Covalent anchoring of copper-Schiff base complex into SBA-15 as a heterogeneous catalyst for the synthesis of pyridopyrazine and quinoxaline derivatives

Article abstract of DOI:10.1016/j.catcom.2012.06.028

The ordered mesoporous silica SBA-15 was covalently anchored with copper (II) Schiff base complex to afford Cu(II)-Schiff base/SBA-15 catalyst. Powder X-ray diffraction, nitrogen adsorption-desorption analyses and TEM image confirm not only the retaining of support properties after anchoring but also the existence of void space in nanometer scale which result in good catalytic activity in the synthesis of pyridopyrazine and quinoxaline derivatives.

Full text of DOI:10.1016/j.catcom.2012.06.028

Catalysis Communications 27 (2012) 49–53
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Covalent anchoring of copper-Schiff base complex into SBA-15 as a heterogeneous
catalyst for the synthesis of pyridopyrazine and quinoxaline derivatives
b,
a
Ghasem Rezanejade Bardajee a, , Reihaneh Malakooti , Fereshteh Jami ,
⁎
⁎
Zeinab Parsaei b, Hassan Atashin b
a
Department of Chemistry, Payame Noor University, PO Box 19395‐3697, Tehran, Iran
b
Nanochemistry Research Laboratory, Department of Chemistry, University of Birjand, PO Box 97175‐615, Birjand, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2012
Received in revised form 25 June 2012
Accepted 29 June 2012
Available online 6 July 2012
The ordered mesoporous silica SBA-15 was covalently anchored with copper (II) Schiff base complex to afford
Cu(II)-Schiff base/SBA-15 catalyst. Powder X-ray diffraction, nitrogen adsorption–desorption analyses and
TEM image confirm not only the retaining of support properties after anchoring but also the existence of
void space in nanometer scale which result in good catalytic activity in the synthesis of pyridopyrazine and
quinoxaline derivatives.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Copper
SBA-15
Schiff base ligand
Heterogeneous catalyst
Pyridopyrazine
Quinoxaline
1. Introduction
Although different approaches have been developed for the synthesis
of these scaffolds, the condensation of appropriate 1,2-diamines
In recent years, heterogenization of homogenous catalysts has
attracted immense consideration [1,2]. As the discovery of ordered
mesoporous silica materials like MCM-41, MCM-48 and SBA-15, at-
tentions of many researchers have been focused on great properties
and applications of these materials especially as a solid support for
catalysts. In particular, SBA-15 is better than others because of the
properties such as larger pore size and thicker silica walls which
result in high hydrothermal stability [3].
Direct attachment of catalytic centers to the SBA-15 is a practical
way to produce heterogeneous catalyst, but it can cause instability
of catalyst because of leaching to the reaction mixture which is a
serious problem from industrial view. Avoiding this problem, grafting
of catalyst by covalent linkage can be used [4]. Moreover, flexible
organic linkages can provide more accessibility to the catalytic center
by increasing mobility of the loaded complex [5]. In addition to these
features, easy recovery and product separation can be possible by
heterogeneous catalysts based on supported Schiff bases complexes
[6,7].
with 1,2-ketones is the most important strategy [10–14], regarding
to the simplicity, conveniences, and generality of the method. In
the remainder of the manuscript, at first we report the synthesis
and characterization of a heterogeneous catalyst based on copper
bearing salen Schiff base ligands covalently anchored into SBA-15
(Cu(II)-Schiff base/SBA-15 or Cu/SBA-15), and then we will study
the catalytic performance of this catalyst for the green and efficient
synthesis of pyrido[2,3-b]pyrazines, pyrido[3,4-b]pyrazines, and
2,3-disubstituted quinoxalines in water.
2. Experimental section
2.1. Anchoring of Cu(II) Schiff base complex onto SBA-15 (Cu/SBA-15)
The Cu(II) Schiff base complex was prepared using procedure in
literature with a slight modification [15]. SBA-15 was synthesized
according to the method previously described by Zhao et al. [16].
Activated SBA-15 (1.5 g) was suspended in the methanol solution
containing Schiff base complex, and the mixture was stirred for
24 h. The solvent was removed using a rotary evaporator, and the
resulting green solid dried at 80 °C overnight. The final product was
washed with MeOH and deionised water until the washings were
colourless to ensure that the non-covalently grafted complex and
physisorbed metal species were removed. Further drying was carried
out in an oven at 80 °C for 8 h. To measure the amount of the copper
Nitrogen containing heterocycles such as pyridopyrazines and
quinoxalines have received considerable attention in pharmaceutical
industry because of their interesting therapeutic properties [8,9].
⁎ Corresponding authors. Tel.: +98 281 3336366; fax: +98 281 3344081.
E-mail addresses: rezanejad@pnu.ac.ir (G.R. Bardajee),
reihaneh.malakooti@gmail.com (R. Malakooti).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.06.028

50
G.R. Bardajee et al. / Catalysis Communications 27 (2012) 49–53
Fig. 2. TEM image of Cu(II)-Schiff base/SBA-15 with ordered channels.
Scheme 1. Preparation of Cu/SBA-15.
3. Results and discussion
loaded into SBA-15, 0.1 g of the catalyst was digested with HNO3 by
stirring at room temperature for a week. Then the mixture was fil-
tered to gain colourless sample. The total of the copper loaded into
SBA-15 was determined by atomic absorption spectroscopy which is
0.14 mmol/g (Scheme 1).
3.1. Characterization of Cu(II)-Schiff base/SBA-15
The X-ray diffraction patterns of SBA-15 and Cu(II)-Schiff base/
SBA-15 are shown in Fig. 1. In the XRD pattern of SBA-15 (Fig. 1b),
the strong peak at 2θ=0.9 belongs to (100) reflection and the latter
two weaker peaks at higher angles corresponding to (110) and
(200) reflections arise from ordered hexagonal unit cell of meso-
porous materials. The XRD pattern of Cu (II)-Schiff base/SBA-15
(Fig. 1a) shows three peaks as same as SBA-15, but with lower inten-
sities due to a decrease in the mesoscopic order and high contrast
matching between the silicate wall and organic moieties located
inside the SBA-15 pores [30]. Also the position of the peaks shifted
to lower angle that indicates the expansion of unit cell due to incor-
poration of the Cu (II)-Schiff base within the pores of SBA-15 [31].
The TEM image shows the retaining of 2D-hexagonal array of uni-
form linear channels with the typical honeycomb like of Cu(II)-Schiff
base/SBA-15 (Fig. 2). It is clearly exhibited lack of pore blocking or
agglomeration of copper metal. Also the pore size can determine to
be 8 0.5 nm for the catalyst.
2.2. General procedure for the synthesis of pyrido[2,3-b]pyrazines, pyrido
[3,4-b]pyrazines and 2,3-disubstituted quinoxalines in water
A round-bottomed flask equipped with a magnet and condenser was
charged with desired 1,2-diamine (1.0 mmol), 1,2-diketone (1.0 mmol),
water (2 mL) and catalyst Cu/SBA-15 (0.01 g (0.0014 mmol)). The
resulting mixture was stirred at reflux temperature for the appropriate
times, and the course of the reaction was monitored using TLC on silica
gel. For separation of the catalyst, the reaction mixture (at the end of
reaction) was filtered and the precipitates on the filter were dissolved
in ethanol or ethyl acetate. These solvents can dissolve the products
(and also organic starting materials), but the catalyst was remained
insoluble. After filtration of the later solution, the catalyst was
recovered.
Nitrogen adsorption/desorption isotherms and also size distribu-
tions of SBA-15 and Cu(II)-Schiff base/SBA-15 are shown in Fig. 3.
Both isotherms exhibited the type IV isotherms in the IUPAC
Spectral and physical data for all heterocycles were compared
with authentic samples and were in accord with the previously
reported data [17–29].
b
a
500
1000
b
a
0.2
0.4
0.6
0.8
1
Relative pressure (p/p0)
Fig. 3. Nitrogen adsorption/desorption isotherms incorporated with pore size distribu-
Fig. 1. XRD patterns of: (a) Cu(II)-Schiff base/SBA-15, and (b) SBA-15.
tions at inset of: (a) Cu(II)-Schiff base/SBA-15, and (b) SBA-15.

G.R. Bardajee et al. / Catalysis Communications 27 (2012) 49–53
51
Table 1
Fig. 4 shows the IR spectra of SBA-15 and Cu(II)-Schiff base/
SBA-15 catalysts. In addition to silica framework band such as
stretching vibrations of (Si\O\Si) bonds in the range of 1000 to
1200 cm−1 (Fig. 4a), Cu(II)-Schiff base/SBA-15 spectrum shows
C_N stretching bands at about 1615 cm−1, symmetry and asymme-
try vibrations of CH2 at 2922 and 2887 cm−1, respectively (Fig. 4b)
[7b]. This result qualitatively confirms the incorporation of Schiff
base complex in the silica support.
Characteristics of support and catalyst: C (M), initial concentration of copper species
(mmol/g); SBET, specific surface area (m2/g); VBJH, pore volume (cm3/g); DBJH, pore
diameter (nm); Ao, cell parameter (nm); D100, interplanar spacing (nm); W, wall thick-
ness (nm).
Materials
SBA-15
C (M) SBET VBJH DBJHa Ao
D100
9.87 2.63
Wb
–
679
445
1.32
0.86
8.78
8.74
11.41
12.23 10.58 3.49
Cu(II)-Schiff base/SBA-15 0.14
a
b
Calculated from the desorption branch. W=Ao−DBJH (Ao=2D100/√3)).
Table 2
Synthesis of desired heterocyclic derivatives catalysed by Cu/SBA-15a.
classification with narrow pore size distributions (see inset in Fig. 3).
The emergence of H1 hysteresis loop and relatively sharp adsorption
and desorption branches in the range of 0.7–0.9 p/p0 can be attrib-
uted to the capillary condensation of nitrogen gas in mesopores.
The incorporation of Schiff base complex in SBA-15 matrix causes a
decreasing in the volume of adsorbed nitrogen, sharpness of hystere-
sis loop, and reveal reduction in the pore volume. Textural parame-
ters of the materials are given in Table 1.
Entry
1
Diamine
Diketone
Product
Yeildb (%)
96
2
96
93
After anchoring of copper complex, a decrease in the specific sur-
face area, pore diameter, pore volume and increase in the wall thick-
ness was observed (Table 1). The TEM image also confirmed the
lack of pore blocking [32]. This issue results in good mass transfer of
substrate during the catalytic test.
3
4
98
97
5
6
7
98
(a)
(b)
94
8
9
97
96
1000
2000
3000
4000
10
11
99c
98c
95c
99c
99c
97c
99c
Wavenumber(cm-1)
Fig. 4. (a) IR spectra of SBA-15, and (b) Cu(II)-Schiff-base/SBA-15.
12
13
100
(a)
(b)
80
14
15
60
40
16
(c)
200
400
600
800
1000
a
All reactions were run under the following conditions: 2,3‐diaminopyridine 1
equiv), benzil (1.0 mmol, equiv) and catalyst (0.01 g) in water
Temperature (oC)
(1 mmol,
1
2
1
(2 mL) were heated for 2 h.
b
Fig. 5. Thermogravimetric analysis results of: (a) SBA-15, (b) Cu(II)-Schiff base/SBA-15,
and (c) Schiff base ligand (Salen).
Isolated yield.
The time of the reaction is 0.5 h.
c

52
G.R. Bardajee et al. / Catalysis Communications 27 (2012) 49–53
Scheme 2. Plausible mechanism for the synthesis of compound 3 in the presence of Pd/SBA-15.
For investigation on the amount of the catalyst loading and stabil-
absorption spectroscopy and no metal traces were found. In addition,
the use of copper acetate as a homogenous catalyst gave compound 3
in 45% yield.
ity of the materials, thermogravimetric analysis was used. The results
are depicted in Fig. 5. SBA-15 shows a slight weight loss due to the
loss of physically adsorbed water. The weight loss between 200 and
700 °C in Fig. 5b was assigned to the decomposition of covalently
bonded Schiff base ligand (about 5.2%) [33]. The amount of ligand
In the next step, the generality of the reaction was examined by
applying different diamines and 1,2-diketones. As one can see in
Table 2, good to excellent yields were obtained for Cu/SBA-15
catalysed reactions and most of them proceed very cleanly and no un-
desirable side products were observed. We found that the reactivity
of diamines is generally dominated by electronic effects. Accordingly,
the hetero-aromatic diamines showed less reactivity in comparison
with aromatic or aliphatic diamines (Table 2, entries 1–9, 13–16). A
plausible mechanism for the synthesis of compound 3 in the presence
of Pd/SBA-15 is shown in Scheme 2.
loading on the pores of SBA-15 was found to be 0.28 mmol/g−1
,
which is in good agreement with the metal content obtained from
atomic absorption spectroscopy.
3.2. Synthesis of heterocyclic derivatives
At first, to find the best efficiency of our catalytic system for the
synthesis of pyridopyrazines and quinoxalines, the conditions for
the reaction of 2,3-diaminopyridine 1 and benzyl 2 as the model reac-
tion were optimized. Different parameters such as solvent (n-hexane,
DMF, ethanol and water), amount of catalyst and temperature was
examined. The first attempt, in the absence of catalyst, gave no yields
of products in refluxing water or ethanol. The same results were
obtained for model reaction in the presence of SBA-15 alone. The
model reaction in the presence of Cu/SBA and in ethanol as solvent
under optimum conditions gave 67% yield. In addition, the yield of
model reaction in the presence of Cu/SBA and in acetic acid solvent
was 89%. In general, the applicability of the catalyst was mainly
found to be influenced by the quantity of the catalyst, solvent and
temperature used in the reaction. The good performance of the reac-
tion in water as reaction medium is a prominent advantage of the
planned procedure which significantly raises its green identifications.
In many reactions, such as the Diels–Alder cycloaddition, significant
rate enhancements are observed in water compared to organic sol-
vents. This acceleration has been attributed to many factors, including
the hydrophobic effect, enhanced hydrogen bonding in the transition
state, and the high cohesive energy density of water (550.2 cal/mL at
25 °C).
In addition, we successfully applied this methodology to
a
large-scale preparation of compound 3. In this regard, the reaction
of 1 (10 mmol) and 2 (10 mmol), using 10 mol % Cu/SBA-15 as cata-
lyst, provided compound 3 in 93% yield.
To test the lifetime and the reusability of the heterogeneous sys-
tem, a series of experiments were planned for the model reaction
under optimized conditions. After completion of the first reaction
with 96% yield, the catalyst was recovered by filtration, washed with
ethanol and dried at 80 °C for 60 min. The recovered catalyst was
employed in another reaction run and we found that the Cu/SBA-15
has outstanding reusability over 6 successive runs (Table 3).
4. Conclusions
We show the synthesis and characterization of a heterogeneous
catalyst by anchoring of copper complex containing N\O chelating
Schiff base ligand through covalent bond within nanoreactor of
SBA-15. This catalyst which can be easily prepared and recycled is
an active catalyst for the synthesis of pyridopyrazine and quinoxaline
derivatives in excellent yields under green conditions.
In a typical procedure using optimized conditions, the reaction of
2,3-diaminopyridine 1 (1.0 mmol, 1.0 equiv) and benzil 2 (1 mmol,
1 equiv) in the presence of Cu/SBA-15 (0.01 g) in refluxing water
for 2 h, afforded compound 3 in 99% isolated yield. Furthermore,
after the applying of the catalyst for the synthesis of compound 3
(model reaction), we tested the hot filtrate of the reaction by atomic
Acknowledgment
Authors are thankful to the University of Pyame Noor and University
of Birjand for the funding of this study.
Appendix A. Supplementary data
Table 3
Reuse of the catalyst for the synthesis of compound 3.
1H and 13C NMR spectra for all products. Supplementary data to
this article can be found online at http://dx.doi.org/10.1016/j.catcom.
2012.06.028.
Run
1
2
3
4
5
6
Yeild (%)
96
96
95
95
94
94

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53
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