Nickel mediated palladium free photocatalytic Suzuki-coupling reaction under visible light irradiation

Article abstract of DOI:10.1039/c9ta13801c

Suzuki coupling is an important, extensively investigated, and manifold approach for C-C bond construction in synthetic chemistry. However, to date, the reaction has been extensively investigated under thermal conditions using palladium-based catalysts. The development of solar light assisted noble-metal free Suzuki coupling represents a green and sustainable approach for such types of reactions. Herein, we describe an efficient, facile, and cost-effective photocatalytic approach for C-C bond formation between aryl halides and phenylboronic acid via Suzuki-cross-coupling using a cobalt(ii) phthalocyanine complex grafted onto a nickel oxide semiconductor at room temperature (RT) and atmospheric pressure. A number of substituted aryl halides have been successfully investigated using the developed protocol and afforded modest to high yields of the desired biphenyls. The possible mechanistic pathway suggested that the formation of photogenerated aryl radical cations from the phenylboronic acid followed by its coupling with aryl radical anions generated from the electron-activated aryl halides resulted in the corresponding biaryls. Importantly, the physical mixture of both components, i.e., CoPc and NiO, showed poor efficiency as compared to the developed hybrid photocatalyst. The recovered photocatalyst could readily be recycled for at least six runs without a significant decrease in the activity.

Full text of DOI:10.1039/c9ta13801c

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DOI: 10.1039/C9TA13801C
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Nickel mediated Palladium free photocatalytic Suzuki-coupling
reaction under visible light irradiation
Pankaj Kumar Prajapati a,b Sandhya Saini a,b, Suman L. Jain*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The Suzuki coupling is an important, extensively investigated, and manifold approach for C–C bond construction in
synthetic chemistry. However, to date, the reaction has been extensively investigated under thermal conditions using
palladium-based catalysts. The development of solar light assisted noble-metal free Suzuki coupling represents a green
and sustainable approach for such types of reactions. Herein, we describe an efficient, facile, and cost-effective
photocatalytic approach for C–C bond formation between aryl halides and phenylboronic acid via Suzuki-cross-coupling
using cobalt(II) phthalocyanine complex grafted onto the nickel oxide semiconductor at room temperature (RT) and
atmospheric pressure. A number of substituted aryl halides have been successfully investigated under the developed
protocol and afforded modest to high yields of the desired biphenyls. The possible mechanistic pathway suggested the
formation of photogenerated aryl radical cation from the phenylboronic acid followed by its coupling with aryl radical
anion generated from the electron-activated aryl halides resulted in the corresponding biaryls. Importantly, the physical
mixture of both components, i.e., CoPc and NiO, showed poor efficiency as compared to the developed hybrid
photocatalyst. The recovered photocatalyst could readily be recycled for at least six runs without significant decrease in
the activity.
www.rsc.org/
accessibility of the noble metals along with higher energy
requirements leave scope for further development of an
efficient noble metal-free and less energy-intensive
1
5, 16
methodology for the Suzuki coupling reaction.
Introduction
In contrast to the thermal catalytic reactions, photo-induced
Since the discovery in 1979, the palladium-catalyzed Suzuki reactions involve the radical-based mechanistic pathways and
cross-coupling reaction has proven its versatility and occur under ambient conditions in the presence of an
convenient approach for C-C bond construction in the organic inexpensive, renewable and abundantly available solar
synthesis.1 The reactions have been extensively utilized in the energy.
-4
17-19
Unlike the thermal catalytic reactions, very few
preparation of a series of industrially relevant compounds, reports are known on the photocatalytic Suzuki reactions. In
5
-7
pharmaceuticals, agrochemicals, and specialty chemicals.
this context, Li et al. described the coupling between aryl
Generally, activated aryl halides such as substituted halides and various boronic acids into corresponding biaryls
bromobenzene and iodobenzene have been effective as under mild conditions using Mott-Schottky Pd@g-C N as a
3
4
8
20
substrates due to their high activity. The use of aryl chlorides photocatalyst. Wang et al. reported palladium supported on
is a challenging task as the associated Ar–Cl bond possesses the conjugated microporous polymer as a photocatalyst to
high activation barrier, which requires high energy afford the biphenyls in bulk. Jiao et al. reported a Mott-
2
1
(
temperature ≥100 ℃) to achieve the formation of targeted C– Schottky type Pd/SiC heterojunction for coupling of various
9
, 10
C bond.
A plethora of reports have been published in the substituted halobenzenes and phenylboronic acid at 30 ℃
2
2
23
prior art mainly using the palladium (Pd) based catalysts, both under visible light irradiation. Sun et al. reported Pd@MOFs
homogeneous and heterogeneous at high reaction catalyzed light assisted coupling of various halobenzenes and
temperatures.1
1-14
However, the expensive nature and limited aromatic boronic acids at 30 °C to afford excellent yields of
biaryls. Chakraborty et al. demonstrated the azobenzene
based colloidal porous organic polymer supported Pd-
a. Synthetic Chemistry and Petrochemicals Area, Chemical and Material Sciences
Division, CSIR-Indian Institute of Petroleum, Haridwar Road, Mohkampur,
Dehradun, India-248005.
nanoparticle catalysed Suzuki coupling reaction under natural
2
4
25
sunlight. Sarina et al. described the enhanced performance
of palladium in Pd-Au alloy nanoparticles for copuling of
phenyl boronic acid with iodobenzene substrtaes under visible
b.
Academy of Scientific and Innovative Research (AcSIR), New Delhi, India-110001.
c.
†Electronic Supplementary Information (ESI) available: Technique used, FE-SEM
images, EDX pattern, and Elemental mapping of the materials, Gas
chromatogram, 1H and 13C NMR spectra of products. See
DOI: 10.1039/x0xx00000x
2
6
illuminance. Xiao et al. synthesized Pd-Au alloy nanoparticles
supported ZrO
2
and studied the coupling between aryl halides
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and phenylboronic acid. Mori et al.27 reported the coupling of NiO, which also matches well with JCPDS card no. 47-1049
View Article Online
3
1
DOI: 10.1039/C9TA13801C
a series of substituted bromobenzenes and phenylboronic acid with face-centered cubic lattice. The hybrid CNO-10 exhibits
with bimetallic Ru-Pd complex under the visible illumination at relevant signals at the identical positions in the diffractogram
2
8
room temperature. Gao et al. reported a highly visible light with almost similar intense peaks indicate that the
active graphene oxide-Pd functionalized plasmonic Ag-AgBr photocatalyst retains the crystallinity after the deposition of
(
GO-Pd@Ag-Br) photocatalyst for biaryls synthesis. Zhang et the cobalt complex (Figure 1b). Furthermore, the absence of
2
9
al. demonstrated the coupling of chemically inert several peaks related to the cobalt suggested the minimum deposition
chlorobenzenes and phenylboronic acid using CeO supported of the cobalt complex on NiO surface.
2
Pd/Au/porous nanorods as a photocatalyst under visible
illumination. However, most of the methods are based on the
expensive palladium or nable-metal-based catalysts; therefore,
it is desired to develop a palladium free, non-noble metal-
Fig. 1. JCPDS card of a) NiO, Powder diffraction pattern of b) CNO-0,
based green, and sustainable approach for C
–C bond
construction via Suzuki cross-coupling reactions.
The present paper describes the first report on the use of
nickel-based photocatalyst comprising nickel oxide grafted
with cobalt(II) phthalocyanine for the photo-induced Suzuki-
cross-coupling of phenylboronic acid and a series of
chlorobenzenes and bromobenzenes at RT (Scheme 1). The
use of low-cost metals (Co, Ni) in the photocatalyst, along with
the favourable synergistic effect of both components to afford
higher product yields, make this protocol a best-suited and
convenient approach for the manufacturing of desired and c) CNO-10 photocatalyst using Cu Kα radiation λ = 0.15418 nm.
biphenyls under mild operational conditions.
FTIR spectral analysis determined the chemical functionalities
in the synthesized materials (Figure 2). FTIR spectrum of the
cobalt complex shows the characteristic stretching and
3
2
bending vibrations (Figure 2a).
The vibrational peaks
-1
between the range 700-1100 cm corresponding to the
phthalocyanine ring. A moderate vibrational peak at 734 cm-1
33
Scheme 1. Photocatalytic Palladium free Suzuki cross-coupling
ascribed the out-of-plane bending for C–H as well as Co–N
3
4,35
-1
bonding vibration.
One more and weak signal at 913 cm
Results and Discussions
Synthesis and Characterization of the Photocatalyst
related to Co–N reveals the bonding of cobalt metal to four
surrounding N-atoms of pyrrole rings of phthalocyanine
moiety. The stretching vibration at 1164 cm and 1521 cm
3
6
-1
-1
The photocatalyst (CNO-10) was synthesized by post-grafting
approach using the CoPc (10 wt.%) immobilized to the NiO
semiconductor, which was obtained via silica-template
assisted protocol as shown in Scheme 2. The cobalt content in
CNO-10 photocatalyst was 0.53 wt.%, as determined by ICP-
AES.
are characteristic of the C–H vibration of the aromatic ring and
37
C=C stretching of phthalocyanine ring, respectively. The peak
-1
at 1636 cm assigned to the C=N stretching vibration and a
-1
remote peak at 3434 cm represented –NH
2
stretching of the
38
pyrrole ring in the complex.
semiconductor, revealed two characteristic peaks at 673 cm
and 1638 cm related to the Ni–O–H stretching and H–O–H
bending vibrations, respectively (Figure 2b). A broad signal at
035 cm attributed to Si–O–Ni stretching frequency;
whereas, the vibrational peak at 1381 cm indicated the
residual nitrate precursor in NiO semiconductor.
peak at 3443 cm- related to –OH stretching indicated the
adsorbed water in the material. After the immobilization, the
nanocomposite revealed the characteristic vibrational
frequencies of both the components, i.e., cobalt complex and
NiO with slight shifting that confirms the successful contact
between the complex and the semiconductor (Figure 2c).
FTIR spectrum of NiO
-1
CoPc
-1
3
9
Ni(NO3)2.6H2O
o
1. H2O, 80 C
-1
1
NaOH
ODAC
o
2
. 500 C, 4 h
1. H2O
o
-1
2
. 60 C, 24 h
4
0, 41
A broad
1
Scheme 2. Synthetic pathways of NiO semiconductor and hybrid CNO-10.
Powder X-ray diffraction determined the crystallinity and
lattice-planes in CNO-0 and CNO-10 (Figure 1). NiO shows
several sharp diffraction peaks in the diffractogram, which
3
0
confirms the crystallinity of the material (Figure 1b). The
diffraction peaks at 2θ of 37.2°, 43.3°, 62.8°, 75.3°, and 79.4°
corresponds to the lattice planes (200), (311), (111), (220) and
(222) reveals the presence of nickel in +2 oxidation state in
2
| J. Name., 2012, 00, 1-3
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shows a total of six signals in the XPS spectra (Figure 4a). The
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DOI: 10.1039/C9TA13801C
two XPS peaks at 881.6 eV and 863.0 eV are the satellite
peaks, while a 856.0 eV and 857.7 eV represented the Ni-2p3/2
2
+
46
and confirm that the nickel presents as Ni in the hybrid. The
rest two signals at 868.0 eV and 874.4 eV corresponding to Ni-
2
+
47
2p1/2 of Ni oxidation state. Cobalt mainly shows two binding
energies for Co-2p3/2 and Co-2p1/2, where the signal Co-2p3/2
splits into the two peaks, at 779.2 eV for octahedral geometry
and 781.7 eV for tetrahedral geometry of Co-2p3/2
,
4
8
respectively (Figure 4b). The signal at 794.7 eV assigned to
Co-2p1/2, which confirmed +2 oxidation state of cobalt in the
4
9
photocatalyst. The XPS spectra of N-1s shows three binding
energies; the signal at 401.3 eV and 400.3 eV confirmed the
C=N and N–H bonding, respectively of the phthalocyanine
moiety, while the third signal at 399.4 eV corresponds to the
pyrrolic C–N bonding, representing the metal-ligand
Fig. 2. FTIR spectra of a) CoPc complex, b) CNO-0, and c) CNO-10
photocatalyst.
5
0
interaction (Figure 4c). The presence of all the elements such
as Ni-2p, Co-2p, and N-1s, clorresponding to the binding
energies 857.9, 781.7, and 400.2 eV, respectively in the survey
scan indicated the well-tuned attachment in the hybrid (Figure
The absorption spectra of the synthesized materials are
represented in figure 3. The cobalt complex shows its
characteristic two bands, B-band and Q-band. B-band arises
4
d).
0 2
due to S →S transition in the UV-region ranging from 250-320
nm; Q-band originated due to the metal to ligand charge
transfer (MLCT), was appeared in the visible region from 500-
4
2
7
20 nm of electromagnetic spectrum (Figure 3a). The
HOMO→LUMO transition of π→π* and n→π* splits the Q-
band into the two sub-bands, i.e., for aggregated and
4
3
monomer form of the complex. The absorption spectra of
CNO-0 (NiO) mainly absorbed in the UV region ranging from
4
4
2
1
55-362 nm (Figure 3b). The UV-Vis spectra of hybrid CNO-
0, owing to the presence of cobalt complex units, exhibit
absorption in the visible region, as depicted in figure 3c.
Furthermore, disappearance of the aggregated CoPc
absorption band at 560 nm, confirmed the well-dispersed
4
5
complex units over the NiO support.
Fig. 4. High-resolution XPS spectra of a) Ni-2p, b) Co-2p, c) N-1s, and d)
XPS survey scan for CNO-10 photocatalyst.
The HR-TEM images of as-synthesized semiconductor and
hybrid, revealed the surface morphology, microstructure, and
the distance between the planes as shown in figure 5. The HR-
TEM images show the hexagonal morphology of NiO particles
(Figure 5a, b). NiO has a 10 nm particle size, which ascribed at
a 10 nm scale (Figure 5a). The interplanar distance of 0.22 nm
corresponding to the crystal plane (111), confirms the
2
+
presence of Ni in the semiconductor (Figure 5b). It also
matches with the crystal plane at 37.2° in the diffraction
pattern. The presence of bright spotted planes in the SAED
pattern suggested the crystallinity of NiO semiconductor
Fig. 3. Ultra-Violet-Visible spectra of a) CoPc complex, b) CNO-0, and c)
CNO-10 photocatalyst.
(Figure 5c). EDX pattern shows the composition of Ni and O-
atoms in NiO semiconductor (Figure 5d). Further, the
emergence of the dark spots in the hybrid indicated the
grafting of CoPc moieties at the surface of NiO semiconductor
The surface chemical properties of the hybrid determined
using X-ray photoelectron spectroscopy (Figure 4). Ni-2p
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(
0
Figure 5e). The distance between the planes of 0.22 nm and conduction band of NiO semiconductor (CB) and retarded the
View Article Online
DOI: 10.1039/C9TA13801C
.25 nm related to (111) and (200) planes, confirmed the electron-hole pair recombination.
2
+
31
presence of Ni in the hybrid (Figure 5b & 5f). There was no
morphological change observed after the immobilization of
the CoPc units, indicating the higher stability of the material.
The diffraction planes (111), (200) and (220) revealed in SAED
pattern confirm the crystallinity of the hybrid (Figure 5g). The
EDX pattern suggested 0.53 wt.% of cobalt content in the
hybrid that is in well agreement with the ICP-AES result (Figure
5
h). Furthermore, the elemental mapping of NiO and hybrid
(CNO-10) suggested the homogeneous dispersion of the cobalt
complex throughout the support (Figure S1). The lower
deposition of Co-metal particles is further verified by the lower
scanned particles in the elemental mapping of the
photocatalyst.
Fig. 6. Photoluminescence spectra of a) CNO-0 semiconductor and b)
CNO-10 photocatalyst.
The thermo-gravimetric analysis (TGA) determined the weight
loss pattern of the synthesized materials with temperature
(Figure 7). The thermogram of CoPc depicts a marginal weight
loss up to 200 °C owing to the evaporation of the adsorbed
moistures in the complex (Figure 7a). The significant decline in
weight occurred at 562 °C attributed to the degradation of the
5
3
phthalocyanine ring in the complex. The thermogram of NiO
shows a minor weight loss below 200 °C due to the absorbed
water molecules. After that, the steady line up to 800 °C shows
the higher strength of the semiconductor support (Figure
5
4
7
b). The maximum weight loss obtained for semiconductor
Fig. 5. a) and b) HR-TEM images, c) SAED Pattern, and d) EDX pattern
for NiO semiconductor. e) and f) HR-TEM images, g) SAED pattern, and was 16%, which exhibits the robust nature of NiO. The hybrid
h) EDX pattern of CNO-10 photocatalyst using the lacey carbon grid of photocatalyst exhibits maximum 20% in between the 450-500
mesh size 200.
°C attributed the degraded phthalocyanine moieties (Figure
c).
7
The electron-hole recombination of photogeneraed charge
carriers was determined by photoluminescence (PL)
spectroscopy (Figure 6). The PL spectra of bare and CoPc
doped NiO excited at 320 nm wavelength at room
temperature. The semiconductor CNO-0 of higher crystallinity
has a stronger band-band PL emission at 439 nm. Also, the
strong band at 439 nm attributed to the surface oxygen
5
1
vacancies in NiO semiconductor (Figure 6a). The fast
recombination of electrons and holes in CNO-0 exhibited an
intense PL signal at 439 nm, ascribed the lower charge
separation, and more moderate activity under visible
illuminance as well. But the high emission indicates the better
visible light-emitting ability of NiO nanocrystals at room
temperature. The hybrid CNO-10 shows much lower intense,
and a broad PL peak at around 435 nm because of blue
emission, which originated from the lower recombination of
the electron-hole pairs and reveals the higher visible light
activity (Figure 6b).52 The lower intensity of the PL peak in the
hybrid due to the deposition of CoPc sensitizer, indicates the
smooth transfer of the electrons from LUMO of CoPc to the
Fig. 7. Thermo-gravimetric analysis of a) CoPc, b) CNO-0, and c) CNO-
0 photocatalyst.
1
4
| J. Name., 2012, 00, 1-3
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The N
2
-sorption analysis determined the bulk properties of the CoPc and CNO-0 remained ineffective under the described
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synthesized materials using the BET theory (Figure 8). CNO-0 conditions. In contrast, the physica
DOI:
l
m
10
ix
.
10in
3
g
9/C9T
of
A13
both
801
C
2
has a high surface area (SBET) of 42.63 m /g with the average components in 1:10 ratio by weight (CoPc and NiO) showed
pore diameter (r ) of 9.25 nm and pore volume (V
) of 0.25 slight activity and afforded only 20% yield of the desired
adsorption-desorption isotherm reveals the type IV
p
p
3
product, but most of the CoPc moieties leached out during the
reaction (Table 1, entry 7). Among the various solvents
cm /g. N
2
isotherm having an H1 hysteresis loop as per the IUPAC
classification (Figure 8a).55 H1 hysteresis loop emerged due to
the cylindrical pore distributions in the material. The mean
pore diameter of 9.25 nm for CNO-0 lies in the mesoporous
range (Figure 8c).56 The mesoporous nature of CNO-0
semiconductor retains in the hybrid with a slight increase in
the BET surface area (45.46 m /g) (Figure 8b). The V
the hybrid are found to be 0.192 m /g and 4.68 nm,
respectively. The isotherm and hysteresis remained almost
same in the hybrid; however, a slight decrease in r and V
p p
obtained. The increased surface area of the hybrid
photocatalyst attributed to the deposition of CoPc at the
p p
exterior of the CNO-0 support; whereas, decreased V and r
values indicates the partial filing of the CoPc units into the
pores of the support (Figure 8d).
studied, a mixed solvent mixture containing DMF/H
2
O was
found to be optimum (Table 1, entry 6.ii).
Table 1. Suzuki Cross-coupling with different photocatalystsa
S. No.
Solvent
Visible
Light
Yield (%)b
CNO-0
2
CoPc
CNO-10
p
and r
p
of
3
1
2
3
4
5
.
.
.
.
.
DMF/H
2
O
No
Yes
Yes
Yes
Yes
0
2
2
0
0
0
< 1
< 1
0
0
DMF
29
23
16
19
2
H O
Toluene
EtOH/H
2
O
0
6
6
6
6
.i.c
.ii.d
.iii.e
.iv.f
-
11
-
-
09
-
47
92
84
69
2
DMF/H O
Yes
-
-
7
.g
.h
DMF/H
DMF/H
2
O
O
Yes
Yes
-
-
-
-
20
0
8
2
aExperimental conditions: chlorobenzene (0.15 mmol), phenylboronic acid (0.165
mmol), K CO (2.25 mmol), photocatalyst (30 mg), solvent 4 mL at 27 ± 2 ℃ and
atmospheric pressure for 8 h under 20-Watt white LED as a visible light source.
2
3
b
c
d
e
f
g
Isolated yield. CNO-5. CNO-10. CNO-15. CNO-20. Physically mixed
h
(CoPc+NiO), Without photocatalyst.
2
Fig. 8. N -sorption isotherm a) & b), and pore size distribution c) & d)
of CNO-0 and CNO-10 photocatalyst, respectively analyzed at
temperature 77 K.
Photocatalytic Suzuki cross-coupling reaction
The photocatalytic efficiency of the synthesized materials, i.e.,
CoPc, CNO-0, CNO-5, CNO-10, CNO-15, and CNO-20 was tested
for the Suzuki coupling between chlorobenzene and
phenylboronic acid at room temperature (27 ℃ ± 2 ℃) under
the white LED light having the wavelength (λ) > 400 nm. Table
1
illustrates the results of the optimization studies. The
reactivity order of the different hybrid samples having variable
amount of CoPc was found to be as CNO-5 < CNO-20 < CNO-15
<
CNO-10 (Figure 9). Among all the synthesized materials,
CNO-10 exhibited the best activity and provided a maximum
5% conversion with a yield of 92% for the coupling product Fig. 9. Effect of variation in the loadings of CoPc on NiO for Suzuki
9
within 8h (Table 1, entry 6.ii). Further, prolonged the time cross-coupling of chlorobenzene and phenylboronic acid.
would not affect the yield of biphenyl. The reduced activity at
higher CoPc loading might be due to confined charge mobility
resulted from the deposition of the excessive complex
molecules on the surface. The reaction did not occur in the
dark under otherwise identical conditions. Similarly, there was
no reaction observed under visible illumination in the absence
of photocatalyst. Subsequently, individual components, e.g.,
After the optimization studies, the reaction was generalized
with a diverse series of substituted chloro and bromoaryls with
phenylboronic acid under the described reaction conditions.
The findings of the experiments are summarized in Table 2. All
the substrates potentially transformed into the corresponding
biphenyls in moderate to high yields using CNO-10 as
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photocatalyst in the presence of 1:1 mixture of DMF and H
Journal Name
2
O
Table 2. Photocatalytic coupling experiments of various cVhileowroAarrteicnleesOnalinned
as reaction media under visible light. Among the various phenylboronic acid under identical conditiona
DOI: 10.1039/C9TA13801C
substituted substrates, chlorobenzene afforded the highest
Sr.
No.
Substrate
Product
Conv.b
95
Yieldc
92
yield of the coupled product. The presence of electron-
withdrawing groups at para-substitution afforded moderate to
good yields (Entry 2-4). 1,4-dichlorobenzene, due to the high
electronegativity and -I effect of chlorine at para-position of
chlorobenzene provided relatively poor yield (83%) of 4-
chloro-1,1’-biphenyl (Entry 2). The presence of the highly
Cl
1
.
Cl
Cl
CN
NO2
2
3
.
.
87
79
83
77
electron-withdrawing –CN and –NO
2
groups at p-position of
Cl
Cl
chlorobenzene reduced the yield of (1,1’-biphenyl)-4-
carbonitrile and 4-nitro-1,1’-biphenyl, respectively (Entry 3 &
4
). The substitution of -OH group at ortho-position decreased
CN
Cl
the product yield of (1,1’-biphenyl)-2-ol to 61% (Entry 5),
mainly due to the steric hindrance. Similarly, 3,4-
dichlorophenol due to the enhanced steric effect afforded
poor yield (52%) of the corresponding coupled product (Entry
4
5
.
.
84
65
81
61
NO2
Cl
HO
OH
6
). The amino-substituted substrate at para-position afforded
slightly higher yield (62%) of (1,1’-biphenyl)-4-amine (Entry 7).
,2-Dichlorobenzene afforded 66% yield of 2-chloro-1,1’-
biphenyl (Entry 8). 3-chloro-N,N-dimethylaniline resulted in
8% yield of N,N-dimethyl-(1,1’-biphenyl)-3-amine (Entry 9). 4-
Chloro-N,N-dimethylaniline retained almost similar yield of
N,N-dimethyl-(1,1’-biphenyl)-4-amine (Entry 10). 4-
Cl
Cl
Cl
1
OH
6
6.
7.
54
52
OH
Cl
NH2
Methylchlorobenzene with an electron-donating methyl group
yielded 59% of 4-methyl-1,1’-biphenyl (Entry 11). The electron-
donating methoxy group at para-position resulted in a 71%
yield of 4-methoxy-1,1’-biphenyl (Entry 12).
67
69
62
66
NH2
Cl
Cl
Cl
8
9
.
.
Cl
N
7
6
2
9
68
66
N
Cl
N
1
0.
N
Cl
6
1
59
1
1
1.
2.
Cl
O
O
74
71
aExperiment: Chloroarene (0.15 mmol), phenylboronic acid (0.165
mmol), K CO (2.25 mmol), photocatalyst CNO-10 (30 mg), DMF (2 mL)
and H O (2 mL) at 27 ± 2 ℃ and atmospheric pressure for 8 h under
0-Watt white LED as visible light source. bDetermined by GC-FID.
2
3
2
2
c]
solated yield.
6
| J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
Journal Name
ARTICLE
The developed protocol also worked efficiently for the cross-
coupling reactions between bromobenzenes and
View Article Online
DOI: 10.1039/C9TA13801C
phenylboronic acid under identical experimental conditions
and gave moderate to high yields of corresponding coupled
products (Table 3). Among the various substrates studied, the
unsubstituted bromobenzene afforded the highest 78% yield
of the biphenyl (Entry 13). In general, the phenyl ring with
electron-withdrawing groups (EWG) favored the reaction than
electron-donating groups (EDG) and provided a better yield of
2
6
2
the desired products. Bromo benzene having –NO group at
para-position produced an almost similar yield of the
corresponding coupled product as the unsubstituted one
(Entry 18). Moreover, bromo benzenes having a substitution of
the electron-donating group afforded much lower yields of the
corresponding biphenyls (Entry 14-17).
Fig. 10. Recycling runs of the photocatalyst CNO-10 up to six cycles for the
Suzuki coupling reaction
.
Table 3. Photocatalytic coupling of different bromoarenes and phenylboronic acid
under identical conditiona
The exact mechanism of the reaction is not known at this
stage. Based on the existing reports we elucidated a possible
mechanism of the reaction as depicted in Scheme 3. Cobalt
phthalocyanine in the hybrid photocatalyst acted as a
photosensitizer that ensures the continuous supply of
electrons to the conduction band (CB) of the NiO under visible
illuminance. Under the light irradiation, CoPc excited and
electrons transferred from HOMO to LUMO. Similarly under
the light irradiation, the charge separation occurred, and the
S. No.
Substrate
Product
Conv.b
Yieldc
78
Br
1
3.
8
4
1
2
Br
OH
39
31
1
1
4.
5.
OH
Br
3
4
6
5
NH2
NH2
Br
photogenerated electrons excited from the valance band (VB)
to CB of NiO.57 The photogenerated holes in the HOMO of the
CoPc converted the Ph–B ̅( OH) into the phenyl radical cation.
3
43
36
77
1
1
1
6.
7.
8.
However, the photoexcited electrons in the LUMO of CoPc
readily transferred to the conduction band of the NiO and
reduced aryl halide to the corresponding aryl anion radical.
Br
4
7
1
9
O
58-60
The holes at the valence band of NiO trapped by the water.
O
In a final step the photogenerated phenyl radical cation
Br
coupled with radical anion to yield corresponding biphenyl as
10,26,61
NO2
depicted in Scheme 3.
The attachment of CoPc units on
NO2
the surface of NiO not only slows down the recombination of
the charge carriers, but also reduces the combined band gap
[a]Experiment: Bromoarene (1.5 mmol), phenylboronic acid (1.65
6
2, 63
of the photocatalyst.
mmol), K
and H O (2 mL) at 27 ± 2 ℃ and atmospheric pressure for 8 h under
0-Watt white LED as visible light source. [ Determined by GC-FID.
2 3
CO (2.25 mmol), photocatalyst CNO-10 (30 mg), DMF (2 mL)
2
b]
2
[
c]
Isolated yield.
The recycling experiments performed to evaluate the stability
and reusability of the photocatalyst CNO-10. After the coupling
reaction of chlorobenzene and phenylboronic acid under
optimized conditions, the photocatalyst CNO-10 was isolated
from the reaction solution by filtration. The recovered
photocatalyst washed with methanol, dried, and used for
subsequent runs under identical conditions. As shown in figure
1
0, the recovered photocatalyst revealed almost consistent
activity for at least six cycles. Most importantly, no leaching
observed during recycling experiments. After six cycles, the
loading of cobalt in the recovered catalyst was 0.51 wt.%,
which was almost equivalent to the fresh photocatalyst (0.53
wt.%).
Scheme. 3. Possible mechanism of the photocatalytic Suzuki cross-
coupling of haloarenes and phenylboronic acid
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ARTICLE
Journal Name
Experimental Section
Materials used
Photocatalytic Coupling Reaction
View Article Online
DOI: 10.1039/C9TA13801C
In the experiment, 1.5 mmol of haloarene, 1.65 mmol of
phenylboronic acid, 2.25 mmol of potassium carbonate, and 4
The chemicals such as cobalt chloride hexahydrate (Across
chemicals), nickel nitrate hexahydrate (Sigma Aldrich),
octadecyl dimethyl (3-trimethoxysilylpropyl) ammonium
chloride (ODAC) (Sigma Aldrich), hydrazine hydrate (Sigma
Aldrich), sodium borohydride (Merck Millipore), sodium
hydroxide (Merck Millipore), phthalic anhydride (TCI
Chemicals), ammonium molybdate (TCI Chemicals) and urea
were purchased and used as received. Cobalt phthalocyanine
2
mL of DMF/H O in 1:1 ratio poured into a 50 mL round bottom
(RB) flask. The flask closed tightly with a septum followed by a
2
flow of N gas for 10 min to create an inert environment in the
vessel with continuous stirring. After that, RB charged with 30
mg of CNO-10 photocatalyst, and the RB was irradiated under
the 20 Watt LED as a white light source. TLC analyzed the
reaction progress after every 1 h interval. The maximum
conversion of the aryl halides under the developed protocol
observed in 8 h; therefore, it is considered as the optimum
reaction time. After 8 h of the experiment, the resulting
mixture was withdrawn using a syringe and filtered through
the Millipore filter (0.45 μm) to remove the photocatalyst. The
6
4
(CoPc) was synthesized by following the literature reported.
All the synthesis and experimental work have been carried out
in de-ionized water and absolute ethanol.
Preparation of NiO (CNO-0)
A slightly modified organosilane-template method used to product was extracted with ethyl acetate and n-hexane in 9: 1
6
5
synthesize nickel oxide (NiO) semiconductor. At first, 1.2 g of ratio through microcolumn fitted with the silica-gel. GC-FID
Ni(NO .6H O poured into 100 mL of de-ionized water with determined the conversion and yield of the coupling products
3
)
2
2
1
13
continuous stirring for 2 h at 40 °C. After the dropwise addition to quantify the reaction products. H & C NMR confirmed the
of the 0.6 g ODAC template, the resulted solution warmed up products by comparing the spectral data of the authentic
to 60 °C with vigorous stirring for 1 h. The pH maintained at compounds.
1
1.6 by adding the 1 M NaOH to above solution. The solution
changed color from green to dark brown, followed by
increasing the temperature slowly up to 80 ℃. Vacuum
filtration used to separate the precipitates through a
membrane filter of 0.45 μm, followed by washing with de-
ionized water and dried at 80 °C for whole night. The dried
material was heated at 500 °C with 1 ℃/min ramping for 4 h in
the presence of air to remove the organosilane-template.
Maximum of silica content in the calcined material (1 g)
removed by washing with 1M NaOH warmed solution and
excess of de-ionized water and ethanol. This material isolated
by vacuum filtration and evaporated the moisture content in a
vacuum oven at 80 °C for 8 h. The as-prepared NiO
semiconductor used further as support for grafting the CoPc
complex.
Conclusions
The present paper demonstrated the first successful palladium
free visible light assisted Suzuki-cross-coupling reaction
between various haloarenes and phenylboronic acid using
cobalt phthalocyanine grafted NiO (CNO) photocatalyst at
room temperature. The developed protocol worked efficiently
for a vast range of substituted chloro and bromobenzenes to
yield the corresponding coupled product in moderate to
excellent yields. Under the developed protocol, the maximum
9
5% conversion of chlorobenzene with a yield of 92% of
biphenyl was achieved. Under similar experimental conditions,
the bare CoPc and NiO did not show any efficiency, whereas a
physical mixture of both components showed a significantly
reduced activity. The CoPc acted as a photosensitizer that
provided a continuous supply of electrons to the CB of the
semiconductor. The presence of the base also played a vital
role and facilitated the formation of the quaternary boronic
acid anion. The developed photocatalyst could easily be
separated and successfully recycled after several runs with
almost consistent efficiency. The present work demonstrates a
sustainable approach for the formation of C-C bonds by using a
cost-effective nickel-based photocatalyst under palladium free
conditions with the added benefits of using abundantly
available solar energy under ambient temperature
compounds.
Preparation of CoPc/NiO (CNO) Photocatalyst
CoPc/NiO hybrid was synthesized by following a simple
dryness method. In the typical synthesis, 10 mL of NiO (1 g)
dispersed de-ionized water sonicated for 1 h, followed by
added 10 mL of a certain amount of CoPc (50, 100, 150, and
2
00 mg) solution with vigorous stirring. The colloidal solution
stirred overnight or 24 h for complete evaporation of water
content from the sample. The dried sample washed thoroughly
with de-ionized water, followed by ethanol to remove excess
or unreacted complex moieties. The washed precipitates were
heated at 60 °C for 24 h and used for the desired cross-
coupling reaction. The different samples containing variable
loading of CoPc coded as CNO-5, CNO-10, CNO-15, and CNO-20
for 5%, 10%, 15%, and 20% of CoPc on NiO support,
respectively. The best-selected photocatalyst (10%CoPc@NiO)
contains 0.53 wt.% of cobalt metal, as ascertained by the ICP-
AES technique.
Acknowledgements
The authors kindly acknowledge, Director CSIR-IIP for his
permission to publish these results. PKP and SS acknowledge
CSIR-New Delhi, India, for their research fellowships. This work
was funded by SERB-DST in the form of a project GAP-3125.
AcSIR-New Delhi, India, is highly acknowledged for the
8
| J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
Journal Name
ARTICLE
registration of Ph.D. Authors are grateful to the Analytical 26.
Section Division for providing analysis of the samples.
Q. Xiao, S. Sarina, E. Jaatinen, J. Jia, D. P. VAierwnoAlrdtic,leHO.nLliniue
and H. Zhu, Green Chem., 2014, 16D, 4O2I:7120-.41023895/.C9TA13801C
K. Mori, M. Kawashima and H. Yamashita, Chem.
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Qu, ACS Catal., 2015, 5, 6481-6488.
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