Synthesis and characterization of dicationic 4,4′-bipyridinium dichloride ordered mesoporous silica nanocomposite and its application in the preparation of 1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives

Abstract

The sol–gel method was used for the synthesis of a dicationic 4,4′-bipyridine silica hybrid nanocomposite. In order to introduce 4,4′-bipyridine into the skeleton framework of an ordered mesoporous silica (SBA-15), first, the N,N′-bis(triethoxysilylpropyl)-4,4′-bipyridinium dichloride precursor was synthesized by the reaction of 3-chloropropyltriethoxysilane with 4,4′-bipyridine to give (TEOS)₂BiPy²⁺ 2Cl⁻. The organic–inorganic hybrid nanocomposite, SBA@BiPy²⁺ 2Cl⁻, was then synthesized by the hydrolysis and polycondensation of the precursor and tetraethyl orthosilicate under mild acidic conditions. The nanocomposite was characterized by FT-IR spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Brunauer–Emmett–Teller (BET). The characteristic results of FT-IR, XRD and TGA confirmed the coexistence of silica and 4,4′-bipyridinium dichloride networks. The catalytic ability of SBA@BiPy²⁺ 2Cl⁻ as a novel environmentally safe heterogeneous nanoreactor for the preparation of 1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives via a one-pot multicomponent method under solvent-free conditions has been described. The catalyst can be reused without an obvious loss of catalytic activity.

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1. Introduction

The application of microporous materials has been greatly restricted due to their small pore sizes. This limitation was overcome by the discovery of mesoporous materials in 1992.¹ In the last decade, ordered mesoporous silicas such as SBA-15 and its functionalized family, with a tunable pore structure and tailored composition, have received considerable interest due to their potential applications such as adsorbents and guest–host chemical supports for large organic molecules.² However, the nature of the neutral framework of SBA-15, with few cavities, poor ion-exchange ability and a lack of active sites, limits its applications in catalysis.³ In this context, very recently, organized hybrid xerogel mesoporous materials, where an organic component is bonded to a polymeric silica skeleton framework, have attracted significant attention due to their outstanding properties.^4,5

Green chemistry has attracted considerable attention for overcoming the problems relating to environmental pollution being encountered by the global population. In this context, multicomponent reactions (MCRs) under solvent-free conditions are gaining more and more attention from organic, medicinal and synthetic chemists, especially in the total synthesis of natural products and medicinal heterocyclic compounds.⁶

1H-Pyrazolo[1,2-b]phthalazine-5,10-dione derivatives, which are nitrogen-containing fused hydrazine-based pyrazole heterocycles, represent an important class of pharmaceutical compounds and exhibit a wide spectrum of biological activities such as anti-inflammatory,⁷ antifungal,⁸ anticancer,⁹ antiviral,¹⁰ antitumor,¹¹ anticoagulant¹² and antibacterial¹³ activity. Several three-component reactions have been reported for the synthesis of these compounds in the presence of an acid or base^14–21 via condensation of an aromatic aldehyde, phthalhydrazide, and malononitrile. Many of the reported synthetic methods have drawbacks, such as harsh reaction conditions, poor yields, prolonged reaction times, application of toxic or hazardous catalysts, or costly reaction procedures. Thus novel methodologies which could overcome these drawbacks are still needed.

Taking all these facts into account, the aim of the present procedure is to highlight the synergistic effects of the combined use of MCRs and solvent-free conditions, with the application of an organized mesoporous tunable pore nanocomposite, SBA@BiPy²⁺ 2Cl⁻, for the development of a green strategy for the preparation of 1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives (Scheme 1).

Scheme 1 One-pot preparation of 1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives.

2. Results and discussion

At first, the N,N′-bis(triethoxysilylpropyl)-4,4′-bipyridinium dichloride precursor was synthesized by the reaction of 3-chloropropyltriethoxysilane with 4,4′-bipyridine to give (TEOS)₂BiPy²⁺ 2Cl⁻. The organic–inorganic hybrid nanocomposite, SBA@BiPy²⁺ 2Cl⁻, was then synthesized by hydrolysis and polycondensation of the precursor and tetraethyl orthosilicate under mild acidic conditions in the presence of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide), Pluronic P123 (Scheme 2). The processes are self-assembly of the amphiphilic surfactant P123 that serves as a template, interaction at the interface between P123 and the mixture of TEOS and (TEOS)₂BiPy²⁺ 2Cl⁻, and the sol–gel process that introduces 4,4′-bipyridine into the skeleton framework of the ordered mesoporous silica. The template is subsequently removed by Soxhlet extraction with ethanol to generate SBA@BiPy²⁺ 2Cl⁻.

Scheme 2 Preparation of SBA@BiPy²⁺ 2Cl⁻.

In order to characterize the catalyst, and to confirm the incorporation of bipyridinium moieties into the skeleton framework of the ordered mesoporous silica, FT-IR spectroscopy was utilized. As is apparent from Fig. 1, the FT-IR spectrum of SBA@BiPy²⁺ 2Cl⁻ shows bands at 807 and 1100 cm⁻¹ due to Si–O–Si bond vibrations and the band at about 960 cm⁻¹ is assigned to Si–OH bonds. Moreover, a broad peak is obvious at 3417 cm⁻¹, which is related to the stretching vibration of the surface OH groups of silica. Furthermore, the quaternary nitrogen has a characteristic peak at 1641 cm⁻¹. Aromatic pyridinium rings are also responsible for the stretching vibrations at 1503 and 1561 cm⁻¹.

Fig. 1 FT-IR spectrum of synthesized mesoporous SBA@BiPy²⁺ 2Cl⁻.

Thermogravimetric (TG) analysis of SBA@BiPy²⁺ 2Cl⁻ is presented in Fig. 2. The TG curve shows two weight losses. The first weight loss, at around 100 °C, is attributed to the loss of physically adsorbed water, representing approximately 0.18 g/g of the sample. The second weight loss is concentrated in the range 300–700 °C, resulting from decomposition of the organic parts of the skeleton framework during hydrothermal treatment. The weight losses of the organic parts are 0.19 g/g of the sample. Therefore, the nanocatalyst is completely stable below 300 °C and can be applied without degradation.

Fig. 2 TGA curve of SBA@BiPy²⁺ 2Cl⁻.

Fig. 3 illustrates SEM images of SBA-15 and SBA@BiPy²⁺ 2Cl⁻. The SEM image (Fig. 3b) shows a dominant lengthy rod-like morphology for SBA@BiPy²⁺ 2Cl⁻, in a bundle arrangement with a diameter of approximately 1 μm. The same morphology is observed for SBA-15 (Fig. 3a), indicating that the morphology was maintained without change.

Fig. 3 SEM images of (a) SBA and (b) SBA@BiPy²⁺ 2Cl⁻.

Fig. 4 shows TEM images of SBA@BiPy²⁺ 2Cl⁻. It can be clearly seen that the uniform mesoporous structure of SBA-15 was not destroyed after the introduction of functionalizing organic parts onto the skeleton framework of SBA.

Fig. 4 TEM images of SBA@BiPy²⁺ 2Cl⁻.

The XRD pattern of SBA@BiPy²⁺ 2Cl⁻ is also provided in Fig. 5. The X-ray diffraction pattern of the SBA-15 material reveals 2D hexagonally structured pores at low angles, whereas no diffraction pattern can be observed at high angles, due to the amorphous nature of the pore walls.²²

Fig. 5 XRD pattern of SBA@BiPy²⁺ 2Cl⁻.

The mesoporous nature of the synthesized sample was studied by its N₂ adsorption–desorption isotherm (Fig. 6b). The sample exhibited a type IV isotherm. BJH pore size distribution analysis (Fig. 6a) shows that the material possesses uniformly sized mesopores centered at ca. 61.8 Å. The BET surface area of SBA@BiPy²⁺ 2Cl⁻ was found to be 339 m² g⁻¹ and its pore volume 0.3 cm³ g⁻¹. The main structural characteristics determined experimentally and the characteristic data for the sample are summarized in Table 1.

Fig. 6 Pore size distribution (a) and nitrogen adsorption–desorption isotherm (b) of SBA@BiPy²⁺ 2Cl⁻.

Table 1 Specific surface area (S_BET), pore diameter and total pore volume

Sample	BET surface area (m^2 g^−1)	Pore diameter (nm)	Pore volume (cm^3 g^−1)
SBA@BiPy^2+ 2Cl^−	339	6.18	0.3

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In addition, the presence of organic components in the skeleton framework was also confirmed by CHN analysis (14.51, 3.23, and 3.62 for C, H, and N respectively).

After characterization of the catalyst, it was decided to evaluate the catalytic activity of SBA@BiPy²⁺ 2Cl⁻ in the preparation of 1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives. A model three-component coupling reaction of phthalhydrazide, malononitrile and aromatic aldehydes under solvent-free conditions at 100 °C in the presence of SBA@BiPy²⁺ 2Cl⁻ was examined.

In order to determine the appropriate catalyst loading, a model reaction was carried out using 0.004–0.03 g of the catalyst at different temperatures under solvent-free conditions (Table 2). It was found that 0.005 g of the catalyst provides a maximum yield in a minimum time. In the next step, the effect of temperature was examined for the model reaction. It was observed that the reaction did not proceed at low temperatures, while the reaction proceeded to the desired product in high yield at elevated temperatures. We were fortunate to find that the reaction proceeded effectively and almost complete conversion to the desired product was observed at 100 °C, affording 3-amino-5,10-dioxo-1-phenyl-5,10-dihydro-1H-pyrazolo[1,2-b]phthalazine-2-carbonitrile in 96% yield within a shorter reaction time (Table 2, entry 6).

Table 2 Optimum conditions for the three-component condensation reaction of phthalhydrazide (1 mmol), malononitrile (1 mmol) and benzaldehyde (1 mmol) under thermal solvent-free conditions

Entry	Catalyst (g)	Temp. (°C)	Time (min)	Yield (%)
1	0.003	60	90	60
2	0.001	60	90	58
3	0.005	60	90	55
4	0.004	60	90	65
5	0.005	90	70	82
6	0.005	100	40	96
7	—	100	60	48
8	0.005	110	40	90

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Subsequently, under the optimal conditions, a 1 : 1 : 1 molar ratio of phthalhydrazide, malononitrile and aromatic aldehyde in the presence of 0.005 g SBA@BiPy²⁺ 2Cl⁻ at 100 °C under solvent-free conditions, the synthetic scope of this procedure was demonstrated by synthesizing a series of 1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives (Table 3). Gratifyingly, a wide range of aromatic aldehydes was well tolerated under the optimized reaction conditions. The times taken for complete conversion and the isolated yields are recorded in Table 3. It is obvious that the electron-withdrawing groups increase the reaction rate due to their effect on the carbonyl group, which makes the aldehyde more electrophilic in the subsequent Michael addition. All the compounds were characterized by the comparison of their physical and spectral data with those of authentic samples.

Table 3 The one-pot three-component condensation reaction of phthalhydrazide (1 mmol), malononitrile (1 mmol), and aromatic aldehydes (1 mmol) promoted by SBA@BiPy²⁺ 2Cl⁻ (0.005 g) under solvent-free conditions at 100 °C

Entry	Substituted benzaldehyde	Product	Time (min)	Yield (%)
1			40	96
2			30	94
3			54	90
4			50	86
5			60	89
6			30	91
7			30	87
8			50	90
9			50	86
10			30	88

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It is postulated that this reaction proceeds through a Knoevenagel condensation of malononitrile and the aromatic aldehyde. In the next step, phthalhydrazide attacks the adduct in a Michael-type reaction, which in the subsequent steps yields the product (Scheme 3). The channels of mesoporous SBA@BiPy²⁺ 2Cl⁻ represent a straightforward nanoreactor for this two-step condensation reaction.

Scheme 3 Postulated mechanism of the reaction.

To investigate the retention of the activity of the catalyst, the catalyst was reused seven times in the one-pot multicomponent condensation of phthalhydrazide, malononitrile, and benzaldehyde at 100 °C under solvent-free conditions. In this procedure, after the completion of each reaction, hot ethanol was added and the catalyst was filtered. The recovered catalyst was washed with ethanol, dried and reused (Fig. 7). As shown in Fig. 7, a steady loss of the catalytic activity of SBA@BiPy²⁺ 2Cl⁻ was observed.

Fig. 7 Reusability of the catalyst in the reaction of phthalhydrazide (1.0 mmol), malononitrile (1.0 mmol), and benzaldehyde (1.0 mmol) at 100 °C under solvent-free conditions.

3. Experimental

3.1. General

All the commercially available chemicals were purchased from the Fluka and Merck companies and used without further purification. Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) [EO₂₀PO₇₀EO₂₀, Pluronic P123, M_w = 5800] was purchased from the Aldrich chemical company. IR spectra were recorded on a BOMEM MB-Series 1998 FT-IR spectrophotometer. ¹H and ¹³C NMR spectra were recorded in DMSO-d₆ on a Bruker Advance DPX 400 MHz spectrometer using TMS as internal standard. Reaction monitoring was carried out by TLC on silica gel Polygram SILG/UV 254 plates. The particle morphology was examined by SEM (LEO 1455VP scanning electron microscope, operating at 1–30 kV) and TEM (ZEISS LEO 906E transmission electron microscope, 80 kV).

3.2. Preparation of N,N′-bis(triethoxysilylpropyl)-4,4′-bipyridinium dichloride precursor, (TEOS)₂BiPy²⁺ 2Cl⁻

In a 25 mL three-necked round-bottom flask, bipyridine (8 mmol, 1.25 g) was added to DMF (5 mL) and stirred to give a clear solution. To this solution, (3-chloropropyl)triethoxysilane (16 mmol, 3.18 g) was added dropwise and the mixture stirred at 90 °C for 72 hours under an argon atmosphere. Afterwards, the white solid, (TEOS)₂BiPy²⁺ 2Cl⁻, was filtered and washed with methanol. This solid was dried for 2 hours in an oven at 90 °C.

3.3. Preparation of dicationic 4,4′-bipyridine silica hybrid nanocomposite, SBA@BiPy²⁺ 2Cl⁻

In a 110 mL round-bottom flask, Pluronic 123 (2.0 g, 0.344 mmol, M_n = 5800) was dissolved in HCl 2 M (62.5 mL) at room temperature. Then tetraethyl orthosilicate (TEOS) (4.2 g, 20 mmol) and (TEOS)₂BiPy²⁺ 2Cl⁻ (0.65 g, 1.0 mmol) were added and the mixture was stirred at 40 °C for 20 hours. The mixture was hydrothermally treated at 95 °C under static conditions for 24 hours. The resulting solid was filtered, washed with excess water and dried at 100 °C. The solid was then Soxhlet extracted with ethanol for 48 hours to remove Pluronic 123 and then dried at 50 °C for 24 hours.

3.4. Typical procedure for the preparation of 1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives

A mixture of phthalhydrazide (1 mmol), malononitrile (1 mmol), aromatic aldehyde (1 mmol), and SBA@BiPy²⁺ 2Cl⁻ (0.005 g) was heated at 100 °C in a test tube under solvent-free conditions (Table 3). Completion of the reaction was indicated by TLC [TLC acetone/n-hexane (3 : 10)]. After completion of the reaction, the insoluble crude product was dissolved in hot methanol and the catalyst was filtered. The crude product was purified by recrystallization from methanol to afford the pure product.

3.5. Selected spectral data

3-Amino-1-(4-chlorophenyl)-5,10-dioxo-5,10-dihydro-1H-pyrazolo[1,2-b]phthalazine-2-carbonitrile. Yellow powder; mp = 273–276 °C; ¹H NMR (400 MHz, DMSO-d₆): δ_H 8.31–8.26 (m, 1H), 8.14 (brs, 2H, NH₂), 8.12–8.08 (m, 1H), 7.99–7.95 (m, 2H), 7.53 (d, J = 8.8 Hz, 2H), 7.45 (d, J = 8.8 Hz, 2H), 6.17 (s, 1H) ppm; ¹³C NMR (100 MHz, DMSO-d₆): 157.3, 154.2, 150.9, 150.7, 137.1, 134.8, 133.9, 133.3, 129.4, 129.3, 128.8, 128.1, 127.7, 126.9, 116.4, 62.7 ppm; IR (KBr): ν = 3377, 3259, 3109, 2187, 1656, 1560 cm⁻¹.

3-Amino-1-(4-nitrophenyl)-5,10-dioxo-5,10-dihydro-1H-pyrazolo[1,2-b]phthalazine-2-carbonitrile. Yellow powder; mp = 227–229 °C; ¹H NMR (400 MHz, DMSO-d₆): δ_H 8.29–8.27 (m, 1H), 8.24 (d, J = 8.6 Hz, 2H), 8.21 (brs, 2H, NH₂), 8.11–8.09 (m, 1H), 7.98–7.97 (m, 2H), 7.82 (d, J = 8.6 Hz, 2H), 6.31 (s, 1H) ppm; ¹³C NMR (100 MHz, DMSO-d₆): 157.4, 154.3, 151.5, 151.3, 148.0, 146.6, 134.9, 134.6, 129.4, 129.0, 128.7, 128.0, 126.0, 124.4, 116.6, 62.8 ppm; IR (KBr): ν = 3440, 3326, 3079, 2165, 1662, 1560, 1520 cm⁻¹.

3-Amino-1-(3-methoxyphenyl)-5,10-dioxo-5,10-dihydro-1H-pyrazolo[1,2-b]phthalazine-2-carbonitrile. Yellow powder; mp = 247–249 °C; ¹H NMR (400 MHz, DMSO-d₆): δ_H 8.27–8.25 (m, 1H), 8.12–8.10 (m, 3H, NH₂, H), 8.0–7.96 (m, 2H), 7.27 (t, J = 7.6 Hz, 1H), 6.99–6.97 (m, 2H), 6.90 (dd, J = 8.0, 1.6 Hz, 1H), 6.12 (s, 1H), 3.76 (s, 3H, OCH₃) ppm; ¹³C NMR (100 MHz, DMSO-d₆): 160.0, 156.9, 153.8, 150.9, 137.8, 134.4, 129.7, 129.2, 129.1, 128.9, 127.2, 126.9, 124.3, 119.3, 114.0, 112.9, 64.1, 55.7 ppm; IR (KBr): ν = 3363, 3261, 3060, 2193, 1655, 1567 cm⁻¹.

3-Amino-1-(3-chlorophenyl)-5,10-dioxo-5,10-dihydro-1H-pyrazolo[1,2-b]phthalazine-2-carbonitrile. Yellow powder; mp = 264–266 °C. ¹H NMR (400 MHz, DMSO-d₆): δ_H 8.25–7.38 (10H, m, Ar and NH₂), 6.13 (1H, s, CH) ppm; ¹³C NMR (100 MHz, DMSO-d₆): 157.4, 154.0, 151.5, 140.9, 135.7, 135.0, 134.1, 129.1, 129.0, 128.3, 127.9, 127.0, 125.8, 122.4, 117.3, 63.0, 61.2 ppm; IR (KBr): ν = 3363, 3265, 2202, 1670, 1660 cm⁻¹.

3-Amino-1-(2-nitrophenyl)-5,10-dioxo-5,10-dihydro-1H-pyrazolo[1,2-b]phthalazine-2-carbonitrile. Yellow powder; mp = 262–264 °C. ¹H NMR (400 MHz, DMSO-d₆): δ_H 8.29–7.60 (10H, m, Ar and NH₂), 6.64 (1H, s, CH) ppm; ¹³C NMR (100 MHz, DMSO-d₆): 156.8, 154.2, 151.9, 148.5, 135.0, 134.5, 134.2, 133.4, 129.8, 129.2, 129.0, 128.5, 127.4, 126.9, 124.4, 106.0, 59.4, 57.8 ppm; IR (KBr): ν = 3385, 3175, 2198, 1701, 1655 cm⁻¹.

4. Conclusion

Herein, for the first time, using a new and facile strategy, the SBA@BiPy²⁺ 2Cl⁻ nanocomposite has been synthesized. Its characterization and catalytic activity as a novel environmentally safe heterogeneous nanoreactor for the preparation of 1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives through a three-component one-pot synthesis under solvent-free conditions have been described. This method offers several advantages, including high yields, application of an inexpensive catalyst, short reaction times, easy workup and performing a multicomponent reaction under solvent-free conditions that are considered to be relatively environmentally benign.

Acknowledgements

We gratefully acknowledge the support of this work by Shahid Chamran University Research Council.

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