Cucurbit[6]uril-Supported Fe3O4 Magnetic Nanoparticles Catalyzed Green and Sustainable Synthesis of 2-Substituted Benzimidazoles via Acceptorless Dehydrogenative Coupling.

A new composite, cucurbit[6]uril (CB[6])-supported magnetic nanoparticles, Fe3O4-CB[6], was synthesized via a co-precipitation method in air and fully characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, thermogravimetric analysis, inductively coupled plasma-mass spectrometry, and vibrating sample magnetometry techniques. It has been found to be a highly efficient, economic, and sustainable heterogeneous catalyst and has been employed for the first time for the synthesis of a series of biologically important 2-substituted benzimidazoles from various benzyl alcohols and 1,2-diaminobenzenes under solvent-free conditions via acceptorless dehydrogenative coupling to afford the corresponding products in good to excellent yields (68-94%). The magnetic nature of the nanocomposite facilitates the facile recovery of the catalyst from the reaction mixture by an external magnet. The catalyst can be reused up to five times with negligible loss in its catalytic activity. All the isolated products were characterized by 1H and 13C{1H} NMR spectroscopy.

Introduction

N-Heterocyclic compounds containing imidazole moieties are of immense interest due to their wide range of biological activities.[1] These benzimidazole-based N-heterocycles are known to exhibit an extensive array of pharmaceutical properties, such as antiulcerative,[2] anticancer,[3] antihypertensive,[4] antipyretic,[5] anthelmintic,[6] antiviral,[7] and anti-HIV[8] activities (Figure ).

Figure 1

Pharmaceutically active compounds containing the benzimidazole moiety.

Various synthetic approaches for 2-substituted benzimidazoles starting from primary-/nitro-amines/imines are summarized in Scheme . Previously, 2-substituted benzimidazoles were synthesized under metal-catalyzed, metal-free (reagent-based), green, and photocatalyzed conditions.[9] However, most of the reported methods required harsh reaction conditions and tedious workup. In a majority of the cases, the product was always contaminated by a stoichiometric amount of wastes or byproducts.[10−12] The low atom economy and lack of functional group compatibility were the inherent problems frequently encountered during the synthesis of imidazoles.[13−15] Therefore, a quest for the development of eco-friendly, green, and sustainable methods for the synthesis of 2-substituted benzimidazoles is highly desirable. In this report, an acceptorless dehydrogenative coupling (ADC) methodology has been proved to be a versatile and sustainable method for the synthesis of 2-substituted benzimidazoles because hydrogen and water are the only byproducts generated under homo- and heterogeneous catalysis.[16−18] The ADC reactions of alcohols and diamines have been employed for the synthesis of several 2-substituted benzimidazoles with spectacular success. Many complexes of noble metals (Ru and Ir)[19−23] and transition metals (Mn, Pd, Cu, Ni, and Co) have been used as catalysts for the ADC reactions.[24−31]

Scheme 1

Reported Approaches for 2-Substituted Benzimidazoles Synthesis

Though these metal-based catalysts were efficient, they suffered from several drawbacks, such as the use of expensive ligands, low thermal stability, high catalyst loading, use of toxic organic solvents, recyclability of the catalyst, and so forth. In order to circumvent the inherited drawbacks associated with homogeneous catalysts, the design and development of economic, green, and sustainable heterogeneous catalysts is of prime importance.[32−34]

Iron (Fe) is the third most earth-abundant metal (about 5% of the earth’s crust). It is one of the cheapest metals when compared to other precious metals. Besides being cost-effective, it is an eco-friendly and biologically compatible transition metal, which shows variable oxidation and spin states.[35,36] These properties of iron makes it a metal of choice for developing a cheap and eco-friendly catalyst for organic synthesis.[37−43] In this regard, Fe3O4 magnetic nanoparticles (MNPs) have evolved as an efficient heterogeneous catalyst for a number of organic transformations.[44−47] Fe3O4 MNPs have a cubic inverse spinel structure, where Fe is present in Fe2+ and Fe3+ cationic states. Fe3O4 forms a cubic close-packed structure along the [110] direction by virtue of its unique magnetic structure.[48] Thus, at present, there is a race for developing MNP-based heterogeneous catalysts.[49−52] Fe3O4 MNPs have been used as the heterogeneous catalyst to facilitate various homocoupling and heterocoupling reactions.[53−57] Other applications of Fe3O4 MNPs include A3-coupling reactions,[58,59] Paal–Knorr reaction,[60] aza-Micheal addition,[61] hydrogenation reactions,[62] oxidation reactions,[63] reduction reactions,[54] and so forth. Unsupported or uncapped Fe3O4 MNPs have a tendency for agglomeration and thus inhibit catalytic properties.[64,65] Fe3O4 MNPs have to be stabilized on a support or encapsulated by a surfactant/polymer or embedded in a matrix/support to achieve a homogeneous distribution, desired catalytic activities, and prevent catalytic leaching.[66,67]

Cucurbit[6]uril, CB[6], is a hexameric macrocyclic molecule synthesized by the acid-catalyzed condensation of formaldehyde with glycoluril.[68] The nonpolar cyclic structure is chemically and thermally stable. Its two identical portals contain carbonyl groups that can interact with NPs/metal ions through noncovalent interactions (Figure ).[69−71]

Figure 2

Ball stick model of CB[6].

CB[6] has been used as a capping agent and an ideal support for various metal NPs in the aerobic oxidation of alcohols,[72] Suzuki and Heck reactions,[73,74] and hydrogenation reactions.[75] Recently, CB[6] has also been used in alkyne–azide cycloaddition reactions (click chemistry) for protein conjugate synthesis invoking supramolecular chemistry.[6]Uril-Promoted Click Chemistry for Protein Modification. J. Am. Chem. Soc.. 2017 ">76] The precedent examples of use of CB[6] in catalysis help us in conceptualizing its use as a support in the present work. In continuation of our ongoing interest on the design and development of cheap, green, sustainable, and magnetically separable catalysts for important organic transformations,[77] we report herein the synthesis of a new nanocomposite Fe3O4–CB[6], where Fe3O4 MNPs are immobilized on a CB[6] support. The catalytic potential of the new composite has also been demonstrated for the first time in the synthesis of a series of pharmaceutically important 2-substituted benzimidazoles from 1,2-diaminobenzenes and benzyl alcohols via ADC reactions under solvent-free conditions.

Experimental Section

Preparation of Fe3O4–CB[6]

Cucurbit[6]uril was synthesized by a previously reported method.[68] The Fe3O4–CB[6] composite was synthesized using the co-precipitation method.[77,78] Aqueous solution of CB[6] (200 mg in 20 mL water) was sonicated at room temperature. A freshly prepared solution of FeCl3·6H2O (1 mmol) and FeCl2·4H2O (0.5 mmol) was added to the CB[6] suspension and heated up to 80 °C under constant stirring. To the above solution, 50 mL of 1 M NaOH solution was added dropwise under vigorous stirring at 80 °C, until the pH reached 10–11 (Scheme ).

Scheme 2

(a) Synthesis of CB[6]; (b) Synthesis of the Fe3O4–CB[6] Nanocomposite

A dark brown precipitate of Fe3O4 magnetic NPs was formed. The solution was then cooled to room temperature, and the precipitate was separated using an external magnet. The nanocomposite thus obtained was washed consecutively three times with water and ethanol. The dark brown precipitate was then dried under vacuum at 80 °C.

General Procedure for the Synthesis of 2-Substituted Benzimidazoles

A 25 mL round-bottomed flask was charged with 1,2-diaminobenzene (1 mmol) and benzyl alcohol (1 mmol), followed by the addition of tBuOK (1 mmol) and Fe3O4–CB[6] (10 mg). The reaction mixture was stirred under solvent-free conditions for 8 h at 120 °C. The progress of the reaction was monitored by TLC at different time intervals. The Rf value of 2-phenyl-1H-benzo[d]imidazole was found to be 0.24 with petroleum ether (40–60 °C) and ethyl acetate (3:2). After the reaction was completed, the reaction mixture was cooled down to room temperature and the nanocomposite was removed by an external magnet. The organic compounds were extracted using water and ethyl acetate (3 × 10 mL). The organic layer was dehydrated over anhydrous sodium sulfate followed by the removal of the solvent under reduced pressure to provide the crude compound. The crude product thus obtained was further purified by column chromatography.

Results and Discussion

The Fe3O4–CB[6] nanocomposite was synthesized by a co-precipitation method (Scheme ).[77,78] The synthesized nanocomposite was fully characterized by various physicochemical techniques such as Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDAX), thermogravimetric analysis (TGA), inductively coupled plasma–mass spectrometry (ICP–MS), and vibrating sample magnetometry (VSM) as described below. All the synthesized 2-substituted benzimidazole derivatives were fully characterized by 1H and 13C{1H} NMR spectroscopy.

The FTIR spectra of the support CB[6] and Fe3O4–CB[6] are depicted in Figure . A peak at 1724 cm–1 is observed in the spectrum of the support CB[6], which corresponds to ν(C=O) frequency.[6]Uril, an Organic Porous Material. Phys. Chem. Chem. Phys.. 2017 ">79] In the spectrum of the nanocomposite, a new peak at 576 cm–1 was observed, which may be assigned to ν(Fe–O).[80] The FTIR spectra of both CB[6] and Fe3O4–CB[6] exhibited a sharp peak at 1724 cm–1 assignable to ν(C=O) frequency. Because, no change in the ν(C=O) frequency was observed, the possibility of covalent interaction between the oxygen of the carbonyl group, and the Fe3O4 MNPs is completely ruled out. It is most likely that the two identical electron-rich carbonyl portals of CB[6] are attached to the Fe3O4 NPs through an electrostatic force of interaction.[74]

Figure 3

FTIR spectra of (a) CB[6] and (b) Fe3O4–CB[6].

The PXRD spectra of support CB[6] and Fe3O4–CB[6] are depicted in Figure . The PXRD spectrum of support CB[6] exhibited peaks at 2θ = 13.9, 15.1, 17.8, and 20.9° consistent with the literature report,[81] whereas the PXRD spectrum of Fe3O4–CB[6] showed peaks at 2θ = 30.21, 35.46, 43.23, 57.12, and 62.62°, which corresponds to diffraction planes (220), (311), (400), (511), and (440), respectively (JCPDS card number 19-0629). The PXRD spectrum of Fe3O4–CB[6] confirmed the immobilization of Fe3O4 MNPs on the support CB[6].[82,83]

Figure 4

PXRD spectra of (a) CB[6] and (b) Fe3O4–CB[6].

The full range convoluted XPS spectrum of Fe3O4–CB[6] and the core level deconvoluted spectrum of Fe 2p are depicted in Figure . The convoluted spectrum Fe3O4–CB[6] exhibited peaks at binding energies of 284.5, 399.2, and 530.6 eV assignable to 1s level of C, N, and O, respectively.[84] The deconvoluted 2p spectrum of Fe3O4 indicates two peaks at binding energies of 709.5 and 723.0 eV, corresponding to 2p3/2 orbitals and 2p1/2 orbitals of Fe2+ ions, respectively. Further, two additional peaks observed at 711.0 and 725.2 eV were assigned to the 2p3/2 orbitals and 2p1/2 orbitals of Fe3+ ions, respectively. The presence of Fe3+ and Fe2+ ions confirmed the formation of the Fe3O4 spinel on the CB[6] support. Two small humps observed at 719.0 and 731.0 eV show the existence of weak satellites, which further confirmed the presence of mixed oxides of iron.[85,86]

Figure 5

(a) Full length convoluted XPS spectra of Fe3O4–CB[6] and (b) deconvoluted XPS spectrum of Fe 2p.

The FESEM images of CB[6] and Fe3O4–CB[6] are depicted in Figure . The surface of CB[6] appeared to be smoother in comparison to the surface of Fe3O4–CB[6]. The FESEM image of Fe3O4–CB[6] clearly exhibited a uniform deposition of Fe3O4 MNPs on the surface of CB[6].

Figure 6

FESEM images of (a) CB[6] and (b) Fe3O4–CB[6].

The HRTEM images of Fe3O4–CB[6] are depicted in Figure . The HRTEM images displayed the homogeneous dispersal of Fe3O4 MNPs onto the CB[6] matrix. The average particle size of the MNPs was found to be ∼12 nm without any agglomeration. On further magnification of the HRTEM image, it was found that Fe3O4 MNPs are cube-shaped. Thus, the HRTEM results are in consonance with the FESEM results.

Figure 7

(a) HRTEM image of Fe3O4–CB[6] at 50 nm and (b) HRTEM image of Fe3O4–CB[6] at 5 nm.

The EDAX spectrum of Fe3O4–CB[6] is depicted in Figure shows the carbon, nitrogen, oxygen, and iron contents in Fe3O4–CB[6] (by atomic %). The iron content in the nanocomposite was found to be 12 (by atomic %). However, the iron content in Fe3O4–CB[6], as determined by the ICP–MS analysis, was found to be 4.53 mmol/g (1.48 mol %).

Figure 8

EDAX spectrum of Fe3O4–CB[6].

The TGA thermogram of Fe3O4–CB[6] (Figure S1) shows three phases of weight loss. The first weight loss (∼6%) up to 150 °C may be due to the loss of adsorbed water molecules, the second weight loss (∼5%) for a temperature range of 150–300 °C can be assigned to the partial decomposition of nanocomposite, and the final weight loss observed beyond 300 °C was probably due to the complete degradation of the nanocomposite.[6]Uril. J. Agric. Food Chem.. 2011 ">81,87]

The VSM study was carried out on Fe3O4–CB[6] to know about the magnetic behavior of the composite. The hysteresis loop of Fe3O4–CB[6] is shown in Figure . The data obtained by a vibrating sample magnetometer at room temperature revealed the ferromagnetic behavior of the nanocomposite. The magnetic saturation (Ms) was observed to be 0.438 emu, which was notably less than that of the pristine Fe3O4 NPs and bulk Fe3O4. The slight decrease in the Ms can be attributed to the shielding by the CB[6] matrix on the surface of Fe3O4 MNPs.[65,78]

Figure 9

Magnetic hysteresis loop of Fe3O4–CB[6].

Catalytic Applications of Fe3O4–CB[6] in the Synthesis of 2-Substituted Benzimidazoles

After the synthesis and characterization of Fe3O4–CB[6], the catalytic potential of the synthesized nanocomposite was explored for the synthesis of a series of pharmaceutically important 2-substituted benzimidazoles. For initial studies, 1,2-diaminobenzene and benzyl alcohol were chosen as the ideal substrates, and a blank reaction (without catalyst) was carried out in a 1:1 molar ratio in toluene in the presence of tBuOK at 120 °C. No product formation could be observed by TLC (Table , entry 1). In order to know the role of the support, the next reaction was attempted in the presence of the support CB[6] (20 mg) under identical conditions. The reaction did not proceed to give any product (Table , entry 2), indicating that the support itself has no role in the catalysis. The reaction was then executed in the presence of the synthesized composite Fe3O4–CB[6] (20 mg) in toluene for 24 h at 120 °C. Gratifyingly, the desired product was obtained in 79% yield (Table , entry 3). Prompted by this result, attempts were made to optimize the reaction conditions, viz, solvent, catalyst loading, base, temperature, and time. The reaction was carried out in various solvents such as CH3CN, ethanol, H2O, and DMSO in the presence of tBuOK (Table , entries 4–7). Surprisingly, the first three reactions were unsuccessful; however, only DMSO afforded the desired product in 52% yield (Table , entry 7). Because solvent screening results were not encouraging, the reaction was performed under solvent-free conditions at 140 °C for 24 h (Table , entry 8). To our delight, the solvent-free reaction afforded the best yield (88%) of the desired product. It was followed by the optimization of the catalyst loading, that is, 15, 10, and 5 mg under solvent-free conditions to afford the desired product in 88, 88, and 86% yields (Table , entries 9–11). It is evident from the results that a decrease in the catalyst loading to 5 mg adversely affected the yield of the product; however, the product yield remained constant with 20, 15, and 10 mg of catalyst. Therefore, 10 mg of catalyst was found to be the optimum for the reaction. Further, the reaction was carried out under solvent-free conditions using four different bases, tBuOK, KOH, NaOH, and NEt3 at 140 °C (Table , entries 10, 12–14). Although the reaction proceeded with KOH and NaOH to afford the product in 81 and 77% yield (Table , entries 12 and 13), the best yield was obtained with tBuOK (Table , entry 10). Surprisingly, the reaction was unsuccessful in the presence of NEt3. A base-free reaction was also carried out but it failed to give any product (Table , entry 15). Finally, time and temperature parameters were also explored for the model reaction (Table , entries 16–21). It is quite clear from Table that a temperature of 120 °C and time 8 h are found to be optimum (Table , entry 20).

Table 1

Optimization of Reaction Conditions Such as Catalyst, Solvent, Base, Temperature, and Time for the Synthesis of 2-Substituted Benzimidazole Using 1,2-Diaminobenzene and Benzyl Alcohola

entry	catalyst	mg	solvent	base	time (h)	temperature (°C)	yieldb (%)
1.			toluene	tBuOK	24	120
2	CB[6]	20	toluene	tBuOK	24	120
3	Fe3O4–CB[6]	20	toluene	tBuOK	24	120	79
4	Fe3O4–CB[6]	20	CH3CN	tBuOK	24	80
5	Fe3O4–CB[6]	20	ethanol	tBuOK	24	80
6	Fe3O4–CB[6]	20	H2O	tBuOK	24	100
7	Fe3O4–CB[6]	20	DMSO	tBuOK	24	140	52
8	Fe3O4–CB[6]	20	neat	tBuOK	24	140	88
9	Fe3O4–CB[6]	15	neat	tBuOK	24	140	88
10	Fe3O4–CB[6]	10	neat	tBuOK	24	140	88
11	Fe3O4–CB[6]	5	neat	tBuOK	24	140	86
12	Fe3O4–CB[6]	10	neat	KOH	24	140	81
13	Fe3O4–CB[6]	10	neat	NaOH	24	140	77
14	Fe3O4–CB[6]	10	neat	NEt3	24	140
15	Fe3O4–CB[6]	10	neat		24	140
16	Fe3O4–CB[6]	10	neat	tBuOK	24	120	88
17	Fe3O4–CB[6]	10	neat	tBuOK	24	110	85
18	Fe3O4–CB[6]	10	neat	tBuOK	12	120	88
19	Fe3O4–CB[6]	10	neat	tBuOK	10	120	88
20	Fe3O4–CB[6]	10	neat	tBuOK	8	120	88
21	Fe3O4–CB[6]	10	neat	tBuOK	6	120	85

Reaction conditions: 1,2-diaminobenzene (1 mmol), benzyl alcohol (1 mmol), base (1 mmol), solvent (3 mL).

Yield after column chromatography.

After discerning the optimum reaction conditions, the substrate scope was explored. Benzyl alcohols having both electron-donating (ED) groups and electron-withdrawing (EW) groups were used. An excellent yield of 94% was obtained when the reaction was performed using 4-methylbenzylalcohol (Table , 3b). A slight decrease in the yield was observed when the reaction was carried out with 3-methylbenzylalcohol (Table , 3c). However, no product could be isolated when 2-methylbenzylalcohol was used. Benzyl alcohols having EW groups, such as, −F, −Cl, −Br, and −NO2 at the para position afforded lower yields of the product as compared to the ED groups (Table , 3d–3g). The scope of the reaction was further extended to furfuryl alcohol, which resulted in an excellent yield of the corresponding product (Table , 3h). The substrate scope was further explored with substituted 1,2-diaminobenzene (4-bromobenzene-1,2-diamine and 4-methylbenzene-1,2-diamine) and a decrease in product yield was observed as compared to nonsubstituted 1,2-diaminobenzene. No marked difference in the yield could be observed when the reaction was carried out with 1,2-diaminobenzene bearing an EW group (4-bromobenzene-1,2-diamine) and that an ED group (4-methylbenzene-1,2-diamine) (Table , 3i–3q). All the secluded organic products were characterized by 1H and 13C{1H} NMR.

Table 2

Substrate Scope of 2-Substituted Benzimidazolesab

Reaction conditions: 1,2-diaminobenzenes (1 mmol), benzyl alcohols (1 mmol), tBuOK (1 mmol).

Yield after column chromatography.

A plausible mechanism is proposed for the synthesis of 2-substituted benzimidazoles (Scheme ) based on an earlier report.[24] Initially, benzyl alcohol undergoes dehydrogenation to produce an aldehyde (I), which further reacts with 1,2-diaminobenzene to yield a monoamine intermediate (II) and water. The monoamine intermediate (II) undergoes an intramolecular nucleophilic addition at the imine carbon to give a benzimidazoline intermediate (III). Intermediate III, so formed, undergoes dehydrogenation leading to the formation of the desired 2-substituted benzimidazoles.

Scheme 3

Plausible Mechanism for the Synthesis of 2-Substituted Benzimidazoles

Recyclability

Facile recovery of the catalyst and its reuse in several catalytic cycles is one of the added advantages of heterogeneous catalysis. To confirm the recyclability, the catalyst was extracted from the reaction mixture using an external magnet after the first catalytic cycle. The retrieved catalyst was washed thoroughly with water and ethyl acetate. Subsequently, it was washed with alcohol and dried under a vacuum at 80 °C and reused for the consecutive five cycles. It is evident from Figure a that a gradual decrease in the catalytic activity, after each cycle, was observed from 88 to 79%. The iron content, based on ICP–MS analysis, of the retrieved nanocatalyst after the fifth cycle, was found to be 4.39 mmol/g in comparison to the iron content in a fresh catalyst, 4.53 mmol/g. It shows a negligible leaching of iron after the fifth cycle. The FESEM image of the retrieved catalyst after the fifth cycle (Figure b) exhibited no noticeable changes in the surface morphology. Further, all the characteristic peaks were observed in the FTIR spectrum of the retrieved catalyst after the fifth cycle (Figure S2). All these observations confirmed the robust nature of the catalyst.

Figure 10

(a) Recyclability of Fe3O4–CB[6] nanocatalyst and (b) FESEM of reused Fe3O4–CB[6] nanocatalyst after the fifth cycle.

Comparison of Fe3O4–CB[6] Nanocatalyst with Previously Reported Catalysts

To find out the advantages of the present work over the earlier reported methodology, the outcome of the synthesized nanocatalyst in the synthesis of 2-substituted benzimidazoles has been compared with the previously reported catalysts, and the results are compiled in Table . It is worth mentioning that our catalyst was found to be a better one for the synthesis of 2-substituted benzimidazoles in terms of product yield and reaction conditions.

Table 3

Comparison of Catalytic Activity of the Fe3O4–CB[6] Nanocatalyst with Previously Reported Catalysts

catalyst	conditions	yield (%)	references
FeNT/FeNS	TBHP/CH3CN/60 °C/24 h	70/55	(27)
NiCl2/1–10 Phen	toluene/140 °C/24 h/tBuOK	52	(26)
Ru–N–C	toluene/KOH/110 °C/24 h/N2	86	(30)
3-nitropyridine	DMSO/NaOtBu/110 °C/16 h	82	(29)
NNS–manganese(l) complex	neat/KOH/140 °C/20 h	77	(24)
Fe3O4–CB[6]	neat/tBuOK/120 °C/8 h	88	this work

Hot Filtration Test

To check the heterogeneous nature of the catalyst, a hot filtration test was conducted for 2-substituted benzimidazoles under the optimized reaction conditions in the presence of catalyst (Figure ). After 5 h, the reaction was paused and the catalyst was separated using an external magnet from the reaction mixture under hot conditions, which resulted in 60% yield of the product. The filtrate was again transferred into a reaction flask and heated at 120 °C for 10 h. No enhancement in the yield of the product could be obtained after the removal of the catalyst from the reaction mixture, which confirmed the heterogeneous nature of the catalyst.[88]

Figure 11

Hot filtration test of 2-substituted benzimidazoles.

Conclusion

In conclusion, CB[6]-supported Fe3O4 MNPs were synthesized to form a nanocomposite Fe3O4–CB[6]. The composition of the nanocomposite was well established by a number of instrumental techniques such as FTIR, PXRD, XPS, FESEM, HRTEM, EDAX, TGA, ICP–MS, and VSM. To the best of our knowledge, the potential of Fe3O4–CB[6], as a heterogeneous catalyst, has been demonstrated for the first time for the synthesis of a series of 2-substituted benzimidazoles from 1,2-diaminobenzenes and benzyl alcohols under solvent-free conditions via ADC reactions. The nanocatalyst can be easily recovered by an external magnet and reused up to five times leading to a sustainable approach toward the synthesis of 2-substituted benzimidazoles. It is shown that the nanocatalyst Fe3O4–CB[6] is cheap and environmentally friendly. The studies open up new avenues for sustainable catalysis.