Synthesis, spectral, thermal, kinetic and antibacterial studies of transition metal complexes with benzimidazolyl-2-hydrazones of o-hydroxyacetophenone, o-hydroxybenzophenone and o-vanillin

Article abstract of DOI:10.4314/bcse.v32i3.3

A novel series of Schiff base metal complexes of the type [M(L)₂].nH₂O, where L= 2-(α-methylsalicylidenehydrazino) benzimidazole (L1), 2-(α-phenylsalicylidenehydrazino)benzimidazole (L2), 2-(o-vanillinidenehydrazino)benzimidazole (L3

Full text of DOI:10.4314/bcse.v32i3.3

Bull. Chem. Soc. Ethiop. 2018, 32(3), 437-450.
 2018 Chemical Society of Ethiopia and The Authors
DOI: https://dx.doi.org/10.4314/bcse.v32i3.3
ISSN 1011-3924
Printed in Ethiopia
SYNTHESIS, SPECTRAL, THERMAL, KINETIC AND ANTIBACTERIAL STUDIES
OF TRANSITION METAL COMPLEXES WITH BENZIMIDAZOLYL-2-
HYDRAZONES OF o-HYDROXYACETOPHENONE,
o-HYDROXYBENZOPHENONE AND o-VANILLIN
Ranjan K. Mohapatra1*, Pradeep K. Das2, Marei M. El-ajaily3, Umakanta Mishra4 and Dhruba
C. Dash4
1Department of Chemistry, Government College of Engineering, Keonjhar-758002, Odisha,
India
2Department of Chemistry, N.C. Autonomous College, Jajpur-755001, India
3Chemistry Department, Faculty of Science, Benghazi University, Benghazi, Libya
4School of Chemistry, Sambalpur University, Jyoti Vihar, Burla, Sambalpur-768019, Odisha,
India
(Received July 27, 2017; Revised November 4, 2018; Accepted November 9, 2018)
ABSTRACT. A novel series of Schiff base metal complexes of the type [M(L)2].nH2O, where L= 2-(α-
methylsalicylidenehydrazino) benzimidazole (L1), 2-(α-phenylsalicylidenehydrazino)benzimidazole
(L2), 2-(o-vanillinidenehydrazino)benzimidazole (L3), M = Cu(II), Co(II), Ni(II) and Zn(II), have
been synthesized and characterized. The results are in consistent with tridentate chelation of ligand with
azomethine nitrogen, ring nitrogen and a deprotonated phenolic oxygen atom. All these compounds have been
screened for their antibacterial activities against B. subtilis, B. stearothermophilus, E. coli and S. typhi.
KEY WORDS: Schiff base, Metal complexes, Spectral studies, Thermal studies, Kinetic studies, Antibacterial
studies
INTRODUCTION
Benzimidazole is a bicyclic compound having imidazole ring containing two nitrogen atoms at
alternate positions fused to the benzene ring. It is an important biologically active heterocyclic
compound. The chemistry of benzimidazole and its derivatives has been of great interest to
medicinal chemists owing to their remarkable antimicrobial [1], antibacterial [2], antioxidant
[3], anticancer [4], anthelmintic [5], cytotoxic [6], analgesic [7], antiulcer [8], antiviral [9] and
anticonvulsant [10] activities. The antifungal [11], antimicrobial [12], antioxidant [13]
and catalytic [14] activities of transition metal complexes with benzimidazole have
also been extensively studied. The benzimidazole metal complexes have been found to
display greater effectiveness than free ligands [15-17] from above counts. In recent years
considerable attention has also been paid to these metal complexes because of their
contribution to cancer therapy [18-19].
Following all these observations and as a part of our ongoing research on coordination
chemistry of multidentate ligands [20-23], we have made an effort to synthesize and perform the
structural studies on the first-row transition metal complexes with some hydrazone
derivatives containing benzimidazole moiety. Moreover, the bio-potency of these
compounds has been investigated against B. subtilis, B. stearothermophilus, E. coli and S.
typhi.
__________
*Corresponding author. E-mail: ranjank_mohapatra@yahoo.com
This work is licensed under the Creative Commons Attribution 4.0 International License

438
Ranjan K. Mohapatra et al.
EXPERIMENTAL
Materials and methods
All chemicals and reagents including 2-hydrazinobenzimidazole, o-hydroxy
acetophenone, o-hydroxy benzophenone, o-vanillin and the metal salts were obtained from
Sigma Aldrich and used without further purification.
Preparation of ligands
Various ligands were prepared by condensing 2-hydrazinobenzimidazole with
respective aldehyde and ketone as follows. Ethanolic solution of 2-
hydrazinobenzimidazole (0.01 mol in 20 mL) was added to ethanolic solution of o-
hydroxyacetophenone/o-hydroxybenzophenone/o-vanillin (0.01 mol in 20 mL), followed by
addition of 2 drops of acetic acid to the above mixture. The resulting solution was
heated for 2 to 3 hours on a water bath. The clear solution was allowed to evaporate at
room temperature and washed with diethyl ether and finally, recrystallized in ethanol.
The sample was dried in vacuo over fused calcium chloride and then analyzed.
(2-hydrazinobenzimidazole) (o-hydroxyacetophenone)
(L1)
Scheme 1. Procedure for preparation of L1.
(2-hydrazinobenzimidazole) (o-hydroxybenzophenone)
(L2)
Scheme 2. Procedure for preparation of L2.
N
O
NH
H
N
H
Piperidine
N
H
N
H2O
+
+
NH
Reflux, 2-3h
N
NH2
OH
H
HO
OCH3
H3CO
(2-hydrazinobenzimidazole) (o-vanillin)
Scheme 3. Procedure for preparation of L3.
(L3)
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Synthesis, spectral, thermal, kinetic and antibacterial studies of transition metal complexes 439
Preparation of the complexes
A hot ethanolic solution of the ligand (0.02 mol in 20 mL) was treated with ethanolic
solution of respective hydrated metal(II) chloride (0.01 mol in 20 mL) in 2:1 molar
ratio. The pH of the resulting mixture was adjusted to 7.5 to 8.0 by adding 0.25 g of
solid NaOH. It was refluxed on a water bath for about 2 hours when colored complexes
(Figure 1) precipitated out in each case. The precipitates were filtered, washed with
ethanol followed by ether and finally dried in vacuo over fused CaCl2.
M = Co(II), Ni(II), Cu(II) and Zn(II)
Figure 1. Proposed structure of the complexes.
Analysis and physical measurements
X = -H, -CH3, -C6H5, Y = -H, - OCH3
The metal contents were determined gravimetrically [24]. The molar conductance measurements
were recorded with a Toshniwal Conductivity Bridge (model CL-01-06, cell constant 0.5 cm-1)
in DMSO. Microanalyses (C, H, N) were executed by using a MLW-CHN micro analyzer. The
room temperature magnetic susceptibility values were determined by Gouy method with
mercury tetrathiocyanatocobalt(II), Hg[Co(SCN)4] as calibrant. The electronic spectra were
obtained in DMSO on a Perkin-Elmer spectrophotometer. FTIR spectra as KBr pellets were
recorded using Varian FTIR spectrophotometer, Australia. Thermo-gravimetric analysis was
done by Netzch-429 thermal analyser in a nitrogen atmosphere with a rate of temperature
increase of 10 oC min-1. The 1H-NMR and 13C-NMR spectra of the complexes were obtained on
JEOL GSX-400 model equipment in DMSO-d6 medium. The ESR spectra were recorded on an
E-112 EPR Spectrophotometer.
In vitro antibacterial activity
The in vitro antibacterial activities of the ligands and their metal complexes were studied at 100
µg mL-1 concentrations by Agar Well Diffusion Assay Technique [25] against B. subtilis
(MTCC-121 and NCIM-2250), B. stearothermophilus (NCIM-2542 and ATCC-12977) (Gram
positive bacteria) and E. coli (NCIM-2109 and ATCC-11775), S. typhi (MTCC-3224, NCIM-
2501 and ATCC-23564) (Gram negative bacteria). Solutions of the compounds in DMSO were
o
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 (in cm). The standard
antibacterial drug ciprofloxacin is also screened as above.
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RESULTS AND DISCUSSION
The analytical data and physical properties of the compounds are listed in Table 1, which are in
good agreement with the calculated values. These reported metal complexes are coloured, non
hygroscopic and soluble in dioxane, DMSO and DMF. They are stable under normal conditions
and all of them decompose above 250 oC.
Table 1. Analytical and physical data of the complexes.
a
S. No.
Compounds
Color
Yields
(%)
Λm
C
Found
H
Found
N
Found
M
Found
(calcd) (calcd) (calcd) (calcd)
1
2
L1
L2
Grey
Light brown
Yellow
80
70
80
61
62
65
62
63
64
60
61
63
60
61
63
-
67.64
(67.67)
73.12
(73.17)
63.76
(63.83)
59.28
(59.31)
59.30
(59.34)
57.15
(57.18)
58.65
(58.69)
65.44
(65.48)
65.48
(65.51)
63.49
(63.53)
64.89
(64.92)
56.12
(56.16)
56.15
5.24
(5.26)
4.83
(4.87)
4.92
(4.96)
4.58
(4.61)
4.59
(4.62)
4.72
(4.76)
4.51
(4.56)
4.34
(4.37)
4.35
(4.38)
4.49
(4.51)
4.30
(4.34)
4.34
(4.38)
4.35
21.01
(21.05)
17.04
(17.07)
19.82
(19.86)
18.42
(18.45)
18.42
-
-
-
-
3
L3
-
4
[Co(L1)2 ].H2O
[Ni(L1)2 ].H2O
Brown
11.39
10.78
10.49
18.21
11.22
12.59
10.31
6.51
8.20
10.19
8.12
11.42
9.65
(9.72)
9.63
(9.67)
10.05
5
Brick red
(18.46)
17.74
6
[Cu(L1)2 ].2H2O Light green
[Zn(L1)2 ].H2O Light yellow
(17.79) (10.10)
18.22 10.62
(18.26) (10.70)
15.25
(15.28)
15.26
(15.29)
14.79
(14.82)
15.11
(15.15)
17.44
(17.47)
17.45
(17.48)
16.85
7
8
[Co(L2)2 ].H2O
[Ni(L2)2 ].H2O
Leaf green
Chocolate
8.01
(8.05)
7.96
(8.01)
8.38
(8.41)
8.80
(8.84)
9.15
(9.20)
9.12
9
10
11
12
13
14
15
[Cu(L2)2 ].2H2O Olive Green
[Zn(L2)2 ].H2O Light yellow
[Co(L3)2 ].H2O
[Ni(L3)2 ].H2O
Brown
Grey
(56.19)
54.23
(54.25)
55.58
(4.38)
4.49
(4.53)
4.29
(9.16)
9.52
(9.58)
10.06
[Cu(L3)2 ].2H2O Bluish black
[Zn(L3)2 ].H2O White
(16.88)
17.26
(55.61)
(4.34)
(17.30) (10.10)
aOhm-1 cm2 mole-1.
IR spectra
2-Hydrazinobenzimidazole exhibits a pair of strong bands at ~3310 and ~3275 cm-l which may
be assigned to asymmetric and symmetric stretching modes of vibrations of the -NH2 group.
Apart from these bands a multiple band system of medium intensity is observed at 3145 and
3100 cm-1 which may be attributed to v(N-H) of benzimidazolyl group and v(N-H) exocyclic,
respectively. Further, absorptions of medium intensity are observed at ~1540 cm-1 and ~1340
cm-1 due to ring stretching vibration modes of cyclic (C=N) and cyclic (C-N) of benzimidazole
moiety. The N-H bending vibrations for primary amine occur at 1600-1575 cm-1. The band
appearing at 1620 cm-1 may be assigned to N-H bending vibrational mode. The bands positioned
at ~1230 cm-l and ~1000 cm-l may be due to the C-N (exocyclic) stretching vibrations and N-N
stretching vibrations, respectively.
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Synthesis, spectral, thermal, kinetic and antibacterial studies of transition metal complexes 441
The most significant changes in the IR spectra of the condensation products of
2-hydrazinobenzimidazole with o-vanillin, o-hydroxyacetophenone or o-hydroxy
benzophenone are the disappearance of the -NH2 stretching and bending vibrations.
The band at ~3300 cm-1 due to phenolic -OH stretching vibration disappears in IR
spectra of metal complexes formed by above ligands which clearly demonstrate the
deprotonation of phenolic (OH) group and coordination to the metal ion through an
oxygen atom. This is further substantiated by the hypsochormic shift of phenolic vC-O
mode around ~1280 cm-1 of the free ligand to ~1500 cm-1 in the spectra of these
complexes. This may be due to partial double bond character assumed by C-O in
coordination to the metal ion as a result of drainage of π-electron of benzene ring
through resonance. The position of the band due to vN-H (exocyclic) however, remains
practically unaltered in the present complexes indicating non-involvement of imino
nitrogen atom in coordination to the metal ion. Further, the non coordination of ring
nitrogen atom (-C=N) of benzimidazole moiety in the complexes is also ascertained by
finding no change in the positions of the characteristic IR bands at ~ 1540 and ~ 1320
cm-1 due to vC=N (cyclic), vC-N (cyclic) mode of vibration, respectively.
On the other hand, the position of benzimidazole vN-H band occurring at ~3150 cm-1
gets shifted by 20-25 cm-1 to lower frequency region indicating the involvement of
-NH group of benzimidazole in coordination to the metal ions. Besides above, the
bands due to vC=N (azomethine) and vN-N vibrations also, shift their position in the metal
complexes, the former shifting to lower frequency by 10–20 cm-1 in all the complexes,
implying participation of azomethine nitrogen atom in coordination. Further, an
additional band at ~2840 cm-1 appears in the IR spectra of the ligand L3 which may be assigned
to stretching vibration of –OCH3 group. The position of this band remains unperturbed in all the
complexes indicating non-involvement of –OCH3 group in co-ordination most probably due to
steric reason. Thus, the introduction of –OCH3 group in ortho position to phenolic –OH does not
produce a remarkable change in the IR spectra of the ligand L3 and its complexes. The
absorption at ~3500 cm-1 may be assigned to vO-H of co-ordinate or lattice water.
Thermal analysis
The complexes formed by the ligands follow
a similar pattern of thermal
decomposition. All the complexes lose water in the temperature range below 100 oC
indicating them to have lattice water in conformity with analytical and IR spectral
data. The loss of water is equivalent to two molecules for Cu(II) complexes while it is
equivalent to one molecule for other complexes [26]. The anhydrous complexes follow
two stage decompositions between 210 – 315 oC and 340 – 510 C. The degradation of
o
organic constituents continues until finally the stable metal oxide is obtained. The
temperature range of decomposition, peak temperature and the possible fragments
removed, have been incorporated in Table 2. The calculation of mass losses from the
thermogram leads us to propose the following stoichiometric decomposition scheme for
metal complexes of various ligands which are presented below. The mass losses due to
the organic constituents have been expressed regarding the mass of the ligands.
1. ML2. nH2O, where, L = L1 (C15N4OH13), M = Co, Ni, Cu and Zn
o
I. ML2.nH2O (s) → ML2(s) + nH2O(g)
( 40 - 110 C)
II. ML2(s) → ML1.004(s) + 0.996L(g) ( 190 – 340 oC)
III. ML1.004 (s) → MO(s) + 0.943L(g) ( 315 – 485 oC)
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Ranjan K. Mohapatra et al.
2. ML2.nH2O, where, L = L2 (C20N4OH15), M = Co, Ni, Cu and Zn
o
I. ML2.nH2O (s) → ML2 (s) + nH2O(g)
( 50 - 110 C)
II. ML2 (s)→ ML1.193 (s)+ 0.807L(g) ( 230 – 350 oC)
III. ML1.193(s) → MO(s) + 1.144L(g) (335 – 500 oC)
3. ML2.nH2O, where, L = L3 (C15N4O2H13), M = Co, Ni, Cu and Zn
o
I. ML2.nH2O (s) → ML2 (s) + nH2O(g)
II. ML2(s) → ML1.061(s) + 0.939 L(g) ( 192 – 335 oC)
III. ML1.061 (s) → MO(s) + 1.004L(g)
(310 – 480 oC)
( 40 - 100 C)
In general, the mass loss in the second stage of degradation in the temperature range 190 –
350 oC is correlated to the detachment of two C7N3H6 moieties from the complexes by cleavage
of -N–N- bond, thereby releasing 2-aminobenzimidazolyl fragments from the anhydrous metal
complexes. This may be the consequence of relatively weak nature of –N-N– bond of the ligand
due to lone pair-lone pair repulsion between two vicinal nitrogen atoms. This step of
degradation requires a little higher temperature in case of complexes with L2 ligands than
others, which is believed to be due to the strengthening of –N–N– bond as a result of
conjugation with -phenyl group. The final stage involves the complete elimination of
the rest of the organic constituents from the complexes leading to the formation of
their respective metal oxides. The temperature range of degradation of the complexes reveals
the following order of thermal stability for complexes with different ligands.
L1 complexes: Zn (II) < Co (II) < Ni (II) < Cu (II)
L2 complexes: Co (II) < Ni (II) < Zn (II) < Cu (II)
L3 complexes: Zn (II) < Co (II) < Ni (II) < Cu (II)
Kinetics of the decomposition of complexes
The kinetic parameters such as the order and the activation energies of the decomposition
processes of the complexes have been evaluated using Horowitz and Metzger procedure [27].
The following equation has been used for the purpose.
ꢆ ꢇ = ꢈꢊꢋ⋕ꢍꢎ ꢏ
(1)
ꢁꢂ ꢄꢂ
ꢆ
ꢉ
ꢃ
ꢅ
lnꢀ
ꢁꢂꢄꢂ
ꢅ
ꢌ
where, W = the weight remaining at a particular temperature, W0 = the initial weight and Wf =
final weight after the completion of the reaction, E# = activation energy in kJ mol−1, R = gas
constant, and Ts = the DTG peak temperature of decomposition and T = any given temperature.
ꢁꢂ ꢄꢂ
ꢆꢆꢇ vs T gives good straight line for each decomposition step, from the
ꢃ
ꢅ
The plot of lnꢀ
ꢁꢂꢄꢂ
ꢅ
slope of which the activation energy E# has been calculated. The order of the reaction ‘n’ for
ꢓ
ꢓꢔꢕ
( )
ꢒ
each decomposition step has been obtained from the equation ꢐꢑ =
, where Cs is the
weight fraction of the substance present at the DTG peak temperature; Ts, which is given by:
ꢐꢑ = ꢀꢁꢂꢄꢂ ꢆꢆꢇ
(2)
ꢅ
ꢁꢂ ꢄꢂ
ꢃ
ꢅ
The Cs values for all the decomposition steps for each complex are found to be in the range
0.28 – 0.33 which suggests [28] that the thermal reactions follow first order kinetics. The kinetic
data of Cu(II) complexes are incorporated in Table 3.
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Synthesis, spectral, thermal, kinetic and antibacterial studies of transition metal complexes 443
Table 2. Thermo-analytical results of metal complexes.
Metal complex Molar Temp. range Peak temp
Mass loss
found calc. (%)
2.75 (2.97)
44.12 (43.50)
40.94 (41.19)
Removed species
Metallic residue
found (Calc.%)
CoO 11.98 (12.35)
mass
(oC)
50 – 95
210 -318
325-462
468-509
(oC)
77
[Co(L1)2].H2O 606.93
H2O
2C7N3H6
C16N2OH14
267
410
485
80
[Ni(L1)2].H2O 606.69 55 – 105
2.79 (2.97)
44.23 (43.51)
40.66 (41.21)
H2O
2C7N3H6
C16N2OH14
NiO 11.93 (12.31)
CuO 12.25 (12.63)
ZnO 13.01 (13.27)
CoO 10.21 (10.25)
NiO 10.15(10.22)
CuO 10.32(10.55)
ZnO 10.95 (10.57)
CoO 11.22 (11.73)
NiO 11.25 (11.69)
CuO 12.01 (12.02)
ZnO 12.20 (12.61)
212 – 330
336 – 470
485 – 515
60 – 110
220 -342
345-485
500-525
275
416
497
85
295
425
514
75
[Cu(L1)2].2H2O 629.5
5.38 (5.72)
42.16 (41.94)
38.94 (39.71)
2H2O
2C7N3H6
C16N2OH14
[Zn(L1)2 ].H2O 613.38 45 – 100
2.69 (2.93)
42.92 (43.04)
39.94 (40.76)
H2O
2C7N3H6
C16N2OH14
195 -310
318 – 452
460 – 502
259
403
478
79
[Co(L2)2 ].H2O 730.93 50 – 100
2.31(2.46)
36.65(36.12)
50.92(51.17)
H2O
2C7N3H6
C26N2OH18
230 – 325
335 -470
480 – 512
[Ni(L2)2 ].H2O 730.69 53 – 103
235 – 332
276
415
493
80
281
422
497
88
301
431
518
85
2.29 (2.46)
36.89 (36.13)
50.52 (51.18)
H2O
2C7N3H6
C26N2OH18
340 – 477
482 – 518
62 – 112
242 – 348
355 – 490
508 – 530
[Cu(L2)2].2H2O 753.5
4.35 (4.78)
35.69 (35.04)
48.98 (49.64)
2H2O
2C7N3H6
C26N2OH18
[Zn(L2)2 ].H2O 737.38 55 – 110
2.31 (2.44)
35.58 (35.80)
51.94 (51.18)
H2O
2C7N3H6
C26N2OH18
239 – 339
348 – 485
497 – 523
42 – 92
205 – 312
320 – 455
460- 502
48 – 95
292
428
505
76
264
405
482
78
[Co(L3)2 ].H2O 638.93
[Ni(L3)2 ].H2O 638.69
[Cu(L3)2].2H2O 661.5
[Zn(L3)2 ].H2O 645.38
2.58 (2.82)
42.18 (41.32)
43.98 (44.14)
H2O
2C7N3H6
C16N2O3H14
2.55 (2.82)
42.12 (41.33)
43.83 (44.15)
H2O
2C7N3H6
C16N2O3H14
209 – 318
325 – 465
472 – 510
55 – 104
215 – 338
344 – 480
491 – 520
44 - 95
270
410
487
82
284
419
508
74
5.08 (5.44)
40.67 (39.91)
41.83 (42.63)
2 H2O
2C7N3H6
C16N2O3H14
2.46 (2.79)
40.68 (40.91)
42.98 (43.70)
H2O
2C7N3H6
C16N2O3H14
192 - 307
312 - 445
453 - 498
256
400
472
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Ranjan K. Mohapatra et al.
Table 3. The kinetic data of thermal decomposition of Cu(II) complexes.
E# (kJmol-1)
Complexes
Decomposition
temperature (oC)
333-383
Cs
[Cu(L1)2].2H2O
83.1
126
247
551
78.4
147
312
504
72.2
134
297
476
0.31
0.30
0.29
0.31
0.28
0.30
0.29
0.29
0.31
0.29
0.28
0.30
493-615
618-758
773-798
335-385
515-621
628-763
781-803
328-377
[Cu(L2)2 ].2H2O
[Cu(L3)2 ].2H2O
488-611
617-753
764-793
Electronic spectra and magnetic properties
The electronic spectra of Co(II) complexes in DMSO contain two main bands, a broad
and a strong band around ~10,500 cm-1 (952 nm) and ~22,050 cm-1 (453 nm),
4
4
respectively. The former band can be attributed to T1g(F)  T2g(F) (v1) and the later
4
4
4
4
to T1g(F)  T1g (P) (v3) transitions, respectively. The T1g(F)  A2g(F) (v2), being a
two electron transition (t5 e2  t3 e4 ) involves significant amount of energy, and
2g
g
2g
g
hence is not observed [29]. The ligand field parameters of the complexes have been
calculated by the crystal field approximation [30]. The spectral data of Co(II) complexes
reveal that the ligand L3 produces the strongest ligand field around Co(II) ion as
evident from the 10 Dq values and the order of ligand field strength is L3 > L1 > L2,
which may be attributed to the presence of electron releasing –OCH3 group in ortho
position to –OH of vanillinidene moiety of L3, -CH3 group (+I effect) in α carbon atom
of the salicylidene part of L1 and electron withdrawing –C6H5 group (-I effect) in the
same position of salicylidene moiety of L2. Further, the inter electron repulsion
parameter (B) and nephelauxetic ratio (β) data [30] suggest that the degree of
covalency in the complexes follow the order [Co(L2)2] H2O > [Co(L1)2] H2O > [Co(L3)2]
H2O. The greater delocalization of d- electron charge cloud of Co (II) in [Co(L2)2] H2O is
ascribed to the presence of a phenyl ring in the α position of the salicylidene moiety of the
ligand which can accommodate the expanded d-electron charge cloud into its π molecular
orbital. The electronic spectral data along with ligand field and nephelauxetic
parameters of Co(II) complexes are presented in Table 4.
The electronic spectral data and the magnetic moment values of Ni(II) complexes are
presented in Table 5. The spectra of the complexes are found to possess a split band system at
~ 8,600 cm-1 (1162 nm) and ~ 10,200 cm-1 (980 nm) followed by two main bands at ~ 15,500
cm-1 (645 nm) and ~ 24,350 cm-1 (410 nm). The last two bands may safely be assigned to
3
3
3
3A2g(F)  T1g(F) (v2) and A2g(F)  T1g(P) (v3) transitions, respectively, under octahedral
field. The transition ratio v3/v2 is however less than expected (~1.7) for octahedral Ni(II)
species. This decrease in value may be due to some tetragonal distortion represented by the
extent of splitting of v1 band. This fact is further supported by the first two bands in the
spectrum, which may be split components of v1 band under D4h geometry. These split
components can be assigned to 3B1g3Eg for band at ~8,600 cm-1 and 3B1g3B2g for the band at
E
A
~10,200 cm-1, respectively. Tetragonal parameters Dt, Dq and Dq of Ni (II) complexes are
calculated by known method [31] and are incorporated in Table 5. In all the complexes the out-
E
of- plane crystal field strength (DqA ) is found to be greater than the in-plane one (Dq ) which
Bull. Chem. Soc. Ethiop. 2018, 32(3)

Synthesis, spectral, thermal, kinetic and antibacterial studies of transition metal complexes 445
suggests that the axial benzimidazolyl nitrogen binds strongly to the metal centre than the
equatorial frame work of two nitrogen and two oxygen in the square plane. Both the axial and
equatorial crystal field strength in all the complexes follow the order [Ni(L3)2] H2O > [Ni(L1)2]
H2O > [Ni(L2)2] H2O. This is consistent with the magnetic moment data which reveal that
[Ni(L2)2] H2O is most paramagnetic owing to the relatively weaker crystal field produced by L2
ligand. The observed µeff values agree reasonably well with the calculated values [32] using the
formula: µeff = µs.o( 1 – αλ/10Dq), where α = 4 and λ = -315 cm-1 for Ni(II) and 10Dq is the energy
3
separation between the ground state and the first excited state corresponding to B1g (3A2g) 
3Eg (3T2g) transition. The magnetic moment observed in this case (2.83 – 3.2 BM) corresponds to
six coordinate octahedral Ni(II) species.
Table 4. Absorption spectra, ligand field parameters, bending parameter(x) and nephelauxetic parameters of
Co(II) complexes.
Complexes
Absorption bands (cm-1)
2 3
4T1g → 4T2g T1g → 4A2g 4T1g → 4T1g (P)
Dq
10Dq
B
β
X
(cm-1)
(cm-1)
(cm-1) (B/B0)
(cm-1)
1
4
[Co(L1)2] H2O
[Co(L2)2] H2O
[Co(L3)2] H2O
10500
10230
11060
_
_
_
22050
21110
22850
1187.3a 11873a
1179.6b 11796b
1155.1a 11551a
1147.5b 11475b
1271.3a 12713a
1241.0b 12410b
848a
853b
797a
808b
859a
876b
0.873a
0.878b
0.821a
0.832b
0.885a
0.902b
1003
990
890
agraphical method, bKonig’s method
Table 5. Electronic spectral data (in cm-1), magnetic moment values and tetragonal parameters of Ni(II)
complexes.
E
A
1(3A2g →3T2g)
2
3A2g →3T1g A2g →3T1g (P)
3
Complexes
Dt
Dq
Dq
µeff
(in cm-1) (in cm-1) (in cm-1) (in BM)
3
3
3B1g3Eg B1g3B2g
[Ni(L1)2].H2O
[Ni(L2)2]. H2O
[Ni(L3)2]. H2O
8600
8250
9070
10200
15500
15150
16100
24350
23860
24950
-182.9
-185.7
-203.4
860
825
907
1180
1150
1263
3.25a
3.24b
3.31a
3.26b
3.20a
3.22b
9875
10850
acalculated, bobserved.
The electronic spectra of all the Cu(II) complexes in DMSO are similar in nature.
Here the spectra is characterized by two bands at ~14,200 cm-1 (704 nm) and ~16,810
2
2
cm-1 (594 nm) which may be attributed to B1g
 
2B2g (v2) and B1g 2Eg (v3)
transitions, suggesting there by a distorted octahedral geometry for these complexes. In
2
this case also the band due to B1g

2A1g is not observed most probably due to
superimposition by v2 band owing to its broadness. The mean value of the two
absorptions (v2 and v3) gives us the tentative 10Dq of the complexes. The absence of
any band below ~10,000 cm-1 rules out the possibility of tetrahedral or pseudo-
tetrahedral geometry. The µeff values in these cases lie in the range 1.79-1.90 B.M.
which is normal for hexa coordinated Cu(II) complexes. The spectral data and
magnetic moment values of Cu(II) complexes are reported in Table 6.
Bull. Chem. Soc. Ethiop. 2018, 32(3)

446
Ranjan K. Mohapatra et al.
Table 6. The electronic spectral bands (in cm-1) and magnetic moment values (µeff) of Cu(II) complexes.
Complexes
1
2
3
10 Dq
(in cm-1)
15505
14910
15725
µeff
(in BM)
1.83
1.91
1.81
2B1g→2A1g
2B1g→2B2g
14200
2B1g→2Eg
16810
15870
16900
[Cu(L1)2] 2H2O
[Cu(L2)2]2H2O
[Cu(L3)2] 2H2O
_
_
_
13950
14550
NMR spectra
The 1H NMR spectrum of the ligands displays a multiplet in the region δ 7.6-8.5 ppm
corresponding to 8/13/7 aromatic protons of the phenyl groups in L1, L2 and L3, respectively
while the signals due to ring NH proton and exocyclic NH proton appear at δ 6.7 ppm and δ 9.0
ppm respectively. Besides the above peaks, a sharp signal at δ 8.7 ppm and δ 4.5 ppm observed
due to azomethine (-N=CH-) protons and -OCH3 group respectively in the ligand L3. A signal is
also observed in case of ligand L1 due to –CH3 protons, at δ 2.5 ppm. The signal due to ring
-NH proton shows a downfield shift appearing at δ 7.0 ppm indicating coordination through the
ring -NH group of benzimidazolyl moiety in the complexes. The disappearance of the signal due
to the phenolic -OH protons and the downfield signal due to the azomethine protons in the
corresponding complexes, indicate the co-ordination of the phenolic oxygen (through
deprotonation) and azomethine nitrogen to the metal ion. The IR spectra corroborated the same
result. In the 13C NMR spectrum, signals appeared in the region 106.6 to 156.3 are due to
aromatic carbons. The signals at 173.1, 161.2 and 19.7 are assigned to C=N (azomethine), C=N
(benzimidazole moiety) and CH3, respectively.
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, which exhibit a well resolved four line spectrum and 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 calculated from the spectrum. The
observed order (g11 = 2.23 > g┴ = 2.08 > 2.0023) indicates that the complex has octahedral
2
2
geometry [33]. This also suggests that the unpaired electron exists in dx -y orbital [34], as is
evident from the value of the exchange interaction term G, estimated from the expression: G =
ꢁꢖ ꢄꢗ.ꢘꢘꢗꢙꢆ
‖
, i.e., G = 3.28 < 4.0. Here, g11 < 2.3 explains the covalent nature of the M→L
(
)
ꢖ
ꢚ
ꢄꢗ.ꢘꢘꢗꢙ
bonding [35]. The covalency parameter α2 (= 0.44) is also calculated by using the following
expression, which also suggests the covalent nature of the copper complexes.
ꢞ
ꢙ
‖
α2 = ꢁꢛ‖ − 2.0023ꢆ+ ꢛꢝ − 2.0023 + ꢈꢘ.ꢘꢙꢟꢎ + 0.04
(3)
(
)
ꢜ
Antibacterial activity
The ligands show moderate activity against all organisms, whereas the complexes are more
active than that of the free ligand. This may be explained on the basis of chelation theory. The
structure of the compounds mainly possess C=N bonds, which coordinate to central metal ion.
We know, coordination reduces polarity [36] of the metal ion due to the partial sharing of its
charge with the donor groups. This process increases the lipophilic nature of the central metal
atom. In addition, many other factors like nature of the ligand, solubility, dipole moment,
conductivity and geometry may be responsible for the higher antibacterial activity of these metal
complexes [37, 38]. The comparative antibacterial activities of the metal complexes with L1, L2
and L3 are shown in Figure 2, 3 and 4.
Bull. Chem. Soc. Ethiop. 2018, 32(3)

Synthesis, spectral, thermal,, kinetic and antibacterial studies of transition metal complexes 447
30
20
10
HMSHBBM
Co-comp.
Zn-comp.
Ni-comp.
Ni-comp
Cu-comp.
Zn-co p.
.
0
HMSHBM
B.
subtilis
mm
B. steear.
E. coli
S. typhi
Figure 2. Comparative aspect
ss
of antibacterial findings with L1 ligand.
40
30
20
10
0
HPSHB
MM
Co-com
p.
p.
p.
Zn-comp.
Ni-comp.
Ni-comm
Cu-com
HPSHBM
B.
subtilis
Zn-comp.
B. st
e
e
ar.
E. coli
S. typhi
Figure 3. Comparative aspect
ss
of antibacterial findings with L2 ligand.
30
20
10
HOVHB
Co-co
M
mmp.
Zn-comp.
Ni-comp.
Ni-co
mmp.
Cu-co
mm
p.
0
HOVHBM
Zn-comp.
B.
subtilis
B. steaar.
E. coli
S. typhi
Figure 4. Comparative aspect
ss
of antibacterial findings with L3 ligand.
Bull. Chem. Soc. Ethiop. 2018, 32(3)

448
Ranjan K. Mohapatra et al.
CONCLUSION
The foregoing observations suggest that all the complexes are neutral in nature which is evident
from their extremely low values of conductivity. Most of the complexes contain water
molecules in their lattice, which certainly do not participate in coordination to the central metal
ions as indicated from their thermal behavior. They are removed at a much lower temperature
than the coordinated molecules. This is further corroborated from IR spectral investigations.
The octahedral geometry of all the complexes are established with tridentate chelation of
ligand with azomethine nitrogen, ring nitrogen and oxygen atoms. All these Schiff bases and
their metal complexes have been screened for their antibacterial activities. The complexes
showed good antibacterial activity compared to the ligands. This may be explained due to
enhanced lipophilic property of the central metal ion as a result of chelation with the ligand
moieties.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the services rendered by Director, Regional Sophisticated
Instrumentation Center, I.I.T., Madras, for recording the spectra.
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