Synthesis, spectral, X-ray crystallography, electrochemistry, DNA/protein binding and radical scavenging activity of new palladium(II) complexes containing triphenylarsine

Article abstract of DOI:10.1016/j.ica.2013.06.038

The reactions of [PdCl₂(AsPh₃)₂] with equimolar amount of salicylaldehyde-4(N)-phenylthiosemicarbaz-one [H ₂-(Sal-ptsc)] (H₂L¹) and 2-hydroxy-1- naphthaldehyde-4(N)-methylthiosemicarbazone

Full text of DOI:10.1016/j.ica.2013.06.038

Inorganica Chimica Acta 405 (2013) 415–426
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, spectral, X-ray crystallography, electrochemistry,
DNA/protein binding and radical scavenging activity of new
palladium(II) complexes containing triphenylarsine
a
a,
⇑
a
b
b
a,
⇑
P. Kalaivani , R. Prabhakaran , M.V. Kaveri , R. Huang , R.J. Staples , K. Natarajan
a
Department of Chemistry, Bharathiar University, Coimbatore 641 046, India
Department of Chemistry, Michigan State University, East Lansing, MI 48824, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2013
Received in revised form 24 June 2013
Accepted 24 June 2013
Available online 2 July 2013
2 3 2
The reactions of [PdCl (AsPh ) ] with equimolar amount of salicylaldehyde-4(N)-phenylthiosemicarbaz-
1
one [H
mtsc)] (H
2
-(Sal-ptsc)] (H
2
L ) and 2-hydroxy-1-naphthaldehyde-4(N)-methylthiosemicarbazone [H
2
-(Nap-
2
2
L ) were carried out in ethanol/dichloromethane medium. The obtained products (1 and 2)
were characterized by variours spectral and analytical techniques. From the X-ray crystallographic stud-
ies, it is inferred that both the ligands coordinate as ONS tridentate dibasic donor by forming more com-
ꢀ
mon five and six member chelate rings. The complex 1 crystallizes in the triclinic space group P1
Keywords:
with two molecules per unit cell, has the dimensions of a = 10.1680(12) Å, b = 10.5535(12) Å,
Palladium(II) complexes
Thiosemicarbazones
X-ray crystallography
Electrochemistry
c = 13.3852(15) Å,
in the monoclinic space group P2(1)/n with four molecules per unit cell, has the dimensions of
a = 14.0981(3) Å, b = 11.2881(2) Å, c = 18.2678(3) Å, = 90°, b = 111.00(10)° and = 90°. The complexes
a = 78.9980(10)°, b = 82.7610(10)° and c = 86.2490(10)°. The complex 2 crystallizes
a
c
DNA/protein binding
Radical scavenging activity
1 and 2 have been tested for their binding towards Herring Sperm (HS)-DNA and BSA (bovine serum albu-
min). The new complexes bound to DNA by electrostatic binding mode and they had a strong binding
affinity with BSA. The mechanism of quenching was found as static. In addition, the free radical bleaching
ability of complexes with DPPH (1,1-diphenyl-2-picryl-hydrazyl) radical was carried out.
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
The chemistry of thiosemicarbazones has gained importance
may be due to their ability to inhibit the biosynthesis of DNA, pos-
sibly by binding with nitrogen base pair of DNA or RNA, which may
hinder or block base replication [9]. In addition, palladium(II) thio-
semicarbazones possess interesting antiproliferative effects on hu-
man cancer cell lines, including tumor cell lines resistant to
cisplatin [10–13] and have also exhibited good antimycobacterial
effects [14]. In continuation with our investigation [15–25] herein
because of their simple preparation, unpredictable complexation
properties and their pharmacological applications [1]. The coordi-
nating ability of thiosemicarbazones is attributed to the extended
delocalization of electron density over the –NH–C(S)–NH–N = sys-
tem, which is enhanced by substitution on the terminal nitrogen
we report the reactions of salicylaldehyde-4(N)-phenylthiosemic-
1
[
1b,2–6]. Variation in the substitution on the azomethine (C2) car-
arbazone (H
2
L ) and 2-hydroxy-1-naphthaldehyde-4(N)-methyl-
2
bon of the thiosemicarbazone ligands influences the mode of their
binding. On introduction of nitrogen and/or oxygen containing
substituents on azomethine carbon, it is anticipated that the bind-
ing ability of thiosemicarbazone becomes more capricious. The
study of the interaction of transition metal complexes with DNA
is a vibrant area of research [7]. The ability to selectively target
and cleave DNA with high affinity and to report on the binding
event by changes in luminescence is of great current interest [8].
An advantage of using transition metal complexes in such studies
can be conveniently varied to suit individual applications. This
thiosemicarbazone [(H
2
L ) with palladium(II) complexes
containing triphenylarsine and their DNA and protein interactions.
2
. Experimental section
.1. Materials and methods
The ligands [H -(Sal-ptsc)] (H
2
L1), [H L2) and
2 2
-(Nap-mtsc)] (H
2
2
2 3 2
palladium metallic precursor [PdCl (AsPh ) ] were synthesized
according to the standard literature procedures [26,27]. All the re-
agents used in this study were analar grade and the solvents were
purified and dried according to the standard procedure [28]. HS-
DNA, ethidium bromide (EB) and BSA were obtained from Sigma
Aldrich and used as received. Infrared spectra were measured as
⇑
Corresponding authors. Tel.: +91 422 2428319; fax: +91 422 2422387.
E-mail addresses: rpnchemist@gmail.com (R. Prabhakaran), k_natraj6@yahoo.
com (K. Natarajan).
0
020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.06.038

4
16
P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
KBr pellets on a Nicolet Avatar Model FT-IR spectrophotometer in
s ? d); 440 nm (MLCT); 1H NMR (DMSO-d6, ppm): d 8.6 (s, 1H,
CH@N), 9.6 (s, terminal –NH), 6.7–7.8 (m, aromatic); C NMR
(DMSO-d6, ppm): 166.4 (C–S), 162.24 (C@N), 153.6 (C-2 Aromatic),
ꢀ1
13
the 400–4000 cm range. Elemental analyses of carbon, hydrogen,
nitrogen, and sulfur were determined using Vario EL III CHNS at the
Department ofChemistry, Bharathiar University, Coimbatore, India.
The electronic spectra of the complexes have been recorded in
dichloromethane using a Systronics 119 Spectrophotometer in
the 800–200 nm range. 1H and C NMR spectra were taken in
DMSO at room temperature with a Bruker 400 MHz instrument
with chemical shift relative to tetramethylsilane. Cyclic voltammo-
grams were recorded on a CH instrument by using platinum wire
working electrode and platinum disc counter electrode. All the
potentials were referenced to the standard Ag/AgCl electrode and
ferrocene was used as external standard. Melting points were re-
corded by using Lab India melting point apparatus.
3 3
152.1 (aromatic AsPh ), 132.08 (aromatic AsPh ), 129.15 (aromatic,
AsPh ), 127.6 (C-11, C-13, aromatic), 126.6 (C-6,aromatic), 121.7
3
(C-5, aromatic), 115.1 (C-10, C-14 aromatic), 112.0 (C-1, aromatic).
The very similar method was followed to synthesize the follow-
ing complex.
13
3
2.4. Synthesis of [Pd(Nap-mtsc)(AsPh )] (2)
It was prepared by the procedure as has been used for (1) with
naphthaldehyde-4(N)-methylthiosemicarbazone [H -(Nap-mtsc)]
2
(0.066 g; 0.253 mmol). Evaporation of the solvent mixture gave
an orange solid which was dissolved in dichloromethane and sub-
jected to column chromatography. A yellowish orange band was
isolated with benzene and on evaporation yielded orange com-
pound. It was recrystallised from hot dimethylformamide which
afforded reddish orange crystals. Yield: 68%. M.p. 260 °C. Anal. Calc.
2.2. Synthesis of the ligands
2
.2.1. Synthesis of salicylaldehyde-4(N)-phenylthiosemicarbazone,
1
[H
2
-(Sal-ptsc)](H
2
L ) [26]
A 4.2 g (0.025 mol) of 4(N)-phenylthiosemicarbazide was dis-
for C31
H
26
N
3
OSPdAs: C, 55.58; H, 3.91; N, 6.27; S, 4.79. Found:C,
3
ꢀ1
solved in 20 cm of hot ethanol and to this was added 2.7 g of
55.49; H, 3.87; N, 6.18; S, 4.20%. FT-IR (cm ) in KBr: 1616 (
m
C@N),
3
ꢀ1
(
1
0.025 mol) salicylaldehyde in 10 cm of ethanol over a period of
0 min with continuous stirring. The reaction mixture was further
1259(
m
C–O), 738 (
m
C–S), 1428, 1074, 688 cm (for AsPh
3
) ; UV–Vis
(CH Cl
2
2
), kmax: 254, 270 nm (intra-ligand transition); 318, 380 nm
1
refluxed for 5 h and allowed to cool whereby a shining yellow com-
pound began to separate which was filtered and washed thor-
oughly with ethanol and then dried in vacuum. The compound
was recrystallized from hot ethanol. The product dissolves in
common organic solvents such as acetone, methanol, ethanol,
dichloromethane, chloroform, DMF and DMSO. Yield: 80%. M.p.
(LMCT s ? d); 430 nm (MLCT); H NMR (DMSO-d6, ppm): d 8.07
(d (J = 11.2), CH@N), 9.29 (s, terminal –NH), 2.84 (d (J = 3.6), –
1
3
CH
(C–S), 158.24 (C@N), 155.6 (aromaticAsPh
AsPh ), 130.12 (aromatic AsPh ), 129.15 (C-5, aromatic), 126.6
(C-8, C-9, aromatic), 123.7 (C-7, aromatic), 118.02 (C-1, aromatic),
28.8 (CH ).
3
), 6.80–7.99 (m, aromatic); C NMR (DMSO-d6, ppm): 163.4
3
), 151.4 (aromatic
3
3
1
1
91 °C. Anal. Calc. for C14
1.82. Found: C, 61.91; H, 4.79; N, 15.42; S, 11.78%. FT-IR (cm
H
13
N
3
OS: C, 61.97; H, 4.83; N, 15.49; S,
3
ꢀ1
)
1
in KBr: 3419 (
m
OH), 1620 (
m
C
@
N
), 1270(mC–O), 815 (
m
C
@
S
); H NMR
2.5. Single crystal X-ray crystallography
(
1
DMSO-d6, ppm): 11.40 (s, 1H, OH), 10.01 (s, 1H, NHCS), 9.59 (s,
1
3
H, NHPh), 8.33 (s, 1H, CH@N), 6.79–7.58 (m, aromatic);
C
Single crystals of [PdCl (AsPh ) ], [Pd(Sal-ptsc)(AsPh )] (1) and
2
3 2
3
NMR (DMSO-d6, ppm): 186.4 (C@S), 162.28 (C@N), 152.1 (C-2, aro-
3 6 6 3
[Pd(Nap-mtsc)(AsPh )] (2) were obtained from C H /CH CN,
matic), 132.08 (C-4, aromatic), 128.92 (C-11, C-13, aromatic), 123.7
2 5 2 2
C H OH/CH Cl mixture and DMF respectively. Single crystal data
(
C-10, C-14, aromatic), 117.4 (C-5, aromatic), 112.0 (C-3, aromatic).
collections and corrections were done at 173 K with Bruker SMART
000 CCD using graphite mono chromated Mo K (k = 0.71073 Å)
1
a
2
.2.2. Synthesis of 2-hydroxy-1-naphthaldehyde-4(N)-
radiation. The structural solution were done by using SHELXS-97
[29] and refined by full matrix least square on F using SHELXL-97
[30].
2
2
methylthiosemicarbazone, [H
The method above described was followed for the preparation.
The ligand [H -(Nap-mtsc)] was prepared from 4(N)-methylthio-
2
-(Nap-mtsc)](H
2
L ) [26]
2
semicarbazide (2.63 g, 0.025 mol) and 2-hydroxy-1-naphthalde-
hyde (4.30 g, 0.025 mol). A cream white compound began to
2.6. DNA binding study
separate out. Yield: 78%. M.p. 220 °C. Anal. Calc. for C13
C, 60.21; H, 5.05; N, 16.20; S, 12.36. Found: C, 60.17; H, 4.99; N,
H
13
N
3
OS:
HS-DNA solutions of various concentrations (0.05-0.5
solved in a tris HCl buffer (pH 7) were added to the palladium com-
plexes 1 and 2 (1 M dissolved in DMSO/H O mixture). Absorption
spectra were recorded after equilibrium at 20 °C for 10 min. The
intrinsic binding constant Kb was determined by using Stern
Volmer Eq. (1) [31,32].
lM) dis-
ꢀ
1
1
1
6.14; S, 12.31%. FT-IR (cm ) in KBr: 3423 (
m
OH), 1640 (
C@S); HNMR (DMSO-d6, ppm): 11.25 (s, 1H,
), 8.02 (s, 1H, CH@N),
m
C@N),
l
2
1
250 (
m
C–O), 795 (
m
OH), 10.65 (s, 1H, NHCS), 9.10 (s, 1H, NHCH
.07–7.71 (m, 3H, aromatic), 2.50 (d (J = 2.4), 3H, CH
DMSO-d6, ppm): 181.4 (C@S), 157.24 (C@N), 135.12 (C-4, aro-
matic), 129.15 (C-6, aromatic), 126.4 (C-8, C-9, aromatic), 122.82
C-1, aromatic), 118.19 (C-3, aromatic), 28.6 (CH ).
3
13
7
(
3
); C NMR
½DNAꢁ=½
e
a
ꢀ
e
f
ꢁÞ ¼ ½DNAꢁ=½
e
b
ꢀ
e
f
ꢁ þ 1=K
b
½e
b
ꢀ
e
f
ꢁ
ð1Þ
(
3
The absorption coefficients
e
a
,
e
f
, and
e
b
correspond to Aobsd/
[
DNA], the extinction coefficient for the free complex and the
2
.3. Synthesis of [Pd(Sal-ptsc)(AsPh
3
)] (1)
extinction coefficient for the complex in the fully bound form,
respectively. The slope and the intercept of the linear fit of the plot
3
An ethanolic (25 cm ) solution of [PdCl
2
(AsPh
3
)
2
] (0.200 g;
of [DNA]/[ ] and 1/K
e
a
ꢀ
e
f
] versus [DNA] give 1/[
e
a
ꢀ
e
f
b
[
e
b
ꢀ
f
e ],
0
.253 mmol) was slowly added to salicylaldehyde-4(N)-phenylthi-
respectively. The intrinsic binding constant K can beobtained from
b
1
osemicarbazone (H
2
L ) (0.068 g; 0.253 mmol) in dichloromethane
the ratio of the slope to the intercept (Table. 4) [31]. Emission mea-
surements were carried out by using a JASCO FP-6600 spectrofluo-
rometer. Tris–buffer was used as a blank to make preliminary
adjustments. The excitation wavelength was fixed and the emis-
sion range was adjusted before measurements. All measurements
were made at 20 °C. For emission spectral titrations, complex
3
(
25 cm ). The mixture was allowed to stand for 4 days at room
temperature. Orange red crystals obtained were filtered, washed
with n-hexane and dried. Yield: 52%. M.p. 218 °C. Anal. Calc. for C32-
H N OSPdAs: C, 56.36; H, 3.84; N, 6.16; S, 4.70. Found: C, 56.30, H,
26 3
3
ꢀ1
.80; N, 6.13; S, 4.62 . FT-IR (cm ) in KBr: 1583 (
m
C@N), 1342(
mC–O),
ꢀ1
7
43 (m
C–S), 1440, 1070, 691 cm (for AsPh
3
) ; UV–Vis (CH
2
Cl
2
),
concentration was maintained constant as 1
l
M and the concen-
kmax: 240, 272 nm (intra-ligand transition); 320, 380 nm (LMCT
tration of HS-DNA was varied from 0.05 to 0.5 l
M. The emission

P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
417
enhancement factors were measured by comparing the intensities
at the emission spectral maxima under similar conditions. In order
to know the mode of attachment of DNA fluorescence quenching
and synchronous measurements were performed using a1 cm
quartz cell on JASCO F 6500 spectrofluorimeter.
experiments of EB-DNA were carried out by adding 0–40
lM palla-
2.8. DPPH (1,1-diphenyl-2-picryl-hydrazyl) radical scavenging assay
dium(II) complexes to the samples containing10 M EB, 10 lM
l
DNA and tris-buffer (pH = 7). Before measurements, the system
was shook and incubated at room temperature for ꢂ5 min. The
emission was recorded at 530–750 nm.
The potential antioxidant activity of new palladium(II) com-
plexes was evaluated by DPPH radical-scavenging assay according
to the procedure described previously with a slight modification
[
35]. DPPH free radicals are used for rapid analysis of antioxidants.
2
.7. Bovine serum albumin binding study
While scavenging the free radicals, the antioxidants donate hydro-
gen and form a stable DPPH-H molecule. Briefly, the various con-
ꢀ1
The protein binding study was performed by tryptophan fluo-
centrations of the complexes (10–50 mg ml ) were added to
5 ml of DPPH (0.1 mM in methanol) and were mixed rapidly. Rad-
ical scavenging capacity was measured in 10 min intervals using
spectrophotometer by monitoring the decrease in absorbance at
517 nm. The IC50 values of DPPH decolonization of the complexes
were calculated. Butylated hydroxyanisole (BHA) and buty-
latedhydroxytoluene (BHT) were used as a positive control.
rescence quenching experiments using bovine serum albumin
BSA, 1 M) as the substrate in phosphate buffer (pH 7). Quenching
of the emission intensity of tryptophan residues of BSA at 346 nm
excitation wavelength at 280 nm) was monitored using com-
(
l
(
plexes 1 and 2 as quenchers with increasing complex concentra-
tion. The possible quenching mechanism has been interpreted
using the following Stern Volmer Eq. (3), the ratio of the fluores-
cence intensity in the absence of (I
corrected fluorescence intensity) the quencher is related to the
concentration of the quencher [Q] by a coefficient Ksv
0
) and in the presence of (Icorr–
3
. Results and discussion
L1 and H L2 with equimolar amount of
2
.
The reactions of H
[PdCl (AsPh ] were carried out in ethanol/dichloromethane mix-
2
I
0
=Icorr ¼ 1 þ KSV½Qꢁ
ð2Þ
2
3 2
)
ture (Scheme 1) which afforded two air and light stable complexes.
They dissolve in common organic solvents such as dimethylform-
amide, dimethylsulfoxide, ethanol, methanol, chloroform and
dichloromethane. The analytical data of the new complexes agree
well with the proposed molecular formulae. The structures of the
complexes (1 and 2) along with their starting material were con-
firmed by X-ray crystallographic study.
In order to correct the inner filter effect the following Eq. (3), is
used.
AexcþAem
corr ¼ Fobs ꢃ 10
2
F
ð3Þ
where Fcorr is the corrected fluorescence value, Fobs the measured
fluorescence value, Aexc the absorption value at the excitation wave-
length, and Aem the absorption value at the emission wavelength
[
33].
The binding constant (K
3.1. IR Spectra
b
) and the number of binding sites (n)
can be determined according to the method [34] using the Scat-
chard Eq. (4):
The IR spectra of free Schiff base ligands showed a strong
ꢀ1
absorption at 1620 and 1640 cm corresponding to
m
C
@
N
azome-
1
2
thine group respectively for [H L ] and [H L ]. This band has been
2 2
log½ðI
0
ꢀ IÞ=Iꢁ ¼ logK
b
þ nlog½Qꢁ
ð4Þ
ꢀ1
lowered by 37 and 24 cm respectively in the complexes indicat-
ing the coordination of azomethine nitrogen atom [16,23]. The
medium intensity band appeared at 815–795 cm due to
where, K
b
is the binding constant for the complex–protein interac-
ꢀ1
tion and n is the number of binding sites per albumin molecule,
which can be determined by the slope and the intercept of the dou-
mC@S of
ꢀ1
the thiosemicarbazones which has been shifted to 743 cm in
ꢀ1
ble logarithm regression curve of log [(I
chronous fluorescence spectra of BSA with various concentrations
of complexes were obtained from 300 to 400 nm when k = 60 nm
and from 290 to 500 nm when k = 15 nm. The excitation and
emission slit widths were 5 and 6 nm, respectively. Fluorescence
0
ꢀ I)/I] versus log[Q]. Syn-
complex 1 and 738 cm in complex 2 indicating the coordination
of thiolate sulfur atom after enolisation followed by deprotonation
D
[18]. A broad band corresponding to mOH appeared at 3419–
ꢀ1
D
3423 cm in the ligands completely disappeared in the spectra
of the new complexes 1 and 2 confirming the coordination of
[
PdCl (AsPh ) ]
2 3 2
Room temperature
Ethanol/ CH Cl
2
2
OH
OH
S
C
S
C
N
N
C
C
H
CH3
N
NH
N
NH
H
H
H
AsPh3
AsPh3
O
O
S
S
Pd
N
Pd
NH
NH
N
N
N
2
1
Scheme 1. Preparation of new palladium(II) complexes.

4
18
P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
phenolic oxygen after deprotonation. This was further supported
by the increase in the value of phenolic C–O stretching frequency
Complex 1 showed two singlets at d 8.6 and 9.6 ppm correspond-
ing to azomethine and terminal –NH protons and complex 2
showed two doublets around d 8.3 and 9.5 ppm corresponding to
azomethine and terminal –NH protons due to the couplingwith a
arsine atom of the triphenylarsine and the restricted rotation on
the C@N bond of the ligand respectively [20]. In addition, multiplet
appeared at d 2.4 ppm in complex 2 corresponding to methyl group
of protons [18]. In the C{1H} NMR spectra of the ligands, the thi-
one (C@S) carbon resonates at 186.4 and 181.4 ppm were com-
pletely disappeared in the complexes and new resonances at
ꢀ1
ꢀ1
from 1270–1250 cm
to 1342–1259 cm
[18,22]. In both the
complexes the characteristic absorption bands for triphenylarsine
were also present in the expectedregion [23].
3.2. Electronic spectra
1
3
The electronic spectra of the complexes 1 and 2 have been re-
corded in dichloromethane and they displayed five to six bands
in the region around 240–440 nm. The bands appeared at the re-
gion 240-320 nm have been assigned to intra ligand transition
and the bands around 340–380 nm have been assigned to ligand
to metal charge transfer transition [36,37] . The bands around
1
66.4 and 163.4 ppm indicate the thione to thiol tautomerisation
and subsequent thiol coordination. The azomethine carbon reso-
nance is observed at 162.28 and 157.24 ppm in the ligands ap-
peared at 164.0 and 163.48 ppm in the complexes. In both
ligands, aromatic carbon atoms of the phenoxy group observed
around 152.1–118.19 ppm are almost comparable with the com-
4
40 nm have been assigned to metal to ligand charge transfer tran-
sition [38].
plexes and the literature values [15]. A signal at 28.6 ppm in the li-
2
gand H
2
L and 27.88 ppm in the complex confirm the presence of
1
3
.3. H NMR spectra
methyl group in the system. In both complexes three signals corre-
sponding to the presence of triphenylarsine observed at 153.6,
The 1H-NMR spectra of H
L1–2 and the corresponding com-
2
1
1
52.1, and 132.08 ppm (complex 1) and 155.6, 151.4, and
30.12 ppm (complex 2) are in the range of the reported values
plexes recorded in DMSO showed all the expected signals. In the
1
–2
spectra of H
2
L
, a singlet appeared in the range 9.11–9.86 ppm
[
15].
has been assigned to N(2)HCS group [20,21]. But in the spectra of
the new complexes 1 and 2, there was no resonance attributable
to N(2)H, indicating the coordination of ligands in the anionic form
after deprotonation at N(2). A sharp singlet corresponding to the
phenolic –OH group has appeared at 11.20–11.41 ppm in the free
ligands. But the same singlet has completely disappeared in com-
plexes 1 and 2 confirming the involvement of phenolic oxygen in
3.4. X-ray crystallography
The crystallographic data, selected bond distances and bond an-
gles are listed in Tables 1 and 2. The ORTEP diagrams with number-
ing scheme of the Palladium precursor and complexes 1 and 2 are
1–2
coordination[17]. In the spectra of H
2
L
and complexes 1 and 2,
in Figs. 1–5. In order to confirm the trans configuration of [PdCl
AsPh ] single crystal X-ray structure of this starting complex was
solved. The single crystals of [PdCl (AsPh ] were obtained in ben-
2
(-
a complex multiplet appeared at 6.60–7.99 ppm due to aromatic
protons of ligands and triphenylarsine [18]. Two singlets observed
at 8.02–8.35 ppm and 8.40–9.59 ppm has been assigned to azome-
3 2
)
2
3 2
)
zene-acetonitrile medium. The starting complexcrystallizes with
one unit of benzene molecule in its lattice. The presence of
1
2
2 2
thine and terminal –NH protons for H L and H L respectively.
Table 1
Crystal data and structure refinement of new Pd(II) thiosemicarbazone complexes.
[
PdCl
2
(AsPh
3
)
2
]
[Pd(Sal-ptsc)(AsPh
CCDC 857201
3
)] (1)
[Pd(Nap-mtsc)(AsPh
CCDC 856729
3
)] (2)
CCDC 857204
Empirical formula
Formula weight
T (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C
39
H
33As Cl Pd
2
2
C
32
H
26 AsN
3
OPdS
31 3
C H26As N OPdS
669.93
173(2)
0.71073
monoclinic
P2(1)/n
828.79
173(2)
0.71073
monoclinic
P2(1)/n
681.94
173(2)
0.71073
triclinic
P1ꢀ
11.651(2)
18.625(4)
16.552(3)
90
105.04(3)
90
3468.8(12)
4
1.587
10.1680(12)
10.5535(12)
13.3852(15)
78.9980(10)
82.7610(10)
86.2490(10)
1397.4(3)
2
14.0981(3)
11.2881(2)
18.2678(3)
90
111.00(10)
90°
2714.02(9)
4
1.640
1.640
1344
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
calc (Mg m3)
1.621
1.945
684
Absorption coefficient (mmꢀ1)
F(000)
2.609
1652
Theta range for data collection (°)
Index ranges
1.68–28.34
1.56–28.26
1.58–36.41
ꢀ15 6 h 6 14, ꢀ24 6 k 6 24,
ꢀ13 6 h 6 13, ꢀ13 6 k 6 13,
ꢀ23 6 h 6 23, ꢀ18 6 k 6 18,
ꢀ
22 6 l 6 22
11611/5950[Rint = 0.0464]
28.34 97.6%
ꢀ17 6 l 6 17
ꢀ30 6 l 6 30
Reflections collected/unique
Completeness to theta
Refinement method
15607/6261[Rint = 0.0433]
28.26 90.4%
42193/13167[Rint = 0.0572]
36.41 99.5%
Full-matrix least-squares on F2
8458/0/409
Full-matrix least-squares on F2
6261/0/456
Full-matrix least-squares on F2
13167/0/344
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
0.576
1.046
1.038
Final R indices [I > 2
R indices (all data)
r
(I)]
R
R
1
= 0.0218, wR
= 0.0322, wR
2
2
= 0.0662
= 0.0752
R
R
1
= 0.0230, wR
= 0.0281, wR
2
= 0.0573
= 0.0593
R
R
1
= 0.0433, wR
= 0.0845, wR
2
2
= 0.0938
= 0.1081
1
1
2
1
Largest difference peak and hole
0.274 and ꢀ1.303
0.330 and ꢀ0.539
1.234 and ꢀ0.727
ꢀ
3
(
e A )

P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
419
Table 2
Selected bond lengths (Å) and angles (°) of new Pd(II) thiosemicarbazone complexes.
[
2
PdCl (AsPh
3 2
) ]
[Pd(Sal-
[Pd(Nap-
ptsc)(AsPh
3
)](1)
3
mtsc)(AsPh )](2)
Bond lengths
Pd–Cl1 2.2896(6)
Pd–Cl2 2.2738(6)
Pd–As1 2.4158(5)
Pd–As2 2.4146(5)
Pd–N9 2.0055(15)
Pd–O1 2.0064(15)
Pd–S1 2.2455(6)
Pd–As1 2.3811(3)
Pd–N1 2.006(19)
Pd–O1 2.027(17)
Pd–S1 2.2351(6)
Pd–As1 2.379(3)
Bond angles
Cl2–Pd–Cl1 178.59(2)
N9–Pd–O1 93.25(6)
N1–Pd–O1 92.18(7)
N1–Pd–S1 85.08(6)
O1–Pd–S1 177.24(5)
N1–Pd–As1 176.51(5)
O1–Pd–As1 90.91(5)
S1–Pd–As1 91.840(18)
As2–Pd–As1 176.55(8) N9–Pd–S1 84.37(5)
Cl1–Pd–As1 93.24(14) O1–Pd–S1 175.70(4)
As1–Pd–Cl2 86.36(14) N9–Pd–As1 176.55(4)
Cl2–Pd–As2 91.59(14) O1–Pd–As1 88.92(4)
As2–Pd–Cl1 88.74(14) S1–Pd–As1 93.63(14)
Fig. 3. Molecular Packing diagram of complex 1 along a axis.
Fig. 1. ORTEP diagram of [PdCl
2
(AsPh
3
2
) ]ꢄC
6 6
H .
3
Fig. 4. ORTEP diagram of [Pd(Nap-mtsc)AsPh ] (2).
mutually trans-triphenylarsine ligands confirm the trans configura-
tion of [PdCl (AsPh ]. The bond angles for As(1)–Pd–As(2)
76.00(14) and Cl(1)–Pd–Cl(2) 178.59(2) indicates deviation of
2
3 2
)
1
the geometry from its original square plane. Complex 1 crystallizes
ꢀ
in triclinic space group P1 having two molecules in the unit cell.
The molecular packing diagram is given in Fig. 3. In this complex,
the ligand is coordinated to palladium as ONS donor by forming
one stable five and another six member ring. The remaining coor-
dination site is occupied by triphenylarsine satisfying the fourth
coordination site. In this complex, the Pd–O, Pd–As and Pd–S bond
lengths of 2.0064(5), 2.3811(3) and 2.2455(6) Å respectively are
close to the reported value for palladium(II) complexes [18,20–
2
2]. The Pd–N bond length of 2.0055(15) is almost consistent with
the value found for dichlorobis(triphenylarsine)palladium(II) [22].
The triphenylarsine and N(9) nitrogen are mutually trans to each
other. The basal plane constituted with AsSNO core and the ligand-
coordinated to palladium with the bite angle of 84.37° causing con-
siderable deviation from the square planar geometry. The suitable
crystals of complex 2 were obtained from dimethylformamide. The
complex 2 crystallizes in the monoclinic space group P2(1)/n hav-
ing four molecules in the unit cell. The packing diagram is given in
Fig. 5. In this complex, the ligand coordinated to metal by losing
3
Fig. 2. ORTEP diagram of [Pd(Sal-ptsc)AsPh ] (1).

4
20
P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
Fig. 5. Molecular packing diagram of complex 2 along the a-axis.
Table 3
Electrochemical data of the new palladium(II) complexes.
Complex Oxidation Pd(II)–Pd(III)
Reduction Pd(II)–Pd(I)
Epa (V) Epc (V)
Ligand oxidation
Ligand reduction
Epa
V)
Epc
(V)
E
(V)
½
D
(mV)
Ep
E
½
(V)
D
(mV)
Ep
Epa
(V)
Epc
(V)
E
(V)
½
D
(mV)
Ep
Epa (V) Epc
(V)
E
½
D
Ep
(
(V)
(mV)
[
Pd(Sal-ptsc)
AsPh )] (1)
Pd(Nap-mtsc)
AsPh )] (2)
0.430 0.380 0.455
50
ꢀ0.390 ꢀ0.320 ꢀ0.355 70
ꢀ0.560 ꢀ0.480 ꢀ0.520 80
1.15
–
1.25
–
1.2
100
ꢀ1.480 ꢀ0.95 ꢀ1.23 530
(
3
[
0.672 0.972 0.822 300
–
–
–
–
–
–
(
3
two protons from its tautomeric thiol form and phenolic –OH. They
act as binegative tridentate ligands via the mercapto sulphur, phe-
nolic oxygen and b-nitrogen atoms by forming one six and another
five member ring with bite angle of 84.37(5)° [N(9)–Pd(1)–S(1)].
The Pd–N 2.0062(19) Å, Pd–S 2.2351(6) Å, Pd–O 2.0270(17) Å and
Pd–As 2.3790(3) Å bond lengths are in the expected region and
agreevery well with the already reported values [18,20–22]. The
complex has the bite angle of 85.06(6) in the PdNSOAs square
planar core. The two trans angles O(1)–Pd–S(1) 177.24(5) and
N(1)–Pd(1)–As(1) 176.51(5) deviate considerably from the ideal
angle causing significant distortion in the square plane of the
complex 2.
Table 4
Binding constant for interaction of complexes (1 and 2) with HS-DNA.
4
ꢀ1
System
K
b
(ꢅ 10
M )
HS-DNA + 1
HS-DNA + 2
1.68 ± 0.15
1.92 ± 0.57
In addition, it exhibited a reversible ligand oxidation and quasi-
reversible ligand reduction at E1/2 of 1.20 and ꢀ1.23 Vwith the
peak to peak separation of 100 and 530 mV respectively. The com-
plex 2 showed a one electron quasi-reversible oxidation corre-
sponding to Pd(II)–Pd(III) and one electron reversible reduction
corresponding to Pd(II)–Pd(I) with a peak to peak separation of
3.5. Electrochemistry
3
00 and 80 mV respectively with the potentials of 0.822 and
0.520 V. The adsorption of the complex on to the electrode sur-
ꢀ
Electrochemical studies of the new palladium(II) complexes
face or slow electron transfer may be the reason for quasi- revers-
ible electron transfer process [39].
have been done in dichloromethane medium by using platinum
wire working electrode and platinum disc counter electrode. All
the potentials were referenced to Ag/AgCl electrode. Ferrocene
was used as an external standard. Voltammetric data are presented
in Table 3. Complex 1 exhibited one electron reversible oxidation
and reduction corresponding to Pd(II)–Pd(III) and Pd(II)–Pd(I) at
3.6. DNA binding studies
3.6.1. Electronic absorption titration
E
ꢀ
1/2 of 0.455 V with the peak to peak separation of 50 and
0.355 V with the peak to peak separation of 70 mV respectively.
The application of electronic absorption spectroscopy is one of
the most useful techniques for DNA-binding properties. The

P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
421
absorption spectra of two new palladium (II) complexes (1
the absence and presence of Herring Sperm HS-DNA (0.05–
.50 M) are recorded, and the absorption spectra of the three
complexes are given in Fig. 6. The absorption spectra of complex
consists of three resolved bands [Intra ligand (IL) and Metal to li-
gand (MLCT) transitions] centered at 250 nm (IL), 389 and 406 nm
l
M) in
showed modest hypochromism (A = 0.0937–0.0927 and A =
0.0978–0.0949 respectively) with negligible shifts in the wave-
length. The binding behavior of complex 2 is quite different. It
exhibited hyperchromism at 268 nm (A = 0.0368–1.770) with
5 nm blue shift. In addition, three resolved charge transfer transi-
tions (CT) centered at 342, 421 and 475 nm showed modest hypo-
chromism with negligible shifts in the wavelength. The observed
hyperchromic effect with red/blue shift suggested that the new
complexes bind to CT-DNA by external contact, possibly due to
0
l
1
(
(
MLCT). As the DNA concentration is increased, the hyperchromism
A = 0.2825–2.800) with a red shift of 12 nm (up to 262 nm) was
observed in the IL band. The MLCT bands at 389 and 406 nm
Fig. 6. Absorption titration spectra of 1 and 2 with increasing concentrations (0.05–0.5 lM) of HS-DNA (tris HCl buffer, pH 7); the inset shows binding isotherms with HS-
DNA.

4
22
P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
1
0
8
6
4
2
2
1
8
6
4
2
0
4
35
440
445
450
455
460
465
375
380
385
390
395
400
Wavelength (nm)
Wavelength (nm)
Fig. 7. Changes in the emission spectra of 1 and 2, with increasing concentrations (0.05–0.5
l
M) of HS-DNA (tris HCl buffer, pH 7);
600
500
400
300
200
100
0
7
6
5
00
00
00
1
660
2
650
640
630
620
610
600
400
6
20
Wavelength (nm)
300
200
100
5
50
600
650
700
750
550
600
650
Wavelength (nm)
700
750
Wavelength (nm)
Fig. 8. The emission spectra of the DNA–EB system (kexc = 515 nm, kem = 530–750 nm), in the presence of complexes 1 and 2. [DNA] = 10
EB] = 10 M. The arrow shows the emission intensity changes upon increasing complex concentration.
lM, [Complex] = 0–40 lM,
[
l
electrostatic binding [40]. The intrinsic binding constant K
b
can be
they can bind weakly to the DNA probably by electrostatic
interaction.
determined by monitoring the changes in the absorbance in the IL
band at the corresponding kmax with increasing concentration of
DNA and is given by the ratio of slope to the Y intercept in plots
of [DNA]/(
e
a
ꢀ
f
e ) versus [DNA] (Insets in Fig. 6, Table 4). From
3
3
.7. Quenching mechanism of BSA by complexes
the binding constant values, it is inferred that the complex 2 exhib-
ited better binding than 1.
.7.1. UV absorption spectra of BSA in the presence of complexes (1
In the emission spectra, Complexes 1 and 2 had fluorescence at
and 2)
UV absorption spectrum is a very simple and applicable method
to explore the structural change, to know the complex formation in
solution and is useful to distinguish the type of quenching exist i.e.,
static or dynamic quenching [41]. The UV absorption spectra of
BSA in the presence of different concentrations of two complexes
4
50 and 390 nm respectively. If HS-DNA solution was added to this
complex solution, fluorescence intensity was increased without
any shift in wavelengths (Fig. 7). The enhanced fluorescence inten-
sity observed for the complexes represents electrostatic binding
mode of HS-DNA.
(Fig. 9) showed that the absorption intensity of BSA was enhanced
3
.6.2. Competitive studies with ethidium bromide
The competitive EB binding studies may be carried out in or-
with the addition of these complexes. There was a little blue shift
of complex–BSA spectrum (from 280 to 277 nm). This phenome-
non indicates the interaction of BSA with the compounds [42]. It
is well known that dynamic quenching only affects the excited
state of fluorophore and does not change the absorption spectrum.
However, the formation of non-fluorescence ground-state complex
induced the change in absorption spectrum of fluorophore. Thus,
possible quenching mechanism of BSA by 1 and 2 was a static
quenching process [43] .
der to examine the binding of complexes with HS-DNA. Emission
spectra of EB bound to HS-DNA in the absence and presence of
complexes 1 and 2 have been recorded. The fluorescence emis-
sion intensities of EB bound to HS-DNA at 620 nm showed
increasing trend with increasing concentration of the complexes
showed in Fig. 8, indicating that they cannot displace EB from
the DNA-EB complex. This observation is often considered that

P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
423
0.4
0.2
0.0
1
2
2
1
1
0
5
0
(
BSA)
BSA+1
BSA+2
5
0
0
10
20
30
40
50
300
Wavelength (nm)
[Q] (µM)
Fig. 9. Absorption spectra of absence and presence of complexes with BSA
(
0
Fig. 11. Plot of I /I vs. [Q].
1 ꢅ 10ꢀ M).
5
1
.2
.0
.8
.6
3
.7.2. Fluorescence quenching studies of BSA
Serum albumin (SA) is the most abundant protein in plasma and
1
1
0
0
2
is involved in the transport of drugs, metal ions, and compounds
through the bloodstream. Bovine serum albumin (BSA) is the most
extensively studied serum albumin, due to its structural homology
with human serum albumin (HSA). Binding to these proteins may
lead to loss or enhancement of the biological properties of the ori-
ginal drug, or provide paths for drug transportation. Since serum
albumins are well known to bind small aromatics, the possible
binding interactions of the palladium(II) salicylaldehyde and 2-hy-
droxy 1-naphthaldehyde thiosemicarbazone complexes with BSA
have been investigated by emission-titration experiments at room
0.4
.2
0.0
0
-
0.2
0.4
0.6
-
-
-
temperature. A solution of BSA (1
concentrations of the complexes (0–50
were recorded in the range of 290–450 nm upon excitation at
80 nm. Thechanges observed on the fluorescence emission spec-
lM) was titrated with various
l
M). Fluorescence spectra
0.8
-
5.4
-5.2
-5.0
-4.8
-4.6
-4.4
-4.2
2
log[Q]
tra of a solution of BSA on the addition of increasing amounts of
the complexes are shown in Fig. 10. Upon the addition of com-
plexes 1 and 2 to BSA, a significant decrease of the fluorescence
intensity of BSA at 346 nm from the initial fluorescence intensity
was observed accompanied by a red shift of 2 and 10 nm, respec-
tively. The observed quenching may be attributed to possible
Fig. 12. Plot of log[(F
0
ꢀ F)/F] vs. log[Q].
changes in protein secondary structure leading to changes in the
tryptophan environment of BSA, thus indicating binding of each
5
4
3
2
1
00
00
00
00
00
0
300
2
1
2
1
00
00
0
3
00
350
400
450
300
350
400
450
Wavelength (nm)
Wavelength (nm)
Fig. 10. The emission spectra of BSA (1
lM; kexc = 280 nm; kemi = 346 nm) in the presence of increasing amounts of complexes 1 and 2 (0–50 lM). The arrow shows the
emission intensity changes upon increasing complex concentration.

4
24
P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
Table 5
log[Q] Eq. (4) (Fig. 12; Tables 5). The values of n at room tempera-
ture are approximately equal to 1, which indicates that there is just
one single binding site in BSA for the complexes 1 and 2.
q
Quenching constant (K ), binding constant (Kbin) and number of binding sites (n) for
the interactions of complexes 1 and 2 with BSA.
/M 1
ꢀ
K
bin/M
ꢀ1
n
Complex
K
q
4
5
5
6
1
2
8.2 ± 0.24 ꢅ 10
3.5 ± 0.35 ꢅ 10
9.2 ± 0.43 ꢅ 10
2.7 ± 0.19 ꢅ 10
1.25
1.28
3.7.4. Synchronous fluorescence spectroscopic studies of BSA
Synchronous fluorescence spectral study was used to obtain
information about the molecular environment in the vicinity of
the fluorophore moieties of BSA [45]. When the difference (
tween the excitation and emission wavelengths is fixed at 15 and
60 nm, and the amount of complexes added to BSA (1 M) is in-
creased, a large decrease in fluorescence intensity with a red-shift
in the tryptophan emission maximum is observed (Fig. 13). In con-
trast, the emission intensity of tyrosine residue increases with red
shift in the emission maximum (Fig. 14). The observation indicated
that complexes mainly bind to tryptophan residues of BSA. It indi-
cates that the hydrophobicity of microenvironment around trypto-
phan residues decreases in the presence of the complexes [46].
Dk) be-
q 0
complex to BSA [44]. The K value obtained from the plot of I /Icorr
4
5
ꢀ1
versus [Q] (2)was found to be 8.2 ꢅ 10 and 3.5 ꢅ 10 M corre-
sponding to complexes 1 and 2 respectively. The observed linearity
in the plots (Fig. 11) indicated the ability of the complexes to
quench the emission intensity of BSA. The strong protein-binding
ability of 2 with enhanced hydrophobicity is consistent with its
strong DNA binding affinity.
l
3.7.3. Binding constants and the number of binding sites
For the static quenching interaction, if it is assumed that there
are similar and independent binding sites in the biomolecule, the
binding constant K and the number of binding sites per albumin
molecule n were determined by the slope and the intercept of
the double logarithm regression curve of log [(I
ꢀ I)/I] versus
3.8. DPPH radical-scavenging assay
b
The free radical scavenging activity of new Pd(II) complexes
was tested by its ability to bleach the stable radical DPPH. This
0
3
3
2
2
1
1
50
00
50
00
50
00
3
2
1
00
00
00
0
1
2
5
0
0
350
300
350
Wavelength (nm)
Wavelength (nm)
Fig. 13. Synchronous spectra of BSA (1
lM) in the presence of increasing amounts of complexes 1 and 2 (0–50
lM) for a wavelength difference of Dk = 60 nm. The arrow
shows the emission intensity changes upon increasing concentration of complex.
3
2
2
1
1
00
50
00
50
00
3
3
2
2
1
1
50
00
50
00
50
00
2
1
50
5
0
0
3
00
350
400
450
500
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 14. Synchronous spectra of BSA (1
lM) in the presence of increasing amounts of complexes 1 and 2 (0–50
l
M) for a wavelength difference of Dk = 15 nm. The arrow
shows the emission intensity changes upon increasing concentration of complex.

P. Kalaivani et al. / Inorganica Chimica Acta 405 (2013) 415–426
425
Fig. 15. DPPH inhibition activity of complexes 1 and 2.
by comparing the activities of complex 1 and 2, the latter exhibited
better activity than former and this may be due to the presence of
stronger electron withdrawing naphthoxy group in 2,favors more
electron deficiency on palladium(II) metal centre which increases
the efficacy of free radical scavenging than that of the phenoxy
complex 1.
Table 6
IC50 values (lM) calculated from DPPH assay of Pd(II) complexes
1
, 2 and standards (BHA and BHT).
Complex
DPPH (IC50 in lM)
1
2
BHA
BHT
20.40 ± 0.24
15.91 ± 0.21
25.60 ± 0.12
45.97 ± 0.26
Acknowledgment
The authors gratefully acknowledge Department of Science
and Technology, India and Council of Scientific and Industrial
Research, India for their financial assistance.
assay provided information on the reactivity of the compound with
a stable free radical. Because of the odd electron, DPPH shows a
strong absorption band at 517 nm in the visible spectrum [35].
As this electron becomes paired off in the presence of a free radical
scavenger, the absorption vanishes, and the resulting decoloniza-
tion is stoichiometric with respect to the number of electrons ta-
ken up. The new palladium(II) complexes exhibited good DPPH
radical-scavenging effect higher than that of butylated hydroxyan-
isole (BHA) and butylated hydroxytoluene (BHT). The free radical
scavenging activity of the complexes increased with an increase
in the concentration of the complex and it is measured interms
of IC50 values (Fig. 15; Table 6) which is an encouraging sign for
this complex to be a good anti-cancer drug. The observed antioxi-
dant activity of the palladium complexes may be due to the neu-
tralization of the free radical character of DPPH either by transfer
of an electron or a hydrogen atom [47].
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2013.06.038.
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[
[
2 3 2
the structure of the palladium precursor [PdCl (AsPh ) ] has also
been solved. The new complexes were introduced to study their
interaction with DNA and protein by taking Herring Sperm (HS-
DNA) and protein BSA as models and found significant activities
on them. The complexes exhibited better DPPH radical scavenging
activity than the conventional standards BHA and BHT. In general,
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