Ethylenediamine-functionalized magnetic Fe3O4@SiO2 nanoparticles: cooperative trifunctional catalysis for selective synthesis of nitroalkenes

Article abstract of DOI:10.1039/c5ra11798d

A magnetically separable trifunctional nanocatalyst Fe₃O₄@SiO₂-NH₂ was synthesized and characterized by TEM, FT-IR, XRD, TGA, and EA. The designed nanocatalyst was found to be highly active for selective synthesis of nitroalkenes with nitromethane and aromatic aldehyde through cooperative trifunctional catalysis of primary amine, secondary amine and Si-OH groups on the surface of the catalyst. Under the optimized conditions, various representative substrates were extended to obtain the corresponding products in moderate or excellent yields. After the reaction, the trifunctional nanocatalyst was easily recovered and recycled by applying an external magnet. In addition, a possible cooperative trifunctional catalysis mechanism was also proposed.

Full text of DOI:10.1039/c5ra11798d

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Ethylenediamine-functionalized
Magnetic
Fe O @SiO
3
4
2
Nanoparticles: Cooperative Trifunctional Catalysis for
Selective Synthesis of Nitroalkenes
ab
ab
ab
ab
ab
Fengjun Xue, Yahao Dong, Peibo Hu, Yanan Deng and Yuping Wei*
a
Department of Chemistry, School of Science, Tianjin University, Tianjin 300072,
P.R.China
b
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin
3
00072, P.R.China
*
Corresponding Author. Tel and Fax: +86 22 27403475; Eꢀmail: ypwei@tju.edu.cn
Abstract A magnetically separable trifunctional nanocatalyst Fe O @SiO ꢀNH was
3
4
2
2
synthesized and characterized by TEM, FTꢀIR, XRD, TGA, and EA. The designed
nanocatalyst was found to be highly active for selective synthesis of nitroalkenes with
nitromethane and aromatic aldehyde through cooperative trifunctional catalysis of
primary amine, secondary amine and SiꢀOH groups on the surface of catalyst. Under the
optimized conditions, various representative substrates were extended to obtain the
corresponding products in a moderate or excellent yield. After reaction, the trifunctional
nanocatalyst was easily recovered and recycled by applying an external magnet. In
addition, a possible cooperative trifunctional catalysis mechanism was also proposed.
Keywords: Nitroalkenes, Magnetically recyclable nanocatalyst,
Henry reaction, Cooperative trifunctional catalysis

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1
Introduction
Since the concept of monomolecular bifunctional catalyst for cooperative catalysis was
1
first proposed in 2003 by Takemoto group , both homogeneous and heterogeneous
catalysts with molecular design and their application in organic transforms have become
the focus of attention. The pursuit for green chemical process impels scientists and
researchers to exploit more recoverable catalysts with multifunctional surface. The
immobilization of homogeneous catalysts on solid supports has been considered as an
2
efficient approach to realize catalyst recycling. It is worth mentioning that the dispersion
3
, 4
of most heterogeneous catalysts in reaction medium are poor. Therefore, the catalytic
activity of the heterogeneous catalyst will be inevitably affected compared to the
corresponding homogeneous catalyst. Due to the large specific surface area and versatile
surface, nanoparticles have attracted much attention for immobilization of homogeneous
5
catalysts. However, nanoparticles are difficult to separate by conventional filtration
6
6
techniques. Magnetic nanoparticles are a promising option. As an ideal support,
magnetic Fe O is nonꢀtoxic, easy to prepare, active for modification and
3
4
5
functionalization. From the point view of sustainable development, magnetically
separable nanocatalysts with multifunctional surface are attracting researches' great
attentions.
7
Nitroalkenes are versatile building blocks because they have been widely used as
1
, 8
9, 10
substrates such as Michael addition , FriedelꢀCrafts alkylation
, 1, 3ꢀdipolar
1
1, 12
13
cycloaddition
and domino reaction . They have also been used as key precursors of
1
4
different targets for scavengers of macrophageꢀgenerated oxidants , antidepressant
1
5
16
drug and rat even human monoamine oxidase inhibitors . The nitro group can be

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1
7, 18
converted into various functional groups.
Several reports about nitroalkenes synthesis
1
7ꢀ26
have been described.
attention due to the large specific surface area and good dispersion.
few nanocatalysts have bifunctional even trifunctional active sites on their surfaces.
The immobilization of amines on silica has received much
1
9ꢀ23
In these reports,
2
1ꢀ22
Additionally, conventional filtration and centrifuge for isolation of these silicaꢀsupported
6
nanocatalysts from the reaction mixture is not so efficient. Magnetic Fe O ꢀsupported
3
4
catalysts with multifunctional surface not only can achieve simple and efficient
separation but also can collaboratively accelerate a single reaction or enable oneꢀpot
reaction sequences.
With this background in mind, in this paper a magnetically separable trifunctional
nanocatalyst (Scheme 1, Fe O @SiO ꢀNH ) was prepared for cooperative catalytic
3
4
2
2
synthesis of nitroalkenes under mild conditions without additional solvents. The reaction
between nitromethane and aromatic aldehyde might experience a cooperative
trifunctional catalytic process which was promoted by primary amine, secondary amine
and SiꢀOH groups on the surface of Fe O @SiO ꢀNH . Under the optimized conditions,
3
4
2
2
various representative substrates were extended. After reaction the separation and
recovery of magnetic Fe O supported catalysts could be achieved using an external
3
4
magnet. Combined with the experimental data and literatures, possible reaction
mechanism for selective synthesis of nitroalkenes through a trifunctional cooperative
1
7ꢀ19, 21, 22, 25ꢀ27
catalysis was proposed.

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Scheme 1 The synthesis of Fe O @SiO ꢀNH
2
3
4
2
2
Experimental Section
2
.1 Materials
Ferric chloride hexahydrate (FeCl ·6H O), ferrous chloride tetrahydrate (FeCl ·4H O),
3
2
2
2
ammonia (25 wt %), trisodium citrate, acetonitrile (CH CN), ethanediamine
3
(
NH CH CH NH ) were purchased from Tianjin Guangfu Fine Chemical Research
2 2 2 2
Institute (Tianjin, P. R. China). Toluene (C H CH ) was obtained from Tianjin Jiangtian
6
5
3
Chemical Technology Co., Ltd. Absolute alcohol, nitromethane were supplied by Tianjin
Real & Lead Chemical Co., Ltd. Tetraethyl orthosilicate (TEOS) was obtained from
Tianjin Kemiou Chemical Reagent Co., Ltd. Chloropropyl triethoxysilane was purchased
from Heowns Biochem Technologies LLC. Dichloromethane (CH Cl ) and toluene
2
2
(
C H CH ) were dried with CaH , distilled under reduced pressure and stored over
6 5 3 2
molecular sieves. Other reagents, unless otherwise stated, were used as received for the

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reaction without further purification. Deionized water was used for all experiments.
.2 Analytical Methods
2
NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer in CDCl using
3
TMS as an internal standard. The morphology of the samples was observed on a Tecnai
G2 F20 transmission electron microscopy (TEM). The content of nitrogen was
determined by a Vario EL cube elemental analyzer. Thermogravimetric analysis (TGA)
was measured with a STA 409 PC thermal analyzer (NETZSCH, Germany). Fourier
transform infrared spectroscopy (FTIR) analysis was performed on a BIOꢀRAD FTS3000
IR Spectrum Scanner. The Xꢀray diffraction (XRD) spectra of the samples were measured
using an Xꢀray diffractometer (BDX3300) with reference target: Cu Ka radiation (λ=1.54
Å), voltage: 30 kV and current: 30 mA. The samples were measured from 10 to 90° (2θ)
ꢀ1
with steps of 4° min . Preparative TLC (20 cm × 20 cm) was performed on Silica Gel 60
F254.
2
.3 Preparation of Fe O Magnetic Nanoparticles (Fe O MNPs)
3 4 3 4
The Fe O MNPs were prepared following the reported chemical coꢀprecipitation
3
4
2
8ꢀ30
methods with slight modification.
First, an ovenꢀdried 500 mL, threeꢀnecked flask
equipped with a Teflon coated mechanical stir bar, constantꢀvoltage funnel was charged
with ferric chloride hexahydrate (21.624 g, 0.08 mol), ferrous chloride tetrahydrate
(
8.419 g,﹥0.04 mol) and sodium citrate dehydrate (C H Na O ·2H O 0.510 g) and then
6 5 3 7 2
sealed tightly with a rubber septum. The flask was evacuated and backfilled with nitrogen
three times under mechanical stirring. Then, 100 mL of distilled water was added via a
dropping funnel under vigorous stirring. The mixture was heated to 60 °C and stirred for

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3
0 min. Finally, 100 mL of aqueous solution containing (25 wt %) ammonium hydroxide
5 mL was added dropꢀwise to the above mixture, and it immediately turned black. The
2
reaction mixture was heated to 80 °C and kept for 2 h and then allowed to cool to room
temperature. The black products were obtained with the help of a magnet and washed
with distilled water, ethanol 3 times respectively and then dried under vacuum at 50 °C.
2
.4 Synthesis of Aminopropyl Modified Silica Coated Fe O Magnetic Nanoparticles
3 4
(
Fe O @SiO ꢀNH )
3 4 2 2
Silica coated Fe O magnetic nanoparticles (Fe O @SiO ) was synthesized according to
3
4
3
4
2
2
8, 29, 31, 32
the procedure reported in the previous literatures.
Chloropropyl modified silica
coated Fe O magnetic nanoparticles (Fe O @SiO ꢀCl) was synthesized according to the
3
4
3
4
2
3
3
procedure reported in the previous literature. The obtained Fe O @SiO ꢀCl (3 g) was
3
4
2
suspended in acetonitrile (200 mL) with sonication about 20 min. To the mixture,
ethylenediamine (10 mL) was added dropꢀwise via a dropping funnel under mechanical
stirring at nitrogen atmosphere and the reaction mixture was refluxed for 12 h. After
cooling to room temperature, the resulted solid (Fe O @SiO ꢀNH ) was separated
3
4
2
2
magnetically and then washed with absolute ethanol several times and dried under
vacuum at 50°C.
2
.5 General procedure for the synthesis of nitroalkenes
Typically, the reactions were carried out as follows: an ovenꢀdried 25 mL flask was
charged with Fe O @SiO ꢀNH (0.1 g) and the aldehyde (0.3 mmol) in CH NO (5 mL).
3
4
2
2
3
2
Then the mixture was stirred at 80 °C under air. After the reaction was completed, the
solid catalyst was separated magnetically, washed with ethyl acetate and dichloromethane

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3
times respectively. Combined the organic layer and dried with anhydrous Na SO , then
2 4
the Na SO was filtered off. After removal of the solvent under reduced pressure, the
2
4
crude product was purified by preparative TLC to give the pure nitroalkene.
.6 Separation of the catalyst and recycling tests
2
After completion of the reaction, the solid catalyst was separated from the reaction
mixture with the help of a magnet and washed with ethyl acetate and dichloromethane 3
times respectively. Without drying, the recovered catalyst was directly used in next run
with new portions of reactants.
3
Results and Discussion
3
.1 Synthesis and characterization of Fe O @SiO ꢀNH
2
3
4
2
Scheme 1 shows the sequence of events in the functionalization of Fe O MNPs with
3
4
ethylenediamine. Firstly, the Fe O MNPs were prepared using reported chemical
3
4
coprecipitation methods with slight modification. Secondly, the surface of Fe O MNPs
3
4
was coated with a silica shell by the hydrolysis and condensation of tetraethyl
orthosilicate (TEOS) in ethanol/ammonia mixture to yield Fe O @SiO . Then treatment
3
4
2
of Fe O @SiO with chloropropyl triethoxysilane obtained chloropropyl modified
3
4
2
Fe O @SiO (Fe O @SiO ꢀCl). Finally, Fe O @SiO ꢀNH was synthesized via the
3
4
2
3
4
2
3
4
2
2
substitution of Fe O @SiO ꢀCl with excess of ethylenediamine. The content of nitrogen
3
4
2
was determined to be 1.27 mmol/g by elemental analysis.
FTIR spectra of Fe O , Fe O @SiO , Fe O @SiO ꢀCl and Fe O @SiO ꢀNH were shown
3
4
3
4
2
3
4
2
3
4
2
2
−
1
in Figure 1. The strong adsorption peak at 581 cm corresponded to the characteristic
−
1
vibration absorption peak of FeꢀO bond. The peak at 3386 cm was assigned to the ꢀOH

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−
1
on the surface Fe O particles. The adsorption peaks located at 1613 and 1398 cm can
3
4
be attributed to the stretching vibration of C = O and CꢀO, which indicates the presence
of carboxyl groups on the surface of Fe O NPs. The new absorption peaks in Figure 1(b)
3
4
−
1
−1
at 1089, 795 and 465 cm were assigned to the SiꢀOꢀSi, the peaks at 960 and 3736 cm
were assigned to Si–OH moieties. These indicate that the silica has been successfully
coated on the surface of Fe O NPs. In Figure 1(c) and (d) the adsorption peaks at 1459
3
4
−
1
−1
cm and 2927 cm were related to CꢀH stretching modes of the propyl groups. The
−
1
−1
absorption peaks at 3421 cm and 1508ꢀ1551 cm were assigned to the stretching
vibration and bending vibration of NꢀH. These suggested that the Fe O @SiO ꢀNH was
3
4
2
2
synthesized successfully.
d
c
b
a
4000
3000
2000
1000
-1
Wavenumber/cm
Fig. 1 FTꢀIR spectra of Fe O (a), Fe O @SiO (b), Fe O @SiO ꢀCl (c) and
3
4
3
4
2
3
4
2
Fe O @SiO ꢀNH (d)
3
4
2
2
The crystalline structures of the Fe O particles, Fe O @SiO and Fe O @SiO ꢀNH were
3
4
3
4
2
3
4
2
2

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characterized by XRD and the results are shown in Figure 2. The experimentally obtained
patterns were compared with the standard Fe O pattern (JCPDS (The Joint Committee
3
4
on Powder Diffraction Standards) card no. 19ꢀ0629). The diffraction peaks at 2θ =30.2°,
5.4°, 43.4°, 53.1°, 57.2° and 62.6° correspond to the (220), (311), (400), (422), (511) and
3
2
8ꢀ31
(
440) Fe O lattice indices respectively.
These results prove that the Fe O obtained
3 4
3
4
has highly crystalline cubic spinel structure. The XRD patterns of Fe O @SiO (b) and
3
4
2
Fe O @SiO ꢀNH (c) show an obvious diffusion peak at about 15.3° which is due to the
3
4
2
2
2
8, 31, 34, 35
effect of amorphous silica and organic groups.
These results indicated that the
surface of Fe O was coated with a silica shell successfully and Fe O @SiO was
3
4
3
4
2
successfully modified by organic groups.
c
b
a
10
20
30
40
50
60
70
80
2θ( degree)
Fig. 2 XRD patterns of Fe O (a), Fe O @SiO (b) and Fe O @SiO ꢀNH (c)
3
4
3
4
2
3
4
2
2
The surface morphology and size of Fe O (a) and Fe O @SiO (b) were observed by
3
4
3
4
2
transmission electron microscopy (TEM) as shown in Figure 3.

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It can be seen that the Fe O particles are successfully packaged by silica. Although the
3
4
presence of multinuclei Fe O cores embeded in SiO shell, this will not affect its
3
4
2
3
application as a support in catalysis.
Fig. 3 TEM images of Fe O (a) and Fe O @SiO (b)
3
4
3
4
2
Thermogravimetric analysis was performed to evaluate the stability of Fe O @SiO ꢀNH ,
3
4
2
2
since the following reaction required heating in an aerobic condition. Figure 4 indicated
the TG spectra of the fresh and spent catalyst recorded up to 1000 °C. As shown in Figure
4
, when Fe O @SiO ꢀNH was heated from room temperature to 80 °C, the loss of
3 4 2 2
weight was only 0.7%, which proved that Fe O @SiO ꢀNH was relatively stable as a
3
4
2
2
catalyst in experimental conditions and it may be attributed to the bound water. Also, the
decomposition of the surface organic groups of catalyst may contribute to the loss part as
the small amount of nitrogen leaching occurred in recycled catalyst.
Xꢀray photoelectron spectrum of Fe O @SiO ꢀNH was shown in Figure 5. The O 1s
3
4
2
2

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peak at 536.0eV and Si 2p peak at 106.4eV corresponded to the SiO ꢀtype material. The
2
C 1s peak at 284.7 eV and the N 1s peak at 399.6 eV confirmed that ethylenediamine was
3
3
successfully grafted to the surface of Fe O @SiO .
3
4
2
a
b
100
90
80
200
400
600
800
1000
o
Temperature/ C
Fig. 4 TGA spectra of the fresh (a) and spent (b) Fe O @SiO ꢀNH
3
4
2
2
O 1s
C 1s
N 1s
Si 2p
1000
800
600
400
200
0
Binding Energy(eV)
Fig. 5 XPS spectrum of Fe O @SiO ꢀNH
2
3
4
2

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3
3
.2 Cooperative catalysis for selective synthesis of nitroalkenes
.2.1 Evaluation of its catalytic activity
Scheme 2 Selective synthesis of nitroalkene in the presence of Fe O @SiO ꢀNH
2
3
4
2
Table 1 Effect factors of cooperative catalysis for selective synthesis of nitroalkenes
a
b
Entry
Amount of catalyst (g) Time (h)
Temperature (°C) Yield (%)
1
2
3
4
5
6
7
8
0.050
0.075
0.100
0.125
0.150
0.100
0.100
0.100
0.100
4
4
4
4
4
4
4
2
3
80
80
80
80
80
30
60
80
80
55
68
97
98
>99
trace
69
65
9
a
87
Reaction conditions: anisaldehyde (0.3 mmol) and nitromethane (5.0 mL), in air.
b
Isolated yield based on aldehyde.
The catalytic activity of prepared Fe O @SiO ꢀNH was tested for selective synthesis of
3
4
2
2
nitroalkenes using anisaldehyde (0.3 mmol, 1a) and nitromethane (5.0 mL, 2a) as a
model reaction without additional solvents (Scheme 2). A wide variety of reaction
conditions such as the amount of catalyst, reaction time and temperature were
investigated as shown in Table 1. The yield increased with the increase of the amount of

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catalyst (Table 1, entries 1ꢀ3). However, no significant improvement of the yield was
observed when the amount of catalyst increased from 0.100 g to 0.150 g. Therefore,
0
.100 g Fe O @SiO ꢀNH (Table 1, entry 3) was considered sufficient to catalyze the
3 4 2 2
reaction. The optimized conditions were determined to be: anisaldehyde (0.3 mmol, 1a),
nitromethane (5.0 mL, 2a) and Fe O @SiO ꢀNH (0.100 g) at 80 °C in air for 4h. The
3
4
2
2
corresponding product 3a was detected in 97% isolated yield.
Under the optimal conditions in Table 1, a range of representative aromatic aldehydes
were applied to evaluate the scope of the reaction as shown in Table 2. The
Fe O @SiO ꢀNH catalyst was effective and selective toward most of the substrates. Both
3
4
2
2
aldehydes bearing electronꢀrich and electronꢀwithdrawing substituents on the aromatic
rings reacting with nitromethane proceeded well and selectively gave the corresponding
nitroalkenes in a moderate or excellent yield. However, the electronꢀwithdrawing
substituted aromatic aldehydes were slightly inferior to the electronꢀrich substituted
aromatic aldehydes even under prolonged reaction times. As expected, the reaction
between nitromethane and heterocyclic aromatic aldehyde was in a moderate yield.

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Table 2 The reaction between nitromethane and aromatic aldehydes.
a
b
Entry
1
Aldehyde
Nitroalkene
Time (h) Yield (%)
4
4
4
4
6
97
2
3
4
5
95
88
97
74
6
7
6
4
87
93
8
9
4
4
89
56
10
4
97
a
Unless otherwise specified, all the reactions were carried out using aromatic aldehydes
or heterocyclic aromatic aldehyde (0.3 mmol), nitromethane (5.0 mL),
Fe O @SiO ꢀNH (0.100 g), under 80 °C in air.
3
4
2
2

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b
Isolated yield based on aldehyde.
Table 3 Reaction conditions for anisaldehyde and nitromethane using supported and
unsupported amines
a
b
Entry
Catalyst (g)
Time (h)
Yield (%)
1
2
3
4
5
NH CH CH NH (0.004 g)
4
4
4
4
4
8
63
2
2
2
2
NH(CH CH ) (0.010 g)
47
2
3 2
NH CH CH NH (0.004 g) + NH(CH CH ) (0.010 g)
70
2
2
2
2
2
3 2
Fe O @SiO @Cl (0.200 g)
trace
95
3
4
2
Fe O @SiO @NH (0.200 g)
3
4
2
2
c
6
Fe O @SiO @NH (0.200 g)
71 (26)
3
4
2
2
a
Reaction conditions: anisaldehyde (0.6 mmol) and nitromethane (5.0 mL), under 80 °C
in air.
b
c
Isolated yield based on anisaldehyde.
Yield of 1,3ꢀdinitroalkanes compounds.
In order to confirm the possible cooperative trifunctional catalysis mechanism, the
following experiments were carried out as shown in Table 3. The content of primary
amine nitrogen in Fe O @SiO ꢀNH (0.200 g) and NH CH CH NH (0.004 g) are equal.
3
4
2
2
2
2
2
2
When ethylenediamine (0.004 g) was used in catalytic reaction, the yield was 63% (Table
, entry 1). In a similar way, when using diethylamine (0.010 g) for catalytic reaction, the
yield was only 47% (Table 3, entry 2). When ethylenediamine (0.004 g) and diethylamine
0.010 g) were used in catalytic reaction together, the yield was up to 70% (Table 3, entry
). These indicated that primary amine and secondary amine collaboratively accelerated
3
(
3
the reaction. In order to eliminate the effect of chlorine substituent and SiꢀOH groups,
Fe O @SiO ꢀCl (0.200g) was used for the reaction between anisaldehyde and
3
4
2

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nitromethane (Table 3, entry 4). Results showed that the separated chlorine substituent or
SiꢀOH groups could hardly catalyze the reaction. When using Fe O @SiO ꢀNH (0.200 g)
3
4
2
2
to catalyze the reaction, the yield was high to 95% (Table 3, entry 5). These results
suggested that the cooperative catalysis of primary and secondary amines was enhanced
by SiꢀOH groups on the surface of Fe O @SiO ꢀNH . In addition, the yield of 1,
3
4
2
2
3
8
ꢀdinitroalkanes compounds 4a could be obtained in 26% by prolonged reaction time to
1
7ꢀ19, 21, 22, 25ꢀ27
h. Combined with the experimental data and literatures
, we speculated that
the reaction between nitromethane and aromatic aldehyde under Fe O @SiO ꢀNH was
3
4
2
2
through a trifunctional cooperative catalysis mechanism in Scheme 3.
Scheme 3 A possible cooperative trifunctional catalysis mechanism
The aromatic aldehyde 1a is activated by both the silanol and primary amine on the
surface of the Fe O @SiO ꢀNH catalyst and reacts with an amino group to form an imine
3
4
2
2
intermediate, the αꢀproton of nitromethane 2a is abstracted by secondary amine or

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another amino group, accompanied by the nucleophilic attack of the deprotonated
nitromethane on imine intermediate, which is also assisted by the silanol group resulting
in the product nitroalkenes 3a. Additionally, the Michael reaction between nitromethane
2
a which is activated by secondary amine or another amino group and nitroalkenes 3a
which might also be activated by the surface silanol group possiblely occurs to obtain the
,3ꢀdinitroalkanes compound 4a. Nucleophilic attack of the deprotonated nitromethane
1
on aromatic aldehyde with electronꢀwithdrawing groups on the aromatic rings might
easily occur without the formation of imine intermediates, resulting in lower selectivity to
2
7
nitroalkene formation.
.2.2 Separation of the catalyst and recycling tests
The recyclability of the Fe O @SiO ꢀNH catalyst was also investigated for the reaction
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2
of anisaldehyde (0.3 mmol, 1a) and nitromethane (5.0 mL, 2a) at 80°C in air for 4h. After
completion of the reaction, the solid catalyst was separated from the reaction mixture
with the help of a magnet and washed with ethyl acetate and dichloromethane 3 times
respectively. Without drying, the recovered catalyst was directly used in next run with
new portions of reactants. The catalytic activity for the third consecutive cycles was 97%,
7
5%, 27% respectively. The yield decreased greatly from 97% to 27% after the third
usage. We have also examined the catalytic activity of the Fe O @SiO ꢀNH catalyst
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recovered at 80 °C. The same results were obtained as above. The total amount of
nitrogen leaching after the third cycle was 4.5% as determined by elemental analysis (the
amount of nitrogen is 1.78% before reaction and 1.70% after three cycles).
Thermogravimetric analysis in figure 4 shows that Fe O @SiO ꢀNH was relatively
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stable in air as a catalyst in experimental conditions. Combining the experiment results

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DOI: 10.1039/C5RA11798D
with literature information, the decrease of catalytic activity might be the result of
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chemical poisoning of the surface of Fe O @SiO ꢀNH catalyst. Although the exact
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deactivation mechanism of Fe O @SiO ꢀNH catalyst remains unclear, the formation of
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aminal seems to play a major role for remarkable catalyst deactivation.
4
Conclusions
In conclusion, a magnetically separable trifunctional nanocatalyst Fe O @SiO ꢀNH was
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designed and synthesized. The catalyst was found to be highly active for selective
synthesis of nitroalkenes with nitromethane and aromatic aldehyde involving cooperative
catalysis of primary amine, secondary amine and SiꢀOH groups. Under the optimized
conditions, various representative substrates were extended to obtain the corresponding
product in a moderate or excellent yield. In addition, a possible cooperative trifunctional
catalysis mechanism was also proposed. The magnetic trifunctional nanocatalyst
Fe O @SiO ꢀNH has the prominent properties as follows：(a) simple and efficient
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separation (b) cooperatively accelerate a single reaction. The molecular design of more
multifunctional surface and further applications to more organic reaction are currently
being investigated in our laboratory.
Acknowledgment
This work was supported by National Natural Science Foundation of China (No.
2
0972109) and National Youth Science Foundation of China (No. 51403151).

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