Synthesis, characterization and in vitro antitumor activity of three organotin(IV) complexes with carbazole ligand

Inorganica Chimica Acta 365 (2011) 302–308

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Synthesis, characterization and in vitro antitumor activity of three organotin(IV) complexes with carbazole ligand Banfeng Ruan b, Yupeng Tian a,c,⇑, Hongping Zhou a,⇑, Jieying Wu a, Rentao Hu d, Chenhao Zhu a, Jiaxiang Yang a, Hailiang Zhu b,⇑ a

Department of Chemistry, Key Laboratory of Inorganic Materials Chemistry of Anhui Province, Anhui University, Hefei 230039, PR China State Key Laboratory of Pharmaceutical Biotechnology, Nanjing University, Nanjing 210093, PR China State key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China d Anhui Research Institute of Chemical Industry, Hefei 230039, PR China b c

a r t i c l e

i n f o

Article history: Received 29 May 2010 Received in revised form 12 September 2010 Accepted 20 September 2010 Available online 24 September 2010 Keywords: Carbazole Organotin(IV) carboxylate Single crystal diffraction Antitumor activity

a b s t r a c t Three novel organotin(IV) complexes with 2-(9H-carbazol-9-yl) acetic acid (HL), of the formulae {[nBu2SnOL]2O}2 (1), [nBuSn(O)OL]6 (2) and [nBu3SnOL]6 (3) were prepared. All compounds were characterized by X-ray crystallography, confirming that complex (1) is tetranuclear one with ladder framework, complex (2) is a hexanuclear organotin(IV) complex with drum structure and complex (3) is a macrocycle with 24-membered stannoxane ring. Furthermore, all complexes were tested in vitro for their cytotoxic activity, using human hepatocellular carcinoma cell line (BEL-7402) and human hepatocellular liver carcinoma cell line (HepG2). Complex (1) displayed the best cytotoxicity and can be pointed out as a promising substrate to be subject of further investigations. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Since the discovery of the antitumor activity of cisplatin during the 1960s, the application of metal complexes in the treatment of numerous human diseases has been a vigorously expanding area in biomedical and inorganic chemistry research [1]. The DNA nitrogen base binding cisplatin, oxalylplatin, nedaplatin and carboplatin, all of which are metallocomplexes of platinum(II), have been proved to be the most effective and widely used coordination compounds as anticancer drugs [2]. In spite of its high activity, the application of cisplatin and its analogues has significant disadvantages that include: (1) poor water solubility, (2) severe side effects that are typical of heavy metals toxicity, and (3) the development of drug tolerance by the tumor [3]. The last two are the major driving force behind current research in the field of novel anticancer agent development. With the aim to avoid, or improve, the problems associated with the use of platinum compounds as therapeutic agents, a substantial investigation of other metals (Au, Ag, Cu, Ti, Ga, Co, and Sn) is underway [4]. In vitro screening of new coordination ⇑ Corresponding authors. Address: Department of Chemistry, Key Laboratory of Inorganic Materials Chemistry of Anhui Province, Anhui University, Hefei 230039, PR China. Tel.: +86 551 5107342 (Y. Tian). E-mail address: [email protected] (Y. Tian). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.09.024

compounds, followed by selecting the best performing anticancer active compounds, is still the best way of identifying potential drug candidates. The QSAR principles, successfully developed for numerous organic and identified natural compounds, are not fully applicable for metal complexes because of their unpredictable stability and kinetics toward ligand substation reactions. There is, however, promising and recent success in using different organotin(IV) derivatives, which have shown acceptable in vivo cytotoxicity and antiproliferative activity as new chemotherapy agents [5]. For example, compound [Sn(C6H5)3{OOCC6H3-3,4(NH2)2}] is very active against HCV29T cells [6]. The in vitro cytotoxic activity of the compound derived from the reaction of potassium b-{[(E)-1-(2-hydroxypheny)ethylidene]amino}propionate with (n-Bu)2SnCl2 with formula {[n-Bu2Sn(L2H)]2O}2, against various cell lines has shown quite promising antiproliferative activity especially compared with cisplatin [7]. The in vitro antiproliferative biological activity of the diorganotin(IV) complexes with 2-mercapto-6-nitrobenzothiazole (MNBT) of formulae [Ph2SnCl(MNBT)], [(PhCH2)2Sn(MNBT)] and [(n-Bu)2Sn(MNBT)2] towards culture cells of Ehrlich ascites carcinoma showed inhibition rates of 78%, 79% and 86%, respectively [8]. The organotin(IV) flufenamates [Bu2(flu)SnOSn(flu)Bu2]2 and [Bu2Sn(flu)2] (Flu = flufenamic acid) exhibited high cytotoxic activity against the cancer cell line A549 [9]. The Et2SnCl2.L [L = N-(2-pyridylmethylene)-4-toluidine (OTC)] has been subjected to investigation for its cytotoxic effect in mouse

B. Ruan et al. / Inorganica Chimica Acta 365 (2011) 302–308

bone marrow cells (BMCs) and human peripheral blood lymphocyte cells (HPBLs) [10]. Organotin(IV) complexes with Schiff bases derived from salicyladehyde and aminopyridines of general formulae Me2SnCl2.2L were screened against the human myleogenous leukaemia K562, cervix (HeLa) and murine L929 fibrosarcoma cell lines and the results were compared with those of the anticancer drugs, cisplatin, carboplatin and oxaliplatin [11]. Organotin(IV) carboxylates comprises the class of tin complexes which has attracted particular attention due to their potential cytotoxicity and biocide activity as well as their industrial and agricultural applications [12–15]. Our recent efforts have focused on the aspects of synthesis and biological activity of new organotin(IV) complexes with different RCOOH (R = carbazole) as ligands because the carbazole and its derivatives have shown pronounced cytotoxicity, anticancer and antimicrobial activity [16–19]. Herein we report the synthesis and characterization of three new organotin(IV) complexes derived from a carbazole carboxylic acid. We also report their in vitro cytotoxicity in hepatocellular carcinoma (BEL-7402) and human hepatocellular liver carcinoma cell line (HepG2), and some preliminary structure–activity relationships are discussed. 2. Results and discussion 2.1. IR The IR spectra of the three organotin(IV) complexes indicate the complete disappearance of the stretching vibration bands of O–H of their free ligand and show characteristic absorptions at 1625– 1706 cm1 assigned to v(C@O). These features are consistent with the double deprotonated carboxylic form of the ligand. For all the complexes, no v(OH) band was detected, in accord with the deprotonation and coordination to the metal of both oxygens of the ligand. The bands in the region 500–600 cm1 were assigned to v(Sn–O). Furthermore, there are two kinds of Sn–O–Sn vibrations in the respective spectra of complexes (1) and (2), which is absolutely absent in complex (3). 2.1.1. 119Sn NMR The values of 119Sn chemical shift for the organotin(IV) compounds may be used to give tentative indications of the environment around tin atoms. For complex (1), the 119Sn NMR data show two signals, indicating the presence of two different types of tin sites of five- and six-coordinated in solution. It also reveals that the tetrameric structure found in the solid state of complex (1) is retained in solution [20]. For complexes (2) and (3), the 119 Sn NMR resonances occur at d 212.4 and 132.4 ppm respectively, which are in accord with their hexa- and penta-coordination [21]. 2.2. Crystal structures of complexes (1–3) The molecular structure of (1) is shown in Fig. 1(a) and the selected bond lengths and angles are given in Table 1. Complex (1) is a centrosymmetric dimmer distannoxane, where a central cyclic four-membered Sn2O2 core is linked to two terminal n-Bu2Sn entities through the l3-O atoms (O(9)). Each pair of the exocyclic Sn atoms and endocyclic Sn atoms are symmetrically bridged by the ligand (L) through the carboxylate oxygens [Sn(1)–O(1) 2.239 and 0 Sn(2)–O(2) 2.259 Å A]. Each exocyclic Sn atom is also coordinated by an isobidentate chelating carboxylate ligand (Sn(2)–O(5) 2.176 0 and Sn(2)–O(6) 2.771 Å A). Thus the central Sn2O2 core is linked to the two outer Sn2O2 rings to result a ladder-framework structure. The coordination number is five and six for Sn(1) and Sn(2), respectively. The metal coordination geometry for Sn(1) is described as

303

intermediate between square pyramidal and cis-trigonal bipyramidal, in which the carboxylato ligand spans equatorial and axial sites and for Sn(2) as distorted octahedral. Distortions from the ideal 0 geometries may be related to the approach 2.978 Å A of the O(5A) atom to Sn(1). This distance is long for primary Sn–O bonding, but represents a type of secondary interaction [22]. As shown in Fig. 1(b) and (c), intermolecular H bonds between adjacent molecules lead 0 to a 1D chain structure and he length is 3.744 Å A (C21O6) and the angle is 143.3° (C21–H21AO6). Furthermore, C–Hp interactions between adjacent 1D0 chains generate a 3D structure and the C–Hp separation is 2.864 Å A. The molecular structure of (2) is shown in Fig. 2 and the selected bond lengths and angles are given in Table 2. The centrosymmetric structure of (2) is built around a drum shaped Sn6O6 central stannoxane core that is made up of two hexameric Sn3O3 rings. These hexameric Sn3O3 rings exist in a puckered chair conformation and form the upper and lower lids of the drum polyhedron. The two Sn3O3 rings are connected further by six Sn–O bonds containing tri-coordinate O atoms and thus the side faces of the drum are characterized by six four-membered Sn2O2 rings. It can be seen that four-membered Sn2O2 rings are not planar; the oxygen atoms are titled toward the cavity of the drum. Thus the interior of the drum can be considered as a crown made of six oxygen atoms in a trigonal antiprismatic arrangement. The two tin atoms in each of the six Sn2O2 rings are bridged by a carboxylate ligand to form a symmetrical bridge between two carboxylate ligands. The Sn–O bond lengths inside the 0 core range between 2.076 (Sn(2)–O(7)) and 2.091 Å A (Sn(1)–O(7)). These distances are comparatively shorter than the Sn–O bonds to 0 the bridging carboxylate ligands (2.162–2.196 Å A). All the six tin atoms are chemically equivalent and are six coordinate with three of the coordination sites occupied by bridging tri-coordinate oxygen atoms. While oxygen atoms from the bridging carboxylate ligands occupy two of the coordination sites, the sixth coordination site is occupied by the n-butyl group [23]. The molecular structure of (3) is shown in Fig. 3 and the selected bond lengths and angles are given in Table 3. The molecule consists of six tri-n-butyltin fragments linked together by six bridging carboxylate groups, and together forms a hexanuclear 24-membered macrocyclic ring. The six tin atoms, twelve oxygen atoms and six carbon atoms that form the macrocycle are almost in plane. The geometry of each of the three pairs of tin atoms is a distorted trigonal bipyramid with the electronegative oxygen atoms occupying the apical positions, and the three n-butyl groups lying in the equatorial plane, with average C–Sn–C angles of 119.9°. Thus, all the tin atoms exist in a distorted trigonal bipyramidal penta-coordinate environment with two oxygen atoms and three n-butyl groups. The sum of the trigonal plane angles is almost 360° (359.85°), which illustrates Sn(1), C(16), C(21), C(25) are coplanar. The O–Sn–O skeleton, with an average value of 177.2°, is bent and slightly larger than that of di- and triorganotin carboxylates with a hexameric structure. There are two kind of Sn–O bonds: Sn(1)–O(1) (2.177 Å) and Sn(1)–O(2) (2.367 Å). The presence of two Sn–O bond lengths around each of the tin centers confirms the asymmetric in bonding. The asymmetric pattern of Sn–O bond lengths around the macrocycle is similar for each independent carboxylate. The Sn(1)–O(2) bond length is 3.497 Å, which implies that a significant interaction of SnO is present in complex (3). Thus, the O–Sn–O angles in (3) are all 177.2°, with a modest deviation from 120° in the C–Sn–C angles and the C–O bonds in (3), C(15)–O(1), 1.250 Å; C(15)–O(2), 1.229 Å are little different from each other [24]. 2.3. Biological activity All of the synthesized compounds were screened for the preliminary in vitro anticancer activity against two different cell lines:

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Fig. 1. (a) Molecular structure of complex (1) (all hydrogen atoms are omitted for clarity). (b) Intermolecular H bonds (C–HO) between molecules in the 1D chains. (c) 1D chains interacted with each other through the C–Hp stacking interactions formed by adjacent molecules to generate a 3D structure.

a human hepatocellular carcinoma cell line (BEL-7402) and human hepatocellular liver carcinoma cell line (HepG2), at four different concentrations. The IC50 values for the compounds were tested, and the results are summarized in Table 5.

The results obtained indicate the order of the antitumor activity as: (1) > (3) > (2) > organotin(IV) precursors. Based on the data analysis, possible structure–activity relationships could be recognized as follows: (i) It is reasonable that the di-n-butyltin(IV)

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B. Ruan et al. / Inorganica Chimica Acta 365 (2011) 302–308 Table 1 Selected bond lengths (Å) and angles (°) in (1).

Table 2 Selected bond lengths (Å) and angles (°) in complex (2).

Bond lengths Sn(1)–O(1) Sn(1)–O(9A) Sn(2)–O(2) Sn(2)–O(6)

2.239(5) 2.168(4) 2.259(6) 2.771(8)

Sn(1)–O(9) Sn(1)–Sn(1A) Sn(2)–O(9) Sn(2)–O(5)

2.050(6) 3.318(1) 2.023(6) 2.176(5)

Bond angles O(1)–Sn(1)–O(9A) Sn(1)–O(9)–Sn(2) O(6)–Sn(2)–O(5) Sn(1)–O(9)–Sn(1A) O(2)–Sn(2)–O(6)

168.7 133. 4 51.5 103.7 136.9

O(1)–Sn(1)–O(9) O(9)–Sn(2)–O(2) Sn(2)–O(9)–Sn(1A) O(9)–Sn(1)–O(9A) O(5)–Sn(2)–O(9)

93.6 88.6 122.3 76.23 83.1

Bond lengths Sn(1)–O(1) Sn(1)–O(7) Sn(1)–O(9) Sn(2)–O(5) Sn(2)–O(8) Sn(3)–O(2) Sn(3)–O(7) Sn(3)–O(9)

2.173(2) 2.091(3) 2.090(2) 2.176(2) 2.100(2) 2.195(3) 2.089(2) 2.087(2)

Sn(1)–O(3) Sn(1)–O(8) Sn(2)–O(4) Sn(2)–O(7) Sn(2)–O(9) Sn(3)–O(6) Sn(3)–O(8)

2.162 (3) 2.078(2) 2.163(3) 2.076(3) 2.090(2) 2.196(2) 2.085 (3)

Bond angles O(3)–Sn(1)–O(8) O(3)–Sn(1)–O(1) O(7)–Sn(1)–O(9)

87.2 79.3 77.6

O(3)–Sn(1)–O(9) O(7)–Sn(1)–O(8)

86.7 105.5

compounds are found to be significantly correlated with the hydrophobic properties [28]; (iii) the activity of the cytotoxic complex can be attributed to the ability of the ligand to form unobstructed H-bonds and/or pp stacking that may facilitate an intracellular uptake of complexes. As the experimental results are preliminary, further study on the antitumor effects of these compounds is highly recommended. 2.4. Conclusions Complexes (1–3) were prepared and fully characterized by a series of spectroscopic methods. Complex (1) displayed good cytotoxicity higher than 5-fluorouracil toward the tumor cell lines in vitro. Complex (1) displayed the best cytotoxicity and can be pointed out as a promising substrate to be subject of further investigations in vivo on animal models. 3. Experimental 3.1. Chemistry

Fig. 2. (a) Molecular structure of complex (2) (all hydrogen atoms and n-butyl groups have been omitted for clarity) and (b) side view of the hexameric stannoxane core in 2 (all the Sn2O2 and Sn3O3 rings puckered).

derivative (1) with a weaker Sn–O bond are more active against the two human tumor cell lines than complexes (2) and (3), because the further ligand replacement with biological ligands is possible 0 [25]. The bond Sn(2)–O(6) with the length of 2.771 Å A in complex (1) is much longer than other Sn–O bonds. It is conceivable that ligand replacement from Sn–O-core cluster to Sn–DNA complex following the Sn–O cleavage for (1) is expected in this case, according to Huber and Saxena [26]; (ii) the polymeric diorganotin complexes with the long carbon chain butyl organotin(IV) precursor are great active ones compared to the known mononuclear diorganotin complexes [27]. The inhibitory potencies of the

3.1.1. Materials and methods The organotin(IV) precursors nBu2SnO, nBuSnO(OH) and n(Bu3Sn)2O were purchased from Aldrich and were used as received. The ligand, 2-(9H-carbazol-9-yl) acetic acid (HL), was synthesized as reported [29]. It had been employed in reactions with the organotin(IV) precursors, see Scheme 1. All the solvents were dried according to reported procedures. Melting points were determined on an Electrothermal Mettler-Toledo (FP62) instrument. 1H and 13C NMR were recorded at Bruker AV 400 spectrometer at 25 °C with TMS and solvent signals allotted as internal standards. 119 Sn NMR spectra (proton-decoupled) were recorded on a Bruker AV 400 spectrometer operating at 150 MHz; resonances are referenced to tetramethyltin (external standard, 119Sn). Elemental analyses were performed on a CHN–O-Rapid instrument and were within ±0.4% of the theoretical values. IR spectra were record on a Nicolet 470 FT-IR spectrophotometer using KBr pellets in the range from 4000 to 400 cm1. 3.1.2. Synthesis of {[nBu2SnOL]2O}2 (1) To a round-bottom-flask containing HL (0.225 g, 1 mmol) dissolved in benzene (15 mL), it was slowly added nBu2SnO (0.249 g, 1 mmol) also in benzene (15 mL). After 24 h of reflux with stirring the solvent was removed in vacuo. The residue was dissolved in CH2Cl2 (10 mL) and filtered. X-ray quality crystals were obtained from a dichloromethane/ethyl ether solution (4:1) at room temperature. Yield: 75.0%, m.p. 260 °C. 1H NMR (400 Hz, CDCl3): 0.64 (t, J = 7.2 Hz, 24H), 0.69–0.85 (m, 48H), 4.64 (s, 8H), 7.17–8.06 (ArH, 32H). 13C NMR (CDCl3): d = 13.51, 26.60, 27.41, 45.86, 68.04, 108.33, 119.43, 120.42, 123.11, 125.74, 140.68. Anal.

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Fig. 3. Molecular structure of complex (3) (all hydrogen atoms have been omitted for clarity).

Table 3 Selected bond lengths (Å) and angles (°) in complex (3).

Table 4 Crystallographic data and structure refinements for complexes (1–3).

Bond lengths Sn(1)–O(1) Sn(1)–O(2)

2.177(8) 3.497(7)

C(15)–O(1) C(15)–O(2)

1.250(1) 1.229(1)

Bond angles O(1)–C(15)–O(2) O(2)–Sn(1)–O(1)

124.3 177.2

C(15)–O(2)–Sn(1)

144.3

Calc. for C88H112O10N4Sn4: C, 56.80; H, 6.07; N, 3.01. Found: C, 56.97; H, 6.05; N, 3.02%. 119Sn NMR (CDCl3): d = 163.4 and 206.8 ppm. IR (KBr): 3434, 3056, 2955, 2925, 2863, 1658, 1597, 1578, 1489, 1458, 1433, 1399, 1380, 1355, 1312, 1266, 1228, 1207, 1153, 1075, 1050, 1020, 999, 964, 921, 872, 847, 789, 748, 719, 701, 679, 635, 622, 535, 421, cm1. 3.1.3. Synthesis of [nBuSn(O)OL]6 (2) Prepared accordingly using HL (0.225 g, 1 mmol) and nBuSnO(OH) (0.210 g, 1 mmol). Yield: 72.0%, m.p. 300 °C. 1H NMR (400 Hz, CDCl3): 0.60 (t, J = 7.2 Hz, 18H), 0.73–0.93 (m, 36H), 4.33 (s, 12H), 7.09 (d, J = 8.0 Hz, 12H), 7.20 (dd, J = 7.2 Hz, 12H), 8.31 (dd, J = 7.2 Hz, 12H), 8.96 (d, J = 7.6 Hz, 12H). 13C NMR (CDCl3): d = 13.27, 25.81, 26.29, 26.88, 46.41, 108.77, 119.28, 120.22, 123.05, 125.62, 140.62, 175.91. Anal. Calc. for C108H114O18N6Sn6: C, 51.96; H, 4.60; N, 3.37. Found: C, 52.19; H, 4.62; N, 3.39%. 119 Sn NMR (CDCl3): d = 212.4 ppm. IR (KBr): 3434, 3052, 2955, 2926, 2868, 1622, 1569, 1486, 1448, 1359, 1321, 1257, 1230, 1205, 1120, 1073, 1032, 1001, 926, 869, 846, 801, 750, 721, 674, 615, 554, 528, 446, cm1. 3.1.4. Synthesis of [nBu3SnOL]6 (3) Prepared accordingly using HL (0.225 g, 1 mmol) and n(Bu3Sn)2O (0.298 g, 0.5 mmol). Yield: 78.0%, m.p. 135 °C. 1H NMR

Formula Molecular weight Crystal system Space group Crystal size (mm3) a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Dcalc (g cm3) l (mm1) F(0 0 0) h Range (°) Reflections collected Reflections unique Parameters Goodness-offit on F2 R1, wR2 [I > 2r(I)] R1, wR2

1

2

3

C88H112O10N4Sn4 1860.58

C108H114O18N6Sn6 2496.19

C156H222O12N6Sn6 3085.72

triclinic

triclinic

rhombohedral

 P1 0.56  0.33  0.18

 P1 0.64  0.27  0.10

 P3 0.56  0.53  0.50

11.950(3) 13.432(4) 16.065(4) 101.282(3) 108.381(3) 109.531(3) 2171.7(10) 1 1.407 0.913 966 1.71–25.10 11259

12.765(3) 14.282(4) 16.465(4) 67.105(3) 75.091(3) 86.360(3) 2670.0(12) 1 1.490 6.159 1248 2.11–26.05 23008

33.290(4) 33.290(4) 12.5582(12) 90.00 90.00 120.00 12053(2) 3 1.163 0.973 4302 1.77–25.30 15855

7535

10458

4869

434 1.001

622 1.022

223 1.005

0.058, 0.1379

0.0320, 0.0660

0.069, 0.1924

0.1182, 0.1933

0.0467, 0.074

0.178, 0.2832

(400 Hz, d6-DMSO): 0.80 (t, J = 7.2 Hz, 54H), 0.94–0.98 (m, 36H), 1.16–1.22 (m, 36H), 1.42 (t, J = 7.6 Hz, 36H), 4.92 (s, 12H), 7.17 (d, J = 7.2 Hz, 12H), 7.39 (dd, J = 8.0 Hz, 24H), 8.12 (d, J = 8.0 Hz,

B. Ruan et al. / Inorganica Chimica Acta 365 (2011) 302–308

R O 4HL +

4nBu2SnO

R'

O

O

O

R

R'

O

Sn R'

R'

Sn

O

O

O

reflux

R

O

Sn

R' Sn

benzene

R'

R'

1

O

R R

O

R'

6HL +

6nBuSnO(OH)

benzene reflux

O

Sn

O

O

R

O

O R'

R' O O O Sn Sn R' Sn O O Sn O O R' O O Sn R O O O R'

R

2 R

R R

6HL +

3n(Bu3Sn)2O

benzene reflux

R' O Sn R' R' O R O R' SnR' R' O R

R' R' Sn R O R' O O R' R' Sn R' O R O R' Sn R' O R' R' O O R Sn R' R'

3

R= N R'= nBu Scheme 1. Synthesis of complexes (1–3).

1 2 3 nBu2SnO nBuSnO(OH) n(Bu3Sn)2O HL 5-Fluorouracila

cell parameters and data collections were performed with Mo Ka radiation (k = 0.71073 Å). Unit cell dimensions were obtained with least-squares refinements, and all structures were solved by direct methods with SHELXL-97. All the non-hydrogen atoms were located in successive difference Fourier syntheses. The final refinement was performed by full-matrix least-squares methods with anisotropic thermal parameters for non-hydrogen atoms on F2. The hydrogen atoms were added theoretically and riding on the concerned atoms. The crystal data and structure refinement were listed in Table 4. 3.3. Cytotoxicity ASSAY against BEL-7402 and HepG2 The cytotoxicity of the prepared complexes against human hepatocellular carcinoma cell line (BEL-7402) and human hepatocellular liver carcinoma cell line (HepG2) was evaluated as described elsewhere with some modifications [30]. Briefly, target tumor cells were grown to log phase in RPMI 1640 medium supplemented with 10% fetal bovine serum. After diluting to 2  104 cells mL1 with the complete medium, 100 lL of the obtained cell suspension was added to each well of 96-well culture plates. The subsequent incubation was permitted at 37 °C, 5% CO2 atmosphere for 24 h before the cytotoxicity assessments. Tested samples at pre-set concentrations were added to 6 wells with 5-fluorouracil co-assayed as a positive reference. After 48 h exposure period, 40 lL of PBS containing 2.5 mg mL1 of MTT (3(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide)) was added to each well. 4 h later, 100 lL extraction solutions (10% SDS-5% isobutyl alcohol-0.01 M HCl) was added. After an overnight incubation at 37 °C, the optical density was measured at a wavelength of 570 nm on an ELISA microplate reader. In all experiments three replicate wells were used for each drug concentration. Each assay was carried out at least three times. The results were summarized in Table 5. Acknowledgements

Table 5 Crytotoxic activities of the synthetic compounds. Compound

307

IC50 (lg/mL) HepG2

BEL-7402

1.93 ± 0.12 9.82 ± 0.34 4.23 ± 0.18 20.26 ± 0.46 24.16 ± 0.29 14.78 ± 0.13 40.34 ± 0.36 19.17 ± 0.38

0.60 ± 0.02 8.25 ± 0.62 3.70 ± 0.10 18.27 ± 0.68 18.18 ± 0.54 15.63 ± 0.20 38.13 ± 0.29 17.43 ± 0.25

Antitumor activities are expressed as IC50 (50% inhibitory concentration) toward the cell lines HepG2 and BEL-7402. Data are average data of triplicate assay. a Used as a positive control.

This work was supported by grants for the National Natural Science Foundation of China (20771001, 20875001and 50873001), Department of Education of Anhui Province (KJ2010A030, KJ2009A52), and the Team for Scientific Innovation of Anhui Province (2006KJ007TD). Appendix A. Supplementary material CCDC 704782, 704781 and 704784 contains the supplementary crystallographic data for complexes (1)–(3). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. References

12H). 13C NMR (d6-DMSO): d = 13.52, 16.63, 26.90, 27.62, 108.50, 119.19, 120.31, 123.15, 125.67, 140.76. Anal. Calc. for C156H222O12N6Sn6: C, 60.72; H, 7.25; N, 2.72. Found: C, 60.76; H, 7.26; N, 2.70%. 119Sn NMR (CDCl3): d = 132.4 ppm. IR (KBr): 3419, 3052, 2921, 2854, 1706, 1590, 1549, 1486, 1456, 1395, 1353, 1324, 1304, 1259, 1228, 1205, 1153, 1122, 1072, 1020, 1000, 923, 875, 845, 798, 750, 720, 704, 673, 624, 420, cm1. 3.2. Crystal structure determination Single crystal X-ray diffraction measurements were carried out on a Siemens Smart 1000 CCD diffractometer equipped with a graphite crystal monochromator situated in the incident beam for data collection at room temperature. The determination of unit

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