Introduction

uzuki coupling reaction between aryl halides and organoboronic acids plays a a vital part in the synthesis of nonsymmetric and symmetric biaryls [1-5], which is widely utilized in many fields, including medicine and cosmetics, pharmacology and polymers, advanced materials, and ligands [6-11]. There are several methods for the synthesis of biaryl compounds in Suzuki reactions using different catalysts [12-15]. Certain heterogeneous catalysts have lower reactivity than homogeneous ones as a result of palladium leaching [16-18]. Homogeneous catalysts suffered from drawbacks such as high operational costs, short life time, tedious work-ups, and the difficulty of separating. Therefore, it is important to design a better heterogeneous catalyst to transform aryl halides into the corresponding biaryls.

In recent years, different supporting materials have been utilized to load MNPs for Suzuki reactions, due to its low cost, thermal, and chemical satiability and easily removal from the reaction mixture [14-15]. But, it is crucial that these active places are accessible.

Among various supports, magnetic nanoparticles have attracted much attention due to their magnetic properties, high stability, and high catalytic activity, as well as inexpensive and simple recovery with an external magnet that can be used for the heterogeneous catalysts [19-25]. Our continuous interest in the development of environmentally sustainable methods for preparing heterogeneous catalysts [22-25], has led us to focus our investigations on the use of Horsetail as the source of biosilica (Scheme 1) for stabilization of palladium magnetic nanoparticles and preparation of nano-Fe₃O₄@SiO₂-Pd (Scheme 2) in Suzuki coupling reaction. Horsetail (Equisetum arvense) is a herbaceous perennial plant which can be used in medicinal chemistry due to its high silica content [26-27].

Scheme 1. The SiO₂ synthesis from Horsetail

Scheme 2. The synthesis of Fe₃O₄@SiO₂-Pd nanoparticles for Suzuki coupling reactions

Experimental

Instruments and reagents

Chemical reagents

Chemical reagents were purchased from the Aldrich and Merck chemical companies.¹H-NMR spectra were recorded on a Bruker Avance DRX-400 spectrometer at 400. FT-IR spectra were recorded on a Nicolet 370 Fourier transform infrared spectrometer (Thermo Nicolet, USA) using pressed KBr pellets. UV-Visible spectral analysis was recorded on a double‐beam spectrophotometer (Hitachi, U‐2900). XRD measurements were carried out using a Philips powder diffractometer type PW 1373 goniometer (Cu Kα = 1.5406 A˚). The scanning rate was 2 ͦ/min in the 2θ range from 10 to 90˚. Scanning electron microscopy was performed on a Cam scan MV2300. EDS (S3700N). Vibrating sample magnetometer was performed using a SQUID magnetometer at 298 K (Quantum Design MPMS XL).

Preparation of Fe₃O₄ magnetic nanoparticles

Fe₃O₄ magnetic nanoparticles were synthesized according to the literature [28-31] using simple chemical coprecipitation. 4.6 g of FeCl₂.4H₂O and 11.3 g of FeCl₃.6H₂O were dissolved in 250 mL of deionized water into a 500 mL round-bottom flask and at 80 ℃ stirred until the solution was clear, and then water solution of ammonia (30 mL, 25%) was added and stirred for 2 h. At the end, black product was separated by magnetic decantation, washed with deionized water for 2 times, and then dried until 60 ℃ in vacuum.

Preparation of biosilica from Horsetail

Firstly, the Horsetail Plant was cleaned with distilled water many times before being dried. at 100 ℃ for 24 h. The reaction was performed in reflux condenser. The Horsetail (5.0 g) was added and acid-leached with HCl 10% and sulfuric acid 30 wt.% solution under stirring at 100 ℃ for 3 h. At the end of the reaction, brown product was filtered and washed 3 times with deionized water and dried for 5 h at 80 ℃. Finally, the product was calcinated for 3 h at 700 ℃ to get Silicon dioxide.

Preparation of nanostructured silica-coated magnetic

The mixture of Fe₃O₄ (1.0 g), ethanol (80 mL), deionized water (40 mL) and NH₃ 25% (4.0 mL) was sonicated for 30 min, and then SiO₂ (1.0 g) was combined after the addition and was refluxed for half a day. At the end of the reaction, the resulting products were washed with DI Wate three times and dried at 65 ℃ in vacuum.

Preparation of functionalized SiO2 magnetic nanoparticles with 3(triethoxysilyl)propane-1-thiol

This compound was prepared by refluxing for 12 h of SiO₂ magnetic nanoparticles (1.0 g) and with 3-(triethoxysilyl) propane-1-thiol (2.0 mmol) in ethanol (5.0 mL). Following the reaction time, the product was separated by centrifugation, washed with DI Wate, and ethanol and dried at 70 ^°C for 5 h.

Preparation of pd magnetic nanoparticles

PdCl₂ (100 mL, 1 mM) and earlier stage product (1.0 g) were refluxed in 10 mL of deionized water for 3 hours. The product was separated and washed with DI Wate, and then the resultant precipitate was re-dispersed in DI Wate and 0.4 mL of newly prepared N₂H₄ aqueous solution (0.25 M) was added and mixture was stirred for 2 hours. Likewise, the solution color changed into black indicating reduction of Pd^II to Pd(0). Finally, the black precipitate was washed with DI Wate and dried at 65  ℃ for 6 hours.

General procedure for synthesis of biaryls via Suzuki coupling reactions

10 mg of the nano-Fe₃O₄@SiO₂-Pd was placed in a 25 mL Schlenk tube, and then 1.0 mmol of the aryl halide in 4 mL of water/ethanol (1:1), 1.1 mmol of phenylboronic acid, 2.0 mmol of K₂CO₃ was added to reaction mixture and stirred at 90 ^°C. After finishing the reaction (checked by TLC), 5 mL ethanol was added and the catalyst was removed by external magnet. After removal of solvent, the remainder was purified by column chromatography. The product structure was confirmed by NMR spectroscopy.

Results and Discussion

Herein, the preparation been describe preparation of the Fe₃O₄@SiO₂-Pd nanocatalyst has been described using Horsetail (Equisetum arvense). The formation of Fe₃O₄@SiO₂-Pd nanocatalyst was confirmed by XRD, EDS, SEM, FT-IR, and VSM analysis.

In a typical synthesis of the Fe₃O₄@SiO₂-Pd nanocatalyst, the Pd nanoparticles (NPs) were synthesized using N₂H₄ as a reducing factor for the conversion of the Pd(II) to Pd(0) and using UV-Vis spectroscopy, the Pd NPs' production was managed. Figure 1 displays the UV-Vis spectra of the Pd(II) and Pd NPs. The Pd(II) solution's yellow hue shifted to a dark brown hue, signifying the Pd NPs production.

Figure 1. The UV-Vis spectra of Pd(II) (a) and Pd(0) (b)

The FT-IR spectra of the SiO₂, Fe₃O₄ and Fe₃O₄@SiO₂-Pd are illustrated in Figure 2a-c. As indicated in Figure 2a, it can be found that the peak attributed to Si-O stretching vibration put at 1000-1150 cm^-1. The signal of 3400 cm^-1 was assigned to stretching of the adsorbed water molecules. The spectrum exhibits absorption band at 1500-1600 cm^-1 for H-O-H bending vibration in water. Figure 2b shows the FT-IR spectrum of Fe₃O₄ the peaks at 1500-1600 cm^-1 and 3400 cm^-1 are related to the Fe-OH and O-H stretching modes of FeOH or adsorbed water. Figure 2c demonstrates the FT-IR spectrum of Fe₃O₄@SiO₂-Pd and the presence of absorbance at 1000-1150, 1500-1600, and 3400 cm^-1 is attributed to Si-O (stretching), H-O-H (bending), and O-H (stretching).

Figure 2. FT-IR spectra of the SiO₂ (a), Fe₃O₄ (b) and Fe₃O₄@SiO₂-Pd (c)

The XRD of the SiO₂, Fe₃O₄ and Fe₃O₄@SiO₂-Pd are demonstrated in Figure 3a-c, respectively. The peak 2θ = 21° is the characteristic peak of SiO₂ (Figure 3a). The diffraction peaks at 2θ value of 29. 7◦, 35.0◦, 42.7◦, 53.0◦, 56.5◦, and 62.1◦ (Figure 3b) corresponding, respectively, to (220), (311), (400), (422), (511), and (440) planes of the cubic Fe₃O₄ (JCPDS 19-0629). From the XRD pattern of the Fe₃O₄@SiO₂-Pd (Figure 3c), any typical peak for Pd nanoparticles is detected, indicating that the Pd NPs are very dispersed on the surface.

Figure 3.  XRD patterns of SiO₂ (a), Fe₃O₄ (b), and Fe₃O₄@SiO₂-Pd (c)

Figure 4 and Figure 5 illustrate SEM images and the morphology of Fe₃O₄ and Fe₃O₄@SiO₂-Pd, spherical shapes and dimensions of the particles.

Figure 4. SEM images of the Fe₃O₄

Figure 5. SEM images of the Fe₃O₄@SiO₂-Pd

Figure 6 depicts the EDS spectrum of the synthesized Fe₃O₄@SiO₂-Pd. EDS analysis reveals a strong signal for the Pd element and confirmed the formation of Fe₃O₄@SiO₂-Pd nanocatalyst.

Fig. 6. The EDS of the Fe₃O₄@SiO₂-Pd

To study the magnetic characteristic of the Fe₃O₄ and Fe₃O₄@SiO₂-Pd, the loops of particles were registered in Figure 7 with the field sweeping from -10000 to +10000 Oe. The hysteresis loop of the Fe₃O₄ showed the highest saturation magnetization (Ms) of 60 emug¹ which is higher than the saturation magnetization of the Fe₃O₄@SiO₂-Pd. The reason of the difference is that low mass fractions of Fe₃O₄ nanoparticles were produced as a result of the thick organic matter's diamagnetic contribution. Therefore, a smaller magnetization value for the Fe₃O₄@SiO₂-Pd nanocatalyst compared to the Fe₃O₄ nanoparticles is reasonable. The Fe₃O₄@SiO₂-Pd nanocatalyst can be efficiently reused by magnet.

Figure 7. The vibrating sample magnetometer (VSM) analysis of the Fe₃O₄ (a) and Fe₃O₄@SiO₂-Pd (b)

The Fe₃O₄@SiO₂-Pd was used for the Suzuki coupling reaction of phenyl boronic acids with different aryl halides. A preliminary screening proved that Fe₃O₄@SiO₂-Pd nanocatalyst is effective in promoting the reaction of phenylboronic acid with 4-bromotoluene. No desired product was gained in the absence of catalyst. Several solvents and base were examined. The reaction was also performed in the presence of different amount of catalyst and the results are presented in Table 1. It was observed that with a rise in catalyst from 5.0 to 10.0 mg, product yield is increased. It was found that a combination of Fe₃O₄@SiO₂-Pd (10 mg) and K₂CO₃ (2.0 equiv.) in the presence of 4-bromotoluene (1.0 equiv) and phenylboronic acid (1.1 equiv) in EtOH/H₂O (1:1) was optimum for an efficient reaction at 90 ^oC (Table 1, entry 5).

Table 1. Effect of amount of catalyst, solvent and base on the Suzuki reaction of 4-bromotoluene with phenylboronic acid^a

Furthermore, the scope of the Fe₃O₄@SiO₂-Pd nanocatalyst was studied with various aryl halides (Table 2). Under these reaction conditions, aryl halides with EWG and EDG substituents completely reacted with phenylboronic acid in the Suzuki reaction in the presence of K₂CO₃ as base in H₂O/EtOH mixture and the Fe₃O₄@SiO₂-Pd as catalyst, affording the related biaryls in good to high yield. We recorded aryl halides containing electron donating groups on the ring giving higher yields than electron withdrawing groups. It can be seen from Table 2, the Fe₃O₄@SiO₂-Pd nanocatalyst can be employed efficiently for different aryl halides coupling reactions. According to the obtained results, substrates containing aryl iodides giving better yields than aryl bromides and chlorides. All of the biaryl compounds were characterized using ¹H-NMR techniques.

Table 2. Effect of Fe₃O₄@SiO₂-Pd nanocatalyst on the Suzuki coupling reaction^a

The distinct advantage of Fe₃O₄@SiO₂-Pd nanocatalyst is its recyclability and reusability. The heterogeneous nature of catalysis is an important feature from a cost viewpoint. For this purpose, Suzuki reaction of phenyl boronic acid with bromobenzene was done under optimized conditions (Table 3). After performing the catalytic reaction, the Fe₃O₄@SiO₂-Pd catalyst was separated by an external magnetic, washed with ethanol to remove all the organic substances, dried, and then reused for another run and found not to lose efficiency even after 4 runs (Table 3).

Table 3. Recyclability of Fe₃O₄@SiO₂-Pd

Conclusion

In summary, the Fe₃O₄@SiO₂-Pd nanocatalyst has been successfully prepared via an efficient, simple, low cost, and fast approach. EDS, XRD, SEM, FT-IR, and VSM techniques proved the formation of Fe₃O₄@SiO₂-Pd nanocatalyst. Furthermore, the Fe₃O₄@SiO₂-Pd nanocatalyst showed an efficient activity for the Suzuki reaction. In addition, the Fe₃O₄@SiO₂-Pd nanocatalyst could be removed without losing the activity by an external magnet that was reused numerous times.

Orcids

Ardeshir Khazaei: https://orcid.org/0000-0001-7990-2266

Mahmoud Nasrollahzadeh: https://orcid.org/0000-0002-4539-3544

Acknowledgements

The authors would like to gratefully thank the Iranian Nano Council and Bu-Ali Sina University for the support of this work.

Citation: R. Khazaei*, A. Khazaei, M. Nasrollahzadeh‎. Horsetail Assisted Green Synthesis of Fe₃O₄@SiO₂-Pd: A Reusable and Highly Efficient Magnetically Separable Catalyst for Suzuki Coupling Reactions.‎ J. Appl. Organomet. Chem., 2023, 3(2), 123-133.