Three-dimensional cubic mesoporous materials with a built-in N-heterocyclic carbene for Suzuki-Miyaura coupling of aryl chlorides and C(sp 3)-chlorides

Article abstract of DOI:10.1016/j.jcat.2010.09.004

New cubic cage-like mesoporous materials with a bulky N-heterocyclic carbene [IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] precursor in the framework were synthesized by a co-condensation of IPr precursor-bridged organosilane and TEOS in the presence of template. N₂ sorption, XRD and TEM characterizations revealed that the mesostructural orderings of the synthesized materials depended on the molar fraction of the bridged organosilane in the initial gel mixture. With the increase in the molar fraction of the organosilane from 2.5% to 15%, the mesostructure of the synthesized material changed from a well-ordered 3D ordered structure to a amorphous structure. FT-IR and solid-state NMR characterizations confirmed that IPr carbene precursor was covalently integrated with the solid materials. Such hybrid materials were able to coordinate Pd(acac)₂, leading to active solid catalysts for Suzuki-Miyaura couplings of less reactive aryl chlorides. The solid catalyst could be reused 8 times without a significant decrease in activity. Furthermore, the solid catalyst was active for the coupling of C(sp³)-chlorides and arylboronic acids.

Full text of DOI:10.1016/j.jcat.2010.09.004

Journal of Catalysis 276 (2010) 123–133
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Three-dimensional cubic mesoporous materials with a built-in N-heterocyclic
carbene for Suzuki–Miyaura coupling of aryl chlorides and C(sp3)-chlorides
a
a
b
b
Hengquan Yang a, , Guang Li , Zhancheng Ma , Jianbin Chao , Zhiqiang Guo
⇑
a School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, PR China
b The Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
New cubic cage-like mesoporous materials with a bulky N-heterocyclic carbene [IPr, 1,3-bis(2,6-diisopro-
pylphenyl)imidazol-2-ylidene] precursor in the framework were synthesized by a co-condensation of IPr
precursor-bridged organosilane and TEOS in the presence of template. N2 sorption, XRD and TEM charac-
terizations revealed that the mesostructural orderings of the synthesized materials depended on the
molar fraction of the bridged organosilane in the initial gel mixture. With the increase in the molar frac-
tion of the organosilane from 2.5% to 15%, the mesostructure of the synthesized material changed from a
well-ordered 3D ordered structure to a amorphous structure. FT-IR and solid-state NMR characterizations
confirmed that IPr carbene precursor was covalently integrated with the solid materials. Such hybrid
materials were able to coordinate Pd(acac)2, leading to active solid catalysts for Suzuki–Miyaura cou-
plings of less reactive aryl chlorides. The solid catalyst could be reused 8 times without a significant
decrease in activity. Furthermore, the solid catalyst was active for the coupling of C(sp3)-chlorides and
arylboronic acids.
Received 10 May 2010
Revised 27 August 2010
Accepted 4 September 2010
Available online 8 October 2010
Keywords:
Mesoporous material
N-heterocyclic carbene
Suzuki–Miyaura coupling
Aryl chloride
SBA-16
Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction
Recently, a new concept for preparing solid catalysts has been
developed with the discovery of hybrid periodic mesoporous mate-
The Pd-catalyzed Suzuki–Miyaura coupling is currently a funda-
mental reaction for the synthesis of polymers, agrochemical and
pharmaceutical compounds [1–3]. The use of aryl chlorides as sub-
strates has proven difficult but is highly desirable in view of the
wide diversity, ready availability and low cost of aryl chlorides in
contrast with aryl bromides and iodides. N-heterocyclic carbene
complex Pd–IPr [IPr = N,N0-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene] was recently found to be the most efficient catalyst for Su-
zuki–Miyaura coupling of aryl chlorides [4–10]. Compared with
the well-established phosphine systems, Pd–IPr complex is more
appealing because it is air-stable and low toxic. Such advantages
make Pd–IPr complex a potential catalyst for practical applications
if this complex can be immobilized in a proper fashion for easy re-
use. However, there is no report on immobilization of the bulky
bis(2,6-diisopropylphenyl)-substituted carbene Pd complex for Su-
zuki–Miyaura coupling although the grafting of small methyl-
substituted carbene complexes on insoluble supports has been re-
ported [11,12], which exhibited low activity toward aryl chlorides
due to lacking bulky electron-donating groups like 2,6-diisopropyl-
phenyl group on the imidazole ring [13–19].
rials. Such materials can be prepared by co-condensation of com-
plex/ligand-bridged trialkoxysilane and tetraalkoxysilane in the
presence of template (namely, a mixture of complex/ligand-
bridged trialkoxysilane and tetraalkoxysilane was hydrolyzed and
condensed in an aqueous medium at a molecular level in one
pot) [20–27]. This method opens a promising avenue for creating
hybrid solid catalysts with open and ordered mesostructures
which facilitate the accessibility of active sites. Compared to the
widely used grafting method [20,28], it endows the solid catalyst
with larger surface area and higher pore volume owing to the
incorporation of organic moieties in the framework instead of
grafting on the surface. Moreover, the co-condensation method
leads to a more homogeneous distribution of active sites through-
out the solid materials and a larger degree of immobilization be-
cause the hydrolysis–condensation of siliceous precursors usually
occurs under strong basic or acidic aqueous conditions that enable
the trialkoxysilyl groups to completely hydrolyze and further con-
dense into SiAOASi linkages [29].
Driven by such advantages, Dufaud et al. synthesized a periodic
mesoporous hybrid material containing a rhodium organophos-
phine complex in the framework [30]. This hybrid material showed
high activity and good recyclability in the hydrogenation of al-
kenes. Corma’s group synthesized periodic mesoporous hybrid
materials with a vanadyl Schiff base and a carbapalladacycle
⇑
Corresponding author. Fax: +86 351 7011688.
E-mail address: hqyang@sxu.edu.cn (H. Yang).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.09.004

124
H. Yang et al. / Journal of Catalysis 276 (2010) 123–133
complex in the frameworks [29,31]. These materials were catalyt-
ically active toward the cyanosilylation of benzaldehyde and
Suzuki–Miyaura coupling of aryl bromides. Yang’s group has re-
cently prepared hybrid mesoporous materials with a built-in
chiral diaminocyclohexane and BINAP, which exhibit good to
excellent performances in the asymmetric hydrogenation of ke-
tones [25,32].
However, in contrast to the ever-expanding families of metal
complex catalysts, the catalytically active hybrid mesoporous
materials prepared with this elegant method are rather limited.
Moreover, the reported, catalytically active hybrid mesoporous
materials are limited to two-dimensional (2D) channel-like hexag-
onal (p6 mm) materials such as MCM-41 and SBA-15 [33,34]. Com-
pared with 2D channel-like hexagonal mesoporous material, the
three-dimensional (3D) cubic mesostructure of SBA-16 (Im3m,
body-centered cubic) was found to be more attractive due to the
3D cage-like mesoporous network (in which each spherical cage
is connected with eight neighboring cages through entrances)
[35–38]. Such a unique mesostructure of SBA-16 was proven to
facilitate the fast diffusions of reaction molecules [39–41]. How-
ever, up to date, there is still no report on the catalytically active
SBA-16 type of hybrid materials synthesized through the co-con-
densation of the bridged organosilane.
methane (135 mL, 1:1). Iodochloride (12.1 g) dissolved in a mix-
ture of methanol–dichloromethane (15 mL, 1:1) was added into
the solution of 2,6-diisopropylaniline [34]. The resultant mixture
was stirred at room temperature for 24 h and then at 40 °C for
16 h. At the end of the reaction, the solvent was evaporated out.
The resultant crude solid product was treated with 40 mL of an
aqueous sodium hydroxide solution (20 wt%) and then extracted
with diethyl ether (3 ꢀ 35 mL). After drying over sodium sulfate,
the solvent was evaporated out. Compound 1 was eventually ob-
tained after distillation under reduced pressure (12.2 g,
yield = 54.4%). 1H NMR (CDCl3): 7.19 (s, 2 H); 3.43 (s, 2 H); 2.77–
2.84 (m, 2H), 1.20 (s, 12).
2.3. Synthesis of bis(4-iodo-2,6-diisopropylphenyl)diazabutadiene (2)
Glyoxal (3.6 g; 40 wt% in water) was added dropwise into
126 mL of methanol containing 15.1 g of 4-iodo-2,6-diisopropylan-
iline. Seventeen drops of formic acid were also added into this sys-
tem as catalyst. The reaction mixture was stirred at room
temperature for 16 h and at 40 °C for another 24 h. The yellow so-
lid was achieved by a filtration and washed with methanol, leading
to the title compound 2 (yield = 70.0%). 1H NMR (CDCl3): 8.01 (s, 2
H); 7.34–7.44 (m, 4 H); 2.79–2.84 (m, 4 H); 1.02–1.17 (m, 24 H).
Herein, we report the successful synthesis of a new type of 3D
cubic mesoporous hybrid materials containing IPr precursor in
the framework. By the coordination of the synthesized hybrid
material with Pd(acac)2, a novel heterogeneous hybrid catalyst
has been derived, which is active and recyclable for Suzuki–Miya-
ura couplings of aryl chlorides and C(sp3)-chlorides.
2.4. Synthesis of 1,3-bis(4-iodo-2,6-diisopropylphenyl)-1H-imidazol-
3-ium trifluoromethane-sulfonate (3)
Compound 2 (3.15 g) was dissolved in 39 mL of dry dichloro-
methane. Chloromethyl pivalate (1.25 g) and silver trifluorome-
thane sulfonate (1.67 g) were added into this dichloromethane
solution. The reaction mixture was heated to reflux for 25 h. After
cooling to room temperature, the formed precipitate (silver chlo-
ride) was removed by filtration. The solution was concentrated,
giving the crude product. The pure compound 3 was obtained after
recrystallization in a mixture of methanol and diethyl ether
(yield = 65.0%). 1H NMR (CDCl3): 10.1 (s, 1 H); 8.51 (s, 2 H); 7.84
(s, 4 H); 2.20–2.23 (m, 4 H); 1.09–1.22 (m, 24 H).
2. Experimental
2.1. Reagents and materials
Pluronic P123 (EO20PO70EO20) and F127 (EO106PO70EO106) were
purchased from Sigma Company. Pd(acac)2 (acac = acetylaceto-
nate) was obtained from Kaida Metal Catalyst & Compounds Co.
Ltd. (China). Various arylboronic acids were obtained from Beijing
Pure Chemical Co. Ltd. Aryl chlorides, ICl and bromides were pur-
chased from Aladdin Company. 2,6-Diisopropylaniline and other
reagents were obtained from Shanghai Chemical Reagent Company
of Chinese Medicine Group. All solvents were of analytical quality.
2.5. Synthesis of 1,3-bis(4-triethoxysilyl-2,6-diisopropylphenyl l)-1H-
imidazol-3-ium trifluoromethane-sulfonate (4)
Compound 3 (2.7 g), Rh(cod)(CH3CN)2BF4 (0.075 g), triethoxysi-
lane (5.5 g) and triethylamine (4.3 g) were dissolved in 60 mL of
dry DMF. The resulting solution was stirred at 80 °C for 8 h under
N2 atmosphere. After cooling, the solvent was removed off by dis-
tillation, and the resultant brown residue was dissolved in 60 mL of
dichloromethane (DCM). After removal of the precipitation by fil-
tration, the DCM solution was washed with cold water
2.2. Synthesis of 4-iodo-2,6-diisopropylaniline (1)
The route for the synthesis of the IPr precursor-bridged orga-
nosilane is described in Scheme 1. 2,6-Diisopropylaniline
(13.19 g) was dissolved in a mixture of methanol and dichloro-
ICl
OHC CHO
NH2
I
NH2
I
N
N
I
1
2
CMP
Ag+CF3SO3
HSi(OEt)3
Catalyst
N+
I
N+
Si(OEt)3
I
N
(EtO)3Si
N
-
-
CF3SO3
-
CF3SO3
3
4
CMP = chloromethyl pivalate; Catalyst = Rh(cod)(CH3CN)2 BF4
Scheme 1. Synthesis routes for IPr precursor-bridged triethoxysilane [IPr, 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene].

H. Yang et al. / Journal of Catalysis 276 (2010) 123–133
125
(5 ꢀ 30 mL). The organic phase was dried over sodium sulfate and
filtered. After concentration, compound 4 was achieved (yield = ca.
80%). 1H NMR (CDCl3): 10.18 (s, 1 H); 8.54 (s, 2 H); 7.52–7.64 (m,
4 H); 3.68–3.95 (m, 12 H); 2.43 (m, 4 H); 1.25–1.32 (m, 42 H).
13C NMR (DMSO) is shown in Section 3.
washed with dry 1,4-dioxane and dried under vacuum. The even-
tually achieved solid was designated as SBA-16-IPr( )–Pd. The
homogeneous catalyst was prepared through the coordination of
v
4 with Pd(acac)2 (the molar ratio is 1:1) for comparison.
2.8. General procedure for Suzuki–Miyaura coupling of aryl chlorides
and arylboronic acids
2.6. Synthesis of the hybrid mesoporous materials
A mixture of aryl chlorides (2 mmol), phenylboronic acid
(2.2 mmol), potassium tert-butoxide (3 mmol), iso-propyl alcohol
(6 mL) and the catalyst SBA-16-IPr(v)–Pd was stirred at 80 °C un-
The cubic mesoporous materials containing IPr (Scheme 2) were
synthesized by the condensation of compound 4 and tetraethyl
orthosilicate (TEOS) in the presence of a mixture of P123 and
F127 as template. A mixture of F127 (0.62 g) and P123 (0.1 g)
was dissolved in a solution of 25 g of distilled water and 5 g of con-
centrated hydrochloric acid (36.5%). After the solution was stirred
at 35 °C for 5 h, total 10.5 mmol of the total siliceous precursors
dissolved in 3 mL of ethanol was added dropwise to the above
solution. The molar fraction of 4 [2M4/(2M4 + MTEOS), wherein
M is the molar numbers] in the total siliceous precursors was
tuned from 2.5% to 5%, 7.5%, 10%, 12.5% and 15%. After stirring
for 40 min, the resultant suspension was transferred into auto-
claves and treated at 35 °C for 24 h under static conditions. After-
ward, the temperature was raised up to 100 °C and the
suspension was kept at this temperature for 24 h. After the hydro-
thermal process, the precipitated solid was isolated by filtration,
washed with water and ethanol, and then dried at room tempera-
ture for 12 h. The template was removed by repeated extractions
with a diluted ethanolic HCl solution under the refluxing condi-
tions (1 g of the hybrid material, 0.5 g of concentrated HCl solution
and 50 mL of ethanol). According to the molar fraction of com-
pound 4 in the total silicon precursors, six materials, SBA-16-
IPr(2.5%), SBA-16-IPr(5%), SBA-16-IPr(7.5%), SBA-16-IPr(10%),
SBA-16-IPr(12.5%) and SBA-16-IPr(15%), were obtained (the num-
ber in parentheses means the molar percent of compound 4 in
the total silicon precursor).
der N2 atmosphere for a given time. At the end of the reaction, the
mixture was cooled down to room temperature and repeatedly ex-
tracted with diethyl ether. The combined organic layers were con-
centrated, and the resulting product was purified by column
chromatography on silica gel. The products were confirmed by
1H NMR (Supplementary material). The purity was confirmed by
GC.
The recycling test for the Suzuki–Miyaura coupling was con-
ducted as follows: for the first run, a mixture of 12 mmol of 40-
chloroacetophenone, 13.2 mmol of phenylboronic acid, 18 mmol
of potassium tert-butoxide, SBA-16-IPr(10%)–Pd (0.5 mol% Pd with
respect to aryl chloride), 36 mL of iso-propyl alcohol was stirred at
80 °C under N2 atmosphere. At the end of the reaction, the system
was cooled down to room temperature and repeatedly extracted
with diethyl ether. The recovered catalyst was washed with diethyl
ether, water, methanol and acetone in sequence and then dried un-
der vacuum. The recovered catalyst was weighed again. The fresh
solvent and substrate were added, but the molar ratio of substrate
to Pd remained the same as that in the first run.
2.9. General procedure for the coupling reactions of benzylic/allylic
chlorides and arylboronic acids
A mixture of benzylic or allylic chlorides (2 mmol), phenylbo-
ronic acid (2.2 mmol), potassium tert-butoxide (3 mmol), iso-pro-
pyl alcohol (6 mL), and the catalyst SBA-16-IPr(10%)–Pd was
stirred at 80 or 50 °C under N2 atmosphere. At the end of reaction,
the mixture was cooled down to room temperature and repeatedly
extracted with diethyl ether. The combined organic layers were
concentrated, and the resulting product was purified by a column
chromatography on silica gel. The products were confirmed by 1H
NMR (Supplementary material).
2.7. Coordination of SBA-16-IPr(
v) materials with Pd(acac)2
SBA-16-IPr( ) (1.0 g; dried at 100 °C for 4 h) was dispersed in
v
30 mL of dry 1,4-dioxane containing 0.02 g of Pd(acac)2
(acac = acetylacetonate). After the mixed system was stirred at
100 °C for 40 h under N2 atmosphere, 1,4-dioxane was removed
by filtration, affording a brown solid. The solid was repeatedly
2.10. Characterization and analysis
Small-angle powder X-ray powder diffraction was performed
on Rigaku (Cu Ka, 40 kV, 30 mA). N2 physical adsorption was car-
ried out on a micrometrics ASAP2020 volumetric adsorption ana-
lyzer (before measurements, samples were out gassed at 125 °C
for 6 h). The Brunauer–Emmett–Teller (BET) surface area was eval-
uated from data in the relative pressure range of 0.05–0.15. The to-
tal pore volume of each sample was estimated from the amount
adsorbed at the highest P/P0 (above 0.99). Pore diameters were
determined from the adsorption branch using the Barrett–Joy-
ner–Halenda (BJH) method. FT-IR spectra were collected on a PE-
1730 infrared spectrometer. Pd content was analyzed with an
inductively coupled plasma-atomic emission spectrometry (ICP-
AES, AtomScan16, TJA Co.). C and N content analysis was con-
ducted on Vario EL (Elementar). X-ray photoelectron spectra
(XPS) were recorded on a Kratos Axis Ultra DLD, and the C1S line
at 284.8 eV was used as a reference. TEM micrographs were taken
with a JEM-2000EX transmission electron microscope at 120 kV.
Solid-state NMR spectra were recorded on an Infinityplus
300 MHz spectrometer: for 13C CP-MAS NMR experiments,
75.4 MHz resonant frequency, 4 kHz spin rate, 4 s pulse delay,
Scheme 2. The structural model for the hybrid mesoporous materials SBA-16-
IPr(v). Note: for clarification, another four entrances are omitted. In fact, each
spherical cage is connected with eight neighboring cages through entrances.

126
H. Yang et al. / Journal of Catalysis 276 (2010) 123–133
Table 1
1.0 ms contact time, hexamethyl benzene as a reference com-
pound; for 13Si CP-MAS NMR experiments, 79.6 MHz resonant fre-
quency, 4 kHz spin rate, 4.0 s pulse delay, TMS as a reference
compound. GC analysis was conducted on a SP-GC6800A.
Physical parameters of SBA-16-IPr(v) materials.
Samples
Sa
Pb
Dc (nm)
C(ꢁ)d
N(ꢁ)d
(m2/g) (cm3/g)
(wt%)
(wt%)
SBA-16-IPr(2.5%)
SBA-16-IPr(5%)
SBA-16-IPr(7.5%)
SBA-16-IPr(10%)
SBA-16-IPr(12.5%) 752
SBA-16-IPr(15%)
753
703
670
645
0.65
0.63
0.58
0.55
0.62
0.58
5.5
5.3
4.7
4.6
8.3 (6.3)
0.5 (0.5)
14.8 (12.0) 0.9 (1.0)
20.2 (17.0) 1.3 (1.5)
24.3 (21.5) 1.8 (2.1)
–
–
3. Results and discussion
4.2
–
–
3.1. Synthesis of IPr precursor-bridged organosilane and the hybrid
materials
576
4.0 (14.0)e
a
b
c
BET surface area.
Single-point pore volume calculated at a relative pressure P/P0 of 0.99.
BJH method from adsorption branch.
Determined by combustion chemical analysis; the data in parentheses are the
In order to incorporate IPr into a material framework through a
hydrolysis–condensation process, we need to derive IPr carbene
precursor with hydrolyzable alkoxysilyl groups. Here, we followed
a modified route for the introduction of two trialkoxysilyl groups
to the terminals of IPr precursor (Scheme 1, compound 4) [34].
At first, iodination of 2,6-diisopropylaniline with iodine chloride
gave 4-iodo-2,6-dimethylaniline (compound 1). In the second step,
compound 1 was reacted with glyoxal, leading to a bis-Schiff’s base
compound 2. Compound 2 was then subjected to a ring-closing
step in the presence of silver salt, yielding an iodo-substituted
IPr precursor (compound 3). This compound was then silylated
with HSi(OEt)3 using a Rh complex as catalyst, eventually affording
IPr precursor-bridged organosilane (compound 4, confirmed by 1H
NMR and 13C NMR). The whole process included four steps, and the
yields for every step were over 50%.
The hybrid materials were fabricated via a modified procedure
for synthesizing SBA-16 [36]. Because compound 4 has a bulky or-
ganic group between two triethoxysilyl groups, it is necessary to
use other small silane compounds such as TEOS to co-condense with
4 for constructing an ordered mesostructure. Previous studies re-
vealed that the presence of bulky group-bridged organosilane usu-
ally influenced the mesostructural ordering and even induced the
mesophase changes [42]. With these considerations, the molar frac-
tion of 4 in the initial total siliceous precursors [(2M4/(2M4 + MTEOS),
wherein M is the molar numbers], was finely tuned from 2.5% to 5%,
7.5%, 10%, 12.5% and 15%. Block copolymer F127 admixed with a lit-
tle amount of block copolymer P123 was chosen as template be-
cause the presence of P123 was favorable to tune the textural
properties. Compound 4 does not dissolve in TEOS. So, a series of or-
ganic solvents were preliminarily screened to find one which can as-
sist the dissolution of 4 in TEOS. Ethanol was found to be a good
solvent. Under our synthesis conditions, 3 mL of ethanol was essen-
tial to dissolve 4 and achieve hybrid mesoporous materials with a
homogeneous appearance. The lack of ethanol led to a solid material
with an unevenly distributed color, suggesting that a phase separa-
tion occurred during the synthesis. The excess of ethanol probably
caused a deterioration of the mesostructural ordering. After the
hydrothermal synthesis, the template filled in the pores was repeat-
edly extracted by refluxing in an acidic ethanol solution. Eventually,
d
theoretical values.
e
The data in parentheses are the second pore diameter.
and a type H2 hysteresis loop at the relative pressure of 0.45–
0.7, which is characteristic of a high-quality cage-like pore struc-
ture of a typical SBA-16. The pore size plot calculated from N2
adsorption branch exhibits a narrow distribution centered at
5.5 nm. The specific surface area and pore volume of SBA-16-
IPr(2.5%) are 753 m2/g and 0.65 cm3/g, respectively (in Table 1),
which fall in the ranges of a typical cubic mesoporous material.
The hysteresis loop shows a slight shift to the lower relative pres-
sure when the molar fraction of 4 in the initial gel mixture in-
creases up to 5%. This is an indication of a decrease in the pore
size (down to 5.3 nm, in Table 1). As the molar fraction of 4 in
the initial gel mixture further increases up to 7.5% and 10%, the
textural parameters such as pore size, specific surface area and
pore volume continue decreasing (Table 1). This trend probably re-
flects the changes in the interactions between the silicon species
and template molecules. Till the molar fraction of 4 increases up
to 12.5%, the nitrogen adsorption/desorption isotherm remains a
type IV of a cage-like mesoporous structure. However, the molar
fraction up to 15% led to considerable changes in the isotherm.
The isotherm shows two steps of less steep capillary condensa-
tion/evaporation, indicating a nonuniform distribution of pore
sizes.
The XRD patterns of SBA-16-IPr(v) materials are displayed in
Fig. S3 (Supplementary material). For the sample SBA-16-
IPr(2.5%), the XRD pattern shows a strong peak at 2h = 0.85°, which
is attributed to the (1 1 0) reflection. Two small peaks at 2h = 1.12°
and 2h = 1.77° can be also observed (inset in Fig. S3, Supplementary
material), which are assigned to the (2 0 0) and (3 1 1) reflections,
respectively. Such assignments for these three diffraction peaks are
in consistent with a cubic Im3m symmetry. When the molar frac-
tion of 4 increases up to 5%, the peaks of the (1 1 0) reflections
along with the (2 0 0) and (3 1 1) reflections can be still observed
although the intensities decrease. When the molar fraction of 4 in-
creases up to 7.5%, 10% and 12.5%, the peak intensity of the (1 1 0)
reflection further decreases and the diffraction peaks for the (2 0 0)
and (3 1 1) reflections become less well resolved. Additionally, the
(1 1 0) peak shifts toward higher angles with the increasing of the
molar fraction of 4, indicating that the dimensions of the unit cell
gradually reduce. The increase in the molar fraction up to 15% leads
to the nearly complete disappearance of these diffraction peaks,
indicating a deterioration of the mesostructural ordering. The re-
sults can be explained by the fact that a higher amount of the bulky
organosilane 4 led to a serious damage of the self-assembly by mi-
celles (accompanied by a phase separation) and the part of amor-
phous silica thus dramatically increased.
six SBA-16-IPr(v) materials were obtained (Scheme 2), wherein v
equals to 2.5%, 5%, 7.5%, 10%, 12.5% or 15% according to the molar
fraction of 4 in the initial gel mixture.
3.2. Structural characterization
In order to investigate the effects of the contents of 4 on the
structure and physical properties of the obtained hybrid materials,
N2 sorption, XRD and TEM were employed to characterize SBA-16-
IPr(
v) materials.
The N2 sorption isotherms of SBA-16-IPr(
v) samples are pre-
sented in Fig. S1 (Supplementary material). The corresponding
pore size distribution curves are displayed in Fig. S2 (Supplemen-
tary material).Their textural parameters measured with N2 sorp-
tion are summarized in Table 1. SBA-16-IPr(2.5%) shows a type
IV isotherm with a steep capillary condensation/evaporation step
TEM investigations show that the synthesized materials have
highly ordered mesoporous structures similar to a typical SBA-16
till the molar fraction of 4 increases up to 10%. Fig. 1 selectively
presents the TEM images for SBA-16-IPr(10%), SBA-16-IPr(12.5%)

H. Yang et al. / Journal of Catalysis 276 (2010) 123–133
127
Fig. 2. FT-IR spectra for SBA-16-IPr(
v) materials. (a) SBA-16; (b) SBA-16-IPr(2.5%);
(c) SBA-16-IPr(5%); (d) SBA-16-IPr(7.5%); (e) SBA-16-IPr(10%).
1350 cmꢂ1) for trifluoromethane sulfonate were not observed. It
can be explained that trifluoromethane sulfonate was exchanged
by Clꢂ due to the presence of significantly excessive HCl under syn-
thesis conditions. So, the eventual compositions of SBA-16-IPr(
materials are proposed as Scheme 2.
v)
The quantitative determinations of 4 incorporated into the ob-
tained materials were made with elemental analysis of C and N
contents. The results are also included in Table 1. Elemental anal-
ysis revealed that the C and N contents gradually increased as the
fraction of 4 in the initial gel mixture increased. The determined C
contents for these four materials were higher with respect to the
theoretical values calculated from the initial molar compositions,
while the detected N contents were lower than their theoretical
values. These results could be explained by the fact that a small
amount of templates remained on the solid materials even after re-
peated solvent extractions due to the difficulty in the complete re-
moval of templates for SBA-16 type of materials [43–45]. Based on
the results of elemental analysis, it is estimated that ca. 4–6% tem-
plates of the initially added amount remained on the solid catalyst
(on the assumption that alkoxy groups of 4 were completely con-
densed), namely the loading of the residue templates is ca. 3–
Fig. 1. TEM images for SBA-16-IPr(v) materials. (a) SBA-16-IPr(10%); (b) SBA-16-
IPr(10%); (c) SBA-16-IPr(12.5%); (d) SBA-16-IPr(15%). The bar is 50 nm.
and SBA-16-IPr(15%). In Fig. 1a and b, SBA-16-IPr(10%) clearly
shows [1 0 0] and [1 1 1] projections of the cubic body-centered
(Im3m) structure. When the molar fraction of 4 increases up to
12.5%, the regular arrays of pore ordering begin deforming
(Fig. 1c). For SBA-16-IPr(15%) sample (Fig. 1d), the irregular pore
arrays and nonuniform pore sizes are observed throughout the
materials, which are consistent with XRD and N2 sorption results.
Above N2 sorption, XRD and TEM results reveal that an Im3m
mesoporous structure can be achieved under the investigated con-
ditions and that the amount of IPr precursor in the initial gel mix-
ture has a significant effect on the mesoporous structure. The
mesostructural ordering decreases as the amount of IPr precursor
increases. Under the current synthesis conditions, 10 mol% is the
up limit for achieving a good Im3m structure with a high specific
surface area and pore volume.
5 lmol/g (F127). The comparison of the theoretical and the deter-
mined N contents reveals that ca. 86–100% of 4 in the initial gel
Q3
3.3. Compositional characterization
To clarify the compositions of the synthesized materials, we
employed FT-IR and solid-state NMR to further characterize the
hybrid materials. Their FT-IR spectra are shown in Fig. 2. For com-
parison, the pure silica SBA-16 was synthesized. Its FT-IR spectrum
is also included in Fig. 2. Compared with the FT-IR spectrum of
pure silica SBA-16, the hybrid materials exhibit new peaks at
2976, 1542 and 1468 cmꢂ1. These three peaks correspond to
CAH stretching, C@N stretching and CAH bending vibrations, sug-
gesting that IPr precursor has been successfully incorporated into
the solid materials. It is worthwhile to note that the intensities
of such three peaks gradually increase as the molar fractions of 4
in the initial gel mixture increase. These results reflect that the
amount of IPr precursor incorporated into the solid materials in-
creases. The characteristic FT-IR signals (at ca. 1600 and
Q4
Q2
2 T3
T
100
50
0
-50
ppm
-100
-150
-200
Fig. 3. 29Si CP-MAS NMR spectrum for SBA-16-IPr(10%).

128
H. Yang et al. / Journal of Catalysis 276 (2010) 123–133
Fig. 4. 13C NMR spectra. (A) 13C NMR spectrum of 4. (B) 13C CP-MAS NMR spectrum for SBA-16-IPr(10%). Star peak is attributed to the solvent.
mixture was incorporated into the hybrid materials. Based on the
determined N content, the IPr carbene precursor present on SBA-
16-IPr(10%) is estimated to be 0.64 mmol/g.
nate with the hybrid material because the formed Pd–IPr complex
was stable in air and catalytically active for Suzuki–Miyaura reac-
tion [9,46]. Pd(acac)2 (2 wt%) with respect to SBA-16-IPr(10%) was
used, which was sufficiently high for the catalytic reaction. The
coordination was conducted in a 1,4-dioxane solution under the
refluxing conditions. ICP-AES analysis revealed that the all Pd(a-
cac)2 in solution was loaded on the hybrid material under the inves-
tigated conditions. XPS spectroscopy was employed to investigate
the coordination states of Pd species on the solid surface. Fig. 5 pre-
sents the XPS spectra of SBA-16-IPr(10%) derived with Pd(acac)2
and pure Pd(acac)2 complex. In Fig. 5A, XPS elemental survey scan
of surface elementals of SBA-16-IPr(10%) derived with Pd(acac)2 re-
veals that silicon, oxygen, carbon, nitrogen, palladium and chlorine
elementals are present in the materials, as initially expected. In
Fig. 5B, Pd(acac)2 exhibits two peaks centered at 338.18 and
343.48 eV, which are assigned to Pd (II) 3d5/2 and Pd 3d3/2, respec-
tively. Compared with the pure compound, the Pd 3d5/2 and 3d3/2
peaks for SBA-16-IPr(10%) derived with Pd(acac)2 slightly shift
down to 337.88 and 343.38 eV, respectively. Such changes may im-
ply that new Pd(II) species were formed on the solid surface.
Figs. 3 and 4 show the 29Si CP-MAS NMR and 13C CP-MAS NMR
spectra of SBA-16-IPr(10%), respectively. In Fig. 3, the 29Si CP-MAS
NMR spectrum of SBA-16-IPr(10%) shows both ‘‘Q” and ‘‘T” silicon
series. The signals at ꢂ110, ꢂ101 and ꢂ92 ppm correspond to Q4
[Si(OSi)4], Q3 [Si(OH)(OSi)3] and Q2 [Si(OH)2(OSi)2] silicon sites,
respectively. The peaks centered at ꢂ81 and ꢂ72 ppm are ascribed
to T3 [SiC(OSi)3] and T2 [SiC(OH)(OSi)2] organosilicon sites, respec-
tively. The presence of ‘‘T” bands indicates that IPr precursor has
been incorporated in the framework through a SiAC linkage. For
comparison, the liquid-state 13C NMR spectrum of 4 and 13C CP-
MAS NMR spectra of SBA-16-IPr(10%) are displayed in Fig. 4. Sim-
ilar to the liquid-state 13C NMR spectrum (Fig. 4A), SBA-16-
IPr(10%) clearly showed the signals for isopropyl groups on the
aromatic rings at 24–30 ppm, and two aryl groups as well as the
central imidazolium rings in the range 125–145 ppm (Fig. 4B).
Their chemical shifts of liquid and solid sates are broadly in agree-
ment with each other. The observed signal at 60 ppm indicates that
a portion of (EtO)3Si-groups were not completely hydrolyzed. It
should be noted that the characteristic signals of template [at
71 ppm, A(CH2CH2O)nA] were not observed. This is an indication
that the content of the residual template is below the detection
limit of solid-state NMR.
3.5. Suzuki–Miyaura reaction
To investigate the catalytic activities of the synthesized materi-
als with different amounts of IPr, we coordinated SBA-16-IPr(
materials with 2 wt% Pd(acac)2, yielding solid catalysts SBA-16-
IPr( )–Pd. The pure silica SBA-16 was also derived with Pd(acac)2
v)
Above characterizations confirm that IPr precursor is covalently
integrated with the hybrid materials.
v
for comparison. Their catalytic activities were examined with Su-
zuki–Miyaura coupling of chlorobenzene and phenylboronic acid
in iso-propyl alcohol at 80 °C under N2 atmosphere. Potassium
tert-butoxide was used as a base to promote this reaction [6,9].
The ratio of Pd to chlorobenzene was kept at 0.5 mol%. The reaction
3.4. Coordination of the hybrid materials with Pd(acac)2
To probe the coordination capacity of IPr carbene in the solid
framework, we chose Pd(acac)2 (acac = acetylacetonate) to coordi-

H. Yang et al. / Journal of Catalysis 276 (2010) 123–133
129
the hybrid material. These primary investigations showed that
the activity of the hybrid solid catalyst was dependent on not only
the IPr content of the solid material but also the structure of the so-
lid material. SBA-16-IPr(10%)–Pd exhibits the highest activity
among the investigated solid catalysts. On this occasion, the ratio
of IPr/Pd is estimated to be 9.8. The decrease in the ratio of IPr/
Pd led to a slight reduction in conversion (the ratio of 6.5 gave a
90% conversion under the same conditions). In view of the stability
of the heterogeneous catalysts, the excessive IPr precursors may be
also desired because the uncoordinated IPr precursors in the
framework as ionic liquids help to stabilize the in situ-formed
Pd(0) nanoparticles against growth [18].
Suzuki–Miyaura couplings of other aryl halides and phenylbo-
ronic acid were also examined. The corresponding results are in-
cluded in Table 3. SBA-16-IPr(10%)–Pd was catalytically active
toward all the substrates examined. This solid catalyst gave biphe-
nyl in an isolated yield of 78% at 80 °C within 24 h (Table 3, entry 1,
Fig.S4 of Supplementary material). Under the same conditions, its
counterpart [treating 4 with Pd(acac)2] gave an 80% yield within
8 h. The decreased activity of the complex after immobilization
may be partially due to the diffusion limitations inside the nanop-
ores. For aryl chloride bearing an electron-withdrawing group, an
89% yield was obtained in a shorter reaction time and the lower
Pd loading led to a relatively lower conversion (Table 3, entry 2).
This catalyst could afford good yields for aryl chlorides bearing
electron-denoting groups such as ACH3 and AOCH3 (Table 3, en-
tries 3 and 4). To our delight, for electron-rich aryl chlorides with
steric hindrances such as 2-methylchlorobenzene and 2,4-dimeth-
ylchlorobenzene, moderate to good yields could also be achieved
(Table 3, entries 5 and 6). Additionally, aryl bromides could be eas-
ily converted to the corresponding products in excellent yields
over this solid catalyst (Table 3, entries 7 and 8).
We further tested its catalytic activities for the coupling of
various arylboronic acids with chlorobenzene. These results are
summarized in Table 4. For electron-rich arylboronic acids, SBA-
16-IPr(10%)–Pd gave good yields (Table 4, entries 1 and 2). For
electron-rich arylboronic acids with moderate steric hindrances,
moderate to good yields were obtained (Table 4, entries 3 and
4). The relatively lower yield was achieved for electron-poor aryl-
boronic acid (Table 4, entry 5). The differences in yields reflect the
electron effects on the activity of the solid catalyst. The above
activity tests suggest that the developed catalyst can represent
one of the most efficient solid catalysts for Suzuki–Miyaura cou-
pling of the challenging aryl chlorides under relatively mild con-
ditions, although its preparation procedure is more complex than
those of the previously reported catalysts such as Pd/C catalyst
[15,16,47–50]. The outstanding activity toward aryl chlorides
Fig. 5. XPS spectra of SBA-16-IPr(10%) derived with Pd(acac)2. (A) The elemental
survey scan of the material. (B) Pd XPS spectra of Pd(acac)2 and SBA-16-IPr(10%)
derived with Pd(acac)2.
results are summarized in Table 2. Only a trace amount of product
was detected for the pure silica SBA-16 derived with Pd(acac)2
within 24 h. Under the same conditions, SBA-16-IPr(2.5%)–Pd
afforded a 38% conversion. It is noteworthy that the increase in
IPr contents leads to a continuous increase in the conversion. A
conversion of 92% could be obtained on the catalyst SBA-16-
IPr(10%)–Pd. The significantly improved conversions verified the
role of IPr carbene in the framework in promoting the coupling
reaction. However, the IPr contents in framework further in-
creased, and the conversion was observed to decrease. For SBA-
16-IPr(15%)–Pd, the conversion decreased down to 79%. The de-
creased activity is probably due to the reduced surface area of
may be attributed to the strong
the framework and the open structure of the solid catalyst. The
strong -donor ligand is necessary to reach a high degree of effi-
r-donor ligand (IPr carbene) in
r
ciency, and the open structure is helpful for the access of the
substrates.
The recyclability of SBA-16-IPr(10%)–Pd was investigated with
the consecutive Suzuki–Miyaura couplings of 40-chloroacetophe-
Table 2
Suzuki–Miyaura coupling reactions catalyzed by SBA-16-IPr(v) materials derived with Pd(acac)2.
0.5 mol% Pd, 80 OC
Cl
B(OH)2
+
iso-propyl alcohol
Samples
SBA-16
<1
SBA-16-IPr(2.5%)
38
SBA-16-IPr(5%)
72
SBA-16-IPr(7.5%)
79
SBA-16-IPr(10%)
92
SBA-16-IPr(12.5%)
89
SBA-16-IPr(15%)
79
Conv.a (%)
a
The conversions were determined with GC.

130
H. Yang et al. / Journal of Catalysis 276 (2010) 123–133
Table 3
Suzuki–Miyaura couplings of various aryl halides and phenylboronic acid over SBA-16-IPr(10%)–Pd.
0.5 mol% Pd, 80 OC
Cl
B(OH)2
+
R
iso-propyl alcohol
R
Entry
1
Substrate
Product
Pd (mol%)
0.5
Time (h)
24
Yielda (%)
78
Cl
2
3
4
5
0.5 (0.1)
0.5
20 (20)
24
89 (69)
71
O
O
Cl
CH3
H3C
Cl
0.5
24
68
OCH3
H3CO
Cl
0.5
24
70
Cl
CH3
CH3
CH3
CH3
6
0.5
24
50b
Cl
CH3
CH3
7
8
0.25
0.25
6
6
90
92
Br
H3COC
Br
COCH3
a
The isolated yield.
Determined with GC.
b
Table 4
Suzuki–Miyaura couplings of various phenylboronic acids and chlorobenzene over SBA-16-IPr(10%)–Pd.
0.5 mol% Pd, 80 OC
B(OH)2
Cl
+
iso-propyl alcohol
R
R
Entry
1
Substrate
Product
Yielda (%)
85
77
72
H3C
B(OH)2
CH3
2
3
OCH3
H3CO
B(OH)2
B(OH)2
B(OH)2
CH3
CH3
4
49
66
OCH3
OCH3
5
F3C
B(OH)2
CF3
a
Isolated yields.

H. Yang et al. / Journal of Catalysis 276 (2010) 123–133
131
none with phenylboronic acid. Due to the unavoidable loss of solid
catalyst during the course of recovery and washing, the reaction
scale was amplified six times to ensure that the catalyst of good
quality was available for us to perform the several consecutive
recycling reactions, as described in Experimental. The recycling re-
sults of SBA-16-IPr(10%)–Pd are summarized in Table 5. Under the
scale-up conditions, the fresh solid gave the product in an 89%
yield within 20 h. After the first reaction cycle, the catalyst could
be recovered by centrifugation along with filtration. After being
washed and dried, the recovered catalyst was directly used for
the next reaction cycle. From the second to the fourth reaction cy-
cle, the 85–89% yields of acetylbiphenyl were achieved without a
prolonged time. Beginning with the fifth reaction cycle, a slight
longer reaction time was needed to complete the reaction, and
81–87% yields were obtained from the fifth to the seventh reaction
cycle. For the eighth reaction cycle, an 84% yield was still attained.
Although the activity of the solid catalyst slightly decreases during
recycling, it represents one of the most robust heterogeneous cat-
alysts for Suzuki–Miyaura coupling of aryl chlorides [15,16,47–49].
After the coupling reaction of chlorobenzene with phenylboronic
Table 5
Recyclability test of SBA-16-IPr(10%)–Pd with the Suzuki–Miyaura coupling reaction.
O
O
0.5% mol Pd, 80 OC
B(OH)2
Cl
+
iso-propyl alcohol
Cycles
1
2
3
4
5
6
7
8
Time (h)
20
89
20
88
20
85
20
89
24
87
26
81
28
83
34
84
Yielda (%)
a
Isolated yield.
Table 6
The coupling reactions of benzylic/allylic chlorides and arylboronic acids over SBA-16-IPr(10%)–Pd.a
CH2
ClCH2
R
0.5% mol Pd
B(OH)2
or
+
or
iso-propyl alcohol
R
Cl
Entry
1
Arylboronic acids
Products
Time (h)
10
Yieldb (%)
88
B(OH)2
CH2
2
3
4
5
6
12
10
12
10
12
71
90
68
92
61
B(OH)2
H3C
CH2
H3C
B(OH)2
B(OH)2
H3C
H3C
H3CO
H3CO
CH2
H3CO
H3CO
B(OH)2
B(OH)2
7
8
9
10
10
10
91
95
89
B(OH)2
B(OH)2
CH2
CH3
CH2
OCH3
CH3
OCH3
F3C
B(OH)2
F3C
CH2
a
For benzylic chloride, the reaction temperature was 80 °C; for allylic chloride, the reaction temperature was 50 °C.
Isolated yield.
b

132
H. Yang et al. / Journal of Catalysis 276 (2010) 123–133
acid proceeded for 6 h (the conversion at 50%), the reaction was
stopped and the filtrate was immediately collected under the hot
conditions. A further 4% increase in conversion was observed after
heating the filtrate at 80 °C for another 18 h (50% of the initial t-
BuOK amount was added to the filtrate). These results may indicate
that the observed conversion was at least partially contributed by
the leached Pd species in the filtrate. Interestingly, the determina-
tion of the Pd content on the catalyst used once reveals that ca. 98%
Pd of the fresh catalyst was kept on the recovered solid catalyst.
The TEM image reveals that the mesoporous structure of the recov-
ered catalyst roughly survived the first reaction cycle and that Pd
nanoparticles are present on the solid catalyst used once as the
homogeneous system (Figs.S5 and S6 in Supplementary material).
Clearly, the Pd particles’ sizes for the homogeneous system are not
uniform. Some of them are larger than those for the heterogeneous
system. The above investigations demonstrate the role of the solid
material in the Pd recovery and stabilizing the Pd nanoparticles.
Additionally, the hybrid solid catalyst exhibited high stability in
air. It was found that there was not decrease in activity after the
catalyst was exposed to air for one month.
Acknowledgments
We acknowledge New Teacher Foundation from Education Min-
istry of China (200801081035), Shanxi Natural Science Foundation
for Youths (2009021009), the Natural Science Foundation of China
(NSF20903064), Shanxi University Innovative Experimental Project
for Undergraduates and Jiangsu Key Lab. of Fine Petrochemistry for
financial supports (KF0802). We also thank Dr. Lei Zhang (BASF,
Nederland, BV, De Meern) for his help with this manuscript.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jcat.2010.09.004.
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We have successfully synthesized new 3D cage-like mesopor-
ous materials containing a well-known NHC ligand (IPr) precursor
in the framework by co-condensation of IPr precursor-bridged tri-
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