Design and synthesis of Fe3O4@SiO2@KIT-6@DTZ-Pd0 as a new and efficient mesoporous magnetic catalyst in carbon–carbon cross-coupling reactions

In this paper, a new type of mesoporous material based on KIT-6 has been introduced. In this aim, magnetic Fe3O4 nanoparticles and mesoporous silica KIT-6 have been combined to obtain mesoporous MNPs. The prepared magnetic mesoporous catalyst has been applied in different carbon–carbon cross-coupling reactions including Mizoroki–Heck, Suzuki–Miyaura, and Stille reactions. This magnetic mesoporous compound is characterized by various techniques including FT-IR, BET, VSM, SEM, XRD, and TGA.

After designing and fabricating Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 , the structure of this magnetic mesoporous material is characterized by various techniques.
To consider the morphology of prepared magnetic mesoporous material, SEM (scanning electron microscopy) was applied. As it can be seen in Fig. 2, Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 particles are spherical with nano-sized particles.
Porosity analysis of Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 studied by nitrogen absorption-desorption technique. Based on this analysis, the specific surface area of Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 (a SBET ) was 71.477 m 2 /g, its monolayer capacity (V m ) was 16.422 cm 3 (STP) g −1 and the total pore volume of the prepared magnetic mesoporous compound was 0.1188 cm 3 /g. The specific surface area was calculated by the Langmuir isotherm.  www.nature.com/scientificreports/ Langmuir isotherm (Fig. 3) illustrated a s,lang = 415.1 m 2 /g and V m = 95.372 cm 3 (stp) g −1 , respectively. Furthermore, the calculations related to BJH diagram from the adsorption and desorption branch of the nitrogen adsorption curve indicated that the pore sizes in these compounds were the same and the average pore diameter of Fe 3 O 4 @ SiO 2 @KIT-6@DTZ-Pd 0 is 2.41 nm.
To measure the exact amount of loaded Pd on the surface of Fe 3 O 4 @SiO 2 @KIT-6@DTZ nanoparticles, induced coupled plasma (ICP) spectroscopy technique was used, which based on this technique, the exact amount of Pd is 1.069 × 10 -3 mol/g.
The TGA diagram for magnetic mesoporous material (Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 ) is shown in Fig. 5. Based on this analysis, the first weight loss under 200 °C (about 10%) is related to the evaporation of physically adsorbed solvents and water. The second weight loss, which is about 9.5%, is between 200 to 800 °C related to the removal of organic moieties on the surface of magnetic mesoporous support. The final weight loss is related to the phase change of Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 nanoparticles.  www.nature.com/scientificreports/ The magnetic properties of the synthesized Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 were investigated by VSM analysis. As displayed in Fig. 6, the magnetic property of nanocatalyst is 3.67 emu/g, which reflects this fact, the nanoparticle surface is coated with SiO 2 and organic groups. Nevertheless, by applying an external magnetic field, the catalyst can be easily separated from the reaction mixture.
Catalytic studies. Mizoroki-Heck cross-coupling reaction. After the characterization of the prepared magnetic mesoporous material, the catalytic activity of this compound was studied in carbon-carbon bond formation reactions.
Initially, the catalytic activity of Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 was examined in Heck reaction. To obtain optimal conditions for this C-C cross-coupling reaction, the reaction of iodobenzene and butyl acrylate was investigated as a model reaction. The reaction was examined in the presence of various solvents such as PEG, DMF, DMSO, CH 3 CN, different bases (KOH, NaOH, Na 2 CO 3 , Li 2 CO 3 ), and different values of catalysts (4, 5, and 6 mg) at various temperature conditions.
The highest yield of product (95%) was obtained in PEG as the solvent, 3 mmol of K 2 CO 3 as the base and 5 mg of catalyst at 100 °C (Table 1).
To develop the efficiency of described catalyst a variety of aryl halides reacted with butyl acrylate under obtained optimal reaction conditions (Scheme 1).  www.nature.com/scientificreports/ The results are summarized in Table 2. As it can be seen, aryl iodides are not significantly different from aryl bromides and chlorides in terms of the reaction yields, and only the reaction time with aryl bromides and aryl chlorides is slightly longer.

Suzuki-Miyaura cross-coupling reaction.
To consider more activity of Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 as the catalyst, the activity of the described catalyst was studied in the Suzuki cross-coupling reaction. The optimization experiment initiated the reaction of iodobenzene with phenylboronic acid in different conditions as a model substrate. Reaction conditions including base, solvent, temperature, the amount of catalyst were screened and the results are summarized in Table 3. The results demonstrated that the highest efficiency was achieved in Ethanol as the solvent, K 2 CO 3 (2.5 mmol), and was 5 mg of catalyst at 75 °C.
The optimal conditions were applied for the reaction of a wide variety of aryl halides with phenylboronic acid (Scheme 2); the results are presented in Table 4.
Stille cross-coupling reaction. Herein in the final part of this research project, Stille cross-coupling reaction of various aryl halides and triphenyl tin chloride was investigated in the presence of Fe 3 O 4 @SiO 2 @KIT-6@ DTZ-Pd 0 . The reaction parameters (including solvent, temperature, type, and amount of base as well as the amount of catalyst) were considered for the cross-coupling of iodobenzene and triphenyl tin chloride. As is evident from Table 5 the highest product yield was obtained in PEG as solvent. Also, the highest yield and the www.nature.com/scientificreports/ shortest time for mentioned cross-coupling reaction was obtained using K 2 CO 3 as the base, 6 mg of Fe 3 O 4 @ SiO 2 @KIT-6@DTZ-Pd 0 at 80 °C.
To spread out the application of described catalyst in Stille reaction; triphenyl tin chloride reacted with a variety of aryl halides (including chloride, bromide, and iodide) under optimal conditions (Scheme 3). The results including yields and reaction times brought in Table 6.
In order to compare the catalytic performance of Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 with the previously reported catalysts, the results of coupling of phenylboronic acid with iodobenzene by the previously reported methods are shown in Table 7. In this comparison, various parameters such as reaction time, reaction conditions, and efficiency with other catalysts were compared. In this work, the C-C bond formation reaction is performed in ethanol as a green solvent and this catalyst almost shows a shorter reaction time and higher performance than the other catalysts.  www.nature.com/scientificreports/ Reusability of the catalyst. According to the principles of green chemistry, the recovery of the catalyst at the end of the reaction and its reusability is a highly significant factor. In this aim, Suzuki cross-coupling reaction of iodobenzene and phenylboronic acid was investigated. After each run nanocatalyst was isolated from reaction media by applying a magnet and washed several times with ethanol, then dried and reused for the next experiment. As it can be seen from Fig. 8, the catalyst is recoverable at least up to four runs, with a negligible decrease in its activity.

Catalyst leaching study.
To perform the hot filtration test, the Suzuki reaction was selected as the model reaction and two types of reactions were performed between iodobenzene and phenylboronic acid under opti-  www.nature.com/scientificreports/ mal reaction conditions. In the first reaction, the biphenyl product was obtained after 6 min (half the reaction time) with a yield of 69. Simultaneously in the second reaction, the same reaction was repeated, but at the half-reaction (after 6 min), the catalyst was removed from the reaction mixture by a magnet and the reaction mixture was allowed to run for another 6 min. The reaction efficiency at this stage was 72%. These experiments confirmed that Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 was necessary to complete the reaction and that confirmed the leaching of palladium during the reaction didn't occur.

Conclusions
In this research project, we have introduced a novel magnetic mesoporous material with two unique properties i.e. high porosity and magnetism. These two factors make this nanomaterial an efficient and versatile catalyst. The catalytic activity of Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 was examined in the variety of cross-coupling reactions   KIT-6@DTZ (1 g) dispersed in 50 mL ethanol via sonication for 20 min. Then, 0.5 g of Pd(OAc) 2 was added to the mixture and stirred for 20 h, at reflux conditions under nitrogen atmosphere. Finally, NaBH 4 (0.6 mmol) was added to the reaction mixture and stirred under the same conditions for 2 more hours. Then, the reaction mixture cooled down to room temperature, and magnetic mesoporous material (Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 ) was isolated by a magnet and washed several times with ethanol.
General procedure for Mizoroki-Heck cross-coupling reaction. To perform Heck reaction, in a 5 mL round bottom flask, aryl halide (1 mmol), butyl acrylate (1.2 mmol), 3 mmol of potassium carbonate, and Fe 3 O 4 @SiO 2 @KIT-6@DTZ-Pd 0 (5 mg) were added to 2 mL of PEG and the mixture stirred at 100 °C. The progress of the reaction was followed by TLC. After completion of cross-coupling reaction, 10 mL water was added