Utilization and simulation of innovative new binuclear Co(ii), Ni(ii), Cu(ii), and Zn(ii) diimine Schiff base complexes in sterilization and coronavirus resistance (Covid-19)

Article abstract of DOI:10.1515/chem-2021-0068

This article aimed at the synthesis and molecular docking assessment of new diimine Schiff base ligand, namely 2-((E)-(2-((Z)-2-(4-chlorophenyl)-2-hydroxyvinyl)hydrazono) methyl)-6-methoxyphenol (methoxy-diim), via the condensation of 1-(4-chloro-phenyl)-2-hydrazino-ethenol compound with 2-((E)-(2-((Z)-2-(4-chlorophenyl)-2-hydroxy vinyl) hydrazono)methyl)-6-methoxyphenol in acetic acid as well as the preparation of new binuclear complexes of Co(ii), Ni(ii), Cu(ii), and Zn(ii). The following synthesized complexes were prepared in a ratio of 2:1 (metal/ligand). The 1H-NMR, UV-Vis, and FTIR spectroscopic data; molar conductivity measurements; and microanalytical, XRD, TGA/DTG, and biological studies were carried out to determine the molecular structure of these complexes. According to the spectroscopic analysis, the two central metal ions were coordinated with the diamine ligand via the nitrogen of the hydrazine and oxygen of the hydroxyl groups for the first metal ions and via the nitrogen of the hydrazine and oxygen of the phenol group for the second metal ions. Molecular docking for the free ligand was carried out against the breast cancer 3hb5-oxidoreductase and the 4o1v-protein binding kidney cancer and COVID-19 protease, and good results were obtained.

Full text of DOI:10.1515/chem-2021-0068

Open Chemistry 2021; 19: 772–784
Research Article
Moamen S. Refat*, Ahmed Gaber, Walaa F. Alsanie, Mohamed I. Kobeasy, Rozan Zakaria,
Kehkashan Alam
Utilization and simulation of innovative new
binuclear Co(II), Ni(II), Cu(II), and Zn(II) diimine
Schiff base complexes in sterilization and
coronavirus resistance (Covid-19)
https://doi.org/10.1515/chem-2021-0068
ligand via the nitrogen of the hydrazine and oxygen of
the hydroxyl groups for the first metal ions and via the
nitrogen of the hydrazine and oxygen of the phenol group
for the second metal ions. Molecular docking for the free
ligand was carried out against the breast cancer 3hb5-oxi-
doreductase and the 4o1v-protein binding kidney cancer
and COVID-19 protease, and good results were obtained.
received January 6, 2021; accepted June 14, 2021
Abstract: This article aimed at the synthesis and molecular
docking assessment of new diimine Schiff base ligand,
namely 2-((E)-(2-((Z)-2-(4-chlorophenyl)-2-hydroxyvinyl)
hydrazono) methyl)-6-methoxyphenol (methoxy-diim),
via the condensation of 1-(4-chloro-phenyl)-2-hydrazino-
ethenol compound with 2-((E)-(2-((Z)-2-(4-chlorophenyl)-
2-hydroxy vinyl) hydrazono)methyl)-6-methoxyphenol in
acetic acid as well as the preparation of new binuclear
complexes of Co(II), Ni(II), Cu(II), and Zn(II). The following
synthesized complexes were prepared in a ratio of 2:1
Keywords: diimine complex, thermogravimetric, COVID-
19 protease, antimicrobial, molecular docking
1 Introduction
1
(metal/ligand). The H-NMR, UV-Vis, and FTIR spectro-
scopic data; molar conductivity measurements; and micro-
analytical, XRD, TGA/DTG, and biological studies were
carried out to determine the molecular structure of these
complexes. According to the spectroscopic analysis, the
two central metal ions were coordinated with the diamine
Our society is currently dealing with one of the most
serious health issues, the Coronavirus. Isatin compounds
were found to possess antiviral properties and were used
in the treatment of HIV [1]. After virus infection, MT-4
cells infected with HIV-1 or infected human cells were
incubated with various concentrations of Isatin deriva-
tives, and the number of practical cells was determined
using the 3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyltetra-
zolium bromide (MTT) technique [1]. In addition, the
Isatin Schiff base compounds have been studied widely
and were found to exhibit significant activity towards
Moloney leukemia virus [2], vaccinia virus [3], rhinovirus
[4], and SARS virus [5]. The mechanism of inhibition
of N-methylisatin-b-40,40-diethylthiosemicarbazone
(MIBDET) has been described based on the growth of
the molecular leukemia virus (MLV): the MLV inhibition
by M-IBDET included blocking the viral RNA translation
rather than RNA interference during the viral transcrip-
tion process [2]. Isatin and Isatin derivatives have also
been examined for their antiviral effectiveness against
acute cardiac acute respiratory syndrome [5]. The Isatin
derivatives showed prohibition activity against SERS-
CoV3C-like protein (3CLpro) at concentrations ranging

* Corresponding author: Moamen S. Refat, Department of
Chemistry, Faculty of Science, Taif University, P.O. Box 11099,
Taif 21944, Saudi Arabia, e-mail: msrefat@yahoo.com,
moamen@tu.edu.sa
Ahmed Gaber: Department of Biology, College of Science, Taif
University, P.O. Box 11099, Taif 21944, Saudi Arabia; Department of
Biology, Center of Biomedical Sciences Research, Taif University,
P.O. Box 11099, Taif 21944, Saudi Arabia
Walaa F. Alsanie: Department of Biology, Center of Biomedical
Sciences Research, Taif University, P.O. Box 11099, Taif 21944,
Saudi Arabia; Department of Clinical Laboratory Sciences, College of
Applied Medical Sciences, P.O. Box 11099, Taif 21944, Saudi Arabia
Mohamed I. Kobeasy: Department of Chemistry, Faculty of Science,
Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
Rozan Zakaria: Department of Chemistry, Faculty of Science,
Port Said University, Port Said 42511, Egypt
Kehkashan Alam: Department of Chemistry, Faculty of Science,
Aligarh Muslim University, Aligarh 202002, India
Open Access. © 2021 Moamen S. Refat et al., published by De Gruyter.
This work is licensed under the Creative Commons Attribution 4.0
International License.

Utilization of new diimine Schiff base complexes against Covid-19
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773
from 0.95 to 17.50 mM. The prohibited properties were range of applications, including pharmaceuticals, medi-
tested using the fluorescence resonance energy transfer cine, and natural activities, among others. It was also
method and confirmed by HPLC analysis. Isatin-thiosemi- used as an oxygen sensor [15–17]. Recently, many research
carbazone as a preventive compound has a substantial articles focused on the Schiff base of thiosemicarbazide
bioactive role against a variety of viral infections [6]. and metal derivatives because of its numerous applica-
Numerous compounds of 5-fluoroisatin and its deriva- tions in the pharmacology field as antiviral, antipara-
tives were synthesized, and they displayed a significant sitic, antimicrobial, anticancer, and anti-HIV [15–17].
inhibition behaviour towards different viruses and inhib- We have, therefore, undertaken the synthesis, characte-
ited the replication process of the vesicular stomatitis rization, structural, thermogravimetric, and antimicrobial
virus in Vero cell lines [7].
studies of a few binuclear complexes formed from Co(II),
The biological activities of Schiff bases of oximes Ni(II), Cu(II), and Zn(II) containing tetradentate 2-((E)-
such as hydrazone, semicarbazones, and thiosemicarba- (2-((Z)-2-(4-chlorophenyl)-2-hydroxyvinyl)hydrazono)
zones have been previously described [8]. Only a few methyl)-6-methoxyphenol (methoxy-diim) Schiff base
variants of these chemicals showed considerable anti- ligand.
viral activity. For example, the 5-acyl-3-methylsulfamyl-
1,2,4-triazine molecule and its derivatives were found to
have antiviral activity against the coxsackie virus B4
in Vero and HeLa cell cultures, with EC50 values of 20 2 Materials and methods
and 45 mg/mL, respectively [8]. The Schiff base silver(I)
complexes with glycine salicylaldehyde showed good
2.1 Synthesis of the methoxy-diim ligand
antiviral activities against the Cucumber mosaic virus
[9–12]. Additionally, nickel(II), platinum(II), and palla-
In this article, the chemicals were tested before use to
dium(II) derivatives, especially with semicarbazone and
emphasize their purity and their analytical structure.
thiosemicarbazone of p-tolualdehyde, have been docu-
The ligand 2-((E)-(2-((Z)-2-(4-chlorophenyl)-2-hydroxy-
mented to show antifungal activities in vitro against several
vinyl)hydrazono) methyl)-6-methoxyphenol (3) was
types of fungi such as Alternaria alternata, Aspergillus
synthesized via two steps. The first step was the reaction
niger, and Fusarium udum [10–13]. In comparison to the
of p-chlorophenacyl bromide (1) with hydrazine hydrate
parent ligands, the metal chelates show an excessive
in ethanol to give 1-(4-chloro-phenyl)-2-hydrazino-
antifungal activity, as determined by examining these
ethenol (2). The second step was the condensation of
derivatives for preventing the growth of fungus. This
the hydrazine derivative (2) with 2-hydroxy-3-methoxy-
result is attributed to the improved delocalization of the
benzaldehyde in acetic acid under reflux to obtain the
p-electrons caused by chelation, which increased the dif-
diimine ligand (3, Scheme 1).
fusion and the entry of the metal complexes to the lipid
membrane, where the metal derivatives then blocked
the enzymes in the microorganisms’ membranes. Salicy-
NH2
laldehyde and 1-amino-3-hydroxyguanidine were used to
determine the Schiff base. Wang et al. [14] used hydroxy-
Br
NH
H2N NH2
EtOH
Cl
Cl
aminoguanidines to modify a few basic structures of
Schiff bases, resulting in the formation of unused substi-
tuted salicylaldehyde Schiff base compounds. For the
first time, all of these compounds were tested against
coronavirus infection and mouse hepatitis virus [14].
At this point, [1-[(30 allyl-20-hydroxy benzylidene)
amino]-3-hydroxyguanidine] is the most active com-
pound in the development of mouse hepatitis virus,
roughly 376 times more active than hydroxylguanidine
and around 564 times more active than hydroxyamino-
guanidines. Schiff bases are versatile carbon-based
molecules that are widely used and produced via the
condensation reaction between various amines and alde-
hydes or ketones, which is known as imine. It has a wide
O
OH
2
1
HO
OCH3
O
2-hydroxy-3-methoxybenzaldehyde
OCH3
HO
N
N
H
Cl
OH
3
Scheme 1: Synthesis of methoxy-diim ligand (3).

774
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Moamen S. Refat et al.
2.2 Compound (2)
46.38, which correspond to [C16H15N2O3Cl]+, [C16H16N2O3]+,
[C14H16N2O]+, [C11H12N2O]+, [C8H9NO]+, [C6H11N]+, and
An equimolar solution of p-chlorophenacyl bromide [C2H7N]+ ions, respectively, that confirm the proposed mole-
(0.01 M) and hydrazine hydrate (0.01 M) was prepared cular structure and formula.
in 50 mL of ethanol; then the mixture was refluxed for
2 h with heating at 70°C. After cooling the mixture, dilute
acetic acid was added to neutralize it. The resulting com-
2.4 Synthesis of solid Co(II), Ni(II), Cu(II), and
pound was filtered, washed with water, and then recrys-
Zn(II) methoxy-diim complexes
tallized in ethanol to obtain compound (2). The melting
point was 98°C; IR (KBr) cm−1 = 3,350 (br-OH), 3,320,
A hot solution of 1 mmol methoxy-diim ligand was pre-
3,117 (NH2), 3,230 (NH), 1,615, 1,605, 1,550 (C]C), and
pared in 30 mL of ethanol to which a few drops of con-
centrated NH4OH were added. Then, 2 mmol solutions
of the following metal(II) salts, CoCl2·6H2O, NiCl2·6H2O,
CuCl2·2H2O, and ZnCl2, were prepared in 20 mL of ethanol.
Then, each metal solution was added gradually to the
above solution of the methoxy-diim ligand. Ammonia
solution was used to neutralize the formed mixtures
(pH = 8–9), refluxed, stirred well for 2 h at 75°C, and
evaporated to half its initial volume. The resulting com-
plex was allowed to cool for a day, separated and washed
with a mixture of methanol and diethyl ether. Then it was
recrystallized in methanol and dried using anhydrous
CaCl2. Thin layer chromatography was performed to con-
firm its purity.
1
1,017 (C–O); H-NMR (DMSO-d6) δ: 3.21 (d, 2H, NH2),
4.20 (q, 1H, NH), 7.28–7.44 (m, 2H, Ar-H), 7.31(d, 1H,
CH), and 14.07 (s, 1H, OH) ppm.
2.3 Compound (3)
A solution was prepared in 30 mL of acetic acid from the
mixture of hydrazine derivative (2) (0.01 M) and 2-hydroxy-
3-methoxybenzaldehyde (0.01 M). Then, the solution was
heated under reflux for 2 h, cooled, and added into water.
The resulting compound was separated out and recrystal-
lized to give (3). M.P. = 140°C; IR (KBr) cm−1 3,449 (br-OH),
3,046 (NH), 1,622 (C]N), 1,575, 1,486 (C]C), 1,390, and
1
1,271 (C–O). H-NMR (DMSO-d6) δ: 6.99 (s, 1H, CH]N),
7.38–7.65 (m, 8H, 2Ar-H), 9.01 (s, 1H, OH), 3.83 (s, 3H,
OCH3), and 11.16 (s, 1H, NH) ppm.
The suggested structural formula for the ligand
(methoxy-diim) (Figure 1) was confirmed by the mass
spectrum. The following molecular ion peaks were recorded
(m/z+): 318, 283.40, 227.86, 188.11, 135.21, 97.51, and
2.5 Instruments
A Jenway 4010 conductivity meter was used to measure
the conductance of complex solutions (10−3 mol/cm3 in
DMF). The ratio of metal in the synthesized metal com-
plexes was determined using the gravimetric method by
Figure 1: Mass spectrum fragmentations of the methoxy-diim ligand.

Utilization of new diimine Schiff base complexes against Covid-19
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775
Table 1: Physicochemical studies for methoxy-diim free ligand and its metal complexes
Compounds
Colour
%Found (% Calcd)
C%
H%
N%
Cl%
M%
diim
I
II
III
IV
Yellow
60.1 (60.28)
30.2 (31.40)
27.546 (28.96)
29.5 (30.84)
32.67 (33.55)
4.43 (4.7)
9.56 (15.07)
13.52 (13.74)
16.78 (16.896)
8.5 (8.99)
11.83 (11.97)
17.88 (18.98)
16.45 (16.49)
17.56 (17.69)
18.05 (18.73)
—
Light brown
Greenish brown
Dark green
Dark brown
4.23 (5.07)
5.12 (5.883)
3.2 (4.66)
18.97 (19.279)
16.9 (17.708)
20.05 (20.41)
22.42 (22.85)
3.89 (4.019)
9.74 (9.786)
oxidizing it to its most stable form as well as chloride hydrazono)methyl)-6-methoxyphenol (methoxy-diim)
ions. The results confirmed the proposed theoretical struc- ligand have been characterized after its synthesis. The
ture for the complexes. A magnetic balance (Sherwood prepared metal complexes are not reactive with atmo-
Scientific Cambridge, England)was used for magnetic mea- spheric components, are non-hygroscopic and dissolve
surements. A Bruker FT-IR spectrophotometer was used to only in DMSO and DMF. The conductivities of the com-
record the infrared spectra in the range of 4,000–400 cm−1; plexes, determined with the DMSO solution, were found
1
the electronic, H-NMR (400 MHz), and thermal studies
were carried out at the Cairo University. The elemental ana-
H3N
H2O
H2O
OCH3
lyses (e.g. carbon, hydrogen and nitrogen) were carried out
using an elemental analyser (Vario EL III Germany) at the
Microanalytical Center, Cairo University. The X-ray diffrac-
tion patterns were obtained using a Bruker AXS configura-
tion X-ray powder diffractometer.
O
Cl
Co
N
H
N
Cl
Cl
O
NH3
OH2
OH2
Co
2.6 Molecular docking
H2O
H3N
H3N
OCH3
O
Ni
Autodock software has been utilized for the docking study.
The structure of the COVID-19 protease (6LU7) was obtained
from the Protein Data Bank (http://www.rcsb.org) [18]. All
of the ligands were optimized using Avogadro version 1.2. In
the AutoDock Tools, we have added Kollman charges and
polar hydrogen atoms to the proteins, and then the grid box
was chosen for the active site of the protein for ligand inter-
actions. The results obtained from docking were visualized
through Discovery Studio [18].
Cl
N
H
N
Cl
Cl
O
NH3
NH3
OH2
Ni
.3H2O
H2O
H3N
H2O
OCH3
O
Cl
Cu
N
H
N
Ethical approval: The study was conducted and approved
by the Ethical Committee approval of Taif University
(1-441-26).
Cl
Cl
O
NH3
OH2
OH2
Cu
OCH3
H3N
Cl
3 Results and discussion
O
Zn
N
H
N
3.1 Elemental analyses and physical
properties
Cl
NH3
Cl
Zn
.2H2O
O
The complexes of Co(II), Ni(II), Cu(II), and Zn(II) with
the 2-((E)-(2-((Z)-2-(4-chlorophenyl)-2-hydroxyvinyl) Figure 2: Proposed structures of 2:1 methoxy-diim complexes.

776
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Moamen S. Refat et al.
Figure 3: Infrared spectrum of the methoxy-diim free ligand.
to be lower than that of the free methoxy-diim ligand a number of distinguished vibration bands attributable
(17.83 μS), except for the Co(II) complex [18]. The results to the functional groups of phenolic (Ar-OH), ketone
of the elemental analysis are given in Table 1. The synthe- (C]O), azomethine (C]N), phenolic (C–O), and carbon
sized coloured solid complexes have formulae [Co2(diim) halogen bond (C–Cl) at frequencies 3,448, 3,065, 1,680,
(H2O)4(NH3)2(Cl)2] (I), [Ni2(diim)(NH3)6(Cl)2]·3H2O (ii), 1,622 (1,397 and 1,253), and 964 cm−1, respectively, which
[Cu2(diim)(H2O)4(NH3)2(Cl)2] (III), and [Zn2(diim)(NH3)2(Cl)2]·H2O (IV) are the essential vibration bands of the methoxy-diim
(Figure 2).
ligand (Figure 3).
The corresponding spectra of Co(II), Ni(II), Cu(II), and
Zn(II) complexes with the methoxy-diim ligand are shown
and summarized in Figure 4 in Table 2. The medium-
strong peak (1,680 cm−1) of the carbonyl group found in
the free methoxy-diim ligand disappeared after com-
plexation. This result can be due to the tautomerism
3.2 Infrared spectral studies
The infrared spectrum of the methoxy-diim free ligand
(Figure 3) was recorded, and it was noted that there are
Figure 4: Infrared spectra of I: Co(II), II: Ni(II), III: Cu(III), and IV: Zn(II) methoxy-diim complexes.

Utilization of new diimine Schiff base complexes against Covid-19
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777
Table 2: Data for infrared spectral bands (cm−1) for the methoxy-diim ligand and its metal complexes
Compounds
ν(O–H)
ν(N–H)
ν(COCH3)
ν(C]O)
ν(C]N)
ν(C–O)
ν(M–O)
ν(M–N)
Methoxy-diim
3,448
3,422
3,446
3,449
3,450
3,065
3,022
3,055
3,023
3,059
2,842
2,833
2,834
2,845
2,835
1,680
—
—
—
—
1,622
1,603
1,606
1,604
1,607
1,397, 1,253
1,321, 1,213
1,323, 1,210
1,323, 1,091
1,302, 1,090
—
—
I
599, 519
603, 520
596, 516
615
474
452
481
456
II
III
IV
change in the methoxy-diim ligand from the keto to there are new peaks in the range of 3,022–3,065 cm−1,
the enol form in alkaline media through the chelation assigned to υ(NH) of the –NH3 group, confirming the
process (Figure 5). In the case of the free diim ligand, participation of the –NH3 group and the lone pair of elec-
the very strong peak at 1,622 cm−1 is due to the C]N trons on its nitrogen atom in the coordination reaction.
stretching frequency of the azomethine group [19]. This In the spectra of the metal complexes, the average vibra-
stretching absorption band is withdrawn to a lower fre- tion peaks are found in the ranges of 615–516 cm−1 and
quency range of 1,603–1,607 cm−1 in the prepared metal 481–452 cm−1, assigned to the vibrational motions of ν
complexes, probably suggesting the coordination of the (M–O) and ν(M–N) bands, confirming the participation
azomethine C]N group by the lone pair of electrons on of oxygen and nitrogen atoms of the phenolic, hydroxyl,
its nitrogen towards the centre metal ions. The peak of azomethine, and hydrazino groups in the coordination
the phenolic –OH group is slightly shifted to higher reaction with the metal [19].
average frequencies than those in the spectrum of the
diim ligand (3,448 cm−1) because of the hydrogen bond
formed among the oxygen atoms of the phenolic and
3.3 Electronic and magnetic studies
nitrogen of azomethine groups (Figure 5). In the spectra
of the synthesized complexes (Figure 4 I–IV), the broad
In the wavelength range of 300–800 nm, the electronic
bands in the frequency range 3,422–3,450 cm−1 are assigned
absorption spectra of the methoxy-diim free ligand and
to the presence of coordinated and uncoordinated water
its metal complexes were recorded in DMSO as a solvent.
molecules [19]. From the spectrum of the free ligand
The electronic spectra of the free methoxy-diim ligand
and its metal complexes, the wavenumber of ν(COCH3)
showed two peaks at 315 and 355 nm, referring to π–π*
(2,842 cm−1) is slightly [19] changed, indicating that
and n–π* electronic transitions [20–22]. The Co(II) com-
the COCH3 group does not take part in the complexation
plex showed three main bands at 300, 355, and 448 nm in
with the metal ion. The average weak absorption peaks
its UV-Vis absorption spectrum: the first absorption peak
was due to the intra-ligand transition of the organic
found in the range 855–735 cm−1 refer to bending vibra-
tions δ(H2O), emphasizing the presence of coordinated
moiety, and both the second and third electronic transi-
water. From the spectra of the methoxy-diim ligand, the
4
4
4
4
tions were due to T1g → T1g(P) and T1g (F) → T2g(F),
respectively [23,24]. The octahedral geometry of the Co(II)
complex was confirmed from the UV-Vis spectrum and
the recorded value of its magnetic moment (5.10 B.M.)
[21]. The electronic spectrum of the Ni(II) complex was
found to have four electronic transition peaks at 300,
320, 360 and 400 nm. The first and second peaks may
be assigned to the ligand-to-metal charge-transfer transi-
tions, while the next two transition bands were assigned
peaks found at 1,397 and 1,253 cm−1 refer to the ν(C–O)
stretching vibrations of the phenolic group. In the case
of the vibration spectra of metal complexes, these
bands are offsite to lower frequencies (1,302–1,321 and
1,090–1,213 cm−1) because of the participation of the phe-
nolic oxygen in the coordination with the metal ions. In
the case of the spectra of the methoxy-diim complexes,
3
3
3
3
to d–d transitions of A2g (F) → T1g (P) and A2g → T1g
(F), respectively. The magnetic moment value of the Ni(II)
complex was recorded at 2.74 B.M. From these results, the
octahedral geometry was confirmed for this complex
[21,22]. The electronic UV-spectrum of the copper(II) com-
plex displayed four electronic peaks at 300, 350, 385, and
410 nm. The first two absorption peaks were assigned to
OCH3
OCH3
O
O
H
N
H
H
N
N
N
H
OH
Cl
Cl
O
Figure 5: Tautomerism change in the methoxy-diim ligand from the
keto to the enol form in the alkaline media.

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Moamen S. Refat et al.
Figure 6: TGA–DrTGA curves of methoxy-diim free ligand (a), cobalt(II) complex (b), nickel(II) complex (c), copper(II) complex (d), and zinc(II)
complex (e).
1
the ligand-to-metal charge-transfer transitions, while the 3.4 H-NMR spectral studies
second two absorption peaks, assigned to the 2Eg → 2T2g
d–d transition, were due to a distorted octahedral geome- 1H-NMR spectra for the methoxy-diim ligand and its dia-
trical structure of the Cu(II) complex [21,22]. The Cu(II) com- magnetic Zn(II) complexes were recorded in d6-DMSO as
plex recorded a magnetic moment of 1.66 B.M., confirming a solvent. The 1H-NMR spectra for the investigated ligand
the deformed octahedral geometry [13]. The electronic spec- (methoxy-diim) and its diamagnetic Zn(II) complexes
trum of the Zn(II) complex was found to have two electronic were recorded in DMSO as a solvent. The chemical shift
transition peaks at 300 and 356 nm. According to the con- of the investigated ligand was as follows: 2.576 (d, 1H,
ductivity, microanalytical studies and data from the spec- N\\NH), 6.997 (s, 1H, CH]N), 7.381–7.423, and 7.534–7.65
trum of the zinc(II) complex, its diamagnetic behaviour and for the two aromatic rings, 9.008 (s,1H, OH) ppm and
tetrahedral geometry were confirmed [21,22].
11.155 (s, 1H, OH) ppm. In the zinc complex (2:1), nearly
Table 3: Thermogravimetric data of the methoxy-diim and its metal complexes
Compounds Steps Temperature range/°C DTG peak/°C Decomposed assignments
Mass loss found (Calc.%)
76.3 (76.4)
diim
1st
46–205
100
335
diim
2nd
205–536
Residue 536–700
Carbon residue
2H2O + CO
2NH3 + 2N2 + 1½Cl2 + 7C2H2 + 2H2
2CoO + carbon residue
3H2O
H2O + 2NH3 + 3N2 + 3H2 + 1½Cl2 + 8C2H2
2NiO
2H2O
1½Cl2 + 5C2H2
2H2O + CO + 2NH3 + H2 + N2
2CuO + carbon residue
H2O + CO
2NH3 + N2 + 1½Cl2 + 6C2H2
2ZnO + carbon residue
23.7 (23.6)
10.6 (10.4)
63.6 (62.9)
25.8 (26.7)
8 (8.2)
I
1st
35–217
57
330
2nd
217–544
Residue 544–700
II
III
1st
40–196
114
377
2nd
196–562
69 (69.1)
Residue 562–700
23 (22.7)
5.9 (5.8)
1st
37–180
60
255
423
2nd
3rd
180–354
354–530
38.2 (38.2)
20.6 (20.6)
35.3 (35.4)
8.7 (8.1)
56.5 (57.1)
34.8 (34.8)
Residue 530–700
IV
1st
40–250
170
399.6
2nd
250–580
Residue 580–700

Utilization of new diimine Schiff base complexes against Covid-19
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779
Table 4: Coats–Redfern (CR) and Horowitz–Metzger (HM) calculations for the methoxy-diim ligand and its metal complexes
Compound
Stage
Method
Parameter
r
E* (J mol−1
)
A (s−1
)
ΔS (J mol−1 K−1
)
ΔH (J mol−1
)
ΔG (J mol−1
)
diim
1st
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
1.80 × 10−1
4.77 × 102
1.56 × 10−1
1.37 × 103
9.89 × 10−2
2.10 × 103
5.32 × 10−2
4.46 × 102
1.16 × 10−1
1.30 × 103
1.79 × 10−1
2.10 × 103
1.02 × 10−1
3.94 × 102
7.07 × 10−2
1.56 × 103
1.17 × 10−1
9.65 × 102
1.32 × 10−1
2.67 × 103
1.17 × 10−1
6.07 × 102
8.31 × 10−2
1.40 × 103
7.27 × 10−9
7.78 × 10−5
4.67 × 10−9
9.95 × 10−5
1.44 × 10−9
8.31 × 10−5
4.47 × 10−9
5.82 × 10−5
2.50 × 10−9
8.59 × 10−5
5.16 × 10−9
7.75 × 10−5
1.03 × 10−8
5.77 × 10−5
1.34 × 10−9
9.18 × 10−5
2.88 × 10−9
7.82 × 10−5
3.86 × 10−9
7.77 × 10−5
4.50 × 10−9
8.29 × 10−5
2.37 × 10−9
7.36 × 10−5
−4.03 × 102
−3.26 × 102
−4.10 × 102
−3.27 × 102
−4.23 × 102
−3.32 × 102
−4.08 × 102
−3.29 × 102
−4.16 × 102
−3.29 × 102
−4.12 × 102
−3.32 × 102
−4.00 × 102
−3.28 × 102
−4.22 × 102
−3.29 × 102
−4.12 × 102
−3.27 × 102
−4.14 × 102
−3.31 × 102
−4.08 × 102
−3.26 × 102
−4.17 × 102
−3.31 × 102
−3.14 × 103
−2.67 × 103
−5.01 × 103
−3.64 × 103
−6.89 × 103
−4.79 × 103
−3.48 × 103
−3.03 × 103
−5.18 × 103
−3.89 × 103
−7.09 × 103
−5.00 × 103
−3.27 × 103
−2.87 × 103
−5.57 × 103
−4.02 × 103
−3.98 × 103
−3.24 × 103
−6.40 × 103
−4.65 × 103
−3.48 × 103
−2.87 × 103
−5.80 × 103
−4.40 × 103
1.49 × 105
1.20 × 105
2.42 × 105
1.94 × 105
3.43 × 105
2.70 × 105
1.67 × 105
1.34 × 105
2.54 × 105
2.01 × 105
3.54 × 105
2.79 × 105
1.54 × 105
1.26 × 105
2.77 × 105
2.16 × 105
1.94 × 105
1.54 × 105
3.12 × 105
2.51 × 105
1.67 × 105
1.33 × 105
2.85 × 105
2.27 × 105
0.9726
0.9692
0.9888
0.9869
0.9518
0.9738
0.9934
0.997
0.9157
0.9589
0.9832
0.9913
0.9809
0.9971
0.9694
0.9561
0.9913
0.9839
0.9832
0.9788
0.954
2nd
3rd
1st
I
2nd
3rd
1st
II
2nd
1st
2nd
1st
III
IV
0.916
0.9887
0.9935
2nd
HM
all of the peaks became weaker and shifted to slightly 10%, calc. = 11%). The second degradation phase was in
lower frequencies. The predicted structural formula of the temperature range 205–536°C with equal weight loss
the zinc(II) complex is debated using the results from
the analysis performed, confirming its tetrahedral geo-
metry as shown in Figure 2.
3.5 Thermogravimetric studies
The TG and DrTGA thermal decomposition curves of the
free methoxy-diim ligand and its metal complexes [Co(II),
Ni(II), Cu(II) and Zn(II)] under a N2 atmosphere at a
heating rate of 10°C/min (Figure 6) are assigned and sum-
marized in Table 3.
3.6 Methoxy-diim free ligand
The melting point of the diim ligand was 100°C after it
underwent concurrent degradation. It was found that
the ligand decomposition takes place in two main degra-
dation steps. The first degradation phase was in the tem-
Figure 7: PXRD pattern of the free ligand, Co(II) and Ni(II) complexes.
perature range 46–205°C with equal weight loss (obs. =

780

Moamen S. Refat et al.
Table 5: PXRD data of the synthesized ligand (methoxy-diim)
3.8 Nickel(II) complex II
Pos. (°2Th.) Height
(cts)
FWHM
d-
Rel.
The thermal degradation of the Ni(II) complex occurred in
two main decomposition phases. The first step in the
decomposition with weight loss (obs. = 8%, calc. =
8.2%) was due to the loss of three uncoordinated water
molecules and two hydrogen molecules in the tempera-
ture range 40–196°C. The second degradation phase with
weight loss (obs. = 69%, calc. = 69.1%) was due to the
loss of chloride, ammonia, water, nitrogen gas, acetylene
gas, and hydrogen gas molecules in the temperature
range 196–562°C. The final residue obtained was 2NiO
molecules until 700°C.
left (°2Th.) spacing (Å) Int. (%)
9.6789
15.4503
17.6460
19.5862
21.3536
25.2967
26.3999
30.5624
45.3754
Ni(II) complex
8.7595
29.19
41.87
12.67
16.96
33.30
58.94
47.30
10.15
7.51
0.2362
0.2755
0.4723
0.4723
0.2362
0.2362
0.3542
0.9446
0.9446
9.13823
5.73523
5.02625
4.53251
4.16117
3.52080
3.37612
2.92513
1.99876
49.53
71.05
21.49
28.77
56.50
100.00
80.26
17.22
12.74
16.34
13.09
0.6298
0.3936
10.09524
2.05229
100.00
80.15
44.1286
Pos. = possible; Rel. Int. = relative intensity; FWHM = full width of
high maximum.
3.9 Copper(II) complex III
The thermal degradation phase of the Cu(II) complex under-
goes three decomposition phases with temperature max-
imum values (DTGmax) of 60 (37–180°C), 255 (180–354°C),
and 423 (354–530°C). The first phase with weight loss
(obs. = 5.9%, calc. = 5.8%) was due to the loss of two water
molecules with a DTGmax = 60°C. The phases from second to
the third degradation with weight loss (obs. = 58.8%, calc. =
58.7%) due to the loss of two water molecules, ammonia
gas, chloride, nitrogen, hydrogen, carbon oxide, and acet-
(obs. = 66.30%, calc. = 65.40%). The remaining residue until
700°C was accompanied by weight loss (obs. = 23.7%, calc. =
23.6%). All of the thermal fragments in all stages were
organic moieties that were converted to gaseous phases.
3.7 Cobalt(II) complex I
From the thermal degradation curve of the Co(II) complex, ylene gas molecules with DTGmax values of 255 and 423°C.
it was found that it decomposes as follows: the first decom- The final residue was CuO.
position phase is in the temperature range 35–217°C with
weight loss (obs. = 10.6 %, calc. = 10.4%), attributed to the
loss of coordinated 2H2O + CO. The second phase degrada-
tion takes place in the temperature range 217–544°C with a 3.10 Zinc(II) complex IV
weight loss (obs. = 63.6%, calc. = 62.9%), attributed to the
loss of chloride, ammonia, nitrogen, hydrogen, and acet- The thermal degradation of the Zn(II) complex was found
ylene gas molecules. The final residue was pure cobalt to take place in two main steps with DTGmax = 170
metal produced from the reduction of cobalt carbonate (40–250°C) and 399.6 (250–580)°C. The first degradation
to cobalt(II) oxide and remained stable up to 700°C.
phase with weight loss (obs. = 8.7%, calc. = 8.1%) was
Table 6: Biological evaluations for methoxy-diim ligand and its metal complexes (concentration: 10 mg/mL)
Compound
Gram-positive bacteria
B. subtilis S. aureus
Gram-negative bacteria
Fungi
E. coli
P. vulgaris
A. flavus
C. albicans
diim
I
II
III
NA
14
15
8
19
26
—
NA
12
18
13
20
24
—
NA
12
11
13
15
30
—
NA
NA
NA
NA
NA
25
—
NA
NA
NA
NA
NA
—
NA
NA
NA
NA
NA
—
IV
Gentamycin
Ketoconazole
16
20

Utilization of new diimine Schiff base complexes against Covid-19
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781
Table 7: Docking calculations for the methoxy-diim free ligand
Est. free energy
of binding
(kcal/mol)
Est. inhibition
constant, Ki (µM) desolv energy
(kcal/mol)
vdW + Hbond +
Electrostatic energy Total intermolec.
Frequency (%) Interact.
(kcal/mol)
energy (kcal/mol)
surface
Results of diim vs 3hb5-oxidoreductase
−6.64 13.54
Results of diim vs 4o1v-protein
−3.54 2.56
Results of diim vs COVID-19 protease-protein
−4.58 441.28 −4.68
−8.14
−5.03
−0.13
−0.00
−0.24
−8.28
−5.03
−4.92
40
10
50
864.29
628.377
566.515
due to the loss of water and carbon oxide with a DTGmax
=
3.12 Powder X-ray diffraction studies of the
free ligand and its complexes with
Co(II) and Ni(II)
170°C. The second thermal decomposition with weight
loss (obs. = 56.5%, calc. = 57.1%) was due to the loss of
chloride, ammonia, nitrogen, and acetylene in the tem-
perature range 250–580°C. The residual product was
2ZnO until 700°C.
The powder XRD pattern of the free ligand (diim) and com-
plexes prepared with different ions are shown in Figure 7.
We have recorded the pattern within the range of 2θ
between 5° and 60°. In the case of methoxy-diim, nine
peaks have been obtained at different diffraction angles,
summarized in Table 5, and therefore diim is crystalline in
nature [25,26]. On the other hand, the complex of diim
with Co(II) does not show any sharp peak in the XRD pat-
tern; instead, it has noise signals that confirm its amor-
phous behaviour. It was noted from the data that only two
peaks at diffraction angles 2θ of 8.75° and 44.12° are
observed in the Ni(II) complex as shown in Table 5.
3.11 Kinetic thermodynamic parameters
From the Arrhenius plots between the rate of decomposi-
tion (ln K) and 1/T, the kinetic values such as activa-
tion energy (E*) were computed. The Coats–Redfern
and Horowitz–Metzger equations were utilized to calcu-
late other thermodynamic parameters such as free energy
ΔG*, the enthalpy ΔH*, and the entropy ΔS* of the process
[23,24]. Since ΔG*, ΔH*, and ΔS* are relevant to the
highest ratios, they are computed by the peak tempera-
ture Ts (Table 4). The prepared activated metal complexes 3.13 Biological assessments
were found to be more orderly structured than the reac-
tants and this was proved by the negative value of ΔS* The free diim ligand and its metal complexes were dissolved
[25] obtained.
in DMSO to obtain a final solution with concentrations of
Figure 8: HB plot of the methoxy-diim ligand vs 3hb5-oxidoreductase protein (a), 4o1v-protein binding (b), and COVID-19 protease binding
protein (c).

782

Moamen S. Refat et al.
Figure 9: Docking interactions of 6LU7 with a: Co(II), b: Ni(II), c: Cu(II), and d: Zn(II) complexes.
The simulation for the docking was accomplished by
Solis & Wets local search method [29,30] and Lamarckian
genetic algorithm (LGA). The docking survey output results
carried out for the 3hb5-oxidoreductase (breast cancer) pro-
tein, 4o1v-protein (kidney cancer), and COVID-19 protease
binding vs the methoxy-diim free ligand are summarized in
Table 7.
The methoxy-diim free ligand displayed good binding
interactions with the amino acids of the protein mole-
cules showing good stability with binding reaction energy
values of −6.64, −3.54, and −4.58 kcal mol−1 for 3hb5-oxi-
doreductase protein, 4o1v-protein, and COVID-19 protease,
respectively (Figure 8a–c).
It can be concluded that the free diim ligand can bind
to the active positions in the 3hb5-oxidoreductase pro-
tein, 4o1v-protein, and COVID-19 protease binding pro-
tein, which means that it has a great binding affinity
towards them.
The active binding site of COVID-19 was studied with
the metal free ligand (diim) and all of the other com-
plexes formed with different ions (Co, Ni, Cu, Zn). It
was found from the docking study that the Ni(II) complex
displays the best free energy of binding of −7.12 kcal/mol
with the protease. The complex interactions of ligand and
COVID-19 are shown in Figure 9.
The free energy of bindings (FEBs) with Co(II), Ni(II),
Cu(II), and Zn(II) were found to be −6.19, −7.12, −6.89, and
−6.79 kcal/mol, respectively. Further, the smaller binding
energy shows better efficiency of binding, which is observed
in the nickel complex [31], and the obtained interactions
with amino acids are Glu-166 and Asn-142. So, it represents
the best binding of COVID-19 protease. However, 3D inter-
actions of Co(II), Ni(II), Cu(II), and Zn(II) complexes are
shown in Figure 10 and the other docked information is
summarized in Table 8.
Figure 10: 3D interaction diagram showing the docking interactions of (a)
Co(II), (b) Ni(II), (c) Cu(II), and (d) Zn(II) complexes with COVID-19 (6LU7).
10 and 50 mg/mL, which was used to measure the inhibi-
tion zone to determine the antibacterial activity of
these compounds. As DMSO was used as a solvent it
was loaded for monitoring. This activity was determined
by the agar well diffusion method three times to ensure
consistency of the results. Bacillus subtilis and Staphylo-
coccus aureus were the Gram-positive bacteria used,
while Escherichia coli and Proteus vulgaris were Gram-
negative bacteria. From the summarized data in Table 6,
we can conclude that the diim metal complexes show
low average activity against B. subtilis, S. aureus, and
E. coli. There was no activity against P. vulgaris for both
the free diim ligand and its metal complexes. Also, they
recorded no activity against Aspergillus flavus and
Candida albicans species during determining their anti-
fungal activity.
3.14 Molecular docking
The docking calculations were performed using the methoxy- 4 Conclusion
diim protein model using docking server after merging
the non-polar hydrogen atoms, defining rotatable bonds, The idea of the present research was based on preparing
adding Gasteiger charges, and other analytical para- some of the new Schiff base compounds from the con-
meters with the assistance of AutoDock tools [27,28]. densation reaction of the salicylaldehyde nucleus with

Utilization of new diimine Schiff base complexes against Covid-19

783
Table 8: Docking interaction parameters
Complexes Binding free energy
Total intermolecular energy
Interacting amino acids
(kcal/mol)
(kcal/mol)
Co(II)
−6.19
−8.58
LYS137, ASP289, GLU298, LEU287, LEU 286, THR
199, TYR239
Ni(II)
Cu(II)
Zn(II)
−7.12
−6.89
−6.79
−9.21
−9.57
−8.29
GLU166, ASN142
GLU14, PRO122, SER121
GLU166, HIS172, GLY170, SER139, GLY138, LYS5,
CYS128
some aromatic amines, as these types of compounds pos- Author contributions: Conceptualization, M.S.R. and M.I.K.;
sess biological properties that are anti-bacteria, anti- methodology, M.S.R., M.I.K. and R.Z.; software, M.S.R.
fungal, and anti-viral. The work of this research project and K.A.; validation, A.G., W.F.A., and M.S.R.; formal
also extends to a theoretical and interactive simulation of analysis, M.S.R., A.G., W.F.A., M.I.K., R.Z., and K.A.;
the prepared compounds and their effect on the Corona investigation, A.G., W.F.A., and R.Z.; resources, M.S.R. and
virus. We were interested in preparing and characterizing M.I.K.; data curation, M.S.R., A.G., W.F.A., M.I.K., R.Z., and
four different metal Cu(II), Ni(II), Zn(II), and Co(II) com- K.A.; writing and original draft preparation, M.S.R., A.G.,
plexes of the diim ligand in a ratio of 2:1 (metal/ligand). and M.I.K.; writing-review and editing, M.S.R., A.G., W.F.A.,
The free ligand and its metal complexes were character- and K.A.; visualization, M.S.R. and A.G.; supervision,
ized by elemental analysis, electronic analysis, FTIR, M.S.R.; project administration, A.G.; funding acquisition,
1H-NMR, XRD, and mass spectroscopy. From the FTIR A.G., W.F.A., and M.S.R. All authors have read and agreed
spectra, it was found that the coordination process took to the published version of the manuscript.
place with the nitrogen atom of the azomethine group
and the oxygen atom of the hydroxyl group for the first Conflict of interest: All authors declare that they have no
metal ion, while the second one formed a coordination conflicts of interest.
bond with the oxygen atom of the phenol group and the
other nitrogen atom of the azomethine group forming (2:1) Data availability statement: The data used to support the
complexes. All complexes had octahedral geometry, findings of this study are included within the article.
except for Zn(II) which had tetrahedral configuration.
The thermal decomposition of both free ligand and its
metal complexes occurs through two main degradation
steps, with metal oxides (CoO, NiO, CuO, and ZnO) as a
final residual product. The conductivity values confirmed
the non-electrolytic behaviour of all of the complexes.
Finally, biological evaluations were run as antibacterial,
antifungal, and molecular docking. The molecular docking
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