Synthesis, characterisation and biological studies of mixed-ligand nickel (II) complexes containing imidazole derivatives and thiosemicarbazide Schiff bases

Article abstract of DOI:10.1016/j.molstruc.2019.126888

Four new mixed-ligand Ni(II) complexes (1–4) containing imidazole (im) or benzimidazole (bz) and tridentate Schiff bases derived from 2,4-dihydroxybenzaldehyde (24D) and 4-methyl-3-thiosemicarbazide (MT24D) or 4-phenyl-3-thiosemicarbazide (PT24D) were syn

Full text of DOI:10.1016/j.molstruc.2019.126888

Journal of Molecular Structure 1198 (2019) 126888
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, characterisation and biological studies of mixed-ligand
nickel (II) complexes containing imidazole derivatives and
thiosemicarbazide Schiff bases
Nurul N.M. Ishak a, Junita Jamsari a, A.Z. Ismail a, Mohamed I.M. Tahir a,
,
, e, *
Edward R.T. Tiekink b, Abhi Veerakumarasivam c d, Thahira B.S.A. Ravoof a
a Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor Darul Ehsan, Malaysia
b Research Centre for Crystalline Materials, School of Science and Technology, Sunway University, No. 5 Jalan Universiti, 47500, Bandar Sunway, Selangor
Darul Ehsan, Malaysia
c Department of Biological Sciences, School of Science and Technology, Sunway University, No. 5 Jalan Universiti, 47500, Bandar Sunway, Selangor Darul
Ehsan, Malaysia
d Medical Genetics Laboratory, Faculty of Medicine and Health Sciences, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor Darul Ehsan, Malaysia
e Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400, UPM Serdang, Selangor Darul
Ehsan, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new mixed-ligand Ni(II) complexes (1e4) containing imidazole (im) or benzimidazole (bz) and
tridentate Schiff bases derived from 2,4-dihydroxybenzaldehyde (24D) and 4-methyl-3-
thiosemicarbazide (MT24D) or 4-phenyl-3-thiosemicarbazide (PT24D) were synthesised and charac-
terised using elemental and spectral analysis including FTIR, UVeVis, 1H NMR, 13C NMR and mass
spectrometry for Schiff bases, while the complexes were additionally analysed using ICP-OES, molar
conductivity, magnetic susceptibility measurements and single crystal X-Ray diffraction (SXRD) analysis.
Magnetic susceptibility indicated a square planar geometry for all the metal complexes while molar
conductance values showed that the complexes were non-electrolytes in DMSO. The molecular geom-
etries of the neutral complex molecule in [Ni(MT24D)(bz)](bz). CH3OH (2′), that is 2 co-crystallised with
a 1,3-benzimidazole molecule as a methanol solvate, and in the cation of [Ni(MT24D)(im)]O2CMe.2H2O
(5), a reaction intermediate for 1, were established by X-ray crystallography. Each featured a trans-N2OS
coordination geometry defined by phenoxide-O, imine-N and thiolate-S (2′) or thione-S (5) donors as
well as the imine-N donors derived from 1,3-benzimidazole (2′) or imidazole (5) molecules. Systematic
variations in geometric parameters were correlated with the form of the tridentate ligand, i.e. di-anionic
(2′) or mono-anionic (5). In the crystal of 2′, supramolecular chains were sustained by hydrogen bonding
Received 1 June 2019
Received in revised form
16 July 2019
Accepted 31 July 2019
Available online 7 August 2019
Keywords:
Mixed-ligand complexes
Single Crystal X-ray diffraction
Imidazole
Thiosemicarbazone
Schiff base
and these were connected into a supramolecular layer by p …
p stacking interactions occurring between
coordinated benzimidazole rings. In the crystal of 5, hydrogen bonding led to a three-dimensional ar-
chitecture. The Schiff bases and mixed-ligand Ni(II) complexes were tested for their cytotoxic activitY, but
all compounds were inactive against the MDA-MB-231 and MCF-7 breast cancer cell lines. Interestingly,
the antibacterial analysis of the compounds showed that the PT24D Schiff base, Ni(MT24D)im,
Ni(MT24D)bz, and Ni(PT24D)bz complexes had specific and selective activity against Staphylococcus
aureus (S. aureus), Bacillus subtilis (B. subtilis), Propionibacterium acne (P. acne) and Enterobacteraerogenes
(E. aerogenes). The DNA binding studies of mixed-ligand Ni(II) complexes against calf thymus DNA
revealed that slight hypochromism was observed in the absorption spectra suggesting
p-p interactions
between the aromatic chromophores and the DNA base pairs where 2 had higher Kb values than 1 thus
indicating stronger interactions.
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
* Corresponding author. Department of Chemistry, Faculty of Science, Universiti
Putra Malaysia, 43400, UPM Serdang, Selangor Darul Ehsan, Malaysia.
E-mail address: thahira@upm.edu.my (T.B.S.A. Ravoof).
In recent years, multi-drug resistance has become a serious
https://doi.org/10.1016/j.molstruc.2019.126888
0022-2860/© 2019 Elsevier B.V. All rights reserved.

2
N.N.M. Ishak et al. / Journal of Molecular Structure 1198 (2019) 126888
issue resulting in significant morbidity and mortality. Thus, there is
a need to develop new types of drugs that can overcome this
growing problem. Transition metal complexes as metallo-drugs are
believed to have great potential as anti-microbial agents [1,2]. Small
molecules like transition metal complexes have been proven to
have strong binding interactions with DNA via both covalent and
non-covalent interaction modes. The interaction modes play a
significant role in the biological activities of metal complexes
especially as antibacterial, antifungal and anticancer agents [3]. For
example, the Cu(II) Schiff base complex derived from 4-
chloroanthranilic and salicylaldehyde showed an intercalative
and benzimidazole with tridentate thiosemicarbazide Schiff bases
and Ni(II) acetate. Since scarce information has been reported on
mixed ligand metal complexes containing imidazole derivatives as
a co-ligand and in view of the previous studies on the biological
relevance of related mixed-ligand metal complexes, we report
herein the synthesis, characterisation, cytotoxicity, antibacterial
activity and DNA-binding studies of mixed ligand metal complexes
derived from tridentate ONS Schiff bases with imidazole
derivatives.
binding mode with CT-DNA with
a high Kb constant of
2. Experimental
1.93 ꢀ 104 L molꢁ1, indicative of strong binding, as compared to
other
similar
complexes
such
as
[Cu(naph-leu)phen]
2.1. Instrumentation and materials
CH3OH$0.5H2O (Kb, 4.87 ꢀ 103 L molꢁ1) and [Cu(sal-
L-val)phen] (Kb,
6.48 ꢀ 103 L molꢁ1) [4]. This has formed the basis of most research
in the synthesis of metal complexes via various methods and
different designs to develop new metal-based drugs.
All chemicals were of analytical grade (A.R. from Alfa Aesar,
Aldrich or Merck) and used without further purification. Elemental
analysis (C, H, N) were performed using a LECO CHNS-932
elemental analyser. Infrared (IR) spectra were recorded on Perki-
nElmer 100 spectrophotometer (4000-280 cmꢁ1). Molar conduc-
tance of 10ꢁ3 M solutions of the mixed-ligand Ni(II) complexes in
DMSO were measured using a Jenway 4310 conductivity meter
Transition metal complexes of thiosemicarbazone ligands have
received considerable interest due to its ability to disrupt DNA
synthesis by causing modification in the reductive conversion of
ribonucleotides to deoxyribonucleotides [5]. Furthermore, by
introducing aldehydes or ketones to thiosemicarbazones, the Schiff
bases that are formed can interact with metal ions to form com-
plexes that have stable four, five or six coordination [6,7]. The
biological activity of thiosemicarbazones is facilitated by their
chelating ability with transition metal ions. Coordination to metal
centre through sulphur and nitrogen donors would form bidentate,
tridentate or even multidentate ligands, thus giving rise to com-
plexes of different geometries and properties that would alter or
enhancing their biological properties [8e12]. Studies also reported
that the biological activity of the metal complexes of
thiosemicarbazone-derived Schiff bases often had higher, and se-
lective bioactivities as compared to the corresponding free thio-
semicarbazones [13e15]. The biological activity of these metal
complexes containing Schiff bases have also been reported to have
enhanced activities when ligated to a co-ligand, forming mixed-
ligand complexes. For instance, the Cu(II) complexes of a tri-
with
a dip-type cell electrode. Magnetic susceptibility was
measured with a Sherwood Scientific MSB-AUTO magnetic sus-
ceptibility balance at 298 K. UVeVis spectra were recorded by using
a Shimadzu UV-2501 PC recording spectrophotometer in the range
of 1000e200 nm. Metal content of the complexes were determined
using Inductive Coupled Plasma-Optical Emission Spectrometry.
Nuclear Magnetic Resonance (1H NMR and 13C NMR) spectra were
recorded using an JNM ECA400 NMR spectrometer.
2.2. Synthesis of 4-methyl-3-thiosemicarbazide-2,4-
dihydroxybenzaldehyde (MT24D) and 4-phenyl-3-
thiosemicarbazide-2,4-dihydroxybenzaldehyde (PT24D)
20 mmol (2.76 g) of 2,4-dihydroxybenzaldehyde was dissolved
in 30 ml ethanol and added to an equimolar solution of 4-methyl-3-
dentate
Schiff
base,
salicylaldehyde-4-methyl-3-
thiosemicarbazone in the presence of imidazole or benzimidazole
exhibited enhanced activity against MCF-7 and MDA-MD-231
breast cancer cell lines [16] as compared to the mono-ligand
metal complex.
thiosemicarbazide
(20 mmol,
2.10 g)
or
4-phenyl-3-
thiosemicarbazide (20 mmol, 3.34 g) in the same solvent. The
mixture was stirred and reduced to half volume from initial vol-
ume. The precipitate that formed was recrystallised with cold
ethanol. The yields were dried over silica gel overnight.
Imidazole derivatives are an important class of heterocycles,
being the core fragment of different natural products and biological
systems. The imidazole ring is biologically relevant as it can mimic
the histidine moiety. It is able to act as a co-ligand in metal com-
plexes, potentially enabling them to bind with biomolecules [17].
They occupy a unique place in the field of medicinal chemistry
owing to their potent biological activity especially as antiprotozoal,
antifungal, and antihypertensive agents [18e21]. Bearing donors
and acceptors capable of hydrogen bonding, imidazole- and
benzimidazole-containing metal complexes often possess inter-
esting supramolecular architectures [22]. In the study of imidazole
derivatives, Brandenburg reported that the compound synthesized
from Cu(II) with salicylideneanthranilic acid and a co-ligand 2-
methylimidazole [Cu(SAA)(MeImH)] had distinct superoxide dis-
mutase activity (SOD) which resulted in 50% inhibition
For MT24D. Yield: (86%). Colour: light yellow, melting point:
215e216 ꢂC. Anal. Calc. Data are given in Table 2, IR, ATR v(cmꢁ1):
3240 v(OeH), 3336 v(NeH), 1616 v(C¼N), 1013 v(NeN), 866 v(C¼S).
1H NMR (400 MHz, DMSO‑d6,
d ppm): 11.41 (s, 1H, NH), 9.45 (q, 1H,
J ¼ 32 Hz, NH), 8.37 (s,1H, CH), 8.35 (s, 2H, OH phenolic), 7.33 (d,1H,
J ¼ 4 Hz, Ar H), 6.74 (d, 1H, J ¼ 8 Hz, Ar H), 6.62 (t, 1H, J ¼ 24 Hz, Ar
H), 2.45 (d, 3H, J ¼ 12 Hz, CH3). 13C NMR (100 MHz, DMSO‑d6,
d
ppm): 177.12 (C8), 146.11 (C5), 144.67 (C3), 140.56 (C7), 121.45
(C6), 118.87 (C4), 116.98 (C1), 116.85 (C2), 31.38 (C9). MS m/z 225
(Mþ1).
For PT24D. Yield: (89%). Colour: yellow, melting point:
205e206 ꢂC. Anal. Calc, Data are given in Table 2, IR, ATR v(cmꢁ1):
3160 v(OeH), 3240 v(NeH),1592 v(C¼N),1022 v(NeN), 825 v(C¼S).
1H NMR (400 MHz, DMSO‑d6,
d ppm): 11.71 (s, 1H, NH), 9.96 (s, 1H,
(IC50 ¼ 35
m
mol dmꢁ3) higher than that of the native enzyme
m
OH phenolic), 9.55 (s, 1H, OH phenolic), 8.94 (q, 1H, J ¼ 32 Hz, NH),
8.43 (s, 1H, CH), 7.51 (d, 2H, J ¼ 8 Hz, Ar H), 7.49 (d, 1H, J ¼ 12 Hz, Ar
H), 7.33 (t, 2H, J ¼ 16 Hz, Ar H), 7.14 (t, 1H, J ¼ 16 Hz, Ar H), 6.78 (d,
1H, J ¼ 16 Hz, Ar H), 6.64 (t, 1H, J ¼ 12 Hz, Ar H). 13C NMR (100 MHz,
(IC50 ¼ 0.004
mol dmꢁ3) [23]. This higher IC50 value was hypoth-
esized to be due to the strong ligand field created by the tridentate
Schiff base that would interfere with the interaction of the com-
plexed copper ion with the superoxide radicals [24].
DMSO‑d6, d ppm): 176.23 (C8), 146.13 (C5), 145.39 (C3), 141.76 (C7),
In the present study, a new series of mixed-ligand metal com-
plexes was formed by reacting single atom donor ligands, imidazole
139.68 (C9), 128 (C10, C14), 126.23 (C10, C11), 125.57 (C1), 121.54
(C6), 119.40 (C4), 118.59 (C2), 117.20 (C12). MS m/z 287 (Mþ1).

N.N.M. Ishak et al. / Journal of Molecular Structure 1198 (2019) 126888
3
Table 1
Crystal data and refinement details for complexes 2’ and 5.
Complex
2′
5
Formula
C16H15N5NiO2S, C7H6N2, CH4O
550.28
green
Triclinic
P1
9.9399(6)
10.4759(7)
12.0659(7)
99.237(5)
93.911(5)
95.581(5)
1229.74(13)
2
1.486
572
0.915
10943
2.4e25.2
5562
C12H14N5NiO2S, C2H3O2.2H2O
446.13
brown
Triclinic
P1
7.8339(7)
10.6523(8)
11.8297(7)
98.363(5)
101.635(6)
100.212(7)
934.61(13)
2
1.585
464
1.192
8119
2.4e25.2
4206
Formula weight
Crystal colour
Crystal system
Space group
a/Å
b/Å
c/Å
ꢂ
ꢂ
a
/
b/
ꢂ
/
g
V/Å3
Z
Dc/g cmꢁ3
F(000)
m
(MoK
Measured data
range/ꢂ
a
)/mmꢁ1
q
Unique data
Observed data (I ꢃ 2.0
s(I))
4207
342
3531
264
No. parameters
R, obs. data; all data
0.045; 0.100
0.051; 0.257
0.069; 0.112
1.05
0.042; 0.099
0.047; 0.550
0.053; 0.106
1.04
a; b in weighting scheme
Rw, obs. data; all data
GoF
Range of residual electron
density peaks/eÅꢁ3
ꢁ0.40 e 0.57
ꢁ0.48 e 0.96
Table 2
Physical properties and elemental analysis of the Schiff bases and mixed-ligand Ni(II) complexes.
Compound (Mol. Wt.)
Color
M.P (ºC)
La
mb (B.M)
(%) Found (Calcd.)
C
H
N
M
MT24D (225.0)
PT24D (287.3)
Ni(MT24D)im (1) (349.0)
Ni(MT24D)bz (2) (399.0)
Ni(PT24D)im (3) (411.0)
Ni(PT24D)bz (4) (461.0)
Light yellow
Yellow
Brown
Brown
Dark brown
Brown
215e216
126e127
>300
>300
>300
3.3
2.3
2.5
2.4
2.9
3.5
e
47.2 (47.9)
58.6 (58.5)
41.9 (41.2)
48.3 (48.0)
50.5 (49.9)
54.2 (54.5)
5.0 (4.9)
4.9 (4.6)
4.1 (3.7)
4.3 (3.8)
4.0 (3.6)
3.9 (3.7)
18.6 (18.6)
14.7 (14.6)
20.7 (20.0)
17.8 (17.5)
16.6 (16.9)
15.4 (15.1)
e
e
e
Diamagnetic
Diamagnetic
Diamagnetic
Diamagnetic
16.2 (16.7)
14.3 (14.6)
14.8 (14.2)
12.9 (12.7)
>300
a
Molar conductance (
U
ꢁ1 cm2 molꢁ1) of 10ꢁ3 M solutions in DMSO.
Magnetic moments at 298 K.
b
2.3. Synthesis of mixed-ligand Ni(II) complexes (1e4)
radiation (
l
¼ 0.71073 Å). Data reduction, including analytical ab-
sorption correction, was accomplished with CrysAlisPro [25]. The
structures were solved by direct-methods [26] and refined (aniso-
tropic displacement parameters, C-bound H atoms in the riding
model approximation) on F2 [27]. The oxygen and nitrogen-bound
hydrogen atoms were located from Fourier difference maps but
refined with distance restraints OeH ¼ 0.84 0.01 Å and
NeH ¼ 0.88 0.01 Å, respectively. The only exception to this was
for the O2w-water-bound hydrogen atoms in 5 which were
included in their as-located positions constrained to OeH ¼ 0.83 Å.
Even restrained refinement of this water molecule proved unstable,
an observation correlated with the relatively large displacement
parameters associated with the O2w oxygen atom. A consequence
of this is that one of the hydrogen bonds formed by this water
molecule is rather weak; it should be noted that the next closest
potential acceptor atom for a hydrogen bond is 3.83 Å distant. A
2 mmol (0.43 g) Ni(CH3COO)2.H2O was dissolved in 20 ml of
methanol. 8 mmol (0.54 g) imidazole or benzimidazole (8 mmol,
0.95 g) in 30 ml dichloromethane was added to this solution. The
mixture was stirred and heated for about 20 min. A solution of
Schiff base, MT24D (2 mmol, 0.45 g) or PT24D (2 mmol, 0.57 g) was
added to the metal-imidazole solution. The mixture was heated
while stirring to reduce the volume to half. The precipitate formed
when the mixture was cooled to room temperature, filtered and
dried over silica gel overnight. For imidazole or benzimidazole to
remain as a co-ligand, a large excess was added in the reaction.
2.4. Single crystal X-ray structure determination of
[Ni(MT24D)(bz)](bz).CH3OH (20), and the cation of [Ni(MT24D)(im)]
O2CMe.2H2O (5)
weighting scheme w ¼ 1/[
s
2(Fo2) þ (aP)2 þ bP] where P ¼ (Fo2 þ 2F2c)/
3) was introduced in each case. In the final cycles of the refinement
of 5, a reflection, i.e. (-6 -4 8), was omitted owing to poor agree-
ment. The molecular structure diagrams was generated with ORTEP
for Windows [28] with 70% displacement ellipsoids and the pack-
ing diagrams were drawn with DIAMOND [29]. Additional data
analysis was made with PLATON [30]. Crystal data and refinement
In addition to crystals of 2′, that is 2 co-crystallised with a 1,3-
benzimidazole molecule as a methanol solvate, a small number of
crystals of a reaction intermediate isolated from the reaction
mixture of 1 and characterised crystallographically, hereafter 5.
Intensity data for 2′ and 5 were measured at T ¼ 100 K on an Oxford
Diffraction Gemini Eos CCD diffractometer fitted with Mo K
a

4
N.N.M. Ishak et al. / Journal of Molecular Structure 1198 (2019) 126888
details are given in Table 1.
dissolved in a mixture of DMSO/buffer (3:7). Absorption titration
measurements were carried out by gradually increasing the DNA
concentration while maintaining the complex concentration
(2 ꢀ 10ꢁ6 M). Absorbance values were recorded after each succes-
sive addition of DNA solution and equilibration using UV spectro-
photometry [34].
2.5. Bioactivity
2.5.1. Cytotoxicity assays
MDA-MB-231 (estrogen receptor-negative human breast cancer
cells) and MCF-7 (estrogen receptor-positive human breast cancer
cells) cell lines were obtained from the National Cancer Institute,
USA. The cells were cultured in tissue culture flasks containing
RPMI 1640 culture medium supplemented with 1% penicillin and
10% fetal bovine serum (FBS). The cytotoxicity was determined by
using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide (MTT) assay [31]. Cytotoxicity was expressed as IC50
which is the concentration of trial compound that inhibits the
cancer cells by 50% as compared to the control. The control that
contained untreated cells were included for each sample. Tamox-
ifen was used as a standard drug.
3. Results and discussion
Schiff bases (MT24D and PT24D) were prepared by the
condensation reaction of 2,4-dihydroxybenzaldehyde with 4-
methyl-3-thiosemicarbazide and 4-phenyl-3-thiosemicarbazide to
form uninegatively-charged tridentate Schiff bases. The mixed-
ligand Ni(II) complexes (1e4) were synthesized by reacting Ni(II)
acetate, imidazole/benzimidazole (excess) and the Schiff bases in a
one-pot reaction. The Schiff bases and mixed-ligand Ni(II) com-
plexes were obtained in good yields with sharp melting points
indicating that the compounds were relatively free of impurities.
The Schiff bases were soluble in most common organic solvents
while the mixed-ligand Ni(II) complexes had limited solubility, and
were only completely soluble in DMSO and THF. The physical
properties and elemental analyses obtained for the Schiff bases and
mixed-ligand Ni(II) complexes are listed in Table 2. All physi-
ochemical and spectroscopic analyses were carried out on the bulk
samples.
2.5.2. Antibacterial activity
Clinical isolates of the following organisms: Methicillin Resis-
tant Staphylococcus aureus (MRSA), Staphylococcus aureus, Bacillus
subtilis, Serratiamarcescens, Propionibacterium acne, Pseudomonas
aeruginosa and Enterobacteraerogenes used in this study were ob-
tained from the culture collection of the Institute of Bioscience,
UPM. The bacterial strains were grown and maintained on nutrient
agar (NA) slant. Antimicrobial activity was based on a clear zone
formed around the disc. Complete inhibition was indicated by a
clear zone, while partial inhibition was by a semi-clear zone.
Streptomycin (100 mg/mL) was used as the reference drug for
antibacterial activity with DMSO as the negative control. The bac-
terial suspension (108 cfu/mL) was inoculated onto the entire sur-
face of a nutrient agar (NA) plate with a sterile cotton-tipped swab
to form an even lawn. After solidification, the filter paper discs
impregnated with the extracts were placed on the NA plates. The
plates were then incubated at 37 ꢂC for 18e24 h. All tests were
performed in triplicate and the antibacterial activity was expressed
as the mean of the inhibition diameters (mm) obtained.
3.1. Magnetic and conductivity data
All the mixed-ligand Ni(II) complexes exhibited diamagnetic
properties which was indicative of d8 Ni(II) complex with a square
planar geometry [3] since the Schiff bases were expected to coor-
dinate with the metal atom through three donor atoms while
imidazole or benzimidazole occupied the fourth position. The
complexes were non-electrolytes in DMSO [35], which fell in the
range of 2.3e3.3 U
ꢁ1 cm2 molꢁ1, indicating that both imidazole/
benzimidazole and ligand did not exist as free ions in solution.
3.2. Electronic and IR spectra
2.5.2.1. Determination of minimum inhibitory concentration (MIC).
The MIC values were determined by the liquid dilution method [32]
for samples that displayed inhibition zones of around 15 mm and
more in the disc diffusion assay. Various concentrations of each
sample (ranging from 0.156 to 20 mg/mL) were prepared in 96-
The infrared spectral data for the ligand and metal complexes
are shown in Table 3 and Figs. S1, S2 and S3 (Supplementary file).
Both ligands successfully coordinated to the metal ion through the
phenoxide-oxygen, azomethine-nitrogen, and thiolate-sulphur
atoms as evidenced by shift of wavenumbers of certain key func-
tional groups in the IR spectra. The absence of the v(SeH) band at
2570 cmꢁ1 in the spectra of the ligands indicated that they existed
as the thione form in the solid-state [34]. The thione forms are
unstable and tend to convert to stable thiolo forms in the presence
of metal ions, by enethiolization in solution [36]. In the infrared
spectrum of MT24D and PT24D ligands, strong bands were
observed at 1616 cmꢁ1 and 1592 cmꢁ1 respectively, representing
v(C¼N) and these bands shifted to lower wavenumbers indicating
microwell plates followed by inoculating 10 ml of the standardized
suspension bacteria. The microwell plates were incubated at 37 ꢂC
for 18e24 h. The MICs of the samples were recorded as the lowest
concentration that could inhibit the visible growth of microor-
ganisms overnight. Each experiment was carried out three times
and was correlated against the controls.
2.5.2.2. Determination of minimum bactericidal concentrations
(MBC). A loopful of sample from each well in the MIC examination
which did not show any growth was inoculated on NA and incu-
bated at 37 ꢂC for 18e24 h. In this test, the lowest concentration of
sample required to kill a particular bacterium was determined as
the MBC.
Table 3
IR spectral bands of the Schiff bases and their mixed-ligand Ni (II) complexes.
Compound v (OeH) v (NeH) v (C¼N)a,b
v (NeN) v (C¼S) v (CeS)
2.5.3. DNA-binding study
MT24D
PT24D
1
2
3
4
3240
3160
3076
3095
3049
3162
3336
3240
3383
3394
3362
3353
1616
1592
1600, 1670 1071
1594, 1748 1130
1596, 1699 1022
1593, 1700 1121
1013
1022
866
825
e
e
The buffer solution was prepared by adding 5 mM Tris-HCl
(Sigma Aldrich) and 25 mM NaCl (Sigma Aldrich) in 500 ml ultra-
pure water (pH ¼ 7.1). The CT-DNA was dissolved in 25 ml of buffer
solution [33]. The concentration of CT-DNA was calculated ac-
cording to Beer-Lambert's Law A ¼ εbc, where ε is the molar
extinction coefficient, 6600 Mꢁ1 cmꢁ1 at 260 nm [4]. The calculated
DNA concentration was 5.136 ꢀ 10ꢁ5 M. The complexes were
e
732
737
742
740
e
e
e
a
C¼N from ligand.
b
C¼N from imidazole/benzimidazole.

N.N.M. Ishak et al. / Journal of Molecular Structure 1198 (2019) 126888
5
that the metal ions coordinated through the azomethine-nitrogen.
A weak v(C¼N) band from imidazole or benzimidazole appeared in
the region 1699-1748 cmꢁ1 indicative of their presence as a co-
with the molecular structures of each of the complex molecules
shown in Fig. 1. The crystallographic asymmetric unit of 2′ com-
prises a neutral complex molecule, a 1,3-benzimidazole molecule
and a solvent methanol molecule. The tridentate di-anion co-
ordinates the nickel(II) centre via thiolate-S1, phenoxide-O3 and
imine-N3 atoms to establish five- and six-membered rings. The
fourth site of the central atom is occupied by the N4 atom derived
from a neutral benzimidazole molecule. The nitrogen atoms are
mutually trans within the resultant N2OS donor set that defines an
approximate square-planar geometry. Selected geometric data are
given in Table 5 from which it can be seen that the maximum de-
viation from the ideal geometry, at least in terms of angles is found
in the O1eNieN3 angle of 95.69(8)ꢂ.
The crystallographic asymmetric unit of 5 comprises a complex
cation, an acetate anion and two water molecules of crystallisation.
The tridentate mono-anion coordinates the nickel(II) centre via
thione-S1, phenoxide-O3 and imine-N3 atoms. The fourth site of
the central atom is occupied by the N4 atom derived from a neutral
imidazole molecule. A very similar description of the coordination
geometry pertains to the complex cation in 5 with the key
ligand.
A broad v(OeH) band was seen in the region of
3049e3240 cmꢁ1 for both ligands and complexes. v(C¼S) band
appeared in the ligands at 866 cmꢁ1 and 825 cmꢁ1 for MT24D and
PT24D respectively and shifted to lower wavenumbers upon
complexation of the ligands and Ni(II) ion in the region of
732e742 cmꢁ1
.
The electronic spectral analysis was carried out in DMSO and the
data is displayed in Table 4. The bands at 261e280 and
300e353 nm were attributed to intra-ligand n-p* and p-p* transi-
tions respectively [37]. Weak d-d transitions displayed at
400e500 nm indicated that the complexes had square planar ge-
ometry where the Ni(II) ion was coordinated to the azomethine-
nitrogen, hydroxyl-oxygen, thiolate-sulphur from the Schiff base,
whereas the fourth position was occupied by the co-ligand, imid-
azole or benzimidazole. The appearance of a strong peak at
347e377 nm additionally supported the ligation of sulphur to the
metal ion via ligand to metal charge-transfer transitions.
3.3. 1H NMR and 13C NMR spectra
To obtain insight and confirm the structure of the tridentate
Schiff bases, 1H NMR spectra of MT24D and PT24D (Figs. S4 and S5,
Supplementary Data) in DMSO‑d6 were obtained. The summary of
the spectral data is shown in Tables S1 and S2 (Supplementary
Data). The 1H spectra MT24D showed a resonance in the downfield
region around
d 11 ppm which was assigned as the proton (H7)
which was attached to the N atom. The existence of the electro-
negative atoms (nitrogen) adjacent this proton deshields the pro-
ton. One singlet at
d 8.35 and two singlets at d 9.55 and 9.96 ppm
were observed correspond to OH phenolic of MT24D and PT24D,
respectively. The aromatic protons were observed as multiplet
ranging
6.64e7.51 ppm corresponding to 8 protons for MT24D and PT24D,
respectively. No resonance was observed at 4.0 ppm which indi-
d 6.62e7.33 ppm corresponding to 3 protons and
d
d
cated the absence of the thiol group which proved that the Schiff
bases were present in their thione tautomeric form in solution [38].
The 13C{1H} NMR spectra and Distortionless Enhancement by
Polarization Transfer (DEPT) experiments (Figs. S6 and S7, Supple-
mentary Data) were used to identify and assign the carbon atoms of
the Schiff bases. The summary of the spectral data of MT24D and
PT24D is shown in Table S2 (Supplementary Data). The signal due to
the methyl group was observed at
group (eC¼S) was assigned at the region
the most deshielded regions. All the other aromatic carbon shifts
were observed in the region of 116.85e128.69 ppm [15].
d
31.38 ppm. The thiocarbonyl
d
176.23e177.12 ppm in
d
3.4. Crystal structures of [Ni(MT24D)(bz)](bz).CH3OH (20) and
[Ni(MT24D)(im)]O2CMe.2H2O (5)
Crystal structure determinations were achieved for 2′ and 5
Table 4
Electronic absorption bands of the Schiff bases and their mixed-ligand Ni (II)
complexes.
Compound
Electronic spectra (in DMSO) lmax(log ε)c
MT24D
280 (3.08); 348 (4.37)
PT24D
279 (4.11); 353 (2.41)
1
2
3
4
261 (4.80); 301 (3.92); 365 (4.13); 406 (3.99)
279 (4.81); 300 (3.93); 369 (4.09); 404 (3.95)
270 (6.43); 304 (6.48); 347 (6.54); 406 (7.61)
280 (4.87); 308 (4.13); 377 (4.23); 411 (4.17)
Fig. 1. Molecular structures of the complex molecules in (a) 2′ and (b) 5, showing atom
labelling schemes.

6
N.N.M. Ishak et al. / Journal of Molecular Structure 1198 (2019) 126888
Table 5
equivalent parameters for the larger chelate ring are 0.0108 Å and
Selected geometric parameters (Å, ꢂ) for 2′ and 5.
0.0151(17) Å for the C5 atom. The dihedral angle between the
chelate rings is 0.861(9)ꢂ. The imidazole ring is inclined with
respect to the rest of the molecule as seen in the dihedral angle of
50.69(7)ꢂ it forms with the five-membered chelate ring.
Complex
20
5
Parameter
NieS1
2.1365(8)
1.8799(18)
1.862(2)
1.897(2)
1.417(3)
1.744(2)
1.350(3)
1.308(3)
1.304(3)
174.82(6)
87.54(7)
89.95(7)
95.69(8)
86.94(8)
176.94(9)
111.4(2)
113.2(2)
117.7(2)
122.8(2)
119.4(2)
2.1545(7)
1.8597(17)
1.853(2)
1.894(2)
1.397(3)
1.715(2)
1.326(3)
1.453(3)
1.301(3)
176.43(6)
88.60(6)
89.25(7)
94.90(8)
87.26(8)
177.47(9)
117.4(2)
115.2(2)
122.78(19)
118.61(19)
118.6(2)
NieO1
NieN3
NieN4
N2eN3
C1eS1
C1eN1
C1eN2
C3eN3
S1eNieO1
S1eNieN3
S1eNieN4
O1eNieN3
O1eNieN4
N3eNieN4
C1eN2eN3
N2eN3eC3
S1eC1eN1
S1eC1eN2
N1eC1eN2
The other obvious difference in conformation between the
complex molecules in 2′ and 5 relates to the relative orientations of
the terminal amine-bound methyl groups. While in both molecules,
this group is co-planar with the coordinated S1 atom with
S1eC1eN1eC2 torsion angles of and 171.5(2) and ꢁ1.87(16)ꢂ in 2
and 5, respectively, in 5, the methyl group is syn to the S1 atom
whereas there is an anti-relationship in 2'.
The mode of coordination of the LHꢁ anion in 5 appears to be
unprecedented in the crystallographic literature according to a
search of the Cambridge Structural Database [39]. Similarly, with a
formal C3eC4 bond in 2′, this is also unprecedented with the
closest structural analogue being a nickel(II) complex where this
bond is substituted by a bond of an aromatic ring [40].
As anticipated from the chemical compositions of 2′ and 5,
significant hydrogen bonding interactions are evident in their
crystals. The geometric parameters characterising these along with
other non-covalent interactions present in the respective crystals
are tabulated in Table 6.
Hydrogen bonding interactions also feature prominently in the
molecular packing of 2'. The hydroxyl group of the complex
molecule forms
difference being the thiolate-S1 donor in 2′ is now a thione-S1 atom
as the tridentate ligand is a mono-anion. The major angular devi-
ation from the square-planar N2OS coordination geometry is again
seen in the O1eNieN3 angle of 94.90(8)ꢂ. The experimental
equivalence of the C13eO3, O4 bond lengths, i.e. 1.244(4) and
1.249(3) Å, respectively, confirms the presence of the acetate
counter-ion in the crystal of 5.
Systematic changes in key geometric parameters are evident in
Table 5, as a result of the different coordination modes of the LHꢁ
and L2ꢁ anions in 5 and 2′, respectively. Most notable is the longer
NieS1 bond length in 5 compared with that in 2′, with concomitant
differences in the NieO1 bond lengths. The short C1eS1 bond
length in 5 is also consistent with the presence of a thione-S1 atom.
Also, the C1eN2 length of 1.308(3) Å in 2′ is consistent with the
formation of an imine bond in the L2ꢁ di-anion. This change is also
reflected in some of the key angles, e.g. a reduction of ca 6ꢂ in the
C1eN2eN3 angle, and a decrease and increase by 5ꢂ and 4ꢂ in the
S1eC1eN1 and S1eC1eN2 angles, respectively going from 5 to 2'.
The tridentate mode of coordination of the L2ꢁ and LHꢁ anions
a
hydroxyl-OeH…N(benzimidazole) hydrogen
bond with a lattice benzimidazole molecule which, in turn, forms a
benzimidazole-NeH…O(methanol) hydrogen bond. The solvent
molecule functions in the same way by forming methanol-
OeH…N(imine)
and
coordinated-benzimidazole-
NeH…O(methanol) hydrogen bonds. The result is a supramolecular
chain along the c-axis direction as shown in Fig. 2(a). While there is
no role formal role for the amine-NeH hydrogen in the hydrogen
bonding scheme, this atom participates in the molecular packing by
…
forming in a NeH
p(C6-benzimidazole) interaction within the
supramolecular chain as detailed in Fig. 2(b). The chains are con-
…
nected into a supramolecular layer in the bc-plane by
p
p
stacking interactions between symmetry related benzimidazole
rings, i.e. (N4,N5,C10eC12) and (C11eC16)i rings with an inter-
centroid separation of 3.6142(16)
Å and angle of inclina-
tion ¼ 1.23(14)ꢂ; symmetry operation (i): 2-x, 2-y, 1-z. Layers stack
along the a-axis direction without directional interactions between
them, Fig. 2(c).
in 2′ and
5 results in the formation of five-membered
In the crystal of 5, the acetate anion spans the two amine-NeH
Ni,S1,C1,N2,N3 and six-membered Ni,O1,N3,C3eC5 chelate rings.
While essentially planar, there are significant deviations from
planarity in the five-membered ring of 2': the r.m.s. devia-
tion ¼ 0.0210 Å with the N3 atom deviating by 0.0290(12) Å from
this plane. Even more pronounced are the deviations in the six-
membered chelate ring with the r.m.s. deviation for the fitted
atoms being 0.0459 Å, and with the N3 and Ni atoms lying
0.0723(14) and 0.0559(10) Å to either side of the plane, respec-
tively; the dihedral angle between the chelate rings ¼ 6.19(10)ꢂ.
Another description for the conformation of the six-membered
chelate ring is one based on an envelope with the Ni atom being
the flap atom, lying 0.152(3) Å out of the plane defined by the
remaining five atoms [r.m.s. deviation ¼ 0.0288 Å]. The coordinated
1,3-benzimidazole molecule [r.m.s. deviation ¼ 0.0125 Å] occupies
a position almost perpendicular to the tridentate ligand as seen in
the dihedral of 88.94(6)ꢂ formed between it and the five-
membered chelate ring. The non-coordinating 1,3-benzimidazole
molecule is also planar [r.m.s. deviation ¼ 0.0125 Å]. A difference
in conformation in the chelate rings is evident in 5. In 5, both rings
are planar with the greatest deviation from the five-membered ring
[r.m.s. deviation ¼ 0.0091] being 0.0137(12) Å for the N3 atom. The
Table 6
Geometric parameters (Å, ꢂ) characterising intermolecular interactions in the crys-
tals of 2′ and 5.a.
A
H
B
H/B
A … B
A-H…B
Symmetry operation
20
O2
O3
N5
N7
N1
5
N1
N2
O2
N5
H2o
H3o
H5n O3
H7n O1
N6
N2
1.90(3)
2.741(3) 172(3)
1-x, 1-y, -z
1-x, 1-y, 1-z
1 þ x, y, z
ꢁ1þx, y, z
1.982(13) 2.805(3) 164(4)
1.927(14) 2.804(3) 175(3)
1.96(3)
2.833(3) 169(3)
3.480(2) 153.7(18) 1 þ x, y, 1 þ z
H1n Cg(1) 2.68(2)
H1n O3
H2n O4
1.937(18) 2.808(3) 173(3)
1.842(17) 2.718(3) 176(3)
1-x, 1-y, 1-z
1-x, 1-y, 1-z
x, y, z
-x, 1-y, -z
x, y, z
x, y, z
H2o
O2w
1.99(2)
1.90(2)
2.01(3)
1.93(3)
1.96
2.825(4) 172(3)
2.759(3) 167(3)
2.852(3) 177(3)
2.763(3) 171(3)
2.678(5) 145
H5n O1w
O1w H1w O1
O1w H2w O4
O2w H3w O3
O2w H4w O4
1-x, -y, -z
x, y, z
2.67
3.438(4) 154
a
Cg(1) is the ring centroid of the C18eC23 ring.

N.N.M. Ishak et al. / Journal of Molecular Structure 1198 (2019) 126888
7
Fig. 2. Supramolecular association in the crystal of 2: (a) a view of the supramolecular chain along the c-axis direction and sustained by OeH/N and NeH/O hydrogen bonding
shown as orange and blue dashed lines, respectively, (b) detail of the amine-NeH …
p
interaction highlighted as a purple dashed line and (c) a view in projection down the c-axis of
…
the unit cell contents of 1 with
p
p interactions shown as purple dashed lines.
atoms via two charge-assisted amine-NeH…O(carboxylate)
hydrogen bonds to form an eight-membered {…OCO/HNCNH}
4. Biological activities
heterosynthon. The presence of hydroxyl-OeH…O2w(water),
4.1. Cytotoxic activities
imidazole-NeH…O1w(water) and water-O1w
O2(phenoxide)
…
hydrogen bond provide further links between the complex cation
and the other constituents of the crystal. Each of the carboxylate-O
atoms forms a water-O-H…O(carboxylate) hydrogen bond, derived
from different water molecules. Finally, the H4w atom participates
in a long hydrogen bond with the carboxylate-O4 atom. A view of
the unit cell contents is shown in Fig. 3. From here, globally, com-
plex cations stack along the crystallographic a-axis and are con-
The Schiff bases and the mixed-ligand Ni(II) complexes were
screened against two breast cancer cell lines, MCF-7 (positive es-
trogen receptor human breast cancer) and MDA-MB-231 (negative
estrogen receptor human breast cancer). The IC50 values (concen-
tration of drug that inhibits the growth of cancer cells by 50% as
compared to the untreated controls) were determined. An IC50
value of less than 0.5
m
M indicates that the complex is strongly
M and more than 5.0
nected into
a
three-dimensional architecture via the
active, whereas IC50 values of 0.5e5.0
m
mM
aforementioned hydrogen bonds which extend laterally from the
columns.
indicate that the complex is moderately active and inactive,
respectively [41]. All the compounds were inactive against the
tested cancer cell lines. This could have been due to the presence of
a hydroxyl group in the Schiff bases of Ni(II) complexes that

8
N.N.M. Ishak et al. / Journal of Molecular Structure 1198 (2019) 126888
Fig. 3. A view in projection down the a-axis of the unit cell contents of 5. The OeH/O and NeH/O hydrogen bonds are shown as orange and blue dashed lines, respectively.
reduced the cytotoxicity, probably due to greater steric hindrance
and intramolecular H-bonding between the compounds [42].
Interestingly, the PT24D and Ni(II) complexes were observed to
have antibacterial potency against the bacterial strains tested.
antibacterial activity as compared to the Schiff bases suggesting
that the metal ions reduced the polarity of the compounds through
the partial sharing of positive charge with the donor atoms of Schiff
bases [43]. Hence,
p-electron delocalisation upon chelation which
also increased the lipophilic character of the central metal atom
allowed the Ni(II) complexes to have a stronger permeating ability
to enter the permeable membrane of the bacterial strains [44]. This
was also observed in similar mixed-ligand Ni(II) complexes with
sap (salicylidene-2-aminophenol) and imidazole which had sig-
nificant antibacterial activities compared to other Ni(II) complexes
containing only 2-carboxybenzaldehyde thiosemicarbazone
[45,46]. The complexes in this study seemed to be more bacterio-
cidal towards the gram-positive bacteria tested. Complex 2 had the
smallest MIC and MBC values against Pseudomonas Aeruginosa
whereas 1 had small MIC and MBC values against a range of both
positive and negative gram bacteria, suggesting that only small
amounts of the metal complex were required to inhibit/kill the
4.2. Antibacterial activities
The Schiff bases, and mixed-ligand Ni(II) complexes were
screened for their antibacterial properties against Staphylococcus
aureus, Bacillus subtilis, Propionibacterium acne (Gram-positive),
and
Pseudomonas
aeruginosa,
Serratiamarcescens,
Enter-
obacteraerogenes (Gram-negative). Streptomycin was used as a
positive control and DMSO was used as negative control. The disc
diffusion assay data is tabulated in Table 7 while the minimum
inhibition concentration (MIC) and minimum bacterial concentra-
tion (MBC) results are recorded in Table 8.
The mixed-ligand Ni(II) complexes were observed to have better
Table 7
Disc diffusion assay of the Schiff bases and mixed-ligand Ni(II) complexes against selected bacterial strains.
Sample (20 mg/mL) Gram-positive bacteria Gram-negative bacteria
Staphylococcus aureus Bacillus subtilis Propionibacterium acne MRSA Pseudomonas aeruginosa Serratiamarcescens Enterobacteraerogenes
MT24D
0
0
0
0
0
0
0
1
2
19
17
12
0
13
24
2
2
2
14
13
10
0
11
29
1
1
1
17
18
14
0
13
19
3
1
1
0
0
10
0
0
0
0
0
0
0
26
7
0
9
11
0
1
1
0
0
14
0
10
25
PT24D
3
4
1
3
1
1
1
2
2
1
0
1
0
0
1
Streptomycin
28
1
21
a
Diameter of 15 mm and above is considered active; 0, inactive.

N.N.M. Ishak et al. / Journal of Molecular Structure 1198 (2019) 126888
9
Table 8
Results of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). (results are in mg/mL).
Sample
Staphylococcus aureus
Bacillus subtilis
Propionibacterium acne
Enterobacteraerogenes
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
1
2
0.63
1.25
e
1.25
2.50
e
10.0
e
20.0
e
0.63
0.16
e
1.25
0.31
e
e
e
e
e
PT24D
4
e
e
10.0
e
20.0
e
10.0
20.0
e
e
e
e
Fig. 5. Electronic spectra of 2 in Tris-HCl buffer upon addition of CT-DNA. The arrow
shows the emission intensity changes upon increasing DNA concentration from 0
DNA to 240 L DNA.
Fig. 4. Electronic spectra of 1 in Tris-HCl buffer upon addition of CT-DNA. The arrow
shows the emission intensity changes upon increasing DNA concentration from 0
DNA to 240 L DNA.
mL
mL
m
m
interaction between 2 and DNA was also better than 1 possibly due
to the presence of aromatic benzimidazole rings where the nitro-
gen atom of benzimidazole interacted well with the nucleobases of
DNA. However, despite good Kb values, both complexes had weaker
interactions as compared to the interaction of the standard,
ethidium bromide with the DNA base pairs (Kb: 7 ꢀ 107 Mꢁ1) [52].
growth of the micro-organism [47].
4.3. DNA-binding studies
Electronic absorption spectroscopy is a universal technique used
to investigate the strength and mode of binding interactions be-
tween compounds 1 and 2 with CT-DNA [4]. The UV spectra of 1
and 2 in the absence and presence of CT-DNA is given in Figs. 4 and
5, respectively. Both spectra exhibited hypochromism of 17.2% and
5. Conclusions
The reaction of Ni(II) acetate with Schiff bases derived from 4-
substituted-3-thiosemicarbazide and 2,4-dihydroxybenzaldehyde
in the presence of imidazole or benzimidazole yielded a new series
of mixed-ligand Ni(II) complexes, 1e4. The Schiff bases coordinated
to the Ni(II) ion via deprotonated ONS donor atoms and a mono-
dentate nitrogen atom from imidazole or benzimidazole, yielding a
distorted square planar geometry as was supported by spectro-
scopic analysis. Indirect evidence for the proposed structures were
provided by crystallographic methods. X-ray crystallography
established the complex in 2′ and complex cation in 5 to exist
within trans-N2OS square planar coordination geometry. In 5, the
sulphur atom is a thione as opposed to a thiolate in 2'. The form
(charge) of the tridentate ligand gave rise to systematic changes in
bond lengths and angles. Considerable hydrogen bonding led to a
three-dimensional architecture in the crystal of 5 but, only one-
dimensional chains in 2'. All the tested compounds were inactive
against the two selected breast cancer cell lines. Although the
complexes had higher activity than their free Schiff bases against
the tested microbes, their Kb values were significantly lower than
23.4% for 1 and 2, respectively indicating that the
omatic chromophore of the complexes interacted with
nucleobases of DNA [48]. The intrinsic binding constant Kb of the
complex with CT-DNA was determined according to the following
equation [4]:
p
orbital of ar-
orbital of
p
.
.
½DNAꢄ ðεa ꢁ εf Þ ¼ ½DNAꢄ ðεb ꢁ εf Þ þ 1=Kbðεb ꢁ εf Þ;
where [DNA] is the concentration of DNA in the base pairs, Kb is the
intrinsic binding constant, εa corresponds to the apparent extinc-
tion coefficient, and εb and εf correspond to the extinction co-
efficients of the bound and free forms of the complex, respectively.
The Kb value obtained for 1 was 4.2 ꢀ 102 Mꢁ1 while for 2 it was
5.7 ꢀ 103 Mꢁ1 which was high, an indication of strong and stable
interactions between the complexes and CT-DNA [49,50].
Comparing the molecular structures of both the complexes, it was
expected that the greater number of coplanar aromatic rings in the
latter could have led to a higher affinity for DNA [51]. The

10
N.N.M. Ishak et al. / Journal of Molecular Structure 1198 (2019) 126888
3675e3679.
the standard, indicating that the complexes had a different mode of
action against the tested cancer cells and microbes in this work.
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