Synthesis, characterization, DFT and antimicrobial study of Co(II), Ni(II), Cu(II) and Zn(II) complexes of novel schiff base ligands derived from 3-Amino-1,2,4-Triazole-5-Thiol

Article abstract of DOI:10.14233/ajchem.2018.21630

Two novel Schiff base ligands are prepared by condensing 2-hydroxy-4-methoxybenzaldehyde with 3-Amino-1,2,4-Triazole-5-Thiol (HL₁) and 2-hydroxy-3-methoxybenzaldehyde with 3-Amino-1,2,4-Triazole-5-Thiol (HL₂). The ligands were synthe

Full text of DOI:10.14233/ajchem.2018.21630

Asian Journal of Chemistry; Vol. 30, No. 12 (2018), 2781-2787
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.21630
Synthesis, Characterization, DFT and Antimicrobial Study of Co(II), Ni(II), Cu(II) and Zn(II)
Complexes of Novel Schiff Base Ligands Derived from 3-Amino-1,2,4-triazole-5-thiol
*
AJAY PRAKASH and RAJESH MALHOTRA
Department of Chemistry, Guru Jambheshwar University of Science & Technology, Hissar-125 001, India
*
Corresponding author: E-mail: ajaychem2012@gmail.com, malhotra_ksrk@yahoo.co.in
Received: 3 August 2018; Accepted: 20 September 2018; Published online: 31 October 2018;
AJC-19148
Two novel Schiff base ligands are prepared by condensing 2-hydroxy-4-methoxybenzaldehyde with 3-amino-1,2,4-triazole-5-thiol (HL
and 2-hydroxy-3-methoxybenzaldehyde with 3-amino-1,2,4-triazole-5-thiol (HL ). The ligands were synthesized and characterized by
H NMR, IR, HRMS and analytical data. The synthesized ligands were complexed with metal salts in 1:2 metal:ligand ratio yielded the
complexes of type [M(L) (H O) ] [where M = Co(II), Ni(II), Cu(II) and Zn(II); x = 2 for Co(II), Ni(II), Cu(II) while x = 0 for Zn(II)]. The
1
)
2
1
2
2
x
metal complexes were characterized by elemental analysis, IR, ESI-mass, UV-visible, ESR, molar conductance, magnetic moment
measurement and thermogravimetric analysis. The results from IR spectra revealed that ligands exist in thione and thiol forms. The
synthesized complexes show non-electrolytic behaviour as indicated by their molar conductance values. The structure of ligand HL and
1
complexes 1-4 were fully optimized using Gaussian 09, taking 6-31g basis set and B3LYP functional. The experimental results based on
various spectroscopic and analytical data predicted an octahedral geometry for Co(II), Ni(II) and Cu(II) complexes while for Zn(II)
complexes the predicted geometry is tetrahedral. in vitroAntibacterial activity against Bacillus macerans (Gram-positive) and Pseudomonas
striata (Gram-negative) and antifungal activity against Rhizoctonia bataticola, Alternaria alternata and Fusarium odum has been evaluated
for all synthesized ligands and their metal complexes.
Keywords: Trasition metal(II) complexes, 1,2,4-Triazoles, DFT study, Biological activity.
Keeping view in mind, the biological importance of 1,2,4-
triazole derived Schiff base ligands, we design and synthesize
ligands viz., 2-[(5-mercapto-1H-[1,2,4]triazol-3-ylimino)methyl]
INTRODUCTION
Ligands having azomethine group called 'Schiff base' serves
as an important class of therapeutic compounds. They can be
easily prepared by the reaction of active carbonyl group with
primary amines. Schiff bases synthesized from an aromatic
aldehyde are more stable as compare to aliphatic aldehydes
because of the conjugation system [1-3]. They serve as a group
of ligands that are not only biological active but also stabilize
metals in various oxidation states [4-7]. Triazoles having amines
and thiol group have been widely studied for their anti-inflamm-
atory and antimicrobial activity and other industrial applications
-
3
5-methoxy phenol (HL
-ylimino)methyl]-6-methoxy phenol (HL
1
) and 2-[(5-mercapto-1H-[1,2,4]triazol-
) by reaction of 3-
2
amino-1,2,4-triazole-5-thiol with 2-hydroxy-4-methoxybenzal-
dehyde and 2-hydroxy-3-methoxybenzaldehyde and their comp-
lexes with Co(II), Ni(II), Cu(II) and Zn(II) ions.
EXPERIMENTAL
Solvents and chemicals used in the present study were of
AnalaR grade. Chemicals were purchased from Sigma-Aldrich,
Fluka and E. Merck and were used as received. The C, H, N analysis
was carried out on Carlo-Erba 1106 elemental analyzer. Condu-
ctivity measurements were carried out by using the ELICO
(CM82T) conductivity bridge. ESI-mass spectra were recorded
on aVG Biotech Quattrro mass spectrometer. Electronic spectra
[
8-11]. 1,2,4-Triazole show a coordination behaviour with a
range of metal ions due to presence of both nitrogen and sulphur
atom [12-14]. Ligands derived from 1,2,4-triazole based moiety
show a range of industrial applications and biological activities
including antimicrobial, anticancer, biomarkers and antitumor
[
15-19].
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2
782 Prakash et al.
Asian J. Chem.
of the metal complexes in DMSO were studied using Shimadzu
UV mini-1240 spectrophotometer. IR spectra in solid state
were recorded on FTIR BX-II spectrophotometer. NMR spectra
H
N
HS
N
N
N
CHO
OH
H N
N
SH
2
in DMSO-d
6
were recorded on a Bruker Advance DPX-300
Methanol
Reflux
+
spectrometer operating at a frequency of 400 MHz. TMS is
used as an internal standard. Thermal analyses were carried out
on a Shimadzu TGA-50H thermal analyzer. The solid state X-
band EPR spectra of Cu(II) complexes were recorded at room
temperature on a E4-EPR spectrometer.
N NH
R
HO
R
Metal Salt
M:L=1:2
HL1: R = 4-OMe
HL : R = 3-OMe
2
Synthesis of ligands: Schiff base HL
by the reaction of 3-amino-1,2,4-triazole-5-thiol with 2-hydroxy-
-methoxybenzaldehyde (1:1) in methanol. On mixing the two
contents the reaction mixture was first stirred for 1 h and then
refluxed for 12 h. The reaction progress was checked by TLC.
The reaction mixture was cooled once the reaction completes.
The solid obtained was filtered and washed with methanol,
1
was synthesized
H
N
HS
O
H
N
N
N
HS
4
R
N
N
N
R
M
N
H O
OH
2
O
O
2
M
O
N
N
N
N
N
R
and then finally dried under vacuum over anhydrous CaCl
The ligand HL was obtained by adopting the similar above
mentioned procedure.
Synthesis of metal complexes: Corresponding metal salt
2
.
SH
N
H
R
N
SH
2
N
H
M = Co(II), Ni(II) and Cu(II)
M = Zn(II)
Scheme-I: Synthesis of Schiff base ligands and their metal complexes
2+
2+
2+
2+
(
1 mmol) MCl
2
·xH O (M = Co , Ni , Cu and Zn ; x = 6 for
2
2+
2+
2+
2+
Co and Ni , x = 2 for Cu and x = 0 for Zn ) was dissolved
in 20 mL of methanol. This solution was added to 20 mL of
hot methanolic solution of the ligand (2 mmol). The reaction
mixture was stirred for 4-8 h, then refluxed for another 4-18 h.
The resulting reaction mixture was cooled overnight at 0 ºC
which yielded a crystalline solid. The precipitate was filtered
off, washed with cold ethanol and finally with dry ether, then
methoxybenzaldehyde (HL
and 2-hydroxy-3-methoxy benzaldhyde (HL
1
), 3-amino-1,2,4-triazole-5-thiol
). The structure
2
1
of these Schiff bases was confirmed by their IR, H NMR and
mass spectrophotometry data. Further, these Schiff bases were
used for the synthesis of metal complexes by taking the
corresponding metal chloride salts in metal:ligand ratio of 1:2.
The monomeric complexes thus obtained are stable in air and
decomposed above 280 ºC. These complexes show poor solu-
bility in methanol and are soluble in DMF or DMSO. The low
value of molar conductance of these complexes suggested their
non-electrolytic nature. Physical measurements and the analy-
tical data of all the synthesized compounds are given in Table-1.
Mass spectra: The HRMS mass spectrum of the ligands
dried under vacuum over P
DFT calculations: The gas phase geometry optimization
of ligand HL and complexes (1-4) was done using Gaussian
9W suite [20-23].
Antimicrobial activity: All the synthesized compounds
4
O10 (Scheme-I).
1
0
were tested for their antibacterial activity against B. macerans
and P. striata by using disc diffusion method [24] and antifungal
activity was done using food poison technique [24] against
the test fungi, R. bataticola, A. alternata and F. odum. The test
compounds were dissolved in DMF in varying concentration.
Streptomycin was used as a standard drug for antibacterial
and chlorothalonil for antifungal study.
HL
was performed and their m/z are listed in Table-1. In the HRMS
spectrum of ligand HL , a peak corresponding to [M+18] appe-
1
, HL
2
and ESI mass spectrum for the metal complexes 1-8
1
ared at m/z value of 268.0196, followed by a series of peaks
due to different fragments. The stability of any fragment relates
directly with the intensity of the corresponding peak. Similarly,
in the mass spectrum of ligand HL (Fig. 1) a peak corresponds
2
RESULTS AND DISCUSSION
to [M+18] appear at m/z value of 268.0198 followed by a series
of peaks. In the mass spectrum of complex 1, a peak corresponds
to [M+1] appears at m/z value of 594.19 (Fig. 2).
Schiff bases were prepared (Scheme-I) by taking equimolar
amount of 3-amino-1,2,4-triazole-5-thiol and 2-hydroxy-4-
TABLE-1
PHYSICAL MEASUREMENTS AND ANALYTICAL DATA OF THE LIGANDS (HL , HL ) AND METAL COMPLEXES (1-8)
1
2
Molar
mass
Elemental analysis (%): Found (calcd.)
No.
m.f.
m/z
Yield (%)
C
H
N
M
HL1
HL2
1
2
3
4
5
6
7
C H N O S
250.05
250.05
593.04
592.05
597.04
562.02
593.04
592.05
597.04
562.02
268.0196
268.0198
594.1900
593.6700
596.7800
562.7300
594.8800
593.4500
599.1200
563.9300
82
79
61
63
65
69
61
63
65
66
47.75 (47.99)
48.22 (47.99)
40.22 (40.48)
40.22 (40.49)
39.91 (40.16)
42.30 (42.60)
40.26 (40.48)
40.25 (40.49)
39.75 (40.16)
42.34 (42.60)
3.86 (4.03)
3.74 (4.03)
3.83 (3.74)
3.62 (3.74)
3.54 (3.71)
3.13 (3.22)
3.56 (3.74)
3.51 (3.74)
3.48 (3.71)
3.10 (3.22)
22.14 (22.39)
22.18 (22.39)
18.63 (18.88)
18.65 (18.89)
18.51 (18.73)
19.60 (19.87)
18.61 (18.88)
18.55 (18.89)
18.37 (18.73)
19.63 (19.87)
–
–
10
10
4
2
C H N O S
10
10
4
2
C H N O S Co
9.72 (9.93)
9.63 (9.89)
10.39 (10.62)
11.34 (11.60)
9.68 (9.93)
9.55 (9.89)
10.31 (10.62)
11.37 (11.60)
2
0
22
8
6 2
C H N O S Ni
20 22 8 6 2
C H N O S Cu
20 22 8 6 2
C H N O S Zn
20 18 8 4 2
C H N O S Co
20 22 8 6 2
C H N O S Ni
20 22 8 6 2
C H N O S Cu
20 22 8 6 2
C H N O S Zn
20 18 8 4 2
8

Vol. 30, No. 12 (2018)
Synthesis of Novel Schiff Base Ligands Derived from 3-Amino-1,2,4-triazole-5-thiol 2783
4
×
10
due to the aromatic protons. The peak for three protons of OCH
appears at 3.78 ppm. All the protons appeared in their expected
range.
3
2
.5
.0
.5
.0
.5
0
1
52.0699
2
1
1
0
268.0198
IR spectra: The key infrared bands of the ligands and
their metal complexes are given in Table-2. The coordination
of ligand to the corresponding metal ions was confirmed on
the basis of shifts in absorption bands of various groups and
absence of certain absorptions. The strong band at 1595-1592
5
0 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950
Counts vs. Mass-to-charge (m/z)
Fig. 1. Mass spectrum of ligand HL
2
-1
cm was assigned to ν(HC=N) linkage. This particular band
TABLE-2
in case of metal complexes shifts to a lower side in the IR spectra
by 15-19 cm thereby indicating the involvement of azomethine
IMPORTANT INFRARED SPECTRAL
BANDS (cm ) AND THEIR ASSIGNMENTS
-1
-
1
nitrogen in complexation [25]. This involvement is further
supported by the presence of a new band in the range of 492-478
Compound
ν(HC=N)
1595
1592
1581
1576
1578
1577
1583
1589
1573
1575
ν(–OH)
ν(M–O)
–
ν(M–N)
–
HL1
HL2
1
2
3
4
5
6
7
3213
3258
-1
cm corresponding to M−N bond for metal complexes. Broad
–
–
-1
band which appears in the range of 3258-3213 cm in ligands
–
–
–
–
–
–
–
–
542
539
598
598
522
545
538
576
488
491
492
490
478
483
478
483
corresponds to ν(OH) of aldehyde moiety disappears after metal
complexation, which indicates the deprotonation of hydroxyl
group in metal complexation. Broad band appears in metal
-1
complexes (1-3 & 5-7) in the range of 3410-3398 cm was
due to ν(OH) of water [26] however, in case of Zn(II) complexes
(
4 & 8) no such band was observed. No shift is observed in
case of ν(SH) and (C=S), indicating the thiol-thione tautomerism
Fig. 3) of ligand and these groups are not involved in coordi-
8
(
1
1
H NMR spectra: The H NMR spectra of the ligands
-1
nation. Similarly, new band in the range of 598-522 cm was
HL
1
and HL
2
were recorded in CDCl
3
with drop of DMSO-d .
6
due to M−O bond. The results of IR spectra revealed that ligands
In case of ligand HL
1
, a singlet corresponding to imine (CH=N)
HL
1
and HL show a uninegative bidentate nature and coordi-
2
proton appears at 8.42 ppm. The signals due to the proton of -OH
and -NH group appear as a singlet at 11.90 and 12.78 ppm,
respectively. Similarly, peak corresponding to -SH proton appe-
ared at 13.93 ppm. Signals in the range of 6.4 to 7.28 ppm were
nation occurs via azomethine nitrogen and deprotonated hydroxyl
group.
Conductance and magnetic susceptibility measurements:
Conductivity measurements for the metal complexes were perfo-
7
3.86
1
00
%
0
101.98
2
78.86
1
78.75
1
77.89
193.84
2
51.94
51.01
32.99
2
1
21.78
2
1
48.97
279.99 353.05
01.00
454.11
594.19
3
3
85.23
475.98
550.51
661.08
50
100
150
200
250
300
350
400
450
500
550
600
6
50
700
750
800
850
900
950 1000
m/z
Fig. 2. Mass spectrum of Co(II) complex of ligand HL
1

2
784 Prakash et al.
Asian J. Chem.
H
N
electronic spectra of Ni(II) complexes display three bands in
H
S
-1
HS
N
the region 10715-11123, 15132-16034 and 23098-24001 cm ,
N
N
N
N
3
3
3
respectively, assigned to the transitions A2g(F) → T2g(F), A2g(F)
→
N
N
3
3
3
H
T1g(F) and A2g(F) → T2g(F). These bands suggests an
octahedral geometry [28] for Ni(II) complexes. In case of Cu(II)
complexes, three bands in the range 12082-13123, 18715-18921
HO
HO
-1
and 22456-23741 cm were observed. These bands may be
R
2
2
2
2
2
R
assigned to B → A (d
x -y
2 2
→ dz ), B → B → and B →
2
1
g
1g
1g
2g
1g
Thiol form
Fig. 3. Tautomeric form of Schiff base
Thione form
2E
(dx2-y2 → dxz, dyz) transitions, respectively, which suggests a
g
distorted octahedral geometry for Cu(II) complexes [28].
Thermogravimetric study:Thermal behaviour of the comp-
lexes (1-4) has been studied using thermogravimetric analysis
from room temperature to 800 ºC at a heating rate of 10 ºC/
rmed at room temperature in DMF solvent and values are given
in Table-3.All the complexes showed molar conductance values
-1
2
-1
min in nitrogen atmosphere. Complex 1 [Co(L
1 2
) (H
2
O) ] shows
2
in the range (4.2-10.2 Ω cm mol ), which suggests their non-
electrolytic nature. In case of Co(II) complexes the obtained
value of magnetic moment (4.96-5.04) B.M. indicates high
spin octahedral geometry with three unpaired electrons. For
Ni(II) complexes, the obtained magnetic moment value of (2.93-
three step decomposition within temperature range of 140-720
ºC. In the first decomposition step, two water molecules are
lost (found 6.10 %, calcd. 6.07 %).A mass loss of 39.12 % (calcd.
3
9.28 %) was observed at second stage of decomposition,
which corresponds to loss of C10 OS. Finally mass loss of
2.18 % (calcd. 42.02 %) was observed due to the loss of
H N
9 4
2
.98) B.M. indicates two unpaired electrons which suggested
4
an octahedral geometry. In case of Cu(II) complexes, the mag-
netic moment values of 1.80-1.83 B.M., indicates one unpaired
electron.
triazole moiety in the third stage of decomposition. As a final
product, it leaves CoO as residue (found 12.60 %, calcd. 12.63
%
). Similar pattern was observed in the TGA curve of metal
TABLE-3
complexes 2-3. However, complex 4 which is Zn(II) complex
show a two step decomposition. The TGA data for the metal
complexes 1-4 are given in Table-4.
EPR spectra: The solid state X-band EPR spectra of poly-
crystalline solid of Cu(II) complexes were recorded in powder
form at room temperature. The ESR spectra of Cu(II) complex
CONDUCTIVITY, MAGNETIC AND ELECTRONIC
SPECTRA OF METAL COMPLEXES
Complex
No.
Ω
µeff
M
2
-1
1
/λ (cm )
-
1
-1
(Ω cm mol )
(B.M.)
1
2
3
5
6
7
4.2
5.4
8.1
10.2
7.4
9.1
4.96
2.98
1.83
5.04
2.93
1.80
10412, 19123
11123, 15132, 23098
13123, 18921, 23741
10998, 19716
10715, 16034, 24001
12082, 18715, 22456
3
at room temperature displayed two g values i.e. g|| = 2.190
and g = 2.053, while for complex 6 the g values are g||
.193 and g = 2.057. From the observed values, it is clear that
> 2.0023 and these parameters suggest that unpaired
⊥
=
2
g
⊥
|| > g
⊥
2
electron lying predominantly in the dx2-y2 orbital giving B1g as
the ground state and are a characteristic of axial symmetry.
The above pattern suggests a tetragonal geometry for the Cu(II)
complexes [29]. The value of exchange interaction term G =
Electronic spectra: The electronic absorption spectral data
of Co(II), Ni(II) and Cu(II) complexes of the ligands HL and
HL are listed in Table-3. The electronic spectra of Co(II) compl-
1
2
exes showed two transitions corresponding 10412-10988 and
(g|| - 2.0023)/(g - 2.0023) gives idea about exchange inter-
⊥
-
1
4
4
4
1
9123-19716 cm to T1g(F) → T2g(F) (ν
1
); T1g(F) → T1g(P)
actions between Cu(II) centers. The value of G < 4.0 (calculated
3.58 for complex 3 and 3.38 for complex 6) indicates consider-
able exchange interactions of Cu-Cu in the solid complexes [30].
(
ν
3
). The transition ν
= ν
2
calculated [27] by using the relation
transition is not observed. The
ν
2
1
+ 10Dq, very close to ν
3
TABLE-4
THERMAL ANALYSIS DATA OF METAL COMPLEXES (1-4)
Possible evolved
species
Residual
species
Mass loss (%)
Compd. No.
Compounds
Stages
Temp. (°C)
Found
Calcd.
st
1
2
3
140-190
190-460
460-720
2H O
6.10
39.12
42.18
12.60
6.0
6.07
39.28
42.02
12.63
6.08
2
nd
C H N OS
10
9
4
1
[Co(L
1
)
2
(H O)
2
2
]
rd
Remaining moiety
CoO
NiO
st
1
2
3
140-180
180-475
475-790
2H O
2
nd
C H N O S
42.12
39.35
12.53
5.98
42.05
39.39
12.48
6.02
10
9
4
2
2
[Ni(L
1
)
2
(H O)
2
2
]
rd
Triazole moiety
st
1
2
3
130-195
195-470
470-780
2H O
2
nd
C H N O S
44.75
36.08
13.19
54.65
31.15
14.20
44.72
36.05
13.21
54.98
30.80
14.22
10
9
3
2 2
3
4
[Cu(L
1
)
2
(H O)
2
]
2
rd
Triazole moiety
CuO
ZnO
st
1
2
110-495
495-720
C H N O S
12
11
3
3 2
nd
[Zn(L
) ]
2
Remaining moiety
2

Vol. 30, No. 12 (2018)
Synthesis of Novel Schiff Base Ligands Derived from 3-Amino-1,2,4-triazole-5-thiol 2785
DFT calculations: The gas phase geometry optimization
of the ligand HL and the metal complexes (1-4) was perfor-
med. Numbering (Fig. 4) and the optimized structures for the
metal complexes along with the ligand are displayed in Fig. 5.
H
N
2
HS
1
C2
N3
H O
MeO
N1
C7' C8'
C1
Initially, the optimized geometry of ligand HL shows planarity
1
C '
6
C ' O ' 2
9
N4 C3
of the molecule. After metal complexation, the triazole moiety
lost planarity and turns out of plane of the paper. The structures
are stable with respect to energy. In complex 1, which is Co(II)
1
C '
5
C4'
M
C4 C5
O1 C9 C6
C8 C7
C '
3
N '
4
H2O
complex of ligand HL , the metal is coordinated in an octahedral
1
environment and binds on one position through nitrogen of
azomethine group and on other through oxygen of deproto-
nated hydroxyl group, the remaining two positions are occupied
by water molecules. The computed bond angle and bond lengths
values are in agreement with the reported M−O and M−N bond
values. The angles around the coordination sphere are in well
agreement with the experimental results.
C '
1
N '
3
OM e
N '
1
C '
2
N '
SH
2
H
M = Co(II), Ni(II), Cu(II) and Zn(II)
Fig. 4. Numbering Scheme of ligand and metal complexes
Fig. 5. Optimized structure of ligand HL (a), Complex 1 (b), Complex 2 (c), Complex 3 (d) and Complex 4 (e)
1

2
786 Prakash et al.
Asian J. Chem.
Similar bonding pattern and parameters values was found
concentration of tested compound, as activity is directly proport-
ional to concentration of tested compounds.
Antifungal activity:All the synthesized compounds were
screened against fungi, Rhizoctonia batatiola, Alternaria alter-
nata and Fusarium odum. Chlorothalonil is used as standard
drug. The antifungal screening data is shown in Table-5. In
in case of Ni(II) complex. Copper(II) complex shows tetragonal
geometry which shows a little variation in their bond length
and bond angle values due to Jahn-Teller distortions. The bond
lengths are slightly elongated and bond angles vary slightly
from the experimental values. Zinc(II) complex stabilized in
tetrahedral geometry. On the basis of analytical, spectroscopic
and theoretical calculation results, following structure has been
proposed for metal complexes (Fig. 6).
between the ligands, ligand HL was more active while among
1
the metal complexes, complex 2 was most active for Rhizoctonia
batatiola, complex 1 was most active for Alternaria alternata
and complex 6 was most active for Fusarium odum. From the
antifungal data, it may be conclude that all the compounds
were moderate to significant active against the tested fungi and
the metal complexes were found to be more active as compare
to free ligand.
H
N
H
N
HS
HS
N
N
N
N
O
R
R
N
N
O
N
O
M
M
Conclusion
H O
OH
O
2
2
N
Two novel Schiff bases HL
1
and HL and their Co(II),
2
N
N
Ni(II), Cu(II) and Zn(II) metal complexes were synthesized.
All the compounds were fully characterized by using various
characterization and analytical techniques. The results obtained
from these techniques concludes that ligands are uninegative
in nature and coordinates in bidentate manner through nitrogen
of azomethine group and oxygen of deprotonated hydroxyl
group. An octahedral geometry was assigned for Co(II), Ni(II)
and Cu(II) complexes while a tetrahaedral one was assigned
for Zn(II) complexes. The metal complexes exhibited significant
antimicrobial activity and the metal complexes are found to
be more active as compare to free ligands against the tested
antibacterial and antifungal strains.
N
R
N
R
SH
SH
N
H
N
H
M = Co(II), Ni(II) and Cu(II)
M=Zn
Fig. 6. Proposed structure of metal complexes
Antibacterial activity: in vitroAntibacterial activity against
Bacillus macerans (Gram-positive) and Pseudomonas striata
Gram-negative) were evaluated for all synthesized compounds.
Streptomycin is the standard drug used for comparison of anti-
bacterial activity of the complexes. The antibacterial screening
data (Table-5) showed that the zone of inhibition is larger for
(
ligand HL
active against Gram-positive bacteria while the metal complex
was most active against Gram-negative bacteria. In general,
an enhancement in the antibacterial activity of the ligand was
observed on complexation. This increase in the activity on comp-
lexation as compare to free ligand is explained on the basis of
chelation theory [31]. Interestingly, the presence of 2D and
2
in between the ligands. Metal complex 5 was most
CONFLICT OF INTEREST
8
The authors declare that there is no conflict of interests
regarding the publication of this article.
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[
[
[
Co(L ) (H O) ]
29
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31
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28
34
1
2
2
2
Ni(L ) (H O) ]
19.1
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20.1
12.3
25.1
27.5
24.8
22.3
–
1
2
2
2
Cu(L ) (H O) ]
1
2
2
2
Zn(L1)2
HL2
7
[
[
[
Co(L ) (H O) ]
13
12
12
10
18
2
2
2
2
Ni(L ) (H O) ]
2
2
2
2
Cu(L ) (H O) ]
–
7
11
2
2
2
2
Zn(L2)2
Streptomycin
standard)
Chlorothalonil
standard)
(
–
–
–
–
–
–
90.0
78.0
48.0
98.0
80.2
46.3
89.0
74.1
46.5
(

Vol. 30, No. 12 (2018)
Synthesis of Novel Schiff Base Ligands Derived from 3-Amino-1,2,4-triazole-5-thiol 2787
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