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Kiyonori Takahashi, Takayoshi Nakamura, Ichiro Hisaki et al.
A proton conductive hydrogen-bonded framework
incorporating 18-crown-6-ether and dicarboxy-o -terphenyl
moieties

Volume 2
Number 17
7 September 2021
Pages 5503–5792

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A proton conductive hydrogen-bonded
framework incorporating 18-crown-6-ether
and dicarboxy-o-terphenyl moieties†
Xin Chen,a Kiyonori Takahashi,
and Ichiro Hisaki *abc

*ab Kenta Kokado,ab

Takayoshi Nakamura*ab

To date, proton-conducting organic crystalline materials based on crown ethers have rarely been
investigated. In this work, we reveal that flexible organic 18-crown-6 derivatives with one or two 4,
4 0 -dicarboxy-o-terphenyl (CT) groups formed four kinds of crystalline frameworks: 1CT-18C6-I (P21/n),
2CT-18C6-I (P1), 2CT-18C6-II (P1) and 2CT-18C6-III (Cmc21). Single crystal X-ray diﬀraction analysis

Received 6th May 2021,
Accepted 29th June 2021

clearly suggested that the water molecules were involved a hydrogen-bonded network for two

DOI: 10.1039/d1ma00411e

frameworks. In particular, a unique one-dimensional (1D) water pathway had formed in 2CT-18C6-III
and the activation energy was evaluated by Arrhenius plots to be 0.14 eV, indicating that the proton

rsc.li/materials-advances

jumps from H3O+ to the neighboring H2O in the hydrogen-bonded network of 2CT-18C6-III.

Graduate School of Environmental Science, Hokkaido University, Sapporo,
Hokkaido 060-0810, Japanb
Research Institute for Electronic Science, Hokkaido University, Sapporo, Hokkaido
001-0020, Japan. E-mail: ktakahashi@es.hokudai.ac.jp, tnaka@es.hokudai.ac.jpcGraduate School of Engineering Science, Osaka University, 1-3 Machikaneyama,
Toyonaka, Osaka 560-8531, Japan. E-mail: hisaki@chem.es.osaka-u.ac.jp† Electronic supplementary information (ESI) available: Synthesis of compounds,
crystal data, crystal structure of 2CT-18C6-II and impedance spectroscopy offrameworks. CCDC 2057700, 2078832, 2078834 and 2078835. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1ma00411e

frameworks (MOFs) are, therefore, promising candidates for proton
conductive materials.11
Proton conduction in PEMs is related to two mechanisms,
namely, the Grotthuss and vehicle mechanisms.12–14 The vehicle mechanism indicates the diffusion of protons with thevehicles. However, the Grotthuss mechanism always occurs ina water-based degenerate system consisting of H3O+ and H2O
within an infinite network of hydrogen bonds. A proton jumps
from H3O+ to a neighbouring H2O as molecule rotations taking
place, which contribute to a high proton conductivity.
As is well known, crown ether derivatives, such as dibenzo18-crown-6-ether (DB18C6) with an excellent ability to bindalkali metal cations and water molecules,15,16 have drawn muchattention as an ion conductive material.17 Theoretical calculation indicates that DB18C6 with H3O+ ions located in thecavities of the crown ether interact with water molecules.18,19
Therefore, crystalline frameworks composed of crown ethers
are expected to show conductivity with the Grotthuss mechanism. In this connection, we were interested in hydrogenbonded organic frameworks (HOFs),20–25 in which molecules
are connected through non-covalent intermolecular interactions such as hydrogen bonds, because of their high crystallinity and high affinity toward protons. To date, some HOFs
have been reported to show proton conductivity.26–32 However,proton conductive HOFs based on crown ethers are hitherto
unknown.
Herein, we newly designed 18-crown-6 derivatives with one
and two 4,4 0 -dicarboxy-o-terphenyl (CT) groups, 1CT-18C6 and2CT-18C6, respectively, as building block molecules to develop
proton conductive HOFs (Fig. 1). A working hypothesis is as© 2021 The Author(s). Published by the Royal Society of Chemistry

Mater. Adv., 2021, 2, 5639–5644 | 5639

Introduction
Under the concept of green sustainable development, new clean fuelsources are absolutely necessary. Proton exchange membrane fuelcells (PEMFCs) are one of the most optimal technologies as analternative to common fossil fuels.1 Proton exchange membranes(PEMs) with an ionic pathway for protons are critical for PEMFCs.
Presently, organic conductive polymers, such as Nafion, sulfonatedand phosphonic-based polyaromatics, and polybenzimidazoles, areemployed as alternative PEM materials.2–7 They exhibit the following
features required for PEMs: (a) the presence of proton-conducting
functionalities,8–10 (b) poor electronic conductivities, and (c) goodchemical and thermal stabilities. However, even the Nafion, achieving the outstanding proton conductivity of 101 S cm1 atroom temperature with 100% relative humidity (RH), still involves
unavoidable defects. Thus, exploration of optimizing electrolytematerials has attracted much attention in this field. Highly crystalline materials with a periodic framework and infinite channels
such as porous coordination polymers (PCPs) and metal–organicaView Article Online

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Scheme 1 (a) Synthesis and (b) crystallization of 1CT-18C6 and 2CT18C6. Reaction conditions: (i) 4-(methoxycarbonyl)phenylboronic acid,
Pd(PPh3)4, K2CO3, THF, water, reflux; (ii) NaOHaq. THF, (iii) HCl.
Fig. 1 (a) The molecular structure of 2CT-18C6 and expected roles of theparts. (b) Related molecules 1CT-18C6, dibenzo-18-crown-6 (DB18C6),
and 4,4 0 -dicarboxy-o-terphenyl (CT).

follows: (1) the 18-crown-6 and carboxy groups can capture
water molecules via hydrogen bonds; (2) the carboxy group also
can form supramolecular synthons to organize the molecules
into crystalline supramolecular frameworks;33 and (3) the rigid
o-terphenyl moiety can form frameworks24,34–36 and provide
space for proton conduction. To date, 2CT-18C6 has been
applied as an organic ligand of MOFs,37 and proton conductive
molecular crystalline materials based on 2CT-18C6 arehitherto-unknown.
2CT-18C6 was revealed to form three kinds of crystalline
frameworks (2CT-18C6-I, -II, and -III). In particular, 2CT-18C6-III
has a unique one-dimensional (1D) water pathway and showed aproton conductivity of 6.75  108 S cm1 at 85% RH. Notably,
the activation energy (Ea) was evaluated by Arrhenius plots to be
0.14 eV, suggesting that the proton jumps from H3O+ to theneighbouring H2O in the hydrogen-bonding networks of 2CT18C6-III. Thermogravimetric (TG) analysis also indicated that
2CT-18C6-III is stable up to 588 K. The present system is the firstexample of the tailoring of structural features of crown ethers witha 1D water pathway.
In this paper, we present the synthesis of crown ether
derivatives, construction of crystalline frameworks, and evaluation of their proton conductivity. Moreover, the relationships
between the structures of water pathways and proton conductivity are discussed.

2CT-18C6, respectively. Conditions to prepare the frameworks areshown in Scheme 1b. 1CT-18C6 dissolved in EtOH at room temperature for 1 day afforded colourless block crystals (1CT-18C6-I).‡
2CT-18C6 forms three forms 2CT-18C6-I, -II, and -III.‡ Slow evaporation of a mixed solution of N,N-dimethylformamide (DMF) andwater at 85 1C for several days successfully obtained two kinds ofblock-shaped P1 crystals (2CT-18C6-I and 2CT-18C6-II) concomitantly. 2CT-18C6 dissolved in EtOH at room temperature afforded acolourless block Cmc21 crystal (2CT-18C6-III).
Crystallography
In a 2CT-18C6-I crystal, a sodium cation is captured at thecentre of the crown ether with a Na+  O distance ranging from2.57 to 2.79 Å. Two DMF molecules are then coordinated to Na+from axial directions. Because of the cation binding, the 18C6
moiety has a boat-shaped conformation with a bending angle of76.41 (Fig. 2a), where the bending angle is defined as a dihedral
angle between the mean planes of the two benzene rings of thedibenzo-18-crown-6-ether moiety. Three of the four carboxy
groups form intermolecular hydrogen bonds and one binds
to a DMF molecule, resulting in the formation of a ribbon like

Synthesis of 1CT-18C6 and 2CT-18C6 is shown in Scheme 1a. 4,
5-Bis(methoxycarbonylphenyl)benzo-18-crown-6 (1) and 4,40 ,5,50 -tetrakis(methoxycarbonylphenyl)dibenzo-18-crown-6 (2) were synthesized through a Suzuki–Miyaura cross-coupling reaction of (4-(methoxycarbonyl)phenyl)boronic acid and the corresponding brominatedcrown ether derivatives. Hydrolysis of 1 and 2 gave 1CT-18C6 and‡ Crystal data for 1CT-18C6-I, C30H36O13; Fw = 604.59; monoclinic, P21/n (#14),
Z = 4, a = 13.6633(2) Å, b = 8.2601(1) Å, c = 26.8789(6) Å, b = 90.208(2)1, V = 3033.54(9)Å3, T = 223 K, D = 1.324 g cm3, 12 904 collected, 5424 unique (Rint = 0.031)
reflections, and the final R1 and wR2 values are 0.059 [I 4 2.0s(I)] and 0.184 (alldata), respectively. CCDC 2057700. Crystal data for 2CT-18C6-I, C58.5H64.5N3.5 Na
O17.5; Fw = 1119.62; triclinic, P1% (#2), Z = 2, a = 11.3113(2) Å, b = 13.4267(2) Å, c =21.7047(4) Å, a = 104.103(2)1, b = 95.267(1)1, g = 111.048(2)1, V = 2924.57 (10) Å3,
T = 293 K, D = 1.271 g cm3, 31 008 collected, 11 017 unique (Rint = 0.026)
reflections, and the final R1 and wR2 values are 0.061 [I 4 2.0s(I)] and 0.209 (alldata), respectively. CCDC 2078832. Crystal data for 2CT-18C6-II, C57H60N3NaO17; Fw= 1082.07; triclinic, P1% (#2), Z = 6, a = 13.1094(8) Å, b = 25.3405(16) Å, c = 26.7457(16)Å, a = 105.281(5)1, b = 90.128(5)1, g = 91.834(5)1, V = 8565.8(9) Å3, T = 293 K, D =1.259 g cm3, 89 237 collected, 32 350 unique (Rint = 0.116) reflections, and the final
R1 and wR2 values are 0.081 [I 4 2.0s(I)] and 0.352 (all data), respectively. CCDC
2078834. Crystal data for 2CT-18C6-III, C48H42O17; Fw = 890.81; orthorhombic,
Cmc21 (#36), Z = 4, a = 27.4687(10) Å, b = 20.3236(8) Å, c = 8.0251(6) Å,V = 4480.1(4) Å3, T = 298 K, D = 1.321 g cm3, 29 283 collected, 4181 unique(Rint = 0.128) reflections, and the final R1 and wR2 values are 0.094 [I 4 2.0s(I)] and0.288 (all data), respectively. CCDC 2078835.

5640 | Mater. Adv., 2021, 2, 5639–5644© 2021 The Author(s). Published by the Royal Society of Chemistry

Results and discussion
Synthesis and crystallization

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Fig. 2 Crystal structure of 2CT-18C6-I. (a) Anisotropic ellipsoid plot with 50% probability. (b) One-dimensional ribbon-like motifs formed throughintermolecular hydrogen bonds. (c) Branched hydrogen-bonded network formed by three carboxy groups. (d) Packing diagram, where the crown ether
moiety and DMF molecules are coloured by purple and orange, respectively.

motif (Fig. 2b). It is noteworthy that the carboxy groups form
no self-complementary dimer, which is often observed inhydrogen-bonded frameworks composed of highly-symmetricrigid p-conjugated molecules,22–25 but branched networks asshown in Fig. 2c. This result clearly indicates that the flexibility
and lower symmetry of 2CT-18C6-I crucially affect the route to
the formation of a hydrogen-bonded network. Packing of theboat shaped molecules provides small inclusion spaces, inwhich DMF molecules are included. In this framework, however, there is no 1D water alignment suitable for proton
conduction. 2CT-18C6-II consists of three crystallographicallyindependent molecules as well as DMF molecules. 2CT-18C6-II
has almost the same structure as 2CT-18C6-I (Fig. S1, ESI†).
In the 2CT-18C6-III crystal, a Cs-symmetric 2CT-18C6 molecule has quite a shallow boat conformation with a bending
angle of 25.21 (Fig. 3a), which is significantly diﬀerent from
2CT-18C6-I. The observed diﬀerences in the conformation areprobably caused by the existence of strongly bound cationicspecies in the macrocycle. The carboxy groups form hydrogenbonds with only water molecules to give the crystal, whichcontains no solvent but water molecules with a host/guest ratioof 1 : 3 stoichiometry (Fig. 3b). One water molecule forms

hydrogen bonds with the crown ether’s oxygen and the carboxy
groups with a distance of 2.86 and 2.69 Å, respectively, and isisolated from neighbouring water molecules (yellow spheres).
The other four molecules (red spheres), on the other hand,form hydrogen bonds with the carboxy groups (O  O distances: 2.72, 2.89, and 2.94 Å) to form a 1D zig-zag alignment
along the c axis (Fig. 3c). The distance between the oxygen
atoms of the neighbouring water molecules is 2.97 and 3.74 Å(Fig. 3d).
1CT-18C6-I also contains only water molecules with a host/
guest ratio of a 1 : 3 stoichiometry. Two water molecules arehydrogen bonded by the crown ether’s oxygen atoms and theothers are by the carboxy groups. In total, the water molecules
are aligned one dimensionally along the b axis. The distance
between oxygen atoms of the neighbouring water molecules
ranges from 2.77 to 3.68 Å. Although these distances are
relatively shorter than that in the case of 2CT-18C6-III, thealignment crosses through the crown ether ring (Fig. 4).
Phase purity and thermal stability
The powder X-ray diﬀraction (PXRD) patterns of crystalline bulk
samples of 1CT-18C6-I and 2CT-18C6-III are in good agreement

Fig. 3 Crystal structure of 2CT-18C6-III. (a) Anisotropic ellipsoid plot with 50% probability. (b) Packing diagram, where two types of water molecules arecoloured with red and orange. (c) Hydrogen bonds among carboxy groups and water molecules. (d) Alignment of water in the channel withintermolecular distances.

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Materials AdvancesFig. 4 Crystal structure of 1CT-18C6-I. (a) Anisotropic ellipsoid plot with 50% probability. (b) Packing diagram, where water molecules are shown by the
space-fill model. (c) Hydrogen bonds among the crown ether, carboxy groups, and water molecules. (d) Alignment of water in the channel withintermolecular distances.

with the ones simulated from their single-crystal data respectively (Fig. 5), indicating high phase purity and good stability.
In the TG curves, 1CT-18C6-I starts losing slowly from room
temperature and displays 8.9% weight loss to 373 K, whichcorresponds to the loss of the water molecules (cal. 8.9%). Thenthe framework is maintained until 588 K (Fig. 5a). Similarly,
2CT-18C6-III shows a 4.5% weight loss of water molecules fromroom temperature to 373 K (cal. 4.5%) and the framework alsocollapses after 588 K (Fig. 5b). On the other hand, variabletemperature-PXRD plots show both 1CT-18C6-I and 2CT-18C6III with an obvious change of structure with the increase of thetemperature, which result from the reorganization of thehydrogen bonds connected with the loss of the water molecules(Fig. 5c and d).
Proton conductivity
The structures of 1CT-18C6-I and 2CT-18C6-III were constructed by a hydrophilic crown ether moiety, assembly ofcarboxy groups, and 1D alignment of water molecules. In

addition, TG and variable temperature-PXRD analyses suggested the frameworks of 1CT-18C6-I and 2CT-18C6-III weremaintained after the removal of water molecules. These results
indicated that there were enough sites for water molecules
supplied at a higher relative humidity. These structural features
could provide an orderly route for proton transport. Therefore,
1CT-18C6-I and 2CT-18C6-III are expected to show proton
conductivity. Due to the limitation of the size of the samples,
a compressed pellet of the samples was used for the measurement for proton conductivity. As a reference, proton conductivity under the same conditions was measured for pelletizedsamples of CT and DB18C6.
Temperature-dependent Nyquist plots under 85% RH for
four samples of 1CT-18C6-I, 2CT-18C6-III, CT, and DB18C6
are summarized in Fig. 6a and b, Fig. S3, and S4 (ESI†),
respectively.
The Nyquist plots (Fig. 6a and b and Fig. S3, ESI†) of 1CT18C6-I, 2CT-18C6-III, and CT are the sum of a straight line anda semicircle in the low and high frequency range, respectively.

Fig. 5 TG plots of as-formed crystals of (a) 1CT-18C6-I and (b) 2CT-18C6-III. Variable temperature PXRD patterns of (c) 1CT-18C6-I and (d) 2CT-18C6-III,
where simulated patterns based on SXRD analysis are shown at the bottom.

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Fig. 6 Proton conductivity for 1CT-18C6-I and 2CT-18C6-III, CT and DB18C6. Nyquist plots for (a) 1CT-18C6-I and (b) 2CT-18C6-III. (c) Arrhenius plot
for proton conductivities under 85 RH%.

On the other hand, each Nyquist diagram of DB18C6 wascomposed of a single semicircle (Fig. S4, ESI†). Since the pellet
sample was used to measure the proton conductivity, theNyquist plot showed the proton conductions which are contributed from a bulk and an interface. Since bulk conduction isusually observed in the higher frequency range than theinterface,11,38 the semicircles in the Nyquist diagram of 1CT18C6-I, 2CT-18C6-III, and CT are attributed to bulk conduction.
For the calculation of proton conductivity, we applied theequivalent circuit model. The model for the conductivity of
DB18C6 is that a resistor and constant phase element (CPE) arearranged in parallel, while the model for that of 1CT-18C6-I,
2CT-18C6-III, and CT is that two parallel pairs of resistors and
CPEs were lined up in series.
Regarding the conductivity estimated from the equivalentcircuit model, an Arrhenius plot was summarized from the bulk
conductivity of 1CT-18C6-I, 2CT-18C6-III, and CT, and the
conductivity of DB18C6 (Fig. 6c). From the Arrhenius plot, theproton conductivities at room temperature and Ea of 1CT-18C6-I,
2CT-18C6-III, CT, and DB18C6 were estimated, and summarizedin Table 1. The proton conductivity at room temperature washigher in the order of 2CT-18C6-III, 1CT-18C6-I, CT, and DB18C6,
and the Ea was also lower in the same order. The proton
conductivities of MFM-511 and MFM-512, which are the MOF
samples with the carboxy group involved in proton transport, are
4.0  108 (87% RH) and 2.5  107 S cm1 (77% RH) at 298 K,
respectively.38 The value of proton conductivity from 2CT-18C6-III
is in the similar order to MFM-511 and MFM-512.
Referring to the proton transport, proton conduction can beclassified as the vehicle and the Grotthuss mechanisms, which

Table 1 Proton conductivity at 300 K and activation energy (Ea) of
1CT-18C6-I, 2CT-18C6-III, CT, and DB18C6, estimated from the Arrheniusplot

1CT-18C6-I
2CT-18C6-III
CT
DB18C6s @300 K/S cm1

Ea/eV

1.95  1010
3.43  1088.69  1011
3.97  1012

0.354
0.140
1.150
0.452

© 2021 The Author(s). Published by the Royal Society of Chemistry

are divided by Ea = 0.4 eV. When Ea is larger and smaller than
0.4 eV, proton conductivity is estimated to be by the vehicle and
Grotthuss mechanisms, respectively.
Since the Ea values of DB18C6 and CT are higher than 0.4 eV,
the proton conducting mechanism of DB18C6 and CT is
attributed to the vehicle one. On the other hand, 1CT-18C6-I
and 2CT-18C6-III had lower Ea values than 0.4 eV, and therefore, the proton conduction of them is due to the Grotthuss
mechanism. Both 1CT-18C6-I and 2CT-18C6-III had continuous
hydrogen-bonded networks, resulting in the eﬀective protonconducting pathways and corresponding to the Grotthuss
mechanism. Notably, 2CT-18C6-III has two unique linear water
pathways. Water molecules in 2CT-18C6-III were not only
arranged along the 1D array of the crown ether unit, but also
arranged in the parts formed by the hydrogen-bonded networks
surrounded by a carboxy group. Therefore, 2CT-18C6-III exhibited the highest proton conductivity of 3.43  108 S cm1
under 85% RH at 300 K.

Conclusions
In this study, we newly applied 18-crown-6 derivatives with one
or two 4,4 0 -dicarboxy-o-terphenyl (CT) moieties as building
block molecules to develop organic crystalline materials with
proton conductivity. The 18-crown-6 and carboxy groups capture water molecules via hydrogen bonds and constructed good
self-adaptability to water molecule frameworks especially for
2CT-18C6-III with a unique 1D water pathway, which provides
available space for proton conduction. Based on this study, it ispossible to develop a novel proton conductive flexible framework with good adaptability to guest molecules. In addition, thehigh chemical stability and superior thermal stability indicate apromising and potential application in PEMFCs.

Conflicts of interest
There are no conflicts to declare.

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Acknowledgements

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5644 | Mater. Adv., 2021, 2, 5639–5644© 2021 The Author(s). Published by the Royal Society of ChemistryThis work was supported by KAKENHI (JP18H01966, JP19H04
557, and JP21H01919) from JSPS and by the Dynamic Alliancefor Open Innovation Bridging Human, Environment and Materials from MEXT Japan. X. C. thanks the Chinese ScholarshipCouncil (CSC) for financial support. The authors thank Prof.
Y. Sagara (currently at Tokyo Institute of Technology) and
N. Tamaoki at RIES, Hokkaido University, for purification ofcompounds by preparative HPLC. X-Ray diﬀraction data,
including preliminary data sets, were partly collected at
BL02B1 and BL40XU in SPring-8 with the approval of JASRI(proposal No. 2019A1161, 2019B1134, and 2020A1117). The
authors thank Dr. Kunihisa Sugimoto and Nobuhiro Yasuda
at SPring-8 for collection of X-ray diﬀraction data. The authors
also thank the reviewers for their accurate comments.

Notes and references