Microwave-assisted fabrication of a mixed-ligand [Cu4(μ3-OH)2]-cluster-based metal–organic framework with coordinatively unsaturated metal sites for carboxylation of terminal alkynes with carbon dioxide

Article abstract of DOI:10.1002/aoc.6288

The development of efficient and stable metal–organic framework (MOF) catalysts with coordinatively unsaturated metal sites for modern organic synthesis is greatly important. Herein, a robust [Cu₄(μ₃-OH)₂]-cluster-based MOF (Cu-MOF) with a mixed-ligand system was successfully fabricated by a microwave-assisted method under mild conditions. The as-prepared Cu-MOF catalyst possessing unsaturated Cu (II) sites exhibited excellent catalytic activity toward the direct carboxylation of 1-ethynylbenzene with CO₂, and various propiolic acid derivatives were synthesized in moderate to good yields under optimized reaction conditions. Furthermore, the catalyst remained stable and could be easily recycled for five sequential runs without incredible decrease in catalytic efficiency.

Full text of DOI:10.1002/aoc.6288

Received: 8 March 2021
DOI: 10.1002/aoc.6288
Revised: 11 April 2021
Accepted: 25 April 2021
F U L L P A P E R
Microwave-assisted fabrication of a mixed-ligand [Cu (μ -
4
3
OH) ]-cluster-based metal–organic framework with
2
coordinatively unsaturated metal sites for carboxylation of
terminal alkynes with carbon dioxide
Wen-Jing Wang
Jun-Feng Qian
| Zhong-Hua Sun
| Ming-Yang He
| Sheng-Chun Chen
| Qun Chen
|
Jiangsu Key Laboratory of Advanced
Catalytic Materials & Technology,
Advanced Catalysis and Green
Manufacturing Collaborative Innovation
Center, Changzhou University,
Changzhou, China
The development of efficient and stable metal–organic framework (MOF) cata-
lysts with coordinatively unsaturated metal sites for modern organic synthesis
is greatly important. Herein, a robust [Cu (μ -OH) ]-cluster-based MOF
(Cu-MOF) with a mixed-ligand system was successfully fabricated by a
microwave-assisted method under mild conditions. The as-prepared Cu-MOF
catalyst possessing unsaturated Cu (II) sites exhibited excellent catalytic
4
3
2
Correspondence
Qun Chen, Jiangsu Key Laboratory of
Advanced Catalytic Materials &
Technology, Advanced Catalysis and
Green Manufacturing Collaborative
Innovation Center, Changzhou
University, Changzhou, 213164, China.
Email: chenqun@cczu.edu.cn
activity toward the direct carboxylation of 1-ethynylbenzene with CO , and
2
various propiolic acid derivatives were synthesized in moderate to good yields
under optimized reaction conditions. Furthermore, the catalyst remained
stable and could be easily recycled for five sequential runs without incredible
decrease in catalytic efficiency.
Funding information
K E Y W O R D S
Jiangsu Key Laboratory of Advanced
Catalytic Materials and Technology,
Grant/Award Numbers: ZZZD201807,
BM2012110; National Natural Science
Foundation of China, Grant/Award
Numbers: 11775037, 21676030
alkyne carboxylation, carbon dioxide, copper (II)-based MOF, microwave synthesis
1
| INTRODUCTION
compounds such as falvones, coumarins, arylidene
[
10]
oxindoles, and alkynylarenes.
To date, many homo-
During the past two decades, carbon dioxide (CO ) has
and heterogeneous catalyst systems for terminal alkyne
carboxylations have been reported in the literature,
2
[11–15]
garnered significant attention as an abundant, low-cost,
nontoxic and renewable C1 source in organic synthe-
and most attention has been focused on noble metal
Ag. For the construction of truly sustainable systems, the
replacement of heterogeneous Ag-based catalysts by
[1–3]
sis.
Although CO is chemically inert and difficult to
2
activate for bond formation, there have also been note-
worthy advances in the development of efficient methods
[
13–15]
cheaper ones is highly desirable.
Several Cu-based
that enable the transformation of CO into value-added
homogeneous catalytic systems using Cu(I) salts (CuI,
CuCl, and CuBr) have been reported in this research
2
[4–9]
chemicals.
In particular, the direct C–H carboxylation
[
16–19]
of terminal alkynes with CO has been recognized as a
area
; however, further application was severely
2
highly valuable strategy for the synthesis of alkynyl
carboxylic acids, because the products are key intermedi-
ates for the synthesis of a variety of biologically active
impeded by either the recycling problems or the metal
contamination of the products. Recently, use of Cu-based
heterogeneous catalytic systems has also proved to be
Appl Organomet Chem. 2021;e6288.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
1 of 12
https://doi.org/10.1002/aoc.6288

2
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WANG ET AL.
[37]
[
35]
[36]
effective in activating the triple bonds of alkynes for the
including MOF-5,
UiO-66,
and HKUST-1,
have
carboxylation with CO . For example, He and Li with
been prepared by the microwave methodology. Very
recently, we have also shown the application of micro-
wave synthesis to rapidly prepare a series of crystalline
blue luminescent coordination polymers with diverse
2
their co-workers reported CuBr loaded on activated
carbon (CuBr@C) as recyclable catalyst for the carboxyla-
[20]
tion with ethylene carbonate as solvent.
Kim's group
[
38]
recently reported that CuCl incorporated in polyvinyl
structures.
2
imidazolium tri-cationic ionic liquid (CuCl @Poly-GLY
With the above considerations in mind, a 3-D tetra-
nuclear Cu-based MOF (Cu-MOF) has been synthesized
rapidly by the reaction of copper (II) nitrate, 4-amino-
1,2,4-triazole, and sodium 5-sulfoisophthalate in aque-
ous solution under microwave irradiation under mild
conditions (Scheme 1). The Cu-MOF featuring a
tetranuclear unit of [Cu (μ -OH) ] possessed good
2
(
1-Vim) (OMs) ) demonstrated an improved catalytic
3 3
[21]
efficiency for coupling of alkyne and CO2,
but the
catalyst synthetic route limits its further application.
Therefore, the exploration of efficient, inexpensive, easily
prepared and robust heterogeneous catalysts for this type
of reaction is still a challenge.
4
3
2
Metal–organic frameworks (MOFs), built from the
assembly of metal centers and multifunctional organic
ligands through metal–ligand coordination bonds, are a
relatively new class of crystalline porous materials,
which can be directly adopted as heterogeneous
catalysts or as catalyst supports/precursors in many
chemical and thermal stability, and was shown to be a
highly active, readily recovered and reused catalyst
for the carboxylation of terminal alkynes with CO2.
This work demonstrates that the potential of
using acid–base mixed-ligand MOFs with high-
nuclearity Cu (II) clusters as a non-precious metal-
[22–26]
important organic transformations.
Recently, a
based heterogeneous catalyst for CO fixation.
2
[13]
[27]
few MOFs, such as MIL-101
UiO-66,
and
[28]
ZIF-8,
have been used as support of reactive silver
nanoparticles to construct MOF-supported silver nano-
particle catalysts for the coupling of alkynes with CO2.
In contrast, the contribution involving the direct catal-
ysis of such conversions by MOF catalysts is still
2 | EXPERIMENTAL SECTION
2.1 | Materials and methods
[29,30]
scarce.
For example, Zhao et al. chose the N,O-
All reagents and solvents were commercially available
and were used as received. Microwave synthesis was per-
formed in MDS-6G (SINEO). Powder X-ray diffraction
(PXRD) patterns were recorded on a Rigaku D/max-2500
diffractometer at 40 kV and 100 mA for a Cu-target tube
(λ = 1.5406 Å). The calculated PXRD patterns were
donor ligand isonicotinic acid (HIN) to synthesize two
cluster-based Gd-Cu heterometallic coordination frame-
works
{[Gd Cu I (CO ) (IN) (HIN) (DMF)(H O)]Á
4
4 3
3 2
9
0.5
2
DMFÁH O} and {[Gd Cu I (IN) (DMF) ]ÁDMF} that
2
n
3
12 12
9
4
n
possess active catalytic sites [Cu I ] and [Cu I ],
12 12
3 2
which could catalyze the alkyne carboxylation in the
obtained from the single-crystal diffraction data using the
[29]
[39]
presence of n-BuI.
workers presented the first example of Cu-based MOF
[Cu (IN) ]ÁDMFÁH O} (Cu (IN)-MOF) applied for the
Very recently, Verpoort and co-
PLATON software.
Thermogravimetric analysis (TGA)
experiments were carried out in the temperature range of
ꢀ
{
25–800 C on a Dupont thermal analyzer under an air
2
2
n
[
30]
ꢀ
À1
direct carboxylation of terminal alkynes with CO2.
atmosphere at a heating rate of 10 C min . Scanning
electron microscopy (SEM) images were taken by a
VEGA 3 TESCAN. The transmission electron microscopy
(TEM) images were obtained on a JEM-2100F microscope
with a limited line resolution capacity of 1.4 Å at an
accelerating voltage of 200 kV. Brunauer–Emmett–Teller
(BET) surface area analysis was performed on a Micro-
Therefore, it is expected that many Cu-based MOFs
could endow them with the ability for this type of
reaction, and thus need further developments.
Developing a fast and efficient approach for the prep-
aration of MOF-based catalysts is of great significance. In
the majority of cases, MOF materials can be produced
through conventional synthetic techniques such as
solvothermal, hydrothermal or diffusion methods over a
1
meritics ASAP 2460 surface area analyzer at 77 K. H
13
NMR and C NMR spectra were recorded on 300- and
500-MHz NMR spectrometers (Bruker AVANCE).
Fourier transform-infrared (FT-IR) spectra of samples in
KBr pellets were recorded utilizing a Nicolet ESP 460 FT-
[31]
period of several days to weeks.
For a long time,
microwave-assisted method has been widely applied in
the synthesis of a great variety of nanomaterials, since
microwave dielectric heating possesses the merits of large
reduction of reaction time, high yields of products, con-
trollable of crystal nucleation and growth, and better
À1
IR spectrometer in the range of 4000–400 cm . X-ray
photoelectron spectra (XPS) were conducted on a PHI
5000 Versa Probe X-ray photoelectron spectrometer
equipped with Al Kα radiation (1486.6 eV).
[32–34]
reproducibility.
So far, some landmarked MOFs

WANG ET AL.
3 of 12
2.2 | Preparation of cu-MOF
3 | RESULTS AND DISCUSSION
The synthesis of Cu-MOF was performed in a micro-
3.1 | Synthesis and characterization of
wave reactor. A mixture of Cu (NO ) Á3H O (0.242 g,
catalyst
3
2
2
1
.0 mmol), 4-amino-1,2,4-triazole (0.040 g, 0.5 mmol),
sodium 5-sulfoisophthalate (0.161 mg, 0.6 mmol),
NaOH (0.048 g, 1.2 mmol), and distilled water (30 ml)
was placed in a 100-ml Teflon-lined container. The
Compared with the conventional hydro/solvothermal
syntheses, the microwave-assisted approach is a facile
and ultrafast method. However, microwave-assisted fabri-
cation of MOFs at room temperature is scarcely reported.
Previously, the single-crystal structure of Cu-MOF was
ꢀ
container was sealed and then irradiated at 60 C in
microwave oven with power 300 W for 15 min. The
reaction product of Cu-MOF (0.238 g) in the form of a
precipitate was centrifuged, washed with distilled water
[
40]
firstly reported by Wang and co-workers,
where
Cu-MOF was hydrothermally synthesized by the reaction
(
10 ml) for three times and dried in vacuum with a
of Cu (NO ) Á3H O, 4-amino-1,2,4-triazole (atrz), sodium
3
2
2
yield of 94% based on Cu. Selected FT-IR data (KBr,
cm ): 3368 br, 3113 m, 1608 s, 1561 m, 1476 w,
5-sulfoisophthalate (NaH SIP) and NaOH under hydro-
2
À1
ꢀ
thermal synthetic conditions at 120 C for 3 days, and the
1
9
436 m, 1369 s, 1278 w, 1,177 w, 1076 w, 1047 w,
28 w, 758 m, 712 m, 614 m.
yield was about 60%. Besides, the chemical properties of
Cu-MOF for potential catalytic applications are not yet
explored, and are of considerable interest since the
acid-based mixed-ligand Cu-MOF contains robust and
coordinatively unsaturated tetranuclear [Cu (μ -OH) ]
2
.3 | General procedure for
4
3
2
[
24]
carboxylation of the terminal alkynes
clusters.
In this work, Cu-MOF could be successfully
ꢀ
obtained by microwave irradiation at 60 C for 15 min to
give a yield of 94%. Moreover, Cu-MOF could also be
isolated under the microwave heating conditions, using
CuCl , Cu (OAc) and CuSO as source of metal atoms
In a typical experiment, a 50-ml stainless steel auto-
clave, equipped with a magnetic stirring bar, was
charged with Cu-MOF (40.6 mg, 1 mol%), Cs CO3
2
2
2
4
(
1.955 g, 6 mmol), 1-ethynylbenzene (0.408 g, 4.0 mmol)
(confirmed by PXRD, IR spectra and elemental analyses).
The morphology and size of the Cu-MOF prepared using
the microwave method were investigated using scanning
electron microscopy (SEM). Figure 1a shows that the
Cu-MOF material is mainly consisted of microparticles
formed from packing of nanosheets. As depicted in
Figure 1b, TEM images revealed that such prepared Cu-
MOF nanoparticles have an ultrathin morphology. The
Brunauer–Emmett–Teller (BET) surface area and pore
size distribution have been measured (Figure S1). The
results indicated that the Cu-MOF has a very small
and DMF (20 ml). Once added, CO2 (0.3 MPa) was
introduced into the reaction mixture under stirring at
1
cooled to room temperature and monitored by high-
performance liquid chromatography (Shimadzu LC-VP)
with a UV detector at 241 nm, a Hypersil ODS2 column
ꢀ
00 C for 16 hr. After the reaction, the mixture was
(
250 mm  4.6 mm, 5 μm), and a mixture of methanol
and H O (3/1, v/v) as eluent with a flow rate of
2
À1
0
.8 ml min . The concentration of 1-ethynylbenzene
and product were calibrated by external standard
method with standard samples.
2
À1
surface area of 42.2 m g and the pore size is very
small. The synthetic reproducibility is high. PXRD
patterns (Figure 1c) confirmed the phase purity of the
as-prepared Cu-MOF. Bulk elemental analysis of
Cu-MOF was consistent with the molecular formula of
{[Cu (μ -OH) (atrz) (SIP) ]Á4H O} . For the IR spectra
The Cu-MOF catalyst was recovered in the solid
residue after centrifugation and separation from the
organic layer (DMF solution). For the other terminal
alkynes, after the reaction, the solid residue was
separated via centrifugation, and the filtrate was
diluted with water (30 ml). The mixture was washed
4
3
2
2
2
2
n
(Figure 1d), a strong and broad band in the range of
À1
with CH Cl2 (20 ml  3) and the aqueous layer
3600–3150 cm indicates the O–H stretching frequen-
2
À1
was acidified with concentrated HCl (6 moL L
)
cies. The medium intensity peaks at ꢁ1561 and
À1
to pH = 1, then extracted with CH Cl2 (20 ml  3).
ꢁ1436 cm are in agreement with the existence of a
2
The combined organic layers were dried over
anhydrous Na SO , filtered and then evaporated. The
triazole ring. The typical strong peaks at ꢁ1608 and
À1
À
ꢁ1369 cm can be attributed to the ν (COO ) and ν
2
4
as
s
À
residue was purified by column chromatography on
silica gel with petroleum ether/ethyl acetate/formic
acid (200/100/1, v/v/v) as the eluent to afford the
propiolic acids.
(COO ) vibrations, respectively. The characteristic
vibrations of νas (SO3 ) in Cu-MOF are in the range of
1278–1177 cm , whereas the ν (SO ) absorptions are
in the range of 1076–1047 cm
À
À1
À
3
s
À1
.

4
of 12
WANG ET AL.
SCHEME 1 Microwave-assisted
synthesis of [Cu (μ -OH) ]-cluster-based
metal–organic framework (Cu-MOF)
4
3
2
FIGURE 1 (a) Scanning electron
microscopy (SEM) images of [Cu (μ -OH)
cluster-based metal–organic framework
Cu-MOF) prepared under microwave
irradiation, (b) transmission electron
4
3
2
]-
(
microscopy (TEM) images of Cu-MOF prepared
under microwave irradiation, (c) powder X-ray
diffraction (PXRD) patterns for as-prepared and
simulated Cu-MOF, and (d) Fourier transform-
infrared (FT-IR) spectrum of Cu-MOF
Cu-MOF is thermally and chemically stable. TG
3.2 | Catalytic activity
analysis showed that the decomposition temperature of
ꢀ
Cu-MOF is about 280 C (Figure 2a). PXRD patterns
Stable copper-based MOFs containing coordinatively
unsaturated copper centers, acting as Lewis acid sites,
can be utilized as effective heterogeneous catalysts in
many important organic transformations. Notably, each
tetranuclear [Cu (μ -OH) ] cluster in Cu-MOF possesses
revealed that the robust structure of Cu-MOF was
ꢀ
retained after the crystalline sample was heated at 250 C
in air for 6 hr (Figure 2b). Cu-MOF is also stable in
various chemical environments (Figure 2c,d), such as
being suspended in dilute acidic or basic aqueous
solution (pH = 2 or 13) at room temperature, in boiling
water, and in various hot organic solvents.
4
3
2
two available unsaturated metal sites, and the activation
process is indeed not required. The catalytic activities of
the as-prepared Cu-MOF were tested using the direct

WANG ET AL.
5 of 12
FIGURE 2 (a) The
thermogravimetric analysis (TGA)
profile of [Cu
metal–organic framework (Cu-MOF).
b) Thermal stability tests for Cu-MOF
monitored by powder X-ray diffraction
PXRD) analysis after the crystalline
samples of Cu-MOF were heated at
4
(μ
3 2
-OH) ]-cluster-based
(
(
ꢀ ꢀ ꢀ ꢀ
1
00 C, 150 C, 200 C, and 250 C in air
for 6 hr, respectively. (c) PXRD patterns
of Cu-MOF after immersing in aqueous
solutions of HCl and NaOH with
different pH values at room temperature
for 24 hr. (d) PXRD patterns of as-
synthesized Cu-MOF and treated
Cu-MOF by immersing in boiling water
and organic solvents, including boiling
chloroform, toluene, acetone,
dichloromethane (DCM), and in hot
ꢀ
DMF at 120 C for 24 hr
carboxylation of 1-ethynylbenzene with CO as a model
of any base (Table 1, entry 13). Entries 1 and 18–21 show
the influence of the catalyst loading on the reaction.
Catalyst loading could not be decreased to 0.5 mol%
because of noticeable decline in the conversion and
selectivity.
2
reaction. In order to obtain the optimized reaction condi-
tions, we have varied different parameters such as sol-
vent, base, amount of base (Table 1), temperature,
reaction time, catalyst loading, and pressure (Figure 3).
All of these parameters play major roles in the carboxyla-
tion reaction as shown in Table 1 and Figure 3.
Initially, various solvents were used to understand
the influence of solvent system on the carboxylation reac-
tion. The reaction was performed in solvents like DMF,
Figure 4 shows the dependence of the conversion and
selectivity on the reaction temperature, reaction time,
catalyst loading and pressure. The conversion markedly
ꢀ
increased at higher temperature and 100 C was found to
be a relatively optimized temperature (Figure 3a). Under
ꢀ
ꢀ
DEC, NMP, DMAC, DMSO, CH CN, dioxane and MB
lower temperature (40 C to 80 C), the carboxylation reac-
tion became slow and cannot be completed. As the
3
(Table 1, entries 1–8). Among these solvents, DMF was
ꢀ
found to be most effective, which may be due to the fact
that DMF is a better solvent for both Cs CO and CO
2
reaction temperature was raised to 120 C, a gradual
decrease in the 1-ethynylbenzene conversion and the
product selectivity was observed, which might be related
to the stability of the propiolic acid product at higher
temperature. It may decompose to 1-ethynylbenzene
through de-carboxylation process when it is over heated.
Meanwhile, the reaction showed moderate conversion in
4 hr of reaction time (Figure 3b). With increasing time of
the reaction, the conversion increases and 16–20 hr are
required to complete reactions. However, a small
decrease in selectivity of propiolic acid product was
observed after prolonging the reaction time to 24 hr,
probably being ascribed to the occurrence of homo-
2
3
[
41]
(
DMF is a weak base) than the other organic solvents.
It is noteworthy that the replacement of Cs CO by other
2
3
bases such as Na CO and K CO under otherwise identi-
2
3
2
3
cal conditions resulted in low conversion (Table 1, entries
and 10), while CsF and CsOAc were inactive to this
9
reaction (Table 1, entries 11 and 12), indicating that
Cs CO is a suitable base for the deprotonation of termi-
2
3
nal alkynes. The amount of Cs CO was varied (Table 1,
2
3
entries 1 and 13–17) and the highest 1-ethynylbenzene
conversion of 91% and high target product selectivity of
9
7% were obtained by using 6 mmol Cs CO (Table 1,
2 3
[30]
entry 1). It can also be observed that a negligible amount
of the propiolic acid product was obtained in the absence
coupling
of
1-ethynylbenzene
(side-reaction).
Figure 3c displays the effect of catalyst loading on the

6
of 12
WANG ET AL.
TABLE 1 Optimization of the
[a]
reaction conditions
[
b]
[c]
[c]
Entry
Solvent
DMF
Base
Base amount (mmol)
Conv. (%)
Select. (%)
1
2
3
4
5
6
7
8
9
Cs
Cs
Cs
Cs
Cs
Cs
Cs
Cs
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
6
6
91
5
97
5
DEC
NMP
6
35
38
72
16
5
87
85
91
32
27
8
DMAC
DMSO
6
6
CH
3
CN
6
dioxane
MB
6
6
3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Na
CO
CsF
2
CO
3
6
19
27
3
33
42
5
10
11
12
13
14
15
16
17
18
19
20
21
K
2
3
6
6
CsOAc
6
2
7
—
—
2
7
12
42
73
93
91
10
51
91
90
Cs
Cs
Cs
Cs
Cs
Cs
Cs
Cs
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
3
33
58
88
85
0
3
3
3
3
3
3
3
4
8
10
6
6
0.5
1.5
2
6
6
a
Reaction conditions: catalyst ([Cu
4
(μ
3
-OH)
2
]-cluster-based metal–organic framework, Cu-MOF, 1 mol%),
ꢀ
1
b
-ethynylbenzene (4 mmol), solvent (20 ml), CO
2
(0.3 MPa), 100 C, 16 hr.
Solvent: DMF (N,N-dimethylformamide), DEC (diethyl carbonate), NMP (1-methyl-2-pyrrolidinone),
DMAC(N,N-dimethylacetamide), and DMSO (dimethylsulfoxide), MB (methylbenzene).
c
Conversion and selectivity were determined by liquid chromatography (LC) analysis.
reaction. When the catalyst loadings was reduced from
mol% to 1 mol%, good to excellent conversion and
selectivity were still obtained. Furthermore, the influence
of pressure on the carboxylation reaction was also
surveyed on the catalytic system (Figure 3d), which
confirmed that the catalyst system proved to be efficient
2-ethylnylthiophene and 3-ethylnylthiophene were also
examined, and the corresponding products were obtained
in good yields (Table 2, entries 17 and 18). Moreover, the
aliphatic alkyne 1-hexyne underwent the reaction to give
71% of the desired product (Table 2, entry 19).
To demonstrate the stability and reusability of the
Cu-MOF catalyst, five recycles were carried out. After
finishing each reaction, the Cu-MOF catalyst was recov-
ered from the reaction mixture by centrifugation and
washed before the next catalytic run. As shown in
Figure 4a, the results revealed that the recovered
Cu-MOF catalyst could maintain its high activity with no
obvious decrease after five consecutive cycles. Further-
more, the PXRD pattern of fifth used catalyst indicated
that the Cu-MOF could retain its crystallinity (Figure 4b).
The FT-IR spectra (Figure 4c) and X-ray photoelectron
2
at a low CO pressure (0.3 MPa).
2
Subsequently, the substrate scope for the Cu-MOF-
catalyzed carboxylation reaction was investigated with
various terminal alkynes under the above-optimized reac-
tion conditions (Table 2). It was found that the Cu-MOF
catalyst is not sensitive to the functionality of the
terminal alkynes. Good yields (71%–88%) were achieved
for aromatic alkynes with both electron-donating and
electron-withdrawing groups (Table 2, entries 1–16).
Additionally, the heterocycle-containing substrates

WANG ET AL.
7 of 12
FIGURE 3 Effect of reaction
temperature (a), reaction time (b),
catalyst loading (c), and the pressure
(
d) on reaction conversion and product
selectivity
FIGURE 4 (a) Recyclability of
[
Cu
4
3 2
(μ -OH) ]-cluster-based metal–
organic framework (Cu-MOF) for the
synthesis of propiolic acid under
standard reaction conditions.
(
b) Powder X-ray diffraction (PXRD)
patterns of Cu-MOF before and after
catalysis. (c) Infrared (IR) spectra of
Cu-MOF before and after catalysis.
(
d) X-ray photoelectron spectra (XPS)
patterns of Cu-MOF before and after
catalysis
spectroscopy (XPS) spectra (Figure 4d) of the recovered
Cu-MOF sample are almost identical to those of the
as-synthesized one. It was observed that the ground
Cu-MOF catalyst with a smaller particle size (60–400
mesh) could accelerate the rate of the carboxylation.
Because the surface area and pore size of the Cu-MOF
catalyst is very small, this reaction should be catalyzed by
the active sites on the surface of Cu-MOF.
To gain insight into the catalytic mechanism, some
control experiments were further conducted (Table 3). In
the absence of the Cu-MOF catalyst, a negligible amount
of the 1-ethynylbenzene conversion was observed
(Table 3, entry 1). The introduction of ligand atrz or
1,3-H BDC did not promote the reaction at all (Table 3,
2
entries 3 and 4). Metal salts such as Cu (NO ) and Cu
3
2
(OAc)2 were found to catalyze the reaction with

8
of 12
WANG ET AL.
[a]
2
TABLE 2 Various terminal alkynes used for the carboxylation with CO using the Cu-MOF catalyst
Entry
Reactant
Product
Yield (%)
1
88
2
3
73
75
4
5
6
80
72
77
7
8
9
81
79
85

WANG ET AL.
9 of 12
TABLE 2 (Continued)
Entry
Reactant
Product
Yield (%)
1
0
82
1
1
1
1
2
3
81
78
76
1
1
4
5
83
76
1
1
6
7
80
72
(
Continues)

10 of 12
WANG ET AL.
TABLE 2 (Continued)
Entry
Reactant
Product
Yield (%)
1
8
75
1
9
71
a
Reaction conditions: catalyst ([Cu
(μ
4 3
-OH)
2
]-cluster-based metal–organic framework, Cu-MOF, 1 mol%), substrate (4 mmol), Cs
2
CO
3
(6 mmol), DMF (20 ml),
ꢀ
CO
b
2
(0.3 MPa), 100 C, 16 hr.
Isolated yield.
TABLE 3 The carboxylation of 1-ethynylbenzene with CO
2
moderate conversion (Table 3, entries 5 and 6), which
implied that the metal Lewis acidic catalysts are favor-
able. For further comparison, we also tested the catalytic
activities of the heterogeneous Cu-based catalysts in the
carboxylation reaction, including Cu-based MOFs
[
a]
with different catalysts
(
Cu-BTC and Cu-FMOF), Cu-doped zeolites (Cu-X,
Cu-Beta and Cu-ZSM-5) and copper oxides (Cu O and
2
[
b]
[b]
CuO) (Table 3, entries 7–13). The results revealed that
the MOF-based catalysts are more efficient than the
metal-doped zeolites and metal oxides as well as metal
salts. Obviously, the catalytic activity of Cu-MOF is
slightly higher than Cu-FMOF and Cu-BTC as well as the
Entry
Catalyst
—
Conv. (%)
Select. (%)
1
2
3
4
5
6
7
8
9
11
91
12
14
44
69
86
78
14
33
50
16
42
38
97
37
39
88
91
93
85
30
52
62
44
56
Cu-MOF
atrz
[
30]
1,3-H
2
BDC
Á3H
ÁH
Cu (IN)-MOF system.
It should be noted that Cu-MOF
Cu (NO
3
)
2
2
O
is constructed from tetranuclear [Cu (μ -OH) ] clusters,
4
3
2
whereas Cu-BTC is based on dinuclear paddle-wheel Cu2
Cu (OAc)
Cu-BTC
2
2
O
clusters, while both Cu-FMOF and Cu (IN)-MOF are
composed of mononuclear Cu unit. Meanwhile, these
MOF-based catalysts all have coordinatively unsaturated
Cu (II) sites. Therefore, the different catalytic activity for
these MOF-based catalysts might be caused by the differ-
ence in their coordination environments of the Cu
(II) center, and Cu-MOF containing high-nuclearity
unsaturated Cu (II) clusters would be beneficial for such
carboxylation reaction.
Cu-FMOF
Cu-X
10
11
12
13
Cu-Beta
Cu-ZSM-5
Cu
2
O
CuO
a
Reaction conditions: catalyst (1 mol%), 1-ethynylbenzene (4 mmol), Cs
2
CO
3
On the basis of the above results and some previous
ꢀ
(
b
6 mmol), solvent (DMF, 20 ml), CO
2
(0.3 MPa), 100 C, 16 hr.
[
21,30,42]
reports,
a possible mechanism over the Cu-MOF
Conversion and selectivity were determined by liquid chromatography (LC)
analysis.
catalyst was proposed and schematically depicted in

WANG ET AL.
11 of 12
SCHEME 2 Proposed reaction mechanism
for the [Cu
4
3 2
(μ -OH) ]-cluster-based metal–
organic framework (Cu-MOF)-catalyzed
carboxylation of terminal alkynes with CO
2
Scheme 2. First, the terminal alkyne coordinates to a
nearby unsaturated copper (II) center of the tetranuclear
unit of Cu-MOF, and the acidity of the alkyne C–H bond
is enhanced. Then, the deprotonation reaction of termi-
nal alkyne by Cs CO leads to the copper (II) acetylide
ACKNOWLEDGMENTS
We gratefully acknowledge the financial supports from
the National Natural Science Foundation of China
(Grants 21676030 and 11775037) and the Jiangsu Key
Laboratory of Advanced Catalytic Materials and
Technology (ZZZD201807 and BM2012110).
2
3
intermediate A. Meanwhile, the insertion of CO into
2
sp-hybridized carbon–copper bond results in the forma-
tion of the copper propiolate intermediate B, which
subsequently reacts with another terminal alkyne and
AUTHOR CONTRIBUTIONS
Wen-Jing Wang: Data curation; formal analysis; investi-
gation; methodology. Zhong-Hua Sun: Data curation;
methodology. Sheng-Chun Chen: Conceptualization;
methodology; project administration; resources; software.
Jun-Feng Qian: Investigation; resources. Ming-Yang
He: Formal analysis; software. Qun Chen: Funding
acquisition; project administration.
Cs CO3 releasing cesium propiolate, simultaneously
2
regenerating copper (II) acetylide intermediate A. At the
end of the reaction, the acidification of cesium propiolate
affords the propiolic acid product.
4
| CONCLUSIONS
DATA AVAILABILITY STATEMENT
In summary, we have prepared a robust tetranuclear
cluster-based Cu (II)-organic framework Cu-MOF
through a simple and fast method assisted by microwave
irradiation. The Cu-MOF material acts as an effective
catalyst for the direct carboxylation of the terminal
The data that support the findings of this study are
available in the supporting information of this article.
ORCID
Wen-Jing Wang https://orcid.org/0000-0001-5093-7377
Zhong-Hua Sun https://orcid.org/0000-0003-1482-0862
Sheng-Chun Chen https://orcid.org/0000-0002-3608-
1581
Jun-Feng Qian https://orcid.org/0000-0002-6127-3390
Ming-Yang He https://orcid.org/0000-0003-1651-7971
Qun Chen https://orcid.org/0000-0001-7841-8990
alkynes with CO with high catalytic activity and selectiv-
2
ity. A wide substrate scope and a good functional groups
tolerance could survive the reaction conditions to afford
the propiolic acids in good to excellent yields. This
catalyst also exhibits good stability and could be readily
recycled for five times without significant decrease in
catalytic activity. This work therefore provides
a
promising strategy for preparing other high-nuclearity
MOF-based catalysts for CO2 conversion, C–C, and
C–heteroatom bond formation, and this approach is
currently underway in our laboratory.
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