Synthesis, Structures, and Thermal Properties of Symmetric and Janus “Lantern Cage” Siloxanes

Article abstract of DOI:10.1002/chem.201805200

Symmetric and asymmetric (Janus-type) new “lantern cage” siloxanes (PhSiO_1.5)₄(Me₂SiO)₄(RSiO_1.5)₄ (R=Ph or iBu) were synthesized through reaction of all-cis-[PhSi(OSiMe₂Br)O]₄ with all-cis-[RSi(OH)O]₄ (R=Ph or iBu). These precursors were obtained by facile two or three-step reactions from commercially available compounds. The spectroscopic properties of the resulting products were fully characterized and they showed high thermal stability and sublimation without decomposition. The crystal structures clearly indicated that the internal vacancy volumes of the lantern cages are considerably larger than that of octaphenylsilsesquioxane (PhSiO_1.5)₈. DFT calculations of the lantern cage showed a distinctly different electronic state from that of octasilsesquioxane. These results suggest that lantern cage siloxanes have a characteristic “field” in the molecule.

Full text of DOI:10.1002/chem.201805200

A Journal of
Accepted Article
Title: Synthesis, Structures, and Thermal Properties of Symmetric and
Janus “Lantern Cage” Siloxanes
Authors: Masafumi Unno, Taishi Uchida, Yasunobu Egawa, Takuto
Adachi, Naoki Oguri, Masato Kobayashi, Takako Kudo,
Nobuhiro Takeda, and Ryoji Tanaka
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To be cited as: Chem. Eur. J. 10.1002/chem.201805200
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Synthesis, Structures, and Thermal Properties of Symmetric and
Janus “Lantern Cage” Siloxanes
Taishi Uchida[a],#, Yasunobu Egawa[a],¶,#, Takuto Adachi[c],#, Naoki Oguri[a],#, Masato Kobayashi[b], Takako
Kudo[b], Nobuhiro Takeda[a], Masafumi Unno[a],*, Ryoji Tanaka[c],*
Dedicated to Prof. Dr. Robert West on the occasion of his 90th birthday
Abstract: Symmetric and asymmetric (Janus-type) new “lantern cage”
siloxanes (PhSiO1.5)4(Me2SiO)4(RSiO1.5 (R = Ph or i-Bu) were
missing valence of the aluminum replacing parts of the silicate
frameworks.[3] The metal cations and solvate water molecules
play an important role in stabilizing the large pores. In contrast,
synthetic zeolite contains large organic cations such as
tetraalkylammonium ions in the cage for charge compensation
and pore stabilization. The organic cations are pyrolyzed at high
temperatures, yielding a usable internal space as a “field”.[4]
In the field of organopolysiloxanes, three-dimensional
siloxane cages (cage silsesquioxanes) can be easily constructed
by hydrolysis and simultaneous condensation of trifunctional
silanes. These cages are thermally stable due to their strong
siloxane bonds and exhibit good processability of the organic
groups on the silicon atoms, opening a number of avenues for
applied materials and fundamental research.[5] Among cage-
shaped siloxanes, cubic octasilsesquioxanes (T8) consisting of
eight silicon atoms is the most common because they can be
selectively formed with high crystallinity. However, the
octasilsesquioxane cage contains a rather small internal space,[6]
which can accommodate only small atoms such as a hydrogen
radical or fluorine anion.[7] Cage silsesquioxanes larger than T8
are difficult to selectively synthesize, and can only be obtained in
small quantities as by-products of condensation or equilibrium
reactions of octasilsesquioxanes.[8] We recently reported that
Janus Cube octasilsesquioxanes with different substituents on
both sides of the cube can be synthesized by stepwise skeleton
assembly from cyclic silanolate and cyclic fluorosiloxanes.[9]
Using this stepwise framework construction, thermodynamically
stable octasilsesquioxanes and more complex and larger cage
siloxanes can be more easily synthesized. In this study, we
describe the synthesis of a “lantern cage” siloxane, named after
the Japanese lantern which can expand and contract easily,
corresponding to a structure in which the top and bottom faces
can easily move with four flexible D (SiMe2O) bridging units. The
structure of the internal space and thermal properties of these
compounds was also determined and is described in detail herein.
)
4
synthesized via reaction of all-cis-[PhSi(OSiMe2Br)O]4 with all-cis-
[RSi(OH)O]4 (R = Ph or i-Bu). These precursors were obtained by
facile two or three-step reactions from commercially available
compounds. The spectroscopic properties of the resulting products
were fully characterized and they showed high thermal stability and
sublimation without decomposition. The crystal structures clearly
indicated that the internal vacancy volumes of the lantern cages are
considerably larger than that of octaphenylsilsesquioxane (PhSiO1.5)8.
DFT calculations of the lantern cage showed a distinctly different
electronic state from that of octasilsesquioxane. These results
suggest that lantern cage siloxanes have a characteristic “field” in the
molecule.
Polycyclic three-dimensional cage compounds contain an interior
space enclosed by the atoms constituting the molecular skeleton.
Depending on the pore size and surrounding atom species, the
space can perform certain chemical functions, such as ion-
molecular recognition and molecular storage, acting as a “field”
for reactions.[1] These cage compounds mainly consist of
crystalline three-dimensional macromolecules, such as zeolites
and silica clathrates. In addition, high performance “field”
chemistry of organic-inorganic hybrid network polymers such as
metal organic frameworks (MOFs) and covalent organic
frameworks (COFs) have been intensively studied.[1,2] In natural
zeolite, the cage contains metal cations to compensate for the
[a]
T. Uchida,# Dr. Y. Egawa,# N. Oguri,# Prof. Dr. N. Takeda, and Prof.
Dr. M. Unno
Department of Chemistry and Chemical Biology
Graduate School of Science and Technology, Gunma University
Kiryu 376-8515, Gunma (Japan)
E-mail: unno@gunma-u.ac.jp, ORCID: 0000-0003-2158-1644
M. Kobayashi, Prof. Dr. T. Kudo
Department of Pure and Applied Science
Graduate School of Science and Technology, Gunma University
Kiryu 376-8515, Gunma (Japan)
T. Adachi# and Dr. R. Tanaka*
[b]
[c]
Inorganic Materials Chemistry Group
Sagami Chemical Research Institute
Ayase 252-1193, Kanagawa (Japan)
E-mail: rtanaka@sagami.or.jp
¶
Present Address: Frontier Research Group, Research and
Development (R&D) Center for Marine Bioscience, Japan Agency
for Marine-Earth Science and Technology (JAMSTEC), 2-15
Natsushima-cho, Yokosuka 237-0061, Kanagawa (Japan)
#
These authors contributed equally to this work T.U., T.A, N.O. and
Y.E. performed synthesis and isolation. Crystallographic work was
done by Y.E., N.T. and R.T.
:
Supporting information for this article can be found under:
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silicon atoms of the D (SiMe2),[14] T (i-Bu-Si),[9,16] and T (Ph-Si)[7b,9]
units, respectively.
Figure 1. Molecular structures of 4 (left) and 6 (right). Thermal ellipsoids are
drawn at 50% probability level. Hydrogen atoms are omitted for clarity.
The crystals of 4 and 6 that were suitable for X-ray
crystallography were obtained by slow solution evaporation. The
crystal structures of lantern cages 4 and 6 are shown in Figure 1.
Scheme 1. Synthesis of Lantern cages 4 and 6.
The unit cell of symmetric lantern cage
4 contains two
crystallographically independent molecules, with almost no
structural difference between the two. The asymmetric lantern
cage 6 has two crystallographically independent molecules in its
unit cell, and except for the disorder of the isobutyl groups, no
structural differences between the independent molecules were
observed. The silicon atoms of 4 and 6 have almost ideal sp3
structure, where the O–Si–O bond angle ranged between 108 and
110° without distortion. In addition, the two cyclotetrasiloxane
rings of lantern cages 4 and 6 are nearly planar. Because of the
additional four D (Me2SiO2) moieties, lantern siloxanes 4 and 6
possess considerably different structures from that of
octasilsesquioxanes. The position of the oxygen atoms in
octasilsesquioxane protrudes outward from the midpoint of the Si-
Si line, but the oxygen adjacent to SiMe2 in 4 and 6 differs. The
oxygen atom preferentially protrudes towards either the center of
the Si6O6 ring or the center of the molecule. These types of
structural features are commonly found in crown ethers,
cryptands,[17] and siloxane with macrocyclic structures,[18] but
have not been previously observed in organic cage siloxanes. In
addition, the distance between the upper and lower Si4O4 rings of
4 (estimated using the Si-Si nonbonding distances) is 5.41 Å
(averaged value). This value is considerably larger than that of
octaphenyloctasilsesquioxane (3.1 Å).[6] Thus, the lantern cage
can be regarded as having an expanded cage-structure by
inserting four D (SiMe2O2) units into the octasilsesquioxane T8
structure. It should be noted that the four planes formed by the
three atoms of O–SiMe2–O were puckered in the same direction.
In addition, both crystal structures of lantern cages 4 and 6
exhibited molecular alignment in nearly the same direction. This
is likely due to the intermolecular offset p-p stacking of the phenyl
groups.
To synthesize a complex skeleton siloxane[9] [10], step-by-
step construction of siloxane frameworks is an effective method.
In the synthesis of lantern cages, similar multistep construction
methods were used. According to
a previously reported
procedure,[11,12] all-cis-tetrasiloxycyclotetrasiloxane
prepared by reacting all-cis cyclic silanol
2
1
was
with
chlorodimethylsilane. The hydrogen atoms on the siloxy groups
were brominated by adding Br2 to afford all-cis-
tetrakis(bromodimethylsiloxy)tetraphenylcyclotetrasiloxane
3.
The condensation of 3 with 1 in the presence of pyridine afforded
a lantern cage 4 in 16% yield (Scheme 1). The cage exhibited
moderate solubility in conventional organic solvents such as THF
and chloroform. In 1965, Brown reported the synthesis of 4 by
reacting 1 with dichlorodimethylsilane in nitromethane with an
extremely low yield (˂1%), and was characterized only by infrared
spectroscopy.[13] We performed full characterization of lantern
cage 4 using a number of spectroscopic measurements and
crystallographic analysis. In the 1H and 13C-NMR spectra, a
simplified spectrum was observed for lantern cage 4, confirming
its highly symmetrical nature. In the 29Si-NMR spectrum, two
signals at –19.2 (D, SiMe2)[14] and –81.2 ppm (T, SiPh)[7b,9] were
observed. The molecular weight of the cage was confirmed by
electrospray ionization-mass spectrometry (ESI-MS).
The synthesis pathway was applicable to the synthesis of
the symmetric lantern cage molecule and the asymmetric Janus-
type
cage
molecule.
Reacting
3
with
all-cis-
tetraisobutylcyclotetrasiloxanetetraol 5[15] in the presence of
triethylamine, the Janus cage 6 was obtained in 6.9% yield as
colorless crystals (Scheme 1). Compound 6 was characterized by
NMR spectroscopy, MALDI-TOF MS, X-ray analysis, and
elemental analysis. In the 29Si-NMR spectrum, three signals were
observed at –21.3, –71.3, and –81.5 ppm corresponding to the
For a cage molecule with centric symmetry, the diameter of
the largest inscribing sphere of the internal vacancies can be
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calculated by subtracting the van der Waals radii of the atoms
closest to the symmetric center. The atoms closest to the vacancy
center in lantern cage 4 is the central eight oxygen atoms, and the
average of the non-bonding distances between opposite oxygen
sites of the central oxygen atoms (e.g. O8 and O8*) in the crystal
structure is 6.44 Å. The van der Waals radius of oxygen was
obtained by subtracting twice the diameter (1.40 Å[19]), resulting in
an inscribable sphere 3.64 Å in diameter. The diameter of the
of the T8 structure with hydrogen substituents[21]. This result
suggests that the internal space of the skeleton may form a "field"
with respect to the donor-acceptor reaction between host and
guest species.
Both lantern cages 4 and 6 exhibited high thermal stability.
In the thermogravimetric (TG) analysis of 4 (Fw: 1328) under a
nitrogen stream, sublimation began at approximately 370 °C
without melting, and most of the mass sublimated at
approximately 500 °C. The residual weight at 900 °C was 7%.
These results show that 4 has high thermal stability and mostly
vaporizes rather than thermally decompose under TG analysis
conditions. In addition, similar behavior was observed in the TG
analysis of asymmetric 6. In the TG analysis of 6 (Fw: 1252), the
5% weight loss temperature was 319 °C and the residual mass at
600 °C was 0%. The evaporation of a large molecule exceeding
1000 during TG analysis is often observed in cage (or ladder)
sphere
inscribable
to
the
internal
vacancies
in
octaphenyloctasilsesquioxane (7) can be similarly estimated from
the oxygen-oxygen atomic distance of the opposite site (2.03 Å).
By comparing these values, the difference between the inscribed
spheres of lantern cage 4 and that of 7 was estimated to be 1.8
times larger in diameter and 5.8 times larger in volume. For the
asymmetric lantern cage 6, the closest interatomic distance
through the symmetric center is 5.85 Å between O7 and O10 and
the largest inscribing sphere was calculated to have a diameter of
3.05 Å. In comparison with 7, the largest inscribing sphere of the
asymmetric lantern cage 6 is 1.5 times larger in diameter.
Presumably, this difference is not essential and it is expected that
the flexible lantern siloxane frameworks are influenced by packing
forces in the crystal.
silsesquioxanes with extremely high thermal stabilities [22]
.
In conclusion, herein we reported the synthesis of two novel
lantern cage siloxanes, consisting of extended silsesquioxanes
via step-by-step construction. The novel materials were
symmetric and asymmetric Janus-type and their cage crystal
structures were significantly expanded compared to that of
octasilsesquioxane. The high thermal stabilities of the prepared
compounds were also demonstrated, and we are currently
exploring the incorporation and recognition of metal cations by
these lantern cages.
Experimental Section
Experimental, spectroscopic, and X-ray crystallographic details are
provided in the Supporting Information. Details of the crystallographic
analysis of 4 and 6 (Figure S1-S2) are also given in the Supporting
Information. CCDC 181067, and 1523637 (4 and 6) contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre.
Figure 2. Calculated molecular orbitals of the hydrogen model (8) (MP2/cc-
pVDZ//B3LYP/cc-pVDZ level): (a) HOMO (–12.34 eV), (b) LUMO (4.19 eV)
To obtain further insight into the electronic structure of
lantern cage skeletons in solution, ab initio molecular orbital and
DFT calculations were performed for a hydrogen-substituted
model compound (8). In the model calculation, the optimized
structure converged to D2 symmetry at the B3LYP/cc-pVDZ level.
The upper and lower Si4O4 rings in the optimized structure were
Acknowledgements
This work was supported by the “Development of Innovative
Catalytic Processes for Organosilicon Functional Materials”
project (PL: K. Sato) from the New Energy and Industrial
Technology Development Organization (NEDO). Y.E. thanks Y.
N. Kanasaki (JAMSTEC) for drawing the beautiful TG-DTA graph.
significantly puckered, adopting a “cradle” type conformation [20]
.
It is expected that the differences in structure between the model
8 and real molecules 4 and 6 can be attributed to the
intermolecular interactions (especially offset p-p stacking) of real
molecules in the crystal. However, collapse of the oxygen of
cyclohexasiloxane ring into the molecular vacancies, as observed
in the crystal structure, could be partially reproduced in the
calculation, indicating that such a conformation is characteristic of
the lantern cage skeletons. The frontier orbitals obtained at the
MP2/cc-pVDZ//B3LYP/cc-pVDZ level in Figure 2 show that the
highest occupied molecular orbital (HOMO) was predominantly
localized on the O atom of the D silicon unit, which corresponds
to a non-bonding pair of electrons, and the Si–H bonds of the D
silicon unit (Figure 2 (a)). On the other hand, the lowest
unoccupied molecular orbital (LUMO, Figure 2 (b) resembles that
Conflict of interest
The authors declare no conflict of interest.
Keywords: Siloxane • Cross-coupling • Structure
elucidation • Janus particle • Synthetic methods
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Entry for the Table of Contents
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Lantern Cage Silsesquioxane:
Symmetric and novel asymmetric
(Janus-type) new “lantern cage”
siloxanes that contains a large
interior space were synthesized,
and structure and electronic
T. Uchida, Y. Egawa, T. Adachi, N.
Oguri, M. Kobayashi, T. Kudo, N.
Takeda, M. Unno*, R. Tanaka*
Page No. – Page No.
Synthesis, Structures, and
Thermal Properties of Symmetric
and Janus “Lantern Cage”
Siloxanes
properties were investigated.
Keywords: Siloxane • Cross-
coupling • Structure elucidation •
Janus particle • Synthetic methods
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