Molecular sieves and mesoporous materials with regular geometries have attracted much attention owing to their remarkable characteristics such as high surface area, high efficiency, large pore volume, and recyclability compared with nonporous objects [1-4]. Mobil composition of matter No. 41 (MCM-41) as a member of a family of mesoporous molecular sieves, which was reported by Beck et al. [5], consists of mesoporous-structured SiO₂ formed by a hexagonal array of unidirectional pore structures [6, 7]. These materials possess unique structural features such as high surface area [8], very narrow pore size distribution [9], and excellent thermal [10], chemical, and mechanical stability [11], which has opened up new possibilities for the design of novel heterogeneous catalysts. Magnetic nanoparticles (MNPs) are an important class of compounds that have received significant attention because of the broad spectrum of biological and medical applications such as drug delivery, biomolecular sensors, magnetic resonance imaging, bioseparation, and magneto-thermal therapy as well as in catalysis, environmental depollution, and in vitro/in vivo applications [12, 13]. MNPs have been widely used in catalytic reactions as heterogeneous catalysts. The advantages of using magnetic nanoparticles as heterogeneous catalysts include ease of recovery from the reaction mixture using an external magnet [14], their large surface area ratio, and ease of preparation. MNPs have been used as a core for the support of catalyst used in organic transformations owing to coupling with organic ligands and inorganic compounds [10]. The design and synthesis of efficient and recoverable catalysts has become an important environmental and economical consideration for industrial applications [15]. One strategy in this area has focused on the consolidation of functionalized mesoporous silica materials with magnetic nanoparticles to form a porous magnetic nanocomposite. These types of magnetic nanocomposites have the properties and advantages of both mesoporous silica and magnetic nanoparticles [16] and have been used for magnetic storage and recording, catalytic applications, sensors, environmental, and biomedical applications [12]. Reactions leading to the formation of C-C bonds catalyzed by palladium complexes, such as Suzuki and Heck coupling reactions, have become powerful methods in modern synthetic organic chemistry for the preparation of natural products, ultraviolet (UV) screens, pharmaceuticals, biologically active compounds, hydrocarbons, polymers, liquid crystal materials, and advanced materials [17-19]. Homogeneous palladium catalysts have been extensively studied in synthetic organic chemistry because of their high selectivity and catalytic activity [20]. Homogeneous catalytic reactions have many disadvantages such as poor reusability and separation of the catalysts from the reaction mixture. These disadvantages can be minimized by immobilizing homogenous catalysts on polymeric organic or inorganic supports [21, 22]. In this study, Pd complexes were anchored to porous magnetic nanocomposites as a catalyst support and employed as excellent and reusable heterogeneous organometallic nanocatalysts for C-C Suzuki and Heck coupling reactions. The main advantage of this catalytic system was the use of PEG as a green solvent to provide the target products in excellent yields in short reaction times. The nanoparticles could be easily isolated from the product solution by using an external magnet after completing the reactions.

The cetyltrimethylammonium bromide (CTAB; 98%), tetraethyl orthosilicate (TEOS), 3-aminopropyltriethoxysilane (3-APTES), FeCl₃·6H₂O, FeCl₂·4H₂O, palladium acetate, phenylboronic acid, and solvents were purchased from Aldrich, Merck, and Flucka and were used as received without further purification. The nanostructures were identified using a Holland Philips X'pert X-ray powder diffraction (XRD) diffractometer at a scanning speed of 2°/min from 1° to 80°. Fourier transform infrared (FT-IR) spectra were recorded with KBr pellets utilizing a VRTEX 70 model Bruker FT-IR spectrometer. Thermogravimetric analyses (TGA) of the samples were carried out between 30 and 800 ℃ using a Shimadzu DTG-60 automatic thermal analyzer. The particle size and morphology were examined by scanning electron microscopy (SEM) using a FESEM-TESCAN MIRA3, as well by high-resolution transmission electron microscopy (TEM) with a JEOL JEM-2100F, 200kV transmission electron microscope. The VSM measurements were performed using a vibrating sample magnetometer (VSM) MDKFD. Magnetization measurements were carried out using an external field of up to 15 kOe at several temperatures. The Pd content was measured using inductively coupled plasma-optical emission spectrometry (ICP-OES).

Magnetic Fe₃O₄ nanoparticles were prepared as follows. A mixture of FeCl₃·6H₂O (2 g) and FeCl₂·4H₂O (0.8 g) were dissolved in 10 mL of deionized water under a N₂ atmosphere. The mixture was added to 100 mL of 1.0 mol/L NH₃ solution containing 0.4 g of CTAB under sonication and a N₂ atmosphere. After 30 min, the reaction was completed, the Fe₃O₄ nanoparticles were isolated by magnetic decantation and dried at 80 ℃ under vacuum [23]. The Fe₃O₄@MCM-41 was prepared by the following method. First, Fe₃O₄ nanoparticles (0.05 g) were dispersed in ethanol (20.0 mL) for 30 min by using ultrasonication. Then, the suspension was centrifuged and the solid was re-dispersed in a mixture of deionized water (10 mL), ethanol (20 mL), and NH₃ (0.5 mL). The system was kept under vigorous mechanical stirring for 30 min, and then TEOS (0.03 g) was added dropwise. The mixture was stirred at room temperature for 6 h. The coated nanoparticles (Fe₃O₄@nSiO₂) were separated by decantation with the aid of a magnet and washed with ethanol and water several times. In the second step, the coated Fe₃O₄@nSiO₂ nanoparticles were dispersed in a mixture of deionized water (40 mL), ethanol (30 mL), NH₃ (0.6 mL), and CTAB (0.15 g). After stirring at room temperature for 30 min, 0.4 g of TEOS (as a silica source) was added slowly to the solution and stirring was continued for another 6 h. After completion of the reaction, the solid products were collected by magnetic separation, washed with plenty of ethanol/deionized water, and subsequently dried in a vacuum oven at 80 ℃ for 12 h. To remove the CTAB template, ethanol (100 mL) and 5 mL of 2 mol/L HCl were added to 0.1 g of the produced nanocomposite, then the mixture was stirred at room temperature for 48 h. The solid products were collected by magnetic separation and subsequently dried in a vacuum oven at 80 ℃ for 24 h. The resulting solid was referred to as Fe₃O₄@MCM-41.

To a suspension of Fe₃O₄@MCM-41(4.8 g) in n-hexane, 4.8 g of 3-aminopropyltriethoxysilane (3-APTES) was added slowly under a N₂atmosphere at 80 ℃ and refluxed for 24 h. The separated solid Fe₃O₄@MCM-41-(SiCH₂CH₂CH₂NH₂)_x was collected by magnetic separation, washed with n-hexane, and dried under vacuum.

For the preparation of Fe₃O₄@MCM-41@P2C, a solution of 0.5 g Fe₃O₄@MCM-41@nPr-NH₂ and pyrrole-2-carbaldehyde (1.2 mmol) in ethanol was refluxed under a N₂ atmosphere for 3 h. The resulting solid was washed with ethanol, collected by magnetic separation and dried under vacuum and designated as Fe₃O₄@MCM-41@P2C. Ultimately, the Fe₃O₄@MCM-41@Pd(0)-P2C was prepared by refluxing a solution of Pd(OAc)₂ (0.63 mmol, 0.168 g) with 0.250 g Fe₃O₄@MCM-41@P2C in ethanol (20 mL) for 20 h. Finally, Pd(Ⅱ) was reduced with NaBH₄ (0.057 mmol, 0.022 g) under the same conditions for another 2 h. The resulting black solid impregnated with the metal complex was separated by a magnet and washed with ethanol to give the Pd(0) complex. The detailed preparation route for Fe₃O₄@MCM-41@Pd(0)-P2C is shown in Scheme 1.

A mixture of an aryl halide (1 mmol), phenylboronic acid (1 mmol), K₂CO₃ (3 mmol), and Fe₃O₄@MCM-41@Pd(0)-P2C (0.004 g, 0.66 mol%) was added to a reaction vessel. The resulting mixture was stirred in PEG-400 at 60 ℃ and the progress of the reaction was monitored by thin-layer chromatography (TLC). After completion of the reaction, the catalyst was separated using an external magnet and washed with ethyl acetate. The reaction mixture was extracted with ethyl acetate and water, and the organic layer was dried over anhydrous Na₂SO₄ (1.5 g). The solvent was evaporated and pure products were obtained in good to excellent yields.

A mixture of an aryl halide (1 mmol), an alkene (1.2 mmol), K₂CO₃ (3 mmol), and Fe₃O₄@MCM-41@Pd(0)-P2C (0.006 g, 0.99 mol%) was stirred in PEG-400 at 120 ℃ and the progress of the reaction was monitored by TLC. After completion of the reaction, the mixture was cooled to room temperature. The catalyst was separated by an external magnet and washed with diethyl ether. The reaction mixture was extracted with water and diethyl ether. The organic layer was dried over Na₂SO₄ (1.5 g) and the solvent was evaporated to obtain the pure products in good to excellent yields.

The structure of the Palladium Schiff base complex anchored onto the surface of magnetic MCM-41 (Fe₃O₄@MCM-41@Pd(0)-P2C) was studied and fully characterized by FT-IR, VSM, EDS, TEM, SEM, XRD, and TGA analysis. Fig. 1 shows the FT-IR spectra of the synthesized Fe₃O₄, Fe₃O₄@MCM-41, Fe₃O₄@MCM-41-NH₂, Fe₃O₄@MCM-41@P2C, and Fe₃O₄@MCM-41@Pd(0)-P2C. The FT-IR spectrum of the Fe₃O₄ nanoparticles (Fig. 1(1)) displayed strong absorptions at 579 cm^-1 and 3000-3500 cm^-1, which wereassigned to the Fe-O and O-H stretching vibrations [22]. The FT-IR spectrum of the Fe₃O₄@MCM-41 sample (Fig. 1(2)) displayed peaks at 460, 958, and 1081 cm^-1, whichcorresponded to bending, symmetric, and asymmetric stretching of the Si-O-Si vibrations, respectively [25]. The presence of the peak at 1550 cm^-1, mainly from NH₂ symmetric bending vibrations, indicated successful attachment of an organic amine onto the Fe₃O₄@MCM-41 surface. The broad band at 3443 cm^-1 was assigned to the hydrogen bonding between the silanol groups and adsorbed water, and the band at approximately 2939 cm^-1 was associated with the CH₂ vibrations corresponding to C-H stretching (Fig. 1 (3)). The Fe₃O₄@MCM-41@P2C exhibited a ν (C=N) stretch at 1637 cm^-1, which shifted to 1629 cm^-1 in the FT-IR spectrum of Fe₃O₄@MCM-41@Pd(0)-P2C, indicating the formation of Pd-ligand bonds [2].

The crystalline structures of the catalyst were analyzed by XRD. The wide angle XRD of the Fe₃O₄@MCM-41 and Fe₃O₄@MCM-41@Pd(0)-P2C samples are shown in Fig. 2(a). The characteristic peaks at 2θ = 30.0°, 35.4°, 43.3°, 54.5°, 57.0°, and 62.8° assigned to magnetite were observed, which is consistent with the standard Fe₃O₄ XRD spectrum and demonstrated that the catalyst had been successfully synthesized without damaging the crystal structure of Fe₃O₄ [25]. Furthermore, the XRD pattern of Fe₃O₄@MCM-41@Pd(0)-P2C contained a series of peaks (39.4°, 45.9°, and 66.9°) which were indexed to Pd camouflaged on the surface of Fe₃O₄@MCM-41 [8]. The comparison study of the XRD patterns of Fe₃O₄@MCM-41 and Fe₃O₄@MCM-41@Pd(0)-P2C showed that after grafting of Pd through the formation of a complex with Fe₃O₄@MCM-41, reflections with weaker intensities were observed. As seen from the small angle XRD patterns in Fig. 2(b), The XRD pattern of Fe₃O₄@MCM-41 showed the presence of three reflection peaks corresponding to d₁₀₀, d₁₁₀, and d₂₀₀ planes at 2θ = 2.36°, 3.94°, and 4.38°, respectively, confirming the presence of the ordered hexagonal mesoporous structure of MCM-41 [23].

Fig. 3 are the SEM images of Fe₃O₄@MCM-41 and Fe₃O₄@MCM-41@Pd(0)-P2C. The Fe₃O₄@MCM-41 and Fe₃O₄@MCM-41@Pd(0)-P2C materials were composed of nearly spherical agglomerations composed of regular uniform nanoparticles.

The energy dispersive spectrum (EDS) indicated the presence of Pd, Fe, Si, O, N, and C species in the catalyst (Fig. 4).

TEM images of the Fe₃O₄@MCM-41@Pd(0)-P2C are shown in Fig. 5. The HRTEM micrograph demonstrated the ordered mesostructures with uniform pore dimension and hexagonal symmetry and agglomeration of many ultrafine spherical-like particles that displayed dark magnetite cores surrounded by a shell.

The magnetic properties of Fe₃O₄@MCM-41 and Fe₃O₄@MCM-41@Pd(0)-P2C were investigated using VSM at room temperature (Fig. 6). VSM measurements of the Fe₃O₄@MCM-41 nanoparticles showed that the saturation magnetization (M_s) of Fe₃O₄@MCM-41@Pd(0)-P2C was 9.42 emu/g (Fig. 6(2)), which is lower than the M_s of Fe₃O₄@MCM-41 (13.02 emu/g) (Fig. 6(1)). Based on these results, the successful grafting of organic layers containing a palladium complex on Fe₃O₄@MCM-41 was verified [26].

The thermogravimetric analysis (TGA) curves of Fe₃O₄@MCM-41and Fe₃O₄@MCM-41@Pd(0)-P2C are shown in Fig. 7. The TGA of the synthesized Fe₃O₄@MCM-41 showed a 3% weight loss in one step at a temperature below 100 ℃ owing to the desorption of water. The Fe₃O₄@MCM-41@Pd(0)-P2C sample showed a mass loss of approximately 2% below 125 ℃ owing to the removal of physically adsorbed water and solvent inside the pore channels. The other weight losses was in the region of 200-550 ℃ (approximately 22%), which was mainly related to the decomposition of the organically modified moieties on Fe₃O₄@MCM-41@Pd(0)-P2C [27]; and above 600 ℃ (about 6%) as a result of the condensation of the silanol groups.

Finally, the exact amount of Pd in the nanostructured catalyst was determined by ICP-OES. The amount of Pd in the immobilized catalyst on Fe₃O₄@MCM-41 was found to be 1.65 mmol/g.

After full characterization of the Fe₃O₄@MCM-41@ Pd(0)-P2C structure, the catalytic activity of this magnetic mesoporous compound was examined for C-C coupling reactions, namely Suzuki and Heck reactions. To optimize the reaction conditions, the Suzuki coupling reaction of iodobenzene (1 mmol) with phenyl boronic acid (1 mmol) was examined using different amounts of catalyst and solvent at a temperature range of 25-60 ℃ (Table 1). When the reaction was carried out in the absence of catalyst, the reaction was slow and resulted in no product, even after 6 h (Table 1, entry 1). The best result was obtained when the reaction was carried out in presence of 4 mg of catalyst in PEG-400 at 60 ℃ (Table 1, entry 3). In the next step, we studied the influence of the solvents PEG-400, EtOH, DMF, 1, 4-dioxane, and H₂O, at 60 ℃ (Table 1, entries 3, 5-8). The best results were obtained when PEG-400 was used. We then investigated the effects of the base on the reaction in the PEG solvent at 60 ℃. Different bases including (Et)₃N, NaHCO₃, Na₂CO₃, KOH, and K₂CO₃ were studied (Table 1, entries 3, 9-12). A high yield and shorter reaction time were obtained when K₂CO₃ was employed as base. Therefore, we chose K₂CO₃ as the base in all of the subsequent coupling reactions. We also studied the effect of temperature (Table 1, entries 3, 13-15). The optimum results were obtained at 60 ℃.

To demonstrate the efficiency and the scope of Fe₃O₄@MCM-41@Pd-P2C as a catalyst, various aryl halides were reacted with phenyl boronic acid under the optimal reaction conditions to produce the corresponding biphenyls in excellent yields and short reaction times (Table 2). As shown in Table 2, the reaction with chlorobenzene was slower than of iodobenzene or bromobenzene and required a much higher temperature and more Fe₃O₄@MCM-41@Pd(0)-P2C catalyst (6 mg, 0.99 mol%) (Table 2, entries 14-16). The aryl halides containing electron-donating and electron-withdrawing functional groups were converted to the corresponding biphenyls in short reaction times with good yield. As shown in Table 2, for the aryl iodides containing electron-donating functional groups, such as Me and OMe, the reaction was completed after shorter reaction times compared with those containing electron-withdrawing substituents.

To extend the catalytic applications of the Fe₃O₄@MCM-41@Pd(0)-P2C catalyst, we investigated its ability to catalyze the Heck C-C coupling reaction. The reaction of aryl halides with two types of vinylic substrates (n-butyl acrylate and acrylonitrile) were employed as the model reaction. The effect of the amount of catalyst, solvent (DMF, 1, 4-dioxane, H₂O, EtOH, or PEG), temperature (room temperature to 120 ℃), and the nature of the base (Et₃N, K₂CO₃, KOH, NaHCO₃, or Na₂CO₃) on the outcome of the coupling of iodobenzene with n-butyl acrylate or acrylonitrile was examined. A summary of the results is shown in Table 3. The reaction did not occur in the absence of Fe₃O₄@MCM-41@Pd(0)-P2C (Table 3, entry 1). Among the different amount of catalyst tested, 6 mg (0.99 mol%) was found to be the most effective amount because it gave the highest product yield. Among the different solvents, the best yields were obtained with PEG-400 using 0.006 g (0.99 mol%) of Fe₃O₄@MCM-41@Pd(0)-P2C (Table 3, entry 4). The reaction was significantly affected by the nature of the base; the best result was obtained using K₂CO₃. The coupling reaction yields were susceptible to temperature changes (Table 3, entries 4, 11-14). The highest product yield was achieved at 120 ℃.

After optimization of the reaction conditions, we examined the catalytic activity of Fe₃O₄@MCM-41@Pd(0)-P2C for other substrates (Table 4). The optimized reaction conditions were applied to a wide range of aryl halides using n-butyl acrylate or acrylonitrile. The aryl bromides and aryl iodides required a shorter reaction time compared with the corresponding aryl chloride (Table 4, entry 10).

To compare the catalytic properties of Fe₃O₄@MCM-41@Pd(0)-P2C with those of other catalysts reported in the literature, we investigated the results of the Suzuki-Miyaura reaction between iodobenzene and phenylboronic acid (Table 5) and the Heck-Mizoroki cross-coupling reactions between iodobenzene and butyl acrylate (Table 6) in the presence of various catalysts with respect to the temperature, the yield of the products, solvent, and reaction time. As shown in Tables 5 and 6, in this work, C-C bond forming through the Suzuki and Heck reaction was carried out in PEG-400 with short reaction times and in good to excellent yields. The new catalyst is also non-toxic, stable, cheap and easy to separate. The recoverability and recyclability of this catalyst was faster and easier than that of the other reported catalysts.

The recovery and reusability of the catalysts is of major importance in green chemistry and heterogeneous catalysis. We investigated the recovery and reusability of the Fe₃O₄@MCM-41@Pd(0)-P2C in the Suzuki-Miyaura reaction between iodobenzene (1 mmol) with phenylboronic acid (1 mmol) in PEG at 60 ℃, and Heck-Mizoroki cross-coupling reaction between iodobenzene (1 mmol) and butyl acrylate (1.2 mmol) in PEG-400 at 120 ℃. After completion of the reaction, as monitored by TLC, the reaction mixture was cooled to room temperature, the catalyst was easily and rapidly separated from the reaction mixture using an external magnet and washed several times with diethyl ether, then used in next run. As shown in Fig. 8, the Fe₃O₄@MCM-41@Pd(0)-P2C could be recycled over seven runs without any significant loss of its catalytic activity.

The heterogeneity of the Fe₃O₄@MCM-41@Pd(0)-P2C catalyst and the leaching of metal in the reaction mixture was examined by carrying out a hot filtration test during the Suzuki reaction between iodobenzene and phenylboronic acid. The yield of the product in half the reaction time (15 min) was 54%. The reaction was repeated and the catalyst was separated from the reaction mixture by hot filtration after 15 min. The filtrate was allowed to react further. The yield of the product under these conditions was 57%. The results from the hot filtration test indicated that no Pd leaching from the sample occurred during the catalytic reaction.

Fe₃O₄@MCM-41@Pd(0)-P2C was prepared and used as a novel, environmentally benign, and heterogeneous nanocatalyst and fully characterized using FT-IR, VSM, EDS, TEM, SEM, XRD, and TGA. The catalyst was successfully used for the Suzuki and Heck reactions under mild experimental conditions. The advantages of this system include the simple experimental procedure, utilization of an inexpensive and readily available catalyst, non-chromatographic purification of the products, simple recrystallization from diethyl ether, and avoiding the use of organic solvents, which is in agreement with green chemistry principles. In addition, this catalyst was easily separated from the reaction mixture by an external magnet and reused seven times without any significant loss of stability and activity.

Acknowledgment

The authors gratefully acknowledge Ilam University Research Council for partial financial support of this study.