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A. Alizadeh et al. / Journal of Molecular Catalysis A: Chemical 372 (2013) 167–174
mixture was stirred at 80 ◦C under an oxygen atmosphere. After
completion of reaction, the reaction mixture was filtered off and
the catalyst rinsed twice with CH2Cl2 (5 ml). The excess of solvent
was removed under reduced pressure to give the corresponding
carbonyl compounds.
metformin/Pd2+ complex would be served as an efficient and
robust catalyst for cross-coupling reactions as well as alcohol
oxidation processes. Here, we report a convenient procedure for
fabrication and characterization of a recoverable nanocatalyst
[SBA-15/Met/Pd(II)] for the Suzuki coupling reaction and aerobic
oxidation of benzyl alcohols.
2. Experimental
All chemicals were purchased from Merck except Pluronic123
and metformin hydrochloride which were obtained from Aldrich. IR
spectra were determined on a Perkin-Elmer 683 instrument. 1H and
13CNMR spectra were recorded on a Bruker (200 MHz) spectrom-
eter in CDCl3 as solvent. X-ray powder diffraction patterns were
obtained on SE1FERT-3003TT.
The schematic pathways for the synthesis of catalyst are
depicted in Scheme 1. First, SBA-15/Met was prepared according
to SBA-15/Met in acetone and the mixture was stirred for 4 h at
room temperature to obtain SBA-15/Met/Pd(II).
The SEM micrographs obtained of the catalyst are shown in
Fig. 1. These micrographs confirmed the formation of a well-
ordered structure for SBA-15/Met/Pd(II) consisting of rod-shaped
particles of 1500–2000 nm in length and 450–550 nm in diameter.
In addition, nitrogen adsorption–desorption isotherms of SBA-
(SBA-15/Met)
The mesoporous material SBA-15 was prepared according to
a literature procedure [29] and then was modified with chloro-
propyl tail groups. A 100 ml of round-bottom flask were introduced
successively 30 ml of anhydrous toluene, 3.0 g of activated sil-
ica, and 1.782 g (9 mmol) of 3-chloropropyl trimethoxysilane. The
solution was refluxed for 24 h under an inert atmosphere, fil-
tered and washed subsequently with toluene, dichloromethane,
and methanol, and dried under reduced pressure at 80 ◦C for 10 h.
Through this simple procedure, the chloropropyl-functionalized
silica was obtained.
15, SBA-15/Met and SBA-15/Met/Pd(II) are shown in Fig.
2
and the corresponding textural parameters calculated by N2
increase in adsorption at relative pressures of 0.6–0.8 for SBA-15
samples attributed to capillary nitrogen condensation according to
ordered pore structures [30].
In another 100 ml round-bottom flask, to a solution of 3 g
(18 mmol) of metformin hydrochloride in 35 ml acetonitrile, 0.72 g
(18 mmol) NaOH was added. After 1 h, 3 g (18 mmol) KI and 3 g of as-
prepared 3-chloropropyl functionalized SBA-15 were added to the
mixture and kept under reflux for 12 h. The solvent was removed
and then 50 ml distilled water was added to the residue and stirred
for 1 h, filtrated and washed with distilled water to afford 3.1 g of
the metformin-functinalized mesoporous SBA-15.
The successful attachment of metformin and subsequent coor-
dination of Pd(II) ions within the mesoporous SBA-15 material can
be investigated employing FT-IR spectroscopy. Fig. 3 shows the FT-
parent SBA-15, (c) SBA-15/Met and (d) SBA-15/Met/Pd(II). Curve
a shows the spectrum of Met.HCl and signals appeared at 1568
and 1630 cm−1 are attributed to the presence of C N stretching
vibrations [31]. The signals appeared at 3200–3500 cm−1 region can
be assigned to the N H stretching of C
N H group on metformin
2.2. Immobilization of Pd(II) ions on the surface of SBA-15/Met
[21]. The unmodified SBA-15 spectrum in curve b shows the typi-
cal silica bands associated with the main inorganic backbone: the
signals appeared at 1050 and 1150 cm−1 is assigned to asymmet-
A mixture of SBA-15/Met (1 g) and palladium acetate (220 mg,
1 mmol) in acetone (10 ml) was stirred at room temperature for 4 h.
The resulting solid was filtered, washed with acetone and THF and
dried in vacu at 80 ◦C for 3 h to give SBA-15/Met/Pd(II).
ric stretching of Si
O
Si, at 1660 cm−1 is related to the angular
vibration of water bonded to the inorganic backbone and signal
appeared at ∼3400 cm−1 can be assigned to O H stretching fre-
quency of Si
O H groups and/or water in atmosphere and within
porous sample. In curves c and d, the sharp band at 1090 cm−1
is corresponding to Si Si anti-symmetric stretching vibration,
2.3. Catalytic testing
O
2.3.1. Suzuki–Miyaura coupling reaction
icant difference in FT-IR spectra of Met.HCl and SBA-15/Met (curves
a and c) was the shift of C N stretching frequencies from 1568 and
1630 cm−1 to 1580 and 1645 cm−1, respectively due to the removal
of HCl when metformin is attached to the silica surface. On the other
hand, the metal–ligand co-ordination [33,34] presumably leads to
a shift of these two peaks again to lower frequencies (1568 and
1630 cm−1). This shift can be observed comparing curves c and d
in Fig. 3. All together, the aforementioned observations confirmed
the immobilization of Pd(II) ions on the surface of SBA-15/Met.
Furthermore, elemental analysis showed that the carbon, hydro-
gen, and nitrogen content of SBA-15/Met was 12.219, 2.520, and
5.194 (wt.%), respectively, which are equivalent to a loading of
∼0.7 mmol of metformin per gram of SBA-15. In addition, the
Pd content of the catalyst estimated by atomic absorption spec-
troscopy was 0.570 0.001 mmol g−1. This indicated that ∼81% of
the anchored metformin moieties have efficiently co-ordinated
with Pd2+ ions providing catalytic active sites.
In a typical reaction, to a solution of 1 mmol of the aryl halide in
5 ml of water/ethanol (1:1) was added 1.1 mmol of phenyl boronic
acid, 276 mg of K2CO3 (2 mmol) followed by 15 mg of the solid cat-
alyst (1 mol%). The mixture was then stirred for the desired time at
80 ◦C. The reaction was monitored by thin layer chromatography
(TLC). After completion of reaction, the reaction mixture was cooled
to room temperature and the catalyst (SBA-15/Met/Pd(II)) was
recovered by centrifuge and washed with ethyl acetate and ethanol.
The combined organic layer was dried over anhydrous sodium sul-
fate and evaporated in a rotary evaporator under reduced pressure.
The crude product was purified by column chromatography.
2.3.2. Aerobic oxidation of benzyl alcohols
A mixture of K2CO3 (1 mmol) and the catalyst (52 mg, ∼3 mol%
of Pd2+) in toluene (5 ml) was prepared in a two necked flask. The
flask was evacuated and refilled with pure oxygen. To this solution,
the alcohol (1 mmol, in 1 ml toluene) was injected and the resulting