Copper nanoparticles loaded polymer vesicles as environmentally amicable nanoreactors: A sustainable approach for cascading synthesis of benzimidazole

Article abstract of DOI:10.1016/j.molliq.2021.116217

This paper reports the synthesis and characterization of Cu nanoparticle loaded, surfactant free metallovesicles (CuNPs@vesicles) as nanoreactors to produce benzimidazoles via cascade reaction. The vesicles exhibited uniform size distribution, spherical m

Full text of DOI:10.1016/j.molliq.2021.116217

Journal of Molecular Liquids 336 (2021) 116217
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Copper nanoparticles loaded polymer vesicles as environmentally
amicable nanoreactors: A sustainable approach for cascading synthesis
of benzimidazole
a
a
a,b,
⇑
Falguni Shukla , Manita Das , Sonal Thakore
a
Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 3960002, India
Institute of Interdisciplinary Studies, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 3960002, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper reports the synthesis and characterization of Cu nanoparticle loaded, surfactant free met-
allovesicles (CuNPs@vesicles) as nanoreactors to produce benzimidazoles via cascade reaction. The vesi-
cles exhibited uniform size distribution, spherical morphology, excellent stability and efficient Cu
loading. This strategy is unique for the fact that, environmental contaminants like nitroaniline have been
utilized as precursor and converted into fine chemicals of commercial significance via non-toxic interme-
diates. CuNPs@vesicles reduces 2-nitroaniline to o-phenylenediamine which further acts as precursor for
benzimidazole synthesis. Thus the reaction occurs via two step cascade pathway comprising reduction
and CAN cross coupling reactions in water. The preliminary studies suggest encouraging results for per-
forming dehydrogenative coupling under relatively mild conditions using CuNPs@vesicles as a catalyst.
All the products are obtained in good to excellent yield with facile catalyst regeneration and recyclability
upto 5 cascading cycles.
Received 6 December 2020
Revised 12 April 2021
Accepted 16 April 2021
Available online 20 April 2021
Keywords:
Self-assembly
Cu NPs
Metallovesicles
Cascade reaction
Benzimidazole
Cross-coupling reaction
Ó 2021 Elsevier B.V. All rights reserved.
1
. Introduction
Polymersomes are assemblies of amphiphilic copolymers in
polymeric vesicles not only protect the catalyst within its
compartments, but also facilitates a facile on-site isolation of reac-
tive compounds enabling cascade reactions. They possess the abil-
ity to convert hydrophobic substrates in water and allow catalyst
recovery. Such polymer vesicles can also encapsulate hydrophobic
molecules and offer surfactant free metallovesicle assemblies [12].
With suitable functionalities polymer vesicles can also support
metal nanoparticles encapsulation resulting in metallovesicles for-
mation. Such metallovesicles can aid to adopt an approach where
reactions can be performed under relatively mild conditions. For
instance Sun et al., have demonstrated the potential of silver dec-
orated homopolymeric vesicles to reduce water insoluble p-
nitrophenol in aqueous media [13]. In yet another report, Jianz-
hong Du and co-workers have reduced 4-nitrophenol (4-NP) using
Au encapsulated vesicles [14]. This is a crucial development, as
nitroaromatics have been listed as ‘‘priority pollutants” by US
EPA and its straightforward hydrogenation into its amino analogue
can be a green strategy [15–19].
aqueous medium which bear resemblance to liposomes. These
polymersomes exhibits unique morphology, with a hydrophobic
membrane and a hydrophilic corona mediating a non-covalent self
assembly. Due to distinct hydrophilic and hydrophobic pockets,
polymersomes are excellent auxiliaries for potential applications
in drug delivery [1–3], and waste water remediation [4]. The suc-
cess of employing polymersomes for aforementioned applications
is due to their stability, robustness and potential of advaced chem-
ical functionalization. This has motivated researchers to design
polymersome derived nanoreactors for applications in catalysis.
Polymersomes with various morphologies like micelles [5,6],
vesicles [4,7,8,9], micro emulsions [10,11] etc. have been known
to serve as nanoreactors for organic transformations. However,
reports of literature suggest that vesicles are more efficient
nanoreactors as compared to other nanoassemblies. This is because
Metallovesicles capable of performing cascade reactions can
further be deployed to convert these amino derivatives into
organic compounds of commercial importance. Thus fabrication
of metallovesicle with appropriate rationale can turn out to bring
a new advent in sustainable chemistry due to their ubiquitous
assistance in synthesis of commercially important compounds.
⇑
Corresponding author at: Department of Chemistry, Faculty of Science, The
Maharaja Sayajirao University of Baroda, Vadodara 3960002, India.
E-mail addresses:
chemistry2797@yahoo.com,
drsonalit@gmail.com
(
S. Thakore).
https://doi.org/10.1016/j.molliq.2021.116217
167-7322/Ó 2021 Elsevier B.V. All rights reserved.
0

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
With this background, our attempts were directed towards the
synthesis of benzimidazole and its derivatives (N-containing hete-
rocyclic compound) as they have significant applications in phar-
maceuticals industries, dyes and chemo sensing UVB filters,
optical devices, etc. Previous research in the area suggest that pre-
cious and rare transition metals (Ru, Rh, Ir, Pd etc.) [20,21]and cop-
per derived hybrid materials are effective catalysts for synthesis of
benzimidazoles. The performance of some homogeneous and
commercial sources and used as received without further purifica-
tion. The solutions were prepared using de-ionized water.
Fourier Transform Infrared Spectroscopy (FTIR) spectra were
recorded on a Bruker Alpha IR spectrophotometer at room temper-
ꢁ1 1
ature in the range of 4000–400 cm
.
H Nuclear Magnetic Reso-
CNMR spectra were recorded on Bruker
Avance 400 MHz spectrometer using tetramethylsilane as an inter-
nal standard and CDCl or DMSO as solvents. High Resolution
1
3
nance (NMR) and
3
heterogeneous catalysts such as {Mo72
ZIF-67 [23], SDS micelles [24], CeCl
ꢀ7H
Cu-Mn B spinel oxide [26], CuFe nanoparticle [27] recently
reported has been summarized in Table S1. The classical approach
of synthesis suffers from various limitations such as use of expen-
sive catalysts, harsh reaction conditions (bases and toxic solvents)
V
30} nanocluster [22], Co-
Transmission Electron Microscopy (HR-TEM) and Energy disper-
sive X-ray spectroscopy (EDS) was recorded on Joel (Jem-2100F)
electron microscope at 200 kV. For HR-TEM analysis, the
CuNPs@vesicles solutions were diluted at ambient temperature
and dispersed on a carbon coated copper grid. The grids were air
dried under ambient temperature environment overnight. Imaging
was recorded using Jeol (Jem-2100F) electron microscope at
200 kV. Dynamic light scattering (DLS) was used to determine
the hydrodynamic diameter and polydispersity of vesicles in the
solution which was performed on Beckman Coulter Delso Nano.
Atomic Force Microscopy (AFM) and Field Emission Gun-
Scanning Electron Microscopy (FEG-SEM) were recorded on NTE-
GRA PRIMA, NT-MDT, Russia and JSM-7600F respectively. The flu-
orescence images were captured on a NikonTI2E live imaging
microscope. UV–vis spectrophotometric determinations were done
using Perkin Elmer Lambda 35 and Fluorescence spectra were
scanned on JASCO FP-6300. The quantification of copper in the cat-
alyst as well as in the supernatant was performed by atomic
absorption spectroscopy (AAS) on an AA 6300: Shimadzu (Japan)
atomic absorption spectrometer using an acetylene flame. The
optimum parameters selected for measurements are: wavelength
¼ 324.7 nm; lamp current ¼ 2 mA; slit width ¼ 0.2 nm; and fuel
3
2
O-CuI [25], bimetallic
2 4
O
[
28], formation of undesirable and hazardous by-products [29,30].
Low yields, difficulties in product separation and lack of catalyst
recyclability are also identified as key limitations [31]. The reaction
also suffers from poor selectivity resulting in the formation of 1, 2-
disubstituted benzimidazole along with 2- substituted benzimida-
zole as a mixture [24].
Recent reports suggest that polymersome mediated catalysis
can help to overcome these limitations. Kaur et al., for instance,
have reported synthesis of benzimidazole derivatives with metal-
losurfactant catalyst, loaded with copper ions, under mild condi-
tions [32]. This is a report on successful micellar catalysis.
However, a cascading synthesis of benzimidazole has not been
reported so far. Based on these findings, we propose the synthesis
of a carboxyl functionalized polymer vesicles to develop metal-
lopolymer nanoreactors loaded with copper nanoparticles (CuNPs).
Compartmentalization of copper ions within the polymeric
vesicle leads to a fine balance between stability and surface activ-
ity. Metallovesicles offer enhanced reactivity and dual properties;
the selectivity of a homogenous catalyst and recyclability of a
heterogeneous catalyst [32]. Further, they are easy to synthesize
effective at low loading, facilitate aqueous catalysis and pose min-
imal toxicity to the environment. Under the influence of ‘‘nano to
nano effect” [33] a cascading reaction pathway can be achieved
within the hydrophobic membrane of metallovesicles wherein
reduction and the cross coupling reactions can occur. Metallovesi-
cles also have the inherent benefits such as ease of separation/pu-
rification of the products along with recyclability of the catalyst.
In the past we have demonstrated the efficiency of hybrid
nanocatalysts for reduction of nitroaromatics [34,35]. Since
nitroaniline is the precursor of benzimidazole, the ultimate aim
was to achieve cascading catalysis as well as selectivity as a result
of structural complexity of metallovesicles that prevents side reac-
tions and improves the reaction rate. Such a surfactant free Cu NPs
encapsulated true polymeric vesicle as a catalyst system for a cas-
cading and sustainable CAN coupling reaction has not been
reported so far to the best of our knowledge.
ꢁ
1
flow rate ¼ 0.2 L min
.
2
.1.1. Synthesis and characterization of amphiphilic polymer DMPA-
HMDI
The polymerization was carried out by adding HMDI (1.19 ml)
to a solution of DMPA (1.0 g, 0.00745 mol) in THF (2 ml). The reac-
tion mixture was maintained at inert atmosphere under continu-
ous flow of nitrogen. To this mixture, a solution of DABCO
ꢁ4
(
33.2 mg, 2.96 ꢂ 10 mol) dissolved in 1 ml THF was added.
The reaction was allowed to proceed for 5 h at 60 °C. After this,
another 1.0 ml of THF was added to obtain a viscous liquid and
the polymer was precipitated using diethyl ether. A sticky polymer
was isolated (for 1 g DMPA, 1.5 mg of polymer was obtained).
2.1.2. Assessment of self-assembly of the amphiphilic polymer
To check self-assembly, the polymer was directly dissolved in
DMSO (0.1 wt% of the polymer) and then subjected to extensive
dialysis for removal of excess of DMSO prior to physical studies.
2.1.3. FTIR studies to prove self-assembly into vesicles:
Polymer solutions were prepared in methanol and water and
the spectra were recorded by placing drops of polymer solutions
between two CaF windows (path length = 0.2 mm). The spectra
were scanned in the range of 4000–600 cm with wavenumber
2
. Experimental section
2
ꢁ1
ꢁ1
precision of 0.005 cm , 24 scans were recorded at 25 °C.
2.1. Materials and methods
Dimethylolpropionic acid (DMPA), Hexamethylene Diiso-
2.1.4. Dye encapsulation studies:
cyanate (HMDI), acetone, aldehydes and alcohols were purchased
from Sigma Aldrich, India. Copper sulphate (CuSO O), Ascorbic
ꢀ5H
acid, o-phenylenediamine were procured from Sisco Research Lab-
oratories (SRL), India. Sodium borohydride (NaBH ), 2-nitroaniline
2-NA), and dimethyl sulfoxide were purchased from spectrochem
India and calcein was purchased from Loba chemie, India. The dial-
ysis bags (MWCO = 3000 Da) were purchased from Sigma Aldrich,
India. Other reagents of analytical grade were purchased from
The polymer solution (1 ml, 0.1 wt%) was encapsulated with
ꢁ3
ꢁ1
4
2
calcein dye (30
lL, 1 ꢂ 10 mol L ) in DMSO and diluted upto
3 ml using water. The resulting solution was subjected to extensive
dialysis against water (MWCO = 3000 Da) for 24 h to eliminate the
possibility of presence of any un-encapsulated dye molecule. This
solution was analyzed using UV–vis spectrophotometric analysis
which shows peak at 484 nm corresponding to k-max of calcein.
Later, emission spectrum was recorded using this solution and
4
(
2

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
compared with absorption matched emission spectra of free dye in
water.
cles solution was collected and used for assessment of recycling
efficiency for the next cascading cycle.
2
.1.5. Urea addition and DLS experiments for disassembly: Proof of
2
.2.5. Experimental procedures for synthesis of benzimidazole using
concept for urethane linkage mediated vesicle formation
aromatic alcohol
For Spectroflourometric studies, 5 mg of solid urea was added to
a calcein encapsulated polymer solution used for studies as per
Section 3.2. Urea was completely dissolved by stirring prior to
spectral measurements. For DLS measurements 0.1 wt% of polymer
solution was treated with 5 mg urea. The DLS spectra were
recorded before and after the addition of urea.
2
-nitroaniline (0.3 mmol) and p-nitrobenzyl alcohol (0.9 mmol)
were taken in a two neck round bottom flask and 5 ml of the cat-
alyst was added to it. N gas was purged into this mixture to create
2
an inert atmosphere. The reaction mass was heated to 170 °C in a
controlled temperature oil bath for 24 h. The progress of the reac-
tion was monitored using thin layer chromatography (TLC). The
reaction mixture was cooled to room temperature after its comple-
tion and extracted in ethyl acetate (3 ꢂ 10 ml). The ethyl acetate
layer was washed with brine (5 ml), dried over sodium sulphate
and concentrated under vacuum. The obtained product was puri-
fied using column chromatography (10–20% ethyl acetate in hex-
ane) to give the desired product.
2.2. Synthesis of CuNPs@vesicle catalyst
The CuNPs@vesicles was prepared by dispersing DMPA-HMDI
(
7 mg) polymer in 10 ml DMSO in Erlenmeyer flask at room tem-
ꢁ1
perature. Further, 1 ml of CuSO
slowly while stirring. The pH of solution was adjusted to 9 by add-
ing few drops ammonia solution. Later 1 ml of ascorbic acid
4 2
ꢀ5H O (0.1 mol L ) was added
ꢁ1
3. Result and discussion
(
0.2 mol L ) solution was added so as to readjust the pH to 3
under continuous stirring. Temperature of the resulting solution
was increased to 80 °C and stirring was continued for 15 min.
The formation of copper nanoparticles was marked by the develop-
ment of a characteristic wine red color in the solution. The
nanoparticles thus prepared were extensively dialyzed against
water for 24 h to remove excess of DMSO, unencapsulated
nanoparticles and unreacted reagents.
3
.1. Synthesis and characterization of catalyst
An amphiphilic co-polymer with carboxyl functionalities was
synthesized using dimethylol propionic acid and hexamethylene
diisocyanate with urethane linkages according to Scheme 1. This
amphiphilic polymer comprises of hydrophilic units of dimethyol
propionic acid and hydrophobic hexamethylene units from HMDI.
The amphiphilic co-polymer was precipitated from the reaction
mass in diethyl ether and purified thrice via precipitation
technique.
The purified polymer was then analyzed with various spectro-
scopic techniques. For instance, the structure of the polymer was
confirmed from H NMR performed by dissolving the polymer in
2.2.1. Experimental procedures for catalytic reaction
The detailed description of the protocol followed for carrying
out desired reactions using CuNPs@vesicles as catalyst is as
follows:
1
2.2.2. General procedure for catalytic formation of o-
DMSOd
urethane linkage. A proton peak at 4.01 represents the C-CH
repetitive unit of Dimethylolpropionic acid, 3.59 ppm due to
OACH ) protons and 2.91–2.93 ppm for (CH ) protons. The
ACH protons corresponding to the repetitive unit of Hexam-
6
. The d value at 7.01 ppm corresponds to N-H proton of
phenylenediamine
The desired precursor o-phenylenediamine (o-PDA) was
obtained by the catalytic reduction of 2-nitroaniline(2-NA) with
2
,
(
2
3
NaBH
.9 ml of catalyst solution was added to 1 ml solution of 10 mM 2-
NA under stirring. To this mixture, 2 ml of freshly prepared NaBH
15.8 mmol) was added as a source of hydrogen and the reaction
4
in the presence of CuNPs@vesicles. In a round bottom flask
2
1
ethylene diisocyanate were observed at 1.04–2.08 ppm. The proton
of free carboxylic acid group appears at 12.54 ppm (S-I Figure S1).
The C NMR demonstrated peaks corresponding to C atom of
4
(
13
was allowed to proceed at 30 °C. The progress of reaction was mon-
itored using UV–vis spectrophotometer. The completion of reac-
tion was marked by complete disappearance of yellow color.
From the black mixture, the product formed was isolated using
ethyl acetate as the solvent followed by the regeneration of the
red metallopolymer catalyst as shown in the video.
urethane linkage and free carboxylic group at d value 156.5 and
1
77 ppm respectively (S-I Figure S4).
The data obtained from NMR spectroscopy corroborates with
the FTIR spectroscopy. The characteristic peaks due to the presence
ꢁ1
of OH group and CH group were observed at 3323 cm
and
ꢁ1
2
956 cm respectively. The formation of urethane linkage was
confirmed by the appearance of peaks at 1264 cm , 1680 cm
and 1580 cm representing CAN, C@O and NAH, NAC@O groups
respectively (S-I Figure S2). An elemental analysis with EDAX
determination suggests that the polymer contains 60 wt% of C,
ꢁ1
ꢁ1
ꢁ1
2.2.3. General procedure for one pot catalytic synthesis of 2-aryl-1H-
benzimidazoles
A mixture of o-phenylenediamine (1 mmol, 1 equiv) and aro-
matic aldehydes (1 mmol, 1 equiv) was added to a round bottom
flask containing CuNPs@vesicles (5 ml, 1440 ppm) and water
5
wt% of N and 34 wt% of O (S-I Figure S3).
After these structural characterizations, the ability of this
(
10 ml). The mixture was stirred at 30 °C and the progress of the
amphiphilic co-polymer to self-assemble into vesicles was evalu-
ated. It was observed that 0.7 mg/ml of polymer concentration
was required for vesicle formation. To make this solution, 7.0 mg
of the purified polymer was dissolved in 5 ml of DMSO. After this
excess of DMSO was removed by extensive dialysis against water
reaction was monitored by thin layer chromatography(TLC) at reg-
ular time intervals. The product precipitated out as solid from the
reaction mass was filtered under vacuum. It was purified by
repeated washings of deionized water and recrystallized using
ethanol.
[
13]. The self–assembly of amphiphilic copolymer is driven by
the colloidal amphiphilicity [36]. To evaluate the formation of vesi-
cles, via self-assembling, different studies were carried out i.e. FTIR,
HR-TEM, AFM, FEG-SEM, DLS, optical imaging and fluorescence
measurements.
The intrachain H-bonding formation assists self-assembly into
vesicles. Vesicular assembly as proposed in Scheme 1 is assumed
2
.2.4. Recycling of the catalyst
While recycling the catalyst, the reaction mixture was filtered.
Upon filteration CuNPs@vesicles solution separated as a filtrate
which was further treated with diethyl ether (3 ꢂ 5 ml) to extract
the traces of unreacted substrates, if any. The purified CuNPs@vesi-
3

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
Scheme 1. Synthesis of amphiphilic DMPA-HMDI co-polymer and subsequent self assembly into vesicle.
to form a bilayer via folding of the hydrophobic polymer backbone.
The folding is driven by intrachain H-bonding of the urethane link-
ages in the polymer network. To support this model, FTIR spectra of
polymer was scanned in methanol and water (Fig. 1(a)). The peak
corresponding to carbonyl stretching of urethane group which
ꢁ1
appeared at 1683 cm in methanol was shifted by approximately
ꢁ1
ꢁ1
41 cm in water and appears at 1642 cm . This demonstrates the
formation of H-bonding in water [37]. The vesicles exhibited a
Fig. 1. Various determinations by characterization techniques to prove self-assembly and disassembly (a) FTIR spectra in methanol and water (b) difference in the
fluorescence intensities of calcein encapsulated vesicle and free calcein in water (c) Urea addition experiment (d) DLS measurements showing disassembly of vesicles and (e)
Calcein encapsulated vesicles showing stability upto 30 days as observed in florescence study.
4

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
spherical morphology and presented diameters in range of 130–
between concentration of nanoparticles and the morphology of
the self-assembling aggregate [41]. Upon an increased nanoparticle
concentration the co-polymers coalesce to form vesicles. With this
purview, for the formation of metal encapsulated vesicles (i.e,
Metallovesicles), the strategy of nanoparticle and copolymers co-
assembly was adopted. For this the carboxylic acids existing as
pendant groups in this amphiphilic co-polymer was employed
for metal encapsulation. Thus the carboxylic group not only ren-
ders adequate hydrophilicity for self assembly but also aids in
metal binding, exhibiting good catalytic activity and recyclability.
The carboxyl functionalities on the bilayer of the vesicles ensures
stronger interactions with the metal precursors to obtain highly
dispersed copper nanoparticles [42,43]. With this rational the cop-
1
60 nm that has been confirmed via HR-TEM and FEG-SEM analy-
sis. This is further confirmed from AFM determination wherein it
was observed that the height of the assemblies (i.e, 20 nm) is less
than its diameter (i.e, 140 nm), suggesting that they have spherical
morphology [38] (Fig. 2).
Vesicular morphology was confirmed by presence of water pool
inside the self assembled polymer [39]. This hypothesis was tested
by the fact that hydrophilic dye molecules can be encapsulated
within the hydrophilic interior. The extensively dialyzed solution
demonstrated characteristics emission peak of calcein with
k
max = 510 nm (Fig. 1(b)), indicating dye encapsulation. This emis-
sion intensity was compared to the absorption matched intensity
of the free dye solution. The reduction in emission observed for cal-
cein encapsulated in the confined water pool within the vesicle, is
attributed to the phenomenon of self-quenching. The dye encapsu-
lated solution examined under fluorescence microscope demon-
strated bilayered green emitting spherical particles.
H-bonding-mediated self-assembled polymer undergoes denat-
uration in the presence of urea because it disturbs the H-bonding
and thus the vesicles disassembled [40]. The self-quenching is
due to confinement of dye molecules in the vesicle, addition of
urea breaks the vesicle causing the release of dye molecules in bulk
water and thus an enhancement in emission intensity (Fig. 1(c)).
This theory was further confirmed by DLS analysis. As shown in
Fig. 1(d) the polymer solution in absence of urea shows peak at
4 2
per sulphate (CuSO .5 H O) was used as the source of copper and
ascorbic acid as reducing agent for subsequent reduction into
nanoparticles (Scheme 2).
Elemental mapping (Fig. 3) confirms the presence and distribu-
tion of Cu in the vesicles while AAS analysis confirmed an encapsu-
lation with 1440 ppm loading. The HR-TEM, FEG-SEM and AFM
showed the size of the copper nanoparticles loaded polymeric vesi-
cles is 160 nm, height was found to be 22 nm and morphology was
observed to be spherical shown in Fig. 2. The formation of metal-
lopolymer occurs as follows: The carboxylic functionalities on the
2
+
vesicular bilayer assists Cu adsorption [42]. This phenomenon
was observed in the FTIR analysis, wherein the spectrum of the
ꢁ1
Cu ion containing vesicle solution shows peak at 447 cm , a char-
2
+
1
32 nm corresponding to presence of vesicles. This peak com-
acteristic of MAO bond [44]. The Cu ions trapped in the vesicles
were then reduced to Cu nanoparticles using ascorbic acid. This
led to disappearance of the MAO bond in FTIR spectrum of metal-
lopolymer (as shown in Fig. 3). The appearance of SPR band in the
range 500 nm to 550 nm confirms formation of CuNPs.
pletely disappears upon urea treatment suggesting urea disassem-
bly. On the contrary a new peak is observed at 32 nm which may
be due to formation of some undefined polymer aggregates. Fur-
ther, calcein encapsulated vesicles captured via a fluorescence
microscope exhibits a bilayered structure and the self-quenching
studies further support their true vesicular nature [37]. The vesi-
cles exhibit excellent kinetic stability upto 30 days as observed
from fluorescence emission intensity of calcein (Fig. 1(e)).
3
3
.2. Role of CuNPs@vesicles as catalyst
.2.1. Catalytic reduction of nitroaromatics
Theoretical simulation studies performed via particle dissipa-
tive particle dynamics suggests that there exists a relationship
The metallovesicles thus formed served as a nanoreactor which
can encapsulate small hydrophobic molecules and whose interior
Fig. 2. (a) Size distribution of the vesicles as observed in DLS histogram (b) optical micrograph of vesicles (Inset: photograph of vial showing polymer solution) (c) DLS
histogram depicting size distribution (d) optical micrograph of CuNPs@vesicles (Inset: photograph of vial showing catalyst solution) (e) HR-TEM (f) FEG-SEM images of
vesicles (g) HR-TEM (h) FEG-SEM images of CuNPs@vesicles (i),(j),(k) & (l) AFM images for vesicles and CuNPs@vesicles respectively. [Concentration of the samples was
maintained as 0.1 mg/mL for all the measurements]
5

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
Scheme 2. Visual changes observed in the process of CuNPs@vesicles formation (i) bare vesicles (ii) binding of Cu2+ ions with vesicles (iii) Color change upon pH adjustment
of the metal encapsulated vesicles (iv) reduction and formation of Cu Nanoparticles.
Fig. 3. (a) Elemental distribution of CuNPs@vesicles as observed in EDAX mapping; C (red), N (green), O (blue) and Cu (purple) (b) FTIR spectra showing binding of Cu2+ ions
(
blue line) in the vesicles and its reduction into nanoparticles (red line) (c) A comparison of UV–vis absorption spectra for bare vesicles and CuNPs loaded vesicles (Inset
digital images showing color transition due to Cu loading. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this
article.)
houses CuNPs wherein an organic transformation can occur. The
nanoreactor was employed for the synthesis of 2-substituted ben-
zimidazole in a one pot environmentally benign synthesis via cas-
cading pathway using easily accessible precursors (Scheme 3).
The investigations began with initial optimizations to assess the
activity of our catalyst towards the reduction of nitroaromatics
using 4-Nitrophenol as model substrate to demonstrate the cat-
alytic capability of CuNPs@vesicles. The UV–vis spectrophotomet-
ric monitoring demonstrated the successful reduction proceeding
via 4-nitrophenolate ion intermediate formation (Fig. 4(a)).
Table 1 shows control experiment details for reduction of 4-
nitrophenol. Control reactions were carried out in presence of only
To understand the role of nanovesicles in catalyzing the organic
transformations, reaction was carried out in the presence of bare
vesicles, bare CuNPs as well as in the presence of only CuSO
4
ꢀ5H
2
O
(the precursor for CuNPs). In all these cases there was no product
formation even after 24 h. This led us to conclude that the Cu
nanoparticle loaded in the polymeric vesicles is the active compo-
nent that drives the reaction which occurs within 30 s.
The reduction of 4-NP was also monitored, further, in the
absence of NaBH by taking only 4-NP and CuNPs@vesicles. Control
4
reactions as shown in Table 1 establish the fact that reduction
doesn’t proceed in the absence of catalyst. The reaction conditions
optimized herein were further used for the reduction of 2-
nitroaniline to yield o-phenylenediamine (o-PDA), which is the
precursor for benzimidazole (S-II Table S3, entry 3). To confirm
4
NaBH . It was observed that even after 72 h reaction does not take
place in the absence of catalyst.
6

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
Scheme 3. CuNPs@vesicles catalyzed reactions for synthesis of benzimidazoles via cascading pathway. [Reaction conditions (a) catalyst 5 ml, NaBH
ml, H O, 30 °C]
4
2
, H O, 30 °C (b) catalyst
5
2
Fig. 4. UV–vis absorption spectra of reduction of (a) 4-Nitrophenol (b) 2-Nitroaniline (c) 2-Nitrophenol.
Table 1
Control experiment for reduction of 4-NP.
Sr.no.
Catalyst
Amount of catalyst (ml)
NaBH
4
(mmol)
Time (seconds)
1
2
3
4
Bare vesicles
1.9
1.9
1.9
1.9
15.8
15.8
15.8
15.8
No reaction
No reaction
No reaction
30
CuSO
CuNPs
CuNPs@vesicles
4
ꢀH
2
O
Reaction conditions: All the reactions were carried out at room temperature (30 °C), 4-Nitrophenol (10 mmol) and water (2 ml).
that the encapsulated metal nanoparticles do not leach out as the
reaction progresses, AAS studies of the reaction mass was carried
out and no leaching was observed.
complete formation of 2-substituted product. This is attributed to
the vesicular nature of the present catalytic system wherein the
amphiphilic polymer and Cu nanoparticles loaded therein func-
tions synergistically to assist the conversion. The reactants are
housed in the vesicular bilayer which helps to enhance its proxim-
ity with Cu NPs and necessary steric hindrance to avoid the forma-
tion of side products [32].
The optimization for CAN coupling reaction was further carried
out by varying several parameters like temperature and catalyst
volume. It was observed that an increase in the volume of
CuNPs@vesicles solution decreases the reaction time with
increased product yield (shown in Fig. 5).
3.2.2. Catalytic synthesis of 2-subtituted benzimidazoles
The CuNPs@vesicles was used to catalyze CAN coupling reac-
tions using o-phenylenediamine and 4-nitrobenzaldehyde as
model substrates, further. We observed that 90% isolated yield of
the product {2-(4-Nitrophenyl)-1H-benzo[d]imidazole} was
obtained within 30 min at ambient temperature (Table 2, entry
1
). In the literature CuNPs have been used for the formation of ben-
zimidazole, but they involve longer reaction time, formation of side
products and use of toxic solvents [45]. However, employing
CuNPs@vesicles for this transformation has enabled to carry out
the reaction in water with significantly lower reaction time for
The results could be understood in terms of increase in number
of catalytically active Cu sites. The constant results beyond 5 ml of
catalyst volume suggest the complete encapsulation of reactants in
7

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
Table 2
Substrate scope of reaction as per Scheme 3 by using o-PDA and substituted aromatic aldehydes via reduction of 2-Nitroaniline.
Sr.No
1
Diamine
Aromatic aldehyde
Product
Time (mins)
30
Yielda (%)
90
E-factor
0.11
2
40
86
0.16
3
4
60
45
88
85
0.14
0.18
5
6
70
60
84
84
0.18
0.19
Reaction conditions: o-phenylenediamine (1.0 mmol), aromatic aldehyde (1.0 mmol), CuNPs@vesicles (5 ml, 1440 ppm), water (15 ml), room temperature (30 °C).
a
Isolated yield.
the CuNPs@vesicles. The reaction was also performed at different
temperatures to understand the effect of temperature on the reac-
tion performance.
It was found that increasing the reaction temperature had neg-
ligible effect on product yield (Fig. 5(a)). However, reaction times
were reduced on increasing both the parameters. The reaction
did not proceed in the absence of catalyst even after 3 h which
highlights the potential role of the catalyst for the benzimidazole
synthesis.
Thus optimized reaction conditions derived from the above
mentioned studies were 5 ml CuNPs@vesicles at 30 °C in water.
Under the optimized conditions 1:1 stoichiometric ratio of the
reactants gave desired 2-substituted benzimidazoles which sug-
gested that the catalyst is regioselective.
The scope of the reaction was extended to other substrates to
synthesize a series of 2-substituted benzimidazole derivatives
using o-PDA and various substituted aromatic benzaldehydes
(Table 2). Interestingly, we observed that the substitution on aro-
Fig. 5. Effect on time and yield for 2-(4-nitrophenyl)-benzimidazole formation using CuNPs@vesicles with variation in(a) reaction temperature(b) catalyst volume (c)
recycling efficiency of the catalyst (d) optical images of catalyst post recycling (Inset: Image showing regenerated catalyst) Reaction conditions for (a) o-phenylenediamine
(
2
1 mmol), 4-nitrobenzaldehyde (1 mmol), CuNPs@vesicles (1440 ppm, 5 ml) &15 ml H O (b) o-phenylenediamine (1 mmol), 4-nitrobenzaldehyde (1 mmol), CuNPs@vesicles
(
1440 ppm), 30 °C (c) o-phenylenediamine (1 mmol), 4-nitrobenzaldehyde (1 mmol), CuNPs@vesicles (1440 ppm, 5 ml), 30 °C.*Isolated yield.
8

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
Fig. 6. (a) FTIR spectrum (b) UV–vis spectrum of recycled catalyst (c) FEG-SEM image (d) optical micrograph of catalyst after 1st recycling cycle (e) FEG-SEM image and (f)
optical micrograph of catalyst after 5th recycling cycle.
matic aldehydes had an impact on the reaction time. Those aldehy-
des having EWG as substituents requires lower reaction times as
compared to aldehydes with ERG. This is attributed to the reaction
mechanism, wherein a nucleophilic attack of o-PDA on aromatic
aldehyde consisting of EWG increases the electrophilicity of car-
bonyl carbon thus favouring the nucleophilic attack. It was, further,
observed that the position of substituent also had an effect on reac-
tion time, for instance the time taken for completion of reaction is
roborative evidences from other analytical techniques like FTIR,
UV–vis spectrophotometry, FEG-SEM analysis support the findings.
FTIR confirms that the presence of peak corresponding to intra-
ꢁ1
chain hydrogen bonding at 1641 cm and appearance of SPR band
of CuNPs in the recycled catalyst confirming that catalyst remains
intact. There is a substantial decrease in the reaction yield after 5th
cycle owing to the distortion in vesicular structure which is evi-
dent from optical micrograph (Fig. 6(f)). AAS analysis confirmed
the absence of leached copper ions in the reaction mixture.
A plausible mechanism was proposed to explain the phe-
nomenon of catalysis. CuNPs@vesicles acts as nanoreactors for
the transformation. These nanoreactors can entrap the reactants
by the virtue of hydrophobic interactions and thus the reactions
can be carried out in water instead of organic solvents. The fluores-
cence microscopy image of hydrophobic dye encapsulated vesicles
provides evidence for their ability to entrap hydrophobic moieties
(S-II Figure S1). The nanoreactors are embellished with catalyti-
cally active CuNPs and when there is a confinement of reactant
molecules in the vesicular bilayer these NPs come in close proxim-
ity with the reactants.
4
0 mins when the reactant is m-nitrobenzaldehyde (Table 2, entry
2) whereas it is 30 mins in case of p-nitrobenzaldehyde (Table 2,
entry 1).
A time difference of 10 mins is related to the steric effect which
also has an influence on the yield (86% for m-nitrobenzaldehyde
and 90% for p-nitrobenzaldehyde). The reaction times in the pre-
sent study are much lower which can be explained on the basis
of the fact that CuNPs@vesicles are nanoreactors bearing catalyti-
cally active Cu (0) sites. The hydrophobic pockets inside the bilayer
structure of CuNPs@vesicles encapsulate the substrates and
enhance their concentration as well as proximity with catalytically
active Cu (0) sites resulting in faster catalysis.
The environmental impact of this catalytic protocol was
assessed through E-factor calculations which measures the amount
of waste generated during a synthetic process. The environmental
viability of this catalytic protocol was assessed in terms of E-factor
calculated as per the following formula:
Further the entrapment of reactant molecules inside the vesic-
ular bilayer causes their concentration at the active site thus caus-
ing a rapid catalysis (as shown in Fig. 7).
This is evident from the experimental data which shows that
low reaction time is required for complete formation of product.
The Cu (0) sites in catalyst activate the carbonyl carbon of the aro-
mass of waste generated during the reaction ðin gÞ
E ꢁ factor ¼
matic aldehyde for nucleophilic attack by ANH
phenylenediamine. The imine formed was further attacked by sec-
ond ANH group resulting in the formation of dihydroimidazole
2
group of o-
mass of the desired product formed ðin gÞ
Since the catalyst was recovered for the usage in new reaction
cycle the mass of catalyst has not been considered for the E-
factor calculations. The values of E-factor have been calculated
for the synthesis of all the benzimidazole derivatives and are given
in Table 2. The E-factor values evaluated for the reactions were
found to be in the range of 0.11–0.19 advocating green nature of
the proposed protocol.
Another advantage of present catalytic system over previous
reports is its recyclability upto 5 cycles with a marginal decrease
in the yield. The optical micrograph of recycled catalyst after 1st
cycle showed spherical morphology indicative of the fact that the
vesicles remain intact even after recycling as shown in Fig. 6. Cor-
2
followed by aromatization to give 2-aryl-1H-benzimidazoles as
product.
To explore the versatility of this catalyst, the catalytic protocol
was extended to dehydrogenative coupling of 2-nitroaniline and p-
nitrobenzylalcohol to form 2-(4-nitrophenyl)-1H-benzo[d]imida
zole (Scheme 4). The aim for selecting this reaction was to impro-
vise the conditions with the help of this catalyst. Conventionally
this reaction is carried out using expensive transition metals (Au,
Pd, Ru etc.) [46,47]. In addition they require harsh conditions like
high temperature (150–170 °C), toxic solvents (toluene, DMF, diox-
ane) and presence of acid or base to yield 70% of product [48].
9

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
Fig. 7. Probable mechanism of catalysis within the nanoreactors: the hydrophobic interior accommodates water insoluble organic substrates.
Scheme 4. Synthesis of 2-(4-Nitrophenyl)-1H-benzo[d]imidazole via dehydrogenative coupling of 2-nitoaniline and p-nitrobenzyl alcohol using CuNPs@vesicles.
product yields. As successfully validated, upon employing
CuNPs@vesicles as catalytic systems, the need for stoichiometric
amounts of reagents and hazardous solvents can be eliminated.
Thus the approach is environmentally benign, as the entire cascade
of reactions proceeds in aqueous medium at ambient conditions.
The important feature of regeneration and recyclability (upto 5
cascade cycles) can be attributed to the robust nature and stability
of vesicular nanostructures. With appropriate optimizations it will
be possible to extend the scope of dehydrogenative coupling at
ambient temperatures using substituted benzyl alcohols and
o-nitroaniline.
Table 3
Optimization of reaction condition (dehydrogenative coupling).
Entry
Catalyst
Temperature (°C)
%Yield
1
2
3
4
5
.
.
.
.
.
30
80
120
150
170
No conversion
No conversion
No conversion
Trace
CuNPs@vesicles
70%
Recently Li et al., have reported a base free catalysis but by
using palladium catalyst in toluene [49]. Herein, we report a con-
venient way to synthesize benzimidazole from o-nitroaniline and
p-nitrobenzylalcohol catalyzed by CuNPs@vesicles.
CRediT authorship contribution statement
Falguni Shukla: Investigation, Methodology, Formal analysis,
Writing - original draft. Manita Das: Visualization, Validation,
Methodology, Writing - review & editing. Sonal Thakore: Concep-
tualization, Supervision, Project administration, Funding
acquisition.
However, we successfully report the formation of desired pro-
duct with 70% isolated yield using in-expensive and recyclable
copper catalyst in water, making the protocol relatively sustain-
able. The reaction did not proceed at lower temperatures, while
at 150 °C traces of the product were obtained (Table 3, entry 4).
Further increase in temperature to 170 °C gave 70% of desired
product under base-free conditions. However extensive studies
and optimizations are required to synthesize the product at lower
temperatures using this catalyst which is a future prospect of the
work.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
4
. Conclusions
Acknowledgements
To conclude, metallovesicles derived from an amphiphilic poly-
mer and loaded with inexpensive metal nanoparticles were effi-
ciently used as catalyst for cascade synthesis of 2-aryl-1H-
benzimidazoles. The catalyst aided regioselective benzimidazole
formation upon using two different approaches. It is remarkable
that low level (1440 ppm) loadings resulted in good to excellent
The authors are grateful to DST-FIST, Department of Chemistry
for NMR facility and DST-FIST, Department of Biochemistry (SR/
FST/LS-II/2017/87) for florescence microscopy. We thank SAIF-IIT
Mumbai for HRTEM and FESEM; CSIR-CLRI for AFM analysis. We
are grateful to Dr. V.K. Aswal from BARC, Mumbai for DLS facility.
10

F. Shukla, M. Das and S. Thakore
Journal of Molecular Liquids 336 (2021) 116217
FS and MD acknowledge Sudeep Pharma AACD and Solvay
Research Fellowship respectively for financial support.
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