocationic form of the complex capable of
further dimerization. Replacing heteroatoms in the
metallocycle, in particular, oxygen by sulfur in the
complex IV results in the formation of structures with
the spin density localized on the sulfur atom. Lack of
delocalization of the electron density over the aromatic
moiety reduces the effectiveness of the antioxidant
action of complex IV. We found that adding
compounds I–IV leads to the inhibition of the oleic
acid oxidation indicating that they possess an
antioxidant activity. The relative content of oleic acid
hydroperoxides in the presence of dithiols I–III is
significantly lower compared to complex IV, and the
free dithiol III shows the maximum effect.
1
m, 553 m, 510 w, 500 w, 490 w, 464 m. H NMR
spectrum (200 MHz, δ, ppm): 7.02 s (2H, C6H2), 7.39–
7.49 m (9H, Ph), 7.65–7.69 m (6H, Ph). 13C NMR
spectrum (50 MHz, δ, ppm): 124.66 (CHAr), 129.40
(Ph), 130.54 (CClAr), 130.88 (Ph), 133.80 (Ph), 137.50
(CSAr), 139.83 (Ph). Mass spectrum (EI): 560, 562,
564, 566 ([M]+: 121Sb, 123Sb, 35Cl, 37Cl).
Measurement of oxidation potentials of com-
pounds I–IV was performed by cyclic voltammetry
(CV) in a three-electrode cell using the potentiostat
IPC-pro, in acetonitrile, under argon. The working
electrode was a stationary glassy carbon (GC)
(platinum) with a diameter of 2 mm, auxiliary elec-
trode was a platinum plate (S = 18 mm2). The
reference electrode was (Ag/AgCl/KCl) with a water-
proof diaphragm. The concentration of the substance is
0.003 M. The number of electrons transferred in the
EXPERIMENTAL
Commercially available reagents: benzene-1,2-
dithiol (Aldrich, 96%), 4-methyl-1,2-ditiobenzene
(Fluka, 97%), 3,6-dichloro-1,2-ditiobenzene (Sigma–
Aldrich, 95%) Ph3SbCl2 (Aldrich, 99%), 1,2-dihyd-
roxybenzene (chemically pure), oleic (cis-9-octa-
decaenoic) acid from Acros Organics, 97% were used
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SYNTHESIS AND PROPERTIES OF TRIPHENYLANTIMONY(V) DITHIOLATE DERIVATIVE
5. Poddel’skii, A.I., Smolyaninov, I.V., Kurskii, Yu.A.,
Berberova, N.T., Cherkasov, V.K., and Abakumov, G.A.,
Russ. Chem. Bull., 2009, vol. 58, no. 3, p. 532.
6. Yang, B., Kotani, A., Arai, K., and Kusu, F., Analytical
Sci., 2001, vol. 17, p. 599.
electrode process was evaluated relative versus
ferrocene as a reference. Potential sweep rate 0.2 V s–1.
The supporting electrolyte 0.1 M Bu4NClO4 (Acros
Organics, 99%).
Oxidation of oleic acid in the presence of
compounds I–IV was carried out in a temperature-
controlled cell (60°C) at a rate of air flow 2–4 mL min–1
for 5 h. Since the oxidation of oleic acid occurs as an
autoxidation, a 2 h preliminary bubbling of air was
performed before adding a compound. Concentration
of additives was 1 mM. Activity of the compounds in
the oxidation of oleic acid was estimated by the
standard method by the amount of isomeric ROOH
hydroperoxides formed [21].
7. Smolyaninov, I.V., Poddel’skii, A.I., Berberova, N.T.,
Cherkasov, V.K., and Abakumov, G.A., Russ. J. Coord.
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ACKNOWLEDGMENTS
This work was financially supported by the Russian
Foundation for Basic Rresearch (grants nos. 11-03-
00589, 11-03-00513, and 12-03-31026), and the
Federal Program “Scientific and scientific-pedagogical
personnel of innovative Russia” for 2009–2013. (State
contract no. 16.740.11.0771).
12. Adamo, C., Subra, R., DiMatteo, A., and Barone, V.J.,
Chem. Phys., 1998, vol. 109, p. 10244.
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Berberova, N.T., and Eremenko, I.L., Russ. J. Coord.
Chem., 2011, vol. 36, no. 1, p. 12.
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