Palladium(II) Complexes of NS Donor Ligands Derived from Steroidal Thiosemicarbazones as Antibacterial Agents

Article abstract of DOI:10.3390/molecules15074784

We have investigated the antibacterial activity of some new steroidal thiosemicarbazones and their Pd(II) metal complexes were prepared by the reaction of the thiosemicarbazones with [Pd(DMSO)₂Cl₂]. The steroidal thiosemicarbazones were prepared by the reaction of thiosemicarbazides with a steroidal ketone. The structures of these compounds were elucidated by IR, ¹H-NMR, ¹³C-NMR, FAB mass spectroscopic methods, elemental analyses and TGA analysis. The antibacterial activity of these compounds were tested in vitro by the disk diffusion assay against two Gram-positive and two Gram-negative bacteria. The results showed that steroidal complexes are better inhibitors of both types of the bacteria (Gram-positive and Gram-negative) as compared to steroidal thiosemicarbazones. Compound Ia displays remarkable antibacterial activity as compared to amoxicillin.

Full text of DOI:10.3390/molecules15074784

Molecules 2010, 15, 4784-4791; doi:10.3390/molecules15074784
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Palladium(II) Complexes of NS Donor Ligands Derived from
Steroidal Thiosemicarbazones as Antibacterial Agents
Abdullah M. Asiri 1,2 and Salman A. Khan 1,*
1
Chemistry Department, Faculty of Science, King Abdul Aziz University, P.O. Box 80203, Jeddah
21589, Saudi Arabia
2
The Center of Excellence for Advanced Materials, King Abdul Aziz University, Jeddah 21589, P.O.
Box 80203, Saudi Arabia
* Author to whom correspondence should be addressed; E-Mail: sahmad_phd@yahoo.co.in.
Received: 13 May 2010; in revised form: 28 May 2010 / Accepted: 9 June 2010 /
Published: 8 July 2010
Abstract: We have investigated the antibacterial activity of some new steroidal
thiosemicarbazones and their Pd(II) metal complexes were prepared by the reaction of the
thiosemicarbazones with [Pd(DMSO)2Cl2]. The steroidal thiosemicarbazones were
prepared by the reaction of thiosemicarbazides with a steroidal ketone. The structures of
1
13
these compounds were elucidated by IR, H-NMR, C-NMR, FAB mass spectroscopic
methods, elemental analyses and TGA analysis. The antibacterial activity of these
compounds were tested in vitro by the disk diffusion assay against two Gram-positive and
two Gram-negative bacteria. The results showed that steroidal complexes are better
inhibitors of both types of the bacteria (Gram-positive and Gram-negative) as compared to
steroidal thiosemicarbazones. Compound Ia displays remarkable antibacterial activity as
compared to amoxicillin.
Keywords: thiosemicarbazone; palladium (II); antibacterial activity; amoxicillin
1. Introduction
The chemistry of coordination metal complexes of thiosemicarbazone ligands have been receiving
considerable attention primarily because of their bioinorganic relevance [1,2]. There have been
attempts [3] to determine structural correlations between metal ion complexes of thiosemicarbazones

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and their wide spectrum of biological applications. Thiosemicarbazones have been the subject of
extensive investigations. In several cases, the pharmacological action of the thiosemicarbazons is
enhanced due to the presence of coordination metal ions [4,5]. It is well authenticated that a NS
bidentate system is present in most of the thiosemicarbazones having carcinostatic potency and
possessing substantial in vitro activity against various human tumour lines [6,7]. The
thiosemicarbazone derivatives of Pd(II) have proven to be more effective as anticancer or anti-
microbial agents than the ligand by itself , probably due to the increased lipophilicity of the complexes
as compared to the free ligands alone [8]. Recent spectral and structural studies of Pd(II) complexes of
N-substituted thiosemicarbazones showed a diversity of molecular geometries of the compounds,
depending on the ligand, the salt used in synthesis and the method of compound synthesis. Substitution
on the terminal N position can also affect the coordination and biological properties. In this paper, we
report herein synthesis, characterization and antibacterial activity of Pd (II) complexes of steroidal
thiosemicarbazones (STSC) against two Gram-positive and Gram-negative bacteria.
2. Results and Discussion
Reaction of steroidal thiosemicarbazones with [Pd(DMSO)Cl2] gave amorphous solid compounds.
All the compounds were isolated in good yields and were stable both in the solid and solution state.
The structures of the ligands and complexes presented in Schemes 1-2 and Figure 2 were established
by comparing spectral data (IR, 1H-NMR, 13C-NMR and FAB mass spectra) with the free ligand, along
with their thermogravimetric analysis.
Scheme.1. Synthesis of the steroidal thiosemicarbazones 1-3.
bromine
solution
Zn
Jones reagent
glacial acetic acid
HO
O
O
Br
(c)
Br
HO
Br
Br
(a)
S
(b)
C2H5OH
R-C-NHNH2
H
N
S
H
N
H
N
Where
R-C-NH-N
R =
(1-3)
(2)
(1)
(3)
Scheme 2. Showing the synthesis of complexes (1a-3a).
CH3OH
[Pd(DMSO)2Cl2] + TSCN
[Pd(TSCN)Cl2]
2 DMSO
+
5 h, reflux
where TSCN = thiosemicarbazones.

Molecules 2010, 15
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2.1. IR spectral studies
Assignments of selected characteristic IR band positions provide significant indication for the
formation of steroidal thiosemicarbazones and their complexes. The thiosemicarbazones can exist as
thione and thiol tautomeric froms IA and IB (Figure 1), respectively.
Figure 1. Structure of thione and thiol form of thiosemicarbazones.
S
S H
R-C-N H-N
R-C = N-N
T h io l fo rm
(IB )
(IA )
T h io n e fo rm
However, the existence of a strong band in the region 1,026–1,038 cm-1 due to v (C=S) and absence
of any band in the region 2,500–2,600 cm-1 due to v (C-SH) suggested that all the thiosemicarbazones
remain in their thione form.
Figure 2. Structure of palladium (II) complexes: (1a) R= -NHC5H9.
N
C l
N H
Pd
C l
C
S
R
A strong band at 1,565-1,578 cm-1 was assigned to the v (C=N) stretch of the azomethine linkage in
the spectra of the free ligands. In the complexes this band shifted to lower frequency by 27–38 cm-1
and this lowering was attributed to the coordination of azomethine nitrogen with metal and formation
of M-N band. The strong band at 1,018–1,024 cm-1 ascribed to v (C=S) of ligands is shifted to lower
frequency (by 8–14 cm-1), indicating the bonding of metal through thionic sulphur. This contention
was further confirmed by the presence of v (Pd-N) and v (Pd-S) bands at 508–521 and 428–437 cm-1 in
the far IR region of the complexes. The broad band observed in region 3,242–3,256 cm-1 due to v
(N-H) stretch is only slightly affected by the coordination.
2.2. NMR spectral analysis
The structures of thiosemicarbazones and their metal complexes were further confirmed from the
1H-NMR spectra, which provide diagnostic tools for the positional elucidation of the protons.

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1
Assignments of the signals are based on the chemical shifts and intensity patterns. The H-NMR
spectra of thiosemicarbazones 1-3 recorded in DMSO-d6 exhibit a broad peak at 9.84–10.48 ppm due
to -NH proton, which indicate that even in a polar solvent they remain in the thione form. The -NH
proton signal of the thiosemicarbazones usually shifts to upfield and appears at 3.38–3.82 ppm in their
respective complexes. This information suggests the adjustment of electronic current upon
coordination of C=S group to the metal ion.
The 13C-NMR spectra of ligands were recorded in DMSO-d6 and the spectral signals were in good
agreement with the proposed structures. The ligands showed signals at 185.8–186.6 ppm due to (C=S)
and 153.8–155.7 ppm assigned to (C=N), respectively complex 182.2–184.5 ppm and
152.2–156.7 ppm assigned to (C=S) and (C=N) respectively. Other carbons in this complex resonate
nearly at the same frequency as that of free ligands as given in the Experimental.
2.3. Thermogravimetric analysis
The TGA profiles of complexes (under nitrogen, 10% min) along with the % weight at different
temperatures were recorded. These complexes do not lose weight up to 240 ºC. Further increment of
temperature causes decomposition of the complexes in two steps. The temperature range for the first
step was 240–312 ºC. In complexes 1a, 2a and 3a, the first fragment corresponded to the loss of
chlorine and sulfur atoms from complexes. The second step starts immediately after the first step and
continues until the complete decomposition of the ligands and formation of the end product as
palladium sulfide (PdS). The total % weight loss of the complexes corresponds to the loss of respective
ligands after considering the transfer of one sulfur atom to the metal ion and the residues correspond to
the palladium sulfide.
2.4. FAB mass analysis
Characteristic peaks were observed in the mass spectra of ligands and their metal complexes, which
followed the similar fragmentation pattern. The spectrum of compound 1 showed a molecular ion peak
(M+.) at m/z 526 and its complex compound 1a showed a molecular ion peak (M+.) at m/z 701. The
characteristic peaks observed within the mass spectra of thiosemicarbazones and their metal complexes
are given in the Experimental section.
2.5. In-vitro anti-bacterial activity
The in vitro antibacterial activity of steroidal thiosemicarbazones and their metal complexes were
assayed by the disk diffusion method using cultures of S. aureus, S. pyogenes, S. typhimurium, and E.
coli [12]. Amoxicillin (30 mg) was used as the standard drug, whereas a DMSO-wetted disk was used
as negative control. Results showed that metal complexes are better antibacterial agent as compared to
the parent thiosemicarbazones. Results are summarized in Table 1.

Molecules 2010, 15
Table 1. Antibacterial activity of steroidal thiosemicarbazones and their complexes,
4788
Positive control A (Amoxicillin), and negative control (DMSO). Measured by the Halo
Zone Test (unit, mm).
Corresponding effect on microorganisms
Compound
S. aureus
S. Pyogenes
S. typhimurium
E. coli
1
15.2 ± 0.4
16.8 ± 0.4
16.4 ± 0.2
14.2 ± 0.5
2
3
13.4 ± 0.4
11.2 ± 0.4
21.5 ± 0.5
16.6 ± 0.4
15.5 ± 0.3
21.0 ± 0.5
-
12.2 ± 0.5
13.5 ± 0.3
23.2± 0.4
15.4 ± 0.2
16.7± 0.4
22.2 ± 0.4
-
11.6 ± 0.4
10.9 ± 0.5
25.6 ± 0.2
16.2 ± 0.2
14.5 ± 0.5
25.2 ± 0.8
-
13.2 ± 0.5
12.2 ± 0.2
21.6 ± 0.4
15.8 ± 0.4
16.8 ± 0.5
20.0 ± 0.2
-
1a
2a
3a
A.
DMSO
3. Experimental
3.1. General
All melting points were measured with a capillary apparatus and are uncorrected. All the
1
compounds were routinely checked by IR, H-NMR, 13C-NMR, mass spectrometry and elemental
analysis. IR spectra were recorded in KBr on a Perkin-Elmer model 1620 FTIR spectrophotometer. 1H-
13
NMR and C-NMR spectra were recorded at ambient temperature using a Brucker Spectroscopin
DPX-600 MHz spectrophotometer in CDCl3 and DMSO. The following abbreviations were used to
indicate the peak multiplicity s- singlet, d- doublet, t- triplet, m- multiplet. FAB mass spectra were
recorded on a JEOL SX102 mass spectrometer using Argon/Xenon (6 kV, 10 mB gas). Column
chromatography was performed on silica gel (Merck). The reactions were monitored by precoated
aluminium silica gel 60F 254 thin layer plates procured from Merck (Germany). Anhydrous sodium
sulfate was used as a drying agent for the organic phase. Compounds a, b, c and thiosemicarbazide
were prepared according to published methods [9].
3.2. Synthesis of thiosemicarbazones: A general method
Steroidal thiosemicarbazones were synthesized (Scheme 1) by refluxing a solution of
thiosemicarbazide (0.03 mol) in methanol (15 mL) and the alcoholic solution of steroidal ketones
(0.03 mol, 11.52 g, 15 mL) at 60 ºC for 5 h with continuous stirring. After cooling the compounds
were filtered and recrystallized from methanol [10].
Cholest-5-en-3-one cyclopentyl thiosemicarbazone (1). C33H55N3S;Yield: 68.5%; m.p. 206–208 ºC; IR
(KBr) ν wax cm-1: 3242 (N-H), 1565 (C=N), 1622 (C=C), 1126 (C-N), 1038 (C=S); 1H-NMR (DMSO-
d6) (δ, ppm): 10.45 (2H, s, -NH), 4.16 (m, 8H, -CH2), 5.34 (1H, s, C6-H), 1.08 (s, C10-CH3), 0.78 (s,
13
C13-CH3), 0.84, 0.96 (other methyl protons); C-NMR (DMSO-d6) (δ, ppm): 186.6 (C=S), 155.7
(C=N), 134.8 (C-NH), 22.6 (C10-CH3), 19.8 (C13-CH3); Mass spectra (M+.) at m/z 526, 457 (M-

Molecules 2010, 15
4789
C5H9), 542 (M-C5H10N), 398 (M- C6H10NS), 383 (M- C6H11NS); Anal. Calc. for C33H55N3S: C, 75.42;
H, 10.47; N, 8.00. Found: C, 75.28; H, 10.32: N, 7.93.
Cholest-5-en-3-one cyclohexyl thiosemicarbazone (2). C34H57N3S; Yield: 72.00%; m.p. 216–218 ºC;
1
IR (KBr) ν
cm-1: 3252 (N-H), 1572 (C=N), 1628 (C=C), 1132 (C-N), 1034 (C=S); H-NMR
wax
(DMSO-d6) (δ, ppm): 9.84 (2H, s, -NH), 4.12 (m, 10H, -CH2), 5.38 (1H, s, C6-H), 1.12 (C10-CH3),
13
0.82 (C13-CH3), 0.88, 1.02 (other methyl protons); C-NMR (DMSO-d6) (δ, ppm): 185.2 (C=S),
154.5 (C=N), 132.5 (C-NH), 23.5 (C10-CH3), 18.9 (C13-CH3); Mass spectra (M+.) at m/z 540, 457(M-
C6H11), 542 (M-C6H12N), 398 (M- C7H12NS), 383 (M- C7H13N2S); Anal. Calc. for C34H57N3S: C,
75.69; H, 10.57; N, 7.79. Found: C, 75.62; H, 10.49: N, 7.75.
Cholest-5-en-3-one cyclooctyl thiosemicarbazone (3). C36H61N3S; Yield: 75.5%; m.p. 228–230 ºC; IR
(KBr) ν wax cm-1: 3256 (N-H), 1578 (C=N), 1632 (C=C), 1138 (C-N), 1026 (C=S); 1H-NMR (DMSO-
d6) (δ, ppm): 10.48 (2H, s, -NH), 4.22 (m, 14H, -CH2), 5.32 (1H, s, C6-H), 1.10 (C10-CH3), 0.80
13
(C13-CH3), 0.90, 1.04 (other methyl protons); C-NMR (DMSO-d6) (δ, ppm): 185.8 (C=S), 153.8
(C=N), 134.8 (C-NH), 24.5 (C10-CH3), 20.2 (C13-CH3); Mass spectra (M+.) at m/z 568, 459 (M-
C8H13), 444 (M-C8H14N), 400 (M- C9H14NS), 385 (M- C9H15N2S); Anal. Calc. for C36H61N3S: C,
76.19; H, 10.75; N, 7.40, Found: C, 76.12; H, 10.68: N, 7.35.
3.3. Preparation of palladium (II) complexes
The metal complexes were prepared by mixing an equimolar ratio of ligand and [Pd (DMSO)2 Cl2]
in refluxing methanol. The solution was kept at 0 ºC overnight, the product was separated by filtration
and finally washed with methanol. Recrystallization was effected in methanol/ DMF (6:4) [11].
Dichloro(cholest-5-en-3-one cyclopentyl thiosemicarbazone)palladium (II) (1a). Pd (C33H55N3S) Cl2);
Yield: 76%; m.p. 256–258 ºC; IR (KBr) v
cm-1: 3442 (N-H), 1538 (C=N), 1528 (C=C), 1152 (C-
wax
N), 1024 (C=S), 516, 434 (Pd-N, Pd-S); 1H-NMR (DMSO-d6) (δ, ppm): 9.54 (2H, s, -NH), 5.58 (1H, s,
C6-H), 4.22 (m, 8H, -CH2), 3.82 (1H, s, NH), 1.12, (C10-CH3), 0.82 (C13-CH3), 0.94, 1.06 (other
13
methyl protons); C-NMR (DMSO-d6) (δ, ppm): 184.5 (C=S), 153.8 (C=N), 133.8 (C-NH), 22.2
(C10-CH3), 20.4 (C13-CH3), 19.4, 19.8 (other methyl carbon); Mass spectra (M+.) at m/z 701, 666 (M-
Cl), 731 (M-Cl2), 597 (M-Pd), 632 (M-C5H9), 617(M- C5H10N), 573 (M- C6H10NS), 558 (M-
C6H11N2S). Anal. Calc. for Pd(C33H55N3S)Cl2: C, 56.38; H, 7.83; N, 5.98, Cl, 10.09, Pd, 15.12. Found:
C, 56.34; H, 7.79; N, 5.95, Cl, 10.02, Pd, 15.09.
Dichloro(cholest-5-en-3-one cyclohexyl thiosemicarbazone)palladium (II) (2a). Pd (C35H57N3S) Cl2;
Yield: 72.00%; m.p. 264–265 ºC; IR (KBr) ν wax cm-1: 3456 (N-H), 1545 (C=N), 1538 (C=C), 1156 (C-
1
N), 1022 (C=S), 508, 428 (Pd-N, Pd-S); H-NMR (DMSO-d6) (δ, ppm): 9.56 (2H, s, -NH), 4.16 (m,
10H, -CH2), 5.52 (1H, s, C6-H), 3.38 (1H, s, NH), 1.16 (C10-CH3), 0.86 (C13-CH3), 0.92, 1.06 (other
methyl protons); 13C-NMR (DMSO-d6) (δ, ppm): 183.5 (C=S), 152.2 (C=N), 131.5 (C-NH), 22.4(C10-
CH3), 18.4 (C13-CH3); Mass spectra (M+.) at m/z 715, 680 (M- Cl), 645 (M-Cl2), 611 (M-Pd) 632 (M-
C6H11), 617 (M-C6H12N), 573 (M- C7H12NS), 558 (M- C7H13N2S); Anal. Calc. for Pd (C34H57N3S) Cl2:
C, 56.95; H, 7.98; N, 5.86, Cl, 9.89, Pd, 14.85. Found: C, 56.89; H, 7.95; N, 5.85, Cl, 9.85, Pd, 14.82.

Molecules 2010, 15
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Dichloro(cholest-5-en-3-one cyclooctyl thiosemicarbazone)palladium (II) (3a). Pd(C36H61N3S)Cl2;
Yield: 76.8%; m.p. 272–273 ºC; IR (KBr) ν wax cm-1: 3438 (N-H), 1532 (C=N), 1522(C=C), 1172 (C-
1
N), 1018 (C=S), 521, 437 (Pd-N, Pd-S); H-NMR (DMSO-d6) (δ, ppm): 9.45 (2H, s, -NH), 4.12 (m,
14H, -CH2), 5.62 (1H, s, C6-H), 3.65 (1H, s, NH), 1.14 (C10-CH3), 0.84 (C13-CH3), 0.96, 1.04 (other
methyl protons); 13C-NMR (DMSO-d6) (δ, ppm): 182.2 (C=S), 156.7 (C=N), 128.4 (C-NH), 23.4
(C10-CH3), 19.5 (C13-CH3); Mass spectra (M+.) at m/z 742, 707 (M-Cl), 672 (M-Cl2), 637 (M-Pd),
633 (C8H13), 618 (M-C8H14N), 574 (M-C9H14NS), 559 (M-C9H15N2S); Anal. Calc. for
Pd(C36H61N3S)Cl2: C, 58.03; H, 8.19; N, 5.64, Cl, 9.52, Pd, 14.29. Found: C, 58.01; H, 8.18: N, 5.62,
Cl, 9.48, 14.26.
3.4. Organism culture and in vitro screening
Antibacterial activity was assayed by the disk diffusion method with minor modifications. S.
aureus, S. pyogenes, S. typhimurium, and E. coli were subcultured in BHI medium and incubated for
18 h at 37 ºC, and then the bacterial cells were suspended, according to the McFarland protocol in
saline solution to produce a suspension of about 10-5 CFU mL-1. Ten μL of this suspension was mixed
with sterile antibiotic agar (10 mL) at 40 ºC and poured onto an agar plate in a laminar flow cabinet.
Five paper disks (6.0 mm diameter) were fixed onto nutrient agar plate. Ten mg of each test compound
was dissolved in DMSO (100 μL) to prepare stock solution and from stock solution different
concentration of 10 (1 μL stock solution + 9 μL solvent), 20 (1 μL stock solution + 4 μL solvent), 25
(1 μL stock solution + 3 μL solvent), 50 (1 μL stock solution + 1 μL solvent), and 100 μg/μL of each
test compound were prepared. These compounds of different concentration were poured over disk
plate on to it. Amoxicillin (30 μg) was used as standard drug (positive control). A DMSO-wetted disk
was used as negative control. The susceptibility of the bacteria to the test compounds was determined
by the formation of an inhibitory zone after 18 h of incubation at 36 ºC. Table 1 reports the inhibition
zones (mm) of each compound and the controls this experiment was repeated two times for each
compound and found same results.
4. Conclusions
This research examined the synthesis, characterization and antibacterial activity of some new
steroidal thiosemicarbazone derivatives and their Pd (II) complexes. In vitro antibacterial activity of
these compounds was tested by the disk diffusion assay against two gram-positive and gram-negative
bacteria. The result showed that cyclopentyl steroidal thiosemicarbazone derivative was found the
most active among all thiosemicarbazones tested. Compound Ia displays remarkable antibacterial
activity as compared to amoxicillin.
Acknowledgements
The authors would like to thank to Head, Chemistry Department, King Abdul Aziz University,
Jeddah Saudi Arabia for providing the research facilities.

Molecules 2010, 15
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distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).