Spectral characterization and antimicrobial activity of some transition metal complexes of 2-(5-nitro-1H-benzimidazol-2-yl)-4-bromophenol

Article abstract of DOI:10.4314/bcse.v33i3.6

MnCl2, FeCl3, Ru(DMSO)4Cl2, CoCl2, NiCl2, PdCl2, CuCl2, AgNO3, CdCl2 and HgCl2 complexes of 2-(5-nitro-1H-benzimidazol-2-yl)-4-bromophenol (HL) were synthesized and characterized. The structures of the complexes were confirmed on the basis of elemental an

Full text of DOI:10.4314/bcse.v33i3.6

Bull. Chem. Soc. Ethiop. 2019, 33(3), 451-466.
 2019 Chemical Society of Ethiopia and The Authors
DOI: https://dx.doi.org/10.4314/bcse.v33i3.6
ISSN 1011-3924
Printed in Ethiopia
SPECTRAL CHARACTERIZATION AND ANTIMICROBIAL ACTIVITY OF SOME
TRANSITION METAL COMPLEXES OF 2-(5-NITRO-1H-BENZIMIDAZOL-2-YL)-4-
BROMOPHENOL
Aydin Tavman1*, Mayram Hacioglu2, Demet Gürbüz1, Adem Cinarli1, M.A. Faruk Oksüzömer3,
A. Seher Birteksöz Tan2
1Istanbul University - Cerrahpasa, Faculty of Engineering, Department of Chemistry, 34320,
Avcilar, Istanbul, Turkey
2Istanbul University, Faculty of Pharmacy, Department of Pharmaceutical Microbiology, 34452,
Beyazit, Istanbul, Turkey
3Istanbul University - Cerrahpasa, Faculty of Engineering, Department of Chemical
Engineering, 34320, Avcilar, Istanbul, Turkey
(Received May 17, 2018; Revised October 11, 2019; Accepted October 21, 2019)
ABSTRACT. MnCl2, FeCl3, Ru(DMSO)4Cl2, CoCl2, NiCl2, PdCl2, CuCl2, AgNO3, CdCl2 and HgCl2 complexes
of 2-(5-nitro-1H-benzimidazol-2-yl)-4-bromophenol (HL) were synthesized and characterized. The structures of
the complexes were confirmed on the basis of elemental analysis, TGA, molar conductivity and magnetic moment
measurements, FT-IR, far-IR, FT-Raman, UV-Visible, NMR and fluorescence spectroscopy. According to the
analytical and spectral data, the ligand acted as bidentate, via the imine nitrogen and the phenolate oxygen atoms
towards to the metal ions except the Ag(I). The complexes were screened for in vitro antibacterial activities
against S. aureus, S. epidermidis, E. coli, K. pneumoniae, P. aeruginosa, P. mirabilis and for antifungal activity
against C. albicans. Ciprofloxacin and fluconazole were used as reference for antibacterial and antifungal
activities of the complexes, respectively. In most cases, the complexes tested showed considerable activity on S.
aureus especially, which is a Gram positive bacterium. In addition, the Hg(II) and Ag(I) complexes were found to
have superior activity toward all of the microorganisms whereas the Pd(II) and Cd(II) complexes showed
considerably antimicrobial effect on S. aureus and C. albicans selectively.
KEY WORDS: Benzimidazole, Bromophenol, Transition metal complexes, Antibacterial, Antifungal
INTRODUCTION
It is known that biological metal ions play key roles in the structural organization and activation
of certain enzymes, which are involved in the transfer of genetic information from DNA,
leading to the synthesis of specific proteins. Transition metal complexes have attracted
attentions of inorganic, metallo-organic as well as bio-inorganic chemists because of their
extensive applications in wide ranging areas from material to biological sciences [1].
Antimicrobial activity of some 2-(1H-benzimidazol-2-yl)-phenol derivatives and their metal
complexes were investigated in detail against some microorganisms. In our previous studies, we
found that various benzimidazolyl-phenol derivatives and some of their complexes showed
antimicrobial activity against several microorganisms [2-6]. For instance, 2-(5-H/Me/Cl/NO2-
1H-benzimidazol-2-yl)-4-Br/NO2-phenols were effective especially on S. aureus; their Ag(I)
and Hg(II) complexes showed high antimicrobial activity toward six bacteria in the study [2].
The Fe(III) complexes of 2-(5-H/Me-1H-benzimidazol-2-yl)-4-Br/NO2-phenols exhibited
considerable activity against S. aureus and S. epidermidis [3]. 2-(5-H/Me/Cl-1H-benzimidazol-
2-yl)-phenols and their Fe(III) complexes showed considerable activity on some bacteria and
fungi [4]. It was reported that the Co(II) and Ni(II) chelates of 2-(1H-benzimidazol-2-yl)-phenol
have antifungal activity against A. alternata and A. niger [5, 6].
__________
*Corresponding author. atavman@istanbul.edu.tr
This work is licensed under the Creative Commons Attribution 4.0 International License

452
Aydin et al.
2-Methoxy-6-(5-H/Me/Cl/NO2-1H-benzimidazol-2-yl)-phenols and their some transition
metal complexes were synthesized and characterized. Some ligands and Cu(II) and Zn(II)
complexes showed antibacterial activity against Gram positive bacteria [7]. It was reported that
4-methoxy-2-(1H-benzimidazol-2-yl)-phenol and its Ag(I) and Cu(II) complexes are effective
on S. epidermidis, S. aureus and B. subtilis. Also, 4-methoxy-2-(5-methyl/chloro-1H-
benzimidazol-2-yl)-phenols showed antibacterial activity toward S. aureus [8]. Ag(I) and Zn(II)
complexes of 2-methyl-6-(1H-benzimidazol-2-yl)-phenol showed antibacterial effect toward K.
pneumoniae, S. epidermidis and S. aureus bacteria whereas the ligand itself had no any activity
[9]. 2-(5-H/Me/F/Cl/NO2-1H-benzimidazol-2-yl)-benzene-1,4-diols (HLX: x = 1-5) and HL1
complexes with Fe(NO3)3, Co(NO3)2, Ni(NO3)2, Cu(NO3)2, Zn(NO3)2 showed considerable
antimicrobial activity [10]. Pd(II), Ag(I) and Au(III) complexes of 4-(1H-benzimidazol-2-yl)-
benzene-1,3-diol showed considerable antibacterial and antifungal activity whereas the ligand
itself has no any activity [11].
Spectral characterizations and antibacterial effect of 2-(5-H/Cl/Me/NO2-1H-benzimidazol-2-
yl)-4-Me/Br-phenols (HL1–HL5) and their some transition metal complexes were reported by us:
while HL1 ligand has considerable antibacterial activity on B. cereus only; it’s Ag(I) complex
show antibacterial effect toward almost to the all bacteria. HL5 and [Zn(L5)2]∙HClO4 exhibit
considerable high antibacterial activity toward K. pneumoniae, B. cereus, S. epidermidis and B.
subtilis [12].
3-(5-H/Me/Cl/NO2-1H-benzimidazol-2-yl)-benzene-1,2-diols (HLX: x = 1-4) ligands and
HL3 complexes with Fe(NO3)3, Cu(NO3)2, Co(NO3)2, Zn(NO3)2 have been synthesized and
characterized. HL1, HL2, HL3 and [Cu(L3)2]∙(H2O)2 show considerable antimicrobial activity
toward S. epidermidis and C. albicans [13].
Coordination compounds exhibit different characteristic properties depend on the metal ion
that they are bonded [14]. These metal complexes have extensive applications in various fields
of human interest on the basis of nature of the metal as well as the type of ligand [15, 16].
Chelation or complexation shows more potent antibacterial effect against some microorganisms
than the respective drug [17-19]. We reported that 2-(5-nitro-1H-benzimidazol-2-yl)-4-
bromophenol (HL) ligand and its Zn(NO3)2, Fe(NO3)3 and Cu(NO3)2 complexes showed strong
antibacterial activity toward S. aureus and S. epidermidis [20, 21]. In this study, we aimed to
synthesize the new transition metal complexes of HL (Figure 1) with MnCl2, FeCl3,
Ru(DMSO)4Cl2, CoCl2, NiCl2, PdCl2, CuCl2, AgNO3, CdCl2 and HgCl2 and to investigate their
structural characteristics and antimicrobial activities toward six bacteria and one fungi, C.
albicans.
Br
H
N
7
3'
6
5'
N
O2N
6'
4
HO
Figure 1. Structure of the ligand.
EXPERIMENTAL
Chemistry and apparatus
All chemicals and solvents were of reagent grade from Sigma-Aldrich, Merck, Alfa Easer, Carlo
Erba and Acros Organics and they were used without further purification.
Analytical data were obtained with a Thermo Finnigan Flash EA 1112 analyzer. Melting
points were determined using an Electrothermal melting-point apparatus. Molar conductivity of
1
the complexes was measured on a WTW Cond315i conductivity meter in DMF at 25 °C. H-
NMR spectra were run on a Varian Unity Inova 500 NMR spectrometer. The residual DMSO-d6
Bull. Chem. Soc. Ethiop. 2019, 33(3)

Spectral characterization and antimicrobial activity of some transition metal complexes 453
signal was also used as an internal reference. FT mid-IR and far-IR spectra were recorded on a
Bruker Optics Vertex 70 spectrometer using Attenuated Total Reflection (ATR) techniques
between 100 and 4000 cm–1. The FT-Raman spectra were also recorded in the same instrument
with a R100/R RAMII Raman module equipped with Nd:YAG laser source operating at 1064
nm line with 200 mW power and a spectral resolution of ±2 cm–1. UV-Visible spectra was
performed on a Perkin Elmer Lambda 25 UV/Visible Spectrometer in MeOH. Fluorescence
spectra were performed on
a
Shimadzu RF-5301 PC Spectrofluorophotometer.
Thermogravimetric studies were made on a TG-60WS Shimadzu, with a heating rate of 10
°C/min and air flowing at the rate of 50 mL/min. Magnetic measurements of the paramagnetic
complexes were carried out on MK1 Sherwood Scientific apparatus at room temperature by
Gouy’s method. Diamagnetic corrections were made using Pascal’s constants.
Synthesis of the ligand
The ligand was prepared according to the literature procedures [22, 23]. 2-Hydroxy-5-
bromobenzaldehyde (2.01 g, 10 mmol) reacted with an equivalent amount of NaHSO3 (1.04 g,
10 mmol) in ethanol (25 mL) at room temperature for 4 h. The mixture was treated with 4-nitro-
1,2-phenylenediamine (1.53 g, 10 mmol) in dimethylformamide (15 mL) and gently refluxed for
2 h. The reaction mixture was then poured into iced water (500 mL) and then a precipitate was
formed. It was filtered and crystallized from ethanol. Yield: 2.04 g (61%).
Synthesis of the complexes
The metal complexes were synthesized by treating an ethanolic solution of the ligand with
an equimolar amount of metal salts under reflux about 2 h except the Ag(I) complex.
[Mn(L)2(H2O)]∙H2O. Equimolar amount of the ligand (167 mg, 0.5 mmol) and MnCl2∙6H2O
(117 mg, 0.5 mmol) were mixed in ethanol (20 mL). This mixture was refluxed for 2 h and then
filtered. After 2 days, a brown precipitate was formed in the filtrate. It was filtered and dried at
room temperature. Yield: 155 mg (65%).
[Fe(L)2(OH)(H2O)]∙2H2O. 135 mg of FeCl3∙6H2O (0.5 mmol) were added to the ligand (167
mg, 0.5 mmol) solution in ethanol (20 mL). The black mixture was refluxed for 2 h and then
filtered. In the filtrate, a black precipitate was formed after 3-4 days staying at room
temperature. It was filtered and dried at room temperature. Yield: 156 mg (68%).
[Ru(L)2(DMSO)2Cl]∙4H2O. 242 mg of Ru(DMSO)4Cl2 (0.5 mmol) were added to the ligand
(167 mg, 0.5 mmol) solution in ethanol (20 mL). The black mixture was refluxed for 2 h and
then filtered. In the filtrate, a black precipitate was formed after a week staying at room
temperature. It was filtered and dried at room temperature. Yield: 180 mg (68%).
[Co(HL)Cl2(H2O)2]∙2H2O. 119 mg of CoCl2∙6H2O (0.5 mmol) were added to the ligand (167
mg, 0.5 mmol) solution in ethanol (20 mL). The mixture was refluxed for 2 h, and a slightly
turbid solution was formed. It was filtered after cooling and left at room temperature. A brown
precipitate was obtained from the filtrate after 2 days. It was filtered and dried at room
temperature. Yield: 185 mg (69%).
[Ni(HL)Cl2(H2O)2]∙4H2O. 120 mg of NiCl2∙6H2O (~0.5 mmol) were added to the ligand (167
mg, 0.5 mmol) solution in ethanol (20 mL). The mixture was refluxed for 2 h, and a slightly
turbid solution was formed as in the Co(II) complex. The brown colored precipitate formed in
the filtrate after 3 days was filtered and dried at room temperature. Yield: 175 mg (70%).
Bull. Chem. Soc. Ethiop. 2019, 33(3)

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Aydin et al.
[Pd(HL)2Cl2]. 88.5 mg of PdCl2 (0.5 mmol) and 74.6 mg of KCl (1.0 mmol) were dissolved in
ethanol (5 mL) and this solution was added to the ligand (167 mg, 0.5 mmol) solution in ethanol
(15 mL). The mixture was refluxed for 2 h and filtered after cooling to room temperature. The
claret red precipitate was filtered off, dried at room temperature. Yield: 195 mg (75%).
[Cu(L)2(EtOH)2]∙H2O. 86 mg of CuCl2∙2H2O (0.5 mmol) were added to the ligand (167 mg, 0.5
mmol) solution in ethanol (20 mL). The mixture was refluxed for 2 h and a slightly turbid
solution was obtained. The brown colored precipitate formed in the filtrate in 2 days was filtered
and dried at room temperature. Yield: 160 mg (74%).
[Ag(HL)(L)]∙H2O. 85 mg of AgNO3 (0.5 mmol) were added to the equimolar ligand solution in
ethanol (20 mL). The mixture was stirred overnight at room temperature. A slightly turbid
solution was formed and filtered. The filtrate was stayed at room temperature. After 2 days a
dirty white precipitate was formed. It was filtered and dried at room temperature. Yield: 121 mg
(64%).
[Cd(HL)(L)Cl]∙H2O. 101 mg of CdCl2∙H2O (0.5 mmol) were added to the equimolar ligand
solution in ethanol (20 mL). The mixture was refluxed for 2 h, and a slightly turbid solution was
formed. It was filtered after cooling. The dirty white crystals were formed in the filtrate after 3
days at room temperature. It was filtered and dried at room temperature. Yield: 165 mg (67%).
[Hg(HL)Cl2]∙2H2O. This complex was prepared in a similar manner to [Cd(L)2]∙H2O complex.
136 mg of HgCl2 (0.5 mmol) was added to the equimolar ligand solution. Yield: 275 mg (63%).
The analytical and physicochemical data of the complexes are presented in Table 1.
Determination of antimicrobial activity
Antimicrobial activity against Staphylococcus aureus ATCC 6538, Staphylococcus epidermidis
ATCC 12228, Escherichia coli ATCC 8739, Klebsiella pneumoniae ATCC 4352, Pseudomonas
aeruginosa ATCC 27853, Proteus mirabilis ATCC 14153 and Candida albicans ATCC 10231
were determined by the microbroth dilutions technique following the Clinical and Laboratory
Standards Institute (CLSI) recommendations [24, 25]. Mueller-Hinton broth (MHB) (Difco,
Detroit, MI, USA) for bacteria, RPMI-1640 medium buffered to pH 7.0 with MOPS (Sigma, St.
Louis, MO, USA) for yeast strain was used as the test medium. Serial two-fold dilutions ranging
from 5000 g/mL to 2.4 g/mL were prepared in MHB. The inoculum was prepared using a 4–6
h broth culture of each bacteria and 24 culture of yeast strains adjusted to a turbidity equivalent
to a 0.5 McFarland standard, diluted in broth media to give a final concentration of 5×105
cfu/mL for bacteria and 0.5×103 to 2.5×103 cfu/mL for yeast in the test tray. The trays were
covered and placed in plastic bags to prevent evaporation. The trays containing MHB were
incubated at 35 °C for 18–20 h and the trays containing RPMI-1640 medium were incubated at
35 ºC for 46–50 h. The minimum inhibitory concentrations (MIC) were defined as the lowest
concentration of compound giving complete inhibition of visible growth. As control,
antimicrobial effects of dimethylsulfoxide (DMSO) were investigated against test
microorganisms. Ciprofloxacin and fluconazole were used to verify the standardization of the
micro-dilution test procedure as reference antimicrobials for bacteria and yeast, respectively.
According to values of the controls, the results were evaluated. The MIC values of the
ciprofloxacin and fluconazole are within the accuracy range in CLSI throughout the study [26].
The experiments were performed in duplicate.
RESULTS AND DISCUSSION
The analytical data and physical properties of the complexes are summarized in Table 1.
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Spectral characterization and antimicrobial activity of some transition metal complexes 455
Table 1. The analytical data and physical properties of the ligand and the complexes.
Elemental analysis:
m.p.
found (calcd) %
H
Color
M
Λ
Compound
°C
C
N
HL
C13H8BrN3O3
46.41
(46.73)
40.74
(41.24)
39.48
(39.37)
35.09
(34.95)
29.42
(29.13)
26.12
(27.31)
37.27
(36.93)
41.99
(42.36)
38.96
(39.37)
38.41
(37.48)
23.92
2.62
(2.46)
2.56
(2.40)
3.01
(2.67)
3.13
(3.32)
2.71
(3.01)
3.94
(3.53)
2.11
(1.91)
3.18
(2.79)
2.38
(2.16)
2.26
(2.06)
1.73
12.81
(12.66)
10.35
(11.10)
10.14
(10.60)
7.87
(8.15)
7.42
(7.84)
6.85
(7.35)
Dark
yellow
277
0
4.00
5.80
1.42
4.66
3.14
0
-
[Mn(L)2(H2O)]∙H2O
C26H18Br2N6O8Mn
[Fe(L)2(OH)(H2O)]∙2H2O
C26H21Br2N6O10Fe
[Ru(L)2(DMSO)2Cl]∙4H2O1
C30H34Br2ClN6O12S2Ru
[Co(HL)Cl2(H2O)2]∙2H2O
C13H16BrCl2CoN3O7
[Ni(HL)Cl2(H2O)2]∙4H2O2
C13H20BrCl2N3O9Ni
[Pd(HL)2Cl2]
C26H16Br2Cl2N6O6Pd
[Cu(L)2(EtOH)2]∙H2O
C28H22Br2N6O8Cu
[Ag(HL)(L)]∙H2O
C26H17Br2N6O7Ag
[Cd(HL)(L)Cl]∙H2O
C26H17Br2ClN6O7Cd
[Hg(HL)Cl2]∙2H2O
C13H12BrCl2N3O5Hg
>35
0
>35
0
>35
0
>35
0
>35
0
>35
0
>35
0
58
43
13
36
123
18
52
67
10
31
Brown
Black
Black
Dark
brown
Dark
brown
Claret
red
9.67
(9.94)
10.73
(10.59)
10.25
(10.60)
10.22
(10.09)
6.27
1.75
0
Brown
Dirty
white
Dirty
white
Dirty
white
158
292
221
0
0
(24.33)
(1.89)
(6.55)
M, magnetic moment, BM; Λ, molar conductivity, 1cm2mol1 (25±1ºC) in DMF. 1S%: 6.09 (6.22). 2Ni%: 7.89
(7.35).
General properties
HL is a bidentate, monodeprotonable and chelating ligand. Spectral characterization of HL and
its Fe(NO3)3, Cu(NO3)2 and Zn(NO3)2 complexes were reported in previous studies [12, 13]. In
this study, MnCl2, FeCl3, Ru(DMSO)4Cl2, CoCl2, NiCl2, PdCl2, CuCl2, AgNO3, CdCl2 and
HgCl2 complexes of the ligand (HL) were synthesized and characterized. Mn(II) Fe(III),
Ru(III), Pd(II), Cu(II), Ag(I) and Cd(II) complexes have 1:1 M:L ratio whereas the Co(II),
Ni(II) and Hg(II) complexes are 1:2 (bis-type).
According to the analytical data, the ligand is monodeprotonated through the phenolic OH
hydrogen atom in most of the complexes. Molar conductivity data shows that the MnCl2, FeCl3,
Ru(DMSO)4Cl2, CoCl2, CuCl2, CdCl2 and HgCl2 complexes are non-ionic (molar conductivities
are in the 10-58 1cm2mol1 range). The Ag(I) complex is a 1:1 ionic complex (67
1cm2mol1) while the Ni(II) complex has 1:2 ionic character (123 1cm2mol1) [27].
The room temperature effective magnetic moment value of the Fe(III) complex, 5.80 BM,
can be considered as indicating stabilization of the species having high ferric spin (s = 5/2) state.
The magnetic moment value of the Cu(II) complex (1.75 BM) are very closer to the expected
value (1.73 BM) for a simple mononuclear Cu(II) complex. The magnetic moment value of
Mn(II) complex, 4.00 BM, may be accepted indicating stabilization of the species having
intermediate spin (s = 3/2) state for d5 ion and can be taken into account as an indication for a
five-coordinated structure and square pyramidal geometry due to theoretical magnetic moment
2
1
value of three unpaired electrons is 3.88 BM (d5 ion in square pyramidal environment: dxz dyz
1
dxy dz21) [28, 29].
The magnetic moment value of Co(II) complex, 4.66 BM, is in the expected range for
octahedral complexes having high-spin state [30]. The magnetic moment value of the Ni(II)
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456
Aydin et al.
complex (3.14 BM) can be considered as evidence showing a distorted octahedral geometry for
[Ni(HL)Cl2(H2O)2]∙4H2O [31, 32].
Surprisingly, the ruthenium complex shows paramagnetic characteristic with a magnetic
moment value 1.42 BM, although Ru(II), is a d6 system, was used for complex preparation. This
can be interpreted that ruthenium undergoes a one-electron oxidation during the course of the
complexation reaction and in view of the ruthenium(III)/ruthenium(II) reduction potential
displayed by this complex, aerial oxygen appears to have served as the oxidant. It can be
interpreted that the magnetic moment value of Ru(III) complex corresponds to one unpaired
electron for d5 electronic configuration despite it is lower than the expected value (1.71 BM)
[33]. It can be proposed capped octahedron or pentagonal bipyramid geometry for Ru(III)
complex.
Thermal analysis
The major features of the thermal analysis of the complexes are summarized in Table 2. TGA
curves of the Mn(II), Ru(III), Co(II), Ni(II) and Cd(II) complexes are shown in Figure 2. The
samples of the complexes were heated from room temperature up to 800 °C. Thermal analysis
results exhibited the valuable information about the water coordination state of the complexes.
Thermal degradation of complexes occurred in three stages. At the first stage, uncoordinated
lattice water was lost through evaporation from 50 to 100 °C (Table 2, first column). At the
second stage, the coordinated water molecules are removed from all the complexes except Pd(II)
complex at temperatures between 100 and 150 °C (Table 2, second column) [34, 35]. In all
complexes, weight losses observed above 250 °C can be explained in terms of cleavage of the
Br and nitro groups and also as decomposition of the anions. Above 350 ºC, all other organic
parts of complexes are oxidized to carbon dioxide, nitrogen oxides and water (third stage). The
decomposition reactions result in the formation of metal oxides.
Table 2. TGA data of the complexes (thermal decomposition).
Temperature (°C) →
Complex
100 150 200 250 300 350 400 450 500 550 600 650 700
Weight loss (%)
[Mn(L)2(H2O)]∙H2O
Fe(III) complexa
Ru(III) complexb
Co(II) complexc
Ni(II) complexd
[Pd(HL)2Cl2]
3.2
4.2
6.5
6.5
4.9
1.1
4.8
4.5
7.6
5.9
7.6
8.9
9.0 12.0 21.2 25.4 48.0 58.7 67.1 80.4 90.6 91.8
8.1 8.8 9.7 74.6 88.5 90.4 90.4 90.4 90.4 90.4
9.3 16.1 22.4 31.3 64.7 81.0 82.4 83.4 84.5 84.5
9.5 11.1 14.5 17.2 20.6 26.6 56.6 64.0 73.5 84.3 84.9 84.9
7.1 13.2 16.6 20.3 25.0 27.0 37.1 47.2 63.4 67.8 79.3 83.3
3.6 10.0 16.4 20.8 24.0 35.4 45.6 50.0 58.9 69.0 76.1 84.5
[Cu(L)2(EtOH)2]∙H2O 2.9
8.5
2.9
2.7
9.6 10.9 14.6 15.3 57.2 80.7 82.1 86.1 89.1 91.2 91.2
4.5 9.9 16.0 19.4 22.1 26.5 31.5 37.9 44.0 53.4 61.5
3.6 5.75 9.3 11.9 15.0 19.4 27.3 33.2 45.7 69.9 93.7
[Ag(HL)(L)]∙H2Oe
[Cd(HL)(L)Cl]∙H2Of
[Hg(HL)Cl2]∙2H2O
2.2
1.9
5.3
7.1 12.7 35.8 38.8 44.8 48.2 52.7 56.8 65.3 74.3 88.9 100
a[Fe(L)2(OH)(H2O)]∙2H2O; b[Ru(L)2(DMSO)2Cl]∙4H2O; [Co(HL)Cl2(H2O)2]∙2H2O; d[Ni(HL)Cl2(H2O)2]∙4H2O; e:
750 °C: 64.5; 800 °C: 69.3; 850 °C: 84.5; : 750 °C: 98.5; 800 °C: 100.
c
f
TGA data are compatible with the suggested structures of the complexes. Molecular weight
ratio of the amount of metal oxide show very good agreement for the proposed structures
according to the TGA data. The weight loss is 100% in the Hg(II) and Cd(II) complexes around
700 and 800 °C, respectively (Figure 3). This anomaly is due to the volatility in the second
phase of decomposition [36, 37] (HgO and CdO decompose around 500 °C and metallic
mercury vapors at 357 °C, and metallic Cd vapors at 767 °C).
Vibrational spectroscopy
The FT-IR (mid-IR and far-IR) and FT-Raman spectral data of the compounds are given in
Table 3. In addition, far-IR spectra of the HL, Ru(III) and Cu(II) complexes in Figure 3 and FT-
Raman spectra of HL and Cd(II) complex are given in Figure 4.
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Spectral characterizationn and antimicrobial activity of some transition metal complexe
ss
457
Interpretation of the Ram
the complexes relative to thatt of free ligand. The characteristic (O–H) and (N–H) vi
frequencies of the ligand exhibit only a medium broad band at 3327 cm1 in the IR sppectra,
caused by double intramolec lar hydrogen bonding between the phenoxyl hydrogen ato and
one of the imine nitrogen atomm [4, 22, 38, 39]. This band is changed significantly upon metal
complexation indicating depr tonation and subsequent involvement of the phenoxyl gr up in
metal coordination in the n(II), Fe(III), Ru(III), Cu(II) and Cd(II) complexes [40 . The
phenolic oxygen atom coordination could also be supported by the appearance of medium bands
a
n, mid-IR and far-IR spectra is based on changes in the specctra of
bbration
uu
mm
oo
oo
MM
]]
at a lower frequency region
medium broad bands betwe
c
ee
a. 535 cm1, assignable to ν(M–OC) vibration frequencies. The
n 3300 and 3400 cm1 in the complexes strongly supp
rt the
oo
presence of the coordinated annd uncoordinated water molecules.
Figure 2. TGA curves of the
MM
n(II), Ru(III), Co(II), Ni(II) and Cd(II) complexes.
The characteristic (C–HH) and (C–H) modes of ring residues in the ligand and the
complexes are observed in thee wave region between 31323076 cm1 and 822–723 cm1
(
Table
3), respectively, particularly in the Raman spectra of the compounds. The CC an
stretching frequencies are expp
dd C=N
ected to appear at between 1631 and 1594 cm1 with their own
characteristics for the ligand and the complexes. Specific differences between spectra of thhe free
ligand and the complexes inn this region support the formation of new complexes. The
coordination of imine nitrogen atom could also be confirmed by appearance of two new bands
around 400 and 200 cm1
42]. Similar vibrational chang
the ligand and the complexes,, the strong or medium bands at the ranges 1514–1524 and 1308–
ma
y be assignable to ν(M–N) and δ(M–N) vibration modes [[8, 41,
ees are detected in the Raman spectra (Table 3). In the IR specctra of
1341 cm1 are assigned to t
h
e symmetric and asymmetric (NO2), respectively. The C–Br
stretching vibration is seen at
tt
he range 525–554 cm1 as medium bands [43].
NMR spectroscopy
1H-NMR spectral data of the ligand and the diamagnetic complexes and their assignme
n
ts are
pectra of the ligand were reported in our previous study [20]. In
glets are observed for the NH and OH protons at 13.41 an 12.37
1
present in Table 4. H-NMR
the HL spectra, two broad si
ss
n
n
dd
Bull. Chem. Soc. Ethiop. 2019, 33(3)

458
Aydin et al.
ppm, respectively. The OH p
r
r
oton appears at higher ppm values than the expected bec
a
use of
the hydrogen bonding betwe
NH protons show considera
connected with the coordinat
bonding mentioned above.
e
b
e
n C=N nitrogen and OH hydrogen atoms [10, 22]. The
le changes in NMR spectra on complexation. This sho
iion of the phenolic OH oxygen atom and removing the hyddrogen
OOH and
uuld be
Figure 3. Far-IR spectra of thee ligand and Ru(III) and Cu(II) complexes.
Figure 4. FT-Raman spectra off HL (a) and its Cd(II) complex (b).
Table 3. The FT-IR (mid- and far-IR) and Raman spectral data of the ligand and the complexes
(frequencies, cm–1).
Compound
HL
FT-IR
3327 m,br, 3085 m, 1630 m, 1597 m, 1583 m, 3074 m, 2978 w, 2887 w, 1630
1518 s, 14 3 s, 1339 s, 1322 s, 1249 s, 1065 m, m, 1605 m, 1526 m, 1442 m,
970 m, 817 m, 737 m, 633 m, 554 m, 468 w, 394 1375 w, 1337 s, 1312 sh, 1269
FT-Raman
8
m, 356 m, 3
2
3 m, 224 w, 144 w, 127 w, 96 m
m, 1244 m, 1130 w, 1060 m,
965 m, 867 w, 840 m, 793 w,
Bull. Chem. Soc. Ethiop. 2019, 33(3)

Spectral characterization and antimicrobial activity of some transition metal complexes 459
690 w, 633 w, 357 w, 270 w,
202 w, 145 m, 72 s
[Mn(L)2(H2O)]∙H2O 3346 m, 3080 m, 2907 w,br, 1631 m, 1603 m, Could not be obtained
1522 m, 1478 m, 1438 m, 1329 s, 1256 m, 1095 m,
1068 m, 822 m, 737 m, 693 m, 633 m, 544 m, 460
m, 382 w, 354 m, 265 w, 207 m, 135 m, 107 m, 87
m
(A very broad band at 3500-2500 cm1 range) 3084
m,br, 3067 m,br, 2927 m,br, 2855 m,br, 1631 m,
1603 m, 1522 m, 1470 m, 1417 m, 1341 s, 1249 m,
1176 m, 878 m, 822 s, 733 m, 652 m, 624 m, 540
m, 427 m, 375 m, 325 m, 281 m, 225 m, 153 m, 78
m
[Fe(L)2(OH)(H2O)]
∙2H2O
1592 m, 1533 m, 1478 m, 1364
w, 1303 s, 1245 m, 878 m, 835
w, 713 m, 656 s, 478 w, 421 m,
370 m, 284 m, 190 w, 129 m
[Ru(L)2(DMSO)2Cl 3390 m,br, 3092 m,br, 3011 m,br, 2923 w, 1627 m, Could not be obtained
] ∙4H2O
1594 m, 1518 m, 1469 m, 1417 m, 1337 s, 1293 m,
1240 m, 1071 m, 1015 m, 990 m, 878 m, 822 s,
736 m, 692 m, 652 m, 535 m, 414 m, 375 m, 299
m, 246 m, 166 m, 143 m, 106 s, 82 w.
[Co(HL)Cl2(H2O)2] 3382 m,br, 3180 m,br, 3129 m,br, 2927 w,br, 1627 Could not be obtained
∙2H2O
m, 1599 m, 1518 m, 1474 m, 1417 m, 1337 s, 1249
m, 1071 m, 982 m, 878 m, 818 s, 737 m, 692 m,
632 m, 544 m, 467 w, 451 m, 393 m, 321 m, 276
m, 208 m, 185 m, 149 s, 116 m
[Ni(HL)Cl2(H2O)2]∙ 3491 m, 3382 m, 3350 m,br, 3080 m,br, 2923 m,br, 3074 w, 2811 w, 1602 s, 1556
4H2O
1627 sh, 1603 m, 1519 m, 1478 m, 1337 m, 1285 m, 1520 m, 1448 m, 1330 m,
m, 1245 m, 1139 m, 1064 m, 882 m, 818 s, 729 m, 1290 m, 1253 m, 1063m, 965
693 m, 564 m, 544 m, 471 m, 427 w, 393 m, 278 w, 839 m, 764 w, 700 w, 629
m, 221 m, 158 m, 116 m, 91 m
w, 75 m
[Pd(HL)2Cl2]
3411 m, 3113 m, 2937 w,br, 1626 m, 1604 m, Could not be obtained
1515 m, 1470 s, 1381 m, 1314 m, 1252 m, 1146 m,
1068 m, 816 s, 738 m, 693 m, 568 m, 525 m,
428m, 333 m, 268 w, 192 m, 159 m, 114 sh, 100 m
3366 m,br, 3328 m,br, 3132 m, 1629 m, 1604 m, Could not be obtained
1537 m, 1515 m, 1473 m, 1311 m, 1252 m, 1149
m, 1057 m, 817 s, 736 m, 644 m, 599 m, 551 m,
426 m, 412 m, 357 m, 321 m, 296 m, 225 m, 156
m, 87 m.
[Cu(L)2(EtOH)2]∙
H2O
[Ag(HL)(L)]∙H2O 3332 m,br, 3094 m,br, 2930 w,br, 1607 m, 1523 m, Could not be obtained
1450 m, 1418 m, 1336 s, 1283 s, 1176 m, 1115 m,
1091 m, 877 m, 815 s, 732 m, 692 w, 623 m, 573
w, 541 m, 433 m, 377 m, 298 m, 239 m, 183 m,
159 m, 142 s, 125 m, 74 m
[Cd(HL)(L)Cl]∙H2O 3350 m, 3084 w, 2866 w, 1626 m, 1601 m, 1524 s, 3083 m, 2823 w, 1600 s, 1521
1477 m, 1436 m, 1331 s, 1285 m, 1258 m, 1229 m, m, 1443 m, 1342 m, 1329 m,
1143 m, 1093 m, 1066 m, 883 m, 818 m, 732 m, 1275 m, 1228 m, 1193 m, 1127
693 m, 657 m, 632 m, 541 m, 474 m, 459 m, 394 m, 1095 m, 1063 m, 958 w, 829
m, 355 m, 326 m, 293 w, 266 m, 207 m, 173 m, 98 m, 692 w, 629 w, 335 w, 238
m.
w, 198 w, 89 m, 69 m
[HgCl2(HL)]∙2H2O 3382 m, 3350 m, 3076 m, 2903 w,br, 1631 m, 1632 m, 1532 m, 1445 m, 1378
1598 m, 1514 s, 1478 m, 1445 m, 1308 s, 1252 s, w, 1326 m, 1310 m, 1269 m,
1059 m, 967 m, 906 m, 817 s, 757 m, 736 m, 692 1245 m, 1060 m, 965 m, 867 w,
m, 632 m, 539 m, 467 m, 427 m, 359 m, 323 m, 840 w, 707 w, 634 w, 474 w,
289 m, 266 m, 239 m, 211 m, 188 m, 147 m, 113 354 w, 145 w, 96 m
m, 83 s.
Bull. Chem. Soc. Ethiop. 2019, 33(3)

460
Aydin et al.
Table 4. 1H-NMR spectral data of the ligand and the diamagnetic complexes (the chemical shift values, H,
ppm, with coupling constants, J, Hz, in DMSO-d6).
The benzimidazole protons
The phenolic protons
Compound
HL
H4
8.51
s,br
8.60
s,br
8.69
s
8.46
s
8.48 d
J=1.5
H6
H7
NH
13.41
s,br
12.46
s,br
10.98
s,br
13.24
s,br
H3’
8.29 d
J=2.2
8.43 d
J=2.0
7.53
H5’
H6’
OH
12.37
s,br
8.14 dd
7.78 d
7.55 dd
7.04 d
J=8.9, 1.5 J=8.8
J=8.8, 2.4 J=8.9
[Pd(HL)2Cl2]
8.18
d,br
7.96
s,br
7.58 dd
J=8.8,2.4
6.98 d
J=8.8
7.52 dd
7.12 d
J=8.8
6.71 d
J=8.8
7.01 d
---
[Ag(HL)(L)]∙H2O
[Cd(HL)(L)Cl]∙H2O
[HgCl2(HL)]∙2H2O
8.24 d
J=8.3
8.10 dd
J=8.8,2.0
8.13 dd
7.55 d
J=8.3
7.76 d
J=8.8
7.78 d
10.20
s,br
12.93
s,br
s
8.23 d
J=2.4
8.25 d
J=2.4
J=8.8, 2.4 J=8.8
7.54 dd 7.02 d
J=8.8, 2.4 J=8.8
--
--
J=8.8, 2.0 J=8.8
The proton H4, neighboring to the nitro group, is observed at 8.51 ppm and it appears at the
8.468.69 ppm range at the complexes (Table 4). The OH proton of Ag(I) complex shows a
different behavior by appearing at higher field according to the other complexes. This may be
relevant with a different coordination of the Ag(I) complex, namely non-chelate structure.
Fluorescence spectroscopy
Excitation and emission spectra of the compounds were obtained in ethanol at room temperature
(excitation wavelength: 354 nm; concentration: ~104 M). The emission spectral data are
presented in Table 5. The fluorescence spectra of HL and its Hg(II), Cd(II), Ag(I), Fe(III) and
Mn(II) complexes are shown in Figure 5.
It was reported that 2-(2’-hydroxyphenyl)benzimidazole (HPBI) and its derivatives can
undergo an excited-state intramolecular proton transfer (ESIPT) from the acidic (hydroxyl
proton) to the basic site (aromatic nitrogen) when photoexcitation changes their charge density
distribution [44–46]. In the ground state, the most stable form of HPBI is usually the enol form.
However, in the first excited singlet state, in which the hydroxyl group is much more acidic and
the nitrogen much more basic than in the ground state, the most stable structure is usually the
keto tautomer. As a result, excitation of the enol form of HPBI tends to be followed by
transformation to the keto form in the excited state via ESIPT reaction [46].
The ligand (HL) shows only a strong emission at 460 nm. All the complexes have
fluorescence characteristic and considerable changes are observed in the fluorescence spectra of
the complexes according to the ligand. The Cd(II) complex exhibited triple fluorescence and
Ni(II), Cu(II), Ru(III), Pd(II) and Hg(II) complexes showed dual fluorescence while the
uncomplexed ligand gave only single fluorescence at 460 nm. A blue shift is observed at Cu(II)
and Ru(III) complexes whereas the other complexes red-shifted with respect to HL. Intensity of
fluorescence generally decreases in the complexes, however in Co(II) and Fe(III) complexes it is
almost same with the ligand, and increasing is observed at Mn(II) and Hg(II) complexes. The
keto structure turned into the enol form as a consequence of complexation. And this change will
affect the fluorescence characteristics of the complexes with respect to HL significantly. It is
observed that all of the compounds have been emitting fluorescence in the visible region.
Especially, dual fluorescence of Ru(III), Ni(II), Pd(II), Cu(II), Cd(II) and Hg(II) complexes is
interesting. Thus, these compounds have potential applications as a luminescent material in
light-emitting devices.
Bull. Chem. Soc. Ethiop. 2019, 33(3)

Spectral characterizationn and antimicrobial activity of some transition metal complexe
ss
461
Figure 5. The fluorescence spectra of HL and Hg(II), Cd(II), Ag(I), Fe(III) and Mn(II)
complexes in ethanol.
Table 5. UV-Visible and emissionn
maximum wavelengths (nm) of the compounds (in ethanol, ~104 M).
Compound
HL
[Mn(L)2(H2O)]∙H2O
UV-vi
230 m
232 m
s
s
ible data*
268 m, 342 sh, 349 m
267 m, 350 m,br
Emission maximum*
460 s
469 s
,
,
,,
[Fe(L)2(OH)(H2O)]∙2H2O 232 m
[Ru(L)2(DMSO)2Cl]∙4H2O 247 m
[Co(HL)Cl2(H2O)2]∙2H2O 227 m
[Ni(HL)Cl2(H2O)2]∙4H2O 232 m
,
,
284 m, 342 m,br, 549 m,vbr
291 sh, 347 m,br, 510 sh, 581 m,br
493 s
394 w, 457 m
,
,
,, 336 sh, 264 m, 347 m,br, 600 w,br, 661 w,br 469 s
,
,
261 sh, 266 m, 338 sh, 353 m
320 m,br, 415 w,br
263 m, 352 sh, 399 m,br,
, 266 m, 349 m
394 w, 460 m
[Pd(HL)2Cl2]
249 m
237 m
230 m
228 m
229, 2
,
,
394 w, 474 w,br
394 w, 452 w,br
492 m,vbr
397 w, 459 m,vbr, 547 w,vbr
397, 465 s
[Cu(L)2(EtOH)2]∙H2O
[Ag(HL)(L)]∙H2O
[Cd(HL)(L)Cl]∙H2O
[HgCl2(HL)]∙2H2O
,,
,
,
,
270 m, 351 m
7 m, 347 m,br
66
* vbr: very broad, sh: shoulder, s: strong, m: medium, w: weak, br: broad.
UV-visible spectroscopy
The UV-visible absorption
temperature. The UV-visible spectral data are given in Table 5. The UV-visible spectra of the
compounds are shown in Fi ure 6. The electronic spectra of the compounds exhibit i tense
gion, which can be assigned to the n→σ*, n→* and →*
50 nm are due to the n→σ* transitions. The compounds exhibit
sspectra of the compounds were obtained in methanol at room
gg
nn
bands in the 200–400 nm r
ee
transitions. The bands below
2
intense intra-ligand absorptionn bands in the 250–350 nm region. The →* transitions of the
aromatic rings are observed i the 250–300 nm region whereas the →* transitions of th C=N
nn
ee
bond are in between 300 and 350 nm [47]. The electronic spectra of the complexes are of little
help in the present case at the visible region, since the d→d transitions are masked by the strong
charge-transfer transition ba
n
n
ds. The very broad bands at 581, 415 and 549 nm for
R
R
u(III),
Pd(II) and Fe(III) complexes, respectively, can be attributed to LMCT (ligand to metal
ccharge
transfer) transitions. The cobaalt complex, [Co(HL)Cl2(H2O)2]∙2H2O, exhibited two weak bands
at 600 nm (16667 cm−1), 661 nm (15129 cm−1) which may be assigned to 4T1g(F)→4T2g(
FF) (ν1)
Bull. Chem. Soc. Ethiop. 2019, 33(3)

462
Aydin et al.
and 4T1g(F)→4T1g(P) (ν3) tra
nnsitions, respectively, and these are suggestive of octaahedral
geometry around the cobalt(II)) ions [48, 49].
Figure 6. The UV-visible specctra of the ligand and complexes in ethanol.
Antimicrobial activity
The results concerning in vitrro antimicrobial activity of the complexes together with the MIC
values are presented in Table 6. Antimicrobial activity of HL and its Fe(NO3)3, Cu(NO3)2 and
Zn(NO3)2 complexes were r ported in previous studies [20, 21]. HL showed considderable
e
activity on S. aureus and S. epidermidis as 2.4 and 19.5 µg/mL, respectively. In adddition,
antibacterial activity of hydroochloride form of HL (HL∙HCl) toward S. epidermidis was 2.4
µg/mL, a remarkable value. Itt was also reported that the Zn(II), Fe(III) and Cu(II) compplexes
was also effective on these twwo bacteria although weaker compared to the HL ligand [2
Actually six bacteria and onne fungus each with multiple, fresh clinical isolates, have been
studied in the antimicrobial a tivity tests. It is considerable (and expected) result that Ag((I) and
00, 21].
c
Hg(II) complexes show stro
n
n
g antimicrobial activity against to all the microorganisms. In
addition, the Ag(I) complex
sshows prominent activity on P. mirabilis and C. albicans (MIC
values are 39 and 19.5 µg/m
L
L
, respectively) whereas AgNO3 and the ligand itself do no
tt show
any activity towards to thesee two microorganisms. This can be considered one of the most
important results of this study.
It is also observed that the Pd(II) and Cd(II) complexes are strongly and selectively efffective
on S. aureus and C. albicans (Table 6). The other complexes  except the Co(II) and u(III)
complexes  generally exhib ted activity towards S. aureus and S. epidermidis that th y are
R
R
ii
ee
Bull. Chem. Soc. Ethiop. 2019, 33(3)

Spectral characterization and antimicrobial activity of some transition metal complexes 463
Gram positive bacteria. The Ni(II) complex is more effective on S. epidermidis (MIC: 9.75
µg/mL) than the ligand (MIC: 19.5 µg/mL). The results of our study indicate that the ligand and
most of the complexes have the potential to generate novel metabolites, by displaying high
affinities towards some of the receptors. The superior antimicrobial activities of the complexes
mentioned above against S. aureus, S. epidermidis and C. albicans are considered to be valuable
contribution to the researches on metal-based drugs field and encourage working further
investigation on these compounds.
Table 6. In vitro antimicrobial activity of the compounds (MIC, µg/mL).
M i c r o o r g a n i s m s
Compound
Saa
Sea
Ecb
Kpb
Pab
Pmb
Ca
c




HL*
2.4
19.5

HL∙HCl*
2.4
19.5
9.8
19.5
4.9
4.9

2.4
156
39
39







39

4.9














156

4.9












39

4.9












39

4.9










78

19.5
39
39

[Zn(L)2]∙2H2O*
[Fe(L)2(H2O)2](NO3) ∙3H2O*
[Cu(L)2] ∙2H2O*
[Mn(L)2(H2O)]∙H2O
[Fe(L)2(OH)(H2O)]∙2H2O
[Ru(L)2(DMSO)2Cl]∙4H2O
[Co(HL)Cl2(H2O)2]∙2H2O
[Ni(HL)Cl2(H2O)2]∙4H2O
[Pd(HL)2Cl2]
[Cu(L)2(EtOH)2]∙H2O
[Ag(HL)(L)]∙H2O
[Cd(HL)(L)Cl]∙H2O
[HgCl2(HL)]∙2H2O
MnCl2∙6H2O









19.5
4.9
4.9
4.9
4.9
4.9


39

4.9

FeCl3∙6H2O







CoCl2∙6H2O







NiCl2∙6H2O







CuCl2∙2H2O







Ru(DMSO)4Cl2
PdCl2
AgNO3


9.8


4.9


4.9


4.9


4.9






CdCl2∙H2O







HgCl2∙H2O
Ciprofloxacin
Fluconazole
0.65
0.25
0.65
0.0625
1.3
0.49
10.4
1.0

1.30
0.0156

2.6

1.0
0.0156 0.0078




*Taken from literatures 20 and 21. Sa Staphylococcus aureus ATCC 6538; Se Staphylococcus epidermidis ATCC
12228; Ec Escherichia coli ATCC 8739; Kp Klebsiella pneumoniae ATCC 4352; Pa Pseudomonas aeruginosa
a
ATCC 1539 Pm Proteus mirabilis ATCC 14153; Ca Candida albicans ATCC 10231. Gram positive; bGram
negative; c,  : No antimicrobial effect at 5000 µg/mL and lower dilutions.
CONCLUSION
In the present study, MnCl2, FeCl3, CoCl2, NiCl2, CuCl2, Ru(DMSO)4Cl2, PdCl2, AgNO3, CdCl2
and HgCl2 complexes of HL were synthesized and characterized by using analytical and
spectroscopic techniques such as molar conductivity, magnetic moment, TGA, UV-Vis, FT-IR,
FT-Raman, NMR and fluorescence spectroscopy. Their antibacterial and antifungal activities
were evaluated by the disk diffusion method against six bacteria and C. albicans as yeast. It was
observed that most of the complexes exhibited considerable activity on S. aureus especially. The
Hg(II) and Ag(I) complexes were found to have superior activity toward all of the
microorganisms whereas the Pd(II) and Cd(II) complexes showed considerably antimicrobial
effect on S. aureus and C. albicans selectively.
Bull. Chem. Soc. Ethiop. 2019, 33(3)

464
Aydin et al.
ACKNOWLEDGMENTS
This work was supported by Scientific Research Projects Coordination Unit of Istanbul
University. Project numbers: 7011, FMP-2016-23012 and BEK-2017-26875.
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