An iron Schiff base complex loaded mesoporous silica nanoreactor as a catalyst for the synthesis of pyrazine-based heterocycles

Article abstract of DOI:10.1007/s11243-013-9772-y

An iron Schiff base complex was encapsulated in SBA-15 mesoporous silica to afford a Fe(III)-Schiff base/SBA-15 heterogeneous nanocatalyst for the synthesis of pyridopyrazine and quinoxaline heterocycles. These reactions proceeded in water with excellent yields. The catalyst was characterized by physico-chemical and spectroscopic methods and found to retain the characteristic channel structures of the SBA-15, allowing good accessibility of the encapsulated metal complex.

Full text of DOI:10.1007/s11243-013-9772-y

Transition Met Chem (2014) 39:47–54
DOI 10.1007/s11243-013-9772-y
An iron Schiff base complex loaded mesoporous silica nanoreactor
as a catalyst for the synthesis of pyrazine-based heterocycles
Reihaneh Malakooti
Somayyeh Hadizadeh
Hadiseh Khanjari
•
Ghasem Rezanejade Bardajee
Hassan Atashin
•
•
•
Received: 2 July 2013 / Accepted: 7 October 2013 / Published online: 23 October 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract An iron Schiff base complex was encapsulated
in SBA-15 mesoporous silica to afford a Fe(III)-Schiff
base/SBA-15 heterogeneous nanocatalyst for the synthesis
of pyridopyrazine and quinoxaline heterocycles. These
reactions proceeded in water with excellent yields. The
catalyst was characterized by physico-chemical and spec-
troscopic methods and found to retain the characteristic
channel structures of the SBA-15, allowing good accessi-
bility of the encapsulated metal complex.
physical stability). Hence, SBA-15 is noteworthy as a solid
material for the support of molecular catalysts, transforming
them into heterogeneous catalysts with the associated
advantages such as easy catalyst separation, possible catalyst
recycling, and high activity and selectivity [1, 2].
Iron(III) complexes of Schiff base ligands have excel-
lent catalytic performances in a wide range of reactions [3].
The covalent anchoring of such complexes onto mesopor-
ous materials with large pore diameters such as SBA-15 is
a fascinating area of current research [4]. The use of
organic spacer to link the iron(III) catalytic center to the
solid support can provide good accessibility without steric
effects. In addition, strong covalent bonds can ensure sta-
bility of the catalyst, ensuring that the Schiff base is
retained during the catalytic cycles [5].
Introduction
Recently, the discovery of ordered mesoporous silica mate-
rials with pore sizes ranging from 2 to 50 nm has attracted
intense attention, due to their features such as high specific
surface area, uniform pore size distribution, wide variety of
surface functionalization, and particularly valuable appli-
cations in catalysis [1, 2]. In Particular, SBA-15 (Santa
BArbara No. 15) is known as one of the most interesting
mesoporous silicas because of its outstanding properties (i.e.,
thicker pore walls, larger pore sizes, and high thermal and
Nitrogen-containing heterocycles such as pyridopyrazines
and quinoxalines have received considerable attention from
the pharmaceutical industry because of their interesting
therapeutic properties [6, 7]. Although different approaches
have been developed for the synthesis of these heterocycles,
thecondensationof 1,2-diamines with 1,2-ketonesis themost
important strategy [8–14]. Here, we describe a simpleprocess
for the preparation of a heterogeneous iron catalyst by teth-
ering a Schiff base complex on SBA-15 (Fe(III)-Schiff base/
SBA-15) and its application as a heterogeneous catalyst for
the synthesis of pyrido[2,3-b]pyrazines, pyrido[3,4-b]pyra-
zines, and 2,3-disubstituted quinoxalines in water.
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-013-9772-y) contains supplementary
material, which is available to authorized users.
R. Malakooti (&) Á S. Hadizadeh Á H. Atashin
Nanochemistry Research Laboratory, Department of Chemistry,
University of Birjand, PO Box 97175-615, Birjand, Iran
e-mail: rmalakooti@birjand.ac.ir;
Experimental
reihaneh.malakooti@gmail.com
Materials and methods
G. R. Bardajee (&) Á H. Khanjari
Department of Chemistry, Payame Noor University,
PO Box 19395-3697, Tehran, Iran
e-mail: rezanejad@pnu.ac.ir
Poly(ethylene glycol)-block-poly(propylene glycol)-block-
poly(ethylene glycol) (P123) triblock co-polymer, iron(III)
123

4
8
Transition Met Chem (2014) 39:47–54
nitrate, salicylaldehyde, (3-aminopropyl) trimethoxysilane
APTMS), tetraethyl orthosilicate (TEOS), HNO , hydro-
added, and the mixture was stirred for 3 h. Finally, the
mixture was filtrated to remove any solid materials, and
then the solvent was evaporated by using a rotary evapo-
rator to afford Schiff base complex compound (0.38 g,
90 % yield).
(
3
chloric acid, 1,2-phenylenediamines, 2,3-diaminopyridine,
,4-diaminopyridine, and 1,2-dicarbonyl compounds were
3
commercially available (from Aldrich, Acros or Merck)
and used as received. The solvents were of analytical grade
and used as received. Silica gel (Merck, grade 9385,
˚
Synthesis of SBA-15
2
30–400 mesh, 60 A) for column chromatography was
used as received. Synthesis of the heterocycles was fol-
lowed by TLC on silica gel plates (Merck, silica gel 60
F254, ready to use), using dichloromethane:methanol (9:1)
or n-hexane:ethyl acetate (1:3) as eluents. The same eluents
were used for column chromatography.
SBA-15 was prepared as previously reported [16]. 2.0 g of
pluronic P123 was first dissolved in a mixture of (15 g)
H O plus 2 M HCl (60 g), and then, 4.25 g of TEOS
2
(
tetraethyl orthosilicate) was added with vigorous stirring
at 40 °C. After that, the mixture was stirred at 40 °C for
4 h and then heated at 100 °C for another 24 h under
Melting points were recorded using a Buchi B540
13
2
1
melting point apparatus and are uncorrected. H and
C
static conditions. Finally, the sample was filtered off,
NMR spectra were recorded at room temperature on Bru-
washed with distilled water, air-dried at room temperature,
ker AC 400 and 500 MHz spectrometers using CDCl or
3
and then calcined at 600 °C in air for 6 h (heating ramp of
1
DMSO-d as the NMR solvents. H NMR spectra are ref-
6
-1
2
°C min ) to remove the template.
1
3
erenced to tetramethylsilane, and C NMR spectra are
referenced from the residual solvent peak. IR spectra were
recorded from samples embedded in KBr pellets using
Jasco 4200 FTIR or Perkin–Elmer IR-781 spectropho-
tometers. X-ray diffraction (XRD; Bruker D8ADVANCED
Immobilization of the complex onto SBA-15
SBA-15 was activated by refluxing in hydrochloric acid
(6 M) for 6 h, to hydrolyze the siloxane Si–O–Si bonds to
Si–OH species for better anchoring of the iron complex.
The sample was then washed thoroughly with deionized
water and heated at 120 °C under vacuum overnight. The
activated SBA-15 (1.5 g) was added to a solution of the
complex (0.38 g) in 20 mL methanol, and the mixture was
stirred for 24 h. The solvent was then evaporated using a
rotary evaporator, and the solid was dried at 80 °C over-
night. The product was washed with methanol and deion-
ized water until the washings were completely colorless.
Finally, the product was dried in an oven at 80 °C for 8 h
(Scheme 1).
˚
with Ni-filtered Cu Ka radiation at 1.5406 A) data were
-
1
recorded with a speed of 28 min and a step of 0.058.
TEM images were collected on a Philips CM10 electron
microscope at 100 kV. N adsorption/desorption isotherms
2
were obtained at 350 °C with a Quantachrome Autosorb-1
apparatus. Before measurements, the samples were out-
gassed at 120 °C for 12 h. The specific surface area and the
pore size distributions were obtained from the desorption
branch of the isotherms, using the Brunauer–Emmett–
Teller (BET) method and Barrett–Joyner–Halenda (BJH)
analysis, respectively. The amount of iron species in the
catalyst was determined with a Shimadzu AA-6300 flame
atomic absorption spectrometer. Elemental analysis (CHN)
was performed by combustion on a Fisons EA1108 ele-
mental analysis apparatus. To assess the amount of
anchored material and its stability on parent SBA-15,
thermogravimetric analysis was carried out with a TGA/
DTA Shimadzu-50 instrument equipped with a platinum
pan. The thermograms so obtained were more or less
constant with increasing temperature from 25 to 1,000 °C
with rate of 10 °C/min in air.
General procedure for the synthesis of heterocycles
A round-bottomed flask equipped with a magnetic stirrer
and condenser was charged with the required 1,2-diamine
(1.0 mmol), 1,2-diketone (1.0 mmol), water (2 mL), and
Fe(III)-Schiff base/SBA-15 catalyst (0.01 g, 0.0014 mmol)
(Scheme 2). The resulting mixture was stirred under reflux
for the appropriate time, and the course of the reaction was
monitored using TLC on silica gel. Finally, the reaction
mixture was cooled and the water was removed under
vacuum. The residue was dispersed in ethanol or ethyl
acetate, and the resulting mixture was filtered to remove
the catalyst. The catalyst was recovered and the product
was purified by column chromatography or recrystallisa-
tion. Spectral and physical data for all the heterocycles
were compared with authentic samples and were in accord
with the previously reported data [17–26].
Synthesis of the complex
The Fe(III) catalyst was synthesized by following the
procedure reported by Chisem et al. [15]. Salicylaldehyde
(
0.12 g, 1 mmol) and APTMS (0.18 g, 1 mmol) were
added to methanol (20 mL). The solution appeared yellow
due to the imine formation, and it was stirred for 3 h at
room temperature. Fe(NO ) (0.12 g, 0.5 mmol) was then
3
3
1
23

Transition Met Chem (2014) 39:47–54
49
CHO
OMe
OMe
Si
+
MeO
MeOH
OMe
N
Si
NH2
OH
OMe
OMe
OH
Fe(NO3)3
OMe
OMe
N
N
Si
OMe
Fe NO3-
SBA-15
O
O
O
O
NO - Fe
3
OMe
Si
N
OMe
OMe
N
Scheme 1 Preparation of Fe(III)-Schiff base/SBA-15
Scheme 2 General procedure for the synthesis of pyrazine-based
heterocycles in water
Results and discussion
Characterization of the supported complex
Both the parent SBA-15 and Fe-Schiff base/SBA-15 cata-
lyst display well-resolved XRD patterns with a sharp peak
and two weak peaks, which are assigned to the (100),
(
110), and (200) reflections, respectively, associated with
Fig. 1 XRD patterns of: a SBA-15 and b Fe(III)-Schiff base/SBA-15
the typical two-dimensional hexagonal symmetry of the
SBA-15 material (Fig. 1). For the supported catalyst, peaks
were observed at about 0.96, 1.56, and 1.76 on the 2h scale
(
Fig. 1b). In comparison with SBA-15, the positions of
framework and organic moieties located inside the channel
and/or a decrease in the mesoscopic order of the materials
[28].
these reflections are shifted to lower angle (or higher
d-value), indicating expansion of the unit cell parameters
due to the incorporation of the complex within SBA-15
In excellent agreement with the XRD data, the TEM
image of the supported complex shows retention of the
hexagonal structure following grafting of the complex onto
SBA-15 (Fig. 2).
[
27]. The marginal reduction in the intensity of the peaks
after binding of the complex could originate from reducing
the average scattering contrast between the silicate
123

5
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Transition Met Chem (2014) 39:47–54
Table 2 Optimizing of the reaction conditions for the reaction of 2,3-
a
diaminopyridine 1 and benzil 2
O
O
N
N
Ph
Ph
NH2
NH2
Fe/SBA-15
+
N
N
1
2
3
b
Entry Catalyst amount Temperature (°C) Time (h) Yield (%)
1
2
3
4
5
6
7
8
a
0
rt
1
1
1
1
2
2
2
2
0
0.01 g
0.01 g
0.01 g
0.01 g
0.1 g
0.1 g
rt
20
60
50
reflux
reflux
reflux
60
68
c,d
98 (99)
98
77
e
0.1 g
reflux
0
Fig. 2 TEM image of Fe(III)-Schiff base/SBA-15
All reactions were run under the following conditions: 2,3-diami-
nopyridine 1 (1 mmol, 1 equiv), benzil 2 (1.0 mmol, 1 equiv) and
catalyst (0.01 g, 0.0,014 mmol) in water (2 mL) were heated for
b
appropriate time. GC yields. Optimum conditions. Isolated yield
c
d
e
in parentheses. 0.1 g of pure SBA-15 was used
Nitrogen adsorption/desorption isotherms and pore size
distributions of the SBA-15 and Fe(III)-Schiff base/SBA-15
are shown in Fig. 3. Both materials exhibited the IV type
isotherms with H1 hysteresis loop, according to IUPAC
classification, characteristic of mesoporous materials [29].
The sharpness of the hysteresis loop indicates the uniformity
of the pore size, which is confirmed by uniform pore size
distributions of the materials as shown in the inset in Fig. 3.
In comparison with SBA-15, a decrease in the surface
area and pore volume was observed for the supported
complex (Table 1), which can be attributed to reduced
structural ordering after incorporation of the complex into
the SBA-15 pores and/or blocking some pores.
The concentration of iron(III) in the catalyst was deter-
mined by atomic absorption spectroscopy as 0.14 mmol/g.
The elemental analysis of supported catalyst showed that the
nitrogen/iron molar ratio was 1.7, which is in good agree-
ment with the expected value of 2. This result confirms
formation of the Fe(III)-Schiff base complex with a N O
Fig. 3 Nitrogen adsorption/desorption isotherms incorporated with
pore size distributions at inset of: a SBA-15 and b Fe(III)-Schiff base/
SBA-15
2
2
ligand environment, as shown in Scheme 1 [30–32].
Fig. S1 (see supporting information) shows the IR
spectra of SBA-15 and the supported catalyst. The IR
spectrum clearly shows bands associated with both the
silica framework and the Schiff base complex, especially, a
Table 1 Characteristics of the materials
a
b
BET
c
Materials
C(M)
S
V
BJH
-
1
strong peak for stretching of the Si–O–Si at 1,100 cm
,
-
bending vibration of Si–O–Si at 800 cm , C = N
1
SBA-15
–
539
472
b
1.01
0.85
-
stretching bands at 1,614 cm , H–C–H bending vibration
1
Fe(III)-Schiff base/SBA-15
0.14
-
1
-1
a
-1
at 1,435 cm , and a broad band at 3,440 cm (hydroxyl
stretching vibrations) [33].
Initial concentration of iron species (mmol g ), specific surface
2
area (m g ), pore volume (cm g
-1
c
3
-1
)
1
23

Transition Met Chem (2014) 39:47–54
51
Table 3 Synthesis of pyrazine-
based heterocycles catalyzed by
Fe(III)-Schiff base/SBA-15 in
b
Yield
%)
Entry Diamine
Diketone
Product
a
water
(
1
2
3
97
97
95
4
5
98
96
6
7
95
95
8
97
96
9
c
1
0
1
99
c
1
97
123

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Transition Met Chem (2014) 39:47–54
Table 3 continued
b
Yield
Entry Diamine
Diketone
Product
(%)
c
c
1
1
2
3
96
99
99
c
1
4
a
All reactions were run under
c
c
the following conditions: 2,3-
diaminopyridine 1 (1 mmol, 1
equiv), benzil 2 (1.0 mmol, 1
equiv), and catalyst (0.01 g,
15
98
99
0
were heated for 2 h. Isolated
.0014 mmol) in water (2 mL)
b
1
6
c
yield. The time of the reaction
was 0.5 h
In order to investigate the iron content and stability of
the material, thermogravimetric analysis was used. The
results are depicted in Fig. S2 (see supporting information).
SBA-15 showed only a slight weight loss due to the loss of
physically adsorbed water. For the supported catalyst, the
Table 4 Investigation on the reusability of the catalyst for the
a
preparation of compound 3
Run
1
2
3
4
5
6
b
Yield (%)
97
97
97
96
96
96
°
weight loss between 200 and 700 C in Fig. S2 was
a
All reactions were run under the following conditions: 2,3-diami-
nopyridine 1 (1 mmol, 1 equiv), benzil 2 (1.0 mmol, 1 equiv), and
catalyst (0.01 g, 0.0014 mmol) in water (2 mL) were heated for 2 h
assigned to decomposition of the covalently bonded Schiff
base ligand (about 5.2 %) [34]. The amount of the ligand
-
1
b
loading on the SBA-15 was found to be 0.28 mmol g
Isolated yield
which is in good agreement with the iron content as
determined from atomic absorption spectroscopy.
The generality of the reaction was examined by applying
different diamines and 1,2-diketones. As shown in Table 3,
good to excellent yields were obtained for most of these
reactions; they generally proceeded very cleanly and no
side products were observed. The reactivities of the dia-
mines appear to be controlled by electronic effects. For
example, the hetero-aromatic diamines showed less reac-
tivity than aromatic or aliphatic diamines (Table 3, entries
1–9, 13–16). Electron-rich systems such as 1,2-phenyl-
enediamine 23 and aliphatic diamines gave higher yields
and required shorter reaction time (Table 3, entries 10–16).
1,2-Diketones showed good reactivity toward the investi-
gated diamines, and the maximum best results were
observed for biacetyl 21 (Table 3, entry 16). Highly
Catalytic synthesis of heterocycles
In the next step, we applied the catalyst for the synthesis of
pyridopyrazine and quinoxaline derivatives. In order to
optimize the efficiency of our catalytic system in water, the
conditions for the reaction of 2,3-diaminopyridine 1 and
benzil 2 as the model reaction were optimized. Different
parameters such as the amount of catalyst and temperature
were examined (Table 2). The good performance of this
system in water is a prominent advantage of the procedure
which significantly raises its green characteristics. Excel-
lent yield were obtained under the optimized conditions
(
Table 2, entry 5).
1
23

Transition Met Chem (2014) 39:47–54
53
Table 5 Comparison of our system with some existing reports in the
literature for the synthesis of compound 3
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A Fe(III)-Schiff base complex was readily synthesized and
anchored onto mesoporous silica SBA-15 to afford a Fe(III)-
Schiff base/SBA-15 heterogeneous catalyst. The catalyst
gives good activity in water for the synthesis of pyrazine-
based heterocycles in excellent yields. Furthermore, this
solid catalyst shows good recoverability and relatively high
stability against leaching of active species and can be reused
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Acknowledgments Authors are thankful to University of Birjand
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