Optical, electrochemical, thermal, biological and theoretical studies of some chloro and bromo based metal-salophen complexes

Article abstract of DOI:10.1016/j.molstruc.2019.127107

Copper, cobalt and zinc-salophen complexes 3–8 were prepared from the reaction of N,N-bis(salicylidene)4-chloro-1,2-phenylendiamine (1) or N,N-bis(salicylidene)4-bromo-1,2-phenylendiamine (2) with their respective acetate salts. All compounds were charact

Full text of DOI:10.1016/j.molstruc.2019.127107

Journal of Molecular Structure 1200 (2020) 127107
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Optical, electrochemical, thermal, biological and theoretical studies of
some chloro and bromo based metal-salophen complexes
,
Zohreh Shaghaghi a *, Nafis Kalantari a, Mina Kheyrollahpoor a, Mehri Haeili b
a Coordination Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Azarbaijan Shahid Madani University, P.O. box 83714-161,
Tabriz, Iran
b Department of Biology, Faculty of Natural Science, University of Tabriz, P. O. box 5166616471, Tabriz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper, cobalt and zinc-salophen complexes 3e8 were prepared from the reaction of N,N-bis(salicyli-
dene)4-chloro-1,2-phenylendiamine (1) or N,N-bis(salicylidene)4-bromo-1,2-phenylendiamine (2) with
their respective acetate salts. All compounds were characterized with common techniques. In additional,
optical, electrochemical, thermal and antibacterial properties of ligands and their complexes were
studied. Schiff base ligands 1 and 2 showed the intense emission band at 476 nm in the fluorescence
spectra. The emission process of these ligands is related to the excited state intramolecular proton
Received 30 September 2018
Received in revised form
7 September 2019
Accepted 20 September 2019
Available online 21 September 2019
transfer phenomenon (and also
p/p* transitions). In general, after the incorporation of copper, cobalt
Keywords:
and zinc metal ions into the structure of ligands 1 and 2, the emission intensity of the ligands totally
quenched. It can be attributed to the changes in the conformational rigidity of the structure of ligands
upon complexation. Thermal stability of complexes was proved by TGA analysis. The cyclic voltam-
mogrames of Cu (II) complexes displayed quasi-reversible one electron Cu(II)/Cu(I) redox process, while
those of their cobalt counterparts exhibited two reversible redox couples of Co(II)/Co(I) and Co(III)/Co(II).
The investigation of antibacterial activity against Staphylococcus aureus, Escherichia coli, Pseudomonas
aeruginosa and carbapenem &colistin resistant Klebsiella pneumoniae showed both Co(II) complexes 4
and 7 have remarkable antibacterial activity. The antibacterial effect of cobalt complexes against
K. pneumoniae is of high interest as the bacterium, is resistant to two most important therapeutic options
for treatment of multi-drug resistant Gram-negative bacteria, including polymyxins and carbapenems.
Moreover, DFT investigations were performed to obtain a better understanding of structure of com-
pounds 1e8. The values of band gap energies revealed ability of all compounds for using in optical
devices. Finally, the electronic spectra of compounds are analyzed by TD-DFT calculations at the B3LYP/6-
311G level for Schiff base ligands and the B3LYP/LANL2DZ level for complexes.
Metal-salophen complexes
DFT calculations
Antibacterial activity
Fluorescence spectroscopy
Cyclic voltammetry
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
these type ligands produce neutral metal complexes when coor-
dinate with divalent transition metal ions [3]. These complexes
Salenes and salophens are one old and common class of diimine
tetradendate ligands in coordination chemistry [1,2]. These ligands
are easily prepared from condensation reaction of ethylenediamine
and 1,2-phenylenediamine or their derivatives with salicylalde-
hyde. Salophen metal complexes are formed by the reaction of both
main and transition metal ions. The preparation method of theses
complexes is easy and simple. In general, the ligand with one N2O2
core coordinates to a metal ion in a deprotonated form. Therefore,
show extensive applications in several fields. Some of these com-
plexes show substantial catalytic activity in a variety of reactions
such as oxidation, epoxidation, hydrogenation, hydroxylation and
cycloaddition reaction of CO2 to propylene oxide [4e11]. A variety
of them exhibit special optical properties [12]. They also found
applications as electroluminescent materials [13,14] and biological
activities such as antifungal, antibacterial, antimicrobial and
interaction with DNA and RNA [15e20]. Metal salophen complexes
show a tendency for square-planer geometry. They act as the bridge
between the two imine groups. Therefore, two axial positions are
available for coordinating solvent molecules or other donating
groups. This makes them suitable for interaction with anions.
* Corresponding author.
E-mail address: shaghaghi@azaruniv.ac.ir (Z. Shaghaghi).
https://doi.org/10.1016/j.molstruc.2019.127107
0022-2860/© 2019 Elsevier B.V. All rights reserved.

2
Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
Anions as Lewis acid can donate one lone pair electron to the metal
center. So, these compounds can be used as anion receptors
[21e23].
complexes 3e8 in optical devices, the band gaps have been calcu-
lated at the DFT level, Finally, the obtained results are correlated
with the experimental data.
Salen and salophen complexes with non-redox active zinc(II)
metal ion can behave as interesting binders towards DNA structures
[24,25]. In some cases, the metal center displays five coordinate
square-pyramidal geometry. Salophens occupy the basal plane
while a solvent molecule occupies the apical position. The zinc
salophen complexes are known to be fluorescent and can be used as
chemosensors for anions or selective receptors for other donating
groups such as amines [26e29]. Also, the lack of redox activity of
theses complexes produces species with low cytotoxicity. This
makes suitable them for using as biological fluorescence probes
[16].
Coordination cobalt complexes with salen or salophen type li-
gands have a low-spin configuration with a square planar donor
atom symmetry. It has been proved that these complexes can act as
catalysts for electroreduction of oxygen that is an important
cathodic reaction in fuel cells and light-driven water oxidation
[30,31]. Recently, the electrocatalytic activity of one bioinspired
ionic liquid tagged Co-salophen metal complex has been demon-
strated towards oxidation of glucose by Senthilkumar and co-
workers [32]. Also, Khoshro et al. [33] have been investigated the
electrochemical carboxylation of benzyl bromide by some cobalt
(II) salophen type complexes. They found that the electrocatalytic
reduction mechanism of benzyl bromide is depended on the elec-
tronic structure of the complexes.
Copper salophen complexes behaves as on-off light switch by
multi-fold fluorescence enhancement or quenching upon satura-
tion, are excellent DNA interacting system [34,35].
Density functional theory is a useful tool for studying the
structural and electronical properties of metal complexes with
organic ligands. The geometry optimization, electronic structures
and theoretical assignments of the UV/Vis spectra of some salen
and salophen complexes have been performed using DFT and TD-
DFT methods in the literature. For example, Cisterna et al. [36]
have been investigated electrochemical and theoretical properties
2. Experimental
2.1. General procedures
All of solvents and materials were used without further purifi-
cations. NMR spectra were recorded on a Bruker Avance DPX-400
MHz spectrometer. FT-IR spectra were prepared with a FT-IR
Spectrometer Bruker Tensor 27 after mixing the samples with
KBr. Electronic absorption spectra were obtained with T 60 UV/vis
Spectrometer PG Instruments Ltd. Fluorescence spectra were car-
ried out using a FP-6200 spectrofluorometer (JASCO Corporation,
Tokyo, Japan, http://www.jasco.co.jp). Cyclic valtammograms were
recorded using an AUTOLAB PGSTAT-100 (potentiostat/galvano-
stat). Thermogravimetric analysis (TGA) were done by a Perkine-
lemersta 6000. Finally, C.H.N analyses were performed on
ElementarVario ELIII.
2.2. General method for the preparation of salophen ligands
4-chloro-1,2-diaminobenzene (1.00 g, 7.00 mmol) or 4-bromo-
1,2-diaminobenzene (0.500 g, 3.00 mmol) in 30 ml ethanol was
slowly added to one ethanol solution of salicylaldehyde (1.49 ml,
14.0 mmol for chloro substitution and 0.568 ml, 6.00 mmol for
bromo substitution) under stirring in room temperature. The so-
lution turned to orange-brown and precipitate was appeared. The
reaction continued for 4 h at room temperature. The precipitate
was filtered and washed with ethanol and recrystallized in ethanol
and then dried [39,40].
2.2.1. Synthesis of N,N-bis(salicylidene)4-chloro-1,2-
phenylendiamine (1)
1H NMR (400 MHz, DMSO‑d6):
d 12.77 (s, 1H, eOH),12.69 (s, 1H,-
OH), 8.99 (s, 1H, eHC]N-), 8.95 (s, 1H, eHC]N-), 7.69 (t, 2H,
J ¼ 5.6 Hz, AreH), 7.63 (d, 1H, J ¼ 2 HZ, AreH), 7.52 (d, 1H, J ¼ 8.8 Hz,
ArH), 7.47 (d, J ¼ 2.0 Hz,1H, ArH), 7.45e7.42 (m, 2H, ArH), 7.02e6.97
(m, 4H, ArH); FT-IR (KBr, cmꢀ1), 3447, 1614, 1578, 1478, 1276, 1224,
1190, 1151,1115, 1085, 920, 866, 81, 752, 697, 652, 591, 503, 437;
elem. Anal.: calc. for C20H15N2O2Cl.0.5C2H5OH: C, 67.47; N, 7.50; H,
4.81; found: C, 67.50; N, 7.90; H, 4.65; m.p: 138-141 ꢁC; Yield:
67.96%.
of
a series of neutral Ni(II) and copper(II) complexes with
unsymmetrically-substituted N2O2-tetradentate Schiff-base li-
gands. They analyzed the electronic structures of the complexes
using DFT and TD-DFT calculations. Consiglio and coworkers have
been reported a computational study of the dimerization process of
an amphiphilic Schiff-base bis (salicyladiminato)zinc(II) complex.
They performed a comparative investigation between experi-
mental and calculated 1H NMR and UVeVis spectra to evaluate the
percentage contribute of each conformer [37]. Recently, the struc-
ture of a new mononuclear Co(II) salophen-type complex and
optimizing of optimized of the molecular geometry in the ground
state by DFT/(U)PBE0/Def2-TZVP level of theory have been reported
by Zarei [38].
In view of the above mentioned factors, the purpose of the
present work is to prepare and investigate some important struc-
tural properties of a series of salophen type complexes with
different substitutions for their ability to be used in several fields
such as precursors for preparation of metal oxide nanostructures,
photovoltaic, electrocatalytic (water oxidation) and catalytic ap-
plications for future works. So, we report on synthesis and spectral
characterization of two salophen-type ligands 1 and 2 resulting
from condensation reaction of salicylaldehyde and 4-chloro-1,2-
diaminobenzen or 4-bromo-1,2-diaminobenzen, respectively, and
on Cu(II) complexes 3 [39] and 6, Co(II) complexes 4 [39] and 7 and
Zn(II) complexes 5 and 8 by treatment ligands 1 and 2 with copper,
cobalt or zinc acetate salts, respectively. The optical, redox, thermal
and antibacterial behavior of all ligands and complexes have been
investigated. In additional, to investigate the potential use of
2.2.2. Synthesis of N,N-bis(salicylidene)4-bromo-1,2-
phenylendiamine (2)
1H NMR (400 MHz, DMSO‑d6):
d 12.77 (s, 1H, eOH),12.73 (s, 1H,-
OH), 8.99 (s, 1H, eHC]N-), 8.95 (s, 1H, eHC]N-), 7.74 (d, 1H,
J ¼ 5.6 Hz, AreH), 7.69 (t, 2H, J ¼ 6.0 Hz, AreH), 7.59 (dd, 1H,
J ¼ 1.6 Hz, ArH), 7.46e7.42 (m, 3H, ArH), 7.01e6.97 (m, 4H, ArH); FT-
IR (KBr, cmꢀ1), 3446, 1614, 1577, 1475, 1458, 1384, 1277, 1191,
1151,1114, 913, 886, 837, 760, 647, 503; elem. Anal.: calc. for
C
20H15N2O2Br.0.25C2H5OH.0.5H2O: C, 59.20; N, 6.74; H, 4.21;
found: C, 59.49; N, 7.13; H, 4.63; m.p: 158-162 ꢁC; Yield: 63.52%.
2.3. General method for the preparation of complexes 3e8
To a stirring dichloromethane solution of ligand 1 (0.710 mmol)
in 15 ml or 2 (0.630 mmol) in 10 ml, one ethanol solution of
Cu(CH3COO)2.1H2O, Co(CH3COO)2.4H2O or Zn(CH3COO)2.2H2O
(0.710 or 0.630 mmol) in 15 ml was slowly added at room tem-
perature, Immediately, the color of solution turned and precipitate
appeared. The reaction continued at room temperature for 4 h and
then the precipitate was filtered and washed with ethanol. The

Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
3
Scheme 1. The route of synthesis of salophen type ligands 1 and 2 and their Cu, Co and Zn complexes (3e8).
Table 1
Molar conductivity values for ligands and complexes (10ꢀ3 M in DMSO).
Compound
Molar conductivity (Cm2 ꢀ1molꢀ1
U )
DMSO
1.475
1.736
1.764
1.767
3.60
1.520
1.538
3.14
1
2
3
4
5
6
7
8
1.627
Fig. 2. d/d transitions of Cu complexes 3 and 6 (10ꢀ2 M in DMSO).
2.3.1. Cu(II) complex 3
FT-IR (KBr, cmꢀ1), 3423, 1608, 1578, 1519, 1485, 1460, 1373, 1338,
1241, 1188, 1150, 1130, 1093, 938.901, 849, 803, 753, 548, 434; elem.
Anal.: calc. for CuC20H13N2O2Cl.0.25H2O: C, 57.62; N, 6.72; H, 3.24.;
found: C, 57.47; N, 7.14; H, 3.65; Decomp: 300 ꢁC; Yield: 64.10%.
2.3.2. Co(II) complex 4
FT-IR (KBr, cmꢀ1), 3446, 1619, 1577, 1542, 1489, 1445, 1384, 1306,
1192, 1158, 1132, 1038, 948.901, 870, 814, 756, 696, 598, 567, 470;
elem. Anal.: calc. for CoC20H13N2O2Cl.2H2O.1.5 CH2Cl2: C, 45.18; N,
4.90; H, 3.50.; found: C, 45.31; N, 4.61; H, 3.52; Decomp: 279 ꢁC;
Yield: 68.96%.
2.3.3. Zn(II) complex 5
1H NMR (400 MHz, DMSO‑d6):
d 9.07 (s, 1H, eHC]N-), 9.040 (s,
1H, eHC]N-), 8.064 (d, 1H, J ¼ 2 Hz, ArH), 7.95 (s, 1H, J ¼ 9.2 Hz,
ArH), 7.46e7.42 (m, 3H, AreH), 7.30e7.26 (m, 2H, AreH), 6.72 (d,
2H, J ¼ 8.4 Hz, ArH), 6.54 (t, J ¼ 8.0 Hz, 2H, ArH); FT-IR (KBr, cmꢀ1),
3423, 1610, 1578, 1525, 1463, 1380, 1316, 1245, 1178, 1150, 1124,
1093, 1057, 930, 898, 852, 804, 751, 589, 540, 507, 424; elem. Anal.:
calc. for ZnC20H13N2O2Cl.2H2O: C, 53.35; N, 6.22; H, 3.77.; found: C,
53.55; N, 6.38; H, 3.21; Decomp: 330 ꢁC; Yield: 65.00%.
2.3.4. Cu(II) complex 6
FT-IR (KBr, cmꢀ1), 3423, 1607, 1571, 1521, 1458, 1372, 1328, 1249,
1187, 1149, 1128, 1080, 1050, 927, 888, 847, 801, 756, 682, 629, 573,
544, 500, 436; elem. Anal.: calc. for CuC20H13N2O2Br.0.5-
H2O.0.5CH2Cl2: C, 48.42; N, 5.51; H, 2.95.; found: C, 48.09; N, 5.93;
H, 3.01; m.p: 303e308 ꢁC; Yield: 68.20%.
Fig. 1. UVevis spectra of compounds 1e8.
filtrated solid was recrystallized with ethanol-dichloromethane
and dried under vacuum.

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Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
2.3.5. Co(II) complex 7
FT-IR (KBr, cmꢀ1), 3446, 1610, 1578, 1525, 1463, 1380, 1316, 1245,
1178, 1150, 1124, 1093, 1057, 930, 898, 852, 804, 751, 589, 540, 507,
424; elem. Anal.: calc. for CoC20H13N2O2Br.2H2O. CH2Cl2: C, 43.97;
N, 4.89; H, 3.31.; found: C, 43.84; N, 4.53; H, 2.69; m.p: 289-293 ꢁC;
Yield: 65.20%.
2.3.6. Zn(II) complex 8
1H NMR (400 MHz, DMSO‑d6):
d 9.08 (s, 1H, eHC]N-), 9.04 (s,
1H, eHC]N-), 8.17 (d, 1H, J ¼ 1.2 Hz, ArH), 7.87 (d, 1H, J ¼ 8.8 Hz,
ArH), 7.54 (d, J ¼ 8.7 Hz, 1H, AreH), 7.44 (dd, J ¼ 7.2 Hz, 2H, AreH),
7.27 (t, 2H, J ¼ 8.0 Hz, ArH), 6.7 (d, 2H, J ¼ 8.8 Hz, ArH), 6.55e6.51
(m, 2H, ArH); FT-IR (KBr, cmꢀ1), 3422, 1610, 1573, 1529, 1460, 1444,
1381, 1321, 1243, 1178, 1151, 11126, 1030, 969, 923, 889, 852, 805,
752, 680, 621, 538, 490; elem. Anal.: calc. for ZnC20H13N2O2Br.2.5-
H2O.2C2H5OH.: C, 48.37; N, 4.70; H, 5.03.; found: C, 48.59; N, 4.23;
H, 5.43; Decomp: 338 ꢁC; Yield: 69.50%.
Fig. 3. d/d transitions of Co complexes 4 and 7(10ꢀ2 M in DMSO).
Fig. 4. Fluorescence spectra of compounds 1e8.

Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
5
Fig. 5. Cyclic voltammograms of ligands 1 and 2 and their Cu and Co complexes recorded in DMSO, internal reference: Cp2Fe0/þ
.
2.4. Evaluation of antibacterial activity
Klebsiella pneumoniae were maintained on Nutrient agar slants agar
slop at 4 ꢁC and sub-cultured for 24 h before use. The antibacterial
activity of ligands and their complexes in DMSO was evaluated
in vitro by agar well diffusion method. Ceftriaxone and DMSO were
The test microorganisms; Staphylococcus aureus, Escherichia coli,
Pseudomonas aeruginosa and carbapenem & colistin resistant

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Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
Table 2
2.6. Theoretical studies
Electrochemical data for ligands 1 and 2 and their complexes with Cu (3 and 6) and
Co (4 and 7).
DFT calculations were carried out using Gaussian 09 version
D.01 program package [41]. Initial structures of all compounds were
drawn in the Gaussian software. Molecular structures were opti-
mized using the B3LYP (Becke’s Three parameter Hybrid Functional
Using the LYP Correlation Functional) approach in conjunction with
the 6-311G basis set for Schiff base ligands and the LANL2DZ basis
set for complexes [42]. No symmetry cbonstrains were applied
during the geometry optimization. For calculating the excited state
properties, time-dependent density functional theory (TDDFT) [43]
was employed at the B3LYP/6-311G level for Schiff base ligands and
the B3LYP/LANL2DZ level for complexes using dimethyl sulfoxide
(DMSO) as solvent and the excitation energies, oscillator strengths
and orbital contribution for the lowest 50 singlet-singlet transitions
at the optimized geometry in the ground state were obtained.
Compound
E
pa(V)
Epc(V)
E1/2/V (DEp (mV))
Ferrocene
1
0.528
0.306
ꢀ0.550
0.489
0.237
ꢀ0.086
0.769
ꢀ1.08
0.014
0.668
ꢀ0.230
0.764
ꢀ1.07
0.007
0.744
0.417
0.472 (111)
e
ꢀ1.63
ꢀ0.630
ꢀ0.679
0.116
ꢀ0.790
ꢀ1.12
ꢀ1.14
ꢀ0.066
ꢀ0.645
ꢀ0.959
e
0.590 (80.0)
e
2
3
4
0.176 (121)
ꢀ0.438 (704)
e
ꢀ1.11 (57.0)
ꢀ0.026 (80.5)
e
6
7
0.594 (729)
e
ꢀ1.12
ꢀ0.066
ꢀ0.628
ꢀ1.09 (53.0)
ꢀ0.029 (73.0)
e
3. Results and discussions
used as positive and negative controls for the antibacterial sus-
ceptibility testing respectively. All compounds were dissolved in
3.1. Synthesis and characterization
300
500
m
m
l DMSO, except for compound 3 which was dissolved in
l. Then 60 l of each reconstituted compound was transferred
Ligands 1 and 2 were synthesized by condensation reaction of
salicylaldehyde and 4-chloro-1,2-diaminobenzene or 4-bromo-1,2-
diaminobenzene in ethanol at room temperature for 4 h. The
complexes 3e8 were prepared by reacting ligands 1 and 2 in CH2Cl2
and 1 equiv. of Cu(II), Co(II) and Zn(II) acetate salts in ethanol
(Scheme 1).
m
to each well measuring 5.5 mm in diameter.
2.5. Electrochemical experiments
3.1.1. FT-TR spectra
In the IR spectra of the schiff-base ligands, a sharp and strong
band is observed at 1614 cmꢀ1, which is assigned to the absorption
of imine stretching vibration. Compounds 1 and 2 also display one
strong and broad band at 3448 and 3450 cmꢀ1 that can be related
to the stertching vibrations of phenolic hydroxyl groups (and also
The electrochemical properties of Schiff base ligands 1 and 2 and
their complexes with copper and cobalt ions were investigated by
cyclic voltammetry. Cyclic voltammetry (CV) was performed using
the solutions of ligands and complexes (10 ml, 10ꢀ3 M) prepared in
DMSO with 0.1 M LiClO4 as a supporting electrolyte and using a
glassy carbon working electrode, a Ag/AgCl reference electrode and
a Pt wire counter electrode under N2 atmosphere at room tem-
perature. The potential was scanned in the range of þ1.50
to ꢀ1.50 V at 50 mV/s potential scan rate.
water molecules). The n(-C]C-) stretching vibrations of aromatic
rings appear at the range of 1473e1578 cmꢀ1. Additionally the
observed peaks between 752 and 822 cmꢀ1 are assigned to the
bromine and chlorine (Figs. S1 and S2) [44,45]. After complexation
with copper, cobalt and zinc metal ions, the absorbance of imine
Fig. 6. The thermogravimetric plots of Schiff-base ligands 1 and 2 as weight loss versus temperature.

Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
7
Fig. 7. The thermogravimetric plots of Cu(II), Co(II) and Zn(II) complexes of ligand 1 (3e5).

8
Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
Table 3
The results of antibacterial activity of the tested compounds, presented as diameter of the zone of inhibition (mm).
Compound
S. aureus
E. coli
P. aeruginosa
carbapenem &colistin resistant K. pneumoniae
1
2
3
4
5
6
7
8
21
19
16
13
15
20
13
13
16
13
13
13
10
27
10
10
19
9.5
10
15
No halo
23
11
No halo
17
No halo
No halo
27
No halo
No halo
22
10
ligands, the imine proton resonances shift to downfield and
appeared at approximately 9.04 ppm and 9.08 ppm which confirm
the coordination of nitrogen atoms of imine groups to the Zn(II)
metal ion center [44,47,48]. Moreover, the aromatic protons are
observed at 6.51e8.18 ppm (Figs. S11 and S12).
3.2. Molar conductivity
The values of molar conductivity of all compounds are sum-
merized in Table 1. The results show that all complexes are non-
electrolyte. Therefore, ligand 1 and 2 act as one dianionic and tet-
radentate ligands with one N2O2 core when bound to the metal ion
center (Scheme 1).
3.3. UV/vis spectroscopy
The UV/Vis spectra of all compounds are recorded in DMSO
solution (10ꢀ4 M) and shown in Fig. 1. In general, the absorption
spectra exhibit intense bands in the 264e338 nm region which can
be related to the
p /p*and n/p* transitions of aromatic rings and
azomethine bonds [44,45]. It should be noted that Schiff base li-
gands exhibit enolimine-ketoamine tautomers in solution [49].
There are several factors such as temperature, solvent, substitution
and complexation that affect keto-enol equilibrium [50,51]. p/p*
Fig. 8. Diameter of the zone of inhibition of compounds 1e8 against studied bacteria
(Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and carbapenem
&colistin resistant Klebsiella pneumoniae.
transition of keto form is usually observed at wavelengths above
400 nm. Here, a low intensity broad shoulder which observed at
around 400 nm, can be assigned to the ketoamine tautomer for
ligands 1 and 2 [49,50]. As shown in Fig. 1, n/p* transitions of
imine groups decrease in intensity and shift to shorter wavelengths
after complexation. Also, the absorption band of the enol forms of
ligands disappear in the complexes. The phenolic hydroxyl groups
are deprotonated when bound to the metal ion center. Therefore,
there is no enolic proton in complexes. Consequently, the absorp-
tion bands which observed for copper complexes 3 and 6 at 427 nm,
for cobalt complexes 4 and 7 at around 400 nm and 480 nm and for
zinc complexes 5 and 8 at 412 nm and 450 nm can be assigned to
vibrations shifts to shorter or higher wavenumbers about
5e8 cmꢀ1. In some cases, these bands decrease in intensity after
complexation. This suggests the coordination of electron pair of
nitrogen atoms of imine groups to the metal ion center. The broad
and strong bands around 3446 and 3422 cmꢀ1 in the spectra of
complexes probably corresponds to the H2O stretching vibration. In
additional, the
n (M-O) and n (M-N) are identified in the range of
434e598 cmꢀ1 [44,45]. Finally, the medium and weak peaks about
752e758 cmꢀ1 and 803-850 cmꢀ1 are related to the bromine and
chlorine, respectively (Figs. S3eS8) [45].
the intra-ligand p/p* transitions of keto tautomer of ligands [50].
On the other hand, MLCT or LMCT is a very common phenomenon
in salophen type complexes that usually appears in the same region
and cannot be ignored [3,44,45,52,53]. It is likely that charge
3.1.2. 1H NMR spectra
transfer bands overlap with some of d/d bands or with the p/p*
In the 1H NMR spectra of ligands, two singlet resonances of two
none-equivalent eOH protons appar at 12.70 ppm and 12.77 ppm
in 1 and 12.74 ppm and 12.77 ppm in 2, while two singlet reso-
nances at approximately 8.95 ppm and 8.99 ppm are assigned to
two non-eqivalent imine protons [46]. The aromatic protons of
compounds 1 and 2 appear in the range of 6.97e7.74 ppm (Figs. S9
and S10).
As expected, the singlet resonances corresponding to the eOH
groups are disappeared in the 1H NMR spectra of Zn complexes 5
and 8, This indicates the phenolic hydroxyl groups are deproto-
nated when bound to Zn(II) metal ion, Also, in the comparison with
transitions [54]. As we will discuss later, in the case of Cu(II)
complexes and especially cobalt(II) complexes, TD-DFT calculations
indicates that the absorption bands in the 400e500 nm region can
also be related to the MLCT and LMCT, while theoretical data does
not support it for Zn complexes. According to the literature, d/d
transitions (lower energy bands) are observed in the range
540e650 nm for square-planner Cu(II) complexes [36,44]. The
appearance of low intensity bands at 511 and 609 nm in 3 and 518
and 600 nm in 6 which related to the d/d transitions, may indicate
square-planner structure for them. (Fig. 2). About Co(II) complexes
4 and 7, the low intensity bands between 598 and 605 nm are

Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
9
Fig. 9. Optimized geometries of Schiff-base ligand 1 and its complexes (3e5).
assigned to the d/d transitions (Fig. 3). Moreover, d/d transitions
are not observed in the visible region for Zn(II) complexes 5 and 8
because of the d10 configuration of Zn(II) ion [55] (note that d/d
transitions are forbidden and exhibit a small absorption coefficient
and not seen in dilute solution of complexes (Fig. 1). Therefore, to
observe these transitions, the stock solutions of complexes in
DMSO were prepared).
ligands totally quenches. It should be noted that the absorption
band of the enol form of ligands disappears in the complexes (Fig.1)
and there is no enolic proton. Consequently, the excited state
intramolecular proton transfer (ESIPT) is not responsible for the
emission intensity in contrast to the free ligands. According to the
literature, it was expected that the absorption bands present in
these complexes at 420e500 nm related to the intra-ligand p/p*
transitions of keto form of ligands and charge transfer transitions
would be responsible for the emission in the complexes [50,59].
Surprisingly, we found that these complexes quench the fluores-
cence intensity of ligands, which means that none of the absorption
peaks leads to a noticeable emission in the complexes compared to
the free ligands. It seems that the incorporation of metal ions af-
fects the conformational rigidity of the structure of ligands and
almost quenches the fluorescence intensity [18]. Since, the com-
plexes do not exhibit a significant emission, the expression of
excitation and emission wavelengths is merely to compare the ef-
fect of different metal ions entering to the ligand structure. (For
example, the typically excitation spectra for ligand 1 and complex 5
are given in Fig. S13).
3.4. Fluorescence study
The emission spectra of ligands and their complexes are given in
Fig. 4. Ligands 1 and 2 exhibit a maximum emission peak at 476 nm
upon excitation at 339 nm and 278 nm, respectively. According to
the literature [49,50,56e58], the emission process of Schiff-base
ligands is related to the
be explained as follows: the excited state which formed by the
* transition of enol form undergoes an ultrafast twist and
p/p* intra-ligand transitions which can
p/p
proton transfer in the time of 100 fs. Then the twists enol for
returns to ground state through a nonradiative manner. The formed
cis-keto state, as a result of excited state proton transfer emit. The
prominent bands in the fluorescence spectra of compounds 1 and 2
show Stokes shifts about 137 nm and 198 nm, respectively. In
general, after the incorporation of copper, cobalt and zinc metal
ions into the structure of both ligands, the emission intensity of
3.5. Cyclic voltammetry
The electrochemical properties of Schiff-base ligands 1 and 2

10
Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
Fig. 10. Optimized geometries of Schiff-base ligand 2 and its complexes (6e8).
and their Cu(II) and Co(II) complexes are investigated using cyclic
voltammetry in DMSO solutions containing 0.1 M LiClO4 as a sup-
porting electrode at room temperature in a potential range of þ1.50
to ꢀ1.50 V at 50 mV/s potential scan rate. The cyclic voltammo-
grams of ligands and complexes are shown in Fig. 5 and electro-
chemical data of all compounds summarized in Table 2. Cu(II)
complexes 3 and 6 exhibit one electron quasi-reversible oxidation
and reduction process [36,39]. The anodic wave which appeared in
the range of ꢀ0.0860 to ꢀ0.230 V, assigned to the quasi-reversible
electron oxidation of Cu(I) to Cu(II) while the chathodic wave in the
range of ꢀ0.790 to ꢀ0.959 V is attributed to quasi-reversible elec-
tron reduction of Cu(II) to Cu (I). Moreover, the observed anodic
wave at 0.769 V for 3 and 0.764 v for 6 corresponds to the oxidation
of respective ligands. Finally, the observed cathodic wave at ꢀ1.12 V
for 3 is assigned to the reduction of respective ligand. Both vol-
tammograms of Co complexes 4 and 7 display two sets of redox
peaks. The peaks at high negative potentials are assigned to Co(II)/
Co(I) redox couple and the peaks at the less negative potentials are
assigned to Co(II)/Co(III) redox couple [32,39,53,60]. For complex 4,
the anodic peak at ꢀ1.083 V and cathodic peak at ꢀ1.14 V are
assigned to reversible redox couple Co(II)/Co(I) and oxidation wave
at 0.014 V and reduction wave at ꢀ0.066 V are attributed to
reversible redox couple Co(III)/Co(II). Additionally, the waves which
observed at 0.668 V and ꢀ0.645 V in voltammogram of 4, are
assigned to the oxidation and reduction of respective ligand,
respectively. About complex 7, observed anodic and cathodic waves
at ꢀ1.07 and ꢀ1.12 V are attributed to reversible redox couple
Co(II)/Co(I) while observed anodic and cathodic waves at 0.007
and ꢀ0.066 V are related to reversible redox couple Co(III)/Co(II).
Finally, the anodic and cathodic peaks at ¼ 0.744 V and ꢀ0.628 V
probably correspond to oxidation and reduction of respective
ligand.
3.6. Thermogravimetric analysis
Thermogravimetric analysis (TGA) are used up to 900 ꢁC at N2
atmosphere to investigate the thermal stability of ligands 1 and 2
and Cu(II) (3 and 6), Co(II) (4 and 7) and Zn(II) (5 and 8) complexes.
Ligands 1 and 2 are stable up to 200 ꢁC with a DTGmax ¼ 245 ꢁC for
1 and 235 ꢁC for 2. The thermogravimetric plots of Schiff-base li-
gands 1 and 2 as weight loss versus temperature have been shown
in Fig. 6 and Fig. S14. Thermal decomposition of ligands take places
in two stages. The first stage is probably related to the omission of
solvent molecule with a mass lose 6.49% (calc. 6.16%) for 1 and 3.15%
(calc. 2.82%) for 2. The second stage is related to vigorous decom-
position of organic residues, with a mass loss of 64.30% (calc.
64.26%) for 1 and 61.33% (calc. 60.75%) for 2.
The TGA plots of Cu(II) complexes 3 and 6 show both complexes

Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
11
Fig. 11. Molecular orbitals for salophen ligand 1 and its complexes with copper (3), cobalt (4) and Zinc (5).
are stable up to 340 ꢁC with a DTGmax ¼ 340.12 ꢁC for 3 and 342 ꢁC
for 6. Decomposition for these complexes involves three steps. The
weight loss in the first stage is probably related to the presence of
the lattice cell water or other solvents in the complexes. The second
stage can be related to the omission of CO molecule of 3 or CO2
molecule of 6 with a mass lose 6.80% (calc. 7.33%) and 9.64%
(calc.9.59%) [61]. The highest weight loss occurs in the range of
600e950 ꢁC, which corresponds to the decomposition of the res-
idue aromatic groups (loss of the C7H5O group in 3 and C7H5N
group in 6) with a mass lose 27.64% (Calc. 27.34%) and 24.62 (Calc.
24.96%) (Fig. 7 and Fig. S15) [60,61].
For Co (II) complex 4 and 7, TGA curves indicate that the loss of
weight starts around 121 and 127 ꢁC (Fig. 7 and Fig. S16). The loss of
weight take places in six steps for 4 and five steps for 7. The weight
loss in first stage can be corresponded to the elimination of water
from lattice cell of the complexes with a mass lose 5.82% (calc.
6.21%) for 4 and 6.91% (calc. 7.37 %) for 7. Similar to Cu(II) com-
plexes, the second stage can be assigned to the release of CO2
molecule of the main body of 4 and 7 with a mass lose 9.02% (calc.
10.79 %) and 8.25% (calc.9.73%), respectively. Decomposition of
organic groups for compounds 4 and 7 starts from 300 ꢁC. The first,
second, third and fourth stages in the temperature range
300e700 ꢁC, theoretically calculated weight losses for 4 are:
14.24%, 8.67%, 8.73% and 7.57%. The thermal analyses for 4 show
experimentally weight losses of 14.30%, 8.03%, 8.73% and 9.56% and
corresponds to the decomposition of some parts of residual aro-
matic rings of Schiff base ligand 1. The first and the second stage of
decomposition of the organic groups, theoretically calculated
weight losses for 7 are 12.74% and 7.0%. The thermal analyses for 7
reveals experimentally weight losses of 12.39%, 5.42%. These stages
involve the decomposition of one of the residual aromatic rings of
the schiff base ligand 2 in two steps. The final decomposition step is
related to the omission of the second residue aromatic ring of the
main body of complex 7 with a mass lose 22.03% (calc. 22.96%).
Zn(II) complex 5 is stable to 203.04 ꢁC while Zn(II) complex 8 is
stable to 196.3 ꢁC. Zn (II) complexes 5 and 8 reveal four and three
decomposition peaks in the thermograms (203.04 ꢁC, 383.81 ꢁC,
527.23 ꢁC and 689.95 ꢁC for 5 and 196.34 ꢁC, 370.20 ꢁC and 710.31 ꢁC
for 8). The weight loss in the first stage is probably corresponds to
the presence of the lattice cell or coordinated water with a mass
lose 6.14% (calc. 6.12%) for 5 and a mass lose 4.92 % (calc. 4.67 %) for
8. The second stage can be related to the omission of CO molecule of
5 with a mass lose 7.47% (calc. 6.77 %) and 8 with a mass lose 4.76%
(calc. 6.10%). The highest weight loss take places about 450e950 ꢁC
that can be related to the removal of one of the phenolate groups
with a mass lose 23.81% (calc. 23.86 %) for 5 and the elimination of
the aromatic ring containing of bromo substitution with a mass lose
39.48 % (calc. 42.52 %) for 8 (Figs. 7 and S17).
3.7. Biological activity
The in vitro antibacterial activity is tested by agar well diffusion
assay using Staphylococcus aureus, Escherichia coli, Pseudomonas
aeruginosa and carbapenem &colistin resistant Klebsiella pneumo-
niae. The results of susceptibility testing, presented as diameter of
the zone of inhibition are given in Table 3. As shown in Fig. 8, both
Co(II) complexes 4 and 7 have remarkable antibacterial activity
against S. aureus as a medically important Gram positive bacterium.
Also, cobalt complexes 4 and 7 reveal the greatest antibacterial
activity against carbapenem & colistin resistant Klebsiella pneu-
moniae. This antibacterial effect is of high interest as the bacterium,
is resistant to two most important therapeutic options for treat-
ment of multi-drug resistant Gram negative bacteria, including
polymyxins and carbapenems. Moreover, Schiff-base ligands 1 and

12
Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
Fig. 12. Molecular orbitals for salophen ligand 2 and its complexes with copper (6), cobalt (7) and Zinc (8).
2 are more active than their copper and zinc complexes against
studied bacteria.
are due to the excitation of electron from HOMO / LUMOþ2 (50%)
and HOMO-5 / LUMO (64%), respectively. The maximum locali-
zation of the HOMO and HOMO-5 are on
bonds while the LUMOþ2 of 1 is mainly consist of
aromatic rings and the LUMO of 2 includes the * orbitals expanded
on aromatic ring and iminic bonds. This transition can be attributed
to the * of aromatic rings. The second excited state for ligands
1 and 2 are due to HOMO / LUMOþ1 (92%) and HOMO-1 / LUMO
(~75%). HOMO-1 of both ligands is mainly formed of (C) orbitals
and iminc bands, while the maximum localization of the LUMO-
þ1of 1 and the LUMO of 2 is on * orbitals of aromatic rings and
iminc bands. Therefore, these transitions can be attributed to the
* of aromatic rings and n/ * of azomethine bonds. Finally,
p
(C) orbitals and iminc
p* orbitals of
3.8. Theoretical studies
p
Density Functional theory (DFT) investigations are performed to
obtain a better understanding of structure and electronic proper-
ties of compounds 1e8. The fully optimized molecular structures of
all ligands and complexes are shown in Figs. 9 and 10. Also, mo-
lecular orbital surfaces for HOMO-1, HOMO, LUMO and LUMOþ1 of
ligands and their complexes are given in Figs. 11 and 12. Analysis of
the frontier orbital molecules of ligands 1 and 2 indicates that the
band gaps are 2.24 eV and 2.22 eV, respectively. The band gaps for
Cu(II) complexes 3 and 6, Co(II) complexes 4 and 7 and Zn(II)
complexes 6 and 8 have similar values: 1.92 eV (alpha) and 1.88 eV
(beta), 1.82 eV (alpha), 1.77 eV (beta) and 1.94 eV. The band gap
values of all complexes shows their potential ability for using in
optical devices such as solar cells.
p/p
p
p
p/p
p
the first excited state of ligands 1 and 2 which related to the pro-
motion of electron from HOMO / LUMO (95%), can be assigned to
the intra-ligand transitions.
3.8.1.2. Complexes. The simulated spectra of all complexes are
shown in Figs. S19 and S20. To investigate the theoretical spectra,
the calculated transitions with major oscillator strength are
considered. In all complexes, the high energy bands are associated
3.8.1. UVevis spectra by TD-DFT calculations
TD-DFTcalculations are performed on ground state of optimized
their structures to predict electronic spectra of compounds 1e8.
Some electronic transitions for ligands and complexes have been
given in Tables S1eS8 from Supplemental Material.
to the intra-ligand transitions (
bands of Cu complex 3 and 6 originate mainly from intra-ligand
transitions and to lesser extend from MLCT(Cu/ *) or LMCT
/Cu) transitions. The low energy bands of Co complexes 4 and 7
corresponds to MLCT Co/ * type and intra-ligand transition and
p/p* and n/p*). The low energy
p
(p
3.8.1.1. Ligands. The simulated spectra of both ligands are given in
Fig. S18. To analyze the theoretical spectra, the calculated transi-
tions with major oscillator strength are considered. The tenth
excited state for ligand 1 and the eighth excited state for ligand 2
p
to lesser expand from LMCT. While, the low energy bands of Zn
complexes 5 and 8 is mainly related to the intra-ligand transitions.
Generally, the theoretical data reveal the good agreement with the

Z. Shaghaghi et al. / Journal of Molecular Structure 1200 (2020) 127107
13
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4. Conclusions
In this study, copper, cobalt and zinc complexes with salophen
type ligands containing chlorine and bromine were synthesized. All
compounds were fully characterized and their optical, electro-
chemical, thermal and antibacterial activity investigated. UVeVis
spectra of all complexes showed the absorption bands in the
400e500 nm region which related to the p/p* transitions of keto
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and Co(II) complexes). Ligands 1 and 2 exhibited one maximum
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ligand p/p* transitions. The results showed that all of complexes
quench the fluorescence of ligands which means that none of the
absorption peaks leads to a noticeable emission in the complexes
compared to the free ligands. The cyclic voltammogrames of Cu (II)
and Co(II) complexes presented one quasi-reversible redox couple
of Cu(II)/Cu(I) and two fine reversible redox couples of Co(II)/Co(I)
and Co(III)/Co (II), respectively. The investigation of antibacterial
activity by agar well diffusion assay showed the Co(II) complexes 4
and 7 have one strong antibacterial effect upon the studies bacteria.
Finally, the molecular geometry of Schiff base ligands and their
complexes in the ground state was obtained by DFT calculations.
Based on optimized structures, electronic absorption spectra of the
ligands and complexes were simulated by TD-DFT method that
displayed good agreement with the experimental data. This study
would pave the way for using of these complexes in several fields
such as precursors for preparation of metal oxide nanostructures,
photovoltaic, electrocatalytic (water oxidation) and biological ap-
plications for future works.
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Acknowledgements
[22] E. Bodo, A. Ciavardini, A.D. 14;Cort, I. Giannicchi, F.Y. 14;Mihan, S. Fornarini,
S. Vasile, D. Scuderi, S. Piccirillo, Anion recognition by uranylesalophen de-
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ab initio modeling, Chem. Eur J. 20 (2014) 11783e11792.
The authors are grateful to Azarbaijan Shahid Madani University
and Tabriz University for financial support of this study. Also, the
authors thank Dr. Ardavan Masoudiasl for helping to the theoretical
studies.
ꢂ
[23] L. Leoni, R. Puttreddy, O. Jurcek, A. Mele, I. Giannicchi, F.Y. Mihan, K. Rissanen,
A.D. 14;Cort, Solution and solid-state studies on the halide binding affinity of
perfluorophenyl-armed uranylesalophen receptors enhanced by anione
interactions, Chem. Eur J. 22 (2016) 18714e18717.
p
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L. Rodríguez, Substituent effects on the biological properties of Zn-salophen
complexes, Inorg. Chem. 52 (2013) 9245e9253.
Appendix A. Supplementary data
[25] A. Arola-Arnal, J. Benet-Buchholz, S. Neidle, R. Vilar, Effects of metal coordi-
nation geometry on stabilization of human telomeric quadruplex DNA by
square-planar and square-pyramidal metal complexes, Inorg. Chem. 47
(2008) 11910e11919.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2019.127107.
[26] M. Cano, L. Rodríguez, J.C. Lima, F. Pina, A.D. Cort, C. Pasquini, L. Schiaffino,
Specific supramolecular interactions between Zn2þ-salophen complexes and
biologically relevant anions, Inorg. Chem. 48 (2009) 6229e6235.
[27] M.E. Germain, T.R. Vargo, B.A. McClure, J.J. Rack, P.G. Van Patten, M. Odoi,
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