Immobilized Lanthanum(III) Triflate on Graphene ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Immobilized Lanthanum(III) Triflate on Graphene Oxide as a New Multifunctional Heterogeneous Catalyst for the One-Pot Five-Component Synthesis of Bis(pyrazolyl)methanes Sara Sobhani,*,† Farzaneh Zarifi,† and Jørgen Skibsted‡ †

Department of Chemistry, College of Sciences, University of Birjand, Birjand 97175615, Iran Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark

‡

S Supporting Information *

ABSTRACT: In this work, immobilized lanthanum(III) triflate on graphene oxide as a new multifunctional heterogeneous catalyst has been synthesized. The structure of immobilized lanthanum(III) triflate on amine-grafted graphene oxide [La(OTf)2-grafted-GO] was established by FT-IR, XPS, and solid-state 13C MAS NMR. The morphology and distribution of elements on La(OTf)2-grafted-GO was determined by FESEM, TEM, and EDS images. La(OTf)2-grafted-GO was tested as the first reusable (Lewis and Brønsted) acid−base multifunctional catalyst for the efficient one-pot fivecomponent synthesis of bis(pyrazolyl)methanes. This multifunctional catalyst exhibits a high synergic effect in this one-pot tandem reaction. Due to the uniform spreading of the active sites along with the crumpling structure of the catalyst, reactants and products can simply move toward the catalytic active sites on the surfaces of the catalyst without significant mass-transfer limitation. These properties increase the catalytic activity of La(OTf)2-grafted-GO. Moreover, the catalyst was readily recycled and reused 9 times. The morphology and structure of La(OTf)2-grafted-GO remained intact after 9 recoveries according to the FT-IR spectrum and TEM image of the used catalyst. KEYWORDS: Cooperative multifunctional catalyst, Acid−base catalyst, Heterogeneous catalyst, Tandem reaction, Graphene oxide

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INTRODUCTION The use of multifunctional catalysts in tandem reactions, in which two or more catalytic reactions take place in one vessel, eliminates the waste of time and yield during synthesis in sequential steps.1,2 Thus, these processes become “green”, sustainable, and attractive with low-cost, step-saving and result in a reduced energy consumption, waste production and consumption of reagents and solvents used.3 It is wellknown that acids and bases are the most attractive catalysts in organic chemistry.4−8 However, the use of acidic and basic functions in a homogeneous system cannot be implemented as they neutralize each other in so-called “wolf-and-lamb” reactions forming inactive salts. In contrast, a heterogeneous multifunctional catalyst may address this challenge by spatially isolated incompatible active organic groups, avoiding their mutual deactivation.9 Silica, clays, and polymers are known supporting materials which have been used for the immobilization of basic and acidic functions.10−12 The presence of both acidic and basic functions on the surfaces of a solid material often requires complex synthetic methods including tedious protection−deprotection steps.13,14 The utilization of acidic supports, e.g., silica, MCM-41, SBA-15, periodic mesoporous organosilicas (PMOs), zeolite, and K10 montmorillonite, which contain inherent silanols as weak acids © 2017 American Chemical Society

may lead to simple procedures for the synthesis of catalysts containing two functions by post-grafting with basic functional groups. However, in mesoporous materials, the pore size is reduced due to the attachment of a layer of functional moiety on the surface.10−12,15,16 Moreover, during functionalization a fraction of acidic silanols are replaced by basic functional groups such as amines, which reduces the concentration of acidic sites on the catalyst. One way to compensate for this reduction in acid concentration is to replace the weakly acidic silanols by stronger acid sites such as sulfonic, carboxylic, or phosphoric acids.17−19 However, this approach has a negative impact on the reactivity of amines, which protonates by the stronger acids. To avoid undesired acid−base interactions, the design of new catalysts that possess separated acidic and basic functional groups is highly desirable. Graphene oxide (GO) is known as a very attractive carbon material since it is inexpensive, highly available, environmentally friendly, and exhibit interesting physical properties such as excellent thermal and mechanical stability.20,21 GO has a two-dimensional structure, which consists of basal planes of Received: December 15, 2016 Revised: April 25, 2017 Published: May 4, 2017 4598

DOI: 10.1021/acssuschemeng.6b03047 ACS Sustainable Chem. Eng. 2017, 5, 4598−4606

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. Synthesis of Immobilized Lanthanum(III) Triflate on Amine-Grafted Graphene Oxide [La(OTf)2-grafted-GO]

honeycomb sp2-bonded carbon with unique surface properties. Epoxy and hydroxyl groups are located on the GO planes, while the carboxylic acids are present on the sheet edges.22,23 Recently, GO has been used as an acidic solid support for the immobilization of basic functional groups to produce acid−base bifunctional catalysts.24,25 These heterogeneous catalysts can be recycled and reused without any difficulty.26−28 Pyrazolones are an important class of heterocyclic compounds, which show different biological and agrochemical activities.29−32 Two general methods exist for the synthesis of 4,4′-(arylmethylene)bis(3-methyl-1-phenyl-1H-pyrazol-5-ols), which are an important group of pyrazolone derivatives: (i) one-pot three-component reaction of aldehydes with 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one (two equivalents) and (ii) one-pot five-component reaction of phenylhydrazine (two equivalents) and acetoacetate derivatives (two equivalents) with arylaldehydes.33−40 However, most of the existing methods have some drawbacks such as the use of environmentally harmful organic solvents, nonrecyclable catalysts, or the need for additional amounts of reagents or difficult workup procedures, which produce large amounts of poisonous waste materials. Thus, from the standpoint of environmentally benign organic synthesis, the design of an efficient and easily recyclable catalyst for the preparation of these important scaffolds is still a challenge. Furthermore, despite the importance of the use of multifunctional catalysts for promoting tandem reactions, there is no report in the literature for the application of multifunctional catalysts for the one-pot five-component synthesis of bis(pyrazolyl)methanes from the reaction of phenylhydrazine and acetoacetate derivatives and arylaldehydes. Taking advantage of multifunctional catalysts in one-pot tandem reactions and further development of work in our laboratory on the introduction of newly synthesized heterogeneous catalysts,41−45 in this paper, we report the synthesis of immobilized lanthanum(III) triflate on amine-grafted graphene oxide [La(OTf)2-grafted-GO] as a new multifunctional catalyst (Scheme 1). This catalyst is characterized by various methods such as FT-IR, XPS, TEM, FESEM, EDS, ICP, and solid-state 13C MAS NMR and employed as the first reusable (Lewis and Brønsted) acid−base trifunctional catalyst for the

Figure 1. FT-IR spectra of (a) amine-grafted GO, (b) Schiff base-GO, and (c) La(OTf)2-grafted-GO.

tandem reaction of phenylhydrazine, ethyl acetoacetate, and arylaldehydes.

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EXPERIMENTAL SECTION

Synthesis of Amine-Grafted GO. GO41 (1.0 g) and diethylenetriamine (10.0 g) were dispersed in ethanol (100 mL), sonicated (30 min), and stirred (24 h) at room temperature. The solid material was then centrifuged, washed three times with ethanol, methanol, and acetone and then dried at 80 °C in a vacuum oven overnight to obtain amine-grafted GO. Synthesis of Schiff Base Supported on Amine-Grafted GO (Schiff base-GO). Salicylaldehyde (0.2 mL) was added slowly to the sonicated mixture of GO (1.5 g) in absolute ethanol (50 mL). The resulting mixture was stirred at 80 °C for 24 h and cooled to room temperature. The suspension was centrifuged and washed with ethanol. The resulting Schiff base-GO was dried at 50 °C in a vacuum oven. Synthesis of Immobilized Lanthanum(III) Triflate on AmineGrafted GO [La(OTf)2-grafted-GO]. Lanthanum(III) triflate (0.7 g) was added to the dispersed mixture of Schiff base supported on aminegrafted GO (1.5 g) in anhydrous acetonitrile (30 mL). The resulting mixture was stirred (24 h) at room temperature. The solid was centrifuged, washed three times with acetonitrile, and dried under vacuum at 90 °C overnight to give La(OTf)2-grafted-GO. 4599


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Figure 2. (a) XPS spectrum of La(OTf)2-grafted-GO. (b) N 1s XPS spectrum of La(OTf)2-grafted-GO. (c) High-resolution spectrum showing the XPS signal of the La 3d in La(OTf)2-grafted-GO.

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General Procedure for the Synthesis of Bis(pyrazolyl)methanes. Ethyl acetoacetate (2 mmol), phenyl hydrazine (2 mmol), and La(OTf)2-grafted-GO (0.5 mol %) was mixed and stirred at 100 °C for 5 min. To this reaction mixture, aldehyde (1 mmol) was added. After stirring for the suitable reaction time (Table 2), EtOH (10 mL) was added, and the separation of the catalyst was carried out by centrifugation. Then, water was added to the organic solution and the precipitated product was filtered. The recycled catalyst was washed with EtOH, dried in an oven under vacuum (80 °C, 1 h), and used again for a sequential run under similar reaction conditions. Characterization. Chemicals were purchased from Merck Chemical Company. 1H NMR spectra were recorded on a Bruker Advance DPX-400 and -500 spectrometers. Chemical shifts are reported in ppm relative to TMS as internal standard. The reactions were monitored by TLC on silica-gel polygram SILG/UV254 plates. Fourier transform infrared spectra were recorded on a Shimadzu FT-IR-8300. TEM images were recorded using TEM microscope (Philips CM30). The morphology of the catalyst was examined by scanning electron microscopy (SEM) and field-emission scanning electron microscopy (FESEM, model Mira 3-XMU). The elemental composition was determined by energy-dispersive spectroscopy (EDS) using SEM (model Mira 3-XMU). The solid-state 13C{1H} CP/MAS NMR spectra were obtained on a Bruker Advance II 400 MHz (9.4 T) spectrometer using a 4 mm CP/MAS NMR probe, a spinning speed of νR = 10.0 kHz, a 2 s relaxation delay, and a CP contact time of 1.5 ms. An OPTIMA 7300DV ICP analyzer was used to determine the content of lanthanum in the catalyst. Surface elemental analysis of the catalyst was performed with an ESCA/AES system. This system was equipped with a concentric hemispherical (CHA) electron energy analyzer (Specs model EA10 plus) suitable for XPS. Elemental analysis (C, H, and N) was performed on a Costech 4010 CHN elemental analyzer.

RESULTS AND DISCUSSION Synthesis and Characterization of La(OTf)2-graftedGO. GO was initially functionalized with an appropriate concentration of diethylenetriamine to give amine-grafted GO. The condensation of salicylaldehyde with amine-grafted GO produced a Schiff base supported on amine-grafted GO (Schiff base-GO). The reaction of lanthanum(III) triflate [La(OTf)3] with the Schiff base-GO in anhydrous acetonitrile led to the formation of La(OTf)2-grafted-GO (Scheme 1). Different techniques such as FT-IR, XPS, TEM, FESEM, EDS, ICP, and solid-state 13C MAS NMR were used for the characterization of La(OTf)2-grafted-GO. By using FT-IR, the chemical nature of La(OTf)2-graftedGO was examined. The FT-IR spectra in Figure 1 show the presence of a peak at 1571 cm−1 that is related to N−H bending vibrations, which is a good indication of the grafting of amine groups on the surface of GO. Stretching and bending vibrations of CH2 appear at about 2902, 2850, and 1420 cm−1. The peaks at 1714, 1621, and 1228 cm−1 are assigned to the CO, CC, and C−OH groups, respectively. A new absorption band at 1664 cm−1, ascribed to the stretching vibrations of CN, is observed in the FT-IR spectrum of the Schiff base-GO. This indicates that the Schiff base is successfully immobilized onto the amine-grafted GO. Moreover, the strong bands at 1033 and 638 cm−1 are attributed to the S−O vibration in La(OTf)3. The electronic properties of the La(OTf)2-grafted-GO was probed by XPS analysis. As shown in Figure 2a, the peaks corresponding to C 1s, O 1s, N 1s, La 3d, F 1s, S 2s, and S 2p are clearly detected in the full XPS spectrum. Figure 2b 4600


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the crumpling structure of the catalyst implies that the reactants and products can easily move toward the catalytically active sites on the surface of the catalyst without significant mass transfer limitation. These properties increase the catalytic activity of La(OTf)2-grafted-GO. The inductively coupled plasma mass spectroscopy (ICP) analysis of the catalyst indicates that the amount of lanthanum immobilized on 1.0 g of La(OTf)2-grafted-GO is 0.12 mmol. The 13C{1H} CP/MAS NMR spectrum of the amine-graftedGO (Figure 4a) contains resonances at 37.1, 45.6, 62.4, and 72.6 ppm, which are assigned to C1, C2, C3, and C4 of the amine group, whereas the broader resonances at 128 and 168 ppm in the aromatic region correspond to CC and COOH groups of the GO. The spectrum of the Schiff-base-GO (Figure 4b) exhibits the same four resonances from C1−C4 and increased intensity in the 110−140 ppm region, reflecting additional intensity from the aromatic ring. Three peaks can be identified in this region at 116, 122, and 130 ppm, which are typical chemical shifts for aromatic-ring carbons. In addition, two peaks at 159 and 165 ppm are identified at higher frequency, which may reflect the aromatic C−OH and CN carbons. The spectrum of La(OTf)2-grafted-GO (Figure 4c) has been acquired exactly as the spectra of the amine-graftedGO and Schiff-base-GO. Similar peaks are detected in the spectrum of the La(OTf)2-grafted-GO. The intensity of the 122 and 165 ppm peaks is higher in the spectrum of the La(OTf)2-grafted-GO, which indicates that the lanthanum(III) triflate unit stabilizes the aromatic C−OH and CN carbons, i.e., more rigid carbons give stronger 13C−1H dipolar couplings and thereby increased CP intensity. Catalytic Performance of the La(OTf)2-grafted-GO. We have investigated the applicability of La(OTf)2-grafted-GO as a multifunctional catalyst for the tandem synthesis of bis(pyrazolyl)methanes. For identification of the best solvent, temperature, and amount of the catalyst, the reaction of phenylhydrazine, ethyl acetoacetate, and 4-nitrobenzaldehyde was studied (Table 1). On the basis of these experiments, 0.5 mol % catalyst, neat conditions, and 100 °C (Table 1, entry 1) were chosen as the optimized reaction conditions. Lower yields of the product were obtained in some organic solvents such as H2O, EtOH, MeOH, and CH3CN. To demonstrate the catalyst function, reactions of phenylhydrazine (2 mmol) and ethyl acetoacetate (2 mmol) in the presence of GO, La(OTf)3, Schiff base-GO, and a physical mixture of Schiff base-La(OTf)2 + GO or under catalyst-free conditions were performed (Figure 5). Pyrazolone was obtained after stirring for 5 min. 4-Nitrobenzaldehyde (1 mmol) was added to the stirring mixture to produce bis(pyrazolyl)methane. Pyrazolone and bis(pyrazolyl)methane were produced in higher yields in the presence of GO or La(OTf)3 compared with that produced under the catalyst-free conditions. These results reveal that carboxylic acids on GO and also La(OTf)3 promote the reaction of phenylhydrazine, ethylacetoacetate, and 4-nitrobenzaldehyde. Schiff base-GO exhibited a higher catalytic efficiency than did GO in the formation of bis(pyrazolyl)methane due to the synergic effect of amine linkers as basic groups and inherent carboxylic acids on the GO. Lower catalytic activity was observed in the presence of a physical mixture of Schiff base-La(OTf)2 + GO. La(OTf)2-grafted-GO catalyzed the reaction to produce a higher yield of pyrazolone and bis(pyrazolyl)methane (45 and 80% yields, respectively). This phenomenon is attributed to the synergistic effects of acidic and

illustrates that the N 1s peak is divided into three components at binding energies of 398.1, 399.0, and 400.9 eV, which are ascribed to the nitrogen of C−NH−C, CN, and NH2, respectively. Furthermore, the high-resolution XPS signal for La 3d (Figure 2c) shows four peaks that can be assigned to La 3d5/2 and La 3d3/2. The peaks at 835.0 and 837.8 eV can be assigned to La 3d5/2, and those at 852.0 and 854.1 eV are attributed to La 3d3/2. The FESEM and TEM images reveal that La(OTf)2-graftedGO has a crumpling structure (Figure 3a−d). The O, N, and La

Figure 3. (a, b) FESEM images and (c, d) TEM images of La(OTf)2grafted-GO. The corresponding quantitative EDS element mappings are shown as an overlay of the key elements (e) and the individual elements of (f) O, (g) N, and (h) La.

elements are uniformly spread on the surface of La(OTf)2grafted-GO according to the EDS images (Figure 3e−h). The uniform distribution of the functional groups along with 4601


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Figure 4. 13C{1H} CP/MAS NMR spectra of (a) amine-grafted GO, (b) Schiff base-GO, and (c) La(OTf)2-grafted-GO acquired at 9.4 T using a spinning speed of νR = 10.0 kHz and a CP contact time of 1.5 ms. Asterisks indicate spinning sidebands.

Table 1. Optimization of the Reaction Conditions for the Synthesis of Bis(pyrazolyl)methanesa

entry 1 2 3 4 5 6 7 8

catalyst (mol %) La(OTf)2-grafted-GO La(OTf)2-grafted-GO La(OTf)2-grafted-GO La(OTf)2-grafted-GO La(OTf)2-grafted-GO La(OTf)2-grafted-GO La(OTf)2-grafted-GO La(OTf)2-grafted-GO

(0.5) (0.5) (0.5) (0.5) (0.5) (0.25) (1) (0.5)

solvent H2O EtOH MeOH CH3CN

T (°C)

time (min)

yield (%)b

100 100 100 100 100 100 100 80

15 30 60 60 30 15 15 15

80 60 68 60 30 65 85 50

Figure 5. Effect of various active groups toward the one-pot fivecomponent reaction of phenylhydrazine, ethyl acetoacetate, and 4-nitrobenzaldehyde. The yields of pyrazolone were obtained after 5 min of stirring phenylhydrazine (2 mmol) and ethyl acetoacetate (2 mmol). The yield of bis(pyrazolyl)methane was obtained after 4-nitrobenzaldehyde (1 mmol) was added to the stirring mixture in 15 min.

a

Phenylhydrazine (2 mmol), ethyl acetoacetate (2 mmol), and La(OTf)2-grafted-GO were stirred for 5 min. 4-Nitrobenzaldehyde (1 mmol) was added to the stirring mixture. bIsolated yield.

basic functional groups presented in La(OTf)2-grafted-GO in the tandem synthesis of bis(pyrazolyl)methane. On the basis of these observations, a mechanism which is supported by the literature33,36,37 for tandem synthesis of bis(pyrazolyl)methanes is outlined in Scheme 2. The carbonyl groups in the ethyl acetoacetate are initially activated by the acidic functional groups of the catalyst to react with phenylhydrazine to form pyrazolone. Then, the activated aldehydes by acidic functional groups of the catalyst undergo a tandem reaction with the activated pyrazolones by basic sites to produce the desired product.

To test the potential synthetic applications of this method, the reaction of benzaldehyde (50 mmol), ethylacetoacetate (100 mmol), and phenylhydrazine (100 mmol) on a larger scale was examined under the optimized reaction conditions. The desired product was obtained in 98% yield in 1 h. To further investigate the applicability of our method, the reactions of different types of aldehydes (1 mmol) with phenylhydrazine (2 mmol) and ethyl acetoacetate (2 mmol) were studied using La(OTf)2-grafted-GO (0.5 mol %) as the catalyst under optimized reaction conditions (Table 2). 4602


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Scheme 2. Possible Cooperative Multifunctional Catalysis Mechanism for the Synthesis of Bis(pyrazolyl)methanes

It is seen from Figure 6 that the reduction in the activity of the catalyst after 9 reuses is negligible. Comparison of the FT-IR spectrum and the TEM image of used catalyst (Figures 7 and 8) with the corresponding data for the fresh catalyst (Figures 1 and 3c,d) indicates that the morphology and structure of La(OTf)2grafted-GO remains intact after nine recoveries. Finally, the activity of La(OTf)2-grafted-GO was compared with those of some reported catalysts in the reaction of phenylhydrazine, ethyl acetoacetate, and benzaldehyde (Table 3). As depicted in Table 3, La(OTf)2-grafted-GO shows the best catalytic activity for the synthesis of bis(pyrazolyl)methane in terms of TON. This promising result should be attributed to the synergic cooperative effect of both acidic and basic groups in La(OTf)2-grafted-GO.

The reactions of various substituted benzaldehydes with phenylhydrazine and ethyl acetoacetate proceed well, and bis(pyrazolyl)methanes 1a−h are obtained in good to excellent yields (75−98%) (Table 2, entries 1−8). Thiophene-2carbaldehyde and furfural as heterocyclic aldehydes underwent the condensation reaction to produce the desired products in 90 and 95% yields, respectively. The reaction of iso-butyraldehyde as an aliphatic aldehyde also proceeds in a satisfactory manner to give the corresponding product in 78% yields (Table 2, entry 11). Interestingly, La(OTf)2-grafted-GO effectively catalyzed the reaction of both carbonyl groups in terephthaldehyde with phenylhydrazine and ethyl acetoacetate (Table 2, entry 12). The recyclability of La(OTf)2-grafted-GO was studied in the reaction of phenylhydrazine and ethyl acetoacetate with benzaldehyde under the optimized reaction conditions. After completion of the reaction (15 min), the catalyst was centrifuged from the reaction mixture, washed with EtOH, dried in vacuum, and reused again.

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CONCLUSIONS In this work, immobilized lanthanum(III) triflate on graphene oxide as a new multifunctional heterogeneous catalyst has been 4603


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ACS Sustainable Chemistry & Engineering Table 2. One-Pot Five-Component Synthesis of Bis(Pyrazolyl)Methanes Catalyzed by La(OTf)2-grafted-GO under Optimized Reaction Conditionsa entry

aldehyde

product

time (min)

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12

C6H5 4-Me−C6H4 4-MeO−C6H4 4-Cl−C6H4 4-NO2−C6H4 4-CN−C6H4 2-OH−C6H4 4-OH−C6H4 2-thienyl 2-furyl −CH(CH3)2 4-OHC−C6H4

1a 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1l

15 10 20 15 15 45 35 15 15 15 40 35

98 90 87 98 80 80 75 90 90 95 78 70c

Figure 8. TEM image of La(OTf)2-grafted-GO after 9 reuses.

Table 3. Catalytic Activity of La(OTf)2-grafted-GO as a Trifunctional Catalyst in Comparison with Various Known Catalysts for the One-Pot Five-Component Condensation Reaction of Phenylhydrazine, Ethyl Acetoacetate, and Benzaldehyde

a

Phenylhydrazine (2 mmol, except for entry 12), ethyl acetoacetate (2 mmol, except for entry 12) and La(OTf)2-grafted-GO (0.5 mol %) stirred at 100 °C. After 5 min, aldehyde (1 mmol) was added to the stirring mixture. bIsolated yield. cReaction conditions: phenylhydrazine (4 mmol), ethyl acetoacetate (4 mmol).

entry

catalyst (mol %)

reaction conditions

yield (%)a TONb

1 2 3 4 5 6

DCDBTSD (10)c HAP@AEPH2−SO3H (1.5)d Na+-MMT-[pmim]HSO4 (5) [MIm]ClO4 (5.4) RHA-SO3H (60 mg)e La(OTf)2-grafted-GO (0.5)

80 °C solvent-free, 80 °C solvent-free, 100 °C solvent-free, 50 °C solvent-free, 80 °C solvent-free, 100 °C

7433 9837 9146 9036 9147 98f

a

7 65 18 16 196

b

Isolated yield. TON = turnover number = moles of product/mol of catalyst. cN,2-Dibromo-6-chloro-3,4-dihydro-2Hbenzo[e][1,2,4]thiadiazine-7-sulfonamide 1,1-dioxide. d2-Aminoethyl dihydrogen phosphate. eSulfonated rice husk ash. fThis work.

five-component reaction of phenylhydrazine, ethyl acetoacetate, and arylaldehydes. The catalyst was easily recycled and reused repetitively. The morphology and structure of La(OTf)2grafted-GO remained intact after 9 recoveries according to the FT-IR spectrum and TEM image of the used catalyst. In this work, a multifunctional catalyst has been used for the first time for the one-pot synthesis of bis(pyrazolyl)methanes. This multifunctional GO may also find promising applications in other one-pot tandem syntheses. Furthermore, similar catalysts can be synthesized using other metal triflates and amines as basic linkers and ligands, which further widen the scope of these materials.

Figure 6. Reusability of La(OTf)2-grafted-GO in the one-pot reaction of phenylhydrazine, ethyl acetoacetate and benzaldehyde under the optimized reaction conditions after 15 min.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03047. NMR data of the products (PDF)

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Figure 7. FT-IR spectrum of La(OTf)2-grafted-GO after 9 reuses under the optimized reaction conditions.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Fax: +98 56 32202065. Tel.: +98 56 32202065.

synthesized. A systematic characterization of the new catalyst has demonstrated that active sites are readily introduced on the surface of graphene oxide (GO) with a homogeneous distribution. Notably, the synthesized multifunctional [(Lewis and Brønsted) acid−base] catalyst showed superior catalytic performance and synergetic catalytic effects in the one-pot

ORCID

Sara Sobhani: 0000-0002-7764-8847 Jørgen Skibsted: 0000-0003-1534-4466 Notes

The authors declare no competing financial interest. 4604


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ACKNOWLEDGMENTS We thank University of Birjand Research Council and Iran National Science Foundation (INSF) for financial support. Access to the Solid-State NMR facilities at the Department of Chemistry, Aarhus University, is also acknowledged.

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