Synthesis, antiradical activity and in vitro cytotoxicity of novel organotin complexes based on 2,6-di-tert-butyl-4-mercaptophenol

Article abstract of DOI:10.1039/c3dt53469c

A series of organotin complexes with Sn-S bonds of formulae Me ₂Sn(SR)₂ (1); Et₂Sn(SR)₂ (2); (n-Bu)₂Sn(SR)₂ (3); Ph₂Sn(SR)₂ (4); R₂Sn(SR)₂ (5); Me₃SnSR (6); Ph₃SnSR (7) (R = 3,5-di-tert-butyl-4-hydroxyphenyl) were synthesized and characterized by elemental analysis, ¹H, ¹³C NMR, and IR. The crystal structures of compounds 1, 4, 5, and 7 were determined by X-ray diffraction analysis. The tetrahedral geometry around the Sn center in the monocrystals of 1, 4, 5, and 7 was confirmed by X-ray crystallography. The high radical scavenging activity of the complexes was confirmed spectrophotometrically in a DPPH-test. The binding affinity of 1-7 and the starting R₂SnCl ₂ (8) towards tubulin through their interaction with SH groups of proteins was studied. It was found that the hindered organotin complexes could interact with the colchicine site of tubulin, which makes them promising antimitotic drugs. Compounds 1-8 were tested for their in vitro cytotoxicity against human breast (MCF-7) and human cervix (HeLa) adenocarcinoma cells. Complexes 1-8 were also tested against normal human fetal lung fibroblast cells (MRC-5). Complexes 2-4 and 8 exhibit significantly lower cytostatic activity against the normal MRC-5 cell line compared to the tumor cell lines MCF-7 and HeLa used. A high activity against both cell lines 250 nM (MCF-7) and 160 nM (HeLa) was determined for the triphenyltin complex 7 while the introduction of hindered phenol groups decreases the cytotoxicity of the complexes against normal cells. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53469c

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Synthesis, antiradical activity and in vitro
cytotoxicity of novel organotin complexes based
on 2,6-di-tert-butyl-4-mercaptophenol†
Cite this: Dalton Trans., 2014, 43,
6880
D. B. Shpakovsky,a C. N. Banti,b E. M. Mukhatova,c Yu. A. Gracheva,a V. P. Osipova,d
N. T. Berberova,c D. V. Albov,a T. A. Antonenko,a L. А. Aslanov,a E. R. Milaeva*a and
S. K. Hadjikakoub
A series of organotin complexes with Sn–S bonds of formulae Me2Sn(SR)2 (1); Et2Sn(SR)2 (2); (n-Bu)2Sn-
(SR)2 (3); Ph2Sn(SR)2 (4); R2Sn(SR)2 (5); Me3SnSR (6); Ph3SnSR (7) (R = 3,5-di-tert-butyl-4-hydroxyphenyl)
1
were synthesized and characterized by elemental analysis, H, 13C NMR, and IR. The crystal structures of
compounds 1, 4, 5, and 7 were determined by X-ray diffraction analysis. The tetrahedral geometry around
the Sn center in the monocrystals of 1, 4, 5, and 7 was confirmed by X-ray crystallography. The high
radical scavenging activity of the complexes was confirmed spectrophotometrically in a DPPH-test. The
binding affinity of 1–7 and the starting R2SnCl2 (8) towards tubulin through their interaction with SH
groups of proteins was studied. It was found that the hindered organotin complexes could interact with
the colchicine site of tubulin, which makes them promising antimitotic drugs. Compounds 1–8
were tested for their in vitro cytotoxicity against human breast (MCF-7) and human cervix (HeLa) adeno-
carcinoma cells. Complexes 1–8 were also tested against normal human fetal lung fibroblast cells
(MRC-5). Complexes 2–4 and 8 exhibit significantly lower cytostatic activity against the normal MRC-5
cell line compared to the tumor cell lines MCF-7 and HeLa used. A high activity against both cell lines 250
nM (MCF-7) and 160 nM (HeLa) was determined for the triphenyltin complex 7 while the introduction of
hindered phenol groups decreases the cytotoxicity of the complexes against normal cells.
Received 10th December 2013,
Accepted 21st February 2014
DOI: 10.1039/c3dt53469c
www.rsc.org/dalton
thymotoxic di-n-butyltin dichloride and tri-n-butyltin chloride
influence macromolecular DNA synthesis in rat thymocytes
Introduction
Organotin compounds show a wide spectrum of biological in vitro.3 Organotin complexes with heterocyclic thioamides
activities despite their high toxicity and nonspecific mode of demonstrate high anticancer and cytotoxic activity which was
action. It is well known that the Sn atom interacts with free correlated with their lipoxygenase inhibitory activity.4–6 The
sulfhydryl groups in proteins, which leads to distortion of the complexes of tri-n-butyltin(IV) and triphenyltin(IV) with 2-thio-
protein structure. Organotins can promote lipid peroxidation barbituric acid were found to exhibit higher cytotoxic activity
in cellular membranes and cause oxidative stress in living than that of cisplatin against cancer cells, in the case of
organisms.1,2 One of the biomolecular modes of organotin human breast adenocarcinoma cells (MCF-7, ER positive), and
action is suggested to be metal-induced apoptosis. The their IC50 values were 272- and 179-fold lower than that of cis-
platin, respectively.7,8 It is suggested that the antiproliferative
activity of organotin complexes correlates with their inter-
action with protein –SH groups.9 The toxicity of organotin
compounds is related with the binding of Sn atoms with
protein SH-groups as well as the induced oxidative stress in
living organisms. In order to lower their toxic effect the use of
antioxidants is suggested. The derivatives of hindered 2,6-di-
alkylphenols are used as antioxidants and models of vitamin E
in industry and medicine. The use of polyfunctional ligands
combining both the antioxidant 2,6-di-tert-butylphenol and
chelating groups for complexation of organotin compounds
was proposed as a way to lower their nonspecific toxicity
aMoscow State Lomonosov University, Department of Chemistry, Lenin Hill 1-3,
Moscow, 119991, Russian Federation. E-mail: milaeva@org.chem.msu.ru;
Fax: +7 495 9328846; Tel: +7 495 9393864
bSection of Inorganic and Analytical Chemistry, Department of Chemistry, University
of Ioannina, Ioannina, Greece
cAstrakhan’ State Technical University, Astrakhan’, Russian Federation
dSouth Research Center, Russian Academy of Sciences, Rostov on Don, Russian
Federation
†Electronic supplementary information (ESI) available: Dose–response survival
curves for 1–7 against MCF-7 and HeLa cells are presented in Fig. S9 and S10.
CCDC 967798–967801. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c3dt53469c
6880 | Dalton Trans., 2014, 43, 6880–6890
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Fig. 1 Ellipsoid plot of 1 with the atom numbering scheme. Hydrogen
atoms are omitted for clarity. Thermal displacement ellipsoids are drawn
at the 50% probability level.
Scheme 1 Organotin complexes based on 2,6-di-tert-butyl-4-
mercaptophenol.
against normal cells.10,11 The cytotoxicity of a series of bis-(3,5-
di-tert-butyl-4-hydroxyphenyl)tin complexes with heterocyclic
thioamides against the MCF-7 cell line exceeds that of cisplatin
(IC50 values was 32-fold lower than that of cisplatin in the
case of R2Sn(MPMT)2; R = 3,5-di-tert-butyl-4-hydroxyphenyl;
MPMTH = 2-mercapto-4-methylpyrimidine).12
We proposed a novel approach to the design of polyfunc-
tional agents which combines the Sn atom and antioxidants
based on 2,6-di-tert-butyl-4-mercaptophenol. It is well known
that hindered phenols 2,6-di-tert-butylphenols are not involved
in complexation via the OH group with large metal ions.
The use of 2,6-di-tert-butyl-4-mercaptophenol as a ligand has
aimed at the simplest way of introducing an antioxidant frag-
ment in an effective S-donor ligand which can form stable Fig. 2 Ellipsoid plot of 4 with the atom numbering scheme. Hydrogen
atoms are omitted for clarity. Symmetry operator i: 1 − x, y, 1.5 − z.
Thermal displacement ellipsoids are drawn at the 50% probability level.
complexes with Sn and hence could lower the toxicity of poten-
tial cytotoxic agents against normal cells. In the present paper,
we report on the synthesis and characterization of various
organotin complexes based on 2,6-di-tert-butyl-4-mercapto-
phenol (Scheme 1).
Crystal structures
Re-crystallization of the complexes from CH3CN solutions gave
colourless crystals which were used for crystallographic analy-
sis. The crystal and molecular structures of complexes 1, 4, 5,
and 7 determined by X-ray diffraction are shown in Fig. 1–4,
while selected bond distances and angles are given in Table 1.
The symmetry operator is (i): 1 − x, y, 1.5 − z. Thermal displa-
cement ellipsoids are given at the 50% probability level for 1,
4, 5, and 7. All compounds were found to be covalent mono-
mers in the solid state with a distorted tetrahedral geometry
around the Sn center, while the 2,6-di-tert-butyl-4-mercapto-
phenol is coordinated to the tin(IV) ion via a S atom. The Sn–S
bond distances in 1–7 range between 2.4033(15) and 2.4228(15)
Å. These are slightly shorter than the corresponding ones
found in the tin(IV) analogues:12 {R2Sn(PMT)2} (Sn1–S2 =
2.4616(14) Å and Sn2–S3 = 2.4631(14) Å, PMTH = 2-mercapto-
pyrimidine), {R2Sn(MPMT)2} (Sn1–S1 = 2.4664(8), Sn1–S2 =
The X-ray crystal structures of the complexes are described
herein. Antiradical properties of the complexes were studied in
order to clarify the mechanism of biological activity. The
in vitro anti-tumor activity of the complexes against human
breast (MCF-7) and human cervix (HeLa) adenocarcinoma
cells was also studied. The complexes were also tested against
normal cells (MRC-5).
Results and discussion
Syntheses
Compounds 1 and 6 were synthesized as was described pre-
viously.13 New organotin(IV) complexes 2–5 and 7 have been
synthesized by interaction of organotin chlorides RnSnCl4−n
(n = 2, 3) with 2,6-di-tert-butyl-4-mercaptophenol (RSH) in
methanol solution in the presence of an equivalent amount of
KOH as shown in Scheme 1. The diorganotin dichloride 8
was obtained by the remetallation reaction of RHgCl and Sn in
o-xylene under reflux.12 Compounds 1–7 are stable in air and
in solution. Compounds 2–5 and 7 were characterized by IR,
1H and 13C NMR spectroscopy and elemental analysis.
2.4265(8) Å, MPMTH
= 2-mercapto-4-methyl-pyrimidine),
{R2SnCl(PYT)} (Sn1–S1 = 2.4504(8) Å and Sn2–S2 = 2.4543(8) Å,
PYTH = 2-mercapto-pyridine) and {R2SnCl(MBZT)} (Sn1–S1 =
2.4796(10) Å, MBZTH = 2-mercapto-benzothiazole).
In the case of 1 the two methyl groups are coordinated with
the tin atom as well as two 2,6-di-tert-butyl-4-mercaptophenol
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Fig. 4 Ellipsoid plot of 7 with the atom numbering scheme. Hydrogen
atoms are omitted for clarity. Thermal displacement ellipsoids are drawn
at the 50% probability level.
Fig. 3 Ellipsoid plot of 5 with the atom numbering scheme. Hydrogen
atoms are omitted for clarity. Thermal displacement ellipsoids are drawn
at the 50% probability level.
A hydrogen bond O4–H4⋯S1ii (symmetry operator ii: x + 1,
y, z) with distance H4⋯S1ii = 2.936 Å was found in the struc-
ture. The bond angles around Sn(IV) vary between 98.0(7) and
114.85(16)° showing a distorted tetrahedral arrangement. In
the case of 7 the bond angles around Sn vary between 102.0(2)
and 114.2(5)°.
Thus, the low steric effects from the methyl and phenyl
groups make Sn(IV) atoms in 1 and 4 more accessible for
coordination with active groups in biological systems. In the
case of 5 with four hindered phenol groups, hydrogen bond
formation with proteins and biosubstrates is maximally feasi-
ble owing to the diphilic character of phenol groups.
ligands (Fig. 1). The bond angles around Sn(IV) vary between
101.2(3) and 117.8(4)°. The whole molecule is twisted in a way
that allows the Sn(IV) atoms to be exposed in the free space
due to the higher distortion of the C–Sn–C bond angle from
the ideal tetrahedral value. In the case of 4 the planes of 2,6-
di-tert-butyl-4-mercaptophenol ligands and two phenyl groups
are angled at 81 and 41 degrees relative to each other, respecti-
vely (Fig. 2). The bond angles around tin(IV) vary between
106.24(13) and 112.1(3)° for C15–Sn–S1i and C15–Sn–C15i,
respectively.
Analysing the crystal structure of 5 with four hindered
phenol groups we have found that the coordination poly-
hedron is much more distorted than the starting diorganotin
compound R2SnCl2.12 The S1–Sn–S2 angle in compound 5 is
the smallest one in the series of 1, 4, 5, and 7 due to the steric
hindrance of phenol groups.
Spectroscopy
(a) Vibrational spectroscopy. The solid state IR spectrum
of ligand RSH shows strong vibrational bands at 1425,
Table 1 Selected bond lengths (Å) and angles (°) for complexes 1, 4, 5, and 7. Symmetry operator i: 1 − x, y, 1.5 − z
Complex, bond lengths (Å)
1
4
5
7
Sn–C30
Sn–C29
Sn–S2
Sn–S1
S1–C1
O1–C4
S2–C15
O2–C18
2.119(7)
2.137(8)
2.404(2)
2.4119(18)
1.801(7)
1.381(8)
1.783(7)
1.382(8)
Sn–C15
Sn–S1
S1–C1
O1–C4
2.126(5)
2.4139(14)
1.794(5)
1.375(6)
Sn–C61
Sn–C41
Sn–S1
Sn–S2
S1–C1
O1–C4
S2–C21
O2–C24
O3–C44
2.081(7)
2.130(7)
2.4228(15)
2.4033(13)
1.773(5)
1.387(6)
1.765(7)
1.375(8)
1.352(9)
Sn–C21
Sn–C15
Sn–C27
Sn–S1
S1–C1
O1–C4
2.116(9)
2.145(10)
2.159(12)
2.413(3)
1.782(10)
1.374(11)
Angles (°)
C30–Sn–C29
C30–Sn–S1
C29–Sn–S1
C30–Sn–S2
C29–Sn–S2
S1–Sn–S2
117.8(4)
111.3(2)
101.2(3)
103.4(4)
114.3(3)
108.93(10)
C15–Sn–C15i
C15–Sn–S1
C15–Sn–S1i
S1–Sn–S1i
112.1(3)
C61–Sn–C41
C61–Sn–S1
C41–Sn–S1
C61–Sn–S2
C41–Sn–S2
S1–Sn–S2
107.3(3)
C21–Sn–C15
C21–Sn–C27
C15–Sn–C27
C21–Sn–S1
C15–Sn–S1
C27–Sn–S1
114.2(5)
114.0(5)
109.0(3)
102.0(2)
109.5(4)
107.8(4)
111.15(13)
106.24(13)
109.97(7)
114.25(18)
114.85(16)
111.97(14)
110.23(15)
98.07(5)
6882 | Dalton Trans., 2014, 43, 6880–6890
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Table 2 Characteristic signals in 1H, 13C NMR spectra of organotin derivatives in CDCl3
1H-NMR δ (ppm)
13C-NMR δ (ppm)
Compound
C(CH3)3
OH
C2H
C1
C2
C3
C4
RSH
1a
2
1.44
1.42
1.44
1.44
5.17
5.13
5.16
5.15
7.19
7.36
7.35
7.36
118.27
120.88
120.95
121.26
128.34
131.49
131.62
131.64
137.00
136.58
136.59
136.54
152.89
153.20
153.00
153.02
3
4
1.29
1.27; 1.36
1.43
5.08
5.00; 5.28
5.10
7.19
7.20; 7.23
7.21
119.29
121.35; 128.42
122.74
132.13
130.94; 132.61
131.15
138.23
136.17; 136.66
136.40
153.19
152.52; 155.50
152.54
5
6a
7
1.22
5.04
7.11
120.12
136.75
137.97
152.93
a Ref. 13.
1234 cm−1 that are assigned to δ vibrations of the aromatic
system, 2871–3000 cm−1 ν(CH), 2573 cm−1 ν(SH) and
3618 cm−1 ν(OH). The IR spectra of the complexes in the
2000–400 cm−1 region have mainly the function of finger-
prints, the vibration bands being too numerous for a reaso-
nably correct assignment. The vibration band is observed in the
IR spectrum of RSH; the thiol ligand in the 3618 cm−1 region
corresponds to the stretching vibrations of the O–H bond of
the hindered phenol non-associated OH group. The ν(SH)
vibration band is not observed due to the Sn–S bond formation
in the spectra of 1–7. Upon coordination of the ligand with a
tin atom in 1–7, the characteristic ν(OH) absorption band
of the non-associated phenol group appeared in the
3620–3640 cm−1 region. The shifts of characteristic ν(OH)
vibrational bands for complexes 1–7 at 3620–3639 cm−1 to a
region of higher frequencies when compared with non-
coordinated ligands confirm S-coordination of the ligand and
the redistribution of electron density in tin complexes. It is
well known that in non-coordinating hindered phenols with
donor substituents in the para-position and in non-polar sol-
vents, the phenol group is not involved in hydrogen bond for-
mation. This causes the OH stretching to be registered in the
region of 3600–3650 cm−1 in the IR spectra of the complexes.14
Scheme 2 Chemical oxidation of 1–7 to phenoxyl radicals.
Table 3 Parameters of ESR spectra of phenoxyl radicals (toluene, PbO2,
293 K)
Number of lines
in spectrum
Radical
g-Factor
a(2H) (G)
RSH′
1′
2′
3′
4′
5′
6′
7′
8′
2.0047
2.0050
2.0045
2.0051
2.0043
2.0046
2.0047
2.0046
2.0041
1.51
1.60
1.47
1.50
3
3
3
3
3
1
3
3
11
1.51
a
1.46
1.53
1.50
a A broad singlet with low intensity was detected.
Thus, in the IR spectrum of 5, the non-associated phenol chemical oxidation of phenol derivatives 1–7 was carried out
group appeared in the 3623–3639 cm−1 region, as expected, in toluene using PbO2 yielding phenoxyl radicals 1′–7′
since hydrogen bonds were found according to structural data (Scheme 2). The presence of either one or up to four phenol
of 5 (see above).
groups in organotin complexes 1–7 might create several radical
(b) NMR spectroscopy. 1H NMR data for complexes 1–7 centers. The intensity of signal of radical 8′ derived from 8 was
are given in Table 2. No signals for SH protons were observed lower than that of radical 1′ that may be due to the influence
in the spectra of thiolates 1–7 in CDCl3 solution. The charac- of electron-acceptor chloride atoms on the distribution of the
teristic signals of tert-butyl protons and hindered phenol unpaired electron in the phenoxyl radical. In general, the orga-
groups are shifted to strong field in the spectra of thiol com- nometallic derivatives with the phenoxyl group can decompose
plexes in comparison with those of the RSH one, confirming by an intramolecular mechanism as was shown for the radical
the ligand–metal coordination. Furthermore, the aromatic formed from RPt(PPh3)2SnCl3.16
C–H protons of 5 show a coupling constant with paramagnetic
The X-band ESR spectra measured at 293 K show the spin
isotopes 117,119Sn (S = 1/2). The value of 3J(SnH) in the case density distribution in the organic ligands. The isotropic
of starting bis-aryltin dichloride 8 was found to be equal to g-values for the radicals 1′–7′ are in the range 2.0041–2.0060
84 Hz,12 which is different from the one of complex 5 (66 Hz).
with hyperfine coupling constants derived from protons
(c) ESR spectroscopy. It is known that the antioxidant (Table 3).
activity of 2,6-di-tert-butylphenols is influenced by the stability
The EPR spectra of radicals 1′–7′ (Fig. 5) exhibit multiplet
of the corresponding phenoxyl radicals.15 For this reason the signal corresponding to the coupling of the unpaired electron
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Dalton Trans., 2014, 43, 6880–6890 | 6883