Nitrogen enriched polytriazine as a metal-free heterogeneous catalyst for the Knoevenagel reaction under mild conditions

Article abstract of DOI:10.1039/c8nj02174k

A nitrogen enriched (52 wt%) nanoporous polytriazine with a high specific surface area up to 850 m² g^-1 was synthesized by an ultra-fast (30 min reaction time) microwave assisted route using the inexpensive precursors cyanuric chlori

Full text of DOI:10.1039/c8nj02174k

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Nitrogen enriched polytriazine as metal-free heterogeneous
catalyst for the Knoevenagel reaction in mild condition
Received 00th January 20xx,
Accepted 00th January 20xx
Monika Chaudhary and Paritosh Mohanty*
Nitrogen enriched (52 wt %) nanoporous polytriazine with high specific surface area up to 850 m2 g-1 is synthesized by an
ultrafast (30 min reaction time) microwave assisted route using inexpensive precursors cyanuric chloride and melamine at
400 W. The metal-free highly electron rich material is used for the heterogeneous base catalysed Knoevenagel reaction
with high yield of ~98% in a very short reaction time of 30 min at room temperature. The reaction condition was optimized
and several aromatic aldehydes have been reacted with malononitrile with a minimum of 85 % yield under mild
experimental conditions.
DOI: 10.1039/x0xx00000x
www.rsc.org/
late due to the control over the textural properties and
functionality driven superior performance as organocatalyst,
electrode materials for supercapacitors, adsorbents for
1. Introduction
The major developments in chemical industries for carrying
out organic or inorganic transformations lie on the
development of efficient catalysts.1-3 This is one of the
prominent research problems that draw attention from
diverse research fields. Traditionally, metal based molecules
and materials have dominated the research area, however,
very recently there is a growing demand for the development
of efficient metal-free organocatalysts owing to their
environmental and cost benefits.4-8 Initially, the metal-free
organocatalysis was focussed on the use of small organic
molecules under homogeneous conditions.9-13 The catalyst
recovery and recyclability are the two major concerns in
homogenous catalysis that further encouraged to develop
their solid catalyst counterparts to carry out the catalysis in the
heterogeneous conditions. Some of the recently explored
metal-free heterogeneous catalysts are functionalized porous
aromatic frameworks,14,15 triazine based polymers,16 porous
organic frameworks (POFs),17 and conjugated microporous
polymers (CMPs).18,19
capturing gases and removing heavy metals.22,24-34
Among these, triazine based high surface area nanoporous
polymers have received a lot of attention owing to their large
nitrogen content, multiple methods of synthesis and
interesting catalytic and adsorption applications.16,23,33-39 The
versatile synthesis methods such as trimerization, Schiff-base
reaction, substitution reaction (both nucleophilic and
electrophilic), Yamamoto coupling, Sonogashira cross-
coupling,
Friedel-Craft’s
reaction,
condensation
polymerization, oxidative coupling and radical polymerization
have made these materials an important class of polymers
with wide range of applications.16,23,33-39 The basicity due to the
lone pair of electrons in the nitrogen and the excess of active
sites exposed have made these materials efficient for catalysis,
gas
sorption
and
as
electrode
materials
for
supercapacitors.16,23,33-39 The Knoevenagel reaction is
considered as one of the very important base-catalysed
reactions for the formation of C-C bond between the carbonyl
compounds and compounds containing acidic methylene
group. For the natural product synthesis, the Knoevenagel
reaction is considered as a key step.16-18 Moreover, several
molecules of therapeutic and pharmacological importance
have been synthesized using the condensation. Various
homogeneous and metal based catalysts have been explored,
however, the recent environmental concerns encouraged the
researchers to explore the organocatalysis pathway for the
Moreover, functionalization of various high surface area
materials such as inorganic-organic hybrid materials, organic
polymers, and mesoporous silicas and organosilicas with
amine groups have shown superior adsorption and catalytic
applications.20-27 Organic amine functionalized high surface
area nanoporous materials have got tremendous interest of
condensations.16-18,27
Mostly,
electron
rich
amine
functionalized organic molecules and materials have been
investigated.4,18,32-40 In this article, we report the
organocatalytic activity of one of the highly electron rich
polymeric materials, polytriazine, for the Knoevenagel
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reaction. Two inexpensive precursors, melamine (C3N6H6) and confirms the condensation.33,38,41 Observation of a small signal
cyanuric chloride (C3N3Cl3) were condensed to form
framework material as shown in Scheme-S1.
a
at 149.5 ppm may be attributed to the side group
functionalities originated from cyanuric chloride.24,38 Further,
the observation of only broad band (instead of multiple bands
observed for primary amine) above 3350 cm-1 in the FTIR
spectrum [Figure 1b] along with the absence of the band at
850 cm-1 due to the C-Cl stretching vibration further
corroborate the NMR results.33,38 The elemental composition
estimated from the CHNSO analysis reveals the high nitrogen
content of 52 wt% in the specimen (C:36.5, N:52.0, H:2.1,
S:1.3). Nearly spherical inter-grown nanoparticles of average
size of 220 nm could be seen in the FESEM image [Figure 1c].
The TEM image [Figure S1] further reveals the porous nature
of the specimen with the pore size in the range of 2 to 6 nm.
The XRD and SAED patterns (Figure S2 and inset of Figure S1)
confirm the X-ray amorphous nature of the specimen.NENP-1
shows good thermal stability upto 350°C in air atmosphere
[Figure S3].
2. Material and methods
2.1. Synthesis of metal free organocatalyst NENP-1
Equimolar mixture of melamine (99 %, Sigma Aldrich, India)
and cyanuric chloride (99 %, Sigma Aldrich, India) was reacted
in 20 ml DMSO at 140 °C with a microwave power of 400 W
and reaction time of 30 min in a microwave reactor in the
presence of triethylamine (TEA) (Fisher Scientific, India) as a
proton absorber. The synthesized product was filtered and
severely washed with, distilled water, THF and dried at 100 at
°C.
2.2. Characterization of metal free organocatalyst NENP-1
The obtained specimen (NENP-1) was investigated by FTIR
(Perkin Elmer Spectrum Two spectrophotometer) and Cross
polarization magic angle spinning (CP-MAS) 13C NMR spectra
(JEOL Resonance JNM-ECX-400II). XRD pattern was obtained
using Rigaku Ultima IV with CuKα source. The microstructure of
the specimen was studied by FESEM (TESCAN MIRA3) and TEM
(TECNAI G2S-TWIN). The elemental analysis (C/H/N/S) was
conducted using Thermo Flash 2000. TGA was performed on
EXSTAR TG/DTA6300. The N2 sorption was carried out using
Autosorb-iQ2 (Quantachrome Instruments, USA).
168
166.7
100
(b)
(a)
80
60
40
20
0
NENP-1
Melamine
# - Triethylamine
- Side bands
*
150
Cyanuric chloride
Cyanuric chloride
180
170
160
150
140
Chemical Shift (ppm)
#
Melamine
149.5
150
*
#
*
4000 3500 3000 2500 2000 1500 1000
Wavenumber (cm-1
500
250
200
100
50
0
-50
)
Chemical shift (ppm)
2.3. Catalytic activity of metal free organocatalyst NENP-1
Here in, NENP-1 is used as a heterogeneous metal free
organocatalyst for the Knoevenagel reaction of malononitrile
0.15
0.10
0.05
0.00
(d)
800
400
0
with various aromatic aldehydes. In
a model reaction,
0
10
20
30
40
Pore width (nm)
benzaldehyde (1 mmol) and malononitrile (1 mmol) were
reacted in 1:1 volume ratio of dioxane-H2O (total volume of
the reaction mixture is 1 mL) at 25°C in the presence of 5 mg of
NENP-1 and the product was isolated after recrystallization
with ethanol (Scheme-S2). Various experimental parameters
such as catalyst loading, reaction time and solvent ratio were
varied to optimize the product yield. Total of eight aromatic
aldehydes have been reacted with malononitrile to conclude
the versatility of the metal free organocatalyst NENP-1.
Synthesized products was characterized by FT-IR (Perkin Elmer
Spectrum Two spectrophotometer) and Cross polarization
Adsorption
Desorption
Relative0P.4ressure (P/P )
0.0
0.2
0.6
0.8
1.0
0
Figure 1. (a) 13C CPMAS NMR spectrum, (b) FTIR spectra, (c) FESEM image (inset:
HRTEM), (d) N2 sorption isotherm of NENP-1. The 13C NMR spectra of melamine and
cyanuric chloride are given in the inset of Figure 1a and PSD in the inset of Figure 1d.
The N2 sorption isotherm measured at 77 K as shown in
Figure 1d is a type-I isotherm with sharp uptake at low
pressure region. A narrow hysteresis extends from the low to
high pressure range indicates the presence of both micro and
mesopores in the specimen with a hierarchical pore structure.
The PSD (inset of Figure 1d) is multimodal and the estimated
pore sizes centered both in the micropore (1.3 nm) and
mesopore (4.1 and 6.9 nm) regions. This confirms the
hierarchy in the pore system. The specific BET surface area
(SABET) of the specimen was estimated to be 850 m2 g-1 with a
total pore volume of 0.89 cm3 g-1.
1
magic angle spinning (CP-MAS) 13C NMR spectra, and H NMR
(JEOL Resonance JNM-ECX-400II). Purity of the products was
investigated by GC (Perkin Elmer GC Clarus 680) and MS
(Perkin Elmer MS Clarus SQ8T).
3. Results and discussion
The nitrogen enriched nanoporous polytriazine (NENP-1)
framework has been synthesized by the condensation of
cyanuric chloride and melamine using a microwave assisted
route at 140 °C with a microwave power of 400 W and short
reaction time of 30 min. The obtained specimen has been
investigated by the 13C CPMAS NMR and FTIR spectroscopy. A
strong signal at 166.7 ppm in the 13C CPMAS NMR spectrum in
Figure 1a originated from the carbon of the triazine ring
Recently, various high surface area microporous materials
have been explored as catalysts for different organic
transformations.4,16-19,27,28,32-40 The major advantages of these
high surface area heterogeneous catalysts are; (i) metal free
catalysts hence, no chance of leaching out of metals, and (ii)
ease of separation from the product compared to the
homogenous catalysts (both metal based catalysts and
organocatalysts).The above advantages made these
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heterogeneous nanoporous polymeric catalysts as safe, green, wt% in 1 ml of the dioxane-H2O mixture (1:1) is ideal for the
environmental friendly and cost effective alternatives to catalytic conversion of 1 mmol each of the reactants (Figure
produce high quality molecules of significant importance.42-44 S7). A 98% yield could be achieved in a short reaction time of
Further, the presence of nitrogen as electron rich entity in 30 min at 25 °C. To the best of our knowledge, the catalytic
some of these materials have shown improved catalytic efficiency is the best among the reported literatures (Table-
activities.16-19
S2). The reaction time was further increased to 60 min to
The material synthesized in this research has high nitrogen check if the yield could be increased. However, there is almost
content and superior textural properties. Moreover, the no change in the yield. Moreover, the product yield of 45 and
presence of both micropores and mesopores in the specimen 60% could be achieved within short reaction time of 2 and 5
forming a hierarchical pore system could be an additional min, respectively. Thus, 30 min is considered as the ideal
advantage. These encouraged us to investigate the reaction time to have an optimum yield. No product formation
Knoevenagel reaction. The Knoevenagel reaction is a base could be observed in the absence of the catalyst, under the
catalysed reaction widely accepted as one of the most identical experimental condition. The progress of the reaction
powerful method for carbon-carbon bond formation that was continuously monitored by TLC at regular interval. After
produces fine chemicals of pharmaceutical importance.
the completion of the reaction in 30 min the catalyst was
separated by filtration and the product was recrystallized using
ethanol.
1
Spectroscopic investigations such as H and 13C NMR, and
FT-IR confirm the successful conversion of benzaldehyde into
α-β unsaturated product as proposed in scheme-1. Absence of
the signals due to the carbonyl carbon of the benzaldehyde
1
and CH2 group of malononitrile in the H and 13C NMR spectra
[Figure S5a & S5b] indicate the complete condensation of the
reactants to form α-β unsaturated product.17,18 Moreover, the
signal at 7.7(s) ppm in the 1H NMR attributed to the benzylic
hydrogen further confirm the product formation.17,18 The FT-IR
spectra further corroborate the results obtained from the
NMR investigation as the carbonyl band at 1700 cm-1 was
absent and a new band at 2270 cm-1 due to the CN stretching
vibration was observed (Figure S4). Further, the product purity
was investigated by GC-MS. The GC chromatograph reveals a
single peak at retention time of 15.03 min and the molecular
ion peak at m/z of 154 in mass spectrum are in good
agreement with molecular weight of product [Figure S6a].
Systematic fragmentation pattern was observed in mass
spectral analysis (Figure S6b). All of the spectra data fairly
match with the recent reports.17,40 Thus, the metal-free
catalyst used in this investigation has shown good
performance for the carbon-carbon bond formation using the
Knoevenagel reaction. In order to further investigate if the
catalyst could be generalized, various aromatic aldehydes were
condensed with malononitrile (Table-1) and the products of all
Scheme 1. Proposed reaction mechanism for Knoevenagel reaction catalyzed over the
metal-free heterogeneous organocatalyst NENP-1.
In this investigation, the NENP-1 was employed as a
heterogeneous catalyst for the Knoevenagel reaction of
malononitrile with various aromatic aldehydes. A model
reaction of benzaldehyde (1 mmol) with malononitrile (1
mmol) was performed. Different experimental parameters
such as reaction time, solvents and catalyst amount was varied
to find out the optimum yield (Table-S1). The reactions were
carried out using tetrahydrofuran (THF), methanol, dioxane
and H2O as co-solvents. It has been observed that a high yield
could be achieved in the mixtures of the solvents such as THF-
H2O, methanol-H2O and dioxane-H2O at the 1:1 volume ratio.
This could be mainly attributed to the better solubility of the
reactant molecules in these mixtures of solvents.18,23
Moreover, the adsorption of water molecule on the N
containing active sites further enhance the basicity of the
system. Among these, a maximum yield was obtained in the
dioxane-H2O mixture and hence, used for further investigation.
It was documented that a solvent mixture tunes the solubility
of the reactants as well control the polarity, which in turn
improve the reactivity and product yield by making available of
the active sites on removing the intermediates if any and the
products formed.18,23,43
1
these reactions were characterized by H NMR, 13C NMR and
GC-MS (Figure S8-S19). The superior efficiency of the catalyst
could be observed as the minimum 80% yield were realized in
a short time span of 30 min for whatever aldehyde may be
condensed.
Effect of catalyst loading is investigated on the yield of
product. It has been observed that a catalyst loading of 4.5
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high yield (up to 98 %) along with very minor loss on
recyclability up to eight cycles, wide substrate adaptability and
low cost, make this catalyst as a potential candidate for
practical applications.
100
80
60
40
20
0
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was financially supported by DST, Govt. of India
with Grant No. DST/TDT/TDP-03/ 2017(G).
1
2
3
4
Cycle number
5
6
7
8
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DOI: 10.1039/C8NJ02174K
bꢀꢁꢂꢃꢄꢅꢆ ꢅꢆꢂꢀꢇꢈꢅꢉ ꢊꢃꢋꢌꢁꢂꢀꢍꢎꢀꢆꢅ ꢍꢏ ꢐꢅꢁꢍꢋꢑꢒꢂꢅꢅ ꢈꢅꢁꢅꢂꢃꢄꢅꢆꢅꢃꢓꢏ ꢇꢍꢁꢍꢋꢌꢏꢁ ꢒꢃꢂ
ꢁꢈꢅ Yꢆꢃꢅꢔꢅꢆꢍꢄꢅꢋ ꢂꢅꢍꢇꢁꢀꢃꢆ ꢀꢆ ꢐꢀꢋꢉ ꢇꢃꢆꢉꢀꢁꢀꢃꢆ
aꢀꢁꢂꢃꢄ / ꢄꢅꢆ ꢄꢇꢈ ꢄꢁꢆ tꢄꢇꢂꢉꢀꢊ aꢀ ꢄꢁꢉꢈꢋ
Cꢅꢁꢌꢉꢂꢀꢁꢄꢍ aꢄꢉꢎꢇꢂꢄꢍꢊ [ꢄꢏꢀꢇꢄꢉꢀꢇꢈꢐ 5ꢎꢑꢄꢇꢉꢒꢎꢁꢉ ꢀꢓ / ꢎꢒꢂꢊꢉꢇꢈꢐ LLÇ wꢀꢀꢇꢃꢎꢎꢐ Üꢉꢉꢄꢇꢄꢃ ꢄꢁꢆ!
"#$%%$ꢐ Lꢁꢆꢂꢄ
9!ꢒꢄꢂꢍ' ꢑꢒꢓꢌꢈ(ꢂꢂꢉꢇ)ꢄꢌ)ꢂꢁꢐ ꢑꢄꢇꢂꢉꢀꢊ $*(+ꢒꢄꢂꢍ)ꢌꢀꢒ
Graphical abstract:
Üꢍꢉꢇꢄ!ꢓꢄꢊꢉ Yꢁꢀꢎ-ꢎꢁꢄ+ꢎꢍ ꢑꢇꢀꢆꢅꢌꢉꢊ .ꢂꢉ
ꢂ+ !ꢈꢂꢎꢍꢆ ꢄꢉ ꢄꢒꢏꢂꢎꢁꢉ ꢌꢀꢁꢆꢂꢉꢂꢀꢁ ꢂꢊ ꢇꢎꢑꢀꢇꢉꢎꢆ ꢅꢊꢂꢁ+
ꢁꢂꢉꢇꢀ+ꢎꢁ!ꢎꢁꢇꢂꢌ ꢎꢆ ꢁꢄꢁꢀꢑꢀꢇꢀꢅꢊ ꢑꢀꢍꢈꢉꢇꢂꢄ/ꢂꢁꢎ ꢄꢊ ꢒꢎꢉꢄꢍ!ꢓꢇꢎꢎ ꢎꢉꢎꢇꢀ+ꢎꢁꢎꢀꢅꢊ ꢀꢇ+ꢄꢁꢀꢌꢄꢉꢄꢍꢈꢊꢉ)