Rationally designed palladium complexes on a bulky N-heterocyclic carbene-functionalized organosilica: An efficient solid catalyst for the Suzuki-Miyaura coupling of challenging aryl chlorides

Article abstract of DOI:10.1039/c1gc15757d

An IPr-functionalized (IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol- 2-ylidene, a N-heterocyclic carbene ligand) mesoporous ethane-silica was synthesized through a co-condensation of IPr-bridged triethoxysilane and bis(triethoxysilyl)ethane. This functionalized mesoporous ethane-silica was characterized with N₂ sorption, XRD, FT-IR, solid NMR and XPS. Owing to high surface area, large pore volume and organically functionalized surface, such a material can be used as a scaffold to design surface NHC-Pd [NHC = N-heterocyclic carbene] complexes. By variation of Pd precursors and/or ancillary ligands, a library of NHC-Pd complexes on the solid surface were tentatively created. It was found that the Pd precursor and ancillary ligands had significant impacts on the activities of the resultant heterogeneous catalysts in the Suzuki-Miyaura coupling of aryl chlorides. Surface NHC-Pd complexes with acetylacetonate (acac) and 3-chloropyridine as ancillary ligands are the most active. A wide range of aryl chlorides even with deactivated and sterically hindered groups were successfully coupled over this solid catalyst under the mild conditions at a relatively low loading of Pd (80 °C, 0.5 mol% Pd). This solid catalyst could be reused 10 times without a significant decrease in activity. This study not only provides the most efficient phosphine-free solid catalyst for the Suzuki-Miyaura couplings of less reactive substrates, but also demonstrates a noteworthy example that IPr-functionalized ethane-silica can be used as a versatile scaffold to create various active surface NHC-metal catalysts.

Full text of DOI:10.1039/c1gc15757d

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PAPER
Rationally designed palladium complexes on a bulky N-heterocyclic
carbene-functionalized organosilica: an efficient solid catalyst for the
Suzuki–Miyaura coupling of challenging aryl chlorides
Guang Li,a Hengquan Yang,*a Wei Lia and Guoliang Zhangb
Received 27th June 2011, Accepted 20th July 2011
DOI: 10.1039/c1gc15757d
An IPr-functionalized (IPr = N,N¢-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, a N-heterocyclic
carbene ligand) mesoporous ethane-silica was synthesized through a co-condensation of
IPr-bridged triethoxysilane and bis(triethoxysilyl)ethane. This functionalized mesoporous
ethane-silica was characterized with N2 sorption, XRD, FT-IR, solid NMR and XPS. Owing to
high surface area, large pore volume and organically functionalized surface, such a material can
be used as a scaffold to design surface NHC-Pd [NHC = N-heterocyclic carbene] complexes. By
variation of Pd precursors and/or ancillary ligands, a library of NHC-Pd complexes on the solid
surface were tentatively created. It was found that the Pd precursor and ancillary ligands had
significant impacts on the activities of the resultant heterogeneous catalysts in the Suzuki–Miyaura
coupling of aryl chlorides. Surface NHC-Pd complexes with acetylacetonate (acac) and
3-chloropyridine as ancillary ligands are the most active. A wide range of aryl chlorides even with
deactivated and sterically hindered groups were successfully coupled over this solid catalyst under
the mild conditions at a relatively low loading of Pd (80 ◦C, 0.5 mol% Pd). This solid catalyst
could be reused 10 times without a significant decrease in activity. This study not only provides the
most efficient phosphine-free solid catalyst for the Suzuki–Miyaura couplings of less reactive
substrates, but also demonstrates a noteworthy example that IPr-functionalized ethane-silica can
be used as a versatile scaffold to create various active surface NHC-metal catalysts.
These rather unique properties make these complexes promising
1. Introduction
catalysts for practical applications if these complexes could be
The Pd-catalyzed Suzuki–Miyaura coupling is currently a fun-
immobilized in a proper fashion for catalyst recovery as well as
damental reaction for the synthesis of polymers, agrochemicals
eliminating the Pd residue in products.
and pharmaceutical compounds.1 Use of aryl chlorides as
Our recent efforts have focused on development of active solid
substrates is highly desirable because of ready availability and
catalysts for the Suzuki–Miyaura coupling of less reactive aryl
low cost in contrast with aryl bromides and iodides. However,
chlorides. The IPr and Mes precursors were successfully incor-
the activation of aryl chlorides is considerably challenging
porated into the framework of mesoporous materials, leading to
especially for heterogeneous catalysts.2 Heterogeneous examples
recoverable catalysts for couplings of aryl chlorides.5 However,
of the Suzuki–Miyaura couplings of deactivated, sterically
the coupling reactions over these solid catalysts were relatively
hindered aryl chlorides are still rare.3 It is well-known that
sluggish. It took 24 h to achieve satisfactory yields for aryl
Pd-phosphine systems can efficiently promote the couplings of
chlorides and a much longer time for deactivated aryl chlorides.
aryl chlorides, but suffer from high toxicity and low stability
The activity is much lower than that of the homogeneous
in air. Recently, N-heterocyclic carbene complexes such as Pd-
IPr [IPr = N,N¢-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]
and Pd-Mes (Mes = N,N¢-bis(2,6-dimethylphenyl)imidazol-2-
ylidene) have been found to not only have comparable activity
counterparts. There is a significant need to improve the catalytic
activity in the light of practical applications.
The recent findings reveal that the surface properties of solid
catalysts have a remarkable influence on the catalytic efficiency
with Pd-phosphine complexes in the Suzuki–Miyaura couplings
and the hydrophobic surface was generally helpful to improve
of aryl chlorides but also be stable in air and low toxic.4
the reaction rate.6 Of various methods for creating solid catalysts
with controlled surface properties such as hydrophobicity and
hydrophilicity, one approach that has been recently developed
but is most preferred and appealing, is the co-condensation
of a ligand or metal complex-bridged trialkoxysilane and a
small bridged organosilane such as bis(triethoxysilyl)ethane
aSchool of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan, 030006, PR China. E-mail: hqyang@sxu.edu.cn;
Fax: +86-351-7011688; Tel: +86-351-7010588
bSchool of Life Science, Shanxi University, Taiyuan, 030006, PR China
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in the presence of template.7 With this method, the complex
or ligand can be homogeneously incorporated in mesoporous
materials through covalent linkages, and its loading on the
solid material can be adjusted by varying their molar fraction
in the initial gel. Because the frameworks mainly consist of
organic units, such a material per se is hydrophobic without extra
modification with organosilane.8 Due to having homogeneously
distributed and accessible functionalized groups, a large surface
area, and organic interfaces, the organosilica materials may be
ideal scaffolds to derive surface metal complexes. However, there
are only a few examples that have demonstrated preparation of
surface metal complexes directly from organosilicas and further
investigated their catalytic performaces.9
Meanwhile, recent studies demonstrated that, besides the
dominant IPr ligand, the ancillary ligations of IPr-Pd com-
plexes were also crucial to their catalytic performances because
the coordination nature of the palladium precatalyst was
responsible for the smooth introduction of the active catalyst
into the catalytic cycle. IPr-Pd(OAc)2,10 IPr-palladacycle,11 IPr-
Pd(allyl)Cl,12 Pd-PEPPSI13 and IPr-Pd(acac)Cl14 have been
found to be active in the Suzuki–Miyaura coupling of aryl
chloride. These achievements of homogeneous catalysis suggest
the possibility that a set of active heterogeneous catalysts could
be created by tailoring IPr-functionalized organosilica.
Herein, aiming to obtain a more active solid catalyst toward
the Suzuki–Miyaura coupling of challenging aryl chlorides
and test the feasibility of creating active surface NHC-metal
complexes, we synthesized hydrophobic mesoporous ethane-
silica with a high loading of IPr ligand in the framework
through a co-condensation of IPr-bridged triethoxysilane and
bis(triethoxysilyl)ethane (as shown in Fig. 1), then attempted
to create a library of surface NHC-Pd complexes (as shown in
Fig. 2) and further investigated their catalytic activity.
Fig. 2 Schematic descriptions of catalyst microstructures and conver-
sions for the reactions of chlorobenzene and phenylboronic acid over the
solid catalysts (isopropyl alcohol/KOtBu as a reaction system, 80 ◦C,
5 h).
boronic acids were from Beijing Pure Chemical Co. Ltd
and from Aladdin Company. Most of the aryl chlorides
and ICl were purchased from Aladdin Company. Pluronic
F127 (EO106PO70EO106) was purchased from Sigma Company.
Bis(triethoxysilyl)ethane was obtained from Gelest Co. (Ger-
many). 2,6-Diisopropylaniline and other reagents were ob-
tained from Shanghai Chemical Reagent Company of Chi-
nese Medicine Group. All solvents were of analytical qual-
ity. 1,3-Bis(4-triethoxysilyl-2,6-diisopropylphenyl)-imidazol-3-
iumtrifluorometh-ane-sulfonate was synthesized by ourselves.5a
Synthesis of IPr-functionalized ethane-silica (ES-IPr)
IPr-functionalized ethane-silica ES-IPr was synthesized
by a co-condensation of IPr-bridged triethoxysilane and
bis(triethoxysilyl)ethane (BTEE) in the presence of F127 as a
template. A mixture of 10.5 mL of distilled water, 4.5 mL of
2 mol L-1 HCl, 1.764 g of NaCl and 0.75 of F127 were stirred
at 40 ◦C for 3 h. 6.4 mmol of bis(triethoxysilyl)ethane was
added into a solution containing 12.5 mL of distilled water and
30 mL of 2 mol L-1 HCl at 40 ◦C. The above F127 solution
and 1.6 mmol of IPr-bridged organosilane dissolved in 3 mL of
methanol were simultaneously added to the solution containing
bis(triethoxysilyl)ethane. After further stirring at 40 ◦C for 20 h,
the resultant suspension was transferred into autoclave and was
further treated under static conditions at 100 ◦C for 24 h. After
this hydrothermal process, the precipitated solid was isolated by
a filtration, washed with water and ethanol, and then dried at
80 ◦C. The template was removed by repeated extractions with
a diluted ethanolic HCl solution under refluxing conditions.
Eventually, IPr-functionalized ethane-silica (denoted as ES-IPr)
was obtained.
Fig. 1 Structural depiction of IPr-functionalized mesoporous ethane-
silica (ES-IPr).
2. Experimental
Reagents and materials
Creation of a library of Pr-Pd sites on the ES-IPr surfaces
PdCl2 was offered by Sigma Company. Pd(OAc)2 and Pd(acac)2
(acac = acetylacetonate) were obtained from Kaida Metal
Catalyst & Compounds Co. Ltd (China). Various aryl-
Catalyst 1:10 0.6 g of ES-IPr (dried at 100 ◦C for 4 h) and 0.24 g
of potassium tert-butoxide were dispersed into 4.8 mL of dry
THF. After the mixed system was stirred at 40 ◦C for 6 h under
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N2 atmosphere, the solvent was filtrated out. The obtained solid
was repeatedly washed with THF and dried under vacuum. This
resultant solid material was re-dispersed in 4.5 mL of dry toluene
containing 0.012 g of Pd(OAc)2. After stirring at 50 ◦C for 5 h,
the solid material was isolated by a centrifugation, and then
washed with dry toluene, yielding catalyst 1 (as shown in Fig. 2).
Catalyst 2:11 0.20 g of catalyst 1 and 0.10 g of acetanilide were
dispersed into 5.5 mL of 1,4-dioxane. After stirring at 70 ◦C for
12 h under N2 atmosphere, the isolated solid was thoroughly
washed with 1,4-dioxane. After drying under vacuum, the solid
was stirred in a NaCl solution for 10 h. The resultant solid was
isolated, washed with 1,4-dioxane and acetone, and then dried
under vacuum, leading to catalyst 2.
were stirred at 80 ◦C. At the end of the reaction the system
was cooled down to room temperature and repeatedly extracted
with diethyl ether. The obtained catalyst was washed with diethyl
ether, water, methanol and acetone in sequence and dried under
vacuum. The recovered catalyst was weighed again. The fresh
solvent and substrates were added, but the molar ratios of
substrates and solvent to Pd remained the same to the first run.
The products were confirmed with GC and NMR. The NMR
data follows.
Biphenyl (CDCl3, 300 MHz, ppm): 7.57 (d, 4 H, J = 9 Hz),
7.41–7.46 (m, 4 H), 7.34 (t, 2 H, J = 7.5 Hz). 4-Acetylbiphenyl
(CDCl3, 300 MHz, ppm): 8.03 (d, 2 H, J = 6 Hz), 7.36–7.69 (m,
7 H); 2.67(s, 3H). 4-Methylbiphenyl (CDCl3, 300 MHz, ppm):
7.59 (d, 2 H, J = 6 Hz), 7.24–7.57 (m, 7 H), 2.40 (s, 3 H).
4-Methoxybiphenyl (CDCl3, 300 MHz, ppm): 7.55–7.60 (m, 4
H), 7.45 (t, 2 H, J = 7.5 Hz), 7.33 (m, 1 H), 7.00 (d, 2 H,
J = 9 Hz), 3.88 (s, 3 H). 2-Methylbiphenyl (CDCl3, 300 MHz,
ppm): 7.11–7.26 (m, 9 H), 2.15 (s, 3 H). 2-Methoxybiphenyl
(CDCl3, 300 MHz, ppm): 7.39–7.42 (m, 2 H), 7.12–7.28 (m,
5 H), 6.78–6.89 (m, 2 H); 3.59 (s, 3 H). 2,6-Dimethylbiphenyl
(CDCl3, 300 MHz, ppm): 7.07–7.35 (m, 8 H), 2.19 (s, 6 H).
2,2¢,6-Ttriimethylbiphenyl (CDCl3, 300 MHz, ppm): 7.16–7.46
(m, 9 H), 1.18 (s, 9 H). 2-Phenylpyridine (CDCl3, 300 MHz,
ppm): 8.64 (d, J = 4.8, 1 H) 7.96–7.98 (m, 2 H), 7.63–7.66 (m.2
H); 7.33–7.46 (m, 3 H); 7.10–7.15 (m, 1 H). 3-Phenylpyridine
(CDCl3, 300 MHz, ppm): 8.82 (s,1 H); 8.55 (s,1 H), 8.78 (d, 2
H), 7.29–7.484 (m, 5 H). 4-Fluorobiphenyl (CDCl3, 300 MHz,
ppm): 7.50–7.53 (m, 4 H); 7.40–7.420 (m, 2 H); 7.32–7.33 (m,
1 H), 7.08–7.12 (m, 2 H). 4-tButylbiphenyl (CDCl3, 300 MHz,
ppm): 7.16–7.46 (m, 9 H), 1.19 (s, 9 H). 1-Phenylnaphthalene
(CDCl3, 300 MHz, ppm): 7.28–7.38 (m, 8 H); 7.68–7.47 (m, 4
H). 4-Trifluoromethylbiphenyl (CDCl3, 300 MHz, ppm): 7.62 (s,
4 H), 7.51–7.54 (d, J = 6.9 Hz, 2 H), 7.30–7.42 (m, 3 H).
Catalyst 3:11 0.28 g of catalyst 1, 0.1 g of acetanilide and 0.36 g
of CF3COOH were dispersed into 7.5 mL of 1,4-dioxane. After
stirring at 70 ◦C for 12 h under N2 atmosphere, the resultant
solid was repeatedly washed with 1,4-dioxane and dried under
vacuum, resulting in catalyst 3.
Catalyst 4:12 0.43 g of ES-IPr (dried at 100 ◦C for 4 h)
and 0.0052 g of Pd(allyl)Cl2 were dispersed into 4 mL of dry
THF. After stirring at 40 ◦C for 11 h, the solid material was
isolated, washed with dry THF, and then dried under vacuum,
giving catalyst 4.
Catalyst 5:13 0.60 g of ES-IPr (dried at 100 ◦C for 4 h),
0.0068 g of PdCl2 and 0.0266 g of K2CO3 were dispersed into
4 mL of 3-chloropyridine. After stirring at 80 ◦C for 24 h under
N2 atmosphere, the solid material was isolated, washed with
3-chloropyridine and 1,4-dioxane, and dried under vacuum,
leading to catalyst 5.
Catalyst 6:5a 1.0 g of ES-IPr (dried at 100 ◦C for 4 h) was
dispersed into 30 mL of dry 1,4-dioxane containing 0.02 g of
Pd(acac)2. After the mixed system was stirred at 100 ◦C for 40 h
under N2 atmosphere, 1,4-dioxane was filtrated out, affording
a brown solid. The solid was repeatedly washed with dry 1,4-
dioxane and dried under vacuum. The eventually achieved solid
was designated as catalyst 6.
Characterization and analysis
By varying the Pd(acac)2 amounts from 2 wt%, to 4
wt%, 6 wt%, 8 wt%, 10 wt% and 12 wt%, six catalysts
Pd(4%)/ES-IPr, Pd(6%)/ES-IPr, Pd(8%)/ES-IPr, Pd(10%)/ES-
IPr and Pd(12%)/ES-IPr were prepared.
Small-angle X-ray powder diffraction was performed on a
D/max 2500 PC diffraction spectrometer (Rigaku, Cu Ka,
40 kV, 30 mA). N2 physical adsorption was carried out on
a micrometritics ASAP2020 volumetric adsorption analyzer
(before the measurements, samples were out gassed at 393 K
for 6 h). The Brunauer–Emmett–Teller (BET) surface area was
determined from data in the relative pressure range of 0.05 to
0.15. The total pore volume of each sample was estimated from
the amount adsorbed at the highest P/P0 (above 0.99). Pore
diameters were calculated from the adsorption branch using
the Barrett–Joyner–Halenda (BJH) method. FT-IR spectra were
collected on a Thermo–Nicolet–Nexus 470 infrared spectrom-
eter. Pd content was analyzed by inductively coupled plasma-
atomic emission spectrometry (ICP-AES, AtomScan16, TJA
Co.). C and N content analysis was conducted 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, 1.0 ms
contact time, hexamethyl benzene as a reference compound; for
General procedure for Suzuki–Miyaura coupling of aryl
chlorides and arylboronic acids
A mixture of aryl chlorides (2 mmol), phenylboronic acid (2.1
mmol), potassium tert-butoxide (3 mmol) and iso-propyl alcohol
(6 mL), and the solid catalyst (0.5 mol% with respect to aryl
chlorides) were stirred at 80 ◦C under N2 atmosphere for a given
time. At the end of reaction, the mixture was cooled down to
room temperature and was repeatedly extracted with diethyl
ether. The combined organic layers were concentrated and the
resulting product was purified by column chromatography on
silica gel. The products were identified by 1H NMR. The purity
was confirmed by GC.
The recycling test for the Suzuki–Miyaura coupling was
conducted as follows. For the first run, the mixture of 12 mmol
of 4¢-chloroacetophenone, 12.6 mmol of phenylboronic acid,
18 mmol of potassium tert-butoxide, Pd(6%)/ES-IPr (0.5 mol%
Pd with respect to aryl chloride), 36 mL of iso-propyl alcohol
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13Si CP-MAS NMR experiments, 79.6 MHz resonant frequency,
4 kHz spin rate, 4.0 s pulse delay, TMS as a reference compound.
GC analysis was conducted on a SP-GC6800A.
3. Results and discussion
Material synthesis and characterization
In order to incorporate IPr precursor into ethane-silica frame-
work, we need to derive IPr precursor with hydrolysable
triethoxysilyl groups. The synthesis of IPr precursor-bridged
triethoxysilane was reported in our recent publications.5a In
order to achieve a hydrophobic surface, IPr precursor-bridged
triethoxysilane admixed with bis(triethoxysilyl)ethane was used
as organosilica precursors. The hydrolysis-condensation of these
two organosilanes was conducted in the presence of Pluronic
F127 as a template. After removal of the template, IPr-
functionalized ethane-silica was obtained. In order to incor-
porate more IPr ligands into the framework, we attempted
to use a high molar fraction of IPr-bridged triethoxysilane in
the initial gel to synthesize materials. However, it was found
that the 20% molar fraction of IPr-bridged triethoxysilane in
the initial gel was an up-limit. Further increasing the molar
ratio led to a significant decrease in the surface area and pore
volume of the resultant materials. The resultant material was
denoted as ES-IPr thereafter, as shown in Fig. 1. IPr and
ethylidene units are expected to be homogeneously incorporated
in the framework of the solid material. Due to the presence
of a large amount of ethylidene units in the framework,
this solid material surface is hydrophobic, and can provide
organic chemical microenvironments for further derivatization
and catalytic reaction.
Fig. 4 Pore size distribution plot of ES-IPr.
surface area and pore volume facilitate further tailoring solid
materials as well as fast mass transport of the reaction molecules.
In Fig. 5, the XRD pattern of ES-IPr displays a diffraction peak
around 0.96◦, although its intensity is not very strong. The N2
sorption and XRD results both confirm that the synthesized
ES-IPr material has characteristics of mesoporous materials to
some extent. The less ordered structures may be explained by
the fact that the presence of the high molar fraction of the bulky
IPr ligand in the initial gel significantly affects the assembly
of the interactions between silicon oligomers and template
molecules. Although the ordering is not so high, the synthesized
material has a high surface area and large pore volume enough
to further modify. Compared with our previously reported
siliceous framework SBA-16 (the maximum molar fraction of
IPr-bridged triethoxysilane in the initial gel is ca. 10%, and
a higher molar fraction leads to a disordered structure),5a the
ethane-framework allows incorporation of a higher loading
of IPr ligand into the mesoporous materials (the maximum
molar fraction of IPr-bridged triethoxysilane in the initial gel
The N2 sorption isotherms of ES-IPr show two steps of steep
capillary condensations at the relative pressures of 0.45–0.65
and 0.8–0.99 (Fig. 3). The corresponding pore size distribution
plot exhibits a bimodal pore distribution (primary mesopore at
4 nm and secondary mesopore at 23 nm) (Fig. 4). The specific
surface area and pore volume are as high as 743 m2 g-1 and
1.15 cm3 g-1, respectively. Our preliminary investigations reveal
that introduction of F127 during the synthesis is a crucial factor
for achieving so high a surface area and pore volume. The high
Fig. 3 N2 adsorption/desorption isotherm of ES-IPr.
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Fig. 5 XRD pattern of ES-IPr.
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is 20%). These differences may be attributed to the different
interaction patterns of the siliceous precursor and template
P123. Organosilane BTEE probably favourably stabilizes the
micelle structure to form the corresponding mesoporous phase,
compared with inorganic siliceous precursor Si(OEt)4.
To check the compositions, we employed FT-IR to charac-
terize the synthesized ethane-silica. The FT-IR spectra is shown
in Fig. 6. The samples clearly exhibit two characteristic peaks
around 2800–3000 and 1542 cm-1. The former corresponds to the
stretching vibrations of C–H bonds and the latter is related to the
vibrations of imidazol ring. The presence of these peaks indicates
that IPr carbene precursor was successfully incorporated into the
framework.
respectively. The Q3 signals imply that a portion of Si–C bonds
in the bridged organosilanes decomposed during the synthesis.
13C CP-MAS NMR spectrum of ES-IPr clearly shows the signals
for ethylidene at 0–10 ppm (Fig. 8), for isopropyl groups on the
aromatic rings at 20–30 ppm, and the central imidazole rings in
the range of 125–145 ppm. The resonance at 59 ppm is attributed
to (EtO)3Si– groups that did not completely hydrolyze.
Fig. 8 13C CP-MAS NMR spectrum of ES-IPr.
The elemental scan by XPS analysis was performed to further
characterize the compositions (in Fig. 9). Silicon, oxygen,
carbon, nitrogen, fluorine, sulphur, and chlorine were detected
on the materials. Chlorine is present on the material. It can be
explained by the possibility that CF3SO3- groups on IPr carbene
precursors were partially exchanged by Cl- during the course of
synthesis and/or template removal. The N content on ES-IPr
was determined to be 2.48 wt%. Namely, the IPr ligand density
on ES-IPr is equal to ca. 0.88 mmol g-1, which is sufficiently high
for performing a further derivatization.
Fig. 6 FT-IR spectrum of ES-IPr.
The 29Si MAS NMR and 13C CP-MAS NMR spectra further
confirm that IPr ligand was integrated with the framework of
ES-IPr through covalent linkages (Fig. 7 and 8, respectively). In
Fig. 7, the 29Si MAS NMR spectrum shows both “T” and “Q”
silicon signals. The presence of strong “T” bands at -62 ppm is
an indication that there are a large number of Si–C linkages
in the framework. The “Q” signals at -110 and -100 ppm
correspond to Q4 [Si(OSi)4] and Q3 [Si(OH)(OSi)3] silicon sites,
Fig. 7 29Si MAS NMR spectrum of ES-IPr.
Fig. 9 XPS spectrum of ES-IPr.
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Table 1 The theoretical and determined Pd contents of ES and ES-IPr derived with different amounts of Pd(acac)2
Entry
ES-IPr
Pd(acac)2 (wt%)a
2%
4%
6%
8%
10%
3.17
2.96
3.2
12%
3.73
2.98
3.1
Theoretical Pd content (wt%)b
Determined Pd content (wt%)c
The molar ratio of IPr/Pd
0.68
0.63
16.9
1.34
1.28
7.4
1.97
1.78
5.3
2.58
2.40
3.9
a With respect to ES-IPr. b The calculated Pd contents. c Determined by ICP-AES.
Creating surface IPr-Pd complexes and activity evaluation
amount on ES-IPr was determined with ICP-AES. The results
are summarized in Table 1. ES-IPr had a good coordination
capacity for Pd(acac)2 and most of the complexes were captured
onto the solid material before the used Pd(acac)2 amounts were
below 10 wt%. For using 10 wt% Pd(acac)2, the determined Pd
content is 2.96 wt%, which is slightly lower than the theoretic
value (3.17 wt%). On this occasion, the ratio of IPr/Pd is
estimated to be 3.2. Further increasing Pd(acac)2 amount did
not result in increasing the Pd content on the solid catalyst. It
should be pointed out that a portion of IPr in the framework
is not accessible for Pd(acac)2 (expressed on the basis of a 1 : 1
Pd/NHC steochiometry).
Homogeneous catalysis has demonstrated that the highest
activity was generally obtained at the IPr/Pd ratio of ca. 1 : 1.15
However, the cases for solid catalysts are more complicated
because the reaction results are associated with diffusion limita-
tions and the non-uniform active sites on the solid surface that
probably change with the variations of the metal loadings. For
these considerations, we tested catalyst 6 derived with different
amounts of Pd. The reaction results are summarized in Fig. 10.
ES-IPr derived with 2 wt% Pd(acac)2 afforded a conversion of
72% under the same conditions. It is noteworthy that the increase
of the Pd amount resulted in a slight increase in conversion.
The maximum conversion of 85% was obtained on ES-IPr
coordinated with 6 wt% Pd(acac)2. Further increasing the
amount of Pd(acac)2 led to a slight decrease in the conversions.
With the defined ES-IPr in hand, we next created a library
of active IPr-Pd sites on the ES-IPr surfaces. As mentioned,
the ancillary ligations of IPr-Pd complexes were also crucial
to their catalytic performances. The reported IPr-Pd complexes
mainly including IPrPd(OAc)2, IPr-palladacycle, IPrPd(allyl)Cl,
Pd-PEPPSI and IPrPd(acac)Cl were selected as targets to be
generated. According to the reported procedures for synthe-
sizing these IPr-Pd complexes, we attempted to prepare the
corresponding surface IPr-Pd complexes by varying palladium
salts and/or ancillary ligations. Their structures are proposed
in Fig. 2. We tried to employ solid 13C NMR to identify their
actual structures and ancillary ligands. However, it did not give
supporting information because the solid 13C NMR routinely
required a relatively high loading of organic groups.
Their activities were examined with the Suzuki–Miyaura cou-
pling of chlorobenzene and phenylboronic acid in the presence
of 0.5 mol% Pd with iso-propyl alcohol as solvent and potassium
tert-butoxide as base. The conversions for each catalyst are
also listed below their catalyst structures in Fig. 2. The control
experiment showed that ES-IPr without loading Pd was almost
inactive in this reaction and only a trace amount of products
were detected. After coordination with Pd(OAc)2, the resultant
catalyst 1 gave a conversion of 22% within 5 h. Treating catalyst 1
with acetanilide and NaCl resulted in catalyst 2. The conversion,
however, was hardly improved in comparison to catalyst 1.
Further modifying catalyst 1 with acetanilide and CF3COOH
led to catalyst 3. Catalyst 3 afforded a significantly increased
conversion (up to 53%) under the same conditions. Catalyst 4,
prepared by using Pd(allyl)Cl2 as Pd precursor, gave a moderate
conversion (up to 66%). Using acetylacetonate (acac) and 3-
chloropyridine as ancillary ligands (corresponding to catalysts
5 and 6, respectively) gave rise to good conversions (71%–72%).
Although the Pd coordination state on ES-IPr is not completely
definite, clearly, the modification of surface NHC-Pd sites with
ancillary ligands can significantly alter the activity like NHC-Pd
complexes in homogeneous cases. Catalysts 5 and 6 outperform
others in activity and the conversions over them reach over 70%.
The above investigations demonstrate that the surface IPr-
Pd complexes with acetylacetonate (acac) and 3-chloropyridine
as ancillary ligands have excellent activities. Because the prepa-
ration process of surface IPr-Pd(acac) complex (catalyst 6) is
relatively simple and the reagent is relatively cheap, catalyst 6
is more promising for practical applications and deserves more
investigation. To investigate the coordination capacity of IPr-
functionalized ethane-silica, we derived ES-IPr with different
amounts of Pd(acac)2 (from 2 wt% to 4 wt%, 6 wt%, 8 wt%,
10 wt% and 12 wt% with respect to ES-IPr). The resultant Pd
Fig. 10 The conversions of chlorobenzene over the solid catalysts with
different molar ratio of IPr/Pd.
The activity of ES-IPr derived with 6 wt% Pd(acac)2 (denoted
as Pd(6%)/ES-IPr, 85% conversion within 5 h) is much higher
than that of our previous catalyst Pd/SBA-16-IPr [Pd(acac)2
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a
Table 2 Suzuki–Miyaura couplings of aryl chlorides and phenyl boronic acid over Pd/ES-IPr coordinated with 6 wt% Pd(acac)2
Entry
1
Substrate
Product
Time (h)
8
Yieldb (%)
87
92
69
92
85
2
3
4
5
4
12
12
12
6
7
12
12
87
72
8
24
56
9
12
12
87
80
10
a 2 mmol of halides, 2.1 mmol of phenyl boronic acid, 3 mmol of KOtBu, 0.5 mol% Pd with respect to chlorides and 4 mL of isopropyl alcohol.
b Isolated yields.
loaded on IPr-functionalized mesoporous material SBA-16,
ca.50% conversion within 6 h].5a The isolated yield of biphenyl
reached up to 87% within 8 h (Table 2, entry1). The activity
of ES-IPr coordinated with Pd(acac)2, is also much higher than
the commercial catalyst Pd/C (1wt% of Pd) (28% conversion
within 24 h under the identical reaction conditions).5a The clear
comparisons highlight the excellent activity of this developed
solid catalyst. The significantly improved activity probably may
be a consequence of hydrophobic surface properties and a high
density of IPr ligands on the ethane-silica surface.
The Suzuki–Miyaura couplings of other aryl halides and
phenylboronic acid over this catalyst were conducted (Table 2).
For aryl chlorides bearing an electron-withdrawing group, yields
up to 92% were obtained in a shorter time (Table 2, entry 2).
To our delight, for deactivated aryl chlorides with electron-
donating groups such as –CH3 and –OCH3, good to excellent
yields were achieved (Table 2, entries 3 and 4). The deac-
tivated aryl chlorides bearing o-substituted electron-donating
groups were also smoothly converted, and 85–87% yields were
obtained (Table 2, entries 5 and 6). It is noteworthy that in
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a
Table 3 Suzuki–Miyaura couplings of arylboronic acids and chlorobenzene over Pd/ES-IPr coordinated with 6 wt% Pd(acac)2
Entry
1
Substrate
Product
Time (h)
8
Yieldb (%)
84
2
3
4
5
6
12
8
83
88
81
89
83
8
8
8
7
8
8
8
80
92
9
12
62
a 2 mmol of halides, 2.1 mmol of arylboronic acid, 3 mmol of KOtBu, 0.5 mol% Pd with respect to chlorobenzene and 4 mL of isopropyl alcohol.
b Isolated yields.
the case of sterically hindered and deactivated aryl chlorides
(2,6-dimethylchlorobenzene) a 72% yield was afforded within
16 h (Table 2, entry 7). More impressively, highly encumbered
biphenyl with three ortho-substituted groups in a good yield was
achieved on the solid catalyst (Table 2, entry 8). Furthermore,
the heteroaromatic substrates such as 2-chloropyridine and 3-
chloropyridine were also successfully coupled, giving 80–87%
yields (Table 2, entries 9 and 10).
We further tested the catalytic performances for the couplings
of various arylboronic acids with chlorobenzene (Table 3). For
arylboronic acids bearing electron-withdrawing groups such as –
F and –CF3, Pd(6%)/ES-IPr gave good yields within 8–12 h (Ta-
ble 3, entries 1 and 2). 81–89% yields were achieved in the cases
of arylboronic acids bearing electron-donating groups such as
methyl, methoxy and tert-butyl groups (Table 3, entries 3–5). The
moderately sterically demanding arylboronic acids were also
smoothly converted to the corresponding products in 80–92%
yields (Table 3, entries 6–8). Remarkably, the coupling reaction
was also effective for arylboronic acid with high steric hindrances
and 62% yield was obtained (Table 3, entry 9). Recently, several
types of heterogeneous catalysts for Suzuki–Miyaura couplings
were reported such as mesoporous silica-supported palladium,
polymer-incarcerated palladium, polyurea-encapsulated palla-
dium and dendrimer-supported palladium catalysts (in ref.
16). Most of these systems successfully catalyzed the Suzuki–
Miyaura couplings of aryl bromides but failed in couplings of
aryl chlorides (especially deactivated aryl chlorides) except only
a few examples. To the best of our knowledge, the developed
catalyst can represent the most efficient phosphine-free solid
catalyst toward the Suzuki–Miyaura reaction of challenging
aryl chlorides in view of its high activity and wide scope of
substrates.
2946 | Green Chem., 2011, 13, 2939–2947
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Table 4 The recyclability test of Pd/ES-IPr coordinated with 6 wt%
Pd(acac)2 with the couplings of 4¢-chloroacetophenone with phenyl-
boronic acida
Science Foundation for Youths (2009021009), the Natural
Science Foundation of China (20903064), Shanxi Univer-
sity Innovative Experimental Project for Undergraduates and
Jiangsu Key Lab. of Fine Petrochemistry for financial supports
(KF0802).
Cycles
1
2
3
4
5
6
7
8
9
10
Time (h)
4
4
93
4
92
4
92
4
93
4
90
5
87
5
89
6
82
7
83
Yieldb (%) 91
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magnified 6 times. b Isolated yields.
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4. Conclusions
A new functionalized mesoporous material with organic frame-
work, a high surface area, large pore volume and a high IPr
loading was synthesized through a co-condensation of IPr-
bridged triethoxysilane and bis(triethoxysilyl)ethane. Such a
material has a good coordination capacity for Pd, and can
be used as a scaffold to design surface NHC-Pd complexes.
By tuning the coordination environments of NHC-Pd sites, a
library of heterogeneous Pd catalyst for the Suzuki–Miyaura
reaction were created. These surface NHC-Pd sites on the
solid surface show remarkable differences in catalytic activity.
Using acetylacetonate (acac) or 3-chloropyridine as a ancillary
ligand leads to the most active solid catalysts. The developed
catalyst represents, so far, the most efficient phosphine-free solid
catalyst toward the Suzuki–Miyaura reaction of challenging aryl
chlorides. A wide range of aryl chlorides even with deactivated
and highly sterically hindered groups were successfully coupled
under the mild conditions (80 ◦C, 0.5 mol% Pd). The good
coordination, facile derivation capacity and excellent catalytic
performances exemplified by IPr-functionalized mesoporous
ethane-silica probably imply that other surface metallic carbene
complexes can be created for more applications.
Acknowledgements
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Lett., 2008, 10, 1609, and references therein.
We acknowledge Specialized Research Fund for the Doctoral
Program of Higher Education (200801081035), Shanxi Natural
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The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 2939–2947 | 2947
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