Synthesis and structural characterization of Pd(II) complexes derived from perimidine ligand and their in vitro antimicrobial studies

Article abstract of DOI:10.1016/j.molstruc.2013.04.064

A novel series of Pd(II) complexes derived from 2-thiophenecarboxaldehyde and 1,8-diaminonaphthalene has been synthesized and characterized by various physico-chemical and spectroscopic techniques viz., elemental analyses, IR, UV-vis, 1H and 13C NMR spectroscopy, and ESI-mass spectrometry. The structure of ligand, 2-(2-thienyl)-2,3-dihydro-1H-perimidine has been ascertained on the basis of single crystal X-ray diffraction. All Pd(II) complexes together with the corresponding ligand have been evaluated for their ability to suppress the in vitro growth of microbes,Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Citrobacter sp., Bacillus subtilis and Stenotrophomonas acidaminiphila and results show that Pd(II) complexes have more significant antimicrobial activity than their corresponding ligand. Fluorescence spectroscopic measurements clearly support that both of the Pd(II) complexes show significant DNA binding with calf thymus DNA.

Full text of DOI:10.1016/j.molstruc.2013.04.064

Journal of Molecular Structure 1047 (2013) 48–54
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and structural characterization of Pd(II) complexes derived
from perimidine ligand and their in vitro antimicrobial studies
a
a
a
a
Mohammad Azam a, , Ismail Warad , Saud I. Al-Resayes , Nabil Alzaqri , Mohammad Rizwan Khan ,
⇑
Raghavaiah Pallepogu b, Sourabh Dwivedi c, Javed Musarrat d, Mohammad Shakir e
a Department of Chemistry, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
b School of Chemistry, University of Hyderabad, Hyderabad 500046, India
c Department of Zoology, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
d Department of Ag. Microbiology, Faculty of Agricultural Sciences, Aligarh Muslim University, Aligarh 202002, India
e Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
Synthesis and characterization of perimidine ligand.
Structure of ligand ascertained on the basis of X-ray crystallography.
Complexes characterized by various spectroscopic studies.
In vitro anti microbial screening showed Pd(II) complexes to be more effective inhibitor of microbial growth.
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 30 April 2013
A novel series of Pd(II) complexes derived from 2-thiophenecarboxaldehyde and 1,8-diaminonaphthalene
has been synthesized and characterized by various physico-chemical and spectroscopic techniques viz.,
elemental analyses, IR, UV–vis, 1H and 13C NMR spectroscopy, and ESI-mass spectrometry. The structure
of ligand, 2-(2-thienyl)-2,3-dihydro-1H-perimidine has been ascertained on the basis of single crystal
X-ray diffraction. All Pd(II) complexes together with the corresponding ligand have been evaluated for
their ability to suppress the in vitro growth of microbes,Escherichia coli, Staphylococcus aureus, Pseudomo-
nas aeruginosa, Citrobacter sp., Bacillus subtilis and Stenotrophomonas acidaminiphila and results show that
Pd(II) complexes have more significant antimicrobial activity than their corresponding ligand. Fluores-
cence spectroscopic measurements clearly support that both of the Pd(II) complexes show significant
DNA binding with calf thymus DNA.
Keywords:
Heterocyclic compound
X-ray structure of perimidine ligand
Pd(II) complexes
Antimicrobial activity
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
an anti-tumor, antiulcer, anti-malarial, and anti-fungal agents.
Their biological activity is probably because of the presence of a
In recent years, the interest in coordination chemistry has been
increasing continuously with the synthesis and characterization of
a large number of transition metal complexes with heterocyclic
systems (cyclic and acyclic) containing nitrogen, oxygen, and sul-
phur donor atoms because of their wide applications in various
fields viz., pharmaceutical drugs, biological activities, efficient
plant protection, analyzing reagents for trace and ultra trace heavy
metal determination and pre-concentration in aqueous media, etc.
[1–4]. Among heterocycles, perimidine and its derivatives have re-
perimidene ring system in nucleic acids, vitamins, co-enzymes
and antibiotics [5–8]. The development of antimicrobial drugs to
treat infectious diseases caused by the pathogens has been one
of the most notable achievements of the past century. Despite
the major breakthrough in modern medicine during the past 10
decades, the successful treatment towards multidrug-resistant
pathogens (i.e., microbial isolates such as fungi and bacteria) has
become a serious problem and remains a significant challenge over
the last 10 years. Therefore, In spite of a large number of antibiotics
and chemotherapeutics available for medical use, the antimicrobial
resistance has created a substantial need for the development of
new classes of antimicrobials that may not be as susceptible to
the bacterial mechanism of resistance developed against the
current range of drugs [9–12]. In continuation of our ongoing
interest in chemistry of perimidine and its complexes [13], we,
ceived considerable attention as they have high
p stacking ability,
electron affinity, and reduction potential, and also exhibit diverse
range of biological activities for example their potential to act as
⇑
Corresponding author. Tel.: +966 596441517.
E-mail address: azam_res@yahoo.com (M. Azam).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.04.064

M. Azam et al. / Journal of Molecular Structure 1047 (2013) 48–54
49
herein, report the synthesis, characterization and crystal structure
2.75; N, 6.47; S, 7.39; Cl, 16.43; Pd, 24.69, IR, 3310 cmꢁ1
822 cmꢁ1 C–S–C), 340 cmꢁ1
(tPd–Cl), UV–vis (nm), 348, 293, 251.
(t
NH),
of 2-(2-thienyl)-2,3-dihydro-1H-perimidine derived from 1,8-dia-
minonaphthalene and 2-thiophenecarboxaldehyde and its Pd(II)
complexes followed by their in vitro antimicrobial and DNA inter-
active studies. Although various 2-substituted 2,3-dihydro-1
H-perimidines have been reported in literature [14]. No one, to
the best of our knowledge, so far has reported the crystal structure
of ligand, 2-(2-thienyl) 2,3-dihydro-1H-perimidine. Therefore, to
add the novelty in the work, we have successfully isolated the crys-
tal of ligand, 2-(2-thienyl)-2,3-dihydro-1H-perimidine and its
Pd(II) complexes followed by their in vitro antimicrobial activity.
(t
2: Yield: 58%, Color: Orange, Mp. 242 °C, Molecular formula
C19H18N2SO4Pd; 1H NMR (CDCl3): d (ppm) 3.65 (NH), 5.69 (CH),
7.11–7.58 (m, Ar–H), 7.60–7.72 (m thiophene protons), 13C NMR
(CDCl3): d (ppm) 62.85 (–CH), 144.11 (N–CH), 52.55 (–OCH3) Anal
Cal; C, 47.85; H, 3.80; N, 5.88; O, 13.42; S,6.72, Pd, 22.31, Found,
C, 47.81; H, 3.75; N, 5.81; O, 13.38; S, 6.67; Pd, 22.25, IR,
3315 cmꢁ1 NH), 828 cmꢁ1
(t (tC–S–C), UV–vis (nm), 345, 291, 250.
2.4. Antimicrobial activity assay of ligand, L and its Pd(II) complexes
The antimicrobial effect of ligand and its complexes was as-
sessed against a variety of microbes including Escherichia coli,
Staphylococcus aureus, Pseudomonas aeruginosa, Citrobacter sp.,
Bacillus subtilis and Stenotrophomonas acidaminiphila employing
agar diffusion technique as reported in literature [15]. In brief,
freshly grown bacterial cultures (1.0 ꢂ 108 CFU mlꢁ1) were plated
on nutrient agar plates. Wells (3 mm diameter) were cut in the
2. Experimental
2.1. Materials and methods
All the reagents used were of analAR grade and were purchased
from Merck and used as received.
Elemental analyses were recorded on a Elementar Varrio EL
analyzer. 1H and 13C NMR spectra of ligand and its Pd(II) complexes
were recorded in CDCl3 using JEOL 400 spectrometer. FT-IR (4000–
400 cmꢁ1) spectra were recorded as KBr pellet on a Perkin Elmer
621 spectrophotometer. Mass spectrometry was performed with
a Micromass Quattro Premier tandem MS fitted with an ESI inter-
face and controlled by MassLynx 4.1 software. MS/MS detection
was performed with electrospray positive ionization mode. Elec-
tronic spectra of the complexes were obtained in dichloromethane
on Pharmacia LKB-Biochem, UV–vis spectrophotometer at room
temperature. Fluorescence measurements were performed on a
spectrofluorimeter Model RF-540 (Shimadzu, Japan) equipped with
a 150 W Xenon lamp and a slit width of 5 nm.
agar plate and 50 ll of compound (100 l
g wellꢁ1) was dispensed
in the centre of each well. DMSO (5%) was used as a control. The
plates were incubated at 37 °C for 24 h and the zones of inhibition
were observed and documented. For quantitative estimation of the
antimicrobial activity, the cells from the Gram negative (E. coli AB
1157) and Gram positive (B. subtilis) bacterial strains were har-
vested in the exponential growth phase and the pellets were sus-
pended in Tris–Cl buffer, pH 7.4. The cells were treated with
increasing concentrations (0–100 l
g mlꢁ1) of ligand and its Pd(II)
complexes, and incubated at 37 °C for 6 h. Subsequently, the cells
were centrifuged and pellet were suspended in Tris–Cl buffer, pH
7.4. The treated cells along with untreated control were serially di-
luted and plated on nutrient agar plates to assay the colony form-
ing ability. Plates were incubated overnight at 37 °C and the
colonies were counted. The percentage survival was determined
and plotted as a function of concentration. Minimum inhibitory
concentrations (MICs) were determined by broth dilution tech-
nique. The nutrient broth containing test compounds and controls
were inoculated within approximately 1.0 ꢂ 108 CFU mlꢁ1 of ac-
tively dividing bacteria cells. The cultures were incubated for
24 h at 37 °C and the growth was monitored visually and
spectrophotometrically.
2.2. Synthesis of 2-(2-thienyl)2,3-dihydro-1H-perimidine, L
A methanolic solution of 2-thiophenecarboxaldehyde (1 mmol)
was added dropwise to the methanolic solution of 1,8-diamino-
naphthalene (1 mmol). The reaction mixture was refluxed for 2 h
resulting into a clear brown colored solution. The resulting colored
solution was concentrated to 1 ml followed by addition of 10 ml
hexane to cause precipitation. Precipitate was isolated and recrys-
tallised in dichloromethane-n-hexane mixture. After few days, yel-
low crystals suitable for X-ray diffraction appeared.
2.5. Single crystal X-ray diffraction analysis
Yield 65%, Color: Yellow, Mp: 138 °C; Molecular formula
C15H12N2S; 1H NMR (CDCl3): d (ppm) 3.35 (NH), 5.70 (–CH),
6.48–7.00 (m, Ar–H), 7.13–7.27 (m thiophene protons), 13C NMR
(CDCl3): d (ppm) 61.58 (–CH), 142.73 (N–CH) Anal. Cal: C, 71.4;
H, 4.79; N, 11.1; S, 12.7% Found: C, 71.35; H, 4.72; N, 11.05; S,
Single crystal X-ray data were collected on a Bruker SMART
APEX CCD area detector system [k(Mo K
a) = 0.71073 Å] at 291 K,
graphite monochromator with a scan width of 0.3°, crystal-
x
detector distance 60 mm, collimator 0.5 mm. The SMART software
was used for the intensity data acquisition and the SAINTPLUS
Software was used for the data extraction [16]. Absorption correc-
tion and an empirical absorption correction using equivalent
reflections were performed with the help of SADABS program
[17]. The structure solution and full-matrix least-squares
refinement against F2 were carried out using SHELXL-97 [18]. All
non-hydrogen atoms were refined anisotropically.
12.63% IR, 3320 cmꢁ1 NH), 840 cmꢁ1
(t (tC–S–C).
2.3. Synthesis of complexes, [PdLCl2] 1 and [PdL(OCOCH3)2] 2
A solution of Pd(II) salt (0.50 mmol) dissolved in 15 ml dichloro-
methane was added dropwise into 10 ml dichloromethane solution
of ligand (0.50 mmol). The resultant reaction mixture was stirred
for half an hour resulting into a clear yellow colored solution.
The resulting solution was concentrated to 1 ml followed by addi-
tion of 10 ml of hexane to cause precipitation. The resulting yellow
colored precipitate was isolated and recrystallised in dichloro-
methane-n-hexane and obtained in analytically pure form. No
crystal was found suitable for single crystal XRD.
1: Yield: 62%, Color: Yellow, Mp: 235 °C, Molecular formula
C15H12N2SPdCl2; 1H NMR (CDCl3): d (ppm) 4.58 (NH), 5.75
(CH), 6.51–7.21 (m, Ar–H), 7.32–7.75 (m thiophene protons), 13C
NMR (CDCl3): d (ppm) 63.81, 144.13 (N–CH) Anal: Cal, C, 41.93;
H, 2.81; N, 6.52; S, 7.46; Cl, 16.50; Pd, 24.76, Found, C, 41.85; H,
2.6. DNA binding assays of ligand, L and complexes 1 and 2
DNA binding experiments were carried out using fluorescence
spectrofluorometry. The purity of calf thymus DNA was confirmed
by taking the ratio of the absorbance values at 260 and 280 nm in
tris–EDTA (10 mM, pH 7.0) buffer, which was found to be 1.8:1,
indicating that the DNA was sufficiently free of protein and other
contaminants. For measurements, free ligand, L and its complexes
1 and 2 were dissolved in 2% DMSO. DNA binding assays of synthe-
sized compounds were carried out in the presence of ethidium

50
M. Azam et al. / Journal of Molecular Structure 1047 (2013) 48–54
bromide. DNA and ethidium bromide (EtBr) were dissolved in tris–
EDTA (10 mM, pH 7.0) buffer at concentrations of 4 and 1
g mlꢁ1
respectively. The concentration of ligand, L and complexes 1 and 2
were 50 M. To perform the experiments, DNA was pretreated
moiety may undergo metallation with formation of carbon–metal
bond by using transition metal complexes with two electron donor
atoms. Cyclometallated complexes constitute an important tool to
l
,
l
activate the C–H r bond stabilized by the additional coordination
with EtBr for 30 min. Then the test solutions were added to this
mixture of EtBr–DNA, and the change in the fluorescence intensity
was measured. The fluorescence was recorded at 485–685 nm after
exciting the solution at 478 nm. The slits were set at 5 nm for exci-
tation and emission. The path length of the sample was 1 cm. To
determine the DNA-binding ability of free ligand, L and complexes
1 and 2, fluorescence intensity data were analyzed by the Stern–
Volmer equation [19]:
of metal to a heteroatom forming transition metal complexes [20–
23]. C–H functionalization has the potential to become a paradigm
shifting for organic synthesis and over the last decade, a large vari-
ety of new C–H functionalization methodologies have been devel-
oped [20–23]. The selective functionalization of sp3 C–H is
challenging because C–H bonds are unreactive and robust. Their
low reactivity can be attributed to the fact that they are strong,
localized, and unpolarized bonds. Transition metal catalysis can
be used to assist in correcting C–H bond to more reactive car-
bon–metal bonds that can subsequently be functionalized to afford
the desired product. In most cases, the intramolecualr activation of
the C–H bond comes about with N-containing ligands, the metal
quite often being palladium. The cyclopallaldated compounds
formed thus way quite often display a characteristic ability as
starting material in organic synthesis and have given way to wide
slate of reaction which proceed with notable regio- and stereose-
lectivity, leading to extensive possibilities for synthetic applica-
tions [20–23]. The synthesized ligand, L and its Pd(II) complexes
were tested in vito against a number of microbes and results
showed complexes to be more effective as compared to their cor-
responding ligand, L. DNA interactive studies have also been car-
ried out with CT-DNA and results suggest that Pd(II) complexes
have significant DNA binding as compared to their corresponding
ligand, L.
F0=F ¼ 1 þ Ksv½Qꢃ
ð1Þ
where F and F0 are the fluorescence intensity with and without the
quencher (complex–DNA), Ksv is the Stern–Volmer quenching con-
stant, and [Q] is the concentration of the quencher.
3. Result and discussion
The perimidene ligand, L has been synthesized by the reaction
of 2-thiophenecarboxaldehyde and 1,8-diaminonaphthalene in
1:1 M ratio in methanolic medium (Scheme 1). Its mononuclear
Pd(II) complexes were synthesized by reaction of ligand, L and
Pd(II) salts in 1:1 M ratio in dichloromethane (Scheme 1). The ana-
lytical data agree well with the proposed composition of ligand and
its complexes. The formation of ligand, L and its Pd(II) complexes
was confirmed on the basis of results of elemental analyses, molec-
ular ion peak in mass spectra, characteristic bands in FT-IR, and
resonance signals in the 1H and 13C NMR spectra, and single crystal
XRD in case of ligand. The geometry around Pd(II) ions in the com-
plexes was confirmed from the positions of absorption bands ob-
served in UV–vis spectra. Complexes so formed are consistently
unstable in solution which can be explained by a coordination in-
duced of the NH bond. This proton is easily removed in solution
resulting in dissociation of the ligand from the metal. Over the last
few decades, the Chemistry of organo palladium complexes has
developed significantly. Literature has also revealed that organic
3.1. IR Investigations
The IR frequencies associated with the main functional group of
the complexes are listed in the experimental section (Fig. 1S). The
results obtained are in accordance with the proposed structures of
the ligand and its Pd(II) complexes [14,24]. The IR spectrum of li-
gand exhibits a strong band at 3320 cmꢁ1 assigned to –NH group
which undergoes a negative shift ca. 10 cmꢁ1 in Pd(II) complexes
indicating the coordination of ligand to Pd(II) ion [25]. The medium
S
O
Methanol
HN
NH
S
NH2 NH2
1,8-diaminonaphthalene
2-thiophenecarboxaldehyde
2-(2-thienyl)2,3-dihydro-1H-perimidinene, L
PdCl2(PhCN)2
CH2Cl2
OR
Pd(OAC)2
S
X
Pd
X
NH
NH
[PdLX2]
X = Cl, OAc
Scheme 1. Synthetic route of the preparation of ligand, 2-(2-thienyl)2,3-dihydro-1H-perimidine and its Pd(II) complexes.

M. Azam et al. / Journal of Molecular Structure 1047 (2013) 48–54
51
intensity band observed at 840 cmꢁ1 in the free ligand assigned to
mC–S–C stretching vibration of thiophene moiety is shifted to lower
values ca. 20 cmꢁ1 in Pd(II) complexes, suggesting the involvement
of the sulphur atom in bonding with Pd(II) ion [26]. The band
assigned to the asymmetric m(C–S) observed at 630 cmꢁ1 is shifted
to lower frequency ca. 25 cmꢁ1 in the complexes confirming the
coordination of sulphur atom [27]. However, it is further confirmed
in the far IR spectra by the appearance of new band at
240–280 cmꢁ1 assignable to Pd–S vibration [28]. IR spectra of
Pd(II) complexes show bands at 340 cmꢁ1 and 365 cmꢁ1 assignable
to Pd–Cl band while the carboxylate bands in complex 2 appear at
1550 cmꢁ1 [24,29].
cated in general positions. The X-ray crystal structure of ligand,
2-(2-thienyl)2,3-dihydro-1H-perimidine shows two crystallo-
graphically independent 2-(2-thienyl)2,3-dihydro-1H-perimidine
in its asymmetric unit and there are four such units in the unit cell
(Z = 4) and the relevant parameters are tabulated in Tables 1S–5S.
An ORTEP representation is shown in Fig. 7S with atom labeling
scheme used and a perspective view of packing pattern is shown
in (Fig. 1). The observed C–C, C–N and C–S bond lengths as well
as the bond angles of the molecule are in the normal range and
are in good agreement with those observed for the corresponding
compounds reported in the literature [32]. The C–S bond lengths
and bond angles are in the range from 1.642(5) to 1.715(3) Å and
111.8° to 123.7°, respectively. Interestingly N–Hꢄ ꢄ ꢄN hydrogen
bonding interaction with H to N separation of 2.33 Å is observed
between N4 and N1 atoms (Fig. 2 and Table 1).
3.2. NMR investigations
The 1H and 13C NMR spectra of ligand and its Pd(II) complexes
provide their important structural information and the results ob-
tained are in close agreement with those obtained by X-ray and IR
studies. The 1H NMR spectrum of ligand shows a singlet at
3.35 ppm (s –NH) which undergoes downfield shift and appear at
4.58 ppm and 3.68 ppm in complexes 1 and 2, respectively [28].
The aromatic protons of free ligand exhibit a multiplet in 6.48–
7.0 ppm region [30], while the resonance signals corresponding
to protons of thiophene moiety appeared in 7.13–7.27 ppm region
(m, 3H) (Fig. 2S) [31]. These valued were found to be shifted down-
field and appear at 6.51–7.21 ppm (m, Ar–H) and 7.32–7.75 ppm
(m thiophene protons) and 7.11–7.58 ppm (m, Ar–H) and 7.60–
7.72 ppm (m thiophene protons) in complexes 1 and 2, respec-
tively (Figs. 2S and 3S). An additional signal corresponding to
methyl proton of -OCOCH3 group was observed at 2.09 pm in com-
plex 2 (Fig. 3S).
3.4. Mass spectrometry
Mass spectra of the perimidene ligand L and its Pd(II) complexes
exhibited molecular ion peak [M+H]+, m/z at 253.39, 430.67, and
477.85 corresponding to their molecular formulae, [C15H12N2S],
[C15H12N2SPdCl2], and [C19H18N2SO4Pd], respectively, as their
calculated m/z being 252.39, 429.66, and 476.85 for their corre-
sponding compounds, [C15H12N2S], [C15H12N2SPdCl2], and
[C19H18N2SO4Pd], respectively (Figs. 3 and 8S and 9S).
3.5. Electronic spectra
Electronic spectra of Pd(II) complexes have been recorded in
CH2Cl2. Both the Pd(II) complexes, 1 and 2, are diamagnetic with
square planar geometry, for which three spin- allowed d–d transi-
tions are expected, corresponding to transitions from the three
lower lying d-orbitals to the empty dx2 ꢁ y2 orbitals [33]. The
The 13C NMR spectral findings further ascertain the 1H NMR
spectral data. The 13C NMR spectrum of ligand shows characteristic
sets of signals belonging to aliphatic and aromatic carbons (Fig. 4S).
These values were deshielded upon complexation (Figs. 5S and 6S).
A signal attributed to –CH3 of acetate group was observed at
52.55 ppm in complex 2 (Fig. 6S).
1
ground state is A2g and the excited states related to these transi-
tions are 1A2g, 1B1g and 1Eg [34]. The most prominent absorption is
around 348, 293 and 251 nm respectively, which could be assigned
to metal-to-ligand charge-transfer transitions (Fig. 10S) [35–37].
3.3. Single crystal X-ray diffraction studies
3.6. Antimicrobial activity
2-(2-thienyl)2,3-dihydro-1H-perimidine crystallizes in the
The antimicrobial effect of ligand and its Pd(II) complexes
cetrosymmetric monoclinic P21/c space group with all atoms lo-
was assessed against a variety of microorganisms including
Fig. 1. A crystal packing of 2-(2-thienyl)2,3-dihydro-1H-perimidine viewed (perspective) along crystallographic b direction.

52
M. Azam et al. / Journal of Molecular Structure 1047 (2013) 48–54
E. coli, S. aureus, P. aeruginosa, Citrobacter sp., B. subtilis and S.
acidaminiphila. The minimum inhibitory concentration of ligand
and its complexes was also determined against these six test
bacterial strains by broth dilution technique (Table 2). The activ-
ities of the compounds were compared with Amoxicillin, as a
standard drug. Since DMSO was used as a solvent, it was also
screened against all organisms and no activity was found. The
complexes 1 and 2 were the most active complexes inhibiting
bacterial growth at 50
as compared to positive control, amoxicillin which exhibited
MIC up to 30
g mLꢁ1. Thus confirms that our complexes show
l l
g mLꢁ1 and 100 g mLꢁ1, respectively
l
better antimicrobial activity as compared to reported palladium
complexes [38,39]. All the tested compounds show lesser activity
than the standard antibiotics. Treatment of cells with these com-
pounds resulted in formation of the zones of inhibition of vari-
able sizes in all the six tested bacterial strains (Figs. 11S and
12S). El-Sherif and co-workers have reported the higher activity
of the complexes as compared to the free ligand which can fur-
ther be understood in terms of the chelation theory. This theory
explains that a decrease in the polarizability of the metal could
enhance the lipophilicity of the complexes [38,40]. comparative
study revealed that complex
1 showed larger zones (16–
19 mm) as compared to complex 2 (9–12 mm) which is larger
as compared to ligand (7–9 mm) with all the tested strains.
The growth inhibiting activity of the ligand and its complexes
was further validated by measuring the reduction in survival of
treated bacterial cells against the control. The results demon-
strate greater antimicrobial activity of ligand and its Pd(II) com-
plexes on E. coli cells vis-a vis B. subtilis cells. The antibacterial
potency was found to be in the order as complex 1 > complex
2 > ligand, both in case of the Gram negative and Gram positive
bacteria, in a concentration dependent manner. The % reduction
in the viability of E. coli and B. subtilis was determined to be
88%, 70%, and 60% and 75%, 62% and 49% with complex 1, com-
plex 2 and ligand, respectively. The data explicitly suggested the
differential antibacterial activity of the tested complexes which
could be applied in the treatment of some common diseases
Fig. 2. The hydrogen bonding interactions with H to N separation of 2.33 Å (shown
as red dotted lines) between two molecules with asymmetric codes #1: 1 ꢁ x, ꢁ1/
2 + y, 1/2 ꢁ z. The other hydrogen atoms are omitted for clarity.
Table 1
Hydrogen bonding parameters (Å, deg).
D–Hꢄ ꢄ ꢄA
d(D–H)
d(Hꢄ ꢄ ꢄA)
D(Dꢄ ꢄ ꢄA)
\DHA
N4–H4Aꢄ ꢄ ꢄN1
0.97(3)
2.33(3)
3.295(5)
165.2(3)
Symmetry transformations used to generate equivalent atoms: #1: 1 ꢁ x, ꢁ1/2 + y,
1/2 ꢁ z.
Fig. 3. ESI-MS spectrum of ligand, L.
Table 2
MICs (
l
g mlꢁ1) of tested bacterial strains.
Bacterial strains
MIC (l )
g mlꢁ1
Ligand
Complex 1
Complex 2
Amoxicillin
Escherichia coli
100
100
100
75
50
75
50
50
50
50
40
30
100
75
100
75
50
75
30
30
30
30
30
30
Staphylococcus aureus
Pseudomonas aeruginosa
Citrobacter sp.
Bacillus subtilis
Stenotrophomonas acidaminiphila

M. Azam et al. / Journal of Molecular Structure 1047 (2013) 48–54
53
activities of the ligand, L and both of its complexes were examined
by screening their ability to inhibit the growth of bacteria espe-
cially E. coli, S. aureus, P. aeruginosa, Citrobacter sp., B. subtilis and
S. acidaminiphila Results suggested complex 1 to be more effective
than its ligand and complex 2 against E. coli and B. subtilis. DNA
interactive studies with the synthesized compounds also suggest
that Pd(II) complexes have more effective DNA binding as
compared to the ligand.
EtBR + DNA
EtBr + DNA+ L
EtBr + DNA+ 1
EtBr + DNA+ 2
EtBr
150
100
50
Acknowledgement
The project was supported by the Research Centre, College of
Science, King Saud University, Riyadh.
Appendix A. Supplementary material
CCDC 823368 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk./data_request/cif. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.molstruc.2013.04.064.
0
560
580
600
620
640
Wavelength (nm)
Fig. 4. Fluorescence emission spectra of ethidium bromide (EB) bound to DNA in
the absence and presence of ligand, L and complexes 1 and 2.
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