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. 2024 Dec 1;146(50):34501–34509. doi: 10.1021/jacs.4c11583

Intricate Low-Symmetry Ag6L4 Capsules Formed by Anion-Templated Self-Assembly of the Stereoisomers of an Unsymmetric Ligand

Shohei Tashiro 1,*, Yoshihiko Yamada 1, Lea Antonia Kringe 1, Yoshiki Okajima 1, Mitsuhiko Shionoya 1,2,*
PMCID: PMC11664504  PMID: 39616534

Abstract

graphic file with name ja4c11583_0006.jpg

Metal–organic cages and capsules exhibit space-specific functions based on their discrete hollow structures. To acquire enzyme-like asymmetric or intricate structures, they have been modified by desymmetrization with two or more different ligands. There is a need to establish new strategies that can desymmetrize structures in a simple way using only one type of ligand, which is different from the mixed-ligand approach. In this study, a strategy was developed to form interconvertible stereoisomers using the unsymmetric macrocyclic ligand benzimidazole[3]arene. Single-crystal X-ray diffraction analysis revealed that the isomers assembled with silver tetrafluoroborate afforded a conformationally heteroleptic Ag6L4 capsule with an intricate structure. The six Ag ions in the capsule were desymmetrized, resulting in significantly different coordination geometries. Remarkably, the capsule encapsulates a single tetrafluoroborate anion via multipoint C–H···F–B hydrogen bonds in both the solid and solution states, suggesting that anions of appropriate size and shape can act as a template for the capsule formation. These results demonstrate that the use of isomerizable and unsymmetric ligands is the effectiveness of constructing highly dissymmetric supramolecular structures from a single ligand.

Introduction

Supramolecular coordination chemistry is a powerful tool for constructing hollow nanostructures in which the nanocavity space exhibits excellent recognition abilities and catalytic properties.1 Of these, metal–organic polyhedra (MOPs) with discrete structures such as cages and capsules can be dissolved in solvents while retaining their hollow structures and have therefore been studied for several molecular functions such as catalysis, separation, and delivery.2 One promising way to further improve these performances is to mimic the intricate and asymmetric higher-order structures of enzymes that perform elaborate functions in biological systems.

In conventional approaches, high-symmetry structures are obtained by the self-assembly of a single ligand,3 sometimes with the aid of a template molecule or ion (Figure 1a).4 Compared to such structures, the construction of desymmetrized cages is one of the current key challenges in this field.5 For instance, utilizing chiral or unsymmetric ligands is a reliable method to create chiral or desymmetrized cavities.6,7 Desymmetrization of structures can also be induced by the incorporation of anions or guest molecules into the cavity.8 In addition, more strategic desymmetrization approaches, i.e., the self-assembly of multiple different ligands into heteroleptic hollow structures, have recently attracted much attention,9 especially in the construction of lantern-shaped M2L4-cages,10 metal–organic tetrahedra,11 and chiral self-sorted cages,12 although the ligand combinations are often very restricted (Figure 1b).

Figure 1.

Figure 1

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Conceptual schemes for the self-assembly of metal–organic cages or capsules. (a) Conventional homoleptic supramolecular structure with high symmetry and (b) heteroleptic structure composed of mixed ligands. (c) Anion-templated self-assembly of four isomeric benzimidazole[3]arene that generates conformationally heteroleptic Ag6L4 capsules with a low-symmetric and intricate structure.

In contrast to the self-assembly of mixed ligands, another promising strategy is to construct conformationally heteroleptic cages13 utilizing a single type of isomerizable ligand that can generate multiple isomers. In this method, only a certain cage can be selectively obtained from a huge library of products by self-sorting14 of ligand isomers through crystallization or the guest-template effects. This strategy is a form of dynamic combinatorial chemistry and is used as an alternative to the rational design of supramolecular structures.15 However, this method has so far yielded only products with roughly symmetric shapes, although the approach does not require mixed ligands and has great potential to easily desymmetrize or transform metal–organic polyhedra using a single ligand.13a

Herein, we report a facile method to synthesize low-symmetry metal–organic capsules with highly intricate structures by reacting silver salts with a benzimidazole[3]arene macrocycle as a ligand. Our recently developed benzimidazole[3]arene (L) has three benzimidazole groups arranged asymmetrically in a cyclic skeleton, giving rise to two enantiomers with a warped bowl-shaped unsymmetric structure in the crystalline state. In contrast, it shows equilibrium interconversion of the (P)- and (M)-enantiomers with an inversion barrier of 11.0 kcal mol–1 in CD2Cl2 as estimated by variable-temperature NMR, leading to 1H NMR patterns corresponding to C1- and Cs-symmetry structures, respectively, at 213 K and above (Figure 1c).16 In this study, solid- and solution-state structural analyses revealed that the reaction of L with an appropriate silver salt selectively affords conformationally heteroleptic low-symmetry metal–organic capsules. These are formed by the selective sorting of four stereoisomers of L, (P)-cis, (M)-cis, (P)-trans, and (M)-trans, from a large library of potential isomers of the product. The product is an enantiomeric mixture of chiral (P)- and (M)-capsules. For example, the (P)-capsule is composed of the (P)-cis- and (P)-trans-isomers of L, forming a conformationally heteroleptic capsule (Figure 1c). In addition, the six silver ions in the structure are not chemically equivalent and are geometrically desymmetrized by different coordination modes. As a result, the capsule has a C1-symmetry in the crystalline state due to distinct differences in coordination geometry, but in solution, the differences averaged out to give it apparent C2-symmetry. Moreover, solution-state NMR analysis suggests that the anion template effect is important for the selective formation of desymmetrized chiral capsules, even when high-symmetry anions are used as templates. Therefore, our strategy of using stereoisomers of only one unsymmetric ligand is a simple and effective method to create supramolecular complexes with highly intricate and desymmetrized structures on the basis of dynamic combinatorial chemistry.

Results and Discussion

The formation of the capsular structure was first revealed by single-crystal X-ray diffraction (ScXRD) analysis (Figure 2). Anti-benzimidazole[3]arene (L) and AgBF4 were mixed in methanol, and nBu4NBF4 was added to the mixture to obtain high-quality crystals. After leaving the mixture at 9 °C for 5 days, colorless block crystals were formed. ScXRD analysis revealed that the crystals consisted of a hexanuclear tetrameric capsule encapsulating one BF4– anion, BF4–⊂[Ag6L4]6+. The guest BF4– anion was completely enclosed in the tetrameric capsular framework via multiple hydrogen bonds between the fluorine atoms of BF4– and the p-phenyl C–H moieties protruding into the inner space (Figure 2b,c).17 Using the Mercury program, the cavity volume was estimated to be 86 Å3 after excluding the included anion, which was identical to the volume estimated using the PLATON/SQUEEZE program.18−20

Figure 2.

Figure 2

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Crystal structure of the conformationally heteroleptic Ag6L4 capsules. (a) Schematic diagram of the diastereoselective sorting of the four isomeric macrocycles L, (P)-cis, (P)-trans, (M)-cis, and (M)-trans, into enantiomeric (P)- and (M)-capsules. (b) Molecular structure and geometric net of BF4–⊂[Ag6((P)-cis-L)2((P)-trans-L)2]6+. The front ligand in the structure on the left is represented as a transparent stick model, showing the BF4– anion surrounded by four ligands. (c) Stick model representation of the C–H···F–B hydrogen bonds (magenta dotted lines) between the p-phenylene moieties and the BF4– anion. (d) Top and side views of (P)-cis-L and (P)-trans-L. (e) Coordination geometry of five types of silver ions in the capsule. (f) Interligand π–π and CH−π interactions (yellow dotted lines) and space-filling model of the close contacts. (g) NCI analysis of the framework of (P)-capsule. The calculations exclude the BF4– anion in the cavity. (h) Crystal packing structure composed of stacked honeycomb packing capsules. (i) Face-to-face contact between the two-sided capsules. Ag: silver; F: aqua; O: red; N: blue; B: apricot; H: white; C: pink; sky blue, orange, and lime for (P)-cis, (P)-trans, (M)-cis, and (M)-trans isomers, respectively.

Focusing on the ligand structure of the capsules, four isomers of ligands with different conformations, (P)-cis-L, (P)-trans-L, (M)-cis-L, and (M)-trans-L, were observed (Figure 2a). The chirality of the ligand is defined by the orientation of the three benzimidazole moieties, and since a capsule can consist of only one stereoisomer of L, either (P) or (M), each capsule can be referred to as a chiral (P)- or (M)-capsule. The (P)- and enantiomeric (M)-capsules comprise crystals as racemates. Next, by looking more closely at the (P)- (or (M)-) capsule, we found that the four (P)-L (or (M)-L) molecules are not structurally equivalent but are divided into two conformational isomers. One conformer with the three methylene groups oriented up–up–down (hereafter referred to as trans-L) has the same conformation as L previously determined by ScXRD.16 By contrast, the other conformer has the three methylene groups oriented up–up–up (hereafter referred to as cis-L) (Figure 2d). The constituents of the (P)- and (M)-capsule parts are therefore represented as BF4–⊂[Ag6((P)-cis-L)2((P)-trans-L)2]6+ and BF4–⊂[Ag6((M)-cis-L)2((M)-trans-L)2]6+, respectively, indicating that the capsules are conformationally heteroleptic chiral capsules. Thus, this self-assembly can be considered as a self-sorting of four isomeric macrocycles based on diastereoselective recognition between cis-L and trans-L.

The six silver ions in the tetrameric capsule are not structurally equivalent and are roughly classified into three different types, AgI(A), AgI(B), and AgI(C), based on their coordination geometries (Figure 2e). Two AgI(A) ions bridge two cis-L molecules to form a cis-dimer, [Ag2(cis-L)2]2+, with a nearly linear N–Ag–N geometry. By contrast, two AgI(B) ions and two trans-L molecules form a trans-dimer, [Ag2(trans-L)2]2+. The cis- and trans-dimers were hybridized via two AgI(B) ions with a three-coordinate trigonal geometry. Two AgI(C) ions are linked by extra benzimidazole nitrogen atoms of the trans-dimer and benzimidazole π-faces of the cis-dimer, supporting the tetrameric capsule structure (Figure 2b). If two AgI(A), two AgI(B), and two AgI(C), respectively, are equivalent, the capsule can exhibit C2-symmetry. However, in fact, their coordination geometries are not equivalent, and therefore, the capsule has C1-symmetry in the crystalline state.

In addition to the coordination bonds with the AgI ions, interligand π–π and CH−π interactions also contribute to stabilizing the tetrameric capsule structure. For example, cis- and trans-L molecules are intricately entangled through multipoint π–π and CH−π interactions to form a conformationally heteroleptic capsule (Figure 2f). The extensive interligand noncovalent interactions were also supported by the noncovalent interaction (NCI) analysis (Figure 2g).21 These results suggest that the formation of this capsule structure requires the concerted action of several types of interactions, including AgI-coordination bonds, multiple hydrogen bonds with the encapsulated BF4– anion, and interligand π–π and CH−π interactions.

The resultant intricate capsule structure is less symmetric and more distorted spherical. In addition, the distorted sphere is bifacial and composed of cis- and trans-faces, with two and four AgI ions exposed on the surface, respectively (Figure 2b). As a result, a honeycomb packing structure is formed within the crystal through face-to-face contacts (Figure 2h). For example, the cis-faces with two AgI(A) are connected via BF4– anions, whereas the trans-faces interact with each other via CH−π or Ag−π interactions to form a distorted honeycomb layer (Figure 2i). Other anions and solvent molecules were located in the voids created by the staggered stacking of the honeycomb layers.

Next, to investigate the formation of capsule structures in solution, a mixed solution of L (0.91 mM) and AgBF4 (1.4 equiv) in methanol was analyzed by NMR spectroscopy. The resultant 1H NMR spectrum was highly complex, with the product exhibiting more than 40 signals, more than twice those of L alone. 1H–1H COSY, ROESY, and 1H–13C HSQC measurements showed that the three methylene signals of L were split into 12 doublet signals, which correspond to six pairs of diastereotopic CH2 moieties. The remaining signals were assigned to six benzimidazole and p-phenylene pairs (Figure 3a). The 1H NMR signals are consistent with a C2-symmetry structure rather than a D2-symmetry capsule, which is the most symmetrical homoleptic tetramer formed from Cs-symmetric L. This loss of symmetry with two C2-axes lost from D2-symmetry is due to the composition of two inequivalent ligands, and the resulting 1H NMR signals suggest that they are consistent with the conformationally heteroleptic capsule composed of cis-L and trans-L (Figure 3c). Note that the symmetry in the solution state (C2) is higher than that in the solid state (C1) due to the fast ligand exchange of methanol bound to AgI(A) and AgI(C) in CD3OD. In particular, it is worth mentioning that the upfield shift of several aromatic C–H signals is reflected in the capsule structure where multiple Ls are unsymmetrically arranged. For example, some signals of p-phenylene are upfield shifted due to π–π stacking, and one of the signals of benzimidazole shows a significant upfield shift due to the shielding effect of the aromatic ring facing the C–H moiety (Figure 3a). Although some ambiguity is unavoidable due to the significant signal overlap, a full assignment of the signals was made based on 2D NMR spectroscopy (Figures S8–S18). It should be noted that the NMR signals were essentially maintained in the solution and showed little change for at least 10 days under ambient light (Figures S27–S30).

Figure 3.

Figure 3

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NMR-based characterization of the capsule formed from L and AgBF4 in CD3OD (300 K, 500 MHz for 1H, 471 MHz for 19F). (a) Changes in 1H NMR spectra before (top) and after complexation (bottom). (b) 1H DOSY. (c) Symmetry analysis of the capsule formation from L. The different panel colors represent ligands with different conformations. (d) 19F NMR spectra of the capsule. The minor 19F signals observed downfield of each BF4– species are due to F species bound to 10B.22

The formation of the capsule structure was also supported by 1H DOSY NMR, 19F NMR, and mass spectrometry. 1H DOSY measurements of the CD3OD solution confirmed all proton signals in the aromatic region assigned to a single species with the diffusion constant estimated to be 3.52 × 10–10 m2 s–1, which corresponds to a hydrodynamic diameter of 2.1 nm, closely matching the size of the capsule (Figure 3b). 19F NMR of the capsule, [BF4–⊂Ag6L4](BF4)5, revealed two signals at −151.6 and −154.8 ppm with an integral ratio of approximately 1:5, which could be assigned to encapsulated and unencapsulated BF4–, respectively (Figure 3d). Electrospray ionization time-of-flight (ESI-TOF) mass spectrometry also showed intense signals of species corresponding to the BF4–-encapsulating capsule [AgnL4(BF4)n-2]2+ (n = 4–6) (Figures S24 and S25). The observation of tetra- and pentanuclear AgI-capsules, in addition to the hexanuclear AgI-capsule, suggests that the two AgI(C) ions, which appear to be weakly bound in the crystalline state, may dissociate in solution while retaining the capsule structure. Taken together, under appropriate conditions, only the tetrameric capsule structures [BF4–⊂AgnL4](BF4)n−1 (n = 4–6) are formed in methanol, which is consistent with the high formation yields of the capsules discussed below. Remarkably, although the emission quantum yield in methanol was significantly decreased from φ = 0.73 for L to 0.13 for L in the capsule, the emission properties of L (λmax = 394 nm at 25 °C) were maintained in the capsule (Figure S26).

Since the inner space of the capsule is occupied by a single BF4– anion in the crystal structure, we next investigated the effect of the anion templating on the capsule formation in solution. For complexation in CD3OD, the effects of several silver salts with different counteranions, namely, AgBF4, AgPF6, AgSbF6, AgClO4, AgOTf (OTf = trifluoromethanesulfonate), AgNO3, and AgOTs (OTs = p-toluenesulfonate), were investigated (Figure 4). Among these, when AgPF6, AgSbF6, or AgClO4 was mixed with L, split 1H NMR signals similar to those with AgBF4 were observed, suggesting that similar capsule structures were formed. This assumption was also supported by 1H DOSY, 19F NMR spectroscopy, and ESI-TOF mass spectrometry, although the 19F signals of SbF6– were too broad to observe (Figures S31–S43). The maximum capsule formation yields in the presence of 1–1.5 equiv of the silver salt were estimated to be 93, 95, 89, or 94% for BF4–⊂capsule, PF6–⊂capsule, ClO4–⊂capsule, or SbF6–⊂capsule, respectively, based on quantitative NMR analysis using an internal standard (Figure 4b and Figures S51–S60). These results are consistent with the metal/ligand ratio of the capsule, Ag6L4, determined by ScXRD. By contrast, in the reaction with AgOTf, AgNO3, or AgOTs, the signal of L broadened and new complicated signals were formed as minor species (Figures S44–S50). When the minor signals can be assigned to the capsular structures, the formation yields of the reactions with AgOTf, AgNO3, or AgOTs are 48, 15, and 7%, respectively (Figures S61–S66). These results indicate that the anion template effect is very important for the capsular structure formation.

Figure 4.

Figure 4

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Anion template effects on the capsule formation. (a) Void (light blue) inside the capsule and its volume. (b) Maximum formation yield of capsules formed from L and silver salts with different counteranions. The yields were estimated based on quantitative NMR analysis with an internal standard. The volume of SbF6– was calculated by density functional theory (DFT) at the RB3LYP/LANL2DZ level, whereas those of the other anions were calculated at the RB3LYP/6-31+G(d) level. (c) Changes in 1H NMR spectra (500 MHz, CD3OD, 300 K) upon successive addition of AgOTs and nBu4NBF4 to L. Bottom: only L; middle: L + AgOTs; top: L + AgOTs + nBu4NBF4.

This difference in the template effect can be well explained by the size and shape of the anions. The effective template anions, SbF6–, PF6–, BF4–, and ClO4–, have a spherical shape with volumes of 86, 75, 55, and 56 Å3, respectively, which fit well into the capsule cavity shape and lead to efficient interactions between the anions and the inner surface of the cationic capsule (Figure 4a,b). Notably, a high formation yield was observed even though the size of SbF6– is almost the same as the volume of the cavity (86 Å3). This result suggests that the cavity size can be tuned within a certain range depending on the anion species included, which is supported by the crystal structure of SbF6⊂capsule with a cavity volume enlarged to 95 Å3 (Figures S39–S41). By contrast, TfO– has the appropriate anion size, but its aspherical shape with polar (SO3) and less polar (CF3) faces makes it difficult for it to efficiently interact with the capsule in the cavity. Conversely, neither NO3– nor TsO– served as a suitable template for capsule formation because their sizes are either too small or too large compared to the cavity volume. In addition, other factors such as the lipophilicity and the negative charge distribution of the anions may also affect the degree of inclusion.23

To directly confirm the anion template effect, we prepared a mixture of L and AgOTs and then added nBu4NBF4 (0.25 equiv for L) to the solution. Before the addition of nBu4NBF4, nearly no capsules were formed, as described above. On the other hand, upon addition of the BF4– salt, the signals drastically changed to a desymmetrized pattern corresponding to the capsule (Figure 4c and Figure S67). This result strongly supports the role of the template anion in stabilizing the supramolecular capsule structure.

Finally, we investigated the postsynthetic anion exchange reaction: a CD3OD solution of the BF4–⊂capsule was mixed with nBu4NPF6 (0.5 equiv for L and 0.33 equiv for BF4–) and analyzed using 19F NMR spectroscopy (Figure 5). The signals of the anion entrapped in the capsules immediately changed from BF4– to PF6–, suggesting that the anion exchange reaction quickly reached equilibrium (Figure 5b). One plausible mechanism for anion exchange is the transient dissociation of L from the capsule, which was supported by observing chemical exchange signals between the capsule and free ligands in NOESY and ROESY analyses (Figures S19 and S20). The equilibrium constant Kex (= [PF6–⊂Ag6L4][BF4–]/[BF4–⊂Ag6L4][PF6–]) for this exchange reaction was estimated by 1H NMR titration experiments, in which the 1H signals of the two different capsules were observed separately because the exchange rate was slower than the NMR time scale. The Kex value was found to be 6.0 ± 1.6 at 300 K (Figures S69 and S70), indicating that the capsule exhibits high selectivity toward PF6– by discriminating the size and shape of the anion in the confined nanospace.24

Figure 5.

Figure 5

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Postsynthetic anion exchange reaction. (a) Schematic representation of an anion exchange reaction between BF4– and PF6–. (b) Changes in the 19F NMR spectrum (471 MHz, CD3OD, 300 K) before (bottom) and after addition of nBu4NPF6 to the solution of BF4–⊂capsule (top). The 19F signals of PF6– are split into doublets due to J-coupling with the bound 31P nucleus.

Conclusions

In conclusion, we have synthesized intricate and low-symmetry metal–organic capsules based on a new facile strategy using dynamic stereoisomers of benzimidazole[3]arene (L) as an unsymmetric ligand. Upon complexation with silver salts, the four stereoisomers of L self-assemble to form racemic, conformationally heteroleptic chiral Ag6L4 capsules. X-ray crystallography revealed that the six silver ions in the capsule are not equivalent and that the capsule adopts C1-symmetry in the crystalline state. This intricate supramolecular structure was found to be stabilized by a combination of various noncovalent interactions, including multipoint CH–F hydrogen bonds with BF4– in the cavity, interligand π–π and CH−π interactions, and Coulombic forces. Furthermore, solution-state analysis by NMR spectroscopy and ESI-MS spectrometry supported the formation of the capsule structure in methanol, although the apparent symmetry in the 1H NMR spectrum increased to C2 due to averaging of similar coordination structures in solution. Solution NMR analysis also allowed us to access the effect of anion templating on capsule formation, showing that anions of suitable size and shape for the inner cavity act as suitable templates. The geometric complementarity between the cavity and the anion also afforded high selectivity for PF6– rather than BF4–, as revealed by postsynthetic anion exchange reactions. These findings demonstrate that heteroleptic capsules, which are usually synthesized with mixed ligands, can also be created using single-ligand stereoisomers. Moreover, our strategy of using an unsymmetric ligand enabled us to find highly desymmetrized capsule structures from the dynamic library, which is the novelty of this study. In the future, this capsule is respected to be further functionalized through several chemical processes, such as optical resolution of the enantiomeric capsules and accumulation of multiple capsules. Therefore, this method combining the stereoisomerism and asymmetry of polytopic ligands could become one of the leading and facile ways to create supramolecular materials with intricate low-symmetry structures.

Acknowledgments

Y.Y. acknowledges World-leading Innovative Graduate Study Program for Materials Research, Information, and Technology (MERIT-WINGS). We would like to thank Editage (www.editage.jp) for English language editing.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c11583.

  • General methods, experimental procedures, and additional data (PDF)

Powder XRD measurement was supported by Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM) of the Ministry of Education, Culture, Sports, Science and Technology MEXT), grant number JPMXP1224UT0209. This work was supported by JSPS KAKENHI, grant numbers JP19K22214 (Challenging Research (Exploratory)) and JP24H01701 (Grant-in-Aid for Transformative Research Areas “Materials Science of Meso-Hierarchy”), Tokuyama Science Foundation to S.T., and Grant-in-Aid for JSPS Fellows, grant number JP23KJ0456 to Y.Y.

The authors declare no competing financial interest.

Supplementary Material

ja4c11583_si_001.pdf (64.4MB, pdf)

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  24. Similarly, another anion exchange reaction from SbF6–⊂capsule to BF4–⊂capsule was observed, and its equilibrium constant was estimated to be 1.9 ± 0.8 at 300 K (Figures S71 and S72). This result suggests that PF6– is the best template anion among them.

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