Spectroscopic, invitro antibacterial, and antifungal studies of Co(II), Ni(II), and Cu(II) complexes with 4-chloro-3-coumarinaldehyde Schiff bases

Article abstract of DOI:10.1080/00958972.2010.541240

A series of metal complexes ML₂ 2H₂O [M = Co(II), Ni(II), and Cu(II)] have been synthesized with Schiff bases derived from 3-substituted-4-amino-5-mercapto-1,2,4-triazole and 4-chloro-3-coumarinaldehyde. The structures of these metal complexes have been proposed from elemental analyses, spectral (IR, UV-Vis, FAB-mass, ESR, and fluorescence), magnetic, and thermal studies. Low molar conductance values in DMF indicate that the metal complexes are non-electrolytes. Cyclic voltammetric studies suggested that the Ni(II) and Cu(II) complexes are single-electron transfer quasi-reversible. The Schiff bases and their metal complexes have been screened for invitro antibacterial (Escherichia coli, Staphylococcus aureus, Bacillus subtilis, and Salmonella typhi) and antifungal activities (Candida albicans, Cladosporium, and Aspergillus niger) by the minimum inhibitory concentration method. The Schiff base I and its Co(II) and Ni(II) complexes exhibit DNA cleavage activity on isolated DNA of S. aureus and A. niger.

Full text of DOI:10.1080/00958972.2010.541240

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Spectroscopic, in vitro antibacterial,
and antifungal studies of Co(II), Ni(II),
and Cu(II) complexes with 4-chloro-3-
coumarinaldehyde Schiff bases
Sangamesh A. Patil a , Shrishila N. Unki a , Ajaykumar D. Kulkarni a
, Vinod H. Naik a , Udaykumar Kamble a & Prema S. Badami b
a P.G. Department of Chemistry, Karnatak University, Dharwad
580003, Karnataka, India
b Department of Chemistry, Shri Sharanabasaveshwar College of
Science, Gulbarga 585102, Karnataka, India
Published online: 23 Dec 2010.
To cite this article: Sangamesh A. Patil , Shrishila N. Unki , Ajaykumar D. Kulkarni , Vinod H. Naik ,
Udaykumar Kamble & Prema S. Badami (2011): Spectroscopic, in vitro antibacterial, and antifungal
studies of Co(II), Ni(II), and Cu(II) complexes with 4-chloro-3-coumarinaldehyde Schiff bases,
Journal of Coordination Chemistry, 64:2, 323-336
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Journal of Coordination Chemistry
Vol. 64, No. 2, 20 January 2011, 323–336
Spectroscopic, in vitro antibacterial, and antifungal studies
of Co(II), Ni(II), and Cu(II) complexes with 4-chloro-3-
coumarinaldehyde Schiff bases
SANGAMESH A. PATIL*y, SHRISHILA N. UNKIy, AJAYKUMAR
D. KULKARNIy, VINOD H. NAIKy, UDAYKUMAR KAMBLEy
and PREMA S. BADAMIz
yP.G. Department of Chemistry, Karnatak University, Dharwad 580003, Karnataka, India
zDepartment of Chemistry, Shri Sharanabasaveshwar College of Science,
Gulbarga 585102, Karnataka, India
(Received 29 April 2010; in final form 19 October 2010)
A series of metal complexes ML2 ꢀ 2H2O [M ¼ Co(II), Ni(II), and Cu(II)] have been synthesized
with Schiff bases derived from 3-substituted-4-amino-5-mercapto-1,2,4-triazole and 4-chloro-3-
coumarinaldehyde. The structures of these metal complexes have been proposed from elemental
analyses, spectral (IR, UV-Vis, FAB-mass, ESR, and fluorescence), magnetic, and thermal
studies. Low molar conductance values in DMF indicate that the metal complexes are non-
electrolytes. Cyclic voltammetric studies suggested that the Ni(II) and Cu(II) complexes are
single-electron transfer quasi-reversible. The Schiff bases and their metal complexes have been
screened for in vitro antibacterial (Escherichia coli, Staphylococcus aureus, Bacillus subtilis, and
Salmonella typhi) and antifungal activities (Candida albicans, Cladosporium, and Aspergillus
niger) by the minimum inhibitory concentration method. The Schiff base I and its Co(II) and
Ni(II) complexes exhibit DNA cleavage activity on isolated DNA of S. aureus and A. niger.
Keywords: Antimicrobial; Coumarin; Electrochemical; Spectroscopic studies; Schiff bases;
Transition metal complexes
1. Introduction
Coumarins are well-known natural products displaying a broad range of biological
activities [1], anticoagulant [2, 3], antibacterial, antifungal [4], antibiotic [5], spasmolytic
[6], anthelmintic [7], diuretic [8], anti-inflammatory [9], antitubercular agents [10],
antihistamic [11], antidepressant [12], and antimalarial [13]. Coumarin derivatives have
been studied to suggest their use as chelating agents [14, 15]. Triazole derivatives
possess antibacterial, fungicidal, hypotensive, and hypothermic activities [16–18].
*Corresponding author. Email: patil1956@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2011 Taylor & Francis
DOI: 10.1080/00958972.2010.541240

324
S.A. Patil et al.
Metal complexes of various Schiff bases have been extensively investigated spectro-
scopically and investigated for their biological properties [19–24]. Interaction of
transition metal complexes with DNA has been studied for use as probes for DNA
structure and potential applications in chemotherapy. Some metal complexes can
induce the breakage of DNA strands. After DNA strands are cleaved, the DNA double
strands break and replication ability of cancer gene is destroyed. A copper complex
could cleave DNA in the presence of ascorbate or hydroquinone [25]; reductants had a
critical influence on DNA cleavage. Very recently, Cu(II) complexes have been reported
to be active in DNA strand scission [26–28].
Comparatively, fewer publications have been devoted to the synthesis of Schiff bases
of 4-chloro-3-coumarinaldehyde [29]. Hence, in continuation with our earlier work on
coumarin Schiff bases [20, 23, 24, 28], here we report the synthesis, spectroscopic, and
biological studies of Co(II), Ni(II), and Cu(II) complexes of Schiff bases derived from
4-chloro-3-coumarinaldehyde and 3-substituted-4-amino-5-mercapto-1,2,4-triazole.
The Schiff bases and their coordinating ability toward metal complexes have been
studied by various spectroscopic (FT-IR, NMR, UV-Vis, ESR, FAB-mass, fluores-
cence, and CV), magnetic, and thermal studies.
2. Experimental
2.1. Analysis and physical measurements
Carbon, hydrogen, and nitrogen were estimated using Elemental Analyzer Truspec
(Leco Corporation, USA). IR spectra of the Schiff bases and their Co(II), Ni(II), and
Cu(II) complexes were recorded on a HITACHI-270 IR spectrophotometer (4000–
250 cmꢁ1) in KBr disks. Electronic spectra of the complexes were recorded in HPLC
grade DMF and DMSO on a VARIAN CARY 50-BIO UV-spectrophotometer from
1
200 to 1100 nm. The H-NMR spectra of ligands were recorded in DMSO-d6 on a
BRUKER 300 MHz spectrometer at room temperature (RT) using TMS as an internal
reference. Fluorescence spectra of Schiff bases and their complexes were recorded on a
HITACHI F-7000 Fluorescence Spectrophotometer (made in Japan) using
10ꢁ3 mol Lꢁ1 in HPLC grade DMF and DMSO at RT. Electrochemistry of the metal
complexes was recorded on a CHI1110A-electrochemical analyzer (made in USA) in
DMF containing 0.05 M n-Bu4NClO4 as the supporting electrolyte. The ESR spectrum
was recorded on a Varian-E-4X-band EPR spectrometer with 3000 G at modulation
frequency of 100 KHz at liquid nitrogen temperature (LNT) using TCNE as ‘‘g’’
marker. FAB-mass spectra were recorded on a JEOL SX 102/DA-6000 mass
spectrometer/data system using argon/xenon (6 kV, 10 A) as the FAB gas. The
accelerating voltage was 10 kV and the spectra were recorded at RT with m-nitrobenzyl
alcohol as the matrix. Thermogravimetric analyses (TGA) were measured from RT to
1000ꢂC at a heating rate of 10ꢂC minꢁ1. The data were obtained using TA Instruments
Water LLC, New Castle, Delaware, USA, Model; DCS Q 20, 2009. Molar conductivity
measurements were recorded on an ELICO-CM-82 T Conductivity Bridge with a cell
having cell constant 0.51. Magnetic moments of the complexes were carried out by
using a Faraday balance.

Coumarinaldehyde Schiff bases
325
2.2. Synthesis
All chemicals and solvents were of AR grade; metal(II) salts were used as their
chlorides. 3-Substituted-4-amino-5-mercapto-1,2,4-triazoles were synthesized by
reported methods [22, 30].
2.2.1. Formylation of 4-hydroxycoumarin. To a well-cooled solution of 4-hydroxy-
coumarin (0.02 mol) in dry DMF (0.2 mol), freshly distilled dry POCl3 (0.02 mol) was
added dropwise with constant stirring at 0–5ꢂC. The reaction mixture was set aside for
30 min and then carefully poured on crushed ice with vigorous stirring. The compound
4-chloro-3-coumarinaldehyde that separated was filtered, washed successively with 5%
NaHCO3 and water, and then recrystallized from acetone–water giving pale yellow
needles. M.p. (yield %); 115ꢂC (80%).
2.2.2. Synthesis of Schiff bases (I and II). The Schiff bases were synthesized by
condensation of 3-substituted-4-amino-5-mercapto-1,2,4-triazole (0.01 mol) and
4-chloro-3-coumarinaldehyde (0.01 mol) dissolved in 30 mL alcoholic medium contain-
ing a few drops of concentrated HCl. The resulting mixture was refluxed for 3–4 h. The
solid product which separated on evaporation of the solvent was filtered, washed with
alcohol, and then recrystallized from EtOH. M.p. (Yield %); 267ꢂC (67%); and 269ꢂC
(65%) of Schiff bases I and II, respectively.
2.2.3. Synthesis of Co(II), Ni(II), and Cu(II) complexes (1–6). An alcoholic solution
(30mL) of Schiff bases (I and II; 2 mmol) was refluxed with 1 mmol of CoCl2 ꢀ 6H2O/
NiCl2 ꢀ 6H2O/CuCl2 ꢀ 2H2O in 30 mL ethanol solution on a steam bath for 1 h. Then,
2 mmol of sodium acetate was added to the reaction mixture and reflux continued for
3 h. The separated complex was filtered, washed thoroughly with water, ethanol, and
ether, and finally dried in vacuum over fused CaCl2.
2.3. Pharmacology
2.3.1. DNA cleavage. Preparation of culture media and DNA isolation of
Staphylococcus aureus and Aspergillus niger microbial strains were done according to
the literature procedure [28]. Nutrient broth (peptone, 10; Yeast extract, 5; NaCl, 10; in
g Lꢁ1) was used for culturing of S. aureus and potato dextrose broth (potato, 250;
dextrose, 20; in g Lꢁ1) was used for the culture of A. niger.
2.3.1.1. Agarose gel electrophoresis. Cleavage products were analyzed by agarose gel
electrophoresis [31]. Test samples (1 mg mLꢁ1) were prepared in DMF. Samples (25 mg)
were added to the isolated DNA of S. aureus and A. niger, incubated for 2 h at 37ꢂC,
and then 20 ꢀL of DNA sample (mixed with bromophenol blue dye at 1 : 1 ratio) was
loaded carefully into the electrophoresis chamber wells along with standard DNA
marker containing TAE buffer (4.84 g Tris base, pH 8.0, 0.5 M EDTA/1 L) and finally
loaded on agarose gel and 50 V of electricity passed through for 30 min. Removing the
gel and staining with 10.0 mg mLꢁ1 ethidium bromide for 10–15 min gave bands
observed under Vilberlourmate Gel documentation system and photographed to

326
S.A. Patil et al.
Cl
O
OH
O
CHO
O
Anhydrous DMF
Dry POCl
0–5°C
3
O
N
N
Refluxed
4–5 h
Ethanol
SH
R
Cl
O
N
N
N
N
CHO
O
SH
R
+
N
2–3 drops
HCl
Cl
CH
O
NH
2
O
R=CH , C H
5
3
2
Scheme 1. Synthesis of Schiff bases I and II.
determine the extent of DNA cleavage. Then, the results are compared with
standard DNA marker.
2.3.2. In vitro antibacterial and antifungal assay
Biological activities of Schiff bases and their complexes were studied by agar and potato
dextrose agar diffusion methods. The antibacterial and antifungal activities were done
at 25, 50, and 100 mg mLꢁ1 concentrations in DMSO using four bacteria (Escherichia
coli, S. aureus, Bacillus subtilis, and Salmonella typhi) and three fungi (A. niger, Candida
albicans, and Cladosporium) by the minimum inhibitory concentration (MIC) method
[32]. The bacterial strains were incubated for 24 h at 37ꢂC and fungal strains for 48 h at
37ꢂC. Standard antibacterial and antifungal drugs (Gentamycine and Fluconazole)
were used for comparison under similar conditions.
3. Results and discussion
Synthesis of the Schiff bases is schematically presented in scheme 1. Schiff bases (I and
II) with CoCl2 ꢀ 6H2O/NiCl2 ꢀ 6H2O/CuCl2 ꢀ 2H2O in EtOH gave octahedral complexes
(1–6), where the coordination sphere of the central metal is completed by two water
molecules. The Co(II), Ni(II), and Cu(II) complexes are colored, stable, non-
hygroscopic, insoluble in common organic solvents and soluble in DMF and DMSO.
Elemental analyses showed that Co(II), Ni(II), and Cu(II) complexes have 1 : 2
stoichiometry ML2 ꢀ 2H2O, where ‘‘L’’ stands for a singly deprotonated ligand. The
molar conductance values are too low to account for any dissociation of the complexes
in DMF, indicating non-electrolytes in DMF (table 1).

Coumarinaldehyde Schiff bases
327

328
S.A. Patil et al.
Table 2. The important infrared frequencies (in cmꢁ1) of Schiff bases and their metal complexes.
Compound no.
ꢁ(OH)
ꢁ(NH)
ꢁ(SH)
Lactonyl ꢁ(C¼O)
ꢁ(C¼N)
ꢁ(M–N)
ꢁ(M–S)
I
–
–
3145
3179
3142
3177
3144
3174
3140
3175
2762
2782
1720
1716
1717
1715
1722
1716
1718
1719
1615
1628
1608
1609
1607
1609
1605
1608
–
–
–
–
II
1
2
3
4
5
6
3410
3427
3410
3419
3433
3434
–
–
–
–
–
–
478
473
475
469
471
470
378
345
368
341
360
358
3.1. IR spectra
IR spectra of the Schiff bases were compared with the spectra of the metal complexes.
IR spectra (table 2) of the Schiff bases show characteristic bands due to ꢁ(NH) and
ꢁ(SH) at 3145–3179 and 2762–2782 cmꢁ1, respectively. Another band at 1090–
1092 cmꢁ1 is assigned to ꢁ(C¼S). These observations suggest that the Schiff bases
exhibit thiol–thione tautomerism (figure 1). Strong bands at 1716–1720, 1615–1628, and
1387–1394 cmꢁ1 in the Schiff bases are assigned to ꢁ(C¼O), ꢁ(C¼N), and phenolic
ꢁ(C–O), respectively.
In comparison with spectra of the Schiff bases, all metal complexes (table 2) exhibit
downward shift of about 10–20 cmꢁ1 of ꢁ(C¼N), indicating the participation of
azomethine nitrogen in coordination to the metal ion. Deprotonation of the thiol is
indicated by the absence of a band in the metal complexes at 2762–2782 cmꢁ1, due to
ꢁ(SH) of Schiff bases, indicating that the Schiff bases are coordinated to the metal
through the thiolate sulfur. This is further supported by the lower frequency band at
752–762 cmꢁ1 in the metal complexes due to ꢁ(C–S). The bands observed at 1716–
1720 cmꢁ1 in the free ligand have been assigned to ꢁ(C¼O). Practically no effect on
these frequencies after complexation indicates non-involvement of these groups in
coordination. The new bands in the region of 469–478 and 341–378 cmꢁ1 in the spectra
of the metal complexes are assigned to stretching frequencies of ꢁ(M–N) and ꢁ(M–S)
bond formation, respectively. The presence of coordinated water in the complexes is
indicated by a broad band in the region 3410–3434 cmꢁ1 and two weak bands at 760–
800 and 710–720 cmꢁ1 due to ꢁ(–OH) rocking and wagging modes of vibrations,
respectively [33].
Thus, IR spectral data provide strong evidence for coordination of Schiff bases to the
metal(II) ions through azomethine nitrogen and thiolate sulfur.
3.2. 1H- and 13C-NMR spectral studies of Schiff bases
1
In the H-NMR spectrum (table 3) of II, a singlet at 13.84 ppm (s, 1H) is ascribed to –
NH and a sharp signal at 10.93 ppm (s, 1H) is attributed to –SH. These observations
suggest that the Schiff bases exist in thiol–thione tautomerism. A singlet at 8.68 ppm
(s, 1H) is assigned to –CH¼N group. The multiplet observed at 6.9–7.7 ppm (m, 4H) is
due to aromatic protons. The quartet and triplet at 2.58 and 1.25 ppm are due to –CH2
and –CH3, respectively.

Coumarinaldehyde Schiff bases
329
H
N
N
N
N
SH
R
R
S
N
N
N
N
Cl
Cl
CH
O
HC
O
O
O
R=CH , C H
2 5
3
Figure 1. Structure of Schiff bases: thiol–thione tautomerism.
Table 3. The important 1H- and 13C-NMR data of Schiff bases.
Schiff base
1H-NMR (DMSO-d6) (ppm)
13C-NMR (DMSO-d6) (ppm)
I
12.56 (s, 1H, NH), 10.92 (s, 1H, SH), 8.67
(s, 1H, CH¼N), 6.9–7.9 (m, 4H, Ar-H),
1.3 (t, 3H, CH3)
91.8, 116.6, 117.2, 124.0, 124.8, 133.5
(coumarin), 154.35 (C¼N), 166.5
(C¼O), 185.3, 179.2 (triazole),
11.30 (CH3)
II
13.84 (s, 1H, NH), 10.93 (s, 1H, SH), 8.68
(s, 1H, CH¼N), 6.9–7.7 (m, 4H, Ar-H),
2.58 (q, 2H, CH2), 1.25 (t, 3H, CH3)
95.2, 116.7, 117.8, 125.3, 125.4, 133.6
(coumarin), 154.6 (C¼N), 166.9
(C¼O), 187, 183 (triazole), 11.33
(CH3), 31.7 (CH2)
In 13C-NMR spectrum (table 3) of the Schiff base II the signal observed at 166.9 ppm
is ascribed to lactonyl carbon (C¼O) of the coumarin. A characteristic signal at
154.6 ppm is assigned to (C¼N). In addition to this, the characteristic signals in the
region 95.2–133.6 ppm are assigned to aromatic carbons and the signals at 11.24 and
31.7 ppm are due to CH3 and CH2, respectively.
3.3. Electronic spectral studies
The cobalt(II) complexes exhibited two distinct bands at 9920–9938 and 18,860–
4
4
20,660 cmꢁ1 corresponding to 4T1g(F) ! T2g(F) (ꢁ1) and 4T1g(F) ! T1g(P) (ꢁ3), which
suggests an octahedral geometry around cobalt(II) [34]. The ꢁ2 band that involves a
two-electron transition is not observed because of its proximity to the strong ꢁ3
transition. Electronic spectra of nickel(II) complexes showed d–d bands in the region
3
3
10,450–10,790, 16,333–16,889, and 26,824–27,620 cmꢁ1, assigned to A2g(F) ! T2g(F)
3
3
3
3
(ꢁ1), A2g(F) ! T1g(F) (ꢁ2), and A2g(F) ! T1g(P) (ꢁ3) transitions, respectively.
Table 4 shows the ligand field parameters (Dq, B0, ꢂ, ꢂꢂ%, ꢁ2/ꢁ1, and LFSE) [35].
Racah parameter B’ is 806.918–826.650 cmꢁ1, less than the free ion value of 1040 cmꢁ1
indicating covalent complex. The ratio ꢁ2/ꢁ1 was 1.563–1.565, well within the range
1.40–1.61 indicative of octahedral geometry [36]. The value of ꢂꢂ%, 23.587, further
supports octahedral geometry around Ni(II) [37].

330
Table 4. Ligand field parameters of Ni(II) complexes.
Transitions (cmꢁ1
Complex no. ꢁ1 ꢁ2 ꢁ3
S.A. Patil et al.
)
meff
Calculated
(BM)
ꢁ2 Calculated Dq
(cmꢁ1 (cmꢁ1) (cmꢁ1
B1
Distortion
(%)
ꢂꢂ
(%)
)
)
ꢁ2/ꢁ1 LSFE
ꢂ
3
4
10,450 16,333 26,824 16,629.76
10,790 16,869 27,620 17,149.75
1045 806.918 1.785 1.563 35.829
1079 826.650 1.520 1.565 36.994
3.171
3.160
0.764 23.587
0.783 21.719
Electronic spectra of Cu(II) complexes showed absorptions in the region 14,555–
2
14,760 cmꢁ1 attributed to T2g 2Eg transition, indicative of distorted octahedral
geometry [38].
3.4. Magnetic studies
Table 1 lists magnetic moments obtained at RT. The magnetic measurement for Co(II)
complexes exhibited magnetic moment values of 4.80–5.02 BM, which agree with
octahedral range 4.3–5.2 BM. Ni(II) complexes showed the magnetic moment values of
3.20–3.28 BM within the range 2.8–3.5 BM [39], consistent with their octahedral
environment. Cu(II) complexes showed magnetic moments of 1.77–1.79 BM, slightly
higher than the spin-only value 1.73 BM expected for one unpaired electron, offering
the possibility of octahedral geometry [40].
.
3.5. ESR spectrum of Cu(C26H16N8S2Cl2O4) 2H2O
ESR spectral studies of Cu(II) complexes provide information of the metal ion
environment. The ESR spectrum of Cu(C26H16N8S2Cl2O4) ꢀ 2H2O recorded in DMSO
at LNT and at RT show g|| and g values of 2.0754 and 2.0551, respectively. These
?
values indicate that the unpaired electron lies predominantly in the dx2ꢁy2 orbital [41].
The trend g|| 4 g 4 2.0023 indicates that the unpaired electron is localized in the d
x2ꢁy2
orbital of Cu(II), characteristic of axial symmetry. Thus, the results suggest that
?
Cu(C26H16N8S2Cl2O4) ꢀ 2H2O possesses distorted octahedral geometry.
3.6. FAB-mass spectral studies of Schiff bases and their complexes
The FAB-mass spectrum of I (figure S1 in Supplementary material) exhibited a
molecular ion peak at m/z 320 which is equivalent to its molecular weight. The
fragmentation peaks at m/z 305, 272, and 166 are ascribed to the cleavage of CH3, SH,
and triazole, respectively. For II, the molecular ion peak at m/z 334 is ascribed to
C14H12N4SClO2.
The FAB-mass spectrum of Co(C26H16N8S2Cl2O4) ꢀ 2H2O (figure S2 in
Supplementary material) is representative of Ni(C26H16N8S2Cl2O4) ꢀ 2H2O and
Cu(C26H16N8S2Cl2O4) ꢀ 2H2O showing molecular ion peaks Mþ at m/z 733, 732, and
737 equivalent to [ML2 ꢀ 2H2O]þ. The molecular ion by loss of two water molecules gave

Coumarinaldehyde Schiff bases
331
fragment ion peaks at m/z 697, 696, and 701 for [ML2]þ. All the metal complexes
undergo demetallation to form [L þ H]þ observed at m/z 320.
3.7. Thermal analyses
The thermal behavior of Co(II), Ni(II), and Cu(II) complexes are all almost the same.
Representative thermogravimetric (TG)/derivative thermogravimetric (DTG) curves
of
Co(C26H16N8S2Cl2O4) ꢀ 2H2O,
Ni(C26H16N8S2Cl2O4) ꢀ 2H2O,
and
Cu(C26H16N8S2Cl2O4) ꢀ 2H2O show the following characteristic features.
The thermal decomposition of Co(C26H16N8S2Cl2O4) ꢀ 2H2O, Ni(C26H16N8
S2Cl2O4) ꢀ 2H2O, and Cu(C26H16N8S2Cl2O4) ꢀ 2H2O took place in three steps, as
indicated by DTG peaks around 185–200ꢂC, 215–255ꢂC, and 300–355ꢂC corresponding
to mass loss of coordinated water molecule, triazole, and coumarinaldehyde, respec-
tively. Hence, the TGA studies confirm the coordination of water to the metal ions.
Metal oxide formed above 560ꢂC. The thermogram of Cu(C26H16N8S2Cl2O4) ꢀ 2H2O is
shown in figure S3 in Supplementary material. The nature of the proposed chemical
change with the temperature range and the percentage of metal oxide obtained are given
in table 5.
3.8. Electrochemistry
Electrochemical properties of the complexes were studied on a CHI1110A-
Electrochemical analyzer in DMF containing 0.05 mol Lꢁ1 n-Bu4NClO4 as the
supporting electrolyte. A cyclic voltammogram of Cu(C26H16N8S2Cl2O4) ꢀ 2H2O
(figure S4 in Supplementary material) displays a reduction peak at Epc ¼ 0.2917 V
with a corresponding oxidation peak at Epa ¼ 0.4579 V. The peak separation of this
couple (DEp) is 0.1662 V at 0.1 V and increases with scan rate. The most significant
feature of the Cu(II) complex is the Cu(II)/Cu(I) couple. The difference between
forward and back peak potentials can provide a rough evaluation of the degree of
reversibility of one-electron transfer reaction. Analyses of cyclic voltammetric responses
with scan rate varying from 50 to 250 mV sꢁ1 gives evidence for quasi-reversible one-
electron oxidation. The ratio of cathodic to anodic peak height was less than one;
however, the peak current increases with increase of the square root of the scan rates,
establishing the electrode process as diffusion controlled [42]. The separation in peak
potentials increases at higher scan rates. These characteristic features are consistent
with quasi-reversibility of the Cu(II)/Cu(I) couple.
The Ni(C26H16N8S2Cl2O4) ꢀ 2H2O complex exhibits
a
reduction peak at
Epc ¼ 0.4541 V with a direct re-oxidation peak at Epa ¼ 0.7490 V corresponding to the
Ni(II)/Ni(I) couple. The peak separation of this couple (DEp) is 0.2251 V at 0.1 V and
increases with scan rate, consistent with quasi-reversibility of the Ni(II)/Ni(I) couple.
3.9. Fluorescence studies
Emission spectra of the Schiff base I and its metal complexes were investigated in DMF
and DMSO. Schiff base I is characterized by an emission band at 535 nm in DMF and
527 in DMSO. Upon addition of 2–3 drops of aqueous alkali (2% NaOH) to I, we

332
Table 5. TG data of Co(II), Ni(II), and Cu(II) complexes of Schiff base I.
Weight loss (%) Metal oxide (%)
S.A. Patil et al.
Decomposition
Complex no. temperature (ꢂC) Observed Calculated Observed Calculated
Inference
1
3
5
190
4.911
4.916
10.23
10.10
10.71
10.25
10.17
10.73
Loss of coordinated water
molecules
Loss of triazole moieties
Loss of coumarin moieties
220
355
34.65
52.38
34.69
52.40
197
4.918
4.920
Loss of coordinated water
molecules
Loss of triazole moieties
Loss of coumarin moieties
225
370
34.69
52.45
34.70
52.47
185
4.888
4.890
Loss of coordinated water
molecules
217
307
34.46
52.10
34.47
52.13
Loss of triazole moieties
Loss of coumarin moieties
observed the ꢃmax values at 509 nm in DMF and 512 in DMSO from formation of
phenoxide anion. The Co(II), Ni(II), and Cu(II) complexes of I are characterized by
emission bands at 468, 442, and 472 nm, respectively in DMF (figure S5 in
Supplementary material). There was a decrease in intensity of fluorescence of Co(II),
Ni(II), and Cu(II) complexes in both DMF and DMSO solutions. Previous studies
reported that transition metal ions decrease the fluorescence quite effectively [24, 28].
Schiff base I and its complexes are fluorescent in nature.
3.10. Pharmacology results
A number of authors [20–24, 43–47] have investigated the biological properties of
transition metal complexes of triazoles as well as coumarin derivatives.
The Schiff bases and their metal complexes were evaluated for antimicrobial activity
(table 6). Schiff bases (I and II) were potentially active against S. aureus and A. niger
and moderately active against S. typhi. Both Schiff bases and their Co(II), Ni(II), and
Cu(II) complexes showed high antibacterial activity against all the bacterial strains.
All the metal complexes possess higher antifungal activity than the Schiff bases.
Khanmohammadi and Abnosi [47] studied the antimicrobial activities of Co(II), Ni(II),
and Cu(II) chelates of 4-amino-3-(4-pyridine)-5-mercapto-1,2,4-triazole and reported
that the activity of metal complexes were higher than the Schiff bases [47]. Bagihalli
et al. [20] reported the in vitro antibacterial and antifungal activities of Co(II), Ni(II),
and Cu(II) complexes with 1,2,4-triazole Schiff bases [33] to be higher than the Schiff
bases due to the change in structure due to coordination. Incorporation of methyl and
ethyl group exhibit differences in activity (5 is more active than 6 toward S. typhi).
The MIC of some selected compounds, which showed significant activity against
selected bacterial and fungal strains, indicated that these compounds were most active
in inhibiting the growth of the tested organisms at 10 mg mLꢁ1 (table 7).

Coumarinaldehyde Schiff bases
333
Table 6. Antimicrobial results of Schiff bases and their metal complexes.
Inhibition against bacteria (%)
Inhibition against fungi (%)
Concentration
(mg mLꢁ1
E.
coli
S.
aureus
B.
subtilis
S.
typhi
C. A.
albicans Cladosporium niger
Compound
)
I
25
50
100
43
51
62
46
66
71
42
54
65
25
29
38
48
51
65
49
50
62
50
52
66
II
25
50
100
42
54
63
48
56
66
43
52
61
27
31
39
46
52
68
47
56
61
44
58
68
1
2
25
50
100
25
50
100
53
60
79
51
63
76
63
71
88
64
72
89
53
62
73
59
69
77
30
35
42
33
40
42
52
66
71
55
75
80
60
68
73
66
71
79
70
72
82
72
72
81
3
4
5
6
25
50
100
56
64
75
63
73
80
59
69
76
33
37
44
59
69
78
58
65
75
66
70
85
25
50
100
53
64
77
66
75
88
59
62
70
36
41
49
54
68
76
61
68
71
68
71
81
25
50
100
58
59
70
66
68
70
50
53
68
31
38
40
16
35
37
35
38
53
30
41
54
25
50
100
56
58
69
61
65
66
53
56
71
32
33
41
14
45
56
33
42
51
45
49
55
Gentamycine
Fluconazole
100
100
100
–
100
–
100
–
100
–
–
100
–
100
–
100
Table 7. MIC (mg mLꢁ1) of selected compounds against selected bacteria.
Compound
E. coli
S. aureus
B. subtilis
S. typhi
C. albicans
Cladosporium
A. niger
I
II
1
2
3
4
5
6
25
25
10
10
10
10
25
25
10
–
10
10
10
25
10
25
25
25
10
10
10
10
25
25
10
–
25
25
10
10
10
10
10
25
10
10
10
10
25
10
10
10
10
25
10
10
10
10
10
25
4100
4100
10
25
4100
10
4100
4100
–
4100
4100
–
4100
4100
–
Gentamycine
Fluconazole
–
–
10
10
10
3.11. DNA cleavage studies
Schiff base I, Co(II) (1), and Ni(II) (3) were studied for their DNA cleavage activity by
agarose gel electrophoresis. Lanes L2, C1, and C2 of figure S6 in ‘‘Supplementary
material’’ represent the cleavage activity of I, 1, and 3, respectively, on isolated DNA

334
S.A. Patil et al.
OH
O
O
2
N
N
R
S
HC
N
Cl
R
N
M
N
Cl
CH
N
S
N
N
OH2
O
O
R=CH , C H
2 5
3
M=Co(II), Ni(II), Cu(II)
Complex
R
M
Formula
no.
CH3
Co(II)
Co(C26H16N S Cl O ) · 2H
2O
1
8
2
2
4
C2H5
CH3
Co(II)
Ni(II)
Ni(II)
Cu(II)
Cu(II)
Co(C28H20N S Cl O ) · 2H
2
3
4
5
6
2O
2O
2O
2O
2O
8
2
2
4
Ni(C26H16N8S2Cl2O4
Ni(C28H20N8S2Cl2O4
) · 2H
) · 2H
C2H5
CH3
Cu(C26H16N S Cl O ) · 2H
8
2
2
4
C2H5
Cu(C28H20N S Cl O
) · 2H
4
8
2
2
Figure 2. Structure of metal complexes.
of S. aureus. The difference in migration was observed compared to control DNA of
S. aureus (Lane Ct1). 1 and 3 show cleavage activity on isolated DNA of A. niger (Lanes
C3 and C4, respectively, of figure S6 in Supplementary material). Control DNA alone
does not show any apparent cleavage whereas I, 1, and 3 show a cleavage. However, the
nature of reactive intermediates involved in DNA cleavage by the complexes is not
clear. The results indicate the important role of metal ion in these isolated DNA
cleavage reactions. We infer that I, 1, and 3 are potent nuclease agents inhibiting growth
of the pathogenic organism by cleaving the genome.
4. Conclusion
The synthesized Schiff bases exhibit thiol–thione tautomerism and act as singly
deprotonated ligands. The metal ion is coordinated through the azomethine nitrogen
and sulfur via deprotonation. Octahedral structures of Co(II), Ni(II), and Cu(II)
complexes are shown in figure 2. The bonding of ligand to metal is confirmed by
analytical, spectral, magnetic, and thermal studies. Electrochemical properties of the
Cu(II) and Ni(II) complexes exhibit one-electron quasi-reversible electron transfer. The
metal complexes have higher antimicrobial activities than the ligands. Schiff base I, 1,
and 3 cleave DNA as compared to control S. aureus and A. niger DNA.

Coumarinaldehyde Schiff bases
335
Acknowledgments
The authors are grateful to the Chairman, Department of Chemistry, Karnatak
University, Dharwad for the facilities. One of the authors, Shrishila N. Unki, thanks
Karnatak University, Dharwad, for grant of Research Fellowship in Science for
Meritorious Students of UGC.
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