Synthesis, spectral characterization and antimicrobial studies of transition metal complexes of benzothiazole based schiff bases

Article abstract of DOI:10.14233/ajchem.2018.21454

A series of transition metal complexes of the type [ML2]·nH2O, where L = 2-(benzothiazolyl-2'-Amino)iminoethanone (HBAE), 2-(benzothiazolyl-2'-Amino)imino-1,2-dimethylethanone (HBME), 2-(benzothiazolyl-2'-Amino)imino-1,2-diphenylethanone (HBPE), M = Cu(II), Co(II), Ni(II) and Zn(II), have been synthesized and characterized on the basis of spectral studies and thermal analyses. The spectroscopic data confirmed octahedral geometrical structures for all the complexes. All these Schiff bases and their metal complexes have been screened for their antibacterial activity against some pathogenic bacteria species (Bacillus subtilis, Bacillus stearothermophilus, Escherichia coli and Salmonella typhi) and antifungal activity against some pathogenic fungi strains (Aspergillus niger and Aspergillus flavus).

Full text of DOI:10.14233/ajchem.2018.21454

Asian Journal of Chemistry; Vol. 30, No. 12 (2018), 2608-2614
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.21454
Synthesis, Spectral Characterization and Antimicrobial Studies of
Transition Metal Complexes of Benzothiazole Based Schiff Bases
1,*
2
3
4
3
P.K. DAS , R.K. MOHAPATRA , S.B. PATJOSHI , M.M. EL-AJAILY and D.C. DASH
1Department of Chemistry, N.C. Autonomous College, Jajpur-755 001, India
2Department of Chemistry, Government College of Engineering, Keonjhar-758 002, India
3School of Chemistry, Sambalpur University, Jyoti Vihar, Burla, Sambalpur-768 019, India
4Department of Chemistry, University of Benghazi, Benghazi, Libya
*Corresponding author: E-mail: pradeepdas1960@gmail.com
Received: 17 May 2018;
Accepted: 20 September 2018;
Published online: 31 October 2018;
AJC-19119
A series of transition metal complexes of the type [ML2]·nH2O, where L = 2-(benzothiazolyl-2'-amino)iminoethanone (HBAE), 2-
(benzothiazolyl-2'-amino)imino-1,2-dimethylethanone (HBME), 2-(benzothiazolyl-2'-amino)imino-1,2-diphenylethanone (HBPE), M = Cu(II),
Co(II), Ni(II) and Zn(II), have been synthesized and characterized on the basis of spectral studies and thermal analyses. The spectroscopic
data confirmed octahedral geometrical structures for all the complexes. All these Schiff bases and their metal complexes have been
screened for their antibacterial activity against some pathogenic bacteria species (Bacillus subtilis, Bacillus stearothermophilus, Escherichia
coli and Salmonella typhi) and antifungal activity against some pathogenic fungi strains (Aspergillus niger and Aspergillus flavus).
Keywords: 2-Hydrazinobenzothiazole, Glyoxal, Diacetyl, Benzil and Transition metal complexes.
Benzothiazole is a bicyclic ring system with multiple
applications. During recent years much attention has been
devoted to the synthesis of new benzothiazole derivatives due
to their diverse chemical reactivity and broad spectrum of
biological activity such as antitumor agents [10], antimicrobial
[11], analgesics [12], anthelmintic [13], anticancer [14,15],
antibacterial [16-18], antifungal [19] and anti HIV [20] activity.
The derivatives of 2-aminobenzothiazole have been used for
the treatment of diabetes, viral infections and as central muscle
relaxants [21]. The activity of some drugs is enhanced when
administered as metal complexes than the free ligands. It was
reported that several benzothiazole derivatives and their metal
complexes showed antifungal and antibacterial activity.
Following all these observations and as a continuation of our
research on the coordination chemistry of multidentate ligands
[22-27], we report here the synthesis and structural studies on the
complexes of Cu(II), Co(II), Ni(II) and Zn(II) with some hydrazone
derivatives containing benzothiazole moiety such as 2-(benzo-
thiazolyl-2’-amino)imino ethanone (HBAE), 2-(benzothiazolyl-
2’-amino)imino-1,2-dimethylethanone (HBME) and 2-(benzo-
thiazolyl-2’-amino)imino-1,2-diphenyl ethanone (HBPE).
INTRODUCTION
Recent years have witnessed a surge in the research interest
in the synthesis and characterization of transition metal
complexes having metal atoms coordinated to sulphur in the
ligand framework. The heavier donor atoms including sulphur
[1], due to their availability of vacant ‘d’orbitals, act as potent
‘d’ electron acceptors from the metal ions through dπ-dπ bond
formation. In view of their immense importance in biological
systems, they are used as synthetic models for many metallo
enzyme reactions [2]. During past several years, the mixed
ligand complexes with sulphur donor atoms have drawn
considerable interest for synthetic chemists owing to their novel
structural features and unusual magnetic properties [3,4]. The
role of coordination compounds containing ‘NOS’ donors as
potential drugs, fungicidal and antibacterial [5] agents, cannot
be overemphasized. Mixed-ligand complexes of transition
metals containing ligands with ‘N, S’and ‘N, O, S’donors are
also known to exhibit interesting antimicrobial [6], anticancer
[7], stereochemical, electrochemical and electronic properties
[8,9].
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Vol. 30, No. 12 (2018)
Synthesis andAntimicrobial Studies of Transition Metal Complexes of Benzothiazole Based Schiff Bases 2609
were recorded on aVarian FTIR Spectrophotometer,Australia
in the range 4000-400 cm-1. The electronic spectra of the comp-
lexes in DMSO were recorded on a Perkin-Elmer Spectro-
photometer. The thermo gravimetric analysis was carried out
EXPERIMENTAL
All the chemicals used are ofAR grade. The solvents were
purified before use by standard procedures.
Synthesis of 2-hydrazinobenzothiazole: To a warm solu-
tion of 2-mercapto benzothiazole (1.67 g, 0.01 mol) in absolute
alcohol (10 mL), hydrazine hydrate (2 mL) was added and the
mixture was refluxed on a water bath for 4 h or until H2S gas
ceased to evolve. Then it was concentrated and cooled, during
which white needle like crystals separated out (Scheme-I). It
was filtered and recrystallized from ethanol.
Synthesis of ligands: The ligands used in the present
investigation are benzothiazolyl-2-hydrazones of aldehydes
and ketones such as glyoxal, diacetyl and benzil which have
been synthesized by condensing 2-hydrazinobenzothiazole
with respective aldehydes and ketones in the following manner.
Ethanolic solution of 2-hydrazinobenzothiazole (0.01 mol in
20 mL) was added to ethanolic solution of glyoxal/diacetyl/
benzil (0.01 mol in 20 mL) and the resulting solution was
refluxed on a water bath for 6 to 8 h. It was concentrated and
allowed to stand overnight when coloured precipitate was
separated out. It was filtered, washed and recrystallized from
ethanol (Scheme-II). The sample was dried in vacuo over fused
calcium chloride and then analyzed.
Synthesis of metal complexes:The complexes were synthe-
sized by refluxing ethanolic solution of the ligands HBAE/
HBME/HBPE (0.02 mol in 20 mL) with a hot ethanolic solu-
tion of corresponding hydrated metal(II) chloride (0.01 mol
in 20 mL) for about 4 h when coloured complexes precipitated
out in each case. The complexes were filtered, washed with
ethanol, then by ether and finally dried in vacuo over fused
CaCl2.
Analysis and physical measurements:The metal contents
in the complexes were determined gravimetrically following
standard procedure [28]. Sulphur was estimated as BaSO4. The
molar conductance measurements were carried out at room
temperature with a Toshniwal Conductivity Bridge (model CL-
01-06, cell constant 0.5 cm-1) using 1 × 10-3 M solution of the
complexes in DMSO. Carbon, hydrogen and nitrogen contents
of the complexes were determined using a MLW-CHN micro
analyzer. The room temperature magnetic moment values of
the compounds have been determined by Gouy method with
Hg[Co(SCN)4] as calibrant. The FTIR spectra in KBr pellets
1
using Netzch-429 thermo analyzer. The H-NMR spectra of
the complexes were recorded in DMSO-d6 medium on JEOL,
GSX-400 model equipment using TMS as reference. The
electronic spin resonance spectra were recorded as polycrys-
talline sample on an E-112 EPR Spectrophotometer with field
set at 3200G, scan range 2.0 × 1 Kg, modulation frequency
100 KHz, microwave frequency 9.4 GHz, receiver gain 10 ×
102 and modulation amplitude 0.63 × 10 G.
Microbicides studies: The antimicrobial activities of the
synthesized compounds were evaluated at 100 µg mL-1 concen-
trations by agar well diffusion assay technique against two
Gram-positive bacteria, i.e., Bacillus subtilis and Bacillus
stearothermophilus and two Gram-negative bacteria, i.e.,
Escherichia coli and Salmonella typhi. Solutions of the ligands
and its metal complexes in DMSO were plated onto the
cultured agar medium and incubated for a period of 24 h at
37 °C. After the incubation period, the plates were observed
for zones of inhibition (cm). The standard drugs ciprofloxacin
and gentamycin are also screened under identical conditions.
The antifungal activities of the same were determined against
two fungal strains, i.e., Aspergillus niger and Aspergillus flavus.
The petri dishes were incubated for 48 h at 28 °C. All the deter-
minations were made three times and the average was recorded.
RESULTS AND DISCUSSION
All the hydrazone ligands and their metal complexes are
insoluble in water and common organic solvents but mode-
rately soluble in ethanol and readily in dioxane, DMF and
DMSO. The complexes formulated from the analytical data
and molar conductance data (Table-1) support the suggested
formulae. They are highly coloured, non hygroscopic, highly
stable under normal conditions and all of them decompose
above 250 °C. The molar conductance of the synthesized metal
-1
complexes in DMSO is in the 7.5 to 13.7 Ω cm2 mol-1 range
indicating them to be non-electrolyte in nature with no anions
outside the coordination sphere. Water molecules present in
the metal complexes as lattice water. This view is confirmed
by TGA studies.
N
N
H2N
NH2
+
+
H2S
SH
NH
2-Hydrazino benzothiazole
NH2
S
S
2-Mercapto (hydrazine)benzothiazole
Scheme-I
R
R
N
R
R
N
O
O
C
C
NH
N
C
H2O
+
+
S
NH
NH2
O
C
S
R = H (Compd. 1), CH3 (Compd. 2), C6H5 (Compd. 3)
Scheme-II: Synthetic route of the ligands

2610 Das et al.
Asian J. Chem.
TABLE-1
ANALYTICAL AND PHYSICAL DATA OF THE COMPOUNDS
Λm (Ohm-1 cm2
Elemental analysis (%): Found (calcd.)
Compounds
Yield (%)
mol-1)
C
H
N
S
M
HBAE
HBME
HBPE
80
72
75
65
62
63
67
62
65
59
62
63
60
61
64
–
–
–
52.64 (52.68)
56.60 (56.65)
70.53 (70.58)
44.49 (44.53)
44.55 (44.58)
42.52 (42.56)
43.94 (43.99)
48.75 (48.79)
48.80 (48.84)
46.80 (46.85)
48.23 (48.26)
63.84 (63.87)
63.87 (63.91)
62.11 (62.14)
63.35 (63.39)
3.38 (3.41)
4.68 (4.72)
4.17 (4.20)
2.44 (2.47)
2.44 (2.48)
3.11 (3.15)
2.40 (2.44)
3.66 (3.69)
3.68 (3.70)
4.22 (4.25)
3.61 (3.65)
3.49 (3.54)
3.53 (3.55)
3.90 (3.94)
3.48 (3.52)
20.43 (20.48)
17.96 (18.02)
11.73 (11.76)
17.30 (17.32)
17.29 (17.34)
16.52 (16.55)
17.07 (17.10)
15.48 (15.52)
15.50 (15.54)
14.87 (14.90)
15.32 (15.35)
10.60 (10.64)
10.61 (10.65)
10.31 (10.35)
10.53 (10.56)
15.58 (15.61)
13.70 (13.73)
8.92 (8.96)
–
–
–
[Co(BAE)2]·H2O
[Ni(BAE)2]·H2O
[Cu(BAE)2]·2H2O
[Zn(BAE)2]·H2O
[Co(BME)2]·H2O
[Ni(BME)2]·H2O
[Cu(BME)2]·2H2O
[Zn(BME)2]·H2O
[Co(BPE)2]H2O
[Ni(BPE)2]H2O
[Cu(BPE)2]2H2O
[Zn(BPE)2]H2O
8.2
12.3
11.5
9.6
7.5
7.8
13.7
11.5
11.8
7.6
10.3
12.2
13.15 (13.19)
13.18 (13.21)
12.58 (12.61)
13.01 (13.03)
11.79 (11.82)
11.81 (11.84)
11.31 (11.35)
11.65 (11.70)
8.09 (8.11)
12.13 (12.16)
12.02 (12.07)
12.48 (12.51)
13.20 (13.24)
10.85 (10.90)
10.78 (10.82)
11.23 (11.27)
11.85 (11.88)
7.43 (7.47)
7.38 (7.42)
7.80 (7.83)
8.14 (8.17)
8.08 (8.11)
7.84 (7.89)
8.03 (8.05)
IR spectra: In the high frequency region, 2-hydrazino-
benzothiazole exhibits a strong band located at about 3230 cm-1
which may be due to symmetric and asymmetric stretching mode
of vibration of –NH2 group [29], being overlapped with ν(N-H)
of secondary amino group. A multiple band system of medium
intensity is observed at about 1616 ν(C=C), 1506 ν(C=N) cyclic
and 1278 ν(C-N) cm-1 which may be due to benzothiazole moiety.
The band observed at about 1610 cm-1 may be assigned to N-H
bending vibration mode of 1° amino group. The bands occurring
at about 1250 and 1030 cm-1 may be due to C-N exo-cyclic stret-
ching and N-N stretching vibrations, respectively. The intense
absorption bands around 950-650 cm-1 are due to C-H out of
plane deformation vibration [29], characteristics of benzothiazole
derivatives. Besides the above bands, a broad band of medium
intensity appears at ~ 870 cm-1 in the IR spectra of 2-hydrazino-
benzothiazole which is assignable to ν(C-S-C) mode of vibration
of thiazole moiety.
Reaction of 2-hydrazinobenzothiazole with glyoxal,
diacetyl and benzil brings about significant changes in the IR
spectra of the products, which are used as ligands. The most
notable features of the spectra are the disappearance of the
–NH2 stretching vibration at about 3230 cm-1 and appearance
of two new bands at about 1677 and 1551 cm-1, respectively
due to ν(C=O) (carbonyl) and ν(C=N) (azomethine). Appea-
rance of ν(C=O) and ν(C=N) bands confirms the condensation
of 2-hydrazinobenzothiazole with glyoxal/diacetyl/benzil in
1:1 proportion with retention of one unreacted carbonyl func-
tion of the parent dialdehyde/diketones. The spectra of the
ligands as well as their metal complexes are very complex with
a profusion of bands in the finger print region. However, there
are several characteristics bands, which can be identified and
used to explain the structure of the ligands and their complexes.
The position of bands due to ν(C=N)(cyclic), ν(C=C) of
the benzothiazole moiety remain practically unchanged in the
present complexes indicating non-participation of ring nitrogen
atom in coordination. However, the red shift in the position of
ν(C-S-C) band by 10-20 cm-1 in all the complexes is indicative
of the participation of ring sulphur atom in coordination to
the metal ions [23] and results due to the electron drainage
from the thiazole moiety to the metal center via resonance. In
the spectra of the complexes the band due to ν(C=O) was found
to be absent and two new bands of medium intensities are
observed at about 1460 and 1300 cm-1 assigned to ν(C=C) and
ν(C-O), respectively, suggesting coordination through enolized
‘O’after deprotonation of enolic –OH group. The bands observed
due to ν(C=N) and ν(N-N) in the ligand are found to be absent
and a new band due to ν(N=N) (azo group) appears at ~ 1585
cm-1 showing there by coordination of one of the nitrogen atoms
of the azo group with the metal ion. This fact is further supported
by the disappearance of band at about 3230 cm-1 due to ν(N-H)
(secondary) vibration. Thus IR spectral investigation provides
evidence of coordination of ligand to the metal center through
conversion of keto form to enol form.
A pair of sharp bands of moderate intensities are observed
in the IR spectra of all the metal complexes, the first band
appearing at about 520 cm-1 and the second one at 450 cm-1.
These bands are distinctly absent in the spectra of ligands.
Considering the sharpness and intensity, higher band is
attributed to ν(M-N) and lower band to ν(M-O) vibrations.
However, the ν(M-S) band could not be studied due to instru-
mental limitation, as it occurs in far IR region. The above obser-
vations lead us to believe that the enolized oxygen, nitrogen
of the azo group and the ring sulphur atom, all three, of the
ligand moiety are involved in coordination to the metal ions.
The band observed at 3500 cm-1 may be assigned to ν(O-H)
of lattice water [30]. The important IR bands of the ligands as
well as their metal complexes are recorded in Table-2.
Thermal analysis: Thermal characteristics of the metal
complexes obtained from the ligands HBAE, HBME and
HBPE are recorded in Table-3.All the complexes follow similar
thermal decomposition pattern. The complexes remain unaffec-
ted up to 40 °C, after which the depression of thermogram
starts and continues up to 120 °C. The loss of weight in this
temperature range corresponds to loss of lattice water, for coor-
dinated water molecules require relatively higher temperatures
to be removed. This is in conformity with our IR spectra and
analytical investigations. In case of Cu(II) complexes the loss
in weight corresponds to weight of two water molecules [31],
while for other complexes it is due to one water molecule.
The anhydrous complexes remain stable up to 300 °C which

Vol. 30, No. 12 (2018)
Synthesis andAntimicrobial Studies of Transition Metal Complexes of Benzothiazole Based Schiff Bases 2611
TABLE-2
INFRARED SPECTRAL DATA OF COMPLEXES (cm-1)
HBAE
–
–
1677
1616
1600
–
1551
1506
–
[Cu(BAE)2]·2H2O
HBME
–
[Co(BME)2]·H2O
HBPE
–
–
1660
1610,1590
1600
–
1550
1510
–
[Ni(BPE)2]·H2O
Assignments
ν(O-H) (lattice water)
ν(C-H) (-CH3)
ν(C=O)
ν(C=C) (cyclic)
δ(N-H)
3500
–
–
1615
–
1585
–
1510
1460
1300
1265
–
3475
2875
–
1620
–
3505
–
–
1605
–
1575
–
1512
1465
1300
1275
–
2880
1667
1620
1605
–
1555
1505
–
1580
–
ν(N=N)
ν(C=N) (azomethine)
ν(C=N) (cyclic)
ν(C=C)
1505
1460
1305
1270
–
–
–
–
ν(C-O)
1278
1030
950-650
870
1275
1040
955-660
875
1278
1035
960-670
870
ν(C-N) (exocyclic)
ν(N-N) (exocyclic)
δ(C-H) (out of plane)
ν(C-S-C)
930-675
855
950-655
860
955-675
860
TABLE-3
IMPORTANT FEATURES OF THERMO GRAVIMETRIC ANALYSIS (TGA)
Water loss (%)
Found
Residue (%)
Found
Temp. range of
water loss (°C)
Decomposition
temperature (°C)
Composition
of the residue
Compounds
Calcd.
3.71
3.71
7.09
3.66
3.33
3.33
6.38
3.29
2.28
2.28
4.44
2.26
Calcd.
15.46
15.37
15.66
16.49
13.86
13.78
14.11
14.80
9.50
[Co(BAE)2]·H2O
[Ni(BAE)2]·H2O
[Cu(BAE)2]·2H2O
[Zn(BAE)2]·H2O
[Co(BME)2]·H2O
[Ni(BME)2]·H2O
[Cu(BME)2]·2H2O
[Zn(BME)2]·H2O
[Co(BPE)2]·H2O
[Ni(BPE)2]·H2O
[Cu(BPE)2]·2H2O
[Zn(BPE)2]·H2O
40-100
50-105
50-119
45-110
45-100
50-110
55-120
50-105
45-110
50-112
50-115
40-105
3.68
3.66
7.06
3.62
3.31
3.29
6.34
3.26
2.24
2.25
4.39
2.23
255
290
285
275
275
272
295
310
282
265
280
300
15.41
15.34
15.62
16.43
13.83
13.74
14.08
14.75
9.46
CoO
NiO
CuO
ZnO
CoO
NiO
CuO
ZnO
CoO
NiO
CuO
ZnO
9.42
9.45
9.74
9.80
10.15
10.19
is indicated by a plateau in the thermogram, following which
further weight loss occurs till around 600 °C due to decompo-
sition of organic constituents of the complexes ultimately
leading to the formation of their corresponding oxides. The
decomposition temperature varies from one complex to other.
Thermal stability of these complexes is in the order:
having ground state configuration t62g e1g with one unpaired
electron is ruled out on the basis of the magnetic moment
values. Further this would envisage a strong Jahn Teller distor-
tion from regular Oh geometry [33] which is contradicted by
our spectral investigation. The electronic spectra of these
complexes in DMSO exhibit a band system at around ~ 9,000
cm-1 (1100 nm) and ~ 20,500 cm-1 (488 nm) corresponding to
HBAE complexes: Co(II) < Zn(II) < Cu(II) < Ni(II)
HBME complexes: Ni(II) < Co(II) < Cu(II) < Zn(II)
HBPE complexes: Ni(II) < Cu(II) < Co(II) < Zn(II)
4
4
4
4T1g(F) → T2g(F) (ν1) transition and T1g(F) → T1g(P) (ν3)
transition, respectively under octahedral symmetry. The 4T1g
→ 4A2g (ν2) transition being formally a two electron transition
involves high energy and hence remains unobserved.
Electronic spectra and magnetic moments: In the
present ligands, bands due to benzothiazolyl ring, azomethine
group, carbonyl group and phenyl group are expected to occur.
However, the spectra of the above ligands exhibit two strong
bands in the region ~ 210-230 nm and ~ 300-375 nm assignable
to π→π* and n→π* transitions, respectively indicating exten-
sive conjugation.
The magnetic moment (µeff) values of Ni(II) complexes
with the above ligands are in fair agreement with six coordi-
nated octahedral Ni(II) species. The electronic spectra of these
complexes in DMSO are found to posses three well resolved
bands observed at around ~ 9,800 cm-1 (1020 nm), ~ 16,400
cm-1 (609 nm) and ~ 25,800 cm-1 (387 nm) assignable to 3A2g(F)
3
→ 3T2g(F) (ν1), 3A2g(F) → 3T1g(F) (ν2) and A2g(F) → 3T1g(P)
The electronic spectral data and µeff of Co(II) complexes
with ligands HBAE, HBME and HBPE are recorded. The mag-
netic moment values for these complexes at room temperature
lie in the range 4.7–4.8 B.M., which are higher than the spin
only values calculated for three unpaired electrons in high spin
(ν3) transitions, respectively under octahedral field [33,34].
The electronic spectral data and room temperature µeff
values of Cu(II) complexes with ligands HBAE, HBME and
HBPE are reported. The µeff value of these complexes lie in the
range 1.75-1.83 B.M. which is well within the range expected
for distorted octahedral Cu(II) species. The electronic spectra
of all these complexes show two main bands at ~ 14,600 cm-1
5
configuration (t2g eg2). This may be attributed to large orbital
contribution, which is reinforced by spin orbit coupling [32].
The possibility of formation of low spin octahedral complexes

2612 Das et al.
Asian J. Chem.
(684 nm) (ν2) and ~ 16,340 cm-1 (611 nm) (ν3) assignable,
a singlet at δ 9.1 ppm of secondary amino group and a twin
peak at δ 2.8 ppm and δ 2.5 ppm assignable to methyl protons
for –N=C-CH3 and O=C-CH3 group, respectively. The proton
2
2
2
2
respectively to B1g → B2g and B1g → Eg transitions under
2
D4h symmetry. In this the band due to 2B1g → A1g (ν1) is not
1
observed most probably due to superimposition by ν2 band
owing to its broadness. In this case tetrahedral or pseudo-
tetrahedral symmetry is ruled out, as no band is observed below
~ 10,000 cm-1.
signal due to secondary amino hydrogen is absent in the H
NMR spectra of the complexes indicating deprotonation of
enolic OH via tautomerization after complexation in confor-
mity with the results of IR investigations, while the positions
of other peaks of the ligand remain practically unaltered.
ESR spectra: The ESR spectra of copper complexes were
recorded in DMSO at 300 K. The nature of the spectra is similar
for all the complexes. Hence the representative spectrum of
[Cu(BME)2]·2H2O is discussed here. The ESR spectrum of
the complex exhibits a well resolved four line spectrum with
no features characteristic for a binuclear complex. This is also
supported by the magnetic moment of copper complex, which
confirms the mononuclear nature of the complex. The spin
Hamiltonian parameters for the copper complex were calcu-
lated from the spectrum. The observed order (A|| = 118 > A⊥ =
38; g|| = 2.34 > g⊥ = 2.06 > 2.0023) indicates that the complex
has distorted octahedral geometry [34] with tetragonal elonga-
The electronic spectral bands along with ligand field and
nephelauxetic parameters of the complexes are presented in
Table-4. The ligand field strength are found to decrease in the
order HBME > HBAE > HBPE for almost all the metal
complexes which is evident from their 10Dq values. This may
be attributed to the + I effect of methyl groups attached to the
ligand system in HBME, that tend to pump electron to the
donor atoms, thereby facilitating stronger coordination to the
metal centers, which acts otherwise incase of HBPE, in which
phenyl groups withdraw electron from the donor centers by
virtue of –I effect. Further, the decrease in B values in the
order [M(BME)2] > [M(BAE)2] > [M(BPE)2] in most of the
metal complexes can be justified by the fact that the deloca-
lization of d-electron charge cloud of the metal ions into the
ligand orbitals gets substantially reduced in the ligand BME
due to +I effect of methyl groups, acting in opposite direction
to the flow of electron by delocalization. On the contrary, the
M→L charge delocalization is accelerated in case of BPE
ligands due to electron withdrawing phenyl groups attached
to the ligands. Consequently, the inter electronic repulsion is
considerably reduced in the corresponding [M(BPE)2] comp-
lexes as compared to their free ion values. This is reflected
in the minimum B values of these complexes. This also reveals
greater degree of covalency in [M(BPE)2] complexes than
[M(BME)2], where the extent of delocalization is minimum.
1H NMR spectra: The 1H NMR spectra of the ligands and
their Zn(II) complexes are recorded in DMSO-d6 medium. The
1H NMR spectrum of the ligand HBPE shows a complex broad
multiplet at δ 7.2-7.9 ppm corresponding to 14 aromatic
protons of the three phenyl groups and a singlet at δ 8.9 ppm
due to proton of secondary amino group. On the other hand
the ligand HBME shows a complex broad multiplet in the
region δ 7.2-7.8 ppm corresponding to 4 aromatic protons and
2
2
tion in which the unpaired electron resides in dx -y orbital [35].
The exchange interaction term G, estimated from the expre-
ssion: G = g|| – 2.0023/g⊥ – 2.0023 is found to be 5.42 which is
greater than 4.0 (i.e. G > 4.0). This suggests the moderate
crystal field strength of the ligands in the complexes. Further
2
the covalency parameter α (=0.73) calculated using standard
equation [36] indicates certain degree of covalency existing
in the complexes.
The following tentative structure (Fig. 1) has been pro-
posed for the present complexes on the basis of the foregoing
observations.
Antimicrobial activity: The free ligands and their metal
complexes have been examined in vitro for their antibacterial
and antifungal activities at 100 µg mL-1 concentrations by using
the agar well diffusion technique. The results are presented in
Table-5. It is observed that metal complexes exhibit greater
activity than the ligands against all organisms. This may be
explained on the basis of chelation theory. Moreover, coordi-
nation reduces the polarity [37,38] of the metal ion mainly
because of the partial involvement of its positive charge with
TABLE-4
ELECTRONIC SPECTRA, MAGNETIC MOMENT, LIGAND FIELD
PARAMETERS AND NEPHELAUXETIC PARAMETERS OF COMPLEXES
Absorption bands (cm-1)
Compound
µeff (BM)
Dq (cm-1)
10Dq (cm-1)
B (cm-1)
β
ν1
4T1g→4T2g
8750
9300
8200
ν2
ν3
4T1g → 4T1g (P)
20100
4T1g → 4A2g
[Co(BAE)2]·H2O
[Co(BME)2]·H2O
[Co(BPE)2]·H2O
4.78
4.72
4.81
–
–
–
997
1069
972
9970
10690
9720
826
878
774
0.85
0.91
0.80
21550
19600
ν1
3A2g→3T2g
9550
9775
9200
ν2
ν3
3A2g →3T1g (P)
25400
3A2g →3T1g
15900
[Ni(BAE)2]·H2O
[Ni(BME)2]·H2O
[Ni(BPE)2]·H2O
2.86
2.79
2.91
955
978
920
9550
9780
9200
843
855
870
0.82
0.83
0.845
16400
15750
25750
24900
ν2
ν3
2B1g→2B2g
14400
2B1g→2Eg
15860
16420
15450
[Cu(BAE)2]·2H2O
[Cu(BME)2]·2H2O
[Cu(BPE)2]·2H2O
1.81
1.79
1.84
–
–
–
1513
1569
1480
15130
15685
14800
–
–
–
–
–
–
14950
14150

Vol. 30, No. 12 (2018)
Synthesis andAntimicrobial Studies of Transition Metal Complexes of Benzothiazole Based Schiff Bases 2613
TABLE-5
ANTIBACTERIAL AND ANTIFUNGAL ACTIVITIES OF THE COMPOUNDS (CONCENTRATION OF 100 µg mL1)
Compound
HBAE
HBME
B. subtilis
16.38
17.75
21.37
20.43
19.71
22.38
23.24
24.85
21.84
25.83
22.85
31.42
27.83
39.53
29.83
–
B. stearothermophilus
13.62
E. coli
18.43
19.54
28.41
21.43
24.65
29.40
28.72
28.49
24.43
28.46
23.48
28.52
26.44
42.62
27.46
–
S. typhi
20.12
21.66
27.25
25.21
23.47
27.27
27.56
25.32
22.37
27.34
25.32
29.34
28.32
45.85
28.32
–
A. niger
7.83
A. flavus
9.27
15.39
24.76
19.35
21.44
25.78
22.87
26.26
23.27
24.19
24.27
32.46
24.18
36.63
25.19
–
12.54
27.85
16.25
21.32
21.86
18.25
20.61
20.31
21.23
20.61
25.47
21.23
36.53
22.23
62.3
11.48
30.72
17.46
20.61
19.73
20.72
19.56
17.58
20.34
17.38
21.38
24.35
34.35
26.34
58.7
HBPE
[Co(BAE)2]·H2O
[Ni(BAE)2]·H2O
[Cu(BAE)2]·2H2O
[Zn(BAE)2]·H2O
[Co(BME)2]·H2O
[Ni(BME)2]·H2O
[Cu(BME)2]·2H2O
[Zn(BME)2]·H2O
[Co(BPE)2]·H2O
[Ni(BPE)2]·H2O
[Cu(BPE)2]·2H2O
[Zn(BPE)2]·H2O
Gentamycin
Ciprofloxacin
53.5
49.2
55.7
59.4
–
–
(azo group) in the complexes suggest, thereby, the coordination
of one of the nitrogen atoms of the azo group with the metal
ion. It is further supported by the presence of ν(M-N) in the
IR spectra of the complexes.
N
S
N
R
R
R
• Although direct evidence of coordination of ‘S’ to the
metal ions could not be obtained from the IR spectra of the
complexes due to limitation of the instrument, the red shift of
ν(C-S-C) band to the extent of 10-20 cm-1 in all complexes
indicates coordination of ring sulphur atom to metal ions.
• The calculation of ligand field parameters from the
absorption bands in the UV-visible spectra of the complexes
reveal that the order of ligand field strength in the complexes
is HBME > HBAE > HBPE. The nephelauxetic parameters
data indicate certain degree of covalency in the complexes which
follow the order [M(BME)2] < [M(BAE)2] < [M(BPE)2]. This
is corroborated by the ESR spectral data in which the value of
C
C
O
N
N
M
C
· nH2O
O
C
R
N
S
N
Fig. 1. M = Co(II), Ni(II), Cu(II) and Zn(II); R = H (compd. 1), CH3 (compd.
2), C6H5 (compd. 3) and n = 1 or 2
the donor groups and possible π-electron delocalization within
the whole chelate ring formed during coordination. This
process further increases the lipophilic nature of the central
metal atom, which subsequently favours its permeation through
the lipid layer of the micro-organism [39,40] more efficiently
thus destroying them more aggressively.
In addition to this, many other factors like nature of the
ligand, solubility, dipole moment, conductivity and geometry,
which are influenced by the metal ion may be responsible for
the antimicrobial activities of these metal complexes [41,42].
2
covalency parameter, α (0.73) lies below 1.0.
• The complexes showed good antimicrobial activity
compared to the ligands. This may be explained due to enhanced
lipophilic property of the central metal ion as a result of chela-
tion with the ligand moieties.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the services rendered
by Director, Regional Sophisticated Instrumentation Center,
I.I.T., Madras, for recording the spectra.
Conclusions
• All the complexes are neutral in nature, which is evident
from their low values of conductivity.
CONFLICT OF INTEREST
• The enolic nature of coordinated ‘O’ is established from
IR spectra due to tautomerism in the system, which gives a
band due to ν(C-O). This is also evidenced by the presence of
ν(C=C), ν(C-N), ν(N=N) and absence of ν(N-H) in the IR
spectra of the complexes. It is further supported by 1H NMR
spectra which shows the absence of proton signal due to
–NH– group in the complexes. The ‘O’coordination is reaffir-
med by the appearance of ν(M-O) in the IR spectra of the
complexes.
The authors declare that there is no conflict of interests
regarding the publication of this article.
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