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Synthesis and characterization of heptacyclic laddersiloxanes and ladder polysilsesquioxane.

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Full Paper
Received: 25 August 2009
Revised: 12 November 2009
Accepted: 12 November 2009
Published online in Wiley Interscience
(www.interscience.com) DOI 10.1002/aoc.1607
Synthesis and characterization of heptacyclic
laddersiloxanes and ladder polysilsesquioxane
Shengho Changa , Tomoe Matsumotoa , Hideyuki Matsumotoa,b
and Masafumi Unnoa,b∗
As a continuation of our previous studies on thermostable materials, heptacyclic laddersiloxanes and ladder polysilsesquioxane
were synthesized. The first heptacyclic laddersiloxanes were obtained by chlorination of pentacyclic laddersiloxanes
prepared using our stereocontrolled synthesis procedure; thereafter, the heptacyclic laddersiloxanes were made to react
with disiloxanediol. Ladder polysilsesquioxane was obtained from cis–trans–cis-tetrabromotetramethylcyclotetrasiloxane by
spontaneous hydrolysis and dehydration. The spectral and thermal properties of these new compounds were investigated. It
c 2010 John Wiley &
was observed that the thermal stability of these compounds increases with the ring number. Copyright Sons, Ltd.
Keywords: silsesquioxane; laddersiloxane; thermal properties; ladder polysilsesquioxane; thermostable materials
Introduction
Ever since they were first reported in literature by Brown et al. in
1960,[1] ladder-type silsesquioxanes have attracted considerable
interest, mainly because of their high thermal stability. Several
reports on ladder-type silsesquioxanes have been published to
date;[2] however, the structures of these compounds are still
controversial. Moreover, the relationship between the structure
and properties of these compounds has not yet been clarified.
In order to establish the properties of ladder-type silsesquioxanes, structure elucidation is necessary. With this objective, we
had previously prepared ladder silsesquioxanes, determined their
structures and investigated the properties. We referred to those
ladder silsesquioxanes whose structures had been determined
as ‘laddersiloxanes,’ and we reported the syntheses and crystallographic analysis of tricyclic laddersiloxanes;[3] pentacyclic
laddersiloxanes;[4] bi-, tri-, tetra- and pentacyclic laddersiloxanes with an all-anti conformation;[5] and extendible pentacyclic
laddersiloxanes.[6] As an extension of this study, we herein report the synthesis of the first heptacyclic laddersiloxanes and
methyl-substituted ladder polysilsesquioxane.
Results and Discussion
Heptacyclic Laddersiloxanes
Appl. Organometal. Chem. 2010, 24, 241–246
∗
Correspondence to: Masafumi Unno, Gunma University, Department of
Chemistry and Chemical Biology, Graduate School of Engineering, 1-5-1 Tenjincho, Kiryu, Gunma 376-8515, Japan. E-mail: unno@gunma-u.ac.jp
a Department of Chemistry and Chemical Biology, Graduate School of
Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515,
Japan
b International Education and Research Center for Silicon Science, Graduate
School of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma
376-8515, Japan
c 2010 John Wiley & Sons, Ltd.
Copyright 241
In 2002, we reported the stepwise synthesis of pentacyclic laddersiloxane, an essential precursor to heptacyclic laddersiloxanes
(Scheme 1).[4] Before this, only tricyclic laddersiloxanes had been
produced. While the reaction yields at each step of the synthesis were satisfactory (>80%), unextendible trans-Ph isomers were
generated. We then developed a stereocontrolled synthesis procedure and separated the extendible pentacyclic laddersiloxane
from the isomers (Scheme 2).[6]
Dephenylchlorination of 1 (mixture of three isomers) resulted
in the formation of tetrachlorides 2 in good yield. Following
this, reaction with (1R,3S)-disiloxanediol gave the first heptacyclic
laddersiloxane 3 (Scheme 3). Since the obtained laddersiloxanes
were also a mixture of isomers, we attempted to separate the
isomers by recycle-type HPLC. However, unlike the case of the
smaller laddersiloxanes, the heptacyclic laddersiloxanes could not
be separated because their retention times are almost the same.
Thus, we measured NMR and mass spectra as a mixture of isomers,
and determined the structure.
The 29 Si NMR spectrum of 3 shows peaks in the region −66.26 to
−64.94 ppm and −34.36 to −33.22 ppm. As shown in Table 1, this
value is in good agreement with the values of other laddersiloxanes
whose structures were determined by X-ray analysis. The peaks
around −65 ppm were attributed to the internal silicon atom, and
those around −34 ppm were attributed to the terminal Si(–Ph)
atom. In addition, mass spectrum showed a peak at 1754 (M+ −
C3 H6 ), and the isotope pattern was similar to the calculated one
(Fig. 1). From these results, the obtained compound was identified
as a heptacyclic laddersiloxane, even though crystallographic
analysis was not possible.
The result of thermogravimetric (TG) analysis in N2 is shown
in Fig. 2; here, the thermostability of heptacyclic laddersiloxanes
can be clearly observed. The Td5 (5% weight loss) temperature
was 326 ◦ C, which is 30 ◦ C higher than the Td5 temperature of
pentacyclic laddersiloxane.[4] At 567 ◦ C, most of the weight was
lost by sublimation. These temperatures are significantly higher
than the Td5 temperatures of cage silsesquioxane, (i-PrSi)8 O12 (Td5
S. Chang et al.
i-Pr
i-Pr
HO Si O Si OH
O
O
HO Si O Si OH
i-Pr i-Pr
Ph Ph
2 i-Pr Si O Si i-Pr
Cl
Cl
pyridine
Ph Ph
2 i-Pr Si O Si i-Pr
Cl
Cl
i-Pr i-Pr i-Pr i-Pr
OH
Si O Si O Si O Si
O
O
O
O
HO Si O Si O Si O Si OH
i-Pr i-Pr i-Pr i-Pr
HO
ŠHCl
i-Pr i-Pr i-Pr i-Pr
Si O Si O Si O Si Ph
O
O
O
O
Ph Si O Si O Si O Si Ph
i-Pr i-Pr i-Pr i-Pr
Ph
i-Pr
i-Pr
i-Pr
1) AlCl3 , HCl
2) H2O
i-Pr
i-Pr
i-Pr
Si O Si Ph
O
O
Ph Si O Si O Si O Si O Si O Si Ph
i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr
Ph
Si
O
O
Si
O
O
Si
O
O
Si
O
O
Scheme 1. Synthesis of pentacyclic laddersiloxane.
Scheme 2. Stereoselective synthesis of laddersiloxanes.
temperature: 200 ◦ C), and the sublimation point was 282 ◦ C. The
results including other silsesquioxanes are summarized in Table 2.
Ladder Polysilsesquioxanes
For the synthesis of polycyclic laddersiloxanes, we chose methyl
groups as the substituents because methyl groups are widely
used for industrial purposes. The synthesis route is shown
in Scheme 4. Cyclotetrasiloxane (MePhSiO)4 is prepared from
dichloromethylphenylsilane as a mixture of isomers; cis–trans–cis
isomer 4 can be separated from these isomers. Subsequent
dephenylhalogenation of 4 with the retained stereostructure
affords the target compound.
As previously reported,[7] tetrabromotetramethylcyclotetrasiloxane 5 was obtained from dichloromethylphenylsilane in three
steps (Scheme 5). The first step afforded a mixture composed
of (MePhSiO)3 , (MePhSiO)4 and (MePhSiO)5 with all isomers.
Fortunately, we were able to obtain only cis–trans–cis-(MePhSiO)4
by recrystallizing this mixture. The structure of tetrabromide
5 was determined by reacting it with diphenylsilanediol; the
242
Scheme 3. Synthesis of heptacyclic laddersiloxane.
www.interscience.wiley.com/journal/aoc
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 241–246
Heptacyclic laddersiloxanes and ladder polysilsesquioxane
Table 1.
29
Si NMR chemical shift of laddersiloxanes
29
Compounds
Si NMR Chemical
shift (ppm)
syn-Tricyclic laddersiloxane[2]
anti-Tricyclic laddersiloxane[5]
all-anti-Pentacyclic laddersiloxane[5]
Pentacyclic laddersiloxanes 1c
Heptacyclic laddersiloxane 3
Ladder polysilsesquioxane (solid state) 6
Hexa(isopropylsilsesquioxane)[3]
−67.2
−66.8
−65.8, −65.2
−66.5, −65.9
−66.2 to −64.9
−64.5
−54.2
Figure 3. TG analysis of ladder polysilsesquioxane 6.
Figure 1. Calculated and observed mass spectrum of heptacyclic laddersiloxane 3.
stabilization, and isolation of tetrols by hydrolysis of tetrabromide
5 failed. In fact, 5 spontaneously decomposed in air to give
a white solid (Scheme 6). On analyzing the structure of 5, it
can be seen that the intermediate is cis–trans–cis-[MeSi(OH)O]4 ,
which was spontaneously dehydrated to afford the ladder
polysilsesquioxane 6.
Similar to a few methyl-substituted silsesquioxanes, ladder
polysilsesquioxane 6 is insoluble in organic solvents. Therefore, we
identified this compound by IR and solid-state NMR spectroscopy.
Two peaks at 1128 and 1032 cm−1 can be observed in the
S–O stretching vibration region. These peaks are much sharper
than those of ladder-like silsesquioxanes,[9] indicating a highly
regulated structure. The MAS 29 Si NMR spectrum shows that 6 has
only one peak at −64.5 ppm. Comparing this value with those in
Table 1, the ladder structure can be confirmed.
The result of TG analysis is shown in Fig. 3. The Td5 temperature
is 642 ◦ C, which is higher than that of any other ladder-like
silsesquioxane reported previously. Furthermore, the weight loss
at 1000 ◦ C was only 13%, which shows that this compound has
high stability.
Properties of Laddersiloxanes
Figure 2. TG analysis of heptacyclic laddersiloxane 3.
Appl. Organometal. Chem. 2010, 24, 241–246
c 2010 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
243
obtained tricyclic laddersiloxane was confirmed to have anticonformation by X-ray analysis.[7] The exclusive formation of
cis–trans–cis isomer of tetrabromide 5 is performed by the lowtemperature conversion with using bromine that is more reactive
than HCl–AlCl3 .
Isopropyl-substituted cyclotetrasiloxanetetrols are stable in air,
and crystallographic analyses have been performed without
problems.[8] However, the methyl groups are not large enough for
The IR spectra of laddersiloxanes, hexasilsesquioxane, and octasilsesquioxane are summarized in Fig. 4. As can be observed
from this figure, it is not easy to distinguish laddersiloxanes from
other silsesquioxanes using only the IR spectra. However, it is true
that ladder polysilsesquioxanes show two absorption bands, at
1050 and 1150 cm−1 , while cage silsesquioxanes show a single
absorption band.
The 29 Si NMR spectra of laddersiloxanes are shown in Table 1.
Only chemical shifts of the internal silicon atoms are indicated. As
seen from this table, the NMR spectra of various laddersiloxanes
are basically similar and can be used for identification.
The thermal properties of laddersiloxanes are summarized with
several previously known silsesquioxanes (Table 2). TG analyses
showed that the elimination of substituents occurs first; thereafter,
when the molecular weight of the compound is not large, sublimation is observed. It is noteworthy that the Td5 temperatures were
higher for the longer laddersiloxanes. Ladder polysilsesquioxanes
show no sublimation, and 88% of the weight remained intact
even when heated to 1000 ◦ C. This result clearly shows that 6
is more stable than any of the previously reported ladder-like
silsesquioxanes.[9]
On comparing 6 with the previously reported ladder-like
silsesquioxanes, we observed that 6 has higher stability and
S. Chang et al.
Me
X
X
O
Si
O Si
Si O
Si O
Me
X X
Me
Me
(MePhSiO)4
MePhSiCl2
Ph
Ph
O
Si
O Si
Si O
Si O
Me
Ph
Ph
Me
Me
Me
mixture of isomers
4
Ladder
Polysilsesquioxanes
X = Cl, Br, or OH
Scheme 4. Synthetic pathway of ladder polysilsesquioxane.
Me
KOH
MePhSiCl2
(MePhSiO)n
n=3,4,5 and over
THF
recrystallization
Me
O
Si
methanol
Ph
Si
O
Ph
4
Ph
Ph
O
Si
Si O
Me
Me
11%
Me
Ph Ph
Br2
Me O Si O
Si
O
O
Si
Si
Me -30°C
Ph
85%
Ph
Me
4
Br
Br
O
Si
O Si
O
Si
Si O
Me
Br
Br
Me
5
Me
Me
Scheme 5. Preparation of tetrabromotetramethylcyclotetrasiloxane.
Table 2. Thermal properties of laddersiloxanes
Table 3. Thermal properties of ladder-like polymers
Td5 (◦ C)
Compounds
(i-Pr2 SiO)4
syn-Tricyclic laddersiloxane[3]
Pentacyclic laddersiloxane[4]
Hexa(isopropylsilsesquioxane)[3]
Octa(isopropylsilsesquioxane)[3]
Heptacyclic laddersiloxane 3
Ladder polysilsesquioxane 6
205
260
296
190
200
326
645
Comments
Td5 (◦ C)
Compounds
◦
Sublimed at 345 C
Sublimed at 390 ◦ C
Sublimed at 423 ◦ C
Sublimed at 252 ◦ C
Sublimed at 282 ◦ C
Sublimed at 567 ◦ C
12% Weight loss at 1000 ◦ C
regularity. As shown in Table 3, the 5% weight loss temperatures
of the previously reported ladder-like polymers are all well below
that of 6.
Ladder
Polysilsesquioxane 6
Ladder-like polymer
(from MeSiCl3 )[10]
Ladder-like polymer
(from MeSiCl3 )[11]
Comments
645 (in N2 )
523 (in air)
Mw = 380–2000
400 (in N2 )
Mw = 2000–6000
stability of the compounds increased with the ring number. Thus,
these interesting compounds can potentially be used to prepare
thermally stable materials.
Experimental
General Aspects
Conclusions
In summary, we have obtained the first heptacyclic laddersiloxane
and ladder polysilsesquioxanes. It was observed that the thermal
Me
Me
O
Si
Br
Preparative recycle-type HPLC was carried out using a JAI LC-908
high-performance liquid chromatograph with a Chemco 7-ODS-H
column (20 × 250 mm). Preparative recycle-type GPC was carried
Br
Si
O
Br
Br
O
Si
Si O
or
Me
Me
5
6
(cyclotetrasiloxane rings are presented as square for clarification)
244
Scheme 6. Preparation of ladder polysilsesquioxane.
www.interscience.wiley.com/journal/aoc
c 2010 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 241–246
Heptacyclic laddersiloxanes and ladder polysilsesquioxane
Figure 4. Infrared spectra of laddersiloxanes and cage silsesquioxanes.
out using a JAI LC-09 with a JAIGEL 1H+2H column (20 × 600 mm)
with THF as the solvent. The Fourier transform nuclear magnetic
resonance (NMR) spectra were obtained using a Jeol model L-500
(1 H at 500.00 MHz, 13 C at 125.65 MHz, and 29 Si at 99.25 MHz). The
chemical shifts were reported as d units (ppm) relative to SiMe4 ,
and the residual solvent peaks were considered as the standard.
Solid-state MAS NMR spectra were measured by a Bruker DMX-300
wide-bore spectrometer operating at 59.6 MHz. The spinning rate
was 3 kHz. Electron impact mass spectrometry was performed
with a Jeol JMS-DX302. The infrared spectra were measured with
a Shimadzu FTIR-8700.
Chlorination of Pentacyclic Laddersiloxanes
Appl. Organometal. Chem. 2010, 24, 241–246
Synthesis of Heptacyclic Laddersiloxanes
A solution of [i-PrPhSi(OH)]2 O [(R,S) (71% HPLC purity) 0.138 g,
0.399 mmol][6] in pyridine (1.5 ml) was added dropwise to a
solution of 2 (0.242 g, 0.193 mmol) in hexane (1.0 ml) at room
temperature. The mixture was stirred for 2 days. The reaction
mixture was added to saturated aqueous NH4 Cl and separated.
The separated aqueous phase was extracted with hexane. The
organic phase was washed with saturated aqueous NH4 Cl, dried
over anhydrous magnesium sulfate, and concentrated. The crude
product was separated by column chromatography (eluent:
hexane–Et2 O = 9 : 1) followed by separation with recycle-type
GPC (eluent: THF) to give heptacyclic laddersiloxanes (3) (22 mg,
6%).
c 2010 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
245
Extendible pentacyclic laddersiloxanes were prepared by the
method reported previously.[6] Hydrogen chloride was passed
through a solution of pentacyclic laddersiloxane 1 (0.33 g,
0.23 mmol, mixture of stereoisomers) and anhydrous aluminum
chloride (0.246 g, 1.8 mmol) in 7 ml of benzene for 1 h at room
temperature. Acetone was added to the reaction mixture to
quench aluminum chloride, and argon gas was bubbled. After
filtration of aluminum chloride, the filtrate was added to saturated
aqueous NH4 Cl and separated. The separated aqueous phase
was extracted with benzene. The organic phase was washed
with saturated aqueous NH4 Cl. The organic phase was dried
over anhydrous magnesium sulfate. Filtration and concentration
gave 9,11,21,23-tetrachloro-1,3,5,7,9,11,13,15,17,19,21,23-dodecaisopropylpentacyclo-[17.5.1.13,17 .15,15 .17,13 ]dodecasiloxanes (2)
(0.287g, 98%).
1 H NMR (CDCl ) δ 0.83–1.11 (m, 84H) ppm; MS (70 eV) m/z (%)
3
1207 [(M+ − C3 H5 , 100], 1205 (M+ − i-Pr, 48), 1161 (4), 1119 (3); IR
(NaCl) ν 2934, 2897, 2870, 1466, 1387, 1367, 1259, 1124, 1045, 999,
908, 889, 770, 737 cm−1 .
S. Chang et al.
29 Si NMR δ−33.22, −33.53, −33.36, −64.94, −65.00, −65.03,
−65.04, −65.07, −65.11, −66.07, −66.26 ppm; MS (70 eV) m/z (%)
1754 (M+ − C3 H6 , 30), 1711 (7), 1668 (5); IR (NaCl) 3071, 3051,
2926, 2868, 2855, 1466, 1429, 1385, 1364, 1259, 1121, 1069, 1038,
997, 920, 889, 773, 719, 700 cm−1 .
Synthesis of Ladder Polysilsesquioxane
4
cis–trans–cis-Tetraphenyltetramethylcyclotetrasiloxane[7]
(504 mg, 0.925 mmol) was placed in the sublimation apparatus
and cooled to −30 ◦ C under Ar. Bromine (778 mg, 4.86 mmol) was
then added with vigorous stirring. After 30 min, all volatiles were
slowly pumped off (0.4 mmHg), giving a light-yellow solid. Further
purification by sublimation was carried out at 0.2 mmHg with an oil
bath maintained at 75 ◦ C. The product was collected in a glove box,
and pure cis–trans–cis-tetrabromotetramethylcyclotetrasiloxane
(5) (437 mg, 85%) was obtained. Tetrabromide 5 (152 mg,
0.27 mmol) was exposed to air, HBr gas generated spontaneously,
and a white solid was obtained. This solid was dried in vacuo to
give ladder polysilsesquioxane (6) (85 mg, quant.).
29 Si NMR (MAS) δ−64.5 ppm; IR (KBr) ν 783, 1032, 1128, 1273,
2970 cm−1 .
[2] S. Hayashida, S. Imamura, J. Polym. Sci. A: Polym. Chem. 1995, 33,
55; E.–C. Lee, Y. Kimura, Polym. J. 1997, 29, 678; E.-C. Lee, Y. Kimura,
Polym. J. 1998, 30, 234; W.-Y. Chen, Y. Lin, K. P. Pramoda, K. X. Ma,
T. S. Shung, J. Polym. Sci. B: Polym. Phys. 2000, 38, 138; K. Deng,
T. Zhang, X. Zhang, A. Zhang, P. Xie, R. Zhang, Macromol. Chem.
Phys. 2006, 207, 404; X. Zhang, P. Xie, Z. Shen, J. Jiang, C. Zhu, H. Li,
T. Zhang, C. C. Han, L. Wan, S. Yan, R. Zhang, Angew. Chem. 2006,
118, 3184.
[3] M. Unno, A. Suto, K. Takada, H. Matsumoto, Bull. Chem. Soc. Jpn.
2000, 73, 215.
[4] M. Unno, A. Suto, H. Matsumoto, J. Am. Chem. Soc. 2002, 124, 1574.
[5] M. Unno,
R. Tanaka,
S. Tanaka,
T. Takeuchi,
S. Kyushin,
H. Matsumoto, Organometallics 2005, 24, 765.
[6] M. Unno, T. Matsumoto, H. Matsumoto, J. Organomet. Chem. 2007,
692, 307.
[7] M. Unno, S. Chang, H. Matsumoto, Bull. Chem. Soc. Jpn. 2005, 78,
1105.
[8] M. Unno, Y. Kawaguchi, Y. Kishimoto, H. Matsumoto, J. Am. Chem.
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[9] C. C. Yang, W. C. Chen, L. M. Chen, C. J. Wang, Proc. Natl Sci. Counc.
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[10] A. Saito, M. Itoh, European Patent 0786489-A1, 1997.
[11] H. Nakashima, Japanese Patent Kokai-H3-227321, 1991.
References
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246
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Copyright Appl. Organometal. Chem. 2010, 24, 241–246
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