Manganese-containing periodic mesoporous organosilica with ionic-liquid framework (Mn@PMO-IL): A powerful, durable, and reusable nanocatalyst for the biginelli reaction

Article abstract of DOI:10.1002/chem.201304349

The catalytic application of a novel manganese-containing periodic mesoporous organosilica with ionic-liquid framework (Mn@PMO-IL) in the Biginelli reaction was investigated. First, the Mn@PMO-IL nanocatalyst was prepared and characterized by TEM, SEM, X-

Full text of DOI:10.1002/chem.201304349

DOI: 10.1002/chem.201304349
Full Paper
&
Nanocatalysts
Manganese-Containing Periodic Mesoporous Organosilica with
Ionic-Liquid Framework (Mn@PMO-IL): A Powerful, Durable, and
Reusable Nanocatalyst for the Biginelli Reaction
Dawood Elhamifar* and Ahmad Shꢀbani[a]
Abstract: The catalytic application of a novel manganese-
containing periodic mesoporous organosilica with ionic-
liquid framework (Mn@PMO-IL) in the Biginelli reaction was
investigated. First, the Mn@PMO-IL nanocatalyst was pre-
pared and characterized by TEM, SEM, X-ray photoelectron
spectroscopy, and nitrogen-sorption analysis. The catalyst
was then used in the one-pot Biginelli condensation of vari-
ous aldehydes with urea and alkyl acetoacetates under sol-
vent-free conditions. The corresponding dihydropyrimidone
products were obtained in high to excellent yields and selec-
tivities at short reaction times. Moreover, the catalyst was re-
covered and successfully reused many times with no notable
decrease in activity and selectivity.
Introduction
On the other hand, the fabrication of ordered mesoporous
organosilicas (OMOs) in recent decades has been an important
achievement in the fields of chemistry and materials science
due to the widespread applications of these materials in catal-
ysis, chromatography, gas storage, solid-phase extraction, and
so forth.[11] Among the different kinds of OMOs, periodic meso-
porous organosilicas (PMOs) have attracted more attention be-
cause of their high surface area, high thermal and mechanical
stability, and uniform distribution of organic functional groups
in their frameworks.[12–14] To date many PMO materials have
been prepared by using different functionalized organic pre-
cursors and successfully applied as supports for the immobili-
zation and stabilization of organic and inorganic catalysts in
chemical processes.[12–16] For example, we recently developed
an ionic-liquid-based PMO (PMO-IL) material and investigated
its performance in the immobilization and stabilization of pal-
ladium and ruthenium catalysts in a number of organic reac-
tions.[16] Our studies showed that the PMO-IL is a powerful and
recoverable support for metal catalysts and could be recovered
and reused several times with no decrease in activity and effi-
ciency.[16] Considering the aforementioned advantages of
PMO-IL nanomaterials and the importance of Biginelli products
in drug synthesis, we have now developed a strategy for
the preparation of a novel manganese-containing PMO-IL
(Mn@PMO-IL) and studied its application in the Biginelli con-
densation of several aldehydes with urea and alkyl acetoace-
tates (Scheme 1). The recoverability, reusability, and stability of
the catalyst were also studied.
The Biginelli reaction involves one-pot condensation of alde-
hydes with b-dicarbonyl compounds and urea to synthesize di-
hydropyrimidones,[1–3] which are very attractive and significant
compounds in organic chemistry due to their pharmacological
and therapeutic properties such as antibacterial, antitumor, an-
tihypertensive, anti-inflammatory, analgesic, anticancer, antivi-
ral, and anti-HIV activity, as well as efficacy as calcium channel
modulators.[2–5] The Biginelli reaction has been traditionally per-
formed under homogeneous conditions with both Brønsted[3,6]
and Lewis[3,7,8] acidic catalysts such as HCl, AcOH, trifluoroacetic
acid, trifluoromethanesulfonic acid, para-toluenesulfonic acid,
TiCl3, BF3·OEt2, ZrCl4, InBr3, InCl3, FeCl3·6H2O, NiCl2·6H2O, BiCl3,
Me3SiCl, LiClO4, LiBr, Mn(OAc)3·2H2O, Cu(OTf)2, CuCl2·2H2O,
La(OTf)3, Yb(OTf)3, LaCl3, and Sc(OTf)3.[3,6–9] However, many of
these homogeneous systems are expensive, harmful, and diffi-
cult to handle as well as having problems of catalyst recovery
and product separation and purification. To overcome these
limitations, several strategies using recoverable heterogeneous
solid acid catalysts have recently been explored.[10] The report-
ed heterogeneous catalysts have advantages of easy separa-
tion and recovery in comparison to their homogeneous coun-
terparts. However, in most cases the activity and efficiency of
the catalysts were low, and this necessitates the design and
synthesis of highly effective recoverable catalysts for this im-
portant three-component reaction.
[a] Dr. D. Elhamifar, A. Shꢀbani
Department of Chemistry, Yasouj University
Yasouj 75918-74831 (Iran)
Results and Discussion
The PMO-IL was first prepared according to our previously re-
ported procedure[16d] and then treated with a substoichiometric
amount of Mn(OAc)3·2H2O in dimethyl sulfoxide to produce
the Mn@PMO-IL nanocatalyst (Scheme 1). The Mn@PMO-IL
Fax: (+98)741-2227574
E-mail: d.elhamifar@yu.ac.ir
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304349.
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structure, which indicates that
loading with manganese acetate
species significantly affects the
surface morphology of the
mesostructure, although the reg-
ularity is maintained. The TEM
image of the Mn@PMO-IL mate-
rial showed a well-ordered rod-
like structure of the channels
with two-dimensional hexagonal
array (Figure 2c). This observa-
tion is in good agreement with
the nitrogen sorption experi-
ment and confirms the uniformi-
ty and stability of the mesostruc-
ture under the applied reaction
conditions. In the next study, the
Mn@PMO-IL nanomaterial was
investigated by X-ray photoelec-
tron spectroscopy (XPS). The XP
spectrum showed peaks at bind-
ing energies of 103.4 eV (Si 2p),
284.5 eV (C 1s), and 532 eV (O
1s) proving the successful incor-
poration of the expected ele-
Scheme 1. Preparation of Mn@PMO-IL and its application in the Biginelli reaction. a) H2O, HCl (2M), P123, 408C,
24 h. b) Static conditions, 1008C, 72 h. c) Soxhlet extraction of surfactant.
nanocatalyst was characterized to confirm the successful im-
mobilization of manganese acetate in its framework.
ments in the material framework (Figure 3). In addition, Mn
2p1/2 and Mn 2p3/2 peaks were observed at 654.2 and 642.6 eV,
respectively (Figure 3a and b), which are reasonably far from
those of manganese metal with Mn 2p1/2 at 649.25 and Mn
2p3/2 at 638 eV.[17] The Mn 2p3/2 peak of MnO2 is observed at
642.2 eV.[17] This means that the manganese in Mn@PMO-ILis
strongly bound to electronegative atoms. In other words, the
Mn electrons of Mn@PMO-IL sample are more strongly bound
than those of MnO2. Elemental analysis of the Mn@PMO-IL cat-
alyst showed that the manganese loading on the solid surface
is 0.3 mmolgÀ1.
Nitrogen adsorption/desorption experiments were per-
formed on the PMO-IL and Mn@PMO-IL materials to compare
their surface areas and pore size distributions (Figure 1,
Table 1). This revealed a type IV isotherm with H1-type hystere-
Table 1. Structural parameters of PMO-IL and Mn@PMO-IL materials de-
termined from nitrogen-sorption experiments.
Sample
BET surface
area [m2 gÀ1
Pore
diameter [nm]
Pore volume
[cm3 gÀ1
]
After characterization, the Mn@PMO-IL nanocatalyst was
used in the Biginelli condensation of benzaldehyde with urea
and ethyl acetoacetate as model reaction (Table 2). This study
illustrated that the progress of the reaction is strongly affected
by catalyst loading and reaction temperature. The dramatic in-
]
PMO-IL
Mn@PMO-IL
655
550
10.4
10.4
1.20
0.94
sis loop for both samples, typical of periodic mesoporous ma-
terials with high regularity. The BET surface area and pore
volume of the Mn@PMO-IL nanocatalyst were both lower than
those of its PMO-IL parent (Table 1), confirming successful im-
mobilization of manganese acetate in the material framework.
BJH measurements showed a sharp peak and a pore diameter
of 10.4 nm for these solid nanostructures, which indicate uni-
form pore size distributions of the materials (Figure 1b,
Table 1). The excellent regularity of the mesochannels after
loading with manganese acetate is proof of the high stability
of the PMO-IL nanostructure during the reaction.
Table 2. Effect of different parameters on the Biginelli reaction of benzal-
dehyde with urea and ethyl acetoacetate.[a]
Entry
Catalyst
Catalyst loading
T [8C]
Yield [%][b]
1
2
3
4
5
6
7
8
9
Mn@PMO-IL
Mn@PMO-IL
Mn@PMO-IL
Mn@PMO-IL
Mn@PMO-IL
PMO-IL
SBA-15
Mn@SBA-15
Mn@SiO2
20 mg (0.6 mol%)
20 mg (0.6 mol%)
20 mg (0.6mol%)
15 mg (0.45 mol%)
10 mg (0.3 mol%)
20 mg
20 mg
0.6 mol%
0.6 mol%
25
45
75
75
75
75
75
75
75
<5
35
97
65
40
26
trace
66
Scanning electron microscopy (SEM) images of PMO-IL and
Mn@PMO-IL materials were recorded to compare their surface
morphologies (Figure 2a and b). These results showed that
PMO-IL has a ropelike structure with high regularity and a di-
ameter of 2–4 mm, while Mn@PMO-IL has a uniform wormlike
62
[a] Reaction conditions: benzaldehyde (1 mmol), ethyl acetoacetate
(1 mmol), urea (1.5 mmol), 45 min. [b] Yields of isolated products.
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Figure 1. a) Nitrogen adsorption/desorption and b) pore size distribution iso-
therms of PMO-IL and Mn@PMO-IL.
crease in the rate of substrate conversion on increasing the
temperature from 25 to 758C confirms that the temperature
directly affects the efficiency of the catalyst (Table 2, entries 1–
3). On increasing the amount of catalyst from 0.3 to 0.6 mol%
a remarkable increase in the yield was observed (Table 2, en-
tries 3–5), which proves that the progress of the reaction is
mainly affected by the catalyst loading. To reveal the individual
effects of the ionic-liquid groups and manganese sites on the
reaction, the efficiency of the Mn@PMO-IL catalyst was com-
pared with those of PMO-IL and an ionic-liquid-free SBA-15
material (Table 2, entry 3 versus entries 6 and 7). In the pres-
ence of PMO-IL, 26% of the product was obtained, while SBA-
Figure 2. a) SEM image of PMO-IL. b) SEM image of Mn@PMO-IL. c) TEM
image of Mn@PMO-IL.
15 gave trace amount of the desired product under the same
conditions as the Mn@PMO-IL catalyst. These results clearly
showed that the reaction cycle is mainly catalyzed by manga-
nese species immobilized on the PMO-IL nanostructure. Fur-
thermore, the higher activity of PMO-IL in comparison to SBA-
15 is attributed to ionic-liquid groups incorporated in the
PMO-IL mesochannels, which may catalyze the reaction
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Table 3. Preparation of dihydropyrimidone derivatives in the presence of
Mn@PMO-IL.[a]
Entry Ar
R
t [min] Yield [%][b] M.p.
45
1
Ph
OEt
97
95
96
90
88
96
97
95
87
95
96
97
97
87
88
84
trace
203–205
211–213
212–214
217–220
252–254
193–195
215–218
204–206
220–221
216
202–204
204–206
193–195
257–259
282–284
228
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Ph
OMe 45
4-Cl-C6H4
2-Cl-C6H4
2-Cl-C6H4
3-Br-C6H4
3-Br-C6H4
4-CN-C6H4
2-NO2-C6H4
4-Me-C6H4
4-Me-C6H4
4-MeO-C6H4
4-MeO-C6H4
2-MeO-C6H4
2-MeO-C6H4
4-OH-C6H4
OEt
OEt
50
65
OMe 65
OEt 55
OMe 55
OEt
OEt
OEt
60
65
45
OMe 45
OEt 45
OMe 45
OEt 55
OMe 55
OEt 60
pyrene-1-carbaldehyde OMe 120
–
[a] Reaction conditions: aldehyde (1 mmol), ethyl/methyl acetoacetate
(1 mmol), urea (1.5 mmol). [b] Yields of isolated products.
cellent yields of the corresponding 1,3-dihydropyrimidinones
at short reaction times. The other aromatic compounds bear-
ing electron-withdrawing groups, namely, 4-chlorobenzalde-
hyde (Table 3, entry 3), 2-chlorobenzaldehyde (Table 3, entries 4
and 5), 3-bromobenzaldehyde (Table 3, entries 6 and 7), 4-cya-
nobenzaldehyde (Table 3, entry 8), and 2-nitrobenzaldehyde
(Table 3, entry 9), also gave excellent yields of Biginelli prod-
ucts. Furthermore, aromatic aldehydes bearing electron-donat-
ing groups, such as 4-methylbenzaldehyde (Table 3, entries 10
and 11), 4-methoxybenzaldehyde (Table 3, entries 12 and 13),
2-methoxybenzaldehyde (Table 3, entries 14 and 15), and 4-hy-
droxybenzaldehyde (Table 3, entry 16), also afforded high
yields of the corresponding coupling adducts under the same
reaction conditions as above. Significantly, the more sterically
demanding ortho-substituted compounds, which are usually
less reactive substrates in organic transformations, were also
smoothly converted to their corresponding products (Table 3,
entries 4, 5, 9, 14, and 15). These results illustrate the high per-
formance of the designed catalyst for the preparation of
a wide range of Biginelli products bearing different functional
groups, which are important in the fields of pharmacology and
chemistry. To study the substrate selectivity of the catalyst, the
reactivity of a bulky aldehyde, namely, pyrene-1-carbaldehyde,
was investigated under the same conditions as the other alde-
hyde substrates (Table 3, entry 17). After 2 h, only a trace
amount of product was observed. This confirms that the reac-
tion mainly occurs inside the pores and proves the high size
selectivity of the catalyst, which is due to the location of the
reactive catalytic centers inside the mesochannels of PMO-IL.
Since recoverability and reusability of the catalyst are impor-
tant in both academic and industrial chemistry, the recyclability
Figure 3. a) XP survey spectrum of Mn@PMO-IL. b) Mn 2p spectrum of
Mn@PMO-IL.
through the acidic hydrogen atom on the imidazolium ring
(ÀNCHNÀ).[9] In another study, the same amount of manganese
was immobilized on SBA-15 and amorphous silica materials to
afford Mn@SBA-15 and Mn@SiO2 catalysts. Comparative effi-
ciency tests were then performed on these catalysts in the Bi-
ginelli reaction of benzaldehyde with urea and ethyl acetoace-
tate under the same conditions as before. For the same reac-
tion time as Mn@PMO-IL, Mn@SBA-15 and Mn@SiO2 gave 66
and 62% yields of the desired product, respectively (Table 2,
entries 8 and 9), which confirms that the efficiency of
Mn@PMO-IL is much better than those of Mn@SBA-15 and
Mn@SiO2. These results reveal that PMO-IL is an effective and
powerful material for immobilization of the Mn catalyst and in-
dicate the valuable effect of the IL moieties in the catalytic
process. These observations may be attributed to the synergis-
tic effect between the PMO-IL and the immobilized manganese
catalyst system. On the basis of these studies, 0.6 mol% of
Mn@PMO-IL catalyst, 758C, and no solvent were chosen as op-
timum conditions. Under these conditions, the activity of the
catalyst was then investigated in the Biginelli reaction of
a number of aldehyde substrates with alkyl acetoacetates and
urea (Table 3). Benzaldehyde (Table 3, entries 1 and 2) gave ex-
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and reusability of the Mn@PMO-IL catalyst were investigated in
the Biginelli reaction of benzaldehyde with urea and ethyl ace-
toacetate under standard reaction conditions. After the com-
pletion of the reaction, 10 mL of hot ethanol was added and
the resulting solution was filtered and the residue was washed
thoroughly with ethanol. The recovered catalyst (RMn@PMO-
IL) was then reused under the same conditions as above for at
least 14 reaction cycles and furnished the desired product in
high to excellent yield and selectivity (Figure 4), indicating the
Figure 5. TEM image of RMn@PMO-IL catalyst after the fifth reaction cycle.
tates, which gave the corresponding products in high to excel-
lent yields and selectivities. Moreover, the catalyst was success-
fully recovered and reused at least 14 times without notable
decrease in activity and selectivity. The nitrogen adsorption/de-
sorption data and TEM image of the recovered nanocatalyst
(RMn@PMO-IL) clearly showed high regularity and uniform
pore size distribution of the mesochannels. These observations
could be attributed to the ionic-liquid nature of the mesostruc-
ture, which protects catalytic manganese species against any
leaching, destabilization, and deactivation. Other advantages
of this catalyst system include high surface area, high conver-
sion rate, solvent-free conditions, easy product separation, and
high catalyst durability. Investigations of other applications of
this catalyst system are underway.
Figure 4. Reusability of the Mn@PMO-IL catalyst in the Biginelli reaction of
benzaldehyde with ethyl acetoacetate and urea.
high stability, durability, and reusability of the catalyst during
the reaction process. This can be attributed to the high surface
area of the PMO-IL nanomaterial and the ionic-liquid nature of
the mesochannels, which effectively immobilize the manga-
nese species and protect them against leaching, agglomera-
tion, and deactivation.[16] Nitrogen adsorption/desorption anal-
ysis of the recovered catalyst after five reaction cycles was also
performed to verify porosity, stability, and regularity of the
mesostructure (Figure S1 of the Supporting Information). The
surface area, pore volume, and pore diameter of RMn@PMO-IL
were 404 m2 gÀ1, 0.85 cm3, and 9.1 nm, respectively (Table S1 of
the Supporting Information). Moreover, the BJH isotherm of
this material showed a single peak with high intensity, indicat-
ing that the regularity of the nanostructure is maintained
under the applied reaction conditions. A TEM image of the
RMn@PMO-IL nanocatalyst showed a uniform rodlike structure
with high regularity, which is in good agreement with the ni-
trogen-sorption data (Figure 5). These observations strongly
confirm high stability of the catalyst during the reaction.
Experimental Section
Preparation of Mn@PMO-IL nanocatalyst
Firstly, the PMO-IL nanostructure was prepared according to our
previous procedure.[16d] Then 1 g of this material was homogene-
ously dispersed in DMSO and treated with Mn(OAc)3·2H2O
(0.5 mmol) at 508C for 6 h and then at 1008C for 2 h. After cooling
the reaction solution to room temperature, the product was fil-
tered and washed thoroughly with ethanol to remove unsupport-
ed Mn(OAc)3·2H2O. The resulting powder was dried at 608C for
12 h under vacuum to give Mn@PMO-IL.
General procedure for the Biginelli reaction with Mn@PMO-
IL nanocatalyst
Conclusion
A manganese-containing imidazolium-based periodic mesopo-
rous organosilica (Mn@PMO-IL) was prepared and character-
ized, and its catalytic performance was investigated in the Bigi-
nelli reaction. Nitrogen-sorption analysis and TEM images
showed high regularity and uniform pore size distribution of
the catalyst. The catalyst was effectively applied in the Biginelli
reactions of various aldehydes with urea and alkyl acetoace-
Aldehyde (1 mmol), alkyl acetoacetate (1 mmol), and urea
(1.5 mmol) were added to a round-bottomed flask with stirring at
258C for 5 min, and then 0.6 mol% of Mn@PMO-IL nanocatalyst
was added and the resulting mixture stirred at 758C in an oil bath
for an appropriate time (see Table 3). After completion of the reac-
tion, 5 mL of hot ethanol was added and the solution was filtered
while hot. The filtrate was cooled to precipitate a solid product.
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The crude product was then recrystallized from 96% ethanol to
give pure crystals in high to excellent yields.
General procedure for the recovery of Mn@PMO-IL nanoca-
talyst in the Biginelli reaction
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Typically, the Biginelli reaction of benzaldehyde (5 mmol), ethyl
acetoacetate (5 mmol) and urea (7.5 mmol), was performed in the
presence of 0.6 mol% of Mn@PMO-IL nanocatalyst under standard
conditions. After the completion of the reaction, 10 mL of hot eth-
anol was added and the solution was filtered, then the residue was
washed thoroughly with ethanol. The obtained material was dried
at 808C and denoted as recovered catalyst (RMn@PMO-IL). The
RMn@PMO-IL was reused under the same conditions as above for
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Acknowledgements
The authors thank the Yasouj University and the Iran National
Science Foundation (INSF) for supporting this work. We also
would like to acknowledge Dr. S. Hajati for his help in XPS
analysis.
Keywords: Biginelli reaction · heterogeneous catalysis · ionic
liquids · manganese · mesoporous materials
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Published online on February 12, 2014
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