The prepared Cu@APS-TDU-PMO (1) was characterized by FTIR, TEM, SEM, XRD, BET, EDX and TGA techniques.
FTIR spectrum of the synthesized PMO and Cu@APS-TDU-PMO (1) are shown in Fig. 1. The absorption band at 3414 cm-1 is attributed to N-H stretching. Two sharp absorption bands at 2928 cm-1 and 2862 cm-1 are assigned to the asymmetric and symmetric stretching of aliphatic C-H bonds, respectively. The absorption bands at 1682 cm-1 and 1654 cm-1 correspond to C=O bond stretching of the urea. The band at 1544 cm-1 can be assigned to the stretching vibration of the C=C bond. Two absorption bands at 1192 cm-1 and 1092 cm-1 are related to the Si-O-Si bonds (Fig. 1a). Also, the absorption band of Cu is clearly observed at 700-800 cm-1 (Fig. 1b).
Thermal gravimetric analysis (TGA) curve (Fig. 2) shows that the weight loss slightly below 100 °C can be assigned to the elimination of adsorbed surface water. Weight loss between 200-300 °C is due to the degradation of small amounts of the unextracted surfactant (P123). Also, weight loss between 300-600 °C is attributed to the removal of the bridge from the Cu@APS-TDU-PMO (1) structure.
FESEM and TEM images show that the Cu@APS-TDU-PMO (1) is composed of a large number of interwoven rods with 40.54-59.13 nm in width. It can also be seen that the morphology of PMO was mostly preserved after deposition of Cu nanoparticles (Fig. 3). TEM images also demonstrate the honeycomb arrangement of mesopores and tubular mesochannels in the Cu@APS-TDU-PMO (1) structure, confirming the formation of a hexagonal mesoporous structure.
There is a peak at 2θ=1.35° in the low-angle XRD pattern, indicating the mesoporous structure of Cu@APS-TDU-PMO (1, Fig. 4a). Also, the wide-angle diffraction signal at 2θ of 20-30°, which is characteristic of mesoporous structures, is observed in the wide-angle XRD pattern of Cu@APS-TDU-PMO (1, Fig. 4b). The diffraction peaks at 2θ of 44.30°, 50.30°, and 75° can be assigned to the reflections of Cu (marked with ●).
EDX analysis confirms the presence of C, N, O, Si, and Cu elements in the Cu@APS-TDU-PMO (1) structure (Fig. 5).
The N2 adsorption-desorption isotherm for Cu@APS-TDU-PMO (1) represents type IV isotherm commonly observed for mesoporous silica structures (Fig. 6). The calculated BET surface area was approximately 276 m2 g-1 which was retained even after the deposition of Cu nanoparticles. The average pore size was about 5.74 nm (Table 1).
Table 1 Structural parameters of the Cu@APS-TDU-PMO (1) determined from nitrogen sorption experiments.
Sample
|
Pore diameter (nm)
|
Surface area (m2 g-1)
|
Vp (cm3 g-1)
|
Cu@APS-TDU-PMO (1)
|
5.74
|
276
|
0.17
|
Catalytic application of the Cu@APS-TDU-PMO (1) for the synthesis of 2-(1H-Tetrazol-5-yl) acrylonitrile derivatives
The catalytic performance of the prepared Cu@APS-TDU-PMO (1) was investigated for the synthesis of 2-(1H-Tetrazol-5-yl) acrylonitrile derivatives. To determine theoptimal reaction conditions for the three components of aromatic aldehyde (2), malononitrile (3), and sodium azide (4), the reaction was conducted using different solvents, temperatures, and catalyst loadings. The optimized reaction conditions are shown in Table 2. Initially, the reaction was performed under different conditions without catalyst. The obtained data showed that the reaction efficiency was negligible after 120 min (Entry 1-7). Then, the reaction was performed in EtOH, DMF, and solvent-free in the presence of 50 mg of catalyst, which solvent-free condition was more efficient (Entry 8-10). The results showed that the optimum amount of catalyst is 30 mg for the reaction, and a lower amount of catalysts leads to reduced efficiency of the reaction (Entry 11-13). Therefore, 30 mg of the catalyst in solvent-free conditions at 110 °C was selected as the optimal reaction condition for the synthesis of 2-(1H-Tetrazol-5-yl) acrylonitrile derivatives.
Table 2 Optimal condition for synthesis of 2-(1H-Tetrazol-5-yl) acrylonitrile derivativesa (5a-i).
 |
Entry
|
Catalyst
|
Catalyst loading (mg)
|
Solvent
|
Temperature (°C)
|
Time (h)
|
yield of (%) 5a
|
1
|
-
|
-
|
Solvent-Free
|
r.t.
|
120
|
Trace
|
2
|
-
|
-
|
H2O
|
r.t.
|
120
|
Trace
|
3
|
-
|
-
|
DMF
|
r.t.
|
120
|
30
|
4
|
-
|
-
|
EtOH
|
r.t.
|
120
|
20
|
5
|
-
|
-
|
EtOH
|
Reflux
|
120
|
20
|
6
|
-
|
-
|
DMF
|
Reflux
|
120
|
40
|
7
|
-
|
-
|
Solvent-Free
|
110
|
120
|
45
|
8
|
Cu@APS-TDU-PMO (1)
|
50
|
EtOH
|
Reflux
|
50
|
38
|
9
|
Cu@APS-TDU-PMO (1)
|
50
|
DMF
|
Reflux
|
50
|
65
|
10
|
Cu@APS-TDU-PMO (1)
|
50
|
Solvent-Free
|
110
|
50
|
90
|
11
|
Cu@APS-TDU-PMO (1)
|
30
|
Solvent-Free
|
110
|
50
|
97
|
12
|
Cu@APS-TDU-PMO (1)
|
20
|
Solvent-Free
|
110
|
50
|
70
|
13
|
Cu@APS-TDU-PMO (1)
|
10
|
Solvent-Free
|
110
|
50
|
55
|
aReaction conditions: aldehydes (2a, 1 mmol), malononitrile (3, 1 mmol), sodium azide (4, 1.2 mmol) and Cu@APS-TDU-PMO (1, 0.03 g) under different conditions.
Benzaldehyde with electron-withdrawing and electron-donating groups was used for the synthesis of 2-(1H- Tetrazol-5-yl) acrylonitrile derivatives, the results of which are summarized in Table 3.
Table 3 Scope of the 2-(1H-Tetrazol-5-yl) acrylonitrile derivatives catalysed using Cu@APS-TDU-PMO (1)a.
Entry
|
Substrate (2)
|
product
|
Time (min)
|
Yield (%)
|
1
|

|

|
50
|
97
|
2
|

|

|
52
|
93
|
3
|

|

|
55
|
92
|
4
|

|

|
60
|
89
|
5
|

|

|
60
|
91
|
6
|

|

|
56
|
93
|
7
|

|

|
53
|
92
|
8
|

|

|
56
|
89
|
9
|

|

|
52
|
92
|
aReaction conditions: aldehydes (2a, 1 mmol), malononitrile (3, 1 mmol), sodium azide (4, 1.2 mmol) and Cu@APS-TDU-PMO (1, 0.03 g) under different conditions.
The proposed mechanism for the synthesis of 2-(1H-Tetrazol-5-yl) acrylonitrile derivatives
The proposed mechanism for the synthesis of 2-(1H-Tetrazol-5-yl) acrylonitrile derivatives is shown in scheme 2. Initially, the carbonyl group of aromatic aldehyde and the nitrile group of malononitrile are activated by the Cu@APS-TDU-PMO (1) catalyst. The Knoevenagel condensation reaction between the aromatic aldehyde and the malononitrile results in intermediate (I) formation. Then, Cu from the catalyst activates N of the intermediate (I), leading to [3+ 2] cycloaddition reaction between intermediate (I) and sodium azide. Subsequently, the catalyst was separated from intermediate (II) using an aqueous solution of HCl (acidic conditions) to obtain intermediate (II). Finally, 2-(1H-tetrazol-5-yl) acrylonitrile derivative as a more desirable tautomer was created via the tautomerization of intermediate (II).
Comparison of the catalytic activity of Cu@APS-TDU-PMO (1) catalyst
Table 4 compares the catalytic activity of Cu@APS-TDU-PMO (1) with other catalysts reported in the literature for the synthesis of 2-(1H-Tetrazol-5-yl) acrylonitrile derivatives. Specific properties of Cu@APS-TDU-PMO (1) catalyst, such as high efficiency, low catalyst loading, short reaction time, elimination of corrosive or expensive reagents, and reusability, make it superior to other reported catalysts.
Table 4 compares the catalytic activity of Cu@APS-TDU-PMO (1) with other catalysts reported.
Entry
|
Catalyst
|
Catalyst loading
|
Tempereture(°C)
|
Time (min)
|
References
|
1
|
Nano-NiO
|
0.06 mmol
|
70
|
360
|
94
|
2
|
Fe3O4@APTMS-DFX
|
0.03 g
|
120
|
60
|
95
|
3
|
Cu-MCM-41
|
0.03 g
|
140
|
720
|
96
|
4
|
Mesoporous ZnS
|
1 mmol
|
120
|
36 h
|
97
|
5
|
Cu@APS-TDU-PMO (1)
|
0.03 g
|
110
|
30
|
This work
|
Recyclability of Cu@APS-TDU-PMO (1)
Performing chemical reactions using recyclable and reusable catalysts is a significant issue in terms of green chemistry and environmental protection. In this study, recyclability of the catalyst was also investigated. For this purpose, the catalyst was separated from the reaction mixture using filtration, then washed with EtOH, and dried at 60 °C. The recycled catalyst was used in four consecutive reactions under optimal conditions for the synthesis of 2-(1H-Tetrazol-5-yl) acrylonitrile derivatives. As shown in Fig. 7, the catalytic activity of Cu@APS-TDU- PMO (1) was slightly decreased from 97 % to 85 %.