Defect engineering: an effective tool for enhancing the catalytic performance of copper-MOFs for the click reaction and the A3coupling

Article abstract of DOI:10.1039/d0cy01946a

A series of Cu(i)-enriched metal-organic frameworks (MOF) of the type CuBTC (BTC = benzene-1,3,5-tricarboxylate) was prepared by a mixed-linker defect engineering technique, namely substituting a portion of a parent linker with truncated pyridine-3,5-dica

Full text of DOI:10.1039/d0cy01946a

Presenting work by Ronald A. Fischer’s group at the
Catalysis Research Center and Department of Chemistry,
Technical University of Munich.
As featured in:
Defect engineering: an effective tool for enhancing the
catalytic performance of copper-MOFs for the click reaction
3
and the A coupling
In our work, we use a kind of Cu(I)-enriched
defect-engineered MOF to catalyse the click reaction and
A3 coupling reaction of phenylacetylene. The coloured
and black-and-white lotus refer to the Cu(I) and Cu(II)
sites in the defect paddlewheel, respectively. The substrate
phenylacetylene tends to coordinate with the Cu(I) site, just
as the fish swim towards the coloured lotus.
See Roland A. Fischer et al.,
Catal. Sci. Technol., 2021, 11, 2396.
rsc.li/catalysis
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Defect engineering: an effective tool for
enhancing the catalytic performance of copper-
MOFs for the click reaction and the A coupling†
Cite this: Catal. Sci. Technol., 2021,
3
11, 2396
a
b
a
and Roland A. Fischer *a
Zhiying Fan, Zheng Wang,
Mirza Cokoja
A series of Cu(I)-enriched metal–organic frameworks (MOF) of the type CuBTC (BTC = benzene-1,3,5-
tricarboxylate) was prepared by a mixed-linker defect engineering technique, namely substituting a portion
of a parent linker with truncated pyridine-3,5-dicarboxylate (PyDC) in the synthesis process. The reduced
carboxyl coordination sites and the emerged Lewis basic pyridyl sites of PyDC spawned mixed-valence
Cu(I)–Cu(II) paddlewheels (PWs) in the defect-engineered CuBTC (DE-CuBTC) structure. Cu(I)-enriched DE-
CuBTC shows significantly enhanced catalytic performance for the click reaction of azide–alkyne
cycloaddition by accelerating the rate determining step of Cu(I)–acetylide intermediate formation. To
further evaluate the catalytic activity of Cu(I)-enriched DE-CuBTC for reactions involving a Cu(I)–acetylide
Received 5th October 2020,
Accepted 16th December 2020
3
intermediate, the A coupling reaction of phenylacetylene, paraformaldehyde and piperidine was studied as
DOI: 10.1039/d0cy01946a
well. This study shows that defect engineering is an effective editing tool of catalytic active sites in
catalysts.
rsc.li/catalysis
Copper(I)-catalysed azide–alkyne cycloaddition (CuAAC),
which is also known as the click reaction, has been
Introduction
The emergence of metal–organic frameworks (MOFs) has a
crucial impact on catalysis chemistry and engineering due to
the large surface area, high porosity, tuneable structure and
extensively employed in many fields such as polymer
2
2–25
chemistry, organic synthesis, and drug discovery.
Usually,
the catalysts for this reaction type are Cu(I) species, which
1
–3
26,27
properties, etc.
originates from their coordinatively unsaturated metal sites
CUS) and/or functional linkers with catalytically active
The catalytic activity of MOFs mainly
operate in the homogeneous phase.
Such catalysts are
prepared by in situ reduction of a Cu(II) salt with a reducing
28
(
agent (e.g. sodium ascorbate), or the direct addition of a
4
–7
29,30
species. The excellent adaptability of MOF structures allows
synthetic efforts to create catalytic active sites artificially by
Cu(I) salt or a Cu(I) complex.
However, homogeneous
Cu(I) catalysts are difficult to separate from the reaction
system, and usually have to be used with additives such as
reducing agents, bases, and stabilizing ligands. Moreover,
molecular Cu(I) catalysts tend to aggregate in the presence of
terminal alkyne and, consequently, lose their activities, which
8
–11
defect engineering and linker modification methods.
Defect engineering,
1
2–14
which generates metal and/or linker
vacancies in the MOFs by partially substituting a pristine
15–21
linker with the so-called defect-generating linker,
has
3
1
attracted great research interest recently due to the easy
accessibility and high efficiency of defect creation. The
introduction of defect-generating linkers into MOFs not only
creates additional CUS but also brings organic groups with
desired functions into the structure. Therefore, defect
engineering is an attractive method to tune the catalytic
properties of MOFs.
further limits their application scope. Hence, it is necessary
to develop more stable and reusable heterogeneous Cu(I)
32–34
catalysts with respect to the environment and economics.
In this regard, MOFs have been applied as recyclable catalysts
3
5–39
for the biphasic solid–liquid CuAAC reactions.
The
formation of Cu(I)–acetylide intermediate is crucial for the
CuAAC reactions. When using the Cu-MOFs as the catalysts,
the rigid MOF skeleton can prevent the aggregation of Cu(I)–
acetylide, thereby extending the lifetime of the active copper
catalyst. However, synthesis of Cu(I)-enriched MOFs is still a
challenge and are normally constructed from expensive and
less-accessible large N-donor linkers such as pyrazolate and
a
Chair of Inorganic and Metal-Organic Chemistry, Catalysis Research Center and
Department of Chemistry, Technical University of Munich, Ernst-Otto-Fischer-
Straße 1, D-85748 Garching bei München, Germany.
E-mail: roland.fischer@tum.de
b
College of Food Science and Engineering, Northwest University, 710127 Xi'an,
4
0–42
pyridine derivatives.
The utilization of defect engineering
Shaanxi, China
methods provides a way of in situ formation of Cu(I) ions in
†
Electronic supplementary information (ESI) available: Characterization of Cu
MOFs, etc. See DOI: 10.1039/d0cy01946a
Cu(II)-based MOFs by employing certain defect-generating
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linkers. Furthermore, to the best of our knowledge, the study Experimental
of Cu(I) sites in copper-based DEMOFs as active centres for
catalysis is still very rare.
Materials
All chemicals for the MOF synthesis and catalytic reactions
were purchased from commercial suppliers and used without
further purification.
In this work, we select one of the best-known Cu(II)-based
MOFs, CuBTC (also known as HKUST-1 or MOF-199, BTC =
benzene-1,3,5-tricarboxylate), as an example to investigate the
concept of defect-engineered MOFs (DEMOFs) in the catalytic
performance of the CuAAC reaction. In our previous works,
we have successfully incorporated a series of truncated
defect-generating linkers with different functional groups
into the Cu(II)–Cu(II) paddlewheel (PW)-containing MOFs to
General methods
Powder X-ray diffraction measurements were performed
using Bragg–Brentano geometry on a PANalytical Empyrean
diffractometer equipped with
a PANalytical PIXcel 1D
1
5,43,44
form the respective DEMOFs,
amount of Cu(I)–Cu(II) deformed PWs in the framework
Fig. 1). Herein, we use this simple approach to prepare a
and found a large
detector. Gas physisorption measurements were performed
1
on a Quantachrome Autosorb IQ2 surface analyzer. H NMR
(
spectra were recorded on a Bruker AV400US spectrometer at
400 MHz. Thermal gravimetric (TG) analysis of the materials
was conducted using a Mettler Toledo TGA/DSC 3+ – MS
instrument at atmospheric pressure under flowing syn-air
series of Cu(I)-enriched defect-engineered CuBTC (DE-CuBTC)
MOFs by employing pyridine-3,5-dicarboxylate (PyDC) as the
defect-generating linker. The phase of DE-CuBTC was
investigated by powder X-ray diffraction (PXRD) and the
presence of Cu(I) was confirmed by X-ray photoelectron
spectroscopy (XPS). Subsequently, the DE-CuBTC materials
were applied as catalysts for the 1,3-dipolar cycloaddition of
benzyl azide and phenylacetylene. We compared the catalytic
performance of the DE-CuBTC catalysts with the activity of
−
1
and a heating rate of 10 K min in a temperature range from
30 to 600 °C. High-resolution X-ray photoelectron
spectroscopy (XPS) measurements were carried out with a
Kratos Axis Supra. Gas chromatography (GC) analysis was
conducted using an Agilent GC-7890B with a flame ionization
detector (FID) and a capillary column HP-5 (length 30 m,
inside diameter 0.32 mm, film 0.25 μm, stationary phase
(5%-phenyl)-methylpolysiloxane).
“defect-free” CuBTC and studied the effect of defect degree in
the DE-CuBTC MOFs on their catalytic activities. The
significantly enhanced activities of DE-CuBTC MOFs show
the beneficial effect of Cu(I) and PyDC, which is in line with
our assumptions. To further evaluate the catalytic activity of
the DE-CuBTC MOFs for reactions involving Cu–alkynyl
Synthesis of CuBTC and DE-Cu-BTC MOFs
Pristine CuBTC was synthesized according to a previously
4
3
3
reported procedure. Typically, Cu(NO
3
)
2
·3H
2
O (1.8 mmol,
intermediates, the A coupling reaction of phenylacetylene,
4
2
38 mg), 1,3,5-benzentricarboxylic acid (H BTC, 1.0 mmol,
10 mg) and 12 ml H O/EtOH (1 : 1 v/v) were added into a
2
3
paraformaldehyde, and piperidine was studied as well.
sealed bottle and kept in a preheated oven at 100 °C for 12 h.
The product was separated by centrifugation and washed
several times with H O and EtOH, respectively. After drying
2
at 70 °C in an oven, the obtained powder was further
activated by heating at 180 °C overnight under dynamic
vacuum.
The CuBTC–PyDC_X (X represents the feeding molar ratio
of PyDC to the total linkers (BTC + PyDC)) samples were
prepared following the reported method with slight
4
5
3 2 2
modification. Cu(NO ) ·3H O (2.42 mmol, 563 mg) and
ligands of H PyDC (n mmol) and H BTC (1.17 − n mmol)
2
3
were dissolved in 25 mL H
2
O/DMF (1 : 1 v/v). Then the
mixture was heated at 80 °C for 7 h. The as-synthesized
materials were collected by centrifugation and washed with
H
2
O and EtOH to remove residual precursor species, and
then dried at 70 °C in an oven. The samples were activated at
80 °C overnight. All the dried MOFs were stored under
1
argon in a glovebox for further use.
Catalytic reactions
CuAAC reaction. The activated MOF catalyst (5 mol%,
calculated based on the Cu content) and a stirring bar were
put into a sealed glass vial with a rubber cap in an argon-
filled glovebox. Then phenylacetylene (0.5 mmol) and 3 mL
Fig. 1 Schematic illustration of the reduced Cu(I)–Cu(II) PWs with a
lowered coordination number of the metal sites. Colour code of the
atoms: Cu(II), blue; Cu(I), green; C, grey; O, red; and N, brown.
Hydrogen atoms are omitted for clarity.
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of EtOH were added into the glass vial with a syringe. The
solution was stirred at 70 °C for 30 min (pretreatment of the
catalyst). Benzyl azide (0.5 mmol) and hexadecane (0.5 mmol,
internal standard) were added, and the mixture was stirred at
mesopores grows. The hierarchical porosity of the DE-CuBTC
MOFs further proves the successful incorporation of defect-
generating ligands. According to previous reports, when PyDC
was incorporated into the framework of CuBTC, some Cu(II)
ions were chemically reduced to Cu(I) for charge balancing
7
0 °C. Aliquots were collected at specific time intervals and
1
5
analysed by GC. Retention times: phenylacetylene 4.71 min,
benzyl azide 6.77 min, hexadecane 10.89 min, and 1-benzyl-4-
phenyl-1H-1,2,3-triazole 15.81 min. The recovered catalyst
was collected by centrifugation, washed with EtOH for 3
times and dried in a vacuum.
To prove that phenylacetylene can coordinate to the Cu(I)
sites in the MOF materials, the activated CuBTC–PyDC_30 (5
mol%, calculated based on Cu content), phenylacetylene (0.5
mmol) and 3 mL of EtOH were added into a glass vial and
stirred for 3 h at 70 °C, and then the material was collected
and re-activated at 150 °C (boiling point of phenylacetylene:
during the solvothermal synthesis.
Hence, X-ray
photoelectron spectroscopy (XPS) measurements were
performed to determine the Cu(I)/Cu ratios in the MOF
samples (ESI,† Fig. S5). The corresponding Cu 2p XPS results
show that the concentration of intrinsic Cu(I) defects is 3.4%
in pristine CuBTC, which is consistent with the previous
4
6
observations. With increasing the doping level of PyDC to
30% in the framework, the Cu(I) concentration in the DE-
CuBTC MOFs reaches 14.9%. When we further increase the
feeding ratio of PyDC to the total linkers to 40%, the Cu(I)
concentration is reduced to 12.7% (Fig. S6a†). A possible
explanation for this is that more “missing PW” type defects
∼
143 °C at 1013 hPa) under dynamic vacuum to remove free
phenylacetylene.
were created when excessive truncated linkers were
3
15,20
A coupling. The MOF catalysts (5 mol%, calculated based
incorporated into the framework.
In this type of defect,
on the Cu content) and paraformaldehyde (1.0 mmol) were
added into a sealed glass vial with a rubber cap in an argon-
filled glovebox. Then, 2 mL of dioxane, 1.0 mmol of
phenylacetylene, 1.0 mmol of piperidine, and hexadecane
the defects could result in the generation of larger pores,
which is in agreement with the PSD results of CuBTC–
PyDC_40 (Fig. S6d†). Therefore, we only investigated the
catalytic performance of CuBTC–PyDC_10–CuBTC–PyDC_30
below.
(1.0 mmol, internal standard) were added with a syringe. The
resulting mixture was stirred at 40 °C. The reaction was
monitored by GC. Retention times: piperidine 3.88 min,
phenylacetylene 4.69 min, hexadecane 10.91 min, and N-(3-
phenyl-2-propynyl)piperidine 11.91 min. The recovered
catalyst was collected by centrifugation, washed with fresh
EtOH, and dried in a vacuum.
CuBTC-Catalysed CuAAC reaction
The catalytic activity of DE-CuBTC MOFs in the CuAAC
reaction was evaluated by studying the cycloaddition of
phenylacetylene with benzyl azide to form 1-benzyl-4-phenyl-
1
H-1,2,3-triazole (Scheme 1). The initial study assessed the
catalytic performance of CuBTC–PyDC_20. The reaction
conditions, starting with a 5% mol catalyst, EtOH, and 70 °C,
were optimised for an efficient catalytic 1,3-dipolar
cycloaddition between phenylacetylene and benzyl azide
Results and discussion
Structure, composition, and porosity of the DE-CuBTC MOFs
The catalysts used in this work were synthesized through
mixing the pristine linker (BTC) with the defect-generating
linker of different molar ratios PyDC under solvothermal
conditions. PXRD shows that all the DE-CuBTC MOFs are
isostructural to the pristine CuBTC (see the ESI,† Fig. S1).
The amount of framework-incorporated PyDC was confirmed
4
7
according to a previous report.
As shown in Fig. 2, the substrates can be nearly
completely converted within 24 h under the optimized
conditions. Moreover, an induction period (IP) of
approximately 20 min could be observed (Fig. 2, a). This was
assumed to be attributed to the slow deprotonation of alkyne
1
by H NMR spectroscopy of the digested samples in NaOD/
3
1
D
2
O (ESI,† Fig. S2 and Table S1). The N
2
adsorption isotherm
to afford the Cu(I)–acetylide species. Therefore, the catalyst
was pretreated with phenylacetylene for 30 min before adding
the benzyl azide. We found that the induction period was not
observed (Fig. 2, b). In all the reactions described below, side
products were not detected.
(77 K) shows that all samples have a high specific surface
area (ESI,† Fig. S3). CuBTC–PyDC_20 and CuBTC–PyDC_30
exhibit a typical type IV adsorption isotherm with a hysteresis
loop, revealing the formation of mesopores. No obvious
hysteresis loop could be found in the isotherm of CuBTC–
PyDC_10 because less defect-generating linkers could not
induce the formation of sufficient mesopores in the
framework. The pore size distribution (PSD) of the pristine
and defect-engineered CuBTC (ESI,† Fig. S4) shows that the
pristine CuBTC only has micropores of around 0.78 nm in
diameter, while the DE-CuBTC MOFs exhibit hierarchical
Fig. 3 shows the performance comparison of the pristine
CuBTC and DE-CuBTC MOFs at different doping levels of
PyDC. In order to eliminate the IP, all the MOF catalysts were
(micro/meso) porosity. Upon integration of defects using the
linker PyDC, mesopores with a 3.8 nm diameter are formed.
When the doping level gradually increases, the proportion of
Scheme
1 The 1,3-dipolar cycloaddition of phenylacetylene with
benzyl azide to form 1-benzyl-4-phenyl-1H-1,2,3-triazole.
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representative sample to perform the leaching tests. After 1 h
reaction time, the solid DEMOFs were separated from the
system by syringe filtration. The reaction continued in the
filtrate for additional 24 h with aliquots collected at specific
time intervals and was analysed by GC. Fig. 4a shows that the
reaction does not proceed after the removal of CuBTC–
PyDC_30, indicating that no homogeneous active Cu species
leached into the reaction solution. After each catalytic cycle,
the catalyst was separated from the reaction mixture by
centrifugation and then washed with ethanol 3 times. The
recyclability results showed that the catalyst lost was around
24% activity after 4 runs (Fig. 4b) The decrease of catalytic
activity is probably due to a minor amount of the catalysts
being lost after each washing/transferring procedure. Fig. 4c
shows that the crystallinity of CuBTC–PyDC_30 is well
maintained after 4 catalytic cycles.
Fig.
2 Reaction profile for the 1,3-dipolar cycloaddition of
phenylacetylene with benzyl azide catalysed by CuBTC–PyDC_20: a)
without the alkyne pretreatment and b) pretreated with
phenylacetylene for 30 min before adding benzyl azide. Reaction
conditions: Cu MOFs (5 mol% Cu), phenylacetylene (0.5 mmol), benzyl
azide (0.5 mmol) and hexadecane (0.5 mmol, internal standard) in
EtOH (3 ml), 70 °C, Ar atmosphere.
Mechanistically, in the CuAAC reaction, the formation of
48,49
Cu(I)–acetylide is
a
rate-determining step,
and the
3
1
binding of azide is also relatively slow. Based on this
information, a possible reaction mechanism for the alkyne–
azide cycloaddition, as catalysed by modified Cu(I)–Cu(II)
PWs in the DE-CuBTC MOFs, is proposed (Scheme 2). In step
pretreated with phenylacetylene for 30 min before adding the
benzyl azide in the subsequent reaction. As expected, all DE-
CuBTC MOFs exhibit much better performance for this
catalytic reaction than the pristine one. Indeed, the reaction
catalysed by pristine CuBTC only affords a conversion of
about 6% after 6 h, while CuBTC–PyDC_30 leads to full
conversion after the same time driven. We emphasize that
our aim was not to prepare heterogeneous catalysts with
absolute high performance, but rather was on the relative
performance of DE-CuBTC and how the catalytic properties
of the MOF materials can be tailored by defect incorporation.
To investigate the stability and recyclability of the DE-
CuBTC catalysts, CuBTC–PyDC_30 was selected as the
1, the phenylacetylene is deprotonated by the pyridyl site of
DE-CuBTC, and then activated by the Cu(I) site. To prove that
phenylacetylene can coordinate to the Cu(I) sites in the MOF
materials, we treated a sample of CuBTC–PyDC_30 with a
solution of phenylacetylene in ethanol, and then the material
was collected and activated at 150 °C under dynamic vacuum
to remove free phenylacetylene. Fig. S8† shows the FTIR
results of CuBTC–PyDC_30 before and after the
phenylacetylene treatment. The material shows additional
−
1
peaks at 2830–2960 cm after the treatment, which can be
assigned to the C–H stretching of the aromatic ring of
5
0
phenylacetylene. Notably, no alkynyl C–H stretching band
−
1
(
∼3300 cm ) is observed, indicating the deprotonation of
phenylacetylene. Therefore, from this study we conclude that
a) phenylacetylene is deprotonated and bound as
(
phenylacetylide to the MOF, and (b) it interacts with the Cu(I)
sites in the framework, since it is well known that Cu(II)–
2
7,31
acetylide is very unstable.
In step 2, the Lewis acidic
Cu(II) site binds to the azido group. The Cu sites in the Cu(I)–
Cu(II) deformed PWs are less sterically hindered compared
with the pristine PWs, which might be beneficial for these
2
2,51,52
two steps.
Subsequently, via an intramolecular ring
closure a copper triazolide intermediate is formed, which is
protonated to afford the triazole product (steps 3 and 4). The
high catalytic activity of DEMOFs (compared with the pristine
CuBTC) can originate from two aspects: (i) the presence of
abundant Cu(I) species for the formation of Cu(I)–acetylide
and (ii) the promotion of alkyne deprotonation by the
defective-generating linker PyDC.
In order to confirm two proposed explanations for the
enhanced catalytic performance over DE-CuBTC, Cu(I)-
enriched and PyDC-free mesoporous CuBTC was prepared by
using a kind of reducing vapor treatment (RVT) to investigate
the effect of PyDC incorporation on the 1,3-dipolar
Fig.
3 Reaction profile for the 1,3-dipolar cycloaddition of
phenylacetylene with benzyl azide catalysed by the pristine CuBTC and
DE-CuBTC MOFs. Reaction conditions: Cu MOFs (5 mol% Cu),
phenylacetylene (0.5 mmol), benzyl azide (0.5 mmol) and hexadecane
(
0.5 mmol, internal standard) in EtOH (3 ml), 70 °C, Ar atmosphere. All
the catalysts were pretreated with phenylacetylene for 30 min before
adding benzyl azide.
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Fig. 4 a) Leaching test of CuBTC–PyDC_30, b) catalyst recycling studies of CuBTC–PyDC_30, and c) the PXRD patterns of the as-synthesized and
reused CuBTC–PyDC_30. Reaction conditions: Cu MOFs (5 mol% Cu), phenylacetylene (0.5 mmol), benzyl azide (0.5 mmol) and hexadecane (0.5
mmol, internal standard) in EtOH (3 ml), 70 °C, Ar atmosphere. All catalysts were pretreated with phenylacetylene for 30 min before adding benzyl
azide.
5
3
3
cycloaddition. This RVT strategy can reduce Cu(II) metal
nodes in the MOFs to Cu(I) by using reducing vapor
Study of the A coupling reaction catalysed by the DE-CuBTC
MOFs
(methanol) and partially break the coordination bonds,
3
A coupling is a one-pot three-component coupling reaction
leading to the creation of hierarchical porosity in the MOFs.
The crystal structure and crystallinity of the reducing vapor
treated MOFs (CuBTC-RVT) were confirmed via PXRD (ESI,†
Fig. S9a). As shown in Fig. S9b,† CuBTC-RVT exhibits a type
IV isotherm with a H2-type hysteresis loop, indicating the
presence of mesopores. The XPS results (Fig. S9e†) show that
the ratio of Cu(I) to the total amount of Cu in CuBTC-RVT is
involving an aldehyde, an alkyne, and an amine to give a
54–56
propargyl amine.
Like the CuAAC reaction, alkyne
deprotonation and the subsequent coordination to Cu(I) lead
57
3
to the corresponding Cu(I)–acetylide.
The A coupling
reaction of phenylacetylene, paraformaldehyde and
piperidine was performed using pristine CuBTC and DE-
CuBTC catalysts. Since the DEMOFs have more Cu CUS,
which facilitates the formation of the Cu–alkynyl
intermediate, these materials should exhibit higher
9
.3%, which is higher than that in the pristine CuBTC (ESI,†
Fig. S5).
Fig. 5 shows that the CuBTC-RVT sample exhibits a better
3
catalytical activities for this reaction. The A coupling was
catalytic performance in the CuAAC reaction, further
confirming that the higher amount of Cu(I) facilitated the
reaction process. Compared with CuBTC–PyDC_20, CuBTC-
performed by using a 5 mol% Cu-based MOF as the catalyst
at 40 °C for 20 h. Note that the reaction temperature was kept
at 40 °C to avoid the reduction of Cu(II) in the MOFs by
RVT has
a
similar porosity and
a
comparable Cu(I)
58
aldehyde. Expectedly, all DE-CuBTC materials show higher
concentration, but no defect-generating linker PyDC with a
Lewis base site was introduced. The catalytic activity of the
activated CuBTC-RVT sample is dramatically lower than that
of CuBTC–PyDC_20, which indicates the vital role of PyDC in
this reaction. This is most presumably due to PyDC
contributing to the deprotonation of alkyne.
activity than pristine CuBTC, with CuBTC–PyDC_30 being the
Fig.
5 Reaction profile for the 1,3-dipolar cycloaddition of
phenylacetylene with benzyl azide catalysed by pristine CuBTC,
CuBTC–PyDC_20 and CuBTC-RVT. Reaction conditions: Cu MOFs (5
mol% Cu), phenylacetylene (0.5 mmol), benzyl azide (0.5 mmol) and
hexadecane (0.5 mmol, internal standard) in EtOH (3 ml), 70 °C, Ar
atmosphere. All the catalysts were pretreated with phenylacetylene for
30 min before adding benzyl azide.
Scheme 2 Proposed catalytic mechanism of the DE-CuBTC catalyzed
1
,3-dipolar cycloaddition of phenylacetylene with benzyl azide.
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Table 1 A3 coupling of piperidine, paraformaldehyde and phenylacetylene, catalysed by different CuBTC materialsa
b,c
Entry
Catalyst
Conversion (%)
1
2
3
4
CuBTC
43
73
88
97
CuBTC–PyDC_10
CuBTC–PyDC_20
CuBTC–PyDC_30
a
Reaction conditions: Cu MOFs (5 mol% Cu), phenylacetylene (1.0 mmol), piperidine (1.0 mmol), paraformaldehyde (1.0 mmol) and
b
c
hexadecane (1.0 mmol, internal standard) in dioxane (2 mL), 40 °C, 20 h, Ar atmosphere. Conversion of phenylacetylene. Determined by GC.
most active among them (Table 1), since it has the highest
degree of defects.
2 A. Dhakshinamoorthy, Z. Li and H. Garcia, Chem. Soc. Rev.,
2018, 47, 8134–8172.
After the reaction, the crystallinity of the DE-CuBTC MOF
was investigated and the results show that the catalyst
structure is retained (see the ESI,† Fig. S10).
3 L. Jiao, Y. Wang, H. L. Jiang and Q. Xu, Adv. Mater., 2018, 30,
1703663.
4 J. Gascon, A. Corma, F. Kapteijn and F. X. Llabres i Xamena,
ACS Catal., 2013, 4, 361–378.
Conclusions
5 D. Yang and B. C. Gates, ACS Catal., 2019, 9, 1779–1798.
6
7
8
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There are no conflicts to declare.
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The authors thank Christian Jandl for his assistance and XPS
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China Scholarship Council (CSC). This work was funded by
the German Research Foundation (DFG) project Fi 502/34-1
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