Green-Synthesized Nickel Nanoparticles on Reduced Graphene Oxide as an Active and Selective Catalyst for Suzuki and Glaser-Hay Coupling Reactions

Article abstract of DOI:10.1002/aoc.5778

A mild and benign methodology to syntheses biaryls and 1,3-diynes has been demonstrated using the nickel nanoparticles supported on reduced graphene oxide (RGO-Ni) as a heterogeneous catalyst which is prepared using green reagents. A series of substituted biaryls and 1,3-diynes has been synthesised in good to excellent yields through C-C homocoupling reaction of arylboronic acids and terminal alkynes respectively using 1,4-dioxane as a benign solvent. The present ligand-free catalytic system proceeds smoothly under mild conditions, avoids noble and stoichiometric metal reagents and tolerates sensitive functional groups. Also has a wide substrate scope and feasible with other nitrogen and sulphur containing heteroaryl boronic acids. Hot filtration test unambiguously proves the true heterogeneity of the catalyst and which support for the further reusability of the catalyst for several times without any change in the activity. The easy preparation and simple magnetic separation, stability and reusability reveal that as-prepared RGO-Ni as a versatile catalyst for the synthesis of polyaromatic compounds both in academia and industries. Highlights: Green-synthesized RGO-Ni nanocomposite used as a heterogeneous catalyst for Suzuki type (C-C) and Glaser–Hay (C ≡ C) homocoupling coupling reactions. Ligand-free catalytic system and avoids noble and stoichiometric metal reagents Short reaction time with a minimum catalyst (nickel) loading RGO-Ni nanocomposite is highly stable, reusable, and magnetically retrievable.

Full text of DOI:10.1002/aoc.5778

Received: 3 March 2020
DOI: 10.1002/aoc.5778
Revised: 10 April 2020
Accepted: 12 April 2020
F U L L P A P E R
Green-Synthesized Nickel Nanoparticles on Reduced
Graphene Oxide as an Active and Selective Catalyst for
Suzuki and Glaser-Hay Coupling Reactions
Karthik Murugan | Devarajan Nainamalai | Pavithara Kanagaraj |
Saravana Ganesan Nagappan | Suresh Palaniswamy
Supramolecular and Catalysis Lab,
A mild and benign methodology to syntheses biaryls and 1,3-diynes has been
demonstrated using the nickel nanoparticles supported on reduced graphene
oxide (RGO-Ni) as a heterogeneous catalyst which is prepared using green
reagents. A series of substituted biaryls and 1,3-diynes has been synthesised in
good to excellent yields through C-C homocoupling reaction of arylboronic
acids and terminal alkynes respectively using 1,4-dioxane as a benign solvent.
The present ligand-free catalytic system proceeds smoothly under mild condi-
tions, avoids noble and stoichiometric metal reagents and tolerates sensitive
functional groups. Also has a wide substrate scope and feasible with other
nitrogen and sulphur containing heteroaryl boronic acids. Hot filtration test
unambiguously proves the true heterogeneity of the catalyst and which sup-
port for the further reusability of the catalyst for several times without any
change in the activity. The easy preparation and simple magnetic separation,
stability and reusability reveal that as-prepared RGO-Ni as a versatile catalyst
for the synthesis of polyaromatic compounds both in academia and industries.
Department of Natural Products
Chemistry, School of Chemistry, Madurai
Kamaraj University, Madurai, 625021,
India
Correspondence
Palaniswamy Suresh, Supramolecular and
Catalysis Lab, Department of Natural
Products Chemistry, School of Chemistry,
Madurai Kamaraj University. Madurai –
625021, India.
Email: ghemistry@gmail.com; suresh.
chem@mkuniversity.ac.in
Funding information
Science and Engineering Research Board,
Grant/Award Number: SR/FT/CS-
53/2011; Council for Scientific and
Industrial Research, Grant/Award
Number: 02(0191)14/EMR-II; University
Grants Commission, New Delhi, India,
Grant/Award Number: 42-291/2013(SR)
Highlights
• Green-synthesized RGO-Ni nanocomposite used as a heterogeneous catalyst
for Suzuki type (C-C) and Glaser–Hay (C ≡ C) homocoupling coupling
reactions.
• Ligand-free catalytic system and avoids noble and stoichiometric metal
reagents
• Short reaction time with a minimum catalyst (nickel) loading
• RGO-Ni nanocomposite is highly stable, reusable, and magnetically
retrievable
K E Y W O R D S
Glaser-Hay reaction, magnetically retrievable catalysis, nickel nanoparticles, reduced graphene
oxide, Suzuki coupling
Appl Organomet Chem. 2020;e5778.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5778

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MURUGAN ET AL.
1 | INTRODUCTION
Mn,[25] Fe,[26] Co,[27] Cu,[28] and W[29] had immobilized
on the surface of graphene basal plane. Even some of the
non-transition metals like Eu,[30] Gd,[31] Al,[32] Ge,[33]
Sn[34]- graphene composite also been studied. Although
these graphene-metal nanocomposites found diverse
material applications. Whereas, Pd, Cu and Au metals
supported on graphene have extensively investigated as a
catalyst in organic coupling reactions.[35] More recently
nickel nanoparticles (Ni NPs) has acquired growing
attention for their potential utility in diverse catalytic
applications.[36] It has been immobilized on various sup-
ports[26] to avoid the formation of large aggregates, to
protect from further oxidation and its surface reactivity.
Notably, Ni NPs supported on reduced graphene oxide
(RGO) has shown significant advantages as a catalyst
because of its distinct reactivity, ease of separation by a
magnetic field and recyclability of the catalyst.[37] The
catalytic proficiency of graphene supported Ni
nanoparticles were explored in very few synthetic trans-
formations.[38] In which, the notable reports on C-C cou-
pling reactions like Kumado-Corriu cross-coupling,[39]
Sonogashira coupling,[40] Silylation of the aldehyde with
phenyl silanes,[41] and C-S coupling reactions.[42] How-
Symmetrical biaryls and 1,3-diynes structural motifs have
exhibited a variety of physical and chemical properties.[1]
They have been found in natural products such as fatty
acids, alkaloids as well as in numerous biologically active
parts of pharmaceutical and agrochemical entities, bioac-
tive molecules,[2] conducting polymer materials,[3] optical
light-emitting materials,[4] and supramolecular architec-
tures.[5] The straightforward approach to obtain biaryls
and 1,3-diynes scaffolds are the direct coupling of
corresponding arylboronic acids and terminal alkynes,
respectively.[6] In this context, the formation of aryl
(Csp2- Csp2) bond and (Csp- Csp) bond formation is an
essential and challenging process to both academic and
industrial catalytic community. One of the promising
approaches for the production of symmetrical biaryls is
transition metal-catalyzed coupling of arylboronic
acids.[7] Whereas Glaser-Hay coupling is the most widely
used procedure for the synthesis of 1,3-diynes through
the oxidative homocoupling of terminal alkynes. Though
a number of modifications were made for the synthesis of
both symmetrical and unsymmetrical diynes, still novel,
and simple methodologies are continuously emerging.[8]
In general, palladium and other noble transition metals
were extensively employed in such C-C coupling reac-
tions.[9] However, a recent search on the alternate transi-
tion metals with economic and simple experimental
aspects, nickel getting more attention, owing to its ele-
gant properties such as stability and unique catalytic
response towards coupling reactions. These properties
represent nickel as an ideal alternative to the existing
noble metals which has already explored in coupling
reactions. Nickel-based homogeneous catalysts have
employed in various coupling reactions such as
Kumada,[10] Stille,[11] Suzuki,[12] Negishi,[13] Chan-
Lam[14] and Glaser-Hay[15] type reactions. However, to
stabilize and sustain its oxidation states as a metal cata-
lyst, complex ligands, additives are required, and unique
or selective reaction atmosphere also essential which
made the coupling reactions are more expensive and
experimentally laborious. Hence, it is an immediate
demand to develop a mild, operationally simple, econom-
ically viable heterogeneous and recyclable catalyst for
coupling reactions.[16]
2
ever, still, the homocoupling of Csp2-Csp as well as Csp-
Csp are not yet investigated with graphene supported Ni
nanoparticles.
In the present work, to unleash the catalytic potential
of the RGO-Ni in coupling reactions under a relatively
milder condition, it has been employed as a catalyst for
the homocoupling of arylboronic acid to biaryls (Suzuki
coupling) as well as homocoupling of terminal alkynes to
1,3-diynes (Glaser-Hay coupling) and its sustainable
nature also explored. The optimization and observed
results are discussed below.
2 | RESULTS AND DISCUSSION
Reduced graphene oxide supported nickel nanoparticles
(RGO-Ni) has prepared by the hydrothermal process by
adopting our previous report.[43] In which 1:3 ratio of GO
and NiCl2.6H2O were taken in water, along with 0.1 M
solution of ascorbic acid. This mixture was placed in a
Teflon lined autoclave in an oven at 120 ꢀC for 16 hr. The
resulting RGO-Ni nanocomposite was obtained as a black
precipitate and it was characterized by various analytical
tools like FT-IR, PXRD, Raman, XPS and TEM
((Figure S1- S5) for details see ESI). To explore the cata-
lytic potential of the as-prepared RGO-Ni nanocomposite
and to develop a benign and alternate methodology for
coupling reactions, synthesis of the biaryl compound has
taken as a prototype reaction. For that phenylboronic
acid (1a) is considered as a model substrate to identify
Graphene materials are a pivotal candidate in the
development of contemporary heterogeneous catalysis. It
acts as a suitable support for the development of
supported heterogeneous metal nanocomposite and
exhibiting superior properties in catalyzed reactions, oxy-
gen reduction reactions, drug delivery, and sensors.[17]
Most of the noble and non-noble transition metals like
Au,[18] Pt,[19] Pd,[20] Ag,[21] Ru,[22] Rh,[23] Ir,[23] Ti,[24]

MURUGAN ET AL.
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the appropriate catalytic parameters such as solvent, tem-
perature, time, base and amount of the catalyst loading.
From the preliminary study, it is understood that the
RGO-Ni nanocomposite catalyzes the homocoupling
when using 1,4-dioxane as a solvent with K3PO4 as a base
at 100 ꢀC in an open vessel yield the corresponding
biphenyl (2a) as a sole product. This encouraging result
from the preliminary study, further directed to screen all
the possible solvent system to choose more appropriate
and compatible solvent works with RGO-Ni. The optimi-
zation results obtained from employing various solvents
and diverse bases which are listed in Table 1. From the
observed experimental results, it could be seen that the
solvents behaved aberrantly in the presence of RGO-Ni
and yield the biphenyl product in different ranges of
yields. While using highly polar solvent, DMSO gives
good yield meantime with another polar solvent DMF
and ACN though the reaction takes place; indeed, the
conversion is not impressive.
solvents such as THF, n-hexane and 1, 4-dioxane, no
reaction was observed with THF and hexane. Among all
the screened solvents the 1,4-dioxane exhibited signifi-
cant superiority over the other solvents, and the reaction
went to completion with a very good yield of 99% of the
homocoupled product (entry 7, Table 1). To improve the
greenness of the reaction, the reaction was carried out
with the aqueous mixture of the 1,4-dioxane solvent sys-
tem; however, the observed result was unfortunate than
the normal solvent conditions and gave a poor yield of
35% (entry 12, Table 1).
Furthermore, to fix the role of the base in the homo-
coupling of phenylboronic acid (1a), various bases were
screened, among which the inorganic base K3PO4
resulted in a better yield with RGO-Ni without any
adverse effect during the product separation and no influ-
ence on the catalyst stability. Other inorganic bases such
as Cs2CO3, K2CO3 and Na2CO3 gave poor yield (entries
13–15, Table 1) and with the organic base triethylamine
(TEA) shown inferior yield (entry 16, Table 1), and the
catalyst was found to be inactive towards coupling in the
absence of a base (entry 8, Table 1). In general, nickel
catalysed coupling reactions required a strong base like
K3PO4 to initiate the coupling process. To fix the exact
amount of the base required in the coupling reaction, the
quantity of the base was screened with the optimised
conditions, and the results revealed that 2.5 equiv.
(1.25 mmol) of the base is adequate for the complete con-
version of the reaction with an excellent yield
(Figure 1a). Similarly, though the initial reaction was per-
formed up to 12 hr to explore the catalytic performance
of the catalyst in the present coupling reaction and also
for solvent and base optimisation, the exact time optimi-
sation with the screened data shown that within 3 hr the
desired coupling was formed without any byproducts.
(Entry 8, Table 1). The optimised reaction time (3 hr) is
minimum for the present nickel catalyzed coupling reac-
tions when compared to the literature reports.[44]
The catalyst is highly inert with the protic polar sol-
vents, such as methanol, ethanol and water and no con-
versions were noted. However, in the case of non-polar
TABLE 1 Reaction parameter optimization of RGO-Ni
Catalyzed Homocoupling of Boronic acids a
Entry
1
Solvent
Base
Yieldb (%)
c
1,4-Dioxane
Methanol
Ethanol
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
Cs2CO3
K2CO3
Na2CO3
Et3N
-
2
-
3
-
4
Water
-
5
DMSO
95
6
DMF
-
7
1,4-Dioxane
1,4-Dioxane
DCM
99
8
-d, 68e, 82f, 99g
9
-
10
11
12
13
14
15
16
THF
trace
Similarly, in the RGO-Ni catalyzed coupling reaction,
the product conversion is susceptible to temperature
changes; A mild drop in temperature dramatically affects
the yield (Figure 1b) within the studied time. And the
ACN
78
35
-
1,4-Dioxane: Water
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
-
ꢀ
temperature optimization showed that at 85 C complete
-
the conversion of phenylboronic acid to the
ꢀ
15
corresponding coupled product. Thus, 85 C was chosen
as the optimum temperature for further reactions.
aReaction Conditions: phenylboronic acid (0.5 mmol), base
(1.5 mmol) and 60 mg of RGO-Ni in 2 ml of solvent at 100 ꢀC up to
With the screened parameters, in order to know the
appropriate amount of catalyst, reactions were performed
with different amount of catalyst loading, since it plays a
crucial role in the enhancement of product conversion
and quantifies the efficiency of the heterogeneous cata-
lyst used. Finally, the studies have shown that 50 mg of
catalyst is adequate to achieve the maximum yield (99%)
12 hr.
bIsolated yield.
cwithout catalyst,
dwithout base,
Yields in
e1 hr, f 2 hr, and
g3 hr.

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MURUGAN ET AL.
FIGURE 1 Optimization of base load (a) and temperature (b) for RGO-Ni catalysed homocoupling
of the homocoupled product with a minimum amount of
nickel loading (9.5 mg, 0.16 mmol). Meanwhile, no cou-
pling product was formed in the absence of RGO-Ni,
which reveals the role of RGO-Ni catalyst towards the
coupling reaction (Figure 2) (Table S1). Finally, all the
above screening studies gave an optimised reaction con-
dition such as K3PO4 as a base, 1,4-dioxane as a medium
the corresponding symmetrical biaryls in 85–99% yields
(2a-2 l, Table 2). Phenylboronic acids with electron-
donating substituents such as methyl and methoxy
groups facilitated the coupling and gave a better yield
within the stipulated reaction time (2b and 2c, Table 2).
Table 2 RGO-Ni nanocomposite catalysed homo-
coupling of arylboronic acida
aReaction conditions: RGO-Ni catalyst (50 mg),
arylboronic acid (0.5 mmol), K3PO4 (1.25 mmol)
ꢀ
of solvent at 85 C, where the RGO-Ni nanocomposite
efficiently catalyses the homocoupling of phenylboronic
acid and selectively promotes the homocoupled symmet-
rical biaryls.
With the optimum reaction conditions, the scope of
the RGO-Ni nanocomposite catalysed homocoupling was
extended to other arylboronic acids, and the results are
presented in Table 2. A diverse arylboronic acids with
either electron-donating or electron-withdrawing substit-
uents smoothly underwent the homocoupling and gave
ꢀ
1,4-dioxane (2.0 ml) at 85 C up to 3 hr. All are isolated
yields.
b TON,
c TOF (h-1).
Meanwhile, when the same substituents present on
the meta position of the aryl ring, a slight decrease in the
yield was noted when compared to its corresponding
para-substituted isomers (2d and 2e, Table 2). On the
other hand, electron-withdrawing substituents such as
chloro and bromo gave good yields irrespective of their
electron nature (2f and 2 g, Table 2). The same trend was
observed with aryl-substituted phenylboronic acid with
an excellent yield of 95% (2 h, Table 2). 2,5-diphenyl
substituted bulkier phenylboronic acid gave attractive
yield (2i, Table 2). These results have shown that the cat-
alyst is not more sensitive towards the steric nature of the
substrate owing to the presence of large surface area as
well as good synergy with aryl motifs. With this promis-
ing results in arylboronic acids, further attempts have
been made on various nitrogen and sulfur-containing
heteroaryl boronic acids, resulted with the excellent for-
mation of symmetrical bipyridines and bithiophenes (2j-
2 l, Table 2), which are being used as versatile building
blocks in synthetic organic and material chemistry. Fur-
thermore, the fused ring system, such as naphthyl
boronic acid and oxiranes were studied for this C-C cou-
pling reaction, gave better yields (2 m-2n, Table 2). Based
on this excellent results and formation of the coupled
FIGURE 2 Catalyst (RGO-Ni) load optimization for
homocoupling reaction

MURUGAN ET AL.
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TABLE 2 RGO-Ni nanocomposite catalysed homocoupling of arylboronic acids
products with various substituents, we concluded that
the acidity of arylboronic acids plays a crucial role in the
coupling reaction, electron-withdrawing groups increase
the acidity of arylboronic acid, which gave the better
yield than the electron releasing group on boronic
acid.[26]
reaction time. Meanwhile, a moderate decrease in the
yield (72%) was noted for the meta-amino substituted
phenylacetylene (4 g, Table 3). On the other hand, halo
substituents such as chloro and fluoro gave good yields
irrespective of their position and electronic nature. From
these flattering results with aromatic alkynes, further
attempts have made on various aliphatic terminal
alkynes, which resulted in reasonable yield. The oxidative
homocoupling of aliphatic terminal alkynes yields only
38 to 42% of the corresponding 1,3-diynes (4 k-4 m,
Table 3). However, the other non-aromatic terminal
alkynes required a quite longer reaction time with infe-
rior to moderate yields. Based on these experimental
observations, it concluded that aryl alkynes are more
reactive towards the homocoupling than aliphatic sys-
tems in RGO-Ni catalyzed Glaser-Hey coupling.
2
Having these impressive results from the CSP2-CSP
homocoupling of arylboronic acids, further directed to
extend our curiosity towards the CSP -CSP coupling of ter-
minal alkynes (Glaser–Hay coupling). Initially, we per-
formed the Glaser-Hay coupling by employing the RGO-
Ni catalyst with our optimised reaction conditions. A
model reaction was performed to establish the catalytic
proficiency of RGO-Ni using phenylacetylene (3a), as a
ꢀ
model substrate and K3PO4 as a base at 85 C tempera-
ture. The observed results are highly encouraging, and
phenylacetylene gave 99% of 1,3-diynes under the identi-
cal screened condition within the optimised time as like
in arylboronic acid system. It was confirmed by GC–MS,
1H and 13C NMR analysis. Changing the reaction param-
eters for RGO-Ni catalysed Glaser-Hay coupling has no
impact on the formation of 1,3-diynes. Hence the
optimised condition for homocoupling of arylboronic
acids has worked smoothly for the coupling of terminal
alkynes also. With the suitable reaction parameters in
hand, the scope of RGO-Ni nanocomposite catalyzed syn-
thesis of 1,3-diynes has been expanded to other terminal
alkynes including aromatic and aliphatic terminal
alkynes, and the results are presented in Table 3. Various
aromatic terminal alkynes with electron-donating substit-
uents smoothly underwent homocoupling and gave the
corresponding 1,3-diynes in 72–99% yields (4a-4 g,
Table 3).
Though RGO-Ni catalyzes the homocoupling of
arylboronic acid as well as terminal alkynes with a mini-
mum Ni loading (9.5 mg, 0.16 mmol) without any exter-
nal ligands, in order to understand the role of nickel
nanoparticles present on the RGO surface control experi-
ments were carried out with other catalytic systems
under the similar optimized conditions and the results
are listed in Table 4.
In control experiments, the precursor of RGO-Ni,
such as graphite, graphene oxide and RGO were tested as
catalysts in the coupling of phenylboronic acid, however,
as individuals they were inactive, and no coupling or any
other products were observed (entries 2–3, Table 4).
Added to that, separately we performed the coupling
reaction with reduced graphene oxide (RGO) and with
homogeneous nickel salt NiCl2.6H2O under the identical
optimised condition, which yields no or poor conversion
even the reaction time extended up to 24 hr (entries 4–5,
Table 4). Similar results again repeated while using the
nickel nanoparticles as a catalyst (entry 6, Table 4). The
results from the above control experiments collectively
indicate that the metal and the carbon support as an indi-
vidual does not possess any catalytic effect on the C-C
coupling of arylboronic acid. The superior catalytic prop-
erties of the nickel and reduced graphene oxide has
unleashed when it exists together as an RGO-Ni
nanocomposite due to the synergy between RGO and
nickel nanoparticles. Also, the high surface of the RGO
and presence of aromatic graphitic basal plane facilitate
the binding of the aryl moiety of the boronic acid through
Table 3 RGO-Ni nanocomposite catalysed Glaser-
Hay coupling of terminal alkynesa
aReaction conditions: RGO-Ni nanocomposite cata-
lyst (50 mg), alkynes (0.5 mmol), K3PO4 (1.25 mmol)
ꢀ
1,4-dioxane (2.0 ml) at 85 C up to 3 hr. All are isolated
yields.
bReaction time was extended to 10 hr.
cTON,
dTOF (h−1
)
Phenylacetylene having electron-donating substitu-
ents at a para position such as methyl, ethyl, tert-butyl,
methoxy and propyloxy groups were facilitates the cou-
pling and gave a good yield of 1,3-alkynes within the

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MURUGAN ET AL.
TABLE 3 RGO-Ni nanocomposite catalysed Glaser–Hay coupling of terminal alkynes
TABLE 4 Control experiments for the catalytic efficiency of RGO-Ni in the homocoupling of phenylboronic acida
Entry
Catalyst
RGO-Ni
Yield b (%)
1
2
3
4
5
6
99
GO
-
Graphite
RGO
-
tracec
38,38c
75d
NiCl2.6H2O
Nickel nanoparticle
aReaction condition: Phenylboronic acid (0.5 mmol), K3PO4 (1.25 mmol) and 50 mg of catalyst in 2 ml of 1,4-dioxane at 85 ꢀC up to 3 hr.
bIsolated yield.
cReaction carried out up to 24 hr.
d9.5 mg of catalyst.

MURUGAN ET AL.
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π-π interaction which delivers the high mass transfer
towards the active nickel sites.
freshly prepared RGO-Ni. The peak position and shape
was not altered in the PXRD pattern of the fifth time
reused catalyst (Figure 4a-b), indicating that the crystal-
linity and oxidation state of the catalyst has maintained
without any oxidised impurities even after five
repeated uses.
The reusability studies investigate the sustainable
nature of the catalyst and greenness of the catalytic pro-
cess. In the present work, the reusability of the catalyst
was assessed from the stability of catalytically active
nickel sites on RGO after completion of the reaction. For
this analysis, RGO-Ni used in the homocoupling of phe-
nylboronic acid was recovered and reused under the
optimised conditions (Figure 3b). Reusability results have
shown that the catalyst is highly stable and withstand for
several reuses. Similarly, to ascertain the stability of the
catalyst after successive reuses, it was recovered by sim-
ple magnetic separation (Figure 3a) followed by washing
and drying and subjected to PXRD and SEM analyses, to
study the structural and morphological changes of the
RGO-Ni nanocomposite before and after the catalytic
reuses.
Furthermore, the morphology of the reused catalyst
was demonstrated using SEM analysis. Figure 4c & 4d
shows the SEM images of fresh and fifth time reused cat-
alysts. From the images, it understands that the morphol-
ogy of both RGO and the immobilized nickel
nanoparticles are highly crystalline with a negligible
deformation in the sheet-like morphology of RGO. It rep-
resents the highly sustainable nature of the catalyst.
Further, the heterogeneity of the catalyst was proved
by the hot filtration test. It is crucial for the carbon-based
heterogeneous nanocomposite-based catalyst. Because
during the course of the reaction some of the
immobilized nanoparticles may be leached into the sol-
vent owing to the influence of any reaction parameters
which promotes the reaction and behaves like
The observed results of the reused catalyst shown that
there is no remarkable change in the PXRD pattern of
the reused catalyst, which matches the pattern of the
FIGURE 3 a) Magnetic separation of the RGO-Ni catalyst; (b) Reusability profile of RGO-Ni in homocoupling of phenylboronic acid.
Reaction conditions: RGO-Ni catalyst (50 mg), phenylboronic acid (0.5 mmol), K3PO4 (1.25 mmol) 1,4-dioxane (2.0 mL) at 85 ꢀC up to 3 hr.
All are isolated yields
FIGURE 4 (a) Powder-XRD pattern of fresh, (b) fifth reused RGO-Ni, (c) SEM image of fresh, (d) fifth reused RGO-Ni

8 of 10
MURUGAN ET AL.
homogeneous system. Hence, a control experiment was
carried out under the optimized conditions, in order to
verify the true heterogeneity of the RGO-Ni. The control
reaction was carried with phenylboronic acid under the
optimized condition, and then the catalyst was removed
entirely from the reaction mixture by external magnate
after 1 hr at 85 C and further the mother liquor alone
was allowed to stir to for the optimized reaction time at
the same temperature (Figure 5).
Before and after the removal of the catalyst, the prod-
uct conversion was quantified using gas chromatography,
and no further product conversion was observed even
after the removal of the catalyst from the reaction
medium. The hot filtration test strongly evidenced that
there is no leaching of nickel nanoparticles from the
RGO–Ni nanocomposite and it is stable under the devel-
oped reaction condition for the homocoupling of
arylboronic acids which proved the heterogeneous man-
ner of the catalyst. Furthermore, the analysis of the
recovered solvent using inductively coupled plasma
optical emission spectroscopy (ICP-OES) confirms the
absence of any detectable nickel species after the reaction
which also supports the true heterogeneity nature of the
catalyst.
Based on the experimental observations and from
reported literature,[8,27,35] a plausible mechanism has
been proposed for the RGO-Ni nanocomposite catalyzed
homocoupling of the arylboronic acid and for terminal
alkyne which are shown in scheme 1. The presence of a
high surface area exposed active sites Ni0 on the
graphene surface, and synergy between the RGO and
nickel nanoparticle catalytically promotes the homo-
coupling. From the recent findings,[27] the possibility of
generating reversible Ni0/Ni2+ entities in RGO-Ni could
significantly support the following proposed mechanism.
Symmetrical biaryls and 1,3-diynes[8,35] were synthesized
via nano Ni-catalyzed homocoupling from nucleophilic
aryl boronic acids and terminal alkynes respectively, in
the presence of an oxidant through the oxidative
coupling.
ꢀ
The proposed mechanism involves the oxidative addi-
tion II in the presence of a base K3PO4 and oxygen from
the air. It leads to the formation of intermediate III,
which subsequently undergoes the reductive elimination,
gives the corresponding coupled product. At the end of
the catalytic cycle, the catalyst gets regenerated and
enters into another new catalytic cycle, which may be
owing to the presence of the π-electrons cloud on the sur-
face of RGO.
The comparison of the catalytic propensity of RGO-
Ni towards the recently reported coupling reactions
(Table 5) emphatically proves its better reactivity and
good sustainable as well as economical nature in the
synthetically important and industrially essential cou-
pling reactions. Remarkably, its unique catalytic behav-
ior towards both homocoupling of aryl boronic acids
and Glaser-Hay coupling augments its demand as a
versatile heterogeneous catalyst among the coupling
reagents.
FIGURE 5 Hot filtration test for RGO-Ni catalyzed
homocoupling of phenylboronic acid
SCHEME 1 Plausible mechanism
for RGO-Ni catalysed homocoupling of
arylboronic acid/aryl alkynes

MURUGAN ET AL.
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TABLE 5 Comparative table for the catalytic efficiency of RGO-Ni in the coupling reactions
S.
No.
Yield
(%)
Catalyst
Condition
Type of Coupling
Ref
[8a]
1
Ni (OAc)24H2O and
AgOTf
110 ꢀC, DMF, 3 hr
Glaser – Hay Heterocoupling
81
[8b]
[35b]
[40]
2
3
4
[Pd(η3-allyl)Cl (PPh3)] 25 ꢀC, NaOAc, H20: Acetone,
Glaser – Hay Homocoupling
Glaser – Hay Homocoupling
Sonogashira coupling
99
99
93
Cu/Cu2O-NPs@rGO
Ni nano- rGO -
80 ꢀC, Cs2CO3, EtOH, 8 hr
120 ꢀC, CuI, K2CO3, NMP,
4 hr
[42]
[45]
5
6
Ni-RGO 40
100 ꢀC K2CO3, DMF, 4 hr
C-S Coupling
92
91
NiCl2.6H2O-n-Bu4NBr
120 ꢀC, NaOH,
Sonogashira coupling
ethyleneGlycol, 12 hr
7.
RGO - Ni
80 ꢀC, K3PO4, 1,4-Dioxane,
3 hr
Homocoupling of boronic acid & Glaser –
Hay coupling
99
This
work
3 | CONCLUSIONS
ORCID
Suresh Palaniswamy https://orcid.org/0000-0002-2827-
The present work discloses the potential catalytic applica-
8552
2
tion of the as-prepared RGO-Ni nanocomposite in Csp
-
2
Csp Suzuki type homocoupling and Csp–Csp Glaser–Hay
coupling reactions. A simple, efficient and greener meth-
odology has been developed for the synthesis of both
symmetrical biphenyls and 1,3-diynes using the heteroge-
neous nickel-based catalytic system with good to excel-
lent yields. This multicatalytic behaviour of the RGO-Ni
nanocomposite has more advantageous when compared
to the existing commercial catalyst due to the utilization
of a non-noble and readily available nickel metal without
any ligand or additives under optimum temperature with
the minimum nickel loading. Also, the RGO-Ni
nanocomposite is highly stable and could be reused for
five times without any significant loss in catalytic activity.
The developed catalytic methodology offers to develop a
commercially viable heterogeneous nanocomposite based
sustainable catalytic system for synthesis of industrially
demanding diverse biaryl and 1,3-diynes compounds.
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ACKNOWLEDGMENTS
We gratefully acknowledge the financial support from
the University Grants Commission, New Delhi, India
(UGC grant No.: 42-291/2013(SR)) Also, we acknowledge
Science and Engineering Research Board SERB), New
Delhi, (Grant No. SR/FT/CS-53/2011) and Council for
Scientific and Industrial Research, New Delhi, India
(CSIR grant No.: 02(0191)14/EMR-II). We thank Univer-
sity Grants Commission under University with Potential
for Excellence program (UGC-UPE), DST-FIST (for LC–
MS) and DST-PURSE (for FT-IR, SEM, and EDX) for the
instrumental facility.

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SUPPORTING INFORMATION
Additional supporting information may be found online
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How to cite this article: Murugan K,
Nainamalai D, Kanagaraj P, Nagappan SG,
Palaniswamy S. Green-Synthesized Nickel
Nanoparticles on Reduced Graphene Oxide as an
Active and Selective Catalyst for Suzuki and
Glaser-Hay Coupling Reactions. Appl Organomet
Chem. 2020;e5778. https://doi.org/10.1002/aoc.
5778