SBA-15-type organosilica with 4-mercapto-N,N-bis-(3-Si-propyl)butanamide for palladium scavenging and cross-coupling catalysis

Article abstract of DOI:10.1002/chem.201002190

This work describes the synthesis of novel functional silica materials with difunctional thiol-amide substructures and featuring regular architectures on a mesoscopic level. The functional materials were synthesised by both one-pot co-condensation and post-grafting approaches. The thiol groups confined in the matrix were found to be efficient for palladium entrapment, leading to highly active and reusable heterogeneous catalysts for Sonogashira and Suzuki-Miyaura cross-coupling reactions. This work evidences the crucial role of both the thiol precursor and the condensation degree of the silica scaffold in view of the design of stable and reusable tailor-made mesoporous catalytic silica materials.

Full text of DOI:10.1002/chem.201002190

DOI: 10.1002/chem.201002190
SBA-15-Type Organosilica with 4-Mercapto-N,N-bis-(3-Si-
propyl)butanamide for Palladium Scavenging and Cross-Coupling Catalysis
Samir El Hankari,[a, b] Abdelkrim El Kadib,[c] Annie Finiels,[a] Ahmed Bouhaouss,[b]
Joꢀl J. E. Moreau,[a] Cathleen M. Crudden,[c] Daniel Brunel,[a, d] and Peter Hesemann*[a]
Abstract: This work describes the syn-
thesis of novel functional silica materi-
als with difunctional thiol–amide sub-
structures and featuring regular archi-
tectures on a mesoscopic level. The
functional materials were synthesised
by both one-pot co-condensation and
post-grafting approaches. The thiol
groups confined in the matrix were
found to be efficient for palladium en-
trapment, leading to highly active and
reusable heterogeneous catalysts for
Sonogashira
and
Suzuki–Miyaura
cross-coupling reactions. This work evi-
dences the crucial role of both the thiol
precursor and the condensation degree
of the silica scaffold in view of the
design of stable and reusable tailor-
made mesoporous catalytic silica mate-
rials.
Keywords: cross-coupling · hetero-
geneous catalysis
·
mesoporous
materials · palladium · template
synthesis
Introduction
For most applications, the introduction of functional
groups on the surface of the material is critical for chemose-
lectivity. Among such functionalised materials, mercapto-
propyl-functionalised nanostructured silica materials have
been studied since the late 1990s. Functionalisation with
thiol groups can be achieved either by post-grafting[6,7] or
one-pot hydrolysis–polycondensation procedures.[8–11] The
resulting sulphur-containing materials show interesting fea-
tures for metal adsorption and therefore have found wide-
spread applications in separation, in particular for the se-
questration of heavy metals such as mercury, palladium or
platinum.[7,9,12] The kinetics of mercury uptake on various
types of thiol-functionalised mesoporous silica have been ex-
amined to reveal the morphologies of the materials on a
mesoscopic length scale and to provide detailed information
concerning the distribution of thiol groups throughout the
material.[13] In particular, it has been shown that the synthet-
ic pathway (post-grafting or co-condensation) has a strong
influence on the binding properties of the immobilised thiol
groups in terms of the degree of accessible thiol sites, ad-
sorption kinetics and stability of the resulting material.[14]
Besides nanostructured silicas, bifunctional thiol-containing
periodic mesoporous organosilicas (PMO) have also been
considered in solid–liquid extraction of heavy metals. More
hydrophobic PMO-type materials obtained from silylated
organic precursors containing p-phenylene[15] or isocyanu-
rate[16] groups on the one side and mercaptopropyl-trialk-
Since their first preparation in the early 1990s,[1] nanostruc-
tured silica-based materials obtained by templating routes[2]
have gained considerable interest due to their good thermal
stability, high porosity and narrow pore-size distribution.
These features make mesoporous silica-based materials
ideal candidates for applications in various fields such as
catalysis,[3] separation[4] and detection.[5]
[a] S. El Hankari, Dr. A. Finiels, Prof. J. J. E. Moreau, Dr. D. Brunel,
Dr. P. Hesemann
Institut Charles Gerhardt de Montpellier
ꢀquipes AM2N and MACS
Ecole Nationale Supꢁrieure de Chimie de Montpellier
8, rue de lꢂEcole Normale
34296 Montpellier Cedex 05 (France)
Fax : (+33)467-1443-53
E-mail: peter.hesemann@enscm.fr
[b] S. El Hankari, Prof. A. Bouhaouss
Laboratoire de Chimie Physique Gꢁnꢁrale I des Matꢁriaux
Nanomatꢁriaux et Environnement
Universitꢁ Mohammed V - Agdal
Facultꢁ des Sciences
Av. Ibn Battouta, Rabat (Maroc)
[c] Dr. A. El Kadib, Prof. C. M. Crudden
Department of Chemistry
Queenꢂs University
AHCTUNGTREGoNNNU xysilane on the other side show similar features in metal
90 Bader Lane
K7L 3N6 Kingston, Ontario (Canada)
sequestration compared with thiol-functionalised mesopo-
rous silica[17] and effectively remove “soft” metal ions such
as Hg2+ and Ag+ from aqueous solutions, whereas lower af-
[d] Dr. D. Brunel
Instituto de Tecnologia Quimica
UPV-CSIC–Universidad Politecnica de Valencia
Av. de los Naranjos s/n
finities were observed for the “harder” metal ions Cd2+
,
Co2+ and Pb2+
.
46022 Valencia (Spain)
Mesoporous silicas containing thiol groups are also inter-
esting intermediates to access materials with anionic sulfonic
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002190.
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Chem. Eur. J. 2011, 17, 8984 – 8994

FULL PAPER
acid groups, which can easily be prepared by oxidation reac-
tions. These materials can be used as ion-exchange resins,[11]
solid acid catalysts[18] or hard templates for the formation of
tube-like carbon replicas.[10]
be considered: 1) the topology and the structure type of the
support influences the mass transfer and diffusion of metal-
lic species;[22,26] 2) the pore size of the material has to be
high enough to ensure diffusion of reactants and products to
and from the active site; it should avoid any mass-transport
limitation;[27] 3) the homogeneous distribution of the active
sites over the whole material (for instance, wall framework
vs. pores) should facilitate the accessibility of the reactants
to the active site and, 4) the hydrophilic–hydrophobic bal-
ance and the surface states of the support should permit the
host–guest interactions and adsorption–desorption process
of the reactants and products.[28] Based on these criteria, we
began to investigate new nanostructured materials based on
sulphur-containing SBA-15-type structures. The use of a new
bis-silylated amido–thiol as a polymerisable precursor al-
lowed us to synthesise novel wall- and pore-functionalised
siliceous materials. Because the nature of the precursors em-
ployed has a pivotal role in the templating process,[29,30] we
were especially interested to study the repercussions of the
use of a disilylated precursor on the morphology of the re-
sulting materials and, consequently, on the accessibility of
the thiol groups and the resulting catalytic activity of these
materials. Compared with previous work employing mono-
dentate grafting sites, the new materials should have greater
stability of the thiolated surface under aqueous conditions,
and should also have different surface chemistry based on
the inclusion of a polar amide in the linker. For the sake of
comparison, grafted material was prepared starting from a
pure inorganic SBA-15 material, and comparisons made to
previously reported MPTMS-functionalised materials. The
accessibility of thiol groups was assessed by scavenging pal-
ladium in THF solution. The resulting palladium-supported
materials were used as catalysts for Sonogashira and
Suzuki–Miyaura cross-coupling reactions and comparisons
made to more conventional MPTMS-functionalised materi-
als.
More recently, Crudden and others showed that thiol con-
taining mesoporous silicas are excellent scavengers and het-
erogeneous ligands for palladium-based catalysts.[14,19,20]
High catalytic activity and recyclability were observed in
Pd-catalyzed cross-coupling reactions such as the Suzuki–
Miyaura and Mizoroki–Heck reactions. In these materials, a
beneficial effect on the catalytic properties was attributed to
the SBA-15-type mesostructure, restricting the growth of
palladium nanoparticles based on the size of the channels.
Whereas Pd adsorbed on amorphous silica agglomerated
into large particles on the exterior of the surface, leading to
catalytic deactivation, palladium particles that formed on
the SBA-15-based support were limited by the size of the
channels to approximately 6 nm.[14] Although the materials
could be reused multiple times, eventually degradation due
to collapse of the walls took place, which was shown to be
caused by the necessary presence of a strong base for these
coupling reactions.[21] The incorporation of aluminium was
shown to dramatically improve stability, but since Pd ap-
pears to reside primarily on the interior of these structures,
a pore wall collapse leads to loss of catalytic activity as it en-
traps Pd inside.[21] Interestingly, this was not the case with
the more open cubic KIT-6-type supports, which remain cat-
alytically active after 6–8 runs, despite the collapse of the
mesostructure.[22] The presence of Pd on the exterior of
these materials was confirmed by TEM studies, and, consis-
tent with the lack of mesoporosity at this point, Pd was ag-
glomerated into large, non-uniform particles. Thus the
design of materials that are able to retain Pd on the interior
surface in the same way that SBA-15-type structures are
able to do, but that are stable towards base-induced pore
collapse is a significant and important challenge in this area.
Detailed studies on this system gave a deep insight in the
reaction mechanism and showed that the reaction likely
occurs in homogeneous solution following a “release and
catch” mechanism.[14,23] Catalytic Pd-species are leached out
of the materials, then they carry out the coupling reaction,
and redeposit at the end of the catalytic cycle. Thiol-func-
tionalised mesoporous silicas are particularly efficient for
the recapture of metallic species at the end of the catalytic
cycle and thus ensure low Pd contamination of the reaction
mixture and products. This feature is of great interest for
the regulatory issue of metal contamination in the context
of pharmaceutical synthesis. Many bioactive substrates are
prepared by Pd-based catalysts that necessitate a time con-
suming work-up and costly effective techniques for metal re-
moval.[24,25] The preparation of highly conjugated materials
for use in organic electronics has even more stringent re-
quirements for metal contamination since the presence of
small quantities of metal can act as locations for short cir-
cuits.
Results and Discussion
Although recent studies have examined the effect of param-
eters such as the method of incorporation of the thiol, the
architecture of the material, and the pore size of the thiol-
functionalised material on catalytic properties and stabili-
ty,[20–22,31] the influence of the structure of the thiol precursor
on the morphology of the formed mesoporous silica and the
catalytic performances of the obtained Pd-impregnated
nanostructured materials has not been examined to date.
Thus, we targeted an original precursor with two points of
attachment through an amide linker, aiming to uniformly
functionalise the silica wall framework. This compound was
used to synthesise amide–thiol-functionalised mesoporous
silica, both through post-grafting and co-condensation ap-
proaches. The catalytic properties of the resulting Pd-im-
pregnated materials were compared to classical mercapto-
propyl-functionalised silica obtained from mercaptopropyl-
triethoxysilane (MPTES) by direct hydrolysis–co-condensa-
In view of designing more sophisticated catalysts for pal-
ladium cross-coupling reactions, several parameters have to
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8985

P. Hesemann et al.
tion with tetraethoxysilane (TEOS) and subsequent Pd-im-
pregnation.
Precursor and materials synthesis and characterisation: The
preparation of the disilylated 4-mercapto-N,N-bis(3-(tri-
ACHTUNGTRENNUNG
Scheme 1. Synthesis of the disilylated thiol–amide precursor 1.
by the ring opening of g-thiobu-
tyrolactone with monosilylated
3-aminopropyltriethoxysilane.[32]
A similar ring opening of g-
thioACHTUNGTRENNUNGbutyrolactone by nucleo-
philic attack of bis(triethoxysi-
lylpropyl)amine afforded the
targeted product 1 after elimi-
nation of volatile products and
starting materials (Scheme 1).
This new bis-silylated com-
Scheme 2. Template-directed hydrolysis/polycondensation of precursor 1 with tetraethyl-ortho-silicate.
1
pound was characterised by H,
13C, infra-red and mass spectro-
scopic analysis. The 13C at-
tached proton test (APT) liquid
NMR spectrum of the precur-
sor 1 is shown in Figure 1. The
presence of the amide group
with its partial double-bond
character results in a doubling
of the 13C and the 1H liquid
NMR spectra. 2D NMR spec-
troscopy (see the Supporting
Information) permitted an ac-
curate attribution of the carbon
centres present in the precur-
sor.
Scheme 3. Synthesis of material SBA-15-G by post-synthesis grafting of precursor 1 onto the SBA-15-type
silica.
Thiol–amide-functionalised
materials were obtained either
by using template-directed hydrolysis/polycondensation of
precursor 1 with various amounts of a silica network precur-
sor (tetraethyl-ortho-silicate, TEOS, Scheme 2) or post-
grafting reactions of precursor 1 onto a previously synthes-
ised SBA-15-type silica (Scheme 3). Whereas post-synthesis
grafting reactions afforded materials in which the whole sily-
lated precursor is grafted onto the surface of the mesopo-
rous silica, one-pot hydrolysis polycondensation reactions
led to the formation of materials in which the siloxy groups
of the thiol precursor are integrated in the silica network
and therefore are an inherent part of the silica scaffold.
In the first series of experiments, the bis-silylated precur-
sor 1 was employed for the synthesis of functional silica-
hybrid materials by using a co-condensation approach. Hy-
drolysis–polycondensation reactions were performed with
thioamide 1:TEOS ratios of 1/2, 1/4, 1/6 and 1/8, to give the
formation of the materials SBA-15-(1/2), SBA-15-(1/4),
SBA-15-(1/6) and SBA-15-(1/8), respectively. Another mate-
rial (SBA-15-(1/8-KCl)) was synthesised in the presence of
potassium chloride to evaluate the influence of the presence
of mineral salts on the formation of nanostructured materi-
als in hydrolysis–polycondensation reactions. All the above
materials were synthesised in the presence of the non-ionic
triblock-copolymer Pluronic P123 as the structure-directing
Figure 1. 13C APT liquid NMR spectrum of precursor 1 (bottom) and 13C
CP MAS NMR spectrum of material SBA-15-G (top).
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SBA-15-Type Organosilica
FULL PAPER
agent following previously published procedures for co-con-
densation reactions of di- and oligosilylated precursors with
TEOS.[30,33] The exact quantities and ratios of precursor 1
and TEOS in the various formulations are given in Table 1.
RSiO3 and SiO4 environments in the materials, which is in
remarkable agreement to the molar ratio of the co-mono-
mers in the reaction mixture (1/8). Besides the quantifica-
tion of the T/Q-signal ratio, the spectra also gave further in-
formation
concerning
the
degree of condensation the sili-
con centres in the materials.
The spectra of SBA-15-(1/8-
KCl) shows nearly equal
Table 1. Molar precursor 1/TEOS ratios and loadings of the amide–thiol-functionalised materials.
Material
Precursor 1
[mg (mmol)]
TEOS
[mg (mmol)]
Surfactant
P123 [mg]
HClaq (1.9m)
[mL]
Loading[a]
SiO2
G
R
G
N
AHCTUNGTERNNUNG ]
[mmolgꢀ1
amounts
of
Q3
(d=
SBA-15-(1/2)
SBA-15-(1/4)
SBA-15-(1/6)
SBA-15-(1/8)
SBA-15-(1/8-KCl)
SBA-15-G
121 (0.23)
540 (1.03)
390 (0.73)
300 (0.58)
300 (0.58)
–
97 (0.46)
850 (4.1)
915 (4.39)
950 (4.55)
950 (4.55)
–
91
2.8
16
16
16
16
–
2.75
2.75
2.23
1.77
ꢀ101.2 ppm) and Q4 (d=
ꢀ110.2 ppm) environments to-
500
500
500
500
–
gether with
a
very small
[b]
2.20 (1.99)[c]
1.48 (1.33)[c]
amount of Q2 species (d=
ꢀ91.1 ppm). This result reflects
the presence of a significant
[a] Values obtained by TGA measurements. [b] KCl amount: 3.17 g (42.5 mmol). [c] Values obtained by 29Si
OP-MAS NMR measurements.
A functionalised material was also synthesised by a post-
grafting reaction. In this synthesis, the bis-silylated precursor
was anchored on the surface of previously synthesised SBA-
15-type silica, giving rise to the material denoted SBA-15-G.
The obtained materials were characterised by means of
solid state NMR spectroscopy and thermogravimetry to de-
termine the loading of the materials. Nitrogen sorption ex-
periments, X-ray diffraction and electron microscopy per-
mitted us to gain detailed insight into the architecture of the
thiol-functionalised materials.
The comparison of the materials obtained by post-synthe-
sis grafting procedures and “one-pot” hydrolysis polycon-
densation procedures revealed different characteristics in
terms of functionalisation degree and morphology.
Firstly, 13C CP-MAS NMR spectroscopy provided
a
Figure 2. 29Si OP MAS NMR spectrum of material SBA-15-(1/8-KCl)
(top) and SBA-15-G (bottom).
method for monitoring the incorporation of the amide–thiol
substructures within the materials. The solid state 13C cross-
polarization magic angle spinning (CP-MAS)-NMR spec-
trum of material SBA-15-G is shown as an example in
Figure 1. The spectrum clearly shows the characteristic sig-
nals of the aliphatic substructures (d=10–50 ppm) and the
signal of the carbonyl group at d=174 ppm, which are both
characteristic for the presence of the thiol–amide precursor
within the materials. The signals at d=17 and 58 ppm indi-
cate the presence of uncondensed ethoxysilyl groups in the
material. The solid state spectrum is in good agreement with
the 13C APT spectrum of precursor 1 (Figure 1, bottom) ob-
tained in liquid NMR spectroscopy.
ꢀ
amount of Si OH groups in the material. In contrast, the Q
resonance in the spectrum of material SBA-15-G is domi-
nated by the Q4 resonance corresponding to completely con-
densed SiACHTUNGRTENNG(U OSi)4 substructures, indicating a highly condensed
silica network. This material was obtained by calcination at
5508C to eliminate the non-ionic structure directing agent
prior to the grafting reaction with precursor 1. For this
reason, the silica scaffold shows approximately 95% con-
densation, which is a considerably higher value than that of
the materials obtained under milder reaction conditions by
“one-pot” hydrolysis–polycondensation procedures.
The shape of the signals corresponding to the T resonan-
ces in Figure 2 reflects the degree of condensation of the
RSiO3 substructures. In the SBA-15-(1/8-KCl) material, a
higher intensity was found for the T3 substructure corre-
The 29Si one pulse (OP)-MAS NMR spectra of materials
SBA-15-(1/8-KCl) and SBA-15-G are shown in Figure 2.
The 29Si OP-MAS technique allows one to quantify the
molar ratio of the different silicon species in the materials.
Both spectra show the T- and Q-resonances characteristic of
the presence of RSiO3 and SiO4 substructures. At first
glance, the shape of the signals reflects the two different
synthetic strategies applied for the synthesis of the two func-
tional materials. In the case of the material SBA-15-(1/8)-
KCl, a ratio of T and Q signals of 19.3/80.7 was found in the
material. This ratio corresponds to a 0.96/8 ratio of the
sponding to fully condensed RSiACHTNUGTRENUNG(OSi)3 sites. This result can
be explained by the fact that the precursor 1 was completely
incorporated within the silica walls during the hydrolysis–
polycondensation reaction and reacted with hydrolyzed
TEOS silica precursors. In contrast, the much broader shape
of the T-resonances in the spectra of material SBA-15-G in-
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P. Hesemann et al.
dicates the presence of nearly equal amounts of T2
(RSi (OSi)3) sites. The
(OSi)2(OH)) and T3 (RSi
Table 2. Surface properties of the materials.
G
ACHTUNGTRENNUNG
Material
XRD
d100 [ꢄ]
Nitrogen sorption
quantification of the T2/T3 ratio in the materials
using the dmfit program[34] shows that the molar
ratio between the different T-species corresponds to
T1/T2/T3 =0.06:0.55:0.39. Thus approximately 39%
of the siloxy groups of the amide–thiol precursor
are anchored to the mesoporous silica walls through
ao [ꢄ]
BET
Pore volume
Average
pore
surface
area
Vp [mLgꢀ1
]
diameter
SBET [m2 gꢀ1
]
Dp [ꢄ][a]
SBA-15-(1/2)
SBA-15-(1/4)
SBA-15-(1/6)
SBA-15-(1/8)
SBA-15-(1/8-KCl)
SBA-15
–
–
–
–
260
442
583
608
568
614
314
–
–
0.64
0.68
0.71
0.93
1.02
86
74
72
89
89
70
92
80
103
98
99
106
104
119
113
114
ꢀ ꢀ
support by three Si O Si bonds. In addition, this
explains the presence of residual ethoxy groups in
the material that can be seen in the 13C CP-MAS
NMR spectrum of the material SBA-15-G.
SBA-15-G
0.48 (0.60)[b]
[a] Pore size evaluated according to the Broekhoff de Boer (BdB) method. [b] Pore
volume versus pure silica weight.
The T/Q ratio found in the 29Si OP-MAS NMR
spectra of materials SBA-15-(1/8-KCl) and SBA-15-
G directly gives the loadings of organic functionali-
ty in these solids. In the case of SBA-15-(1/8-KCl),
a T/Q ratio of 0.193:0.807 was found, corresponding to a
loading of 1.99 mmolgꢀ1 SiO2. The material SBA-15-G has a
slightly lower degree of functionalisation with a T/Q ratio of
0.138:0.862, corresponding to a loading of 1.33 mmolgꢀ1
SiO2.
The characterisation of the materials by thermogravime-
try gave additional quantitative information concerning the
functionalisation degree of the materials. When the organic
grafting was estimated from TGA measurements, the de-
composed weight/residual (SiO2) weight at 7008C stems
from organic grafted species on silica based on 1 g of silica.
We used the following Equation (1) for the determination
of the amount of immobilised organic entities. The results
obtained are given in Table 1.
diffraction, nitrogen sorption and electron microscopy. The
textural properties of these materials are summarised in
Table 2.
Small-angle X-ray diffraction experiments gave the first
information concerning the architectures of the materials.
The diffractograms of the materials SBA-15-(1/2), SBA-15-
(1/6), SBA-15-(1/8) and SBA-15-(1/8-KCl) are given in
Figure 3. The results are summarised in Table 2. It is clear
nthiol ¼ org: wt %=½100ꢀðorg: wt %ꢀH2O wt %ÞꢁðMthiolꢀ2H2OÞ
ð1Þ
The results obtained from TGA measurements show that
the TEOS/precursor ratio in the hydrolysis–polycondensa-
tion mixture has a direct influence on the loading of the re-
sulting materials. The loadings of the materials are in the
range of 2.75 mmolgꢀ1 SiO2 for the material SBA-15-(1/2)
and 1.77 mmolgꢀ1 SiO2 in the case of the sample SBA-15-(1/
8). Post-synthesis grafting led to a material with the lowest
loading in this series (1.48 mmolgꢀ1 SiO2). As expected, the
use of higher amounts of the functional precursor in the hy-
drolysis–polycondensation mixtures yielded materials con-
taining higher concentrations of immobilised organic sub-
structures. The results of the TGA experiments are in good
agreement with the results obtained from 29Si OP-MAS
NMR spectroscopy. The slightly higher loadings determined
from TGA measurements can be explained by the presence
of residual ethoxy groups in the materials as already con-
cluded from 13C CP-MAS NMR spectra of the solids. The
degree of condensation of the T- and Q-silicon sites, as de-
termined by 29Si solid state NMR spectroscopy, clearly re-
flect the different synthetic pathways employed in the syn-
thesis of the materials.
Figure 3. X-ray diffractograms of the materials SBA-15-(1/2), SBA-15-(1/
6), SBA-15-(1/8) and SBA-15-(1/8-KCl), obtained by co-condensation of
precursor 1 with TEOS
that the TEOS/precursor ratio in the hydrolysis–polycon-
densation mixture has a significant influence on the archi-
tecture of the materials synthesised by co-condensation re-
actions. Whereas the SBA-15-(1/2) materials do not show
any diffraction pattern and are completely amorphous, in-
creasing the ratio of TEOS in the hydrolysis–polycondensa-
tion mixtures led to materials with high structural regularity.
The materials SBA-15-(1/6) and SBA-15-(1/8) display inter-
mediate regularity. The diffractograms of these materials
show intense (100) and weak (200) reflections, indicating
that the materials have essentially worm-like or lamellar ar-
chitectures. The addition of inorganic salts such as sodium
chloride to the reaction mixture favours the formation of
nanostructured materials with highly regular architectures.[35]
In the following, we addressed the determination of the
morphologies of the obtained materials by means of X-ray
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SBA-15-Type Organosilica
FULL PAPER
We have already employed this so-called “salt effect” to
obtain nanostructured silica materials containing covalently
tethered guanidinium sulfonimide ion pairs.[33] Consequently,
the diffractogram of the material SBA-15-(1/8-KCl), syn-
thesised in the presence of potassium chloride in the hydrol-
ysis–polycondensation mixture, displays the 100, 110 and
200 reflections characteristic of nanostructured materials
with hexagonal architectures on a mesoscopic length scale
(Figure 3). It is noteworthy that the addition of potassium
chloride to the hydrolysis–polycondensation mixture led to
an increase of the pore–pore distance as can be seen from
the shift of the (100) reflection from 2q=0.96/0.998 (a0 =
106/104 ꢄ) in the case of the materials SBA-15-(1/6) and
SBA-15-(1/8) to 2q=0.86 (a0 =119 ꢄ) observed for the ma-
terial SBA-15-(1/8-KCl).
0.7 indicating essentially mesoporosity with an average pore
diameter of 86 ꢄ. Slightly increased porosity and pore
volume but very similar architecture on a mesoscopic length
scale were found for the material SBA-15-(1/8).
The beneficial effect of mineral salts expected from previ-
ous reports[33,35,36] was evidenced in the case of material
SBA-15-(1/8-KCl), which shows a very sharp adsorption
step at p/p0 =0.78 indicating very narrow pore-size distribu-
tion centred at 89 ꢄ. The nitrogen adsorption in the case of
the material SBA-15-(1/8) occurs at lower pressures and is
considerably flatter, which suggests a smaller average pore
size and broader pore-size distribution in this material
(Figure 5). Finally, the materials SBA-15-(1/8) and SBA-15-
On the other hand, post-synthesis grafting reactions do
not affect the architectures of nanostructured silica materi-
als. The pristine SBA-15 silica and the material SBA-15-G
show very similar diffractograms with nearly identical cell
parameters (Table 2).
The results of the X-ray diffraction experiments were con-
firmed by nitrogen sorption experiments. The nitrogen sorp-
tion isotherms of the materials SBA-15-(1/2), SBA-15-(1/4),
SBA-15-(1/6) and SBA-15-(1/8) are shown in Figure 4. The
Figure 5. Nitrogen sorption isotherms of the materials SBA-15-(1/8) (&)
and SBA-15-(1/8-KCl) (ꢁ).
(1/8-KCl) show similar specific surface area as it can be seen
from the very similar shape of the isotherms at low pres-
sures (p/p0 <0.4; SBA-15-(1/8): SBET =608 m2 gꢀ1; SBA-15-(1/
8-KCl): SBET =568 m2 gꢀ1) but considerably different pore
volumes (SBA-15-(1/8): 0.71 cm3gꢀ1 vs. SBA-15-(1/8-KCl):
0.93 cm3gꢀ1).
This increase of the structural regularity in the case of the
material SBA-15-(1/8-KCl), which is induced by the so-
called “salt-effect”, can be rationalised by a swelling of the
micelles together with a significant increase of the precur-
sor/template interaction at the interface.[33,35,36] PEO-PPO-
PEO triblock copolymers such as Pluronic P123 form mi-
celles in water with poly(propyleneoxide) (PPO) cores sur-
rounded by a shell of hydrated poly(ethylene oxide) (PEO)
end blocks.[37] The presence of inorganic salts in the hydroly-
sis–polycondensation mixture induces dehydration of ethyl-
ene oxide layer, which decreases the hydrophilicity of PEO
and enhances the hydrophobicity of PPO.[38] Thus, the less
hydrophilic organosilicate precursor interacts in a favoura-
ble way with the template resulting in long-range-ordered
domains of organosilica–surfactant–silica mesostructures.
From TGA measurements, it can be seen that the “salt-
effect” increases the amount of organic molecules integrated
in the wall of the materials. However, sufficient silica con-
tent is needed in order to “relax” the wall-framework of
~
Figure 4. Nitrogen sorption isotherms of the materials SBA-15-(1/2) ( ),
^
SBA-15-(1/4) (ꢁ), SBA-15-(1/6) ( ), SBA-15-(1/8) (&).
isotherms of the materials SBA-15-(1/8) and SBA-15-(1/8-
KCl) are compared in Figure 5. Finally, the isotherms of the
pristine silica SBA-15, used for post-grafting reactions, and
the grafted material SBA-15-G are shown in Figure 7. The
surface properties of these materials are summarised in
Table 2.
In the series of materials obtained by co-condensation re-
actions, materials SBA-15-(1/2) and SBA-15-(1/4) show ni-
trogen uptake over a large p/p0 range, which indicates that
these two materials display a low structural regularity and
large pore-size distribution. Highly porous and structured
materials were obtained starting from a 1:TEOS ratio of 1/6.
The material SBA-15-(1/6) has a specific surface area SBET
of 583 m2 gꢀ1. The adsorption–desorption isotherm of this
material shows a hysteresis loop in the range of p/p0 =0.6–
Chem. Eur. J. 2011, 17, 8984 – 8994
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8989

P. Hesemann et al.
Figure 6. SEM pictures of materials a) SBA-15-(1/2) and b) SBA-15-(1/6); scale bar represents 6 mm. c) TEM image of material SBA-15-(1/8-KCl).
these hybrid materials. Consequently, materials with low
silica content are amorphous or low-structured and show
low porosity. In contrast, materials with high silica content
display the typical features of SBA-type materials, that is,
highly regular architectures on a mesoscopic length scale,
mesoporosity and a very narrow pore-size distribution.
These results clearly indicate that the molar TEOS/1 ratio
in the hydrolysis–polycondensation mixture together with
the presence of mineral salts are the most important param-
eters for the formation of nanostructured silica phases by
co-condensation approaches for the synthesis of thiol func-
tionalised materials from TEOS and the thiol-amide precur-
sor 1. The material SBA-15-(1/8-KCl) shows the highest
structural regularity in this series.
Finally, the results obtained from nitrogen sorption and
X-ray diffraction experiments were confirmed by transmis-
sion and scanning electron microscopy. An SEM image of
material SBA-15-(1/2) given in Figure 6 shows the presence
of agglomerated particles without any particular shape and
diameters of approximately 1 mm. The particle shape ob-
served in materials SBA-15-(1/6), SBA 1/8 and SBA-15-(1/8-
KCl) is very different. Here, elongated particles and fibres
(300–400 nm thick and up to 20 mm long) can be seen from
the SEM images. The TEM image of material SBA-15-(1/8-
KCl) (Figure 6c) reveals regular architectures on a meso-
scopic length scale as already indicated from X-ray diffrac-
tion experiments.
The isotherms of materials SBA-15 and SBA-15-G, ob-
tained through grafting of the precursor 1 on the surface of
SBA-15, are shown in Figure 7. It is clear that the grafting
reaction strongly affects the texture of the materials. The
material SBA-15-G shows decreased values in terms of spe-
cific surface areas, pore volume and average pore diameter
compared to the pristine silica SBA-15 (Table 2). The varia-
tion of the pore volume when standardised versus a pure
silica weight of 1.02–0.6 mLgꢀ1 SiO2 results from the space
occupied by the organic tether lining, and would correspond
to a loading density of 1.67 mmolgꢀ1 SiO2 assuming the or-
ganic components are in a pseudo-liquid-phase state, which
is in relatively good agreement with the value determined
by TGA and 29Si MAS NMR data. On the contrary, the ar-
chitecture of these materials is not affected as indicated by
Figure 7. Nitrogen sorption isotherms of the materials SBA-15 and SBA-
15-G.
the similar shape of the adsorption–desorption isotherms.
This result is confirmed by the results of X-ray diffraction
experiments with these two materials showing nearly identi-
cal cell parameters of the materials before and after grafting
(Table 2).
Catalytic tests: To get a better understanding on the accessi-
bility of thiol groups of the new reported materials SBA-15-
(1/8-KCl) and SBA-15-G, we assessed their palladium scav-
enging ability. The materials were impregnated with palladi-
um acetate in THF solution according to Scheme 4. Pd-
uptake of the solid could easily be followed by immediate
decolouration of the solution and the appearance of an
orange colour on the mesoporous silica. Inductively coupled
plasma (ICP)-MS analysis indicates that 99.92 and 99.97%
of palladium was removed from the solution to SBA-15-(1/
8-KCl) and SBA-15-G, respectively. Initially, the quantity of
palladium to be loaded was calculated based on the sulphur
content of the material to give a S/Pd ratio of 2:1 since this
ratio was shown to give the most active catalyst with the
lowest leaching in catalytic studies.[14]
Having ascertained the capacity of thiolated groups to
scavenge palladium, we directed our attention to study their
catalytic behaviour in two important carbon–carbon bond-
forming reactions: the Sonogashira[39,40] and Suzuki–
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SBA-15-Type Organosilica
FULL PAPER
shira reaction, despite the fact
that this reaction is used exten-
sively in the preparation of
molecules for pharmaceutical
and electronic applications. In
this regard, we investigated the
scope of our materials for the
Sonogashira cross-coupling re-
action (Table 4). Common pro-
tocols for the Sonogashira reac-
Scheme 4. Pd impregnation of the materials SBA-15-(1/8-KCl) and SBA-15-G.
Table 3. Catalytic results in Suzuki–Miyaura-coupling reactions involving the haloarenes 2–4 and the Pd-con-
Miyaura[41] cross-couplings. We
began our study with the
taining materials SBA-15-(1/8-KCl)-Pd, SBA-15-G-Pd and SBA-15-SH-Pd.[a]
Suzuki–Miyaura
reaction,
which consists of coupling an
aryl halide with an organobor-
on compound. To obtain
a
Entry
Substrate
Catalyst
t
TOF
[hꢀ1
Yield
[%][b]
Pd leaching
Pd leaching
[%]
[h]
N
]
[mg][c]
more accurate assessment of
catalytic activity, electronically
deactivated aryl bromides were
chosen rather than the more
easily coupled aryl iodides. We
selected three coupling part-
ners: 1-bromo-4-acetophenone
2, 1-bromo-4-nitrobenzene
and 3-bromo-quinoline
(Scheme 5).
1
2
3
4
5
6
7
8
9
2
2
2
3
3
3
4
4
4
SBA-15-(1/8-KCl)-Pd
SBA-15-G-Pd
SBA-15-SH-Pd
SBA-15-(1/8-KCl)-Pd
SBA-15-G-Pd
SBA-15-SH-Pd
SBA-15-(1/8-KCl)-Pd
SBA-15-G-Pd
SBA-15-SH-Pd
3.5
3
7
3
3
3
3
2.25
4.5
60
96
88
84
116
92
135
154
92
96
99
96
94
98
98
88
98
93
3.7
5.0
0.66
2.4
4.2
1.5
2.9
4.0
1.9
1.4
1.9
0.25
0.9
1.6
0.58
1.1
1.5
0.7
3
4
[a] Arylbromide (1 equiv), phenylboronate (1.5 equiv). K2CO3 (2 equiv), Pd (1 mol%). [b] Calculated by GC
using dimethoxybenzene as the internal standard. [c] Total Pd leached during the reaction.
tion involve the use of a copper source to assist the transme-
tallation of the alkyne.[39,42] We chose to employ copper-free
conditions to avoid the possible contamination by a second
metal. Much to our delight, the desired products were ob-
tained selectively without side reactions and in high yield
(up to 92%, Table 4, entry 4). Similar to the Suzuki–
Scheme 5. Aryl bromides 2–4 used as substrates in Suzuki–Miyaura and
Sonogashira coupling reactions.
Miyaura cross-coupling, 1-bromo-4-nitrobenzene
3 was
found to be the most reactive substrate in this series result-
ing in complete consumption of the starting material in
40 min.
The results obtained with our catalysts SBA-15-(1/8-KCl)-
Pd and SBA-15-G-Pd are systematically compared with the
commonly used mercaptopropyl-mesoporous silica denoted
as SBA-15-SH-Pd, obtained from mercaptopropyl-triethoxy-
silane (MPTES) by direct hydrolysis–co-condensation with
tetraethoxysilane (TEOS) and subsequent Pd impregnation
(Table 3). Under our conditions, no homo-coupling product
was detected. Among the three coupling candidates, 1-
bromo-4-nitrobenzene was the most reactive substrate be-
cause of its capacity to undergo a fast oxidative addition.
Even though the three materials lead to quantitative cross-
coupling, SBA-15-G-Pd and SBA-15-(1/8-KCl)-Pd are slight-
ly more reactive compared with SBA-15-SH-Pd based on
turn-over frequency (TOF), although a full kinetic assess-
ment would need to be done to determine the exact magni-
tude of the differences.
An interesting feature of these thiolated materials resides
in their high capacity to recapture leached palladium at the
end of the reaction.[43] After recovering the materials by
simple filtration, we were interested to assess the palladium
contamination in the obtained biphenyl products. Palladium
leaching was found to be low, despite the presence of func-
tionality in the coupling partners. As expected, the Pd leach-
ing was higher for the Sonogashira coupling compared with
the products of the Suzuki–Miyaura cross-coupling. This can
be attributed to the acetylene fragment, which is known to
bind palladium and retain it in the product.[24] However, in
all cases, the level of leaching is typically in the order of 1%
of the added catalyst in the Suzuki–Miyaura reaction and
about 2–3% of the added catalyst in the case of the more
challenging Sonogashira reaction This corresponds to a total
leaching of 5–10 mg of Pd.
Another cross-coupling reaction that has been less ad-
dressed in the context of thiolated materials is the Sonoga-
Chem. Eur. J. 2011, 17, 8984 – 8994
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8991

P. Hesemann et al.
terial with only SiO4 (Q4 sites)-
walls and low density of silanol
groups at the surface. Its ro-
bustness and hydrophobicity
allow this material to more effi-
ciently resist the harsh alkaline
reaction conditions.
Moreover, the enhanced sta-
bility of material SBA-15-G
compared with material SBA-
15-(1/8-KCl) in catalyst recy-
cling experiments may also
have its origin in the surface
roughness of the materials.
Indeed, corrugation of the pore
surface of SBA-15-type materi-
als has been evidenced by sev-
eral research groups.[44] Materi-
als synthesised at low tempera-
ture 408C<T<1008C) display
Table 4. Catalytic results in Sonogashira-coupling reactions involving the haloarenes 2–4 and the Pd-contain-
ing materials SBA-15-(1/8-KCl)-Pd, SBA-15-G-Pd and SBA-15-SH-Pd.[a]
Entry
Substrate
Catalyst
t
TOF
[h-1]
Yield
[%][b]
Pd leaching
Pd leaching
[%]
[h]
N
[mg][c]
1
2
3
4
5
6
7
8
9
2
2
2
3
3
3
4
4
4
SBA-15-(1/8-KCl)-Pd
SBA-15-G-Pd
SBA-15-SH-Pd
SBA-15-(1/8-KCl)-Pd
SBA-15-G-Pd
SBA-15-SH-Pd
SBA-15-(1/8-KCl)-Pd
SBA-15-G-Pd
SBA-15-SH-Pd
8
26
35
16
92
348
98
92
176
106
93
96
92
92
96
96
96
88
88
5.6
5.7
7.4
7.2
6.5
9.0
7.7
10.0
8.3
2.1
2.17
2.8
7.5
8.5
0.66
0.66
2
1
1.5
3.5
2.7
2.45
3.41
2.9
3.8
3.14
[a]Arylbromide (1 equiv), phenylacetylene (1.5 equiv), NaOAc (2 equiv), Pd (1 mol%). [b]Calculated by GC
using dimethoxybenzene as an internal standard. [c]Total Pd leached during the reaction.
For large scale applications, the high cost of palladium
makes its recovery and reuse highly desirable. Hence, new
heterogeneous candidates have to be assessed in terms of
activity and selectivity, metal leaching resistance and catalyst
lifetime and reusability considerations. With this aim, the re-
cyclability of the three catalysts was investigated in the
Suzuki–Miyaura coupling of 1-bromo-4-nitrobenzene 3 with
PhBpin (pinacol ester of phenylboronic acid, Figure 8). Gen-
erally, the three catalysts show deactivation upon reuse,
which is expected since the silica suffers from low stability
under the strongly basic conditions of the Suzuki–Miyaura
and Sonogashira cross-coupling reactions. Here, SBA-15-
SH-Pd shows the strongest deactivation. In this field, it has
been reported that end-capping of exposed silanols prevents
the material from structural damage and extends its activi-
ty.[21] Recycling experiments clearly indicate that SBA-15-(1/
8-KCl)-Pd is less sensitive than SBA-15-SH-Pd because the
wall was occupied by bulky organic groups that increase the
hydrophobicity of the material. The most resistant and reus-
able material in this series was SBA-15-G-Pd, which afford-
ed a yield of 64% of the coupling product after four runs.
As mentioned above, SBA-15-G consists in a condensed ma-
a corrugated pore surface while transversal secondary
porous channels were observed with materials synthesised at
temperatures ꢂ1308C. The parent SBA-15-type silica used
for the synthesis of the SBA-15-G-Pd catalyst was synthes-
ised at elevated temperature and therefore features trans-
versal mesoporosity. In contrast, the lower temperature
(708C) applied in the synthesis of material SBA-15-(1/8-
KCl) may lead to materials displaying a higher microporous
contribution as a result of higher corrugated pore surface.
Conclusion
We have reported the synthesis of novel mesoporous silica-
based materials featuring SBA-15 architectures and surfaces
functionalised with amide–thiol groups. The spectroscopic
properties of the materials clearly reflect the synthetic strat-
egies employed for the synthesis of the materials either by
post-synthetic grafting or co-condensation reactions. We par-
ticularly focussed on the formation of nanostructured mate-
rials through “one-pot” hydrolysis–polycondensation reac-
tions. Materials with high structural regularity were obtained
from hydrolysis–polycondensation reactions of reaction mix-
tures containing both at least six equivalents of TEOS and
potassium chloride, indicating that the formation of struc-
tured materials is governed by siloxy–surfactant interactions.
The catalytic properties of the materials prepared in this
study also reflect the synthetic pathways employed and the
composition of the materials on a molecular scale. Com-
pared with conventional thiol-functionalised materials ob-
tained from mercaptopropyl-trialkoxysilane precursors, the
new thiol-functionalised silica materials reported herein
show significantly improved recyclability. This result can be
attributed to the higher hydrophobicity of the new materials,
induced by the use of the bulkier bis-silylated amide–thiol
precursor. In this series, the grafted material SBA-15-G-Pd
shows the highest performance in terms of catalytic activity
Figure 8. Recyclability of the materials SBA-15-(1/8-KCl)-Pd, SBA-15-G-
Pd and SBA-15-SH-Pd in Suzuki–Miyaura coupling reaction of 1-bromo-
4-nitrobenzene with PhBpin.
8992
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Chem. Eur. J. 2011, 17, 8984 – 8994

SBA-15-Type Organosilica
FULL PAPER
(m, 18H), 1.26 (t, 1H), 1.57 (m, 4H), 1.88 (q, 2H), 2.38 (t, 2H), 2.53 (q,
2H), 3.19 (m, 4H), 3.75 ppm (m, 12H); 13C NMR (CDCl3) d=7.4, 7.5,
18.1, 20.9, 22.4, 24.1, 29.3, 30.9, 48.2, 50.1, 58.1, 58.3, 171.3 ppm; FTIR-
and recyclability. The high stability of this material can be
attributed to the high degree of condensation of the silica
centres in the mesoporous framework. These results clearly
highlight the importance of this feature in the stability and
recyclability of silica-based catalysts. Thus parameters such
as silanol concentration, degree of condensation, hydrophi-
licity/-phobicity of the immobilised organic substructure and
surface roughness, which are rarely taken into consideration,
can have a significant impact on the catalytic properties of
heterogeneous silica-based catalysts. For this reason, our re-
sults will permit the engineering of more efficient catalysts.
For example, recent studies in the Crudden laboratory dem-
onstrated that incorporation of aluminium species in the
silica walls of mesoporous materials resulted in a more
robust and reusable material.[21] This strategy, in combina-
tion with the use of more highly condensed catalysts such as
those reported herein, can be applied to the preparation of
new catalysts with even higher catalyst lifetimes and recycla-
bility.
AHCTUNGTRENNUNG
(KBr) n˜max =2973, 2930, 2886, 1646, 1107, 610 cmꢀ1; HRMS (ESI+): m/z:
calcd for C10H25NO7SSi2 of the completely hydrolyzed compound:
360.0969 [M+H]+; found 360.0961.
Material syntheses: The materials were synthesised following standard
procedures for the preparation of nanostructured mesoporous silica ma-
terials of the SBA-15-type.[33] Hydrolysis–polycondensation procedures
were performed at 708C in the presence of Pluronic P123 (Aldrich) as
structure-directing agent using the quantities indicated in Table 1.
Catalysis: In Suzuki–Miyaura coupling reactions, the Pd catalyst (1
mol% Pd; S/Pd=2:1), the bromoarene (0.25 mmol), K2CO3 (70 mg,
0.50 mmol), PhBpin (pinacol ester of phenyl boronic acid, 75 mg,
0.375 mmol) and 1,4-dimethoxybenzene were weighed into a sealed tube
with a stirrer bar and flushed with Ar. Dry DMF (2.5 mL, degassed by
bubbling Ar) and distilled water (0.125 mL) were added using a syringe.
The solution was heated at 808C with stirring while monitoring the reac-
tion progress by GC-FID.
In the Sonogashira reaction, the same protocol was adopted. Phenylace-
tylene (1.5 equiv with respect to the starting aryl halide) was used as
alkyne. Na2CO3 was used instead of K2CO3 in two molar ratios. The reac-
tion temperature was 1008C.
Experimental Section
General details: Bis(triethoxysilylpropyl)amine was purchased at ABCR.
g-Thiobutyrolactone and the surfactant Pluronic P123 (EO20PO70EO20)
were purchased from Aldrich. 1H and 13C spectra in solution were record-
ed on Bruker AC 250 or Bruker Avance 400 spectrometers at room tem-
perature. Deuterated chloroform was used as solvent for liquid NMR ex-
periments and chemical shifts are reported as d values in parts per mil-
lion relative to tetramethylsilane. IR samples were prepared as KBr pel-
lets. FT-IR spectra were measured on a Perkin–Elmer 1000 FT-IR spec-
trometer. MS-ESI were measured on a Water Q-Tof mass spectrometer.
Solid state 13C and 29Si CP MAS NMR experiments were recorded on a
Varian VNMRS 400 MHz solid spectrometer using a two channel probe
with 7.5 mm ZrO2 rotors. The 29Si Solid NMR spectra were recorded
using both CP MAS and One Pulse (OP-) sequences with samples spin-
ning at 6 kHz. CP MAS was used to get high signal to noise ratio with
5 ms contact time and 5 s recycling delay. For OP experiments, p/6 pulse
and 60 s recycling delay were used to obtain quantitative information on
the silane–silanol condensation degree. The 13C CP MAS spectra were
obtained using 5 ms contact time, 5 s recycling delay and 5 kHz spinning
rate. The number of scans was in the range 1000–3000 for 29Si OP-MAS
spectra and of 2000–4000 of 13C-CP-MAS spectra. Nitrogen sorption iso-
therms at 77 K were obtained with a Micromeritics ASAP 2020 appara-
tus. Prior to measurement, the samples were degassed for 10 h at 1208C.
The surface areas (SBET) were determined from BET treatment in the
range 0.04–0.3 p/p0 and assuming a surface coverage of nitrogen molecule
estimated to 13.5 ꢄ2.[45] Pore-size distributions were calculated from the
desorption branch of the isotherms using the BdB method.[46] The pore
width was estimated at the maximum of the pore-size distribution. TEM
images were obtained using JEOL 1200 EX II (120 kV). XRD experi-
ments were carried out with a Bruker D8 Advance AXS (powder diffrac-
tion geometry Bragg–Brentano) diffractometer using CuKa radiation.
TGA experiments were performed with a Netzsch TG 209C apparatus.
The samples were heated under an air stream from 50 to 8008C with a
Acknowledgements
S.E.H. and P.H. thank the program AVERROES (Erasmus Mundus Pro-
gramme of the European Community) for a doctoral scholarship (S.E.H)
and post-doctoral fellowship (P.H). Financial support of the “Reseau de
Recherche 3, CPDD” of the CNRS is gratefully acknowledged. A.E.K.
and C.M.C. thank the Natural Science and Engineering Council of
Canada (NSERC) and Johnson and Johnson for support of this work.
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.
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a
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Received: July 30, 2010
Revised: March 3, 2011
Published online: July 5, 2011
8994
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ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 8984 – 8994