Synthesis and characterization of unsymmetrical double-decker siloxane (Basket Cage)

Article abstract of DOI:10.3390/molecules24234252

The one-pot synthesis of an unsymmetrical double-decker siloxane with a novel structure via the reaction of double-decker tetrasodiumsilanolate with 1 equiv. of dichlorotetraphenyldisiloxane in the presence of an acid is reported herein for the first time. The target compound bearing all phenyl substituents on the unsymmetrical siloxane structure was successfully obtained, as confirmed by 1H-NMR, ¹³C-NMR, ²⁹Si-NMR, IR, MALDI-TOF, and X-ray crystallography analyses. Additionally, the thermal properties of the product were evaluated by TG/DTA and compared with those of other siloxane cage compounds.

Full text of DOI:10.3390/molecules24234252

molecules
Communication
Synthesis and Characterization of Unsymmetrical
Double-Decker Siloxane (Basket Cage)
Rungthip Kunthom, Nobuhiro Takeda and Masafumi Unno *
Department of Chemistry and Chemical Biology, Graduate School of Science and Technology, Gunma University,
Gunma, Kiryu 376-8515, Japan; t172a002@gunma-u.ac.jp (R.K.); ntakeda@gunma-u.ac.jp (N.T.)
* Correspondence: unno@gunma-u.ac.jp; Tel.: +81-277-30-1230
Academic Editor: Vadim A. Soloshonok
Received: 6 November 2019; Accepted: 20 November 2019; Published: 22 November 2019
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: The one-pot synthesis of an unsymmetrical double-decker siloxane with a novel structure
via the reaction of double-decker tetrasodiumsilanolate with 1 equiv. of dichlorotetraphenyldisiloxane
in the presence of an acid is reported herein for the first time. The target compound bearing all phenyl
substituents on the unsymmetrical siloxane structure was successfully obtained, as confirmed by
1H-NMR, 13C-NMR, 29Si-NMR, IR, MALDI-TOF, and X-ray crystallography analyses. Additionally,
the thermal properties of the product were evaluated by TG/DTA and compared with those of other
siloxane cage compounds.
Keywords: basket shape; unsymmetrical structure; functional siloxane; all phenyl groups;
well-defined compound
1. Introduction
Many scientists have been interested in silsesquioxane cage materials with a well-defined
structure because such materials have a low dielectric constant and excellent thermal stability [
Additionally, these compounds find a wide range of applications in polymers [ 11], sensors [12],
catalysis [ 13], organic light-emitting diodes [14], host-guest complexes [15 16], and porous
materials [17]. Most of the double-decker silsesquioxanes (DDSQ) [10 11 14 18 26] and polyhedral
1–7].
8–
7–
9
,
,
,
, , –
oligomeric silsesquioxanes [7–9,12,13,15–17,27–29] have a symmetrical structure with the same organic
functional group on both sides. In contrast, only a few studies on unsymmetrical silsesquioxane or
siloxane cages have been reported because of the difficulty in obtaining a perfect Janus structure
via a simple hydrolytic condensation of two organomonosilane precursors. Due to its unique
chemical/physical properties related to type, number, and position of organic functional groups on
a single molecule, the synthesis of asymmetrical structures has recently become an essential subject
in chemistry [30–35]. Since the first report by the Laine group [30], to the most recent by the Kuroda
group [31], on the synthesis of Janus cube octasilsesquioxane by the hydrolytic condensation of
trimethoxy groups on cyclotetrasiloxanes rings, only our group has successfully synthesized a Janus
cube from two different organic functional groups on cyclotetrasiloxane precursors [32]. Furthermore,
we also expanded to the synthesizing of an elongated core-sized Lantern Cage siloxane with the
similar capping reaction between tetrakis(bromodimethylsiloxy)tetraphenylcyclotetrasiloxane and
cyclotetrasiloxanetetraol [33].
Recently, the Maleczka Jr. and Lee group obtained an unsymmetrical DDSQ with the introduction
of a trichloromethylsilane on one side of the DDSQ. This caused difficulty in controlling the synthesis,
and purification required a vast difference in polarity between the product and the undesired
compound [34]. One year later, this group succeeded in synthesizing an unsymmetrical DDSQ by using
Molecules 2019, 24, 4252; doi:10.3390/molecules24234252
www.mdpi.com/journal/molecules

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p-MeO-C6H4B(OH)2 as the protecting group for each side of the reactive silanol group [35]. Developing
an effective synthesis procedure for unsymmetrical DDSQ materials remains an interesting topic.
To date, the synthetic pathway of a novel unsymmetrical cage with one-side capping as a model
compound for highly regulated silsesquioxanes has not been studied. Meanwhile, a siloxane material
bearing phenyl groups has attracted attention for photochemical applications because of its high
refractive index and high temperature stability [18,36]. In this work, we prepared a novel well-defined
unsymmetrical DDSQ via the formation of DDSQ-ONa with 1 equiv. of dichlorotetraphenyldisiloxane
(ClPh2Si)2O for the first time. This method allows for control of the unsymmetrical closed-cage
DDSQ structure with a disiloxane substituent, in addition to being simple and inexpensive; moreover,
the reaction proceeds to completion in a short time, with the chloride ligand being an excellent
leaving group.
2. Results and Discussion
Dichlorodisiloxane
dichlorodiphenylsilane in the presence of water for 3 h. The product obtained after distillation
was a viscous colorless oil in 35% yield. The synthesis procedure for DDSQ-ONa ( ) is described in
detail in previous reports [19 21 38]. A 2-propanol solution of trimethoxyphenylsilane with NaOH and
water was refluxed for 4 h, followed by continuous stirring at room temperature for 15 h. After filtration,
a white solid was obtained in 58% yield. Afterward, unsymmetrical DDSQ ( ) was synthesized by a
capping reaction between and to obtain the closed DDSQ T8D2-Ph structure, as shown in Scheme 1.
Compound was slowly added to the reaction mixture of in 1:1 molar ratio and stirred at room
1 [37] was freshly prepared by the hydrolytic condensation of
2
,
,
3
2
1
1
2
temperature to obtain the expected intermediate, one-side capped DDSQ, with the residual reactive
siloxanolate group (Si-O−). After 3 h, an acid was directly added to the reaction mixture to induce
a one-pot reaction, and hydrolytic condensation led to the formation of
1 h. Subsequent to the extraction and drying of the crude product, a significant challenge in product
purification was isolation from several unwanted byproducts. Compound was isolated as a white
3
since heating at 60 ◦C for
3
solid in 15% yield by column chromatography (CHCl3/hexane: 4:6; Rf = 0.41). The desired compound
was not moisture-sensitive and could be recrystallized by slow evaporation from a CH2Cl2/2-propanol
(1:1) solvent mixture to obtain colorless crystals in 12% yield. The structure of the compound was
determined based on spectroscopic data and X-ray crystallography.
Scheme 1. Synthesis of unsymmetrical double-decker silsesquioxanes (DDSQ) (3).
1
The NMR spectrum of
3 in CDCl3 (Figure S1) supports the proposed structure of 3 with
proton resonances around 7.13–7.60 ppm attributed to the phenyl groups. Spectra of 13C (Figure S2)
and 13C DEPT-90 (Figure S3) NMR showed resonances for the phenyl carbons at 127.5–134.5 ppm.
The 29Si{1H}-NMR spectrum of
3
(Figure 1) showed four singlet signals at
−
45.4,
−
76.6,
−
78.2, and
76.6, 78.2,
78.4 ppm, which were derived from DDSQ, were shifted significantly upfield relative to the T
−
78.4 ppm assignable to four types of silicon atoms in a cage. Silicon resonance signals at
−
−
and
−

Molecules 2019, 24, 4252
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type silicon, while the introduction of tetraphenyldisiloxane showed a downfield-shifted signal at
−45.4 ppm [18,37]. The 29Si-NMR chemical shifts of related compounds are summarized in Table 1.
Figure 1. 29Si{1H}-NMR (119.2 MHz, CDCl3) spectrum of 3 with labeled silicon positions.
Table 1. 29Si{1H}-NMR data of all-phenyl siloxane compounds.
Compound
Si Unit Type
Signal (ppm)
T8-Ph [39]
T12-Ph [40]
sym-DDSQ-Ph (T8D2) [18,38]
(ClPh2Si)2O [37]
3
T
T
T, D
D
−78.4
−77.0, −80.6
−45.4, −78.1, −79.4
−18.6
T, D
−45.4, −76.6, −78.2, −78.4
The IR spectrum of 3 was shown in Figure S6. Strong Si-O-Si stretching absorption [18] at 1090 and
1113 cm−1 confirmed that the DDSQ core consisted of disiloxane, which was unperturbed even after
acid addition during the synthesis. Meanwhile, notable absorption bands appeared at 3007–3072 cm−1
(Ar-H) and 1134 cm−1 (Si-Ph). Additionally, the structure of 3 was supported by the MALDI-TOF mass
spectrum (Figure S5), which showed molecular ions at m/z 1452.48 g mol−1 [M + Na]+, calculated value
1452.16.
The crystal structure (Figure 2) from X-ray crystallography analysis strongly supported the
formation of 3. There are two different siloxane bonds (Si-O-Si) that differ in their positions linked
to two decks of tetracyclic siloxane: 1) Si(2)-O(5)-Si(6) and Si(4)-O(7)-Si(8), which connect the pairs
of cyclotetrasiloxane decks in the middle; and 2) Si(3)-O(6)-Si(7), which join at the end positions.
The average Si-Si distances are 3.220(3) and 3.237(6) Å, which fall within the range commonly found
in siloxanes. Moreover, the average Si-O-Si angle in 3 is about 144.9(4)–165.2(5) Å, which is larger
than that of T8-Ph (149(5) Å) [40] and symmetrical double-decker silsesquioxanes (140.1(1)–165.2(1)
Å) [18] due to the constraints imposed by the ring structure. In these three cases, the Si-O-Si bonds
are linked to form a six-faced well-defined structure, but compound 3 forms a structure with eight-
and twelve-membered rings. These results show that bond expansion occurred due to the disiloxane
linkage. The resulting structure is similar to a basket in the view of DDSQ as the core and the flexible
disiloxane as the handle.

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Figure 2. X-ray crystallography data. (
of in perspective view. Ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted
for clarity. Black: carbon; blue: silicon; red: oxygen; green: chlorine. ( ) Side view of with number
labels. Hydrogen atoms and phenyl rings are omitted for clarity. Yellow: silicon; red: oxygen.
a) ORTEP (Oak Ridge Thermal- Ellipsoid Plot Program) diagram
3
b
3
Thermogravimetric analysis (TGA) of 3 (Figure 3) was performed under air at a heating rate
◦
◦
◦
of 10 C/min, over the temperature range of 30 C to 1000 C. The Td5 (5% weight loss) values in
Table 2 show that this material has high thermal stability up to 417 ◦C, but its stability is significantly
lower than that of the octaphenylsilsesquioxane (T8-Ph) with a symmetrical cage structure and
di(diphenylsilyl)-bridged double-decker octaphenylsilsesquioxane (Sym-T8D2-Ph) because of the
loose structural packing. At a high temperature of 1000 ◦C, a small amount of residual silica is left;
about 22%, was observed. It is noteworthy that compound 3 showed similar thermal stability to
T8-Ph even containing d-siloxane moiety. This fact also indicates that 3 is the promising precursor of
thermally-stable well-defined materials.
Figure 3. TG/DTA results obtained under nitrogen atmosphere. Heating rate is 10 ◦C/min.
Table 2. TG/DTA results obtained under nitrogen atmosphere.
Compound
Si+O Ratio (%)
Td5 (◦C)
439
Residue at 1000 ◦C (%)
T8-Ph [18]
sym-DDSQ-Ph (T8D2)
40
35
35
50
N/A
22
425
[18]
3
417

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3. Materials and Methods
3.1. Materials and Instruments
Reaction solvents: toluene, 2-propanol, and tetrahydrofuran (THF) were purchased from FUJIFILM
Wako Chemical Industry, Ltd. (Osaka, Japan). Pure phenyltrimethoxysilane and dichlorodiphenylsilane
were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) and used without
further purification. Sodium hydroxide was purchased from Kanto Chemical Co. Inc. (Tokyo,
Japan). Triethylamine was distilled and stored over potassium hydroxide under argon atmosphere,
with protection from light. Toluene was dried over calcium hydride (CaH2) and stored under argon
atmosphere with 4 Å activated molecular sieves until use.
Fourier transform nuclear magnetic resonance (NMR) spectra were recorded on JEOL JNM-ECA
(Tokyo, Japan) 400 (1H at 399.78 MHz) and JEOL JNM-ECS 600 (13C at 150.91 MHz and 29Si at
119.24 MHz) instruments, in CDCl3 using SiMe4 (TMS) as an internal standard. Chemical shifts were
reported as
δ units (ppm), and the residual solvent peaks were used as standards. Elemental analyses
were performed by the Center for Material Research by Instrumental Analysis (CIA), Gunma University,
Japan. MALDI-TOF mass analysis was carried out with a Shimadzu AXIMA Performance instrument
(Kyoto, Japan) using 2,5-dihydroxybenzoic acid (dithranol) as the matrix and AgNO3 as the ion
source. Infrared spectra were obtained using a Shimadzu FTIR-8400S instrument. TGA analyses were
performed on a Rigaku thermal gravimetric analyzer (Thermoplus TG-8120, Tokyo, Japan). All samples
were heated from 30 ◦C to 1000 ◦C at a heating rate of 10 ◦C min−1, under N2. The crystal data were
collected on a Rigaku XtaLab P200 diffractometer with multi-layer mirror monochromated Mo Kα
radiation (λ = 0.71075 Å). The structures were solved by direct methods (SHELXS-974), and refined by
full-matrix least-squares procedures on F2 for all reflections (SHELXL-97). All the non-hydrogen atoms
were refined anisotropically. All hydrogens were positioned using AFIX instructions. All calculations
were carried out using Yadokari-XG2009 (Sendai, Japan).
3.2. Synthesis of Unsymmetrical T8D2, 3
1,3-Dichloro-1,1,3,3-diphenyldisiloxane ((ClPh2Si)2O,
octasilsesquioxanolate (DDSQ-ONa, ) were freshly prepared according to reported procedures [21
Compound (0.15 mL, 0.86 mmol) was added dropwise to a solution of (DDSQ-ONa,
1
) and tetrasodiooctaphenyltetracyclo-
2
,
37].
1
2
) (1.0 g, 0.86
◦
mmol), triethylamine (0.24 mL, 1.72 mmol), and dried toluene (10 mL) over 15 min at 0 C under
argon atmosphere. Stirring was further continued for 3 h at room temperature. Subsequently, 1 M
HCl (aq) (2 mL) in toluene (20 mL) was added to this solution and stirring was continued for 1 h at
60 ◦C. Then, the organic layer was washed with water (5 mL), aqueous sat. NaHCO3 (5 mL), and water
(10 mL). The organic layer was dried over anhydrous Na2SO4, filtered, and evaporated. The crude
product was obtained as the mixture between the white solid of the target compound and the viscous
liquid of undesired byproduct. The target product (Rf = 0.41) was isolated by column chromatography
(chloroform/hexane (4:6)) and recrystallized from a mixture of dichloromethane and 2-propanol (1:1)
to obtain colorless crystals (0.15 g; 12% yield).
1
Spectral data for 3: H-NMR (400 MHz, CDCl3):
δ
7.11–7.19 (m, 20H), 7.26–7.46 (m, 24H), 7.49–7.51
127.46 (CH), 127.56
(m, 4H), 7.58–7.60 (m, 8H), 7.69–7.71 (m, 4H). 13C-NMR (150.9 MHz, CDCl3):
δ
(CH), 127.70 (CH), 127.85 (CH), 129.88 (CH), 130.23 (CH), 130.31 (CH), 130.40 (C), 130.70 (CH), 131.98
(C), 133.97 (CH), 134.09 (CH, C; overlapped), 134.14 (CH), 134.43 (C), 134.48 (CH) ppm. 29Si-NMR
(119.2 MHz, CDCl3): δ −45.4,
−
76.6,
−
78.2, 78.4 ppm. IR (KBr): 494, 521, 696, 729, 741, 999, 1028,
−
1090, 1113, 1134, 1431, 1595, 3007, 3049, 3072 cm−1. MALDI-TOF MS (m/z): 1452.48 ([M + Na]+, calcd.
1452.16).
4. Conclusions
Tetraphenylsiloxyl-bridged double-decker octaphenylsilsesquioxane (
3), an unsymmetrical DDSQ,
was successfully synthesized starting from DDSQ-ONa under mild conditions and characterized using

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various techniques. The Td5 value of
3
at 417 ◦C indicated its high thermal stability. Because of its
unique structure within all-phenyl groups, this compound may be advantageous for the encapsulation
of ions and photochemical applications. Additionally, this simple synthesis protocol would be useful
as a prototype for the preparation of organofunctional siloxane compounds. The disiloxane precursor
with other reactive and/or unreactive organic functional groups could be introduced in the future.
Supplementary Materials: Additional characterization data are available as electronic supporting information:
Figure S1 1H-NMR (400 MHz, CDCl3) spectrum of ; Figure S2 13C{1H}-NMR (150.9 MHz, CDCl3) spectrum
3
of
CDCl3) spectrum of
thermogram of
3
; Figure S3 13C-DEPT-90 NMR (150.9 MHz, CDCl3) spectrum of
3
; Figure S4 29Si-{1H} NMR (119.2 MHz,
; Figure S7 TG/DTA
at a heating rate of 10 ◦C/min under nitrogen; Figure S8 ORTEP diagram of
3 with atomic and
3
; Figure S5 MALDI-TOF MS spectrum of ; Figure S6 IR spectrum of 3
3
3
number labels; Figure S9 Packing diagram for 3; Table S1 Crystal data and structure refinement for 3; Table S2
Bond lengths [Å] and bond angles [◦] for 3.
Author Contributions: Conceptualization, R.K. and M.U.; methodology, R.K.; validation, M.U. and N.T.; formal
analysis, R.K. and N.T.; resources, M.U.; writing—original draft preparation, R.K.; writing—review and editing,
M.U. and N.T.; visualization, N.T.; supervision, M.U.; project administration, M.U.; funding acquisition, M.U.
Funding: This work was partly 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). APC was sponsored by MDPI.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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