Design, synthesis, characterization, and catalytic properties of g-C3N4-SO3H as an efficient nanosheet ionic liquid for one-pot synthesis of pyrazolo[3,4-b]pyridines and bis(indolyl)methanes

Article abstract of DOI:10.1016/j.molliq.2020.112625

A novel sulfonic-functionalized ionic liquid, C₃N₄ (C₃N₄-SO₃H (IL)), was prepared and its structure was characterized using FTIR, TEM, XRD, CHNS elemental analysis and FESEM-EDX mapping. Catalytic act

Full text of DOI:10.1016/j.molliq.2020.112625

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Design, synthesis, characterization, and catalytic properties of g-
C3N4-SO3H as an efficient nanosheet ionic liquid for one-pot
synthesis of pyrazolo[3,4-b]pyridines and bis(indolyl)methanes
Hojat Veisi, Pourya Mohammadi, Turan Ozturk
PII:
S0167-7322(19)35580-1
DOI:
https://doi.org/10.1016/j.molliq.2020.112625
MOLLIQ 112625
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
9 October 2019
8 January 2020
30 January 2020
Please cite this article as: H. Veisi, P. Mohammadi and T. Ozturk, Design, synthesis,
characterization, and catalytic properties of g-C3N4-SO3H as an efficient nanosheet ionic
liquid for one-pot synthesis of pyrazolo[3,4-b]pyridines and bis(indolyl)methanes, Journal
of Molecular Liquids(2020), https://doi.org/10.1016/j.molliq.2020.112625
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Design, Synthesis, Characterization, and Catalytic Properties of g-C3N4-SO3H
as an Efficient Nanosheet Ionic Liquid for One-pot Synthesis of Pyrazolo[3,4-
b]pyridines and Bis(indolyl)methanes
Hojat Veisi,a,* Pourya Mohammadi,a,* Turan Ozturkb,c
aDepartment of Chemistry, Payame Noor University, Tehran, Iran.
bDepartment of Chemistry, Istanbul Technical University, Maslak, Istanbul 34469, Turkey
cTUBITAK-UME, Chemistry Group Laboratories, PO Box 54, 41471, Gebze, Kocaeli, Turkey
Emails: hojatveisi@yahoo.com (H. Veisi); pourya.mohammadi93@yahoo.com (P. Mohammadi)
Abstract. A novel sulfonic-functionalized ionic liquid, C3N4 (C3N4-SO3H (IL)), was prepared
and its structure was characterized using FTIR, TEM, XRD, CHNS elemental analysis and
FESEM-EDX mapping. Catalytic activity of the liquid was studied for the syntheses of
pyrazolo[3,4-b]pyridine and bis(indolyl)methane derivatives under moderate conditions. The
catalyst was demonstrated to be reused at least 5 runs without considerable decrease in its
activity. Some of the important advantages of the methodology were proved to be good yields,
easy work-up, short reaction time and broad scope of substrates.
Key words: g-C3N4, acidic ionic liquid, pyrazolo[3,4-b]pyridine, bis(indolyl)methane
Introduction
Utilization of ionic liquids (ILs) as efficient catalysts has been one of the most important
development in the field of green chemistry. In spite of their goodness advantages, they could
not find enough attention, mainly due to some disadvantages, such as hard to separate from the
reaction mixture, high viscosity, high cost and uneconomic reuse procedures [1,2]. In order to
overcome these problems, the supported ionic liquid catalyzres (SILC) with their high catalytic
activity, were introduced as heterogeneous ionic liquids. So far, various methods have appeared
in the literature on design and synthesis of efficient SILCs, such as mesoporous silicas [3-5],
silica gels [6-8], polymers [9-11] and magnetic nanoparticles [12-14].
Recently, use of carbon nanostructures has attracted enormous attention as one of the most
promising candidates as the catalytic substrate due to their unique properties such as high
conductivity, high mechanical strength, high surface-to-volume ratio, and good physicochemical
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properties [15,16]. As a carbon nanomaterial, graphitic carbon nitride (g-C3N4) is one of the non-
metal substrates, being nontoxic with high chemical and thermal stability, resistant to acidic and
basic conditions and various solvents. These unique properties are due to the presence of
nitrogen atoms in the carbon architecture of graphitic carbon nitride g-C3N4, which can be
prepared easily from cheap nitrogen-rich precursors such as thiourea, urea, cyanimide and
melamine through pyrolysis [17,18]. Because of these great features, nowadays, g-C3N4 is
widely applied in different studies such as degradation of pollutants [19-22], water splitting
[23,24] and oxidation [25,26] and reduction reactions. However, so far, g-C3N4 has not been
widely applied as a catalyst support or catalyst in organic syntheses [27-29] Therefore, its use as
a catalyst in organic synthesis could be beneficial as the development of new methods for the
synthesis of heterocyclic compounds plays a significant role in organic syntheses.
Pyrazolo[3,4-b]pyridines are among the important heterocyclic compounds in organic and
medicinal chemistry. They show a wide spectrum of biological and pharmacological activities,
such as antiviral [30] and antibacterial [31] activities, and xanthine oxidase and oncogenic Ras
inhibitions [32]. On the other hand, some highlighted procedures for the preparation of
pyrazolopyridones have disadvantages such as low yields, long reaction times, harsh reaction
conditions and use of expensive catalysts and reactants [33]. Thus, more effective and novel
methods are desired for the preparation the heterocycles.
Another important heterocyclic compounds are bis(indolyl)methanes and their derivatives. They
are significant intermediates in R&D, particularly, in pharmaceutical industry [34,35].
In continuation to our endeavor in the sustainable development using nanocatalysis [36], in this
work, preparation and applications of g-C3N4-SO3H (IL) in the syntheses of the above mentioned
heterocycles, selected considering their importance in organic and medicinal chemistry, are
presented. A cheap raw material, urea, was used for the preparation of IL. Thermal treatment of
urea produced g-C3N4 in bulk, which was followed by addition of chlorosulfonic acid to obtain
the catalyst IL (Scheme 1). Its composition and morphology were analyzed in detail, and then it
was applied as an ionic liquid acidic catalyst for one-pot synthesis of pyrazolo[3,4-b]pyridines
and bis(indolyl)methanes. The catalyst was proved to be reused at least 5 runs without
considerable decrease in its catalytic activity.
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Scheme 1. The preparation method of ionic liquid g-C3N4-SO3H (IL)
Experimental
Synthesis of g-C3N4/SO3H
Initially, the graphitic carbon nitride (g-C3N4) was prepared by thermal treatment of urea, for
which urea powder (10 g) in a Muffle was heated at 550 ºC for 3 h to obtain the crude product.
Carbon nitride (0.5 g), without further purification, was dispersed by sonication in dry CH2Cl2
(30 mL), after which, applying Zolfigol’s method [37], the suspension was placed in an ice bath
o
and ClSO3H (1 mL) was added dropwise under vigorous stirring at 0-5 C. After that, the
reaction mixture was stirred for 12 h at room temperature, and, finally, the product (g-C3N4-
SO3H, IL) was collected by filtration, which was washed with ethanol/water (1/1) mixture for
several times and dried in an oven at 50 °C for 12 h (0.52 g).
Typical experimental procedure
a) Synthesis of pyrazolo[3,4-b]pyridine derivatives:
To a vigorously stirred mixture of Meldrum's acid (1 mmol), 5-methylpyrazol-3-amine (1
mmol), aldehyde (1 mmol) and g-C3N4-SO3H (10 mg) in ethanol/water (5 mL, 4/1 ratio) at 70
oC. The progress of the reaction was checked by thin-layer chromatography (TLC, n-
hexane/ethyl acetate, 5:1). After the complete of the reaction, the catalyst was collected by
filtration and washed several times with acetone and dried in an oven at 50 °C for 12 h to be used
for the next run. The solvent of the filtrate was evaporated under reduced pressure, and the crude
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product was crystallized from ethanol and dried in a vacuum oven to obtain the title compound.
All the products are known compounds in the literature [33].
b) Synthesis of bis(indolyl)methane derivatives:
A mixture of indole (2 mmol), aldehyde (1 mmol) and g-C3N4-SO3H (10 mg) in a mixture of
ethanol/water (4/1) was stirred at 70 oC until the complete of the reaction (monitored by TLC, n-
hexane/ethyl acetate, 5:1). The catalyst was then separated from the reaction mixture by
filtration, which was washed several times with acetone and dried in an oven at 50 °C for 12 h to
be used for the next run. The solvent of the filtrate was evaporated under reduced pressure, and
the crude product was crystallized from ethanol and dried in a vacuum oven to obtain the title
compound.
Results and discussion
Characteristics of the g-C3N4-SO3H (IL)
FT-IR
The functionalization of g-C3N4 by -SO3H was explained using FT-IR analyses (Fig. 1). The
spectra of g-C3N4-SO3H revealed that, as expected, the original C-N network of g-C3N4 remained
almost unchanged after the functionalization. The broad peaks between 3000-3500 cm-1 were
attributed to -NH2 and N-H stretching vibrations of the both compounds, and the hydrogen
bonding interactions of g-C3N4. The absorption peaks, observed at 1305 cm-1 and 1604 cm-1, are
due to the C-N and C=N stretching vibration, respectively, and the peak at 807 cm-1 belongs to
tri-s-triazine units. After functionalization of g-C3N4 with SO3H, some new peaks appeared, such
as the peaks at 1051 cm-1 and 1152 cm-1 from S=O symmetric and asymmetric stretchings,
respectively. The broad peaks at 3000-3500cm-1 were attributed to O-H stretching vibration, and
hydrogen bonding interactions of g-C3N4. Overlap of the asymmetric SO2 and S=O stretching
bands with g-C N bands makes the peaks not observable, which, as expected, may lead to the
̓
4
3
conclusion that the functionalization of g-C3N4 did not change its C-N network.
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g-C3N4-SO3H
g-C3N4
4000 3600 3200 2800 2400 2000 1600 1200 800
Wavenumber (cm-1)
400
Figure 1. The FTIR of g-C3N4 and g-C3N4-SO3H
XRD
In order to investigate the crystalline property of g-C3N4-SO3H, an XRD analysis was conducted
(Fig. 2.). A peak with a high intensity at 2θ = 27.2°, related to the (002) plane of g-C3N4, was
attributed to interlayers of g-C3N4. Moreover, the peak at 2θ = 13.1° was interpreted to be due to
the (100) plane of g-C3N4, possessing in-plane structure packing motif of tri-s-triazine units.
Thus, the XRD pattern of g -C3N4-SO3H demonstrated that the peak positions of g-C3N4 did not
change during the functionalization. Therefore, observing the peaks in their positions led to the
conclusion that functionalization of g-C3N4 did not destruct the C-N crystalline structures.
2500
(002)
2000
1500
(100)
1000
500
0
10
20
30
40
50
60
70
80
2 Theta (degree)
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Figure 2. The XRD of g-C3N4-SO3H
FESEM-EDX mapping
The morphology and elemental analysis of g-C3N4-SO3H were performed applying FESEM-
EDX mapping, which demonstrated a porous structure with g-C3N4-SO3H. As the structure of g-
C3N4-SO3H was agglomerated, its sheets were not observed clearly. This might be due to the
electrostatic interactions of g-C3N4-SO3H layers (Fig. 3a). While the EDX analysis of g-C3N4-
SO3H confirmed the presence of C, N, S and O atoms (Fig. 3b), its elemental mapping analysis
showed the presence of these elements. Moreover, in order to further confirm the presence of the
SO3H groups, CHNS elemental analysis was performed, which indicated the presence of sulfur
atom (S= 10.78 %) in good match with EDX analysis. The homogeneity of S, O elements in the
mapping was concluded that the functionalization of g-C3N4 is successful (Fig. 4).
Figure 3. The FESEM-EDX of g-C3N4-SO3H
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Figure 4. The image mapping of g-C3N4-SO3H
TEM
A TEM image of g-C3N4-SO3H is depicted in Fig. 5. A sheet-like structure with wrinkles was
observed with g-C3N4-SO3H. This might be due to the electrostatic interactions between of g-
C3N4-SO3H layers.
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Figure 5. The TEM image of g-C3N4-SO3H
Measurement of the acidic active cites (-SO3H)
In order to determine the amount of SO3H groups of g-C3N4-SO3H (IL), a standard acid-base
back titration method was applied. The catalyst (0.30 g) was added into NaOH solution (0.01
mol/L, 50 mL) and stirred at room temperature for 1h. Then, it was back titrated with HCl (0.01
mol/L). The total acidity (-SO3H) of g-C3N4-SO3H (IL) catalyst was obtained to be 3.1 mmol/g.
Density of the –SO3H groups, determined through the back titration method, was found to be in
good match with the percentage of S (10.78%), obtained from elemental analysis.
Catalytic activity on the heterogeneous g-C3N4-SO3H
In order to determine the catalytic activity of the catalyst, syntheses of pyrazolo[3,4-b] pyridines
and bis(indolyl) methanes as model compounds were performed. Initially, the synthesis of
pyrazolo[3,4-b]pyridines was attempted as a model reaction using 5-methylpyrazol-3-amine,
Meldrum’s acid and benzaldehyde as reactants in the presence of g-C3N4-SO3H to obtain the best
reaction conditions. Impacts of different parameters such as temperature, solvent and amount of
the catalyst were examined (Table 1). The results indicated that the condition having 15 mg of
catalyst, a solvent mixture of EtOH/H2O (3/1) and 70 oC of the reaction temperature is the best to
obtain the highest yield (98%) (Table 1, entry 12). The increase of the amount of the catalyst to
20 mg did not change the yield significantly (96%, entry 7). The reaction was also conducted in
the absence of the catalyst, which resulted in a very low yield of 12% (entry 10).
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Table 1. Optimization of reaction conditions of 3-methyl-4-phenyl-2,4,5,7-tetrahydropyrazolo[3,4-
b]pyridine-6-one in the presence of g-C3N4-SO3H .a
Entry Solvent
Catalyst (mg)
Temperature
Time (min)
Yield
(%)
79
42
55
64
75
95
96
82
62
12
98
(ºC)
25
25
25
25
25
25
25
25
25
25
50
70
1
EtOH
15
15
15
15
15
15
20
10
5
120
240
120
120
120
90
90
90
90
180
60
2
H2O
3
CH2Cl2
4
Toluene
5
CH3CN
6
7
8
9
10
11
12
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
0
15
15
45
98
a
Reaction conditions: 5-methylpyrazol-3-amine (1 mmol), aldehyde (1 mmol), Meldrum's acid (1 mmol), and
solvent (3 mL).
Applying the determined optimal conditions, various aldehyde derivatives, possessing electron-
withdrawing and electron-donating groups, were subjected to the synthesis of a wide range of
pyrazolo[3,4-b]pyridines in excellent yields (Table 2). Unusual electronic effect of any
substituent was not observed.
Table 2. Synthesis of pyrazolo[3,4-b]pyridines in the presence of g-C3N4-SO3H.a
Entry
Aldehyde
Yield (%)b
Mp (°C)
Mp (ºC)ref
1
2
3
4
5
6
7
8
C6H5CHO
98
98
96
92
96
96
96
82
303-305
>300
>300
266-268
>300
>300
304–30633
4-Me-C6H4CHO
4-MeO-C6H4CHO
3-NO2-C6H4CHO
4-Cl-C6H4CHO
3,4,5-(MeO)3-C6H2CHO
2-thienylaldehyde
Butanal
>30033
308–31033
264-26633
>30033
>30033
300–302
258–260
303–30533
259–26033
aGeneral procedure: 5-methylpyrazol-3-amine (1 mmol), aldehydes (1 mmol), Meldrum's acid (1 mmol),
Fe3O4/PEG-SO3H (10 mg), EtOH (3 mL), 60 minutes at 70 ºC.
bYield based on 1H NMR data.
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After obtaining successful yields with the synthesis of pyrazolo[3,4-b]pyridines, synthesis of
bis(indolyl)methanes was attempted. Similarly, optimum reaction conditions were initially
determined, which indicated the same reaction conditions of the previous reactions, i.e. 15 mg of
catalyst, EtOH/H2O (4/1) solvent mixture and 70 oC reaction temperature (Table 3).
Table 3. Optimization of reaction conditions of bis(indolyl)methanes in the presence of g-C3N4-SO3H.a
Entry Solvent
Catalyst (mg)
Temperature (ºC)
Time (min)
Yield (%)
81
1
EtOH
15
15
15
15
15
15
20
10
5
25
25
25
25
25
25
25
25
25
25
50
70
90
90
90
90
90
90
80
90
90
120
75
60
2
H2O
47
61
69
78
94
96
85
68
3
CH2Cl2
4
Toluene
5
CH3CN
6
7
8
9
10
11
12
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
0
15
15
19
98
98
a
Reaction conditions: benzaldehyde (1 mmol), indol (2 mmol), and solvent (3 mL).
A wide range of bis(indolyl)methane derivatives was then synthesized applying the best reaction
conditions to various aldehyde derivatives (Table 4). Contrary to the previous results, better
yields were obtained with the aldehydes having electron-withdrawing and electron-donating
substituents.
After obtaining successful results with the syntheses of pyrazolo[3,4-b]pyridines and
bis(indolyl)methanes in the presence of g-C3N4-SO3H, a possible reaction mechanism was
suggested (Scheme 2).
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Table 4. Synthesis of bis(indolyl)methane derivatives in the presence of g-C3N4-SO3H.a
Entry Aldehyde
Yield (%)b Mp (°C) Mp (ºC)ref
1
2
3
4
5
6
7
8
C6H5CHO
98
98
97
94
97
97
96
59
123-124
92-93
190-192
266-268
86-88
225-227
151–153
Oil
124–12534
4-Me-C6H4CHO
4-MeO-C6H4CHO
3-NO2-C6H4CHO
4-Cl-C6H4CHO
3,4,5-(MeO)3-C6H2CHO
2-thienylaldehyde
Butanal
93-9434
192–19334
264-26634
87-8934
226-22734
152-15334
-
aGeneral procedure: aldehydes (1 mmol), indol (2 mmol), g-C3N4-SO3H (15 mg), EtOH (3 mL), 60 minutes at 70 ºC.
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Scheme 2. The suggested mechanism for synthesis of pyrazolo[3,4-b]pyridines and
bis(indolyl)methane by g-C3N4-SO3H were presented
One of the important parameters of the stability of a catalyst is its reusability. Thus, the
reusability of the catalyst g-C3N4-SO3H was tested in the presence of 5-methylpyrazol-3-amine,
Meldrum's acid and benzaldehyde applying the above optimized reaction conditions (Fig. 6).
After the reaction was complete, the catalyst was simply filtered, washed with acetone and dried
o
at 50 C. It was then reused for the next reaction. Reusability test was also performed for the
synthesis of bis(indolyl)methane in the same optimized reaction conditions. The catalyst was
determined to be used at least five times without any considerable decrease in its activity, which
was further confirmed by TEM and elemental analyses (Fig. 7). The elemental analyses showed
the presence of “S” in 10.24%, which is comparable with the analysis of the original catalyst, i.e.
10.78%. Moreover, TEM image demonstrated the presence of sheet structure of the catalyst
without any noticeable change compare with the original one.
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PP
BM
100
80
60
40
20
0
1
2
3
4
5
Cycle
Figure 6. The recyclability of catalyst for synthesis of pyrazolo[3,4-b]pyridines (PP) and
bis(indolyl)methanes (BM).
Figure 7. The TEM image of g-C3N4-SO3H after 5 runs.
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Conclusions
We have presented the preparation of a recoverable porous ionic liquid catalyst, g-C3N4-SO3H,
and its application for the synthesis of a range of pyrazolo[3,4-b] pyridine and
bis(indolyl)methane derivatives through a highly effective and environmentally friendly one-pot
methodology. The catalyst was proved to be used at least five times without a considerable loss
in its activity. Its one-pot reaction resulted in the preparation of the heterocycles in high yields.
Therefore, the catalyst demonstrated to produce good to excellent yields, have simple work-up,
and could be used many times.
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Conflict of Interest
The authors report no conflicts of interest in this work.
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Highlights
 A novel sulfonic-functionalized ionic liquid of C3N4 (C3N4-SO3H (IL)) was prepared.
 The products were produced in excellent yields under mild conditions.
 Low metal loading was used in the reaction.
 The catalyst can be reused several consecutive cycles.
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