Bimetallic Au-Pd nanochain networks: Facile synthesis and promising application in biaryl synthesis

Article abstract of DOI:10.1039/c7nj00998d

A simple and facile method is described for the morphology-controlled synthesis of bimetallic Au-Pd alloyed nanochain networks (NNCs) that show high catalytic performance for the Ullmann intermolecular homocoupling of aromatic or alkyl halides (X = I, Br, Cl) in aqueous media and can be reused at least five times, which can also be applied to intramolecular homocoupling.

Full text of DOI:10.1039/c7nj00998d

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Bimetallic Au-Pd Nanochain Networks: Facile Synthesis and
Promising Application in Biaryl Synthesis
Zheng-Jun Wang,a Xia Wang,a Jing-Jing Lv,b Jiu-Ju Feng,b Xinhua Xu,a* and Ai-Jun Wang,b* Zhiwu
Liang a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A simple and facile way is described for morphology-controlled synthesis of bimetallic Au-Pd alloyed nanochain networks
(NNCs) that show high catalytic performance for the Ullmann intermolecular homo-coupling of aromatic or alkyl halides
(X=I, Br, Cl) in aqueous media and can be reused for at least five times, which can be also applied to intramolecular
homo-coupling.
www.rsc.org/
nanomaterials are superior in terms of catalytic activity,
recyclability, and atom effectiveness in ligand-free coupling
reactions.12 Consequently, widespread attention has been paid
to the synthesis of Pd-based nanomaterials. Particularly so
with the synthesis of bimetallic nanocatalysts that show
performance higher than that of the single counterparts in
terms of catalytic activity, stability, and selectivity.13
There are numerous reports on Pd-M (M represents a
transition metal) nanocatalysts such as Pd-Au,14 Pd-Rh,15
Pd-Pt,16 Pd-Ag,7 Pd-Fe,18 Pd-Co,19 Pd-Ni,20 and Pd-Cu.21 It was
reported that Au promotes the electron transfer of Pd for
better catalytic activity.22 We envisaged that with such
synergistic effect, Au-Pd nanocatalysts can be used for efficient
synthesis of biaryls under mild conditions.
There are numerous methods for the generation of Au-Pd
nanocatalysts. Teng et al. prepared Pd68Au32 non-random alloy
with Au-rich core and Pd-rich shell by means of galvanic
replacement.23 Fang and co-workers synthesized bimetallic
Introduction
Au@Pd NPs by
a
seed-mediated method.24 Lee et al.
Biaryls are commonly found in biologically active natural
products.1 They are key building blocks of pharmaceuticals,2
commercial dyes,3 and organic materials (Scheme 1).4 There
are effective ways to synthesize biaryls with environmental
and economic concerns, such as by means of the Suzuki,5
Negishi,6 Stille,7 Hiyama,8 and Kumada cross-coupling
reactions.9
constructed core-shell Au-Pd nanoparticles with well-defined
octahedral shape through simultaneous reduction of Au and
Pd precursors.25 From the above examples, one can reckon
that the wet-chemical co-reduction approach is a popular way
to synthesize Au-Pd nanocatalysts with controlled
structures.12b, 22a Nonetheless, most of them involve the use of
polymer or surfactant such as octadecylamine (ODA),
Recently, palladium (Pd)-based nanomaterials have been
demonstrated as an effective catalyst for C-C bond
formation.10 Compared to other bulk catalysts, the Pd-based
cetyltrimethylammonium
chloride
(CTAC),
and
polyvinylpyrrolidone (PVP) as stabilizer, inevitably making the
synthesis procedures and separation processes complicate and
aState Key Laboratory of Chemo/Biosensing and Chemometrics, College of time consuming. Therefore, it is of high significance to develop
Chemistry and Chemical Engineering, Hunan University, Changsha 410000, China
a simple and facile way for morphology-controlled synthesis of
bCollege of Geography and Environmental Science, College of Chemistry and Life
Science, Zhejiang Normal University, Jinhua 321004, China
Pd-based nanocatalysts that show high catalytic performance.
Herein, we report a one-pot wet-chemical approach for the
construction of Au-Pd nanochain networks (denoted herein as
*Corresponding
Authors:
aTel./Fax:
+86
731
8882-1546;
e-mails:
xhx1581@hnu.edu.cn b Tel./Fax: +86 579 82282269; e-mails: ajwang@zjnu.cn
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See Au-Pd NNCs). The architecture is constructed through the
DOI: 10.1039/x0xx00000x
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co-reduction of suitable Au and Pd precursors using fringes with inter-planar d-spacing distances of 0.234 and
4-aminopyridine (4-AP) as structure-directing agent which can 0.239 nm that correlate well with the (111) crystal planes of
be easily removed afterward from the system. And the AuPd alloy, close to those of the PdAu nanowire networks
as-prepared Au-Pd NNCs was explored for the Ullmann reported by Hong et al.28 Figure 2 shows the typical images of
reaction of aryl halides under mild conditions. It was observed HAADF-STEM and HAADF-STEM-EDS mapping images. The
that the catalyst exhibits high activity and good stability HAADF-STEM image further confirms the porous
applicable for
a
broad range of substrates, giving nanostructures of Au-Pd NNCs (Figure 2A). As seen from the
good-to-excellent yield of target products.
HAADF-STEM-EDS mapping images (Figure 2B and C), the
Au-Pd NNCs sample shows similar densities of Au and Pd
atoms throughout the whole architecture, reflecting high
homogeneity of alloy nature.25
Results and Discussion
The composition and crystalline structure of Au-Pd NNCs
were determined by EDS and XRD analysis. As illustrated in
Figure 3A, the product mainly consists of Au and Pd elements.
Furthermore, the atomic ratio of Au to Pd is 51.9:48.6 in the
alloy, close to be 1:1, which agrees well with the theoretical
composition, revealing the formation of alloyed Au-Pd
nanocrystals in the present work.29 The XRD (Figure 3B)
pattern shows clear diffraction peaks at 39.2°, 45.6°, 66.5°, and
79.9° that can be well indexed to the (111), (200), (220), and
(311) crystal planes of face-centered cubic (fcc) Au-Pd alloy,30
respectively. Moreover, these peaks coincidently show up
among the peaks of pure Au (JCPDS-04-0784) and Pd
(JCPDS-46-1043),31 once again indicating the formation of
Au-Pd alloy.32
Synthesis and characterization of Au-Pd NNCs
A facile co-reduction strategy was employed to fabricate Au-Pd
NNCs in the presence of 4-AP. The as-prepared nanomaterial
was characterized by TEM (Figure 1A and B). One can see that
the sample possesses plenty of uniform and well-defined
streptococcus-like nanochain networks, without any detection
of isolated nanoparticles. The interlaced networks are
composed of three-dimensional porous nanostructures that
are high in specific surface area. The concentrations of 4-AP
play an important role in tuning the morphology of Au-Pd
NNCs, which can be confirmed by the control experiments
under standard conditions except varying its concentrations (SI,
Figure S1). The low concentration of 4-AP (5 mM) leads to the
severe aggregated product without any specific morphology
(SI, Figure S1A). When the 4-AP concentration is increased to
70 mM, it obtained the aggregated nanochains (SI, Figure S1B),
different from the standard case by using 40 mM 4-AP (Figure
1), due to the high affinity induced by the too much adsorbed
4-AP on the surfaces.26 They are formed via the oriented
attachment mechanism with the selectively adsorption of 4-AP
molecules on the specific crystal planes, in well accordance to
the pervious reported work.27
A
A
B
B
100 nm
20 nm
Pd-L
0.234 nm
D
E
C
C
0.239 nm
1 nm
5 nm
Au-M
Figure 1. TEM (A, B), and HR-TEM (C-E) images of Au-Pd NNCs.
Figure 2. HAADF-STEM (A) and HAADF-STEM-EDS mapping (B, C) images of
Au-Pd NNCs.
As shown in Figure 1 C-E, HR-TEM images were taken from
the indicated positions. One can see well-resolved lattice
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Au-Pd NNCs
Element Weight% Atomic%
Table 1. Optimization of the experimental conditions for the Ullmann
coupling reaction of bromobenzene.a
Pd-JCPDS 46-1043
Au-JCPDS 04-0784
A
Cu
B
Pd
66.7
33.3
51.9
48.6
C
Au
Au
Cat
Base
Pd
Br
2
Sol., Temp., Time
Au
10
Pd
20
Temp. Time Yield
0
30
40
30
45
60
θ / degree
75
90
Entry
Solvent
Base
(oC)
80
80
80
80
80
80
80
80
80
80
80
80
80
80
25
40
60
80
80
80
80
80
80
(h)
(%)b
2
Energy / KeV
D
Au 4f
1
2
Ethanol
Toluene
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Na2CO3
CsF
4
80
Pd 3d
C
7/2
5/2
Au 4f
5/2
Au
0
Pd 3d
3/2
0
Pd
4
19
2+
Pd
3
DMSO
4
76
4
Acetone
4
82
90
88
86
84
82
80
345
340
335
330
Binding Energy / eV
Binding Energy / eV
5
Acetonitrile
H2O
4
35
Figure 3. EDS spectrum (A), XRD pattern (B), high-resolution Pd 3d (C) and
6
4
13
Au 4f (D) XPS spectra of Au-Pd NNCs.
7
Ethanol/H2Oc
H2O
4
86
XPS experiments were conducted to examine the
composition and valence state of Au-Pd NNCs (Figure 3C, D).33
Evidently, metallic Pd0 and Au0 are the predominant species,
revealing effective co-reduction of the Au and Pd precursors.13
Besides, the Au 4f7/2 and 4f5/2 peaks of Au-Pd NNCs are lower
in binding energy (ca. 83.03 and 87.00 eV) than those of pure
Au (84.0 and 87.7 eV).29 In contrast, the Pd 3d5/2 peak at
around 335.22 eV of Au-Pd NNCs is slightly higher in binding
energy than that of pure Pd (335.00 eV).34 This is attributed to
the modification of Pd electronic structure under the influence
of Au in the Au-Pd alloy. As reported by Liu et al., there is
improvement of catalytic activity over Au-Pd alloy in
comparison with the individual metals.35
8
4
83
9
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc
Ethanol/H2Oc,d
Ethanol/H2Oc,e
Ethanol/H2Oc
4
80
10
11
12
13
14
15
16
17
18
19
20
21
22
23
4
65
NaOH
NaHCO3
Et3N
4
78
4
69
4
64
KOAc
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
4
77
4
trace
25
4
Catalytic activity of Au-Pd NNCs for Ullmann coupling reaction
Symmetrical di- or poly-aromatic compounds are important in
chemical industry because of their potential applications in
molecular devices, organic conductors and optical materials.36
On the other hand, the synthesis devised by Ullmann is one of
the most simple and effective ways to prepare numerous
symmetrical and unsymmetrical biaryls and polyaryls.37 In this
regard, the catalytic activity of Au-Pd NNCs was evaluated as a
catalyst for Ullmann coupling.
4
37
0.5
1
17
44
2
61
4
trace
39
4
First, we optimized the reaction conditions for the Ullmann
coupling reaction and the results are presented in Table 1.
Bromobenzene (1 mmoL, 1 equiv) was chosen as the substrate
in the presence of K2CO3 (1 mmoL, 1 equiv). Because the rate
of the coupling reaction is solvent-dependent,38 we also
investigated the use of ethanol, toluene, DMSO, acetone,
acetonitrile and H2O, as well as co-solvent ethanol-H2O in the
reaction (Table 1, entries 1-7). It was found that the use of
ethanol-H2O brings good yield (Table 1, entry 7), and the result
is attributed to the higher solubility of inorganic base in the
co-solvent that increases the collision probability of
reactants.39 Meanwhile, bases such as K2CO3, Cs2CO3, Na2CO3,
CsF, NaOH, NaHCO3, Et3N, and KOAc were screened (Table 1,
entries 8-14), and 1 equiv of K2CO3 is the best choice, leading
to 86% yield of target product over the Au-Pd NNCs catalyst.
4
98
a. Reaction conditions: bromobenzene (1 mmoL, 1 equiv), catalyst (Au-Pd
NNCs, 4 mg), base (1 mmoL, 1 equiv), solvent (2 mL); b. Isolated Yield; c.
Ethanol/H2O=1:1; d. Catalyst (Au-Pd NNCs, 1 mg); e. Catalyst (Au-Pd NNCs,
2 mg).
We investigated the effects of the reaction temperature and
time on product yield (Table 1, entries 15-20, 23). With
increase of reaction temperature from 25 to 80 °C, the product
yield increases from trace amount to 98% within four hours.
Besides, the time-dependent experiments show that from 0.5
to 4 h there is gradual increase of product yield. The results
suggest that the reaction comes to completion after four hours
and the product yield can be as high as 98%. Next, the
influence of Au-Pd NNCs amount on the reaction was
investigated. When the amount of Au-Pd NNCs is lowered
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from 4 to 2 mg, there is drastic decrease of product yield. Scheme 2. Complex compounds with biaryl units synthesized
Based on the above results, the optimized reaction parameters by intramolecular homocoupling.
for Ullmann reaction are as follow: aromatic halide (1.0 mmoL,
Au-Pd NNCs
K2CO3
1 equiv), Au-Pd NNCs (4 mg) and K2CO3 (1 mmoL, 1 equiv)
N
Eq. 1
N
Ethanol/H2O
80 oC, 4 h
sealed under air atmosphere in a 10 mL Schlenk tube
Br
Br
2q, 82%
containing ethanol/H2O (v/v = 1/1) solution. And the most
appropriate reaction temperature and time is 80 °C and 4 h,
Inspired by the results, the catalytic system was extended
respectively.
to the intramolecular homocoupling of compounds that are
Table 2 shows the efficiency and scope of Ullmann reaction
more complex. As described in Scheme 2 and characterized by
over Au-Pd NNCs. The results reveal that Au-Pd NNCs has high
1H/13C NMR and mass spectrum (seen in SI), there is 2q in 82%
catalytic activity for the homo-coupling of an extensive
yield. It is apparent that the Au-Pd NNCs system can be used
spectrum of aromatic halides under the optimized conditions.
for the synthesis of complex compounds that contain biaryl
Compared to aryl chloride, the reactants with C-Br and C-I
units.
bonds are more inclined to the formation of Ar-Ar products. It
is because the bond energy decreases in the sequence of C-Cl, Possible mechanism and recovery of the Au-Pd NNCs catalyst
C-Br, and C-I (Table 2, 2a, k).40 The electron-donating groups
When the radical inhibitor 2,2,6,6-tetramethyl-1-piperidinyloxy
such as –CH3, -OCH3 and -OH are efficiently converted to the
(TEMPO) was added to the reaction media, the product yield
corresponding
biaryls
(Table
2,
2b-e,
i
).
With
stays around 97% (Scheme 3). It is clear that the catalytic
system does not involve a free-radical process.42 On the basis
of the above experimental results and those of Pd-catalyzed
electron-withdrawing groups such as –NO2, -Cl and -F (Table 2,
2f, k, p), there is coupling of aryl bromide to offer the
corresponding product in good yields. Even with the sterically
hindered aromatic halides, the target products are obtained
with satisfactory yield (Table 2, 2g, h, j). As for the aliphatic
aromatic halides that are much more inert than the aryl
halides,41 the desired products are obtained in good yield
(Table 2, 2l and 2m). The catalyst was also used to synthesize
heteroaromatic symmetrical molecules such as 1,
1'-biisoquinoline (Table 2, 2n and 2o), and the product yield is
satisfactory.
Ullmann reaction reported previously,43 we propose
a
plausible mechanism. Scheme 4 shows a catalytic cycle for the
Ullmann coupling of aromatic halides. The catalytic cycle
includes the following steps. First, Au-Pd NNCs provides a
platform for the oxidative addition of Ar-X, giving Pd-complex
intermediate I. Then, another Ar-X is adsorbed on the catalyst
surface for the formation of intermediate II by means of
oxidative addition. Subsequently in the presence of base K2CO3
which provides an alkaline environment and helps the leaving
group more easily to leave,44 smoothly giving the complex
species III which undergoes a reductive elimination step to
provide Ar-Ar product and regenerate the catalyst. However,
further experiments should be rigorously performed to
confirm the reaction mechanism.
Table 2. Substrate scope of the Ullmann reaction catalyzed by Au-Pd
NNCs.a,b
Au-Pd NNCs
R
R
R
14 examples
Isolated yield
77 - 98%
K2CO3
X
2
Ethanol/H2O
80 oC, 4 h
1
2
Scheme 3. Preliminary mechanistic study.
Au-Pd NNCs
OCH3
H3CO
OCH3
K2CO3
TEMPO (3 equiv)
H3CO
X = I, 98%
Br, 98%
Cl, 87%
Br
2
X = Br, 98%
Cl, 85%
Eq. 2
2a,
2d, 92%
2h, 88%
2b,
2c, 96%
Ethanol/H2O
80 oC, 4 h
OCH3
97%
Scheme 4. Possible reaction mechanism of the Ullmann reaction catalyzed
by Au-Pd NNCs.
O2
N
NO2
H3CO
2e, 81%
2i, 83%
2g, 85%
2k, 87%
2f, 90%
2j, 90%
X
R
R
Cl
O
O
C
R
HO
OH
Au-Pd
Cl
2l, X= I, 92%, 8 h
H
N
N
III
Au-Pd
F
F
N
X
R
N
H
I
Au-Pd
R
2p, 89%
2n, X= Cl, 77%, 8 h
2m, 83%, 6 h
2o, 82%, 6 h
R
X-
a, Reaction conditions: aromatic halides (X=Br, except
special illustration, 1 mmoL, 1 equiv), Au-Pd NNCs (4 mg),
K2CO3 (1 mmoL, 1 equiv), ethanol/H2O=1:1, 80 oC. b: Isolated
yield.
X
X
Au-Pd
X
R
K2CO3
R
R
II
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Recycle experiments were carried out under the adopted 1 mL of H2PdCl4 (100 mM) were added dropwise into the 4-AP
conditions to investigate the stability of Au-Pd NNCs. To solution, followed by the quick addition (1 mL) of
ensure the same amount of catalyst in each run, several freshly-prepared NaBH4 solution (1 M), and the mixture was
parallel experiments were conducted under identical magnetically stirred for 30 min. Finally, the precipitate was
conditions. As displayed in Figure 4, the product yield is 91% in collected by centrifugation, thoroughly washed with ethanol
the fifth cycle. The XRD of the used Au-Pd NNCs was also given and water for the removal of 4-AP, and then dried at 60 °C in
in Figure S1, which still retained the original Au-Pd alloyed vacuum for future use.
crystallization. Moreover, metal leaching of the catalyst before
Characterization
and after reaction was studied by inductively coupled plasma
The morphology of the samples was characterized by scanning
electron microscopy (SEM, LEO-1530), transmission electron
microscopy (TEM), and high-resolution TEM (HR-TEM), as well
as by high angle annular dark field-scanning transmission
mass spectroscopy (ICP-MS). The Au and Pd contents of the
Au-Pd NNCs was found to be 33.4 and 67.3 (mg/L) before
reaction and 32.2 and 66.7 (mg/L) after five reaction cycles (SI,
Table S1), which is a negligible amount of metal loss actually.
These results demonstrate that the Au-Pd NNCs catalyst has
good reusability and stability.
electron microscopy (HAADF-STEM) over
a JEM-2100F
transmission electron microscope operating at an acceleration
voltage of 200 kV coupled with selective area electron
diffraction (SAED) and energy dispersive X-ray spectrometer
(EDS). X-ray diffraction (XRD) analysis was conducted on a
Rigaku Dmax-2000 diffractometer using Cu Kα radiation source
(λ = 0.15418 nm). The elemental mappings were recorded over
We used commercial Pd/C catalyst which is popular in
organic synthesis to evaluate the performance of the Au-Pd
NNCs catalyst under the standard conditions.45 As shown in
Figure 4, the Au-Pd NNCs is superior to the commercial Pd/C
catalyst both in efficiency and recyclability. The better catalytic
activity of Au-Pd NNCs is ascribed to the large specific surface
area of the porous networks and the synergistic effect
between Au and Pd.46
the scanning transmission electron microscope with
a
high-angle annular dark-field detector (HITACHI S-5500). X-ray
photoelectron spectroscopy (XPS) analysis was performed on a
K-Alpha XPS (ThermoFisher, E. Grinstead, UK) with Al Kα
radiation (1486.6 eV photons) for excitation. The inductively
coupled plasma mass spectroscopy (ICP-MS) was obtained on
an ICP-MS analyzer (180-80, Hitachi, Japan).
1H NMR and 13C NMR spectra were employed to
characterize the organic compounds. The 1H NMR spectra
were recorded on a Bruker Avance 400 MHz spectrometer
having the sample dissolved in CDCl3 [Internal standard: CDCl3
(for 13C, δ = 77.00. The 13C NMR spectra were acquired on a
Bruker Avance 100 MHz spectrometer with the sample
dissolved in CDCl3 [Internal standard: CDCl3 (for 13C, δ =
77.00)]. The mass analysis data were obtained over a MAT 95
XP equipment of Thermo Finnigan Corporation. The relevant
physical properties, 1H NMR and 13C NMR spectra and mass
data of corresponding products are provided as
Supplementary Information (SI).
Figure 4. Comparison of the Au-Pd NNCs and commercial Pd/C catalysts in
the Ullmann coupling reaction of bromobenzene.
Au-Pd NNCs catalytic activity for Ullmann reaction
Experimental
Bromobenzene (Ar–Br) was chosen as a typical reactant used
to examine the catalytic properties of Au-Pd NNCs in Ullmann
reaction. Aromatic halides (1.0 mmoL, 1 equiv), Au-Pd NNCs
and a base were added into a 10 mL Schlenk tube containing
ethanol/H2O solution. The tube was then sealed and the
mixture magnetically stirred for a desired period at a selected
temperature, and the reaction was monitored by thin layer
chromatography (TLC). Afterwards, the reaction mixture was
cooled down to room temperature and the catalyst was
removed by filtration, followed by washing thoroughly with
ethyl acetate and water. The combined organic layer was dried
by Na2SO4, and the filtered residue was purified by flash
column chromatography using silica gel.
Chemicals
Chlorauric acid (HAuCl4·4H2O), palladium chloride (PdCl2),
sodium borohydride (NaBH4, 98%), 4-AP and commercial Pd/C
were purchased from Shanghai Energy Chemical Reagent Co.
Ltd (Shanghai, China). All chemicals were of analytical grade
and used without further purification. The H2PdCl4 (100 mM)
solution was prepared by adding 0.4 mL of concentrated
hydrochloric acid to dissolve PdCl2. Twice distilled water was
used to prepare the aqueous solutions throughout the
experiments.
Synthesis of Au-Pd NNCs
Typically,32 188 mg of 4-AP was dissolved in 50 mL of water
under stirring at 0 °C (using an ice-water bath) to obtain the
4-AP solution (40 mM). Then, 4.16 mL of HAuCl4 (24.3 mM) and
Conclusions
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Langmuir, 2010, 26, 1137-1146; (d) P. C. Pandey, R. Singh and A. K. Pandey,
Electrochim. Acta, 2014, 138, 163-173; (e) P. C. Pandey and R. Singh, RSC Adv.,
With the use of Au-Pd NNCs fabricated by
a facile
chemical-wet route, the Ullmann coupling of aromatic halides
proceeds smoothly under mild conditions. The catalyst displays
excellent activity for a broad scope of substrates and can be
reused 5 times without showing obvious decline of product
yield. The synthesis of Au-Pd NNCs to catalyze the Ullmann
coupling reaction would engender new dimensions for the
development of multi-metallic catalysts for the formation of C
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Page 7 of 7
New Journal of Chemistry
View Article Online
DOI: 10.1039/C7NJ00998D
Graphical Abstract
Au-Pd NNCs
K2CO3
R
R
R
X
Ethanol/H2O
80 oC, 4 h
X = Cl, Br, I
R = Aryl, Alkyl,
Heterocycyl
14 examples
Isolated yield
77 - 98%
100 nm
A facile approach was developed for the synthesis of Au-Pd NNCs that shows
excellent catalytic performance for Ullmann coupling reaction.