Metal-organic gels as functionalisable supports for catalysis

Article abstract of DOI:10.1039/b822104a

Modification of Fe-tricarboxylate based metal-organic gels yielded tert-butyl substituted or phosphine-functionalised gels in alcohols or DMF, which were prepared from 5-tert-butylisophthalic acid and 5- diphenylphosphanylisophthalic acid, respectively. Owing to their permeability, such phosphine-functionalised metal-organic gels can act as a new type of functionalisable porous scaffold. The phosphorus-containing gel was subsequently functionalised with Pd(ii) by immersion in a solution of Pd(COD)Cl₂ (COD = 1,5-cyclooctadiene). The subsequently functionalised Pd(ii)-immobilised gel and its xerogel/aerogel showed high activity in the catalysis of Suzuki C-C coupling comparable to unsupported complexes. The gel/xerogels could be reused under ambient atmosphere.

Full text of DOI:10.1039/b822104a

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Metal–organic gels as functionalisable supports for catalysisw
Jianyong Zhang,*a Xiaobing Wang,a Lisi He,a Liuping Chen,a Cheng-Yong Sua
and Stuart L. Jamesb
Received (in Montpellier, France) 11th December 2008, Accepted 22nd January 2009
First published as an Advance Article on the web 24th February 2009
DOI: 10.1039/b822104a
Modification of Fe-tricarboxylate based metal–organic gels yielded tert-butyl substituted
or phosphine-functionalised gels in alcohols or DMF, which were prepared from
5-tert-butylisophthalic acid and 5-diphenylphosphanylisophthalic acid, respectively. Owing to their
permeability, such phosphine-functionalised metal–organic gels can act as a new type of
functionalisable porous scaffold. The phosphorus-containing gel was subsequently functionalised
with Pd(II) by immersion in a solution of Pd(COD)Cl2 (COD = 1,5-cyclooctadiene). The
subsequently functionalised Pd(II)-immobilised gel and its xerogel/aerogel showed high activity in
the catalysis of Suzuki C–C coupling comparable to unsupported complexes. The gel/xerogels
could be reused under ambient atmosphere.
through coordination of the phosphines themselves to metal
Introduction
cations. In such a strategy, metal ions interconnect the phos-
phine ligands to form a 3D gel network. Reported examples
include a Ag(I) gel10 and a Pd(II) gel.11 The second strategy
requires synthesis of a gel network, functionalised with free
phosphine groups which are available for coordination. It
could then be post-modified with a variety of transition metal
ions. Owing to the importance of phosphines in catalysis, this
strategy may offer a versatile entry into transition metal gel
catalysts.
Solid-supported metal catalysts have been actively studied in
recent years,1 and various methods have been developed to
synthesise them.2 Networked structures studied in this
regard include zeolites, organic polymers and coordination
polymers.2,3 Supramolecular gels are viscoelastic solid-like
materials comprised of a 3D interconnected network which
contain a large amount of solvent, usually up to 97%, making
them highly permeable. Supramolecular gels are normally
formed through solvophobic effects, hydrogen bonding,
coordination bonding, van der Waals attraction etc., some-
times by highly organized self-assembly.4 They behave like soft
solids and are of interest for applications in food, cosmetics,
medicine, controlled release, optoelectronics, and so on.
Although gels could also have some desirable characteristics
for heterogeneous catalysis, such as rapid mass transfer or
permeability, high surface areas, easy removal, and so on, gels
have received limited attention in this area.5,6 Notably, gels
functionalised with free ligand sites which can subsequently be
metallated to generate catalytic centres have been rarely
described so far.4i,5a
Although a large number of gels have been discovered in the
last decade, phosphine-functionalised gels remain unknown to
our knowledge. Herein we show that a previously known
coordination polymer gel system can be functionalised with
free phosphine groups, further metallated with palladium(II)
centres, and employed for catalytic use.
Results and discussion
Metal–organic gels (MOGs) have been shown to be con-
veniently prepared by mixing solutions of benzene-tricarboxylic
acid (H3BTC, Scheme 1) and Fe(NO3)3.12 These gel networks
are believed to be connected via Fe-carboxylate coordination
bonds to form a 3D matrix. Polymeric chains might also form
if one of the carboxylate groups were replaced by a different
functional group. 5-tert-butylisophthalic acid (H2BuBDC), a
dicarboxylic acid, is thus a possible substitute for H3BTC
(although introduction of hydrophobic groups can potentially
also affect the formation of gel networks13). The phosphine-
functionalised diacid, 5-diphenylphosphanylisophthalic acid
Phosphines are widely used in transition metal homogeneous
catalysis,7 for example, palladium catalysed C–C coupling.8
Palladium complexes bearing phosphines, able to stabilise
Pd(0) intermediates, have been found to be particularly active
in cross-coupling reactions.9 However, phosphine-based gels
are almost unknown. There are two strategies to synthesise
phosphorus-functionalised metallogels. The first is self-assembly
a MOE Laboratory of Bioinorganic and Synthetic Chemistry, School
of Chemistry and Chemical Engineering, Sun Yat-Sen University,
Guangzhou, 510275, China. E-mail: zhjyong@mail.sysu.edu.cn;
Fax: 86 020 8411 5178
b Centre for the Theory and Application of Catalysis (CenTACat),
School of Chemistry and Chemical Engineering, Queen’s University
Belfast, David Keir Building, Stranmillis Road, Belfast, Northern
Ireland, UK BT9 5AG
w Electronic supplementary information (ESI) available: IR and XPS
spectra. See DOI: 10.1039/b822104a
Scheme 1
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(H2PBDC)14 can be considered similarly. The phosphine
group should not interfere with the Fe-carboxylate network
due to poor HSAB matching.15
Gelation study
Gelation tests were performed by mixing solutions of
Fe(NO3)3 and H2BuBDC or H2PBDC at RT. The gelation
ability was evaluated in various solvents. Gel formation is
confirmed if the sample does not flow when the test tube is
inverted. Both H2BuBDC and H2PBDC displayed gelation
abilities when reacting with ferrous nitrate (Table 1). Turbid
orange–brown gels were obtained when Fe(NO3)3 solution
(0.50 mmol, 5 ml) and H2BuBDC solution (0.50 mmol, 5 ml)
were mixed together. The gels were formed in MeOH (GBu1),
EtOH (GBu2) or DMF (GBu3) after 1 min, 2–3 h, 3–5
days, respectively. When THF solutions of Fe(NO3)3 and
Fig. 1 Metal–organic gels formed from solutions of H2PBDC and
Fe(NO3)3 in ethanol (GP1), left, and n-butanol (GP2), right.
Table 2 Comparison of Dn for the gels/xerogelsa
n(OH)/cmꢁ1
n n
asym(CO2)/cmꢁ1 sym(CO2)/cmꢁ1 Dn/cmꢁ1
H2PBDC 3413
1635
1616
1614
1610
1610
1432
1432
1434
1437
1437
203
184
180
173
173
gP1
gP2
3427
3413
3428
3387
H2BuBDC were mixed together,
a precipitate formed
gP3
gP1-Pd
immediately, which subsequently formed a gel (GBu4) after
12–24 h. Similar orange–brown gels were also formed when
H2PBDC was used in place of H2BuBDC. The gels were able
to form in a series of solvents, including EtOH (GP1), n-BuOH
(GP2) and DMF (GP3) after 3–5 days, 3–5 days or 15 days,
respectively (Fig. 1). Generally the gelation with H2PBDC is
slower than with H2BuBDC, and the gelation in DMF is
slower than in alcohols.
a
The xerogels of GP1, GP2, GP3 are referred as gP1, gP2, gP3,
respectively.
by Fe(III).19 The partial oxidation was also confirmed by MS.
However, the PQO presence in the gels could not be proved
due to overlapping. The X-ray photoelectron spectroscopy
(XPS) analysis exhibited the presence of Fe and P in the
xerogel of GP1. A signal at 711.2 eV in the Fe 2p3/2 region
and a signal at 132.5 eV in the P 2p region were evident in the
spectrum.
FT-IR spectra confirmed the presence of carboxylate in the
metal coordination sphere (Table 2). The bands due to the
coordinated carboxylates are shifted to lower frequencies with
respect to the free ligands. The absorption bands characteristic
of the carboxylate ligands in the wet gels appear generally
around 1651–1624 cmꢁ1 for the asymmetric nasym(CO2) vibra-
tion and the bands for the symmetric stretch overlap with
bands due to the solvents. For the xerogels, strꢁong bands at
around 1384 and 1120 cmꢁ1 are due to NO3 and P–Ar,
respectively. The difference between nasym(CO2) and nsym(CO2)
of around 180 cmꢁ1 suggests that the carboxylate ligands are
chelating-bridging.16 For the xerogels lower PQO stretching
frequencies were found at around 1160 cmꢁ1, compared with
PQO absorption at around 1176 cmꢁ1 due to oxidized
H2PBDC,17 indicating that some coordinated PQO groups
were present.18 The phosphine may be partially oxidised
during formation of these xerogels, which is possibly catalysed
Gels are generally understood to comprise one-dimensional
(1-D) fibrous aggregates,4,20 which cross-link into 3-D net-
works, with few exceptions.11,21 SEM was used to examine the
morphology of the xerogels of H2BuBDC and H2PBDC
(Fig. 2). All the gels have similar globular or block-like
morphologies, with characteristic features of 2–10 mm, rather
than the more common fibrillar morphologies of gel-phase
materials. Such morphologies can be expected however from
the random 3-D growth of coordination polymer particles.
Table 1 Gelation ability in various solvents (1 + 1 ml) for Fe(NO3)3
(0.05 mmol) and 5-tert-butylisophthalic acid/5-diphenylphosphanyl-
isophthalic acid (0.05 mmol)
Solvent
Resulta
Time
Solvent
Resulta
5-tert-Butylisophthalic acid (H2BuBDC)
MeOH
EtOH
DMF
THF
G (GBu1)
G (GBu2)
G (GBu3)
G (GBu4) + P 12–24 h
3–5 days
1 min
2–3 h
H2O
CH3CN
P
P
P
S
3–5 days DMSO
HOCH2CH2OH
n-BuOH Weak G
5-Diphenylphosphanylisophthalic acid (H2PBDC)
3–5 days MeOH
3–5 days DMSO
EtOH
G (GP1)
n-BuOH G (GP2)
S
S
S
DMF
G (GP3)
15 days
DMA
Fig. 2 SEM images of xerogels of GBu2 (top two), and GP2 (bottom
a
S: solution; G: gel; P: precipitate.
two), prepared by slow evaporation of wet gels in air.
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The X-ray diffraction diagrams for gels GP1–GP3, and their
xerogels confirmed such random growth, showing only an
amorphous halo (broad ‘‘hump’’).
The above results with H2BuBDC/H2PBDC confirm that
gels can form by self assembly of Fe3+ with functionalised
dicarboxylic acids. Some gels spontaneously fragmented on
standing and all were found to fragment if shaken vigorously,
suggesting a rigid rather than flexible structure. The gels were
not observed to redissolve upon heating, i.e. they are thermally
irreversible.
Postmodification of metal–organic gel
The phosphine gel is of interest for catalytic studies. The
phosphorus centres should be dispersed throughout the
whole gel medium, and accessible to molecular reactants. A
Pd(II)-functionalised gel was prepared by placing an aceto-
nitrile solution containing excess Pd(COD)Cl2 in on top of the
phosphine gel. After diffusion for ca. 10 days, the supernatant
was decanted and the gel was washed several times with
acetonitrile until the washings were colourless. The FT-IR
spectrum of the Pd-loaded gel (GP1-Pd) generally displays
similar bands to that of GP1 in addition to bands of included
acetonitrile. The XPS surface analysis of GP1-Pd confirmed
the presence of Fe, P, Pd and Cl in the xerogel of GP1-Pd. The
spectrum showed a signal at 711.4 eV in the Fe 2p3/2 region
and a signal at 132.3 eV in the P 2p region. Analysis in the
Pd 3d5/2 region showed a signal at 337.8 eV and a signal at
198.7 eV in the Cl 2p region, confirming attachment of the Pd
atoms to the gel network. The P/Pd/Cl ratio was calculated to
be about 1 : 1 : 2. The weight percentage of Pd atoms in the
GP1-Pd xerogel was determined to be 1.0% by inductively
coupled plasma (ICP) spectroscopic analysis. Compared with
that in the Fe-BTC gel12 without phosphorus (0.05%), a much
higher content of Pd has been loaded in GP1.
Fig.
4
N2 adsorption–desorption isotherm of GP1-Pd (circle,
adsorption; triangle, desorption).
exhibits a N2-physisorption isotherm of type III, indicative of
macroporous solids (Fig. 4).22 No hysteresis is observed
between the desorption and adsorption isotherms of the
aerogel, indicating that there is little irreversible interaction
between N2 and the pores. The BET surface area is 22.1 m2 gꢁ1
and total pore volume is 0.074 mL gꢁ1. The low BET surface
area of the aerogel may result from the closure of micropores
in the drying process. The results of the adsorption isotherm
are consistent with SEM microscopy. The aerogel may thus
exhibit a colloidal microstructure rather than a polymeric
one.23
The thermostability of the GP1-Pd aerogel was studied by
thermogravimetric analyses (TGA) as depicted in Fig. 5.
The TG curve exhibits two types of weight losses from
RT to 600 1C. The weight losses of 7.3 and 6.9% in the
temperature range of RT–140 and 140–240 1C, respectively,
are attributed to the loss of water and organics absorbed
on the particle surface and those physisorbed and chemisorbed
in the pores. The coordination network may be anticipated
to collapse from 240 to 590 1C shown by the weight loss
of 52.9%.
To obtain its more precise microstructure, the gel GP1-Pd
was dried by supercritical CO2 extraction so that the solvent in
the network of the gel was removed and an aerogel was
formed. The XRD pattern showed that the aerogel
was amorphous. The SEM images show that the aerogel was
composed of interconnected macroporous networks of globular
or block-like nanometer-sized particles of diameter ca.
100–300 nm (Fig. 3). The microstructure is similar to the
xerogels without loading of Pd, although the component
particles are significantly smaller. An adsorption isotherm of
the solid was performed using N2 gas as the adsorbate at
liquid nitrogen temperature. BET analyses show that the solid,
which was dried in vacuum for 20 h at RT before analysis,
Catalytic activity
GP1-Pd was found to be an active catalyst for the Suzuki C–C
cross-coupling of aryl halides/bromopyridines with arylboronic
acids. Catalytic reactions were performed under slow stirring
with the gel catalyst packed in a filter paper bag to avoid
mechanical degradation, and the results are given in Table 3
and 4. The reaction of iodobenzene with phenylboronic acid
Fig. 3 SEM images of the GP1-Pd supercritical aerogel.
Fig. 5 TG curve for the GP1-Pd aerogel.
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Table 3 Suzuki cross-coupling of aryl halides with phenylboronic acid catalysed by gel GP1-Pda
Entry
X, R
T/1C
Solvent
Yieldb (%) (t/h)
1
2
3
4
5
6
X = I, R = H
X = I, R = H
X = I, R = H
X = Br, R = H
X = Br, R = OMe
X = Br, R = COMe
60
60
RT
60
60
60
MeOH
H2O
MeOH
MeOH
MeOH
MeOH
99 (0.5), 100 (1.0)
39 (0.5), 51 (1.0), 66 (1.5), 81 (2.0)
69 (0.5), 74 (1.0), 82 (2.0), 91 (3.0), 98 (3.5)
71 (0.5), 74 (3.0)
26 (0.5), 35 (1.0), 39 (3.0)
97 (0.5), 99 (1.0)
a
Reaction conditions: 0.67 mmol of aryl halide, 1.5 mol equiv. of PhB(OH)2, 0.2 mmol% of Pd(II) of the catalyst, 3 mol equiv. of Na2CO3, in 8 ml
b
of CH3OH or H2O under air. GC yield.
Table 4 Suzuki cross-coupling of bromopyridines with boronic acids catalysed by gel GP1-Pd in methanol at 60 1Ca
Entry
Ar0X
ArB(OH)2
Yielda (%) (t/h)
1
2
3
4
4-Bromopyridine
4-Bromopyridine
3-Bromopyridine
2-Bromopyridine
3,5-Difluorophenylboronic acid
Phenylboronic acid
3,5-Difluorophenylboronic acid
3,5-Difluorophenylboronic acid
68 (0.5), 87 (1.0), 92 (1.5), 95 (2.0)
79 (0.25), 98 (0.5), 499 (1.0)
38 (0.5), 42 (1.0), 53 (2.0), 71 (3.0)
32 (0.5), 35 (1.0), 52 (2.0), 53(3.0)
a
Determined by GC.
(1.5 mol equiv.) in the presence of 0.2 mmol% of Pd(II) of
GP1-Pd under air, proceeded efficiently in methanol to give
biphenyl in 99% yield at 60 1C after 0.5 h, and in 98% yield at
RT after 3.5 h, while use of water as solvent at 60 1C also gave
a good yield (81%) after 2.0 h. As expected, bromobenzene
was less reactive, and its coupling with phenylboronic acid
gave a yield of 74% at 60 1C after 3.0 h in methanol, while a
relatively modest yield (39%) was achieved in the coupling
between phenylboronic acid and 4-bromoanisole, a deactivated
electron-rich aryl bromide, and the use of 4-bromoaceto-
phenone, an activated electron-poor one, resulted in a near
quantitative yield after 1.0 h. The catalyst is also effective
towards the coupling of bromopyridines with boronic acids.
4-Bromopyridine could be coupled with phenylboronic
acid easily giving near-quantitative yields after 1.0 h.
The coupling of different isomers of bromopyridine with
3,5-difluorophenylboronic acid gave 4-(3,5-difluorophenyl)-
anisole, 3-(3,5-difluorophenyl)anisole and 2-(3,5-difluoro-
phenyl)anisole in 95% (after 2.0 h), 71% (3.0 h) and 53%
(3.0 h) yield, respectively.
acid (1.5 mol equiv.) in the presence of 0.2 mmol% of Pd(II) of
the xerogel (without filter paper bag) under air at 60 1C
gave biphenyl in 88% yield after 0.5 h, and in 93% yield
after 1.0 h. When the xerogel was subjected to the reaction
of 4-bromopyridine with 3,5-difluorophenylboronic acid
under similar conditions, 4-(3,5-difluorophenyl)anisole was
obtained in 32% (after 0.25 h), 58% (0.5 h), 70% (1.0 h),
78% (1.5 h), 81% (2.0 h) and 87% (2.5 h) yield. These results
indicate that the xerogel showed poorer catalytic activity than
the GP1-Pd gel.
The catalytic activity of the supercritical aerogel was also
studied (Table 5, entry 1). The aerogel exhibited slightly lower
activity than the wet gel (36% vs. 40% after 1.0 h) in the
reaction of bromobenzene with phenylboronic acid. The
aerogel was observed to show increasing activity at higher
temperature (153 1C) in refluxing DMF. Higher yields was
obtained, 58% after 20 min and 62% after 0.5 h for the
reaction of 4-bromoanisole and phenylboronic acid, and
498% within 0.5 h for the coupling of 3,5-difluorophenyl-
boronic acid and 3- or 4-bromopyridine.
As control experiments, we examined the catalytic activity
of Pd(COD)Cl2 towards iodobenzene, and the yield of biphenyl
at 60 1C was 97% after 0.5 h in the presence of 1 mmol% of
Pd(II). A yield of 26% was obtained from the reaction of
4-bromoanisole and phenylboronic acid after 3.0 h. The
coupling of 4-bromopyridine with phenylboronic acid gave
4-phenylpyridine in 55% yield after 0.5 h, and 70% after 1.0 h.
Comparison of the results of GP1-Pd and Pd(COD)Cl2 shows
that the gel has higher catalytic activity than the unsupported
homogeneous catalyst.
Table 5 Suzuki cross-coupling of bromobenzene with phenylboronic
acid using GP1-Pd and the aerogel as recovered catalystsa
Yieldb (%)
(Cat.: wet gel)
Yieldb (%)
(Cat.: aerogel)
Entry
Run
1
2
3
1st
2nd
3rd
40
20
9
36
13
2
a
0.5 mmol of bromobenzene, 0.75 mmol of PhB(OH)2, 1.5 mmol of
Na2CO3, 0.2 mmol% of Pd(II) of wet gel/aerogel in a filter paper bag,
The xerogel, obtained after the solvent in GP1-Pd was
allowed to evaporate slowly in air at RT, was also examined
for its activity. The reaction of iodobenzene with phenylboronic
in 6 mL of methanol (60 1C) under ambient atmosphere for
b
1 h. Determined by GC.
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Encouraged by the good catalytic activity of GP1-Pd, we
examined its recyclability in the reaction of bromobenzene/
bromopyridine with phenylboronic acid. The gels/xerogels/
aerogels packed in filter bags were readily separated after
catalysis. In the reactions of aryl halides, the recovered catalyst
could be subjected to the next reactions after the first reac-
tion. There was no leaching into solution observed for the
xerogels, while a small amount of leaching was observed for
the gels/aerogels. Palladium black was observed within the
filter paper bags, and the catalytic activity of wet gel/aerogel
was diminished in the following reuse, as shown in the reaction
of bromobenzene with phenylboronic acid (Table 5).
polymer supports, or silica/alumina supports, which are
usually prepared by sol–gel processes.23,26 Such gel networks
may also be further readily modified by loading with other
metals, such as Pt, Rh, Ir and so on. It would also be
interesting to explore combining the catalytic abilities of both
the supporting and supported metal ions. Studies along these
lines are currently in progress in our lab.
Experimental
All starting materials and solvents were obtained from
commercial sources and used without further purification.
5-Diphenylphosphanylisophthalic acid was synthesised
following the literature method.14 Infrared spectra were
measured on a Nicolet Avatar 330 FT-IR spectrometer with
KBr pellets except that those of gels were measured with liquid
film on KBr plates. Scanning electron micrographs were
recorded by using a Jeol JSM-6330F instrument. Gel samples
were evaporated slowly to dryness in air, dried in vacuum, and
applied to aluminium stubs. Prior to examination the gels were
coated with a thin layer of gold. X-Ray photoelectron spectro-
scopy was measured on a Thermo-VG Scientific ESCALAB
250 spectrometer, under a pressure of B2 ꢂ 10ꢁ9 mbar, fitted
with X-ray monochromated Al-Ka radiation (photo-energy =
1486.6 eV, 15 kV, 150 W, spot size = 500 mm). Pd elemental
analyses were performed by inductively coupled plasma–
atomic emission spectroscopy using a TJA IRIS HR ICP
instrument.
It is noteworthy, however, that no palladium black was
visually seen in the reactions of bromopyridines with boronic
acids. The gel or xerogel maintains its catalytic activity under
similar conditions after five reuses at least, as shown in the
reaction of 4-bromopyridine with 3,5-difluorophenylboronic
acid (Table 6). The reactions in the 1st and 5th cycled run gave
4-(3,5-difluorophenyl)anisole in 85% and 80% yield, respec-
tively, catalysed by the wet gel, and in 41 and 55% yield,
respectively, catalysed by the xerogel. In addition, the results
show again that the xerogel has poorer catalytic activity than
the GP1-Pd gel under similar conditions.
Conclusions
In conclusion, we have demonstrated that
a known
Fe–H3BTC gel system can be readily modified to give new
phosphine-functionalised gels. These are novel metal–organic
gels with free phosphorus donors24 available for further
coordination. The subsequently functionalised Pd(II)-immobilised
gel and its xerogel/aerogel showed high activity in the catalysis
of Suzuki C–C coupling comparable to unsupported com-
plexes. The gel/xerogel could be reused in the reactions of
bromopyridines under ambient atmosphere as shown in the
reaction of 4-bromopyridine with 3,5-difluorophenylboronic
acid. These show that metal–organic gels can act as a new type
of functionalisable porous scaffold despite their potential
complexity in some aspects, for example, sensitivity to
mechanical stress, which however may be overcome by specific
postmodification.25 Due to the large range of metal ions and
organic ligands available, metal–organic gels have much
further chemistry to explore. MOG-supported catalysts may
have tailored properties compared with conventional organic
Synthesis of metal–organic gels
(a) Fe(NO3)3 (303 mg, 0.75 mmol) and 5-tert-butylisophthalic
acid (111 mg, 0.50 mmol) were each dissolved in ethanol (5 ml
each), and the two solutions were rapidly mixed together, then
left to stand. An turbid orange gel (GBu2) was formed after
1 min. Similar gels were also obtained in MeOH or DMF.
(b) 5-Diphenylphosphanylisophthalic acid (0.05 mmol) and
Fe(NO3)3 (0.05 mmol) in ethanol (1 ml each) were mixed
together, and then left to stand. An turbid orange gel (GP1)
was formed after 3–5 days. Similar gels were obtained in
n-BuOH or DMF.
Post-modification of metal–organic gel
An acetonitrile solution containing an excess of Pd(COD)Cl2
was allowed to stand above a sample of gel GP1. After slow
diffusion for ca. 10 days, the supernatant was decanted and
the gel was washed several times with acetonitrile until the
washings were colourless, yielding a gel, GP1-Pd.
Table 6 Suzuki cross-coupling of 4-bromopyridine with 3,5-difluoro-
phenylboronic acid using recovered GP1-Pd as catalysta
Yieldb (%)
(Cat.: wet gel)
Yieldb (%)
(Cat.: xerogel)
Formation of aerogel
Entry
Run
The wet gel GP1-Pd was extracted with liquid CO2 (ca. 270 g)
in a supercritical high pressure Soxhlet extractor (0.75 L) for
3 h to remove solvent from the gel. The extraction temperature
were kept at 35.0 1C. When the autoclave was depressurized
slowly for 1 h, a supercritical aerogel was obtained.
1
2
3
4
5
1st
85
83
80
82
80
41
43
45
49
55c
2nd
3rd
4th
5th
a
0.5 mmol of 4-bromopyridine hydrochloride, 0.55 mmol of
3,5-difluorophenylboronic acid, 2.0 mmol of Na2CO3, 0.2 mmol%
of Pd(II) of wet gel/xerogel in a filter paper bag, in 6 mL methanol
General procedure for the coupling reactions of aryl halides with
boronic acids catalysed by gel/xerogel
b
(60 1C) under ambient atmosphere for 1 h. Determined by GC.
The yield increased due to catalyst’s smaller size.
A typical procedure is given for the reaction shown in
entry 1 of Table 3. Iodobenzene (136.7 mg, 0.67 mmol),
c
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phenylboronic acid (121.9 mg, 1.0 mmol), Na2CO3 (212.0 mg,
2.0 mmol), MeOH (8 ml) and GP1-Pd (0.2 mmol% of Pd(II))
were introduced to a flask under ambient atmosphere. The
mixture was slowly stirred at 60 1C. H2O (10 ml) was added
into the resultant mixture and extracted three times with Et2O
(10 ml). The ethereal extract was evaporated to dryness and
analyzed by GC/MS and NMR spectroscopy. For recovering,
the gel was packed in a filter paper bag, and the xerogel/
aerogel was either centrifuged after reaction or packed in a
filter paper bag.
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Acknowledgements
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We thank Mr Yunfei Hao for assistance in synthesis and the
SRF for ROCS, SEM, the NSF of China (Grant 20525310,
20773167, 20731005) and the 973 Program of China (Grant.
2007CB815302) for support. We thank the Leverhulme Trust
(grant no. F/00203/H) for partial support of this work.
13 (a) X. Yang, R. Lu, T. Xu, P. Xue, X. Liu and Y. Zhao,
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