Shape-controlled bridged silsesquioxanes: Hollow tubes and spheres

Article abstract of DOI:10.1002/chem.200390183

A new approach for the morphological control of bridged silsesquioxanes has been achieved by the hydrolysis of silylated organic molecules bearing urea groups. The urea groups are responsible for the auto-association of the molecules through intermolecular hydrogen-bonding interactions. The self-assembly leads to supramolecular architectures that have the ability to direct the organization of hybrid silicas under controlled hydrolysis. The hydrolysis of the chiral diureido derivatives of trans-(1,2)-diaminocyclohexane 1 under basic conditions has been examined. The solid-state NMR spectra (²⁹Si and ¹³C) showed the hybrid nature of these materials with wholly preserved Si-C covalent bonds throughout the silicate network. Hybrid silicas with hollow tubular morphologies were obtained by the hydrolysis of the enantiomerically pure compounds, (R,R)-1 or (S,S)-1, whereas the corresponding racemic mixture, rac-1, led to a hybrid with ball-like structures. The tubular shape is likely to result from a combination of two phenomena: the auto-association abilities and a self-templating structuration of the hybrid materials by the organic crystalline precursor. Electronic microscopy techniques (SEM and TEM) gave evidence for the self-templating pathway. The formation of the ball-like structures occurs through a usual nucleation growth phenomenon owing to a higher solubility of the corresponding crystals in the same medium.

Full text of DOI:10.1002/chem.200390183

FULL PAPER
Shape-Controlled Bridged Silsesquioxanes: Hollow Tubes and Spheres
[a]
Joe»l J. E. Moreau,*Luc Vellutini, Michel Wong Chi Man, and Catherine Bied
Dedicated to Professor Dr. Walter Siebert on the occasion of his 65th birthday
Abstract: A new approach for the mor-
phological control of bridged silses-
quioxanes has been achieved by the
hydrolysis of silylated organic molecules
bearing urea groups. The urea groups
are responsible for the auto-association
of the molecules through intermolecular
hydrogen-bonding interactions. The
self-assembly leads to supramolecular
architectures that have the ability to
direct the organization of hybrid silicas
under controlled hydrolysis. The hydrol-
ysis of the chiral diureido derivatives of
trans-(1,2)-diaminocyclohexane 1 under
basic conditions has been examined. The
solid-state NMR spectra (29Si and 13C)
showed the hybrid nature of these
structures. The tubular shape is likely to
result from a combination of two phe-
nomena: the auto-association abilities
and a self-templating structuration of
the hybrid materials by the organic
crystalline precursor. Electronic micros-
copy techniques (SEM and TEM) gave
evidence for the self-templating path-
way. The formation of the ball-like
structures occurs through a usual nucle-
ation growth phenomenon owing to a
higher solubility of the corresponding
crystals in the same medium.
À
materials with wholly preserved Si C
covalent bonds throughout the silicate
network. Hybrid silicas with hollow
tubular morphologies were obtained by
the hydrolysis of the enantiomerically
pure compounds, (R,R)-1 or (S,S)-1,
whereas the corresponding racemic mix-
ture, rac-1, led to a hybrid with ball-like
Keywords: bridged silsesquioxanes
¥ hybrid silica ¥ nanostructures ¥
self-assembly ¥ sol-gel
controlled materials with the aim of creating new composites
with structuration at different scales.
Introduction
The increasing demand for new techniques to afford mini-
aturized devices has stimulated materials chemists to direct
their research interest towards the design of nanostructured
materials with tailored chemical and physical properties.
Hybrid organic inorganic materials produced by the sol-gel
process[1 3] are developing into a considerable field of interest
for the materials community and offer the opportunity to
achieve such nanostructured materials. Bridged silsesquiox-
anes were introduced more than a decade ago[4, 5] and
represent attractive hybrid materials with a wide variety of
Organogels consist of a self-assembly of organic mole-
cules[12] and have been used as templating agents to transcribe
their supramolecular architectures to produce inorganic
16]
silicas.[13
Helical fibers were successfully obtained by the
hydrolysis of tetraethoxysilane (TEOS) in the presence of
d, 14]
such templates.[13a
Interestingly, a chirality transcription
from a chiral supramolecular template to silicas was achieved
leading to right- and left-handed tubular helices. Hollow
tubular[13e,f] and vesicular silicates[17] have also been produced
by the sol-gel hydrolysis of TEOS with organogels. This
method has been applied to other inorganic materials. The
synthesis of a helical morphology with cadmium sulfide
(CdS)[15] and also with several transiton metal oxides (Ti, Ta,
tunable properties, because of the organic fragment cova-
11]
lently linked to the silicate framework.[4
The solid-state
properties and morphology depend not only on the organic
component, but also on the synthetic conditions of the
hydrolysis condensation process (solvent, temperature, cat-
alyst, etc.). Growing attention is being paid to shape-
]
V)[16 have recently been reported. In all these cases, the
materials are purely inorganic.
The development of surfactant-templated inorganic meso-
structured materials[18 20] has led to considerable focus on the
25]
formation of highly organized, mesoporous hybrid silicas.[21
[a] Prof. Dr. J. J. E. Moreau, Dr. L. Vellutini,
Dr. M. Wong Chi Man, Dr. C. Bied
A bioinspired organic inorganic hybrid silsesquioxane de-
veloped as bilayers of vesicular nanoparticles, with the use of
monosilylated lipid vesicles, by means of this approach.[25]
In the recent years, periodic, mesoporous, bridged silses-
¬ ¬
¬
¬
Heterochimie Moleculaire et Macromoleculaire (UMR-CNRS 5076)
¬
Ecole Nationale Superieure de Chimie de Montpellier
¬
8, rue de l'ecole normale
¬
34296 Montpellier cedex 5 (France)
quioxanes have been obtained by a similar templated
Fax : (‡33)4-67-14-72-12
32]
route.[26
Interestingly highly ordered, hybrid mesoporous
E-mail: jmoreau@cit.enscm.fr
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1594 1599
silicates with well-defined external morphologies have been
achieved by this method.[33]
respectively to (R,R)-1, (S,S)-1 and rac-1 (Scheme 1) in high
yield (84 93%).
Recently, we developed a new approach based on hydrogen
bonding of bis-urea groups to morphologically-controlled
bridged silsesquioxanes.[34 35] The use of appropriate silylated
organic molecules capable of forming supramolecular archi-
tectures led, after hydrolytic condensation, to lamellar solids
with architectures at different scales.[34] Interestingly, a chiral
silylated diureidocyclohexyl derivative (Scheme 1) led to
helical hybrids with controlled handedness according to the
configuration of the organic substructure.[35] Whereas in acidic
Compounds (R,R)-1 and (S,S)-1 gelate common organic
solvents (cyclohexane, toluene, and THF) at very low
concentration (1 5 mgmLÀ1), whereas a greater amount is
needed for rac-1 (ꢀ40 mgmLÀ1) to form the organogel.[34, 36]
This phenomenon typically occurs because of the auto-
association of the molecules through hydrogen bonding of
the urea groups in these solvents. Higher intermolecular
associations in the case of (R,R)-1 and (S,S)-1 relative to rac-1
are evidenced by the FTIR spectra of the compounds in
chloroform. Characteristic vi-
brations are observed at 1574
(dNH), 1631 (nCO
)
3313 cmÀ1
SiO1.5
Si(OEt)3
(nNH) for (R,R)-1, whereas the
corresponding absorptions are
at 1570 (dNH), 1638 (nCO) and
3348 cmÀ1 (nNH) for rac-1. The
frequency difference between
the amide I (nCO) and amide II
(dNH) bands of (R,R)-1 (Dn ˆ
57 cmÀ1) is smaller than that of
rac-1 (Dn ˆ 68 cmÀ1). More-
over, the NH stretch band
(dNH) is located at a lower
frequency value for (R,R)-1
than that of rac-1. These obser-
vations are in accord for a
better aggregation of (R,R)-1
through hydrogen bonding than
Si(OEt)3
2
O
O
NH
NH
O
O
NH
NH
NH
NH
NH2
NH2
H2O/EtOH
NaOH
NCO
NH
NH
SiO1.5
Si(OEt)3
n
(R,R)-1, (S,S)-1, or rac-1
(R,R)-1HS, (S,S)-1HS, or rac-1HS
Scheme 1. Synthesis of the molecular precursors and hybrid silicas.
aqueous medium right- or left-handed helical fibers formed
from the hydrolysis of the pure enantiomers (R,R)-1 or (S,S)-
1, respectively, the hydrolysis of the racemic mixture of
precursors rac-1 in the same medium led to a featureless
granular solid. This chirality transcription from the enantio-
pure precursors to the hybrid material was found to be
dependent on the hydrolysis condensation conditions. The
morphologies of these hybrids do not only arise from the
intrinsic properties of the organic fragment, but also depend
on the reaction medium and the temperature at which the
reaction is performed. These results prompted us to explore
the role of the reaction conditions for the hydrolysis of these
hybrid precursors. We examined the hydrolysis of these
compounds in the presence of a base (NaOH) in a solvent
mixture (ethanol and water). From these studies, we report, in
this paper, the synthesis of tubular and spherical hybrid silicas
by the hydrolysis of 1 (Scheme 1) according to a new
mechanism involving both templating and self-association
properties.
rac-1.[37, 38a] The easier gel formation in the case of the pure
enantiomers is attributable to the presence of much favored
intermolecular hydrogen bonding, as already observed in
similar derivatives between the urea groups.[38] The resulting
supramolecular architectures usually develop as fibers or
ribbons and have the ability to form an entangled network in
solvents to end up with an organogel.
Synthesis of the tubular hybrid silica: The hydrolysis of (R,R)-
1 or (S,S)-1 was performed in a 2:5 volume ratio of ethanolic/
aqueous medium to give the corresponding hybrid silicas
(R,R)-1HS and (S,S)-1HS, respectively (Scheme 1). The
precursor was first completely dissolved in dry ethanol, and
when water was added a white precipitate appeared. An
aqueous solution of NaOH (3m) was then immediately added
to attain pH 12, and the mixture was heated at 808C for
6 days. It should be noted that the precipitate was not
completely soluble in the reaction mixture, but after 1 hour
the formation of a significant volume fraction of solid was
observed. After filtration, the white solid was washed
successively with water, ethanol, and acetone, and then dried
at 1108C for 6 h.
Hybrid silicas (R,R)-1HS and (S,S)-1HS display identical
characteristics. The elementary analyses confirm the presence
of the organic moieties in the silica network (N/Si ˆ 2).
Solid-state NMR spectra show that these materials consist
of an organic inorganic network and are represented for
(R,R)-1HS in Figure 1. The 29Si NMR spectrum (Figure 1a)
exhibits a low intensity signal at À56.8 ppm and a predom-
Results and Discussion
Synthesis of the molecular precursors: The molecular pre-
cursors 1 were synthesized in CH2Cl2 by the reaction of 3-
isocyanatopropyltriethoxysilane with the pure enantiomers,
trans-(1R,2R)- and trans-(1S,2S)-diaminocyclohexane, and
also with the corresponding racemic compound leading
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J. J. E. Moreau et al.
FULL PAPER
of fiberlike structures with a rough outer surface. No chiral
morphology could be perceived from these analyses.
The transmission electronic microscopy (TEM) images of
(R,R)-1HS are shown in Figure 3 and confirms the hollow
tubular structures. Here again, long fiberlike structures 15
25 nm wide can be distinguished. It seems likely that these
solids consist of a longitudinal juxtaposition of these fibers.
Figure 1. Solid-state NMR of (R,R)-1HS: a) 29Si and b) 13C.
2
À
inant one at 67.6 ppm, assigned to SiC(OH)(OSi)2 (T ) and
SiC(OSi)3 (T3) units, respectively; no SiO4 unit (Q type) is
observed. The 13C CP-MAS spectrum (Figure 1b) exhibits a
3
ˆ
peak at 161 ppm (C O) and several sp carbon atoms (55.2,
42.8, 34.1, 25.3 and 10.3 ppm) characteristic of the organic
fragment.
The existence of hydrogen bonding between the urea
groups in these materials is evidenced by FTIR studies with
vibrations at nNH ˆ 3341, nCO ˆ 1634, and dNH ˆ 1574 cmÀ1.
These values are similar to those of the helical silsesquiox-
anes.[35] Powder X-ray diffraction measurements showed these
hybrids to be completely amorphous.
Figure 3. TEM images of tubular hybrid silicas (R,R)-1HS or (S,S)-1HS.
Scanning electronic microscopy (SEM) images of (R,R)-
1HS or (S,S)-1HS reveal similar tubular structures (Fig-
ure 2a). Under higher magnification these tubes appear
hollow and rectangular channels are often observed (Fig-
ure 2b and c). The sizes range from 0.5 2 mm for the external
width and the length is up to 15 mm. The diameter of the
channel is 0.3 1.6 mm and the shells are 0.1 0.6 mm thick.
Interestingly, the inner part of the tubes consists of a lining up
For a better understanding of the formation of the tubes, we
analyzed the precipitate that formed at the initial stage in the
EtOH/H2O mixture before the addition of NaOH. It was
recovered by filtration and was identified as microcrystals of
the nonhydrolyzed precursor by NMR spectroscopy. At this
stage no hydrolysis had occurred. SEM images of this
precipitate show rectangular rodlike crystals (Figure 4) that
are 2 12 mm long and 0.3 1.5 mm wide; they show an
interesting size and shape similarities with the observed
hollow structures in the final hybrid solid after hydrolysis.
Figure 4. SEM images of microcrystals of (R,R)-1.
It is conceivable that the incipient crystallization is respon-
sible for the hollow tubular morphologies of (R,R)-1HS or
(S,S)-1HS. A possible mechanism might involve, at the early
stages, the hydrolysis condensation at the surface of the
crystalline precursor (R,R)-1 as soon as NaOH is added.
Ethanol, which is a better solvent than water for (R,R)-1, is
gradually released and facilitates the in-situ solubilization of
the crystals in solution. The dissolved compound, which forms
self-assembled fibrous structures,[35, 36] tends to condense at
Figure 2. SEM images of tubular hybrid silicas (R,R)-1HS or (S,S)-1HS:
a) magnification Â3000; b) and c) magnification Â50000.
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Bridged Silsesquioxanes
1594 1599
the external surface, causing a thickening of the walls of the
materials. On this basis we presume that the hollow tubular
structure results from a self-templating process of the hybrid
materials by the organic crystalline precursor itself. Recently,
organic crystals were used as external templates for the
formation of hollow tubular silica. It formed by sol gel
deposition of silica on the outer surface of organic crystals
The solid-state NMR spectra of the corresponding material,
rac-1HS, were identical to those of the enantiopure materials
with similar elementary analysis results (N/Si ˆ 2). The hybrid
obtained from rac-1 exhibits weaker hydrogen-bonding
interactions than the related hybrids arising from the pure
enantiomers as observed by FTIR for the vibrations of the
urea groups (3352, 1640, and 1573 cmÀ1). This is shown by a
greater Dn value between the nCO and the dNH and also by the
higher nNH value[37, 38a] relative to the values observed for the
tubular materials.
40]
that were subsequently removed.[39
To further corroborate our assumption for the existence of
the self-templating mechanism, we again performed the
hydrolysis reaction and stopped it simply by filtration after
one hour. The resulting sample was washed with water and
dried. By carefully examining the resulting solid through
TEM techniques, we were able to obtain pictures in which
clear partial hydrolysis of the enantiopure compound has
occurred (Figure 5).
An SEM image exclusively shows spheres with a 0.3 2 mm
diameter (Figure 6). These spheres are completely dense:
only dark balls are observed from TEM analysis.
Figure 6. SEM image of spherical hybrid silica rac-1HS.
Figure 5. TEM image of tubular sample after 1 h hydrolysis of (R,R)-1.
The different shapes are probably related to the aggrega-
tion mode during the hydrolysis condensation steps of the
precursors. While (R,R)-1 and (S,S)-1, which exhibit strong
self-assembly properties,[35] tend to form fiberlike aggregates
during acid-mediated hydrolysis, rac-1, with much weaker
auto-association, grows up into a featureless solid.[36] Fur-
thermore, the complete solubility of rac-1 at the initial stage of
the reaction is in full agreement with a nontemplating
pathway for the formation of the spherical solids.
A rigid rodlike stucture can be seen that is not completely
transparent contrarily to the fully hydrolyzed relating hybrid
solid (Figure 3). In this picture, it looks likely that the
solubilization of the precursor occurs at both ends after an
initial hydrolysis at the external surface of the crystal. The
transparent parts of the solid correspond to areas in which the
organic crystalline template has completely dissolved. The
inner dark part could represent nondissolved (R,R)-1 and the
outer one can be assigned to the condensed hybrid silica.
These results give additional support for the crystal self-
templating process for the formation of the hollow tubes in
the case of the pure enantiomers.
Conclusion
The tubular structures represent the first shape-controlled
hybrid silicas obtained by a self-templating method. Only
inorganic hollow tubes were previously obtained by the sol
gel hydrolysis of metallic alkoxides and by using an external
cristalline molecule as template. Our approach combines the
crystal nature and the auto-association abilities of the
precursor to generate interesting structured materials. The
auto-association properties of the main organic group play an
important role for the shape of the final solid: the strongly
associated enantiopure precursors afforded hollow tubes,
whereas, under the same reaction conditions, only spheres
formed from the racemic mixture with weaker hydrogen-
bonding interactions. The complete solubilization of rac-1 at
the beginning of the reaction accounts for the nontemplating
effect during the hydrolysis condensation step. The hollow
tubes represent a new class of hybrid materials with well-
defined organic substructures regularly spread on the molec-
ular scale over the whole silica network. The incorporation of
chiral units in the walls of the tubular hybrid silica may lead to
materials with interesting properties and which may find
applications in various domains such as molecular recogni-
Synthesis of the spherical hybrid silica: For comparison we
also performed the hydrolysis of the racemic mixture of
precursors rac-1 in exactly the same reaction conditions
leading to rac-1HS.
The compound rac-1 formed crystals that look similar in
shape to those obtained from (R,R)-1 or (S,S)-1, as observed
by SEM images (not shown here) after the addition of water
to the ethanolic solution in which rac-1 was completely
dissolved. After the addition of the aqueous solution of
NaOH, the mixture was heated to 808C. In contrast to the
pure enantiomers, a complete dissolution of the microcrystals
occurred followed by the formation of a colloidal solution
after 5 min; this then turned into a white precipitate with time.
This observation suggests that the initial hydrolysis con-
densation in the case of rac-1 proceeds through a conventional
liquid-state reaction and, hence, precludes any possibility for
organic crystalline self-templating of the precursor to occur as
for the pure enantiomers. The particulates only formed and
grew according to the usual nucleation growth pathway.
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J. J. E. Moreau et al.
FULL PAPER
1108C of the gel for 6 h led to a white powder. (R,R)-1HS: IR (KBr): nÄ ˆ
1574 (dNH), 1634 (nCO), 3341 cmÀ1 (nNH); 13C CPMAS NMR: d ˆ 10.3, 25.3,
34.1, 42.8, 55.2, 161.0 ppm; 29Si CPMAS NMR: d ˆ À56.8, À67.6 ppm (T2
and T3 units); elemental analysis (%) calcd for completely condensed
silsesquioxane C14H26N4O5Si2: C 43.50, H 6.78, N 14.49, Si 14.53; found: C
41.58, H 6.83, N 13.48, Si 13.50.
tion, chromatographic separation, or enantioselective cata-
lysts.[6a,b,d] The development of this method for the preparation
of other hybrid silicas with targeted functionalities should
bring to the development of new hybrid materials with
sought-after properties.
Hybrid silica from rac-1: The same procedure as above was applied. In this
case, the white precipitate formed after addition of water to the
corresponding ethanolic solution rapidly dissolved into the solution when
the mixture was dipped into the oil bath. A white precipitate reappeared
after 15 min. The mixture was left standing at the same temperature under
static conditions. The same workup was applied. IR (KBr pellet): nÄ ˆ 1574
(dNH), 1640 (nCO), 3352.2 cmÀ1 (nNH); 13C CPMAS NMR: d ˆ 10.8, 25.2, 34.7,
42.8, 54.2, 159.6 ppm; 29Si CPMAS NMR: d ˆ À58.2, À66.9 ppm (T2 and T3
units); elemental analysis (%) calcd for completely condensed silsesquiox-
ane C14H26N4O5Si2: C 43.50, H 6.78, N 14.49, Si 14.53; found: C 42.12, H
6.77, N 13.60, Si 14.15.
Experimental Section
General information and techniques: The syntheses of the molecular
precursors were carried out under nitrogen atmosphere by using a vacuum
line and schlenk techniques. trans-(1R,2R)- and trans-(1S,2S)-diaminocy-
clohexanes were resolved according to literature[41] from a commercial cis/
trans mixture of (1,2)-diaminocyclohexane, which was purchased from
Aldrich, as was 3-isocyanotopropyltriethoxysilane. Dichloromethane was
distilled from P2O5 and ethanol from magnesium turnings. Melting points
were determined on an electrothermal apparatus (IA9000 series) and are
uncorrected. Optical rotations were measured using a Perkin Elmer
(Norwalk, CT) 214 polarimeter with solutions in a 1 dm cell in CHCl3. IR
data were obtained on a Perkin Elmer 1000 FT-IR spectrophotometer.
Elemental analyses were carried out by the ™Service Central d'Analyse du
CNRS∫ in Vernaison (France).
1H, 13C, and 29Si NMR spectra in solution were recorded on Bruker AC-200
and AC-250 spectrometers at room temperature with deuterated chloro-
form as solvent and TMS as internal reference. 1H, 13C, and 29Si solid-state
NMR spectra were obtained from Bruker FT-AM 200 or FT-AM 400
spectrometers by using cross-polarization and magic-angle spinning
techniques (CP-MAS) and TMS as reference for the chemical shifts. Mass
spectra were measured on a JEOL MS-DX 300 mass spectrometer.
Acknowledgement
¡
Financial support from the ™Ministere de la Recherche de France∫ (ACI
¡
¬
2000–Physicochimie de la matiere condensee) and the CNRS (Pro-
¬
¬
¬
gramme materiaux–Auto-assemblage de materiaux nanostructures) are
gratefully acknowledged. We also thank Dr. F. Di Renzo for fruitful
discussions and Mr. L. Datas for preliminary SEM measurements.
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(anticathode Cu) and registered from q ˆ 1 to 2 08.
The TEM images were obtained with JEOL 200 CX and JEOL JEM 2010
microscopes. SEM images were realized on a JEOL 6300F apparatus.
General method for the preparation of (R,R)-1, (S,S)-1, and rac-1: These
molecular precursors were prepared in a similar manner. In a typical
synthesis trans-(1R,2R)-diaminocyclohexane (114 mg, 1 mmol) was dis-
solved in CH2Cl2 (30 mL) under a nitrogen atmosphere in a dried schlenk
tube. 3-Isocyanotopropyltriethoxysilane (519 mg, 2.1 mmol) was slowly
added by means of a syringe at room temperature, and the reacting mixture
was stirred for 2h. The solvent was evaporated and the white precipitate
(R,R)-1 was washed with pentane. The solution was filtered to remove the
solvent and the excess of 3-isocyanatopropyltriethoxysilane. The remaining
white solid was then dried in vacuo.
¬
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(R,R)-1: Yield: 548 mg, 91%; m.p. 2108C; [a]D ˆ ‡4.3 (c ˆ 0.03 in chloro-
form); IR (KBr pellet): nÄ ˆ 1597 (dNH), 1635 (nCO), and 3336 cmÀ1 (nNH);
1H NMR (200 MHz, CDCl3): d ˆ 0.6 (t, 4H; SiCH2), 1.2(t, 18H; C H3), 1.5
2(m, 12H; C H2), 3 3.5 (m, 6H; NCH, NCH2), 3.8 (t, 12H; OCH2), 4.9 and
5.3 ppm (2m, 4H; 2N H); 13C NMR (200 MHz, CDCl3): d ˆ 7.6 (CH2Si),
18.2( CH3), 23.6, 25 and 33.3 (3CH2), 43 (CH2N), 54.6 (CHN), 58.3 (CH2O),
159.2ppm ( CO); 29Si NMR (250 MHz, CDCl3): d ˆ À46 ppm; MS (FAB):
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washed successively with water, ethanol, and acetone. Careful drying at
¬
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Received: December 6, 2002 [F4639]
Chem. Eur. J. 2003, 9, No. 7
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0947-6539/03/0907-1599 $ 20.00+.50/0
1599