Immobilized CuO hollow nanospheres catalyzed alkyne-azide cycloadditions

Article abstract of DOI:10.1166/jnn.2010.2530

An approach for gram-scale synthesis of uniform Cu₂O nanocubes by a one-pot polyol process was used. The CuO hollow nanostructures were prepared by adding aqueous ammonia solutions with Cu₂O nanocube colloidal solutions. CuO hollow n

Full text of DOI:10.1166/jnn.2010.2530

Copyright © 2010 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 10, 6504–6509, 2010
Immobilized CuO Hollow Nanospheres Catalyzed
Alkyne-Azide Cycloadditions
Hyuntae Kang1, Hyun Seok Jung1, Jee Young Kim1, Ji Chan Park2, Mijong Kim2,
Hyunjoon Song2ꢀ∗, and Kang Hyun Park1ꢀ∗
1Department of Chemistry and Chemistry Institute for Functional Materials,
Pusan National University, Busan 609-735, Korea
2Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea
An approach for gram-scale synthesis of uniform Cu2O nanocubes by a one-pot polyol process
was used. The CuO hollow nanostructures were prepared by adding aqueous ammonia solutions
with Cu2O nanocube colloidal solutions. CuO hollow nanospheres on acetylene black (CuO/AB),
were synthesized and used for the catalytic [3+2] cycloaddition of azides with terminal alkynes to
provide products in good yields with high regioselectivity. The CuO/AB was readily separated by
centrifugation and could be reused ten times under the present reaction conditions without any loss
of catalytic activity. Transition metals loaded onto acetylene black are useful reagents for a wide
variety of organic transformations. Moreover, these heterogeneous systems are promising industrial
catalysts.
Keywords: Copper Oxide, Acetylene Black, Charcoal, Heterogeneous, Catalyst, Click Reaction.
Delivered by Publishing Technology to: York University
IP: 130.63.0.29 On: Sat, 06 Dec 2014 14:24:55
Copyright: American Scientific Publishers
1. INTRODUCTION
of product isolation and purification.13 Moreover, these het-
erogeneous systems are promising industrial catalysts. For
example, commercially available Pd/C is frequently used
in debenzylation, hydrogenation, and C–C bond-formation
reactions in the laboratory and industry and has recently
been developed as a catalysts for coupling, hydrosilylation,
and cycloaddition reactions, respectively.
This paper studies the click reaction using CuO on
acetylene black based on the strength of these hetero-
geneous catalysts. Special attention was given to acety-
lene black out of the above-mentioned solid supports.
Acetylene black is a special type of carbon black formed
by an exothermic decomposition of acetylene and is char-
acterized by the highest degree of aggregation and crys-
talline orientation when compared with all types of carbon
black. The carbon content is approximately 99.9%. Acety-
lene black must not be confused with the carbon black
produced as a by-product during the production of acety-
lene in the electric arc process. Acetylene black is widely
used in battery systems possessing excellent electric con-
ductivity, large specific surface areas and strong adsorptive
abilities, as well as in supports.14
The impossibility in recovering and recycling homoges-
neous catalysts is a task of great economic and environ-
mental importance in the chemical and pharmaceutical
industries, especially when expensive and/or toxic heavy
metal complexes are employed.1 The development of cat-
alysts anchored to solid supports has been one of the
areas of most intense research activity over the past years.
Immobilization of homogeneous catalysts onto various
insoluble supports, carbon black,2 activated carbon,3 car-
bon nanotubes,4 carbon film,5 C60,6 acetylene-black(AB),7
charcoal,8 polymer,9 alumina,10 porous silica materials with
high surface areas,11 or onto soluble supports,12 are usually
the methods of choice since the immobilized catalysts can
be easily recovered via simple filtration after the reaction
(Scheme 1). A drawback of homogeneous catalysis is the
impossibility of catalyst recovery and recycling. Immobi-
lization of the catalysts onto an insoluble matrix can pro-
vides a simple solution to this problem. The possibility of
recovering and recycling catalysts, which are often expen-
sive, bring with it positive effects from an economical and
environmental point of view. A further benefit is the ease
Click chemistry is a chemical philosophy introduced
in 2001 by Sharpless, and is important in understanding
the behavior of low-weight molecules.15 Click chemistry
∗Authors to whom correspondence should be addressed.
6504
J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 10
1533-4880/2010/10/6504/006
doi:10.1166/jnn.2010.2530

Kang et al.
Immobilized CuO Hollow Nanospheres Catalyzed Alkyne-Azide Cycloadditions
measured by inductively coupled plasma-atomic emission
spectrometry (ICP-AES).
Carbon
Black
2.2. General Procedure for [3+2] Cycloaddition of
Soluble
Support
Activated
Carbon
Azides with Terminal Alkynes
In a 10 mL pressure tube Schlenk was placed 33.0 mg of
CuO hollow nanospheres on acetylene black (CuO/AB),
Carbon
Nanotube
Silica
benzyl azide (0.1 mL, 0.84 mmol), phenylacetylene
(0.13 mL, 1.18 mmol) and 2.5 mL H2O/t-BuOH
(1.7 mL:0.8 mL). The reaction mixture was stirred at 50 ꢀC.
After 5 h, the CuO/AB was recovered by centrifugation and
the clean solution analyzed by 300 MHz NMR.
Immobilization of
homogeneous catalysts
Acetylene-
Black (AB)
Alumina
2.3. Synthesis of CuO Hollow Nanospheres
Polymer
Charcoal
The CuO hollow nanospheres were synthesized by a
controlled oxidation reaction of Cu2O nanocubes. Typi-
cally, Cu2O nanocubes were prepared by a polyol process
in 1,5-pentanediol (PD, Aldrich, 96%) in the presence of
poly(vinyl pyrrolidone) (PVP, Aldrich, Mw= 55,000). The
PVP (5.3 g) dissolved in 45.0 mL of 1,5-pentanediol (PD,
Aldrich, 96%), was slowly heated to 240ꢀ under a nitro-
gen atmosphere. Then, 4.0 mmol of Cu(acac)2 (STREM,
98%), dissolved in 15 ꢀmL of PD, was injected into the hot
PVP solution at 240 C and the mixture allowed to stir
for 15 min at the same temperature. The yellowish col-
Carbon
C60
Film
Scheme 1. Immobilization of homogeneous catalysts on various
supports.
has had a substantial impact on organic synthesis, drug
discovery, and biological applications.16 The formation
of a triazole-containing carbon-heteroatom bonds through
Huisen [3+2] cycloadditions is a representative example of
Delivered by Publishing Technology to: York University
loidal dispersion was cooled to room temperature and was
IP: 130.63.0.29 On: Sat, 06 Dec 2014 14:24:55
a click reaction. The desired triazole-forming cycloaddition
precipitated by adding acetone followed by centrifugation
at 8,000 rpm for 20 min. The precipitated Cu2O particles
were washed with ethanol several times and re-dispersed in
ethanol. To obtain the CuO hollow nanospheres, an aque-
ous ammonia solution (2.0 mL, 3.7 M) was added into
25.0 mL of the Cu2O cube dispersion in ethanol (16.0 mM
with respect to the precursor concentration). The mixture
was subjected to stirring at room temperature for 2 h. After
the reaction, the final products were collected by centrifu-
gation at 6,000 rpm for 20 min.
Copyright: American Scientific Publishers
typically requires high temperatures and usually results in
a mixture of the 1,4 and 1,5 regioisomers (Eq. (1)).
N
R1
N
R1
N
N
N
N
R2
R1
(1)
N3
+
+
R2
R2
2. EXPERIMENTAL PROCEDURE
2.1. General Remarks
2.4. Immobilization of CuO Hollow Nanospheres on
Acetylene Carbon Black (CuO/AB) and
Charcoal (CuO/C)
Reagents were purchased from Aldrich Chemical Co.
and Strem Chemical Co. and used as received. Reaction
products were analyzed by 1H-NMR. 1H-NMR with spectra
obtained on a Varian Mercury Plus (300 MHz). Chem-
ical shift values were recorded as parts per million rel-
ative to tetramethylsilane as an internal standard, unless
otherwise indicated, and coupling constants in Hertz. Reac-
tion products were assigned by comparison with the lit-
erature value of known compounds. The CuO and CuO
nanoparticles immobilized on acetylene black were char-
acterized by TEM (Philips F20 Tecnai operated at 200 kV,
KAIST). Samples were prepared by placing a few drops of
the corresponding colloidal solution on carbon-coated cop-
per grids (Ted Pellar, Inc). The X-ray powder diffraction
(XRD) patterns were recorded on a Rigaku D/MAX-RB
(12 kW) diffractometer. The copper loading amounts were
The acetylene carbon black (STREM, 99.99%, 1.2 g) was
mixed with 100 mL of the CuO hollow nanosphere dis-
persion in ethanol (17.0 mM), and the reaction mixture
sonicated for 1 h at room temperature. After 1 h, the
product CuO/AB was washed with ethanol several times
and vacuum dried at room temperature. For the synthesis
of CuO/C, the mixture solution of charcoal (0.8 g) and
50.0 mL of CuO hollow nanosphere dispersion in ethanol
(50.0 mM) was refluxed for 4 h. After 4 h, the black sus-
pension was cooled to room temperature and precipitated
by centrifugation. The product CuO/C was washed with
ethanol thoroughly and dried in a vacuum oven at room
temperature.
J. Nanosci. Nanotechnol. 10, 6504–6509, 2010
6505

Immobilized CuO Hollow Nanospheres Catalyzed Alkyne-Azide Cycloadditions
Kang et al.
3. RESULTS AND DISCUSSION
metal on the acetylene black was determined by induc-
tively coupled plasma-atomic emission spectroscopy (ICP-
AES). The CuO hollow spheres on the acetylene black
showed excellent activity towards a wide range of azides
and acetylenes.
In the present study, an approach for gram-scale synthesis
of uniform Cu2O nanocubes by an one-pot polyol process
was developed.17 The CuO hollow nanospheres were pre-
pared by adding an aqueous ammonia solution to the Cu2O
nanocube colloidal solution. The CuO hollow nanospheres
were immobilized onto acetylene black (AB) or charcoal.
As such, these immobilized CuO hollow nanospheres over-
came the issue of reuse.18
3.2. Reaction Test
Research studies testing catalyst effectiveness have
employed benzyl azide and phenylacetylene as the bench-
mark substrates. The cycloaddition reaction of ben-
zyl azide (0.1 mL, 0.84 mmol) and phenylacetylene
(0.13 mL, 1.18 mmol) with CuO/AB in H2O/t-BuOH
(1.7 mL:0.8 mL), off the shelf, afforded 1,4-disubstituted
1,2,3-triazoles as a single regioisomer. This CuO nano-
structure catalyzed the reaction sequence that regiospecif-
ically unites azides and terminal acetylenes to give only
1,4-disubstituted 1,2,3-triazoles.
As shown in Table I, the reaction was carried out at 25–
50 ꢀC using benzyl azide and phenylacetylene as the bench-
mark substrate (Table I). The best results were obtained
when t-BuOH was used as the solvent under mild, room
temperature conditions.19 Improved results were possible
with a solvent mixture of 2:1 rather than t-BuOH and H2O
independently, indicating that both solubility and hydro-
scopic properties are portant factors. First, no reaction
3.1. Catalyst Characterization
The Cu2O nanocubes prepared by a polyol process were
transformed into CuO hollow nanospheres by a controlled
oxidation reaction using an aqueous ammonia solution.
The addition of ammonia solution (2.0 mL, 3.7 M) into
Cu2O colloidal solution yielded CuO hollow nanospheres
through a dissolution–precipitation process. The transmis-
sion electron microscopy (TEM) image in Figure 1(a)
shows the regular hollow shape of the CuO particles.
CuO hollow spheres were obtained as (103 8)-nm-sized,
highly monodisperse nanoparticles (Fig. 1(d)). The crys-
talline features of the hollow spheres are represented in
the XRD data (Fig. 1(c)). The main peaks at ꢀ = 35ꢀ
and 39ꢀ are assigned to the reflections of the (002)/(11–1)
and (111)/(200) planes in the CuO phase (JCPDS No.
Delivered by Publishing Technology to: York University
occurred without a catalyst. When the Cu2O nanocubes
48-1548). The CuO hollow particles were immobilized on
IP: 130.63.0.29 On: Sat, 06 Dec 2014 14:24:55
(5 mol%) were used, 1-benzyl-4-phenyl-1H-1,2,3-triazole
acetylene carbon black by simple sonication method at
room temperature. The TEM image in Figure 1(b) shows
that the immobilized CuO hollow spheres are well dis-
persed and isolated with approximately 100 nm in an aver-
age diameter, maintaining the original size and structure
of CuO hollow spheres. The absolute amount of copper
Copyright: American Scientific Publishers
ꢀ
was obtained in 100% conversion at 25 C within 3 h
(entry 2). When the CuO on AB (1 mol%) caꢀtalyst was
used, less than a 1% yield was found under 25 C for 3 h
(entry 3). Furthermore, when 3 mol% of the catalyst was
used, less than a 1% yield was achieved under the same
Table I. Optimization of click reaction catalyzed by various CuO NPs.
(a)
(b)
Entry
Cat (mol%)
Temp (ꢀC) Time (h) Conv (%)a
1
2
3
4
5
6
7
8
Blank
CuO (5 mol%)
50
25
25
50
25
50
50
40
30
50
50
50
50
50
50
50
50
50
50
50
5
3
3
5
5
5
3
5
5
5
5
5
5
5
5
5
5
5
5
5
7
100
>1
22
>1
100
60
23
1.1
90
CuO on AB (1 mol%)
CuO on AB (1 mol%)
CuO on AB (3 mol%)
CuO on AB (3 mol%)
CuO on AB (3 mol%)
CuO on AB (3 mol%)
CuO on AB (3 mol%)
CuO on AB (3 mol%)
CuO on AB (5 mol%)
Recovered from # 6
Recovered from # 12
Recovered from # 13
Recovered from # 14
Recovered from # 15
Recovered from # 16
Recovered from # 17
Recovered from # 18
Recovered from # 19
9
(d)
(c)
10
11
12
13
14
15
16
17
18
19
20
96
100
100
100
100
100
98
100
100
100
Fig. 1. (a) TEM image, (c) XRD pattern, (d) size distribution diagrams
of the CuO hollow nanospheres, and (b) TEM image of the CuO hollow
nanospheres on acetylene black. The scale bars represent 200 nm.
aDetermined by 1H-NMR. Yields are based on the amount of benzyl azide used.
6506
J. Nanosci. Nanotechnol. 10, 6504–6509, 2010

Kang et al.
Immobilized CuO Hollow Nanospheres Catalyzed Alkyne-Azide Cycloadditions
conditions (entry 5). In general, it was found that increasing
the reaction temperature and time were an effective means
of increasing conversion (22% for 50 ꢀC, 5 h). Finally, the
optimum reaction conditions were established: click reac-
tion of benzyl azide (0.1 mL, 0.84 mmol ) and phenylacety-
lene (0.13 mL, 1.18 m mol) with CuO on AB (26.0 mg,
3 mol%) in H2O:t-BuOH(2:1) (2.5 mL) to afford 1,2,3-
triazole. In the case of using another heterogeneous cata-
lyst (CuO on charcoal), 90% or 96% conversion was found
under the same conditions (Table I, entries 10 and 11).
Remarkably, after the reaction, the CuO on AB were
separated by centrifugation and reused ten times under
the same reaction conditions without any catalytic activ-
ity loss. An inductively coupled plasma-mass spectrometry
(ICP-AES) study showed that the copper that bled from
the catalyst was negligible. These results confirm that the
catalytic system presented here satisfies the conditions for
heterogeneous catalysts of easy separation, recyclability,
and persistence.
As shown in Figure 2, the structure of the CuO hol-
low nanospheres on acetylene black (CuO/AB) remained
unchanged after the reaction, demonstrating catalyst
recyclability.
Furthermore, in the case of using various terminal
alkynes, good results were achieved (Table II). Hydroxy-
substituted alkynes such as propargyl alcohol, 2-methyl-
3-butyn-2-ol, and 1-phenyl-2-propyn-1-ol also gave
the expected adducts of (1-benzyltriazol-4-yl)metha-
nol, 2-(1-benzyl-1H-1,2,3-triazol-4-yl)propan-2-ol, and
(1-benzyl-1H-1,2,3-triazol-4-yl)(phenyl)methanol as a sin-
gle regioisomer in good to high yields (Table II, entries I,
II, and III.). Acetylenes conjugated with an ester group,
Table II. [3+2] Cycloaddition of benzyl azides with terminal alkynes
in the presence of CuO hollow nanospheres.
(a)
Delivered by Publishing Technology to: York University
IP: 130.63.0.29 On: Sat, 06 Dec 2014 14:24:55
Copyright: American Scientific Publishers
(b)
Fig. 2. TEM images of CuO/AB, before (a) and after the fifth cycle.
(b) The scale bars represent 100 nm.
J. Nanosci. Nanotechnol. 10, 6504–6509, 2010
6507

Immobilized CuO Hollow Nanospheres Catalyzed Alkyne-Azide Cycloadditions
Kang et al.
such as methyl propargyl ether and phenyl propargyl
ether, reacted without incident with the benzyl azide. The
corresponding triazoles 1-benzyl-4-(methoxymethyl)-1H-
1,2,3-triazole and 1-benzyl-4-(phenoxymethyl)-1H-1,2,3-
triazole were obtained in high yields (Table II, entries 4
and 5). The reaction with alkynes containing electron-
withdrawing substituents such as ethyl propiolate and
methyl propiolate gave distinctively high yields (Table II,
entries 6 and 7). The reactions with aliphatic alkynes such
as ethynyltrimethylsilane, 1-hexyne, and 1-octyne, were
relatively sluggish (Table II, entries 8, 9, and 10).
In a second series of experiments, various azides bear-
ing different groups were submitted to phenylacetylene
(Table III). When the phenyl group was directly linked
to the reactive azide, phenyl azide or its analogue with a
p-Cl, Ph-O group gave expected 1,4-diphenyl-1H-1,2,3-
triazole, 1-(4-Chlorophenyl)-4-phenyl-1H-1,2,3-triazole,
and 1-(4-phenoxyphenyl)-4-phenyl-1H-1,2,3-triazole as
single regioisomers with 100% conversion yield (Table III,
entries 1, 2, and 3). In some case, electrondonating or
-withdrawing groups on the benzyl azides greatly affected
reactivity. Electron-withdrawing groups disfavoured the
reaction, with lower obtained yields. Within these groups,
fluorine and nitrogen dioxide exhibited the largest effect
(Table III, entries 4 and 5). Interestingly, the result showed
low conversion yield when the methoxy group, one of
the electron-donating groups, was located on para and
meta position while 100% conversion yield was shown
when it is located on ortho position. As in the other series
(Table III, entries 1 versus 10), electronic effects of azides
were more pronounced and they significantly altering the
reactivity. For instance, it is highly reactive when azides
are bonded directly to the phenyl group while only a low
conversion yield is observed when it is indirectly bonded
as in (2-azidoethyl)benzene. Indeed, ethyl 2-azidoacetate
was obtained in high yields without any problem (Table III,
entry 6). Nevertheless, a single regioisomer was still
produced and the substitution and yields of the isolated
products remained excellent.
Table III. [3+2] Cycloaddition of various azides with phenylacetylene
in the presence of CuO hollow nanospheres.
4. CONCLUSION
In summary, oxidation of Cu2O nanocubes has been con-
trolled to yield CuO hollow nanospheres through a sequen-
tial dissolution–precipitation process. As expected, CuO
Delivered by Publishing Technology to: York University
hollow nanospheres on acetylene black (CuO/AB) have
IP: 130.63.0.29 On: Sat, 06 Dec 2014 14:24:55
Copyright: American ScbieeenntiufiscePdufobrlisthheercsatalytic [3+2] cycloaddition of azides
with terminal alkynes to provide products in good yields
with high regioselectivity. The CuO/AB was readily sepa-
rated by centrifugation and could be reused ten times under
the present reaction conditions without any loss in catalytic
activity. Transition metals loaded onto acetylene black are
useful reagents for a wide variety of organic transforma-
tions. Moreover, these heterogeneous systems are promis-
ing industrial catalysts.
Acknowledgments: This study was supported by the
Research Fund Program of Research Institute for Basic
Science, Pusan National University, Korea, 2009, Project
No. RIBS-PNU-2009-108.
References and Notes
1. D. J. Cole.-Hamilton, Science 299, 1702 (2003).
2. T. N. Murakami, S. Ito, Q. Wang, M. K. Nazeeruddin, T. Bessho,
I. Cesar, P. Liska, R. Humphry.-Baker, P. Comte, P. Péchy, and
M. Grätzel, J. Electrochem. Soc. 153, A2255 (2006).
3. K. Imoto, K. Takatashi, T. Yamaguchi, T. Komura, J. Nakamura, and
K. Murata, Sol. Energy Mater. Sol. Cells 79, 459 (2003).
4. K. Suzuki, M. Yamaguchi, M. Kumagai, and S. Yanagida, Chem.
Lett. 32, 28 (2003).
5. A. Kay and M. Grätzel, Sol. Energy Mater. Sol. Cells 44, 99 (1996).
6. T. Hino, Y. Ogawa, and N. Kuramoto, Carbon 44, 880 (2006).
7. A. Freund, J. Lang, T. Lehmann, and K. A. Starz, Catal. Today
27, 279 (1996).
6508
J. Nanosci. Nanotechnol. 10, 6504–6509, 2010

Kang et al.
Immobilized CuO Hollow Nanospheres Catalyzed Alkyne-Azide Cycloadditions
8. K. H. Park and Y. K. Chung, Adv. Synth. Catal. 347, 854 (2005).
9. T. J. Dickerson, N. N. Reed, and K. D. Janda, Chem. Rev. 102, 3325
(2002).
10. A. Gniewek, J. J. Ziokowski, A. M. Trzeciak, M. Zawadzki,
H. Grabowska, and J. Wrzyszcz, J. Catal. 254, 121 (2008).
11. B. R. Bodsgard and J. N. Burstyn, Chem. Commun. 647 (2001).
12. L. A. Thomson and J. A. Ellmann, Chem. Rev. 96, 555 (1996).
13. L. V. Dinh and J. A. Gladysz, Chem. Commun. 8, 998 (2004).
14. W. Y. Li, C. S. Li, C. Y. Zhou, H. Ma, and J. Chen, Angew. Chem.
Int. Ed. 45, 6009 (2006).
15. H. C. Kolb, M. G. Finn, and K. B. Sharpless, Angew. Chem. Int. Ed.
40, 2004 (2001).
16. J. F. Pritchard, M. J. Komet, M. L. J. Reimer, E. Mortimer, B. Rolfe,
and M. N. Cayen, Nat. Rev. Drug Discovery 2, 542 (2003).
17. J. C. Park, J. Kim, H. Kwon, and H. Song, Adv. Mater. 21, 803
(2009).
18. J. Y. Kim, J. C. Park, H. Kang, H. Song, and K. H. Park, Chem.
Commun. 46, 439 (2010).
19. H. A. Orgueria, D. Fokas, Y. Isome, P. C. Chan, and C. M. Baldino,
Tetrahedron Lett. 46, 2911 (2005).
Received: 26 November 2009. Accepted: 27 November 2009.
Delivered by Publishing Technology to: York University
IP: 130.63.0.29 On: Sat, 06 Dec 2014 14:24:55
Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 10, 6504–6509, 2010
6509