Preparation, characterization and catalytic application of nano-Fe3O4@SiO2@(CH2)3OCO2Na as a novel basic magnetic nanocatalyst for the synthesis of new pyranocoumarin derivatives

We present a study on the synthesis, characterization and application of sodium carbonate tag silica-coated nano-Fe3O4 (Fe3O4@SiO2@(CH2)3OCO2Na) as a novel and efficient heterogeneous basic catalyst. The described catalyst was fully characterized via FT-IR, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and field emission scanning electron microscopy (FE-SEM). The reported novel magnetic nanocatalyst presents an excellent activity and catalytic performance for the synthesis of a novel series of pyranocoumarins through the reaction of dialkyl acetylenedicarboxylates and 5,7-dihydroxy coumarin derivatives at 100 °C under solvent-free conditions.


Introduction
In recent years, there has been a pronounced tendency to utilize heterogeneous catalysts. 1 The reason why heterogeneous catalysts are attractive is their unique properties including easy separation, low toxicity, air tolerance, easy handling, and reusability. [2][3][4] Fe 3 O 4 magnetic nanoparticles (MNPs) have been widely used in design of environmentally benign heterogeneous catalysts. 5 The reason for the above choice is their large surface areas, good textural properties, supermagnetism, high coercivity and low Curie temperature as well as non-toxicity. [6][7][8] A striking feature of magnetic nanocatalysts is that they can be readily separated using an external magnet, which achieves a simple separation of the catalyst without ltration. 9 Also, they possess high potential active sites for loading of other functional groups. 10 To prevent Fe 3 O 4 nanoparticles from undergoing oxidation in an air atmosphere and in order to increase the surface area and simplify the surface functionalization, a protective shell of silica can be formed onto their surface. 11,12 Pyranocoumarins with excellent chemical and physical characteristics have acquired substantial attention and show great practical values in many elds, such as medicine discovery, dye chemistry, and materials chemistry. Structural diversity associated with pyranocoumarins has resulted in a large number of new molecular entities, which have been found to be useful as anti-cancer, anti-oxidant, anti-inammatory, anti-allergic, hepatoprotective, anti-viral, anti-carcinogenic agents, enzyme inhibitor, and precursor of toxic substances. [13][14][15][16] In view of the biological, industrial, and synthetic importance of pyranocoumarins, several methods for their synthesis have been reported. [17][18][19] Although most of these processes have distinct advantages, the use of high temperatures, environmentally harmful catalysts, harsh reaction conditions, long reaction times and large quantity of volatile organic solvents limit the use of these methods. Therefore, the search for the possibility synthesis of these compounds under mild reaction conditions with recoverable effective heterogeneous catalyst is still in high demand.
In continuation of our efforts to design, synthesis and application of novel nanocatalyst, 20,21 we have immobilized sodium carbonate on silica-coated Fe 3 O 4 magnetic nanoparticles (Fe 3 O 4 @SiO 2 @(CH 2 ) 3 OCO 2 Na) and then investigated its performance as novel strong, recoverable, and stable basic nanocatalyst for synthesis of new pyranocoumarin derivatives.
instrument. The morphology of the particles was studied by Field Emission Scanning Electron Microscopy (FE-SEM) in a MIR-A3TESCAN-XMU FE-SEM instrument.

Synthesis of Fe 3 O 4 MNPs
A mixture of FeCl 3 $6H 2 O (2.3 g, 8.7 mmol) and FeCl 2 $4H 2 O (0.86 g, 4.3 mmol) was dissolved in deionized water (100 mL). The solution was heated to 90 C under nitrogen atmosphere and stirred about 30 min. Subsequently, sodium hydroxide solution (10 mL, 25%) was added dropwise to the solution until the brown color solution turned out to the black. Aer approximately 1 h, the black precipitate isolated in a magnetic eld from the reaction mixture, repeatedly washed with deionized water several times to remove the remaining impurities. 22

Preparation of Fe 3 O 4 @SiO 2
Dried Fe 3 O 4 nanoparticles (0.5 g) was suspended in a mixture of ethanol (20 mL) and NH 3 $H 2 O (5 mL, 25%) followed by the addition of tetraethoxysilane (TEOS) (0.3 g) to the solution and the mixture was ultrasonicated for 2 h. Next, the mixture was degassed and stirred for 24 h. 23 Procedure for the synthesis of Fe 3 O 4 @SiO 2 @(CH 2 ) 3 Cl The prepared Fe 3 O 4 @SiO 2 (0.5 g) was dispersed in dry toluene (80 mL) by sonication for 15 min, then, 3-chloropropyltriethoxysilan (0.121 g, 0.5 mmol) was added to the mixture and heated to 100 C under reux condition and N 2 atmosphere. Aer 12 h, Fe 3 O 4 @SiO 2 @(CH 2 ) 3 Cl was separated magnetically and washed with deionized water and ethanol. 24 Preparation of Fe 3 O 4 @SiO 2 @(CH 2 ) 3 OCO 2 Na Fe 3 O 4 @SiO 2 @(CH 2 ) 3 Cl (0.5 g) was dissolved in DMSO (50 mL) by sonication. Then Na 2 CO 3 (1 g) was added to the above mixture and heated to the 90 C under reux condition for 24 h. Next, the obtained black precipitate was separated by a normal magnet an washed with distilled water and ethanol and dried overnight under vacuum at 60 C.

General procedure for the synthesis of pyranocoumarins 4
In a round bottom ask, a mixture of 5,7-dihydroxy coumarins 2 (1 mmol), dialkyl acetylene dicarboxylate 3 (1.5 mmol) and prepared magnetic nanocatalyst 1 (0.0005 g) was heated to 100 C. The progress of the reaction was monitored by TLC (n-hexane : EtOAc/ 3 : 1 (v/v)). Aer completion of the reaction which was distinguished by disappearing of starting materials' spots on TLC, the reaction mixture was diluted by methanol and the catalyst was easily separated magnetically using an external magnet. The desired product was puried using column chromatography (using ethylacetate : n-hexane, 3 : 1 as mobile phase).

Results and discussion
Due to reasonable needs to clean and green heterogeneous basic catalysts, the magnetic nanocatalyst Fe 3 O 4 @SiO 2 @(CH 2 ) 3 OCO 2 Na (1) was synthesized following the procedure shown in Scheme 1. The structure of prepared nanocatalyst 1 was studied and fully characterized using FT-IR, EDS, XRD, and FE-SEM analysis. These results provided the evidences that the expected structure was successfully achieved. Fig. 1 presents the X-ray diffraction (XRD) patterns of the magnetic nanocatalyst 1. As can be seen, six    25 The characteristic peaks conrming the presence of -OCO 2 Na is appeared around 10, 34, 36 and 43 (2q) which some of them is overlapped by the Fe 3 O 4 peaks. However, the appearance of a peak at 10.53 conrms the connection of -OCO 2 Na in nanocatalyst. 26 Additionally, a broad peak at 13-28 in XRD pattern of catalyst indicates the existence of amorphous silica in the structure of the catalyst. 27 The structure of prepared nanocatalyst 1 was conrmed by the FT-IR spectra (Fig. 2). The sample exhibits a peak at 632 cm À1 band that is due to the stretching vibration mode associated to the Fe-O absorption band. The stretch found at 466 cm À1 is related to the presence of Fe-O-Si bond in the sample. 28 Furthermore, the spectra present the O-H stretching vibrational around 3431 cm À1 and two absorption peaks at 798 cm À1 and 1084 cm À1 which corresponds to the symmetric and asymmetric stretching vibration of Si-O. 29 The band at 2928 cm À1 is attributed to the alkyl chain -CH 2 . 30 The presence of the -OCO 2 Na group is conrmed by the bands at 1636 cm À1 and 1407 cm À1 . 26 The prepared magnetic nanocatalyst was analyzed by using an energy dispersive spectrometer (EDS). According to the Fig. 3, it is seen that Fe 3 O 4 @SiO 2 @(CH 2 ) 3 OCO 2 Na contains all expected elemental cases including Si, O, Fe, C and Na.
The FE-SEM image of Fe 3 O 4 @SiO 2 @(CH 2 ) 3 OCO 2 Na was shown in Fig. 4. The image demonstrates uniform-size particles with near spherical morphology. As it comes from FE-SEM analysis the average diameter of obtained nanoparticles is around 25 nm.
To test the stability of the catalyst structure, the recycled nano catalyst was examined by XRD analysis; the diffraction patterns and relative intensities of all peaks matched well with those of the primary catalyst (Fig. 5).
Having successfully prepared the new nanocatalyst 1, further studies were performed for the synthesis of a novel class of pyranocoumarins 4 via the condensation of 5,7-dihydroxycoumarins 2 and dialkyl acetylenedicarboxylates 3 (Scheme 2). The structure of product was conrmed by FT-IR, 1 H NMR, 13 C NMR, and elemental analysis. Initially, 5,7-dihydroxycoumarins (2) was synthesized according to the reported procedure. 31 In order to obtain the optimal experimental conditions, we set up a model reaction between 5,7-dihydroxy-4-methyl coumarin and dimethyl acetylenedicarboxylate (DMAD). The model reaction was conducted in the presence of several homogeneous and heterogeneous catalysts at various conditions. It was observed that the best result was achieved using Scheme 3 Proposed mechanism for the synthesis of pyranocoumarin 4.