New magnetic nanocatalyst containing a bis-dicationic ionic liquid framework for Knoevenagel condensation reaction

Article abstract of DOI:10.1007/s11164-019-03747-x

Bis[(3-aminopropyl)triethoxysilane]dichloride immobilized on magnetic nano-γ-Fe₂O₃@SiO₂ has been prepared. After characterization, the reagent was used for efficient promotion of the Knoevenagel reaction, achieving high re

Full text of DOI:10.1007/s11164-019-03747-x

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03747-x
New magnetic nanocatalyst containing a bis‑dicationic
ionic liquid framework for Knoevenagel condensation
reaction
Reyhaneh Karimi‑Chayjani1 · Nader Daneshvar1 · Farhad Shirini1,2
Hassan Tajik1,2
·
Received: 4 December 2018 / Accepted: 12 January 2019
© Springer Nature B.V. 2019
Abstract
Bis[(3-aminopropyl)triethoxysilane]dichloride immobilized on magnetic nano-γ-
Fe2O3@SiO2 has been prepared. After characterization, the reagent was used for
efcient promotion of the Knoevenagel reaction, achieving high reaction rates and
yields. Also, the recovery and reusability of the catalyst were excellent for the stud-
ied reaction.
Keywords Bis-dicationic magnetic nanoparticles · Nano-γ-Fe2O3 · Arylidene
malononitriles · Ethyl-3-aryl-2-cyanoacrylates · Water-mediated reaction
Introduction
In recent decades, magnetic nanoparticles have received attention due to their
important properties such as superparamagnetism, coercivity, high surface area, and
low Curie temperature [1]. These properties make them suitable for use in the felds
of drug delivery and magnetic resonance imaging (MRI) [2], biosensors [3], and
other felds, especially catalysis [4, 5].
Usually, nanoparticles are covered with organic (such as starch [6], alginates [7],
chitosan [8] polyacrylic acid [9], polyethyleneimine [10], and polyethylene glycol
[11]) or inorganic (including silver [12], gold [13], graphene oxide [14–16], silicon
and silica [17]) coatings, to extend their ability and increase their surface.
Various iron oxides have important applications in industry as pigments, polish-
ing agents, and catalysts as well as in electrochemical cells [18]. Moreover, iron
oxide nanoparticles have properties distinct from the bulk material. The applications
*
Farhad Shirini
shirini@guilan.ac.ir
1
2
Department of Chemistry, University Campus 2, University of Guilan, Rasht 41335-19141, Iran
Department of Chemistry, College of Sciences, University of Guilan, Rasht, Iran
Vol.:(0123456789)
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R. Karimi-Chayjani et al.
of iron oxide nanoparticles have led to the introduction of various methods for their
synthesis and characterization. One of the most famous methods for preparation of
coated γ-Fe2O3 is the modifed Stöber method, using which the size of the resulting
nanoparticles can be tuned by adjusting the pH, solvent, temperature, and concentra-
tions of the starting materials [19].
Even though ionic liquids have many benefts, their use also has some disadvan-
tages. Their high viscosity prevents their high surface from taking part in reactions.
Their separation and reusability (especially in homogeneous catalysis) are also dif-
fcult. Also, use of large amounts of ionic liquids is not afordable [20, 21]. Because
heterogeneous catalysts are preferred in chemical industry to achieve required reac-
tions, use of supported ionic liquids represents a good method to beneft from their
advantages as well as heterogeneous catalysis [22]. These advantages increase when
the catalyst can be separated using an external magnet, to reduce the cost of separa-
tion and obtain highly pure products [23].
Knoevenagel condensation is one of the most important reactions for C–C bond
formation [24–27]. From the viewpoint of green chemistry, this reaction is ecof-
riendly because its byproduct is water [28]. This reaction has been used to obtain
fne chemicals and many other useful compounds [29]. This reaction plays a key role
in synthesis of some bioactive compounds [30, 31]. Also, the Knoevenagel reaction
is one of the key steps in preparation of some medicines such as entacapone [32]
and atorvastatin [33]. Because of the importance of the Knoevenagel condensation
of an aldehyde and malononitrile (or ethyl cyanoacetate) as a basic and key step to
obtain more important molecules, various methods and catalysts have been reported
to promote this reaction [34–39].
In spite of their considerable advantages, these methods are accompanied by
drawbacks such as high toxicity, low reusability, use of large amounts of catalyst,
use of organic solvents, high cost, long reaction time, and low yield. Therefore,
introduction of new methods to avoid such disadvantages has received attention
from many researchers in organic chemistry.
Experimental
Materials
All chemicals were purchased from Merck, Aldrich, and Fluka chemical companies
and used without any further purifcation. All products were separated and char-
acterized based on their physical properties and comparison with reported values.
Both purity determination of the substrates and reaction monitoring were carried
out by thin-layer chromatography (TLC) using silica gel SIL G/UV 254 plates.
Melting points were determined using a Büchi B-545 apparatus. Fourier-transform
infrared (FT-IR) spectra were recorded using a PerkinElmer (USA) Spectrum BX
series spectrophotometer (KBr disks). Field-emission scanning electron microscopy
(FESEM) was performed using a TESCAN (Czech Republic) MIRA III instru-
ment. X-ray difraction (XRD) patterns of samples were recorded on a Philips (The
Netherlands) PW1730. Thermogravimetric analysis (TGA) was carried out on a TA
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New magnetic nanocatalyst containing a bis-dicationic ionic…
1
(USA) model Q600 instrument. H and 13C nuclear magnetic resonance (NMR)
spectra were recorded using a Bruker (Germany) Ultrashield 400 MHz instrument
with deuterated dimethylsulfoxide (DMSO) as solvent. Vibrating sample magnetom-
eter (VSM) was recordedon a Lakeshore 7200 (USA) instrument.
Catalyst preparation
Preparation of γ‑Fe2O3 NPs
To a solution of FeCl2·4H2O (4 g), FeCl3·6H2O (6.35 g), and HCl (1.7 mL) in deion-
ized water (50 mL), NH3 solution (1.5 M, 250 mL) was added dropwise under argon
atmosphere at room temperature to reach pH 11. The resulting black mixture was
stirred at room temperature for 1 h until its color changed to brown. The synthesized
magnetic nanoparticles were separated from the reaction medium using an external
magnet, then washed with deionized water to reach pH 7. Finally, the residue was
placed in an oven at 250 °C for 3 h to obtain reddish-brown powder of γ-Fe2O3
nanoparticles [40, 41].
Preparation of γ‑Fe2O3@SiO2 NPs
First, 1.0 g of synthesized γ-Fe2O3 nanoparticles was dispersed by sonication in a
solution of ethanol and water (80.0 mL) [4:1], then tetraethylorthosilicate (3.0 mL)
and ammonia 28 % (3.0 mL) were slowly added to it, and the reaction mixture was
stirred for 8 h at room temperature. After that, the obtained solid was separated
using an external magnet, washed several times with deionized water and ethanol,
and dried in an oven at 60 °C for 12 h. The synthesized silica-coated nano-γ-Fe2O3
was characterized by FT-IR spectroscopy, XRD analysis, FESEM, and TGA, whose
results are presented in the next sections [42].
Preparation of bis‑dicationic ionic liquid N1,N4‑bis(3‑(triethoxysilyl)propyl)
butane‑1,4‑diaminium dichloride, [Bis‑APTES]Cl2
In a 50-mL round-bottomed fask, a mixture of (3-aminopropyl)triethoxysilane
(10.0 mmol) and 1,4-dichlorobutane (5.0 mmol) was stirred in refuxing acetonitrile
(10 mL) for 12 h. During this process, a white solid gradually settled out. After that,
the white precipitate was fltered and washed with ethanol, and dried at 70 °C for
12 h.
Preparation of γ‑Fe2O3@SiO2@[Bis‑APTES]Cl2 NPs
In a 50-mL round-bottomed fask, 1.0 g γ-Fe2O3@SiO2 was dispersed in dry tolu-
ene (20 mL) by sonication, then 1.1 g of synthesized N1,N4-bis(3-(triethoxysilyl)pro-
pyl)butane-1,4-diaminium dichloride was added to it and stirred for 48 h at refux
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R. Karimi-Chayjani et al.
H O
2
FeCl2.4
+
FeCl3.6
Cl
Cl
γ
-Fe2O3
H O
2
OEt
EtO Si
OEt
NH3
H O
2
NH2
HCl,
2
O
TEOS,H2
EtOH
NH3
Acetonitrile
Reflux
EtO
Si
SiO2
EtO
Si
OEt
OEt
H2
N
Cl
Fe3O4
EtO
EtO
Cl
γ
-Fe2O3
N
H2
250 o
C
3h,
Toluene
Reflux
SiO2
SiO2
O
H2
O
Cl
N
γ
-Fe2O3
N
Si
O
γ
-Fe2O3
γ
-Fe2O3
O
Si
Cl
H2
O
O
Scheme 1 Preparation of γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs
Fig. 1 Stepwise preparation of γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs
temperature. Finally, the brown precipitate was separated using an external magnet,
washed with ethanol several times, and dried at 70 °C for 12 h (Scheme 1; Fig. 1).
Preparation of products
Preparation of arylidene malononitrile and ethyl‑3‑aryl‑2‑cyanoacrylate derivatives
In a 10-mL round-bottomed fask, a mixture of aldehyde (1.0 mmol), malononi-
trile (or ethyl cyanoacetate) (1.1 mmol), and γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs
(5 mg) in water (3.0 mL) was stirred at 80 °C for appropriate time. After reaction
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New magnetic nanocatalyst containing a bis-dicationic ionic…
completion as monitored by TLC [n-hexane:EtOAc (7:3)], the precipitate was dis-
solved in ethanol or acetone (20 mL). Then, the nanocatalyst was removed using an
external magnet. After that, the solvent was evaporated and the solid residue was
washed with a few drops of cold ethanol and washed with water. The expected prod-
uct was obtained in high yield purity without need for any extra purifcation steps.
Physical and spectral data of newly synthesized compound
2‑((1‑Methyl‑1H‑pyrrol‑2‑yl)methylene)malononitrile Yellow crystals; m.p. 156–
158 °C; IR (KBr, cm−1): 3386, 3118, 3032, 2924, 2854, 2212, 1586, 1491, 1402,
1318, 1183, 1170, 752; 1H NMR (400 MHz, DMSO-d6) δ (ppm)=3.80 (s, 3H), 6.46
(ddd, J1 =4.4 Hz, J2 =2.0 Hz, J3 =0.4 Hz, 1H), 7.40 (dd, J1 =4.4 Hz, J2 =1.2 Hz,
1H), 7.51 (t, J=1.2 Hz, 1H); 13C NMR (100 MHz, DMSO-d6) δ (ppm)=34.0, 67.7,
112.6, 115.2, 115.9, 119.0, 127.2, 134.3, 145.
Results and discussion
Various catalysts have been designed and introduced for chemical reactions by our
research group over more than two decades, each showing at least one distinguished
advantage in comparison with other introduced ones [43–47]. Among them, nano-
catalysts and nanomagnetic catalysts are a unique feld for our research group [48,
49]. In continuation of these studies and for the frst time, we introduce herein a bis-
dicationic acidic ionic liquid using an aliphatic spacer and its immobilization on sil-
ica-coated magnetic nano-γ-Fe2O3 for efcient promotion of synthesis of arylidene
malononitrile and ethyl-3-aryl-2-cyanoacrylate derivatives.
Characterization of γ‑Fe2O3@SiO2@[Bis‑APTES]Cl2 NPs
FT‑IR spectroscopy
The FT-IR spectrum of γ-Fe2O3 (a) showed a peak centered at 3424 cm−1 related
to hydroxyl groups of the surface of γ-Fe2O3 and adsorbed water. Strong stretching
vibrations of Fe–O appeared at 559 cm−1 and 638 cm−1, while these peaks were
weakened in the FT-IR spectrum of γ-Fe2O3@SiO2 (b). Also, for this compound,
the strong broad stretching peak in the region of 1000–1300 cm−1, which is related
to Si–O bonds, shows that γ-Fe2O3 was successfully coated by silica. In the FT-IR
spectrum of the obtained N1,N4-bis(3-(triethoxysilyl)propyl)butane-1,4-diamin-
ium dichloride (c), the broad peak at 1000–1300 cm−1 is related to Si–O stretch-
ing vibrations. In this spectrum, peaks at 2800–3000 cm−1 are related to stretching
C–H vibrations. N–H stretching vibration is indicated at 3433 cm−1. Also, acidic
vibrations related to H+ cause peak broadening at 2600–3300 cm−1, which proves
the acidic nature of the IL and its successful preparation. The FT-IR spectrum of
γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs (d) shows the Fe–O stretching medium
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R. Karimi-Chayjani et al.
vibration peaks at 565 cm−1 and 640 cm−1. In this spectrum, the broad peak at
1000–1300 cm−1 is due to Si–O–Si symmetric and asymmetric stretching vibrations.
Overall, these results clearly show that, in the FT-IR spectrum of Fe2O3@SiO2@
[Bis-APTES]Cl2 NPs, peaks related to the bis-dicationic link are added to the FT-IR
peaks of Fe2O3@SiO2 (Fig. 2).
XRD analysis
Figure 3 shows the XRD patterns of γ-Fe2O3@SiO2 (a) and γ-Fe2O3@SiO2@[Bis-
APTES]Cl2 (b). Peaks at around 2θ=30.4°, 35.8°, 43.5°, 54.0°, 57.5°, and 63.2°,
which can be assigned to (220), (311), (400), (422), (511), and (440), are readily
recognized from the XRD patterns. Observed difraction peaks match well with the
structure of maghemite. The broad peak from 2θ=19° to 28° is attributed to amor-
phous-phase silica in the shell of the silica-coated γ-Fe2O3 nanoparticles (γ-Fe2O3@
SiO2) (a). Certainly, this peak for γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs is broader,
indicating the presence of ionic liquid in the catalyst structure (b) [50] (Fig. 3).
TGA
The TGA curve of γ-Fe2O3@SiO2 shows two distinguished weight loss peaks. First,
a decrease in weight (2 %) related to desorption of surface water is observed from
room temperature to 130 °C. In continuation, 3 % weight loss is seen between 130
and 700 °C, which indicates the thermal stability of the compound (a). In the case
Fig. 2 FT-IR spectra of γ-Fe2O3 (a), γ-Fe2O3@SiO2 NPs (b), [Bis-APTES]Cl2 (c), and γ-Fe2O3@SiO2@
[Bis-APTES]Cl2 NPs (d)
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New magnetic nanocatalyst containing a bis-dicationic ionic…
Fig. 3 XRD patterns of γ-Fe2O3@SiO2 NPs (a) and γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs (b)
of γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs, decomposition started at 340 °C and
18 % weight loss was observed up to 700 °C, which in comparison with γ-Fe2O3@
SiO2 (5 %) is a higher amount to the same temperature. This diference is caused by
decomposition of ionic liquid on the surface of the catalyst, confrming successful
immobilization of the organic part on Fe2O3@SiO2 (b) [50] (Fig. 4).
Fig. 4 TGA curves of γ-Fe2O3@SiO2 NPs (a) and γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs (b)
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R. Karimi-Chayjani et al.
Fig. 5 FESEM image of γ-Fe2O3 NPs
Fig. 6 FESEM image of γ-Fe2O3@SiO2 NPs
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New magnetic nanocatalyst containing a bis-dicationic ionic…
FESEM analysis
Field-emission scanning electron microscopy (FESEM) revealed γ-Fe2O3 and
γ-Fe2O3@SiO2 microspheres with size of around 18 and 35 nm, respectively
(Figs. 5, 6). Comparison between γ-Fe2O3@SiO2 and γ-Fe2O3@SiO2@[Bis-
APTES]Cl2 NPs showed that, in the case of the prepared catalyst, the free state of
the particles was changed, becoming interconnected and attached (Figs. 7, 8). This
shows that ionic liquid links are successfully formed between the γ-Fe2O3@SiO2
nanoparticles. Also, these results indicate that the particles were connected in difer-
ent directions.
Vibrating‑sample magnetometry (VSM) analysis
The synthesized nanomagnetic catalyst possessed a high superparamagnetic char-
acteristic. Magnetic measurements were performed in an applied magnetic feld at
room temperature, sweeping the feld from −12,000 to +12,000 Oe using a VSM to
measure the magnetic properties. The magnetic saturation of γ-Fe2O3@SiO2@[Bis-
APTES]Cl2 NPs was found to be 40 emu/g. The catalyst showed high magnetization
ability, sufcient for magnetic separation using a conventional magnet (Fig. 9).
Fig. 7 FESEM image of γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs, revealing a severe change in surface
morphology
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R. Karimi-Chayjani et al.
Fig. 8 FESEM image of γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs, indicating their size
Fig. 9 Magnetization curve of γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs
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New magnetic nanocatalyst containing a bis-dicationic ionic…
Table 1 Optimization of typical Knoevenagel reaction
Entry
Catalyst (mg)
Solvent
Temp.
Time (min.)
Yield (%)c
1
2
3
4
5
6
7
8
–
5
5
5
5
5
5
5
5
5
5
2
8
5
–
–
80 °C
80 °C
Refux
Refux
RT
150
120
120
120
120
100
55
20
6
1
1
3
1
15a
25a
35a
60a
55a
75b
81b
85b
90b
98b
90
CHCl3
CH3CN
EtOH
EtOH
H2O
H2O
H2O
H2O
H2O
Refux
RT
50 °C
70 °C
80 °C
90 °C
80 °C
80 °C
Refux
9
10
11
12
13
14
H2O
H2O
H2O/EtOH (1:1)
95b
96b
88b
3
Reaction conditions: 4-chlorobenzaldehyde (1.0 mmol), malononitrile (1.1 mmol), solvent (3.0 mL), and
required amount of catalyst
aReaction not completed
bYields refer to isolated products
R
γ
-Fe2O
-NPs
(5 mg)
3@SiO2@[Bis-APTES]Cl 2
R
Ar
CHO
Ar
o
H2
O
80 C
(3 mL),
CN
R=
CN
2Et
CN CO
,
Scheme 2 Preparation of arylidene malononitrile and ethyl-3-aryl-2-cyanoacrylate derivatives
Catalytic activity
Optimization and derivation steps
After preparation and on the basis of the results obtained from the structural stud-
ies, we investigated the catalytic ability of γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs
in reactions needing acceleration using an acidic catalyst. For this purpose, Knoev-
enagel reaction between various aldehydes and malononitrile (or ethyl cyanoacetate)
in presence of this reagent was studied.
Optimization step
First, to optimize the conditions, the reaction of 4-cholorobenzaldehyde and malo-
nonitrile was selected as a model. Performing this reaction in absence of catalyst
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R. Karimi-Chayjani et al.
and solvent and in refuxing chloroform did not result in a remarkable product.
Carrying out the reaction in refuxing acetonitrile and room-temperature ethanol
did not lead to reaction completion, while use of refuxing ethanol led to reac-
tion completion but after long reaction time. Replacement of the organic solvent
with water led to the best result at 80 °C. Using a mixture of water and ethanol
(1:1) caused a slight decrease in the reaction rate and yield. Also, alteration of the
amount of catalyst showed that the optimum amount was 5 mg for 1 mmol scale
reaction (Table 1).
After the optimization step and determination of the best conditions (Scheme 2),
various aldehydes with electron-donating or electron-withdrawing groups were used
for this reaction, all leading to corresponding products. No noticeable diference was
observed between the various types of aldehydes in this reaction, but the reaction
rate and yield were lower when malononitrile was replaced with ethyl cyanoacetate
(Table 2).
Reusability of catalyst
Figure 10 shows the magnetic property of the catalyst which facilitates the recycling
process.
To determine the reusability of γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs, as one
of its distinguished advantages, reusability curves were prepared for the mentioned
reaction. First, a typical reaction (Table 2, entry 4) was selected. After completion of
the reaction, the product was dissolved by addition of ethanol or acetone and heat-
ing. Then, the fask was placed on an external magnet, and the solution was poured
into a beaker while the catalyst remained in the fask. The same reaction was per-
formed in the same fask without addition of extra catalyst. This test was repeated
for six times for this reaction. The results confrmed the excellent recyclability and
reusability of the catalyst in the studied reaction (Fig. 11).
Comparison of catalytic activity
To illustrate the catalytic ability of the synthesized nanoparticles in the mentioned
reactions, the results obtained from the reaction of 4-chlorobenzaldehyde and malo-
nonitrile in presence of γ-Fe2O3 @SiO2@[Bis-APTES]Cl2 NPs are compared with
some other catalysts introduced in literature in Table 3. In all cases, γ-Fe2O3@
SiO2@[Bis-APTES]Cl2 NPs showed one or more advantages. Meanwhile, the high
reusability of the nanocatalyst introduced herein with respect to many other types is
also noticeable.
Reaction mechanism
The proposed mechanism begins with activation of both aldehyde and nitrile com-
pounds using acidic hydrogen of the catalyst. After that, the activated aldehyde is
attacked by the activated nitrile compound to obtain intermediate I. Then, with the
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New magnetic nanocatalyst containing a bis-dicationic ionic…
Table 2 Synthesis of arylidene malononitrile and ethyl-3-aryl-2-cyanoacrylate derivatives
1 3

R. Karimi-Chayjani et al.
Table 2 (continued)
*Yields refer to isolated products
1 3

New magnetic nanocatalyst containing a bis-dicationic ionic…
Fig. 10 Recycling catalyst using external magnet
Fig. 11 Reusability curves for preparation of arylidene malononitrile derivative (Table 2, entry 4)
1 3

R. Karimi-Chayjani et al.
Table 3 Comparison between some catalysts and γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs in the men-
tioned reaction
Entry Catalyst [Ref.]
Amount
of catalyst
(mg)
Condition
Time (min) Yield (%) Product
1
Fe3O4@SiO2@
CuO–Fe2O3 [34]
30
H2O/refux
7
91
2
Fe3O4@SiO2-3N
[35]
50
Toluene/75 °C 20
90
3
4
5
6
Ni NPs [36]
[TBA][Leu] [37]
Taurine [38]
10
9
25
5
S.F.
45
60
7
95
97
98
96
H2O/50 °C
H2O/refux
PEG/H2O
MNPs–Guanidine
150
[52]
7
8
Melamine [53]
This work
10
5
S.F. R.T
H2O/80 °C
15
1
90
98
H
H
N
H
HN
C
O
H
Ar
H
R
R
H
Ar
H
CN
-H
2O
Ar
H
OH CN
(I)
R
H
R
Scheme 3 Proposed mechanism for the mentioned reaction in presence of γ-Fe2O3@SiO2@[Bis-
APTES]Cl2 NPs
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New magnetic nanocatalyst containing a bis-dicationic ionic…
interaction of the catalyst, a molecule of water is eliminated from intermediate I,
leading to the corresponding product (Scheme 3).
Conclusions
Compounds containing triethoxysilane moiety have been used for many years as
linkers to attach organic compounds to mineral supports. We used this reagent as
both the cationic part and linker to prepare γ-Fe2O3@SiO2@[Bis-APTES]Cl2 NPs
by immobilization of this reagent on the surface of γ-Fe2O3@SiO2 NPs. After
characterization using various techniques, the prepared compound was applied as
a catalyst in synthesis of arylidene malononitrile and ethyl-3-aryl-2-cyanoacrylate
derivatives. This reaction was catalyzed well in the presence of this new magnetic
bis-dicationic reagent to give the desired target molecules. The results also con-
frmed the high efciency and considerable reusability of the prepared catalyst in
the studied reactions. It should be mentioned that, since this nanocatalyst is not
a hygroscopic material, it is very suitable for use and storage in humid climates.
Acknowledgements We are grateful to the Research Council of the University of Guilan for partial sup-
port of this research.
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