Generation of Cu nanoparticles on novel designed Fe₃O₄@SiO₂/EP.EN.EG as reusable nanocatalyst for the reduction of nitro compounds

Abstract

In this work, copper nanoparticles loaded on Fe₃O₄@SiO₂, which we have named Fe₃O₄@SiO₂/EP.EN.EG@Cu for short, have been designed, prepared and characterized by XRD, FTIR, TEM, SEM, EDS, TGA, ICP, successfully. The catalytic performance was investigated in the reduction of aromatic nitro compounds in the presence of NaBH₄ aqueous solution as the source of hydrogen at 50 °C. The Fe₃O₄@SiO₂/EP.EN.EG@Cu catalyst was readily removed from the reaction mixture by applying an external magnet and reused for five times without significant loss of catalytic activity.

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Introduction

In the past decades, controlled assembly of nanomaterials has attracted much attention. Well-known copper-based catalysts in organic reactions have been given much attention by materials scientists and chemists.^1–3 Homogeneous copper-based catalysts have been used in many organic transformations, but many problems are associated with the separation and reusability of the catalyst after the reaction that prevent their industrial applications. Therefore, the heterogenization of homogeneous catalysts to provide supported catalysts with highly active sites and high surface area has become an important protocol. Therefore, design and utilization of a heterogeneous nanocatalyst which can be practically separated easily is desirable.^4–7 Nanocatalysis has shown improved catalytic activity compared to bulk catalysts. This is due to an increased number of active sites for the smaller versions of the catalysts and high surface to volume ratio as compared to their larger counterparts.^8–11 In family of nanocatalysis, the magnetic nanoparticles have attracted increasing attention because they possess interesting properties which could see potential use in catalysis including nanomaterial-based catalysts,¹² magnetic resonance imaging,¹³ drug delivery systems,¹⁴ hyperthermia treatment of cancer cells.¹⁵ In addition, magnetic nanoparticles are easily separated from the reaction mixture by an external magnet and this is described as the more sufficient strategy than conventional techniques such as filtration or centrifugation.^16,17 Unfortunately, naked magnetic nanoparticles are often unstable, readily aggregated and usually don't constitute strong covalent bonds with functionalization molecules. Encapsulation of magnetic nanoparticles in inert matrices such as silica (SiO₂) can overcome these drawbacks.¹⁸ Magnetic nanoparticles have been extensively applied in various organic reaction, such as C–C bonding formation,¹⁹ dihydroxylation of olefins,²⁰ oxidation reactions²¹ and hydrogenation reaction.²² Hydrogenation of nitroarene is one of the most useful methods to production of aromatic amines. The amine group are often significant intermediates in the fabrication of many pharmaceuticals, agrochemicals, dyes, polymers, photographic and rubber materials.²³ There are common methods to reduce a nitro precursor: metal/acid reduction,^24–26 catalytic hydrogenation,²⁷ homogeneous catalytic transfer hydrogenation²⁸ and heterogeneous catalytic transfer hydrogenation.^29,30 However, these approaches have several major drawbacks: metal/acid system has shown poor selectivity and this methodology is environmentally dangerous, catalytic hydrogenation is undesirable due to use of H₂ gas at high pressure and temperature, the separation of homogeneous or heterogeneous catalytic transfer hydrogenation, from the final product is difficult.^31,32 Therefore, it is necessary to find new catalysts for hydrogenation of nitro arenes under mild conditions. In this paper we wish, to report preparation of functionalized magnetic nanoparticles Fe₃O₄@SiO₂/EP.EN.EG and in situ generation of Cu NPs as excellent catalytic efficiency and the catalytic activity has been estimate by the reduction of nitroarenes to amin with NaBH₄ under mild aqueous condition.

Results and discussion

As shown in Scheme 1, the Fe₃O₄@SiO₂/EP.EN.EG@Cu nanocatalyst was prepared in 4 steps: in first step the Fe₃O₄@SiO₂ magnetic nanoparticles was synthesized by Stöber sol–gel method,³³ in step 2, EP.EN.EG linker as novel designed ligand covalently bonded on surface hydroxyl groups of silica coated magnetic nanoparticles. In step 3, the ligands was activated with NaOH and then Cu(OAc)₂ was complexed to this N, N, O type ligand. Finally in step 4, Cu nanoparticles was generated with NaBH₄ reduction procedure. The synthesized Fe₃O₄@SiO₂/EP.EN.EG@Cu was characterized by FT-IR, thermo gravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDS), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM).

Scheme 1 Preparation process of the Fe₃O₄@SiO₂/EP.EN.EG@Cu.

FTIR was used to characterize the surface constructure of the synthesized materials, as shown in Fig. 1. The strong absorption bands at 573 cm⁻¹ can be attributed to the stretching vibration of the Fe–O bond, which confirms the existence of Fe₃O₄. The broad characteristic band 3444 cm⁻¹ is related to stretching vibration O–H, demonstrating the presence of hydroxyl groups on the surface of Fe₃O₄ MNPs (Fig. 1a). Comparision with Fig. 1a and b shows, that the intensity of Fe–O bond significantly weakens and a strong absorption peak at 1098 cm⁻¹ and weak absorption peak at 953 cm⁻¹ was appeared. This can be assigned to asymmetric vibration of the Si–O–Si bond and the symmetric stretching of the Si–OH bond, respectively, which suggests that Fe₃O₄ was effectively coated with a silica layer. The epoxy rings which are anchored on the surface of silica coated magnetic nanoparticles are characterized by the methylene C–H stretching³⁴ at 3050 cm⁻¹ and C–O–C vibration stretching³⁵ at 1260–1240 cm⁻¹. In both cases, vibration frequency was covered by the broad band of stretching vibration O–H and asymmetric vibration of the Si–O–Si bond, respectively. The absorption bond of stretching vibration O–H and asymmetric vibration of the Si–O–Si bond are getting more broadened which is clearly observed in the Fig. 1c in comparison to Fig. 1b. As shown in Fig. 1d broad absorption band in the 3436 cm⁻¹ are ascribed to the stretching vibration of O–H and –NH₂ and the band at 3700 cm⁻¹ are due to stretching of the N–H bonds or an overtone peaks of C–O stretching bands at 1233 cm⁻¹. Increasing of the intensity of the O–H and N–H stretching vibration can suggest the attachment of the second epoxy ring to catalyst (as shown in Fig. 1e).

Fig. 1 FTIR spectrum of Fe₃O₄ (a), Fe₃O₄@SiO₂ (b), Fe₃O₄@SiO₂.EP (c), Fe₃O₄@SiO₂.EP.EN (d), Fe₃O₄@SiO₂.EP.EN.EG (e).

The thermal stability of the Fe₃O₄@SiO₂/EP.EN.EG was characterized by thermo gravimetric analysis (TGA). TGA curves in Fig. 2 demonstrate the weight loss around 100 °C which was related to the loss of absorbed water molecules on the support. The weight loss of 13% in the temperature range of 190–600 °C which was attributed to the decomposition of organic parts that were grafted on the surface of Fe₃O₄@SiO₂ and amount of anchored content was evaluated to be 0.6 mmol g⁻¹. Therefore, the result confirms that the ligand grafting had been successfully achieved.

Fig. 2 TG analysis of Fe₃O₄@SiO₂/EP.EN.EG.

The crystal structure of Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂/EP.EN.EG@Cu were characterized by XRD. As shown in Fig. 3, six characteristic peaks at 2θ = 30.31, 35.64, 43.31, 53.86, 57.24, 62.83, which attributed to the (220), (311), (400), (422), (511), (440) crystal planes confirm well with the standard Fe₃O₄ (cubic phase). Fig. 3b Shows a new broad peak at 2θ = 23 which can be attributed to the amorphous silica shell. Compared with curve b, we find that no remarkable change occurs in the number of peaks in curve c, only a slight decline in the intensity of the diffraction peaks indicating retained crystal structure of the Fe₃O₄ core after immobilization of Cu nanoparticles. No peaks of Cu nanoparticles was observed in the XRD pattern due to the disperse of Cu nanoparticles in the catalysis matrix. So, Cu nanoparticles cannot lead to the formation of regular crystal lattice. The presence of Cu nanoparticles was verified through the analysis of ICP and EDS techniques.

Fig. 3 XRD pattern of Fe₃O₄ (a), Fe₃O₄@SiO₂ (b) and Fe₃O₄@SiO₂/EP.EN.EG@Cu (c). Fe₃O₄@SiO₂/EP.EN.EG@Cu catalyst after five recoveries (d).

Fig. 4a shows the SEM image of Fe₃O₄@SiO₂/EP.EN.EG@Cu nanocatalyst. Based on Fig. 4a spherical morphology of nanocatalyst is clearly observed. Moreover, to further investigate the morphologies and particle size of Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂/EP.EN.EG@Cu, TEM images of catalyst were studied. TEM images of Fig. 4b indicate the spherical structure of Fe₃O₄@SiO₂ and each nanocomposite contains several Fe₃O₄ nano particles in one SiO₂ shell, which results in the nano composites demonstrating strong magnetic properties.³⁶ also the average size of MNPs is about 20 nm. As expected, the as prepared Fe₃O₄@SiO₂/EP.EN.EG@Cu has the spherical shape (Fig. 4c), which indicates, spherical structure has not significantly influenced after immobilization of Cu nanoparticle on the surface of Fe₃O₄@SiO₂ support. In addition, the attendance of Fe, Si and Cu elements are shown in EDS spectrum (Fig. 4d). It also confirmed our design to the existence of Fe₃O₄, SiO₂ and Cu nanoelements in this catalyst. The copper amounts of catalyst were determined by inductively coupled plasma (ICP) 2.29 mmol g⁻¹.

Fig. 4 SEM images of Fe₃O₄@SiO₂/EP.EN.EG@Cu (a). TEM images of the Fe₃O₄ @SiO₂ (b). Fe₃O₄@SiO₂/EP.EN.EG@Cu (c). EDS spectrum of the Fe₃O₄@SiO₂/EP.EN.EG@Cu (d).

For magnetic designed materials, the magnetic responsivity is very important. Therefore, the magnetic properties of the Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂/EP.EN.EG@Cu were analyzed by VSM (vibrating sample magnetometer) at room temperature, as shown in Fig. 5. As can be seen, the saturation magnetization (M_s) value of the above magnetic samples are 63.6, 37.8 and 29.4, respectively. These results indicated the decrease in M_s values for Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂/EP.EN.EG@Cu in comparison to uncoated Fe₃O₄. These decreases are due to increase in mass and size after coating the SiO₂ shell and anchoring the Cu nanoparticles. Also, in the magnetization curves no noticeable remanence or coercivity was observed, which confirms superpara magnetic character. So, the new magnetic materials can be easily separated by an external magnet.

Fig. 5 Magnetization curve of Fe₃O₄ (a), Fe₃O₄@SiO₂ (b), Fe₃O₄@SiO₂/EP.EN.EG@Cu (c).

The catalytic performance of the novel Fe₃O₄@SiO₂/EP.EN.EG@Cu as a magnetically nanocatalyst was investigated for the reduction of aromatic nitro compounds. The reduction reaction of para-nitrophenol in the presence of NaBH₄ as the source of hydrogen (eqn (1)) was chosen as a model reaction to optimize the reaction conditions. At first, the effect of solvent and molar ratio of hydrogen sources on the model reaction were examined (Table 1, entries 1–10). It was found that water was the best solvent for the model reaction when 3.0 mmol NaBH₄ as hydrogen source were used and the product was obtained in 98% yield (Table 1, entry 5). A rise in the amount of NaBH₄ from 0.1 mmol to 3.0 mmol caused increment in the rate of reduction of para-nitrophenol (Table 1, entries 1–5). Reduction was very slow in aprotic solvents such as THF, DMF (Table 1, entries 7–8) and non-polar solvent like toluene (Table 1, entry 10). The employment of a protic solvents for example EtOH, CH₃OH, H₂O was the most suitable for this catalytic system (Table 1, entries 5–7), H₂O was selected for the nitroarene reduction because it is generally considered as inexpensive, safe, and environmentally green solvent.

Table 1 Reduction of para-nitrophenol in the presence of different amounts of NaBH₄ in various solvents by Fe₃O₄@SiO₂/EP.EN.EG@Cu

Entry	Hydrogen source	Solvent	Time (min)	Yield [%]
1	NaBH4 (0.1 mmol)	H2O	60	40
2	NaBH4 (0.5 mmol)	H2O	60	40
3	NaBH4 (1 mmol)	H2O	45	60
4	NaBH4 (2 mmol)	H2O	45	90
5	NaBH4 (3 mmol)	H2O	15	98
6	NaBH4 (3 mmol)	EtOH	20	95
7	NaBH4 (3 mmol)	CH3OH	30	90
8	NaBH4 (3 mmol)	THF	120	20
9	NaBH4 (3 mmol)	DMF	120	40
10	NaBH4 (3 mmol)	Toluene	240	20

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The influence of catalyst amount was investigated on the model reaction. No progress in the reduction of aromatic nitro compounds was observed neither in the absence of catalyst nor in the presence of Fe₃O₄@SiO₂/EP.EN.EG. Results in Table 2 shows that, the increasement of the amount of catalyst from 0.001 to 0.05 g per mmol of substrate, (Table 2, entries 3–7) results an increased reaction yield and the maximum yield (98%) has been achieved in the presence of 0.05 g of catalyst, (Table 2, entry 7). The results are in good agreement with significant rise in the number of active sites.

Table 2 Yield% dependence on the amount of Fe₃O₄@SiO₂/EP.EN.EG@Cu

Entry	Amount of catalyst (g)	Time (min)	Yield (%)
a In the presence of NaBH4.
1^a	—	15	0
2	Fe3O4@SiO2/EP.EN.EG	15	0
3	0.001	240	50
4	0.005	120	50
5	0.01	90	70
6	0.03	45	85
7	0.05	15	98

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As shown in Table 3, we employed the optimized conditions for the conversion of nitro aromatic containing different functional group into the corresponding amino compounds (Table 3, entries 1–15). The results of Table 3 indicate, electron with drawing/donating substituents do not have a remarkable influence on the reaction times and yields. Sensitive functional groups, such as –OH, –Cl, –CH₃, –COOH, –CN, –COCH₃, –OCH₃ and –NHCOCH₃ remained intact during the reduction of nitro group (Table 3, entries 6–14). Dinitro compounds were successfully transformed to the corresponding diamines, which are important precursor for many heterocyclic compounds and used as a component of engineering polymers and composites (Table 3, entries 3–5). Aldehyde carbonyl groups in the nitro compounds were not compatible with the reaction condition and reduction happens for both functional groups (Table 3, entry 15). According to steric effect that has been observed, the existence of electron with drawing/donating groups, ortho or meta to the nitro group increased the reaction time than at the para position. Therefore, these investigation indicated Fe₃O₄@SiO₂/EP.EN.EG@Cu catalyst system was also effective for the reduction of nitro aryl compounds with high yields and short reaction times in water.

Table 3 Reduction of various nitro aromatic compounds over Fe₃O₄@SiO₂/EP.EN.EG@Cu catalyst

Entry	Reactant	Product	Time (min)	Yield^a (%)	Mp (°C)
a Isolated yield.
1			15	85	—
2			8	98	100–102 °C (Lit. 101–104 °C)^37
3			10	98	100–102 °C (Lit. 101–104 °C)^37
4			8	95	64–66 °C (Lit. 63–65 °C)^37
5			5	100	139–140 °C (Lit. 139–141 °C)^37
6			10	95	40–41 °C (Lit. 40–41 °C)^38
7			10	95	57–59 °C (Lit. 57 °C)^39
8			5	98	62–64 °C (Lit. 62–66 °C)^40
9			Immediately	100	203–205 (Lit. 203–204 °C)^41
10			15	98	185–187 °C (Lit. 187–189 °C)^42
11			20	90	202–204 °C (Lit. 204–205 °C)^43
12			15	95	104–106 °C (Lit. 102–104 °C)^44
13			5	100	182–185 °C (Lit. 185–186 °C)^45
14			5	95	169–170 °C (Lit. 172–173 °C)^45
15			Immediately	100	90–92 °C (Lit. 90–92 °C)^38

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The reduction of p-nitrophenol to p-aminophenol using NaBH₄ as reductant and the Fe₃O₄@SiO₂/EP.EN.EG@Cu as the catalyst was studied by UV-Vis spectroscopy. The presence of both the reactant p-nitrophenolate anion (λ_max = 400 nm) and the product p-AP (λ_max = 300 nm) can be convincingly demonstrated by the UV/Visible absorption spectroscopy (Fig. 6a). Linear relationships of ln(C_t/C₀) versus reaction time was shown in Fig. 6b. Therefore, the reduction of p-nitrophenol follows the pseudo-first order kinetics. The rate constant k is calculated to be 3.2 × 10⁻³ s⁻¹.

Fig. 6 UV/Visible spectrum of the absorption spectra of mixture of p-nitrophenol and NaBH₄ in presence of Fe₃O₄@SiO₂/EP.EN.EG@Cu at different time (a), plots of ln(C_t/C₀) versus reaction time (b). Reaction condition: p-nitrophenol (5 × 10⁻³ M), NaBH₄ (0.5 mM) and catalyst (0.08 mol%) at 50 °C.

The reuse ability of a catalyst is very important for a catalytic process. The Fe₃O₄@SiO₂/EP.EN.EG@Cu catalyst was separated by an external magnet, was washed with ethanol to remove the organic impurities, was dried at room temperature and the catalyst was reused for at least 5 times. The result is presented in Table 4 and indicates that the catalytic activity of Fe₃O₄@SiO₂/EP.EN.EG@Cu did not decrease after 5 consecutive cycles.

Table 4 The catalyst reusability for the reduction of para-nitrophenol

Run	Time (min)	Yield (%)
1	15	98
2	15	98
3	15	98
4	15	95
5	15	95

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The kinetic reusability of Fe₃O₄@SiO₂/EP.EN.EG@Cu in the reduction of p-nitrophenol was studied (Fig. 7). Results demonstrate that Fe₃O₄@SiO₂/EP.EN.EG@Cu can be reused for at least 5 times. Based on these results, the reaction was not completed even after 60 min for recycle 6. The catalyst was recovered by an external magnetic field and then was washed with water and ethanol to remove any absorbed products.

Fig. 7 Plots of ln(C_t/C₀) versus reaction time for each of Fe₃O₄@SiO₂/EP.EN.EG@Cu recyclability.

Comparison of TEM image, SEM image (Fig. 8) of reused catalyst with fresh catalysts (Fig. 4a–c) showed that the morphology and structure of Fe₃O₄@SiO₂/EP.EN.EG@Cu catalyst was remained intact after five recoveries. The XRD pattern of reused Fe₃O₄@SiO₂/EP.EN.EG@Cu catalyst is shown in Fig. 3d, revealed that the Fe₃O₄ retain its magnetite crystalline structure after the 5 runs. In additionally, new characteristic diffraction peaks at 2θ = 36.88, 42.48 of Cu₂O can be observed after 5 times reused of catalyst, which indicated to the reaction between small amount of Cu nanoparticles and the air O₂ due to successive performing. Furthermore, the copper amounts of catalyst after five times reuse was determined by inductively coupled plasma (ICP) 1.22 mmol g⁻¹.

Fig. 8 TEM image of Fe₃O₄@SiO₂/EP.EN.EG@Cu after five times reuse (a), SEM image of Fe₃O₄@SiO₂/EP.EN.EG@Cu after five times reuse.

The catalytic activity of Fe₃O₄@SiO₂/EP.EN.EG@Cu was compared with various reported catalysts in the literature^46–50 in the case of reduction of 4-nitrophenol. It was observed that Fe₃O₄@SiO₂/EP.EN.EG@Cu promotes the yield of products and reduces the reaction time effectively in comparison with other catalysts. So, Fe₃O₄@SiO₂/EP.EN.EG@Cu can be considered as efficient catalyst in reduction of nitro aryl compounds (Table 5).

Table 5 Comparison of various catalysts for reduction of 4-nitrophenol to 4-aminophenol

Entry	Catalyst	Temp (°C)	Time (h)	Yield (%)	Ref.
1	CuBr2	RT	3.5	88	46
2	Cu NPs	50	5	66	47
3	Ru(0)-supported on AT-Mont	Reflux	12	65	48
4	NiCo2O4	80	1.5	90	49
5	Ni/SiO2	100	7	100	50
6	Fe3O4@SiO2/EP.EN.EG@Cu	50	15 (min)	98	Present work

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Experimental

The melting point of the products was determined with an Electrothermal Type 9100 melting point apparatus. The FT-IR spectra were recorded on an Avatar 370 FT-IR Thermal Nicolet spectrometer. Mass spectra were recorded on a 5973 Network Mass Selective Detector. The ¹HNMR spectra were recorded on a Bruker AC 400 MHz instrument in DMSO-d₆. Termogravimetric analysis (TGA) was performed on a Shimadzu Thermogravimetric Analyzer (TG-50). Transmission electron microscope (TEM) images were acquired on a TEM microscope Leo 912 AB120 kV Zeiss Germany. Elemental compositions were determined with a Leo 1450 VP scanning electron microscope equipped with an SC7620 energy dispersive spectrometer (SEM-EDS) presenting a 133 eV resolution at 20 kV. Inductively coupled plasma (ICP) was a obtained using a Varian, VISTA-PRO, CCD, Australia. X-ray diffraction (XRD) patterns were collected using a Bruker D4 X-ray diffractometer with Ni-filtered Cu Kα radiation (40 kV, 30 MA). The UV-Vis spectra were recorded on a Philips PUB 700 spectrophotometer with a quartz cuvette (path length = 1 cm) at 50 °C.

Synthesis of Fe₃O₄@SiO₂

The synthesis of core–shell Fe₃O₄@SiO₂ microspheres was carried out by a Stöber sol–gel method.³³ An aqueous dispersion of Fe₃O₄ nanoparticles (6 ml, 0.05 g ml⁻¹) were dissolved into a mixture of absolute ethanol (560 ml), deionized water (140 ml). Ammonia solution (28 wt%, 10 ml) was then added to the mixture under ultrasound for 20 min. Then, 8.0 ml tetraethyl orthosilicate (TEOS) was dropped into the mixture under stirring for 12 h. The acquired silica coated Fe₃O₄ nanoparticles were separated by using a magnet, consequently washed with ethanol several times and dried under vacuum.

General procedure for the preparation Fe₃O₄@SiO₂/EP.EN.EP

Fe₃O₄@SiO₂ (1 g) was added to the solution of epibromohydrin (1 ml) in ethanol (3 ml) and the mixture was stirred at 60 °C. After 5 h, the solvent was removed and the resulting solid was washed with ethanol and dried at 80 °C. Then the dried precipitate was suspended in ethylenediamine (5 ml). The reaction mixture was stirred at 60 °C for 24 h, after completion the product was separated by an external magnet, washed with MeOH and dried at 80 °C, the dried powder was mixed and stirred in epibromohydrin (1 ml) and ethanol (3 ml) at 60 °C for 5 h, the resulting solid material was separated by an external magnet, washed with MeOH and dried at 80 °C.

General procedure for the preparation of copper nanoparticles loaded on magnetic core (Fe₃O₄@SiO₂/EP.EN.EG@Cu)

In first step, Fe₃O₄@SiO₂/EP.EN.EP (0.5 g) was activated at room temperature for 1 h in the presence of 5 ml NaOH solution(0.1 mol l⁻¹), after separation, the activated catalyst (Fe₃O₄@SiO₂/EP.EN.EG) washed with water and ethanol several times. In second step, Cu(OAc)₂ (0.25 g) dissolved in absolute EtOH (5 ml) and stirred at room temperature for 4 h with 0.5 g activated catalyst. Then, solution of sodium borohydride (10.0 ml, 0.15 mol l⁻¹) was added dropwise to the mixture and stirred under Ar atmosphere for 2 h. Finally, nanocatalyst collected by magnetic separation, washed with MeOH and dried under vacuum at room temperature for 8 h.

General procedure for reduction of nitro compounds in the presence of Fe₃O₄@SiO₂/EP.EN.EG@Cu

Suspension of NaBH₄ (3 mmol) in H₂O (3 ml) was added to nitro compound (1 mmol), then stirred at 50 °C in presence of Fe₃O₄@SiO₂/EP.EN.EG@Cu (0.05 g) in water. The reaction progress was monitored by TLC, after the completion of the reaction the catalyst was taken out with external magnet and washed with ethanol. The reaction mixture was extracted with ethyl acetate and the combined organic layer was dried with Na₂SO₄. The product was purified by column chromatography by using hexane-ethyl acetate as solvent system in different concentration to obtain the pure compound.

p-Toluidine (Table 3, entry 6). FT-IR (KBr, cm⁻¹): 3417, 3336, 3220, 3009, 2913, 2859, 1623, 1515, 1268, 1176, 813. ¹H NMR (400 MHz, DMSO-d₆): 6.83 (d, J = 12 Hz, 2H, Ph), 6.49 (d, J = 12 Hz, 2H, Ph), 4.78 (s, 2H, NH₂), 2.14 (s, 3H, Me). MS (EI): m/z (%) 107 [M⁺].

N,N′-(4-Amino-1,2-phenylene)diacetamide (Table 3, entry 11). FT-IR (KBr, cm⁻¹): 3395, 3331, 3227, 3056, 1654, 1590, 1511, 1438, 1360, 1281. ¹H NMR (400 MHz, DMSO-d₆): 9.22 (s, 2H, NH), 8.26–8.23 (m, 1H, Ph), 7.85 (dd, J = 12 Hz, J = 3.2 Hz, 1H, Ph), 6.74 (d, J = 12 Hz, 1H, Ph), 6.51 (s, 2H, NH₂), 2.08 (s, 3H, Me).

4-Amin acetophenon (Table 3, entry 12). FT-IR (KBr, cm⁻¹): 3395, 3331, 3227, 3056, 1654, 1590, 1511, 1438, 1360, 1281. ¹H NMR (400 MHz, DMSO-d₆): 7.67 (d, J = 12 Hz, 2H, Ph), 6.56 (d, J = 12 Hz, 2H, Ph), 6.04 (s, 2H, NH₂), 2.39 (s, 3H, Me). MS (EI): m/z (%) 135 [M⁺].

4-Aminobenzoic acid (Table 3, entry 13). FT-IR (KBr, cm⁻¹): 3460, 3381, 3363, 3235, 2978, 2823, 2671, 2594, 1665, 1624, 1600, 1572, 1442, 1421, 1312, 1291. ¹H NMR (400 MHz, DMSO-d₆): 12.0 (br, 1H, OH), 7.62 (d, J = 12 Hz, 2H, Ph), 6.54 (d, J = 8 Hz, 2H, Ph), 5.88 (s, 2H, NH₂). MS (EI): m/z (%) 137 [M⁺].

Conclusion

In summary, we report the preparation and characterization of novel designed Fe₃O₄@SiO₂/EP.EN.EG@Cu catalyst. This catalyst demonstrate excellent catalytic performance in reduction of aromatic nitro compounds. The advantages of this catalytic system are: (a) ease of preparation, use and separation of catalyst (b) the mild reaction conditions, (c) short reaction times and (d) high yields of substituted amines.

Notes and references

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