Boronate self-assemblies with embedded Au nanoparticles: Preparation, characterization and their catalytic activities for the reduction of nitroaromatic compounds

Article abstract of DOI:10.1039/c2jm34797k

Sequential boronate esterification of benzene-1,4-diboronic acid with pentaerythritol induced hierarchical molecular self-assembly to produce mono-dispersed flower-like microparticles. ATR-FT-IR, PXRD, ¹³C-CP- MAS and ¹¹B-DD-MAS NMR spectra indicate that the particles consist of zigzag-shaped packing structures of polymeric 2,4,8,10-tetraoxa-3,9- diboraspiro[5.5]undecane. Au nanoparticles (Au NPs) with a mean diameter of 2.7 nm were successfully deposited on the microparticles by the deposition reduction (DR) method. It is noteworthy that the resulting novel hybrids exhibited an efficient catalytic activity for the reduction of nitroaromatic compounds; in particular, high chemoselectivity in the hydrogenation of 4-nitrostyrene to the corresponding aniline was attained without reduction of the vinyl bond. Careful investigation of the catalyst suggested a synergistic effect between Au NPs and the boronate support in the selective hydrogenation. These findings strongly suggest that boronate self-assemblies are advantageous as support materials for preparation of heterogeneous catalysts based on polymer-Au hybrids. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm34797k

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PAPER
Boronate self-assemblies with embedded Au nanoparticles: preparation,
characterization and their catalytic activities for the reduction of
nitroaromatic compounds†
Yusuke Matsushima,a Ryuhei Nishiyabu,a Naoto Takanashi,a Masatake Haruta,a Hideaki Kimurab
a
*
and Yuji Kubo
Received 20th July 2012, Accepted 26th September 2012
DOI: 10.1039/c2jm34797k
Sequential boronate esterification of benzene-1,4-diboronic acid with pentaerythritol induced
hierarchical molecular self-assembly to produce mono-dispersed flower-like microparticles. ATR-FT-
IR, PXRD, 13C-CP-MAS and 11B-DD-MAS NMR spectra indicate that the particles consist of zigzag-
shaped packing structures of polymeric 2,4,8,10-tetraoxa-3,9-diboraspiro[5.5]undecane. Au
nanoparticles (Au NPs) with a mean diameter of 2.7 nm were successfully deposited on the
microparticles by the deposition reduction (DR) method. It is noteworthy that the resulting novel
hybrids exhibited an efficient catalytic activity for the reduction of nitroaromatic compounds; in
particular, high chemoselectivity in the hydrogenation of 4-nitrostyrene to the corresponding aniline
was attained without reduction of the vinyl bond. Careful investigation of the catalyst suggested a
synergistic effect between Au NPs and the boronate support in the selective hydrogenation. These
findings strongly suggest that boronate self-assemblies are advantageous as support materials for
preparation of heterogeneous catalysts based on polymer–Au hybrids.
As part of our ongoing program to develop boronate 3D
Introduction
systems, we reported that pyridine-assisted boronate esterifica-
tion of simple diboronic acid and tetrahydroxybenzene induced
polymeric self-assembly, producing well-defined submicro-
spheres.10 We realized that a judicious combination of plural
dihydroxyboryl and multiple hydroxyl components would enable
us to produce self-organized boronate microstructures with a
variety of morphologies. Furthermore, the surface of such
ensembles is composed of boronate esters with Lewis-acidic
character, which opens up new possibilities for the use of scaffold
entities in various applications. In this context, we envisaged how
such self-assemblies might have application in the fabrication of
organic–inorganic hybrid catalysts.11 The development of new
types of catalysts in fine chemical synthesis is urgently required
from the viewpoint of sustainable chemistry and green organic
synthesis; in particular, gold nanoparticle (Au NP)-catalyzed fine
chemical synthesis has a potential for practical use in the
industry.12 To explore such catalytic activity, not only metal
oxides such as TiO2, Al2O3, ZrO2, and CeO2, but also activated
carbon and organic polymers have been employed as support
materials. While several types of polymer-supported Au NPs
have been recently investigated in various catalytic reactions,13
the catalytic performance for hydrogenation is still in its infan-
cy.13g Therefore, the development of solid organic materials with
embedded Au NPs is worthwhile for the application.14
Considerable attention has been devoted to the use of boronic
acid complexation with diols and their congeners for developing
boron-containing multicomponent systems wherein the boronate
ester linkage serves as the intercomponent bond of supramolec-
ular architectures.1 Careful adjustment of the structure-directing
potential allows well-defined self-assembly involving not only
macrocycles,2 capsules,3 and gels4 but also reversible polymers5
and boronic acid-attached polymers.6 Moreover, boronic acids
recognize the diol motif through boronate esterification. There-
fore, boronic acid-appended systems have been explored as
sensing protocols for diol derivatives involving saccharides.7 In
the field of materials science, boronate ester-linked periodic
covalent organic frameworks (COFs) with two-dimensional
(2D)8 and 3D9 porous crystals have been prepared. Taking into
account such superior properties of boronic acids in supramo-
lecular chemistry, hierarchical assemblies formed by sequential
boronate esterification is worthwhile to be explored.
aDepartment of Applied Chemistry, Graduate School of Urban
Environmental Science, Tokyo Metropolitan University, Minami-ohsawa,
Hachioji, Tokyo 192-0397, Japan
bBrukerBioSpin K. K., 3-9, Moriya-cho, Kanagawa-ku, Yokohama-shi,
Kanagawa 221-0022, Japan
† Electronic supplementary information (ESI) available: Preparation of
Au/SiO2, additional characterization and catalytic data. See DOI:
10.1039/c2jm34797k
Herein we report well-defined boronate self-assemblies BP
arising from sequential dehydration of benzene-1,4-diboronic
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acid (1) with pentaerythritol (2), being characterized using
spectroscopic and electron microscopic data. Obtained mono-
dispersed flower-like microparticles were allowed to deposit Au
NPs having almost a uniform diameter of 2.7 ꢀ 0.6 nm on the
surface by the deposition reduction (DR) method using HAuCl4,
producing hybrid Au–BP(DR). And then, the catalytic activities
were investigated for the reduction of nitroaromatic compounds.
It should be noted that the hybrid system exhibited an efficient
catalytic performance for the chemoselective hydrogenation of
4-nitrostyrene. A synergistic effect between Au NPs and the
boronate support has been discussed by not only quantitative
comparison among related hybrids but also ATR-FT-IR
measurements.
relative to tetramethylsilane (TMS; d ¼ 0 ppm). 11B dipolar
decoupling (DD)-MAS NMR spectra were also measured by a
Bruker Avance III 400 spectrometer. The observation frequency
was 128.4 MHz for 11B NMR. The Hahn echo pulse sequence
was used for the reduction of the baseline distortion. Spectra
were obtained by using an acquisition time of 3.3 ms, a recycle
delay of 10 s, and an MAS speed of 10 kHz. The 11B chemical
shifts were calibrated by using boron phosphate (BPO4; d ¼
ꢁ3.3 ppm (ref. 16)) as an external standard relative to boron
trifluoride etherate (BF3$O(C2H5)2; d ¼ 0 ppm). Powder X-ray
diffraction (PXRD) data were collected using a Rigaku RINT-
TTR III X-ray diffractometer with Cu Ka radiation. Thermal
gravimetric analysis (TGA) was conducted using a SHIMADZU
DTG-60H with a scan rate of 5 ꢂC minꢁ1 and a scan range from
22.7 to 878.2 ꢂC under a N2 flow (50 mL minꢁ1). An alumina pan
was used. Zeta potential measurements for BP and poly-
ethyleneimine (PEI)-coated BP in methanol (0.5 mg mLꢁ1) were
performed using an ELSZ-2 (OTSUKA ELECTRONICS)
instrument. Atomic absorption spectroscopy for determination
of the amount of Au deposited on Au–BP was performed using a
SHIMADZU AA-6200. For preparation of sample solutions,
Au–BP (8.6 mg) was added to aqua regia (2 mL) and the mixture
was allowed to stand for 12 h to complete the digestion of gold.
UV/vis absorption spectra for monitoring the reduction of 4-NP
were recorded on a Shimadzu UV-3600 spectrophotometer at
Experimental
Materials
Benzene-1,4-diboronic acid (1), pentaerythritol (2), tetrahydrofuran,
methanol, toluene (dehydrated), anisole, 4-nitrophenol (4-NP),
4-nitrostyrene (3), 4-aminostyrene (4), 4-ethylnitrobenzene (5),
4-ethylaniline (6), and diethyl ether were used as received.
Hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4$4H2O),
dimethyl(acetylacetonate)gold(III) complex (Me2Au(acac)), and
linear polyethyleneimine (Mw: 25 000) were purchased from
Kanto Chemical, Tri Chemical Laboratories, and Polymer-
sciences, respectively. Au/SiO2 catalysts, the amount of loaded Au
being 1.6 wt%, were prepared according to the procedure reported
by Corma et al.15 The TEM image and histogram of the particle
size distribution are shown in Fig. S1, ESI.†
ꢂ
25 C and a quartz cell with 1 mm path length was used. Iden-
tification and quantitative analysis of the products for catalytic
tests were performed using a SHIMADZU GC-2010 equipped
with a SHIMADZU PARVUM2 (GC-MS) with a Shinwa
Chemical ULBON HR-1 capillary column (0.25 mm i.d., 30 m)
and an Agilent Technologies 7890A (GC) with an HP-5 capillary
column (0.32 mm i.d., 30 m), respectively.
Characterization
Field-emission scanning electron microscopy (FE-SEM) was
performed using a JEOL JSM-7500F microscope (accelerating
voltage of 5 kV). Transmission electron microscopy (TEM) was
performed and high resolution TEM (HR-TEM) images were
taken using a JEOL JEM-3200FS microscope (accelerating
voltage, 300 kV). As for FE-SEM measurements, boronate
particles BP collected on a PTFE membrane filter (pore size,
0.1 mm, Advantec Toyo Kaisha, Ltd) by filtration was employed.
The sample was coated with Au on an EIKO IB3 ION
COATER. TEM measurement was conducted as follows: a
suspension of Au–BP was put on a carbon-meshed copper grid
(elastic carbon substrate on STEM 100 Cu grids, Okenshoji Co.
Ltd) and the excess solution was immediately sucked by using a
filtration paper. The resultant grid was dried under vacuum and
measured without staining. Infrared spectra were recorded on a
SHIMADZU IR Affinity-1 equipped with an attenuated total
reflection (ATR) apparatus. Solid-state 13C cross-polarization-
magic angle spinning (CP-MAS) NMR spectra were measured
by a Bruker Avance III 400 spectrometer. The observation
frequency was 100.6 MHz for 13C NMR. The spectrometer is
equipped with a 4 mm MAS probehead capable of producing an
MAS speed of 15 kHz. Spectra were obtained by using a 1H–13C
CP contact time of 2 ms, an acquisition time of 34.4 ms, a recycle
delay of 5 s between scans, and an MAS speed of 9 kHz. Two-
Preparation of boronate microparticles (BP)
Benzene-1,4-diboronic acid (1, 415 mg, 2.5 mmol) and pentaer-
ythritol (2, 341 mg, 2.5 mmol) were mixed in THF (250 mL) and
the resultant mixture was then allowed to stand for 48 h at room
temperature. In this period, the mixture turned into a turbid
suspension. The resultant solids were isolated by filtration and
dried under vacuum. In this way, 457 mg of BP was obtained
with 80% yield.
Preparation of hybrid BP–Au nanoparticles by the deposition
reduction (DR) method: Au–BP(DR)
To a dispersion solution of boronate microparticles (BP)
(400 mg, 1.74 mmol per unit (ref. 17)) in methanol (200 mL) was
added linear polyethyleneimine (150 mg, 3.48 mmol per unit
(ref. 17)) at room temperature. The mixture was sonicated for
5 min using an ultrasonic bath and was then allowed to stand for
25 min at room temperature. Filtration and drying under
vacuum afforded white solids (371 mg). The white solids were
dispersed in methanol (183 mL) and HAuCl4$4H2O (15.5 mg,
0.038 mmol in 2 mL methanol) was then added to the dispersion
solution. The resultant yellow dispersion was sonicated for
5 min. After being allowed to stand for 25 min, pale yellow solids
were collected by filtration and were then dispersed in methanol
(185 mL). Subsequent addition of NaBH4 (155 mg, 4.10 mmol)
1
pulse phase modulation (TPPM) was used for H decoupling to
ensure the sharper peaks. The 13C chemical shifts were calibrated
by using adamantine (d ¼ 29.5 ppm) as an external standard
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into the solution afforded pale purple solids, being collected by
filtration. In this way, 309 mg of Au–BP(DR) was obtained, the
amount of loaded Au being 2.3 wt%.
Preparation of hybrid BP–Au nanoparticles by the solid grinding
(SG) method: Au–BP(SG)
A mixture of boronate particles (BP) (480 mg, 2.09 mmol per
unit (ref. 17)) and Me2Au(acac) (acac ¼ acetylacetonate)
(15.8 mg, 0.049 mmol) was ground in an agate mortar in air at
room temperature for 20 min. The resultant white solids were
reduced in a stream of 10 vol% H2 in N2 at a flow rate of 50 mL
minꢁ1 at 120 ꢂC for 2 h to afford pale purple solids. In this way,
150 mg of Au–BP(SG) was obtained, the amount of loaded Au
being 2.0 wt%.
Fig. 1 FE-SEM image of boronate microparticles BP formed by aging a
solution of 1 and 2 in THF.
spectroscopy equipped with an attenuated total reflection (ATR)
apparatus capable of negating the influence of water (Fig. 2); the
O–H stretching frequencies at 3279 and 3310 cmꢁ1 were
removed. In addition, a characteristic intense signal observed at
658 cmꢁ1 was assigned to the boronate ester of 1 and 2 and the
absorbance at 1310 cmꢁ1 was attributed to a B–O stretch.10 The
solid-state 13C-CP-MAS NMR spectrum was also recorded to
verify intact building units in the microparticles; the assignment
of peaks in the 13C NMR spectrum is shown in Fig. 3a. Not only
the signal (d ¼ 135.3 ppm) arising from aromatic carbon but also
three signals of methylene carbon (d ¼ 62.9, 64.5 ppm) and spi-
rocarbon (d ¼ 35.2 ppm) were clearly obtained, indicating that
polymeric 2,4,8,10-tetraoxa-3,9-diboraspiro[5.5]undecane was
formed through sequential boronate esterification of 1 with 2.
Further assessment was made by 11B-DD-MAS NMR analysis
and significant signals were obtained in the region 9.7 to 15.8
ppm, which were higher field shifted than those of typical sp2-
hybrized trigonal boron. The line shapes from quadripolar 11B
spectra have been known to be highly sensitive to the chemical
and geometrical bonding environment of boron.8a,18 Thus, it
might be interpreted on the basis of phenyl-boron p-stacking
interactions between polymers in the solid state (vide infra).
Powder X-ray diffraction (PXRD) (Fig. 4a) showed charac-
teristic peaks at 16.86, 21.06, and 25.06ꢂ, corresponding to 5.25,
Catalytic reduction of 4-nitrophenol
A dispersion solution of Au–BP (Au–BP(DR); 0.44 mg, [Au] ¼
1.1 ꢃ 10ꢁ4 M, Au–BP(SG); 0.50 mg, [Au] ¼ 1.0 ꢃ 10ꢁ4 M) in
methanol and a solution of 4-nitrophenol (8.4 ꢃ 10ꢁ3 M) in
methanol were successively added to methanol. To the resultant
mixture, a freshly prepared solution of NaBH4 (1.7 ꢃ 10ꢁ1 M) in
methanol was added. In this way, the initial molar ratio of Au/
4-nitrophenol/NaBH4 in methanol (3.5 mL) was set up to be 1/
90/1250 (1.4 ꢃ 10ꢁ5 M/1.3 ꢃ 10ꢁ3 M/1.8 ꢃ 10ꢁ2 M). Absorption
spectra of the reaction mixture were taken at a fixed interval at
25 ꢂC using a 1 mm cell. The rate constants of the reduction
process were determined by recording the change in absorbance
at 400 nm as a function of time at 25 ꢂC using a 1 mm cell.
Control experiments employing BP (0.44 mg) and
HAuCl4$4H2O ([Au] ¼ 1.4 ꢃ 10ꢁ5 M) were carried out under
similar conditions, respectively.
Catalytic hydrogenation of 4-nitrostyrene
Au–BP (50 mg, Au–BP(DR); Au 1.1 mol%, Au–BP(SG); Au
1.0 mol%), 4-nitrostyrene (0.5 mmol), dry toluene (3 mL) and a
magnetic stirring bar were placed in a stainless steel autoclave.
Hydrogen was then introduced into the autoclave until the
pressure reached 0.5 MPa. The mixture was stirred at 100 ꢂC for
22 h. After the reduction, hydrogen was removed and the reac-
tion mixture was extracted with diethyl ether, filtered, and
analyzed by GC-MS and GC using anisole as an internal stan-
dard. A control experiment using Au/SiO2 (71 mg, Au 1.1 mol%)
was performed under similar conditions.
Results and discussions
Boronate support materials were easily prepared in 80% yield by
aging a solution of benzene-1,4-diboronic acid (1) and pentaer-
ythritol (2) in THF at room temperature for 48 h. The FE-SEM
image shows the formation of monodispersed flower-like boro-
nate microparticles BP (Fig. 1). The particle size distribution was
determined from 1000 particles observed with a FE-SEM; the
histogram constructed indicates that the average diameter of the
particles was 2.3 mm with a standard deviation of 0.3 mm (Fig. S2
in the ESI†). The isolated particles were insoluble in organic
solvents such as methanol and toluene. Particle component
analysis was performed using infrared (IR) absorption
Fig. 2 ATR-FT-IR spectra of 1, 2, and boronate microparticles BP. A
characteristic intense peak at 658 cmꢁ1 (*) was assignable to boronate
ester.
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Fig. 4 (a) PXRD spectrum and (b) plausible stacking structure of BP.
polyethyleneimine (PEI) as a binder for Au NPs deposition.21 As
a result, a zeta potential of +45.10 mV was obtained for the PEI-
coated boronate microparticles22 due to the presence of positively
charged PEI chains on the surface of BP. The DR method was
carried out using PEI-coated boronate microparticles to afford
the product Au–BP(DR) as a pale purple solid. This material was
characterized by TEM (Fig. 5a) and atomic absorption spec-
trometry. The space fringes of the Au NPs could be detected in
the HR-TEM image to be d ¼ 0.225 nm (Fig. 5b), being well
consistent with the {111} facet of Au. The histogram of the size
distribution suggested that highly dispersed Au NPs (2.3 wt%)
with a mean diameter of 2.7 nm and a standard deviation of
0.6 nm were loaded on the surface (Fig. 5c).
Fig. 3 (a) 13C-CP-MAS and (b) 11B-DD-MAS NMR spectra of BP,
where ssb represents spinning side bands.
ꢀ
4.21, and 3.55 A, respectively. It should be noted that the peaks
ꢀ
of 3.55 A are related to the phenyl-boron p-stacking distance.
These results suggest the zigzag-shaped packing structures as
shown in Fig. 4b. Although in THF (ET(30) ¼ 37.2) spherical
aggregation of such boronate polymers was obtained, the use of
solvent with a higher polarity such as DMSO (ET(30) ¼ 45.0)
resulted in agglomeration with a different morphology (Fig. S3 in
the ESI†), thus indicating that the shape in such aggregates is
sensitive to the bulk solvent and possibly due to interfacial free
energy between the aggregation and the bulk solvent used.19
Stability of the microparticles was assessed by using thermog-
ravimetric analysis (TGA) which indicated no loss of mass up to
As an alternative method for the direct deposition of Au NPs
on the boronate support, the solid grinding (SG) method was
also employed.13e Volatile organogold complex, Me2Au(acac),
and BP were ground in an agate mortar at room temperature.
Theꢂmixture was then treated in a stream of 10 vol% H2 in N2 at
120 C for 2 h to obtain the Au–BP(SG) hybrid (Fig. 6a) where
the HR-TEM image shows the space fringes (d ¼ 0.235 nm)
ascribable to the {111} facet of the Au NPs (Fig. S5 in ESI†). In
this case, compared to the DR method, relatively large Au NPs,
their diameter being 5.5 ꢀ 1.7 nm, were deposited on the surface
(Fig. 6b). By employing different techniques for Au deposition
on the boronate support, we could obtain Au–BP hybrids with
different size distributions of Au NPs, which should be useful to
assess size-dependency of Au NPs in the hybrids for the catalytic
performance (vide infra).
ꢂ
300 C (Fig. S4 in the ESI†), being advantageous as a polymer
support for depositing Au NPs.
For direct deposition of Au NPs onto the BP, the deposition
reduction (DR) method was employed;20 a solution of HAuCl4 in
methanol was added to a turbid solution of BP in methanol,
followed by filtration, dispersion in methanol and reduction with
NaBH4. However, no change in the color of the mixture was
observed, indicating that Au NPs were not deposited on the
surface of BP. The zeta potential of BP wꢁas determined to
be ꢁ33.24 mV, which suggests that AuCl4 (anion) hardly
binds to the surface. To overcome this difficulty, we used
Reduction of 4-nitrophenol (4-NP) with NaBH4 to 4-amino-
phenol (4-AP) has been often used as a test reaction to investigate
the catalytic performance of candidate catalysts.13f,h,23 We
measured the catalytic activity of Au–BP(DR) in this reaction
using UV/vis spectroscopy. The reaction kinetics was simulta-
neously recorded by absorption spectra because the
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Fig. 5 (a) TEM image of Au–BP(DR). The inset shows the corre-
sponding microparticle. (b) HR-TEM image of a lattice structure
observed for {111} facet of the Au NP. (c) The size distribution of 1000
particles in the TEM images.
Fig. 7 (a) UV/vis absorption spectra for the reduction of 4-NP with
NaBH4 over Au–BP(DR) in methanol at 25 ꢂC. (b) Plot of ln(At/A0)
versus time for the reduction of 4-NP. Reaction conditions: Au–BP(DR)
(circles), Au–BP(SG) (inverted triangles), HAuCl4 (squares), BP (trian-
gles), [Au] ¼ 1.4 ꢃ 10ꢁ5 M, [4-NP] ¼ 1.3 ꢃ 10ꢁ3 M, and [NaBH4] ¼ 1.8 ꢃ
10ꢁ2 M.
moles of 4-NP consumed per mol of Au–BP(DR) per h under the
present reduction conditions, was estimated to be 1013 hꢁ1
.
Fig. 6 (a) TEM image of Au–BP(SG). The inset shows the corre-
sponding microparticle. (b) The size distribution of 1000 particles in the
TEM images.
Compared to conventional polymer-supported Au nano-
particles,13f,h,23 Au–BP(DR) exhibited an efficient catalytic
activity for the reduction of nitrophenol. In contrast, under
similar conditions Au–BP(SG) did not facilitate the reduction
reaction of 4-NP with NaBH4 as inferred from Fig. 7b.24 It is
difficult to understand its low catalytic activity on the only basis
of size-dependency of Au NPs for the catalytic reaction. The FE-
SEM image of Au–BP(SG) (Fig. S7, ESI†) led us to consider that
grinding-induced change in morphology of the surface would
affect its catalytic activity.
characteristic peaks of both 4-NP and 4-AP can be easily
distinguished from each other. The absorbance at 400 nm,
assignable to 4-nitrophenolate ions, significantly decreased upon
addition of NaBH4 in the presence of Au–BP(DR); the isosbestic
point between the two peaks was also observed, suggesting that
two principal species are responsible for the conversion (Fig. 7a).
The reaction was almost completed within 320 s under the
following conditions: Au(0)/4-NP/NaBH4 (mol/mol/mol) ¼ 1/90/
1250. A linear relationship was observed between ln(At/A0) vs.
reaction time as shown in Fig. 7b, and the pseudo-first order rate
constant (k) was estimated to be 1.6 ꢃ 10ꢁ2 sꢁ1. No reaction was
observed under similar conditions in the presence of HAuCl4 as
well as boronate microparticles BP, demonstrating the effective
catalytic activity of Au–BP(DR) for the reduction. The turnover
frequency (TOF) of the catalyst, calculated by the number of
We decided to employ Au–BP(DR) in the hydrogenation of
nitroarenes to anilines to assess its catalytic performance. This
hydrogenation is of fundamental importance to industry.25 In
particular, the chemoselective reduction of nitroaromatic
compounds containing other reducible functional groups is
challenging26 because conventional reduction methods usually
afford a mixture of the possible reduced compounds. To the best
of our knowledge, there is no report on the polymer-supported
Au NPs that show an efficient catalytic activity in chemoselective
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Table 1 Hydrogenation of 3 over Au–BP hybrid catalysts for 22 ha
hydrogenation of nitrostyrenes to the corresponding amino-
styrenes. We evaluated Au–BP(DR) as a catalyst in the hydro-
Yieldc (%)
genation under the following conditions:
a solution of
4-nitrostyrene 3 (0.5 mmol) and Au–BP(DR) (Au: 1.1 mol%) in
toluene (3 mL) was stirred under 0.5 MPa H2 at 100 ꢂC (Fig. 8).
The reactant 3 and products (4, 5, and 6) were analyzed by using
gas chromatography (GC). As a result, the yield of 4-amino-
stylene 4 increased while the concentration of 4-nitrostyrene 3
decreased as a function of time. It is interesting to note that other
possible reduction products such as 4-ethylnitrobenzene 5 and
4-ethylaniline 6 were present at negligible levels. Subsequently,
Au–BP(DR) showed a high selectivity in the hydrogenation of 3,
affording the desired 4 in 91% yield with a reaction conversion of
96% over 22 h (Table 1). A remarkable result is that 99% che-
moselectivity has been achieved in terms of –NO2 reduction. This
is the first demonstration of chemoselective reduction of nitro-
styrene using polymer-supported Au NPs, while retaining the
C]C bond in the reactant. On the other hand, when Au–BP(SG)
was employed in the reduction of 3 under similar conditions, a
relatively poor yield of 4 (24%) was obtained (Table 1), its
reaction conversion being 33% for 22 h. Despite chemoselectivity
being maintained, the lower catalytic activity exhibited with
respect to Au–BP(DR) may be attributable to not only the
increase in the size of the Au NPs but also the grinding-induced
change in morphology on the surface (vide supra). As a control
Au/SiO2 was employed under similar conditions. Although the
reaction proceeded with a higher conversion (64%) than that for
Au–BP(SG), a poor yield of 4 (5%) was observed for the
hydrogenation.
Catalyst
Methodb Size of Au (nm) Conversion (%)
4
5
6
Au–BP(DR) DR
Au–BP(SG) SG
Au/SiO2
2.7 ꢀ 0.6
5.5 ꢀ 1.7
6.9 ꢀ 2.7
96
33
64
91
24
5
1
0
0
0
0
0
d
a
Reaction conditions: 3 (0.5 mmol), catalyst (Au–BP(DR); 50 mg, Au ¼
1.1 mol%, Au–BP(SG); 50 mg, Au ¼ 1.0 mol%, Au/SiO2; 71 mg, Au ¼
b
1.1 mol%, toluene (3 mL), 100 ꢂC. DR: deposition reduction, SG:
c
d
solid grinding. GC yield. The amount of loaded Au (1.6 wt%) for
Au/SiO2 was determined using inductively coupled plasma atomic
emission spectrometry (ICP-AES, Rigaku CIROS CCD).
interactions between substrate 3 and Au–BP(DR) using ATR-
FT-IR. Fig. 9a displays the characteristic intense signal of the
boronate ester (B–O stretching frequency) of Au–BP(DR) at
1314 cmꢁ1. The signal was lower-shifted by 6 cmꢁ1 upon addition
of 3. This shift suggests an intermolecular interaction between the
nitro group and the boron on the surface of Au–BP(DR). In
addition, our careful measurements allowed us to detect slightly
shifted N–O symmetric stretching frequency of 3. Thus, we
assume that the nitro group of 3 may be favorably adsorbed on
the boronate support. From these results, we propose a plausible
reaction mechanism (Fig. 9b): Au atoms of Au–BP(DR), that
In an effort to better understand the highly chemoselective
hydrogenation of 3 to 4 using Au–BP(DR), we investigated
Fig. 9 (a) ATR-FT-IR spectra of (A) 3, (B) Au–BP(DR), and (C)
the mixture of 3 with 2 equiv. of Au–BP(DR). (b) A plausible
reaction mechanism for chemoselective hydrogenation of 3 to 4 using
Au–BP(DR).
Fig. 8 Time course of the reduction of 3 with H2 over Au–BP(DR)
(squares, 4-nitrostyrene (3); circles, 4-aminostyrene (4); triangles, 4-eth-
ylnitrobenzene (5); inverted triangles, 4-ethylaniline (6)).
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J. Mater. Chem., 2012, 22, 24124–24131 | 24129

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of 1 and 2 in THF. Obtained particles was found to be stable and
served as a support to mainly deposit Au NPs with diameter in
the range of 1–3 nm when the DR method was employed for
deposition. The prepared hybrid exhibited an excellent catalytic
activity for the reduction of nitroaromatic compounds. Most
notably, we have highlighted that our boronate hybrid system
can serve as the first polymer-supported Au NPs catalyst for
highly chemoselective hydrogenation of 4-nitrostyrene to 4-
aminostyrene. This result has presented evidence for the superi-
ority of boronate microparticles as catalytic support materials.
We believe that the availability of boronic acids and catechols or
diols could lead us to prepare a variety of analogous boronate
self-assemblies at the microlevel, which would offer unique and
versatile platforms for catalyst construction. Considering such
potential, further research is currently underway in our
laboratory.
Fig. 10 Reusability of Au–BP(DR) hybrids as catalysts for the reduction
of 4-NP with NaBH4 confirmed by the conversion (bar graph) and TOFs
(line graph) in five reaction runs.
Acknowledgements
may be located at low coordinated positions such as corners or
edges,27 activate H2 decomposition to produce Au–H species,28
leading to favorable participation in the reduction of the nitro
group of the substrate adsorbed on the boronate surface.
The existence of Au NPs as a majority on the surface of the
boronate support material BP should be an important factor to
produce Au–H as a reducing agent.13g On the other hand, BP
promotes the access of the nitro reactant into the active site of the
Au NPs. In this way, the synergistic effect between Au NPs and
boronate support gives rise to an efficient catalytic performance
with complete chemoselectivity for the reduction of nitrostyrene
to the corresponding aminostyrene. It is worth noting that the
reduction of the C]C bond was completely suppressed by the
catalysts used.
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports and
Culture of Japan (no. 23750167, 24350075), the Yamada Science
Foundation and the Tokuyama Science Foundation. The
authors thank Dr T. Ishida of Kyushu University for her kind
support in this study.
Notes and references
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