Synthesis and characterization of bentonite-based NiO nanoparticles as bi-functional heterogeneous catalyst for efficient synthesis of 1,8-dioxo-decahydroacridines

Abstract

A straightforward, expeditious, robust, and efficient synthesis of NiO@Bentonite nanocatalyst was done using a simple microwave method. The X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), Brunauer–Emmett–Teller (BET), thermal behavior analysis, and vibrating sample magnetometer (VSM) techniques were used to characterize the physicochemical properties of the NiO@Bentonite nanocomposites. The outcomes demonstrated that the NiO nanoparticles are evenly distributed throughout the Bentonite surface and that the NiO@Bentonite nanocomposites have a high specific surface area and rich pore structure. The following report details the investigation of this catalyst for the preparation of 1, 8-dioxodecahydroacridine heterocycles in a one-pot, three-component reaction of aromatic aldehydes, dimedone, and aniline. After the conditions were optimized, the results demonstrated that this reaction could be carried out in an aqueous medium with a good yield.

Introduction

Nickel transition metal oxide as a p-type semiconductor with a large band gap energy in the range of 3.6–4.0 eV¹ has become more popular among other transition metal oxides as heterogeneous catalysts due to its excellent oxidation–reduction properties, low cost, high stability^2,3, and its application in various fields such as wastewater dye degradation, catalysts, lithium-ion batteries, magnetic materials, gas sensors, cell separation, magnetic resonance imaging, drug delivery, and biomedical detection^4–11.

Because it possesses significant qualities such as commercial availability, chemical and mechanical stability, a fairly high loading capacity, excellent physicochemical structure, and ease of functionalization, bentonite is a precursor for solid acid catalysts in organic transformations¹². In the modern world of chemistry, combining several materials results in unique benefits and advantages, including cost-effectiveness, time savings, atom economy, selectivity, environmental friendliness, and ease of experimentation¹³.

One of the problems is particle aggregation when using nanoparticles as catalysts; however, stabilizers can help¹⁴. Small nanoparticle dispersion and reducing the optimum amount of NiO can be accomplished with bentonites as an appropriate support¹⁵. Clays high in bentonites include a lot of smectites. Smectites, which are categorized as 2:1 phyllosillicates, consist of two tetrahedral [SiO₄]⁴⁻ sheets sandwiched by an octahedral [AlO₃(OH)₃]⁶⁻ sheet. Because lower-charged cations are isomorphously substituted for cations inside sheets (Si⁴⁺ with Al³⁺ or Fe³⁺ in tetrahedral sheets and Al³⁺ with Mg²⁺, Fe²⁺, or Fe³⁺ in octahedral sheets), the smectite layers have a negative net charge. Within the interlamellar area, hydrated cations counteract this negative charge¹⁶.

Multicomponent reactions (MCRs) have shown great promise in synthesizing vast libraries of structurally diverse compounds from readily available starting materials¹⁷. These one-pot condensations of at least three separate starting materials are characterized by their high atom economy, ease of use, shorter reaction times, reduced costs, accessibility to a vast range of chemicals and complicated molecules, and quick reaction times¹⁸.

Acridine-1,8-dione (ADD), with its many applications in biological, pharmaceutical, materials, industrial, and synthetic organic chemistry fields, has emerged as the most prominent fused 1,4-DHP molecule in recent decades (Fig. 1). It is a common heterocycle in many facets of life due to its N-heteroatom, acridine ring framework, and diversified substitution options.

Fig. 1.

Structure of substituted acridine-1,8-dione.

1,4-DHPs (functionalized 1,4-dihydropyridines) and fused symmetrical polycyclic aromatic molecules are the precursors of 1,8-dioxo-decahydroacridine derivatives. According to research from the 1980s, from the Hantzsch reaction of 1,3-dicarbonyl compounds and aldehydes with a nitrogen source arises this useful scaffold¹⁹. These heterocycles are noteworthy due to their electrical characteristics, bichromophoric structure, possible biological activity, and occurrence in several essential natural compounds and manufactured dye-stuffs^20,21. Presently, several members of this family are employed in the treatment of Alzheimer's disease, strong anti-cancer medications (such as amsacrine derivatives), cardiovascular disorders such as hypertension and diabetes, platelet antiaggregatory activities, cytostatic and antitumor activities (e.g., nitracrine derivatives)^22,23, and so forth. A few acridine derivatives' structures are shown in Fig. 2³¹. In addition, these materials can be used as laser dyes^21,24, whereby they act as photoinitiators²⁵.

Fig. 2.

Structures of some known xanthenes and acridines.

Several approaches are improved for the synthesis of 1,8- dioxo-decahydroacridines via the traditional heating in organic solvents²⁶, under MW irradiation²⁷, and using Lewis and Bronsted acids as catalysts, for example, Amberlyst-15²⁸, sulfonic acid functionalized silica (SBSSA)²⁹, ZnO nanoparticles³⁰, nano-Fe₃O₄³¹, silica bonded N-propyl sulfamic acid (SBNPSA)³², ionic liquids³³ such as 1-methyl imidazolium trifluoroacetate ([Hmim]TFA)³⁴, [TEAPS]₃PW₁₂O₄₀³⁵, Bronsted acidic imidazolium salts containing perfluoroalkyl tails³⁶, p-dodecylbenzenesulfonicacid (DBSA)³⁷, PMA-SiO₂³⁸, Ceric ammonium nitrate (CAN)³⁹, Choline chloride: Urea⁴⁰, salicylic acid⁴¹, MNPs-N-propyl-benzoguanamine-SO₃H⁴², SO₄²⁻/ZrO₂⁴³ leading to acridines.

Even though several of these methods have potential applications, they have drawbacks such as weak catalyst non-recoverability, hazardous organic solvents, low yields, and difficult reaction conditions. Therefore, to circumvent these limitations and improve the existing reaction conditions, it is still necessary to introduce simple, effective, efficient, and environmentally friendly processes. Furthermore, with simple, easily accessible, recyclable catalysts and novel nanocatalysts under mild conditions.

In continuation of our recent works on the multicomponent synthesis of heterocyclic compounds^44–47, in the present research, we applied NiO@Bentonite nanocomposite as an effective, recyclable, and robust catalyst to facilitate the formation of acridines from three-component Hantzsch condensation reactions of aldehydes, dimedone, and anilines.

Experimental section

Chemicals and reagent

The Merck Corporation provided the chemicals employed in this investigation, which included nickel nitrate, ethanol, sodium hydroxide, dimedone, aryl aldehydes, and aniline. Since all reagents are of analytical grade, no additional purification is required. Water that has been double-distilled was utilized to prepare the solution.

Material characterization

Every XRD result was gathered using a Philips PC-APD X-ray diffractometer (XRD, Netherlands). Energy Dispersive Spectroscopy (SEM–EDS analysis; EM 3200 SEM and KYKY; China) was utilized to analyze the heterogeneous catalyst. A thermoanalyzer (TG 209F3 NETZSCH) was used to analyze thermal behavior in N2 between ambient temperature and 350 °C. A TriStar II Plus surface area and porosity analyzer operating at 77 K was used to quantify N2 adsorption–desorption isotherms (BET). Magnetization measurements were carried out with a Lakeshore (model 7407) under magnetic fields at room temperature. The reaction progress and purity of the materials were investigated by thin-layer chromatography (TLC). Melting points were measured and uncorrected by Electro thermal 9100 apparatus. IR spectra were recorded on a Perkin-Elmer 240-C spectrophotometer operating with KBr discs. ¹H and ¹³C NMR spectra were also provided with a Bruker AC (250 MHz for ¹H NMR and 62.5 MHz for ¹³C NMR) in DMSO-d₆ as solvents and tetramethylsilane (TMS) as the internal standard.

Fabrication of nanocatalyst

Fabrication of NiO nanoparticles

To prevent the creation of contaminants in the finished product, an aqueous solution of nickel nitrate (0.5 M) (100 mL) in deionized distilled water was used as the solvent to prepare NiO nanoparticles. Then an appropriate amount of NaOH solution with a concentration of 0.1 M was added dropwise in the aforementioned solution until the pH value of the mixture reached 9–10 and the obtained mixture was stirred for two hours. Following the completion of the reaction, the precursor was centrifuged for 15 min at 3000 rpm, cleaned several times with distilled water and ethanol, dried for 8 h at 80 °C, and then calcined for 4 h at 400 °C⁴⁸.

Fabrication of NiO@Bentonite nanocatalysts as heterogeneous catalyst

2.0 mmol (0.149 g) of the NiO nanoparticles and 0.593 g of the bentonite have been dispersed in deionized water under ultra-sonication for 30 min. The mixture was transferred to a glass vial to be irradiated in the microwave. Subsequently, the glassy vial was placed straight into the 180 W microwave oven and was exposed to microwave radiation for 90 min. After that, the finished product was washed with distilled water and was dried for ten hours at 80 °C.

General procedure for synthesizing 1,8-dioxo-decahydroacridine derivatives

In a round-bottom flask, the three-component reaction of aromatic aldehyde (4 mmol), aniline (4 mmol), and at 40 °C, dimedone (8 mmol) was used in a solvent of water: ethanol (1:1) with a 15-weight percent NiO@Bentonite catalysis. Thin-layer chromatography (TLC) was used to examine the reaction's progress in ethyl acetate: solvent n-hexane (3:7). Following the end of the reaction, to washout the produced precipitates, appropriate amount of ethanol was introduced into the flask and stirred on a magnetic stirrer for a couple of minutes to separate the product from the insoluble catalyst particles, Then, the obtained mixture was filtered to have the product pass through the paper filter while keeping the catalyst on the paper. The catalyst was several times washed with ethanol and then dried in an oven at 40 °C. Following the dehydration, the recycled catalyst was used for the next reactions. The pure product could be obtained following the evaporation of the ethanol in the mixture and washing the precipitates with another 15 mL of ethanol.

Selected spectral data

3,3,6,6-tetramethyl-9-(3-nitrophenyl)-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4a): Yield: 97%, m.p: 193–196 °C; IR (KBr, cm⁻¹): 3087, 2961, 2872, 1597, 1527, 1460, 1378, 1042–1347, 703–893; ¹H NMR (250 MHz, DMSO-d₆, ppm) δ: 1.03 (s, 6H, 2CH₃), 1.10 (S, 6H, 2CH₃), 2.33 (brs, 8H, 4CH₂), 5.41 (s, 1H, CH), 7.40–7.53 (m, 5H, H-Ar), 7.77 (brs, 1H, H-Ar), 7.94 (brs, 3H, H-Ar); ¹³C NMR (62.5 MHz, DMSO-d₆) δ: 27.8, 29.2, 32.8, 33.7, 43.6, 48.0, 50.2, 51.7, 101.8, 102.7, 115.17, 121.7, 122.4, 124.2, 130.1, 130.8, 134.8, 136.5, 137.5, 145.7, 148.6, 148.7, 149.2, 188.8, 197.3 ppm.

9-(4-hydroxy-3-methoxyphenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4b): m.p.: 221–225 °C; Yield: 94%; ¹H NMR (250 MHz, DMSO-d₆) δ: 1.03 (s, 12H, 4CH₃), 2.30 (brs, 8H, 4CH₂), 3.61 (s, 3H, OCH₃), 5.75 (s, 1H, CH), 6.33–6.63 (m, 8H, H-Ar), 8.61 (brs, 1H, OH).

9-(4-chlorophenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4c): m.p.: 148–149 °C; Yield: 92%; ¹H NMR (250 MHz, DMSO-d₆) δ: 1.00 (s, 12H, 4CH₃), 2.30 (brs, 8H, 4CH₂), 5.91 (s, 1H, CH), 6.84–7.21 (m, 9H, H-Ar).

9-(4-hydroxyphenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4d): m.p.: 282–284 °C; Yield: 95%; ¹H NMR (250 MHz, DMSO-d₆) δ: 0.69 (s, 6H, 2CH₃), 0.84 (s, 6H, 2CH₃), 1.68–2.48 (m, 8H, 4CH₂), 4.92 (s, 1H, CH), 6.58–7.56 (m, 9H, H-Ar), 9.05 (brs, 1H, OH).

9-(4-methoxyphenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4e): m.p.: 217–218 °C; Yield: 98%; ¹H NMR (250 MHz, DMSO-d₆) δ: 1.04 (s, 6H, 2CH₃), 1.08 (s, 6H, 2CH₃), 2.17–2.62 (m, 8H, 4CH₂), 3.68 (s, 3H, OCH₃), 6.78 (d, J = 7.5 Hz, 2H, H-Ar), 6.98 (d, J = 7.5 Hz, 2H, H-Ar), 7.16–7.40 (m, 5H, H-Ar).

9-(5-bromo-2-hydroxyphenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4f.): m.p.: 247–251 °C; Yield: 98%; ¹H NMR (250 MHz, DMSO-d₆) δ: 0.94 (s, 6H, 2CH₃), 1.01 (s, 6H, 2CH₃), 1.97–2.55 (m, 8H, 4CH₂), 5.00 (s, 1H, CH), 6.90–7.26 (m, 8H, H-Ar), 10.62 (brs, 1H, OH).

9-(2-hydroxynaphthalen-1-yl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4g): m.p.: 288–290 °C; Yield: 99%; ¹H NMR (250 MHz, DMSO-d₆) δ: 0.96 (s, 6H, 2CH₃), 1.05 (s, 6H, 2CH₃), 2.55–2.65 (m, 8H, 4CH₂), 5.33 (s, 1H, CH), 7.19 (dd, J₁ = 5 Hz, J₂ = 2.5 Hz, 2H, H-Ar), 7.38 (brs, 4H, H-Ar), 7.72 (d, J = 10 Hz, 2H, H-Ar), 7.81 (t, J = 3.7 Hz, 1H, H-Ar), 8.18 (dd, J₁ = 3.7 Hz, J₂ = 2.5 Hz, 2H, H-Ar), 10.48 (brs, 1H, OH).

3,3,6,6-tetramethyl-9-(2-nitrophenyl)-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4h): m.p.: 188–190 °C; Yield: 97%; ¹H NMR (250 MHz, DMSO-d₆) δ: 1.01 (s, 6H, 2CH₃), 1.05 (s, 6H, 2CH₃), 2.05 (brs, 4H, 2CH₂), 2.20 (brs, 4H, 2CH₂), 5.71 (s, 1H, CH), 7.13 (d, J = 5 Hz, 1H, H-Ar), 7.33–7.45 (m, 5H, H-Ar), 7.60 (d, J = 7.5 Hz, 1H, H-Ar), 7.78 (d, J = 10 Hz, 1H, H-Ar), 7.87 (d, J = 7.5 Hz, 1H, H-Ar),

9-(4-(dimethylamino)phenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4i): m.p.: 168–170 °C; Yield: 99%; ¹H NMR (250 MHz, DMSO-d₆) δ: 1.02 (s, 12H, 4CH₃), 2.29 (brs, 8H, 4CH₂), 2.79 (s, 6H, 2CH₃), 5.73 (s, 1H, CH), 6.59 (brs, 5H, H-Ar), 6.77 (d, J = 5 Hz, 2H, H-Ar), 6.95 (d, J = 5 Hz, 2H, H-Ar).

9-(2-hydroxy-3-methoxyphenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4j): m.p.: 228–229 °C; Yield: 94%; ¹H NMR (250 MHz, DMSO-d₆) δ: 0.96 (s, 6H, 2CH₃), 1.02 (s, 6H, 2CH₃), 1.97–2.47 (m, 8H, 4CH₂), 3.75 (s, 3H, OCH₃), 5.01 (s, 1H, CH), 6.50 (d, J = 5.7 Hz, 4H, H-Ar), 6.75–6.88 (m, 4H, H-Ar), 10.33 (brs, 1H, OH).

9-(4-bromophenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4 k): m.p.: > 300 °C; Yield: 98%; ¹H NMR (250 MHz, DMSO-d₆) δ: 1.00 (s, 12H, 4CH₃), 2.30 (brs, 8H, 4CH₂), 5.90 (s, 1H, CH), 6.82–7.57 (m, 9H, H-Ar).

9-(3-hydroxyphenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4l): m.p.: 231–233 °C; Yield: 94%; ¹H NMR (250 MHz, DMSO-d₆) δ: 0.92 (s, 6H, 2CH₃), 1.04 (s, 6H, 2CH₃), 2.30 (brs, 4H, 2CH₂), 2.38–2.48 (m, 4H, 2CH₂), 5.52 (s, 1H, CH), 6.52 (s, 1H, H-Ar), 6.99 (brs, 2H, H-Ar), 7.16–7.22 (m, 4H, H-Ar), 7.39 (d, J = 2.5 Hz, 2H, H-Ar).

9-(2-chlorophenyl)-3,3,6,6-tetramethyl-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4m): m.p.: 220–223 °C; Yield: 92%; ¹H NMR (250 MHz, DMSO-d₆) δ: 1.01 (s, 6H, 2CH₃), 1.08 (s, 6H, 2CH₃), 2.03–2.48 (m, 8H, 4CH₂), 5.58 (s, 1H, CH), 6.88 (d, J = 3.7 Hz, 2H, H-Ar), 7.07 (brs, 5H, H-Ar), 7.28 (brs, 2H, OH).

3,3,6,6-tetramethyl-10-phenyl-9-(3,4,5-trimethoxyphenyl)-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione (4n): m.p.: 243–244 °C; Yield: 95%; ¹H NMR (250 MHz, DMSO-d₆) δ: 1.04 (s, 12H, 4CH₃), 2.32 (brs, 8H, 4CH₂), 3.62 (s, 9H, 3OCH₃), 5.82 (s, 1H, CH), 6.23 (brs, 5H, H-Ar), 6.36–6.42 (m, 2H, H-Ar).

Results and discussion

Characterization of Co-MOF@Ag₂O nanocomposite

Figure 3a and b show the XRD diffractograms of NiO nanoparticles and NiO@Bentonite nanocomposite, respectively. From Fig. 3a, it can be observed that the diffraction peaks of pure NiO catalyst located at 2θ = 37.2°, 43.2°, 62.7°, 75.3° and 79.2° correspond to (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) respectively (PDF#89-7390)⁴⁹. In addition, the single particle sizes of all the NiO are estimated to be about 19.14 nm by Scherer Formula. XRD peaks at 6.1°, 19.7°, 20.7°, 26.6°, 28.44°, 36.5°, 42.3°, 45.8°, 50.0°, 54.8°, 59.8°, 68.1°, 73.5°, 75.7° and 91.1° revealed of bentonite (Fig. 3b). The main characteristic peak of bentonite occurred at 6.1°. This peak represents the d₀₀₁ reflection of bentonite, which is consistent with previously reported work. Strong peaks at 20.7° and 26.6° indicate SiO₂ crystallites in bentonite. The peak at 59.8° represents an octahedral sheet of bentonite structure⁵⁰.

Fig. 3.

XRD pattern of (a) NiO nanoparticles and (b) NiO@Bentonite nanocomposite.

FESEM images of bentonite-based NiO nanocatalyst are shown in Fig. 4. FESEM image of NiO@Bentonite nanocomposite exhibited a hexagon shape with an average particle size around 250 nm of bentonite. It showed that NiO is homogeneously distributed over the surface of bentonite.

Fig. 4.

(a) FESEM image of NiO@Bentonite nanocomposite and (b) high resolution FESEM image of NiO@Bentonite nanocomposite.

EDS analysis of NiO@Bentonite nanocatalyst confirms the presence of O, Si, Fe, K, Na, Ca, Mg, and Ni in the catalyst (Fig. 5). EDS mappings further demonstrated that all elements were evenly distributed throughout the nanostructure. These results illustrated that NiO@Bentonite nanocatalyst was successfully synthesized (Fig. 5).

Fig. 5.

EDX spectra of NiO@Bentonite nanocomposite.

Mapping analysis of the surface of the nanocatalyst showed that all elements are evenly distributed throughout the nanostructure. These results indicated the successful synthesis of NiO@Bentonite nanocatalyst (Fig. 6).

Fig. 6.

Elemental mapping of NiO@Bentonite nanocomposite.

Surface areas of NiO@Bentonite nanocomposite were determined via the BET analysis. The BET surface area, corresponding pore volume and mean pore diameter of the nanocomposite were computed to be 111.68 m² g⁻¹, 0.4019 cm³ g⁻¹, and 14.396 nm. Therefore the high surface area and pore volume of NiO@Bentonite nanocomposite can provide larger space and enhance the catalytic activity of the catalyst.

Figure 7 illustrates the magnetic features of NiO@Bentonite nanocomposite that were measured by the application of VSM, with a plot that seemed to be positioned between magnetization (M) versus magnetic field (H) of 15,000 to − 15,000 at room temperature. Various parameters such as maximum magnetization (Mm), coercivity (Hc), and remanent magnetization (Mr) are 0.2316 emu/g, 50 (Oe), and 0.0048 emu/g, respectively. Magnetization in the case of NiO@Bentonite nanocatalyst seemed to linearly increase, while saturation was not perceived to face any alteration as the magnetic intensity was heightened Up to 15,000 Oe. This observation could be due to electronic arrangements caused by uncompensated spins on the surface of the sample, indicating the presence of superparamagnetic. The magnetic property will change if a decrease in the particle size is induced, which is acknowledged as superparamagnetic. Hence, the observed superparamagnetic behavior is probably due to the decrease in particle size (19.14 nm) which was also obtained during the XRD results. The method used in the production of NiO nanoparticles can lead to a reduction in particle size and show superparamagnetic behavior⁴⁹.

Fig. 7.

VSM magnetization curves of NiO@Bentonite nanocomposite.

As shown in Fig. 8, the thermogravimetric diagrams from DSC have been displayed for observation. The DSC curve has demonstrated that the decomposition of NiO@Bentonite nanocomposite can be revealed through several steps: Two stages of weight loss have been detected through the DSC curve, which has been accompanied by one exothermic and two endothermic peaks in the DSC curve. A small exothermic peak observed in the DSC curve indicates the occurrence of an exothermic reaction at 27.8 °C due to moisture evaporation and water decomposition/dehydration. The strong endothermic peaks observed in the DSC curve at around 36 °C with an endothermic peak at 100 °C could be related to the removal of water. Therefore, obtained data show high thermal stability in elevated temperatures.

Fig. 8.

Thermogravimetric analysis of NiO@Bentonite nanocomposite.

The FT-IR spectra of NiO, bentonite, and NiO@Bentonite are shown in Fig. 9. The FTIR spectra of NiO nanoparticles (NiO NPs) (curve 9a) showing characteristic absorptions at 468, 627, and 649 cm⁻¹ that are attributing to the nickel oxygen (Ni–O) bond stretching vibrational mode and shows the formation of NiO nanoparticles. The broadness of peaks indicates that the NiO NPs are crystalline in nature⁵⁰. Also, the peaks at 1420, 1575, and 3444 cm⁻¹ may be because of bending and stretching vibrations of adsorbed water molecules absorbed on the nanoparticle's surface from the open atmosphere when FT-IR analysis was carried out⁵¹.

Fig. 9.

IR (KBr, υ/cm⁻¹) curve of (a) Ag₂O, (b) 2,6-pyridinedicarboxylic acid linker, (c) synthesized Co-MOF and (d) Co-MOF@Ag₂O nanocomposite.

The spectra of the bentonite (curve 9b) are similar to that of amorphous silica, showing absorption peaks at 468, 525, 796, 1039, and 1075 arising from the bending and stretching of ${\text{SiO}}_4^{2-}$ tetrahedra⁵². The absorption strips at 468 and 525 cm⁻¹ are attributed to Si–O–Si and Al–O–Si bending vibrations, respectively. The peak at 620 cm⁻¹ is due to coupled Al–O and Si–O out-of-plane vibrations⁵². The absorption observed at 796 cm⁻¹ corresponds to Tetrahedral SiO₄ bending. The band observed at 922 cm⁻¹ is attributed to the OH deformation mode of Al–OH–Al or Al–Al–OH. The absorption of sharp strips at 1039 cm⁻¹ is a characteristic of layered silicate montmorillonite minerals and is attributed to the triply degenerate Si–O stretching (in-plane) vibration. The absorption observed at 1638 cm⁻¹ corresponds to the asymmetric OH stretching (deformation state) of water and is a structural part of the mineral. A single broad absorption at 3442 cm⁻¹ followed by a strong absorption observed at 3621 cm⁻¹ in the mineral is attributed to water at the stretching frequencies of the mineral and OH silanol groups (Si–OH–Al), respectively⁵³. This indicates the possibility of the hydroxyl linkage between octahedral and tetrahedral layers.

Absorption 649, 627 and 468 cm⁻¹ of NiO NPs (curve 9a) in the IR spectrum of the catalyst appeared in the areas 693, 629, and 468 cm⁻¹ (curve 9c). Also, absorptions 3621, 3442, 1039, 922, 796, 525 and 468 cm⁻¹ of bentonite (curve 9b) appeared in the IR spectrum of the catalyst in the areas 3623, 3444, 1037, 918, 795, 525 and 468 cm⁻¹ (curve 9c). The presence of absorptions of NiO NPs and bentonite in the IR spectrum of the catalyst indicates their presence in the structure of the catalyst. Also, the partial displacement of the adsorption of NiO NPs and bentonite in the spectrum of the catalyst indicates the interaction of nanoparticles with bentonite and the formation of the catalyst.

Synthesizing 1,8-dioxo-decahydroacridine derivatives in presence of NiO@Bentonite catalyst

The synthesis of 1,8-dioxo-decahydroacridine is carried out through a three-component one-stage reaction of different aromatic aldehydes, dimedone, and a nitrogen source like aniline in a solvent of ethanol: water (1:1) at 40 °C in presence of 20 wt.% NiO@Bentonite catalyst. The obtained products were solid (Fig. 10).

Fig. 10.

Preparation reaction of 3,3,6,6-tetramethyl-9,10-diphenyl-1,8-dioxodecahydroacridine in the presence of NiO@Bentonite.

In this research, the NiO@Bentonite nanocomposite was investigated as a heterogeneous environment-friendly catalyst for increasing the rate and yield of the reaction in synthesizing 1,8-dioxo-decahydroacridines. In this regard, to optimize the reaction conditions, the effects of different parameters such as solvent type, temperature, and catalyst dosage on the model reaction of 3-nitrobenzaldehyde with dimedone and aniline as an amine derivative were investigated.

To optimize the solvent for the studied reaction, first, the model reaction was tested with different solvents, including water, ethanol, ethanol: water (1:1), methanol, methanol: water (1:1), acetonitrile, and free-solvent conditions in presence of 15 wt.% NiO@Bentonite nanocatalyst at reflux temperature. According to the results, the mixture of ethanol: water (1:1) solvents was found to be the best solvent for this reaction. Then, to check for the effect of catalyst dosage, amounts of 5–25% catalysts were applied, also, the progress of the reaction was investigated in free-catalyst condition according to Table 1, the best result was achieved with a catalyst dosage of 20 wt.% (see 9th row in Table 1). Once finished with optimizing the solvent type and catalyst dosage, the reaction temperature was subjected to optimization. For this purpose, the model reaction was conducted at three temperatures, namely ambient temperature, 40 °C, and 80 °C in the presence of the optimal amount of 20 wt.% nanocatalyst and optimal solvent of EtOH: H₂O (1:1). The results obtained in Table 1 show that 40 °C is the optimal temperature (Table 1, entry 14). Investigations indicated that any increase in the catalyst dosage tends to enhance the reaction performance while shortening the reaction time. In the meantime, no further improvement in the reaction yield was seen when the catalyst dosage was increased beyond 20 wt.%.

Table 1.

Optimizing reaction conditions for the preparation of 1,8-dioxodecahydroacridine derivatives in the presence of Ni@Bentonite catalysis.

Entrya	Catalyst	Solvent	Tem (°C)	Time (min/h)	Yield (%)b
1	NiO@Bentonite 0.15 wt. %	EtOH	Reflux	8 min	84
2	NiO@Bentonite 0.15 wt. %	EtOH: H2O	Reflux	6 min	97
3	NiO@Bentonite 0.15 wt. %	CH3OH	Reflux	12 min	69
4	NiO@Bentonite 0.15 wt. %	CH3OH: H2O	Reflux	8 min	75
5	NiO@Bentonite 0.15 wt. %	CH3CN	Reflux	10 min	75
6	NiO@Bentonite 0.15 wt. %	H2O	Reflux	12 min	88
7	NiO@Bentonite 0.15 wt. %	–	Reflux	17 min	83
8	NiO@Bentonite 0.25 wt. %	EtOH: H2O	Reflux	3 min	97
9	NiO@Bentonite 0.2 wt. %	EtOH: H2O	Reflux	3 min	97
10	NiO@Bentonite 0.1 wt. %	EtOH: H2O	Reflux	10 min	80
11	NiO@Bentonite 0.05 wt. %	EtOH: H2O	Reflux	17 min	76
12	–	EtOH: H2O	Reflux	24 h	N. R
13	NiO@Bentonite 0.2 wt. %	EtOH: H2O	r.t	25 min	86
14	NiO@Bentonite 0.2 wt. %	EtOH: H2O	40	3 min	97
15	NiO@Bentonite 0.2 wt. %	EtOH: H2O	80	3 min	97

^aReaction conditions: 3-nitrobenzaldehyde (4 mmol), aniline (4 mmol), dimedone (8 mmol) and NiO@Bentonite catalyst (0.121 g, 20% wt.) under different conditions.

^bIsolated yields after purification.

In summary, the optimal conditions for the synthesis of 1,8-dioxo-decahydroacridine were identified as follows. Aldehyde, aniline and dimedone reacted in the molar ratio of 1:1:2 in the presence of 20 wt% NiO@Bentonite catalyst in ethanol: water (1:1) as solvent at 40 °C.

To demonstrate the efficiency of the NiO@Bentonite catalyst under the mentioned optimal conditions, derivatives of 1,8-dioxo-decahydroacridine were prepared and extracted (Table 2). In this research, aromatic aldehydes with donating and withdrawing groups were used. Considering the results, the electronic effects were found to impose no significant impact on the yield and rate of the reaction produced compounds were identified based on their melting point and FTIR analysis. In addition, to further ensure the findings, the 3,3,6,6-tetramethyl-9-(3-nitrophenyl)-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H) diene (4a) was characterized through the ¹H NMR and ¹³C NMR spectroscopy.

Table 2.

Preparation of 1,8-dioxodecahydroacridine derivatives using NiO@Bentonite nanocomposite as catalyst.

Entrya	R (aldehyde)	Product	Time (min)	Yield (%)b	$m.p.{(}^{\circ }\mathrm{C})$
					Found	Reported [ref.]
1	3-NO2C6H4-	4a	3	97	193–196	194–19654
2	3-OCH3-4OHC6H3	4b	2	94	221–225	224–22655
3	4-ClC6H4-	4c	5	92	148–149	148–15154
4	4-OHC6H4-	4d	3	95	282–284	284–28640
5	4-OCH3C6H4-	4e	3	98	217–218	219–22154
6	5-Br-2-OHC6H3-	4f	10	98	247–251	248–25050
7	2-OH-1-naphthaldehyde	4g	12	99	288–290	New
8	2-NO2C6H4-	4h	4	97	188–190	187–18942
9	4-N(CH3)2C6H4-	4i	2	99	168–170	166–16842
10	2-OH-3-OCH3C6H3-	4j	2	94	228–229	227–23054
11	4-BrC6H4-	4k	6	98	> 300	> 30040
12	3-OHC6H4-	4l	4	94	231–233	230–23242
13	2-ClC6H4-	4m	4	92	248–250	249–25256
14	3,4,5-(OCH3)3C6H2-	4n	2	95	222–224	New

^aReaction conditions: 3-nitrobenzaldehyde (4 mmol), aniline (4 mmol), dimedone (8 mmol) and NiO@Bentonite catalyst (0.121 g, 20% wt.) under different conditions.

^bIsolated yields after purification.

Considering the great environmental and commercial importance of the recyclability of catalysts, we studied the recyclability and reusability of the heterogeneous NiO@Bentonite catalyst in the reaction of 3-nitrobanzaldehyde, dimedone, and aniline under optimal conditions (ethanol:water (1:1) solvent, 40 °C, 20 wt.% catalyst dosage for producing the 4a compound. To this end, upon the end of the reaction, an appropriate amount of ethanol was added to the reaction vessel, and the mixture was subjected to stirring on a magnetic stirrer for a few minutes. Given the insolubility of the catalyst in hot ethanol, it could be easily separated through a simple filtration. The separated catalyst was several times washed with the ethanol-and-water solvent and then dried in the oven at 40 °C for 8 h before reusing. Finally, it was figured out that the catalyst can be easily reused with no significant change in its catalytic activity for at least 4 successive rounds (Fig. 11).

Fig. 11.

Recovery diagram of NiO@Bentonite catalyst in the synthesis of 1,8-dioxodecahydroacridine derivatives.

Table 3 presents some of the previously reported methods for the synthesis of the 3,3,6,6-tetramethyl-9-(3-nitrophenyl)-10-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H) diene (4a), where one can compare them in terms of the required reaction time, use/no use of organic solvents, and the reaction yield. The limitations suffered by some of the proposed methods (e.g., reflux temperature, long reaction time, etc.) motivated us to develop and present a new procedure for the synthesis of acridines.

Table 3.

Comparison of different methods for the preparation of 1,8-dioxodecahydroacridines.

Entry	Catalyst	Amount of catalyst	Conditions	Time (min/h)	Yield (%)	Ref
1	SO42−/ZrO2	15 wt%	Ethanol, 70 °C	8–15 h	90	43
2	β‐cyclodextrin	10 mol%	H2O, 80 °C	62 min	93	58
3	CTAB	10 mol%	H2O, Reflux	1.5 h	80	59
4	Amberlyst-15	200 mg	CH3CN, Reflux	5 h	81	28
5	Cu-doped ZnO nanocrystal	10 mol%	Solvent-free, 90 °C	1.5 h	90	60
6	CoFe2O4/OCMC/Cu (BDC)	0.002 g	EtOH, r.t	10 min	97	61
7	ChCl: Urea	5 mL	ChCl: Urea, 80 °C	30 min	95	40
8	Fe3O4@SiO2-HBP-FeCl3	9 mg	Solvent-free, 80 °C	30	95	62
9	Pt NPs@rGO	8 mg	EtOH/H2O, 90 °C	60 min	94	63
10	Salicylic acid	20 mol%	PEG-200, 80 °C	15 min	90	41
11	MNPs-N-propyl-benzoguanamine-SO3H	6 mg	EtOH: H2O (3:1), reflux	20 min	94	42
12	NiO@Bentonite	20 wt.%	EtOH: H2O (1:1), 40 °C	3 min	97	This work

Based on the results of this table, the proposed methodology in the present article is preferable for its shorter reaction time. Moreover, with the synthesis of the NiO@Bentonite catalyst being done on a natural bed of bentonite and the acridine synthesis process being practiced in an almost green solvent of ethanol–water, the proposed method can be acknowledged as an environmentally friendly method of green chemistry.

The proposed mechanism for the studied reaction is shown in the following scheme. As a Lewis acid, NiO@Bentonite catalyst boosts the electrophilic effects of carbonyl groups on the dimedone and aldehyde by establishing strong dative coordinate bonds, pushing the oxygen atom of the dimedone toward taking the relevant enolic form and hence activating the oxygen atom of the aldehyde in the vicinity of this catalyst, making it a good electrophile center. Next, the enolic dimedone attacks, via its carbon atom to the carbon atom of the activated aldehyde group. Upon the Knoevenagel compressive reaction, the unsaturated α and β intermediates (alkene intermediates) are produced. Afterward, the second dimedone molecule attacks, via its active methylene and produces unsaturated α and β intermediates following a proton exchange. Finally, the aniline molecule comes into play with a nucleophilic attack, by which a water molecule is eliminated and an intermediate is formed that after tautomerization (imine to enamine and enol to keto), intramolecular ring closure and elimination of a water molecule takes the form of the final 1,8-dioxo-decahydroacridine product (Fig. 12).

Fig. 12.

Formation mechanism of 1,8-dioxodecahydroacridine product.

Conclusion

In the present work, we have successfully functionalized the surface of bentonite via microwave irradiation by NiO nanoparticles as a heterogeneous catalyst and characterized by XRD, FESEM, EDX, BET, VSM, and TGA techniques. The synthesized NiO@Bentonite nanocatalyst was used in a three-component reaction to prepare 1,8-dioxo-decahydroacridine from three main reactants, namely dimedone, aldehyde and aniline. Optimization of reaction conditions showed the best solvent, temperature, and catalyst amount was H₂O: EtOH (1:1), 40 °C, and 20 wt. % respectively. In the presence of the NiO@Bentonite catalyst, this reaction was carried out in a green environment and the product was easily separated from the reaction mixture, thus reducing the cost and time required for the product. In addition, recyclability, reusability, and environment-friendliness are other advantages of this procedure.

Author contributions

E.F.: Investigation Roles/Writing-original draft. E.S.: conceptualization, data curation, formal analysis, investigation, methodology, project administration, resources, supervision, validation, visualization, writing-review & editing. M.Y.: formal analysis, methodology, supervision, validation, visualization, writing-review & editing. This article has been read by all authors and agreed to be published.

Funding

The authors received no specific funding for this work.

Data availability

The original contributions presented in the study are included in the article/Supplementry material; further inquiries can be directed to the corresponding author.

Competing interests

The authors declare no competing interests.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-71898-y.