Nano-Fe₃O₄ encapsulated-silica supported boron trifluoride as a novel heterogeneous solid acid for solvent-free synthesis of arylazo-1-naphthol derivatives

Abstract

Nano-Fe₃O₄ encapsulated-silica supported boron trifluoride (Fe₃O₄@SiO₂–BF₃) as a new type of green heterogeneous solid acid was prepared by the immobilization of BF₃·Et₂O on the surface of Fe₃O₄@SiO₂ core–shell nanoparticles and characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), field emission-scanning electron microscope (FE-SEM), energy dispersive X-ray (EDS), and transmission electron microscope (TEM). Then, this super solid acid was used as an acidic reagent for the synthesis of aryl diazonium salts as a starting reactant, followed by its diazo coupling with 1-naphthol in a basic solvent-free medium at room temperature. The main advantages of this clean method were high yields, short reaction times, the possibility of performing it at room temperature, and no need of corrosive and toxic liquid acids and solvents. In addition, the long-term stability of aryl diazonium salts supported on Fe₃O₄@SiO₂–BF₃ magnetic nanoparticles (MNPs) at room temperature was one of the most important results of this procedure.

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Introduction

In recent years, solvent-free organic reactions¹ have attracted great interest because of their various advantages such as high efficiency and selectivity, easy separation and purification, mild reaction conditions, reduction in waste, simplicity in progress and handling, and benefits to the industry as well as the environment. Aromatic azo compounds constitute a very important class of organic dyes because of their widespread applications in many areas of technology and medicine. They are well-known for their use as colorants in textile industries,² digital printing and photography.³ They are also applied as chiral receptors,⁴ liquid crystals,⁵ new glassy materials,⁶ chiral switches in photochemistry,⁷ dyes for drugs, food, cosmetic, biomedicine,⁸ and molecular recognition.⁹ The synthesis of azo dyes requires some special conditions, such as low temperature and concentrated liquid acid, in addition to high costs because it leads to the corrosion of the reactors and equipments.¹⁰ Nowadays, solid supported reagents, rather than individual reagents, have resolved these problems and improved the activity and selectivity.¹¹ Such reagents not only simplify the purification processes, but they also help prevent the release of reaction residues into the environment.¹² In this regard, nanostructured solid acids exhibit higher activity and selectivity than their corresponding bulk materials due to their particular physical and chemical properties, especially large surface to volume ratio.¹³ Recently, Fe₃O₄ MNPs has appeared as a new type of efficient catalyst support due to its low toxicity, ease of surface modification, and unique physical properties, including their high surface area and superparamagnetism.¹⁴ To prevent the aggregation of Fe₃O₄ nanoparticles, its surface is usually modified with a silica layer, taking advantage of the fact that they are biocompatible and hydrophilic and also because the surface silanol groups can easily react with various organic and inorganic materials to achieve certain purposes, especially in the field of catalysis.¹⁵ As a continuation of our efforts on the development of heterogeneous solid acids in organic transformations,¹⁶ we herein report the preparation and characterization of a novel and eco-friendly magnetic solid acid Fe₃O₄@SiO₂–BF₃, as well as its utility for the synthesis of arylazo-1-naphthol dyes in a solvent-free environment at room temperature. In this method, special cold conditions are not required for the stabilization of aryl diazonium salts. The reaction easily takes place at room temperature, and the resulting diazonium salt can remain on the solid substrate for several months. To the best of our knowledge, this research is the first report about the long-term stability of aryl diazonium salts supported on the surface of MNPs and their use as the reactant for the synthesis of arylazo-1-naphthols in solvent-free conditions. The findings of this research may have implications for effective synthesis on a larger scale in dyeing and medical industries.

Results and discussion

This research was performed in three stages. Initially, Fe₃O₄@SiO₂–BF₃ MNPs was synthesized and identified by FT-IR, XRD, VSM, FE-SEM, EDS, and TEM. In the second stage, aryl diazonium salts were synthesized in the presence of Fe₃O₄@SiO₂–BF₃ nanoparticles at room temperature and their structure and stability was investigated. The third stage was the diazo coupling of aryl diazonium salts supported on Fe₃O₄@SiO₂–BF₃ with basic 1-naphthol under solvent-free grinding condition.

Synthesis and characterization of Fe₃O₄@SiO₂–BF₃ nanoparticles

Fe₃O₄@SiO₂–BF₃ core–shell nanoparticles, with Fe₃O₄ spheres as the core and silica supported BF₃ as the shell, were prepared by a simple and convenient method. First, Fe₃O₄ nanoparticles were prepared by the chemical co-precipitation of FeCl₂·4H₂O and FeCl₃·6H₂O in an ammonium hydroxide solution. To improve the chemical stability of Fe₃O₄ and prevent self-aggregation, its surface engineering was successfully performed by the suitable deposition of SiO₂ on Fe₃O₄ surface via the ammonia-catalyzed hydrolysis of tetraethylorthosilicate (TEOS). Next, the Fe₃O₄@SiO₂ spheres served as support for the immobilization of BF₃ groups by the simple stirring of Fe₃O₄@SiO₂ and BF₃·Et₂O in ethanol. All three steps were carried out under sonication at room temperature (Scheme 1).

Scheme 1 Preparation of Fe₃O₄@SiO₂–BF₃ MNPs under sonication at room temperature.

To identify the molecular structures of Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂–BF₃ MNPs, FT-IR analysis of three mentioned samples was performed (Fig. 1). Fe₃O₄ was identified by a stretching vibration of the Fe–O absorption peak at 576 cm⁻¹, O–H stretching vibration at 3429 cm⁻¹ and O–H deformed vibration at 1625 cm⁻¹ in Fig. 1a. The FT-IR spectrum of Fe₃O₄@SiO₂ (Fig. 1b) displayed characteristic peaks at 1093 and 800 cm⁻¹, corresponding to the symmetrical and asymmetrical vibrations of Si–O–Si, respectively. The weak band at 466 cm⁻¹ corresponded to the Si–O–Fe stretching vibrations of the Fe₃O₄@SiO₂ core–shell, overall confirming the presence of SiO₂ in the sample. The successful covalent linking of the BF₃·Et₂O on the surface of Fe₃O₄@SiO₂ core–shell was proved by the appearance of a new band at 1457 cm⁻¹, which originates from the absorption of B–O (Fig. 1c). The absorption band of B–F was masked by the Si–O band. Moreover, the ethanolic OH and existing moisture in BF₃·Et₂O caused a broad O–H stretching band at the wavenumber of 3221 cm⁻¹. The observation of Fe₃O₄ absorption peaks in both Fig. 1b and c implies that Fe₃O₄ MNPs do not change chemically or physically after the coating and surface modification processes. According to this information and from the reported structure of BF₃–SiO₂ in the literature,¹⁷ the final structure of nano Fe₃O₄@SiO₂–BF₃ is predicted and displayed in Scheme 2.

Fig. 1 FT-IR spectra of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂ and (c) Fe₃O₄@SiO₂–BF₃.

Scheme 2 The structure of Fe₃O₄@SiO₂–BF₃ MNPs.

Fig. 2 shows the XRD powder diffraction patterns of three synthesized MNPs. The data for the Fe₃O₄ nanoparticles at 2θ of 30.22°, 35.61°, 43.25°, 53.58°, 57.30°, 62.89°, and 74.66° (Fig. 2a) correspond to the standard Fe₃O₄ powder diffraction data. Moreover, the relatively sharp peaks observed indicate the phase purity of Fe₃O₄ nanoparticles, which are consistent with the presence of a cubic inverse spinel structure of Fe₃O₄. The XRD pattern of the Fe₃O₄@SiO₂ (Fig. 2b) is in good agreement with that of Fe₃O₄ phase, except for a broad peak around 2θ of 20°–30° corresponding to the amorphous phase of SiO₂. This indicates that the MNPs obtained after the coating process are composed of Fe₃O₄ core and amorphous SiO₂ shell. The XRD pattern of the modified Fe₃O₄@SiO₂ with BF₃·Et₂O in Fig. 2c is nearly the same as in Fig. 2b, meaning that the surface modification by BF₃ group has little effect on the XRD pattern of Fe₃O₄@SiO₂ nanoparticles because of the shielding effect of Fe₃O₄ and SiO₂ peaks. However, the changes in the intensity of peaks in spectra Fig. 2b and c, as well as the increase of noise in Fig. 2c verify the linking of BF₃ on the surface of Fe₃O₄@SiO₂ core–shell MNPs. Furthermore, the characteristic peaks of Fe₃O₄ are observed in three samples, thus indicating that the binding process did not induce any phase change.

Fig. 2 XRD patterns of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, and (c) Fe₃O₄@SiO₂–BF₃.

The magnetic properties of synthesized Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂–BF₃ nanoparticles were assessed by VSM at room temperature. The magnetization curve in Fig. 3 indicates magnetization as a function of the applied magnetic field. The saturation magnetization of Fe₃O₄@SiO₂ nanoparticles in Fig. 3b was about 29.15 emu g⁻¹, and it was reduced to 27.44 emu g⁻¹ after supporting with BF₃·Et₂O (Fig. 3c). Both of these values were considerably lower than the initial saturation magnetization of Fe₃O₄ nanoparticles (59.2 emu g⁻¹) in Fig. 3a. The decrease of the saturation magnetization after the surface coating of Fe₃O₄ confirms the presence of a diamagnetic outer shell (SiO₂ or SiO₂–BF₃) at the surface of the Fe₃O₄ particles.

Fig. 3 Magnetization curves of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, and (c) Fe₃O₄@SiO₂–BF₃.

The high magnification FE-SEM images of the purified MNPs are displayed in Fig. 4a–c. These images clearly showed the surface morphology of the three types of synthesized MNPs with a nearly spherical shape. The elemental components of three MNPs were characterized by EDS analysis. Fig. 4d shows the EDS of Fe₃O₄ nanoparticles; the particles contain Fe and O elements. The presence of Fe, O and Si elements in Fig. 4e verified the coating of the Fe₃O₄ core by SiO₂ shell. The appearance of two new signals related to F and C elements in Fig. 4f confirmed the supporting of BF₃·Et₂O on Fe₃O₄@SiO₂ core–shell nanoparticles, according to Scheme 2.

Fig. 4 FE-SEM images of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, (c) Fe₃O₄@SiO₂–BF₃ and EDS spectra of (d) Fe₃O₄, (e) Fe₃O₄@SiO₂, (f) Fe₃O₄@SiO₂–BF₃.

TEM images of Fe₃O₄ and Fe₃O₄@SiO₂–BF₃ are displayed in Fig. 5. As demonstrated in Fig. 5a, Fe₃O₄ nanoparticles have a spherical morphology. In Fig. 5b, two regions with different electron densities can be distinguished, confirming that the Fe₃O₄ nanoparticles were successfully coated with a thin layer of a different phase. However, it can be observed that the sample is nearly in a core–shell structure. An electron dense region (black colour), which corresponds to the Fe₃O₄ cores, and a less dense or more translucent region (ash colour) surround these cores, which corresponds to the SiO₂–BF₃ shell. From the size distribution histograms, the average size of 9 nm for Fe₃O₄ and 13 nm for Fe₃O₄@SiO₂–BF₃ nanoparticles could be estimated.

Fig. 5 TEM images of (a) Fe₃O₄, and (b) Fe₃O₄@SiO₂–BF₃ nanoparticles.

Finally, Fe₃O₄@SiO₂–BF₃ was identified by using the techniques described above, and applied for the synthesis of arylazo-1-naphthol derivatives.

Synthesis and characterization of aryl diazonium salts

Aryl diazonium salts are usually synthesized in the presence of a liquid acid dissolved in water at low temperatures between 0 and 5 °C because temperatures above 5 °C generally promote phenol formation in aqueous media. Thus, the synthesis of aryl diazonium salts has limitations and drawbacks such as the control and maintenance of the low-temperature, use of toxic liquid acids that are incompatible with environment, and most importantly the instability of aryl diazonium salts at room temperature. We tried to resolve these problems by the development of a green and simple procedure for the synthesis of aryl diazonium salts followed by their diazo coupling with 1-naphthol.

First, to evaluate the efficiency of Fe₃O₄@SiO₂–BF₃ MNPs in the diazotization reaction, an initial optimization of the type and amount of acidic reagent was performed via the diazotization of 4-chloroaniline as a model substrate. A range of parameters, such as the stability of 4-chlorophenyl diazonium salt at room temperature, the reaction time of diazotization, and the yield of the resulting 4-chlorophenylazo-1-naphthol are reported in Table 1.

Table 1 Comparison of the efficiency of various acids in the synthesis of 4-chlorophenyl diazonium salt

Entry	Acid (wt%)	Stability at r.t.	Time of diazotization	Yield^a (%)
a The yields refer to the total isolated yield of 2-(4-chlorophenylazo)-1-naphthol and 4-(4-chlorophenylazo)-1-naphthol after adding fresh 4-chlorophenyl diazonium salt into basic 1-naphthol.b Silica phosphoric acid.c Silica chloride.
1	SPA^b (10)	∼2 days	35 min	57
2	SC^c (10)	∼2 days	30 min	61
3	Fe3O4@SiO2	—	No reaction	—
4	BF3·Et2O	—	No reaction	—
5	Fe3O4@SiO2–BF3 (5)	>13 months	12 s	89
6	Fe3O4@SiO2–BF3 (10)	>13 months	6 s	95
7	Fe3O4@SiO2–BF3 (15)	>13 months	8 s	90
8	Fe3O4@SiO2–BF3 (20)	>13 months	8 s	87

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Solid acids, such as silica phosphoric acid and silica chloride, gave 4-chlorophenylazo-1-naphthol with modest yields: 57% and 61%, respectively (Table 1, entries 1 and 2). The stability of aryl diazonium salt supported on the above mentioned solid acids was at maximum after 2 days. Fe₃O₄@SiO₂ and BF₃·Et₂O sources were also tested individually. There was no reaction in the presence of neither Fe₃O₄@SiO₂ nor BF₃·Et₂O (Table 1, entries 3 and 4), while Fe₃O₄@SiO₂–BF₃ with different loadings afforded improved yields: from 87% to 95% for 4-chlorophenylazo-1-naphthol (Table 1, entries 5–8). These results clearly indicate that the presence of Brönsted acid sites on the solid acid surface is essential for promoting diazotization. Moreover, the rate of diazotization in the presence of Fe₃O₄@SiO₂–BF₃ MNPs was considerably higher than silica phosphoric acid and silica chloride.

In another comparative study (Table 1, entries 5–8), the effect of Fe₃O₄@SiO₂ loading by BF₃·Et₂O on the acidic performance of Fe₃O₄@SiO₂–BF₃ was investigated. Although the time of diazotization and the stability of aryl diazonium salt supported on Fe₃O₄@SiO₂–BF₃ nanoparticles with different loadings in the same conditions were approximately identical, but 10 wt% Fe₃O₄@SiO₂–BF₃ resulted in the highest yield of the corresponding azo dye (Table 1, entry 6). In conclusion, 10 wt% Fe₃O₄@SiO₂–BF₃ was selected as the most ideal acid to be used for the synthesis of arylazo-1-naphthol dyes among those listed in Table 1.

In addition, with more investigations it was found that aryl diazonium salts supported on Fe₃O₄@SiO₂–BF₃ nanoparticles underwent no significant changes at room temperature for several months. To explore the reason of this unusual stability, the FT-IR spectrum of 4-chlorophenyl diazonium salt was studied (Fig. 6).

Fig. 6 FT-IR spectrum of 4-chlorophenyl diazonium salt supported on Fe₃O₄@SiO₂–BF₃ MNPs.

In this spectrum, the ethanolic OH and the existing moisture in BF₃·Et₂O caused a broad O–H stretching band at the wavenumber of 3419 cm⁻¹. The appearance of a new band at 2289 cm⁻¹ clearly demonstrated N≡N stretching vibration and verified the formation of 4-chlorophenyl diazonium salt. The absorption bands of B–O and Si–O vibrations were observed at 1442 and 1085 cm⁻¹, respectively. Aromatic C–H bending vibrations, C–Cl and Fe–O stretching bands were revealed at 829, 634, and 586 cm⁻¹, respectively.

According to this information, the structure of aryl diazonium salts supported on MNPs was predicted. Scheme 3 reveals that in this theoretical structure, the aryl diazonium cations are located on the surface of negatively charged particles called as Fe₃O₄-silica trifluoroborate nanoparticles. Thus, the presence of bulky anions and charge–charge interactions between nitrogen and boron atoms are the possible reasons for the unusual stability of these salts.

Scheme 3 The probable structure of aryl diazonium salt supported on Fe₃O₄@SiO₂–BF₃ nanoparticles.

As a result, the high conversions of aniline derivatives to aryl diazonium salts and their long-term stability showed that the Fe₃O₄@SiO₂–BF₃ has strong and sufficient acidic sites, which are responsible for the excellent performance in the synthesis of aryl diazonium salts.

After the optimization of the conditions, aniline derivatives, including electron-withdrawing and electron-donating substituents, were ground with NaNO₂ and Fe₃O₄@SiO₂–BF₃ nanoparticles. Aryl diazonium salts were obtained in a very short time with an excellent conversion at room temperature (Scheme 4). Indeed, due to the high acidic strength of Fe₃O₄@SiO₂–BF₃ nanoparticles, the reaction occurred so rapidly that the substituent type could not affect the time of diazotization.

Scheme 4 Diazotization of aromatic amines in the presence of Fe₃O₄@SiO₂–BF₃ nanoparticles.

It is important to note that Fe₃O₄@SiO₂–BF₃ in diazotization reaction acts as a two-function reagent, such that on one side, its acidic protons convert the nitrite anions (NO₂⁻) in NaNO₂ to nitrosonium cations (NO⁺) to promote diazotization, while on the other side, its bulky anions facilitate the stability of aryl diazonium cations.

Synthesis of arylazo-1-naphthol dyes

The most common synthetic route to arylazo-1-naphthol compounds involves the coupling of aryl diazonium salts with 1-naphthol in a basic solution. Using water for the preparation of 1-naphthoxide salt appears to be necessary; however, similar to the previous step, it causes the formation of some phenoxide salts and its attack to diazonium salt to form phenolic azo dyes. To prevent this problem, the reaction was performed under solvent-free conditions toward green chemistry. Thus, we ground solid 1-naphthol with some NaOH in a mortar. The moisture absorbed by the reaction mixture during the grinding appears to be sufficient for the formation of a homogeneous mixture. Moreover, the higher concentrations of reactants in the absence of a solvent at room temperature usually leads to a more favourable kinetics than in solution.¹⁸

In this reaction, the diazo coupling of aryl diazonium salts with 1-naphtoxide led to two products, in which 4-arylazo-1-naphthol derivatives were the major products and 2-arylazo-1-naphthol derivatives were the minor products. The resulting two main products were easily separated by a flash column chromatography and identified by ¹H-NMR and FT-IR spectroscopic methods. The molar ratios of these two dyes were determined from the integral intensities of OH/NH signals in ¹H-NMR spectrum (ratio 5 : 1 for 4-arylazo-1-naphthol to 2-arylazo-1-naphthol). The total yields of two products are shown in Table 2.

Table 2 Solvent-free synthesis of arylazo-1-naphthol derivatives at room temperature

Entry	Amine	2-Arylazo-1-naphthol (a′)	4-Arylazo-1-naphthol (b′)	Yield^a (%)	MPfound (Lit.) in a′/found (Lit.) in b′ (°C)
a Total isolated yield of 2-arylazo-1-naphthols and 4-arylazo-1-naphthols after chromatography.
1				92	181–183 (180.5–181)^19/176–178 (177–178)^20
2				89	116–118/157–159 (159)^20
3				93	127–129 (127.5–128)^19/173–175 (173)^20
4				91	157–159 (156)^19/160–162 (162–163)^20
5				90	116–118 (117–118)^21/201–203 (200)^20
6				96	148–150 (151)^19/211–213 (211.5)^20
7				87	184–186 (186)^21/138–140
8				89	137–139 (138)^22/207–209 (208)^20
9				92	172–174/188–190 (189.5–190.5)^20
10				90	152–154/228–230 (227.5–228)^20
11				95	187–189 (187)^21/229–231 (229.5)^20
12				85	216–218 (216)^23/258–260 (256–257)^20

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It is also important to note that the electronic effects of aryl diazonium salt substituents do not play significant role in the rate of diazo coupling step. However, the coupling reaction occurred rapidly regardless of the substituent effects.

Altogether, in this research, we investigated the application of Fe₃O₄@SiO₂–BF₃ MNPs as a strong and useful solid acid reagent for the synthesis of azo dyes based on 1-naphthol via a green procedure.

Experimental

Materials and apparatus

Chemicals and solvents were purchased from Merck and Sigma-Aldrich Companies. Melting points were obtained with a micro melting point apparatus (Electrothermal, Mk3) and are uncorrected. ¹H-NMR spectra were recorded on a Bruker DRX-400 Avance spectrometer. Tetramethyl silane (TMS) was used as an internal reference, and CDCl₃ and DMSO-d₆ were used as solvents. FT-IR spectra were obtained on a Nicolet Magna 550 spectrometer. The ultrasonic equipment used for the synthesis of MNPs (Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂–BF₃) was a Sonica 2200ETH S3 SOLTEC ultrasonic bath (Italy) with a working frequency of 40 KHz. XRD patterns were acquired using a Philips Xpert MPD diffractometer equipped with a Cu Kα anode (λ = 1.54 Å) in the 2θ range from 10° to 80°. The magnetization of the samples was recorded as a function of the applied magnetic field sweeping between ±10 kOe at room temperature. All the measurements were performed on a vibrating sample magnetometer device (Meghnatis Daghigh Kavir Co.; Kashan Kavir; Iran). The morphology of the synthesized samples was studied by a Mira II LMU Tescan FE-SEM made in the Czech Republic. The elemental composition of the three above mentioned MNPs was investigated by EDS spectroscopy (SAMX, France). The average size of Fe₃O₄ and Fe₃O₄@SiO₂–BF₃ MNPs was analyzed by TEM using a Philips CM120 with a LaB₆ cathode at an accelerating voltage of 120 kV.

Synthesis of Fe₃O₄@SiO₂–BF₃ MNPs

The synthesis of Fe₃O₄ nanoparticles was carried out according to a known procedure using a chemical co-precipitation method with a little modification of the methodology already reported in the literature.²⁴ Briefly, FeCl₃·6H₂O (8.0 g, 0.029 mol) and FeCl₂·4H₂O (3.5 g, 0.017 mol) with a molar ratio of approximately 2 : 1 were dissolved in 38 mL of deoxygenated 0.4 M HCl solution. Then, 375 mL of deoxygenated 0.7 M ammonia solution was quickly added into the reaction mixture with sonication and under nitrogen atmosphere. This resulted in the immediate formation of a black precipitate of Fe₃O₄ (magnetite). The sonication of magnetite dispersion was continued for 30 min. Finally, the precipitates were collected using an external magnetic field and washed for several times with distilled water and ethanol. The synthesized Fe₃O₄ MNPs were suspended in 50 mL of distilled water to be used in the next steps.

A modified Stöber sol–gel process²⁵ was used for coating magnetite nanoparticles with a silica shell. Typically, 50 mL of magnetite suspended in water was added to 250 mL ethanol and sonicated at room temperature for 20 min under nitrogen flow. Then, 11.85 mL PEG 200, 50 mL distilled water, and 25 mL NH₃ (28%) were added, after 15 min, 5 mL of TEOS was introduced into the suspension and the mixture was again sonicated for 6 h. Fe₃O₄@SiO₂ nanoparticles were centrifuged at 3000 rpm for 10 min. The solvent was discarded and nanoparticles were washed three times with water and ethanol and then dried in vacuum at room temperature.

In the final stage, BF₃·Et₂O (0.45 mL) was added drop-wise to a slurry containing Fe₃O₄@SiO₂ core–shell nanoparticles (4.5 g) and ethanol (15 mL). The mixture was sonicated for 1 h at room temperature. The resulting suspension was filtered and dried at room temperature to obtain a brown solid named nano Fe₃O₄@SiO₂–BF₃ (10 wt%).

Typical procedure for the synthesis of arylazo-1-naphthol dyes

For the synthesis of arylazo-1-naphthol derivatives, we mixed aromatic amines (2 mmol) with sodium nitrite (3 mmol) and Fe₃O₄@SiO₂–BF₃ nanoparticles (0.3 g) in a mortar with a pestle by rapid grinding. The progress of reaction was monitored by TLC (ethyl acetate/n-hexane). Moreover, for the preparation of 1-naphtoxide salt, 2 mmol of 1-naphthol and 10 mmol NaOH were ground in another mortar. Then, aryl diazonium salt was added to 1-naphthoxide salt and mixing and grinding were performed for a short time (about 3 min). After the completion of the reaction and formation of two main products (2-arylazo-1-naphthol and 4-arylazo-1-naphthol), the mixture was washed with distilled water (3 × 10 mL) and acetone (4 × 5 mL). Two final products were separated by flash column chromatography with silica mesh of 230–400 (40–63 μm).

Conclusion

Fe₃O₄@SiO₂–BF₃ MNPs as a novel heterogeneous solid acid reagent was prepared by the support of BF₃·Et₂O on the surface of Fe₃O₄@SiO₂ core–shell nanoparticles and characterized by various methods. Then, some brilliant arylazo-1-naphthol derivatives were successfully synthesized by the diazotization of aniline derivatives in the presence of Fe₃O₄@SiO₂–BF₃ MNPs and their diazo coupling with 1-naphthol at room temperature. The use of Fe₃O₄@SiO₂–BF₃ and a solvent-free procedure simplified the synthesis; in addition the procedure did not require special conditions, such as liquid acids and low temperature, and the method was compatible with environment. The procedure gave efficient yields using short reaction times, making it attractive to synthesize a variety of important dyes. The structure and stability of the aryl diazonium salt supported on Fe₃O₄@SiO₂–BF₃ MNPs were also studied.

Acknowledgements

The authors are grateful to the University of Kashan for supporting this work by Grant no. 159189/41.

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