Zn (II)-Schiff base covalently anchored to CaO@SiO2: A hybrid nanocatalyst for green synthesis of 4H-pyrans

Article abstract of DOI:10.1002/aoc.6394

In this study, the Zn (II)-Schiff base covalently anchored onto the surface of CaO@SiO₂ nanoparticles. The structure of this hybrid nanomaterial (CaO@SiO₂-NH₂-Sal-Zn) was characterized using the analytical techniques such

Full text of DOI:10.1002/aoc.6394

Received: 16 May 2021
DOI: 10.1002/aoc.6394
Revised: 14 July 2021
Accepted: 14 July 2021
R E S E A R C H A R T I C L E
Zn (II)-Schiff base covalently anchored to CaO@SiO2: A
hybrid nanocatalyst for green synthesis of 4H-pyrans
Fatemeh Sameri
| Mohammad Ali Bodaghifard
| Akbar Mobinikhaledi
Department of Chemistry, Faculty of
Science, Arak University, Arak, Iran
Abstract
In this study, the Zn (II)-Schiff base covalently anchored onto the surface of
CaO@SiO2 nanoparticles. The structure of this hybrid nanomaterial
(CaO@SiO2-NH2-Sal-Zn) was characterized using the analytical techniques
such as Fourier transform infrared spectroscopy, field emission-scanning
electron microscopy, X-ray powder diffraction, energy-dispersive X-ray
spectroscopy, transmission electron microscopy, thermogravimetric analysis,
and inductively coupled plasma atomic mass spectrometry. The catalytic
activity of this organic–inorganic hybrid nanostructure was investigated
through the green synthesis of 4H-pyran derivatives via one-pot three-
component condensation reaction which high yields of desired products
obtained under green media. The reusability experiments confirmed the
stability and high performance of this hybrid nanomaterial under the reaction
conditions. The retrievability of this catalyst was proved by recycling six times
in reactions without significant loss in its activity. Green conditions, use of
available materials, easy work-up procedure, and high yield of products, low
reaction times, and simple purification are some advantages of this protocol
over earlier ones.
Correspondence
Akbar Mobinikhaledi, Department of
Chemistry, Faculty of Science, Arak
University, Arak 38156-8-8349. Iran.
Email: a-mobinikhaledi@araku.ac.ir;
akbar_mobini@yahoo.com
Funding information
Arak University
K E Y W O R D S
4H-pyran, catalyst, chicken-egg shells waste, Schiff base
1 | INTRODUCTION
developments in green chemistry is the widespread
application of heterogeneous nanocatalysts in MCRs[6–9]
According to the principals of green chemistry, the use of
green reaction conditions including nontoxic stating
materials and elimination of hazardous materials has
become a huge challenge in academic and industry
aspects.[1]
Multicomponent reactions (MCRs) are a green and
powerful strategy for the synthesis of different biologi-
cally active organic compounds. These reactions have
many useful advantages over traditional multistep
processes including short reaction times, low costs, high
efficiency, atom economy, and environmentally
friendliness.[2–5] Furthermore, one of the most critical
because homogeneous catalysts suffer from separation
difficulty over reaction mixture. Heterogeneous catalysts
compared to homogeneous ones have several advantages
like milder experimental conditions, simple separation,
easy recycling, and recovery of catalyst.[10–12] Conse-
quently, the improvement of conventional approaches in
an eco-friendly manner to prepare the heterogeneous
nanocatalysts is in great demand.[13,14]
In recent years, the considerable attention to
developing of green technologies in organic synthesis led
to biosynthesis of nanoparticles using environmentally
benign raw materials and biomolecular surface
Appl Organomet Chem. 2021;e6394.
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© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6394

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SAMERI ET AL.
functionalization.[12,15] CaO obtained from waste
chicken-egg shells is one of the feedstock for synthesis of
green catalysts, which has several advantages including
the ability to form a nanoporous structure, biodegrad-
able, recyclable, biocompatible, high catalytic activity,
and the most positive point in reducing environmental
pollution associated with the wrong disposal of waste
chicken-egg shells.[16,17]
Surface-functionalized natural-based nanoparticles
have emerged as suitable supports for synthesis of
organic–inorganic hybrid catalysts. The surface of CaO
nanoparticles can be easily functionalized with different
organic groups to protect the inorganic solids core from
chemical reactions and improve the behavior and overall
performance. The immobilization of Schiff base onto the
surface of CaO nanoparticles may produce such a hybrid
nanoparticle, which can be used as a green catalyst for
synthesis of several organic compounds.
Based on the above considerations and also due to our
interests in synthesis of heterocyclic compounds,[21–23]
we wish to report an environmentally benign procedure
for synthesis of some dihydropyrano[c]chromenes,
dihydropyrano[3,2-c]pyrazoles and tetrahydrobenzo[b]
pyrans, via one-pot three-component reactions of related
materials in the presence of a novel prepared CaO@SiO2-
NH2-Sal-Zn hybrid nanomaterial as an efficient and green
catalyst (Scheme 1).
2 | EXPERIMENTAL
2.1 | General information
All reagents and solvents were purchased from Merck
and Fluka companies. Nuclear magnetic resonance
(NMR) spectra were recorded on
a
300-MHz
Recently, the use of multicomponent reactions as a
suitable method for synthesis of 4H-pyran motifs has also
gained great attention due to their biological and
pharmacological activities. These compounds perform
diverse therapeutic activities, such as anti-inflammatory,
spasmolytic, antimicrobial, anticoagulant, anticancer,
and anti-anaphylactic properties.[18–20]
Brucker-Avance spectrometer using tetramethylsilane
(TMS) as an internal standard. IR spectra were obtained
by ALPHA-BRUKER spectrophotometer using KBr
pellets. The particle size and external morphology of the
nanoparticles as well as their components were assayed
by field emission-scanning electron microscopy
(FE-SEM) analyses. The energy-dispersive X-ray (EDX)
SCHEME 1 Synthesis of 4H-pyrans in
the presence of CaO@SiO2-NH2-Sal-Zn as a
catalyst

SAMERI ET AL.
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and the X-ray powder diffraction (XRD) were carried out
by MIRA III and PW1730 instruments, respectively.
Transmission electron microscopy (TEM) was performed
using a Philips EM208S 100KV. Thermogravimetric anal-
ysis (TGA) curves were recorded using a Q600TA ana-
lyzer and inductively coupled plasma atomic mass
spectrometry (ICP-MS) carried out by Agilent 7500.
2.5 | Preparation of the shift base-
functionalized nanoparticles (CaO@SiO2-
NH2-Sal)
CaO@SiO2-NH2
nanoparticles
(1 g)
and
of
salicylaldehyde (2 mmol) were dispersed in dry toluene
(50 ml) by ultrasonic vibrations. The reaction mixture
was refluxed for 24 h under N2 atmosphere. The solid
CaO@SiO2-NH2-Sal was filtered, washed with toluene,
and dried under vacuum at 80ꢀC.
2.2 | Synthesis of silica-coated CaO
nanoparticles (CaO@SiO2)
Nano-CaO particles were prepared according to the
reported method in the literature from eggshells
waste.[24] Silica-coated CaO nanoparticles were prepared
by the Stober method.[22] Briefly, nano-CaO (1 g) was
sonicated in a mixture of ethanol (40 ml), deionized
water (6 ml), and 1.5 ml of 25 wt% concentrated aqueous
ammonia for 20 min at room temperature. Then, 1.4 ml
of tetraethyl orthosilicate (TEOS) was added and the
resulting mixture stirred for 22 h at room temperature
under N2 atmosphere. Finally, CaO@SiO2 core-shell
2.6 | Preparation of Zn (II)-decorated
CaO@SiO2-NH2-Sal nanoparticles
(CaO@SiO2-NH2-Sal-Zn)
A
mixture of CaO@SiO2-NH2-Sal (0.5 g), ZnCl2
(0.5 mmol) and ethanol (20 ml) in a dry flask was put in
an ultrasonic bath for 20 min and refluxed for 24 h under
N2 atmosphere. Then, the catalyst was separated from
the solution using a centrifuge, washed with ethanol, and
dried in the vacuum dryer at 80ꢀC for 24 h.
nanoparticles were successively separated using
a
centrifuge, which washed several times by ethanol and
deionized water and dried for 24 h by vacuum dryer.
2.7 | General procedure for the synthesis
of 4H-pyrans in the presence of the hybrid
nanocatalyst (CaO@SiO2-NH2-Sal-Zn)
2.3 | Synthesis of chloropropyl-modified
silica-coated CaO nanoparticles
(CaO@SiO2-cl)
A
mixture
of
malononitrile
(1 mmol),
4-hydroxycoumarin (1 mmol) /or 3-methyl-1-phenyl-
2-pyrazolin-5-one (1 mmole) /or 5,5-dimethyl-
cyclohexan-1,3-dione (1 mmol) and aromatic aldehyde
The CaO@SiO2 (1 g) was dispersed in dry toluene
(50 ml) by ultrasonic vibrations for 20 min. Then,
(3-chloropropyl)trimethoxysilan (1.8 ml, 10 mmol) was
added dropwise to the suspension and the mixture
refluxed for 24 h under N2 atmosphere. The resulted
product was separated by centrifuge, washed several
times with toluene, and dried under vacuum conditions
at 80ꢀC.
(1 mmol) and CaO@SiO2-NH2-Sal-Zn (0.01 g) in
a
round-bottomed flask was mechanically stirred at room
temperature for an appropriate time (Table 2). After com-
pletion of the reaction, which was confirmed by thin-
layer chromatography (TLC), hot ethanol (10 ml) was
added and stirring continued at room temperature for
10 min. The catalyst was separated easily by centrifuge
and the solution cooled to room temperature to precipi-
tate the product. The solid product was filtered and rec-
rystallized from EtOH to obtain pure product.
2.4 | Synthesis of triethylenetetramine-
functionalized silica-coated CaO
nanoparticles (CaO@SiO2-NH2)
3 | RESULTS AND DISCUSSION
Dispersion of CaO@SiO2-Cl (1 g) in dry toluene (50 ml)
was carried out by ultrasonic waves for 30 min. Subse-
quently, triethylenetetramine (2.1 ml, 15 mmol) and
triethylamine (0.1 ml) were added dropwise to the reac-
tion mixture and refluxed for 24 h under N2 atmosphere.
The resulted solid product was separated from the solu-
tion using a centrifuge, washed with toluene, and finally
dried at 80ꢀC.
3.1 | Preparation and characterization of
CaO@SiO2-NH2-Sal-Zn catalyst
Synthesis of Zn (II)-Schiff base covalently stabilized onto
the surface of CaO@SiO2 nanoparticles was carried out
carefully as shown in Scheme 2. Firstly, the silica-coated
CaO nanoparticle was prepared from clean eggshells

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SAMERI ET AL.
SCHEME 2 The schematic pathway for
preparation of CaO@SiO2-NH2-Sal-Zn
waste using our previously reported method.[22] Subse-
quently, (3-chloropropyl)trimethoxysilan was reacted
with silica-coated CaO to form CaO@SiO2-Cl.
CaO@SiO2-NH2 was prepared by the simple procedure
between triethylenetetramine and CaO@SiO2-Cl. In the
next step, the reaction of salicylaldehyde with
CaO@SiO2-NH2 afforded CaO@SiO2-NH2-Sal, which
then converted to CaO@SiO2-NH2-Sal-Zn by treatment
with ZnCl2. The structure of CaO@SiO2-NH2-Sal-Zn was
established by Fourier transform infrared spectroscopy
(FT-IR), FE-SEM, TEM, energy-dispersive X-ray spectros-
1481 cmꢁ1 is attributed to asymmetric C-O stretching of
carbonate ion. CaO@SiO2 spectrum shows distinguished
vibration bands in FT-IR (Figure 1b). The peaks observed
around 968, 876, and 462 cmꢁ1 are attributed to symmet-
ric stretching, in-plane bending, and rocking mode of the
Si–O–Si groups, respectively. The twisting vibration mode
of adsorbed H–O–H in the silica shell is seen at
1641 cmꢁ1. The stretching vibration mode of Si-OH is
apparent in the range 3200–3500 cmꢁ1.[[22]] The weak
adsorption bands at 2862 and 2936 cmꢁ1 are related to
the symmetric and asymmetric stretching modes of C–H
bonds which indicating the definite attach of
(3-chloropropyl)trimethoxysilan groups (Figure 1c). The
stretching vibration of amine group in CaO@SiO2-NH2 is
observed at 3173 and 3287 cmꢁ1 (Figure 1d). The bending
vibrations of N–H groups appeared at 1597 cmꢁ1, and the
band at 1632 cmꢁ1 proved the existence of imine group
(Figure 1e). The two absorption bands around 546 and
511 cmꢁ1 indicate the existence of Zn-O and Zn-Cl vibra-
tion, respectively, which overlapped with vibration band
for CaO@SiO2 (Figure 1f).[25,26] On the other hand, the
shifting of C=N vibrational band to lower frequency
(1625 cmꢁ1) in the spectrum of CaO@SiO2-NH2-Sal-Zn
copy
(EDS),
X-ray
diffraction
(XRD),
and
thermogravimetry analysis (TGA), and inductively
coupled plasma atomic mass spectrometry (ICP-MS)
techniques.
Figure
1
shows FT-IR spectra of (a) CaO,
(b) CaO@SiO2, (c) CaO@SiO2-Cl, (d) CaO@SiO2-NH2,
(e) CaO@SiO2-NH2-Sal, and (f) CaO@SiO2-NH2-Sal-Zn.
In Figure 1a, the strong vibration band around 529 cmꢁ1
shows the characteristic adsorption of Ca-O from pure
CaO.[24] The broad absorption bands around 3400–
3600 cmꢁ1 assigned to OH stretching vibration and physi-
cally adsorbed water (H2O).[16] The weak band at

SAMERI ET AL.
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FIGURE 1 Fourier
transform infrared spectroscopy
(FT-IR) spectra for (a) CaO,
(b) CaO@SiO2, (c) CaO@SiO2-
Cl, (d) CaO@SiO2-NH2,
(e) CaO@SiO2-NH2-Sal, and (f)
CaO@SiO2-NH2-Sal-Zn
FIGURE 2 Field emission-scanning electron microscopy (FE-SEM) (a) and transmission electron microscopy (TEM) (b) images of
CaO@SiO2-NH2-Sal-Zn hybrid nanomaterials
(Figure 1f) is in support of zinc complex band forma-
tion.[27] These spectra clearly confirm the structure of
CaO@SiO2-NH2-Sal-Zn hybrid nanomaterials.
The EDS analysis for CaO@SiO2-NH2-Sal-Zn nano-
material is presented in Figure 3. The presence of Si, O,
Ca, N, C, and Zn is clearly matched with the structure for
CaO@SiO2-NH2-Sal-Zn nanostructure. Also, the increas-
ing intensity of Si compared to that of Ca confirms that
CaO nanoparticles were trapped by the SiO2 shell.
The X-ray diffraction (XRD) patterns were
implemented to investigate structural properties of CaO
and CaO@SiO2-NH2-Sal-Zn (Figure 4). The crystalline
structure of CaO nanoparticles caused diffraction peaks
at 2θ = 32.50ꢀ, 37.65ꢀ, 54.16ꢀ, 64.54ꢀ, and 67.71ꢀ which
are attributed to the Miller indices (hk l) at (111), (200),
(202), (311), and (222), respectively.[24] The position and
The FE-SEM technique and TEM were employed for
studying the size and morphology of CaO@SiO2-NH2-
Sal-Zn (Figure 2). As shown in Figure 2, these
nanoparticles are spherical with an average size of about
38–44 nm. The observed agglomeration in the FE-SEM
images is due to high-specific surface area of
nanoparticles, which led to aggregation. TEM analysis
shows dark regions coated by bright outer shells, which
indicates silica as a shell is successfully anchored to the
CaO as a core.

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SAMERI ET AL.
FIGURE 3 The energy-dispersive X-ray spectroscopy (EDS) analysis of CaO@SiO2-NH2-Sal-Zn hybrid nanomaterials
FIGURE 4 The X-ray diffraction (XRD)
patterns of (a) CaO nanoparticles and
(b) CaO@SiO2-NH2-Sal-Zn hybrid
nanomaterials
intensity of all peaks agree with the standard calcite CaO
sample (JCPDS card no.77-9574). The broad peak around
2θ = 18ꢀ to 24ꢀ can be indexed to amorphous silica phase
of CaO@SiO2-NH2-Sal-Zn (Figure 4b).[28] The average
size of the crystals can be estimated by the (200) XRD
peak and applying Scherrer's formula (D = 0.9λ/β cos θ).
In this formula, D denotes the average crystalline size, λ
represents the wavelength (200) of the incident X-ray, β
is the full width of the (200) powder peak by the radian
subtended at half maximum intensity, and θ corresponds
to the Bragg diffraction angle of the (200) peak in
degrees. The peak at 2θ = 37.65ꢀ (200) is selected to cal-
culate the crystallite size. The average crystallite size of
CaO@SiO2-NH2-Sal-Zn is about 35 nm by using
Scherrer's equation, which is according to the particle
sizes obtained from FE-SEM data.
TGA and derivative thermogravimetry (DTG) of the
CaO@SiO2-NH2-Sal-Zn nanoparticles are presented in
Figure 5. The thermal decomposition of this nanostruc-
ture shows two weight loss steps. The first step, a weight
loss of about 6% from room temperature to nearly 150ꢀC,
is due to the removal of organic solvents, water and sur-
face hydroxyl groups (Figure 5). The second step weight
loss (27.5%), beyond 150ꢀC to nearly 649ꢀC, is attributed
to the decomposition of the organic tags. Finally, after
649ꢀC, decomposition of calcium carbonate to CaO and
CO2 is observed. Based on this mass loss, the attaching of
NH2-Sal-Zn on the surface of CaO@SiO2 is confirmed.

SAMERI ET AL.
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FIGURE 5 Thermo-
gravimetic analysis (TGA/DTG)
of CaO@SiO2-NH2-Sal-Zn
TABLE 1 The optimization of reaction conditions for synthesis of 2-amino-4-(4-nitrophenyl)-5-oxo-4H,5H-pyrano[3, 2-c]chromene-
3-carbonitrile
Entry
Catalyst (g)
Solvent
Temp (ꢀC)
Time (min)
Yield (%)[a]
1
-
-
25
25
25
25
50
100
25
25
25
25
25
25
25
150
30
20
10
10
10
15
60
60
30
30
20
60
trace
88
95
97
97
95
97
80
82
70
75
92
72
2
CaO@SiO2-NH2-Sal-Zn)0.005)
CaO@SiO2-NH2-Sal-Zn (0.008)
CaO@SiO2-NH2-Sal-Zn (0.01)
CaO@SiO2-NH2-Sal-Zn (0.01)
CaO@SiO2-NH2-Sal-Zn (0.01)
CaO@SiO2-NH2-Sal-Zn (0.02)
CaO (0.01)
-
3
-
4
-
5
-
6
-
7
-
8
-
9
CaO@SiO2 (0.01)
-
10
11
12
13
CaO@SiO2-NH2-Sal-Zn (0.01)
CaO@SiO2-NH2-Sal-Zn (0.01)
CaO@SiO2-NH2-Sal-Zn (0.01)
CaO@SiO2-NH2-Sal-Zn (0.01)
H2O
EtOAc
EtOH
CHCl3
Note: Isolated yield.
aReaction conditions: malononitrile (1, 1 mmol), 4-nitrobenzaldehyde (2, 1 mmol) and 4-hydroxycoumarin (3, 1 mmol).
3.2 | Synthesis of 4H-pyrans catalyzed by
CaO@SiO2-NH2-Sal-Zn
The different solvents (EtOH, CHCl3, EtOAc, and H2O at
room temperature) as well as solvent-free medium,
temperatures, and catalyst amounts were examined on
model reaction, and the results are shown in Table 1.
The solvent-free medium, room temperature, and use
of 10 mg of hybrid catalyst (CaO@SiO2-NH2-Sal-Zn)
serve as the best condition with respect to the green
nature and clean work-up procedure for this synthesis
(Table 1, entry 4).
In order to determine the best results in terms of reaction
temperature, solvent, and catalyst amount, initially, the
reaction of malononitrile (1), 4-nitrobenzaldehyde (2)
and 4-hydroxycoumarin (3) as a model reaction was
examined for synthesis of 2-amino-4-(4-nitrophenyl)-
5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile
6f.

8 of 12
SAMERI ET AL.
These results encouraged us to investigate the scope
times and without formation of side products (Table 2).
The characterization data for desired products are pres-
ented in the Supporting information.
The comparison of the catalytic efficiency of
CaO@SiO2-NH2-Sal-Zn in the present work with previ-
ously reported methods is shown in Table 3. The pres-
ented data confirm that the CaO@SiO2-NH2-Sal-Zn is a
of this method for synthesis of 4H-pyran, in the presence
of various aldehydes with both electron-withdrawing and
electron-releasing substituents under optimized condi-
tion. It was found that the reactions for all of the various
substrates proceed efficiently to obtain the desired prod-
ucts in good to excellent yields in relatively short reaction
TABLE 2 Synthesis of the 4H-pyran derivatives in the presence of CaO@SiO2-NH2-Sal-Zn
Product Time (min) Yield (%)[a] M.P (Obs.) (ꢀC) M.P (Lit.) (ꢀC)
Entry Ar/Aldehyde
1
C6H5
6a
6b
6c
30
25
20
25
20
10
20
25
30
20
25
35
30
25
30
25
30
15
15
20
15
20
25
40
25
30
25
20
25
20
20
25
93
91
92
92
90
97
89
91
90
94
88
90
92
90
92
89
92
95
89
93
94
95
90
85
93
93
83
91
92
93
95
95
267–269
269–271
280–281
250–252
230–232
260–262
293–295
273–274
258–260
245–247
246–248
223–225
263–264
138–139
170–172
141–143
158–161
170–172
183–186
165–167
185–187
187–189
196–197
235–237
191–193
225–227
216–218
282–283
210–214
213–215
260–262
209–211
271–273[30]
273–274[30]
283–284[30]
255–256[30]
229–231[29]
263–264[31]
295–297[32]
274–276[33]
262–263[34]
248–250[31]
250–252[35]
228–230[36]
263–265[30]
-
170–173[37]
145–146[18]
161–163[38]
169–171[39]
186–187[40]
164–167[38]
188–193[38]
186–190[37]
205–207[41]
236–238[42]
193–194[42]
228–230[43]
217–219[43]
280–282[44]
210–214[22]
214–215[45]
265–266[46]
209–211[22]
2
2-ClC6H4
3
2,4-Cl2C6H3
2-NO2C6H4
4
6d
6e
6f
5
3-NO2C6H4
6
4-NO2C6H4
7
2-BrC6H4
6 g
6 h
6i
8
3-BrC6H4
9
3-OHC6H4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
3-OEt-4-OHC6H3
4-FC6H4
6j
6 k
6 L
6 m
6n
7a
7b
7c
2,5-(OMe)2C6H3
2-naphthaldehyde
3-phenoxybenzaldehyde
C6H5
2-ClC6H4
3-ClC6H4
4-ClC6H4
7d
7e
7f
4-FC6H4
3-BrC6H4
2,4-Cl2C6H3
3-OEt-4-OHC6H3
2-naphthaldehyde
terephthalaldehyde
[1,10-biphenyl]-4-carbaldehyde
C6H5
7 g
7 h
7i
7j
7 k
8a
8b
8c
2-ClC6H4
2,3-Cl2C6H3
4-FC6H4
8d
8e
8f
1-naphthaldehyde
Pyridine-4-carbaldehyde
2,20-(butane-1,4-diylbis (oxy)
dibenzaldehyde
8 g
33
[1,10-biphenyl]-4-carbaldehyde
8 h
30
83
163–166
-
Note: Isolated yield.
aReaction conditions: malononitrile (1, 1 mmol), arylaldehyde (2, 1 mmol), 4-hydroxycoumarin (3, 1 mmol) /or 3-methyl-1-phenyl-2-pyrazolin-5-one (4, 1
mmole) /or 5,5-dimethyl-cyclohexan-1,3-dione (5, 1 mmol), and catalyst (0.01 g).

SAMERI ET AL.
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TABLE 3 The comparison of CaO@SiO2-NH2-Sal-Zn with other catalysts reported in the literatures for synthesis of 2-amino-
4-(4-nitrophenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile
Entry
Catalyst
Condition
H2O/r.t
Time (min)
Yield (%)[a]
Ref.
[47]
1
2
3
4
5
6
7
8
Fe3O4@GO-naphthalene-SO3H
Hexamethylenetetramine (HMT)
S-proline
10
91
94
82
95
96
88
97
97
[32]
[34]
[34]
[48]
[49]
[30]
EtOH/reflux
H2O/EtOH-reflux
H2O/EtOH-reflux
H2O/60ꢀC
15
360
480
360
90
(Diammonium hydrogen phosphate)DAHP
SDS (Sodium dodecyl sulfate)
[Sipim]HSO4
s.f/100ꢀC
Ni (II)-Schiff base/SBA-15
CaO@SiO2-NH2-Sal-Zn
H2O/70ꢀC
10
s.f/r.t
10
Present work
Note: Isolated yield.
aReaction conditions: malononitrile (1, 1 mmol), 4-nitrobenzaldehyde (2, 1 mmol) and 4-hydroxycoumarin (3, 1 mmol).
comparable, suitable, and efficient catalyst for the synthe-
sis of desired 4H-pyrans. The application of CaO@SiO2-
NH2-Sal-Zn as a hybrid catalyst in solvent-free medium
at room temperature led to the greener condition for this
synthetic protocol. The porosity and high surface area of
nanostructure (CaO@SiO2-NH2-Sal-Zn) and Lewis
acidity of Zn ions increase the catalytic performance of
the prepared hybrid nanomaterial.
The plausible mechanism for the synthesis of
dihydropyrano[c]chromene derivatives 6 in the presence
of CaO@SiO2-NH2-Sal-Zn is similar to that reported in
the literature.[29] As illustrated in Scheme 3,
a
Knoevenagel condensation between malononitrile and
catalyst-activated aldehyde generates arylidene
malononitrile (intermediate I). The nucleophilic attack
of 4-hydroxycoumarin on intermediate I, via Michael
addition, affords the intermediate II. The reaction is
followed by consecutive intermolecular cyclization to
give the intermediate III, which gives the desired product
after tautomerization (Scheme 3). Similar mechanisms
for the synthesis of dihydropyrano[3,2-c]pyrazoles 7 and
SCHEME 3 The possible mechanism for one-pot synthesis of
dihydropyrano[c]chromenes using CaO@SiO2-NH2-Sal-Zn as a
catalyst
tetrahydrobenzo[b]pyrans
8
in the presence of
CaO@SiO2-NH2-Sal-Zn are proposed.
of fresh and reused catalyst after six runs were compared
by FT-IR (Figure 7) and XRD patterns (Figure 8). The
activity and structure of reused catalyst (CaO@SiO2-NH2-
Sal-Zn) prove the high stability and recyclability of this
catalyst. The exact amount of zinc loaded on CaO@SiO2-
NH2-Sal was evaluated using the ICP/MS analysis.
According to the results obtained by ICP/MS technique,
the exact amount of zinc in the hybrid nanocatalyst was
estimated to be 0.81 mmol gꢁ1for fresh CaO@SiO2-NH2-
Sal-Zn and 0.73 mmol gꢁ1 for catalyst after six runs.
These results verify that the leaching of zinc in the reac-
tion mixture is negligible, and this catalyst has good
stability.
3.3 | Catalyst reusability
The possibility of recycling CaO@SiO2-NH2-Sal-Zn as a
catalyst was investigated in the model reaction under the
optimized conditions. Upon completion of the reaction,
10-ml hot ethanol was added and the catalyst was
collected by centrifuge, washed with ethanol, dried in a
vacuum oven, and reused for next reaction. The recycled
catalyst can be used in the model reaction for six times
without any additional treatment and significant loss of
its initial catalytic activity (Figure 6). The structure

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SAMERI ET AL.
FIGURE 6 Recyclability of
CaO@SiO2-NH2-Sal-Zn for
synthesis of model reaction
FIGURE 7 Fourier transform infrared
spectroscopy (FT-IR) spectra of the fresh catalyst
and the six-time reused catalyst
FIGURE 8 X-ray diffraction (XRD) spectra
of the fresh catalyst and the six-time reused
catalyst
4 | CONCLUSION
Schiff base complex. The novel Zn-complex decorated
hybrid nanostructure (CaO@SiO2-NH2-Sal-Zn) was char-
acterized successfully using various techniques, and its
catalytic activity was considered in the synthesis of
In this work, surface-modified CaO nanoparticles (from
biogenic wastes) were functionalized with the Zn (II)-

SAMERI ET AL.
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under solvent-free condition. Besides, using this catalyst
benefits many features including generality, synthesis of
pure products in high yields, short reaction times, green
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ACKNOWLEDGMENTS
We gratefully acknowledge the financial support of this
work by the research council of Arak University.
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AUTHOR CONTRIBUTIONS
Fatemeh
Sameri:
Investigation;
methodology.
Mohammad Ali Bodaghifard: Validation. Akbar
Mobinikhaledi: Project administration; supervision.
CONFLICT OF INTEREST
The authors have no any conflict of interests to declare.
[22] F. Sameri, A. Mobinikhaledi, M. A. Bodaghifard, Silicon
2021, 1.
[23] F. Sameri, A. Mobinikhaledi, M. A. Bodaghifard, Res. Chem.
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DATA AVAILABILITY STATEMENT
The data that support the findings of this study are
available in the supporting information of this article.
ORCID
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Fatemeh Sameri https://orcid.org/0000-0002-2492-0480
Mohammad Ali Bodaghifard https://orcid.org/0000-
0001-9732-4746
Akbar Mobinikhaledi https://orcid.org/0000-0002-9732-
7282
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How to cite this article: F. Sameri, M.
A. Bodaghifard, A. Mobinikhaledi, Appl Organomet
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6394
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