Magnetic cobalt oxide supported organosilica-sulfonic acid as a powerful nanocatalyst for the synthesis of tetrahydrobenzo[a]xanthen-11-ones

A novel core–shell structured magnetic cobalt oxide supported organosilica-sulfonic acid (Co3O4@SiO2/OS-SO3H) nanocomposite is prepared through a low-cost, simple, and clean method. The characterization of Co3O4@SiO2/OS-SO3H was performed by using Fourier transform infrared (FT-IR) spectroscopy, thermal gravimetric analysis (TGA), powder X-ray diffraction (PXRD), energy dispersive X-ray (EDX) spectroscopy, scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and transmission electron microscopy (TEM). The TGA and FT-IR results illustrate the high stability of the designed nanocomposite. The SEM image showed a size of about 40 nm for the Co3O4@SiO2/OS-SO3H nanoparticles. Furthermore, according to the result of VSM analysis, the saturation magnetization of this nanocomposite was about 25 emu/g. This novel material was used as an efficient nanocatalyst for the synthesis of biologically active tetrahydrobenzo[a]xanthen-11-one derivatives. These products were obtained in high to excellent yields under green conditions. The recoverability and reusability of this catalyst were also investigated under applied conditions.

It is also important to note that, in recent years, the use of metal and metal oxide-based heterogeneous catalysts in organic transformations has been developed [59][60][61][62] .However, some of these catalytic systems suffer from problems of non-recoverability of catalyst, harsh reaction conditions, and the use of toxic organic solvents.
In view of the above and according to our experience in the preparation of magnetic nanocatalysts, herein, for the first time, a novel core-shell structured magnetic cobalt oxide supported organosilica-sulfonic acid (Co 3 O 4 @ SiO 2 /OS-SO 3 H) nanocomposite is successfully prepared through a simple method.This namomaterial contains the advantages of magnetic nanoparticles such as easy separation and also the advantages of heterogeneous catalysts such as easy recoverability.The Co 3 O 4 @SiO 2 /OS-SO 3 H nanocomposite was characterized by using TGA, FT-IR, VSM, SEM, TEM, PXRD, and EDX analyses.The catalytic efficiency of this material was studied in the synthesis of biologically active xanthenes giving the desired products in high to excellent yields.

Experimental
Synthesis of Co 3 O 4 @SiO 2 .Magnetic Co 3 O 4 nanoparticles were firstly synthesized through a reduction procedure as follows: CoCl 2 .6H 2 O (1.32 g) was added in 25 mL absolute EtOH while stirring at room temperature (RT).Then, ethanol-dissolved pluronic P123 (0.6 g in 7 mL EtOH) was added to the above solution.After complete mixing, NaBH 4 (1.47 g) was added and the resulted combination was stirred for 10 min at RT.The obtained material was magnetically separated and washed completely with warm EtOH and water to remove pluronic P123 and other impurities.The product was dried at 65 °C for 5 h and called magnetic cobalt oxide (Co 3 O 4 ).For the preparation of Co 3 O 4 @SiO 2 , the Co 3 O 4 NPs (1 g) were dispersed in ethanol (60 mL), while ammonia (5.3 mL, 60% wt%) was added drop-wise.Then, tetraethylorthosilicate (TEOS, 1 mL) was slowly added and the resulted mixture was stirred at RT for 16 h.Finally, the magnetic solid product was collected using a magnet, washed with water and ethanol, dried at 70 °C for 6 h, and called Co 3 O 4 @SiO 2 nanocomposite.
Preparation of Co 3 O 4 @SiO 2 /OS-SH.To prepare the Co 3 O 4 @SiO 2 /OS-SH MNPs, the Co 3 O 4 @SiO 2 nanomaterial (0.5 g) was added to a solution containing water (12 mL) and ethanol (50 mL).The resulting mixture was stirred at RT for 30 min.Then, ammonia (2 mL, 25% wt%) was added and it was stirred at RT for another 10 min.Next, tetraethylorthosilicate (TEOS, 1 mL) and 1,2-bis(triethoxysilyl)methane (BTEM, 1 mL) were added drop-wise, and the obtained mixture was stirred at RT for 16 h.The product was magnetically separated, washed with absolute ethanol and water, and dried at 70 °C for 6 h.After that, the resulting material (1 g) was dispersed in dried toluene (25 mL), while (3-mercaptopropyl)trimethoxysilane (0.7 mmol) was added.This mixture was refluxed for 24 h.The final product was magnetically separated, washed with absolute ethanol and water, dried at 70 °C for 6 h, and denoted as Co 3 O 4 @SiO 2 /OS-SH.
Preparation of Co 3 O 4 @SiO 2 /OS-SO 3 H.For this, Co 3 O 4 @SiO 2 /OS-SH (0.5 g) was first dispersed in methanol (20 mL).Then, hydrogen peroxide (5 mL, 35%) was added and the resulted mixture was stirred at ambient temperature for 24 h.The product was separated by using an external magnetic field.After that, this was added to a flask containing sulfuric acid solution (25 mL, 2 M) and stirred at RT for 3 h.The resulted material was separated, washed with ethanol and water, dried at 70 °C for 5 h, and denoted as Co 3 O 4 @SiO 2 /OS-SO 3 H.

Synthesis of tetrahydrobenzo[a]xanthen-11-one derivatives.
For this purpose, a mixture of benzaldehyde (1 mmol), dimedone (1 mmol), 2-naphthol (1 mmol), and Co 3 O 4 @SiO 2 -SO 3 H nanocatalyst (0.015 g) was stirred at 60 °C.The reaction progress was monitored by TLC.At the end of the reaction, the nanocatalyst was collected by a magnet and pure products were obtained after recrystallization in ethanol.

Results and discussion
The Co 3 O 4 NPs were synthesized through the reduction of CoCl 2 .6H 2 O in the presence of NaBH 4 .Next, Co 3 O 4 @ SiO 2 nanocomposite was prepared via a sol-gel process.In the following, the surface of Co 3 O 4 @SiO 2 was modified with a layer of organosilica through co-condensation of BTEM and TEOS to give Co 3 O 4 @SiO 2 /OS nanomaterial.
In fact, the SiO 2 /OS shell was prepared to protect Co 3 O 4 NPs against oxidation and destruction by acid.Moreover, the organosilica (OS) layer also increases the surface lipophilicity of the material improving the performance of the designed catalyst in organic reactions.After that, Co 3 O 4 @SiO 2 /OS was modified with MPTMS groups to deliver Co 3 O 4 @SiO 2 /OS-SH nanocomposite.Finally, to obtain the Co 3 O 4 @SiO 2 /OS-SO 3 H nanocatalyst, the SH moieties of the Co 3 O 4 @SiO 2 /OS-SH nanocomposite were oxidized to SO 3 H in the presence of H 2 O 2 (Fig. 1).
Figure 2 shows the FT-IR spectra of Co 3 O 4 , Co 3 O 4 @SiO 2 , Co 3 O 4 @SiO 2 /OS, and Co 3 O 4 @SiO 2 /OS-SO 3 H.The characteristic peaks at 3400 and 620 cm −1 , for all materials, are related to the O-H and Co-O bonds, respectively (Figs. 2a-d).The intense absorption peaks at 1081 and 928 cm −1 are, respectively, related to unsymmetrical and symmetrical vibrations of the Si-O-Si bonds (Figs.2b-d).Also, the peaks observed at 2825-2961 cm −1 can be assigned to the vibrations of aliphatic C-H bonds (Figs.2c,d).The peak observed at 1107 cm −1 is related to S=O bond, which is partially overlapped with the silica peaks, confirming the successful oxidation of SH-SO 3 H (Fig. 2d).
Figure 3  Also, the VSM analysis of the Co 3 O 4 @SiO 2 /OS-SO 3 H nanocomposite showed a saturation magnetization of about 25 emu/g (Fig. 5).This result proves the good magnetic properties of the prepared nanocomposite which is a very important characteristic in the catalytic processes.
The SEM image of the Co 3 O 4 @SiO 2 /OS-SO 3 H nanocomposite is demonstrated in Fig. 6.As shown, spongelike particles with spherical morphology and an average size of 40 nm are observed for this material.
TEM image of Co 3 O 4 @SiO 2 /OS-SO 3 H nanocomposite confirmed that the structure of the designed nanocomposite is almost spherical.Also, the image demonstrates a dark core (Co 3 O 4 ) enclosed by a gray silica/organosilica layer confirming the core shell structure of this nanocomposite (Fig. 7).
Thermal stability of the prepared nanocomposite was also investigated by thermal gravimetric analysis (TGA).The first weight loss below 150 °C is corresponded to the removal of adsorbed water and alcoholic solvents.The second weight loss at 151-220 °C is related to elimination of supported propansulfonic acid moieties.The main weight loss observed at 225-600 °C is corresponded to decomposition and removal of organic groups in the shell framework.These results confirm high thermal stability of the designed catalyst (Fig. 8).
After the characterization of the designed nanocomposite, its catalytic activity was studied in the synthesis of tetrahydrobenzo[a]xanthen-11-ones.To optimization of the conditions, the condensation between benzaldehyde, dimedone, and 2-naphthol was selected as a test model.The study showed that the presence of the catalyst is necessary for the progress of the reaction and by using 0.015 g of Co 3 O 4 @SiO 2 /OS-SO 3 H, the highest yield was  resulted (Table 1, entries 1-4).The study also showed that among EtOH, toluene, CH 2 Cl 2 , and CH 3 CN solvents, in EtOH the best result is obtained (Table 1, entry 3 vs.entries 5-7).Also, the study of the temperature effect illustrated that the best temperature for this process is 60 °C (Table 1, entry 3 vs.entries 8-11).Accordingly, the use of 0.015 g catalyst, ethanol solvent, and 60 °C were selected as optimum conditions (Table 1, entry 3).Next, the substrate scope of this catalytic system was studied under optimal conditions.This demonstrated that all aldehyde substrates containing both electron-donor and electron-acceptor substituents successfully react with dimedone and 2-naphthol in the presence of Co 3 O 4 @SiO 2 /OS-SO 3 H nanocatalyst to give corresponding tetrahydrobenzo[a]xanthen-11-one in high yields (Table 2).
Then, the recoverability and reusability of the Co 3 O 4 @SiO 2 /OS-SO 3 H nanocatalyst were investigated.For this, the reaction between benzaldehyde, dimedone, and 2-naphthol by using Co 3 O 4 @SiO 2 /OS-SO 3 H nanocatalyst under optimal conditions was selected as a test model.After completion of the process, Co 3 O 4 @SiO 2 /OS-SO 3 H was separated and reused in another reaction under the same conditions as the first run.These steps were repeated and the results displayed that Co 3 O 4 @SiO 2 /OS-SO 3 H can be recovered and reused for at least seven runs with no important decrease in its activity (Table 3).
To prove the stability of the catalyst structure during the reaction, the recovered catalyst, after the fifth run, was washed several times with ethanol and characterized by using EDX and PXRD analyses.The PXRD analysis of the recovered catalyst showed a pattern with seven peaks at 2θ = 23.2°,30.1°, 35.5°, 41.2°, 47.5°, 60° and 71.6° (SI, Fig. 1S).This result is in good agreement with the PXRD pattern of the fresh nanocatalyst confirming the high stability of the crystalline structure of magnetic Co 3 O 4 @SiO 2 /OS-SO 3 H NPs under applied conditions.
The EDX analysis of the recovered catalyst, after the fifth run, also showed the presence of expected C, O, Si, S, and Co elements in a wt% of 18.65, 42.18, 15.71, 0.9, and 22.56, respectively (SI, Fig. 2S).These results are  approximately the same as those of fresh catalyst confirming the high stability of the composition of the designed catalyst under applied conditions.The activity of Co 3 O 4 @SiO 2 /OS-SO 3 H nanocatalyst was compared with a number of catalysts reported in the synthesis of tetrahydrobenzo[a]xanthen-11-ones (Table 4).The result showed that our catalyst is better than others in terms of recovery times, reaction temperature, and yield of product.These findings can be ascribed to high stability, good lipophilicity, and the magnetic properties of the Co 3 O 4 @SiO 2 /OS-SO 3 H nanocatalyst.
A plausible mechanism for the synthesis of tetrahydrobenzo

Conclusion
In summary, a novel core-shell structured magnetic cobalt oxide supported organosilica-sulfonic acid nanocomposite was synthesized and called Co 3 O 4 @SiO 2 /OS-SO 3 H.The high thermal and chemical stability of the designed nanocomposite were confirmed by using EDX, TGA and FT-IR techniques.The SEM and TEM images illustrated a spherical morphology for this material.The good magnetic property of this material was  confirmed by VSM.The Co 3 O 4 @SiO 2 /OS-SO 3 H nanocomposite was used as an effective catalyst in the synthesis of tetrahydrobenzo[a]xanthen-11-ones under mild reaction conditions.The desired xanthene products were obtained in high to excellent yield and selectivity at a relatively short reaction time.The catalyst was also recovered and reused several times with no significant decrease in its efficiency.Some applications of Co 3 O 4 @SiO 2 / OS-SO 3 H in other chemical processes are underway in our laboratory.
illustrates the PXRD pattern of Co 3 O 4 @SiO 2 /OS-SO 3 H nanocomposite.As shown, the diffraction peaks of Co 3 O 4 NPs are appeared at 2θ = 23.2°,30.1°, 35.5°, 41.2°, 47.5°, 60° and 71.6° proving the high stability of crystalline structure of the Co 3 O 4 NPs during the synthesis of Co 3 O 4 @SiO 2 /OS-SO 3 H nanocomposite.The EDX analysis was used to investigate the presence of O, C, Co, Si, and S in the structure of the Co 3 O 4 @ SiO 2 /OS-SO 3 H nanocomposite.As shown in Fig. 4, the signals of C, O, Si, S and Co elements are clearly seen in weight% of 18.9, 42.62, 15.85, 0.93, 21.7 and 21.7, respectively.This confirms the successful incorporation/immobilization of cobalt oxide and silicasulfonic acid moieties into/onto the framework of the designed nanocomposite.
[a]xanthen-11-ones in the presence of Co 3 O 4 @ SiO 2 /OS-SO 3 H is shown in Fig. 9. Firstly, the aldehyde is activated by the catalyst to give intermediate [I].Next, the activated aldehyde and 2-naphthol react with each other via a Knoevenagel condensation to deliver intermediate [II].In the next step, the nucleophilic attack of dimedone to the intermediate [II], through a Michael-type addition, gives intermediate [IV].Finally, an intramolecular cyclization is performed to give the desired product.

Table 1 .
The effect of different parameters in the synthesis of tetrahydrobenzo[a]xanthen-11-one.

Table 4 .
Comparison the efficiency of the Co 3 O 4 @SiO 2 /OS-SO 3 H nanocatalyst with that of formerly reported catalytic systems in the synthesis of tetrahydrobenzo[a]xanthen-11-ones.