Magnetic silica/graphene oxide nanocomposite supported ionic liquid–manganese complex as a powerful catalyst for the synthesis of tetrahydrobenzopyrans

A novel magnetic silica/graphene oxide nanocomposite supported ionic liquid/manganese complex (Fe3O4@SiO2-NH2/GO/IL-Mn) is prepared, characterized and its catalytic application is investigated. The Fe3O4@SiO2-NH2/GO/IL-Mn catalyst was synthesized via chemical immobilization of graphene oxide on Fe3O4@SiO2 nanoparticles followed by modification with ionic liquid/Mn complex. This nanocomposite was characterized by using SEM, TGA, FT-IR, PXRD, EDX, TEM, nitrogen adsorption–desorption, and VSM analyses. The catalytic application of Fe3O4@SiO2-NH2/GO/IL-Mn was studied in the synthesis of tetrahydrobenzo[b]pyrans (THBPs) in water solvent at RT. This nanocatalyst was successfully recovered and reused at least eight times without a significant decrease in its activity.


Preparation of Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn
For this, 1 g of Fe 3 O 4 @SiO 2 -NH 2 /GO /IL was dispersed in 20 mL of DMSO under ultrasonic irradiation.Then, 0.5 mmol of Mn(OAc) 3 .4H 2 O salt was added and the resulting mixture was stirred at 80 °C for 2 h.The product was separated by using a magnet, washed with ethanol, dried at 70 °C for 6 h and denoted as Fe 3 O 4 @SiO 2 -NH 2 / GO/IL-Mn.

Synthesis of THBPs using Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn nanocatalyst
For this purpose, the Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn catalyst (0.8 mol%), malononitrile (1 mmol), benzaldehyde (1 mmol) and dimedone (1 mmol) were added in distilled water (10 mL).The resulting mixture was vigorously stirred at RT.The progress of the reaction was monitored by using TLC.After the completion of the reaction, the catalyst was separated by using a magnet.Then, ethyl acetate (20 mL) was added to the residue and the obtained mixture was washed three times with water in a decanter to remove some impurities.Finally, the obtained ethyl acetate solution was placed in an ice bath to crystalize/precipitate the desired pure products.

Results and discussion
The preparation of the Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn nanocomposite includes four steps (Fig. 1).Firstly, the magnetic Fe 3 O 4 nanoparticles were modified with TEOS and APTES to give Fe 3 O 4 @SiO 2 -NH 2 NPs.Secondly, this material was chemically reacted with GO to give Fe 3 O 4 @SiO 2 -NH 2 /GO nanocomposite.Thirdly, the Im-based ionic liquid was chemically grafted on the surface of Fe The functional groups of the GO, Fe 3 O 4 @SiO 2 -NH 2 and Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn materials were determined by using a Fourier transform infrared (FT-IR) spectrometer (Fig. 2).For all samples, the strong peak at 3394 cm −1 is due to the O-H bonds of the material surface (Fig. 2a-c) 40 .Moreover, the peaks at 1724, 1519, 1288 and 1049 cm −1 are, respectively, associated to carboxyl C=O, aromatic C=C, epoxy C-O and alkoxy C-O bonds of GO (Fig. 2a-c) 41 .For the Fe 3 O 4 @SiO 2 -NH 2 and Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn materials, the signals  www.nature.com/scientificreports/ at 2825 and 2923 cm −1 are attributed to the C-H bonds of the aliphatic groups (Fig. 2b and c) 42 .Moreover, for the latter materials, the peak at 593 cm −1 is assigned to the Fe-O bond (Fig. 2b and c) 43 .For Fe 3 O 4 @SiO 2 -NH 2 / GO/IL-Mn, the signal at 1627 cm −1 is attributed to C=N bond of ionic liquids (Fig. 2c) 41,44 .In addition, for both Fe 3 O 4 @SiO 2 -NH 2 and Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn nanomaterials, the strong signals at 1083 and 1215 cm −1 are assigned to the Si-O-Si vibrations 45,46 .
The surface morphology of Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn was studied by using SEM technique.The spherical nanoparticles of Fe 3 O 4 @SiO 2 NPs and also the graphene oxide layers were clearly seen in the SEM image (Fig. 3).This confirms the successful formation of the Fe 3 O 4 @SiO 2 -NH 2 /GO composite during applied conditions.
The TEM analysis of the designed catalyst was also performed to investigate its structure.This analysis showed the catalyst to be composed of spherical Fe 3 O 4 @SiO 2 NPs and GO layers (Fig. 4).
The EDX analysis showed the signals of carbon, nitrogen, oxygen, silicon, manganese and iron elements in the prepared nanocomposite (Fig. 5).This is in good agreement with the FT-IR results, confirming the successful immobilization of IL-Mn complex on Fe 3 O 4 @SiO 2 -NH 2 /GO composite.
The EDX-mapping analysis of the Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn nanocatalyst is shown in Fig. 6.As seen, all desired elements of C, O, N, Fe, Si and Mn are very well distributed in the material.This is also in good agreement with the FT-IR and EDX results, indicating the successful formation of the designed Fe 3 O 4 @SiO 2 -NH 2 / GO/IL-Mn nanocomposite.
The powder XRD analysis of Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn showed six signals at 2θ of 30, 35.5, 43.1, 54, 57.2, and 63.5 degree, corresponding to the Miller indices of 220, 311, 400, 422, 511 and 440, respectively (Fig. 7).These signals are attributed to the spinel structure of magnetic iron oxide NPs, 47,48 confirming the high stability of the magnetite NPs during modification processes.Also, the peak at 2θ = 19° is related to silica layer of the designed catalyst 49,50 .www.nature.com/scientificreports/According to the VSM analysis, the saturation magnetization of the designed Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn material was found to be 40 emu/g (Fig. 8), confirming its high magnetic properties.This characteristic is very important in the fields of adsorption and catalysis.
Thermal stability of the Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn nanocatalyst was investigated by using thermal gravimetric analysis (TGA, Fig. 9).The first weight loss at temperatures between 10 to 110 °C (3%) is related to the removal of water and alcoholic solvents 39 .The second weight loss at 111-210 °C (4%) is attributed to the removal of the parts of functional groups that are located on the surface of the material.The main weight loss at temperatures more than 220 °C is related to the complete removal of the ionic liquids and also some parts of GO.
The nitrogen adsorption-desorption isotherms of the Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn nanocomposite showed a type II curve with a pronounced H3 hysteresis loop, according to the IUPAC classification 51 .The BET specific surface area and total pore volume of the material were calculated to be about 386.5 m 2 /g and 0.35 cm 3 /g, respectively.In addition, the BJH pore size distribution analysis showed a peak with good intensity centered at average pore diameter of about 4.8 nm (Fig. 10).
After preparation and characterization, the catalytic activity of Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn was investigated in the synthesis of THBPs at room temperature (RT).For this, the reaction between benzaldehyde, dimedone and  www.nature.com/scientificreports/malononitrile was selected as a test model (Table 1).The effect of various parameters such as catalyst loading and solvent was investigated to obtain the best conditions.In the absence of a catalyst, no product was obtained after 3 h, proving the catalyst is necessary for the development of this reaction (Table 1, entry 1).After addition of the catalyst, the reaction was progressed effectively and the best result was obtained in the presence of 0.8 mol% of Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn (Table 1, entries 2-4).It is important to note that increasing the amount catalyst to 1 mol% did not result in a significant change in the reaction yield (Table 1, entry 5).In order to demonstrate the effect of the Mn-centers on the catalytic process, the catalytic activity of Mn-free Fe 3 O 4 @SiO 2 -NH 2 /GO/IL nanocomposite was also investigated.This experiment showed that the Mn-free material gave no yield of the desired product, verifying the process is actually catalyzed by catalytic Mn sites (Table 1, entry 6).This catalytic  www.nature.com/scientificreports/system was also significantly affected by the solvent.Yields of 58%, 82%, 53% were obtained in toluene, EtOH and also under solvent-free media, respectively.Pleasingly, in water, the best yield was obtained (Table 1, entry 4).Accordingly, 0.8 mol% of catalyst, water solvent and RT were identified as the optimal conditions (Table 1, entry 4).
With the optimum conditions in hand, various aldehyde derivatives containing both electron and electron donating substituents were used as substrate (Table 2).All of these aldehydes delivered the desired products in high yield at short time.It was also found that Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn offers high turnover number (TON) and turnover frequency (TOF) for all products, confirming the high ability of the present catalytic system to synthesis a wide range of biologically active THBPs.
The recoverability and reusability of Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn were also investigated in the reaction model.For this, after finishing of the reaction, the catalyst was easily separated by using a magnet.Then, it was reused in the next run under the same conditions as the first run.These steps were repeated and it was found that the catalyst could be recovered and reused for at least eight times with no significant decrease in efficiency (Fig. 11).These findings confirm high performance and very good stability of the designed catalyst under applied conditions.www.nature.com/scientificreports/Next, a leaching test was performed in the model reaction to investigate the nature of the Fe 3 O 4 @SiO 2 -NH 2 / GO/IL-Mn nanocatalyst under the applied conditions.For this, after the conversion was about 45% complete, the catalyst was magnetically removed.Then, the progress of catalyst-free residue was monitored.Interestingly, after 120 min, no notable conversion was observed.This proves no leaching of Mn species in the reaction solution under the applied conditions and also the heterogeneous nature of the designed catalyst.
Furthermore, the reactivity of the catalyst was investigated under optimal conditions.For this purpose, the model reaction was carried out and its progress was monitored using TLC.After the completion of the reaction, the starting materials were again added to the reaction vessel in the same proportion as the first run.These steps were repeated and the results showed that the activity of the Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn nanocatalyst is maintained for at least seven runs without a significant decrease in performance (Table 3).
In the next, in order to study the chemical and structural stability of the catalyst under applied conditions, the FT-IR and XRD analyses of the recovered catalyst were performed after fifth run.As shown in Fig. 12, the FT-IR spectrum of the recovered Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn showed a pattern similar to the FT-IR of fresh nanocatalyst, proving the high stability of the designed material under the applied reaction conditions.
The PXRD of the recovered Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn also illustrated six peaks at 2θ of 30, 35.5, 43.1, 54, 57.2, and 63.5, which are in good agreement with the PXRD pattern of the fresh nanocatalyst, proving the high stability of the crystalline structure of Fe 3 O 4 NPs during the reaction process (Fig. 13).
Finally, the performance of Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn nanocomposite was compared with some previous catalytic systems in the synthesis of THBPs (Table 4).The results showed that our catalyst is better in terms of reaction conditions, catalyst loading and recovery times.These findings may be attributed to the magnetic

Conclusion
In this study, for the first time, a manganese-containing IL-modified Fe 3 O 4 @SiO 2 -NH 2 /GO nanocomposite was prepared, characterized and used as a novel catalyst for the synthesis of THBPs.The high chemical and thermal stability of the designed catalyst were confirmed by using FT-IR, TGA and EDX analyses.The PXRD and VSM analyses showed high magnetic properties of the designed catalyst.The SEM and TEM analyses also confirmed

Table 1 .
58fect of solvent and catalyst loading in the synthesis of THBPs at RT. nature of Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn as well as the positive effect of chemically immobilized ionic liquids in the stabilization of the catalytically active Mn-species.A plausible mechanism for the synthesis of THBPs using Fe 3 O 4 @SiO 2 -NH 2 /GO/IL-Mn is outlined in Fig.14.At first, the malononitrile and the Mn-activated aldehyde are condensed through the Knoevenagel condensation to give intermediate 1. Intermediate 2 is then delivered via a Michael-type addition between the enol form of dimedone and intermediate 1.An intramolecular cyclo-condensation is performed on intermediate 2 to give intermediate 3. Finally, the intermediate 3 is converted to the desired product 4 through a tautomerization process58.