Antioxidant and antimicrobial properties of nickel(II), cobalt(III), and zinc(II) complexes of a Schiff base ligand

Article abstract of DOI:10.1007/s11243-016-0069-9

A series of mononuclear Ni(II), Co(III), and Zn(II) complexes based on a Schiff base ligand, 4-chloro-6-[2-(pyrazin-2-ylmethylene)hydrazinyl]pyrimidine (HL), have been synthesized and characterized by physicochemical and spectroscopic methods. The complexes were found to have superior radical scavenging potency against DPPH, superoxide anion, hydroxyl, and ABTS radicals than free HL. The antibacterial effects of the complexes against Gram-positive and Gram-negative bacteria were also higher than that of free HL.

Full text of DOI:10.1007/s11243-016-0069-9

Transition Met Chem
DOI 10.1007/s11243-016-0069-9
Antioxidant and antimicrobial properties of nickel(II), cobalt(III),
and zinc(II) complexes of a Schiff base ligand
Qi-Meige Hasi1,2 Yan Fan1,2 Xiao-Xia Feng1,2 Xiao-Qiang Yao1,2
Jia-Cheng Liu1,2
•
•
•
•
Received: 15 March 2016 / Accepted: 1 June 2016
Ó Springer International Publishing Switzerland 2016
Abstract A series of mononuclear Ni(II), Co(III), and
Zn(II) complexes based on a Schiff base ligand, 4-chloro-
6-[2-(pyrazin-2-ylmethylene)hydrazinyl]pyrimidine (HL),
have been synthesized and characterized by physico-
chemical and spectroscopic methods. The complexes were
found to have superior radical scavenging potency against
DPPH, superoxide anion, hydroxyl, and ABTS radicals
than free HL. The antibacterial effects of the complexes
against Gram-positive and Gram-negative bacteria were
also higher than that of free HL.
e.g., some types of neoplasia, inflammation processes, and
atherosclerosis [1–9]. In addition, microbial infections, in
particular those caused by bacteria, remain a primary rea-
son for mortality and morbidity all over the world [10].
Therefore, there is an impetus to search for new metal
complexes which possess both antioxidant and antimicro-
bial activities.
Schiff base metal complexes have aroused considerable
attention due to their wide biological activity, which can
include antibacterial, antioxidant, and antitumor properties
[11–15]. It is well known that some drugs have higher
activities when administered as metal complexes compared
to the free ligand [16]. Given these considerations, in this
work we have synthesized a new Schiff base ligand,
4-chloro-6-[2-(pyrazin-2-ylmethylene)hydrazinyl]pyrim-
idine (HL, Scheme 1), and some of its metal complexes.
Furthermore, the biological properties of the free ligand
and its metal complexes have been investigated.
Introduction
Free radicals are involved in the processes of normal
oxygen metabolism in the human body, including electron
transfer in the mitochondrial respiratory chain, cell differ-
entiation, immune response, and vasodilatation. An
imbalance between formation and detoxification of free
radical species results in oxidative stress. This condition
can cause serious damage to proteins, lipids, and DNA,
potentially leading to the development of serious diseases,
Experimental
Materials and measurements
All chemicals and solvents were commercial products and
used without further purification. Elemental analyses for C,
1
H, and N were obtained on a Vario EL instrument. H
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-016-0069-9) contains supplementary
material, which is available to authorized users.
NMR spectra were recorded on a Varian Mercury Plus-400
spectrometer, in DMSO, at 400 MHz. 13C NMR spectra
were measured on a Varian Mercury Plus-400 spectrome-
ter, in DMSO, at 100 MHz. FTIR spectra were recorded in
the range 4000–400 cm-1 on an FTS 3000 (United States
DIGILAB) spectrometer using KBr pellets. Thermogravi-
metric analysis (TGA) experiments were carried out on a
PerkinElmer TG-7 analyzer, heating from 25 to 800 °C at a
& Jia-Cheng Liu
jcliuchem@163.com
1
College of Chemistry and Chemical Engineering, Northwest
Normal University, Lanzhou 730070, People’s Republic of
China
2
State Key Laboratory of Coordination Chemistry, Nanjing
University, Nanjing 210093, People’s Republic of China
123

Transition Met Chem
Scheme 1 Synthesis of HL.
a Hydrazine, triethylamine,
dioxane, yield 70.8 %. b 1-
(Pyrazin-2-yl) ethanone, acetic
acid, MeOH, yield 86.3 %
rate of 10 °C/min under N2 atmosphere. Powder X-ray
diffraction (PXRD) patterns were obtained on a Philips PW
1710 - BASED diffractometer at 293 K.
refinements of complexes 1–5 are given in Table 1.
˚
Selected bond lengths (A) and angles (°) are given in
Tables S1–S5.
Single-crystal X-ray studies
Synthesis of HL
Single-crystal X-ray diffraction analyses of complexes 1–5
were carried out on a Bruker Smart Apex CCD area
detector diffractometer with graphite-monochromated Mo
A stirred solution of 1-(pyrazin-2-yl)ethanone (1.24 g,
0.01 mol) in methanol (20 ml) was added dropwise to a
solution of 4-chloro-6-hydrazinylpyrimidine (1.44 g,
0.01 mol) in methanol (30 ml) plus three drops of glacial
acetic acid. The mixture was refluxed for 4 h and then
cooled to room temperature. The precipitated solid was
filtered off and washed with cold methanol to obtain a
˚
Ka radiation (k = 0.71073 A) at 296 K using the x-scan
technique. The structures were solved by direct methods
and refined with the full-matrix least-squares technique
using the SHELXL-97 software package. All of the non-
hydrogen atoms were refined anisotropically, and all the
hydrogen atoms were fixed geometrically at calculated
distances and refined isotropically to ride on the parent
atoms. The detailed crystallographic data and structure
1
white solid. Yield: 86.3 %. H NMR (400 MHz, DMSO)
dppm: 11.16 (s, 1H), 8.62–8.53 (m, 5H), 9.46 (s, 1H), 7.48
(s, 1H), 2.40 (s, 3H). (Fig. S1). 13C NMR (100 MHz,
DMSO) dppm: 162.94, 159.80, 158.21, 150.26, 148.14,
Table 1 Crystal data and structure refinement for complexes 1–5
Complex
1
2
3
4
5
Empirical formula
Formula weight
Temperature (K)
Cryst system
C20H16Cl2N12Ni
554.04
C40H17Cl8Co3N24
1311.30
296 (2)
Monoclinic
C2/c
C20H16Cl2N12Zn
560.74
C20H18Cl2N12NiO
572.05
C20H17Cl2CoN13O3
617.30
296 (2)
Monoclinic
C2/c
296 (2)
292 (2)
Monoclinic
P21/n
293 (2)
Triclinic
P¯ı
Triclinic
P¯ı
Space group
˚
a (A)
18.298 (3)
9.914 (2)
14.088 (3)
90
14.2644 (13)
11.4471 (9)
31.543 (4)
90
9.6768 (13)
10.3560 (13)
14.7389 (19)
89.612 (11)
73.026 (12)
67.330 (12)
1294.2 (3)
2
12.4971 (3)
13.2900 (4)
14.2246 (4)
90
8.9761 (8)
9.0478 (9)
17.4887 (14)
75.277 (8)
80.196 (7)
65.445 (9)
1246.1 (2)
2
˚
b(A)
˚
c(A)
a (deg)
b (deg)
c (deg)
101.47 (2)
90
93.346 (12)
90
98.318 (2)
90
3
˚
V (A )
2504.6 (9)
4
5141.8 (9)
4
2337.66 (11)
4
Z
Dc (g/cm3)
1.469
1.694
1.439
1.625
1.645
F(000)
1128.0
2636.0
568.0
1168.0
626.0
h min, max(deg)
GOF
0.736,0.791
1.051
0.986,1.000
1.044
0.741,0.867
1.021
0.912,0.946
1.068
0.878, 0.9021
1.032
R1 [I [ 2r(I)]a
0.0548
0.0533
0.0471
0.0347
0.0590
(1640)
(3535)
(3920)
(3693)
(4066)
wRb2 (all data)
0.1324
0.1090
0.1364
0.0881
0.1792
(2364)
(5062)
(5066)
(4596)
(5055)
a
R1 = R||Fo| - |Fc||/R|Fo|
wR2 = [Rw(F2o - Fc2)2/Rw(F2o)2]1/2
b
123

Transition Met Chem
143.83, 143.13, 142.63, 102.99, 11.91. (Fig. S2). ESI–MS
(DMF, positive ion mode): m/z = 249.07([M ? H]?),
C10H9ClN6 theoretical mass: 248. (Fig. S3). IR/cm-1
(KBr):1565 (s), 1369 (m), 3150 (m), 3438 (m).
C20H17Cl2CoN13O3: C, 38.9; H, 2.8; N, 29.5 %. Found: C,
38.9; H, 2.7; N, 29.4 %. IR/cm-1 (KBr):1441 (s), 1565 (s),
1598 (s), 3432 (m).
Antioxidant assays
Synthesis of [Ni(L)2] (1)
The antioxidant properties of free HL and its metal com-
plexes were determined by DPPH radical, superoxide
radical, hydroxyl radical, and ABTS radical scavenging
methods. The DPPH (2,2-diphenyl-2-picryl-hydrazyl) rad-
ical scavenging activities of the test compounds were
assayed using the method of Elizabeth [17], with slight
modifications. Each test compound was dissolved in
DMSO, and the solution of DPPH was prepared in
methanol. The reaction mixture contained 2 ml of 0.2 mM
DPPH solution and 100 ll of the test compound solution
(the final concentration: Ci(i=1–5) = 5, 10, 15, 20, 25 lM).
The reaction mixtures were incubated at 37 °C for 20 min
in the dark. The decrease in absorbance of DPPH was
measured at 517 nm every 5 min. As a control, the
absorbance of a blank solution of DPPH (2 ml) was also
determined at 517 nm. The suppression ratio was calcu-
lated by the following equation:
A methanol solution (10 ml) of Ni(OAc)2Á4H2O (24.9 mg,
0.1 mmol) was added to a methanol solution (10 ml) of HL
(24.8 mg, 0.1 mmol). The resulting solution was stirred for
15 min at room temperature, then filtered, and evaporated
slowly to give dark green crystals after 2 weeks. Yield:
57.2 %. Anal. Calcd. For C20H16Cl2N12Ni: C, 43.4; H, 2.9;
N, 30.3 %. Found: C, 43.3; H, 2.9; N, 30.3 %. IR/cm-1
(KBr):1559 (s), 1441(s), 1585 (m), 3438 (m).
Synthesis of {[Co(L)(HL)]CoCl4} (2)
An ethanol solution (10 ml) of CoCl2Á6H2O (23.8 mg,
0.1 mmol) was added to an ethanol solution (10 ml) of HL
(24.8 mg, 0.1 mmol). Green crystals were obtained by
slow evaporation of the resulting solution within 5 days in
49.8 % yield. Anal. Calcd. For C40H34Cl8Co3N24: C, 36.6;
H, 2.6; N, 25.6 %. Found: C, 36.6; H, 2.6; N, 25.6 %. IR/
cm-1 (KBr): 1598 (s), 2920 (m), 3444 (m).
Suppression ratio ð%Þ ¼ ½ðA0 À AiÞ=ðA0ފ  100 %
where Ai = the absorbance in the presence of the test
compound and A0 = the absorbance in the absence of the
test compound.
Synthesis of [Zn(L)2] (3)
The hydroxyl radical scavenging activities of the test
compounds were investigated using the Fenton reaction
[18]. Each test compound was dissolved in DMF and then
added to a reaction mixture containing 2.0 ml of 100 mmol
phosphate buffer (pH 7.4), 1.0 ml of 0.10 mmol safranine,
1 ml of 1.0 mmol EDTA-Fe(II), and 1 ml of 3 % H2O2.
The resulting mixtures were incubated at 37 °C for 60 min
in the dark, and then the absorbance was measured at
520 nm (Ai, A0, Ac). The suppression ratio was calculated
by the following equation:
The preparation of complex 3 was similar to that of 2,
except that Zn(OAc)2 (18.3 mg, 0.1 mmol) was used
instead of CoCl2Á6H2O. Yellow crystals were obtained by
slow evaporation of the resulting solution within two weeks
in 55.6 % yield. Anal. Calcd. For C20H16Cl2N12Zn: C,
42.8; H, 2.9; N, 30.0 %. Found: C, 42.9; H, 2.9; N, 30.0 %.
IR/cm-1 (KBr):1565 (s), 1435 (s), 1605 (m), 3438(m).
Synthesis of {[Ni(L)2]H2O} (4)
Complex 4 was also prepared by the same method as of 2,
but using Ni(OAc)2Á4H2O (24.9 mg, 0.1 mmol) instead of
CoCl2Á6H2O. Dark green crystals were obtained by slow
evaporation of the resulting solution within three weeks in
59.2 % yield. Anal. Calcd. For C20H18Cl2N12NiO: C, 42.0;
H, 3.2; N, 29.4 %. Found: C, 42.0; H, 3.1; N, 29.4 %. IR/
cm-1 (KBr):1560 (s), 1438 (s), 1353 (m), 3375 (m).
Suppression ratio ð%Þ ¼ ½ðAiÀA0Þ=ðAcÀA0ފ  100 %
where Ai = the absorbance in the presence of the test
compound; A0 = the absorbance in the absence of the test
compound; and Ac = the absorbance in the absence of the
test compounds, EDTA-Fe(II) and H2O2.
The superoxide radical scavenging activities of the test
compounds were measured using the standard testing sys-
tem of NBT/VitB2/MET [19]. Solutions of the test com-
pounds were prepared in DMF. The reaction mixture
contained 2.5 ml of 100 mmol phosphate buffer (pH 7.8),
1.0 ml of 50 mmol MET, 1.0 ml of 0.23 mmol NBT,
0.5 ml of 33 lM VitB2, and the test compound. The
solutions of MET, VitB2, and NBT were prepared with
phosphate buffer (pH 7.8) in the dark. After incubating the
Synthesis of {[Co(L)2]NO3} (5)
The preparation of complex 5 was similar to that of 2,
except that Co(NO3)2Á6H2O (29.1 mg, 0.1 mmol) was used
instead of CoCl2Á6H2O. Red-brown crystals were obtained
by slow evaporation of the resulting solution within
3 weeks in 60.2 % yield. Anal. Calcd. For
123

Transition Met Chem
mixture at 30 °C for 10 min and illuminating with a fluo-
rescent lamp for 3 min, the absorbances (Ai) of the samples
were measured at 560 nm. A sample without any test
compound was used as the control. The suppression ratio
was calculated according to the following equation:
asymmetric unit contains one Ni2? center and two L-
ligands. As shown in Fig. 1a, the nickel is six-coordinated
by six nitrogen atoms from two deprotonated ligands, in
which N1 from the pyrimidine ring, N4 from the imino
backbone, and N5 from the pyrazine ring provide an
octahedral coordination geometry. As shown in Fig. 1b, the
zero-dimensional structure of 1 leads to a 1D chain struc-
ture by means of hydrogen bonds involving the chlorine
atoms. The hydrogen bond distance (C–ClÁÁÁH) is
Suppression ratio ð%Þ ¼ ½ðA0 À AiÞ=ðA0ފ  100 %
where Ai = the absorbance in the presence of the test
compound and A0 = the absorbance in its absence.
˚
The ABTS (2,20-azino-bis(3-ethylbenzothiazoline-6-
sulfonic acid)) radical scavenging activities of the test
compounds were studied according to the following pro-
cedure [20]. ABTS was dissolved in 10 ml of double-dis-
tilled water at a concentration of 0.22 mM, and cationic
radical was produced by reacting with 10 ml of
0.2623 mM potassium persulfate for 12 h in the dark. The
resulting solution was diluted with phosphate buffer (pH 6)
until the absorbance was 0.70 0.02 at 737 nm. The
reaction mixture containing 2.5 ml of ABTS solution and
0.5 ml of the test compound solution was incubated at
37 °C for 30 min in the dark. The absorbance was then
measured at 737 nm (Ai, A0). The suppression ratio was
calculated according to the following equation:
2.8625(16) A, and the angle (C–HÁÁÁCl) is 179.41 (35)°.
Structure of {[Co(L)(HL)]CoCl4} (2)
X-ray diffraction analysis shows that complex 2 crystallizes
in the monoclinic space group C2/c. As depicted in Fig. 2,
the two ligands are each coordinated to the Co1 center
through one imide, one pyrimidine, and one pyrazine
nitrogen donors to form an octahedral cation. A tetrahedral
[CoCl4]2- anion maintains electroneutrality in this case.
Structures of [Zn(L)2] (3) and {[Ni(L)2](H2O)} (4)
Complex 3 crystallizes in the triclinic space group P¯ı. As
presented in Fig. 3, the metal atom is coordinated by two
tridentate L- ligands, leading to a distorted octahedral
geometry. Complex 4 crystallizes in the monoclinic crystal
system of the P21/n space group. The asymmetric unit
(Fig. S5) is composed of one independent Ni2? ion, two
terdentate L- ligands, and one water ligand.
Suppression ratio ð%Þ ¼ ½ð1ÀAiÞ=ðA0Þ Â 100 %
where Ai is the absorbance measured after addition of the
sample and A0 is the absorbance of the uninhibited radical
cations.
For each of the above assays, the antioxidant activity is
given as the 50 % inhibitory concentration (IC50) [21].
Crystal structure of {[Co(L)2](NO3)} (5)
Antimicrobial assays
The single-crystal X-ray diffraction study reveals that com-
plex 5 crystallizes in the triclinic space group P¯ı. As shown in
Fig. S6, the metal atom is coordinated by two tridentate
ligands, forming an octahedral cation, and the nitrate coun-
terion is not coordinated. The metal–nitrogen bond lengths
The antibacterial activities of the compounds were evalu-
ated against Bacillus subtilis and Staphylococcus aureus
(Gram-positive) and Escherichia coli and Proteus vulgaris
(Gram-negative) by the filter paper diffusion method [22].
Each test compound was dissolved in DMSO. Under
aseptic conditions, circular filter papers (6.0 mm in diam-
eter) were immersed in test solutions of different concen-
trations for 2 h. The filter papers were then drained off and
pasted onto a Petri plate containing the bacterial suspen-
sion. The Petri plates were incubated at 37 °C for 24 h. The
antimicrobial activity was evaluated by measuring the zone
of growth inhibition against the test microorganisms.
˚
range from 1.8784(34) to 1.9411(33) A and are significantly
shorter than for complexes 1, 3, and 4, due to the decreased
ionic radius and electrophilicity of CoIII compared to CoII.
One nitrate counterion was found in the asymmetric unit,
consistent with the oxidation of CoII to CoIII [26]. This is
further verified by the Co–N bond distances, which fall in the
range reported for CoIII–N, compared to significantly longer
distances for the divalent state [23–26]. The bond lengths are
shortest for the metal–imino nitrogen atoms N4 and N3 and
longest for the metal–pyridine nitrogen atoms N2 and N9. The
same trends are also found for complexes 1–4.
Results and discussion
Crystal structure of [Ni(L)2] (1)
PXRD and thermal analyses of the complexes
Single-crystal X-ray diffraction analysis reveals that com-
In order to confirm the phrase purities of the complexes,
PXRD experiments have been performed. As shown in
plex 1 crystallizes in the monoclinic space group C2/c. The
123

Transition Met Chem
Fig. 1 a Crystal structure of
complex 1 with 30 % thermal
ellipsoids probability
(symmetry code: A = 1 - x, y,
0.5 - z). All hydrogen atoms
are ignored for clarity. b The 1D
chain structure of complex 1
Fig. 2 Crystal structure of complex 2 with 30 % thermal ellipsoids
probability (symmetry code: A = 1 - x, y, 0.5 - z). All hydrogen
atoms are ignored for clarity
Fig. 3 Crystal structure of complex 3 with 30 % thermal ellipsoids
probability. All hydrogen atoms are ignored for clarity
Fig. S7, there are good agreements between the simulated
and experimental patterns, confirming the phase purities of
the bulk samples. Because of the preferred orientations of
the powder samples, different intensities are observed.
The thermal stabilities of the complexes have been
investigated under nitrogen. As shown in Fig. S8, com-
plexes 1–5 all exhibit similar thermal stability curves in the
temperature range from 20 to 800 °C. Complex 1 under-
goes a 15.66 % weight loss in the range of 321–335 °C,
which may be ascribed to the loss of organic components.
The residue is then stable up to 335 °C, after which
decomposition is continuous. Complex 2 has excellent
thermal stability, showing no weight loss until 332 °C,
after which decomposition occurs. Complex 3 displays
weight losses of 8.57 and 4.7 % between 184–197 and
299–311 °C, respectively. Complex 4 shows a weight loss
of 2.2 % between 139 and 251 °C, which may be assigned
to the departure of the water ligand, followed by
123

Transition Met Chem
decomposition above 310 °C. Finally, complex 5 shows the
onset of decomposition at 287 °C.
complexes have better DPPH radical scavenging activity
than free HL. The Co(III) complex shows the highest
scavenging effect of all.
DPPH radical scavenging activity
Hydroxyl radical scavenging activity
The DPPH radical scavenging experiment is extensively
used to assess the antioxidant properties of test compounds.
The DPPH radical is relatively stable, but in the presence
of a compound capable of donating hydrogen atoms, the
radical is destroyed leading to a color change from purple
to yellow. As shown in Fig. 4, the inhibitory effects of the
test compounds on DPPH radical are concentration-related
such that the suppression ratio increases with increasing
sample concentration. The IC50 values of HL and com-
plexes 1–5 against DPPH radicals are 24.75, 18.72, 9.32,
15.74, 20.60, and 10.27 lM, respectively, showing that the
Figure 5 compares the inhibitory effects of HL and its
Á
metal complexes on OH radicals. Here also, the suppres-
sion ratio of the test compounds increases with concen-
tration. The IC50 values for HL and complexes 1–5 are
29.48, 15.66, 5.75, 9.64, 13.09, and 6.19 lM, respectively.
As before, the Co(III) complex has the highest scavenging
activity.
Fig. 6 Scavenging effects of HL and complexes 1–5 on O2Á- radicals
Fig. 4 Scavenging effects of HL and complexes 1–5 on DPPH
radicals
Fig. 7 Scavenging effects of HL and complexes 1–5 on ABTS
radicals
Á
Fig. 5 Scavenging effects of HL and complexes 1–5 on OH radicals
123

Transition Met Chem
Fig. 8 Antimicrobial effects of HL and complexes 1–5 on Escherichia coli (a), Staphylococcus aureus (b), Proteus vulgaris (c), and Bacillus
subtilis (d)
Superoxide anion scavenging activity
In vitro antibacterial activities
The superoxide anion scavenging activities of the test
compounds are shown in Fig. 6. The IC50 values for free
HL and complexes 1–5 are 23.66, 12.50, 5.93, 7.84, 11.41,
and 6.36 lM, respectively, showing that the complexes are
again more active than free HL.
The antibacterial activities of free HL and its complexes
are shown in Fig. 8. The antibacterial effects against all for
bacteria are concentration-related such that the inhibition
zone diameter increases with increasing sample concen-
tration. Complexes 2 and 5 were found to have the highest
antibacterial effects, compared to the other complexes and
free HL.
ABTS radical scavenging activity
The ABTS radical scavenging experiment is usually used
to evaluate the total antioxidant properties of a compound.
As shown in Fig. 7, the suppression ratio of HL
(IC50 = 49.63 lM) is the least of all these compounds,
while complexes 2 and 5 (IC50 values for 2 and 5 are 9.03
and 9.65 lM, respectively) are more effective than the
other complexes (IC50 values for 1, 3, and 4 are 27.01,
15.27, and 24.61 lM, respectively).
Conclusion
A series of complexes based on a Schiff base ligand,
4-chloro-6-[2-(pyrazin-2-yl-methylene)hydrazinyl] pyrim-
idine, have been prepared and characterized. These metal
complexes exhibit greater radical scavenging activities
than the free Schiff base. This study may provide useful
123

Transition Met Chem
10. Elke F, Hasan S, Hanna GS, Dimitris AS, Reza GA (2011)
Biomacromolecules 12:3528–3539
information for the development of new antioxidants and
therapeutic agents. Preliminary studies show that these
complexes also possess antibacterial activities.
11. Satheesh CE, Raghavendra Kumar P, Sharma P, Lingaraju K,
Palakshamurthy BS, Raja Naika H (2016) Inorg Chim Acta 442:1
12. Medici S, Peanaa M, Nurchi VM, Lachowicz JI, Crisponi G,
Zoroddua MA (2015) Coord Chem Rev 284:329
13. Patel MN, Patel CR, Joshi HN (2013) Inorg Chem Commun
27:51
Acknowledgments This work was supported by grants from the
Natural Science Foundation of China (Nos. 21461023 and 21361023),
Fundamental Research Funds for the Gansu Universities.
14. Raja DS, Bhuvanesh NSP, Natarajan K (2012) Dalton Trans
41:4365
15. Sathyadevi P, Krishnamoorthy P, Butorac RR, Cowley AH,
Bhuvanesh NSP, Dharmaraj N (2011) Dalton Trans 40:9690
16. Nahid S, Zeinab G, Saba H (2012) Bioinorg Chem Appl
10:126451
17. Elizabeth K, Rao MNA (1990) Int J Pharm 58:237–240
18. Winterbourn CC (1981) Biochem J 198:125
19. Winterbourn CC (1979) Biochem J 182:625
20. Re R, Pellegrini N, Proteggente A, Pannala A, Yang M, Evans
CR (1999) Biol Med 26:1231
21. Li Y, Yang ZY, Wu JC (2010) Eur J Med Chem 45:5692–5701
22. Liang YT, Yang GP, Liu B (2015) Dalton Trans 44:13325–13330
23. Kime NE, Ibers JA (1969) Acta Crystallogr B 25:168–169
24. Min KS, DiPasquale AG, Golen JA, Rheingold AL, Miller JS
(2007) J Am Chem Soc 129:2360
References
1. Abdel Aziz AA, Elbadawy HA (2014) Spectrochim Acta A
124:404
2. Sathiyaraj S, Sampath K, Butcher RJ, Pallepogu R, Jayabalakr-
ishnan C (2013) Eur J Med Chem 64:81
3. Suksrichavalit T, Prachayasittikul S, Nantasenamat C, Na-
Ayudhya CI, Prachayasittikul V (2009) Eur J Med Chem 44:3259
4. Chen WJ, Guo P, Song J, Cao W, Bian J (2006) Bioorg Med
Chem Lett 16:3582
5. Asayama S, Mizushima K, Nagaoka S, Kawakami H (2004)
Bioconjugate Chem 15:1360
6. Imlay JA (2002) J Biol Inorg Chem 7:659
25. Kil SM, Antonio GD, Arnold LR, Henry SW, Joel SM (2009) J
Am Chem Soc 131:6229–6236
26. Gru¨nwald KR, Volpe M, Cias P, Gescheidt G (2011) Inorg Chem
50:7478–7488
7. Dreher D, Junod AF (1996) Eur J Cancer 32:30
8. Dexter DT, Sian J, Rose S (1994) Ann Neurol 53:38
9. Ray PS, Maulik G, Cordis GA, Bertelli AAE, Bertelli A, Das DK
(1999) Free Radical Biol Med 27:160
123