Nanomagnetic CoFe2O4@SiO2-EA-H3PO4 as a zwitterionic catalyst for the synthesis of bioactive pyrazolopyranopyrimidines and dihydropyrano[2,3-c]pyrazoles

This study presents the development of a phosphoric acid-based zwitterionic catalyst immobilized on CoFe2O4 nanoparticles [CoFe2O4@SiO2-EA-H3PO4]. The structure of the nanocatalyst CoFe2O4@SiO2-EA-H3PO4 was identified by applying several spectroscopic techniques, i.e. FT-IR, SEM, TEM, XRD, EDX, elemental Mapping, VSM, TGA, and BET techniques. The catalytic efficiency of CoFe2O4@SiO2-EA-H3PO4 was evaluated in the water-based multicomponent synthesis of pyrazolopyranopyrimidine and dihydropyrano[2,3-c]pyrazole derivatives. Subsequently, an exploration of the antibacterial properties of the compounds was conducted. The catalytic system offers several advantages, encompassing high efficiency, brief reaction duration, uncomplicated operation, and facile recycling of the catalyst.


Introduction
2][3] In this sense, MCRs are recognized for their ability to create complex molecules in a single-pot reaction with high yields, short reaction times, minimal waste, and low purication costs.This makes MCRs a powerful tool used to construct novel and structurally diverse compounds. 4,57][8] Pyrazolopyranopyrimidine and dihydropyrano [2,3-c]pyrazoles exhibit notable promise in the realms of organic synthesis and medicine, owing to their prospective role as fundamental building blocks for pharmaceutical compounds. 9As a result, in order to synthesize such compounds, numerous techniques have been developed. 6,10reen catalysis, a subset of green chemistry, emphasizes the pressing need to develop and utilize catalysts that are environmentally friendly. 12,13In this regard, an ideal catalyst should be cost-effective, highly active, efficient, selective, stable, easily recoverable, and recyclable. 14Moreover, hybrid organic-inorganic materials have attracted considerable attention as heterogeneous catalysts in the eld of organic synthesis. 15In recent years, there has been an increase in using heterogeneous catalysts due to their ability to be separated from the reaction mixture with ease, despite the potential lower activity as compared to homogeneous catalysts. 16,17Furthermore, the use of magnetic nanoparticles and their coated analogues has gained popularity due to their various advantages, i.e. effortless separation with the help of an external magnet, low toxicity, a highly active surface, easy recovery, and remarkable stability. 18hosphoric acid stands out as one of the most crucial industrial catalysts, nding extensive application in a diverse range of organic transformations since 2004. 19Notably, various methods have been applied aiming to recycle phosphoric acid in these transformations. 20However, these methods are very costly and complex.It is worth mentioning that zwitterions are an efficient class of catalysts that include two different ions in their structure. 21As a part of our continuing efforts toward the development of solid acid catalysts, an efficient method is described for the preparation of a novel phosphoric acid-based zwitterionic catalyst immobilized on CoFe 2 O 4 MNPs as the catalyst for the synthesis of important Ncontaining heterocycles in water and at room temperature. 22The overuse of antibiotics in the market has led to various health issues, including antibiotic resistance; therefore, discovering new drugs to combat microbial infections is crucial. 23The synthesized compounds were evaluated for their antibacterial properties against both Gram-negative and Gram-positive bacteria in our research.

General methods
The chemicals and solvents utilized in this study were obtained from Merck and Sigma-Aldrich Chemical Companies and were used as received.
2.2.Typical method for the synthesis of the CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 nanomagnetic catalyst The CoFe 2 O 4 @SiO 2 core-shell was synthesized using the method that has been previously documented. 24In the subsequent step, 1.5 g of nanoparticles obtained from the previous step were dispersed in 50 mL of H 2 O for 10 min.The dispersion was then combined with 2 g of 2-chloroethylamine hydrochloride, followed by the addition of 1.5 g of NaHCO 3 .The mixture was subjected to reux temperature for 24 h and 2 h, respectively.The resulting CoFe 2 O 4 @SiO 2 -EA product was extracted using a magnet, rinsed with DI water and, then, dried.In the concluding step, the CoFe 2 O 4 @SiO 2 -EA-MNP was dispersed in 10 mL of CH 2 Cl 2 using ultrasound for 10 min.Subsequently, 1.5 mL of phosphoric acid was added dropwise to the reaction mixture, followed by stirring at room temperature.Aer 12 h, the nanoparticles were extracted with a magnet, washed with CH 2 Cl 2 and EtOH, and then dried to produce the CoFe 2 O 4 @-SiO 2 -EA-H 3 PO 4 nanocatalyst outlined in Scheme 1.A thoroughly stirred mixture comprising 1 mmol of ethyl acetoacetate and 1 mmol of hydrazine hydrate was supplemented with 1 mmol of barbituric acid or malononitrile, 1 mmol of aromatic aldehyde, and 20 mg of the catalyst.The reaction was monitored using TLC (applying n-hexane/ethyl acetate (1 : 2) as eluent) and, then, the mixture was diluted with hot ethanol.Subsequently, the catalyst was effortlessly removed using a magnet.Following that, the resulting substance which was subjected to recrystallization, using ethanol, was puried and, then, dried in an oven.The identication of the known compounds was done by comparing their melting points with those of authentic samples.Moreover, in certain instances, the analysis was conducted using FT-IR, 1 H NMR, and 13 C NMR, as illustrated in ESI Fig. S1-S33.†

Antibacterial activity
The cup plate technique was used to test the in vitro antimicrobial activity of the synthesized derivatives against Gramnegative Escherichia coli ATCC-25922 and Gram-positive Staphylococcus aureus ATCC-25923.The minimum inhibitory concentration (MIC) of the compounds was determined using the broth micro-dilution method, wherein concentrations of 1000 and 5000 mg were tested for all derivatives.The compounds exhibiting antibacterial activity were selected for MIC determination.

Results and discussion
Following the synthesis of CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 , the structure of the nanomagnetic catalyst was veried through the utilization of FT-IR, SEM, TEM, XRD, EDX, elemental Mapping, VSM, TGA, and BET techniques.MNPs is conrmed by the stretching frequencies detected at 1089 cm −1 (Fig. 1B).The graing of 2-chloroethylamine on the CoFe 2 O 4 @SiO 2 surface is veried by distinct peaks for methylene groups (CH 2 ) at 2897-2949 cm −1 and a broadening peak at 3435 cm −1 for the NH 2 group (Fig. 1C).The ndings validate that the CoFe 2 O 4 crystal structure remains unaltered even aer the addition of organic and inorganic layers onto its surface.Through the application of Debye-Scherrer's calculation, it has been established that the nanoparticles exhibit a dimension of approximately 17.43 nm, aligning closely with the ndings from TEM analysis.

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3.1.5.EDX analysis.To determine the elemental composition of the CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 catalyst, EDX spectroscopy was employed.The analysis conrmed the presence of Fe, Co, O, Si, C, P, and N elements, as predicted (Fig. 5).
3.1.6.EDX elemental mapping analysis.EDX mapping imaging was used to examine the elemental composition of the CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 MNP catalyst.The analysis revealed a signicant amount of cobalt, iron, silicon, and oxygen elements in the CoFe 2 O 4 @SiO 2 material.Signicantly, the distribution of carbon, nitrogen, and phosphorus conrmed the presence of the desired zwitterion functionalities (EA-H 3 PO 4 ) on the nanomagnetic support surface, as illustrated in Fig. 6.   3.1.9.N 2 adsorption/desorption analysis.The surface area and pore volume of CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 were studied applying the Brunauer-Emmett-Teller (BET) method (Fig. 9).The BET specic surface area (SSA) and total pore volume (TPV) were ascertained to be 110.31m 2 g −1 and 2.36 cm 3 g −1 , respectively, which indicate a high surface area for catalysis applications.

Catalysis study
Our research endeavors center around the development of efficient synthetic pathways for heterocycles, employing magnetic nanoparticle catalysts.Using CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 MNPs as the catalyst, a one-pot, four-component reaction was conducted that involved ethylacetoacetate, hydrazine hydrate, barbituric acid, and aromatic aldehydes to produce pyrazolopyranopyrimidines. The reaction conditions were optimized through studying the model reaction while using 4-chlorobenzaldehyde and varying the solvent type and amount of the catalyst.The best reaction conditions were achieved using 20 mg of the catalyst in H 2 O solvent at room temperature (entry 3, Table 1).
Table 2 presents the results of synthesizing pyrazolopyranopyrimidine derivatives using different benzaldehyde and barbituric acid combinations under optimized reaction conditions, resulting in good yields and short reaction times.The synthesis of these derivatives was inuenced by both electronwithdrawing and electron-releasing groups.The synthesized derivatives using benzaldehydes, featuring electronwithdrawing groups, exhibited shorter synthesis times and higher yields in comparison to those with electron-donating groups.
To explore the scope of the reaction, malononitrile was used in place of barbituric acid to synthesize dihydropyrano[2,3-c] pyrazole products.The optimal reaction conditions, previously

Plausible mechanism for the synthesis of pyrazolopyranopyrimidine and dihydropyrano[2,3-c]pyrazoles
A plausible mechanism for the synthesis of pyrazolopyranopyrimidine, based on the previously reported reactions, is shown in Scheme 2. Initially, the condensation of ethyl

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acetoacetate with hydrazine resulted in the formation of pyrazole (I), which can subsequently be converted to its corresponding enolate form (II).In the presence of the nanocatalyst, Knoevenagel condensation takes place between benzaldehyde and barbituric acid, leading to the formation of intermediate (III).Subsequently, compound (II) reacts with intermediate (III) through a Michael addition, resulting in the formation of intermediate (IV).Ultimately, through the reaction of the intramolecular intermediate (IV) and the elimination of a water molecule, compound 5a was synthesized.The mechanism for the formation of dihydropyrano[2,3-c]pyrazoles closely parallels the synthesis mechanism of pyrazolopyranopyrimidine, with the substitution of malononitrile for barbituric acid. 30

Antibacterial activity
Table 4 and Fig. 8 show the antibacterial results of pyrazolopyranopyrimidine derivatives, with compounds 6b, 6c, 6e, 6f and 6k being the most active antibacterial agents.The structure-activity relationship revealed that amide groups at the pyrimidine ring contribute to the antibacterial activity.It should be mentioned that chlorine and methoxide derivatives increased the antibacterial properties of the compound.In addition, the substitution of barbituric acid with thiobarbituric acid or dimethylbarbituric acid decreased the antibacterial properties of pyrazolopyranopyrimidine with similar derivatives.The antibacterial activity of the dihydropyrano[2,3-c]pyrazole derivatives against Gram-positive and Gram-negative bacteria was evaluated as shown in Table 5, wherein no signicant effect was observed against the tested strains.The substitution of barbituric acid with malononitrile resulted in the elimination of the antibacterial effect observed in the targeted derivatives (Fig. 10).

Catalyst recycling and reusing test
The model reaction was used to examine the recovery and recyclability of CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 MNPs.Upon completion of the reaction, an external magnetic eld was applied to facilitate the separation of the catalyst from the reaction mixture.The catalyst was subsequently washed with EtOH, dried, and reused in subsequent reactions.This catalyst showed little deactivation aer six uses, which is an important aspect of green chemistry.The reactions were conducted under optimal conditions, the results of which are presented in Fig. 11.To evaluate the durability and reusability potential of the CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 particles, some specic analyses were used applying FT-IR, SEM, VSM, TGA, and BET (Fig. 12).
The results of the various tests conducted on the recycled catalyst conrm that the catalytic components attached to the CoFe 2 O 4 support and its morphology remain stable throughout the reaction and recycling process.The N 2 adsorption/ desorption isotherms of the regenerated catalyst reveal a BET specic surface area (SSA) and total pore volume (TPV) of approximately SSA = 96.64m 2 g −1 and 1.98 cm 3 g −1 , respectively.These values indicate a minimal decrease as compared to the fresh catalyst, affirming its stability throughout the catalytic process.

Hot-ltration and leaching test
In order to investigate the heterogeneity of the catalyst in the reaction medium, a hot ltration test was performed on the model reaction.The catalyst was separated using a magnet at the midway point of the reaction and, then, the resulting ltrate solution was employed to resume the process.Following the hot ltration experiment, no change in the yield of the product was witnessed, which is comparable to the catalyst-free reaction conditions.In addition, ICP-OES analysis of the ltrate solution showed that there was almost no phosphor leaching into the reaction medium.These results indicate the heterogeneous nature and no leakage of the catalyst in the course of the reaction.

Comparison
In order to investigate the activity of the CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 nanocatalyst in synthesizing pyrazolopyranopyrimidine, this system was compared to previous studies concerning the model reaction (product 6b), the results of which are illustrated in Table 6.These results demonstrate that the utilization of the CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 nanocatalyst for the synthesis of pyrazolopyranopyrimidine offers advantages such as high yield, reduced reaction time, and lower temperature as compared to other catalysts.
In conclusion, this study demonstrated that the CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 nanocomposite is an efficient and highly reusable nanomagnetic catalyst for the synthesis of pyrazolopyranopyrimidines and dihydropyrano [2,3-c]pyrazole via a one-pot fourcomponent reaction under mild conditions with minimal energy consumption.Some of the important advantages of this catalytic system are as follows: high reusability of the nanocatalyst, high purity and yield of the products, environmentally friendly conditions and simple operation.

3 . 1 . 2 .
Scanning electron microscopy (SEM) studies.The SEM image of CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 MNPs is shown in Fig. 2. The obtained images indicate that the nanoparticles are spherical and fall within the size range of nanomaterials.3.1.3.Transmission electron microscopy (TEM) studies.Transmission electron microscopy (TEM) was used to examine the morphology, size and shape of the CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 MNPs.The TEM images indicate that the catalyst has a spherical form and a core-shell structure, with the average particle size at around 15-20 nm which is illustrated in Fig. 3. Furthermore, the TEM results were consistent with those obtained from SEM images.3.1.4.XRD analysis.Fig. 4 depicts the XRD pattern of CoFe 2 O 4 @SiO 2 -EA-H 3 PO 4 MNPs.The plots exhibit six peaks at 2q = 30.21°,35.76°, 43.36°, 54.06°, 57.61°and 63.16°, which correspond to the standard plot of CoFe 2 O 4 MNPs.

Fig. 10
Fig. 10 The antibacterial activity of synthesized derivatives against Gram positive bacteria (S. aureus).