An effective photothermal dual-responsive Pd1Cu4/Ce: XOy catalyst for Suzuki-Miyaura coupling reactions under mild conditions

Article abstract of DOI:10.1039/c9nj06195a

The Suzuki-Miyaura coupling reaction is one of the effective methods for forming C-C bonds in modern organic synthesis. However, most of the current catalysts can only effectively catalyze the aryl iodides and aryl bromides, and the catalytic activity for the inexpensive aryl chlorides is relatively low. Herein, the Pd₁Cu₄/Ce_xO_y catalysts can not only exhibit excellent performance in the conventional thermal reactions of aryl bromides and arylboronic acids, but also effectively activate aryl chlorides in Suzuki-Miyaura coupling reactions under visible light irradiation at room temperature. The Ce_xO_y carriers can provide photogenerated electrons to enrich the electron density of the Pd nanoparticles, which can effectively activate the strong C-Cl bonds of the aryl chlorides. Meanwhile, the photogenerated holes of Ce_xO_y can activate arylboronic acids. The presence of Pd and Cu₂O can further enhance the absorption of visible light by Ce_xO_y. Catalysts prepared in this work could provide a great promise for using inexpensive metals to synergistically catalyze Suzuki-Miyaura coupling reactions under mild conditions.

Full text of DOI:10.1039/c9nj06195a

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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DOI: 10.1039/C9NJ06195A
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An effective photothermal dual-responsive Pd1Cu4/CexOy catalyst
for Suzuki−Miyaura coupling reaction under mild conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Bo Liua, b, Tong Xua, b, Chunping Lia, b, Jie Bai a, b,
*
DOI: 10.1039/x0xx00000x
The Suzuki−Miyaura coupling reaction is one of the effective methods for forming C-C bonds in modern organic synthesis.
However, most of the current catalysts can only effectively catalyze the aryl iodides and aryl bromides, and the catalytic
activity for the inexpensive aryl chlorides is relatively low. Herein, the Pd1Cu4/CexOy catalysts can not only exhibit excellent
performance in the conventional thermal reactions of aryl bromides and arylboronic acids, but also effectively activate aryl
chlorides in Suzuki-Miyaura coupling reactions under the visible light irradiation at room temperature. The CexOy carriers
can provide photogenerated electrons to enrich the electron density of the Pd nanoparticles, which can effectively activate
the strong C-Cl bonds of the aryl chlorides. Meanwhile, the photogenerated holes of CexOy can activate arylboronic acids.
The presence of Pd and Cu2O can further enhance the absorption of visible light by CexOy. Catalysts prepared in this work
could provide a great promise for using inexpensive metals to synergistically catalyze the Suzuki-Miyaura coupling
reactions under the mild conditions.
application of cuprous oxide for this reaction has been the
focus of organic chemists and played an important role in the
development of green chemistry.21-23 Therefore, the use of
1. Introduction
The Suzuki-Miyaura (SM) coupling reaction is one of the
effective methods for forming C-C bonds in modern organic
synthesis, so it is widely used in theoretical research and
industrial production.1-5 The reaction generally employs an
arylboronic acid and an aryl halide as a substrate. Among the
aryl halides, aryl iodides and aryl bromides have higher
activity, and aryl chlorides are difficult to be activated due to
the presence of strong C-Cl bonds, so aryl iodides and aryl
bromides usually used in SM coupling reactions.6-10 Metal
palladium and its oxidation are the most efficient and
commonly used catalytic materials for SM coupling
reactions.11-14 However, since the price of the precious metal is
relatively high, it is desirable to reduce the amount of
palladium by introducing a part of the inexpensive metal, and
to synergize with the palladium in the reactions.
Copper metal and its compounds are the most low-cost
materials among the commonly used catalysts for the
reactions such as Pd, Ni and their compounds. Therefore, from
the economical point of view, Cu could be employed as a
synergistically catalyzed metal.15-20 Among various compounds
of Cu, cuprous oxide is inexpensive, low toxicity, and high
catalytic activity in the carbon-carbon coupling reactions. The
cuprous oxide as a synergistic catalyst for palladium is a viable
solution. However, it is worth to notice that most catalysts
which are currently used in the SM coupling reactions are hot
catalysts. Their catalytic activities on the aryl iodide and aryl
bromide are relatively high, but still not satisfy the reactions
when used inexpensive aryl chloride as the substrate.24, 25
A
viable solution for the problems above mentioned is to
promote the oxidative addition step of the SM coupling
reaction by enriching the electron density of the catalytic
active center, thereby achieve the activation of the aryl
chloride under a mild condition. A photocatalytic method in
which photogenerated electrons are injected into Pd
nanoparticles under visible light irradiation can effectively
enrich the electron density of Pd nanoparticles. For example,
Sun et al. synthesized Au-Pd nanostructures and demonstrated
that the Au nanocrystal core acts as a plasmonic component
for efficient light absorption, and the Pd nanoparticles act as
the catalysts for SM reactions.26 Moreover, the
photogenerated electrons of the SiC surface and excited
electrons of the Cu particles in the PdCu alloy could be
transferred to Pd under light irradiation, which was confirmed
by Wang et al.27 In recent years, the application of rare earth
oxides as photocatalysts to various chemical reactions has
become a research hotspot. Among them, resource-rich and
inexpensive cerium oxide is widely used as a carrier material in
heterogeneous catalysis.28-32 Although the band gap of cerium
oxide is very wide, the precious metal Pd and the Cu2O
a. a Chemical Engineering College, Inner Mongolia University of Technology, Hohhot,
010051, People’s Republic of China
b Inner Mongolia Key Laboratory of Industrial Catalysis, Hohhot, 010051, People’s
Republic of China
Fax:+86-471 6575722 ; Tel:+86-471 6575722
E-mail address: baijie@imut.edu.cn
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semiconductor with narrow band gap can effectively improve examined by field emission scanning electron microscopy (FE-
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the visible light absorption range of the composite material,33- SEM, FEGQUANTAN 650). The crystaDlOI:s1t0r.u10ct3u9/rCe9NoJ0f61t9h5eA
36
and suppress the recombination of photogenerated composites was characterized by X-ray Diffractomer (XRD,
electrons and hole pairs, thereby effectively utilizing them D/Max-2500). The instrument was equipped with Cu-Ka
under visible light irradiation,37,38 In addition, the cerium radiation and the range of 2θ angle was from 5° to 90°. The X-
element has a ground state electron ([Xe]4f15d16s2) in the 4f ray photoelectron spectroscopy (XPS, Escalab 250xi) was used
orbital, which gives cerium oxide excellent redox to determine the elemental composition, content and
characteristics. As the most common lattice defects, oxygen chemical state of the samples. Thermogravimetric Analysis
vacancies of cerium oxide can also enhance its redox (TGA, STA449 F3) was used to observe the weight change of
properties.39-42 These properties of cerium oxide are extremely the samples with increasing temperature. The absorption
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important for the SM coupling reactions.
In this work, Pd1Cu4/CexOy composite was prepared by Ultraviolet–visible Spectrophotometer (UV-vis, SHIMADZU,
electrospinning, high temperature calcination, and UV-2700). The specific surface areas and poresizes of the
capacity of different catalysts for visible light was detected by
hydrogenation reduction. The morphologies and components samples were calculated by Brumauer−Emmett−Teller (BET)
of the composite were characterized by FE-SEM, TEM, TGA, and Barrett–Joyner–Halenda (BJH) methods. The content of Pd
XRD, XPS, BET and ICP. Its catalytic performance was evaluated in the samples was detected by Agilent ICP-OES 730. The gas
for SM coupling reaction. This composite can not only exhibit chromatograph (GC, SHIMADZU, GC-2010) was employed to
excellent performance in the conventional thermal reactions analyze the yield of the products in the SM coupling reactions.
of aryl bromides and arylboronic acids, but also effectively
2.4 Catalytic Activity Measurement
activated aryl chlorides under visible light irradiation at room
temperature.
The Pd1Cu4/CexOy catalysts were employed in SM coupling
reactions to evaluate the catalytic activity. Typically, aryl
halides, arylboronic acids, bases and the catalysts were added
to the mixed solution of EtOH/H2O. After the reactions, the
solution was added into a separatory funnel containing ethyl
acetate and distilled water for extraction. The organic layer
solution was detected by GC and the yield of the products was
calculated. The catalysts were separated by centrifugation and
washed several times with ethanol and distilled water, and
then dried for subsequent cycle reactions.
2. Experimental
2.1. Chemicals
Palladium chloride (Pd
(99.0%), N, N-Dimethylformamide (DMF, 99.5%)
ethanol (99.7%)
(99.5%) were obtained from Sinopharm Chemical Reagent
Co.,Ltd. Polyvinylpyrrolidone (PVP, Mw = 1, 390, 000) was
purchased from Gobrkie New Material Technology Co.,Ltd.
Cerium nitrate (99.0%) was provided from Tianjin Xingfu Fine
Chemical Research Institute.
≧
59%), Copper Chloride Dihydrate
Anhydrous
Bromobenzene (99.5%) and Chlorobenzene
，
，
3. Results and discussion
To investigate the relationship between sample weight and
temperature change, the TG analysis of PdCl2-CuCl2-
Ce(NO3)3/PVP nanofibers is performed under the analytical
condition for TGA is from 45 °C to 800 °C at a heating rate of
10 °C/min. The results are shown in Fig. 1. There are three
stages in weight loss in the air. When the temperature raises
from 50 °C to 274 °C, the weight loss of the sample is about
14.57% which belongs the crystal water of CuCl2·2H2O and
Ce(NO3)3·6H2O. On the second step, due to the massive
decomposition of PVP, the dramatic weight loss of the sample
which is about 23.15% occurs between the the temperature
ranging from 274°C to 276 °C. With the temperature increasing
from 276 °C to 420 °C, the weight loss is 47.03%, which is
because of the decomposition of the cerium nitrate and the
residual PVP. The results indicate that the calcination
temperature of the catalysts in the tube furnace is determined
to be 500 °C.
The morphologies of the samples are observed by the FE-
SEM, as the Fig. 2 shows. In Fig. 2a, A smooth surface is found
on the PdCl2-CuCl2-Ce(NO3)3/PVP nanofibers with diameters
ranging from 200 nm to 300 nm. The FE-SEM images also show
that after the high temperature calcination and hydrogenation
reduction (Fig. 2b), the surface wrinkled coaxial Pd1Cu4/CexOy
nanotublular structure is obtained. This structure is beneficial
2.2 Catalyst Preparation
The
Pd1Cu4/CexOy
high
catalysts
were
fabricated
calcination
via
and
electrospinning,
temperature
hydrogenation reduction methods. In a typical experiment,
0.018 g of PdCl2 and 0.069 g of CuCl2·2H2O were first added to
1.35 g of ethanol and 20.25 g of DMF mixed solution, which
was stirred continuously for 8 h. Then, 1.35 g of Ce(NO3)3·6H2O
and 2.25 g of PVP were put into the above solution by stirring
for 12 h to obtain the precursor solution. The precursor
solution was prepared into nanofibers by electrospinning at 20
cm distance and 15 kV voltages. The nanofibers were placed in
a tube furnace and raised to 250 °C at a heating rate of 5
°C/min under air atmosphere for 3 h, and then heated to 500
°C for 4 h. Finally, the calcined samples were hydrogenated at
a pressure of 3 MPa and a temperature of 120 °C for 12h to
achieve the Pd1Cu4/CexOy catalysts.
2.3 Catalyst characterization
The transmission electron microscope (TEM) and high
resolution transmission electron microscopy (HRTEM) images
of the samples were taken by JEM-2100 (JEOL, Japan). The
surface morphological characterization of the samples was
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shown in Table S1.
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The specific surface area and pore siDzOeI:d1i0st.1r0ib39u/tCio9NnJo06f1t9h5eA
samples are analyzed by BET and BJH calculation methods. The
N2 physisorption isotherm of the samples is shown in Fig. 3a,
we can observe that it is a type IV isotherm curve. The samples
exhibit double hysteresis loops in the isotherm ranging from
P/P0 = 0.35 to 0.85 and from 0.85 to 0.98, which may be due to
the different pore structures of the inner and outer nanotubes.
The specific surface area of the samples can be calculated by
the BET analysis which is 91 m2/g. The pore sized distribution
profiles of the samples are presented in Fig. 3b, the average
pore diameter of the samples calculated by the BJH analysis is
16 nm, indicating that the samples are mesoporous materials.
The high specific surface area and large pore size can expose
more active sites and increase the contact area with the
substrate, thereby enhance the catalytic activity of
Pd1Cu4/CexOy.
The XRD pattern of the Pd1Cu4/CexOy is shown in Fig. 4a.
The diffraction peaks at 2θ = 28.5o, 33.0 o, 47.5 o, 56.2 o, 69.3 o,
76.9 o, 78.8 o, and 88.3 o are corresponding to the crystal faces
of (111), (200), (220), (311), (222), (400), (331), (420) and (422)
of cubic fluorite CeO2 (JCPDS 43-1002), respectively, indicating
a high quality of crystallinity in CeO2 nanotubes. The diffraction
peaks of Pd and Cu are not detected because of their low
contents. The Ce 3d XPS spectrum is shown in Fig. 4b, the 3d5/2
and 3d3/2 spin-orbital components are represented by V and U
respectively. The Vʹ/Uʹ component is the feature of Ce+3 and
the V/U, Vʹʹ/Uʹʹ, and Vʹʹʹ/Uʹʹʹ components are the features of
Ce+4. These results confirm that the CexOy has a mixed valence,
and the atomic ratio of Ce+3/Ce+4 is 0.31 which is calculated by
the peak area ratio. The presence of Ce+3 is advantaged to
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Fig. 1 TGA curves of PdCl2-CuCl2-Ce(NO3)3/PVP nanofibers.
to the increasing of the specific surface area. In comparison
with the initial electrospinning fibers in Fig. 2a, the diameters
of the Pd1Cu4/CexOy samples are slightly reduced because the
PVP is removed during the high temperature calcination.
To further observe the morphologies of the samples, the TEM
images are obtained. As can be seen from Fig. 2c and d, the coaxial
nanotubular structure can be clearly observed. Moreover, granular
and rugged surfaces are found in Fig. 2e, which indicates the
increasing of the specific surface area of the samples, and more
active site exposure compared with the smooth surfaces. In
addition, the HRTEM image is obtained, as shown in Fig. 2f, the
nanoparticles with the lattice spacing of 0.20nm, 0.21nm, 0.23nm
and 0.32nm are corresponding to the PdO (102) plane, Cu2O (100)
plane, Pd (111) plane and CeO2 (111) plane, respectively, indicating
the Pd/PdO, Cu2O and CeO2 of the Pd1Cu4/CexOy have good
crystallinity. As is seen from the element mapping images of
Pd1Cu4/CexOy, the Pd and Cu clusters supported on CexOy are
evenly distributed. The ICP-OES results of the Pd1Cu4/CexOy are
increase the oxygen vacancy of CexOy，which promotes the
redox in SM coupling reactions. As shown in Fig. 4c, the
binding energies of Cu in Pd1Cu4/CexOy are 952.1 eV for Cu
2p1/2 and 932.3 eV for Cu 2p3/2. Due to the binding energies of
Cu0 and Cu+1are around 951.9 eV, 932.4eV and 952.4eV,
932.7eV respectively, the Cu0 and Cu+1 cannot only be
distinguished by using the XPS spectrum. Hence, we further
investigate the state of Cu by the Auger electron spectrum.
The inset of Fig. 4c shows the binding energy of the Auger
peak is around 570 eV which corresponds to Cu2O. The Pd 3d
XPS spectrum is shown in Fig. 4d, there are two states of the
Pd species. The binding energies of Pd+2 are 342.5 eV for Pd
3d3/2 and 337.2 eV for Pd 3d5/2, respectively. Similarly, the
binding energies of 340.2 eV and 335.0 eV belong to Pd0. The
percentage contents of PdO and Pd are 73% and 27%,
Fig. 2 FE-SEM images of PdCl2-CuCl2-Ce(NO3)3/PVP (a) and Pd1Cu4/CexOy (b), TEM
images of Pd1Cu4/CexOy (c,d,e), HRTEM image of Pd1Cu4/CexOy (f) and element mapping
images of Pd, Cu, Ce and O for the Pd1Cu4/CexOy.
Fig. 3 N2 adsorption−desorption isotherms (a) and pore size distribution curves (b) of
Pd1Cu4/CexOy.
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Fig. 6 The effect of various catalysts on biphenyl yield. Reaction conditions: 1 mmol of
bromobenzene, 1.2 mmol of phenylboronic, 1mg of catalysts, 3 mL of water, 4 mL of
alcohol, 1.5 mmol of sodium hydroxide, 30 min, 70 °C.
Fig. 4 XRD pattern of Pd1Cu4/CexOy (a), XPS spectra of CexOy (b), Cu (c), Pd (d) in
Pd1Cu4/CexOy. The inset in (c) is the Auger electron spectrum of Cu in Pd1Cu4/CexOy.
reactions is shown in Figure 5c. It can be observed from the
results that inorganic bases can be more effective than the
organic bases in the reactions. Among the inorganic bases, the
NaOH and KOH are the stronger ones which can enhance the
yield. The recycling results of the reactions are shown in Fig. 5d.
After five cycles of the reactions, the catalysts can still
maintain their excellent catalytic performance, which proves
that they have a good stability.
In comparison of the Pd1Cu4/CexOy and Pd1Cu4/CNF,
Pd1Cu4/CexOy catalyst displayed a better catalytic performance
in the reactions of bromobenzene with phenylboronic acid, as
shown in Fig. 6, this may be due to the CexOy has a higher
specific surface area and richer oxygen vacancies than CNFs.
Moreover, the catalytic performances of Pd1/CexOy and
Cu4/CexOy are also investigated. The yields of biphenyl in the
reactions catalyzed by the Pd1/CexOy and the Cu4/CexOy were
85% and 3%, respectively. However, the yields of biphenyl
employing the Pd1Cu4/CexOy catalysts in the reactions are 99%,
which proves that Cu2O acts synergistically with Pd. When the
content of Cu continues to increase, we can observe the yields
of biphenyl degradation of the Pd1Cu8/CexOy catalysts.
Additionally, we investigate the catalytic activities of the
Pd1Cu4/CexOy in the reactions of chlorobenzene with
phenylboronic acid. In Fig. 7a, curve A shows the time course
of biphenyl yield in the thermal reactions of chlorobenzene
with phenylboronic acid. Due to the strong chemical inertness
of the C-Cl bonds, we can observe that the reactions at 70 °C
for 6 h, the yield of biphenyl is still low (< 20%). In order to
activate the strong C-Cl bonds, we introduce the visible light
irradiation. The time course of biphenyl yield in the reactions
of chlorobenzene with phenylboronic acid under visible light
irradiation is shown in the curve B. The biphenyl yield is greatly
improved indicating that the photogenerated electrons can
effectively activate the C-Cl bond under a visible light
irradiation. The catalysts can keep a high activity and good
crystalline structure after five cycles of reactions (Fig. 7d and
S1). As can be seen from Fig. 8a and b, after five cycles, the
morphology of the catalysts is not destroyed, and the
nanoparticles on carriers have no obvious agglomeration.
respectively. It demonstrates that PdO is the major part of the
Pd/PdO catalytic active center.
In Fig. 5, the effect of different reaction conditions on
biphenyl yield is investigated. The yield of biphenyl increases
from 91% to 99% in a shorter time range from 10 minutes to
30 minutes, indicating that the Pd1Cu4/CexOy catalysts have an
excellent catalytic efficiency in the reactions of bromobenzene
with phenylboronic acid (Fig. 5a). Then the effect of
temperature on biphenyl yield is studied (Fig. 5b). When the
temperature rises from 25 °C to 70 °C, the yield of biphenyl is
Fig.
5 (a) The time course of biphenyl yield by Pd1Cu4/CexOy catalysts. Reaction
conditions: 1 mmol of bromobenzene, 1.2 mmol of phenylboronic, 1mg of catalysts, 3
mL of water, 4 mL of alcohol, 1.5 mmol of sodium hydroxide, 70 °C. (b) The effect of
temperature variation on biphenyl yield. Reaction conditions:
1 mmol of
bromobenzene, 1.2 mmol of phenylboronic, 1mg of catalysts, 3 mL of water, 4 mL of
alcohol, 1.5 mmol of sodium hydroxide, 30 min. (c) The effect of different bases on
biphenyl yield. (d) Recycling experiments of the Pd1Cu4/CexOy catalysts in the reactions
of bromobenzene with phenylboronic acid.
increasing. However, the yield does not change when the
temperature rises to 80 °C. The effect of various bases in the
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reactions at the room temperature in dark. When visible light
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is introduced, the yield of biphenyl achiDeOveI:s10m.10o3r9e/Ct9hNaJn069199%5A.
After adding 0.25mmol of KBrO3, the yield of biphenyl
decreases slightly from 99% to 94%. These demonstrate that
the presence of photogenerated electrons in the catalysts can
promote the reactions. While the amount of KBrO3 increases
to 2.5mmol, the yield of biphenyl is only 8%, which indicates
that when the photogenerated electrons are removed,
Pd1Cu4/CexOy exhibits no catalytic activity in the reactions even
under the visible light irradiation. Additionally, to determine
whether the KBrO3 could destroy the catalysts, the catalyst
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Table 1 The capture experiment of photogenerated electrons and holes.
KBrO3
(mmol)
MeOH
(mL)
Yield
(%)
Entry Visible light
Fig. 7 (a) The time course of biphenyl yield by Pd1Cu4/CexOy catalysts. AReaction
conditions: 0.25mmol of chlorobenzene, 0.3mmol of phenylboronic acid, 20mg of
catalysts, 1 mL of water, 2 mL of alcohol, 0.375 mmol of sodium hydroxide, 70 °C.
BReaction conditions: 0.25mmol of chlorobenzene, 0.3mmol of phenylboronic acid,
20mg of catalysts, 1 mL of water, 2 mL of alcohol, 0.375 mmol of sodium hydroxide,
room temperature, visible light. (b) UV−vis absorption spectra of various catalysts. (c)
The effect of various catalysts on biphenyl yield. Reaction conditions: 0.25mmol of
chlorobenzene, 0.3mmol of phenylboronic acid, 20mg of catalysts, 1 mL of water, 2 mL
of alcohol, 0.375 mmol of sodium hydroxide, room temperature, visible light, 6h. (d)
Reusability study of the Pd1Cu4/CexOy catalysts in the reactions of chlorobenzene with
phenylboronic acid under visible light irradiation.
1
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7
--
+
+
+
+
+
+
--
--
0.25
--
--
--
--
--
--
--
--
--
99
94
8
97
86
44
0.5
2
--
Reaction conditions: 0.25mmol of chlorobenzene, 0.3mmol of phenylboronic acid, 20mg
of catalysts, 1 mL of water, 2 mL of alcohol, 0.375 mmol of sodium hydroxide, room
temperature, 6h.
The UV-vis absorption spectra of various catalysts are
shown in Fig. 7b. The visible light absorption range of pure
CexOy is around 400-425 nm. As can be seen from Fig. 7b, a
red-shift occurs in the absorption spectrum of CexOy, because
the energy band structure of the CexOy semiconductor is
changed after the introduction of noble metal Pd. Owing to
the narrow-band gap Cu2O semiconductor is loaded on the
wide-band gap CexOy, the sensitization of the Cu2O to the
CexOy can expand the absorption range of visible light.
Therefore, due to the presence of the noble metal Pd and the
semiconductor Cu2O, the visible light absorption range of the
Pd1Cu4/CexOy is tremendously improved. The synergy of
various components in Pd1Cu4/CexOy catalysts for the reactions
under visible light irradiation is also investigated by employing
Pd1/CexOy, Cu4/CexOy, Pd1Cu4/CexOy and Pd1Cu4/CNF as
catalysts (Fig. 7c). Despite the Cu4/CexOy catalysts have a little
activity in the thermal reactions of bromobenzene with
phenylboronic acid at 70 °C (Fig. 6), they can still activate the
strong C-Cl bonds of chlorobenzene under visible light
system after removing KBrO3 has been employed to the
Fig. 8 (a) FE-SEM images and (b) TEM images of five cycle experiments.
irradiation. Meanwhile, Pd1Cu4/CexOy exhibits
a better
catalytic performance than Pd1/CexOy in the reactions,
indicating the synergistic effect of Cu2O. By comparing the
catalytic performance of Pd1Cu4/CexOy and Pd1Cu4/CNF, it is
confirmed the CexOy carrier is essential to the SM coupling
reactions when they are under visible light irradiation.
To further demonstrate the promotion of photogenerated
electrons and holes in the reactions of chlorobenzene with
phenylboronic acid, various amounts of electron scavenger
KBrO3 and hole scavenger MeOH are added to the reactions.
As shown in Table 1, no biphenyl product is obtained in the
reactions, and the yield of biphenyl can recover to more than
97%. This indicates that the presence of KBrO3 will not destroy
the catalysts.
Then the effect of photogenerated holes is also studied.
Arylboronic acid can combine with OH− in the basic
environment and then be adsorbed on the basic sites of CexOy.
When the photogenerated holes are diffused to the basic sites,
the adsorbed molecules can be oxidized with the cleavage of
C-B bond.43 The role of photogenerated holes in the reaction is
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Table 2 Scope of Pd1Cu4/CexOy catalysts for the SM coupling reactions
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DOI: 10.1039/C9NJ06195A
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Entry
1a
Aryl halide
Arylboronic acid
Product
Yield (%)
TOF (h-1)
12,276
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99
99
12,276
2a
3a
76
99
9424
12,276
4a
5a
97
12,028
99
99
12,276
12,276
6b
7b
8b
78
99
9673
12,276
9b
10b
53
6572
aReaction conditions: 1 mmol of bromobenzene, 1.2 mmol of phenylboronic, 1mg of catalysts, 3 mL of water, 4 mL of alcohol, 1.5 mmol of sodium hydroxide, 70 °C,
30min. bReaction conditions: 0.25mmol of chlorobenzene, 0.3mmol of phenylboronic acid, 20mg of catalysts, 1 mL of water, 2 mL of alcohol, 0.375 mmol of sodium
hydroxide, room temperature, visible light, 6h.
demonstrated by the introduction of methanol as a hole which indicates that the photogenerated holes can effectively
scavenger. The yield of biphenyl decreased from 99% to 86% activate the arylboronic acid.
and 44% after adding 0.5 and 2 mL methanol, respectively,
The reaction substrates are further extended to various
aryl halides and arylboronic acids to determine the scope of
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the capture experiments. The final products are achieved
when the activated aryl chlorides meet with the oxidized
arylboronic acids. The Pd1Cu4/CexOy catalysts are of great
significance in the use of inexpensive metals to synergistically
catalyze the SM coupling reactions under the mild conditions.
View Article Online
DOI: 10.1039/C9NJ06195A
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Conflicts of interest
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There are no conflicts to declare.
Acknowledgments
The authors gratefully acknowledge the support from the
National Natural Science Foundation of China (No. 21766022).
Scheme 1 The proposed catalytic mechanism of Pd1Cu4/CexOy catalysts in the SM
coupling reactions under visible light irradiation.
Notes and references
1
E. Fernandez, H. Garrido, M. Boronat, A. L. Perez, A. Corma, J.
reaction systems catalyzed by Pd1Cu4/CexOy (Table 2). It can be
observed from the results that the Pd1Cu4/CexOy catalysts have
a good universality for the most of the substrates. However,
since there are strong electron-withdrawing groups on aryl
halide including -OCH3 (Entry 3a, Entry 8b) and-NO2 (Entry 10b),
the yields of the products can only reach 76%, 78% and 53%,
respectively. The 1H NMR and 13C NMR spectra of all the
products are provided in supporting information.
The proposed catalytic mechanism is shown in Scheme 1.
The presence of Pd and Cu2O can expand the visible light
absorption range of CexOy, further promote its production of
photogenerated electrons, and thereby enrich the electron
density of Pd nanoparticles. The Pd nanoparticles with high
electron density can effectively activate the aryl halides and
promote the oxidative addition reactions by accelerating the
formation of the ligand ArPdIIX.26 Meanwhile, the arylboronic
acids can also be activated by photogenerated holes of
CexOy.43 When the oxidized arylboronic acids meet with the
activated aryl halides, the final products of the coupling
reactions are obtained.
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In conclusion, the Pd1Cu4/CexOy can not only exhibit excellent
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bromides and arylboronic acids, but also effectively activate
aryl chlorides for SM coupling reactions under the visible light
irradiation at room temperature. The synergy of each
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DOI: 10.1039/C9NJ06195A
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The Pd1Cu4/CexOy catalysts can efficiently catalyze the Suzuki reactions under both heating and
visible light irradiation conditions.