A palladium(II) complex: Synthesis, structure, characterization, electrochemical behavior, thermal aspects, BVS calculation and antimicrobial activity

Article abstract of DOI:10.1016/j.poly.2013.03.064

The reaction of equimolar proportions of Na₂[PdCl₄] and 1-(4-methylimidazol-5-yl) phenylhydrazonopropane-2-one oxime (LH), a 1:1 Schiff-base condensate of 1-hydrazono-1-phenyl-propan-2-one oxime (1) and 4-methylimidazole-5-carboxalde

Full text of DOI:10.1016/j.poly.2013.03.064

Polyhedron 56 (2013) 211–220
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A palladium(II) complex: Synthesis, structure, characterization, electrochemical
behavior, thermal aspects, BVS calculation and antimicrobial activity
Chiranjan Biswas a, Miaoli Zhu b, Liping Lu b, Sudipta Kaity c, Mousumi Das c, Amalesh Samanta c,
Jnan Prakash Naskar a,
⇑
a Department of Chemistry, Jadavpur University, Calcutta 700 032, India
b Institute of Molecular Science, Key Laboratory of Chemical Biology and Molecular Engineering of the Education Ministry, Shanxi University, 92 Wucheng Road, Taiyuan,
Shanxi 030006, People’s Republic of China
c Division of Microbiology, Department of Pharmaceutical Technology, Jadavpur University, Calcutta 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of equimolar proportions of Na2[PdCl4] and 1-(4-methylimidazol-5-yl) phenylhydrazono-
propane-2-one oxime (LH), a 1:1 Schiff-base condensate of 1-hydrazono-1-phenyl-propan-2-one oxime
(1) and 4-methylimidazole-5-carboxaldehyde, in methanol gives rise to [Pd(L)(Cl)] (2) in a satisfactory
yield. The title monomeric palladium(II) complex, 2, has been characterized by C, H and N microanalyses,
1H and 13C NMR, FAB-MS, FT-IR, UV–Vis spectra, molar electric conductivity measurements and room
temperature magnetic susceptibility measurements. The X-ray crystal structures of 1 and 2 have been
determined. The structure of 1, a precursor of the ligand (LH), shows that the methyl and phenyl groups
are in an ‘anti’ disposition. Compound 1 crystallizes in the monoclinic space group P21 with a = 6.9503(5),
b = 6.4535(3), c = 10.1631(6) Å, V = 455.04(5) Å3 and Z = 2. The structure of 2 reveals that it is a distorted
square-planar palladium(II) compound. The palladium center is in an ‘N3Cl’ coordination chromophore.
Received 9 January 2013
Accepted 26 March 2013
Available online 10 April 2013
Keywords:
Oxime
Palladium
Structure
BVS
Redox
Complex
2 crystallizes in the tetragonal space group I41/a with a = 22.5744(3), b = 22.5744(3),
Cytotoxicity
c = 13.2967(2) Å, V = 6776.05(16) Å3 and Z = 16. The thermal and electrochemical aspects of 2 have been
studied. Electrochemical studies in DMF show a Pd(II) to Pd(III) oxidation at 0.573 V, along with a reduc-
tion of Pd(II) to Pd(I) at ꢀ0.757 V versus Ag/AgCl. A Bond-Valence Sum (BVS) model calculation was per-
formed to assign the oxidation state of the palladium center in 2. The in vitro antimicrobial activity of 2
was tested against both Gram positive and Gram negative bacteria. Complex 2 exhibits satisfactory bac-
teriostatic activity.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
chemistry [11,12]. Apart from catalytic aspects, Pd(II) compounds
have inherent cytotoxic behavior. On the basis of the structural
In terms of annual demand, palladium is one of the most impor-
tant platinum group metals [1]. Pd(II) complexes have attracted
renewed interest due to their wide-spread applications in organic
syntheses [2], catalytic processes [3], biological and pharmacolog-
ical usage [4]. The palladium catalyzed Suzuki–Miyaura cross-cou-
pling reaction represents one of the most widely used processes for
the synthesis of biaryls [5–8]. In addition, palladium-catalyzed
Heck reactions of aryl halides with alkenes have also become one
of the most powerful tools in organic synthesis for the making of
the carbon–carbon bond [9,10]. The use of Pd-catalysts in Suzu-
ki–Miyaura and Heck-Mizoroki cross-coupling reactions in aque-
ous media stems interest from a viewpoint of green sustainable
analogy (d8 ions in a square-planar geometry) and the thermody-
namic difference with Pt(II) complexes, there is much interest in
the study of palladium(II) complexes as potential anticancer drugs,
especially those bearing the chelating ligands [13–15]. In this con-
text it is interesting to note that the first monomeric Pd(II) complex
having excellent antiproliferative activity, as demonstrated by
in vitro experiments, was a mixed ligand Pd(II) complex [16]. The
associated ligand was, as usual, a thiosemicarbazone based ligand
with triphenyl phosphine as an ancillary ligand [17,18]. Curiously
such type of bioactivity of a monomeric Pd(II) compound with an
N,O-donor oxime-based ligand is unprecedented. Herein we wish
to report the synthesis, characterization, structure, thermal and
redox behavior of a monomeric Pd(II) complex synthesized from
an oxime-based ligand. The antimicrobial activity of the ligand,
1-(4-methylimidazol-5-yl) phenylhydrazonopropane-2-one oxime
(LH), and its palladium(II) compound, 2 was investigated against
⇑
Corresponding author. Fax: +91 33 2414 6223.
E-mail addresses: asamanta61@yahoo.co.in (A. Samanta), jpnaskar@rediffmail.
com (J.P. Naskar).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.03.064

212
C. Biswas et al. / Polyhedron 56 (2013) 211–220
seventeen bacterial cell lines. Semi-empirical BVS calculations
were also undertaken to assign the oxidation state of the central
palladium ion.
compounds separated. These were filtered, washed with 5 mL of
chilled methanol and subsequently dried in a vacuum desiccator
over fused CaCl2. The product (1) is partly soluble in chloroform, ace-
tone and also in methanol. Yield: 872 mg (80%). M.p.:
162–164 °C. C9H11N3O (177.088): Anal. Calc. for C9H11N3O: C,
60.99; H, 6.26; N, 23.72. Found: C, 60.84; H, 6.27; N, 23.82%. FTIR
2. Experimental
(KBr)
m
[cmꢀ1]: 3389(s) [
(C@N) of oxime], 1208(s) [
m
(OH)], 3283(s) [
m
(NH2)], 1569(s) [
m(C@N)
2.1. Materials and measurements
of imine], 1467(s) [
m
m
(N–O)]. 1H NMR
(300 MHz, CDCl3, TMS) d [ppm]: 7.86 (2H, d, ortho-hydrogens of
the phenyl group attached to C2 and C6), 7.47 (2H, t, meta-hydro-
gens of phenyl ring attached to C3 and C5), 7.42 (1H, t, para-hydro-
gen of phenyl ring attached to C4), 7.18 (2H, s, hydrogens of the free
amine group), 2.15 (3H, s, methyl proton attached to C9), UV–Vis
All chemicals were of analytical reagent grade and used without
further purification. Palladium chloride and 4-methylimidazole-5-
carboxaldehyde were purchased from Sigma–Aldrich Chemicals
Pvt. Ltd. 1-Hydrazono-1-phenyl-propan-2-one oxime was
prepared by a procedure reported herein. The melting point was
determined by an electro-thermal digital melting point apparatus
(SUMSIM India) and is uncorrected. Microanalyses were performed
with a Perkin-Elmer 2400II elemental analyser. FTIR spectra (KBr
disc) [(vs) = very strong, (s) = strong, (m) = medium and
(w) = weak] were recorded with a Nicolet Magna-IR spectropho-
tometer (Series II). UV–Vis spectra (in DMF) were recorded on a
Shimadzu UV-160A spectrophotometer. 300 MHz 1H NMR spectra
of the ligand precursor, 1 and LH (in CDCl3: reference, TMS) were
recorded on a Bruker DPX300 spectrometer and the 13C NMR spec-
trum (observed frequency: 126 MHz) of LH (in DMSO-d6 with
reference to TMS) on a Bruker DPX500 spectrometer. 500 MHz 1H
and 13C NMR spectra (observed frequency: 126 MHz for 13C) of
the Pd(II) compound 2 were recorded in DMSO-d6 with reference
to TMS on a Bruker DPX500 spectrometer. The FAB mass spectra
in the positive ionization mode were recorded in m-nitrobenzyl
alcohol (MNBA) matrix on a JEOL Mass spectrometer (Model:
JMS-700), Japan. Solution conductivity measurements of 2 were
carried out in DMF and DMSO at room temperature on a Systronics
(India) direct reading conductivity meter (Model 304). Cyclic
voltammetric (CV) experiments were performed under nitrogen
in dry and degassed DMF using a BAS Epsilon electrochemical
workstation at 298 K. The conventional three-electrode assembly
is comprised of a Glassy Carbon (GC) working electrode, a plati-
num-wire auxiliary electrode and an Ag/AgCl reference electrode.
The supporting electrolyte is n-Bu4NClO4 (0.1 M). Thermal analyses
(TGA/DTA) were performed under a dynamic nitrogen atmosphere
(DMF) kmax[nm] (
e
max[Mꢀ1 cmꢀ1]): 273 (1.1504 ꢁ 104).
Crystals suitable for X-ray structure determination were ob-
tained by slow aerial evaporation of a moderately concentrated
solution of 1 in methanol.
2.1.1.2. 1-(4-Methylimidazol-5-yl) phenylhydrazonopropane-2-one
oxime, LH. 100 mg (0.565 mmol) of 1-hydrazono-1-phenyl-pro-
pan-2-one oxime (1) was dissolved in 15 mL of hot methanol to
get a light yellow solution. To this solution was added 62 mg
(0.564 mmol) of solid 4-methylimidazole-5-carboxaldehyde,
which was thoroughly dissolved. The reaction mixture was heated
under reflux for 4 h. Within 15 min of refluxing, a yellow color was
obtained. After refluxing, the dark yellow reaction mixture was left
in air for slow evaporation. After 2 days, the precipitated yellow
compound that was obtained was filtered, washed thoroughly with
diethyl ether and then dried in a vacuum desiccator over fused
CaCl2. The product, LH, is soluble in DMSO, chloroform and meth-
anol upon heating, but insoluble in acetonitrile and dichlorometh-
ane even on heating. Yield: 100 mg (66%). M.p.: 192–194 °C.
C
14H15N5O (269.12): Anal. Calc. for C14H15N5O: C, 62.43; H, 5.62;
N, 26.01; Found: C, 62.54; H, 5.49; N, 26.31%. FTIR (KBr)
[cmꢀ1]: 3293(vb) [
(OH)], 1616(s) [ (C@N)], 1462(s) [ (C@N of
oxime)], 1188(s) [
(N–O)]. 1H NMR (300 MHz, CDCl3, TMS) d
m
m
m
m
m
[ppm]: >12 (methyl protons of imidazole ring attached to C14),
9.82 (oximato proton), 8.47(methylene proton adjacent to the
imine attached to C10), 7.90 (ortho-hydrogens of the phenyl ring
attached to C2 and C6), 7.61 (NH proton of the imidazole ring),
7.53 (CH proton of the imidazole ring attached to C12), 7.46
(meta-hydrogens of the phenyl ring attached to C3 and C5), 7.43
(para-hydrogen of the phenyl ring attached to C4), 2.19 (methyl
protons attached to C9). 13C NMR (126 MHz, DMSO-d6, TMS) d
[ppm]: 162.9(C10), 155.7(C8), 154.9(C7), 135.3(C12), 134.1(C13),
129.2(C1), 128.6(C4), 127.5(C2,C6), 127.3(C3,C5), 127.0(C11),
(20.00 mL/min) on
a Perkin-Elmer instrument (Model: Pyris
Diamond TG/DTA). Magnetic susceptibility was determined at
room temperature with a PAR 155 vibrating sample magnetometer
fitted with a walker scientific L75FBAL magnet. The magnetometer
was calibrated with Hg[Co(SCN)4].
The microorganisms used in the present antimicrobial study
consisted of 17 bacterial strains, namely Escherichia coli ATCC
25938, Salmonella typhi 62, Pseudomonas AMRI 100, Vibrio cholerae
VC 20, Klebsiella pneumoniae 714, Shigella dysenteriae 1, Staphyloc-
cus aureus 29737, Bacillus cereus 11778, Pseudomonas aeruginosa
25619, Bacillus subtilis 6633, Bacillus pumilus 14884, Bacillus
bronchiseptica 4617, Streptococcus epidermidis 12228, Klebsiella
pneumoniae 10031, Micrococcus luteus 10240, Shigella sonnei NK
4010 and Salmonella typhimurium NTCC 74. The bacterial strains
were obtained from the Division of Microbiology, Department of
Pharmaceutical Technology, Jadavpur University, Kolkata, India.
11.1(C9), 10.0(C14). UV–Vis (MeOH) kmax[nm]
(e -
max[Mꢀ1
cmꢀ1]) = 320 (2.269 ꢁ 104), 233 (1.201 ꢁ 104), 203(1.996 ꢁ 104).
2.1.2. Preparation of [Pd(L)(Cl)] (2)
20 mg (0.074 mmol) of LH was dissolved in 10 mL of methanol to
get a yellowish solution, then 21.88 mg (0.074 mmol) of Na2[PdCl4]
dissolved in 5 mL of methanol was added into the ligand solution
dropwise with stirring at room temperature. The color of the
solution gradually changed to dark brown. The resulting reaction
mixture was left in air for slow evaporation. After 24 h shining rod
shaped brown crystals which deposited were filtered, washed
thoroughly with diethyl ether and dried in a vacuum desiccator over
fused CaCl2. Some of the brown crystals were fit for X-ray structure
determination. Yield: 13 mg (43%). M. p.: >220 °C. C14H14ClN5OPd
(410.15): Anal. Calc. for C14H14ClN5OPd: C, 40.97; H, 3.44; N, 17.07.
2.1.1. Preparation of the ligand
2.1.1.1. Preparation of 1-hydrazono-1-phenyl-propan-2-one oxime
(1). 1.0 g (6.132 mmol) of 1-phenyl-1,2- propanedione-2-oxime
was dissolved in 5 mL of dry methanol to give a colorless solution.
Then 0.3 mL (6.132 mmol) of hydrazine hydrate was added drop-
wise with constant stirring to the oxime solution. Gradually the
solution became light yellow and the resulting solution was stirred
for 3 h. At the end of stirring, it was left in air for slow evaporation.
After 24 h, shining light yellow needle shaped crystalline
Found: C, 40.76; H, 3.21; N, 17.25%. FT-IR (KBr)
m
m
[cmꢀ1]: 3038 (br)
(C@N) of oxime],
(Pd–N)]. 1H NMR (500 MHz, DMSO-d6,
[m
(NH)], 1609(vs) [
m
(C@N) of imine], 1451(m) [
1206(s) [
m(N–O)], 441(s) [
m
TMS) d [ppm]: 8.24 (1H, s, hydrogen of imine carbon attached to
C10), 7.69 (1H, s, imidazole ring proton attached to C12), 7.48 (3H,

C. Biswas et al. / Polyhedron 56 (2013) 211–220
213
Table 1
d, meta and para hydrogens of phenyl group attached to C3, C5 and
Crystal data and structure refinement for compounds 1 and 2.
C4), 7.35 (2H, d, ortho-hydrogens of the phenyl group attached to C2
and C6), 2.33 (3H, s, imidazole ring methyl protons attached to C14)
1.47 (3H, s, methyl protons attached to C9). 13C NMR (126 MHz,
Compound
1
2
CCDC No.
903577
C9H11N3O
177.21
903578
C14H14ClN5OPd
410.15
DMSO-d6, TMS)
d [ppm]: 176.8(C10), 147.7(C8), 141.6(C7),
Empirical formula
Formula weight
T (K)
138.2(C12), 137.5(C13), 133.3(C1), 130.0(C4), 128.9(C2,C6),
298(2)
298(2)
128.7(C3,C5), 118.9(C11), 12.5(C9), 9.3(C14). UV–Vis (DMF) kmax[-
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
0.71073
monoclinic
P21
0.71073
tetragonal
I41/a
nm] (
e
max[Mꢀ1 cmꢀ1]): 315 (1.012 ꢁ 104), 319 (1.014 ꢁ 104), 502
(3.452 ꢁ 103). FAB-MS (positive ion mode in m-nitrobenzyl alcohol
matrix) (m/z): 374.1 for [Pd(L)]+ (100%). KM (DMF): 58
X
-1cm2molꢀ1
M
6.9503(5)
6.4535(3)
10.1631(6)
93.418(4)
455.04(5)
2
22.5744(3)
22.5744(3)
13.2967(2)
90
6776.05(16)
16
(Non-conducting).
ing).
K
(DMSO): 11
X
ꢀ1 cm2 molꢀ1 (Non-conduct-
b (Å)
l
= Diamagnetic.
c (Å)
b [o]
V (Å3)
2.2. Crystal structure determinations
Z
qCalcd (g/cm3)
Absorption coefficient
1.293
0.089
1.608
1.260
Shining rod shaped brown single crystals of 2 suitable for X-ray
crystallography were grown from the reaction mixture. Single
crystals suitable for X-ray crystallographic analysis were selected
following examination under a microscope. Intensity data were
collected at 298(2) K for 1 and 2 on a Bruker Smart Apex II diffrac-
tometer equipped with a 1 K CCD instrument using a graphite
(mmꢀ1
F(000)
)
188
3264
Crystal size (mm)
h (°)
0.12 ꢁ 0.04 ꢁ 0.06
0.20 ꢁ 0.20 ꢁ 0.15
2.01–25.50
1.78–25.04
Limiting indices
ꢀ8 < h < 8, ꢀ7 < k < 7, ꢀ26 < h < 24,
ꢀ11 < l < 12
6566/1712 (0.0301)
100.0
ꢀ22 < k < 26,
monochromator with Mo Ka radiation (k = 0.71073 Å). Cell param-
ꢀ15 < I < 15
Reflections collected/
25760/3001 (0.0335)
eters were determined using SMART software [19]. Data reduction
and corrections were performed using SAINTPLUS [19]. Absorption
corrections were made via SADBAS [20]. The structures were solved
by direct methods with the program SHELXS-97 and refined by
full-matrix least-squares methods on all F2 data with SHELXL-97
[20]. The non-H atoms were refined anisotropically. Hydrogen
atoms attached to C, N and O atoms were added theoretically
and treated as riding on the concerned atoms. The final cycle of
full-matrix least-squares refinement was based on observed reflec-
tions and variable parameters. Large void spaces for 2 accounting
in total for 1745.2 Å3 per unit cell, i.e. some 31.2% of the total
volume, were examined using PLATON [21]. The reflection data
were subjected to the SQUEEZE routine in PLATON before the final
refinement and this suggested the presence of 156 electrons per
unit cell within the voids, but we failed to define some solvent
molecules. A summary of the data collection and structure refine-
ment for 1 and 2 is given in Table 1. Selected bond lengths, bond
angles and hydrogen-bond geometries are given in Tables 2 and
3 respectively.
unique (Rint
)
Completeness to
theta = 25.50 (%)
100
Data/restraints/parameters 1712/1/119
3001/0/199
R1, all data, R1 [I > 2
r
(I)]
0.0499, 0.0560
0.0254, 0.0343
0.0677, 0.0726
1.064
wR2, all data, wR2 [I > 2
r(I)] 0.1382, 0.1452
Goodness-of-fit (GOF) on F2 1.049
Largest difference in peak 0.243 and ꢀ0.195
0.366 and ꢀ0.212
and hole (e Åꢀ3
)
Table 2
Selected bond distances (Å) and angles (°) for 1 and 2.
Compound 1
C1–C7
C8–C9
N1–O1
C2–C1–C7
C7–N2–N3
1.499(3)
1.498(3)
1.403(3)
120.8(2)
117.6(2)
C7–C8
N2–N3
C7–N2
N1–C8–C7
C8–N1–O1
1.464(3)
1.363(3)
1.293(3)
113.5(2)
114.1(2)
Compound 2
Pd1–N1
Pd1–N2
Pd1–N4
Pd1–Cl1
N1–O1
N1–C8
N2–C7
N2–N3
N3–C10
N4–C12
N4–C11
C1–C2
N1–Pd1–N2
N1–Pd1–N4
N2–Pd1–N4
N1–Pd1–Cl1
N2–Pd1–Cl1
N4–Pd1–Cl1
O1–N1–C8
O1–N1–Pd1
C8–N1–Pd1
C7–N2–N3
C7–N2–Pd1
N3–N2–Pd1
1.984(2)
2.001(2)
2.016(2)
2.3109(8)
1.275(3)
1.327(4)
1.321(4)
1.372(3)
1.283(4)
1.311(4)
1.379(4)
1.381(4)
80.28(9)
171.42(10)
91.91(9)
95.61(7)
172.84(7)
92.51(7)
119.7(2)
125.43(19)
114.8(2)
116.1(2)
112.97(19)
130.53(18)
C1–C7
C2–C3
C3–C4
C4–C5
C5–C6
C7–C8
C8–C9
C10–C11
C11–C13
C12–N5
C13–C14
C1–C6
C10–N3–N2
C12–N4–C11
C12–N4–Pd1
C11–N4–Pd1
N2–C7–C8
N2–C7–C1
C8–C7–C1
N1–C8–C7
N1–C8–C9
C7–C8–C9
N3–C10–C11
1.499(4)
1.377(4)
1.363(5)
1.372(5)
1.373(5)
1.426(4)
1.494(4)
1.440(4)
1.370(4)
1.332(4)
1.493(4)
1.379(4)
117.8(2)
106.2(2)
132.2(2)
120.90(19)
116.6(3)
122.7(3)
120.7(3)
114.4(3)
119.6(3)
125.8(3)
133.1(3)
2.3. Studies of antimicrobial activity
The antimicrobial activity of the ligand LH as well as the Pd(II)
compound 2 were examined by in vitro testing. Seventeen microor-
ganisms, both Gram positive and Gram negative, were used for this
screening. Amoxycillin, a versatile antibacterial clinical therapeutic
agent, was used as the reference material [22]. Compound 2 was
primarily tested for screening by agar dilution [23], then suscepti-
ble organisms were selected and the Minimum Inhibitory
Concentrations (MIC) were determined as per the NCCLS protocol
[24] and by an INT assay (p-iodo-nitro-teterazolium chloride).
Finally, considering the best result against a particular organism,
the effect of 2 on bacterial growth rate was studied.
2.3.1. Preparation of inoculums
These strains were grown in Mueller-Hilton Agar (Merck India
Ltd.) at 37 °C for 24 h and the suspension was prepared by match-
ing a 0.5 McFarland standard [25].
2.3.2. Drug solution
A drug solution of the synthetic drug was prepared by dissolv-
ing analytically pure crystals of 2 in 1% DMSO solution along with
sterile distilled water and the volume was made up to 2 mL.
2.3.3. Determination of MIC
The determination of the MIC was done by the agar disk diffu-
sion method [26] and confirmed by an INT assay. For disk diffusion,

214
C. Biswas et al. / Polyhedron 56 (2013) 211–220
Table 3
Hydrogen bonds (Å and o) for 1 and 2.
D–Hꢃ ꢃ ꢃA
d(D–H)
d(Hꢃ ꢃ ꢃA)
d(Dꢃ ꢃ ꢃA)
\(D–H–A)
Symmetry code
Compound 1
O1–H1ꢃ ꢃ ꢃN2
N3–H3Bꢃ ꢃ ꢃN1
0.82
0.86
2.17
2.28
2.894(3)
2.937(3)
147.8
133.6
x, y + 1, z
x, y ꢀ 1, z
Compound 2
N5–H5Aꢃ ꢃ ꢃCl1
N5–H5Aꢃ ꢃ ꢃO1
0.86
0.86
2.80
2.01
3.385(3)
2.755(3)
126.8
144.4
y ꢀ 1/4, ꢀx + 5/4, ꢀz + 5/4
yꢀ1/4, ꢀx + 5/4, ꢀz + 5/4
a 0.1 mL standard suspension of bacteria (2 x 106 CFU/mL) was
spread over Muller-Hinton agar plates and disks containing drugs
of different concentrations were placed on every type of microor-
ganism containing plate. The inoculated plates were incubated at
37 °C for 24 h and observed for a zone of inhibition. The obtained
MIC values were confirmed by an INT assay where two sets of
nutrient broth were prepared to which 0.1 mL standard suspension
of bacteria (2 x 106 CFU/mL) were added. To one set, a specific
from the presence of a positive FAB-MS peak in m-nitrobenzyl
alcohol (MNBA) matrix at 374.1 (Calc. 374.5; 100% intensity) corre-
sponding to [Pd(L)]+ for 2. The C, H and N microanalytical data of the
single crystals of 2 also corroborate the above formulation. The
compound is diamagnetic, suggesting a square-planar disposition
of the palladium center in 2. The synthetic scheme is outlined below
(see Scheme 1):
The diamagnetic nature of 2 enables us to detect all the carbon
nuclei in its 500 MHz 13C NMR spectrum in DMSO-d6 (Fig. 1 as
Supplementary Data). The bands associated in the infrared spec-
amount of drug (10, 25, 50, 100, 200 and 400 lg/mL) was added,
and another set was taken as a control, then finally p-iodo-nitro-
tetrazolium chloride (INT assay) was added and incubated at
37 °C for 24 h.
trum of free LH with
m
1616(s) [
m(C@N)], 1462(s) [m(C@N of oxime)]
and 1188(s) cmꢀ1
[
m(N–O)] appear respectively at wavenumbers
1609, 1451 and 1206 cmꢀ1 in 2. The
m(Pd–Cl) vibrational modes,
which typically range between 300 and 400 cmꢀ1 [27] could not
be observed within the window of the spectrometer used (4000–
3. Results and discussion
400 cmꢀ1) here. However, the
m(Pd–N) mode is discerned at
3.1. Synthesis and formulation
441 cmꢀ1 [28].
The electronic spectra of Pd(II) complexes are indicative of their
square-planar geometries. In the electronic spectrum of 2 in DMF,
only one d–d band at 502 nm was observed, which is assigned to
The ligand employed for the present work is 1-(4-methylimida-
zol-5-yl) phenylhydrazonopropane-2-one oxime (LH), a Schiff-base
ligand. LH has been synthesized by the Schiff-base condensation of
equimolar proportions of 1-hydrazono-1-phenyl-propan-2-one
oxime (1) and 4-methylimidazole-5-carboxaldehyde in methanol.
The reaction of LH with Na2[PdCl4] in a 1:1 M proportion in
methanol gives rise to the brown monomeric Pd(II) compound,
[Pd(L)(Cl)] (2). The formation of the compound was established
2
2
2
the dz to dx
transition [18]. This is indicative of a low-spin
ꢀy
complex of a d8 metal ion with a square-planar geometry. The
molar electrical conductivities of ꢂ10ꢀ3 M solutions of complex 2
in DMF (58
X ) and DMSO (11 X )
ꢀ1 cm2 molꢀ1 ꢀ1 cm2 molꢀ1
indicate the non-electrolytic nature of 2 in both the solvents [29].
Scheme 1. Ligand and compound structures along with the atom numbering for NMR spectroscopic assignments.

C. Biswas et al. / Polyhedron 56 (2013) 211–220
215
Fig. 1. 13C NMR spectrum of [Pd(L)Cl] (2) in DMSO-d6.
for an oxime moiety can be found in 1-phenyl-1-{N(4)-methylthi-
osemicarbazone}-2-oximepropane (H2Po4M) [30]. The crystal
structure of H2Po4M was found to have the oxime and thiosemicar-
bazone moieties on opposite sides of the carbon–carbon backbone.
The relative dispositions of the functionalities as found in H2Po4 M
is akin to our case. In 1, the oxime and the hydrazone moieties are on
the opposite sides of the C7–C8 bond. The C8–N1–O1 angle of the
oxime function is 114.1(2)°. The magnitude of the torsion angle
around C7–C8 seems to be very informative. A close value of the tor-
sion angle, C–C@N–O to 0° corresponds to the cis configuration;
while it is 180° for a trans configuration of the oxime function with
respect to the C–C connectivity. In 1, the C7–C8–N1–O1 torsion
angle is 179.32(17)°, as found from X-ray crystallography. Thus
the oxime group in 1 adopts a clear trans-planar arrangement with
respect to the C7-C8 linkage. Again, the torsion angle C8–N1–O1–H1
is 172.49 °. The O-H bond may be syn-planar or anti-periplanar to
C@N. Though the anti-periplanar conformation is common, a few
cases are also known where the syn-planar situation (torsion angle
close to 0°) are also known [31]. This difference in conformation
arises due to rotation around the C–C single bond, the connector
in a heterodiene system. In our case this connector is the C7–C8
Fig. 2. The structure of 1 with ellipsoids at 50% probability.
0
bond. This bond length is 1.464(3) ÅA, a value considerably shorter
0
than the theoretical C(sp2)–C(sp2) single-bond (1.48 ÅA). For oximes
3.2. Molecular structures
having co-planar conjugated double bonds, this bond length lies in
0
the range of 1.47–1.50 ÅA [32]. Thus the shrinkage of the C7–C8 bond
length in 1 from the ideal C–C single bond length may be due to the
development of a partial double bond character due to delocaliza-
tion in this heterodiene system.
3.2.1. 1-hydrazono-1-phenyl-propan-2-one oxime (1)
The solid state structure of 1 was established through X-ray crys-
tallography. The molecular structure and the atom numbering
scheme for 1 is shown in Fig. 2. A packing diagram of the hydrogen
bonded polymeric network is shown in Fig. 3. Selected metrical
parameters for this structure are summarized in Table 2. The
compound crystallizes in the monoclinic space group P21 with
two molecular weight units accommodated per unit cell. The N2–
N3 bond distance is 1.363(3) Å. The trans angles C9–C8–C7 and
C1–C7–C8 are 121.1(2)° and 118.82(19)° respectively. The C8–N1
In a monoxime having two double bonds (inclusive of the oxime
moiety), three possible isomers are possible, syn,syn, syn,anti and
anti,anti [33]. 1 has two such centers due to the presence of the
C8@N1 and C7@N2 double bonds. With respect to these, 1 is the
syn-methyl and syn-phenyl isomer, as found in its solid state struc-
ture (Fig. 2).
The O1–H1ꢃ ꢃ ꢃN2i0 (i x, y + 1, z) hydrogen bond in0 1 (Fig. 3), with
donor O1–H1 (0.82 ÅA) and acceptor H1ꢃ ꢃ ꢃN2i (2.17 ÅA), has been de-
tected by X-ray crystallography (Table 3). Though the position of
the H atom, as deciphered from X-ray crystallography, has been
and N1–O1 bond distances of the oxime function in 1 respectively
0
have values of 1.276(3) and 1.403(3) ÅA. Similar magnitudes of the
C–N and N–O bond distances, 1.290(6) and 1.397(5) ÅA respectively,
0

216
C. Biswas et al. / Polyhedron 56 (2013) 211–220
2
1
Fig. 3. The packing diagram of 1 showing the H-bonding motifs: R2 (6) and C1 (6).
are only a few examples of X-ray crystal structures with O–Hꢃ ꢃ ꢃN
bonds in which the H atom has at least similar distances to O
0
and N, with O–H and Hꢃ ꢃ ꢃN distances of 1.23 and 1.29 ÅA respec-
tively [34,35]. However the first O–Hꢃ ꢃ ꢃN hydrogen bond with a
centered proton, as detected by Neutron Diffraction (ND) at 20 K,
0
has an Oꢃ ꢃ ꢃN distance of 2.506 ÅA [36]. To date, this is the shortest
known O–Hꢃ ꢃ ꢃN hydrogen bond for which ND data is available. In
0
our case this O–Hꢃ ꢃ ꢃN distance is 2.894 ÅA, a value significantly lar-
0
ger than 2.506 ÅA. Thus based on this distance criterion alone, the
O1–H1ꢃ ꢃ ꢃN2 hydrogen bond in 1 is weak indeed. In light of the sim-
ilar distance criterion, N3–H3Bꢃ ꢃ ꢃN1ii (ii x, y ꢀ 1, z) is also weak.
3.2.2. [Pd(L)(Cl)] (2)
The solid state structure of 2 was determined through X-ray
crystallography. The molecular structure and the atom labeling
scheme for 2 is shown in Fig. 4. A packing diagram along with
the H-bonded network of 2 is shown in Fig. 5. Selected metrical
parameters for the structure of 2 are also summarized in Table 2.
The compound crystallizes in the tetragonal space group I41/a,
with sixteen molecules in the unit cell. The palladium center in 2
Fig. 4. The structure of [Pd(L)(Cl)] (2) with ellipsoids at 50% probability.
calculated (riding model), it seems that the H atom of the O1–
is nested in an ‘N3Cl’ core. The Pd1–N1, Pd1–N2 and Pd1–N4 bond
0
H1ꢃ ꢃ ꢃN02 hydrogen bond in 1 is closer to the O1 center by a value
0
distances are 1.984(2), 2.001(2) and 2.016(2) ÅA respectively. A
monomeric palladium(II) complex having an ‘N3Cl’ coordination
of 0.7 ÅA from the baricentre of the O1–N2 bond (2.894 ÅA). There
4
Fig. 5. H-bonding motif: R4 (28) in 2.

C. Biswas et al. / Polyhedron 56 (2013) 211–220
217
Fig. 6. CV of 2 in DMF at a scan rate of 200 mV sꢀ1. The analyte concentration was 6.095 ꢁ 10ꢀ4 (M).
environment for Pd is known [37], with which we can compare our
atmosphere. The resulting cyclic voltammogram (CV) is shown in
Fig. 6 and the peak potentials are tabulated in Table 4. On the po-
sitive side of the Ag/AgCl reference electrode, 2 exhibits one anodic
wave (Couple-I) with a peak potential of 0.573 V versus Ag–AgCl.
Pd–N bond lengths. Here the three Pd–N bond distances are
0
1.9796, 2.0117 and 2.1065 ÅA, and these values are comparable to
2. Again, all these values match well with the literature value
(2.02 to 2.04 Å) [38].
The associated peak current, ipa, is 4.49 lA. The corresponding
The cis-(N1–Pd1–N2, N2–Pd1–N4, N1–Pd1–Cl1, N4–Pd1–Cl1)
and the trans-(N1–Pd1–N4, N2–Pd1–Cl1) angles lie respectively
in the ranges 80.28–95.61 and 171.42–172.84°, deviating copiously
from 90° and 180°. Thus the geometry around the palladium(II)
cathodic response is discernable in the subsequent reverse cycle
with a peak potential value of 0.443 V versus Ag–AgCl, having a
peak current, ipc, of 4.21 lA. Thus this oxidation can safely be as-
signed as metal centered. A comparison of the voltammetric peak
currents with those of the ferrocene-ferrocenium couple (0.44 V
versus Ag/AgCl) under the same experimental conditions estab-
lishes that the present oxidative response in 2 involves one
center in 2 is quite distorted. The palladium center is displaced
0
by 0.029(1) ÅA from the mean square-plane defined by N1, N2, N4
and Cl1. The core is nevertheless essentially planar. Additionally,
in order to estimate the geometric shape of 2, the Addison angular
electron. So, E at + 0.508 V versus Ag–AgCl corresponds to a Pd(II)
½
structural parameter (
dinated complexes, this parameter s4 is defined as
+ b)]/141 where is the largest angle and b is the second largest
s
) has been calculated [39,40]. For four coor-
to Pd(III) oxidation. The ratio of ipc to ipa is 0.937, a value close to
unity. Thus this oxidative response is quasi-reversible in nature.
This observed value of the Pd(III)/Pd(II) redox couple is comparable
with the value reported earlier [41,42].
s4 = [360-
(a
a
angle around the central metal atom in question. A value of 0 for s4
is the signature for a perfect square-planar geometry, while it is
Our palladium(II) complex again shows only one electrochemi-
unity for
a
regular tetrahedral core [40]. Taking N2-Pd1-
cal response at a E value of -0.516 V (Couple II) on the negative
½
Cl1 = 172.84° (
a
) and N1-Pd1-N4 (b) = 171.42°, the s4 value comes
side of the Ag/AgCl reference electrode. Again, comparison of the
voltammetric peak currents of couple II with those of the ferro-
cene-ferrocenium couple (0.44 V versus Ag/AgCl) under the same
experimental conditions establishes that the present reductive
responses in 2 involve one electron only. Thus the above reductive
response can be assigned as Pd(II) to Pd(I). The cathodic peak po-
out as 0.112 for 2. This value is close to 0. Thus the geometry
around the palladium center is square-planar. However, it is
slightly distorted. The sum of the angles around the palladium is
360.31°, a value quite close to 360°. This is due to the chelate ef-
fects around the Pd ion, with 5- and 6-membered planar rings,
which bring about stability to the structure. The packing diagram
of 2 shows the intermolecular hydrogen bonding. This H-bonding
consists exclusively of classical N–Hꢃ ꢃ ꢃO bonds.
tential and peak current of couple II are ꢀ0.757 V and 16.67
lA,
respectively. The subsequent anodic response in the reverse cycle
can be discernable at ꢀ0.274 V versus Ag–AgCl with an anodic
peak current, ipa, of only 5.04 lA. The ratio of ipc to ipa is 3.307. This
value shows a considerable deviation from unity. The cathodic
3.3. Electrochemistry
peak current, ipc, increases with the square root of the scan rate
(m
1/2), but not in proportionality. Again, the cathodic peak poten-
The redox behavior of the title compound 2 has been studied by
tial, Epc, shifts more negatively with the increase in sweep rate,
cyclic voltammetry in DMF at a GC working electrode under N2
m. Judged on these criteria, the metal centered reduction for 2 at
Table 4
peak II is said to be quasi-reversible in nature [43]. Those CV effects
might be due to involvement of ligand orbital electrons as well.
However the ligand LH does not show any voltammogram in the
potential range of interest here under identical conditions, namely
solvent, three-electrode assembly, supporting electrolyte, scan rate
etc., in which the title palladium(II) compound displays voltammo-
grams. Thus the following plausible metal-centered redox steps
might be operative here:
Cyclic voltammetric data for 2.
Entry
Epc (V)
Epa (V)
D
E (V)
E°
ipc
(
l
A)
ipa (lA)
½
Couple I
Couple II
0.443
ꢀ0.757
0.573
ꢀ0.274
0.13
0.483
0.508
ꢀ0.516
4.21
16.67
4.49
5.04
Epc is the peak potential for the cathodic wave, Epa is the peak potential for the
anodic wave, E = 0.5 (Epc + Epa), ipc is the peak current for the cathodic wave, ipa is
½
the peak current for the anodic wave.

218
C. Biswas et al. / Polyhedron 56 (2013) 211–220
Fig. 7. TG/DTA of [Pd(L)Cl] (2).
0
Couple I: [PdIII(L)(Cl)]+ + eꢀ ? [PdII(L)(Cl)]0
Couple II: [PdII(L)(Cl)]0 + eꢀ ? [PdI(L)(Cl)]ꢀ
E
E
= 0.508 V
= ꢀ0.516 V
where rij is the length of the bond in ÅA and ro is a parameter
½
0
characteristic of the bond. With the same unit in ÅA, like rij, r0 is
known as the bond valence parameter. This parameter is geometry
and coordination number specific. The oxidation number Ni of the
atom i is simply the algebraic sum of these s values of all the bonds
(n) around the atom I, following equation (2)
½
3.4. BVS analysis
With a view to assigning the oxidation state of the central pal-
ladium center in our complex 2, we have taken the initiative to
perform calculations based on the Bond Valence Sum (BVS) meth-
od [44,45]. BVS relates the bond lengths around a metal center to
its oxidation state. Developed historically from the concept of bond
number, this method was originally propounded by Pauling [46].
Later I. D. Brown and co-workers further fleshed it out [46]. The
advantage of this approach is that the bond length is a unique func-
tion of bond valence. Generally for a particular bond type, the bond
valence diminishes exponentially as the bond length increases In
this semi-empirical method, the valence s of a bond between two
given atoms i and j is related by an empirical relation (1)
n
X
Ni ¼
Sij
ð2Þ
i¼1
This Ni is known as the BVS of the ith atom. Thus if r0 is known
for a particular bond type, the BVS can be calculated from the crys-
tallographically determined rij values. Taking r0 values for Pd–N
and Pd–Cl bonds of 1.81 and 2.05 Å respectively [47], the BVS for
the palladium center in 2 comes out as 2.288 valence units (vu).
This value closely approaches the error limit of 0.250 vu as
proposed earlier by Thorp [48,49]. Thus an oxidation number of
+2 can safely be assigned to the palladium center in
computationally.
2
Sij ¼ exp½ðro ꢀ rijÞ=0:37ꢄ
ð1Þ
Table 5
Representation of MIC values of drug 2 on the tested organisms.
3.5. Thermal behavior
Organisms
MIC (
400
lg/mL) values of the drug
The thermal behavior (TGA and DTA) of 2 have been studied
(Fig. 7) within the temperature range 30.0–900.0 °C in a dynamic
N2 atmosphere (flow rate = 20.0 mL/min). 3.970 mg of 2 was
heated in a platinum crucible at a heating-rate of 15.0 °C per
200
100
50
25
10
AM
E.coli ATCC 25938
S. typhi 62
Pseudomonas AMRI 100
V. cholerae 20
K. pneumoniae 714
S. dysenteriae 1
S. aureus 29737
B. cereus 11778
P. aeruginosa 25619
B. subtilis 6633
B. pumilus 14884
B.bronchiseptica 4617
S. epidermidis 12228
K.pneumoniae 10031
M. luteus 10240
+
+
+
+
+
+
+
+
+
ꢀ
+
+
+
+
+
+
+
+
ꢀ
+
ꢀ
+
+
+
+
+
ꢀ
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
ꢀ
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
10
25
100
25
100
1
ꢀ
ꢀ
minute. Dried and purified a-Al2O3 powder was used as a reference
+
+
in our case. Thermogravimetric (TG) analysis confirms that 2 is
thermally stable up to ca. 300 °C. The thermogram shows sequen-
tial events of weight loss. Within the temperature range
300–312.5 °C, the experimental mass loss of 9.4% is in fair confor-
mity with the theoretical loss (8.7%) for the complete removal of
the bonded chloride atom per formula unit. The corresponding dif-
ferential thermal analysis (DTA) curve shows one exothermic peak
at ca. 315 °C. The second weight loss is associated within the tem-
perature range 313–534 °C. This is due to ligand combustion. The
experimental weight loss was 52.3% against the calculated loss of
56.8%. This combustion is accompanied by a large exothermic heat
loss. The respective DTA peak is found at 450 °C. Up to 900 °C, the
mass loss is virtually complete. The final decomposition product is
+
+
+
+
+
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
+
5
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
+
25
100
25
50
50
5
100
5
50
25
ꢀ
ꢀ
S. sonnei NK 4010
S. typhimurium NTCC 74
+
+
+
+
‘‘+’’ represents no antimicrobial activity and ‘‘ꢀ’’ shows antimicrobial activity (AM –
amoxycillin).

C. Biswas et al. / Polyhedron 56 (2013) 211–220
219
Table 6
Representation of the zone diameter of the MIC values of drug 2 on tested organisms.
4. Conclusions
Organisms
Zone diameter of MIC (mm)
Here we have synthesized and structurally characterized a
neutral and diamagnetic Pd(II) compound (2) from an oxime-based
ligand, 1-(4-methylimidazol-5-yl) phenyl-hydrazonopropane-2-
one oxime (LH). The X-ray crystal structures of both 1-hydrazon-
o-1-phenyl-propan-2-one oxime (1) and 2 have been determined.
The palladium center is in a square-planar ‘N3Cl’ coordination
sphere. The BVS method of calculations were also undertaken to
assign the oxidation state of palladium and to substantiate the
stability of 2. Electrochemical studies in DMF show a Pd(II) to
Pd(III) oxidation at 0.573 V, along with a reduction of Pd(II) to
Pd(I) at ꢀ0.757 V versus Ag/AgCl. Compound 2 shows antimicro-
bial activity against a good number of both Gram positive and
Gram negative bacteria. Thus our title compound 2 may be devel-
oped as a new antimicrobial agent.
V. cholerae 20
B. cereus 11778
10 0.1
15 0.2
14 0.2
10 0.2
10 0.2
13 0.2
10 0.1
15 0.2
P. aeruginosa 25619
B. subtilis 6633
B. pumilus 14884
B. bronchiseptica 4617
S. epidermidis 12228
M. luteus 10240
elemental palladium, as indicated by the weight of 11.8% (Theo.
12.6%). It is pertinent to note that for palladium complexes, the
final residue in the thermal analysis is metallic palladium [50,51].
3.6. Antimicrobial activity
Acknowledgements
Our compound 2 shows antimicrobial properties. However, the
ligand, LH does not. The drug 2 was effective against 8 tested
organisms among the listed organisms in Table 5.
J.P.N. gratefully acknowledges the University Grants Commis-
sion (UGC), New Delhi, India for financial support [F. No. 41-220/
2012 (SR)]. M.L.Z and L.P.L. appreciated the NSFC (Grant Nos.
21171109 and 21271121), SRFDP (Grant Nos. 20111401110002
and 20121401110005), the Natural Science Foundation of Shanxi
Province of China (Grant Nos. 2010011011-2 and 2011011009-1)
and the Shanxi Scholarship Council of China (2012-004). We solicit
to bestow our gratitude to the reviewers for helping us in the
course of a revision of this paper.
The lowest MIC value of 2 was found at a concentration of
25
drug’s inhibitory effect was against V. cholerae VC 20 at a concen-
tration 25 g/mL and Micrococcus luteus 10240 and Streptococcus
epidermidis 12228 at 100 g/mL.
lg/mL and the highest concentration was 400 lg/mL. The
l
l
From the results against the microorganisms of the test samples
we can conclude that 2 shows a broad spectrum, i.e. effective on
both Gram positive and Gram negative organisms. 2 is more effec-
tive on enteric organisms.
Appendix A. Supplementary data
The zone of inhibition of the test samples was evaluated by the
well diffusion method [24]. 0.1 mL of bacterial solution
(2 ꢁ 106 CFU/mL) was transferred to Mueller–Hinton agar plates.
The bacterial suspensions were uniformly spread over a number
of plates using a sterile glass spreader. For each type of organisms
a particular concentration of drug solution was impregnated into
the agar plates of the screened 8 microorganisms (Table 6). All
the agar plates were then incubated at 37 °C for 24 h. The sensitiv-
ity was evaluated by measuring the presence of a clear zone of
inhibition on the agar surface around the wells.
CCDC 903577 and 903578 contains the supplementary crystal-
lographic data for 1 and 2. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2013.03.064.
The killing rate of the tested compounds was determined by
using viable cell count experiments [52]. The microbial suspension
was diluted with sterile distilled water up to 2 ꢁ 106 CFU/mL. Two
sets of test were taken as a control. Both sets of test tubes were
incubated at 37 °C and at regular intervals of 0, 2, 4, 6 and 18 h,
a 0.1 mL sample was withdrawn and diluted 100 times in a sterile
test tube with sterile distilled water and spread over sterile tubes
containing Muller-Hinton broth. In one set of test tubes, 1 mL of
bacterial suspension and the drug sample in the MIC range were
added and the another set contained a bacterial suspension
without the drug sample agar plates. Then the spread plates were
incubated overnight at 37 °C. After incubation the colonies were
counted in the plates and the reduction in colonies of the test
samples were calculated by comparing to the colonies in the con-
trol plates.
The results for each type of the 8 microorganisms screened
previously have been shown graphically with the respective
numerical logarithmic values of the number of colonies after incu-
bation in Figs. 8–15 (Supplementary data). Pertinent data are
summarized in Table 7 (Supplementary data).
By analyzing the bacterial growth curves, we can conclude that
our Pd(II) compound (2) inhibits growth of the tested bacterial
strains at the MIC and exerts its effect as a bacteriostatic agent.
The probable mechanism of action of 2 against the above microor-
ganisms can only be deciphered by further studies.
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