Co-crystals of 9,9′-bianthracene-10,10′-dicarboxylic acid with linear bidentate basic ligand molecules: Synthesis, crystal structure, and properties based on the layer structure exfoliated by water

Article abstract of DOI:10.1039/c9ce01460h

Co-crystals of 9,9′-dianthracene-10,10′-dicarboxylic acid (1) with the linear bidentate bases 9,9′-biacridine (bac), phenazine (phez), 4,4′-dipyridine (dpy), and DABCO (dabco) were synthesized. In the crystal structures of the co-crystals of 1 and bac, phez, and dpy, one-dimensional (1D) structures were formed through the hydrogen bonding between the acid and the base. For each combination of the acid and the bases, several kinds of polymorphic co-crystals with a similar structure were obtained. Among them, a combination of 1 and dabco gave three kinds of co-crystals which had a layered structure. In one of the co-crystals, a phenomenon where layers of the crystals could be exfoliated by dropping water was observed. The two-dimensional (2D) fingerprinting plot of the co-crystals showed that the major interaction to form the 1D structure was O-H?N hydrogen bonding and that to form the layered structure was the CH-π interaction, and the influence of the latter interaction on the robustness of the crystal was stronger than the former in the co-crystals.

Full text of DOI:10.1039/c9ce01460h

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Co-crystals of 9,9′-bianthracene-10,10′-
dicarboxylic acid with linear bidentate basic ligand
molecules: synthesis, crystal structure, and
properties based on the layer structure exfoliated
by water†
Cite this: DOI: 10.1039/c9ce01460h
Misaki Okayasu,
Shoko Kikkawa,
Hidemasa Hikawa
and Isao Azumaya
*
Co-crystals of 9,9′-dianthracene-10,10′-dicarboxylic acid (1) with the linear bidentate bases 9,9′-biacridine
bac), phenazine (phez), 4,4′-dipyridine (dpy), and DABCO (dabco) were synthesized. In the crystal
(
structures of the co-crystals of 1 and bac, phez, and dpy, one-dimensional (1D) structures were formed
through the hydrogen bonding between the acid and the base. For each combination of the acid and the
bases, several kinds of polymorphic co-crystals with a similar structure were obtained. Among them, a
combination of 1 and dabco gave three kinds of co-crystals which had a layered structure. In one of the
co-crystals, a phenomenon where layers of the crystals could be exfoliated by dropping water was
observed. The two-dimensional (2D) fingerprinting plot of the co-crystals showed that the major
interaction to form the 1D structure was O–H⋯N hydrogen bonding and that to form the layered structure
was the CH–π interaction, and the influence of the latter interaction on the robustness of the crystal was
stronger than the former in the co-crystals.
Received 17th September 2019,
Accepted 19th November 2019
DOI: 10.1039/c9ce01460h
rsc.li/crystengcomm
as the main interactions when constructing a framework
structure. However, aromatic–aromatic interactions work
Introduction
Recently, the field of the construction of framework
strongly when some parts of the molecules “fit” together.
1–4
structures, such as metal–organic frameworks (MOFs),
That is, when the molecules efficiently combine to fill some
5
–7
14,15
covalent-organic frameworks (COFs),
organic frameworks (HOFs),
or hydrogen-bonded
part of the space, this gives a stable crystal structure
even
8–10
has been significantly
if each interaction is weak. Thus, using the CH–π/π–π
interaction by adding a broad aromatic ring can be a
common strategy to design crystal structures, even if it is not
developed. Various kinds of bonds and intermolecular
interactions are used to construct these frameworks, and they
can be carefully tailored by the researchers to yield
frameworks for various functions. In this field, constructing a
structure with two or more components is a powerful strategy
to achieve a diversity of functionalized materials.
1
6–20
the main interaction.
In line with this strategy, we have
previously reported functional crystals in which one-
dimensional (1D) structures are self-assembled using a
2
1
coordination linkage and CH–π/π–π interactions.
In addition to the relatively strong interactions, such as
hydrogen bond coordination or ionic interactions, the weaker
interactions can be exploited to yield a precise construction
of the framework structures. The CH–π or π–π interaction
Various interactions are also exploited in the design of a
co-crystal. If two or more compounds are used in
constructing crystalline materials, the obtained crystals are
2
2–25
called co-crystals.
Co-crystals have been studied
(
so-called aromatic–aromatic interactions), if it works alone,
especially in the pharmaceutical field because co-crystals with
different coformers will naturally exhibit different properties,
1
1–13
is weaker than the other interactions,
many examples of aromatic–aromatic interactions being used
and there are not
2
6–28
such as stability, solubility, and bioavailability.
From the
viewpoint of materials science, the phenomenon of co-
crystallization has shown great potential for providing a wide
variety of crystalline materials by a combination of two or
more compounds which will mutually interact.
To design co-crystals, the three-dimensional (3D) shape of
each component is also important in addition to the
intermolecular interactions. That is, the concepts of the
Faculty of Pharmaceutical Sciences, Toho University, 2-2-1 Miyama, Funabashi,
Chiba 274-8510, Japan. E-mail: isao.azumaya@phar.toho-u.ac.jp;
Fax: +81 47 472 1585; Tel: +81 47 472 1589
†
Electronic supplementary information (ESI) available. CCDC 1952757–1952763.
For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/c9ce01460h
1
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molecular synthon and embrace are important in crystal
engineering, which are based on intermolecular interactions
of the 3D shapes of the molecule. These concepts have
enabled the design of various crystal structures, like
2
9
retrosynthesis in synthetic chemistry.
Based on these concepts, we have synthesized a novel
building block that can interact with neighboring molecules
by two types of interactions (Scheme 1). Dicarboxylic acid 1
has carboxy groups that can form hydrogen bonding (acid–
base interactions), and anthracene rings that have the
potential to form CH–π/π–π interactions. Therefore,
compound 1 is versatile as a building block for designing
functional crystalline materials including co-crystals.
Fig. 1 Structures of linear bidentate base ligand molecules.
relative to tetramethylsilane (TMS) (0 ppm) in CDCl3 and
coupling constants are expressed in hertz (Hz). The following
abbreviations are used: s = singlet, d = doublet, dd = double
doublet, ddd = double double doublet. Mass spectroscopy
(MS) and high resolution mass spectroscopy (HRMS) were
performed on a JEOL JMS-AM SYSTEM II-50.
In this study, we synthesized compound 1 and its various
co-crystals with linear bidentate basic ligand molecules
(Fig. 1), 9,9′-biacridine (bac), phenazine (phez), 4,4′-
dipyridine (dpy), and DABCO (dabco), using several types of
Single crystal X-ray diffraction analysis
weak intermolecular interactions, mainly hydrogen bonds
Single crystals were mounted on loop for the X-ray
measurements. Diffraction data were collected on an X-ray
(acid–base interactions) and CH–π/π–π interactions. In the co-
crystals of 1 and the base molecules, molecule 1 and the base
molecule generally aligned alternately through O–H⋯N acid–
base interactions (hydrogen bonds) to form a 1D structure,
and the bianthracene moieties were assembled by CH–π
interactions, which worked perpendicularly between the
aromatic rings to form a 2D sheet.
diffractometer (Rigaku XtaLAB P200) equipped with
a
rotating anode X-ray source (Mo-Kα, λ = 0.71075 Å: 1dpy,
1dabco-b, 1dabco-c, Cu-Kα, λ = 1.54187: 1, 1bac, 1phez,
1dabco-a) and a hybrid photon counting detector (PILATUS
200 K) at 93 K. The frame data were integrated, and the
absorption correction was calculated using the Rigaku
CrystalClear, CrysAlisPro program package. The structures
were solved by direct methods or Patterson methods
Experimental
Materials
(
SHELXT Version 2014/5 (ref. 30)), and refined by full-matrix
2
least-squares fitting on F (SHELXL2013, SHELXL Version
014/7, 2018/3). All non-hydrogen atoms were refined
Anthraquinone was purchased from TCI Co., Ltd., Tokyo,
Japan. Zinc powder was purchased from KANTO Chemical
Co., Inc., Tokyo, Japan. n-BuLi, bromine, and solvents (ethyl
acetate, tetrahydrofuran, HCl, acetic acid, 1,2-dichloroethane,
and methanol) were purchased from FUJIFILM Wako Pure
Chemical Corporation. All commercially available reagents
and solvents were used without further purification.
2
anisotropically and hydrogen atoms were theoretically added.
The crystal data and the structure refinements are
summarized in Table 1. Crystallographic data have been
deposited with the Cambridge Crystallographic Data Center
as supplementary publication numbers CCDC 1952757–
1952763.
Analytical techniques
3
1,32
Synthesis of 9,9′-bianthracene
Fourier transform infrared spectroscopy (FT-IR) was
performed on a JASCO FT/IR-4100 spectrometer using KBr
Anthraquinone (25.03 g, 0.12 mol) was dissolved in acetic
acid (600 mL). Zinc powder (55.06 g, 0.84 mol) was added to
the solution under stirring using a mechanical stirrer, and
the reaction mixture was heated to 90 °C. Concentrated HCl
1
tablets. Proton nuclear magnetic resonance ( H-NMR) and
1
3
carbon-13 NMR ( C-NMR) spectra were recorded on a JEOL
JNM-ECS400 (400 MHz) spectrometer. Chemical shifts (δ) are
Scheme 1
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Table 1 Crystallographic data and details of diffraction experiments for the co-crystals
Cocrystal
1
1bac
1phez
1dpy
1dabco-a
1dabco-b
1dabco-c
Formula
C H O
80 78 14
C
28
H
17NO
2
C H N O
42 26 2 4
C H N O
40 26 2 4
C H N O
44.8 39.6 3.6 8.2
C H N O
42 42 4 11
42 43 4 11
C H N O
Formula weight
1263.49
399.45
622.68
598.66
759.62
778.81
779.82
Crystal system
Space group
Triclinic
P¯1
Orthorhombic Orthorhombic Monoclinic Triclinic
Monoclinic Orthorhombic
Pbcn
Pbca
1
P2 /c
P¯1
Cc
Pna2
1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
9.18848IJ15) 11.97202IJ19)
18.3718(2) 12.27700IJ18)
19.60910IJ18) 12.64035IJ17)
98.7879(9) 90
92.6442IJ10) 90
97.7172IJ13) 90
12.1391(2)
12.75610IJ12)
38.9751(4)
90
90
90
14.3335(5)
11.8876(4)
16.9672(6)
90
96.463(3)
90
10.3818(5)
10.9676(2)
18.7727(5)
95.299(2)
98.714(3)
91.279(3)
10.2344(8)
33.477(2)
11.5077IJ10)
90
93.536(7)
90
33.4734IJ10)
10.5712(4)
11.1961(4)
90
90
90
3
V/Å
Z
3234.19(7)
2
1857.88(5)
4
6035.20IJ13)
8
2872.64IJ17) 2102.39IJ12)
4
3935.2(5)
4
3961.8(2)
4
2
T/K
μ/mm
93
93
93
0.711
1.146
79 408
4986
93
93
0.70
1.593
55 347
4608
93
0.096
0.995
31 178
4291
93
0.095
1.04
32 412
5920
−1
0.713
1.029
43 357
0.713
1.050
21 955
1537
0.090
1.024
45 664
4351
a
2
GOF on F
Reflections collected
Independent reflections I > 2σ(I) 10 419
Restraints/parameters
0/859
0.0269
0.0802
0.2417
0/166
0.0349
0.0453
0.1307
0.26/−0.36
1952758
0/441
0/423
0.0822
0.053
0.1176
0.26/−0.30
1952760
64/585
0.1150
0.1379
0.4352
1.29/−0.41
1952761
2/517
1/521
R
int
0.0593
0.0954
0.2908
0.59/−0.36
1952759
0.1157
0.1064
0.2994
0.55/−0.37
1952762
0.0596
0.0560
0.1536
0.42/−0.35
1952763
b
R
1
[on F, I > 2σ(I)]
c
2
wR
2
(on F , all data)
−
3
Largest diff. peak/hole/e Å
CCDC no.
0.62/−0.56
1952757
P
P
P
P
a
2
2 2
1/2
b
c
2
o
GOF = [ wIJF
o
− F
c
) /(N
o
− N
v
)] (N
o
; number of observations, N
v
; number of variables).
R
1
=
||F
o
| − |F
c
||/ |F
o
|. wR
2
= [ (wIJF
−
P
2
2
2 2 1/2
F
c
) )/ wIJF
o
) ]
.
(
150 mL) was dropped into the solution using a dropping
reaction mixture for 30 min at room temperature. 0.2%
CO (10 mL) was added under stirring for 2 h, and the
funnel over 30 min. The mixture was kept at 90 °C for 2 h
under stirring. After cooling to room temperature, the
reaction mixture was added to water (50 mL) under ice
cooling. The solid was collected by filtration and washed with
water. The solid was dissolved in toluene (500 mL), and
heated to 120 °C under reflux. The solution was filtered
whilst hot, and the filtrate was allowed to stand overnight.
K
2
3
mixture was neutralized with concentrated HCl. The white
solid was collected by vacuum filtration and washed with
water (49.73 mg, 71% yield). The crystals were crystallized
from tetrahydrofuran. M.p. > 295 °C (dec.). FT-IR (KBr,
−
1
cm ): 3382, 1684, 1561, 1488, 1441, 1392, 1320, 1290, 1247,
1
1026, 860, 770, 679, 599, 472. H NMR (400 MHz, 313 K,
9
5
8
,9′-Bianthracene was obtained as yellow crystals (22.37 g,
DMSO-d ): δ 8.24 (d, J = 8.7, 4H), 7.48 (t, J = 7.3, 7.8, 4H) 7.18
(t, J = 8.7, 6.4, 4H), 6.91 (d, J = 8.7, 4H); C NMR (100 MHz,
6
1
13
2% yield). H NMR (400 MHz, 298 K, CDCl
3
): δ 8.68 (s, 2H),
.15 (d, J = 8.6, 4H), 7.44 (ddd, J = 8.3, 6.6, 1.2, 4H), 7.14
6
313 K, DMSO-d ): 170.5, 130.5, 130.5, 126.7, 126.7, 126.1,
+
(ddd, J = 8.8, 6.3, 1.2, 4H), 7.08 (dd, J = 8.8, 1.26, 4H).
126.0, 125.9, 125.5. HRMS (EI, m/z) calcd. for C H O [M ] =
3
0
18
4
4
C
7
42.1200. Found 442.1205. Anal. calcd. for C30
·(H
1.64; H, 4.52; N, 0.00%.
25 7.5
H O =
3
1,32
H O
30 18 4
2
O)3.25: C, 71.93; H, 4.9; N, 0.00%. Found: C,
Synthesis of 10,10′-dibromo-9,9′-bianthracene
9
,9′-Bianthracene (3.50 g, 9.87 mmol) was dissolved in 1,2-
dichloroethane (70 mL). Bromine (1.1 mL, 2 eq.) was slowly
dropped into the solution, and the reaction mixture was
stirred for 3 h. After cooling in an ice bath, the yellow solid
was collected by vacuum filtration, and washed with 1,2-
Crystallization
Co-crystals of 1 with linear bidentate bases, bac, phez, dpy,
and dabco, were synthesized by liquid/liquid diffusion or
grinding. Three co-crystals of 1 and dabco were obtained:
1
dichloroethane (3.66 g, 73% yield). H NMR (400 MHz, 298 K,
CDCl ): δ 8.69 (ddd, J = 9.4, 0.9, 0.4, 4H), 7.58 (ddd, J = 9.0,
3
1dabco-a, 1dabco-b and 1dabco-c.
6
8
.4, 1.2, 4H), 7.18 (ddd, J = 9.0, 6.4, 1.2, 4H), 7.07 (ddd, J =
.8, 0.8, 0.4, 4H).
−
5
1bac. Solutions of compound 1 (16.9 mg, 3.8 × 10 mol) in
−5
tetrahydrofuran (7 mL) and bac (5.0 mg, 1.4 × 10 mol) in
acetonitrile (3 mL) were prepared. The solution of compound 1
was dispensed into five glass vessels (200 μL per vessel), then
the solution of bac (200 μL) was layered onto each solution of
compound 1. After the solution at room temperature (rt) was
left for 1 week, yellow crystals were obtained.
Synthesis of 9,9′-dianthracene-10,10′-dicarboxylic acid (1)
0,10′-Dibromo-9,9′-bianthracene (81.16 mg, 0.16 mmol) was
1
dissolved in anhydrous diethyl ether (3 mL). The mixture was
stirred for 15 min at −15 °C under an atmosphere of argon.
n-BuLi (0.3 mL, 1.6 M) was added dropwise, and then the
mixture was stirred for 1.5 h. CO2 was bubbled into the
−
6
1phez. Solutions of compound 1 (4.4 mg, 9.9 × 10 mol)
in ethyl acetate (2 mL) and phez (1.9 mg, 1.0 × 10 mol) in
−
5
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n-hexane (2 mL) were prepared. The solution of compound 1
was dispensed into eight glass vessels (200 μL per vessel),
and then the solution of phez (200 μL) was layered onto each
solution of compound 1. After the solution at rt was left for 3
days, yellow crystals were obtained.
−
6
1
dpy. Compound 1 (2.4 mg, 5.4 × 10 mol) and dpy (2.2
−
5
mg, 1.4 × 10 mol) were ground using an agate mortar. A
solution of the mixture in methanol (600 μL) was prepared.
The solution of compound 1 and dpy was dispensed into
three glass vessels (200 μL per vessel). After the solution at rt
was left for 5 days, red crystals were obtained.
−6
1
dabco-a. Solutions of compound 1 (2.8 mg, 6.3 × 10 mol)
−5
in ethyl acetate (2 mL) and dabco (4.2 mg, 3.7 × 10 mol) in
tetrahydrofuran (2 mL) were prepared. The solution of
compound 1 was dispensed into eight glass vessels (200 μL per
vessel), then methanol (100 μL) and the solution of dabco (200
μL) were layered onto each solution of compound 1. After the
solution at rt was left for 1 week, red crystals were obtained.
Fig. 2 Crystal structure of 1: (a) view along the a-axis, (b) b-axis, and
(c) c-axis. Colors: C, gray; O, red. The aromatic H atoms are omitted
for clarity.
−
−
6
1
dabco-b. Solutions of compound 1 (1.2 mg, 2.7 × 10
mol) in ethyl acetate (6 mL) and dabco (2.4 mg, 2.1 × 10
5
mol) in tetrahydrofuran (6 mL) were prepared. The solution
of compound 1 was dispensed into 24 glass vessels (200 μL
per one vessel), and then the solution of dabco (200 μL) was
layered onto each solution of compound 1. After the solution
at rt was left for 1 week, red crystals were obtained.
interactions formed a straight 1D structure where the molar
ratio of 1 and the base molecule was 1 : 1. In all these
cocrystals, the 1D structures are constructed with hydrogen
bonds that are further assembled into extended structures by
CH–π and π–π interactions.
In co-crystal 1bac (Fig. 3), molecules 1 and bac were
disordered at the same position and the positions of each
atom of COOH⋯N were the same because they had very
similar molecular shapes. Regardless, it was proposed that
they should align alternately judging from the demand of
their chemical structure. Additionally, co-crystal 1bac
crystallized in the orthorhombic system and space group
Pbcn. The distance and angle of the joint part in the
−
6
1
dabco-c. Solutions of compound 1 (0.8 mg, 1.8 × 10
mol) in tetrahydrofuran (10 mL) and dabco (1.2 mg, 1.1 ×
−
5
1
0
mol) in ethyl acetate (2 mL) were prepared. The solution
of compound 1 was dispensed into eight glass vessels (200
μL per vessel), and then the solution of dabco (200 μL) was
layered onto each solution of compound 1. After the solution
at rt was left for 1 week, red crystals were obtained.
Results and discussion
Crystal structures of the co-crystals were determined by single
crystal X-ray diffraction (SXRD) analysis. 2D fingerprint plots
as Hirshfeld surfaces were calculated using Crystal Explorer
3
3–35
1
7.5.
Crystal structure of 1
A crystal of compound 1 was obtained from tetrahydrofuran–
water by vapor diffusion (Fig. 2). The crystal structure in the
P ¯1 space group had two molecules of 1, five tetrahydrofuran
2
molecules, and one H O molecule in the asymmetric unit.
The torsion angles of C10–C11–C16–C17, C10–C11–C16–C29,
C40–C41–C46–C59, and C40–C41–C46–C47 were +94.74°,
−
88.50°, +87.38°, and −93.10°. No CH–π or π–π interactions
were observed between bianthracene moieties of the
neighboring molecules.
1
D structures and their assemblies in co-crystals: 1bac, 1phez
Fig. 3 Crystal structure of 1bac: (a) 1D structure consisting of 1 and
bac, (b) view from the top of 1D structures, and (c) view from the side
of 1D structures. Colors: C, gray; N, violet; O, red. The aromatic H
atoms and disordered atoms are omitted for clarity.
and 1dpy
All three co-crystals formed 1D coordination polymers. 1 and
the base molecule aligned alternately through acid–base
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structure (O1–H1⋯N1) were 2.665 Å and 170.96°. The C10–
C11–C11–C12 torsion angles were +71.31° and −107.56°.
Co-crystal 1phez crystallized in the orthorhombic system
and space group Pbca (Fig. 4). The asymmetric unit consisted
of one molecule of 1 and one molecule of phez. Molecule 1
and phez formed 1D chains through acid–base interactions,
which were aligned parallel to the c-axis. The distance and
angle between atoms associated with hydrogen bonding were
as follows: O1–H1⋯N1 (2.710 Å, 176.73°) and O3–H3⋯N2
(2.708 Å, 174.96°). The void space of the cocrystal was 247.7 Å
in the unit cell, which corresponded to 4.1%. The torsion
angles of C10–C11–C16–C17 and C10–C11–C16–C29 were
+
72.21° and −108.95°.
Co-crystal 1dpy crystallized in the monoclinic system and
1
space group P2 /c (Fig. 5). The asymmetric unit consisted of
one 1 and one dpy. The torsion angles of C10–C11–C16–C17,
C10–C11–C16–C29, C32–C33–C38–C37, and C32–C33–C36–
C37 were −81.35°, 102.13°, 44.75°, and −136.42°. The
distances and angles of the joint part in the structure were
Fig. 5 Crystal structure of 1dpy: (a) 1D structure consisting of 1 and
dpy, (b) view from the top of the 1D structures, and (c) view from the
side of the 1D structures. Colors: C, gray; N, violet; O, red. The
aromatic H atoms are omitted for clarity.
2
.605 Å and 169.37° (O1–H1⋯N1), and 2.612 Å and 174.69°
(O3–H3⋯N2).
Co-crystals 1dabco-a, 1dabco-b, and 1dabco-c grew as
crystals with a thin square shape and were composed of 1,
dabco, and a different number of H O molecules. Very
2
similar layer structures were formed in these co-crystals,
which consisted of molecules of 1 and dabco. The molecules
of 1 in the layer structure assembled perpendicular to the
direction of the bianthracenyl axis by the CH–π and π–π
interaction of the anthracene skeletons. This layer is referred
to as a bianthracene layer. In co-crystals 1dabco-a–c, dabco
molecules filled the void between the bianthracene layers.
Lattice parameters were very similar in these cocrystals. The
difference among these co-crystals was based on the number
of water molecules contained and the way the bianthracene
layers were stacked. The bianthracene layers were connected
by O–H⋯N hydrogen bonding of 1, dabco, and H O
2
molecules. Judging from the atom distance concerning the
acidic proton, a proton transfer between 1 and dabco
occurred. This is supported by the fact that if a ΔpK value
a
between a protonated and an acid base is more than 4, both
3
6
of them tend to exist in ionic forms and that the pK
protonated dabco and anthracene-9-carboxylic acid
corresponding to a partial structure of 1) are 8.82 (ref. 37)
a
of
(
3
8
and 3.77.
2
Since there were dabco and a lot of H O
molecules between the layers, these piles of bianthracene
layers were physically soft and fragile.
The structures and differences of 1dabco-a, 1dabco-b, and
1dabco-c were as follows. Crystal 1dabco-a crystallized in the
triclinic system and space group P ¯1 (Fig. 6(a)). The
asymmetric unit consisted of one molecule of 1, two dabco,
one ethyl acetate, and three H O molecules. The disorders
2
observed in 1dabco-a were complicated because the water
molecule was also involved in the disorders. The dabco
molecule was partly replaced with H O (only one molecule
2
could be assigned because of its low occupancy), and the best
result for structural optimization was obtained when the
occupancy rates were 0.8 (dabco) and 0.2 (H O). The ethyl
2
acetate molecule was differently ordered corresponding to
whether the neighbouring molecule would be dabco or H O.
2
The layered structures were stacked along the c-axis. The
thickness of the layer was 12.5 Å, and the distance between
layers was 6.1 Å. In the crystal structure of 1dabco-a, not only
the layer structures but also a chained 1D structure were
observed. The 1D structure included one molecule of 1 and
Fig. 4 Crystal structure of 1phez: (a) 1D structure consisting of 1 and
phez, (b) view from the top of 1D structures, (c) view from the side of
1
D structures. Colors: C, gray; N, violet; O, red. The aromatic H atoms
are omitted for clarity.
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Fig. 6 Crystal structure of (a) 1dabco-a, (b) 1dabco-b, (c) 1dabco-c. Description of the stacking pattern of the bianthracene layer.
one dabco molecule, while the other dabco molecules existed
as guest molecules with ethyl acetate between the 1D chains.
The guest dabco molecules and ethyl acetate molecules were
disordered. The 1D chains were aligned parallel to the c-axis.
The distances and angles of atoms involved in hydrogen
bonding were 2.527 Å and 153.4°(O1⋯H1A⋯N1), 2.566 Å and
Two dimensional fingerprint plot analysis
The intermolecular interactions in these co-crystals were
analyzed using Hirshfeld surface analyses (Fig. 8). The
relative contributions by hydrogen bonds and those by CH–π
interactions were determined. The contribution of hydrogen
bonds was shown as the sum of H⋯N contacts and O⋯H
contacts taking into account that a proton transfer could
occur between 1 and dabco (the second line of the small
square figures in Fig. 8(a)). Also, the contribution of CH–π
interactions was shown as the sum of H⋯C contacts and
C⋯H contacts (third line of the small square figure in
1
69.61° (O3⋯H3A⋯N2). The torsion angles of C10–C11–C16–
C29 and C10–C11–C16–C17 were +85.85° and −93.15°.
dabco-b crystallized in the monoclinic system and space
1
group C2/c (Fig. 6(b)). The layered structures were stacked
along the b-axis. The neighboring upper and lower layers in
1
dabco-b staggered to the a-axis direction. The thickness of
Fig. 8(a)). Among the CH–π interactions, the H⋯C contacts
the layer was 11.8 Å, and the distance between layers was 5.0
11–13
show the T-shape CH–π interaction
between anthracene
Å. The distance and angle of O1⋯H1A⋯N1 were 2.645 Å and
rings and the C⋯H contact shows the CH–π interaction
between the anthracene ring and guest dabco. First, the
contribution of the hydrogen bond appeared in 1phez
1
52.24°. The torsion angles of C10–C11–C16–C29 and C10–
C11–C16–C17 were +82.81° and −92.21°.
dabco-c crystallized in the orthorhombic system and
1
space group Pna21 (Fig. 6(c)). The layered structures were
stacked along the a-axis. The layers of 1dabco-c were
inverted, rotated by 180° and slid in the a and c-axis
directions. Moreover, the thickness of the layer was 12.0 Å,
the distance between layers was 4.7 Å. The distance and angle
of O2⋯H1⋯N1 were 2.748 Å and 157.82°. The torsion angles
of C10–C11–C16–C29 and C10–C11–C16–C17 were +88.19°
and −88.15°.
Next, the face indices were calculated from the structural
analysis and were assigned to the crystal appearance. The
wide surface (100) matched with the horizontal surface of the
layer (Fig. 7). 1dabco-a, 1dabco-b, and 1dabco-c grew with a
thin square shape. Although the square shape became large
as the crystal grew, the crystal did not become thick.
Fig. 7 Crystal face and crystal appearance.
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Fig. 8 (a) 2D fingerprint plots for 1phez, 1dpy, 1dabco-a, 1dabco-b, and 1dabco-c, (b) percentage contributions to the Hirshfeld surface area of
the intermolecular contacts in the crystal structures of 1phez, 1dpy, 1dabco-a, 1dabco-b, and 1dabco-c, (c) positions and directions of
intermolecular contacts.
(
17.3%), 1dpy (16.4%), and 1dabco-a (15.6%) judging from
they remained unchanged. Although the details of the
exfoliation mechanism remain unclear, water molecules
between the layer of 1 may be relatively movable and allow
penetration of added water molecules through the gap
between the layers. All the exposed carboxyl groups can then
bond with water molecules through hydrogen bonding, and
the surface of the layer may become coated with water
molecules. However, because the inside of the layer consists
of aromatic rings (lipophilic site) through the CH–π or π–π
interactions, the layers may be robust against water (Fig. 10).
The hydrogen bonded dabco molecules bind the layers of
the shape of the Hirshfeld surface and the ratio of their
contribution, which meant that 1D chained structures were
constructed in the crystals. The ratios were larger than those
of 1dabco-b (11.9%) and 1dabco-c (11.9%) that have layered
structures. Next, a contribution of CH–π interaction contacts
significantly appeared in 1dabco-a, 1dabco-b, and 1dabco-c
(40.2, 33.7, and 34.1%) because the Hirshfeld surface was
spread broadly centered around distances of 1.2/1.8 (d /d ) Å
e
i
and the ratios of their contributions were much larger than
those of 1phez (31.4%) and 1dpy (24.6%). That is, to
maintain the crystal framework, the contribution of the sum
of CH–π interactions between anthracene moieties and those
between the anthracene moiety and guest dabco was larger
than the sum of the hydrogen bonding interactions, in the
order 1dabco-a < 1dabco-b < 1dabco-c.
Exfoliation of the layer structure by water
We tried to determine the characteristic of the layer structure
of the co-crystals. When the co-crystal of 1dabco-c was
exposed to water, a phenomenon where the layers of the co-
crystal were immediately exfoliated was observed. The
exfoliation method is shown in Fig.
9 and a video
presentation is provided in the ESI.† The layers were not
exfoliated using other usual solvents such as methanol,
chloroform,
n-hexane,
or
tetrahydrofuran.
Similar
phenomena were observed in 1dabco-a and 1dabco-b. When
we dropped water on the crystals of 1bac, 1phez and 1dpy,
Fig. 9 Microscopy visualization of the time-dependent change in the
single crystals of 1dabco-c for 13 seconds after dropping water.
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Acknowledgements
This research was supported by the Nagai Memorial Research
Scholarship from the Pharmaceutical Society of Japan (M. O.).
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