Strategy used to synthesize high activity and low Pd catalyst for Suzuki coupling reaction: An experimental and theoretical investigation

Article abstract of DOI:10.1039/c9cp06802c

Unveiling the reaction mechanism is significant for developing high-performance catalysts. In this paper, a series of precisely controlled Pd_xM_147-x (M = Cu, Pt, Au, Rh, Ru) dendrimer encapsulated nanoparticles (DENs) has been successfully synthesized. The mechanisms of Pd_xM_147-x as catalysts for Suzuki cross-coupling reactions were investigated by combining experimental and theoretical methods. The experimental results indicate that Pd₇₄Cu₇₃ DEN shows similar activity to Pd₁₄₇ DEN and excellent substrate adaptability under mild reaction conditions. Moreover, the Cu component can play an important role in tuning the catalytic activity of Pd_xCu_147-x DEN. Density functional theory (DFT) calculations illustrate that the similar activities of the Pd₁₄₇ and Pd₇₄Cu₇₃ DENs originate from the comparable energy barriers of the rate-determining steps. The partial density of states (PDOS) and electron density differences demonstrate that Cu decreases the intensities of the valence orbitals of the top and edge Pd atoms and weakens orbital interactions between the intermediates and Pd₇₄Cu₇₃ DEN, leading to low desorption energies of the products. This work can provide a promising strategy to reduce the cost of Pd catalysts in Suzuki cross-coupling reactions.

Full text of DOI:10.1039/c9cp06802c

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Strategy used to synthesize high activity and low
Pd catalyst for Suzuki coupling reaction: an
experimental and theoretical investigation†
Cite this:DOI: 10.1039/c9cp06802c
ab
b
a
a
a
b
Xiao Yan,‡ Yafei Luo,‡ Wei Liu, Li Liang, Ya Gan, Zhongzhu Chen,
b
c
b
d
a
Zhigang Xu, Hua Wan, Dianyong Tang, * Hubing Shi* and Jianping Hu
*
Unveiling the reaction mechanism is significant for developing high-performance catalysts. In this paper,
a series of precisely controlled Pd 147ꢀx (M = Cu, Pt, Au, Rh, Ru) dendrimer encapsulated nanoparticles
DENs) has been successfully synthesized. The mechanisms of Pd 147ꢀx as catalysts for Suzuki cross-
x
M
(
x
M
coupling reactions were investigated by combining experimental and theoretical methods. The experimental
results indicate that Pd74Cu73 DEN shows similar activity to Pd147 DEN and excellent substrate adaptability
under mild reaction conditions. Moreover, the Cu component can play an important role in tuning the
catalytic activity of Pd Cu147ꢀx DEN. Density functional theory (DFT) calculations illustrate that the similar
x
activities of the Pd147 and Pd74Cu73 DENs originate from the comparable energy barriers of the rate-
determining steps. The partial density of states (PDOS) and electron density differences demonstrate that
Cu decreases the intensities of the valence orbitals of the top and edge Pd atoms and weakens orbital
interactions between the intermediates and Pd74Cu73 DEN, leading to low desorption energies of the
products. This work can provide a promising strategy to reduce the cost of Pd catalysts in Suzuki cross-
coupling reactions.
Received 18th December 2019,
Accepted 19th February 2020
DOI: 10.1039/c9cp06802c
rsc.li/pccp
8
and controlled reactivity via employing various ligands, many
drawbacks are also found, including the lack of recyclability,
1
. Introduction
Suzuki coupling, as an efficient C–C cross-coupling reaction, has difficulty of product separation and metal contamination of the
1
,2
attracted extensive attention in the field of organic synthesis,
and has emerged as a widely used tool for forming C–C bonds.
During the past decades, numerous homogeneous catalysts
product.
The reasonable heterogenization of homogeneous catalysts
3
–6
5,9,10
can efficiently overcome these drawbacks,
which has stimulated
have been developed for palladium complex-catalyzed carbon– the development of heterogeneous catalysts for C–C cross-coupling
1
1–19
carbon cross-coupling reactions. Although the developed homo- reactions.
geneous catalysts possess high stability, easily accessible properties
El-Shall and co-workers reported the development of
7
a new family of highly active Pd nanoparticle catalysts supported
on partially reduced graphene oxide nanosheets (Pd/PRGO).
18
The Pd/PRGO catalyst shows excellent catalytic activity for Suzuki
coupling with a turnover number (TON) of 7800 and a remarkable
a
College of Pharmacy and Biological Engineering, Sichuan Industrial Institute of
Antibiotics, Key Laboratory of Medicinal and Edible Plants Resources Development
of Sichuan Education Department, Chengdu University, Chengdu 610106,
P. R. China. E-mail: hjpcdu@163.com
ꢀ1
turnover frequency (TOF) of 230 000 h at 120 1C under micro-
wave heating. Khanna et al. provided insights into the nature of
the interfacial interactions between metal nanoparticles and defect
sites on the graphene surface, demonstrating that the cross-
b
Collaborative Innovation Center of Targeted Therapeutics and Innovation,
Chongqing Key Laboratory of Kinase Modulators as Innovative Medicine,
Chongqing Engineering Laboratory of Targeted and Innovative Therapeutics,
International Academy of Targeted Therapeutics and Innovation,
Chongqing University of Arts and Sciences, Chongqing 402160, P. R. China.
E-mail: tangdy2008@163.com
12
coupling reactions occur at the catalyst surface. Subsequently,
they provided a fundamental understanding of how a graphene
support may activate both oxidative and reductive reaction steps in
the catalytic cycle of Suzuki–Miyaura palladium-catalyzed cross-
coupling reactions via employing computational and experimental
c
College of Mathematics and Informatics, South China Agricultural University,
Guangzhou 510642, P. R. China
d
Laboratory of Tumor Targeted and Immune Therapy, Clinical Research Center for
Breast, West China Hospital, Sichuan University, Chengdu 610041, P. R. China.
E-mail: shihb@scu.edu.cn
15
investigations. These studies are significant for designing
high-performance heterogeneous catalysts for C–C cross-coupling
reactions. However, there is a crucial problem with the hetero-
genization of homogeneous catalysts; that is, the question of the
†
‡
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cp06802c
These authors contributed equally to this work.
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high cost of Pd cannot be avoided. Moreover, compared with Sigma-Aldrich Co., the precursor CuCl
2
from Shanghai Macklin
homogeneous catalysts, heterogeneous catalysts generally exhibit Biochemical Co. Ltd, RuCl
3
from Sigma-Aldrich Co., RhCl
3
6
lower turnover numbers and consume larger quantities of precious from Aladdin Co., NaAuCl from Bidepharm Co., and K PtCl
4
2
13
metal. Therefore, in order to reduce the cost of Pd catalysts, from Adamas-beta Co. In addition, 4-bromobenzotrifluoride,
adding earth-abundant and less expensive transition metals to Pd phenylboronic acid, 4-methoxyphenylboronic acid, 4-trifluoro-
catalysts is a promising strategy.
methylphenylboronic acid, 4-tolyboronic acid, 4-chlorophenyl-
there is a feasible boronic acid and 4-tert-butylphenylboronic acid were purchased
2
0–27
According to previous investigations,
method that can be employed to reduce the cost of Pd catalysts; from Bidepharm Co., 1-bromo-4-nitrobenzene from Innochem
that is, the synthesis of Pd-based bimetallic nanoparticles (NCs) Co., 4-bromoacetophenone, 1-bromo-4-tert-butylbenzene, 4-bromo-
in place of pure Pd as a C–C cross-coupling catalyst. Due to the toluene and 4-bromoanisole from Energy Chemical Co., and the
28–30
tunable properties and synergistic effects,
bimetallic catalysts standard substance biphenyl from Tokyo Chemical Industry Co.
can exhibit promising prospects as potential alternative catalysts. All chemicals were used without any purification, and 18 MO cm
In general, compared to monometallic NCs, bimetallic NCs may Millli-Q deionized water was employed for all solutions.
possess much higher catalytic activities, which can be attributed to
the advantageous combination of properties. For example, Gao
and co-workers synthesized high-performance Pd-based alloy
2
.2 Pd-based NPs synthesis
The G6-OH(Pd M ) alloy DENs were obtained by the following
x
147ꢀx
3
2 2
nano-catalysts for the direct synthesis of H O , combining density
3
two-step process: metal ions are first complexed with dendrimers
in solution, then reduced by NaBH . Here, the synthesis method
for G6-OH(Pd147) is similar to Crooks’ work. Briefly, 147 equiv. of
freshly prepared 5.88 mM Na PdCl stock solution was added to a
.0 mM solution of G6-OH(Pd147) PAMAM dendrimer, and the pH
31
functional theory (DFT) calculations and Sabatier analysis. This
study indicated that the valence electrons of Pd-shell atoms could
be adjusted to the optimal range to enhance the activity and
selectivity of the nanocluster simultaneously via introducing
dopants with suitable electronegativity. Shishido et al. reported
that monometallic Pd and Au catalysts were totally ineffective,
however Pd–Au alloy nanoparticle catalysts with a low Pd/Au molar
4
34
2
4
2
value was adjusted to 3.0 using 0.1 M HCl. The solution was stirred
for 30 minutes, and again for 15 minutes after adding a 10-fold
excess of NaBH from a 60 mM stock solution. It’s worth noting
32
4
ratio showed high activity. To sum up, it is necessary to design
novel Pd-based bimetallic NCs as C–C cross-coupling catalysts to
reduce the cost of Pd catalysts and improve the turnover number.
For C–C cross-coupling, however, so far there is a lack of
systematic exploration of the activities of Pd-based bimetallic
NCs. It is not clear if there is a trend in Pd-based bimetallic NCs
when the second component metal is in the same row or line in
the periodic table of the elements. In order to reach this goal, in
this investigation, a series of highly dispersed and random
that the NaBH
will be greatly weakened. The freshly prepared G6-OH(Pd147
catalyst was first bubbled with N for 10 min, and then water;
unreacted NaBH and other impurities were removed by means of
ultrafiltration centrifugation. Then 10-fold deionized water was
added to the concentrated solution to remove the salts and
impurities. After repeating this step twice, the preparation process
of the catalyst was complete.
4
must be freshly prepared, or its reducing capacity
)
2
4
The synthesis strategy of G6-OH(Pd
x
M147ꢀx) is similar to that
x 147ꢀx
component Pd-based bimetallic NCs, namely Pd M ,
n+
of G6-OH(Pd147), except for the order and time of M ions added
M = Pd, Pt, Au, Rh, Ru, Cu, were synthesized. The activities of
these bimetallic NCs and the mechanisms of C–C cross-coupling
reactions are unveiled in detail by combining experimental studies
and density functional theory (DFT) calculations. By performing
this investigation, two scientific questions should be answered.
2+
to the solution. First, Pd ions were added to the solution and
2+
stirred for 15 minutes, permitting the complexation of Pd to the
3
5
interior of the dendrimer. Then, the pH value was adjusted to
n+
7
–8 with dilute aqueous NaOH, favoring the complexation of M
to completely form G6-OH(Pd M ), and also gaining the
x
147ꢀx
x
One is that a comparison of activities between Pd M147ꢀx and pure
2
+
n+
desired Pd /M ratio by stirring for another 15 minutes. Finally,
a 10-fold excess of NaBH was added from a 60 mM stock
solution and stirred for another 15 min. After that, the solution
was bubbled with N for 10 min and we think that the colloidal
metal is G6-OH(Pd 147ꢀx) alloy NPs. It is worth noting that
G6-OH(Pd 147ꢀx) is not molecular in nature; it represents the
Pd147 should be illustrated and the relationship between the
activity of Pd-based bimetallic NCs and the ratio of the second
transition metal should be elucidated. Another is that the differ-
4
2
ences in the natural properties and activities between Pd M
x
147ꢀx
x
M
and pure Pd147 should be expounded by analyzing the electronic
characteristics. We hope that this investigation will have a
significant influence on the current understanding of hetero-
geneous catalytic C–C Suzuki coupling reactions and future
catalyst designs for this reaction.
x
M
nanoparticles mainly distributed at this size which are composed
of 147 metal atoms.
2.3 Characterization
UV-vis spectra were acquired using a T9 UV-vis spectrophoto-
meter. Transmission electron microscopy (TEM) and line profile
analysis by energy dispersive X-ray spectroscopy of the
2
. Experimental
2.1 Chemicals
G6-OH(Pd74Cu73) catalysts were carried out using a JEOL JEM-
1
Poly-amidoamine (PAMAM) dendrimer (G6-OH) was purchased 2100F with an accelerating voltage of 200 keV. The H NMR
1
3
from Weihai CY Dendrimer Technology Co., Na
2
PdCl
4
from (400 MHz) and C NMR (100 MHz) information of the isolated
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product were recorded using a Bruker Avance 400 spectrometer
3
with CDCl as the solvent.
2.4 Computational details
In this study, Pd147 and four different configurations of alloy
NPs (containing 73 copper atoms and 74 palladium atoms)
3
6
were established. The SIESTA package with Troullier–Martins
norm-conserving pseudopotentials and numerical local basis
3
7
sets were used for all atoms. SIESTA has shown excellent
performance for medium and large systems. Large-scale atomic
simulation with a neural network potential (LASP) package was
3
8
employed to explore the global potential energy surface. The
orbital-confining cutoff was determined from an energy shift of
0.010 eV. The energy cutoff for the real space grid was used to
represent the density and was set at 150 Ry. The optimized
2
+
2+
3
9,40
Fig. 1 UV-vis absorption spectra of G6-OH(Pd
)
147 and G6-OH((Pd
73) complexes before and after reduction with NaBH
G6-OH blank.
74
) -
double-z plus polarization (DZP) basis set
was employed for
2+
(
Cu
)
4
, with 2.0 mM
all atoms. The lattice constant was set at 28.0 Å to ensure a
sufficient vacuum gap between periodic images. The exchange–
correlation interactions were described by a generalized gradient
41,42
approximation (GGA) Perdew–Burke–Ernzerhof (PBE)
functional
2
+
2+
chemical reduction of the G6-OH(Pd )74(Cu )73 complexes,
the LMCT bands were replaced by broad, monotonically decreas-
ing bands. These characteristic absorption peaks agree with
Rothe’s results and are attributed to the interband transitions of
with gamma points. Because the investigated complexes are large-
sized molecules, dispersion terms were introduced through the D2
43
formulation. The quasi-Newton L-BFGS method was used for
geometry relaxation until the maximal force on each relaxed atom
50
ꢀ1
spherical metal nanoparticles. Similar results are also found for
the reduction of G6-OH((Pd ) (Rh ) ); G6-OH((Pd ) (Pt ) );
was less than 0.05 eV Å . The transition state search adopted the
2+
3+
2+
2+
44,45
74
73
2+
74
73
double-ended surface walking method.
2+
3+
3+
G6-OH((Pd ) (Ru ) ) and G6-OH((Pd ) (Au ) ) (Fig. S1, ESI†).
74
73
74
73
The adsorption peaks of G6-OH(Pd Ru ) and G6-OH(Pd Au )
74
73
74 73
can also be attributed to the interband transitions of spherical
metal nanoparticles. Moreover, the reduced G6-OH(Pd74Rh)73
G6-OH(Pd74Pt73); G6-OH(Pd74Ru73) and G6-OH(Pd74Au73) corre-
3
. Results and discussion
;
3.1 Particle synthesis and characterization
5
1–55
As described in the Pd-based NPs synthesis section, G6-OH(Pd147); spond to previous reports.
G6-OH(Pd74Cu73); G6-OH(Pd74Ru73); G6-OH(Pd74Rh73); G6-OH- was produced.
73
This proves that G6-OH(Pd74M )
(Pd74Pt73) and G6-OH(Pd74Au73) were synthesized and char-
Taken together, although the UV-vis spectra cannot provide
acterized. The UV-vis absorption spectra are shown in Fig. 1 sufficient information to infer structural details of the reduced
and Fig. S1 (ESI†). G6-OH(Pd147) and G6-OH(Pd Cu ) show product, they could reflect the proposed synthetic pathway.
74
73
significant differences in the synthesis process. The G6-OH(Pd147
)
In order to unveil the morphological properties of the Pd147
73 (M = Cu, Pt, Au, Rh, Ru) DENs, TEM analyses were
or 73 equiv. of performed and the results are shown in Fig. 2 and Fig. S2 and S3
Cu to the interior of the dendrimer, respectively. The UV-vis (ESI†). The TEM images (Fig. 2 and Fig. S2, ESI†) demonstrate
spectra of these precursors exhibit strong absorption bands at that the Pd147 and Pd74 73 (M = Cu, Pt, Au, Rh, Ru) DENs are
and G6-OH(Pd74Cu73) DENs were prepared via first complexing and Pd74
M
2ꢀ
2ꢀ
4
1
47 equiv. of PdCl
4
and 74 equiv. of PdCl
2+
M
B230 and B280 nm and an isosbestic point at B260 nm (Fig. 1, highly dispersive and well-ordered. The corresponding TEM
red and green lines). The corresponding peaks originate from the data indicate that the initial Pd147 DENs have an average
ligand-to-metal charge transfer (LMCT) bands associated with the diameter of 1.57 ꢁ 0.2 nm (Fig. 2c), which is consistent with
4
6
56
dendrimer/metal-ion complexes. The presence of an isosbestic Anderson’s experimental data and the calculated diameter of a
2
+
point may imply that all Pd ions have been complexed with 147-atom, closed-shell Pd cuboctahedron. After the replacement
the dendrimer interior because Pd ions are not associated of Pd by Cu, Pt and Ru, the TEM analyses (Fig. 2d and Fig. S3,
2
+
with the dendrimer absorbed in this region interior, which is ESI†) indicate comparable average diameters of 1.6 ꢁ 0.2,
4
7
consistent with Crooks previously published results. Like the 1.59 ꢁ 0.2 and 1.59 ꢁ 0.2 nm, respectively. However, when
2
+
G6-OH(Pd )147 complexes, there are LMCT bands at B230 and replaced with Au and Rh, the TEM results (Fig. S3, ESI†) exhibit
2
+
2+
48
B280 nm for the G6-OH((Pd ) (Cu ) ) precursors.
an average increase/decrease of 0.7/0.1 nm to 2.27 ꢁ 0.2 and
7
4
73
2
+
After reduction of the G6-OH(Pd )
complex, the LMCT 1.47 ꢁ 0.2 nm, respectively.
1
47
band disappears and the broad-band absorption associated
Moreover, high-resolution TEM (HR-TEM) was also employed
with the nanoparticles is observed (blue line). It indicates that to measure the average lattice spacings of Pd147 and Pd74
the additional NaBH added yields monometallic NPs which contain (M = Cu, Pt, Au, Rh, Ru) DENs and the related HR-TEM images
an average of 147 atoms: G6-OH(Pd147). However, following are plotted in Fig. 3 and Fig. S4 (ESI†). The HR-TEM images
49
M
73
4
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Fig. 4 EDS spectrum of the Pd74Cu73 DENs.
The UV-vis and TEM analyses demonstrate that the Pd147
and Pd74 73 (M = Cu, Pt, Au, Rh, Ru) DENs were efficiently
Fig. 2 TEM images of the Pd147 (a) and Pd74Cu73 (b) DENs; size distribution synthesized. Besides, the TEM results could provide valuable
M
histograms of the Pd147 (c) and Pd74Cu73 (d) DENs.
and useful information for constructing models for theoretical
calculation.
3
.2 Catalytic activities of Pd147 and Pd74
Rh, Ru) DENs for Suzuki coupling reaction
The catalytic performances of Pd147 and Pd M (M = Cu, Pt,
M73 (M = Cu, Pt, Au,
74
73
Au, Rh, Ru) DENs were initially evaluated for Suzuki coupling of
bromobenzene with phenylboronic acid under mild conditions
(
Fig. 5). Herein, three different evaluation parameters were
considered, including HPLC yield (Fig. 5a), turnover number
TON) (Fig. 5b) and turnover frequency (TOF) (Fig. 5c). For the
(
HPLC yield, an external standard method was employed to
calculate the product concentration and the corresponding
standard concentration curve is shown in Table S2 and Fig. S7
Fig. 3 HR-TEM images of the Pd147 and Pd74Cu73 DENs (scale bars =
5
nm).
(
ESI†). The turnover number (TON) and turnover frequency (TOF)
5
9
were computed via the methodology proposed in Karak’s work.
The Pd74Cu73 and Pd147 DENs were proven to be effective
catalysts which displayed HPLC yields of 79.9% and 80.9% after
(Fig. 3 and Fig. S4, ESI†) reveal changes in the components of
different Pd147 and Pd M (M = Cu, Pt, Au, Rh, Ru) DENs. The
74
73
0
.75 h reaction time, TONs of 6389 and 6546, and TOFs of 8519
lattice spacings of 2.18, 2.12, 2.27, 2.26, 2.42, 2.21 Å could be
assigned to the same planes of the Pd147 and Pd74 73 (M = Cu,
Pt, Au, Rh, Ru) DENs. Note that the lattice spacings of the
ꢀ1
and 8728 h , respectively. Thus, Pd74Cu73 and Pd147 DENs
M
5
7
Pd74Pt73 and Pd74Au73 DENs are consistent with Henkelman’s
and Xia’s work.
5
8
Furthermore, with the purpose of unveiling the components
of bimetallic Pd M (M = Cu, Pt, Au, Rh, Ru) DENs, energy-
7
4
73
dispersive X-ray spectroscopy (EDS) was used and the corres-
ponding images are shown in Fig. 4 and Fig. S5 (ESI†). The EDS
image in Fig. 4 indicates that Pd and Cu simultaneously exist in
the same nanoparticle, demonstrating that the nanoparticle is
a Pd–Cu alloy. The atomic ratios of Pd : M are given in Table S1
(
ESI†); the atomic ratio of Pd : Cu is 52.17% : 47.85%, near the
expected atomic ratio of 1 : 1 (74 : 73). Likewise, the EDS images
Fig. S5, ESI†) illustrate that the Pd Pt , Pd Rh , Pd Ru
73
(
7
4
73
74
73
74
and Pd Au DENs exhibit binary metallic properties. The
7
4
73
corresponding atomic ratios of Pd : M (M = Pt, Rh, Ru, Au) are
Fig. 5 (a) High performance liquid chromatography (HPLC) yield, (b) turn-
over number (TON) and (c) turnover frequency (TOF) for the Suzuki cross-
46.01% : 53.99%, 48.36% : 51.64%, 52.48% : 47.52% and 52.01% :
7.99%; the measured compositions are generally consistent with
4
coupling reaction catalyzed by Pd74M73 DENs. Except where marked ‘‘CH’’,
the expected values.
the source of heat is microwave heating.
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Table 1 The catalytic activity of Pd74Cu73 DENs for Suzuki coupling using
a
various substrates
0
b
ꢀ1
Entry
R
R
Time (h)
Yield (%)
TON
TOF (h )
1
2
3
4
5
6
7
8
9
H
CF
t-Bu
CH
CH
OCH
NO
H
H
H
H
H
H
H
H
H
H
Cl
t-Bu
0.75
0.75
0.75
0.75
0.75
0.75
0.75
1.50
0.75
0.75
1.50
99.9
79.4
82.5
93.9
98.4
99.7
78.5
95.6
99.5
94.0
32.1
9990
6304
8250
8817
9683
9940
6162
9139
9900
8836
1030
13 320
8406
11 000
11 756
12 910
13 253
8216
3
3
3
CO
3
Fig. 6 The conversion rate vs. the Pd : Cu ratio.
2
6093
13 200
11 781
687
1
1
0
1
CH
CF
3
outperformed all of the investigated Pd74M73 (M = Pt, Au, Rh, Ru)
H
3
DENs. Moreover, the Pd147 and Pd74Cu73 DENs catalyzed Suzuki
cross-coupling reactions by conventional heating were analyzed.
Both present high catalytic activity, but at the same time, the
a
Reaction conditions: aryl halide (588 mmol), aryl boric acid (588 mmol),
% mol catalyst, K CO = 1.176 mmol, EtOH : H O = 3 : 1 (v/v, 15 mL).
1
2
3
2
The reaction temperature was preset at 50 1C to protect the NPs from
reaction yield of microwave heating is higher than that of high temperature acceleration of aggregation. The reaction was heated
b
1
13
by microwave reactor. Isolated yields, H NMR and C NMR informa-
tion of the product; see Section S4 in ESI.
conventional heating. Obviously, microwave heating could accel-
6
0–62
erate the reaction,
used for all the reactions.
To illustrate the role of Cu and further cut the cost of boronic acid was modified by an electron-withdrawing group,
catalytic activity for Pd Cu147ꢀx DENs, various Pd
Cu147ꢀx DENs such as –CF , the reaction time was prolonged and the yield, TON
were synthesized containing ratios of Pd : Cu = 1 : 3, 1 : 2, 1 : 1.5, and TOF were reduced correspondingly.
: 1, 1.5 : 1, 2 : 1, 3 : 1 and 1 : 0. The relationship between the
conversion rate for Suzuki coupling and the Pd : Cu ratio in
Pd Cu DENs is elucidated to determine the optimal ratio
so for this reason microwave heating was
x
x
3
1
3
.4 Mechanism of Suzuki cross-coupling reaction and energetic
span model analysis catalyzed by Pd147 and Pd74Cu73 DENs
of Pd : Cu. As shown in Fig. 6, the conversion rate for the Suzuki The above experimental results show that the Pd147 and Pd Cu
x
147ꢀx
74
73
coupling significantly increases with an increase of the Pd DENs exhibit comparable activity for the Suzuki cross-coupling
component in Pd Cu147ꢀx DENs, implying that Pd plays an reaction. To further unveil the inherent factors, the mechanism
important role in the catalytic activity. When the Pd : Cu ratio is of the Suzuki cross-coupling reaction was explored in detail with
close to 1 : 1 (74 : 73), the catalytic activity of Pd Cu147ꢀx DENs the help of density functional theory (DFT). According to TEM
for the Suzuki coupling nearly reaches the maximum value and analysis and the previous report, the Pd147 and Pd Cu DENs
x
x
6
3
74
73
the conversion rate is 83%. This result indicates that the activity are modeled as cuboctahedron structures in this research. For
of Pd Cu DENs with a Pd : Cu ratio near 1 : 1 (74 : 73) is the Pd Cu DENs, the Pd :Cu ratio is near 1 : 1 (74: 73) and the
x
147ꢀx
74
73
almost equal to that of Pd147 DENs for Suzuki coupling. Since the distribution of Pd/Cu is uncertain. These alloyed DENs were
metal Cu is cheaper than Pd, this study can provide an efficient constructed by randomly assigning each atom to the constituent
64
strategy to design low-cost catalysts for Suzuki coupling.
metals by referring to Henkelman’s work. Table S3 (ESI†) shows
four configurations of Pd74Cu73 DENs and the energy differences
are no more than 2.0 eV, which means that the Pd74Cu73 DENs
structure may influence the reaction path. As a result, four
3
.3 Catalytic activity of Pd Cu DENs for Suzuki coupling
74 73
using various substrates
Bromobenzene and phenyboronic acid modified by electronic- configurations of Pd74Cu73 DENs were analyzed. After comparing
donating groups or electron-withdrawing groups were chosen the energy differences between the Pd Cu DENs, the adsorptions
74
73
to investigate whether the Pd Cu DENs possess high activity of bromobenzene on Pd147 and Pd Cu DENs are taken into
7
4
73
74
73
for Suzuki coupling. The yields, TON and TOF parameters are consideration. As we all know, in the heterogeneous reaction
given in Table 1. As shown in Table 1, whether the substrates mechanism, the Pd catalysts are mainly distributed on the
were modified by different electron-donating or electron- surface of the Pd NPs, and recent studies point to catalytic recycling
65
withdrawing substituents in the Suzuki coupling, the Pd74Cu73 taking place at the defect/edge sites of Pd nanoparticles. For the
DENs still show high catalytic activity, except for the –CF Pd147 DENs, the adsorption of bromobenzene was explored in
3
substituent on phenylboronic acid. The reaction times, yields, detail, including at two different top sites and three different bridge
TONs and TOFs of these Suzuki cross-coupling reactions are sites. The adsorption energies are collected in Table S4 (ESI†).
ꢀ
1
0
.75–1.5 h, 32.1–99.9%, 1030–9990 and 687–13 320 h , respectively. As shown in Table S4 (ESI†), compared with top adsorption, the
This demonstrates that the Pd Cu DENs can effectively catalyze adsorptions of bromobenzene at bridge sites possess larger
74
73
Suzuki coupling using various substrates. Based on these experi- adsorption energies, indicating that bromobenzene is inclined
ments, there was no byproduct generation. Obviously, the to adsorb on bridge sites. Similarly, for the adsorption of bromo-
homo-coupling pathway can be excluded. However, when phenyl- benzene on four different Pd74Cu73 DENs (i.e. configurations
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1
–4), the stable adsorption configurations can be achieved when adsorption on the Cu atoms. Subsequently, the formation of
ꢀ1
the Br and C atoms adsorb on Pd (Table S5, ESI†). The KBr occurs, which is endothermic by about 65.0 kcal mol and
ꢀ1
corresponding adsorption energies are ꢀ65.9, ꢀ60.4, ꢀ60.8 and 60 kcal mol . Finally, the cleavage of the C–B bond in the
ꢀ
1
ꢀ
ꢀ
61.2 kcal mol , respectively.
adsorbed Ph–B(OH)3 can be achieved via TS2. The corres-
ꢀ
1
Based on the above discussion of the adsorption of bromo- ponding energy barriers are 15.4/8.2 kcal mol , respectively.
benzene and phenylboronic acid, the mechanisms of Suzuki An interesting phenomenon has been found; when Pd74Cu73
coupling are explored by using potential energy profiles (Fig. 7 DENs act as the catalyst, the energy barrier of the transmetalation
6
6,67
ꢀ1
and Fig. S8, ESI†). As presented in Maseras’s work,
Suzuki step is reduced from 26.6 to 8.2 kcal mol . The same results can
coupling contains three steps, namely oxidative addition, trans- be obtained for the other three configurations; the energy barriers
ꢀ1
metalation and reductive elimination. The oxidative addition are 9.7, 15.3 and 23.2 kcal mol for configurations 1, 3 and 4.
process involves cleavage of the C–Br bond of bromobenzene Hence, Cu atoms could cut down the energy barrier of the
and formation of the C–Pd bond. This step needs to overcome transmetalation step and accelerate the Suzuki reaction.
ꢀ
1
an energy barrier of 12.8 kcal mol on Pd147 DEN. Never-
Along with the dissociation of B(OH) from the catalyst, the
3
theless, for the Suzuki coupling reactions catalyzed by Pd74Cu73 reductive elimination process would lead to the formation of
DENs, the oxidative addition process needs to overcome energy biphenyl via TS3 (Fig. 7 and Fig. S8, ESI†). In the case of Pd147
ꢀ
1
barriers of 17.8, 11.8, 23.7 and 25.1 kcal mol , respectively acting as the catalyst for Suzuki coupling, the energy barrier of
ꢀ
1
(for configurations 1–4).
TS3 is 26.6 kcal mol . Combined with the above investigation, the
3ꢀ
ꢀ1
For the transmetalation process, Ph–B(OH) first adsorbs energy barriers of the transmetalation processes (26.6 kcal mol
on the Pd/Cu atoms of Pd147 and Pd Cu DENs to form B–Pd are larger than those of the oxidative addition (12.8 kcal mol ) and
bonds or B–Cu bonds. Two adsorption options are discussed in reductive elimination processes (25.6 kcal mol ), indicating that
Fig. 7b. In terms of the adsorption energy, the adsorption of the transmetalation process is the rate-determining step for the
phenylboronic acid on the Pd atoms is more stable than Suzuki coupling catalyzed by Pd147 DENs. As for the energy barriers
of Pd74Cu73 DENs, that of reductive elimination is much higher
)
ꢀ1
74
73
ꢀ1
than those of the transmetalation and oxidative addition processes.
From the point of view of the energy barriers, an interesting
conclusion can be drawn; for Pd74Cu73 DENs (configurations 1, 2,
3, 4) as the catalyst, the rate-determining step for Suzuki coupling is
the reductive elimination process. Additionally, once the biphenyl–
NPs complex is generated, the complex will be divided into
biphenyl and NPs (Pd147 and Pd74Cu73 DEN) with largely
ꢀ
1
endothermic energies of 106.2 and 79.2 kcal mol , respectively.
Obviously, Pd74Cu73 DENs acting as the catalyst is more efficient
than Pd147 DENs in transmetalation processes. What’s more,
Pd74Cu73 DENs show even better performance than Pd147 DENs
in the biphenyl desorption process. Presumably, this is the
reason why Pd147 and Pd Cu DENs exhibit highly similar
74
73
catalytic activity when the number of catalytic active sites is
reduced. This result is supported by reactant conversion, with
the reaction times of Pd147 and Pd74Cu73 DENs and the TON/
TOF relationships with the reaction times illustrated in Fig. S7
(ESI†). Seen from the conversion with reaction times (Fig. S7A
and B, ESI†), both Pd147 and Pd74Cu73 DENs present nearly the
same catalytic activity. Meanwhile, at the first 9 min, the TOF
of Pd Cu DENs is much higher than that of Pd147 DENs
7
4
73
(Fig. S7C and D, ESI†), because the desorption of biphenyl from
Pd147 DENs is more difficult than from Pd74Cu73 DENs.
By exploring the mechanism of Suzuki coupling and analyzing
the energy barriers, it is found that the energy barriers of the rate-
determining step are different. This result is consistent with the
experimental results; that is, Pd147 and Pd74Cu73 DENs exhibit
nearly the same catalytic activity.
3
.5 Analyses of partial density of states (PDOS) and charge
Fig. 7 Potential energy profile of Suzuki cross-coupling reaction catalyzed
by Pd147 (a) and Pd74Cu73 DENs (b, configuration 2). OH(Cu) means the –OH of
phenylboronic acid adsorbs on the surface of Cu atoms and OH(Pd) means the
density differences
Herein, the d orbitals of the partial density of states (PDOS) and
charge density differences were calculated to expound the
–OH of phenylboronic acid adsorbs on the surface of Pd atoms.
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similar activity of the Pd147 and Pd74Cu73 DENs, the comparable used to explain the reduced desorption energy of biphenyls on
energy barriers of the transmetalation steps and the biphenyl Pd74Cu73 alloy NPs.
decomposition processes. In this article, the PDOS discussion
In addition to the PDOS investigation, the charge density
focuses only on the valence orbitals of the top and edge Pd differences were also computed to illustrate the orbital inter-
atoms due to the interactions between the substrate and the actions and the results are shown in Fig. 8b and c and Fig. 8e–g.
catalyst. As plotted in Fig. 8a and d, near the Fermi level, the Fig. 8b, c and e show the charge density differences of the
bands of the top and edge Pd atoms in the Pd74Ru73 (pink line) transmetalation steps. The charge transfer process can be obtained
and Pd74Rh73 (green line) DENs moved to the left, toward the in this step; phenylboronic acid was activated by base and the
lower energy region, when compared with Pd147 DENs; the lower charge accumulated around the C and B atoms. Compared with
bands lead to lower d-band centers, which are adverse to the Pd147 DENs, the charges accumulate on the surface of C and B
6
8–72
adsorption of substrate.
For the Pd Pt (blue line) and atoms more easily and promote the cleavage of the C–B bond; Cu
74 73
Pd Au (cyan line) DENs, although the bands of the top and atoms play a charge donor role in this step. Next, the electron
7
4
73
edge Pd atoms are similar to those of Pd147 DENs, the intensities density difference plots for the adsorption of biphenyl on Pd147 and
of the bands are smaller or larger than those of Pd147 DENs. Pd74Cu73 DENs illustrate the biphenyl decomposition processes
However, in contrast to Pd74Ru73, Pd74Rh73, Pd74Pt73 and (Fig. 8f and g).
Pd74Au73, the bands of the top and edge Pd atoms of Pd74Cu73
Fig. 8f and g show an obvious charge transfer between
DENs are similar to those of Pd147 DENs. However, for Pd74Cu73 biphenyl and Pd147/Pd74Cu73 DEN. For Pd147 DEN, the charges
DENs, the band strengths of the valence orbitals of the top and are transferred from Pd147 DEN to biphenyl. In contrast, for
edge Pd atoms are weaker than those of Pd147 DENs near the Pd Cu DEN, the charges are transferred from biphenyl to
74
73
Fermi level. Therefore, the interactions between the valence Pd Cu DEN. The interaction between the dz
2
orbital of Pd
7
4
73
orbitals of the top and edge Pd atoms of the Pd Cu DENs with and the p orbital of C leads to the formation of a Pd–C bond.
7
4
73
the valence orbitals of the substrate may be decreased, leading Compared with the Pd74Cu73 DENs, the increased interactions
to the decrease of adsorption energies (see Fig. 8). This can be between the d orbitals of Pd and the p orbitals of C cause the
z
2
stronger adsorption of biphenyl on Pd147 DENs.
According to the PDOS and electron density difference
analyses, compared to the Suzuki cross-coupling catalyzed by
Pd147 DENs, Cu and Pd have a synergistic effect in the mechanism
of the Suzuki cross-coupling reaction catalyzed by Pd Cu DENs
74
73
(see Fig. 8). It originates from two aspects. One is that Cu can
decrease the intensities of the valence orbitals of the top and edge
Pd atoms near the Fermi level. Another is that Cu plays a charge
donor role in the transmetalation process, leading to weak orbital
interactions between the intermediates and Pd74Cu73 DENs.
4. Conclusion
A series of Pd74M73 (M = Cu, Pt, Au, Rh, Ru) DENs were
synthesized and their catalytic activities were investigated in
Suzuki coupling reactions. Similar conversion rates for Suzuki
coupling were observed for Pd147 and Pd74Cu73 DENs, with
HPLC yields of 79.9%/80.9% after 0.75 h reaction time, TONs of
ꢀ
1
6
389/6546, and TOFs of 8519/8728 h , respectively. DFT com-
putations are helpful to elucidate the mechanism of the Suzuki
coupling reaction and TOF. The calculated results indicate that
although comparable energy barriers of the rate-determining step
can be achieved by Pd147 and Pd74Cu73 DENs, the adsorption
energies of substrates on Pd74Cu73 DENs are slightly lower than
those for Pd147 DENs. The PDOS of top and edge Pd atoms and the
electron density differences demonstrate that Cu synergizes with the
Fig. 8 (a and d) The PDOS of the Pd atoms at the top (right) and edge sites
left); the Fermi level is set to zero. (b), (c) and (e) Electron density
(
difference plots for the transmetalation processes catalyzed by Pd147 and Pd –Cu
catalyst in the Suzuki coupling reaction. The Pd atoms
x
147ꢀx
Pd74Cu73 DENs; (b and c) represent phenylboronic acid adsorbed on Pd
facilitate the reaction process and act as a catalyst in the Suzuki
cross-coupling reaction. In comparison, Cu atoms favor the trans-
metalation and desorption processes in the catalytic cycle. This
work provides a general strategy for designing and synthesizing
cheap bimetallic catalysts to boost the Suzuki coupling reaction.
and Cu atoms on the surface of Pd74Cu73 DENs, whereas (e) represents the
adsorption of phenylboronic acid on Pd147 DENs. (f) and (g) Electron
density difference plots for the adsorption of biphenyl on Pd147 and
Pd74Cu73 DENs; yellow contours represent charge accumulation, and
green contours denote charge depression; cutoff = 0.002.
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Conflicts of interest
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Chem. Soc., 2002, 124, 13856–13863.
The authors declare no competing financial interest.
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Acknowledgements
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This work was supported by the National Key Research and
Development Program of China (No. 2016YFC0906000 [2016
YFC0906003]), the National Natural Science Foundation of China
2
(Grant No. 21573030), and Project of Chongqing Key Laboratory of
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