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Original article
12 2022
:15;
104311
doi:
10.1016/j.arabjc.2022.104311

Immobilized sulfonic acid functionalized ionic liquid on magnetic cellulose as a novel catalyst for the synthesis of triazolo[4,3-a]pyrimidines

,
 Department of Chemistry, Yasouj University, P. O. Box 353, Yasouj 75918-74831, Iran
Received: , Accepted: ,
© The Author(s) 2022
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The Brønsted acidic ionic liquid 1-(propyl-3-sulfonate) vinyl imidazolium hydrogen sulfate [IL] was supported on modified magnetic cellulose. The physical structure, composition, and functional groups of the novel supported ionic liquid catalyst were characterized via XRD, FT-IR, EDS, SEM, VSM, TGA, TEM, and BET techniques. Owing to the combination of nano-support features and flexible imidazolium linkers, it acted as a “quasi-homogeneous” catalyst to catalyze the preparation of triazolo[4,3-a]pyrimidine derivatives by a one-pot three-component reaction of active methylene compounds (ethyl cyanoacetate or malononitrile), aminotriazole and aryl aldehydes. The catalyst shows good catalytic activity for the synthesis of triazolo pyrimidines after six times of recycling.

Keywords

Cellulose
Fe3O4
Immobilized catalyst
Ionic liquid
Triazolo[4,3-a]pyrimidine
1

1 Introduction

From significant important aspects of green chemistry are the expansion and usage of green solvents. Nowadays, ILs is introduced as green solvents in alternative to conventional volatile organic solvents. ILs, liquid salts, were formed wholly of cations and anions and are mainly defined as being liquid beneath an ideal amount, such as 100 °C or at room temperature (Morais et al., 2022; Minea and Sohel Murshed, 2021; Greer et al., 2020). Ionic liquids (ILs) as eco-friendly reaction media have attracted increasing attention due to their particular properties, such as tunable acidity, selective dissolvability, very low viscosity, negligible vapor pressure, wide liquid range, high thermal stability, and easiness of product separation (Han et al., 2022; Singh and Savoy, 2020; Tajbakhsh et al., 2013; Miao et al., 2011a, 2011b). The strongly acidic ionic liquids (Brønsted-type and Lewis-type) have been exploited as efficient catalysts for many reactions and generally can afford reusability, higher yields, and selectivity against traditional acid catalysts (Azizi and Shirdel, 2016; Cole et al., 2002; Tayebee et al., 2015). Among them, SO3H-functionalized ILs with a hydrogen sulfate counteranion have been intensively studied as a class of dual acidic functionalized ILs during the last years since the existence of both SO3H-functional groups and the hydrogen sulfate counteranion can increase their acidities. Ionic liquids based on imidazolium cation, because of their high stability, are also widely used in organic reactions in laboratories and industries (Tayebee et al., 2017; Elavarasan et al., 2020; Miao et al., 2011a, 2011b). However, immobilizing ionic liquids on various solid supports is one of the efficient ways to overcome these problems. An exciting combining feature of ionic liquid with those of supporting material will develop novel performances when the synergistic effects appear (Li et al., 2014a, 2014b). Therefore, stabilized ionic liquid phase catalyst is an emerging concept in heterogeneous catalysis. Mainly, polymer-supported ionic liquids facilitate easy catalyst recovery, high selectivity, recyclability, and less contamination of products with a catalyst (Patil et al., 2022; Pei et al., 2022). The heterogeneous catalysts, due to easy segregation from the products, recyclability and reusability, and much stability, received remarkable attention (Saeedi and Rahmati, 2022). The heterogenization of ILs on appropriate porous carriers (Oliveira et al., 2019; Sun et al., 2017; Wang et al., 2020), suitable magnetic nanoparticles (Veisi et al., 2021; Zolfigol et al., 2016; Sheykhan et al., 2011; Zhang et al., 2012a, 2012b), immobilized on solid supports by either a physical coating of ionic liquids on Al2O3 (Li et al., 2017; Khoshnevis et al., 2013), SiO2 (Jamshidi et al., 2018; Testa et al., 2010; Niknam et al., 2011) and TiO2 (Atghia and Sarvi-Beigbaghlou, 2013; Tabrizian et al., 2016) or covalent attachment of ionic liquids to the support surface, would be a viable and appealing approach to fabricate an efficient solid catalyst with superior activity and stability (Safari and Ahmadzadeh, 2017). In this sense, cellulose, one of the most critical natural organic polymers, can be used as nanocomposite, support materials, and emulsions because of various advantages such as biocompatible, renewability, flexibility, dimensional stability, and ability to modify its surface chemistry and hydrophilicity (Kamel and Khattab, 2021; Hamdy et al., 2017; Sabaqian et al., 2017; Abdelhameed et al., 2020). The hydroxyl groups are reactive on the surface of cellulose, which helps to modify their surface with organic and inorganic groups chemically and increase cellulose-based substances (Marghaki et al., 2022; Hasan et al., 2021; Niu et al., 2021).

In recent years, green chemistry has been one of the essential aspects of chemists' experimental and industrial efforts (Ruijter and Orru, 2013; Wu et al., 2014; Cioc et al., 2014). Due to their atom efficiency and significant diversity, multi-component reactions (MCRs) have occupied an essential part of the green chemistry world (Jarrahi et al., 2021; Zhi et al., 2019; Sunderhaus and Martin, 2009; Biggs-Houck et al., 2010; Zareai et al., 2012). The multi-component reaction is a powerful synthetic strategy in modern chemistry in which three or more simple components as starting reagents are involved in a one-pot system to achieve new complex molecules at a less processing procedure in comparison to the step-by-step approach with usually few side-products (Váradi et al., 2016; Liu et al., 2019; Abdollahi-Basir et al., 2019; Jalili et al., 2021). Multi-component reactions have been widely used to synthesize critical heterocyclic compounds, which have many biological activities (Slobbe et al., 2012; Maleki et al., 2015; Wei et al., 2021). The pyrimidine family is one of the essential nitrogen-containing heterocycles due to their presence in many natural and biologically active products (Amin et al., 2009; Rashad et al., 2010; Singh et al., 2019). It is well known that the condensation of triazole and pyrimidine gives rise to the formation of bicyclic heterocycles known as triazolo pyrimidines, which exhibit a wide range of biological properties. Triazolo pyrimidines can be applied in various synthetic pharmacophores (Kolos et al., 2011; Gladkov et al., 2012; El-Gendy et al., 2008). Furthermore, they are valuable building blocks in the structure of many herbicidal drugs, such as penoxsulam, diclosulam, flumetsulam, azafenidin, and floransulan. Triazolopyrimidines are synthetic analogs of purines and nucleosides (Navarro et al., 1998; Magan et al., 2004; Magan et al., 2005). [1,2,4]Triazolo[1,5-a]pyrimidines, a subtype of purine bioisosteric analogs, were also reported to possess potential anti-tumor activities, especially those bearing functional groups at C-5, C-6, or C-7 positions (Traxler et al., 1996; Rusinov et al., 1986). Several synthetic strategies have been reported for the preparation of triazolo pyrimidine derivatives, most of which are based on the modification of the classical Biginelli reaction (Bhatt et al., 2016). Although some of these procedures are efficient, a number of them have limiting factors, including long reaction time, side reactions, rigid workup, high-temperature conditions, and non-recyclable reagents (Lauria et al., 2002).

In continuation of our efforts to synthesize a new heterogeneous nanocatalyst, in this study (keshavarz et al., 2022), we have reported the preparation and characterization of Fe3O4/MPC-[IL] as a supported Bronsted acidic ionic liquid catalyst. Moreover, the catalytic activity and reusability of the ionic liquid catalyst were examined for the synthesis of triazolo[4,3-a]pyrimidine derivatives.

2

2 Experimental

2.1

2.1 Methods and materials

All chemicals, as well as all solvents, were purchased from Merck, Aldrich-Sigma, and Fluka chemical companies and were used without further purification. Fourier-transform infrared spectrum was carried out in the region 400–4000 cm−1 by an FT-IR JASCO 6300D instrument. NMR spectra were taken with a Bruker 400 MHz Ultrashield spectrometer at 400 MHz (1H) and 100 MHz (13C) using DMSO‑d6 as solvent. Melting points were determined in open capillaries using an electrothermal KSB1N-apparatus (Kruss, Germany). X-ray diffraction analysis was studied using a Rigaku Ultima IV, Japan diffractometer operated. Scanning electron microscopy was carried out by SEM: KYKY-EM3200 instrument operated at 26 kV. EDS was determined using the TESCAN vega model instrument. Thermo gravimetric (TGA) analyses were regulated using a Perkin Elmer STA 6000 instrument. The vibration sample-magnetometry (VSM) was monitored by the Kavir Magnet VSM. The surface area was investigated using the Brunauer-Emmett-Teller (BET) technique.

2.2

2.2 Synthesis of Fe3O4 NPs

FeCl3·6H2O (2.3 g, 8.7 mmol) and FeCl2·4H2O (0.86 g, 4.3 mmol) were mixed in distilled water (30 ml) under stirring and N2 atmosphere at 80 °C for 30 min. Afterward, NaOH solution (10 ml, 25 %) was added dropwise to the mixture until the brown color solution turned out to the black and then was stirred for one h. The resulting product was collected using an external magnet and was washed with distilled water, and dried in an oven (Farahi et al., 2017).

2.3

2.3 Synthesis of 3-mercaptopropylcellulose (MPC)

The mixture of cellulose (1 g) and 3-mercaptopropyltrimethoxysilane (3 ml) in anhydrous toluene (40 ml) was stirred for 15 min at room temperature and then refluxed for 24 h. After this period, the mixture was filtered and repeatedly washed with anhydrous toluene and dried to obtain 3-mercaptopropylcellulose (MPC) (Carvalho et al., 2021).

2.4

2.4 Preparation of Fe3O4/MPC

In a round-bottomed flask (100 ml), the prepared Fe3O4 (1 g) was well dispersed in an aqueous solution (40 ml) containing NaOH (7 wt%) and urea (12 wt%) (ultrasonication for 10 min). Then 3-mercaptopropylcellulose (MPC) (1 g) was added to the flask, and the mixture was stirred for 1.5 h at −12 °C. After freezing for one h, the 3-mercaptopropylcellulose was dissolved fully. Ultimately, Fe3O4/MPC was collected with an external magnet, washed with deionized water, and dried in a vacuum oven for 24 h.

2.5

2.5 Preparation of acidic Bronsted ionic liquid [IL]

First, 1,4-butanesultone (12.2 g) was added to 1-vinyl imidazole (9.4 g) slowly at 0 °C. Then, the mixture was stirred at room temperature for about 24 h until it turned solid. The obtained solid was washed with diethyl ether and dried in a vacuum at 50 °C. Second, the prepared solid salt (2 g) was dissolved in H2O (5 ml) in a 100 ml round bottom flask, and equal molar sulfuric acid was slowly dropped into the flask at 0 °C. Then the mixture was heated up to 60 °C gradually and then stirred for 12 h. Finally, the formed liquid was washed with diethyl ether and dried in a vacuum at 50 °C for six h.

2.6

2.6 Preparation of Fe3O4/MPC-[IL] (1)

For the synthesis of supported ionic liquid, the mixture of Fe3O4/MPC (1 g), [IL] (5 mmol), anhydrous toluene (100 ml), and azodiisobutyronitrile (AIBN) (5 mol %) were refluxed under N2 atmosphere for 30 h. Finally, the product was washed with diethyl ether and dried under a vacuum.

2.7

2.7 General procedure for the synthesis of 5

A mixture of aldehyde (1 mmol), ethyl cyanoacetate or malononitrile (1 mmol), 3-amine-1H-1,2,4-triazole (1 mmol), and catalyst 1 (0.003 g) were stirred under solvent-free conditions at 100 °C. After completion of the process, monitored by TLC, hot ethanol (10 ml) was added, and the Fe3O4/MPC-[IL] was separated using an external magnet. Then, the pure product was obtained by recrystallization from EtOH. The recycled catalyst was washed with distilled water (10 ml) and ethanol (10 ml) and then dried at 100 °C. Finally, it was reused in subsequent runs.

2.8

2.8 Spectral data

Ethyl 5-amino-7-phenyl-7,8-dihydro-[1,2,4]triazolo[4,3-a]pyrimidine-6-carboxylate (5a). FT-IR (KBr) ( υ ¯ max, cm−1): 3396, 3375, 3326, 2746, 1687, 1565, 1490, 1110. 1H NMR (400 MHz, DMSO‑d6): δ = 9.25 (s, 1H), 8.08 (d, 1H, J = 8 Hz), 7.66 (s, 2H), 7.57–7.64 (m, 5H) ppm, 5.37 (s, 1H), 3.93 (q, 2H, J = 8 Hz), 0.84 (t, 3H, J = 8 Hz). 13C NMR (100 MHz, DMSO‑d6): δ = 162.28, 155.60, 133.90, 131.84, 131.28, 129.82, 116.09, 103.10, 24.96, 14.46.

Ethyl 5-amino-7-(4-chlorophenyl)-7,8-dihydro-[1,2,4]triazolo[4,3-a]pyrimidine-6-carboxylate (5b). FT-IR (KBr) ( υ ¯ max, cm−1): 3412, 3253, 3115, 2872, 1692, 1532, 1485, 1203, 1H NMR (400 MHz, DMSO‑d6): δ = 9.25 (s, 1H), 8.08 (d, 1H, J = 8 Hz), 7.65 (s, 2H), 7.45–7.58 (m, 5H) ppm, 5.41 (s, 1H), 4.38 (q, 2H, J = 7 Hz), 0.92 (t, 3H, J = 7 Hz).13C NMR (100 MHz, DMSO‑d6): δ = 163.83, 160.63, 154.82, 134.82, 130.96, 129.87, 129.04, 110.16, 61.09, 57.01, 14.02 ppm.

Ethyl 5-amino-7-(4-bromophenyl)-7,8-dihydro-[1,2,4]triazolo[4,3-a]pyrimidine-6-carboxylate (5c). FT-IR (KBr) ( υ ¯ max, cm−1): 3427, 3389, 3098, 2923, 1677, 1587, 1488, 1178. 1H NMR (400 MHz, DMSO‑d6): δ = 9.16 (s, 1H), 8.39 (s, 2H), 7.41–7.43 (m, 5H) ppm, 7.08 (d, 1H, J = 3.6 Hz), 5.21 (s, 1H), 3.98 (q, 2H, J = 6.4 Hz), 1.33 (t, 3H, J = 6.6 Hz). 13C NMR (100 MHz, DMSO‑d6): δ = 169.6, 156, 146, 141.99, 132.5, 129, 126.8, 62.1, 40.6, 27.5 ppm.

Ethyl 5-amino-7-(4-nitrophenyl)-7,8-dihydro-[1,2,4]triazolo[4,3-a]pyrimidine-6-carboxylate (5d). FT-IR (KBr) ( υ ¯ max, cm−1): 3480, 3430, 3198, 2917, 1680, 1604, 1504, 1054. 1H NMR (400 MHz, DMSO‑d6) δ = 9.09 (s, 1H), 8.10 (d, 1H, J = 8 Hz), 7.83–7.91 (m, 4H), 5.99 (s, 1H), 4.43 (q, 2H, J = 7.2 Hz), 1.03 (t, 1H, J = 3.2 Hz) ppm. 13C NMR (100 MHz, DMSO‑d6) δ = 163.62, 154.95, 149.60, 144.21, 134.01, 131.06, 130.42, 124.86, 124.46, 114.11, 63.57, 59.84, 14.00 ppm.

Ethyl 5-Amino-7-(3-nitrophenyl)-7,8-dihydro-[1,2,4]triazolo[4,3-a]pyrimidine-6-carboxylate (5e). FT-IR (KBr) ( υ ¯ υ ¯ max, cm−1): 3444, 3328, 3097, 2888, 1697, 1623, 1531, 1272. 1H NMR (400 MHz, DMSO‑d6): δ = 8.09 (d, 1H, J = 8 Hz), 7.80–7.83 (q, 4H), 6.41 (s, 1H), 5.69 (s, 1H), 4.03 (q, 2H, J = 8 Hz), 0.92 (t, 3H, J = 8 Hz) ppm. 13C NMR (100 MHz, DMSO‑d6): δ = 163.63, 154.55, 150.36, 147.93, 134.84, 131.17, 130.83, 125.30, 123.78, 123.13, 113.63, 63.19, 59.06, 13.86 ppm.

Ethyl 5-amino-7-(2,4dichlorophenyl)-7,8-dihydro-[1,2,4]triazolo[4,3-a]pyrimidine-6-carboxylate (5f). FT-IR (KBr) ( υ ¯ max, cm−1): 3489, 3421, 3085, 2878, 1699, 1586, 1474, 1106. 1H NMR (400 MHz, DMSO‑d6): δ = 11.67 (s, 1H), 8.11 (d, 1H, J = 8 Hz), 7.87 (s, 2H), 7.26–7.77 (t, 3H) ppm, 4.16 (s, 1H), 2.48 (q, 2H, J = 8 Hz), 1.25 (t, 3H, J = 7 Hz). 13C NMR (100 MHz, DMSO‑d6): δ = 160.6, 153.6, 137.6, 130.2, 120.8, 127, 67.1, 40.4, 21.6 ppm.

Ethyl 5-amino-7-(4-methylphenyl)-7,8-dihydro-[1,2,4]-triazolo[4,3-a]pyrimidine-6-carbonitrile (5 g). FT-IR (KBr) ( υ ¯ max, cm−1): 3347, 3262, 3185, 3118, 2921, 2192, 1660, 1633, 1531, 1482, 1363, 1286, 1214, 1157. 1H NMR (400 MHz, DMSO‑d6) δ = 8.75 (d, 1H, J = 1.6 Hz) ppm, 7.71 (s, 1H), 7.21 (s, 2H), 7.18 (s, 4H), 5.29 (d, 1H, J = 2.4 Hz), 2.28 (s, 3H). 13C NMR (100 MHz, DMSO‑d6) δ = 153.92, 151.83, 146.93, 140.24, 137.26, 129.18, 126.00, 119.06, 56.06, 53.70, 20.64 ppm.

Ethyl 5-amino-7-(4-isopropylphenyl)-7,8-dihydro-[1,2,4]-triazolo[4,3-a]pyrimidine-6-carbonitrile (5 h). FT-IR (KBr) ( υ ¯ max, cm−1): 3378, 3295, 3181, 3118, 2964, 2186, 1656, 1627, 1523, 1479, 1367, 1284, 1211, 1151. 1H NMR (400 MHz, DMSO‑d6) δ = 8.80 (d, 1H, J = 1.6 Hz) ppm, 7.76 (s, 1H), 7.24 (s, 4H), 7.31 (s, 2H), 5.33 (d, 1H, J = 2 Hz), 2.92 (s, 1H), 1.23 (d, 6H, J = 6.8 Hz). 13C NMR (100 MHz, DMSO‑d6) δ = 153.91, 151.83, 148.21, 146.95, 140.71, 126.59, 125.99, 119.11, 55.97, 53.68, 33.11, 23.81 ppm.

3

3 Results and discussion

In this work, we report the preparation of Fe3O4/MPC-[IL] with high catalytic activity and its application in the synthesis of triazolo [4,3-a]-pyrimidines. The details of the preparation of magnetic nanoparticles supported by an acidic ionic liquid are presented in Scheme 1. Initially, the cellulose surface was functionalized with commercially available 3-mercaptopropyltrimethoxysilane (MPTMS) through siloxane linkages (MPC). Then, Fe3O4/MPC was fabricated through chemical modification of Fe3O4 nanoparticles with MPC in the presence of urea/NaOH. Afterward, the precursor ionic liquid was prepared using the reaction of 1-vinyl imidazole with 1,4-butane sultone, followed by treatment with sulfuric acid [IL]. Finally, Fe3O4/MPC-[IL] was synthesized by a radical grafting copolymerization using the reaction of [IL] with Fe3O4/MPC. The prepared Fe3O4/MPC-[IL] was characterized using XRD, FT-IR, SEM map, EDS, SEM, VSM, TGA, and BET techniques.

Preparation of Fe3O4/MPC-[IL] (1).
Scheme 1
Preparation of Fe3O4/MPC-[IL] (1).

Characterization by X-ray diffraction (XRD) was performed to investigate the structure of cellulose, Fe3O4, and Fe3O4/MPC-[IL] in a range of 2θ = 10-80° (Fig. 1). The analysis of the XRD pattern of cellulose (Fig. 1a) shows three characterization peaks at 2θ = 16.2°, 18° and 24° assigned to the (1 1 0), (1 1 1) and (2 0 0) planes of crystalline cellulose (Jokar et al., 2021; Zhang et al., 2019; Karami et al., 2018). In XRD pattern of Fe3O4, six characteristic peaks at 2θ = 30.26°, 35.7°, 43.5°, 53.59°, 57.5° and 63.26° were corresponding to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) crystal planes of a pure Fe3O4 with a spinal structure (Fig. 1b) (Dang et al., 2018; Ghanbari et al., 2018; Sadeghzadeh et al., 2014). XRD diffraction patterns were presented in Fig. 1c for the new catalyst Fe3O4/MPC-[IL]. As shown, on the path to the new catalyst synthesis, the diffraction pattern of cellulose was changed after functionalization with different layers and Fe3O4. In the pattern XRD, the Fe3O4/MPC-[IL] nanocatalyst show that severities of peaks declined; however, comparing Fig. 1c with Fig. 1a and 1b confirms the stabilization of the Fe3O4 nanoparticles with IL on cellulose and Fe3O4/MPC-[IL] nanocatalyst was successfully synthesized.

XRD patterns of a) cellulose, b) Fe3O4, and c) Fe3O4/MPC-[IL].
Fig. 1
XRD patterns of a) cellulose, b) Fe3O4, and c) Fe3O4/MPC-[IL].

The FT-IR analyses of cellulose, MPC, Fe3O4, Fe3O4/MPC, [IL], and Fe3O4/MPC-[IL] were performed to prove their synthesis at each step. In Fig. 2a, the broad peak at 3350 cm−1 is attributed to the stretching vibration of the hydroxy groups of cellulose. The band at 1462 cm−1 is related to the bending vibrations of H—C—H and O—C—H (Fig. 2a) (Liu et al., 2021; Heidari and Aliramezani, 2021). The peaks of symmetric and asymmetric vibrations of the Si-O bond were seen at 973 and 1140 cm−1. The typical peak of the thiol group (S—H) was observed at 2550 cm−1 (Fig. 2b) (Jankauskaite et al., 2020; Shang et al., 2018; Loof et al., 2016). The spectra of the blank Fe3O4 show that the peak at 580 cm−1 could be due to the Fe–O vibration (Fig. 2c) (Maleki et al., 2017; Zhang et al., 2019). The presence of the characteristic absorption peak of Fe-O in all compared spectra is a confirmation of how nanoparticles of Fe3O4 have remained during the process (Fig. 2d and Fig. 2f). The IR curve of IL shows typical bands at 1563 cm−1 can be assigned to the imidazolium ring (C⚌C, C⚌N). The peaks at around 3143 cm−1 correspond to the C—H stretching vibration of imidazole moiety (sp2) (Zhang et al., 2012a, 2012b; Zhang et al., 2018). The bands around 1039 cm−1 and 1165 cm−1 were associated with the signals of C—S and S⚌O bonds, indicating the existence of the -SO3H group (Fig. 2e) (Qiao et al., 2006). However, the peak of S—H at 2565 cm−1 and end-group C⚌C totally at 1647 cm−1 disappeared, while the peak of the imidazole ring at 1563 cm−1 remained after a radical chain transfer reaction between modified cellulose and ionic liquid occurred (Fig. 2f), which convinced us that the [(CH2)3SO3HVIm]HSO4 ionic liquid was successfully grafted on functionalized cellulose via the route shown in Scheme 1.

FT-IR spectra of a) cellulose, b) MPC, c) Fe3O4, d) Fe3O4/MPC, e) [IL] and f) Fe3O4/MPC-[IL].
Fig. 2
FT-IR spectra of a) cellulose, b) MPC, c) Fe3O4, d) Fe3O4/MPC, e) [IL] and f) Fe3O4/MPC-[IL].

The element mapping analysis of the Fe3O4/MPC-[IL] nanocatalyst was shown in Fig. 3. As can be seen, all elements (C, Si, Fe, O, S, and N) are uniformly distributed on the surface of the catalyst, which indicates the successful immobilization of expected elements on cellulose.

Elemental mapping analysis of Fe3O4/MPC-[IL] nanocatalyst.
Fig. 3
Elemental mapping analysis of Fe3O4/MPC-[IL] nanocatalyst.

The EDX analysis confirmed the presence of the desired elements in the Fe3O4/MPC-[IL] nanocatalyst. As shown, the peaks of C, Fe, Si, O, N and S are observed in the EDX spectrum, and these elements confirm that the magnetite NPs and ionic liquid are successfully immobilized onto functionalized cellulose surface and the desired catalyst has been synthesized (Fig. 4).

Energy dispersive X-ray spectroscopy (EDS) result for Fe3O4/MPC-[IL] nanocatalyst.
Fig. 4
Energy dispersive X-ray spectroscopy (EDS) result for Fe3O4/MPC-[IL] nanocatalyst.

The morphology and size of the Fe3O4 and Fe3O4/MPC-[IL] were investigated using scanning electron microscopy (SEM) (Fig. 5). These images confirm the formation of the desired nanoparticles with spherical morphology. The average nanoparticles diameter of Fe3O4/MPC-[IL] is around 56–73 nm.

FE-SEM images of a) Fe3O4/MPC-[IL] and b) Fe3O4.
Fig. 5
FE-SEM images of a) Fe3O4/MPC-[IL] and b) Fe3O4.

The transmission electronic microscopy (TEM) images of Fe3O4/MPC-[IL] nanocatalyst are shown in Fig. 6. These images indicate magnetite NPs black cores surrounded by a gray shell of modified cellulose. Furthermore, the TEM images showed that nanoparticles were composed of relatively small and almost spherical particles.

TEM images of the Fe3O4/MPC-[IL] nanocatalyst.
Fig. 6
TEM images of the Fe3O4/MPC-[IL] nanocatalyst.

The magnetic properties of Fe3O4 and Fe3O4/MPC-[IL] nanoparticles were investigated using the VSM technique. As shown in Fig. 7, VSM measurements for Fe3O4 nanoparticles show that the saturation magnetization is 53.03 emu/g, while the saturation magnetization of Fe3O4/MPC-[IL] is decreased to 15.2 emu/g. The free MNPs show a higher magnetic valence in comparison with functionalized Fe3O4, which this result is due to the coated cellulose and the ionic liquid that joined to support.

VSM analysis of Fe3O4 and Fe3O4/MPC-[IL].
Fig. 7
VSM analysis of Fe3O4 and Fe3O4/MPC-[IL].

In the next step, the thermal stability of the catalyst was studied using TGA (Fig. 8). These were performed from 30 to 900 °C, showing the TGA curve of Fe3O4/MPC-[IL], which this analysis confirms the stability and presence of fixed groups on nanostructures. As can be seen in the TGA curve of the prepared Fe3O4/MPC-[IL], it has an initial weight loss of about 3.5 % up to 180 °C, which was attributed to desorption of physically adsorbed solvents, surface hydroxyl groups, and structural water. The second and significant weight loss occurs between 180 and 550 °C, showing a weight loss of 30 %, which may be related to the decomposition of organic groups, amine groups, and removal of the sulfuric acid group on the surface of the nanocatalyst. The last weight loss between 550 and 900 °C, with an observed weight loss of 4 %, is related to the immobilized organic groups grafting to the cellulose surface and confirms the stability of the synthesized nanocatalyst.

TGA curve of Fe3O4/MPC-[IL] 1.
Fig. 8
TGA curve of Fe3O4/MPC-[IL] 1.

In the following, the surface area of the solid acid catalyst Fe3O4/MPC-[IL] was investigated using the N2 adsorption–desorption curve (Fig. 9). According to the IUPAC classification, the Fe3O4/MPC-[IL] nanocomposite reveals a type IV isotherm with an H1 hysteresis loop. Based on this analysis, the area of the surface 23.92 m2g−1, the total volume of the pores 5.49 cm3g−1 and the mean pores diameter 6.79 nm were obtained.

BET of Fe3O4/MPC-[IL] catalyst.
Fig. 9
BET of Fe3O4/MPC-[IL] catalyst.

After successful preparation and characterization, Fe3O4/MPC-[IL] was applied as an effective nanocatalyst in the synthesis of triazolo[4,3-a]-pyrimidines derivatives 5 (Scheme 2).

Synthesis of triazolo [4,3-a]-pyrimidines derivatives 5 in the presence of nanocatalyst 1.
Scheme 2
Synthesis of triazolo [4,3-a]-pyrimidines derivatives 5 in the presence of nanocatalyst 1.

To optimize the reaction conditions, the effect of catalyst, solvent and temperature was investigated in the reaction of benzaldehyde (1 mmol), ethyl cyanoacetate (1 mmol), and 3-amine-1H-1,2,4-triazole (1 mmol) as the model reaction. In the first step, the effect of catalyst loading in the reaction progress was studied under solvent-free conditions, and the desired product was not produced without a catalyst. The best result was conducted in the presence of 0.003 g of catalyst 1. Furthermore, the model reaction was performed by 0.003 g of Fe3O4/MPC-[IL] in some solvents such as EtOH, EtOH: H2O, MeOH, H2O, DMF, DMSO, and toluene. As can be seen, considerable acceleration is observed chiefly in reactions performed under solvent-free conditions. Next, the effect of the temperature was studied. We performed the reaction at different temperatures, and the reaction at 100 °C gave the best result. The results of this study are summarized in Table 1. According to these results, using Fe3O4/MPC-[IL] (0.003 g) as a catalyst under solvent-free conditions at 100 °C would be the best choice.

Table 1 Optimization of the reaction conditions.a
Entry Catalyst 1 (g) Solvent Temp. (°C) Yield (%)b
1 – – 25 –
2 – – 80 5
3 – – 90 8
4 – – 100 9
5 0.001 – 100 60
6 0.003 – 100 90
7 0.005 – 100 85
8 0.007 – 100 85
9 0.003 Ethanol reflux 25
10 0.003 EtOH:H2O reflux 34
11 0.003 Methanol reflux 38
12 0.003 H2O 100 43
13 0.003 DMF 100 20
14 0.003 DMSO 100 35
15 0.003 Toluene 100 23
16 0.003 – 80 75
17 0.003 – 110 90
18 0.003 – 120 85
a Reaction conditions: benzaldehyde (1 mmol), ethyl cyanoacetate (1 mmol), 3-amine-1H-1,2,4-triazole (1 mmol), time: 20 min. b Isolated yields.

To investigate the generality of this protocol, different aryl aldehydes containing both electron-donating and electron-withdrawing groups were employed in the reaction. The reaction proceeded smoothly to afford the desired products 5 in good to excellent yields. Furthermore, under similar conditions, aryl aldehydes and 3-amine-1H-1,2,4-triazole were reacted with malononitrile in the presence of catalyst 1. The obtained results are summarized in Table 2.

Table 2 Synthesis of compounds 5 in the presence of Fe3O4/MPC-[IL].a
Entry Aldehyde Product 5 Yield (%)b M. p. (°C) References
5a C6H4CHO
96		 189-191c –
5b 4-Cl C6H4CHO
92

 190-191c –
5c 4-Br C6H4CHO
93		 184-184c –
5d 4-NO2 C6H4CHO
92		 194-196c –
5e 3-NO2 C6H4CHO
95		 190-192c –
5f 2,4-Cl2 C6H3CHO
96		 243-245c –
5 g 4-CH3 C6H4CHO
92		 243–245 Ablajan et al., (2012)
5 h 4-i-Pr C6H4CHO
93		 218-220c –
5i 4-NO2 C6H4CHO
92		 245–247 Ablajan et al., (2012)
5j 2-Cl C6H4CHO
94		 263–266 Ablajan et al., (2012)
5 k 4-Br C6H4CHO
95		 264–266 Ablajan et al., (2012)
5 l 4-Cl C6H4CHO
97		 257–258 Ablajan et al., (2012)
a Reaction conditions: aryl aldehyde (1 mmol), ethyl cyanoacetate or malononitrile (1 mmol), 3-amine-1H-1,2,4-triazole (1 mmol), catalyst 1 (0.003 g), solvent-free, 100 °C. b Isolated yield. c Novel compound.

According to the reported mechanisms in the literature (Ablajan et al., 2012), we proposed a plausible mechanism for the synthesis of [1,2,4]-triazolo[4,3-a]-pyrimidines (5) (Scheme 3). Initially, aldehyde (2) is activated by the acidic surface sites of the catalyst. Next, the catalyst-activated aldehyde is attacked by 3-amine-1H-1,2,4-triazole (3), followed by the elimination of water to form intermediate I. Then, compound 4 performs a Michael-type addition to intermediate I to give intermediate II. The desired product 5 was obtained after the intramolecular cyclization and tautomerization intermediate III (Scheme 3).

Proposed mechanism for synthesizing 5 in the presence of Fe3O4/MPC-[IL] as a catalyst.
Scheme 3
Proposed mechanism for synthesizing 5 in the presence of Fe3O4/MPC-[IL] as a catalyst.

In the subsequent study, a leaching experiment was performed in the model reaction. When the reaction progress reached 50 %, hot EtOH (5 ml) was added, and the catalyst was magnetically separated and removed. After removing the solvent, the reaction of residue was screened under optimal conditions. Interestingly, no remarkable product was obtained in the reaction progress, indicating that the catalyst operates heterogeneously. The most important advantage of the applied catalyst is recoverability and reusability. It is important to note that the magnetic property of this catalyst facilitates its efficient recovery from the final products. Furthermore, we examined the recyclability of Fe3O4/MPC-[IL] in the model reaction. After completion of the reaction, EtOH (5 ml) was added to the mixture, and the catalyst was filtered and washed with distilled water three times by ethanol, followed by drying at 100 °C. Applying the recovered catalyst for six successive runs in the model reaction generated the product, having a low reduction in yield (Fig. 10). These experiments indicate the high stability and durability of this nanocatalyst under the applied conditions.

Reusability of the heterogeneous nanocatalyst 1 for the synthesis of 5a.
Fig. 10
Reusability of the heterogeneous nanocatalyst 1 for the synthesis of 5a.

FT-IR spectrums of the Fe3O4/MPC-[IL] and recycled catalyst are shown in Fig. 11. This spectrum confirms good stability of the structure of Fe3O4/MPC-[IL] after recycling. The structural properties of the Fe3O4/MPC-[IL] and recycled catalyst were analyzed by XRD. This spectrum indicates structural stability; as shown in Fig. 12, the position and relative intensities of all peaks confirm this well.

FT-IR spectrum of a) Fe3O4/MPC-[IL] and b) recycled Fe3O4/MPC-[IL].
Fig. 11
FT-IR spectrum of a) Fe3O4/MPC-[IL] and b) recycled Fe3O4/MPC-[IL].
The XRD pattern of a) Fe3O4/MPC[IL] and b) recycled Fe3O4/MPC[IL].
Fig. 12
The XRD pattern of a) Fe3O4/MPC[IL] and b) recycled Fe3O4/MPC[IL].

Next, we compared the efficiency of Fe3O4/MPC[IL] nanocatalyst with previous catalysts utilized recently in the synthesis of triazolo[4,3-a]pyrimidine derivatives (Table 3). As shown, Fe3O4/MPC[IL] is a catalyst with high durability, stability, recycling times, reaction time, and increased product yield better than the other catalysts.

Table 3 The comparison study between the efficiency of the Fe3O4/MPC[IL] heterogeneous catalyst with that of other catalysts in the synthesis of triazolo[4,3-a]pyrimidine.
Catalyst Conditions Time (min) Yield (%) Recovery times References
Bi2O3/FAp Cat. (30 mg), EtOH, r. t. 25–35 92–96 5 Kerru et al., 2020
TMDP Cat. (10.5 mg), Solvent-free, 50 °C 35–70 71–93 – Zaharani et al., 2020
TMDPS Cat. (1 ml), Neat, r. t. 105–150 73–94 – Zaharani et al., 2020
NaOH Cat. (20 mol %), Ultrasonic, 25–35 °C EtOH 60 35–88 – Ablajan et al., 2012
mPMF Cat. (20 mg), 600 rpm, Solvent-free, r. t. 90 76–94 5 Khaligh and Mihankhah, 2022
3-vinyl-1-(4-sulfobutyl) imidazolium Cat. (0.01 g), Solvent-free, 100 °C 20 – – This work
IL Cat. (1 ml), Solvent-free, 100 °C 20 – – This work
Fe3O4/MPC[IL] Cat.(0.003 g), Solvent-free, 100 °C 15–20 90–97 6 This work

4

4 Conclusions

Herein, given one of the bases of green chemistry, we introduced Fe3O4/MPC[IL] as a novel Fe3O4-cellulose-supported ionic liquid. Structural characterization was performed using XRD, FT-IR, EDX, SEM, VSM, TGA, and BET. Finally, this new nanocatalyst as an efficient heterogeneous catalyst with the ability to be recyclable and reusable, biocompatible, easy separation, wide substrate tolerance, high atom economy, mild reaction conditions, good to excellent yields, and short reaction times for the synthesis of triazolo[4,3-a]pyrimidine derivatives are the major important features of this new catalytic system. Also, the catalyst can be quickly recovered using a simple external magnet and reused several times without significantly losing its catalytic activity. The results of this study give us hope that it will be used in the future to synthesize other reactions and organic compounds.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2022.104311.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary Data 1

Supplementary Data 1


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