Synthesis, characterization, and biological activity of metal complexes of 4-ethyl vanillideneamino-3-methyl-5-mercapto-1,2,4-triazole

Article abstract of DOI:10.1080/00958972.2010.505283

A series of transition metal-triazole derivatives have been synthesized and characterized by microanalytical, thermal, magneto-chemical, and spectral studies. The synthesized complexes were screened for biological activities. The complexes of Co(II) and Ni(II) emerged as a good antibacterial agent against Gram-negative bacteria such as Escherichia coli, Pseudomonas aeruginosa, Salmonella typhi, Bacillus subtilis, and Shigella flexneri.

Full text of DOI:10.1080/00958972.2010.505283

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Synthesis, characterization, and
biological activity of metal complexes
of 4-ethyl vanillideneamino-3-
methyl-5-mercapto-1,2,4-triazole
A.S. Ramasubramanian a , B. Ramachandra Bhat a & R. Dileep a
a Catalysis and Material Chemistry Division, Department of
Chemistry , National Institute of Technology Karnataka ,
Srinivasnagar, Surathkal–575025, Karnataka, India
Published online: 28 Jul 2010.
To cite this article: A.S. Ramasubramanian , B. Ramachandra Bhat & R. Dileep (2010) Synthesis,
characterization, and biological activity of metal complexes of 4-ethyl vanillideneamino-3-
methyl-5-mercapto-1,2,4-triazole, Journal of Coordination Chemistry, 63:17, 3108-3116, DOI:
10.1080/00958972.2010.505283
To link to this article: http://dx.doi.org/10.1080/00958972.2010.505283
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Journal of Coordination Chemistry
Vol. 63, No. 17, 10 September 2010, 3108–3116
Synthesis, characterization, and biological activity
of metal complexes of 4-ethyl vanillideneamino-
3-methyl-5-mercapto-1,2,4-triazole
A.S. RAMASUBRAMANIAN, B. RAMACHANDRA BHAT*
and R. DILEEP
Catalysis and Material Chemistry Division, Department of Chemistry, National Institute of
Technology Karnataka, Srinivasnagar, Surathkal – 575025, Karnataka, India
(Received 29 December 2009; in final form 5 May 2010)
A series of transition metal–triazole derivatives have been synthesized and characterized by
microanalytical, thermal, magneto-chemical, and spectral studies. The synthesized complexes
were screened for biological activities. The complexes of Co(II) and Ni(II) emerged as a good
antibacterial agent against Gram-negative bacteria such as Escherichia coli, Pseudomonas
aeruginosa, Salmonella typhi, Bacillus subtilis, and Shigella flexneri.
Keywords: Ethyl vanillin; Metal complexes; Schiff base; Biological studies
1. Introduction
Schiff bases are widely used as analytical reagents since they allow simple and
inexpensive determination of several organic and inorganic substances [1]. They also
form stable complexes with metals that perform an important role in biological systems
[2]. Transition metal Schiff-base complexes have many applications in catalysis,
photonics, antitumor, and antibacterial activity [3] and are also used as enzyme models
[4]. Schiff bases derived from salicylaldehyde are polydentate ligands, coordinating as
deprotonated or neutral forms.
Triazole chemistry is becoming more important due to its excellent biological activity.
Triazole antifungal drugs include fluconazole, isavuconazole, itraconazole, voricona-
zole, pramiconazole, and posaconazole. The triazole plant protection fungicides include
epoxiconazole, triadimenol, propiconazole, metconazole, cyproconazole, tebuconazole,
flusilazole, and paclobutrazol. Studies have been reported on the synthesis and
characterization of metal complexes of Schiff bases involving 1,2,4-mercapto triazoles
with salicylaldehyde, p-nitro benzaldehyde, o-nitrobenzaldehyde, p-toluyl aldehyde, and
cinnamaldehyde [5–12]. Transition metal complexes of pentadentate ligands [13]
coordinate through deprotonated oxygen, thioenolic sulfur, and azomethine nitrogen.
*Corresponding author. Email: chandpoorna@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.505283

Ethyl vanillin
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Many transition metal complexes of Schiff bases containing 1,2,4-mercapto triazole
exhibit greater antibacterial activity against Gram-positive and Gram-negative bacteria.
Coordinating metal ions have pronounced effect on the antibacterial activity of the
ligand [14, 15].
In this study, the synthesis and characterization of Co(II), Ni(II), Pd(II), and Zn(II)
complexes of Schiff-base ethylvanillidene-4-amino-5-mercapto triazole have been
undertaken. The antibacterial activities of the ligands and their metal complexes have
been screened in vitro against bacteria such as Escherichia coli, Pseudomonas aeruginosa,
Salmonella typhi, Bacillus subtilis, and Shigella flexneri.
2. Experimental
2.1. Methods and materials
The synthesized complexes were analyzed for metal content using an Atomic
Absorption Spectrometer model Varian AA 55B. Carbon–hydrogen–nitrogen–sulfur
(CHNS) analysis was carried out on Thermo Flash EA 112 series. Magnetic
susceptibilities were measured by a Sherwood Scientific Magnetic Susceptibility meter
(UK). Thermogravimetric analyses (TGA) were carried out on a Universal V4.1D TA
Instruments TGA analyzer, infrared (IR) spectra in KBr pellets on a Nicolet Fourier
transform-infrared (FT-IR) from 4000 to 400 cmꢀ1, UV-Vis spectra on a Shimadzu
Pharmaspec, molar conductance measured on an Elico conductivity meter and
1H-NMR spectra were recorded on DSX-300/AMX-400/DRX-500/AV500 NMR
spectrometers.
2.2. Synthesis of 4-ethyl vanillideneamino-3-methyl-5-mercapto-1,2,4-triazole
This ligand (shortly EVAMMT) was synthesized in two stages. In the first stage,
thiocarbohydrazide was refluxed with glacial acetic acid for 2 h [5]. The white-colored
product, 3-methyl-4-amino-5-mercapto-1,2,4-triazole (MAMT) was filtered, washed,
and recrystallized. In the second stage, the MAMT (0.02 mmol) was dissolved in ethyl
alcohol and refluxed with (0.002 mmol) of ethyl vanillin for 4 h. The reaction mixture
was cooled, filtered, washed, and recrystallized from alcohol. The ligand was
1
characterized by elemental analysis, IR, UV, and H-NMR. Yield: 89%, m.p.: 214ꢁC.
1H-NMR (400 MHz, ppm, from TMS in DMSO): 11.2 (s, 1H, –SH), 8.1
(s, 1H, –CH¼N–), 10.3 (s, 1H, –OH), 6.24–8.76 (m, 4H, Ar), 3.8 (q, 2H, –OCH2–),
2.3 (s, 3H, –CH3, triazole), and 1.8 (t, 3H, –CH3). FT-IR (KBr pellets, in cmꢀ1): 3150
(N–H), 1604 (C¼N azomethine), and 1430 (thioamide), 800 (C–S thiomide). Elemental
Anal. Calcd (%): C (52.4), H (5.1), N (20.8), and S (11.7); Found (%): C (52.2), H (5.1),
N (21.0), and S (11.6).
2.3. Synthesis of complexes
EVAMMT complexes were prepared by refluxing a solution of 0.004 mmol EVAMMT
in ethyl alcohol with 0.002 mmol solution of respective transition metal salt

3110
A.S. Ramasubramanian et al.
(NiCl2 ꢂ 6H2O, CoCl2 ꢂ 6H2O, PdCl2, and ZnCl2 Á 2H2O). The complexes formed were
filtered, washed with ether, and dried in vacuo. EVAMMT complexes of Ni(II), Co(II),
Pd(II), and Zn(II) were slightly soluble in dimethylformamide (DMF) and soluble
in DMSO.
2.3.1. bis(1,2,4-triazole-3-methyl-4 ethylvanillideneamino 5-thiolato)zinc(II). 1H-NMR
(400 MHz, ppm, from TMS in DMSO): 8.5 (s, 1H, –CH¼N–), 10.8 (s, 1H, –OH),
6.24–8.76 (m, 4H, Ar), 3.8 (q, 2H, –OCH2–), 2.3 (s, 3H, –CH3, triazole), and 1.8 (t, 3H,
–CH3). FT-IR (KBr pellets, in cmꢀ1): 2987 (N–H), 1591 (C¼N azomethine), 1433
(thioamide), and 800 (C–S thiomide). Elemental Anal. Calcd (%): C (46.5), H (4.5),
N (18.2), and S (10.3); Found (%): C (46.3), H (4.4), N (18.1), and S (10.2).
2.3.2. Diaquobis(1,2,4-triazole-3-methyl-4 ethylvanillideneamino 5-thiolato)nickel(II).
1H-NMR (400 MHz, ppm, from TMS in DMSO): 8.6 (s, 1H, –CH¼N–), 10.7 (s, 1H,
–OH), 6.24–8.76 (m, 4H, Ar), 3.8 (q, 2H, –OCH2–), 2.3 (s, 3H, –CH3, triazole), and 1.8
(t, 3H, –CH3). FT-IR (KBr pellets, in cmꢀ1): 3525 (N–H), 1591 (C¼N azomethine),
1450 (thioamide), 733 (C–S thiomide). Elemental Anal. Calcd (%): C (47.3), H (4.3),
N (18.4), S (10.4); Found (%): C (47.11), H (4.23), N (18.36), and S (10.3).
2.3.3. Diaquobis(1,2,4-triazole-3-methyl-4 ethylvanillideneamino 5-thiolato)cobalt(II).
1H-NMR (400 MHz, ppm, from TMS in DMSO): 8.5 (s, 1H, –CH¼N–), 10.8 (s, 1H,
–OH), 6.24–8.76 (m, 4H, Ar), 3.8 (q, 2H, –OCH2–), 2.3 (s, 3H, –CH3, triazole), and 1.8
(t, 3H, –CH3). FT-IR (KBr pellets, in cmꢀ1): 3500 (O–H), 3197 (N–H), 1597 (C¼N
azomethine), 1445 (thioamide), and 681 (C–S thiomide). Elemental Anal. Calcd (%):
C (47.3), H (4.3), N (18.4), and S (10.7); Found (%): C (47.2), H (4.3), N (18.6),
and S (10.5).
2.3.4. bis(1,2,4-Triazole-3-methyl-4 ethylvanillideneamino 5-thiolato)palladium(II).
1H-NMR (400 MHz, ppm, from TMS in DMSO): 8.5 (s, 1H, –CH¼N–), 10.6 (s, 1H,
–OH), 6.24–8.76 (m, 4H, Ar), 3.8 (q, 2H, –OCH2–), 2.2 (s, 3H, –CH3, triazole), and 1.8
(t, 3H, –CH3). FT-IR (KBr pellets, in cmꢀ1): 3200 (N–H), 1591 (C¼N azomethine),
1440 (thioamide), and 750 (C–S thiomide). Elemental Anal. Calcd (%): C (47.5),
H (4.6), N (17.0), and S (9.8); Found (%): C (47.3), H (4.5), N (17.1), and S (9.6).
2.4. Biological activity
Preliminary screening of Co(II), Ni(II), and Zn(II) complexes of EVAMMT for their
in vitro antibacterial activity against pathogenic strains of bacteria such as E. coli,
P. aeruginosa, S. typhi, B. subtilis, and S. flexneri using plate technique was performed.
The bacteria were cultured (15 mm diameter) in a previously sterilized Mueller–Hinton
agar medium in a petri dish and used as an inoculum for the study. The components to
be tested were dissolved in DMF to a final concentration of 0.5% and 1% and soaked
in filter paper disks of 5 mm diameter and 1 mm thickness. These disks were placed on

Ethyl vanillin
3111
the previously seeded plates and incubated at 35 ꢃ 2ꢁC for 24 h. The diameter (mm) of
the inhibitory zone around each disk was measured after 24 h.
3. Results and discussion
3.1. UV spectra
Electronic spectra of the synthesized complexes (10ꢀ3 mol Lꢀ1 in DMSO) are given in
table 1. The Co(II) complex shows three bands at 7273, 14,285, and 19,638 cmꢀ1 due to
4
4
4
4
4
4T1g(F) ! T2g(F) (ꢀ1), T1g(F) ! A2g(F) (ꢀ2), and T1g(P) ! T1g(F) (ꢀ3) transitions,
respectively, suggesting octahedral environment around Co(II). The ꢀ3 : ꢀ2 value for the
compound is 2.70, leading to (from appropriate energy level diagram)
Dq=B0 ¼ 0:96
and to,
4
4T1gðFÞ ! T2gðFÞ=B0 ¼ 7273=B0 ¼ 8:2
B0 ¼ 887
Hence, Dq ¼ 851.5.
Consequently, the spectral parameters of the compounds are: 10Dq ¼ 8515 cmꢀ1
,
B0 ¼ 887 cmꢀ1, and ꢁ ¼ 0.79. The reduction of Racah parameter from the free ion value
of 1120 to 887 cmꢀ1 and the value of ꢁ (0.79) indicates covalence in the compound. The
Ni(II) complex shows three bands at 13,984, 16,090, and 23,772 cmꢀ1, which are
assigned to the spin-allowed transitions, 3A2g(F) ! T2g(F) (ꢀ1), 3A2g(F) ! T1g(F) (ꢀ2),
3
3
3
3
and A2g(F) ! T1g(P) (ꢀ3), respectively, suggesting an octahedral geometry [16–18].
The ꢀ1 : ꢀ2 value for the present compound is 1.74 and corresponds to usual range
(1.6–1.82). The spectral parameters are 10Dq ¼ 8565 cmꢀ1, B0 ¼ 874 cmꢀ1, ꢁ ¼ 0.81, and
the ꢁ0 value is 19%. The reduction of Racah parameter from the free ion value of 1080
to 874 cmꢀ1 and ꢁ value of 0.81 indicates the covalent nature of the compound [19, 20].
The above discussion very strongly indicates octahedral geometry around the central
Table 1. Analytical results.
Molar
conductance (mS)
Frequency
(cmꢀ1
B
B
(%)
Complex
ꢂeff
)
Assignments
–
10Dq (cmꢀ1) (cmꢀ1
)
ꢁ
EVAMMT
Zn-EVAMMT
Ni-EVAMMT
–
21.0
26.6
–
Diamagnetic
3.12
–
16,090 3A2g(F) ! T2g(F)
8565 874 0.81 19
8515 887 0.79 21
3
13,984 3A2g(F) ! T1g(F)
3
25,000 3A2g(F) ! T1g(P)
3
4
Co-EVAMMT
Pd-EVAMMT
20.8
22.2
5.20
14,285 4T1g(F) ! A2g(F)
7273 4T1g(F) ! T2g(F)
4
23,772 4T1g(P) ! T1g(F)
4
Diamagnetic

3112
A.S. Ramasubramanian et al.
metal ion in these two complexes (Ni and Co), requiring occupation of two
coordination sites by H2O.
3.2. FT-IR spectra
IR positions of the ligand and the complexes given in table 2 have bands in the region of
2900 cmꢀ1, assigned to ꢀ(C–H). The thioamide moiety in the ligand is confirmed by four
thioamide bands present in the IR spectra of the ligand [21]. In the complexes, the
thioamide band IV at 800 cmꢀ1 shifts lower by 30–40 cmꢀ1; thioamide band III at
1180 cmꢀ1 shifts 20–30 cmꢀ1 higher; thioamide band I at 1500 cmꢀ1, and thioamide
band III have less intensity compared to the ligand. These systematic shifts support
bond formation through both nitrogen and sulfur [22–26]. Ni(II) and Co(II) complexes
exhibit a broad band at 3500 and 910 cmꢀ1 indicating that the metal ion is coordinated
to water.
3.3. 1H-NMR spectra
Ni(II) and Co(II) complexes have been investigated in the context of dynamic
combinatorial libraries. The kinetic lability of the d7 cobalt(II) center and d 8 nickel(II)
center combined with the strength of the Co–N and Ni–N bonds allows for the
construction of thermodynamically stable, but labile, libraries of complexes from
the reaction of cobalt and nickel salts with mixtures of ligands. The 1H-NMR
spectra of these complexes are paramagnetically shifted, but only modestly, broadened
and are characteristic of the individual complexes providing a fingerprint of
the speciation [27].
The 1H-NMR spectra of the free ligands and complexes have been recorded
in DMSO. The ligands exhibit singlets at 11.24. The disappearance of these signals
in the spectra of the complexes clearly suggests deprotonation of the functional group
during the complexation and coordination through sulfur. The singlet at 8.1 ppm in the
ligand is altered to that in the complex at 8.5 ppm. In spectra of the ligands, multiplets
due to aromatic protons appear in the range ꢃ 6.24–8.76 ppm and remain unchanged in
spectra of the complexes. The peak corresponding to ꢀO–H– at 10.8 ppm in the free
ligand is unaltered in the complexes. Also, the peaks at 1.8 ppm (–CH3) and 3.8 ppm
(–OCH2–) in the ligand are unaltered in the complexes. A peak due to –CH3 (triazole) at
2.3 ppm in the ligand remains unchanged in the complexes.
3.4. Magnetic susceptibility measurements
At room temperature, Ni(II) and Co(II) complexes exhibit paramagnetic behavior (3.12
and 5.20 B.M., respectively) which are in good agreement with the theoretical values for
octahedral complexes [28–31]. All other complexes are diamagnetic.
3.5. TGA studies
TGA analyses of Ni(II) and Co(II) complexes show weight loss starting from 100ꢁC
indicating coordinated water investigated in nitrogen and dynamic air atmospheres

Ethyl vanillin
3113

3114
Table 3. Antibacterial activity of metal(II) complexes.
B. subtilis (mg) S. typhi (mg) P. aeruginosa (mg)
A.S. Ramasubramanian et al.
E. coli (mg)
S. flexneri (mg)
Compound
30
50
30
50
30
50
30
50
30
50
Ligand
–
12
9
20
25
–
8
18
14
24
27
–
–
7
16
12
18
20
–
9
16
10
20
24
–
11
20
11
22
26
–
5
13
13
13
22
–
9
18
15
21
24
–
7
10
8
12
13
–
9
13
12
15
18
–
Ni(II) complex
Co(II) complex
Zn(II) complex
Ampicillin
14
10
16
18
–
DMF
Zone inhibition are given in mm.
using TGA and differential thermal analysis (DTA). The complexes decompose in
three or four decomposition steps. The first step is elimination of water. The
endothermic peaks at 60–100ꢁC correspond to the evolution of the crystal water
(DH ¼ 100–250 J gꢀ1), while those at 110–160ꢁC are consistent with coordinated water
(DH ¼ 150–330 J gꢀ1) [32, 33]. Afterwards, continuous decomposition sets in with
varying decomposition peaks. Some decomposition could be correlated with decom-
position products. Decomposition of all complexes in dynamic air ends with oxide
formation. The percentage weight of the residue agrees well with oxide formation.
However, higher percentages of the residue than calculated for oxide have been
observed in nitrogen flow. This may be attributed to contamination of the oxide with
some carbon. From 250ꢁC, decomposition of the complex starts and ends at around
900ꢁC.
The molar conductance values of all the complexes in DMF are less than 30 mS at
room temperature confirming that the complexes are non-ionic. From the analytical
data, it was confirmed that the complexes were ML2.
3.6. Biological activity
Preliminary in vitro antimicrobial activities of the investigated compounds were tested
against bacteria E. coli, P. aeruginosa, S. typhi, B. subtilis, and S. flexneri. Ampicillin
was used as a standard. Ni(II), Co(II), and Zn(II) complexes showed good activity.
From table 3, it is clear that the inhibition zone of the metal chelates is higher than that
of the ligand. Such increased activity of the metal chelates is due to the lipophilic nature
of the metal ions in complexes [34]. The increase in activity of metal chelates, with
increase in concentration, is due to the effect of metal ions on cell process [35].
Furthermore, the mode of action of the compounds may involve the formation of a
hydrogen bond through the azomethine nitrogen with the active centers of cell
constituents, resulting in interference with the normal cell process [36].
4. Conclusions
On the basis of analytical and spectral data, octahedral structures have been proposed
for Co(II) and Ni(II) complexes (figure 1a), square planar geometry for Pd(II) complex

Ethyl vanillin
3115
N
(a)
N
N
M
N
OH
N
H5C2O
HO
S
H2O
OC2H5
OH2
S
N
N
N
N
S
(b)
N
N
M
N
OH
N
H5C2O
HO
OC2H5
S
N
N
N
N
(c)
N
N
OH
N
H5C2O
HO
S
M
N
OC2H5
S
N
N
N
Figure 1. (a) M ¼ Ni(II), Co(II); (b) M ¼ Pd(II); and (c) M ¼ Zn(II).
(figure 1b), and tetrahedral structure for Zn(II) (figure 1c). Zn(II), Co(II), and Ni(II)
complexes show antibacterial activity against Gram-negative bacteria such as E. coli,
P. aeruginosa, S. typhi, B. subtilis, and S. flexneri.
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