Synthesis, spectroscopic and radical scavenging studies of palladium(II)-hydrazide complexes

Article abstract of DOI:10.1016/j.saa.2013.05.064

In present study, a series of palladium(II) complexes with biologically active hydrazide ligands have been synthesized, characterized and screened for their antioxidant (superoxide and DPPH radical scavenging) properties. Spectral studies (FT-IR, EI-mass, ¹³C and ¹H NMR spectroscopy) and physico-chemical measurements including elemental analysis, magnetic susceptibility and conductivity measurements represented square planar structure for all complexes. Substituted and unsubstituted benzohydrazides (1-4) have shown monodentate behavior forming complexes of general formula [PdL ₂Cl₂]. However, pyridine-carbohydrazides (5 and 6) were coordinated in bidentate fashion of [PdLCl₂] general formula producing stable five-membered chelate ring. All palladium complexes were found to be considerably more potent inhibitors of DPPH free radical compared to free hydrazides. These complexes are even stronger DPPH scavengers than standard antioxidant propyl gallate. The complexes have also shown good superoxide scavenging ability compared to inactive free hydrazides, however complexes are weaker superoxide scavengers than ascorbic acid, a standard superoxide inhibitor. An interesting structure activity relationship has been evaluated.

Full text of DOI:10.1016/j.saa.2013.05.064

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 683–689
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopic and radical scavenging studies of
palladium(II)-hydrazide complexes
Qurrat Ul Ain a,1, Uzma Ashiq a,1, Rifat Ara Jamal a,1, Mohammad Mahrooof-Tahir b,c,
⇑
a Department of Chemistry, University of Karachi, Karachi 75270, Pakistan
b Department of Chemistry and Earth Sciences, Qatar University, P.O. Box 2713, Doha, Qatar
c Department of Chemistry, St. Cloud State University, 720 Fourth Ave. S., St. Cloud, MN 56301, United States
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
A new generation of palladium(II) hydrazide complexes are studied.
Characterization is based on the basis of their physical, spectral and analytical data.
The superoxide and DPPH radical scavenging properties are also determined.
DPPH and superoxide radical scavenging is enhanced upon coordination.
a r t i c l e i n f o
a b s t r a c t
Article history:
In present study, a series of palladium(II) complexes with biologically active hydrazide ligands have been
synthesized, characterized and screened for their antioxidant (superoxide and DPPH radical scavenging)
properties. Spectral studies (FT-IR, EI-mass, 13C and 1H NMR spectroscopy) and physico-chemical mea-
surements including elemental analysis, magnetic susceptibility and conductivity measurements repre-
sented square planar structure for all complexes. Substituted and unsubstituted benzohydrazides (1–4)
have shown monodentate behavior forming complexes of general formula [PdL2Cl2]. However, pyridine-
carbohydrazides (5 and 6) were coordinated in bidentate fashion of [PdLCl2] general formula producing
stable five-membered chelate ring. All palladium complexes were found to be considerably more potent
inhibitors of DPPH free radical compared to free hydrazides. These complexes are even stronger DPPH
scavengers than standard antioxidant propyl gallate. The complexes have also shown good superoxide
scavenging ability compared to inactive free hydrazides, however complexes are weaker superoxide
scavengers than ascorbic acid, a standard superoxide inhibitor. An interesting structure activity relation-
ship has been evaluated.
Received 24 January 2013
Received in revised form 21 April 2013
Accepted 19 May 2013
Available online 30 May 2013
Keywords:
DPPH radical
Superoxide
Antioxidants
Palladium(II)
Hydrazide
Published by Elsevier B.V.
Introduction
The coordination chemistry of NAO donor ligands is an interest-
ing area of research [5,6]. Hydrazides [RACOANHANH2] and their
Metals have been used in medicines for centuries. After the
success of cis-platin as an anticancer agent, an interest in the study
of the palladium(II) based chemotherapies has been advocated due
to significantly similar coordination properties of Pt(II) and Pd(II)
[1]. Numerous palladium complexes with comparable antitumor
activity and enhanced antiviral, antibacterial and antifungal
activity have been synthesized [1–4]. These promising results
encourage further research in this field, for further applications.
analogues have continued to attract interest in the literature
because of their ability to readily coordinate to a variety of transi-
tion metals using carbonyl oxygen and amino nitrogen as donor
atoms forming five-membered ring [7–9]. There are strong and
documented evidences that a large number of hydrazides and their
complexes exhibit diverse biological activities including
antimycobacterial, antitumour, antiinflammatory, trypanocidal,
leishmanicidal, anti-HIV, antidiabetic, antimalarial, anticonvulsant,
antifungal and anthrax lethal factor inhibiting properties [10–23].
Reactive oxygen species (ROS) includes free radicals, which are
the products of several normal biological pathways [24], attack
biomolecules leading to the onset of variety of degenerative dis-
eases such as cancer, Parkinson’s disease, heart diseases, chronic
inflammation and premature aging [25]. Superoxide anion ðOÅ2Þ is
a predominant cellular free radical, which is produced by variety
⇑
Corresponding author at: Department of Chemistry, St. Cloud State University,
720 Fourth Ave. S., St. Cloud, MN 56301, United States. Tel.: +1 320 308 3198/+974
6680 4352; fax: +1 320 308 6041.
E-mail addresses: ainychem85@gmail.rcom (Q.U. Ain), uzzmma@yahoo.com
(U. Ashiq), rifat_jamal@yahoo.com (R.A. Jamal), mmahroof@stcloudstate.edu,
mmahroof@qu.edu.qa (M. Mahrooof-Tahir).
1
Tel.: +92 21 99261301 8x2290; fax: +92 21 99243190.
1386-1425/$ - see front matter Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.saa.2013.05.064

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Q.U. Ain et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 683–689
of cytosolic and membrane-bound enzymes like xanthine oxidase
[26]. The administration of exogenous superoxide dismutase
(SOD) as a therapeutic against oxidative injury is limited by its size,
price, charge and rapid degradation with consequent short life-
times in biological systems [27,28]. To circumvent such limita-
O
C
O
C
SOCl2 , CH3OH
B
R
OH
Room temp. , 5-6 h
R
OCH3
A
tions,
a great interest has been developed towards metal
complexes as SOD mimics with high OÅ2 scavenging activities for
their effect on preventing pathological diseases due to oxidative
stress [29,30]. DPPH (1,1-diphenyl-2-picryl hydrazyl) is a stable
free radical which provides a simple, rapid and convenient method
of screening of samples for their radical scavenging activity or anti-
oxidative potential [31].
Reflux , 4-5h C2H5OH, N2H5OH
O
C
NH2
R
N
H
C
Combination of physiologically active ligand and metal in one
molecule may result in synergism of their effect and reduced tox-
icity of resulting complexes [32]. Despite the above interesting
structural and biological properties, very little is reported on syn-
thesis and antioxidant properties of Pd(II) complexes. Studies of
the coordination behavior of hydrazides towards Pd(II) center upon
modification of their electronic and steric properties is also insuf-
ficient. Hence, there is a high demand for synthesis, structural
and biological studies of substituted hydrazides and their com-
plexes with palladium. These considerations have given sufficient
impetus for us to proceed in this direction.
1 - 11
7
8
9
10
11
R = 2-F-C6H4
R = 3-F-C6H4
R = 4-F-C6H4
R = C6H4-CH2
R = C6H4-NH
1
2
3
4
5
6
R = C6H5
R = 2-OCH3-C6H4
R = 3-OCH3-C6H4
R = 4-OCH3-C6H4
R = 4-pyridyl
R = 3-pyridyl
Scheme 1. Synthesis of ligands 1–11.
The present work describes the synthesis and characterization
of Pd(II) complexes with hydrazides as well as their superoxide
and DPPH radical scavenging activities to uncover the chemistry
and biochemistry of Pd(II)-hydrazide complexes.
Benzohydrazide (1). Colorless compound; 1H NMR (400 MHz,
DMSO): 7.80 (d, 2H, J = 6.8 Hz, H-2/H-6), 7.52 (dd, 2H, J = 6.8 Hz,
H-3/H-5), 7.46 (dd, 1H, J = 7.6 Hz, J = 6.8 Hz, H-4); 13C NMR
(300 MHz, DMSO): d 163.09, 160.35, 134.12, 131.10, 128.14; EI
MS: 136 (47, M+), 121 (2), 107 (4), 105 (95), 83 (2), 77 (84), 63
(4), 51 (100); Anal. Calc. for C7H8N2O (136): C 61.76, H 5.88, N
20.58; found: C 61.73, H 5.91, N 20.58.
Experimental
Materials
2-Methoxybenzohydrazide (2). Colorless compound; 1H NMR
(400 MHz, DMSO): 7.69 (dd, 1H, J = 7.6 Hz, J = 2.0 Hz, H-6), 7.44
(dd, 1H, J = 7.6 Hz, J = 8.0 Hz, H-4), 7.09 (dt, 1H, J = 8.0 Hz, H-5),
7.01 (d, 1H, J = 10.1 Hz, H-3); 13C NMR (300 MHz, DMSO): d
164.65, 156.74, 131.88, 130.07, 122.25, 120.35, 111.79, 55.68; EI
MS: 166 (37, M+), 152 (3), 135 (100), 121 (17), 105 (8), 92 (52),
77 (80), 64 (25), 51 (36); Anal. Calc. for C8H10N2O2 (166): C
57.83, H 6.02, N 16.86; found: C 57.85, H 6.03, N 16.83.
All reagent grade chemicals were purchased from Sigma–
Aldrich, BDH or Merck and used without further purification.
All the solvents were distilled before use.
Physical measurements
FT-IR spectra were recorded in the range of 4000–400 cmꢁ1 on
KBr disks using a Shimadzu 460 IR spectrophotometer. 1H NMR
and 13C NMR spectra in DMSO were recorded on a Bruker 300 spec-
3-Methoxybenzohydrazide (3). Colorless compound; 1H NMR
(400 MHz, DMSO): 7.4 (d, 1H, J = 7.6 Hz, H-6), 7.3 (d, 1H,
J = 6.4 Hz, H-5), 7.3 (s, 1H, H-2), 7.0 (d, 1H, J = 7.5 Hz, H-4); 13C
NMR (300 MHz, DMSO): d 165.56, 159.10, 134.68, 129.38, 119.16,
116.87, 112.07, 55.18; EI MS: 166 (19, M+),150 (20), 135 (100),
134 (18), 107 (30), 91 (40), 77 (51), 64 (33), 50 (37); Anal. Calc.
for C8H10N2O2 (166): C 57.83, H 6.02, N 16.86; found: C 57.81, H
6.01, N 16.89.
trometer using TMS as internal standard at 300 or 400 MHz for 1
H
NMR and 300 MHz for 13C NMR experiments. EI-mass analysis of
ligands was done on a Finnigan MAT 311-A apparatus. Magnetic
susceptibility data for powdered complexes were collected at room
temperature utilizing a magnetic susceptibility balance Sherwood
MSB Mk1 using sealed-off MnCl2 solution as calibrant. Elemental
(CHN) analysis was performed on Perkin Elmer 2400 series II
CHN/ S analyzer and Pd contents were determined by complexo-
metric EDTA back titration using xylenol orange as an indicator
and sodium nitrite as a selective masking agent [33]. Conductance
measurements were made using a Hanna (HI-8633) conductivity
meter.
4-Methoxybenzohydrazide (4). Colorless compound; 1H NMR
(300 MHz, DMSO): 7.79 (d, 2H, J = 9.0 Hz, H-2/H-6), 6.96 (d, 2H,
J = 8.8 Hz, H-3/H-5); 13C NMR (300 MHz, DMSO):
d 165.60,
161.41, 128.69, 125.49, 113.50, 55.27; EI MS: 166 (19, M+), 151
(5). 135 (100), 133 (8), 107 (30), 92 (38), 77 (51), 64 (33), 50
(37); Anal. Calc. for C8H10N2O2 (166): C 57.83, H 6.02, N 16.86;
found: C 57.81, H 6.01, N 16.89.
Isonicotinic benzohydrazide (5). Colorless compound; 1H NMR
(300 MHz, DMSO): 8.70 (dd, 2H, J = 6.0 Hz, J = 1.5 Hz, H-2/H-6),
7.73 (dd, 2H, J = 6.0 Hz, J = 1.5 Hz, H-3/H-5); 13C NMR (300 MHz,
DMSO): d 163.86, 150.16, 140.22, 120.95; EI MS: 137 (77, M+),
107 (19), 106 (97), 83 (3), 78 (97), 51 (100); Anal. Calc. for
C6H7N3O (137): C 52.55, H 5.10, N 30.65; found: C 52.52, H 5.13,
N 30.65.
General procedure for the synthesis of the hydrazide ligands
Different substituted carboxylic acids (20 mmol) (A) were
stirred with thionyl chloride (100 mmol) in dry methanol (75 ml)
for 5–6 h to synthesize corresponding methyl esters (B) (Scheme
1). After extraction of esters in chloroform, solvent was evaporated
and esters (66 mmol) were refluxed with hydrazine hydrate
(330 mmol) in ethanol (75 ml) for 4–5 h. A solid was obtained
upon removal of the solvent by rotary evaporation. The resulting
solid was washed with hexane to afford hydrazide ligand (C). The
spectral and analytical data are given below.
Nicotinic benzohydrazide (6). Colorless compound; 1H NMR
(300 MHz, DMSO): 8.95 (d, 1H, J = 1.8 Hz, H-2), 8.67 (dd, 1H,
J = 3.6 Hz, J = 1.2 Hz, H-6), 8.14 (dd, 1H, J = 6.0 Hz, J = 1.5 Hz, H-4),
7.48 (dd, 1H, J = 7.5 Hz, J = 3.6 Hz, H-5); 13C NMR (300 MHz,
DMSO): d 164.31, 151.73, 148.06, 134.64, 128.88, 123.46; EI MS:

Q.U. Ain et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 683–689
685
137 (40, M+), 121 (7), 106 (67), 93 (4), 80 (3), 78 (71), 64 (4),
51(100); Anal. Calc. for C6H7N3O (137): C 52.55, H 5.10, N 30.65;
found: C 52.53, H 5.11, N 30.66.
The analytical data of 2-fluorobezohydrazide (7), 3-fluorobe
zohydrazide (8), 4-fluorobezohydrazide (9), 2-phenylacetohydraz-
ide (10), 4-phenylsemicarbazide (11) will be reported elsewhere.
O
C
R
O
NH2
Pd
Cl
Cl
Cl
C
Cl
R
N
H
Pd
H
N
HN
R
N
H2N
C
O
H2
General procedure for the synthesis of the palladium(II)-hydrazide
complexes
5a
6a
R = 4-Pyridyl
R = 3-Pyridyl
1a
R = C6H5
2a
3a
4a
R = 2-OCH3-C6H4
R = 3-OCH3-C6H4
R = 4-OCH3-C6H4
The complexes 1a–4a were prepared in CH3CN while complexes
5a and 6a were synthesized in H2O. Complexation was carried out
at room temperature. A solution of metal salt PdCl2, (1 mmol) in
water or CH3CN (30 ml) was added slowly with stirring to a solu-
tion (30 ml) of hydrazide (2 mmol). The reaction mixture was stir-
red for about 3 h, during which time solid complex was separated.
The solid complex was filtered and washed with water or acetoni-
trile to remove the unreacted metal salt and ligand. Physical and
analytical data of complexes 1a–6a is given in Table 1.
Fig. 1. Structures of palladium(II) complexes.
Effective magnetic moments (
leff) were ranged from 0.185 to
0.257, showing diamagnetic behavior of complexes which may
be due to square planar geometry of complexes as previously re-
ported square planar Ni2+ (d8 system) complexes are diamagnetic
[34]. The diamagnetic nature of Pd(II) complexes is also evident
from sharp well defined signals in their NMR spectra. Molar con-
ductivities of complexes in their fresh DMSO solutions were in
Antioxidant properties
The test procedures for DPPH radical scavenging [20] and
superoxide radical scavenging [19] have already been reported.
the range of 1.01–10.32
X
ꢁ1 cm2 molꢁ1 indicating the non electro-
lytic nature of all complexes [35]. The CHN and Pd contents of com-
plexes 1a–4a are consistent with formula PdL2Cl2 while the
complexes 5a and 6a are assumed to have formula PdLCl2.
Insight in the DPPH antioxidant activity of Pd(II)-hydrazide complexes
through UV–Visible spectroscopy
The strength of metal–ligand bond depends on
which is favored by increased effective negative charge and or
bonding. -back bonding involves the donation of electron rich d
(or p) orbitals of metal into the electron deficient -orbitals of
unsaturated ligand. Hence metal acts as -electron donor while
unsaturated ligand is -electron acceptor [36]. In hydrazides ami-
no nitrogen is stronger -donor and soft base as compared to car-
bonyl oxygen, hence always coordinated with Pd2+, a soft acid.
r-bonding,
Concentration of complex 4a was varied (0–56
DMSO (1:2) solvent system while keeping the final concentration
of DPPH constant (69 M). After placing the series of solutions in
lM) in ethanol–
p-
p
l
p
dark for 30 min, each solution was scanned from 250 to 800 nm
on UV–Visible spectrophotometer.
p
p
r
Results and discussion
However carbonyl group could involve in both
r-bonding and p-
back bonding with Pd2+, if carbonyl group becomes electron defi-
cient. The replacement of a carbon atom of benzohydrazide with
more electronegative nitrogen atom may decrease the electron
Synthesis and physico-chemical properties
Hydrazide ligands (1–11) were synthesized according to
Scheme 1. The structures of six synthesized ligands (1–6) were
determined by IR, 1H NMR, 13C NMR mass and micro analytical
data (as detailed in experimental part). Combination of all physical,
chemical and spectral measurements have been used for the
prediction of chemical formula as well as structure of complexes
(1a–6a) shown in Fig. 1. The analytical and spectral data of ligands
(7–11) along with their complexes (7a–11a) will be reported
elsewhere.
Physical, chemical and analytical data for the characterization
of complexes (1a–6a) is given in Table 1. All the complexes are
yellow colored, non hygroscopic, amorphous and fairly stable at
room temperature. Complexes are immediately soluble in organic
coordinating solvents like DMF, DMSO and THF.
density on the neighboring carbonyl group promoting its
ing with electron rich Pd2+ center. The strong
-bonding of car-
bonyl group in pyridinecarbohydrazides along with -bonding by
p-bond-
p
r
amino group could account for their bidentate behavior as seen
in 5a and 6a. In case of benzohydrazides, the greater electron den-
sity at carbonyl group may prevent it from
p-bonding and since
carbonyl oxygen is poor -donor and a hard base, the benzohyd-
r
razides herein behave as monodentate ligands having only amino
nitrogen available for coordination with Pd2+. The presence of
intramolecular H-bonding in benzohydrazides could also account
for their monodentate behavior. In short, the electronic properties
of hydrazides could be used to tune the desired coordination
behavior of ligands towards Pd2+ center.
Table 1
Physical and analytical data of complexes 1a–6a.
a
Compound
Molar mass
Yield%
leff (B.M.)
KM
Elemental analysis, calc. (found)
Pd %
C %
H %
N %
1a
2a
3a
4a
5a
6a
449.64
509.69
509.69
509.69
314.47
314.47
45
73
80
85
81
80
0.203
0.219
0.257
0.252
0.185
0.193
9.75
1.01
1.93
1.19
10.32
3.17
23.67 (23.2)
20.89 (19.3)
20.89 (21.2)
20.89 (21.0)
33.90 (34.2)
33.90 (34.1)
37.38 (38.18)
37.68 (36.34)
37.68 (37.94)
37.68 (37.73)
22.90 (22.52)
22.90 (21.57)
3.56 (3.10)
3.92 (3.17)
3.92 (4.04)
3.92 (3.78)
3.10 (2.89)
3.10 (2.87)
12.46 (12.36)
10.99 (10.78)
10.99 (11.57)
10.99 (11.23)
13.30 (13.38)
13.30 (13.39)
a
Molar conductance values in
X .
ꢁ1 cm2 molꢁ1

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Q.U. Ain et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 683–689
Table 3
Spectroscopy
Comparison of NMR data of hydrazide ligands 1–6 and their Pd(II) complexes 1a–6a.
Compound 1H NMR (d ppm)
13C NMR (d ppm)
IR spectroscopy
IR spectral data of the ligands and their Pd(II) complexes are gi-
ven in Table 2. All the ligands exhibit a pair of fairly intense NAH
stretching bands in the range of 3372–3113 cmꢁ1 attributed to
asymmetric and symmetric hydrazinic amino group modes. The li-
NH2 NH
Aromatic protons C@O Aromatic carbons
1
1a
2
2a
3
3a
4
4a
5
5a
6
4.55 10.40 7.44–7.80
5.96 10.03 7.46–7.85
163.09 127.54–160.35
165.21 127.03–160.21
164.65 111.79–156.74
162.63 111.87–157.34
165.56 112.07–159.10
165.57 112.21–159.13
165.60 113.50–161.41
165.61 113.76–162.16
163.86 120.95–150.16
153.61 114.50–152.64
164.31 123.46–151.73
155.70 123.01–155.70
4.52
7.01
4.47
9.19 6.99–7.69
9.79 7.08–7.97
9.73 7.00–7.62
gands also displayed
a sharp band ranging from 3216 to
3011 cmꢁ1, tentatively assigned to NAH vibration of imino group
[37]. The sharp peaks in this region for ligands are indicative of
hydrogen bonded ANH protons. All the complexes showed consid-
erable positive or negative shift in bands (3499–3095 cmꢁ1) origi-
nating from amino NAH stretching frequency as compared to free
ligands. This indicates the coordination of terminal amino nitrogen
with Pd(II) [38,39]. The sharp increase or decrease in amino NAH
stretching frequency is due to increase or decrease in the stretch-
ing force constant of amino NAH respectively upon complexation.
The bands originating from imino NAH stretching in complexes are
in the range of 3204–3020 cmꢁ1. The bands attributed to imino
NAH in two complexes (5a and 6a) is broader than that observed
for other complexes. It can be hence inferred that in the pyridine-
carbohydrazide complexes (5a and 6a), there is no hydrogen bond-
ing but in all other complexes intramolecular NAH O hydrogen
bonding is present.
6.20 10.06 7.01–8.10
4.40
6.86
9.58 6.94–7.80
9.82 6.98–7.80
4.63 10.10 7.44–9.01
7.10 11.15 7.84–8.91
4.60
9.94 7.46–8.95
6a
6.95 11.11 7.30–9.15
also located by taking 1H NMR spectra of all the compounds in a
mixture of DMSO and D2O. A signal at about 4.5 ppm (observed
in all the ligands in pure DMSO) disappeared in the presence of
D2O showing exchange of NH2 protons with deuterium. The signals
for NH2 protons in ligands appeared in (4.40–4.63 ppm) range
which is considerably shifted in (5.96–7.10 ppm) range on
complexation with Pd+2 whereas the signals for aromatic protons
in ligands appeared in (6.94–9.01 ppm) range which is shifted in
(6.98–9.15 ppm) range upon complexation.
Other bands in the ligands and complexes due to NH2 bending,
aromatic C@C stretching and CAN stretching have also been iden-
tified [40–42] and shown in Table 2. All the ligands showed a
strong carbonyl stretching absorption around 1656 17 cmꢁ1
,
13C NMR spectroscopy
which is close to the reported value as amide have C@O absorp-
tions in the range from 1670 to 1640 cmꢁ1 [42,43]. For complexes
5a and 6a, the position of C@O stretching band shifts considerably
indicating the coordination of carbonyl oxygen with metal center
[3,37,38,44]. Coordination of carbonyl oxygen with Pd2+ is absent
in complexes 1a–4a as there is negligible shift in C@O stretching
frequency of respective ligands after complexation (Table 2). The
results suggest monodentate behavior in complexes 1a–4a with li-
gands having nitrogen (of hydrazinic amino group) available for
coordination while the carbonyl oxygen is supposed to be involved
in intramolecular hydrogen bonding with imino NH group. In the
complexes 5a and 6a, the ligands are thought to be coordinated
in bidentate fashion utilizing amino nitrogen and carbonyl oxygen
as donor atoms.
13C NMR spectra of ligands and their complexes in DMSO (Table
3) confirm the bonding modes explained in 1H NMR and IR spectra.
The number of carbon peaks in ligands and complexes agree well
with their expected values. The aromatic carbon signals in the
ligands are appear in the range of 111.79–161.41 ppm and the
carbon of AOCH3 group in compounds 2–4 and 2a–4a appeared
at 55.18–55.68 ppm, as detailed in Experimental part. The most
downfield signal in compounds 1–6 (163.09–165.60 ppm) is as-
signed to carbonyl carbon atom [46]. This C@O carbon signal is
shifted upfield by 9–11 ppm in 5a and 6a, which indicates the
coordination of carbonyl oxygen to the Pd(II) [43]. For the rest of
complexes (1a–4a) the position of C@O signal remains almost un-
changed and thus excludes their coordination of C@O group to the
Pd2+ (Table 3). Other aromatic carbon atoms in the free ligands and
corresponding complexes resonate in nearly the same region.
Based on the data presented above, the structure of all the
complexes (1a–6a) has been proposed as square planar as depicted
in Fig. 1. The pyridinecarbohydrazide ligands (5 and 6) have shown
bidentate behavior (OAN donor) forming 1:1 Pd-hydrazide
complexes while benzohydrazide ligands (1–4) were coordinated
in monodentate fashion (N-donor) producing 1:2 metal–ligand
complexes.
1H NMR spectroscopy
1H NMR spectra of the hydrazide ligands (1–6) and their com-
plexes (1a–6a) in DMSO revealed a singlet between 9.19 and
11.15 ppm attributed to NH group (Table 3). This indicates that li-
gands remain in their ketone (protonated) form in polar solvent
even after complexation and hence the metal (Pd2+) is not directly
bonded to NH group [37,45]. The signal due to NH2 protons was
Table 2
IR dataa of hydrazides 1–6 and their palladium(II) complexes 1a–6a.
Compound
NH2
Imino NH
C@O
NH2 bending
C@C
CAN
Other ms
1
1a
2
2a
3
3a
4
4a
5
5a
6
3299, 3199
3252, 3095
3372, 3304
3448, 3320
3294, 3207
3306, 3198
3314, 3188
3240, 3213
3304, 3113
3491, 3194
3321, 3207
3499, 3198
3011
3020
3216
3204
3071
3069
3061
3099
3011
3092
3015
3074
1661
1660
1655
1654
1639
1640
1651
1649
1668
1674
1673
1664
1616
1624
1615
1599
1583
1616
1622
1609
1633
1611
1651
1611
1578
1549
1503
1475
1529
1591
1568
1541
1556
1543
1595
1537
1348
1320
1242
1244
1252
1254
1254
1259
1333
1319
1338
1323
1142, 906, 797, 697, 505
1220, 902, 798, 694, 526
949, 760, 654, 590, 429
955, 748, 687, 594, 426
1038, 959, 698, 542, 573
1032, 945, 681
1113, 837, 656, 611, 500
1115, 845, 648, 613, 467
1136, 995, 845, 675, 436
1232, 1038, 847, 689, 540
1121, 953, 708, 631, 525
1119, 912, 691, 613, 503
6a
a
All values are in cmꢁ1
.

Q.U. Ain et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 683–689
687
Table 4
DPPH radical-scavenging activities of hydrazides 1–11 and their Pd(II) complexes
1a–11a.
Compound IC50
(
l
M)
133 0.06
0.02 5a
49 0.76
0.09 6a
121 0.84
10 0.09 7a
694 0.23
0.04 8a
Compound IC50 (lM) Compound IC50 (lM)
1
5
86 0.30
11 0.05
56 0.21
23 0.66
>1000
9
72 0.20
11 0.08
73 0.75
25 0.32
372 0.21
28 0.25
30 0.06
260 0.94
1a
2
8
9a
6
10
2a
3
9
10a
11
7
3a
4
12 0.06
32 0.70
11a
PGa
Pd saltb
8
4a
9
9
0.02
a
Propyl gallate (pos. control).
PdCl2.
b
Antioxidant properties
DPPH radical scavenging
Fig. 3. UV–Visible bands of DPPH, Pd(II) complex 4a and hydrazide ligand 4.
Free radical scavenging abilities of hydrazides and their palla-
dium complexes were determined by measuring the decrease in
the absorbance of purple stable free radical DPPH at 517 nm due
to the formation of its reduced diamagnetic stable form DPPH-H
(1,1-diphenyl-2-picryl hydrazine), which is yellow in color [47].
The DPPH radical-scavenging activities of the hydrazide ligands
(1–11) and their Pd(II) complexes (1a–11a) are shown in Table 4
and Fig. 2. The IC50 values of the complexes are in the range of
The observation that free hydrazide ligands did not show more
inhibition in contrast to the corresponding Pd complexes demon-
strates the essential role of the metal center in terms of inhibitory
potential. It is also interesting to note that the radical scavenging
potential of palladium salt, PdCl2, is very low (IC50 = 260 lM)
8–28 lM and those of the free hydrazide ligands 32–1000 lM.
Reactivity of different compounds towards DPPH radical is appar-
ently affected by the substituents on the ring of benzohydrazide li-
gands and their complexation with Pd(II). Among the hydrazide
ligands, compound 8 having meta-fluoro substituent on benzene
ring has lowest IC50 value (32
lM), which is comparable to that
of propyl gallate (30 M), a standard DPPH inhibitor. Other hydra-
l
zides are relatively less free radical scavengers than standard
inhibitor. All the complexes of Pd(II) have shown promising antiox-
idant activity as compared to corresponding free hydrazide ligands.
These results are in agreement with the previous studies with co-
balt and copper complexes [48,49]. It is interesting to note that all
the Pd(II) complexes exhibit potent antioxidant behavior as their
IC50 values are very lower compared to that of standard i.e. propyl
gallate. Since DPPH radical requires electron to pair off, it can react
with compounds having ability to donate an electron or hydrogen
radical [50]. The electron removal site in the hydrazides could be
a
or b hydrazinic nitrogen. Pd(II) provides an electron rich (d8 sys-
tem) center, which may increase the electron density on ligand
upon complexation, which seems to be responsible for marked in-
crease in the antioxidant activity of ligands after complexation.
Fig. 4. UV–Visible spectral monitoring of reaction between DPPH and complex 4a
ranging 3–16 lM.
140
260
>1000
372
694
120
100
80
60
40
20
0
Compounds
Fig. 2. Bar graph of hydrazide and their palladium(II) complexes showing DPPH scavenging activity.

688
Q.U. Ain et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 683–689
O2N
O2N
H
H
N
.
N
N
NO2
N
NO2
H
O2N
O2N
DPPH-H
H+
.
e
e
DPPH
O2N
N
N
NO2
O2N
DPPH
Scheme 2. Redox reactions for DPPH.
Table 5
compared to Pd(II)-hydrazide complexes. This suggests the impor-
tant role of hydrazide ligands coordinated with palladium centers
in DPPH activity.
Superoxide scavenging activities of hydrazidesa 1–11 and their Pd(II) complexes
1a–11a.
Compound IC50 (lM) Compound IC50 (lM) Compound IC50 (lM)
UV–Visible spectral studies indicate two step mechanism for
the reaction of DPPH with Pd-hydrazide complex. Fig. 3 shows
three peaks at 519 nm, 331 nm and 258 nm characteristic for DPPH
[51]. As the concentration of Pd(II)-hydrazide complex (4a)
increases, the visible band of DPPH starts disappearing with an
increase in absorption at 428 nm (Fig. 4). The new band is charac-
teristic of DPPHꢁ anion [52,53], which is produced by gaining an
electron from Pd(II)-hydrazide complex (Scheme 2). The intensity
1a
2a
3a
4a
47 1.2
84 2.0
64 0.8
70 1.0
5a
6a
7a
56 0.7
162 2.0
57 0.9
90 1.5
32 1.0
9a
10a
77 1.2
74 1.0
56 0.5
56 0.4
11a
8a
Pd saltc
AAb
a
b
c
All hydrazide ligands are inactive against SOD.
Ascorbic acid (pos. control).
PdCl2.
of band at 428 nm reaches to its maximum at 16 lM concentration
of the complex indicating complete conversion of DPPH to its anion
(Fig. 4). Further addition of Pd-hydrazide complex results in
disappearance of DPPHꢁ band which (Fig. 5). is attributed to the
conversion of DPPHꢁ anion to DPPH-H on accepting a H+ from
the complex [54]. The study revealed the two step mechanism
for the scavenging reaction of DPPH by Pd(II)-hydrazide complex
involving an electron transfer followed by a proton transfer.
Absence of any band corresponding to Pd(III) and Pd(IV) indi-
cates that Pd center is not involved in redox reaction. The very poor
antioxidant behavior of precursor metal salt (PdCl2) also confirms
that Pd(II) is not directly involved in the redox reaction between
DPPH and Pd(II)-hydrazide complex. However, the presence of
Pd2+ (electron rich d8 system) significantly facilitates the removal
of H from
a or b hydrazinic nitrogen of ligand in the complex as
compared to free hydrazide ligand. Electron transfer reactions are
known to be significantly accelerated by the presence of metal ions
[55,56]. The ease in the H transfer from ligand may involve the
coordination of DPPHꢁ anion with metal ion [54].
Superoxide dismutase activity
Superoxide scavenging ability of free hydrazide ligands, their
Pd(II) complexes and Pd(II) ion was determined using non-enzy-
matic PMS–NADH–NBT system. In this, OÅ2 anions derived from dis-
solved oxygen by PMS–NADH coupling reaction reduce NBT. The
decrease in absorbance at 560 nm due to consumption of OÅ2 anions
in the reaction mixture indicate antioxidative potential. The O2Å
scavenging data is shown in Table 5.
It is apparent from the results that inhibitory effect of tested
compounds (excluding ligands) on OÅ2 anions is dose-dependent
and percent inhibition of OÅ2 anions increases with increasing sam-
ple concentration in the tested range. The free hydrazide ligands
have shown no SOD mimetic activity whereas their Pd(II) com-
plexes are better superoxide scavengers exhibiting IC50 values in
the range from 47 to 162 lM. The results indicate that the com-
plexation of hydrazide ligands with Pd(II) ion play dominant role
in enhancing SOD like activity of hydrazides. However, Pd(II) com-
plexes are relatively less potent inhibitor of OÅ2 anions as compared
to standard antioxidant, ascorbic acid (IC50 = 32
lM). The precursor
Fig. 5. UV–Visible spectral monitoring of reaction between DPPH and complex 4a
ranging 16–56 lM.
Pd(II) has also shown good superoxide scavenging potential

Q.U. Ain et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 115 (2013) 683–689
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