SnO2 nanoparticles as an efficient heterogeneous catalyst for the synthesis of 2H-indazolo[2,1-b]phthalazine-triones

A new protocol for the synthesis of 2H-indazolo[2,1-b]phthalazine-trione derivatives is described via a one-pot three-component condensation reaction of phthalhydrazide, dimedone or 1,3-cyclohexanedione and aromatic aldehydes catalyzed by SnO2 nanoparticles as a heterogeneous catalyst under solvent-free conditions. The SnO2 nanoparticles (NPs) were characterized by FT-IR, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analyzer (EDAX). The advantages of the protocol are the shorter reaction time, simple work-up procedure and reusable catalyst.

antifungal [11], anticancer [12] and anti-inflammatory [13] activities. In addition, phthalazine derivatives also have excellent optical properties [14,15]. Due to the notable significances, we would like to establish a new facile protocol for the synthesis of phthalazine scaffolds.
In recent years, the literature reports reveal that only a limited number of solvent-free protocols are available for the synthesis of phthalazine derivatives using the condensation reaction of phthalhydrazide, dimedone, and aromatic aldehydes with various catalysts [16][17][18][19][20][21][22][23][24][25]. However, the reported methods have some limitations such as nonbiodegradable and expensive metal catalysts, corrosive reaction media and longer reaction times due to which new synthetic protocols may be suggested for the synthesis of phthalazine derivatives via one-pot multicomponent reactions. Multicomponent reactions (MCRs) are more essential to construct the complex organic molecule in a single step without isolation of any intermediates [29,30]. MCRs with heterogeneous catalyst and solvent-free process are more attractive to the researchers, because they will minimize the waste and usage of hazardous chemicals [31,32]. Moreover, the heterogeneous catalysts have more enrapture to the researchers because of their selectivity and reactivity in organic transformations [33][34][35] and they provide more advantages over homogeneous catalysts due to the more active sites. In addition, they are environmental friendly, cost effective, insoluble in solvents and reusable leading to easy work-up procedures. In general, the nanomaterials exhibit superior mechanical, thermal, chemical, electrical, optical and catalytic properties. Therefore, we extend our present research work by employing SnO 2 nanoparticles as a heterogeneous catalyst for the synthesis of phthalazine derivatives. Tin oxide (SnO 2 ) has a broad spectrum of applications which includes its use in electrochemical [36] and photo-catalytical processes [37], gas sensors [38,39], optical electronic devices [40] and solar cells [41]. Additionally, the SnO 2 NPs have an excellent catalytic behavior in the synthesis of various heterocyclic compounds [42][43][44][45].
The present research work utilizes SnO 2 NPs as a reusable catalyst for the synthesis of 2H-indazolo[2,1-b]phthalazine-trione derivatives. To the best of our knowledge, there is no literature report available for the SnO 2 NPs-catalyzed synthesis of 2H-indazolo[2,1-b]phthalazinetriones.

General
All reagents and common solvents were purchased from Loba chemie and Sigma-Aldrich and used without further purification. The products were isolated and characterized by physical and spectral analysis. 1 H NMR and 13 C NMR spectra were recorded on Bruker (DRX Avance-400) spectrometer in the presence of tetramethylsilane as an internal standard. The IR spectra were recorded on Bruker (ATR FT-IR) apparatus using Zn-Se technique. Reactions were monitored by TLC using precoated silica gel plates and melting points were determined on an electro thermal apparatus, and are uncorrected. Powder X-ray diffraction (XRD) was carried out on a Philips diffractometer of X'pert Company with monochromatized Cu Ka radiation (k = 1.5406 Å ). Field emission scanning electron microscopy (FE-SEM) and the X-ray dispersive analysis were recorded by FE-SEM (LEO 1455VP), operated at a 15 kV accelerating voltage.
General procedure for the preparation of the SnO 2 nanoparticles The SnO 2 NPs were prepared by taking 2 g (0.1 M) of stannous chloride dihydrate (SnCl 2 Á2H 2 O) in 50 ml of distilled water and dissolved. Simultaneously, an aqueous ammonia solution was added drop by drop under continuous stirring for 15-20 min while maintaining the pH of the solution between 9 and 11. The resulting gel was washed with deionized water until the pH reached 7, filtered and dried at 80°C for 24 h to remove water. Then, the powdered sample was calcinated at 600°C for 2 h and the finally obtained powder sample was characterized with FT-IR, powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and X-ray dispersive analyses.
General procedure for the synthesis of 2Hindazolo[2,1-b]phthalazine-trione derivatives An equimolar mixture of phthalhydrazide (1.0 mmol), aromatic aldehyde (1.0 mmol), 5,5-dimethyl-1,3-cyclohexanedione or 1,3-cyclohexanedione (1.0 mmol) and 10 mol % of SnO 2 NPs was magnetically stirred over an oil bath at 80°C under solvent-free condition for an appropriate time. The completion of the reaction was monitored by TLC using ethyl acetate-hexane (1:1) as the eluent. After completion of the reaction, the reaction mass was cooled to room temperature and 3 ml of chloroform was added and stirred well. The product was soluble in chloroform and the catalyst was recovered by simple filtration. The catalyst was washed with 3 ml of acetone and dried at 100°C. Finally, the recovered catalyst was subjected to the next run of the reaction. The solvent was removed under vacuum to get the crude product, which on further recrystallization with ethanol afforded the pure product (compared with literatures Ref. [16][17][18][26][27][28]). The pure products were characterized with FT-IR, 1 H and 13 C NMR spectroscopic techniques.

Characterization of SnO 2 NPs
The SnO 2 NPs were synthesized using an already reported procedure [38]. The synthesized SnO 2 NPs were characterized by Fourier transform-infrared spectroscopy, powder X-ray diffraction, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy.

FT-IR and XRD analysis
We have compared the FT-IR spectra of as-synthesized form and calcinated samples of SnO 2 NPs (Fig. 1). Figure 1a shows a broad band at 3460.78 cm -1 which represents the presence of water molecule bound to Sn surface. The band at 1573.15 cm -1 is due to the bending vibration of water molecules and the peak at 1423.15 cm -1 represents the unreactive SnCl 2 . The band at 625 cm -1 indicates the Sn-OH. Furthermore, the calcinated sample has shown a band at 646.31 cm -1 corresponds to Sn-O-Sn and the band present at 553.94 cm -1 is due to the stretching vibration of Sn-O. The intensity of peaks due to water molecules is very much decreased in the case of calcinated sample. The observed results reveal that the Sn-O band is not present in the fresh sample. Figure 1c shows the IR spectrum of the SnO 2 catalyst after the sixth run of the reaction in which there is no change in the peak position and intensity when compared to that of the calcinated catalyst supporting its reusability results. From the X-ray diffraction spectrum, the formations of SnO 2 NPs are confirmed (Fig. 2). All the diffraction peaks are in good The average crystallite size of the SnO 2 NPs was calculated using Scherrer formula as, (D = kk/bcosh), where D is the average crystallite size, k is the Scherrer constant, k is the wavelength of the X-ray beam, b is the (p/2) FWHM (Full width at half maximum) and h is the diffraction angle. The average crystallite size of the SnO 2 NPs was found to be 23.4 nm. Meanwhile, these results were in good agreement with the literature report [46].

FE-SEM and EDAX analysis
The field emission scanning electron microscopy (FE-SEM) that shows the morphology and size of the SnO 2 NPs is presented in Fig. 3. The morphology and size of the individual particles could not be measured due to their extremely small size and heavy aggregation. In general, the formation of agglomerated nanoparticles can be explained by nucleation-aggregation-agglomeration-growth mechanism proposed by Rodriguez-Clemente et al. [47]. The particle size of the SnO 2 NPs is in the range of 44-71.79 nm approximately and also the SnO 2 NPs have been aggregated in spherical morphology. Finally, the energy dispersive X-ray spectrum exhibits the atomic weight % of tin 33.33 and 66.67% of oxygen present in the SnO 2 NPs (Fig. 4).

Catalytic activity
The formation of SnO 2 NPs made us to utilize the NPs as a catalyst for the one-pot synthesis of 2H-indazolo[2,1b]phthalazine-triones under various reaction conditions.
We have optimized the amount of catalyst required to catalyze the reaction where the temperature and solvent have also been screened. Initially, we have performed the model reaction with a mixture of 4-chlorobenzaldehyde (1 mmol), dimedone (1 mmol) and phthalhydrazide (1 mmol) without catalyst and solvent and found that there was no reaction even after five hours ( With the optimized catalyst, solvent-mediated reaction did not succeed well which may be due to the interruption of solvent molecules for the direct contact between the reactants and the catalyst ( Table 1, entries [15][16][17]. We have compared the yield of the product and the reaction time with previous literature in Table 2, entries 1-10. Apparently, some of the reported catalysts afforded good yields but most of the catalysts were hazardous to the environment and the catalyst quantity needed was also more in addition to longer reaction time. The catalytic activity of SnO 2 NPs may be due to the presence of more acidic (Sn 4? ) and the basic (O 2-) surface sites [37] that could readily induce the organic transformations. We have concluded that 10 mol % of SnO 2 NPs at 80°C was more favorable under solventfree condition to provide excellent yield of the target product with shorter reaction time (Table 1, entry 9) and the optimization results are presented in Table 1. The reusability of the catalyst was also successfully checked up to seven runs.
Using the above optimized conditions, we have taken various aromatic aldehydes having different electron withdrawing and donating substituents in the ortho, meta The plausible reaction mechanism proposed for the synthesis of 2H-indazolo[2,1-b]phthalazine-trionederivatives is represented in Scheme 2. Initially, the SnO 2 NPs catalyzed the Knoevenagel condensation reaction between dimedone (I) and aromatic aldehyde (II) providing III.  This step is facilitated by the activation of carbonyl group in aromatic aldehyde by SnO 2 NPs. Michael addition of phthalhydrazide to III provides IV which on cyclization afforded the target product (V).

Reusability of the catalyst
Next, we have also checked the reusability of SnO 2 NPs. The reusability of the catalyst was investigated with the model reaction using an equimolar mixture of 4-chlorobenzaldehyde (1 mmol), dimedone (1 mmol) and phthalhydrazide (1 mmol) performed with 10 mol % of SnO 2 NPs under solvent-free condition. After completion of the reaction, the reaction mass was cooled to room temperature and 3 ml of chloroform was added and stirred well. After that the product soluble in chloroform was separated by filtration. The recovered catalyst was washed with 3 ml of acetone to remove any organic residues present in the catalyst. The recovered catalyst was dried at 100°C and subjected to the consecutive runs with same experimental condition. The yield of the product and the reaction time were not much affected up to fifth run after that a slight decrement in the yield of the product was observed. From these results, it has been concluded that 10 mol % of SnO 2 NPs can be efficiently reused for more than five to six consecutive runs without loss in its catalytic Reaction condition: 4-chlorobenzaldehyde (1 mmol), dimedone or 1,3-cyclohexanedione (1 mmol) and phthalhydrazide (1 mmol) and 10 mol % SnO 2 NPs, solvent-free condition a Isolated yield   (Fig. 5). After the sixth run, the recovered SnO 2 NPs were further subjected to FE-SEM analysis which was shown in Fig. 6.

Conclusion
We have demonstrated the SnO 2 NPs as an effective catalyst for the synthesis of 2H-indazolo[2,1-b]phthalazine-triones derivatives via one-pot three-component condensation reaction of the aromatic aldehydes, dimedone/1,3-cyclohexanedione and phthalhydrazide under solvent-free condition at 80°C. The SnO 2 NPs have remarkable advantages such as cheap and efficient catalyst. The excellent yield of the product without using column chromatography, shorter reaction time and easy work-up procedure and the reusability of the catalyst are the added advantages of this method. The present protocol seems to be a better alternative compared to other reported protocols.
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