Green synthesis of 1,2,3-triazoles via Cu2O NPs on hydrogen trititanate nanotubes promoted 1,3-dipolar cycloadditions

Article abstract of DOI:10.1002/aoc.4752

Cu₂O nanoparticles supported on hydrogen trititanate nanotubes (Cu₂O/HTNT) catalysts have been efficiently catalyzed the multicomponent synthesis of 1,2,3-triazoles in water at room temperature from different azide precursors, for example organic halides, sulfonates and anilines. The catalysts were synthesized by hydrothermal & wet-impregnation methods and was characterized by HR-TEM, EDS, XRD, XPS, N₂-adsorption desorption and ICP-MS analysis. The catalyst could be recycled by centrifugation and reused up to seven cycles. The 1-benzyl-4-(4-chlorophenyl)-1H-1,2,3-triazole (25) structure was proven by single crystal X-ray diffraction studies.

Full text of DOI:10.1002/aoc.4752

Received: 11 September 2018
DOI: 10.1002/aoc.4752
Revised: 2 November 2018
Accepted: 13 November 2018
F U L L P A P E R
Green synthesis of 1,2,3‐triazoles via Cu2O NPs on hydrogen
trititanate nanotubes promoted 1,3‐dipolar cycloadditions
Rajendra Prasad Reddy Bhoomireddy1 | L.G. Bhavani Narla2 |
Vasu Govardhana Reddy Peddiahgari1
1 Department of Chemistry, Yogi Vemana
Cu2O nanoparticles supported on hydrogen trititanate nanotubes (Cu2O/
HTNT) catalysts have been efficiently catalyzed the multicomponent synthesis
of 1,2,3‐triazoles in water at room temperature from different azide precursors,
for example organic halides, sulfonates and anilines. The catalysts were synthe-
sized by hydrothermal & wet‐impregnation methods and was characterized by
HR‐TEM, EDS, XRD, XPS, N2‐adsorption desorption and ICP‐MS analysis.
The catalyst could be recycled by centrifugation and reused up to seven cycles.
The 1‐benzyl‐4‐(4‐chlorophenyl)‐1H‐1,2,3‐triazole (25) structure was proven by
single crystal X‐ray diffraction studies.
University, Kadapa 516 003, Andhra
Pradesh, India
2 Department of Humanities and Sciences,
SV College of Engineering, Kadapa
516003, Andhra Pradesh, India
Correspondence
Vasu Govardhana Reddy Peddiahgari,
Department of Chemistry, Yogi Vemana
University, Kadapa ‐ 516 003, Andhra
Pradesh, India.
Email: pvgr@yogivemanauniversity.ac.in
KEYWORDS
Funding information
alkynes, copper nanoparticles, hydrogen trititanate nanotubes, multicomponent reactions, triazoles
UGC‐CSIR; CSIR, Grant/Award Number:
02(248)/15/EMR‐II
1 | INTRODUCTION
surface area and decrease the possibility of agglomera-
tion.[8] A number of solid supported copper nano cata-
1,4‐Disubstituted 1,2,3‐triazoles are privileged heterocy-
cles, displaying huge spectrum of biological properties
and are widely used as pharmaceuticals.[1] Many 1,2,3‐
triazoles have found medicinal applications, such as
anti‐bacterial,[2] anti‐HIV activity[1a, 3] and selective β3‐
adrenergic receptor agonism.[4] Also, 1,2,3‐triazoles are
found in fungicides, herbicides and dyes.[5] Copper cata-
lyzed azide‐alkyne cycloaddition (CuAAC) is the most
convenient protocol for the construction of these 1,2,3‐
triazole derivatives.[6]
Since the discovery of the CuAAC reaction, huge
efforts have been paid to maximize the general efficiency
of the reaction. For instance, a reduction in the quantity
of copper in solution gives priority, particularly for bio-
logical applications, due to its promising toxicity.[7] In
this sense, support materials in heterogeneous catalysts
offer several advantages over homogeneous catalysts, for
example mechanical strength, recovery and recycling, sta-
bility, activity and selectivity. They also help in uniform
distribution of the active catalytic particles over a large
lysts have been developed over the last few years for the
title click reaction.[9] Unfortunately, many of them suffer
from metal leaching after several cycles.[10] Furthermore,
most of the described methodologies involve pre‐formed
organic azides. In situ formation of organic azides in the
presence of the alkyne (three‐component CuAAC reac-
tion) minimizes the difficulties accompanying with the
explosive nature of azides. This transformation is very
interesting, especially when performed under green con-
ditions,i.e., in water.[9k–o] So, it is necessary to develop
new catalysts that are able to promote the three‐
component CuAAC reaction in water.
The advances in nanoscience and nanotechnology
offer new opportunities for the construction of more
effective, “green”, and sustainable nanostructured cata-
lytic systems. Among them, one dimensional hydrogen
trititanate nanotubes (HTNTs) identified as attractive cat-
alysts and supports due to its unique physicochemical
properties viz., a large surface‐to‐volume‐ratio, good
dispersibility and increased accessibility of surface
Appl Organometal Chem. 2018;e4752.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
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BHOOMIREDDY ET AL.
hydroxyl groups, Lewis and Brønsted acid sites.[11] Also,
the HTNT surface is highly hydrophilic (due to the pres-
ence of hydroxyl groups) which stabilizes the dispersion
of the HTNT in water, which facilitate the deposition of
metal or metal oxide NPs on the HTNT surface. More-
over, the synergetic activity of HTNT and metal NPs
enhances the stability and catalytic activity of metal NPs
on HTNT. Nowadays, HTNTs have been extensively used
in several organic transformations as solid‐acid catalyst
(in Friedel‐crafts alkylation,[11a] acetylation[12] and aldol
condensation[13]) and catalyst support (in cross‐coupling
reactions,[14] hydrogenation,[15] oxidation[16] and epoxi-
dation[17]). Inspired by the importance of HTNT as
catalyst and support in organic synthesis, we prepare
the Cu2O nanoparticles (Cu2O NPs) supported on
HTNT (Cu2O/HTNT) catalysts and explore their catalytic
performance in water mediated three‐component CuAAC
reaction.
In this present study, we have conducted the three‐
component CuAAC reaction in water at room tempera-
ture using novel Cu2O loaded HTNTs heterogeneous
catalysts. To our delight, the scope of the reaction was
tested with different azide precursors, such as halides,
sulfonates and anilines (Tables 2 and 3 and Scheme 1).
2 | RESULTS AND DISCUSSIONS
To test the activity of prepared Cu2O/HTNT catalysts in
CuAAC reaction, we have chosen the benzyl chloride
and phenylacetylene as model substrates. Remarkably,
in the first attempt the reaction has given good yield with
Cu2O/HTNT‐5 in the water (Table 1, entry 1). Among the
trialed catalysts, Cu2O/HTNT‐7 was found to be the best
catalyst (Table 1, entry 3). To optimize the amount of
the catalyst, we performed a set of reactions with differ-
ent amounts of catalysts (Table 1, entry 10–12). 20 mg
of the Cu2O/HTNT‐7 was enough for this reaction
(Table 1, entry 11). The superior catalytic activity of
SCHEME 1 Cu2O/HTNT‐7 catalyzed CuAAC reaction with
different azide precursors
TABLE 1 Catalytic activities of copper catalysts for CuAAC reactiona
S.No
Catalyst
Catalyst loading (mg)
Copper content (mg)
Cu2O content (mol%)
t (h)
Yield (%)b
1
Cu2O/HTNT‐5c
Cu2O/HTNT‐6d
Cu2O/HTNT‐7e
Cu2O/HTNT‐8f
Cu2O Nps
50
50
50
50
50
50
50
50
‐‐‐
25
20
15
2.5
3.0
0.4
0.47
0.55
0.63
35
8.0
7.0
79
91
98
98
50
21
43
00
00
98
98
91
2
3
3.5
5.0
4
4.0
5.0
5
44.5
40.0
16.7
‐‐‐
24.0
24.0
24.0
24.0
24.0
5.0
6
CuO Nps
63g
7
CuI
26h
8
HTNT
‐‐‐
9
‐‐‐
‐‐‐
‐‐‐
10
11
12
Cu2O/HTNT‐7e
Cu2O/HTNT‐7e
Cu2O/HTNT‐7e
1.75
1.4
0.28
0.22
0.17
5.0
1.05
5.0
aBenzyl chloride (5.0 mmol), sodium azide (6.0 mmol) and phenylacetylene (5.0 mmol) in water (5.0 mL) stirred at room temperature.
bIsolated yield.
c5 wt% copper loaded HTNT.
d6 wt% copper loaded HTNT.
e7 wt% copper loaded HTNT.
f8 wt% copper loaded HTNT.
gCuO mol%.
hCuI mol%.

BHOOMIREDDY ET AL.
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FIGURE 1 (a) HR‐TEM image of HTNT, (b, c) HR‐TEM image of Cu2O/HTNT‐7 catalyst, (d) SAED pattern of Cu2O NPs taken from b, (e)
size distribution of the Cu2O NPs (f) recovered Cu2O/HTNT‐7 after seven cycles
FIGURE 2 EDS analysis of Cu2O/
HTNT‐7 catalyst
Cu2O/HTNT‐7 may be due to high dispersion of the cata-
lyst in water, accompanied by interaction of alkynes with
Cu2O NPs.
of 4.6 nm (see size distribution Figure 1e). The selected
area electron diffraction (SAED) pattern taken from the
nanoparticles in Figure 1b revealed that Cu2O NPs were
polycrystalline (Figure 1d).
Different techniques were chosen for the characteriza-
tion of the optimized catalyst. HR‐TEM image of HTNT
(Figure 1a) showed a bundle of one dimensional ran-
domly aligned tubes having length ranging between
200–400 nm with open ends. The HR‐TEM image of sin-
gle nanotube (Figure 1a, inset) displays cylindrical shape
with hollow inside. It consists of 4–5 layers on the wall;
inner and outer diameters of the tubes have 3–5 nm and
8–12 nm, respectively. The HR‐TEM image of Cu2O/
HTNT‐7 demonstrated that Cu2O NPs were effectively
formed on the HTNT (Figure 1b and 1c) and the Cu2O
NPs were in spherical shape with an average diameter
Figure 2 reveals the standard energy‐dispersive spec-
trum (EDS) taken on the Cu2O/HTNT‐7. Only Ti, O
and Cu elements were observed, which point out that
these nanoparticles could be copper oxide. The Cu, Ti
and O atomic ratio were estimated to be 3.58%, 31.35%
and 65.07%, respectively. This information is not enough
to figure out that the nanoparticles contain only Cu2O.
Thus, an additional characterization was needed.
XRD data were taken to analyze the crystal identity of
HTNTs and also the effect of Cu2O on the crystal struc-
ture of HTNTs (Figure 3). XRD pattern of HTNT shows

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BHOOMIREDDY ET AL.
FIGURE 5 N2‐adsorption desorption isotherm of Cu2O/HTNT‐7
catalyst and corresponding BJH pore size distribution curve (inset)
FIGURE 3 XRD pattern of HTNT, Cu2O/HTNT‐7 and recovered
Cu2O/HTNT‐7 after seven cycles
interposition of Cu2O in the HTNTs has no impact on
the crystal structure of HTNTs, the core structure is
protected.
a series of weak and broad diffraction peaks with 2θ
values 24.5 (202), 27.7 (112) and 48.6 (303), corresponding
to the monoclinic H2Ti3O7 structure (JCPDS No. 47–
0561).[18] The specific diffraction peak at 2θ = 10.0 (001)
depicts the presence of layered crystal structure. In addi-
tion, there are three peaks with 2θ values at 36.6, 42.2
and 61.7 in Cu2O/HTNT‐7, corresponding to (111), (200)
and (220) crystal planes of Cu2O, respectively (JCPDS
No. 88–1175).[18] Obviously, Cu and CuO phases are not
present in Cu2O/HTNT‐7. The comparison of XRD
patterns of HTNTs and Cu2O/HTNT‐7 denotes, the
The XPS survey spectra of the Cu2O/HTNT‐7 con-
firmed the presence of Ti, O and Cu elements, as shown
in Figure 4. In the high resolution Ti 2p spectrum
(Figure 4b), the two peaks are placed at binding energies
of 458.8 and 464.4 eV. These peaks can be owing to the Ti
2p3/2 and Ti 2p1/2 spin‐orbital splitting photoelectrons of
Ti
.
4+ [19] A peak at 529.8 eV in the XPS spectrum of O 1 s
(Figure 4c) was ascribed to O 1 s. In high resolution Cu
FIGURE 4 XPS survey spectrum of Cu2O/HTNT‐7 catalyst

BHOOMIREDDY ET AL.
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TABLE 2 Click synthesis of 1,2,3‐triazoles catalyzed by Cu2O/HTNT‐7 with organic halides as the azide precursorsa
Unless otherwise stated all the compounds are prepared from chlorides.
aHalide (5.0 mmol), sodium azide (6.0 mmol), alkyne (5.0 mmol) and Cu2O/HTNT‐7 (20.0 mg) in water, stirred at room temperature for 5 h. Isolated yields in
parentheses.
breactions conducted with chloride and bromide, when X = Br the reaction was completed in 2.5 hr.
cReaction at 50 °C.
FIGURE 6 ORTEP view of compound
25

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BHOOMIREDDY ET AL.
TABLE 3 Click synthesis of 1,2,3‐triazoles catalyzed by Cu2O/HTNT‐7 with organic sulfonates as the azide precursorsa
aBenzyl sylfonate (5.0 mmol), sodium azide (6.0 mmol), alkyne (5.0 mmol) and Cu2O/HTNT‐7 (20.0 mg) in water stirred at room temperature for 8 h. Isolated
yields in parentheses.
bX = Ioslated yields of triazole prepared from mesylate, triflate and tosylate respectively.
TABLE 4 Recyclability and reusability of Cu2O/HTNT‐7
method. The lowering of the surface area is in agreement
with observed structural transformations (Figure 1) and
previous data.[22]
Catalysta
Cycle No.
1st
2nd
3rd
4th
5th
6th
7th
Under the optimized conditions, we examined the
scope of the CuAAC reaction employing halides and
alkynes (Table 2). Benzyl bromide reacted faster than
the chloride counterpart although in excellent yield in
both cases (4). Electron‐withdrawing or electron‐donating
substituents on benzyl chloride afforded corresponding
triazoles in near quantitative yields (5–14). Ethyl
α‐bromoacetate (an activated organic halide) also offers
an excellent yield (15). Cinnamyl bromide or chloride
readily participated in the CuAAC reaction, leading to
16 with 95% yield. Deactivated alkyl halides worked quite
well in the title reaction, but it requires heating at 50 °C
(17–19). The CuAAC reaction was also attempted with
3‐(bromomethyl) thiophene, 2‐(bromomethyl) thiophene
and 2‐(bromomethyl) pyridine, the former substrate was
disappointed (20–22), It quickly gave a complicated
mixture of side‐products (not identified), turning the
reaction mixture black and viscous. Furthermore,
phenylacetylenes bearing electron‐withdrawing (4‐CN,
4‐NO2, 4‐Cl) and electron‐donating (4‐Me, 4‐MeO)
groups, 2‐ethynylpyridine, 1‐octyne, 5‐hexynenitrile and
N,N‐diethyl‐2‐butyn‐1‐amine reacted smoothly with
excellent yields (23–31). Likewise, bistriazole was formed
in 83% yield from 1,6‐heptadiyne and two equivalents of
benzyl chloride (32).
Yield (%)b
98
98
96
95
95
93
91
aBenzyl chloride (5.0 mmol), sodium azide (6.0 mmol), phenylacetylene
(5.0 mmol) and Cu2O/HTNT‐7 (20.0 mg) in water stirred at room tempera-
ture for 5 hr.
bIsolated yield.
2p spectrum (Figure 4d), there are only two peaks posi-
tioned at binding energies of 932.8 eV and 951.8 eV.[20]
These peaks can be attributed to the Cu 2p3/2 and Cu
2p1/2 spin‐orbital splitting photoelectrons of Cu+.
Furthermore, the Cu 2p1/2 and Cu 2p3/2 satellite peaks
for Cu2+ were not observed, that provides clear evidence
for the successful coating of solely Cu2O on the surface
of HTNT.
Figure 5 illustrates the N2‐adsorption desorption iso-
therm and pore size distribution of the Cu2O/HTNT‐7.
The adsorption desorption isotherm corresponds to the
classical type IV curve with an H3 hysteresis loop that
evidences the presence of a mesoporous framework in
the Cu2O/HTNT‐7. Cu2O/HTNT‐7 having a lower BET
surface area and reduced average pore diameter of
HTNTs 210 m2.g−1 and 6.2 nm, respectively correspond-
ing to the HTNT (286 m2.g−1 and 10.5 nm; taken from our
previous publications[21]) synthesized from hydrothermal

BHOOMIREDDY ET AL.
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The structure of compound 25 was proven by a single
crystal X‐ray crystallography (Figure 6). The crystals
were grown in ethanol by slow evaporation technique.
The X‐ray intensity data were collected on a Bruker
Kappa APEX II diffractometer using Mokα radiation at
T(K) = 150.0(1). From the diffraction data, it was
observed that the title molecule C15H12ClN3 crystallizes
in monoclinic with C 2/c space group with unit cell
dimensions, a = 17.5929(11) Å, b = 5.7024(3) Å,
c = 25.6144(16) Å and Z = 8. The crystal data of the
compound are shown in Table S1 (See supporting
information).
hydrogen trititanate nanotubes in water at room temper-
ature. A variety of azide precursors, such as halides,
amines and sulfonates could be successfully formed the
corresponding 1,2,3‐triazoles in high yields. The catalyst
is reusable and promotes the CuAAC reaction by acting
as a heterogeneous catalyst. Further research to extend
the substrate scope and better understand the catalysis
is under way and will be reported eventually.
4 | EXPERIMENTAL SECTION
4.1 | Hydrothermal synthesis of HTNT
In order to widen the applicability of this method, we
wish to use benzyl mesylates as azide precursors. Due to
the broad existence of alcohols in nature and in the
synthetic world, they have drawn chemist's attention.
We were delighted to introduce sulfonates derived from
alcohol as the azide precursors in CuAAC reaction. These
participated quite nicely in this reaction and gave good
yields, despite reacting slower than halides (Table 3).
Lastly, we explored the feasibility of the method
using aryl azide precursors instead of activated azide
precursors. The four‐component reaction between
phenylacetylene and in situ generated aryl azide (from
aromatic amine, t‐BuONO and together with NaN3)
worked similarly well at 80 °C (Scheme 1). Aryl iodide
also reacted smoothly in the title reaction with 0.3 eq of
DBU as a base at 70 °C (Scheme 1). The successful
transformation of these reactions strongly reflected the
efficient and promising application of Cu2O/HTNT‐7
catalyst.
Recyclability and reusability of Cu2O/HTNT‐7 were
carried out under optimized reaction conditions and also
the catalyst was retrieved via centrifugation. Pure catalyst
was obtained, washing thrice with water, followed by
drying in a hot air oven at 100 °C for 12 hr. The retrieved
catalyst was reused 7 times without any appreciable
change in its catalytic activity (Table 4). Very negligible
leaching of copper (≤ 1 ppm) was observed by ICP‐MS
analysis. Further, to check the morphology and crystal
structure of recycled catalyst, after 7th cycle characterized
by HR‐TEM and XRD. HR‐TEM images reflected the
unchanged morphology of the catalyst (Figure 1f) and
no variations in crystal structures of HTNT and Cu2O
were detected from XRD (Figure 3). This recycling
experiment recommends that, no changes in the active
Cu2O and strong interaction with the HTNT support.
Hydrogen trititanate nanotubes were prepared via a
hydrothermal method. Typically, 6 g anatase TiO2
particles were dispersed in NaOH aqueous solution
(10 M, 80 ml) and the mixture was sonicated for 30 min
and then transferred into a 100 mL teflon‐lined stainless
steel autoclave and kept at 150 °C for 48 hr. The white
precipitate in the autoclave was washed repeatedly with
deionized water followed by the precipitates dispersed
in 0.1 N HCl (100 ml) under stirring for 24 hr, and then
collected by centrifugation and washed repeatedly with
deionized water and dried at 80 °C for 12 hr to obtain
hydrogen trititanate nanotubes (HTNTs).[23]
4.2 | Synthesis of Cu2O/HTNT catalysts
The Cu2O doped HTNTs were synthesized by impregna-
tion method. Typically, 0.5 g of HTNTs was suspended
with Cu (NO3)2.3H2O dissolved in 20 ml of distilled water
and placed in 50 ml glass beaker. The resulting mixture
was magnetically stirred for 24 hr at room temperature
and then heated at 100 °C for 4 hr to remove water.
The acquired powder was crushed with a mortar and kept
for dry at 80 °C for 12 hr. The samples with calculated Cu
weight percentages of 5%, 6%, 7% and 8% were prepared
and denoted as Cu2O/HTNT‐ Cu wt% depending on the
copper amount.[23]
4.3 | General procedure for Cu2O/HTNT‐7
catalyzed Click reaction
To a suspension of Cu2O/HTNT‐7 (20 mg) in water (5.0 ml)
were successively added NaN3 (390 mg, 6.0 mmol),
the azide precursor (organic sulfonate/halide, 5.0 mmol)
and the alkyne (5.0 mmol). The reaction medium was
stirred at room temperature for 5 hr/halides and
8 hr/sulfonates. Centrifugation was employed to separate
the catalyst and washed thrice with water and dried at
100 °C in hot air oven for 12 hr. The resulting solution
3 | CONCLUSIONS
In conclusion, a novel approach for the CuAAC reaction
has been developed using a Cu2O NPs supported on

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BHOOMIREDDY ET AL.
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over Na2SO4 and concentrated under vacuum to furnish
the resulting 1,2,3‐triazole as pure product that does not
require any further purification. The prepared triazoles
were structurally assigned by their NMR (1H & 13C)
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ACKNOWLEDGEMENTS
We are grateful for the financial support from CSIR
(02(248)/15/EMR‐II). B.R.P. Reddy thanks to UGC‐CSIR
for a Junior Research Fellowship. We are thankful to
Prof. M. V. Shankar, Materials and Nanotechnology,
Yogi Vemana University, Kadapa for his fruitful scientific
discussions on nano catalysts characterization.
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CONFLICT OF INTEREST
All authors to declare that there are no conflicts.
ORCID
Vasu Govardhana Reddy Peddiahgari
https://orcid.org/
0000-0001-8837-0847
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How to cite this article: Bhoomireddy RPR,
Narla LGB, Peddiahgari VGR. Green synthesis of
1,2,3‐triazoles via Cu2O NPs on hydrogen
trititanate nanotubes promoted 1,3‐dipolar
cycloadditions. Appl Organometal Chem. 2018;
e4752. https://doi.org/10.1002/aoc.4752