Synthesis, experimental and theoretical characterizations of a 1,2,4-triazole Schiff base and its nickel(II) complex

Article abstract of DOI:10.1016/j.molstruc.2018.11.073

A new Schiff base, HL, derived from condensation reaction of 2,4-dihydroxybenzaldehyde and 4-amino-3-methyl-1H-1,2,4-triazole-5(4H)-thione together with its Ni(II) complex have been synthesized and characterized by several spectroscopies methods. The opti

Full text of DOI:10.1016/j.molstruc.2018.11.073

Accepted Manuscript
Synthesis, experimental and theoretical characterizations of a 1,2,4-triazole Schiff
base and its nickel(II) complex
Hamide Raouf, S. Ali Beyramabadi, Sadegh Allameh, Ali Morsali
PII:
S0022-2860(18)31385-1
DOI:
https://doi.org/10.1016/j.molstruc.2018.11.073
MOLSTR 25902
Reference:
To appear in: Journal of Molecular Structure
Received Date: 10 July 2018
Revised Date: 15 November 2018
Accepted Date: 22 November 2018
Please cite this article as: H. Raouf, S.A. Beyramabadi, S. Allameh, A. Morsali, Synthesis, experimental
and theoretical characterizations of a 1,2,4-triazole Schiff base and its nickel(II) complex, Journal of
Molecular Structure (2018), doi: https://doi.org/10.1016/j.molstruc.2018.11.073.
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ACCEPTED MANUSCRIPT

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Synthesis, experimental and theoretical characterizations of a 1,2,4-triazole Schiff base and its
nickel(II) complex
*
Hamide Raouf, S. Ali Beyramabadi , Sadegh Allameh, Ali Morsali
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
*
Corresponding author (Beyramabadi):
E-mail address: beiramabadi6285@mshdiau.ac.ir; beiramabadi@yahoo.com.
1

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Abstract
A new Schiff base, HL, derived from condensation reaction of 2,4-dihydroxybenzaldehyde and 4-
amino-3-methyl-1H-1,2,4-triazole-5(4H)-thione together with its Ni(II) complex have been
synthesized and characterized by several spectroscopies methods. The optimized geometries,
assignment of the IR bands and NMR chemical shifts as well as exploring of the frontier orbitals of
two synthesized compounds have been calculated using the density functional theory (DFT)
procedures. the most stable tautomer of the HL Schiff base and kinetics of its tautomerization have
-
been explored theoretically. In the square planar structure of the [Ni(L)Cl] complex, the anionic L
acts as a NOS tridentate ligand. The chloro ligand is positioned trans to the azomethine nitrogen.
The DFT-computed IR frequencies and NMR chemical shifts are good in agreement with their
corresponding experimental values, approving validity of the optimized geometries for two
investigated compounds. The [Ni(L)Cl] complex exhibits high catalytic activities in a three-
component cyclocondensation for synthesis of the tetrahydrobenzo[b]pyrans.
Keywords: 1,2,4-triazole; Schiff base; Nickel; dihydroxybenzaldehyde; DFT; Catalyst.
2

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1. Introduction
The Schiff base ligands are easily synthesized and form complexes with almost all metal ions. The
Schiff bases and their metal complexes have gained importance in medicinal and pharmaceutical
fields due to various biological activities such as anti-inflammatory [1, 2], analgesic [2-4],
antimicrobial [1, 5, 6], antioxidant [5, 6], antifungal [6], DNA binding [5, 7], antitubercular [8, 9],
anticancer [7, 10], anthelmintic [11], insulin enhancing properties [12] and so forth.
The aromatic substitutions in each of the amine and aldehyde reactants result in more stable
Schiff base as product of their condensation reaction [13]. The 1,2,4-triazoles are aromatic
compounds that incorporate into a wide variety of important drugs. The fluconazole, itraconazole,
alprazolam, vorozole, letrozole, anastrozole, ribavirin and rizatriptan are some examples of drugs
containing the 1,2,4-triazole moiety in their structures [14-19]. The antimicrobial, analgesic, anti-
inflammatory, antileishmanial, cytotoxicity, anticancer and antioxidant properties of the 1,2,4-
triazoles have been widely investigated [20-27]. Hitherto, many 1,2,4-triazole Schiff bases and their
metal complexes have been synthesized and investigated [6, 25, 26, 28-33].
On the other hands, synthesis, characterization and biological activities of several Schiff bases of
the 2,4-dihydroxybenzaldehyde and their metal complexes have been reported [34-42]. These
findings promoted us to synthesis and characterization of a new Schiff-base derived from the 2,4-
dihydroxybenzaldehyde and 4-amino-3-methyl-1H-1,2,4-triazole-5(4H)-thione (=HL Schiff base)
together with its [Ni(L)Cl] complex. The computational methods are very applicable in
characterization of the chemical compounds as well as investigation on the kinetics and mechanism
of the chemical reactions [34, 43-47]. Herein, tautomerism of the HL Schiff base, geometries
optimization, assignment of the IR and NMR spectra and the NBO analysis of two synthesized
compounds have been explored theoretically using the valuable DFT approaches. In addition,
catalytic behavior of the [Ni(L)Cl] complex in synthesis of the several polyhydroquinolines was
tested.
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2. Experimental
All of used chemicals were obtained from the Merck Company, which were used without any
further purification. Melting points have been determined using a Stuart SMP3 melting point
apparatus. A Bruker Tensor 27 spectrophotometer was employed to record the IR spectra, where the
1
13
KBr disks were used. The H-NMR (300 MHz) and C-NMR (75 MHz) spectra were acquired on a
Bruker spectrometer in DMSO solution. The elemental analyses for C, H, N and S were carried out
2
+
using a Heraeus elemental analyzer CHN-O-Rapid. Percentage of the Ni ion in complex was
measured on a Hitachi 2-2000 atomic absorption spectrophotometer.
2
.1. Synthesis of the HL (C H N O S)
10 10 4 2
A solution of 1 mmol (0.130 g) of 4-amino-3-methyl-1H-1,2,4-triazole-5(4H)-thione in 2 mL
ethanol was added to a solution of 1 mmol (0.138 g) of 2,4-dihydroxy benzaldehyde in 2 mL
ethanol. The reaction mixture was stirred for 80 min at reflux condition. Then, the light pink
product was filtered and washed with ethanol to give the HL Schiff base (m.p.: 282 � , yield: 90%).
2.2. Synthesis of the [Ni(L)Cl] complex
A solution of 1 mmol (0.250 g) of the HL Schiff base in 2 mL methanol was added slowly to a
methanolic solution of 1 mmol (0.238 g) NiCl .6H O. The reaction mixture was refluxed for 2 h to
2
2
give the [Ni(L)Cl] complex. The dark green precipitate was filtered off and washed with ethanol
m.p.: 291 � , yield: 80%).
(
3. Computational details
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In this work, all of the DFT calculations have been performed by using the Gaussian 03 software
package [48]. The B3LYP functional [49] and the 6-31+G(d,p) basis set were used, except for the
Ni atom, where the LANL2DZ basis set was used. The used functional and basis sets are widely
applicable and are frequently employed for the DFT investigations on the organic compounds and
their metal complexes (for example [50-53]). Firstly, all of the geometries were fully optimized.
Based on the frequency calculations, it has been confirmed that the optimized geometries have no
imaginary frequency, except for the transition states (TSs) which have only one imaginary
frequency. For evaluation of energies, sum of the electronic energies and the zero-point corrections
(E+ZPE) have been considered. The IR, NMR and NBO calculations were done on the optimized
geometries. Usually, the DFT-computed vibrational frequencies are higher than the experimental
1
13
ones, which were improved by using a scale factor of 0.9614 [54]. The H-NMR and C NMR
chemical shifts were computed by employing the GIAO method [55] with respect to
tetramethylsilane (TMS). All of the structures were visualized by using the Chemcraft 1.7 program
[
56].
4. Results & Discussion
The elemental analyses of the HL Schiff base (Anal. Calc.: C, 47.99; H, 4.03; N, 22.39; S, 12.81 %.
Found: C, 48.21; H, 4.17; N, 22.09; S, 12.29 %) and its Ni(II) complex (Anal. Calc.: C, 34.31; H,
2.59; N, 16.00; S, 9.16 %. Found: C, 34.58; H, 2.42; N, 15.86; S, 9.43 %.) confirm the proposed
C H N O S and C H ClN O SNi formulas for the HL Schiff base and its [Ni(L)Cl] complex,
1
0
10
4
2
10
9
4
2
2
+
respectively. Calculated and measured values for the Ni ion in structure of the [Ni(L)Cl] complex
are 18.68 and 19.01 %, respectively, confirming validity of the proposed for the complex. On the
other hands, the molecular ion peaks of the mass spectra of the HL ligand and its [Ni(L)Cl]
complex are appeared at 251 and 343 m/z, respectively. These values are additional evidence for
confirmation of the proposed formulas for the HL ligand and [Ni(L)Cl] complex. In structure of the
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-
2+
investigated complex, the L ligand and Ni metal ion show the 1:1 ratio, which is in consistent
with the reported structure for the complexes of similar tridentate ligands [25, 27, 57-59].
4.1. Geometries optimization
Two different intramolecular proton transfers may be occurred in structure of the HL Schiff base:
between the phenolic –OH group and azomethine nitrogen and another one between the sulfur
substitution and –NH moiety of the triazole ring. Since, the HL Schiff base could exist as four
different isomers, which are named as OHNH, OHSH, NHNH and NHSH. Optimized geometries of
these isomers are shown in Fig. 1.
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Figure 1. Optimized geometries of the OHNH, OHSH, NHNH and NHSH tautomers of the HL
Schiff base.
Calculated energies of the four possible tautomers of the HL Schiff base are gathered in Table 1.
As seen, the OHNH species is the most stable tautomer of the HL Schiff base. The calculated
values for the Gibbs free energy difference between the tautomers (∆G ) approve these matter, too.
-1
Due to low activation energy (23.58 kJ.mol ) the OHNH tautomer of the HL is tautomerized to the

→
NHNH tautomer. While, the OHNH
OHSH tautomerization cannot be occurred, because of its

←
-1
high barrier energy (187.31 kJ.mol ) (Table 3). Since, its predicted that the methanol solution of the HL
Schiff base involves both of the OHNH and NHNH tautomers.
Table 1. Relative electronic energies (E+ZPE) and the ∆G for investigated species of the HL Schiff base (in
-1
term of kJ.mol ).
Species
E+ZPE
∆G
OHNH
OHSH
0
0
50.63
24.54
73.43
-
53.23
26.04
76.80
23.58
187.31
NHNH
NHSH
TSOHNH-OHSH
TSOHNH-NHNH
-
In the optimized geometry of the OHNH tautomer, there is a strong intramolecular hydrogen
bond between the H9 atom of the phenolic –OH group and N4 azomethine nitrogen. The H9…..N4
H-bond length is 177.1 pm. Transfer of the H9 proton to the N4 imine nitrogen results in the OHNH

→
NHNH tautomerization. Transition state of this tautomerization is named as the TSOHNH-
←

8

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NHNH, optimized geometry of which is shown in Fig. 2. As seen in Fig. 2 and Table 2, cleavage of
the O1-H9 bond and formation of the N4-H9 bond is clear in structure of the TSOHNH-NHNH
species. Optimized geometry of the TSOHNH-OHSH species is shown in Fig. 2, which is related to

→
OHSH tautomerization.
the transition state of the OHNH
←

9

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Figure 2. Optimized geometries of the TSOHNH-NHNH and TSOHNH-OHSH.
In the optimized geometries for the OHNH and NHNH tautomers, the HL Schiff base has not a
planar structure, but two aromatic rings make approximately a dihedral angle of 50 and 80° to each
other, respectively. The calculated values for the C4-N4-N1-C2 and C4-N4-N1-C2 dihedral angles
confirm this matter (Table 2).
-
For complexation, the HL Schiff base loses the H9 phenolic proton to produce the L anion. The
-
2+
L is a tridentate ligand that is coordinated to the Ni ion via the N4 azomethine nitrogen, the O1
phenolate oxygen and the sulfur atom. Another coordination position of the square planar structure
of the [Ni(L)Cl] complex is occupied by the chloro ligand. Optimized geometry of the investigated
complex is shown in Fig. 3, structural parameters of which are gathered in Table 2. regarding the
calculated dihedral angles such as the C4-N4-N1-C2 and C4-N4-N1-C2 (Table 2), the [Ni(L)Cl]
complex exhibits more planar structure compared with the free HL ligand.
In the optimized geometries of the free HL ligand and its Ni(II) complex, the C4=N4 imine
group lies in the same plane with the benzene ring (Table 2). In comparison with the free HL Schiff
base, coordination of the N4 azomethine nitrogen to the Ni(II) ion elongates the C4=N4 bond
1
0

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length. But, deprotonation of the O1 phenolic oxygen decreases the C7-O1 bond length in structure
-
of the Ni(II) complex. Sulfur is another donating atom of the L ligand, coordination of which
causes to increase in the C1-S bond length (Table 2).
The [Ni(L)Cl] complex has a square planar geometry. Calculated dihedral angles approves that
2
+
all of the donating atoms lie in a same plane with the Ni metal ion. However, the triazole ring is
slightly out of the square plane (Table 2). The computed structural parameters for the HL Schiff
base and its Ni(II) complex are in consistent with the corresponding values in the similar
compounds [6, 22, 24, 25, 27, 28, 31, 59-62].
Figure 3. Optimized geometry of the [Ni(L)Cl] complex.
Table 2. Selected structural parameters of the investigated species.
Species
OHNH
NHNH
OHSH
NHSH
TSOHNH-NHNH
TSOHNH-SHNH
[Ni(L)Cl]
Bond length (ppm)
O1-H9
98.7
177.1
134.7
177.4
103.4
126.7
98.7
177.2
134.5
177.7
103.4
126.6
132.3
117.5
129.8
98.3
179.5
134.8
-
-
H9-N4
O1-C7
128.6
1
1

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C7-C5
C5-C4
C4-N4
N4-N1
N1-C1
C1-S
142.2
143.9
129.7
138.7
138.9
168.5
301.6
100.9
135.0
137.0
135.7
-
147.1
138.5
134.0
137.0
139.4
167.5
304.3
100.9
134.8
137.5
126.7
-
142.3
143.6
129.8
139.6
136.9
176.1
134.8
266.7
131.0
138.9
135.6
-
147.1
138.4
134.1
137.5
137.9
175.9
134.8
266.4
130.4
139.6
135.6
-
145.1
140.5
132.1
137.5
139.1
167.9
303.8
100.9
134.9
137.3
135.6
-
142.1
144.1
129.4
138.0
136.2
173.4
171.1
137.8
132.2
137.9
135.8
-
144.8
140.5
132.2
138.7
137.0
169.5
-
S-H1
N3-H1
N3-C1
N2-N3
C10-O2
Ni-N4
Ni-S
-
134.8
136.7
136.0
193.1
225.6
186.9
-
-
-
-
-
-
Ni-O1
Angle (°)
-
-
-
-
-
-
N4-H9-O1
H9-O1-C7
144.9
108.5
122.1
122.5
121.1
116.2
101.9
129.9
64.3
132.5
106.1
121.3
120.6
122.7
121.3
101.5
128.6
63.3
144.7
108.7
122.1
122.3
121.5
115.7
110.2
121.6
79.9
132.3
106.3
121.2
120.6
122.8
121.6
109.6
121.9
79.9
148.6
104.9
120.4
118.7
118.5
120.3
101.6
128.8
63.6
144.6
108.6
122.1
122.8
121.4
118.2
107.5
144.2
106.5
108.2
110.5
-
-
O1-C7-C5
124.0
122.1
124.8
118.4
109.5
124.0
-
C7-C5-C4
C5-C4-N4
C4-N4-N1
N1-C1-N3
N1-C1-S
S-H1-N3
N3-C1-S
128.2
110.3
130.0
109.6
128.2
108.7
128.5
108.6
129.6
109.8
132.2
109.5
N1-C2-N2
Dihedral angle (°)
N4-H9-O1-C7
O1-C7-C9-C10
C7-C9-C10-O2
O1-C7-C5-C4
C7-C5-C4-N4
C5-C4-N4-N1
C4-N4-N1-C2
C4-N4-N1-C1
N4-N1-C1-S
N4-N1-C1-N3
N3-N2-C2-C3
C2-N1-C5-C7
1.9
179.9
-179.9
0.4
5.8
179.8
-179.8
0.6
-0.5
-179.8
179.4
-0.2
-0.9
-179.9
179.9
-0.1
3.4
179.6
-179.5
1.2
1.2
179.9
-179.9
0.2
-
177.9
-179.1
4.3
-2.1
-1.7
1.2
0.3
-2.5
-0.9
0.1
177.4
-55.5
136.2
7.0
174.8
103.7
-81.4
3.7
-178.1
67.8
-178.7
90.5
175.0
114.6
-73.0
5.9
178.7
165.3
-18.8
-1.9
177.4
-22.5
163.2
1.3
-124.3
8.4
-94.8
3.4
-171.5
-8.5
-176.5
179.7
-96.3
-172.2
177.8
-122.2
-176.5
179.5
-101.9
-174.3
179.9
-84.00
-177.3
-179.5
-21.3
178.4
-177.0
160.0
-60.5
4.2. NMR analysis
1
13
The H- and C-NMR spectrum of the HL ligand and [Ni(L)Cl] complex were recorded in DMSO
solvent. The experimental and DFT-calculated NMR chemical shifts (δ) of them are listed in Table
3. The atom positions of the ligand and complex are numbered as in Figs. 1 and 3, respectively. The
chemical shifts were computed for both of the OHNH and NHNH tautomers of the HL Schiff base.
1
2

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As seen in Table 3, the experimental chemical shifts of the Schiff base are closer to the DFT-
calculated chemical shifts for the OHNH tautomer than the NHNH one. There is good consistency
between the DFT-computed chemical shifts and the corresponding experimental values, confirming
suitability of the optimized geometries for the OHNH tautomer of the HL Schiff base and the
[
Ni(L)Cl] complex. The only exception is the H10 phenolic proton. The calculated chemical shift
for the phenolic H10 proton is very lower than its experimental value. This proton can be engaged
in intermolecular hydrogen bond, which affects its chemical shift. On the other hands, the DFT
calculated values are obtained from the isolated molecule in vacuum, while the experimental results
are obtained from DMSO solution. Clearly, the solvent-solute interactions change the NMR
chemical shifts.
1
13
Table 3. Experimental and computational H-NMR and C-NMR chemical shifts of the HL Schiff base and
the [Ni(L)Cl] complex in DMSO solution, δ [ppm].
HL
Theo.
[Ni(L)Cl]
HL
[Ni(L)Cl]
Atom
Atom
position
Theo.
position
Exp.
Exp.
Theo.
Exp.
Exp.
Theo.
OHNH
11.09
9.16
8.62
7.23
6.57
6.25
4.35
2.26
2.26
1.97
NHNH
13.46
7.07
8.56
6.70
6.13
5.83
4.28
2.18
2.23
1.94
OHNH
169.82
168.74
166.52
164.08
148.60
136.85
111.15
108.39
101.76
11.37
NHNH
162.24
172.88
179.43
170.70
151.53
139.36
116.40
112.02
105.51
10.86
H9
H5
H1
H6
H7
H8
H10
H2
H3
H4
12.81
10.40
9.71
7.69
6.43
6.41
10.31
2.32
2.32
2.32
-
-
C4
C1
C7
C10
C2
C6
C5
C8
C9
C3
163.71
162.98
161.73
160.98
148.46
130.24
110.32
109.03
102.85
11.25
163.77
163.01
161.78
161.02
130.12
110.29
109.06
105.44
102.95
16.20
171.08
170.44
168.30
164.12
140.55
128.14
114.00
112.90
109.07
16.17
10.39
9.71
7.69
6.41
6.41
10.29
2.32
2.32
2.32
8.64
8.72
6.91
6.35
6.71
4.35
2.57
2.86
2.44
4.3. Vibrational spectroscopy
By comparison of the experimental and theoretical IR vibrational frequencies, the observed IR
vibrational frequencies of the HL Schiff base and its Ni(II) complex have been identified. The
vibrational frequencies have been computed for the OHNH tautomer of the HL Schiff base and the
[
Ni(L)Cl] complex. The obtained results are gathered in Table 4.
1
3

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-1
A very strong absorption band at 1660-1500 cm region of the IR spectrum accounts as an
critical signal in identification of coordination mode of the Schiff bases to the metal ions [25, 27-29,
-1
3
1, 33, 44, 45, 61, 63-65]. A strong band at 1603 cm of the IR spectrum of the HL Schiff base is
related to the symmetrical stretching mode of C4=N4 bond. By coordination of the N4 azomethine
2
+
nitrogen to the Ni ion, the υ(C4=N4) shifts to lower wavenumber in the IR spectrum of the
[
Ni(L)Cl] complex [23, 25, 27, 33, 44, 45, 61, 63-68]. In comparison with the free HL Schiff base,
deprotonation of the O1-H9 phenolic group increases electron density in the C7-O1 bonding region
of the [Ni(L)Cl] complex. Because of stronger bond, the υ(C7-O1) shifts to higher energies in the
IR spectrum of the [Ni(L)Cl] complex (Table 4).
Overlapping of the O-H, N-H, S-H and C-H stretching vibrations results in a band broading in
-1
3
600-2000 cm spectral region of the IR spectra [25, 27, 28, 31, 33, 44, 45, 61, 64-66, 68, 69].
Deconvolution of this spectral region of the HL ligand and its Ni(II) complex has been listed in
Table 4. In this region, the most intensive band of the HL ligand and [Ni(L)Cl] complex is related
to the υ(O1-H9) and υ(N3-H1) stretching vibrations, respectively.
-1
Table 4. Experimental and DFT-calculated IR vibrational frequencies (cm ) of the HL ligand and [Ni(L)Cl]
complex.
Experimental frequencies
Calculated frequencies
OHNH tautomer
[Ni(L)Cl]
Vibrational assignment
HL ligand
[Ni(L)Cl]
Intensity
Intensity
Frequency
Frequency
-
1
-1
(
km.mol )
(km.mol )
-
-
-
-
-
-
-
369
498
612
-
28
7
υ(Ni-Cl)
477 (w)
594 (w)
-
-
-
υ(Ni-O1)
-
14
-
υ(Ni-N4)
6
7
9
93 (m)
98 (w)
76 (w)
719
810
994
1076
149
53
24
36
δout of plane(O1-H9)
δout of plane(C-H) of the benzene ring
υ(C1-S)
818
990
1097
47
5
1
123 (m)
1
123 (m)
26
υ(N2-N3)
1
4

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1
1
1
1
096
158
253
318
148
301
248
211
142
162
162
363
190
1139
1197
1228
1290
1344
1466
1428
1470
1514
350
266
122
53
δin plane(C-H) of the aromatic rings
1
1
170 (s)
295 (s)
1171 (s)
υ(C10-O2)
υ(N1-N4)
1295 (s)
υ(C1-N1)
1
1
324 (m)
413 (m)
1373
423
1421
137
93
υ(N1-C2)
1
υ(C=C) of the benzene ring
υ(C7-O1)
1
416 (s)
59
1
1
431
519
289
99
υ(C1-N3)
υ(C4=N4) + υasym(C=C) of the benzene ring
1
502 (s)
1503 (s)
1
561
136
1560
113
υ(C2-N2)
1
1
603 (vs)
630 (vs)
1599 (vs)
1628 (vs)
2943 (s)
1587
1626
2931
839
425
9
1577
1624
833
238
5
υ(C4=N4)
υ(C=C) of the benzene ring
υsym(C-H) of the methyl group
υ(C4-H5)
2
937 (m)
2938
3
3
024
026
9
3086
4
10
6
3036
6
υasym(C-H) of the methyl group
υ(C1-H) of the aromatic hydrogens
υ(O1-H9)
3
171 (s)
445 (s)
3173 (s, br)
3
057-3083
3052-3076
-
9
3
3
201
330
737
221
-
3536
205
υ(N3-H1)
3
426 (s)
3
3655
201
3666
105
υ(O2-H10)
4.4. NBO analysis
Fig. 4 shows the 3D-distribution map of the highest-occupied-molecular orbital (HOMO) and the
lowest-unoccupied-molecular orbital (LUMO) of the HL Schiff base and [Ni(L)Cl] complex. The
HOMO and LUMO frontier orbitals of the HL Schiff base are mainly localized on the triazole and
benzene rings, respectively. Since, the HOMO-LUMO electron transition of the HL Schiff base can
*
be considered as the
π
→
π
transition between these aromatic rings. In the [Ni(L)Cl] complex, the
HOMO orbital is mainly localized on the triazole ring, while the LUMO one is mainly located on
the benzene ring. Both of the Ni atom and chloro ligand contribute in both of the HOMO and
LUMO orbitals.
1
5

ACCEPTED MANUSCRIPT
The energy difference between the HOMO and LUMO frontier orbitals, energy gap, plays a
critical role in such cases as electric properties, electronic spectra and photochemical reactions of
the chemical compounds. Calculated value for the energy gaps of the HL and [Ni(L)Cl] species are
equal to 3.67 and 3.68 eV, respectively. These large energy gaps confirm stability of these
compounds [33, 65, 69-71].
HOMO of the HL
LUMO of the HL
HOMO of the [Ni(L)Cl]
LUMO of the [Ni(L)Cl]
Figure 4. The HOMO and LUMO frontier orbitals of the HL Schiff base and its [Ni(L)Cl] complex.
5. Catalytic performance of the [Ni(L)Cl] complex
1
6

ACCEPTED MANUSCRIPT
Previously, we reported catalytic behavior of the oxovanadium(IV) complexes in synthesis
polyhydroquinoline derivatives [44]. Herein, the same methods were used for investigation of the
catalytic properties of the [Ni(L)Cl] complex in synthesis of the polyhydroquinoline derivatives
(Fig. 5).
O
Ar
O
O
O
[Ni(L)Cl]
OEt
OEt
ArCHO
+
+
4
NH OAc
+
N
H
O
O
4
1
3
5a-f
2a-f
Figure 5. Catalytic synthesis of the polyhydroquinolines by the [Ni(L)Cl] complex.
Initially, in order to find the optimal conditions, reaction of the dimedone, benzaldehyde (2a),
ethyl acetoacetate and ammonium acetate has been done in the presence of various amounts of the
Ni(L)Cl] catalyst. 1 mmol of each of the reactants has been used. The reaction has been
[
investigated in different solvents and under solvent-free conditions used as a model reaction. the
obtained results are gathered in Table 5. In the absence of the catalyst, the product 5a was obtained
in trace amount after 180 min. The best result has been obtained at 0.03 g of the [Ni(L)Cl] catalyst
under solvent-free conditions and 80°C (Table 5, Entry 7).
Due to solubility of the catalyst in ethanol, the filtrate was evaporated under reduced pressure
and the [Ni(L)Cl] catalyst was recycled by a simple filtration. The separated catalyst reused in
model reaction without appreciable reduction in the catalytic activity. The resulted yields for the
first, second and third experiments were almost consistent as 95, 93 and 89 %, respectively.
1
7

ACCEPTED MANUSCRIPT
Table 5. Effects of the solvent, temperature and amount of the [Ni(L)Cl] catalyst on the model reaction.
Entry
Catalyst (g)
Solvent
Temp (°C
)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
----
0.01
0.02
0.03
0.01
0.02
0.03
0.03
0.03
0.10
0.05
0.05
0.05
----
----
----
----
----
----
----
----
----
110
180
40
----
55
61
65
75
83
95
96
96
50
80
60
55
70
70
40
70
40
80
30
80
30
80
30
90
30
100
30
1
1
1
1
0
1
2
3
H O
2
Reflux
Reflux
Reflux
Reflux
90
EtOH
MeOH
CHCl3
80
120
100
To evaluate generality of this method, the reaction was explored with a wide range of aromatic
aldehydes under the optimized reaction conditions (entry 7 of Table 5). The obtained results are
listed in Table 6. As seen, significant yields are observed for all of the used aldehydes. For example,
selected spectral data of the 5a and 5c species are shown in below.
Ethyl 4-phenyl-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate (5a)
1
H NMR (300 MHz, DMSO-d ): δ 0.8 (s, 3H, CH ), 1.0 (s, 3H, CH ), 1.2 (t, 3H, CH3, OEt), 2.3 (s,
6
3
3
3
1
H, CH ), 2.0 (m, 4H, 2CH ), 4.0 (q, 2H, OCH ), 4.9 (s, 1H, CH), 7.1-7.4 (m, 5H, arom-H), 9.1 (s,
3 2 2
H, NH). IR (KBr disc): 1617 υ(C=O, keton), 1699 υ(C= O, ester), 3290 cm υ(NH).
-1
Ethyl4-(4-methoxyphenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-
carboxylate (5c)
1
H NMR (300 MHz, DMSO-d ): δ 0.9 (s, 3H, CH ), 1.0 (s, 3H, CH ), 1.2 (t, 3H, CH3, OEt), 2.1 (s,
6
3
3
3
4
H, CH ), 2.3 (m, 4H, 2CH ), 2.5 (s, 3H, OCH ), 4.0 (q, 2H, OCH ), 4.8 (s, 1H, CH), 6.8-7.3 (m,
3 2 3 2
H, arom-H), 9.1 (s, 1H, NH). IR (KBr disc): 1607 υ(C=O, keton), 1700 υ(C= O, ester), 3277 cm
-1
υ(NH).
Table 6. Synthesis of polyhydroquinolines under optimized conditions.
Entry
Ar
Product
Time (min)
Yield (%)
Melting pint (°C) [23]
1
8

ACCEPTED MANUSCRIPT
Found
Reported
1
2
3
4
5
6
C H
5a
30
35
32
31
29
34
95
92
220-222 214-216
252-255 259-260
260-262 257-259
241-242 245-247
263-266 257-260
234-236 239-241
6
5
4-BrC H
5b
5c
5d
5e
5f
6
4
4-MeOC H
90
95
90
88
6
4
4-NO C H
4
2
6
4-MeC H
6
4
4-HOC H
6
4
6. Conclusion
The HL Schiff base and its Ni(II) complex were newly synthesized and characterized using several
spectroscopic methods as well as valuable DFT methods. Based on the obtained results, the
-
complex has a [Ni(L)Cl] formula, where the L anion acts as a tridentate ligand. Three donor atoms
-
of the L ligand are the sulfur atom, azomethine nitrogen (N4) and phenolate oxygen (O1). Another
coordination site of the square complex is occupied by the chloro ligand.
The HL Schiff base could exist as four possible tautomers. Kinetics and mechanism of its
tautomerism have been investigated by employing the DFT methods. The OHNH is the most stable
tautomer in methanol solution of the HL Schiff base.
The optimized geometry of the [Ni(L)Cl] complex is more planar than the free HL Schiff base.
Going from the free HL ligand to the [Ni(L)Cl] complex, coordination of the N4 atom to the Ni(II)
elongates the C4=N4 bond of the azomethine group; inversely, deprotonation of the phenolic group
decreases the C7-O1 bond length.
The IR and NMR spectra of the HL and [Ni(L)Cl] species have been interpreted theoretically.
Good agreement between the DFT-calculated and experimental values confirms suitability of the
optimized geometries for these compounds. High energy gaps between the HOMO and LUMO
frontier orbitals confirm stability of the HL and [Ni(L)Cl] species.
1
9

ACCEPTED MANUSCRIPT
The [Ni(L)Cl] complex exhibits a considerable catalytic effects in in synthesis of the
tetrahydrobenzo[b]pyrans via a three-component cyclocondensation reaction of the dimedone,
malononitrile and some aromatic aldehydes. No considerable yield of the product was detected in
any of the used solvent. The optimum conditions for reaction of the benzaldehyde were 0.05 g of
the [Ni(L)Cl] catalyst, T=110 °C and t=15 min in free solvent condition.
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
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A new 1,2,4-triazole Schiff base and its Ni(II) complex have been newly synthesized.
Optimized geometries and assignments of the IR and NMR spectra have been computed using
the DFT methods.
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The synthesized Schiff base (HL) acts as a NOS tridentate ligand.
th
The chloro ligand occupies the 4 coordination position of the square [Ni(L)Cl] complex.
Catalytic behavior of the [Ni(L)Cl] complex has been investigated.