Synthesis, characterization and cytotoxic activity of palladium (II) carbohydrate complexes

Article abstract of DOI:10.1007/s12039-012-0340-3

Carbohydrate containing pyridyl triazole ligands, 5-deoxy-1,2-O- isopropylidene-5-(4-(2-pyridyl)-1H-1,2,3-triazole-1-yl)-α-D-xylofuranose (2a), 3-O-Benzyl-5-deoxy-1,2-O-isopropylidene-5-(4-(2-pyridyl)-1H-1,2,3-triazol- 1-yl)-α-D-xylofuranose (2b), methyl-

Full text of DOI:10.1007/s12039-012-0340-3

c
J. Chem. Sci. Vol. 124, No. 6, November 2012, pp. 1405–1413. ꢀ Indian Academy of Sciences.
Synthesis, characterization and cytotoxic activity of palladium (II)
carbohydrate complexes
S BHAVYA DEEPTHIa, RAJIV TRIVEDIa,∗, P SUJITHAb, C GANESH KUMARb,
B SRIDHARc and SURESH K BHARGAVAd,e
aOrganometallic Chemistry Group, Inorganic and Physical Chemistry Division, bChemical Biology
Laboratory, Natural Products Chemistry Division, cLaboratory of X-Ray Crystallography, CSIR-Indian
Institute of Chemical Technology, Hyderabad 500 607, India
dRMIT-IICT Research Centre, Indian Institute of Chemical Technology, Hyderabad 500 607, India
eAdvanced Materials and Industrial Chemistry Group, School of Applied Sciences, Royal Melbourne
Institute of Technology University, Melbourne, VIC, 3001, Australia
e-mail: trivedi@iict.res.in, rtrajiv401@gmail.com
Abstract. Carbohydrate containing pyridyl triazole ligands, 5-deoxy-1,2-O-isopropylidene-5-(4-(2-pyridyl)-
1H-1,2,3-triazole-1-yl)-α-D-xylofuranose (2a), 3-O-Benzyl-5-deoxy-1,2-O-isopropylidene-5-(4-(2-pyridyl)-1H-
1,2,3-triazol-1-yl)-α-D-xylofuranose (2b), methyl-5-deoxy-2,3-O-isopropylidene-5-(4-(2-pyridyl)-1H-1,2,3-
triazol-1-yl)-β-D-ribofuranoside, (2c) and 6-deoxy-1,2:3,4-di-O-isopropylidene-6-(4-(2-pyridyl)-1H-1,2,3-
triazol-1-yl)-α-D-galactopyranose (2d) were prepared by the ‘click’ reaction of 2-ethynyl pyridine with the
corresponding azides. The palladium complexes were synthesised by the reaction of pyridyl triazole ligands
with [Pd(COD)Cl2] in dichloromethane. All the compounds were characterized by NMR, IR, mass and elemen-
tal analysis. Structural characterization of the ligand 2a was done by X-ray crystallography. The ligands and
complexes were tested for their cytotoxic activity on different cell lines like A549 (human alveolar adenocar-
cinoma cells), Neuro2a (mouse neuroblastoma cells), HeLa (cervical carcinoma cancer cells), MDA-MB-231
(human breast adenocarcinoma cells) and MCF7 (human breast adenocarcinoma cells). The complexes showed
considerable cytotoxicity while the ligands were non-toxic on the tested cell lines.
Keywords. Carbohydrate; pyridyl triazole; palladium complexes; cytotoxicity.
1. Introduction
In order to make the palladium complexes as good
cytotoxic agents, it is very important to stabilize the
complexes by a strong coordinating nitrogen ligand
and a suitable leaving group, which should be non-
labile to maintain the structural integrity of the drug
in vivo for long period of time.20,21 It is evident from
literature that the palladium complexes derived from
diamines, amine–imine, di-imines22–26 have shown
excellent cytotoxicity. Metal complexes derived from
the click chelators are known for their cytotoxicity27
and have proven versatility in the radiolabelling of
biomolecules.28 Among various triazole functionalities,
(2-pyridyl)-substituted 1,2,3-triazoles are well-known
for their metal binding studies29–31 and are synthesised
easily by the well-known click reaction procedure.32
In recent years, synthesis of metallodrugs containing
biologically important ligands, especially carbohydrate
residues has become an interesting area in antitumour
therapy.33 Certain carbohydrate containing drugs such
as bleomicin and adriamicine are used in antitumour
therapy. Compared to the conventional medicines, car-
bohydrates containing metal drugs are more water-
soluble. The resemblance of the carbohydrate fragments
Bioinorganic chemistry is a rapidly developing field
dealing with the synthesis and biological investigation
of inorganic complexes.1–5 In inorganic complexes, the
central metal ion has a significant role in the develop-
ment of anticancer metallodrug. Discovery of cytotoxic
activity of cisplatin and its analogue such as carbo-
platin has led to an extensive search of new biologically
active metal drugs. Several cisplatin related platinum-
based drugs have been synthesised, however high gene-
ral toxicity and development of platinum-resistance to
the drug has become major determinant to its effi-
cacy. Development of palladium (II) containing metal-
lodrug has gained importance due to significant sim-
ilarity in the coordination behaviour of platinum (II)
and palladium (II).6–18 However, most of the palladium
complexes are less cytotoxic compared to their plat-
inum counterparts, due to faster hydrolysis of palladium
complexes leading to highly reactive species that are
unable to reach their pharmacological target.19
∗For correspondence
1405

1406
S Bhavya Deepthi et al.
with the molecules involved in various biological func- vigorously at room temperature till the disappear-
tions enhance the tolerance levels of the organism and ance of starting materials. The reaction mixture was
these fragments are active constituents of glycopro- quenched with saturated NH4Cl solution and extracted
teins, glycolipids and nucleotides. Several carbohydrate with CH2Cl2 (2 × 5 mL). The combined organic lay-
containing metallodrugs are studied for their cytotoxi- ers were dried over Na2SO4 and the removal of sol-
city34–36 and radiolabelling techniques.37
vent under vacuum resulted crude mass, which can be
Our recent results on the biological studies of purified using silica column (hexane/ethyl acetate).
ferrocene-carbohydrate conjugates38,39 have lead to the
design and study of the cytotoxic activity of metal com-
plexes containing carbohydrate ligands. Hence, here
we present the detailed synthesis and characteriza-
tion of the carbohydrate triazole ligands and their Pd-
complexes together with the crystal structures of ligand
2a. The cytotoxic activity of the compounds on all the
tested cell lines are reported and discussed.
2.2a 5-Deoxy-1,2-O-isopropylidene-5-(4-(2-pyridyl)-
1H-1,2,3-triazole-1-yl)-α-D-xylofuranose, 2a: Treat-
ment of 5-azido-5-deoxy-1,2-O-isopropylidene-α-D-
xylofuranose, (0.298 g, 1.39 mmol) 1a and 2-ethynyl
pyridine (0.14 g, 1.39 mmol) resulted 2a as a pale yel-
low solid. Yield: 0.38 g (81%); mp: 170◦C; [α]D (c
30
0.1, CHCl3) = −24.86; Anal. calc. for C15H18N4O4 :
C 56.60, H 5.70, N 17.60; found: C 56.71, H 5.18, N
1
17.94; H NMR (CDCl3 + d6-DMSO, 75.5 MHz): δ
2. Experimental
1.28 (s, 3H, CH3 of CMe2), 1.41 (s, 3H, CH3 of CMe2),
4.19 (d, 1H, J = 1.79 Hz, H-3), 4.51 (m, 2H, H-5),
4.59–4.65 (m, 1H, H-2), 4.77 (dd, 1H, J = 4.72 and
9.25 Hz, H-4), 5.58 (d, 1H, J = 3.58 Hz, OH), 5.97
(d, 1H, J = 3.39 Hz, H-1), 7.25 (t, 1H, J = 7.17 and
5.09 Hz, Py), 7.79 (dt, 1H, J = 1.3 and 6.4 Hz, Py),
8.09 (d, 1H, J = 7.9 Hz, Py), 8.37 (s, 1H, triazole), 8.57
(d, 1H, J = 4.15 Hz, Ar); 13C (75.5 MHz, CDCl3 + d6-
DMSO): δ 25.43, 26.03, 48.53, 73,28, 78.56, 84.68,
110.79, 119.20, 122.62, 136.0, 147.2, 148.57, 149.43;
ESI-MS (in CH3OH): m/z 318 [M+1]+; IR (KBr,
cm−1): 3447, 3107, 2984, 1640, 1261, 1027, 804.
2.1 Materials and methods
The complex formation reactions were performed by
standard schlenk techniques under argon atmosphere.
All the solvents were purified by standard proce-
dures. 2-Ethynyl pyridine was purchased from Aldrich
Co. and the azido sugars were prepared according
to the literature procedures.40,41 Melting points were
determined on Toshniwal melting point apparatus and
are uncorrected. Optical rotations were determined on
JASCO P120 polarimeter. 1H spectra were recorded for
CDCl3/CDCl3 and d6-DMSO solutions on 500 MHz
(Innova 500) and 300 MHz (Avance 300) instruments.
Chemical shifts for protons were reported by taking
TMS as the internal reference. 13C NMR spectra were
recorded at 75.5 MHz (Avance 300) and the carbon
shifts are referenced to the 13C signal of CDCl3 at
77.0 ppm. The coupling constants (J) were expressed in
Hz. Infrared spectra were recorded on a Thermo Nicolet
Nexus 670 Spectrometer using KBr discs. The UV-Vis
spectra were recorded on Varian Carry 500 spectropho-
tometer over the range 240–400 nm using 1 cm path
length cuvettes. Compounds 2e and 3e were prepared
according to the literature procedures.29
2.2b 3-O-Benzyl-5-deoxy-1,2-O-isopropylidene-5-(4-
(2-pyridyl)-1H-1,2,3-triazol-1-yl)-α-D-xylofuranose, 2b:
Treatment of 5-azido-3-O-benzyl-5-deoxy-1,2-O-
isopropylidine-α-D-xylofuranose, (0.423 g, 1.39 mmol)
1b and 2-ethynyl pyridine (0.14 g, 1.39 mmol) resulted
2b as a pale yellow solid. Yield: 0.49 g (86%); mp:
30
110–115◦C; [α]D (c 0.5, CHCl3) = −75.0; Anal.
calc. for C22H24N4O4: C 64.69, H 5.92, N 13.72.
1
Found: C 65.01, H 6.29, N 13.81; H NMR (CDCl3,
300 MHz): δ 1.28 (s, 3H, CH3 of CMe2), 1.41 (s,
3H, CH3 of CMe2), 3.96 (d, 1H, J = 2.26 Hz, H-3),
4.54–4.58 (m, 3H, H-4, H-5), 4.60 (d, 1H, J = 3.77,
H-2), 4.69 (bs, 1H, CH2Ar), 4.72 (d, 1H, J = 2.26 Hz,
CH2Ar), 5.90 (d, 1H, J = 3.77 Hz, H-1), 7.15 (t, 1H,
J = 5.21 Hz, Py), 7.31–7.35 (m, 5H, Ar), 7.74 (dt,
1H, J = 1.2 and 6.04 Hz, Py) 8.14 (s, 1H, triazole),
8.17 (d, 1H, J = 3.02, Py), 8.54(d, 1H, J = 4.53,
Py); 13C NMR (CDCl3, 75.5 MHz): δ 26.26, 26.79,
49.22, 72.08, 78.74, 81.60, 82.00, 105.23, 112.0, 120.1,
122.6, 123.17, 128.01, 128.28, 128.66, 136.64, 136.78,
149.31, 148.35, 150.4; ESI-MS (in CH3OH): m/z 431
2.2 General procedure for the preparation of
2-pyridyl-carbohydrate triazoles (2a–d)
To a suspension of azido sugars, 1a–d (1.39 mmol) in
1:1 mixture of water and tert-butyl alcohol (8 mL), 2-
ethynylpyridine (0.14 g, 1.39 mmol) was added drop-
wise. To this solution, sodium ascorbate (0.05 g,
0.2 mmol) and CuSO4.5H2O (0.02 g, 0.09 mmol) were
added. The reaction mixture was allowed to stir

Palladium complexes of carbohydrate ligands
1407
[M+Na]+; IR (KBr, cm−1): 2967, 1727, 1622, 1453, dissolved in dichloromethane (5 mL). The mixture
1074, 787.
was allowed to stir at room temperature for 2 h and
the resulting precipitate was collected by filtration.
The solid was washed with diethyl ether and dried in
vacuum to get the desired palladium complex.
2.2c Methyl-5-deoxy-2,3-O-isopropylidene-5-(4-(2-
pyridyl)-1H-1,2,3-triazol-1-yl)-β-D-ribofuranose, 2c:
Treatment of methyl-5-azido-5-deoxy-2,3-O-isopropylidene-
β-D-ribofuranoside, (0.304 g, 1.39 mmol) 1c and
2-ethynyl pyridine (0.14 g, 1.39 mmol) resulted 2c as
2.3a Synthesis of [PdCl2(2a)]: Treatment of xylo-
furanose triazole, (0.053 g, 0.17 mmol) 2a with
[Pd(COD)Cl2] (0.05 g, 0.17 mmol) resulted 3a as pale
yellow solid. Yield: 0.074 g (78%); mp: 285–288◦C;
Anal. calc. for C15H18Cl2N4O4Pd : C 36.35, H 3.66,
white solid. Yield: 0.36 g (77%); mp: 180–185◦C; [α]D
30
(c 0.5, CHCl3) = −19.6; Anal. calc. for C16H20N4O4:
C 57.82, H 6.07, N 16.86. Found: C 57.51, H 5.78, N
1
16.36; H NMR (CDCl3 + d6-DMSO, 500 MHz): δ
1
N 11.30; found: C 35.92, H 3.11, N 11.15; H NMR
(CDCl3 + d6-DMSO, 75.5 MHz): δ 1.27 (s, 3H, CH3
of CMe2), 1.40 (s, 3H, CH3 of CMe2), 4.21 (d, 1H,
J = 3.02 Hz, H-3), 4.48 (m, 2H, H-5), 4.53–4.65
(m, 1H, H-2), 4.86 (dd, 1H, J = 3.39 and 10..95 Hz,
H-4), 5.60 (d, 1H, J = 4.91 Hz, OH), 5.90 (d, 1H,
J = 3.58 Hz, H-1), 7.65 (t, 1H, J = 6.98 and 7.17 Hz,
Py), 8.13 (t, 1H, J = 6.98 and 6.79 Hz, Py), 8.21 (d,
1H, J = 7.36 Hz, Py), 9.09 (d, 1H, J = 5.66 Hz, Py),
9.15 (s, 1H, triazole); ESI-MS(in CH3OH+DMSO):
m/z 478 [M-OH]+; IR (KBr, cm−1): 3500, 3087, 2984,
2930, 1624, 1455, 1379, 1074, 1017, 777.
1.31 (s, 3H, CH3 of CMe2), 1.44 (s, 3H, CH3 of CMe2),
3.41 (s, 3H, OCH3), 4.51–4.56 (m, 1H, H-4), 4.59–4.65
(m, 2H, H-5), 4.67 (d, 1H, J = 4.88 Hz, H-3), 4.83
(d, 1H, J = 4.88 Hz, H-2), 4.98 (d, 1H, J = 1.22 Hz,
H-1), 7.25 (t, 1H, J = 6.1 and 4.88 Hz, Py), 7.81 (t,
1H, J = 7.32 Hz, Py), 8.10 (d, 1H, J = 7.32, Py),
8.4 (s, 1H, triazole), 8.86 (d, 1H, J = 4.53, Py); 13C
NMR (CDCl3 + d6-DMSO, 75.5 MHz) δ: 23.56, 25.02,
51.60, 53.92, 80.22, 83.69, 83.48, 108.42, 110.98,
118.42, 121.87, 121.45, 135.56, 148.72, 146.45,
147.93; ESI-MS (in CH3OH): m/z 333 [M+1]+; IR
(KBr, cm−1): 2967, 1727, 1622, 1453, 1074, 787.
2.3b Synthesis of [PdCl2(2b)]: Treatment of xylo-
furanose triazole, (0.069 g, 0.17 mmol) 2b with
[Pd(COD)Cl2] (0.05 g, 0.17 mmol) resulted 3b as pale
yellow solid. Yield: 0.074 g (75%); mp: 270–275◦C;
Anal. calc. for C22H24Cl2N4O4Pd: C 45.11, H 4.13, N
2.2d 6-Deoxy-1,2:3,4-di-O-isopropylidene-6-(4-(2-
pyridyl)-1H-1,2,3-triazol-1-yl)-α-D-galactopyranose,
2d: Treatment
diisopropylidine-α-D-galactopyranose
of
6-azido-6-deoxy-1,2:3,4-O-
(0.396 g,
1.39 mmol) 1d and 2-ethynyl pyridine (0.14 g,
1.39 mmol) resulted 2d as white solid. Yield: 0.43 g
(79%); mp: 185–190◦C; [α]D 30 (c 0.5, CHCl3) = −43.7;
Anal. calc. for C19H24N4O5: C 58.75, H 5.77, N 8.28.
1
9.56. Found: C 46.26, H 3.92, N 9.81; H NMR (d6-
DMSO, 500MHz): δ 1.29 (s, 3H, CH3 of CMe2), 1.41
(s, 3H, CH3 of CMe2), 4.13 (d, 1H, J = 1.22 Hz, H-3),
4.57 (d, 2H, J = 2.26 Hz, CH2Ar), 4.63–4.72 (m, 3H,
H-4 and H-5), 4.90 (d, 1H, J = 2.41, H-2), 5.90 (d, 1H,
J = 2.41 Hz, H-1), 7.27 (m, 1H, Ar), 7.34 (m, 4H, Ar),
7.49 (t, 1H, J = 6.10 and 7.32 Hz, Py), 8.08 (t, 1H,
J = 7.32 Hz, Py), 8.12 (d, 1H, J = 7.32, Py), 9.02 (s,
1H, triazole), 9.05 (d, 1H, J = 4.88, Py); ESI-MS (in
CH3OH+DMSO): m/z 549 [M-Cl]+; IR (KBr, cm−1):
3090, 2934, 1623, 1456, 1377, 1078, 1027, 779.
1
Found: C 58.45, H 5.32, N 8.34; H NMR (CDCl3,
500 MHz): δ 1.28 (s, 3H, CH3 of CMe2), 1.34 (s, 3H,
CH3 of CMe2), 1.42 (s, 3H, CH3 of CMe2), 1.48 (s, 6H,
CH3 of CMe2), 3.50 (dd, 1H, J = 5.0 Hz, H-3), 4.16
(dd, 1H, J = 2.0 and 6.0 Hz, H-6), 4.23–4.28 (m, 2H,
H-6 and H-2), 4.48 (m, 1H, H-4), 4.60 (m, 1H, H-5),
5.47 (d, 1H, J = 5.0 Hz, H-1), 7.16 (t, 1H, J = 5.0 Hz,
Py), 7.72 (dt, 1H, J = 2.0 and 6.0 Hz, Py), 8.14 (d,
1H, J = 8.0 Hz, Py), 8.28 (s, 1H, triazole), 8.53 (d,
1H, J = 4.0 Hz, Py); 13C NMR (CDCl3, 75.5 MHz):
δ 24.46, 24.82, 25.91, 50.45, 67.0, 70.25 70.73, 70.98,
76.57, 76.99. 109.02, 109.88, 120.19, 122.67, 123.50,
136.76, 147.99, 149.35, 150.35; ESI-MS (in CH3OH):
m/z 389 [M+1]+; IR (KBr, cm−1): 3109, 2984, 2921,
1641, 1379, 1219, 1080, 1007, 814.
2.3c Synthesis of [PdCl2(2c)]: Treatment of ribo-
furanose triazole, (0.056 g, 0.17 mmol) 2c with
[Pd(COD)Cl2] (0.05 g, 0.17 mmol) resulted 3c as pale
yellow solid. Yield: 0.067 g (78%); mp: 280–285◦C;
Anal. calc. for C16H20Cl2N4O4Pd: C 37.70, H 3.96, N
10.99. Found: C 38.14, H 3.72, N 10.82; 1H NMR (d6-
DMSO, 300 MHz): δ 1.32 (s, 3H, CH3 of CMe2), 1.45
(s, 3H, CH3 of CMe2), 3.39 (s, 3H, OCH3), 4.62–4.71
(m, 3H, H-3 and H-5), 4.83 (m, 2H, H-2 and H-4), 5.01
(d, 1H, J = 3.32 Hz, H-1), 7.54 (t, 1H, J = 3.32 Hz,
2.3 General procedure for the preparation of
palladium complexes (3a–d)
Under an argon atmosphere, ligand 2a–d (0.17 mmol) Py), 7.64 (t, 1H, J = 7.76 Hz, Py), 8.13 (d, 1H,
and [Pd(COD)Cl2] (0.053 g, 0.17 mmol), were J = 3.32, Py), 9.09 (d, 1H, J = 5.54, Py), 9.13 (s,

1408
S Bhavya Deepthi et al.
1H, triazole); IR (KBr, cm−1): 3078, 2987, 2926, 1622, HTB-22), Neuro2a derived from mouse neuroblastoma
1380, 1205, 1096, 1060, 865.
cell line (ATCC No. CCL-131) were obtained from
American Type Culture Collection, Manassas, VA,
USA. All tumour cell lines were maintained in a Modi-
fied DMEM medium supplemented with 10% fetal
bovine serum, along with 1% non-essential amino acids
without L-glutamine, 0.2% sodium bicarbonate, 1%
sodium pyruvate and 1% of antibiotic mixture (10,000
units penicillin and 10 mg streptomycin per mL).
The cells were washed and resuspended in the above
medium and 100 μL of this suspension was seeded in
96 well flat bottom plates. The cells were maintained at
37◦C in a humidified 5% CO2 incubator (Model 2406
Shellab CO2 incubator, Sheldon, Cornelius, OR). After
24 h incubation, the cells were treated for 2 days with
test compounds at concentrations ranging from 0.1 to
100 μM in DMSO (1% final concentration) and were
assayed at the end of the 2nd day. Each assay was per-
formed with two internal controls: (1) an IC0 with cells
only, (2) an IC100 with media only. After 48 h incuba-
tion, the cells were subjected to the MTT colorimetric
assay (5 mg mL−1). The effect of the different test
compounds on the viability of tumour cell lines was
measured at the wavelength of 540 nm on a multimode
reader (Infiniteꢀ M200, Tecan, Switzerland). The IC50
values (50% inhibitory concentration) were calculated
from the plotted absorbance data for the dose-response
curves. IC50 values (in μM) are expressed as the
average of two independent experiments.
2.3d Synthesis of [PdCl2(2d)]: Treatment of galac-
topyranose triazole (0.066 g, 0.17 mmol) 2d with [Pd
(COD)Cl2] (0.05 g, 0.17 mmol) resulted 3d as yellow
solid. Yield: 0.72 g (75%); mp: 240–245◦C; Anal. calc.
for C19H24Cl2N4O5Pd: C 40.34, H 4.28, N 9.90. Found:
C 40.17, H 4.71, N 8.99; 1H NMR (DMSO, 500 MHz):
δ 1.29 (s, 3H, CH3 of CMe2), 1.38 (s, 3H, CH3 of
CMe2), 1.44 (s, 3H, CH3 of CMe2), 1.48 (s, 3H, CH3
of CMe2), 4.28 (d, 1H, J = 8.21 Hz, H-3), 4.35–
4.38 (m, 2H, H-6), 4.48–4.51 (m, 2H, H-4 and H-5),
4.63–470 (m, 1H, H-2), 5.50 (d, 1H, J = 4.16 Hz, H-
1), 7.56 (t, 1H, J = 5.0 Hz, Py), 8.09–8.14 (m, 2H,
J = 7.180 Hz, Py), 8.98 (s, 1H, triazole), 9.14 (d,
1H, J = 4.10 Hz, Py); ESI-MS (in CH3OH): m/z 531
[(M-Cl)+1]+; IR (KBr, cm−1): 3085, 2983, 2925, 1624,
1456, 1380, 1069, 785.
2.4 X-ray crystallography
X-ray data for the compounds was collected at room
temperature using a Bruker Smart Apex CCD diffrac-
tometer with graphite monochromated MoKα radi-
ation (λ = 0.71069 Å) with ω-scan method. Prelim-
inary lattice parameters and orientation matrices were
obtained from four sets of frames. Integration and scal-
ing of intensity data was accomplished using SAINT
program. The structure was solved by direct methods
using SHELXS97 and refinement was carried out by
full-matrix least-squares technique using SHELXL97.
Anisotropic displacement parameters were included
for all non-hydrogen atoms. All O-bound H atoms of
2a were located in difference Fourier maps and their
positions and isotropic displacement parameters were
refined. Distance restraints were applied to O–H with
a set values of 0.89(2) Å. All other H atoms were
positioned geometrically and treated as riding on their
parent C atoms [C-H = 0.93–0.98 Å and Uiso(H) =
1.5 Ueq(C) for methyl H or 1.2Ueq(c) for other H
atoms]. The methyl groups were allowed to rotate but
not to tip.
3. Results and discussion
3.1 Synthesis and characterization
The carbohydrate derived azido substrates for the inter-
molecular 1,3-dipolar cycloaddition were prepared by
S2N displacement reaction of the corresponding tosylates
or mesylates with sodium azide in DMF at reflux tem-
peratures.40,41 The corresponding triazoles 2a–d were
prepared by the click reaction26 between sugar azide
(1a–d) and 2-ethyl pyridine in the presence of cop-
per(II) sulphate and sodium ascorbate in a mixture of
1:1 tBuOH–H2O (scheme 1), in good yield. Pyridyl tri-
azole containing benzyl group was prepared according
to the literature procedure.29 The spectroscopic data and
micro analytical data of all compounds were consistent
with the proposed structures.
2.5 In vitro cytotoxic testing
Cell lines used for testing in vitro cytotoxicity included
HeLa derived from human cervical cancer cells (ATCC
No. CCL-2), A549 derived from human alveolar ade-
nocarcinoma epithelial cells (ATCC No. CCL-185),
MDA-MB-231 derived from human breast adenocar-
cinoma cells (ATCC No. HTB-26), MCF7 derived
from human breast adenocarcinoma cells (ATCC No
In the 1H NMR spectrum, the anomeric proton of the
furanose/pyranose ring appeared as doublet at around δ
5.83–5.97 ppm and the triazole proton appeared as sin-
glet around δ 8.13 ppm. Protons of the gem-dimethyl
group appeared as two singlets at δ 1.21 and δ 1.34 ppm
in the case of furanose derived triazoles 2a–c and four

Palladium complexes of carbohydrate ligands
1409
Scheme 1. Synthesis of pyridyl triazole ligands and their palladium complexes.
singlets in the case of galactose derived triazole 2d. as a sharp singlet around 8.5–9.0 ppm, this down field
In the case of ribofuranosyl pyridyl triazole 2c, the shift confirmed the complex formation.
protected methyl group appeared as a sharp singlet at
3.2 Crystal structure of 2a
δ 3.39 ppm. The absence of IR bands corresponding
to azide (2095 cm−1) and alkyne groups (2150 cm−1)
The molecular structure of the pyridyl triazole 2a
further confirmed the presence of triazole ring. Com-
was determined by means of X-ray diffraction studies
pounds exhibited their characteristic stretching bands
and its crystallographic data and structural refinement
for the C–N stretching at around 1620–1640 cm−1.
parameters are given in table 1.
The palladium complexes were prepared by react-
Pale yellow crystals of 2a were obtained by the slow
ing triazoles 2a–e with [Pd(COD)Cl2] in anhy-
evaporation of ethyl acetate solution. The compound
drous dichloromethane for 2 h at room temperature
crystallized in the orthorhombic, space group P212121.
(scheme 1). All the complexes were obtained as pale
The atom numbering scheme is given in figure 1, which
yellow powders and are soluble only in high polar sol-
shows a perspective view of the compound 2a.
vents such as DMF and DMSO. The complexes have
The pertinent bond length and bond angle data for
1
been fully characterized by H NMR, ESI-MS, IR and
this compound is listed in table 2. The crystallographic
elemental analysis. Attempts to grow single crystals of
structure reveals the orientation of the pyridine with
complexes yielded only a microcrystalline solid that
respect to the sugar–triazole moiety. The sugar binds
was unsuitable for X-ray crystallographic analysis.
to N(1) atom of triazole through the C(5) carbon atom
The characterization of the palladium complexes
with a bond angle of 117.2◦ and the pyridine ring has
an almost ‘anti’ conformation with respect to the tria-
1
was done by means of H NMR spectra recorded in
1
d6-DMSO solutions. The H NMR spectra of com-
zole ring (figure 1) as reported for the pyridyl triazole
plex 3e is obtained similar to the reported values.29
ligands.29 The N(2)–N(3) distance of the 1,2,3-triazole
1
In H NMR spectra of complexes 3a–d, a down field
is 1.32 Å, which is shorter than the N(3)–C(10)
shift was observed for anomeric proton of carbohy-
(1.36 Å) and N(2)–N(1) (1.35 Å bonds) bond distances,
drate scaffold as well as for the pyridyl and triazole
respectively.
protons. The anomeric proton of the sugar moiety
appeared as doublet around 5.01–5.90 ppm depending
on the sugar moiety and the protons corresponding to
the two gem-isopropylidiene units of compounds 3a–c,
3.3 Electronic spectroscopy
appeared as two singlets in the region 1.27–1.31 ppm The formation of palladium complexes was further con-
and 1.40–1.45 ppm. For compound 3d, four singlets firmed by means of absorption spectroscopic studies in
were observed for the four isopropylidiene units at 1.29, 0.5 mM DMSO solution (figure 2) over a wavelength
1.38, 1.44 and 1.48 ppm. The triazole proton appeared range of 250–400 nm. The absorption spectra of the

1410
S Bhavya Deepthi et al.
Table 1. Crystallographic parameters for ligand 2a.
Crystal parameters
CCDC No.
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a/(Å)
852721
C15H18N4O4
318.33
294(2)
0.71069
Orthorhombic
P212121
7.2530 (6)
b/(Å)
11.0360 (4)
c/(Å)
19.5230 (10)
90.0
90.0
90.0
1562.70 (16)
4
1.353
0.100
α ()
β (◦)
γ (◦)
Volume (Å3)
Z
Density (calc., mg m−3
)
M (mm−1
F (000)
)
672
Crystal size (mm3)
θ range
0.12 × 0.17 × 0.12
2.09 to 25.00◦
−8 to 8, −13 to 13, −23 to 23
14691
h, k, l ranges
Reflection collected
Independent reflection [Rint]
Completeness to θ = 25.00◦
Refinement method
1605 (0.0307)
100.0%
Full-matrix least square on F2
1605/0/214
Data/restraints/parameters
Goodness-of-fit-on F2
1.123
Final R indices [I>2σ(I)]
R indices (all data)
Absolute structure parameter
R1 = 0.0372, wR2 = 0.0982
R1 = 0.0411, wR2 = 0.1015
0.004 (12)
Largest diff. peak and hole (e. Å)
0.408 and −0.122
complexes closely resemble the UV spectra of palla- 280 nm and 310 nm characteristic to typical π → π∗
dium triazole complexes reported in the literature.29 All charge transfer of triazole and metal to ligand charge
the complexes exhibited two absorption maxima around transfer transitions (MLCT), respectively. In the UV
spectra, an intense π → π∗ transition is observed,
whereas the MLCT transition is observed as a weak
transition band.
Table 2. Selected bond lengths and bond angles for the
compound 2a.
Bond lengths (Å)
Bond Angles (◦)
C(10)-C(11)
C(10)-N(3)
N(1)-N(2)
N(2)-N(3)
C(9)-N(1)
N(1)-C(5)
C(11)-N(4)
1.481(3)
C(4)-C(5)-N(1)
N(1)-C(9)-C(10)
N(1)-N(2)-N(3)
N(3)-C(10)-C(11)
N(3)-C(10)-C(9)
C(11)-C(10)-C(9)
N(4)-C(11)-C(10)
113.8(2)
105.3(2)
107.3(2)
108.2(2)
120.8(2)
131.0(2)
117.2(2)
1.359(3)
1.347(3)
1.316(3)
1.345(3)
1.465(3)
1.352(3)
Figure 1. A view of crystal structure of 2a, showing the
atom labelling scheme. Displacement ellipsoids are drawn at
the 30% probability level and the H-atoms are represented by
circles of arbitrary radii.

Palladium complexes of carbohydrate ligands
1411
2.0
1.5
1.0
0.5
0.0
adenocarcinoma cells). The concentration of com-
pounds at which 50% of cell growth was inhibited
(IC50) was calculated and shown in table 3.
3a
3b
3c
3d
The palladium xylofuranose complex (3a) exhibited
good cytotoxicity on all the cell lines with IC50 < 10 μM,
but the free ligand 2a was found to be non-toxic.
The pyridyl triazole ligand derived from benzyl pro-
tected xylofuranose (2b), exhibited considerably high
IC50 values ranging from 11.9–43.7 μM, whereas its
palladium complex exhibited good activity with IC50
< 9 μM on all the cell lines except on A549. The ribofu-
ranose triazole ligand also found to be non-toxic but the
complex 3c showed excellent cytotoxicity toward A549
(IC50 = 7.68 μM) and Neuro2a (IC50 = 7.1 μM) cell
lines. Similar to ribofuranose triazole, galactopyranose
triazole (2d) was also found to be non-toxic, whereas its
palladium complex 3d exhibited cytotoxicity towards
A549, HeLa and hormone independent breast cancer
cells MDA-MB-231. In order to understand the impor-
tance of carbohydrate scaffold in these types of ligands,
benzyl pyridyl triazole (2e) and its palladium com-
plex (3e) were prepared and tested for their cytotox-
icity. Interestingly, the pyridyl triazole ligand without
carbohydrate moiety (2e) and the corresponding palla-
dium complex (3e) did not show any inhibitory activ-
ity on cancer cell lines, indicating the importance of the
carbohydrate scaffold in these types of complexes.
From these results it is evident that except ligand 2b,
all the carbohydrate pyridyl triazole ligands are non-
toxic in nature but their palladium complexes exhi-
bited excellent cytotoxicity. These results also show the
significance of the presence of metal and carbohydrate
structure in determining the cytotoxicity. The excel-
lent cytotoxicity of carbohydrate complexes may be
300
400
Wavelength (nm)
Figure 2. UV-Vis spectra of palladium complexes 3a–d in
0.5 mM DMSO solutions.
3.4 Cytotoxicity of ligands and their metal complexes
(MTT assay)
Cytotoxicity of the free triazole ligands 2a–e and their
palladium complexes 3a–e were investigated on the
basis of the measurement of in vitro growth in 96 well
plates by cell mediated reduction of tetrazolium salt to
form water insoluble formazan crystals according to the
literature procedures42 using doxorubicin and cisplatin
as standards. The compounds were tested for cytotoxi-
city against five different tumour cell lines: A549 (human
alveolar adenocarcinoma epithelial cells), Neuro2a
(mouse neuroblastoma cells), HeLa (human cervical
carcinoma cancer cells), MDA-MB-231 (human breast
adenocarcinoma cells) and MCF-7 (human breast
Table 3. Cytotoxicity of triazoles 2a–e and metal complexes 3a–e.
Test
IC50(μM)
HeLa MDA-MBA-231
Compound
A549
Neuro 2a
MCF-7
a
a
a
a
a
2a
-
-
-
-
-
2b
2c
2d
2e
43.7
11.9
18.5
35.0
39.4
a
a
a
a
a
-
-
-
-
-
-
-
-
-
-
a
a
a
a
a
a
a
a
a
a
-
-
-
-
-
3a
3b
3c
3d
6.4
-
7.8
5.6
7.1
8.3
7.9
-
9.9
7.9
-
5.5
8.9
a
a
a
a
7.68
6.9
-
-
-
a
a
-
-
9.2
12.1
a
a
a
a
a
3e
-
-
-
Doxorubicinb
Cisplatinb
1
1
<1
<1
1.2
<1
<1
<1
1
<1
aNo inhibiton, bstandard

1412
S Bhavya Deepthi et al.
attributed to the liphophilic character of the carbohy-
drate scaffold. The carbohydrate scaffold may be visu-
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4. Conclusions
In conclusion, synthesis of pyridyl-sugar triazole li-
gands derived from different pentose and hexose sugar
moieties and the corresponding palladium–triazole
complexes is described. It was observed that the free
ligands are non-toxic towards the tested cell lines and
the insertion of metal leads to the considerable cyto-
toxicity. The metal complexes derived from xylofura-
nose moiety exhibited good cytotoxicity compared to
the metal complexes of ribofuranose and galactpyra-
nose moieties. The study clearly indicates that the inser-
tion of metal into a non-toxic carbohydrate functional
group imparts cytotoxic activity to the compound. Fur-
ther experiments involving structural modifications and
other biological studies to determine the role of these
compounds on the apoptotic and proliferative pathways
in tumour cell lines are in progress.
Supporting Information
Supporting Information contain H, 13C NMR spectra
1
of the ligands, 1HNMR spectra of the metal complexes
and crystallographic date for compound 2a. CCDC
852721 contains supplementary Crystallographic data
for the compound 2a. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/data_request/cif.
or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Cen-
tre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033.
Tables S1–S6 and figures S1–S12 can be seen in the
website of Journal of Chemical Sciences (www.ias.ac.
in/chemsci).
Acknowledgements
This work was financially supported by in-house project
MLP-0008. Authors SBD and PS thank the Council for
Scientific and Industrial Research (CSIR), New Delhi
for the award of research fellowship.
28. Mindt T L, Struthers H, Brans L, Angelov T,
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J. Am. Chem. Soc. 128 15096
29. Schweinfurth D, Pattacini R, Strobel S and Sarkar B
2009 Dalton Trans. 9291
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