Synthesis, spectral characterization, crystal structure and antibacterial activity of nickel(II), copper(II) and zinc(II) complexes containing ONNO donor Schiff base ligands

Article abstract of DOI:10.1016/j.molstruc.2021.130112

In the present study, a series of mononuclear Ni(II), Cu(II) and Zn(II) complexes with ONNO donor salen-type Schiff base ligands, derived from different 3,5-dihalosalicylaldehyde with polymethylenediamines of varying chain length, have been prepared. All

Full text of DOI:10.1016/j.molstruc.2021.130112

Journal of Molecular Structure 1233 (2021) 130112
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Synthesis, spectral characterization, crystal structure and antibacterial
activity of nickel(II), copper(II) and zinc(II) complexes containing
ONNO donor Schiff base ligands
Hadi Kargara,∗, Amir Adabi Ardakanib, Muhammad Nawaz Tahirc, Muhammad Ashfaqc,
Khurram Shahzad Munaward,e
a Department of Chemical Engineering, Faculty of Engineering, Ardakan University, P.O. Box 184, Ardakan, Iran
b Department of chemistry, Ardakan Branch, Islamic Azad University, Ardakan, Iran
c Department of Physics, University of Sargodha, Punjab, Pakistan
d Department of Chemistry, University of Sargodha, Punjab, Pakistan
e Department of Chemistry, University of Mianwali, Mianwali, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study, a series of mononuclear Ni(II), Cu(II) and Zn(II) complexes with ONNO donor salen-
type Schiff base ligands, derived from different 3,5-dihalosalicylaldehyde with polymethylenediamines of
varying chain length, have been prepared. All the synthesized compounds were characterized by using
various spectroscopic (FT-IR and 1H NMR) and analytical (CHN etc.) techniques. Furthermore, three com-
plexes (ZnL1, NiL3 and NiL4) were successfully characterized by single crystal X-ray diffraction. The data
revealed that the geometry around Zn(II) was distorted square pyramidal for ZnL1, while NiL3 and NiL4
complexes adapted slightly distorted octahedral geometries around Ni(II). The antibacterial activities of
the synthesized ligands and their respective complexes were elaborated by screening them against two
strains of Gram positive (Staphylococcus aureus and Bacillus cereus) and two strains of Gram negative (Es-
cherichia coli and Pseudomonas aeruginosa) bacteria. It was found from the measurements of zones of
inhibition that metal chelates are marginally more effective than free ligands.
Received 17 January 2021
Revised 7 February 2021
Accepted 8 February 2021
Available online 16 February 2021
Keywords:
Schiff base complex
Spectral characterization
X-ray structure
Antibacterial activity
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
These salen-type ligands are considered among the most ex-
tensively studied ligands in coordination chemistry [7]. This is due
The species having imine bonds (–C=N−) are familiar with
many names like imines, anils, Schiff bases and azomethines. These
species can be prepared easily by the condensation of primary
amine and aldehyde or ketones in a particular solvent [1]. These
moieties are useful as chelating agents because of their structural
diversity, easy mode of preparation and have a variety of applica-
tions in industry as well as in medicine [2–5].
to their adaptability, broad range of complexing behavior, catalytic,
enzymatic, magnetic and pharmacological applications [8–11]. The
beauty of salen type ligands is the facile reformation of the ligand’s
backbone and chelated transition metals ions which lead to the en-
hancement of antiproliferative, antioxidant and chemical properties
of parent organic ligands [12–14]. Coordination of salen-type lig-
ands with the metal atoms of 3d series specifically copper, nickel
and zinc possess tunable electrical, spectroscopic, crystallographic,
enzymatic and catalytic properties, and have rich structural func-
tionality reliant on specific applications [15–20].
A particular category of chelating Schiff base, salen, is synthe-
sized when salicylaldehyde and diamine are condensed together
in a 2:1 ratio. Salen-type ligands have the coordination number of
four (ONNO), which can chelate with metal ion by making two co-
valent and two coordinate covalent bonds to adapt a trans planar
geometry in addition to vacant axial position for ancillary ligand,
analogous to porphyrins [6].
It was revealed from the literature survey that Ni complexes
exhibit excellent antimicrobial, DNA binding activities, and also
played a key role as catalyst in redox enzymes and hydrogena-
tion reactions [21–22]. Copper complexes are active pharmacolog-
ical species and are termed as potent inhibitors against cancer
cells by stopping the uncontrolled extension of cell division. Cop-
per complexes can crack the cell DNA via oxidative mechanism and
hence can be considered as alternative to Cis-Platin [23–27]. Zinc,
∗
Corresponding author.
E-mail address: h.kargar@ardakan.ac.ir (H. Kargar).
https://doi.org/10.1016/j.molstruc.2021.130112
0022-2860/© 2021 Elsevier B.V. All rights reserved.

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
– –
=
a very effective bioactive metal, found in the human body and
is a part of many enzymes like carbonic anhydrase, carboxypep-
tidase and alcohol dehydrogenase. A brief overview of literature is
evident about the role of zinc complexes as antidepressant, anti-
inflammatory and antimicrobial agents [28,29]
υ C H (aromatic): 3066, υ C H (aliphatic): 2931, 2899, υ HC N:
–
–
1639, υ C=C and C N: 1577, 1510, 1433, 1381, υ C O: 1296, υ Zn-
N: 547, υ Zn-O: 432. 1H NMR [d6-DMSO, δ (ppm)]: 8.45 (s, 2 H,
– =
H C N), 7.68 (d, 2 H, J = 2.7 Hz, H arom.), 7.44 (d, 2 H, J = 2.7 Hz,
H arom.), 3.75 (s, 4 H, CH2–N).
Keeping in mind about the large-scale applications and con-
tinuation of our previous work, here we planned to prepare new
dihalogenated salen-type multidentate (ONNO) ligands and their
complexes with Cu(II), Ni(II) and Zn(II) to elaborate their structural
diversities and biological potential.
[Ni(L2)], Calculated for C16 H10I4N2NiO2: C 23.19, H 1.22, N
3.38%. Analysis found: C 22.28, H 1.01, N 3.27. FT-IR (KBr, cm−1):
–
–
=
υ C H (aromatic): 3047, υ C H (aliphatic): 2918, 2850, υ HC N:
=
–
–
1614, υ C C and C N: 1568, 1492, 1433, 1369, υ C O: 1321, υ Ni-
N: 545, υ Ni-O: 408.
[Cu(L2)], Calculated for C16 H10CuI4N2O2: C 23.06, H 1.21, N
3.36%. Analysis found: C 23.19, H 1.17, N 3.23. FT-IR (KBr, cm−1):
2. Experimental
–
–
=
υ C H (aromatic): 3037, υ C H (aliphatic): 2908, 2850, υ HC N:
–
–
1616, υ C=C and C N: 1564, 1487, 1427, 1371, υ C O: 1296, υ Cu-
2.1. Materials and physical measurements
N: 545, υ Cu-O: 415.
[Zn(L2)], Calculated for C16 H10I4N2O2Zn: C 23.08, H 1.21, N
3.35%. Analysis found: C 22.98, H 1.27, N 3.42. FT-IR (KBr, cm−1):
The solvents, relevant precursors and other chemicals used in
the present study were of analytical grade and purchased from
Merck, Acros organics and Sigma Aldrich. Dynalon SMP10 equipped
with a digital thermometer was used to measure the melting
points of all the synthesized compounds. Microanalysis of the com-
–
–
=
υ C H (aromatic): 3053, υ C H (aliphatic): 2924, 2852, υ HC N:
–
–
1635, υ C=C and C N: 1564, 1506, 1425, 1402, υ C O: 1305, υ Zn-
N: 574, υ Zn-O: 424. 1H NMR [d6-DMSO, δ (ppm)]: 8.37 (s, 2 H,
– =
H C N), 7.93 (d, 2 H, J = 2.4 Hz, H arom.), 7.56 (d, 2 H, J = 2.4 Hz,
–
plexes were done using a Heraeus CHN O-FLASH EA 1112 ele-
H arom.), 3.73 (s, 4 H, CH2–N).
mental analyzer. FT-IR spectra were recorded on a FT-IR Prestige
21 spectrophotometer from 4000–400 cm−1 using KBr pellets. 1H
NMR spectroscopy in DMSO–d6 (400 MHz, Bruker) was carried out
by using tetramethylsilane (TMS) as internal standard. Chemical
shifts for proton resonances are reported in ppm (δ) relative to
tetramethylsilane and DMSO–d6.
[Ni(L3)], Calculated for C18 H14 Br4N2NiO2: C 32.33, H 2.11, N
4.19%. Analysis found: C 32.21, H 2.08, N 4.23. FT-IR (KBr, cm−1):
–
–
=
υ C H (aromatic): 3070, υ C H (aliphatic): 2922, 2852, υ HC N:
=
–
–
1616, υ C C and C N: 1579, 1508, 1444, 1384, υ C O: 1325, υ Ni-
N: 532, υ Ni-O: 418.
[Cu(L3)], Calculated for C18 H14 Br4CuN2O2: C 32.10, H 2.10, N
4.16%. Analysis found: C 32.01, H 2.13, N 4.25. FT-IR (KBr, cm−1):
–
–
=
2.2. General procedure for the synthesis of the Schiff bases (H2Lx)
υ C H (aromatic): 3068, υ C H (aliphatic): 2920, 2850, υ HC N:
=
–
–
1622, υ C C and C N: 1579, 1510, 1440, 1384, υ C O: 1315, υ Cu-
N: 549, υ Cu-O: 420.
(x = 1–4)
[Zn(L3)], Calculated for C18 H14 Br4N2O2Zn: C 32.01, H 2.09, N
4.15%. Analysis found: C 32.14, H 2.05, N 4.03. FT-IR (KBr, cm−1):
Same procedure was used for the synthesis of salen-type Schiff
base ligands as reported earlier with some modifications [30].
2 mmol of 3,5-dihalosalicylaldehyde was dissolved in 20 mL of
ethanol in a round bottom flask (100 mL) to make a homogeneous
solution. To this, 1 mmol of ethanolic solution (15 mL) of respec-
tive diamine was added and the reaction mixture was refluxed for
1–3 h by putting the flask over the water bath. On cooling to the
room temperature the precipitates appeared which were filtered
and washed thrice with absolute ethanol and finally dried in a des-
iccator containing anhydrous calcium chloride.
–
–
=
υ C H (aromatic): 3051, υ C H (aliphatic): 2931, 2891, υ HC N:
=
–
–
1620, υ C C and C N: 1574, 1502, 1444, 1409, υ C O: 1315, υ Zn-
N: 551, υ Zn-O: 420. 1H NMR [d6-DMSO, δ (ppm)]: 8.34 (s, 2 H,
– =
H C N), 7.67 (d, 2 H, J = 2.6 Hz, H arom.), 7.41 (d, 2 H, J = 2.6 Hz,
H arom.), 3.36 (br, 4 H, CH2–N), 1.85 (br, 4 H, CH2–C).
[Ni(L4)], Calculated for C18 H14 I4N2NiO2: C 25.24, H 1.65, N
3.27%. Analysis found: C 25.13, H 1.67, N 3.18. FT-IR (KBr, cm−1):
–
–
=
υ C H (aromatic): 3053, υ C H (aliphatic): 2906, 2866, υ HC N:
=
–
–
1612, υ C C and C N: 1566, 1500, 1435, 1406, υ C O: 1319, υ Ni-
N: 530, υ Ni-O: 410.
[Cu(L4)], Calculated for C18 H14 CuI4N2O2: C 25.10, H 1.64, N
3.25%. Analysis found: C 25.25, H 1.61, N 3.16. FT-IR (KBr, cm−1):
2.3. General procedure for the synthesis of the complexes
Acetates of Ni(II), Cu(II) and Zn(II) were used to prepare the
aimed metal complexes (Scheme 1). Equimolar amounts of salen-
type Schiff base ligands (H2Lx) were mixed with respective salt of
metal acetates after complete dissolution in methanol and refluxed
for 3 h till precipitation occurs. Each solid product was separated
by filtration, washed by MeOH (30 mL) and after that kept under
CaCl2-dryness in a specific desiccators.
–
–
=
υ C H (aromatic): 3051, υ C H (aliphatic): 2922, 2850, υ HC N:
=
–
–
1622, υ C C and C N: 1565, 1544, 1435, 1404, υ C O: 1317, υ Cu-
N: 545, υ Cu-O: 414.
[Zn(L4)], Calculated for C18 H14 I4N2O2Zn: C 25.04, H 1.63, N
3.24%. Analysis found: C 24.93, H 1.67, N 3.13. FT-IR (KBr, cm−1):
–
–
=
υ C H (aromatic): 3035, υ C H (aliphatic): 2931, 2868, υ HC N:
=
–
–
1620, υ C C and C N: 1564, 1492, 1431, 1400, υ C O: 1313, υ Zn-
N: 555, υ Zn-O: 410. 1H NMR [d6-DMSO, δ (ppm)]: 8.27 (s, 2 H,
[Ni(L1)], Calculated for C16 H10Br4N2NiO2: C 30.00, H 1.57, N
4.37%. Analysis found: C 31.13, H 1.51, N 4.46. FT-IR (KBr, cm−1):
– =
H C N), 7.91 (d, 2 H, J = 2.4 Hz, H arom.), 7.53 (d, 2 H, J = 2.4 Hz,
–
–
=
υ C H (aromatic): 3066, υ C H (aliphatic): 2947, 2924, υ HC N:
H arom.), 3.56 (br, 4 H, CH2–N), 1.85 (br, 4 H, CH2–C).
–
–
1620, υ C=C and C N: 1581, 1506, 1442, 1373, υ C O: 1321, υ Ni-
N: 549, υ Ni-O: 410.
2.4. Crystallographic methods
[Cu(L1)], Calculated for C16 H10Br4CuN2O2: C 29.77, H 1.56, N
4.34%. Analysis found: C 29.64, H 1.49, N 4.21. FT-IR (KBr, cm−1):
For single crystal XRD inspection of ZnL1, NiL3 and NiL4
complexes, Bruker Kappa APEXII CCD X-ray diffractometer was
utilized possessing a graphite monochromated Mo-Kα radiation
–
–
=
υ C H (aromatic): 3062, υ C H (aliphatic): 2933, 2916, υ HC N:
–
–
1628, υ C=C and C N: 1577, 1504, 1433, 1377, υ C O: 1301, υ Cu-
˚
N: 547, υ Cu-O: 420.
(λ = 0.71073 A). Suitable single crystals were obtained from pyri-
[Zn(L1)], Calculated for C16 H10Br4N2O2Zn: C 29.69, H 1.56, N
4.33%. Analysis found: C 29.57, H 1.51, N 4.47. FT-IR (KBr, cm−1):
dine and DMF solution and fixed on a glass fiber for collecting
reflections on Bruker Apex-II software [31]. In order to solve raw
2

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
Scheme 1. Synthetic route of complexes.
data, SHELXS97 was employed [32] and then refined by utilizing
anisotropic displacement parameters for all non-hydrogen atoms.
International tables for X-ray Crystallography were utilized for
atomic factors [33]. All the H-atoms were added to the structure
from residual peaks and refined with relative isotropic displace-
ment parameters. For refinement purposes, SHELXL-2018/3 and
WinGX-2014.1 programs were utilized [34,35]. ω-scans method
was employed for data collection and integrated using Bruker
SAINT [36] software package. For pictorial view of ZnL1, NiL3 and
NiL4 ORTEP-3 [37] and platon were employed [38].
2.5. Biological activity evaluation
Evaluation of biological potential was carried out by in vitro
screening of the synthesized compounds against the panel of
two gram negative, Escherichia coli PTCC1394 and Pseudomonas
aeruginosa PTCC1074 and two gram positive, Staphylococcus aureus
PTCC1431 and Bacillus cereus PTCC1015 bacterial strains by adapt-
ing the disk diffusion method. This method is very convenient,
efficient and has very low cost as compared to other procedures
[39,40].
3

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
Fig. 1. ORTEP diagram of ZnL1 drawn at the probability level of 30%. H-atoms are displayed by small circle of arbitrary radii.
Fig. 2. ORTEP diagram of NiL3 drawn at the probability level of 30%. H-atoms are displayed by small circle of arbitrary radii. Only major parts of disordered moieties are
shown for clarity.
The most convenient solvent, dimethyl sulfoxide (DMSO), was
used to prepare the stock solutions of the synthesized compounds
because it has the ability to dissolve almost all kinds of transi-
tion metal complexes. Furthermore, it also acts a negative control
and shows zero activity against the tested bacterial strains. The
stock solutions were prepared by dissolving 20 mg of the sample
in 20 mL of DMSO (1 mg/mL). Then the dilution method was ap-
plied to attain the concentration of 50 μg/mL for the preparation
of laboratory samples. The population of bacterial strains was sub
cultured by using an agar as nutrition media. Erythromycin and
Ampicillin were chosen as standard antibacterial drugs to act as
positive control for the sake of comparison under the provision of
similar conditions.
cally in the petri dishes containing nutrient agar media inoculated
with the above mentioned four bacteria separately and then in-
cubated for 24 h at 37 °C. The diameters of zones of inhibitions
created by the samples were measured in mm after an incubation
period of 24 h.
3. Results and discussion
3.1. Syntheses
A series of four salen-type Schiff base ligands H2L1−H2L4 and
their respective nickel(II), copper(II) and zinc(II) complexes were
synthesized (Scheme 1). After synthesis various spectro analytical
techniques were employed to elucidate the structural features of
the synthesized compounds. It was found that all compounds were
non-hygroscopic, stable at room temperature and have bright col-
Paper discs, with a diameter of 7 mm, made up of Whatman
filter paper number 1 were sterilized before use in an autoclave.
Then these sterilized paper discs were thoroughly saturated with
the desired synthesized compounds or DMSO and placed asepti-
4

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
Fig. 3. ORTEP diagram of NiL4 drawn at the probability level of 30%. H-atoms are displayed by small circle of arbitrary radii. Only major parts of disordered moieties are
displayed for clarity.
Table 1
˚
to 2.086 (6) A. Bond angles in equatorial plane range from 76.5
Physical data of all complexes.
(3)° to 97.62 (2)° whereas the bond angles of apical atom (N2)
with the atoms of the equatorial plane range from 101.05 (2)° to
Compounds/molecular formula
Mol wt
Yield%
M. pt °C
Color
˚
NiL2/C16H10I4N2NiO2
NiL3/C18H14Br4N2NiO2
NiL4/C18H14I4N2NiO2
CuL1/C16H10Br4CuN2O2
CuL2/C16H10CuI4N2O2
CuL3/C18H14Br4CuN2O2
CuL4/C18H14CuI4N2O2
ZnL1/C16H10Br4N2O2Zn
ZnL2/C16H10I4N2O2Zn
ZnL3/C18H14Br4N2O2Zn
ZnL4/C18H14I4N2O2Zn
828.58
668.63
856.63
645.43
833.43
673.48
861.49
647.26
835.27
675.32
863.32
74
76
74
77
84
81
83
76
75
65
68
301
>400
261
295
304
318
263
351
291
341
257
Red
103.2 (2)°. Zn-atom is deviated by 0.48 A from the basal plane (N1/
Orange
N1i/O1/O1i) indicating that the base of the geometry is not a per-
fect square. Thus, a distorted square pyramidal geometry is formed
in ZnL1. In (C1-C7/C8A/N1/O1/Br1/Br2), the (E)−3-iminoprop-1-en-
1-ol group A (C1/C6/C7/N1/O1), 3,5-dibromobenznene ring B (C1-
C6/Br1/Br2) and pyridine ring C (C9-C11/C9i/C10i/N2) are found to
be planar with respective r.m.s deviation of 0.0100, 0.0252 and
Dark red
Green
Dark green
Brown
Orange
Light yellow
Yellow
˚
0.0007 A. The C-atom that is directly attached with the N-atom
Light yellow
Yellow
of moiety A is found to be disordered over two sets of sites with
occupancy of each part is 1/2. The dihedral angles among A/B, A/C
and B/C are 2.11 (2)°, 77.7 (2)° and 75.9 (2)°, respectively. The in-
vestigations of dihedral angles infer that group A and ring B are
ors. A brief summary of some physical and analytical data is pre-
sented in Table 1.
mainly parallel to each other. The molecules are interconnected
through C H O and C H…Br bonding to form one dimensional
infinite chain along [100] crystallographic direction, where CH is
from disordered C-atom which acts as donor (Fig. S1, Table 3). Be-
sides H-bonding, a weak π-π stacking interaction is found which
assists in further stabilization of crystal packing as displayed in Fig.
S2.
…
–
–
The tetradentate (ONNO) chelating nature of the ligands
(H2L1–4) was confirmed by FT-IR and 1H NMR spectroscopic data.
3.2. Crystal structure descriptions of complexes
In NiL3 (Fig. 2, Table 2), the central Ni-atom is coordi-
nated by two N & two O-atoms from a chelating ligand, and
two O-atoms from non-chelating ligands. In coordination sphere,
Ni1-N1 and Ni1-O1 bond length ranges from 2.046 (4) to
In ZnL1 (Fig. 1, Table 2), the central Zn-atom is coordinated
by one chelating ligand (C1-C7/C8A/N1/O1/Br1/Br2), one symmet-
rically generated chelating ligand (C1i-C7i/C8Ai/N1i/O1i/Br1i/Br2i))
(i) (x, 1/2-y, z) and one non-chelating ligand (C9-C11/C9i/C10i/N2)
containing two symmetry related C-atoms. Equatorial locations are
occupied by two N-atoms and two O-atoms of the chelating ligand
whereas apical location is occupied by N-atom (N2) of the pyridine
ring. The bond lengths in coordination sphere range from 1.983 (4)
˚
˚
2.038 (4) A and 1.986 (4) to 2.155 (4) A, respectively, whereas
bond angles range from 83.18 (2)° to 96.44 (2)°. Bond angles
and bond lengths are such that
a slightly distorted octahe-
dral geometry is formed. The (E)−3-iminoprop-1-en-1-ol group
5

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
Fig. 4. 1H NMR spectra of the H2L1 and H2L2 (Bottom) ligands in CDCl3 and their ZnL1 and ZnL2 (Top) complexes in DMSO.
A (C1/C6/C7/N1/O1), 3,5-dibromobenznene ring B (C1-C6/Br1/Br2),
basal plane C (C8/C10/C11/N1) of butane-1,4-diamine moiety (C8-
C11/N1/N2), (E)−3-iminoprop-1-en-1-ol group D (C12-C14/N2/O2)
and 3,5-dibromobenznene ring E (C13-C18/Br3/Br4) are found to
be planar with respective r.m.s deviation of 0.0307, 0.0151, 0.0555,
4.4 (2)°, respectively. The investigation of dihedral angles shows
that group A and ring B are mainly parallel to each other. Vari-
ous weak intramolecular H-bonding of type C H…O are found that
–
assist in the stable configuration of the crystal structure. Various S
(5) loops are found with the distance between donor hydrogen and
˚
˚
0.0189 and 0.0208 A. The apical (C9/N2) atoms are deviated by
acceptor oxygen ranges from 2.35 to 2.52 A as displayed in Fig. S3
˚
−0.6789 (7), 1.157 (6) A, respectively below from the basal plane
and specified in Table 3. The molecules are interconnected by weak
…
–
B. The dimethylformamide moiety of the first non-chelating lig-
and (C19-C21/N3/O3) is disordered with occupancy ratio of 0.608
(1): 0.392(1). The major part F (C19A-C21A/N3A) and the minor
part G (C19B-C21B/N3B) are found to be planar with respective
C H π interaction to form infinite chains along [010] direction as
displayed in Fig. S4.
In NiL4 (Fig. 3, Table 2), the central Ni-atom is coordinated by
two N and two O-atoms from a chelating ligand and two O-atoms
from non-chelating ligands. In coordination sphere, Ni1-N1 and
˚
r.m.s deviation of 0.0512 and 0.0312 A, the dihedral angle of 23.7
˚
(4)° is found between F and G moieties. The dimethylformamide
moiety of the second non-chelating ligand (C22-C24/N4/O4) is also
disordered with occupancy ratio of 0.608 (1): 0.392(1). The major
part H (C22A-C24A/N4A) and the minor part I (C22B-C24B/N4B)
are found to be planar with respective r.m.s deviation of 0.0025
Ni1-O1 bond length ranges from 2.038 (4) to 2.050 (4) A and
˚
1.985 (4) to 2.271 (3) A, respectively whereas bond angles range
from 83.13 (2)° to 97.27 (2)°. Bond angles and bond lengths are
such that
a slightly distorted octahedral geometry is formed.
The (E)−3-iminoprop-1-en-1-ol group A (C1/C6/C7/N1/O1), 3,5-
diiodobenznene ring B (C1-C6/I1/I2), basal plane C (C8/C11/C12/N1)
of butane-1,4-diamine moiety (C8-C11/N1/N2), (E)−3-iminoprop-
1-en-1-ol group D (C12/C13/C18/N2/O2) and 3,5-diiodobenznene
ring E (C13-C18/I3/I4) are found to be planar with respective r.m.s
˚
and 0.0583 A, the dihedral angle of 41.2 (2)° is found among
H and I moieties. Ring Puckering Analysis shows that the first
chelating ring (C1/C6/C7/N1/Ni1) adapts nearly half chair confor-
˚
mation with puckering amplitude Q = 0.314(3) A, θ = 59.7(7)°
˚
and ϕ = 14.4 (10)° whereas the second chelating ring (C12-
deviation of 0.0347, 0.0155, 0.0866, 0.0302 and 0.0342 A. The
˚
C14/N2/O2/Ni1) adapts nearly S-form with puckering amplitude
apical (C11) atom is deviated by 0.4709 (9) A from the basal plane
˚
Q = 0.090(4) A, θ = 63(3)° and ϕ = 329(3)°. The dihedral angles
B. In the first non-chelating ligand (C19/C20A/C21A/N3/O3), the
terminal C-atoms are disordered with occupancy ratio 0.56(3):
among A/B, A/C, C/D and D/E are 1.6 (2)°, 48.7 (2)°, 57.4 (2)° and
6

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
Table 2
Crystal data and structure refinement parameters for ZnL1, NiL3 and NiL4 complexes.
Identification code
ZnL1
NiL3
NiL4
Chemical formula
Formula weight
Temperature (K)
Radiation type
C
21H15Br4N3O2Zn
C24H28Br4N4NiO4
814.85
C26.25H33.25I4N4.75NiO4.75
726.37
296
1057.63
296
296
Mo Kα
0.71073
Orthorhombic
Pnma
Mo Kα
Mo Kα
0.71073
Triclinic
˚
Wavelength (A)
0.71073
Crystal system
Triclinic
¯
P1
¯
P1
Space group
˚
a (A)
7.3940(9)
23.504(4)
13.083(2)
90
9.5133(5)
12.5026(7)
15.2114(9)
110.469(3)
93.407(3)
103.105(3)
1631.69(16)
2
9.6767(5)
12.7208(8)
16.1316(10)
68.776(2)
89.955(2)
76.598(2)
1793.05(19)
2
˚
b (A)
˚
c (A)
α (°)
β (°)
γ (°)
90
90
3
˚
Volume (A )
2273.8(6)
4
Z
Calculated Density (Mg/m3)
Absorption coefficient (mm−1
F(000)
2.122
1.659
1.959
)
8.134
5.525
4.022
1392
800
1004
Crystal Shape
Needle
Light yellow
0.28 × 0.16 × 0.16
Needle
Prism
Crystal color
Green blue
0.42 × 0.22 × 0.20
Light yellow
0.30 × 0.25 × 0.20
Crystal size (mm)
Data Collection
Diffractometer
Bruker KAPPA APEXII CCD
Diffractometer
Bruker KAPPA APEXII CCD
Diffractometer
Bruker KAPPA
APEXII CCD
diffractometer
multi-scan
Absorption correction
multi-scan
multi-scan
(SADABS;
(SADABS;
(SADABS;
Bruker, 2007)
19,052, 2547, 1193
Bruker, 2007)
24,576, 7089, 4099
Bruker, 2007)
27,356, 7823, 5890
No. of measured, independent
and observed [I> 2s(I)] reflections
Rint
0.089
0.038
0.030
Theta range for data collection
Index ranges
3.031° to 27.000°
−9 ≤ h ≤ 9, −30≤ k ≤ 30,
−16≤ l ≤ 16
0.639
2.224° to 27.142°
−11≤ h ≤ 12, −16≤ k ≤ 15,
−20≤ l ≤ 20
0.642
1.773° to 27.107°
−12≤ h ≤ 12, −16≤ k ≤ 16,
−20≤ l ≤ 20
0.641
−1
˚
(sin θ/λ)max (A
)
Refinement
R[F2> 2σ(F2)], wR(F2), S
No. of reflections
No. of parameters
No. of restraints
0.046, 0.106, 1.00
0.048, 0.131, 1.02
0.045, 0.122, 1.04
2547
7089
7823
155
415
424
10
234
175
H-atom treatment
H-atom parameters constrained
H-atom parameters constrained
H-atom parameters constrained
−3
˚
ꢀρmax, ꢀρmin (e A
)
0.51, −0.68
1.03, −0.97
1.62, −1.84
3.3. lH NMR spectra
0.44(3). All the atoms of the second non-chelating ligand are
disordered with occupancy ratio 0.56(3): 0.44(3). The dihedral
angles among A/B, A/C, C/D and D/E are 1.76 (3)°, 62.95 (2)°,
17.3 (3)° and 6.33 (2)°, respectively. The investigation of dihedral
angle shows that group A and ring B are almost parallel to each
other; similarly, moiety D and ring E are also almost parallel
to each other. In the asymmetric unit, 75% solvent molecule of
dimethylformamide is there which is also disordered over two
set of locations with occupancy ratio of 0.417(3): 0.333(3) re-
fined by making anisotropic displacement parameters of all the
disordered atoms equal. Various weak intramolecular H-bonding
The 1H NMR spectra of the synthesized salen type Schiff bases
were taken in deuterated chloroform, while deuterated dimethyl
sulfoxide (DMSO–d6) was used to record the spectra of zinc com-
plexes (ZnL1-ZnL4). The details of chemical shift values of the
tested compounds are presented in experimental section. One of
the important chemical shift (δ) values in the form of singlets
present at 14.15, 14.39, 14.74 and 14.92 ppm in the 1H NMR spectra
of the ligands H2L1, H2L2, H2L3 and H2L4 respectively, confirm the
presence of aromatic OH. These values appeared very down field
in the recorded spectra due to the involvement of phenolic pro-
tons in hydrogen bonding. These phenolic peaks vanished on the
coordination of ligands with the zinc metals as evident from Figs.
4 and 5.
of type C H…O are found that assist in the stable configuration
–
of the crystal structure. Various S (5) loops are found with the
distances between donor hydrogen and acceptor oxygen ranges
˚
from 2.41 to 2.66 A as displayed in Fig. S5 and specified in Table 3.
The Bis(dimethylformamide)-(N,N’-bis(3,5-diiodosalicylidene)−1,4-
Another key evidence of complexation of ligands with metals
is the slight down field shifts of signals of azomethine protons at
8.45, 8.37, 8.34 and 8.27 ppm for ZnL1, ZnL2, ZnL3 and ZnL4 com-
plexes, respectively. Hence, phenolic oxygen and azomethine ni-
trogen are the binding sites available for coordination with zinc
metal. The protons of aromatic rings of Zn(II) complexes appeared
in the range of 7.41–7.93 ppm in the form of doublets. The –CH2–N
protons of the ZnL1 and ZnL2 complexes appeared as a singlet at
δ = 3.75 and 3.73 ppm, respectively. The broad signals at δ = 3.36,
diaminobutane)-nickel(II) molecules are jointed with solvent
molecules through weak C H…O bonding as displayed in Fig. S6
–
and specified in Table 2. Furthermore, Bis(dimethylformamide)-
(N,N’-bis(3,5-diiodosalicylidene)−1,4-diaminobutane)-nickel(II)
molecules are jointed among themselves through weak C H…N
–
and C H…I bonding as given in Table 3. Another weak interaction
–
…
–
of type C H π is found which assists in further strengthening of
crystal packing as displayed in Fig. S7.
7

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
Fig. 5. 1H NMR spectra of the H2L3 and H2L4 (Bottom) ligands in CDCl3 and their ZnL3 and ZnL4 (Top) complexes in DMSO.
3.56 and 1.85 ppm are assigned to the –CH2–N and –CH2–C pro-
tons of the ZnL3 and ZnL4 complexes, respectively.
Fig. 8 in the form of histogram. A careful observation of the results
indicated that tested compounds were active against the screened
microbial species. The tested compounds bear comparable activ-
ities against parent standard drugs, Erythromycin and Ampicillin.
The results suggested, a little bit, enhanced antibacterial activity of
complexes in comparison with the free ligands. This amplification
in the activities against bacterial pathogens can be assumed on the
basis of Overtone’s concept and Tweedy’s chelation theory [41].
The decrease in the number of active bacterial cells on the treat-
ment with these compounds may be attributed on the basis of cell
permeability and lipophilicity. The ligands were found to be active
probably due to the presence of hydroxyl function and azomethine
chromophore which may lead to the formation of hydrogen bond-
ing with active centers of the cells constituting the cell membrane
and hence the permeability was increased [42]. A slight increase
in the activity of the ligands on chelation might be due to change
in the polarity of cation on account of hybridization of filled or-
bitals of ligands with empty “d” orbitals of the metals [43,44]. The
increased lipophilicity can be addressed owing to the delocaliza-
tion of π electrons on chelation which covers the whole chelate
ring which leads to the facilitated perforation through lipid bilayer
membrane to destroy the pathogens [45]. Another concept is that
these metal complexes can also affect the process of respiration of
3.4. FT-IR spectra
The comparison between the selected bands in the FT-IR spec-
tra of Schiff base ligands and their metal complexes are presented
in experimental part and shown in Figs. 6 and 7. In the FT-IR spec-
=
tra of complexes, the ν (-CH N) and ν (C–O) bands shifted to
lower and higher wavenumbers, respectively, in comparison with
their corresponding free ligands thereby indicating a coordina-
tive interaction between the iminic nitrogen and phenolic oxygen
atoms with central metals. This coordination could also be con-
firmed by the appearance of weak bands located at the lower
wavenumbers which are assigned to (M–N) and (M–O) at about
530 to 574 cm−1 and 409 to 432 cm−1 respectively.
3.5. Antibacterial activity
The antibacterial action of the synthesized compounds and
standard drugs was checked by screening against four species of
bacterial strains and the data is summarized in Table 4 and in
8

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
Fig. 6. FT-IR spectra of the H2L1 and H2L2 ligands and their corresponding complexes.
Table 4
Table 3
Antibacterial activity of Schiff base ligands and its complexes.
1
3
Hydrogen-bond geometry (A, °) for ZnL , NiL and NiL4 complexes.
˚
Diameter of zone of inhibition (mm)
ZnL1
D—H···A
D—H
H···A
D···A
D—H···A
Compound
E. coli
P. aeruginosa
S. aureus
B. cereus
C8A—H8A1···Br1i
C8B—H8B2···O1ii
C9—H9···Br2iii
C10—H10···Br1iv
D—H···A
0.97
0.97
0.93
0.93
D—H
0.93
0.96
0.97
0.93
0.96
C—H
0.96
0.96
0.96
D—H
0.93
0.97
0.93
0.96
0.96
0.96
0.93
0.93
0.96
0.93
0.96
0.93
0.96
C—H
0.96
0.96
2.63
2.56
2.98
3.08
H···A
2.52
2.39
2.46
2.44
2.35
H···π
2.77
2.91
2.96
H···A
2.58
2.47
2.41
3.18
3.33
2.68
2.66
1.98
2.65
3.08
3.22
3.18
2.00
H···π
2.84
2.55
3.432 (17)
3.505 (16)
3.633 (7)
3.738 (7)
D···A
140
H2L1
NiL1
CuL1
ZnL1
H2L2
NiL2
CuL2
ZnL2
H2L3
NiL3
CuL3
ZnL3
H2L4
NiL4
CuL4
ZnL4
12
14
13
14
12
14
14
13
15
17
16
16
11
13
13
12
–
12
13
14
14
12
13
13
15
13
15
14
15
12
13
13
13
–
22
24
23
24
26
27
27
29
21
22
23
24
25
26
27
27
–
19
22
21
23
22
24
25
26
23
25
26
25
22
24
24
25
–
166
129
129
NiL3
D—H···A
119
C19A—H19A···O1
C20A—H20C···O3
C8—H8B···O4
3.07(2)
2.780 (12)
3.112 (7)
2.87 (2)
2.764 (12)
C···π
104
124
C22A—H22A···O2
C24A—H24A···O4
C—H···π
108
105
C—H···π
54
3.603(18)
3.373 (15)
3.373 (15)
D···A
39
31
NiL4
D—H···A
D—H···A
139
C7—H7···O5Bvii
C8—H8B···O3
3.43 (3)
3.123 (7)
2.886 (7)
3.94 (2)
4.28 (4)
3.52 (5)
3.14 (2)
2.63 (3)
3.40 (4)
3.91 (4)
4.00 (3)
3.98 (4)
2.56 (5)
C···π
125
Control (DMSO)
Standard (Erythromycin)
Standard (Ampicillin)
C19—H19···O2
112
16
11
17
14
25
27
24
26
C20A—H20B···I4i
C20B—H20E···I4i
C20B—H20F···N3viii
C22A—H22A···O1
C22B—H22B···O2
C23B—H23D···O5Bviii
C25A—H25A···I2ix
C26A—H26B···I2ix
C25B—H25B···I1x
C27B—H27F···I5A
C—H···π
137
170
147
113
126
135
150
cells, leading to the blockage in the synthesis of proteins causing
the death of organisms [46,47].
139
146
115
It is worth noting from the zones of inhibition, created by
tested compounds, that ligands and their respective metal com-
plexes are moderately more potent inhibitors of gram positive
rather than gram negative bacterial strains. This is because gram
positive bacteria have thick peptidoglycan layer without outer lipid
membrane while gram negative bacteria have thin peptidoglycan
layer and have an outer lipid membrane [48].
C—H···π
159
C24A—H24A···Cg1vi
3.76(3)
C20A—H20A···Cg2vi
3.42(3)
150
Symmetry codes: (i)
x
+
1, y, z; (ii)
x
+
1/2, y, −z
+
3/2;
(iii) −x + 1, −y + 1, −z + 1; (iv) x + 1/2, y, −z + 1/2: (v) –x,1-y,2-z; (vi) –x,-
y,1-z; (vii) x, y − 1, z + 1; (viii) −x, −y + 1, −z + 1; (ix) −x + 1, −y, −z + 1;
(x) x + 1, y + 1, z − 1.
9

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
Fig. 7. FT-IR spectra of the H2L3 and H2L4 ligands and their corresponding complexes.
Fig. 8. A histogram show the antibacterial evaluation of the investigated Schiff base ligands and its complexes.
4. Conclusion
corresponding metal complexes showed that the all compounds
were more toxic towards gram positive strains than gram negative
strains.
In the current research work, we have synthesized and charac-
terized some new mononuclear metal complexes containing ONNO
donor salen-type Schiff base ligands. The synthesized complexes
were characterized by using various physicochemical methods. Fur-
thermore, the molecular structures of certain complexes have been
confirmed by single crystal X-ray diffraction. The results showed
that the ligand to metal stoichiometry in complexes is 1:1. A dis-
torted square pyramidal geometry for ZnL1 and slightly distorted
octahedral geometries for NiL3 and NiL4 were proposed for Schiff
base metal complexes. The in vitro biological screening experi-
ments inferred that the compounds showed satisfactory inhibitory
effects against most of the tested microbes and metal complexes
were found to be faintly more active than the free Schiff base
ligands. Also, the inhibition zones of the Schiff base ligands and
Appendix A. Supplementary data
CCDC No. 2036048 (for ZnL1), CCDC No. 2036049 (for NiL3) and
CCDC No. 2039058 (for NiL4) contain the supplementary crystal-
lographic data for this contribution. Copies of the data may be
obtained free of charges on application to CCDC, 12 Union Road,
Cambridge CB2 1 EB, UK (Fax: +44–1223–336–033; E-mail: de-
posit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Authors’ statement
We would like to make a statement about the author’s main
contribution: HK and AAA synthesized all the compounds, and
characterized them by different techniques; AAA was responsible
10

H. Kargar, A.A. Ardakani, M.N. Tahir et al.
Journal of Molecular Structure 1233 (2021) 130112
for biological data acquisition and analysis; MNT, MA and KSM
collected the single-crystal X-ray diffraction data and determined
the structures. HK wrote the manuscript with input from all au-
thors. All authors discussed the results and commented on the
manuscript.
[17] R.P. Kashyap, J.M. Bindlish, P.C. Jain, Crystallographic data of
bis(N-allylsalicylaldiminato) copper (II) monohydrate, Indian J. Chem. 11
(1973) 388–389.
[18] J.M. Bindlish, S.C. Bhatia, P. Gautam, P.C. Jain, Crystal and molecular structure
of bis(N-allylsalicylaldiminato)-palladium(II), Indian J. Chem., Sect. A16 (1978)
279–282.
[19] H. Kargar, R. Kia, A.A. Ardakani, M.N. Tahir, Bis(dimethylformamide-
ꢀ
ꢀ
κO){4,4 ,6,6 -tetrachloro-2,2-[butane-1,4-diyl(nitrilomethanylylidene)]
ꢀ
ꢀ
diphenolatoκ4O,N,N ,O }nickel(II), Acta Crystallogr. E68 (2012) m997.
Declaration of Competing Interest
The authors declare that they have no conflict of interest.
Acknowledgment
ꢀ
ꢀ
ꢀ
[20] H. Kargar, R. Kia, A.A. Ardakani, M.N. Tahir, {4,4 ,6,6 -Tetraiodo-2,2 -
[propane-1,3-diylbis(nitrilomethanylylidene)]-diphenolato-κ4O,N,N ,O }nickel(II),
ꢀ
ꢀ
Acta Crystallogr. E68 (2012) m1090.
[21] H. Kargar, A.A. Ardakani, M.N. Tahir, M. Ashfaq, K.S. Munawar, Synthesis, spec-
tral characterization, crystal structure determination and antimicrobial activity
of Ni(II), Cu(II) and Zn(II) complexes with the Schiff base ligand derived from
3,5-dibromosalicylaldehyde, J. Mol. Struct. 1229 (2021) 129842.
The support of this work by Ardakan University is gratefully ac-
knowledged.
[22] R.N. Patel, S.P. Rawat, M. Choudhary, V.P. Sondhiya, D.K. Patel, K.K. Shukla,
D.K. Patel, Y. Singh, R. Pandey, Synthesis, structure and biologi-
cal activities of mixed ligand copper(II) and nickel(II) complexes of
ꢀ
N -(1E)-[(5-bromo-2-hydroxyphenyl)methylidene]benzoylhydrazone,
Inorg,
Supplementary materials
Chimica Acta 392 (2012) 283–291.
[23] H. Kargar, R. Behjatmanesh-Ardakani, V. Torabi, M. Kashani, Z. Chavoshpour–
Natanzi, Z. Kazemi, V. Mirkhani, A. Sahraei, M.N. Tahir, M. Ashfaq, K.S. Mu-
nawar, Synthesis, characterization, crystal structures, DFT, TD-DFT, molecular
docking and DNA binding studies of novel copper (II) and zinc (II) complexes
bearing halogenated bidentate N,O-donor Schiff base ligands, Polyhedron 195
(2021) 114988.
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130112.
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