Preparation and Characterization of 3-Amino- ¹H-1,2,4-Triazole Grafted on the Surface of Silica Nanoparticles Support (SNP_s-AT ) for the Synthesis of Pyrano[2,3-c]Pyrazole Derivatives as the Novel, Effective, and Reclaimable Catalyst

The main objective of the current research, 3-amino-¹H‐1,2,4‐triazole immobilized on 3-chloropropyl joined SiO₂ nanoparticles as a recyclable, environment-friendly, and novel heterogeneous base catalyst. It was characterized by fourier transform infrared, X-ray diffraction patterns, field emission scanning electron microscopes, and energy-dispersive X-ray spectroscopy. This new material as a green and efficient catalyst was used for the synthesis of pyrano [2,3-c]pyrazoles derivatives through the reaction of aromatic aldehydes, malononitrile, and 3-methyl-1-phenyl-2-pyrazoline-5-one. The reactions did in solvent-free conditions at 60°C and the related pyran derivatives prepared in excellent yields. Also, this catalyst could be used several times without decreasing the activity.

About 1.5% of the Gross National Product (GNP) in the world is related to catalyst technology and plays a significant role in economic advancement and the promotion of the chemical industry [1]. On the other hand, in today's modern world, the inclination to use tinier and more efficient systems in the field of nanoscience and technology by researchers causes them to synthesize smaller materials with improved properties. The application of nanomaterials as heterogeneous catalysts has attracted a lot of consideration due to their economic, environmental, and structural specifications [2], and as the forerunner of green chemistry plays an important role in achieving selectivity, excellent activity, desired flexibility, chemical and thermal stabilization [3]. In recent research, attempts to the synthesis of supported nanocatalysts on various substrates such as charcoal, alumina, magnetic nanoparticles, silica, and polymers, as green materials have obtained great attention [4]. SiO₂ nanomaterials have unique and adjustable physiochemical properties with a high surface area to volume ratio, excellent chemical, thermal, and mechanical stability. Hence, They have high potential chemical reactivity [5]. Former articles have authenticated that substituted pyrano[2,3-c]pyrazoles are an appealing category of heterocyclic compounds due to biological and medicinal activities [6]. In specific, their advantages have resulted in fungicidal, anticancer, antibacterial, antitumor, antiplatelet, and antioxidant of these compounds [7]. So far, there have been many reports for the synthesis of the pyrano[2,3-c]pyrazole. The common method is condensation reaction of aryl aldehydes, malononitrile, and 3-methyl-1-phenyl-2-pyrazoline-5-one in the presence of various catalysts including HDBAC, HTMAB, MDOs, D and L-Proline, [Dsim]AlCl4, silicotungstic acid, isonicotinic acid, gamma-Alumina. Each of the above methods has its own merits, while most of these techniques suffer from disadvantages such as long reaction times, low product yields, use of excess amounts of catalyst, application of hazardous solvents, and toxic effluents [8,9].

In continuation of research on developing supported catalysts done by Karami group, several green catalysts were reported such as STA [4], SSC [10] Fe₃O₄@TiO₂@(CH₂)₃OWO₃H [6], Fe₃O₄@SiO₂@(CH₂)₃OMoO₃H [11], MCM-41-HWO₄ [12], phthalhydrazide-MCM-41 [13]. Herein, we have reported a new heterogeneous nanocatalyst using Azoles because other researcher groups have used them in the synthesis of catalysts, too [14,15]. Accordingly, we grafted 3-amino-¹H-1,2,4-triazole (3-AT) on 3-chloropropyl, which is coated on silica nanoparticles, and used in the multi-component reactions in organic chemistry. The SiO₂ NPs were initially synthesized through the sol-gel process. Then, SiO₂@(CH₂)₃Cl was formed via the reaction of SiO₂ NPs with 3-chloropropyl tri methoxy silane according to the previous report [13]. Finally, as expected, in the reaction of SiO₂@(CH₂)₃Cl with 3-amino-¹H-1,2,4-triazole (3-AT), the chloride group was replaced by (3-AT), to obtain nano-catalyst SiO₂@(CH₂)₃-(3-AT) (SNP_s-AT) (Scheme 1). Chemical analysis of SiO₂@(CH₂)₃(3-AT) bases catalyst was determined using fourier transform infrared (FT-IR), X-ray diffraction patterns (XRD), field emission scanning electron microscopes (FE-SEM), and energy-dispersive X-ray spectroscopy (EDS).

To prove the produced catalyst, FT-IR spectra of the synthesized particles (SiO₂, SiO₂@(CH₂)₃Cl and SNP_s-AT) are from wavelength 400 to 4000 cm-1, shown in figure 1. FT-IR spectra of SiO₂ (Figure 1a) illustrate the absorption bands at the region 809, 1103 cm^-1 assigned to Si–O–Si stretching symmetric and Si–O–Si stretching asymmetric, respectively. The bending vibrations Si–O–Si appear at 462 cm^-1 [16]. FT-IR spectra of the SiO₂@ (CH₂)₃Cl structure (Figure 1b) in comparison with that of SiO₂ (Figure 1a) confirm that the CH₂ of the alkyl chains is in the 2854 cm^-1 for symmetric CH₂ and 2923 cm^-1 for asymmetric stretching peaks [17]. Compare with FT-IR spectra of figure 1b, SiO₂ @ (CH₂)₃ (3-AT), shown in figure 1c, has been formed. Moreover, two peaks that appear at 1442 cm^-1 and 1645 cm-1are related to N = N and C = N stretching vibration bands of the (3-AT) material immobilized on the surface catalyst [15,18].

Figure 1: The FT-IR spectrum of SiO₂ (a), SiO₂@(CH₂)₃Cl (b), SNP_s-AT (c).

Scheme 1: The FT-IR spectrum of SiO₂ (a), SiO₂@(CH₂)₃Cl (b), SNP_s-AT (c).

EDS is an analytical technique used for the elemental or chemical characterization of a sample. According to figure 2, it shows the presence of elements Si,O,C, and N in the SNP_s-AT that agrees with our predictions. Also, the successful bonding of the 3-AT groups is fully confirmed by the presence of nitrogen.

Figure 2: EDX spectrum a of SNP_s-AT.

XRD is one of the most significant, non-destructive instruments used to study the structure, composition, and quality control. The presence of amorphous SiO₂ structure with a wide scattering centered in the range of (20° < 2θ < 32°) was observed in figure 3a. The XRD pattern in figure 3b displays the nanoparticles SNP_s-AT and in comparison with figure 3a, the existance of nitrogen in the structure confirmed the formation of SNP_s-AT [19].

Figure 3: XRD patterns of SiO₂ (a), SNP_s-AT (b).

FE-SEM images show the morphology and size of the novel catalyst SNP_s-AT. The result proves that the nanoparticles are spherical with average diameters of 37.96-64.91 nm (Figure 4). All this evidence demonstrates that the novel nanocatalyst SNP_s-AT was produced successfully. Continuing this research, we have tested the catalytic activity through one-pot, the multicomponent reaction for the synthesis of pyrano[2,3-c] pyrazoles derivatives (4a-r). In this work, the one-pot reaction was performed between a mixture of 3-methyl-1-phenyl-2-pyrazoline -5-one (1) (1mmol, 0.174 g), malononitrile (2) (1.25 mmol, 0.08 g) and aromatic aldehydes (3a-r) (1 mmol, 0.14 g) and SiO₂@(CH₂)₃ (3-AT) as catalyst under different conditions (Scheme 2). As a replacement of toxic organic solvents, solvent-free reactions were shown to be an impressive technique in chemist organic. frequently, solvent-free conditions lead to a significant decrease in reaction times, increased production, and regioselectivity, and stereoselectivity of reactions [20,21].

Figure 4: Fe-SEM image of SNP_s-AT.

Scheme 2: Novel SNP_s-AT as catalysts for the synthesis of pyrano [2,3-C] pyrazoles.

At first, the optimized conditions for the model reaction should be obtained. The model reaction in the absence of catalyst checked out but the favorable product was not created even after a long time reaction. The model reaction was performed by various amounts of SNP_s-AT at the range of 25-70ºC. Results are shown in table 1. According to table 1, the products were produced efficiently utilizing 0.003g of the catalyst (Table 1). Also, maximized product yield obtained at 60ºC (Table 1). To investigate the solvent effect, the model reaction was performed using 0.003g nanocatalyst SNP_s-AT in solvent-free conditions and several polar and nonpolar solvents including water, ethanol, carbon tetrachloride, tetrahydrofuran, and acetonitrile conditions (Table 1). Using the solvents couldn’t increase the yields of the product, while under solvent-free conditions, product yield was in the range of 85-95 percentage (Table 1). In this way, the optimized conditions are shown in entry 11 of table 1, for pyrano[2,3-c] pyrazoles derivatives which have been synthesized by SNP_s-AT catalyst in solvent-free condition at 60ºC. Under these conditions, the modal reaction was studied in an extensive range of aromatic aldehydes containing both electron-donating and electron-withdrawing groups completely. Acceptable efficiency of the products was obtained by both aromatic aldehydes groups in the short reaction times showed in table 2. According to (Scheme 3), a possible mechanism for the synthesis of product 4 is brought in the following steps. the first step includes the Knoevenagel condensation between aryl aldehyde (3a-o) and malononitrile (2) using a base catalyst. Then, Michael's addition occurred between intermediate (6) and 2-benzylidene malononitrile (5). Finally, cyclization and tautomerization of intermediates (7) and (8) give the expected product (4) (Scheme 3,4).

Scheme 3:

Scheme 4: Novel SNP_s-AT as catalysts for the synthesis of pyrano [2,3-C] Pyrazoles.

Table 1: Optimization of the model reaction for the synthesis of pyranopyrazoles 4a-r a.
Entry	Catalyst (g)	Solvent	Time (min)	Temp (ºC)	Yield (%) b
1	No Catalyst	No solvent	240	25	No product
2	0.005	EtOH	120	Reflux	85
3	0.005	H2O	120	Reflux	70
4	0.005	THF	120	Reflux	55
5	0.005	CCl4	120	Reflux	48
6	0.005	CH3CN	120	Reflux	40
7	0.005	Solvent-free	60	25	40
8	0.005	Solvent-free	60	60	95
9	0.005	Solvent-free	60	70	95
10	0.004	Solvent-free	30	60	93
11	0.003	Solvent-free	30	60	90
12	0.003	Solvent-free	30	70	93
13	0.002	Solvent-free	60	60	75
abenzaldehyde 1 mmol, 3-methyl-1-phenyl-2-pyrazoline-5-one 1 mmol, malononitrile 1.25 mmol bIsolated yield.

Table 2: Synthesis of pyrano [2,3-c] pyrazoles derivatives 4a-r using SNPs-AT a.
Entry	Product	Time (min)	Yield (%)b	Mp. (ºC) [lit]
4a		15	97	172-173 (171-173) [22]
4b		25	86	175-177 (176-177) [22]
4c		20	88	190-192 (190-192) [22]
4d		20	90	171-173 (172-174) [23]
4e		25	90	174-175 (174-175) [23]
4f		30	92	186-188 (185-186) [23]
4g		40	80	322-325 (320-322) [6]
4h		50	90	316-318 (314-315) [6]
4i		35	88	169-170 (169-170) [23]
4j		30	80	169-171 (169-171) [23]
4k		25	89	160-162 (160-161) [23]
4l		35	80	144-146 (144-146) [23]
4m		30	80	159-161 (159-160) [24]
4n		30	92	159-160 (159-160) [25]
4o		25	85	158-159 (157-159) [26]
4p		30	82	166-168 (166-167) [26]
4q		30	89	183-185 (183-184) [23]
4r		20	94	193-195 (194-196) [23]
aReaction conditions: 3-methyl-1-phenyl-2-pyrazolin-5-one (1 mmol), arylaldehyde (1 mmol), malononitrile (1.25 mmol), and SNPs-AT (0.003 g), solvent-free, 60°C. bIsolated yields.

Materials and methods

Tetraethyl Orthosilicate (TEOS), 3-Chloropropyl Tri Methoxy silane (CPTMO), 3-Amino-¹H-1,2,4-Triazole (3-AT), and another chemical material in this research were buying from Fluka and Merck chemical companies in high-level purity. The reaction progress was controlled by utilizing Thin Layer Chromatography (TLC) (silica-gel SIL G/UV254 plates, n-Hexane/ EtOAc: 2/1). Melting points were determined by a KRUSS model measuring device. ¹H NMR spectra were recorded with a Bruker 400 Ultrashield and ¹³C NMR spectra were recorded at 100 MHz, with DMSO-d6 as the solvent and TMS as the internal standard. Fourier Transform Infrared (FT-IR) spectra were taken with a JASCO FT-IR/680 plus spectrometer, using KBr pills. Energy Dispersive Spectroscopy (EDS) was recorded exploitation TESCAN Vega model instrument. Energy Dispersive X-Ray (XRD) spectroscopy was performed using a Bruker AXS (D₈ Advance) model instrument, with Cu-Ka radiation (λ = 0.15418 nm). The measurement in 2θ ranging from 10º to 80º at the rate of 0.05º min-1 was done. Field Emission Scanning Electron Microscopy determination for the particle size and morphology of the samples through a MIRA3TESCAN-XMU FE-SEM instrument under an acceleration voltage of 26 kV was observed.

A general manner for preparation of SiO₂ nanoparticle: The SiO₂ nanoparticles applied in this process are prepared using the sol-gel method. These nanoparticles were made by mixing TEOS (6.2 ml) with ethanol (100 ml) as the solvent and ammonium hydroxide (6.5 ml) as a catalyst to create alkaline conditions under magnetic stirring for 15 h at room temperature. The mixture was filtered by Centrifuge (4000 rpm, 30 min), washed three times with ethanol solution and water, and then white powder dried at 60°C for 12 h in the vacuum [27].

Method for the synthesis of 3-chloropropyl-SiO₂ NPs: (1g) SiO₂ nanoparticles were dispersed in dried toluene (60 mL) by ultrasonic for 15 min. Then, in the reflux method, 3-chloropropyl tri methoxy silane (2 ml) was added dropwise into a round bottom flask under moderate stirring and argon atmosphere for 24 h. Finally, the mixture was cooled to room temperature, separated by Centrifugation (4000 rpm, 10 min), and washed with water and ethanol several times. The 3-chloropropyl-SiO₂ NPs were dried at 80°C for 6 h in the vacuum [13].

Synthesis of 3-amino ¹H‐1,2,4‐triazole (3-AT) -3-chloropropyl- SiO₂ NPs: 3-Amino-¹H‐1,2,4‐Triazole (3-AT) (0.42 g, 5 mmol) in the presence of K2CO3 (0.69 g, 5 mmol) was added into a suspension of SiO₂@(CH₂)₃Cl powder (1.0 g) and dry toluene (30 mL) under stirring and refluxed at 110°C for 12 h. After completion of the reaction, the mixture was filtered and the obtained powder washed several times with ethanol and water, then the solid powder was dried at 100°C in the Vacuum for 24 h.

The general procedure for the synthesis of pyranopyrazole derivatives using SiO₂@ (CH₂)₃- (3-AT)(SNP_s-AT) as nanocatalyst: Aryl aldehydes (1 mmol), 3-methyl-1-phenyl-2-pyrazoline-5-one (1mmol), and malononitrile (1.25 mmol) in the presence (0.003 g) of SNP_s-AT catalyst were combined under solvent-free conditions and heated at 60°C in an oil bath. The reaction completion was followed using TLC (n‐hexane: ethyl acetate, 2:1). At the end of the reaction, the mixture was cooled to room temperature, then hot ethyl acetate (5 mL) was added, and the catalyst was collected by Centrifuge instrument. Extra purification and recrystallization were obtained by adding hot ethanol. Several selected spectral data of compounds (4c-4h) are given below.

6-amino-3-methyl-4-(3-nitrophenyl)-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4c): Mp. 190–192°C, White crystals. IR spectrum, ῡ, cm–1:3454, 3359 (NH2), 2923 (CH arom.), 2854 (CH, CH3), 2190 (CN), 1653, 1594 (C=N), 1446, 1351, 1267, 1184, 1069 (C-O, CN).

6-amino-4-(4-chlorophenyl)-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4d): Mp. 171–173°C, White crystals. IR spectrum, ῡ, cm–1:3454, 3330 (NH2), 3068 (CH arom.), 2883 (CH, CH3), 2195 (CN), 1655, 1594 (C=N), 1455, 1396, 1285, 1177, 1069 (C-O, CN), 692 (C-Cl).

6-amino-4-(4-methoxyphenyl)-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4e):  Mp. 174–175°C, White crystals. IR spectrum, ῡ, cm–1: 3392, 3321 (NH2), 3023 (CH arom.), 2883 (CH, CH3), 2197 (CN), 1658, 1590 (C=N), 1455, 1392, 1250, 1173, 1128 (C-O, CN).

6-amino-4-(2,4-dichlorophenyl)-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4f): Mp. 186-188°C, White crystals. IR spectrum, ῡ, cm–1: 3457, 3324 (NH2), 3023 (CH arom.), 2883 (CH, CH3), 2191 (CN), 1660, 1595 (C=N), 1469, 1394, 1267, 1167, 1126 (C-O, CN), 756 (C-Cl).

6-amino-4-(2,4-dihydroxyphenyl)-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (4g): Mp. 320–322°C, White crystals. IR spectrum, ῡ, cm–1:3424, 3309, 3183 (OH, NH2), 2923 (CH arom.), 2854 (CH, CH3), 2211 (CN), 1590, 1511 (C=N), 1400, 1351, 1255, 1189, 1045 (C-O, CN), 842, 800. ¹H NMR spectrum (DMSO-d₆), δ, ppm: 9.68 s (¹H, OH), 7.92 s (¹H, OH), 7.76 s (2H), 7.52–7.75 m (2H), 7.25–7.27 m (2H), 6.89 s (2H, NH2), 6.45 d (J = 4 Hz, 2H), 6.32 s (¹H), 5.53 s (¹H, CH), 1.82 s (3H, ring CH3). ¹³C NMR spectrum, δ, ppm: 166.6, 162.2, 161.9, 159.7, 155.8, 153.6, 138.2, 126.3, 123.1, 117.1, 114.3, 108.8, 107.1, 103.8, 101.3, 75.7, 42.1, 26.1. Anal. Calcd. for C₂₀H₁₆N₄O₃: C, 66.66;H, 4.48;N, 15.55. Found: C, 66.58; H, 4.53; N, 15.60.

6-amino-4-(5-bromo-2-hydroxyphenyl)-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazole-5 carbonitrile (4h): Mp. 314-315°C, Yellow crystals, IR spectrum, ῡ, cm–1: 3455, 3347, 3212 (OH, NH2), 2935 (CH arom.), 2210 (CN), 1646, 1608, 1554 (C=N), 1477, 1388, 1280 (CN), 1222, 1087 (C-O), 802, 470 (C-Br). ¹H NMR spectrum (DMSO-d6), δ, ppm: 8.71 s (¹H, OH), 7.71 d (J=4 Hz, 2H), 7.52 t (J=4 Hz, 2H), 7.36 s (¹H), 7.27-7.30 m (2H), 6.83–6.93 m (2H, NH2), 6.61 d (J= 4 Hz,¹H), 5.46 s (¹H, CH), 2.04 s (3H, CH3). ¹³C NMR spectrum, δ, ppm: 162.9, 162.3, 158.2, 158.1, 151.6, 140.9, 137.2, 127.6, 121.3, 118.3, 118.0, 118.0, 117.2, 116.7, 97.6, 75.2, 47.5, 23.8. Anal. Calcd. for C₂₀H₁₅BrN₄O₂: C, 56.75; H, 3.57; N, 13.24. Found: C, 56.70; H, 3.51; N, 13.29.

Figure 5: Reuse of the catalyst in the synthesis of pyrano [2,3-c] pyrazole derivatives.

In this study, SNP_s-AT as a novel, eco-friendly and efficient nanocatalyst was synthesized. The characterization and catalytic activity were done by FT-IR, EDS, XRD, Fe-SEM analysis. SNP_s-AT has used for the synthesis of pyrano[2,3-c] pyrazoles derivatives. This procedure has unique advantages including excellent yields, reusability of the catalyst, short reaction times, simple protocol, that cause the present methodology will be effective.

We gratitude to thank the research council of the Yasouj University of Iran for the financial support of this project.

Conflict of Interests

The authors declare no conflict of interests.

1. Wilson K, Clark JH. Solid acids and their use as environmentally friendly catalysts in organic synthesis. Pure Appl Chem. 2000;72(7):1313-1319. doi: 10.1351/pac200072071313.
2. Amarloo F, Zhiani R, Mehrzad J. Novel Fe₃O₄/SiO₂/PPA magnetic nanoparticles: Preparation, characterization, and first catalytic application to the solvent- Free Synthesis of Tetrahydrobenzo [a] xanthene-11-ones. Russ J Org Chem. 2019;55:1584-1590. doi: 10.1134/S1070428019100191.
3. Karami B, Kiani M, Hosseini SJ, Bahrami, M. Synthesis and characterization of novel nanosilica molybdic acid and its first catalytic application in the synthesis of new and known pyranocoumarins. New J Chem. 2015;39(11):8576-8581. doi: 10.1039/C5NJ01302J.
4. Karami B, Ghashghaee V, Khodabakhshi S. Novel Silica Tungstic Acid (STA): Preparation, characterization and its first catalytic application in synthesis of new benzimidazoles. Catal Commun. 2012;20:71-75. doi: 10.1016/j.catcom.2012.01.012.
5. Biricik H, Sarier N. Comparative study of the characteristics of nano silica - , silica fume - and fly ash - incorporated cement mortars. Mater Res. 2014;17:570-582.  doi: 10.1590/S1516-14392014005000054.
6. Gholtash JE, Farahi M. Tungstic acid-functionalized Fe₃O₄@TiO₂: preparation, characterization and its application for the synthesis of pyrano [2,3-c]pyrazole derivatives as a reusable magnetic nanocatalyst. RSC Adv. 2018;8:40962-40967. doi: 10.1039/C8RA06886K.
7. Nasr MN, Gineinah MM. Arch Pharm Pharm Med Chem. 2002;335:289-295.
8. Mishra M, Nizam A, Jomon KJ, Tadaparthi K. A new facile ultrasound-assisted magnetic nano-[CoFe2O4]-catalyzed one-pot synthesis of pyrano [2,3-c]pyrazoles. Russ J Org Chem. 2019;55:1925-1928. doi: 10.1134/S1070428019120194.
9. Moosavi-Zare AR, Zolfigol MA, Mousavi-Tashar A. Synthesis of pyranopyrazole derivatives by in situ generation of trityl carbocation under mild and neutral media. Res Chem Intermed. 2016;42:7305-7312. doi: 10.1007/s11164-016-2537-4.
10. Eskandari K, Karami B, Khodabakhshi S. Novel Silica Sodium Carbonate (SSC): Preparation, characterization and its first catalytic application to the synthesis of new dihydropyrano [2,3-c] pyrazoles. Catal Commun. 2014;54:124-130. doi: 10.1016/j.catcom.2014.05.029.
11. Khosravian F, Karami B,  Farahi M. Synthesis and characterization of molybdic acid immobilized on modified magnetic nanoparticles as a new and recyclable catalyst for the synthesis of chromeno[4,3-b]chromenes. New J Chem. 2017;41:11584-11590. doi: 10.1039/C7NJ02390A.
12. Karami B, Farahi M, Akrami S, Elhamifar D. Tungstic acid-functionalized MCM-41 as a novel mesoporous solid acid catalyst for the one-pot synthesis of new pyrrolo[2,1-a]isoquinolines. New J Chem. 2018;42:12811-12816. doi: 10.1039/C8NJ02699H.
13. Akrami S, Karami B, Farahi M. Preparation and characterization of novel phthalhydrazide-functionalized MCM-41 and its application in the one-pot synthesis of coumarin-fused triazolopyrimidines. RSC adv. 2017;7:34315-34320. doi: 10.1039/C7RA06240K.
14. Garkoti C, Shabir J, Mozumdar S. An imidazolium based ionic liquid supported on Fe₃O₄@SiO₂ nanoparticles as an efficient heterogeneous catalyst for N-formylation of amines. New J Chem. 2017;41:9291-9298. doi: 10.1039/C6NJ03985E.
15. Nasrollahzadeh M, Sajjadi M, Khonakdar HA. Synthesis and characterization of novel Cu(II) complex coated Fe₃O₄@SiO₂ nanoparticles for catalytic performance. J Mol Struct. 2018;1161:453-463. doi: 10.1016/j.molstruc.2018.02.026.
16. Azarshin S, Moghadasi J, AAboosadi Z. Surface functionalization of silica nanoparticles to improve the performance of water flooding in oil wet reservoirs. Energy Explor Exploit.   2017;35:685-697. doi: 10.1177%2F0144598717716281.
17. Nasseri MA, Zakerinasab B, Samieadel MM. Sulfamic acid supported on Fe₃O₄@SiO₂ superpara magnetic nanoparticles as a recyclable heterogeneous catalyst for the synthesis of quinolones. RSC Adv. 2014;4:41753-41762. doi: 10.1039/C4RA06699E.
18. Wang J, Zheng S, Shao Y, Liu J, Xu Z, Zhu D. Amino-functionalized Fe(3)O(4)@SiO(2) core-shell magnetic nanomaterial as a novel adsorbent for aqueous heavy metals removal. J Colloid Interface Sci. 2010 Sep 1;349(1):293-9. doi: 10.1016/j.jcis.2010.05.010. Epub 2010 May 7. PMID: 20542278.
19. Márquez F, Herrera GM, Campo T, Cotto M, Ducongé J, Sanz JM, Elizalde E, Perales Ó, Morant C. Preparation of hollow magnetite microspheres and their applications as drugs carriers. Nanoscale Res Lett. 2012;7:210. doi: 10.1186/1556-276X-7-210.
20. Khazaei A, Zolfigol MA, Moosavi-Zare AR, Zare A, Khojasteh M, Asgari Z, Khakyzadeh V, Khalafi-Nezhad A. Organocatalyst trityl chloride efficiently promoted the solvent-free synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]-xanthen-11-ones by in situ formation of carbocationic system in neutral media. Catal Commun. 2012;20:54-57. doi: 10.1016/j.catcom.2012.01.001.
21. Imanzadeh GH, Zare A, Khalafi-Nezhad A, Hasaninejad A, Zare AM, Parhami A. Microwave-assisted Michael addition of sulfonamides to α,β-unsaturated esters: A rapid entry to protected β-amino acid synthesis. J Iran Chem Soc. 2007;4:467-475. doi: 10.1007/BF03247234.
22. Jin TS, Zhao RQ, Li TS. An one-pot three-component process for the synthesis of 6-amino-4-aryl-5-cyano-3-methyl-1-phenyl-1,4-dihydropyrano[2,3-c]pyrazoles in aqueous media (06-1864DP). Arkivoc. 2006;11:176-182. doi: 10.3998/ark.5550190.0007.b18.
23. Reddy GM, Raul Garcia J. Synthesis of pyranopyrazoles under eco-friendly approach by using acid catalysis. J Heterocycl Chem. 2017;54:89-94. doi: 10.1002/jhet.2544.
24. Niknam K, Borazjani N, Rashidian R, Jamali A. Silica-bonded N-propylpiperazine sodium n-propionate as recyclable catalyst for synthesis of 4H-pyran derivatives. Chin J Catal. 2013;34:2245-2254. doi: 10.1016/S1872-2067(12)60693-7.
25. Farahi M, Karami B, Sedighimehr I, Tanuraghaj HM. Chinese Chem Lett. 2014;25:1580-1582. doi: 10.1016/j.cclet.2014.07.0126.
26. Aliabadi RS, Mahmoodi NO. Green and efficient synthesis of pyranopyrazoles using [bmim][OH−] as an ionic liquid catalyst in water under microwave irradiation and investigation of their antioxidant activity. RSC adv. 2016;6:85877-58884. doi: 10.1039/C6RA17594E.
27. Zhang YP, Lee SH, Reddy KR, Gopalan AI, Lee KP. Synthesis and characterization of core-shell SiO₂ nanoparticles/poly(3-aminophenylboronic acid) composites. J Appl Polym Sci. 2007;104:2743-2750. doi: 10.1002/app.25938.