Thermal and spectral studies of palladium(II) complexes

Article abstract of DOI:10.1007/s10973-006-7608-0

Palladium(II) complexes of type [Pd(L)Cl₂] [where L=2-aminopyridine-N-thiohydrazide (L¹), (2-aminopyridine-N-thio)-1,3- propanediamine (L²), benzaldehyde 2-aminopyridine-N-thiohydrazone (L³) and salicylaldehyde-2-aminopyridine-N-thiohydrazone (L ⁴)] have been synthesized. The thiohydrazide, thiodiamine and thiohydrazones can exist as thione-thiol tautomer and coordinate as a bidentate N-S ligand. The ligands found to act in bidentate fashion. The complexes have been characterized by elemental analysis, IR, mass, electronic, ¹H NMR spectroscopic studies, and TG/DTA study. Antifungal studies of some complexes were also carried out. Various kinetic and thermodynamic parameters like order of reaction (n), activation energy (E _a), apparent activation entropy (S ^# ) and heat of reaction (ΔH) have also been carried out for one complex.

Full text of DOI:10.1007/s10973-006-7608-0

Journal of Thermal Analysis and Calorimetry, Vol. 90 (2007) 2, 509–515
THERMAL AND SPECTRAL STUDIES OF PALLADIUM(II) COMPLEXES
A. K. Mishra1, S. B. Mishra2, N. Manav1 and N. K. Kaushik1*
1Department of Chemistry, University of Delhi, Delhi 110007, India
2Department of Chemistry, University of the Free State, 9866, South Africa
Palladium(II) complexes of type [Pd(L)Cl2] [where L=2-aminopyridine-N-thiohydrazide (L1), (2-aminopyridine-N-thio)-1,3-propane-
diamine (L2), benzaldehyde 2-aminopyridine-N-thiohydrazone (L3) and salicylaldehyde-2-aminopyridine-N-thiohydrazone (L4)] have
been synthesized. The thiohydrazide, thiodiamine and thiohydrazones can exist as thione-thiol tautomer and coordinate as a bidentate
N–S ligand. The ligands found to act in bidentate fashion.
The complexes have been characterized by elemental analysis, IR, mass, electronic, 1H NMR spectroscopic studies, and TG/DTA
study. Antifungal studies of some complexes were also carried out. Various kinetic and thermodynamic parameters like order of reac-
tion (n), activation energy (Ea), apparent activation entropy (S#) and heat of reaction (DH) have also been carried out for one complex.
Keywords: characterization, palladium, synthesis
Introduction
Experimental
Materials and methods
The thiohydrazides, thiodiamines and thiohydrazones
have aroused considerable interest in chemistry and
biology due to their significant antibacterial, anti-
malarial, antineoplastic and antiviral activities [1–4].
They are significantly affected by substitution at the
moiety’s N(4) position [5, 6]. The chemistry of transi-
tion metal complexes of thiohydrazides and
thiohydrazones has been receiving considerable at-
tention largely because of their pharmacological
properties [7]. The combination of thiohydrazides and
thiohydrazones with agents like platinum(II) or palla-
dium(II) that damage DNA produces synergistic inhi-
bition of tumor growth and may lead to improvements
in the effectiveness of cancer chemotherapy [8–10].
The ligands with sulphur, nitrogen and oxygen
donor atoms in their structures can act as good chelat-
ing agents for the transition and non-transition metal
ions [11–18]. The biological active compounds and the
thermochemical data in literature, concerning the
thiohydrazides and thiohydrazones, are essentially
originated from techniques such as thermogravimetry,
differential thermal analysis and differential scanning
calorimetry [19–21]. The main aim of the present work
is to synthesize complexes of Pd(II) and to study the
thermal decomposition. We have also studied the
antifungal activity of some complexes. Thermody-
namic parameters such as activation energy (Ea), ap-
parent activation entropy (S#) and enthalpy change
(DH) for the dehydration and decomposition reactions
of the complexes have been evaluated.
All the reagents used were AR grade.
The analysis of CHNS/O contents of ligands and
metal complexes were done on Elementar
Analysensysteme GmbH Vario El-III. IR spectra
were recorded on Perkin-Elmer spectrum 2000 FTIR
spectrometer using KBr disc. Electronic spectra were
recorded using DMSO as solvent on Shimadzu
UV-Visible spectrophotometer Model 1601. Conduc-
tance measurements were carried out on Digital
Conductometer Model PT-827, India in DMSO sol-
vent. Model Jeol SX102/DA-600 (KV 10MA) was
used for recording mass spectra of the ligands using
methanol as solvent. 1H NMR was recorded in
d6-DMSO on Brucker spectrospin 300 spectrometer.
TG/DTA curves for the complexes were re-
corded on Shimadzu, model 60 WS thermal analyzer,
in static air at a heating rate of 10°C min–1. The plati-
num crucible was used with alumina as the reference
material.
Preparation of thiohydrazides
Preparation of 2-aminopyridine-N-thiohydrazides,
(2-aminopyridine-N-thio)-1,3-propanediamine, benz-
aldehyde-2-aminopyridine-N-thiohydrazone
salicylaldehyde-2-aminopyridine-N-thiohydrazone
were prepared by modified [3–6], literature
method [22].
and
*
Author for correspondence: narenderkumar_kaushik@yahoo.co.in
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

MISHRA et al.
Preparation of 2-aminopyridine-N-thiohydrazide (L1)
Preparation of (2-aminopyridine-N-thio)-1,3-pro-
panediamine (L2)
In a three-necked round bottle flask 4.71 g (0.05 mol)
of 2-aminopyridine dissolved in 20 mL methanol
taken and chilled it. To this, a chilled solution of 2.8 g
(0.05 mol) potassium hydroxide in 1 mL water and
10 mL methanol was mixed with constant stirring.
The mixed solution was treated with an ice-cold solu-
tion of 3.02 mL (0.05 mol) carbon disulphide (density
1.26 g cm–3) in 3 mL methanol. The temperature of
the reaction mixture was maintained below 10°C by
keeping flask in a freezing mixture of common salt
and ice. During the process, a yellowish-white crys-
10.4 g (0.05 mol) 2-aminopyridine dithiocarbamate
prepared as earlier was suspended in 15 mL methanol
and treated with freshly prepared potassium
chloroacetate [(0.05 mol){potassium chloroacetate
was obtained by dissolving 4.73 g chloroacetic acid in
3 mL ice-cold water and mixing it in 5 mL aqueous
solution of 2.8 g potassium hydroxide}]. The temper-
ature of the reaction mixture was kept at about 40°C
for an hour and the contents were left overnight at
room temperature. After 24 h methanolic solution of
4.36 mL (0.05 mol) 1,3-propanediamine (density
0.85 g cm–3) was added to the reaction mixture. The
content was then heated on a water bath for about
45 min when the desired product began to separate
out. It was cooled in ice for 24 h and filtered.
(2-aminopyridine-N-thio)-1,3-propanediamine thus
obtained was recrystallized from methanol and dried
under vacuum over CaCl2 at room temperature.
The reactions taking place in the preparation are
shown below
talline
precipitate
of
2-aminopyridine
dithiocarbamate separated. It was filtered, washed
with ice-cold aqueous methanol. The product was
then suspended in 10 mL methanol and treated with
freshly prepared potassium chloroacetate [{4.73 g
chloroacetic acid (0.05 mol) in 3 mL ice-cold water
and mixing it in 5 mL aqueous solution of 2.8 g potas-
sium hydroxide}]. The temperature of the reaction
mixture was kept at about 40°C for an hour and the
contents were left overnight at room temperature. Af-
ter 24 h methanolic solution of 2.44 mL (0.05 mol)
hydrazine hydrate (density 1.026 g cm–3) was added
to the reaction mixture. The content was then heated
on a water bath for about 45 min when the desired
product began to separate out. It was cooled in ice for
24 h and filtered. 2-aminopyridine-N-thiohydrazide
thus obtained was recrystallized from methanol and
dried under vacuum over CaCl2 at room temperature.
The reactions taking place in the preparation are
shown below
Scheme 2 CHNS – analysis; found (calculated) %; C: 52.68
(51.40), H: 6.63 (6.67), N: 25.59 (26.67),
S: (14.93) 15.20. Mass spectra (CH3OH);
m/z: 94.52 (M+), 210.89
Preparation of thiohydrazones
The thiohydrazones were prepared by refluxing the
thiohydrazides with corresponding aldehydes or ke-
tones in methanol.
Preparation of benzaldehyde-2-aminopyridine-N-
thiohydrazone (L3)
5.04 g (0.03 mol) of 2-aminopyridine-N-thio-
hydrazide and 3.05 mL (0.03 mol) of benzaldehyde
(density 1.044 g cm–3) were refluxed in methanol for
3 h. On cooling yellowish mass obtained was filtered
Scheme 1 CHNS – analysis; found (calculated) %; C: 43.46
(42.86), H: 4.28 (4.77), N: 34.23 (33.33),
S: (18.67) 19.0. Mass spectra (CH3OH);
m/z: 94.76 (M+), 168.76
510
J. Therm. Anal. Cal., 90, 2007

PALLADIUM(II) COMPLEXES
and washed with cold methanol. It was recrystallized
from hot methanol.
Analysis
Estimation of chloride
Chloride was determined gravimetrically as silver
chloride [23]. The sample was acidified with
5N HNO3 and 1% silver nitrate solution was added,
till the precipitation was complete. The precipitate
was filtered through a G-4 sintered glass crucible,
dried at 110°C and weighed as silver chloride.
Estimation of metal [23]
Scheme 3 CHNS – analysis; found (calculated) %; C: 61.17
(60.93), H: 4.73 (4.70), N: 20.99 (21.87),
S: (11.86) 12.50. Mass spectra (CH3OH);
m/z : 94.25 (M+), 256.73
The synthesized palladium complexes were decom-
posed by nitric acid and it was precipitated as palla-
dium dimethylglyoxime complex by adding 1% solu-
tion of dimethylglyoxime in 95% ethanol till
palladium was completely precipitated. The orange
yellow precipitates of palladium dimethylglyoxime,
washed with water and dried at 100°C to constant
mass and weighed as Pd(C4H7O2N2)2.
Preparation of salicylaldehyde-2-aminopyridine-N-
thiohydrazone (L4)
5.04 g (0.03 mol) of 2-aminopyridine-N-thio-
hydrazide and 3.15 mL (0.03 mol) of salicylaldehyde
(density 1.164 g cm–3) were refluxed in methanol for
3 h. On cooling yellowish mass obtained was filtered
and washed with cold methanol. It was recrystallized
from hot methanol.
In vitro antifungal activity
Most of the compounds have been screened in vitro
against Aspergillus fumigatus, Aspergillus flavus and
Aspergillus niger. Among several methods [24] avail-
able, the one method [25, 26] that is common in use in
recent times has been adopted.
Microbroth dilution assay
The susceptibility of the fungi to various fractions of
compounds was assayed by microbroth dilution
method. Sabouraud dextrose medium was dissolved
in glass double distilled water and autoclaved at
10 psi for 15 min. A volume of 90 mL of medium was
added to the wells of cell culture plates (Nunc
Nunclon). The different concentrations in the range of
1000–15 mg mL–1 of various fractions were prepared
in duplicate wells and then the wells were incubated
with 10 mL of conidial suspension containing 1·104
conidia. The plates were incubated at 37°C and exam-
ined macroscopically after 48 h for the growth of
Aspergillus mycelia. The activity was represented as
–ve if growth was there and +ve if medium appeared
clear without any visible growth of Aspergillus
fumigatus, Aspergillus flavus and Aspergillus niger.
Scheme 4 CHNS – analysis; found (calculated) %; C: 56.96
(57.35), H: 4.26 (4.41), N: 19.47 (20.59),
S: (11.98) 11.76. Mass spectra (CH3OH);
m/z: 94.16 (M+), 272.43
Preparation of complexes
Preparation of thiohydrazide[Pt(L)2Cl2] complexes
where L=L1, L2, L3 and L4
The corresponding ligand L [where L=L1 (0.084 g,
0.5 mmol), L2 (0.105 g, 0.5 mmol), L3 (0.128 g,
0.5 mmol) and L4 (0.141 g, 0.5 mmol) in methanol
and added with constant stirring to 1 N HCl solution
of palladium chloride (0.089 g, 0.5 mmol). The solu-
tion was stirred for 4–5 h. The colour of solution
changed yellow to yellowish orange. It was washed
with double distilled water several times and dried in
desiccator over CaCl2 under vacuum.
Spore germination inhibition assay
The basic method for spore germination inhibition
was modified and used to evaluate the activity of vari-
ous test fractions against fungi. The Aspergillus
fumigatus, Aspergillus flavus and Aspergillus niger
were grown on Sabouraud dextrose agar plates and
their homogenous conidial suspension was prepared
J. Therm. Anal. Cal., 90, 2007
511

MISHRA et al.
Table 1 Elemental analysis of the complexes
Found (calculated)/%
Complexes
C
H
N
S
Cl
Metal
21.36
(20.87)
2.31
(2.32)
15.94
(16.23)
9.17
(9.28)
29.54
(29.58)
30.75
(30.72)
Pd(L1)Cl2
Pd(L2)Cl2
Pd(L3)Cl2
Pd(L4)Cl2
28.18
(27.91)
3.67
(3.62)
14.43
(14.47)
7.89
(8.27)
18.19
(18.35)
27.08
(27.39)
36.12
(36.03)
2.78
(2.77)
12.84
(12.93)
7.26
(7.39)
16.12
(16.39)
24.60
(24.48)
34.63
(34.74)
2.63
(2.67)
12.53
(12.47)
7.08
(7.13)
15.58
(15.81)
23.89
(23.61)
Table 2 Electronic spectra of the complexes
in the Sabouraud maltose broth. The conidia were
counted and their number in the suspension was ad-
justed to 1·104 mL–1. Various concentrations of the
test samples in 90 mL of culture medium were pre-
pared in 96-well flat bottom micro-culture plates
(Nunc Nunclon) by double dilution method. The
wells were prepared in duplicates for each concentra-
tion. The wells were inoculated with 10 mL of
conidial suspension containing 100±5 conidia. The
plates were incubated at 37°C for 10 h and then exam-
ined for spore germination under inverted microscope
(Nikon Diphot). The number of germinated and
non-germinated conidia was recorded. The present
spore germination inhibition (PSGI) was calculated
using following formula:
Complexes
Pd(L1)Cl2
lmax (nm)
log (e)
270
308
389
3.44
2.64
2.07
284
326
394
3.79
2.82
2.16
Pd(L2)Cl2
Pd(L3)Cl2
Pd(L4)Cl2
281
351
403
3.85
3.41
2.38
293
356
410
4.36
3.74
2.91
No. of conidia germinated in drug treated well
PSGI=100–
×100
No. of conidia germinated in control well
Infrared spectra
In the present studies, the IR spectra (Table 3) of the
thiohydrazide, thiodiamine and thiohydrazones con-
tain groups –NH–C=S as a potential bond forming
site. Complexes of the ligands containing thioamide
group have been extensively studied. The thioamide
Results and discussion
Elemental analysis
Elemental analysis (Table 1) reveals the purity of the
complexes. All complexes are soluble in DMSO. The
molar conductance values of the isolated complexes
measured in DMSO are found to be less than
15 ohm–1 cm2 mol–1 suggesting their non-electrolytic
nature.
group displays four prominent IR bands due to n(C=N)
,
n(N–H), n(C–N) and n(C=S). The thioamide band I have
been observed in the region of 1460–1630 cm–1 which
is due to n(C=N) and n(N–H) stretching vibrations. The
thioamide band II is observed in the range of
1300–1380 cm–1. The band has major contribution
from n(N–H) and minor contributions from n(C=N)
stretching vibrations. The thioamide band III
1210–1290 cm–1 has major contributions from n(C=S)
vibration. All these IR bands are shifted on complex
formation due to increased double bond character of
C–N group on complexation. Thioamide IV
750–900 cm–1 is due to n(C=S) as major contributor and
n(C–N) as minor. This is shifted to lower frequency on
complexation indicates the coordination to metal ion
is through thioamide sulphur C=S. This shift is ~
120–150 cm–1, if coordination is through thiol sul-
phur [29] and 30–40 cm–1, if coordination is through
the thione sulphur [30].
Electronic spectra
The electronic spectra (Table 2) of the thiohydrazides
(L1), thiodiamines (L2), thiohydrazones (L3 and L4)
show spectral bands because of p®p* and n®p*
transition. On complexation these bands are shifted.
Strong charge transfer transitions may interfere and
prevent the observation of all the expected bands
[27, 28]. Strong bands ~340 nm is assignable to a
combination of metal ligand charge transfer
(M®LCT) and d–d band. The very intense band ~390
is assignable to combination of sulphur®metal
charge transfer (Lp®MCT) and d–d bands.
512
J. Therm. Anal. Cal., 90, 2007

PALLADIUM(II) COMPLEXES
Table 3 IR spectra of the complexes
Wavenumber/cm–1
Complexes
nC=N
nN–N
nC=S
nM–N
nM–S
nM–Cl
L1
Pd(L1)Cl2
L2
–
–
1025
1026
1032
1022
1026
1020
1036
1032
876
856
880
–
461
–
–
379
–
–
262
–
–
Pd(L2)Cl2
L3
Pd(L3)Cl2
L4
–
808
890
761
888
789
470
–
380
–
273
–
1633
1635
1614
1624
475
–
390
–
298
–
Pd(L4)Cl2
462
389
297
Table 4 Antifungal study of the complexes
A. fumigatus/mg mL–1
A. flavus/mg mL–1
A. niger/mg mL–1
S. no.
Complexes
MDA
250
–
PSGI
250
–
MDA
PSGI
250
–
MDA
PSGI
500
–
1
2
3
4
Pd(L1)Cl2
Pd(L2)Cl2
Pd(L3)Cl2
Pd(L4)Cl2
250
–
500
–
500
250
500
250
250
500
250
500
500
500
500
500
where MDA = micro dilution activity and PSGI = percent spore germination inhibition
8.3 (s, 1Hc, –CH), 7.4–6.26 (m, 5H, –Ar–H), 11.3 (br s, 1Hd,
–OH)
[Pd(L4)Cl2] d(ppm) 8.5–7.41 (m, 4H, Py–H), 10.2 (br s, 1Ha,
–NH), 9.11 (br s, 1Hb, –NH),
8.2 (s, 1Hc, –CH), 7.8-6.9 (m, 5H, –Ar–H), 11.23 (br s, 1Hd,
In all the Pd(II) complexes the metal nitrogen vi-
bration, n(M–N) are assigned to the new bands [31] in
the far IR between 460–490 cm–1, while in the region
between 350–390 cm–1 gives metal–sulphur, n(M–S)
band stretching [32]. The band at ~330–270 cm–1 is
assigned due to n(Pd–Cl) stretching vibrations.
–OH)
The 1H NMR spectrum of thiohydrazides,
thiodiamines and thiohydrazones [33, 34] shows two
signals at d ~ 9.0–10.3 and d ~ 4.0 ppm, due to the
presence of NH protons which are lost on D2O ex-
change. This is observable in the complexes also sug-
gesting that hydrogen bonding to the solvent occurs in
the complexes as well as free ligands. The resonance
assigned to aldehyde –CH is generally shifted upfield,
indicating coordination of azomethine nitrogen [35].
NMR spectra
1H NMR spectra of ligands and complexes were re-
corded in d6–DMSO taking TMS as internal stan-
dards.
L1
d(ppm) 7.73–6.4 (m, 4H, Py–H), 9.16 (br s, 1Ha,
–NH), 3.43 (br s, 2Hb, –NH2)
[Pd(L1)Cl2] d(ppm) 8.2–7.78 (m, 4H, Py–H), 9.19 (br s, 1Ha,
–NH), 3.73 (br s, 2Hb, –NH2)
L2
3.3 (t, 4Hb, –CH2), 1.8 (m, 2Hc, –CH2),
3.67 (br s, 2Hd, –NH2)
d(ppm) 7.7–6.6 (m, 4H, Py–H), 9.12 (br s, 1Ha, –NH),
In vitro antifungal study
In the current study (Table 4) of some synthesized
was tested against pathogenic Aspergillus fumigatus,
Aspergillus flavus and Aspergillus niger. The mini-
mum inhibitory concentrations (MICs) by microbroth
dilution assays (MDA) and percent spore germination
inhibition assays (PSGIA) being 250–500 mg mL–1
(Table 4). Amphotericin B was used as reference drug
for fungi. The activity of compounds did not appear to
be very high. But it is known fact that Aspergillii have
hard chitinous outer wall and therefore, higher con-
centration of fungicidal compounds may be often re-
quired to kill the fungi.
[Pd(L2)Cl2] d(ppm) 8.37–7.5 (m, 4H, Py–H), 9.27 (br s, 1Ha,
–NH), 3.45 (t, 4Hb,
–CH2), 1.83 (m, 2Hc, –CH2), 3.8 (br s, 2Hd, –NH2)
L3
d(ppm) 7.7–6.51 (m, 4H, Py–H), 9.6 (br s, 1Ha, –NH),
9.16 (br s, 1Hb, –NH), 8.23 (s, 1Hc, –CH),
7.3–6.4 (m, 5H, –Ar–H)
[Pd(L3)Cl2] d(ppm) 7.7–6.51 (m, 4H, Py–H), 9.6 (br s, 1Ha,
–NH), 9.21 (br s, 1Hb, –NH),
8.1 (s, 1Hc, –CH), 7.7–6.8 (m, 5H, –Ar–H)
L4
d(ppm) 7.21–6.42 (m, 4H, Py–H), 10.42 (br s, 1Ha,
–NH), 9.1 (br s, 1Hb, –NH),
J. Therm. Anal. Cal., 90, 2007
513

MISHRA et al.
Table 5 Thermal data of the complexes
TG (Coats–Redfern method)
Step
No.
Complexes
DH/kJ g–1
Temperature
range/°C
Ea/kJ mol–1
n
S#/J K–1 mol–1
I
29–450
1
1
17.35
8.15
3.96
0.32
55.58
85.12
Pd(L1)Cl2
II
450–842
Thermal study
Conclusions
The TG and DTA study in air atmosphere have been
carried out for one complex. Thermal studies were uti-
lized to elucidate the number of kinetic and thermody-
namic parameters. From TG curve, order of reaction
(n), activation energy (Ea), apparent activation entropy
(S#) were enumerated by the Coats–Redfern method
[36]. From the DTA curves, the heat of reaction was
calculated. Thermal data were tabulated in Table 5.
The shifts observed from IR, electronic and NMR
spectra on complexation, indicates the coordination
to metal ion is through nitrogen and sulphur. In the
antifungal activity against Aspergillus fumigatus,
Aspergillus flavus and Aspergillus niger, the mini-
mum inhibitory concentrations (MICs) by microbroth
dilution assays (MDA) and percent spore germination
inhibition assays (PSGIA) was found to be
250–500 mg mL–1. The thermal data (TG/DTA) of the
complex indicates that for all the two steps, the reac-
tion order is found to be one and heat of reaction for
the first step are found to be higher. On the basis of
these spectroscopic studies the probable structure of
the complexes is:
[Pd(L1)Cl2)] complex
TG curve (Fig. 1) shows two-step decomposition. The
first decomposition step (29–450°C) corresponds to
the loss of all organic moieties for which observed
and calculated mass losses are 48.19 and 48.69% re-
spectively. The final step starts at 450°C and the final
observed mass loss at 842°C is 68.05%, which corre-
sponds to the palladium metal residue, calculated
mass loss for which is 68.8%.
The DTA profile (Fig. 2) shows the only one
exotherm at 348°C corresponds to the oxidation of or-
ganic moieties.
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Fig. 2 DTA curve of [Pd(L1)Cl2] complex
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PALLADIUM(II) COMPLEXES
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Received: March 23, 2006
Accepted: December 6, 2006
OnlineFirst: February 26, 2007
DOI: 10.1007/s10973-006-7608-0
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