Ramazani Magnetically recoverable nanostructured Pd complex of dendrimeric type ligand on the MCM-41 : Preparation , characterization and catalytic activity in the Heck reaction


 A palladium complex of a dendrimer type ligand of aminoethylacrylamide immobilized onto the mesoporous channels of MCM-41 with magnetic core was prepared and characterized using various techniques such as XRD, TEM, BET, FT-IR, TGA, and VSM. The prepared nanostructured material was found as a magnetically recoverable catalyst for Heck reaction of aryl halides and vinylic C–H. The catalyst is easily recoverable with an external magnet and is reusable with different leaching amounts depending to loading of Pd. A hot filtration test was also performed and gave evidence that Palladium in heterogeneous samples can dissolve and then redeposit on the surface of the support material.

Mesoporous silica, is a very widely used support, due to bearing a very high surface area and uniform array of hexagonal channels (Abdollahi-Alibeik and Heidari-Torkabad, 2012; Abdollahi-Alibeik and Rezaeipoor-Anari, 2016;Hashemi-Uderji et al., 2018b). Moreover, silanol groups on the surface of MCM-41 are useful for immobilization of organic modifiers (Abdollahi-Alibeik and Pouriayevali, 2011). Linkage of modifier with the silanol groups onto the inner surface of hexagonal channels can provide active catalytic sites and therefore hexagonal channels of MCM-41 can act as nanoreactor. Recently, a wide variety of transition metal complexes have been anchored on the surface of MCM-41 and have been applied as catalysts in various organic transformations (Chang et al., 2013;Gao et al., 2015).
Dendrimers are particularly well-suited for hosting metal nanoparticles because the dendrimer templates are of fairly uniform composition and structure, and therefore they yield well-defined nanoparticle replicas (Crooks et al., 2001). Immobilizing of metal complex of denderimeric ligands on the solid supports is of practical importance due to the ability of loading a large amounts of metal ions on the surface of the support, as well as their recoverability and reusability (Isfahani et al., 2013). Many dendrimer functionalized solid support have been used as heterogeneous catalysts in organic reactions such as SO 3 H-dendrimer functionalized magnetic nanoparticles , HPA-dendrimer functionalized magnetic nanoparticles (Jamshidi et al., 2018), magnetic multi-walled carbon nanotubes functionalized with polyamidoamine (PAMAM) dendrimers (Adibian et al., 2020), dendritic chiral auxiliaries on silica (Chung and Rhee, 2002;Gharibpour et al., 2017;Isfahani et al., 2013;Tarahomi et al., 2019). MCM-41-supported coppercomplexed dendrimer (Gharibpour et al., 2017), Pd nanoparticles on the ternary hybrid system of graphene oxide, Fe 3 O 4 nanoparticles, and pamam dendrimer (Tarahomi et al., 2019), Ce(IV) immobilized on halloysite nanotube-functionalized dendrimer (Ataee-Kachouei et al., 2020).
However, due to importance of recovery and reusability of solid catalysts, improvement of new magnetically recoverable catalytic system is also desirable.
In this research, our strategy is developing a heterogeneous catalyst by immobilizing a denderimeric type ligand on the surface of MCM-41 (MMNP, Scheme 1) with various loading amounts of palladium and application of metal complex in Heck reaction in order Scheme 1: Steps of preparing the MMNP catalyst.
to study of the catalytic activity and also leaching of Pd (Scheme 2).

The catalyst characterization
MCM-41 with magnetic core (MNPs) was prepared according to the presented method in Scheme 1, and functionalization with dendrimer type ligand and then a complexation was carried out according to presented method in Scheme 1. To confirm successful functionalization of the synthesized MNPs@MCM-41, the MMNP catalyst was characterized using FT-IR spectroscopy in the range of 400-4000 cm −1 and the corresponding spectra are shown in Figure 1 Formation of this bond was confirmed by FT-IR spectra of MNPs@MCM-41-NH 2 with a characteristic band at 1736 cm -1 (Figure 1c). After the formation of the amide group, carbonyl peak of this group is appeared at 1653 cm -1 in MNPs@MCM-41-NE 2 -EDA spectrum (Figure 1d). This carbonyl peak is shifted to 1657 cm -1 in the spectrum of palladium complex (Figure 1e). This result confirms Scheme 2: Heck reaction in the presence of the MNNP catalyst. formation of complex between Pd and amide ligand on the MCM-41.
The FT-IR spectra of reused catalyst after the first run to the third run were studied to investigate effect of possible leaching of Pd from ligand and also possible deformation of ligand ( Figure 2). Results show that all of main peaks of the ligand remained unchanged. The carbonyl peak in 1657 cm -1 is unchanged in all three spectra. This result confirms that there is not significant leaching of Pd from the ligand.
Low angle powder X-ray diffraction (XRD) patterns of all samples: pure MCM-41 (a), MNPs@MCM-41 (b) and MMNP (c) were displayed in Figure 3. The XRD analysis of MCM-41 revealed three characteristic Bragg reflections peaks at low angle pattern (2θ = 2°-6°). The presence of these three Bragg reflexes can be indexed highly ordered pore system with a high porosity in a hexagonal mesoporous structure of MCM-41, corresponding to d 100 , d 110 , and d 200 planes at 2θ = 2.4°, 4.0°, and 4.6°, respectively (Figure 3a). On the other hand, an overall decreased intensity, some broadened reflections of d 100 and a decrease in 2θ based on the Bragg's equation (an increase in the d-spacing) are observed at low angle XRD pattern of MNPs@MCM-41 ( Figure 3b) compared to pure MCM-41, which are due to the decrease in ordered structure degree of hexagonal mesostructures after incorporation of magnetic iron oxide into the MCM-41 framework. At low angle XRD pattern of MMNP (Figure 3c) compared to Fe 3 O 4 @MCM-41, the organometallic functionalization of palladium complex dendrimer reduces the intensity of d 100 , increases the d-spacing and shifts it to the higher angle (2θ = 1.84°), corresponded to blockage of the mesoporous channels by palladium complex dendrimer. These changes often observed in mesoporous silica materials.
High-angle powder X-ray diffraction ( The transmission electron microscopy (TEM) image of MMNP was shown in Figure 5. TEM image of MMNP shows that the ordered hexagonal mesoporous structure of MCM-41 channels is remained unchanged  The superparamagnetic behavior of FMNC was confirmed by magnetization curves of MNPs@MCM-41 and MMNP ( Figure 6). The magnetic hysteresis loops were accomplished by an applied magnetic field at room temperature (300 K). There was no apparent hysteresis detected, zero coercivity and remanence on the loop of magnetization, suggesting that both of MNPs@MCM-41 and MMNP exhibited a superparamagnetic feature. The magnetic saturation values of MNPs@MCM-41 and MMNP were 4.05 emu/g and 2.50 emu/g (Figures 6a,b), respectively. There was some decrease in saturation magnetization value after surface modification of MNPs@ MCM-41 (MMNP, Figure 6b), in comparison with MNPs@ MCM-41 (Figure 6a), which confirms modified organic matter on the MNPs@MCM-41 surface and relative low mass ratio of MNPs in the MMNP sample. The isolation of the MMNP was achieved using an external permanent magnet beside the reactor wall, making the resumption of the MMNP very straightforward, which is necessary for magnetic separation (Figure 7).
The mesoporous structure of MNPs@MCM-41 was confirmed by nitrogen adsorption-desorption isotherms. In this isotherms a sharp inflection at p/p° = 0.2-0.4 is related to the nitrogen capillary condensation in the uniform mesopores, which confirms the mesoporous structure of synthesized MNPs@MCM-41. A sharper hysteresis was observed at higher p/p° (p/p° > 0.8) related to the nitrogen condensation within the formed voids by nanoparticles ( Figure 8a). Figure 8b shows the nitrogen adsorption-desorption isotherms of the MMNP catalyst that a mesoporous inflection was not observed at the medium p/p° partial pressure region (p/p° = 0.2-0.4). This means that organic maters occupied the inner voids of the MMNP@MCM-41. According to the obtained results from N 2 sorption, the surface area (S BET ), pore volume (V pore ), and pore diameter (D pore ) of MMNP@MCM-41 are 783.24 m 2 /g, 1.00 cm³/g, and 7.4 nm, respectively. There is some decrease amount of S BET and V p in Table 2 which identified cavities filling by organic matter.
The thermal stability of the MMNP was studied by thermogravimetric analysis (TGA). TGA measurement was conducted by the heating of 47 mg MMNP sample in   the range of 25-800°C ( Figure 9). TGA curve presents two exothermic peaks in the range of 25-280°C and 280-700°C.
The exothermic peak at 25-280°C contains 7.4% weight loss which corresponded to the removal of physically adsorbed moisture and other solvents. The second exothermic peak at 280-700°C contains 17% weight loss can be assigned to the decomposition of organometallic dendrimer on the surface of MMNP@MCM-41. According to the TGA curve, organometallic dendrimer on the MMNP@MCM-41 surface can be thermally stable up to 280°C.

Catalytic activity
The catalytic performance of MMNP catalyst was investigated in the reaction of p-bromonitrobenzene and ethyl acrylate as a model reaction. The results summarized in Tables 3 and 4. The reaction was carried out under the optimum conditions using various Pd loading, catalyst amount, solvents, bases, temperatures and the reacting components ratios. The effect of the palladium loading on the catalytic activity of MMNP was studied by using the catalyst with different loading amounts of Pd with mass ratios of ligand:PdCl 2 = 100:5, 100:10, 100:15 and 100:20 in the Heck model reaction for three runs (Table 3). As shown in Table 3, MMNP with loading mass ratio of ligand:PdCl 2 = 100:10 had the best catalytic behavior rather than other loading mass ratios in terms of reaction time and palladium loss percent after third runs. In the loading mass ratio of ligand:PdCl 2 = 100:5, the reaction was not complete at all, so the palladium amount in this loading was not enough for this reaction (Table 3, entry 1). In 100:10 loading, the reaction time is 1.5 times in the second cycle, also palladium loss percent after second cycle is about 9.4%, which is negligible compared to the fresh catalyst (Table 3, entry 2). While for the 100:15 and 100:20 loading, the reaction times are 8.4 and 14 times respectively in the second cycle, and palladium loss percent after second cycle is about 9.1 and 7.6%, respectively, compared to the fresh catalyst (Table 3, entries 3 and 4). The reaction times at 100:15 and 100:20 fresh catalysts were better than 100:10, but the reaction times after second cycle for 100:15 and 100:20 were worse than 100:10, while the palladium loss for 100:15 and 100:20 after second cycle were less than 100:10. These results suggest that a part of palladium chloride used in the construction of 100:15 and 100:20, formed complexes at MMNP catalyst and    the excess palladium chloride have been settled on the catalyst surface. So the Heck reaction in the presence of 100:15 and 100:20 was done with palladium-complex and free palladium particles at fresh catalyst and after the first run, free palladium particles deactivated and still existed on the catalyst surface so it seems less palladium loss in the loading mass ratios of ligand:PdCl 2 = 100:15 and 100:20 rather than 100:10 (Table 3, entries 2-4). After selecting the best loading of palladium chloride on dendrimer ligand (ligand:PdCl 2 = 100:10), the reaction was carried out under the optimum conditions using various catalyst amount, solvents, bases, temperatures, and the reacting components ratios using model reaction (p-bromonitrobenzene with ethyl acrylate as the model reaction). The solvents had significant effects on the MMNP catalytic activity. Among the different polar and nonpolar solvents (H 2 O, CH 3 CN, CH 3 CN:H 2 O, DMF, DMF:H 2 O, and toluene) or solvent-free condition; the polar solvent such as dimethylformamide (DMF) gave higher yield than nonpolar solvent such as toluene (Table 4, entries 2-9) and solvent-free condition was not appropriate (Table 4, entry 1). Amount of DMF also affected on the product yield. As shown in Table 4 (entries 8 and 26-29), with decreasing DMF amount from 4 mL to 0.5 mL, it was observed that the suitable amount of DMF is 1 mL. The bases (such as TEA, Na 3 PO 4 , K 3 PO 4 , and K 2 CO 3 ) used in the Heck reaction had great influences on the MMNP catalytic activity. As shown in Table 4 (entries 8 and 10-13) the organic base (TEA) gave a better result than inorganic bases (Na 3 PO 4 , K 3 PO 4 and K 2 CO 3 ), whereas, there is no reaction progress in the absence of base. Interesting results were obtained by changing in the amount of base. The use of lower or more than 3 mL TEA, caused weakening in the MMNP catalytic activity (Table 4, entries 8 and 14-18). When the Heck reaction was accomplished at different temperatures in the presence of MMNP, it was found that temperature at 130°C is the best. The Heck reaction in the absence of the catalyst was not carried out at all. Notwithstanding, the amounts of 10 and 20 mg of MMNP showed good results for the Heck reaction and 40 mg of the catalyst had negative effect on the Heck reaction. At first it seemed that 2 mg is the optimal amount of the catalyst, but was not enough for other derivatives, so finally; 10 mg MMNP (0.42 mol% palladium) was selected as the optimal amount of the catalyst (Table 4, entries 8 and 22-25). The ethylacrylate usage in amount of 2, 2.5 and 3 mmol (as an olefin) was studied in Heck reaction and as shown in Table 4 (entries 24, 30, and 31) the amount of 1:2.5 mmol of aryl halide:olefin was the best reacting component ratio. More interestingly, reaction atmosphere changing from inert to air had better effect on MMNP catalytic activity (Table 4, entry 24 and 32).
To investigate the application of MMNP as catalyst in Heck reaction, a variety of olefins were coupled with different aryl halides in the presence of 10 mg MMNP (0.42 mol% palladium) using TEA as a base in DMF.  These results are illustrated in Table 5. Aryl bromide with electron-donating groups has excellent yield compared with aryl bromide with electron-withdrawing groups and their activity were NO 2 > CHO > H > CH 3 > OCH 3 (Table 5, entries 1, 4 and 8-10). Halobenzene activities were I > Br > Cl (Table 5, entries 6, 8 and 11). The order of activity for olefins is ethyl acrylate > acrylonitrile > styrene (Table 5, entries 1-7). The scope of the procedure was also investigated using aliphatic halides as starting material. Results show that alkyl halides react with olefins as well as aryl halides   and high yields of products have been achieved (Table 5, entries, 12-16). The reuse of the MMNP catalyst was investigated in the Heck coupling reaction of p-bromonitrobenzene with ethyl acrylate as a model reaction. After completion of the reaction (monitored by TLC; eluent: EtOAc:n-hexane, 20:80) MMNP catalyst could be easily separated from reaction mixture using an external permanent magnet, washed with warm ethanol (3 × 3 mL) and dried at 100°C for 1 h, then reused in the model reaction at the same reaction condition. The results of reusability study are demonstrated in Table 6. As shown in Table 6 (entries 1-3), it was found that there is a low increase in the reaction time and low decrease in the product yield. These results suggest that the palladium leaching of MMNP catalyst can be ignored. To confirm this issue, the Pd loading of the recovered catalyst after second cycle was determined using atomic absorption spectroscopy and 0.38 mol% palladium was detected (Table 6, entry 3). Thus, 9.5% of Pd leaching (Table 6, the difference between entries 1 and 3) is a reason for ignored deactivation of the catalyst after the second cycle. It is noticeable that after each run, the catalyst easily and completely was separated by external magnet and there were no problems due to the reduction of magnetic properties.
The general methods for the leaching study is hot filtration test or split test. Especially in the case of the Heck reaction, how the reaction starts with a solid catalyst has received a great deal of attention. In principle, the two discussed possibilities were that either the reaction is catalyzed by a heterogeneous surface mechanism or that dissolved palladium, which is generated by metal leaching (Bucsi et al., 2017).
To improve the understanding of the dynamic leaching process under Heck reaction conditions, Köhler et al. (2002) reported that a large amount of Pd was dissolved into the solvent during the Heck coupling of chloroarenes and then the Pd species were redeposited on the surface of the support material. After further investigation, it was demonstrated that palladium is dissolved simultaneously with the start of the reaction, the highest reaction rate matches the highest palladium concentration in solution, and palladium is redeposited on the support again after the reaction is completed (Gnad et al., 2020;Heidenreich et al., 2002;Zhao et al., 2002). It has been confirmed that solid palladium catalysts act as reservoirs for active Pd species in solution and the dissolved Pd species tend to readsorb onto the support after the reaction has been completed.
In the present study, in order to investigate whether the MMNP catalyzed Heck reaction takes place based on the heterogeneous surface or due to Palladium leaching from MMNP, we accomplished hot filtration test to differentiate between heterogeneous and homogeneous activity based on the comparison of observed results before and after MMNP catalyst separation using an external permanent magnet in the reaction temperature. The reaction was interrupted at 30 min and then the filtrate was collected from the hot reaction mixture. The reaction was then conducted for another 105 min using the filtrate in the absence of the solid catalyst. Results show that after removal of the solid catalyst, the reaction mixture retained its activity and after 135 min, the reaction was split at a conversion of 30%. This may be related to the presence of active dissolved Pd.
It may be established that the results of the hot filtration test were in accordance with those of the recycling experiments (Table 1). The atomic absorption spectroscopy data for palladium amount in MMNP after the general Heck reaction (Table 6, entry 1) shows that the palladium leached is about 2.4% that is negligible. As shown in Table 1, compare of the Pd loading on the fresh catalyst and reused catalyst after run 1 and also run 2, show that there is not considerable leaching. This result and also result of hot filtration confirms that Palladium in heterogeneous samples can dissolve and then redeposit on the surface of the support material.
We recommend that the MMNP catalyst is superior in terms of palladium consumption amount compared to some other catalysts that had been used in the Heck reaction. A comparative study of the MMNP catalytic activity with other reported heterogeneous palladium base in Heck reaction is reported in Table 7. The results show that our work in terms of reaction time, yield and the palladium consumption amount is superior to the other catalysts. Furthermore, other advantages of our method are lack of usage of an inert atmosphere and moderate reaction temperature, with respect to the others. However, it should be emphasized that the results reported in Table 7 were obtained under very different reaction conditions and therefore the results could not be compared correctly.

Conclusion
Finally, we have introduced a new magnetically recyclable catalyst with magnetite core and dendrimeric palladium complex on the MCM-41 (Fe 3 O 4 @MCM-41-NE 2 -EDA-Pd (MMNP), Si:Fe 3 O 4 = 40:1, loading mass ratio of ligand:PdCl 2 = 100:10). The catalyst is efficient in the Heck reaction at 130°C in DMF as solvent. The catalyst leaching was studied for various loading of ligand:PdCl 2 and results showed less leaching for 100:10 ratio and also mild decrease in activity for this sample. High products yields, simple workup and easy recovery of the catalyst are some advantages of this catalyst.

Material and methods
All chemicals were commercial and used as revived. Progress of the reactions were determined by thin layer chromatography and all yields refer to isolated products. NMR spectra for 1 H and 13 C were recorded on a Bruker DRX-400 AVANCE (400 MHz for 1 H and 100 MHz for 13 C) spectrometer in CDCl 3 . The melting point of solid products were determined by Buchi melting point model B-540. Infrared spectra of all products and the catalyst were recorded on a Bruker FT-IR Equinox 55 spectrophotometer in KBr disks. Thermogravimetric analysis (TGA) was performed using a thermal gravimetric analysis  (Tamami et al., 2013) a Isolated yield.
General procedure for the Heck reaction A 5 mL round bottom flask was charged with MMNP (10 mg), an appropriate amount of DMF (1 mL), aryl halide (1.0 mmol), olefin (2.5 mmol) and TEA (303 mg, 3.0 mmol) and a magnetic stir bar. The mixture was stirred for an appropriate time under reflux condition. The reaction progress was monitored by TLC (n-hexane:acetone, 80:20 as eluent). After the reaction completion, the mixture was cooled to ambient temperature and the catalyst was detached from the reaction mixture with an external magnet. The resultant remaining mixture was diluted with H 2 O (9 mL) followed by extraction with chloroform (3 × 3 mL). The organic layer was dried over Na 2 SO 4 and the solvent was evaporated under reduced pressure to obtain crude product. The corresponding pure products were obtained by preparative thin layer chromatography on silica gel using n-hexane:ethylacetate (95:5) as eluent or by recrystallization (ethanol:water).