Surface enriched palladium on palladium-copper bimetallic nanoparticles as catalyst for polycyclic triazoles synthesis

Article abstract of DOI:10.1016/j.jcat.2019.07.063

Transition-metal nanoparticles (NPs) gained interest as a catalyst due to their high reactivity and recyclability. Monometallic NPs are less catalytic active compared to homogeneous counterparts as less number of catalytic active metal atoms are present on the surface of NPs and metal atoms present in the core part of the NPs are not utilized. It is necessary to synthesize a stable, reusable, highly reactive and economic catalyst which contain a more number of catalytic active precious metal atoms on the surface of NPs. Here in we have synthesized metal-carbon bond stabilized, alloy structured Pd/Cu bimetallic nanoparticles using binaphthyl moiety as a stabilizer (Pd/Cu-BNP). The enrichment of catalytically active precious Pd atoms over inexpensive Cu on the surface was observed with a narrow size distribution of the nanoparticles (average diameter 3 ± 0.5 nm). The Pd/Cu-BNP was used as an efficient and recyclable catalyst for the synthesis of polycyclic triazoles through domino alkyne insertion and C[sbnd]H bond functionalization reaction sequence.

Full text of DOI:10.1016/j.jcat.2019.07.063

Journal of Catalysis 377 (2019) 673–683
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Surface enriched palladium on palladium-copper bimetallic
nanoparticles as catalyst for polycyclic triazoles synthesis
⇑
Rajib Saha, Dhanarajan Arunprasath, Govindasamy Sekar
Department of Chemistry, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Transition-metal nanoparticles (NPs) gained interest as a catalyst due to their high reactivity and recycla-
bility. Monometallic NPs are less catalytic active compared to homogeneous counterparts as less number
of catalytic active metal atoms are present on the surface of NPs and metal atoms present in the core part
of the NPs are not utilized. It is necessary to synthesize a stable, reusable, highly reactive and economic
catalyst which contain a more number of catalytic active precious metal atoms on the surface of NPs.
Here in we have synthesized metal-carbon bond stabilized, alloy structured Pd/Cu bimetallic nanoparti-
cles using binaphthyl moiety as a stabilizer (Pd/Cu-BNP). The enrichment of catalytically active precious
Pd atoms over inexpensive Cu on the surface was observed with a narrow size distribution of the
nanoparticles (average diameter 3 ± 0.5 nm). The Pd/Cu-BNP was used as an efficient and recyclable cat-
alyst for the synthesis of polycyclic triazoles through domino alkyne insertion and CAH bond function-
alization reaction sequence.
Received 11 March 2019
Revised 23 July 2019
Accepted 31 July 2019
Available online 27 August 2019
Dedicated to Prof. P. Selvam on the occasion
of his 60th birthday.
Keywords:
Bimetallic nanoparticles
Palladium
Copper
Ó 2019 Published by Elsevier Inc.
Nano catalysis
Polycylic triazoles
CAH functionalization
1
. Introduction
Transition-metal based nanoparticles have received much
beneficial synergistic effect between the electronic and geometric
properties which attributes its unique enhanced selectivity, reactiv-
ity and stability of the bimetallic nanoparticles (BMNPs) [10–16].
Moreover, these BMNPs are known to have better stability, robust-
ness and less susceptibility to aggregation and deactivation during
the catalytic process compared to the corresponding monometallic
counterparts. The continuous research in this field has led to the
development of several BMNPs having specific morphologies such
as random alloy, core-shell and porous structure [17–25]. For exam-
ple, a bimetallic Rh/Fe nanoparticles stabilized by phenylazome-
thine dendrimers (TPP-DPA G4) was elegantly synthesized and
utilized for hydrogenation of olefins and nitroarenes [26].
The concept of covalent stabilization of nanoparticles was
recently pioneered by Mirkhalaf and co-workers for Au and Ag
monometallic NPs [27]. However, the synthesis of BMNPs stabi-
lized by metal-carbon covalent bonds has not been attempted so
far. We questioned whether it could be possible to apply our
monometallic Pd-BNP synthesis strategy to achieve BMNPs. Being
non-toxic, cost-effective and environmentally benign, copper drew
our attention to be chosen as a second component to prepare the
economic BMNPs. Herein, we describe the synthesis of surface
enriched Pd on Pd/Cu-BNP which are stabilized by the simple
and effective binaphthyl moiety and its applications in the synthe-
sis of densely substituted tricylic triazoles under mild reaction
conditions.
attention recently due to their great potential in catalyzing various
types of organic transformations as well as being stable and reusa-
ble than traditional homogenous metal catalysts [1,2]. However,
most of the highly stable transition metal nanoparticles have not
been used as catalysts for organic synthesis as they are less
reactive. In this context, recently our research group reported a
Pd-C(binaphthyl) covalent bond stabilized palladium nanocatalyst
(
Pd-BNP) with a narrow size distribution which generally shown
efficient catalytic activity in annulation, CAH activation, reductive
Heck, reduction, and cross-coupling reactions [3–8]. The catalyst
was prepared by grafting binaphthyl moiety over Pd during the
2 4 4
reduction of K PdCl using NaBH in toluene as solvent.
In general, monometallic NPs are less catalytic active than its
homogeneous counterparts due to surface phenomenon, i.e. active
metal component present on the surface of NPs could only catalyze
the reaction [9]. The straightforward approach to improve the cat-
alytic properties of monometallic NPs is to include a second metal
to enrich the density of active sites on the surface of the catalysts.
The presence of hetero metal-metal interactions could induce the
⇑
Corresponding author.
E-mail address: gsekar@iitm.ac.in (G. Sekar).
https://doi.org/10.1016/j.jcat.2019.07.063
021-9517/Ó 2019 Published by Elsevier Inc.
0

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R. Saha et al. / Journal of Catalysis 377 (2019) 673–683
Fig. 1. Examples of biologically important fused 1,2,3-triazoles.
-
1
1
,2,3-Triazoles are important nitrogen containing heterocyclic
are free from the diazonium salt (Fig. 2a). The peaks at 3050 cm
ꢀ1
motifs due to its rich and intriguing biological and physical proper-
ties. Specifically, compounds containing fused triazoles have
become
substances such as chemotherapeutic and cardiovascular drugs
28–31]. Some of the biologically active molecules containing
fused 1,2,3-triazole ring are shown in Fig. 1. For these reasons,
more efforts have been devoted to develop different methods to
synthesize fused triazoles [32,33]. Lautens research group recently
reported synthesis of fused triazole through multi-component
strategy using Herrmann-Beller palladacycle precatalyst [34].
Despite of all the advances made to synthesize fused triazoles, still
efforts are required to develop more sustainable, milder, yet
environment-friendly approach to construct poly functionalized
triazoles.
and 2929 cm in Pd/Cu NPs are due to the stretching vibrations
of aromatic CAH bond. The peaks at 1130, 812, 745 and
are due to CAH bending and peaks at 1609 and
1500 cm are due to C@C vibrations, all of which confirmed the
ꢀ
ꢀ
1
a
key structural core of potential pharmacological
626 cm
1
[
presence of the binaphthyl moiety in the Pd/Cu NPs. The character-
ꢀ
1
istic peaks of CuO at 536 and 482 cm are absent in synthesized
Pd/Cu nanoparticles which implies that NPs is free from CuO. Fur-
ther, the presence of PdAC covalent bond in Pd/Cu-BNP was con-
ꢀ1
firmed, as the peak presence at 545 cm
which is known as
characteristic peak for C(sp2)-Pd bond stretching vibration [36]
(Fig. 2b).
In the 1H NMR, broad multiplet peaks were observed at 6.29–
8.30 ppm corresponds to aromatic CAH (Fig. 2c). In solid state
1
3
C NMR, a broad peak was found at 110–145 ppm which are cor-
responds to aromatic sp carbons (Fig. 2d). The both H NMR and
solid state C NMR results confirmed that the aromatic binaphthyl
moiety is present in Pd/Cu-BNPs nanoparticles.
2
1
1
3
2
. Results and discussion
The parallel beam X-ray diffraction was used to further confirm
the crystalline nature of Pd/Cu-BNP. Fig. 3 showed the Bragg
diffraction of Pd/Cu-BNP and broad peaks were observed due to
small particle size (2–5 nm). Distinct peaks were observed at
2
.1. Synthesis of bimetallic nanocatalysts
The preparation of Pd/Cu-BNP involves two step process follow-
ing our procedure for the monometallic Pd NPs as shown in
Scheme 1. Initially, 2,2 -binaphthalene-bisdiazonium tetrafluorob-
4
0.3, 47.1 and 69.0 which might be corresponded to the Pd
0
(
1 1 1), Pd(2 0 0) and Pd(2 2 0) crystal planes respectively (Fig. 3,
0
0
orate was prepared from the 1,1 -binaphthyl-2,2 -diamine
BINAM) using HBF and NaNO at 0 °C. Then, simultaneous reduc-
tion of Pd(OAc) and Cu(OAc) (1:1) using NaBH was carried out in
top). No credible peak was observed for the Cu in Pd/Cu-BNP which
illustrates that the surface is enriched with Pd atoms. The bimetal-
lic Pd/Cu-BNP showed a slight deviation of peak from monometal-
lic Pd-BNP due to the less atomic radius of Cu than Pd. Further,
using the Debye-Scherrer equation the particle size of Pd/Cu-BNP
was calculated and found the average particle size is 3.8 nm (see
SI, Fig. S7).
The high resolution TEM images of Pd/Cu-BNP (Fig. 4a) showed
that most of the particles are spherical in shape and well dispersed
without any apparent aggregation. The average size distribution of
Pd/Cu-BNP is 3.00 ± 0.5 nm (Fig. 4b). A well-defined lattice fringe of
0.22 nm was seen which is ascribed to the Pd/Cu (1 1 1) lattice
plane (c and d).
(
4
2
2
2
4
the presence of diazonium salt. The heat generated from the reduc-
tion leads to the generation of binaphthyl radical which stabilizes
the nanoparticles via a strong PdAC [3,27,35] and CuAC covalent
bond linkages. The resulting Pd/Cu-BNP was found to be highly
stable under air and no physical changes occurred over time.
2
.2. Characterization of synthesize Pd/Cu bimetallic nanocatalysts
1
The newly developed Pd/Cu-BNP was characterized using IR, H
1
3
NMR, C NMR, TEM, ICP, TGA, powder XRD and XPS analyses (See
SI for more details).
The comparison of FTIR spectra of the corresponding BINAM
diazonium salt and Pd/Cu-binaphthyl NPs, the absence of absorp-
The high angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) equipped with energy dispersive
X-ray (EDX) elemental mapping analysis was used to examine
the micro structure, elemental distribution and the interaction of
ꢀ1
tion band at 2260 cm in the Pd/Cu-BNPs revealed that the NPs
Scheme 1. Synthesis of Pd/Cu-BNP.

R. Saha et al. / Journal of Catalysis 377 (2019) 673–683
675
Fig. 2. Selected characterizations of Pd/Cu-BNP. (a and b) FTIR spectra of Pd/Cu-BNP (c and d) 1HNMR and solid state 13CNMR of Pd/Cu-BNP.
Fig. 3. Powder XRD patterns of Pd-BNP (bottom), Cu-BNP (middle), Pd/Cu-BNP (top).

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R. Saha et al. / Journal of Catalysis 377 (2019) 673–683
Fig. 4. (a) HRTEM images of Pd/Cu-BNP, (b) Histogram of particles size distribution of Pd/Cu-BNP (c) well-defined lattice fringe of 0.22 nm for Pd/Cu-BNP, (d) SEAD image of
Pd/Cu-BNP.
the metal to BNP of the synthesized nanoparticles (Fig. 1). The
coloured elemental mapping images showed that palladium atoms
The next attention was turned towards testing the catalytic per-
formance of the Pd/Cu-BNP in domino synthesis of triazoles via
carbometalation and CAH functionalization. It must be noted that
the catalytic activity investigation of most of the developed BMNPs
are restricted to simple hydrogenation reactions of nitroarenes and
olefins [40–42]. The usage of BMNPs in domino reactions which
involves multiple bond formation in a single step has never been
investigated.
(
(
green, Fig. 5c), copper atoms (blue, Fig. 5d) and carbon atoms
cyan, Fig. 5e) were uniformly distributed with a characteristic
alloy structure. This analysis indicated the presence of Pd, Cu and
C in the same nanoparticle. It can also be observed in overlay image
that, a very thin shell of Pd was noted on the surface which con-
firms the surface enrichment of the Pd atoms in the Pd/Cu-BNP
(Fig. 5f, also see SI and Fig. S4). Further, palladium enrich surface
was confirmed from the EDX line profile shown in Fig. 5g, h and
i. Both, the elemental mapping and the EDX line profile were con-
firmed that the palladium surface enrich Pd/Cu alloy nanoparticles
is surrounded by binaphthyl moiety.
The XPS pattern of Pd/Cu-BNP showed that the binding energy
of most of the palladium species shifted towards higher binding
energy region (Pd 3d5/2 at 337.35) and for copper shifted towards
lower binding energy region (Cu 2p3/2 at 928.36) (Fig. 6). The
abnormal shift in binding energy could be explained as the metallic
bonded (PdACu) electron shifted from palladium to copper in Pd/
Cu bimetallic NPs because Cu has higher electron affinity
2.3. Investigations of reaction conditions and scope
Our effort to synthesize polycyclic triazoles through sustainable
manner began by choosing triazole-bearing aryl iodide (3a) and
diphenylacetylene (4a) as model substrates. The initial reaction
was carried out in the presence of 2.5 mol% Pd/Cu-BNP (10 wt%
Pd and 5 wt% of Cu by ICP-OES analysis) and 3 equiv. of Et N in
DMF as solvent. The Pd/Cu-BNP has shown excellent catalytic
activity as the desired product via domino carbopalladation
and C(sp2)-H functionalization was isolated in 79% yield (Table 1,
entry 1).
3
(
1.24 eV) compared to the Pd (0.56 eV). As a results, electron defi-
Then, different metal NPs were screened, concentrating impact
of their catalytic activity. The other bimetallic Pd/Cu-QNP and Pd/
Cu-PNP provided 74% and 67% yield of 5a. Our monometallic Pd-
BNP furnished the product in 65% yield (entry 6). Cu-BNP was
unable to give 5a (entry 4). Notably, the mixture of Pd-BNP and
cient Pd showed higher binding energy and electron rich Cu
showed lower binding energy. In literature, this type of binding
energy shift is observed some other nano alloy type nanoparticles
such as Pt/Co, Pt/Ru, Pd/Ag and Pd/Cu [37–39].

R. Saha et al. / Journal of Catalysis 377 (2019) 673–683
677
Fig. 5. Selected Characterizations of Pd/Cu-BNP. (a and b) HAADF of Pd/Cu-BNP, STEM-EDX elemental mapping of HRTEM image, (c) Pd, (d) Cu, (e) C (f) Pd/Cu-BNP (overlay),
Line mapping (g) HAADF, (h) The line profile of Pd, Cu and C (i) Line profile along with particles.
Cu-BNP (1:1) has also shown less catalytic activity (67%, entry 5).
Other catalysts such as Pd-PNP, Pd-QNP and commercial NPs fur-
nished the product in 53–66% yields (entries 7–9). Screening of
various catalysts manifested that bimetallic Pd/Cu-BNP is more
effective for the present transformation than other catalysts due
to the presence of more number of active metal components (Pd)
on the surface which increases the efficiency.
To further improve the yield of tricyclic triazole 5a, other
parameters were screened and results are summarized in Table 2.
The other bases including organic bases (DBU and DIPEA) and inor-
Then the effect of solvent was investigated. Polar aprotic sol-
vents such as DMSO and DMA afforded the desired product in
62% and 50% yields respectively (entries 5 and 6). Interestingly,
the reaction occurred in water medium albeit with poor yield
(20%, entry 7). The reaction temperature influenced the reaction
outcome as decreasing the reaction temperature to 110 °C led to
an increase in the yield to 81% (entry 10). In absence of base reac-
tion did not provide any yield of product 5a (entry 11). This results
implies that the reaction catalyzed by base can be rule out and Pd/
Cu-BNP is crucial for these methodology. The changes in catalyst
loading did not help to increase the yield (entries 12 and 13).
Surprisingly, the maximum yield of 93% was observed when the
ganic bases (KF and K
2 3 3
CO ) were found less efficient than Et N for
the current transformation (entries 1–4).

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R. Saha et al. / Journal of Catalysis 377 (2019) 673–683
Fig. 6. XPS pattern of Pd/Cu-BNP for Pd (left) and Cu (right).
Table 1
a
Screening of different metal NPs.
a
All the reactions were carried out on 0.25 mmol scale and isolated yields are shown.
reaction was carried out in presence of 50 mol% AcOH presumably
due to assistance of caboxylate ligand in the CAH activation (entry
tion conditions were tolerant with electron-donating and
electron-withdrawing substituents, as in all the cases the desired
products were isolated in good to excellent yields. At first, triazoles
with electron-donating groups such as 4-Me, 4-pentyl, 4-OMe and
3-Me were employed with 4a and the desired products were
obtained in 84–91% yields (5b-e). Triazoles with aliphatic
1
4). Further changing the amount of AcOH did not improve the
yield (entries 15 and 16).
With the optimized reaction conditions in hand, the substrate
scope for this domino reaction was surveyed (Table 3). The reac-

R. Saha et al. / Journal of Catalysis 377 (2019) 673–683
679
Table 2
Optimization of the reaction conditions.
Entry
Base
Solvent
Temp. (°C)
Yield (%)a,b
1
2
3
4
5
6
7
8
9
DBU
DIPEA
KF
DMF
DMF
DMF
DMF
DMSO
DMA
120
120
120
120
120
120
120
120
130
110
110
110
110
110
110
110
8
52
60
9
2 3
K CO
Et
Et
Et
Et
Et
Et
–
3
N
N
N
N
N
N
62
50
20
00
55
81
00
66
72
93
86
90
3
3
3
3
3
2
H O
THF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
10
11
12
13
14
15
16
c
d
e
f
Et
Et
3
N
N
3
Et
3
N
N
N
Et
Et
3
g
3
a
b
c
d
e
f
Reaction conditions: 3a (0.25 mmol) and 4a (0.375 mmol).
Isolated yields are shown.
1
3
5
2
1
.5 mol% of Pd/Cu-BNP.
.0 mol% of Pd/Cu-BNP.
0 mol% AcOH was used.
5 mol% of AcOH was used.
.0 equiv. of AcOH was used.
g
side-chains such as n-butyl, t-butyl and cylopropyl were found to
be suitable substrates as the corresponding products (5f-h) were
isolated in 84%, 80% and 82% yields respectively.
Next the scope of this transformation was explored with vari-
ous substituted diarylacetylenes. 4-Me and 4-OMe substituted
diphenylacetylenes afforded the corresponding products 5i and
plete conversion of 7, the Pd/Cu-BNP and 4a were added to afford
the annulated product 5a in 61% yield (Scheme 2b). The one-pot
reaction through in-situ generation of both the coupling partners
also yielded the desired product in 48% yield highlighting the prac-
ticability of the current methodology (Scheme 2c).
5
j in 84% and 85% yields respectively. Electron-withdrawing sub-
stituents such as F, CF , CO Et and CN were found to be compatible
3
2
2.5. Investigation of catalyst stability in reaction medium
as the products 5 k-o were isolated in 68–90% yields. 2-Thienyl
attached acetylene was also found to be reactive under the opti-
mized reaction conditions and gave yield of corresponding product
Further efforts were made to examine the recoverability and
reusability of the Pd/Cu-BNP catalyst. The model reaction was con-
ducted at 1 mmol scale using triazole 3a with 4a under the opti-
mized reaction conditions. The catalyst was recovered and reused
up to 4 catalytic cycles and desired product 5a was isolated in
the yields of 92%, 90%, 87% and 86% in successive runs which
proved the preserved activity of the catalyst (Fig. 8). To confirm
that the reaction is catalyzed by heterogeneous Pd/Cu-BNP and
not by the leached homogeneous Pd/Cu, the heterogeneous test
such as hot centrifugation test and mercury poisoning test were
conducted. In hot centrifugation test, the Pd/Cu-BNP was removed
from reaction mixture by centrifuging in the middle of the reaction
at two different time intervals from two different set of reactions.
After that, both the filtrates (liquid portions) were allowed to con-
tinue stir at 110 °C. It was observed that the reaction did not pro-
ceed in both the liquid portions. It is clear that active species is Pd/
Cu-BNP and not the leached out Pd or Cu atoms (Fig. 8).
5
p in 77%. The substrates containing divergent substituents were
also efficiently converted into the annulated tricyclic triazoles in
good yields (5q-w, 75–88%). However, 2-pyridine attached triazole
and aliphatic acetylene did not provide the desired product under
the optimized reaction conditions. The unsymmetrical diary-
lacetylene (1-methoxy-4-(phenylethynyl)benzene) gave insepara-
ble mixture of two regioisomer ratio in 1:1.4 of 5y in 88% yield.
(
see SI data of 5y). Notably, the structure of 5f and 5p were con-
firmed unambiguously by single crystal X-ray analysis (Fig. 7).
2.4. Application in the gram scale and one-pot synthesis
For the practical utility of the present methodology, the scala-
bility of the reaction was investigated. A gram-scale reaction using
model substrates 3a and 4a was carried out and it gave the corre-
sponding polycyclic triazole 5a in good yield (88%) without atten-
uation of the yield of pilot-scale reaction (Scheme 2a). Importantly,
the synthesized triazole 5a was derivatized through alkylation
reaction in 75% yield. Then, a one-pot reaction through in-situ gen-
eration of triazole 3a via click reaction was investigated. After com-
HRTEM image of fresh catalyst and reused catalyst showed that
particles size are similar which confirmed that the synthesized Pd/
Cu-BNPs is stable in reaction medium (Fig. 9). Also, it was further
confirmed from mercury poisoning test that the reaction is cat-
alyzed by heterogeneous Pd/Cu nanocatalyst, not by leaching of
homogeneous Pd/Cu.

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R. Saha et al. / Journal of Catalysis 377 (2019) 673–683
Table 3
a
Substrate scope of the domino reaction.
a
All the reactions were carried out on 0.25 mmol scale and isolated yields are shown.
2.6. Preliminary investigations of reaction mechanism
3
erated carboxylate anion from the reaction of AcOH with Et N
undergoes a ligand exchange with iodide to give the intermediate
C. Then, carboxylate ligand-assisted CAH bond activation occurs
via concerted metallation deprotonation (CMD) process [43] to
give the intermediate E which upon reductive elimination expels
the desired product 5 with a regeneration of active catalytic spe-
cies for the next catalytic cycle.
Based on the literature precedent [34], we propose the follow-
ing mechanism as shown in Scheme 3. Initially, an oxidative addi-
tion occurs between Pd/Cu-BNP and triazole bearing aryl iodide 3
to give the intermediate A and subsequent migratory insertion of
diarylacetylene produces the intermediate B. Next, an in-situ gen-

R. Saha et al. / Journal of Catalysis 377 (2019) 673–683
681
Fig. 7. ORTEP diagram of 5f (left) and 5p (right). The thermal ellipsoids are shown at 50% probability level (CCDC: 1852522 and 1936036).
Scheme 2. Experiments to investigate the practicability of the Pd/Cu-BNP catalyzed domino reaction.
3
. Conclusion
sequence. In addition, the catalyst has shown recyclability and
was also found to be heterogeneous in nature. However, a detailed
mechanistic study, biologically activity of synthesized tricyclic tri-
azoles and efforts to make chiral bimetallic nanoparticles are cur-
rently underway in our laboratory.
In conclusion, we have successfully synthesized metal-carbon
covalent bond stabilized Pd/Cu bimetallic nanoparticles for the
first time using simple binaphthyl moiety as stabilizers. The syn-
thesized Pd/Cu-BNP was characterized by various physical meth-
ods and the surface enrichment of Pd atoms in the Pd/Cu-BNP
with a narrow size distribution was observed. The XPS analysis
confirmed that the synthesized Pd/Cu-BNPs contain the electron
deficient Pd centre and electron rich Cu centre. Moreover, this
newly developed electron deficient Pd nanomaterial shown excel-
lent catalytic activity for the synthesis of polycyclic triazoles via
domino carbopalladation and C(sp2)-H bond functionalization
4. Methods
4.1. General procedure for Pd/Cu-BNP catalyzed synthesis of polycyclic
triazoles
Under the open atmosphere, 1,2,3-triazole 3 (0.25 mmol), diaryl
acetylene 4 (1.5 equiv.), 2.5 mol% of Pd/Cu-BNP (10 wt% Pd and

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R. Saha et al. / Journal of Catalysis 377 (2019) 673–683
Fig. 8. Recyclability of the Pd/Cu-BNP (left) and Hot-filtration test (right).
Fig. 9. HRTEM image of fresh (left) and reused (right) Pd/Cu-BNP.
5
wt% Cu by ICP-OES analysis) and DMF (1.5 mL) were successively
3
added to oven dried reaction tube. Then Et N (1.5 mmol) and AcOH
(
50 mol%) were added and closed with a glass-stopper. The reac-
tion tube was then immersed in pre-heated oil bath with magnetic
pellet and stirred at 110 °C. The reaction was heated with stirring
till complete consumption 3. After cooling down to room temper-
ature, the solvent was removed under reduced pressure. Water
was added to the reaction mixture and extracted with ethyl acetate
(
3 ꢁ 5 mL). Brine wash (1 ꢁ 10 mL) was given to the combined
organic extractions and dried over anhydrous Na SO . Removal of
2
4
solvent and silica gel column separation of crude using hexanes
and ethyl acetate mixture (4:1) afforded the corresponding tri-
cyclic triazoles 5a-w.
Acknowledgements
G.S thanks IIT Madras for exploratory research project (CHY/16-
1
7/846/RFER/GSEK). R.S thanks IIT Madras for senior research fel-
lowship and D.A thanks CSIR, New Delhi for senior-SPM fellowship.
We thanks to Prof. T. Pradeep for XPS and HRTEM analysis. We
would like to thanks department of metallurgical and materials
engineering at IIT Madras for STEM analysis.
Scheme 3. Proposed catalytic pathway.

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683
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Appendix A. Supplementary material
Supplementary data to this article can be found online at
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