Nickel nanoparticles supported on reduced graphene oxide sheets: a phosphine free, magnetically recoverable and cost effective catalyst for Sonogashira cross-coupling reactions

Abstract

In this study, we have developed a cost effective and one-pot strategy for the synthesis of a heterogeneous catalyst consisting of a Ni nanoparticle–reduced graphene oxide composite for Sonogashira cross-coupling reactions. Several characterization tools were employed to characterize the Ni nanoparticle–reduced graphene oxide composite, which indicated that magnetic Ni nanoparticles of a size range of 1–4 nm were uniformly anchored on the reduced graphene oxide nanosheets without using any surfactant or stabilizing agent. Different types of aryl halides and phenylacetylenes were coupled under the optimized reaction conditions with excellent yields to give biphenylacetylenes. The ferromagnetic behaviour of the Ni nanoparticle–graphene composite resulted in it being easily separable from the reaction mixture and the composite was reusable, up to six times, without losing its catalytic activity. The fresh as well as the reused catalyst for the Sonogashira cross-coupling reaction was well characterized using analytical techniques which showed that the Ni nanoparticles were well dispersed on the reduced graphene oxide nanosheets without agglomeration, and the size and morphology of the catalyst remained unchanged after use in the catalytic reaction.

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Introduction

Carbon nanomaterials in catalytic research have received great attention from researchers all over the globe for the last two decades. The catalytic performance of the catalyst, particularly for heterogeneous catalysts, mainly depends on the properties and structure of the support material. After the receiving of the Nobel Prize by Andre Geim and Konstantin Novoselov for physics in 2010 for the their ground-breaking discovery of the 2D material graphene, it is receiving prime importance as one of the ideal supports among all the carbonaceous materials in the heterogeneous catalysts area due to its outstanding properties.¹ Graphene sheets possess a unique 2D crystal structure which is easily blended with metals, metal oxides, polymers etc.^1,2 In recent years, heterogeneous catalysts of metal nanoparticles have experienced enormous progress compared to homogenous catalysts in terms of stability, selectivity and reusability. In that regard, metal nanoparticles supported on graphene have attracted significant attention due to the high corrosion resistance, large surface to volume ratio and excellent dispersive nature. In the last two decades, tremendous effort has been expended by researchers for the development of metal nanoparticle–graphene composite materials with controlled size, shape, crystallinity and functionality, due to their potential applications in a wide range of fields including supercapacitors,³ field effect transistors,⁴ hydrogen storage,⁵ sensors,⁶ photocatalysis,⁷ solar cells,⁸ molecular imaging,⁹ water treatment,¹⁰ catalysis^1,11 and drug delivery.¹² But in the area of catalysis, metal nanoparticle–graphene composite materials are still to be explored for other applications.¹ The use of a metal–graphene composite material as a heterogeneous catalyst has many advantages: (a) using graphene as a support prevents agglomeration and leaching of the metal nanoparticles due to interactions between the metal atoms and residual oxygen containing functional groups present on the surface of the graphene which results in an increase in the surface to volume ratio, (b) the presence of the 2D structure in graphene results in a superior catalytic performance of the catalyst because the reactant molecules can absorb to both faces of the catalyst, (c) due to the unique electronic properties of graphene, electron transfer can take place between the graphene and the supported metal nanoparticles which in turn greatly affects the selectivity for the desired product, and (d) π–π interactions between aromatic moieties of the reactant molecules and the graphene support enhance the adsorption capacity of the reactant molecules onto the surface of the catalyst.

In view of these advantageous properties of metal–graphene composite materials for heterogenous catalysis, some metal nanoparticles have been designed on the surface of graphene sheets for catalytic applications such as CO oxidation, oxidation of alcohols, degradation of organic pollutants, hydrogenation of C=C and C=O bonds, selective reduction of nitroarenes, Fischer–Tropsch synthesis and coupling reactions.^1,13 However, most of these reports deals with the problem of separation of the catalyst from the reaction mixture, which lead to trace amounts of metal contaminants in the product. This problem can be easily overcome by designing a magnetically separable heterogeneous catalyst. In this regard, the development of environmentally friendly, cost effective, practical, and efficient catalytic processes with catalyst reusability has attracted worldwide attention in the field of catalysis. Therefore, in this report, we have decorated magnetic Ni nanoparticles onto the surface of reduced graphene oxide sheets (rGO), which showed excellent ferromagnetic properties and thereby results in effective magnetic separation of the catalyst after completion of the reaction. Magnetic separation of the catalyst is more effective than filtration or centrifugation as it prevents loss of the catalyst. Magnetic separation of the catalyst from the reaction system is simple, cost-effective and favorable for industrial applications. The magnetic Ni nanoparticle–rGO composite material is also a low cost heterogeneous catalyst compared to having a noble metal (Au, Ag, Pt and Pd) anchored on the graphene sheets.

The Sonogashira cross-coupling reaction is one of the most important carbon–carbon bond formation reactions in organic synthesis. This coupling reaction has been extensively used for the synthesis of various pharmaceuticals, bioactive compounds, natural products, molecular organic materials and engineered materials.^14–16 This reaction has been well-developed and admirable results can be obtained with Pd-complexes with phosphine ligands.^17–20 However, the most commonly used phosphine ligands are sensitive to air and moisture, which means an inert atmosphere is required as a prerequisite during handling, and even a trace amount of such a ligand may act as an inhibitor in some metal-catalyzed asymmetric reactions.²¹ Therefore, the development of a ligand- and additive-free Pd catalyst is of immense interest. On the other hand, the use of a stable and reusable heterogeneous catalyst to replace the homogenous catalyst for Sonogashira cross-coupling reactions is of great importance in sustainable chemistry. Various heterogeneous Pd nanoparticles have been developed and efficiently used for the Sonogashira cross-coupling reaction.^22–24 Although some of these catalysts are highly efficient, most of them gave a low yield of the coupling product even in the presence of different additives.^25–27 To enhance the efficiency of the catalyst, bimetallic nanoparticles comprising Pd metal with other non-noble metals such as copper, nickel, iron and cobalt are used in the Sonogashira cross-coupling reaction. Bimetallic nanoparticles such as Ag–Pd@rGO,²⁸ Pd/Co alloy NPs,²⁹ Pd–Co/G alloy NPs,³⁰ Pd/Cu mixed NPs,³¹ Pd/Ni core shell NPs,³² hollow Pd–Co nanospheres,³³ nano-Pd/PdO/Cu systems,³⁴ Pd/Cu nano-alloys,³⁵ and rGO–Cu₄₈Pd₅₂ alloy nanoparticles³⁶ are notable examples that were recently reported for this reaction.

Nickel is a promising and cheaper alternative to the use of a Pd-based catalyst for the Sonogashira cross-coupling reaction. Among the literature found for Sonogashira cross-coupling reactions of aryl halides with phenylacetylenes catalyzed by nickel, Yin et al. have reported the use of a mesoporous silica supported Ni(II) organometallic complex as a reusable catalyst for the Sonogashira cross-coupling reaction.³⁷ Beletskaya et al. reported a homogenous Ni(II) species as an efficient catalyst for the Sonogashira cross-coupling reaction.³⁸ Farjadian et al. showed that poly(vinylpyridine)-grafted silica containing Ni nanoparticles is an efficient catalyst for the Sonogashira cross-coupling reaction of aryl halides and phenylacetylene.³⁹ Wang et al. reported a Ni(0) powder catalyzed Sonogashira cross-coupling reaction in the presence of cuprous iodide and triphenylphosphine.⁴⁰ Recently, a Ni–Cu system has been developed by Bakherad and his co-workers for the Sonogashira cross-coupling reaction of terminal acetylenes with aryl iodides in the presence of sodium lauryl sulphate.⁴¹ Most of these previous reports require either the use of phosphine ligands and surfactant or an inert atmosphere during the reaction.

As a part of our continuous efforts in the synthesis of metal nanoparticle–graphene composite materials and their application in the catalytic field,^42–44 herein we have reported the synthesis of magnetically separable Ni nanoparticles on rGO sheets under ligand-free conditions and their application in Sonogashira cross-coupling reactions in the presence of cuprous iodide. To the best of our knowledge, there is no report on Sonogashira cross-coupling reactions catalyzed by magnetic Ni nanoparticles anchored on rGO. In view of this we have developed a heterogeneous catalyst of a very cheap, magnetically recoverable and reusable Ni nanoparticle–rGO composite for the Sonogashira cross-coupling reaction.

Results and discussion

Characterization of the Ni nanoparticle–rGO composite

The formation of Ni nanoparticles on the rGO sheets was confirmed using analytical tools like X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDS) and vibrating sample magnetometer (VSM) analysis. The XRD analysis, as shown in Fig. 1, revealed 2θ values of 44.73°, 52.13° and 76.84° corresponding to d values of 2.02, 1.75 and 1.24 Å that were assigned to the well resolved (111), (200) and (220) crystallographic planes of the Ni nanoparticles, respectively. The positions and relative intensities of the diffraction peaks matched well with the standard XRD data of Ni nanoparticles (JCPDS card No. 01-071-4655). It also confirmed the absence of NiO and Ni(OH)₂ diffraction peaks in the XRD pattern of the Ni nanoparticle–rGO composite. It was also noticed that a broad peak appeared at ∼25°, suggesting that the GO is completely reduced in the presence of the hydrazine hydrate.

Fig. 1 Powder XRD diffractogram of the Ni nanoparticle–rGO composite.

Thermogravimetric analysis (TGA), as shown in Fig. 2, provided information on the reduction of GO to rGO and the formation of Ni nanoparticles on the rGO sheets at the same time. The major weight loss of 37.72% that occurred at a temperature around 200 °C is attributed to decomposition of the labile oxygen-containing functional groups present in GO. The weight loss in this region dramatically decreases to 9.72% after formation of the Ni nanoparticle–rGO composite material because of the reduction of the oxygen containing functional groups present in GO, such as carbonyl, hydroxyl, epoxy and carboxyl groups. The weight loss found above 600 °C for both the Ni nanoparticle–rGO composite material and GO results from pyrolysis of the carbon skeleton of the rGO nanosheets.

Fig. 2 TGA curves of (a) GO and (b) the Ni nanoparticle–rGO composite.

The catalytic activity of the catalyst mainly depends on the size and shape of the nanoparticles distributed on the support in a heterogeneous catalysis system. In this regard, we have examined the morphology of the Ni nanoparticles anchored on the rGO sheets (Fig. 3). Graphene sheets of micron size are clearly visible in the TEM images and the Ni nanoparticles are uniformly distributed on those sheets. Ni nanoparticles of a mean diameter of 2.7 nm with a narrow particle size distribution are embedded in the rGO sheets and are spherical in nature. The size of our synthesized Ni nanoparticles is very small in comparison to other previous reports of the synthesis of Ni nanoparticles.^45–49 The small size and uniform distribution of the synthesized Ni nanoparticles result from a strong interaction between the surface of the rGO sheets and Ni nanoparticles.⁵⁰ Lu et al. proposed that the interaction between the graphene and Ni is attributed to partially occupied d-orbitals which are localized in the vicinity of the Fermi level.⁵¹ This result clearly fulfils our aim to prepare very small-sized nanoparticles and thereby a large surface to volume ratio in order to get excellent catalytic activity. Xu et al. recently reported Ni nanoparticles of an average size of 9.7 nm decorated on graphene sheets with narrow size distribution.⁵² Wu et al. also demonstrated the synthesis of a Ni nanoparticle–graphene composite using a solvothermal method via electrostatic induced spread adsorption. Ni nanoparticles of an average size of ∼55 nm were well distributed on the graphene sheets.⁴⁵ For another synthesis method reported by Tian et al., it was observed that Ni nanoparticles of an average size of 8 nm were homogeneously decorated on rGO sheets in the presence of poly(N-vinyl-2-pyrolidone) (PVP).⁴⁸ However, Ni nanoparticles of an average size of ∼27 nm were decorated on the rGO sheets without using the stabilizing agent PVP due to agglomeration of the Ni nanoparticles. The surface morphology and elemental composition of the obtained composite material were examined using SEM-EDS (shown in Fig. 4(a)–(c)). The crumpled and rippled structure of GO which results from deformation upon exfoliation is partially destroyed in the composite material due to the reduction of a large amount of oxygen containing functional groups (Fig. 4a and b). Although the rGO sheets were layered in structure, it is irregular and folded where the spherical Ni nanoparticles are uniformly distributed. The EDS analysis (shown in Fig. 4c) clearly confirmed sufficient loading of the Ni nanoparticles onto the surface of the rGO sheets with an insignificant amount of oxygen, which remains due to the presence of unreduced oxygen containing functional groups.

Fig. 3 TEM images of the Ni nanoparticles on rGO nanosheets (a–c). TEM image along with particle size distribution (a), SAED image of Ni nanoparticles (d).

Fig. 4 SEM images of the Ni nanoparticles on rGO nanosheets (a and b); EDS analysis of the Ni nanoparticle–rGO composite material (c).

The FTIR spectra of GO and the Ni nanoparticle–rGO composite are shown in Fig. 5. As shown in Fig. 5, prominent peaks at 3131, 1728, 1583, 1436, and 1042 cm⁻¹ occur for GO and are attributed to the stretching vibrations of O–H, C=O, C=C, C–O–H and C–O–C, respectively. The intensities of these peaks decreased significantly after the formation of the Ni nanoparticles on the rGO sheets due to the reduction of these oxygen containing functional groups present in GO.

Fig. 5 FTIR spectra of GO and the Ni nanoparticle–rGO composite.

To investigate the magnetic properties of our synthesized Ni nanoparticle–rGO composite material, magnetic measurements were performed at room temperature in terms of field dependent magnetization measurements (M–H). The results for the saturation magnetization (M_s), remanent magnetization (M_r) and coercivity (H_c) are listed inside Fig. 6. The hysteresis behaviour and the magnetic parameters clearly reveal a ferromagnetic interaction of the synthesized Ni nanoparticles on rGO nanosheets.^53–56 As shown in Fig. 6, the Ni nanoparticle–rGO composite shows a saturation magnetization of 43.54 emu g⁻¹ and the reduction of this saturation magnetization value in comparison to bulk nickel is due to the increase in the surface to volume ratio resulting from a decrease in particle size.⁵⁴ The remanence magnetization (M_r) of the sample was found to be 6.92 emu g⁻¹ and the coercivity (H_c) 197.54 Oe, which is greater than for bulk nickel. The increase in the coercivity value in comparison to bulk nickel confirms the rule of H_c ∝ 1/D for the multidomain ferromagnetic nanoparticles.⁵⁵ Thus, the excellent ferromagnetic behaviour of the Ni nanoparticle–rGO composite fulfils our aim to develop a more efficient and easily separable catalyst for catalysis reactions.

Fig. 6 Variation of magnetization (M) with magnetic field (H) at room temperature for the Ni nanoparticles supported on rGO sheets.

Catalysis study

After complete characterization of the Ni nanoparticle–rGO catalyst, it was utilized as an effective catalyst for the Sonogashira cross-coupling reaction of aryl halides with phenylacetylenes in the presence of CuI. Initially, we optimized the reaction conditions using bromobenzene 1e and phenylacetylene 2a as model substrates. Different solvents as well as bases were screened and the results are summarized in Table 1. We first examined the effect of the solvent on this coupling reaction using K₂CO₃ as the base. The results revealed that N-methyl-2-pyrrolidone (NMP) was the best solvent for this coupling reaction (Table 1, entry 7). Then we examined different bases such as Na₂CO₃, KOH, NaOH and K₃PO₄. The use of a strong base such as NaOH and KOH gave a low yield of the product (Table 1, entries 10 and 13), whereas Na₂CO₃ and K₃PO₄ showed almost identical results. Additionally, we performed the coupling reaction at different temperatures, and 120 °C was found to be the optimum reaction temperature for this reaction.

Table 1 Optimization studies for the Ni nanoparticle–rGO catalyst in the Sonogashira cross-coupling reaction^a

Entry	Solvent	Base	Temperature (°C)	Yield^b (%)
a Reaction conditions: bromobenzene (1 mmol), phenylacetylene (1.2 mmol), CuI (0.08 mmol), catalyst (25 mg, 0.15 mmol Ni), base (3 mmol), solvent (5 mL), 4 h.b Isolated yield.
1	H2O	K2CO3	110	15
2	DMF	K2CO3	60	40
3	DMF	K2CO3	120	45
4	Toluene	K2CO3	120	30
5	NMP	K2CO3	60	70
6	NMP	K2CO3	100	85
7	NMP	K2CO3	120	93
8	DMSO	K2CO3	100	80
9	DMSO	K2CO3	120	80
10	NMP	KOH	120	50
11	NMP	Na2CO3	120	92
12	NMP	K3PO4	120	90
13	NMP	NaOH	120	58

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After having optimized the reaction conditions, we explored the versatility and efficiency of our catalyst for the Sonogashira cross-coupling reaction using different aryl halides and phenylacetylenes. The results are shown in Table 2. As shown in Table 2, aryl halides such as those containing bromide and iodide efficiently coupled with phenylacetylenes to give an excellent yield of the desired product in spite of the electron-rich, electron-poor and electron-neutral nature of the halide components. The reaction conditions are notably compatible with the presence of a nitro group on the aryl ring. Having established a range of aryl bromides and aryl iodides for use as the coupling partner, we next examined the scope of the cross-coupling reaction using aryl chlorides (Table 2, entries 14 and 15). Under our optimized conditions, the aryl chlorides coupled with terminal alkynes and gave a good yield of the corresponding products. However, the chloroaryl substrates required a long reaction time and high reaction temperature compared to the iodide and bromide substrates in order to get a comparable yield. Additionally, our catalyst system chemoselectively reacted with the bromide, when both chloro- and bromo-groups were present on the same substrate (Table 2, entry 10). Our catalyst system was also applied to a heteroaryl halide, 2-bromopyridine, and the coupling product 3i was obtained in 86% yield (Table 2, entry 16). To further explore the scope of our Ni nanoparticle–rGO catalyst in the Sonogashira cross-coupling reaction, we choose a heteroaryl substrate having both bromo- and iodo-substituents. In this regard, 2-bromo-5-iodopyridine was treated with two equivalents of phenylacetylene under our catalytic conditions. The product 3j was obtained with an excellent yield (Scheme 1).

Table 2 Sonogashira cross-coupling of various aryl halides and phenylacetylenes^a

Entry	Aryl halide (1a–m)	Alkyne (2a or 2b)	Product (3a–i)	Yield^b (%)
a Reaction conditions: aryl halide (1 mmol), phenylacetylene (1.2 mmol), CuI (0.08 mmol), catalyst (25 mg, 0.15 mmol Ni), K2CO3 (3 mmol), NMP (5 mL), 120 °C, 4 h. NP: nanoparticle.b Isolated yield.c The reaction was performed at 140 °C for 16 h.
1				95
2				94
3				95
4				95
5				93
6				91
7				93
8				93
9				88
10				91
11				93
12				93
13				93
14				72^c
15				70^c
16				86

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Scheme 1 Ni nanoparticle–rGO catalysed Sonogashira cross-coupling reaction of 2-bromo-5-iodopyridine with phenylacetylene.

Furthermore, we have compared our Ni nanoparticle–rGO catalyst with other previously reported heterogeneous as well as homogenous catalysts containing Ni to highlight the advantages of our catalyst in the Sonogashira cross-coupling reaction (Table 3). From these comparison results, we found that our synthesized catalyst is more advantageous with respect to yield and reaction conditions. Moreover, for most of the previously reported cases, the scope of the catalyst in the Sonogashira cross-coupling reaction is limited to only aryl iodides with phenylacetylenes.

Table 3 Comparison of the catalytic activity of the Ni nanoparticle–rGO catalyst with other homogenous and heterogeneous catalysts containing Ni

Catalyst	Conditions	Yield (%)	Ref.
Ni nanoparticle–rGO	K2CO3, NMP, CuI, 120 °C, 4 h	70–95	This work
Si–P4VPy–Ni^0	K2CO3, NMP, CuI, 120 °C, 1.5–10 h	40–90	39
Pd70Ni30/MWCNTs	NaOH, pyrrolidine, 120 °C, 1 h	60–74	57
Ni(PPh3)2Cl2	K2CO3, [Cu(CH3CN)4]BF4, reflux	57	58
NiCl2·6H2O–n-Bu4NBr	NaOH, ethylene glycol, 120 °C, 1–12 h	46–91	59
Ni(PPh3)2Cl2	K2CO3, CuI, dioxane : H2O, reflux, 4 h	93–100	38
Ni(0)–CuI–PPh3	KOH, isopropanol, 80–120 °C, 5 h	56–98	40
Ni(PPh3)2Cl2/CuI	Cs2CO3, H2O, surfactant, 60 °C, 2–6 h	70–92	41
Ni-PPh2–PMOs(Ph)	K2CO3, CuI, dioxane/H2O, N2 protection	63–75	37

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At present, the exact mechanism of the reaction is not clear. However, we propose here a plausible mechanism for the Ni nanoparticle–rGO catalyzed Sonogashira cross-coupling reaction, as shown in Scheme 2. We believe that the Ni nanoparticle–rGO undergoes oxidative addition with the aryl halide to form a Ni(II) reactive species, which readily transmetalates with Cu-phenylacetylene to form intermediate A. Finally, the desired coupling product is formed from the resulting intermediate A via reductive elimination.^37,58,60

Scheme 2 Proposed mechanism for the Ni nanoparticle–rGO catalyzed Sonogashira cross-coupling reaction.

Reusability of the Ni nanoparticle–rGO heterogeneous catalyst

The reusability of our Ni nanoparticle–rGO catalyst for the Sonogashira cross-coupling reaction was also investigated. The excellent magnetic behaviour of our synthesized Ni nanoparticles allows them to accumulate onto the magnetic stirring bar as soon as the magnetic stirring is stopped. Therefore, after completion of the reaction, the reaction mixture could be simply and efficiently separated from the catalyst without using any filtration or centrifugation. After separating the catalyst, it was washed with water followed by acetone (2 to 3 times) and dried in an air oven, and then directly used for the subsequent reaction. To check the reusability of the catalyst, bromobenzene 1e and phenylacetylene 2a were used as substrates for the Sonogashira cross-coupling reaction. As shown in Fig. 7, the recovered catalyst was consecutively used six times without significant loss of its activity. Although different types of heterogeneous catalysts of metal nanoparticles have been reported for Sonogashira cross-coupling reactions as discussed in the introduction part, to the best of our knowledge no reports are available on characterization of the catalyst after performing the reaction. We characterized the Ni nanoparticle–rGO heterogeneous catalyst using XRD and TEM after performing the catalytic reaction, as shown in Fig. 8. The average size of the nickel nanoparticles after performing the reaction was found to be ∼3 nm, which is very close to 2.7 nm, the average size of the nanoparticles before the reaction. Also we have found the same crystallite size for the Ni nanoparticles for both the fresh and reused catalyst by using the Scherrer equation using PDXL software for the XRD. Therefore, the XRD as well as the TEM analysis clearly demonstrate that the size and morphology of the Ni nanoparticle–rGO catalyst remain unchanged after performing the catalytic reaction. Moreover, the nickel content of the recovered catalyst was also determined using ICP-AES which suggested a negligible difference to the catalyst before it was used in the organic catalysis reaction.

Fig. 7 Reusability of the Ni nanoparticle–rGO catalyst for the Sonogashira-cross coupling reaction.

Fig. 8 Characterization of the Ni nanoparticle–rGO catalyst after performing the reaction (a) XRD pattern, (b–d) TEM images.

Since leaching of the nanoparticles from the support is a common problem in catalysis, we have examined the leaching of the Ni nanoparticles from the rGO support by performing a hot filtration test. For this test we considered the Sonogashira-cross coupling reaction of bromobenzene 1e with phenylacetylene 2a. After continuing the reaction for 1.5 h, the catalyst was separated and the conversion to 1,2-diphenylethyne 3a was determined using GC and was found to be 45%. After that, the filtrate part was further heated for another 5 h to check the progress of the reaction. From the results obtained by GC, it was found that no further conversion was observed after separation of the catalyst. This clearly proves that no Ni nanoparticles were leached from the catalyst when performing the reaction.

Conclusion

In conclusion, the present work reports the decoration of very small and uniform sized ferromagnetic Ni nanoparticles onto the surface of rGO sheets. The synthesized composite material shows excellent catalytic activity for the Sonogashira cross-coupling reaction. The catalyst could be easily magnetically separated from the reaction mixture without any leaching of the nanoparticles. The size and morphology of the reused catalyst were again characterized using TEM and XRD, which suggested that the size and shape of the Ni nanoparticles remained unchanged without undergoing any agglomeration of the particles. In addition, the use of Ni nanoparticles as a catalyst for the Sonogashira cross-coupling reaction makes the catalytic process more cost effective. In view of these advantages, the present work represents a new protocol for the synthesis of biphenylacetylenes in an efficient way.

Experimental section

Materials and methods

The materials used for the synthesis of the Ni nanoparticle–rGO composite were graphite powder (<20 μm, Sigma-Aldrich), potassium permanganate (>99%, E-Merck, India), sulfuric acid (AR grade, Qualigens, India), H₂O₂ (30%, Qualigens, India), hydrochloric acid (AR grade, Qualigens, India), hydrazine hydrate (80%, LobaChemie, India) and NiCl₂ (>97%, E-Merck, India). All the substrates required for the Sonogashira cross-coupling reactions were purchased from Sigma Aldrich, USA and used without any further purification.

Characterization techniques

Powder XRD spectra of the samples were obtained using a Rigaku, Ultima IV X-ray diffractometer from 5–100° for 2θ using a Cu-Kα source (l = 1.54 Å). The TGA of the samples was performed at a rate of a 5 °C rise in temperature per minute using a TA-SDT (model: Q600DT, TA Instruments, USA). The TEM images were obtained using a JEOL JEM-2011 electron microscope, Transmission Electron Microscope, Japan. A VSM (USA) operated at room temperature was used to investigate the magnetic properties of the composite material. SEM-EDS analysis was performed using a Carl ZEISS Field Emission SEM with an Oxford EDS to determine the elemental composition of the composite material. FTIR spectra were recorded in the frequency range of 400–4000 cm⁻¹ using KBr discs in a Perkin-Elmer system 2000 FT-IR spectrophotometer. All the NMR spectra were recorded using a Bruker Advance DPX 300 or 500 MHz spectrometer. The chemical shifts are reported on the δ scale (ppm) downfield from tetramethylsilane (δ = 0.0 ppm) using the residual solvent signal at δ = 7.26 ppm (¹H) or δ = 77 ppm (¹³C) as an internal standard. Gas chromatography analyses were performed using a Chemito GC-8610, FID gas chromatograph fitted with a Porapak Q column (2 m × 1/8′′ O. D., SS) and the data were analyzed using Winchrom GC data processing software. The Ni content in the nanocomposite catalyst was determined using the EPA 200.7 method of acid digestion followed by inductively coupled plasma analysis (ICP-AES, Perkin Elmer, Optima 5300 V). Multilevel calibration of the Ni was performed using a metal standard for ICP (Sigma). The calibration curve was linear with an R² value of 0.99999.

Synthesis of the Ni nanoparticle–rGO composite material

NiCl₂ (0.338 g) was dissolved in deionised water (10 mL) in a round bottom flask. Then 15 mL of hydrazine hydrate (80%) was mixed with 10 mL of deionised water, which was added to the above solution and heated to 75 °C for 15 min with stirring. Then 20 mL of an aqueous suspension of GO (0.012 g L⁻¹) was added to the above mixture followed by addition of NaOH (20 mg) and then the mixture was subjected to ultrasonication for 10 min. Finally, the reaction mixture was vigorously stirred at 80 °C until a black precipitate of the composite material was obtained. The solid material was separated by a simple filtration and washed with ethanol several times followed by water (2 to 3 times), and then dried in an air oven at 60 °C overnight.

General procedure for the Sonogashira cross-coupling reaction

To a round-bottom flask containing the aryl halide (1 mmol), phenylacetylene (1.5 mmol), K₂CO₃ (3 mmol) and CuI (0.08 mmol), a suspension of the catalyst (25 mg, 0.15 mmol of Ni) in N-methyl-2-pyrrolidone (5 mL) was added. The whole reaction mixture was stirred at 120 °C for 4 h. After completion of the reaction (monitored using TLC), the catalyst was separated from the reaction mixture using an external magnet and the reaction mixture was poured into water. The organic product was extracted with ethyl acetate (3 × 10 mL). The combined organic phases were dried over Na₂SO₄ and concentrated under vacuum. The crude products were purified by column chromatography using silica gel (60–120 mesh) with EtOAc/hexanes as the eluent to obtain the desired coupling product.

Acknowledgements

The authors acknowledge the Department of Science and Technology, New Delhi for financial support (DST no. SR/FT/CS-136/2011, CSIR-NEIST project no. GPP-269, DST no. INT/RUS/RFBR/P-193 and CSIR-NEIST project no. GPP-0301) and the Director, CSIR-North East Institute of Science and Technology, Jorhat, India for their interest in this work and the facilities. The authors are also thankful to SAIF, NEHU, Shillong for the use of the TEM facility. NH acknowledges UGC, New Delhi, for a JRF grant.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22601e

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