INTRODUCTION
Nano catalysts exhibit greater reactivity, selectivity, and efficiency than their bulk counterparts. Having a high surface area-to-volume ratio allows for an increased number of active sites, facilitating speedier reactions. The following enhances the sustainability of chemical processes and makes them a compelling subject for chemists [1-6]. Metal-based nano-catalysts are crucial in numerous chemical reactions and industrial processes [7-9]. Nano Catalysts, especially those using palladium nanoparticles, are commonly utilized in coupling processes [10-13]. Palladium-based Nano Catalysts can catalyze various coupling reactions, such as Suzuki-Miyaura, Heck, Sonogashira, and Buchwald-Hartwig reactions [14-17]. These reactions are crucial for creating medicines, agrochemicals, and innovative materials by efficiently building complex organic compounds. P123 is a symmetric triblock copolymer containing polyethylene oxide (PEO) and polypropylene oxide (PPO). When placed in a selective solvent, P123 can form both spherical and cylindrical micelles. At temperatures over 288 °K, the PPO block is hydrophobic. Below 288 °K, it becomes soluble in water, forming micelles of PEO-PPO-PEO triblock copolymers. P123 has been used to produce mesoporous materials [18-19].
Mesoporous organosilica substrates are hybrid materials with unique porous structures and organic functional groups incorporated into their silica framework. They have the properties of both inorganic silica and organic compounds [20]. These substrates have mesopores with widths ranging from 2 to 50 nm, making them appropriate for varied applications.  The large surface area, well-defined pore structure, and unique inorganic and organic properties make them excellent candidates for catalyst supports, adsorbents, drug delivery systems , and sensors [21-24].
The Suzuki reaction has become popular in pharmaceuticals due to its high selectivity, gentle reaction conditions, broad substrate compatibility, adaptability, and capacity to create complex and physiologically active chemicals. Combining boronic acids and halides allows for a diverse range of functional groups and substitution patterns. This coupling reaction allows for fast synthesis of aryl- and vinyl-substituted chemicals and complex molecules, with exquisite structural control [25-27]. Bi-aryls, produced by the Suzuki reaction, have been extensively studied in organic chemistry due to their distinct structural features and diverse properties. They are chemically stable and can be produced using numerous techniques, including transition metal-catalyzed cross-coupling. These chemicals are effective in medical chemistry, materials science, and other fields of research. Their aromatic characteristics and functionalization capabilities make them valuable building blocks for creating complex compounds with specialized features [28, 29].
This study utilized nano Pd@L-Serine-PMO as a reusable heterogeneous catalyst for the Suzuki-Miyaura C-C coupling reaction under mild conditions (Fig. 1). This reaction was carried out at 40 °C using water and ethanol as green solvents. The usage of water and ethanol enhances sustainability and follows green chemistry principles. K2CO3 served as the base of the reaction.    

MATERIALS AND METHODS
Preparation of PMO
P123 (3.9 g) was added to a mixture of KCl (18 g), 2M HCl (94 mL), and H2O (20 mL). The mixture was stirred for 5 hours at 40 °C. Then, 1,2-bis (tri methoxy silyl) ethane (1.35 g), and tetra ethyl orthosilicate (7.35 g) were added and stirred continuously for 24 h. The product was separated by simple filtration, transferred to an autoclave, and placed in an oven at 100 °C for 70 h (Fig. 2).

Preparation of L-Serine-PMO
A mixture of PMO (0.5 g) and L-serine (1 g) was added to H2O (15 mL) and refluxed under an N2 atmosphere. The mixture was cooled to room temperature after 24 h, and the product was separated by centrifugation, washed with water, and dried at 40 °C to produce L-Serine-PMO (Fig. 3).

Preparation of nano Pd@L-Serine-PMO
L-Serine-PMO (0.25 g) in H2O (25 mL) was stirred for 30 minutes at room temperature. PdCl2 (1 g) was added to the reaction mixture and stirred continuously for 2 h. Then, NaBH4 (0.75 g) was added, and stirring was continued under an N2 atmosphere for 24 h. Finally, the product was separated by centrifugation (Fig. 4) and identified by FTIR, SEM, EDX, XRD, and TGA.

General procedure for the suzuki C-C coupling reaction
A mixture of aryl halide (1 mmol), phenylboronic acid (2 mmol), K2CO3 (2 mmol), and an optimized amount of nano Pd@L-serine-PMO (0.10 g) was stirred at 40 °C in 2 mL H2O/EtOH (1:1) as the solvent reaction. The progress was monitored by TLC (n-hexane/ethyl acetate: 7/3). After completion of the reaction, the nano catalyst was separated via filtration, the product was extracted with ethyl acetate, and the solvent was evaporated. The pure product was obtained via recrystallization from hot ethanol and was characterized by its melting point and spectroscopic data. 

RESULTS AND DISCUSSION
Identification of nano Pd@L-Serine-PMO
The FT-IR spectra of L-serine, L-serine-PMO, and nano Pd@L-serine-PMO are presented in Fig. 5(A-C). As can be seen in Fig. 5A, the absorptions at approximately 3450 and 3350 cm-1 are related to the asymmetric and symmetric NH2 of L-serine, respectively. The broad peak at ~3200 cm-1 is attributed to O-H vibration. The O-H absorption of acid is recorded in the range of 2400–3050 cm-1. The absorptions of C=O are recorded at about 1700 cm-1. For L-serine-PMO (Fig. 5B), broadband was recorded at 3400 cm-1, which corresponds to the molecular water hydrogen bond and SiOH groups. The 1630 cm-1 peak is related to the vibration of molecular water. The bands observed around 1090 and 800 cm-1 are attributed to the asymmetric and symmetric modes of Si-O-Si bridging sequences, respectively. In the spectrum of nano Pd@L-serine-PMO, some peaks have been shifted to lower wave numbers, which confirmed the existence of palladium nanoparticles in the structure of the catalyst (Fig. 5C).
The morphology of the nano Pd@L-serine-PMO structure was studied by SEM. The results are presented in Fig. 6(a, b). According to these figures, the nano catalyst has a rod-like and uniform structure, and the obtained nanorods are well dispersed. The diameter of the nanorods and their length are approximately 500 nm and 1.5 mm, respectively. Nanoparticles are spherical, and their average size is ~32 nm.
The elements of the catalyst were identified by EDS analysis. The results are shown in Fig. 7. According to this figure, the presence of Si, C, N, O, and Pd confirmed the structure of nano Pd@L-serine-PMO.
Fig. 8 displays the low-angle powder X-ray diffractogram of nano Pd@L-serine-PMO. It can be observed that nano-Pd@L-serine-PMO exhibits an XRD pattern with one very intense diffraction peak (100) and two weak peaks (110) and (200), which are characteristic of a two-dimensional hexagonal p6mm structure with excellent uniformity.
The TGA curves of PMO and nano-Pd@L-serine-PMO are shown in Fig. 9(a, b). According to these figures, PMO and nano-Pd@L-serine-PMO show 25% and 35% weight loss, respectively. The increase in the loss weight for nano Pd@L-serine-PMO compared with that for PMO confirmed that the nano catalyst was synthesized successfully because of the presence of organic groups of L-serine in the structure of the nano catalyst.

Optimization of the reaction conditions for Suzuki coupling
The Suzuki C-C coupling reaction was studied in the presence of nano-Pd@L-serine-PMO. The reaction of 4-bromo-toluene and phenylboronic acid was chosen as the model reaction and tested under various conditions (including different solvents, bases, temperatures, and amounts of nano catalyst). The results are summarized in Table 1. The best efficiency was obtained using 0.10 g of nano-Pd@L-serine-PMO in 1:1 H2O/EtOH as the solvent and K2CO3 as the base at 40 ⁰C (Table 1, Entry 13). There was no reaction in the absence of a nano catalyst (Table 1, Entry 1), and the yield is traced in the absence of a base (Table 1, Entry 15).
The results for the Suzuki C-C coupling reaction using nano-Pd@L-Serine-PMO according to optimized condition (Table 1, entry 13) are summarized in Table 2.

Reusability of Catalyst
The reusability of the nano catalyst was studied via optimized condition (table 1, entry 13) and the results have been illustrated in Fig. 10. After the completion of the reaction, the nano catalyst was separated by filtration from the reaction mixture, washed several times with EtOAc, and dried. The recovered nano catalyst was used in the next run. The results show that the nano catalyst can be reused five times without any considerable decrease in its catalytic activity. 
The catalytic activity of nano Pd@L-Serine-PMO for the synthesis of 3a was compared with that of other catalysts. The results are presented in Table 3. According to this table, nano-Pd@L-serine-PMO is also a highly efficient catalyst, and it has advantages such as high yield, short reaction time, and low temperature.

Mechanism of the reaction
The proposed mechanism for the Suzuki C-C coupling reaction in the presence of nano-Pd@L-serine-PMO is shown in Fig. 11. According to this mechanism, the aryl halide is converted to intermediate A in the presence of the nano catalyst. Then, B is produced by adding the base. In the next step, C is produced via the reaction of intermediate B with phenylboronic acid. Finally, by removing Pd, bi-aryl D is synthesized as the final product.

CONCLUSION
This study evaluated nano Pd@L-serine-PMO, a new heterogeneous nano-catalyst, using FTIR, SEM, EDS, XRD, and TGA. This nano catalyst served its purpose during the Suzuki C-C coupling reaction. Various aryl halides and boronic acids were applied to create important biphenyl compounds. This procedure has advantages such as mild reaction conditions, use of green solvents, clean workup, high yields, reusability of the catalyst, and heterogeneous conditions.

ACKNOWLEDGEMENTS
The authors are so grateful to the University of Kashan.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.