Oxotitanium(IV) complexes of some bis-unsymmetric Schiff bases: Synthesis, structural elucidation and biomedical applications

Article abstract of DOI:10.1002/aoc.4876

A number of oxotitanium(IV) complexes of the type TiOL with bis-unsymmetric dibasic tetradentate Schiff base (LH₂) containing ONNO donor atoms have been synthesized. Mono-Schiff base (OPD-HNP) was prepared by the condensation of 1:3 molar ratio of 2-hydroxy-1-naphthaldehyde (HNP) with o-phenylenediamine (OPD). Dibasic unsymmetric tetradentate diamine Schiff bases were prepared by the reaction of OPD-HNP with 2-hydroxyacetophenone, 2-hydroxypropeophenone, benzoylacetone, acetylacetone and ethylacetoacetate. Further, titanylacetylacetonate was reacted with these ligands to obtain their metal complexes. On the basis of analytical and physiochemical data, the formation of complexes as TiOL was suggested having square pyramidal geometry. Quantum mechanical approach also confirmed this geometry. The assessment of the synthesized ligands and their complexes showed that some behave as good inhibitors of mycelial growth against selected phytopathogic fungi but weak inhibitors against some selected bacteria. A few of them also showed antioxidant properties.

Full text of DOI:10.1002/aoc.4876

Received: 5 September 2018
DOI: 10.1002/aoc.4876
Revised: 12 November 2018
Accepted: 27 November 2018
F U L L P A P E R
Oxotitanium(IV) complexes of some bis‐unsymmetric Schiff
bases: Synthesis, structural elucidation and biomedical
applications
Mohammad Nasir Uddin1
| Zainul Abedin Siddique1 | Nobuyuki Mase2 |
Monir Uzzaman1,2 | Wahhida Shumi3
1 Department of Chemistry, University of
A number of oxotitanium(IV) complexes of the type TiOL with bis‐
Chittagong, Chittagong 4331, Bangladesh
2 Department of Applied Chemistry and
Biochemical Engineering, Shizuoka
University, 3‐5‐1, Johoku, Hamamatsu
432‐8011, Japan
3 Department of Microbiology, University
of Chittagong, Chittagong 4331,
Bangladesh
unsymmetric dibasic tetradentate Schiff base (LH2) containing ONNO donor
atoms have been synthesized. Mono‐Schiff base (OPD‐HNP) was prepared by
the condensation of 1:3 molar ratio of 2‐hydroxy‐1‐naphthaldehyde (HNP)
with o‐phenylenediamine (OPD). Dibasic unsymmetric tetradentate diamine
Schiff bases were prepared by the reaction of OPD‐HNP with
2‐hydroxyacetophenone, 2‐hydroxypropeophenone, benzoylacetone, acety-
lacetone and ethylacetoacetate. Further, titanylacetylacetonate was reacted
with these ligands to obtain their metal complexes. On the basis of analytical
and physiochemical data, the formation of complexes as TiOL was suggested
having square pyramidal geometry. Quantum mechanical approach also con-
firmed this geometry. The assessment of the synthesized ligands and their com-
plexes showed that some behave as good inhibitors of mycelial growth against
selected phytopathogic fungi but weak inhibitors against some selected bacte-
ria. A few of them also showed antioxidant properties.
Correspondence
Mohammad Nasir Uddin, Department of
Chemistry, University of Chittagong,
Chittagong 4331, Bangladesh.
Email: nasircu72@gmail.com;
mnuchem@cu.ac.bd
KEYWORDS
quantum mechanical approach, Schiff base, square pyramidal geometry, titanium(IV) complex
1 | INTRODUCTION
base ligand, 2‐(2‐nitrophenylthio)‐N‐((pyridine‐2‐yl)
methylene)benzenamine, and its Mn(II), Ni(II), Cu(II)
Ligands that contain an azomethine group (>C═N─) and
connected to an aryl or alkyl group but not hydrogen are
called Schiff bases.[1] They can be synthesized easily by
the condensation of carbonyls (aldehyde/ketone) and
amines.[2] Nowadays, the research on Schiff bases is
increasing due to their ease of synthesis, structural vari-
ety, varied coordinating ability, thermal stability and cat-
alytic activities.[3,4] The study of metal complexes of Schiff
bases is also increasing because of their anticancer, anti-
tumor, antimicrobial, antifungal, herbicidal and antibi-
otic properties.[5–11] An unsymmetric tridentate Schiff
and Zn(II) complexes were synthesized and characterized
using various physicochemical and spectroscopic
techniques and crystal X‐ray diffraction.[12] Fasina et al.
studied the synthesis, characterization and biological
activity of unsymmetric N2O2 Schiff bases obtained from
the condensation of o‐phenylenediamine with various
salicylaldehydes.[13] A novel Schiff base, Z‐3‐((2‐((E)‐(2‐
hydroxynaphthyl)methylene)amino)‐5‐nitrophenylimino)
‐1,3‐dihydroindin‐2‐one, was synthesized from the con-
densation of 2‐hydroxy‐1‐naphthaldehyde and isatin with
4‐nitro‐o‐phenylenediamine.[9]Previously, titanium(IV)
Appl Organometal Chem. 2019;e4876.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4876

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UDDIN ET AL.
complexes of unsymmetric Schiff bases derived from
ethylenediamine and o‐hydroxyaldehyde/ketone have
been synthesized and their antimicrobial properties have
been evaluated.[14] The work presented here attempted to
prepare unsymmetric Schiff bases from o‐pheny-
lenediamine and 2‐hydroxy‐1‐naphthaldehyde with
various carbonyl compounds, namely 2‐hydro-
xyacetophenone, 2‐hydroxypropeophenone, benzoylace-
tone, acetylacetone and ethylacetoacetate. Complexes of
titanium(IV) with these Schiff bases were prepared and
characterized. On the basis of experimental evidence and
computational chemistry study, the geometry of the com-
plexes was elucidated. In addition, the Schiff bases and
their titanium(IV) complexes were subjected to an assess-
ment of microbial activity (antibacterial, antifungal) and
antioxidant properties.
2.3 | Quantum mechanical calculations
Quantum mechanical methods have an important role in
the calculation of thermal, electrical and geometrical
properties and allow a more accurate interpretation of
stability and reactivity of molecules. The quantum
mechanical calculation was implemented using density
functional theory employing Becke's (B) exchange func-
tional combining Lee, Yang and Parr's (LYP) correlation
functional in the Gaussian 09 program package for all
ligands and complexes.[15–17] Pople's 6‐31G(d) basis set
was used to optimize all the ligands and complexes.[18]
Molecular orbitals were calculated using the same level
of theory. Hardness (η) and softness (S) of all molecules
were also calculated from the energies of frontier highest
occupied molecular orbitals (HOMOs) and lowest unoc-
cupied molecular orbitals (LUMOs) considering Parr
and Pearson interpretation[19] of density functional the-
ory and Koopmans theorem[20] on the correlation of ion-
ization potential (I) and electron affinities (E) with
HOMO and LUMO energies (ε). The following equations
are used to calculate hardness (η) and softness (S):
η = (εLUMO − εHOMO)/2; S = 1/η.
2 | EXPERIMENTAL
2.1 | Materials
All chemicals used were of analytical grade and used as
procured. 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 of analytical grade and further
2.4 | Schiff base ligand synthesis
The tetradentate diamine Schiff bases were prepared
using the usual condensation techniques with the follow-
ing two steps.
purified
using
standard
procedures.
Titanylacetylacetonate (TiC10H14O5; purity 99.50%) was
used for the synthesis of Schiff base complexes of Ti(IV).
2.4.1 | Synthesis of OPD‐HNP
o‐Phenylenediamine (OPD) and 2‐hydroxy‐1‐naphthal-
dehyde (HNP) were used in a ratio of 3:1 for the
2.2 | Instrumentation
Melting point and decomposition temperatures were
determined using an electronic melting point apparatus.
Fourier transform infrared (FT‐IR) spectral analyses were
recorded using an FT‐IR spectrophotometer (model 8900,
Shimadzu, Japan) with KBr as matrix in the range 400–
4000 cm−1. Electrical conductivity measurements were
performed using a HANNA conductivity meter (model
HI 8820N). UV–visible spectral measurements were
conducted with a Shimadzu UV–visible spectrophotome-
ter (model 1800) using a 1 cm cell in the research labora-
tory of the Department of Chemistry, University of
1
Chittagong. H NMR of several Schiff base ligands and
their complexes were recorded with a Bruker NMR spec-
trometer from Wazed Miah Science Research Centre
(WMSRC), Jahangirnagar University, Dhaka, Bangla-
desh. Mass spectra were obtained using an Agilent 6460
Triple Quad LC/MS by dissolving samples in DMSO as
solvent.
FIGURE 1 Reaction scheme of (a) mono Schiff base ligand HNP‐
OPD, (b) bis Schiff base ligand HNP‐OPD‐HAP and (c) complex
TiO[HNP‐OPD‐HAP]

UDDIN ET AL.
3 of 15
preparation of mono Schiff base ligand OPD‐HNP
(Figure 1). OPD (4.33 g, 40 mmol) was dissolved in
25 ml of rectified spirit (RS) to which HNP (6.89 g,
40 mmol) was added dropwise with continuous stirring
in a round‐bottom flask with a reflux condenser. The
mixture was heated at 60–65°C to reflux for 2 h and then
cooled to room temperature. The product was filtered off,
washed with RS then dried and stored in a desiccator over
silica gel along with calcium chloride.
dropwise to HNP‐OPD (10 mmol, 2.55 g) dissolved in
30 ml of RS with continuous stirring to synthesis
unsymmetric bis Schiff base ligands (Figure 2). The solu-
tion was refluxed for for 1 h. The mixture was cooled and
filtered off, washed with RS then dried and stored in a
desiccator over silica gel along with calcium chloride.
Precipitate of some ligands was formed immediately and
some were formed after cooling.
2.5 | Synthesis of Ti(IV) complexes
2.4.2 | Synthesis of unsymmetric bis Schiff
An amount of 5 mmol (1.30 g) of titanylacetylacetonate
(TiC10H14O5) dissolved in RS (25 ml) was added gradually
dropwise with continuous stirring to a solution of ligand
(5 mmol) in RS (25 ml) at room temperature. The reac-
tion mixture was then heated to reflux for 3 h. Colored
precipitates of the prepared complexes were formed
bases
A total (10 mmol) of 2‐hydroxyacetophenone (HAP; 1.20
ml),[21] 2‐hydroxypropiophenone (HPP; 1.37 ml),
benzoylacetone (BA; 1.50 ml), acetylacetone (AA; 1.00
ml) or ethylacetoacetate (EAA; 1.30 ml) was added
FIGURE 2 1H NMR spectrum of representative ligand HNP‐OPD‐HAP
FIGURE 3 Mass spectrum of complex [TiO(HNP‐OPD‐HAP)]

4 of 15
UDDIN ET AL.
(Figure 3). After completion of reflux the dense mixture
was transferred to a 100 ml beaker in hot condition and
allowed to cool to room temperature. Then the product
was carefully wrapped with clean, sterile soft paper and
kept in a dry place for 48 h at a temperature near 15°C.
After 48 h, the product was filtered off, washed with a
small amount of cold RS then dried and stored in a desic-
cator over silica gel along with calcium chloride.Figure 1
shows the reaction scheme of representative mono Schiff
base ligand HNP‐OPD, bis Schiff base ligand HNP‐OPD‐
HAP and complex TiO[HNP‐OPD‐HAP]. The prepared
ligands and their Ti(IV) complexes are presented in
Table 1.Figures S1 and S2 of the supporting information
show the reaction scheme of the synthesis of all ligands
and complexes, respectively.
2.6 | Antimicrobial activities
The prepared ligands and complexes were tested in vitro
against some human pathogenic bacteria, Escherichia
coli, Bacillus subtilis, Salmonella paratyphi and Staphylo-
coccus aureus, and two selected phytopathogenic fungi,
Aspergillus niger and Aspergillus flavus, using two
methods: one was the liquid medium method[22] and
the other was the disc diffusion method.[23] All strains
were collected from the Department of Microbiology,
University of Chittagong. The identity of all of the strains
was confirmed.
2.6.1 | Antibacterial activities
For the determination of antibacterial activities the liquid
culture method[22] was conducted by measuring absorp-
tion with a spectrophotometer at a wavelength of
560 nm. Nutrient agar (NA) was used as the basal
medium for culture of bacteria and DMSO was used as
a solvent for the initial preparation of the desired com-
pound solution.[24] NA medium was prepared using the
following composition: beef extract (3 g), peptone (5 g),
NaCl (0.5 g), agar (15 g) and distilled water (1000 ml). A
total of 1000 ml of distilled water was placed in a beaker
and 15 g of agar powder, 3 g of beef extract, 5 g of peptone
and 0.5 g of NaCl were added slowly and mixed thor-
oughly with a glass rod. The mixture was heated to boil-
ing for 10 min. After 10 min of boiling, the medium was
transferred to 250 ml conical flasks at a volume of
200 ml per flask. The conical flask was closed with a cot-
ton plug and autoclaved at 121°C and 15 psi pressure for
15 min. Culturing of the various microorganisms was
then performed.

UDDIN ET AL.
5 of 15
2.6.2 | Antifungal activities
2.7.2 | Experimental procedure[27]
Potato dextrose agar (PDA) slants were prepared for
maintenance and culture preservation. Small portions
of mycelia of the collected fungal isolates were trans-
ferred to the freshly prepared slants with the help of
sterilized needles. The slants were incubated at
27 2°C for 3 to 5 days. Glass Petri plates were steril-
ized and the melted sterilized PDA medium was poured
at the rate 10 ml per Petri plate. Solutions (0.10%) were
prepared by dissolving 0.001 g of solid samples in 1 ml
of DMSO. Sterilized PDA liquid medium (50 ml) was
dispensed into 200 ml conical flasks. Each of the flasks
was labeled and 100 μL of test compound (0.1% concen-
tration) was added.[25] The flasks were shaken to mix
thoroughly. Then 100 μL of 12 test samples each were
added in each separate conical flask and shaken to mix
thoroughly. After that, 1 ml of fungal suspension was
added carefully to each of 13 conical flasks. All the con-
ical flasks were incubated at 27 2°C for 5 days. A con-
trol conical flask was prepared with DMSO solvent to
observe the result. After 5 days incubation, the growth
of the fungal biomass was observer and recorded.[26]
The parameters of visual observation of spore formation,
record of change in pH and biomass measurement were
checked and measured.
Standard solution (0.5 ml) was taken in a test tube.
Then 2.5 ml of diluted FCR and 2.5 ml of sodium car-
bonate (7.5%) solution were added into the test tube.
The test tube was incubated for 20 min at 25°C to com-
plete the reaction. Then the absorbance of the solution
was measured at 760 nm using a spectrophotometer
against blank. The total phenolic content of standard
in different fractionates in gallic acid equivalents
(GAE) was calculated using the following formula:
C = (c × V)/m, where C is the total content of phenolic
compounds, mg g−1 standard, in GAE; c the the concen-
tration of gallic acid established from the calibration
curve, mg ml−1; V the volume of extract, ml; and m
the weight of ssmple, g.
3 | RESULTS AND DISCUSSION
3.1 | Physical properties
Ligands and complexes were characterized by their
1
physical properties, FT‐IR, H NMR and mass spectral
analysis, UV–visible spectroscopy and magneto‐ and
conductometric studies. All the ligands and complexes
were stable at room temperature and non‐hygroscopic
in nature. On heating, all the ligands dissolved below
200°C but the complexes were decomposed at tempera-
tures above 200°C. All the ligands were soluble in
CHCl3, DMSO and DMF but the complexes were insol-
uble in water but soluble in DMSO and DMF.
2.7 | Assessment of antioxidant
properties: total phenolic content
A number of Schiff base ligands and their Ti(IV) com-
plexes were used as test chemicals to determine whether
these have good antioxidant properties or not.
3.2 | FT‐IR spectra
The FT‐IR spectra of compounds were obtained in the
range 400–4000 cm−1. For the present systems, the FT‐
IR spectra of the ligands and metal complexes provided
information about their formation in respect to the pres-
ence and coordination of the ligands. The peak between
3350 and 3376 cm−1 has been assigned to the υO─H
stretching frequency in the spectrum of free Schiff base,
which is not found for any complexes, indicating complex
formation due to deprotonation and coordination with
the central metal atom. A band at 1610–1620 cm−1
assigned to the υC═N stretching frequency in the spectra
of free Schiff bases was shifted to a lower frequency rang-
ing from 20 to 30 cm−1 in all complexes.[2,13] Another
band near 1621 cm−1 indicates the presence of υC═O
group found for both ligands and complexes which was
shifted to a lower frequency due to coordination of oxy-
gen with central metal atom.[2,5] The band appearing at
around 1536–1562 cm−1 has been assigned to aromatic
2.7.1 | Preparation of reagents
An amount of 2 ml of Folin–Ciocalteu reagent (FCR)
was taken in a beaker and 18 ml of distilled water was
added for 10 times dilution. An amount of 7.5 g of
Na2CO3 (7.5% solution) was taken in a 100 ml volumet-
ric flask. After dissolving Na2CO3 with a small amount
of distilled water, the volume was made up to the mark
by adding distilled water. Gallic acid (1 mg) was dis-
solved into 1 ml of distilled water to prepare a stock
solution of 1 mg ml−1 or 1000 μg ml−1. Serial dilution
was performed in order to prepare different solution
concentrations as required. Blank solution was prepared
consisting of 5 ml of FCR, 1 ml of ethanol and 4 ml of
sodium carbonate solution (7.5%).

6 of 15
UDDIN ET AL.
υC═C. The bands appearing at 1320–1365 cm−1 have
been assigned to the υC─N mode.[12] The bands observed
for the present complexes in the range 1301–1330 cm−1
have been assigned to υC─O.[5,7] A band at 1188–
1190 cm−1 is due to the presence of υTi═O in all com-
plexes.[14] Bands in the regions 562–584 and 452–
453 cm−1 have been assigned to υTi─N and υTi─O vibra-
tions, respectively.[6,12]The some physical properties and
FT‐IR spectral bands of the prepared ligands and com-
plexes are summarized in Table 1. FT‐IR spectra of a rep-
resentative ligand (HNP‐OPD‐EAA) and its complex,
[TiO (HNP‐OPD‐EAA)], are given in Figure S3 and S4,
respectively.
observed for any of the complexes. The disappearance of
the singlet signals for the OH proton in the 1H NMR spec-
tra of the complexes indicating the enolic oxygen atoms
are coordinated to the central metal atom via deproton-
ation.[28] The same result was confirmed from the FT‐IR
spectra. The presence of a sharp singlet for the ─CH═N
proton at 8.83 to 9.27 ppm showed a clear downfield shift
compared to the free ligand and indicated that the mag-
netic environment is equivalent for all such protons.
The downfield chemical shift observation showed the
coordination of the azomethine nitrogen to central
atom.[27] This downfield shift observation may be
assigned to the de‐shielding of protons as a result of the
reduction of electron density after coordination.[29] The
multiplets of aromatic protons appeared within the range
7.45–8.93 ppm and they were not affected by chelation.
The methyl (H3C─C═) signal appeared at 2.13–
2.65 ppm for ligands and complexes.[30] In the case of
ligands (L1, L5) triplet of H3C─ appeared at 1.23–
1.33 ppm. Singlet at 4.56–4.88 ppm was observed for
ligands L3–L5 due to their enolic form that showed
downfield shift after complexation. On the basis of the
physical and spectral data for the Schiff bases and the
complexes discussed above, it is assumed that the metal
ions are bonded to the Schiff bases via the phenolic oxy-
gen and the imino nitrogen.
3.3 | 1H NMR spectra
1
The H NMR spectra were recorded in DMSO solvent at
room temperature using tetramethylsilane as an internal
standard. The 1H NMR data and assignments of the
1
ligands and complexes are presented in Table 2. The H
NMR spectrum of ligand HNP‐OPD‐HAP is shown in
Figure 2. The free ligands exhibit a peak between 10.85
and 12.03 ppm, which is due to hydrogen‐bonded pheno-
lic (L1, L2)/enolic (L3–L5) protons. This peak is due to
hydrogen‐bonded phenolic protons and the integration
is generally less than 2.0 due to this intramolecular
hydrogen bonding. The enol hydroxylic proton appears
as an unsaturated alcohol, but intramolecular hydrogen
bonding causes it to behave like a carboxylic acid, shifting
its resonance far downfield to 12.00 ppm. The chemical
shift observed for the OH protons in the ligands was not
3.4 | Mass spectra
The mass spectra of HNP‐OPD‐HAP and [TiO(HNP‐
OPD‐HAP)] show molecular ion peaks at m/z 380.4 and
TABLE 2 1H NMR spectra of Schiff base ligands in CDCl3 and complexes in DMSO
Chemical shift (ppm)
Ligand
Ar─OH/R─OH
N═CH─
Ar─H
═CH─
─CH2─
─CH3
m/z value
HNP‐OPD‐HPP
HNP‐OPD‐HAP
HNP‐OPD‐BA
HNP‐OPD‐AA
HNP‐OPD‐EAA
12.03
11.12
10.85
11.15
11.35
8.87
9.17
9.18
9.27
9.08
7.45–8.32
7.71–8.91
7.87–8.71
7.55–8.93
7.97–8.52
—
3.11(q)
—
1.28 (t)
2.51
395.50
380.47
407.51
345.45
373.46
—
4.70
4.82
4.88
3.37(s)
3.41(s)
2.44
2.23
4.20(q)
3.45(s)
1.29(t)
2.27(s)
Complex
TiO(HNP‐OPD‐HPP)
TiO(HNP‐OPD‐HAP)
TiO(HNP‐OPD‐BA)
TiO(HNP‐OPD‐AA)
TiO(HNP‐OPD‐EAA)
—
—
—
—
—
8.83
7.41–8.21
—
—
3.01(q)
—
1.33(t)
475.37
461.34
485.38
424.32
455.33
9.15
9.07
9.16
9.01
7.80–8.60
7.83–8.25
7.50–8.83
7.87–8.50
2.65
2.64
2.13
4.56(s)
—
4.61(s)
4.75(s)
—
4.07(q)
1.23(t)
2.18(s)

UDDIN ET AL.
7 of 15
462.3, respectively, which correspond to the molecular
weight of the respective compound, [M + 1], supporting
the structure of the complexes and confirming the
[TiOL]‐type stoichiometry of the metal chelates.
Figure 3 shows a representative mass spectrum of com-
plex [TiO(HNP‐OPD‐HAP)]. The series of peaks at
128.7, 149.7, 277.7, 228.3, 254.9, 332.2, 346.5 and 380.4
may correspond to various fragments of HNP‐OPD‐HAP
and their intensity gives an idea of the stability of the
fragments. Fragmentation scheme is shown in Figure 4.
Again the mass spectrum of [TiO(HNP‐OPD‐HAP)]
shows a series of peaks at 99, 238.6, 327, 405.8, 432.1,
444.4 and 462.3 which may correspond to various frag-
ments. Intensity of fragments gives an idea of their stabil-
ity. The m/z values of all ligands and complexes are
summarized in Table 2. These mass spectral values indi-
cated the formation of the title ligands and complexes.
The spectra, however, did not look explicit as titanium
has five isotopes ranging from 46Ti to 50Ti.
3.5 | Electronic spectra
The electronic spectra of the prepared Schiff base metal
complexes were obtained in DMSO solutions recorded
in the range 250–500 nm. The electronic spectral data
for the prepared ligands and their Ti(IV) complexes are
presented in Table 3. Figure S4 shows the electronic spec-
tra of ligand EAA and its complex. The spectrum of the
Schiff base ligand exhibits three main peaks at around
255, 318 and 428 nm. The first and second peaks at
around 255 and 318 nm were attributed to aryl π–π*
and imino π–π* (C═N) transitions, respectively.[27] The
first band of the other complexes of the Schiff base was
not significantly affected by chelation. The second band
was shifted to a shorter wavelength on chelation. The
third band in the spectra of ligand (ca 428 nm) was
assigned to n–π* transitions.[29,30] This band was shifted
to a longer wavelength along with an increase in its
intensity. This shift may be attributed to the donation of
the lone pairs of the nitrogen atoms of the Schiff base
ligand to the metal ion (N–M). Similar observation was
made for the prepared complexes. The electronic spectra
of the complexes showed bands almost the same to those
FIGURE 4 Mass fragmentation pattern of HNP‐OPD‐HAP
TABLE 3 Electronic spectral data, conductance values and elemental analytical data of Schiff base ligands and their Ti complexes
Elemental contenta
Electronic spectral
bands (nm)
Molar conductance,
Ligand/complex
Solvent
λm (Ω−1 cm2 mol−1
)
T (%)i
C (%)
H (%)
HNP‐OPD‐HPP
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
428, 318, 255
—
—
—
—
—
3
—
78.87 (79.08)
78.65 (78.84)
79.81 (79.70)
80.03 (80.12)
70.36 (70.50)
67.89 (68.06)
67.63 (67.51)
68.79 (68.88)
67.55 (67.59)
60.27 (60.23)
5.45 (5.57)
5.31 (5.25)
5.32 (5.41)
6.29 (6.38)
5.41 (5.34)
4.31 (4.36)
3.96 (4.05)
4.22 (4.25)
4.77 (4.89)
4.17 (4.10)
HNP‐OPD‐HAP
427, 318, 256
—
HNP‐OPD‐BA
428, 319, 254, 202
536, 450, 397, 318, 273
426, 318, 262, 254
419, 334, 259
—
HNP‐OPD‐AA
—
HNP‐OPD‐EAA
—
TiO(HNP‐ OPD‐HPP)
TiO(HNP‐ OPD‐HAP)
TiO(HNP‐ OPD‐BA)
TiO(HNP‐ OPD‐AA)
TiO(HNP‐ OPD‐EAA)
10.34 (10.44)
10.63 (10.77)
10.21 (10.17)
11.69 (11.72)
10.85 (10.92)
426sh, 420, 340, 275, 271
410, 342, 255, 207
460sh, 419, 334, 255
538, 418, 332, 265
18
16
2
2
aFigures in parentheses indicate calculated values.

8 of 15
UDDIN ET AL.
observed for the free ligands. There is no evidence of any
d–d transition for the complexes in the visible region. The
bands in the longer wavelength region may be attributed
to the ligand‐to‐metal charge transfer transition.[27] The
complexes are colored only through these intense charge
transfer absorptions tailing in from the UV. Being a d0
system, the present complex systems should not show
any d–d but rather charge transfer transitions. The elec-
tronic spectral behavior of the complexes suggests that
they are genuine d0–Ti(IV) species rather than any Ti(III)
species.
structures. Mainly data are used in obtaining information
about the nature of complexes in solution. Most useful
data are generally obtained in non‐aqueous solvents such
as nitrobenzene, acetonitrile, acetone, dimethylsulfoxide
and chloroform.[12,24] Generally for transition metal com-
plexes, which are insoluble in aqueous medium, conduc-
tance data are obtained in non‐aqueous solvents. The
conductivity of the Ti(IV) complexes was measured in
DMSO. The molar conductivity values of the prepared
Ti(IV) complexes are given in Table 3. All the complexes
show low molar conductance values, which suggested
that they are non‐electrolyte in nature.[31–33] Thus it is
evident that there are no inorganic anions present outside
the coordination sphere. From the conductance data, it is
clear that complexes synthesized from o‐hydroxy aro-
matic aldehyde, aromatic diamine and Ti(IV) are non‐
electrolyte in nature.
3.6 | Conductance measurements
Conductance measurements of coordination compounds
are performed to aid in the interpretation of possible
FIGURE 5 (a) Optimized structures, (b) frontier molecular orbitals and (c) MEP of HNP‐OPD‐HPP (L1) and TiO(HNP‐OPD‐HPP) (C1)

UDDIN ET AL.
9 of 15
this analysis, free energies of both ligand and complex
are negative, i.e. they are thermally stable, which indi-
cates that no energy is required for the interaction to pro-
ceed and the binding will occur spontaneously
(exothermic).[34] The calculated dipole moments of the
ligand and complex show a significant change. Elevated
level of dipole moment enhances the polar nature of a
molecule. The dipole moment of L1 is found to be
3.642 D whereas for the complex it is found to be
2.095 D. Increased dipole moment can promote hydrogen
bond and non‐bonded interactions in drug receptor com-
plexes which in turn can lead to increased binding affin-
ity.[35] Therefore, L1 with increased dipole moment could
result in increased binding affinity with receptor proteins
in biological systems.
3.7 | Magnetic properties
Negative values of magnetic susceptibility for all com-
plexes indicate their diamagnetic nature. It is expected
as 3d°4s° for the Ti(IV) complexes possessing the 4+
oxidation state. It is concluded that the prepared
oxotitanium(IV) complexes are independent of field
strength and temperature and the oxotitanium(IV) com-
plexes contain no unpaired electrons in their ground
states. This is consistent with diamagnetic behavior
expected for the electronic spectra of Ti(IV) complexes.
4 | COMPUTATIONAL CHEMISTRY
STUDY
4.1 | Thermodynamic properties
4.2 | Frontier molecular orbitals
All the ligands and complexes were designed depending
on previous characterization evidence such as UV–
visible, FT‐IR, 1H NMR and mass spectral analyses.
Geometries of the ligands and complexes with metals
were constructed using the structures experimentally sug-
gested in this study. Thermodynamic properties, internal
electronic energy, enthalpy, free energy and dipole
moment were calculated on the basis of computational
investigation for ligand L1 and its complex as representa-
tive molecules. Initially, the ligand was optimized by
means of equilibrium geometry calculations. The experi-
mental data obtained in this study have amply been
proven to give very good ground‐state geometries. No
constraints were imposed on the structure during the
geometry optimizations.[27] The optimized structures of
the ligand (L1) and its complex (C1) are presented in
Figure 5a. The stoichiometry, electronic energy, enthalpy,
free energy and dipole moment of ligand and complex are
reported in Table 4. Thermochemical values indicate a
unique dimension for the stability and reactivity of mole-
cules. In this investigation, the free energies of L1 and C1
are −1263.598 and −2272.926 Ha, respectively. Here,
higher negative value of C1 compared to L1 suggests that
C1 is energetically and configurationally more stable. In
Electronic absorption relates to the transition from the
ground to the first excited state and is mainly described
by one‐electron excitation from the HOMO to the
LUMO.[36] Chemical hardness and softness depends on
the HOMO–LUMO gap: small HOMO–LUMO gap is
indicative of high chemical reactivity and low kinetic sta-
bility; high kinetic stability and low chemical reactivity
are associated with large HOMO–LUMO gap.[37] Frontier
molecular orbitals, HOMO–LUMO gap and related
energy of L1 are shown in Figure 5b. Calculated
HOMO–LUMO gap as given in Table 4 has the values
3.73 and 3.48 eV for ligand L1 and its complex C1, respec-
tively. This indicates their softness, which may contribute
to higher activity.
4.3 | Molecular electrostatic potential
To predict the reactive sites for electrophilic and nucleo-
philic attack of investigated molecules, molecular electro-
static potential (MEP) calculation was performed at the
same level of theory. Red color represents the maximum
negative region as the preferred site for electrophilic
attack, blue color indicates the maximum positive region
TABLE 4 Thermodynamic and frontier molecular orbital data of ligand L1 and complex C1
Name
Molecular formula
Electronic energy (eV)
Enthalpy
Gibbs free energy (eV)
Dipole moment (D)
L1
C26H22N2O2
C26H20N2O3Ti
HOMO−1
−1263.517
−2272.806
−1263.516
−2272.802
LUMO+1
−1263.598
−2272.926
Gap (eV)
3.642
2.095
C1
Name
HOMO
LUMO
Hardness
1.86
1.74
Softness
L1
C1
−5.7114
−5.7462
−5.4052
−5.5949
−1.6751
−2.1170
−1.1730
−1.9426
3.73
3.48
0.53
0.57

10 of 15
UDDIN ET AL.
as the preferred site for nucleophilic attack and green
color represents the region of zero potential.[37] From
the MEP map, regions having negative potential are over
electronegative atoms (oxygen atoms) and those having
positive potential are over hydrogen atoms. Negative
potential is −0.2363 au (deepest red) for oxygen atoms
and positive potential localized on the hydrogen atoms
has a value of +0.1742 au (deepest blue) in L1. For com-
plex C1, the negative value is −0.7665 au and the positive
value is +0.0899 au. The MEP surface provides a visual
representation of the reactive sites, as shown in Figure 5c.
4.5 | Equilibrium geometries
The bond distances and angles help us to understand the
geometry and the change of structure due to the addition
of different functional groups. Values of bond distances
and angles are presented in Table 6, supporting the geom-
etry suggested by the experimental evidence.
4.6 | Atomic partial charge
Two different methods (Mulliken and natural bond
orbital) have been used for ligand (L1) and complex
(C1) to compute the partial charges of atoms including
both quantum chemical and empirical schemes.[39] In
L1 (Figure S5), all the hydrogen atoms show positive
charge and greater positive charges are found for H‐45
(0.51462e) and H‐46 (0.49032e) compared to the other
hydrogen atoms due to the presence of electronegative
oxygen atoms (O‐42 and O‐43). In the case of complex
C1 (Figure S5), all the hydrogen show positive charge
and the highest positive value is found for Ti atoms as
expected. Electronegative atoms (such as O, N) show con-
sistent sign of charge in both methods.
4.4 | Vibrational spectral analysis
The FT‐IR spectra for the present compounds were
recorded in the range 400–4000 cm−1, indicating regions
of absorption due to the respective vibrations and the har-
monic vibrational frequencies of ligand and its complex.
These were calculated at the B3LYP level using the 6‐
31G(d) basis set. Calculated frequencies are scaled by
multiplying by a scaling factor 0.9613.[38] Experimental
frequencies agree with calculated frequencies as shown in
Figure 6 (Figure S3). Vibrational frequencies of ligand
and its complex are given in Table 5.
FIGURE 6 Comparison of experimental FT‐IR spectrum with calculated spectrum of C1

UDDIN ET AL.
11 of 15
TABLE 5 Vibrational frequencies (cm−1) of ligand L1 and complex C1
Calculated FT‐IR
Experimental
Molecule
FT‐IR
Scaled
Unscaled
Assignment
L1
3350
1610
1320
1557
3047
3586
1609
1192
1570
3087
3730
1674
1240
1633
3211
υO─H
υC═N
υC─N
υC═Ca
υC─Ha
C1
1600
1361
1307
1188
452
1597
1357
1341
1026
532
1661
1412
1395
1067
553
υC═N
υC─N
υC─O
υTi═O
υTi─O
υTi─N
υC═Ca
565
1547
560
1528
583
1590
TABLE 6 Selected bond distances and bond angels of ligand L1 and complex C1
Bond distances
Bond angels
L1, HNP‐OPD‐HPP
C1, TiO(HNP‐OPD‐HPP)
L1, HNP‐OPD‐HPP
C1, TiO(HNP‐OPD‐HPP)
Atom
number
Bond distance
(Å)
Atom
number
Bond distance
(Å)
Atom
number
Bond angle
(°)
Atom
number
Bond angle
(°)
C1–C2
1.4201
1.4110
1.4023
1.2809
1.2964
1.4282
1.5412
1.4783
1.5175
1.4341
1.4431
1.4008
1.4183
1.3579
0.9709
1.0078
1.4676
1.3388
C22–H26
C22–C17
C17–C18
C17–N38
C18–N37
N37–Ti44
N38–Ti44
O41–Ti44
O39–Ti44
O40–Ti44
C30–O39
C6–O40
1.0851
1.4013
1.4130
1.4182
1.4093
2.1696
2.2189
1.6131
1.9093
1.9474
1.3094
1.2968
1.4319
1.4318
1.4142
1.0878
1.3933
C1–N39–C38
C2–N40–H37
C1–C2–N40
124.63
120.45
117.57
120.77
117.44
122.93
119.57
121.15
121.37
105.92
123.95
119.73
121.68
119.90
119.23
109.26
C1–C2–C3
121.94
119.47
124.35
123.51
107.71
110.03
106.09
103.04
99.69
C1–N39
C1–C6–C5
C2–N40
C22–C17–N38
C17–N38–C42
C17–N38–Ti44
C13–N37–Ti44
N37–Ti44–O41
N38–Ti44–O41
O39–Ti44–O40
O40–Ti44–O41
N37–C43–C5
C32–C31–C30
C29–C30–O39
C4–C5–C46
C37–N40
C38–N39
C13–C14
C41–C47
C14–C38
C38–C41
C23–C24
C24–C25
C25–C26
C21–C26
C26–O42
O42–H46
O43–H45
C25–C37
C13–O43
C2–C1–N39
N39–C38–C14
N39–C38–C41
C14–C38–C41
C13–C14–C38
C15–C14–C38
C13–O43–H45
N40–C37–C25
C37–C25–C24
C37–C25–C26
C25–C26–O42
C21–C26–O42
C26–O42–H46
110.07
126.84
117.63
118.48
120.48
128.25
128.80
C31–C30
C6–C1
C3–C10
C42–N38–Ti44
C43–N37–Ti44
C2–H9
C19–C20
phytopathogenic fungi A. niger and A. flavus. Antioxidant
assessment was performed through the total phenolic
content method.[22,23] The ligands were designated by
L1, L2, L3, L4 and L5 and the complexes by C1, C2, C3,
C4 and C5.
5 | BIOCHEMICAL APPLICATIONS
In the present investigation, all the ligands and com-
plexes were evaluated against human pathogenic bacteria
E. coli, B. subtilis, S. paratyphi and S. aureus and two

12 of 15
UDDIN ET AL.
Overall, it is seen that the growth of bacteria for a few
samples (L1, C1, L2, L3 and L5) was slightly lower than
the growth of bacteria for the control. This means that
these ligands and complexes are showing inhibiting activ-
ity rather than inducing activity against the test organisms,
i.e. they have antibacterial properties. For remaining sam-
ples (C2, C3, L4, C4 and C5), growth of bacteria was higher
than the growth of bacteria for the control. This means that
these ligands and complexes are showing inducing activity
rather than inhibiting activity against the test organisms.
5.1 | Antibacterial activity
Antibacterial activity was assessed by the liquid culture
method.[22] Bacterial growth from 3 to 21 h incubation
in the presence of ligands and complexes was investi-
gated. The optical densities for E. coli, S. aureus, B. subtilis
and S. paratyphi were recorded for all ligands and com-
plexes at two concentrations of 0.10 and 0.80%. Figure 7
depicts the bacterial growth for representative ligand L1
at 0.80%. At higher concentration (0.80%) the growth of
organisms is inhibited during the incubation of 3 to
21 h. But, at lower concentration (0.10%) nearly the same
activity as the control was found against the tested organ-
isms. Figure 8 depicts the bacterial growth of representa-
tive complex C1 at higher concentration (0.80%). It is seen
that in the case of E. coli and S. aureus, the growth is
inhibited at 18 and 21 h. But at lower concentration
(0.10%), the growth of all tested organisms was induced
during the incubation of 3 to 21 h.
5.2 | Antifungal activity
5.2.1 | Liquid media method
Antifungal activities of the prepared ligands and com-
plexes were assessed by the liquid media method by mea-
suring pH of the solutions and dry weight measured by
FIGURE 7 Antibacterial activity of ligand L1 at a concentration of 0.80%
FIGURE 8 Antibacterial activity of complex C1 at a concentration of 0.80%

UDDIN ET AL.
13 of 15
weighing fungal biomass against A. niger and A. flavus.
From the experimental data it is seen that all the ligands
and complexes inhibit the growth of A. flavus showing
antifungal activity. Figure S6 shows pH data and Figure
S7 shows the biomass of antifungal experiments of pre-
pared ligands and complexes (0.1%) against A. flavus.
Again from the further experiment it is seen that L1,
L4, C1, C2, C3, C4 and C5 reduce the biomass weight
compared to the control in the case of A. niger indicating
their antifungal activity.
From Table 7, it is clear that almost all the synthesized
ligands and complexes have antioxidant activity, i.e. the
Schiff base ligands and their metal complexes can act as
antioxidants.
6 | DISCUSSION OF SYNTHESIS,
GEOMETRY AND ANTIMICROBIAL
STUDY
All the prepared ligands and their titanium complexes
have been synthesized and characterized. The ligands
have the potential to act as dibasic tetradentate ligands
in forming TiOL complexes. All ligands and complexes
were soluble in DMF and DMSO. Most of the complexes
have a melting point higher than 200°C. FT‐IR spectral
bands observed between 1640 and 1675 cm−1 for the free
ligands were due to νC═O and those at 1600–1632 cm−1
were due to νC═N of the azomethine group.
Downshifting of these values and bands observed in the
range 420–470 cm−1 due to νTi─O indicate complex for-
mation. Three main peaks were found in the electronic
spectra for the ligands at around 270, 320 and 370 nm.
There was no evidence of any d–d transition for the TiOL
complexes in the visible region. Absence of δ(─OH) peak
5.2.2 | Disc diffusion method
The antibacterial activities of the complexes were studied
and detected using the disc diffusion method. The inhibi-
tion zones of the test organisms for ligands (L1–L5) and
complexes (C1–C5) are presented in Figure S8. The per-
centage inhibition of mycelia growth of fungi due to the
effect of studied ligands and complexes was compared
with that for a standard antibiotic, nystatine. The results
are presented graphically in Figure 9. The same trends
of inhibition for respective ligands and complexes were
found as those obtained in liquid cultured media. It is evi-
dent that ligands of higher hydrophobicity (long organic
chain or phenyl ring) in the asymmetric part showed
higher inhibition.
1
characteristic of ligands in the H NMR spectra f com-
plexes also indicates complex formation. Mass spectral
data of the ligands and complexes showed relatively cor-
rect fragmentation patterns.The elemental analytical data
(Table 3) indicate that the complexes have 1:1 (metal‐to‐
ligand) stoichiometry. The Schiff base ligands possess a
planar configuration with respect to the positions of
donor atoms. Therefore, square pyramidal geometry of
the complexes is proposed. From quantum mechanical
5.3 | Antioxidant properties
The phenolic contents were calculated by the standard
curve constructed for gallic acid: y = 0.0019x + 0.0039
(R2 = 0.9981). Calibration data for standard solution of
total phenolic content of gallic acid are presented in
Table S1 and Fig. S9 shows the respective calibration
curves. The total phenolic content of experimental sam-
ples is summarized in Table S2. The total phenolic con-
tent of experimental samples was calculated in GAE.
TABLE 7 Total phenolic content summary of ligands and
complexes
Total phenolic
Identification
code
content (mg GAE
(g dry weight)−1
)
Sample
L1
L2
L3
L4
L5
C1
C2
C3
C4
C5
HNP‐OPD‐HPP
486.73 3.193
74.26 0.555
HNP‐OPD‐HAP
HNP‐OPD‐BA
20.20 0.830
64.49 0.187
455.30 13.089
292.55 5.539
41.67 0.894
23.50 0.827
33.97 0.700
305.84 1.920
HNP‐OPD‐AA
HNP‐OPD‐EAA
TiO(HNP‐OPD‐HAP)
TiO(HNP‐OPD‐HAP)
TiO(HNP‐OPD‐BA)
TiO(HNP‐OPD‐AA)
TiO(HNP‐OPD‐EAA)
FIGURE 9 Percentage inhibition of mycelia growth of fungi for
studied compounds using the cup plate technique

14 of 15
UDDIN ET AL.
calculations, geometry was optimized for a representative
ligand (L1) and its complex (C1). They are found to be
thermally stable at room temperature and safe for topical
uses.Accuracy of the results of all microbial measure-
ments was evaluated statistically comparing with 0.05
confidence level. For t‐tests each of the microbes was con-
sidered as type one population having two different con-
ditions such as control condition and test condition.
Probability was found in the range 0.07–0.48 for L1
(Figure 7) and 0.001–0.429 for C1 (Figure 8) in their anti-
fungal assessment against tested organisms. Figure S6
shows the antifungal activity of the prepared ligands
and complexes tested against A. flavus by the liquid
media method by measuring three different pH values
(initial, test and final) of the solutions. Probability check
was performed by t‐tests of these pH values. Probability
was found in the range 0.088–0.153 for all ligands and
complexes. Probability was found in range 0.005–0.132
in dry weight measured technique for three different
weights of fungal biomass against A. flavus (Figure S7).
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How to cite this article: Uddin MN, Siddique
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Oxotitanium(IV) complexes of some bis‐
unsymmetric Schiff bases: Synthesis, structural
elucidation and biomedical applications. Appl
Organometal Chem. 2019;e4876. https://doi.org/
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