In recent decades, nanostructured catalysts have attracted a lot of attention due to their high-efficiency in organic reactions. Although nanocatalysts have a wide range of advantages including controllable size, biocompatibility and high efficiency for practical applications, however, their separation and reconstruction are often fraught with limitations and difficulties (Wei et al., 2012; Gawande et al., 2013; Gawande et al., 2015; Chen et al., 2021). The introduction of magnetic iron oxide nanoparticles suffer some of these problems and has led to the discovery of important criteria for the design of many novel and modern catalytic processes (Gawande et al., 2013; Kong et al., 2013; Garkoti et al., 2017). Especially, the use of Fe₃O₄ MNPs in the catalytic industry, which is based on principles of green chemistry, is very attractive in this matter. The unique properties of magnetic NPs such as biocompatibility and easy magnetic separation, has led other sciences such as chemistry, physics, pharmacy and medicine to pay particular attention to these particles. Therefore, the use of magnetic nanocatalysts not only saves time but also prevents problems such as catalyst degradation, catalyst oxidation and the preparation of organic waste (Gawande et al., 2015; Mirhosseini-Eshkevari et al., 2015; Zhang et al., 2017; Karimi et al., 2018; Kargar et al., 2020; Mousavi et al., 2021). Also, magnetite nanoparticles have wide applications in the drug delivery, cancer treatment, purification of water contaminated with heavy metals and radioactive materials, magnetic resonance imaging, etc. However, the use of iron oxide magnetic nanoparticles suffers from problems such as aggregation and oxidation, which has limited their range of application. Surface modification of iron oxide MNPs is a practical technique to prevent the aggregation and oxidation of these NPs that is achieved through the use of noble metals, metal oxides, silica and organic polymers (Neysi et al., 2019; Neysi et al., 2020; Li et al., 2021; Tang et al., 2021). Among different species, silica is the most common shell for the modification of the surface of magnetite NPs. On the other hand, periodic mesoporous organosilicas (PMOs) are a desirable class of organic-inorganic composite materials that have emerged as an ideal shell for MNPs due to their excellent properties such as high surface area, high lipophilicity and high thermal and mechanical stability (Liu, 2017; Zhao et al., 2017; Zhou et al., 2017; Elhamifar et al., 2018; Norouzi et al., 2018; Norouzi et al., 2019; Liu et al., 2022). Some of recently reported in this matter are Fe₃O₄@SiO₂@PMO (Dai et al., 2017), Fe₃O₄ @SiO₂@Am-PMO(Norouzi and Elhamifar, 2021), Fe₃O₄@RF@void@ PMO(IL)/Cu (Shaker and Elhamifar, 2021) and Fe₃O₄@MePMO-IL/Pd (Shaker and Elhamifar, 2020b).

In recent decades, Schiff-base ligands have attracted a lot of attention in the chemical and materials sciences due to their easy synthesis, easy complexation with the most of transition metal ions, good solubility and high catalytic properties. Moreover, Schiff-base ligand is considered as a linker between the catalytically active center and the solid materials to increase the catalytic activity (Ghorbani-Choghamarani et al., 2015; Zhao et al., 2018; Amirmahani et al., 2020; Zhou et al., 2020; Lashkari et al., 2021; Mazraati et al., 2021). Some of recently developed reports in this matter are Fe₃O₄@MCM-41-SB/Pd (Shaker and Elhamifar, 2020a), Fe₃O₄@BOS@SB/In (Mirbagheri and Elhamifar, 2019), Cu/SB-Fe₃O₄ (Elhamifar et al., 2017) and BPMO@ ISB/Mn (II) (Norouzi and Elhamifar, 2019).

The Suzuki reaction is an example of Pd-catalyzed cross-coupling processes where the coupling species are an aryl-boronic acid and an aryl or vinyl halide. The Suzuki products are widely used in the pharmaceutical industry, natural and pharmaceutical compounds, conductive polymers, sensors and dyes. Therefore, in recent years many researchers have studied and evaluated the optimization of this reaction using efficient catalytic systems (Dong et al., 2021; Favalli et al., 2021; Kempasiddaiah et al., 2021; Kim et al., 2021). Some of recently reported catalysts in this matter are Fe₃O₄@SiO₂@NHC@Pd-MNPs (Akkoç et al., 2021), Fe₃O₄/Pd (Veisi et al., 2021), Fe₃O₄@SiO₂/glucosamine-Pd (Eslahi et al., 2021), SiO₂-NH₂@Pd (dpa)Cl₂ (Aghahosseini et al., 2021), Fe₃O₄/SiO₂ -NH₂@CS/Pd (Veisi et al., 2020), Fe₃O₄@MCM-41-SB/Pd (Shaker and Elhamifar, 2020a), PEt@ IL/Pd (Kargar and Elhamifar, 2020), GO-N₂S₂/Pd (Zarnegaryan and Elhamifar, 2020) and GO–SB/Pd (Zarnegaryan et al., 2019). In view of the above, in the present work, for the first time, a novel Fe₃O₄@Et-PMO supported Pd-Schiff base complex is prepared, characterized and its catalytic application is studied in the Suzuki reaction. Importantly, the present catalytic system has the advantages of magnetic Fe₃O₄ NPs, mesoporous materials and heterogeneous catalysts in the same time.

The synthesis method for the Fe₃O₄@PMO/SB-Pd is shown in Figure 1. For this, at first magnetite NPs were coated with periodic mesoporous organosilica shell via CTAB-directed hydrolysis and co-condensation of TEOS and BTEE. After CTAB removal, the obtained Fe₃O₄@PMO was modified with Schiff-base groups to deliver Fe₃O₄@PMO/SB. The Fe₃O₄@PMO/SB nanocomposite was finally treated with Pd(OAc)₂.4H₂O to give Fe₃O₄@PMO/SB-Pd catalyst. The chemical and structural properties of the Fe₃O₄@PMO/SB-Pd catalyst were investigated using FT-IR, VSM, SEM, EDX, TEM and PXRD analyses.

The FT-IR spectra of Fe₃O₄, Fe₃O₄@PMO, Fe₃O₄@PMO/Pr-NH₂ and Fe₃O₄@PMO/SB-Pd are depicted in Figure 2. The signals at 588 and 3,300–3,450 cm⁻¹ are, respectively, assigned to Fe-O and O-H bonds. Also, for Fe₃O₄@PMO, Fe₃O₄@PMO/Pr-NH₂ and Fe₃O₄@PMO/SB-Pd, the peaks observed at 823 and 1076 cm⁻¹ are related to the Si-O-Si bonds. Moreover, the peaks at 2,921 and 2,853 cm⁻¹ are for C-H bonds of Et-PMO and propyl groups. Interestingly, for the Fe₃O₄@PMO/SB-Pd nanocomposite, the peaks at 1100, 1428 and 1623 cm^-1 are, respectively, for the C-O, C=C and C=N bonds of the SB complex. These results confirm the successful formation and high stability of Et-PMO and SB groups into/onto the material framework.

The wide-angle PXRD of the Fe₃O₄@PMO/SB-Pd nanocatalyst is illustrated in Figure 3. This analysis revealed the signals at 2Ɵ = 30.3, 35.7, 43.4, 53.8, 57.7 and 63.0 degrees that are, respectively, due to the reflections of 220, 311, 400, 422, 511 and 440, confirming the crystalline structure of MNPs is stable and not changed during preparation of Fe₃O₄@PMO/SB-Pd (Figure 3).

The low-angle PXRD analysis of Fe₃O₄@PMO/SB-Pd nanocatalyst showed a peak at 2.2° that is attributed to the mesoporous structure of the PMO shell (Figure 4).

Scanning electron microscopy (SEM) analysis of Fe₃O₄@PMO/SB-Pd nanocatalyst revealed spherical particles for the designed catalyst (Figure 5). The average particle size of Fe₃O₄@PMO/SB-Pd NPs was about 50 nm according to the particle size distribution histogram.

The transmission electron microscopy image of the Fe₃O₄@PMO/SB-Pd nanocatalyst demonstrated that the Fe₃O₄@PMO/SB-Pd nanocatalyst has a core-shell structure with a black core (magnetite NP) and a gray shell (mesoporous layer) (Figure 6).

The magnetic properties Fe₃O₄@PMO/SB-Pd were determined using vibrating sample magnetometer analysis (VSM). This analysis illustrated that the magnetic saturation of the Fe₃O₄@PMO/SB-Pd nanocatalyst is about 40 emu g⁻¹, which is a confirmation of its high magnetic property (Figure 7).

The presence of the desired elements in the Fe₃O₄@PMO/SB-Pd nanocatalyst was confirmed using EDX analysis. This showed the signals of the elements of Fe, Si, C, N, Pd and O in the catalyst, which confirms the high stability of the expected organic and inorganic groups onto/into material framework (Figure 8).

The elemental mapping analysis also indicated the uniform distribution of all elements in the material framework (Figure 9).

Finally, TGA was done to study the stability of Fe₃O₄@PMO/SB-Pd. The first weight loss at 25°C–150°C is attributed to removal of water and organic solvents remaining in the synthesis process. The next weight loss in the range of 200°C–300°C is related to the decomposition of the p123 surfactant, which remains after the extraction process. The main weight loss, which appears at 301°C–850°C is due to the decomposition and removal of incorporated/immobilized organic functional groups (ethylene and Schiff-base) onto/into the structure of Fe₃O₄@PMO/SB-Pd nanocomposite (Figure 10).

Subsequently, the catalytic activity of Fe₃O₄@PMO/SB-Pd in the Suzuki reaction was investigated. The condensation between iodobenzene and PhB(OH)₂ was selected to achieve the best conditions (Table 1). At first, the effect of catalyst loading was investigated. As shown, the reaction yield is increased with the increasing amount of catalyst in which the highest product yield is obtained using 0.08 mol% of Fe₃O₄@PMO/SB-Pd (Table 1, entries 1–3). Also, the temperature effect study showed that at 50°C under ultrasonic conditions the best result is obtained (Table 1, entries 2, 4–6). The study also confirmed that the result of ultrasonic bath is much better than oil bath under the same conditions (Table 1, entry 2 versus 7). Various solvents were also examined (Table 1, entries 2, 8–10), which, water as environmentally-friendly solvent gave the highest yield. Among different bases of NaOAc, NaOH, K₂CO₃ Et₃N and base-free media, K₂CO₃ was the best (Table 1, entry 2 vs. entries 11–14). Accordingly, the use of 0.08 mol% of Fe₃O₄@PMO/SB-Pd and K₂CO₃ in H₂O at 50°C under ultrasonic irradiations were chosen as optimum conditions. In the next step, the Suzuki reaction was performed using Pd-free Fe₃O₄@PMO and Fe₃O₄@PMO/SB nanomaterials under the same conditions as the Fe₃O₄@PMO/SB-Pd nanocatalyst. Interestingly, in the latter cases no product was obtained confirming the process is catalyzed by supported Pd species (Table 1, entries 15, 16). It is well-known that in the Pd-catalyzed Suzuki reaction, the active catalytic species are Pd (0). In the present study, although the oxidation state of supported Pd is (II), however, during the reaction conditions this converts to active Pd (0) to successfully catalyze the Suzuki process (Karimi et al., 2010; Karimi et al., 2011; Elhamifar et al., 2013).

After optimization, the efficiency of Fe₃O₄@PMO/SB-Pd nanocatalyst was investigated in the synthesis of biphenyl products via Suzuki reaction. As shown in Table 2, all arylhalides including aryl-iodide, bromide and chloride, with different substituents, have been used as substrate giving corresponding coupling adducts in good to high yield and selectivity. These results show the high efficiency of Fe₃O₄@PMO/SB-Pd for synthesis a wide-range of Suzuki products.

The reusability and recoverability of the Fe₃O₄@PMO/SB-Pd nanocatalyst were investigated under optimal conditions in the Suzuki reaction between PhB(OH)₂ and PhI. For this, after completion of the reaction, the catalyst was recovered and reused in the next run. The results showed that the nanocatalyst can be reused and recovered at least eleven times with only a slight decrease in the product yield after each run (Figure 11).

In the next, the PXRD analysis of the recovered catalyst was performed to study its chemical stability under applied conditions (Figure 12). As shown, the pattern of this analysis is the same as PXRD of the fresh catalyst confirming high stability of the crystalline structure of the iron oxide NPs during the reaction conditions.

The leaching test of Fe₃O₄@PMO/SB-Pd was also investigated under optimal conditions in the Suzuki reaction between PhB(OH)₂ and PhI. After about 50% progress, the reaction was stopped and the catalyst was magnetically removed. The reaction of the residue was monitored and the result revealed no progress after 2 h. This indicates that the Fe₃O₄@PMO/SB-Pd nanocatalyst acts heterogeneously and also confirms that the palladium moieties are well stabilized on the material.

Finally, the catalytic activity of Fe₃O₄@PMO/SB-Pd nanocatalyst was compared with various catalysts that have recently been used in the Suzuki reaction. As shown in Table 3, the new catalyst possesses better performance than others in terms of temperature, reaction rate and recyclability.

In summary, the Fe₃O₄@PMO/SB-Pd catalyst was successfully prepared and employed in the Suzuki reaction. The FT-IR, EDX and TGA analyses showed high stability of organic and palladium moieties on material framework. The VSM and XRD analyses proved high magnetic properties of the designed catalyst. The SEM and TEM analyses also showed a spherical morphology for Fe₃O₄@PMO/SB-Pd. This nanocatalyst was effectively employed in the Suzuki reaction giving coupling products in high yield. Fe₃O₄@PMO/SB-Pd was also recovered and re-employed several times with no significant reduction in its performance.