New acyclic 1,2,4-triazole-based Schiff base hydrazone: Synthesis, characterization, spectrophotometric and computational studies

Article abstract of DOI:10.1016/j.saa.2012.12.033

A new 1,2,4-triazole-based Schiff base hydrazone with N, O, S donor set of atoms, H₄L, has been prepared by condensation reaction of N,N′-bis(3-formyl-5-methylsalicylidene)ethane-1,2-diamine, H₂L, with 4-amino-3-(4-pyridyl)-5-mercapto-1,2,4-triazole. The structure of H ₄L was characterized by using FT-IR, UV-Vis and ¹H NMR spectroscopic methods as well as elemental analysis data. The formation constants of copper(II), cadmium(II), mercury(II) and silver(I) complexes of H₄L in DMSO were calculated using a hard model chemometrics method applying the spectrophotometric data. The protonation constants of H ₄L were also measured in DMSO-water (1:10) mixture. Furthermore, ¹H chemical shifts of H₄L were studied by the gauge independent atomic orbital (GIAO) and continuous set of gauge transformations (CSGTs) methods at the level of density functional theory using B3LYP/6-311++G^- basis sets in gas phase. The computed chemical shifts are in reasonably good agreement with the experimental data.

Full text of DOI:10.1016/j.saa.2012.12.033

Accepted Manuscript
New acyclic 1,2,4-triazole-based Schiff base hydrazone; Synthesis, characteri‐
zation, spectrophotometric and computational studies
Hamid Khanmohammadi, Malihe Erfantalab, Golamhassan Azimi
PII:
S1386-1425(12)01258-9
http://dx.doi.org/10.1016/j.saa.2012.12.033
SAA 10085
DOI:
Reference:
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Spectrochimica Acta Part A: Molecular and Biomo‐
lecular Spectroscopy
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Accepted Date:
25 October 2012
10 December 2012
10 December 2012
Please cite this article as: H. Khanmohammadi, M. Erfantalab, G. Azimi, New acyclic 1,2,4-triazole-based Schiff
base hydrazone; Synthesis, characterization, spectrophotometric and computational studies, Spectrochimica Acta
Part A: Molecular and Biomolecular Spectroscopy (2012), doi: http://dx.doi.org/10.1016/j.saa.2012.12.033
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New acyclic 1,2,4-triazole-based Schiff base hydrazone;
Synthesis, characterization, spectrophotometric and
computational studies
Hamid Khanmohammadi 1, Malihe Erfantalab1, Golamhassan Azimi2
1Faculty of Science, Department of Chemistry, Arak University, Arak, 38156-8-8349, Iran
2Central Research Laboratory, Isfahan University, Isfahan, 8174673441, Iran
Abstract
A new 1,2,4-triazole-based Schiff base hydrazone with N, O, S donor set of atoms, H4L,
has
been
prepared
by
condensation
reaction
of
N,N'-bis(3-formyl-5-
methylsalicylidene)ethane-1,2-diamine, H2L, with 4-amino-3-(4-pyridyl)-5-mercapto-1,2,4-
1
triazole. The structure of H4L was characterized by using FT-IR, UV-Vis and H NMR
spectroscopic methods as well as elemental analysis data. The formation constants of
copper(II), cadmium(II), mercury(II) and silver(I) complexes of H4L in DMSO were
calculated using a hard model chemometrics method applying the spectrophotometric data.
The protonation constants of H4L were also measured in DMSO-water (1:10) mixture.
1
Furthermore, H chemical shifts of H4L were studied by the gauge independent atomic
orbital (GIAO) and continuous set of gauge transformations (CSGT) methods at the level of
density functional theory using B3LYP/6-311++G* basis sets in gas phase. The computed
chemical shifts are in reasonably good agreement with the experimental data.
Keywords: Schiff base, Chemometrics method, Hydrazone, 1,2,4-Triazole, stability
constant, GIAO.

Corresponding author. h-khanmohammadi@araku.ac.ir Tel: +98-861-2777401; Fax: +98-861-
4173406.
1

1. Introduction
Recently, there is a renewed interest in synthesis and spectrophotometric properties of
mixed multi-donor Schiff base hydrazones including heterocyclic moieties such as triazole,
tetrazole, oxadiazole, pyridazine and so on [1-6]. Among them, Schiff base hydrazones
bearing triazole moiety have attracted considerable attention due to their impressive,
chemical and physical properties [7-10] biological activities [11-14], photochromic
properties [15,16] and also their analytical applications [17]. On the other hand, 1,2,4-
triazole-based hydrazones have long been involved in investigations of metal ion
recognition because of number and nature of their donor atoms, and also their ability to
accommodate the preferred coordination geometry of the metal ions [18,19]. Furthermore,
several metal complexes of 1,2,4-triazole-based ligands were found to display enhanced
physico-chemical properties and also biological activity over the uncomplexed free ligands
[17,18]. In view of this, several articles devoted to the study of stability constants of metal
complexes of triazole-based ligands in the last decade [20-23]. However, a survey of
literature reveals that the study available on formation constants of metal complexes of
1,2,4-triazole-based Schiff base hydrazones is rare [24].
Various methods such as potentiometric titration [25], conductometry [26] and
spectrophotometric determination [27] have been used for determination of stability
constants of metal complexes. Among of these, spectrophotometric methods are more
sensitive than others and are suitable for determination of stability constants of metal
complexes in solution under different experimental conditions [27]. However, spectrum’s
overlapping of various chemical species involved in the equilibria is an important problem.
At this condition, determination of stability constants by classical methods is very hard and
can cause great uncertainties on the obtained results. This problem can be easily overcome
2

using chemometrics methods [28,29], where one can analyze whole spectra, thereby
utilizing all spectral information.
The present paper is devoted to the design and synthesis of new 1,2,4-triazole-based Schiff
base hydrazone, H4L, (Scheme 1) and also spectrophotometric determination of stability
constants of its Cu(II), Cd(II), Hg(II) and Ag(I) complexes. The H4L has been prepared by
condensation reaction of N,N'-bis(3-formyl-5-methylsalicylidene)ethane-1,2-diamine, H2L,
with 4-amino-3-(4-pyridyl)-5-mercapto-1,2,4-triazole [30]. The prepared compound has
been characterized by spectroscopic methods (1H NMR, UV–Vis and IR) as well as
elemental analysis data. Also, we have tried to develop (i) the electronic properties of 1,2,4-
triazole-based ligands, and (ii) the calculation of stability constants of metal complexes for
different mole ratio of ligand: metal. Furthermore, in order to compare experimental and
theoretical NMR data, which may be helpful for correct assignments and understanding the
relationship between chemical shielding and molecular structure, complete analyses of the
1
1H NMR spectra of H4L is reported. For this reason, the H NMR isotropic chemical
shieldings of H4L were systematically studied by the GIAO/DFT (Gauge Independent
Atomic Orbitals/ Density Functional Theory) [31,32] and CSGT/DFT (Continuous Set of
Gauge Transformations/ Density Functional Theory) [33,34] methods with 6-311++G*
basis set in gas phase.
2. Experimental
2.1. Materials
All chemicals and solvents were of reagent grade and purchased commercially. All
solutions were prepared fresh daily. 2,6-diformyl-4-methylphenol [35], N,N'-bis(3-formyl-
5-methylsalicylidene)ethane-1,2-diamine (H2L) [36] and 4-amino-5-(4-pyridyl)-3-
mercapto-1,2,4-triazole [30] were prepared as described previously.
2.2. Instrumentation
3

1H NMR spectra were obtained with a Bruker Avance 300 MHz spectrometer. Absorption
spectra were obtained with a Perkin-Elmer Lambda spectrophotometer in the range 300-600
nm, using 1cm path quartz cells. The measurements were performed at 25 ( 0.5) ºC. The
pH measurements were made using a Metrohm 691 pH meter equipped with a glass calomel
combined electrode. Elemental analyses were performed on an Elementar Vario EL III
elemental analyzer. Infrared spectra were recorded as pressed KBr discs, using a Unicom
Galaxy Series FT-IR 5000 spectrophotometer (4000-400 cm-1). All calculations were
performed in MATLAB 7.8 (R2009a) and Microsoft Excel 2007.
2.3. Theoretical calculations
1
The structure data and H chemical shifts of H4L in gas phase were calculated using
Gaussian-03 [37] series of programs. A starting molecular mechanics structure for the DFT
calculations was obtained using the Hyper Chem 5.02 program [38]. The geometry of the
prepared compound was fully optimized at the B3LYP/6-31G* level. Vibrational frequency
analyzes, calculated at the same level of theory, indicate that the optimized structure is at
the stationary points corresponding to global minima without any imaginary frequency. The
calculations of the computed magnetic isotropic shielding tensors were performed using
GIAO/DFT [31,32] and CSGT/DFT [33,34] methods at the B3LYP/6-311++G* basis set.
2.4. Preparation of H4L
To a suspension of 4-Amino-5-(4-pyridyl)-3-mercapto-1,2,4-triazole (1 mmol) in DMF (5
mL), was added a solution of N,N'-bis(3-formyl-5-methylsalicylidene)ethane-1,2-diamine,
H2L, (0.5 mmol) in DMF (5 mL). The mixture was refluxed for 3 h and then filtered whilst
hot. The crude orange product was collected and washed with ethanol and dried in air. Yield
50%, m.p. >300 oC, IR (KBr, cm-1); 2920, 2856, 1637 (C=N), 1606, 1582 ((C=N)py), 1471
1
(C=C), 1255 (N-N), 1271 (C-O), 1217, 1091, 831 (C=S), 592. H NMR (DMSO-d6, ppm);
H 2.28 (s, 6H), 4.01 (s, 4H), 7.50 (s, 2H), 7.85 (br,s, 6H), 8.66 (s, 2H), 8.75 (br,s, 4H), 9.98
4

(s, 2H), 14.58 (br, 4H). Anal. Calc. for C34H30N12O2S2: C, 58.10; H, 4.30; N, 23.92; S, 9.12
%. Found: C, 57.90; H, 4.60; N, 23.70; S, 9.10 %.
2.5. Thermodynamic calculations
The pH of the solution (DMSO/water, 1/10 V/V) containing H4L (25 mL of 5.0×10-5 M)
was gradually adjusted in the range of 1.5 to 10 using hydrochloric acid and potassium
hydroxide solutions. Then the absorbance spectra of the solution were recorded in
wavelength range of 300-600 nm. All experiments were carried out at the temperature
(25 0.5) °C. Also, the effect of the addition of metal ions to the DMSO solution of H4L was
studied. The concentration of H4L in DMSO was kept constant, and the metal ions were
progressively added to the solution. After each addition, UV-Vis spectra of the solution
were recorded.
3. Result and discussion
3.1. Characterization
The IR spectrum of H4L containing vibrational bands at 1637, 1606, 1582 and 1255 cm−1
which are assigned for νs and νas of the azomethine, pyridine C=N and N–N bands,
respectively [6,39]. The band at 831 cm−1 can be assigned to the C=S stretching modes of
1
mercapto substitute [6]. H NMR spectrum of H4L contains slightly broad signals in the
region 14.58 ppm assigned to NH/SH and OH protons, as were confirmed by deuterium
exchange when D2O was added to DMSO-d6 solution. The CH=N imine protons exhibit a
singlet resonance in the region 9.98 ppm (See supplementary material).
3.2. Analysis of experimental data
In order to obtain some information about the stoichiometry, stability and selectivity of H4L
toward different two and monovalent metal ions, we investigated the complexation of H4L
with Cu2+, Cd2+, Hg2+ and Ag+ ions spectrophotometrically in DMSO. It was found that the
addition of metal ions to the H4L solution affected on absorption spectrum of the ligand.
5

The changes in the UV–Vis spectrum of H4L upon addition of Cu2+, Cd2+, Hg2+ and Ag+
ions are presented in Figures 1-4, respectively.
The maximum wavelength of the ligand was observed at 353 nm. The maximum
wavelengths of complexes were observed at 417, 359, 360 and 358 nm for Cu2+, Cd2+, Hg2+
and Ag+, respectively. Notably, the CT1 band at ca. 353 nm decreased gradually, and the
new band at ca. 420-435 nm appeared and simultaneously grew with increasing metal ions
concentration. Also, the clean isobestic points at ca. 380-400 and ca. 460 nm appear after
addition of metal ions, indicating interaction between metal ions and ligand.
The conditional stability constants were calculated by using hard-modeling analysis of
spectrophotometric-mole ratio data. The computational procedure was performed using
some Matlab codes written in our laboratory. The same procedure was applied to calculate
protonation constants of the ligand by analysis of recorded spectrophotometric-pH metric
data. The calculated spectra profile and concentration profiles are shown in supplementary
material. The best model of complex equilibrium in solution for each metal ion was selected
based on the degree of calculated fitting and experimentally recorded spectra (i.e. total
residual sum of squares), the shape of spectra profiles (molar absorptivity) and meaningful
concentration profiles (the non-negativity and unimodality restrictions). The stability
constants values as well their standard deviation are listed in Table 1. The data show the
sequence of the stability constants of complexes vary as Cu(II) > Hg(II) > Ag(I) > Cd(II).
This may be due to different geometry tendencies of the metal ions. The high stability
constant of ML for copper complex makes it superior as a suitable ligand for developing an
analytical method for selective determination of copper ion.
3.3. pH dependence of the electronic absorption spectra of H4L in DMSO
UV-Vis absorption electronic spectrum of H4L in DMSO: water (1:10) at different pH
values were carried out to understand the role of the phenolic functions in the acid-base
6

behavior of this ligand. The involvement of the phenol and triazole groups in the
protonation steps is well depicted in absorption spectra. At acidic pH, the λmax at 354 nm is
observable, while at alkaline pH the spectra exhibit two λmax at 308 and 432 nm,
respectively. The different behavior of the chromophore can be attributed to the
deprotonation of the phenolic and triazole groups, respectively, which are in agreement with
data reported in the literatures [39,40,22]. In other words, the change in λmax is due to the
presence of various forms at low and high pH values. By monitoring of the spectra from
acidic to basic pH, the band with λmax = 432 nm starts to appear from pH≈5.5 and reaches
its maximum absorbance at pH≈10, where protonated species are prevalent in solution. This
means that the deprotonation of hydroxyl and triazole groups start in pH=5.5-10. We can
conclude that the bands centered at 432 and 308 nm are arising from the deprotonated
ligand; while the band located at 354 nm is mainly arising from the protonated species,
Figure 5. The pKa values and their standard deviation are 3.61
and 6.81
for
phenol and triazole moieties of H4L, respectively. The calculated spectra profile and
concentration profiles are shown in supplementary file.
3.4. Computational studies
The fully optimized geometries for different configurations of keto-enol and thione-thiol
structures of H4L (totally, for 8 structures) were calculated with the 6-31G* basis set at
B3LYP level of theory [41,42]. Theoretical calculations show that the thione form, Figure
6, is stable than others in gas phase (see supplementary material). The validity of stable
structure was confirmed by performing a frequency analysis. The selected bond lengths,
bond angles and torsion angles of the optimized structure of H4L are given in Table 2.
Moreover, in order to comparison between experimental and theoretical NMR data, which
may be helpful in making correct assignments and understanding the relationship between
1
chemical shielding and molecular structure, complete analyses of the H NMR spectra of
7

1
H4L is reported. For this reason, H NMR chemical shifts of H4L in gas phase were
calculated (Table 3) using Gauge Independent Atomic Orbitals/Density Functional Theory
(GIAO/DFT) [31,32] and Continuous Set of Gauge Transformations/ Density Functional
Theory (CSGT/DFT) [33,34] methods with B3LYP/6-311++G* basis set. The following
conclusions can be straightforwardly derived based on presented data. On an absolute scale,
the computed NMR chemical shifts for H4L at the DFT level of theory are in acceptable
agreement with the experimental data. Differences between the calculated and measured
values may be a result of solvent interactions, particularly in the case of hydrogen-bonding
atoms (OH groups). The GIAO/DFT computed magnetic isotropic shielding tensors
correlate better with the experimental data than the CSGT/DFT ones. The correlation
between the experimental 1H chemical shifts and the computed magnetic isotropic shielding
tensors is linear (Figure 7 and Table 3) and it is described by the equation:
exp= a+ bσ
Parameters a and b for all complexes are:
1H= -0.9534σ + 31.596 (R2= 0.9886) for GIAO method
and
1H= -0.7511σ + 30.286 (R2= 0.9804) for CSGT method
Conclusions
1
Synthesis, FT-IR, H NMR and UV-Vis data of new acyclic triazole-based Schiff base
1
hydrazone, H4L, is reported. The H NMR chemical shifts of H4L in gas phase was also
systematically studied by GIAO/DFT and CSGT/DFT methods at B3LYP/6-311++G(d)
basis set. The GIAO/DFT computed magnetic isotropic shielding tensors correlate better
with the experimental data than the CSGT/DFT ones.
8

Moreover, the conditional stability constants were calculated by using hard-modeling
analysis of spectrophotometric-mole ratio data. The data show the sequence of the stability
constants of Cd(II), Cu(II), Hg(II) and Ag(I) complexes of H4L vary as Cu(II) > Hg(II) >
Ag(I) > Cd(II). The complex geometry may be affected on this variation. The high stability
constant of ML for copper(II) complex makes H4L superior as a suitable ligand for
developing an analytical method for selective determination of copper(II) ion.
Acknowledgments
We are grateful to the Arak University for financial support of this work.
Supplementary material
Supplementary data associated with this article can be obtained free of charge via
http://www.araku.ac.ir/~h_khanmohammadi/SupportingInformation-9.doc
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12

Table 1. Results of logKâ for Metal-Ligand Complexes a
Metal / Ligand
Metal / Ligand
logKâ
logKâ
6.10
3.98
9.09
ML
ML
Cu2+
10.83
M2L
M2L
Ag+
16.70
0.05
13.00
0.01
M2L2
SSE b
M3L
SSE
5.39
10.72
0.01
3.20
8.92
0.01
ML
ML
Cd2+
Hg2+
M2L
SSE
ML2
SSE
a L, Schiff base; b SSE, Sum of Squares Errors.
13

Table 2. Selected calculated bond lengths (Å) and bond angles () for H4L.
Bond lenghts
Bond lenghts
Bond lenghts
1.396
1.392
1.292
N(29)-N(30)
N(26)-N(24)
N(29)-C(28)
1.359
1.395
1.312
N(26)-C(79)
N(26)-C(28)
N(24)-C(22)
C(79)-S(80)
N(30)-C(79)
1.683
1.359
Bond angles
Bond angles
Bond angles
110.1
101.6
131.8
126.4
N(29)-N(30)-C(79)
N(24)-N(26)-C(79)
N(24)-N(26)-C(28)
N(26)-N(24)-C(22)
115.0
125.7
124.6
112.8
N(29)-C(28) -N(26)
N(30)-C(79)-N(26)
N(26)-C(79)-S(80)
N(30)-C(79)-S(80)
N(30)-N(29)-C(28)
C(79)-N(26)-C(28)
104.5
108.5
14

b
H
a
H
N
N
c
e
j
d H3C
OH HO
CH3
H
S
H
f
N
N
N
N
N
N
S
h
g
N
HN
N
NH
i
Table 3.Selected hydrogen chemical shifts (, ppm) in d6-DMSO and calculated magnetic
isotropic shielding tensors for more stable isomer of H4Lin gas phase
Hydrogen
Hydrogen
Hydrogen
Hydrogen
a
1
2
3
4.02
27.92
26.96
c
7.50
24.63
25.01
e
7.87
23.39
24.65
g
7.87
23.98
24.79
b
1
2
3
9.98
22.27
22.71
d
2.20
29.48
28.69
f
8.66
23.73
24.60
h
8.75
23.27
22.95
1; Experimental chemical shifts, 2; computed magnetic isotropic shielding tensors at GIAO B3LYP/6-311++G* for all
atoms and 3; computed magnetic isotropic shielding tensors at CSGT B3LYP/6-311++G* for all atoms
15

Figure Captions:
Scheme 1. Synthesis of acyclic triazole-based Schiff base hydrazone (H4L)
Figure 1: Experimental absorption spectra for the Cu2+ complex with H4L. The analytical
concentration of H4L is 5.0×10-5(mol·L-1), The concentrations of Cu2+ ion is: (1) 0.0; (2) 2.0×10-
6; (3) 1.0×10-6; (4) 1.8×10-5;(5) 2.6×10-5;(6) 3.4×10-5;(7) 4.2×10-5; (8) 5.0×10-5; (9) 5.8×10-5; (10)
6.6×10-5; (11) 7.4×10-5; (12) 8.2×10-5; (13) 9.0×10-5; (14) 9.8×10-5 and (15) 1.0×10-4(mol·L-1).
Figure 2: Experimental absorption spectra for the Cd2+ complex with H4L. The concentration of
ligand and Cd2+ ion is the same as described in Figure 1.
Figure 3: Experimental absorption spectra for the Hg2+ complex with H4L. The concentration of
ligand and Hg2+ ion is the same as described in Figure 1.
Figure 4: Experimental absorption spectra for the Ag+ complex with H4L. The analytical
concentration of H4L is 5.0×10-5(mol·L-1), The concentrations of metal is: (1) 0.0; (2) 2.0×10-6;
(3) 1.0×10-6; (4) 1.8×10-5;(5) 2.6×10-5;(6) 3.4×10-5;(7) 4.2×10-5; (8) 5.0×10-5; (9) 5.8×10-5; (10)
6.6×10-5; (11) 7.4×10-5; (12) 8.2×10-5; (13) 9.0×10-5; (14) 9.8×10-5; (15) 1.0×10-4;(16) 1.1×10-4;
(17) 1.2×10-4; (18) 1.3×10-4; (19) 1.4×10-4; (20) 1.5×10-4(mol·L-1).
Figure 5: The electronic absorption spectra of H4L in DMSO/water (1:10) solution at pH 1.5–
10.0.
Figure 6: B3LYP/6-31G* geometry optimized structure of the H4L.
Figure 7: Plot of experimental chemical shifts vs. magnetic isotropic shielding tensors from (a)
the CSGT B3LYP/6-311++G* and (b) GIAO B3LYP/6-311++G* calculations for H4L.
16

N
N
N
N
N
N
HO
CH3
2
H C
3
OH
O
+
HS
O
NH2
H
H
H2L
DMF
3 hours reflux
H
H
N
N
HO
CH3
H3C
OH
H
H
S
N
N
N
N
N
N
S
N
N
N
H
N
H
H4L
Scheme 1
17

Figure 1.
Figure 2.
18

Figure 3.
Figure 4.
19

Figure 5.
Figure 6.
20

(a)
(b)
Figure 7.
21

Formation constants of copper(II),
cadmium(II), mercury(II) and
silver(I) complexes of new
Hamid Khanmohammadi , Malihe
Erfantalab, Golamhassan Azimi
Spectrochimica Acta Part A.. xxx (2012)xxx
bisiminated
1,2,4-triazole-based
Schiff base hydrazone in DMSO
have been determined by hard
model chemometrics method.
New acyclic 1,2,4-triazole-based
Schiff base hydrazone; Synthesis,
characterization, spectrophotometric
and computational studies
1
Furthermore, H chemical shifts of
the prepared ligand were studied
by the GIAO/DFT and CSGT/DFT
methods using B3LYP/6-311++G*
basis sets.
1

Research highlights:
> Synthesis of new acyclic 1,2,4-triazole-based Schiff base hydrazone is reported > Formation
constants of metal complexes were determine by hard model chemometrics method > The 1H
chemical shielding of ligand was studied by GIAO and CSGT methods at DFT level of theory.