Synthesis of Fe3O4@SiO2-PMA-Cu. Immobilization of catalysts on the surface of Fe3O4 nanoparticles, compared with nonmagnetic supports, increases the dispersion of effective sites of catalyst, as well as provides the sufficient magnetic properties for easy separation of catalyst from the reaction mixture and thus improves the activity of the surface modified catalyst. To protect the catalyst surface against oxidants and corrosive agents and also to prevent aggregation of its particles, the surface of Fe3O4 was coated with silica layer. In addition, the silica layer through its shell thickness, stabilizes the catalyst and controls its particle size and interparticle interactions, as well as improving its surface effects.
Supporting polyoxometalates onto solid materials and decorating them into suitable porous supports such as metal oxides and MNPs is one of the most effective methods for improving their performance, which is achieved by increasing their active centers and reusability of these heterogeneous materials 61. The heterogenization of phosphomolybdic acid on silica coated nanomagnetic materials enabled us to overcome the limitations involved in the separation and recycling of homogeneous PMA. On the other hand, heteropolyacids such as PMA (H3PMo12O40) have unique structures with a wide range of coordination positions comprising oxygen atoms, which are appropriate for anchoring the single atoms such as copper particles 62. Since there are several possible coordination sites on the surface of PMA, it was selected as the support to trap the single metal atoms of copper in this study. The use of atomic catalysts leads to saving the quantity and cost of precious metals, because they increase the efficiency and activity of the catalyst dramatically.
The nanoparticles of Fe3O4@SiO2-PMA-Cu were synthesized in a four-step procedure (Figure 2). First, Fe3O4 was prepared using solid-state reaction of FeCl2·4H2O, FeCl3·6H2O, NaOH, and NaCl in an agate mortar. The crude powder was calcined at 700 °C, and then Fe3O4 particles were acquired with high purity. Coating of silica layer on the surface of Fe3O4 nanoparticles was achieved by sonication of a Fe3O4 suspension in an alkaline NH3·H2O solution of tetraethyl orthosilicate (TEOS). Then, PMA was added to a suspension of Fe3O4@SiO2 in ethanol, while being dispersed by sonication. In order to synthesize Fe3O4@SiO2-PMA-Cu, the prepared particles of Fe3O4@SiO2-PMA were added to a solution of CuCl2⋅2H2O in water and then the KBH4 powder was gradually added, while the mixture was strongly stirred. Eventually, the dark brick-red sediment of Fe3O4@SiO2-PMA-Cu was separated magnetically and then washed with distilled water, and dried at room temperature under air atmosphere.
The different techniques such as FT-IR, X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDS), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), vibration sample magnetometer (VSM), and inductively coupled plasma optical emission spectrometry (ICP-OES) analyses were applied for characterization of new synthesized Fe3O4@SiO2-PMA-Cu nanocatalyst.
Catalyst characterization
Vibration sample magnetometer (VSM). To confirm magnetic property of the synthesized Fe3O4@SiO2-PMA-Cu, VSM analysis was carried out (Fig. 3). As it is revealed in Fig. 1, the saturation magnetization (Ms) of the magnetic catalyst was 2.95 emug− 1 and hysteresis phenomenon was not found. The magnetization curve quickly rises without showing any remanence or coercivity, and the sample displays a typical superparamagnetic behavior of soft magnetic materials at room temperature. The superparamagnetic property of these nanoparticles is a vital feature in their application, because it prevents accumulation and aggregation of particles and enables them to re-disperse in the absence of a magnetic field immediately. The saturation magnetization (Ms) amount of the Fe3O4@SiO2-PMA-Cu MNPs was appropriate and the separation of the catalyst nanoparticles was easily carried out by using an external magnet.
FT-IR spectrum. Figure 4 shows the FT-IR spectrum of Fe3O4@SiO2-PMA-Cu nanocatalyst. The absorption peaks at 3449 and 3358 cm− 1 are assigned to the stretching vibration of H2O molecules and indicates the OH groups on the surface of the magnetic nanoparticles and hydroxyl groups in PMA structure respectively. The band around 1627 cm− 1 corresponds to the bending mode of H2O molecules. The presence of SiO2 is confirmed by the stretching and bending vibrations of Si-O. The absorption bands at 803 and 1097 cm− 1 are related to Si–O–Si stretching vibrations, and the band observed at 478 cm− 1 belongs to the bending vibration of SiO2 [59]. The peak at 950 cm− 1 belongs to the Mo-O stretching vibrations, which confirms the existence of phosphomolybdic acid 63. The formation of Fe3O4 nanoparticles was confirmed by the absorption band at 561 cm− 1 which corresponds to the vibration of metal oxide bonds (Fe-O).
X-ray diffraction (XRD). The X-ray diffraction (XRD) pattern of the synthesized Fe3O4@SiO2-PMA-Cu magnetic nanocomposite is shown in Fig. 5. As can be seen, the particle structure of Fe3O4@SiO2-PMA-Cu is amorphous due to the coating of the ferrite surface with layers of silica, phosphomolybdic acid and copper nanoparticles, and the wide peaks confirm the small size of the particles at the nanoscale. In the XRD pattern of nanocomposite, all the peaks of Fe3O4, SiO2, PMA and Cu nanoparticles are detectable. The lines (220), (311), (400), (422), (511), (440), (620) and (533) related to 2Ө = 32.88, 35.64°, 44.11°, 53.43°, 57.60°, 64.01°, 71.95° and 75.01° respectively, are assigned to the diffraction of Fe3O4 crystals. These peaks are compatible with the standard data (JCPDS: 00-43-0317) 64. The significant diffraction peaks are observed in case of PMA at 2Ө = 8.95°, 18.49°, 26.43°, 27.65° and 28.84° corresponding to (011), (202), (141), (311) and (312) crystallographic planes respectively (JCPDS: 00-043-0317) 65. The broad peak at 2Ө = 22.90° is related to the amorphous SiO2 shell on the surface of Fe3O4 59. The copper diffraction peaks were compared with the standard sample (JCPDS 04-0836), and peaks appearing at 2Ө = 43.71°, 50.70°, and 74.32° corresponding to the (111), (200), and (220) planes respectively, revealed the excellent agreement of the synthesized sample with its standard sample 31.
TEM, FESEM and EDS of Fe3O4 @SiO2 -PMA-Cu.TEM and FESEM techniques were used to determine the morphology and size distribution of the nanocatalysts. TEM images of the Fe3O4@SiO2-PMA-Cu nanocomposite are shown in Fig. 6. TEM images show that black and spherical Fe3O4 nanoparticles were synthesized at the nanoscale and coated with a dark gray silica layer, and the silica layer is entirely coated with phosphomolybdic acid. The PMA layer is visible in light gray. The TEM images also display that very small spherical Cu nanoparticles have been successfully deposited on the PMA layer and completely surround the outer surface of the catalyst.
Figure 7 shows FESEM images of Fe3O4@SiO2-PMA-Cu that approve the formation of nancomposite, and small amounts of agglomerates were observed in the Fe3O4@SiO2-PMA-Cu surface due to the modification of the catalyst surface with non-magnetic layers and decreased magnetic properties. The information obtained from the FESEM images is consistent with the XRD and TEM data.
The chemical composition and percentage of nanocomposite elements were acknowledged using EDS data (Fig. 8). In this spectrum, Cu, Mg, Fe, and O signals are detectable. The weight percentage of the elements indicates that the expected nanocomposite has been successfully synthesized. In addition, the percentage of copper nanoparticles in the structure of the composite is remarkable. The exact concentration of Fe, Mo, and Cu was also determined by ICP-OES and the resulting amounts were 31.16, 9.2 and 25.84 wt% respectively. These values are consistent with EDS data.
Synthesis of β-thiol-1,2,3-triazoles in the presence of Fe3O4@SiO2 -PMA-Cu nanocatalyst. The reaction of styrene episulfide, sodium azide and phenyl acetylene was chosen as model reaction and the synthesis of 2-phenyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)ethane-1-thiol was optimized under different conditions. The various empirical factors such as temperature, catalyst quantity, solvent, reaction time and the amount of reactants were examined, and the acquired results were provided in Table 1. The desired result in terms of product yield, time and reaction conditions was achieved by use of styrene episulfide (1 mmol), sodium azide (1.2 mmol), phenylacetylene (1 mmol) and Fe3O4@SiO2-PMA-Cu (0.1 g) as catalyst in water at 55 ºC (Table 1, entry 4). According to the results of experiments, the presence of catalyst was essential to accomplish the reaction and no reaction was performed in the absence of Fe3O4@SiO2-PMA-Cu even after 10 h (entry 1). The catalyst amount was optimized using different quantities of Fe3O4@SiO2-PMA-Cu nanocomposite (0.05, 0.08, 0.1 and 0.2 g), and 0.1 g of catalyst gave the eligible outcome. The product yield and reaction time were strongly influenced by the concentration of catalyst, so that by increasing the amount of catalyst from 0.05 to 0.1 g, the product yield and reaction rate increased dramatically (entries 2–4). The higher amount of catalyst had no effect on the product yield (entry 5).
The effect of various polar and non-polar solvents on reaction was examined. The polar solvents such as H2O, CH3CN, EtOH, MeOH, EtOAc and DMF were efficient and useful whereas non-polar solvents were not appropriate for this purpose (entries 6–13). Water as a green and eco-friendly solvent was the most privileged choice because the yield of the product in water was higher than all other solvents (entry 4).
In order to investigate the effect of temperature, the reaction was tested at different temperatures. The reaction result was not desirable at room temperature (25 ºC) and the product yield was low after 9 h (entry 14). As a result of raising the temperature to 45°C, the experimental data improved and the product yield increased by 72%, and the reaction time was reduced to 4 hours (entry 15). Further raising the temperature to 55 ° C significantly improved the product yield as well as reduced the reaction time (entry 4).
In order to study the catalytic activity of nanocomposite components in the reaction of styrene episulfide, sodium azide and phenylacetylene to give 2-phenyl-2-(4-phenyl-1H-1,2,3-triazol-1-yl)ethane-1-thiol, the reaction was evaluated separately using PMA, Cu and Fe3O4 under the optimal conditions. The results of experiments indicated that although the presence of Fe3O4 and PMA leads to increase the efficiency and catalytic activity of the nanocomposite, but the fundamental catalytic role is played by copper nanoparticles (entries 16, 17 and 18).
Table 1. Reaction of styrene episulfide with phenylacetylene and sodium azide catalysed by Fe3O4@SiO2-PMA-Cu under different conditionsa
Entry
|
Fe3O4@SiO2-PMA-Cu (g)
|
Solvent
|
Time (h)
|
Temperature (°C)
|
Yield (%)b
|
1
|
-
|
H2O
|
10
|
55
|
0
|
2
|
0.05
|
H2O
|
6
|
55
|
45
|
3
|
0.08
|
H2O
|
6
|
55
|
75
|
4
|
0.1
|
H2O
|
1.5
|
55
|
98
|
5
|
0.2
|
H2O
|
1.5
|
55
|
98
|
6
|
0.1
|
CH3CN
|
4
|
82
|
60
|
7
|
0.1
|
EtOH
|
4
|
78
|
65
|
8
|
0.1
|
MeOH
|
4
|
65
|
65
|
9
|
0.1
|
EtOAc
|
4
|
77
|
70
|
10
|
0.1
|
DMF
|
4
|
100
|
60
|
11
|
0.1
|
THF
|
15
|
60
|
0
|
12
|
0.1
|
n-Hexane
|
15
|
68
|
0
|
13
|
0.1
|
CCl4
|
15
|
77
|
0
|
14
|
0.1
|
H2O
|
9
|
25
|
40
|
15
|
0.1
|
H2O
|
4
|
45
|
72
|
16c
|
0.1
|
H2O
|
6
|
55
|
Trace
|
17d
|
0.1
|
H2O
|
6
|
55
|
Trace
|
18e
|
0.1
|
H2O
|
2
|
55
|
91
|
aAll reactions were performed using styrene episulfide (1 mmol), phenylacetylene (1mmol) and sodium azide (1.2 mmol). bIsolated yields. cCatalysed by Fe3O4 nanoparticles. dCatalysed by PMA. eCatalysed by Cu particles |
To evaluate the generalizability of the proposed synthetic method, the synthesis of 1,2,3-triazoles was examined using different thiiranes with electron donating and withdrawing substituents and cyclic thiiranes in the presence of phenyl acetylene, sodium azide and Fe3O4@SiO2-PMA-Cu nanocatalyst (Table 2, entries 1–8). Moreover, the reactivity of aliphatic terminal alkynes as well as 4-methoxyphenyl acetylene with styrene episulfide was investigated in this reaction and the results were satisfactory (entries 9–11). Different triazole derivatives were synthesized from the corresponding thiiranes in high yields without the formation of any by-products.
Recycling of Fe3O4@SiO2-PMA-Cu. The catalyst recovery was evaluated under the optimized reaction conditions. The magnetic nanoparticles were first collected with a magnet, then thoroughly washed several times with ethyl acetate and distilled water. After washing, they were dried under air atmosphere and reused several times in successive cycles without losing their activity or magnetic property (Fig. 9). The VSM, XRD, FESEM and TEM techniques were used to confirm the structure of the recycled catalyst (Fig. 10). The data obtained from the recovered catalyst and the freshly prepared sample were compared and the results revealed that the catalyst morphology remained constant after several reuses. The ICP-OES analysis of the supernatant liquid after separating the catalyst was applied to determine the leaching extent of Fe, Mo, and Cu during the reaction, and according to the results, no traces of Fe, Mo and Cu metals were observed in the supernatant liquid.
Hot filtration test. In order to confirm the heterogeneous nature of catalyst, a hot filtration test was carried out for reaction of styrene episulfide under the optimized conditions. For this purpose, the catalyst was filtered after 30 min at 100°C and the filtrate was again transferred back into the reaction vessel and reaction was continued for further 3 h. However, no reaction was performed under these conditions and no triazole product was obtained, indicating the absence of copper particles in the reaction vessel.
Comparison of Fe3O4@SiO2-PMA-Cu catalytic activity with other catalysts. The synthesis of 1,2,3-triazole from thiiranes has not been reported so far, except in one recent case 31. The advantages of the presented synthetic method were manifested by comparison of click reaction of styrene episulfide, phenyl acetylene and sodium azide with the other reported procedure in the literature. In viewpoints of temperature, reaction time, recoverability and product yield the present procedure is more preferable. The reaction is performed in the presence of Fe3O4@SiO2-PMA-Cu in a shorter time and the product is obtained with higher yield. In addition, the need for a lower temperature to complete the reaction also indicates the higher efficiency of the new nanocatalyst.
The possible mechanism for the synthesis of β-thiol-1,4-disubstituted-1,2,3-triazoles in the presence of Fe3O4@SiO2-PMA-Cu catalyst. The proposed mechanism for synthesis of β-thiol-1,2,3-triazole is consist of two possible pathways (A and B) 31,66. In both paths, Fe3O4@SiO2-PMA-Cu plays the role of catalyst (Fig. 11). First, the catalyst facilitates the ring opening of thiirane and then accelerates 1,3-dipolar cycloaddition reaction and formation of triazoles. Pathway A shows that initially, a non-covalent interaction between metal and azide is created, followed by activation of thiirane ring with Fe3O4@SiO2-PMA-Cu catalyst. Then, azide is transferred from the catalyst to thiirane and 2-azido-2-arylethanthiol is generated through the ring opening. In this step, the thiirane rings bearing aryl substituents prefer to be opened from the more hindered position because the benzyl carbocation resulting from SN1 type of mechanism (α-cleavage) is more stable; however, the regioselective ring opening of thiiranes with alkyl and allyl groups is carried out from the less hindered carbon via SN2 type of mechanism (β-cleavage). In the pathway A, in order to confirm the catalytic role of Fe3O4@SiO2-PMA-Cu in the preparation of 2-azido-2-arylethanthiol from styrene episulfide and sodium azide, the reaction was performed in the absence of catalyst, and only a very small amount of ring opened product was produced. During the reaction, gas chromatography (GC) and thin layer chromatography (TLC) runs of the reaction mixture were utilized to monitor the consumption of styrene episulfide and sodium azide and the formation of 2-azido-2-phenylethanthiol intermediate. FT-IR spectrum was used for characterization of 2-azido-2-arylethanthiol through stretching frequency of 2097 cm− 1 corresponding to the azide (Fig. 12). In pathway B, first, the intermediate (I) is generated by insertion of copper nanoparticles of catalyst into the C-H bonds of phenylacetylene. The intermediate (II) is then obtained from the in situ reaction of 2-azido-2-phenylethanthiol produced in pathway A and intermediate (I). Next, The Cu-C-triazole (IV) is formed from the 1,3-dipolar cycloaddition between azide and carbon-carbon triple bond of intermediate (II). The formation of acetylide intermediate (I) was confirmed by mixing phenylacetylene and Fe3O4@SiO2-PMA-Cu in a separate vessel and evaluating the pH of the water as solvent. A 0.7 Unit decrease in pH after 15 min showed that the coordination of phenylacetylene to copper led to the formation of acetylide intermediate (I) and release the terminal proton to the water. The consumption of phenylacetylene as well as the vanishing of the 2-azido-2-arylethanthiol intermediate, were controlled through the GC and TLC of the reaction mixture. Finally, the Cu-C bond of intermediate (IV) was protonolyzed by aqueous media to afford the β-thiol-1,2,3-triazole (V).