Supramolecular aggregates of silanols and solid-state synthesis of siloxanes

Article abstract of DOI:10.1080/15421400590958584

Cocrystallization of all-cis-tetraisopropylcyclotetrasiloxanetetraol and diisopropylsilanediol gave single crystals, which were proved to have a nano-size tube-like aggregate structure. Six molecules of the cyclic tetraol and four molecules of the dial form an asymmetric unit by means of hydrogen bonding. The structure and properties of the aggregates are discussed in detail, and also the possibility of solid-state synthesis of siloxanes is described.

Full text of DOI:10.1080/15421400590958584

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Supramolecular Aggregates
of Silanols and Solid-State
Synthesis of Siloxanes
a
a
Masafumi Unno , Keisuke Takada , Yasuaki
a
a
Kawaguchi & Hideyuki Matsumoto
a
Department of Nano-Material Systems, Graduate
School of Engineering , Gunma University , Kiryu,
Gunma, Japan
Published online: 31 Aug 2006.
To cite this article: Masafumi Unno , Keisuke Takada , Yasuaki Kawaguchi &
Hideyuki Matsumoto (2005) Supramolecular Aggregates of Silanols and Solid-State
Synthesis of Siloxanes, Molecular Crystals and Liquid Crystals, 440:1, 259-264, DOI:
10.1080/15421400590958584
To link to this article: http://dx.doi.org/10.1080/15421400590958584
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Mol. Cryst. Liq. Cryst., Vol. 440, pp. 259–264, 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590958584
Supramolecular Aggregates of Silanols and
Solid-State Synthesis of Siloxanes
Masafumi Unno
Keisuke Takada
Yasuaki Kawaguchi
Hideyuki Matsumoto
Department of Nano-Material Systems, Graduate School of Engineering,
Gunma University, Kiryu, Gunma, Japan
Cocrystallization of all-cis-tetraisopropylcyclotetrasiloxanetetraol and diisopropyl-
silanediol gave single crystals, which were proved to have a nano-size tube-like
aggregate structure. Six molecules of the cyclic tetraol and four molecules of the
diol form an asymmetric unit by means of hydrogen bonding. The structure and
properties of the aggregates are discussed in detail, and also the possibility of
solid-state synthesis of siloxanes is described.
Keywords: crystal structure; hydrogen bonding; nano-size tube; silanol; solid-state
synthesis
INTRODUCTION
In recent years, we have reported reactions starting from silanols
[
1–5], and showed that all-cis-tetraisopropylcyclotetrasiloxanetetraol
(
[i-PrSiO(OH)] , 1) was a versatile precursor of siloxanes of various
4
structures. As shown in Figure 1, cage silsesquioxanes (octasilses-
quioxane and hexasilsesquioxane) and ladder siloxanes (tricyclic and
pentacyclic) were prepared. Regarding the structure of 1, we have
determined its configuration (all-cis form) by X-ray crystallography,
and demonstrated its sphere-like packing structure on hydrogen bond-
ing with water [3]. In 2000, we showed that the co-crystallization of
This work was supported by Grants-in-aid (Nos. 09239103 and 09304058) from the
Ministry of Education, Science, Sports, and Culture of Japan.
Address correspondence to Hideyuki Matsumoto, Department of Nano-Material
Systems, Graduate School of Engineering, Gunma University, Kiryu, Gunma 376-8515,
Japan. E-mail: matumoto@chem.gunma-u.ac.jp
259

2
60
M. Unno et al.
FIGURE 1 The structure of all-cis-cyclotetrasiloxanetetraol 1 and derived
cage siloxane and ladder siloxane structures.
1
and diisopropylsilanediol 2 afforded the nano-size supramolecular
aggregate 3 by means of hydrogen bonding [6]. As an extension, here
we elaborate the structure and thermal properties of 3, and explore
the possibility of solid-state reaction yielding nano-sized siloxane
tubes.
RESULTS AND DISCUSSION
Cyclic silanol 1 was prepared from isopropyltrichlorosilane in 22%
yield [3], and silanediol 2 was obtained from diisopropyldichlorosilane
in 81% by hydrolysis with aniline in water-ether. The co-crystallization
was effected by slow evaporation from ether-hexane solution of both
ꢀ
compounds. The X-ray crystallography was performed at –100 C, the
structure was solved, and the space group determined to be P2 . As
1
shown in Figure 2, the asymmetric unit contains six molecules of
1
and four of the diol 2. Each molecule was connected through hydrogen
bonding, and the material composed an infinite length nano-size
aggregate. Figure 3 indicates the top and side views of a space filling
model. The structure resembles a micelle, however, the hydrophilic
core (hydroxyl groups and siloxane oxygens) was surrounded by the
hydrophobic isopropyl groups.
ꢀ
The melting point of 3 was 180–185 C, and higher than those of
ꢀ
ꢀ
1
(165–169 C) and 2 (106–109 C). The result of thermogravimetric
analysis of 1–3 is shown in Figure 4. Tetraol 1 starts losing weight
ꢀ
ꢀ
from 150 C, and completely sublimed at 578 C. Diol 2 lost all its weight
ꢀ
at 149 C. On the other hand, the thermal profile of 3 is like an addition
of 1 and 2, except that the weight loss starts at 82 C, and is much
ꢀ

Supramolecular Aggregates of Silanols
261
FIGURE 2 The asymmetric unit of the co-crystalline adduct 3 contains six
molecules of 1 and four of diisopropylsilanediol 2.
FIGURE 3 Top and side views of a space-filling model of the adduct 3.

2
62
M. Unno et al.
FIGURE 4 TG-DTA results for the compounds 1–3.

Supramolecular Aggregates of Silanols
263
FIGURE 5 The dehydration of diphenyldisilane diol under solid state
conditions.
ꢀ
higher than that of 2 (54 C). Unfortunately, no indication of dehydra-
tive condensation in the solid state was observed. The calculated
amount of weight loss when all the silanols condense is 9.3%, but
the profile of 3 shows that one of the components (2) starts to sublime
before condensation occurs.
Although the crystal is stable in air, dissolution in polar solvents
cleaves the hydrogen bonding and the aggregate form is not observed
any more. It is probable that the structure could be maintained in non-
polar solvents, but 3 was insoluble in such solvents. If we can trans-
form 3 into a stable siloxane, the resulting material promises
desirable properties as a nano-sized silicone tube. As the structure is
not maintained in solution, transformation of 3 to a stable siloxane
tube is only possible through solid-state reaction.
For solid-state dehydration of alcohols, Toda et al. reported the facile
and high-yield transformation of aryl alcohols using a pestle and
mortar in the presence of HCl gas, Cl CCOOH, or p-toluenesulfonic
3
acid [7]. We applied this reaction to the dehydration of diphenylsilane-
diol as a model reaction. As shown in Figure 5, the target compound
was obtained in 49% in the case of p-toluenesulfonic acid. Unfortu-
nately, this reaction could not be performed for 3, and resulted in the
recovery of starting materials. All the model reactions including ther-
mal reaction, microwave reaction, or under HCl gas gave no reactions.
We are now seeking more effective dehydration reaction conditions.
REFERENCES
[
1] Unno, M., Shamsul, B. A., Saito, H., & Matsumoto, H. (1996). Synthesis of
hexasilsesquioxanes bearing bulky substituents: Hexakis(1,1,2-trimethylpropyl-
silsesquioxane) and hexakis(tert-butylsilsesquioxane). Organometallics, 15, 2413–
2
414.

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[
2] Unno, M., Shamsul, B. A., Arai, M., Takada, K., Tanaka, R., & Matsumoto, H. (1999).
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of silanols. Appl. Organomet. Chem., 13, 1–8.
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4
the tetrahydroxycyclotetrasiloxane [(i-Pr)(OH)SiO] . Chem. Lett., 489–490.
[
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Jpn., 73, 215–220.
[
[
[
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