Synthesis, physicochemical properties, and in vitro antibacterial screening of palladium(II) complexes derived from thiosemicarbazone

Article abstract of DOI:10.1002/cbdv.201200128

A new series of Pd^II complexes derived from thiosemicarbazone has been synthesized. The synthesized Pd^II complexes have been characterized on the basis of elemental analyses, FT-IR, ¹H- and ¹³C-NMR, UV/VIS, and thermal studies. A square-planar geometry has been assigned around Pd^II ions on the basis of results obtained from UV/VIS studies. The thiosemicarbazone ligand and its Pd^II complexes have been screened against Gram-positive (Bacillus subtilis and Staphylococcus aureus) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria in vitro as growth-inhibiting agents, and the results revealed significant antibacterial activities. Copyright

Full text of DOI:10.1002/cbdv.201200128

CHEMISTRY & BIODIVERSITY – Vol. 10 (2013)
1109
Synthesis, Physicochemical Properties, and in vitro Antibacterial Screening of
Palladium(II) Complexes Derived from Thiosemicarbazone
by Mohammad Azam*a), Ismail Warada), Saud I. Al-Resayesa), M. Rafiq Siddiquia), and M. Ovesb)
a) Department of Chemistry, King Saud University, P.O. Box 2455, Riyadh 11451, KSA
(phone: þ966-596441517; e-mail: azam_res@yahoo.com)
b) Department of Agricultural Microbiology, Faculty of Agricultural Sciences, Aligarh Muslim
University, Aligarh 202002, India
A new series of PdII complexes derived from thiosemicarbazone has been synthesized. The
synthesized PdII complexes have been characterized on the basis of elemental analyses, FT-IR, 1H- and
13C-NMR, UV/VIS, and thermal studies. A square-planar geometry has been assigned around PdII ions
on the basis of results obtained from UV/VIS studies. The thiosemicarbazone ligand and its PdII
complexes have been screened against Gram-positive (Bacillus subtilis and Staphylococcus aureus)
and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria in vitro as growth-inhibiting
agents, and the results revealed significant antibacterial activities.
Introduction. – Thiosemicarbarbazones (TSCs) are very promising molecules in
coordination chemistry because of their pharmacological features, which include
notably their antibacterial, antiviral, antimalarial, antileprotic, and anticancer activities
[1–3]. Among TSCs, heterocyclic TSCs have received considerable attention due to
their potential biological activities [4]. TSCs, being well-known chelating ligands,
coordinate to the metal ion through S- and one of the hydrazine N-atoms (N(2) or
N(1)). Coordination through N(2) results in an unusual four-membered chelating ring,
while that through the hydrazine N(1)-atom leads to a more stable five-membered
chelate [4][5]. TSCs possess the ability to adopt various coordination modes, leading to
a rich structural diversity of these complexes [6–8]. It has been reported that the
biological activity of TSCs is due to their ability to form tetradentate chelates with
essential heavy metal ions bonded to S- and two N-atoms. Structural alterations that
hinder a TSC to act as a chelating agent with metal ions tend to destroy or reduce its
medicinal activity [1][9]. In view of the wide spectrum of biological applications of
TSCs, including activity against diseases such as TB, leprosy, malaria, as well as a range
of bacterial infections, the chemistry of TSCs and their complexes has attracted prolific
attention over the past decade [10][11]. It has been reported that several metal
complexes of TSCs, particularly with Cu, Pt, Pd, Re, and Ru, exhibited marked and
diverse biological activities [12][13]. Among them, PdII and PtII complexes with TSC
are active against cisplatin-resistant human tumor cell lines, probably because their
mode of action involves interstrand cross-links with DNA instead of intrastrand cross-
links, which is the major coordination mode of cisplatin [14]. Moreover, PdII complexes
with N-containing ligands have been the subject of intensive biological evaluation in
the search for less toxic and more selective anticancer therapies [15][16]. However,
ꢀ 2013 Verlag Helvetica Chimica Acta AG, Zꢁrich

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PdII N,S-chelates with inert ligands (e.g., S or N) were suggested to be more effective
antitumor agents than those of other metals because of their proper lability to bring the
metal to the target (DNA) and allow their interaction. In this respect, PtII chelates are
kinetically inert, while those of other metals such as NiII, ZnII, and CuII do not have
sufficient thermodynamic stability [17]. The N,S-donor ligands used to prepare
antitumor and antimicrobial PdII complexes were mostly TSCs and dithiocarbazates,
and possess antiviral, antimalarial, antifungal, antimicrobial, and antitumor activities,
and their mechanisms of action, most probably involve the inhibition of ribonucleotide
reductase, converting ribonucleotides to deoxyribonucleotides [18][19]. Herein, we
report the synthesis, structural characterization of PdII complexes derived from
thiosemicarbazone, followed by their in vitro antibacterial screening against Gram-
positive (Bacillus subtilis and Staphylococcus aureus) and Gram-negative (Escherichia
coli and Pseudomonas aeroginosa) bacteria.
Result and Discussion. – Synthesis. Targeted PdII complexes were synthesized by the
reaction of TSC ligand with [PdCl2(PhCN)2] and [Pd(OAc)2] in 1: 1 molar ratio
(Scheme). The formation of PdII complexes were ascertained by elemental analyses,
1H- and 13C-NMR, FT-IR, and thermal studies. The complexes so formed are soluble in
common organic solvents. The overall geometry around PdII ions has been assigned on
the basis of bands observed in electronic spectra.
Scheme. Synthesis of Ligand (L), and Its PdII Complexes 1 and 2
IR-Spectral Investigations. The IR spectrum of the free ligand showed a strong
absorption band at 1585 cmꢀ1 assigned to n(C¼N) which was shifted to lower wave
number of ca. 1475 cmꢀ1 in the spectra of complexes, indicating the coordination of
azomethine N-atom to Pd ion (Fig. 1) [20][21]. The moderate strong band at 1250 cmꢀ1
assigned to n(C¼S) in the free ligand disappeared completely and, instead, a new band
appeared at 785 cmꢀ1 attributed to the enolization of NHꢀC¼S group [22][23]. The
missing of SꢀH band between 2600–2800 cmꢀ1 indicated that the ligand remained in its
thione form [24]. The spectrum of ligand showed three bands at 3190, 3395, and
3310 cmꢀ1 assignable to asymmetric and symmetric vibration of NꢀH, and to nNH2,
respectively [25]. The n(NꢀH) band of free ligand disappeared completely in the
spectrum of the PdII complex indicating the deprotanation of the NH group and
coordination via the thiolate S-atom. On the other hand, bands attributed to
asymmetric and symmetric vibrations of NH2 group underwent very small changes,
indicating no interaction between PdII ion and terimal NH2 group [26][27]. The bands
assigned to n(PdꢀS) and n(PdꢀCl) appeared at 400 and 340 cmꢀ1, respectively [28].
The medium intensity band observed at 850 cmꢀ1 in the spectrum of the free ligand

CHEMISTRY & BIODIVERSITY – Vol. 10 (2013)
1111
assigned to n(CꢀSꢀC) stretching vibration of thiophene moiety was shifted to ca.
825 cmꢀ1 in the spectrum of PdII complexes, further evidencing the involvement of the
S-atom in bonding with PdII ion [29]. The bands at 1595 and 1380 cmꢀ1 observed in
complex 2, attributed to antisymmetric and symmetric stretching vibrations of the
COO group, indicated the monodentate nature of the COO moiety. This was further
confirmed by frequency separation between these two modes (nas(CO2) – ns(CO2)). In
general, the difference between asymmetric (nas(CO2)) and symmetric (ns(CO2))
absorption frequency below 200 cmꢀ1 evidences a bidentate COO moiety, while that
greater than 200 cmꢀ1 implies the unidentate COO moiety [30].
Fig. 1. IR Spectra of ligand and its complex 1
NMR Studies. The 1H- and 13C-NMR spectra of ligand and its PdII complexes have
1
been recorded in (D6)DMSO and CDCl3, respectively. The H-NMR spectrum of the
ligand showed a broad singlet at 11.47 ppm attributed to hydrazine H-atom which
disappeared in the spectra PdII complexes, indicating the deprotonation of ligand and
subsequently the replacement of the H-atom by PdII ion, inducing the shifts of NH2 H-
atoms to lower field in the spectra of PdII complexes. The signal for azomethine H-atom
of the free ligand appeared at 8.24 ppm, while the multiplets for thiophene H-atoms
were detected in the region of 7.10–7.12 ppm. It is interesting to note the presence of
two singlets for NH2 H-atoms at 7.65 and 7.7 ppm, respectively, in the spectrum of the
free ligand, indicating that the free rotation around C¼N bond was hindered because of
its partial C¼C bond character [31][32]. The coordinating mode of ligand was
1
confirmed by comparing H-NMR data of the ligand with those of PdII complex
(Fig. 2). A significant downfield shift of the azomethine H-atom signal in the spectra of

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CHEMISTRY & BIODIVERSITY – Vol. 10 (2013)
PdII complexes [33] with respect to the corresponding free ligand confirmed the
coordination of azomethine N-atom to PdII ion, while slight deshielding in the H-atoms
of thiophene moiety further confirmed the coordination of ligand to PdII ion (Figs. 2
and 3). [34]. An additional signal at 2.08 ppm assigned to AcO (acetate) H-atom was
observed in the spectrum of complex 2 (Fig. 3). The 13C-NMR spectrum of ligand
showed a sharp signal at 177.99 ppm corresponding to C¼S group which underwent
deshielding in PdII complexes, indicating the thiolate-like coordination rather than
thione [35]. Various signals attributed to azomethine and thiophene C-atom appeared
at 165.96, 153.50, 148.61, 139.70, and 127.98, respectively, which were deshielded in PdII
complexes, confirming the coordination of PdII ion with ligand (Figs. 4 and 5) [34]. Two
additional signals appearing at 178.78 and 25.50 ppm corresponding to C¼O and Me C-
atom, respectively, were observed in the spectrum of complex 2 (Fig. 5).
Fig. 2. 1H-NMR Spectra of ligand and complex 1
Electronic Spectra. The electronic spectra in the ultraviolet and visible ranges (UV/
VIS) of the PdII complexes were recorded in 0.25ꢁ10ꢀ3 m solution of CH2Cl2 (Figs. 6
and 7), and all spectra indicated square-planar geometry. The spectra exhibited
prominent bands at ca. lmax 260 (e 10.80 mꢀ1 cmꢀ1) and ca. lmax 350 nm (e 8.0 mꢀ1 cmꢀ1),
accompanied by a weak shoulder attributed to n-p* transitions, and they were
associated with azomethine functions of the TSC moiety [36–38].
Thermal Studies. Thermal stabilities of complexes 1 and 2 were examined by
thermogravimetry (TG) in N2 atmosphere at a heating rate of 208 minꢀ1 in the
temperature range of 20–8008. The TG curve showed three steps of weight loss. The
first stage of the complexes 1 and 2 at 1258 involved the loss of hydrated H2O molecules,
followed by the second step which included loss of chloride and acetate ions, and
coordinated H2O up to 2958. This step was followed by a third step at which the whole

CHEMISTRY & BIODIVERSITY – Vol. 10 (2013)
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Fig. 3. 1H-NMR Spectrum of complex 2
Fig. 4. 13C-NMR Spectrum of ligand
Fig. 5. 13C-NMR Spectrum of complex 2

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CHEMISTRY & BIODIVERSITY – Vol. 10 (2013)
Fig. 6. Electronic spectrum of complex 1 in 0.25ꢁ10ꢀ3 m solution of CH2Cl2
Fig. 7. Electronic spectrum of complex 2 in 0.25ꢁ10ꢀ3 m solution of CH2Cl2
organic moiety decomposed at temperature up to 6708, and, finally, metal oxide
formed.
Crystallography of the Ligand. Molecular structure of ligand together with atom
numbering is shown in an ORTEP diagram (Fig. 8). Selected bond lengths are listed in

CHEMISTRY & BIODIVERSITY – Vol. 10 (2013)
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Table 1. The ORTEP diagram shows two molecules which are independent of each
other. There are two independent molecules and plenty of intermolecular H-bonds. In
the crystal structure of ligand, only the (E)-isomer of imine bond is observed. It is
known that the TSC group can present a thione>thiol tautomerism. The C(12)ꢀS(3)
and C(6)ꢀS(4) bond distances are similar to those found in other TSCs which evidence
the thione form in the solid state [39], indicating that the ligand is in that form. The
C(11)ꢀN(3) and C(5)ꢀN(1) bond distances are in accordance with N¼C bond
character. The difference in the C(1)ꢀC(2) compared to the C(3)ꢀC(4) bond also
reflects the double-bond character of the C(1)ꢀC(5) bond.
Fig. 8. ORTEP Diagram showing the molecular structure of the ligand
Table 1. Selected Bond Distances and Angles of the Ligand (L)
Bond
Distance [ꢂ]
Bonds
Angle [8]
S(3)ꢀC(12)
S(4)ꢀC(6)
S(1)ꢀC(1)
S(1)ꢀC(4)
S(2)ꢀC(7)
S(2)ꢀC(10)
C(11)ꢀN(3)
C(11)ꢀC(10)
1.695(2)
1.703(2)
1.718(2)
1.729(2)
1.718(2)
1.728(2)
1.278(3)
1.447(3)
N(4)ꢀC(12)ꢀS(3)
N(1)ꢀC(5)ꢀC(4)
C(11)ꢀN(3)ꢀN(4)
C(5)ꢀN(1)ꢀN(2)
119.33(17)
120.1(2)
114.76(19)
115.37(19)
Antibacterial Activity. The antibacterial activities of all the compounds were
determined against both Gram-positive (Bacillus subtilis and Staphylococcus aureus)
and Gram-negative (Escherichia coli and Pseudomonas aeroginosa) bacteria (Table 2).
Ligand (L), and its complexes 1 and 2 exhibited significant activities against Gram-
positive strains compared to Gram-negative strains. Gram-positive bacterial stain
showed highest sensitivity against L, with observed maximum zones of inhibition of 25
and 18 mm on overnight-incubated plates of B. subtilis and S. aureus, respectively.
Maximum growth inhibitions of Gram-negative bacterial stains E. coli and P.

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CHEMISTRY & BIODIVERSITY – Vol. 10 (2013)
aeruginosa were detected at 22 and 14 mm on nutrient agar palates after overnight
incubation under optimum conditions. Antibacterial activities of synthesized com-
pounds (L>1>2) were significant compared to the positive control antibiotics
tetracycline.
Table 2. Antibacterial Activities of Synthesized Compounds at 100 mg/ml Concentration
Compound
Zone of inhibition [mm]a)
B. subtilis
S. aureus
E. coli
P. aeroginosa
L
1
2
25ꢂ1.05
16ꢂ0.95
18ꢂ1.25
18ꢂ1.10
18ꢂ1.23
14ꢂ1.48
25ꢂ1.54
22ꢂ1.33
22ꢂ1.10
19ꢂ0.90
15ꢂ0.75
20ꢂ1.50
14ꢂ1.84
15ꢂ1.36
11ꢂ1.02
15ꢂ0.75
Tetracycline
a) Mean zone of inhibition (millimeterꢂstandard deviation).
Conclusions. – The synthesis and spectral characterization of PdII complexes
derived from thiosemicarbazone were described. The in vitro antibacterial studies were
performed on ligand and its PdII complexes, revealing significant activities.
This work is supported by the Research Center, College of Science, King Saud University, Riyadh,
KSA.
Experimental Part
General. All the reagents used were of AR (anal. reagent) grade and were purchased from Merck
and used as received. Thermal analyses were carried out with a SDT-Q 600 instrument in a He atm.
Electronic spectra of PdII complexes in CH2Cl2 were recorded on Pharmacia-LKB Biochem 4060 UV/
VIS spectrophotometer at r.t. FT-IR (4000–400 cmꢀ1) Spectra were recorded as KBr discs on a Perkin-
1
Elmer 621 spectrophotometer; ˜n in cmꢀ1. H- and 13C-NMR spectra for ligand and its PdII complexes
were recorded in (D6)DMSO and CDCl3, resp., using Bruker Avance II 400 MHz and JEOL 400 MHz
NMR spectrometer, resp.; d in ppm, J in Hz. Elemental analyses were conducted on a Elementar Vario
EL analyzer.
Synthesis of Thiosemicarbazone Ligand, L. A MeOH soln. of thiophene-2-carboxaldehyde (1 ml)
was added dropwise to the MeOH soln. of thiosemicarbazide (1 ml). The mixture was refluxed for 2 h
resulting in a clear yellow-colored soln. The soln. was kept for evaporation at r.t. After 4 d, yellow crystals
suitable for X-ray diffraction were obtained.
(2E)-2-(Thiophen-2-ylmethylidene)hydrazinecarbothioamide (L). Yield: 85%. M.p. 1378. IR (KBr):
3395, 3310, 3190, 1585, 785. 1H-NMR ((D6)DMSO): 11.47 (s, NH); 8.24 (s, CH¼N); 7.65, 7.7 (2s, NH2).
13C-NMR ((D6)DMSO): 177.99 (C¼S); 165.96 (CH¼N). Anal. calc. for C6H7N3S2 (185.27): C 38.89, H
3.80, N 22.68, S 34.61; found: C 38.65, H 3.72, N 22.50, S 34.56.
Synthesis of Complexes, [PdLCl] and [PdL(OAc)], 1 and 2, Resp. A soln. of PdII salt (1 ml)
dissolved in 15 ml of CH2Cl2 was added in 10 ml of CH2Cl2 soln. of L (1 ml). The resulting mixture was
stirred for 0.5 h resulting in a colored soln. The soln. was concentrated to 1 ml, followed by addition of
10 ml of hexane to cause precipitatation. The resulting colored precipitate was collected and
recrystallized in CH2Cl2/hexane to give the complex in anal. pure form.
Chloro{2-[(thiophen-2-yl-kS)methylidene]hydrazinecarbothioamide-k2N2,S}palladium(1þ) Chlor-
1
ide (1). Yield: 65%. M.p. 2458. IR (KBr): 3397, 3314, 1475. H-NMR (CDCl3): 8.90 (s, CH¼N); 7.82,
7.9 (2s, NH2); 7.5–7.77 (m, thiophene). 13C-NMR (CDCl3): 177.97 (PdꢀSꢀC); 170.58 (PdꢀN¼CH). Anal.

CHEMISTRY & BIODIVERSITY – Vol. 10 (2013)
1117
calc. for C6H7Cl2N3PdS2 (362.59): C 19.87, H 1.95, Cl 19.55, N 11.59, S 17.68, Pd 29.35; found: C 19.79, H
1.83, Cl 19.49, N 11.52, S 17.60, Pd 29.23.
(Acetato-kO){2-[(thiophen-2-yl-kS)methylidene]hydrazinecarbothioamide-k2N2,S}palladium(1þ)
Acetate (2). Yield: 70%. M.p. 2378. IR (KBr): 3392, 3320, 1470. 1H-NMR ((D6)DMSO): 9.92 (s, CH¼N);
8.00, 7.61 (2s, NH2); 7.24–7.65 (m, thiophene); 2.08 (s, AcO). 13C-NMR ((D6)DMSO): 178.78
(PdꢀOCO); 177.94 (PdꢀSꢀC); 173.81 (PdꢀN¼CH). Anal. calc. for C10H13N3O4PdS2 (409.78): C 29.31,
H 3.19, N 10.25, O 15.61, S 15.65, Pd 25.97; found: C 29.25, H 3.15, N 10.18, O 15.52, S 15.58, Pd 25.85.
Crystallographic Data Collection and Refinement for the Ligand1). A yellow crystal suitable for X-
ray diffraction was obtained by slow evaporation of MeOH. Single-crystal data were collected using
graphite-monochromated MoKa radiation (l 0.71073 ꢂ) on a Bruker SMART APEX CCD diffrac-
tometer at 293 K. The data integration and reduction were processed with SAINT program. The
structure was solved by direct methods using SIR 97 and refined on F2 by the full-matrix least-square
technique by using the program contained in SHELXL-97 packages [40][41]. Parameters associated with
unit cell dimensions, intensity data collection, and refinement for the crystal are compiled in Table 3.
Table 3. Crystallographic Data of 2-(thiophen-2-ylmethylidene)hydrazine-1-carbothioamide (L)
Empirical formula
Formula weight
Temp. [K]
C24H28N12S8
741.06
293(2)
Wavelength [ꢂ]
Crystal system
Space group
Unit cell dimensions:
a [ꢂ]
0.71073
Monoclinic
P2(1)/n
13.411(4)
b [ꢂ]
5.7754(16)
c [ꢂ]
21.300(6)
a [8]
90
b [8]
96.311(4)
g [8]
90
V [ꢂ3]
1639.7(8)
Z
2
Dx(calc.) [Mg/m3]
Absorption coefficient [mmꢀ1
F(000)
1.501
0.584
768
]
Crystal size [mm]
q Range for data collection [8]
Index ranges
0.24ꢁ0.22ꢁ0.18
1.89–26.50
ꢀ16ꢃhꢃ16, ꢀ2ꢃkꢃ7, ꢀ26ꢃlꢃ26
Reflections collected
8775
Independent reflections [R(int.)¼0.0319]
Completeness to q¼25.008
Absorption correction
3364
99.2%
None
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
0.9022; 0.8726
Full-matrix least-squares on F2
3364/1/231
1.040
Final R indices [I>2s(I)]
R Indices (all data)
Largest diff. peak and hole [e ꢂꢀ3
R1 ¼0.0375, wR2 ¼0.0874
R1 ¼0.0446, wR2 ¼0.0944
0.340; ꢀ0.368
]
1
)
CCDC-779194 contains the supplementary crystallographic data for this article. These data can be
obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.

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CHEMISTRY & BIODIVERSITY – Vol. 10 (2013)
Antimicrobial Assay of PdII Complexes. The antimicrobial-activity assays for the ligand (L) and its
PdII complexes were performed by modified KirbyꢀBauer (1957) agar well diffusion method [42]. Fresh
overnight-grown cultures of test organisms (106 CFU/ml) were used to evaluate the antibacterial
activities. Test bacterial culture (100 ml) was spread over the nutrient agar palates by sterilized glass
spreader in sterilized condition. Plates were left standing for 5 min to let the culture get absorbed. The 8-
mm-size wells were prepared in nutrient agar plates with help of sterile micropipette tip back point. Wells
were sealed with one drop of molten agar (0.8% agar) to prevent leakage from the bottom of the plate.
The wells were loaded with 100 ml of test compounds (30 mg/100 ml). Solvent blank (DMSO) was used as
negative control. Antibiotic tetracycline (30 mg/disc) was used as a positive control. Plates were
incubated at 35ꢂ28 for 36 h. The antibacterial activity was determined by measuring the zone of
bacterial growth inhibition.
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Received April 9, 2012