A new, higher yielding synthetic route towards dodecaphenyl cage silsesquioxanes: Synthesis and mechanistic insights

Article abstract of DOI:10.1039/c2dt30659j

Cage dodecaphenylsilsesquioxane (T12-Phenyl) was synthesized in a one batch, mildly basic aqueous solution under room temperature conditions using a trialkoxysilane precursor. Significant improvements in synthetic yield (>95%) were observed compared with previous reports. Kinetic studies of the hydrolysis of phenyltrimethoxysilane were conducted and the condensation was monitored by ²⁹Si NMR which revealed the presence of a transient, intermediary T1 species as the pathway to dodecaphenylsilsesquioxane spherulites, and the tendency for T12 structures over T8, T10, and other substructures was explained through MM2 simulations.

Full text of DOI:10.1039/c2dt30659j

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PAPER
A new, higher yielding synthetic route towards dodecaphenyl cage
silsesquioxanes: synthesis and mechanistic insights†
Albert S. Lee,a,b Seung-Sock Choi,a He Seung Lee,a Kyung-Youl Baeka,b and Seung Sang Hwang*a,b
Received 22nd March 2012, Accepted 11th June 2012
DOI: 10.1039/c2dt30659j
Cage dodecaphenylsilsesquioxane (T12-Phenyl) was synthesized in a one batch, mildly basic aqueous
solution under room temperature conditions using a trialkoxysilane precursor. Significant improvements in
synthetic yield (>95%) were observed compared with previous reports. Kinetic studies of the hydrolysis
of phenyltrimethoxysilane were conducted and the condensation was monitored by 29Si NMR
which revealed the presence of a transient, intermediary T1 species as the pathway to
dodecaphenylsilsesquioxane spherulites, and the tendency for T12 structures over T8, T10, and other
substructures was explained through MM2 simulations.
these aqueous conditions has made control of the hydrolysis–
condensation equilibrium difficult. The use of alkoxy silane
Introduction
Cage silsesquioxane compounds, of molecular formula (RSiO3/2)n,
where n = 6, 8, 10, 12, have received considerable interest as
building blocks for organic–inorganic hybrid materials due to
their nano-sized 3-dimensional structure having numerous
organic arms as platforms for further chemical modification.1–12
Advantageous properties such as excellent thermal stability,
optical transparency, chemical stability, and excellent mechanical
properties1,2 have brought about applications in a plethora of
research fields including microporous materials for chromato-
graphy applications,4 light emitting diodes,5,6 low dielectric con-
stant materials for semiconductor devices,7,8 and nanocomposite
hybrid materials.9 In particular, phenylsilsesquioxanes, known
for their superior thermal stability, have been the center of atten-
tion since Brown’s comprehensive study in the 1960s,13 and
Andrianov’s studies in the 1970s.14
However, syntheses of singular isomers of cage silsesquioxane
compounds have been arduous15,16 due the many variables
involved, including monomer type, catalyst species, water con-
centration, solvent, and reaction temperature. The hydrolysis of
trichlorosilane monomers under aqueous conditions has been
shown to produce cage, partial cage, and sol-type silsesquiox-
anes,13 but low yields, long reaction times exceeding months,
high temperatures, and additional steps16 to isolate the different
isomers have rendered these syntheses tedious for large scale
production. Moreover, the production of hydrochloric acid under
monomers has made cage silsesquioxane syntheses much milder,
but with similar drawbacks of having multiple isomeric products
in low yields,2 with the preferential formation of the more
common octa-substituted T8 compound.2,15 Additionally, the
hydrolysis–condensation reactions of trialkoxysilanes yield pro-
ducts that depend highly on the catalyst. Acid-catalyzed reactions
often favor hydrolysis, with incomplete condensations being the
problem in obtaining high yields of well-defined cage structures.
Similarly, base-catalyzed reaction conditions facilitate highly
condensed products, but with unhydrolyzed byproducts that are
difficult to separate.
Today, while many synthetic recipes exist for different cage
silsesquioxane isomers,2,18–20 the direct synthesis of the T12
cage formation has been overlooked. In this study, we report the
facile synthesis of isomerically pure dodecaphenylsilsesquioxane
(T12-Phenyl) in high yield under mildly basic aqueous con-
ditions at room temperature. Kinetic studies, along with 29Si
NMR spectroscopy helped elucidate the mechanism of cage
structure formation during the hydrolysis–condensation reaction
(Fig. 1).
aNanohybrids Research Center, Korea Institute of Science and
Technology, Hwarangno 14-gil 5, Seong-Buk Gu, Seoul 136-791, Korea.
E-mail: sshwang@kist.re.kr
bNanomaterials Science and Engineering, University of Science and
Technology, 217 Gajungro, 176 Gajung-dong, Yuseong-Gu, Daejeon,
Korea 305-333
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt30659j
Fig. 1 Synthetic scheme for T12-Phenyl.
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Characterization
FT-IR spectra were measured using Perkin-Elmer FT-IR system
(Spectrum-GX) using solvent cast films on KBr pellets. MAL-
DI-TOF MS were recorded using a 4800 MALDI TOF/TOF ana-
lyzer (Applied Biosystems Inc) with dithranol and aqueous NaCl
1
as the matrix and ionization aid, respectively. H NMR spectra
and 29Si NMR spectra were recorded in CDCl3 or THF-d8 at
25 °C on a Varian Unity INOVA (1H: 600 MHz, 13C:
150.66 MHz, 29Si: 119.2 MHz) with a pulse delay time of 10 s.
Experimental
Synthesis of cage dodecaphenylsilsesquioxane (T12-Phenyl)
Typically, a solution of potassium carbonate (40 mg, 0.30 mmol)
dissolved in distilled water (4.8 ml, 0.27 mol), with THF (80 g)
as solvent was prepared. To this solution, phenyltrimethoxysilane
(PTMS) (15.86 g, 80 mmol) was added dropwise. The reaction
mixture was stirred vigorously for 14 days at room temperature,
or until a white precipitate formed. Afterwards, the volatiles
were evaporated under reduced pressure and the white resinous
material was dissolved in methylene chloride and extracted with
deionized water several times. After drying the organic layer
with magnesium sulfate and the evaporation of methylene chlor-
ide, white crystals (11 g, 98% yield) were obtained. Recrystalli-
zation using a 9 : 1 methylene chloride : ethanol cosolvent
system yielded spectroscopically pure dodecaphenylsilsesquiox-
ane (T12-Phenyl) in excellent yield (>95%). Reaction times
were defined by the precipitation time as an indication of the for-
mation of the T12-phenyl compound. The product was character-
ized using FT-IR, 1H NMR, 13C NMR, 29Si NMR,
MALDI-TOF MS, and WAXD. 1H NMR: 7.08–7.8 ppm, (m,
5H); 13C NMR: 127.6 ppm, 129.4 ppm, 130.2 ppm, 135.9 ppm;
29Si NMR: −78.2 ppm, −80.1 ppm.
Fig. 2 Isolated yields of T12-Phenyl by varying PTMS concentration.
Fig. 3 MALDI-TOF mass spectrum of T12-Phenyl.
facilitate the selective formation of T12-Phenyl over other sub-
structures. Other similar studies that have shown improvements
in yield have either used phase transfer catalysts15 or obtained a
mixture of isomeric products T8, T10, and T12.16 The utility of
this synthetic method is shown through the use of mild trialkoxy-
silane starting materials and room temperature conditions to
obtain high yields and isomerically pure product.
Kinetic studies
FT-IR spectrum (ESI Fig. 1†) confirmed the structure of the
T12-Phenyl compound. While ladder-like and sol-gel derived
materials have two sharp Si–O–Si siloxane stretching vibration
modes ranging from 1100–1250 cm−1, cage silsesquioxanes
such as T12 have only one peak.20 The singular, sharp peak at
1145 cm−1 of the obtained product was assigned to the Si–O–Si
stretching vibration mode of cage silsesquioxane.
Hydrolysis of phenyltrimethoxysilane. The same synthesis
was carried out as above, except for the use of D2O as the water
source and THF-d8 as the solvent. At various time intervals, the
reaction mixture was quenched with an ice bath and small ali-
quots were taken for 1H NMR studies.
However, because FT-IR cannot distinguish between T8 and
T12 isomers, MALDI-TOF MS and WAXD studies were con-
ducted. MALDI-TOF results (Fig. 3) confirmed that the syn-
thesized silsesquioxane was obtained as a singular isomeric T12
structure. Moreover, wide angle X-ray diffraction patterns of the
synthesized T12-Phenyl (ESI Fig. 2†) gave crystal lattice peaks
that agreed very well with those reported elsewhere in the litera-
Results and discussion
In this study, a literature procedure17 for the synthesis of high
molecular weight polysilsesquioxanes was modified for the pre-
ferential formation of T12-Phenyl. By diluting the initial reaction
mixture by incrementally increasing the amount of solvent to
give a molar concentration of 0.35 M from 8.89 M, the selective
formation of T12-Phenyl in high yields (>95%) was observed
(Fig. 2). The yields represented in Fig. 2 depict the maximum
isolated yield of T12-Phenyl after all cage-rearrangement reac-
1
ture and of product’s obtained commercially.23 Furthermore, H
NMR and 13C NMR spectra of the obtained T12-Phenyl are
reported (ESI Fig. 3 and 4†).
Kinetic studies were conducted not only to determine the
hydrolysis–condensation rates, but to identify which intermedi-
ary species exist, if any. Fig. 4 shows the hydrolysis of phenyltri-
methoxysilane at various time intervals. As shown, the methoxy
groups of phenyltrimethoxysilane were hydrolyzed to deuterated
tions have occurred, confirmed by the H and 29Si NMR. The
1
effect of increasing the relative amount of solvent in this syn-
thesis was the increase in reaction time from 5 days to a
maximum of 14 days. Through this simple modification of the
reaction conditions, a slow condensation rate was induced to
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Fig. 6 Mechanism studies of T12-Phenyl using 29Si NMR at (a)
0 days, (b) 3 days (c) 10 days (d) 14 days (e) recrystallized T12-Phenyl.
Fig. 4 1H NMR spectra showing the hydrolysis of phenyltrimethoxysi-
lane under basic, aqueous conditions at (a) 0 h, (b) 4 h, (c) 8 h, and (d) 24 h.
decreased with ongoing intermolecular condensation to give T3
structured species attributed to the cage and cage-like structures
at around −75 to −80 ppm. At 14 days, full condensation was
achieved and T12-Phenyl was obtained in excellent yield. The
detection of this T1 species at −62.4 ppm is interesting because
previous reports have shown that the tendency for the formation
of T8 cage silsesquioxanes over the T12 analogues was due to
the stability of the T2 tetraol species Si4O4R4(OH)4,2,23–25 which
have been synthesized by several groups.19,21
From these 29Si NMR studies, a proposed reaction pathway
for T8 and T12 is illustrated in Fig. 7. While the reaction
pathway for T8 compounds has been proposed elsewhere2,22 that
the tetraol T2 species is an intermediate, our results indicated
that the reaction pathway for T12 compounds is through the T1
species. While several efforts were made to detect larger molecu-
lar weight oligomers preceding the T12-Phenyl product, no other
intermediary species were able to be detected other than the T1
species. Moreover, comprehensive simulation studies22 have
shown that T1 dimer formation is energetically favorable and is
a transition state stabilized by hydrogen bonding. Thus we were
able to conclude that the T1 tetraol species was the major inter-
mediate in the reaction pathway of T12-Phenyl.
The amount of solvent was found to be critical for improving
the yield of T12-Phenyl. The effect of increasing the solvent
amount is that the intermolecular condensation of T1 does not
occur completely between the two adjacent silanol groups
leading to the T2 tetraol species, but rather condensation occurs
relatively slowly compared to that of T8-Phenyl. This decrease
in the kinetic rate of condensation is to be expected given the
relatively lower solution concentration.
Fig. 5 Hydrolysis kinetics of PTMS under basic, aqueous conditions.
silanols, giving quantitative byproducts of deuterated methanol.
1H NMR showed that all the methoxy groups were hydrolyzed at
24 h. Moreover, the rate of hydrolysis was found to be pseudo-
first order with respect to phenyltrimethoxysilane under these
conditions (Fig. 5). These results were a confirmation of theoreti-
cal studies conducted by Kudo and Gordon,23 which detailed the
rate-determining step as the first hydrolysis of a trialkoxysilane
under quantitative amounts of water.
On the basis of kinetics studies of the hydrolysis of phenyltri-
methoxysilane, the condensation of silanols groups was moni-
tored by 29Si NMR (Fig. 6). From 24 h to 72 h, condensation
between silanols occurred in which a detectable T1 species was
observed (Fig. 6b). While several efforts to isolate this T1
species through liquid chromatography and silylation were made,
the kinetic instability of the silanol groups made it difficult to
isolate these species. This T1 species at −62.4 ppm was tenta-
tively assigned as the T1 dimer, Si2OPh2(OH)4, which was first
reported by Brown,26 and later synthesized and isolated by
Seto.27 At 10 days, the intensity of the T1 peak substantially
Moreover, the relatively slow rate of condensation can explain
the propensity for T12 cage formation over T8 or T10 cage
structures in this mild synthesis, as a slow rate of condensation/
rearrangement reactions would lead to the most energetically
favored product. MM2 simulations of the models depicted
in Fig. 8 of the three most common cage silsesquioxanes T8,
T10, and T12 showed that the T12 total energy was
−41.8978 kcal mol−1, a comparatively low value compared to
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Fig. 7 Proposed reaction pathways for T8 and T12-Phenyl cage silsesquioxanes.
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Conclusion
A new and high yielding synthesis of dodecaphenylsilsesquiox-
ane (T12-Phenyl) was carried out using potassium carbonate as
base. The isomerically pure product was obtained in significantly
greater yields (>95%) than in previous studies utilizing a facile
synthetic method. The synthesized T12-Phenyl was spectrosco-
pically pure compared with those synthesized elsewhere and
1
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This research was supported by a grant from the Fundamental
R&D Program for Core Technology of Materials funded by the
Ministry of Knowledge Economy, Republic of Korea and par-
tially by the Nanohybrids Research Center of Korea Institute of
Science and Technology (KIST).
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This journal is © The Royal Society of Chemistry 2012