Integrating CuO?Fe2O3 Nanocomposites and Supramolecular Assemblies of Phenazine for Visible-Light Photoredox Catalysis

Article abstract of DOI:10.1002/asia.201901687

A photoredox catalytic ensemble consisting of CuO-Fe₂O₃ nanocomposites and oligomeric derivative of phenazine has been developed. The prepared system acts as an efficient photoredox catalyst for C?N bond formation reaction via SET mechanism under ‘green’ conditions (aerial environment, mixed aqueous media, recyclable), requiring less equivalents of base and amine substrate. The present study demonstrates the significant role of supramolecular assemblies as photooxidants and reductants upon irradiation and their important contribution towards the activation of the metallic centre through energy transfer and electron transfer pathways. The potential of oligomer 4: CuO-Fe₂O₃ has also been explored for C?C bond formation reactions via the Sonogashira protocol.

Full text of DOI:10.1002/asia.201901687

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Integrating CuO-Fe2O3 Nanocomposites and Supramolecular
Assemblies of Phenazine for Visible Light Photoredox Catalysis
Authors: Lovjot Kaur, Harnimarta Deol, Manoj Kumar, and Vandana
Bhalla
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Integrating CuO-Fe O Nanocomposites and Supramolecular
2
3
Assemblies of Phenazine for Visible Light Photoredox Catalysis
Lovjot Kaur, Harnimarta Deol, Manoj Kumar and Vandana Bhalla*
Lovjot Kaur, Harnimarta Deol, Manoj Kumar and Vandana Bhalla*
Department of Chemistry
UGC Sponsored Centre for Advanced Studies-II,
Guru Nanak Dev University, Amritsar 143005, Punjab, India
E-mail. vanmanan@yahoo.co.in
Supporting information for this article is given via a link at the end of the document.
Abstract: Photoredox catalytic ensemble consisting of CuO-Fe
2
O
3
radical and aryl radical. We hypothesized that by developing a
photoredox catalytic ensemble which could act as photooxidants
in an excited state and reductants at ground, the need for
excessive amine substrate/base can be controlled. Thus, we
planned to develop a catalytic system consisting of photoredox
supramolecular assemblies and metallic center. For the
recyclable metallic center, we focused on the development of
nanocomposites consisting of copper and iron metallic species.
Further, we planned to stabilize the nanocomposites using
supramolecular assemblies which besides acting as light-
harvesting antenna should also have the potential to serve as
reductants at ground state and oxidants at excited state. Hence,
we focused on synthesizing donor-acceptor platform (D-A) as
such systems are known to show good redox potential and
strong absorption in the visible region. Since the HPB
derivatives show maximum absorption in the range of 300-350
nanocomposites and oligomeric derivative of phenazine has been
developed. The prepared system acts as an efficient photoredox
catalyst for C-N bond formation reaction via SET mechanism under
‘green’ conditions (aerial environment, mixed aqueous media,
recyclable) requiring less equivalents of base and amine substrate.
The present study demonstrates the significant role of
supramolecular assemblies as photooxidants and reductants upon
irradiation and their important contribution towards the activation of
the metallic centre through energy transfer and electron transfer
2 3
pathways. The potential of oligomer 4: CuO-Fe O has also been
explored for C-C bond formation reactions via Sonogashira protocol.
Introduction
1
1
nm , we planned to substitute HPB scaffold with dibenzo [a, c]
phenazine (DBPZ) scaffold which is known for high absorption in
The formation of carbon-carbon and carbon-heteroatom bonds
via palladium-catalyzed coupling reaction is an outstanding tool
for the preparation of pharmaceutically active compounds and
13
the visible region. Furthermore, we incorporated donor amino
groups at the periphery in the designed molecule 3 (DBPZNH
due to their known affinity toward soft metal ions such as
copper. Interestingly, DBPZNH formed fluorescent aggregates
in mixed aqueous media which showed high affinity towards
2
)
1
-4
functional materials.
Though the palladium-based catalytic
systems are efficient but the toxic nature of palladium is a big
hurdle towards the development of sustainable synthetic
approaches. Besides its toxic nature, high cost and sensitive
2
2+
3+
Cu and Fe ions and serve as reactors for reducing these
metal ions simultaneously to furnish nanocomposites. During the
5
handling of the palladium complexes encouraged the organic
chemists to develop easy to handle and relatively economical
reduction of metal ions, the assemblies of derivative DBPZNH
2
6
copper-based catalytic systems. Despite the huge economical
are oxidized to generate oligomeric species 4 which show high
absorption in the visible region, good photostability, low band
gap and high redox potential in the ground state as well as in the
excited state (vide infra). The approach being presented in this
manuscript for preparation of CuO-Fe O nanocomposites is
2 3
rapid, economic and ‘green’ in comparison to the other reported
and environmental advantages of copper-based catalytic
systems, some of the elements of reaction protocols such as the
requirement of high temperature, additional ligands,
a
stoichiometric amount of metal reagents and strong bases
remained unaddressed which need great attention to achieve
7
-9
‘
green’ organic transformations.
To overcome the problems
approaches (Table S1). The supramolecular ensemble
associated with thermal heating, a variety of photocatalysts
comprising of economical and benign Ni complexes and
consisting of oligomeric species
4
2 3
and CuO-Fe O
nanocomposites showed high efficiency in C-N bond formation
reactions. To our pleasure, the target product was achieved in
1
0,11
photoactive ligands have been reported
(Scheme S1). Most
of the reported systems act efficiently in the presence of organic
solvents and under inert conditions. Recently, from our lab, we
reported a photocatalytic system based on nanocomposites
92% yield (Scheme 3, entry vii). Various control experiments
confirm the contribution of supramolecular assemblies in the
catalytic efficiency through electron/energy transfer processes.
Interestingly, the need of high equivalents of amine substrate
and base could be controlled under optimized conditions. The as
prepared supramolecular ensemble presents a good example of
supramolecular catalysis as each component of the ensemble is
contributing towards the high catalytic activity of the system.
Furthermore, the catalytic system also work efficiently in the C-C
(
2 2 3
Cu O-Fe O ) using assemblies of hexaphenylbenzene based
1
2
derivative (Scheme S2), which could overcome the need for
the thermal heating but the reaction protocol still needed high
amount of amine as substrate (5.0 equiv.) and base (2.5 equiv.)
to achieve product in higher yield. Excess substrate amine/base
is generally needed in C-N coupling reactions for activation of
the metallic center for generation of intermediates i.e. amino
1
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bond formation reaction via Sonogashira coupling in the
presence of visible light, mixed aqueous media and under aerial
conditions. The catalytic system could be recycled for four
cycles without any change in the yield for C-C/C-N coupling
reactions. To the best of our knowledge, this is the first report
highlighting the contribution of supramolecular assemblies in
enhancing the catalytic efficiency of the nanocomposites. In
comparison to other reported systems, the catalytic system
being presented in this manuscript could catalyze C-N as well as
C-C bond formation reactions more efficiently (Table S2 and
S3).
gradual increase in intensity with time (30 min) (Figure S9).
These spectral changes are accompanied by the color change
of the solution from light yellow to brown which indicates the
3
+
formation of CuO NPs. Further, upon addition of Fe ions (0-30
equiv.) to the solution of DBPZNH (5µM) in H O/THF (5/5, v/v)
solvent mixture, a broad band appeared around 550 nm which is
attributed to SPR band of Fe nanoparticles (Figure S10). A
2
2
2
O
3
visible color change from light yellow to intense yellow was
observed to the naked eye. These studies indicate the potential
of assemblies of DBPZNH
2
for the preparation of CuO and
Under the same conditions as used above for
Cu and Fe ions, we also tested the UV-vis and fluorescence
response of assemblies of DBPZNH toward other metal ions
1
2, 15
2 3
Fe O NPs.
2
+
3+
.
2
2
+
2+
2+
2+
2+
2+
+
2+
2+
such as (Fe , Co , Pb , Zn , Ni , Pd , Ag , Hg ,and Mg )
as their chloride salts but no significant change in the
absorbance behavior was observed in the presence of these
metal ions (Figure S11).
Results and Discussion
The Suzuki Miyaura coupling between 11, 12-dibromodibenzo
1
4
15
[
a,c]phenazine 1 and boronic ester of aniline 2 furnished the
2+
3+
Cu , Fe
(b)
(a)
(i)
(ii)
SPR band at 550
nm Fe NPs
2
O
3
Scheme 1. Synthesis of derivative 3 DBPZNH
2
SPR band at 750
nm CuO NPs
yellow colored compound 3 named as DBPZNH
2
in 80% yield
has been
(
Scheme 1). The structure of compound DBPZNH
2
characterized by various spectroscopic and analytical
techniques (Figure S63-S66).
The UV-vis spectrum of DBPZNH in THF exhibits absorption
2
bands at 305 nm due to π-π * transitions of phenazine moiety
and at 440 nm attributed to intramolecular charge transfer (ICT)
state (Figure S1). Upon increasing the water fraction (10% to
2+
Figure 1. (a) UV-vis spectra of derivative DBPZNH
2
upon addition of Cu and
3+
Fe ions simultaneously in H
3+
2
O/THF (5/5) mixture. (b) Visible colour change
2+
2
upon addition of Cu and Fe in the solution of DBPZNH derivative.
99%) in the THF solution, a red shift of 15 nm was observed in
the absorption band at 440 nm which suggests the formation of
J- aggregates (Figure S2). The fluorescence spectrum of
Having established, the potential of the assemblies of DBPZNH
to generate CuO and Fe NPs, we prepared nanocomposites
of CuO–Fe using the supramolecular assemblies of
DBPZNH as a common platform. We simultaneously added the
solution of Cu ions (0-50 equiv.) and Fe ions (0-30 equiv.) to
the solution of DBPZNH in H O/THF (5/5, v/v) solvent mixture
and kept the solution for 30 min at room temperature. The
absorption spectrum of the above solution shows absorption
bands at 550 nm and 750 nm with the visible color change of the
solution from light yellow to dark brown (Figure 1). These
2
DBPZNH
2
in THF exhibits dual emission bands at 430 nm
2 3
O
attributed to locally excited state (LE) and at 580 nm attributed to
twisted intramolecular charge transfer (TICT) state. A large
Stokes shift in solvent dependent studies and enhancement in
the emission intensity of the band at 580 nm with a blue shift on
increasing the temperature confirms the presence of the TICT
2 3
O
2
2
+
3+
2
2
1
5
state
(Figure S3 and S4). The fluorescence spectra of
DBPZNH
studies
2
in mixed aqueous media and viscosity dependent
indicate the aggregation-induced emission
characteristics of the molecule (Figure S5 and S6). The
transmission electron microscopic (TEM) image of DBPZNH in
O/THF (5/5) solvent mixture show the presence of spherical
spectral changes indicate that assemblies of DBPZNH
2
could
2
simultaneously reduce both ions to generate CuO–Fe O
2 3
1
2, 15
H
2
nanocomposites.
shaped aggregates (Figure S7). Further, the dynamic light
scattering studies (DLS) indicate that the average size of
aggregates is in the range of 200-500 nm (Figure S8).
To shed more light on the mechanistic pathway for the formation
of metal nanoparticles using supramolecular assemblies of
DBPZNH
DBPZNH
2
, we examined the interactions between assemblies of
and Cu(II)/Fe(III) ions under inert and aerial
Next, we evaluated the binding affinity of assemblies of
2
2
+
2+
2+
DBPZNH
2
3+
toward various metal ions such as Zn , Cu , Hg ,
atmosphere. For this, we added Cu(II) ions to the assemblies of
the DBPZNH and stirred the solution under the inert
2
+
2+,
2+,
2+,
2+,
2+,
2+,
2+
+
3+
Fe , Fe , Co Pb Pd Ni Cd Ba Mg , Ag , and Al
2
2
+
by UV- vis spectroscopy. Upon gradual addition of Cu ions (0-
0 equiv.) to the solution of DBPZNH (5µM) in H O/THF (5/5,
v/v) solvent mixture, a new band is observed at 720 nm with the
atmosphere. After 20 minutes, a small portion of the solution
was withdrawn and its absorption spectrum was recorded which
shows the formation of a new band at 555 nm corresponding to
5
2
2
2
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1
5
Cu(0) species. When the solution was left under aerial
conditions for half an hour, the band at 555 nm disappeared and
a new band appeared at 720 nm corresponding to CuO NPs
spectrum of the residue obtained after evaporating the solvent
shows the broadening of the aromatic protons with the
appearance of additional peaks at 6.9 and 7.5 ppm which
1
5
(
Figure S12). A similar experiment was carried out with Fe (III)
ions under inert and aerial conditions. Initially, the bands at 263
and 350 nm corresponding to Fe(0) species (Figure S13),
were observed which upon exposure to aerial conditions
disappeared and a new band corresponding to Fe NPs
indicates the oxidation of DBPZNH
GPC analysis of the sample corroborates the oxidation of
DBPZNH leading to the formation of oligomeric oxidized
2
(Figure S21). Further, the
1
6,17
2
species 4 (Figure S22). The DLS experiment indicates the size
of aggregates of oligomeric species in the range of 300- 600 nm
2
O
3
appears at 550 nm. These results clearly show that initially
Cu(0)/Fe(0) NPs are produced and air oxidation leads to the
(Figure S23). The powder XRD analysis of oxidized species
0
shows
a
broad peak at 2ϴ=25 corresponding to the
2
0
formation of respective metal oxide NPs i.e. CuO and Fe
2
O
3
.
characteristic peak of polyanilines (Figure S24). The FT-IR
spectrum shows
-1
The TEM image of supramolecular assemblies of DBPZNH
2
in
a broad stretching band at 3500 cm
2
+
3+
mixed aqueous media in the presence of Cu and Fe ions
show the presence of spherical shaped nanocomposites
supported over oxidized species (Figure 2). The DLS studies
confirm the average size of nanoparticles in the range of 30–50
corresponding to primary and secondary amines in the
1
5
oligomeric species 4 of DBPZNH
2
(Figure S25). We believe
itself undergoes
that upon reduction of metal ions the DBPZNH
2
oxidation to form oligomeric species 4 (Scheme 2).
2
nm in the H O/THF (5/5) solvent mixture (Figure S14). From
thermogravimetric analysis (TGA), the organic part was found to
be 16% by weight in the supramolecular ensemble (Figure S15).
The inductively coupled plasma- optical emission spectrometry
(
ICP-OES) studies showed the presence of 44.5 wt % of copper
and 22.5 wt % of the iron in the CuO-Fe nanocomposites
Figure S16). The X-ray diffraction peaks of precipitates
2
O
3
(
(a)
(b)
2 3
Scheme 2. Possible schematic representation of formation of CuO-Fe O
nanocomposites by using DBPZNH in mixed aqueous medium.
2
Next, we examined the photophysical properties of oligomeric
species 4 in THF/H O solvent mixture. The oligomeric species 4
absorbs strongly in the visible region with maxima at 400 nm
Figure S26). Further, we checked the photostability of oxidized
species, by exposing the solution of oligomeric species 4 in
mixed aqueous media to visible light irradiation for 6 h. To our
pleasure, only a slight change in the absorption properties was
observed which indicates high photostability of oxidized species
4 (Figure S27). The fluorescence spectrum of the oxidized
species 4 in mixed aqueous medium shows an emission band at
2
Figure 2. (a) TEM image of CuO-Fe
scale bar 500 nm). (b) TEM image of CuO-Fe
using DBPZNH (scale bar 100 nm).
2
O
3
NPs supported over organic part
(
(
2 3
O
nanocomposites formed by
2
obtained by evaporating THF/H
and FeCl ) at 23.98, 33.06, 35.48, 38.62, 48.36, 53.88, 57.44,
1.22, 63.86, 65.80, 67.84, 72.06 and 74.74 support the
2 2 2
O solution of DBPZNH , CuCl
3
6
1
8
formation of CuO and Fe
2
O
3
NPs (Figure S17). The high-
2 3
500 nm. Interestingly, upon the addition of bare CuO-Fe O
2
1, 22
resolution XPS spectrum shows binding energy peaks at 933
and 953 eV corresponding to Cu 2p3/2 and Cu 2p1/2 spin–orbital
components confirmed the formation of CuO nanocomposites,
respectively. Further, the binding energies peaks at 712 and 723
eV corresponding to Fe 2p1/2 and Fe 2p3/2 indicate the formation
nanocomposites
to the solution of oligomeric species 4, 60%
quenching of the emission was observed (Figure S28). Further
good spectral overlap was observed between the emission of
oligomeric species 4 and the absorption spectrum of hybrid
metal nanocomposites (Figure S29). These results indicate the
possibility of energy transfer from oligomeric species 4 to
nanocomposites. From cyclic voltammetry studies, the reduction
potential of oligomeric species 4 was found to be -1.6 V and the
HOMO-LUMO gap was calculated to be 2.48 eV (Figure S30).
The excited state reduction potential as calculated by the Rehm
Weller equation was found to be +1.00 V. These studies indicate
the potential of oligomeric species 4 to act as photooxidant in
excited state and photoreductant in the ground state. To confirm
the possibility of electron transfer from amine substrate to
assemblies, we examined the emission behavior of oxidized
species in the presence of morpholine (Figure S31). To our
1
5,19
of Fe
2
O
3
nanocomposites (Figure S18).
The FT-IR studies
show bands corresponding to Cu-O and Fe-O stretching at 462
-1
-1
cm and 536 cm , respectively, (Figure S19) which suggests
12,15
the formation of the CuO-Fe
hysteresis measurement of the same material shows the super-
paramagnetic nature of generated CuO-Fe nanocomposites
2
O
3
nanoparticles.
The magnetic
2
O
3
1
2
(
Figure S20).
To elucidate the structure of oxidized species, we slowly
evaporated the solution of DBPZNH containing CuCl and
FeCl . After 2 days, precipitates were formed which were filtered
and washed with organic solvents (THF/CHCl
2
2
3
1
3
). The H NMR
3
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delight, the emission spectra was fully quenched, which confirms
the potential of assemblies to act as photooxidant in the
presence of morpholine substrate as donor.
The strong absorption of the supramolecular ensemble in the
visible region with sufficient redox potential (ground as well as
excited state) and good photostability of oligomeric species 4
motivated us to explore the photocatalytic efficiency of the
12). The model reaction was also carried under dark conditions
to understand the importance of visible light, however, the
desired product was obtained only in traces which supports the
role of visible light in the reaction (Table S4, Entry 13). In the
next part of the investigation, we carried out various control
experiments to examine the role of each component of the
supramolecular ensemble (Scheme 3). The model reaction was
independently carried out in the presence of bare nanoparticles,
2
3
supramolecular
ensemble
of
2 3
oligomer4@CuO-Fe O
2
1
nanocomposites in C-N cross coupling reaction under visible
light irradiation. We chose a reaction between 1-iodobenzene (1
in case of CuO NPs the yield of the final product was found to
2
2
2 3
be 30% (Scheme 3, entry i) and with Fe O NPs product was
equiv.) and morpholine (5 equiv.) in the glycerol/H
solvent mixture as the model reaction in the presence of
supramolecular ensemble of oligomer4@CuO-Fe O
3
nanocomposites (0.5 mol%) as photoredox catalyst under aerial
conditions (Scheme S3). To our pleasure, the reaction was
complete in 4 h to furnish the product in 92% yield (Table S4,
Entry 1). The TON and TOF values were found to be 18.4, 4.6
which shows the high activity of the catalytic system (Table S5).
Next, the model reaction was repeated in different solvents such
as glycerol, ethanol, DMF, DMSO, and water (Table S4, Entries
2
O (5/5)
obtained in 25% yield in 12 h (Scheme 3, entry ii). We also
carried out the reaction in the presence of oligomeric species 4
but the yield of the desired product was found to be 25%
(Scheme 3, entry iv). Although the target product is furnished in
low yield but these studies indicate the capacity of each
2
2
-6). The desired product was obtained in comparable yields in
case of glycerol, glycerol/ H O (5/5) and ethanol. We chose
glycerol/H O (5/5) mixture as the reaction medium for carrying
2
2
out further reactions (Table S4, Entry 1). Further, the effect of
switching of the base was investigated on the photocatalytic C-
N cross coupling reaction (Table S4, Entries 7, 8). Interestingly,
no significant change in the model reaction was observed by
2 3
switching the base. We chose K CO as the base for further
investigation due to easy accessibility. To optimize the amount
of base required for coupling reactions, we carried out the model
reaction by reducing the amount of base from 2.5 equiv. to 1.0
equiv. The product was obtained in almost same yield but
Scheme 4. The C–N coupling reaction between substituted aryl halides and
cyclic amines in
2 2 3
H O/Glycerol (5/5), using oligomer4@CuO-Fe O
nanocomposites under visible light irradiation.
component of an ensemble to carry out reactions in the isolated
state. We observed an increase in the yield of the desired
product upon taking the NPs and oxidized species together.
However, the reaction did not proceed in the absence of a
catalytic system under optimized conditions (Scheme 3, entry
viii).
After optimizing the reaction conditions, we investigated the
substrate scope with regards to amines and aryl halides.
Secondary and cyclic amines like pyrrolidine and piperidine were
also examined as coupling partners (Scheme 4).
We also examined several reactions by changing the amines
from aniline and diphenylamine with aryl halides under optimized
conditions. Interestingly, in all the cases target compounds were
obtained in excellent yields (Scheme S4).
Heterocyclic amines were also examined as substrates and all
the tested amines furnished the desired products in good to
excellent yields (Scheme 5). Interestingly, in all the reactions no
column chromatography was needed for the purification of the
Scheme 3. Schematic illustrations of C-N cross coupling using CuO NPs,
Fe O NPs, and oligomeric species 4 separately as photocatalyst.
2 3
time increased from 4 to 7 hours (Table S4, Entry 9). But in the
absence of base desired product was obtained only in trace
amount which suggests that basic media is indispensable for
carrying out coupling reaction (Table S4, Entry 10). Next, we
repeated the model reaction using a lower amount of morpholine
(
from 5.0 equiv. to 3.0 equiv.) under optimized conditions and
the desired product was obtained in 90% yield (Table S4, Entry
1). Upon further reducing the amount of morpholine from 3.0
equiv. to 2.0 equiv., the yield decreased to 65% (Table S4, Entry
1
4
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products except derivatives 11a, 12a. The TONs and TOFs of all
the desired products has been determined and these values
shows the high efficiency of the catalytic system in catalyzing C-
N cross coupling reactions (Table S5).
no significant change in the morphology (Figure S35 a),
however, the TEM image of the recovered catalyst after the 4
cycle shows slight change in the morphology (Figure S35 b).
After the fourth cycle, the yield reduced to 75% (Figure S36)
which was due to slight change in the PXRD of recovered
catalyst (Figure S37) and leaching of 1.006 ppm of copper in
th
Further, to investigate the reaction mechanism of C-N coupling
2 3
reactions catalyzed by oligomer4@CuO-Fe O nanocomposites,
we carried out the model reaction in the presence of radical
residual
solvent
confirmed
by
atomic
absorption
1
1
scavenger 2, 2, 6, 6-tetramethyl-1-piperidinoxyl (TEMPO)
Scheme 6a). The yield of the desired product was drastically
spectroscopy(AAS) (Figure S38).
(
Encouraged by the above findings which demonstrate high
efficiency of as prepared catalytic ensemble in C-N bond
formation reactions, we examined the efficiency of the
decreased and the formation of the adduct of 4-iodobenzene
and TEMPO was detected (Figure S32). This result indicates
that the reaction proceeds through a single electron transfer
pathway (SET). Furthermore, the outcome of control reactions
supramolecular
ensemble
of
2 3
oligomer4@CuO-Fe O
nanocomposites in C-C bond formation reactions via
Sonogashira protocol. The reaction between 1-iodobenzene and
phenylacetylene was chosen as a model reaction. The model
reaction was carried out under optimized conditions of C-N
(a)
Supramolecular ensemble
Oligomer4:CuO-Fe NPs
(b)
2 3
O
Energy transfer
Electron transfer
Scheme 5. The C–N coupling reaction between substituted aryl halides and
heterocyclic amines in H
2 2 3
O/glycerol (5/5), using oligomer4@CuO-Fe O NPs
under visible light irradiation.
Energy
transfer
between an aryl halide and morpholine (3.0 equiv.) (Table S4,
Entry 11, 12) clearly shows that at least 3.0 equivalents of amine
substrate are required for a complete transformation at a faster
rate. Thus, substrate amine is contributing towards the
enhancement of catalytic activity. Further, in the FT-IR spectra
of the reaction mixture of morpholine and iodobenzene in the
-
1
presence of oligomer4@CuO-Fe
(
2
O
3
NPs, no peak at 3295 cm
(c)
2
4
corresponding to N-H of morpholine) was observed (Figure
S33). Based upon all the experimental results, we believe that
upon visible light irradiation, the supramolecular assemblies
interact with amine and accept the electron from amine in the
excited state. Since the energy of the conduction band of CuO
NPs is lower than that of LUMO of the assemblies, the transfer
of electrons from LUMO of assemblies to the conduction band of
NPs is favored. As a result of which electron density is increased
2
4
on the surface of metallic species (Scheme 6b) for activation of
C-X bond and N-H bond which results in the formation of the
final product. Thus, supramolecular assemblies of oligomeric
species 4 are assisting the metallic center via energy transfer
and electron transfer processes for carrying out C-N cross
12
coupling reaction (Scheme 6c).
Further, we chose a reaction between 1-iodobenzene and
morpholine as a model reaction to determine the recyclability of
Scheme 6. (a) The model reaction in the presence of TEMPO as radical
scavenger. (b) Schematic representation for energy transfer and electron
transfer by supramolecular assemblies of oligomeric species 4. (c) The
proposed mechanism for C-N coupling catalysed by oligomer4@CuO-Fe O
2 3
NPs as photocatalyst under visible light irradiation.
the
oligomer4@CuO-Fe
CuO-Fe nanocomposites could be easily separated from the
in-situ
generated
supramolecular
ensemble
of
2
O
3
nanocomposites as the catalyst. The
2
O
3
reaction mixture due to their magnetic nature and were recycled
in the next round of the model reaction (Figure S34). The yield of
the desired product remained almost same in the four cycles.
coupling. As expected, the desired product was obtained in 85%
yield after 4 h (Scheme S5 entries viii). In literature, costly metal-
rd
The TEM image of the recovered catalyst after 3 cycle show
5
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Chemistry - An Asian Journal
10.1002/asia.201901687
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based complexes (such as gold and palladium) have been
utilized as photocatalysts for carrying out Sonogashira
successful in isolating the adduct between iodobenzene and
TEMPO (Figure S39) which support the radical mechanism for
C-C coupling reactions catalyzed by the supramolecular
2
6
coupling. Recently, TiO
2
as photocatalyst in combination with
CuO NPs has also been reported as photocatalyst for carrying
2 3
ensemble of oligomer4@CuO-Fe O nanocomposites under
2
7
out Sonogashira reaction (Scheme S6). Besides the cost of
photocatalyst, the reaction conditions required the presence of
additional bases and inert atmosphere. In comparison,
visible light.
2
8
Further, to understand the activation of acetylene , we recorded
the FTIR of the phenylacetylene and reaction mixture of a model
reaction containing the reactants (iodobenzene and
phenylacetylene) and catalytic system. The FT-IR spectrum of
oligomer4@CuO-Fe
product in excellent yield under aerial conditions using K
the base. Next, we carried several experiments to examine the
role of each component (CuO NPs, Fe NPs and assembly) in
the reaction (Scheme S5 entries i, ii, iv). These studies show
that CuO and Fe NPs in combination with oxidized species 4
2
O
3
nanocomposites furnished the desired
2
CO as
3
-1
the reaction mixture showed no peak at 3287 cm
2
O
3
corresponding to acetylenic hydrogen (Figure S40). On the basis
of the above experiments and literature reports, we proposed
the mechanism for C-C coupling reaction. We believe that upon
2
O
3
could catalyze Sonogashira reaction more efficiently. These
studies reveal relatively less contribution of supramolecular
assemblies in catalyzing the C-C bond formation reactions. We
believe that due to the absence of activating stimulus (electron-
rich amine as the donor) the potential of assemblies to activate
C-X bond is decreased. To confirm this assumption, we
repeated the model reaction using oligomeric assemblies 4 and
2 3
irradiation of light, the oligomer4@CuO-Fe O activates C-H
bond of alkyne and metal acetylide complex (A*) is generated
which acts as a photocatalyst. Simultaneously the catalytic
system generates aryl radical which finally interacts with metal
acetylide leading to formation of the desired alkynylated product
(Scheme 8b).
(
a)
(
b)
Scheme 7. The Sonogashira coupling reaction between substituted aryl
halides and phenylacetylene in H
2
O/Glycerol (5/5) utilizing oligomer4@CuO-
Fe nanocomposites as photocatalyst under visible light irradiation.
2 3
O
triethylamine as the base. The target product was achieved in
0% yield and in presence of in situ generated oligomer4@CuO-
Fe nanocomposites the target product was furnished in 92%.
2
2
O
3
These findings support our assumption regarding the activation
of assemblies by donor substrate amine (Scheme S5 entries v
and ix).
With these optimized conditions in hand, we checked the
substrate scope with regard to different aryl halides. The
Sonogashira reaction was examined using aryl halides having
bromo and chloro as the leaving group and phenylacetylene
Scheme 8. (a) The model reaction in the presence of TEMPO. (b) The
proposed mechanism for Sonogashira coupling catalyzed by oligomer4@CuO-
using supramolecular ensemble of oligomer4@CuO-Fe
2
O
3
2 3
Fe O nanocomposites as photocatalyst under visible light irradiation.
nanocomposites as photocatalyst. Interestingly, reactions with
bromobenzene and chlorobenzene went smoothly to furnish the
desired products in good yield (Scheme S7).
Further, we checked the scope with substituted aryl halides
having electron-withdrawing as well as electron-donating
groups. All the substrates furnished target products in good to
excellent yields (Scheme 7). All the products were purified by
recrystallization from the organic solvents. The TONs and TOFs
We also checked the recyclability of the system for C-C coupling
reaction. The catalytic activity remained unchanged for four
cycles. After the 4 cycle, a slight decrease in the yield of the
th
desired product was observed (Figure S41).
of the desired products had also been calculated which show the Conclusion
high efficiency of the catalyst for carrying out C-C coupling
reactions (Table S6).
2 3
In conclusion, the integration of CuO-Fe O nanocomposites
To get insight into the reaction mechanism, we carried out the
reaction in the presence of TEMPO (Scheme 8a). We were
and assemblies of oligomeric species 4 led to the development
of a highly efficient recyclable photoredox catalytic system for C-
6
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Chemistry - An Asian Journal
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FULL PAPER
N/C-C coupling reaction under green conditions. The reactions
proceed through the SET mechanism and oligomeric species 4
contribute significantly towards the activation of nanocomposites
through electron/energy transfer processes. The catalytic cycle
could be recycled for four cycles without any loss in the yield of
the target compound in model reaction. All the target
compounds could be purified by recrystallization and no column
chromatography was needed.
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2 3
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7
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Chemistry - An Asian Journal
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FULL PAPER
Entry for the Table of Contents
The supramolecular assemblies of DBPZNH
2
act as a nanoreactor for the formation of CuO-Fe
O
2 3
NPs in aqueous media at room
temperature. Further, the supramolecular ensemble catalyses C-C and C-N couplings under mild conditions such as aerial
atmosphere, mixed aqueous media, visible light. The as-prepared supramolecular ensemble presents a good example of
supramolecular catalysis as each component of the ensemble is contributing towards the high catalytic activity of the system. The
system also illustrates the potential of oligomeric species to harvest visible radiations as well as generate the activated species for
the formation of C-N bond in the isolated state.
8
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