Ligational behavior of Schiff bases towards transition metal ion and metalation effect on their antibacterial activity

Article abstract of DOI:10.1016/j.saa.2012.06.026

New Schiff bases pyrazine-2-carboxylicacid (phenyl-pyridin-2-yl-methylene)- hydrazide (Hpch-bp) HL₁ and pyrazine-2-carboxylicacid (pyridin-2-ylmethylene)-hydrazide (Hpch-pc) HL₂ derived from condensation of pyrazine carboxylic hydrazide (Hpch) with 2-benzoyl pyridine (bp) or pyridine 2-carbaldehyde (pc) and their transition metal complexes of type ML_(1-2)2 have been synthesized, where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II). Characterization of ligands and their metal complexes was carried out by elemental analysis, conductimetric studies, magnetic susceptibility, spectroscopic techniques (IR, UV-VIS, NMR, ESR, Mass) and thermogravimetric analysis. The physico-chemical studies revealed octahedral geometry or distorted octahedral geometry around metal ion. These azomethine Schiff base ligands acted as tridentate coordinating through carbonyl, azomethine and pyridine nitrogen present in the ligand. The thermodynamic and thermal properties of the complexes have been investigated and it was observed on the basis of these studies that thermal stability of complexes follows the order Mn < Zn < Cu < Co < Ni. The ligands and their complexes were tested for in vitro antibacterial activity at different concentrations against bacteria viz. Gram positive Bacillus subtilis, Micrococcus luteus and Gram negative Pseudomonas aeruginosa, Pseudomonas mendocina. A marked enhancement in biocidal activity of the ligands under similar experimental conditions was observed as a consequence of coordination with metal ions. The trend of growth inhibition in the complexes was found to be in the order: Cu > Mn > Ni > Co > Zn.

Full text of DOI:10.1016/j.saa.2012.06.026

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 397–405
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Ligational behavior of Schiff bases towards transition metal ion
and metalation effect on their antibacterial activity
⇑
Jai Devi, Nisha Batra, Rajesh Malhotra
Department of Chemistry, Guru Jambheshwar University of Science and Technology, Hisar 125001, Haryana, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
Synthesis of azomethine Schiff base
ligands and their metal complexes.
Ligands and metal complexes were
characterized by (IR, UV–VIS, NMR,
ESR, Mass) and TGA.
"
"
"
In vitro antibacterial activity at
different concentrations of
compounds were studied.
Biocidal activity of the metal
complexes increased upon
coordination with metal ions.
a r t i c l e i n f o
a b s t r a c t
Article history:
New Schiff bases pyrazine-2-carboxylicacid (phenyl-pyridin-2-yl-methylene)-hydrazide (Hpch-bp) HL1
and pyrazine-2-carboxylicacid (pyridin-2-ylmethylene)-hydrazide (Hpch-pc) HL2 derived from conden-
sation of pyrazine carboxylic hydrazide (Hpch) with 2-benzoyl pyridine (bp) or pyridine 2-carbaldehyde
(pc) and their transition metal complexes of type ML(1–2)2 have been synthesized, where M = Mn(II),
Co(II), Ni(II), Cu(II) and Zn(II). Characterization of ligands and their metal complexes was carried out
by elemental analysis, conductimetric studies, magnetic susceptibility, spectroscopic techniques (IR,
UV–VIS, NMR, ESR, Mass) and thermogravimetric analysis. The physico-chemical studies revealed octa-
hedral geometry or distorted octahedral geometry around metal ion. These azomethine Schiff base
Received 13 March 2012
Received in revised form 5 June 2012
Accepted 19 June 2012
Available online 28 June 2012
Keywords:
Transition complexes
Azomethine
Antibacterial
Inhibition
ligands acted as tridentate
coordinating through carbonyl, azomethine and pyridine nitrogen
N
O
N
present in the ligand. The thermodynamic and thermal properties of the complexes have been investi-
gated and it was observed on the basis of these studies that thermal stability of complexes follows the
order Mn < Zn < Cu < Co < Ni. The ligands and their complexes were tested for in vitro antibacterial activ-
ity at different concentrations against bacteria viz. Gram positive Bacillus subtilis, Micrococcus luteus and
Gram negative Pseudomonas aeruginosa, Pseudomonas mendocina. A marked enhancement in biocidal
activity of the ligands under similar experimental conditions was observed as a consequence of coordi-
nation with metal ions. The trend of growth inhibition in the complexes was found to be in the order:
Cu > Mn > Ni > Co > Zn.
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
metal coordination chemistry even after almost a century since
their discovery. This class of compounds has been explored as
The Schiff base have been most widely used versatile ligands, in
their neutral or deprotonated forms to form stable complexes with
most of the transition metal ions and still play an important role in
models for biological systems [1] and has attracted attention of
biological chemists due to their wide ranging pharmacological
properties including antimicrobial [2–4], anti-tuberculosis and
anti-tumor activity [5–7], herbicidal, insecticidal and as plant
growth regulators. It has been reported that many compounds pos-
sess modified pharmacological and toxicological potentials when
⇑
Corresponding author. Tel.: +91 9896608818; fax +91 1662276240.
E-mail address: malhotra_ksrk@yahoo.co.in (R. Malhotra).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.06.026

398
J. Devi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 397–405
combined with metal ions [8]. The remarkable enhancement in the
biological activity of acid hydrazides and their corresponding aroyl
hydrazones is due to their ability to bind with transition metal ions
in living systems. Tridentate and tetradentate Schiff base ligands
are of particular interest not only for existing in lactam (keto) or
lactim (enol) tautomeric forms giving variations to donating prop-
erties of these ligands to form complexes with unusual coordina-
tion numbers [9], high thermodynamic stability and kinetic
inertness but also for their ability to exhibit catalytic and magnetic
properties and to mimic biological sites upon complexation with
metal ions. The activity of metal complexes in biological systems
depends on the ease of cleavage of bond between metal ion and
the ligand for which it is important to understand their coordination
behavior and relationship between metal and the ligands in various
biological systems. Considering the importance and properties of
the Schiff base hydrazones and their metal complexes, present work
describes synthetic, spectroscopic, thermal aspects and particularly
antibacterial activity of transition metal complexes of the Schiff
base obtained from condensation of pyrazine carboxylic hydrazide
with 2-benzoyl pyridine or pyridine 2-carbaldehyde.
Experimental
Materials and methods
All the chemicals used were obtained through Aldrich and used
as such without any further purification. IR spectra were recorded
on Shimadzu IR affinity-I 8000 FT-IR spectrometer using KBr disc
having wavelength range of 4000–400 cmꢀ1 1H NMR and 13C
.
NMR spectra were recorded on Bruker Avance II 300 MHz NMR
spectrometer and the chemical shifts were reported in parts per
million relative to TMS as internal standard in CDCl3. UV spectra
were recorded on UV–VIS–NIR Varian Cary-5000 spectrometer in
DMF. Magnetic susceptibilites of complexes were measured by
Gouy’s method, using Hg [Co(SCN)4] as the calibrant at room tem-
perature. ESR spectra of the copper complexes were recorded on a
Varian E112 X-band spectrometer using tetracyanoethylene
(TCNE) as the internal standard. Molar conductance measurements
of a 10ꢀ3 M solution of metal complexes in DMF at room temper-
ature were carried out using a model-306 Systronics conductivity
bridge. Elemental analysis was carried out on Perkin Elmer 2400.
Thermogravimetric (TG) analysis of samples at a heating rate of
10 °C minꢀ1 was carried out using Perkin Elmer Diamond TG/DTA
thermogravimetric analyzer instrument under high purity argon
atmosphere at a flow rate of 20 ml minꢀ1. Mass spectra were re-
corded on API 2000 (Applied Biosystems) mass spectrometer
equipped with an electrospray source and a Shimadzu Prominence
LC.
Synthesis of the Schiff bases
Pyrazine-2-carboxylic acid (phenyl-pyridin-2-yl-methylene)-
hydrazide (Hpch-bp) HL1. To the solution of pyrazine carboxylic
acid hydrazide (0.55 g, 4 mmol) in methanol (30 mL) was added
dropwise with continuous stirring, a solution of 2-benzoyl pyridine
(0.73 g, 4 mmol) in the same solvent. The reaction mixture was
then refluxed for 5 h. The solid product so obtained was filtered
and recrystallised in hot methanol. The same method was adopted
to prepare other ligand (Hpch-pc) HL2.
Synthesis of the metal complexes
Metal complexes were prepared by dropwise addition of 10 mL
methanolic solution of the metal salt to 20 mL methanolic solution
of Schiff base (2 mmol) with continuous stirring. The resulting

J. Devi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 397–405
399
Fig. 1. Scheme for synthesis of ligand and their metal complexes.
Table 2
Infrared spectra of compounds.
Sr. no.
Ligand/complexes
t
(N–H)
t(C–O)
t(C-O)
t
(C@N)
t
(M–O)
t(M–N)
t(Py)
1
2
3
4
5
6
7
8
HL1
HL2
3350
3361
1681
1695
–
–
1580
1577
1547
1556
1545
1538
1549
1544
1553
1542
1536
1546
–
–
–
–
693
693
700
695
694
702
699
699
694
696
701
698
Mn(L1)2
Co(L1)2
Ni(L1)2
Cu(L1)2
Zn(L1)2
Mn(L2)2
Co(L2)2
Ni(L2)2
Cu(L2)2
Zn(L2)2
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1074
1086
1065
1090
1083
1078
1072
1071
1086
1074
434
441
435
427
438
431
439
428
429
442
501
499
503
509
501
509
511
505
499
509
9
10
11
12
Table 3
1H and 13CNMR of ligands and their zinc complexes.
Ligands 1H NMR (CDCl3) d in ppm
13C NMR (CDCl3) d in ppm
HL1
15.08(s, NH proton), 9.50(s, 1H, C3–H), 8.93 (J = 3.3, d, C6–H), 8.77(J = 2.1, d, C6– 162.47 (C@O), 152.53 (C@N), 149.20 (C2), 148.85(C6), 148.18(C2),
0
H), 8.62(J = 2.1, d, C5–H), 7.84 (J = 7.8, d, 1H, C3–H), 7.67 (J = 9.0, J = 3.3, dd, C4-H, 147.90(C5), 137.84(C3), 137.46(C6), 136.94(C4), 135.97(C5 ), 129.63–
0
C5-H), 7.48–7.28 (m, 5H, Ph-H)
128.45(Ph-C), 127.10(C3
)
HL2
15.77(s, NH proton), 12.20(s, –CH proton), 9.20(s, C3-H), 8.82 (J = 2.8, d, C6–H), 162.33(C@O), 152.33(C@N), 148.80(C2), 148.54(C6), 148.03(C2),
0
0
8.76 (J = 3.3, d, C6-H), 8.61(J = 3.3, d, C5-H), 7.80 (J = 2.7, d, C3-H), 7.56 (J = 9.4,
147.71(C5), 137.96(C3), 136.13(C6), 135.48(C4), 135.04(C5 ), 126.05(C3
)
J = 2.1, dd, C4-H, C5-H)
Zn(L1)2
Zn(L2)2
9.15(s, C3-H), 8.81 (J = 4.8, d, C6-H), 8.30(J = 6.0, d, C6–H), 8.04 (J = 6.0, d, C5-H), 163.34(C@O), 154.32(C@N), 149.28(C2), 149.05(C6), 147.78(C2),
0
7.50 (J = 3.3, d, C3-H), 7.43 (J = 7.8, J = 2.1, dd, C4-H, C5–H), 7.65–7.56 (m, 5H,
Ph-H)
147.50(C5), 138.44(C3), 137.84(C6), 137.04(C4), 136.07(C5 ), 130.08–
0
128.45(Ph-C), 128.10(C3
)
12.29(s,-CH proton), 9.08(s, 1H, C3-H), 8.85 (J = 3.8, d, C6-H), 8.50(J = 4.2, d,
162.44(C@O), 153.45(C@N), 148.28(C2148.15(C6), 147.97(C2), 147.50(C5),
0
0
C6–H), 8.44 (J = 4.2, d, C5-H), 7.75 (J = 4.4, d, C3-H), 7.52 (J = 9.0, J = 2.1, dd, C4-H, 138.92(C3), 136.84(C6), 135.04(C4), 135.01(C5 ), 127.10(C3
)
C5–H)

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solution was refluxed for 2 h. The precipitate separated out during
refluxing was filtered and washed with hot methanol, finally with
petroleum ether and then dried.
activation. Inoculation was done with the help of micropipette
with sterilized tips, 100 l of activated strain was placed onto
the surface of agar plate, spread over the whole surface and then
two wells having diameter of 10 mm were dug in media. Sterilized
stock solutions were used for the application in the well of inocu-
l
Methodology of antibacterial activity
lated agar plates. In this well of inoculated agar plates 100 ll of
solution was poured and incubated at 37 °C for 48 h. Activity was
determined by measuring the diameter of zone showing complete
inhibition and has been expressed in mm. All this experiments
were performed in triplicate.
In vitro antibacterial activity of ligands and their complexes
were assessed against a series of Gram positive Bacillus subtilis,
Micrococcus luteus and Gram negative Pseudomonas aeruginosa,
Pseudomonas mendocina using agar plate disc method [10–11].
Stock solution was made by dissolving compound in 10 mL of
DMSO. The media was made by dissolving Nutrient agar (15 g) in
1 L distilled water. The mixture was autoclaved for 15 min at
120 °C and then dispensed into sterilized Petri dishes, allowed to
solidify and then used for inoculation. Target microorganisms cul-
tures were prepared separately in 15 ml of liquid Nutrient agar for
Results and discussion
The ligands have been synthesized by condensation of pyrazine
carboxylic hydrazide with 2-benzoyl pyridine or pyridine 2-carbal-
dehyde. The homogeneity of these compounds was regularly
checked by TLC. The reaction of these ligands with metal salts in
1:2 molar ratio in dry methanol afford metal complexes of type
ML(1–2)2. All these metal complexes have been obtained as colored
solid, stable on prolonged exposure to air and insoluble in most of
common organic solvents expect in DMF and DMSO. The analytical
data of the ligands and complexes together with other physical
properties are summarized in Table 1. The molar conductivity of
metal complexes in DMSO has low value 1.8–2.7 ohmꢀ1 cm2 molꢀ1
indicating non electrolytic nature. The geometry of these metal
complexes has been determined using spectroscopic techniques
(IR, Electronic, ESR and Mass). The ligand was chelated to all the
metal ions in enolic form with the replacement of one hydrogen
atom as shown in Scheme (Fig. 1).
Table 4
Electronic spectra of compounds.
Sr. no.
Ligands/
complexes
Absorption
(cmꢀ1
Band
assignment
Geometry
)
1
2
3
4
HL1
HL2
Mn(L1)2
Co(L1)2
31,250
30,959
25,839
23,415
15,391
9826
24,839
15,977
10,873
25,510
16,492
23,445
25,745
23,126
15,267
9785
INCTb
–
–
INCTb
n?p⁄
Octahedral
Octahedral
4T1g(F)?4T1g(P)
4T1g(F)?4A2g(F)
4T1g(F)?4T2g(F)
3A2g(F)?3T1g(P)
3A1g(F)?3T1g(F)
3A2g(F)?3T2g(F)
5
6
Ni(L1)2
Cu(L1)2
Octahedral
p
N?Cu⁄
Distorted
2Eg(D)?2T2g(D)
LMCT
Octahedral
Octahedral
Octahedral
Octahedral
Infrared spectra
7
8
9
Zn(L1)2
Mn(L2)2
Co(L2)2
n?p⁄
IR spectra of complexes were analysed in comparison to free li-
gands (HL1&HL2) and the coordination sites were ascertained on
the basis of shifts in the frequencies of various groups and/or from
the intensity lowering. In the IR spectrum of the ligands strong
band at 1681–1695 cmꢀ1 and 3350–3361 cmꢀ1 were due to
4T1g(F)?4T1g(P)
4T1g(F)?4A2g(F)
4T1g(F)?4T2g(F)
3A2g(F)?3T1g(P)
3A1g(F)?3T1g(F)
3A2g(F)?3T2g(F)
10
Ni(L2)2
24,779
15,945
10,783
25,467
16,473
24,547
Octahedral
t(C@O) and t(N–H) bands, respectively. The absence of these
11
12
Cu(L2)2
Zn(L2)2
p
N?Cu⁄
Distorted
Octahedral
Octahedral
bands in the spectra of complexes indicated that hydrazone ligand
has coordinated to metal center through carbonyl oxygen after
deprotonation. This evidence was strengthened by presence of
new stretching vibrations between 1097 and 1050 cmꢀ1 attributed
2Eg(D)?2T2g(D)
LMCT
b
INCT – intranuclear charge transfer; LMCT – ligand to metal charge transition.
Fig. 2. ESR spectra of Cu(II) complex at 300 K.

J. Devi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 397–405
401
to
t
(C–O) in the complexes. The IR spectra of complexes displayed
carbonyl and azomethine carbons were slightly shifted downfield
in comparison to the corresponding signals of these groups in the li-
gands thereby confirming the complexation with zinc metal ion. It
was observed that DMSO did not have any coordinating effect on
the spectra of zinc complexes.
absorption bands at 1536–1556 cmꢀ1 which can be assigned to
C@N streching frequencies of coordinated ligands, whereas for free
ligands these bands were at 1577–1580 cmꢀ1. The shift to lower
frequencies (as compared to free ligand) indicated donation of
the lone pair of electrons on azomethine nitrogen to metal center,
this has been strengthened by the shift of t(N–N) band from 1050
Electronic spectra
to 1070 cmꢀ1 in free ligands to 1025–1070 cmꢀ1 in complexes and
supported by the presence of M–O and M–N bands at 427–
440 cmꢀ1 and 480–510 cmꢀ1, respectively [12] in the complexes.
Low energy pyridine ring in plane and out of plane vibrations ob-
served in spectrum of ligands at 693–698 cmꢀ1 were shifted to
higher frequencies at 694–702 cmꢀ1 on complexation indicated
that heterocyclic nitrogen (1) of pyridine ring was involved in coor-
dination [13] (Table 2).
Electronic spectra of complexes were recorded in DMF and com-
pared with that of corresponding ligands. In the electronic spectra of
Mn(II) complexes band due to n?p⁄ transition was appeared in the
range of 25,745–25,839 cmꢀ1 confirming their octahedral geometry
whereas d–d transition was not observed probably for low intensi-
ties for these complexes. In the electronic spectra of Co(II) com-
plexes and Ni(II) complexes three bands in the vicinity of
10,000 cmꢀ1, 17,000 cmꢀ1, 24,000 cmꢀ1 (Table 4) have been as-
signed due to T1g(F)?4T2g(F), T1g(F)?4A2g(F), T1g(F)?4T1g(P)
and 3A2g(F)?3T2g(F), 3A1g(F)?3T1g(F), 3A2g(F)?3T1g(P) respectively,
which was in accordance with octahedral geometry around cobalt
and nickel ion [14]. In the electronic spectra of Cu(II) complexes
bands in vicinity of 25,000 cmꢀ1 and 16,000 cmꢀ1 were due to
charge transfer and 2Eg?2T2g transition, respectively. In the elec-
tronic spectra of Cu(II) complexes broadness of band occur due to
Jahn-Teller distortion, indicating the distortion from octahedral
geometry [15]. In the electronic spectrum of Zn(II) complexes only
one band was observed due to LMCT transition.
4
4
4
1H and 13C NMR spectra
The NMR (1H and 13C) spectra of ligands/zinc(II) complexes were
recorded in CDCl3/CDCl3 containing small amount of DMSO-d6, with
tetramethylsilane as internal reference and data is given in Table 3.
1H NMR of ligands showed singlet at 15.08 ppm due to azomethine
proton and one singlet at 9.50 ppm due to proton attached to C3. The
aromatic protons appeared as doublet and multiplet in the range at
8.93–7.28 ppm. A comparative study of ligands and Zn(II) com-
plexes revealed the ligational behavior of the ligand. The absence
of peak at 15.08 ppm indicated deprotonation of –NH proton (via
enolization) and the shift in protons of pyridine ring were also
observed. In 13C NMR of ligand carbonyl carbon showed signal at
162.57 ppm, azomethine carbon at 152.53 ppm and aromatic car-
bons in between 149.20 and 127.10 ppm. The signals due to
ESR spectra of copper complexes
ESR spectra of copper complexes were recorded in DMSO at
300 K (Fig. 2). The observed value for Cu(II) complex with ligand
HL1 was g\ = 2.07 and g|| = 2.29 while for Cu(II) complex with li-
gand HL2 it was g\ = 2.06 and g|| = 2.26 (Table 5). The anisotropic
G
value have been calculated by using G = (g|| ꢀ2.0023)/
Table 5
ESR data of copper(II) complexes.
(g\ ꢀ2.0023) giving idea for exchange interactions between Cu(II)
centers. Since in these complexes G = 4.24 and 4.46 indicated that
there was no exchange in the copper complexes and hence dis-
torted octahedral geometry proposed for Cu(II) complexes [16].
Moreover, the value of g|| > g\ > 2.0023 which is in accordance with
Complex
g
g\
gav
G
k
a2
b2
k
Cu(L1)2
Cu(L2)2
2.31
2.29
2.07
2.06
2.15
2.13
4.54
4.24
618
535
0.65
0.63
0.97
0.96
Fig. 3. TG/DTG curve of complex Zn(L1)2.

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J. Devi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 397–405
axially elongated octahedral geometry and indicated that unpaired
electron present in dx2ꢀy2 ground state of Cu(II) [17]. The spin orbi-
tal coupling constant k was calculated using the relation gav = 2(1–
state, which was orbitally triply degenerated and would cause an
angular moment contribution to magnetic moment, Ni(II) com-
plexes showed magnetic moment 2.8 BM corresponding to two un-
paired electrons indicating a triplet ground state having octahedral
geometry around metal ion. Cu(II) complexes showed magnetic
moment 1.9 BM which was due to one unpaired electron and sug-
gested octahedral geometry of the complexes and Zn(II) complexes
were found to be diamagnetic as expected for d10 configuration.
2k/10Dq), gav=1/3(g|| + 2g\) which is less than Cu(II) k (832 cmꢀ1
)
free ion complex supported the covalent character of M–L bond
in complex. The molecular orbital coefficients, in-plane bonding
(b2) and in-plane 2) have been calculated using rela-
bonding (
p
r
a
tions [18] (Table 5). The observed value of covalency parameter
a2 (0.65) indicated that the complex has some covalent character
in ligand environment, if the value of
covalent bonding, while the value of
a
2 = 0.5, it indicates complete
a
Mass spectra
2 = 1.0 suggest complete io-
2 = 0.65 and b2 = 0.97, it was evi-
bonding (
2), where
bonding (b2) is almost ionic [19].
nic bonding. From these value of
dent that there is an interaction in in-plane
as in-plane
a
Mass spectra of Schiff base ligands and their complexes showed
molecular ion peaks which were in agreement with their molecular
formula (Table 1). The molecular ion peak for the ligand HL1
(C17H13N5O) and its corresponding cobalt complex (CoC34H24-
N10O2) observed at m/z 303.8 and 663.4, respectively. The purity
of the ligands and their complexes were checked using LC-mass
technique.
r
a
p
Magnetic susceptibility measurements
The observed magnetic moment for the transition metal com-
plexes are generally diagnostic of the coordination geometry
around metal ion. The effective magnetic moment was calculated
Thermogravimetric analysis
using the relation l
eff = 2.828(XmT)1/2 BM, where Xm is molar sus-
ceptibility. Mn (II) complexes showed magnetic moment values
in the range 5.8–5.9 BM which was expected for high spin d5 sys-
tem having octahedral geometry. The effective magnetic moment
values of 4.4–4.5 BM for Co(II) complexes corresponded to the
presence of three unpaired electrons indicating a quartet ground
Thermal behaviors of complexes were investigated by TG/DSC
techniques. The thermogravimetric analysis for the complexes
was carried out within temperature ranging from room tempera-
ture to 800 °C. The TG/DTG curve of complex ZnL(1)2 is given in
Fig. 3.
Table 6
Kinetic parameters of complexes.
Sr. no.
1
Compounds
Mn(L1)2
Decomposition range (ꢀC)
E (KJ molꢀ1
)
D
H (KJ molꢀ1
)
D
S (J molꢀ1 Kꢀ1
)
D
G (KJ molꢀ1
)
50–280
34.62
87.31
92.12
68.1
92.08
116.04
62.4
104.21
128.04
59.4
95.21
112.1
45.76
90.44
103.21
40.32
78.12
102.16
69.72
78.94
94.32
103.02
81.04
124.08
81.13
105.04
91.05
61.84
107.6
95.16
105.6
83.58
103.4
136.3
103.4
105.6
ꢀ9.87
72.06
85.51
105.96
68.1
92.08
280–500
500–780
50–280
280–500
500–780
50–280
280–500
500–780
50–280
280–500
500–780
50–280
280–500
500–780
50–280
280–500
500–780
ꢀ15.73
ꢀ18.36
ꢀ13.04
ꢀ45.81
ꢀ32.14
ꢀ25.4
2
3
4
5
6
Co(L1)2
Ni(L1)2
Cu(L1)2
Zn(L1)2
Zn(L2)2
116.04
62.4
104.21
128.04
85.91
111.86
104.58
108.05
91.71
114.31
138.88
108.61
115.87
ꢀ43.51
ꢀ58.12
ꢀ17.4
ꢀ13.53
ꢀ15.58
ꢀ11.28
ꢀ21.35
ꢀ18.34
ꢀ12.04
ꢀ14.05
ꢀ20.06
Table 7
In vitro antibacterial activity of ligands and their complexes.
Compounds
Zone of inhibition (mm)
Gram +ve
Gram –ve
B. subtilis
M. Luteus
P. aeruginosa
P. mendocina
25
50
100
200
25
50
100
200
25
50
100
200
25
50
100
200
1
2
3
4
5
6
7
8
HL1
HL2
9
8
12
10
18
15
17
20
13
16
12
16
19
13
21
13
11
20
16
19
22
15
19
15
18
21
14
25
15
14
21
19
21
23
17
20
17
19
22
17
26
11
8
13
10
18
15
19
17
13
15
13
15
16
11
20
15
11
21
17
20
21
16
18
14
17
20
13
23
18
14
21
18
21
22
18
20
16
20
22
16
25
8
10
11
17
13
16
18
11
15
11
13
17
10
23
11
12
19
14
18
20
13
16
12
14
19
11
25
13
14
21
17
20
23
16
19
15
18
22
14
28
10
8
11
9
13
10
20
16
19
22
14
15
14
16
20
13
27
18
13
23
18
22
24
17
18
16
19
22
14
30
10
15
11
14
16
9
13
10
11
15
9
Mn(L1)2
Co(L1)2
Ni(L1)2
Cu(L1)2
Zn(L1)2
Mn(L2)2
Co(L2)2
Ni(L2)2
Cu(L2)2
Zn(L2)2
Streptomycin
17
13
14
18
12
14
10
15
16
10
19
16
13
16
15
12
13
10
13
15
10
19
17
13
16
18
11
11
10
12
15
10
22
18
15
17
20
13
13
12
14
18
11
25
9
10
11
12
13
20

J. Devi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 397–405
403
ligand HL1 follows the order Mn < Zn < Cu < Co < Ni. Complexes of
ligand HL1 with Zn(II) ion were more stable as compared to complex
of ligand HL2 with same metal ions.
All the complexes were stable up to 250 °C as no mass loss was
observed below 250 °C, eliminated the possibility of water mole-
cule outside the coordination sphere.
In general, the stages of thermal decomposition of the com-
plexes can be written as shown:
Antimicrobial results
50ꢀ250 ꢂC
MðL1ꢀ2Þ2
No hydrated water
ƒƒƒƒƒƒƒ!
Determination of in vitro antimicrobial activity of the hydra-
zones and their corresponding metal complexes are given in Table 7
and their graphical representation in Figs. 4 and 5. Antibacterial
studies were done by the agar plate disc method on the following
strains i.e., Gram positive Bacillus subtilis, Micrococcus luteus and
Gram negative Pseudomonas aeruginosa, Pseudomonas mendocina
using different concentration of ligands and their complexes (25,
[M=Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)]
250ꢀ500 ꢂC
MðL1ꢀ2Þ2
Intermediate ðunstableÞ
ƒƒƒƒƒƒƒ!
500ꢀ800 ꢂC
Intermediate
Metal oxide
ƒƒƒƒƒƒƒ!
50, 100, 200 lg/mL). Streptomycin was used as a standard drug
for antibacterial activity.
Intermediates formed in the subsequent steps ultimately have
undergone violent decomposition to give the respective metal oxi-
des. The thermodynamic activation parameters of decomposition
processes of dehydrated complexes namely activation energy (E),
It may be concluded from the antimicrobial activity data that:
(i) the antimicrobial activity of ligands and their complexes
was due to the presence of toxophorically important imine
or carbonyl groups. The mode of action of these compounds
may involve the formation of hydrogen bond through
enthalpy (
DH), entropy (DS) and free energy of decomposition
(D
G) (Table 6), were evaluated by employing the Coats–Redfern
method [20] and were helpful in assigning the strength of the com-
plexes. It was found that the thermal stabilities of the complexes of
Fig. 4. Antibacterial activity data of ligands and their transition metal complexes (1–12 as in Table 7).

404
J. Devi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 397–405
Fig. 5. Average antibacterial activity data of ligands and their transition metal complexes.
azomethine/carbonyl group with the active center of cell
constituents, there by resulting in interference with normal
cell process.
Conclusion
Tridentate
Schiff base ligands have been synthesized
N
N
O
(ii) A marked enhancement of in vitro biocidal studies of the
ligands was exhibited on coordination with metal ion against
all microorganisms strains under tested identical experimen-
tal conditions. The increase in antimicrobial activity may be
explained on basis of fact that on chelation the polarity of
metal ion is reduced due to overlap of ligand orbital and shar-
ing of positive charge of metal ion with donor groups. Further
it increases delocalization of chelate ring and increases the
lipophilicity of complexes. This increased lipophilicity
enhances penetration of complexes there by disturbing the
respiration process of cell and blocking the synthesis of
proteins, which further restricts growth of organisms.
(iii) Ligand HL1 was found to be more potent as compared to HL2
(except in Pseudomonas aeruginosa) which may be due to
fact that phenyl ring is electron releasing thereby increasing
electron density over azomethine nitrogen and therefore
have comparatively stronger interactions with active centers
of cell constituents.
and coordinated to divalent metal ions through azomethine nitro-
gen, carbonyl oxygen and nitrogen of pyridine ring forming stable
complexes of ML2 type with octahedral geometry for Mn(II), Co(II)
and Ni(II) ions while with Cu(II) ions distorted octahedral geome-
try has observed. Ligands and their complexes were evaluated for
antimicrobial activity (in vitro) of the hydrazones and their
corresponding metal complexes. These complexes were found to
be more active than respective ligands under identical conditions.
Among these complexes Copper(II) complexes were found to be
most active, compared to other complexes.
Acknowledgments
Nisha Batra is thankful to Department of Science and Technol-
ogy, Panchkula Haryana for providing HSCST-JRF fellowship vide
letter No. HSCST/4117 on dated 11/11/10.
Appendix A. Supplementary data
(iv) Complexes of HL1 with divalent transition metal ion were
found to be more active as compared to complexes of HL2.
Among these complexes Copper(II) complexes were found
to be most active, compared to other complexes. The trend
of growth inhibition in the complexes was found to be in
the order: Cu > Mn > Ni > Co > Zn.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.06.026.
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of Gram (ꢀ) strains have more antigenic properties due to
the presence of an outer lipid membrane of lipopoly-
saccharides.
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