Immobilization of CdCl2 on filamentous silica nanoparticles as an efficient catalyst for the solvent free synthesis of some amidoalkyl derivatives

Abstract Due to desirable properties of filamentous silica nanoparticles (KCC-1), these compounds can be considered as appropriate support for the preparation of heterogeneous catalysts. In this study, immobilized CdCl2 on modified KCC-1 surfaces was fruitfully prepared. The structure of prepared catalyst was confirmed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), brunauer-emmett-teller (BET) and energy dispersive X-Ray (EDX) analysis. The efficiency of obtained solid acid catalyst was studied for the synthesis of some amidoalkyl naphthole and amidoalkyl naphthoquinones through a multicomponent reaction in solvent free conditions. The structure of all obtained products was characterized by analytical and spectroscopic methods.


Introduction
In recent years, great progress has been made in the manufacture of nonporous materials with different physical and chemical properties. Nano porous materials which can be morphology-controlled (like silica), have a critical role in the growth of technologies and solving challenges in the fields of catalyst, energy and chemical process. 1-4 A novel silica nanoparticles which have filamentous morphology (KCC-1) with a great physical and surface features, low toxicity, good stability, ease of modification and good inner surface availability, presents special activities in many fields like catalysis, sensors, solar energy absorption and biomedical usages. [5][6][7] KCC-1 nanoparticles (KCC-1 NPs) are similar to narrow sheets with a thickness of 3.5 to 5.2 nm; hence different names have been applied in the literature as fibrous, string, lamellar, dendritic, etc. Due to fibrous morphology of KCC-1 NPs, which give accessibility to them from all sides, they are very unique supports and allow that more materials such as metals, metal oxides and organic molecules to be loaded on their surfaces without blocking the pores (channels). [6][7][8][9] KCC-1 NPs are a great alternative to common silica materials such as SBA-15, MCM-41 and mesoporous silica nanoparticles (MSNs), Although it has lower surface area than SBA-15 and MCM-41, but the accessibility of KCC-1 surface area is significantly better than popular silica. 7,10-12 These surface characteristics turn them a terrific support for the preparation of different catalysts to increase the yield of reactions and reducing the consumption of energy which is accordance to green chemistry. Green chemistry has attracted a lot of attention in recent years particularly in preparation of complex organic molecules by multi-component reactions in water (as solvent) or solvent-free conditions. 13,14 Today, multicomponent reactions owing to possessing a short, simple and cost-effective pathway, offers significant merits rather stepwise methods for the synthesis of organic and pharmaceutical molecules. [15][16][17][18][19] Naphthoquinone, a category of quinones, is a significant heterocycle with some great properties like anticancer, antimalarial, antiallergic and antibacterial that have been caused to its extended applications in medical field. 20 Up to now, Amido alkyl b-naphthols have been synthesis with different homogeneous and heterogeneous catalysts such as SiO 2 -ZnCl 2 , 20 sulfonic acid functionalized imidazolium salts, 21 Trityl chloride, 22 nano-Co-[4-chlorophenyl-salicylaldimine-methyl pyranopyrazole]Cl 2, 23 ascorbic acid, 24 PEG-based dicationic acidic ionic liquids, 25 adipic acid 26 Sulfanilic acid-functionalized silicacoated magnetite nanoparticles, 27 Brønsted acidic ionic liquid, 28 PbS nanoparticles, 29 AgI nanoparticles, 30 ZS-1 Zeolite, 31 1-methylimidazolium tricyanomethanide 32 and Zirconocene dichloride. 33 In this study, immobilized CdCl 2 on functionalized KCC-1 (KCC-1/ECH-Meg/CdCl 2 ) was successfully synthesized and used as an efficient heterogeneous catalyst for the preparation of some amidoalkyl b-naphthol and amidoalkyl naphthoquinone derivatives under solvent free conditions.
In this regard, initially, KCC-1 NPs were prepared using tetraethylorthosilicate as a precursor of silica and CTAB as a driving agent. Then, epichlorohydrin and meglumine were applied to activate the KCC-1 surfaces and finally, CdCl 2 2.5H 2 O was added to immobilize the CdCl 2 on the surfaces of KCC-1 NPs (Scheme 1). Cadmium chloride is recognized as a low-price, mild, and moisture-resistant Lewis acid for different organic reactions. 34
FT-IR spectra were recorded in a spectrophotometer (Perkin-Elmer 781). The thermogravimetric analysis (TGA) curves were carried out using a V5.1A DUPONT 2000. To investigate the surface morphology of the catalyst FE-SEM images and EDX analyses provided by a Sigma ZEISS, Oxford Instruments Field Emission Scanning Electron Microscope. The morphology of prepared catalysts was investigated using TEM by a Philips CM 120, Netherlands and microscope with an accelerating voltage of 150 kV. X-ray diffraction was performed using a Philips X'pert MPD diffractometer with a Cu operating at a current of 100 mA and a voltage of 45 kV, with the Cu-Ka radiation (k ¼ 0.154056 nm) at the 2q range of 10-80 and scanning at the speed of 0.05 Scheme 1. Synthesis of amido alkyl derivatives in the presence of KCC-1/ECH-Meg/CdCl 2 degree per minute. NMR spectra (Bruker 400 MHz) were used to confirm product structure by DMSO-d 6 as a solvent on a Bruker DRX-400 spectrometer.

Synthesis of filamentous silica nanoparticles (KCC-1 NPs)
The synthesis of KCC-1 was done based on reported method. 35 Briefly, 0.75 g cetyl trimethyl ammonium bromide (CTAB) and 0.23 g urea were dissolved in 75 mL deionized water and stirred for 1 hour until all of the CTAB was dissolved. Then, a solution of tetraethoxysilane (TEOS) (3.75 g), cyclohexane (75 mL), and pentanol (5 mL) was added drop wise to the above solution of CTAB and urea in water. Obtained mixture was stirred at room temperature for an hour and then transferred into a Teflon lined hydrothermal reactor (200 mL). The reactor was placed in an oven at 120 C for 6 h. After that, the mixture was allowed to cool to room temperature. In next step, prepared solid was isolated by centrifugation (30 min, 6000 rpm) and washed with deionized water and ethanol thoroughly and dried for 12 h at 40 C. Eventually, the resulted solid was calcined at 550 C with 5 C ramping for 6 h in air to obtain KCC-1 NPs as product.

Synthesis of KCC-1 modified with epichlorohydrine and meglumine (KCC-1/ECH-Meg)
0.1 g of KCC-1 in 3.2 ml of epichlorohydrin/ethanol with a volume ratio of (1:2), was mixed for 5 hours at 60 C. Then, 1.1 g of meglumine was added and the resulted mixture was stirred for 6 hours at 60 C. Achieved mixture was let to cool to room temperature and the precipitate was separated by centrifugation (10 minutes, 6000 rpm) and washed with deionized water and ethanol and dried at 60 C for 24 hours.

Immobilization of CdCl 2 on KCC-1 NPs modified with epichlorohydrine and meglumine (KCC-1/ECH-Meg/CdCl 2 )
To immobilize CdCl 2 on KCC-1/ECH-Megl, 0.1 g of synthesized of KCC-1/ECH-Meg was dispersed in 3 ml absolute ethanol and the solution of CdCl 2 2.5H 2 O (0.5 g in 5 ml ethanol) was added to the mixture and refluxed for 18 h. The obtained solid was filtered by centrifugation (10 minutes, 6000 rpm) and washed with deionized water and ethanol and dried at 80 C for 8 hours.

Results and discussion
To prepare the intended catalyst, at first step, tetraethyl orthosilicate (TEOS) and cetyl trimethyl ammonium bromide (CTAB) were mixed in suitable solvents. Then epichlorohydrin and meglumine was used to activate the suface of obtained KCC-1. Finally, CdCl 2 2.5H 2 O salt was utilized to embaded the cadmium chloride on the surface of catalyst. As can be seen in Scheme 2, CdCl 2 bonded to hydroxyl groups of meglumine to produce a four dentate structure 38,39 (Scheme 2).
FT-IR analysis of KCC-1, KCC-1/ECH-Meglumine and KCC-1/ECH-Meglumine/CdCl 2 are shown in Figure 1. The spectrum of KCC-1 (Figure 1a) demonstrates a wide peak in the region of 3400-3500 cm À1 which is assigned to the stretching vibration of the OH groups and peaks in the region about of 1100 cm À1 , 804 and 461 cm À1 are related to the asymmetric stretching of Si-O-Si, symmetrical stretching of Si-OH and bending vibration of Si-O-Si respectively. In the spectrum (b), in addition to peaks related to KCC-1, the peak appeared in the region of 1092-1200 cm À1 is ascribed to C-N and C-O which is overlapped with symmetrical stretching of Si-O-Si. In the spectrum (c) as well as mentioned peaks, it shows a weak peak at about 700 cm À1 and a stronger peak at 1600 cm À1 which are relevant to the Cd-O and Cd-Cl linkages.
Surface morphology of KCC-1 and KCC-1/ECH-Meglumine/CdCl 2 was investigated by FESEM analysis (Figure 2a-d). According to obtained images, it is clear that KCC-1 NPs have spherical structure (a, b) and its morphology has not been changed after modification. Also, prepared catalyst comprise of uniform spherical nanoparticles without notable aggregation. Indeed, modification the KCC-1 NPs surfaces has significantly reduced the accumulation of spherical nanostructures (c, d). Furthermore, the EDX image of prepared catalyst displays the presence of nitrogen, cadmium and chlorine atoms in the structure of prepared catalyst (Figure 3).
TEM images (Figure 4) shows the morphology of KCC-1 NPs (a, b) and prepared catalyst (c, d). The images demonstrated the spherical needle-shaped morphology for KCC-1 and prepared catalyst.
The X-ray diffraction pattern of KCC-1 NPs and KCC-1/ECH-Meg/CdCl 2 can be seen in Figure 5. It is clear that the XRD pattern of KCC-1 displays a broad peak at 2h ¼ 23 confirming its amorphous structure. 40 In the spectrum of obtained catalyst, in addition to the mentioned peak at 2h ¼ 23 (with a lower intensity in comparison to bare KCC-1), there are some other peaks at 2h ¼ 17. 32 , 28.09 , 29.4 , 33.19 , 35.05 , 38.72 . 49.71 , 53.04 regarding to the 002, 100, 101, 102, 004 103, 110, 112 plates which are associated to the CdCl 2 stabilized on the KCC-1 surfaces.
The N 2 adsorption-desorption isotherms and also BJH-plots of synthesized catalyst are shown in Figure 6. The achieved results demonstrated that KCC-1 and KCC-1/ECH-Meg/CdCl 2 had 571.85 and 114.8 m 2 /g specific surface area respectively which is in accordance with other similar works. 41 Furthermore, the total pore volume of KCC-1 and KCC-1/ECH-Meglumine/CdCl 2 certified the mesoporous structure for them. Also the reduce of surface area and total pore volume of  KCC-1/ECH-Meglumine/CdCl 2 , demonstrated the successfully modification of raw KCC-1 (Table 1).

Catalytic performance of KCC-1/ECHMeg/CdCl 2
To attain the optimum conditions for the multicomponent synthesis of amidoalkyl derivatives, the reaction of benzaldehyde (0.1 mmol), b-naphthol (0.1 mmol) and acetamide (0.2 mmol) was intended as a model reaction. In this regard, all of the effectual parameters containing the solvent, temperature and catalyst were investigated. So, at first, the efficacy of the synthesized catalyst in different solvents was examined ( Table 2, entries 1-8). Obtained results show that when the reaction was performed under solvent-free conditions, the highest yield of product was achieved. In  continue, to optimize the amount of catalyst and also the reaction temperature, the model reaction was done in the presence of different amounts of catalyst under various temperatures. Obtained results show that the best yield of reaction was achieved using the 0.025 g of KCC-1/ ECH-Megl/CdCl 2 in the solvent free conditions at 100 C (entry 16).  In order to investigate the catalytic activity of KCC-1/ECH-Meg/CdCl 2 , the reaction was extended to substituted aromatic aldehydes, which was reacted with benzamide (or acetamide) and b-naphthol (or 2-hydroxy-1,4-naphthoquinone) under optimized conditions. As evident from the results ( Table 3), presence of electron-withdrawing groups on aromatic aldehydes causes to higher yield of product while bearing electron-donating groups resulted the lower yield of products. Overall, obtained results show the high activity and efficiency of prepared solid acid catalyst for the synthesis of some amidoalkyl derivatives. The structure of the synthesized compounds was confirmed with FT-IR, 1 H NMR and 13 C NMR analysis.
To compare the efficiency of prepared catalysts with the other catalysts in the similar works, the reaction conditions for the synthesis of product 5 g and 6a was considered. As can be seen in Table 4, prepared catalyst show the best yield of products after short reaction times both for the synthesis of amidoalkyl b-naphthol and amidoalkyl naphthoquinones (entry 15, 17).

Proposed mechanism
The following pathway presented the reaction mechanism for the synthesis of amidoalkyl b-naphthol and amidoalkyl naphthoquinones synthesis (Scheme 3). As can be seen, in first step, the nucleophilic attack of b-naphthol (or 2-hydroxy-1,4-naphthoquinone) to the activated aldehyde   Solvent-free/ 100 C, 6 min., 90% This work a 2-methylpyridinium trifluoromethane sulfonate b 3-methyl-1-sulfonic acid imidazolium tetrachloroferrate c magnetite nanoparticles-supported dodecylbenzenesulfonic acid d Magnetic nanoparticle-supported ionic liquid e Reaction conditions for the synthesis of 6a Scheme 3. Proposed mechanism for the synthesis of amidoalkyl b-naphthols and amidoalkyl naphthoquinones.

Reusability of the catalyst
Separation and reusing of the heterogeneous catalysts in the subsequent reaction cycles is one of the most important issues in applying the heterogonous catalysts. In the mentioned multicomponent reaction, after the completion of reaction, the reaction mixture was cooled to room temperature and washed with hot water to remove the remained amide. Then, 3 mL acetone was added to the reaction mixture and filtered off for separation the catalyst. Then the separated catalyst was washed several times with ethanol and water and dried at 80 C and utilized for reusing in repeated cycles. As can be seen in Figure 7, after 5 cycles of reaction, there is no considerable changes in the yield of product and the catalyst efficiency.

Conclusion
In this research, an efficient solid acid catalyst (KCC-1/ECH-Meg/CdCl 2 ) for the environmentally friendly synthesis of amidoalkyl b-naphthols and amidoalkyl naphthoquinones in solvent-free conditions is reported. The synthesized catalyst has high efficiency and reusability in the consequent cycles for the synthesis of amidoalkyl derivatives through the multicomponent reaction. Moreover, Green reaction conditions, short reaction time, easy, low cost and reusable catalyst caused to this method considered as an attractive and unique route for the synthesis of amidoalkyls in the presence of heterogeneous acid catalyst.