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

Article abstract of DOI:10.1039/c5ra22601e

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.

Full text of DOI:10.1039/c5ra22601e

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Nickel nanoparticles supported reduced graphene oxide sheets: A
phosphine free, magnetically recoverable and cost effective catalyst for
Sonogashira cross-coupling reaction
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Najrul Hussainac, Pranjal Gogoibc*,Puja Khared and Manash R. Dasac*
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aMaterials Science Division, CSIRꢀNorth East Institute of Science and Technology, Jorhatꢀ
785006, Assam, India.
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bMedicinal Chemistry Division, CSIRꢀNorth East Institute of Science and Technology,
Jorhatꢀ785006, Assam, India.
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cAcademy of Scientific and Innovative Research, CSIR, India
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dAgronomy & Soil Science Division, CSIRꢀCentral Institute of Medicinal and Aromatic
Plants, Near Kukrail Picnic Spot, Lucknowꢀ226015, Lucknow India
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Abstract
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In this study, we have developed a cost effective and oneꢀpot strategy toward the synthesis of
heterogeneous catalyst of Ni nanoparticlesꢀ reduced graphene oxide composite for
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Sonogashira cross-coupling reaction. Several characterization tools were employed to
characterize the Ni nanoparticleꢀreduced graphene oxide composites, which indicates that
magnetic Ni nanoparticles of the size range of 1ꢀ4 nm are uniformly anchored on the reduced
graphene oxide nanosheets without using any surfactant or stabilizing agent. Different types
of aryl halide and phenyl acetylenes were coupled under optimized reaction condition with
excellent yields to give biphenylacetylenes. The ferromagnetic behaviour of the Ni
nanoparticleꢀgraphene composite demonstrated the easy separable from the reaction mixture
and reusable up to six times without losing its catalytic activity. The fresh as well as reused
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catalyst in the Sonogashira cross-coupling reaction was well characterized by analytical
techniques which show that Ni nanoparticles were well dispersed on the reduced graphene
oxide nanosheets without agglomeration and size and morphology of the catalyst remains
unchanged after used in the catalytic reaction.
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Keywords: Ni nanoparticle, reduced graphene oxide, Sonogashira cross-coupling reaction
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___________________________________________________________________________
*To whom correspondence should be addressed: Manash R. Das (mnshrdas@yahoo.com), Pranjal
Gogoi (gogoipranj@yahoo.co.uk)
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Introduction
Carbon nanomaterials in catalytic research have received a great attention by the researchers
of all over the globe for last two decades. The catalytic performance of the catalyst
particularly heterogeneous catalyst is mainly depends on the properties and structure of the
support materials. After receiving the Nobel Prize by Andre Geim and Konstantin Novoselov
in physics in 2010 for the their ground breaking discovery of 2D material graphene, it is
receiving a prime importance as one of the ideal support among all the carbonaceous
materials in the heterogeneous catalysts area due to its outstanding properties.1 Graphene
sheets possesses unique 2D crystal structure which easily blended with metals, metal oxides
or polymers etc.1, 2 In recent years, heterogeneous catalyst of metal nanoparticles experienced
an enormous progress than homogenous catalyst in terms of stability, selectivity and
reusability. In that case metal nanoparticles supported on graphene have attracted significant
attention due to its high corrosion resistance, large surface to volume ratio and their excellent
dispersive nature. In the last two decades, tremendous efforts have been devoted by the
researcher for the development of metal nanoparticleꢀgraphene composite material 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
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sensors,6 photocatalysis,7 soalr cells,8 molecular imaging,9 water treatment,10 catalysis
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and drug delivery. But in the area of catalysis, metal nanoparticleꢀgraphene composites
materials are still to be explored as other applications.1 The use of metalꢀgraphene
composite material as a heterogeneous catalyst has many advantages: a) the graphene as a
support prevents the agglomeration and leaching of the metal nanoparticles due to the
interaction between the metal atoms and residual oxygen containing functional groups
present on the surface of the graphene which results in increase in the surface to volume ratio,
b) The presence of 2D structure in graphene results in the superior catalytic performances of
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the catalyst because the reactant molecules can absorb to both faces of the catalyst, c) Due to
the unique electronic properties of graphene, the electron transfer can take place between the
graphene and supported metal nanoparticles which in turn greatly affects the selectivity of the
desired product, d) The πꢀπ interaction between the aromatic moieties of reactant molecules
and the graphene support enhances the adsorption capacity of the reactant molecules on to the
surface of the catalyst.
In view of these advantageous properties of metalꢀgraphene composite materials in
heterogenous catalysis, some metal nanoparticles are designed on to the surface of graphene
sheets for their catalytic applications such as CO oxidation, oxidation of alcohols,
degradation of organic pollutants, hydrogentation of C=C and C=O bond, selective reduction
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of nitroarenes, fisherꢀtropsch synthesis and coupling reactions.
However, most of these
reports deal with the problem of the separation of the catalyst from the reaction mixture
which lead to trace amount of metal contaminates on the product. This problem can be easily
overcome by designing magnetically separable heterogeneous catalyst. In this regard, the
development of environmentally friendly, cost effective, practical, and efficient catalytic
processes and its reusability have been 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 shows excellent ferromagnetic properties and
thereby results in the effective magnetic separation of the catalyst after completion of the
reaction. The magnetic separation of the catalyst is more effective than the filtration or
centrifugation as it prevents the loss of the catalyst. Magnetic separation of the catalyst from
the reaction system is simple, costꢀeffective and favorable for industrial applications. The
magnetic Ni nanoparticleꢀrGO composite material is also a low cost heterogeneous catalyst
compared to the noble metal (Au, Ag, Pt and Pd) anchored on the graphene sheets.
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On the other hand, Sonogashira crossꢀcoupling reaction is one of the most important carbonꢀ
carbon bond formation reactions in organic synthesis. This coupling reaction have
extensively used for the synthesis of various pharmaceuticals, bioactive compound, natural
products, molecular organic materials and engineered materials.14,15,16 This reaction has
developed and admirable results can be obtained with Pdꢀcomplexes with phosphine ligands.
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However, the most commonly used phosphine ligands are sensitive to air and moisture,
which require inert atmosphere as prerequisite during handling and even a trace amount of
such ligand may act as inhibitor in some metalꢀcatalyzed asymmetric reaction.21 Therefore,
the development of ligandꢀand additiveꢀfree Pd catalyst is of immense interest. On the other
hand, the use of stable and reusable heterogeneous catalyst to replace the homogenous
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catalyst for the Sonogashira cross-coupling reaction is of great importance in sustainable
chemistry. Various heterogeneous Pd nanoparticles have been developed and efficiently used
coupling reaction.22ꢀ24 Although some of these catalysts are highly
for the Sonogashira cross
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efficient, most of them gave low yield of coupling product even in the presence of different
additives.25ꢀ27 To enhance the efficiency of the catalyst, bimetallic nanoparticles comprising
Pd metal with other nonꢀnoble metals such as copper, nickel, iron and cobalt are used in the
Sonogashira crossꢀcoupling reaction. Bimetallic nanoparticles such as Ag–Pd@rGO,28
Pd/Co alloy NPs,29 PdꢀCo/G alloy NPs,30 Pd/Cu mixed NPs,31 Pd/Ni core shell NPs,32 hollow
PdꢀCo nanospheres,33 nano Pd/PdO/Cu system,34 Pd/Cu nano alloys,35 rGOꢀCu48Pd52 alloy
nanoparticles 36 are the notable examples which are recently reported for this reaction.
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Nickel is promising and cheaper alternative to the use of Pdꢀbased catalyst for the
Sonogashira crossꢀcoupling reaction. Among the literature found for the Sonogashira crossꢀ
coupling reaction of aryl halides with phenyl acetylenes catalyzed by nickel, Yin et al.
reported the use of mesoporous silica supported Ni(II) organometallic complex as reusable
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catalyst for Sonogashiracrossꢀcoupling reaction.37 Beletskaya et al. reported homogenous
Ni(II) species as efficient catalyst for Sonogashira crossꢀcoupling reaction.38 Farjadian et al.
showed that poly(vinylpyridine)ꢀgrafted silica containing Ni nanoparticle is an efficient
catalyst for the Sonogashira crossꢀcoupling reaction of aryl halides and phenyl acetylene.39
Wang et al. reported Ni(0) powder catalysis Sonagashira crossꢀcoupling reaction in presence
of cuprous iodide and triphenylphosphine .40 Recently, NiꢀCu system has developed by
Bakherad and his coꢀworkers for the Sonogashira crossꢀcoupling reaction of terminal
acetylenes with aryl iodides in presence of sodium lauryl sulphate.41 Most of these previous
reports require either the use of phosphine ligand and surfactant or inert atmosphere during
the reaction.
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As a part of our continuous efforts for the synthesis of metal nanoparticle − graphene
composites materials and application in catalytic field,42ꢀ44 herein we have reported the
synthesis of magnetically separable Ni nanoparticles on to rGO sheets under ligand free
condition and its application in the Sonogashira crossꢀcoupling reaction in presence of
cuprous iodide. To the best of our knowledge there is no report on the Sonogashira crossꢀ
coupling reaction catalyzed by magnetic Ni nanoparticles anchored on rGO. In view of this
we have developed a heterogeneous catalyst of very cheap, magnetically recoverable and
reusable Ni nanoparticleꢀrGO composites for the Sonogashira crossꢀcoupling reaction.
Results and discussion
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Characterization of Ni nanoparticles-rGO composites
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The formation of Ni nanoparticles on the rGO sheets was confirmed by using analytical tools
like Xꢀray diffraction (XRD), Fourier transform Infrared spectrocscopy (FTIR),
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Thermogravimetric analyses (TGA), Transmission Electron Microscope (TEM), Scanning
electron microscopeꢀ Energyꢀdispersive Xꢀray spectroscopy (SEMꢀEDS) and Vibrating
Sample Magnetometer (VSM) analysis. XRD analysis as shown in Fig. 1 revealed that the 2θ
values 44.73o, 52.13o and 76.84o corresponding to d values 2.02, 1.75 and 1.24 Å are
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 nanoparticle (JCPDS card No. 01ꢀ071ꢀ4655).
It is also confirm that the absence of the NiO and Ni(OH)2 diffraction peak in the XRD
pattern of the Ni nanoparticleꢀrGO composites. It is also noticed that a broad peak at ~25o
suggesting that the GO is completely reduced in presence of the hydrazine hydrate.
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Thermogravimetric analysis (TGA) as shown in Fig. 2 which provided the information about
the reduction of GO to rGO and the formation of Ni nanoparticles onto rGO sheets at the
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same time. The major weight loss of 37.72% occurred at temperature around 200 C is
attributed to the decomposition of the labile oxygenꢀcontaining functional groups present in
GO. The weight loss in this region dramatically decreases upto 9.72% after formation of Ni
nanoparticlesꢀrGO composites material because of the reduction of the oxygen containing
functional groups present in GO such as carbonyl, hydroxyl, epoxy and carboxyl groups. The
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weight loss found above 600 C for both Ni nanoparticlesꢀrGO composites material and GO
are results from the pyrolysis of the carbon skeleton of rGO nanosheets
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 regards
we have examined the morphology of the Ni nanoparticle anchored on the rGO sheets (Fig.
3). The graphene sheets of micron size are clearly visible in the TEM images and Ni
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nanoparticles are uniformly distributed onto those sheets. Ni nanoparticles of the mean
diameter 2.7 nm with narrow particle size distribution are embedded in the rGO sheets and
spherical in nature. The size of our synthesized Ni nanoparticles is very small in comparison
to the other previous reports of the synthesis of Ni nanoparticles.45ꢀ49 The small size and
uniform distribution of the synthesized Ni nanoparticles is results from the strong interaction
between the surface of rGO sheets and Ni nanoparticles.50 Lu et. al proposed that the
interaction between the graphene and Ni is attributed to the partially occupied dꢀorbital which
are localized on to the vicinity of the Fermi level.51 This result clearly fulfils our aim to
prepare very small sized nanoparticles and thereby large surface to volume ratio in order to
get excellent catalytic activity. Xu et al. recently reported the Ni nanoparticle of the average
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size 9.7 nm decorated on the graphene sheets with narrow size distribution . Wu et al. also
demonstrated that the synthesis of the Ni nanoparticleꢀgraphene composite by solvothermal
method via electrostatic induced spread adsorption. Ni nanoparticle of average size ~55 nm is
well distributed on the graphene sheets.45 Another synthesis method reported by Tian et al.
observed that Ni nanoparticles of the average size 8 nm homogeneously decorated on the
rGO sheets in presence of the poly(Nꢀvinylꢀ2ꢀpyrolidone) (PVP).48 However, the Ni
nanoparticles of average size of ~27 nm is decorated on the rGO sheets without using
stabilizing agent PVP due to agglomeration of the Ni nanoparticles. The surface morphology
and elemental composition of the composite material was examined by SEMꢀEDS (shown in
Fig. 4 (aꢀc)). The crumpled and rippled structure of GO which results from deformation upon
the exfoliation is partially destroyed in the composite material due to the reduction of large
amount of oxygen containing functional groups (Fig. 4a,b). However, 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 confirms the sufficient
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loading of Ni nanoparticles onto the surface of rGO sheets with an insignificant amount of
oxygen which remains due to the presence of unreduced oxygen containing functional group.
The FTIR spectra of GO and Ni nanoparticlesꢀrGO composite is shown in Fig. 5. As shown
in Fig. 5, the prominent peaks at 3131, 1728, 1583, 1436, and 1042 cmꢀ1 of GO 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 formation of Ni nanoparticles on rGO
sheets due to the reduction of these oxygen containing functional groups present in GO.
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To investigate the magnetic properties of our synthesized Ni nanoparticles−rGO composite
material, magnetic measurements were performed at room temperature in terms of field
dependent magnetization measurement (MꢀH). The results of saturation magnetization (Ms),
remanent magnetization (Mr) and the coercivity (Hc) are listed inside the Fig.6. The hysteresis
behaviour and the magnetic parameters clearly reveal the ferromagnetic interaction of the
synthesized Ni nanoparticles on rGO nanosheets.53ꢀ56 As shown in Fig. 6, the Ni
nanoparticles−rGO composites shows saturation magnetization of 43.54 emu/g and the
reduction of this saturation magnetization value in comparison to bulk nanoparticle is due to
the increase in surface to volume ratio resulting from decrease in particle size.54 The
Remanence magnetization (Mr) of the sample is found to be 6.92 emu/g and the coercivity
(Hc) of 197.54 Oe which is greater than the bulk nickel. The increase in coercivity value in
comparison to bulk nickel confirms the rule of HC ∝ 1/D of the multidomain ferromagnetic
nanoparticles.55 Thus, the excellent ferromagnetic behaviour of Ni nanoparticles−rGO
composite fulfils our aim to develop more efficient and easily separable catalyst in catalysis
reaction.
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Catalytic study
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After complete characterization of the Ni nanoparticlesꢀrGO catalyst, it was utilized as an
effective catalyst for the Sonogashira crossꢀcoupling reaction of aryl halides with phenyl
acetylenes in presence of CuI. Initially, we optimized the reaction conditions using
bromobenzene 1e and phenyl acetylene 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 solvents on this coupling reaction by using K2CO3 as a base. The results revealed that Nꢀ
methylꢀ2ꢀpyrrolidone (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
strong bases such as NaOH and KOH gave low yield of the product (Table 1, entry 10, 13),
whereas Na2CO3 and K3PO4 showed almost identical results. Additionally, we run the
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coupling reaction at different temperatures and 120 C was found the optimum reaction
temperature for this reaction.
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Table 1: Optimization studies for the Ni nanoparticlesꢀrGO catalyst in the Sonogashira crossꢀ
coupling reaction a
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Entry
Solvent
Base
Temperature (OC) Yield (%) b
1
2
3
4
5
6
7
8
9
10
11
12
13
H2O
DMF
DMF
Toluene
NMP
NMP
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOH
Na2CO3
K3PO4
NaOH
110
60
120
120
60
100
120
100
15
40
45
30
70
85
93
80
NMP
DMSO
226
227
228
DMSO
NMP
NMP
NMP
NMP
120
120
120
120
120
80
50
92
90
58
a Reaction conditions: Bromobenzene (1 mmol), Phenyl acetylene (1.2 mmol), CuI (0.08
mmol), catalyst (25 mg, 0.15 mmol Ni), base (3 mmol), Solvent (5 mL), 4 h. b Isolated
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After having the optimized reaction conditions, we explored the versatility and efficiency of
our catalyst for the Sonogashira crossꢀcoupling reaction using different aryl halides with
phenyl acetylenes. The results are shown in Table 2. As shown in Table 2, the aryl halides
such as bromides and iodides efficiently coupled with phenyl acetylenes to give the excellent
yield of the desired product in spite of electronꢀrich, electronꢀpoor and electronꢀneutral nature
of the halides. The reaction conditions are notably compatible with nitro group on the aryl
ring. Having established a range of aryl bromide and aryl iodide as coupling partner, we next
examined the scope of the crossꢀcoupling reaction with aryl chloride (Table 2, entry 14 and
15). Under our optimized conditions, arylchlorides coupled with terminal alkynes and gave
good yield of corresponding products. However, the chloroaryl substrate requires long
reaction time and high reaction temperature as compared to iodides and bromides in order to
get the comparable yield. Additionally our catalyst system chemoselectively reacts with
bromide, when both chloroꢀ and bromoꢀ groups were present on the same substrate (Table 2,
entry 10). Our catalyst system was also applied to the heteroaryl halide, 2ꢀbromopyridine and
the coupling products 3i were obtained in 86 % yields (Table 2, entries 16). To further extend
the scope of our Ni nanoparticlesꢀrGO catalyst in the Sonogashira crossꢀcoupling reaction,
we choose a heteroaryl substrate having both bromo and iodoꢀsubstituents. In this regard, 2ꢀ
bromoꢀ5ꢀiodopyridine was treated with three equivalents of phenylacetylene under our
catalytic conditions. The product 3j was obtained with excellent yield (Scheme 1).
Furthermore, we have compared our Ni nanoparticlesꢀrGO catalyst with other previously
reported heterogeneous as well as homogenous catalyst of Ni to highlight the advantages of
our catalyst in the sonogashira crossꢀcoupling reaction (Table 3). From this comparison
results, we have found that our synthesized catalyst is more advantageous with respect to
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yield and reaction condition. Moreover, in most of these cases, the scope of their catalyst in
Sonogashira crossꢀcoupling reaction is limited to only aryl iodides with phenyl acetylenes.
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Table 2: Sonogashira cross-coupling of various aryl halides and phenyl acetylenes a
Entry
1
Aryl halide
1(a-m)
Alkynes
2(a-b)
Product
3(a-i)
Yield b
(%)
95
2
3
4
94
95
95
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6
7
8
93
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93
93
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88
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72 c
70 c
86
a
Reaction conditions: Aryl halide (1 mmol), Phenyl acetylene (1.2 mmol), CuI (0.08 mmol),
catalyst (25 mg, 0.15 mmol Ni), K2CO3 (3 mmol), NMP (5 mL, 120 C, 4 h. b Isolated Yield.
o
C
The reaction was performed at 140 oC for 16h. NP: nanoparticle
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Scheme 1. Ni nanoparticlesꢀrGO catalysed Sonogashira crossꢀcoupling reaction of 2ꢀbromoꢀ
5ꢀiodopyridine with phenyl acetylenes.
Table 3: Comparison of catalytic activity of Ni nanoparticles-rGO catalyst with other
homogenous and heterogeneous catalyst of Ni
Catalyst
Conditions
Yield
(%)
70ꢀ95
Ref
Ni nanoparticleꢀrGO
K2CO3, NMP, CuI, 120 oC, 4 h
This work
SiꢀP4VPyꢀNi0
Pd70Ni30/MWCNTs
Ni(PPh3)2Cl2
NiCl2.6H2Oꢀ nꢀBu4NBr
Ni(PPh3)2Cl2
Ni(0)ꢀCuIꢀPPh3
Ni(PPh3)2Cl2/CuI
NiꢀPPh2ꢀPMOs(Ph)
K2CO3, NMP, CuI, 120 oC, 1.5ꢀ10 h
40ꢀ90
60ꢀ74
57
46ꢀ91
93ꢀ100
56ꢀ98
70ꢀ92
63ꢀ75
39
57
58
59
38
40
41
37
NaOH, Pyrrolidine, 120 oC, 1h
K2CO3, [Cu(CH3CN)4]BF4, reflux
NaOH, Ethylene Glycol, 120 oC,1ꢀ12 h
K2CO3, CuI, Dioxane:H2O, reflux, 4 h
KOH, isopropanol, 80ꢀ120 oC, 5 h
Cs2CO3, H2O, Surfactant, 60 oC, 2ꢀ6 h
K2CO3, CuI, Dioxane/H2O, N2 protection
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At present, the exact mechanism of the reaction is not clear. However, we proposed here a
plausible mechanism for the Ni nanoparticlesꢀrGO catalyzed Sonogashira crossꢀcoupling
reaction as shown in scheme 2. We believed that, the Ni nanoparticlesꢀrGO undergoes
oxidative addition with aryl halide to form Ni(II) reactive species which readily
transmetalated with Cuꢀphenylacetylene to form intermediate A. Finally the desired coupling
product is formed from the resulting intermediate A via reductive elimination. 37, 58, 60
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Scheme 2. Proposed mechanism for the Ni nanoparticlesꢀrGO catalyzed Sonogashira crossꢀ
coupling reaction
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Reusability of the Ni nanoparticle-rGO heterogeneous catalyst
The reusability of our Ni nanoparticles−rGO catalyst for the Sonogashira crossꢀcoupling
reaction was also investigated. The excellent magnetic behaviour of our synthesized Ni
nanoparticles allows themselves to accumulate onto the magnetic stirring bar as soon as the
magnetic stirring was 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 subsequent
reaction. To check the reusability of the catalyst, bromobenzene 1e and phenyl acetylene 2a
was used as substrates for the Sonogashira crossꢀcoupling reaction. As shown in Fig. 7, the
recovered catalyst was consecutively used for six times without loss of its significant activity.
Although, the different type of heterogeneous catalyst of metal nanoparticles are reported for
Sonogashira crossꢀcoupling reaction as discussed in the introduction part, but to the best of
our knowledge no reports are available of characterization of the catalyst after performing the
reaction. We characterized the Ni nanoparticleꢀrGO heterogeneous catalyst by 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 the
2.7 nm, the average size of the nanoparticles before the reaction. Also we have found the
same crystallite size of the Ni nanoparticles for both the fresh and reused catalyst by Sherrer
equation using PDXL software in XRD. Therefore, the XRD as well as TEM analysis clearly
demonstrate that the size and morphology of the Ni nanoparticleꢀrGO catalyst remain
unchanged after performing the catalytic reaction. Moreover, the nickel content of the
recovered catalyst was also determined by ICPꢀAES which suggest negligible difference with
the catalyst before using in organic catalysis reaction.
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Since the leaching of the nanoparticles from the support is a common problem in catalysis,
we have examined the leaching of Ni nanoparticles from the rGO support by performing hot
filtration test. For that we have considered the Sonogashiraꢀcross coupling reaction of
bromobnezene 1e with phenylacetylene 2a. After continuing the reaction for 1.5 h, the
catalyst was separated and the conversion to 1,2ꢀdiphenylethyne 3a was determined by GC
and was found to be 45 % yield. After that, the filtrate part was further heated for another 5h
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 nanoparticle was leached from the catalyst after performing the reaction.
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Conclusion
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In conclusion, the present works 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 crossꢀcoupling reaction. The
catalyst could be easily magnetically separable from the reaction mixture without any
leaching of the nanoparticles. The size and morphology of the reused catalyst was again
characterized by TEM and XRD which suggest that size and shape of the Ni nanoparticles
remain unchanged without undergoing any agglomeration of the particles. In addition, the use
of Ni nanoparticle as catalyst for the Sonogashira crossꢀcoupling 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.
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Experimental Section
Materials and Methods
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Materials used for synthesis of Ni nanoparticlesꢀrGO composites are graphite powder
(<20ꢁm, SigmaꢀAldrich), potassium permanganate (>99 %, EꢀMerck, 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 %, EꢀMerck,
India). All the substrates required for Sonogashira crossꢀcoupling reaction were purchased
from sigma Aldrich, USA and used without any further purification.
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Characterization techniques
Powder XRD spectra of the samples were taken on a Rigaku, Ultima IV Xꢀray diffractometer
from 5–100o 2θ using CuꢀKa source (l = 1.54 A˚). TGA of the samples were performed at a
rate of 5 °C rise in temperature per minute by using TAꢀSDT (model: Q600DT, TA
Instruments, USA). TEM images were taken from JEOL JEMꢀ2011 electron microscope,
Transmission Electron Microscope, Japan.VSM, USA operated at room temperature to
investigate the magnetic properties of the composite material. SEMꢀEDS analysis was
performed by using Carl ZEISS Field Emission SEM with Oxford EDS to determine the
elemental composition of the composite material. FTIR spectra were recorded in the
frequency range of400−4000 cm−1 on KBr discs in a Perkin–Elmer system 2000 FTꢀIR
spectrophotometer. All NMR spectra were taken by using Bruker Advance DPX 300 or 500
MHz spectrometer. 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 internal standard. Gas Chromatography analyses were performed with the help
of Chemito GCꢀ8610, FID gas chromatograph fitted with Porapak Q column (2 m × 1/8 // O.
D., SS) and data were analyzed by Winchrom GC data processing software. Ni content in the
nanocomposite catalyst was determined using the EPA 200.7 method of acid digestion
followed by Inductively Coupled Plasma Analysis (ICPꢀAES, Perkin Elmer, Optima 5300 V).
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Multilevel Calibration of the Ni was performed using Metal Standard of ICP (Sigma). The
calibration curve was linear with R2 value of 0.99999
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Synthesis of Ni nanoparticle-rGO composite materials
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 oC for 15 min with stirring. Then 20 mL of an aqueous
suspension of GO (0.012 gLꢀ1) was added to above mixture followed by addition of NaOH
(20 mg) and subjected to ultrasonication for 10 min. Finally, the reaction mixture was
vigorously stirred at 80 oC until the black precipitate of the composites material was obtained.
The solid material was separated by simple filtration and washed with ethanol for several
times followed by water (2 to 3 times) and then dried in an air oven at 60 oC for overnight.
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General procedure for the Sonogashira cross-coupling reaction
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In a roundꢀbottom flask having aryl halide (1 mmol), phenyl acetylene (1.5 mmol), K2CO3 (3
mmol) and CuI (0.08 mmol), a suspension of the catalyst (25 mg, 0.15 mmol of Ni) in Nꢀ
o
methylꢀ2ꢀpyrrolidone (5 mL) was added. The whole reaction mixture was stirred at 120 C
for 4 h. After completion of the reaction (monitored by TLC), the catalyst was separated from
the reaction mixture by using an external magnet and the reaction mixture was poured into
water. The organic product was extracted with ethyl acetate (3x10 mL). The combined
organic phase were dried over Na2SO4 and concentrated in vacuum. The crude products were
purified by column chromatography using silica gel (60ꢀ120 mesh) with EtOAc/hexanes as
eluent to obtain the desired coupling product.
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Acknowledgement:
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The authors acknowledged the Department of Science and Technology, New Delhi for
financial support (DST No. SR/FT/CSꢀ136/2011, CSIRꢀNEIST Project no. GPP–269 and
DST No. INT/RUS/RFBR/Pꢀ193 and CSIRꢀNEIST Project No. GPPꢀ0301) and the Director,
CSIRꢀNorth East Institute of Science and Technology, Jorhat, India for the interest in this
work and facilities. The authors are also thankful to SAIF, NEHU, Shillong for the use of
TEM facility. NH acknowledges to UGC, New Delhi, for JRF grant.
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Figures
Fig. 1 Powder XRD diffractogram of Ni nanoparticles-rGO composites

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Fig. 2 TGA curve of (a) GO and (b) Ni nanoparticles -rGO composites

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a
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Diameter (nm)
b
c
d
Fig. 3 TEM images of Ni nanoparticle on rGO nanosheets (a-d). HRTEM image along with
particle size distribution (a)

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Fig. 4 SEM images of Ni nanoparticles on rGO nanosheets (a-b); EDS analysis of Ni
nanoparticles-rGO composite material

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Fig. 5 FTIR spectra of (a) GO and (b) Ni nanoparticles -rGO composite

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

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93 %
100
80
60
40
20
0
93 %
90 %
88 %
80 %
68 %
1st
2nd
3d
4th
5th
6th
Run
Fig. 7 Reusability of the Ni nanoparticles-rGO catalyst for the Sonogashira-cross coupling
reaction

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