Sulfonic acid-functionalized MIL-101(Cr) as a highly efficient heterogeneous catalyst for one-pot synthesis of 2-amino-4H-chromenes in aqueous medium

Article abstract of DOI:10.1039/c5ra28135k

Sulfonic acid-functionalized MIL-101(Cr) was established to be an active heterogeneous catalyst for the one-pot synthesis of 2-amino-4H-chromenes via a three-component condensation reaction between resorcinol, malononitrile and a diverse range of aromatic

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Sulfonic acid-functionalized MIL-101(Cr) as a highly efficient heterogeneous catalyst
for one-pot synthesis of 2-amino-4H-chromenes in aqueous medium
Mrinal Saikiaa,b, Lakshi Saikiaa,b*
aMaterials Science Division, CSIR-North East Institute of Science and Technology
Jorhat – 785006, Assam, India
bAcademy of Scientific and Innovative Research, New Delhi, India
*Corresponding author. Tel.: +91 376 2370 081; fax. +91 376 2370 011
*E-mail address: l.saikia@gmail.com
Abstract
Sulfonic acid-functionalized MIL-101(Cr) was established to be an active heterogeneous
catalyst for one-pot synthesis of 2-amino-4H-chromenes via three component condensation
reaction between resorcinol, malononitrile and a diverse range of aromatic aldehydes in
aqueous medium. Here, sulfonation of the parent MIL-101(Cr) has been carried out through a
post synthetic modification (PSM) technique. The resulting MIL-101(Cr)-SO3H has been
characterized by employing powder X-ray diffraction, N2 sorption, FESEM, EDS, FT-IR and
TGA studies. Furthermore, the catalyst can easily be recovered and recycled without much
change in catalytic activity.
Introduction
Multicomponent reaction (MCR) strategy has been gaining much attention in recent
years owing to the growing concerns of the development of new synthetic routes in chemical
processes using non toxic reagents, catalysts and solvents. In comparison to the conventional
synthetic route, MCR type reactions have shown some advantages like facile one-pot
character, high substrate scope, required less reaction time and energy, avoiding intermediate
isolation etc. A variety of biologically active heterocyclic compounds like benzoxanthene,
benzopyrans and benzochromene have been synthesized by employing this MCR strategy.
With its unique characteristics of showing anticancer1, antibacterial2, antimicrobial3,
antiviral4 and anti-inflammatory5 activities benzochromene has found to be an important class
of biologically active heterocyclic compound which has also been used as the building block
of several human drugs, usually employed for the treatment of neurodegenerative diseases
like Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, amyotrophic lateral
sclerosis, AIDS associated dementia and Down’s syndrome. In addition, these compounds
have made a significant contribution in the area of pigments, cosmetics and potential

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biodegradable agrochemicals6. Therefore synthesis of these compounds has attracted an
emerging interest in recent research area.
Reports are available for the synthesis of amino chromene derivative over various
lewis acid and base catalyst like NaOH7, K2CO3 , InCl3 , TiCl410, MgO11, Et3N12 and
heteropolyacids. Although these catalysts display a very high activity and selectivity, yet the
inherent drawback associated with these catalysts is their recovery and reusability due to their
homogeneous character. Therefore, development of heterogeneous catalytic system is always
a demandable task for sustainable development of chemical procedures owing to their facile
recovery and reusability. A few heterogeneous system have been developed for the synthesis
of 2-amino-4H-chromene employing the MCR strategy like triazine functionalized
mesoporous organocatalyst (TFMO-1)13, nano-sized magnesium oxide, Mg/Al hydrotalcite,
nano-structured diphosphate (Na2CaP2O7)14, tungstic acid functionalized mesoporous SBA-
1515. Moreover, Fe3O4@CS-SO3H NPs has been employed for the synthesis of 2-amino-4H-
chromen-4-yl phosphonate derivatives16.
8
9
Metal-organic framework has been found to be gaining much importance owing to
their potential application in the field of gas storage, separation and catalysis17-20. In the field
of heterogeneous catalysis, it plays a promising role because of its high surface area, regular
and accessible pores and controllable structures. In particular, metal-organic framework like
MIL-101(Cr) is observed to be a suitable candidate for catalysis perspective due to its thermal
and chemical robustness21, possess coordinatively unsaturated metal sites (CUSs) that can act
as a mild Lewis acid22, introduction of functional group through post synthetic modification
(PSM) approach can be possible due to its excellent chemical stability. This chemical
stability again makes this material a suitable candidate in various important applications23-30
.
However, in case of ultra-stable MOFs the lack of functional and selective sites inhibits their
uses in catalysis. In order to counteract this shortcoming, functionalization of MOFs has been
carried out by encapsulating or loading different active sites like heteropoly acid31-42, sulfonic
acids into the matrix of MOFs via post-grafting, one-pot synthesis or premodification of
organic linker with functionalized ligand36,40,43-48. Among the different functionalized MIL-
101(Cr), sulfonic acid functionalized MIL-101(Cr) is evaluated as one of the potential solid
acid catalyst49 for various important organic transformations46,47,49-53. As the mentioned
chemical transformation is basically an acid catalyzed reaction, therefore we adopt a protocol
of synthesizing 2-amino-4H-chromene through three component condensation reaction
between resorcinol, aromatic aldehyde and malononitrile in aqueous medium utilizing MIL-
101(Cr)-SO3H as solid acid catalyst. Herein we synthesized MIL-101(Cr)-SO3H by using a

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post synthetic modification approach54 and the material has been thoroughly characterized by
PXRD, FE-SEM, XPS, N2 sorption, FT-IR and TGA studies. The catalyst shows excellent
catalytic activity in short reaction time and can be recycled up to forth catalytic cycles
without much change in catalytic activity.
Experimental
Synthesis of MIL-101(Cr)
MIL-101(Cr) was prepared according to the reported method by Férey et al21. In a
typical synthesis, a mixture of 3 mmol of Cr(NO3)3·9H2O, 3 mmol of terephthalic acid
(H2bdc) and 0.6 ml of 5 M HF (3 mmol) is undergo hydrothermal treatment in 15 ml H2O at
220°C for 8 h in a Teflon-lined autoclave bomb. After equilibration at ambient temperature,
the resulting Cr-MIL-101 solid was filtered to remove the unreacted colourless crystals of
H2bdc and purified by double treatment with DMF at 60 °C for 3 h and then triple treatment
with ethanol at 70 °C for 2.5 h. Finally the green solid was separated by centrifugation, dried
in an air oven at 70 °C overnight and kept it in a vacuum desiccator.
Synthesis of MIL-101(Cr)-SO3H
1 g of activated MIL-101(Cr) was stirred in 30 ml of dichloromethane at 0 °C for 20
minutes. Then 0.3 g of ClSO3H taken in 10 ml dichloromethane was added drop-wise to the
mixture under continuous magnetic stirring. After 30 minutes, the solid product was
separated through filtration, washed it with distilled water and acetone, soaked in ethanol for
24 h at 70 °C. The resultant material was then separated by centrifugation and dried under
vacuum. The S content of the synthesized MIL-101(Cr)-SO3H was found to be 0.13 mmol g-
1
.
General procedure for the synthesis of 2-amino-4H-chromenes
A mixture of aromatic aldehyde (1 mmol), resorcinol (1 mmol) and malononitrile (1
mmol) was reflux in water for the certain period of time in presence of MIL-101(Cr)-SO3H
(0.37 mol %). The progress of the reaction was monitored through TLC. After completion of
the reaction, the reaction mixture was allowed to cool to room temperature, added 10 ml
ethylacetate to the mixture and stirring for 45 minutes. The catalyst was then separated
through filtration, washed it with acetone by several times and then dried. The filtrate was
washed with water and dried over anhydrous Na2SO4 and solvent was evaporated in
rotavapor. The crude product was further purified by re-crystallization in ethanol to afford
pure 2-amino-4H-chromenes. The products were characterized by (1H, 13C) NMR
spectroscopy and all give satisfactory results (ESI).

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Result and discussion
MIL-101(Cr)-SO3H has been synthesized by employing a post synthetic modification
route involving the sulfonation of the MIL-101(Cr) framework with chlorosulfonic acid in
dichloromethane. No apparent modification in the crystallinity of the parent MIL-101(Cr) has
been observed after sulfonation of the frameworks with chlorosufonic acid as indicated by the
PXRD pattern of MIL-101(Cr)-SO3H (Fig. 1(black)).The pattern matches well with the
patterns of the precursor MIL-101(Cr) (Fig. 1 (red)) with slight changes of the intensities,
indicating the preservation of the structure of MIL-101(Cr) after inserting the sulfonic acid
group onto the framework. FT-IR spectra of MIL-101(Cr)-SO3H (Fig. S1(b), ESI) with its
unfunctionalized counterpart was shown in (Fig. S1(a), ESI). The successful functionalization
of MIL-101(Cr) with the acid moiety was well represented by the presence of new vibrational
band at 1129 and 1287 cm-1 along with a shoulder at 1426 cm-1 which can be assigned to the
O=S=O symmetric and asymmetric stretching mode of vibration55. The other vibrational
bands around 1400-1600 cm-1 could be attributed to the framework (O-C-O)- groups,
establishing the presence of dicarboxylate within MIL-101(Cr).
N2 physisorption analysis (Fig. 2) of both the bare (Fig. 2(a)) and acid functionalized
MIL-101(Cr) (Fig. 2(b)) has been carried out at 77k. MOF-101(Cr)-SO3H exhibits a similar
typical Type-I isotherm to that of the parent MIL-101(Cr). However, an appreciable decrease
in surface area and pore volume from 2787 m2g-1 and 1.58 cm3g-1 to 2206 m2g-1 and 1.27
cm3g-1 has been observed for the functionalized MOF due to the partial blockage of cavities
of the parent MOF by the sulfonic acid groups.
FE-SEM images (Fig. S2, ESI) illustrate the preservation of the characteristic cubic
symmetry of MIL-101(Cr) after incorporating the acid moiety into the mesocages. Energy-
dispersive x-ray spectroscopy (EDS) (Fig. 3) analysis confirms the successful incorporation
of -SO3H group into the framework while the elemental mapping of Cr, S, O elements of
MIL-101(Cr)-SO3H clearly demonstrate that -SO3H group is well distributed into the cages of
MIL-101(Cr). XPS spectrum (Fig. 4) also shows a good agreement with this result, exhibit a
typical S2p band (Fig. 4 (c)) at 168.2 eV corresponding to –SO3H group anchored with the
linkers of the parent MOF. Furthermore, no change in the peak position has been observed
with respect to the other elements present in the bare MIL-101(Cr) (Fig. 4(b)), establishing
the conservation of the integrity of MIL-101(Cr) after the post synthetic modification.
The thermal stability of MIL-101(Cr)-SO3H (Fig. S3(a), ESI) was analyzed through
thermogravimetric analysis (TGA), operated in the temperature range from room temperature

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to 800 °C with a heating rate of 10 °C min-1 under N2 atmosphere. The first weight loss of
11.56 % appeared below 150 °C is due to the loss of adsorbed water on MIL-101(Cr)-SO3H.
The second weight loss of 7.77 % appeared between 150 – 350 °C, may be attributed to the
decomposition of sulfonic acid moiety anchored within the cages of MIL-101(Cr). The third
weight loss step of 44.73 % is due to the decomposition of MIL-101(Cr) framework. In
addition, another weight loss step of 10.1 % has been observed above 550 °C, establishing
the decomposition of chromium sulphate56.
We have estimated the loading of -SO3H group on to the framework of MIL-101(Cr)
by adopting a back titration method involving the addition of a standard NaOH solution (0.01
N) to a known quantity of MIL-101(Cr)-SO3H. After treating with NaOH, it has been
assumed that all the -SO3H groups are converted into -SO3 Na+ form and the excess NaOH
-
left in the solution is allowed to titrated with a known strength of oxalic acid (0.01 N). From
the titrametric experiment it may easily be estimated the exact amount of NaOH consumed
during the titration or the equivalent amount of -SO3H group attached with the linkers of
MIL-101(Cr). The result shows that 1 g of MIL-101(Cr)-SO3H contains 0.15 mmol –SO3H
group.
We have explored the catalytic activity of the synthesized MIL-101(Cr)-SO3H in a
three component condensation reaction between aromatic aldehydes, resorcinol and
malononitrile in an aqueous medium. In presence of bare MIL-101(Cr), a low yield of desired
product was formed confirming the need of strong acid moiety on the framework structure to
perform the condensation reaction. The effect of different dosage of the catalyst on the
mentioned condensation reaction has been monitored by taking the condensation between
benzaldehyde, resorcinol and malononitrile as a model reaction. From the result (Table 1) it
was observed that with increase in the mol% of the catalyst (Entry1, 2 table 1), the yield of
the product increases with the maximum yield of 82 % was obtained when the reaction was
carried out with 0.37 mol% catalyst (Entry 3, table 1). This may be due to the increase in the
number of active sites on the catalyst with increase in catalyst dosage. No significant increase
in product yield was obtained with further increase in the mol% of the catalyst (Entry 4,
table1), therefore, 0.37 mol% catalyst has been considered as the optimum catalyst amount to
conduct the three component condensation reaction.
The solvent effect on the condensation reaction between benzaldehyde, resorcinol and
malononitrile has been studied by carry out the reaction in various types of solvents exhibit
different polarities under reflux condition. From the result (Table S1, ESI) it has been
concluded that the desired product formation in polar protic solvents like ethanol, methanol

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and water (Entry1, 2, 4, table S1, ESI) was found to much higher than that of the polar aprotic
solvent like acetonitrile, THF, DCM, (Entry 3, 5, 6, table S1, ESI) where we have observed
the knoevenagel condensation adduct as the major product. This behaviour may be attributed
to the more polar character of the product to that of reactant molecules. However, under
solvent free condition (Entry 7, table S1, ESI) the desired product yield was found to be low
that clearly demonstrate that water is the suitable solvent (Entry 4, table S1, ESI) for carrying
out the three component condensation reaction that furnish the highest yield of the product
under the optimized condition.
The influence of the temperature on the one pot three component condensation
reaction between benzaldehyde, resorcinol and malononitrile has been invested (Table 2).
The results show that at room temperature the reaction cannot proceed even running the
reaction for 12 h, while with increase in temperature (Entry 2, 3 and 4, table 2) the amount of
desired product increases with a maximum yield 82 % was obtained at 100 °C (Entry 5, table
2). We have also found a very low yield of the desired product in the absence of catalyst at
100 °C, owing to the thermal condensation reaction at higher temperature. Therefore, taking
all these aspects we have chosen 100 °C as the optimum temperature to carry out the further
studies.
The substrate scope of the catalyst has been examined by employing various aromatic
aldehydes containing electron-withdrawing, electron donating substituents. The results are
summarized in table 3. It has been observed that MIL-101(Cr)-SO3H is very active to all kind
of aromatic aldehydes. In fact, we do not observe any pronounced substituent effect attached
to the phenyl ring over the reaction as both the electron withdrawing (Entry 8-10, table 3) and
donating groups (Entry 2-7, table 3) show the condensation reaction with almost equal
efficiencies. However, ortho substituted aldehydes display a lower yield of product as
compared to its meta and para analogues, may be ascribed to the electronic effect. In
addition, the heteroaromatic aldehydes like pyridine-2-carbaldehyde (Entry 11, table 3) furan-
2-carbaldehyde (Entry 12, table 3) are also quite compatible with this present catalytic
system. Overall MIL-101(Cr)-SO3H has been established to be a highly flexible catalyst for
the proposed one-pot three component condensation reaction.
The efficiency of the present catalytic system has been examined by taking the
condensation reaction between 4-chlorobenzaldehyde, resorcinol and malononitrile as the
representative case. From the results (shown in table S2, ESI) it has been observed that most
of the catalysts available in the literature are homogeneous in nature that displays the basic
shortcoming of their recovery and reusability. The prime advantages of our protocol are high

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product yield %, facile recovery and reusability, low catalyst dosage, high TON etc.
Furthermore, the high surface area of the material provides a better dispersion of catalytically
active sites that actually increases the yield of the product.
A plausible reaction mechanism has been proposed based on the available literature
and our experimental data. Actually, the reaction was proceed through Knoevenagel
condensation reaction between aromatic aldehyde and malononitrile followed by Michael
addition of resorcinol with the Knoevenagel product ((I) scheme1). The final step of the
reaction is an intra-molecular cyclization ((II) scheme 1) and tautumerization to afford the
desired 2-amino-4H-chromenes ((III) scheme 1). The catalyst MIL-101(Cr)-SO3H containing
the acid sites increases the electrophilicity of the aldehydic carbon which undergo
dehydration to give the Knoevenagel product. The Cr (III) sites presented in the parent MOF
may increase the electrophilicity of the –CN group of the Knoevenagel product to facilitate
the Michael addition which subsequently undergoes intra-molecular cyclization and
tautomerization to yield the 2-amino-4H-chromene derivative.
The heterogeneous nature of MIL-101(Cr)-SO3H was examined by performing a hot
filtration test involving the reaction between benzaldehyde, resorcinol and malononitrile
under the optimized reaction condition. After 1h of the reaction the catalyst was removed
from the reaction mixture by filtration (product yield 40 %) and allowed the reaction to
proceed with the filtrate for further 2 h. No increase in the product yield beyond 40 % was
observed after 2 h reaction time. This result clearly confirms the heterogeneous nature of
MIL-101(Cr)-SO3H during the course of the reaction.
The reusability of the catalyst was monitored by taking the condensation reaction
between benzaldehyde, resorcinol and malononitrile as the model reaction. After completion
of the reaction the catalyst was recovered from the reaction mixture through filtration,
washed it with acetone for several times and then dried in an oven at 70 °C for 3 h. The
activated MIL-101(Cr)-SO3H was then subsequently employed for the next catalytic run
under identical condition. The results (Fig. 5) show that the catalyst can perform effectively
up to fifth catalytic cycle without any significant loss of its catalytic activity. The crystallinity
of the recovered catalyst is well maintained as indicated by the PXRD analysis (Fig. 1(blue)).
FT-IR analysis (Fig. S1(c)) also shows a good agreement with the PXRD results. The
vibrational bands corresponding to the bare MIL-101(Cr) is well preserved in the recovered
catalyst. FE-SEM image of the recovered catalyst (Fig. S2 (c)) clearly demonstrate the
preservation of the integrity of the parent MOF. We have calculated the S content of the
recovered catalyst and it was found to be 0.10 mmol g-1. All these results strongly

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recommend that a heterogeneous process was involved in the mentioned organic
transformation and also establishing the chemical robustness of the catalyst.
Conclusions
In summary, sulfonic acid functionalized MIL-101(Cr) have been synthesized by
adopting a post synthetic modification technique using chlorosulfonic acid as sulfonation
reagent. The synthesized MIL-101(Cr)-SO3H exhibits a high surface area and acidity and
thus can be effectively employed as a heterogeneous catalyst for one-pot three component
condensation reaction between aromatic aldehydes, resorcinol and malononitrile in aqueous
medium to synthesize 2-amino-4H-chromenes. Furthermore, the catalyst can easily be
recovered and reused without any significant loss of its catalytic activity. Therefore,
considering all these aspects it can be concluded that MIL-101(Cr)-SO3H acts as a potent
heterogeneous catalyst for this chemical transformation and also established its potential
utility as acid catalyst for various liquid phase important organic transformations.
Acknowledgements
The authors are grateful to the Director of CSIR-North East Institute of Science and
Technology, Jorhat, Assam, India for his kind permission to publish the work. We are
thankful to Dr P. Sengupta, Head of Materials Science Division for support, and non network
project MLP-6000 (WP-1) and network project (CSC-0408) project for financial assistance.
M. S. acknowledges DST New Delhi for an INSPIRE fellowship.
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Captions
Fig1 PXRD patterns of MIL-101(Cr) (red), MIL-101(Cr)-SO3H and recovered MIL-101(Cr)-
SO3H (blue)
Fig 2 N2 adsorption isotherm of (a) MIL-101(Cr), (b) MIL-101(Cr)-SO3H
Fig 3 a) EDS analysis of MIL-101(Cr)-SO3H, b) mapping of different elements of MIL-
101(Cr)-SO3H
Fig 4 XPS spectra of (a) MIL-101(Cr)-SO3H, (b) MIL-101(Cr) and (c) S2p band of -SO3H
group
Fig 5 Catalyst recycling test for MIL-101(Cr)-SO3H
Table1 Effect of catalyst amount
Table 2 Effect of temperature
Table 3 MIL-101(Cr)-SO3H catalyzed one- pot three component condensation reactions with
different aromatic aldehydes
Scheme1 Plausible reaction mechanism for the condensation reaction between aromatic
aldehyde, resorcinol and malononitrile in the presence of MIL-101(Cr)-SO3H

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2
4
6
8
10
12
14
16
18
20
2-Theta(degree)
Fig. 1 PXRD pattern of MIL-101(Cr) (red), MIL-101(Cr)-SO3H (black) and recovered MIL-
101(Cr)-SO3H (blue)
1200
(a)
1000
(b)
800
600
400
200
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure(P/P
)
o
Fig. 2 N2 adsorption isotherm of (a) MIL-101(Cr), (b) MIL-101(Cr)-SO3H

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Fig. 3 a) EDS analysis of MIL-101(Cr)-SO3H, b) mapping of different elements of MIL-
101(Cr)-SO3H

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80000
70000
60000
50000
40000
30000
20000
10000
0
O1s
(a)
(b)
C1s
Cr2p
S2p
0
200
400
600
800
1000
1200
Binding energy(eV)
7500
7000
6500
6000
5500
5000
4500
4000
-SO H
(C)
3
160
162
164
166
168
170
172
174
Binding energy (eV)
Fig. 4 XPS spectra of (a) MIL-101(Cr)-SO3H, (b) MIL-101(Cr) and (c) S2p band of -SO3H
group

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100
80
60
40
20
0
1
2
3
Run
4
5
Fig. 5 Catalyst recycling test for MIL-101(Cr)-SO3H
Table1 Effect of catalyst amounta
Entry Catalyst amount (mol%) Yield (%)b
1
2
3
4
0.15
0.23
0.37
0.45
48
62
82
83
aReaction conditions: benzaldehyde (1 mmol), resorcinol (1 mmol) and malononitrile (1
mmol), water= 5 ml, temperature= 100°C, time =3h
b Isolated yields are based on benzaldehyde
Table 2 Effect of temperaturea
Entry
Temperature (°C)
Room temperature
Yield(%)b
No desired product
1
2
3
40
65
20
45
68
82
4
5c
85
100
aRecation condition: benzaldehyde (1 mmol), resorcinol (1 mmol) and malononitrile (1
mmol), time =3h
b Isolated yields are based on benzaldehyde, cWithout catalyst at 100°C yield= 6%

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Table 3 MIL-101(Cr)-SO3H catalyzed one- pot three component condensation reactions with
different aromatic aldehydesa
Entry Aromatic aldehydes
1
Time (h)
3
Product
Yield (%)b
82
2a
2b
7
7
76
78
80
78
80
2
3
4
2c
2d
2e
2f
4
4.5
3.5
H3CO
CHO
5
6
3.5
3
76
81
2g
2h
7
8
4
74
77
2i
2j
9
3.5
10
6
7
72
70
2k
2l
11
12
aReaction conditions: aromatic aldehydes (1mmol), resorcinol (1 mmol), malononitrile(1
b
mmol), water=5 ml, MIL-101(Cr)-SO3H (0.37 mol%). Isolated yields are based on
1
aromatic aldehydes. All compounds have been characterized by H and 13C NMR
(available in the ESI)

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Scheme1 Plausible reaction mechanism for the condensation reaction between aromatic
aldehyde, resorcinol and malononitrile in the presence of MIL-101(Cr)-SO3H

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Table of contents
MIL-101(Cr)-SO3H was found to be an active heterogeneous catalyst for one-pot synthesis of
2-amino-4H-chromenes in aqueous medium.
MIL-101(Cr)-SO3H