Metal-Ion-Induced Switch of Liquid-Crystalline Orientation of Metallomacrocycles

Article abstract of DOI:10.1002/chem.201601941

A new series of shape-persistent imine-bridged macrocycles were synthesized based on dynamic covalent chemistry. The macrocycles had an alternating sequence of dibenzothiophene and N,N′-bis(salicylidene)-ethylenediamine (salen) tethering branched alkyl ch

Full text of DOI:10.1002/chem.201601941

DOI: 10.1002/chem.201601941
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Liquid Crystals
Metal-Ion-Induced Switch of Liquid-Crystalline Orientation of
Metallomacrocycles
Shin-ichiro Kawano, Takashi Hamazaki, Atsushi Suzuki, Kenshin Kurahashi, and
Kentaro Tanaka*[a]
Chem. Eur. J. 2016, 22, 1 – 11
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Abstract: A new series of shape-persistent imine-bridged
macrocycles were synthesized based on dynamic covalent
chemistry. The macrocycles had an alternating sequence of
dibenzothiophene and N,N’-bis(salicylidene)-ethylenediamine
(salen) tethering branched alkyl chains. The macrocycles and
tetranuclear metallomacrocycles bearing long and branched
alkyl chains exhibited thermotropic columnar liquid-crystal-
line phases over a wide temperature range and the metallo-
macrocycles greatly depended on the characteristics of the
coordinated metal ions. The metal-free macrocycle showed
a liquid-crystalline phase with a lamellar structure and poor
birefringence. In sharp contrast, the macrocyclic Ni complex
showed a columnar oblique liquid-crystalline phase, whereas
the Pd and Cu complexes showed columnar liquid-crystalline
phases with a lamellar structure. The macroscopic organiza-
tion and thermal properties of the corresponding liquid-crys-
talline metallomacrocycles were significantly dependent on
the subtle structural differences among the planar macrocy-
cles, which were revealed by single-crystal X-ray crystallo-
graphic analysis of the macrocycles with shorter alkyl chains.
ture in flowable media. In pioneering works, Moore[13] and
Hçger[14] reported shape-persistent macrocycles composed of
phenylacetylene units in macrocyclic frameworks, which exhib-
ited thermotropic LC properties. However, there is a demand
for utilizing heteroatoms or components other than hydrocar-
bons to design such columnar LC macrocycles with additional
functions such as specific recognition, storage, construction of
arrays, transportation, and chemical reactions of entrapped
molecules.[15–17]
Introduction
Liquid crystals are functional anisotropic fluids and are suscep-
tible to not only external stimuli but also their own molecular
structures. In some soft materials, structural transformations at
the molecular scale can change the structural and thermal
properties of the liquid crystals in macroscopic domains. Under
external stimuli, chiral dopants for the cholesteric mesophase,[1]
molecular motors,[2] and photochromic molecules on the sur-
face[3] have been known to induce reorientation of liquid crys-
tals. In contrast, it was recently reported that even an intramo-
lecular structural change in topologically entangled supra-
molecular complexes[4] could control liquid-crystalline (LC)
properties, leading to novel designs for unconventional liquid
crystals. In this regard, metal-containing liquid crystals are fas-
cinating molecules because transition-metal ions could lead to
distinctive properties involving binding of LC ligands,[5,6a]
metal–metal[6b] and metal–ligand interactions,[6c,d] and the lumi-
nescence of lanthanide complexes,[6e] rendering the LC materi-
als promising for functional materials exhibiting magnetic,[7]
electrical,[6d,8] and electro-optical properties.[9] Moreover, even
among the transition-metal ions with the same coordination
geometry, subtle structural differences in the metal ions (here-
after called “atomic scale differences”) could induce macro-
scopic organization of the assembly.[10] For example, it has
been reported that the phase structures and thermal proper-
ties of LCs with metallomesogens such as N,N’-bis(salicylidene)-
ethylenediamines (salens),[10] porphyrins,[8b,c,11] phthalocya-
nines,[12] and supramolecular architectures[4a] significantly de-
pended on the coordinated metal ions with the same coordi-
nation geometry.
Recently, we reported shape-persistent imine-based macro-
cycles with discrete inner pores exhibiting well-developed ther-
motropic columnar LC phases.[18] The macrocycles are formed
spontaneously through efficient dynamic imine formation[19,20]
so that the resultant planar macrocycle self-assembled into col-
umnar LC phases over a wide range of temperatures. More-
over, after incorporation of transition-metal ions into the N,N’-
bis(salicylidene)-o-phenylenediamine (salphen) moieties of the
macrocycle, it was found that the resultant metallomacrocycles
significantly changed the thermal behavior of the columnar LC
phases. The metallomacrocycles with an inner cavity as large
as 1 nm in diameter showed thermotropic columnar LC phases
in a reversible fashion.[18]
Herein, we report our strategy to induce macroscopic LC or-
ganization by controlling these atomic scale differences.[10,21] In
LC metallomacrocycles, macroscopic organization of columnar
LC phases could be effectively manipulated by tuning the
design of the macrocyclic mesogen on the atomic scale. This
design strategy would be effective to conveniently make
a new series of LC metallomacrocycles to form flowable nano-
spaces in the LC phase (Figure 1).
Incidentally, it is a challenging problem to construct well-de-
fined, nanometer-sized spaces in LC materials in conjunction Results and Discussion
with flowable properties and the ability to undergo phase tran-
Synthesis of macrocycles composed of salen and dibenzo-
thiophenes
sitions. In particular, columnar liquid crystals based on shape-
persistent macrocyclic compounds have gathered increasing
attention as they are expected to exhibit a nanochannel struc-
In the present study, we designed a series of novel metalloma-
crocycles composed of dibenzothiophenes with tethered
branched alkyl chains. Compared with rather rigid salphen
moieties,[18] we presumed that the more flexible salen in the
macrocyclic mesogen would be susceptible to structural
change induced by the metal coordination. The synthesis of
the LC metallomacrocycles is outlined in Scheme 1. After care-
[a] Dr. S.-i. Kawano, T. Hamazaki, A. Suzuki, K. Kurahashi, Prof. Dr. K. Tanaka
Department of Chemistry, Graduate School of Science
Nagoya University, Nagoya 464-8602 (Japan)
E-mail: kentaro@chem.nagoya-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201601941.
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was isolated with a high yield of 65%. The MALDI-TOF mass
spectrum showed a prominent single signal at m/z=5985.8
([M+H]+), indicating that 4:4 condensation selectively oc-
1
curred. The H NMR spectrum of H81c in CDCl3 showed a set
of proton signals for the dibenzothiophene and 1,2-substituted
ethylenediamine moieties, including a new singlet signal for
the imines at 8.71 ppm, suggesting that the macrocycle has
a D4h symmetric structure. The efficiency of the cyclization de-
rives from two factors: the flexibility of salen and the right
angle geometry of the imines connected to dibenzothiophene.
Similarly, macrocycles H81a and H81b, with other peripheral
alkyl chains, were synthesized with good yields, regardless of
the side chain. Tetranuclear nickel(II), palladium(II), and cop-
per(II) complexes derived from H81 were also prepared by
mixing the macrocyclic ligands with the corresponding metal
acetates (Scheme 1). These macrocycles and their metal com-
plexes were soluble in halogenated solvents, toluene, and THF.
Figure 1. Schematic illustration of the macroscopic change of thermotropic
columnar liquid crystals composed of macrocycles or metallomacrocycles
triggered by atomic scale differences in the flexible macrocycle.
ful choice of the molecular design, a dibenzothiophene with
dialdehydes at the 2- and 7-positions, 2, and a series of ethyle-
nediamines with branched alkyl chains 3a–c were synthesized
according to the synthetic schemes shown in Schemes S1 and
S2 in the Supporting Information. An imine-based cyclization
reaction was utilized as the key step in the synthesis of macro-
cycles H81a–c by 4:4 condensation between 2 and 3a–c. Ini-
tially, as reported previously by the Nabeshima[19] and MacLa-
chlan[20] groups as well as our group,[18] we used o-phenylene-
diamine derivatives containing tethered alkyl chains instead of
ethylenediamine derivatives 3a–c. The 1H NMR spectrum of
the reaction mixture indicated that the target macrocyclic
compound was obtained in moderate yields (less than 37%);
however, it was difficult to isolate the target macrocycle from
the mixture of the intermediates owing to the strong aggrega-
tion tendency of the macrocycle. Unlike the efficient synthesis
of our macrocycle composed of carbazole and salphen,[18] the
result indicates that moderate solubility was necessary to iso-
late the target macrocycles. In contrast, when 3c was mixed
Thermotropic liquid-crystalline properties of macrocycles
and metallomacrocycles
To gain insight into the macroscopic characteristics of the mac-
rocycles, we have investigated the LC properties of the macro-
cycles with longer tethered alkyl chains (both metal-free and
metalated 1b and 1c). The thermal properties were investigat-
ed below the decomposition temperatures, by using thermog-
ravimetric analysis (TGA; Figures S1 and S2 in the Supporting
Information). The macrocycles with shorter alkyl chains, H81b,
and those with Ni and Pd complexes (Ni41b and Pd41b)
showed solid-state characteristics below the decomposition
temperatures (Figures S3, S4, and S5 in the Supporting Infor-
mation). These results could be attributed to the relatively in-
sufficient number of alkyl chains attached to the rigid macrocy-
clic mesogens.[14,18] However, H81c and metal complexes Ni41c,
Pd41c, and Cu41c exhibited thermotropic LC properties owing
to the presence of much longer side chains containing ether
units. Although H81c showed an extremely weak birefringence
without texture corresponding to LC ordering under polarized
1
with 2, a H NMR spectrum of the reaction mixture indicated
that the 4:4 macrocycle was formed predominantly with only
trace amounts of byproducts. After purification by gel permea-
tion chromatography (GPC), the desired 4:4 macrocycle H81c
Scheme 1. Synthesis of the macrocycles and the metallomacrocycles. Reaction conditions: (a) Na2SO4 or NEt3, ethanol, reflux, (b) M(OAc)2, CHCl3/ethanol.
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Figure 2. POM images of the macrocycle (a) H81c at 308C, metallomacrocy-
cles, (b) Ni41c at 258C, (c) Pd41c at 1708C, and (d) Cu41c at 1508C.
optical microscopy (POM, Figure 2a), the POM observation and
differential scanning calorimetry (DSC, Figure S6 in the Sup-
porting Information) revealed that H81c forms a LC phase from
À48C to 848C. Grazing incidence X-ray diffraction (GIXRD) anal-
ysis of H81c showed diffraction peaks indexed as (001), (002),
and (003), indicating a lamellar structure in the LC phase (Fig-
ure 3a). The d-spacing of 33.7 ꢁ can be modeled as a layered
structure of the macrocycles with interdigitation of the side
chains between the layers, and the layered structure fits in the
space filling model. On the basis of the relatively weak birefrin-
gence in the POM images and the GIXRD results, it was sug-
gested that the orientation of the macrocycle was obscure in
the layered mesophase.[22]
Figure 3. GIXRD patterns for (a) H81c at 608C, (b) Ni41c at 1008C, and
(c) Pd41c at 1808C on glass substrates.
ters also take square-planar configurations similar to the Ni
complex. DSC analysis of Pd41c showed a higher clearing
point of 2358C. Below the clearing point, LC phases of Pd41c
were observed between 1538C and 2358C (Figure 2c). The
higher transition temperatures to the LC phase of Pd41c sug-
gest stronger association forces for Pd41c compared with
those of Ni41c. This may be related to the lower solubility of
Pd41a in CDCl3 than Ni41a. Furthermore, compared with the
less ordered lamellar LC phase of H81c (Figure 2a), a distin-
guished broken fan-shaped texture indicating a columnar ar-
rangement was observed in the POM image of Pd41c (Fig-
ure 2c). GIXRD patterns of Pd41c suggested that the complex
adopted a lamellar structure in the LC phases (Figure 3c). The
spacing of 30.4 ꢁ was comparable with the column diameter
estimated from space filling molecular modeling of Pd41c. In-
terestingly, a broad but significant diffraction with a spacing of
11.1 ꢁ was observed in the in-plane direction to the substrate
(see the Supporting Information, Figures S8 and S9), indicating
that there was a repetitive stacking distance of the macrocycle
within the layer in the LC phase. Therefore, we assigned the LC
phase as the lamellar-columnar mesophase, ColL, which is con-
sistent with the POM observation.[22a,b,24] When Pd41c was fur-
ther cooled from 1538C to À108C, it showed a harder crystal-
line phase without fluid properties. The GIXRD results indicated
that the lamellar structure was retained in this phase, implying
that the fluid columnar structure appeared frozen (Figure S8 in
the Supporting Information).[25,26b] Cu41c exhibited bright bire-
In contrast to H81c, Ni41c exhibited strong birefringence
and a clear fan-shaped texture in the POM image upon cooling
from the clearing point between the LC phase and the isotrop-
ic liquid of 1618C (Figure 2b). Although the POM texture did
not exhibit a chiral LC phase, which was expected from associ-
ation of the chiral macrocycles, the fan-shape texture suggest-
ing formation of a columnar association of the macrocycle. As
Ni41c was shearable, the phase between the two transitions at
À178C and 1618C in the DSC (Figure S6 in the Supporting In-
formation) was assigned as the LC phase. As shown in Fig-
ure 3b, GIXRD analysis of Ni41c showed several diffraction
peaks in the small-angle region and they were indexed by ret-
rostructural analysis. The analysis indicated that Ni41c takes on
an oblique columnar phase (Colob) in which the columns are
organized into monoclinic packing, with the following estimat-
ed unit cell parameters: a=67.6 ꢁ, b=65.2 ꢁ, and g=85.28
(Figure 2b and 3b).[22a,b,23] The columnar structure of Ni41c has
higher order that that of H81c, probably owing to the planar
macrocyclic structure induced by coordination of nickel(II) ions
with square-planar geometry. The temperature at which the
isotropic point was reached was also significantly higher for
Ni41c than for H81c.
Similarly, thermal and structural characterizations of the LC
phases of Pd41c and Cu41c were performed. These metal cen-
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fringence and a clear texture consisting of small domains of
the LC phase (Figure 2d). Cu41c formed an LC phase between
1258C and 1898C and a frozen crystalline phase below 1258C
(Figure 4, Figure S4 in the Supporting Information). The GIXRD
patterns of Cu41c suggested that the LC phase was comprised
of a lamellar structure (Figure S8 in the Supporting Informa-
tion). From the results above, we concluded that Pd41c and
Cu41c exhibit thermotropic columnar LC phases in which the
columnar assemblies were arranged into lamellar structures.
These assembled structures could be ascribed to the presence
of columnar structures composed of rigid macrocyclic meso-
gens. It is noteworthy that the LC phases and the self-assem-
bled structures were strongly influenced by the coordination
of square-planar metal ions with the square-planar geometry
to the macrocyclic host. Unlike the related compounds previ-
ously reported,[18] ordering of both the columns and the ther-
mal properties of the metallomacrocycles were drastically influ-
enced by the atomic scale differences in the macrocyclic meso-
gens (Figure 4). The metallomacrocycles showed bright bire-
fringence with clear fan-shaped textures, which represent the
formation of columnar LC phases with large domain sizes,
whereas H81c forms a less-ordered LC phase with weak bire-
fringence. It should be noted that the macrocyclic nickel(II)
complex formed a well-developed columnar oblique LC phase,
whereas the palladium(II) and copper(II) complexes formed la-
mellar-columnar structures in the mesophase. These thermal
and structural variations among the metallomacrocycles occur
as a result of the difference in the flowability of the nickel(II)
complex and the palladium(II) and copper(II) complexes. The
palladium(II) and copper(II) complexes solidified at higher tem-
peratures owing to their higher association ability, and the la-
mellar-columnar structures froze before transitioning to more
ordered structures upon cooling from the clearing points.
Thus, we presumed that the subtle structural differences be-
tween the complexes, such as enhanced planarity and/or the
size of the metal ions in the macrocyclic mesogens induced by
square-planar metal complexation, lead to drastic differences
in the macroscopic order of their LC phases. Therefore, in the
subsequent sections, we discuss the structural analyses of the
metallomacrocycles to reveal the switching of the macroscopic
assembled structure.
Solid-state NMR spectroscopy of macrocycles and metallo-
macrocycles
To gain further insight into the structural and dynamic proper-
ties of the macrocycles, solid-state NMR experiments were car-
ried out by using the magic angle spinning technique.[26,14d]
The cross-polarization magic angle spinning (CPMAS) 13C NMR
experiments were performed at room temperature, at which
metal-free macrocycles H81a and H81b are in the solid state,
whereas H81c forms an LC phase. As shown in Figure S10 in
the Supporting Information, crystalline H81a forms well-re-
solved crystallographic split signals in the aromatic region
(100–180 ppm). The spin-lattice relaxation time (T1C) of H81a
was estimated to be more than 17 s for the aromatic carbons
and 1–3 s for the alkyl chains (Table S5 in the Supporting Infor-
mation). As investigated in polyethylenes, the T1C of the crystal-
line and noncrystalline components are on the order of 103 s
and 10À1 s, respectively.[27] These characteristics indicate that
the aromatic core has nonequivalent and static environments
in the solid state on the NMR timescale. In contrast, the aro-
matic carbon spectra of crystalline H81b with branched N,N-di-
octylamide chains became broader, indicating less ordered
packing of the macrocycle. Other than the assignment of each
signal, however, the number and chemical shifts of the carbon
signals were similar to those in CHCl3 solution. These results
suggest that the macrocyclic framework of H81b did not expe-
Figure 4. 2D packing structures of (a) H81c, (b) Ni41c, and (c) Pd41c in LC phases. (d) Phase diagrams of macrocycles H81c, Ni41c, Pd41c, and Cu41c traced on
the second heating cycle of DSC with a heating rate of 108CminÀ1. Phase notation: Cr, crystal; L, LC phase with lamellar structure; Colob, oblique columnar LC
phase; ColL, lamellar columnar LC phase; ColL(F), frozen glassy phase with lamellar structure; Iso, isotropic liquid.
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rience any deformation of the mesogen, but retained the sym-
metric environment to some extent through the packing.[26] LC
H81c showed a similar tendency as that shown by H81b in the
aromatic region; however, significantly sharp signals appeared
in the aliphatic region (10–40 ppm). It was also observed that
the T1C of H81c was 21–23 s for the aromatic carbons, whereas
that of the aliphatic carbons was much shorter at 0.61–0.70 s.
Moreover, the aromatic signals of H81c at 100–180 ppm ap-
peared more pronounced in the cross-polarization spectrum
than in the direct-excitation spectrum (see the Supporting In-
formation, Figure S11).[26] Such characteristics of the spectrum
reflect a rather dynamic and liquid-like environment around
the alkyl chains, as found in other discotic LC molecules,
whereas the macrocyclic mesogen remains less mobile.[14d,28,29]
The 13C NMR spectra and relaxation behavior of the columnar
LC nickel complex Ni41c were similar to those of the lamellar
LC H81c. The T1C of Ni41c was 12–13 s for the aromatic car-
bons, whereas that of the aliphatic carbons was much shorter
at 0.61–0.70 s. These results indicate that the molecular-level
structures as well as the dynamic properties of LC Ni41c on the
NMR timescale are similar to those of the lamellar LC H81c
(Figure S10 in the Supporting Information). Notably, all the
solid-state NMR signals of these nickel(II) complexes showed
diamagnetic properties, indicating that the Ni center retains
the square-planar configuration without any axial coordination
by neighboring macrocycles even in the condensed solid or LC
phases.
Indeed, this tendency was also observed in other metallosalen
complexes with a similar configuration, as reported by Saal-
frank et al., who found the following differences in the chemi-
cal shifts of the geminal protons: Dd=64 Hz for metal-free
salen and Dd=32 Hz for the Ni–salen complex.[31] Following
Saalfrank et al., we investigated the vicinal coupling constant
of Hd. However, the Hd signals for H81c and Ni41c were hidden
by other proton signals owing to the side chains. Fortunately,
for Pd41c, the Hd signal was isolated and the vicinal coupling
constant of Jdd’ was found to be 0 Hz, indicating that the meth-
ylene groups showing the He signal were oriented in the axial
direction.[31] This prediction is in good agreement with the
single-crystal structures (Figure 7 and Figure 8). The phenom-
ena may imply a change in the flexibility of the macrocyclic
mesogens through metal coordination, which is discussed in
the next section.
1H NMR spectroscopy of macrocycles and metallomacrocy-
cles
To understand the molecular structure in detail, we performed
structural analysis of the macrocycles in solution. All signals of
these compounds were assigned by using H-1H COSY, NOESY,
1
1
Figure 5. Partial H NMR spectra of (a) H81c, (b) Ni41c, and (b) Pd41c
and HMQC techniques. Similar to macrocycle H81c, the
1H NMR spectra of metal complexes Ni41c and Pd41c in CDCl3
supported the D4h symmetric structures of the metallomacrocy-
cles (Figure 5). Intriguingly, chemical shifts of the proton sig-
nals in the aromatic region were significantly affected by the
coordination of the metal ions to the macrocyclic ligands. The
imine signals at 7.69 ppm for Ni41c and at 7.98 ppm for Pd41c
appeared upfield compared with that of H81c, which were
consistent with those of other salen complexes.[30,31]
(600 MHz, CDCl3, 298 K).
UV/Vis absorption and circular dichroism of macrocycles
and metallomacrocycles
The optical properties of H81c and the Ni, Pd, and Cu com-
plexes were investigated by UV/Vis absorption and circular di-
chroism (CD) spectroscopies in CHCl3 (3.0ꢂ10À6 m). H81c exhib-
ited absorption maxima at 257, 316, and 359 nm in the UV/Vis
absorption spectrum (Figure 6a). The latter two absorptions
were accompanied by shoulder bands of shorter wavelengths.
A broad absorption band at 420 nm in the visible region was
also observed, probably owing to the n–p* transition associat-
ed with the azomethine group.[32a] In contrast, metal com-
plexes Ni41c, Pd41c, and Cu41c showed strong bands in the
400–500 nm region with absorption coefficients of 6.5ꢂ104–
1.2ꢂ105 mÀ1 cmÀ1, followed by weak and broad absorption
bands at around 500–580 nm. The characteristics and absorp-
tion coefficients were consistent with those of other metallosa-
len complexes.[32]
Furthermore, it is noteworthy that several signals at around
3.50–4.50 ppm, which were assigned to the protons in proximi-
ty to the metallosalen, were more affected compared with
H81c. In particular, signals of the geminal protons, He and He’
in H81c were significantly split (Dd=137 Hz, Figure 5a), indi-
cating that these protons might be under different magnetic
environments owing to the distorted chiral salen moiety on
the NMR timescale. In contrast, the differences in the chemical
shift between the two sets of the signals, He and He’, of the
metallomacrocycles Ni41c (Dd=86 Hz) and Pd41c (Dd=28 Hz)
were smaller than that of the metal-free H81c. This result may
indicate that the magnetic environments of the geminal pro-
tons are similar to each other owing to the restricted configu-
ration of the metallosalen moieties through coordination.
The CD spectra of the macrocycles provided significant de-
tails about the chiral salen moieties (Figure 6). For H81c, posi-
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tive CD bands were observed in the p–p* transition region.
Furthermore, all the metallomacrocycles exhibited positive CD
intensities in the 400–500 nm region. Both properties are con-
sistent with the R,R configuration of the metallosalen moiet-
ies.[32] Although the metallomacrocycles investigated in the
present study showed similar tendencies as H81c, Ni41c
showed a weaker Cotton effect than those of the other macro-
cycles. These results show that the Ni complex possessed a cer-
tain planarity with a square-planar configuration, as observed
in the simple model complexes,[32] and are consistent with the
structural investigation unveiled by the single-crystal analysis
of the model compounds (Figure 7 and Figure 8).
shorter alkyl chains. The square-planar geometry of the metal
complexes affects the planar configuration of the macrocyclic
mesogen. In fact, the average dihedral angle between the p
planes of the neighboring dibenzothiophenes was 8.858 (Fig-
ure 8a, shown in blue, Table 1) and the dihedral angle of the
mean planes of OÀC=CÀC=N atoms was only 7.488 (Figure 8a,
shown in red, Table 1), whereas the corresponding parameters
for H81a were 10.38 (Figure 8b, shown in blue, Table 1) and
13.08 (Figure 8b, shown in red, Table 1). These differences in
the parameters reflect the planarity of the macrocycles (Fig-
ure 8c and d). Standard vertical deviations of all the sp2 atoms
in the macrocycles from the mean planes were calculated as
s=0.32 for Ni41a and s=0.88 for H81a (Table 1). These pa-
rameters were sufficiently consistent with the experimental re-
sults of the UV/Vis and CD data recorded in solution. There-
fore, metallomacrocycle Ni41a is more planar than metal-free
H81a. Furthermore, the packing structures of the metalloma-
crocycles significantly differ from that of H81a (Figure 7c and
d). In fact, the metallomacrocycles self-assembled with each
other, where the metallosalen moieties gathered in anti-parallel
fashion, whereas H81a stacked dibenzothiophenes rather than
the distorted salen moieties (see the Supporting Information).
It seems reasonable to assume that the differences in the pla-
narity among the macrocycles directly affected the differences
in the liquid-crystalline phases. Such an enhanced planarity of
the macrocycle was also observed in the other metallomacro-
cycles, that is, Pd41a and Cu41a (see Table 1, and the Support-
ing Information). Although it was somehow unexpected that
the single-crystal X-ray structures of Pd41a and Cu41a were
almost similar to that of Ni41a, we presumed that the subtle
structural difference at the submolecular scale involving size
and intermolecular interactions of the square-planar metal ions
might induce macroscopic differences among the self-assem-
bled LC macrocycles (see the Supporting Information, Fig-
ure S17, Table S7).
X-ray crystallographic analyses of the macrocycles and met-
allomacrocycles
Single-crystal X-ray analyses of the model macrocycles with
short alkyl chains showed the detailed structures of the macro-
cycles. The crystals were grown by slow evaporation of sol-
vents from a CHCl3 and hexane solution for H81a and Pd41a or
from a CHCl3, hexane, and toluene solution for Ni41a and
Cu41a. As expected, Ni41a adopts a flat macrocyclic structure,
in which all the sulfur atoms were arranged on the outside
fringe of the macrocycle and the Ni ions were embedded in
the salen ligands on the corners of the macrocycle, leading to
an electronically neutral tetranuclear metallomacrocycle. The
diameter of the vacant inner cavity of the macrocycle was elu-
cidated to be 10.1 ꢁ, which is comparable to that of a related
macrocycle reported previously.[18] In the packing structure of
Ni41a, the metallomacrocycles are aligned in an alternating
and overlapping pattern and the cavities are filled by the side
chains of the neighboring macrocycles (Figure 7a). The confor-
mations of the carbon atoms connected peripherally to the
1
ethylenediamine moieties were consistent with the H NMR re-
sults discussed in Figure 5. The molecular packing in the crystal
is completely different from the one-dimensional stacked col-
umnar assembly in the liquid-crystalline phases owing to the
Figure 6. CD (top) and UV/Vis (bottom) spectra of the macrocycle (a) H81c, and metallomacrocycles (b) Ni41c, (c) Pd41c, and (d) Cu41c recorded at 293 K in
CHCl3 (3.0ꢂ10À6 m).
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Conclusion
We have demonstrated the orientation switch of the LC phase
of a macrocyclic compound induced by metal complexation.
The macrocyclic framework, consisting of dibenzothiophenes
and salens, forms tetranuclear metal complexes with square-
planar metal ions to afford planar mesogens, the structures of
which are similar to that of the metal-free macrocycle. Howev-
er, the atomic scale differences among 2H+, NiII, CuII, and PdII
when bound to the salen ligands induced a significant variabil-
ity in the macroscopic LC organization on the phase structure
and transition behavior. Single-crystal X-ray structural analyses
of model macrocycles with shorter alkyl chains revealed that
the mesogen moieties of the metallomacrocycle formed more
rigid and planar conformations than those of the metal-free
macrocycle owing to the square-planar configurations of the
transition-metal ions. This showed that the metallomacrocycles
had highly ordered LC phases and higher transition tempera-
tures. In particular, the macrocyclic nickel(II) complex formed
a well-developed columnar oblique LC phase, which reversibly
underwent a phase transition, whereas the palladium(II) and
copper(II) complexes formed the lamella-columnar structure in
the mesophase. This difference is derived from the difference
in the flowability among the metallomacrocycles. The palladiu-
m(II) and copper(II) complexes solidified at higher tempera-
tures owing to the higher association ability and the lamellar-
columnar structures froze before transition to more ordered
structures. As the nanospace in a liquid crystal would be influ-
enced by the assembly of the macrocycles based on the struc-
ture and fluidity, the metal-ion-induced switch of LC orienta-
tion at the atomic scale differences could produce functional
diversity and generate much higher order nanospaces in flowa-
ble media.
Figure 7. X-ray crystal structures of (a) the metallomacrocycle Ni41a and
(c) the macrocycle H81a. ORTEP diagrams with thermal ellipsoids at the 50%
probability level. The packing structures of (b) Ni41a and (d) H81a shown as
space filling models.
Figure 8. Comparison of macrocyclic mesogen structures of (a) Ni41a and
(b) H81a determined by X-ray crystallography. Hydrogen atoms and side
chains were omitted for clarity. The mean planes formed by the OÀC=CÀC=
N atoms in the salen moieties are shown in red, whereas the p planes of the
dibenzothiophenes are shown in blue. The dihedral angles between these
adjacent planes are summarized in Table 1. The structures of each salen
moiety in the macrocyclic mesogens of Ni41a and H81a are highlighted in
(c) and (d), respectively.
Experimental Section
Materials
Synthetic procedures were carried out under an atmosphere of dry
nitrogen unless otherwise specified. All reagents and solvents were
purchased at the highest commercial quality available and used
without further purification, unless otherwise stated. Detailed syn-
Table 1. Crystallographic data for the macrocycles.
H81a·CHCl3
108H132Cl12N8O16S4
P21
2
Ni41a·CHCl3·hexane·toluene
Pd41a
Cu41a·CHCl3·hexane·toluene
Moiety formula
Space group
Z
C
C128H161Cl15N8Ni4O16S4
P212121
4
C104H120Pd4N8O16S4
P212121
4
C127H160Cl12Cu4N8O16S4
P212121
4
Dihedral angle [8][a]
Dihedral angle [8][b]
s[c]
10.3
13.0
0.88
8.85
7.48
0.32
11.0
6.64
0.32
9.52
7.12
–
[d]
Inner diameter [ꢁ]
R1, RW
GOF
10.1
0.1346, 0.3514
1.345
9.22
0.1434, 0.3861
1.616
9.63
0.1085, 0.2868
1.039
9.61
0.2181, 0.4767
1.475
[a] The average dihedral angles between the p planes of two adjacent dibenzothiophenes in the macrocycles. [b] The average dihedral angles between
the mean planes composed of N=CÀC=CÀO atoms in the salen moieties. [c] The standard deviation of the distance from the mean plane composed of the
sp2 atoms (C, N, O) in the macrocycles. [d] This characterization was not performed owing to the moderate quality of the crystal.
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thesis and analytic data are described in the Supporting Informa-
tion. 1H and 13C NMR spectra were recorded with a JEOL JNM-
ECS400 or JNM-ECS600 spectrometer at a constant temperature of
298 K. Tetramethylsilane (TMS) was used as an internal reference
for 1H and 13C NMR measurements in CDCl3. MALDI-TOF-MS was
performed with an ultraflex III, Bruker Daltonics, and dithranol or
a-cyano-4-hydroxycinnamic acid (CHCA) was used as the matrix. El-
emental analyses were performed with a Yanaco MT-6 analyzer.
Silica gel column chromatography and thin-layer (TLC) chromatog-
raphy were performed by using Merck silica gel 60 and Merck silica
gel 60 (F254) TLC plates, respectively. GPC was performed by using
a JAI LC-9204 equipped with JAIGEL columns.
(Nagoya University) for his technical assistance in the X-ray dif-
fraction measurements. We also appreciate Dr. S. Saito (Kyoto
University), Dr. Y. Segawa (Nagoya University), and Dr. T. Matsu-
moto (Nagoya University) for helping us to solve the single-
crystal X-ray analyses.
Keywords: dynamic covalent chemistry · liquid crystals ·
macrocycles · metallomesogens · nanospace
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Acknowledgments
This work was supported by a JSPS KAKENHI Grant-in-Aid for
Scientific Research (A) (Number 15H02167) to K.T. and a JSPS
KAKENHI Grant-in-Aid for Scientific Research (C) (No. 15K05473)
for S.K. We thank Dr. Kinichi Oyama of the Chemical Instrumen-
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University, for elemental analysis. We also thank Mr. T. Hikage
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Received: April 26, 2016
Published online on && &&, 0000
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FULL PAPER
Atomic scale difference induces mac-
roscopic organization: The macroscop-
ic organization and thermal properties
of liquid-crystalline metallomacrocycles
are significantly dependent on the coor-
dinated metal ions. The subtle structural
differences among the planar macrocy-
cles play a role in the macroscopic or-
ganization as revealed by single-crystal
X-ray crystallographic analyses of the
macrocycles with shorter alkyl chains.
&
Liquid Crystals
S.-i. Kawano, T. Hamazaki, A. Suzuki,
K. Kurahashi, K. Tanaka*
&& – &&
Metal-Ion-Induced Switch of Liquid-
Crystalline Orientation of
Metallomacrocycles
Metallomacrocycles
Tetranuclear metal complex formation of the macrocyclic
ligand induced the orientation switch and phase transition
of the liquid-crystalline macrocycle depending on the metal
ions. Even though Ni2+, Pd2+, and Cu2+ ions form square-
planar complexes with a salen ligand (N,N’-bis(salicylidene)-
ethylenediamine) and difference of the ionic sizes among
these metal ions are much smaller than the size of the
macrocycle, the “atomic-scale differences” among 2H+, Ni2+
Cu2+, and Pd2+ when bound to the salen ligands induced
a significant variability in the macroscopic liquid crystalline
,
organization on the phase structure and transition behavior.
For more details, see the Full Paper by K. Tanaka et al. on
page && ff.
Chem. Eur. J. 2016, 22, 1 – 11
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These are not the final page numbers! ÞÞ