Green synthesis of graphene oxide (GO)-anchored Pd/Cu bimetallic nanoparticles using: Ocimum sanctum as bio-reductant: An efficient heterogeneous catalyst for the Sonogashira cross-coupling reaction

Article abstract of DOI:10.1039/d0ra01189d

To explore the synergism between two metal centers we have synthesized graphene oxide (GO) supported Pd/Cu?GO, Pd?GO and Cu?GO nanoparticles through bio-reduction of Pd(NO₃)₂ and CuSO₄·5H₂O using Tulsi (Ocimum sanctum) leaf extract as the reducing and stabilizing agent. The graphene oxide (GO) was obtained by oxidation of graphite following a simplified Hummer's method. The as-prepared nanomaterials have been extensively characterized by FTIR, powder X-ray diffraction (PXRD), HRTEM, TEM-EDS, XPS, ICP-AES and BET surface area measurement techniques. The morphological study of Pd/Cu?GO revealed that crystalline bimetallic alloy type particles were dispersed on the GO layer. The activity of Pd?GO, Cu?GO and Pd/Cu?GO as catalysts for the Sonogashira cross-coupling reaction have been investigated and it was found that the Pd/Cu?GO nanostructure showed highly superior catalytic activity over its monometallic counterparts, substantiating the cooperative influence of the two metals. The inter-atom Pd/Cu transmetalation between surfaces was thought to be responsible for its synergistic activity. The catalyst showed higher selectivity towards coupling of aryl iodides with both aliphatic and aryl alkynes resulting in moderate to excellent isolated yield of the desired products (45-99%). The products have been characterized by GC-MS and ¹H-NMR spectroscopic techniques and compared with authentic samples. The Pd/Cu?GO catalyst could be easily isolated from the reaction products and reused for up to at least ten successive runs effectively.

Full text of DOI:10.1039/d0ra01189d

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Green synthesis of graphene oxide (GO)-anchored
Pd/Cu bimetallic nanoparticles using Ocimum
sanctum as bio-reductant: an efficient
heterogeneous catalyst for the Sonogashira cross-
coupling reaction†
Cite this: RSC Adv., 2020, 10, 23108
b
Samim Sultana,a Swapna Devi Mech,a Farhaz Liaquat Hussain,b Pallab Pahari,
a
a
*
*
Geetika Borah
and Pradip K. Gogoi
To explore the synergism between two metal centers we have synthesized graphene oxide (GO) supported
Pd/Cu@GO, Pd@GO and Cu@GO nanoparticles through bio-reduction of Pd(NO3)2 and CuSO4$5H2O
using Tulsi (Ocimum sanctum) leaf extract as the reducing and stabilizing agent. The graphene oxide
(GO) was obtained by oxidation of graphite following a simplified Hummer's method. The as-prepared
nanomaterials have been extensively characterized by FTIR, powder X-ray diffraction (PXRD), HRTEM,
TEM-EDS, XPS, ICP-AES and BET surface area measurement techniques. The morphological study of Pd/
Cu@GO revealed that crystalline bimetallic alloy type particles were dispersed on the GO layer. The
activity of Pd@GO, Cu@GO and Pd/Cu@GO as catalysts for the Sonogashira cross-coupling reaction
have been investigated and it was found that the Pd/Cu@GO nanostructure showed highly superior
catalytic activity over its monometallic counterparts, substantiating the cooperative influence of the two
metals. The inter-atom Pd/Cu transmetalation between surfaces was thought to be responsible for its
synergistic activity. The catalyst showed higher selectivity towards coupling of aryl iodides with both
aliphatic and aryl alkynes resulting in moderate to excellent isolated yield of the desired products
Received 7th February 2020
Accepted 11th June 2020
1
(45–99%). The products have been characterized by GC-MS and H-NMR spectroscopic techniques and
DOI: 10.1039/d0ra01189d
compared with authentic samples. The Pd/Cu@GO catalyst could be easily isolated from the reaction
rsc.li/rsc-advances
products and reused for up to at least ten successive runs effectively.
toxic phosphine or other ligands and a stoichiometric amount
of base under inert conditions, which make it economically and
1. Introduction
environmentally malignant. Additionally, the use of copper as
co-catalyst exhibits several drawbacks such as generation of
Hay/Glaser type homo-coupling products of terminal alkynes
along with the desired coupling product, thereby decreasing the
efficiency of its formation.2–6 On the other hand, a few
successful examples of copper-free Sonogashira coupling
require expensive aminophosphine ligands.7 A large number of
reports on the Sonogashira reaction used homogeneous palla-
dium catalysts, while comparatively advantageous heteroge-
neous palladium catalysts have received less attention. The
difficulties with the homogeneous catalysis are the isolation of
the catalyst from the products and incapability of its application
in the consecutive runs. Hence the necessity of heterogeneous
approaches is highly preferred from the perspective of easy
handling, easy catalyst separation and reusability.4,5,8–11
The palladium catalyzed Sonogashira cross-coupling reaction is
one of the most powerful and reliable routes in modern
synthetic organic chemistry for the formation of C sp2–C sp
bonds between aryl halides and terminal alkynes.1 Aryl alkynes
are versatile intermediates in the synthesis of natural products,
pharmaceuticals, and agro-chemicals and in materials science.
The traditional methods of Sonogashira cross-coupling reaction
use an organic solvent such as an amine, benzene, acetonitrile,
THF, DMSO or DMF with a palladium catalyst and a copper co-
catalyst (enhancing the reaction efficiency) in the presence of
aDepartment of Chemistry, Dibrugarh University, Dibrugarh-786004, Assam, India.
E-mail:
samimsultana786@gmail.com;
swapnadevimech@gmail.com;
hussain786@gmail.com;
ppahari@gmail.com; geetikachem@yahoo.co.in;
pradip54@gmail.com
It is well-known that catalysis takes place on metal surface
and nanoparticles (NPs) are much more reactive due to their
small sizes and large surface areas than the particulate metal
counterpart. As a result, heterogeneous catalysts in the form of
bApplied Organic Chemistry Group, Chemical Science and Technology Division, CSIR-
North East Institute of Science and Technology, Jorhat-785006, Assam, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra01189d
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nanoparticles have gained tremendous importance in the elds as Pd–Ni on ZIF-8,22 Pd–Cu nanoparticles on magnesium oxide,23
like catalysis. In recent years, graphene and its derivatives have hollow Pd–Fe nanospheric catalyst,24 Cu–Pd on poly-4-vinyl-
attracted the attention of researchers all over the world due to pyridine,25 gold–copper ferrite nanoparticles on silica micropar-
their unique two-dimensional structure, huge surface area, high ticles,26 Pd–Cu on graphene quantum dots modied Fe3O4
dispersibility and other excellent properties.12 Because of their nanoparticles,27 hollow Pd–Co nanospheres,6 Pd–Co nano-
inertness, large surface area, availability and stability, the particles on graphene,28,29 Pd–Cu nanoparticles on macroporous
chemically derived graphene such as graphene oxide (GO) and ion-exchange resin,7 Pd–Cu nano alloys on montmorillonite,30
reduced graphene oxides (rGO) serve as ideal candidates as Pd–Co nanoparticles on polypropylenimine graed graphene,31
support in catalysis.13 Several studies on catalysis using GO as Pd–Au on carbon,32 CuPd nanoparticles on reduced graphene
support have been reported so far, viz. epoxidation of styrene,14 oxide,2 etc. However, most of the above mentioned protocols
aqueous phase Suzuki cross-coupling reaction,12,15 Tsuji–Trost suffered from serious disadvantages, viz. tedious experimental
allylation,16 hydrogenation of p-nitro phenol and hydrogen procedure, long reaction times, use of expensive, harsh and toxic
generation,17 Sonogashira cross-coupling reaction2 etc.
chemicals (e.g. hydrazine hydrate, sodium borohydride, dime-
The bimetallic alloy nanoparticles comprising a noble metal thylformamide, ethylene glycol, surfactants, organic ligands etc.)
with an additional cheaper (rst-row transition) metal such as and so on. Thus, from the standpoint of the reduction of envi-
iron, cobalt, nickel and copper is an effective and low cost suit- ronmental burdens and cost effectiveness, environmentally
able alternative showing synergistic catalytic effects between the benign methods of nanoparticles synthesis are highly
two distinct metals thereby decreasing the precious metal appealing.19,33–35 Biosynthesis of metal nanoparticles using plant
loading.2,18 Despite a number of reports on bimetallic nano- materials can be used without maintaining cell culture and can
particles, palladium (with high surface-to-volume ratio, highly suitably be scaled to large-scale nanoparticle synthesis, that's why
active surfaces, unique size, shape-dependent optoelectronic and receiving apex priority over other biological methods.36,37 Various
anti-cancer properties)19 and copper (super strong materials, with plant extracts, such as Tulsi (Ocimum sanctum),33,38 tea,37 coffee,39
antibacterial, antifungal, sensing capacity, interaction with other C. Camphora,40 Magnolia,36 soyabean41 etc. have been used for
nanoparticles due to the high surface area–volume ratio)20,21 biosynthesis of Pd and Cu nanoparticles. Tulsi (Ocimum sanctum)
bimetallic nanoparticles have received signicant attention in is a traditional medicinal plant, readily available in various parts
the eld of catalysis. There are several methods for the synthesis of India. Tulsi leaves mainly contain phenolic acids, alkaloids,
of metal nanoparticles using toxic and expensive chemicals. In avonoids, polyphenols, terpenoids etc. having excellent
the recent past, Sonogashira cross-coupling reaction with various reducing and stabilizing properties are responsible for the
heterogeneous bimetallic nanoparticles have been reported, such formation and stabilization of metal nanoparticles,
Scheme 1 Plausible reaction scheme for the components of plant extract for bio reduction of metal ions to nanoparticles.
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respectively.38,42 The tentative reaction scheme for the synthesis
of Pd/Cu@GO nanocomposite through bio-reduction process by
the components of Tulsi plant extract has been shown in
Scheme 1.43
As per our knowledge, there is a single report of green
synthesis of Pd@Fe3O4 nanoparticles and its application in
Sonogashira cross-coupling reaction44 however, it was associ-
ated with low fault, such as, long reaction time (24 h), use of
high reaction temperature, toxic solvent, inert reaction medium
etc. Contrastingly, there is no report available till date for the
synthesis of bio-reduced graphene oxide supported bimetallic
Pd/Cu nanocomposite and investigation of its heterogeneous
catalytic potentiality in Sonogashira cross-coupling reaction in
ethanol. From this point of view and in continuation of our
previous works as well,33,45–49 here we report the synthesis and
characterization of ligand-free, Pd–Cu bimetallic nanoparticles
supported on graphene oxide by bio reduction method and its
application in Sonogashira cross-coupling reaction.
2. Experimental
2.1 General information
Fig. 2 XRD patterns of (a) GO and (b) Pd/Cu@GO.
The chemicals used for the synthesis are of analytical reagent
grade. All the chemicals were obtained commercially and were
used as received without further drying or purication. Graphite
was obtained from Otto, Biochemika; H2SO4, KMnO4 were
received from Fine Chemical Ltd. (Finer). Phenyl acetylene, 1-
hexyne, 4-ethynyltoluene, 3-iodotoluene, 4-iodotoluene, bro-
mobenzene, 4-bromotoluene, 4-bromoanisole, 4-bromoaniline
and 1-iodo-3-nitrobenzene were purchased from Sigma-Aldrich.
4-ethynylanisole, 3-ethynylaniline, iodobenzene, CuSO4, NaOH,
K2CO3, NaNO3, Pd(NO3)2, NaHCO3, Et3N and H2O2 were
procured from different rms, such as TCI (Tokyo Chemical
Industry), Merck Specialities, Lobachemic, Research-Lab Fine
Chem. Industries, Spectrochem, Rankem etc.
ray diffraction (PXRD) patterns were recorded on a Rigaku
˚
Ultima IV diffractometer with Cu-Ka (l ¼ 1.541 A radiation). TEM
micrographs were obtained from SAIF, NEHU, Shillong using JEM-
100 CX II and CSIR-NEIST, Jorhat using JEM-2100 Plus Electron
Microscope. X-ray photoelectron spectrum (XPS) was performed at
CSIR-NEIST, Jorhat using ESCALAB Xi+. Inductively Coupled
Plasma Atomic Emission Spectrometric (ICP-AES) analyses were
obtained from SAIF, IIT Bombay on ARCOS, Simultaneous ICP
Spectrometer. The Brunauer–Emmett–Teller (BET) surface area
measurements at liquid nitrogen temperature were recorded at
BIT, Bangalore on Quantachrome Novae 2200. GC-MS of the
products were performed on Agilent Technologies GC system 7820
FT-IR spectra were recorded in KBr pellets in a Shimadzu IR
prestige-21 FT-IR spectrophotometer (400–4000 cmꢀ1). Powder X-
Fig. 1 FTIR spectra of (a) GO and (b) Pd/Cu@GO.
23110 | RSC Adv., 2020, 10, 23108–23120
Fig. 3 XRD patterns of (a) Pd@GO and (b) Cu@GO.
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1
coupled with a mass detector 5975 and SHRXI-5MS column. H (95%, 120 mL) in a 500 mL round bottom ask and stirred in an
NMR spectra of the products were recorded in CDCl3 using TMS as ice bath (0–5 ꢁC) for 30 min. KMnO4 (15 g) was added very slowly
the internal standard using Bruker Ascend 500 MHz spectrometer. to the suspension in a manner that the reaction temperature
ꢁ
was maintained below 20 C. The ice bath was then removed
and the mixture was stirred for overnight until its color turns to
2.2 Methods
hasty brown. To the mixture, 150 mL of water was added slowly
2.2.1 Synthesis of graphene oxide (GO) from graphite
(simplied Hummer's method). Graphene oxide (GO) was
synthesized from oxidation of graphite.50,51 In this method,
graphite powder (5 g) and NaNO3 (2.5 g) were mixed with H2SO4
ꢁ
and the reaction temperature was maintained at 98 C for one
day. H2O2 (30%, 50 mL) was added slowly to the resulting
mixture, whereupon color of the solution turns to bright yellow.
Fig. 4 TEM images of (a) GO and (b and c) Pd/Cu@GO with SAED patterns inset and (d) TEM-EDS (e) Pd@GO with SAED (f) Cu@GO with SAED.
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2.2.2 Synthesis of Pd/Cu nanoparticles. A mixture of
aqueous solution of 0.05 N Pd(NO3)2 (0.23 g Pd(NO3)2 in 40 mL
water) and 0.05 N CuSO4$5H2O (0.249 g CuSO4$5H2O in 40 mL
water) was taken in a round bottomed ask and stirred for
30 min. Aer that, 5 g of Tulsi (Ocimum sanctum) leaves were
collected, washed thoroughly with distilled water, dried using
blotting paper, crushed in a mortar, boiled in 50 mL water for
5 min, centrifuged and the ltrate was collected as the leaf
extract. 6 mL of as-prepared leaf extract was added to the
solution containing Pd(NO3)2 and CuSO4$5H2O, allowed to
stand for 5 min followed by stirring at room temperature for one
day. The resulting mixture was centrifuged and the nano-
particles were collected.
2.2.3 Synthesis of graphene oxide anchored Pd/Cu nano-
composite (Pd/Cu@GO). To the suspension of Pd/Cu nano-
particles in water, 4 g of graphene oxide was added and the
mixture was stirred for 6 days. Aer 6 days, the mixture was
centrifuged, washed with water and acetone successively for
several times and dried in air at room temperature. The product
obtained was characterized and its application was studied.33
Fig. 5 Elemental dot mapping of Pd/Cu@GO nanocomposite.
It was puried by washing through centrifugation with 5% HCl
followed by deionized water for several times. It was then dried
in vacuum oven at room temperature and graphene oxide was
obtained as powder.
Fig. 6 (a) Deconvoluted high-resolution XPS spectrum of Pd 3d core level (b) deconvoluted high-resolution XPS spectrum of Cu 2p core level
and (c) deconvoluted high-resolution XPS spectrum of O1s core level for Pd/Cu@GO catalyst.
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2.2.4 General procedure for Sonogashira cross-coupling
reaction using Pd/Cu@GO catalyst. A mixture of aryl halide
(1 mmol), alkyne (1 mmol), Et3N (10 mL), ethanol (10 mL) and
5 mg of the catalyst (0.0001176 wt% Pd and 0.0002515 wt% Cu)
was taken in a 25 mL round bottomed ask and was vigorously
RSC Advances
stirred at 75 ꢁC for the required time. The progress of the
reaction was monitored by TLC (Thin Layer Chromatography)
using hexane as the eluent at different intervals of time. As the
reaction completes (monitored by GC-MS), the catalyst was
separated from the reaction mixture by simple ltration. The
ltrate was treated with water–ethyl acetate mixture (water-
: ethyl acetate ¼ 1 : 1), followed by washing with brine and
dehydrated with anhydrous Na2SO4. To get the desired product,
the residue was puried by silica gel chromatography using
hexane as the eluent. While recycling the catalyst, aer each
cycle it was isolated from the reaction mixture by simple
centrifugation, washed with water and ethyl acetate for several
Table 1 BET surface area measurements of GO and Pd/Cu@GO
nanocomposite
Surface area Pore volume Pore diameter
Serial no. Materials
(m2 gꢀ1
)
(cc gꢀ1
)
(nm)
ꢁ
times and dried in an oven at 110 C for overnight. The recov-
ered catalyst was then subjected for next runs under identical
reaction conditions.
1
2
GO
154
374
0.362
0.507
3.54
1.49
Pd/Cu@GO
nanocomposite
Table 2 Optimization of reaction conditions for Sonogashira cross-coupling reaction of iodobenzene and phenyl acetylenea
Entry
Catalyst (amount in mg)
Base
Solvent
Time (h)
GC-MS (% conversion)
Isolated yield (%)
1
2
3
4
5
6
7
8
—
5
5
10
10
10
10
10
10
5
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
C2H5OH
C2H5OH
H2O
C2H5OH
H2O
C2H5OH
H2O
C2H5OH
H2O
24
24
24
24
24
24
24
24
24
24
5
—
—
—
—
—
—
—
82
80
—
100
100
100
—
—
—
—
—
b
—
b
—
76c
9
71c
c
10
11
12
13
H2O
—
3
5
10
Et3N
Et3N
Et3N
C2H5OH
C2H5OH
C2H5OH
97c
99c
99c
5
5
a
Reaction conditions: iodobenzene (1 mmol), phenyl acetylene (1 mmol), base (1 mmol/10 mL), solvent (10 mL), catalyst (Pd/Cu@GO,
0.0001176 wt% Pd and 0.0002515 wt% Cu), r.t. ¼ 29 ꢁC in air unless otherwise noted. b At 65 ꢁC. c At 75 ꢁC.
Table 3 Base optimization for Sonogashira cross-coupling reaction of iodobenzene and phenyl acetylenea
Entry
Base
GC-MS (% conversion)
Isolated yield (%)
1
2
3
4
5
6
Et3N
NaOH
KOH
NaHCO3
Na2CO3
K2CO3
100
45
43
—
—
71
99
37
28
—
—
63
a
Reaction conditions: iodobenzene (1 mmol), phenyl acetylene (1 mmol), base (1 mmol/10 mL), solvent (ethanol, 10 mL), catalyst (Pd/Cu@GO,
0.0001176 wt% Pd and 0.0002515 wt% Cu), temperature ¼ 75 ꢁC.
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shiing. These observations are indicative of the presence of
some type of interactions between Pd and Cu anchored on GO,
rather than a physical mixture of Pd and Cu deposited on GO.
3.1.3 ICP-AES analysis. The ICP-AES analysis of the catalyst
provided that our catalyst Pd/Cu@GO contains 0.0001176 wt%
of Pd and 0.0002515 wt% of Cu.
3. Results and discussion
3.1 Characterization
3.1.1 FT-IR study. The FTIR spectrum of GO (Fig. 1a) shows
signicant peaks at 3128 and 2812 cmꢀ1 corresponding to
aromatic C–H and carboxylic acid O–H stretching vibrations,
respectively. The peaks at 1733 and 1611 cmꢀ1 could be attrib-
uted to anti-symmetric and symmetric stretching vibrations of
carboxyl C]O respectively, while band at 1403 cmꢀ1 could be
3.1.4 HR-TEM analysis. The morphology and metal
dispersion degree of GO and Pd/Cu@GO have been investigated
with HR-TEM analysis (Fig. 4). The SAED patterns inset repre-
sent their crystallinity. The sheet like morphology of GO and the
effective anchoring of Pd and Cu nanoparticles onto the surface
of GO sheets are evident from their representative TEM micro-
graphs (Fig. 4a and b). In the cross-sectional view of HRTEM
image of Pd/Cu@GO (Fig. 4c), the crossing of interplanar
fringes clearly indicates the presence of Pd and Cu as mixture
rather than its monometallic counterparts. While comparing
TEM images of Pd@GO and Cu@GO (Fig. 4e and f) with the
catalyst (Fig. 4b) morphological changes were distinctly
observed. Fig. 5 demonstrates the elemental dot mapping
images indicating that Cu and Pd nanoparticles are homoge-
neously distributed throughout the surface of graphene oxide
with Cu in higher weight% than that of Pd as observed from the
TEM-EDS (Fig. 4d).
3.1.5 X-ray photoelectron spectroscopy (XPS) analysis. XPS
measurements were done on Pd/Cu@GO nanocomposite to
investigate the chemical state of Pd and Cu on the surface of the
catalyst (Fig. 6). The core level spectrum for Pd 3d was charac-
terized by a pair of relatively narrow peaks corresponding to the
5/2 and 3/2 spin–orbit components located at 337.68 and
342.96 eV, respectively could be assigned to Pd0.30,59,60 The Cu
2p3/2 and Cu 2p1/2 peaks at 935 eV and 955 eV, respectively were
characteristic of Cu(II).61 Additionally, the presence of Cu(II)
could also be ascertained by the strong shake up peak observed
assigned to C]C stretching vibration. The C–O–C stretching
vibration appears at 1076 cmꢀ1
.
The FTIR spectrum of Pd/
52,53
Cu@GO (Fig. 1b) shows corresponding peaks with a clear
downeld shi of Dn ¼ ꢂ16–91 cmꢀ1, suggesting immobiliza-
tion of Pd/Cu nanoparticles onto the GO sheet by some elec-
trostatic interactions.7
3.1.2 XRD study. The crystalline nature of GO and Pd/
Cu@GO can be conrmed from the powder X-ray diffraction
patterns (Fig. 2). Fig. 2a shows the characteristic diffraction
peaks of GO at 2q ¼ 10.02ꢁ and 42.5ꢁ indicating insertion of
oxygen rich functional groups within the graphite layers.51,52,54
The broad peak between 20ꢁ and 30ꢁ could be assigned to
disorderedly stacked graphene oxide.55 The XRD pattern of Pd/
Cu@GO exhibits characteristic diffractions of Pd (111) and (200)
planes of face-centered cubic lattice at 2q ¼ 40.07ꢁ and 46.7ꢁ
(ref. 2 and 56) which were shied from the reported 2q values at
40.2ꢁ and 42.78ꢁ of Pd (111) and (200) planes, respectively.2,57,58
Likewise, (111) and (112) planes of CuO have also been observed
which shi to 2q ¼ 33.62ꢁ and 42.72ꢁ from the reported value of
35.39ꢁ and 46.25ꢁ.7 Moreover, on comparing the XRD pattern of
Pd/Cu@GO (Fig. 2b) with that of Pd@GO [2q ¼ 40.2ꢁ (111),
42.78ꢁ (200), 67.9ꢁ (220), 82ꢁ (311) and 86.35ꢁ (222)]2,57,58 and
Cu@GO [2q ¼ 42.89ꢁ (112)]20 (Fig. 3), it has been observed that
in case of Pd/Cu@GO (Fig. 2b) the peaks undergoes some
Table 4 Solvent screening for Sonogashira cross-coupling reaction of iodobenzene and phenyl acetylenea
Entry
Solvent
GC-MS (% conversion)
Isolated yield (%)
1
2
3
4
5
6
7
8
C2H5OH
H2O
i-PrOH
CH3OH
DCM
100
—
89
57
—
—
—
50
—
—
—
10
—
99
—
81
32
—
—
—
32
—
—
DMF
DMSO
CH3CN
CHCl3
CCl4
C2H5OH
C2H5OH
C2H5OH
9
10
11
12
13
b
—
7c
—
d
a
Reaction conditions: iodobenzene (1 mmol), phenyl acetylene (1 mmol), base (Et3N, 10 mL), solvent (10 mL), catalyst (Pd/Cu@GO, 0.0001176 wt%
Pd and 0.0002515 wt% Cu), otherwise noted. GO. c Nano Pd@GO. d Nano Cu@GO, temperature ¼ 75 ꢁC.
b
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Table 5 Sonogashira cross-coupling reaction of various aryl halides and alkynes catalyzed by Pd/Cu@GOa
GC-MS (%
conversion)
Isolated yield
(%)
Entry
1
R1
H
R2
X
I
Product
Time (h)
5
C6H5
100
92
99
89
78
99
2
3
4
3-Methyl
4-Methyl
3-Nitro
C6H5
C6H5
C6H5
I
I
I
5
5
3
83
100
5
H
C4H9
Br
5
100
99
6
7
8
4-Methyl
4-Methoxy
4-Amino
C4H9
C4H9
C4H9
Br
Br
Br
5
5
5
88
60
96
84
53
93
9
3-Nitro
H
C4H9
I
I
5
5
48
83
45
79
10
4-MeC6H4
11
12
3-Methyl
4-Methyl
4-MeC6H4
4-MeC6H4
I
I
5
5
81
75
76
72
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Table 5 (Contd.)
GC-MS (%
conversion)
Isolated yield
(%)
Entry
13
R1
H
R2
X
I
Product
Time (h)
5
4-MeOC6H4
76
71
14
15
16
3-Methyl
4-Methyl
3-Nitro
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
I
I
I
5
5
5
63
72
97
57
68
91
17
H
3-NH2C6H4
I
5
63
57
18
19
3-Methyl
4-Methyl
3-NH2C6H4
3-NH2C6H4
I
I
5
5
61
60
54
53
20
3-Nitro
3-NH2C6H4
I
5
73
65
a
Reaction conditions: aryl halide (1 mmol), alkyne (1 mmol), base (Et3N, 10 mL), solvent (ethanol, 10 mL), catalyst (Pd/Cu@GO, 0.0001176 wt% Pd
and 0.0002515 wt% Cu), temperature ¼ 75 ꢁC.
at 943.7 eV.61,62 A clear shiing of Cu 2p3/2 peak to higher candidate for adsorption of the reactant molecules over its
binding energy (935 eV) in the as-synthesized catalyst might be surface indicating enhanced catalytic activity.63
due to the formation of Cu–O–C type surface structure.62 The
O1s spectrum showed a peak at 532.6 eV with a shi of ꢂ2 eV
towards higher binding energy. This component might be due
4. Sonogashira cross-coupling
reaction catalyzed by Pd/Cu@GO
nanocomposite
to the presence of metal–oxygen bond and may be attributed to
small particle with a higher number of defect sites and low
oxygen coordination.
3.1.6 Surface area measurement. The BET surface area,
In order to obtain the best reaction condition, we have chosen
pore volume and pore diameter of GO and Pd/Cu@GO nano-
iodobenzene and phenyl acetylene as the model substrates and
composite are shown in Table 1. It has been observed that upon
have conducted a series of reactions under different conditions
immobilization of Pd and Cu nanoparticles onto the surface of
as presented in Table 2. First of all, in absence of any catalyst,
GO its surface area and pore volume increased, making it a good
iodobenzene (1 mmol), phenyl acetylene (1 mmol), K2CO3 (base,
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Scheme 2 Plausible mechanism for Sonogashira cross coupling reaction.
Table 6 Comparison of catalytic activity of our catalyst (Pd/Cu@GO) with previously reported Pd–Cu based catalysts for Sonogashira cross-
coupling reaction
Reaction
Temperature
time (h)
Isolated
Entry
1
Catalyst
Substrates
reaction condition
(reusability)
Solvent
Ethanol
yield (%)
Reference
Pd/Cu@GO
Aryl iodides and bromides
with aliphatic and aromatic
terminal alkynes
Aryl iodides and bromides
with phenyl acetylene
Aryl iodides and bromides
with aliphatic and aromatic
terminal alkynes
Aryl iodides, bromides and
chloride with aliphatic and
aromatic terminal alkynes
Aryl iodides with aliphatic
and aromatic terminal
alkynes
Aryl iodides, bromides and
chloride with aromatic
terminal alkynes
75 ꢁC magnetic
stirring
3–5 (10
cycles)
45–99
Author's
present
work
2
3
rGO–CuPd
120 ꢁC magnetic
stirring
1–5 (5 cycles)
DMF
70–96
77–95
2
Pd/Cu–ARF(II)
80 ꢁC magnetic
stirring N2
3–16 (5
cycles)
CH3CN
7
atmosphere
60–120 ꢁC magnetic
stirring
4
5
6
7
MgO@PdCu
6–24 (11
cycles)
DABCO
and DMF
10–99
57–97
76–99
68–98
23
30
37
67
MMT@Pd/Cu
65 ꢁC magnetic
stirring N2
16
Ethanol
atmosphere
PdCu@GQD@Fe3O4
Pd–CuFe2O4@SiO2
50–110 ꢁ
C
8–48 (10
cycles)
DABCO,
toluene
or DMA
DABCO,
DMA
Aryl iodides and bromides
with aromatic terminal
alkynes
50 ꢁ
C
24 (5 cycles)
ꢁ
1 mmol) and ethanol (solvent, 10 mL) were taken and carried 2–5). Even increase in temperature to 65 C did not afford any
out the reaction for 24 h which did not lead to any product positive results (Table 2, entries 6 & 7). But it delighted us when
ꢁ
formation (Table 2, entry 1). Introduction of the catalyst at room increasing the temperature to 75 C gave 76 and 71% isolated
temperature also showed no product formation (Table 2, entries yield of products (Table 2, entries 8 & 9). This showed that both
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nanostructure showed highly superior catalytic activity over its
monometallic counterparts, substantiating the presence of
some type of cooperative inuence between Pd and Cu.
The scope and limitations of Pd/Cu@GO were studied with
various aryl halides and alkynes under the optimized reaction
condition (Table 5). Our results showed that coupling of phenyl
acetylene with aryl iodides bearing electron donating group
(–CH3) afforded lower yields (Table 5, entries 2 and 3) compared
to that bearing electron withdrawing group (–NO2) (Table 5,
entry 4). This might be due to the increase of electron density
around the aromatic ring making it difficult to remove the iodo-
group from the substrate.2 It is remarkable to note that coupling
of phenyl acetylene with aryl bromides for 8 h duration resulted
no product formation, while coupling of 1-hexyne with aryl
bromides gave noticeable isolated yields (Table 5, entries
5–8).We have examined a series of coupling reactions between
1-hexyne with aryl iodides, and interestingly only nitro deriva-
tive can afford product formation with moderate isolated yield
(Table 5, entry 9). In contrast to that, 4-ethynyltoluene, 4-ethy-
nylanisole and 3-ethynylaniline gave good isolated yields when
coupled with aryl iodides (Table 5, entries 10–20), while with
aryl bromides did not lead to product formation.
Fig. 7 Recycling of the catalyst.
4.1 Probable mechanism
The plausible mechanism for Sonogashira cross-coupling
reaction follows the pathway64,65 as shown in Scheme 2. In the
rst step, oxidative addition of aryl halide with the active Pd
species (I) takes place to generate the species (II). Cu(II) is in situ
reduced to Cu(I) in the presence of reducing solvent (ethanol)
and reducing components of plant extract. Cu(I) activates the
terminal alkyne by generating transient alkynyl copper inter-
mediate which facilitates the transmetalation with the previ-
ously formed species (II) to give (III). The desired coupled
product is obtained by isomerization of (III) followed by
reductive elimination of (IV).
The present catalytic system appears to be more efficient in
the Sonogashira reaction for several reasons: (i) use plant
extract instead of toxic chemicals as the reducing agents, (ii)
does not release any toxic bi-products into the environment, (iii)
cost effective, (iv) use of mild reactions conditions, completely
ligand free, no additional co-catalyst is required, and (v) the
catalyst is recyclable without any apparent leaching.19,66 Table 6
highlighting the improved catalytic performance and reaction
conditions as well, of the present catalytic protocol compared to
that of similar Pd and Cu-based heterogeneously catalytic
systems reported so far. To the best of our knowledge, our
Pd/Cu@GO nanocomposite could provide highly attractive and
environment friendly option for the synthesis of aryl alkynes.
Fig. 8 Hot Filtration Test.
temperature and amount of catalyst played an important role in
this reaction (Table 2, entries 11–13). Hence, 5 mg of the catalyst
ꢁ
at 75 C was found to be the suitable amount to carry out the
reaction effectively.
As base is an inevitable part for Sonogashira cross-coupling
reaction, we have performed the reaction in presence of
a number of bases as shown in Table 3. Our investigations
showed that Et3N acts as the most suitable base for this reaction
(Table 3, entry 1).
Solvent screening with a number of solvents showed ethanol
is the best solvent in this case. The DMF, DMSO and CH3CN are
capable of coordinating as a ligand with the metals and thereby,
the stability of the catalyst might be affected by competing with
the ligands. Moreover, since our catalytic system contains Cu as
Cu2+, therefore, during catalytic reactions Cu2+ must be reduced
to Cu+ by some means (Cu+ is the active species to carry out
Sonogashira reaction). Therefore, ethanol featuring a reducing
property is most effective in our case (Table 4, entry 1). Our
investigations showed that 5 mg of the catalyst (Pd/Cu@GO,
0.0001176 wt% Pd and 0.0002515 wt% Cu) at 75 ꢁC in Et3N
(10 mL) and ethanol (10 mL) is the best reaction condition for
this reaction. It is noteworthy to mention that we have synthe-
sized Pd@GO and Cu@GO following the same synthetic
procedure as described for Pd/Cu@GO nanocomposite and
examined their catalytic efficiency too for the said reaction
under the identical optimized condition (Table 4, entries 12 and
13). These results clearly demonstrated that Pd/Cu@GO
4.2 Recycling of the catalyst
The reusability is the key parameter to evaluate the sustain-
ability and efficiency of a heterogeneous catalyst. That is why we
have carried out the recyclability of Pd/Cu@GO for Sonogashira
cross-coupling reaction between iodobenzene and phenyl acet-
ylene under the optimized reaction condition. Aer completion
of the reaction, it was easily separated from the reaction mixture
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by centrifugal precipitation, washed thoroughly with water and
ethyl acetate and then dried. It was applied for subsequent runs
and continued up to tenth cycle. The slight decrease in product
yield aer the h cycle might be indicative of very gradual
catalyst deterioration. Aer all, the catalyst was found to be
quite active even up to tenth cycle giving 93% isolated yield
(Fig. 7).
In order to establish the heterogeneous nature of the cata-
lyst, we have performed a hot ltration test (Fig. 8) using
iodobenzene and phenyl acetylene as the model substrates
under the optimized reaction condition. Aer 1 h, the reaction
was stopped and the catalyst was ltered off from the reaction
mixture. The percentage of product formation was analyzed
with GCMS (45% conversion). The reaction was continued with
the ltrate obtained (without any solid catalyst) for another 4 h
and found no product formation (studied with GCMS), indi-
cating non-leaching of metals during the catalytic cycle. More-
over, the ICP-AES analysis of the ltrate showed Pd or Cu
content below detection level (i.e. <0.01 ppm) which further
enumerates non-leaching of either Cu or Pd metal during the
reactions, thereby affirming its heterogeneous nature and
stability.
References
1 K. Ramesh and G. Satyanarayana, J. Organomet. Chem., 2019,
890, 58.
2 S. Diyarbakir, H. Can and O. Metin, ACS Appl. Mater.
Interfaces, 2015, 7, 3199.
3 S. K. Das, M. Sarmah and U. Bora, Tetrahedron Lett., 2017, 58,
2094.
4 Z. I. Dehimat, S. Yasar, D. Tebbani and I. Ozdemir, Inorg.
Chim. Acta, 2018, 469, 325.
5 Y. B. Platonova, A. N. Volov and L. G. Tomilova, J. Catal.,
2019, 373, 222.
6 H. Zhong, J. Wang, L. Li and R. Wang, Dalton Trans., 2014,
43, 2098.
7 D. Sengupta, J. Saha, G. De and B. Basu, J. Mater. Chem. A,
2014, 2, 3986.
8 H. Li, Z. Zhu, J. Liu, S. Xie and H. Li, J. Mater. Chem., 2010, 20,
4366.
¨
´
´
9 A. Komaromi, G. L. Tolnai and Z. Novak, Tetrahedron Lett.,
2008, 49, 7294.
10 J. T. Guan, T. Q. Weng, G. A. Yu and S. H. Liu, Tetrahedron
Lett., 2007, 48, 7129.
11 M. J. Mio, L. C. Kopel, J. B. Braun, T. L. Gadzikwa, K. L. Hull,
R. J. Brisbois, C. J. Markworth and P. A. Grieco, Org. Lett.,
2002, 4, 3199.
12 C. Bai, Q. Zhao, Y. Li, G. Zhang, F. Zhang and X. Fan, Catal.
Lett., 2014, 144, 1617.
13 S. Sabater, J. A. Mata and E. Peris, ACS Catal., 2014, 4, 2038.
14 H. Su, Z. Li, Q. Huo, J. Guan and Q. Kan, RSC Adv., 2014, 4,
9990.
15 J. H. Park, F. Raza, S. J. Jeon, H. I. Kim, T. W. Kang, D. B. Yim
and J. H. Kim, Tetrahedron Lett., 2014, 55, 3426.
16 Q. Zhao, Y. Zhu, Z. Sun, Y. Li, G. Zhang, F. Zhang and X. Fan,
J. Mater. Chem. A, 2015, 3, 2609.
17 W. Sun, X. Lu, Y. Tong, Z. Zhang, J. Lei, G. Nie and C. Wang,
Int. J. Hydrogen Energy, 2014, 39, 9080.
18 K. W. Cheah, M. J. Taylor, A. Osatiashtiani, S. K. Beaumont,
D. J. Nowakowski, S. Yusup, A. V. Bridgwater and
G. Kyriakou, Catal. Today, 2019, DOI: 10.1016/
j.cattod.2019.03.017.
5. Conclusion
The present study reports a sustainable, green approach to
integrate Pd with Cu on graphene oxide and exploiting its
catalytic potency for Sonogashira cross-coupling reaction. The
benign reaction conditions, use of bio-reducing agents and its
easy availability, easy separability, excellent retrievability (at
least for ten consecutive runs) attesting its superiority. Only
0.0001176 wt% Pd and 0.0002515 wt% Cu is sufficient to
produce excellent isolated yields is the added advantage of Pd/
Cu@GO catalyst. This bio-reduced, GO anchored Pd/Cu bime-
tallic nanocomposite catalytic protocol has been applied for the
1st time in Sonogashira cross-coupling reactions under ligand
free, eco-friendly conditions. The remarkably high catalytic
activity of Pd/Cu@GO over its monometallic counterparts
(Pd@GO and Cu@GO) clearly demonstrating to the cooperative
inuence between Pd and Cu in the nanostructure.
19 H. Veisi, A. Rashtiani and V. Barjasteh, Appl. Organomet.
Chem., 2016, 30, 231.
20 J. Ramyadevi, K. Jeyasubramanian, A. Marikani,
G. Rajakumar and A. A. Rahuman, Mater. Lett., 2012, 71, 114.
Conflicts of interest
´
˜
´
21 J. S. Cerda, H. E. Gomez, G. A. Nunez, I. A. Rivero, Y. G. Ponce
The authors declare that there is no conict of interest
regarding the publication of this article.
´
and L. Z. F. Lopez, J. Saudi Chem. Soc., 2016, 21, 341.
22 Y. Wang, Y. Zeng, X. Wu, M. Mu and L. Chen, Mater. Lett.,
2018, 220, 321.
´
23 M. Gholinejad, M. Bahrami, C. Najera and B. Pullithadathil,
Acknowledgements
J. Catal., 2018, 363, 81.
The authors thank SAIF, NEHU and STIC, Cochin for TEM,
ACMS, IIT, Kanpur for XPS, SAIF, IIT Bombay for ICP-AES, CSIR-
NEIST, Jorhat for TEM-EDS and BIT, Bangalore for BET surface
area measurement facilities. The authors express their gratitude
to UGC, New Delhi, India for the nancial support under the
scheme SAP-DRS-I (2016–2021) and S. Sultana thanks UGC-
Maulana Azad National Fellowship, Delhi for nancial support.
24 H. Li, Z. Zhu, H. Li, P. Li and X. Zhou, J. Colloid Interface Sci.,
2010, 349, 613.
25 C. Evangelisti, A. Balerna, R. Psaro, G. Fusini, A. Carpita and
M. Benfatto, ChemPhysChem, 2017, 18, 1921.
´
26 M. Gholinejad, J. Ahmadi and C. Najera, ChemistrySelect,
2016, 3, 384.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 23108–23120 | 23119

View Article Online
RSC Advances
Paper
27 M. Gholinejad, J. Ahmadi, C. Najera, M. Seyedhamzeh, 48 R. Gogoi, R. Saikia and G. Borah, J. Organomet. Chem., 2019,
F. Zareh and M. K. Zareh, ChemCatChem, 2017, 9, 1442. 897, 80.
28 Y. S. Feng, X. Y. Lin, J. Hao and H. J. Xu, Tetrahedron, 2014, 49 C. Pathak and G. Borah, Mater. Res. Express, 2019, 6, 1250i1.
70, 5249.
50 F. T. Thema, M. J. Moloto, E. D. Dikio, N. N. Nyangiwe,
L. Kotsedi, M. Maaza and M. Khenfouch, J. Chem., 2013,
150536.
29 M. Dabiri and M. P. Vajargahy, Appl. Organomet. Chem.,
2017, 31, e3594.
30 W. Xu, H. Sun, B. Yu, G. Zhang, W. Zhang and Z. Gao, ACS 51 N. Xiang, J. Huang, H. Zhao, C. Liu and X. Liu, Z. Phys.
Appl. Mater. Interfaces, 2014, 6, 20261. Chem., 2016, 230, 1711.
31 A. Shaabani and M. Mahyari, J. Mater. Chem. A, 2013, 1, 9303. 52 C. Ma, K. Yang, L. Wang and X. Wang, J. Appl. Biomater.
32 C. Rossy, J. Majimel, E. Fouquet, C. Delacote, M. Boujtita, Funct. Mater., 2017, 15, 1.
C. Labrugere, M. T. Delapierre and F. X. Felpin, Chem.–Eur. 53 N. Hussain, P. Gogoi, P. Khare and M. R. Das, RSC Adv., 2015,
J., 2013, 19, 14024. 5, 103105.
33 N. Gogoi, P. Bordoloi, G. Borah and P. K. Gogoi, Catal. Lett., 54 H. Yang, J. Jiang, W. Zhou, L. Lai, L. Xi, Y. M. Lam, Z. Shen,
2017, 147, 539. B. Khezri and T. Yu, Nanoscale Res. Lett., 2011, 6, 531.
34 S. Momeni and I. Nabipour, Appl. Biochem. Biotechnol., 2015, 55 N. Shang, C. Feng, H. Zhang, S. Gao, R. Tang, C. Wang and
176, 1937. Z. Wang, Catal. Commun., 2013, 40, 111.
35 A. Kalaiselvi, S. M. Roopan, G. Madhumitha, C. Ramalingam 56 G. Wu, X. Wang, N. Guan and L. Li, Appl. Catal., B, 2013, 136–
and G. Elango, Spectrochim. Acta, Part A, 2015, 135, 116. 137, 177.
36 H. J. Lee, G. Lee, N. R. Jang, J. H. Yun, J. Y. Song and 57 A. H. Al-Marri, M. Khan, M. R. Shaik, N. Mohri, S. F. Adil,
B. S. Kim, NSTI Nanotech, 2011, 1, 371.
37 S. Lebaschi, M. Hekmati and H. Veisi, J. Colloid Interface Sci.,
2017, 485, 223.
M. Kuniyil, H. Z. Alkhathlan, A. Al-Warthan, W. Tremel,
M. N. Tahir, M. Khan and M. R. H. Siddiqui, Arabian J.
Chem., 2016, 9, 835.
38 V. D. Kulkarni and P. S. Kulkarni, Int. J. Chem. Stud., 2013, 1, 58 W. Sun, X. Lu, Y. Tong, Z. Zhang, J. Lei, G. Nie and C. Wang,
2321. Int. J. Hydrogen Energy, 2014, 39, 9080.
39 M. N. Nadagouda and R. S. Varma, Green Chem., 2008, 10, 59 X. Zhang, D. Wu and D. Cheng, Electrochim. Acta, 2017, 246,
859. 572.
40 J. Huang, Q. Li, D. Sun, Y. Lu, Y. Su, X. Yang, H. Wang, 60 M. K. Mohan, K. R. Sunajadevi, N. K. Daniel, S. Gopi,
Y. Wang, W. Shao, N. He, J. Hong and C. Chen,
Nanotechnology, 2007, 18, 105104.
S. Sugunan and N. C. Perumparakunnel, Bull. Chem. React.
Eng. Catal., 2018, 13, 286.
41 J. I. Abd-Elkareem, H. M. Bassuony, S. M. Mohammed, 61 S. M. Lomnicki, H. Wu, S. N. Osborne, J. M. Pruett,
H. M. Fahmy and N. R. Abd-Elkader, J. Bionanosci., 2016,
10, 15.
R. L. McCarley, E. Poliakoff and B. Dellinger, Mater. Sci.
Eng., B, 2010, 175, 136.
42 P. K. Saikia, R. P. Bhattacharjee, P. P. Sarmah, L. Saikia and 62 T. Tsoncheva, G. Issa, T. Blasco, M. Dimitrov, M. Popova,
D. K. Dutta, RSC Adv., 2016, 6, 110011.
S. Hernandez, D. Kovacheva, G. Atanasova and
J. M. L. Nieto, Appl. Catal., A, 2013, 453, 1.
63 S. Verma, R. B. N. Baig, M. N. Nadagouda and R. S. Varma,
Green Chem., 2016, 18, 1327.
64 M. H. P. Temprano, J. A. Casares and P. Espinet, Chem.–Eur.
J., 2012, 18, 1864.
43 M. Nasrollahzadeh, M. Sajjadi, J. Dadashi and H. Ghafuri,
Adv. Colloid Interface Sci., 2020, 276, 102103.
44 M. Nasrollahzadeh, S. M. Sajadi, A. Rostami-Vartooni and
M. Khalaj, J. Mol. Catal. A: Chem., 2015, 396, 31.
45 S. Sultana, G. Borah and P. K. Gogoi, Appl. Organomet. Chem.,
2019, 33, e4595.
´
65 R. Chinchilla and C. Najera, Chem. Rev., 2007, 107, 874.
46 S. Sultana, G. Borah and P. K. Gogoi, Catal. Lett., 2019, 149, 66 C. Ha, N. Zhu, R. Shang, C. Shi, J. Cui, I. Sohoo, P. Wu and
2142. Y. Cao, Chem. Eng. J., 2016, 288, 246.
47 S. Sultana, S. Bordoloi, S. Konwer, G. Borah and P. K. Gogoi, 67 M. Gholinejad and J. Ahmadi, ChemPlusChem, 2015, 80, 973.
Appl. Organomet. Chem., 2020, e5582.
23120 | RSC Adv., 2020, 10, 23108–23120
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