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)

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]Cl₂being 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.

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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

15^ꢀC 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 76^ꢀC); bis(2-hydroxy-

1-napthaldehyde)hexanediamine,

HNP-hn-HNPH₂

1

of H NMR, FTIR, UV-Visible and mass spectra, mag-

(greenish, yield 90%, mp 164^ꢀC); 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).

78^ꢀC);

bis(2-hydroxyacetophenone)hexanediamine,

HAP-hn-HAPH₂(greenish yellow, yield 90%, mp

88^ꢀC);

bis(2-hydroxypropiophenone)hexanediamine,

HPP-hn-HPPH₂(yellowish, yield 90%, mp 110^ꢀC);

bis(ethylacetoacetate)hexanediamine, EAA-hn-EAAH₂

(light brownish, yield 65%, mp 68^ꢀC).

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 HNO₃was

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 HNO₃acid followed by evaporation to

dryness, and metal content was estimated spectrophoto-

metrically. A standard calibration curve was made using

TiO₂as 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 15^ꢀC 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

>200^ꢀC); C2-Ti[(HNP-hn-HNP(H₂O)₂Cl₂] (deep brown-

ish, yield 75%, mp >200^ꢀC); C3-Ti[(AA-hn-AA)(H₂O)₂]

Cl₂(pale brownish, yield 70%, mp >200^ꢀC); C4-Ti[(HAP-

hn-HAP)(H₂O)₂]Cl₂(yellowish orange, yield 60%, mp

>200^ꢀC); C5-Ti[(HPP-hn-HPP) (H₂O)₂]Cl₂(orange, yield

60%, mp >200^ꢀC); C6-Ti[(EAA-hn-EAA)(H₂O)₂]Cl₂

(whitish orange, yield 55%, mp >200^ꢀC).

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 CDCl₃or 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 CDCl₃or DMSO, respectively.

Conductivity measurement was performed on a Hanna

conductivity meter (Model-HI 8820 N) dissolving com-

plexes (of the order of 10⁻³M) in dimethylsulfoxide

(DMSO) at 25^ꢀC.

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)₂Cl₂], (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 35^ꢀC, 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

120–150^ꢀC 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 CHCl₃in cold. Though all are soluble in DMSO,

acetone, and DMF on heat, Sal-hn-SalH₂is 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⁻¹indicating 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⁻¹have been

assigned due to ν_{C N}, ν_{C C}, ν_{C O}, and ν_{C C}, respec-

tively.^[33,34]The bands observed at 1130–1134 cm⁻¹were

assigned to the stretching frequency of the phenolic

ν(C O). Bands observed at ꢁ3048–3178 cm⁻¹were

assigned to the presence of aromatic stretching of the

C H bond. Furthermore, the presence of bands at

ꢁ2862–2889 and 2927–2940 cm⁻¹were 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⁻¹is assigned

for the presence of CH₂bonds ν( CH₂), out of

plane deformation of CH₂bonds.^[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 CDCl₃against tetramethylsilane (TMS) as

1

internal reference is presented in Table S1. The H NMR

of the prepared Schiff base ligands had shown signals for

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UDDIN ET AL.

methyl protons, methylene protons, aromatic protons,

and hydroxyl and azomethine protons. The ¹H NMR

spectra of AA-hn-AAH₂and EAA-hn-EAAH₂displayed

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-HAPH₂displayed the phenolic

O H protons at 12.37–14.40 ppm. Azomethine protons

( N CH) in Sal-hn-SalH₂and 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⁻¹had 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-HNPH₂ligand 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 200^ꢀC, 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, CHCl₃and 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]Cl_2,Ti[(HAP-

hn-HAP)(H₂O)₂]Cl₂, Ti[(HPP-hn-HPP)(H₂O)₂]Cl₂and 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⁻¹than the free ligands.^[13,40]

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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⁻¹is absent at the

spectrum of complexes, which has been overlapped with

H₂O coordination. Bands at 3421–3468 cm⁻¹due 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⁻¹attributed

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⁻¹in 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-SalH₂and 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 d⁰state 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⁻¹cm²mol⁻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⁻¹cm²mol⁻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-HNPH₂and 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 200^ꢀC. 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‐HNPH₂and Ti[(HNP‐hn‐HNP)Cl₂].2H₂O obtained by “density functional theory,”

(b.) binding affinity, nonbonding interaction, and (c.) pharmacokinetic properties of HNP‐Hn‐HNPH₂and 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 (E_l‐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

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-HNPH₂and

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-HNPH₂was 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-HNPH₂ligand 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-

HNPH₂was −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-HNPH₂ligand. 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-HNPH₂was 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-HNPH₂indicates

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-

HNPH₂and 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

480.72

276.28

274.60

275.72

135.17

134.61

559.61

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

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-SalH₂did 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-HNPH₂have 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-HNPH₂would 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 IC₅₀values 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 IC₅₀values 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.

17 of 17

[29] V. L. Singleton, J. A. Rossi, Am J EnolVitic. 1965, 16, 144.

REFERENCES

[30] P. Prieto, M. Pineda, M. Aguilar, Anal. Biochem. 1999, 269

(2), 337.

[31] T. Hatano, H. Kagawa, T. Yasuhara, T. Okuda, Chem. Pharm.

Bull. 1988, 36, 1090.

[1] T. Acter, N. Uddin, J. Das, A. Akhter, T. R. Choudhury, S.

Kim, Sci. Total Environ. 2020, 138996.

[2] T. M. Abd El-Aziz, J. D. Stock, Infect. Genet. Evol. 2020, 83,

104327.

[3] R. J. Fessenden, J. S. Fessenden, Organic chemistry,

Brooks/Cole publishing Company, USA 1998.

[4] L. H. Abdel-Rahman, A. M. Abu-Dief, M. Basha, A. A. H.

Abdel-Mawgoud, Appl. Organomet. Chem. 2017, 31(11), e3750.

[5] L. H. Abdel-Rahman, M. S. Adam, A. M. Abu-Dief, H.

Moustafa, M. Basha, A. H. Aboria, B. S. Al-Farhan, H. E.

Ahmed, Appl. Organomet. Chem. 2018, 32, e4527.

[6] L. H. Abdel-Rahman, A. M. Abu-Dief, M. R. Shehata, F. M.

Atlam, A. A. H. Abdel-Mawgoud, Appl. Organomet. Chem.

2019, 33(4), e4699.

[32] S. Kalidas, C. Sanders, X. Ye, T. Strauss, M. Kuhn, Q. Liu,

D. P. Smith, Mech. Dev. 2008, 125(5–6), 475.

[33] K. Nakamoto, Infrared and Raman Spectra of Inorganic and

Coordination Compounds, Wiley Interscience, New york, NY,

USA 1970.

[34] L. H. Abdel-Rahman, A. M. Abu-Dief, M. O. Aboelez, A. A. H.

Abdel-Mawgoud, J. Photochem. Photobiol. B 2017, 170, 271.

[35] S. I. Al-Saeedi, L. H. Abdel-Rahman, A. M. Abu-Dief, S. M.

Abdel-Fatah, T. M. Alotaibi, A. M. Alsalme, A. Nafady, Cata-

lysts 2018, 8, 452.

[36] M. N. Uddin, Z. A. Siddique, N. Mase, M. Uzzaman, W.

Shumi, Appl. Organomet. Chem. 2019, 33, e4876.

[37] A. M. Abu-Dief, H. M. El-Sagher, M. R. Shehata,

ApplOrganometal Chem 2019, 33, e4943.

[38] A. M. Abu-Dief, L. H. Abdel-Rahman, A. A. Abdelhamid,

A. A. Marzouk, M. R. Shehata, M. A. Bakheet, O. A.

Almaghrabi, A. Nafady, SpectrochimicaActa Part a: Mol. And

Biomol. Spect. 2020, 228, 117700.

[7] A. M. Abu-Dief, L. H. Abdel-Rahman, A. A. H. Abdel-

Mawgoud, Appl. Organomet. Chem. 2020, 34(2), e5373.

[8] H. Schiff, Justus Liebigs Ann. Chem. 1864, 131(1), 118.

[9] D. Chaturvedi, M. Kamboj, Chem. Sci. J. 2016, 7(2).

[10] L. R. Chen, Y. C. Wang, Y. W. Lin, S. Y. Chou, S. F. Chen,

L. T. Liu, Y. T. Wu, C. J. Kuo, T. S. Chen, S. H. Juang, Bioorg.

Med. Chem. Lett. 2005, 15, 3058.

[11] A. M. Abu-Dief, I. M. A. Mohamed, Beni-Suef Uni. J. Basic and

Appl. Sci. 2015, 4, 119.

[12] M. N. Uddin, D. A. Chowdhury, K. Hossain, J. Chin. Chem.

Soc. 2012, 59, 1520.

[13] A. M. Ajlouni, Z. A. Taha, W. Al Momani, A. K. Hijazi, Inorg.

Chim. Acta 2012, 388, 120.

[14] H. A. Bayoumi, Characteristics Studies of Hexamethylene

Diamine Complexes, Hindawi Publishing Corporation 2013 1.

[15] J. Huang, B. Lian, Y. Qian, W. Zhou, Macromolecules 2002, 35,

4871.

[39] M. N. Uddin, D. A. Chowdhury, M. M. Rony, M. E. Halim,

Modern Chem. 2014, 2(2), 6.

[40] M. N. Uddin, D. A. Chowdhury, N. Mase, M. F. Rashid, M.

Uzzaman, A. Ahsan, N. M. Shah, J. Coord. Chem. 2018, 71,

3874. https://doi.org/10.1080/00958972.2018.1533125

[41] J. Aihara, J. Phys. Chem. A 1999, 103, 7487.

[42] D. E. Manolopoulos, J. C. May, S. E. Down, ChemPhysLett

1991, 181, 105.

[43] L. Ferreira, R. dos Santos, G. Oliva, A. Andricopulo, Molecules

2015, 20, 13384.

[44] R. C. Wade, P. J. Goodford, Prog. Clin. Biol. Res. 1989, 289, 433.

[45] A. Gleich, B. Kaiser, W. Honscha, H. Fuhrmann, A.

Schoeniger, Cytotechnology 2019, 71, 231.

[16] A. I. Vogel, Quantitative Inorganic Analysis, 4thEdn ed.,

Longman 1978.

[17] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785.

[18] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,

M. A. Robb, J. R. Cheeseman, Gaussian 09, Revision A.02,

Gaussian Inc, Wallingford CT 34, Wallingford CT 2009.

[19] H. Kruse, L. Goerigk, S. Grimme, J. Org. Chem. 2012, 77,

10824.

[20] J. L. Sussman, D. Lin, J. Jiang, N. O. Manning, J. Prilusky, O.

Ritter, E. E. Abola, Acta Crystallogr Sect D Biol Crystallogr.

1998, 54, 1078.

[46] M. C. Sanguinetti, M. Tristani-Firouzi, Nature 2006, 440, 463.

SUPPORTING INFORMATION

Additional supporting information may be found online

in the Supporting Information section at the end of this

article.

[21] G. Stockwell, Biochemistry 2003, 42, 225.

[22] N. Guex, M. C. Peitsch, Electrophoresis 1997, 18, 2714.

[23] M. Uzzaman, M. N. Uddin, DARU J. Pharm. Sci. 2019, 27, 71.

https://doi.org/10.1007/s40199-019-00243-w

[24] S. Dallakyan, A. J. Olson, in Chemical Biology: Methods and

Protocols, (Eds: J. E. Hempel, C. H. Williams, C. C. Hong),

Springer New York, New York, NY 2015 243.

[25] Version ADS. 2017.

[26] F. Cheng, W. Li, Y. Zhou, J. Shen, Z. Wu, G. Liu, P. W. Lee, Y.

Tang, J. Chem. Inf. Model. 2012, 52, 3099.

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.

1002/aoc.6067

[27] N. K. Ishikawa, M. C. M. Kasuya, M. C. D. Vanetti, Brazilian

J. Micro. 2001, 32, 206.

[28] M. N. Uddin, S. Khandaker, M. Uzzaman, M. S. Amin, W.

Shumi, M. A. Rahman, M. R. Sheikh, J. Mol. Struct. 2018,

1166, 79e90.

Article Doi

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)

DOI: 10.1002/aoc.6067

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Article abstract of DOI:10.1002/aoc.6067

Full text of DOI:10.1002/aoc.6067

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