Decoration of Graphene Oxide with Cobalt(II) Coordinated Silica and its Catalytic Activity for the Synthesis of Functionalized Indenopyrazolones

We synthesized functionalized f-SiO 2 @GO@Co catalyst through decorating graphene oxide surface using SiO 2 sphere with the help of ethylenediamine ligand and chelation with CoCl 2 .6H 2 O for increasing the catalyst activity to produce heterogenous catalyst. The heterogenous catalyst was characterized by FT-IR, XRD, SEM, Raman spectra, and TGA. We assessed activity of the catalyst in the synthesis of indenopyrazolones and results demonstrated high activity for the catalyst. The ability of the catalyst to increase the yield and reduction in reaction time as well as high catalytic activity, and recycling are prominent advantages of the catalyst.


Introduction
In last few years, applying carbon-based materials as a catalyst have received considerable attention [1].
Graphene is one forms of carbon as a single layer that its crystalline structure is two-dimensional. Graphene was rst discovered in 2004 by Geim and Novoselov [2]. Graphene oxide is an oxidized form of graphene with a two-dimensional (2D) honeycomb structure. A monolayer graphene oxide which is a layer of graphite has various oxygen-containing groups like hydroxyl groups, epoxides, and carboxyl group via oxidation of graphite crystals. The presence of oxygen functional groups on the surface of graphene oxide increase chemical interactions so, graphene oxide can be participated as a desirable support or catalyst in chemical reactions [3][4][5].
Functionalization of graphene oxide is more bene cial for biomedical, electrochemical, and chemical applications [1]. It can be processed using functionalization of oxygen-containing groups on the basal plane of graphene oxide with different electroactive species [6,7].Different methods have been developed for the synthesis of graphene oxide but the common method is Hummer with oxidation of graphite using KMnO 4 under acidic conditions [8]. The llers like SiO 2 are often used as corrosion resistant coatings.
Then, the proper dispersion of SiO 2 in graphene oxide/epoxy coating can improve corrosion resistance [9,10]. Also, for improving activity and stability of the catalysts metal-based catalysts are used. Due to the high cost of noble metals, the cheaper metals replace for this purpose such as Fe, Ni, and Co [11].
Nitrogen heterocyclic compounds have attracted a researcher's interest because they have more applications in biological and other sciences [12]. Pyrazoles are a famous series of ve-membered nitrogen heterocycles containing two adjacent nitrogen atoms [13]. Compounds containing pyrazole ring and its derivatives often exhibit different physiological and pharmacological properties such as anticonvulsant [14], antioxidant [15], anticancer [16], and fungicides [17]. Moreover, some compounds with pyrazole ring are used as ligands in transition-metal-catalyzed cross-coupling reactions [18,19].
In the current literature, we focus on preparation, characterization and application of an e cient heterogenous catalyst based on cobalt(II) coordination on f-SiO 2 functionalized graphene oxide(f-SiO 2 @GO@Co) (Scheme1) for the synthesis of cis-3-aryl-3a,8b-dihydro-3a,8b-dihydroxy-1-phenylindeno [1,2-c] Synthesis of graphene oxide 1 g graphite powder and sodium nitrate (0.5 g) were added into 25 ml of acid sulfuric and stirred for 10 minutes. Under magnetic stirring, potassium carbonate (3 g) was slowly added into the mixture. Then, the mixture was heated to 35 o C and stirred for further 30 min. After that, 45 ml deionized water was added to the mixture and temperature was then raised up to 95 o C and stirred for 15 min. Next, add 150 ml deionized water and 10 ml hydrogen peroxide 30% to the solution. The resulting solid phase was ltered and repeatedly washed with hydrochloric acid and deionized water for several times. The obtained solid was graphite oxide and dried at temperature 60 o C for 12 h. The resulting solid was dispersed in deionized water by ultrasonication for making graphene oxide. At the end, the nal solid was recovered by centrifugation and dried for 24 h in 60 o C. The nal brown solid is graphene oxide.

Synthesis of spherical SiO 2 nanoparticles
The mixture of distilled water (20 ml) and ethanol (50 ml) were sonicated for 30 min. Then, 3 ml TEOS was added dropwise within 5 min followed by addition of 0.1 mmol pvp into the mixture under stirring. Thereafter, 0.1 ml ethylenediamine was added dropwise into the mixture as precipitating agent, under ultrasonic. After 30 min, the produced SiO 2 product isolated by centrifugation and washed with ethanol and water three times. The nal product was dried at 80 o C for overnight.
Synthesis of SiO 2 @ CPTES (0.5 ml, 5 mmol) 3-chloropropyl triethoxysilane (CPTES) was added dropwise to a stirred solution of SiO 2 (1 g) in dry toluene (30 ml) and re uxed for 24 h. After completion of the reaction, the impure product was separated and washed three times with toluene and dried under 120 °C in a vacuum oven for 8 h to obtain the white powder as SiO 2 @CPTES.

Synthesis of SiO 2 @ Ethylenediamine (f-SiO 2 )
In a 50 ml round-bottomed ask, Ethylenediamine (0.3 g, 1 mmol) was added to the suspension of SiO 2 @CPTES (1 g) in absolute ethanol (30 ml) and heated under re ux for 24 h. The resulting solid was collected by ltration and washed successively with ethanol several times and dried at 90 o C overnight.
Synthesis of f-SiO 2 @GO 0.04 g graphene oxide powder was dispersed in 20 ml deionized water by sonication, then SiO 2 @Ethylenediamine (f-SiO 2 ) (0.16 g) was added to the mixture and sonicated for 20 min. The solution was stirred at 85 o C in an oil bath for 12 h. Lastly, the resulting product was collected by centrifugation, washed with deionized water and ethanol three times, and then dried at 60 o C.

Synthesis of f-SiO 2 @GO@Co
As-prepared f-SiO 2 @GO (0.1 g) with 0.01%wt CoCl 2 were dispersed into an absolute ethanol under ultrasound irradiation for 5 min, and then reacted for 24 h at room temperature. The nal catalyst was collected and washed with ethanol and deionized water. The product was dried at room temperature for several hours to obtain f-SiO 2 @GO@Co catalyst.
General procedure for the synthesis of cis-3-aryl-3a,8bdihydro-3a,8b-dihydroxy-1 phenylindeno[1,2-c]pyrazol-4(1H)-ones A mixture of aldehyde (1 mmol) and phenylhydrazine (1 mmol) and 15 mol% f-SiO 2 @GO@Co as a catalyst in ethanol (5 ml) were stirred at 60 o C until an intermediate was formed. Next, ninhydrin (1 mmol) was added to the mixture reaction and allowed to stir until the completion of reaction (monitoring by TLC). After that, the heterogeneous catalyst was separated and the crude product was collected and washed with n-hexane and ethyl acetate to achieve the pure nal product.

Results And Discussion
Characterization of catalyst FT-IR spectra for the catalyst preparation steps are reported in Fig. 1 describes hydroxyl group vibrations. In SiO 2 @Cl and SiO 2 @Ethylenediamine spectra, the bands at around 950 cm -1 is related to ethoxy moieties vibrations. Functionalized graphene oxide shows the characteristic peak in 1102 cm -1 which reveals that functionalized SiO 2 was successfully grafted on the graphene oxide.
XRD patterns of the catalyst in different steps are depicted in Fig. 2. GO sheets show a characteristic peak around 12 o which proves the synthesis of graphene oxide. In comparison graphene oxide and functionalized graphene oxide, the new broad peak at 2θ = 25 o is related to amorphous SiO 2 which shows surface functionalization of graphene oxide. The small peak at 2θ = 44 o correspond to cobalt in XRD pattern of nal catalyst con rm successful modi cation of the GO surface.
The SEM images of GO(a) and GO@f-SiO 2 @Co (b) has been represented in Fig. 3. The SEM image of graphene oxide clearly shows the layer sheet structure of graphene oxide and GO@f-SiO 2 @Co exhibits the surface modi cation of graphene oxide with functionalized silica nanoparticles.
The presence of Si, Co, C, N in EDS spectrum of the catalyst (Fig. 4) con rm decoration of graphene oxide surface with functionalized SiO 2 .
The Raman spectra for GO and GO@f-SiO 2 are shown in Fig. 5. The characteristic peaks for Go at 1362 and 1595 cm -1 are attributed to D and G bands, respectively. Also, the spectrum of GO@f-SiO 2 also shows these peaks which con rm the presence of graphene oxide in the structure. In addition, after functionalization of GO slight increase in the ratio of I D /I G , indicating more transition from sp 2 to sp 3 from grafting of f-SiO 2 on the graphene oxide.
According to the differential thermal analysis (DTA)/Thermogravimetric analysis (TGA) for the nal catalyst (Fig. 6), the primary stage of decomposition occurred at 220 o C and continued to 800 o C with 18% weight loss in endothermic condition according to the curve of DTA. which is attributed to decomposition of the organic functional groups on the graphene oxide surface.

Catalyst reusability
The important point for a proper catalyst is recovery and recycling. The reusability of f-SiO 2 @GO@Co was investigated in the model reaction between phenylhydrazine, ninhydrin, and 2-nitrobenzaldhyde. For checking the reusability, after the completion of the reaction, the catalyst was recovered using ltration, and washed with ethanol to remove impurities and then dried. The recovery of the catalyst was excellent with an average yield (96%) after ve times subsequent use. As shown in Fig. 7 the catalyst activity without considerable loss was approximately same for ve cycles.

Analysis and characterization of synthesized compound
In order to optimize the reaction conditions, different parameters such as temperature, solvent and catalyst loading were assessed on the model reaction between phenylhydrazine, ninhydrin, and 2nitrobenzaldehyde. Firstly, the reaction was conducted in MeCN without catalyst in r.t. and, no product was formed after 24 h. When the reaction was carried out in the presence of f-SiO 2 @GO@Co in MeCN at 60 o C, no signi cant yield was formed after 24 h (50%). Also, the reaction was carried out in the presence of f-SiO 2 @GO in ethanol and the product was obtained in 75% after 24 h (Table 1, entry4). Then it was tested in ethanol in different temperatures in the presence of f-SiO2@GO@Co as a catalyst ( Table 1). The results revealed the yield of product increase at 60 o C ( Table 1, entry7). According to results shown in Table 1, the best performance of the catalyst was in the presence of ethanol as a solvent. Furthermore, we investigated the effect of catalyst loading (Table 2) and the yield improvement was found with increasing the catalyst from 3 to 15 wt%. It should be mentioned, increasing the more amount of catalyst did not affect on the yield. The performance of the 15 wt% f-SiO 2 @GO@Co in the reaction rate in the presence of ethanol as e cient solvent and various aldehydes are given in Table 3.
To con rm the accuracy of desired products (4a-k), we used FT-IR, 1 H NMR, and 13 CNMR. The IR spectrum of the compound 4i exhibits the peak at 3461 cm -1 that is attributed to the stretching vibrations of hydroxyl groups. The strong peak at around 1710 cm -1 indicates the presence of carbonyl group. It shows singlet peaks at δ = 7.93 ppm and δ = 7.31 ppm due to hydroxyl groups. The protons on the aromatic rings appear between δ = 8.37 to 7 ppm. In addition, the peak for carbonyl group in 13 13

Conclusion
In summary, we have synthesized GO@f-SiO 2 @Co as a heterogenous and recoverable catalyst which was an e cient catalyst for synthesis of indenopyrazolones derivatives. Results showed the catalyst with high catalytic activity provided excellent yields in a shorter reaction time under mild conditions.

Declarations
Ethics approval and consent to participate Not applicable

Consent for publication
The authors declare that the copyright belongs to the journal Availability of data and materials