Non-Covalent Supported of l-Proline on Graphene Oxide/Fe3O4 Nanocomposite: A Novel, Highly Efficient and Superparamagnetically Separable Catalyst for the Synthesis of Bis-Pyrazole Derivatives

A superparamagnetic graphene oxide/Fe3O4/l-proline nano hybrid that was obtained from the non-covalent immobilization of l-proline on graphene oxide/Fe3O4 nanocomposite was used as a new magnetically separable catalyst for the efficient synthesis of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol) derivatives. The prepared heterogeneous catalyst was characterized using FTIR, TGA, DTG, XRD, TEM, SEM, and elemental analysis techniques. Short reaction times (5–15 min), excellent yields (87–98%), and simple experimental procedure with an easy work-up are some of the advantages of the introduced catalyst.


Introduction
In recent years, carbocatalysis has attracted much attention, as it is cheap and can be easily obtained. Catalysts based on carbon constituents, such as activated carbon and carbon nanotubes, have also been used as catalyst supports [1]. More recently, graphene and graphene oxide (GO) are emerging as a novel class of carbocatalysts, as well as catalyst supports in organic synthesis [2,3]. GO has been proved as a favorable support due to its inherent properties, such as high chemically stability, large surface area, and good availability [4]. Most notably, GO has functional groups, mainly including epoxide, hydroxyl, and carboxyl on the edge, top, and bottom surface of each sheet that even after sufficient reduction, cannot be completely removed which makes GO a suitable support for metal and metal oxide nanoparticles [5]. Similar to carbon nanotubes, these functional groups can offer a platform for various chemical reactions. Recently, GO owing its surface decorated myriad oxygenated functions and conductivity with very high surface area has been used in sensors, nanoelectronics, contaminant removal, drug delivery, as metal-free catalysts or as supports for immobilizing active species for facilitating synthetic transformations. Among the various uses of GO, assembling other inorganic materials, like

IR Spectra
The non-covalent immobilization of L-proline on the surface of GO/Fe3O4 nano hybrid can be confirmed by characterizing the pure GO, pristine L-proline, and GO/Fe3O4/L-proline nano hybrid using FT-IR spectroscopy, as shown in Figure 1. The FT-IR spectrum of GO displays distinguishing stretching vibration bands of carboxylic groups at round 3390, 1735, and 1224 cm −1 , and stretching vibration band at 1050 cm −1 denotes to epoxy C-O groups (Figure 1a). The FT-IR spectrum of pristine L-proline shows typical stretching frequencies, including N-H asymmetric stretching at 3055 cm −1 and carboxylate (COO − ) asymmetric and symmetric stretching at 1622 and 1380 cm −1 , respectively ( Figure 1c). These bands are appeared as new peaks in the FT-IR spectrum of GO/Fe3O4/L-proline nano hybrid when compared to the FT-IR spectrum of pure GO sheet (Figure 1a,b). The carboxylate (COO − ) asymmetric and symmetric stretching of L-proline are presented in GO/Fe3O4/L-proline nano hybrid spectrum and seemed to move to lower positions at 1615 and 1375 cm −1 ,respectively (Figure 1b). The -Fe-Ostretching peak of GO/Fe3O4/L-proline appeared at 588 cm −1 . Furthermore, the band at 3056 cm −1 corresponding to the asymmetric stretching vibration of the N-H group in L-proline is around 2800 cm −1 in FT-IR spectrum of GO/Fe3O4/L-proline. On the other hand, the wavenumbers of the characteristic bands of GO have also been decreased to lower locations and the individual band at 1735 cm −1 related to the carboxyl group in the spectrum of GO sheet vanished after L-proline was loaded. All of these results from the comparison of FT-IR spectra propose that L-proline organocatalyst has been successfully and effectively loaded on the surface of GO/Fe3O4 nano hybrid through hydrogen bonding interaction between carboxylic and secondary amine groups of L-proline with hydroxyl, carboxyl, and epoxy groups on the GO sheets of GO/Fe3O4 nano hybrid.

IR Spectra
The non-covalent immobilization of L-proline on the surface of GO/Fe 3 O 4 nano hybrid can be confirmed by characterizing the pure GO, pristine L-proline, and GO/Fe 3 O 4 /L-proline nano hybrid using FT-IR spectroscopy, as shown in Figure 1. The FT-IR spectrum of GO displays distinguishing stretching vibration bands of carboxylic groups at round 3390, 1735, and 1224 cm −1 , and stretching vibration band at 1050 cm −1 denotes to epoxy C-O groups (Figure 1a). The FT-IR spectrum of pristine L-proline shows typical stretching frequencies, including N-H asymmetric stretching at 3055 cm −1 and carboxylate (COO − ) asymmetric and symmetric stretching at 1622 and 1380 cm −1 , respectively ( Figure 1c). These bands are appeared as new peaks in the FT-IR spectrum of GO/Fe 3 O 4 /L-proline nano hybrid when compared to the FT-IR spectrum of pure GO sheet (Figure 1a,b). The carboxylate (COO − ) asymmetric and symmetric stretching of L-proline are presented in GO/Fe 3 O 4 /L-proline nano hybrid spectrum and seemed to move to lower positions at 1615 and 1375 cm −1 , respectively ( Figure 1b). The -Fe-O-stretching peak of GO/Fe 3 O 4 /L-proline appeared at 588 cm −1 . Furthermore, the band at 3056 cm −1 corresponding to the asymmetric stretching vibration of the N-H group in L-proline is around 2800 cm −1 in FT-IR spectrum of GO/Fe 3 O 4 /L-proline. On the other hand, the wavenumbers of the characteristic bands of GO have also been decreased to lower locations and the individual band at 1735 cm −1 related to the carboxyl group in the spectrum of GO sheet vanished after L-proline was loaded. All of these results from the comparison of FT-IR spectra propose that L-proline organocatalyst has been successfully and effectively loaded on the surface of GO/Fe 3 O 4 nano hybrid through hydrogen bonding interaction between carboxylic and secondary amine groups of L-proline with hydroxyl, carboxyl, and epoxy groups on the GO sheets of GO/Fe 3 O 4 nano hybrid.

TGA and DTG Analysis
Thermogravimetric analysis (TGA) and differential thermal analysis (DTG) associated to the decomposition profiles of the pristine L-proline and GO/Fe3O4/L-proline nano hybrid under air atmosphere deliver additional proof for the immobilization of L-proline on the surfaces of GO/Fe3O4 nanocomposite (Figures 2 and 3). The pristine L-proline shows two distinct steps of weight loss in the combined TG-DTG curves ( Figure 2). The first weight loss centers around 60 °C, which is due to a loss of water. The second large weight loss assigned to the successive cleavage of the L-proline existed at 215-250 °C. The weight loss extends up to ca. 600 °C until the L-proline is almost entirely decomposed under air flow. Figure 3 illustrates the TG-DTG curves of GO/Fe3O4/L-proline nano hybrid. The first step (weight loss = ca. 10 wt %) before 100 °C is referred to the elimination of the surface-adsorbed water and interlayer water molecules. The second step contains the weight loss located at 125 °C and the trailed weight loss at 220 °C. The two weight loss peaks are assigned to the successive pyrolysis of the labile oxygen-containing functional groups (weight loss = ca. 20 wt %). This content of the highly rich oxygenated species is the main cause for the efficient loading of L-proline on the surface of GO/Fe3O4 nanocomposite. The third step, in the range of 310-750 °C (ca. 25 wt %), is reliable to refer to the successive cleavage of the L-proline moiety [36], since the weight loss in this range is nearly equal to the content of L-proline moiety calculated from the elemental analysis (24.5 wt %). The non-removable residue belongs to the remaining carbon and Fe3O4 nanoparticles. The increased decomposition temperature of the pristine L-proline from 215-250 °C to 310-750 °C range, when loaded on GO/Fe3O4 nano composite, suggests that the guest/host interaction was well done through the oxygen-containing functional groups of the GO sheet in GO/Fe3O4 support with carboxylic and secondary amine groups of L-proline (Scheme 1). This observation is also in accordance with the carboxylate asymmetric and symmetric stretching vibration of L-proline, which has been shifted to lower positions in FT-IR spectrum of GO/Fe3O4/L-proline ( Figure 1b). Moreover, this increased thermal stability clearly confirms that the L-proline is successfully loaded on the GO/Fe3O4 support by forming the non-covalent hydrogen bond, rather than physical adsorption because this remarkable thermal stability could not be gained by physical adsorption. The high loading of L-proline on the surface of GO/Fe3O4 support (25 wt %), together with the unique non-covalent hydrogen binding behaviors between L-proline and GO/Fe3O4 support, makes the GO/Fe3O4/L-proline nano hybrid an efficient and stable catalyst in the reaction system.

TGA and DTG Analysis
Thermogravimetric analysis (TGA) and differential thermal analysis (DTG) associated to the decomposition profiles of the pristine L-proline and GO/Fe 3 O 4 /L-proline nano hybrid under air atmosphere deliver additional proof for the immobilization of L-proline on the surfaces of GO/Fe 3 O 4 nanocomposite (Figures 2 and 3). The pristine L-proline shows two distinct steps of weight loss in the combined TG-DTG curves ( Figure 2). The first weight loss centers around 60 • C, which is due to a loss of water. The second large weight loss assigned to the successive cleavage of the L-proline existed at 215-250 • C. The weight loss extends up to ca. 600 • C until the L-proline is almost entirely decomposed under air flow. Figure 3 illustrates the TG-DTG curves of GO/Fe 3 O 4 /L-proline nano hybrid. The first step (weight loss = ca. 10 wt %) before 100 • C is referred to the elimination of the surface-adsorbed water and interlayer water molecules. The second step contains the weight loss located at 125 • C and the trailed weight loss at 220 • C. The two weight loss peaks are assigned to the successive pyrolysis of the labile oxygen-containing functional groups (weight loss = ca. 20 wt %). This content of the highly rich oxygenated species is the main cause for the efficient loading of L-proline on the surface of GO/Fe 3 O 4 nanocomposite. The third step, in the range of 310-750 • C (ca. 25 wt %), is reliable to refer to the successive cleavage of the L-proline moiety [36], since the weight loss in this range is nearly equal to the content of L-proline moiety calculated from the elemental analysis (24.5 wt %). The non-removable residue belongs to the remaining carbon and Fe 3 O 4 nanoparticles. The increased decomposition temperature of the pristine L-proline from 215-250 • C to 310-750 • C range, when loaded on GO/Fe 3 O 4 nano composite, suggests that the guest/host interaction was well done through the oxygen-containing functional groups of the GO sheet in GO/Fe 3 O 4 support with carboxylic and secondary amine groups of L-proline (Scheme 1). This observation is also in accordance with the carboxylate asymmetric and symmetric stretching vibration of L-proline, which has been shifted to lower positions in FT-IR spectrum of GO/Fe 3 O 4 /L-proline ( Figure 1b). Moreover, this increased thermal stability clearly confirms that the L-proline is successfully loaded on the GO/Fe 3 O 4 support by forming the non-covalent hydrogen bond, rather than physical adsorption because this remarkable thermal stability could not be gained by physical adsorption. The high loading of L-proline on the surface of GO/Fe 3 O 4 support (25 wt %), together with the unique non-covalent hydrogen binding behaviors between L-proline and GO/Fe 3 O 4 support, makes the GO/Fe 3 O 4 /L-proline nano hybrid an efficient and stable catalyst in the reaction system.

XRD
Comparison of the XRD pattern of the synthesized GO with the XRD pattern of pure graghite indicated that the interlayer spacing between GO sheets is much more than graghite sheets (more than two times). Oxidation leads to the abundant formation of hydroxy, epoxy and carboxyl groups on the edge, top, and bottom surface the each sheet and increasing of the interlayer spacing between the GO sheets. This created interlayer spacing is a good opportunity for both Fe3O4 nanoparticles and L-proline organocatalyst to occupy interlayers and interact with GO functional groups, which is the cause that the loading amount of immobilized L-proline on GO is more than on graghite. The main peaks of pristine L-proline (denoted as *, 2θ) are shown in Figure 4a. Figure

XRD
Comparison of the XRD pattern of the synthesized GO with the XRD pattern of pure graghite indicated that the interlayer spacing between GO sheets is much more than graghite sheets (more than two times). Oxidation leads to the abundant formation of hydroxy, epoxy and carboxyl groups on the edge, top, and bottom surface the each sheet and increasing of the interlayer spacing between the GO sheets. This created interlayer spacing is a good opportunity for both Fe3O4 nanoparticles and L-proline organocatalyst to occupy interlayers and interact with GO functional groups, which is the cause that the loading amount of immobilized L-proline on GO is more than on graghite. The main peaks of pristine L-proline (denoted as *, 2θ) are shown in Figure 4a. Figure

XRD
Comparison of the XRD pattern of the synthesized GO with the XRD pattern of pure graghite indicated that the interlayer spacing between GO sheets is much more than graghite sheets (more than two times). Oxidation leads to the abundant formation of hydroxy, epoxy and carboxyl groups on the edge, top, and bottom surface the each sheet and increasing of the interlayer spacing between the GO sheets. This created interlayer spacing is a good opportunity for both Fe 3 O 4 nanoparticles and L-proline organocatalyst to occupy interlayers and interact with GO functional groups, which is the cause that the loading amount of immobilized L-proline on GO is more than on graghite. The main peaks of pristine L-proline (denoted as *, 2θ) are shown in Figure 4a.

TEM and SEM
SEM was used to see the morphology of GO nanoplatelets. images of the prepared GO nanoplatelets show crumpled thin layers with wrinkles and folds on the surface of GO ( Figure 5). The morphology of GO/Fe3O4/L-proline and the reused GO/Fe3O4/L-proline after eight runs was determined by TEM. As Figure 6 shows, Fe3O4 nanoparticles are chemically deposited on GO; although, some Fe3O4 aggregation is detected. The results of TEM confirmed that the nano-sized organic-inorganic hybrid material was prepared. The morphology of the reused catalyst did not show any significant change even after eight reaction cycles, which proved its robustness ( Figure 7).
Size data of Fe3O4 nanoparticles were achieved by the counting of almost 500 particles from the TEM images of GO/Fe3O4/L-pro catalyst. Particle size and size distribution of Fe3O4 nanoparticles are represented in Figure 8. The number-frequency histogram illustrates the frequency of existence versus the size range of Fe3O4 nanoparticles. As it can be seen from Figure 8, the size distribution of the particles is twisted to the larger end of the particle-size scale. The mean size of Fe3O4 nanoparticles was obtained to be 24 nm with a standard deviation of 12.2.

Optimization of the Reaction Conditions
After the preparation and characterization of GO/Fe3O4/L-proline catalyst, its catalytic activity was investigated in a one-pot pseudo three-component reaction for the synthesis of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol) derivatives. To search for the optimal conditions, the reaction of benzaldehyde and two equivalents of 3-methyl-1-phenyl-2-pyrazolin-5-one was selected as the model reaction to examine the effect of GO/Fe3O4/L-proline catalyst (0.01-0.1 g), under a variety of conditions ( Table 1).
The present optimization studies revealed that the best result was achieved by caring out the reaction in the presence of 0.05 g of GO/Fe3O4/L-proline under the reflux condition in ethanol (entry 10). The use of larger amounts of the catalyst (0.1 g, entry 11) did not improve the yield, while decreasing its amount led to decreased yield (Table 1, entries 8, 9). Using the optimized reaction conditions, the efficiency of this approach was explored for the synthesis of a wide variety of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol) derivatives (Scheme 2, Table 2).

Optimization of the Reaction Conditions
After the preparation and characterization of GO/Fe 3 O 4 /L-proline catalyst, its catalytic activity was investigated in a one-pot pseudo three-component reaction for the synthesis of 4,4 -(arylmethylene)bis(1H-pyrazol-5-ol) derivatives. To search for the optimal conditions, the reaction of benzaldehyde and two equivalents of 3-methyl-1-phenyl-2-pyrazolin-5-one was selected as the model reaction to examine the effect of GO/Fe 3 O 4 /L-proline catalyst (0.01-0.1 g), under a variety of conditions ( Table 1).
The present optimization studies revealed that the best result was achieved by caring out the reaction in the presence of 0.05 g of GO/Fe 3 O 4 /L-proline under the reflux condition in ethanol (entry 10). The use of larger amounts of the catalyst (0.1 g, entry 11) did not improve the yield, while decreasing its amount led to decreased yield (Table 1,

Optimization of the Reaction Conditions
After the preparation and characterization of GO/Fe3O4/L-proline catalyst, its catalytic activity was investigated in a one-pot pseudo three-component reaction for the synthesis of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol) derivatives. To search for the optimal conditions, the reaction of benzaldehyde and two equivalents of 3-methyl-1-phenyl-2-pyrazolin-5-one was selected as the model reaction to examine the effect of GO/Fe3O4/L-proline catalyst (0.01-0.1 g), under a variety of conditions ( Table 1).
The present optimization studies revealed that the best result was achieved by caring out the reaction in the presence of 0.05 g of GO/Fe3O4/L-proline under the reflux condition in ethanol (entry 10). The use of larger amounts of the catalyst (0.1 g, entry 11) did not improve the yield, while decreasing its amount led to decreased yield (Table 1, entries 8, 9). Table 1. Investigation of catalytic activity of GO/Fe3O4/L-pro for the synthesis of -bis-pyrazole (3a) under various Conditions. Using the optimized reaction conditions, the efficiency of this approach was explored for the synthesis of a wide variety of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol) derivatives (Scheme 2, Table 2). Using the optimized reaction conditions, the efficiency of this approach was explored for the synthesis of a wide variety of 4,4 -(arylmethylene)bis(1H-pyrazol-5-ol) derivatives (Scheme 2, Table 2). All of the reactions delivered excellent product yields and accommodated a wide range of aromatic aldehydes bearing both electron-donating and electron-withdrawing substituents (3a-r).

Conditions Temperature ( • C) GO/Fe 3 O 4 /L-pro (g) Time (min) Yield (%) a
Molecules 2018, 23, x 9 of 16 All of the reactions delivered excellent product yields and accommodated a wide range of aromatic aldehydes bearing both electron-donating and electron-withdrawing substituents (3a-r).

Scheme 2.
Synthesis of bis-pyrazole derivatives catalyzed by GO/Fe3O4/L-proline nano hybrid. The recovery of a catalyst is highly preferred for a greener process. For this purpose, the reusability of GO/Fe3O4/L-proline was studied for eight consecutive cycles (fresh + seven cycles) for the synthesis of 4,4′-(phenylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3a). From Figure 9, it can be observed that GO/Fe3O4/L-proline catalyst can be reused up to eight runs without the need to reload, and the yield difference between the first and eighth runs is only 5%, indicating that the catalyst efficiency is almost completely maintained during eight consecutive runs. The nitrogen content of the fresh and reused catalyst was measured using the elemental analysis and the comparison of the nitrogen contents suggested that the catalyst lost only 5% of its nitrogen content after eight runs. This is solid proof for the very low leaching account of L-proline organocatalyst from GO/Fe3O4/ L-proline catalyst into the reaction mixture during eight runs, and also, confirms that the catalytic ability of GO/Fe3O4/L-proline has almost completely remained stable after eight runs, which is in agreement with the recyclability study. The TEM image of the recovered catalyst after eight runs did not reveal any significant change in the morphology of the reused catalyst, which proved its robustness while maintaining catalytic activity (Figure 7). Superparamagnetic properties and simple recovery of the catalyst are shown in Figure 9. GO/Fe3O4/L-proline catalyst can be recovered from the reaction mixture simply by applying an external magnet and its superparamagnetic properties remain constant even after eight runs.  The recovery of a catalyst is highly preferred for a greener process. For this purpose, the reusability of GO/Fe 3 O 4 /L-proline was studied for eight consecutive cycles (fresh + seven cycles) for the synthesis of 4,4 -(phenylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3a). From Figure 9, it can be observed that GO/Fe 3 O 4 /L-proline catalyst can be reused up to eight runs without the need to reload, and the yield difference between the first and eighth runs is only 5%, indicating that the catalyst efficiency is almost completely maintained during eight consecutive runs. The nitrogen content of the fresh and reused catalyst was measured using the elemental analysis and the comparison of the nitrogen contents suggested that the catalyst lost only 5% of its nitrogen content after eight runs. This is solid proof for the very low leaching account of L-proline organocatalyst from GO/Fe 3 O 4 /L-proline catalyst into the reaction mixture during eight runs, and also, confirms that the catalytic ability of GO/Fe 3 O 4 /L-proline has almost completely remained stable after eight runs, which is in agreement with the recyclability study. The TEM image of the recovered catalyst after eight runs did not reveal any significant change in the morphology of the reused catalyst, which proved its robustness while maintaining catalytic activity (Figure 7). Superparamagnetic properties and simple recovery of the catalyst are shown in Figure 9. GO/Fe 3 O 4 /L-proline catalyst can be recovered from the reaction mixture simply by applying an external magnet and its superparamagnetic properties remain constant even after eight runs.  Figure 10 shows the powerful superparamagnetic property of fresh GO/Fe3O4/L-pro by using an external magnet. For more detailed study, magnetic properties of fresh GO/Fe3O4/L-pro and the reused GO/Fe3O4/L-pro after eight runs were investigated using VSM ( Figure 11). The achieved results illustrated that the catalyst samples have appropriate property for magnetic actuations, but there were some decrease in the values of saturation magnetization after eight consecutive runs, which possibly originated from the decrease of loading amount of Fe3O4 nanoparticles on the GO surface during using under eight repeated heating condition (Figure 11a,b).   Figure 10 shows the powerful superparamagnetic property of fresh GO/Fe 3 O 4 /L-pro by using an external magnet. For more detailed study, magnetic properties of fresh GO/Fe 3 O 4 /L-pro and the reused GO/Fe 3 O 4 /L-pro after eight runs were investigated using VSM ( Figure 11). The achieved results illustrated that the catalyst samples have appropriate property for magnetic actuations, but there were some decrease in the values of saturation magnetization after eight consecutive runs, which possibly originated from the decrease of loading amount of Fe 3 O 4 nanoparticles on the GO surface during using under eight repeated heating condition (Figure 11a,b).  Figure 10 shows the powerful superparamagnetic property of fresh GO/Fe3O4/L-pro by using an external magnet. For more detailed study, magnetic properties of fresh GO/Fe3O4/L-pro and the reused GO/Fe3O4/L-pro after eight runs were investigated using VSM ( Figure 11). The achieved results illustrated that the catalyst samples have appropriate property for magnetic actuations, but there were some decrease in the values of saturation magnetization after eight consecutive runs, which possibly originated from the decrease of loading amount of Fe3O4 nanoparticles on the GO surface during using under eight repeated heating condition (Figure 11a,b).  It is worth noting that the introduced GO/Fe3O4/L-proline nano hybrid is a collection of three most applied catalysts, including GO, Fe3O4 nanoparticles, and pristine L-proline organocatalyst. Each of these catalysts has been separately used for several organic transformations. Hence, we became encouraged to compare the catalytic efficiency of GO/Fe3O4/L-proline nano hybrid with each of its component catalysts for the preparation of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol) derivatives. The reaction of benzaldehyde and two equivalents of 3-methyl-1-phenyl-2-pyrazolin-5-one was selected again as the model reaction (Table 3). On the base of the CHN analysis and TG (disscused above), the loading amount of L-proline in 0.05 g of GO/Fe3O4/L-proline nano hybrid is 0.01 g. To compare the catalytic activity of pristine L-proline with L-proline on GO/Fe3O4 nanocomposite, the equivalent amount of pristine L-proline (0.01 g) was also used as catalyst (entry 1 vs. 4). Table 3. Catalytic activity of GO/Fe3O4/L-proline nano hybrid in comparison with pure GO, Fe3O4 nanoparticles, prestine L-proline, and GO/Fe3O4 nanocomposite. The results show that the L-proline immobilized on GO/Fe3O4 nanocomposite has better catalytic activity than pristine L-proline with the same loading amount of L-proline. This observation confirms that the catalytic activity of L-proline is improved when is immobilized on GO/Fe3O4 nanocomposite. Moreover, GO/Fe3O4/L-proline is a more powerful catalyst than its other component catalysts regarding the reaction time and the yield of the product (entry 1 vs. 2, 3 and 5). Finally, to show the efficiency of this method in comparison with other reported procedures, we selected the reaction of benzaldehyde and two equivalents of 3-methyl-1-phenyl-2-pyrazolin-5-one for the It is worth noting that the introduced GO/Fe 3 O 4 /L-proline nano hybrid is a collection of three most applied catalysts, including GO, Fe 3 O 4 nanoparticles, and pristine L-proline organocatalyst. Each of these catalysts has been separately used for several organic transformations. Hence, we became encouraged to compare the catalytic efficiency of GO/Fe 3 O 4 /L-proline nano hybrid with each of its component catalysts for the preparation of 4,4 -(arylmethylene)bis(1H-pyrazol-5-ol) derivatives. The reaction of benzaldehyde and two equivalents of 3-methyl-1-phenyl-2-pyrazolin-5-one was selected again as the model reaction (Table 3). On the base of the CHN analysis and TG (disscused above), the loading amount of L-proline in 0.05 g of GO/Fe 3 O 4 /L-proline nano hybrid is 0.01 g. To compare the catalytic activity of pristine L-proline with L-proline on GO/Fe 3 O 4 nanocomposite, the equivalent amount of pristine L-proline (0.01 g) was also used as catalyst (entry 1 vs. 4). Table 3 It is worth noting that the introduced GO/Fe3O4/L-proline nano hybrid is a collection of three most applied catalysts, including GO, Fe3O4 nanoparticles, and pristine L-proline organocatalyst. Each of these catalysts has been separately used for several organic transformations. Hence, we became encouraged to compare the catalytic efficiency of GO/Fe3O4/L-proline nano hybrid with each of its component catalysts for the preparation of 4,4′-(arylmethylene)bis(1H-pyrazol-5-ol) derivatives. The reaction of benzaldehyde and two equivalents of 3-methyl-1-phenyl-2-pyrazolin-5-one was selected again as the model reaction (Table 3). On the base of the CHN analysis and TG (disscused above), the loading amount of L-proline in 0.05 g of GO/Fe3O4/L-proline nano hybrid is 0.01 g. To compare the catalytic activity of pristine L-proline with L-proline on GO/Fe3O4 nanocomposite, the equivalent amount of pristine L-proline (0.01 g) was also used as catalyst (entry 1 vs. 4). Table 3. Catalytic activity of GO/Fe3O4/L-proline nano hybrid in comparison with pure GO, Fe3O4 nanoparticles, prestine L-proline, and GO/Fe3O4 nanocomposite. The results show that the L-proline immobilized on GO/Fe3O4 nanocomposite has better catalytic activity than pristine L-proline with the same loading amount of L-proline. This observation confirms that the catalytic activity of L-proline is improved when is immobilized on GO/Fe3O4 nanocomposite. Moreover, GO/Fe3O4/L-proline is a more powerful catalyst than its other component catalysts regarding the reaction time and the yield of the product (entry 1 vs. 2, 3 and 5). Finally, to show the efficiency of this method in comparison with other reported procedures, we selected the reaction of benzaldehyde and two equivalents of 3-methyl-1-phenyl-2-pyrazolin-5-one for the The results show that the L-proline immobilized on GO/Fe 3 O 4 nanocomposite has better catalytic activity than pristine L-proline with the same loading amount of L-proline. This observation confirms that the catalytic activity of L-proline is improved when is immobilized on GO/Fe 3 O 4 nanocomposite. Moreover, GO/Fe 3 O 4 /L-proline is a more powerful catalyst than its other component catalysts regarding the reaction time and the yield of the product (entry 1 vs. 2, 3 and 5). Finally, to show the efficiency of this method in comparison with other reported procedures, we selected the reaction of benzaldehyde and two equivalents of 3-methyl-1-phenyl-2-pyrazolin-5-one for the synthesis of 4,4 -(phenylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) (3a) as a representative model. This comparison is shown in Table 4. It is clear from the data that our method has shorter reaction times and provides higher yields of the products.

Experimental
All of the chemicals were purchased from Merck and Aldrich. GO was prepared using modified Hummers method from flake graphite (Merck Company, Darmstadt, Germany). The reaction progresses were monitored by thin layer chromatography (TLC; silica-gel 60 F 254 , n-hexane: AcOEt). The 1 H-NMR spectra were obtained on a Bruker Instrument DPX-400 Avance 2 model spectrometer (Bruker, Billerica, MA, USA). The vario El CHNS (made in Germany) was used for elemental analysis. Scanning electron micrographs (SEM) were obtained using a Cambridge S-360 instrument with an accelerating voltage of 20 kV (Cambridge, Austin, TX, USA). The powder X-ray diffraction (XRD) pattern was obtained by a Bruker AXS (D8, Avance) (Avance, San Antonio, TX, USA) instrument employing the reflection Bragg-Brentano geometry with CuKa radiation. A continuous scan mode was used to collect 2θ from 5 • to 40 • . Fourier transform infrared (FT-IR) spectra were obtained as potassium bromide pellets in the range 400-4000 cm −1 using an AVATAR 370 Thermo Nicolet spectrophotometer. The thermogravimetric and differential thermogravimetric (TG-DTG) analysis was performed on Netzsch STA449c (Kimia Sanat Ara, Tehran, Iran). The sample weight was ca. 10 mg and was heated from room temperature up to 600 • C with 10 • C/min using alumina sample holders. The structure of the products was confirmed on the basis of IR, NMR spectroscopic data, and elemental analysis.

General Procedure for the Preparation of GO
A flask containing graphite (1 g) and NaNO 3 (0.75 g) were placed in an ice-water bath. H 2 SO 4 (75 mL) was added with stirring, and then KMnO 4 (4.5 g) was slowly added during about 1 h. After strongly stirring for 3 days at room temperature, 5% H 2 SO 4 (140 mL) aqueous solution was slowly added over for about 1 h with stirring, keeping the temperature at 98 • C. Then, the temperature was reduced to 60 • C, 3 mL of H 2 O 2 (30 wt % aqueous solution) was added, and the mixture was stirred for 2 h at room temperature. The prepared GO was suspended in pure water to give a brown dispersion, which was subjected to dialysis to absolutely remove the remaining salts and acids. The resultant purified GO powders were collected by centrifugation and air-dried [33,34].