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Self-assembly of two pairs of homochiral M2L4 coordination capsules with varied confined space using Tröger's base ligands
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Yi Zhoua, Wei Zhanga, Rong Fua, Jiaxin Donga, Yuxuan Liua, Zihang Songa, Han Hanb, c, *, Kang Caia, *
Chinese Chemical Letters | 2025, 36(2) : 109865
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Chinese Chemical Letters | 2025, 36(2): 109865
• Communication •
Self-assembly of two pairs of homochiral M2L4 coordination capsules with varied confined space using Tröger's base ligands
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Yi Zhoua, Wei Zhanga, Rong Fua, Jiaxin Donga, Yuxuan Liua, Zihang Songa, Han Hanb, c, *, Kang Caia, *
Affiliations
  • a College of Chemistry, Nankai University, Tianjin 300071, China
  • b Department of Chemistry, Northwestern University, Evanston, IL 60208, United States
  • c Department of Chemistry, The University of Hong Kong, Hong Kong SAR 999077, China
Published: 2025-02-15 doi: 10.1016/j.cclet.2024.109865
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Chiral coordination molecular cages/capsules with discrete nanoconfined chiral cavities demonstrate significant potential applications across various fields. In this study, we utilized Tröger's base as the building block to design and synthesize two pairs of enantiopure ligands. These ligands were then self-assembled with Pd(Ⅱ) ions through chiral self-sorting coordination, resulting in the formation of two pairs of homochiral M2L4-type coordination molecular capsules. Notably, due to differences in the substitution positions on the Tröger's base, these two pairs of enantiomeric coordination molecular capsules exhibited distinct levels of cavity closures, cavity sizes, and host-guest recognition properties. This research offers valuable insights into the construction of novel chiral molecular capsules and the regulation of confined cavities.

Metal-organic capsule  /  Chirality  /  Self-sorting  /  Tröger's base  /  Host-guest recognition
Yi Zhou, Wei Zhang, Rong Fu, Jiaxin Dong, Yuxuan Liu, Zihang Song, Han Han, Kang Cai. Self-assembly of two pairs of homochiral M2L4 coordination capsules with varied confined space using Tröger's base ligands[J]. Chinese Chemical Letters, 2025 , 36 (2) : 109865 - . DOI: 10.1016/j.cclet.2024.109865
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Metal-organic cages/capsules (MOCs) based on coordination interactions have long been a focal point in supramolecular chemistry and related fields due to their modular constructions, thermodynamically driven quantitative self-assembly and confined cavity environments [1-10]. The introduction of chiral ligands enables the preparation of chiral MOCs, whose chiral cavity environments are useful for applications like chiral molecule recognition [11-14], chiral separation [15,16], and asymmetric catalysis [17-19]. Consequently, the construction and study of chiral MOCs have garnered increasing attention [20-22]. However, challenges in building chiral MOCs and functional research persist considering limited types of chiral building blocks and difficulties in obtaining enantiopure ligands.
As a chiral organic base, Tröger's base (TB) can be synthesized via a one-step condensation between aromatic amines and formaldehyde [23]. Notably, Belowich et al. have demonstrated that enantiopure TB derivatives can be conveniently obtained through crystallization-induced asymmetric transformation (CIAT) utilizing dibenzoyl-tartaric acid (DBTA) [24]. Moreover, TB's V-shaped, inward-bending conformation make it particularly suited for constructing MOCs with aromatic shells. In contrast to MOCs with wirelike frameworks [25], MOCs featuring aromatic shells [26-33] offer more favorable noncovalent interactions and a stronger solvophobic effect, thereby providing advantages for guest-encapsulation. Thus, we believe TB is an ideal molecular building block for constructing chiral MOCs, a potential that has, however, been rarely explored [34].
Herein, we used enantiopure TB as the building block and synthesized two TB-based pyridine ligands, L1 and L2, through substitution at different positions on TB. Inspired by Clever et al. [35,36], L1 and L2 were further coordinated with Pd(Ⅱ) to form two homochiral M2L4-type MOCs, namely, MOC-1 and MOC-2 (Fig. 1). It is noteworthy that self-sorting assembly occurred with racemic ligands RR-L1 and SS-L1, forming homochiral capsules R8-MOC-1 and S8-MOC-1, while a mixture of two different ligands also leaded to narcissistic self-sorting, producing MOC-1 and MOC-2, respectively. Crystallographic analysis revealed that the substitution differences on TB resulted in MOCs with varied cavity sizes and closure extents. Significantly, the fully enclosed cavity of MOC-1 was capable of encapsulating one [2.2]paracyclophane (PCP) guest to form a stable 1:1 complex with slow exchange kinetics on the 1H NMR timescale. These findings provide insights for the establishment of novel chiral molecular capsules and the control of confined cavities.
Tröger's base ligands L1 and L2 were synthesized through one-step Suzuki coupling, initiated from their enantiopure precursors 1 and 2, along with pyridine derivatives (Scheme 1). The synthetic routes for precursors 1 and 2 and their chiral resolutions based on CIAT strategy are elaborated in Schemes S1 and S2 (Supporting information). L1 is characterized by a methoxy group at positions 2 and 8 in the Tröger's base skeleton, while L2 features modification on the pyridine ring with a pendant methoxyethoxy chain to enhance the solubility. Upon reacting enantiopure L1 or L2 with Pd(CH3CN)4(BF4)2 at a molar ratio of 1:0.6 in CD3CN at 80 ℃ for 3 h, a clear and colorless solution was obtained, followed by the precipitation of the corresponding chiral MOC-1 or MOC-2 as a white powder after the addition of diethyl ether. Significant shifts of several resonances in the 1H NMR spectrum of S8-MOC-1, were observed when compared to ligand L1 (Figs. 2a and b), suggesting the coordination of pyridine to Pd(Ⅱ). The proton 1 associated with pyridine units displayed an upfield shift due to shielding effect within the capsule's cavity. The other protons, influenced by the electron-withdrawing effect of Pd(Ⅱ), exhibited downfield shifts. Similar spectral characteristics were observed for S8-MOC-2 (Figs. 2d and e), suggesting the quantitative formation of a single compound with high symmetry derived from the complexation of the ligand with Pd(Ⅱ). Consistently, diffusion-ordered 1H NMR spectroscopy (DOSY) corroborated this by showing uniform diffusion coefficients D = 5.248×10–10 m2/s for all protons in S8-MOC-1, which, according to the Stocks-Einstein equation, indicated a radius size of 11.53 Å (Fig. 2c, Figs. S39 and S40 in Supporting information). Similarly, the DOSY spectrum of S8-MOC-2 showed a single band with a diffusion coefficient D = 4.898×10–10 m2/s, yielding a calculated radius of 12.33 Å in line with the ligand length (Fig. 2f, Figs. S41 and S42 in Supporting information).
Additionally, the enantiomers of both homochiral capsules were characterized using circular dichroism (CD) spectroscopy, exhibiting perfect mirror-image CD signals (Fig. 3). Comprehensive analysis of the CD and UV–vis spectra revealed strong Cotton effects for R8-MOC-1 at 290 nm (positive) and 340 nm (positive), while the enantiomer S8-MOC-1 demonstrates opposite Cotton effects at the same wavelengths. Similarly, the enantiomers of MOC-2 exhibited strong Cotton effects at 254, 266, 290 and 333 nm.
Conclusive evidence for the formation of M2L4 homochiral capsules was obtained from their single-crystal structures. By slow evaporation of diisopropyl ether into acetonitrile solutions of the capsules, single crystals of S8-MOC-1 (Fig. 4a) and S8-MOC-2 (Fig. 4b) were obtained. Both crystal structures contained two Pd2+ ions respectively, each in a square planar N4 coordination environment, bridged by four ligands with electron-rich aromatic rings forming the capsule shells. In S8-MOC-1, the distance between two Pd2+ ions was 11.05 Å, slightly shorter than that (11.12 Å) in S8-MOC-2. This difference can be attributed to varied substitution sites on the TB skeletons. The directional nature of the Tröger's base backbone leaded to twisted capsule shells, yet maintained an overall orientation around a C4-symmetry axel. While the structures were expected to display high symmetry, their symmetry was slightly reduced on account of substitution effect. S8-MOC-1 belonged to tetragonal I422 chiral space group, while S8-MOC-2 was assigned to P1 space group, influenced by its more flexible pendant hydrophilic chain. Notably, the pyridine segment at different connecting sites on TB resulted in two ligands exhibiting different tilt angles when coordinating with Pd(Ⅱ), leading to the formation of two MOCs with distinct cavity sizes and different closure degrees for the confined spaces. As indicated by MoloVol software calculations [37], S8-MOC-1 and S8-MOC-2 displayed cavity volumes of 280 Å3 and 382 Å3, respectively. Besides, S8-MOC-1′s cavity was devoid of counterions, whereas S8-MOC-2′s cavity encapsulated a counterion (BF4–) and several solvent molecules (Fig. S44 in Supporting information). Moreover, due to the larger tilt angle of the ligands and the presence of methoxy groups, S8-MOC-1 formed a fully enclosed capsule as depicted in surface overlay images, while S8-MOC-2 displayed four narrow open windows with widths of ca. 3.5 Å.
We subsequently explored the chiral self-sorting assembly behaviors using racemic ligands RR-L1 and SS-L1 to coordinate with Pd(Ⅱ). The 1H NMR spectra only displayed a single set of signals, indicating that enantiomeric ligands undergo self-recognition to form homochiral capsules R8-MOC-1 and S8-MOC-1 (Figs. S46-S48 in Supporting information), instead of the occurrence of social or comprehensive self-sorting. To evaluate the influence of non-covalent interactions on ligand bending angles, additional self-sorting studies involving different ligands were also conducted. Theoretical analysis suggested that two Tröger's base ligands with similar backbones might achieve similar bending angles, potentially leading to heteroleptic structures containing both L1 and L2 [38,39]. However, the formation of heteroleptic capsule was found to be unfavorable when an equimolar SS-L1 and SS-L2 were mixed with Pd(CH3CN)4(BF4)2 in CD3CN (Fig. 5a). The 1H NMR spectra revealed two clear sets of signals corresponding to compounds S8-MOC-1 and S8-MOC-2 (Figs. 5b-d). Additionally, the DOSY spectra also showed two distinct bands corresponding to the smaller-radius MOC-1 and the larger-radius MOC-2, despite being closely spaced (Fig. 5e). Comparable results were observed using ligands RR-L1 and SS-L2 with opposite chirality (Figs. S52-S54 in Supporting information). These results demonstrated that narcissistic self-sorting had occurred.
The variances in sizes and closure extents of the cavities in MOC-1 and MOC-2 also resulted in distinct host-guest recognition properties. MOC-1 was capable of recognizing a size-matched guest, [2.2]paracyclophane (PCP, molecular volume: 198 Å3), while MOC-2 was not. Upon suspending PCP (0.70 equiv. relative to S8-MOC-1) in a 2.0 mmol/L CD3CN solution of S8-MOC-1 at 80 ℃ overnight, a white suspension was obtained. In the 1H NMR spectrum (Fig. 6b), there were four distinct sets of peaks. Two sets of resonances corresponded unequivocally to the unbound S8-MOC-1 (Fig. 6a, blue circles) and the free guest PCP (Fig. 6c, red triangles), while the other two were notably shifted compared with those associated with the free host and guest, suggesting the formation of host-guest complexes. These results indicated that the unbound capsule, free guest, and host-guest complexes coexisted in the solution, with the host-guest binding process featuring slow exchange kinetics on the 1H NMR timescale. Compared to free PCP, the protons in the encapsulated guest displayed considerable upfield shift (Δδ > 1.4 ppm) due to shielding effect inside the cavity, with methylene protons b separated into two sets of peaks. Further, the calculation of integral area determined 1:1 binding between the host and guest (Fig. S55 in Supporting information), revealing a binding constant of 5.0 × 103 L/mol. The formation of the host-guest complex was further substantiated by the DOSY spectra (Fig. S56 in Supporting information). Spatial correlation signals were observed between the aromatic peak (Ha') of the bound guest and the pyridine proton peak (H1) that was oriented towards the interior of the cavity, as evidenced in the ROESY spectrum (Fig. S57 in Supporting information). Furthermore, the structure of the host-guest complex was optimized using the GFN2-xTB calculation method (Fig. S58 in Supporting information) [40]. Rebek's investigations elucidate that optimal binding is attained when the guest molecule's dimensions match precisely with the inner cavity of the capsule, with the packing coefficient reaching an ideal value of approximately 55%. Despite the substantial volume of PCP, which poses a theoretical encapsulation challenge within S8-MOC-1, we observed a remarkable 1:1 binding efficiency, accompanied by a packing coefficient of 71%. This intriguing outcome is presumably facilitated by intermolecular forces, including [C—H···π] and [π···π] interactions, during the encapsulation process. These forces impart enhanced stability to the host-guest complex, thereby accommodating the encapsulation of guest molecules with larger volumes.
In summary, two enantiopure Tröger's base ligands have been designed and self-assembled with Pd(Ⅱ) to form two M2L4-type chiral metal-organic capsules. Differences in substitution sites on the TB's backbone resulted in varied sizes and closure degrees of the confined cavities in MOC-1 and MOC-2. Single crystal X-ray analysis of the two S-configured homochiral capsules revealed cavity volumes of 280 Å3 and 382 Å3 for MOC-1 and MOC-2, respectively. Narcissistic chiral self-sorting occurred when mixing two racemic chiral ligands for coordination. The fully enclosed capsule MOC-1 was able to encapsulate one [2.2]paracyclophane guest to form a stable 1:1 host-guest complex with slow exchange kinetics on the 1H NMR timescale, while MOC-2, featuring a larger cavity and open windows, was incapable of encapsulating this guest. This research offers valuable insight into the construction of novel chiral molecular capsules and the control of confined cavities. More comprehensive investigation of the host-guest recognition capabilities for both MOCs, particularly regarding the potential application of the chiral cavities in differentiation [41] and separation [42] of chiral guests, is a promising avenue for future exploration.
Declaration of competing interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statement
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Yi Zhou: Data curation, Formal analysis, Investigation, Methodology, Project administration, Validation, Visualization, Writing – original draft. Wei Zhang: Formal analysis, Methodology, Writing – review & editing. Rong Fu: Software, Validation. Jiaxin Dong: Investigation, Validation. Yuxuan Liu: Investigation. Zihang Song: Methodology. Han Han: Formal analysis, Resources, Writing – review & editing. Kang Cai: Conceptualization, Project administration, Resources, Supervision, Writing – review & editing.
Acknowledgments
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This work is supported by the National Natural Science Foundation of China (No. 22271164), the Fundamental Research Funds for the Central Universities and Nankai University (NKU). This work also made use of the IMSERC at Northwestern University, which has received support from the State of Illinois and International Institute for Nanotechnology (IIN).
During the preparation of this work, the authors used GPT in order to polish the language. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication.
Supplementary materials
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Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109865.
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Year 2025 volume 36 Issue 2
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Article Info
doi: 10.1016/j.cclet.2024.109865
  • Receive Date：2024-01-20
  • Online Date：2025-11-12
  • Published：2025-02-15
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History
  • Received：2024-01-20
  • Revised：2024-03-26
  • Accepted：2024-04-07
Affiliations
    a College of Chemistry, Nankai University, Tianjin 300071, China
    b Department of Chemistry, Northwestern University, Evanston, IL 60208, United States
    c Department of Chemistry, The University of Hong Kong, Hong Kong SAR 999077, China
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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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