Synthesis, functionalization and properties of incompletely condensed half cube silsesquioxanes as a potential route to nanoscale Janus particles

Article abstract of DOI:10.1016/j.crci.2009.10.007

Multiple literature reports describe the synthesis of [RSiO(OH)]₄ or [RSiO(ONa)]₄ compounds. Surprisingly, in the majority of cases the OH or ONa groups all lie on the same face of the cyclomers or half cubes. Consequently, it appears possible to couple R containing half cubes with R' containing half cubes to form bi-functional (Janus) silsesquioxane cages or cubes. We report here synthesis of [PhSiO(ONa)]₄ and [p-IPhSiO(ONa)]₄ half cubes. We thereafter discuss efforts to react the [PhSiO(ONa)]₄ salt with (R = Me, vinyl, and cyclohexyl) RSiCl3 in MeOH to produce the compounds {PhSiO[OSi(OMe)2R]}₄ species which were characterized using traditional spectroscopic procedures. These compounds were then subjected to acid catalyzed hydrolysis tohydrolytically remove the OMe groups to generate Janus cubes. While it is possible to isolate the target compounds, which were also characterized in detail, the yields were lower than expected perhaps because of competitive hydrolytic cleavage of the Si-O-SiR(OMe)₂ linkage.

Full text of DOI:10.1016/j.crci.2009.10.007

C. R. Chimie 13 (2010) 270–281
Full paper/ Mémoire
Synthesis, functionalization and properties of incompletely
condensed ‘‘half cube’’ silsesquioxanes as a potential
route to nanoscale Janus particles
Michael Z. Asuncion a, Marco Ronchi a, Haya Abu-Seir b, Richard M. Laine a, ,b
*
a Macromolecular Science and Engineering, Department of Materials Science and Engineering,
University of Michigan, Ann Arbor, Michigan 48109-2136, United States
b Dipartimento di Chimica Inorganica Metallorganica, Analitica dell’Universita à di Milano, Unita ‘di Ricerca dell’INSTM,
Centro di Eccellenza CIMAINA dell’Universita à di Milano e Istituto di Scienze e Tecnologie Molecolari del CNR (ISTM),
20133 Milano, Italy
Received 1 February 2009; accepted after revision 6 October 2009
Available online 12 November 2009
Abstract
Multiple literature reports describe the synthesis of [RSiO(OH)]4 or [RSiO(ONa)]4 compounds. Surprisingly, in the majority of
cases the OH or ONa groups all lie on the same face of the cyclomers or half cubes. Consequently, it appears possible to couple R
containing half cubes with R’ containing half cubes to form bi-functional (Janus) silsesquioxane cages or cubes. We report here
synthesis of [PhSiO(ONa)]4 and [p-IPhSiO(ONa)]4 half cubes. We thereafter discuss efforts to react the [PhSiO(ONa)]4 salt with
(R = Me, vinyl, and cyclohexyl) RSiCl3 in MeOH to produce the compounds {PhSiO[OSi(OMe)2R]}4 species which were
characterized using traditional spectroscopic procedures. These compounds were then subjected to acid catalyzed hydrolysis
tohydrolytically remove the OMe groups to generate Janus cubes. While it is possible to isolate the target compounds, which were
also characterized in detail, the yields were lower than expected perhaps because of competitive hydrolytic cleavage of the Si-O-
SiR(OMe)2 linkage. To cite this article: M.Z. Asuncion et al., C. R. Chimie 13 (2010).
# 2009 Published by Elsevier Masson SAS on behalf of Académie des sciences.
Keywords: Silsesquioxanses; Bifunctional; Janus; Nanoparticles
1. Introduction
phenomena compared to the macroscale. In fact, the
essential challenge is to control matter at these
dimensions because the ability to tailor global proper-
ties precisely is dependent on manipulating component
organization at the finest length scales. Indeed the
construction of materials nanometer-by-nanometer
should lead to the design of a variety of materials with
well-defined nanoarchitectures and predictable beha-
viors.
The synthesis and perfect assembly of 2- and 3-D
structures from molecular components (nanobuilding
blocks) is of immense current interest due to the
potential to realize novel properties in nanosized and
nanostructured materials, since materials reduced to the
nanoscale often exhibit very different properties or
Consequently highly symmetrical nanobuilding
blocks are required, particularly those that offer both
diverse functionality coupled with ease of synthesis,
* Corresponding author.
E-mail address: talsdad@umich.edu (R.M. Laine).
1631-0748/$ – see front matter # 2009 Published by Elsevier Masson SAS on behalf of Académie des sciences.
doi:10.1016/j.crci.2009.10.007

M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
271
since it follows that breaks in periodicity (defects) in
assembled 2- and 3-D structures should be minimized
with higher component symmetry. Since the energy
required to create a defect is offset by a gain in entropy,
in theory defects can be minimized by using highly
symmetrical and well-ordered components that mini-
mize the entropy of a given system. It can be expected
then that misaligned but highly symmetrical compo-
nents would require the least energy (associated with
the least movement) to reorient and align with other
assembled components. Accordingly, cubic structures
(offering the highest symmetry) would also require the
least energy to assemble [1]. Note that defects will still
occur due to temperature-dependent entropic effects
(TDS).
Still another perhaps more important issue is the fact
that assembly is often driven by kinetics rather than
thermodynamics. Thus, for example in studies on the
assembly of supramolecular structures, it is possible to
actually find assembled structures wherein on edge of
the structure is completely random and the opposite
side is perfectly ordered [2]. Alternately, if the energy
of two similar structures is very close, then one can
observe stacking faults resulting from twinning [3]. In a
similar vein, if surface orientation energies are very
similar one can also drive the assembly of two different
polymorphs at surfaces that promote assembly [4].
Finally, control of solvent effects assembly conditions
and temperature can drastically change the types of
assembly observed [5]. Thus, although high symmetry
is the best place to start, it is important to recognize that
this alone can only be considered an optimal starting
point but not a guarantee of perfectly controlled
assembly.
Cubic silsesquioxanes are one of the few groups of
3-D molecules with structures that offer high symmetry,
ease of synthesis and/or modification, and octafunc-
tionality such that each octant in Cartesian space
contains one functional group. It is the positioning of
functional groups, the variety possible, and their nm size
that provide unique opportunities to build nanocompo-
sites in 1-, 2- or 3-D, one nm at a time [6–37]. In
addition, the core adds the rigidity and heat capacity of
silica (in essence ‘‘the smallest single crystal sand
particle’’) making these compounds thermally robust at
elevated temperatures.
Fig. 1. Suggested structures of a: [RSiO(OH)]4 ‘‘half cube’’; b: [PhSiO(ONa)]4 half cube salt; c: [p-IPhSiO(ONa)]4 half cube salt.

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M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
As nanoconstruction sites, completely condensed
2. Experimental section
silsesquioxanes can be further functionalized by direct
chemical modification of the organic moieties to form
larger, well-defined structures. However, similar ‘‘bot-
tom-up’’ construction possibilities may also exist by
employing incompletely condensed silsesquioxanes as
nanobuilding blocks. Recently, incompletely condensed
silsesquioxanes ((RSiO1.5)a(H2O)0.5b, where a, b, n are
integers: a + b = 2n, b < a + 2) have attracted attention
as models for silica-supported systems, ligands for
homogeneous catalysts, and comonomers in silses-
quioxane-siloxane polymers [38]. Of particular interest
to us is the all cis-(RSiO1.5)4(H2O)2 ‘‘half cube’’
(Fig. 1a), since it represents a proposed intermediate in
the formation of completely condensed cubic silses-
quioxanes [39,40].
2.1. Materials
All trichlorosilanes were purchased from Gelest, Inc.
and used without further purification. All other
chemicals were purchased from Fisher or Aldrich and
used as received. Octaphenyl-silsesquioxane (OPS) and
octaiodophenylsilsesquioxane (I8OPS) were synthe-
sized following methods described in the literature
[42,43]. All work was performed under N2 unless
otherwise stated.
2.2. Analytical methods
2.2.1. Gel permeation chromatography
In principle, such incompletely condensed half cube
silsesquioxanes may provide potential access to
‘‘perfectly’’ defined surfaces via ‘‘Janus’’ (two-faced)
cubes and may also help explain the unique emission
characteristics of stilbene silsesquioxane derivatives,
which are red-shifted up to 80 nm (0.75 eV) [41]. The
synthesis of a Janus cube such as suggested by Fig. 2
could help support (or refute) the existence of 3-D
excited state conjugation through the center of the
silsesquioxane cage, which DFT HOMO-LUMO
calculations have deemed quite possible. [42].
In addition, bi-functional Janus cubes in principle
can be tailored to modify surfaces and/or thin films with
nanometer-length control of individual layers. In
addition, one could also introduce bifunctionality
allowing layer-by-layer coatings of hydrophobic/hydro-
philic or hard/flexible layers with defined length scales.
Janus cubes could also serve as interfaces between
different domains or to guide self-assembly wherever
complementary or dissimilar chemical functionalities
are used. Here we describe efforts to develop routes to
Janus cubes through studies on the functionalization of
compounds like that shown in Fig. 1b.
All GPC analyses were done on a Waters 440 system
equipped with Waters Styragel columns (7.8 Â 300, HT
0.5, 2, 3, 4) with RI detection using Optilab DSP
interferometric refractometer and THF as solvent. The
system was calibrated using polystyrene standards and
toluene as reference.
2.2.2. NMR analyses
All 1H and 13C-NMR analyses were done in CDCl3
(or CD3OD) and recorded on a Varian INOVA 400
spectrometer. 1H-NMR spectra were collected at
400.0 MHz using a 6000 Hz spectral width, a relaxation
delay of 3.5 s, a pulse width of 388, 30 k data points, and
CHCl3 (7.27 ppm) as an internal reference. 13C-NMR
spectra were collected at 100 MHz using a 25000 Hz
spectra width, a relaxation delay of 1.5 s, 75 k data
points, a pulse width of 408, and CDCl3 (77.23 ppm) as
the internal reference.
2.2.3. Thermal gravimetric analysis (TGA)
Thermal stabilities of materials under N2 or air were
examined using a 2960 simultaneous DTA-TGA
Instrument (TA Instruments, Inc., New Castle, DE).
Samples (5–10 mg) were loaded in platinum pans and
ramped to 1000 8C (10 8C/min/N2). The N2 or airflow
rate was 60 mL/min.
2.2.4. FTIR spectra
Diffuse reflectance Fourier transform (DRIFT)
spectra were recorded on a Mattson Galaxy Series
FTIR 3000 spectrometer (Mattson Instruments, Inc.,
Madison, WI). Optical grade, random cuttings of KBr
(International Crystal Laboratories, Garfield, NJ) were
ground, with 1.0 wt% of the sample to be analyzed. For
DRIFT analysis, samples were packed firmly and
leveled off at the upper edge to provide a smooth
Fig. 2. Proposed structure of tetrastilbene cubic silsesquioxane
[StilbeneSiO1.5]4[RSiO1.5].

M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
273
1
surface. The FTIR sample chamber was flushed
continuously with N2 prior to data acquisition in the
range 4000–400 cm-1.
Characterization data: H NMR (400 MHz, CDCl3):
0.2 (36H, s, CH3), 7.1 (4H, m, Ar-H), 7.3 (4H, m,
Ar-H), 7.4 (8H, m, Ar-H) ppm.
2.2.5. Melting point
2.3.3. Synthesis of tetra(p-iodo)phenyl sodium salt
from I8OPS
Melting point determinations were performed on
samples using a Mel-Temp 3.0 (Laboratory Devices,
Inc. Dubuque, IA) with a ramp rate of 5 8C/min.
To a dry 250 mL round bottom flask under N2 and
equipped with a magnetic stir bar and reflux condenser
was added 3.40 g I8OPS (1.66 mmol, 13.28 mmol Si),
0.53 g NaOH (13.25 mmol), and 100 mL n-BuOH. The
solution was stirred at 110 8C for 12 h, during which
time it turns colorless after 40 mins. The hot solution
was gravity filtered to remove the insolubles and
allowed to cool to room temperature. The solvent was
removed slowly under reduced pressure at 35 8C for
ꢀ5 h at which point white needle-like crystals form.
The solid was filtered and dried in vacuo at 50 8C for
6 h, giving 2.82 g [(74%) – Fig. 1c]. Characterization
2.2.6. Matrix-assisted laser-desorption/Time-of-
flight spectrometry
MALDI-TOF was performed on a Micromass
TofSpec-2E equipped with a 337 nm nitrogen laser in
positive ion reflection mode using poly(ethylene glycol)
as the calibration standard, 1,8,9-anthracenetriol as the
matrix, and AgNO3 as the ion source. Samples were
prepared by mixing solutions of five parts dithranol
(10 mg/mL in THF), five parts sample (1 mg/mL in
THF) and one part AgNO3(10 mg/mL in water) and
blotting the mixture on the target plate.
1
data: H NMR (400 MHz, CD3OD): 7.3 (2H, m, meta
Ar-H), 7.6 (2H, m, ortho Ar-H).
2.3. Synthetic methods
2.3.4. Synthesis of tetramethyltetraphenyl
dimethoxy derivative
2.3.1. Synthesis of tetraphenyl sodium salt from
OPS
To a 500 mL round bottom flask under N2 and
equipped with magnetic stir bar was added a suspension
of 5.00 g of tetraphenyl sodium salt (8.22 mmol) in
100 mL MeOH. 4.25 mL of CH3SiCl3 (36.2 mmol) was
dissolved in 100 mL of hexane and added to the MeOH
suspension via addition funnel over 30 minutes. The
heterogeneous reaction mixture was allowed to stir for
24 h at room temperature at which point insoluble NaCl
formed. The salt was filtered from the reaction mixture
and the hexane layer was separated, dried over Na2SO4,
and rotary evaporated to give viscous light-yellow oil.
The oil was dried in vacuo at 80 8C for 6 h to give 6.20 g
(82%). Characterization data: 1H NMR (400 MHz,
CDCD3): 0.4 (3H, s, CH3), 3.2 (6H, s, OCH3), 7.2 (3H,
m, Ar-H); 7.6 (2H, m, Ar-H) ppm; GPC: Mn 658; Mw
684; PDI 1.09.
To a dry 250 mL round bottom flask under N2 and
equipped with a magnetic stir bar and reflux condenser
was added 5.00 g OPS (4.84 mmol, 38.8 mmol Si),
1.71 g NaOH (42.6 mmol), and 150 mL n-BuOH. The
solution was stirred at 110 8C for 24–48 h, during which
time it turns yellowish green. The hot solution was
gravity filtered to remove the insolubles and allowed to
cool to room temperature. The solution was then placed
ina freezerandafter24 h white needle-like crystalsform.
The solid was filtered and dried in vacuo at 50 8C for 6 h,
giving4.97 g[(80%)–Fig. 1b]. Characterizationdata:1H
NMR (400 MHz, DMSO): 7.1 (1H, m, para Ar-H), 7.2
(2H, m, meta Ar-H), 7.7 (2H, m, ortho Ar-H).
2.3.2. Reaction of tetraphenyl sodium salt and
Me3SiCl3
2.3.5. Hydrolysis of tetramethyltetraphenyl
To a 250 mL round bottom flask under N2 and
equipped with magnetic stir bar was added 8.23 mL
Me3SiCl (65.0 mmol) and 2.83 mL pyridine
(35.0 mmol) in 50 mL of toluene. 5.00 g of tetraphenyl
sodium salt (8.22 mol) was added in one portion and
the resulting mixture was refluxed for 1.5 h. The
mixture was allowed to cool to room temperature and
the insolubles were gravity filtered. The toluene filtrate
was washed with water and dried over sodium sulfate.
Benzene was removed in vacuo at 50 8C for 12 h
to give 6.23 g (90.0%) of white crystalline solid.
dimethoxy derivative
To a 25 mL round bottom flask under N2 and
equipped with magnetic stir bar was added 0.500 g
(0.516 mmol) of tetramethyltetraphenyl dimethoxy
derivative dissolved in 10 mL of hexane. 5 mL of
H2O and 1 mL of concentrated HCl were added
dropwise to the solution and allowed to react 24 h at
room temperature. The white insoluble precipitate
(0.344 g, 85%) was filtered and dried in vacuo at
80 8C for 12 h. Characterization data: TGA (air,
1000 8C): Found 61.2%; Calculated 61.2%; Td5%

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M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
450 8C; IR: C = H (3048–2939), CH3 (2838), C = C (Ar
dropwise to the solution and allowed to react 24 h at
room temperature. The white insoluble precipitate
(0.377 g, 92%) was filtered and dried in vacuo at 80 8C
for 12 h. Characterization data: TGA (air, 1000 8C):
Found 51.5%; Calculated 57.7%; Td5% 424 8C; IR:
C = H (3048–2950), C = C (1630), C = C (Ar ring,
1593), Si-O (1134), cm-1; MALDI-TOF: m/z (Ag+
adduct) = 755, 900, 941, 1060, 1080, 1130, 1180, 1230
[AgSi8O12(C6H5)4 (C2H3)4) 941].
ring, 1591), Si-CH3 (1269), Si-O(1132) cm-1.
2.3.6. Bromination of tetramethyltetraphenyl Janus
cube
To a 50 ml Schlenk flask under N2 and equipped with
a magnetic stir bar was added granular iron (0.060 g,
1.00 mmol), tetramethyl tetraphenyl silsesquioxane
(2.00 g, 2.56 mmol), and CH2Cl2 (15 mL). The suspen-
sion was cooled to 0 8C and bromine (3.30 g,
21.0 mmol) was slowly added. The suspension was
stirred for two days and then more bromine (0.1 mL)
and iron (0.02 g) were added and stirred for an
additional two days. The suspension was filtered and
the organic layer was washed with a sodium bicarbonate
solution and then dried over Na2SO4. Solvent was
removed under reduced pressure and the solid was
collected, washed with water (3 Â 10 mL), acetone
(3 Â 10 mL) and diethylether (3 Â 10 mL) and dried in
vacuo to give 1.2 g as white solid (64%). Characteriza-
tion data: IR: C = H (2870), Si-C (1274), Si-O (1142,
1041), cm-1; MALDI-TOF: m/z (Ag+ adduct) = 897
[Si8O12(C6H5)3(CH3)4], 970 [Si8O12(C6H4Br)(C6H5)3
(CH3)4], 1205 [Si8O12(C6H4Br)4(CH3)4].
2.3.9. Synthesis of tetracyclohexyltetraphenyl
dimethoxy derivative
To a 500 mL round bottom flask under N2 and
equipped with magnetic stir bar was added a suspension
of 5.00 g of tetraphenyl sodium salt (8.22 mmol) in
100 mL MeOH. 6.45 mL of C6H9SiCl3 (36.2 mmol)
was dissolved in 100 mL of hexane and added to the
MeOH suspension via addition funnel over 30 min. The
heterogeneous reaction mixture was allowed to stir for
24 h at room temperature at which point insoluble NaCl
formed. The salt was filtered from the reaction mixture
and the hexane layer was separated, dried over Na2SO4,
and rotary evaporated to give a viscous light-yellow oil.
The oil was dried in vacuo at 80 8C for 6 h to give 7.76 g
(76%). Characterization data: 1H NMR (400 MHz,
CDCl3): 0.8 (3H, m, aliphatic-H), 1.2 (4H, m, aliphatic-
H), 1.7 (4H, m, aliphatic-H), 3.5 (7H, m, OCH3), 7.4
(2H, m, Ar-H), 7.7 (3H, m, Ar-H) ppm; GPC: Mn 934;
Mw 1037; PDI 1.18.
2.3.7. Synthesis of tetraphenyltetravinyl dimethoxy
derivative
To a 500 mL round bottom flask under N2 and
equipped with magnetic stir bar was added a suspension
of 5.00 g of tetraphenyl sodium salt (8.22 mmol) in
100 mL MeOH. About 4.60 mL of C2H3SiCl3
(36.2 mmol) was dissolved in 100 mL of hexane and
added to the MeOH suspension via addition funnel over
30 min. The heterogeneous reaction mixture was
allowed to stir for 24 h at room temperature at which
point insoluble NaCl formed. The salt was filtered from
the reaction mixture and the MeOH layer was separated,
dried over Na2SO4, and rotary evaporated to give a
viscous colorless oil. The oil was dried in vacuo at 80 8C
for 6 h to give a white crystalline solid 6.91 g (87%).
3. Results and discussion
The objectives of the current study are to develop
straightforward routes to half cube silsesquioxanes that
in turn may be tailored by simple chemistry. The
resulting modified half cubes could then be hydrolyzed
to form fully condensed Janus cubes with different
moieties on each defined ‘‘face’’ of the cube. For
example, any effort to functionalize just four of the
eight p-iodo groups in I8OPS, [43] see reaction (3)
below, would lead not only to a statistical distribution of
functional groups at the corners rather than functiona-
lizing only one side, it would also lead to sets of
products where the average degree of substitution
would be four.
1
Characterization data: H NMR (400 MHz, CD3OD):
3.2 (6H, s, OCH3), 5.9 (3H, m, -CH = CH2), 7.2 (3H, m,
Ar-H), 7.6 (2H, m, Ar-H) ppm; GPC: Mn 705; Mw 741;
PDI 1.05.
2.3.8. Hydrolysis of tetraphenyltetravinyl
dimethoxy derivative
Thus, one can only expect to form a true Janus cube
(where each face has one specific functionality)
exclusively via a half cube intermediate. Previous work
[39,43–48] by others and members of our team [44]
focused on using acid-catalyzed hydrolysis of aromatic
trichloro- or triethoxysilanes to form half cubes as
suggested in reaction (1).
To a 25 mL round bottom flask under N2 and
equipped with magnetic stir bar was added 0.500 g
(0.492 mmol) of tetraphenyltetravinyl dimethoxy deri-
vative dissolved in 10 mL of MeOH. Five millitres of
H2O and 1 mL of concentrated HCl were added

M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
275
However, the susceptibility of incompletely con-
densed half cubes (Fig. 1a or the product of reaction (1))
to further condensation even at room temperature in the
solid state severely limits their utility and makes
subsequent modification of the phenyl groups (i.e. by
electrophilic aromatic substitution) essentially impos-
sible without encountering extensive undesirable side
reactions. In the past, the incorporation of tailorable
moieties on the half cube required prior functionaliza-
tion of the starting monosilane via multiple step
reactions involving both air and moisture sensitive
intermediates [44]. We report here an alternative route
that permits access to desired perfectly functionalized
half cube intermediates and initial efforts to produce
Janus cages.
A similar approach was extended to the NaOH
cleavage of I8OPS to produce [p-IPhSiO(ONa)]4,
Fig. 1c and reaction (3), which incorporates a
modifiable para iodo on the aromatic ring. This
compound is readily synthesized in one step in high
yield (see experimental section) and eliminates the need
for Grignard modification of a triethoxysilane. The
structure of the I4Ph4 sodium salt was confirmed by 1H
NMR and the absence of a para aromatic proton (due to
p-iodo substitution) is observed in the spectrum. A
single crystal XRD data set was collected and while the
data at this time are not suitable for publication, the
structure clearly shows the I-Ph-groups entirely cis on
one face and the half cube in a distorted boat
conformation [45,47].
As shown by Feher et al., [39] Shchegolikhina et al.
[46] and Pizzotti et al. [44], these salts react
quantitatively with in the presence of an amine base.
In principle, reaction of the nucleophilic oxygen with
produces a trimethylsilyl ‘‘face’’ that protects the half
cube from further hydrolysis. This would allow
subsequent modification of the iodophenyl group by
various Pd-catalyzed coupling reactions [43] (Heck,
Sonogashira, Suzuki, etc.) and exclude unwanted side
reactions. In principle the trimethylsilyl protecting
4. Synthesis of tetra(p-iodo)phenyl
tetrasilesquioxane (I4Ph4) sodium salt
Shchegolikhina et al. [46] reported the high yield
synthesis of the sodium salt of tetraphenyl tetrasiles-
quioxane [(Ph4 half cube) – Fig. 1b] from OPS, reaction
(2). The structure of the Ph4 half cube salt was
1
corroborated by H NMR and reacts as expected (see
below) based on the literature [46].

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M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
group could then be removed by displacement with
either fluoride or oxygen nucleophiles to regenerate the
hydroxyl group. This remains an area for future
investigation.
tions) to form the Me4Ph4 Janus cube (Fig. 3) in 85%
yield. Supporting TGA data is shown in Fig. 4. The
ceramic yield (61.23%) is essentially identical to theory
(61.21%), which is a strong indication that the product
5. Synthesis of tetramethyltetraphenyl (Me4Ph4)
silsesquioxane
formed is indeed Me4Ph4 silsesquioxane. Unfortu-
nately, the yields of this material are rather low, 10%.
The correlation of the ceramic yield of the fully
condensed product to theory seems to indicate that all of
the -O(Me)Si(OCH3)2 groups of the starting material
are cis on the same face of the half cube, or else
subsequent reaction with aqueous acid would lead to the
formation of high MW oligomeric condensation
products. Thus the TGA ceramic yield seems to support
that the phenyl and methyl groups are located on
opposite sides of the cube.
In addition to the TGA, the melting point of the
Me4Ph4 cube(112–114 8C)formed inFig. 3 isessentially
the same (116–118 8C) as reported by Andrianov et al.
[47] formed by the reaction of cis-1, 3, 5, 7-tetrahydroxy-
1, 3, 5, 7-tetraphencylcyclotetrasiloxane with tetra-
methylcylclotetrasiloxane in low concentrations.
The FTIR of the Me4Ph4 cube, Fig. 5, exhibits
characteristic aromatic = C-H stretching vibrations
In an effort to make fully condensed Janus cubes, the
Ph4 half cube sodium salt was reacted with MeSiCl3 in
the presence of methanol at room temperature to form
the tetramethyltetra-phenyl (Me4Ph4) dimethoxy half
cube derivative [PhSi(O)O(Me)Si(OCH3)2]4 reaction
1
(4). H NMR spectroscopy confirmed the structure of
the product and revealed that the remaining chloro
groups are replaced by methoxy groups due to reaction
with methanol solvent. GPC analysis reveals a single
peak with a molecular weight lower than expected
(Mn = 658 and Mw = 684 vs. 969 amu theory), but such
discrepancies are typical of silsesquioxane compounds
due to their smaller hydrodynamic radii compared to
polystyrene standards. Mass determination by MALDI-
ToF was unsuccessful and such compounds may not be
readily ionizable.
Hydrolysis of Me4Ph4 dimethoxy half cube deriva-
tive and subsequent cage closure was achieved by
reaction with H2O and conc. HCl (at dilute concentra-
(ca. 3070–2970 cm-1), aliphatic C-H stretching vibra-
tions (ca. 2490 cm-1), Si-C bond vibrations (1270 cm-1),
and Si-O bonds attributed to the silsesquioxane cage

M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
277
Fig. 3. [PhSiO1.5]4[MeSiO1.5] Janus silsesquioxane cube.
MALDI spectrometry data was inconclusive because
effective sample preparation requires THF solubility to
allow intimate mixing with aqueous AgNO3 solution.
Unfortunately, recent efforts to reproduce the
synthesis of the hydrolysis product have been incon-
sistent (even under the exact same experimental
conditions) and have sometimes led to insoluble
products with lower-than-expected ceramic yields (ꢀ
45–55% – indicating higher-than-expected Mws) and
no melting points. This seems to indicate that
oligomerization of the dimethoxy derivative occurs
and hydrolysis at the methoxy-substituted silicon
proceeds competitively via bi-molecular (or higher-
molecular) pathways even at highly dilute concentra-
tions of acid and/or water. The inconsistency in forming
the intended product signifies a more complicated
hydrolysis mechanism than originally predicted.
Recently we tried a variety of hydrolysis conditions
and varied solvents, concentrations of reactants,
reaction time, temperature, etc. with no clear insight
into hydrolysis optimization. Amore precise and
rigorous study into the optimization of the hydrolysis
conditions to form the Me4Ph4 Janus cube exclusively
via a unimolecular pathway is necessary.
Fig. 4. TGA (air) of [PhSiO1.5]4[MeSiO1.5].
(1130 cm-1). Perhaps most importantly, no O-H bond
vibrations (ca. 3200–3400 cm-1) are seen signifying a
lack of unreacted -OH groups and a completely
condensed compound.
We have not been able to gather evidence for the
structure of the Me4Ph4 cube by NMR or MALDI-TOF
because it is highly insoluble in both nonpolar and polar
organic solvents (CHCl3, DMSO, THF, MeOH, etc.).
As a result of these shortcomings, we attempted to
brominate the phenyl moieties of the Me4Ph4 cube,
reaction (5) in order to make the compound more
soluble (and thus more easily isolable). We found
evidence for the Ag+ adduct of both mono- [Me4(Br-
Ph)1Ph3] and tetra-brominated [Me4(Br-Ph)4] Janus
cube. However, the very limited solubility of Me4Ph4 in
solution gave moderate yields (64%) of a mixture of
soluble products, of which we are uncertain as to the
amounts of mono- and tetra-brominated Janus cube and/
or higher-MW brominated species. Furthermore,
bromination of phenyl silsesquioxanes always gives
rise to a mixture of isomers and varying degrees of
substitution [48]. No further attempt was made to
isolate solely the tetra-brominated Janus cube and
represents an area for further investigation.
Fig. 5. FTIR of [PhSiO1.5]4[MeSiO1.5].

278
M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
6. Synthesis of [PhSiO1.5]4[vinylSiO1.5
]
Again we attempted to hydrolyze the Vinyl4Ph4
dimethoxy derivative with H2O and HCl as previously
to form the Vinyl4Ph4 Janus cube (Fig. 7). The
MALDI spectrum of the hydrolysis product is shown
in Fig. 8. While the spectrum indicates the presence of
the MW (Ag+ adduct) of the proposed structure,
higher MW peaks in the MALDI also suggests the
existence of oligomeric or polymeric condensation
products in addition to the desired compound.
Moreover, since the resulting product is insoluble in
both aqueous and organic solvents, GPC and 1H NMR
analyses of the hydrolyzed product are impractical in
this case.
Similarly the tetraphenyltetravinyl dimethoxy deri-
vative [PhSi(O)OSi(Vinyl)(OCH3)2]4 was formed by
reaction of the Ph4 half cube sodium salt with
vinyltrichlorosilane at room temperature in the presence
of methanol, reaction (6). Unlike the Me4Ph4 dimethoxy
derivative (which is a viscous oil), the Vinyl4Ph4
derivative is a white crystalline solid. The structure of
the Vinyl4Ph4 dimethoxy derivative was confirmed
1
by H NMR and is shown in Fig. 6. The spectrum
exhibits characteristic aromatic (7.0–8.0 ppm), vinyl
(ꢀ5.9 ppm), and methoxy (ꢀ3.2 ppm) peaks with the
correct integration (1.9:1.0:22 actual vs. 1.7:1.0:2.0
theory, respectively). GPC analysis showed a single
peak consistent with the proposed molecular weight.
As in the case of Me4Ph4 derivative, the hydrolysis of
the Vinyl4Ph4 dimethoxy derivative may proceed via
bimolecular pathways because of the competing
Fig. 6. 1H NMR of [PhS1.5]4[vinylSiO1.5]. (CD3OD).
Fig. 7. [PhSiO1.5]4[vinylSiO1.5].

M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
279
Fig. 8. MALDI of tetravinyltetraphenyl (Vinyl4Ph4) Janus cube (m/z Ag+). 941 Da is the expected Mw of the proposed product.
reactivity of the methoxy-substituted silicon. Again,
further optimization of hydrolysis conditions is needed.
However, the Vinyl4Ph4 Janus cube is definitely a
product that forms during hydrolysis, albeit one of many
products that may or may not be easily isolated.
the tendency of the Cyhexyl4Ph4 dimethoxy derivative
and acid to phase separate at the expense of intimate
mixing. Perhaps this problem can be overcome through
the use of a miscible organic acid (e.g. glacial acetic
acid).
7. Synthesis of
8. Conclusions
[PhSi(O)OSi(Cyclohexyl)(OCH3)2]4
Half cube sodium salts are readily synthesized in one
step and greater than 70% yields from OPS and I8OPS.
In particular, the I4half cube sodium salt shows potential
for further modification at the iodo moiety by a host of
Pd-catalyzed substitution reactions (after protection of
the hydroxyl group with Me3SiCl). We have also shown
evidence for the syntheses of the Me4Ph4, Vinyl4Ph4
and Cyhexyl4Ph4 dimethoxy derivatives and their
respective hydrolysis products. However, further inves-
tigation into the optimization of the hydrolysis reactions
to form the Janus cubes is needed because we have also
observed the formation of unwanted higher MW
products. Base-catalyzed hydrolysis may also be
considered as an alternative to acid-catalyzed condi-
tions. The purity of the dimethoxy derivatives as
The Ph4 half cube sodium salt was reacted with
cyclohexyltrichlorosilane in MeOH in order to make
the Cyhexyl4Ph4 dimethoxy derivative reaction (7).
Again, the structure of the intended product was
1
supported by H NMR and GPC analyses. Unfortu-
nately, we were unsuccessful in hydrolyzing the
Cyhexyl4Ph4 dimethoxy derivative to form the
intended, fully-condensed cube. We attempted a
variety of hydrolysis conditions and were only able
to isolate a high MW liquid product(s). We surmise that
the mechanism of hydrolysis with aqueous HCl is
especially inefficient in this case because of the
presence of bulky alkyl groups, which when hydro-
lyzed most likely form oligomeric products because of

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M.Z. Asuncion et al. / C. R. Chimie 13 (2010) 270–281
assessed by 1H NMR and GPC suggest that the critical
stage of Janus cube formation occurs during hydrolysis.
Regardless, while such optimization may not be trivial,
the dimethoxy derivatives still remain a potential route
to nanoscale Janus cubes.
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Acknowledgements
This work was supported by NSF through grant CGE
0740108 and in part by Canon Ltd, and Mayaterials Inc.
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