Palladium(II)@Zirconium-Based Mixed-Linker Metal–Organic Frameworks as Highly Efficient and Recyclable Catalysts for Suzuki and Heck Cross-Coupling Reactions

Article abstract of DOI:10.1002/cctc.201600774

Two series of amine-functionalized Zr-based mixed-linker metal–organic frameworks (MOFs; UiO-66-Mix and UiO-67-Mix) have been synthesized with different ratios of 1,4-benzenedicarboxylate/2-amino-1,4-benzenedicarboxylate and 4,4′-biphenyldicarboxylic acid/2-amino-biphenyl-4,4′-dicarboxylic acid incorporated into their structures. The pendant amino groups were postmodified by the condensation reaction with pyridine-2-carboxaldehyde, and Pd metal centers were anchored onto the MOFs by coordination to the pyridylimine moieties. The two series of heterogeneous Pd catalysts (UiO-66-Mix-PI-Pd and UiO-67-Mix-PI-Pd) were utilized for the Suzuki and Heck cross-coupling reactions in ethanol/water under mild reaction conditions and they showed a remarkably high catalytic activity. The porosity of the MOFs and the density of the metal binding sites of the Pd catalysts had great influence on the Suzuki and Heck cross-coupling reactions. The best results were achieved with a very low Pd loading (0.054 mol %), and the use of UiO-67-3-PI-Pd catalyst led to the complete conversion of reaction substrates. Moreover, the catalysts were readily recovered and recycled in at least 10 cycles without significant leaching or loss of catalytic activity. The catalysts were generally applicable for different reaction substrates with various substituents in both Suzuki and Heck reactions.

Full text of DOI:10.1002/cctc.201600774

DOI: 10.1002/cctc.201600774
Full Papers
Palladium(II)@Zirconium-Based Mixed-Linker Metal–
Organic Frameworks as Highly Efficient and Recyclable
Catalysts for Suzuki and Heck Cross-Coupling Reactions
Rong Sun, Bing Liu, Bo-Geng Li, and Suyun Jie*[a]
Two series of amine-functionalized Zr-based mixed-linker
metal–organic frameworks (MOFs; UiO-66-Mix and UiO-67-Mix)
have been synthesized with different ratios of 1,4-benzenedi-
carboxylate/2-amino-1,4-benzenedicarboxylate and 4,4’-biphe-
nyldicarboxylic acid/2-amino-biphenyl-4,4’-dicarboxylic acid in-
corporated into their structures. The pendant amino groups
were postmodified by the condensation reaction with pyri-
dine-2-carboxaldehyde, and Pd metal centers were anchored
onto the MOFs by coordination to the pyridylimine moieties.
The two series of heterogeneous Pd catalysts (UiO-66-Mix-PI-
Pd and UiO-67-Mix-PI-Pd) were utilized for the Suzuki and
Heck cross-coupling reactions in ethanol/water under mild re-
action conditions and they showed a remarkably high catalytic
activity. The porosity of the MOFs and the density of the metal
binding sites of the Pd catalysts had great influence on the
Suzuki and Heck cross-coupling reactions. The best results
were achieved with a very low Pd loading (0.054 mol%), and
the use of UiO-67-3-PI-Pd catalyst led to the complete conver-
sion of reaction substrates. Moreover, the catalysts were readily
recovered and recycled in at least 10 cycles without significant
leaching or loss of catalytic activity. The catalysts were general-
ly applicable for different reaction substrates with various sub-
stituents in both Suzuki and Heck reactions.
Introduction
Palladium-catalyzed cross-coupling reactions, one of the most
powerful tools for the formation of CÀC bonds, such as Suzuki
and Heck reactions, are of great interest from both academic
and industrial points of view.[1] The significance of Pd catalysis
was emphasized by the award of the 2010 Nobel Prize to
Heck, Suzuki, and Negishi for their pioneering work in this
field.[2] As a result of the limited availability and high price of
Pd, environmental concerns and economic considerations
make it essential to develop Pd catalysts with the full utiliza-
tion of each Pd active site with excellent recyclability and neg-
ligible metal leaching to reduce the cost of the catalysts and
pollution to the environment.[3]
erogeneous catalysts because they contain inherent catalytical-
ly active sites within the frameworks or as supporting materials
for various catalysts.[6] There have been many reports about
MOFs as heterogeneous catalysts or catalyst supports applied
in a variety of organic transformations, which include cross-
coupling, oxidation, and multicomponent reactions.[7] Postsyn-
thetic modification (PSM) is an attractive and powerful tool for
the introduction of functional groups into MOFs by incorporat-
ing organic linkers or unsaturated metal sites.[8] For this pur-
pose, the basicity and reactivity of pendant amino groups has
resulted in the exploitation of amino-functionalized MOFs for
this purpose.[9] Heterogeneous catalysts prepared by the intro-
duction of metal centers onto amino-functionalized MOFs have
been applied in coupling reactions.[10] The dilution of function-
alization through the use of a mixed-linker approach is an ef-
fective method to tune activity by varying the composition of
mixed-linker MOFs.[11]
As a result of their high surface area, porosity, and chemical
tenability, metal–organic frameworks (MOFs) have emerged as
an important class of functional porous materials with distinct
structural properties that are constructed by metal ions or clus-
ters and bridging organic ligands.[4] The advantages of MOFs
have led to the intensive examination of their properties for
potential applications in several areas, which include gas stor-
age, sensors, thin films, optics, drug carriers, and catalysis.[5]
Heterogeneous catalysis is superior to homogeneous catalysis
because it leads to easier separation, reusability, minimized
waste, and cleaner products. MOFs have been pursued as het-
In our ongoing investigations into the application of mixed-
linker MOFs for Suzuki and Heck cross-coupling reactions, Zr-
based MOFs have drawn our attention because of their excel-
lent thermal, aqueous, and acid stability.[11a,12] UiO-66- and UiO-
67-type MOFs were chosen for the incorporation of Pd active
centers because of their high specific surface areas and unpre-
cedented stability.[13] Herein, we report the quantitative PSM of
two series of mixed-linker MOFs (UiO-66-Mix and UiO-67-Mix)
by the reaction with pyridine-2-carboxaldehyde and the further
coordination of Pd to the pyridylimine moieties to afford heter-
ogeneous Pd catalysts (UiO-66-Mix-PI-Pd and UiO-67-Mix-PI-
Pd). The Suzuki and Heck cross-coupling reactions catalyzed by
these heterogeneous Pd catalysts were investigated in detail,
[a] R. Sun, Dr. B. Liu, Prof. B.-G. Li, Prof. S. Jie
State Key Laboratory of Chemical Engineering
College of Chemical and Biological Engineering
Zhejiang University Hangzhou 310027 (P.R. China)
E-mail: jiesy@zju.edu.cn
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600774.
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which included the optimization of reaction conditions, differ-
ent catalysts, various reaction substrates, and recyclability.
(H2BPDC)/2-amino-biphenyl-4,4’-dicarboxylic acid (H2ABPDC) in
DMF at 808C overnight according to a reported procedure.[14]
The difference between the FTIR spectra was hard to observe
because of the similarities of the structures (Figures S1 and S3).
To determine the molecular ratio of ABDC/BDC in UiO-66-Mix
or ABPDC/BPDC in UiO-67-Mix, the mixed-linker MOFs were di-
Results and Discussion
Synthesis and characterization
1
gested in NaOH/D2O solution and analyzed by using H NMR
To optimize the catalytic efficiency of UiO-66-Mix-PI-Pd and
UiO-67-Mix-PI-Pd, the Pd content in the MOFs was controlled
by varying the feed ratio of the mixed linkers during the prep-
aration of the MOFs. The mixed-linker MOFs (UiO-66-Mix and
UiO-67-Mix) were prepared from ZrCl4 and different propor-
tions of 1,4-benzenedicarboxylate (H2BDC)/2-amino-1,4-benze-
nedicarboxylate (H2ABDC) or 4,4’-biphenyldicarboxylic acid
spectroscopy.[15] The signal intensities of ABDC in UiO-66-Mix
or ABPDC in UiO-67-Mix increased gradually with the increase
of the feed ratio of H2ABDC/H2BDC or H2ABPDC/H2BPDC
(Figure 1), and the corresponding quantitative analysis is sum-
marized in Table 1. Except for UiO-67-2, there was a slightly
lower amount of amino groups in UiO-66-Mix and UiO-67-Mix
than that in the feed (Table 1), which was consistent with the
Figure 1. 1H NMR spectra of decomposed samples of a) UiO-66-Mix and b) UiO-67-Mix.
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Table 1. Synthetic mixed-linker MOFs and their postsynthetic modification.
MOF precursors
Modified Pd catalysts
Sample
Feed ratio of H2ABDC/H2BDC or
H2ABPDC/H2BPDC
Ratio of ABDC/BDC or
ABPDC/BPDC in mixed-linker MOFs[a]
Sample
Theoretical Pd content[b]
Pd content[c]
[mmolgÀ1
]
[mmolgÀ1
]
UiO-66-1
UiO-66-2
UiO-66-3
UiO-66-4
UiO-66-5
UiO-67-1
UiO-67-2
UiO-67-3
UiO-67-4
UiO-67-5
0:1
1:11
1:5
1:3
1:2
0:1
1:11
1:5
1:3
1:1
0:1
UiO-66-1-PI-Pd
UiO-66-2-PI-Pd
UiO-66-3-PI-Pd
UiO-66-4-PI-Pd
UiO-66-5-PI-Pd
UiO-67-1-PI-Pd
UiO-67-2-PI-Pd
UiO-67-3-PI-Pd
UiO-67-4-PI-Pd
UiO-67-5-PI-Pd
0
0.004
0.35
0.44
0.66
0.87
0.007
0.16
0.27
0.51
0.83
1:12.8
1:5.2
1:3.2
1:2.1
0:1
1:9.9
1:5.8
1:3.3
1:1.0
0.41
0.50
0.82
1.06
0
0.18
0.31
0.62
1.08
1
[a] Determined by using H NMR spectroscopy. [b] The theoretical Pd content was calculated with the assumption that every pyridine site was coordinated
with Pd.[6e] [c] Measured by using ICP-AES.
reported results for other mixed-linker MOFs.[11d,15] The structur-
al integrity of UiO-66-Mix and UiO-67-Mix was confirmed by
using powder X-ray diffraction (PXRD; Figure S2).
MOFs, and the results showed that the content of Pd in the
catalysts increased with the increase of the content of amino
groups in UiO-66-Mix and UiO-67-Mix (Table 1). The amount of
active sites introduced by PSM could be controlled by adjust-
ing the feed ratio of the mixed linkers (H2ABDC/H2BDC or
H2ABPDC/H2BPDC) during the synthesis of UiO-66-Mix and
UiO-67-Mix. Notably, only traces of Pd entered the pores of the
MOFs by physical absorption, which could be confirmed from
the content of Pd in UiO-66-1-PI-Pd and UiO-67-1-PI-Pd.
Externally, all the MOFs showed no signs of degradation and
maintained their original appearance after PSM. A comparison
of the PXRD patterns of the mixed-linker MOFs before and
after PSM showed no apparent loss of crystallinity after the
modification, although the incorporation of Pd resulted in
a slight variation in the intensity of the diffraction peaks
(Figure 2 and Figures S2, S4, and S5).
UiO-66-Mix and UiO-67-Mix are porous MOFs decorated with
pendant amino groups, which are available to undergo chemi-
cal transformations. The free ÀNH2 groups were thus modified
covalently through a condensation reaction with pyridine-2-
carboxaldehyde (Py-CHO) to yield pyridylimine moieties (UiO-
66-Mix-PI and UiO-67-Mix-PI; Scheme 1). The further coordina-
tion reaction between UiO-66-Mix-PI or UiO-67-Mix-PI and an
excess of dichloro(1,5-cyclooctadiene)palladium(II) ((cod)PdCl2)
in CH2Cl2 formed the Pd catalysts UiO-66-Mix-PI-Pd and UiO-
67-Mix-PI-Pd, in which PdII was anchored onto MOFs
(Scheme 1). The products of each step were characterized by
using PXRD (Figures S4 and S5), BET analysis (Figures S6 and
S7), and thermogravimetric analysis (TGA; Figures S8 and S9).
Inductively coupled plasma atomic emission spectroscopy
(ICP-AES) was used to determine the loading of Pd on the
The N2 adsorption–desorption experiments showed that the
BET surface area of UiO-66-Mix was lower than that of UiO-67-
Scheme 1. Postsynthetic modification of UiO-66-Mix and UiO-67-Mix to yield UiO-66-Mix-PI-Pd and UiO-67-Mix-PI-Pd, respectively.
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Figure 3. a) N2 adsorption isotherms at 77 K and b) DFT pore size distribu-
tion plots of UiO-67-3, UiO-67-3-PI, and UiO-67-3-PI-Pd.
Figure 2. PXRD patterns of a) UiO-66-3, UiO-66-3-PI, and UiO-66-3-PI-Pd and
b) UiO-67-3, UiO-67-3-PI, and UiO-67-3-PI-Pd.
5008C (Figure S8). After the coordination of Pd, the thermal
stability of the materials decreased slightly compared to that
of the corresponding pyridylimine ligands and mixed-linker
MOF precursors (Figure S9). The final residue was a mixture of
ZrO2 and a small amount of PdO.
Mix because of the shorter linkers (Figure S6). As expected, the
PSM is accompanied by a reasonable decrease of the BET sur-
face area (Figure 3a). The BET surface area of the final Pd cata-
lysts decreased gradually with the increase of the content of
amino groups in the mixed-linker MOFs (Figure 4a and
Table 2), whereas the final UiO-67-Mix-PI-Pd catalysts still had
large BET surface areas. Calculations based on the N2 sorption
isotherm at 77 K by the DFT method were also performed to
reveal that the pore size distributions of all the MOFs were
1.2–1.5 nm (Figures 3b and 4b). The N2 sorption investigation
further confirms that all the MOFs were highly porous.
Catalytic Suzuki and Heck cross-coupling reactions
Effect of different catalysts with different Pd contents and
porosities
All the synthetic heterogeneous Pd catalysts, which included
the UiO-66-Mix-PI-Pd and UiO-67-Mix-PI-Pd series, were ex-
plored in Suzuki and Heck cross-coupling reactions. The effect
of different catalysts with different Pd contents and porosities
on the Suzuki and Heck cross-coupling reactions was investi-
gated first (Table 2). We performed model reactions between
bromobenzene and phenylboronic acid for the Suzuki reaction
and between bromobenzene and styrene for the Heck
reaction.
TGA was performed by using desolvated powdered samples
in the temperature range of 50–6508C, and the results demon-
strated their good thermal stability and architectural robust-
ness (>4008C). A high content of amino groups resulted in
a slightly lower decomposition temperature of the Pd catalysts
(Figure 5). The UiO-66-Mix and UiO-67-Mix series showed simi-
lar thermal stabilities. The first slow weight loss at 50–3008C
was caused by the removal of residual solvent and coordinated
DMF, and the second rapid weight loss was caused by the
combustion of the organic parts of the frameworks at 400–
As a result of the larger BET surface area of the UiO-67-Mix
series, the UiO-67-Mix-PI-Pd catalysts were much more active
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nylethene in the Heck reactions. The high porosity provided
more space to allow the reactant molecules to approach the
Pd centers and the possibility of cis-to-trans isomerization,
which led to a higher catalytic activity and better selectivity. If
we compare the results obtained with the UiO-66-Mix-PI-Pd
and UiO-67-Mix-PI-Pd series separately, the conversion of bro-
mobenzene and the selectivity in the Heck reactions increased
firstly and then decreased with the increase of the Pd content.
These results suggested that the CÀC cross-coupling reactions
were affected simultaneously by the metalation and the poros-
ity of the heterogeneous catalysts. Among the Pd catalysts,
UiO-67-3-PI-Pd, which has a low Pd content and high surface
area, gave the best results for both the Suzuki and Heck reac-
tions. Importantly, to determine the role of the pyridylimine
moieties in the catalysts, the amine-free and pyridylimine-free
catalytic systems were also tested (entries 11–13 in Table 2),
and the results indicated that the pyridylimine moieties are im-
portant to promote the CÀC cross-coupling reactions. The het-
erogeneous Pd catalysts with a mixed-linker MOF matrix had
a much better catalytic performance for both the Suzuki and
Heck reactions than homogeneous Pd catalysts, such as
Pd(OAc)2, PdCl2, (cod)PdCl2, and tris(dibenzylideneacetone)-
dipalladium(0) (Pd2(dba)3; entries 14–18 in Table 2).
Effect of the amount of Pd loading using UiO-67-3-PI-Pd
The effect of the Pd loading on the catalytic behavior in the
Suzuki and Heck coupling reactions was investigated with UiO-
67-3-PI-Pd (Table 3). No coupling product was formed with
UiO-67-3, UiO-67-3-PI, or without catalyst. The addition of
a small amount of Pd (2 mg, 0.018 mol%) caused the fast for-
mation of the coupling product (entry 1 in Table 3), and an in-
crease of the Pd loading led to a higher conversion of bromo-
benzene and E selectivity in the Heck reaction. If 6 mg of UiO-
67-3-PI-Pd catalyst (0.054 mol%) was used, almost complete
conversions (2 h for Suzuki and 4 h for Heck) were obtained
and the selectivity for E-1,2-diphenylethene in the Heck reac-
tion reached 100% (entry 3 in Table 3).
Figure 4. a) N2 adsorption isotherms at 77 K and b) DFT pore size
distribution plots of UiO-67-Mix-PI-Pd catalysts.
Optimization of the Suzuki reaction conditions using UiO-67-
3-PI-Pd
With 6 mg of UiO-67-3-PI-Pd catalyst (0.054 mol%), the Suzuki
reaction between bromobenzene and phenylboronic acid was
optimized by investigating the effect of various reaction condi-
tions, such as solvent, base, and reaction temperature
(Table S2). Although
a mixed solvent of DMF/H2O gave
a higher conversion, EtOH/H2O (4:1) was considered as the per-
fect solvent in view of environmental factors and toxicity.
Among all the tested bases, K2CO3 gave the highest conver-
sion, and the Suzuki reaction did not take place in the absence
of base. Organic bases, such as triethylamine, resulted in
a much lower conversion because of their poor solubility in
EtOH/H2O. A higher reaction temperature was favorable to
speed up the Suzuki reaction, and the complete conversion of
bromobenzene was obtained at 808C.
Figure 5. TGA curves of UiO-67-Mix-PI-Pd in the temperature range of 50–
6508C.
than the UiO-66-Mix-PI-Pd catalysts with a similar Pd content
for both the Suzuki and Heck reactions. In addition, the UiO-
67-Mix-PI-Pd catalysts led to a better selectivity for E-1,2-diphe-
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Table 2. Effect of catalysts with different Pd contents and porosities on the Suzuki[a] and Heck[b] reactions.
Entry
Cat.
BET surface
area
Pd
Suzuki
Heck
Conversion[e]
[%]
content[c]
[mmolgÀ1
Conversion[e]
[%]
E selectivity[e]
[%]
[m2 gÀ1
]
]
1
2
3
4
5
6
7
8
UiO-66-1-PI-Pd
UiO-66-2-PI-Pd
UiO-66-3-PI-Pd
UiO-66-4-PI-Pd
UiO-66-5-PI-Pd
UiO-67-1-PI-Pd
UiO-67-2-PI-Pd
UiO-67-3-PI-Pd
UiO-67-4-PI-Pd
UiO-67-5-PI-Pd
UiO-67-1/Pd(COD)Cl2 (1:1)
UiO-67-3/Pd(COD)Cl2 (1:1)
UiO-67-3/Py-CHO (1:1)
Pd(COD)Cl2/Py-CHO (1:1)
Pd(COD)Cl2
1007
826
611
437
268
2106
1641
1321
1198
877
–
0.004
0.35
0.44
0.66
0.87
0.007
0.16
0.27
0.51
0.83
1.75[d]
1.75[d]
0
1.75[d]
3.50[d]
5.64[d]
4.45[d]
2.18[d]
3
62
78
51
43
4
39
61
49
38
1
62
69
98
95
93
88
81
100
100
98
96
40
44
0
2
76
95 (94[f])
92
93 (91[f])
9
85
77
11
9
0
7
10
11
12
13
14
15
16
17
18
83
18
21
0
17
16
18
26
–
–
–
–
–
–
–
23
25
48
32
55
8
PdCl2
Pd(OAc)2
Pd2(dba)3
11
19
3
5
[a] Suzuki reaction conditions: bromobenzene (3.0 mmol), phenylboronic acid (3.3 mmol), K2CO3 (3.0 mmol), Pd catalyst (4 mg), EtOH (4 mL), H2O (1 mL),
808C, 2 h. [b] Heck reaction conditions: bromobenzene (3.0 mmol), styrene (3.3 mmol), K2CO3 (3.0 mmol), Pd catalyst (4 mg), EtOH (4 mL), H2O (1 mL),
808C, 4 h. [c] Determined by using ICP-AES. [d] Calculated according to the formula of corresponding palladium compound. [e] Conversion of bromoben-
zene and selectivity for E-1,2-diphenylethene determined by using GC. [f] Yield of isolated product analyzed by using 1H NMR spectroscopy (Figure S12).
Table 3. Effect of the amount of Pd loading using UiO-67-3-PI-Pd catalyst on Suzuki[a] and Heck[b] reactions.
Entry
Cat.
Amount of
catalyst
[mg]
Pd
Suzuki
Heck
loading[c]
[mol%]
Conversion[d]
[%]
Conversion[d]
[%]
E selectivity[d]
[%]
1
2
3
4
5
6
7
8
UiO-67-3-PI-Pd
UiO-67-3-PI-Pd
UiO-67-3-PI-Pd
UiO-67-3-PI-Pd
UiO-67-3-PI-Pd
UiO-67-3-PI
2
4
6
8
10
6
0.018
0.036
0.054
0.072
0.090
–
69
95
100
100
100
–
58
93
98
100
100
–
91
100
100
100
100
–
UiO-67-3
No catalyst
6
–
–
–
–
–
–
–
–
–
[a] Suzuki reaction conditions: bromobenzene (3.0 mmol), phenylboronic acid (3.3 mmol), K2CO3 (3.0 mmol), EtOH (4 mL), H2O (1 mL), 808C, 2 h. [b] Heck re-
action conditions: bromobenzene (3.0 mmol), styrene (3.3 mmol), K2CO3 (3.0 mmol), EtOH (4 mL), H2O (1 mL), 808C, 4 h. [c] mol% Pd loading with respect
to bromobenzene. [d] Conversion of bromobenzene and selectivity for E-1,2-diphenylethene determined by using GC.
Suzuki reactions with different substituents using UiO-67-3-PI-
Pd
boronic acid derivatives with various substituents were investi-
gated under the optimized conditions (0.054 mol% Pd loading,
K2CO3, VEtOH/VH O : 4/1, 808C; Table 4).
2
The scope and generality of the developed protocol with re-
spect to the Suzuki reactions between aryl halides and phenyl-
The stronger electronegativity of the X atom on aryl halides
resulted in a stronger CÀX bond and a lower catalytic activity.
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Table 4. Suzuki reactions with different substituents catalyzed by UiO-67-
3-PI-Pd.[a]
Table 5. Heck reactions with different substituents catalyzed by UiO-67-
3-PI-Pd.[a]
Entry
X
R1
R2
t
[h]
Conv.[b]
[%]
TOF[c]
[hÀ1
]
Entry
X
R1
R3
t
[h]
Conv.[b]
[%]
E select.[b]
[%]
TOF[c]
[hÀ1
]
1
2
3
4
5
6
7
8
Cl
Cl
Br
I
Br
Br
Br
Br
Br
Br
Br
Br
Br
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
OCH3
F
6
10
2
86
100
100
100
93
265
185
926
1852
861
880
1210
1235
1235
1235
1235
1235
843
1
2
3
4
5
6
7
8
Cl
Cl
Br
I
Br
Br
Br
Br
Br
Br
Br
Br
Br
H
H
H
H
CH3
CH3O
F
NO2
H
H
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
8
12
4
87
100
100
100
93
100
100
100
100
100
100
100
100
100
100
100
100
100
231
154
463
1852
463
463
463
926
617
617
463
463
926
1
2
2
1.5
1.5
1.5
1.5
1.5
1.5
2
CH3
OCH3
CH3CO
F
NO2
CN
H
1
4
4
4
2
3
3
4
95
98
96
100
100
100
100
100
91
100
100
100
100
100
97
9
10
11
12
13
9
4-CH3Ph
4-OCH3Ph
COOH
COOCH3
COOCH2Ph
10
11
12
13
H
H
4
2
100
[a] Reaction conditions: aryl halide (3.0 mmol), substituted phenylboronic
acid (3.3 mmol), K2CO3 (3.0 mmol), UiO-67-3-PI-Pd (6 mg, 0.054 mol% Pd
loading), EtOH (4 mL), H2O (1 mL), 808C. [b] Conversion of aryl halide de-
termined by using GC. [c] Turnover frequency: moles (converted)/moles
(Pd)/time.
[a] Reaction conditions: aryl halide (3.0 mmol), olefin (3.3 mmol), K2CO3
(3.0 mmol), UiO-67-3-PI-Pd catalyst (6 mg, 0.054 mol% Pd loading), EtOH
(4 mL), H2O (1 mL), 808C. [b] Conversion of aryl halide determined by
using GC. [c] Turnover frequency: moles (converted)/moles (Pd)/time.
As a result, the conversion of chlorobenzene was 86% in 6 h
and reached 100% in 10 h, whereas the complete conversion
of iodobenzene was achieved in 1 h (entries 1–4 in Table 4). In
general, the expansion of the Suzuki reaction to a wide range
of substituted aryl bromides and phenylboronic acid deriva-
tives as the reaction substrates gave the excellent yields for
the cross-coupling product. Electron-withdrawing groups (R1;
such as CH3COÀ, ÀF, ÀNO2, and ÀCN) on the para position of
aryl bromides produced a much higher activity than electron-
donating groups (such as ÀCH3 and ÀOCH3; entries 5–10 in
Table 4). The substituents on the para position of phenylboron-
ic acids (R2) also had a great influence on the Suzuki reactions,
but the inverse trend was observed (in the order of ÀCH3 > À
OCH3 > ÀH> ÀF; entries 11–13 in Table 4).
the same position slowed reaction to some extent (entries 5
and 6 in Table 5). Electron-donating groups on the para posi-
tion of styrene derivatives could speed up the Heck reaction,
and complete conversions of bromobenzene were obtained in
3 h (entries 9 and 10 in Table 5). Both acrylic acid and acrylates
were also tolerated by this catalyst under the above condi-
tions, and excellent results were obtained (entries 11–13 in
Table 5). Importantly, only the E isomeric product was obtained
in all cases.
Heterogeneity test
To verify whether UiO-67-3-PI-Pd was truly heterogeneous or
not, the catalyst was removed from the reaction medium by
filtration after 1 h, and the reaction in the filtrate was contin-
ued under the same conditions and monitored by using GC
(Figure 6). Both the Suzuki and Heck reactions stopped com-
pletely at 67 and 44% conversion, respectively, after the re-
moval of catalyst, which indicated that the catalytic cross-cou-
pling reactions were truly heterogeneous and occurred over
the solid catalyst rather than in the solution. In addition, the
liquid reaction medium was collected by hot filtration after the
reaction and analyzed by using ICP-AES along with the recov-
ered UiO-67-3-PI-Pd catalyst. No dissolved Pd ions were detect-
ed in the solution, and the Pd content of the recovered UiO-
Heck reactions with different substituents using
UiO-67-3-PI-Pd
The expansion of the Heck reaction to a wide range of substi-
tuted aryl bromides and olefins as the reaction substrates was
also investigated with UiO-67-3-PI-Pd, and excellent yields
were obtained over a broad range of different reaction sub-
strates (Table 5). The Heck reaction between iodobenzene and
styrene was much faster than that of chlorobenzene and bro-
mobenzene (in the order of I>Br>Cl; entries 1–4 in Table 5). If
we consider the Heck reactions of the substituted bromoben-
zenes and styrene, electron-withdrawing groups (R1, especially 67-3-PI-Pd catalyst (0.266 mmolgÀ1) was almost identical to
ÀNO2) on the para position of aryl bromides could promote
the reactions and decrease the completion time greatly (en-
tries 7 and 8 in Table 5). However, electron-donating groups on
that of the fresh catalyst (0.269 mmolgÀ1), which implies that
no leaching of the active species took place during the
reaction.
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Figure 7. Reusability of UiO-67-3-PI-Pd in the Suzuki and Heck reactions.
Reaction conditions: bromobenzene (10.0 mmol), phenylboronic acid
(11.0 mmol) or styrene (11.0 mmol), K2CO3 (10.0 mmol), UiO-67-3-PI-Pd
(20 mg, 0.054 mol% Pd loading), EtOH (13.2 mL), H2O (3.3 mL), 808C, 2 h for
each cycle in the Suzuki reaction or 4 h for each cycle in the Heck reaction.
Figure 6. Conversion [%] versus time relationships of the Suzuki and Heck
reactions between bromobenzene and phenylboronic acid/styrene using
UiO-67-3-PI-Pd. a) Normal Suzuki reaction; b) Suzuki reaction with the re-
moval of the catalyst after 1 h; c) Normal Heck reaction; d) Heck reaction
with the removal of the catalyst after 1 h. Reaction conditions: bromoben-
zene (3.0 mmol), phenylboronic acid (3.3 mmol) or styrene (3.3 mmol), K2CO3
(3.0 mmol), UiO-67-3-PI-Pd (6 mg, 0.054 mol% Pd loading), EtOH (4 mL), H2O
(1 mL), 808C.
Recycling of the UiO-67-3-PI-Pd catalyst
For the development of environmentally benign procedures,
the ability to recover and reuse the UiO-67-3-PI-Pd catalyst was
investigated. UiO-67-3-PI-Pd is a thermostable and air-stable
solid so there was no need to perform the reaction under an
inert atmosphere. The reusability of UiO-67-3-PI-Pd was evalu-
ated for both the Suzuki and Heck reactions. To recover, reuse,
and characterize the catalyst, the recycling reactions were per-
formed on a larger scale (10.0 mmol of bromobenzene). After
the reaction finished, the solid catalyst was separated from the
reaction mixture by centrifugation. The catalyst was recovered
and reused several times without any loss of catalytic activity.
The results of the catalyst recycling in the Suzuki and Heck re-
actions in up to 10 successive cycles are shown in Figure 7.
The high catalytic activity of the recovered catalyst also dem-
onstrated that the Pd leaching of the catalyst was very low.
X-ray photoelectron spectroscopy (XPS) of UiO-67-3-PI-Pd
(Figure 8) revealed the 3d5/2 and 3d3/2 peaks of PdII species that
appear at binding energies of 338.3 and 343.6 eV, respectively,
Figure 8. XPS spectra of UiO-67-3-PI-Pd before and after the Suzuki reaction.
Reaction conditions: bromobenzene (10.0 mmol), phenylboronic acid
(11.0 mmol), K2CO3 (10.0 mmol), UiO-67-3-PI-Pd catalyst (20 mg, 0.054 mol%
Pd loading), EtOH (13.2 mL), H2O (3.3 mL), 808C, 2 h.
which confirmed the +2 oxidation state of Pd.[16] The absence
of peaks at binding energies of 335.6 and 340.7 eV, which cor-
respond to Pd0, further demonstrated the nonexistence of
Pd0.[17] After the Suzuki coupling reaction, the XPS spectra of
UiO-67-3-PI-Pd showed the same Pd3d peaks, which suggests
that the oxidation state of the immobilized Pd did not change
after the repeated reactions. Notably, the weak intensity of the
Pd peaks is because of the low ratio of Pd/Zr. The other two
peaks at binding energies of 347.0 and 333.5 eV are ascribed
to Zr3p1/2 and Zr3p3/2, respectively.
The recovered catalyst was further characterized by using
PXRD, and the results confirmed that the crystallinity of UiO-
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Figure 9. SEM images of a) UiO-67-3-PI-Pd before the Suzuki reaction and b) the recovered UiO-67-3-PI-Pd catalyst after 10 cycles of the Suzuki reaction.
Figure 10. EDX analysis of a) UiO-67-3-PI-Pd before the Suzuki reaction and b) the recovered UiO-67-3-PI-Pd catalyst after 10 cycles of the Suzuki reaction.
67-3-PI-Pd was well retained during the recycling studies (Fig-
ure S10). Additionally, the morphological changes of UiO-67-3-
PI-Pd were observed by using SEM before and after the multi-
ple cycles (Figure 9). The results of N2 adsorption–desorption
on the recovered catalyst showed a reasonable decrease of the
BET surface area (from 1321 to 719 m2 gÀ1), but the recovered
catalyst still retained its porosity (Figure S11). The energy dis-
persive X-ray spectroscopy (EDX) analysis data of UiO-67-3-PI-
Pd before and after the multiple cycles are given in Figure 10
and show the presence of Pd metal and chloride ions in UiO-
67-3-PI-Pd before and after the multiple cycles.
high yields with a low Pd loading (0.054 mol% Pd loading).
The UiO-67-3-PI-Pd catalyst offers a number of advantages,
which include high porosity, high activity, recyclability up to 10
times, mild conditions, and short reaction times in an environ-
mentally benign solvent system, which could be regarded as
a very effective catalyst for both Suzuki and Heck reactions
from the point of view of green chemistry. The combination of
the advantages of homogeneous Pd catalysts and solid MOFs
in the UiO-67-3-PI-Pd catalytic system may open new opportu-
nities in the development of highly active heterogeneous Pd
catalysts for a variety of Pd-catalyzed transformations.
Conclusions
Experimental Section
Two series of Zr-based mixed-linker metal–organic framework
(MOF) supported Pd catalysts (UiO-66-Mix-PI-Pd and UiO-67-
Mix-PI-Pd) were designed and synthesized by postsynthetic
strategies, and their structural integrities were preserved after
the modification. It is possible to control the density of the
active centers introduced by adjusting the feed ratio of the
linkers that contain functional groups during the synthesis of
MOFs. The resulting heterogeneous UiO-66-Mix-PI-Pd and UiO-
67-Mix-PI-Pd catalysts were active for both Suzuki and Heck
cross-coupling reactions. The successful application of UiO-67-
3-PI-Pd to a wide range of various reaction substrates demon-
strated its universality in both Suzuki and Heck reactions for
the synthesis of versatile biphenyl and styrene derivatives in
Materials
H2BDC, H2ABDC, H2BPDC, dimethyl (1,1’-biphenyl)-4,4’-dicarboxy-
late (Me2BPDC), 1,5-cyclooctadiene (COD), SnCl2·2H2O, ZrCl4,
Pd(OAc)2, PdCl2, and pyridine-2-carboxaldehyde were obtained
from J&K Scientific Ltd. and used as received. All other chemical
chemicals were obtained commercially and used without further
purification unless otherwise stated. [(cod)PdCl2][18] and Pd2(dba)3
were prepared according to the published procedures. H2ABPDC
was prepared according to a literature method from Me2BPDC.[20]
[19]
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308CminÀ1 to a final temperature of 3008C and maintained for
3 min. The GC sample was prepared by diluting 1 mL of the reac-
tion solution with 0.2 mL of dichloromethane.
Preparation of UiO-66-Mix-PI-Pd and UiO-67-Mix-Pd
catalysts
Synthesis of UiO-66-Mix and UiO-67-Mix
UiO-66-Mix and UiO-67-Mix were synthesized according to a report-
ed procedure,[14] and the amounts of the reagents used are listed
in Table S1. ZrCl4, DMF, and concentrated HCl aq. were mixed and
sonicated for 20 min until all the materials were dissolved fully. The
ligands and the residual DMF were then added, and the mixture
was sonicated for an additional 20 min before it was heated at
808C for 15 h. The resulting solids were collected by filtration and
washed with DMF (2ꢁ30 mL) and EtOH (2ꢁ30 mL). The samples
were dried under vacuum at 1208C for 24 h.
Quantitative analysis of the linkers in the mixed-linker
MOFs
Before the measurement (1H NMR performed on a Bruker DMX-400
instrument), the UiO-66-Mix or UiO-67-Mix samples (20 mg) were
digested with 600 mL of a NaOH/D2O solution (1m) and allowed to
stand for 24 h. This hydroxide-based procedure only dissolves the
organic portion of the MOFs (linkers and pore-filling solvent), and
the inorganic component of the MOFs forms zirconium hydroxide
sinks to the bottom and is not measured.
Synthesis of UiO-66-Mix-PI and UiO-67-Mix-PI
Catalytic CÀC coupling reactions
Pyridine-2-carboxaldehyde (0.428 g, 4.0 mmol) was dissolved in dry
CH3CN (15 mL). Freshly desolvated UiO-66-Mix or UiO-67-Mix
(1.0 g) was immersed in this solution, and the reaction mixture was
allowed to stand for 10 d. The resulting solid was collected by cen-
trifugation, washed with dry CH3CN (5ꢁ20 mL) to remove unreact-
ed pyridine-2-carboxaldehyde, and dried under vacuum at 1208C
for 12 h to yield UiO-66-Mix-PI or UiO-67-Mix-PI as a light yellow
solid.
Typical procedure for the Suzuki reaction
The Suzuki coupling reaction was performed in a glass batch reac-
tor. Aryl halide (3.0 mmol) and phenylboronic acid (0.402 g,
3.3 mmol) were dissolved in EtOH (4 mL). This mixture was then
added to a solution of K2CO3 (0.415 g, 3.0 mmol) in water (1 mL)
under stirring and then heated to 808C in an oil bath. After all the
substrates were dissolved, the catalyst was added. The reaction
was monitored by using GC at regular intervals.
Synthesis of UiO-66-Mix-PI-Pd and UiO-67-Mix-PI-Pd
UiO-66-Mix-PI-Pd and UiO-67-Mix-PI-Pd were prepared by stirring
desolvated UiO-66-Mix-PI or UiO-67-Mix-PI (1.0 g) with different
amounts of Pd(cod)Cl2 (2:1 molar ratio of Pd to ÀNH2) in dry CH2Cl2
(15 mL). The reaction was allowed to stand at RT for 2 d. The solid
was isolated by filtration and washed with CH2Cl2 (3ꢁ20 mL) over
2 h and with CH2Cl2 (3ꢁ20 mL) over 3 d to remove any unanch-
ored Pd species, and dried under vacuum at 1208C to yield UiO-
66-Mix-PI-Pd or UiO-67-Mix-PI-Pd as a light yellow or light green
solid, respectively.
Typical procedure for the Heck reaction
A similar procedure to the Suzuki reaction was employed except
for the replacement of phenylboronic acid by olefin (3.3 mmol).
Typical catalytic procedure for catalyst recycling
The reactions were performed under the same reaction conditions
as described above, except that the recovered catalyst was used.
For each cycle, the catalyst was recovered by centrifugation and
then washed thoroughly with diethyl ether, a large amount of
water to remove the base, and finally dichloromethane. The
recovered catalyst was dried under vacuum at 1208C overnight.
Characterization
The FTIR spectra of samples were recorded by using a Nicolet 5700
spectrometer using KBr disks in the range of 4000–400 cmÀ1. The
crystallinity of the materials was characterized by using PXRD by
using a PANalytical B.V.-Empyrean diffractometer using a CuKa radi-
ation source. TGA was performed by using a Pyris 1 TGA analyzer
Acknowledgements
in flowing air from 50 to 6508C with a heating rate of 108CminÀ1
.
The work was supported by the Zhejiang Provincial Natural Sci-
ence Foundation of China (No. LY16B040001) and the National
Natural Science Foundation of China (No. 21536011).
N2 adsorption–desorption isotherms were measured at 77 K by
using a Quantachrome Autosorb-1-C instrument. Before BET analy-
sis, the samples were degassed at 1008C for 24 h. The pore size
distribution curves were determined from the adsorption iso-
therms by the DFT method. ICP-AES was performed by using an
IRIS Intrepid 2 XSP Inductive Coupled Plasma Emission Spectrome-
ter. Samples were first heated slowly to 5008C, dissolved in HCl
after 5 h, and finally diluted to the preset volume. XPS was per-
formed by using an ESCALAB-250Xi X-ray photoelectron spectrom-
eter (ThermoFisher, UK) equipped with an AlKa radiation source
with an energy resolution of 1 eV for the survey and 0.1 eV for in-
dividual characteristic peaks. SEM was performed by using an Utral
55 SEM microscope (CorlzeisD, Germany), which was equipped
with EDX to analyze the elemental composition. GC was performed
by using an Agilent GC 6890 spectrometer with an Agilent HP5-MS
column with dimensions of 30 mꢁ0.25 mmꢁ0.25 mm. The initial
temperature was set to 458C for 1 min and followed by heating at
Keywords: cross-coupling · heterogeneous catalysis · metal–
organic frameworks · palladium · zirconium
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Received: June 27, 2016
Revised: July 28, 2016
Published online on && &&, 0000
ChemCatChem 2016, 8, 1 – 12
www.chemcatchem.org
11
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ

FULL PAPERS
R. Sun, B. Liu, B.-G. Li, S. Jie*
&& – &&
Palladium(II)@Zirconium-Based Mixed-
Linker Metal–Organic Frameworks as
Highly Efficient and Recyclable
Catalysts for Suzuki and Heck Cross-
Coupling Reactions
In the mix: It is possible to control the
density of the introduced active centers
by adjusting the feed ratio of the linkers
that contain functional groups during
the synthesis of metal–organic frame-
works. The prepared heterogeneous Pd
catalysts are very efficient for Suzuki
and Heck cross-coupling reactions with
a very low Pd loading and could be re-
cycled in at least 10 cycles.
&
ChemCatChem 2016, 8, 1 – 12
www.chemcatchem.org
12
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!