Effective one-pot synthesis of tetrahydrobenzo[b]pyran derivatives using nickel Schiff-base complex immobilized on iron oxide nanoparticles

Article abstract of DOI:10.1002/aoc.5683

Nickel Schiff-base complex immobilized on silica-coated Fe₃O₄ as a heterogeneous catalyst was designed and characterized by different techniques, such as Fourier transform infrared (FT-IR), X-ray powder diffraction (XRD), field emiss

Full text of DOI:10.1002/aoc.5683

Received: 27 December 2019
DOI: 10.1002/aoc.5683
Revised: 9 March 2020
Accepted: 12 March 2020
F U L L P A P E R
Effective one-pot synthesis of tetrahydrobenzo[b]pyran
derivatives using nickel Schiff-base complex immobilized
on iron oxide nanoparticles
Hossein Maleki | Jamshid Rakhtshah | Behrouz Shaabani
Department of Inorganic Chemistry,
Abstract
Faculty of Chemistry, University of
Tabriz, Tabriz, Iran
Nickel Schiff-base complex immobilized on silica-coated Fe O as a heteroge-
3 4
neous catalyst was designed and characterized by different techniques, such
as Fourier transform infrared (FT-IR), X-ray powder diffraction (XRD), field
emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray
spectroscopy (EDX), inductively coupled plasma (ICP) and vibrating sample
magnetometry (VSM) thermogravimetric analysis (TGA), and Brunauer–
Emmett–Teller (BET). The synthesized nanocatalyst has been explored as a
new and efficient recyclable heterogeneous catalyst for the one-pot three-
component synthesis of tetrahydrobenzo[b]pyran derivatives. The reaction
proceeds smoothly to supply the respective products in excellent yields and
low reaction times. The catalyst can be easily recovered by a magnetic field
and reused for eight consecutive reaction cycles without significant loss of
activity.
Correspondence
Behrouz Shaabani, Department of
Inorganic Chemistry, Faculty of
Chemistry, University of Tabriz, Tabriz,
Iran.
Email: shaabani.b@gmail.com
K E Y W O R D S
3 4
Fe O nanoparticles, immobilized complex, solvent-free, Tetrahydrobenzo[b]pyran
1
| INTRODUCTION
be used in the pharmaceutical field. They have possessed
significant attention in the biological properties such as
Nowadays, environmentally benign and effective syn-
thetic procedures have great importance in organic syn-
theses. Thus, development of multicomponent reactions
spasmolytic,
diuretic,
anticoagulant,
anticancer,
antitumor,
[
5]
[6]
[7]
antianaphylactic,
antibacterial,
[
8]
antiallergic, antileishmanial, antifungal, hypotensive,
[
9]
[10]
(
MCRs) by their convergence, facile execution, generally
antiviral and insulin-sensitizing activity.
The con-
[1]
high yield, high atom economy, great efficiency and
procedural convenience in the construction of complex
structures from three or more reactants, has become
the goal of present-day chemical reactions Strecker
established the first MCR in 1850 for the synthesis of
amino acids. However, in the past decade, there have
been enormous developments in three-component reac-
tions, and great efforts have been and still are being made
ventional synthetic method for the preparation of these
compounds includes a three-component condensation of
malononitrile and dimedone with different aromatic
aldehydes. According to the literature, various methods
have emerged for the synthesis of these compounds in
the presence of different catalysts such as SO /MCM-
[2]
[3]
2−
4
[
11]
[12]
[13]
[14]
41,
exchange resin,
Fe O @SiO @TiO
2
NH H PO /Al O
THAM,
urea,
ion-
4
2
4
15]
2 3,
[
[16]
2,
SO H-functionalized nano-TiO
3
[4]
[17]
to find and develop new MCRs.
and
However, many of these methodolo-
gies could be faced with one or more disadvantages likes
Manganese
Schiff-
3
4
2
[
18]
Tetrahydrobenzo[b]pyrans have belonged to the class
of oxygen-containing heterocycle compounds which can
base@MWCNTs.
Appl Organomet Chem. 2020;e5683.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 15
https://doi.org/10.1002/aoc.5683

2
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MALEKI ET AL.
the use of expensive catalysts, prolonged reaction times,
and non-reusability of the catalyst. Therefore, there is a
need to develop new methods using reusable catalytic
systems for this type of reaction.
In recent years Schiff-base transition metal complexes
have been emerged in the development of catalytic reac-
tion, especially in organic reactions due to their unique
features such as their stability to moisture and high tem-
perature. They can be used in both homogeneous and
heterogeneous reactions such as hydrogenation of
G/UV 254 plates. Infrared (IR) spectra were recorded
on (Tensor 27, Bruker, Germany) in the range of
400–4000 cm− by the KBr disk method. The crystallo-
graphic characteristics of the samples were analyzed by
X-ray diffraction (XRD) (PANalytical X'Pert PRO, Ger-
many, Cu Kα radiation: 0.15406 nm; 45 kV, 40 mA).
Nanostructure observations were conducted with a
field emission scanning electron microscope (FE-SEM)
using a Tescan Mira3 microscope (Czech Republic).
TG analysis was performed using a TA Q-600 instru-
ment. The specific surface area (SBET), pore size, and
pore volume of the materials were determined by N2
physisorption studies on a BELSORP MINI II instru-
ment. Melting points are uncorrected and were
recorded on a Buchi B-545 apparatus in open capillary
tubes. Magnetic measurement of materials was investi-
gated with a vibrating sample magnetometer (VSM-4
inch, Daghigh Meghnatis Kashan Co., Kashan, Iran) at
room temperature.
1
[
19]
[20–22]
organic
reduction,
substrates,
oxidation
reactions,
[23]
synthesis of bis (indolyl) methanes, Diels–
[24]
[25]
Alder reactions,
multi-component reactions.
system, the catalyst faces with problematic separation
hydrosilylation of ketones
and
[26–28]
In the homogeneous
[
29]
from the product for reuse.
For solving this problem,
several insoluble materials have frequently been applied
as heterogeneous catalytic supports for these
[30–32]
complexes.
Magnetic nanoparticles (MNPs) are one
of these supports which have attracted a lot of attention
in the field of catalysis due to unique features like mag-
netization, high activities, and easy recovery in heteroge-
2.3 | Synthesis of magnetite Fe O4
nanoparticles
3
[33–37]
neously catalytic processes.
Due to being unstable
under acidic conditions, the magnetic core is usually
[
38,39]
protected with an outer shell such as silica
mers.
or poly-
Silica is one of the sufficient candidates, which
terminate the surface with abundant Si-OH bond and
The Fe O4 NPs were synthesized based on the
3
[29]
[42]
method reported in the literature.
For this purpose,
an aqueous solution of FeCl .6H O (2 mmol) and
3
2
[
27,40]
facilitates the immobilization of desired compounds
on magnetic nanoparticles.
FeCl .4H O (1 mmol) were mixed in deionized water
2 2
ꢀ
and stirred for 10 min at 80 C. Then 15 ml of con-
centrated ammonia (25%) was added rapidly to the
solution. The black mixture was stirred under N2
atmosphere for another 2 hr then cooled to room
temperature. The black precipitate was separated with
the help of a magnet. The resulted magnetic particles
were washed several times with water until the pH of
In continuation of our research on the design and
synthesis of Schiff-base complexes immobilized on
[26,41]
MNPs,
we decided to synthesis and characterized
new nickel Schiff-base complex supported on MNPs as a
benign and recoverable catalyst in the synthesis of
tetrahydrobenzo[b]pyran derivatives. It is noteworthy
that different aldehydes have been used in the synthesis
of the mentioned derivatives.
ꢀ
the runoff was around 7 and dried at 60 C under
vacuum.
2
2
| EXPERIMENTAL
.1 | Materials
2.4 | Synthesis of silica-coated iron oxide
nanoparticle (Fe O @SiO )
3
4
2
The Fe O @SiO
2
according to the Stober method.
nanoparticles were prepared
3
4
[
43]
All of the reagents and solvents involved in synthesis
were purchased from Merck Chemical Company. All the
solvents were distilled, dried, and purified by standard
procedures.
Magnetic
nanoparticles were dispersed under ultrasonication for
20 min in a mixture of 100 ml of ethanol, 10 ml of
deionized water and 2.5 ml of 25 wt% concentrated
aqueous
ammonia
solution
and
2
ml
of
tetraethylorthosilicate (TEOS). After stirring the mix-
ture for 6 hr at room temperature, the obtained brown
solid was collected from the solution by using an
external magnet and washed several times with etha-
nol and dried under vacuum.
2
.2 | Instrumentation
Reaction progress and purity determination of the
compounds were checked by TLC using silica gel SIL

MALEKI ET AL.
3 of 15
2
.5 | Synthesis of magnetite
was added. The mixture was stirred for 4 hr under
reflux condition. The residue was washed with ethanol
and dried under vacuum. Then, a solution of the
corresponding Schiff-base ligand (2 mmol) in ethanol
nanoparticles coated by
3
2@
-chloropropyltrimethoxysilane(Fe O @SiO
3 4
Cl)
and Ni(OAc) (1 mmol) were mixed and stand for 4 hr
2
1
g
of Fe O @SiO
2
was dispersed in the
at room temperature. Finally, the solid precipitate was
collected and dried under vacuum at room tempera-
ture. Then the product was purified by recrystallization
from ethanol, affording the pure nickel Schiff-base
complex.
3
4
3
-chloropropyltrimethoxysilane CPTMS (2.5 ml) dis-
persed in 50 ml ethanol under reflux condition. The
obtained particles were collected using a magnet and
washed with ethanol three times.
2
.6 | Synthesis of nickel Schiff-base
2.7 | Synthesis of nickel Schiff-base
complex immobilized on silica-coated
Fe O (Fe O @SiO @NiSB)
complex (NiSB)
3
4
3
4
2
To a stirred solution of 2-pyridine carboxaldehyde
(
1 mmol, 0.107 g) in absolute EtOH (20 ml) 2-amino
The prepared nickel Schiff-base complex was added to
the ethanolic solution of as-synthesized
phenol (1 mmol, 0.109 g) in absolute EtOH (10 ml)
SCHEME 1 The sequence
of events in the preparation of
3 4 2
Fe O @SiO @NiSB
3 4
FIGURE 1 FT-IR spectra of Fe O (a),
Fe
Fe
3
O
O
4
4
@SiO
@SiO
2
(b), Fe
3 4 2
O @SiO @Cl (c),
3
2
@NiSB (d), and NiSB (e)

4
of 15
MALEKI ET AL.
Fe O @SiO @Cl (1 g) containing a few drops of
Ni Schiff-base (NiSB) are shown in Figure 1. The particu-
lar absorption peak of Fe-O bond at around 561 cm
3
4
2
ꢀ
−1
triethylamine and stirred at 60 C for 4 hr. The
resulted precipitate was separated from the solution by
an external magnet and washed several times with eth-
anol and dried in the air.
confirms the accurate synthesis of iron oxide
−1
nanoparticles. Also, the band has seen at 3361 cm can
be related to the stretching vibrations of O-H bond of the
surface of the nanoparticle (Figure 1a). The bands appe-
aring at the range of 1000–1100 cm− affirm the silica
modification on the nanoparticles (Figure 1b). The weak
1
2
.8 | General procedure for the
bands at 2922 cm− and 2830 cm assigned to the
1
−1
synthesize of tetrahydrobenzo[b]pyran
derivatives
A mixture of aromatic aldehyde (1 mmol), 1,3-dione
(
(
1 mmol), malononitrile (1 mmol), and nanocatalyst
0.002 g) in a round bottom flask was placed, and the
subsequent mixture was firstly stirred magnetically under
solvent-free conditions at room temperature. After com-
pletion of the reaction, which was identified by TLC (n-
hexane/ethyl acetate: 5/2), the nanocatalyst was recov-
ered magnetically by adding 5 ml ethyl acetate to solve
the product and then it was purified via recrystallization
from ethanol.
2.9 | General procedure for reusing
experiments
To reuse the catalyst, after finishing each run, 3 ml of hot
ethyl acetate was added to the reaction mixture and
heated to dissolve the product. Then, the nanocatalyst
was separated from the reaction mixture by an external
magnet and washed with ethanol to reused in the next
reaction eight times.
3
3
| RESULT AND DISCUSSION
.1 | Characterization of nickel Schiff-
base complex immobilized on Fe O as a
3
4
heterogeneous catalyst
The reactions leading to the nickel Schiff-base complex
immobilized on MNPs are summarized in Scheme 1.
The structure of nickel Schiff-base complex
immobilized on Fe O as a heterogeneous catalyst was
3
4
studied and characterized by Fourier transform infrared
FT-IR), X-ray powder diffraction (XRD), field-emission
(
scanning electron microscopy (SEM), energy-dispersiveX-
ray spectroscopy (EDX), inductively coupled plasma
(ICP), vibrating sample magnetometry (VSM) analysis,
thermogravimetric analysis (TGA) technique, and
Brunauer–Emmett–Teller (BET) method.
The FT-IR spectra of Fe O4 nanoparticles,
FIGURE 2 XRD patterns of Fe O (a), Fe O @SiO (b),
3
3
4
3
4
2
Fe O @SiO , Fe O @SiO @Cl, Fe O @SiO @NiSB, and
Fe
3
O
4
@SiO
2
@Cl (c), and Fe
3
O
4
2
@SiO @NiSB (d)
3
4
2
3
4
2
3
4
2

MALEKI ET AL.
5 of 15
[
26]
stretching mode of the CH groups of chlorosilane cou-
modified nanoparticle.
Also, in the FT-IR spectra of
2
pling agent. Also, the band appeared at 1639 cm− illus-
trates the C-N stretching vibration bond, which validates
the attachment of the Schiff-base complex on the
1
neat nickel Schiff-base, the vibrational stretching fre-
quency of C=N bond appears at 1639 cm . Altogether,
the aforementioned results confirmed the formation of a
−1
FIGURE 3 SEM images of
3 4 2
Fe O @SiO @NiSB at different
magnifications (a: 500 nm and b: 100 nm)
3 4 2
FIGURE 4 EDX spectra of the Fe O @SiO @NiSB
3 4 2
FIGURE 5 EDX-map of elements in the structure of Fe O @SiO @NiSB

6
of 15
MALEKI ET AL.
FIGURE 6 The magnetic hysteresis loops
of Fe
Fe
d)
3
O
4
MNPs (a) Fe
3
O
4
@SiO
2
(b),
3
O
4
@SiO @Cl (c), and Fe
2
3
O
4
2
@SiO @NiSB
(
silica layer around the Fe O4 nanoparticles and the
nickel Schiff-base functionalization of this core-shell
structure.
2 0}, {3 1 1}, {4 0 0}, {4 2 2}, {5 1 1}, {4 4 0} and {5 5 3}
(Figure 2a). Figure 2b exhibited XRD broadened pattern
because of its noncrystalline pattern at 2θ: 20–30 , which
3
ꢀ
Figure 2 shows the X-ray diffraction (XRD) patterns
of initial Fe O , Fe O @SiO , Fe O @SiO @Cl, and
confirms Fe O @SiO structure. Also, the XRD pattern of
3
4
2
nickel complexes immobilized on Fe O (Figure 2d) was
3
4
3
4
2
3
4
2
3 4
nickel complex immobilized on silica-coated Fe O in the
the same with neat Fe O , clearly displaying that crystal-
3 4
3
4
ꢀ
range of 10–80 . The XRD pattern of Fe O show eight
linity and morphology were not changed after grafting.
The signals of Ni metal were not detected in XRD, indi-
cating that the Ni species is highly dispersed on
3
4
ꢀ
peaks at 2θ = 18, 30.4, 35.5, 43.1, 53.5, 57.3, 63 and 74.3 ,
corresponding to Miller index values {h k l} of {1 1 1}, {2
[
26]
ferrites.
The
morphology
of
the
synthesized
Fe O @SiO @NiSB was studied by scanning electron
3
4
2
microscopy technique (SEM). As can be seen in Figure 3,
most of the prepared nanoparticles are spherical shaped
and have an average diameter of about 35 nm. Energy-
dispersive X-ray spectroscopy (EDX) from the obtained
nanomaterials (Figure 4) provided the attendance of fore-
tasted elements in the structure of the nanocatalyst,
namely iron (Fe), silicon (Si), oxygen (O), chlorine
(
Cl) and nickel (Ni). The EDX-map of elements in the
structure of the nanocatalyst (Figure 5) displays that the
distribution of elements in the structure of the catalyst is
monotonous, and it affirms the stable connection of the
nickel Schiff-base complexes into the supports. The Ni
content of the immobilized complex supported on MNPs
3 4 2
FIGURE 7 TGA curve of Fe O @SiO @NiSB
TABLE 1
Material
Textural properties of fresh and recovered Fe
3
O
4
@SiO
2
@NiSB
Surface area (m g−1)
2
Pure volume (cm g−1)
3
Pore diameter (nm)
12.797
Fresh Fe
3
O
4
@SiO
2
@NiSB.
@NiSB.
69.653
61.606
16.003
14.154
Recovered Fe
3
O
4
@SiO
2
12.657

MALEKI ET AL.
7 of 15
TABLE 2
solvent-free medium
Effects of nanocatalyst concentration and temperature on the synthesis of tetrahydrobenzo[b]pyran derivatives (4) under
a,b
ꢀ
b
Entry
Amount of nanocatalyst (mg)
Reaction temperature ( C)
Reaction time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
Catalyst-free
r.t.
100
r.t.
60
60
20
20
10
10
10
10
10
10
25
30
63
70
93
93
93
93
95
95
Catalyst-free
2
2
4
4
4
4
6
6
100
r.t.
50
75
100
r.t.
1
0
100
Reaction conditions:
a
4
-Chlorobenzaldehyde (1) (1 mmol), dimedone (2) (1 mmol), and malononitrile (3) (1 mmol), Solvent-free condition;
b
Isolated yields.
TABLE 3
Effects of solvent on the synthesis of
tetrahydrobenzo[b]pyran derivatives (4) under room temperature
measured using ICP analysis, the value for which is about
a,b
and 4 mg of nanocatalyst
−1
0
.28 mmol g .
The magnetic properties of the nanocatalyst were
studied using a vibrating sample magnetometer (VSM)
at room temperature. The magnetization curves mea-
sured for Fe O , Fe O @SiO , Fe O @SiO @Cl, and
3
4
3
4
2
3
4
2
Fe O @SiO @NiSB are compared in Figure 6. It could
3
4
2
be seen from the loops that saturation magnetization
−
1
of the mentioned material is 43, 38, 33, and 31 emu g ,
respectively. Decreasing in saturation magnetization of
the nanocatalyst compared to the bare nanoparticles
related to the formation of silica shell and nickel
Schiff-base complexes moieties during the modification
process.
Entry
Solvent
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
Solvent-free
10
30
20
25
40
40
60
60
93
75
85
74
40
45
50
48
2
H O
2 5
C H OH
TGA analysis is shown in Figure 7, which exhibits the
stability of the Fe O @SiO @NiSB as nanocatalyst under
CH
CH
CH
3
3
2
CN
CO
3
4
2
2
Et
the air atmosphere. It is clearly observed that that
ꢀ
nanocatalyst could be used up to 120 C. The weight loss
Cl
2
ꢀ
(
6%) below 100 C is likely due to the loss of hydroxyl
Toluene
Benzene
surface groups and solvent molecules. The weight loss in
ꢀ
the range of 200–400 C suggests that nickel complex and
Reaction conditions:
-Chlorobenzaldehyde (1) (1 mmol), dimedone (2) (1 mmol), and
malononitrile (3) (1 mmol), Room temperature, Amount of
nanocatalyst (4 mg);
Isolated yields.
other organic linkers are decomposed in this range. On
a
4
ꢀ
the other hand, the weight loss at >400 C was related to
the thermal crystal phase transformation from Fe O to
3
4
b
γ-Fe O .
2
3

8
of 15
MALEKI ET AL.
TABLE 4
Synthesis of tetrahydrobenzo[b]pyrans via tandem Knoevenagel-Michael cyclocondensation reactions in the presence of
a,b
3 4
nickel Schiff-base complexes immobilized on Fe O
b
ꢀ
M.p ( C) [Ref.]
Entry
1
Ar
R'
Product
Time (min)
Yield (%)
[
44]
C
6
H
5
Me
5
96
228–230
[
45]
2
4-Cl-C
6
H
4
Me
10
93
214–216
[
17]
3
2,3-Cl
2
-C
6
H
3
Me
10
95
257–259
[
45]
4
2,4-Cl
2
-C
6
H
3
Me
6
97
193–195
[
[
46]
45]
5
6
4-Br-C
6
H
4
Me
Me
7
3
95
98
196–197
179–181
2 6 4
4-NO -C H

MALEKI ET AL.
9 of 15
TABLE 4 (Continued)
b
ꢀ
M.p ( C) [Ref.]
Entry
Ar
R'
Product
Time (min)
Yield (%)
[
[
[
47]
47]
45]
7
8
9
3-NO
2
-C
6
H
4
Me
Me
Me
5
95
97
94
210–212
227–229
223–225
2-NO
2
-C
6
H
4
5
4-OH-C
6
H
4
10
[
45]
1
0
4-CN-C
6
H
4
Me
7
96
225–227
[
44]
1
1
4-CH
3
O-C
6
H
4
Me
12
95
198–200
Continues)
(

10 of 15
MALEKI ET AL.
TABLE 4 (Continued)
b
ꢀ
Entry
Ar
R'
Product
Time (min)
Yield (%)
M.p ( C) [Ref.]
[
44]
1
2
4-CH
3
-C
6
H
4
Me
14
92
212–214
[
47]
1
3
2-Furyl
Me
25
85
221–233
[
18]
1
4
4-Cl-C
6
H
4
H
7
95
267–269
[
18]
1
5
4-NO
2
-C
6
H
4
H
5
98
254–256
[
18]
1
6
4-OH-C
6
H
4
H
8
93
256–258

MALEKI ET AL.
11 of 15
TABLE 4 (Continued)
b
ꢀ
M.p ( C) [Ref.]
Entry
Ar
R'
Product
Time (min)
Yield (%)
Reaction conditions:
a
Aldehyde derivatives (1) (1 mmol), dimedone (2) (1 mmol), and malononitrile (3) (1 mmol), Amount of nanocatalyst: 4 mg, Solvent-free
condition, Room temperature;
b
Isolated yields.
FIGURE 8 Magnetic
separation and recycling of
Fe
heterogeneous nanocatalyst in
0 min
3 4 2
O @SiO @NiSB as a
6

12 of 15
MALEKI ET AL.
Table 1, displays the surface area, pore volume, and
pore diameter analysis of Fe O @SiO @NiSB before and
observed in terms of the yield in the absence of catalyst
after 60 min, which displays that the nanocatalyst has an
essential role in the reaction progress. The highest effi-
ciency of the reaction was obtained by applying 4 mg
nanocatalyst for 10 min at room temperature. It is note-
worthy that in the presence of catalyst (4 mg and more),
differences in temperature rise and catalyst amount did
not improve the product yield.
In the next step, the effect of different solvents was
investigated in the synthesis of tetrahydrobenzo[b]pyran
derivatives. According to the outcome, it is evident that
the highest yield in the shortest reaction time pertains to
the solvent-free medium comparing with the rest of the
solvents studied (Table 3).
3
4
2
after catalytic reaction by the Brunauer–Emmett–Teller
BET) analyzer. The surface area of recovered
(
2
−1
nanocatalyst was found to be 61.606 m g , which was
slightly lower than the fresh nanocatalyst
69.653 m g ). The results reveals that the recovered
2
−1
(
nanocatalyst was lower in the surface area, pore volume,
and pore diameter compared to fresh nanocatalyst indi-
cating the incorporation of tetrahydrobenzo[b]pyran
derivatives over the surface of nanoparticles as well as a
little inside the pores.
3
.2 | Application of Fe O @SiO @NiSB
To extend the scope and demonstrate the generality
of the present method, we studied the reaction of differ-
ent aromatic aldehydes with malononitrile and dimedone
under the optimized reaction conditions, and the results
are summarized in Table 4. The effect of substituents on
the aromatic ring shows estimated strong effects in terms
of yields under these reaction conditions. The aromatic
aldehydes, including those with electron-releasing sub-
stituents and electron-withdrawing substituents on their
aromatic ring, give excellent yields (85–98%). The reac-
tions are completed within 3–25 min under solvent-free
conditions. Also, reaction times of aromatic aldehydes
having electron-withdrawing groups are slightly faster
3
4
2
as a heterogeneous nanocatalyst in the
synthesis of tetrahydrobenzo[b]pyrans
derivatives
After the successful preparation and characterization of
synthesized new nanocatalyst, to optimize the reaction
conditions, we considered the effect of the catalyst in the
synthesis of tetrahydrobenzo[b]pyran derivatives. For this
purpose, the reaction of 4-chlorobenzaldehyde,
dimedone, and malononitrile using various amounts of
the nanocatalyst under solvent-free conditions was exam-
ined as model reaction (Table 2). A low change was
FIGURE 9 Study of Ni Schiff base complex supported on Fe
3
O
4
after eight catalytic cycles: FT-IR spectrum (a), XRD pattern (b), VSM
analysis (c), and TGA curve

MALEKI ET AL.
13 of 15
than those having electron-donating groups. The
electron-withdrawing groups (such as nitro and halide)
are found to activate the aldehyde towards nucleophilic
attack and increase the reaction rate.
Moreover, the recyclability of the nickel Schiff-base
complex immobilized on MNPs as a nanocatalyst was
significant loss of activity and even after these runs. The
experimental results suggest that the reaction involves a
heterogeneous process, and there was no significant
change in the catalytic activity of nanocatalyst after its
catalytic application. It is observed from spectral studies
(Figure 9) that there is no change in the nature of the cat-
alyst even after eight cycles. The FT-IR analysis exhibited
identical peaks for both the fresh and recovered
nanocatalyst (Figure 1d & 9a). Likewise, the XRD pattern
of nanocatalyst before (Figure 2d) and after (Figure 9b)
the reactions show identical shapes. The magnetic
established
using
the
condensation
of
4
-chlorobenzaldehyde, dimedone, and malononitrile. The
results displayed in Figure 8, approve that the catalytic
activity of the catalyst remains within the limits of experi-
mental error for eight continuous runs without any
SCHEME 2 The probable mechanism for the synthesis of tetrahydrobenzo[b]pyrans via tandem Knoevenagel- Michael
cyclocondensation reaction using Fe @SiO @NiSB as a nanocatalyst
3
O
4
2
a
TABLE 5
3 4
Comparison of various Fe O based catalysts in the synthesis of tetrahydrobenzo[b]pyrans with our catalyst
b
Nanomagnetic catalyst (amount of
Entry catalyst)
Temp
( C)
Time
(min)
Yield
(%)
ꢀ
Year Solvent
Ref
[
48]
1
2
3
4
5
6
7
8
9
Fe
Fe
Fe
Fe
3
3
3
3
O
O
O
O
4
4
4
4
NPs/MWCNTs (5 mg)
2014
2014
2015
2015
2016
2017
2017
2019
EtOH/MW
Reflux
Reflux
4
40
7
83
90
96
93
95
88
89
96
96
[
49]
@SiO
@SiO
@SiO
2
2
2
@NH-NH
2
-PW (30 mg)
n
H
H
2
2
O
[45]
-imid-PMA (15 mg)
O/Ultrasonic R.T.
[
[
[
[
[
17]
50]
51]
52]
53]
@TiO
H (30 mg)
-SCMNPs (30 mg)
AIL@MNP (60 mg)
2
(10 mg)
Solvent-free
EtOH/H
100
Reflux
60
20
60
15
25
35
5
Fe3-xTi
x
O
4
@SiO
3
2
O
H
2
PO
4
Solvent-free
Solvent free
90
Fe
3
3
O
4
4
@SiO
@SiO
2
2
-guanidine-PAA (50 mg)
@NiSB (4 mg)
H
2
O
70
Fe
O
Solvent free
R.T.
-
Reaction conditions:
a
Benzaldehyde (1) (1 mmol), dimedone (2) (1 mmol), and malononitrile (3) (1 mmol);
Isolated yields.
b

14 of 15
MALEKI ET AL.
property of recovered nanocatalyst has been compared
with fresh nanocatalyst using a vibrating sample magne-
tometer technique. VSM measurements for recovered
with an external magnet that makes it an instrumental
alternative to the previous methodologies for the scale-up
of these one-pot three-component reactions.
−
1
nanoparticles is about 31 emu g (Figure 9c). We investi-
gated the results for the TGA analysis of nanocatalyst in
Figure 9d. The results verified identical pathways for
both fresh (Figure 7) and recovered nanocatalyst. The
ICP analysis of the product after isolation of catalyst
showed no loss of metal (nickel) during the catalytic reac-
tion, indicating that no metal leaching occurred.
ORCID
Behrouz Shaabani https://orcid.org/0000-0001-5576-
4604
A
suggested mechanism for the synthesis of
tetrahydrobenzo[b]pyran derivatives in the presence of
Fe O @SiO @NiSB is illustrated in Scheme 2. Initially,
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4
| CONCLUSION
In summary, a green, mild, and eco-friendly nickel
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tetrahydrobenzo[b]pyran derivatives using nickel
Schiff-base complex immobilized on iron oxide
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