Synthesis, crystal structure and investigation of the catalytic and spectroscopic properties of a Zn(II) complex with coumarin-hydrazone ligand

Article abstract of DOI:10.1016/j.poly.2018.12.023

A Zn(II) complex with coumarin-hydrazone ligand, [ZnCl(HL)(CH₃OH)_0.76(H₂O)_0.24]·0.24(CH₃OH) (1), was synthesized from the reaction of equimolar amount of ZnCl₂·4H₂O and (E)-2-hyd

Full text of DOI:10.1016/j.poly.2018.12.023

Accepted Manuscript
Synthesis, crystal structure and investigation of the catalytic and spectroscopic
properties of a Zn(II) complex with Coumarin-hydrazone ligand
Sanam Aslkhademi, Nader Noshiranzadeh, Mirabdullah Seyed Sadjadi,
Kheirollah Mehrani, Nazanin Farhadyar
PII:
S0277-5387(18)30828-3
https://doi.org/10.1016/j.poly.2018.12.023
POLY 13640
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
19 September 2018
13 December 2018
15 December 2018
Please cite this article as: S. Aslkhademi, N. Noshiranzadeh, M.S. Sadjadi, K. Mehrani, N. Farhadyar, Synthesis,
crystal structure and investigation of the catalytic and spectroscopic properties of a Zn(II) complex with Coumarin-
hydrazone ligand, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.12.023
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Synthesis, crystal structure and investigation of the catalytic and
spectroscopic properties of a Zn(II) complex with Coumarin-
hydrazone ligand
a
,b
,a
Sanam Aslkhademi , Nader Noshiranzadeh* , Mirabdullah Seyed Sadjadi* ,
a
c
Kheirollah Mehrani , Nazanin Farhadyar
a
Department of Chemistry, Science and Research Branch, Islamic Azad University,
Tehran, Iran
Department of Chemistry, Faculty of Sciences, University of Zanjan 45371-38791,
b
Zanjan, Iran
c
Department of Chemistry, Varamin-Pishva Branch, Islamic Azad University, 33817-
7
4895,Varamin, Iran
Abstract
A Zn(II) complex with coumarin-hydrazone ligand,
[
ZnCl(HL)(CH
3
OH)0.76(H
2
O)0.24]∙0.24(CH
3
OH) (1), was synthesized from the
reaction of equimolar amount of ZnCl
2
∙4H
2
O and (E)-2-hydroxy-N'-(1-(2-oxo-2H-
chromen-3-yl)ethylidene)benzohydrazide (H L) in methanol solvent. The complex
2
was characterized by elemental analysis, spectroscopic methods and single crystal X-
ray diffraction studies. Single crystal X-ray analysis indicated that the Zn(II) ion in
complex 1 has a distorted square pyramidal environment and the coumarin-hydrazone
−
ligand is coordinated as a mononegative tridentate ONO-donor ligand, (HL) . The
photoluminescent studies indicated that complex 1 has emission at about 470 nm. The
catalytic potential of complex 1 was investigated for preparing tetrazole based
compounds from the azide-nitrile cycloaddition reactions. The catalytic reactions
were carried out in water as a green solvent. The results indicated that complex 1 is
an active catalyst for preparing tetrazole compounds. The effects of temperature and
electronic properties of the substituent connected to the nitrile group on the activity of
this catalytic system were also considered.
Keywords: Zn(II) complex; Crystal structure; Coumarin-hydrazones; Catalytic
properties; Spectroscopic studies
1

1
. Introduction
Zinc, as the second most abundant metal ion the human body, is an important bioessential
trace element which has been recognized as a crucial element in a large number of
proteins and enzymes [1]. Zinc plays vital roles in several biological processes (like gene
expression, metabolism of cells, apoptosis, etc.) and it is responsible for some neurological
disorders like Parkinson and Alzheimer diseases [2]. Due to this, specific detection of zinc ion
has attracted considerable attention during recent years and a number of methods have been
developed for Zn detection [3]. Among these methods, fluorescence spectroscopy by the use
of fluorescent chemosensors has been introduced as one of the advanced techniques for
detection of Zn(II) ion which is also known as low cost, high selective and sensitive method
[
4]. Fluorescence properties of zinc(II) complexes, especially in the function of light-emitting
materials (LEDs), have been intensively explored and the results indicate that the
fluorescence intensity and properties of Zn(II) complexes are sensitive to the structure and
donor/acceptor potentials of the employed organic ligands [5]. As catalyst, zinc complexes
have been frequently used in phenol oxidation [6], CO
polymerization [8], Friedel-Crafts acylation [9] and 3+2 nitrile-azide cycloaddition reactions
10].
2
fixation [7], ring opening
[
Tetrazoles are a class of important five membered nitrogen rich heterocyclic compounds
which contain one carbon and four nitrogen atoms [11]. Although such heterocyclic systems
are not found in nature, they show broad range of biological and pharmaceutical activities
[
[
12]. By having low toxicity and high lipophilicity, tetrazoles have been used as clinical drugs
13], HIV-inhibitors [14], anti-inflammatories [15], antibiotics [16] and antimicrobial
compounds [17]. Furthermore, tetrazoles have also been used as explosives and gas-
generating materials [18] and they have been employed as important synthons in synthetic
organic chemistry [19]. In inorganic and organometallic chemistry, tetrazoles have been
utilized as efficient ligands for the formation of interesting multinuclear complexes and
coordination polymers [20]. Due to widespread applications of tetrazoles, their preparation
2

has been the subject of intense investigation and several methods and catalytic systems have
been introduced for production of tetrazoles. Among the various reported methods in
literature, the 3+2 cycloaddition reaction of a nitrile with azide in the presence of a catalyst is
the most common and conventional method to synthesize a tetrazole ring [21]. The results of
studies indicated that the zinc based compounds are more effective catalysts for production of
tetrazoles. It is expected that the development of new zinc based catalysts for 3+2 azide-nitrile
cycloaddition reactions can lead to new effective catalytic systems for preparing this class of
heterocyclic compounds.
Coumarins are a class of heterocyclic compounds which display important biological
activities and have been observed in many plants [22]. Recently, it has been reported
that the coumarin Schiff-base derivatives are effective fluorescent chemosensors for
Zn(II) [23]. Some of the coumarin derivatives, like 3-acetylcoumarins, have high
potential to be used in coordination chemistry by the reaction with suitable amines to
form interesting Schiff base ligands [24]. Coumarin-hydrazone Schiff base ligands,
obtained from the reaction of 3-acetylcoumarin with hydrazones, can act as interesting
tridentate ONO-donor ligands in preparing transition metal complexes. By
considering the high stability of hydrazone complexes [25], coumarin-hydrazone
complexes can be efficient and stable catalysts in various catalytic reactions.
Moreover, some of the hydrazones have also shown good fluorescent properties [26].
Therefore, it is predictable that the combination of coumarin and hydrazone
compounds can show interesting optical properties. However, considering literature
including Cambridge Structural Database (CSD) indicates that they are less studied
and there are only three reports about coumarin-hydrazone complexes [27]. Recently,
we have also reported some transition metal complexes and coordination polymers
with coumarin-naphthohydrazone ligand that showed a rare coordination mode in the
formation of coordination polymers [28].
3

In this paper, we report synthesis, characterization, spectroscopic properties, crystal
structure and catalytic activity of a Zn(II) complex with coumarin-hydrazone ligand.
2
. Experimental
2
.1. Materials and instrumentations
Zinc(II) chloride tetrahydrate, ethyl acetoacetate, salicylaldehyde, piperidine and
acetic acid were purchased from Merck and used as received. Solvents of the highest
grade commercially available (Merck) were used without further purification. 3-
Acetylcoumarin was prepared according to previous report [28]. FT-IR spectra were
recorded as KBr disks with a Bruker FT–IR spectrophotometer. UV–Vis solution
spectra were recorded on a thermo-spectronic Helios Alpha spectrophotometer. The
zinc content of the complex was measured by atomic absorption analysis using Varian
Spectra AA-220 equipment. The elemental analyses (carbon, hydrogen and nitrogen)
1
of the compounds were obtained from a Carlo ERBA Model EA 1108 analyzer. H
1
3
and C NMR spectra in DMSO-d
spectrometer and the chemical shifts are indicated in ppm relative to tetramethylsilane
TMS). Thermogravimetric analysis was performed with a Mettler Toledo TGA-DSC
6
solution were measured on a Bruker 250 MHz
(
instrument. Photoluminescence spectra of the ligand and complex were recorded on a
Perkin-Elmer Luminescence Spectrometers (equipped with a xenon lamp) by
absorption and emission modes. The spectra were recorded at room temperature in the
range of 200–900 nm.
2
.2. Synthesis of (E)-2-hydroxy-N'-(1-(2-oxo-2H-chromen-3-
yl)ethylidene)benzohydrazide (H L)
2
4

A methanol solution (10 mL) of 3-acetylcoumarin (0.565 g, 3.0 mmol) was added to a
methanolic solution of 2-hydroxybenzohydrazide (0.456 g, 3.0 mmol) and the
solution was refluxed for 2 h at 70 °C. The obtained yellow solid was separated,
filtered off and washed with cold methanol and finally recrystallized from ethanol.
Yield: 92.0% (890 mg). Anal. Calc. for C18
.38; N, 8.69%. Found: C, 67.13; H, 4.32; N, 8.74%. FT-IR (KBr, cm ): 3400 (m,
br), 3178 (m), 3161 (m), 3070 (m), 3052 (br, m), 3031 (br, m), 2950 (w), 2929 (w),
H
14
N O
2 4
(MW = 322.31): C, 67.07; H,
-1
4
1
1
735 (m), 1712 (m), 1690 (vs), 1609 (vs), 1603 (vs), 1576 (s), 1557 (vs), 1487 (s),
456 (vs), 1409 (m), 1371 (m), 1334 (s), 1308 (s), 1297 (s), 1252 (w), 1230 (s), 1215
(s), 1159 (vs), 1140 (s), 1120 (s), 1092 (m), 1032 (m), 967 (m), 938 (s), 879 (w), 825
(w), 808 (w), 779 (s), 769 (vs), 754 (s), 737 (m), 699 (m), 629 (w), 597 (w), 554 (w),
1
5
41 (w), 535 (w), 529 (w), 492 (w), 461 (m), 413 (m). H NMR (250.13 MHz,
DMSO-d
6
, 25 °C, TMS): δ = 2.26 (s, 3H, −CH
3
), 6.80-7.01 (m, 3H, aromatic), 7.38-
7
.51 (m, 3H, aromatic), 7.64 (t, 1H, J = 7.25 Hz, aromatic), 7.85 (d, 1H, J = 7.25 Hz,
1
3
aromatic), 8.26 (s, 1H), 11.24 (s, 1H, NH), 11.63 ppm (s, 1H, OH). C NMR (62.90
MHz, DMSO-d ): δ = 16.1 (−CH ), 116.5, 117.5, 118.2, 119.2, 119.7, 125.3, 129.7,
31.1, 132.9, 133.9, 142.2, 147.5, 150.4, 153.9, 157.6, 159.8 (C=O, ester), 162.9 ppm
6
3
1
-5
-1
-1
(
C=O, amide). UV-Vis (in CH
41200), 360 (3400).
3
OH, c = 2.5×10 M, λmax [nm] with ε [M cm ]): 235
(
2
.3. Synthesis of complex [ZnCl(HL)(CH
Complex 1 was synthesized by the reaction of the ligand H
and ZnCl ∙4H O (0.208 g, 1.00 mmol) in methanol by using thermal gradient method
3
OH)0.76(H
2
O)0.24]∙0.24(CH
3
OH) (1)
2
L (0.322 g, 1.00 mmol)
2
2
in a branched tube. The above mentioned amounts of the materials were placed in the
main arm of a branched tube. Methanol was carefully added to fill the arms, the tube
5

was sealed and the reagents containing arm was immersed in oil bath at 60 °C, while
the other arm was kept at ambient temperature. After twenty day, block yellow
crystals were deposited in the cooler arm. Yield 86.4% (0.396 g). Anal. Calc. For
C
19
H
17.47ClN O5.24Zn (MW = 458.40): C, 49.77; H, 3.84; N, 6.11; Zn, 14.26. Found:
2
-1
C, 49.87; H, 3.80; N, 6.02; Zn, 14.20%. FT-IR (KBr, cm ): 3440 (m, br), 3059 (w),
925 (w), 1666 (vs), 1607 (s), 1591 (s), 1560 (s), 1513 (s), 1496 (s), 1468 (s), 1454
m), 1442 (m), 1371 (s), 1356 (m), 1335 (m), 1260 (s), 1218 (m), 1186 (s), 1165 (m),
147 (w), 1124 (m), 1077 (m), 1034 (m), 979 (m), 966 (m), 929 (m), 860 (w), 838
2
(
1
(m), 828 (w), 764 (vs), 727 (w), 700 (m), 670 (m), 643 (m), 591 (m), 580 (m), 537
1
(
m), 529 (w), 485 (m), 441 (w), 426 (m). H NMR (250.13 MHz, DMSO-d
6
, 25 °C,
), 6.92-7.89 (m, 8H, aromatic),
.32 (s, 1H), 12.02 ppm (s, 1H, OH). C NMR (62.90 MHz, DMSO-d ): δ = 16.8
−CH ), 49.0 (−OCH ), 116.5, 117.2, 117.6, 118.0, 119.2, 119.7, 125.5, 129.8, 130.6,
33.4, 133.7, 134.4, 143.6, 147.4, 153.6, 159.2 (C=O, ester), 160.3 ppm (N=C−O).
TMS): δ = 2.28 (s, 3H, −CH
3
), 3.12 (s, 3H, −OCH
3
1
3
8
6
(
3
3
1
-5
-1
-1
UV-Vis (in CH OH, c = 2.5×10 M, λmax [nm] with ε [M cm ]): 220 (45000), 310
3
(38000), 410 (30000).
2
.4. Single crystal X-ray crystallography
Data collection for X-ray structure determination of complex 1 was performed on a
Kuma R four-circle diffractiometer with a Sapphire CCD detector using graphite
monochromatized MoKα radiation. Data were collected at 100(1) K using an Oxford
Cryosystems cooler. Data collection, cell refinement, data reduction and absorption
Pro
corrections were carried out with the Xcalibur R software, CrysAlis [29]. The
structure was solved by direct methods with SHELXS and refined by a full-matrix
2
least-squares technique on F using SHELXL-2016 [30] with anisotropic thermal
6

parameters for the non-H-atoms. The H atoms were found in difference Fourier maps,
but in the final refinement cycles they were repositioned in their calculated positions
and refined using a riding model, with C–H = 0.95–0.99 Å, and with Uiso(H) =
1
.2Ueq(CH) or 1.5Ueq(CH
maps, refined isotropically and then were constrained to ride on their parent atoms
AFIX 3 instruction in SHELXL-2016). Two different ligands share the same
coordination site in the complex molecule: CH OH (with s.o.f. = 0.76) and H
s.o.f. = 0.24) which in the case of H O a methanol molecule is also located beside the
complex molecule. Thus, the crystals of complex 1 are generated from two different
complex molecules, [ZnCl(HL)(CH OH)] and [ZnCl(HL)(H O)]∙CH OH, and the
formula [ZnCl(HL)(CH OH) in the final model
3
), except for the OH atoms, which were located in Fourier
(
3
2
O (with
2
3
2
3
3
OH)0.76(H
2
O)0.24]∙0.24(CH
3
corresponds to the existence of these two complexes, randomly occupying the
crystallographic sites of the unit cell in a 0.76:0.24 ratio. The structure plots were
prepared with DIAMOND [31]. Details of structure refinements are given in Table 1.
Table 1.
2
.4. Catalytic activity of complex 1 in preparation of tetrazole derivatives
The catalytic reactions for production of tetrazole derivatives were carried out in a 25
mL round bottom flask at aerial atmosphere. For this purpose, sodium azide (2 mmol)
was added to a flask and was dissolved in distillated water (5 mL). Then, the target
benzonitrile derivative (1 mmol) was added to the flask. Finally, complex 1
(0.01−0.15 mmol) was added to the flask and the mixture was stirred at 90 °C. The
progress of the reaction was monitored by thin-layer chromatography (TLC) until the
starting benzonitrile was finished. After completion of the reaction, 1 mmol of diluted
HCl was added drop-wise to the reaction mixture and simultaneously, white
7

precipitates were slowly formed. After completion of the precipitation, the reaction
mixture was treated with ethyl acetate (30 mL). The mixture was stirred vigorously
and the precipitates were dissolved in ethyl acetate. The resultant organic layer was
separated and, once again, the aqueous solution was extracted with ethyl acetate. The
organic solvent was evaporated to give the solid compound. The isolated yield of
precipitates in the completed reactions was in the range of 95-96%. The final product
was washed several times with distillated water and recrystallized in ethanol.
Obtained products were characterized by elemental analysis and spectroscopic
methods.
3
. Results and discussion
3
.1. Synthesis and spectroscopic characterization
The hydrazone Schiff base ligand, H L (Scheme 1), was prepared by the reaction of 3-
2
acetylcoumarin with 2-hydroxybenzohydrazide at 1 :1 molar ratio in methanol. The
elemental analysis, FT-IR and NMR spectroscopic data confirmed synthesis of the
ligand. In FT-IR spectrum of H
L (Fig. S1) the broad bands at 3400 and 3178 cm−1
2
are due to phenolic OH and amidic NH groups respectively, which the broadness
indicates they are involved in the hydrogen bond interactions. The bands at 1609,
-1
1
690 and 1735 cm are due to C=N, amidic C=O and esteric C=O functionalities,
1
13
respectively [32]. H and C NMR spectra of the ligand (Figs. S2-S3) in DMSO-d
6
1
confirmed the proposed structure for H
2
L. In H NMR spectrum of H
2
L the broad
singlet peaks at δ 11.63 and 11.24 ppm are due to the phenolic O–H and amidic N–H
groups, respectively. The peak of methyl hydrogen atoms is located at δ 2.26 ppm and
1
3
the peaks at  6.90–7.85 ppm are due to the aromatic hydrogen atoms. In C NMR
spectrum of H
2
L (Fig. S3), the peaks at δ 162.9, 159.8 and 153.9 ppm are related to
8

amidic C=O, esteric C=O and azomethine carbon (C=N) groups, respectively, and the
peak of methyl group (–CH ) appeare at δ 16.1 ppm.
3
Scheme 1.
Complex 1 was synthesized through the reaction of H
2
L with ZnCl
2
∙4H O in
2
methanol solution and the obtained crystalline product was then characterized by FT-
IR, NMR) and single crystal X-ray studies. Comparison of the FT-IR spectra of
complex with free ligand (Fig. S5) provides evidences for coordination of ligand to
the Zn(II) ion in complex 1. In the FT-IR spectrum of complex (Fig. S4) the shifts of
the C=N and esteric C=O vibration bands confirms coordination of azomethine
nitrogen and esteric oxygen atoms to the metal ion [28,[33]]. The bands related to NH
and amidic C=O groups have been eliminated which confirm the coordination of
ligand to the metal ion as mono-negative ligand in the iminol form. The broad peak at
-1
about 3400 cm in the FT-IR spectrum of complex 1 can be attributed to the presence
of OH groups involved in hydrogen bond interactions [34]. The NMR spectrum of
complex 1 (Fig. S6) confirmed the coordination of ligand to the metal ion and its
elemental analysis is also in agreement with the structure obtained from single crystal
X-ray analysis.
The electronic spectrum of H L displays two absorption bands at 235 and 360 nm
2
which based on extinction coefficients, they are assigned to intraligand π → π* (235
nm) and n → π* (360 nm) transitions. The UV-Vis spectrum of complex 1 shows
three bands at 220, 310 and 410 nm which can be attributed to the intraligand and
charge transfer (CT) transitions. The shifts of the absorption bands in the UV-Vis
spectrum of complex relative to bands of free ligand confirm the coordination of
ligand to the metal ion.
9

3
.2. Fluorescence spectroscopy
By considering the luminescence properties of coumarins and zinc complexes, the
luminescence properties of H L and complex 1 were investigated. The luminescence
behavior of H L was investigated by using excitation wavelengths of 330 and 410 nm
at room temperature. As shown in Fig. 1a, when the excitation wavelength was 330
nm, the maximum emission was observed for H L at 413 nm. Moreover, H
2
2
2
2
L
exhibited an intense fluorescent emission at 475 nm upon excitation at 410 nm. The
intensity of emission at 475 nm is higher than the intensity of emission at 410 nm.

These emissions can be assigned to the – transfers in the structure of H
2
L [35].
The emission spectra of complex 1 by excitation wavelengths of 270, 330 and 410 nm
are shown in Fig. 1b. By excitation at 270 and 330 nm, a broad emission peak was
observed at about 460 nm. An intense emission was observed at 475 nm when the
excitation wavelength was 410 nm. According to the previous reports [36], the
1
0
photoluminescence properties of d complexes are usually due to intraligand
emissions and metal-to-ligand or ligand-to-metal charge transfer (MLCT or LMCT)
transitions. The luminescence mechanism in complex 1 is mainly due to the
coordination of the ligand to the Zn(II) and can be attributed to intraligand emissions.
The red shift in the emission spectrum of complex 1 relative to free ligand may be
caused by the coordination of ligand to the metal ion.
Fig. 1.
3
.3. X-ray structure of complex 1
The molecular structure of complex 1 determined by single crystal X-ray analysis is
shown in Fig. 2. Selected bond lengths and angles around the metal ion are presented
in Table 2. Diffraction studies indicated that the obtained block yellow crystals are
1
0

crystallized in monoclinic system (P2
mononuclear Zn(II) complex. In complex 1, the Zn(II) ion is five coordinated, as
ZnClO N], with a nitrogen and two oxygen atoms provided by coumarin-hydrazone
1
/c space group) and they are related to a neutral
[
3
ligand, a chloro anion and one oxygen atom from coordinated methanol or water
molecule. The coordination geometry around the Zn(II) in complex 1 can be
described as distorted square pyramidal geometry with the τ value of 0.298 (where τ =
the difference between the two largest angles/60; τ = 1 for ideal trigonal–bipyramidal
and τ = 0 for ideal square pyramidal). Three ONO-donor atoms of the ligand and the
oxygen atom of coordinated methanol/water molecule form the equatorial plane of
square pyramidal geometry and the axial position is filled by chloro ligand at a
distance of 2.2469(9) Å. Relative to this plane, the zinc atom is displaced toward the
apical chloro ligand by 0.636 Å. The ONO-donor ligand forms a five-membered and a
six membered chelate rings with the bite angles of 78.65(5) (O3–Zn1–N1) and
8
3.93(4) (O1–Zn1–N1), respectively. In complex 1 the Zn–O, Zn–Cl and Zn–N
bonds are in the normal range reported for Zn(II) complexes [37].
Comparing the bond lengths of ester C=O bond in the crystal structure of complex 1
(1.2271(16) Å) with free 3-acetylcoumarin (1.207–1.211 Å) [38] indicates that it is
longer in complex 1 which can be attributed to the coordination of oxygen to the
metal core. This is also in agreement with the red shift of υ(C=O) in the FT-IR
spectrum of complex 1 in comparison to the free ligand. Moreover, the N2−C12
(1.3382(17) Å) and O3−C12 (1.2742(17) Å) bond lengths in complex 1 are
respectively shorter and longer than the bond length of similar group in the free
hydrazone ligands (C−N ≈ 1.35 Å and C=O ≈ 1.23 Å) [39]. Moreover, the hydrogen
atom of phenolic OH group is involved in intramolecular hydrogen bond interactions
with the amidic nitrogen atom (N2). These observations confirm the coordination of
1
1

ligand to metal ion as mononegative ligand by elimination of the amidic hydrogen
atom.
There are some strong and directed intermolecular hydrogen bond interactions in the
crystal structure of the complex 1 (Table 3, Fig. 3). The hydrogen atoms of the
coordinated methanol or water molecules form strong hydrogen bonds with the
oxygen atom of the phenolic ring. As a result, 1D polymeric chain is formed by this
interaction (see Fig. 3). Moreover, the uncoordinated methanol molecule is stabilized
in the crystal packing by forming O−H∙∙∙O interaction with the coordinated water
molecule. Also, there are strong hydrogen bond interactions between chloro ligand
and hydrogen atoms from the neighboring groups (see Fig. S7).
Fig. 2.
Fig. 3.
Table 2.
Table 3.
3
.4. Thermal gravimetric analysis of complex 1
Thermogravimetric analysis (TGA) was performed on a crystalline sample of
complex 1 from ca. 30 to 800 °C at a rate of 10 °C/min under nitrogen atmosphere
(
see Fig. S8). The results of the thermal analysis demonstrate that the methanol and
water molecules were eliminated at the first step in the range of 60-150 °C (observed
.58%, calculated 7.92%). In the next step the chloro ligand is eliminated in the range
8
of 250-340 °C (observed 8.34%, calculated 7.74%). Finally, the Schiff base ligand is
removed in the range of 350–700 °C to give the final product which is estimated to be
zinc oxide.
1
2

3
.5. Catalytic activity of complex 1 in preparing tetrazoles
By considering the widespread applications of tetrazole derivatives in various fields
12-20], we investigated the catalytic activity of complex 1 in nitrile–azide
[
cycloaddition reaction to produce tetrazole based compounds (see Scheme 2). For this
purpose, two-component reaction between benzonitrile and sodium azide was
selected as a model reaction to investigate the catalytic activity of complex 1 in
preparing tetrazoles. All of the reactions were done in water as a green solvent. In the
preliminary step, benzonitrile was treated with sodium azide at 90 °C in the absence
of complex 1 which the results revealed that the presence of catalyst is essential for
this reaction. When complex 1 (0.05 mmol) was added to the reaction flask, the
reaction was started and the progress of the reaction was monitored by TLC until
benzonitrile was consumed. The results of catalytic studies indicated that the reaction
was completed after 7 h at 90 °C. By decreasing temperature to 70 and 50 °C, the
conversion was considerably decreased in 7 h because, by TLC, considerable amount
of benzonitrile was observed in the reaction mixture which indicated the reaction was
not completed after 7 h. This matter indicates that temperature is one of the effective
parameters in the activity of this catalytic system. The effect of the amount of catalyst
was investigated by using 0.02, 0.05, 0.10 and 0.15 mmol of complex 1. The reaction
was completed in 7, 6 and 5.5 h in the case of 0.05, 0.10 and 0.15 mmol of complex
1
, respectively. By using 0.02 mmol of complex, the reaction did not finish after 12 h.
These results indicate that the reaction was completed in shorter time by increasing
the amount of catalyst. However, there is not a linear relationship between the amount
of catalyst and the catalytic activity. By considering the results, 0.05 mmol of
complex 1 was considered as the optimum amount of complex for this catalytic
reaction.
1
3

Scheme 2.
To explore the potential of complex 1 in the catalytic production of tetrazoles, 4-
nitrobenzonitrile, 4-methoxybenzonitrile and 4-cyanopyrdine were selected as
substrates. The catalytic reactions were performed at 90 °C in the presence of 0.05
mmol of complex 1 and the summary of results is collected in Table 4. Generally,
these substrates were also involved in the cyclization reaction with sodium azide in
the presence of complex 1. However, considering the results indicated that the
reaction was completed in a shorter time in the case of 4-methoxybenzonitrile. On the
other hand, the reaction was completed after about 9 hours in the case of 4-
nitrobenzonitrile and 4-cyanopyrdine. This matter indicates that the electronic
demands of the group connected to the CN moiety have considerable effect on the
activity of this catalytic system. The substrate with electron donating substituent
showed higher activity in comparison to the substrates containing electron
withdrawing groups. Low conversion of substrates containing electron withdrawing
groups can be attributed to the low coordination ability of these substrates to the
metal ion which decreases the speed of the catalytic reaction.
Table 4.
In general, the complete mechanism of the reaction is not fully clear. However, on the
basis of the obtained results and also by considering the reported mechanisms for the
synthesis of tetrazoles [40], it is clear that the key step in this reaction is the
cyclization reaction between azide and benzonitrile derivatives which are coordinated
to the Zn center. A general mechanism for this catalytic reaction is shown in Scheme
3
. In the first step, the azide anion can coordinate to the metal ion by elimination of a
1
4

NaCl and the benzonitrile group can coordinate to the vacant coordination site of the
Zn(II) by the nitrogen atom. In this step, the cycloaddition reaction can take place
between the coordinated azide and bezonitrile to the Zn(II) ion to generate the
tetrazole ring. After cycloaddition, the obtained tetrazole ring is replaced by the attack
of next azide ligand and the tetrazole product is released as sodium salt. The obtained
Zn(II)-hydrazone-azide complex can involve in the next cycle to continue the
catalytic reaction. By adding hydrochloric acid to the reaction mixture, the obtained
sodium salt of tetrazole reacts with HCl to form NaCl and neutral tetrazole compound
which is insoluble in the aqueous solution and precipitates in the reaction media.
Scheme 3.
Conclusion
In summary, a Zn(II) complex of coumarin-hydrazone ligand was synthesized and
characterized by elemental analysis, spectroscopic methods and single crystal X-ray
studies. The photoluminescent behavior of complex was also studied which revealed
that complex 1 has high photoluminescent properties. The catalytic properties of the
complex in cyclization reactions were investigated in preparing tetrazoles from the
reaction of sodium azide and benzonitriles. The catalytic studies indicated that
complex 1 is an active catalyst for green synthesis of tetrazole compounds and the
electronic properties of substrate has considerable effect on the activity of this
catalytic system.
Acknowledgments
The authors are grateful of Islamic Azad University and the University of Zanjan for
their financial assistance.
1
5

Appendix A. Supplementary material
CCDC 1864764 contains the supplementary crystallographic data for complex 1.
These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with
this article can be also found, in the online version, at doi:
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1
8

Fig. 1. Emission profile of a) H L; b) complex 1 in methanol
2
Fig. 2. Molecular structure of complex 1 with atom numbering scheme. The Green
dashed lines show hydrogen bond interactions.
Fig. 3. 1D polymeric chain obtained by O−H∙∙∙O hydrogen bond interactions
Scheme 1. Synthesis pathway and molecular structure of H L
2
Scheme 2. Catalytic azide–nitrile cycloaddition reaction in the presence of complex 1.
Scheme 3. Proposed general mechanism for catalytic cycloaddition reaction of azide
and benzonitrile derivatives in the presence of complex 1.
Table 1. Crystal data and structure refinement parameters for 1
Table 2. Selected bond lengths (Å) and angles () for complex 1
Table 3. Hydrogen bonding parameters (Å,) for complex 1
Table 4. Comparing the catalytic activity of some nitriles in cycloaddition reaction in
the presence of complex 1
1
9

Table 1. Crystal data and structure refinement parameters for 1
Complex 1
net formula
C
18.77
H
16.53ClN25Zn·0.235(CH O)
4
−1
M
r
/g mol
458.40
crystal size/mm
T/K
0.56 × 0.35 × 0.29
100
Radiation
Diffractometer
crystal system
Space group
Crystal shape, color
a/Å
Mo Kα
KM4CCD
Monoclinic
P2 /c
1
Block, yellow
7.460(3)
b/Å
c/Å
18.545(4)
13.839(3)
9
5.16(3)
3
V/Å
Z
1906.8(10)
4
1.597
1.46
−3
calc. density/Mg m
μ/mm
−1
F(000)
θrange
h,
k,
l
937.4
3.0–36.9
−12 →1 2
−29 →3 0
−23 →2 2
0.054
0.046
0.132
1.00
R
int
R(Fo b2 s
)
R
S
w
(F )
Abs. correction
Analytical
Mixed
31193
9032
7397
286
hydrogen refinement
measured reflections
independent reflections
reflections with I > 2σ(I)
Parameters
Restraints
Δρmax, Δρmin (e Å )
0
−3
0.70, -1.11
2
0

Table 2. Selected bond lengths (Å) and angles () for complex 1
Bond
Length/Å
2.2469(9)
2.0944(12)
2.0804(11)
2.0174(12)
2.077(5)
1.909(18)
1.2271(16)
1.2742(17)
1.3060(17)
1.3382(17)
1.3836(16)
Angles
deg/º
Zn1—Cl1
Zn1—N1
Zn1—O1
Zn1—O3
Zn1—O1M
Zn1—O1W
O1—C1
O3—C12
N1—C10
N2—C12
N1—N2
O1—Zn1—Cl1
O1—Zn1—N1
O1—Zn1—O3
O1—Zn1—O1M
O3—Zn1—Cl1
O3—Zn1—N1
O3—Zn1—O1M
O1M—Zn1—Cl1
O1M—Zn1—N1
N1—Zn1—Cl1
O1W—Zn1—Cl1
O1W—Zn1—O1
O1W—Zn1—N1
O1W—Zn1—O3
100.06(4)
83.93(4)
151.67(5)
84.25(16)
107.58(4)
78.65(5)
91.82(15)
110.21(16)
133.77(16)
115.82(4)
104.3(6)
85.5(6)
139.7(6)
93.4(6)
2
1

Table 3. Hydrogen bonding parameters (Å,) for complex 1
D—H···A
D—H
H···A
D···A
D—H···A
O4—H4···N2
C17—H17···O1
O1M—H1M···O4
C1M—H1MC···O3
O2M—H2M···Cl1
O1W—H1WA···O4
0.84
0.95
0.85
0.98
0.84
0.86
0.86
1.79
2.51
2.01
2.46
2.24
2.04
1.83
2.5388 (16)
3.4151 (19)
2.769 (5)
3.111 (3)
2.997 (6)
147
160
148
124
149
160
167
i
ii
iii
ii
2.863 (19)
2.677 (19)
O1W—H1WB···O2M
Symmetry codes: (i) −x+1, y−1/2, −z+3/2; (ii) −x+1, y+1/2, −z+3/2; (iii) x+1, y, z.
2
2

Table 4. Comparing the catalytic activity of some nitriles in cycloaddition reaction in
the presence of complex 1a
Entry
1
Substrate
Product
Time (h)
7
2
3
6.5
9
4
9.5
a
Reaction condition: solvent = water; temperature = 90 °C; complex 1 = 0.05 mmol.
2
3

Scheme 1. Synthesis pathway and molecular structure of H L
2
2
4

Scheme 2. Catalytic azide–nitrile cycloaddition reaction in the presence of complex
1
.
2
5

Scheme 3. Proposed general mechanism for catalytic cycloaddition reaction of azide
and benzonitrile derivatives in the presence of complex 1.
2
6

Fig. 1. Emission profile of a) H L; b) complex 1 in methanol
2
2
7

Fig. 2. Molecular structure of complex 1 with atom numbering scheme. The
Green dashed lines show hydrogen bond interactions.
2
8

Fig. 3. 1D polymeric chain obtained by O−H∙∙∙O hydrogen bond interactions
2
9