Cadmium complexes and cocrystals of indium complexes of benzothiazole derivatives and anticancer activities of the cadmium complexes

Polyhedron 54 (2013) 285–293

Contents lists available at SciVerse ScienceDirect

Polyhedron journal homepage: www.elsevier.com/locate/poly

Cadmium complexes and cocrystals of indium complexes of benzothiazole derivatives and anticancer activities of the cadmium complexes Tapan Karmakar a, Yuting Kuang b,1, Nouri Neamati b,1, Jubaraj B. Baruah a,⇑ a b

Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India Department of Pharmacology and Pharmaceutical Sciences, University of Southern California, School of Pharmacy PSC304, 1985 Zonal Avenue, Los Angeles, CA 90089, USA

a r t i c l e

i n f o

Article history: Received 16 December 2012 Accepted 15 February 2013 Available online 6 March 2013 Keywords: Benzothiazole Cadmium Indium Fluorescence Crystal structure Cytotoxicity

a b s t r a c t Cadmium and indium complexes [Cd(L1)(O2CCH3)2(H2O)] (1) [Cd(L2)2(O2CCH3)2(H2O)] (2), [L1In (H2O)Cl3]L1 (3) and [L1In(MeOH)Cl3]L1 (4) (where L1 = 2-(pyridin-2-yl)benzo[d]thiazole and L2 = 2-(pyridin-4-yl)benzo[d]thiazole) are synthesized and characterized. The cadmium complex 1 is hexacoordinated in which L1 act as N,N-chelate. The two acetate ligands of the complex bind to cadmium in monodentate and bidentate fashion. In the complex 2, the ligand L2 binds to cadmium ion through the nitrogen atom of the pyridine ring. The indium complexes 3 and 4 are hexa-coordinated; each has one L1 as chelating ligand and another L1 as molecule of crystallization. Fluorescence study revealed that the ligand L1 can detect indium(III) ions in the presence of cadmium(II) ions. In cell-based cytotoxicity assays, the ligands are biologically inactive, while the cadmium complexes 1 and 2 are cytotoxic in pancreatic cancer cell lines Mia Paca-2, BxPC-3 and Panc-1, with IC50 values as low as 16.0 lM. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The thiazole units are found in many naturally occurring compounds [1–12]. Thiazole ring is also a part of vitamin B1, penicillin and coenzymes [13,14]. Some thiazole derivatives are valuable medicines [15–17]. Thiazoles are used as neuroprotectors [18,19] and antioxidants [20,21]. They also form biologically potent metal complexes [22–29]. Thiazole derivatives are used in inorganic chemistry for building polydentate ligands [30]. 2,6-Bis(benzthiazol-2-yl)pyridine is an example of widely studied thiazole ligand [31–38]. Analogous chelate complexes of 2-(pyridin-2yl)benzo[d]thiazole (L1) with iridium, ruthenium and rhodium ions are inorganic anticancer drugs [39]. Fluorescent nature of these compounds provides avenues to study them as potential drugs [40–42]. A large numbers of heavy metal complexes of L1 [43– 52] are studied; but so far cadmium and indium complexes of the ligand is not studied. Thus, we have chosen to study cadmium(II) and indium(III) complexes of the ligand, both these ions have important status in the field of quantum dots to probe biological study [53–57]. Cadmium is a toxic metal, have lesser value in medicinal chemistry [58], but study on the drug activity of such compound would pave way to understand the structural requirements or the role of a metal ion in drugs. The ligand L1 is a well

⇑ Corresponding author. Tel.: +91 361 2582311; fax: +91 361 2690762. E-mail addresses: [email protected] (N. (J.B. Baruah). 1 Tel.: +1 323 442 2341; fax: +1 323 442 1390.

Neamati),

[email protected]

0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2013.02.068

known chelator for first row transition metal ions [59–64]. The ligand L1 is also known to form interesting coinage metal complexes [65,43]. The interaction of copper(II) complexes of L1 with DNA have been reported [59]. The metal complexes of benzoxazoles show anticancer activity [64]. In this study we present cadmium(II) and indium(III) complexes of L1 and L2 (Fig. 1) and the cytotoxicity of two cadmium complexes. 2. Experimental All the reagents and solvents were purchased commercially and were used without further purification, unless otherwise stated. The ligands L1 and L2 are prepared by condensing corresponding pyridine aldehyde with 2-aminothiophenol as reported in literature [65,43]. 1H NMR spectra were recorded by a Varian 400 MHz FTNMR spectrometer. The FT-IR spectra were recorded using a PerkinElmer spectrum one spectrometer in KBr pellets in the range 4000 to 400 cm1. The UV–Vis spectra and fluorescence spectra were recorded using PerkinElmer Lambda 750 spectrometer and PerkinElmer LS 55 spectrometer, respectively. Complex 1: To a methanol solution (20 ml) of L1 (0.053 g, 0.25 mmol), cadmium(II) acetate dihydrate (0.06 g, 0.25 mmol) was added and stirred at room temperature for 2 h. The solution was kept for crystallization and after 1 week yellow needle shape crystals were formed. Isolated yield: 74%. IR (KBr, cm1): 3432 (bs), 2924 (m), 2851(w), 1570 (m), 1492 (w), 1412 (m), 1157 (w), 1017 (w), 759 (w). 1H NMR (400 MHz, DMSO-d6): 8.72 (d, 4.4 Hz, 1H), 8.33 (d, 8 Hz, 1H), 8.16 (d, 8 Hz, 1H), 8.1 (d, 8.4 Hz, 1H), 8.04

286

T. Karmakar et al. / Polyhedron 54 (2013) 285–293

N

S

S

N

N N

2-(pyridin-2-yl)benzo[d ]thiazole (L1)

2-(pyridin-4-yl)benzo[d ]thiazole (L2)

Fig. 1. Structure of the ligands.

(t, 8 Hz, 1H), 7.58 (m, 2H), 7.49 (t, 7.6 Hz, 1H), 1.82 (s, 6H). Elemental Anal. Calc. for C16H16N2O5SCd; C, 41.71; H, 3.50; N, 6.08. Found: C, 41.58; H, 3.51; N, 5.66%. Molar conductance (103 M in methanol, RT) = 36.2 S cm2 mol1. Complex 2: Complex 2 was obtained by crystallization of a methanol solution (20 ml) containing L2 (0.106 g, 0.5 mmol) and cadmium(II) acetate dihydrate (0.06 g, 0.25 mmol). Isolated yield: 66%. IR (KBr, cm1): 3431 (bm), 1566 (s), 1479 (w), 1419 (s), 1346 (w), 1218 (w), 1122 (w), 1014 (m), 981 (w), 933 (w), 825 (m), 755 (m), 675 (s), 621 (m), 479 (w). 1H NMR (400 MHz, DMSO-d6): 8.79 (bs, 2H), 8.23(d, 8 Hz, 1H), 8.15(d, 8 Hz, 1H), 8.05(d, 4.8 Hz, 2H), 7.61(t, 7.2 Hz, 1H), 7.54(t, 8 Hz, 1H), 1.82(s, 3H). Elemental Anal. Calc. for C28H22N4O5S2Cd: C, 50.12; H, 3.30; N, 8.35. Found: C, 49.92; H, 3.21; N, 8.28%. Molar conductance (103 M in methanol, RT) = 39.6 S cm2 mol1. Complex 3: A solution of L1 (0.053 g, 0.25 mmol) in acetonitrile (20 ml), anhydrous indium(III) chloride (0.055 g, 0.25 mmol) was stirred for 4 h and the resulting mixture was filtered; the transparent liquid was kept for crystallization. After 1 week brown crystals appeared. Isolated yield: 65%. IR (KBr, cm1): 3375 (s), 3069 (w), 2923 (w), 1585 (w), 1492 (m), 1454 (m), 1433 (s), 1324 (m), 1248 (w), 1166 (w), 1076 (w), 1057 (w), 997(m), 784 (m), 759

(s), 741 (m). 1HNMR (400 MHz, DMSO-d6): 8.67 (d, 4.8 Hz, 1H), 8.27 (d, 7.6 Hz, 1H), 8.1 (d, 8 Hz, 1H), 8.04 (d, 8.4 Hz, 1H), 7.98 (t, 3.6 Hz, 1H), 7.53 (m, 2H), 7.44 (t, 8 Hz, 1H). Elemental Anal. Calc. for C24H16Cl3N4OS2In: C, 43.56; H, 2.44; N, 8.47. Found: C, 43.48; H, 2.29; N, 8.29%. Molar conductance (103 M in DMSO, RT) = 17.2 S cm2 mol1. Complex 4: The complex 4 was obtained from the reaction of L1 (0.053 g, 0.25 mmol) and anhydrous indium(III) chloride (0.055 g, 0.25 mmol) in a mixed solvent of methanol and acetonitrile (20 ml, 1:1 v/v). On standing, after 1 week brown crystals were formed. Isolated yield: 68%. IR (KBr, cm1): 3399 (bs), 3065 (w), 2923 (w), 1630 (w), 1494 (s), 1476 (w), 1454 (m), 1433 (s), 1324 (m), 1265 (w), 1163 (w), 1075 (w), 1050 (w), 998 (m), 784 (m), 760 (s), 740 (m), 625 (w). 1HNMR (400 MHz, DMSO-d6): 8.72 (d, 4.4 Hz, 1H), 8.32 (d, 7.6 Hz, 1H), 8.16 (d, 7.2 Hz, 1H), 8.09 (d, 8 Hz, 1H), 8.03 (t, 7.6 Hz, 1H), 7.57 (m, 2H), 7.49 (t, 8 Hz, 1H), 3.16 (s, 3H). Molar conductance (103 M in DMSO, RT) = 37.2 S cm2 mol1.

2.1. Structure determination The X-ray data were collected using a Bruker SMART APEX-II CCD diffractometer, with Mo Ka radiation (k = 0.71073 Å) at 298 K, with increasing x (width of 0.5 per frame) at a scan speed of 3 s/frame. The SMART software was used for data acquisition. Data integration and reduction were done with SAINT and XPREP software. Structures were solved by direct methods using SHELXS-97 and refined with full matrix least-squares on F2 using SHELXL-97 software. All the non-H atoms were refined in the anisotropic approximation against F2 of all reflections. The H-atoms were placed at their calculated positions and refined in the isotropic approximation. The

Table 1 Crystallographic parameters of the complexes. Compound No.

Complex 1

Complex 2

Complex 3

Complex 4

Formulae CCDC No. Mol. wt. Crystal system Space group Temperature (K) Wavelength (Å) a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Density (Mg m3) Abs. Coeff. (mm1) F(0 0 0) Total No. of reflections Reflections, I > 2r(I) Maximum 2h (°) Ranges (h, k, l)

C16H16N2O5SCd 890720 460.77 triclinic  P1 296(2) 0.71073 8.4722(6) 10.3502(7) 10.9933(8) 106.012(3) 95.986(4) 106.060(3) 873.24(11) 2 1.752 1.399 460 3072

C28H22N4O5S2Cd 890722 673.03 triclinic  P1 296(2) 0.71073 8.5391(3) 12.5833(4) 14.6876(7) 110.966(4) 95.317(4) 105.792(3) 1385.99(9) 2 1.613 0.984 680 5014

C24H16Cl3N4OS2In 902272 661.70 monoclinic P21/c 296(2) 0.71073 11.9173(4) 19.6123(7) 11.0515(4) 90.00 99.0690(10) 90.00 2550.73(16) 4 1.723 1.431 1312 4521

C25H20Cl3N4O1S2In 902273 677.74 monoclinic C2/c 296(2) 0.71073 20.5911(13) 11.9245(10) 23.3032(17) 90.00 102.856(5) 90.00 5578.4(7) 8 1.614 1.311 2704 4785

2696 50.5 10 6 h 6 9; 11 6 k 6 12; 13 6 l 6 13 97.1 full-matrix least-squares on F2 3072/8/236

4405 50.5 9 6 h 6 10; 15 6 k 6 14; 10 6 l 6 17 99.8 full-matrix least-squares on F2 5014/0/371

3970 50.48 13 6 h 6 14; 23 6 k 6 23; 13 6 l 6 13 98 full-matrix least-squares on F2 4521/0/316

2585 49.98 23 6 h 6 24; 8 6 k 6 13; 27 6 l 6 27 97.6 full-matrix least-squares on F2 4785/0/330

1.125

1.096

1.071

1.163

0.0536 0.0590 0.1575 0.1513

0.0356 0.0423 0.0648 0.0681

0.0375 0.0417 0.1306 0.1343

0.0683 0.1318 0.1579 0.1838

Complete to 2h (%) Refinement method Data/restraints/ parameters Goodness-of-fit (GOF) (F2) R indices [I > 2r(I)] R indices (all data) WR2 indices [I > 2r(I)] WR2 indices (all data)

287

T. Karmakar et al. / Polyhedron 54 (2013) 285–293

S N

N

Cd(OAc)2

S

N

O

MeOH/H2O

O

L1

S N Cd(OAc)2 N L2

MeOH

O

Cd

.... i

O H2O 1

O

S N N

N

Cd

H2O O

O N O

N S

.... ii

2 Scheme 1. The cadmium complexes of L1 and L2.

crystallographic parameters of the complexes 1–4 are tabulated in Table 1. 2.2. MTT assay Cytotoxicities of compounds were evaluated with 3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay [66,67]. Briefly, cells were seeded in 96-well tissue culture plates at the concentration of 4000 cells/well. After overnight attachment, cells were treated with a continuous exposure to studied compounds or DMSO (0.2% v/v) for 72 h. MTT solution was then added to each well for a final concentration of 0.3 mg/ml MTT. After 4 h incubation at 37 °C, media from each well was removed and DMSO was added to dissolve the formazan crystals formed by live cells. Absorbance was measured at 570 nm using a microplate reader (Molecular devices, Sunnyvale, CA). Cytotoxicities of the compounds are presented as percentage inhibition of cell proliferation against DMSO treated controls:

Inhibition of cell proliferation ð%Þ ¼ 1  ðODcompound  ODblank Þ=ðODcontrol  ODblank Þ: IC50 values for the cytotoxic compounds are determined from a plot of percentage inhibition of cell proliferation versus log compound concentration. 2.3. Colony formation assay Cells were seeded in 96-well tissue culture plates at the concentration of 200 cells/well. After overnight attachment, cells were treated with compounds under consideration or DMSO (0.2% v/v) for 24 h, and left in culture in fresh media until colonies were formed, typically 7–10 days after compound removal. Colonies were then fixed and stained with crystal violet solution (1% formaldehyde, 0.05% crystal violet, 1% methanol in distilled H2O). 3. Results and discussion The reaction of cadmium(II) acetate with L1 results in the formation of a mononuclear complex [Cd(L1)(O2CCH3)2(H2O)] (1) (Eq. (i) of Scheme 1). The complex 1 is a neutral complex as it is reflected in its very low molar conductance. The cadmium(II) ion in the complex has one monodentate and one bidentate acetate ligands. Besides this, the L1 coordinates in N,N-bidentate fashion to the cadmium(II) and the six coordination place is occupied by a water molecule. The Cd–N1, Cd–N2 bond distances and \N1– Cd–N2 bond angle are found to be 2.35 Å, 2.42 Å and 70.3°,

respectively, this suggests that the ligand (L1) and cadmium(II) form a five membered chelated structure. The relevant bond parameters in the complex are listed in Table 2. The coordinated ligand L1 adopts cisoid geometry, whereas the free ligand has transoid geometry. The tendency of the L1 to form chelate structure through formation of coordination bond by two nitrogen atoms of the ligand with various metal ions are reported in literature [43-52,59-65]. The ligand L1 has structural similarity in terms of binding sites with 2,20 -bipyidine. The binding constant of 2,20 bipyridine with cadmium was earlier determined [68]. It has been reported that the first binding constant 2,20 -bipyridine to bind to cadmium(II) is greater than the second, thus the formation of 1:1 complex is favored. In addition to these cadmium coordination polymer with thiophene dicarboxylate linkers having one 2,20 bipyridine per cadmium ion [69] have been reported. In this polymer sulfur atom of thiophene ring does not coordinate to cadmium ion. Similar 1:1 metal to 2,20 -bipyridine stoichiometry in mixed ligand polymer of cadmium, 2,20 -bipyridine and thiocarboxylate is also known [70]. There are 1:1 Rh or Ir complexes of 2-(pyridine2-yl)thiazole, this ligand has a very close structural resemblance to L1. In these complexes the ligand form chelate with metal ions through N,N-binding sites [39]. The ligand L1 itself forms 1:1 complexes with metal ions such as Cr, Mo, W and in all these cases the N,N binding mode is prevalent [49]. The IR spectra of the complex 1 show the O-H stretching from coordinated aqua ligand at Table 2 The bond angles and distances of complexes 1 and 2. Complex 1

Complex 2

M–L

dM–L (Å)

M–L

dM–L (Å)

Cd–O1 Cd–O2 Cd–O3 Cd–O5 Cd–N1 Cd–N2

2.402(4) 2.368(4) 2.313(5) 2.327(5) 2.351(4) 2.418(5)

Cd–O1 Cd–O2 Cd–O3 Cd–O4 Cd–O5 Cd–N2 Cd–N4

2.369(2) 2.483(3) 2.467(3) 2.354(3) 2.278(3) 2.360(3) 2.353(3)

\L–M–L

Angle (°)

\L–M–L

Angle (°)

\O3–Cd–O5 \O3–Cd–N1 \O5–Cd–N1 \O5–Cd–O2 \O5–Cd–O1 \N1–Cd–O1 \N1–Cd–N2

86.85(18) 90.16(16) 163.52(18) 96.12(16) 87.53(18) 92.88(16) 70.35(16)

\O5–Cd–O2 \O3–Cd–O2 \O5–Cd–N2 \O3–Cd–N2 \O5–Cd–N4 \O3–Cd–N4 \O2–Cd–N4 \O5–Cd–O4 \N2–Cd–O4 \N4–Cd–O4 \O3–Cd–O1

137.19(11) 140.38(9) 96.15(11) 98.39(9) 92.68(11) 85.80(9) 82.84(9) 134.12(10) 85.16(9) 90.76(10) 165.63(9)

288

T. Karmakar et al. / Polyhedron 54 (2013) 285–293

Fig. 2. The crystal structure of the cadmium complex (a) 1 (ORTEP 30% thermal ellipsoids); (b) 2 (ORTEP 50% thermal ellipsoids; hydrogen atoms are omitted for clarity).

N N S S

N

S

CH3CN/CH3OH

N

S

N

N In

Cl

N

InCl3

O

Cl

Cl 4

InCl3

S

CH3CN/H2O

N

L1

H

N

N In

Cl

Cl

OH2 Cl

3

CH3 Scheme 2. Indium complexes of L1.

3432 cm1. The free ligand L1 has C@C and C@N bond stretching at 1565 cm1 and 1584 cm1, respectively. Upon coordination these absorptions appear as broad unresolved peak at 1570 cm1 which is also the region of absorption for acetate. All these results establish the identity of the complex 1. The reaction of 2-(pyridin-4-yl)benzo[d]thiazole (L2) with cadmium(II) acetate leads to a mononuclear seven-coordinated complex [Cd(L2)2(O2CCH3)2(H2O)] (2) as shown in Eq. (ii) of Scheme 1. The crystal structure of the complex is shown in Fig. 2b. In the complex 2 both the acetate ligands bind to the cadmium ion as chelates. The acetates are present at the opposite side across the cadmium ion. The two nitrogen atoms of the pyridine part of the ligand bind at the two apices of a pentagonal bipyrimidal structure. The pentagon is constituted by an aqua ligand and the acetate ligands. Seven-coordinated carboxylate complexes of cadmium(II) are common [71]. The complex shows IR absorption

at 3431 cm1 due to O-H stretching of coordinated water molecules and sharp absorption at 1566 cm1 due the chelating acetate groups (Scheme 2). The reaction of indium(III) chloride with L1 in acetonitrile or methanol/acetonitrile solvent led to two different complexes, namely [L1In(H2O)Cl3]L1 (3) or [L1In(MeOH)Cl3]L1 (4), respectively. These two complexes are in the form cocrystals with the corresponding ligands. These are similar to conventional solvated complexes, but in the present examples the ligands are fluorescent, which make more avenues for study. The ligand L1 molecules have dual roles in the formation of these indium cocrystals; it participates in formation of a neutral complex which holds a molecule of L1 through weak interaction outside the coordination sphere. The crystal structure and the crystal packing of the complex 3 are shown in Fig. 3a and b. The indium(III) ion in the complex 3, has a six coordinated geometry. Three chloride ions occupy facial

T. Karmakar et al. / Polyhedron 54 (2013) 285–293

289

Fig. 3. The structure of the (a) indium complex 3 (30% thermal ellipsoids) and (b) its packing pattern. Some relevant bond distances (Å) and bond angles (°) in 3 are: In1–O1, 2.271(3); In1–N2, 2.330(3); In1–N1, 2.351(3); In1–Cl2, 2.409(10); In1–Cl3, 2.431(10); In1–Cl1, 2.461(10). \O1–In1–N2 85.57(11); \O1–In1–N1, 80.43(10); \N2–In1–N1, 71.46(11); \O1–In1–Cl2, 87.01(8); \N2–In1–Cl2, 169.60(8); \N1–In1–Cl2, 100.12(8); \N2–In1–Cl3, 87.64(8); \N1–In1–Cl3, 90.27(8); \Cl2–In1–Cl3, 98.70(4); \O1–In1– Cl1, 89.19(8); \N2–In1–Cl1 89.74(9); \Cl2–In1–Cl1, 97.43(4); \Cl3–In1–Cl1, 98.31(4). (c) The structure of the complex 4 and (30% thermal ellipsoids). Some relevant bond distances (Å) and bond angles (°) are: In1–O1, 2.253(9); In1–N1, 2.344(10); In1–N2, 2.345(10); In1–Cl1, 2.417(4); In1–Cl3, 2.436(4); In1–Cl2, 2.449(3). \O1–In1–N2, 82.2(3); \O1–In1–N1, 81.0(3); \N2–In1–N1, 71.9(3); \O1–In1–Cl1, 87.2(3); \N2–In1–Cl1, 167.8(2); \N1–In1–Cl1, 100.5(3); \O1–In1–Cl3, 91.5(2); \N2–In1–Cl3, 90.1(3); \N1–In1– Cl3, 161.3(3); \Cl1–In1–Cl3, 96.23(13); \O1–In1–Cl2, 166.3(3); \N2–In1–Cl2, 89.7(2); \N1–In1–Cl2, 86.1(3); \Cl1–In1–Cl2, 99.47(13); \Cl3–In1–Cl2, 99.51(13). (d) The packing pattern of the complex 4.

290

T. Karmakar et al. / Polyhedron 54 (2013) 285–293

Fig. 4. (a) Changes in fluorescence emission (kex = 300 nm) of L1 (2 ml, 103 M in methanol) upon addition of cadmium(II) acetate (102 M in methanol, 10 ll per aliquot). (b) Changes in fluorescence emission (kex = 300 nm) of L1 (2 ml, 103 M in methanol) upon addition of cadmium(II) acetate (102 M in methanol, 10 ll per aliquot) followed by addition of indium(III) chloride (102 M in methanol, 10 ll per aliquot).

Fig. 5. Solid state fluorescence emission (kex = 300 nm) of (i) L1 and (ii) complex 1.

positions, and another face is occupied by two chelating nitrogen atoms from L1 and a water molecule. The complex interacts with p-stacking interaction through (dp–p(C12–C16) = 3.20 Å and dp– 1 p(C12–C17) = 3.36 Å) and planar nature of the L makes it easy to get accommodated in the lattice. The water molecule attached to the indium(III) stabilizes the packing pattern of the complex through H-bond interactions. Due to poor crystal quality we could not locate the hydrogen atom on the aqua ligand to provide exact details on such interactions. The structure of the complex 4 is also determined. The structure of complex 4 is similar to complex 3; both are hexa-coordinated complex but difference is that the com-

plex 4 has a methanol ligand instead of the aqua ligand. While in the complex 4, the uncoordinated ligand L1 interacts with the methanol ligand. The major interactions are through O1–H  N3 (dD  A, 2.771 Å, and \D–H  A, 174.65°). There are also strong pinteractions between the L1 outside and inside coordination sphere as evident from their parallel disposition with a separating distance 3.314 Å. In the complexes 3 and 4 the free ligand has a transoid geometry, whereas coordinated ligand has cisoid geometry. The transoid geometry is observed in the structure of the free ligand [65]. The energy required to interconvert cisoid and transoid geometry [31] is about 1–2 kcal/mol; so dispersive forces such as

T. Karmakar et al. / Polyhedron 54 (2013) 285–293

Table 3 IC50 values of complexes 1 and 2 in pancreatic cancer cell lines. Values were calculated from three independent experiments. IC50 (lM)

Complex 1 Complex 2

Mia Paca-2

BxPC-3

Panc-1

24.7 ± 3.7 16.0 ± 2.6

67.7 ± 3.5 49.7 ± 4.0

39.7 ± 0.6 31.3 ± 1.5

interaction of solvent and/or the chelate formation would cause such change in the geometry. It is found that dissolving 4 in mixed solvent water/acetonitrile (1:1, v/v) results in the formation of 3. It may be mentioned that the indium sulfides intercalates 2,20 -bipyridine molecules and in these intercalated form the free 2,20 -bipyridine adopts transoid geometry [72]. Both the complexes 3 and 4 are non-ionic in nature. The IR spectra of complex 3 has a sharp O–H stretching at 3375 cm1 whereas the IR of the complex 4 shows a very broad and sharp OH stretching at 3399 cm1 from the ligating methanol hydroxy group. The 1H NMR spectra of the two complexes show the differences in their ligand dissociation abilities. The obvious difference in the 1 H NMR spectrum of the compound 4 from the corresponding 1H NMR spectrum of 3 is the signal of the methyl group of the ligated methanol; which appears at 3.16 ppm. The aromatic region of the complex 3 has seven peaks from the ligand. Whereas similar peaks are obtained from 4; but in the case of 4 there are also a set of extra peaks in this region. The peaks at 7.43–7.46 (triplet), 7.50–7.56 (multiplet), 7.97–8.00(triplet) ppm in complex 4; comprises of another set of repetitive non-overlapping identical signals next to them (occurs in pairs), which is due (please refer to Supplementary materials) to the distinctions of the signals of the ligands outside and inside the coordination sphere in complex 4. These signals look like doublets of triplet in the case of a triplet that appears in 3. This happens due to coordination effect of the metal ion causing diamagnetic shielding on the ligands and these make two sets of peaks from protons in the close proximity of the metal ions of undissociated and dissociated ligands. The rest of the 1H NMR signals appearing at 8.03–8.06 (doublet), 8.09–8.12(doublet), 8.27– 8.29 (doublet), 8.67–8.68 (doublet) are identical in both the complexes. These doublets are from the protons on the pyridine ring. The coordinated and free ligand peaks of the complex 4 can be distinguished, so, it may be suggested that they do not exchange in complex 4 or exchange very slowly. The formation of the cadmium complex 1 was monitored from the changes in the fluorescence emission of the ligand upon interaction with cadmium(II) ions. The ligand L1 has an absorption maximum at 308 nm due to p–p⁄ transition of benzthioimidazole. The assignment is on the basis of earlier assignment on analogous ligands [31]. When a solution of the ligand is excited at 300 nm (a

291

wavelength slightly away from the kmax to avoid saturation) it shows an emission peak at 386 nm. The emission spectra of the ligand L1 changes on addition of the cadmium ions. When a solution of cadmium(II) acetate is added to a solution of the ligand, this emission peak quenches and a new emission peak at 438 nm with an isoemissive point at 420 nm arises (Fig. 4). The new emission peak at 438 nm resembles with the emission peak of the complex 1. The fluorescence emission at the L1 at 386 nm is also quenched on addition of indium(III) ions. The relative changes in fluorescence emission of the ligand by the ions having similar electronic configuration to that of indium(III) ions, such as aluminum(III) and galium(III) are different. The galium(III) ions have the least effect. Aluminum(III) ions also affect the fluorescence in an insignificant manner. The quenching of fluorescence of a solution of L1 containing aluminum(III) ions by indium(III) ions is also not significant, this suggests the interference of aluminum ions in the quenching process. There are reports on estimation of aluminum(III), galium(III) and indium(III) ions simultaneously [73–75]. Those methods were shown to be suitable to detect aluminum(III) ions in the presence of galium(III) and indium(III) ions. Methods based on competitive complexation [76–79] were used to separate aluminum(III) ions from galium(III) ions and indium(III) ions from aluminum(III) ions. The gallium and indium ions are obtained from bauxite ores while processing zinc-blende, thus detection of indium(III) ions in a sensitive way is important [80]. As our system is not superior to detect indium(III) ions in the presence of aluminum(III) or galium(III) ions or vice versa, we studied selectivity of the ligand for binding to cadmium(II) ions versus indium(III). It is found that the new fluorescence emission peak at 438 nm formed by addition of cadmium ion to a solution of L1, shifts to 386 nm on addition of indium ions (Fig. 4b). This wavelength corresponds to the emission maximum of the ligand L1 and suggests that the indium(III) replaces the ligand associated to cadmium(II) ions. As mentioned earlier the indium complex of L1 fast equilibrates in solution as bound and free state, once the ligand is anchored to an indium, it facilitates the release of the bound ligands to its solution, thereby the solution shows the characteristic fluorescence of the free ligand. Thus detection of indium ions by the ligand in the presence of cadmium(II) ions is possible. The ligand L2 is weakly fluorescent; it shows a new emission peak at 410 nm upon interaction with cadmium(II) acetate (refer to Supplementary figures). The formation of new peak is followed by an isoemissive point at 445 nm; this isoemissive point occurs due to the formation of the complex 2 in solution. The fluorescence emission of ligand L2 is not affected by indium(III) ions; accordingly we also could not isolate complex of indium(III) of this ligand. A solution of complex 3 in dimethylsulfoxide emits at 397 nm whereas a similar solution of the complex 4 emits at 386 nm (kex = 300 nm). This fluorescence emission intensity of complex 3

Fig. 6. Colony formation assay for the ligands and complexes 1 and 2.

292

T. Karmakar et al. / Polyhedron 54 (2013) 285–293

is 13% lower to emission intensity of complex 4 at identical concentration. The excitation is carried out at p–p⁄ energy level in the complexes in which the environment around the central metal ions are similar. The observation of lower intensity in complex 3 is ascribed to the presence of water molecule in the complex, a relatively higher ability of fluorescence quenching by water is often observed [81,82]. The solid state fluorescence emission of the ligand L1 and the complex 1 were also independently recorded. The ligand shows an emission peak at 489 nm whereas the complex 1 shows two emissions at 412 nm and 510 nm, respectively (Fig. 5). Thus, there is a large shift in the emission spectra of the ligand as well as the complex in solid state from solution, which points out the effect of packing in solid to provide different orientations. The organometallic complexes of 2-(pyridine-2-yl)thiazole with Rh, Ir shows anticancer activity [39]. The complex 2 on the other hand has structural resemblance with some monodentate imidazole containing ruthenium complexes having anticancer activity [82]. There are also other metal complexes with different ligands having similar structural features to L1 and L2 which are good anticancer agents [83,84]. Thus, due to structural resemblances of 2-(pyridine-2-yl)thiazole with L1 to form 1:1 complex with cadmium and also the structural similarity of complex 2 with cytotoxic imidazole containing metal complexes, we studied cytotoxicity of the complexes. To understand the medicinal potentials the free ligands L1, L2 and the cadmium complexes 1 and 2, each of them were independently tested in pancreatic cancer cell lines for evaluation of cytotoxicity. Since indium is not a very good chelator for ligand L1 as mentioned above, we did not consider such studies with the indium complexes. MTT and colony formation assays were performed in Mia Paca-2, BxPC-3 and Panc-1 cell lines. Interestingly, no cytotoxicity was detected with ligands L1 or L2 at the concentration of 100 lM in all three cell lines; while both complexes 1 and 2 showed significant inhibition of cell proliferation in MTT assays (Table 3). The cadmium containing complexes were most potent in Mia Paca-2 cells, with IC50 values at 24.7 lM for complex 1 and 16.0 lM for complex 2. Complexes 1 and 2 could also significantly inhibit formation of cancer cell colonies in colony formation assay, while ligands L1 or L2 did not show any activity at 50 lM (Fig. 6). Correlating with results from MTT assays, complex 2 is more potent than complex 1 in all cell lines, and both complexes showed best activities in Mia Paca-2 cells, with IC50 values around 25 lM. From the two cytotoxicity assays, ligands L1 or L2 were not cytotoxic in pancreatic cancer cell lines, but the metal complexes contributed to significant increases in cytotoxicity, suggesting medicinal potentials for the cadmium containing complexes as anticancer agents. For comparison, in human ovarian cancer cell line the IC50 values of cis-platin varies in the range of 1.6–8.6 lM, and the different 2(pyridine-2-yl)thiazole metal complexes shows IC50 values in the range of 124–300 lM [39]. Although cadmium is a toxic material, the IC50 values obtained for complexes 1 and 2 are encouraging to further study of related systems. In conclusion the present study shows that the L1 is a chelating ligand for cadmium and indium ions. However, the complexes differ in their structures, in the latter case cocrystals with the ligand are observed. The ligand L2 is not a chelating ligand; it binds to cadmium ion through the nitrogen of pyridine. The change in fluorescence emission of the L1 upon interaction with metal ions enables one to detect indium ions in the presence of cadmium ions. Although the ligands L1 and L2 are not cytotoxic, their cadmium complexes 1 and 2 show anticancer activities in pancreatic cancer cell lines. Appendix A. Supplementary data CCDC 890720, 890722, 902272 and 902273 contain the supplementary crystallographic data for this paper. These data can be

obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336033; or e-mail: [email protected]. The 1H NMR spectra and fluorescence studies of indium compounds are available. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2013.02.068.

References [1] Z. Jin, Nat. Prod. Rep. 22 (2005) 196. [2] G.R. Pettit, Y. Kamano, C.L. Herald, A.A. Tuinman, F.E. Boettner, H. Kizu, J.M. Schmidt, L. Baczynskyj, K.B. Tomer, R.J. Bontems, J. Am. Chem. Soc. 109 (1987) 6883. [3] B.S. Davidson, Chem. Rev. 93 (1993) 1771. [4] N. Fusetani, S. Matsunaga, Chem. Rev. 93 (1993) 1793. [5] P. Wipf, Chem. Rev. 95 (1995) 2115. [6] V.S. Aulakh, M.A. Ciufolini, J. Org. Chem. 74 (2009) 5750. [7] A. Dondoni, A. Marra, Chem. Rev. 104 (2004) 2557. [8] G. Haberhauer, F. Rominger, Eur. J. Org. Chem. (2003) 3209. [9] D. Dhanak, L.T. Christmann, M.G. Darcy, A.J. Jurewicz, R.M. Keenan, J. Lee, H.M. Sarau, K.L. Widdowson, J.R. White, Bioorg. Med. Chem. Lett. 11 (2001) 1445. [10] M. Vinícius, N. de Souza, J. Sulfur Chem. 26 (2005) 429. [11] J.S. Lomas, J.-C. Lacroix, J. Vaissermann, J. Chem. Soc., Perkin Trans. 2 (1999) 2001. [12] P. Pornsuriyasak, N.P. Rath, A.V. Demchenko, Chem. Commun. (2008) 5633. [13] J.-H. Julliard, R. Douce, Proc. Natl. Acad. Sci. U.S.A. 88 (1991) 2042. [14] C. Fuganti, R. Rigoni, Biotechnol. Lett. 15 (1993) 1163. [15] H.D. Troutman, L.M. Long, J. Am. Chem. Soc. 70 (1948) 3436. [16] V.E. Borisenko, A. Koll, E.E. Kolmakov, A.G. Rjasnyi, J. Mol. Struct. 783 (2006) 101. [17] Y. Liu, L. Zhang, J. Gong, H. Fang, A. Liu, G. Du, W. Xu, J. Enzyme Inhib. Med. Chem. 26 (2011) 506. [18] C. Ramalingan, S. Balasubramanian, S. Kabilan, M. Vasudevan, Eur. J. Med. Chem. 39 (2004) 527. [19] G.T. Zitouni, S. Demirayak, A. Ozdemir, Z.A. Kaplancikli, M.T. Yildiz, Eur. J. Med. Chem. 39 (2003) 267. [20] M. Karatepe, F. Karatas, Cell Biochem. Funct. 24 (2006) 547. [21] A.K. Jain, R.K. Singla, B. Shrivastav, Pharmacol. Online 2 (2011) 1072. [22] M. Van Beusichem, N. Farrell, Inorg. Chem. 31 (1993) 634. [23] H. Yu, L. Shao, J. Fang, J. Organomet. Chem. 692 (2007) 991. [24] E. Matczak-Jon, T. Kowalik-Jankowska, K. Slepkura, P. Kafarski, A. Rajewska, Dalton Trans. 39 (2010) 1207. [25] M.A. Neelakantan, S.S. Marriappan, J. Dharmaraja, T. Jeyakumar, K. Muthukumaran, Spectrochim. Acta, Part A 71 (2008) 628. [26] F. Tellez, A. Pena-Hueso, N. Barba-Behrens, R. Contreras, A. Flores-Parra, Polyhedron 25 (2006) 2363. [27] F. Tellez, A. Flores-Parra, N. Barba-Behrens, R. Contreras, Polyhedron 23 (2004) 2481. [28] A. Giusti, G. Peyronel, Spectrochim. Acta, Part A 38 (1982) 975. [29] H.D. Yin, S.W. Chen, J. Organomet. Chem. 691 (2006) 3103. [30] T. Weidner, N. Ballav, M. Zharnikov, A. Priebe, N.J. Lonh, J. Maurer, R. Winter, A. Rothenberger, D. Fenske, D. Rother, C. Bruhn, H. Fink, U. Siemeling, Chem. Eur. J. 14 (2008) 4346. [31] M. Boca, R. Boca, G. Kickelbick, W. Linert, I. Svoboda, H. Fuess, Inorg. Chim. Acta 338 (2002) 36. [32] J. Garcia-Lozano, J. Server-Carrio, E. Coret, J.-V. Folgado, E. Escriva, R. Ballesteros, Inorg. Chim. Acta 245 (1996) 75. [33] S.B. Sanni, H.J. Behm, P.T. Beurskens, G.A. van Albada, J. Reedijk, A.T.H. Lenstra, A.W. Addison, M. Palaniandavar, J. Chem. Soc., Dalton Trans. (1988) 1429. [34] W. Shuangxi, Z. Ying, Z. Fangjie, W. Qiuying, W. Liufang, Polyhedron 11 (1992) 1909. [35] S. Wang, Y. Zhu, Y. Cui, L. Wang, Q. Luo, J. Chem. Soc., Dalton Trans. (1994) 2523. [36] R. Boca, P. Baran, L. Dlhan, H. Fuess, W. Haase, F. Renz, W. Linert, I. Svoboda, R. Werner, Inorg. Chim. Acta 260 (1997) 129. [37] S. Ruttimann, C.M. Moreau, A.F. Williams, G. Bernardinelli, A.W. Addison, Polyhedron 11 (1992) 635. [38] S. Wang, Y. Cui, R. Tan, Q. Luo, J. Shi, Q. Wu, Polyhedron 13 (1994) 1661. [39] M. Gras, B. Therrien, G. Suss-Fink, A. Casini, F. Edafe, P.J. Dyson, J. Organomet. Chem. 695 (2010) 1119. [40] E. Tomat, S.J. Lippard, Curr. Opin. Chem. Biol. 14 (2010) 225. [41] M. Rouffet, C. Augusto, F. De Oliveira, Y. Udi, A. Agarwal, I. Sagi, J.A. McCammon, S.M. Cohen, J. Am. Chem. Soc. 132 (2010) 8232. [42] D. Martin, M. Rouffet, S.M. Cohen, Inorg. Chem. 49 (2010) 10226. [43] X.-F. He, C.M. Vogels, A. Decken, S.A. Westcott, Polyhedron 23 (2004) 155. [44] S. Haneda, Z. Gan, K. Eda, M. Hayashi, Organometallics 26 (2007) 6551. [45] M.N. Patel, P.A. Dosi, B.S. Bhatt, Spectrochim. Acta, Part A 86 (2012) 508. [46] C.K. Choudhary, R.K. Choudhary, L.K. Mishra, J. Indian Chem. Soc. 79 (2002) 761. [47] C. Richardson, R.F. Keene, P.J. Steel, Aust. J. Chem. 61 (2008) 183. [48] S.S. Mandal, P.C. Ghorai, S. Ray, H.K. Saha, J. Indian Chem. Soc. 71 (1995) 807.

T. Karmakar et al. / Polyhedron 54 (2013) 285–293 [49] P. Datta, D. Sardar, A.P. Mukhopadhyay, E. Lopez-Torres, C.J. Pastor, C. Sinha, J. Organomet. Chem. 696 (2011) 488. [50] S. Tzanopoulou, C.I. Pirmettis, G. Patsis, C. Raptopoulou, A. Terzis, M. Papadopoulos, M. Pelecanou, Inorg. Chem. 45 (2006) 902. [51] R. Czerwieniec, A. Kapturkiewicz, J. Lipkowski, J. Nowacki, Inorg. Chim. Acta 358 (2005) 2701. [52] S. Hu, D. Shi, T. Huang, J. Wan, Z. Huang, J. Yang, C. Xu, Inorg. Chim. Acta 173 (1990) 1. [53] S. Ghaderi, B. Ramesh, A.M. Seifalian, J. Drug Target. 19 (2011) 475. [54] B.A. Rzigalinski, J.S. Strobl, Toxicol. Appl. Pharmacol. 238 (2009) 280. [55] Q. Huo, Colloids Surf. 59B (2007) 1. [56] Z.Y. Li, Y. Xia, Nano Lett. (2010) 243. [57] V. Biju, T. Itoh, A. Anas, A. Sujith, M. Ishikawa, Anal. Bioanal. Chem. 391 (2008) 2469. [58] G.F. Nordberg, Environ. Health Perspect. 54 (1984) 213. [59] P.U. Maheswari, M. van der Ster, S. Smulders, S. Barends, G.P. van Wezel, C. Massera, S. Roy, H. den Dulk, P. Gamez, J. Reedijk, Inorg. Chem. 47 (2008) 3719. [60] L. Yanmei, C. Yongheng, O. Zhibin, C. Shi, Z. Chuxiong, L. Xueyi, Chin. J. Chem. 30 (2012) 303. [61] K. Marjani, M. Mousavi, D.L. Hughes, Transition Met. Chem. 34 (2009) 85. [62] L.J. Carlson, J. Welby, K.A. Zebrowski, M.M. Wilk, R. Giroux, N. Ciancio, J.M. Tanski, A. Bradley, L.A. Tyler, Inorg. Chim. Acta 365 (2011) 159. [63] E.K. Beloglazkina, I.V. Yudin, A.G. Majouga, A.A. Moiseeva, A.I. Tursina, N.V. Zyk, Russ. Chem. Bull. 55 (2006) 1803. [64] M.L. Mckee, S.M. Kerwin, Bioorg. Med. Chem. 16 (2008) 1775. [65] N. Kundu, A. Audhya, Sk.Md.T. Abtab, S. Ghosh, E.R.T. Tiekink, M. Chaudhury, Cryst. Growth Des. 10 (2010) 1269.

293

[66] T. Mosmann, J. Immunol. Methods 65 (1983) 55. [67] J. Carmichael, W. DeGraff, A. Gazdar, J. Minna, J. Mitchell, Cancer Res. 47 (1987) 936. [68] K. Yamasaki, M. Yasuda, J. Am. Chem. Soc. 78 (1956) 1324. [69] X.-Z. Sun, Z.-L. Huang, H.-Z. Wang, B.-H. Ye, X.-M. Chen, Z. Anorg. Allg. Chem. 631 (2005) 919. [70] S.-Z. Ge, Q. Liu, S. Deng, Y.-Q. Sun, Y.-P. Chen, J. Inorg. Organomet. Polym. (2013), http://dx.doi.org/10.1007/s10904-012-9814-5. [71] K. Bania, N. Barooah, J.B. Baruah, Polyhedron 26 (2007) 2612. [72] Y.-P. Zhang, X. Zhang, W.-Q. Mu, W. Luo, G.-Q. Bian, Q.-Y. Zhu, J. Dai, Dalton Trans. 40 (2011) 9746. [73] M.D. Prat, R. Compano, M. Granados, E. Miralles, J. Chromatogr. 746 (1996) 239. [74] S. Vojta, L. Jancar, L. Sommer, J. Fluoresc. 18 (2008) 339. [75] B.D. Karcher, J. Krull, J. Chromatogr. Sci. 25 (1987) 472. [76] T. Williams, N.W. Barnett, Anal. Chim. Acta 264 (1992) 297. [77] E. Merian, Metals and Their Compounds in the Environment: Occurrence, Analysis and Biological Relevance, VCH, Weinheim, New York, 1991. [78] S. Doose, H. Neuweiler, M. Sauer, ChemPhysChem 10 (2009) 1389. [79] L. Guo, H. Hu, R. Sun, G. Chen, Talanta 79 (2009) 775. [80] Z. Xu, N.J. Singh, J. Lim, J. Pan, H.N. Kim, S. Park, K.S. Kim, J. Yoon, J. Am. Chem. Soc. 131 (2009) 15528. [81] S. Tobita, K. Ida, S. Shiobara, Res. Chem. Intermed. 27 (2001) 205. [82] C.G. Hartinger, S. Zorbas-Seifried, M.A. Jakupee, B. Kynast, H. Zorbas, B.K. Keppler, J. Inorg. Biochem. 100 (2006) 891. [83] A.M. Pizaro, A. Hatemariam, P.J. Sadler, Top. Organomet. Chem. 32 (2010) 21. [84] L. Ronconi, P.J. Sadler, Coord. Chem. Rev. 251 (2007) 1633.