Nickel stabilized by triazole-functionalized carbon nanotubes as a novel reusable and efficient heterogeneous nanocatalyst for the Suzuki–Miyaura coupling reaction

Abstract

An interesting nanotube based nickel nanocatalyst was successfully prepared through “click” reaction of azide-functionalized nanotube with propargyl alcohol followed by immobilization of nickel nanoparticles. The as-prepared nanocatalysts behave as very efficient heterogeneous catalysts in the Suzuki–Miyaura cross coupling reaction in terms of activity and recyclability.

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Introduction

Over the last few decades, carbon–carbon bond formation reactions such as Suzuki, Heck, and Sonogashira and many others are important as they provide access to an important class of organic compounds that are prevalent moieties in many important compounds of the biological, pharmaceutical, and polymer industries and in medicinal chemistry.¹ Among the many different coupling protocols, palladium–phosphine complexes are still one of the most frequently investigated metals for such reactions.^2–6 However, the use of palladium catalysts has some major limitations including contamination of products, moisture-sensitive nature, high cost and toxicity.⁷ One promising solution to overcome this issue is to employ available and cost-effective transition metals rather than their palladium analogs.⁸ Recently, nickel catalysis has attracted considerable attention as a viable alternative to Pd in cross-coupling reactions.⁹ These reactions are mostly performed under homogeneous Ni catalytic systems which have some major limitations such as using strong reducing agents, the lack of recyclability, high catalyst loading (up to 10 mol%), instability at high temperatures. Most of the issues associated with homogenous catalyst can be solved by immobilizing the catalyst in various heterogeneous supports.¹⁰ Among the different catalyst supports, multi-walled carbon nanotubes (MWCNTs) have recently attracted significant attention due to the high surface area, high mechanical strength, ultra-light weight, high chemical and thermal stability.^11–15 However, the difficulty for functional groups to attach on it due to their strong chemical stability behaviour and a lack of dispersibility in organic solvents or in aqueous media is the major drawback of pristine carbon nanotube for most applications. In order to overcome this problem, two approaches for modifying CNT have been developed, including covalent and noncovalent functionalization. The non-covalent functionalisation involves van der Waals interaction, p–p stacking interactions and the adsorption or wrapping of various functional molecules around the nanotubes,^16–19 and the covalent functionalisation based on attachment of chemical of molecules onto the sp² carbon atoms of the CNTs.²⁰ Many different covalent methods, such as acid treatment, esterification, amidation, anionic coupling and click coupling, have been developed for the covalent modification of the sidewalls of CNT.^21–24 In the past few years, the copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC), known as the “click reaction”, has received unrivalled attention because of its high quantitative yield, high tolerance of functional groups, compatibility with water, mild and simple experimental conditions, high chemoselectivity with no side products.²⁵ Thus, the “click” process has become an invaluable approach for the efficient introduction of new functionalities onto CNTs.^26–29 As a part of our efforts to investigate on the application of heterogeneous catalytic systems in cross-coupling reaction,^30–32 herein, we describe an interesting and facile approach for the functionalization of multi-walled carbon nanotubes using click reaction for the preparation of Ni nanoparticles, as well as catalytic properties the latter for performing Suzuki–Miyaura cross coupling reaction.

Result and discussion

Preparation and characterization of the catalysts

A schematic illustration of the preparation strategy of the MWCNTs-supported nickel catalyst was shown in Scheme 1. First, pristine nanotubes were treated with strong acids, in which hydroxyl moieties are created on nanotubes. In the following step, the oxidized MWCNTs were further reacted with 3-azidopropyltrimethoxysilane to introduce an N₃ group, which is subsequently reacted with propargyl alcohol via click chemistry to furnish the modified MWCNTs. Finally, the immobilization of Ni was carried out by the reduction of NiCl₂·6H₂O in the presence of NaBH₄ as reducing agent, in H₂O as the solvent system.

Scheme 1 The nanocatalyst preparation.

The catalytic system was well characterized using techniques including elemental analysis (EA), inductively coupled plasma analysis (ICP), FT-IR, TEM, EDX, XRD and SEM. FT-IR Spectroscopy was used to confirm the modifications of the MWCNTs surface. As shown in Fig. 1b, the absorption bond at 1092 cm⁻¹ corresponds to the Si–O–C stretching vibration and the peak at 2100 cm⁻¹ is attributed to N₃ stretching, which demonstrates that the azide groups have been successfully coupled to the surface of the nanotube. As shown in Fig. 1c, the characteristic vibration of azide completely was disappeared after the click reaction between propargyl alcohol and N₃-MWNTs. In Fig. 1d, the intensity of the peaks is weaker than of observed for the free ligand (Fig. 1c), supporting the assumption that the ligand coordinates to the metal ion. According to the results of elemental analysis, the loading of the azide group on the carbon nanotube surface was determined to be 1.24 mmol g⁻¹. The particle size, morphology, and distribution of nickel nanoparticles supported on MWCNTs were investigated using scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) imaging techniques (Fig. 2). As seen in the typical TEM image of the catalyst, the NiNPs were of spherical shape with relatively good monodispersity and have average diameter ranging about 8 nm (Fig. 2a and b). The information about the surface morphology of functionalized carbon nanotube was also investigated by scanning electron microscopy (SEM). As shown in Fig. 2d, the nanotubes are aggregated and have maintained their nanotube nature after surface modification.

Fig. 1 FT-IR spectra of 1 (a), 3 (b), 4 (c), 5 (d).

Fig. 2 TEM images of nanocatalyst (a and b), SEM photographs of pure MWCNTs (c) and nanocatalyst (d).

As clearly observed, the EDS spectrum showed the elemental composition after the grafting reaction, confirming the presence of C, O, N, Si, Cl and Ni species in the catalyst (Fig. 3). Meanwhile, the quantitative determination of the metal content was determined by ICP analysis and was found to be 0.417 mmol g⁻¹ for this catalyst. Fig. 4 presents the XRD-diffraction patterns of the pristine MWCNTs and Ni(0)/MWCNTs. As can be seen (Fig. 3a), the XRD pattern of MWCNT displays two prominent peaks at around 2θ = 26 and 43° correspond to the (002) and (100) plane of hexagonal graphitic structure on the mesoporous carbon. The observed diffractions in sample Ni(0)/MWCNTs displays three additional diffraction peaks at 44.5°, 51.6° and 76.9° (JCPDS, no. 03-1051) belong to the diffraction peaks of Ni(111), Ni(200) and Ni(220) reflections, respectively.

Fig. 3 SEM-EDX spectrum of the catalyst.

Fig. 4 XRD pattern of pure MWCNTs (a), nanocatalyst (b).

Catalytic tests

To evaluate the catalytic performance of the nanocatalyst and to determine optimal conditions for Suzuki reaction, phenylboronic acid and bromobenzene was selected as model substrates and were examined under different parameters such as bases, solvent, temperature conditions and catalyst loading. In our first set of experiments, the model reaction was studied in the presence of K₂CO₃ as base and 20 mg of catalyst in different solvents at 100 °C. The results showed that the yield of product was enhanced to 93% when 1,4-dioxane was used as solvent. Subsequently, we tested the impact of various bases on the efficiency of this procedure because of its great influence in this type of coupling reaction. On the basis of this study, K₃PO₄ was chosen as the most effective and suitable base for the reaction and the desired product was obtained in 97% yield after 6 h (Table 1, entry 5). To obtain the proper temperature for the cross-coupling, the model reaction was also conducted at different temperatures and it was found that reaction temperature plays a substantial role on this transformation. As you can see in Table 1, the improvement in reaction results was achieved at 100 °C. Finally, we investigated the effect of catalyst loading by employing different amounts of catalyst. As you can see, the good reaction time and higher yield were observed when the reaction was carried out with 20 mg of catalyst, while less than this amount led to a decrease in the percentage yield of the product. After optimization, Ni(0)/MWCNTs catalyzed Suzuki reactions were examined with various types of aryl halides (chlorides, bromides and iodides). Results summarized in Table 2 show that aryl iodides and aryl bromides were converted effectively to the desired products with good yields in short reaction times. Also, the less active chlorobenzene and its derivatives showed lower yields and needed a longer time to obtain a moderate yield. We also investigated the electronic effect on the yields and reaction times of the reactions. Overall, the coupling reaction of phenylboronic acid with electron withdrawing aryl halides gave better conversions in shorter reaction times. Steric factor of the procedure was surveyed using 2-bromoacetophenone as hindered substituted aryls (Table 2, entry 6). It was observed that an increasing steric hindrance of ortho substituents can cause a decrease in the reaction conversion than those obtained with para-substituted ones.

Table 1 Effect of different parameters for the reaction of bromobenzene with phenylboronic acid^a

Entry	Solvent	T (°C)	Base	Yield^b
a Reaction conditions: bromobenzene (1 mmol), phenylboronic acid (1.2 mmol), base (2 mmol), amount of catalyst (20 mg), time (6 h) and solvent (5 mL).b Isolated yield.c Amount of catalyst used: 10 mg.d Amount of catalyst used: 15 mg.e Amount of catalyst used: 18 mg.
1	DMF	100	K2CO3	76
2	Toluene	100	K2CO3	90
3	DMSO	100	K2CO3	85
4	Dioxane	100	K2CO3	93
5	Dioxane	100	K3PO4	97
6	Dioxane	100	KOH	64
7	Dioxane	100	NaHCO3	71
8	Dioxane	100	K3PO4	75^c
9	Dioxane	100	K3PO4	83^d
10	Dioxane	100	K3PO4	91^e
11	Dioxane	90	K3PO4	86
12	Dioxane	80	K3PO4	71

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Table 2 Suzuki cross-coupling reaction of various aryl halides with phenylboronic acid^a

Entry	Ar–X	Product	Time (h)	Yield^b	TON^c
a Reaction conditions: aryl halide (1 mmol), phenylboronic acid (1.2 mmol), base (2 mmol), solvent (4 mL), catalyst (20 mg), 100 °C.b Isolated yield.c TON: calculated using the 0.417 mmol g^−1 nickel (obtained by ICP for Ni/MWCNT).
1			6	97	116
2			5	96	115
3			5	97	116
4			6	87	104
5			6.5	85	101
6			10	63	75
7			4	96	115
8			4	93	111
9			4	93	111
10			3	98	117
11			9	84	100
12			7	88	105
13			9	78	93

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Reusability and heterogeneity test

The level of recovery of the catalyst is highly preferable for its industrial applications. In order to investigate this issue, the reusability of Ni(0)/MWCNTs was examined by using bromobenzene and phenylboronic acid as model substrates. For this, after the completion of the reaction, the catalytic system was separated from the reaction mixture by centrifugation, washed thoroughly with water and ethanol, dried and reused in subsequent reactions. As shown in Fig. 5, the described catalyst could be recovered seven times without a noticeable loss of yield and catalytic performance until the fifth reuse. However, the yield of the reaction in the seventh run decreased to 62%. On the other hand, TEM image of used nanocatalyst (after 7 runs) indicated the structure of Ni/MWCNTs has no significant change in comparison with fresh catalyst and the catalyst preserves its structure Fig. S1, (ESI†). The comparison between the XRD measurements of the fresh and the reused catalyst indicated that the metallic state of nickel was kept during the reaction progress and Ni nanoparticles do not removed from the MWCNTs surface after the catalytic reaction Fig. S2, (ESI†). The SEM images of the seventh cycle of the catalyst are not considerably different from the fresh catalyst, indicating that the morphology of catalyst remained unchanged after seven recoveries Fig. S3, (ESI†). Also, the obtained ICP analysis from the aqueous phases of reaction mixture after recycling experiment showed only a low amount of Ni metal (0.094 ppm) was leaching out from the catalyst. These results indicate that the attachment between nickel nanoparticles and carbon nanotube surface is sufficiently strong. To probe this point that the nature of the active species originated from the supported nickel nanoparticles and not from leached Ni, a hot-filtration test was conducted by coupling of bromobenzene and phenylboronic acid. The reaction was stopped and the catalyst was then removed from the solution after 1 h (24%, analyzed by GC), the filtrate was carried out further. No further progress in product yield was detected even up to 6 h upon catalyst removal. The ICP analysis only showed 0.04 ppm nickel in this solution, indicating that metal leaching into the solution is negligible and the catalyst was mainly heterogeneous in nature. So, these results confirmed the high catalytic activity and good stability of the catalytic system. To further evaluate the efficiency of our catalyst, a comparison was made with the reported heterogeneous Ni-catalysts in the Suzuki reactions (Table 3). It can be clearly seen that our catalyst is superior to others in terms of reaction condition, reaction time, yield, recovery and reusability. The high catalytic performance could be attributed to the high dispersion of the NiNPs onto the outer surface of the nanotube.

Fig. 5 Reusability of the catalyst in the Suzuki–Miyaura.

Table 3 Comparison of catalytic activity of the Ni(0)/MWCNT catalyst with literature precedents using Ni based homogeneous and heterogeneous catalysts for Suzuki–Miyaura cross coupling reaction

Entry	Catalyst	Aryl halide	Conditions	Yield (%) (ref.)
1	Ni(0)/MWCNT		K3PO4, dioxane 100 °C, 6 h	97 (this work)
2	Ni(0)/MWCNT		K3PO4, dioxane 100 °C, 5 h	96 (this work)
3	Octahedral nickel(II) benzoylhydrazone complex		Toluene, K2CO3 reflux, 8 h	83 (^36)
4	Ni(II) mounted on graphite (Ni/Cg)		K3PO4, PPh3, LiBr, dioxane, 180–200 °C, 9 h	87 (^37)
5	Magnetic Fe–Ni alloy		NaOH, PCy3, dioxane, 120 °C, 12 h	95 (^38)
6	Monodisperse Ni and NiO nanoparticles		DMSO, 135 °C	98 (^39)
7	HPMC stabilized Ni nanoparticles		Ethylene glycol, Cs2CO3, 100 °C, 20 h	99 (^40)

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Experimental

All chemical reagents were purchased from Merck Chemical Company and used without further purification. ¹H-NMR spectra were recorded on a Bruker 400 spectrometer using deuterated CDCl₃ as solvent and tetramethylsilane (TMS) as internal standard. X-ray diffraction (XRD) powder patterns were obtained using an X’PERT MPD, with Cu Kα radiation (40 kV, 30 mA). Transmission electron microscopy (TEM) images were obtained using a Philips CM10 microscope. FT-IR spectroscopy (JASCO FT-IR 680-Plus spectrophotometer) was employed for characterization of products. Also we used inductive coupled plasma Perkin Elmer Optima 7300 DV. Gas chromatography (GC) (BEIFIN 3420 gas chromatograph equipped with a Varian CP SIL 5CB column: 30 m, 0.32 mm, 0.25 mm) was used for consideration of reaction conversions and yields. Scanning electron micrographs of the catalyst were taken on [FE-SEM, HITACHI (S-4160)].

Synthesis of 3-azidopropyltrimethoxysilane

3-Chloropropyltrimethoxysilane (5 mmol) is poured into a solution of sodium azide (5 mmol) and tetrabutylammonium bromide (1 mmol) in dry acetonitrile (75 mL) and the mixture is stirred under reflux for 48 h under a nitrogen atmosphere. After the reaction, the solvent was evaporated under reduced pressure. The crude mixture was then dissolved in Et₂O (30 mL) filtered and washed twice with Et₂O. The solvent was removed to obtain 3-azidopropyltriethoxysilane (AzPTES).³³

Synthesis of azide-functionalized multi-walled carbon nanotubes

The functionalization of MWCNTs surface was obtained via oxidation with a mixture of concentrated sulfuric acid and nitric acid. Briefly, 0.1 g of pristine MWCNTs was dispersed in 50 mL of concentrated H₂SO₄/HNO₃ (3 : 2 v/v) solution at 50 °C and stirred for 20 h. The solution was filtered, washed several times with deionized water to reach pH = 7 and dried under vacuum at 100 °C for 24 h. Afterward, the resulting functionalized nanotubes were suspended in ethanol via ultrasonication for 30 min and 3-azidopropyltriethoxysilane was then added. The solution was refluxed for 4 h under nitrogen atmosphere. After that, the obtained product was washed thoroughly with water followed by acetone and dried under vacuum at 80 °C.³⁴

Click coupling between propargyl alcohol and N₃-MWNTs

Azide functionalized MWCNTs (0.5 g) were dispersed in DMF/THF (1 : 1, v/v) solution via ultrasonication for 10 minutes and propargyl alcohol (2 mmol) was added. Then CuI (0.1 mmol) was added to the above solution. The reaction mixture was stirred for 72 h at room temperature. The completion of the reaction was checked by IR. The solid particles was filtered and washed several times with Et₂O, H₂O and then acetone and finally dried in vacuum at 60 °C.³⁵

Preparation of Ni(0)/MWCNT

5 mL aqueous solution of NaBH₄ (3 mmol) was dropwise to an aqueous suspension containing 0.5 g of the modified carbon nanotubes and 0.2 mmol of NiCl₂·6H₂O and mechanically stirred at room temperature for 24 hours. The resulting solid of Ni–MWCNT was separated from the mixture by centrifugation, washed several times with water and ethanol, respectively to remove unreacted NiCl₂ and dried under vacuum to obtain a black solid powder.

Conclusions

In summary, we have applied nickel nanoparticles supported on modified multiwalled carbon nanotubes as highly selective, economical and efficient heterogeneous catalysts in the Suzuki cross-coupling reaction. The corresponding Suzuki products were obtained in good yields under relatively mild reaction conditions. Also, the catalyst could be reused for seven consecutive runs without a noticeable loss of its catalytic performance until the fifth reuse.

Acknowledgements

We gratefully acknowledge the funding support received for this project from the Isfahan University of Technology (IUT), IR of Iran, and Isfahan Science and Technology Town (ISTT), IR of Iran. Further financial support from the Center of Excellence in Sensor and Green Chemistry Research (IUT) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23004k

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