Synthesis and characterization of 1,2,3-triazole containing Fe(II) sensor

Article abstract of DOI:10.14233/ajchem.2020.22494

A new bis(1,2,3-triazolyl) imine based probe was designed and synthesized. Chemical structure of the probe was confirmed by IR, ¹H and ¹³C NMR spectroscopy. The probe was investigated for its recognition abilities in aqueous-organic

Full text of DOI:10.14233/ajchem.2020.22494

Asian Journal of Chemistry; Vol. 32, No. 5 (2020), 1033-1038
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2020.22494
Synthesis and Characterization of 1,2,3-Triazole Containing Fe(II) Sensor
*
G. SENTHILKUMARAN and S. SENTHIL
Department of Chemistry, Government Arts College (Autonomous), Salem-636007, India
*Corresponding author: E-mail: shendils@gmail.com
Received: 14 October 2019;
Accepted: 3 December 2019;
Published online: 29 April 2020;
AJC-19830
A new bis(1,2,3-triazolyl) imine based probe was designed and synthesized. Chemical structure of the probe was confirmed by IR, 1H and
13C NMR spectroscopy. The probe was investigated for its recognition abilities in aqueous-organic mixture against various cations and
anions. It shows a highly selective colorimetric response to Fe(II) ion by changing the colour from colourless to brownish pink. Chemo-
1
sensitivity of the probe was investigated by absorption spectrometric titration with the Fe(II) ions. H NMR titration studies indicated
imine nitrogen and one of the nitrogen in triazole ring was involved in complex formation with Fe2+ ion. Energy optimization studies by
DFT method exhibits a marginal energy gap between ligand and Fe(II) complex (0.1166 eV) confirms the formation of metal ligand
complex.
Keywords: Colorimetric chemo sensor, Chromogenic Fe2+ sensor, 1,2,3-Triazole, Absorption spectroscopy, Energy minimization.
that also contains pyridine based binding sites. A few research
papers published with other binding sites like fluoran dye [22],
imidazole [23], pyrimidine [24], fluorogenic and functionalized
gold nanoparticle sensing probes have also been reported for
Fe2+ ions [25-28]. 1,2,3-Triazole is an important chelating system
with strong chemical stability and excellent biocompatibility
as it mimics the peptide linkage of the protein [29]. It has been
very much used as a linker between chemical and biological
entities [30]. 1,2,3-Triazole moiety is established as effective
chemosensor for various ions like Cu2+ [31], Al3+ [32], Zn2+
[33], Hg2+ [34], Fe3+ [35], Ca2+ [36], Pb2+ [37]. In this paper,
we report a colorimetric sensor based on 1,2,3-triazole as an
integral part of the sensing molecule. This sensor is highly
selective towards Fe2+ and produces a distinct pink colour when
in contact with the analyte.
INTRODUCTION
Iron is the most important, essential and abundantly available
metal ion in vertebrates and plants, involved many biological
processes like the construction of haemoglobin [1], electron
transport processes [2,3], oxygen uptake and transport in the
human cellular metabolic system [4-6]. It is also toxic when
in excess in many ways for example Parkinson’s [7], alzheimer
diseases [8], anaemia [9], kidney disorder [10,11] and liver
damage [12], etc. Though Fe2+ plays an important role in natural
processes and a dietary requirement of most organism, it create
hurdles to achieve good bleaching level and uniform dying in
textile processing [13]. It is also considered a potent conta-
minant in ground water [14]. Comparatively, reports for Fe2+
selective sensor are very limited; hence it is necessary to
develop an efficient sensor system to detect Fe2+ ions. Among
the various available methods to detect Fe2+, chromogenic
analysis gain attraction due to its simplicity and portability.
Majority of the chromogenic sensor systems so far reported
for Fe2+ are based on complex formation using electron donor
in moieties like pyridine [11], bipyridine [12,13] and terpy-
ridine [14-18] based ligands. But in those heterocyclic donors,
most of them are sensible to more than one ion [19]. Apart
from that, few Schiff base sensors were also reported [19-21]
EXPERIMENTAL
All the chemicals were purchased from Loba Chemicals
or SRL, India and used as received. FTIR spectra were recorded
on Agilent FT-IR Spectrometer with KBr pellets. 1H and 13C
NMR spectra were recorded on a Bruker 400 NMR Spectro-
meter Using TMS as internal standard and CDCl3 as solvent.
UV/Vis measurements were recorded on Elico double beam
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1034 Senthilkumaran et al.
Asian J. Chem.
spectrophotometer at room temperature. Column chromato-
graphy was performed on silica gel (60-200 mesh) from Merck
Company.All the reactions were followed by TLC using Merck
0.2 mm silica gel 60 GF254 analytical aluminum plates. Cation
solutions were freshly prepared as 1 × 10-3 M stocks from the
corresponding perchlorate salts and anion solutions were pre-
pared at the same concentration from its quaternary ammonium
or sodium (N3–, acetate) salts. Appropriate amount of ligand
was dissolved in 8:2 water:acetonitrile mixture and sonicated
to ensure complete dissolution. This solution was diluted in
0.1 M HEPES buffer at pH 7.2 at room temperature.
Synthesis of benzazide (A):Aniline (0.01 mol) is dissolved
in 30 mL of conc. HCl and mixture was cooled (0-5 C). A
solution of NaNO2 (0.012 mol) in water (5 mL) was added in
a drop-wise manner to the reaction mixture in such a way that
the temperature of the mixture is controlled between 5-7 °C.
After being stirred well for 10 min, a cold solution of NaN3
(0.02 mol) in water (20 mL) was added to the reaction mixture
and stirred for 0.5 h. An oily layered was formed was then
extracted with ether several times and the combined extracts
are washed with NaHCO3, brine solution and dried over
Na2SO4. The solvent was removed under vacuum, a yellow oil
(A) obtained was pure enough for further steps.
(triazole CH stretching) and 3032 (aromatic CH– stretching)
and 1730 (C=O stretching).
Synthesis of (1E,1'E)-N,N'-(ethane-1,2-diyl)bis(1-(1-
phenyl-1H-1,2,3-triazol-4-yl)methanimine) (D): Compound
C (0.002 mol) was dissolved in ethanol (50 mL) and ethylene
diamine (0.001 mol) and 3 drops of glacial acetic acid were
added with stirring. The mixture was refluxed in an oil bath
for another 6 h and concentrated to half of its volume. The
mixture was kept at the refrigerator for overnight. A white
solid obtained was filtered and recrystallized in hot ethanol
yielded the target compound D.
1H NMR (CDCl3, δ ppm): 7.48 to 7.82 (d, 3H, aromatic H)
and 8.58 (s, 1H, triazole) and 8.87 (s, 1H, imine) and 3.75 (CH2).
13C NMR (CDCl3, δ ppm): 120.75, 121.06, 126.60 (aromatic
carbon) and 136.6 (aromatic ipso carbon) and 144.4 (triazole
ipso carbon) and 29.71 (methylene carbon) and 147.52 (imine).
FT-IR (cm–1): 3060 (triazole CH stretching) and 3123 (aromatic
CH–stretching) and 1640 (C=N stretching).
Binding titration: The stock solutions of ligand (D) were
prepared by dissolving compound D in CH3CN/water (2:8,
v/v) containing HEPES buffer (pH = 7.2). The cationic stocks
were prepared in distilled water and taken in fives times to the
concentration of ligand for UV-visible absorption spectral
analysis. For titration experiments, each time 3 mL solution
of compound D was filled in a quartz cell and added the metal
ion solutions by stepwise addition of different equivalents using
a micro-syringe.After each addition of analyte ion, the solution
was well stirred to ensure complete reaction.
1H NMR (CDCl3, δ ppm): 6.9 to 7.24 (d, 3H, aromatic
H). 13C NMR (CDCl3, δ ppm): 134, 130, 118 (aromatic carbon),
137 (aromatic ipso carbon). FT-IR (cm-1): 2105 (–N3), 3030
(aromatic CH– stretching).
Synthesis of (1-phenyl-1H-1,2,3-triazol-4-yl)methanol
(B): The synthesized compound A (0.001 mol) was dissolved
in dry THF and added with propargyl alcohol (0.001 mol)
along with CuI (0.001 mol) and triethylamine (0.5 mL). The
reaction mixture was stirred at room temperature for 12 h and
followed by TLC and the solution was filtered after comple-
tion of the reaction. THF was removed under vacuum yielded
pale brown coloured crude product. It was purified by column
chromatography over silica gel using a chloroform-methanol
mixture (9:1).
RESULTS AND DISCUSSION
The target imine ligand (D) was synthesized from starting
compound aniline through four steps. The scheme of the synthe-
sis of the target ligand is presented in Fig. 1.All the compounds
were sufficiently purified before proceeding to the next step
except in the first step because the product benzazide (A)
obtained was pure enough and starts decomposing on storage.
Aniline on diazotization followed by nucleophilic substitution
with azide produces azido benzene, which on further reaction
with propargyl alcohol under click reaction conditions [38]
yielded methanolic derivative of 1,2,3-triazole. It was further
oxidized to get 1,2,3-triazole containing aldehyde functionality.
The target ligand was prepared by the reaction of triazole
containing aldehyde with aliphatic diamine. Spectroscopic
investigation of all the precursors and final compound confirms
the chemical structure of the synthesized compounds. The
disappearance of azide stretching frequency at 2105 cm-1 and
appearance a new weak stretching absorption at 3109 cm-1 in
IR spectrum of compound B indicated the conversion of azide
group to 1,2,3-triazole ring and further it was confirmed by
observing a resonating singlet for one proton around 8.5 δ
ppm in compounds B, C and D. Formation imine linkage was
confirmed by observing a singlet at 8.9 ppm.
1H NMR (CDCl3, δ ppm): 7.47 to 7.88 (d, 3H, aromatic
H) and 8.66 (s, 1H, triazole) and 4.66 (s, 2H, CH2) and 5.3 (s,
1H, -OH). 13C NMR (CDCl3, δ ppm): 120, 128, 129 (aromatic
carbon) and 136 (aromatic ipso carbon) and 149 (triazole ipso
carbon) and 54 (methylene carbon). FT-IR (cm–1): 3109
(triazole CH stretching) and 3030 (aromatic CH– stretching)
and 2920 (CH2-stretching) and 3350 (OH stretching).
Synthesis of 1-phenyl-1H-1,2,3-triazole-4-carbaldehyde
(C): Compound B (0.0017 mol) was dissolved in dry dichloro-
methane (30 mL) and pyridinium chloro chromate (0.002 mol)
was added and stirred at room temperature. The progress of
the reaction was checked by TLC. The reaction mixture was
filtered after the reaction was completed. The solvent in the
filtrate was removed under vacuum. The residue was purified
by column chromatography over silica gel using chloroform
as eluent. The product was collected as a pale white solid.
1H NMR (CDCl3, δ ppm): 7.52 to 7.65 (d, 3H, aromatic
H) and 7.98 (s, 1H, triazole) and 9.55 (s, 1H, CHO). 13C NMR
(CDCl3, δ ppm): 120.67, 126.09, 129.45 (aromatic carbon)
and 135.94 (aromatic ipso carbon) and 147.52 (triazole ipso
carbon) and 184.92 (aldehydic carbon). FT-IR (cm–1): 3110
In order to investigate the recognition abilities of the ligand
D, it has been carried out a series of experiments in CH3CN/
H2O (2:8/v:v) aqueous/organic HEPES buffered solution at
pH 7.0. The experiments were carried out by dissolving the
ligand in the aqueous/organic mixture at 1.23 × 10-3 M concen-
tration buffered at pH 7.0 with HEPS and sonicated for 2 min

Vol. 32, No. 5 (2020)
Synthesis and Characterization of 1,2,3-Triazole Containing Fe(II) Sensor 1035
NH2
N3
OH
1.NaNO2/HCl
2.NaN3
N
N
N
OH
CuI/TEA/THF
B
A
PCC/CH2Cl2
N
N
NH2
N
H2N
N
N
N
N
N
N
D
N
N
O
C
Fig. 1. Scheme of the synthesis of ligand D
Fig. 2. Colour change while adding various metals ions to ligand solution
for ensuring complete dissolution. Then it was added with
various aqueous perchlorate cation solutions (Al3+, Ca2+, Mg2+,
Mn2+, Cd2+, Fe3+, Co2+, Ni2+, Cu2+, Hg2+, Zn2+, Pb2+) and
quaternary ammonium salts of halogens, SCN- and the sodium
salt of N3 and OAc–, approximately five times the concentration
of ligand. The mixtures were gently shacked and leave it for
another 1 min. The ligand was responded only with Fe2+ ion
by changing its colour from colourless to pink (Fig. 2). All
other cation and anion solutions did not bring any changes to
naked eye observation and ligand was sensitive to only Fe2+.
Further, the recognition was studied using electronic
spectra. UV-visible spectra of ligand and its recognition are
presented in Fig. 3. UV-visible spectrum of compound D
exhibits λmax at 300 nm due to its π-π* transition of aromatic
ring π electrons. Upon adding different cations it produces
the change in absorption pattern according to its binding nature.
No significant change in spectra was observed except in the
case of Cu2+ and Fe2+. Cu2+ produces a slight red shift λmax
from 300 nm to 320 nm. But it does not produce any significant
colour change while in the case of Fe2+ it exhibited a remarkable
colour change from colourless to dark pink and in electronic
absorption spectra; a new λmax appears in low intensity at 520
nm. The red shift, while the ligand interacts with Fe(II) ion,
was significant around 220 nm due to d-d transition in L-Fe
complex. It has been noted that there is no change in the higher
energy region of the spectra suggested that the π electrons of
the ligand have not participated in the complex formation and
only non-bonding electrons on nitrogen atom might be involved.
The binding property of ligand D with Fe2+ was further
studied by UV-visible titration experiments (Fig. 4). Upon
incremental addition of Fe2+, the absorption peak at 512 nm
gradually increases which indicated the formation of a D-Fe2+
complex. The addition of EDTA to a mixture of ligand D and
Fe2+ bring back to colourless solution instantly and diminution
of the absorption intensity at 512 nm decreases to initial level
Ligand
2.5
2.0
1.5
1.0
0.5
0
Ligand + Ag
Ligand + Al
Ligand + Ca
Ligand + Cd
Ligand + Co
Ligand + Cu
Ligand + Hg
Ligand + K
Ligand + Mg
Ligand + Na
Ligand + Ni
Ligand + Pb
Ligand + Zn
Ligand + Fe(II)
Ligand + F
Ligand + HSO4
Ligand + OAc
Ligand + N3
Ligand + I
Fe(II)
Ligand + SCN
Ligand + Cl
180
280
380
480
580
Wavelength (nm)
Fig. 3. Electronic spectral studies of ligand D and various metal ions
was observed, which indicated the regeneration of the ligand
D. The absorption band was again reappearing by the addition
of Fe2+ again. Such reversibility and regeneration are important
for the fabrication of devices to sense the ions.
Fig. 5 shows the change in absorbance intensities at 512
nm while adding Fe2+ ion with ligand D. With the incremental
addition of Fe2+, the absorbance intensity of ligand D at around
512 nm was gradually increased and it was evidenced by observing

1036 Senthilkumaran et al.
Asian J. Chem.
ligand D, which on addition of Fe2+ ion shifted to downfield at
8.7 ppm. Meanwhile, the imine proton initially appeared at
8.9 ppm for pure ligand was shifted to slight up field. The
shift of methylene and imine proton attached to electronegative
nitrogen upon addition of metal ion might be due to the loss
of electron density of non-bonding lone pair of nitrogen for
the complex formation with metal ion. Meanwhile the triazole
ring proton was shifted to downfield by deshielding effect.
This indicated the ligand-Fe2+ complex could be formed through
the participation of imine nitrogen and triazole nitrogen of the
Ligand. Further Job’s plot showed a 1:1 stoichiometric ratio
of D-Fe2+ complex (Fig. 4) indicated the formation of the mono-
nuclear complex.
Density functional theory (DFT) calculations have been
used to understand the behaviour of ligand D with the Fe2+
ions. Highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) for ligand D and its
metal complex D-Fe2+ have been generated from optimized
structures (Fig. 7). The structural optimization and the com-
putational calculations were carried out using Gaussian09
quantum chemistry package and results were viewed with
GaussView 5. In this study, density functional theory supported
B3LYP/6-31G(d,p) model chemistry was implemented for
Schiff base ligands and LANL2DZ basis set to optimize the
geometry of the metal complex. The metal atom present in the
complex was treated under LANL2DZ whereas other atoms
are manipulated with B3LYP/6-31G(d,p) level of theory. HOMO
and LUMO pictures of ligand indicated that, the electron
density of the ligand was evenly distributed all over but in the
case of Fe2+ complex the electron density was accumulated on
the metal centre shows the metal draws up the electron density
that causes deshielding effect of protons attached to the nitrogen
atom. In addition, donation of electron pair of one of the nitrogen
in triazole causes upfield shift in NMR spectrum, which was
evidenced in Fig. 6. Band gap energy of the ligand D was
calculated as -0.2788 eV and for the metal complex as 0.16211
eV. This clearly indicated that theoretically, the formation of
Ligand-Fe complex formation is feasible.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
2.5
0 Eq
0.1 Eq
0.2 Eq
0.3 Eq
0.4 Eq
0.5 Eq
0.6 Eq
0.7 Eq
0.8 Eq
0.9 Eq
1.0 Eq
1.1 Eq
3 Eq
2.0
1.5
1.0
0
5
10
Fe(II)/Ligand
0.5
0
Fe(II)
5 Eq
10 Eq
20 Eq
50 Eq
180
280
380
480
580
Wavelength (nm)
Fig. 4. Binding titration study of the ligand with Fe2+ ion
the colour of the solution gradually becomes colourless to dark
pink. The increase in absorbance intensity attained saturation
after the addition of slightly more than one equivalent of Fe2+
ions. Upon further addition, the intensity did not increase,
which clearly indicated that the formation of 1:1 complex.
The infrared spectra of the ligand and its complex with
Fe(II) ion are shown in Fig. 6. Absorbance at 1595, 1498 and
1636 cm-1 confirmed the presence of 1,2,3-triazole and imine
C=N groups respectively. A new strong absorption in the case
of metal complex at 1145, 1085 cm-1 shows metal coordination
through nitrogen of triazole ring occurs by the formation of
C-N-Fe(II) bond.
To get further information for the binding mode of ligand
D with Fe2+, 1H NMR titration study was carried out (Fig. 6).
Upon the addition of 0.4, 0.8 and 1.1 equivalent of Fe2+ to
ligand D, a peak resonating for methylene at 3.85 ppm was
shifted to up field (3.25 ppm) and the resonating peak of a
proton ‘Hc’ in the 1,2,3-triazole ring appeared at 8.6 ppm in
Conclusion
1,2,3-Triazole based new Schiff base ligand was success-
fully synthesized through four steps. In all stages of synthesis,
intermediates and final compounds were thoroughly investi-
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
4000
3800
3600
3400
3200
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
Wavenumber (cm–1)
Fig. 5. IR spectra of ligand and its Fe2+ complex

Vol. 32, No. 5 (2020)
Synthesis and Characterization of 1,2,3-Triazole Containing Fe(II) Sensor 1037
Ha
Hb
Hb
Hc
N
N
Hc
N
N
N
4
N
N
N
Ha
L + 1.1 eq Fe2+
3
L + 0.8 eq Fe2+
2
L + 0.4 eq Fe2+
1
L + 0 eq Fe2+
10.0 9.8 9.6 9.4 9.2 9.0 8.8 8.6 8.4 8.2 8.0 7.8 7.6 7.4 7.2 7.0
ppm
4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2
Fig. 6. 1H NMR spectral titration with Fe2+ ion
N
N
N
N
Fe2+
Fe
N
N
N
N
N
N
N
N
N
N
N
N
LUMO
∆
E = -0.2788 eV
∆E = -0.16211 eV
HOMO
Fig. 7. Molecular modelling of ligand D and ligand-Fe2+ complex

1038 Senthilkumaran et al.
Asian J. Chem.
18. H. Kim,Y. Seo,Y.Youn, H. Lee, M.Yang and C. Kim, ChemistrySelect,
4, 1199 (2019);
gated using various spectral techniques and confirmed the
chemical structure of the synthesized compounds. The synthe-
sized ligand was investigated for its ion recognition abilities
and found that it was very selective towards Fe2+ ion. Fe2+ exhibit
distinct colour change detected by naked eye. Recognition
ability was further confirmed by titration experiments and
found to be 1:1 complex formation. 1H NMR titration studies
indicated imine nitrogen and nitrogen in triazole ring was
involved in complex formation with Fe2+ ion. Molecular modeling
studies also support the formation of ligand-Fe2+ metal complex
formation.
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