Synthesis, crystal structure, electrochemistry and studies on protein binding, antioxidant and biocidal activities of Ni(II) and Co(II) hydrazone complexes

Article abstract of DOI:10.1016/j.poly.2011.09.021

Bivalent transition metal hydrazone complexes of the composition [Ni(L ₁)₂] (1), [Co(L₁)₂] (2), [Ni(L ₂)₂] (3) and [Co(L₂)₂] (4) have been synthesised from the reactio

Full text of DOI:10.1016/j.poly.2011.09.021

Polyhedron 31 (2012) 294–306
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, crystal structure, electrochemistry and studies on protein binding,
antioxidant and biocidal activities of Ni(II) and Co(II) hydrazone complexes
P. Sathyadevi a, P. Krishnamoorthy a, M. Alagesan a, K. Thanigaimani b, P. Thomas Muthiah b,
N. Dharmaraj a,
⇑
a Department of Chemistry, Bharathiar University, Coimbatore 641 046, India
b School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2011
Accepted 16 September 2011
Available online 22 September 2011
Bivalent transition metal hydrazone complexes of the composition [Ni(L1)2] (1), [Co(L1)2] (2), [Ni(L2)2] (3)
and [Co(L2)2] (4) have been synthesised from the reactions of [MCl2(PPh3)2] (where M = Ni or Co) with
hydrazones derived from 2-acetyl pyridine and carboxylic acid hydrazides of benzhydrazide (HL1) or thi-
ophene-2-carboxylic acid hydrazide (HL2), respectively. Structure of the ligands HL1 and HL2 and their
corresponding complexes with Ni(II) and Co(II) ions were proposed based on the elemental analysis,
infrared and 1H NMR spectral methods. Single crystal X-ray diffraction study of complex 1 revealed a dis-
torted octahedral geometry around the metal ion provided by two units of the ligand. To explore the
potential medicinal value of the new complexes, binding interaction of all the complexes with bovine
serum albumin (BSA) was studied at normal physiological conditions using fluorescence and UV–Vis
spectral techniques. The number of binding sites (n) and binding constant (Ka) were calculated according
to the double logarithm regression equation. The results of synchronous fluorescence spectrum showed
that binding of metal hydrazones with BSA induced conformational changes in BSA. The in vitro antiox-
idant and antimicrobial potentials of the new chelates were also carried out.
Keywords:
Metal hydrazone complex
Crystal structure
Protein interaction
Antioxidant
Antimicrobial
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
stable drug–protein complex that can exert important effect on the
distribution, free concentration and metabolism of the drug in the
Metal ions, especially those of bivalent type, distribute in human
blood and play an essential structural role in binding with many
proteins. They have a definite ability to bind proteins and affect
the binding of drugs to albumin through their coordinate bonds.
Investigation on the effect of metal ions on drug–albumin binding
is useful to understand the transport and mechanism of the drug
in the body [1,2]. Bovine serum albumin (BSA) is a protein with
hydrophobic patches that could be the initial targets of their asso-
ciation to biomolecules [3]. Among various bio-macromolecules,
serum albumins are the most abundant soluble proteins in the cir-
culatory system of a wide variety of organisms and have many
indispensable physiological functions. Serum albumins often in-
crease the apparent solubility of hydrophobic compounds in plas-
ma and modulate their delivery to cells. Therefore, the absorption,
distribution, metabolism and excretion properties, as well as the
stability and toxicity of chemical substances can be significantly af-
fected because of their binding to serum albumins [4]. The interac-
tions of drugs with any protein normally result in the formation of a
blood stream. Thus the drug–albumin complex may be considered
as a model for gaining fundamental insights into the drug–protein
interactions. In this regard, bovine serum albumin (BSA) was stud-
ied extensively in the past years, partly because of its structural
homology with human serum albumin (HSA) [5,6]. Moreover, there
is evidence of conformational changes of serum albumin induced
by its interaction with low molecular weight molecules that in turn
affect the secondary and tertiary structure of albumins [7]. Many
drugs including anti-coagulants, tranquilizers, anti-allergic drugs,
anti-cancer drugs etc., are transported in the blood while bound
to albumin [8]. This has stimulated much interest in the nature of
binding sites and investigations of whether fatty acids, natural
metabolites and drugs compete with one another for binding to
the protein [9]. Therefore, studies on the binding of chemicals with
protein will aid interpretation of the metabolism and transport pro-
cess of them and will help to explain the relationship between the
structure and the function of the protein.
Hydrazones are important class of ligands with interesting liga-
tion properties due to the presence of several coordination sites
[10] and are widely applied in the field of insecticides, medicines
and analytical reagents due to their excellent bioactivity [11]. Var-
ious important properties of carbonic acid hydrazides along with
⇑
Corresponding author. Tel.: +91 422 2428316; fax: +91 422 2422387.
E-mail address: dharmaraj@buc.edu.in (N. Dharmaraj).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.09.021

P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
295
their applications in medicine and analytical chemistry have led to
increased interest in their complexation characteristics with tran-
sition metal ions [12]. In this respect, the formation of metal com-
plexes plays an important role to enhance their biological activity
[13].
Studies have also shown that the azomethine nitrogen which
has a lone pair of electrons in a sp2 hybridised orbital has consid-
erable biological importance. Hydrazone derivatives are found to
possess antimicrobial, anti-tubercular, anti-convulsant and anti-
inflammatory activities [14–19]. Some hydrazone analogs have
been investigated as potential oral iron chelating drugs for the
treatment of genetic disorders such as thalassemia and have also
been suggested as possible metal chelating agents for treating neu-
rodegenerative disorders such as Alzheimer disease [20–22].
emission spectra were recorded on Jasco FP 6600 spectrofluorom-
eter using methanol as a solvent. Cyclic voltammetric study of the
complexes was conducted in methanol using CH instrument
electrochemical analyzer with tetrabutylammonium perchlorate
(TBAP) as supporting electrolyte. All the solutions were purged
with nitrogen prior to measurements at room temperature. A
three-electrode configuration that comprised of glassy carbon as
a working electrode, platinum wire as an auxiliary electrode and
an Ag/AgCl electrode as a reference electrode were used.
The X-ray diffraction data of the complex 1, were collected at
293 K with MoK
a radiation (k = 0.71073 Å) using a Bruker Smart
APEX II CCD diffractometer equipped with graphite monochroma-
tor. The structure was solved by direct method and refined using
full-matrix least squares (on F2) SHELXS97 and SHELXL97 [23] and
R2
R2
R2
H
N
R3
N
R3
N
R3
R1
N
R1
N
R1
N
-H
O
O
O
General formula of deprotonable acyl− /aroyl− hydrazones and mesomerism of the anion
obtained by deprotonation.
Detailed literature survey reveals that attempts have not been
made so far to investigate the interaction of metal hydrazone che-
lates 1–4 with BSA. In continuation of our interest in the chemistry
and biology of transition metal hydrazone chelates, we carried out
a systematic work to synthesize, characterize and to investigate
the interaction of Ni(II) and Co(II) hydrazone complexes with
BSA. The nature of binding between the chelates and BSA were
monitored through fluorescence and synchronous spectroscopy.
In vitro antioxidant and antimicrobial activities of the free ligands
as well as the metal chelates involved in this study have also been
assessed and compared with that of commercial standards.
the graphics were produced using PLATON97 [24]. All the non-hydro-
gen atoms were refined anisotropically and the hydrogen atoms
were positioned geometrically and refined as riding model.
2.3. Synthesis of starting metal complexes
The precursor metal complexes [NiCl2(PPh3)2] and [CoCl2
(PPh3)2] were prepared according to the literature methods [25,26].
2.4. Synthesis of[N0-(1-pyridin-2-yl)ethylidene)benzohydrazide] (HL1)
and[N0-(1-pyridin-2-yl)ethylidene)thiophene-2-carbohydrazide] (HL2)
ligands
2. Experimental
2.1. Materials
The hydrazone ligands HL1 and HL2 were prepared by mixing
equimolar (10 mmol) amounts of 2-acetylpyridine with benzhy-
drazide and thiophene-2-carboxylic acid hydrazide in ethanol
(50 ml) as shown in Scheme 1. The reaction mixture was then re-
NiCl2ꢀ6H2O, CoCl2ꢀ6H2O, triphenylphosphine (Sigma–Aldrich
chemie), 2-acetyl pyridine and thiophene-2-carboxylic acid hydra-
zide (Alfa Aesar) were obtained commercially and used without
any purification. Bovine serum albumin (BSA) was obtained from
Himedia. Solvents used for the synthesis were purified by standard
methods. All the other chemicals and reagents used for protein
binding, antioxidant and antimicrobial studies were of high quality
and commercially purchased from reputed suppliers.
CH3
O
O
H2N
+
R
N
H
N
EtOH
Reflux, 5h
2.2. Physical measurements
CH3
N
CH3
N
H
N
R
N
R
Microanalyses (% C, H and N) were performed on a Vario EL III
CHNS analyzer. Melting points were determined with Raaga appa-
ratus. Infrared spectra of the samples were recorded as KBr pellets
on a Nicolet Avatar model in the range of 400–4000 cmꢁ1. 1H NMR
spectra of the ligands were recorded with a Bruker AMX 500
instrument operating at 500 MHz using CDCl3 as a solvent and
TMS as an internal standard. Electronic spectra were recorded on
Jasco V-630 spectrophotometer using methanol as a solvent. The
N
O
N
OH
Keto form
Enol form
S
where, R =
(or)
Scheme 1. Synthesis of hydrazone ligands.

296
P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
Pathway 1
Ph3P
Cl
M
N
O
N
N
R
CH3
Reflux, 5h
MeOH / Alc.KOH
CH3
[MCl2(PPh3)2]
+
CH3
H
N
N
R
R
N
MeOH / Alc.KOH
Reflux, 5h
R
N
MeOH / Alc.KOH
Reflux, 5h
N
O
N
O
M
M
Cl
CH3
H
Cl
Cl
Pathway 2
N
N
Pathway 3
N
O
MeOH / Alc.KOH Reflux, 5h
CH3
N
R
N
where, M = Ni(II) (or) Co(II)
S
N
O
O
N
M
N
and R =
(or)
N
R
CH3
Pathway 4
Scheme 2. Synthesis of metal hydrazone complexes.
fluxed on a water bath for 5 h and cooled to room temperature. The
hydrazones formed were filtered, washed and recrystallized from
ethanol and the purity was checked by TLC.
For complex 1: Yield, 53%. M.p. >250 °C. Anal. Calc. for
28H24N6O2Ni1: C, 62.83; H, 4.52; N, 15.70. Found: C, 62.59; H,
4.63; N, 15.79%.
C
For HL1: Yield, 90%. M.p. 168 °C. Anal. Calc. for C14H13N3O1: C,
70.21; H, 5.43; N, 17.56. Found: C, 69.89; H, 5.18; N, 17.41%.
For HL2: Yield, 88%. M.p. 166 °C. Anal. Calc. for C12H11N3O1S1: C,
58.75; H, 4.52; N, 17.12; S, 13.07. Found: C, 58.46; H, 4.58; N,
17.27; S, 13.25%.
For complex 2: Yield, 50%. M.p. >250 °C. Anal. Calc. for
28H24N6O2Co1: C, 62.75; H, 4.51; N, 15.69. Found: C, 62.89; H,
4.33; N, 15.54%.
For complex 3: Yield, 51%. M.p. >250 °C. Anal. Calc. for
24H20N6O2S2Ni1: C, 52.67; H, 3.68; N, 15.35; S, 11.72. Found: C,
C
C
52.91; H, 3.77; N, 15.22; S, 11.49%.
For complex 4: Yield, 48%. M.p. >250 °C. Anal. Calc. for
2.5. Synthesis of new hydrazone metal complexes
C24H20N6O2S2Co1: C, 52.64; H, 3.68; N, 15.34; S, 11.71. Found: C,
52.50; H, 3.76; N, 15.19; S, 11.57%.
Complexes [Ni(L1)2] (1), [Co(L1)2] (2), [Ni(L2)2] (3) and [Co(L2)2]
(4) have been synthesised from the reactions of equimolar amount
of ligand HL1 (0.239 g; 1 mM) or HL2 (0.245 g; 1 mM) with
[NiCl2(PPh3)2] (0.653 g; 1 mM) or [CoCl2(PPh3)2] (0.653 g; 1 mM)
in 40 ml of methanol in the presence of methanolic KOH (Scheme
2). The mixture was then continuously refluxed for 5 h. After cool-
ing the solution to room temperature, the precipitate formed was
collected by filtration, washed thoroughly with methanol and dried
overnight in a vacuum desiccator. The purity of all the complexes
was checked by TLC. Micro-analytical data and yield are presented
below in this section. Complex 1 dissolved in methanol and chloro-
form mixture under slow evaporation resulted in the formation of
red colored crystals suitable for X-ray diffraction study.
2.6. Protein binding studies
Binding of metal hydrazone complexes with bovine serum albu-
min (BSA) was studied using fluorescence spectra recorded with
excitation at 280 nm and corresponding emission at 344 nm
assignable to that of bovine serum albumin (BSA). The excitation
and emission slit widths and scan rates were constantly main-
tained for all the experiments. Samples were carefully degassed
using pure nitrogen gas for 15 min by using quartz cells
(4 ꢂ 1 ꢂ 1 cm) with high vacuum Teflon stopcocks. Stock solution
of BSA was prepared in 50 mM phosphate buffer (pH 7.2) and

P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
297
stored in the dark at 4 °C for further use. Concentrated stock solu-
tions of the copper complex were prepared by dissolving them in
DMSO: phosphate buffer (1:99) and diluted suitably with phos-
phate buffer to required concentrations. 2.5 ml of BSA solution
For the above three assays, all the tests were run in triplicate
and various concentrations of the ligands and their complexes
were used to fix a concentration at which the test compounds
showed in and around 50% of activity. In addition, the percentage
(1
l
M) was titrated by successive additions of a 2
l
l stock solution
of activity was calculated using the formula, % of activ-
of metal hydrazone complexes (10ꢁ3 M) using a micropipette. Syn-
chronous fluorescence spectra were also recorded using the same
concentration of BSA and complexes as mentioned above with
ity = [(A0 ꢁ AC)/A0] ꢂ 100. A0 and AC are the absorbance in the ab-
sence and presence of the tested compounds, respectively. The
50% of activity (IC50) can be calculated using the percentage of
activity results.
two different
Dk (difference between the excitation and emission
wavelengths of BSA) values of 15 and 60 nm.
2.8. Antimicrobial activity
2.7. Antioxidant activity
The in vitro antibacterial and antifungal screening of the hydra-
zone ligands (HL1) and (HL1) and their corresponding metal hydra-
zone complexes 1, 2, 3 and 4 have been carried out against human
pathogenic gram positive bacteria (Staphylococcus aureus and
Enterococcus faecalis) and gram negative bacteria (Klebsiella pneu-
moniae, Escherichia coli and Salmonella typhium) and fungi (Tricho-
phyton rubrum, Candida albicans (yeast like)) using disc diffusion
method [30]. The bacterial and fungal strain were inoculated into
the nutrient agar broth and sabouraud dextrose agar broth and
incubated for 24 and 48 h, respectively in petriplates. In the disc
diffusion method, sterile nutrient agar for bacteria and potato dex-
trose agar medium for fungi were separately inoculated with the
test microorganisms. The compounds to be tested were dissolved
in DMSO to a final concentration of 2.5, 5.0 and 7.5 mM and soaked
in filter paper disc of 5 mm diameter and 1 mm thickness. The
plates were incubated at 32 °C for bacteria (18–24 h) and at
25 °C for fungus (72 h). The diameter (mm) of inhibition zone
around each disc was determined after 24 h (bacteria) and 48 h
(fungus). Triplicate plates were maintained in both the assays.
Streptomycin and Nystatin were used as standards for antibacterial
and antifungal studies respectively.
2.7.1. Hydroxyl radical scavenging activity
The hydroxyl radical scavenging activity of the compounds was
investigated using the Nash method [27]. In vitro hydroxyl radicals
were generated by Fe3+/ascorbic acid system. The detection of hy-
droxyl radicals was carried out by measuring the amount of form-
aldehyde formed from the oxidation reaction with DMSO. The
formaldehyde produced was estimated spectrophotometrically at
412 nm. A mixture of 1.0 mL of iron – EDTA solution ferrous
ammonium sulfate (0.331 mM) and EDTA (0.698 mM), 0.5 mL of
EDTA solution (0.048 mM) and 1.0 mL of DMSO (10.83 mM) (v/v)
in 0.1 M phosphate buffer, pH 7.4) sequentially added with solu-
tion of test compounds with different concentrations in the test
tubes. The reaction was initiated by adding 0.5 mL of ascorbic acid
(1.25 mM) and incubated at 80–90 °C for 15 min in a water bath.
After incubation, the reaction was terminated by the addition of
1.0 mL of ice-cold TCA (107 mM). Subsequently, 3.0 mL of Nash re-
agent was added to each tube and left at room temperature for
15 min. The reaction mixture without sample was used as control.
The intensity of the color formed was measured spectrophotomet-
rically at 412 nm against reagent blank. The IC50 values of the sam-
ples were also calculated.
3. Results and discussion
2.7.2. Nitric oxide radical scavenging activity
Nitric oxide scavenging activity was determined based on the
reported method [28], where sodium nitroprusside in aqueous
solution at physiological pH spontaneously generates nitric oxide,
which interacts with oxygen to produce nitrite ions that can be
estimated using Greiss reagent. Scavengers of nitric oxide compete
in this process leading to reduced production of nitrite ions. For the
experiment, sodium nitroprusside (10 mM) in phosphate buffered
In this study, we treated the respective metal precursor com-
plexes and the heterocyclic hydrazone ligands in 1:1 molar ratio
(Scheme 2) with the following expectations that these hydrazones
could behave as
(i) A uninegative bidentate N,O chelating system to yield a four
coordinated complex (pathway 1), or
(ii) A uninegative tridentate chelating system through N,N,O
donor atoms to form yet another type of four coordinated
complex (pathway 2), or
(iii) An unusual neutral tridentate N,N,O chelating system to
yield a five coordinated metal hydrazone complex with
two chloride ions for charge compensation (pathway 3).
saline was mixed with 100 lL of test samples of variable concen-
trations and incubated at room temperature for 150 min. The same
reaction mixture without the sample but the equivalent amount of
the solvent served as the control. After the incubation period,
0.5 mL of Griess reagent containing sulfanilamide (5.8 mM),
H3PO4 (20 mM) and N-(1-naphthyl) ethylenediamine dihydrochlo-
ride (0.39 mM) was added. The absorbance of the chromophore
formed was measured at 546 nm.
Unexpectedly, the heterocyclic hydrazone ligands [HL1] and
[HL2] involved in this work deceived all our expectations and
formed stable six coordinated metal chelates with 1:2 metal–li-
gand stoichiometry (pathway 4) without any other side products.
The analytical data of all the complexes presented elsewhere in
the manuscript also supported the 1:2 compositions.
2.7.3. Lipid peroxidation
The lipid peroxidation was evaluated by the modified method of
Draper and Hadley [29], 1990 in ex vivo. Rat liver was cut into small
species and stored in deep freezer. One gram of liver was homog-
enized with 10 ml of saline buffer. From that, 200
nate was taken and added with 100 L of test compounds with
different concentrations and 50 L of 0.07 M FeSO4. The content
lL of homoge-
l
l
3.1. Solid structure of the complex [Ni(L1)2] (1)
was incubated for 30 min at 37 °C and sequentially added with
1.5 ml of acetic acid (198.5 mM), 1.5 ml of thiobarbituric acid
(8 mM) and 50 lL of trichloroacetic acid (0.2 mM), respectively.
The reaction mixture was further incubated in boiling water bath
for 60 min and allowed to cool at room temperature and added
with 5 ml of butanol. Then the contents were centrifuged and
supernatant was collected and read at 532 nm.
Molecular structure of [Ni(L1)2] (1) together with the atom
labeling scheme is depicted in Fig. 1. The crystallographic data with
selected bond lengths and bond angles are listed in Tables 1 and 2.
The compound crystallized in monoclinic lattice with space group
Cc. Ni(II) center exhibits a distorted octahedral geometry compris-
ing of two equivalent monoanionic ligands coordinated in a

298
P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
Table 2
0
Selected bond lengths (ÅA) and bond angles (°).
C28H24N6Ni1O2 (1)
Bond lengths
Bond angles
Ni1–N1
Ni1–N3
Ni1–O1
Ni1–N6
Ni1–N4
Ni1–O2
C2–C1–N2–N1
N2–N1–C8–C10
C16–C15–N5–N4
N4–C22–C24–C25
1.986(2)
2.095(2)
2.097(2)
2.094(2)
1.989(2)
2.102(2)
ꢁ179.5(2)
ꢁ178.2(2)
ꢁ177.6(2)
168.8(2)
O1–Ni1–N1
N1–Ni1–N3
O2–Ni1–N4
N4–Ni1–N6
O1–Ni1–N3
O2–Ni1–N6
N4–Ni1–N1
N1–Ni1–N6
N1–Ni1–O2
N3–Ni1–N4
N4–Ni1–O1
O1–Ni1–O2
N3–Ni1–N6
76.34(7)
78.27(7)
76.60(7)
78.07(8)
154.04(7)
154.27(7)
174.24(8)
105.96(8)
99.63(7)
106.12(7)
99.58(7)
96.21(7)
89.46(8)
occupy the coordination sites with the ligand–metal–ligand bite
angles varying between 78.27(7)° [N3–Ni1–N1], 76.34(7)° [O1–
Ni1–N1], 76.60(7)° [O2–Ni1–N4] and 78.07(8)° [N4–Ni1–N6].
The two central coordinating bonds [Ni1–N1] 1.986(2)] Å and
[Ni1–N4] 1.989(2) Å are comparable to the basal planar bonds
[Ni1–O1] 2.097(2) Å, [Ni1–N3] 2.095(2) Å, [Ni1–N6] 2.094(2) Å
and [Ni1–O2] 2.102(2) Å confirming that all the bonds in basal
plane are almost equal in length but the axial bonds are little longer
than that of the former. The [N4–Ni1–N1] trans angle was found as
174.24(8)° but the other trans angles [O1–Ni1–N3] 154.04(7)° and
[O2–Ni1–N6] 154.27(7)° are constrained within the meridional li-
gands. These observations suggested that the coordination geome-
try around the nickel ion had undergone much distortion from a
perfect octahedron. A perspective view of unit cell packing of com-
plex 1 is shown in Fig. 2.
Fig. 1. Molecular structure of complex 1 showing the atom-numbering scheme
with ellipsoid of 25% probability.
Table 1
Crystal data and structure refinement data of complex 1.
Empirical formula
C28H24N6Ni1O2 (1)
Name
[Ni(L1)2]
535.24
monoclinic
Cc
293(2)
0.71073
4
Formula weight
Crystal system
Space group
Temperature (K)
Wavelength (Å)
Z
3.2. Infrared spectra
In the absence of a powerful technique such as X-ray crystallog-
raphy, IR spectra has proven to be the most suitable technique to
Unit cell dimensions
a (Å)
b (Å)
10.2389(2)
19.6419(6)
12.2648(3)
90
91.538(10)
90
c (Å)
a
(°)
b (°)
(°)
c
Color
Red
Dcalc (Mg/m3)
F(000)
1.442
1112
Crystal size (mm)
hkl Limits
0.22 ꢂ 0.20 ꢂ 0.17
ꢁ14 6 h 6 14
–28 6 k 6 29
–18 6 l 6 17
2.07° to 32.41°
7663
R1 = 0.0364, wR2 = 0.1036
R1 = 0.0427, wR2 = 0.1077
1.046
h Range for data collection
Reflections collected
R indices (final)
R indices (all data)
Goodness-of-fit on F2
meridional fashion using cis pyridyl nitrogen, trans azomethine
nitrogen and cis enolate oxygen atoms positioned very nearly per-
pendicular to each other.
The ligand has undergone keto–enol tautomerism and it is well
known that the imino tautomers can exist as two geometrical iso-
mers, syn (Z) and anti (E), but in this crystal, only the E isomer has
been observed. The torsion angles C2–C1–N2–N1, ꢁ179.5(2)°,
N2–N1–C8–C10, ꢁ178.2(2)°, C16–C15–N5–N4, ꢁ177.6(2)° and
N4–C22–C24–C25, 168.8(2)° observed supports the E conformation
of the ligand on coordination. The coordination around nickel is dis-
torted octahedral with two oxygen atoms and four nitrogen atoms
Fig. 2. Packing diagram of the unit cell of complex 1.

P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
299
Table 4
give enough information to elucidate the nature of bonding of the
Characteristic electronic and emission spectral data.
ligands to the metal ions.
IR spectra of the ligands showed characteristic stretching vibra-
tions in the region of 3178 & 3170, 1656 & 1643, 1540 & 1573 and
Complex
kmax (nm)
Emissionc data (kmax (nm))
1
2
3
4
370a, 289b, 248b
354a, 281b, 232b
387a, 309b, 268b, 241b
368a, 267b, 243b
496
461
447
416
1060 & 993 cmꢁ1 due to m(N–H)
, m(C@O), m(C@N) and m(N–N) vibrations of
HL1 and HL2 ligands, respectively. In the IR spectra of the com-
plexes, we observed the disappearance of bands due to C@O and
N–H stretching vibrations and the appearance of new bands attrib-
uted to m(C–O) and m(–C@N–N@C–) in the range of 1299–1306 and
1591–1598 cmꢁ1 & 1490–1497 cmꢁ1 [31], respectively due to
enolisation followed by deprotonation prior to coordination. Fur-
ther, the stretching frequency of C@N slightly shifted to lower
wave number, which supports that azomethine nitrogen atoms
were coordinated to the central metal atom [32]. In addition, the
a
b
c
LMCT.
ILCT.
Excitation at LMCT.
characteristic m(N–N) stretching of the free ligands HL1 and HL2 (ob-
1.2
1.0
0.8
0.6
0.4
0.2
0.0
served at 1060 or 993 cmꢁ1) undergoes a positive shift to a higher
wave number upon complexation due to diminished repulsion be-
tween the lone pairs of adjacent nitrogen atoms [33–35]. All the
above mentioned IR spectral data are summarized in Table 3.
3.3. Electronic spectra
The electronic spectra of the complexes recorded in methanol
displayed three to four bands in the region 232–387 nm (Table
4). The bands appeared in 232–309 nm region in the spectra of
all the complexes is assigned to the intra-ligand
p–
p⁄ transition
due to the formation of conjugated double bond after complexa-
tion to the metal ions [36]. In addition to the above absorptions,
all the new hydrazone complexes showed another band in the re-
gion 367–387 nm aroused out the ligand to metal charge transfer
(LMCT) transitions [37] and a representative spectrum for the com-
plex 1 is shown in Fig. 3. The nature and position of the electronic
spectral absorptions of all the complexes indicated an octahedral
geometry around metal ions in these complexes.
350
400
450
500
550
Wavelength (nm)
Fig. 3. Absorption (––) and emission (—) spectra of complex 3.
new photoluminescent materials. Emission properties of the metal
hydrazone complexes in methanol were examined at room temper-
ature. When an excitation wavelength corresponding to the lowest
energy absorption was used, no emission band was observed. How-
ever, when the samples are excited using higher wavelength, all the
complexes showed strong emission. The emission maxima of the
complexes were observed in the 416–496 nm range and are listed
in Table 4 and a representative spectrum for the complex 1 is shown
in Fig. 3 (dashed line). The wavelength of emission maxima of all
the complexes have experienced a positive shift of about 48–
126 nm compared to those of excitation maxima. The observed CT
luminescence in these complexes may be due to the presence of
imine functional group. The luminescence observed in these com-
3.3.1. 1H NMR spectra
1H NMR spectra of the hydrazone ligands were assigned on the
basis of observed chemical shift and relative intensities of the sig-
nals. The 1H NMR spectra of both the ligands HL1 and HL2 dis-
played sharp singlets owing to the –NH protons and the methyl
protons of –CH3–C@N at 9.07 or 9.38 ppm and at 2.48 or
2.53 ppm respectively. The signals corresponding to the protons
of respective benzoyl, thiophene and pyridyl moieties were ob-
served as multiplets in the range of 7–8 ppm. A representative
spectrum for the HL2 ligand is shown in Fig. 4. All these observa-
tions supported the structure of the ligands assigned on the basis
of infrared vibrations.
plexes are a molecular property, attributed to the
p–
p⁄ transition
of the N-arylmethylenebenzhydrazones [38].
3.4. Emission spectra
3.5. Electrochemical studies
Synthesis of coordination complexes using functional ligands
and transition-metal centers can be a useful method to obtain
Electrochemical properties of the ligands HL1 and HL2 and cor-
responding metal complexes 1–4 were studied in methanol with
tetrabutylammonium perchlorate as supporting electrolyte at a
scan rate of 100 mV sꢁ1. The ligands used in this work were not
reversibly reduced or oxidized in the potential range applied,
hence the observed redox process were assigned to the metal cen-
tered only. The cyclic voltammogram of the complex 1 showed the
peaks at +0.8965 and +0.8024 V was shown in Fig. 5 (Table 5). Ano-
dic response obtained at +0.8965 V is believed to be due to
Ni(II) ? Ni(III) oxidation and the cathodic response at +0.8024 V
is due to the Ni(III) ? Ni(II) reduction. The peak-to-peak separation
Table 3
IR spectral frequencies of the free ligand and its metal(II) complexes.
Ligand/complex
m(N–H)
m(C@O)
m(C@N)
m(C–O)
m(N–N)
HL1
1
3178
1656
1540
1593
1496
1591
1491
1573
1596
1497
1598
1490
1060
1099
1299
1301
2
1098
HL2
3
3170
1643
1298
1300
993
1029
4
1306
1028
value (DEP = 94 mV) and the ratio of ic/ia due to cathodic and ano-
dic sweeps at different scan rates (25–200 mV sꢁ1) revealed that
this process can be at best as reversible oxidation. The
m
in cmꢁ1
.

300
P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
Fig. 4. 1H NMR spectrum of the hydrazone ligand HL2.
The higher magnitude of
DEP as well as its increase while increas-
3.0x10-5
2.5x10-5
2.0x10-5
1.5x10-5
1.0x10-5
ing the scan rate confirmed that the electrochemical process is a
quasi-reversible oxidation attributed to the slow electron transfer
and adsorption of the complexes on to the electrode surface.
3.6. Protein binding studies
3.6.1. Fluorescence spectroscopy
Qualitative analysis of binding of chemical compounds to BSA
can be detected by examining fluorescence spectra. Generally, the
fluorescence of protein is caused by three intrinsic characteristics
of the protein, namely tryptophan, tyrosine and phenyl alanine res-
idues. Fluorescence quenching refers to any process that decreases
the fluorescence intensity from a fluorophore due to variety of
molecular interactions including excited-state reactions, molecular
rearrangements, energy transfer ground-state complex formation
and collisional quenching. Here, the change in fluorescence inten-
sity is related to both the concentration and nature of the quencher.
Therefore, the quenched fluorophore serves as an indicator to
determine the ability of quenching agent. It is obvious that BSA
has a strong fluorescence emission peaked at 344 nm. When BSA
was titrated with different amounts of metal hydrazone complexes,
a remarkable intrinsic fluorescence decrease of the protein was ob-
served with a hypsochromism of about 3, 10, 4 and 15 nm respec-
tively for complexes 1, 2, 3 and 4 (Fig. 6). Fluorescence quenching
is described by the Stern-Volmer equation [39]: I0/I = 1 + KSV [Q];
where I0 and I represent the fluorescence intensities in the absence
and presence of quencher, respectively. KSV is the Stern-Volmer
quenching constant and [Q] is the quencher concentration. KSV va-
lue obtained from the plot of I0/I vs [Q] was found to be
5.0x10-6
0.0
-5.0x10-6
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Potential (V)
Fig. 5. Cyclic voltammogram of complex 1.
Table 5
Cyclic voltammetric data of metal hydrazone complexes.
Complex
M(III)–M(II)
Epa (V)
Epc (V)
E1/2 (V)
D
Ep (mV)
1
2
3
4
0.8965
0.6846
0.8456
0.8141
0.8024
0.1153
0.1030
0.1773
0.8495
0.4000
0.4740
0.4957
94
569
743
636
1.6506 ꢂ 105, 1.9323 ꢂ 105, 2.3203 ꢂ 105 and 4.8025 ꢂ 105 Mꢁ1
,
Supporting electrolyte: NBu4ClO4; solvent: MeOH;
Epc are anodic and cathodic potentials respectively; E1/2 = 0.5(Epa + Epc); scan rate:
100 mV sꢁ1
D
Ep = Epa ꢁ Epc, where Epa and
corresponding to the respective metal hydrazone chelates 1, 2, 3
and 4 (Fig. 6).
.
voltammogram of the complexes 2, 3 and 4 showed anodic peaks
at +0.6846, +0.8456 and +0.8141 V, respectively attributed to the
oxidation of M(II) ? M(III) with a corresponding reduction peak
of M(III) ? M(II) at +0.1153, +0.1030 and +0.1773 V with a peak
3.6.2. Absorption spectral studies
UV–Vis absorption measurement is a simple but effective meth-
od to detect complex formation in solution phase. Quenching can
occur by different mechanisms either by dynamic or static. Dy-
namic quenching refers to a process in which the fluorophore
to peak separation (DEP) of 569, 743 and 636 mV, respectively.

P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
301
1000
750
500
250
0
1000
2.7
2.4
2.1
1.8
1.5
1.2
2.4
2.1
1.8
1.5
1.2
1
2
750
0
2
4
6
8
0
2
4
6
8
[Q] x 10-6
M
[Q] x 10-6
M
500
250
0
300
350
400
450
300
350
400
450
Wavelength (nm)
Wavelength (nm)
1000
750
500
250
0
1000
800
600
400
200
0
5
4
3
2
3.0
2.7
2.4
2.1
1.8
1.5
1.2
4
3
1
0
2
4
6
8
0
2
4
6
8
[Q] x 10-6
M
[Q] x 10-6
M
300
350
400
450
300
350
400
450
Wavelength (nm)
Wavelength (nm)
Fig. 6. Emission spectrum of BSA (1 ꢂ 10ꢁ6 M; kexi = 280 nm; kemi = 344 nm) as a function of concentration of the complexes 1, 2, 3 and 4 (0.8, 1.6, 2.4, 3.2, 4.0, 4.8, 5.6, 6.4, 7.2
and 8.0 ꢂ 10ꢁ6 M). Arrow indicates the effect of metal complexes on the fluorescence emission of BSA. (Inset: Stern-Volmer plot of the fluorescence titration data
corresponding to the complex 1, 2, 3 and 4).
and the quencher come into contact during the transient existence
of the excited state while the later type of quenching refers to
fluorophore–quencher complex formation in the ground state. In
order to validate the quenching mechanism, the absorption spectra
of BSA in the absence and presence of hydrazone complexes were
determined. After the addition of appropriate amount of hydrazone
complexes 1, 2, 3 and 4 to BSA resulted in an increase in the inten-
sity of absorption band of BSA observed at 278 nm suggesting that
an interaction between metal hydrazones and BSA occurred and
the possible quenching mechanism of fluorescence of BSA by these
chelates is a static one [40]. A representative absorption spectrum
of pure BSA and BSA-complex 4 is shown in Fig. 7.
in the present case, K is the binding constant for the metal hydra-
zones–BSA and n is the number of binding sites per albumin mol-
ecule, which can be determined from the slope and the intercept of
the double logarithm regression curve of log [(F0 ꢁ F)/F] versus log
[complex] based on the above equation. Binding constants
obtained from the plot of log [(F0 ꢁ F)/F] versus log [Q] (Fig. 8) cor-
responding to complexes 1, 2, 3 and 4 were calculated to be
3.064 ꢂ 103, 6.4969 ꢂ 104, 6.628 ꢂ 104 and 3.3768 ꢂ 105 Mꢁ1
,
respectively.
3.6.4. Synchronous measurements
Influence of metal chelates on the conformational changes of
BSA was assessed by synchronous fluorescence spectra. Synchro-
nous fluorescence measurements provide information about the
molecular micro-environment in the vicinity of the functional fluo-
rophore groups [42]. Synchronous fluorescence spectra are ob-
tained by simultaneously scanning excitation and emission
3.6.3. Binding constants and the number of binding sites
For the static quenching interaction, if it is assumed that there
are similar and independent binding sites in the biomolecule, the
binding efficiency of the test compounds with the BSA can be
determined according to the method described by using the fol-
lowing equation [41]: log [(F0 ꢁ F)/F] = log [K] + n log [Q], where,
monochromators. As
sion wavelength is 15 nm, synchronous fluorescence offers charac-
teristics of tyrosine residues, while with k as 60 nm, it provides
Dk between excitation wavelength and emis-
D

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P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
0.4
0.3
0.2
0.1
0.0
1
2
3
4
BSA + complex 4
BSA
0.50
0.25
0.00
-0.25
-0.50
-5.0
-5.2
-5.4
-5.6
log [Q]
-5.8
-6.0
-6.2
250
275
300
325
350
Wavelength (nm)
Fig. 8. Plot of log [(F0 ꢁ F)/F] vs log [Q] for complexes 1, 2, 3 and 4.
Fig. 7. The absorption spectra of BSA (1 ꢂ 10ꢁ5 M) and BSA-complex
(BSA = 1 ꢂ 10ꢁ5 M and complex 4 = 1 ꢂ 10ꢁ6 M).
4
crease in the fluorescence intensity with a blue shift was ob-
served in case of complexes 2, 3 and 4 but only a decrease in
intensity without any shift corresponding to complex 1 (Figs.
9B, 10B, 11B and 12B). These results suggest that the hydrazones
did influence the micro-environment around both tyrosine and
tryptophan residues of BSA as observed with other hydrazone
complexes [26].
the characteristic information of tryptophan residues [43]. The
changes in maximum emission position correspond to the change
in the polarity around the chromophore molecule. Thus, the envi-
ronment of amino acid residues can be studied by measuring the
shift in wavelength of emission maximum. The synchronous
fluorescence spectroscopy of BSA upon addition of metal chelates
gained at
Dk = 15 nm and Dk = 60 nm are shown in Figs. 9–12.
It appears from Figs. 9A, 10A, 11A and 12A that the emission
intensity corresponding to tyrosine residue increases with a red
shift of 1 nm in the case of interaction with complex 1 whereas
the same band showed a decrease in the intensity of emission
without any wavelength shift upon interaction with complex 2.
However, in case of complexes 3 and 4, a decrease in emission
intensity with a red shift of about 1 nm was observed. Regarding
the changes in the emission intensity due to tryptophan residues
upon interaction with the above said complexes, a gradual de-
3.7. Antioxidant studies
3.7.1. Hydroxyl radical scavenging activity
Hydroxyl radical is highly reactive oxygen centered radical
formed from the reactions of various hydroperoxides with tran-
sition metal ions. Among all the free radicals, hydroxyl radical is
by far the most potent and therefore the most dangerous oxygen
metabolite and hence the elimination of this radical is one of the
major aims of antioxidant administration [44]. It attacks
250
A
B
200
200
150
100
50
150
100
50
0
0
280
300
320
340
360
300
320
340
360
380
Wavelength (nm)
Wavelength (nm)
Fig. 9. Synchronous spectra of BSA (1 ꢂ 10ꢁ6 M) as a function of concentration of the complex 1 (0.8, 1.6, 2.4, 3.2, 4.0, 4.8, 5.6, 6.4, 7.2 and 8.0 ꢂ 10ꢁ6 M) with wavelength
difference of
Dk = 15 nm (A) and Dk = 60 nm (B). Arrow indicates the change in emission intensity w.r.t various concentration of complex 1.

P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
303
250
A
B
200
150
100
50
200
150
100
50
0
0
300
320
340
360
380
280
300
320
340
360
Wavelength (nm)
Wavelength (nm)
Fig. 10. Synchronous spectra of BSA (1 ꢂ 10ꢁ6 M) as a function of concentration of the complex 2 (0.8, 1.6, 2.4, 3.2, 4.0, 4.8, 5.6, 6.4, 7.2 and 8.0 ꢂ 10ꢁ6 M) with wavelength
difference of
D
k = 15 nm (A) and
D
k = 60 nm (B). Arrow indicates the change in emission intensity w.r.t various concentration of complex 2.
250
200
150
100
50
250
A
B
200
150
100
50
0
0
280
300
320
340
360
300
320
340
360
380
Wavelength (nm)
Wavelength (nm)
Fig. 11. Synchronous spectra of BSA (1 ꢂ 10ꢁ6 M) as a function of concentration of the complex 3 (0.8, 1.6, 2.4, 3.2, 4.0, 4.8, 5.6, 6.4, 7.2 and 8.0 ꢂ 10ꢁ6 M) with wavelength
difference of
Dk = 15 nm (A) and Dk = 60 nm (B). Arrow indicates the change in emission intensity w.r.t various concentration of complex 3.
proteins, DNA, polyunsaturated fatty acid in membranes and
most biological molecules [45]. Hydroxyl radical is known to
be capable of abstracting hydrogen atoms from membrane lipids
and brings about peroxidic reaction of lipids. Scavenging activity
of the free ligands and corresponding metal hydrazones on hy-
droxyl radical has been investigated and their inhibitory effect
containing thiophene moiety in their molecular architecture,
the one containing cobalt ion as the metal center (complex 4)
showed very high activity than that of nickel ion under identical
experimental conditions.
3.7.2. Nitric oxide radical scavenging activity
ꢀ
on OH is shown in Fig. 13. By comparing the antioxidant activity
of the ligands (IC50 value for HL1 and HL2 are 166.25 and
Nitric oxide (NO) is a diffusible free radical which plays many
role as an effectors in diverse biological systems including neuro-
nal messenger, vasodilation, antimicrobial and antitumor activities
[46]. Nitric oxide inhibitors were shown to have beneficial effects
on some aspect of inflammation and tissue damage seen in inflam-
matory diseases [47]. In order to understand the ability of the
hydrazone ligands and their corresponding metal complexes to-
wards the reactive radical species, NO scavenging assay of them
was carried in various concentrations and the results are presented
in Fig. 13 with the corresponding IC50 values of the ligands HL1 and
121.30
(IC50 values of 1, 2,
l
M, respectively) with that of the metal complexes
and are 35.26, 29.88, 31.70 and
3
4
25.28
l
M, respectively) it became clear that the metal complexes
ꢀ
possess higher scavenging activity towards OH than the respec-
tive parent ligands. Among all the complexes, complexes 3 and 4
showed more scavenging potential than the rest of the metal
hydrazone complexes, which may be due to the presence of thi-
ophene moiety in the former. However, among the complexes

304
P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
250
B
200
150
100
50
A
200
150
100
50
0
0
280
300
320
340
360
300
325
350
375
Wavelength (nm)
Wavelength (nm)
Fig. 12. Synchronous spectra of BSA (1 ꢂ 10ꢁ6 M) as a function of concentration of the complex 4 (0.8, 1.6, 2.4, 3.2, 4.0, 4.8, 5.6, 6.4, 7.2 and 8.0 ꢂ 10ꢁ6 M) with wavelength
difference of
D
k = 15 nm (A) and Dk = 60 nm (B). Arrow indicates the change in emission intensity w.r.t various concentration of complex 4.
180
160
140
120
100
80
166.25
HL1
1
2
HL2
3
4
121.30
78.63
66.91
60
51.65
47.65
35.26
29.88
31.70
28.39
40
25.28
23.65
25.97
17.48
20
9.11
8.80
8.75
8.12
0
Hydroxyl
Nitric Oxide
Lipid peroxidation
Radicals
Fig. 13. Trends in the inhibition of hydroxyl, nitric oxide and lipid peroxidation radicals by the hydrazone ligands and corresponding metal complexes.
HL2 (IC50 values are 51.65 and 47.65
1, 2, 3 and 4 (IC50 values are 9.11, 8.80, 8.75 and 8.12
l
M) and the metal complexes
high scavenging activity due to the presence of thiophene moiety
and Co(II) ion as described in the previous section reports. All the
test compounds compared with the standard compound butyl-
ated hydroxy anisole (BHA) showed superior activity in all the
analysis. However, the results indicated that the metal complexes
exhibited greater antioxidant activity than the free ligands HL1
and HL2.
lM). How-
ever, the results indicated that the metal complexes exhibited
greater antioxidant activity than the free ligands HL1 and HL2.
Among the tested complexes, complex 4 displayed high scavenging
activity due to the presence of thiophene moiety and Co(II) ion as
described in the previous section.
In general, the antioxidant activity of the ligands HL1, HL2
and its M(II) complexes [where M = Ni or Co] against the free
radicals i.e., OH, NO and lipid peroxidation was found to de-
crease in the order of 4 > 3 > 2 > 1 > HL2 > HL1 in these experi-
ments. Further, the results obtained against the different
radicals confirmed that the complexes are more effective to ar-
rest the formation of the nitric oxide than the rest of the radi-
cals. The observed lower IC50 values in antioxidant assays did
demonstrate that these complexes have the potential as drugs
to eliminate the radicals.
3.7.3. Lipid peroxidation assay
The mechanisms involved in many human diseases such as
hepato toxicities, hepatocarcinogenesis, diabetes, malaria, acute
myocardial infarction, skin cancer include lipid peroxidation as
a main source of membrane damage [48]. Lipid peroxidation
was assessed as formation of lipid hydroperoxides and TBARS.
In order to examine the possible mechanisms of action we have
studied the abilities of HL1, HL2, 1, 2, 3 and 4 to prevent oxidative
damage of rat liver. The inhibition of lipid peroxidation by HL1,
HL2, 1, 2, 3 and 4 (the IC50 values are 78.63, 66.91, 28.39,
25.97, 23.65 and 17.48
l
M, respectively) is displayed in Fig. 13.
3.8. Antimicrobial studies
Overall, the results demonstrate that the complex 4 has a greater
capacity to prevent oxidative deterioration of lipids than HL1, HL2,
1, 2 and 3. Among the tested complexes, complex 4 displayed
Some important factors such as the nature of metal ion, nature of
the ligand, coordinating sites, geometry of the complex,

P. Sathyadevi et al. / Polyhedron 31 (2012) 294–306
305
Table 6
Antibacterial activity data of hydrazone ligands and corresponding metal complexes.
a
Ligand/
Diameter of inhibition zone (mm)
complex
S. aureus
K. pneumoniae
7.5 mM 2.5 mM 5 mM
0.7
0.7 10 0.9 14 0.5
0.4 14 0.6 17 1.1 10 0.5 12 0.4 18 1.2
0.7 0.5 0.9
0.3 13 1.2 19 0.9 0.7 15 0.2 20 0.5
0.5 14 0.9 19 0.4 10 0.7 17 0.7 23 0.2
E. coli
S. typhium
E. faecalis
2.5 mM 5 mM
7.5 mM 2.5 mM 5 mM
7.5 mM 2.5 mM 5 mM
7.5 mM 2.5 mM 5 mM
7.5 mM
0.4
0.7 10 0.2 13 0.8 16 0.7
0.5 16 0.5 10 0.3 15 0.4 20 0.7
1.1 0.2
HL1
1
2
HL2
3
4
3
5
0.3
5
0.4
7
9
0.4 10 0.3
7
9
8
0.3 14 0.2 18 0.8
9
5
0.2 12 0.9 14 0.8
9
5
0.6
4
7
7
5
8
4
7
0.8 11 0.8 14 0.4
0.5 12 0.7 16 0.4
8
8
9
7
5
0.7 11 0.4 15 0.4 13 0.7 18 0.9 22 0.6
0.5 11 0.8 18 1.2 15 0.3 16 0.6 25 0.4
S
11 1.1 16 1.5 21 0.9 10 0.5 16 0.7 23 0.8 13 0.6 19 0.9 27 0.8 11 0.4 18 0.5 25 0.5 14 0.8 21 0.7 29 0.9
S = Streptomycin.
a
Average of triplicates.
Table 7
plexes is only very modest and also they could not reach the effec-
tiveness of standard antimicrobial drugs such as streptomycin and
nystatin.
Antifungal activity data of hydrazone ligands and corresponding metal complexes.
a
Ligand/
Diameter of inhibition zone (mm)
complex
Trichophyton rubrum
Candida albicans
7.5 mM 2.5 mM 5 mM
1.1
4. Conclusion
2.5 mM 5 mM
7.5 mM
HL1
1
2
HL2
3
4
6
0.6
6
Four new octahedral Ni(II) and Co(II) complexes of heterocyclic
hydrazone ligands were synthesised and characterized by various
spectral data. All the complexes were found to have the metal to
ligand molar ratio of 1:2 instead of the expected 1:1 composition.
The single crystal X-ray crystallographic study of complex 1 re-
vealed the distorted octahedral configuration around the metal
ion. The interaction between metal chelates and BSA in buffer solu-
tion studied by UV–Vis, fluorescence and synchronous fluorescence
spectroscopic methods did suggest that the metal hydrazone che-
lates possess strong ability to quench the BSA fluorescence mainly
through a static quenching process. The results of synchronous
fluorescence measurements revealed that the present metal hydra-
zone chelates did influence the micro-environment around both
tyrosine and tryptophan residues of BSA. The results of antioxidant
and antimicrobial studies revealed that the metal complexes are
more effective than that of the respective free ligands under iden-
tical experimental conditions. Among the four different complexes
synthesised, complex 4 containing thiophene moiety in the ligand
compartment with cobalt ion as the metal center showed higher
activity than the corresponding nickel counter part with respect
to all kinds of studies undertaken in the present work.
5
5
0.7 11 0.7 15 0.4
0.2 14 0.4 19 0.9
8
0.3 13 0.5
11 0.5 16 0.7
0.9
9
0.4 13 0.7
5
8
8
0.4 17 0.5 22 0.2
0.3 19 0.8 25 0.3
5
5
0.4 13 0.3 16 0.7
0.7 14 0.2 18 0.2
S
13 0.7 21 0.3 30 1.2 11 0.9 18 0.8 24 0.4
S = Nystatin.
Average of triplicates.
a
concentration, hydrophilicity and lipophilicity have considerable
influence on antibacterial activity. Certainly, steric and pharmaco-
kinetic factors also play a decisive role in deciding the potency of
an antimicrobial agent. Heterocyclic ligands with multifunctional
groups have a greater chance of interaction either with nucleoside
bases (even after complexation with metal ion) or with biologically
essential metal ions present in the biosystem and can be promising
candidates as bactericides [49]. Thus antibacterial property of metal
complexes cannot be ascribed to chelation alone, but it is an intri-
cate blend of several contributions.
The results of antibacterial studies of the free ligands and their
corresponding Ni(II) and Co(II) complexes were given in Table 6.
From the data presented in the table, it is clear that the metal com-
plexes 1–4 were more active than the ligands HL1 and HL2.
The antifungal activity of the free hydrazone ligand showed a
considerable enhancement upon coordination with the metal ions
against the same fungal strains. In our study, both the hydrazone
ligands did not show any activity against Trichophyton rubrum
and Candida albicans at all experimental concentrations. However,
the metal complexes derived from them showed significant anti-
fungal activity against Trichophyton rubrum and Candida albicans
(Table 7). It was evident from the data that this activity signifi-
cantly increased upon coordination that can be rationalized on
the basis that their structures mainly possess an additional C@N
bond due to enolisation during chelate formation. Among all the
complexes, the complex 4 possessed very high activity due to the
presence of thiophene moiety and biologically essential metal
ion, Co(II) ion. Moreover, coordination reduces the polarity
[50,51] of the metal ion mainly because of the partial sharing of
its positive charge with the donor groups [52] within the chelate
ring system formed during coordination. This process, in turn, in-
creases the lipophilic nature of the central metal atom, which fa-
vors its permeation more efficiently through the lipid layer of the
microorganism [53,54] thus destroying them more aggressively.
However, the antimicrobial activity of the tested ligands and com-
Acknowledgement
We sincerely thank Dr. S. Abraham John, Gandhigram Univer-
sity, Gandhigram, India for carrying out the electrochemical mea-
surements. We are also grateful to Department of Science and
Technology (DST) for the award of Senior Research Fellowship
(SRF) to P. Sathyadevi. PTM is thankful to DST-India (FIST Pro-
gramme at School of Chemistry, Bharathidasan University) for
the use of Bruker Smart APEX II diffractometer.
Appendix A. Supplementary data
CCDC 771884 contains the supplementary crystallographic data
for this paper. 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.
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