Green synthesis of predominant (1 1 1) facet CuO nanoparticles: Heterogeneous and recyclable catalyst for N-arylation of indoles

Article abstract of DOI:10.1016/j.molcata.2012.03.017

Well faceted CuO nanoparticles were synthesized by thermal-assisted green strategy at reflux temperature in a short period of time. A possible growth mechanism of such highly faceted nanostructures based on typical biomolecule-crystal interactions in aque

Full text of DOI:10.1016/j.molcata.2012.03.017

Journal of Molecular Catalysis A: Chemical 359 (2012) 28–34
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Green synthesis of predominant (1 1 1) facet CuO nanoparticles: Heterogeneous
and recyclable catalyst for N-arylation of indoles
Nikhil V. Suramwara, Sanjay R. Thakareb,∗, Nandkishor N. Karadec, Niren T. Khatya
a Department of Applied Chemistry, Priyadarshini College of Engineering and Technology, Hingna, Nagpur, Maharashtra 440019, India
b Nanotechnology Laboratory, Department of Chemistry, Science College, Congress Nagar, Nagpur, Maharashtra 440012, India
c Department of Chemistry, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, Maharashtra, 440033, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Well faceted CuO nanoparticles were synthesized by thermal-assisted green strategy at reflux temper-
ature in a short period of time. A possible growth mechanism of such highly faceted nanostructures
based on typical biomolecule-crystal interactions in aqueous solution is tentatively proposed. The large
surface area (223.36 m2/g) and rich exposed active sites are expected to endow such nanostructures
with excellent performances in catalysis as demonstrated here for remarkable catalytic activity with
respect to the N-arylation of indoles. Nanoparticles were characterized by X-ray diffraction, scanning
electron microscopy and transmission electron microscopy. Both the activity and selectivity of the N-
arylation reactions could be tuned by varying the concentration of CuO nanoparticles. Nanoparticles
catalyst were recycled and reused for further catalytic reactions with minimal loss in activity. A variety
of indole derivatives afforded corresponding N-arylation product with excellent yields (up to 98%).
© 2012 Elsevier B.V. All rights reserved.
Received 21 January 2012
Received in revised form 12 March 2012
Accepted 15 March 2012
Available online 28 March 2012
Keywords:
Nanoparticles
CuO
N-Arylation
Starch
Indoles
1. Introduction
environmental benignity [10–12]. How to improve catalytic perfor-
mance is become more important and ranks as one of the hot topic.
Catalyst plays a significant role in the production of chemicals
today and nanomaterials have the potential for improving effi-
ciency, selectivity, and yield of catalytic process. The higher surface
to volume ratio means that much more catalyst is actively partici-
pating in the reaction. The potential for cost saving is tremendous
from a material, equipment, labour, and time standpoint. Higher
selectivity means less waste and fewer impurities, which could
lead to safer and reduced environmental impact. It was reported
that catalytic performance of nanocrystals finely tuned either by
their composition or by their shape which mediates the elec-
tronic structure [1,2]. The composition of nanocrystals was useful
in determination of surface atomic arrangement and coordination
[3–5]. When nanocrystals with different shapes were applied to
electron transfer reaction the average rate constant of reaction
increased exponentially with the percentage of surface atoms at the
corners and edges increases [6,7]. Several groups intensively inves-
tigated the effect of shape of nanocrystals on their catalytic activity
and stability and found that some specific facets are believed to be
most catalytically active [8,9].
A fundamental goal of materials science is to design and synthesize
materials with controllable shape, size and tailored functional-
ity. The preparation of high purity, monodispersed nanocrystalline
CuO using sonochemical [13], microwave irradiation [14], and
precipitation-pyrolysis [15] methods were well reported. Major
drawback of these methods was the formation of multifaceted
CuO nanoparticles. Hence, in this paper, a facile one-step synthetic
route is describe to construct face-centred-cubic (fcc) CuO nanos-
tructures assisted by starch as a stabilizer at reflux temperature
in a short reaction time. In comparison with reported protocols
our method is green, environment-friendly, facile and time-saving.
These particles have spherical morphology having predominant
(1 1 1) facets, which are expecting to be chemically and dynamically
active, therefore catalytically active. To the best of our knowledge,
no studies have investigated the catalytic activity of predominant
(1 1 1) facet nanometre sized CuO in N-arylation of indoles.
2. Experimental
CuO nanoparticles as a catalyst for C–N, C–S, C–O cross-coupling
reactions and C-arylation reaction have been well-documented
because it is safe, cheap, having high theoretical capacity and
2.1. Preparation of predominant (1 1 1) facet spherical shape CuO
nanoparticles
All the reagents used in this synthesis were of analytical grade
and used without further purification. CuO nanoparticles were
prepared by decomposition of copper ammonia complex. The
experimental procedure is briefly described as follows. The blue
∗
Corresponding author. Tel.: +91 9011097694; fax: +91 0712 2440955.
E-mail address: sanjaythakare@yahoo.co.uk (S.R. Thakare).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2012.03.017

N.V. Suramwar et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 28–34
29
Table 1
Optimization of reaction condition for the N-arylation of indole with iodobenzene.
Entry
CuO quantity (mol%)
Base (1 equiv.)
Solvent
Reaction time (h)
Yielda (%)
1
2
3
4
5
6
7
8
9
10
10
10
10
2.5
05
05
00
05
05
05
03
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
–
K2CO3
KOH
DBU
CH2Cl2
CH3CN
DMSO
Toluene
DMF
DMF
DMF
DMF
DMF
24
24
9
24
12
8
24
24
24
10
10
10
31
59
97
76
65
98
13
00
65
81
73
88
10
11
12
DMSO
DMSO
DMSO
Et3N
K2CO3
a
Isolated yield.
solution of copper ammonia complex was stirred with 4 g starch as
a stabilizing agent for 1 h followed by addition of 100 ml of 0.25 M
NaBH4 solution and resultant solution was refluxed for 1 h. Dur-
ing reflux the color of solution changed from blue to colorless and
then it turned slowly to brick red which indicated the formation of
CuO nanoparticles. The nanoparticle separated out by centrifuge at
3000 rpm and washed with water two to three times.
N,N-dimethyl formamide (10 ml) was taken in a round bottom flask
fitted with a reflux condenser. The resulting mixture was refluxed
for the time as mentioned in Table 1. The progress of the reaction
was monitored by TLC. After completion of reaction, mixture cooled
to room temperature and diluted with 10 ml of water and extracted
with ethyl acetate (2× 10 ml). The combined organic extracts were
washed with brine and dried on anhydrous sodium sulphate. Sol-
vent was evaporated under reduced pressure and the crude product
was purified by flash column chromatography on silica gel using
ethyl acetate and petroleum ether (1:9) as eluent to yield analyti-
cally pure product. Aqueous layer was centrifuged to recover CuO
nanocatalyst.
2.2. Sample characterization
The powder X-ray diffraction (XRD) measurements were car-
ried out on a PHILIPS HOLLAND, XRD system using Cu K␣ radiation
˚
(ꢀ = 1.54056 A). The morphology of the CuO nanoparticles was
investigated via scanning electron microscopy (SEM), (model JSM-
5400, JEOL) and transmission electron microscopy (TEM) (Philips
CM200). The BET surface area was determined by specific surface
area analyzer (ASAP 2000, micrometrics) with nitrogen gas as an
adsorbate.
3. Results and discussion
3.1. Structural analysis
The X-ray diffraction analysis of prepared CuO nanoparticles
was carried to identify the product (Fig. 1).
2.3. Catalytic test
All diffraction peaks are index with the corresponding planes of
CuO. The XRD spectra showing an intense peak at 36.4 is having
plane (1 1 1), which is the crystal plane of CuO. The low intensity
peaks at 29.6, 61.7 which match well with the plane (1 1 0), (1 1 3).
Indole (0.351 g, 3 mmol), iodobenzene (0.673 g, 3.3 mmol),
CuO nanoparticles (0.0105 g), K2CO3 (0.414 g, 3 mmol) and
Fig. 1. XRD spectra of CuO nanoparticles.

30
N.V. Suramwar et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 28–34
Fig. 2. SEM images of CuO nanoparticles.
All these planes of CuO with FCC crystal structure are very close
to those in the JCPDS File No.5-0661. No other diffraction peaks
arising from metallic Cu or Cu2O present in the XRD pattern, which
indicate the high phase purity of synthesized sample. The crystal
size of products calculated by Scherrer formula was 40 nm.
The BET surface area of the synthesized CuO nanoparticles was
determined and found to be 223.36 m2/g which is significantly
higher. The higher surface area of CuO nanoparticles plays the major
role in the catalytic activity of CuO nanoparticles.
In order to obtained detail information about the microstruc-
ture and morphology of the synthesized sample, SEM and TEM
observations were carried out and shown in Figs. 2 and 3. SEM
images (Fig. 2) shows the spherical morphology of CuO nanoparti-
cles. Careful observation of these image shows that particle surfaces
are not smooth but some small round structures formed on the
surface.
On the basis of XRD, SEM and TEM analysis we proposed the
mechanism of CuO formation.
3.2. Growth mechanism
According to the Gibbs–Curie–Wulff theorem, the shape of a
crystal is determined by the relative surface free energy of indi-
vidual crystallographic faces. The final crystal shape results from
minimizing the total free energy of the system [16]. For fcc single
crystal, surface energies associated with different crystallographic
planes are usually different, and a general sequence may hold, ␥
(1 1 1) < ␥ (1 1 0) < ␥ (1 1 3). Generally, the selective adsorption of
surfactants, ions, ligands or polymers on a given crystal plane can
inhibit the crystalline growth along one specific direction, leading
to a preferential growth along another direction [17–19]. Therefore,
the introduction of an additive with a selective adsorption func-
tion is widely used for the synthesis of anisotropic nanocrystals
in a solution phase. Based on these, a possible mechanism is pro-
posed (Scheme 1). It is believed that the growth of spherical shape
completed by two steps, i.e. nucleation and growth respectively.
In our synthetic route, starch plays a crucial role in controlling
The formation of small structures on bigger one may increase its
surface area and indirectly the catalytic activity of CuO nanoparti-
cles. TEM images of the CuO nanoparticles are shown in Fig. 3, which
shows the synthesized CuO nanoparticles having the size around
40–50 nm match exactly with the XRD analysis.
Fig. 3. TEM images of CuO nanoparticles.

N.V. Suramwar et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 28–34
31
Scheme 1. Possible growth mechanism for single facet CuO nanoparticles.
the shape of nanocrystals. The functional hydroxyl group ( OH)
of starch molecules, which is a hydrophilic group can provide coor-
dination sites, namely nucleation sites. When Cu2+ ions enter into
aqueous solution, there is electrostatic attraction between the posi-
tively charged Cu2+–ammonia complex and the negatively charged
hydroxyl groups of starch, thus forming complex structure hav-
ing spherical morphology. The introduction of NaBH4 in a solution
leads a rapid redox reaction results in the formation of spherical
Cu crystal seeds, followed by CuO conversion after reflux. It has
been revealed that a slow reaction is favourable for the formation
of anisotropic nanocrystals [20]. Here, the existence of Cu2+–starch
complex inside the cavity will reduce the reduction rate of Cu2+
ions. Under the slow reaction, as reaction progress forward starch
molecules seem to be preferentially adsorbed on the (1 1 0) planes
of the nuclei, and consequently the area ratio of (1 1 1) to (1 1 0)
increased. An increase in ratio facilitates the formation of spheri-
cal shape CuO nanocrystals based on the idea that an increase in
the area ratio of (1 1 1) to (1 1 0) results in the evolution of particle
shapes to a spherical shape [21].
copper sources, bases, ligands, and other additives, several mild
and efficient methods have been reported for the N-arylation of
indoles [34,35]. Ligands based on diamines [36], oxime-phosphine
oxide [37], aminoacids [38], phosphoramidite [39], and proline
have been introduced to promote copper-catalyzed N-arylation of
indoles and other heterocycles [40]. Recently, a transition metal
free N-arylation of indole using strong base with longer reaction
time has been reported [41].
The wide scope of CuO nanoparticles catalyst led us to
investigate transformations involving less reactive nitrogen nucle-
ophiles such as indoles. It is worth to mention that there was
only one example in the literature utilizing commercially avail-
able multi-facet CuO nanoparticles for the N-arylation of indole
with lesser yield (45%) and longer reaction time (23–36 h) [10].
Herein, we describe our progress in the predominant (1 1 1) facet
nano CuO-catalysed N-arylation of indoles which overcome a
number of problems associated with the techniques reported
earlier.
The reaction of indole with iodobenzene was considered as a
model reaction to find out the optimum reaction conditions for
Besides, the failure of producing spherical shape CuO in the
absence of starch further confirmed the special role of starch
in the formation of such unique spherical shape CuO as shown
in Fig. 4.
3.3. Catalytic activity
The formation of C N bond is one of the most important
reactions in numerous syntheses of intermediates and targets
with biological and pharmaceutical impact [22–26]. N-aryl indoles
are of interest as angiotensin, antagonists [27], melatonin recep-
tor, partial agonists [28], antipsychotic agents [29] and synthetic
intermediates used in the preparation of other biologically active
heterocyclic agents [30]. Various strategies were developed for
the N-arylation of indoles. The century old copper-catalyzed Ull-
mann reaction has limitations because of harsh reaction conditions,
requirement of stoichiometric amounts of copper reagents, long
reaction times, and low yields [31,32]. The palladium-catalyzed N-
arylation is an alternative method under mild reaction conditions
[33]. The copper catalyzed Ullmann reactions have seen a grad-
ual expansion in the past few years and by the correct choice of
Fig. 4. TEM images of CuO nanoparticles produced in the absence of starch.

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N.V. Suramwar et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 28–34
Table 2
CuO nanoparticle catalysed N-arylation of indole with iodoarenes.a,b
I
R1
R1
CuO nano (5mol%)
K2CO3 (1 equiv)
N
N
H
DMF/reflux/ 8h
R2
R2
3
1
2
.
N
N
N
N
3d (82%)b
3c (80%)b
3b (95%)
NC
3a (98%)
O2N
O2N
N
N
N
3e (95%)
3g (82%)b
3f (95%)
Br
NC
N
N
N
N
NO2
3i (85%)b
NO2
3k (98%)
N
3j (97%)
N
3h (95%)
H3CO
Br
H3CO
N
N
3n (85%)b
3o (98%)
3m (95%)
3l (97%)
Isolated yields.
a
Reaction conditions: indole (3 mmol), iodoarene (3.3 mmol), K2CO3 (3 mmol), CuO nano 5 mol%, DMF (4 ml) at reflux for 8 h
unless noted otherwise.
b
12 h.
catalytic N-arylation process using CuO nanoparticles. The varia-
tions in the parameters like solvent, bases and the quantity of nano
CuO catalyst were carried out and the results are summarized in
Table 1.
The data of Table 1 shows that the N-arylation of indole was
effectively possible in various solvents with different bases. As the
concentration of catalyst was increased from 2.5 mol% to 5 mol%,
the yield of the product was increased from 65% to 98% (Table 1,
X
CuO nano (5mol%)
K2CO3 (1 equiv)
(I = 98%),
(Br = 74%),
(Cl = 60%)
N
N
DMF/reflux, 8 h
H
X = Cl, Br, I
Scheme 2. CuO nanoparticle catalyzed N-arylation of indoles using different aryl halide.

N.V. Suramwar et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 28–34
33
Table 3
Recyclability of CuO nanoparticles as a catalyst.
CuO nano (5 mol%)
I
K2CO3 (1 equiv)
N
N
H3C
H
DMF/reflux, 8 h
CH3
(95%)
.
Entry
Cycle
Yield (%)a
1
2
3
4
5
1st
95
94
94
90
85
2nd
3rd
4th
5th
a
Isolated yield.
selectivity of CuO nanoparticles as catalyst in the N-arylation of
indole.
CuO nano (5mol%)
I
K2CO3 (1 equiv)
No reaction
To find out whether any copper traces are in the aqueous solu-
tion after reaction, we done ICP study and electrochemical study
of aqueous solution after the centrifugation but there were no any
traces of copper found in the solution. The absence of Cu traces in
the solution confirmed that the reaction was heterogeneous and
occurring in the surface of nanoparticle and there was no leaching
of metal species involved during the reaction.
N
CH3
DMF/reflux, 8 h
Scheme 3. Reaction of 1-methyl indole with aryl iodide.
entries 5 and 6). It is important to mention that N-arylation of
indole did not take place in absence of CuO nanoparticle (Table 1,
entry 8). The combination of K2CO3 as the base and 5 mol% of CuO
nanoparticle as the catalyst in DMF solvent was found to be the best
choice to get maximum yield of the product (Table 1, entry 6). With
these optimum reaction conditions in hand, the scope and general-
ity of the N-arylation method was investigated by the variations of
iodoarenes and indoles with different substituent’s and the results
are summarized in Table 2.
The effect of different aryl halide on the yield of product was also
studied. The reactions were performed with bromobenzene and
chlorobenzene as a source of aryl halide under optimised reaction
conditions (Scheme 2). The order of reactivity of aryl halide was
found to be iodobenzene > bromobenzene > chlorobenzene.
To ensure that there was no C-arylation taken place in the reac-
tion, the reaction was carried out with 1-methyl indole and found
that there was no product formed (Scheme 3). This shows the high
3.4. Recyclability of catalyst
Finally the stability and activity of the catalyst was tested in
the recycle-use experiments. The CuO nanoparticles recycled for
five times without loss of its catalytic activity (Table 3). The used
catalyst was then separated by centrifugation and washed with hot
water two-three times before recycled in next reaction. It was found
that during recycle experiments there was not much loss in yield
of the product which shows the recyclability and reusability of cat-
alyst without significant loss of its catalytic activity. In addition,
TEM analysis of the CuO nanoparticles, before and after the reaction
showed identical particle shape and size (Figs. 5 and 3).
4. Conclusion
In summary, we developed an environment-friendly procedure
for rapid synthesis of highly faceted CuO with FCC nanostruc-
tures. Compared with the reported protocols, our method is green,
environment-friendly, facile, and time-saving. The synthesized CuO
nanoparticles with spherical nanostructures featured extremely
strong reflection peaks of the (1 1 1) plane and possess small round
structures on the bigger one. This kind of morphology endow them
with excellent catalytic performance, as demonstrated here for
the remarkable catalytic activity with respect to coupling reaction
between indole and iodoarene in a ligand free condition. This inter-
esting result highlights the advantage of such CuO nanostructures
over the bulk counterpart and places a solid foundation for the
feasible and promising application of such highly faceted nanoma-
terials in catalysis. To the best of our knowledge, it is the first report
on nanometre-sized predominant (1 1 1) facet CuO acting as a cata-
lyst for N-arylation of indoles and simultaneously is a good example
for the combination of green chemistry and functional materials.
Acknowledgement
The authors gratefully acknowledge UGC, New Delhi (No. F-
39-696/2010 (SR) and F-14-11/2008 (inno./ASIST)) for financial
Fig. 5. TEM image CuO nanoparticles after recycled for 4th time in the reaction.

34
N.V. Suramwar et al. / Journal of Molecular Catalysis A: Chemical 359 (2012) 28–34
assistance to carry out this work through Major research project
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