L. Zhang et al. / Tetrahedron 69 (2013) 9237e9244
9243
cycles and the activity only decreased slightly from the fourth cycle,
and the framework of the recovered catalyst was completely un-
changed after five runs (Fig. 1c), proving that the catalyst possessed
high chemical activity and good stability. Additionally, the same
yield could be obtained after the catalyst was kept in air for several
weeks, further confirming the high activity and stability of the
catalyst.
using CDCl
3
or DMSO-d
6
as solvent and chemical shift values were
expressed in parts per million (ppm) from tetramethylsilane (TMS)
as an internal standard.
ScBTC NMOFs were synthesized by a surfactant-assisted micro-
wave irradiation method from a mixture of trimesic acid (53 mg,
0.25 mmol), scandium nitrate aqueous solution (0.4 M, 625
triethylamine (100 L) and cetrimonium bromide (CTAB,
mL),
m
0
.03 mmol) in ca. 5 mL distilled water. This solution was heated by
ꢀ
3
. Conclusions
microwave in sealed vessels at 300 W for 15 min at 150 C. The
resulting particles were isolated by centrifugation (4500 rpm,
5 min), washed with ethanol and water for several times by soni-
cation followed by centrifugation and then dried under vacuum at
In summary, catalytic Pd nanoparticles have been successfully
synthesized and further supported by using nanoscale scandium-
organic framework as a novel template and support through a mi-
crowave-assisted method. It is found that ScBTC NMOFs act as solid
solvent to support and disperse Pd NPs via electrostatic interaction
rather than PdeO covalent bonding. Thus the synthesized Pd-ScBTC
NMOFs exhibits high catalytic activity and good stability in the
Suzuki cross-coupling reaction, and superior performances are
observed compared to either the bulk MOFs or active carbon sup-
ported Pd catalyst. Notably, the catalyst shows negligible metal
leaching and can be recovered without significant activity loss after
several catalytic cycles. In view of the highly porosity, size minia-
turization and the resulting high surface area, nanoscale MOFs as
support materials for the loading of catalytic particles ensures
competitive advantage over bulk materials for enhanced catalytic
activity. A key issue for our future studies will be to identify NMOFs
with higher chemical stability and robustness allowing for
a broader range of catalytic reactions.
ꢀ
45 C for 12 h. Yield: 75.4% (based on Sc).
ScBTC MOFs were synthesized under hydrothermal condition
within a Teflon-lined Parr autoclave from a mixture of trimesic acid
(53 mg, 0.25 mmol), scandium nitrate aqueous solution (0.4 M,
625 mL) and triethylamine (100 mL) in ca. 5 mL distilled water. After
the reaction mixture was stirred to homogeneity, the autoclave was
ꢀ
sealed and heated for 2 days at 150 C under autogenous pressure.
After reaction, the precipitate was isolated by centrifugation
(4500 rpm, 5 min), washed with distilled water (3ꢃ10 mL) and
ethanol (3ꢃ5 mL) alternately, and then dried under vacuum at
ꢀ
45 C for 12 h. The resulting product was obtained as white powder
in 87.9% yield (based on Sc).
ScBTC NMOFs supported Pd catalyst was prepared as follows:
ꢀ
0.1 g of the activated ScBTC NMOFs (150 C under dynamic vacuum
for 12 h) and an appropriate amount of chloropalladinic acid
(H PdCl ) solution containing ca. 5 wt. % Pd were sonicated in
2 4
a sonication bath until a homogeneous yellowish-white suspension
was obtained, and then stirred for another 12 h at room tempera-
ture. The solution was placed inside a conventional microwave after
4
4
. Experimental section
.1. Synthesis and characterization
adding 100
mL of the reducing agent hydrazine hydrate (HH). The
microwave oven was then operated at full power (300 W, 2.45 GHz)
2
ꢂ
Starting materials and solvents were purchased from commer-
for 5 min. After reaction, the yellow solution of PdCl -NMOFs
4
cial resources and used as received. The synthesis of materials was
assisted by a microwave apparatus (Biotage, InitiatorÔ Eight). The
chemical composition of the impregnated material was determined
by an inductively coupled plasma optical emission spectrometer
changed to a black color, and the precipitate was centrifugally
isolated and washed with water and ethanol alternatively, then
dried under vacuum. Pd-ScBTC NMOFs were obtained as gray
powder in 75.0% yield (based on Pd).
(
ICP-OES, Jobin Yvon Ultima2). Structural characterization of ScBTC
ScBTC MOFs supported Pd catalyst was prepared as the same
procedure as the Pd-ScBTC NMOFs mentioned above.
NMOFs and Pd-ScBTC NMOFs were recorded by powder X-ray
diffraction (PXRD) using a Rigaku X-ray diffractometer (Miniflex II,
Cu K
a, 30 kV, 15 mA). Microstructural analyses were performed by
4.2. Catalytic reaction
scanning electron microscopy (JEOL JSM-6700F SEM) and trans-
mission electron microscopy (JEOL JEM-2010 TEM, operating at
4.2.1. General procedure for the Pd-ScBTC NMOFs catalyzed Suzuki
coupling reaction. Typically, all solid reagents were weighted into
an oven-dried 25 mL Schlenk flask, which was equipped with
a magnetic stir bar, septum, and a condenser. The liquid compounds
were then introduced by syringe, with the aryl/heteroaryl halide
always as the last addition. The flask was immersed in an oil bath
and stirred at desired temperature, while the extent of the reaction
was monitored by TLC analysis. When the reaction was complete,
the solid was removed by centrifugation and washed with ethanol
and water alternately and recovered for further consecutive runs.
The filtrate was collected and extracted with diethyl ether
(3ꢃ10 mL), and the ether layer was washed with a saturated brine
solution. The extract was dried with anhydrous sodium sulfate, and
then concentrated under reduced pressure. The residue was puri-
fied by flash chromatography on silica gel using a certain pro-
portional mixture of petroleum ether and ethyl acetate as an eluent,
and followed by a recrystallization process to provide the desired
biaryl derivatives. The structures of all the products were un-
2
00 kV with a point resolution of 0.23 nm). The texture properties
were determined from nitrogen adsorption/desorption isotherms
at 77 K measured with a Micromeritics ASAP 2020 apparatus. The
specific surface area was obtained by multiple-point Bru-
nauereEmmetteTeller (BET) method, the total pore volume was
computed from the amount of gas adsorbed at p/p
pore diameter distribution was determined by HorvatheKawazoe
0
¼0.99, and the
(
HK) model applied to the adsorption branch of the isotherms.
ꢀ
Thermogravimetric analysis (TGA) was performed from 30 C to
1
ꢀ
000 C in a nitrogen atmosphere with a Netzsch STA449C in-
strument at a heating rate of 10 K/min. X-ray photoelectron spec-
troscopy (XPS) measurements were carried on a Thermo Scientific
ESCALAB 250. Binding energies were calibrated by using the con-
tainment carbon (C1s¼284.6 eV). Fourier transform infrared (FTIR)
spectra was obtained by using a PerkinElmer Spectrum One in-
strument with sample prepared as KBr pellets at wavenumbers
ꢂ1
range from 400 to 4000 cm . Gas chromatography (GC) and gas
1
chromatographyemass spectrometer (GCeMS) analysis of the re-
action mixture and the isolated products were performed on
a Varian 430-GC system by using an external standard method and
ambiguously established on the basis of their spectral analysis ( H
13
NMR and C NMR spectral data, see the Supplementary data).
1
13
Varian 450-GC/240-MS, respectively. H and C NMR spectra were
recorded on a BRUKER BIOSPIN AVANCE III (400 MHz) spectrometer
4.2.2. The reuse of the catalyst. After the first run of the coupling
reaction between iodobenzene and phenylboronic acid, the catalyst