Core-shell magnetic mesoporous N-doped silica nanoparticles: solid base catalysts for the preparation of some arylpyrimido[4,5-b]quinoline diones under green conditions

Article abstract of DOI:10.1039/d0ra06546c

Nowadays, the application of solid base catalysts as a perfect replacement for homogenous basic catalysts has attracted the attention of researchers. In this study, core-shell magnetic mesoporous N-doped silica nanoparticles N(xwt%)-MSN, as a heterogeneou

Full text of DOI:10.1039/d0ra06546c

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Core–shell magnetic mesoporous N-doped silica
nanoparticles: solid base catalysts for the
preparation of some arylpyrimido[4,5-b]quinoline
diones under green conditions†
Cite this: RSC Adv., 2020, 10, 35397
a
b
Shekofeh Neamani,a Leila Moradi
and Mingxuan Sun
*
Nowadays, the application of solid base catalysts as a perfect replacement for homogenous basic catalysts
has attracted the attention of researchers. In this study, core–shell magnetic mesoporous N-doped silica
nanoparticles N(x wt%)-MSN, as a heterogeneous base catalyst, were synthesized. The N(x wt%)-MSN
composite was fabricated by adding different amounts of diethanolamine as a source of nitrogen,
besides using tetraethylorthosilicate as a precursor of silica. The as-prepared catalyst was employed
efficiently for the synthesis of some arylpyrimido[4,5-b]quinoline-dione derivatives under green
conditions. The highly efficient catalyst N(1.3 wt%)-MSN was characterized via XRD, FESEM, HRTEM, BET
and XPS techniques, and the results of these analyses proved that the nitrogen was doped into the silica
structure. Also, the results demonstrated the core–shell structure of the as-synthesized composite.
Received 28th July 2020
Accepted 31st August 2020
DOI: 10.1039/d0ra06546c
rsc.li/rsc-advances
passing ammonia gas on the surface of mesoporous silica under
harsh reaction conditions. Doping nitrogen by this technique
Introduction
could be performed at high temperatures, approximately 1173–
1423 K; however, such methods require high-tech tools.17,18
Another less developed method for nitrogen doping in different
frameworks is using a precursor containing nitrogen atoms. In
this method, during the synthesis of the intended framework,
a precursor containing nitrogen atoms is added to the mixture.
Then, the mixture is calcined in a furnace to produce N-doped
structures. In 2019, Ghiaci et al. prepared nitrogen-doped
carbon networks by the pyrolysis of salen complexes in the
argon atmosphere.20 Caruso et al. in 2016 synthesized porous
nitrogen-modied titania (N-TiO2) using diethanolamine (DEA)
and titanium(IV) butoxide. The prepared N-TiO2 was applied for
dye removal at room temperature.21 In our previous work in
2020, we reported the preparation of nitrogen-doped silica
materials using DEA as a precursor of nitrogen in the silica
framework. The prepared catalyst was applied as a basic
heterogeneous catalyst for the synthesis of some dihydropyr-
idine derivatives.1
Recently, core–shell structures have attracted considerable
attention due to their great applications as a catalyst in reac-
tions. Unlike typical catalysts, the core–shell materials can
incorporate several functions into a system for particular uses.
Between the core–shell catalysts, composites containing
magnetic cores have numerous applications such as selective
separation, synthesis of ne chemicals, drug delivery, chroma-
tography, and biological sensors.22,23
The development of new and efficient nanostructured base
catalysts with accessible active sites is an important eld in
chemistry.1,2 Up to now, various types of heterogeneous solid
base catalysts based on hydrotalcites,3,4 zeolites,4 basic ionic
liquids4 and organic/inorganic hybrids5 have been reported. In
this regard, use of mesoporous silica (with unique characteris-
tics) as a support for the synthesis of heterogeneous basic
catalysts has attracted much consideration.6–8 Some important
properties of mesoporous silica include high surface area,
thermal stability, narrow pore size distribution, controllable
porosity and morphology.9–13 The surface modication of mes-
oporous silica frameworks with basic characteristic fabricated
using basic organosilane reagents has been reported.14,15 Irre-
versible participation of the anchored organic species in the
reaction and also, structural destruction at elevated tempera-
tures in air atmosphere or in the presence of oxidative agents
limit the application of such catalysts.1,5,16 Doping nitrogen in
the exible silicate framework and placing it in the form of
bridged Si–NH–Si groups is a novel technique for incorporating
basic sites into silica nanoparticles.17–19 These N-doped catalysts
show appropriate basic activity similar to the typical base
catalysts in reactions. The N-doped silica could be obtained by
aDepartment of Organic Chemistry, Faculty of Chemistry, University of Kashan, P.O.
Box 8731753153, Kashan, I. R. Iran. E-mail: l_moradi@kashanu.ac.ir
bSchool of Materials Engineering, Shanghai University of Engineering Science, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra06546c
In this study, to prepare the magnetic nitrogen-doped silica
core–shells, we used DEA as a source of nitrogen, and tetraethyl
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Scheme 1 Synthesis of the N(x wt%)-MSN composite.
orthosilicate (TEOS) for the in situ synthesis of N-doped silica
The amount of doped nitrogen in the as-prepared catalysts
shell on the magnetic cores. The as-prepared catalyst was used was investigated by an elemental analysis. According to the
as a strong heterogeneous base catalyst for the synthesis of obtained data, when 0.5, 1.0 and 1.5 mL of DEA were used, 0.6,
some quinoline derivatives under green conditions [4,5-b]. The 1.3 and 1.7 wt% of N atoms were doped, respectively, in the
quinoline derivatives are abundant in nature and exhibit mesoporous silica nanostructures. Based on the experimental
numerous biological properties such as antitumor, antima- results from the synthesis of some arylpyrimido[4,5-b]
larial, antibacterial, anthelmintic, anti-inammatory, anti- quinoline-dione derivatives, the catalyst containing 1.0 mL of
platelet and antiasthmatic.24–30
the DEA precursor (N(1.3 wt%)-MSN) shows the best perfor-
mance as a heterogeneous catalyst. Accordingly, the structural
properties of N(1.3 wt%)-MSN has been discussed further.
The morphology of the N(1.3 wt%)-MSN catalyst was inves-
tigated via FESEM (Fig. 1). According to the results, generally,
two different types of morphologies (spherical and amorphous
aggregates) are observed for the as-synthesized catalyst. The
presence of aggregated structures can be related to the
increased hydrolysis rate of the silica precursor in the reaction
medium. In fact, the presence of DEA during the synthesis of
silica nanoparticles causes an effect on the hydrolysis rate of
silica precursors, leading to amorphous silica with spherical
shapes.
Results and discussion
Catalyst characterization
In this work, magnetic N-doped silica nanoparticles were
prepared and their efficiency was investigated in the synthesis
of arylpyrimido[4,5-b]quinoline dione derivatives. Scheme 1
shows the preparation method of the core–shell magnetic
mesoporous N-doped silica nanoparticles or N(x wt%)-MSN in
which x represents the amount of doped nitrogen in the nal
structure. During the preparation of mesoporous silica nano-
particles (using TEOS precursor and different amounts of DEA),
the DEA molecules are trapped into the silica frameworks.
Calcining the as-obtained samples at 550 ꢀC resulted in the
doping of different amounts of doped nitrogen atoms into the
structure of silica magnetic nanoparticles (Scheme 1).
The HRTEM analysis of N(1.3 wt%)-MSN is presented in
Fig. 2. It can be seen that the obtained images conrm the core–
shell morphology of the magnetic mesoporous N-doped silica
nanostructures. Moreover, it is clear that magnetic
Fig. 1 FESEM of N(1.3 wt%)-MSN.
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Fig. 2 HRTEM analysis of synthesized N(1.3 wt%)-MSN.
nanoparticles are well trapped within the mesoporous silica an amorphous silica structure both in MSN and N(1.3 wt%)-
spherical shells to form core–shell structures. MSN can be seen. The amorphous peak in the N-doped meso-
To investigate the crystallinity of the structure of the porous silica nanoparticle showed a shi to a lower 2q,
magnetic N-doped nanoparticles, powder X-ray diffraction compared to that in mesoporous silica nanoparticles. This
(XRD) patterns of MSN and N(1.3 wt%)-MSN were obtained. As proved a change in the morphology and d-spacing of the silica
shown in Fig. 3, a broad peak around 2q z 21–26ꢀ, representing structure, which is a result of doping nitrogen into the silica
Fig. 3 XRD patterns of the as-synthesized N(1.3 wt%)-MSN and MSN.
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Fig. 4 (a) Nitrogen adsorption–desorption isotherms of MSN and N(1.3 wt%)-MSN, (b) BJH plots of MSN and N(1.3 wt%)-MSN.
frameworks. The other peaks in the XRD patterns of MSN and low concentration of nitrogen atoms in the N(1.3 wt%)-MSN
N(1.3 wt%)-MSN were associated with Fe3O4 nanoparticles and composite, the deconvolution of it does not offer specic
the presence of some Fe2O3 nanoparticles. The over oxidation of information about the oxidation states of nitrogen. However, it
Fe3O4 nanoparticles in the furnace caused the formation of is clear that the Si–N–H bond at 400.2 eV has the maximum
some Fe2O3 nanoparticles.18,31,32
involvement.32–35
In Fig. 4, nitrogen adsorption–desorption isotherms of MSN
and N(1.3 wt%)-MSN nanocomposite, and BJH plots of them are
compared. According to the obtained results, MSN shows
a surface area of 361 m2 gꢁ1 with a total pore volume of 0.37 cm3
gꢁ1 and a mean pore diameter of 4.1 nm. However, N(1.3 wt%)-
MSN showed a surface area of 387 m2 gꢁ1, a total pore volume of
0.35 cm3 gꢁ1 and a mean pore diameter of 3.6 nm. Conse-
quently, the as-prepared N(1.3 wt%)-MSN composite had
a higher surface area with a less mean pore diameter, which
established the presence of nitrogen atoms in the nal meso-
porous silica structure.
Finally, to establish the nitrogen doping into the silica
frameworks, X-ray photoelectron spectroscopy was performed
on the N(1.3 wt%)-MSN composite (Fig. 5). The deconvolution
of the Si 2p spectrum reveals two main peaks at 103.3 and
104.5 eV related to Si–N and Si–O bonds, respectively (Fig. 5a).
The presence of Si–N bonds successfully proved the doping of
nitrogen atoms in silica frameworks. In Fig. 5b, the obtained N
1s spectrum of N(1.3 wt%)-MSN composite is shown. Due to the
Catalytic performance
To obtain proper and green conditions for the synthesis of some
arylpyrimido[4,5-b]quinoline derivatives, the reaction of barbi-
turic acid, 3-nitrobenzaldehyde and 4-methoxy aniline was
considered as a model reaction. All of the effective factors
including the solvent, catalyst and temperatures were studied
for determining the optimized conditions. In this regard, the
efficiency of the as-synthesized solid base catalysts and some of
the homogeneous basic catalysts in different solvents was
investigated. The model reaction was accomplished in various
solvents using some homogenous base catalysts.
Results from Table 1 show that when Et3N or L-proline was
used as a catalyst, the yield of the product was low (in various
solvents), and there no product was formed when the reaction
was performed under catalyst-free conditions (entry 1) and
when MSN was used as a catalyst (entry 14). The study on the
relationship between the wt% of N atoms doped in the catalyst
structure and catalyst efficiency was performed in reactions
Fig. 5 XPS analysis for N(1.3 wt%)-MSN: (a) deconvolution of the Si 2p, (b) spectrum of N 1s.
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Table 1 Optimization of the reaction conditions for the synthesis of arylpyrimido[4,5-b]quinolinesa
Entry
Solvent
Base
Temperature
Yield (%)
1
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
EtOH
EtOH
EtOH
H2O
CH3CN
EtOH
EtOH
EtOH
—
Reux
Reux
Reux
Reux
Reux
Reux
Reux
Reux
N. R.
62
65
58
52
56
70
59
70
76
87
72
N. R.
75
82
71
86
Et3N (20 mol%)
L-Proline (20 mol%)
L-Proline (20 mol%)
L-Proline (20 mol%)
N(0.6 wt%)-MSN (20 mg)
N(1.3 wt%)-MSN (20 mg)
N(1.7 wt%)-MSN (20 mg)
N(1.3 wt%)-MSN (20 mg)
N(1.3 wt%)-MSN (20 mg)
N(1.3 wt%)-MSN (20 mg)
N(1.3 wt%)-MSN (20 mg)
MSN (20 mg)
EtOH/H2O (9 : 1)b
EtOH/H2O (7 : 3)b
EtOH/H2O (5 : 5)b
EtOH/H2O (3 : 7)b
EtOH/H2O (5 : 5)b
EtOH/H2O (5 : 5)b
EtOH/H2O (5 : 5)b
EtOH/H2O (5 : 5)b
EtOH/H2O (5 : 5)b
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
80 ꢀ
70 ꢀ
90 ꢀ
C
C
C
C
C
C
C
C
C
N(0.6 wt%)-MSN (20 mg)
N(1.7 wt%)-MSN (20 mg)
N(1.3 wt%)-MSN (20 mg)
N(1.3 wt%)-MSN (20 mg)
a
Reaction conditions: barbituric acid (1 mmol), 3-nitrobenzaldehyde (1 mmol) and 4-methoxy aniline (1 mmol) in 10 mL solvent at the reux for
12 h. b Amounts of the applied volume for each solvent.
using 20 mg of every as-synthesized catalyst. It can be seen in that the presence of electron-donating groups (such as methoxy)
entries 7–9 that N(1.3 wt%)-MSN shows a higher yield in EtOH. on aniline increases the yield of products, whereas the presence
In the next step, using water as a co-solvent in the reaction of electron withdrawing groups reduces the reaction yield. Also,
medium was investigated. According to the obtained results, benzaldehydes bearing electron withdrawing groups produced
the volume ratio of 1 : 1 of H2O/EtOH was better than that of higher reaction yields and the presence of electron donating
absolute ethanol when N(1.3 wt%)-MSN was used (entry 12). It groups in the structures lead to lowering of the progress of the
can be seen in entry 16 that the catalytic performance of reaction. FTIR, 1H NMR, and 13C NMR spectra of the as-
N(1.7 wt%)-MSN was lower than that of N(1.3 wt%)-MSN. This synthesized products are presented in the ESI.†
could be related to increasing the basicity of the reaction
medium during the synthesis of N-doped mesoporous silica
nanoparticles, causing it to form the catalyst with different
morphologies and catalytic activities.
Proposed mechanism
According to other similar works,24 the following mechanism
for the multicomponent synthesis of arylpyrimido[4,5-b]
quinolone-dione is suggested (Scheme 2). It can be seen in
Scheme 2 that the Knoevenagel condensation of barbituric acid
To investigate the effect of temperature on the progress of
the reaction, the catalytic activity of N(1.3 wt%)-MSN was
studied at different temperatures. The obtained results showed
and benzaldehyde in the presence of
a basic catalyst
ꢀ
that the catalyst exhibited more activity at 80 C (entry 12).
(N(1.3 wt%)-MSN) resulted in the formation of intermediate I.
Furthermore, the Michael addition of the aniline through
a nucleophilic attack on structure I produced intermediate II.
Subsequently, cyclocondensation and water removal in inter-
mediate III afforded the desired product.
Consequently, the as-synthesized N(1.3 wt%)-MSN
composite showed the best yield (87%) in 10 mL H2O/EtOH with
ꢀ
a volume ratio of 1 : 1 at 80 C (entry 12).
For extended studies on the catalytic activity of N(1.3 wt%)-
MSN in the synthesis of arylpyrimido[4,5-b]quinolines, the
reaction of barbituric acid (or thiobarbituric acid) and aniline
and benzaldehyde derivatives was examined in the presence of
Reusability of the catalyst
20 mg of the catalyst (Table 2). The reaction yields exactly prove Reusability of the heterogeneous catalysts was studied through
subsequent runs. In this way, aer the completion of the
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Table 2 Investigation of the catalytic activity of N(1.3 wt%)-MSN for the synthesis of some arylpyrimido[4,5-b]quinolone-dione derivativesa
Entry
Time (h)
Dicarbonyl
Aldehyde
Aniline
Product
Yieldb (%)
87/75/82c
1
9
2
11
82
3
4
5
14
81
79
77
8
10
6
16
89
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Table 2 (Contd.)
Entry
Time (h)
Dicarbonyl
Aldehyde
Aniline
Product
Yieldb (%)
7
12
10
85
8
9
82
75
14
11
10
78
11
24
<20
a
Reaction conditions: dicarbonyl (1 mmol), aldehyde (1 mmol), aniline (1 mmol) in 10 mL EtOH/H2O (5 : 5) in the presence of 20 mg N(1.3 wt%)-
MSN at 80 ꢀC. b Isolated yield. c Yield of product using N(0.6 wt%)-MSN and N(1.7 wt%)-MSN, respectively.
reaction, the solid base catalyst was separated using a magnetic that there was no considerable change in the catalyst perfor-
eld, washed with water and ethanol several times and applied mance and yield of the desired product even aer four cycles
once more in the same reaction. The obtained results exhibited (Fig. 6).
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Scheme 2 The suggested mechanism for the synthesis of arylpyrimido[4,5-b]quinolone-diones.
by using a Carlo ERBA model EA 1108 analyzer. X-ray photo-
electron spectroscopy (XPS) analysis was performed to prove the
nitrogen doping into the silica structure using a Perkin Elmer
RBD upgraded system (PHI-5000C ESCA) with Al/Mg-K radia-
tion. FTIR spectra were recorded using the KBr pellet method in
the range of 400–4000 cmꢁ1 on a Perkin-Elmer 781 spectro-
photometer, and an impact 400 Nicolet FTIR spectrophotom-
eter was employed to assign the functional groups of the as-
synthesized derivatives of 5-arylpyrimido[4,5-b]quinoline-
diones. Furthermore, investigation on the structural conrma-
tion of 5-arylpyrimido[4,5-b]quinoline-diones derivatives was
carried out by 1H NMR and 13C NMR methods, which were
performed in the DMSO-d6 solvent on a Bruker DRX-400 spec-
trometer. The melting points of all 5-arylpyrimido[4,5-b]
quinoline-diones derivatives were determined using an Elec-
trothermal Mk3 apparatus.
Experimental
Materials and methods
All of the chemicals were obtained from Sigma-Aldrich and
Merck companies and used without further purication. To
investigate the phase and crystalline structure of the as-
synthesized catalysts, the N2 adsorption/desorption analysis
(BET) was employed to calculate the specic surface area of the
obtained catalysts using an automated gas adsorption analyzer
ꢀ
(Tristar 3000, Micromeritics) at ꢁ196 C. X-ray diffraction was
performed using a Philips X'pert MPD diffractometer with a Cu
operating at a current of 100 mA and a voltage of 45 kV, with the
Cu-Ka radiation (l ¼ 0.154056 nm) at the 2q range of 10–80ꢀ and
scanning at the speed of 0.05 degree per minute. The surface
morphology of the as-prepared catalysts was studied using
a Mira3-XMU eld emission scanning electron microscope
(FESEM) with a scanning electron electrode operating at 15 kV.
The morphology of the N-doped mesoporous silica nano-
particles was observed using high-resolution TEM on a FEI
Tecnai F20 USA microscope at an accelerating voltage of 200 kV.
Also, to measure the amounts of nitrogen amount in the as-
synthesized core shells, the elemental analysis was performed
Synthesis of magnetic nanoparticles
The synthesis of magnetic nanoparticles was performed
following our previous work.1 Initially, 0.0845 g FeCl2$4H2O and
0.2 g FeCl3$6H2O were dissolved into 50 mL of deionized water
ꢀ
at 85 C. Then, 0.6 mL of ammonia was added to the obtained
solution and stirred under the nitrogen atmosphere at 85 ꢀC for
30 min. Aer ltering, the resulting magnetic nanoparticles
were washed with DI water and ethanol, and nally dried at
room temperature.
Synthesis of core–shell magnetic mesoporous N-doped silica
nanoparticles (N-MSN)
0.1 g of the as-prepared magnetic nanoparticles and 0.7 g CTAB
(as a structural directing agent) were added to 160 mL of
deionized (DI) water and sonicated for 10 min to form a good
dispersion. Next, the mixture was added to 220 mL of ethanol
comprising 1.2 mL of the NH3 solution. Aerward, DEA as
a source of nitrogen in different amounts (0.5, 1.0 and 1.5 mL)
was added in the solution. Then, the TEOS solution (0.4 mL
TEOS in 10 mL ethanol) was added slowly to the mixture under
stirring. The resultant solution was stirred for 12 h. Eventually,
Fig. 6 Reusability of the N(1.3 wt%)-MSN catalyst in the multicom-
ponent reaction.
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the prepared magnetic solid was separated by a magnet and
washed with ethanol several times. The attained solid was
calcined in the furnace at 550 ꢀC for 6 h (in an air atmosphere at
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¨
core–shell magnetic mesoporous N-doped silica nanoparticles
were synthesized and labeled as N(x wt%)-MSN (x ¼ the content
of nitrogen in the core–shell magnetic mesoporous N-doped
silica nanoparticles, as measured by the elemental analysis:
0.6, 1.3 and 1.7 wt%).
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To prepare the magnetic mesoporous silica nanoparticles (or,
namely, MSN), all the procedures were followed as per the
above-described method. However, the difference was that DEA
was not used. The synthesized solid was named as MSN.
Catalyst performance for the synthesis of arylpyrimido[4,5-b]
quinoline derivatives
For preparing the arylpyrimidoquinoline derivatives, 1 mmol of
aniline derivatives, 1 mmol of barbituric acid and 1 mmol of
benzaldehyde in the presence of 20 mg of the synthesized
catalyst in water and ethanol solvents (at a volumetric ratio 1 : 1
at 80 ꢀC) were reacted. The reaction progress was monitored by
TLC, and aer the completion of the reaction, the magnetic
catalyst was removed from the reaction medium by a magnet.
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Conclusion
In this study, mesoporous magnetic N-doped silica nano-
particles as a heterogeneous basic catalyst (N(x wt%)-MSN) was
prepared. In the presented process, DEA was used as a nitrogen
source and TEOS as a silica precursor. The as-prepared catalyst
was applied successfully for the synthesis of some arylpyrimido
[4,5-b]quinoline-dione derivatives in high yields. The nitrogen
doping by this method is a new route for developing highly
efficient solid base catalysts.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
The Research Council of Kashan University should be appreci-
ated for supporting this work.
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