Thermosalience in Macrocycle-Based Soft Crystals via Anisotropic Deformation of Disilanyl Architecture

Article abstract of DOI:10.1021/jacs.0c03643

We describe here the preparation of soft crystals using disilanyl macrocycle C4 possessing four p-phenylenes circularly connected by four flexible disilane bonds. Single crystals of C4 exhibited a reversible thermal single-crystal-to-single-crystal (SCSC)

Full text of DOI:10.1021/jacs.0c03643

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Article
Thermosalience in macrocycle-based soft crystals
via anisotropic deformation of disilanyl architecture
Kenichiro Omoto, Toyotaka Nakae, Masaki Nishio, Yoshinori Yamanoi, Hidetaka
Kasai, Eiji Nishibori, Takaki Mashimo, Tomohiro Seki, Hajime Ito, Kazuki Nakamura,
Norihisa Kobayashi, Naofumi Nakayama, Hitoshi Goto, and Hiroshi Nishihara
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c03643 • Publication Date (Web): 22 Jun 2020
Downloaded from pubs.acs.org on June 22, 2020
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Thermosalience in macrocycle-based soft crystals via anisotropic
deformation of disilanyl architecture
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†
, ‡
†
†
†,
§
Kenichiro Omoto, Toyotaka Nakae, Masaki Nishio, Yoshinori Yamanoi, * Hidetaka Kasai, Eiji
§
∥
∥
∥, ⊥
#
Nishibori, Takaki Mashimo, Tomohiro Seki, Hajime Ito,
Kobayashi, Naofumi Nakayama, Hitoshi Goto, and Hiroshi Nishihara *
Kazuki Nakamura, Norihisa
#
¶
Δ
†,
†
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
‡
Division of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama-cho, Ikoma 630-
0
192, Japan
§
Division of Physics, Faculty of Pure and Applied Sciences, Tsukuba Research Center for Energy Materials Science
(TREMS), University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan
∥
Division of Applied Chemistry & Frontier Chemistry Center, Faculty of Engineering, Hokkaido University, Sapporo,
Hokkaido 060-8628, Japan
⊥
Institute for Chemical Reaction Design and Discovery (WPI-ICReDD), Hokkaido University, Sapporo, Hokkaido, 060-
8
628, Japan
#
Department of Image and Materials Science, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-
ku, Chiba 263-8522, Japan
¶
CONFLEX Corporation, 3-23-17 Takanawa, Minato-ku, Tokyo 108-0074, Japan
Δ
Department of Computer Science and Engineering, Toyohashi University of Technology, 1-1 Tempaku-cho, Toyohashi,
Aichi 441-8580, Japan
ABSTRACT: We describe here the preparation of soft crystals using disilanyl macrocycle C4 possessing four p-phenylenes
circularly connected by four flexible disilane bonds. Single crystals of C4 exhibited a reversible thermal single-crystal-to-
single-crystal (SCSC) phase transition behavior between two crystal phases accompanied with remarkable mechanical motion
(
thermosalient effect), as revealed by thermal analyses and X-ray diffraction measurements. Detailed structural analyses
implied flexibility of parallelogram disilanyl architecture and molecular packing mode via weak intermolecular interactions
facilitated structural transformation (parallel crank motion) of macrocycles concerted in crystal, thus resulting in SCSC phase
transition accompanied with anisotropic shrinking/elongation of the cells to induce the thermosalient effect. This work
explores a new area of organosilicon chemistry, and presents a potential utility of disilanyl macrocycles as soft crystals.
1. Introduction
Therefore, exploring the various factors that control the
formations and phase transitions of soft crystals is required
to get insight about their design direction (e.g., topology,
flexibility, and intermolecular interactions).
Molecular crystals are widely investigated as functional
solid materials because they exhibit chemical/physical
properties specific to the cooperative interactions among
their highly ordered component molecules.1 Examples
exhibiting a single-crystal-to-single-crystal (SCSC) phase
transition under simple external stimuli (e.g., heat, light, or
2
pressure) have attracted attention and recently been
proposed as ‘soft crystals’. The remarkable changes in the
3
photophysical, mechanical, and electrochemical properties
of soft crystals, induced by external stimuli affecting their
molecular packing, make them a promising new category of
2
environment-responsive
materials.
In particular,
conformational changes of molecules in crystals have been
recently attracted high attention, because molecular-level
motion can be integrated into significant macroscopic
2
motion in such precise and long-range ordering systems.
However, understanding and predicting the crystal phase
transitions and polymorphism remain challenging.
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Figure 1. Crystal structure of C4α: a) side view and b) front view measured at 0 °C. Crystal structure of C4β: c) side view
and d) front view measured at −183 °C. Hydrogen atoms in a–d) and methyl groups in a) and c) are omitted for clarity.
Atoms in b) and d) are ORTEP or ball-and-stick models drawn at 50% probability level, respectively. C, gray; Si, blue.
Molecular packing and temperature dependency of the crystal structures of e) C4α (C, orange; Si, blue, 0 °C) and f) C4β
(
C, purple; Si, pale blue, −183 °C) along the a-axis. g) Comparison of y/x ratio of C4α (0 °C) and C4β (−183 °C).
The circular structures of macrocycles have been widely thermal phase transition accompanied by mechanical
investigated because of their potential use in molecular
recognition, catalysis, and optical materials. Macrocycles
motion (thermosalient effect), where crystals suddenly
jump and/or explode during the thermal phase
4
2
b,10
possessing a flexible molecular skeleton have been
particularly studied as they can change their conformation
in response to external factors (e.g., solvent, temperature,
or presence of guest molecules), enabling switching/tuning
transition.
Detailed structural analyses suggested that
the flexibility of the parallelogram structure of C4 with
rotatable Si–Si bonds and the molecular packing mode via
weak intermolecular interactions promoted parallel crank
motion of macrocycles concerted in crystal, resulting in
SCSC phase transition.
5
of their functionalities. Although conformational flexibility
of macrocycles has mainly been studied in solution so far,
they can also provide a cue to develop soft crystals by
designing molecular skeleton which realize high
crystallinity and conformational flexibility at the same
2. Results and discussion
Disilanyl macrocycle C4 was prepared by sequentially
connecting aromatic rings and disilane units (Scheme 1).
Firstly, mono-disilyl linear precursors (1,2-bis(4-
halophenyl)-1,1,2,2-tetramethyldisilanes, 2a and 2b) were
prepared by mono-lithiation of the corresponding 1,4-
dihalobenzene 1a and 1b by n-BuLi and subsequent
reaction with 1,2-dichloro-1,1,2,2-tetramethyldisilane. A
tris-disilyl linear precursor 3 possessing two hydrodisilyl
moieties at the both terminals was prepared by dilithiation
of 2b by n-BuLi and subsequent reacting with excess
equivalent of 1,2-dichloro-1,1,2,2-tetramethyldisilane
6
time. However, it is still challenging to utilize
conformational flexibility of macrocycles to the SCSC phase
transition, since intrinsic flexible molecular motion can be
prevented within crystals due to the packing effect. To
utilize conformational flexibility of macrocycle to SCSC
phase transition, it is required to design crystals which
realize conformational change in concerted manner among
multiple molecules in the crystals.
This work describes the crystal phase transition behavior of
disilanyl macrocycle C4 composed of four disilane (Si–Si)
4
followed by hydrogenation with LiAlH . The resulting
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,8
bonds and four p-phenylenes (Scheme 1). Incorporation
of silicon atoms into π-conjugated system has been
developed as an effective means to create novel emissive
materials, and disilane-bridged arenes generally possess
flexible molecular structures owing to the σ-bond character
product 3 is stable enough to be isolated by recrystallization
and stored in ambient condition for prolonged time. After
intermolecular cyclization reaction between 2a and 3 via
Pd-catalyzed
coupling
reaction
and
subsequent
purification, tetrameric macrocycle C4 was isolated in 5%
9
of Si–Si units. The bulky tetrahedral structure of Si atoms
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yield as a stable product. The yield of C4 was not high
suppresses the formation of strong π–π interactions,
providing a possibility of conformational changes in the
solid state. We envision that macrocyclic structures based
on aryl disilanes have a potential to exhibit crystal
polymorphism due to the flexibility and structural
bulkiness of the Si–Si bonds which allows crystallization in
various conformer. Herein, we found that C4 exhibits a SCSC
because of the side reaction in the cyclization reaction
which provided oligomeric silane compounds as
byproducts. C4 was characterized by high resolution ESI-
+
TOF mass measurement (m/z = 791.3033 as [C4 + Na] ) and
1
NMR spectroscopy. The H NMR spectra in CDCl
only two signals (CH and Ar–H) at 300 and 220 K, which
suggests a D4h-symmetrical structure with free rotation of
3
showed
3
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aromatic rings along Si–Si and Si–C bonds (Fig. S7). The up-
heating from −196 °C at 10 °C/min caused the crystals to
start moving again at above approximately −150 °C (Figs.
2b, S17b, S18b, and Movie S2).
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field shift of aromatic signals of C4 (δ = 7.06 ppm) compared
with those of precursor 3 (δ = 7.39–7.33 ppm) suggests
shielding effect by the neighboring aromatic rings owing to
1
3
In our observation, single crystals seem to exhibit
thermosalient effect from C4α to C4β accompanied with
cracking along (010) or (001) planes. Such plane specific
cracking might reflect relatively weak intermolecular
interaction (no CH– interaction) in the b- and c-axes
directions in crystals (Fig. S12). Stress accumulation which
induces cracking in these specific directions might be one of
the plausible driving forces for the thermosalience in our
system.
1
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its compact cyclic structure.
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The thermo-responsive behavior of C4α and C4β was
also studied by differential scanning calorimetry (DSC) and
variable temperature-powder X-ray diffraction (VT-PXRD).
The DSC profile of C4α shows an exothermic peak during
cooling and an endothermic peak during subsequent
heating at −130 °C ~ −140 °C, matching well the above
1
3
observations of mechanical movement (Fig. 3). These
peaks in the DSC traces are zig-zag pattern, which is typical
to the cases of already reported thermosalient effect of the
10b
single crystals.
The VT-PXRD measurements of the
corresponding crystals showed reversible changes between
the diffractions of C4α (high-temperature phase) and C4β
(low-temperature phase) at −130 °C ~ −150 °C (Figs. 4 and
S14). |ΔH| values (transition enthalpy) were calculated as
Scheme 1. Synthesis of C4. i) n-BuLi, Et
,2-dichlorotetramethyldisilane, −78 °C to rt, iii) n-BuLi,
THF, −78 °C, iv) 1,2-dichlorotetramethyldisilane, THF,
78 °C to rt, v) LiAlH , 0 °C to rt, vi) Pd(P(t-Bu) , (i-
Pr) EtN, toluene, 0 °C.
2
O, −78 °C to rt, ii)
1
2
3.0 × 10 cal/mol from the both endo- and exothermic peaks
−
4
3 2
)
in the DSC chart. Such small |ΔH| values are consistent with
SCSC phase transition accompanied with subtle structural
changes. Overall, these observations suggest the
thermosalient effect originates from the reversible crystal
2
The X-ray diffraction analyses of C4 revealed its circular
molecular structure and the crystal polymorphism (α- and
β-phases, hereafter abbreviated as C4α and C4β,
respectively) based on deformation of the cyclic skeleton.
First, single crystals of C4α were obtained as colorless
blocks upon slow evaporation of a solution of C4 in
14
phase transition between C4α and C4β.
2 2
CH Cl /toluene at room temperature. The crystal structure
of C4α at 0 °C contained four p-phenylenes circularly
arranged via Si–Si bonds (Fig. 1a,b). Each Si–Si bond stands
orthogonally to the π-planes of adjacent p-phenylenes in
i
anti- and syn-configurations alternatively to afford a C -
symmetrical chair-like conformation. In this crystal, C4
forms a columnar arrangement along the a-axis (Fig. 1e).
Notably, there is no intermolecular π–π stacking among the
2 2
macrocycles. Bulky SiMe –SiMe moieties standing almost
orthogonally to adjacent π-planes seem to hamper close
contact among them (similar to many reported examples of
aryl disilanes), resulting in a specific molecular packing
through isotropic intermolecular interactions (e.g., CH–π or
9
van der Waals interactions, Fig. S11).
The second phase, C4β (Fig. 1c,d) was obtained from
C4α via a thermal phase transition accompanied by abrupt
movements (e.g., jumping, explosion, or rolling), namely
2
b,10
thermosalient effect.
Upon cooling C4α crystals on a
variable temperature stage from 30 °C to −196 °C at 10
°C/min, the crystals initially remained intact down to −150
°
C. However, from around −155 °C they started jumping or
exploding (Figs. 2a, S17a, S18a, and Movie S1), thus
suggesting the onset of thermosalient effect at around this
temperature. Thermosalient effect in the resultant C4β
crystals were also observed during subsequent heating:
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Figure 4. PXRD patterns (synchrotron X-ray, λ = 1.08 Å)
of a) C4α and b) C4β measured at −133 °C and −153 °C,
respectively.
The molecular structure of C4β was clarified based on
PXRD profiles at −183 °C and compared with that of C4α
Figs. 1, S8 and Table S1). The molecular structure (Fig.
c,d) and packing mode of C4β (Fig. 1f) are basically same
as those of C4α (Fig. 1a,b,e). In both crystals, the C4
molecules display chair-shaped conformers that stacked
with each other via CH–π interactions along the a-axis. The
thermal phase transition did not alter the space group and
(
1
Figure 2. Microscopic images on a variable temperature
stage illustrating the thermosalient effect of C4α or C4β
subjected to a) cooling and b) heating processes
between 30 °C and −196 °C. Crystals at a) −155 °C during
cooling and b) −50 °C during heating. Crystals exhibiting
thermosalient effect are marked with red arrow.
Representative images were shown in the figures
selected as clear thermosalient phenomena. Movement
of crystals during whole cooling and heating cycles can
be seen in the Supporting Information (Figs. S17 and
S18) and Supplementary Movies.
crystal
system,
but
drastic
anisotropic
shrinking/elongation of the cells was observed in the b- and
c-axes (b-axis: ca. –1.4 Å (−11%), c-axis: ca. +1.1 Å (+8%),
Table S1). Similar great anisotropic changes in lattice
dimensions have been reported previously in thermosalient
crystals and have been proposed as one of the possible
2
b,10c,i
mechanisms underlying the mechanical movement.
When projected on the (100) planes in both crystal systems,
C4 forms a parallelogram structure with four Si–Si bonds at
the apexes standing almost perpendicular to the (100)
planes (Fig. 1e,f). The aspect ratio of the parallelogram
structure (defined as y/x; where y = Si3−Si3’ and x =
Si1−Si1’, Fig. 1g) changed from 1.03 to 1.32 as C4α changed
to C4β accompanied by rotations around the Si–Si bonds
and changes in the Si–Si–π angles (Fig. 1a,c and Fig. S8). It
should be noted that parallel crank motion of the
parallelogram structure seems to realize concerted
structural change of macrocycles packed within crystals.
Such structural change of macrocycles seems to be
facilitated by rotation of the flexible Si–Si bonds at the
apexes and the absence of strong π–π interaction. This
allows a thermal crystal phase transition with drastic lattice
elongation/contraction
and
minimal
molecular
rearrangement leading to thermosalient behavior.
According to Naumov’s classification of thermosalient
materials, C4α and C4β can be categorized as class II, in
which bulky and flexible molecules are shielded from strong
intermolecular interactions in crystals showing
Figure 3. DSC trace of single crystals of C4. Scan rate:
5 °C/min under N
2
. The inset shows the magnified
trace around transition temperature.
10b
conformational changes leading to thermosalient effect.
Finally, we performed the molecular structure
optimization in each crystalline environment using
1
5
16
CONFLEX 8.B with the MMFF94s force field to obtain
some insight into the crystal energy difference between
C4α and C4β. The crystal energies were obtained after
optimization of each conformer with fixed lattice constants
based on X-ray crystallographic data as the initial packing
models. These optimized structures are in good agreement
with those in X-ray structures (Fig. S19, Table S2). The
calculated energies are listed in Table S3. The
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In conclusion, C4 exhibited a thermal SCSC phase
intramolecular energy of C4α (158.7 kcal/mol) is smaller
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than that of C4β (161.2 kcal/mol). This result suggests that
the conformation of C4α is more stable than that of C4β. In
fact, the conformation (e.g. Si–Si–π angels) of C4α is similar
to that of the minimum-energy structure optimized under
isolated condition using MMFF94s, whereas that of C4β
deviates from it (Fig. S19, Table S2). The relationship of
stability between C4α and C4β is also supported by
quantum chemical calculations (Fig. S20 and Table S4). The
intermolecular energy is dominated by van der Waals
energy and charge energy is almost negligible. The van der
Waals energy of C4α (–48.0 kcal/mol) is smaller than that
of C4β (–45.8 kcal/mol) probably due to a looser packing of
macrocycle in C4α than in C4β (Fig. S11).
transition accompanied by salient effect. Detailed structural
analysis revealed that C4 formed parallelogram structure
within the crystals with flexible Si–Si bonds at the apexes.
The concerted parallel crank motion of macrocycles packed
within the crystal was realized by flexibility of the disilanyl
architecture and the molecular packing via weak
intermolecular interactions, thus resulting in anisotropic
shrinking/elongation of the cells to induce the
thermosalient effects. Our result provides a design direction
to utilize intrinsic conformational flexibility of molecules to
SCSC transformation accompanied with mechanical
motions.
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ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on the
ACS
Publications
website
at
DOI:
pubs.acs.org/doi/10.1021/jacs.
Abbreviations, experimental section, crystal structure of C4,
powder X-ray analysis, visual observation of thermosalient
behavior, and theoretical calculation (PDF). CIF files of C4α and
C4β (cif). Observations of thermosalienece (movie).
AUTHOR INFORMATION
Corresponding Author
*
5
To whom correspondence should be addressed. TEL: +81-3-
841-4348. E-mail: yamanoi@chem.s.u-tokyo.ac.jp;
nisihara@rs.tus.ac.jp.
ORCID
Yoshinori Yamanoi: 0000-0002-6155-2357
Hiroshi Nishihara: 0000-0002-6568-5640
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Figure 5. a) Changes in intermolecular energy of the
virtual crystals of C4α with the extension of
intermolecular distance. b) Crystal packing with cell
axes and c) face-indexing graphics of C4α.
This work was financially supported in part by the Mikiya
Science and Technology Foundation (Y.Y.), a grant from the
Institute for Fermentation, Osaka (IFO; No. G-2019-3-001;
Y.Y.), the Naohiko Fukuoka Memorial Foundation (Y.Y.), the
Yashima Environment Technology Foundation (Y.Y.), Tonen
General Sekiyu Research/Development Encouragement &
Scholarship Foundation (Y.Y.), a Grant-in-Aid for Young
Scientists (No. 19K15530; K.O.), a Grant-in-Aid for Scientific
Research (C) (No. 19K05627; Y.Y.), and Scientific Research on
Innovative Area “Soft Crystal: Science and Photofunctions of
Flexible Response Systems with High Order” (area 2903, Nos.
17H06369 (Y.Y.), 18H04499 (E.N.), 17H06370 (H.I.),
17H06377 (N.K.), and 17H06373 (H.G.)) from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan. The
synchrotron radiation experiments were performed at BL02B2
of SPring-8 (Proposal Nos. 2018B0074 and 2019A0066). We
are grateful to Dr. Hiroshi Sato for useful discussions.
To discuss relationship between the directions of
intermolecular interaction and that of crystal cracking
during the thermosalient phenomenon, we estimated the
intermolecular energies of virtual crystals in which the
lattice constants a, b, and c were extended from original C4α
crystal structure by 0.1 Å (Fig. S21). As shown in Figure 5a,
the increment of energies along with b- and c-axes
directions are smaller than that in the a-axis direction,
suggesting smaller intermolecular interaction in these two
axes direction than a-axis. This result is consistent with the
above-mentioned visual observation of the thermosalient
phenomenon which often accompanied with cracking along
(
010) or (001) planes (Figs. 2 and 5b,c).
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2, 10237–10240. (c) Shima, T.; Muraoka, T.; Hoshino, N.;
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Structure–Kinematic Aspects of the Self-Actuation and Motility of
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Dericioglu, A. F.; Reddy, C. M.; Naumov, P. Kinematic and
Mechanical Profile of the Self-Actuation of Thermosalient Crystal
Twins of 1,2,4,5-Tetrabromobenzene: A Molecular Crystalline
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13843−13850. (d) Nath, N. K.; Panda, M. K.; Sahoo S. C.; Naumov, P.
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(12) Similar up-field NMR chemical shifts of aromatic hydrogens
were also observed in the case of smaller disilane-bridged
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(
macrocycles. For instance, the
octamethyltetrasila[2.2]cyclophane (δ = 6.79 ppm) locates upper
field than that of acyclic substructure, 1,4-
H
NMR signals of
bis(pentamethyldisilanyl)benzene (δ = 7.38 ppm). Sekiguchi, A.;
Yatabe, T.; Kabuto, C.; Sakurai, H. 1,1,2,2,9,9,10,10-Octamethyl-
1,2,9,10-tetragerma[2.2]paracyclophane:
the
first
[2.2]paracyclophane bridged by two Ge–Ge bonds. J. Organomet.
Chem. 1990, 390, C27–C31.
1
850‒1858. (e) Panda, M. K.; Runčevski, T.; Sahoo, S. C.; Belik, A.
(13) Single crystals exhibit the thermosalient effect with wide
range temperature by the visual observation (Figs. 2, S17, S18, and
Movies S1–S4). One of the possible reasons is different size, shape,
and orientation of C4 single crystals which would cause various
heating/cooling conduction to the crystals from the temperature
controllers in each measurement/observation. Due to these
factors, the temperature of the thermosalient effect of single
crystals depends on measurement/observation methods. Similar
phenomena (e.g. crystal-size dependency of the temperature of
thermosalient) were recognized in previously reported
A.; Nath, N. K.; Dinnebier, R. E.; Naumov, P. Colossal positive and
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Figure 1. Crystal structure of C4α: a) side view and b) front view measured at 0 °C. Crystal structure of
C4β: c) side view and d) front view measured at −183 °C. Hydrogen atoms in a–d) and methyl groups in a)
and c) are omitted for clarity. Atoms in b) and d) are ORTEP or ball-and-stick models drawn at 50%
probability level, respectively. C, gray; Si, blue. Molecular packing and temperature dependency of the
crystal structures of e) C4α (C, orange; Si, blue, 0 °C) and f) C4β (C, purple; Si, pale blue, −183 °C) along
the a-axis. g) Comparison of y/x ratio of C4α (0 °C) and C4β (−183 °C).
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Figure 2. Microscopic images on a variable tempera-ture stage illustrating the thermosalient effect of C4α or
C4β subjected to a) cooling and b) heating process-es between 30 °C and −196 °C. Crystals at a) −155 °C
during cooling and b) −50 °C during heating. Crystals exhibiting thermosalient effect are marked with red
arrow. Representative images were shown in the fig-ures selected as clear thermosalient phenomena.
Movement of crystals during whole cooling and heat-ing cycles can be seen in the Supporting Information
(Figs. S17 and S18) and Supplementary Movies.
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Figure 3. DSC trace of single crystals of C4. Scan rate: 5 °C/min under N2. The inset shows the magnified
trace around transition temperature.
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Figure 4. PXRD patterns (synchrotron X-ray, λ = 1.08 Å) of a) C4α and b) C4β measured at −133 °C and
−153 °C, respectively.
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Figure 5. a) Changes in intermolecular energy of the virtual crystals of C4α with the extension of
intermolecular distance. b) Crystal packing with cell axes and c) face-indexing graphics of C4α.
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Scheme 1. Synthesis of C4. i) n-BuLi, Et2O, −78 °C to rt, ii) 1,2-dichlorotetramethyldisilane, −78 °C to rt,
iii) n-BuLi, THF, −78 °C, iv) 1,2-dichlorotetramethyldisilane, THF, −78 °C to rt, v) LiAlH4, 0 °C to rt, vi)
Pd(P(t-Bu)3)2, (i-Pr)2EtN, toluene, 0 °C.
1
22x107mm (300 x 300 DPI)
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TOC
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