Asymmetric aryl polyhedral oligomeric silsesquioxanes (ArPOSS) with enhanced solubility

Article abstract of DOI:10.1016/j.jorganchem.2011.03.035

Four new asymmetric Polyhedral Oligomeric SilSesquioxanes (POSS) with the formula (Aryl)Phenyl₇Si₈O₁₂, where Aryl = 1-naphthyl, 2-naphthyl, 9-phenanthrenyl, and 1-pyrenyl, have been synthesized in reasonable yield and high purity. These compounds were characterized with ¹H, ¹³C, ²⁹Si NMR and elemental combustion analysis. These compounds possess polycyclic aromatic functionality, which disrupts symmetry to improve solubility in organic solvents and aromatic polymers, without significant impact on thermal stability.

Full text of DOI:10.1016/j.jorganchem.2011.03.035

Journal of Organometallic Chemistry 696 (2011) 2676e2680
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Asymmetric aryl polyhedral oligomeric silsesquioxanes (ArPOSS) with
enhanced solubility
,
Brian M. Moore a, Sean M. Ramirez b, Gregory R. Yandek a, Timothy S. Haddad b, Joseph M. Mabry a
*
a Air Force Research Laboratory, Space & Missile Propulsion Division, Edwards Air Force Base, CA 93524, USA
b ERC Inc., Air Force Research Laboratory, Space & Missile Propulsion Division, Edwards Air Force Base, CA 93524, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new asymmetric Polyhedral Oligomeric SilSesquioxanes (POSS) with the formula (Aryl)Phenyl7
-
Received 15 February 2011
Received in revised form
24 March 2011
Si8O12, where Aryl ¼ 1-naphthyl, 2-naphthyl, 9-phenanthrenyl, and 1-pyrenyl, have been synthesized in
reasonable yield and high purity. These compounds were characterized with 1H, 13C, 29Si NMR and
elemental combustion analysis. These compounds possess polycyclic aromatic functionality, which
disrupts symmetry to improve solubility in organic solvents and aromatic polymers, without significant
impact on thermal stability.
Accepted 25 March 2011
Keywords:
POSS
Published by Elsevier B.V.
Thermal Stability
Solubility
Silsesquioxane
Aromatic
Polycyclic
1. Introduction
a greater material design space in the context of nanoparticle
assembly. However, the availability of thermally stable, inert POSS
The synthesis of hybrid organic-inorganic materials that
combine the diversity and ease of processing of organic polymers
with the thermo-chemical stability and oxidative resistance of
ceramics remains a goal of material researchers worldwide. Poly-
hedral Oligomeric SilSesquioxanes (POSS) have emerged as effec-
tive multi-functional, highly tailorable additives, capable of
improving polymer performance [1e4]. Each of these nanoparticles
features an inorganic SiO1.5 core, as well as an organic corona,
which helps to determine overall solubility. Compounds based on
this architectural framework have received a great deal of attention
as nearly ideal hybrid materials due to the synergy of the silses-
quioxane cage and organic character at the molecular level [5]. In
contrast to most other forms of nanoscale reinforcement, POSS
compounds have been shown to improve processing characteristics
when either blended into polymer hosts or incorporated by copo-
lymerization [6e10]. Although both techniques have distinct
advantages, inert blending is generally the preferred method,
offering facile modification of commercial polymers without the
necessity for polymer synthesis and balancing stoichiometry.
Furthermore, blending techniques generally provide access to
additives for the purpose of reinforcing high temperature polymers
by this method is limited. Further development would be of benefit
to a range of potential products requiring lightweight materials
for energy efficiency, aerospace, and durable infrastructure
applications.
Aryl-functionalized silsesquioxanes, such as phenyl8Si8O12
(Ph8Si8O12), have been in existence for decades, appearing well-
suited for the preparation of high-performance, aromatic nano-
composites [11e15]. However, the high symmetry and low dipole
moments of Ph8Si8O12 promote highly-efficient crystalline packing.
This is manifested in poor solubility in organic solvents, and
a neutral response to mechanical shear, thus severely limiting
incorporation into polymers [16,17]. To circumvent these limita-
tions, several research groups have focused on the modification of
aryl-functionalized POSS compounds. For example, Sellinger et al.
functionalized vinyl8Si8O12 with aromatic photo-luminescent
compounds via Heck coupling [18,19]. Laine et al. reported the
synthesis of (para-iodophenyl)8Si8O12 as a platform for coupling
additional organic moieties to POSS cages [20]. Their work high-
lighted the ability to produce soluble symmetric POSS cages. Other
work by Shi et al. attached Ph8Si8O12 to polybenzimidazole via an
in-situ Friedel-Crafts acylation copolymerization reaction [21].
They found that copolymerized Ph8Si8O12 was more thoroughly
* Corresponding author. Tel.: þ1 661 275 5857.
dispersed in the polymer host than physically blended Ph8Si8O12
.
E-mail address: joseph.mabry@edwards.af.mil (J.M. Mabry).
0022-328X/$ e see front matter Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2011.03.035

B.M. Moore et al. / Journal of Organometallic Chemistry 696 (2011) 2676e2680
2677
Herein, we report the synthesis of aromatic POSS compounds, via
a “corner-capping” methodology, which overcome the limitations
of Ph8Si8O12 through geometric consideration of the POSS cage
periphery. These compounds demonstrate enhanced solubility in
organic solvents and aromatic polymers, enabling a more facile
route to the fabrication of high temperature nanocomposites.
2. Results and discussion
Aromatic POSS structures were synthesized by the “corner-
capping” of phenyl7Si7O9(OH)3 (1) with aryl trichlorosilanes. The
desired aryl trichlorosilanes were synthesized by reaction of an
aryl Grignard or lithium reagent with SiCl4 under reaction
conditions similar to those previously reported [22]. The aryl tri-
chlorosilanes (ArSiCl3) (Ar ¼ phenyl, 1-naphthlyl, 2-naphthyl, 9-
phenanthrenyl, and 1-pyrenyl) were coupled with 1 under basic
conditions to yield the desired, well-defined (Aryl)phenyl7Si8O12
structures (Scheme 1).
The desired products of these reactions were separated from
byproducts and starting materials via filtration and methanol
washings. Previous work has proven this synthetic strategy as
an effective technique for coupling trichlorosilanes to silanols
[10,23e25]. These POSS cage structures were confirmed on the basis
of multinuclear NMR (1H, 13C, and 29Si) and elemental combustion
analysis (CHN). The absence of silanol NMR peaks was used to
confirm reaction completion. The initial chemical shifts of the 29Si
NMR peaks for 1 are ꢀ69.08, ꢀ77.54, and ꢀ78.51 ppm, respectively,
in a ratio of 3:1:3. For comparison, the chemical shift of the single
peak for symmetric phenyl8Si8O12 (2) is ꢀ78.07 ppm. When the
corner-capping reaction is complete on the asymmetric compounds,
the 29Si NMR peaks are shifted with respect to the additional
substituent aryl group. For example, the peaks of (1-naphthyl)phe-
nyl7Si8O12 (3) are shifted to ꢀ77.37, ꢀ78.05, ꢀ78.14, and ꢀ78.17 ppm,
respectively, in a ratio of 1:3:1:3 (Fig. 1). Similar peak shifts in 29Si
NMR spectra were observed for the remaining corner-capped
compounds, though the order of peak integration is occasionally
different.
Scheme 1. Synthesis of corner-capped POSS cages.
The influence of the prescribed peripheral asymmetry of these
compounds on solubility was determined by visual measurements
in five organic solvents, including chloroform (CHCl3), tetrahydro-
furan (THF), phenyl ether (PE), toluene (Tol), and dimethyl form-
amide (DMF), and is summarized in Table 1. The poor solubility of 2
has been previously documented [17], exhibiting only limited
solubility (w1 mg/mL) in CHCl3 and thus, serving as the benchmark
for this work. As expected, the substitution of a single polycyclic
aromatic ring on the POSS cage improves solubility substantially in
most cases. However, it is difficult to predict the influence of each
aromatic group on the observed solubility, as well as in which
solvent the solubility will be affected. Modification with a 1-naph-
thyl group to produce 3 results in solubility in CHCl3 and THF
exceeding 100 mg/mL and the observance of more finite solubility in
the other investigated solvents. Modification with a 2-naphthyl
group to produce (2-naphthyl)phenyl7Si8O12 (4) increases solubility
in PE and DMF, but reduces solubility in THF and CHCl3, when
compared to 3. Substitution with a phenanthrenyl group to produce
(9-phenanthrenyl)phenyl7Si8O12 (5) results in comparable solubility
Fig. 1. 29Si NMR spectra of 1 (bottom) and 3 (top).

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B.M. Moore et al. / Journal of Organometallic Chemistry 696 (2011) 2676e2680
in THF to that of 2, as well as the highest solubility in DMF and
toluene of any compound examined. Limited solubility gains in
comparison with the other polycyclic aromatic groups are observed
with pyrenyl substitution, with (1-pyrenyl)phenyl7Si8O12 (6)
exhibiting the lowest solubility of the four corner-capped POSS
cages in the examined solvents, albeit a noteworthy improvement
over that of 2.
samples in a nitrogen atmosphere at a scan rate of 10 ꢁC/min. The
results of this study are summarized in Table 2. In comparison with
2, each of these compounds exhibits a lower temperature, corre-
sponding to a 5% weight loss. The temperature trend appears to
decrease with geometric size of the corner cap group, ranging from
a reduction of 7% for 3 to 17% for 6. A detailed description of the
thermal properties of these compounds in relation to chemical
architecture will be provided in a subsequent publication.
Table 1
Solubility (mg/mL) of aromatic POSS compounds in organic solvents.
Table 2
Five percent weight loss temperatures (ꢁC) of aromatic POSS compounds in nitrogen
measured by TGA.
#
CHCl3
THF
PE
Tol
DMF
a
a
a
a
a
a
2
3
4
5
6
1
104
55
52
7
2
3
4
5
6
120
28
117
10
15
53
25
4
24
35
57
5
465
432
406
399
385
27
5
3. Conclusions
a
Insoluble.
Replacing a single phenyl ring on 2 with one of several polycyclic
aromatic groups results in compounds that exhibit improved solu-
bility in organic solvents and aromatic polymers, without significant
sacrifices in thermal stability. These improvements, relative to the
state-of-the-art materials, are realized through a disruption of
symmetry and reduced ordering in the forms of crystallization and/
or aggregation. Frameworks containing a single unique pendent
group were prepared and isolated. These compounds offer new
opportunities to blend ArPOSS with a variety of organic/polymer
materials, a possibility that was previously impractical due to the
insolubility of 2. Future work will focus on the influence of soluble
ArPOSS compounds on polymer nanocomposite properties.
To examine the phase behavior of this new class of asymmetric
POSS nanoparticles with representative aromatic polymer,
a
attempts were made to solubilize both 2 and 3 in polyetherimide
(Ultem 1000, PEI). Chloroform solutions were prepared at a solute
concentration of 5 weight percent PEI and POSS concentration of 5
weight percent with respect to PEI. Subsequent films from these
solutions were drop cast onto glass substrates, dried under vacuum,
and annealed at 220 ꢁC, above the glass transition temperature of
PEI, to promote equilibrium phase formation. The overall results of
this study are depicted in Fig. 2. In accordance with the solubility
study, the solution containing asymmetric 3 is transparent, indi-
cating superior solubility in chloroform, in contrast to that con-
taining 2, which is cloudy.
Qualitatively, the dried and annealed films were visually
observed to be of contrasting appearance. The film containing 2
was significantly more opaque, suggesting a greater degree of
phase separation. Examination of the films by microscopy revealed
aggregated particles of 2 at length scales greater than 1 mm in some
regions of the PEI host. Phase separation in the film containing 3
appears to be limited to less than 5 mm in the post-annealed state,
revealing improved solubility in the predominately aromatic
polymer.
4. Experimental
4.1. Materials
Phenyl7Si7O9(OH)3 was obtained from Hybrid Plastics, while
additional silicon-containing organic compounds were purchased
from Gelest. Remaining chemicals were purchased from Aldrich. All
chemicals were used without further purification unless otherwise
noted. All reactions were performed under an atmosphere of dry
nitrogen. Flasks were oven-dried and allowed to cool under
nitrogen prior to use.
In order to examine the thermal stability of compounds 2-6,
thermogravimetric analysis (TGA) was conducted on 5e10 mg
Fig. 2. Inside: Photographs of PEI solutions containing 2 (left) and 3 (right) in chloroform. Outside: Optical micrographs of drop cast films from PEI solutions containing 2 (left) and
3 (right).

B.M. Moore et al. / Journal of Organometallic Chemistry 696 (2011) 2676e2680
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4.2. Characterization
1-naphthyltrichlorosilane (5.63 g, 0.0215 mol) in THF (50 mL) was
then slowly added. A dilute solution of triethylamine (6.84 g,
0.0676 mol) in THF (100 mL) was then added over a 90 min period
under vigorous stirring. The reaction was allowed to proceed
overnight. The solution was then filtered and the volume reduced
under dynamic vacuum. The product was dissolved in ether and an
aqueous wash (4:1) was performed to remove water-soluble
byproducts. The solution was again reduced under vacuum and
the remaining oil was dissolved in THF. The solution was precipi-
tated in methanol and then filtered to obtain a 92% yield of product
(21.0 g, 0.0194 mol). 1H NMR (CDCl3, ppm) 8.51 (m, 1H-nap), 8.04
(dd, J ¼ 6.8 Hz, J ¼ 1.3 Hz, 1H-nap), 7.98 (d, J ¼ 8.3 Hz, 1H-nap), 7.81
(m, 15H-nap/ph), 7.39 (m, 24H-nap/ph). 13C{1H} NMR (CDCl3, ppm)
136.45 (C-nap), 135.36 (CH-nap), 134.22, 134.21, 134.18 (3:1:3,
CH-ph), 133.16 (C-nap), 131.51 (CH-nap), 130.83, 130.80, 130.77
(3:1:3, CH-ph), 130.17,130.13,130.03 (3:1:3, C-ph), 128.65 (CH-nap),
128.34 (CH-nap), 128.23 (C-nap), 127.91, 127.88, 127.85 (3:1:3, CH-
ph), 126.50 (CH-nap), 125.73 (CH-nap), and 124.83 (CH-nap). 29Si
{1H} (CDCl3, ppm) ꢀ77.37, ꢀ78.05, ꢀ78.14, and ꢀ78.17 (1:3:1:3).
Combustion Anal. (Calcd): C, 57.68 (57.64); H, 3.81 (3.91).
1H, 13C, and 29Si NMR spectra were obtained on Bruker 300-MHz
and 400-MHz spectrometers using 5 mm o.d. tubes. Sample
concentrations were approx. 10% (w/v) in chloroform-d. Combus-
tion analysis was performed by Atlantic Microlab, Inc. Norcross, GA.
Thermogravimetric analysis (TGA) was performed on a TA Instru-
ments Q5000 using 5e10 mg of material, at a scan rate of 10 ꢁC/min
under a nitrogen atmosphere.
4.3. General synthesis of chlorosilane compounds
4.3.1. 1-Naphthyltrichlorosilane
Under a dry nitrogen atmosphere, a solution of 1-bromonaph-
thalene (27.7 g, 0.134 mol) in THF (175 mL) was added slowly to
a suspension of magnesium turnings (3.9 g, 0.16 mol) in THF
(15 mL) that had previously been activated with an iodine crystal.
After cooling to room temperature, this Grignard reagent was
added via canula to a THF (70 mL) solution of SiCl4 (25.1 g, 0.148
mol) and stirred overnight. The mixture was evaporated to dryness,
extracted with hexane and filtered to remove Mg halide. The
product was distilled at 120 ꢁC under dynamic vacuum to give a 67%
yield (23.6 g, 0.0902 mol) of product. 1H NMR (CDCl3, ppm) 8.46
(dd, 1H), 8.21 (dd, J ¼ 6.8 Hz, J ¼ 1.2 Hz, 1H), 8.09 (d, J ¼ 8.4 Hz, 1H),
7.96 (d, J ¼ 7.6 Hz, 1H), 7.69 (t, 1H), 7.62 (t, 1H), 7.57 (t, 1H). 13C{1H}
NMR (CDCl3, ppm) 135.51 (CH), 134.42 (C), 134.07 (CH), 133.58 (C),
129.37 (CH), 127.91 (C), 127.45 (CH), 127.16 (CH), 126.56 (CH), and
124.65 (CH). 29Si{1H} (CDCl3, ppm) ꢀ0.17 (s).
4.4.2. (2-Naphthyl)phenyl7Si8O12 (2-NapPh7Si8O12) (4)
Yield 90%. 1H NMR (CDCl3, ppm) 8.31 (s, 1H-nap), 7.84 (m, 18H-
nap/ph), 7.45 (m, 23H-nap/ph). 13C{1H} NMR (CDCl3, ppm) 135.91
(CH-nap), 134.51 (C-nap), 134.22 (CH-ph), 132.59 (C-nap), 130.82,
130.80 (3:4, CH-ph), 130.14 (C-ph), 129.58 (CH-nap), 128.46 (CH-
nap), 127.89 (CH-ph), 127.71 (CH-nap), 127.47 (C-nap), 127.29 (CH-
nap), 127.06 (CH-nap), and 126.03 (CH-nap). 29Si{1H} (CDCl3,
ppm) ꢀ77.94, ꢀ78.14, and ꢀ78.18 (1:3:4). Combustion Anal. Calcd:
C, 57.28 (57.64); H, 3.87 (3.91).
4.3.2. 2-Naphthyltrichlorosilane
Yield 31%. 1H NMR (CDCl3, ppm) 8.40 (s, 1H), 7.96 (d, 2H), 7.89
(d, 1H), 7.82 (dd, J ¼ 8.4 Hz, J ¼ 1.5 Hz, 1H), 7.62 (m, 2H). 13C{1H}
NMR (CDCl3, ppm) 135.49 (CH), 135.06 (C), 132.28 (C), 128.90 (CH),
128.54 (C), 128.52 (CH), 128.34 (CH), 127.85 (CH), 127.34 (CH), and
127.02 (CH). 29Si{1H} (CDCl3, ppm) ꢀ0.87 (s).
4.4.3. (9-Phenanthrenyl)phenyl7Si8O12 (PhenPh7Si8O12) (5)
Yield 79%. 1H NMR (CDCl3, ppm) 8.74 (d, J ¼ 7.6 Hz, 1H-phen),
8.68 (d, J ¼ 8.3 Hz, 1H-phen), 8.51 (dd, J ¼ 7.8 Hz, J ¼ 1.2 Hz, 1H-
phen), 8.31 (s,1H-phen), 7.84 (m, 15H-phen/ph), 7.64 (m, 4H-phen),
7.43 (m, 21H-ph). 13C{1H} NMR (CDCl3, ppm) 138.35 (CH-phen),
134.24, 134.23 (3:4, CH-ph), 133.89 (C-phen), 131.66 (C-phen),
130.83 (CH-ph), 130.59 (C-phen), 130.18, 130.14, 130.03 (3:1:3,
C-ph), 129.97 (C-phen), 129.21 (CH-phen), 127.91, 127.88 (3:4, CH-
ph), 127.11 (C-phen), 126.86 (CH-phen), 126.60 (CH-phen), 126.43
(CH-phen), 122.92 (CH-phen), and 122.40 (CH-phen). 29Si{1H}
(CDCl3, ppm) ꢀ77.28, ꢀ78.06, ꢀ78.12, and ꢀ78.18 (1:3:3:1).
Combustion Anal. (Calcd): C, 59.10 (59.33); H, 3.84 (3.91).
4.3.3. 9-Phenanthrenyltrichlorosilane
Yield 58%. 1H NMR (CDCl3, ppm) 8.76 (m, 1H), 8.68 (d, J ¼ 8.4 Hz,
1H), 8.48 (s, 1H), 8.43 (m, 1H), 7.99 (dd, J ¼ 7.9 Hz, J ¼ 0.5 Hz, 1H),
7.75 (m, 3H), 7.67 (t, 1H). 13C{1H} NMR (CDCl3, ppm) 139.02 (CH),
132.30 (C), 131.36 (C), 130.48 (C), 130.10 (CH), 129.77 (C), 129.45
(CH), 128.18 (CH), 127.25 (CH), 127.19 (CH), 127.14 (CH), 126.77 (C),
123.49 (CH), and 122.56 (CH). 29Si{1H} (CDCl3, ppm) ꢀ0.51 (s).
4.3.4. 1-Pyrenyltrichlorosilane
4.4.4. (1-Pyrenyl)phenyl7Si8O12 (PyPh7Si8O12) (6)
Under a dry nitrogen atmosphere, n-BuLi (10.6 mL, 1.6 M) in
hexanes was added dropwise to a cooled (ꢀ60 ꢁC) solution of 1-bro-
mopyrene (5.01 g, 0.018 mol) inTHF/Et2O (1:1) (80 mL) and stirred for
2 h at ꢀ60 ꢁC. The solutionwas cooled to ꢀ90 ꢁC and a THF solution of
SiCl4 (8.66 g, 0.051 mol) (10 mL) was added slowly and stirred for 24 h
at room temperature. The reaction mixture was evaporated to
dryness, washed in Et2O (100 mL), and filtered to remove any
unreacted 1-bromopyrene and LiBr. The filtrate was collected and
evaporated to dryness to give a 48% yield of 1-trichlorosilylpyrene
a yellow powder.1H NMR (CDCl3, ppm) 8.66 (d, J ¼ 9.3 Hz,1H), 8.59 (d,
J ¼ 8.1 Hz, 1H), 8.30 (m, 5H), 8.11 (m, 2H). 13C{1H} NMR (CDCl3, ppm)
136.18 (C), 135.85 (C), 133.13 (CH), 132.06 (C), 131.30 (C), 131.22 (CH),
130.31 (CH),127.96 (CH),127.67 (CH),127.61 (CH),126.65 (CH),125.61
(C), 125.01 (CH), and 124.39 (C). 29Si{1H} (CDCl3, ppm) ꢀ1.64 (s).
Yield 29%. 1H NMR (CDCl3, ppm) 8.72 (d, J ¼ 9.2 Hz, 1H-py), 8.43
(d, J¼ 7.6Hz,1H-py), 8.21 (m, 2H-py), 8.12 (m, 3H-py), 8.04(m, 2H-py),
7.80 (m,14H-ph), 7.37 (m, 21H-ph). 13C{1H} NMR (CDCl3, ppm) 136.09
(py-C), 134.29, 134.24 (3:4, CH-ph), 133.34 (py-C), 133.30 (py-CH),
131.14(py-C),130.87,130.81(4:3,CH-ph),130.74(py-C),130.24,130.20,
130.10 (3:1:3, C-ph), 128.66 (py-CH), 128.11 (py-CH), 127.95, 127.91
(3:4, CH-ph), 127.78 (py-CH), 127.47 (py-CH), 125.91 (py-CH), 125.59
(py-CH), 125.42 (py-CH), 125.27 (py-C), 124.60 (py-C), 124.50 (py-C),
and 124.04 (py-CH). 29Si{1H}(CDCl3, ppm)ꢀ76.80,ꢀ77.82, and ꢀ77.93
(1:3:4). Combustion Anal. (Calcd): C, 59.77 (60.18); H, 3.75 (3.83).
Acknowledgment
We gratefully acknowledge the Air Force Office of Scientific
Research and the Air Force Research Laboratory, Propulsion Direc-
torate for financial support.
4.4. General synthesis of POSS compounds
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0.0211 mol) was dissolved in THF (150 mL). A solution of
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