Design, Preparation and Characterization of MoO3H-functionalized Fe3O4@SiO2 Magnetic Nanocatalyst and Application for the One-pot Multicomponent Reactions

Molybdic acid-functionalized silica-based Fe3O4 nanoparticles (Fe3O4@SiO2-MoO3H) are found to be a powerful and magnetically recyclable nanocatalyst. The morphology and structure of this nanocatalyst were investigated by Fourier transform infrared spectroscopy (FT-IR), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), thermo gravimetric analyses (TGA), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) techniques. The high catalytic activity of this catalyst was investigated in the synthesis of pyrano[2,3-c]chromenes, representing potent biologically active compounds. The catalyst can be readily separated by applying an external magnet device and recycled up to 8 times without significant decrease in its catalytic activity, which makes it highly beneficial to address the industrial needs and environmental concerns. Fe3O4@SiO2-MoO3H has many advantages, such as low cost, low toxicity, ease of preparation, good stability, high reusability and operational simplicity.


Introduction
Nowadays, the design and synthesis of efficient, reusable, easily separable, low toxicity, low cost, and insoluble acidic nanocatalysts have become an important area of research in chemistry. 1 The use of nanoparticles as heterogeneous catalysts has attracted considerable attention because of the interesting structural features and high levels of catalytic activity associated with these materials. 2 Magnetic nanoparticles (MNP) are widely applied in various fields, such as magnetic resonance imaging (MRI) contrast agents, biomedical science, bioseparation and hyperthermia. [3][4][5][6] Transition metal nanoparticles are used as efficient catalysts for various synthetic organic transformations due to their high surface area-to-volume ratio and coordination sites which are mainly responsible for their catalytic activity. 7 Because the Fe 3 O 4 nanoparticles will aggregate quickly into large bunches and therefore lose their unique properties, various surface modification methods have been developed to modify the surface of naked Fe 3 O 4 nanoparticles to improve the dispersibility, stability, biocompatibility and biodegradability for specific purposes. The resulting modified Fe 3 O 4 nanoparticles have been extensively used for various applications. 8 Among them, the silica coating is a very good surface modifier, Kiani et al.: Design, Preparation and Characterization ... because of its excellent stability, biocompatibility, nontoxicity and ease of furthered conjugation with various functional groups, thus enabling the coupling and labeling of biotargets with high selectivity and specificity. [9][10][11] Development of MCRs can lead to new efficient synthetic methodologies to afford many small organic compounds in the field of modern organic, bioorganic, and medicinal chemistry. 10 Hence, MCRs are considered as a pivotal theme in the synthesis of many important heterocyclic compounds, such as pyranocoumarin derivatives nowadays. 12 In continuation of our research on the introduction of recoverable catalysts in organic synthesis, [13][14][15][16] recently, we disclosed that Fe 3 O 4 @SiO 2 -MoO 3 H can be used as a novel magnetic nanocatalyst for the synthesis of 1,8-dioxodecahydroacridine derivatives. 17 In this work, we demonstrate high catalytic activity of this new catalyst in the synthesis of pyrano[2,3-c]chromenes as potent biologically active compounds.
It is also interesting to note that the catalyst can be recovered and reused several times.

1. General
The chemicals were purchased from Merck and Aldrich chemical companies. The reactions were monitored by TLC (silica gel 60 F 254, hexane : EtOAc). Fourier transform infrared (FT-IR) spectroscopy spectra were recorded on a Shimadzu-470 spectrometer, using KBr pellets and the melting points were determined on a KRUSS model instrument. 1 H NMR spectra were recorded on a Bruker Avance II 400 NMR spectrometer at 400 MHz, with DMSO-d 6 used as the solvent and TMS as the internal standard. X-Ray diffraction (XRD) pattern was obtained by Philips X Pert Pro X diffractometer operated with a Ni filtered Cu Kα radiation source. Transmission electron microscopy (TEM) images of the electrocatalyst were recorded using a Philips CM-10 TEM microscope operated at 100 kV. Field emission scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) analyses were carried out on a Philips XL30, operated at a 20 kV accelerating voltage. Thermogravimetric analyses (TGA) were conducted on a Rheometric Scientific Inc. 1998 thermal analysis apparatus under a N 2 atmosphere at a heating rate of 10 °C/min. The magnetic measurement was carried out in a vibrating sample magnetometer (Model 7407 VSM system, Lake Shore Cryotronic, Inc., Westerville, OH, USA) at room temperature.

General Procedure for the Preparation of nano-Fe 3 O 4 (1)
FeCl 3 · 6H 2 O (20 mmol) and FeCl 2 · 4H 2 O (10 mmol) were dissolved in distilled water (100 mL) in a three-necked round-bottom flask (250 mL). The resulting transparent solution was heated at 90 °C with rapid mechanical stirring under N 2 atmosphere for 1 h. A solution of concentrated aqueous ammonia (10 mL, 25 wt%) was then added to the solution in a drop-wise manner over a 30 min period using a dropping funnel. The reaction mixture was then cooled to room temperature and the resulting magnetic particles collected with a magnet and rinsed thoroughly with distilled water.

3. General Procedure for the Preparation of nano-Fe 3 O 4 @SiO 2 (2)
Nano-Fe 3 O 4 @SiO 2 (2) was synthesized according to a previously published literature method. Magnetic nano particles (1.0 g) were initially diluted via the sequential addition of water (20 mL), ethanol (60 mL) and concentrated aqueous ammonia (1.5 mL, 28 wt%). The resulting dispersion was then homogenized by ultrasonic vibration in a water bath. A solution of TEOS (0.45 mL) in ethanol (10 mL) was then added to the dispersion in a drop-wise manner under continuous mechanical stirring. Following a 12 h period of stirring, the resulting product was collected by magnetic separation and washed three times with ethanol.

4. General Procedure for the Preparation of nano-Fe 3 O 4 @SiO 2 -OMoO 3 H (3)
To an oven-dried (125 °C, vacuum) sample of nano-Fe 3 O 4 @SiO 2 60 (2 g) in a round bottomed flask (50 mL) equipped with a condenser and a drying tube, thionyl chloride (8 mL) was added and the mixture in the presence of CaCl 2 as a drying agent was refluxed for 48 h. The resulting dark powder was filtered and stored in a tightly capped bottle. To a mixture of Fe 3 O 4 @SiO 2 -Cl (1 g) and sodium molybdate (0.84 g) n-hexane (5 mL) was added. The reaction mixture was stirred under refluxing conditions (70 °C) for 4 h. After completion of the reaction, the reaction mixture was filtered and washed with distilled water, and dried and then stirred in the presence of 0.1 N HCl (20 mL) for an hour. Finally, the mixture was filtered, washed with distilled water, and dried to afford nano-Fe 3 O 4 @SiO 2 -OMoO 3 H.
The resulting MNP acid catalyst was characterized by XRD, FT-IR, TEM, SEM, TGA and EDX. 17 The transmission electron microscopy (TEM) image of Fe 3 O 4 @SiO 2 -MoO 3 H powder reveals the spherical Fe 3 O 4 @SiO 2 -MoO 3 H powder with an average particle sizes of about 10-30 nm (Fig. 1a).
Surface morphology, particle shape and size distribution features of Fe 3 O 4 @SiO 2 -MoO 3 H nanoparticles were examined by FE-SEM (Fig. 1b).
The successful incorporation of molybdate groups was also confirmed by EDAX analysis (Fig. 1c), which showed the presence of Fe, Si, Mo and O elements.  The thermogravimetric analysis (TGA) was used to study the thermal stability of the acid catalyst (Fig. 3). The first weight loss which occurred below 150 °C, displayed a mass loss that was attributable to the loss of adsorbed solvent or trapped water from the catalyst. A weight loss of approximately 5% weight occurred between 300 and 500 °C which can be attributed to the loss of molybdate groups covalently bound to silica surface. Thus, it can be concluded that the catalyst is stable up to 300 °C.   pond to the spinel structure of Fe 3 O 4 , which can be assigned to the diffraction of the (220), (311), (400), (422), (511), and (440) planes of the crystals, respectively. 18 Fig.  2b shows the XRD pattern of Fe 3 O 4 @SiO 2 -OMoO 3 H demonstrating that the crystalline structure of the Fe 3 O 4 particles was retained after the deposition of SiO 2 layers. The broad peak at around 2θ = 20° to 27° indicates the presence of amorphous silica in Fe 3 O 4 @SiO 2 -OMoO 3 H. The intensity of this peak increased with the introduction of molybdate on the silica-coated magnetic nanoparticles, which can be attributed to the amorphous molybdate supported on the composite. The XRD results showed that the Fe 3 O 4 @SiO 2 particles have been successfully coated with molybdate.
Typical magnetization curves for Fe 3 O 4 nanoparticles and Fe 3 O 4 @SiO 2 -MoO 3 H are shown in Fig. 4. Room temperature specific magnetization (M) versus applied magnetic field (H) curve measurements of the sample indicate a saturation magnetization value (Ms) of 20.30 emu g -1 , lo-wer than that of bare MNPs (59.14 emu g -1 ) due to the presence of coated shell.
In order to explore the catalytic efficiency of The influence of the solvent was studied when the model reaction was performed using Fe 3 O 4 @SiO 2 -Mo- After optimization of the reaction conditions, in order to extend the scope of this reaction, a wide range of aromatic aldehydes was used with 3 and 5 ( Table 2). All the products were characterized by comparison of their spectra and physical data with those reported in the literature. [22][23][24][25] As shown in Table 2, the new catalyst fortunately also works very well for the preparation of a vast variety of pyrano [2,3-c]coumarin derivatives 7a-n. The present method not only affords the products 7 in excellent yields, but also avoids the problems associated with catalyst cost, handling, safety and pollution.

Conclusions
In summary, we found Fe 3 O 4 @SiO 2 -OMoO 3 H to be an effective acidic magnetic nanocatalyst which successfully catalyzed the reaction between 4hydroxycoumarin, various aromatic aldehydes and malononitrile to produce new and known pyrano [2,3-c] chromens of potential synthetic and pharmaceutical interest. High catalytic activity under solvent free conditions, high yields, a clean process, reusable several times without loss of activity or selectivity simple catalyst preparation, easy separation after the reaction by a magnet and green conditions are the advantages of these protocols.

Acknowledgement
We acknowledge the research council of Yasouj University.