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

Abstract

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.

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1 Introduction

Since the concept of a monomolecular bifunctional catalyst for cooperative catalysis was first proposed in 2003 by Takemoto group,¹ both homogeneous and heterogeneous catalysts with molecular design and their application in organic transformations have become the focus of attention. The pursuit of green chemical processes impels scientists and researchers to exploit more recoverable catalysts with multifunctional surfaces. The immobilization of homogeneous catalysts on solid supports has been considered as an efficient approach to realize catalyst recycling.² It is worth mentioning that the dispersion of most heterogeneous catalysts in reaction medium are poor.^3,4 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 catalysts.⁵ However, nanoparticles are difficult to separate by conventional filtration techniques.⁶ Magnetic nanoparticles are a promising option.⁶ As an ideal support, magnetic Fe₃O₄ is non-toxic, easy to prepare, active for modification and functionalization.⁵ From the point view of sustainable development, magnetically separable nanocatalysts with multifunctional surface are attracting researches' great attentions.

Nitroalkenes are versatile building blocks⁷ because they have been widely used as substrates such as Michael addition,^1,8 Friedel–Crafts alkylation,^9,10 1,3-dipolar cycloaddition^11,12 and domino reaction.¹³ They have also been used as key precursors of different targets for scavengers of macrophage-generated oxidants,¹⁴ antidepressant drugs¹⁵ and rat even human monoamine oxidase inhibitors.¹⁶ The nitro group can be converted into various functional groups.^17,18 Several reports about nitroalkenes synthesis have been described.^17–26 The immobilization of amines on silica has received much attention due to the large specific surface area and good dispersion.^19–23 In these reports, few nanocatalysts have bifunctional even trifunctional active sites on their surfaces.^21,22 Additionally, conventional filtration and centrifuge for isolation of these silica-supported nanocatalysts from the reaction mixture are not so efficient.⁶ Magnetic Fe₃O₄-supported catalysts with multifunctional surface not only can achieve simple and efficient separation but also can collaboratively accelerate a single reaction or enable one-pot reaction sequences.

With this background in mind, in this paper a magnetically separable trifunctional nanocatalyst (Scheme 1, Fe₃O₄@SiO₂–NH₂) was prepared for cooperative catalytic 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 Si–OH groups on the surface of Fe₃O₄@SiO₂–NH₂. Under the optimized conditions, various representative substrates were extended. After reaction the separation and recovery of magnetic Fe₃O₄ supported catalysts could be achieved using an external magnet. Combined with the experimental data and literatures, possible reaction mechanism for selective synthesis of nitroalkenes through a trifunctional cooperative catalysis was proposed.^{17–19,21,22,25–27}

Scheme 1 The synthesis of Fe₃O₄@SiO₂–NH₂.

2 Experimental section

2.1 Materials

Ferric chloride hexahydrate (FeCl₃·6H₂O), ferrous chloride tetrahydrate (FeCl₂·4H₂O), ammonia (25 wt%), trisodium citrate, acetonitrile (CH₃CN), ethanediamine (NH₂CH₂CH₂NH₂) were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, P. R. China). Toluene (C₆H₅CH₃) was obtained from Tianjin Jiangtian 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 (CH₂Cl₂) and toluene (C₆H₅CH₃) were dried with CaH₂, distilled under reduced pressure and stored over molecular sieves. Other reagents, unless otherwise stated, were used as received for the reaction without further purification. Deionized water was used for all experiments.

2.2 Analytical methods

NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer in CDCl₃ using 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 BIO-RAD FTS3000 IR Spectrum Scanner. The X-ray diffraction (XRD) spectra of the samples were measured using an X-ray diffractometer (BDX3300) with reference target: Cu Kα radiation (λ = 1.54 Å), voltage: 30 kV and current: 30 mA. The samples were measured from 10 to 90° (2θ) with steps of 4° min⁻¹. Preparative TLC (20 cm × 20 cm) was performed on Silica Gel 60 F254.

2.3 Preparation of Fe₃O₄ magnetic nanoparticles (Fe₃O₄ MNPs)

The Fe₃O₄ MNPs were prepared following the reported chemical co-precipitation methods with slight modification.^28–30 First, an oven-dried 500 mL, three-necked flask equipped with a Teflon coated mechanical stir bar, constant-voltage 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 (C₆H₅Na₃O₇·2H₂O, 0.510 g) and then 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 30 min. Finally, 100 mL of aqueous solution containing (25 wt%) ammonium hydroxide 25 mL was added drop-wise to the above mixture, and it immediately turned black. The 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 Fe₃O₄ magnetic nanoparticles (Fe₃O₄@SiO₂–NH₂)

Silica coated Fe₃O₄ magnetic nanoparticles (Fe₃O₄@SiO₂) was synthesized according to the procedure reported in the previous literatures.^28,29,31,32 Chloropropyl modified silica coated Fe₃O₄ magnetic nanoparticles (Fe₃O₄@SiO₂–Cl) was synthesized according to the procedure reported in the previous literature.³³ The obtained Fe₃O₄@SiO₂–Cl (3 g) was suspended in acetonitrile (200 mL) with sonication about 20 min. To the mixture, ethylenediamine (10 mL) was added drop-wise 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 (Fe₃O₄@SiO₂–NH₂) was separated 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 oven-dried 25 mL flask was charged with Fe₃O₄@SiO₂–NH₂ (0.1 g) and the aldehyde (0.3 mmol) in CH₃NO₂ (5 mL). 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 3 times respectively. Combined the organic layer and dried with anhydrous Na₂SO₄, then the Na₂SO₄ was filtered off. After removal of the solvent under reduced pressure, the crude product was purified by preparative TLC to give the pure nitroalkene.

2.6 Separation of the catalyst and recycling tests

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 Fe₃O₄@SiO₂–NH₂

Scheme 1 shows the sequence of events in the functionalization of Fe₃O₄ MNPs with ethylenediamine. Firstly, the Fe₃O₄ MNPs were prepared using reported chemical coprecipitation methods with slight modification. Secondly, the surface of Fe₃O₄ MNPs was coated with a silica shell by the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol/ammonia mixture to yield Fe₃O₄@SiO₂. Then treatment of Fe₃O₄@SiO₂ with chloropropyl triethoxysilane obtained chloropropyl modified Fe₃O₄@SiO₂ (Fe₃O₄@SiO₂–Cl). Finally, Fe₃O₄@SiO₂–NH₂ was synthesized via the substitution of Fe₃O₄@SiO₂–Cl with excess of ethylenediamine. The content of nitrogen was determined to be 1.27 mmol g⁻¹ by elemental analysis.

FTIR spectra of Fe₃O₄, Fe₃O₄@SiO₂, Fe₃O₄@SiO₂–Cl and Fe₃O₄@SiO₂–NH₂ were shown in Fig. 1. The strong adsorption peak at 581 cm⁻¹ corresponded to the characteristic vibration absorption peak of Fe–O bond. The peak at 3386 cm⁻¹ was assigned to the –OH on the surface Fe₃O₄ particles. The adsorption peaks located at 1613 and 1398 cm⁻¹ can be attributed to the stretching vibration of C=O and C–O, which indicates the presence of carboxyl groups on the surface of Fe₃O₄ NPs. The new absorption peaks in Fig. 1(b) at 1089, 795 and 465 cm⁻¹ were assigned to the Si–O–Si, the peaks at 960 and 3736 cm⁻¹ were assigned to Si–OH moieties. These indicate that the silica has been successfully coated on the surface of Fe₃O₄ NPs. In Fig. 1(c) and (d) the adsorption peaks at 1459 cm⁻¹ and 2927 cm⁻¹ were related to C–H stretching modes of the propyl groups. The absorption peaks at 3421 cm⁻¹ and 1508–1551 cm⁻¹ were assigned to the stretching vibration and bending vibration of N–H. These suggested that the Fe₃O₄@SiO₂–NH₂ was synthesized successfully.

Fig. 1 FT-IR spectra of Fe₃O₄ (a), Fe₃O₄@SiO₂ (b), Fe₃O₄@SiO₂–Cl (c) and Fe₃O₄@SiO₂–NH₂ (d).

The crystalline structures of the Fe₃O₄ particles, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂–NH₂ were characterized by XRD and the results are shown in Fig. 2. The experimentally obtained patterns were compared with the standard Fe₃O₄ pattern (JCPDS (The Joint Committee on Powder Diffraction Standards) card no. 19-0629). The diffraction peaks at 2θ = 30.2°, 35.4°, 43.4°, 53.1°, 57.2° and 62.6° correspond to the (220), (311), (400), (422), (511) and (440) Fe₃O₄ lattice indices respectively.^28–31 These results prove that the Fe₃O₄ obtained has highly crystalline cubic spinel structure. The XRD patterns of Fe₃O₄@SiO₂ (b) and Fe₃O₄@SiO₂–NH₂ (c) show an obvious diffusion peak at about 15.3° which is due to the effect of amorphous silica and organic groups.^28,31,34,35 These results indicated that the surface of Fe₃O₄ was coated with a silica shell successfully and Fe₃O₄@SiO₂ was successfully modified by organic groups.

Fig. 2 XRD patterns of Fe₃O₄ (a), Fe₃O₄@SiO₂ (b) and Fe₃O₄@SiO₂–NH₂ (c).

The surface morphology and size of Fe₃O₄ (a) and Fe₃O₄@SiO₂ (b) were observed by transmission electron microscopy (TEM) as shown in Fig. 3.

Fig. 3 TEM images of Fe₃O₄ (a) and Fe₃O₄@SiO₂ (b).

It can be seen that the Fe₃O₄ particles are successfully packaged by silica. Although the presence of multinuclei Fe₃O₄ cores embedded in SiO₂ shell, this will not affect its application as a support in catalysis.³

Thermogravimetric analysis was performed to evaluate the stability of Fe₃O₄@SiO₂–NH₂, since the following reaction required heating in an aerobic condition. Fig. 4 indicated the TG spectra of the fresh and spent catalyst recorded up to 1000 °C. As shown in Fig. 4, when Fe₃O₄@SiO₂–NH₂ was heated from room temperature to 80 °C, the loss of weight was only 0.7%, which proved that Fe₃O₄@SiO₂–NH₂ was relatively stable as a 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.

Fig. 4 TGA spectra of the fresh (a) and spent (b) Fe₃O₄@SiO₂–NH₂.

X-ray photoelectron spectrum of Fe₃O₄@SiO₂–NH₂ was shown in Fig. 5. The O 1s peak at 536.0 eV and Si 2p peak at 106.4 eV corresponded to the SiO₂-type material. The C 1s peak at 284.7 eV and the N 1s peak at 399.6 eV confirmed that ethylenediamine was successfully grafted to the surface of Fe₃O₄@SiO₂.³³

Fig. 5 XPS spectrum of Fe₃O₄@SiO₂–NH₂.

3.2 Cooperative catalysis for selective synthesis of nitroalkenes

3.2.1 Evaluation of its catalytic activity. The catalytic activity of prepared Fe₃O₄@SiO₂–NH₂ was tested for selective synthesis of 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 catalyst (Table 1, entries 1–3). 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 Fe₃O₄@SiO₂–NH₂ (Table 1, entry 3) was considered sufficient to catalyze the reaction. The optimized conditions were determined to be: anisaldehyde (0.3 mmol, 1a), nitromethane (5.0 mL, 2a) and Fe₃O₄@SiO₂–NH₂ (0.100 g) at 80 °C in air for 4 h. The corresponding product 3a was detected in 97% isolated yield.

Scheme 2 Selective synthesis of nitroalkene in the presence of Fe₃O₄@SiO₂–NH₂.

Table 1 Effect factors of cooperative catalysis for selective synthesis of nitroalkenes

Entry^a	Amount of catalyst (g)	Time (h)	Temperature (°C)	Yield^b (%)
a Reaction conditions: anisaldehyde (0.3 mmol) and nitromethane (5.0 mL), in air.b Isolated yield based on aldehyde.
1	0.050	4	80	55
2	0.075	4	80	68
3	0.100	4	80	97
4	0.125	4	80	98
5	0.150	4	80	>99
6	0.100	4	30	Trace
7	0.100	4	60	69
8	0.100	2	80	65
9	0.100	3	80	87

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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 Fe₃O₄@SiO₂–NH₂ catalyst was effective and selective toward most of the substrates. Both aldehydes bearing electron-rich and electron-withdrawing substituents on the aromatic rings reacting with nitromethane proceeded well and selectively gave the corresponding nitroalkenes in a moderate or excellent yield. However, the electron-withdrawing substituted aromatic aldehydes were slightly inferior to the electron-rich substituted aromatic aldehydes even under prolonged reaction times. As expected, the reaction between nitromethane and heterocyclic aromatic aldehyde was in a moderate yield.

Table 2 The reaction between nitromethane and aromatic aldehydes

Entry^a	Aldehyde	Nitroalkene	Time (h)	Yield^b (%)
a Unless otherwise specified, all the reactions were carried out using aromatic aldehydes or heterocyclic aromatic aldehyde (0.3 mmol), nitromethane (5.0 mL), Fe3O4@SiO2–NH2 (0.100 g), under 80 °C in air.b Isolated yield based on aldehyde.
1			4	97
2			4	95
3			4	88
4			4	97
5			6	74
6			6	87
7			4	93
8			4	89
9			4	56
10			4	97

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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 Fe₃O₄@SiO₂–NH₂ (0.200 g) and NH₂CH₂CH₂NH₂ (0.004 g) are equal. When ethylenediamine (0.004 g) was used in catalytic reaction, the yield was 63% (Table 3, 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 3). These indicated that primary amine and secondary amine collaboratively accelerated the reaction. In order to eliminate the effect of chlorine substituent and Si–OH groups, Fe₃O₄@SiO₂–Cl (0.200 g) was used for the reaction between anisaldehyde and nitromethane (Table 3, entry 4). Results showed that the separated chlorine substituent or Si–OH groups could hardly catalyze the reaction. When using Fe₃O₄@SiO₂–NH₂ (0.200 g) 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 Si–OH groups on the surface of Fe₃O₄@SiO₂–NH₂. In addition, the yield of 1,3-dinitroalkanes compounds 4a could be obtained in 26% by prolonged reaction time to 8 h. Combined with the experimental data and literatures,^{17–19,21,22,25–27} we speculated that the reaction between nitromethane and aromatic aldehyde under Fe₃O₄@SiO₂–NH₂ was through a trifunctional cooperative catalysis mechanism in Scheme 3.

Table 3 Reaction conditions for anisaldehyde and nitromethane using supported and unsupported amines

Entry^a	Catalyst (g)	Time (h)	Yield^b (%)
a Reaction conditions: anisaldehyde (0.6 mmol) and nitromethane (5.0 mL), under 80 °C in air.b Isolated yield based on anisaldehyde.c Yield of 1,3-dinitroalkanes compounds.
1	NH2CH2CH2NH2 (0.004 g)	4	63
2	NH(CH2CH3)2 (0.010 g)	4	47
3	NH2CH2CH2NH2 (0.004 g) + NH(CH2CH3)2 (0.010 g)	4	70
4	Fe3O4@SiO2@Cl (0.200 g)	4	Trace
5	Fe3O4@SiO2@NH2 (0.200 g)	4	95
6	Fe3O4@SiO2@NH2 (0.200 g)	8	71 (26)^c

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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 Fe₃O₄@SiO₂–NH₂ catalyst and reacts with an amino group to form an imine intermediate, the α-proton of nitromethane 2a is abstracted by secondary amine or 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 2a which is activated by secondary amine or another amino group and nitroalkenes 3a which might also be activated by the surface silanol group possibly occurs to obtain the 1,3-dinitroalkanes compound 4a. Nucleophilic attack of the deprotonated nitromethane on aromatic aldehyde with electron-withdrawing groups on the aromatic rings might easily occur without the formation of imine intermediates, resulting in lower selectivity to nitroalkene formation.²⁷

3.2.2 Separation of the catalyst and recycling tests. The recyclability of the Fe₃O₄@SiO₂–NH₂ catalyst was also investigated for the reaction of anisaldehyde (0.3 mmol, 1a) and nitromethane (5.0 mL, 2a) at 80 °C in air for 4 h. 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%, 75%, 27% respectively. The yield decreased greatly from 97% to 27% after the third usage. We have also examined the catalytic activity of the Fe₃O₄@SiO₂–NH₂ catalyst 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 Fig. 4 shows that Fe₃O₄@SiO₂–NH₂ was relatively stable in air as a catalyst in experimental conditions. Combining the experiment results with literature information, the decrease of catalytic activity might be the result of chemical poisoning of the surface of Fe₃O₄@SiO₂–NH₂ catalyst.³⁶ Although the exact deactivation mechanism of Fe₃O₄@SiO₂–NH₂ catalyst remains unclear, the formation of aminal seems to play a major role for remarkable catalyst deactivation.

4 Conclusions

In conclusion, a magnetically separable trifunctional nanocatalyst Fe₃O₄@SiO₂–NH₂ was 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 Si–OH 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 Fe₃O₄@SiO₂–NH₂ has the prominent properties as follows: (a) simple and efficient 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.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 20972109) and National Youth Science Foundation of China (No. 51403151).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11798d

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