Stable silanetriols that contain tert-alkoxy groups: Versatile precursors of siloxane-based nanomaterials

Article abstract of DOI:10.1002/chem.200700914

Novel tert-alkoxysilanetriols (ROSi(OH)₃, R = adamantyl and 3-ethyl-3-pentyl) have been prepared from the corresponding tert- alkoxytrichlorosilanes and successfully used as molecular building blocks to produce ordered siloxane-based nanomateri

Full text of DOI:10.1002/chem.200700914

FULL PAPER
DOI: 10.1002/chem.200700914
Stable Silanetriols That Contain tert-Alkoxy Groups: Versatile Precursors of
Siloxane-Based Nanomaterials
[
a]
[c]
[a]
Jumpei Suzuki, Atsushi Shimojima,* Yasuhiro Fujimoto, and
[
a, b, c]
Kazuyuki Kuroda*
Abstract: Novel tert-alkoxysilanetriols
ROSi(OH)3, R=adamantyl and 3-
upon heating, which led to the forma-
tion of ordered silica–organic nano-
composites with laminated morpholo-
gies. On the other hand, silylation of
the tert-alkoxysilanetriols with chloro-
trimethoxysilane enabled us to synthe-
size well-defined oligomeric alkoxysi-
lanes (ROSi
A
H
R
U
G
A
H
R
U
G
(
and polycondensation of these oligo-
mers followed by acid treatment gave
microporous silica with narrow pore
size distributions. Thus, tert-alkoxy
groups serve not only as protecting
groups of siloxane species to regulate
hydrolysis and polycondensation, but
also as templates to generate micro-
pores thereby providing unique syn-
thetic pathways for the design of or-
dered silica-based materials.
ethyl-3-pentyl) have been prepared
from the corresponding tert-alkoxytri-
chlorosilanes and successfully used as
molecular building blocks to produce
ordered siloxane-based nanomaterials.
Controlled hydrolysis of the alkoxytri-
chlorosilanes led to the formation of
crystalline powders of alkoxysilane-
triols that were stable under ambient
conditions. Solid-state polycondensa-
tion of the alkoxysilanetriols occurred
Keywords: building
block
approach · microporous materials ·
nanostructures · silanes · sol–gel
processes
Introduction
condensation of SiOH groups is probably the most studied
process that allows the production of siloxane-based materi-
[1,2]
Molecular routes to inorganic solids with designed structures
have received increasing attention in modern materials
chemistry. The polymerization of silicic acids or silicates by
als under mild conditions.
Tetrafunctional silanes, such as
Si(OR) and SiCl , are widely used as precursors. However,
4
4
concurrent hydrolysis and polycondensation generally lead
to the formation of amorphous silica with randomly cross-
linked networks. Control of the reaction kinetics is therefore
crucial to tailor the structures of the final products. Al-
though the use of organic structure-directing agents or tem-
plates has made it possible to control silica structures on
[
a] J. Suzuki, Dr. Y. Fujimoto, Prof. K. Kuroda
Department of Applied Chemistry, Waseda University
Ohkubo-3, Shinjuku-ku, Tokyo, 169-8555 (Japan)
Fax : (+81)3-5286-3199
[3–5]
various length scales,
the molecular design of the alkoxy-
E-mail: kuroda@waseda.jp
or chlorosilanes that serve as single precursors of ordered
silica-based materials is still a great challenge.
[
b] Prof. K. Kuroda
Kagami Memorial Laboratory for Materials Science and Technology
Waseda University, Nishiwaseda-2, Shinjuku-ku, Tokyo, 169-0051
It is known that several kinds of silanetriols can be ob-
tained by controlled hydrolysis of the corresponding tri-
chloro- or trialkoxysilanes in which organic groups are
bonded to silicon atoms through SiÀC, SiÀN, or SiÀO
(
Japan)
+
[
c] Dr. A. Shimojima, Prof. K. Kuroda
Core Research for Evolutional Science and Technology (CREST)
Japan Science and Technology Agency (JST)
Honcho 4-1-8, Kawaguchi-shi
[6–12]
bonds.
Polycondensation is retarded by the steric effects
Saitama, 332-0012 (Japan)
of the organic groups and also by hydrogen-bonding be-
tween silanol groups. Such molecules have been used as mo-
lecular building blocks in the construction of well-defined
+
[
] Present address: Department of Chemical System Engineering
The University of Tokyo, Hongo-7
Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax : (+81)3-5800-3806
[7,8]
oligomeric siloxane and cagelike metallosiloxane species.
Furthermore, solid-state polycondensation of crystalline sila-
netriols offers an interesting approach to ordered siloxane-
E-mail: shimoji@chemsys.t.u-tokyo.ac.jp
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
[11,12]
based materials.
We have previously reported the for-
Chem. Eur. J. 2008, 14, 973 – 980
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973

mation of layered silica–organic hybrids by solid-state poly-
condensation of lamellar assemblies of alkylsilanetriols de-
Results and Discussion
[10]
rived from long-chain alkyltriethoxysilanes. More recently
we synthesized long-chain n-alkoxysilanetriols that under-
went spontaneous polycondensation and cleavage of SiÀOÀ
Formation of alkoxysilanetriols 1e and 1a from alkoxytri-
chlorosilanes: The solutions obtained after hydrolysis of tert-
alkoxytrichlorosilanes were analyzed by liquid-state NMR
[13]
29
C linkages to form layered silica–alcohol nanocomposites.
spectroscopy. The Si NMR spectra of the hydrolyzed solu-
Alkoxysilanetriols are particularly important as molecular
building blocks because alkoxy groups and silanols can be
used in chemical reactions. One of the aims of research in
this field is to produce porous solids with well-defined pore
sizes by using alkoxy groups as templates that can be hydro-
lyzed under mild conditions. To produce spherical or cylin-
drical assemblies with three-dimensional silica frameworks,
additional siloxane units should be attached to the SiÀOH
tions of 3-ethyl-3-pentoxy- and 1-adamantoxytrichlorosi-
lanes (Figure 1) exhibit single signals at d=À77.8 and
groups of alkoxysilanetriols to enlarge the size of the head
group. Recently we demonstrated that oligomeric alkoxysi-
lanes in which three trialkoxysilyl groups are attached to an
[14]
alkylsilane unit formed cylindrical assemblies. The design
of similar oligomeric species with alkoxy groups instead of
alkyl groups will open a new route to porous silica. Howev-
er, the chemical modification of long-chain alkoxysilane-
triols is difficult because of the instability of alkoxysilane-
triols under ambient conditions.
2
9
Figure 1. Liquid-state Si NMR spectra of hydrolyzed solutions of 3-
ethyl-3-pentoxytrichlorosilane (bottom) and 1-adamantoxytrichlorosilane
(
top).
Herein, we report the synthesis of stable tert-alkoxysilane-
triols as effective molecular building blocks for producing
silica-based nanomaterials with well-regulated structures,
morphologies, and porosities. We chose highly bulky alkoxy
groups, 3-ethyl-3-pentoxy and 1-adamantoxy groups
À76.9 ppm, respectively, which are very different from those
of the tert-alkoxytrichlorosilanes (d=À52.0 and À49.3 ppm).
These signals can be tentatively assigned to ROSi(OH)3
based on the fact that the signal of the silicic acid monomer
(
Scheme 1), as protecting groups of siloxane species to pro-
(
Si(OH) ) appears at around
4
d=À72 ppm and is shifted up-
field when one hydroxy group
is substituted by an alkoxy
[1]
group. The differences in the
chemical shifts of the tert-alkox-
ysilanetriols compared with
those of n-alkoxysilanetriols
[
13]
(
for example, d=À73.4 ppm)
can be attributed to a higher
electron-donating ability of the
tert-alkoxy groups. The broad
signal centered at d=À110 ppm
is due to the glass tube used for
the measurement.
Scheme 1. Design of various silica nanomaterials by using tert-alkoxysilanetriols as building blocks.
The retention of the tert-
alkoxy groups was clearly con-
1
3
duce alkoxysilanetriols (1e and 1a, respectively). Solid-state
polycondensation of the molecular crystals of these alkoxysi-
lanetriols, induced by heating, led to ordered alkoxylated
silica materials (2e and 2a). Furthermore, the high stability
of the alkoxysilanetriols enabled us to design novel oligo-
meric alkoxysilanes (3e and 3a) by silylation. These mole-
cules formed xerogels with ordered structures (4e and 4a);
removal of the alkoxy groups of 4e and 4a gave micropo-
rous silica with controlled pore sizes.
firmed by C NMR spectroscopy (Figure 2). The spectra ex-
hibit signals arising from the alkoxy groups along with
strong signals from the solvent (THF). For the hydrolyzed
solutions of 3-ethyl-3-pentoxy- and 1-adamantoxytrichlorosi-
lanes, the chemical shifts of the a-carbon atoms (d=76.7
and 70.7 ppm, respectively) are different to those of either
the tert-alkoxytrichlorosilanes (d=90.0 and 80.8 ppm) or the
tert-alkyl alcohols (d=74.7 and 68.2 ppm). Taking into ac-
count the fact that the a-carbon signals of n-alkoxy groups
appear 1.0–1.5 ppm downfield of those for the correspond-
[13,15,16]
ing alcohols,
it is reasonable to assign these signals to
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Chem. Eur. J. 2008, 14, 973 – 980

Siloxane-Based Nanomaterials
FULL PAPER
low temperature (À208C) and both polycondensation of the
silanol groups and cleavage of SiÀOÀC bonds rapidly oc-
curred at room temperature. In contrast, 1e and 1a were
readily crystallized at room temperature by adding a poor
solvent (hexane) and were stable for a long period of time
(
at least for 1month). This suggests that hydrophobic inter-
actions between the alkoxy groups are not the main factor
stabilizing alkoxysilanetriols. Furthermore, our preliminary
experiments revealed that tert-butoxy- or 2-methyl-2-
heptoxytrichlorosilanes do not afford stable silanetriols. It is
A
H
R
N
therefore concluded that bulky alkoxy groups with more
than two carbon atoms in each of the side-chains are essen-
tial for the formation of stable alkoxysilanetriols. The rigidi-
ty of the alkoxy groups appears to have little effect because
both 3-ethyl-3-pentoxy- and adamantoxysilanetriols were
obtained under similar conditions.
1
3
Figure 2. Liquid-state C NMR spectra of hydrolyzed solutions of 3-
ethyl-3-pentoxytrichlorosilane (bottom) and 1-adamantoxytrichlorosilane
(
top).
These results clearly show that the stability of alkoxysila-
netriols is dominated by the steric effects of the alkoxy
groups. Similar behavior has also been reported for organo-
silanetriols with stable SiÀC bonds. It is known that molecu-
the a-carbon atoms of tert-alkoxy groups. From these NMR
data we conclude that tert- alkoxysilanetriols 1e and 1a
were selectively formed by partial hydrolysis of the corre-
sponding alkoxytrichlorosilanes.
lar crystals of organosilanetriols are stabilized by steric re-
pulsion between bulky organic groups and also by hydro-
Importantly, alkoxysilanetriols 1e and 1a were isolated as
crystalline precipitates by adding hexane to concentrated
[6]
gen-bonding between adjacent silanol groups. For example,
tert-butyl- and cyclohexylsilanetriols are stable in the solid
state, whereas n-alkylsilanetriols undergo polycondensa-
2
9
solutions of these triols. The solid-state Si MAS NMR
0
spectra of 1e and 1a show sharp Q signals with no signals
2
3
4
[10]
being observed in the Q , Q , and Q regions (Figure 3a).
tion. It is plausible that the bulky organic groups attached
to the silicon atoms hinder the attack of neighboring silanol
groups to form SiÀOÀSi bonds. In addition, the presence of
bulky alkoxy groups leads to an increase in the lateral dis-
tances between silanetriol molecules, which should be unfav-
orable for condensation.
Thermal polycondensation of alkoxysilanetriols 1e and 1a:
Although alkoxysilanetriols 1e and 1a are stable at room
temperature, they underwent solid-state polycondensation at
higher temperatures. Thermal treatment of 1e and 1a was
performed at 100 and 2008C, respectively. These tempera-
tures are still lower than the decomposition temperatures of
the tert-alkoxy groups (ꢀ250 and ꢀ3808C, respectively).
The samples became insoluble in THF after thermal treat-
ment for one day (designated as 2e and 2a), which suggests
that polycondensation of the silanol groups had occurred. In
2
9
Figure 3. Solid-state Si MAS NMR spectra of A) 3-ethyl-3-pentoxysila-
netriol (1e) and B) 1-adamantoxysilanetriol (1a) for the compounds as-
synthesized (a) and after thermal treatment (b).
These precipitates were soluble in THF and the liquid-state
Si and C NMR spectra showed signals assigned to 3-
29
13
29
the Si MAS NMR spectra of 2e and 2a (Figure 3b), three
ethyl-3-pentoxy- and 1-adamantoxysilanetriols (see the Sup-
porting Information). These results provide evidence that
the polycondensation of 1e and 1a is strictly retarded both
in the solid-state and in THF. To the best of our knowledge,
these are the first stable alkoxysilanetriols to be synthesized
even though several stable organosilanetriols with organic
groups linked to the silicon atom through stable SiÀC bonds
new signals appeared at around d=À90, À100, and
0
À110 ppm, and the Q signal disappeared. These signals gen-
2
3
4
x
erally correspond to the Q , Q , and Q sites (Q =(Si-
[1]
1
A
H
R
U
G
2
3
Q , and Q sites with tert-alkoxy groups may overlap them
29
because tert-alkoxy groups induce upfield shifts of the Si
signals. The formation of siloxane networks was also con-
firmed by IR spectroscopy (data not shown). The bands due
[6–11]
have been previously reported.
À1
À1
to SiÀOH (925 cm ) and OH (3230 cm ) almost disap-
peared, and bands due to SiÀOÀSi networks (1060–
Factors stabilizing alkoxysilanetriols: We have previously re-
ported the formation of n-alkoxysilanetriols with long alkyl
À1
À1
1220 cm and ꢀ460 cm ) appeared.
[13]
13
chains (C –C ). Hydrophobic interactions between alkyl
Further information was obtained from the solid-state C
12
20
groups appeared to play a crucial role. However, crystalline
precipitates of n-alkoxysilanetriols were only obtained at a
cross-polarization (CP)/MAS NMR spectra of tert-alkoxysi-
lanetriols 1e and 1a, and of 2e and 2a, as shown in
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975

K. Kuroda, A. Shimojima et al.
Figure 4. All of the signals have been assigned to the 3-
ethyl-3-pentyl and 1-adamantyl groups with the signals be-
coming broader upon thermal treatment. It appears that the
Figure 5. Powder XRD patterns of alkoxysilanetriols a) 1e and c) 1a, and
those obtained after thermal treatment b) 2e and d) 2a.
1
3
Figure 4. Solid-state C CP/MAS NMR spectra of A) 3-ethyl-3-pentoxy-
silanetriol (1e) and B) 1-adamantoxysilanetriol (1a) for the compounds
as-synthesized (a) and after thermal treatment (b).
gions roughly correspond to twice the extended lengths of
tert-alkoxysilanetriol molecules which suggests that they
have bilayer structures.
The layered structures of the alkoxysilanetriols were also
suggested by their specific particle morphologies. The SEM
image of 1e shows irregular, angular particles with laminat-
ed surfaces, whereas 1a is shown to be composed of ran-
domly shaped flakelike particles several micrometers in size
(Figure 6a and c). No major change in the morphologies of
alkoxy groups remain intact in the products owing to the
lack of a-carbon signals in the NMR spectra that arise from
the corresponding alcohols. However, a combination of ther-
mogravimetric (TG) and CHN analyses revealed that the
molar ratios of alkoxy groups per SiO decreased to around
2
0.40 and 0.34 for 2e and 2a, respectively. This suggests that
some SiÀOÀC linkages were cleaved and the resulting alco-
hols eliminated from the samples at high temperatures. The
alkoxy groups can be cleaved either by the condensation of
SiÀOR and SiÀOH groups (i.e., alcohol-producing conden-
sation) or by hydrolysis with water formed by the condensa-
[13]
tion of SiÀOH groups.
Structural considerations: Figure 5 shows the powder XRD
patterns of 1e and 1a and those obtained after thermal
treatment (2e and 2a). The powder XRD patterns of alkox-
ysilanetriols 1e and 1a exhibit several sharp diffraction
peaks that are indicative of their crystalline nature. The
XRD pattern of 1e shows the strongest peak with a d spac-
ing of 1.29 nm accompanying higher order reflections. On
the other hand, compound 1a exhibits a rather complicated
pattern consisting of three peaks close together with d spac-
ing values of 1.68, 1.53, and 1.41 nm and many other peaks
in the higher 2q regions. After thermal treatment, broad
peaks with d spacing values of 1.56 and 1.65 nm were ob-
served for 2e and 2a, respectively, which implies that the
crystalline order of alkoxysilanetriols was partly retained
even after polycondensation.
Figure 6. FE-SEM images of alkoxysilanetriols a) 1e and c) 1a, and those
obtained after thermal treatment b) 2e and d) 2a.
The crystal structures of several organosilanetriols have
previously been elucidated by X-ray analyses, and those
with tert-butyl and cyclohexyl groups were found to adopt a
double-sheet structure in which the organic groups point to-
1e and 1a was observed even after thermal treatment (Fig-
ure 6b and d). Layered hybrids with similar flakelike mor-
phologies have been prepared by the self-assembly of sever-
[6,7]
[10,17,18]
wards each other.
Although the detailed crystal structures
al organoalkoxysilanes that have stable SiÀC bonds,
of 1e and 1a have not yet been clarified, the layered struc-
ture of these samples was suggested by the slight increase in
d spacing when treated with decyl alcohol and also by the
collapse of their structures after calcination (at 5008C for
but 2e and 2a are unique because the organic groups are at-
tached through hydrolyzable SiÀOÀC bonds.
The slight increases in the d spacings upon thermal poly-
condensation are presumably as a result of increases in the
thickness of the siloxane layers, as expected from the forma-
8
h). The d spacings of the XRD peaks in the lowest 2q re-
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Siloxane-Based Nanomaterials
FULL PAPER
2
3
4
tion of the Q , Q , and Q units (Figure 3b). Because of the
ing from methanol (d=49.8 ppm) had appeared. It was also
shown that the tert-alkoxy groups were partially hydrolyzed
to 3-ethyl-3-pentanol and 1-adamantanol. However, the hy-
drolysis rate appeared to be much lower and therefore most
of the alkoxy groups (at least 70%, as estimated from the
signal intensity ratio) were preserved. This may be owing to
steric hindrance of the bulky alkoxy groups, which prevents
nucleophilic attack of water molecules on silicon atoms.
large volume of bulky alkoxy groups relative to the SiÀOÀSi
moiety, partial cleavage and removal of alkoxy groups upon
thermal treatment should play an important role in satisfy-
ing the spatial restriction demanded for the formation of
cross-linked siloxane networks.
Molecular design of 3e and 3a: A significant feature of tert-
alkoxysilanetriols is that they can be used as building blocks
for further modification. Figure 7 shows the liquid-state
2
9
Figure 8 shows the Si MAS NMR spectra of 4e and 4a
prepared by casting and drying hydrolyzed solutions of 3e
and 3a, respectively. Both spectra exhibit three signals at
2
9
Figure 7. Liquid-state Si NMR spectra (in CDCl
3
) of 3e (bottom) and
2
9
Figure 8. Solid-state Si MAS NMR spectra of 4e (bottom) and 4a (top)
prepared by casting and drying of hydrolyzed solutions of 3e and 3a, re-
spectively.
3
a (top) prepared by trimethoxysilylation of 1e and 1a, respectively.
2
9
Si NMR spectra of 3e and 3a prepared by trimethoxysily-
1
lation of 1e and 1a, respectively. Both spectra show Q and
Q signals with intensity ratios of 3:1which suggests that
d=À91.1, À99.8, and À108.6 ppm with intensity ratios of
1:5:4 and 1:5:3, respectively. As mentioned above, the sili-
con atoms bonded to tert-alkoxy groups appear upfield from
those bonded to n-alkoxy or hydroxy groups. However, con-
sidering that 75% of the silicon units arise from the
3
three trimethoxysilyl groups are bonded to tert-alkoxysilyl
3
cores. The appearance of the Q signals (at d=À109.6 and
À108.1 ppm for 3e and 3a) upfield from that generally ob-
served at around d=À100 ppm has been attributed to the
presence of tert-alkoxy groups bonded to silicon atoms. The
branched ÀSi
A
T
E
N
2
3
4
to assign these signals to the Q , Q , and Q units rather
1
3
1
2
3
C NMR spectra of 3e and 3a exhibit signals arising from
the a-carbon atoms of tert-alkoxy groups (d=81.0 and
2.9 ppm, respectively) in addition to the methoxy signal at
than to the Q , Q , and Q units that have tert-alkoxy
groups.
7
Partial cleavage of the alkoxy groups was suggested by
1
3
d=51ppm (see the Supporting Information). These data
demonstrate the successful synthesis of novel oligomeric al-
koxysilanes with well-defined structures and two kinds of
alkoxy groups.
the C CP/MAS NMR spectra (see the Supporting Informa-
tion). The a-carbon atoms of the 3-ethyl-3-pentoxy and 1-
adamantoxy groups appear at d=82.5 and 74.4 ppm in the
spectra of 4e and 4a, respectively. Other signals observed at
d=77.3 and 69.0 ppm might be due to 3-ethyl-3-pentanol
and 1-adamantanol, respectively, formed by cleavage of SiÀ
OÀC linkages. The organic content of 4e and 4a was evalu-
Hydrolysis and polycondensation of 3e and 3a: After hy-
drolysis and polycondensation of 3e and 3a in THF under
1
2
acidic conditions, several signals appeared in the Q , Q , and
ated by elemental analysis in combination with TG analysis.
The number of residual alkoxy groups (and/or alcohol mole-
3
29
Q regions of the Si NMR spectra (see the Supporting In-
formation). These signals may be assigned to cyclic species
formed by hydrolysis of the methoxy groups and subsequent
intramolecular polycondensation of the silanol groups, as we
recently reported for similar oligomeric alkoxysilanes with
n-alkyl groups bonded to the silicon atom through stable
cules) per 4 SiO was 0.62 and 0.89 for 4e and 4a, respec-
2
tively, which confirms that alkoxy groups were partly lost
during synthesis, possibly through condensation in the
drying process.
[
14b]
SiÀC bonds.
In fact, complete hydrolysis of the methoxy
Removal of alkoxy groups from 4e and 4a: The large silox-
ane units of 3e and 3a resulted in the formation of ordered
three-dimensional siloxane networks that are preserved
even after the removal of alkoxy groups. It is of particular
1
3
groups was confirmed by the C NMR spectra (data not
shown) in which the signals of the methoxy groups (d=
51.1 ppm) had almost disappeared and a strong signal aris-
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977

K. Kuroda, A. Shimojima et al.
importance that alkoxy groups can be removed by hydroly-
sis at room temperature as well as by calcination. Acid-cata-
lyzed hydrolysis appears to be suitable for the removal of
organic groups without the rearrangement of siloxane net-
works. The removal of organic groups from silica–organic
hybrids that contain CÀC triple bonds adjacent to silicon
atoms has previously been achieved by chemical treatment
in the presence of fluoride ions, however, the rearrangement
[19,20]
of siloxane networks also occurred.
Figure 9 shows the XRD patterns of 4e and 4a and those
recorded after acid treatment or calcination. The as-synthe-
sized XRD patterns of 4e and 4a exhibit broad peaks that
Figure 10. Nitrogen adsorption isotherms for A) 4e and B) 4a after acid
treatment. The insets show pore size distribution curves obtained by the
SF method.
create well-regulated pores. As
expected from the broad XRD
peaks, a disordered, wormhole-
like pore architecture was ob-
served by TEM (Figure 11).
On the other hand, when
alkoxy groups were removed by
calcination, 4e gave a solid with
a
very small surface area
Figure 11. TEM image of 4e
after acid treatment.
2
À1
Figure 9. Powder XRD patterns of A) 4e and B) 4a for the compounds
as-synthesized (a), after acid treatment (b), and after calcination (c).
(10 m g ). Thermal shrinkage
of siloxane networks might
leave no pores and/or only
closed pores. In contrast, micro-
correspond to d spacings of 2.0 and 2.3 nm, respectively,
which suggests the products have an ordered structure.
These peaks are still observed after acid treatment or calci-
nation. The slight decreases in the d spacings are as a result
of shrinkage of the siloxane networks by further polycon-
densation of the silanol groups. The complete removal of
the organic groups was confirmed by IR spectroscopy (see
the Supporting Information); the absorption bands arising
from the tert-alkoxy groups (typically a CÀH stretching vi-
porous silica was obtained from 4a. This result is probably
owing to the higher thermal stability of adamantyl groups.
The networks could be retained while shrinkage of the net-
works occurred, thus leaving voids even after calcination.
However, the BET surface area was smaller (370 m g )
than that of the material produced after acid treatment.
These results proved that the removal of alkoxy groups as a
molecular template under mild conditions is a promising
way to produce microporous silica materials.
2
À1
À1
bration at 2890–2980 cm ) had almost disappeared. The
À1
bands due to SiÀOH stretching (ꢀ925 cm ) are more
prominent after acid treatment than after calcination owing
Conclusion
to the formation of new SiÀOH groups as a result of hydrol-
ysis of alkoxy groups.
We have demonstrated the first successful synthesis of stable
alkoxysilanetriols that contain tert-alkoxy groups and their
use as novel molecular building blocks in the construction of
ordered silica-based materials. Solid-state polycondensation
of crystalline alkoxysilanetriols led to the formation of al-
koxylated silica with ordered nanostructures that could be
utilized as a new class of nanofillers for composite materials.
Well-defined oligomeric alkoxysilanes were also obtained by
trimethoxysilylation of alkoxysilanetriols. Hydrolysis and
polycondensation of these oligomers led to the formation of
nanostructured networks that gave microporous solids by
hydrolysis of the tert-alkoxy groups under acidic conditions.
This is a sort of molecular imprinting technique that allows
The generation of micropores upon removal of alkoxy
groups was confirmed by nitrogen adsorption measurements.
Figure 10 shows the adsorption–desorption isotherms for 4e
and 4a after acid treatment. Although as-synthesized 4e
and 4a are almost nonporous, after acid treatment they
show type I isotherms that are characteristic of microporous
silica. The Brunauer–Emmet–Teller (BET) surface areas in-
2
À1
creased to 350 and 670 m g for 4e and 4a, respectively.
The pore sizes estimated by the Saito–Foley (SF) method
were around 0.6 nm in both cases. This value corresponds
well to the molecular size of the alkoxy groups, which im-
plies that the alkoxy groups acted as molecular templates to
978
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 973 – 980

Siloxane-Based Nanomaterials
FULL PAPER
the design of microporous silica with well-regulated pores.
The simple sol–gel processing of the oligomeric precursors is
suitable for morphological control, which is very important
for various applications. The molecular design of further al-
koxysilanetriols with bulky alkoxy groups is underway, the
aim of which is to create a diverse class of materials with or-
dered structures at the molecular level.
3 h. The solutions were then cast onto glass substrates and air-dried to
give 4e and 4a as thick films. They were pulverized for characterization.
Removal of the alkoxy groups was carried out either by acid treatment
or by calcination at 5008C for 8 h in air. For the acid treatment, 4e (or
4
a) (0.1g) was dispersed in a mixture of THF ( 10 mL) and an aqueous
solution of 0.1n HCl (0.1mL). After stirring the dispersion at room tem-
perature for 1d, precipitates were recovered by centrifugation and dried
for 1d under vacuum.
2
9
13
Characterization: Liquid-state Si and C NMR spectra were obtained
by using a JEOL Lambda-500 spectrometer with resonance frequencies
of 99.25 and 125.65 MHz, respectively. The sample solutions were put
into 5 mm glass tubes and tetramethylsilane (TMS) was added as an in-
Experimental Section
ternal reference; CDCl
3
or [D
8
]THF was used to obtain lock signals. A
2
9
small amount of [Cr(acac)
A
H
R
U
G
3
] was also added as a relaxation agent for Si
2
9
Materials: Tetrachlorosilane (SiCl
-pentanol (C COH, Tokyo Kasei, 99%), and 1-adamantanol
15OH, Aldrich, 99%) were used for the synthesis of the alkoxytri-
4
, Tokyo Kasei Co. Ltd., 98%), 3-ethyl-
nuclei. Solid-state Si magic-angle spinning (MAS) NMR spectroscopy
was performed by using a JEOL JNM-CMX-400 spectrometer at a reso-
nance frequency of 79.42 MHz with a pulse width of 458 and a recycle
3
2 5 3
H )
10
(C H
1
3
chlorosilanes. Other chemicals, which included aniline, pyridine, dehy-
drated tetrahydrofuran (THF), and n-hexane (all from Kanto Chemical
Co.), were used as received.
delay of 100 s. Solid-state C CP/MAS NMR spectra were obtained by
using the same spectrometer at a resonance frequency of 100.54 MHz
with a contact time of 1.5 ms and a recycle delay of 5 s. The chemical
2
9
13
shifts in both the Si and C NMR spectra were referenced to tetrame-
thylsilane at 0 ppm. Powder X-ray diffraction (XRD) patterns were re-
corded by using a Mac Science M03XHF22 diffractometer with Mn-fil-
tered FeKa radiation. Field-emission scanning electron microscopy (FE-
SEM) was performed on samples coated with Pt/Pd by using a Hitachi S-
Synthesis of tert-alkoxytrichlorosilanes (ROSiCl
chlorosilanes were synthesized by the reaction of SiCl
cohols (3-ethyl-3-pentanol and 1-adamantanol). Typically, a solution of
one of the alcohols in THF was added slowly to a solution of SiCl in
hexane and the mixture was stirred at room temperature for 1.5 h. The
gaseous HCl generated by the reaction was allowed to leave the vessel.
3
): The tert-alkoxytri-
4
and tert-alkyl al-
4
4
500S microscope at an accelerating voltage of 15 kV. FTIR spectra of
the products in KBr pellets were obtained by using a Perkin–Elmer Spec-
An excess of SiCl
generation of Cl Si(OR)
tures revealed the formation of tert-alkoxytrichlorosilanes as the predom-
inant species. After removal of hexane and unreacted SiCl under re-
4 4
(SiCl /tert-alkyl alcohol, 5:1) was used to suppress the
À1
2
9
trum One spectrometer with a nominal resolution of 0.5 cm . TG analy-
2
2
.
Si NMR spectroscopic analyses of the mix-
sis was carried out with a RIGAKU TG8120 instrument under a flow of
À1
dry air at a heating rate of 10 Kmin . The SiO
2
content in the products
4
was determined by the residual weight after heating to 9008C and the
amounts of organic constituents were determined by CHN analysis
(Perkin–Elmer PE-2400). Nitrogen adsorption measurements were per-
formed by using an Autosorb-1instrument (Quantachrome Instruments,
Inc) at 77 K. Samples were preheated at 1208C for 3 h under a pressure
duced pressure, the residue was distilled under vacuum to yield alkoxytri-
chlorosilanes as clear and colorless liquids.
1
3
3
(
-Ethyl-3-pentoxytrichlorosilane: C NMR (125.7 MHz, CDCl
3
): d=8.06
g), 30.37 (b), 90.10 ppm (a); Si NMR (99.3 MHz, CDCl ): d=
À52.78 ppm.
-Adamantoxytrichlorosilane: C NMR (125.7 MHz, CDCl
2
9
3
À2
of 1”10 Torr.
1
3
1
3
): d=31.28
g), 35.84 (d), 44.91(b), 80.86 ppm (a); Si NMR (99.3 MHz, CDCl ): d=
À49.41ppm.
Synthesis of tert-alkoxysilanetriols (1e and 1a): tert-Alkoxytrichlorosilane
2
9
(
3
(
(
1
ROSiCl
40 mL), aniline, and water (molar ratio of ROSiCl
:3.3:3.3) in an ice bath. Aniline acts as an accepter of the HCl generated
3
, 1g) in THF (10 mL) was added dropwise to a mixture of THF
Acknowledgements
3
/aniline/H O=
2
The authors are grateful to Dr. Dai Mochizuki and Mr. Yoshiaki Hagi-
wara (Waseda University) for solid-state NMR measurements and also to
Mr. Yoshiyuki Kuroda and Mr. Ryutaro Wakabayashi (Waseda Universi-
ty) for TEM measurements. This work was supported in part by a Grant-
in-Aid for the 21st Century COE Program “Practical Nano-Chemistry”
and the Global COE program “Practical Chemical Wisdom” from
MEXT, Japan. The A3 Foresight Program “Synthesis and Structural Res-
olution of Novel Mesoporous Materials” supported by the Japan Society
for the Promotion of Science (JSPS) is also acknowledged.
by hydrolysis of the SiÀCl groups, and therefore, maintains the solution
at neutral pH. After stirring for 1.5 h, precipitates of aniline hydrochlo-
ride were removed by filtration. The resulting clear solution was concen-
trated to about 10 vol% under reduced pressure. The addition of hexane
(
50 mL) to this solution led to the formation of precipitates that were
separated by filtration, washed with hexane, and dried under vacuum to
give 1e and 1a as white powders.
Synthesis of trimethoxysilylated derivatives (3e and 3a) of the tert-alkox-
ysilanetriols: The silylating agent (chlorotrimethoxysilane (ClSi
was synthesized by adding methanol (40 mL) to SiCl (38 mL) under a
flow of nitrogen (3:1molar ratio of MeOH/SiCl ) and the mixture was
stirred for 20 min. Si NMR spectroscopy confirmed that the resulting
liquid was a mixture of chlorotrimethoxysilane (ClSi(OMe) ) and tetra-
methoxysilane in an approximate molar ratio of 7:3. This mixture was di-
rectly used for silylation because tetramethoxysilane is relatively inert to-
wards silylation to cause little or no side reactions. Trimethoxysilylation
of the alkoxysilanetriols was performed by adding a solution of 1e (or
A
H
R
U
G
3
(OMe) ))
4
4
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Received: June 16, 2007
Published online: November 8, 2007
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27, 14108–14116.
980
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 973 – 980