Titanium (IV) complexes of some tetra-dentate symmetrical bis-Schiff bases of 1,6-hexanediamine: Synthesis, characterization, and in silico prediction of potential inhibitor against coronavirus (SARS-CoV-2)

Article abstract of DOI:10.1002/aoc.6067

Symmetrical bis-Schiff bases (LH₂) have been synthesized by the condensation of 1,6-hexanediamine (hn) and carbonyl or dicarbonyl. One of the synthesized Schiff bases has been subjected to the molecular docking for the prediction of their potentiality against coronavirus (SARS-CoV-2). Molecular docking revealed that tested Schiff base possessed high binding affinity with the receptor protein of SARS CoV-2 compared with hydroxychloroquine (HCQ). The ADMET analysis showed that ligand is non-carcinogenic and less toxic than standard HCQ. Schiff bases acting as dibasic tetra-dentate ligands formed titanium (IV) complexes of the type [TiL(H₂O)₂Cl₂] or [TiL(H₂O)₂]Cl₂ being coordinated through ONNO donor atoms. Ligands and complexes were characterized by the elemental analysis and physicochemical and spectroscopic data including FTIR, ¹H NMR, mass spectra, UV-Visible spectra, molar conductance, and magnetic measurement. Optimized structures obtained from quantum chemical calculations supported the formation of complexes. Antibacterial, antifungal, and anti-oxidant activity assessments have been studied for synthesized ligands and complexes.

Full text of DOI:10.1002/aoc.6067

Received: 18 July 2020
DOI: 10.1002/aoc.6067
Revised: 29 September 2020
Accepted: 30 September 2020
F U L L P A P E R
Titanium (IV) complexes of some tetra-dentate symmetrical
bis-Schiff bases of 1,6-hexanediamine: Synthesis,
characterization, and in silico prediction of potential
inhibitor against coronavirus (SARS-CoV-2)
Mohammad Nasir Uddin1
| Md. Shaharier Amin1
| Md. Saifur Rahman1
|
Sonia Khandaker1
| Wahhida Shumi2 | Md. Atiar Rahman3
|
Sheikh Mahbubur Rahman4
1Department of Chemistry, University of
Chittagong, Chittagong, Bangladesh
2Department of Microbiology, University
of Chittagong, Chittagong, Bangladesh
3Department of Biochemistry & Molecular
Biology, University of Chittagong,
Chittagong, Bangladesh
Symmetrical bis-Schiff bases (LH2) have been synthesized by the condensation
of 1,6-hexanediamine (hn) and carbonyl or dicarbonyl. One of the synthesized
Schiff bases has been subjected to the molecular docking for the prediction of
their potentiality against coronavirus (SARS-CoV-2). Molecular docking rev-
ealed that tested Schiff base possessed high binding affinity with the receptor
protein of SARS CoV-2 compared with hydroxychloroquine (HCQ). The
ADMET analysis showed that ligand is non-carcinogenic and less toxic than
standard HCQ. Schiff bases acting as dibasic tetra-dentate ligands formed tita-
nium (IV) complexes of the type [TiL(H2O)2Cl2] or [TiL(H2O)2]Cl2 being coor-
dinated through ONNO donor atoms. Ligands and complexes were
characterized by the elemental analysis and physicochemical and spectro-
4UL Laboratories, Dhaka, Bangladesh
Correspondence
Mohammad Nasir Uddin, Department of
Chemistry, University of Chittagong,
Chittagong, Bangladesh.
Email: nasircu72@gmail.com;
mnuchem@cu.ac.bd
1
scopic data including FTIR, H NMR, mass spectra, UV-Visible spectra, molar
conductance, and magnetic measurement. Optimized structures obtained from
quantum chemical calculations supported the formation of complexes.
Antibacterial, antifungal, and anti-oxidant activity assessments have been
studied for synthesized ligands and complexes.
K E Y W O R D S
anti-oxidant activity, bis-Schiff bases, coronavirus (SARS-CoV-2), inhibitors, titanium
(IV) complexes
1 | INTRODUCTION
remdesivir, chloroquine, and hydroxychloroquine that
are previously designed antiviral drugs are rec-
A new infectious disease affected millions of people
throughout the world that is caused by severe acute
respiratory syndrome coronavirus-2 (SARS-CoV-2). It
has recently been declared as a pandemic by the
World Health Organization.[1] It is a challenging pro-
cess though vital to discover fruitful drugs against
SARS-CoV-2. Having no alternatives, favipiravir,
ommended in some countries though still they are
under investigation by the US Food and Drug Admin-
istration as a treatment for COVID-19.[2]
Schiff bases containing azomethine group >C N
show broad range of applications in medicine due to their
pharmacological properties.[3] The >C N linkage in
azomethine derivatives is essential for biological
Appl Organomet Chem. 2020;e6067.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6067

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UDDIN ET AL.
activity.[4–7] Several azomethines were reported to possess
remarkable antimicrobial, anticancer, and diuretic activi-
ties.[8] Antiviral property of Schiff bases was reported by
Chaturvedi and Kamboj.[9] Antiviral activities of Schiff
base prepared from isatin against moloney leukemia
virus, vaccinia, rhino virus, and SARS virus have been
reported.[10]
Structures, stereochemistry, and mechanism of the
formation of the Schiff base complexes and their ana-
logues have been reviewed.[11] Tetra dentate Schiff base
ligands are synthesized by the condensation of
β-diketones or o-hydroxyaldehydes or ketones with
diamines.[12] The antimicrobial activity of nine Ln (III)
complexes of the Schiff base ligand derived by the con-
2 | METHODS AND MATERIALS
2.1 | Chemicals
All chemicals used in these analyses were of analytical
grade and used as procured. 1,6-hexanediamine (hn),
2-hydroxyacetophenone (HAP), 2-hydroxynapthaldehyde
(HNP), and ethyl acetoacetate (EAA) (purity 99.50%)
were from Aldrich Chemical, Germany. Salicylaldehyde
(Sal) (purity 99.00%) from Riedel-de-Haen, acetylace
-tone (AA) (purity 99.50%) from Merck, Germany,
and succinic acid (SA) (purity 99.00%) from Sigma,
Germany, were collected. All necessary precautions
were taken to exclude moisture and oxygen during
the synthesis and handling of the compounds. The
solvents (DMSO, DMF, MeOH, and EtOH) were
analytically graded and further purified by using
standard procedures. TiCl4.6H2O (purity 99.50%)
was used for the synthesis of Schiff base complexes of
V (IV).
densation
of
2-hydroxy-1-naphthaldehyde
with
1,6-hexanediamine was explored by Ajlouni et al.[13]
Bayoumi and his co-workers[14] synthesized and charac-
terized several hexamethylenediamine-metal complexes
showing octahedral geometry. Despite the potential of
the Schiff base ligands, there has been comparatively
only a few works done using titanium (IV) ion. Coordina-
tion chemistry of titanium dominated by 4+ oxidation
state is either achieved in the form of oxo-titanium (IV),
TiO2+ or in the form of non-oxotitanium (IV), Ti4+ ion
when all electrons of valance are lost. Jiling Huang et al.
reported the synthesis of two kinds of titanium
(IV) complexes with Schiff base as the mixed ligands.[15]
Uddin et al. prepared and characterized the titanium
(IV) complexes of the unsymmetrical Schiff base ligands
of ethylenediamine and salicylaldehyde, o-hydrox-
yacetophenone, and o-hydroxynapthaldehyde when
unsymmetrical ligands are synthesized through in situ
partial displacement of the symmetrical bis-Schiff
bases.[12]
2.2 | Synthesis of Schiff bases
Solid 1,6-hexanediamine (hn; 20 mmol) was dissolved
in 50 ml of methanol. To which methanolic solution
of 40 mmol (1:2 ratio) of aldehyde/ketones
(salicylaldehyde-Sal, 2-hydroxy-1-napthaldehyde-HNP,
acetylacetone-AA,
2-hydroxyacetophenone-HAP,
2-hydroxy propiophenone-HPP, and ethylacetoacetate-
EAA) was added drop wise with continuous stirring,
and the mixture was heated to reflux for 2 h. After
completion of the reflux, the dense mixture was trans-
ferred in a 150 ml beaker and allowed to cool at room
temperature. After cooling, product mixture was kept
at a dry place for 24 h at a temperature near about
15ꢀC when colored solid product was seen clearly. This
was then filtered off, washed with methanol, dried,
and preserved in the desiccator over silica gel along
with calcium chloride. Figure 1 shows the schematic
presentation of synthesis of a representative ligand,
The number of the pure coordination compounds,
especially the chelate compounds with non-
oxotitaniumTi (IV), is limited. This research has been
designed to study some chemical behavior of titanium
(IV) through the formation of complexes of
tetradentate
Schiff
bases
prepared
from
1,6-hexanediamine with different aldehydes/ketones,
expected to have either six-coordinated octahedral or
eight-coordinated unusual geometry with non-
oxotitanium (IV). Prepared Schiff bases and titanium
(IV) complexes were characterized with the evidence
Sal-hn-SalH2.
Prepared
Schiff
bases
are
bis(salicylaldehyde)hexanediamine, Sal-hn-SalH2 (yel-
lowish, yield 75%, mp 76ꢀC); bis(2-hydroxy-
1-napthaldehyde)hexanediamine,
HNP-hn-HNPH2
1
of H NMR, FTIR, UV-Visible and mass spectra, mag-
(greenish, yield 90%, mp 164ꢀC); bis(acetylacetone)
hexanediamine, AA-hn-AAH2 (reddish, yield 85%, mp
netic and conducting behavior, and quantum chemical
calculations. Antibacterial, antifungal, and anti-oxidant
activity of the prepared ligands and complexes also has
been assessed. In context to the recent demands
molecular docking approach has been applied for
screening of a Schiff base as potent inhibitor of SARS-
CoV-2 main protease (PDBID: 6Y84).
78ꢀC);
bis(2-hydroxyacetophenone)hexanediamine,
HAP-hn-HAPH2 (greenish yellow, yield 90%, mp
88ꢀC);
bis(2-hydroxypropiophenone)hexanediamine,
HPP-hn-HPPH2 (yellowish, yield 90%, mp 110ꢀC);
bis(ethylacetoacetate)hexanediamine, EAA-hn-EAAH2
(light brownish, yield 65%, mp 68ꢀC).

UDDIN ET AL.
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2.4.1 | Chemical analysis
Identification of chloride
About 2 g of solid sample was taken in a 25 ml volumet-
ric flask and concentrated. Concentrated HNO3 was
added to digest organic portion of the sample complex,
then heated. The mixture was kept about 30 min. The
presence of chloride ion in complexes has been identified
by both dry and wet tests.[16]
Quantitative estimation of titanium
Metal estimation is important to decide the geometry and
structure of Schiff base complexes. Complexes were
brought into solution by decomposing the complexes
with concentrated HNO3 acid followed by evaporation to
dryness, and metal content was estimated spectrophoto-
metrically. A standard calibration curve was made using
TiO2 as the source to Ti4+ according to the standard pro-
cedure.[16] The calibration equation y = 624.26x + 0.0053
obtained with R2 = 0.9999 was used to find the titanium
concentrations of unknown solutions.
FIGURE 1 Schematic presentation of synthesis of a
representative ligand, Sal-hn-SalH2
2.3 | Synthesis of Ti (IV) complexes
A colorless solution of TiCl4.6H2O (10 mmol) in ethanol
was added gradually drop wise with continuous stirring
to a solution of ligands (10 mmol) in ethanol at room
temperature. The reaction mixture was heated to reflux
for 2.5 h. After completion of the reflux, the dense mix-
ture was transferred in a beaker and allowed to cool at
room temperature and then carefully wrapped with
clean, sterile soft paper and kept at a dry place for 48 h at
a temperature of about 15ꢀC when the product was seen
clearly. This was then filtered off, washed with ethanol,
dried, and preserved in the desiccator. Solid complexes
were dry and amorphous. Prepared complexes are C1-Ti
[(Sal-hn-Sal)(H2O)2Cl2] (light pink, yield 55%, mp
>200ꢀC); C2-Ti[(HNP-hn-HNP(H2O)2Cl2] (deep brown-
ish, yield 75%, mp >200ꢀC); C3-Ti[(AA-hn-AA)(H2O)2]
Cl2 (pale brownish, yield 70%, mp >200ꢀC); C4-Ti[(HAP-
hn-HAP)(H2O)2]Cl2 (yellowish orange, yield 60%, mp
>200ꢀC); C5-Ti[(HPP-hn-HPP) (H2O)2]Cl2 (orange, yield
60%, mp >200ꢀC); C6-Ti[(EAA-hn-EAA)(H2O)2]Cl2
(whitish orange, yield 55%, mp >200ꢀC).
Instrumental analysis
Electronic absorption spectra were run on Shimadzu
UV-Visible recording spectrophotometer (Model-1800)
using 1 cm cell, and magnetic susceptibility of Ti
(IV) complexes was measured with magnetic susceptibil-
ity (Balance, Sherwood Scientific, Cambridge, UK) at
room temperature at advanced research laboratory,
Department of Chemistry, University of Chittagong,
Bangladesh. 1H NMR of Schiff bases and complexes were
recorded by Bruker NMR spectrometer from WazedMiah
Science Research Center (WMSRC), Jahangirnagar Uni-
versity, Savar, Dhaka, Bangladesh, by dissolving samples
in CDCl3 or DMSO, respectively. Mass spectrum of Schiff
bases and their Ti (IV) complexes were recorded by the
process “MS Scan” through “Agilent 6460 Triple Quad
LC/MS” by dissolving in CDCl3 or DMSO, respectively.
Conductivity measurement was performed on a Hanna
conductivity meter (Model-HI 8820 N) dissolving com-
plexes (of the order of 10−3 M) in dimethylsulfoxide
(DMSO) at 25ꢀC.
2.4 | Characterization techniques
2.5 | Computational details
The following physical and spectral methods, for exam-
ple, solubility, melting point, infrared, UV-Visible, mass
and 1H NMR spectroscopy, molar conductivity, and mag-
netic susceptibility measurements were investigated for
the characterization of the ligands and complexes. Ele-
mental analytical data and quantum mechanical calcula-
tions also applied for these purposes.
2.5.1 | Quantum chemical calculations
by Gaussian 09
Quantum mechanical (QM) methods allow us to calcu-
late different types of energies precisely and interpret var-
ious types of complicated interaction between ligands
and metal in the formation of complexes. In the present

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UDDIN ET AL.
work, density functional theory with Becke's and
exchange functional combining Lee, Yang, and Pare's
correlation functional[17] in Gaussian 09 program pack-
age was used for complexes.[18] PBEPBE/SDD basis set
has been employed to optimize the complexes to eluci-
date their thermodynamic properties such as thermal
energy, enthalpy, Gibb's free energy, entropy, dipole
moment, and their electronic properties, for example,
frontier molecular orbital features (HOMO, LUMO, and
HOMO-LUMO gap), hardness, and softness.[19] The fol-
lowing equations are used to calculate the hardness (η)
and softness (S); hardness = [LUMO-HOMO]/2; soft-
ness = 1/hardness.
2.6 | Biomedical assessments
Selected Ti (IV) complexes, (S1) Ti[(AA-hn-AA)(H2O)2]
Cl2, (S2) Ti[(HNP-hn-HNP)(H2O)2 Cl2], (S3) Ti[Sal-hn-
Sal)(H2O)2Cl2], (S4) Ti[(HPP-hn-HPP)(H2O)2Cl2] pre-
pared from Schiff bases (S5) AA-hn-AAH2, (S6) Sal-hn-
SalH2, (S7) HNP-hn-HNPH2, (S8) HPP-hn-HPPH2,
respectively, were subjected to evaluate their biomedical
assessments, for example, anti-bacterial and anti-fungal
activity and antioxidant property. Three human patho-
genic bacteria, Bacillus cereus, Salmonella paratyphi, and
Staphylococcus aureus, and three fungi, Aspergillus niger,
Penicillium sp., and Candida albicans were collected from
the microbiology laboratory, Department of Microbiol-
ogy, University of Chittagong, Bangladesh.
2.5.2 | Preparation of receptor protein
The crystal structure (3D) of COVID-19 Mpro (PDB ID:
6LU7) was collected from online protein data bank
(PDB) and used as receptor protein.[20] The water mole-
cules, hetero atoms, and unwanted molecules were
removed from chain A of receptor protein by PyMol (ver-
sion 2.2) software package[21] and saved in pdb format.
Swiss-Pdb Viewer software (version 4.1.0)[22] was used to
implement for energy minimization of protein. Finally,
the protein was ready for molecular docking with opti-
mized molecules considering molecule as ligand and pro-
tein as macromolecule.
2.6.1 | Bacterial activity assessment
The “liquid culture method”[27] was applied for the detec-
tion of antibacterial activities. Preparation of nutrient
ager (NA) medium, stock culture, preservation of stock
culture, and preparation of bacterial suspension were
performed according to the procedure stated in
literature.[27]
In this method, 10 ml of liquid NA was taken in each
of the test tubes first. Then it was sterilized in autoclave
for 15 min. Then different concentration of test samples
(0.1%, 0.05%, and 0.01%) was taken at different amount
ratio (25, 50, 75, and 100 μl) in different test tubes. After
that 100 μl of bacterial suspension was taken in each test
tubes, and this mixture was thoroughly mixed with con-
tinuous shaking. After a while, it was kept in the incuba-
tor at temperature near 35ꢀC, and absorbance readings
were taken at 600 nm after 3, 6, and 24 h with UV–
Visible spectrophotometer using 1 cm cell. All the absor-
bance readings were taken at least three times (n = 3).
Mean value and standard deviation of absorbance were
taken into account. During recording, absorbance after
24 h saline water was added with the mixture to prevent
over growth of bacterial species. During each interval
(3, 6, and 24 h) of incubation, the mixture was cen-
trifuged, and the absorbance was taken of the clear
supernatant.
2.5.3 | Molecular docking simulation,
analysis, and visualization
In computer-aided drug design, molecular docking simu-
lation helps to predict binding affinity and mode(s) of
ligand and protein.[23] Molecular docking was performed
utilizing PyRx software (version 0.8),[24] and all rotatable
bonds were transferred to non-rotatable involving center
gird box size 26.290, 12.6046, and 58.9551 Å along with x,
y, and z direction, respectively. Accelrys Discovery Studio
(version 4.5)[25] was utilized to perform non-bonding
interactions.
2.5.4 | ADMET analysis
ADMET is important to analyze the pharmacodynamics
of the proposed molecules which could be used as drugs.
AdmetSAR online database was used to predict the
absorption, distribution, metabolism, excretion, and tox-
icity.[26] Utilizing both structure data file) and simplified
molecular-input line entry system strings, the result was
analyzed.
2.6.2 | Fungal activity assessment
The anti-fungal activity of the test complexes was studied
using PDA as basal medium.[28] DMSO was used as sol-
vent to prepare desired solution (0.1%) of the test com-
pounds. Proper control was maintained with DMSO in
each case of fungi. Potato dextrose agar (PDA) medium

UDDIN ET AL.
5 of 17
was used throughout the study for the growth and main-
tenance of fungi. Test tube slants of PDA medium were
prepared for the maintenance of cultures. Preparation of
liquid potato dextrose medium, stock culture, preserva-
tion of stock culture, and preparation of fungal suspen-
sion were performed according to the procedure stated in
literature.[28]
effectiveness of various antioxidant substances
according to Kalidas et al.[32] with slight modification.
3 | RESULTS AND DISCUSSION
3.1 | Characterization of ligands
At first, 20 ml of sterilized “liquid potato dextrose”
medium was taken in 50 ml conical flasks. Ten conical
flasks were prepared for one batch of fungus in the same
manner. Then 200 μl of chemical control (DMSO) and
different samples were added in each separate conical
flask and shaken well to mix thoroughly. A conical flask
was kept without chemical control (DMSO) or test sam-
ples considered as blank for identifying general growth of
each fungus in “liquid potato dextrose” medium. After
that, the fungal inoculums (6 mm mycelial block) were
placed carefully at each of 10 conical flasks. Initial pH of
one batch (chemical control and test samples) was taken
before 3 days. All the conical flasks were incubated at
room temperature on the laboratory desk for 3 days.
After 3 days, growth of fungus in each conical flask was
observed, and then it was filtered. The filtrate was used
for the pH measurement. Data found were analyzed to
understand the change in pH and to establish a co-
relation between pH change of the culture medium and
the fugal inhibition/inducing activity of test samples. The
biomass collected after filtration was kept in the oven at
120–150ꢀC to make it dry, and then weight of dry mass of
each conical flask was measured.
These prepared ligands were characterized by melting
point determination, solubility test, UV-Visible, FTIR, 1H
NMR, mass spectra, and quantum chemical calculations.
All ligands are insoluble in methanol or ethanol but solu-
ble in CHCl3 in cold. Though all are soluble in DMSO,
acetone, and DMF on heat, Sal-hn-SalH2 is soluble in
cold in DMSO. Sharp melting points of all ligands have
been recorded. Elemental analysis (% C, H) results sup-
port their formation (Table S1).
The infrared spectra of the Schiff bases were recorded
using KBr disc in the range of 500–4000 cm−1, and data
are presented in Table S1. The important features
observed in the infrared spectra of the prepared Schiff
base ligands are
a
weak broad band around
3368–3422 cm−1 indicating the presence of ( OH) group
in hydrogen bonding.[33,34] The present Schiff base
ligands contain the νC N modes showing its characteris-
tic sharp band at 1558–1655 cm−1. On the basis of various
studies, bands observed in the region of ꢁ1562–1620,
1242–1400, 1130–1134, and 1500–1510 cm−1 have been
assigned due to νC N, νC C, νC O, and νC C, respec-
tively.[33,34] The bands observed at 1130–1134 cm−1 were
assigned to the stretching frequency of the phenolic
ν(C O). Bands observed at ꢁ3048–3178 cm−1 were
assigned to the presence of aromatic stretching of the
C H bond. Furthermore, the presence of bands at
ꢁ2862–2889 and 2927–2940 cm−1 were assigned to the
presence of both symmetric and asymmetric stretching of
C H bonds in both alkyl and phenyl environment of the
ligands. The presence of bands in the region of
ꢁ630–791, ꢁ864–924, and ꢁ1315–1585 cm−1 is assigned
for the presence of CH2 bonds ν( CH2 ), out of
plane deformation of CH2 bonds.[35]
2.6.3 | Anti-oxidant activity assessment
Selected Schiff base ligands and Ti (IV) complexes were
used as test chemicals to justify whether these have good
anti-oxidant property or not. Then four experiments were
done to evaluate the anti-oxidant property of test
samples.
1. Total phenolic content determination (TPC): Total
phenolic content of different fraction was determined
by a method established by Singleton and Rossi[29]
with some modification.
2. Total anti-oxidant capacity (TAC): The antioxidant
activity was evaluated by the phosphomolybdenum
method according to the procedure described by
Prieto et al.[30]
3. DPPH free radical scavenging activity: DPPH
(2,2-diphenyl-1-picrylhydrazyl) radical scavenging
assay was done by Hatano et al.[31]
4. Hydroxyl radical scavenging activity: It is commonly
used to evaluate the free radical scavenging
The UV-Visible spectra of the Schiff base ligands were
recorded in DMSO solution. All the ligands have showed
the electronic spectral absorption in the region of near
ultraviolet between 200 and 400 nm. These spectra are
reproduced in Table S1. UV-Visible spectra of the ligands
at a range of 252–315 nm and 330–406 nm are attributed
to the π-π* (benzoid and azomethine) and n-π*
(azomethine) transition, respectively.[36,37]
The 1H NMR spectral data of the free ligands
recorded in CDCl3 against tetramethylsilane (TMS) as
1
internal reference is presented in Table S1. The H NMR
of the prepared Schiff base ligands had shown signals for

6 of 17
UDDIN ET AL.
methyl protons, methylene protons, aromatic protons,
and hydroxyl and azomethine protons. The 1H NMR
spectra of AA-hn-AAH2 and EAA-hn-EAAH2 displayed
the R OH protons and ethylene proton ( C CH) pro-
tons at 12.01–12.70 and 3.75–4.17 ppm, respectively. The
1H NMR spectra of Sal-hn-SalH2, HNP-hn-HNPH2, HPP-
hn-HPPH2, and HAP-hn-HAPH2 displayed the phenolic
O H protons at 12.37–14.40 ppm. Azomethine protons
( N CH) in Sal-hn-SalH2 and HNP-hn-HNPH2
appeared at 8.54–8.85 ppm.[13,38] The signals due to
methyl protons were observed at 1.21–1.83 ppm, and the
methylene protons were observed between 1.56 and
3.72 ppm for all ligands.[13,14,38] In most of the cases, both
methyl and methylene protons interfered each other.
hydrogen in the sample produced due to higher ioniza-
tion potential or some other reasons.
It was observed that all ligands were insoluble in
methanol, a solvent used for the preparation of ligands
confirming their successful formation. Major IR spectral
band such as υC N was found in the range of
1558–1655 cm−1. Also, a broad peak in the range of
3368–3422 cm−1 had confirmed the presence of υ( OH)
in the ligand structure, indicating that ( OH) group is
present in ortho position. Electronic spectral evidences
had ensured the occurrence of π-π* and n-π* transitions
1
for the ligands. Evidences from H NMR spectroscopy
had suggested the proper presence of δCH2, δCH3,
δ(C N), and δ( OH) spectral peak in relatively proper
ppm range. Mass spectral evidences of ligands had shown
relatively correct fragmentation pattern that comply with
their molecular mass. All these spectral evidences and
elemental analysis date ensured the formation of desired
ligands.
1
Figure 2 presents the H NMR spectrum of a representa-
tive ligand, Sal-hn-SalH2.
The mass spectra of the free ligands confirmed the
proposed formula of ligands by showing a series of peak
at different mass-to-charge (m/z) ratios. DMSO was used
as the dissolving media for sample.[13,39] Molecular ion
peaks (m/z) of all ligands are tabulated in Table S1. The
mass fragmentation pattern of HNP-hn-HNPH2 ligand is
depicted in Figure 3. Mass spectral evidence of “HNP-hn-
HNPH2” ligand had shown the fragmentation pattern in
the (m/z ratio) range 425.20–152.30. Slight deviation m/z
of molecular ion peak may be due to spectrum handling
or monitoring error or may be due to the presence of
impurities or may be due to isotopic abundance of
3.2 | Characterization of complexes
3.2.1 | Physico-chemical analysis
All the prepared complexes were quite hygroscopic in
nature. So these were transferred to dry desiccator as
early as possible after synthesis. Also, sweet aroma was
FIGURE 2 1H NMR spectrum of a
representative ligand, Sal-hn-SalH2

UDDIN ET AL.
7 of 17
FIGURE 3 Mass fragmentation pattern of
HNP-hn-HNPH2
found during condensation and storing, whereas pre-
pared ligands had bitter scent or nothing at all. It is seen
that there is a clear difference between the melting point
of prepared ligands and complexes that confirms their
successful formation. Melting points of all complexes
exceeded 200ꢀC, which are higher than their respective
ligands. All complexes are mostly insoluble in ethanol
and in methanol used as solvent in their synthesis. In
acetone, CHCl3 and DMF complexes are insoluble or par-
tially soluble. In DMSO, all are soluble on heat. At the
mouth of a test tube glass rod soaked with concentrated
NH4OH created white fume, indicating the presence of
chloride in all complexes in qualitative identification. It
was also confirmed by wet test for qualitative chloride
identification. In case of complexes, Ti[(Sal-hn-Sal)
(H2O)2Cl2] and Ti[(HNP-hn-HNP)(H2O)2Cl2], formed
white curd like ppt. were less dense compared with other
complexes such as Ti[(AA-hn-AA)(H2O)2]Cl2, Ti[(HAP-
hn-HAP)(H2O)2]Cl2, Ti[(HPP-hn-HPP)(H2O)2]Cl2 and Ti
[(EAA-hn-EAA)(H2O)2]Cl2. From this particular test, it
was seen that chloride ion might be present at the outer
sphere in the Ti (IV) complexes.
3.2.2 | Infrared spectra
The infrared spectra of the Ti (IV) complexes were
recorded using KBr disc in the range of 400–4000 cm−1
,
and data are presented in Table S2. A vibrational spec-
trum provides valuable information regarding the nature
of functional group attached to the metal ion in a com-
plex. Attention has been focused on a limited number of
bands that provide considerable structural information in
order to suggest the most probable manner of coordina-
tion of the ligand with the metal atom.[33] FTIR spectrum
of a representative complex, [Ti (Sal-hn-SalH2) (H2O)Cl2]
has been presented in Figure 4. The azomethine nitrogen
νC N stretching frequency of the complexes appeared in
the range of 1508–1635 cm−1, which is shifted to lower by
approximately 20–50 cm−1 than the free ligands.[13,40]

8 of 17
UDDIN ET AL.
FIGURE 4 FTIR spectrum of a
representative complex, [Ti (Sal-hn-
SalH2)(H2O)2Cl2]
These bands are shifted to lower wave numbers indicat-
ing the involvement of azomethine nitrogen in coordina-
tion to the metal ion.[34–39] In the complexes, νC O
(multiplet) ppm. The disappearance of hydroxyl signal
and lower shift of azomethyl signals indicates that O H
and CH N groups are involved in chelation.[13,37,40]
1
frequency was found in the range of 1245–1292 cm−1
,
Shifting of the bands/peaks in case of complexes than H
which is shifted to higher than free ligands. It is due to
NMR of ligands (both in lower or higher energy level)
indicates that the desired complex formation was
successful.
the coordination of titanium as confirmed by the pres-
ence of νTi O at 370–451 cm−1 [28]
The broad νO H
.
band at the range of 3368–3422 cm−1 is absent at the
spectrum of complexes, which has been overlapped with
H2O coordination. Bands at 3421–3468 cm−1 due to
νO H indicates that the central metal is coordinated by
water molecule.[13,40] Coordination sites are further
supported by the assignment of 476–552 cm−1 attributed
to νTi N. Also, appearance of a bond at 370–421 cm−1
attributed to C O Ti moiety suggests bonding through
phenolic oxygen. The presence of νTi Cl is confirmed by
absorption band at 652 cm−1 in the complexes, Ti[(Sal-
hn-Sal)(H2O)2Cl2] and Ti[(HNP-hn-HNP)(H2O)2Cl2].
3.2.4 | Electronic spectra
The electronic absorption spectra are often very helpful
in the evaluation of results furnished by other methods of
structural investigation. The UV-Visible spectra of the Ti
(IV) Schiff base complexes were recorded in DMSO solu-
tion in the range of 200–600 nm regions. These spectral
bands are tabulated in Table S2. UV-Vis spectra of a rep-
resentative ligand (red), Sal-hn-SalH2 and its complex
(black), Ti[(Sal-hn-Sal)(H2O)2Cl2] have been presented
in Figure S1. Bands around 305–325 nm is assigned as an
n-π* transition. Also, bands around 368–423 nm is
assigned as LMCT.[37,39] Observed bands in the range of
226–261 nm are designated as π-π* type transitions.[40]
The electronic absorption bands due to the n-π* transi-
tion of the non-bonding electrons present on the nitrogen
of the azomethine group in the Schiff bases are shifted to
the lower energy.[13,14] From these results, it is said that
the imine nitrogen atom appears to be coordinated to
the metal ion.[28] These bands were shifted to a shorter
wavelength by chelation, which confirms the complex
formation. It was difficult to separate spectral bands that
occur in complexes due to LMCT from n-π* transition
bands. Most importantly, the absence of any peak above
500 nm (indicating d-d transition) confirms d0 state of Ti
(IV) complexes.
3.2.3 | 1H NMR spectra
The 1H NMR spectral data presented in Table S2 of the Ti
(IV) complexes were recorded in DMSO against TMS as
internal reference. The 1H NMR spectra of the Ti
(IV) complexes showed signals for methyl, methylene,
ethylene, aromatic, and azomethine protons. The 1H
NMR spectra displayed the aromatic protons at 6.35–8.53
(multiplet). Azomethine protons ( N CH) generated
from
aromatic aldehydes showed
signals
at
8.51–8.71 ppm. The ethylene protons ( C CH) were
observed at 4.01–4.69 ppm due to the enolization of
diketone upon coordination. The signal due to methyl
protons was observed at 1.11–1.50 ppm, and the methy-
lene protons were observed between 1.45 and 3.67

UDDIN ET AL.
9 of 17
3.2.5 | Mass spectra
isotopic abundance of hydrogen, carbon, or titanium in
the sample produced due to higher ionization potential
or some other reasons.[28] Mass spectral evidence clearly
confirms the six coordinated octahedral structure of both
electrolytic and non-electrolytic complexes derived from
Schiff bases.
The electron impact mass spectra of the prepared Ti
(IV) complexes confirmed the proposed formula by show-
ing a series of peak at different mass-to-charge (m/z)
ratios. DMSO was used as the dissolving media for the
sample. The molecular ions or parent peaks (m/z) are
mentioned in Table S2 for respective complexes. Frag-
mentation pattern of mass spectra of two complexes is
discussed herein. Figure 5 shows the mass spectra of a
representative complex, Ti[(HNP-hn-HNP)(H2O)2Cl2].
The complex, Ti[(HNP-hn-HNP)(H2O)2Cl2], had shown a
peak at 579.00 due to molecular ion (parent peak). The
series of peaks in the range of 563.10, 514.20, 487.30,
473.50, 447.50, 365.30, 352.70, 325.311, and 277.10 were
corresponding to various fragments, and their intensity
gives an idea of the stability of these fragments. The com-
plex, Ti[(AA-hn-AA)(H2O2]Cl2, had shown a peak at
437.20 due to molecular ion (parent peak) instead of
435.33. The series of mass spectral peaks at the range of
425, 409, 387, 349, 302, 289, 274, 244, 230, and 202 were
corresponding to various fragments, and their intensity
gives an idea of the stability of these fragments. Percent
deviation of parent peak was estimated in the range of
−0.07 to 0.38. Slight deviation of parent peak might be
due to spectrum handling or monitoring error or may be
due to the presence of impurities or may be due to
3.2.6 | Molar conductance
Mainly molar conductance values are used in obtaining
information about the nature of the complex in solution.
The interpretation of the molar conductance study results
provides information of possible structures of coordina-
tion compounds. Generally, for the transition metal
complexes, which are insoluble in aqueous medium, con-
ductance data are obtained in non-aqueous medium.
Most useful data are generally obtained in non-aqueous
solvents such as nitrobenzene, acetonitrile, acetone,
dimethylformamide
(DMF),
dimethylsulphoxide
(DMSO), and chloroform.[13,39] The conductivity of the
prepared Ti (IV) complexes was measured in
dimethylsulphoxide. Molar conductance data of prepared
Ti (IV) Schiff base complexes are enlisted below in
Table S2. From the molar conductance data, it is said that
complexes C1 and C2 did show molar conductance value
of 32 and 11 (ohm−1 cm2 mol−1), respectively, which is
FIGURE 5 The mass spectra of a
representative complex, Ti[(HNP-hn-HNP)
(H2O)2Cl2]

10 of 17
UDDIN ET AL.
completely different than the remaining complexes
having conductance values in the range of 61–97
(ohm−1 cm2 mol−1). The values of molar conductance
suggested that complexes C1 and C2 are non-
electrolyte in nature, and the remaining complexes are
electrolyte in nature. This is in agreement with the
proposed structure for the complexes on the basis of
analytical data. Thus, it might be said that complexes
derived from Schiff bases of ortho-hydroxyketones
or diketones and Ti (IV) complexes (C3–C6) are
electrolyte in nature, but complexes derived from
Schiff bases of ortho-hydroxy aldehydes and their Ti
(IV) complexes (C1 and C2) are non-electrolyte in
nature.
Ti[HPP-hn-HPP)(H2O)2]Cl2, and Ti[(AA-hn-AA)(H2O)2]
Cl2, Ti[(EAA-hn-EAA)(H2O)2]Cl2, suggested chloride
ions to be outer sphere. 1,6-Hexanediamine possessing
two symmetrical amino groups formed bis-Schiff bases
with orthohydroxyaldehydes/diketones. Ligands acting
as a dibasic tetradentate ligand through ONNO donors
satisfy both primary and secondary valencies of Ti
(IV) having four donor atoms in a plane. Estimation of
Ti4+ as shown in Table S2 confirms the 1:1 stoichiom-
etry of complexes having evidences that the prepared
complexes are mononuclear/monometallic. Along with
mass spectral analysis confirmed their geometry as
shown in Figure 6.
3.3 | Frontier molecular orbital
3.2.7 | Thermal study
All the complexes are designed depending on character-
ization evident such as UV-Vis, FTIR, 1HNMR,
and mass spectroscopy. Quantum mechanical (QM)
methods allow us to calculate different types of energies
precisely and interpret various types of complicated
interaction between ligands and metal in the formation
of complexes. In the present work, density functional
theory (DFT) with Becke's and exchange functional
combining Lee, Yang, and Pare's correlation func-
tional[17] in Gaussian 09 program package were used
for complexes.[18] PBEPBE/SDD basis set has been
employed to optimize the ligands and complexes to elu-
cidate their thermodynamic properties such as thermal
energy, enthalpy, Gibb's free energy, entropy, dipole
moment, and their electronic properties, for example,
frontier molecular orbital features (HOMO, LUMO,
and HOMO-LUMO gap), hardness, and softness.[19]
Thermodynamic properties of HNP-hn-HNPH2 and Ti
[(HNP-hn-HNP)(H2O)2Cl2] obtained by “density func-
tional theory” in Gaussian 09 program package are dis-
cussed as representative compounds, and the values of
thermodynamic properties are tabulated in Table 1.
Hardness and softness of this ligand and its complex
were also calculated from the energies of frontier
HOMOs and LUMOs considering Parr and Pearson
interpretation[41] of harness in DFT and Koopmans
theorem[42] on the correlation of ionization potential
(IP) and electron affinities (EA) with HOMO and
LUMO energy. Energy gap between HOMO and LUMO
of optimized structures is obtained 7.70285 eV for
ligand and 1.9012 eV for complex. Geometry of the
complexes proposed on the basis of experimental evi-
dences was confirmed by the optimized structures as
shown in Figure 7 obtained by quantum chemical cal-
culation for a representative ligand, HNP-hn-HNPH2
and its complex, Ti[(HNP-hn-HNP)(H2O)2Cl2].
Thermal analysis is used to evaluate the stability of the
metal complexes and the existence of hydrated or coordi-
nated water molecules. Sharp melting points of com-
plexes could not be detected. Decomposition of
complexes is observed at/above 200ꢀC. At this tempera-
ture, certain amount of complexes was heated till gaining
a constant weight. Calculated mass lost was 7.36%, 6.32%,
8.17%, 6.88%, 6.85%, and 7.43% for C1, C2, C3, C4, C5,
and C6, respectively, which are in close agreement with
their theoretical mass loss of 7.51%, 6.21%, 8.27%, 7.10%,
6.72%, and 7.27%, respectively. These results comply with
the elimination of two coordinated water molecules from
complexes.[5,6]
3.2.8 | Magnetic susceptibility
The magnetic susceptibility of the complexes was found
to be in the range of diamagnetic substances. Thus, it can
be said that all formed complexes are diamagnetic in
nature.[36,40]
All physic-chemical characteristics data suggested
the highest possibility of the formation of Ti
(IV) complexes with Schiff bases derived from
1,6-hexanediamine and carbonyl compounds. All chem-
ical and characterization data gave us significant idea
about the nature and property of prepared complexes.
Two important conclusions can be reached about the
geometry of formed complexes from these studies
stated above. Mass spectral analysis confirmed the
following eight coordinated geometry as Ti[(HNP-hn-
HNP)(H2O)2Cl2] and Ti[(Sal-hn-Sal)(H2O)2Cl2]. Molar
conductance data supported the non-electrolytic nature
of these types of complexes. The electrolytic nature of
the remaining complexes, Ti[HAP-hn-HAP)(H2O)2]Cl2,

UDDIN ET AL.
11 of 17
FIGURE 6 Molecular formula and
geometry of prepared complexes
3.4 | Biomedical assessment results
S. paratyphi whereas sample S7 (HNP-hn-HNPH2) did
show stimulating effect against all tested organisms and
S4, Ti[(HPP-hn-HPP) (H2O)2]Cl2, did show stimulating
effect against C. albicans. After all, it is said that prepared
ligands did show better activity (both stimulating and
inhibiting) than prepared Ti complexes of these ligands.
3.4.1 | Discussion on bacterial activity
assessment
Bacterial activity assessment of prepared Ti
(IV) complexes (S1–S4) of Schiff base ligands (S5–S8) did
show very low activity against all four tested organisms.
Comparison of activity of sample (S6) at different concen-
tration and different incubation period against different
organism has been shown in Figure S2. In some cases,
higher absorbance value was obtained for 75 μl concen-
tration than that of 100 μl concentration. This effect can
be said as the minimum amount of concentration that
have the ability to enhance aerial growth of bacteria at
maximum level which otherwise can be termed as satura-
tion point for test organisms or the optimum chemical
used by the organism. Sample S5 (AA-hn-AAH2) and S6
(Sal-hn-SalH2) did show low inhibition activity against
3.4.2 | Discussion of fungal activity of
test samples
Fungal activity against Aspergillus niger
The prepared ligands and complexes were tested by
liquid media method by measuring pH of the solutions
against Aspergillus niger. Figure 8 showed that in the
presence of test samples, fungal biomass showed lower
pH (acidic) except sample S5 after 3 days incubation.
Control species (test organism) in liquid medium tends to
be neutral at pH value, whereas chemical control species

12 of 17
UDDIN ET AL.
TABLE 1 (a.) Thermodynamic properties of HNP‐hn‐HNPH2 and Ti[(HNP‐hn‐HNP)Cl2].2H2O obtained by “density functional theory,”
(b.) binding affinity, nonbonding interaction, and (c.) pharmacokinetic properties of HNP‐Hn‐HNPH2 and hydroxychloroquine with SARS
Cov‐2
a. Thermodynamic properties
b. Binding affinity, nonbonding interaction
Property
HNP‐hn‐
HNPH2
Ti[(HNP‐hn‐HNP) Name
(H2O)2Cl2]
Binding affinity
(kcal/mol)
Residues in
contact
Gln110
Interaction
type
Distance
(Å)
Electronic
energy
−652.7109
−1400.953
HNP‐hn‐
−6.3
H
2.4452
HNPH2
(Hartree)
Enthalpy
(Hartree)
−652.8021
−653.3249
−1400.952
−1401.031
Gln110
Phe294
C
3.0672
3.8712
Gibb's free
energy
Pp
(Hartree)
Dipole moment
(D)
1.9703
7.309
Phe294
Pp
3.8836
?HOMO (eV)
?LUMO (eV)
Gap (El‐Ev) eV
Hardness
−8.5669
−0.8640
7.70285
3.8514
−5.2115
−3.3103
1.9012
7.309
HCQ
−5.8
Tyr154
Gln110
Thr111
Asp153
Gln110
Thr111
Arg298
H
H
H
C
C
C
A
2.47092
2.80338
2.89271
2.43213
2.56382
2.29355
3.77516
Softness
0.2596
1.0519
c. Pharmacokinetic parameters
Drug
Blood brain
barrier
Human intestinal
absorption
P‐glyco protein
inhibitor
hERG
Carcinogen
Rat acute toxicity LD50
(mol/kg)
HNP‐hn‐HNPH2 +(0.6626)
HCQ +(0.5602)
+(0.9282)
+(0.9892)
NI(0.7147)
NI(0.7297)
SI(0.6178)
NC(0.8182)
NC(0.8370)
2.2838
2.6348
WI(0.6798)
Abbreviations: A, alkyl; C, carbon hydrogen bond; H, conventional hydrogen bond; NC, non‐carcinogen; NI, non‐inhibitor; PA, Pi‐alkyl; WI,
weak inhibitor.
became acidic (after 3 days); however, in the presence of
test ligands and complexes, the fungal medium became
acidic that means that pH of the sample S1, S2, S3, S4,
S6, and S7 became 2.30, 2.53, 2.32, 4.74, 5.41, and 4.41,
respectively, but in case of S5, pH of the sample tends to
be less acidic or neutral by showing pH value 6.29.
Figure S3 showed that the test sample expressed more or
less biomass after 3 days incubation. From the biomass of
Aspergilus niger species, it is seen that S4 and S6 have the
inducing capacity to enhance the aerial growth of this
species, whereas S1 and S2 have similar activity as the
control. On the other hand, S3, S5, and S7 have the inhi-
bition ability.
pH (acidic or weak acidic) after 3 days incubation. Con-
trol species (test organism) in liquid medium tends to be
neutral at pH value, whereas chemical control species
became acidic (after 3 days); however, in the presence of
test ligands and complexes, the fungal medium became
acidic that means that pH of the sample S1, S2, S6, and
S7 became 3.62, 2.88, and 3.90, respectively, and in case
of sample S1, S3, S4, and S5, pH became less acidic. pH
values of sample S1, S3, S4, and S5 became 5.57, 5.06,
5.40, and 5.42, respectively. The fungal biomass result,
which was measured by dry weight measured technique,
showed that the test sample expressed more or less
biomass after 3 days incubation. From the biomass of
Penicillium sp., it is seen that sample S2, S4, S5, S5, S6,
and S7 have the inducing capacity to enhance the aerial
growth of this species, whereas sample S1 and S4 have
the inhibition ability.
Fungal activity against Penicillium sp.
The prepared ligands and complexes were tested by liq-
uid media method by measuring pH of the solutions
against Penicillium sp. The results showed that with the
presence of test samples, fungal biomass showed lower
From this study, it can be said that prepared ligands
did show better inhibition activity than Ti (IV) complexes

UDDIN ET AL.
13 of 17
FIGURE 7 Optimized structure,
HOMO, and LUMO of HNP-hn-HNPH2 and
Ti[(HNP-hn-HNP)(H2O)2Cl2] after DFT
calculations
their permeation through the lipid layer of the cell mem-
branes of microorganisms. But in general, it can be said
that prepared ligands and complexes did show stimulat-
ing activity, which is supported by lowering of pH value
of the liquid medium after certain incubation period
(3 days). The titanium (IV) complexes of Schiff bases
derived from aroylhydrazine also showed higher induc-
ing capacity against Salmonella paratyphi and Bacillus
cereus.[28] In case of stimulating activity, it is supposed
that microorganisms might consume the ligands by deg-
radation and have their nutrients.
FIGURE 8 Fungal activity evaluation of test samples against
Aspergillusniger by pH measurement
3.5 | In silico studies
of respective ligands. Similar results were investigated for
oxotitanium (IV) complexes of Schiff bases prepared by
the condensation of 2-hydroxy-1-naphthaldehyde with
o-phenylenediamine.[36] It can best be explained by
chelation theory that states that the influence of positive
charge of metal π-electrons of Schiff bases get delocalized
over the whole chelate.[5,6] It is the chelation that
increases lipophilic property of complexes and assists
3.5.1 | Binding affinity and interactions
In order to find potent SARS-CoV-2 (main protease
6LU7) inhibitor, HNP-Hn-HNPH2 was selected because
Schiff bases are already known to exert antiviral activity
against many RNA viruses.[10] The molecular docking
was done to predict the binding affinity and pose of

14 of 17
UDDIN ET AL.
HNP-Hn-HNPH2 ligand against SARS-CoV-2 main pro-
tease (6LU7). Similarly, hydroxyl chloroquine (HCQ)
was docked with same receptor protein as standard
(Figure 9). Stronger binding was predicted between
ligand and receptor protein through negative value of
binding affinity.[43] The binding affinity of HNP-Hn-
HNPH2 was −6.3 kcal/mol as presented in Table 1.
Corresponding value of standard (HCQ) was
−5.8.0 kcal/mol indicating the higher binding affinity of
HNP-Hn-HNPH2 ligand. This highest binding affinity
(−6.3 kcal/mol) of the ligand predicted that it might
serve as better inhibitor standard (HCQ). However,
ligand is partially located into the receptor protein
similar to (HCQ). Hydrogen bond is considered as a sig-
nificant factor for RNA structure in biological system
and less than 2.3 Å increases the binding affinity.[44]
Most significant hydrogen bonds of HNP-Hn-HNPH2
with Gln110 amino acid residue of protein in a distance
2.4452 was identified which is closer than standard
HCQ (Table 1). No significant interaction with the Tyr,
Thr catalytic dyad like HCQ was demonstrated for
HNP-Hn-HNPH2. Besides, Pi-Pi hydrophobic interac-
tions with Phe294 and 6LU7 receptor protein was
depicted (Figure 10). This special type of Pi-pi stacked
interaction with Phe294 is supposed to increase binding
affinity in HNP-Hn-HNPH2-6LU7 complex. Hence,
HNP-Hn-HNPH2-6LU7 complex was stabilized, and
HNP-Hn-HNPH2 was fitted within the substrate
binding pocket of 6LU7 Mpro having different binding
modes, hydrogen bond interactions, and hydrophobic
interactions.
3.5.2 | ADMET analysis
The ADMET predicted that the ligand HNP-Hn-HNPH2
shows positive response to both blood brain barrier and
human intestinal absorption as hydroxychloroquine
(HCQ) as shown in Table 1. The ligand was found to be
non-carcinogenic and does not inhibit the P-glycoprotein.
Therefore, it has protective capacity to interrupt the
absorption, permeability, and retention of the drugs.[45]
Ligand along with hydroxychloroquine shows type III
acute oral toxicity. Ligand HNP-Hn-HNPH2 indicates
good inhibitor for human ether-a-go-go-related gene
(hERG) that can lead to short QT syndrome.[46]
3.6 | Discussion on anti-oxidant property
assessment
The compounds are considered to have antioxidant prop-
erties in those that possess biological properties such as
antiapoptosis, antiaging, anticarcinogen, antiinflamation,
antiatheros-clerosis, cardiovascular protection, and
improvement of endothelial function, as well as inhibi-
tion of angiogenesis and cell proliferation activities.
1. Higher TPC value of any compound indicates higher
anti-oxidant property. Method used here to estimate
TPC in this research is F-C method, which may suffer
from a number of interfering substances. But the pro-
posed geometry suggested for the ligands and com-
plexes suggesting that all the test samples should have
FIGURE 9 Docked conformations and
nonbonding interactions of HNP-Hn-
HNPH2 and hydroxychloroquine at
inhibition bounding site of receptor
protein 6LU7

UDDIN ET AL.
15 of 17
FIGURE 10 Molecular
docking of HNP-Hn-HNPH2
with receptor protein
6LU7.6LU7-HNP-Hn-HNPH2
complex depicting the possible
hydrogen bonding and
hydrophobic Pi-Pi interactions
with Gln 110 and Phe
294 amino acid residue,
respectively
TABLE 2 Total phenolic content and total anti-oxidant capacity of test samples
Total phenolic content (mg GAE/g)
Total anti-oxidant capacity (mg AAE/g)
Eq.
Average
conc.
RSD
(%)
Eq.
Average
conc.
RSD
(%)
Name of test sample
conc. (μg/ml)
TPC
conc. (μg/ml)
TAC
Ti[(AA-hn-AA)(H2O)2]
Cl2
481.28
481.28
480.72
276.28
274.60
275.72
135.17
134.61
134.61
559.61
559.06
559.06
109.06
108.50
110.17
93.50
481.09 0.32 300.68 0.07
4.09
3.95 0.24
9.25 0.53
14.15 0.92
31.06 0.53
27.43 0.53
4.74
5.98
4.09
3.68
Ti[(HNP-hn-HNP)
(H2O)2Cl2]
275.53 0.85 172.21 0.31
9.55
11.56 5.68
16.85 6.47
38.83 1.69
34.29 1.92
8.64
9.55
Ti[(Sal-hn-Sal)
(H2O)2Cl2]
134.80 0.32 84.25
0.24
15.00
13.18
14.27
31.36
30.45
31.36
27.73
26.82
27.73
Ti[(HPP-hn-HPP)
(H2O)2]Cl2
559.24 0.31 349.53 0.56
HPP-hn-HPPH2
AA-hn-AAH2
HNP-hn-HNPH2
Sal-hn-SalH2
109.24 0.85 68.28
0.77
0.90
93.68 0.85
58.55
92.94
94.61
187.38
187.38
185.17
127.39
125.72
127.94
186.64 1.27 116.65 0.68
127.02 1.15 79.39
0.91
Abbreviations: TAC, total anti-oxidant capacity; TPC, total phenolic content determination.

16 of 17
UDDIN ET AL.
higher values of TPC. TPC of samples (S1–S8) suggest
tetradentate one coordinated through ONNO donor
atoms lying in a plane with respect to metal ion. Stim-
ulating effect was found for sample HNP-hn-HNPH2
against all studied test organisms (C. albicans, S. para-
typhi, S. aureus, and B. cereus), whereas AA-hn-AAH2
and Sal-hn-SalH2 did show low inhibition activity
against S. paratyphi. Prepared ligands and complexes
did show fungal stimulating activity against Aspergillus
niger and Penicillium sp. Some of the prepared ligands
and complexes have potential to be an anti-oxidant
agent. The results of molecular docking revealed that
HNP-Hn-HNPH2 have the best binding affinity with
the receptor protein compared with the approved med-
icine, hydroxychloroquine. The ADMENT analysis
showed that ligand is non-carcinogenic and less toxic
than standard HCQ. However, these in silico studies
proved that HNP-Hn-HNPH2 would be a good candi-
date as inhibitor for SARS-CoV-2. But it is required to
perform in vitro and in vivo studies to establish a solid
experimental evidence of its activity as inhibitor for
COVID-19.
that all of these have moderate value as shown in
Table 2. But among all of these, S1, S2, S4, and S6 are
highly potential as these all have TPC value more
than 100 mg GAE/g dry weight. It is decided to say all
the test samples (S1, S2, S4, and S6 are prominent
among test samples) can show better anti-oxidant
property.
2. The phosphomolybdate method is quantitative,
because total anti-oxidant capacity (TAC) is expressed
as ascorbic acid equivalents. In our experiment, TAC
values of S4 and S8 are 38.83
0.663 and
34.29 0.663 mg AAE/g dry weights as shown in
Table 2, though it is in the range of pro-oxidative
property. But due to the presence of water molecule in
the structures of S1–S4, it may support the lower value
of TAC. Thus, it is decided to say that S2 and S4
are potential for anti-oxidant property by this
measurement.
3. The DPPH assay is considered to be based on an ET
reaction, and hydrogen atom abstraction is a marginal
reaction pathway. It is said that the lower the absor-
bance value of test sample, the higher the DPPH free
radical scavenging activity is. Many antioxidants that
react quickly with peroxyl radicals may react slowly
or may even be inert to DPPH due to steric in accessi-
bility. Higher absorbance value of samples might be
due to the presence of color compound or compound
with the self-absorbing power. In this DPPH method,
the IC50 values of sample S4 and S7 and ascorbic acid
(standard) are 1.34, 5.25, and 1.77 (μg/ml), respec-
tively. The values were highly encouraging as the cut
off value of ascorbic acid is 1000. It can be referring to
be potential (S4 and S7) as an anti-oxidant agent.
4. The hydroxyl radical is an extremely reactive free radi-
cal formed in biological system and has been impli-
cated as a highly damaging species in free radical
pathology, capable of damaging almost every mole-
cule found in living cell. The hydroxyl radical can
cause oxidative damage to DNA, lipids, and proteins.
In this method, the IC50 values of sample S5, S6, S7,
and S8 are 1681.31, 104.77, 3952.65, and 534.20
(μg/ml), respectively. Thus, S6 is said to be highly
promising among the test samples studied for
“hydroxyl radical scavenging activity.”
FUNDING
No funding was available.
CONFLICT OF INTEREST
The authors declare that they have no conflict of interest.
AUTHOR CONTRIBUTIONS
Mohammad Nasir Uddin: Conceptualization; method-
ology; project administration; supervision. Shaharier
Amin: Formal analysis. Md. Saifur Rahman: Software.
Sonia Khandaker: Formal analysis. Wahhida Shumi:
Data curation; formal analysis. Atiar Rahman: Formal
analysis. Sheikh Mahbubur Rahman: Resources.
DATA AVAILABILITY STATEMENT
All data and materials are available upon request.
ORCID
Mohammad Nasir Uddin https://orcid.org/0000-0003-
1235-2081
Md. Shaharier Amin https://orcid.org/0000-0001-7316-
2439
Md. Saifur Rahman https://orcid.org/0000-0003-1738-
6950
Sonia Khandaker https://orcid.org/0000-0003-4025-
6230
4 | CONCLUSION
Wahhida Shumi https://orcid.org/0000-0001-7593-6792
Md. Atiar Rahman https://orcid.org/0000-0002-4902-
8923
Sheikh Mahbubur Rahman https://orcid.org/0000-
0002-2230-4932
The prepared Schiff bases and their Ti (IV) complexes
have been characterized on the basis of physic-chemical
and instrumental analysis. Ligands acted as dibasic

UDDIN ET AL.
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How to cite this article: Uddin MN, Amin MS,
Rahman MS, et al. Titanium (IV) complexes of
some tetra-dentate symmetrical bis-Schiff bases of
1,6-hexanediamine: Synthesis, characterization,
and in silico prediction of potential inhibitor
against coronavirus (SARS-CoV-2). Appl
Organomet Chem. 2020;e6067. https://doi.org/10.
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