Synthesis, biological and in silico studies of a tripodal schiff base derived from 2,4,6-triamino-1,3,5-triazine and its trinuclear Dy(III), Er(III), and Gd(III) salen capped complexes

Article abstract of DOI:10.3390/molecules26144379

A tripodal Schiff base ligand, 2,4,6-Tris(4-carboxybenzimino)-1,3,5-triazine (MT) and its trinuclear Dy(III), Er(III), and Gd(III) complexes were synthesized. These were characterized using UV-visible, IR,¹H, and¹³C NMR spectroscopies, elemental analysis, and molar conductivity meas-urements. The spectral studies indicate that the ligand is hexadentate and coordinates to the Ln(III) ions through the oxygen atoms of the carboxylic group. The trinuclear complexes were characterized as being bridged by carboxylate anions to the Dy(III), Er(III), and Gd(III) salen centers and displaying a coordination number of six. Biological studies revealed that MT is more active against the test micro-organisms relative to the trinuclear complexes. Acute toxicity studies revealed that MT is safe and has a wide range of effective doses (ED50). In vivo antimalarial studies indicate that MT could serve as an effective antimalarial agent since it has parasitemia inhibition of 84.02% at 50 mg/kg and 65.81% at 25 mg/kg, close to the value (87.22%) of the standard drug—Artesunate. Molecular docking simulation studies on the compounds against SARS-CoV-2 (6Y84) and E. coli DNA gyrase (5MMN) revealed effective binding interactions through multiple bonding modes. The binding energy calculated for Er(III)MT-6Y84 and Er(III)MT-5MMN complexes showed active molecules with the ability to inhibit SARS-CoV-2 and E. coli DNA gyrase.

Full text of DOI:10.3390/molecules26144379

molecules
Article
Synthesis, Biological and In Silico Studies of a Tripodal Schiff
Base Derived from 2,4,6-Triamino-1,3,5-triazine and Its
Trinuclear Dy(III), Er(III), and Gd(III) Salen Capped Complexes
Uchechukwu Susan Oruma 1, Pius Oziri Ukoha 1, Chiamaka Peace Uzoewulu 2 , Joseph Chinedum Ndefo 3,
Sabastine Chinweike Ugwuoke 4, Nkechinyere Nwanneka Ukwueze 2, Tochukwu Emmanuella Eze 2,4
,
Lilian Chinenye Ekowo 2, Florence Uchenna Eze 2 , Uchenna Vivian Chinaegbomkpa 2,5
,
Sunday Nwankwo Okafor 6,* and Chigozie Julius Ezeorah 2,
*
1
Coordination Chemistry and Inorganic Pharmaceuticals Unit, Department of Pure and Industrial Chemistry,
University of Nigeria, Nsukka 410001, Nigeria; susan.oruma@unn.edu.ng (U.S.O.);
pius.ukoha@unn.edu.ng (P.O.U.)
Department of Pure and Industrial Chemistry, University of Nigeria, Nsukka 410001, Nigeria;
chiamaka.uzoewulu@unn.edu.ng (C.P.U.); nkechinyere.ukwueze@unn.edu.ng (N.N.U.);
tochyodike@gmail.com (T.E.E.); chinenye.ekowo@unn.edu.ng (L.C.E.); florence.ali@unn.edu.ng (F.U.E.);
uchenna.chinaegbomkpa@futo.edu.ng (U.V.C.)
Department of Science Laboratory Sciences, University of Nigeria, Nsukka 410001, Nigeria;
joseph.ndefo@unn.edu.ng
Department of Biochemistry, University of Nigeria, Nsukka 410001, Nigeria;
sabastine.ugwoke.88417@unn.edu.ng
2
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
3
4
Citation: Oruma, U.S.; Ukoha, P.O.;
Uzoewulu, C.P.; Ndefo, J.C.;
Ugwuoke, S.C.; Ukwueze, N.N.; Eze,
T.E.; Ekowo, L.C.; Eze, F.U.;
5
6
Department of Chemistry, Federal University of Technology, Owerri 460114, Nigeria
Department of Pharmaceutical and Medicinal Chemistry, University of Nigeria, Nsukka 410001, Nigeria
Correspondence: sunday.okafor@unn.edu.ng (S.N.O.); chigozie.ezeorah@unn.edu.ng (C.J.E.);
Tel.: +234-803-655-6699 (S.N.O.); +234-703-299-6667 (C.J.E.)
Chinaegbomkpa, U.V.; et al.
*
Synthesis, Biological and In Silico
Studies of a Tripodal Schiff Base
Derived from 2,4,6-Triamino-
1,3,5-triazine and Its Trinuclear Dy(III),
Er(III), and Gd(III) Salen Capped
Complexes. Molecules 2021, 26, 4379.
https://doi.org/10.3390/
Abstract: A tripodal Schiff base ligand, 2,4,6-Tris(4-carboxybenzimino)-1,3,5-triazine (MT) and its
trinuclear Dy(III), Er(III), and Gd(III) complexes were synthesized. These were characterized using
UV-visible, IR, 1H, and 13C NMR spectroscopies, elemental analysis, and molar conductivity mea-
surements. The spectral studies indicate that the ligand is hexadentate and coordinates to the Ln(III)
ions through the oxygen atoms of the carboxylic group. The trinuclear complexes were characterized
as being bridged by carboxylate anions to the Dy(III), Er(III), and Gd(III) salen centers and displaying
a coordination number of six. Biological studies revealed that MT is more active against the test
micro-organisms relative to the trinuclear complexes. Acute toxicity studies revealed that MT is safe
and has a wide range of effective doses (ED50). In vivo antimalarial studies indicate that MT could
serve as an effective antimalarial agent since it has parasitemia inhibition of 84.02% at 50 mg/kg
and 65.81% at 25 mg/kg, close to the value (87.22%) of the standard drug—Artesunate. Molecular
docking simulation studies on the compounds against SARS-CoV-2 (6Y84) and E. coli DNA gyrase
(5MMN) revealed effective binding interactions through multiple bonding modes. The binding
energy calculated for Er(III)MT-6Y84 and Er(III)MT-5MMN complexes showed active molecules with
the ability to inhibit SARS-CoV-2 and E. coli DNA gyrase.
molecules26144379
Academic Editors: Antonella
Dalla Cort and Jan Janczak
Received: 19 May 2021
Accepted: 14 July 2021
Published: 20 July 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Keywords: tripodal schiff base; trinuclear Ln(III) complexes; antimicrobial activity; antimalarial
activity; docking
Copyright:
© 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1. Introduction
The emergence of resistant strains of the malaria parasite has necessitated the con-
tinued search for other effective, safe, and cheap antimalarial agents. The resistance of
the malaria parasite to chloroquine and other anti-malarials has driven scientists into an
intensive search for more effective agents against the scourge. Currently, there are reports
of parasite resistance to the newly developed Artemisinin in some regions [1]. Additionally,
Molecules 2021, 26, 4379. https://doi.org/10.3390/molecules26144379
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 4379
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the WHO warns that COVID-19 disruption to anti-malaria programs could cause deaths in
Sub-Saharan Africa from malaria to double [2]. Thus, there is an urgent need for increased
efforts in antimalarial drug discovery especially in Africa [3].
1,3,5-Triazine(s-triazine) analogs demonstrate a wide range of biological applications
such as antimalarial, antimicrobial, antiviral, anticancer, antituberculosis, anti-HIV, an-
tileishmanial, anti-inflammatory agents, insecticidal, and herbicidal [
they are mostly used in the pharmaceutical, plastic, textile, and rubber industries [11
They have also been used as dyestuffs, optical bleaches, explosives, and surface-active
agents [14 17]. Additionally, cycloguanil, a derivative of 1,3,5-trazine, has been noted to be
4–
10]. Moreover,
–13].
–
a cyclic metabolite of the antimalarial drug proguanil [18].
The s-triazine core has been applied in the development of less toxic bioactive com-
pounds [19]. Consequently, melamine (2,4,6-triamino-s-triazine) based dendrimers are
widely used in cancer and in many biological treatments [20]. Additionally, in the field
of neuroleptics, s-triazines showed the most potent antimethamephetamine action with
less toxicity than chlorpromazine [21]. Gupta and co-workers have reported thioether-
linked s-triazine to 1,2,4-triazine derivative bearing piperidine substituent as potent against
Leishmania donovani with less toxicity compared with standard pentamidine and sodium
stibogluconate, respectively [22]. Comparison of s-triazine based drugs with other non-
triazine based drugs has shown that s-triazine based drugs are more potent [19].
The literature has shown that s-triazine has been combined with other heterocycles
with the aim of developing more potent drug-like molecules [23–27]. However, there is
no report of s-triazine scaffolds containing lanthanides. In recent years, lanthanides and
lanthanide compounds have attracted a great deal of interest because they have applica-
tions in medicinal inorganic chemistry and in material science [28]. In medicine, lanthanide
complexes are exploited as contrast agents for magnetic resonance imaging (MRI) and as
radiotherapeutic drugs [28,29]. Due to the catalytic, magnetic, and luminescent properties
of lanthanide complexes with organic ligands, they have found application in electrolu-
minescent devices and diodes, lasers, cathode ray tubes, sensors, dosimeters, biological
fluoro-immunoassays, imaging agents, organic light emitting diodes (OLEDS), display ap-
plication, decoration purposes, and telecommunication [30
desired functions have been designed and synthesized using different kinds of organic lig-
ands [32 33]. Moreover, lanthanides have an interesting, but not well understood, biological
,31]. Lanthanide complexes with
,
role in living organisms as trace elements [34]. Our group recently reported neodymium(III)
and thallium(III) complexes of N-benzothiazol-2-yl)-4-chlorobenzenesulphonamide as po-
tential antibacterial agents [35]. Recently, Taha et al. (2011) [36] reported that Nd, Dy,
Sm, Pr, Gd, Tb, and Er complexes with bis-(salicylaldehyde)-1,3-propylenediimine Schiff
base ligand possessed high antibacterial activity against Shigella dysenteriae, Pseudomonas
aeruginosa, and Proteus vulgaris (Gram-negative bacteria).
In view of the noted biological properties of s-triazines and lanthanide(III) complexes,
we synthesized and characterized 2,4,6-Tris(4-carboxybenzimino)-1,3,5-triazine (MT) and
its trinuclear Dy(III), Er(III), and Gd(III) salen capped complexes. Their in vitro antimicro-
bial, in vivo antimalarial activities, and acute toxicity were also investigated. Additionally,
in silico studies were carried to have a view of the interaction of the synthesized compounds
with SARS-CoV-2 (6Y84) and E.coli DNA gyrase (5MMN).
2. Results and Discussion
2,4,6-Tris(4-carboxybenzimino)-1,3,5-triazine (MT) was synthesized by the reaction of
2,4,6-triamino-1,3,5-triazine and 4-carboxybenzaldehyde (Scheme 1). MT was identified
I
using FTIR, H, and 13C-NMR elemental analysis. It is soluble in acetone, chloroform,
ethylacetate, DMF, and DMSO.

Molecules 2021, 26, 4379
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Scheme 1. Synthesis of MT.
[Dy/Er/Gd(salen)]2O were used as Ligand complexes (LC) because they were able
to coordinate to MT. The reaction of the LCs with MT gave rise to the tripodal trinu-
clear complexes, [{Dy/Er/Gd(salen)}3(MT)].3H2O (Scheme 2). These tripodal trinuclear
complexes are the first examples of s-triazine based trinuclear complexes bridged to the
Dysprosium(III), Erbium(III), and Gadolnium(III) centers by COO-. All complexes are
stable at room temperature and soluble in DMSO and DMF but insoluble in water. The
analytical data of MT and its trinuclear complexes are in good agreement with the proposed
molecular formula as shown in Table 1. Molar conductivity measurements in methanol at
room temperature show that the compounds are non-electrolytes [37].
Scheme 2. Synthesis of the tripodal trinuclear complexes; M = Dy, Er, Nd.
Table 1. Elemental and physical data of 2,4,6-Tris(4-carboxybenzimino)-1,3,5-triazine(MT) and its Ln(III) complexes.
Elemental Analysis % Calc. and Found
Λ
m
2
◦
Compound
Yield g (%)
M.p. ( C)
C
H
N
Color
(Ω−1cm mol
)
−1
Calc. Found Calc. FoundCalc. Found
C27H18O6N6 (MT)
White
-
(2.25) 86.21
(0.27) 58.70
346 a
325 a
62.07 61.95 3.45 3.40 16.09 15.90
C75H57N12O12Dy3.3H2O
(Dy(III)MT)
Yellow
4.60
48.43 48.30 3.39 3.50 9.04 8.80
48.08 48.10 3.37 3.50 8.97 8.60
48.86 48.90 3.42 3.60 9.12 8.90
C75H57N12O12Er3.3H2O
(Er(III)MT)
Yellow
Yellow
6.70
(0.28) 59.57
(0.30) 65.22
324 a
352 a
C75H57N12O12Gd3.3H2O
(Gd(III)MT)
18.40
a
= decomposition temperature.

Molecules 2021, 26, 4379
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2.1. Electronic Spectra
The UV/Vis absorption spectra of MT and its complexes (10−4 moldm−3) were carried
out in methanol at room temperature, see Table 2 and Figure S1. The spectra are shown in
supporting documents. The absorption band of MT showed two bands at 233 and 291 nm
assigned to π–π* transitions of the benzene rings. In the Ln complexes, these bands are red
shifted, supporting the coordination of MT to the Ln ions (Figures S2–S4).
Table 2. Electronic absorption data of 2,4,6-tris(4-carboxybenzimino)-1,3,5-triazine (MT) and its
Ln(III) complexes.
λ
max
3
3
−1
ε × 10 (mol−1dm cm
)
Compound
Band Assignment
−1
cm
nm
233
291
42,918
34,364
2.88
1.82
π–π*
π–π*
MT
Dy(III)MT
Er(III)MT
Gd(III)MT
348
347
350
28,736
28,818
28,571
10.9
9.72
9.82
n–π*
n–π*
n–π*
2.2. Infrared Spectra
The relevant stretching frequencies of MT and its Ln(III) complexes are shown in
Table 3, Figures S5–S8. The absorption band due to the carboxylic acid C=O was obse−rv1ed
−1
at 1674 cm in MT [38
,
39]. This band shifted to higher frequencies of about 22–28 cm in
the complexes, suggesting coordination of the ligand complexes via the carboxylic acid
C=O of MT. This was further supported by the vibrations of the COO− group observed at
1410–1412 cm−1 in the complexes and at 1391 cm−1 in the ligand [40]. The IR spectrum of
MT showed two medium bands for C=N(a) and C=N(b) at 1501 and 1573 cm−1, respectively.
−1
However, in the complexes, three bands were observed: C=N(a) bands at 1592–1596 cm
,
C=N(b) bands at 1628–1636 cm−1, and C=N(c) bands at 1536–1545 cm−1 respectively. The
C=N(a) and C=N(b) stretching vibration in the complexes shifted to higher wavenumbers
in comparison to the same transition in the ligand indicating delocalization of the double
bond of the tripodal Schiff base in coordinating with the ligand complexes. While the
C=N(c) band, which was absent in the tripodal Schiff base ligand but present in the
complexes, confirms that the ligand complexes were actually capped to the tripod. Similar
−1
observation has been made in literature [38
tripodal–trinuclear complexes were assigned to
of 489–437 cm−1 were assigned to ν (Ln–N) [42].
–
40]. Bands in the range of 579–571 cm in the
ν
(Ln–O) [36,41] while bands in the range
Table 3. IR band assignments (cm−1) for 2,4,6-tris(4-carboxybenzimino)-1,3,5-triazine (MT) and its Ln(III) metal complexes.
−
Compound
νC-H ar
νC=O
νC=N
νC–C
νCOO
νC–N
νLn–O
νLn–N
1501(m) a
1573(m) b
MT
-
1674(s)
1422(m)
1391(m)
1410(s)
1168(m)
-
-
1594(m) a
1631(s) b
1542(m) c
1475(m)
1448(m)
1151(m)
1125(m)
489(m)
437(m)
Dy(III)MT
Er(III)MT
Gd(III)MT
3129(sh)
3158(w)
3125(m)
1698(m)
1696(m)
1702(m)
579(m)
575(m)
571(m)
1596(s) a
1632(s) b
1543(s) c
1151(m)
1125(w)
1477(m)
1483(m)
1410(s)
1412(s)
439(m)
437(m)
1592(m) a
1636(m) b
1540(s) c
1153(m)
Where a = from triazine ring; b = from azomethine linkage; c = from salen.

Molecules 2021, 26, 4379
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1
2.3. H and 13C-NMR Spectra
1
The H and 13C-NMR spectra of MT and its Ln(III) complexes are presented in
1
Tables 4 and 5 respectively. The H NMR spectrum of MT revealed a singlet peak at
10.17 ppm due to carboxylic proton (Figure S9). This peak disappeared in the complexes.
The signal due to azomethine protons was observed between 9.77 and 8.29 ppm in the
compounds. The signals in the range of 6.25–7.99 ppm in the compounds were assigned
to aromatic protons. The signal due to ethylene protons appeared only in Dy(III)MT
at 5.82 ppm. The signal due to DMSO was observed at 2.46 ppm in Dy(III)MT (Figure S10)
.
The spectra for Er(III)MT and Gd(III)MT showed only solvent peak while that of Dy(III)MT
showed only three peaks, probably due to the extent of paramagnetism of the Ln (III)
ions [42,43].
Table 4. 1H-NMR data of 2,4,6-tris(4-carboxybenzimino)-1,3,5-triazine (MT) and its Ln(III)
Complexes (ppm).
Compound
OHCarboxylic
CH=N
Haromatic
CH2=CH2
DMSO
9.77(1H,s)
8.29(1H,s)
7.99–
6.25(4H,m)
MT
10.17(1H,s)
-
2.50
Dy(III)MT
Er(III)MT
Gd(III)MT
-
-
-
-
-
-
7.65(4H,m)
5.82(4H,s)
2.46
-
-
-
-
-
-
Table 5. 13C-NMR data of 2,4,6-tris(4-carboxybenzimino)-1,3,5-triazine (MT) and its Ln(III) Complexes (ppm).
DMSO
Peak
Compound Carboxylic Carbon
Azomethine Carbon
Carbons on Triazine Ring
Aromatic Carbons
MT
193.47
167.47, 165.75
139.07, 136.98
130.32, 129.92
39.89
Dy(III)MT
Er(III)MT
Gd(III)MT
-
-
-
-
-
-
-
-
-
-
-
-
39.58
-
-
13
The
C-NMR of MT gave signal at 193.47 ppm attributed to carboxylic carbon
44]. The signal due to azomethine carbon was observed at 165.75 and 167.47
(Figure S11)
[
ppm in MT [45]. The signal at 139.07 and 136.98 ppm in MT has been assigned to carbons
on the triazine ring. Carbons on the benzene ring are present at 130.32 and 129.92 ppm in
MT. The spectra of the complexes revealed only the peak due to DMSO because of their
paramagnetic nature (Figure S12) [42,43].
2.4. In Vitro Antimicrobial Activity
The results of the in vitro antimicrobial screening carried out on the compounds are
given in Table 6. Ciprofloxacin, Tetracycline, Gentamicin, and Fluconazole were used
as positive control while sterile DMSO served as negative control. These drugs have
been chosen because they have the same mechanism of action, which is by inhibiting
nucleic acid synthesis [46]. The structures of these drugs are shown in Supplementary
Materials (Figure S13). Ciprofloxacin (C17H18FN3O3) belongs to fluoroquinolnes and
inhibits bacteria growth by preventing Deoxyribonucleic acid (DNA) synthesis before
mitosis. Tetracycline (C22H24N2O8) inhibits the multiplication of bacteria by binding to
a subunit of the ribosomes, thereby inhibiting protein and nucleic acid synthesis and
consequent death of the bacterium [47,48]. Gentamycin (C21H43N5O7) belongs to the
class of aminoglycosides and acts by preventing protein synthesis, thereby inhibiting the
synthesis of nucleic acids (DNA replication or RNA synthesis) and causes death of the
bacterium. Fluconazole is an antifungal drug (C13H12F2N6O) and belongs to synthetic

Molecules 2021, 26, 4379
6 of 15
triazoles. Fluconazole inhibits fungal cytochrome P–450, an enzyme responsible for fungal
sterol synthesis, thereby causing fungal cell walls to weaken [48].
Table 6. Inhibition zone diameter (IZD in mm) of the compounds against typed strains (ATCC
CULTURES) micro-organisms.
50 µg/mL
B. c
(ATCC
14579)
S. a
(ATCC
6538P)
P. a
(ATCC
9027)
E. c (ATCC
Compound
C. a
A. n
6749)
MT
5
5
2
3
15
1
12
6
3
5
8
31
2
21
15
14
12
Dy(III)MT
Er(III)MT
Gd(III)MT
1
2
5
3
9
2
25 µg/mL
7
2
MT
-
-
-
-
10
-
2
-
19
-
15
2
-
Dy(III)MT
Er(III)MT
Gd(III)MT
-
-
-
-
3
-
4
-
-
12.5 µg/mL
MT
-
-
-
-
5
-
3
-
3
-
6
-
10
-
Dy(III)MT
Er(III)MT
Gd(III)MT
-
-
-
-
-
-
-
-
-
-
Key: B. c = Bacillus cereus, S. a = Staphylococcus aureus, P. a = Pseudomonas aeruginosa, E. c = Escherichia coli,
C. a = Candida albicans. A. n = Aspergillus niger, (-) = no zone of inhibition observed.
The results obtained (Table 6) show that the compounds inhibited the growth of
Bacillus cereus, Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans,
and Aspergillus niger with inhibition zone diameter (IZD) in the range of 2–5, 1–15, 1–12, 2–8,
2–31, and 2–21 mm respectively. This reflects that the compounds exhibit higher activity
against fungi (Candida albicans and Aspergillus niger) relative to the bacteria strains used.
Among the test bacteria, the compounds were most active against Staphylococcus aureus
followed by Pseudomonas aeruginosa. It was observed from the results (Table 6) that the
activity of MT is higher than that of the trinuclear complexes. Hence, it could be inferred
that the activity of the trinuclear complexes was not enhanced after anion coordination.
The inhibition zone diameter (IZD in mm) of the controls is displayed in Table S1.
From Table S1, the inhibition zone diameters (IZD) of the controls are higher than that of
the compounds.
The minimum inhibitory concentration (MIC) of the compounds and controls against
Bacillus cereus, Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans,
and Aspergillus niger are displayed in Table 7. From Table 7, the MIC of the compounds is
found to be in the range of 50 for Bacillus cereus, 5.57–50 for Staphylococcus aureus, 6.27–50
for Pseudomonas aeruginosa, 7.3–50 for Escherichia coli, 2.6–50 for Candida albicans, and 2.3–50
for Aspergillus niger.

Molecules 2021, 26, 4379
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Table 7. Minimum inhibitory concentration (MIC) of the compounds and controls against test bacteria
and fungi.
MIC (µg/mL)
B. c
(ATCC
14579)
S. a
(ATCC
6538P)
P. a
(ATCC
9027)
E. c
(ATCC
6749)
Compound
C. a
A. n
MT
50
50
50
50
5.57
50
6.27
7.3
50
50
25
2.60
50
2.30
25
Dy(III)MT
Er(III)MT
Gd(III)MT
50
50
50
50
50
25
50
50
50
Controls (µg/mL)
T
F
1.90
6.25
1.50
1.40
1.80
6.25
0.70
2.70
0.63
6.25
0.92
0.71
2.15
2.80
0.65
2.60
2.10
0.64
2.00
2.50
0.58
0.74
6.25
0.64
CP
G
Legend: T = Tetracycline, F = Fluconazole, CP = Ciprofloxacin, G = Gentamycin; B. c = Bacillus cereus,
S. a = Staphylococcus aureus, P. a = Pseudomonas aeruginosa, E. c = Escherichia coli, C. a = Candida albicans,
A. n = Aspergillus niger.
From Table 7, the MIC of the controls is found to be in the range of 1.40–6.25 µg/mL for
Bacillus cereus, 0.70–6.25 µg/mL for Staphylococcus aureus, 0.63–6.25
aeruginosa, 0.65–2.80 g/mL for Escherichia coli, 0.64–2.50 g/mL for Candida albicans, and
0.58–6.25 µg/mL for Aspergillus niger.
The MIC of MT against Staphylococcus aureus was 5.57
tamycin was 2.70 g/mL. Among all the test compounds, MT was found to be the
most active against Candida albicans and Aspergillus niger (MIC = 2.60 and 2.30 g/mL,
respectively). However, the standard antifungal drug Fluconazole was more active
µg/mL for Pseudomonas
µ
µ
µg/mL while that of Gen-
µ
µ
against Candida albicans and Aspergillus niger (MIC = 0.64 and 0.74 µg/mL, respectively)
relative to MT.
2.5. Acute Toxicity
The acute toxicity test was recorded for MT alone as shown in Table S2. MT was
selected because it showed higher sensitivity against the test micro-organisms. For mice
administered with MT, no animal died within 24 h after administration (Table S2). This
implies that MT is safe and has a wide range of effective doses (ED50).
2.6. In Vivo Antimalarial Studies
The suppressive antiplasmodic effect of methanolic solutions of MT samples on albino
mice are displayed in Table S3 while Table 8 gives the percentage parasitemia inhibition.
Table 8. Percentage of parasitemia inhibition.
Drug/Dose (mg/kg)
MT 25
% Parasitemia
10.6667 ± 0.88192
5.0000 ± 1.52753
4.00 ± 0.58
%Inhibition (PC)
65.81
84.02
87.22
00.00
MT 50
Artesunate 5
Dist. Water 5 mL/kg
31.33 ± 3.38
Artesunate is an antimalarial drug with a molecular formular of C19H28O8 and was
used as standard because of the presence of –COOH group in both Artesunate and MT.
The structure is shown in Figure S14. Its mechanism of action is by inhibiting cytochrome

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oxidase of the malaria parasite surface membrane—food vacuole membrane and mitochon-
drial membrane—thereby blocking the supply of nutrients from the host cell cytoplasm [49].
It was observed from Table S3 that the effect of the samples on weight (Wt), PCV, and Hb of
the infected mice treated did not show an orderly pattern of dose dependent effect. How-
ever, the effect was significant compared to the negative control. MT shows a significant
dose dependent reduction of the effect of the sample on PCV, though in a negative manner.
The effect of MT on Hb concentration shows the same effect as in PCV.
The results in Table 8 showed a general dose dependent significant parasitemia
inhibition compared with the negative control with MT inhibition of 84.02% at 50 mg/kg
close to the value (87.22%) of the standard drug—Artesunate at 5 mg/kg. At 25 mg/kg, MT
gave 65.81%. This implies that MT can be used as an antimalarial drug after further tests.
2.7. In Silico Studies
COVID-19 is a deadly disease that has led to the loss of so many lives all over the world.
This disease is still ravaging the human race irrespective of the fact that some vaccines
have been discovered. There is need for discovery of more drugs. It is therefore of
paramount importance that drug-like molecules should target the SARS-CoV-2 6Y84 so
as to identify compounds with high binding affinity, hence they could effectively combat
COVID-19.
Table 9 shows the binding free energies of the ligand and the complexes for both
SARS-CoV-2 and antibacterial in silico studies. The binding energy reveals a strong binding
affinity the ligand and the complexes have with the receptors. MT showed the highest
binding affinity with E. coli DNA gyrase while the complex, Er(III)MT demonstrated the
highest binding affinity to the SARS-CoV-2 receptor (Figure 1). To gain further insights
into the nature and type of chemical interactions involved, the binding poses of these
compounds in the active binding sites of the receptors were analyzed and are shown in
Figures 2–4. The atoms of the ligand, MT interacted with various amino acid residues of
5MMN. Significant interactions include H-bonding interaction between the O-atom of MT
and VAL 160 through a distance of 3.02 Å. Pi-cation was observed between the pi-electrons
of the phenyl group and ARG 76 (4.03 Å). Other amino acid residues involved include
ALA 47, ILE 78 ILE 94, and PRO 79. Figure 3 depicts a well-fitted complex, Er(III)MT in the
active binding site of 6Y84. The atoms of the complex interacted mostly with the amino
acid residues through various hydrophobic interactions.
Table 9. Free binding energy of compounds against SARS-CoV-2 (6Y84) and E. coli DNA
Gyrase (5MMN).
SARS-CoV-2.
Compound
E. coli DNA Gyrase (5MMN)
(6Y84)
−5.69
−7.30
−7.98
−7.55
−6.31
−6.30
-
MT
Dy(III)MT
Er(III)MT
Gd(III)MT
CQ
−7.03
−5.82
−7.03
−6.59
-
HCQ
-
Cipro
−5.40

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Figure 1.
(a) E. coli DNA Gyrase B 24 kDa ATPase domain in complex with 1-ethyl-3-[8-methyl-5-(2-methyl-pyridin-4-yl)-
isoquinolin-3-yl]-urea. (b) SARS-CoV-2 main protease with unliganded active site.
Figure 2. Binding interactions of MT with amino acid residues of E. coli DNA gyrase (5MMN). Key
to bond type: orange dotted line = pi-cation bond; green dotted lines = H-bond; purple dotted line =
pi-alkyl.

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Figure 3. Binding pose of Er(III)MT in the active binding site of 6Y84.
Figure 4. 2D interaction of MT with the amino acid residues of E. coli DNA gyrase (5MMN).

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3. Materials and Methods
3.1. Materials
All the chemicals used were of analytical reagent grade, purchased from Zayo–Sigma
and were used as supplied without further purification. The melting points of the com-
pounds were determined using Fischer Jones melting point apparatus and were uncor-
rected. Molar conductance measurements were carried out using 10−4 mol/L solutions
of the complexes in methanol at room temperature using a WTW-LF 90 conductivity
meter. Electronic spectra (in methanol) were recorded on a UV-Vis 1800 SHIMADZU
spectrophotometer. Infrared spectra of the compounds were performed using KBr discs
on a Perkin–Elmer (Waltham, MA, USA) 100 series version 10.03.08 FTIR spectropho-
1
tometer. The H and 13C-NMR spectra of the compounds were recorded on a Bruker
(Billerica, MA, USA) DPX 300 spectrometer in DMSO-d6 at 300.13 MHz and 75.47 MHz
respectively. Elemental analysis for C, H, and N were carried out using an LECO–CHN–932
analyzer.
3.2. Synthesis of 2,4,6-Tris(4-carboxybenzimino)-1,3,5-triazine(MT)
The method reported by Uysal and Ucan (2009) [39] was adopted. Melamine (0.63 g,
0.005 mol) was dissolved in benzene (5 cm3) stirred for 1 h, then 4-carboxybenzaldehyde
(2.25 g, 0.015 mole) was added and refluxed for 4 h. A white precipitate was obtained,
filtered, and recrystallized from a mixture of methanol and water, dried and stored
over CaCl2.
3.3. Synthesis of Ligand Complexes
The ligand complexes were prepared by addition of concentrated ammonia solution
to a solution of [Dy/Er/Gd(salen)]2O in absolute ethanol, stirred at 50 ◦C until a pH of 12
was achieved [50,51].
3.4. Synthesis of [{Dy/Er/Gd(salen)}3(MT)].3H2O
[Dy/Er/Gd(salen)]2O (0.00037 mol) was suspended in hot absolute ethanol (25 cm3)
and a solution of MT (0.13 g, 0.00025 mol) in absolute ethanol was added while stirring.
The reaction mixture was boiled under reflux for 4 h. The light yellow solid formed was
washed with water and dried over CaCl2 (Scheme 2).
3.5. In Vitro Antimicrobial Activity
The tripodal ligand and its trinuclear complexes were tested in vitro for their antimicro-
bial activities against American Type Culture Collection (ATCC) bacteria strains obtained
from Rockville, MD, USA, by the Department of Microbiology, University of Nigeria,
Nsukka while the fungi strains were isolated under clinical conditions. The typed bacte-
ria culture was comprised of Gram-positive bacteria: Staphylococcus aureus (ATCC 6538P)
and Bacillus cereus (ATCC 14579); Gram-negative bacteria: Escherichia coli (ATCC 6749)
and Pseudomonas aeruginosa (ATCC 9027). The fungi strains used were Candida albicans and
Aspergillus niger. The bacteri◦a strains were tested for sterility on nutrient agar and then
grown in nutrient broth at 37 C for 24 h while the fungal strains were tested on Sabourand
◦
Dextrose Agar (SDA) and cultured in Sabourand Dextrose Liquid medium at 25 C for 24 h.
The overnight cultures were subsequently diluted and suspensions were made in normal
saline and adjusted to 0.5 McFarland standards [52].
3.6. Antimicrobial Assay
The antimicrobial activities of all the synthesized compounds were determined by
the agar cup diffusion technique [53]. The nutrient agar and SDA plates were inoculated
with 0.1 mL broth culture of the test bacteria or fungi. Using a sterile cork borer, wells
(5 mm in diameter and 2.5 mm deep) were bored into the inoculated agar. Fresh stock
solutions (1000
µg/mL) of the synthesized compounds were prepared in DMSO. The stock
solution was further diluted with sterilized distilled water to 12.5, 25, and 50
µg/mL for

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antimicrobial evaluation. The wells were filled with 100
µL of the test compounds by
means of a sterile micropipette. Standard antibiotics, namely Ciprofloxacin, Tetracycline,
Gentamycin, and Fluconazole were used as positive control while sterile DMSO served as
negative control. Subsequently, 12.5, 6.25, and 3.125
µ
g/mL of each positive control were
◦
prepared in DMSO. The bacteria plates were incubated at 37 C for 24 h while fungal plates
were incubated at 25 ◦C for 24 h. Inhibition zone diameter (IZD) around each well was
measured in millimeters and recorded. The graph of IZD2 against the log of concentration
was plotted for each plate containing a specific compound and a micro-organism. The
anti-log of the intercept on x-axis is the MIC.
3.7. Determination of Acute Toxicity (LD50)
All experiments involving the use of mice were conducted in compliance with NIH
guidelines for care and use of animals [54]. MT was used for the test because it showed
higher sensitivity against the tested micro-organisms. The oral acute toxicity of the ethanolic
solutions of the samples were estimated in albino mice (100–250 g) by upper-level lethal
dose (LD50) described by Lorke’s method [55]. A total of four mice of both sexes were
employed and an acclimatization period of 24 h was allowed. The samples were weighed
and dissolved in 3% ethanol. The ethanolic solutions of the samples were administered
orally at doses of 1000, 1600, 2900, and 5000 mg/kg. The animals were monitored for 24 h
and the number of deaths per group recorded. Then, the mice were observed continuously
for one hour after treatment, intermittently for three hours, and thereafter over a period of
24 h. The mice were observed for gross behavioral changes such as feeding, hair erection,
lacrimation, mortality, and other signs of toxicity manifestation. The mice were given
access to food and clean water during the study.
3.8. In Vivo Studies
The in vivo antimalarial assay was done based on a 4-day suppressive test using
mice. The evaluation of the antimalarial activity against the methanolic solutions of the
samples and Artesunate sensitive Plasmodium berghei (NK 65) was carried out according to a
standard protocol of Peter’s 4-day suppressive test [56]. Each of the 12 healthy experimental
mice was inoculated intraperitoneally on the first day (Day 1). The infected mice were
weighed and randomly divided into four groups of three mice each and four hours post
inoculation were treated orally thereafter 24, 48, and 72 h post inoculation. For groups 1 and
2, 25 mg of sample/kg of mouse and 50 mg of sample/kg of mouse was administered orally
for four consecutive days. For group 3, 5 mg of Artesunate/kg of mouse was administered
while group 4 was given 5 mL of distilled water/kg of mouse for four consecutive days.
On day 4 post inoculation, one drop of blood was taken from the tail of each experimental
mouse and smeared on a microscope slide to make a thin film [57]. The thin films were
fixed with methanol, stained with 10% Giemsa solution at pH 7.2 for 10 min, and examined
microscopically. Parasitemia level was determined by counting the number of parasitized
erythrocytes out of 100 erythrocytes per field in 4 random fields under a light microscope at
magnification ( 100) while average percentage parasitemia suppression was determined
×
by comparing the parasitemia in the control group with the treated group.
pt
pc
Average percentage suppression = pc −
× 100
where pc = average parasitema in the control group, pt = average parasitemia in the treated
group [58].
3.9. In Silico Study
The 3D structures of the receptors of SARS-CoV-2 (6Y84) and E. coli DNA gyrase
(5MMN) as shown in Figure 1 were retrieved from the Protein Data Bank. The 3D struc-
tures of the ligand and complexes drugs drawn using MarvinSketch 17.2.6.0 were energy
minimized and docked into the active binding sites of these receptors. Docking protocols

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were validated by reproducing the PDB crystal structures in silico. The various binding
poses produced were analyzed with Discovery Studio vs16.1.0.15350.
DNA gyrase, a type II topoisomerase, is found in all bacteria. They consist of two
subunits: GyrA and GryB of E. coli gyrase. These enzymes are responsible for catalyzing
topological changes in DNA and have proved to be drug targets for therapeutic agents.
4. Conclusions
A tripodal Schiff base ligand derived from 2,4,6-triamino-1,3,5-triazine and its novel
trinuclear Dy(III), Er(III), and Gd(III) salen capped complexes were synthesized and char-
acterized based on various physico-chemical and spectral studies. The ligand was found to
be tripodal and to coordinate to the ligand complexes via the carboxylic group. In vitro
antimicrobial tests indicated that the tripodal ligand, MT is more active against the test
micro-organisms relative to the trinuclear complex. Acute toxicity studies reveal that MT is
safe and has a wide range of effective doses (ED50). In vivo antimalarial studies indicate
that MT could serve as an effective antimalarial agent since it has parasitemia inhibition
of 84.02% at 50 mg/kg and 65.81% at 25 mg/kg close to the value (87.22%) of the stan-
dard drug—Artesunate at 5 mg/kg. The chemical interactions of our synthesized ligand
and complexes with SARS-CoV-2 and DNA gyrase revealed multiple effective bonding
interactions. Through docking simulation, the binding energy calculated for the two drug
targets showed active molecules with the ability to inhibit SARS-CoV-2 (COVID-19) and
DNA gyrase.
Supplementary Materials: Figure S1: Electronic absorption spectrum of MT, Figure S2: Electronic
absorption spectrum of Dy(III)MT, Figure S3: Electronic absorption spectrum of Er(III)MT, Figure S4:
Electronic absorption spectrum of Gd(III)MT, Figure S5: Infrared Spectrum of MT, Figure S6: Infrared
Spectrum of Dy(III)MT, Figure S7: Infrared Spectrum of Er(III) MT, Figure S8: Infrared Spectrum
1
1
of Gd(III)MT, Figure S9: H-NMR Spectrum of MT, Figure S10: H-NMR Spectrum of Dy(III)MT,
Figure S11: 13C-NMR Spectrum of MT, Figure S12: 13C-NMR Spectrum of Dy(III)MT, Figure S13:
Structures of the drugs used as standard, Figure S14: Structure of Artesunate, Table S1: Inhibition
Zone Diameter (IZD in mm) of the Controls, Table S2: Determination of Acute Toxicity (LD50), Table
S3: Anti-plasmodic Effect of Samples on Albino mice.
Author Contributions: Conceptualization, U.S.O. and P.O.U.; methodology, U.S.O. and P.O.U.;
software, C.J.E., U.S.O., J.C.N., and F.U.E.; validation, C.J.E., U.S.O., T.E.E., and C.P.U.; formal
analysis, U.S.O. and P.O.U.; investigation, U.S.O., P.O.U., C.J.E., T.E.E., and N.N.U.; resources, U.S.O.
and P.O.U.; data curation, U.S.O., F.U.E., J.C.N., and U.V.C.; writing—original draft preparation,
U.S.O. and S.N.O.; writing—review and editing, C.J.E., L.C.E., N.N.U., and S.C.U.; visualization,
U.S.O., S.C.U., and C.P.U.; supervision, P.O.U.; project administration, U.S.O. and P.O.U. All authors
have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
WHO. Artemisinin and Artemisinin-Based Combination Therapy Resistance. 2017. Available online: http://www.who.int/
malaria/publications/atoz/artemisinin-resistanceapril2017/en/ (accessed on 20 January 2021).
WHO. Malaria Could Make a Comeback Thanks to COVID-19. 2020. Available online: https://www.weforum.org/agenda/2020
/04/malaria-treatment-rise-africa-coronavirus/ (accessed on 20 January 2021).
Fidock, D.A.; Rosenthal, P.J.; Croft, S.L.; Brun, R.; Nwaka, S. Antimalarial Drug Discovery: Efficacy Models for Compound
Screening. Nat. Rev. Drug Discov. 2004, 3, 509–520. [CrossRef]
Ojha, H.; Gahlot, P.; Tiwari, A.K.; Pathak, M.; Kakkar, R. Quantitative structure activity relationship study of 2, 4, 6-trisubstituted-
s-triazine derivatives as antimalarial inhibitors of Plasmodium falciparum dihydrofolate reductase. Chem. Biol. Drug Des. 2011, 77,
57–62. [CrossRef] [PubMed]

Molecules 2021, 26, 4379
14 of 15
5.
6.
7.
Modh, R.P.; Patel, A.C.; Chikhalia, K.H. Design, synthesis, antibacterial, and antifungal studies of novel 3-substituted coumarinyl-
triazine derivatives. Heterocycl. Commun. 2013, 19, 343–349. [CrossRef]
Machakanur, S.S.; Patil, B.R.; Badiger, D.S.; Bakale, R.P.; Gudasi, K.B.; Annie, B.S. Synthesis, characterization and anticancer
evaluation of novel tri-arm star shaped 1, 3, 5-triazine hydrazones. J. Mol. Struct. 2012, 1011, 121–127. [CrossRef]
Lozano, V.; Aguado, L.; Hoorelbeke, B.; Marleen, R.; María-José, C.; Dominique, S.; Jan, B.; Ana, S.-F.; María-Jesús, P.-P. Targeting
HIV entry through interaction with envelope glycoprotein 120 (gp120): Synthesis and antiviral evaluation of 1, 3, 5-triazines with
aromatic amino acids. J. Med. Chem. 2011, 54, 5335–5348. [CrossRef]
8.
9.
Moni, S.; Kuldeep, C.; Rahul, S.; Preeti, V.; Manish, K.S.; Abhisheak, S.; Suman, G.; Jitendra, K.S.; Jawahar, L.; Preeti, C.; et al.
Discovery of a new class of natural product-inspired quinazolinone hybrid as potent antileishmanial agents. J. Med. Chem. 2013
56, 4374–4392.
,
Chiara, D.; Massimo, C.; Margherita, G.; Roberto, F.; Simona, S.; Giovanni, S.; Rita, M. Evaluation of in vitro anti-inflammatory
activity of some 2-alkyl-4, 6-dimethoxy-1, 3, 5-triazines. J. Pharm. Pharmacol. 2010, 58, 219–226.
10. Zhao, H.; Liu, Y.; Cui, Z.; Beattie, D.; Gu, Y.; Wang, Q. Design, synthesis, and biological activities of arylmethylamine substituted
chlorotriazine and methylthiotriazine compounds. J. Agric. Food. Chem. 2011, 59, 11711–11717. [CrossRef]
11. Uysal, S.; Koc, Z.E. Synthesis and Characterization of Dendrimeric Melamine Cored [salen/salophenFe(III)] and [salen/salophenCr(III)]
Capped Complexes and Their Magnetic Behaviors. J. Hazard. Mater. 2010, 175, 532–539. [CrossRef]
12. Wu, J.; Chen, L.; Fu, T.; Zhao, H.; Guo, D.; Wang, X.; Wang, Y. New application for aromatic Schiff base: High efficient
flameretardant and anti-dripping action for polyesters. Chem. Eng. J. 2018, 336, 622–632. [CrossRef]
13. Agathian, K.; Kannammal, L.; Meenarathi, B.; Kailash, S.; Anbarasan, R. Synthesis, characterization and adsorption behavior of
cotton fiberbased Schiff base. Int. J. Biol. Macromol. 2018, 107, 1102–1112. [CrossRef]
14. Naz, A.; Arun, S.; Narvi, S.S.; Alam, M.S.; Singh, A.; Bhartiya, P.; Dutta, P.K. Cu(II)-carboxymethyl chitosan-silane schiff
basecomplex grafted onnano silica: Structural evolution, antibacterial performance and dyedegradation ability. Int. J. Biol.
Macromol. 2018, 110, 215–226. [CrossRef] [PubMed]
15. Al-Hamdani, A.A.S.; Balkhi, A.M.; Falah, A.; Shaker, S.A. Synthesis and investigation of thermal properties of vanadyl complexes
with azo-containing Schiff-base dyes. J. Saudi Chem. Soc. 2016, 20, 487–501. [CrossRef]
16. Lu, F.; Astruc, D. Nanomaterials for removal of toxic elements from water. Coord. Chem. Rev. 2018, 356, 147–164. [CrossRef]
17. Diem, H.; Matthias, G. Amino Resins. In Ullmann’s Encyclopedia of Industrial Chemistry, 7th ed.; Wiley: Weinheim, Germany, 2006;
pp. 1–20.
18. Tripathi, K.D. Essentials of Medical Pharmacology, 5th ed.; Jaypee Brothers Medical Publishers Ltd.: New Delhi, India, 2003; p. 78.
19. Shah, D.; Modh, R.P.; Chikhalia, K.H. Privileged s-triazines: Structure and pharmacological applications. Future Med. Chem. 2014
6, 463–477. [CrossRef]
,
20. Hatfield, S.E. Applications of Triazine Chemistry: Education, Remediation, and Drug Delivery. Ph.D. Thesis, Texas A&M
University, College Station, TX, USA, 2007.
21. Tobe, A.; Kobayashi, T. Pharmacological studies on triazine derivatives V. Sedative and neuroleptic actions of 2-amino-4-(4-(2-
hydroxyethyl)-piperazin-1-yl)-6- trifluoromethyl-s-triazine (TR-10). Jpn. J. Aerosp. Med. Psychol. 1976, 26, 559–570. [CrossRef]
22. Leena, G.; Naresh, S.; Aditya, V.; Saumya, S.; Suman, G.; Neena, G.; Prem, M.S.C. Synthesis and biological evaluation of new
[1,2,4] triazino[5,6-b] indol-3-ylthio-1,3,5-triazines and [1,2,4] triazino [5,6-b] indol-3-yl thio-pyrimidines against Leishmania
donovani. Eur. J. Med. Chem. 2010, 45, 2359–2365.
23. Hashmi, S.Z. Synthesis of Pharmacologically Important s-Triazine Derivatives. J. Pharmacol. Res. Rev. 2016, 1, 1–9.
24. Sonikan, J.; Pankaj, K.J.; Shalu, S.; Devarapalli, K.; Jaya, D. Anticancer s-triazine derivatives: A synthetic attribute. Mini-Rev. Org.
Chem. 2020, 17, 904–921.
25. Guo, F.J.; Hyun, S.B.; Hiroyuki, N.; Jong-Dae, L. O-Carboranylalkoxy-1,3,5-Triazine Derivatives: Synthesis, Characterization,
X-ray Structural Studies, and Biological Activity. Molecules 2018, 23, 2194.
26. Cascioferro, S.; Parrino, B.; Spanò, V.; Carbone, A.; Montalbano, A.; Barraja, P.; Diana, P.; Cirrincione, G. 1,3,5-Triazines: A
promising scaffold for anticancer drugs development. Eur. J. Med. Chem. 2017, 142, 523–549. [CrossRef]
27. Nie, Z.; Perretta, C.; Erickson, P.; Margosiak, S.; Lu, J.; Averill, A.; Almassy, R.; Chu, S. Structure-based design and synthesis
of novel macrocyclic pyrazolo[1,5-a] [1,3,5]triazine compounds as potent inhibitors of protein kinase CK2 and their anticancer
activities. Bioorg. Med. Chem. Lett. 2008, 18, 619–623. [CrossRef]
28. Salehzadeh, S.; Nouri, S.M.; Keypour, H.; Bagherzadeh, M. Synthesis of Gadolinium(III) and Samarium(III) Complexes of New
Potentially Heptadentate(N4O3) Tripodal Schiff Base Ligands, and a Theoretical Study. Polyhedron 2005, 24, 1478–1486. [CrossRef]
29. Casellato, U.; Tamburini, S.; Tomasin, P.; Vigato, P.A.; Botta, M. Lanthanide(III) Complexes with a Podand Schiff Base Containing
an N4O3 Coordination Site. Inorg. Chim. Acta 1996, 247, 143–145. [CrossRef]
30. Mikhalyova, E.A.; Yakovenko, A.V.; Zeller, M.; Gavrilenko, K.S.; Lofland, S.E.; Addiso, A.W.; Pavilshchuk, V.V. Structure, Magnetic
and Luminescence Properties of Lanthanide Complexes Ln2(Salphen)3.H2O (Ln = Pr, Nd, Sm, Eu, Gd, Tb,Dy; H2Salphen =
N,N1-Bis(salicylidene)-1,2-phenylenediamine). Inorg. Chim. Acta 2014, 414, 97–104. [CrossRef]
31. Chen, R.X.; Gao, T.; Sun, W.B.; Li, H.F.; Wu, Y.H.; Xu, M.M.; Zou, Y.; Yan, P.F. Salen Homonuclear and Heteronuclear Lanthanide(III)
Complexes with Near-Infrared (NIR) Luminescence. Inorg. Chem. Comm. 2015, 56, 79–82. [CrossRef]

Molecules 2021, 26, 4379
15 of 15
32. Liu, H.; Zhang, Y.; Feng, W.X.; Su, P.Y.; Shi, G.X.; Zhang, Z.; Fan, D.D.; Lu, R.; Lu, X.Q.; Wong, W.K.; et al. Photo-luminescent
Hetero-tetranuclear Zn2Ln2 (Ln = Nd, Yb, Er, Gd, Eu or Tb) Complexes Self-assembled from the Benzimidazole-based HL and
bpe. Inorg. Chem. Comm. 2013, 35, 213–216. [CrossRef]
33. Chen, C.; Chen, C.; Yan, P.; Hou, G.; Li, G. Structure and Electrochemistry of Salen Cerium(IV) Complexes Tuned by Multiform
Counter Ions. Inorg. Chim. Acta 2013, 405, 182–187. [CrossRef]
34. Oruma, U.S.; Ukoha, P.O.; Asegbeloyin, J.N. Synthesis, Characterization and Biological Studies of S-1,3-Benzothiazol-2-
ylthiophene-2-carbothioate and its Ce(IV) and Nd(III) Complexes. Asian J. Chem. 2014, 26, 7622–7626. [CrossRef]
35. Obasi, L.N.; Oruma, U.S.; Al-Swaidan, I.A.; Ramasami, P.; Ezeorah, C.J.; Ochonogor, A.E. Synthesis, Characterization and An-
tibacterial Studies of N-(Benzothiazol-2-yl)-4-chlorobenzenesulphonamide and its Neodymium(III) and Thallium(III) Complexes.
Molecules 2017, 22, 153. [CrossRef]
36. Taha, Z.A.; Ajlouni, A.M.; Al-Hassan, K.A.; Hajazi, A.K.; Faiq, A.B. Synthesis, Characterization, Biological Activity and Fluores-
cence Properties of Bis-(salicylaldehyde)-1,3-propylenediimine Schiff base Ligand and Its Lanthanide Complexes. Spectrochim.
Acta Part A 2011, 81, 317–323. [CrossRef] [PubMed]
37. Ali, I.; Wani, W.A.; Saleem, K. Empirical formulae to molecular structures of metal complexes by molar conductance. Synth. React.
Inorg. Met. Chem. 2013, 43, 1162–1170. [CrossRef]
38. Uysal, S.; Koc, Z.E. Synthesis and Characterization of Dopamine Substitue Tripodal Trinuclear [(salen/salophen/salpropen) M]
(M = Cr(III), Mn(III), Fe(III) ions) Capped S-triazine Complexes: Investigation of their Thermal and Magnetic Properties. J. Mol.
Struc. 2016, 1109, 119–126. [CrossRef]
39. Uysal, S¸.; Uçan, H.I. The synthesis and characterization of melamine based Schiff bases and its trinuclear [salen/salophenFe (III)]
and [salen/salophenCr (III)] capped complexes. J. Incl. Phenom. Macrocycl. Chem. 2009, 65, 299–304. [CrossRef]
˙
40. Uysal, S¸.; Koç, Z.E.; Çelikbilek, S¸.; Uçan, H.I. Synthesis of star-shaped macromolecular schiff base complexes having melamine
cores and their magnetic and thermal behaviors. Synth. Commun. 2012, 42, 1033–1044. [CrossRef]
41. Lekha, L.; Raja, K.K.; Rajagopal, G.; Easwaramoorthy, D. Synthesis, spectroscopic characterization and antibacterial studies of
lanthanide (III) Schiff base complexes containing N, O donor atoms. J. Mol. Struct. 2014, 1056, 307–313. [CrossRef]
42. Bertini, I.; Luchinat, C.; Parigi, G. Magnetic Susceptibility in Paramagnetic NMR. Prog. Nucl. Magn. Reson. Spectrosc. 2002, 40,
249–273. [CrossRef]
˙
43. Is¸çi, B.; Uysal, S. The synthesis and characterization of [M(salen/salophen/saldeta)][M=Cr(III), Mn(III) or Fe(III)] capped
s-triazine cored tripodal trinuclear Schiff bases complexes. J. Incl. Phenom. Macrocycl. Chem. 2018, 92, 281–299. [CrossRef]
44. Silverstein, R.M.; Webster, F.X.; Kiemle, D.J. Spectrometric Identification of Organic Compounds, 7th ed.; John Wiley & Sons: Hoboken,
NJ, USA, 2005; pp. 200–228.
45. Ukoha, P.O.; Oruma, U.S. Synthesis and Antimicrobial Studies of N, N1-Bis(4- Dimethylaminobezylidene)ethane-1,2-diamine
(DAED) and its Nickel(II) and Platinium(IV) complexes. J. Chem. Soc. Niger. 2014, 39, 102–107.
46. Uchechukwu, S.O.; Pius, O.U.; Lydia, R.; Mohamed, I.E.; Lawrence, N.O.; Ponnadurai, R.; Klaus, J. Synthesis, Characterization,
Antimicrobial Screening, and Computational Studies of a Tripodal Schiff Base Containing Pyrimidine Unit. J. Heterocycl. Chem.
2018, 55, 1119–1129.
47. Kostova, I.; Manolov, I.; Momekov, G. Cytotoxic Activity of New Neodymium (III) Complexes of Bis-coumarins. Eur. J. Med.
Chem. 2004, 39, 765–775. [CrossRef]
48. Wolters, K. Clinical Pharmacology Made Incredibly Easy, 3rd ed.; Lippincott Williams & Wilkins: Philadelphia, PA, USA, 2009.
49. Artesunate. Artesunate Chemical Properties, Usage, Production. 2016. Available online: www.chemicalbook.com/
ChemicalProductProperty_EN_CB3157307.htm (accessed on 8 August 2017).
50. Kopel, P.; Sindelar, Z.; Klicka, R. Complexes of Iron(III) Salen and Saloph Schiff Bases with Bridging Dicarboxylic and Tricarboxylic
Acids. Transit. Met. Chem. 1998, 23, 139–142. [CrossRef]
51. Gembicky, M.; Boca, R.; Renz, F. A Heptanuclear Fe(III)-Fe(III)6 System with Twelve Unpaired Electrons. Inorg. Chem. Comm.
2000, 3, 662–665. [CrossRef]
52. Cheesbrough, M. District Laboratory Practice in Tropical Countries; Cambridge University Press: Cambridge, UK, 2006; pp. 393–394.
53. Alli, A.; Ehinmidu, J.; Ibrahim, Y. Preliminary phytochemical screening and antimicrobial activities of some medicinal plants
used in Ebiraland. Bayero J. Pure Appl. Sci. 2011, 4, 10–16. [CrossRef]
54. Council, N.R. Guide for the Care and Use of Laboratory Animals; National Academies Press: Washington, DC, USA, 2010.
55. Lorke, D. A new approach to practical acute toxicity testing. Arch. Toxicol. 1983, 54, 275–287. [CrossRef]
56. Peters, W. The four-day suppressive in vivo antimalarial test. Ann. Trop. Med. Parasitol. 1975, 69, 155–171. [CrossRef]
57. Saidu, K.; Onah, J.; Orisadipe, A.; Olusola, A.; Wambebe, C.; Gamaniel, K. Antiplasmodial, analgesic, and anti-inflammatory
activities of the aqueous extract of the stem bark of Erythrina senegalensis. J. Ethnopharmacol. 2000, 71, 275–280. [CrossRef]
58. Ukwe, V.C.; Epueke, E.A.; Ekwunife, O.I.; Okoye, T.C.; Akudor, G.C.; Ubaka, C.M. Antimalarial activity of aqueous extract and
fractions of leaves of Ageratum conyzoides in mice infected with Plasmodium berghei. Int. J. Pharm. Sci. 2010, 2, 33–38.