8 promotes self-bromination to produce [o-BrPhSiO 1.5]8: Further bromination gives crystalline [2,5-Br 2PhSiO1.5]8 with a density of 2.32 g cm -3 and a calculated refractive inde

Article abstract of DOI:10.1039/c1jm11536g

We describe here the synthesis and characterization of the octa-, hexadeca- and tetraicosa-brominated derivatives of octaphenylsilsesquioxane, [PhSiO _1.5]₈ or OPS, which offer high 3-D symmetry. Surprisingly OPS catalyzes its own bro

Full text of DOI:10.1039/c1jm11536g

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PAPER
[PhSiO1.5]8 promotes self-bromination to produce [o-BrPhSiO1.5]8: further
bromination gives crystalline [2,5-Br2PhSiO1.5]8 with a density of 2.32 g cmꢀ3
and a calculated refractive index of 1.7 or the tetraicosa bromo compound
[Br3PhSiO1.5]8†
a
Mark F. Roll, Priyanka Mathur, Kunio Takahashi, Jeffrey W. Kampf and R. M. Laine
c
c
b
ac
*
Received 11th April 2011, Accepted 26th May 2011
DOI: 10.1039/c1jm11536g
We describe here the synthesis and characterization of the octa-, hexadeca- and tetraicosa-brominated
derivatives of octaphenylsilsesquioxane, [PhSiO1.5]8 or OPS, which offer high 3-D symmetry.
Surprisingly OPS catalyzes its ownbromination in refluxing CH2Cl2, in the absence of a traditional Lewis
acid catalyst, leading unexpectedly and with high selectivity ($85%) to the ortho-bromo product,
[o-BrPhSiO1.5]8. It appears that the electrophilic character of the cage polarizes the Br2 molecule
promotingbromination at the ortho position. Thereafter, withthe introduction of a Lewis acidcatalystor
simply through prolonged bromination without a catalyst, it is possible to double the number of Br’s to
produce primarily crystalline [2,5-Br2PhSiO1.5]8 in yields of 80+%. Extended catalytic bromination
provides the disordered tetraicosa-brominated compound [Br3PhSiO1.5]8 in low yields. The latter two
compounds have bromine densities of 9.5 and 8.4 Brper nm3, respectively, some of the highest densities of
Br compounds found in the literature. Crystalline [2,5-Br2PhSiO1.5]8 offers single crystal densities of
2.32 g cmꢀ3 and a calculated refractive index of 1.7 where sapphire is 1.76. The brominated compounds
were characterized using NMR, FTIR, TGA and single crystal X-ray diffraction. This set of compounds
offers the potential via catalytic cross-coupling reactions to produce spherical molecules with some of the
highest degrees of functionality possible for any first generation star/hyperbranched cores.
derivatives described in early detailed work by Eaborn and
Lickess.10 Cubic symmetry in 3-D and octa-functionality, such
that each octant in Cartesian space contains one functional
group, in combination with the proper choice of functionality, is
particularly desirable to facilitate assembly of functional
materials.11–14
Introduction
The ability to assemble 2- and 3-D structures from molecular
components or nano-building blocks (NBs) is of great current
interest.1–9 Altering the chemical constitution of these NBs
through synthetic means would presumably permit tuning of the
coupling and interaction between blocks allowing one to tailor
global properties exactly at the finest length scales.10
Cubic silsesquioxanes, Q8 [RMe2SiOSiO1.5 8
]
and T8
[RxC6H5ꢀxSiO1.5]8 SQs or POSS compounds,14,15 with D4R-like
silica core structure, are easily prepared in comparison to
cubanes, as well as other hydrocarbons or other hybrid alter-
natives.4,5 High thermal stability (ꢁ500 ꢂC in air for OPS), octa-
functionality (with a possibility of forty functional groups for
OPS) and a wide variety of possible chemical modifications make
OPS an excellent starting point for the development of a wide
variety of NBs.13,14
Although there are numerous literature examples of molecules
possessing high degrees of 2-D symmetry, only a small number of
molecular classes possessing structure and functionality with
high 3-D symmetry are known, such as cubanes,4,5 dodecahedral
boranes,8,9 tetraphenyladamantanes,6,7 and tetrasilylmethane
aMacromolecular Science and Engineering, University of Michigan, Ann
Arbor, MI, 48109-2136, USA. E-mail: talsdad@umich.edu; Tel: +1 734
764-6203
As discussed previously, the commercially available phenyl-
trichlorosilane (PhSiCl3) is easily hydrolyzed and condensed to
form polyphenylsilsesquioxane (PPS).14,17 Strong inorganic bases
(e.g. KOH) can be used to equilibrate PPS in refluxing toluene by
rapid rearrangement, driving the formation of the cubic octamer
(PhSiO1.5)8, OPS.14,17 In this process, the highly insoluble OPS
precipitates continuously from the reaction solution and may be
isolated by filtration of the reaction solution at the end of the
desired reaction period.17 As part of our efforts to develop a wide
bDept of Chemistry, University of Michigan, Ann Arbor, MI, 48109-2136,
USA
cMaterials Science and Engineering, University of Michigan, Ann Arbor,
MI, 48109-2136, USA
† Electronic supplementary information (ESI) available: Detailed
spectroscopic characterization of the various compounds prepared in
this work. CCDC reference numbers 821599, 821600, 821601. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1jm11536g
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variety of SQ based NBs, we now report the development of well
defined pathways to octa-, hexadeca- and tetraicosa-bromo OPS
derivatives that offer the potential to introduce 8, 16 or even 24 of
the same or different functional groups in essentially the same
volume via many different types of catalytic cross-coupling
reactions. Furthermore, we find that these compounds have very
interesting properties in their own right.
The THF layer was extracted, dried with sodium sulfate and
subsequently injected into the instrument.
Synthetic methods
Peroxide oxidation of the silica core. Cleavage reactions were
conducted as discussed previously.17
Br8OPS. To a dry 100 mL Schlenk flask under N2 was added
5.0 g OPS (4.85 mmol, 38.5 mmol phenyl) followed by 10 mL
dichloromethane. A condenser was attached followed by an
addition funnel. The flask was then heated to 60 ꢂC in an oil bath.
To the addition funnel were added 3.3 mL of Br2 (64 mmol) and
finally an additional 2 mL of CH2Cl2, to wash the graduated
cylinder. Following this, the N2 flow was stopped, and a vent to
a bubbler containing aqueous base was added. At this point, the
Br2 solution was added dropwise, to the dispersion of OPS. The
solution was stirred for 1 d. At this point, the flask was placed in
an ice bath and a saturated 2 : 1 sodium metabisulfite : sodium
carbonate was added to the top of the solution with vigorous
stirring, until the Br2 color disappeared (ꢁ10 to 20 mL). The
mixture was transferred to a separatory funnel, and the organic
layer was extracted and washed sequentially with a 50 mL
portion of water. The organic layer was removed, and charcoal
and celite were added and stirred for 10 min. The black mixture
was filtered, then dried with sodium sulfate, and filtered again to
give a clear, colorless liquid. The CH2Cl2 was removed by rotary
evaporation, the resulting solid was redissolved in ꢁ10 mL of
THF, and subsequently precipitated into ꢁ200 mL stirred, cold
methanol. The precipitate is recovered by filtration and dried
under vacuum to yield 6.8 g (4.1 mmol) of white powder (84%
yield).
Experimental
Materials
OPS was prepared using literature methods16,17 or was received as
a gift from Mayaterials Inc. Bromine and other synthetic
reagents were purchased from Sigma Aldrich and used as
received. Dichloromethane was purchased from Burdick-Jack-
ꢀ
son. Solvents were dried over 4 A molecular sieves.
Analytical methods
1
Nuclear Magnetic Resonance (NMR). All H NMR spectra
were collected from samples dissolved in CDCl3/CS2 and recor-
ded on a Varian INOVA 400 MHz spectrometer. 1H NMR
spectra were collected at 400 MHz using a spectral width of 6000
Hz, a relaxation delay of 0.5 s, 30k data points, a pulse width of
38ꢂ, and CHCl3 (7.24 ppm) as the internal reference. 13C NMR
spectra were collected at 100 MHz using a spectral width of
25 141.4 Hz, a relaxation delay of 1.5 s, 75k data points, a pulse
width of 40 deg with 256 repetitions and CHCl3 (77.23 ppm) as
the internal reference.
Thermogravimetric analyses (TGA/DTA). All TGA/DTA
analyses were run on a 2960 simultaneous DTA-TGA Instru-
ment (TA Instruments, Inc. New Castle, DE). Samples (15–
25 mg) were loaded in alumina pans and ramped at 10 ꢂC minꢀ1
o-Br8OPS. To a dry 1 L round-bottom flask under N2 was
added 60 g OPS (58 mmol, 464 mmol phenyl) followed by
600 mL CH2Cl2. A condenser was attached followed by an
addition funnel. The flask was then heated to 60 ꢂC in an oil bath.
To the addition funnel was added 43 mL of bromine (834 mmol)
and finally an additional 2 mL of CH2Cl2, to wash the graduated
cylinder. Following this, the N2 flow was stopped, and a vent to
a bubbler containing aqueous base was added. At this point, the
Br2 solution was added dropwise, to the dispersion of OPS. The
solution was stirred for 7 d. The flask was then removed from
heat and allowed to settle overnight. A saturated sodium meta-
bisulfite was added to the top of the solution with vigorous
stirring, until the Br2 color disappeared (ꢁ15 mL). The mixture
was transferred to a separatory funnel, and the organic layer was
extracted and washed sequentially with two 50 mL portions of
water. The organic layer was removed, and charcoal and celite
were added and stirred for 10 min. The black mixture was
filtered, then dried with sodium sulfate, and filtered again to give
a clear, colorless liquid. Most of the CH2Cl2 was removed by
rotary evaporation, leaving a viscous white oil. The flask was
capped and placed in a freezer. The resulting solid was triturated
in ethyl acetate and the solid recovered by filtration and dried in
air, providing 30 g of white, microcrystalline powder. Oxidative
cleavage of this material indicated that only 85% of the substi-
tution was ortho to the silicon. A 12 g portion of the material was
recrystallized twice from 100 mL of hot m-xylene, and once from
to 1000 C in dry air at a flow rate of 60 mL minꢀ1
.
ꢂ
Matrix-Assisted Laser Desorption/Ionisation Time-of-Flight
(MALDI-TOF) mass spectrometry. MALDI-TOF was done on
a Micromass TofSpec-2E equipped with a 337 nm nitrogen laser
in positive-ion reflectron mode using poly(ethylene glycol) as
a calibration standard, dithranol as the matrix and AgNO3 as the
ion source. Samples were prepared by mixing solutions of 5 parts
matrix (10 mg mLꢀ1 in THF), 5 parts sample (1 mg mLꢀ1 in
THF), and optionally 1 part AgNO3 (2.5 mg mLꢀ1 in water) or
1 part LiOTf (2.5 mg mLꢀ1 in water) and blotting the mixture on
the target plate.
The resulting chromatograms were averaged and smoothed
once using the Savitzky–Golay algorithm. The baseline was
subtracted using a 99th order polynomial, and the spectra were
centered using a channel width of half the full width at half-
maximum.
Gas chromatography/mass spectroscopy. GC/MS data were
collected on a Thermo/electron TraceMS using splitless injection.
Samples for analyses were prepared by reacting 0.8 mL of
MeOH/H2O/NH4F solution with 10 mg of sample in 1.5 mL
ꢂ
THF at 60 C for 2 h. The resulting reaction mixture was sepa-
rated into two phases by the addition of 5 mL of saturated brine.
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o-dichlorobenzene. The resulting large, colorless crystals were
solvent exchanged with 2 portions of 15 mL of ethyl acetate,
triturated, recovered by filtration and dried under vacuum,
yielding 1.1 g of pure, white, microcrystalline octa-ortho powder
(ꢁ2% of theoretical). The remaining material (80 g, ꢁ85%) is
recovered by precipitation of a THF solution into MeOH.
generation of HBr. The reaction was stirred for an additional
ꢁ24 h. Then the viscous reaction solution was diluted with
100 mL of dichloromethane and quenched with 150 mL of water
with a 10 g of sodium metabisulfite and 20 g of sodium carbonate
was added to the top of the solution with vigorous stirring, until
the solution largely decolorized. The biphasic mixture was added
to ꢁ500 mL of CS2 (safety note: CS2 is a highly flammable,
volatile solvent with a low auto-ignition point) and transferred to
a separatory funnel. After the aqueous layer was found to be
neutral, the organic layer was separated and charcoal and celite
were added and stirred for 10 min. The black mixture was filtered
to give a clear, colorless liquid, which was subsequently dried
over sodium sulfate, and filtered again. The carbon disulfide was
diluted with ꢁ50 mL of heptane and the solvent was removed by
rotary evaporation, allowing the material to crystallize out of
solution. The resulting solid was triturated in ꢁ15 mL of tetra-
hydrofuran and recovered by filtration. The solid was then
recrystallized 3ꢃ from hot o-dichlorobenzene. The filtrate was
removed and the solid was washed with THF and methanol and
dried to yield 1.2 g of a white microcrystalline solid (8% yield).
Single crystals for XRD analysis were grown by slow cooling of
saturated hot o-dichlorobenzene solutions.
Br16OPS. To a dry 100 mL Schlenk flask under N2 was added
5.0 g OPS (4.85 mmol, 38.5 mmol phenyl) followed by 0.5 g
FeBr3 (1.7 mmol) and 10 mL CH2Cl2. The red dispersion was
stirred for 10 minutes at room temperature before placing the
flask in an ice bath. A condenser was attached followed by an
addition funnel. To the addition funnel were added 5.0 mL of
bromine (98 mmol) and finally an additional 2 mL of CH2Cl2, to
wash the graduated cylinder. Following this, the nitrogen flow
was stopped, and a vent to a bubbler containing aqueous base
was added. At this point, the bromine solution was added
dropwise, to the dispersion of OPS. The solution was stirred for
1 d. Then the flask was placed in an ice bath and ꢁ5 mL of
a saturated solution of 2 : 1 sodium metabisulfite : potassium
phosphate was added to the top of the solution with vigorous
stirring, until the solution largely decolorized.
The viscous dispersion was dissolved in ꢁ100 mL of carbon
disulfide (safety note: CS2 is a highly flammable, volatile solvent
with a low auto-ignition point) and transferred to a separatory
funnel. After the aqueous layer was found to be neutral, the
organic layer was separated, charcoal and celite were added and
stirred for 10 min. The black mixture was filtered to give a clear,
colorless liquid, then dried with sodium sulfate, and filtered
again. The carbon disulfide was diluted with ꢁ50 mL of heptane
and the solvent was removed by rotary evaporation. The
resulting solid was triturated in ꢁ10 mL of ethyl acetate and
recovered by filtration. The solid was washed with an additional
ꢁ5 mL of ethyl acetate. The filtrate was removed and the solid
was washed with methanol to yield 7.1 g (3.1 mmol, 64% yield) of
a white microcrystalline solid. This microcrystalline solid may be
further purified by stirring in THF overnight at a concentration
of 1 g per 5 mL and isolating the insoluble portion in 25% overall
yield. The filtrate was diluted to ꢁ25 mL of solvent and precip-
itated into ꢁ200 mL stirred, cold methanol. The precipitate was
recovered by filtration and dried under vacuum to yield 1.9 g
(0.82 mmol, 17% yield) of white powder. Total yield: 81%.
Materials property analyses
Refractive indices, n, were estimated using the formula given by
Liu and Ueda.18 Rm is the sum of the molar refractivities of the
SQ compound only (i.e. neglecting the solvent) and Vm is the
volume of the corresponding crystalline unit cell.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 þ 2ðRm=VmÞ
n ¼
1 ꢀ ðRm=VmÞ
The mass energy-transfer coefficient, mtr/r, was calculated using
the NIST standard NISTIR 5632 for each of the compounds
using the tabulated values therein, again neglecting contributions
from the solvent molecules.19 The densities were calculated using
the volume of the corresponding crystalline unit cell. The
formula is given above.19
Results and discussion
In the following sections we discuss the synthesis and charac-
terization of the brominated OPS derivatives in the order of the
degree of bromination.
Br24OPS. To a dry 100 mL Schlenk flask under N2 was added
5.0 g OPS (4.85 mmol, 38.5 mmol phenyl) followed by 10 mL
CH2Cl2. The red dispersion was stirred for 10 minutes at room
temperature before placing the flask in an ice bath. A condenser
was attached followed by an addition funnel. To the addition
funnel were added 8.5 mL of Br2 (165 mmol) and finally an
additional 2 mL of CH2Cl2, to wash the graduated cylinder.
Following this, the N2 flow was stopped, and a vent to a bubbler
containing aqueous base was added. At this point, the Br2
solution was added dropwise to the dispersion of OPS. The
solution was stirred for ꢁ5 h until the solid OPS entered the
solution. At this point, the mixture was cooled with an ice bath
and the magnetic stirrer was turned off. Then 0.50 g FeBr3
(1.7 mmol) was added quickly and the vent reattached. 15 mL of
dichloromethane was added via a pressure equalized addition
funnel as the stirring was restarted leading to the vigorous
Octabromination of OPS
Brominated aromatics, such as the tetra(p-bromophenyl)ada-
mantane7,20 and the hexa(p-bromophenyl)benzene,21,22 have been
used as starting points for the synthesis of complex supramo-
lecular systems, like our proposed BrxOPS nano-building
blocks.23 The development of catalytic C–C and C–heteroatom
bond forming reactions via cross-coupling over the past few
decades has multiplied the types of functionality that may be
incorporated.24–33 Examples of these reactions applied to BrxOPS
NBs have in part been described previously,23,34 with more results
reported in the accompanying report.35
We recently developed very selective routes to [p-IPhSiO1.5]8
(>95% para), [p-IPhSiO1.5]10 (>90% para), and [p-IPhSiO1.5]12
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(>90% para),36,37 despite our original misgivings about the
potential to synthesize highly symmetrical NBs via electrophilic
modification of OPS. This success provided the impetus to
explore and develop methods whereby it is possible to selectively
brominate OPS.
If the SQ cage is an electron withdrawing substituent, as sug-
gested by Feher and Budzichowski, significant meta substitution
should occur due to deactivation of the ortho and para positions.
However in this case, oxidative Si–C cleavage of the octa(bro-
mophenyl)silsesquioxane shows no meta substituted phenols.
More surprisingly, the ortho substitution is preferred to substi-
tution at the more sterically available para- position, 85% to 15%,
as expected based on unpublished work by K. Takahashi.41 This
contrasts sharply with our iodination studies where we observe
nearly quantitative para substitution.
There are now several theoretical studies43–45 that suggest that
the cage core is highly electrophilic in keeping with Feher and
Budzichowski’s analysis of the 13C NMR values for substituents
directly linked to the cage. For the most part these studies suggest
that the electrophilic character is localized within the cage;
however, recent studies by Mabry and Bowers et al.44 and Park
et al.45 suggest that this electrophilic character extends beyond
the cage interior. Indeed Park et al. find that halide anion
complexation on the square faces of octasilsesquioxanes is
Aromatic bromination is almost always conducted in the
presence of Lewis acid catalysts, often iron compounds. The
general mechanism involves Br2 coordination with the Lewis acid
to generate species that behave like Br+ that serve as the elec-
trophilic intermediate that attacks aromatic groups.38,39 Iron and
iron trichloride catalyzed bromination of OPS was explored by
Brick et al.23 and He et al.34 The bromination of OPS by IBr was
reported by Erben et al.40
These studies gave products that were relatively disordered
with respect to substitution patterns and number, in part because
of the insolubility of OPS, which leads to extended bromination
of the first brominated species as they become more soluble with
increasing bromination. However, following our success with
iodination,36,37 we concluded it might be possible to more selec-
tively brominate OPS. Serendipitously, a significant preference
for ortho-bromination was found in the bromination of OPS by
bromine alone in dilute methylene chloride solution at room
temperature.41
energetically favored by 10–60 kcal molꢀ1 45
. Thus in solution at
room temperature, we can speculate that diatomic bromine
polarizes spontaneously at the cage face, leading to relatively
higher concentrations of Br+ close to the ortho position. In other
words, the SQ cage itself appears to drive the bromination
reaction acting as its own Lewis acid, perhaps implying that it
may serve as a Lewis acid for other types of reactions.
Though benzene does not react with bromine in the absence of
a Lewis acid catalyst,38 we find that the addition of OPS to neat
bromine or bromine/methylene chloride solution (ꢁ25%) leads to
rapid bromination of the aromatic rings. While it is known that
bromination of phenols and other activated aromatic systems
occurs without the presence of a catalyst,38 the SQ cage should be
deactivating.42 Work published by Feher and Budzichowski
suggests that the cage has electron withdrawing character similar
to a CF3 group based on an analysis of Hammett constant
correlations.42 In principle, this should substantially reduce the
rate of electrophilic substitution by reducing the electron density
in the aromatic ring, which contrasts with our findings.
Thus, we now report that careful addition of bromine to
a suspension of OPS in methylene chloride (1 g OPS per 5 mL
CH2Cl2) at a bromine to phenyl molar ratio of 1.65 : 1, followed
by heating to reflux, provides octa-brominated OPS with
a narrow distribution in substitution number (Scheme 1).
Fig. S1† shows a MALDI-TOF spectrum of the as-prepared
material.
During the work-up, trituration of the crude product in ethyl
acetate at room temperature (see Experimental) followed by
filtration gives a microcrystalline fraction in ꢁ10% of the total
yield by mass. The filtrate is reduced and precipitated into 5
volumes of cold methanol. If the bromination reaction
(Scheme 1) volume is quadrupled (ꢁ1 g OPS per 20 mL CH2Cl2)
and the length of the reaction increased to one week, the ethyl
acetate insoluble material comprises roughly 40% by mass of the
total. This microcrystalline material on recrystallization from hot
m-xylene gives large, colorless crystals suitable for single crystal
X-ray diffraction. A MALDI-TOF spectrum of the recrystallized
material is shown in Fig. 1. The remaining soluble material
(ꢁ85% of the calculated yield) is precipitated into cold methanol
and isolated as a fine white powder, Br8OPS, with an average
degree of ortho substitution of 85%. The mass spectrum of this
Thereafter we determined the bromine substitution pattern
using Fꢀ/H2O2 to oxidatively cleave the Si–C bonds to produce
a mixture of phenols.23 GC-MS was used to determine the ratio
of ortho to para isomers by integrating the responses for each.
Scheme 1 Bromination of octa(phenyl)silsesquioxane (OPS) showing
approximate 85 : 15 ortho : para substitution pattern.
Fig. 1 MALDI-TOF spectrum of octa(o-bromophenyl)silsesquioxane
(dithranol/AgNO3).
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Table
silsesquioxane$2.5m-xylene
1
Crystal
structure
data
for
octa(o-bromophenyl)
less crystalline material shows a range of number substitutions
from 7–9, as in the material prepared at the original concentra-
tion (Fig. S1†).
o-Br8OPS
1
The H NMR in Fig. S2† reveals four primary multiplets,
centered at 7.77, 7.47, 7.25 and 7.23 ppm. In contrast to Feher
and Budzichowski’s42 suggestion that the cage offers electron-
withdrawing character equivalent to a CF3 group, peak corre-
lation to o-bromonitrobenzene appears to be qualitatively better
than o-bromobenzotrifluoride.46 The largest downfield shift
indicates the SQ cage offers electron withdrawing character
intermediate between CF3 and NO2 groups. The 13C NMR
spectrum in Fig. S3† can be assigned in accord with assignments
made for o-bromonitrobenzene from the spectral database for
organic compounds (SDBS) and consideration of resonance
integrations.46
Space group
Unit cell dimensions
I4(1)/a, tetragonal
ꢀ
a ¼ 20.5738(10) A
ꢀ
b ¼ 20.5738(10) A
ꢀ
c ¼ 36.261(4) A
a ¼ 90ꢂ
b ¼ 90ꢂ
g ¼ 90ꢂ
3
ꢀ
Unit cell volume
Z
R-Factor, [I > 2s(I)]
15 348.5(19) A
8
5.3% (5.3%)a
a
Before the use of SQUEEZE to account for disordered solvent.
Single crystal X-ray diffraction analysis demonstrates that o-
Br8OPS crystallizes in the tetragonal space group I4(1)/a (Fig. 2
and Table 1). The unit cell contains eight molecules with a total
another position, and thus more deactivated. Previously, we
reported that iron powder catalysts dramatically accelerate
bromination allowing substitution up to an average of sixteen.23
It was also observed that as the substitution number reached
sixteen, insoluble product precipitated from the reaction solu-
tion.23 Indeed, once isolated, this product was found to be
insoluble in common solvents, including ethyl acetate, toluene,
THF and acetone.
3
ꢀ
volume of 15 348.5 A including 2.5 m-xylene solvate molecules,
which are disordered. The partial occupancy of the disordered
xylenes was modeled using the SQUEEZE sub-routine of the
PLATON program suite.47
As apparent in Fig. 2, the ortho substitution pattern of the
bromine provides functionality directly adjacent to the cage face,
which is reported in the accompanying report.35 This is in direct
contrast to the para substitution, which we have previously
reported for the iodinated compounds.36,37
However, Olsson and Axen brominated thiophenylSQ some
fifty years ago, and noted its remarkable solubility in carbon
disulfide.48 We find that Br16OPS material is also soluble in
carbon disulfide. (Note: CS2 is a highly flammable, volatile
solvent with a low auto-ignition point.)
Hexadecabromination of OPS
Optimization of the synthesis of this microcrystalline isomer
indicated that slow addition of bromine to a stirring suspension
While the degree of substitution of the uncatalyzed bromination
may be increased by adding more bromine, we find that the
average degree of bromination does not exceed fourteen. It could
be that an extended reaction time is necessary to reach unreacted
ortho sites on rings which are already mono-brominated in
ꢂ
of OPS and homogeneous FeBr3 in CH2Cl2 at 0 C leads to an
instantaneous, vigorous reaction. After the suspended OPS
reacts and dissolves, a new precipitate forms as the reaction
continues. After 24 h, the residual bromine is quenched. The
recovered insoluble material is dissolved in CS2, and the organic
layer is washed twice with brine (Scheme 2).
Oxidative cleavage indicates the product is essentially pure 2,5-
dibromophenol (>95%). Fig. 3 shows a MALDI-TOF MS
analysis of the crystalline material, indicating the desired hex-
adecabrominated isomer. The 1H NMR spectrum of the material
in CDCl3/CS2 shows two multiplets (Fig. S3†). Starting again
with the conclusions of Feher and Budzichowski, peak assign-
ments are made by comparison to 2,5-dichlorobenzotrifluoride
and 2,5-dibromonitrobenzene.46 Again, the downfield shifts of
the aromatic protons indicate that the SQ cage possesses electron
withdrawing character intermediate between the CF3 and NO2
groups.
Fig.
2 50% Thermal ellipsoid plot of octa(o-bromophenyl)
silsesquioxane$2.5m-xylene. Hydrogen atoms and m-xylene solvates are
omitted for clarity. Full XRD data can be found in the ESI†.
Scheme 2 Synthesis of octa(dibromophenyl)silsesquioxane.
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Table 2 Crystal structure data for octa(2,5-dibromophenyl)SQ$CS2
Br16OPS
ꢁ
P1, triclinic
Space group
Unit cell dimensions
ꢀ
ꢀ
ꢀ
a ¼ 12.7172(9) A
b ¼ 12.7392(9) A
c ¼ 13.0890(9) A
a ¼ 97.592(1)ꢂ
b ¼ 109.075(1)ꢂ
g ¼ 116.231(1)ꢂ
3
ꢀ
Unit cell volume
Z
R-Factor, [I > 2s(I)]
1699.3(2) A
1
2.7%
the same number of equivalents of bromine were added to
ꢂ
Fig. 3 MALDI-TOF MS of octa(2,5-dibromophenyl)silsesquioxane
a stirring suspension of OPS in methylene chloride at 0 C, and
(dithranol/AgNO3).
allowed to react without the iron catalyst until the evolution of
HBr slowed.
X-Ray quality crystals of 2,5-Br16OPS$CS2 were grown by
slow evaporation of a saturated solution of the compound in
1 : 1 : 1 carbon disulfide/dichloromethane/dodecane. Single
crystal X-ray diffraction analysis revealed that 2,5-Br16OPS
At this point the reaction mixture was cooled and stirring was
stopped as FeBr3 catalyst was added. Once the vent and N2 were
reinstalled, the ice-bath was removed and the reaction allowed to
continue. The reaction continues vigorously, and is quenched
after 24 h.
ꢁ
crystallizes in the triclinic space group P1 (Fig. 4 and Table 2).
The unit cell contains one molecule with a total volume of
3
The obtained product is completely soluble in CH2Cl2,
ꢁ1 g mLꢀ1 at room temperature, in addition to CS2. The
Fꢀ/H2O2 cleavage reaction indicates a 2 : 1 ratio of two isomers
of dibromophenol assigned to 2,5-dibromophenol and
3,4-dibromophenol, respectively, by 1H NMR.45 Presumably this
increase in mixed points of substitution prevents any significant
crystallization from occurring. The MALDI-TOF spectrum
shows a narrow distribution in substitution number (Fig. S6†).
This observation implies that iron-catalysis strongly directs to
2,5-di-substitution in comparison to the uncatalyzed reaction. By
initiating bromination in the absence of the iron catalyst and
carrying it beyond the Br8OPS composition, no single substitu-
tion pattern dominates, reducing the symmetry of the resulting
macromolecule. This reduction in symmetry, and therefore
crystallinity, increases solubility in the later stages of
substitution.
1699.3 A , and a density of 2.318 g cmꢀ3 with a bromine density
ꢀ
of 9.5 Br per nm3. The single CS2 solvate is disordered over two
crystallographic sites.
The structure clearly shows that under the iron catalyzed
conditions, dibromination of OPS gives predominantly ortho and
meta substitution. This second meta bromo functionality in
addition to the ortho substitution pattern found in the o-Br8OPS
allows for better comparisons of the optical and electronic
structure/property relationships of the derivatives of o-Br8OPS
and Br16OPS discussed in the accompanying report.35
It was previously assumed that the iron catalyst promotes
a dynamic rearrangement of the substitution pattern throughout
the reaction period, allowing for the kinetically driven precipi-
tation of the crystalline 2,5-isomer.23 As a test of this hypothesis,
Tetraicosabromination of OPS
The addition of catalytic amounts of FeBr3 after an initial stage
of bromination without catalyst increases the degree of bromi-
nation beyond sixteen (Scheme 3, Fig. 5 and 6), providing highly
insoluble white powders. Attempts to increase the degree of
bromination to twenty-four led to significant Si–C cleavage as
a side-reaction, leaving a silanol functionalized corner, as
detected in the crude reaction products by MALDI-TOF (Fig. 5).
It appears that the tri-brominated corners are readily cleaved
under the highly acidic (due to HBr produced), oxidizing (due to
excess Br2) conditions. Repeated recrystallization of the reaction
product from hot o-dichlorobenzene removes the cleaved prod-
ucts and allows isolation of crystalline material in low yields,
<10%.
GC/MS analysis of the oxidative cleavage products shows two
primary tribromophenols and a tetrabromobenzene isomer.
MALDI-TOF analysis indicates a broader distribution in the
degree of bromination (Fig. 5 and S7†). In addition, molecules
Fig. 4 50% Thermal ellipsoid plot of octa(2,5-dibromophenyl)silses-
quioxane CS2 solvate. Hydrogen atoms and carbon disulfide solvates are
omitted for clarity. Full XRD data can be found in the ESI†.
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1
The H NMR in Fig. S8† shows only two poorly resolved
multiplets at 7.88 and 7.84 ppm, with a minor broad peak at
7.435 ppm. In contrast, the proton resonances for 1,2,5-tri-
bromo-3-trifluoromethylbenzene are found at 8.14 and
7.90 ppm.49 The resonance from residual chloroform is seen at
7.222 ppm.
Single crystals suitable for X-ray diffraction were grown from
dilute solutions of hot o-dichlorobenzene, and the refined crystal
structure shows a solid solution, with multiple substitution
patterns, with 2,4,5-tribromination as the most probable, based
on partial occupancies (Fig. 6 and Table 3). The crystal structure
is relatively open, hosting disordered o-dichlorobenzene solvent
molecules, and has a lower density (2.0 g cmꢀ3, with a bromine
density of 8.4 Br per nm3) than the Br16OPS.
Scheme 3 Synthesis of tetraicosabrominated OPS.
The partial filling factors indicate an average of 24 bromines,
giving 24 possible sites for functionalization by cross-coupling
reactions as discussed in the adjoining report.35 One area of
interest is the nature of electronic interactions of conjugated
functionality under the sterically congested arrangement as
shown in Fig. 6.
These highly brominated materials may be valuable as high
dielectric materials, or high refractive index materials, due to the
high Z value of bromine.18,50 In addition, Pd or Ni catalyzed
cross-coupling reactions24–33 should allow the synthesis of
multiple types of highly stable materials with eight to twenty-four
functional groups in nearly the same volume, as briefly demon-
strated earlier.23 The possibility of using multiple functional
groups tethered to the same molecule should provide access to
a wide variety of nanocomposite materials with highly tunable
mechanical, optical, and/or electronic properties.
Fig. 5 MALDI-TOF MS of tetraicosabrominated OPS (Ag+/dithranol).
Comparison of the bromo derivatives
Table 4 compares the properties of the three crystalline
compounds. The found ceramic yields are not dramatically
different from those expected from theory, except for Br8OPS.
The octabrominated compound shows a mass-loss onset at ca.
450 ꢂC. The TGA traces of the hexadeca- and tetraicosa-
brominated compounds show stabilities >500 ꢂC in air
(Fig. S8†). In contrast, the TGA trace of the Br16OPS in nitrogen
shows no evidence of sublimation. The mass loss seen is indica-
tive of the loss of bromine and aromatic groups from the SQ
corners.
Table 3 Crystal structure data for tetraicosabrominated OPS$5o-
dichlorobenzene
Br24OPS
ꢁ
P1, triclinic
Space group
Unit cell dimensions
Fig. 6 50% Thermal ellipsoid plot of tetraicosabrominated OPS$5o-
dichlorobenzene (R-factor of 6.3%). Hydrogen atoms and o-dichloro-
benzene solvates are omitted for clarity. Dashed bonds indicate partially
occupied bromines. Full XRD data can be found in the ESI†.
ꢀ
ꢀ
ꢀ
a ¼ 15.5726(10) A
b ¼ 20.4685(13) A
c ¼ 20.6449(13) A
a ¼ 65.3990(10)ꢂ
b ¼ 74.6160(10)ꢂ
g ¼ 82.1720(10)ꢂ
3
ꢀ
Unit cell volume
Z
R-factor, [I > 2s(I)]
5766.3(6) A
2
with degrees of substitution $24 reveal the addition of a water
molecule to the structure. Since silanols are not detected by
FTIR, it appears that Si–O–Si bond opening occurs under the
influence of the laser.
6.3% (12.7%)a
a
Before the use of SQUEEZE to account for disordered solvent.
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Table 4 Characterization data for the brominated OPS derivatives
Mw MALDI-TOF
found (% intensity)
Ceramic
yield (theor.)
Ceramic
yield (found)
Sample
Mw calc. (Ag+ ion)
Br8OPS
Br16OPS
Br24OPS
1772.55
2403.72
3034.89
1772.1 (100%)
2402.2 (100%)
3032.3 (63%)
29%
21%
17%
23%
19%
16%
Fig. S9† provides DSC analyses showing melting points
ranging 250–400+ ꢂC. While the as-prepared Br8OPS melts at ca.
260 ꢂC, the crystalline o-Br8OPS melts at ca. 360 ꢂC, almost
100 ꢂC higher. We believe that this is due to the higher degree of
purity and crystallinity of the material. When heated under
nitrogen at ambient pressure o-Br8OPS is found to volatilize at
ca. 410 ꢂC. Accounting for the low ceramic yields seen in Table 4.
of hexabromoethane, 1.86, and tetrabromomethane, 1.59.52 The
estimated RI for Br16OPS is equal to the highest values reported
for highly brominated polymers. The index of refraction of poly
(pentabromophenylmethacrylate) is 1.71,53 while a series of
methacrylates with brominated carbazole side chains had indices
of refraction ranging from 1.68–1.72.18
[2,5-Br2PhSiO1.5]8
melts at ca. 375 ꢂC. Cooling the melted
materials yields opaque, glassy solids, indicating slow crystalli-
zation from the melt. Together, these melting point temperatures
are below or far below the onset of decomposition found in the
Conclusions
We report here the syntheses of three new, crystalline OPS
TGA analyses. [Br3PhSiO1.5]8
has a melting point which
derivatives:
[o-BrPhSiO1.5]8,
[2,5-Br2PhSiO1.5]8
and
approaches its decomposition temperature, so an accurate
melting point determination was not possible.
[Br3PhSiO1.5 8
] via two different bromination mechanisms. We
succeeded in obtaining single crystal X-ray structures of the three
compounds, allowing the estimation of physical properties
including refractive index and X-ray absorption. The bromine
densities of the crystals range from 4.2 Br per nm3 for o-Br8OPS
to 9.5 Br per nm3 for Br16OPS to 8.4 Br per nm3 for Br24OPS.
These derivatives include the octabromophenylsilsesquioxane
in 80% yield with z85% ortho substitution, and a crystalline
octa-ortho-brominated OPS derivative in low yield <5%. Of
particular importance is the fact that OPS promotes polarization
of the Br2 molecule indicating unexpected electronic and/or
electrostatic interactions from what normally might be viewed as
an insulating, inert structure.
The surprising volatility of the o-Br8OPS in comparison to the
lower mass parent compounds OPS (which shows no evidence of
volatility under heating prior to decomposition >500 ꢂC) implies
that the solid-state orientation of the bromophenyl groups on
each corner of the molecule significantly reduces intermolecular
interactions at these temperatures, allowing adjacent molecules
to dissociate and evaporate. The crystal structures of the hex-
adeca bromo and tetraicosa bromo compounds do show signif-
icant halogen–halogen intermolecular contacts, which will be
discussed at a later date.51
FTIR spectra are found in Fig. S11 and S12†. Characteristic
nSi–O–Si bands are seen at 1000–1200 cmꢀ1. In the correspond-
ing aromatic nC–H region (3100–3000 cmꢀ1), the nC–H intensi-
ties diminish as the degree of bromination increases. However,
distinct differences are also seen in the aromatic regions between
1500–1250 cmꢀ1 and 850–700 cmꢀ1. The FTIR spectrum of
A Br16OPS derivative consisting of two isomers, 2,5- and 3,4-
dibromo substituted OPS, was isolated in 80% yield and pure
crystalline octa(2,5-dibromophenyl)SQ in 40% yield. A Br24OPS
derivative consisting of crystalline solid solution of substitutional
isomers was also isolated in 10% yield, and its single crystal X-ray
structure refined. Each derivative could provide access to highly
polyfunctional molecules, if suitable cross-coupling reactions can
be found.23 The sterically confined geometry might allow the
observation of intramolecular exciplexes or other photophysical
phenomena.44
[o-BrPhSiO1.5]8
shows a feature at 755 cmꢀ1, in the aromatic
dC–H region (Fig. S11 and S12†). In each spectrum, peaks at
1415–1428 cmꢀ1 and 1580–1555 cmꢀ1 can be associated with
aromatic nC–C bands.23,38
Using formulae provided in the Experimental section, the
X-ray mass energy-transfer coefficients for o-Br8OPS, Br16OPS
and Br24OPS are estimated by the standard method19 as
(respectively) 143, 163 and 174 cm2 gꢀ1 at 8 keV (ꢁCu-Ka) using
the atom populations of the SQ molecules and volumes of the
unit cell, and ignoring the contributions of solvent molecules.
For comparison bromine has a mass energy-transfer coefficient
of 212 cm2 gꢀ1. The refractive indices are estimated by summing
the molar refractivities of the SQ molecules and using the crys-
tallographic unit cell volume18 as 1.5, 1.7 and 1.45 for o-Br8OPS,
Br16OPS and Br24OPS, respectively.
These compounds have the potential to serve as high dielec-
trics due to their high electron densities. They suggest the
possibility of tunable high dielectric characteristics by modifying
the reaction chemistry. They may also serve as nano-building
blocks with up to twenty-four functional groups.
We have shown that the degree of bromination in the OPS
system may be increased from eight to sixteen to twenty-four
bromines per OPS molecule, increasing the electron density and
volume of the molecule. However, as this degree of substitution
reaches twenty-four, Si–C cleavage reactions begin to dramati-
cally reduce the yield of crystalline material. The crystal structure
of tetraicosabrominated OPS has been refined, and it shows
a solid solution of substitutional positions. The Br16OPS and
Br24OPS show thermal stability in excess of 450 ꢂC, and all three
derivatives show melting behavior under DSC analysis.
The lower calculated RI for Br24OPS arises because of the
lower packing density of the unit cell. The estimated RI for
Br16OPS is close to that of sapphire (n ¼ 1.76) indicating that this
crystalline hybrid material has one of the highest RIs ever found
for a brominated molecule, namely between the refractive indices
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20 W. Guo, E. Galoppini, R. Gilardi, G. I. Rydja and Y.-H. Chen, Weak
intermolecular interactions in the crystal structures of molecules with
tetrahedral symmetry: diamondoid nets and other motifs, Cryst.
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21 R. Rathore and C. L. Burns, Preparation of hexakis(4-bromophenyl)
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Unique characteristics of these crystal structures will be
detailed at a later date. In particular, the variation in densities
from the o-Br8OPS (1.67 g cmꢀ3) and the Br24OPS (2.0 g cmꢀ3), to
densely packed structures, in the case of the Br16OPS
(2.32 g cmꢀ3). At the heart of these effects will be the increasing
number of bromines packed into the 1 nm diameter of the OPS
molecule.
23 C. M. Brick, R. Tamaki, S.-G. Kim, M. Z. Asuncion, M. Roll,
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