Ag-grafted covalent imine network material for one-pot three-component coupling and hydration of nitriles to amides in aqueous medium

Article abstract of DOI:10.1039/c4ra07622b

A nitrogen rich porous covalent imine network material (CIN-1) has been successfully employed for grafting silver nanoparticles (Ag NPs). The Ag NPs grafted CIN-1, Ag-CIN-1 has been characterized by elemental analysis, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectroscopy (DRS), thermogravimetric analysis (TGA) and EPR spectroscopic studies. Ag-CIN-1 acts as a truly heterogeneous catalyst in the hydration of nitriles to amides and the A³ coupling reactions between the alkyne, amine and aldehyde to produce propargylamines by using water as a green solvent. This journal is

Full text of DOI:10.1039/c4ra07622b

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Ag-grafted covalent imine network material for
one-pot three-component coupling and hydration
of nitriles to amides in aqueous medium†
Cite this: RSC Adv., 2014, 4, 47593
Noor Salam,a Sudipta K. Kundu,b Rostam Ali Molla,a Paramita Mondal,a
b
a
*
*
Asim Bhaumik and Sk. Manirul Islam
A nitrogen rich porous covalent imine network material (CIN-1) has been successfully employed for grafting
silver nanoparticles (Ag NPs). The Ag NPs grafted CIN-1, Ag-CIN-1 has been characterized by elemental
analysis, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform
infrared spectroscopy (FT-IR), UV-vis diffuse reflectance spectroscopy (DRS), thermogravimetric analysis
(TGA) and EPR spectroscopic studies. Ag-CIN-1 acts as a truly heterogeneous catalyst in the hydration of
nitriles to amides and the A3 coupling reactions between the alkyne, amine and aldehyde to produce
propargylamines by using water as a green solvent.
Received 25th July 2014
Accepted 17th September 2014
DOI: 10.1039/c4ra07622b
www.rsc.org/advances
polymeric porous materials bearing active legend moieties
within the framework are very demanding.13
Introduction
Due to strict environmental regulations environmentally
benign, efficient, economical and green synthesis routes for the
synthesis of organic ne chemicals have become more
demanding today. Over the past decade, transition metal cata-
lyzed multi-component reactions are a powerful synthetic tools to
access complex structures from simple precursors via one-pot
synthetic pathways. In this context, the three-component coupling
of aldehydes, alkynes, and amines (A3 coupling) is one of the best
examples of such a process and this has received much attention
in recent times.14 During the last few decades, there are several
reports on highly efficient A3-coupling in organic media, water,
ionic liquid, or under solvent-free conditions catalyzed by copper,
silver, gold, iron, nickel or iridium based catalysts via catalytic
C–H activation to afford various propargylamines.15 The resultant
propargylamines are versatile intermediates for organic
synthesis16 and important structural elements of natural prod-
ucts, making agrochemicals and potential drug molecules.17
In this context it is also pertinent to mention that hydration
of nitriles into the corresponding amides is another important
reaction in organic synthesis. This is because of the facts that
the amides are versatile synthetic intermediates used in the
production of pharmacological products, polymers, detergents,
lubricants and drug stabilizers.18 Conventional catalytic systems
employed for this reaction require hazardous organic solvents
in the presence of homogeneous strong acid and base cata-
lysts.19–21 However, major drawbacks of these classical methods
are: (i) requirement of harsh reaction conditions, which could
be harmful for the sensitive functional groups, and (ii) difficult
to stop the reactions at the amide stage and further hydrolysis to
the carboxylic acid oen takes place, especially in basic media.
In contrast to these catalytic processes performed in organic
High surface area porous organic polymers (POPs), which can
be designed through a wide range of chemical reactions
between the reactive functional organic moieties of the
precursor organic monomers, have attracted increasing interest
over the years.1–4 Synthesis of these microporous organic poly-
mers with a rational design at the molecular level using cheap
and simple processes is highly desirable. These porous poly-
mers are primarily employed as gas storage,5,6 and heteroge-
neous catalysis materials.7,8 A large number of POPs has been
invented till date like, polymers of intrinsic micro porosity9 and
conjugated microporous polymers10 for efficient and robust
heterogeneous catalysts because of their synthetic diversity,
presence of a wide range of donor sites in the framework and
high mechanical stability together with high surface areas.11
Especially, incorporation of an active functionality bearing
nitrogen-containing groups in the polymeric network is very
demanding as it could help to bind an active metal sites at the
surface of the material, and thus to enhance the catalytic
performance as well as the stability of the supported materials.12
Nitrogen-rich covalent organic frameworks can act as a catalytic
support to immobilize an active metal and thus immobilized
metal sites in heterogeneous molecular catalysts based on
aDepartment of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, West
Bengal, India. E-mail: manir65@rediffmail.com; Fax: +91-33-2582-8282; Tel: +91-
33-2582-8750
bDepartment of Materials Science, Indian Association for the Cultivation of Science,
Kolkata-700032, India. E-mail: msab@iacs.res.in; Fax: +91-33-2473-2805; Tel: +91-
33-2473-4971
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07622b
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Scheme 1 Schematic diagram of the Ag nanoparticles grafted porous covalent imine network (Ag-CIN-1).
solvents, and despite the growing interest to develop environ- was obtained from Spectrochem, India and used as received. All
mentally benign and safety processes, metal catalysts able to other chemicals used for this investigation purposes were of
promote such a transformation in pure aqueous media are much analytical grade produced by Merck, India unless mentioned
scarcer because of its low cost, availability, and nontoxic nature.22,23 otherwise.
Melamine and 1,4-piparazinedicarbaldehyde have been
chosen for this Schiff-base condensation reaction, which could
lead to the incorporation of piparazine units on edges and
Preparation of the catalyst
Synthesis of covalent imine network materials (CIN-1). The
covalent imine network materials (CIN-1) was prepared
according to the literature method.24 A 50 ml round-bottom
ask tted with a condenser and a magnetic bar was charged
with piparazine dicarbaldehyde (1.1 mmol) and melamine (0.79
mmol) and dimethyl sulfoxide (10 ml). The ask was degassed
by dry nitrogen and reuxed at 180 ꢀC for 72 h in the inert
atmosphere. Aer cooling to room temperature the precipitated
off-white product was isolated by ltration over a Buchner
funnel and washed with dry ethanol and excess dry acetone,
THF and dichloromethane sequentially to remove dimethyl
sulfoxide completely from the porous framework. Finally, the
product was dried in vacuum to obtain the desired solvent-free
powder.
melamine at vertices (Scheme 1) and thus this synthesis
approach results in an uniform arrangement of both nitrogen-
rich components in the network. Nitrogen-rich porous organic
polymeric framework could offer a large number of metal
binding sites and improved thermal stability as the porous
covalent imine network (CIN-1) material synthesized through the
Schiff-base condensation reaction of a rigid triamine and rela-
tively exible dialdehyde has been found to bind active Pd sites
strongly at its surface.24 Inspired by the above observations
herein, we have graed Ag-nanoparticles at the surface of the
microporous network of CIN-1 and subsequently explored its
high catalytic performance in the hydration of nitriles to amide
and A3 coupling reaction by using water as the reaction medium.
Synthesis of colloidal Ag nanoparticles. In
a typical
synthesis, 0.1 ml of an aqueous solution of 1% AgNO3 was
added to 10 ml of water containing 0.5 mg TRIS and was stirred
for 2 min. Then, 0.25 ml of an aqueous solution of NaBH4
Experimental section
Materials and reagents
Melamine and 1,4-piparazine dicarbaldehyde were purchased (0.08%) was added drop-wise under stirring. The stirring was
from Sigma-Aldrich. Silver nitrate (AgNO3) was also purchased continued for another 10 min, and the resulting nanocolloid
ꢀ
from Universal Chemicals, India. Sodium borohydride (NaBH4) was stored at 4 C.
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Synthesis of Ag-CIN-1 nanocatalyst in situ. A total of 50 mg of
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ionization detector and identied by Trace DSQ II GC-MS
equipped with a 60m TR-50 MS capillary column.
CIN-1 was dispersed in 10 ml of TRIS-stabilized Ag-NPs and
stirred for 1 h at room temperature. The colour of the colloidal
nanoparticles gradually disappeared while stirring. The super-
natant solution was colourless aer 1 h of stirring at room
temperature, whereas the colour of CIN-1 changed to black,
indicating the loading of Ag-NPs onto the surface of CIN-1. Aer
centrifugation, black coloured Ag-NP containing microporous
polymer Ag-CIN-1 was obtained. This microporous polymer
material was washed further with copious amounts of water and
dried at room temperature. The loading of Ag-NPs onto CIN-1
was further conrmed by spectral measurements.
Results and discussion
The imine-functionalized nitrogen-rich Ag-CIN-1 has been
synthesized by Schiff base condensation reaction of rigid tri-
amine and relatively exible dialdehyde followed by graing
with Ag NPs at its surface through impregnation (Scheme 1). Ag-
CIN-1 material was thoroughly characterized by powder XRD,
electron microscopy, elemental microanalysis, EPR, thermal
analysis together with UV-vis spectroscopic studies.
One-pot synthesis of propargylamines catalyzed by Ag-CIN-1
nanocatalyst. In a 50 ml RB ask, benzaldehyde (1.0 mmol), X-ray diffraction
phenyl acetylene (1.5 mmol), piperidine (1.2 mmol), Ag-CIN-1
Fig. 1A and B shows wide angle powder XRD pattern of CIN-1 and
catalyst (0.05 g) and water (5 ml) was added. The mixture was
stirred at 40 ꢀC for 12 h. The reaction mixtures were collected at
different time intervals and identied by GC-MS and quantied
by GC. Aer the completion of the reaction, the catalyst was
ltered off and washed with water followed by acetone and
dried in oven. The ltrate was extracted three times with ethyl
acetate (3 ꢁ 20 ml) and the combined organic layers were dried
with anhydrous Na2SO4 by vacuum. The ltrate was concen-
trated and the resulting residue was puried by column chro-
matography on silica gel to provide the desired product.
Hydration of nitriles using Ag-CIN-1 nanocatalyst. In an oven
dried 50 ml RB ask Ag-CIN-1 catalyst (0.05 g), water (5.0 ml),
Ag-CIN-1 material respectively. Fig. 1B shows three characteristic
Bragg diffraction peaks for the silver nanoparticles with 2q value
of 38.02, 44.12, 64.29 degrees, which correspond to the face-
centered cubic (fcc) phase of the silver (111), (200), (220) respec-
tively.25 This result suggests that in Ag-CIN-1 the Ag nanoparticles
are embedded in the covalent imine network of CIN-1.
Electron paramagnetic resonance study
A typical EPR spectrum of Ag-CIN-1 recorded at 298 K in solid
state is shown in Fig. 2. It shows axially symmetric signal. The g
value for the corresponding signal is 1.99, which could be
ꢀ
and the corresponding nitrile (1 mmol) were stirred at 100 C
for 3 h. The reaction mixtures were collected at different time
interval and identied by GC-MS and quantied by GC analysis.
Aer the completion of the reaction, the catalyst was ltered off
and washed with water followed by acetone and dried in oven.
The ltrate was extracted three times with ethyl acetate (3 ꢁ 20
ml) and the combined organic layers were dried with anhydrous
Na2SO4 under high vacuum. The ltrate was concentrated and
the resulting residue was puried by column chromatography
on silica gel to obtain the desired product.
Characterization techniques
Powder X-ray diffraction (XRD) patterns of different samples
were analyzed with a Bruker D8 Advance X-ray diffractometer
using Ni-ltered Cu Ka (l ¼ 0.15406 nm) radiation. Trans-
mission electron microscopy (TEM) images of the mesoporous
polymer were obtained using a JEOL JEM 2010 transmission
electron microscope operating at 200 kV. X-ray photoelectron
spectroscopy (XPS) was performed on an Omicron nanotech
operated at 15 kV and 20 mA with a monochromatic Al Ka X-ray
source. Thermogravimetric analysis (TGA) was carried out using
a Mettler Toledo TGA/DTA 851e. UV-Vis spectra were taken
using a Shimadzu UV-2401PC doubled beam spectrophotom-
eter having an integrating sphere attachment for solid samples.
The EPR (electron paramagnetic resonance) spectra of the silver
catalyst were recorded for the solid sample at room temperature
by a JES-FA200 ESR spectrometer (JEOL). The reaction products
were quantied (GC data) by Varian 3400 gas chromatograph
Fig. 1 Wide angle powder XRD pattern of CIN-1 (A) and Ag-CIN-1
equipped with a 30m CP-SIL8CB capillary column and a ame material (B).
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attributed to the presence of Ag-nanoparticles26 in zero oxida- and 3d3/2 are at 367.26 eV and 373.22 eV, respectively. These
tion state, embedded into covalent imine network.24
binding energy values correspond to Ag (0) nanoparticles
anchored with nitrogen sites in the polymer network since the
reported binding energy of Ag0 (3d5/2) is 368.3 eV.27 Little shi in
binding energy suggests stronger binding of Ag0 sites with the
N-sites of the covalent imine framework.
Electron microscopic analysis
In Fig. 3, HR-TEM images of Ag-CIN-1 material at different
magnications are shown. It shows the presence of spherical
spots of dimension ca. 15–40 nm throughout the specimen,
which could be assigned to Ag-nanoparticles in Ag-CIN-1. The
FFT diffractogram of a selected area of the grid is shown in the
inset of the Fig. 3C. Diffraction spots in this FFT pattern sug-
gested crystalline feature of the Ag-nanoparticles bound into the
covalent imine network.
Thermal analysis
The quantitative determination of the organic content and the
framework stability of the Ag-CIN-1 samples are obtained from
the thermogravimetric (TG) and differential thermal analysis
(DTA) under N2 ow. TGA-DTA curve of Ag-CIN-1 materials are
shown in Fig. S1.† The TGA of this material showed the rst
weight loss below 100 ꢀC due to desorption of physisorbed
water. This was followed by a gradual decrease in the weight
aer 250 ꢀC. Thus this thermal analysis data suggested that Ag-
CIN-1 microporous material is stable ca. 250 ꢀC.
XPS analysis
In Fig. 4 X-ray photoelectron spectroscopic prole of Ag-CIN-1
material is shown. The observed Ag binding energies for 3d5/2
UV-vis spectroscopy study
In Fig. 5, UV-vis spectrum of the as-prepared Ag-CIN-1 is shown.
The silver nanoparticle thus formed showed an absorption peak
at 370 nm corresponding to the surface plasmon resonance
(SPR) of Ag NPs, suggesting the presence of Ag NPs on the CIN-1
surface.28
Catalytic activity of Ag-CIN-1 for ‘one-pot’ A3 coupling
reactions
The catalytic activity of Ag-CIN-1 for the A3-coupling reaction
was investigated using benzaldehyde, phenylacetylene and
piperidine as a model reaction using water as reaction medium
ꢀ
at 40 C (Scheme 2).
Fig. 2 EPR spectrum of Ag-CIN-1.
It has been observed that solvents play crucial role on the
reactivity of the A3 coupling reactions.29 Our interest was to
replace organic solvents by water in this reaction because it
offers green chemical route. Although in toluene, DCM, THF
and DMF, the catalyst system afforded the corresponding
coupling product in moderate yields (Table 1, entries 2–5),
MeOH, acetone and chloroform were proven to be poor solvents
for the same process (entries 6–8). The reaction did not occur
Fig. 3 HR-TEM images of the Ag-CIN-1 material.
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Fig. 4 XPS spectrum of Ag-CIN-1.
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Table 1 The effects of various variables in the three-component
coupling reaction (A3 coupling) catalyzed by the Ag-CIN-1a
Entry
Solvent
Temperature (ꢀC)
Yieldb (%)
1
H2O
40
98
2
DMF
40
45
3
THF
40
62
4
DCM
40
70
5
6
7
8
Toluene
MeOH
Aectone
Chloroform
H2O
40
40
40
40
40
40
76
10
32
36
Trace
Trace
Trace
9c
10d
11
H2O
H2O
RT (25 ꢀC)
a
Reaction conditions: benzaldehyde (1.0 mmol), phenyl acetylene (1.5
mmol), piperidine (1.2 mmol), Ag-CIN-1 catalyst (0.05 g) and solvent
Fig. 5 UV-Visible absorbance of Ag-CIN-1 material.
b
c
(5 ml). Yield determined by GC and GCMS analysis. Metal free
CIN-1 ligand. d Without catalyst.
under catalyst-free conditions and also in the presence of metal-
free CIN-1 material (entry 9 and 10) or when the reaction was
carried out under room temperature (25 ꢀC) (entry 11). From the
above discussions, it can be seen that the best yield of the A3
coupling reaction was observed when water was used as solvent
para-positions could be effectively reacted under the present
reaction conditions (Table 2, entries 12–13). Note that the
reactions proceeded smoothly to give the corresponding prop-
argylamines in a good yield. Good yields were observed when
cyclic dialkylamines such as pyrrolidine and morpholine and
also aromatic amine were used (entries 14 and 15), whereas the
diethylamine afforded a lower conversion (entry17).
ꢀ
at 40 C.
To expand the scope of this A3-coupling, various aldehydes,
alkynes and amines were used as substrates under the opti-
mized reaction conditions. The results are summarized in Table
2. From the Table 2 it is clear that the conditions were equally
applicable to the coupling of a variety of aromatic and aliphatic
aldehydes including heterocyclic aldehydes with various amines
utilizing phenylacetylene, giving the corresponding propargyl-
The catalytic hydration of nitrile using Ag-CIN-1 nanocatalyst
The hydration of nitriles is one of the most imperative tech-
nologies for the large-scale synthesis of amides, which are a very
signicant group of compounds in the chemical and pharma-
ceutical industry. The catalytic activity of microporous Ag-CIN-1
was tested for the hydration of benzonitrile under aqueous
conditions without organic solvents. Here, we have explored the
catalytic efficiency of the Ag-CIN-1 nanocatalyst in hydration of
ꢀ
amines in high yields at 40 C.
Both aromatic and aliphatic aldehydes, including those
bearing functional groups such as alkoxy, chloro and bromo
additions, were able to undergo the corresponding three-
component-coupling. Irrespective of the electronic nature of the
substituent, aromatic aldehydes reacted smoothly to give the
corresponding products in good yields (entries 2–6). On the
other hand, aliphatic aldehydes (entries 7–9) reacted rapidly
and gave excellent yields without any trimerization. Aryl alde-
hyde possessing an electron-withdrawing group afforded
slightly lower yield than the electron-donating group. It is worth
noting that 4-urobenzaldehyde gave lower yield of the coupled
product despite long reaction times (entry 16). In addition,
heteroaromatic aldehydes displayed high reactivity and gave
good yields of products (entries 10–11). We also found that a
variety of terminal aromatic alkynes with substituted groups in
ꢀ
nitriles in green solvent water at 100 C (Scheme 3).
For hydration of nitriles over Ag-CIN-1, the reaction param-
eters are optimized using benzonitrile as a model substrate and
the results are summarized in Table 3. Without any catalyst and
catalyst alone without any solvent (Table 3, entries 10 and 11) no
reaction occurred at 100 ꢀC. Among the different solvents
employed to optimize the reaction conditions, only water has
shown highest catalytic activity (Table 3).
Under the optimized reaction conditions, the protocol is also
extended to various nitriles. The scope of the present Ag-CIN-1
nanocatalyst system with regard to various kinds of nitriles was
Scheme 2 Three-component synthesis of propargylamines catalyzed by Ag-CIN-1 nanocatalyst.
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Table 2 The three-component (A3) coupling reaction catalyzed by the Ag-CIN-1 nanocatalysta
Entry
1
Aldehyde
Amine
Alkyne
Yieldb (%)
98
2
3
4
5
6
7
8
87
91
81
74
79
74
78
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Table 2 (Contd.)
Entry
Aldehyde
Amine
Alkyne
Yieldb (%)
9
81
10
11
12
13
81
85
91
86
14
65
15
16
73
65
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Table 2 (Contd.)
Entry
Aldehyde
Amine
Alkyne
Yieldb (%)
17
31
a
Reaction conditions: aldehyde (1.0 mmol), alkyne (1.5 mmol), Amine (1.2 mmol), Ag-CIN-1 catalyst (0.05 g) and water (5 ml). b Yield determined by
GC and GCMS analysis. Products were identied by comparison of their 1H-NMR spectral data those reported in the literature.
yield the nal product (entries 2–6). Usually, the hydration of
heteroaromatic nitriles is more difficult and the reaction rates
are much lower than those of common nitriles because of their
strong coordination to the metal centers. In the present study,
hydration of 3-cyanopyridine (78%) and also furan-2-carbon-
itrile (83%) afforded the corresponding amides in near quan-
titative yields (entries 7 and 8). The less reactive aliphatic
nitriles could also be hydrated to the corresponding aliphatic
amides (entries 9 and 10).
Scheme 3 Ag-CIN-1-catalyzed hydration of benzonitrile to benza-
mide in water.
Table 3 Optimization of reaction conditions in synthesis of benza-
Recycling efficiency of the catalyst
mide from benzonitrile catalyzed by Ag-CIN-1 nanocatalysta
For a heterogeneous catalyst, it is important to examine its ease
of separation, recoverability and reusability. The reusability of
Ag-CIN-1 nanocatalyst was investigated in ‘one-pot’ A3-coupling
reactions and hydration of various nitriles under the optimized
reaction conditions. Aer each run, ethyl acetate was added to
dilute the reaction mixture and the organic layer was dried with
anhydrous Na2SO4 by vacuum. The ltrate was concentrated
and the resulting residue was puried. Aer a simple wash
using ethyl acetate and dried, the catalyst was reused for the
next run and almost consistent activity was observed for next
ve consecutive cycles. As seen from Fig. 6, the recycled catalyst
did not show any appreciable change in the activity which
indicates that the catalyst is stable and can be regenerated for
repeated use.
Entry
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Toluene
5
5
5
5
3
3
3
3
3
3
3
—
Trace
—
Trace
20
24
72
51
Acetonitrile
Dichloromethane
THF
Ethanol
Methanol
Methanol/water (1 : 2)
Methanol/water (1 : 1)
Water
Water
—
9
96
10c
11
No reaction
No reaction
a
Reaction condition: Ag-CIN-1 catalyst (0.05 g), solvent (5.0 ml),
b
benzonitrile (1 mmol). Yield determined by GC and GCMS analysis.
c
Without catalyst.
Characterization of reused catalyst
The reused catalyst have been characterized by employing wide
angle powder XRD, TEM and EPR studies to clarify if any change
occur in the catalyst aer catalysis. In the wide angle powder
XRD pattern of the reused catalyst (Fig. S2†) three characteristic
Bragg diffraction peaks for the Ag nanoparticles are observed,
which suggest that the face-centered cubic (fcc) phase of the
silver are well preserved during the course of the catalytic
reaction. The TEM images of the reused Ag-CIN-1 aer the
examined. The results are summarized in Table 4. A large
numbers of aromatic nitriles (entries 1–8) as well as an aliphatic
nitrile (entries 9 and 10) are smoothly hydrated to the corre-
sponding amide in excellent yield. Aromatic nitriles carrying
either electron-donating such as p-methyl and p-methoxy or
electron-withdrawing such as p-bromo, chloro and p-nitro
groups exhibit comparable reactivity and react efficiently to
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Table 4 Hydration of various nitriles catalyzed by Ag-CIN-1 nanocatalysta
Entry
Nitrile
Amide
Yieldb (%)
1
96
2
3
82
86
4
5
78
74
6
7
8
82
78
83
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Table 4 (Contd.)
Entry
Nitrile
Amide
Yieldb (%)
9
63
10
73
a
Reaction condition: Ag-CIN-1catalyst (0.05 g), water (5.0 ml), nitrile (1 mmol) temperature (100 ꢀC). b Yield determined by GC and GCMS analysis.
Heterogeneity test
To examine whether silver was being leached out from the solid
support to the solution, experiment has been carried out in the
one-pot three-component coupling reaction with our supported
Ag-CIN nanocatalyst. A typical ltration test was performed in
the one-pot three-component coupling reaction of benzalde-
hyde, phenyl acetylene and piperidine (A3-coupling) to investi-
gate whether the reaction proceeded in a heterogeneous or a
homogeneous fashion. For the rigorous proof of heterogeneity,
a test was carried out by ltering catalyst from the reaction
mixture aer 4 h and the ltrate was allowed to react up to the
completion of the reaction (6 h). In this case no change in
conversion was observed, which suggests that the catalyst is
heterogeneous in nature. No evidence for leaching of silver or
Fig. 6 Recycle efficiency of Ag-CIN-1 nanocatalyst for A3-coupling
reaction and hydration of nitrile.
decomposition of the complex catalyst was observed during the
catalytic reaction. It was noticed that aer ltration of the
catalyst from the reactor at the reaction temperature, coupling
reactions do not proceed further. Atomic absorption spectro-
metric analysis of the supernatant solution of the reaction
mixture thus collected through ltration also conrmed the
absence of silver ions in the liquid phase. Thus, results of the
hot ltration test suggested that silver was not being leached
out from the solid porous polymer catalyst during the cross-
coupling reactions.
fourth catalytic cycle (Fig. S3†) show that Ag nanoparticles are
well embedded in CIN-1 polymer aer the reaction. We have
taken the EPR spectra of the reused catalyst aer the fourth
catalytic reaction cycle at 77 K temperature in solid state
(Fig. S4†). It also shows axially symmetric signal with g value
1.97. The above results reveal that Ag-CIN-1 catalyst is very
stable during the catalysis.
Table 5 Comparison of catalytic activity of the present catalyst in synthesis of propargylamines and hydration of nitriles with other reported
systems
Entry
Catalyst
Reaction conditions
Yield (%)
Ref.
1
2
3
4
5
6
7
8
Nanoferrite–[Ru(OH)]x
Ni NPs/HT
AgHAP
Ag-CIN-1 nanocatalyst
NHC silver complexes
Polymer-supported NHC–Ag(I)
Au@PMO-IL
Water, 130 ꢀC, 30 min
Water, 120 ꢀC, 10 h
Water, 140 ꢀC, 3 h
Water, 100 ꢀC, 3 h
Dioxane, 100 ꢀC, 12 h
CH2Cl2, RT, 24 h
85
85
94
96
81
92
87
98
29
30
31
This study
32
32
34
CHCl3, 60 ꢀC, 11 h
Water, 40 ꢀC, 12 h
Ag-CIN nanocatalyst
This study
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3 R. Dawson, A. I. Cooper and D. J. Adams, Prog. Polym. Sci.,
2012, 37, 530–563.
Comparison with other reported systems
Synthesis of propargylamines and hydration of nitriles over a
variety of heterogeneous catalysts have been summarized in
Table 5. It provides a comparative catalytic activity of Ag-CIN-1
over related catalytic systems reported in the literature.30–35
From Table 5, it is seen that present catalyst exhibited higher
yields compared to the other reported system.
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Conclusions
From the above results we can conclude that a facile synthesis of
silver graed imine-functionalized nitrogen-rich porous
organic network (Ag-CIN-1) can be developed from the
precursor organic monomer building blocks through an easy
chemical route and the resulting material showed excellent
catalytic activity towards ‘one-pot’ A3-coupling reactions and
hydration of various nitriles in green solvent. The high amount
of nitrogen in the CIN-1 support makes the silver nanoparticles
resistant to aggregation and also helps to preserve their catalytic
activity and stability during recycling. Thus the catalytic process
is green and environmental friendliness and it offers a number
of advantages, such as excellent stability, easy separation from
the reaction mixture by ltration, reusability for several times
with oen minimal loss of activity. Thus, these key ndings
reported herein based recyclable heterogeneous catalysis would
open-up advanced, economical and environmentally sustain-
able routes for the synthesis of propargylamines and hydration
of various nitriles.
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Acknowledgements
SMI acknowledges Department of Science and Technology
(DST) and Council of Scientic and Industrial Research (CSIR),
New Delhi, India for funding. NS is thankful to the UGC, New
Delhi and SKK are thankful to the CSIR, New Delhi, India for his
research fellowship. RAM acknowledges UGC, New Delhi, India
for his Maulana Azad National Fellowship. PM acknowledges
UGC, New Delhi, India for her D.S. Kothari Fellowship. AB
thanks DST for providing instrumental facilities through DST
Unit on Nanoscience and DST-SERB project grants. We
acknowledge Department of Science and Technology (DST) and
University Grant Commission (UGC) New Delhi, India for
providing support to the Department of Chemistry, University
of Kalyani under PURSE, FIST and SAP programme.
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