Crystal structures of sandwich-type potassium cation complexes formed from benzo-15-crown-5-based ligand containing a chloromaleimide moiety

Yuto Hirayama a, Akitaka Ito *bc and Rika Ochi *adef
aTOSA Innovative Human Development Programs, Kochi University, 2-5-1, Akebono-cho, Kochi 780-8520, Japan. E-mail: ochi@kochi-u.ac.jp
bSchool of Engineering Science, Kochi University of Technology, Kami, Kochi 782-8502, Japan
cResearch Center for Molecular Design, Kochi University of Technology, Kami, Kochi 782-8502, Japan
dResearch and Education Faculty, Kochi University, 2-5-1, Akebono-cho, Kochi 780-8520, Japan
eGraduate School of Integrated Arts and Sciences, Kochi University, 2-5-1, Akebono-cho, Kochi 780-8520, Japan
fFaculty of Science and Technology, Kochi University, 2-5-1, Akebono-cho, Kochi 780-8520, Japan

Received 9th March 2025 , Accepted 1st May 2025

First published on 1st May 2025


Abstract

We developed a novel benzo-15-crown-5 (B15C5)-based organic ligand containing an N-phenylchloromaleimide moiety. Two geometrically different potassium cation (K+) complexes with a B15C5/K+/B15C5 sandwich structure were prepared and characterised via single-crystal X-ray diffraction.


Crown ethers, which are macrocyclic poly(ethylene glycol) molecules, have been widely used as representative host molecules owing to their ability to encapsulate cations (e.g. metal ions and ammonium).1 The host–guest interactions of crown ethers with these cations enable the development of materials that exhibit catalytic activity,2 ion transport activity,3 optical properties,4 magnetic properties,5 and phosphorescent properties.6 Remarkably, crown ethers possess unique and specific inclusion modes for guest species that correspond to the diameters of the cavities within their cyclic structures. For example, benzo-15-crown-5 (B15C5) forms a 2[thin space (1/6-em)]:[thin space (1/6-em)]1 sandwich complex with potassium cations (K+; 2.76 Å), i.e. B15C5/K+/B15C5,7 whereas the cavity of B15C5 (1.70–2.20 Å) is suitable for the formation of a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 chelate complex with sodium cations (Na+; 2.04 Å). Such coordination-mode variation arises from the flexibility of these cyclic structures containing oxygen donor atoms for coordination.

By exploiting the inclusion characteristics of B15C5, we prepared a 2-anilino-3-chloromaleimide (AC)-based chromophore in which the anilino moiety was derivatised to a B15C5 moiety to serve as the K+-chelating unit (B15C5-AC-C6-COO, Fig. 1A).8B15C5-AC-C6-COO interacted effectively with K+ in an aqueous solvent (0.2 M Tris-HCl buffer, pH 8.0) to form a gel-like molecular assembly (i.e. supramolecular hydrogel), resulting in a colour change from orange to yellow. The formation of this supramolecular hydrogel was attributed to molecular assembly induced by the formation of the aforementioned B15C5/K+/B15C5 sandwich complex. Although the interactions between B15C5-AC-C6-COO and K+ were monitored by 1H nuclear magnetic resonance spectroscopy in CD3OD and the structure of the host–guest complex was evaluated by density functional theory calculations, the structure was not experimentally determined because of the low crystallinity. Therefore, the molecular-scale mechanism of the assembly process is not yet well understood.


image file: d5ce00252d-f1.tif
Fig. 1 Chemical structures of B15C5-ACC-C6-COO (A, previous work8) and B15C5-AC-Ph (B, this work).

To determine the inclusion mode of the B15C5-AC structure with K+, we reviewed the literature on the crystal structures of complexes in which K+ is encapsulated by B15C5-based organic ligands. The B15C5 structure exhibited multiple inclusion modes for K+ depending on the ligand structure and counterions (some of which are shown in Fig. 2).9 For example, a direction difference between two benzo moieties in a B15C5/K+/B15C5 complex in Fig. 2A, B and C are ∼180°, ∼50°, and ∼120°, respectively. Furthermore, in some B15C5 systems, the complexes are oligomerised by another complex unit and/or via noncovalent interactions (Fig. 2A and C). This variation in the inclusion mode suggests the importance of developing a crystalline B15C5-AC analogue and its K+ complex formed by the addition of KCl, as in our previous studies.


image file: d5ce00252d-f2.tif
Fig. 2 Reported crystal structures of B15C5/K+/B15C5-type complexes: benzo-15-crown-5 (B15C5, A),9i bis(15-crown-5)stilbene (B)9e and hexylureidobenzo-15-crown-5 (C).9f Purple represents potassium; red, oxygen; blue, nitrogen; gray, carbon; and white, hydrogen.

In this study, we designed and synthesised a novel B15C5-AC analogue, B15C5-AC-Ph (Fig. 1B and Scheme S1, ESI) and obtained crystal structures of an inclusion complex of this ligand with KCl. We attributed the low crystallinity of the previously reported B15C5-AC-C6-COO to (1) diversification of possible conformations as a result of the flexibility of the alkyl (C6) chain and (2) electrostatic repulsion arising from the terminal carboxylate moiety. Furthermore, owing to the presence of this carboxylate moiety, the 2[thin space (1/6-em)]:[thin space (1/6-em)]1 complex was anionic at pH 8.0 and the flexible Tris·H+ used as the buffer was a possible countercation. Thus, to increase the crystallinity, we replaced the alkyl chain containing a terminal carboxyl group (C6-COO) with a rigid, charge-neutral phenyl (Ph) group. We expected that the Ph group would induce a regular molecular orientation via π–π and/or CH–π interactions. The chloro group was selected as a halogeno group introduced in the maleimide backbone. The halogeno group is thought to play roles in the molecular packing and the absorption change upon the molecular assembly. The former was demonstrated in our recent publication, where a bromo group substituted for a chloro group contributed to crystallisation via the strong halogen bond.10 We have expected that the chloro group exhibiting weaker halogen bond enlarges the impact of the B15C5 moiety and complexation with K+ on the molecular packing.

Single crystals of non-clathrate B15C5-AC-Ph (1) were prepared by evaporation from a methanol/water mixture (ESI). The crystal structure is shown in Fig. 3, and the crystallographic data (Table 1) indicated that 1 crystallised in the monoclinic P21/n space group. The benzo–AC and AC–N-Ph dihedral angles in a B15C5-AC-Ph molecule are 40.2(2)° and 45.8(2)°, respectively. The ether oxygens of two B15C5 moieties hydrogen-bonded to the secondary amines of the AC–Ph moieties (light blue broken lines in Fig. 3B). Furthermore, the Ph group on the ring nitrogen in the maleimide (N-Ph) moiety interacted with the benzene ring of the B15C5 moiety in another B15C5-AC-Phvia CH–π interactions (light green broken lines in Fig. 3B). Thus, B15C5-AC-Ph participated in three-dimensional packing (Fig. 3C), and in the crystal structure of 1, the N-Ph group influenced the molecular orientation and crystallinity despite the flexible ether structure.


image file: d5ce00252d-f3.tif
Fig. 3 Crystal structures of B15C5-AC-Phe. Monomer (A), molecular interactions (B) and packing (C). Red represents oxygen; blue, nitrogen; gray, carbon; and white, hydrogen.
Table 1 Crystallographic data for crystals 1–3
Compound B15C5-AC-Ph (1) B15C5-AC-Ph⊃K+ (2) B15C5-AC-Ph⊃K+ (3)
a R 1 = R = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [(∑w(|Fo|2 − |Fc|2)2)/∑w(Fo2)2]1/2.
Formula C24H25ClN2O7 C48H50ClKN4O14·(CH3OH)1.75(H2O)2.4 C48H50Cl3KN4O14·(H2O)2.8
Formula weight 488.91 1151.17 1151.17
Crystal system Monoclinic Triclinic Monoclinic
Space group P21/n P[1 with combining macron] P21/n
a 14.6695(5) 12.0536(4) 15.1444(4)
b 8.4232(2) 14.4733(5) 23.9026(5)
c 19.7536(6) 17.4640(5) 15.8158(6)
α/deg. 90 90.685(2) 90
β/deg. 108.105(4) 105.563(2) 103.663(3)
γ/deg. 90 108.209(3) 90
V3 2319.99(13) 2772.69(16) 5563.2(3)
Z 4 2 4
Crystal size/mm3 0.281 × 0.081 × 0.021 0.235 × 0.111 × 0.018 0.237 × 0.166 × 0.032
T/K 113.15 113.15 103.15
D c/g cm−3 1.400 1.379 1.371
F 000 1024.0 1206.0 2395.0
λ 0.71073 0.71073 0.71073
μ (Mo Kα)/mm−1 0.213 0.315 0.315
R 1 [I > 2.00σ(I)]a 0.0384 0.0689 0.0549
R (all reflections)a 0.0532 0.0941 0.0845
wR2 (all reflections)b 0.1000 0.2030 0.1561
GOF 1.055 1.043 1.022
Number of observations 16[thin space (1/6-em)]615 34[thin space (1/6-em)]566 38[thin space (1/6-em)]344
Number of variables 4744 11[thin space (1/6-em)]304 11[thin space (1/6-em)]373
CCDC number 2428279 2428376 2428377


The reaction of 15C5-AC-Ph with KCl in a methanol/water mixture afforded two crystallographically different B15C5-AC-Ph⊃K+ crystals, namely, [K(B15C5-AC-Ph)2·(CH3OH)1.75(H2O)2.4]n (2) and [K(B15C5-AC-Ph)2·(H2O)2.8]n (3) (ESI). The crystals used in this study were obtained from a sample bottle from a different batch that was prepared under identical conditions. 2 crystallised in the triclinic P[1 with combining macron] space group (Table 1). In the crystal structure of 2 (Fig. 4), two molecules of B15C5-AC-Ph and one K+ formed a B15C5-AC-Ph/K+/B15C5-AC-Ph sandwich complex. In this sandwich complex, the K–O distances were 2.79–3.00 Å, and direction difference between the benzo moieties was 43°. The AC–Ph moieties of each B15C5-AC-Ph ligand were oriented in the same direction relative to B15C5 and the chloro groups were oriented in the opposite direction relative to the central axis of the complex (Fig. 4A). The dihedral angles between benzo and AC moieties being 44.2(3)° and 45.8(3)° resulted in parallel orientation of two maleimides. In contrast, the N-Ph groups are not parallel to each other owing to the maleimide–N-Ph dihedral angles (80.1(4)° and 55.9(5)°). The sandwich complexes dimerised by forming π–π interactions between the N-Ph group and the benzene ring of the B15C5 moiety (light green broken lines in Fig. 4B) and hydrogen bonds between the maleimide oxygen (C[double bond, length as m-dash]O) and the secondary amine hydrogen (NH; light blue broken lines in Fig. 4B). Furthermore, these dimers contained hydrogen bonds between the maleimide oxygen and CH2 moieties in B15C5 and with incorporated solvents (H2O and MeOH; Fig. 4C).


image file: d5ce00252d-f4.tif
Fig. 4 Crystal structures of B15C5-AC-Phe⊃K+ [K(B15C5-AC-Ph)2·(CH3OH)1.75(H2O)2.4]n (2). Sandwich complex (A), molecular interactions (B, counterion and solvents omitted) and packing (C). Purple represents potassium; red, oxygen; blue, nitrogen; gray, carbon; and white, hydrogen.

In the crystal structure of 3, which belonged to the monoclinic P21/n space group (Table 1), the formation of a B15C5-AC-Ph/K+/B15C5-AC-Ph sandwich complex (K–O distances: 2.59–3.15 Å, benzo direction difference: 48°), similar to that in 2, was confirmed (Fig. 5A). In contrast to the crystal structure of 2, as structures overlaid in Fig. 6, the chloro groups in 3 were oriented in the same direction relative to the central axis of the complex. The AC and N-Ph moieties in the sandwich complex were stacked via π–π interactions (dihedral angles for benzo–AC: 46.1(3)° and 46.4(3)°, those for AC–N-Ph: 47.5(4)° and 50.5(4)°). The sandwich complexes were packed via complex CH–π interactions and hydrogen bonds via the incorporation of water molecules (Fig. 5B and C). This geometric variation of the K+ complex of B15C5-AC-Ph was attributed to the asymmetry of the AC moiety, the presence of non-covalent interactions with the N-Ph moiety and the flexibility of the B15C5 moiety.


image file: d5ce00252d-f5.tif
Fig. 5 Crystal structures of B15C5-AC-Phe⊃K+ [K(15C5-AC-Ph)2·(H2O)2.8]n (3). Sandwich complex (A), molecular interactions (B, counterion and solvents omitted) and packing (C). Purple represents potassium; red, oxygen; blue, nitrogen; gray, carbon; and white, hydrogen.

image file: d5ce00252d-f6.tif
Fig. 6 Overlaid structure comparison between [K(B15C5-AC-Ph)2·(CH3OH)1.75(H2O)2.4]n (2, green) and [K(B15C5-AC-Ph)2·(H2O)2.8]n (3, purple).

Here we compare the halogen bonds in the crystals. As mentioned in the molecular design section, halogen bonds are an important factor in inducing molecular packing in this system. In crystal 1, the distance between the two Cl atoms in the B15C5-AC structure was 4.434 Å and the two C–Cl⋯Cl angles of them were 95.50°, indicating the presence of type-I halogen bonds.11 In crystal 2, there are no halogen bonds because the Cl atoms are oriented in opposite directions. In crystal 3, the distance between the two Cl atoms in the B15C5-AC structure was 3.922 Å and the two C–Cl⋯Cl angles of them were 75.17° and 107.96° respectively, indicating the presence of type-II halogen bonds.11 These results suggests that the halogen-bonding ability is not dominant in the present molecular system and that such weak interactions afford flexibility in the molecular packing pattern. Recently, an analysis of halogen–halogen (X1⋯X2, X = Cl, Br and I) contacts in solids using a large data set reported that type-II contacts occur most frequently in iodinated derivatives, less frequently in brominated derivatives and least frequently in chlorinated derivatives.11b,12 This suggests that the Cl⋯Cl contacts observed in this molecular system may be primarily driven by molecular assembly due to potassium encapsulation in B15C5-AC structure.

Conclusions

In summary, we determined the crystal structures of a non-clathrate crystal of B15C5-AC-Ph (1) and two types of K+ clathrate crystals of B15C5-AC-Phe⊃K+ (2 and 3). It was hypothesised that the N-Ph group induced efficient molecular packing and increased the crystallinity by providing an interaction site. Notably, two K+ complexes in which the chloro groups were arranged in the different directions were successfully characterised, revealing different crystal structures. The molecular assembly patterns of the B15C5-AC backbone were relatively vague and flexible, making these structures potentially useful for the design of flexible molecular assemblies, such as supramolecular gels, liquid crystals and soft crystals. Crystalline phase transition might occur due to changes in the external environment (e.g. solvent removal by heating) as well. We are currently investigating the physical properties of this molecular system as soft materials, such as supramolecular gels, liquid crystals and soft crystals that respond to external stimuli.

Data availability

The data supporting this article have been included as part of the PDF and CIF files in the ESI.

Author contributions

Conceptualization: Y. H. and R. O.; synthesis and structural analysis: Y. H.; single-crystal XRD analysis: A. I.; writing—original draft: Y. H. and R. O.; writing—review and editing: A. I. and R. O.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful to Dr. Nobuyuki Tamaoki, Dr. Shin-ichiro Noro, Dr. Takayoshi Nakamura and Dr. Kiyonori Takahashi (Hokkaido University), and Dr. Masayuki Izumi and Dr. Masanobu Mori (Kochi University) for fruitful discussions. This work was partially supported by the JSPS KAKENHI Grant Number JP24K08637 and the Cooperative Research Programme of ‘Network Joint Research Center for Materials and Devices’ from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We would like to thank MARUZEN-YUSHODO Co., Ltd (https://kw.maruzen.co.jp/kousei-honyaku/) for the English language editing.

Notes and references

  1. (a) B. Valeur and I. Leray, Coord. Chem. Rev., 2000, 205, 3 CrossRef CAS; (b) M. Takeuchi, M. Ikeda, A. Sugasaki and S. Shinkai, Acc. Chem. Res., 2001, 34, 865 CrossRef CAS PubMed; (c) G. W. Gokel, W. M. Leevy and M. E. Weber, Chem. Rev., 2004, 104, 2723 CrossRef CAS PubMed; (d) G. Yu, K. Jie and F. Huang, Chem. Rev., 2015, 115, 7240 CrossRef CAS PubMed; (e) Y.-F. Zhang, F.-F. Di, P.-F. Li and R.-G. Xiong, Chem. – Eur. J., 2022, 28, e202102990 CrossRef CAS PubMed; (f) Z. Duan, F. Xu, X. Huang, Y. Qian, H. Li and W. Tian, Macromol. Rapid Commun., 2022, 43, 2100775 CrossRef CAS PubMed; (g) K. Takahashi, T. Nakamura and T. Akutagawa, Coord. Chem. Rev., 2023, 475, 214881 CrossRef CAS.
  2. (a) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450 RSC; (b) M. Yoon, R. Srirambalaji and K. Kim, Chem. Rev., 2012, 112, 1196 CrossRef CAS.
  3. (a) M. Yoon, K. Suh, S. Natarajan and K. Kim, Angew. Chem., Int. Ed., 2013, 52, 2688 CrossRef CAS PubMed; (b) P. Ramaswamy, N. E. Wong and G. K. H. Shimizu, Chem. Soc. Rev., 2014, 43, 5913 RSC.
  4. V. Stavila, A. A. Talin and M. D. Allendorf, Chem. Soc. Rev., 2014, 43, 5994 RSC.
  5. M. Kurmoo, Chem. Soc. Rev., 2009, 38, 1353 RSC.
  6. (a) R. E. Morris and P. S. Wheatley, Angew. Chem., Int. Ed., 2008, 47, 4966 CrossRef CAS PubMed; (b) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724 CrossRef CAS PubMed; (c) Y. He, W. Zhou, G. Qian and B. Chen, Chem. Soc. Rev., 2014, 43, 5657 RSC.
  7. (a) S. Shinkai, T. Nakaji, T. Ogawa, K. Shigematsu and O. Manabe, J. Am. Chem. Soc., 1981, 103, 111 CrossRef CAS; (b) A. Robertson, M. Ikeda, M. Takeuchi and S. Shinkai, Bull. Chem. Soc. Jpn., 2001, 74, 883 CrossRef CAS; (c) S.-Y. Lin, S.-W. Liu, C.-M. Lin and C.-H. Chen, Anal. Chem., 2002, 74, 330 CrossRef CAS PubMed; (d) Y. Ishii, Y. Soeda and Y. Kubo, Chem. Commun., 2007, 2953 RSC; (e) P. G. A. Janssen, P. Jonkheijm, P. Thordarson, J. C. Gielen, P. C. M. Christianen, J. L. J. van Dongen, R. W. Meijer and A. P. H. J. Schenning, J. Mater. Chem., 2007, 17, 2654 RSC; (f) P. Mi, X.-J. Ju, R. Xie, H.-G. Wu, J. Ma and L.-Y. Chu, Polymer, 2010, 51, 1648 CrossRef CAS; (g) A. A. Sobczuk, S. Tamaru and S. Shinkai, Chem. Commun., 2011, 47, 3093 RSC; (h) X. Wang, J. Hu, T. Liu, G. Zhang and S. Liu, J. Mater. Chem., 2012, 22, 8622 RSC; (i) G. Sun, J. Pan, Y. Wu, Y. Liu, W. Chen, Z. Zhang and J. Su, ACS Appl. Mater. Interfaces, 2020, 12, 10875 CrossRef CAS PubMed.
  8. Y. Chabatake, T. Tanigawa, Y. Hirayama, R. Taniguchi, A. Ito, K. Takahashi, S. Noro, T. Akutagawa, T. Nakamura, M. Izumi and R. Ochi, Soft Matter, 2024, 20, 8170 RSC.
  9. (a) D. Wang, X. Sun, H. Hu, Y. Liu, B. Chen, Z. Zhou and K. Yu, Polyhedron, 1989, 8, 2051 CrossRef CAS; (b) P. D. Beer, C. G. Crane and M. G. B. Drew, J. Chem. Soc., Dalton Trans., 1991, 3235 RSC; (c) P. D. Beer, M. G. B. Drew, R. J. Knubley and M. I. Ogden, J. Chem. Soc., Dalton Trans., 1995, 3117 RSC; (d) G. J. Kirkovits, R. S. Zimmerman, M. T. Huggins, V. M. Lynch and J. L. Sessler, Eur. J. Org. Chem., 2002, 3768 CrossRef CAS; (e) S. P. Gromov, A. I. Vedernikov, N. A. Lobova, L. G. Kuz'mina, S. S. Basok, Y. A. Strelenko, M. V. Alfimov and J. A. K. Howard, New J. Chem., 2011, 35, 724 RSC; (f) M. Barboiu, D. Dumitrescu and A. van der Lee, Cryst. Growth Des., 2014, 14, 3062 CrossRef CAS; (g) T. Mäkelä and K. Rissanen, Dalton Trans., 2016, 45, 6481 RSC; (h) J. P. Shupp, A. R. Rose and M. J. Rose, Dalton Trans., 2017, 46, 9163 RSC; (i) P. Wei, X. Zhang, J. Liu, G.-G. Shan, H. Zhang, J. Qi, W. Zhao, H. H.-Y. Sung, I. D. Williams, J. W. Y. Lam and B. Z. Tang, Angew. Chem., Int. Ed., 2020, 59, 9293 CrossRef CAS PubMed.
  10. K. Yamashita, A. Ito, M. Ishida, Y. Shintani, M. Ikeda, S. Hadano, M. Izumi and R. Ochi, Soft Matter, 2025, 21, 2124 RSC.
  11. (a) G. R. Desiraju and R. Parthasarathy, J. Am. Chem. Soc., 1989, 111, 8725 CrossRef CAS; (b) G. Cavallo, P. Metrangolo, R. Milani, T. Pilati, A. Priimagi, G. Resnati and G. Terraneo, Chem. Rev., 2016, 116, 2478 CrossRef CAS PubMed.
  12. A. Mukherjee, S. Tothadi and G. R. Desiraju, Acc. Chem. Res., 2014, 47, 2514 CrossRef CAS PubMed.

Footnote

Electronic supplementary information (ESI) available: Synthesis of compounds and measurements (Fig. S1–S5). CCDC 2428279, 2428376 and 2428377. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5ce00252d

This journal is © The Royal Society of Chemistry 2025
Click here to see how this site uses Cookies. View our privacy policy here.