Post-synthetically modified MOF for the A3-coupling reaction of aldehyde, amine, and alkyne

Article abstract of DOI:10.1039/c8cy00662h

A new heterogeneous NHC catalyst (Ag-NHC-MOF) was synthesized by the post-synthetic modification of an azolium-containing metal-organic framework. The structure of the catalyst was well defined and fully characterized using various techniques such as PXRD, FE-SEM, HR-TEM, XPS, FT-IR, TGA, BET, ¹H-NMR, ¹³C-NMR, EDS, and ICP. The catalyst showed an excellent activity towards the A³-coupling reaction with a broad substrate scope. The A³-coupling reaction was performed under ambient conditions and full conversion was reached within just 1 hour for the model reaction. The heterogeneous silver NHC MOF (Ag-NHC-MOF) catalyst can be recycled and reused at least four times without a significant loss of activity. In comparison with previously reported heterogeneous catalysts applied for the A³-coupling reaction, the Ag-NHC-MOF demonstrates a superior performance for the A³-coupling reaction.

Full text of DOI:10.1039/c8cy00662h

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Post-synthetic modified MOF for A3-coupling reaction of
aldehyde, amine, and alkyne
Meruyert Kassymova,a Alaric de Mahieu,b Somboon Chaemchuen,a Patrick Demeyere,b
Bibimaryam Mousavi,*a Serge Zuykov,d Mekhman S. Yusubov,c and Francis Verpoort,*acd
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A new heterogeneous NHC catalyst (Ag-NHC-MOF) was synthesized by the post-synthetic modification of an azolium-
containing metal-organic framework. The structure of the catalyst was well defined and fully characterized using various
techniques such as PXRD, FE-SEM, HR-TEM, XPS, FT-IR, TGA, BET, 1H-NMR, 13C-NMR, EDS, and ICP. The catalyst showed an
excellent activity towards A3-coupling reaction with a broad substrates scope. The A3-coupling reaction was performed
under ambient conditions and full conversion was reached within just 1 hour for the model reaction. The heterogeneous
silver NHC MOF (Ag-NHC-MOF) catalyst can be recycled and reused at least four times without significant loss in the
activity. In comparison with previously reported heterogeneous catalysts applied for the A3-coupling reaction, the Ag-NHC-
MOF demonstrates a superior performance for A3-coupling reaction.
www.rsc.org/
PSM of MOFs is the employment of N-heterocyclic carbene
(NHC) precursor (Fig 1).
Introduction
Metal-Organic Frameworks (MOFs) are one of the new groups
of crystalline materials that emerged only two decades ago
while due to their various specific properties a rapid grew in
their development is observed. Their advantages such as
enormous pore size, high surface area, selective adsorption of
smaller molecules, and low density make them potential
candidates for different applications like gas storage,1
purification,2 molecular sensing,3 drug delivery and catalysis.
Metal-organic frameworks (MOFs), particularly at the nano-
scale, have been used as effective heterogeneous catalysts
because of their tunable nature resulting from their metallic
cores and malleable organic linkers.4 Various competent
strategies have been applied for construction of MOFs as
catalysts to improve their catalytical activity at the molecular
level.5 These strategies are categorized as follows: unsaturated
metal parts at active sites,6 chemical transformations, and
immobilization of other metal nanoparticles inside the pores of
the framework,7,8 employment of designed linkers9 and post-
synthetic modification (PSM).10 Among them, the post-
synthetic modification can be developed on both the metal
cores and organic ligands of MOFs.11 One good example for
Fig 1 1,3-bis(4-carboxyphenlyl)imidazolium chloride
N-Heterocyclic carbenes (NHCs) alone are still powerful
catalysts for a valuable series of chemical reactions involving
carbon−carbon, carbon−nitrogen, and carbon−oxygen bond
formation or deletion.11 Therefore, integrating NHCs into a
MOF would combine various advantageous properties such as
generation of the cavities that are appropriate for binding
anionic guests, availability of the protons of the azolium which
is then accessible for active metals, heterogeneous, and easy
separation. This would result in multiple and different
catalytical sites close together in one structure that generally
cannot be formed simultaneously in catalytical systems.11
Regarding these advantages, multiple synthetic methods have
been developed to create NHC linkers inside MOFs. For
example, in one method, the NHC-complexes are pre-
synthesized as organic linkers and utilized for the synthesis of
MOFs.12 While in another method, the N-heterocyclic carbene
complexes generated via MOF synthesis.13-15 However, one of
the easiest and practical methods is PSM of frameworks
containing azolium based linkers16,17 which results in the
immobilization of homogeneous catalysts and also increases
the performance of the final MOF.18 In this work, crystalline
MOF (
1
) was synthesized according to the previous work of
Sen and coworkers.19
1
consists of the [Zn8O] clusters and
angular N-heterocyclic carbene (NHC) containing ligands with
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two aromatic carboxylates (Fig 2). During the post-synthetic
modification step, first, the NHC carbon of the linker was
Experimental
1
Materials and methods
deprotonated, forming a stable carbene. The metal is then
added to the persistent carbene, to form a novel post-
synthetic modified azolium based MOF.
All chemicals were purchased from commercially available
sources and were used as received without further
purification. The materials were synthesized according to the
previous reports with slight modifications.
Ligand synthesis. The compounds N,N-bis(4-carboxyphenyl)
ethylenediimine (LA) and 1,3-bis(4-carboxyphenyl) imidazolium
chloride (H2L+Cl¯) were synthesized according to the previous
reports19,47 with slight modification†.
MOF synthesis.
19
1: [{(Zn0.25)8(O)}Zn6(L)12(H2O)29(DMF)69(NO3)2]n
Fig 2 Structure of the 1 consisting of the [Zn8O] clusters with
six metallomacrocycles and NHC moieties around it8
1
was synthesized following the procedure reported in the
literature8,19 †.
Post-synthetic modification of MOF. The post-synthetic
modification of 1 was performed similarly to a previous report by
our group.8 Ag(OAc) (different amounts) and 1 (60.0 mg, 0.1 mmol)
was introduced to a 150 mL two-necked round bottom flask
containing 60 mL of CH2Cl2 (Table 1). The reaction mixture was
stirred at ambient temperature for 12 h and then refluxed for 24 h
at 39°C under an inert N2 atmosphere. The light-brown solid was
collected by filtration and washed with CH2Cl2 (3 x 5 mL), MeOH, (2
x 5 mL) and Et2O (2 x 5 mL) and dried in air.
In this regard, the new Ag(I)-NHC MOF was synthesized and
fully characterized and utilized as
a
highly active
heterogeneous catalyst for the A3-coupling reaction. Notably,
this is the first report of using the active site Ag(I)-NHC in MOF
catalyst for A3-coupling application.
It should be noted that the A3-coupling reaction is a three-
component reaction involving aldehyde, alkyne, and amine to
produce propargylamine (Fig 3). This reaction attracted many
attentions due to the utilization of simple precursors to
produce biologically active propargylamines. Propargylamines
are an attractive class of compounds with a wide range of
applications as enzyme inhibitors,20,21 antitumor antibiotics,12
pharmaceutical agent22 and so on. They can be applied for
powerful synthetic methods as a key construction block in
various nitrogen-containing heterocycles, like pyrrolidines,23
pyrrole,24 pyrrolophanes,25 aminoindolizines,26 natural
products,27, 28 as well as in therapeutic drug molecules.29-33
Table 1. Various amounts of silver acetate used in the synthesis of
1-Ag
Entry
Silver MOF
1- Ag (1)
Silver Acetate
MOF-1
1
2
3
4
25.0 mg, 0.15 mmol
20.0 mg, 0.12 mmol
12.5 mg, 0.075 mmol
7.5 mg, 0.045 mmol
60.0 mg, 0.1 mmol
60.0 mg, 0.1 mmol
60.0 mg, 0.1 mmol
60.0 mg, 0.1 mmol
1-Ag (0.8)
1-Ag (0.5)
1-Ag (0.3)
A3-Coupling reaction. In a typical procedure phenylacetylene (110
mg, 1.1 mmol), paraformaldehyde (30 mg, 1.0 mmol),
diisopropylamine (111 mg, 1.1 mmol), and 1-Ag catalyst (5 mg)
were added to a 10 mL of Schlenk tube. The mixture was stirred at
room temperature for 1 h. After the reaction was completed, the
mixture was washed with ethyl ether (3 x 5 mL) and filtered. The
compounds were concentrated and the residue was purified by
flash chromatography on silica gel (eluant: hexane/ethyl acetate =
3:1, V/V) to give the desired product as a colourless oil (208.87 mg,
97% isolated yield).
Fig 3 A3-coupling reaction scheme
Various heterogeneous catalytical systems such as silver
nanoparticles31,34,35 supported by copper (I), nickel
complexes,36 nanocrystalline copper (II) oxide,37,38 zinc dust,39
Iron (III) oxide nanoparticles,40,41, gold nanoparticles supported
by zinc oxide,42 gold nanoparticles supported by magnesium
oxide,43 copper(I) metal-organic frameworks,44 and so on have
been reported previously for the synthesis of different
propargylamines. However, the wide use of these catalysts in
the industry is inhibited due to the high cost, difficulty in the
recovery from the reaction mixture, and high reaction
temperature and tedious conditions45,46. In this work, we
present Ag(I)-NHC-MOF as a simply synthesized MOF, which
shows high catalytical activity towards A3-coupling reactions.
To the best of our knowledge, Ag(I)-NHC-MOF shows superior
catalytical performance compared to all previously reported
catalysts for A3-coupling reaction.
Recycling test. An important issue to be evaluated in
heterogeneous catalysis is the reusability. The 1-Ag exhibited good
reusability in terms of the A3-coupling reaction. For this purpose,
after carrying out the first reaction, the catalyst was separated by
filtration and washed with diethyl ether and dried in the air.
Afterward, it was reused directly without further purification. The
recovered catalyst was used for further reactions and showed the
consistent activity for at least 4 consecutive cycles (Table 6, entries
1–4).
2 | J. Name., 2012, 00, 1-3
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Characterization
After the synthesis of
1, a post-synthetic modification was
Powder X-ray diffraction spectra (PXRD) were recorded on an
Empyrean instrument from PANalytical with a monochromatic Cu
Kα radiation at ambient conditions. 1H and 13C-NMR spectra were
recorded using a Bruker 500 MHz NMR spectrometer (500 MHz and
100 MHz, respectively) in CDCl3. Fourier transform infrared (FT-IR)
spectra were collected on a SCIMITAR FTS 2000 spectrometer. A
JEOL (JSM-5610LV, 0.5–35 kV) Scanning electron microscope (SEM)
was applied to study the texture of the material. TEM analyses were
carried out on a Philips CM20 microscope operated at 200 kV. The
element distribution map analysis was obtained by an S4800 field
emission scanning electron microscope (FE-SEM, Hitachi, Japan)
equipped with an X-Max 50 energy-dispersive X-ray spectroscopy
(EDS, Oxford Instruments, Britain) with an accelerating voltage of 10
kV. The binding energy was collected using an X-ray photoelectron
spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA), and C 1s =
297.98 eV was used as a reference. The content of Ag was
determined by inductively coupled plasma optical emission spectra
(ICP, Varian VISTAMPX). MS spectra were determined using a Trace
MS 2000 organic mass spectrometer, and the signals were given in
m/z. Thermogravimetric analyses (TGA-DSC) were performed using
a Netzsch (STA449c/3G) instrument (heating rate of 10 °C/min, N2
flow). The micro pore surface measurements were obtained from
the Brunauer-Emmett-Teller (BET) analysis.
performed. According to the obtained results, the major
diffraction peaks of the PXRD pattern were maintained, hence
it can be concluded that
without collapse of the framework and change in the
structure. In addition, new peak around 8.9-9.0
1 can tolerate such modification
a
2
theta/degrees appeared (Fig 5), which corresponds to the
silver attached to the metal-organic framework.
1-Ag(0.3)
1-Ag(0.5)
1-Ag(0.8)
1-Ag(1.0)
1
10
15
20
25
30
2 Theta/degrees
Fig 5 PXRD pattern of 1 and 1-Ag with various amounts of silver.
Results and discussion
The energy dispersive X-ray spectroscopy (EDS) analysis of the
In order to observe the local geometry and phase purity of the post-synthetic modified MOF clearly showed the existence of
crystalline framework of the synthesized compounds, X-ray the silver in the framework (Figure S1 and S2). The elemental
diffraction (PXRD) was performed and the results are shown in mapping of the 1-Ag(1) indicates the presence of Ag, Zn, C and
Fig 4 and 5. Fig 4 shows that the structure of the metal organic O as shown in Figure S1. According to Figure S1, the
framework, its homogeneity and phase purity match well with distribution of the silver is not uniform in the framework. The
the PXRD patterns previously reported.8,19 Moreover, the PXRD rod-like structures contain definitively more silver, while the
pattern exhibited a long-range crystalline structure proving stone-like structures are composed of the Zn, C, O, and Ag.
that the framework was formed with an unusual secondary Thereafter, the texture and morphology of the silver modified
building unit (SBU). To be more precise, the SBU coordinated
1 were investigated using the field emission scanning electron
to six azolium linkers creating a three-dimensional structure microscope (FE-SEM) (Fig 6). According to Fig 6 (a), it is
proving that synthesized MOF
results found in the literature.8,19
1 is in accordance with the confirmed that the modified MOF
1 had two different
structures: stone-, and rod-like structures that were
distributed randomly. The stone-like particles contained
mostly the zinc framework (1) which was in accord with
previous results,8,19 whereas the rod-like structures were
composed of silver agglomerates (Fig 6 (a)). Subsequently, it
was decided to reduce the amount of silver added during post-
synthetic modification step, in order to avoid agglomerations
1 simulated
of silver on 1.
1 as synthesized
10
15
20
25
30
2 Theta/degrees
Fig 4 PXRD pattern of 1 simulated and as synthesized
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Fig 8 EDS spectrum of 1-Ag(0.5)
The quantitative amount of silver in the 1-Ag samples was
obtained by inductively coupled plasma mass spectrometry
(ICP-MS). The results illustrated that by decreasing the silver
Fig 6 FE-SEM images of (a) 1-Ag(1), (b) 1-Ag(0.5) and HR-TEM acetate amount, the attached silver to the framework
images of (c) MOF
1, (d) 1-Ag(1), (e) 1-Ag(0.5), (f) 1-Ag(0.3)
reduced, which is in agreement with PXRD results (Fig 5).
According to the ICP-MS analyses, the silver content of 1-
After obtaining the results of PXRD, FE-SEM, and HR-TEM Ag(1), 1-Ag(0.8), 1-Ag(0.5), 1-Ag(0.3) amounts to 15.62 wt%,
analysis, the procedure of the post-synthetic of 1 was 14.20 wt%, 11.41 wt%, and 6.23 wt% respectively. On the
modified. Therefore, various amounts of silver acetate were other hand, (Table S1,†), the percentage of Zn did not change
added during the PSM but lower compared to the original 1- remarkably confirming that the main structure of the
Ag(1). Three different ratios of the silver acetate to MOF were framework remained unaffected.
examined. The new amounts of the silver were decreased to
0.8, 0.5, and 0.3 from the initial amount of 1-Ag(1)
.
Reducing the amount of silver acetate for PSM, affected the
distribution of the silver in the metal-organic framework. From
the PXRD analysis it is obvious that by stepwise decreasing the
silver amount, the corresponding peak of silver declined
respectively (Fig 5). Further characterization of the new
samples was performed by HR-TEM and FE-SEM and EDS
analysis. The high-resolution transmission electron microscopy
1173 cm-1
1701 cm-1
Ligand H2L+Cl-
1-Ag(0.5)
1317 cm-1
1655 cm-1
of the original MOF
1 showed the uniform structure (Fig 6 (c)).
1
After the application of the PSM procedure and decreasing the
amount of silver acetate during the synthesis, uniform
distribution of areas of silver (black dots) in the metal-organic
framework is observed using TEM (Fig 6 (e), (f)). FE-SEM
analysis of the newly synthesized 1-Ag(0.5) and 1-Ag(0.3)
exhibited the uniform stone-like morphology (Fig 6 (b)). From
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
EDS mapping and analysis of the newly synthesized silver Fig 9 FT-IR spectra of the ligand (H2L+Cl-),
1 and 1-Ag(0.5).
catalyst 1-Ag(0.5) (Fig 7 and 8) a uniform distribution of the
silver in the framework is observed. Hence, silver ions are Further evidence of silver coordination to the framework is
attached to the metal-organic framework. collected by the FT-IR spectra of the ligand, , and 1-Ag(0.5)
1
and shown in Fig 9. The strong absorption band at 1701 cm-1
corresponds to the ν(C=O) stretching vibration, of
uncoordinated carboxylate acids, the -COOH vibration is not
observed in
groups were totally deprotonated during the synthesis of the
. Furthermore, in , as well as in 1-Ag(0.5) two new peaks, at
1
and 1-Ag samples8 evidencing that all carboxyl
1
1
1655 cm-1 and 1317 cm-1, corresponding to the stretching
vibration of coordinated carboxylate anions, are observed.
Those two peaks are not observed in the FT-IR spectrum of the
ligand, which confirms the coordination between COO- from
the ligand and Zn(II) from zinc nitrate.8 Moreover, the FT-IR
spectra of
1
and 1-Ag(0.5) matched well, which implies that
remained unchanged during the post-
Fig 7 EDS mapping of 1-Ag(0.5) indicating the presence of Zn, the structure of
1
O, C and Ag
synthetic modification. In addition, there is a sharp peak in the
spectrum of the ligand, around 1173 cm-1, which corresponds
to the C-H vibration at the C2 position of the imidazolium ring,8
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which decreases dramatically after the PSM. This proofs that
the carbonic carbons of the imidazolium moieties were
deprotonated by silver acetate and that silver was coordinated
to the respective carbon.
In addition, the TGA analysis of the initial MOF
1 and post-
synthetic modified 1-Ag(0.5) showed no change in the thermal
stability of the framework after PSM as the TGA curve pattern
remained the same (Figure S4). The BET surface area of the
1
and 1-Ag(0.5) are 103.01 and 92.35 m2/g respectively (Table
S2), which is in accordance with previously reported data.8
Afterwards, another evidence of silver anchored to framework
was obtained from the oxidation state of silver and zinc in the
framework analysed by X-ray photoelectron spectra (XPS)
analysis. In the general XPS survey (Fig 10 (a)) the existence of
Ag, Zn, C, O, and N in the synthesized 1-Ag(0.5) is illustrated
and the signal at 400.08 eV corresponds to the binding energy
N 1s of the imidazolium moieties.48-50 According to the high-
resolution spectra of Ag 3d region (Fig 10 (b)), there is a
doublet related to the binding energy of Ag 3d5/2 and Ag 3d3/2
at 367.38 eV and 373.38 eV respectively. These binding
energies are the characteristics of the Ag(I) cation.51-55 In case
of the presence of silver metal nanoparticles peaks at 368.4 eV
and 374.4 eV, corresponding to the binding energy of Ag 3d5/2
and Ag 3d3/2 respectively should be observed.47-50,56 The
absence of these binding energies confirmed that there were
silver metal nanoparticles present in our 1-Ag(0.5) 47-50,56
.
Furthermore, no trace of Ag(OAc) was detected as well, due to
the absence of the binding energy peak at 384.4 eV, which is
the characteristic for the Ag 3d5/2 level of binding energy of
silver acetate.47-50,56 Fig 10 (c) demonstrates the high-
resolution XPS spectrum for Zn. The corresponding peaks of Zn
2p3/2 and Zn 2p1/2 at 1020.98 eV and 1043.88 eV are
characteristic binding energies for Zn(II) cations, which are
coordinated with carboxylic groups to form the SBUs.57-59
Furthermore, the XPS analysis of carbon (Fig 10 (d)) revealed
binding energy peaks at 283.68 eV, 287.28 eV, and 290.78 eV,
which are assigned to the binding energies of the 1s of C=C of
aromatic carbon, imidazolium carbon (C-H) and COO- of
carboxylate respectively.49,60-62 The XPS analysis data are
Fig 10 (a) XPS survey spectra; high-resolution spectra of (b) Ag 3d
region, (c) Zn 2p region, (d) C 1s region of 1-Ag(0.5).
Activity test
The above-synthesized catalysts were applied for the A3-
consistent with the reported values for post-modified 1 8
.
coupling
reaction
between
phenylacetylene,
paraformaldehyde, and diisopropylamine (as a model reaction)
at room temperature in CH2Cl2. In order to study the catalytical
activity, the reaction was studied using several catalysts, such
as no catalyst, 1, 1-Ag(0.3), 1-Ag(0.5), and 1-Ag(0.8) (Table 2).
(a)
According to the obtained results, no conversion was observed
when the reaction was performed without any catalyst (Table
2, Entry 1), on the other hand, the reaction can occur with the
help of both
catalyzed by 1-Ag outperforms
time of 1 h (Table 2, Entry 4 and 5), whereas reaction catalyzed
by reaches >99% conversion only after 24 h (Table 2, Entry
2). The different silver-containing catalysts (1-Ag(0.3) 1-
1
and 1-Ag catalysts. However, the reaction
1
with respect to the reaction
1
,
Ag(0.5), and 1-Ag(0.8)) were able to catalyze the reaction
showing a good activity within short period of time. However,
1-Ag(0.5) was selected as the catalyst for further investigations
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due to the better distribution of the silver ions inside the
framework and better performance.
100
80
60
40
20
Table 2 Effect of the catalyst on the A3-coupling reactiona
H
N
O
Catalyst
H2
C
+
+
N
H
H
Conversion
(%) b
Isolated
Yield (%)
Entry
Catalyst
Solvent
Time
1-Ag(0.5)
1
1
2
3
4
5
No catalyst
1
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
2 h
24 h
2 h
0
0
>99
>99
>99
>99
96
97
97
97
1-Ag(0.3)
1-Ag(0.5)
1-Ag(0.8)
0
5
10
15
20
25
30
35
40
1 h
Amount of catalyst (mg)
1 h
Reaction conditions: phenylacetylene (1.1 mmol), paraformaldehyde (1.0 mmol), Fig 11 Effect of the amount of the catalyst on A3-coupling
a
diisopropylamine (1.1 mmol), CH2Cl2 (2.0 mL), N2 atmosphere, 1 or 1-Ag catalyst, room
temperature. b Conversions and yields were determined by 1H NMR spectroscopy
reactiona
Reaction conditions: phenylacetylene (1.1 mmol), paraformaldehyde (1.0 mmol),
(a
diisopropylamine (1.1 mmol), CH2Cl2 (2.0 mL), N2 atmosphere,
1
1
or 1-Ag(0.5) catalyst, room
)). b Conversions were determined by 1H NMR
It is evident that 1-Ag(0.5), is a much better-performing
catalysts for the A3-coupling reaction compared to
showed poor activity toward this reaction.
temperature (1 h (for 1-Ag(0.5) and 24 h (for
spectroscopy)
1 which
The effect of the solvent on the A3-coupling reaction was
studied and the results are summarized in Table 4. Among the
solvents tested, CH2Cl2, acetone, and CH3CN were the most
suitable solvents for the A3-coupling reaction (Table 4, entry 1-
3). Reaction in THF and without any solvent generated the
desired product, however, residues of substrates were still in
the reaction mixture (Table 4, entry 6, and 8). Poor results
were obtained when the reaction was carried in toluene (Table
4, entry 7). Furthermore, the reaction did not proceed in the
DMSO and DMF (Table 4, entry 4, and 5).
In order to optimize the reaction time, a series of reactions
were performed. According to the obtained results, the full
conversion of the substrates into the desired product occurred
after 24 hours when 1 was used as a catalyst, in the absence of
any silver source (Table 3, entry 4). On the other hand, it was
found out that the three-component reaction can occur just in
1 hour in the presence of 1-Ag(0.5) (Table 3, entry 9). Hence, it
is clearly demonstrated that silver plays a pivotal role in the
catalysis of the A3-coupling reaction. This can be assigned to
the presence of efficient Ag-NHC active site inside the
framework, which is known for its ability to promote three-
coupling reaction.31
Table 4 Effect of the solvent on A3-coupling reaction using
silver-MOF catalysta
Table 3 Effect of the time on A3-coupling reaction using
1-Ag catalystsa
1 and
H
O
Catalyst
H2
C
N
+
+
N
H
H
Entry
Catalyst
1-Ag(0.5)
1-Ag(0.5)
1-Ag(0.5)
1-Ag(0.5)
1-Ag(0.5)
1-Ag(0.5)
1-Ag(0.5)
1-Ag(0.5)
Solvent
CH2Cl2
Conversion (%) b
Isolated Yield (%)
H
1
>99
78
97
75
89
----
----
59
25
50
O
Catalyst
H2
C
N
+
+
N
2
3
4
5
6
7
8
Acetone
CH3CN
DMSO
H
H
92
----
----
62.5
29
DMF
Entry
Entry
THF
Toluene
No solvent
53
1
2
3
4
5
1
1
1
1
1
2 h
6 h
30%
57%
6
7
8
9
1-Ag(0.5)
1-Ag(0.5)
1-Ag(0.5)
1-Ag(0.5)
15 min
30 min
45
22
a
67
83
Reaction conditions: phenylacetylene (1.1 mmol), paraformaldehyde (1.0 mmol),
diisopropylamine (1.1 mmol), CH2Cl2 (2.0 mL), N2 atmosphere, 1-Ag(0.5) catalyst, room
15 h
24 h
36 h
86%
temperature. b Conversions and yields were determined by 1H NMR spectroscopy
>99%
>99%
1 h
>99
a
In order to examine the scope of the A3-coupling reaction
using this 1-Ag catalyst, the study was extended to different
combinations of the substrates. The results are shown in Table
5. Phenylacetylene was used as a model substrate in the
beginning. Various cyclic, heterocyclic and acyclic secondary
aliphatic amines were reacted with phenylacetylene and para-
formaldehyde and afforded excellent conversions to the
desired product at room temperature within 1 hour (Table 5,
Entry 1,5, 7-10, 17). In addition, aromatic amines in
Reaction conditions: phenylacetylene (1.1 mmol), paraformaldehyde (1.0 mmol),
diisopropylamine (1.1 mmol), CH2Cl2 (2.0 mL), N2 atmosphere, 1 or 1-Ag(0.5) catalyst, room
temperature. b Conversions were determined by 1H NMR spectroscopy
Fig 11 summarizes the results of the effect of the catalyst
amount on the three-component coupling reaction. In order to
do so, the amount of the catalyst was gradually decreased
from 35 mg down to 2.5 mg. Experimental results showed that
A3-coupling reaction can successfully occur using 5 mg of the
silver catalyst, whereas 30 mg of
reactants to products (Fig 11).
1 is required to convert
6 | J. Name., 2012, 00, 1-3
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a
Reaction conditions: phenylacetylene (1.1 mmol), paraformaldehyde (1.0 mmol),
diisopropylamine (1.1 mmol), CH2Cl2 (2.0 mL), N2 atmosphere, 1 or 1-Ag(0.5) catalyst, room
combination with alkyne and aldehyde produced the desired
propargylamines with high yields (Table 5, Entry 8).
temperature. b Conversions and yields were determined by 1H NMR spectroscopy
To expand and examine further the scope of the A3-coupling
reaction, aliphatic and aromatic alkynes were used as one of
the substrates. A number of alkynes were combined with
aldehydes and amines under A3-coupling reaction conditions.
Aromatic alkynes containing bromo-, and hydroxy groups
demonstrated a good reactivity along with aliphatic ones
(Table 5, Entry 17-20). Under the applied reaction conditions,
cyclic and acyclic aliphatic aldehydes showed good reactivity
(Table 5, Entry 1-4). Good yields were afforded also when
combining aromatic aldehydes with electron-donating and
electron-withdrawing functional groups (Table 5, Entry 12-16).
The hot filtration test was performed under the optimized
reaction conditions, in order to investigate the heterogeneous
nature of the silver catalyst. Phenylacetylene (1.1 mmol),
paraformaldehyde (1.0 mmol) and diisopropylamine (1.1
mmol) were added together with 1-Ag(0.5) (5 mg) and 2 mL of
DCM under room temperature. After 20 min 60% conversion
of reactants was observed and the catalyst was filtered out
from the mixture, leaving the reaction to proceed for 1h.
During that time, no further formation of the product was
observed, which clearly shows that no leaching of silver(I)
occurred during reaction.
Table 5 A3-coupling reaction catalyzed by 1-Ag(0.5)a
In order to examine the decomposition of the catalyst and its
recyclability, several reactions were performed. Thereafter,
the catalyst was filtered and washed with diethyl ether (2 mL x
3), air-dried and reused again without further purifications.
The recovered catalyst was used again in the next cycle and an
almost consistent activity was noticed for 4 consecutive cycles
(Table 6). A slight loss of the activity of catalyst was observed
and can be attributed to the filtration process, by which
catalyst was collected after each reaction cycle. There is a
possibility that during filtration some of the particles of the
catalyst were lost, which lead to the slight reduction in the
yield.
Conversion
(%)b
Isolated
Entry
Alkyne
Aldehyde
CH2O
Amine
Yield (%)
H
N
C6H5C≡CH
C6H5C≡CH
C6H5C≡CH
C6H5C≡CH
C6H5C≡CH
C6H5C≡CH
C6H5C≡CH
C6H5C
>99
94
93
90
96
93
90
90
92
89
85
84
82
83
79
78
79
86
82
85
97
91
90
87
93
89
87
86
89
87
83
82
79
80
77
75
76
81
79
82
1
H
N
C6H5CHO
2
O
H
N
3
H
H
N
C6H12CHO
CH2O
4
Table 6 Successive trials by using 1-Ag(0.5) catalyst
5
N
H
H
O
Catalyst
H2
C
N
+
+
C6H5CHO
CH2O
6
N
N
H
H
H
HN
O
7
Cycle
Conversion (%)
Isolated Yield (%)
1
2
3
4
>99
>99
96
97
96.5
94
H
N
≡CH
CH2O
8
NH2
94
91
C6H5C≡CH
C6H5C≡CH
C6H5C≡CH
C6H5C≡CH
CH2O
9
a
Reaction conditions: phenylacetylene (1.1 mmol), paraformaldehyde (1.0 mmol),
diisopropylamine (1.1 mmol), CH2Cl2 (2.0 mL), N2 atmosphere,
temperature.
1 or 1-Ag(0.5) catalyst, room
NH
NH
NH
NH
NH
CH2O
10
11
12
13
14
15
16
17
18
19
20
b Conversion by 1H-NMR
C6H5CHO
Fig 12 represents the PXRD pattern for 1-Ag(0.5) before and
after the catalytical activity. It can be seen that the structural
integrity of the metal organic framework of catalysts was not
affected significantly as the main peaks characterizing the
material remained unchanged. However, the minor differences
were observed on the PXRD pattern. The possible reason is
that during the catalytical synthesis some residues of the
substrates were being trapped in the pores of the catalyst,
which lead to the decrease of the surface area of reused
catalyst, as confirmed by BET analysis (Table S2, Entry 3) and
caused the appearance of additional signals on the PXRD
pattern. FE-SEM images confirm that the morphology is not
affected by the reaction (Figure S5). The silver catalyst can be
reused without the loss of the activity as well as without any
significant collapse of the structure. Additionally, silver
leaching of the catalyst was examined as well. Inductively
coupled plasma (ICP) analysis indicated that silver content
p-NO2-
C6H5CHO
p-Br-
C6H5C
≡CH
C6H5CHO
p-Cl-
C6H5C≡CH
C6H5C≡CH
C6H5C≡CH
C6H5C≡CH
C6H5CHO
p-OH-
NH
C6H5CHO
p-CH30-
NH
C6H5CHO
Ph
HN
HN
HN
HN
CH2O
CH2O
CH2O
CH2O
Ph
Ph
Ph
p-Br-
C6H5C≡CH
Ph
p-OH-
C6H5C≡CH
Ph
Ph
Ph
C5H10C≡C
H
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slightly changed from 11.41 % before the reaction to 11.33 % II.46,71 On the other hand, the iminium ions are formed from
after the catalytical reaction (Table S1).
aldehyde and amine, through the coordination of the carbonyl
group of aldehyde to Zn (II). This iminium ion reacts with zinc
acetylide intermediate II to give intermediate III. Finally, the
nucleophilic attack of the acetylide ion on the iminium carbon
generates the desired product and regenerates the initial
catalyst.46,71
1-Ag(0.5) after the reactions
The second path involves the silver side of the metal-organic
framework. The synthesized silver metal-organic framework
(
1-Ag(0.5)) contains the NHC moiety that is attached to the
silver. Therefore, it can be assumed that reaction follows the
following mechanism , which is slightly modified from the
originally proposed mechanism by Li.63 In the beginning, the
Ag(I) inserts into the C-H of the alkyne, followed by an
elimination of the H which will generate the silver alkynide and
the amine hydrogen halide salt.32,45,72 Thereafter, the amine
hydrogen halide salt reacts with the aldehyde to give the
iminium halide, which then reacts with the silver alkynide to
produce the desired product and regenerate the initial
1-Ag(0.5) before the reactions
10
15
20
25
30
2 Theta/degrees
Fig 12 PXRD pattern of the 1-Ag(0.5) catalyst before and after catalyst. 32,45,72 Further studies to shed light on the mechanism
the reaction
of A3-coupling reaction are currently ongoing in our laboratory.
To stress out the good catalytical performance and efficiency
of the silver-MOF catalyst a comparison to various catalysts in
the literature was made. The recently reported efficient
catalysts are summarized in Table 7 below. Among reported
catalysts, the high yields were obtained using the following
catalysts: PS-supported NHC-Ag(I) (Table 7, entry 1),
ZnII/HAP/Fe3O4 (Table 7, entry 4), Ag nanoparticles (Table 7,
entry 7), Cu2(BPDC)2(BPY) (Table 7, entry 11) and Cu[(2-
pymo)2] (1%) (Table 7, entry 12). Obviously, the mentioned
catalysts are very active, however, some of them require long
reaction time (Table 7, entry 1-3. 7-10), and others perform
only at high temperatures (Table 7, entry 2-4, 7-11). Besides
those, MOF−Cu2I2(BTTP4) catalyst demonstrated good
reaction conditions, but the yield of the product was slightly
lower than for the synthesized silver catalyst. From the Table
7, it is obvious that most of the methods reported in the
literature are suffering from various drawbacks related to
reaction conditions. In comparison to that, the synthesized 1-
Ag(0.5) is superior to the other catalyst because it can
overcome mentioned weaknesses. The advantages of the
heterogeneous 1-Ag(0.5) catalyst are a minimum time of
reaction to achieve the desired yield of the product, easy to be
separated from the mixture, easy to be recycled and re-used
without further purification.
Conclusions
To sum up, a novel, efficient and environmentally friendly
heterogeneous silver catalyst was synthesized and successfully
applied for the three-component coupling reaction between
alkyne, aldehyde, and amine. The reaction proceeds at room
temperature within 1 hour using a low 1-Ag(0.5) catalyst
loading. The reaction gives the desired propargylamine
product in high yields without using high temperatures and
long reaction times. It was demonstrated that the 1-Ag(0.5) is
superior to the other reported catalyst due to the mild
reaction conditions, easy separation, and high recyclable
features. The catalyst can be used for several consecutive
cycles without significant loss in the activity and without
collapse of the metal-organic framework structure. The
provided mechanism demonstrates that both
1 and 1-Ag(0.5)
can be applied for the catalysis of the A3-coupling reaction to
synthesize propargylamines from a variety of alkynes, amines,
and aldehydes, which have a great potential to be applied in
biochemistry, in medicine as pharmaceuticals agents and in
chemistry as a basis for the synthesis of variety of nitrogenous
compounds.
According to our obtained results, the three-coupling reaction
can be catalyzed by both unmodified and PSM metal-organic
frameworks (Table 3). Fig 13 demonstrates the plausible
mechanism of the
1
and 1-Ag(0.5) to catalyze the A3-coupling
reaction. The mechanism suggests that reaction can occur by
two different pathways. First, the reaction can undergo the
mechanism that is activated by zinc metal in the
framework.46,71 Here, at first the alkyne is coordinated to Zn(II)
to produce the alkyne complex I. After, the catalyst is inserted
into the C-H bond of the terminal alkyne, the acetylide
complex is deprotonated and gives zinc acetylide intermediate
8 | J. Name., 2012, 00, 1-3
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Table 7 Comparison between efficiency of Silver-MOF catalyst and other catalysts for A3-coupling reaction
Entry
1
Catalyst
Time
24 h
Temp
r.t.
Solvent
Solvent-free
Water or organic
solvents
Recyclability
12 cycles
Yield (%)
97
Ref.
19
PS-supported NHC-Ag(I)
20
2
AgI
14 h
100°C
No data
75
21
22
23
24
25
26
27
3
4
5
6
7
8
9
Pd@MOF1
ZnII/HAP/Fe3O4
MOF−Cu2I2(BTTP4)
IRMOF-3-Gl-Cu
Ag nanoparticles
MCM-TS CuI solid catalyst
Au-nanoparticles
Tetra-nuclear macrocyclic
Cu(II)
12-30 h
4-7 h
2 h
90°C
110°C
r.t.
toluene
4 cycles
7 cycles
4 cycles
4 cycles
3 cycles
7 cycles
4 cycles
91
95
90
91
95
92
92
Solvent-free
CH3CN
6 h
40°C
CHCl3
10-20 h
4-10 h
3-13 h
100°C
80°C
PEG
Toluene
75–80°C
CH3CN
28
10
12 h
80-120°C
Dioxane
4 cycles
93.5
complex
29
30
11
12
13
Cu2(BPDC)2(BPY)
Cu[(2-pymo)2] (1%)
Silver-MOF
3 h
21 h
1 h
80°C
40°C
r.t.
Toluene
Dioxane
DCM
7 cycles
5 cycles
4 cycles
94
99
97
This work
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Fig 13 Proposed mechanism for A3-coupling reaction catalyzed by
1 and 1-Ag(0.5)
Enhancement Program grant (VIU-195/2018). M.K. like to
express deep appreciation for the Chinese Scholarship Council
(CSC) for the financial support for Masters study CSC No
2015GXZ414.
Conflicts of interest
There are no conflicts to declare
Notes and references
Acknowledgements
1
S. Chaemchuen, N. A. Kabir, K. Zhou and F. Verpoort,
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S. Chaemchuen, Z. Luo, K. Zhou, B. Mousavi, S. Phatanasri,
The authors gratefully acknowledge ‘‘State Key Lab of
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for financial support. F.V. and M.S.Y. acknowledge the support
from the Russian Foundation for Basic Research (N° 18-29-
04047) and Tomsk Polytechnic University Competitiveness
2
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DOI: 10.1039/C8CY00662H