PVP coated copper-iron oxide nanocomposite as an efficient catalyst for Click reactions

Article abstract of DOI:10.1016/j.tetlet.2015.05.011

A highly efficient Polyvinylpyrrolidone (PVP) coated copper-iron oxide magnetic nanoparticles as catalyst has been developed for the Huisgen 1,3-dipolar cycloaddition reactions between organic azides and terminal alkynes which is known to be the best exam

Full text of DOI:10.1016/j.tetlet.2015.05.011

Accepted Manuscript
PVP coated copper-iron oxide nanocomposite as an efficient catalyst for Click
reactions
Neha Joshi, Shaibal Banerjee
PII:
S0040-4039(15)00813-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.05.011
TETL 46283
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
15 April 2015
2 May 2015
4 May 2015
Please cite this article as: Joshi, N., Banerjee, S., PVP coated copper-iron oxide nanocomposite as an efficient catalyst
for Click reactions, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.05.011
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Graphical Abstract
PVP coated copper-iron oxide nanocomposite as an
efficient catalyst for Click reactions
Leave this area blank for abstract info.
Neha Joshi and Shaibal Banerjee*

1
Tetrahedron Letters
journal homepage: www.els evier.com
PVP coated copper-iron oxide nanocomposite as an efficient catalyst for Click
reactions
Neha Joshia , Shaibal Banerjeea, ∗
a Department of Applied Chemistry
Defence Institute of Advanced Technology (Deemed University)
Girinagar, Pune – 411025, India.
ARTICLE INFO
ABSTRACT
A highly efficient Polyvinylpyrrolidone (PVP) coated copper-iron oxide magnetic nanoparticles
as catalyst has been developed for the Huisgen 1,3-dipolar cycloaddition reactions between
organic azides and terminal alkynes which is known to be the best example in click chemistry.
The in situ generated Cu (I) species is the catalytically active species for click reaction. Wide
ranges of 1,2,3 triazoles ave been synthesized in good yields by use of the catalyst at room
temperature. This nanocatalyst can be recovered by applying external magnetic field which
results in easy separation of catalyst without filtration and can be reused for three times without
loss of its significant activity. This catalyst was characterized by FT-IR, XRD, XPS, FESEM
and TEM.
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
Click
Copper-iron oxide
Nanocatalyst
PVP
2009 Elsevier Ltd. All rights reserved.
Triazole
Introduction
salt, or in situ reduction of Cu (II) by sodium ascorbate.18
However, a profound impact on the development of new era of
1,2,3-triazoles are five membered nitrogen containing
heterocyclic compounds having aromatic character which are
stable to light, moisture and oxygen. Various compounds
consisting 1,2,3-triazole moiety shows biological activities such
as antiviral,1 fungicidal,2 antibacterial,3 antiallergic,4 anticancer5
and anti-HIV.6 Sharpless7 and Meldal8 independently showed
that copper (I) species can be used as catalyst for Husigen 1,3
cycloaddtion for terminal alkyne and organic azide under mild
conditions towards 1,2,3 triazole synthesis. Cu(I) acts as an
efficient and regioselective catalyst for this reaction yielding 1,4-
disubstituted 1,2,3- triazoles. It is a classic example in click
chemistry.9,10 Click reactions have diverse applications in areas
such as synthetic chemistry, supramolecular chemistry,11 polymer
and materials sciences,12 bioconjugation13 and combinatorial
chemistry.14 S. Kotha et al. have synthesised di-triazole based
peptide as fluorescent chemosensor for Zn2+ ions using copper
nanoparticles based materials as catalyst is a new challenge for
researchers. In recent years, nanoparticles have become the
centre of attention for its use as catalyst in organic reactions19 as
the nanoparticles provide high catalytic surface area which helps
to enhance the rate of reaction. Orgueira et al. have used Cu(0)
nanosize activated powder and amine hydrochloride salt as
catalyst for click reaction.20 Giorgio Molteni et al have reported
mixed Cu/Cu-oxide nanoparticles as catalyst for click, 1,3-
dipolar cycloaddition between azide and alkynes. 21
In order to discover more reusable catalysts and to improve
the efficiency of catalyst, solid supported catalysts have been
developed for catalytic applications.22,23,24,25 Solid support mostly
entails soluble and insoluble polymers,26,27 zeolites28.29,30 or
magnetic materials.31,32,33 In addition to this, Cu(0) on charcoal, 34
Cu(0) nanoparticles35,36,37,38 or CuO nanostructures39 have also
successfully exhibited activity for click reaction. Veerakumar et
al. have synthesized 1,2,3-triazoles and thioethers by using
highly dispersed silica-supported copper nanoparticles as
catalyst.40 In pursuit of a green catalyst, a development has been
done in chemical synthesis which minimizes the pollution with
respect to the increasing environmental issues.16 Although, the
use of heterogeneous catalyst system is being exploited to
catalyzed Huisgen cycloaddition.14d
.
Himo et al. have reported that copper metal alone can catalyse
the click reaction owing to long reaction time and high catalyst
loading.15 Copper (I) containing oxide nanoparticles16 and copper
nanoclusters17 have been used to catalyze Cu(I) azide-alkyne
click reactions. Most of these procedures describe
homogeneous Cu(I) source –either by direct addition of a Cu(I)
a
overcome the problem of separation of catalyst from the reaction.
41
———
∗ Corresponding author. Tel.: (+91) 20 2430 4164; fax: (+91) 20 24389318; e-mail: shaibal.b2001@gmail.com

2
Tetrahedron Letters
Recently, magnetic nanoparticles have emerged as viable and
promising support for immobilization with applications in
catalytic transformations such as robust, readily available and
high surface area heterogeneous catalyst.42,43 These particles can
be separated easily from the reaction medium by applying
external magnetic field thus, representing easy and
environmentally benevolent means of catalyst recovery. Hudson
et al. have reported the synthesis of magnetic bi-metallic copper-
iron nanoparticles as heterogeneous catalyst for click reactions.44a
P´erez et al reported Bimetallic nickel and copper on magnetite
has been used as catalyst for multicomponent azide-alkyne
cycloaddition.44b Moreover, magnetic nanoparticles coated with
polymer as catalysts have gained much interest in organic
reactions as they tend to minimize agglomeration. The nature of
supporting materials on which nanoparticles are stabilized plays
an important role in catalysis as it provides highly active catalyst
surface, which increases the rate of reaction and helps to decrease
the use of amount of catalyst in reaction.19
In this paper, we report the synthesis of new
Polyvinylpyrrolidone (PVP) coated copper-iron oxide (Cu-Fe3O4)
nanoparticles (NPs) as catalyst for click reactions. Our first
attempt was to synthesize of PVP coated copper-iron oxide (Cu-
Fe3O4) nanoparticles, where this catalyst can be magnetically
recovered and reused. Further, PVP is used as stabilizing agent
which prevents the agglomeration of nanoparticles. PVP is
widely used due to its easy availability, low cost and
biodegradable nature. Sarkar et al. have reported the synthesis of
highly stable PVP coated copper NPs as catalyst for click
reactions.19 We report the development and synthesis of new PVP
coated Cu-Fe3O4 nanocomposite as catalyst and scrupulously its
application in click chemistry.
Scheme 2: 1,3-dipolar cycloaddition of azides and terminal alkynes
catalyzed by PVP coated Cu-Fe3O4 nanocomposite.
Normally click reactions are reported at elevated temperature,
however by using the above nanocomposite, the click reaction
proceeded under room temperature. This is particularly useful for
the synthesis of energetic and biologically active triazoles where
some groups are thermally sensitive. Energetic triazole is a
relatively new field and we have synthesized various energetic
triazoles like 3j, 3k, 3l by incorporating explosophores like NO2,
CF3, cyclopropane. Incorporating trimethylsilyl (TMS) and
bromide (Br) groups offer scope for further functionalization of
triazoles (3b, 3c, 3i & 3n). We have explored the functional
group compatibility of this process by incorporating groups such
as electron withdrawing, electron donating and heterocycles.
Electron donating substituents take comparatively less time to
complete the reaction, than electron withdrawing groups (Entry 6
& 8).
In this click reaction, Cu (I) is considered the most active
species in the mechanism and forms Cu (I) - acetylide complex
with terminal alkynes which further reacts with azide to form
1,2,3- triazoles.7 As reported by Sarkar19a et al, in nanoparticle
state, copper may be present in zero as well as in +1 state because
of its unsatisfied valences. Further, thermodynamic instability of
Cu(I), it would have been oxidized to Cu(II) as most stable
form.20 To support our hypothesis, we performed an experiment
in absence of sodium ascorbate (Entry 4) and observed that even
after the reaction time is increased to two fold, the conversion
factor was less when compared to the presence of sodium
ascorbate. Hence it may be assumed that sodium ascorbate is
acting as reducing agent, which reduces Cu(II) to Cu(I) and thus,
enhancing the rate of reaction and yield of the product.
Synthesis of PVP coated copper-iron oxide nanocomposite
The copper-iron oxide nanoparticles were prepared from ferric
nitrate nonahydrate and copper nitrate trihydrate using
trioctylamine by reflux method (Scheme 1). Trioctylamine is
playing a dual role of reducing as well as hydrolyzing agent.45
The synthesized Cu-Fe3O4 nanoparticles were further coated with
PVP as a supporting matrix. These PVP coated Cu-Fe3O4
nanocomposite was used as a catalyst for the click reactions.
Characterization of Catalyst
The FT-IR spectrum of Cu-Fe3O4 (Figure 1(a)) exhibit a broad
band at v max./ cm-1 ~580 corresponding to the stretching vibration
of the Fe–O bonds is seen which is assigned to spinel form of
iron oxide (Fe3O4).46 The peak at 578 cm-1 (Figure 1(b)) suggests
the formation of the magnetite form of iron oxide. The band at
~2900 cm-1 is due to characteristic stretching vibration of C-H
band. The band at 1650 cm-1 is attributed to C=C bond. The sharp
peaks at 1290 and 1440 cm-1 of PVP coated Cu-Fe3O4 (Figure
1(b)) corresponds to C-N stretching and C-H bending vibrations
of PVP respectively, which signifies that Cu-Fe3O4 nanoparticles
are uniformly embedded in polymer matrix. A broad band at
3400 cm-1 is ascribed to the stretching mode of surface H2O
Scheme 1: Synthesis of PVP coated copper-iron oxide
nanocomposite
molecules 47
.
1,3-dipolar cycloaddition of azides and terminal alkynes
catalyzed
In a typical copper catalysed cycloaddition reaction (Scheme 2),
1.0 equivalent of azide was mixed with 1.5 equivalent of alkyne
with 0.05 mol % of catalyst and 0.1 equivalent of sodium
ascorbate in t-butanol-water (1:3) medium. The reaction was
carried out in shaker at room temperature. We observed good
conversions and yields for a variety of azide-alkyne combinations
(Table 1). The resulting products of 1,4-disubstituted 1,2,3-
1
triazoles were purified and characterized by FT-IR, H NMR, 13
C
NMR, 19F NMR and mass spectrometry.

3
Table 1: Huisgen 1,3-dipolar cycloaddition of orgnic azides and terminal alkynes carried out in Shakera
R1 1
Yield [ %] b
55
R2 2
Compound 3
Time [h]
3
Entry
1
N
N
N
3a
N
Si
Si
Si
Si
83
2
5
N
N
N
67
84
41
3
3
7
3
4
5
N
N
NO2
NO2
3c
N
N
N
N
N
NO2
NO2
3d
NO2
NO2
N
N
3e
3f
MeO
N
N
84
3
6
MeO
MeO
Me
MeO
N
N
100
92
7
8
6
1
N
N
NO2
NO2
Me
Br
N
N
3h
N
N
44
NO2
9
10
11
7
7
7
Br
NO2
N
N
3i
3j
N
N
85
80
N
N
MeO
N
N
N
N
N
NO2
CF3
NO2
3k
3l
F3C
N
12
13
14
8
1
100
73
N
N
N
N
N
NO2
Br
N
NO2
Br
3m
3n
N
N
4
91
N
a Reaction conditions: 1.0 equivalent of azide, 1.5 equivalent of alkyne with 0.05 mol % of catalyst and 0.1 equivalent of sodium ascorbate in t-butanol-water
(1:3) at room temperature.
b Yield of isolated and purified products.

4
Tetrahedron Letters
In order to know the exact oxidation state of copper in
catalyst, as-synthesized Cu-Fe3O4, PVP coated Cu-Fe3O4
nanocomposite (catalyst) and used catalyst were characterized by
X-ray photoelectron spectroscopic analysis (XPS) (Figure 3).
XPS spectra of Cu-Fe3O4 (Figure 3 (a)) shows that the catalyst
surface is composed of Cu(0) and Cu (II) species. The Cu 2p
peaks at 953.7 eV was assigned to Cu(0)49 and 934.2 eV with
shakeup satellite peak at 942.3 eV were assigned to Cu (II)16, 19b
.
The XRD result of Cu-Fe3O4 does not show the existence of Cu
(II), while XPS indicates the presence of Cu (II) ions on the
nanoparticles surface, suggesting that Cu (II) is present only on
the surface of the nanoparticles as a thin amorphous outer shell.
Figure 3 (b) shows the XPS spectra of catalyst (binding
energies were observed at 934.4, 954.3 and satellite peak at 944.1
eV) and figure 3 (c) shows the XPS spectra of used catalyst
(binding energies were observed at 934.9, 954.7 and satellite
peak at 943.6 eV). This confirms that even after the reaction, the
catalyst (copper) is present in Cu (0) and Cu (II) form and thus
remains unaltered.
The field emission scanning electron microscopy (FESEM)
images are shown in Figure 4. The SEM image shows that Cu-
Fe3O4 nanoparticles are well incorporated in polymer matrix
(Figure 4 (b)). There is no significant change observed in SEM
image of catalyst even after the 3rd cycle (Figure 4 (b)). Energy
dispersive X-ray (EDX) studies showed the composition of Cu-
Fe3O4. It clearly indicates the presence of copper (Cu), iron (Fe)
and oxygen (O) metals in the Cu-Fe3O4 nanoparticles, presented
in Figure 5. No other peak related to any impurity has been
detected in the EDX, which confirms that the as-synthesized Cu-
Fe3O4 nanoparticles are composed only with copper, iron and
oxygen.
Figure 1: FT-IR spectrum of (a) Cu-Fe3O4 and (b) PVP coated Cu-
Fe3O4
X-ray diffraction (XRD) measurements of the Cu-Fe3O4, PVP
coated Cu-Fe3O4, pure PVP and used catalyst is depicted in
Figure 2. XRD of Cu-Fe3O4 nanoparticles exhibit peaks at 2θ
values 30.43, 35.78, 43.85, 57.31 and 62.93° correspond to the
(220), (311), (400), (511) and (440) planes, respectively in
accordance with the magnetite phase as per reported data (JCPDS
89-4319). In addition to this peaks at 43.85, 50.60 and 74.26° are
also observed which corresponds to Cu nanoparticles (JCPDS 04-
0836). The XRD pattern of pure PVP (Figure 2(c)) shows a broad
peak at ~22.37° which confirms the amorphous nature of
[48]
The particle size of Cu-Fe3O4 nanoparticles was obtained by
transmission electron microscopy (TEM) which was found to be
~10 nm. TEM image of nanocomposite (Figure 6) elucidates that
these nanoparticles are well encrusted in polymer. The specific
surface area of Cu-Fe3O4 nanoparticles and catalyst measured by
BET method was 48.42 m2/g and 0.88 m2/g respectively. The
turnover number (TON) of PVP coated Cu-Fe3O4 catalyst
reached upto 8200 and turnover frequency (TOF) upto 2157 h-1.
Literature report on synthesized 1,4- disubstituted 1,2,3-triazoles
using polymer-bound Cu-catalyst showed TON in the range of
200–375,50 TON upto 8200 for nanoporous copper catalyst in
click reaction49 and TON 800 and TOF 505 h-1. 51 Hence it can
be said that this catalyst is superior as compared to some reported
catalysts.
polymer.
Figure 2(b) illustrates the XRD pattern of PVP
coated Cu-Fe3O4 in which a broad peak corresponding to pure
PVP along with peaks assigned for Cu-Fe3O4 are also observed.
In XRD of used catalyst (Figure 2(d)), the peak corresponding to
PVP seems to be crystalline which has masked the other peaks,
so it is difficult to locate the exact phase of copper and iron
oxide.
In click reactions, Cu (I) is considered the most active species
in the mechanism and forms Cu (I) - acetylide complex with
terminal alkynes which further reacts with azide to form 1,2,3-
triazoles.7 As reported by Sarkar et al19a, in nanoparticle state,
copper may be present in zero as well as in +1 state because of its
unsatisfied valences. Further, thermodynamic instability of Cu(I),
it would have been oxidized to Cu(II) as most stable form.20 To
support our hypothesis, we performed an experiment in absence
of sodium ascorbate (Entry 4) and observed that even after the
reaction time is increased to two fold, the conversion factor was
less when compared to the presence of sodium ascorbate. XPS
results also support our hypothesis. Figure 3 (c) shows the XPS
spectra of used catalyst where Cu 2p peak at 934.9 eV confirms
the +2 oxidation state of copper. Hence, it may be assumed that
sodium ascorbate is acting as reducing agent, which reduces
Cu(II) to Cu(I) in-situ and in reversible manner. Thus, enhancing
the rate of reaction and yield of the product.
Figure 2. XRD patterns of: (a) Cu-Fe3O4 (b) Catalyst (before
reaction) (c) Pure PVP and (d) Catalyst (after reaction). In this
figure, P = polymer (PVP), F = Fe3O4, C = copper.

5
Figure 3. XPS spectra of: (a) Cu-Fe3O4 (b) Catalyst (before reaction) (c) Catalyst (after reaction).
Figure 4: FESEM images of (a) Cu-Fe3O4 (b) PVP coated Cu-Fe3O4 (before reaction) (c) PVP coated Cu-Fe3O4 after 3rd cycle.
Element Weight%
Atomic%
O K
32.41
54.92
12.67
100
63.14
30.65
6.21
Fe K
Cu K
Total
Figure 5: EDX of Cu-Fe3O4

6
Tetrahedron Letters
Figure 6: TEM images of (a) Cu-Fe3O4 and (b) PVP coated Cu-Fe3O4
Apart from conventional reaction condition, we have also
attempted to synthesize the triazoles by ultrasonication method
(Table 2). The same reactions (Entry 4 & 7) carried out on shaker
were performed in sonicator (frequency 40 KHz). Ultrasonication
decreased the reaction time as compared to reactions carried on
shaker. This technique furnished good yields of 1,2,3- triazole
products at short time.
Table 2: Huisgen 1,3-dipolar cycloaddition of orgnic azides and terminal alkynes carried out in ultrasonicator batha
a Reaction conditions: 1.0 equivalent of azide, 1.5 equivalent of alkyne with 0.05 mol % of catalyst and 0.1 equivalent of sodium ascorbate in t-butanol-water
(1:3) at room temperature.
b Yield of isolated and purified products.
Conclusions & Perspective
Reusability study of the Catalyst
In conclusion, we have developed a simple and convenient
route for the preparation of copper-iron oxide nanoparticles. We
To ensure the reusability of the catalyst, we performed a set of
have successfully synthesized a new and effective PVP coated
experiments using PVP coated Cu-Fe3O4 nanocomposite for the
Cu-Fe3O4 nanocomposite which can expeditiously act as catalyst
click reaction between 4-ethynyltoluene and benzyl azide. After
for 1,3-dipolar cycloadditions of azides and terminal alkynes.
the completion of the first reaction, the catalyst was separated by
This catalyst can be separated magnetically and can be reused for
applying external magnet. Then the catalyst was washed with
at least 3 times. We have also explored another route for
methanol and dried in oven which showed magnetic property.
synthesis of 1,2,3-triazoles by ultrasonication method which was
Further a fresh reaction was performed using this catalyst under
found to be very useful as a quick alternative to conventional
the same reaction condition. Likewise, the catalyst was recovered
reaction.
and reused for at least 3 times without losing its activity. The
yield of the catalyst recovered was found to be decreased which
may be due to loss of polymer because of its solubility in reaction
mixture. But it is noteworthy that no significant change was
observed in the catalytic activity of the catalyst. Results are
summarized in Table 3.
Acknowledgements
Authors would like to thank, Vice Chancellor, DIAT,
for providing the infrastructural facilities. Financial support from
ER&IPR, DRDO, New Delhi through “DRDO-DIAT
programme on Nano Materials” is greatly acknowledged.
Authors would also like to thank Dr. Hema Singh and Mr. K.
Durga Bhaskar Yamajala for their valuable discussions.
Table 3: Reusability study of the PVP coated Cu-Fe3O4
nanocomposite as catalyst for Huisgen 1,3-dipolar cycloaddition
reactions between benzyl azide and 4-ethynyltoluene.a
% Yieldb
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