Phosphoric acid functionalized graphene oxide: A highly dispersible carbon-based nanocatalyst for the green synthesis of bio-active pyrazoles

2.1 Materials

Graphite flakes (325 mesh), polyphosphoric acid (PPA, 105% H₃PO₄ basis) and phosphoric acid (PA, analytical grade) were purchased from Sigma-Aldrich. All common reagents for the preparation of pyranopyrazoles were obtained from commercial suppliers and used without further purifications.

2.2 Characterizations

The chemical composition of the catalyst was characterized with Raman spectrometer (LabRAM HR Evo, Horiba), Fourier Transform Infrared attenuated total reflection spectroscopy (FTIR-ATR Thermo Fisher Scientific Nicolet iS50 spectrometer) and X-ray photoelectron spectroscopy (XPS, PHI Quantera II Scanning XPS Microprobe). The localized chemical composition (O/C and P/C ratios) and surface morphology were evaluated by Scanning Electron Microscope (SEM/EDX, Gemini SEM500). XRD diffraction pattern was obtained with the PANalytical Empyrean diffractometer with PIXcel^3D detector over a range of 2θ  = 4 − 70°. The peak position was used to calculate interlayer spacing according to Bragg’s equation (λ = 2dsinθ).

The structure of the prepared pyrazoles was confirmed with ¹H and ¹³C NMR spectroscopy at 400 and 100 MHz, respectively (BrukerAQS-AVANCE spectrometer using TMS as an internal standard in DMSO-d₆ or CDCl₃), FTIR-ATR and elemental analysis (Perkin Elmer 2004 (II) CHN analyzer). The melting points were measured using a capillary tube method with a Barnstead Electrothermal 9200 apparatus.

2.3 Preparation of phosphonated catalyst

The catalyst was prepared in a high yield procedure with three steps.

2.4 General procedure for the synthesis of dihydropyrano[2,3-c]pyrazole derivatives

A mixture of aromatic aldehydes (2 mmol), malononitrile (0.13 g, 2 mmol), ethyl acetoacetate (0.26 g, 2 mmol), hydrazine hydrate (2.5 mmol) and phosphonated GO nanocatalyst (5 mol% – of aldehydes) was refluxed at water media for an appropriate period of time. After completion of the reaction which was monitored by thin layer chromatography (TLC), the reaction mixture was cooled to room temperature and the precipitated organic product was decanted, washed with water (3 × 10 ml) and recrystallized from proper solvents to give the pure products. Then, the separation of the catalyst was carried out by centrifugation. The recycled catalyst was washed with ethanol (5 ml), dried in an oven under vacuum and used again for a sequential run under similar reaction conditions.

3.1 Synthesis and characterization of the nanocatalyst

Phosphonated GO was prepared from graphite in a three-step procedure of oxidation, exfoliation and phosphonation as shown in Fig. 1. The oxidation and exfoliation steps are well established methods to prepare graphene oxide (Hummers and Offeman, 1958; Kovtyukhova et al., 1999) Parameters of phosphonation step could be varied to achieve desired level of functionalization.

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Fig. 1 Synthesis route of phosphonated GO.

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Successful exfoliation of Graphite was confirmed with XRD analysis as shown in the Fig. 2. The representative peaks of exfoliated GO and phosphonated GO was seen around 2θ = 9.7. The magnitude of XRD interlayer spacing shows that the distance among carbon layers have been increased slightly upon phosphonation and the value is around 1 nm. This enhancement can be due to the non-planar nature of phosphonic acid groups. The broad peak at 2θ around 25° of phosphonated graphene is due to the sample preparation as the nanocatalyst was deposited on a glass substrate.

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Fig. 2 XRD pattern of graphite, exfoliated GO and phosphonated GO.

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Fig. 3 shows the changes in the IR spectrum upon phosphonation. As shown, the characteristic peaks for O—H stretching at 3400 cm⁻¹, C⚌O stretching at around 1690 cm⁻¹, C⚌C at 1580 cm⁻¹ and C—O stretching at 1220 cm⁻¹ have been seen for GO. Upon phosphonation, P—O, C_ring—P and P ⚌ O peaks have been overlapped and seen as a broad peak centered at 1040 cm⁻¹. Peak around 1350 cm⁻¹ may also be related to phosphoric acid groups. In addition, C⚌O stretching peak was shifted to around 1715 cm⁻¹ upon phosphonation due to less involvement of carboxylic acids in the hydrogen bonding as the phosphonic acid groups are more involved in the stronger hydrogen bonds. Extended easily polarizable hydrogen bonding in phosphonated GO resulted in a broad bond around 2900–3600 cm⁻¹. Additionally, a weak-broad peak ending around 620 cm⁻¹ could be attributed to the P—C stretching.

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Fig. 3 FT-IR spectra of GO and phosphonated GO.

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As shown in Fig. S1, phosphonation reaction further confirmed by the XPS spectra as the P 2p and P 2 s peaks were clearly found in the narrow scan at around 133.5 and 191.0 eV, respectively. To confirm the detailed structure, narrow scan of the C 1s and O 1s were deconvoluted as shown in the Fig. 4. C 1s revealed a peak at 284.4 eV which is attributed to the presence of C—C bond in the carbon structure of GO. The other peaks centered at 287 and 288.4 eV are assigned to C⚌O and C(O)OH. The small peak at 285.9 eV is assigned for C—P bond which indicates that the phosphonate groups have been attached to C atoms from the P sides. Deconvoluted O 1s revealed 3 peaks centered at 531.8, 532.8 and 535.8 eV corresponding to the C⚌O and P⚌O, C—O and P—O, and C(O)OH, respectively. It could be concluded that both forms of hydrolytically stable P—C bond and P—O—C of limited stability are formed during phosphonation.

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Fig. 4 High-resolution XPS spectra of phosphonated catalyst: (a) C 1s; (b) O 1s.

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EDX elemental analysis was used to calculate the phosphonation level. As shown in the Fig. 5 and summarized in the Table 1, the ratio of O/C has changed from 0.498 in GO to 0.31 in GO-acid-I and increased to 0.44 in GO-acid-III. On the other hand, the ratio of P/C has been increased from 0 in pristine graphene oxide to 0.125 in GO-acid-III. As shown in the SEM micrographs, as-prepared phosphonated catalyst and GO have almost the same graphene-like structures.

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Fig. 5 SEM and EDX analysis of GO (a, c) and GO-PO₃H₂ (b, d).

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The calculated ration of I_D/I_G bands intensity in the Raman spectroscopy is another indication of the extent of defects on graphene materials. Fig. 6 shows the Raman spectra of the GO and GO-PO₃H₂ with two clear broad peaks at 1355 and 1580 cm⁻¹ attributed to D and G bands, respectively. As seen, the intensity of D band (representing symmetry A_1g mode typically results from the presence of sp³ carbons or defects on the graphene sheets) is increasing in compared to G band (representing the E_2g mode of pristine sp² carbon atoms). In addition, such broad D and G bands are clear indicative of a disordered chemical environment. An increment of the I_D/I_G ratio upon phosphonation from 0.815 to almost 1 (in Table 1) indicates the formation of large edges and defects on the phosphonated substrates. In addition, the other peaks assigned to 2D (two-phonon double resonance process) and S3 can be also seen at 2697 and 2947 cm⁻¹. In the expanded 2D band area in Fig. S2, sharp band centered at 2697 cm⁻¹ was seen for GO with a full width half maximum (FWHM) of around 60 cm⁻¹. This peak is indication of 2–3 layers GO. Upon phosphonation, the FWHM of the peak is clearly decreasing to around 40 cm⁻¹. Such changes could be also considered as an indication of enhanced interlayer spacing.

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Fig. 6 Raman spectra of the GO and phosphonated GO.

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3.2 Catalytic investigation of phosphonated GO nanocatalyst

The catalytic activity of the developed acid catalyst was evaluated in the synthesis of pyrano[2,3-c]pyrazoles. Such pyrazoles play an essential role as biologically active compounds and represent an interesting template in medicinal chemistry and pharmaceutical ingredients, such as anti-microbial, analgesic, anti-inflammatory, vasodilator and anti-fungicidal activities (Maity et al., 2017; Xie et al., 2016b) Varying grades of success have been reported for catalyzing the preparation of pyrano[2,3-c]pyrazoles using zirconium oxide,(Saha et al., 2015) ionic liquids,(Zakeri et al., 2017) silica-supported polyphosphoric acid (Vekariya et al., 2016) and NH₄H₂PO₄/Al₂O₃ (Maleki and Ashrafi, 2014) However, numerous limitations of low yields, use of expensive reagents, long reaction times, tedious work-up procedures with homogeneous catalyst, and co-occurrence of several side reactions have been reported. Consequently, the development of a general multicomponent reaction using an inexpensive, cleaner and green procedure leading to the pyrano[2,3-c]pyrazole derivatives is highly desirable.

Among the developed catalyst with various phosphonic acid content, GO-acid-II was selected for evaluation of the catalytic activity due to the easier recycling compared to GO-acid-III and more active sites compared to GO-acid-I. Fig. 7 represents the schematic of the multicomponent reaction assisted with GO-acid-II nanocatalyst.

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Fig. 7 The preparation of pyrano[2,3-c]pyrazoles using phosphonated GO as a nanocatalyst.

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The catalytic activity of the prepared nanocatalyst was evaluated in different conditions for the reaction of hydrazine hydrate, ethyl acetoacetate, benzaldehyde and malononitrile. As summarized in Table 2, several reactions were carried out to optimize the amount of required catalyst, best solvent and reaction time. The reaction under solvent-free condition at 80 and 120 °C afforded the products in low yields and reflux in water was drastically enhanced the yield. The screening of different solvents revealed that the phosphonated GO is more effective in polar-protic solvents such as ethanol and water compared to organic solvents (e.g. CHCl₃, CH₂Cl₂, DMF, THF and CH₃CN). The optimized conditions of 5 mol% catalyst, water as a reaction media and 15 min reaction time was achieved.

To explore the activity of the catalyst toward different substitutes, the generality of the multicomponent reaction was extended to a library of pyrano[2,3-c]pyrazole derivatives 5a-l under the optimized reaction conditions. As summarized in Table S1 in supporting information, a variety of aryl aldehydes (e.g. —NO₂, —CN, —F, —Cl, —Br, —OMe, —OH) containing either electron withdrawing or electron donating groups afforded 5a-l in good to excellent yields. The spectroscopy details of the prepared pyrazoles were summarized in the supporting information. The high yield of 90% in short reaction time of 15 min was achieved due to presence of concerted functional sites on the catalysts: both hydrophilic phosphoric acid and hydrophobic carbon sites together with acidic and basic sites of phosphate groups. As can be seen in Table S1, the electronic effects of the substituent were not considerably altered the yields of the products. However, relatively low yields of the products were observed in case of phenyl hydrazine.

In a plausible mechanism, the formation of pyrano[2,3-c]pyrazole derivatives proceeds via the simultaneous formation of two intermediates, pyrazolone A via cyclo condensation and arylidenemalononitrile B by Knoevenagel condensation reaction as shown in Fig. 8. Intermediate A was formed by the attack of phenylhydrazine to carbonyl group of the activated ethyl acetoacetate followed with the loss of water and continued with intramolecular nucleophilic attack by another amine group of phenylhydrazine to the next carbonyl group of ethyl acetoacetate. In the next step, formation of the intermediate B was occurred via Knoevenagel condensation between malononitrile and activated benzaldehyde, followed by loss of water molecules. Subsequently, intermediate A undergoes Michael addition with B in the presence of catalyst. The reaction was followed by intramolecular nucleophilic attack and cyclization to afford the expected dihydropyrano[2,3-c]pyrazole derivative. As shown, the amphoteric nature of the phosphonic acid is playing a role in the proposed mechanism and both of.

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Fig. 8 Plausible mechanism for the formation of pyrano[2,3-c]pyrazole derivatives.

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3.3 Reusability study of the phosphonated catalyst

The recovery and reusability are among the most important criteria in developing alternative catalysts. Even though the catalyst dispersion is one of the important advantages of phosphonated materials, it can negatively affect the catalyst recovery. In the synthesis of pyrazoles, the catalyst was collected after each reaction by centrifuging at low temperature and amount of retrieved catalyst was taken as an indication of the recovery. The recovered catalyst was tested for the similar reaction under the identical conditions and the reaction yield was considered as an indication of catalyst stability. As summarized in the Table 3, the catalyst can be reused at least six times with retention of its catalytic performance. However, a slight decrease in recovery was observed mainly after the first run. It could be due to the solubility of highly functionalized and/or small sized nanocatalysts.