Ultrafine hybrid Cu2O-Fe2O3 nanoparticles stabilized by hexaphenylbenzene-based supramolecular assemblies: A photocatalytic system for the Ullmann-Goldberg coupling reaction

Article abstract of DOI:10.1039/c8gc02527d

Ultrafine hybrid Cu₂O-Fe₂O₃ NPs have been prepared using the supramolecular assemblies of hexaphenylbenzene (HPB) derivative 3 as nanoreactors and stabilizers. The as-prepared hybrid Cu₂O-Fe₂O₃ NPs serve as an efficient and recyclable photocatalytic system for carrying out C-N coupling between aryl halides and various amines (aliphatic, aromatic and N-heterocyclic) at room temperature in mixed aqueous media under visible light irradiation. Amazingly, Cu₂O-Fe₂O₃ NPs also exhibited high efficiency in the reactions involving the synthesis of biologically important N-substituted carbazole derivatives. The work being presented in this article demonstrates the preparation of a 'dip strip' coated with the as-prepared catalytic system and utilization of this paper strip as a recyclable and portable heterocatalytic system for carrying out the Ullmann-Goldberg coupling.

Full text of DOI:10.1039/c8gc02527d

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PAPER
Ultrafine hybrid Cu O–Fe O nanoparticles
2
2 3
stabilized by hexaphenylbenzene-based
supramolecular assemblies: a photocatalytic
system for the Ullmann–Goldberg coupling
reaction†
Cite this: DOI: 10.1039/c8gc02527d
Gurpreet Singh,
Manoj Kumar * and Vandana Bhalla
*
Ultrafine hybrid Cu
2
O–Fe
2
O
3
NPs have been prepared using the supramolecular assemblies of hexa-
O–Fe
phenylbenzene (HPB) derivative 3 as nanoreactors and stabilizers. The as-prepared hybrid Cu
2
2 3
O
NPs serve as an efficient and recyclable photocatalytic system for carrying out C–N coupling between
aryl halides and various amines (aliphatic, aromatic and N-heterocyclic) at room temperature in mixed
2 2 3
aqueous media under visible light irradiation. Amazingly, Cu O–Fe O NPs also exhibited high efficiency
Received 10th August 2018,
Accepted 22nd October 2018
in the reactions involving the synthesis of biologically important N-substituted carbazole derivatives. The
work being presented in this article demonstrates the preparation of a ‘dip strip’ coated with the as-pre-
pared catalytic system and utilization of this paper strip as a recyclable and portable heterocatalytic
system for carrying out the Ullmann–Goldberg coupling.
DOI: 10.1039/c8gc02527d
rsc.li/greenchem
high temperature, presence of additional ligands, high equiva-
lents of bases and organic solvents as the reaction media.
Introduction
The development of facile methods for introducing nitrogen Further, the use of stoichiometric amounts of metal reagents
functionality into organic frameworks has been one of the such as CuI restricted the large scale industrial application of
2
1
major research topics in synthetic organic chemistry owing to these reported systems. Thus, the development of new sus-
the ubiquity of heterocyclic compounds in natural products tainable and green synthetic methods for carrying out C–N
1
–5
and medicinally important derivatives.
Conventionally, coupling is a challenge yet to be achieved.
Our research work involves the development of supramole-
palladium-catalyzed Buchwald–Hartwig coupling of amines
and aryl halides is a powerful tool for the synthesis of cular ensembles having metal nanoparticles as catalytic
6
N-heterocyclic compounds. Although this method of amin- centers for carrying out various carbon–carbon and carbon–
ation is efficient, the reaction conditions require an inert heteroatom bond formation reactions under mild
2
3–27
atmosphere, high temperature and high loading of the palla- conditions.
In continuation of our efforts in this direction,
dium catalyst for obtaining the target molecule in good to we were then interested in the development of more efficient,
excellent yields. For making this coupling reaction eco- greener, recyclable and economical catalytic systems for C–N
friendly, a variety of catalytic systems based on transition bond formation. In the literature, several research groups have
7
8–11
12–14
15–19
metal ions such as gold, copper,
nickel,
cobalt
reported the utilization of benign, economical copper metal-
2
0
and zinc have been developed over the last decade. Despite based catalytic systems for carrying out C–N bond formation
the different beneficial aspects of these reported catalytic reactions, which work efficiently only under an inert atmo-
systems, it was not possible to overcome the requirement of sphere at a high temperature in the presence of additives and
8
–11,28
organic solvents.
Recently, an Ir/Cu-based photocatalytic
system has been reported for carrying out C–N coupling reac-
Department of Chemistry, UGC Sponsored Centre for Advanced Studies-II,
Guru Nanak Dev University, Amritsar 143005, Punjab, India.
E-mail: vanmanan@yahoo.co.in, mksharmaa@yahoo.co.in
tion in organic solvents under the irradiation of a blue LED
2
2
and in the presence of a strong organic base (Scheme 1A).
These reports inspired us to develop a copper-based photo-
catalytic system which could overcome the limitations of pre-
viously reported systems such as the inert atmosphere, pres-
ence of additives, organic solvents and utilization of a blue
†
Electronic supplementary information (ESI) available: The contents of the ESI
1
13
section include H, C NMR and ESI-MS spectra of compounds; UV-vis and
SEM and TEM images; powder XRD analysis; VSM analysis; DLS studies; XPS
studies; and IR spectroscopy. See DOI: 10.1039/c8gc02527d
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cally active iron species in bimetallic magnetic nano-
composites will improve the recyclability of the catalytic
system in C–N bond formation reactions. Unfortunately, the
photophysical properties of derivative 1 remained unchanged
in the presence of ferric ions. To enhance the affinity of the
HPB derivative towards iron, we introduced morpholine
groups at the periphery of the molecule through imine lin-
kages. Morpholine moieties are known to have affinity toward
3
3
ferric ions. We envisaged that morpholine groups along with
imine linkages will enhance the affinity of the molecule
Scheme 1 Photocatalytic systems for the Ullmann–Goldberg coupling
reaction.
3
+
2+
towards Fe and Cu ions. As expected, derivative 3 formed
spherical fluorescent supramolecular aggregates in the mixed
aqueous media, which exhibited sensitive response toward
2
+
3+
Cu and Fe ions and served as reactors and stabilizers for
the preparation of ultrafine hybrid Cu O–Fe NPs. Further,
LED as the radiation source for carrying out C–N bond for-
mation reactions. We envisioned that a supramolecular ensem-
ble developed in mixed aqueous media having copper as the
catalytic center and supramolecular assemblies as the visible
light harvesting centers could be a good choice for the prepa-
ration of an efficient catalytic system for carrying out C–N
bond formation reactions under mild conditions. For the
development of such a photocatalytic system, we planned to
have a hexaphenylbenzene (HPB) core as the scaffold of the
supramolecular assemblies. HPB derivatives are known to emit
in the wavelength range of 400–550 nm (ref. 29 and 30); hence,
a good overlap is expected between the emission spectrum of
2
2 3
O
supramolecular aggregates of the HPB derivative in combi-
nation with hybrid Cu O–Fe O NPs act as efficient photocata-
2
2 3
lysts for Ullmann–Goldberg coupling under mild conditions
Scheme 1B). The photocatalytic system being presented in
this article has several advantages over the literature reports
Table S1 in the ESI†). First, the aggregates of derivative 3 act
(
(
as nanoreactors for the preparation of ultrafine nanomaterials
in the absence of any additional reducing and stabilizing
agents. Previously, there were a few reports for the preparation
3
4,35
of Cu
2
O–Fe
2
O
3
alloy NPs;
however, these methods involve
the utilization of harsh conditions such as the use of reducing/
capping agents, organic solvents, high temperature, longer
2
the HPB derivative and the absorption spectrum of Cu O NPs,
which is a must for possible energy transfer between supramo-
lecular assemblies and metal NPs during the photocatalytic
reaction time and an inert atmosphere to yield the Cu
2
O–
Fe O alloy NPs (Table S2 in the ESI†) of bigger size. In this
2
3
2
process. Recently, we have reported Ag@Cu O core–shell NPs
context, the method being reported in this article is con-
venient, fast and ‘green’. We believe that the preparation of
ultrafine (5–7 nm) Cu O–Fe O magnetic nanocomposites is
stabilized by the supramolecular assemblies of HPB derivative
1
(Chart 1) and utilized them as an efficient photocatalytic
3
1
2
2 3
system for carrying out C–H activation reactions.
unprecedented in the literature. Additionally, during the for-
mation of nanomaterials, the imine linkages were hydrolyzed,
which restored the aldehyde groups at the periphery of deriva-
tive 3. To the best of our knowledge, this is the first report of
alloy nanoparticle induced hydrolysis of the Schiff base to
generate the aggregates of the HPB derivative having aldehyde
groups at their periphery. Second, the as-prepared ultrafine
As a test of our hypothesis, we planned to examine the
applicability of this system for carrying out C–N bond for-
mation reactions under photocatalytic conditions. We chose
the reaction between iodobenzene and morpholine as the
model reaction. The reaction proceeded; however, the target
product was obtained only in 40% yield (vide infra). This study
2
suggests that the Ag@Cu O system is not an efficient catalytic
system for C–N bond formation. We envisaged that by switch-
2 2 3
hybrid Cu O–Fe O nanoparticles act as an efficient catalytic
system for carrying out the visible light mediated C–N coupling
reaction of aromatic halides with various amines (primary/sec-
ondary aliphatic, aromatic and N-heterocyclic amines).
Additionally, being magnetic in nature, the separation of the
present catalytic system was very easy and it exhibited recycl-
ability for six runs. The good recyclability of the catalytic
system clearly demonstrates the efficiency of the supramolecu-
lar aggregates in preventing the leaching of nanomaterials.
Previously, palladium- and CuI-based catalytic systems have
been used for carrying out reactions between aryl halides and
ing Ag NPs with some other metallic species which could
assist Cu O in C–N bond formation reactions, the yield of the
2
target compound may be improved. Among all the other metal
ions, the iron metal ion is of our choice as the catalytic partner
of copper due to its known catalytic efficiency in C–N bond for-
3
2
mation. We further expected that the presence of magneti-
aliphatic amines. However, these reactions required the pres-
t
ence of strong bases such as NaO Bu, Cs
2
CO
3
, ligands, organic
solvents (DMSO, DMF, isopropanol, and toluene), high temp-
2
8,36–39
erature and longer reaction time.
Third, most impor-
tantly, the as-prepared catalytic system exhibited high
Chart 1 Structure of HPB derivative 1.
efficiency in the preparation of N-substituted carbazoles. The
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development of simple and efficient methods for the prepa- Photophysical properties of derivative 3
ration of N-substituted derivatives of carbazole is important
The UV-vis spectrum of derivative 3 in THF exhibits an absorp-
tion band at 305 nm due to the π–π* transition. On adding
water fractions up to 90% (volume fraction) to the THF solu-
tion, the intensity of the absorption band increased with a
slight red shift (∼6 nm) from 305 to 311 nm (Fig. S1 in the
ESI†). Further, a level-off tail was also observed in the visible
due to the potential applications of these compounds as build-
ing blocks for the preparation of molecules having material
and pharmaceutical applications. In the literature, an iridium-
based complex has been used as a photocatalyst for carrying
out N-arylation of carbazoles under strong basic conditions
using DMSO as a solvent under a blue LED and inert atmo-
41
region due to the formation of aggregates. The THF solution
2
2
sphere for a prolonged reaction time (18 h). Very recently, an
Ir/Ni-based catalytic system has been reported which could not
catalyze the reaction involving amines lacking in α-hydrogens
even in the presence of DABCO as the base and DMSO as the
of derivative 3 is non-emissive when excited at 305 nm;
however, an emission band at 440 nm was observed on
addition of 10% water fraction (Φ = 0.06) to the THF solution.
On increasing the water fraction up to 60% (volume fraction),
the emission band red-shifted to 455 nm with an enhance-
ment in the emission intensity (Φ = 0.45) (Fig. S2 in the ESI†).
We also studied the fluorescence behaviour of derivative 3 in
different solvent mixtures of DMSO and glycerol. It was
observed that upon increasing the volume fraction of glycerol
from 0% to 99%, the emission intensity of derivative 3 was
enhanced (Fig. S3 in the ESI†). This enhancement in the emis-
sion intensity upon increasing the viscosity of the solvent
clearly suggests that the restriction to intramolecular rotation
is the reason behind the aggregation induced emission
enhancement (AIEE) phenomenon. The absorption and fluo-
rescence studies suggest the formation of J-aggregates of
derivative 3 in the mixed aqueous media. The concentration-
1
2
2 2 3
solvent. In contrast to this, the as-prepared Cu O–Fe O
nanocatalytic system works efficiently in mixed aqueous media
under mild reaction conditions and requires a shorter reaction
time period (6 h) for the completion of the reaction. Fourth,
the ‘dip strip’ prepared by dipping a filter paper strip into the
solution of a supramolecular ensemble worked as a heterocata-
lytic system for carrying out C–N bond formation reactions.
The dried coated strip could be reused as a heterocatalytic
system for carrying out C–N bond formation reactions up to
three cycles.
Results and discussion
Synthesis and characterization of hexaphenylbenzene
1
dependent H NMR studies of derivative 3 (Fig. S4 in the ESI†)
show a slight down-field shift of all the aromatic protons,
which also supports the formation of J-aggregates (Scheme 3).
The transmission electron microscopy (TEM) image of com-
derivative 3
4
0
Condensation of HPB derivative 1 with 4-(2-aminoethyl)mor-
pholine 2 in dichloromethane (DCM)/ethanol (EtOH) (2 : 8)
furnished compound 3 in 75% yield (Scheme 2). The structure
of compound 3 was confirmed from its spectroscopic data.
pound 3 in H O/THF (6/4) showed the presence of spherical
2
aggregates having an average size in the range of 400–450 nm
(
Fig. S5 in the ESI†) in the mixed aqueous media. The
1
The H NMR spectrum of compound 3 showed one singlet at
dynamic light scattering (DLS) studies also indicate the
average size of aggregates in the range of 400–500 nm (Fig. S6
in the ESI†) in H O/THF (6/4) solvent mixture.
2
8
7
.26 ppm corresponding to imino protons, four doublets at
.66, 7.47, 7.17, and 6.94 ppm corresponding to aromatic
protons, two multiplets – one at 6.86–6.85 ppm corresponding
to aromatic protons and another at 3.78–3.69 ppm corres-
ponding to methylene protons, two triplets at 2.70 and
2 2 3
Preparation of ultrafine hybrid HPB@Cu O–Fe O NPs
2
.54 ppm corresponding to methylene protons. A parent ion The presence of imino moieties and nitrogen atoms in deriva-
+
peak for [M + H] was observed at m/z = 967.4964 in the mass tive 3 prompted us to examine the binding ability of derivative
spectrum. The IR spectrum of derivative 3 showed character- 3 toward different metal ions, such as Zn , Cu , Hg , Au ,
istic peaks of CvN and C–N bonds at 1630 cm
1
2
+
2+
2+
3+
−
1
2+
3+
2+
2+
2+
2+
2+
2+
2+
+
and Fe , Fe , Co , Pb , Pd , Ni , Cd , Ba , Mg , Ag , and
116 cm− , respectively. These spectroscopic data corroborate Al as both their perchlorate and chloride salts using UV–vis
1
3+
3
+
structure 3 for this compound (Fig. S75–S78 in the ESI†).
spectroscopy. Upon gradual addition of Fe ions (0–60 equiv.)
to the solution of derivative 3 (5 μM) in H O/THF (6/4, v/v), a
2
new absorption band at 500 nm was observed, which corres-
ponds to the surface plasmon resonance (SPR) band of Fe O
2
3
4
2
nanoparticles (Fig. S7 in the ESI†). The color of the solution
changed from colorless to yellow, which was clearly visible to
the naked eye. The TEM image of the solution showed the for-
mation of the rod-shaped nanoparticles (Fig. S8 in the ESI†).
2
+
Further, upon gradual addition of Cu ions (0–120 equiv.) to
the solution of derivative 3 (5 μM) in H O/THF (6/4, v/v), new
2
absorption bands were observed at 258 and 715 nm, respect-
ively, within 30 min (Fig. S9 in the ESI†). These absorption
bands correspond to the surface plasmon resonance (SPR)
Scheme 2 Synthesis of hexaphenylbenzene-based derivatives 3.
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4
3,44
band of the Cu
2
O nanoparticles.
The color of the solution
changed from colorless to light reddish, which was clearly
visible to the naked eye. The TEM image clearly shows the for-
mation of spherical nanoparticles (Fig. S10 in the ESI†). No
change in the absorption spectra was observed in the presence
of other metal ions (Fig. S11 in the ESI†). In the fluorescence
3
+
spectra, upon addition of 60 equiv. of Fe ions to the solution
of derivative 3 (5 μM) in the H O/THF (6/4, v/v) mixture, the
2
intensity of the emission band at 455 nm gradually decreased
2
+
(
(
Fig. S12 in the ESI†). Likewise, upon addition of Cu ions
120 equiv.) to the solution of aggregates of 3, the intensity of
the emission band at 455 nm gradually decreased (Fig. S13 in
the ESI†). These spectral changes indicate the interactions
3
+
2+
between the aggregates of derivative 3, Fe ions and Cu
ions.
Keeping in mind the potential of the aggregates of deriva-
tive 3 to generate Cu O and Fe O NPs, we planned to prepare
2
2
3
hybrid nanoparticles of Cu
2
2 3
O–Fe O using the supramolecular
assemblies of derivative 3 as a common platform. To the solu-
tion of supramolecular assemblies of derivative 3 in H O/THF
2
2
+
3+
(
(
6/4, v/v), we added Cu ions (0–120 equiv.) and Fe ions
0–60 equiv.), simultaneously and kept the solution for 30 min
at room temperature (Fig. 1B). The absorbance spectra of this
solution showed the absorption bands at 258 nm and in the
region of 700–750 nm characteristics of Cu
band in the region of 490–530 nm was observed corresponding
to the Fe nanoparticles (Fig. 1A). These spectral changes
2
O NPs. A broad-
2 3
O
indicate that the supramolecular assemblies of derivative 3
interact with metal ions and serve as reactors for the reduction
2 2 3
of metal ions into their nanoforms to generate Cu O–Fe O
2+
3+
Fig. 1 (A) UV-vis spectra of derivative 3 upon addition of Cu and Fe
ions simultaneously in the H O/THF (6/4) mixture. The inset figure shows
the color change upon addition of Fe and Cu ions. (B) Schematic re-
presentation for the preparation of hybrid Cu O–Fe NPs.
nanohybrid materials. The colour of the solution changed
from colourless to yellowish to dark brown (Fig. S14 in the
ESI†).
2
3+
2+
2
2 3
O
Characterization of the catalytic system
The TEM studies of the supramolecular assemblies of deriva-
3
+
2+
tive 3 in the presence of Fe and Cu ions showed the pres-
ence of spherical-shaped Cu O–Fe nanohybrid materials
2
2 3
O
supported on the assemblies of the HPB derivative (Fig. 2A).
The average size of the hybrid Cu O–Fe O nanoparticles was
2
2 3
found to be in the range of 5–7 nm. The high-resolution TEM
image showed d spacings of 0.21 nm and 0.24 nm (ref. 45)
corresponding to the (200) and (111) planes of Cu O NPs and
2
d spacings of 0.25 and 0.27 nm (ref. 46) corresponding to the
2 3
(110) and (104) planes of Fe O NPs (Fig. 2B and C). The
energy-dispersive X-ray spectroscopy (EDX) showed the pres-
ence of Fe, Cu and O elements in the nanohybrid material
(Fig. S15 in the ESI†). The TEM-EDS line-scanning profile con-
firmed the formation of a nanohybrid of Cu O–Fe O NPs
2
2 3
(
Fig. S16 in the ESI†). The DLS studies also indicated the pres-
ence of particles having an average size in the range of 5–8 nm
Fig. S17 in the ESI†). The powder X-ray diffraction (XRD)
measurement showed the presence of diffraction peaks at
9.60, 36.46, 42.24, 52.64, 61.42, and 73.63 attributed to the
110), (111), (200), (211), (220) and (311) crystal planes of cubic Cu
(
Fig. 2 (A) TEM images show ultrafine hybrid Cu
posited on the HPB derivative, (B) SAED and (C) HR TEM ultrafine hybrid
O–Fe NPs.
2 2 3
O–Fe O NPs de-
2
(
2
2 3
O
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Fig. 3 (a) XPS spectrum of the Cu 2p region, (b) XPS spectrum of the Fe
2p region, and (c) XPS spectrum of the 1s region of O of ultrafine hybrid
Cu O–Fe NPs.
2
2 3
O
(
primitive) Cu
2
O NPs, respectively. The diffraction peaks were
observed at 33.40 and 35.70 (2θ) corresponding to the (104)
3
4
and (110) planes of Fe
2 3
O NPs, respectively (Fig. S18 in the
ESI†). In the FT-IR spectrum of hybrid nanoparticles, peaks at
1
5
6
80 cm− corresponding to the Fe–O bond of Fe O NPs and at
2 3
Scheme 3 Schematic representation of the preparation of hybrid
Cu O–Fe O NPs using the aggregates of derivative 3.
28 cm− corresponding to Cu
1
O NPs were observed
47
2
2
2
3
(Fig. S19 in the ESI†). The XPS studies were further employed
to investigate the compositions and elemental states of the
samples. The high-resolution Cu 2p XPS spectrum (Fig. S20 in
the ESI†) shows the presence of two main peaks at 932.9 and under lab conditions in dry THF. The absorption spectrum of
52.7 eV, which correspond to the Cu 2p3/2 and Cu 2p1/2 spin– the reaction mixture did not show the formation of nano-
9
orbital components of Cu species, respectively (Fig. 3a). particles upon addition of metal ion solution (Fig. S24 in the
1
Further, the binding energies of the Fe 2p1/2 and Fe 2p3/2 are ESI†). After the usual workup, the H NMR spectrum of the
found to be 709.3 and 723 eV, respectively (Fig. 3b), which recovered compound clearly indicated the presence of deriva-
confirm the formation of Fe
hybrid Cu O–Fe NPs is shown in Fig. 3c. The super- studies suggest the important role played by supramolecular
paramagnetic nature of the hybrid Cu O–Fe O NPs was aggregates in aqueous media as reactors for the generation of
2 3
O NPs. The O 1s region of the tive 3 and no formation of aldehyde was observed. These
3
5
2
2 3
O
2
2 3
4
8
3+
2+
observed via magnetic hysteresis measurements (Fig. S21 in NPs. Afterwards, we examined the role of Fe and Cu ions,
the ESI†). separately in the formation of hydrolyzed species. We per-
To understand the mechanism of reduction of metallic formed two separate experiments in mixed aqueous media
species by the aggregates of derivative 3, we performed a reac- under lab conditions. Interestingly, in both the cases, clea-
3
+
2+
tion between the Fe , Cu ions and supramolecular assem- vage of imino linkages was observed. Further, to gain more
blies of derivative 3 in H O/THF (6/4, v/v) at room temperature insight into the mechanism of cleavage of imine linkages (i.e.
2
under lab conditions. After the formation of Cu
30 min), the solution of the aggregates of derivative 3 contain- of imine linkages is a simultaneous process or the cleavage is
ing the nanoparticles was slowly evaporated. After two days, induced by alloy NPs), we prepared bare Cu O–Fe O NPs and
2 2 3
O–Fe O NPs whether the formation of hybrid nanoparticles and cleavage
(
2
2 3
brown precipitates appeared which were separated by fil- performed the same reaction in mixed aqueous media in the
tration and then washed with THF. The filtrate was evapor- presence of these NPs. Amazingly, bare Cu O–Fe NPs
ated to obtain the solid residue. The presence of a signal at could not cleave the imine linkages. These observations
0.00 ppm corresponding to the aldehyde proton and the clearly suggest that the formation of hybrid Cu O–Fe NPs
2
2 3
O
1
2
2 3
O
absence of a signal at 8.26 ppm corresponding to the imino and the cleavage of the imine linkages go side by side. Next,
1
proton in the H NMR spectrum of the solid residue (Fig. S22 we examined the photophysical behaviour of derivative 1 in
in the ESI†) suggested the hydrolysis of imino moieties the H
during the formation of hybrid Cu O–Fe NPs (shown in band at 455 nm. Upon addition of bare hybrid NPs, the emis-
Scheme 3). Further, in the IR spectrum of the hydrolyzed sion intensity of the solution of derivative 1 decreased by
2
O/THF (6 : 4) mixture. Derivative 1 showed an emission
2
2 3
O
−
1
species, a sharp peak was observed at 1697 cm
ponding to the CvO stretching of the aldehyde group was observed between the fluorescence emission spectrum of
Fig. S23 in the ESI†). To gain insight into the metal NP- derivative 1 and the absorption spectrum of Cu O–Fe O NPs,
corres- 74% (Fig. S25 in the ESI†). Further, a good spectral overlap
(
2
2 3
induced cleavage of the imine linkage, we performed a series which suggests the possibility of energy transfer between
of experiments. First, we carried out a reaction between hydrolyzed species 1 and Cu
derivative 3 and metal ions (Fe and Cu ions) separately ESI†).
2
O–Fe
2
O
3
NPs (Fig. S26 in the
3
+
2+
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9
50
Photocatalytic activity of the ultrafine hybrid Cu
for the Ullmann–Goldberg coupling
2
O–Fe
2
O
3
NPs
2 2 3 2 2 3
bare Cu O NPs, bare Fe O NPs and bare Cu O–Fe O
3
4
NPs by hydrothermal methods. We evaluated the catalytic
efficiency of these metal nanoparticles in the C–N coupling of
iodobenzene and morpholine under visible light. The desired
product was obtained in 30% yield in the presence of Cu O
NPs, 20% yield in the presence of Fe
the presence of Cu O–Fe NPs (Table 1, entries 10–12). The
above results highlight the important contribution of supra-
molecular assemblies towards the catalytic efficiency of hybrid
Cu O–Fe O NPs in the coupling reaction. We also carried out
2 2 3
The possibility of energy transfer between the supramolecular
assemblies and nanoparticles prompted us to investigate the
catalytic efficiency of the as-prepared hybrid catalytic system in
the Ullmann–Goldberg coupling reaction under visible light
irradiation. The reaction between iodobenzene (4a, 0.4 mmol)
2
2 3
O NPs and 32% yield in
2
2 3
O
and morpholine (5, 2 mmol) in the H
mixture in the presence of 0.5 mol% hybrid HPB@Cu
2
O/EtOH (1 : 1) solvent
2 3
O–Fe O
2
nanoparticles under aerial conditions and visible light
irradiation was screened as the model reaction. To our delight,
the reaction was complete in six hours and the desired
product 6 was obtained in 90% yield (Table 1, entry 1). Next,
we repeated the model reaction in different solvents such as
triethylene glycol (TEG), glycerol, EtOH, water and the
the model reaction in the presence of the aggregates of HPB
derivative 1, but the reaction did not proceed (Table 1, entry
1
3). This result shows that the hybrid Cu
posite is the catalytic center. Further, the reaction in the pres-
ence of the assemblies of derivative 1 and bare Cu O–Fe
NPs furnished the desired product only in 25% yield (Table 1,
entry 14).
Next, we investigated the effect of the halide (Br and Cl)
leaving group on the progress of the reaction. Interestingly, in
the presence of hybrid HPB@Cu O–Fe O NPs as a catalyst,
2 2 3
O–Fe O nano-com-
2
2 3
O
EtOH : H O mixture (Table 1, entries 1–5). The desired product
2
was obtained in comparable yields in all these solvents;
however, the yield of the final product was low in aqueous
media. Thus, we chose H O : EtOH (1 : 1) as the reaction
2
2
2 3
medium for carrying out further reactions. Further, we also
the Ullmann–Goldberg coupling involving bromobenzene and
chlorobenzene went smoothly to yield the products (Scheme 4)
in high yields. In the case of bromobenzene, the product was
obtained in 85% yield, while the yield was reduced to 70% in
the case of chlorobenzene.
investigated the effect of various bases such as K CO , Cs CO
3
2
3
2
and t-BuOK on the photocatalytic efficiency of the Ullmann–
Goldberg coupling reaction (Table 1, entries 6 and 7).
Interestingly, switching of the base has no significant effect on
the yield of the reaction; however, in the absence of alkaline
conditions, the desired product was obtained only in traces
(Table 1, entry 8). Thus, a basic medium is essential for carry-
ing out the reaction. We chose K CO as the base due to its
2
3
more availability and economic viability. We also performed
the model reaction under dark conditions and the coupled
product was obtained in traces (Table 1, entry 9). This study
clearly emphasizes the important role of visible radiation in
the feasibility of the reaction. Further, to understand the role
Scheme 4 C–N coupling reaction between aryl halides and morpho-
line in H O/EtOH (1 : 1), using hybrid HPB@Cu O–Fe O NPs under
of supramolecular assemblies along with hybrid Cu O–Fe O
2 3
2
2
2
2
3
nanoparticles in the Ullmann–Goldberg coupling, we prepared visible light irradiation.
Table 1 Optimization of the reaction conditions for the model reaction between iodobenzene and morpholine
Entry
Catalytic system
Time (h)
Base
Solvent
O/EtOH (1 : 1)
EtOH
Triethylene glycol
Glycerol
Light/dark
Isolated yield
1
2
3
4
5
6
7
8
9
HPB@Cu
HPB@Cu
HPB@Cu
HPB@Cu
HPB@Cu
HPB@Cu
HPB@Cu
HPB@Cu
HPB@Cu
2
O–Fe
2
O–Fe
2
O–Fe
2
O–Fe
2
O–Fe
2
O–Fe
2
O–Fe
2
O–Fe
2
O–Fe
2
2
2
2
2
2
2
2
2
O
3
O
3
O
3
O
3
O
3
O
3
O
3
O
3
O
3
NPs
NPs
NPs
NPs
NPs
NPs
NPs
NPs
NPs
6
6
6
6
10
6
6
8
24
6
6
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
3
H
2
Light
Light
Light
Light
Light
Light
Light
Light
Dark
Light
Light
Light
Light
Light
90%
92%
80%
83%
65%
90%
90%
Traces
Traces
30%
20%
32%
No reaction
25%
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O
Cs
2
CO
3
O/EtOH (1 : 1)
O/EtOH (1 : 1)
O/EtOH (1 : 1)
O/EtOH (1 : 1)
O/EtOH (1 : 1)
O/EtOH (1 : 1)
O/EtOH (1 : 1)
O/EtOH (1 : 1)
O/EtOH (1 : 1)
t-BuOK
—
K
K
K
K
K
K
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
1
1
1
1
1
0
1
2
3
4
Bare Cu
Bare Fe
Bare bimetallic Cu
2
O
2
O
3
2
O–Fe
O
2 3
NPs
6
6
6
HPB derivative 1 (hydrolyzed species)
HPB derivative 1 (hydrolyzed species) and bare Cu
2
O–Fe
O
2 3
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Next, we examined the substrate scope with regard to
various aryl halides and cyclic amines under the optimized
reaction conditions. We carried out the reaction between sub-
stituted aryl halides and morpholine(5)/piperidine(9)/pyrroli-
dine(10) in the presence of hybrid HPB@Cu O–Fe O NPs
2
2 3
(Scheme 5). Aryl halides having electron-donating and elec-
tron-withdrawing groups furnished the desired products in
excellent yields. Both substituted aryl iodides and aryl bro-
mides worked well under the optimized reaction conditions.
These results show that the as-prepared hybrid HPB@Cu
2
O–
Fe O NPs act as an efficient and greener catalytic system for
2
3
carrying out the Ullmann–Goldberg reaction under visible
light irradiation.
Further, different substituted aryl iodides were successfully
3
treated with an aqueous solution of NH (14) to obtain the
corresponding substituted anilines in high yields under the
optimized reaction conditions (Scheme 6; 18a–18d). The
efficiency of the hybrid HPB@Cu
2
O–Fe
2
O
3
NPs toward
different primary and secondary aliphatic amines was also
investigated. To our delight, the C–N coupling of aryl iodides
with primary aliphatic amines such as n-butylamine (15),
n-octylamine (16) and secondary 2-aminooctylamine (17) furn-
ished N-arylated amines in excellent yields (Scheme 6; 19, 20a–
c, and 21).
Scheme 6 C–N coupling reaction between substituted aryl iodides and
The Ullmann–Goldberg coupling reaction using hybrid ammonia/aliphatic amines in
2
H O/EtOH (1 : 1), using hybrid
HPB@Cu O–Fe NPs under visible light irradiation.
2
2 3
O
HPB@Cu O–Fe O NPs was further extendable to primary and
2
2 3
secondary aromatic amines. We carried out the reactions
between aniline (22) and diphenylamine (23) with aryl iodides
under the optimized reaction conditions. The substituted
diphenylamines and triphenylamines were furnished in good
to excellent yields (Scheme 7).
Next, we examined the reactivity of N-heterocyclic amines
toward aryl halides in the presence of hybrid HPB@Cu O–
2
2 3
Fe O NPs. We carried out the Ullmann–Goldberg coupling
Scheme 5 C–N coupling reaction between substituted aryl halides and
cyclic amines in H O/EtOH (1 : 1), using hybrid HPB@Cu O–Fe NPs
under visible light irradiation.
Scheme 7 C–N coupling reaction between substituted aryl iodides and
aniline/diphenylamine in H O/EtOH (1 : 1), using hybrid HPB@Cu O–
Fe NPs under visible light irradiation.
2
2
2
O
3
2
2
2 3
O
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reaction between carbazole (26) and various substituted aryl
iodides under the optimized reaction conditions. Interestingly,
N-substituted carbazole derivatives having electron-donating
and electron-withdrawing groups were obtained in good yields
(Scheme 8). Recently, the synthesis of N-substituted carbazole
derivatives has been catalyzed by an iridium-based photo-
t
catalytic system in the presence of a copper salt and LiO Bu
2
2
under blue LED irradiation in DMSO as the reaction media.
On the other hand, hybrid HPB@Cu O–Fe NPs act as a sus-
2
2 3
O
tainable and reusable photocatalytic system for the synthesis
of N-substituted carbazole under mild conditions to afford the
target derivative in high yield.
Scheme 10 C–N coupling reaction between substituted aryl iodides
We also examined the photocatalytic efficiency of hybrid
HPB@Cu O–Fe O NPs in the reaction between heteroaryl
and indole in H
2 2 2 3
O/EtOH (1 : 1), using hybrid HPB@Cu O–Fe O NPs
2
2 3
under visible light irradiation.
bromide (28) and carbazole (26). The reaction was complete in
3 h under the optimized reaction conditions to furnish the
target compound in good yield (Scheme 9).
Further, the hybrid HPB@Cu O–Fe NPs were utilized in
1
2
2 3
O
the reaction between indole (30) and substituted aryl iodides.
The reaction was complete in 10 h and the desired products
were obtained in good yields (Scheme 10). Interestingly,
N-substituted indole 31b having an electron-withdrawing
group (–NO ) was obtained in a higher yield in comparison
2
with indoles 31a and 31c having neutral and electron-donating
groups.
Inspired by these results, we checked the catalytic efficiency
2 2 3
of the hybrid HPB@Cu O–Fe O NPs in the reaction of 1-iodo-
4
-nitrobenzene (7a) with other N-heterocyclic amines such as
Scheme 11 C–N coupling reaction between 1-iodo-4-nitrobenzene
and different N-heterocyclic amines in H O/EtOH (1 : 1), using hybrid
HPB@Cu O–Fe NPs under visible light irradiation.
Scheme 8 C–N coupling reaction between substituted aryl iodides and
carbazole H O/EtOH (1 : 1), using hybrid HPB@Cu O–Fe under visible
light irradiation.
2
2
2
2 3
O
2 3
O
2
benzimidazole (32), indazole (34), pyrrole (36) and imidazole
38). The desired products were furnished in excellent yields
(
under the optimized reaction conditions (Scheme 11).
We also examined the applicability of the as-prepared
2 2 3
hybrid HPB@Cu O–Fe O NPs for the synthesis of derivative
41, which is an important precursor of different materials
having applications in the OLED industry. The reaction
between the pyrazine derivative (40) and carbazole (26) in the
presence of the as-prepared catalytic system in mixed aqueous
Scheme 9 C–N coupling reaction between 2-pyridinyl bromide and
carbazole in H O/EtOH (1 : 1), using hybrid HPB@Cu O–Fe NPs
under visible light irradiation.
2
2
2 3
O
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Scheme 12 C–N coupling reaction between the derivatives of pyrazine
and carbazole in H
2 2 2 3
O/EtOH (1 : 1), using hybrid HPB@Cu O–Fe O NPs
under visible light irradiation.
media furnished the desired product in 60% yield (Scheme 12)
Fig. S44 and S45 in the ESI†). Earlier, this compound had
(
been synthesized using palladium acetate as a catalyst under
an inert atmosphere at a high temperature (140 °C) in the pres-
5
1
ence of DMSO as the solvent.
Further, we chose the reaction between iodobenzene 5a and
morpholine 6 as the model reaction to examine the recyclabil-
ity of the photocatalytic system. The as-prepared hybrid
HPB@Cu O–Fe O NPs could be easily separated from the
Scheme 13 Proposed mechanism of photocatalytic C–N coupling
catalyzed by HPB@Cu O–Fe O NPs (irradiation with a 60 W tungsten
2 2 3
2
2 3
aqueous layer of the reaction mixture by simple magnetic sep-
aration and the isolated catalytic system was reused up to six
times. After the sixth cycle, the yield of the final product
reduced to 70% (Fig. 4). The TEM image of the recycled cata-
lytic system showed a change in the size and morphology of
the hybrid NPs (Fig. S27 in the ESI†). We also carried out
atomic absorption studies (AAS) of the residual liquid left after
the recycling of the catalyst and found that only 1.91 ppm of
Cu had leached into the solution.
filament bulb).
strip clearly shows the deposition of the supramolecular
ensemble on the filter paper (Fig. S29 in the ESI†). The EDX
pattern of the dip-coated paper strip also confirmed the depo-
2 2 3
sition of the hybrid HPB@Cu O–Fe O NPs (Fig. S30 in the
ESI†). The as-prepared coated filter paper strip was used as a
heterocatalytic system for carrying out the reaction between
iodobenzene (4a) and morpholine (5). To our delight, the reac-
tion was complete in 7 h to furnish the desired product in
Mechanism of the photocatalytic Ullmann–Goldberg coupling
catalyzed by ultrafine hybrid HPB@Cu
To understand the mechanism of C–N coupling reaction cata-
lyzed by hybrid HPB@Cu O–Fe NPs, we investigated the
2 2 3
O–Fe O NPs
86% yield. After the completion of the reaction, the coated
paper strip was removed from the reaction mixture, air dried
and reused as a catalytic system for the second run by follow-
ing the same procedure. The coated paper strip could be
reused up to three cycles for carrying out the C–N coupling
reaction and after the 3 cycle the desired product was furn-
ished in 65% yield (Fig. 5). The SEM image and EDX profile of
the paper strip after the 3 cycle showed a decrease in the
coated area of the paper strip (Fig. S31 and S32 in the ESI†).
We believe that the partial removal of the coated material
2
2 3
O
model reaction of 4a with 5 in the presence of the radical sca-
venger 2,2,6,6-tetramethyl-1-piperidinoxyl (TEMPO, 2 equiv.).
The presence of TEMPO completely inhibited the reaction. The
ESI MS studies of the reaction mixture suggested the for-
mation of an adduct between benzene and TEMPO (Fig. S28 in
the ESI†). These studies support the radical mechanism for
the reaction as shown in Scheme 13.
rd
rd
Further, we also prepared a ‘dip strip’ by dip coating a filter
paper strip in the catalytic solution. The SEM image of the
Fig. 5 Recyclability of the dip-coated paper strip with hybrid
Fig. 4 Recyclability of hybrid HPB@Cu
2
O-Fe
2
O
3
NPs for carrying out
HPB@Cu
2 2 3
O–Fe O NPs for carrying out C–N coupling reaction between
C–N coupling reaction between morpholine and iodobenzene.
morpholine and iodobenzene under visible light.
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during the recyclability experiment is the reason behind the after 30 min. 0.5 mol% of this solution was used as such in
decreased efficiency of the catalytic system.
each catalytic experiment.
Synthesis of derivative 3
Conclusion
A clear solution of derivative 1 (0.1 g, 0.13 mmol) and 4-(2-
A HPB-based derivative 3 has been designed and synthesized, aminoethyl)morpholine 2 (0.035 g, 0.27 mmol) in dry dichloro-
which forms supramolecular assemblies in aqueous media. methane (DCM)/ethanol (EtOH) (2 : 8) was refluxed at 70 °C.
The supramolecular assemblies of derivative 3 act as nanoreac- After the completion of the reaction, precipitates were formed
tors and stabilizers for the synthesis of ultrafine hybrid (48 h) which were filtered and recrystallized from methanol to
1
HPB@Cu
capping agents in mixed aqueous media at room temperature. NMR (300 MHz, CDCl
The ultrafine hybrid HPB@Cu O–Fe O NPs serve as an 6 Hz, 4H, Ar–H), 7.47 (d, J = 6 Hz, 4H, Ar–H), 7.17 (d, J = 9 Hz,
2
O–Fe
2
O
3
NPs without using any reducing and afford the white compound 3 in 78% yield; mp: >250 °C; H
3
) δ = 8.26 (s, 2H, HCvN), 7.66 (d, J =
2
2 3
efficient, recyclable photocatalytic system for carrying out C–N 4H, Ar–H), 6.94 (d, J = 9 Hz, 4H, Ar–H), 6.86–6.85 (m, 20H, Ar–
coupling between substituted aryl halides and various H), 3.78–3.69 (m, 12H, CH ), 2.70 (t, J = 7.5 Hz, 4H, CH ), 2.54
, 125 MHz) δ = 161.63,
2
2
1
3
2 3
primary/secondary aliphatic, aromatic and N-heterocyclic (t, J = 4.5 Hz, 8H, CH ), C NMR (CDCl
amines at room temperature in the mixed aqueous media 142.97, 140.63, 140.34, 140.51, 139.76, 136.73, 134.74, 131.97,
under visible light irradiation and aerial conditions. The ultra- 131.39, 128.33, 126.85, 126.70, 126.60, 125.26, 66.99, 59.34,
+
fine hybrid HPB@Cu O–Fe O NPs act as a green photo- 59.09, 54.04; mass m/z = 967.4964 [M + 1] .
2
2 3
catalytic system for the synthesis of medicinally important
N-substituted carbazole derivatives. The use of visible light as
an energy source to carry out C–N coupling reactions over
thermal conditions makes this approach greener and sustain-
able. Further, the as-prepared coated filter paper strip was
General procedure for the photocatalytic Ullmann–Goldberg
coupling between aryl-halide and cyclic/acyclic amines
catalyzed by hybrid HPB@Cu O–Fe O NPs
2
2 3
used as a heterocatalytic system for carrying out C–N bond for- A mixture of aryl-halide (0.4 mmol), amine (2 mmol) and
mation reactions with reusability up to three cycles.
K
2
CO
3
(1 mmol) in H
2
O : EtOH (1 : 1) in the presence of hybrid
HPB@Cu
2
O–Fe NPs (0.5 mol%) as a photocatalyst was
2 3
O
stirred at room temperature for 3–9 h under visible light. After
the completion of the reaction (monitored by TLC), the
Experimental section
General experimental methods and materials
organic part was extracted with CHCl
3
and the combined
5
2
organic layer was dried over anhydrous sodium sulphate and
distilled under reduced pressure. Products 11a–e were purified
by recrystallization from a methanol/hexane(1 : 5) mixture. In
the case of other cyclic, acyclic aliphatic and aromatic amine
derivatives, the products (11f, 12a–d, 13a–d, 18a–d, 19, 20a–c,
The general experimental methods, quantum yield calcu-
lations, and materials used are the same as those reported
earlier by us.
5
2
21, 24a–d, and 25a–b) were purified by column chromato-
UV-vis and fluorescence titrations
1
graphy on silica gel. All the products were identified by
H
A 10− (M) stock solution of compound 3 was prepared by dis-
solving 9.67 mg of compound 3 in 10.0 ml of THF. 15 µl of this
stock solution was further diluted with 1800 µl water (pH =
3
NMR spectroscopy (Fig. S33–S61 in the ESI†).
7
.05) to prepare a 3.0 ml solution of derivative 3 (5.0 µM) and General procedure for the photocatalytic Ullmann–Goldberg
this solution was used for each UV-vis and fluorescence experi- coupling reaction between aryl halide and N-heterocyclic
ment. The aliquots of freshly prepared standard solutions of amines catalyzed by hybrid HPB@Cu O–Fe O NPs
2
2 3
2
+
2+
2+
3+
2+
metal perchlorates [M(ClO )X; M = Zn , Cu , Hg , Au , Fe ,
4
2+
A mixture of arylhalides (0.4 mmol), N-heterocyclic amines
2 mmol) and K CO (1 mmol) in H O : EtOH (1 : 1) in the pres-
ence of HPB@Cu O–Fe NPs (0.5 mol%) as a photocatalyst
was stirred at room temperature for 10–13 h under visible
light. After the completion of the reaction (monitored by TLC),
3
+
2+
2+
2+
2+
2+
2+
+
3+
Fe , Co , Pb , Pd , Ni , Cd , Ba , Mg , Ag , and Al ; X =
(
2
3
2
−
1
−3
1
–3], chlorides (MClY; Y = 1–3) and CuCl (10 M to 10 M)
2
2
2 3
O
in distilled water were added to the 3.0 ml solution of com-
pound 3 taken in a quartz cuvette and their spectra were
recorded.
3
the organic part was extracted with CHCl and the organic
layer was dried over anhydrous sodium sulphate and distilled
under reduced pressure to give a solid residue. The products
Synthesis of hybrid HPB@Cu O–Fe O NPs
2
2 3
The hybrid HPB@Cu
2
O–Fe
2
O
3
NPs were prepared by adding (27a–c, 29, 31a, and 31c) were purified by column chromato-
−
2
an aqueous solution of 180 μL of CuCl (10 M) and 90 μL of graphy on silica gel using EtOAc : hexane (1 : 19) as an eluent,
2
−
2
FeCl
3
(10 M) to 3.0 ml of 5 μM solution of derivative 3 in a while nitroderivatives (31b, 33, 35, 37, and 39) were purified by
H
2
O : THF (6 : 4) solvent mixture. The resulting solution was recrystallization from hexane. All the products were identified
1
stirred at room temperature. The solution turned dark brown by H NMR spectroscopy (Fig. S62–S72 in the ESI†).
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Procedure for the synthesis of derivative 41
15 Y.-C. Teo and G.-L. Chua, Chem. – Eur. J., 2009, 15, 3072.
1
1
1
6 B. Y.-H. Tan and Y.-C. Teo, Org. Biomol. Chem., 2014, 12, 7478.
7 G. Toma and R. Yamaguchi, Eur. J. Org. Chem., 2010, 6404.
8 N. Mukherjee, T. Chatterjee and B. C. Ranu, Eur. J. Org.
Chem., 2015, 4018.
A mixture of derivative 40 (0.4 mmol), carbazole (26) (2 mmol)
2 3 2
and K CO (1 mmol) in H O : EtOH (1 : 1) in the presence of
hybrid HPB@Cu O–Fe O NPs (0.5 mol%) as a photocatalyst
2
2 3
was stirred for 18 h at room temperature under visible light.
After the completion of the reaction (monitored by TLC), the
19 H. Ibrahim and M. D. Bala, J. Organomet. Chem., 2015, 794,
301.
organic part was extracted with CHCl and the organic layer
3
2
0 R. Ayothiraman, S. Rangaswamy, P. Maity, E. M. Simmons,
G. L. Beutner, J. Janey, D. S. Treitler, M. D. Eastgate and
R. Vaidyanathan, J. Org. Chem., 2017, 82, 7420.
was dried over anhydrous sodium sulphate and distilled under
reduced pressure to give a solid residue. The product (41) was
purified by column chromatography on silica gel using
EtOAc : hexane(1 : 19) as an eluent. All the products were
2
1 D. S. Surry and S. L. Buchwald, Chem. Sci., 2010, 1, 13.
1
22 W.-J. Yoo, T. Tsukamoto and S. Kobayashi, Org. Lett., 2015,
7, 3640.
identified by H NMR spectroscopy (Fig. S73 and S74 in the
1
ESI†).
2
2
2
2
2
3 H. Deol, S. Pramanik, M. Kumar, I. A. Khan and V. Bhalla,
ACS Catal., 2016, 6, 3771.
4 M. Kaur, S. Pramanik, M. Kumar and V. Bhalla, ACS Catal.,
Conflicts of interest
2017, 7, 2007.
5 R. Chopra, K. Sharma, M. Kumar and V. Bhalla, J. Org.
Chem., 2016, 81, 1039.
6 S. Kaur, M. Kumar and V. Bhalla, Chem. Commun., 2015,
There are no conflicts to declare.
51, 16327.
Acknowledgements
7 S. Pramanik, V. Bhalla and M. Kumar, Chem. Commun.,
2014, 50, 13533.
V. B. is thankful to the Science and Engineering Research
Board (SERB), New Delhi (ref. no. EMR/2014/000149) for finan- 28 F. Y. Kwong, A. Klapars and S. L. Buchwald, Org. Lett.,
cial support. We are also thankful to the UGC (New Delhi) for
2002, 4, 581.
University with Potential for Excellence” (UPE) and DST for 29 S. Pramanik, H. Deol, V. Bhalla and M. Kumar, ACS Appl.
“
the FIST programme.
Mater. Interfaces, 2018, 10, 12112.
30 G. Singh, S. I. Reja, V. Bhalla, D. Kaur, P. Kaur, S. Arora and
M. Kumar, Sens. Actuators, B, 2017, 249, 311.
3
1 R. Chopra, M. Kumar and V. Bhalla, Chem. Commun., 2016,
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4
4
4
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