CuO nanostructures of variable shapes as an efficient catalyst for [3 + 2] cycloaddition of azides with terminal alkyne

Article abstract of DOI:10.1039/c6ra20725a

CuO nanostructures of variable shapes: CuO nanospheres (5-10 nm), CuO nanorods (W × L = 24-27 nm × 124-140 nm) and CuO nanowires (W × L = 8-10 nm × 230-270 nm) have been synthesised to study the effect of shape of the catalyst on the Cu(i)-catalysed "click" azide-alkyne cycloaddition. Cu(i) species were generated in situ by the reduction of CuO nanostructures in the presence of sodium ascorbate. CuO nanowires exhibited highest catalytic efficiency for the cycloaddition reaction between azide and terminal alkyne, featuring short reaction time, soft reaction conditions and complete regioselectivity. We have further extended the study by using azides with varying functional groups (-OCH₃ and -NO₂) and studied the effect of shape of the nanostructures on the rate of the reaction and yield of the triazole products. The activity trend observed was: CuO-NW > CuO-NR > CuO-NS, irrespective of the presence of electron withdrawing or donating groups on the azide.

Full text of DOI:10.1039/c6ra20725a

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Page 1 of 23
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CuO nanostructures of variable shapes as an efficient catalyst for [3+2] cycloaddition of
azides with terminal alkyne
Anupamjeet Kaur, Sukhmani Mann, Bhupesh Goyal, Bhupender Pal* and Deepti Goyal*
Department of Chemistry, School of Basic and Applied Sciences,
Sri Guru Granth Sahib World University, Fatehgarh Sahib–140406, Punjab, India
*Corresponding authors
Eꢀmail address: bhupenderpal@thapar.edu; deeptig@iitbombay.org
Abstract
CuO nanostructures of variable shapes: CuO nanospheres (5ꢀ10 nm), CuO nanorods (W×L =
24ꢀ27 nm×124ꢀ140 nm) and CuO nanowires (W×L= 8ꢀ10 nm×230ꢀ270 nm) have been
synthesised to study the effect of shape of the catalyst on the Cu(I)ꢀcatalyzed “click” azide–
alkyne cycloaddition. Cu(I) species were generated in situ by the reduction of CuO
nanostructures in the presence of sodium ascorbate. CuO nanowires exhibited highest
catalytic efficiency for the cycloaddition reaction between azide and terminal alkyne,
featuring short reaction time, soft reaction conditions and complete regioselectivity. We have
further extended the study by using azides with varying functional groups (–OCH and –NO )
3
2
and studied the effect of shape of the nanostructures on the rate of the reaction and yield of
the triazole products. The activity trend observed was: CuOꢀNW> CuOꢀNR> CuOꢀNS,
irrespective of the presence of electron withdrawing or donating groups on the azide.
Introduction
Oneꢀdimensional nanoparticles (NPs) [nanowires (NW), nanorods (NR), nanobelts (NB) etc.]
having high surface area and small size have attracted remarkable attention in recent years
1
due to their wide applications in catalysis. The catalytic activity and selectivity of the
nanocatalysts is directly affected by their shape as NPs with different shapes have different
facets and different ratios of the number of atoms on the corners and edges to those on the
2
facets. Furthermore, the nanoparticle catalysed reactions provide the advantages of high
atom efficiency, simplified isolation of products, easy recovery and recyclability of the
catalysts. Transition metal oxides NPs, predominantly cupric oxide (CuO), have attracted
considerable attention in catalysis due to square planar coordination of Cu atom to the
1

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3
neighbouring oxygen atoms and a monoclinic crystal structure, unlike other 3d metal oxides.
Moreover, CuO NPs are highly stable, nonꢀtoxic, environmental friendly and can be easily
recycled. CuO NPs have been used as heterogeneous catalyst in many important chemical
4
5
6
reactions such as nitroꢀaromatic reduction, oxidations of carbon monoxide, Cꢀarylation,
7
8
epoxidation of olefins and crossꢀcoupling reactions. The present article is intended to use
CuO nanostructures of variable shapes as catalyst in Cu(I)ꢀcatalyzed Huisgen azide–alkyne
cycloaddition.
9
Huisgen 1,3ꢀdipolar cycloaddition of terminal alkynes and organic azides is a classical
1
0
example of click chemistry, that delivers 1,2,3ꢀtriazole with high regioselectivity under
1
1
12
Cu(I) catalysis as demonstrated by Sharpless and Meldal independently. Compounds
1
3
containing 1,2,3ꢀtriazole moiety found extensive applications as energetic binders, antiꢀ
1
4
15
16
microbial coatings,
chemical sensors,
biodegradable polymers,
pharmaceutical
1
7
18
19
chemistry, bioꢀconjugation and peptide modifications.
Traditionally, in copper catalyzed azide–alkyne cycloaddition, Cu(I) salts were used directly
or generated in situ by reduction of Cu(II) salts [e.g. SharplessꢀFokin catalyst (CuSO /sodium
The conventional catalysts though the most often used are associated with
4
11b, 20
ascorbate)].
difficulties like large required catalyst amount, difficulty in handling and catalyst
recyclability and slow reaction rate etc.. The use of Cu(I) or Cu(II) NPs as catalyst for the
above reaction has been active lately with numerous review articles published in the past
2
1
22
23
decade. Orgueira et al. and Rothenberg et al. have reported the pioneer work in this area
by demonstrating the use of Cu(0) nanoclusters and NPs as catalyst in azide–alkyne
cycloaddition. However, the catalyst cannot be recovered and reused due to the dissolution of
2
4
NPs in the reaction mixture. In another report, Himo et al. have reported copper metal
alone as a catalyst in cycloaddition reaction owing to long reaction time and high catalyst
loading. Mixed Cu/CuO NPs have been used as catalyst for cycloaddition reaction by Molteni
2
5
et al. in 2006. In 2013, Bai and Chen’s group have successfully synthesised a triazole drug
by using Cu O NP as catalyst for azideꢀalkyne cycloaddition and further demonstrated that
Cu O NP are more efficient catalyst than CuSO
have reported a highly efficient, recyclable amphiphilic dendrimer nanoreactor for partꢀperꢀ
2
2
6
2
4
/sodium ascorbate. Recently, Astruc et al.
2
7
million (ppm) Cu(I) catalyzed azideꢀalkyne cycloaddition in water. Solid support catalysts
have been explored by various research groups for enhanced efficiency and reusability of the
2
8
catalyst. Various heterogeneous systems such as copper supported on soluble and insoluble
2
9
30
31
32
33
polymers, zeolities, charcoal, silicaꢀtype material (SABꢀ15), graphene nanosheet and
2

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3
4
Amberlyst Aꢀ21 etc. have been explored for catalysis in azide–alkyne cycloaddition and
success have been achieved to develop highly efficient catalytic systems which work with
3
2,35
considerably low amount of Cu(I) catalyst.
The attachment of support imparts higher
surface for the reaction and hence enhance the catalytic efficiency. An alternative approach is
to have CuO nanostructures of suitable shapes having higher surface area to avoid the use of
solid support for the catalyst. In 2010, Park and Song et al. have highlighted the effect of
wellꢀdefined CuO NPs of variable shapes (urchin, hollow spheres, and hollow cubes) on the
azide–alkyne cycloaddition and documented that CuO hollow spheres gave best activity
3
6
owing to large surface area of the catalyst. There are enormous reports on the use of Cu(I)
21
or Cu(II) NPs as catalyst in azide–alkyne cycloaddition reaction, and yet a comparative
account of their catalytic activity as a function of shape is rarely investigated. In this regard,
we plan to synthesise CuO nanostructures of variable shapes (spheres, rods and wires) with
specifically exposed crystal planes, to evaluate the effect of shape of the catalyst on the
Cu(I)ꢀcatalyzed “click” azide–alkyne cycloaddition. Moreover, aryl azides bearing
substituents with different electronic demands have been investigated. We further explored
the catalyst reusability and tried to understand the mechanism of azide–alkyne cycloaddition
catalysed by CuO nanostructures.
Results and Discussion
Structural, optical and morphological characterisation of CuO nanostructures
Powder Xꢀray diffraction (XRD) patterns of CuO nanostructures are shown in Fig. 1 (panels
a–c). XRD patterns indicated that all peaks indexed to monoclinic CuO (space group C2/c)
crystal phase as per JPCDS Card No. 80–1268. Absence of peaks corresponding to Cu(OH)
2
and Cu O confirmed high purity of synthesised CuO NPs. Different intensities and broadness
2
of XRD peaks observed for variable CuO nanostructures suggested differences in crystallite
size of CuOꢀNS (~5 nm), CuOꢀNR (~10 nm) and CuOꢀNW (~25 nm) as calculated by the
DebyeꢀScherrer equation d= kλ/βcos θ using the average values of (0 0 2) (2θ= 35.5°) and (2
0
0) (2θ= 38.8°) planes, where k is the DebyeꢀScherrer constant (0.89), θ is diffraction angle
and β is full width at halfꢀmaximum. Presence of strongest peaks at lower Millerꢀindexed
002) and (200) reflections indicated the preferential crystal planes of the CuOꢀNR and CuOꢀ
(
NW. The difference in intensity of related peaks for the variable CuO nanostructures may be
due to texture effect as well as dimension/morphological alteration in certain direction.
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Similar results highlight the variation of morphology and growth direction leading to the
3
7
change in intensity of the XRD peaks were reported by Singh et al.
CuOꢀNS
CuOꢀNR
CuOꢀNW
(a)
(b)
(c)
20
40
(degrees)
60
2θ
Fig. 1. Powder Xꢀray diffraction (XRD) patterns of (a) CuOꢀNS (b) CuOꢀNR and (c) CuOꢀ
NW.
The absorption spectra of CuOꢀNS (Fig. 2) showed a broad peak at 290 nm with absorption
edge at 426 nm. The calculated bandꢀgap for synthesised CuOꢀNS is E
g
= 2 eV, which is
3
8
much larger than the reported E value for bulk CuO (1.65 eV). The blueꢀshift in E value
g
g
was attributed to the enhancement of quantum confinement effect resulting from decrease in
size of the synthesised CuOꢀNS (5ꢀ10 nm). The broad absorption band observed in the
absorption spectra was probably due to dominant surfaceꢀrelated defects for intraꢀgap states.
38
Ovchinnikov et al. have reported the role of electronic defects due to the presence of
+
dopants or valence defects (O vacancies and Cu ) in CuO NPs that results in the development
of intraꢀband states. The redꢀshift observed in the absorption spectra of CuOꢀNR and CuOꢀ
NW (Fig. 2) was due to their large size as compare to CuOꢀNS.
4

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0.8
0.6
0.4
CuOꢀNW
0.2
CuOꢀNR
CuOꢀNS
0.0
400
600
800
1000
Wavelength (nm)
Fig. 2. UVꢀvisible absorption spectra of CuOꢀNS, CuOꢀNR and CuOꢀNW.
Panels a–c of Fig. 3 shows typical Transmission electron microscopy (TEM) images for
CuOꢀNS, CuOꢀNR and CuOꢀNW. The CuOꢀNS are uniformly dispersed with average
diameter of 5ꢀ10 nm. The TEM image of CuOꢀNR (Fig. 3b) shows sharpꢀtopped rod like
structures of width 24ꢀ27 nm and length 124ꢀ140 nm. The TEM image of CuOꢀNW indicates
that the nanowires have width 8ꢀ10 nm with length 230ꢀ270 nm are randomly agglomerated
as observed in Fig. 3c. To further confirm the purity of synthesised CuO NPs, Energyꢀ
dispersive Xꢀray (EDX) analysis was carried out. The EDX spectra (Fig. 3d) of CuO NPs
showed peaks corresponding to copper and oxygen only, indicating 100% purity of the
synthesised CuO NPs. To confirm the valence state of copper present in the synthesised CuO
nanostructures Xꢀray photoelectron spectroscopy (XPS) analysis were done. A high intensity
peaks at 933.5 eV, 953.2 eV and satellite peaks could be assigned to the binding energies of
3
9
Cu(II) (Fig. 4).
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Fig. 3. TEM images of (a) CuOꢀNS (b) CuOꢀNR (c) CuOꢀNW (d) EDX spectra of CuO NPs.
6

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Fig. 4. The XPS spectra of synthesised CuOꢀNW and the inset shows the Cu 2p core level
binding energy spectrum.
The observed BrunauerꢀEmmettꢀTeller (BET) surface area for CuOꢀNS, CuOꢀNR and CuOꢀ
2
ꢀ1
NW were 41, 53 and 61 m g respectively, which was in accordance with their difference in
geometric dimension and surface morphology (Table 1).
Table 1. Summary of surface morphology and surface area of variable CuO nanostructures
2
ꢀ1
CuO nanostructures
CuOꢀNS
TEM size (nm)
5ꢀ10
Surface area (m g )
41
53
61
CuOꢀNR
W×L= 24ꢀ27×124ꢀ140
W×L= 8ꢀ10×230ꢀ270
CuOꢀNW
Effect of shape of CuO nanostructures on azide–alkyne cycloaddition
CuO nanostructures of variable shapes (NS, NR and NW) have been used as catalyst in the
cycloaddition reaction of azides and terminal alkynes (Scheme 1). Firstly, to standardised the
reaction conditions, series of experiments were conducted on the model reaction between
phenyl azide and phenylacetylene. Best yields were obtained in the presence of 5 mol% of
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catalyst (synthesised CuO nanostructures) loading and H
temperature.
2
O/tꢀBuOH (3:1) as solvent at room
N3
N
N
N
CuO nanostructures
Na-ascorbate
H O:t-BuOH (3:1)
2
3
3
3
a R= H
b R= -OCH3
c R= -NO2
R
rt
R
1
2a R= H
b R= -OCH3
c R= -NO2
2
2
Scheme 1. [3+2] cycloaddition of phenylacetylene (1) and azide (2a–c) catalysed by CuO
nanostructures (NS, NR and NW).
To investigate the effect of shape of CuO nanostructures on catalysis, phenylacetylene and
phenyl azide were reacted in the presence of CuOꢀNS, CuOꢀNR and CuOꢀNW (Table 2, entry
1–3). The results indicated that the CuOꢀNW showed best catalytic activity yielding the
desired triazole 3a as single regioisomer and in quantitative yield. Formation of compound 3a
1
was confirmed by comparing its melting point (mp) and H NMR values with the reported
40
data. The obtained results (Table 2, entry 1–3) for catalytic activity indicated that the
anisotropy in the particle shape dominates over the isotropic particles and hence, CuOꢀNW
and CuOꢀNR were found to be more active than CuOꢀNS. CuOꢀNW resulted in quantitative
yield of 3a in 1h due to much exposed surface active atoms. The relatively lower reactivity of
CuOꢀNS as compared to CuOꢀNR and CuOꢀNW could be explained on the basis of effective
surface area. The BET surface area of CuO nanostructures measured by nitrogen desorption
2
ꢀ1
experiments were 41, 53 and 61 m g for CuOꢀNS, CuOꢀNR and CuOꢀNW respectively, and
it was quite reasonable that the catalytic activities depend on the active surface area of the
catalyst.
We further extended the study by using aryl azides bearing substituents with different
3 2
electronic demands (–OCH and –NO ) and studied the effect of shape of the CuO
nanostructures on the rate of reaction and yield of the triazole products (Table 2, entry 4–9).
Observed results indicated that the desired triazole 3b was obtained in 95% yield in the
presence of CuOꢀNW as catalyst, whereas the –NO
2
substituted triazole 3c was obtained in
comparatively less yield (67%), although CuOꢀNW provide best conversion as compared to
40
41
CuOꢀNS and CuOꢀNR in all cases. The formation of compounds 3b and 3c was confirmed
1
by comparing their mp and H NMR data with literature values. Overall, the activity trend
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observed was: CuOꢀNW> CuOꢀNR> CuOꢀNS, irrespective of the presence of electron
withdrawing or donating groups on the azide.
Table 2. [3+2] cycloaddition of phenylacetylene with various azides in the presence of CuO
nanostructures of variable shapes
a
Entry
R
H
H
Catalyst (5 mol%) Time (h)
Product
Yield (%)
1
2
CuOꢀNS
CuOꢀNR
2
2
85
89
3
3
3
a
a
a
3
4
5
6
7
8
H
CuOꢀNW
CuOꢀNS
CuOꢀNR
CuOꢀNW
CuOꢀNS
CuOꢀNR
1
2
100
80
87
95
40
58
OCH
OCH
OCH
3
3
3
3
3
3
b
b
b
1.5
1
NO
NO
2
2
4
3
3
c
c
4
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9
NO2
CuOꢀNW
3
67
3
c
a
Yields were of isolated and purified products.
b
Reaction conditions: 1.0 equiv. of alkyne, 1.1 equiv. of azide, 5 mol% catalyst, 0.1 equiv. sodium ascorbate in
O/tꢀBuOH (3:1) at room temperature.
H
2
Plausible catalytic pathway with synthesised CuO nanostructures
The plausible reaction pathway for the azide–alkyne cycloaddition has been shown in
Scheme 2. In the first step, Cu(II) is reduce to Cu(I) inꢀsitu by sodium ascorbate. The Cu(I)
species thus generated will react with an alkyne to give Cuꢀacetylide species A. The
formation of the Cuꢀacetylide species A (greenish yellow colour) between the synthesised
CuOꢀNW and alkyne in the presence of sodium ascorbate has been ascertain by Infrared (IR)
ꢀ1
42
ꢀ1
spectra (Fig. S1–S3). The vibrational absorption of
at 1931 cm (Lit. 1930 cm ,
43
ꢀ1
44
ꢀ1
Lit. 1940 cm , Lit. 1936 cm ) was observed in the heterogeneous composite of
phenylacetylene, CuOꢀNW and sodium ascorbate. However, no such peak was observed in
the absence of sodium ascorbate and the catalyst colour remains unchanged. These results
indicate the formation of Cuꢀacetylide species A by the reaction of Cu(I) and alkyne. Similar
IR studies were reported by Uozumi and coworkers with their catalyst to establish the
formation of Cuꢀacetylide complex during Cu(I)ꢀcatalyzed azide–alkyne cycloaddition
4
4
reaction. In the XPS spectra of greenish yellow colour solid (Cuꢀacetylide species A)
binding energy peaks appeared at a value different from those of CuO, suggesting that the
4
5
new peaks could be assigned to Cu(I)ꢀacetylide species (Fig. S4–S5). Later, the azide attack
the Cuꢀacetylide A via 1,3ꢀdipolar cycloaddition followed by protonation to yield the triazole
and regeneration of the Cu(I) catalyst. Although, more recently various kinetic
21a,46
47
experiments
and computation studies have indicated the formation of bis(copper)
complex as intermediate in the Cu(I) catalyzed azideꢀalkyne cycloaddition. In a recent study,
Bertrand and coworkers have isolated the monoꢀ and bisꢀcopper acetylide complexes and
demonstrated that although both species are active in the catalytic cycle, the dinuclear
4
8
complex is involved in the kinetically favoured pathway. The author’s also highlighted the
role of anionic ligands (LCuX; anionic ligand X= Cl, OAc, OPh. OTf etc.) in each individual
step of Cu catalyzed azideꢀalkyne cycloaddition reaction. Based on the postulated mechanism
2
(Scheme 2), relatively low yield observed in case of –NO substituted azide (3c, 67%) could
1
0

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be justified. During the step c (Scheme 2) the azide coordinate with the complex A, the
presence of electron withdrawing substituents on the azide will withdraw the electron density
on the azide nitrogen and will decrease the reactivity.
Scheme 2. Plausible catalytic pathway for alkyne–azide cycloaddition catalysed by CuO
nanostructures.
Reusability study of the catalyst (CuOꢀNW)
The reusability of the catalyst (CuOꢀNW) was demonstrated by a set of experiments between
phenylacetylene and pꢀmethoxyphenyl azide (Table 3). After the first cycle, the catalyst was
separated by centrifugation, washed with water and ethanol and dried in oven. Further, the
recovered catalyst was used in a fresh reaction batch under the same reaction conditions. In a
similar way the catalyst was recovered and used for at least 3 cycles without considerable
loss in catalytic activity. The slight loss in catalytic activity may be due to loss of catalyst
during washing, recovery process and leaching of the catalyst. To ascertain the amount of
leached catalyst in the reaction mixture atomic absorption spectrophotometry (AAS) analysis
was carried out. The AAS analysis showed that only 0.33 ppm of Cu leached after the
reaction. To investigate whether the leached metal ions from the catalyst have contributed in
catalytic activity, we have examined the reaction with or without catalyst. The click reaction
1
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was performed under optimized reaction condition but the catalyst was removed after 10
minutes by filtration. The reaction was again started and further conversion was monitored
for another 3 hours. As observed in Fig. S6, no significant gain in triazole yield was observed
to rule out any considerable homogeneous contribution in catalytic activity. Thus, the
reaction is essentially catalyzed by solid catalyst and the leached metal ions did not contribute
to catalytic activity.
Table 3. Reusability studies with CuOꢀNW as catalyst in cycloaddition reaction between
phenylacetylene and pꢀmethoxyphenyl azide
a
Entry Run/cycle
Product
3b
Time (h) Yield (%)
st
1
2
3
1 run
1
1
94
89
78
nd
2 run
3b
rd
3 run
3b
1.5
a
Yields were of isolated and purified products.
Reaction conditions: 1.0 equiv. of phenylacetylene, 1.1 equiv. of pꢀmethoxy phenyl azide,
mol% catalyst, 0.1 equiv. sodium ascorbate in H O/tꢀBuOH (3:1) at room temperature.
b
5
2
Conclusions
In conclusion, we have synthesised CuO nanostructures of variable shapes (CuOꢀNS, CuOꢀ
NR and CuOꢀNW) and demonstrated the effect of shape of CuO nanostructures on the
catalytic efficiency in azide–alkyne cycloaddition. We observed that nanocatalysis is shape
dependent and anisotropy in the particle shape showed dominance over the isotropic particles
and hence CuOꢀNW and CuOꢀNR were found to be more active than the isotropic CuOꢀNS.
The catalyst could be easily recovered and reused upto three cycles in azide–alkyne
cycloaddition without considerable loss in catalytic efficiency.
Experimental Section
Chemicals: Cupric nitrate [Cu(NO
3
)
2
·3H
2
3 2
O], cupric acetate [Cu(CH COO) ], glacial acetic
acid (CH COOH), sodium hydroxide (NaOH), ethanol (CH
3
3
CH OH) and aqueous ammonia,
2
tertiary butanol, sodium ascorbate, phenylacetylene were purchased from Sigma Aldrich and
used without further purification. Various aryl azides were prepared from the corresponding
4
9
aryl amines by using the standardized literature procedure. Deꢀionized water (DI water) was
1
2

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obtained using an ultrafiltration system (MilliꢀQ, Milipore) with a measured conductivity of
−1
o
3
5 mho cm at 25 C.
Synthesis of CuO nanostructures
Schematic representation of synthesis of various CuO nanostructures (NS, NR and NW) has
been shown in Scheme 3.
Synthesis of CuOꢀNS: In a roundꢀbottomed flask, an aqueous solution of 0.02 M
o
Cu(CH
3
COO)
2
was mixed with 1 mL of glacial acetic acid and refluxed at 100 C for 3 h with
vigorous stirring. The reaction mixture was maintained at pH 6ꢀ7 with the addition of 0.1 M
5
0
NaOH solution. On addition of NaOH black precipitates were formed and the reaction
mixture was allowed to cool down. After cooling, the precipitates were centrifuged and
washed with ethanol and water and dried in oven for 12 h.
3 2 2
Synthesis of CuOꢀNR: A 0.03 M solution of Cu(NO ) .3H O in 40 mL ethanol was added
slowly into 40 mL ethanolic NaOH (0.5 M) solution in a roundꢀbottomed flask and refluxed
o
at 80 C. After about 20 minutes, 60 mL of DI water was added to the above reaction mixture
o
51
dropwise. The resultant product was refluxed together with mother liquor at 80 C for 24 h.
The black precipitates obtained were filtered and washed with DI water and ethanol several
times; the solid sample was dried under vacuum overnight at room temperature.
5
2
3 2 2
Synthesis of CuOꢀNW: In a 100 mL DI water 1g of Cu(NO ) .3H O was dissolved and
then 30 mL aqueous ammonia solution was added with constant stirring. The pH of the above
solution was maintained at 9ꢀ10 by dropwise addition of 1 M NaOH solution. The blue
2
coloured Cu(OH) precipitate formed were filtered and washed several times with ethanol.
The solid product was added to 30 mL DI water, sealed in a Teflonꢀlined stainless autoclave
o
and kept at 130 C for 10 h. Finally, black precipitates of CuO were washed with DI water
o
and ethanol and dried at 60 C overnight.
1
3

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Scheme 3. Schematic representation for the synthesis of CuOꢀNS, CuOꢀNR and CuOꢀNW.
Characterisation of CuOꢀNS, CuOꢀNR and CuOꢀNW: The crystallographic information of
the samples were investigated by powder Xꢀray diffraction (XRD) using a Panalytical’s
1
4

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X’Pert Pro with Cu Kα (λ = 1.5406 Å) radiation. The absorbance spectra were recorded by
UVꢀvisible spectrophotometer (Perkin Elmer). The morphological characterisation has been
performed by Transmission electron microscopy (TEM) (Hitachi 7500 model). The Xꢀray
photoelectron spectroscopy (XPS) measurements were carried out using a ESCA+ (Omicron
nanotechnology, Oxford instrument Germany) equipped with monochromator Aluminium
Source (Al Kα radiation; hν = 1486.7 eV). The instrument was operated at 15 kV and 20 mA
with pass energy of 20 eV (for short scans) and 50 eV (for survey scans). The Brunauerꢀ
EmmettꢀTeller (BET) surface area was measured by (Smart Sorb 92ꢀ93) using 150 mg of
o
sample preheated at 150 C for 1h. The metal ion concentration was estimated by atomic
absorption spectrophotometry (AAS) (GBC 932 AA, Australia).
General Experimental: All reactions were monitored by thin layer chromatography (TLC)
using an appropriate solvent system for development. Yields reported are isolated yields. IR
spectra were recorded with Bruker ALPHA FTꢀIR spectrometer. The coupling constants (J)
are given in hertz (Hz) and chemical shifts are reported in parts per million (ppm). The
1
abbreviations s, d, t and m refer to singlet, doublet, triplet and multiplet respectively. H
1
3
NMR (500 MHz, CDCl or DMSOꢀd6) and C NMR (125 MHz, CDCl or DMSOꢀd6)
3
3
spectra were recorded on a Bruker NMR spectrometer. HRMS data were recorded with
electrospray ionization (ESI) on Bruker MaXis Impact spectrometer. Melting points were
recorded with a Perfit apparatus.
General procedure for [3+2] cycloaddition of azides and terminal alkynes catalysed by
CuO nanostructures of variable shapes
A 25 mL round bottom flask was charged with CuO nanostructures (5 mol%), sodium
ascorbate (0.1 equiv), phenylacetylene (1.0 equiv.), and azide (1.1 equiv.) in H
2
O/tꢀBuOH
(3:1). The resulting mixture was stirred at rt, until TLC indicated completion of reaction. The
heterogenous catalyst was recovered by centrifugation. The mixture was diluted with water
and ethyl acetate. The organic layer was separated, and the aqueous phase was extracted with
ethyl acetate (2 × 10 mL). The combined organic layer was washed with aq NH
4
OH (0.2%)
and brine, later dried over Na SO and concentrated in vacuo to give the corresponding
2
4
triazole product. The crude product was easily purified by flash chromatography on silica gel.
We observed good conversions and yields for variety of azide–alkyne combinations (Table
1
13
2
). The triazoles were characterized by H NMR, C NMR and HRMS.
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Preparation of compound 3a: According to the general procedure, alkyne 1 (40 mg, 0.39
mmol), phenyl azide 2a (51.2 mg, 0.43 mmol), CuO nanostructures
(
NS/NR/NW) (1.5 mg, 0.02 mmol, 5 mol%) and sodium ascorbate
7.7 mg, 0.04 mmol) in H O/tꢀBuOH (3:1)was stirred at rt till the
(
2
completion of the reaction (2h with CuOꢀNS, 2h with CuOꢀNR and 1h with CuOꢀNW
respectively). The crude reaction mixture was purified by column chromatography (10%
ethyl acetate/petroleum ether) to give the desired compound 3a as a white solid (73.3 mg.
85%) with CuOꢀNS, (76.8 mg, 89%) with CuOꢀNR and (86.3 mg, 100%) with CuOꢀNW
o
40
respectively. R
f
: 0.53 (20% ethyl acetate/ petroleum ether). Mp: 171–173 C. (Lit. 170–
1
1
72). H NMR (500 MHz, CDCl
3
): δ = 8.20 (s, 1H), 7.91 (d, J = 7.4 Hz, 2H), 7.79 (d, J =
13
7
.9Hz, 2H), 7.55 (t, J = 7.6 Hz, 2H), 7.47ꢀ7.45 (m, 3H), 7.37 (t, J = 7.4 Hz, 1H) ppm. C
NMR (125 MHz, CDCl
3
): δ = 117.84, 120.72, 126.04, 128.62, 128.97, 129.11, 129.97,
+
1
30.39, 137.24 ppm. HRMS (ESI) m/z: calcd. for C H N [M+H] 222.1026, found:
14
12
3
222.1026.
Preparation of compound 3b: According to the general procedure, alkyne 1 (40 mg, 0.39
mmol), pꢀmethoxyphenyl azide 2b (63.9 mg, 0.43 mmol),
CuO nanostructures (NS/NR/NW) (1.5 mg, 0.02 mmol, 5 mol%)
and sodium ascorbate (7.7 mg, 0.04 mmol) in H O/
2
tꢀBuOH (3:1) was stirred at rt till the completion of the reaction (2h with CuOꢀNS, 1.5h with
CuOꢀNR and 1h with CuOꢀNW respectively). The crude mixture was purified by column
chromatography (20% ethyl acetate/ petroleum ether) to give the desired compound 3b as a
white solid in (78.4 mg, 80%) with CuOꢀNS, (85.3 mg, 87%) with CuOꢀNR and (93.1 mg,
95%) with CuOꢀNW respectively. R
f
: 0.32 (15% ethyl acetate/ petroleum ether). Mp: 167–
o
40
o
1
1
69 C. (Lit. 167–168 C). H NMR (500 MHz, CDCl ): δ = 8.11 (s, 1H), 7.89 (d, J = 7.15
3
Hz, 2H), 7.67 (d, J = 8.9 Hz, 2H), 7.44 (t, J = 7.45 Hz, 2H), 7.35 (t, J = 7.4 Hz, 1H), 7.02 (d,
1
3
J = 8.9 Hz, 2H), 3.86 (s, 3H) ppm. C NMR (125 MHz, CDCl
3
): δ = 55.78, 114.93, 118.04,
122.31, 125.96, 128.48, 129.05, 130.47, 130.63, 148.33, 159.98 ppm. HRMS (ESI) m/z:
+
14 3
calcd. for C15H N O [M+H] 252.1131, found: 252.1131.
Preparation of compound 3c: According to the general procedure, alkyne 1 (40 mg, 0.39
mmol), pꢀnitrophenyl azide 2c (70.5 mg, 0.43 mmol),
CuO nanostructures (NS/NR/NW) (1.5 mg, 0.02 mmol, 5
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6

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2
mol%) and sodium ascorbate (7.7 mg, 0.04 mmol) in H O/tꢀ
BuOH (3:1) was stirred at rt till the completion of the reaction (4h with CuOꢀNS, 4h with
CuOꢀNR and 3h with CuOꢀNW respectively). The crude mixture was purified by column
chromatography (20% ethyl acetate/ petroleum ether) to give the desired compound 3c as a
pale yellow solid in (41.5 mg, 40%) with CuOꢀNS, (60.2 mg, 58%) with CuOꢀNR and (69.6
f
mg, 67%) with CuOꢀNW respectively. R : 0.46 (20% ethyl acetate/ petroleum ether). Mp:
o
41
o
1
2
35–237 C. (Lit. 236–238 C) H NMR (500 MHz, DMSOꢀd6): δ = 9.48 (s, 1H), 8.50ꢀ
8.48 (m, 2H), 8.27ꢀ8.25 (m, 2H), 7.96ꢀ7.95 (m, 2H), 7.53ꢀ7.51 (m, 2H), 7.42ꢀ7.39 (m, 1H)
13
ppm. C NMR (100 MHz, DMSOꢀd6): δ = 120.44, 121.00, 125.93, 126.14, 129.15, 129.59,
+
130.19 ppm. HRMS (ESI) m/z: calcd. for C14
H
11
N
4
O
2
[M+H] 267.0877, found: 267.0877.
IR experiments for the detection of {L•Cu(C≡CC
6
H
)}:
5
In a 10 mL round bottom flask, CuOꢀNW (12 mg, 0.12 mmol), phenylacetylene 1 (120 mg,
.17 mmol), and sodium ascorbate (48 mg, 0.35 mmol) was added in H O/tꢀBuOH (3:1) as
1
2
solvent. The reaction vessel was stirred at room temperature for 30 min. The colour of the
CuOꢀNW gradually changes from black to greenish yellow. The reaction mixture was
filtered, and the resulting greenish yellow solid was washed with water and hexane and dried
6 5
in vacuo to deliver the greenish yellow colour solid {L•Cu(C≡CC H )}, which shows a
ꢀ1
42
ꢀ1
43
ꢀ1
.44
vibrational absorption of Cu–C≡C at 1931 cm (Lit. 1930 cm , Lit. 1940 cm , Lit.
ꢀ1
1
936 cm ). On the other hand, the reaction of CuOꢀNW, phenylacetylene 1 in the absence of
sodium ascorbate didn’t show any change in colour of the catalyst and also no peak was
ꢀ1
observed at 1930–1940 cm in the IR spectrum.
Supporting Information
1
13
H/ C NMR and HRMS spectra of synthesised compounds and Fig. S1ꢀS6.
Acknowledgements
D. G. and B. G. gratefully acknowledges Science and Engineering Research Board (SERB),
Department of Science & Technology (DST), Government of India for the award of SERB
Young Scientists Research Grant (Sanction No: SERB/F/4167/2015ꢀ16 and SB/FT/CSꢀ
013/2014). A. K. is thankful to SERBꢀDST, India for the award of fellowship. The authors
acknowledge Department of Chemistry, Sri Guru Granth Sahib World University, Fatehgarh
Sahib, Punjab, India for providing the research facilities.
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Keywords
Azide–alkyne cycloaddition, triazoles, CuOꢀnanowires, CuOꢀnanorods, CuOꢀnanospheres
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Graphical abstract
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Page 23 of 23
RSC Advances
View Article Online
DOI: 10.1039/C6RA20725A
CuO nanowires exhibited highest catalytic efficiency for the cycloaddition reaction between
azide and terminal alkyne, featuring short reaction time, soft reaction conditions and
complete regioselectivity.
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