Novel tetranuclear triarylantimony(v) complexes with (±)-mandelic acid ligands: Synthesis, characterization, in vitro cytotoxicity and DNA binding properties

Article abstract of DOI:10.1039/c3dt50221j

Four novel tetranuclear organoantimony(v) complexes [R₃SbL] ₄, in which LH = (±)-mandelic acid and R = phenyl (1), 4-fluorophenyl (2), 3-fluorophenyl (3), 3,4,5-trifluorophenyl (4), were synthesized and characterized. The complexes displayed rapid, low micromolar in vitro cytotoxicity against a range of epithelial tumour cells and efficient CT-DNA binding.

Full text of DOI:10.1039/c3dt50221j

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Novel tetranuclear triarylantimony(V) complexes with
( )-mandelic acid ligands: synthesis, characterization,
in vitro cytotoxicity and DNA binding properties†
Cite this: DOI: 10.1039/c3dt50221j
Received 22nd January 2013,
Accepted 16th April 2013
Jin Jiang,a Handong Yin,*a Fangli Wang,a Zhong Han,a Fei Wang,a Shuang Chengb
and Min Hong*a,c
DOI: 10.1039/c3dt50221j
www.rsc.org/dalton
Four novel tetranuclear organoantimony(V) complexes [R3SbL]4, in dioxo cyclometallates are presented. Thereby motivated, we
which LH = ( )-mandelic acid and R = phenyl (1), 4-fluorophenyl herein make use of a chiral ( )-mandelic acid ligand to coordi-
(2), 3-fluorophenyl (3), 3,4,5-trifluorophenyl (4), were synthesized nate the triarylantimony ion in the +V oxidation state and have
and characterized. The complexes displayed rapid, low micromolar prepared a series of novel tetranuclear triarylantimony(V) com-
in vitro cytotoxicity against a range of epithelial tumour cells and plexes with (Sb4(μ-OCO)4) geometry. These complexes are
efficient CT-DNA binding.
found to exhibit prominent cytotoxic activities in vitro. They
are designed to have three bound aryl groups with or without
fluoro-substituents, which are subsequently found to be crucial
for their cytotoxic activities. In addition, the DNA-binding
properties of these complexes with calf thymus DNA (CT-DNA)
have also been investigated by fluorescence spectroscopy.
The triarylantimony complexes used in this study are
shown in Scheme 1, which were synthesized by reactions of
triarylantimony dichlorides (denoted as a: a1–a4 hereafter) and
the ( )-mandelic acid ligand (denoted as b hereafter) in the
presence of sodium ethoxide, and the fluoro-substituted triaryl-
antimony dichloride was prepared according to the modified
literature method (see Scheme S1 and Fig. S1 in ESI†).10 All the
titled complexes (denoted as c: c1–c4 hereafter) were character-
The clinical success of cisplatin in the treatment of several
human malignant tumours has motivated major research
efforts toward the discovery of alternative metal complexes
with potential as anticancer drugs,1 which are expected to
overcome the remaining problems such as severe side effects
and resistance phenomena. Besides platinum, complexes of
other transition metals, such as gold2 and ruthenium,3 have
been reported to exhibit promising anticancer activities. In
contrast, the reports on the anticancer properties of the main-
group metal complexes are limited. Recent studies have shown
that a series of main-group metal complexes displays potent
in vitro or in vivo antitumour activities,4 in some cases being
more effective than cisplatin in in vitro tests.5,6
The synthesis, crystal structure and cytotoxic activity of
compounds containing multiple coordination with heavier
Group 15 (such as Sb, Bi) elements have been among the most
exciting targets in the chemistry of main-group elements.7,8 As
a contribution towards understanding the stereochemistry of
nucleophilic substitution at tetrahedral Group 15 centers,9 the
solid-state geometries of all the known five or six coordinated
ized by H and 13C NMR, elemental analysis. The H and 13C
NMR spectra of c1–c4 revealed one set of sharp signals corre-
sponding to ligand b, showing the presence of only one
species in CDCl3 solution. However, the electrospray ionization
mass spectra (ESI-MS) of c1–c4 show that complexes 1–4 split
into several fragments and the ligands depart from (R3)3Sb
1
1
aShandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell
Technology, School of Chemistry and Chemical Engineering, Liaocheng University,
Liaocheng, 252059, P. R. China. E-mail: handongyin@163.com,
hongminlcu@163.com; Fax: (+)86 635 8239121
bSchool of Agriculture, Liaocheng University, Liaocheng, 252059, P. R. China
cState Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical
Engineering, Nanjing National Laboratory of Microstructures, Nanjing University,
Nanjing 210093, China
†Electronic supplementary information (ESI) available: Synthesis and character-
ization of complexes c1–c4; cytotoxicity experiments and competitive ethidium
displacement experiments; ESI figures/table; references. CCDC 888519, 931285,
905087, 931642, 915546, 915547 and 915548. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c3dt50221j
Scheme 1 Preparation of the compounds studied.
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and leave vacant coordinative positions in methanol compared
to those in the crystal structures of c1–c4. In addition, mole-
cular structures of c1–c4 were unambiguously determined by
the single-crystal X-ray diffraction analysis (Fig. 1–4) together
with the partial atomic numbering scheme. The compounds
crystallized in different space groups although with similar
geometry structures.
Usually, both triaryl antimony dicarboxy esters11 and cyclo-
metallate compounds12 could be prepared by the metathetical
reaction of the dihalide SbAr3X2 (X-Br or Cl) with the corre-
sponding α-hydroxy carboxylic ligands under the different
reaction conditions. Excitingly, by the common method we
obtain compounds c1–c4, which contain two interactions at the
same time, that is, di-carboxy esters and 1,3-transannular
interactions, with the central Sb atoms hexa-coordinated. So
far, the vast majority of the Sb(III and V) complexes tested are
mononuclear. Although tetranuclear or polymeric species have
also been investigated,13,14 no examples of organoantimony
have been reported (Sb4(μ-OCO)4).
Fig. 3 Molecular structure of complex c3.
Fig. 4 Molecular structure of complex c4.
The unit cell of the complexes, except for c3, contains one
independent molecule of c (Scheme 1) and one ethyl ether
molecule. We take complex c1 as an example to describe their
structural characteristics. It is composed of four antimony
metal centers, in which each one is bonded to three phenyl
ligands and coordinated to the tridentate O2CC(O)(H)Ph
moiety. Through the 1,3-transannular interaction, it forms
four similar cyclometallations (Scheme 1). The central part
of the structure remains essentially planar (the coordination
environment of Sb2 for example), as the antimony atom is only
slightly bent out of the plane defined by O4–C46–C45–O5
atoms (0.4257 (0.0118) Å). During the di-carboxy ester inter-
action, it forms a chelate ring, which contains sixteen
members (Scheme 2). Representing the μ-OCO group as M, the
stereostructure of the chelate ring, that is, boat conformation,
is presented evidently.14 The outside of the compact chelate
ring is protected by phenyl groups. Therefore, there are no
close intermolecular contacts with other cyclic molecules or
with ethyl ether molecules. The high solubility and the rela-
tively high stability of c1 may be explained by the coordinative
Fig. 1 Molecular structure of complex c1. Hydrogen atoms are omitted and the
phenyl group of ligand b is replaced by a carbon atom for clarity.
Fig. 2 Molecular structure of complex c2.
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Sb(V) precursors and the ligand. In contrast, cisplatin is less
cytotoxic to these cells (IC50 > 100 μM).
Additionally, we chose four aryl groups, phenyl, 4-fluoro-
phenyl, 3-fluorophenyl and 3,4,5-trifluorophenyl, as organic
ligands bound with antimony centers in Sb(V) complexes to
investigate the electronic influence on their cytotoxic activities.
As we can see, complex c4 exhibits the highest activity com-
pared with other complexes, and complex c2, which has
fluorine atoms in the para-position, shows better activity than
complex c3 with fluorine atoms in the meta-position. This
means that the cytotoxicity tends to increase as the quantity
and the electron-withdrawing ability of F atom increase, that
is, 3,4,5-3F-substituted > 4-F-substituted > 3-F-substituted.
Using the rat hepatocytes cell line (BRL), the cytotoxicity of
all complexes and their precursors to non-cancerous origi-
nated cells were also examined. Among all the complexes, only
c1 is less cytotoxic towards the rat hepatocytes cells than
towards the examined cancerous cells. Meanwhile, for c1, the
cytotoxicity is decreased after the formation of the organo-
antimony(V) compound compared to their Sb(V) salt precursor.
On the other hand, the participation of fluorine atoms exacer-
bates the lethality of the complexes to normal cells. Fluoro-
substituted complexes c2–c4 all exhibit higher cytotoxicity to
the normal cells. Therefore, to reduce their toxicity towards
normal cells, and to ensure high cytotoxic activity of fluori-
nated complexes, will be the focus of future research.
DNA is a key biological target for many metal-based antitu-
mour drugs, and distortions and damage of DNA structures
are often associated with anticancer activity.18 Thus, it is
important to understand the DNA binding of complexes and
their possible relationship to cytotoxicity in tumour cell
lines.19 Polynuclear organoantimony(V) species may exhibit a
higher cytotoxic activity than related mononuclear com-
pounds, possibly due to their ability to interact with DNA. The
interaction of compounds c1–c4 with DNA has been evaluated
by the ethidium bromide (EB)–DNA system with limited EB
bound to excess of DNA, which can be used to distinguish
intercalating and non-intercalative ligands.20 Competitive
binding of other intercalators leads to a loss of fluorescence
because of the depletion of the EB–DNA complex.21 As shown
in Fig. 5 and Fig. S3 in ESI,† the fluorescent intensity of EB in
the bound form is remarkably quenched upon adding c1–c4,
respectively. The Stern–Volmer DNA binding constant (KSV) is
evaluated to be 0.58, 1.50, 0.47, and 0.18 for complexes c4, c3,
c2, and c1, respectively, which are in accordance with the
extent of displacement of EB by the complexes. Complexes c2–
c4 show higher KSV values. These results show that the position
and quantity of fluoro-substituents of the metal salts play an
important role in the DNA binding ability of complexes
Scheme 2 The chelate ring structure of compounds c1–c4.
saturation of the atoms in the ring and their shielding by the
phenyl groups.
In many cases, the polynuclear backbones of molecular
organometallic compounds can indeed be regarded as frag-
ments that are cut off from a solid-state network.15 The chelate
ring framework structure of compound c1 is dictated by the
bonding abilities of the oxygen atoms located at the corners of
the metal-centered polyhedral building blocks shown in
Scheme 2. In the formation of the solid-state structure, oxygen
plays a structure-directive role. The final outcome of the struc-
ture is dictated by the Sb–M–Sb bond angles that determine
how the polyhedral units can be assembled into polynuclear
structures. Since the Sb–M–Sb bond angles are quite flexible, a
great structural diversity is possible.
By means of MTT assays,16,17 the in vitro cytotoxicities of
complexes c1–c4 along with their metal salts a1–a4, the free
ligand b and the clinical antitumour agent cisplatin towards a
panel of established human cancer cell lines including lung
carcinoma (A549), colon carcinoma (HCT-116) and colon
adenocarcinoma (Caco-2), which are well-known cisplatin-
resistant cancer cell lines, were measured. Because most of the
compounds showed inhibition activity at the concentration of
>10 μM, herein biological results were merely listed at the con-
centration of 10 μM in Fig. S2 in ESI.† In the meantime, the
IC50 (half maximal inhibitory concentration) values for com-
pounds c1–c4 obtained after 48 h of drug treatment were calcu-
lated from the dose–survival curves, which are summarized in
Table 1. The viability assay shows that the cell proliferation
can also be inhibited by Sb(V) chloride salts to some extent,
which exhibit a relatively weak activity, and for the ligand, it is
non-active absolutely to the determined cancer cells. However,
the cytotoxic activities of complexes c1–c4 are markedly
improved, possibly due to the cooperative interaction of their
Table 1 IC50 (μM) of all compounds against three human tumour cell lines and
one normal cell line
A549
3.7 0.4
HCT-116
6.4 0.6
Caco-2
17.0 0.7
BRL
30.7 1.1
c1
c2
c3
c4
a1
a2
a3
a4
b
3.7 0.5
7.2 0.6
1.8 0.3
3.0 0.5
3.7 0.5
1.3 0.3
4.2 0.6
11.6 0.5
1.3 0.4
40.5 3.2
26.6 2.7
29.8 2.3
26.5 2.0
6.6 0.5 besides their molecular structures and the Sb atom. In
7.0 0.5
addition, we also determined the DNA binding properties of
four organoantimony salts (a1–a4) and the free ligand b (see
3.7 0.3
18.0 2.2
36.6 2.9
25.0 2.4
26.3 2.6
25.6 2.0
>100
>100
45.0 3.2
24.0 2.1
20.0 2.0
12.2 1.4
>100
>100
22.8 2.0 Fig. S3 in ESI†). a3 followed the same trend as the four com-
19.1 1.9
11.4 1.6
>100
>100
plexes. Especially, the fluorescence intensity of EB–DNA after
adding a1, a2, a4 and b became intricate unexpectedly. On the
basis of all of the fluorescence studies, we conclude that
>100
>100
Cisplatin
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Rev., 2009, 253, 1682–1691; (c) C. F. Shaw III, Chem. Rev.,
1999, 99, 2589–2600.
3 (a) I. Kostova, Curr. Med. Chem., 2006, 13, 1085–1107;
(b) R. W.-Y. Sun, M. F.-Y. Ng, E. L.-M. Wong, J. Zhang, S. S.-
Y. Chui, L. Shek, T.-C. Lau and C.-M. Che, Dalton Trans.,
2009, 10712–10716; (c) M. J. Clarke, F. Zhu and
D. R. Frasca, Chem. Rev., 1999, 99, 2511–2533.
4 (a) R. Contreras, A. Flores-Parra, E. Mijangos, F. Tellez,
H. Lopez-Sandoval and N. Barba-Behrens, Coord. Chem.
Rev., 2009, 253, 1979–1999; (b) S. H. VanRijt and P. Sadler,
Drug Discovery Today, 2009, 14, 1089–1097.
5 X. M. Shang, X. G. Meng, E. C. B. A. Alegria, Q. S. Li,
M. F. C. Guedes da Silva, M. L. Kuznetsov and
A. J. L. Pombeiro, Inorg. Chem., 2011, 50, 8158–8167.
6 H. Yin, C. Yue, M. Hong, J. Cui, Q. Wu and X. Zhang,
Eur. J. Med. Chem., 2012, 58, 533–542.
7 (a) P. Naredi, D. D. Heath, R. E. Enns and S. B. Howell,
Cancer Res., 1994, 54, 6464–6468; (b) L. Balázs and
H. J. Breunig, Coord. Chem. Rev., 2004, 248, 603–621.
8 I. I. Ozturk, S. K. Hadjikakou, N. Hadjiliadis,
N. Kourkoumelis, M. Kubicki, M. Baril, I. S. Butler and
J. Balzarini, Inorg. Chem., 2007, 46, 8652–8661.
9 S. Kumaraswamy, C. Muthiah and K. C. Kumara Swamy,
J. Am. Chem. Soc., 2000, 122, 964–965.
10 R. F. De Ketelaere, F. T. Delbeke and G. P. Van Der Kelen,
J. Organomet. Chem., 1971, 30, 365–368.
11 H. Barucki, S. J. Coles, J. F. Costello and M. B. Hursthouse,
Chem.–Eur. J., 2003, 9, 2877–2884.
12 H. Barucki, S. J. Coles, J. F. Costello and M. B. Hursthouse,
J. Organomet. Chem., 2001, 622, 265–273.
Fig. 5 Emission spectra from EB bound DNA in the absence (---) and in the
presence of [complex c1] 0–125 μM concentration, [DNA] 25 μM and [EB] 3 μM.
The arrow shows changes in the emission intensity upon addition of increasing
concentration of the complex. Inset: plots of I0/I vs. r (r = [Complex]/[DNA]) with
experimental data points. λex = 258 nm.
organoantimony(V) complexes c1–c4 and metal salt a3 can bind
to CT-DNA in an intercalative mode, but the interactions
between DNA and a1, a2, a4 and b are more complicated.
In short, the present article describes the syntheses and
structural characterization of four new complexes of the type
[Sb4(μ-OCO)4] containing ( )-mandelic acid ligands. Prelimi-
nary in vitro cytotoxic studies reveal that they display remark-
ably high cytotoxicity toward platin-resistant cancer cell lines.
In addition, this is a significant report on the DNA-binding
properties of organoantimony(V) complexes, which could bind
with CT-DNA via an intercalative mode, and it supports the
conclusion that DNA is the target for the antimony(V) com-
plexes c1–c4 and the interaction intensity is influenced by the
position and quantity of fluoro-substitutents on antimony(V)
complexes. These results may provide the grounds for estab-
lishing new structure–pharmacological activity relationships
for DNA-targeting complexes as novel anticancer drugs.
The authors would like to thank the National Natural Foun-
dation of China (21105042) and the Natural Science Foun-
dation of Shandong Province (ZR2011BM007, ZR2010BQ021,
ZR2011BQ015) for financial support. Additionally, this work
was supported by the Shandong “Tai-Shan Scholar Research”
Fund.
13 J. Bordner, G. O. Doak and T. S. Everett, J. Am. Chem. Soc.,
1986, 108, 4206–4213.
14 (a) M. Galakhov, M. Mena and C. Santamaria, Chem.
Commun., 1998, 691–692; (b) R. Andrés, M. Galakhov,
M. P. Gomez-Sal, A. Martin, M. Mena and C. Santamaria,
Chem.–Eur. J., 1998, 4, 1206–1213.
15 H. W. Roesky, I. Haiduc and N. S. Hosmane, Chem. Rev.,
2003, 103, 2579–2595.
16 (a) F. Denizot and R. Lang, J. Immunol. Methods, 1986, 89,
271–277; (b) P. Skehan, R. Storeng, D. Scudiero, A. Monks,
J. McMahon, D. Vistica, J. T. Warren, H. Bokesch, S. Kenney
and M. R. Boyd, J. Natl. Cancer Inst., 1990, 82, 1107–1112.
17 P. Price and T. McMillan, Cancer Res., 1990, 50, 1392–1396.
18 C. X. Zhang and S. J. Lippard, Curr. Opin. Chem. Biol., 2003,
7, 481–489.
19 J.-H. Wen, C.-Y. Li, Z.-R. Geng, X.-Y. Ma and Z.-L. Wang,
Chem. Commun., 2011, 47, 11330–11332.
20 T. C. Jenkins, in Methods in Molecular Biology, ed. K. R. Fox,
Humana Press Inc., Totowa, New Jersey, 1997, vol. 90, pp.
205–207.
References
1 M. A. Jakupec, M. Galanski, V. B. Arion, C. G. Hartinger
and B. K. Keppler, Dalton Trans., 2008, 183–194.
21 H. Yin, H. Liu and M. Hong, J. Organomet. Chem., 2012,
713, 11–19.
2 (a) V. Milacic and Q. P. Dou, Coord. Chem. Rev., 2009, 253,
1649–1660; (b) R. W.-Y. Sun and C.-M. Che, Coord. Chem.
Dalton Trans.
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