Self-assembly of cucurbit[8]uril-based polypseudorotaxanes using host–guest interactions

Abstract

Two inclusion complexes formed on self-assembly of the Q[8] host with dihexyl-4,4′-bipyridinium (HV²⁺) dibromide and 1,3-bis(4-butylpiperazin-1-yl)-propane (C₃PA²⁺) dibromide guests have been characterized by X-ray crystallography, which clearly shows how the hosts and the guests interlock with each other using supramolecular interactions, eventually generating novel polypseudorotaxanes.

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(Pseudo)rotaxanes in which the wheel-like components encircle axle-like components through noncovalent interactions have received much attention in recent years. The interest is stimulated not only by their topologically intriguing structures but also by their potential applications in preparation of molecular devices and machines.¹ To date, a number of macrocyclic hosts such as cyclodextrins, calix[n]arenes, cyclophanes and pillar[n]arenes have been synthesized to construct simple (pseudo)rotaxanes, then further to construct interlocked supermolecules, such as poly(pseudo)rotaxanes, catenanes and molecular necklace.² In general, there are two non-covalent synthetic methodologies for preparing interlocked supermolecules: (1) joining the linear axle component of the (pseudo)rotaxanes with metal ions as ‘linkers’ to assemble into a interlocked system is the most reported synthetic strategy; (2) using intramolecular or intermolecular host-stabilized charge-transfer (CT) complex formation is the other effective synthetic strategy. For both synthetic methodologies, special moieties such as coordinating groups and D–A groups are required for the participating guests.

Cucurbit[n]urils^3–5 (n = 5–8, 10, 14, abbreviated as Q[n]), featuring a hydrophobic cavity and polar carbonyl groups surrounding the portals, have the remarkable property of encapsulating hydrophobic or/and cationic guest molecules in aqueous solution, and have been extensively employed in developing interlocked supramolecular systems.^6–10 For example, Kim and co-workers have successfully synthesized a series of transition-metal-directed 1D, 2D and 3D poly(pseudo)rotaxanes and molecular necklaces using the first approach.⁸ They also prepared a variety of polyrotaxanes, molecular necklaces, and rotaxane dendrimers through the second approach.⁹

We have recently reported that Q[8] can selectively encapsulate a pair of alkyl terminal chains of two guests in its hydrophobic cavity under appropriate condition.¹¹ The interesting finding prompted us to wonder whether two alkyl terminal chains of one guest could be respectively encapsulated into two hosts to form interlocked supermolecules, utilizing the host–guest interaction as ‘supramolecular glue’ (Scheme 1). In order to address this question, we thus designed and synthesized two ‘string’ guests dihexyl-4,4′-bipyridinium (HV²⁺) dibromide and 1,3-bis(4-butylpiperazin-1-yl)-propane (C₃PA²⁺) dibromide, both containing a pair of terminal alkyl chains (see the ESI†). Herein we report, for the first time, the self-assembly of two inclusion complexes of Q[8] with HV²⁺ and C₃PA²⁺. Interestingly, X-ray diffraction analysis of both inclusion complexes reveal that each Q[8] ‘bead’ is threaded on a pair of normal terminal alkyl chains from two different ‘string’ guests through host–guest interaction, forming novel 1D polypseudorotaxanes.

Scheme 1 Formation of interlocked supermolecule through host–guest interaction.

Slow evaporation of hydrochloric acid aqueous solution containing a 1 : 1 mixture of HV²⁺ dibromide and Q[8] resulted in colorless crystals of the inclusion complex Q[8]·HV²⁺, which crystallises in the monoclinic system with the C2/c space group.¹² The asymmetric unit of the inclusion complex Q[8]·HV²⁺ consists of two crystallographically distinct half Q[8] moieties, one HV²⁺ guest, two chloride anions, and 34 lattice water molecules. Crystal structure analysis reveals a unique binding behaviour between Q[8] and HV²⁺ in the solid state, which is distinctly different from that in the aqueous solution.¹³ As shown in Fig. 1a, HV²⁺ guest adopts an unconventional contorted conformation when bound within the cavities of two adjacent Q[8] hosts. It is interesting to note that one of the hexyl chains entirely engulfed into the Q[8] host while for the other hexyl chain only four carbons (C49, C50, C51 and C52) were encapsulated into the adjacent Q[8] host. As suggested in scheme 1, the whole viologen nucleus resides outside of the portal of the Q[8]. At the same time, each Q[8] host includes two hexyl chains from two HV²⁺ guests, giving rise to the formation of a novel 1D polypseudorotaxane (Fig. 1b). In the aqueous solution of Q[8] host and HV²⁺ guest, however, all the guest protons experience considerable upfield shift (see S2 in ESI†), which suggest that the HV²⁺ guest were encapsulated into the cavity of Q[8] host. Since the cavity of Q[8] host is not large enough to accommodate the whole HV²⁺ guest, we conclude that the HV²⁺ guest is easily in and out of the cavity of Q[8] host because of the large portals of Q[8], and what we observed signals were average signals of the free and included guests.

Fig. 1 (a) Stick representation showing one HV²⁺ guest bound within two Q[8] hosts; (b) stick representation showing Q[8] hosts and HV²⁺ guests link to form linear 1D polypseudorotaxane through host–guest interaction.

As can be seen in Fig. 2, the neighboring polypseudorotaxanes are offset by about one-half of the repeating unit and contact each other through hydrogen bonding, producing a 2D layer structure, which is parallel to the ab-plane of the unit cell. In the solid-state, the adjacent layers stack along the c-axis and rotate through an angle of 64.5° to maximize the interactions between the layers. Such a staggered arrangement produces infinite 1D channels along the c axis, in which the chloride anions and water molecules are entrapped (Fig. 3). PLATON calculations indicated that the solvent accessible volume within the inclusion complex Q[8]·HV²⁺ is 7549.0 ų, which is 35.4% of the total unit cell volume.¹⁴

Fig. 2 Two 2D closed packed layers of polypseudorotaxane chain view down c-axis (on ab plane). The thin and the thick lines show the upper and lower layer, respectively.

Fig. 3 Spacefilling diagram of inclusion complex Q[8]·HV²⁺ viewed down the c axis. O = red, C = grey and N = light blue. Solvate water molecules and chloride anions are omitted for clarity.

After numerous attempts, we also obtained the single crystal of the inclusion complex Q[8]·C₃PA²⁺ suitable for X-ray analysis, which crystallized in the space group of P2₁/c.¹⁵ The X-ray crystal structures of this inclusion complex (Fig. 4) clearly shows that both butyl groups of the C₃PA²⁺ guest are encapsulated entirely within the Q[8] host and each Q[8] host contained a pair of butyl groups. Two piperazine units connected by three carbon chains all reside at the portal of the Q[8] host. As a result, the Q[8] ‘bead’ threaded on a pair of C₃PA²⁺ ‘strings’ forms an infinite 1D polypseudorotaxane chain (Fig. 4) whereas the adjacent Q[8] hosts aren't parallel as suggested in Scheme 1. Moreover, neighboring chains form a 2D layer on the ab plane with a distance between two contacting chains of 17.5 Å as shown in Fig. 5. In the solid-state, adjacent sheets stack along c-axis and rotate an angle of 90° to maximize the interactions between the layers. Of course, the interchain space is filled with bromide anions and water molecules and they interact with each other to form hydrogen-bonding network. In the aqueous solution, the ¹H NMR spectra of Q[8] host and C₃PA²⁺ guest (see S5 in ESI†) suggest that the binding behavior of Q[8] host and C₃PA²⁺ guest is similar to that found in inclusion complex Q[8]·HV²⁺.

Fig. 4 Stick representation of the inclusion complex Q[8]·C₃PA²⁺ showing the formation of an infinite polypseudorotaxane.

Fig. 5 Two 2D closed packed layers of polypseudorotaxane chain view down c-axis (on ab plane). The thin and the thick lines show the upper and lower layer, respectively.

The driving force of the formation of these two polypseudorotaxanes may be due to the ion–dipole interactions between each of the quaternized nitrogens in the guests and the carbonyl oxygens at the portals of Q[8] hosts, instead of previously reported coordination bonds or charge-transfer interactions. Additionally, hydrophobic effect should be taken into account, which facilitates the Q[8] cavity to accommodate the alkyl chains of both guests. Size and shape complementarity of the receptor and the guest may also contribute to the stability of the complexes. It is worth mentioning that a significant ellipsoidal deformation of the Q[8] host was observed in both inclusion complexes. Obviously, in order to accommodate two alkyl chains and to provide a complementary shape, the Q[8] host has to modulate its conformation spontaneously during the encapsulation process. For the inclusion complex Q[8]·HV²⁺, the largest O⋯O diameter of the carbonyl portals has been changed from 8.785 to 11.292 Å (Fig. 6). While in the inclusion complex Q[8]·C₃PA²⁺, the corresponding datum is 10.868 Å. Furthermore, by comparing these data with those of assembly 1–3 we previously reported,¹¹ we found that the more carbon atoms are encapsulated, the more severe the Q[8] is deformed.

Fig. 6 Comparison structures of normal Q[8] and distorted Q[8]. O = red, C = grey and N = light blue.

In summary, two novel polypseudorotaxanes, in which the repeating units are linked by host–guest interactions (including ion–dipole interaction and hydrophobic interaction) between Q[8] ‘bead’ and guest ‘string’, are synthesized successfully and have been unequivocally characterized by X-ray crystallography. The present polypseudorotaxane not only enriches the world of supramolecular architectures but also provides a novel approach to design and construct other interlocked supermolecules. Such designed interlocked supermolecules may find interesting applications in drug delivery, catalysis and as sensors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC; no. 21101037 and 21371004), the “Chun-Hui” Fund of the Chinese Ministry of Education, the Science and Technology Fund of Guizhou Province, and the International Cooperation Projects of Science and Technology Agency of Guizhou province (Grant no. 20127005). All are gratefully acknowledged.

References

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12. Crystal data for Q[8]·HV²⁺: (C₄₈H₄₈N₃₂O₁₆)·(C₂₂H₃₄N₂)²⁺·(H₂O)₃₄·2Cl⁻, M_r = 2339.14, monoclinic, space group C2/c, a = 44.361(3) Å, b = 19.8427(10) Å, c = 31.175(2) Å, β = 128.983(6)°, V = 21331(2) ų, Z = 8, D_c = 1.457 g cm⁻³, F(000) = 9936, GOF = 1.006, R₁ = 0.0674 (I > 2σ(I)), wR₂ = 0.1697 (all data). For complex Q[8]·HV²⁺, some algorithms such as FLAT, DAMP and DFIX are used in the refinement process to model the pyridinium moiety because the pyridinium moiety bound in the Q[8] suffers some degree of disorder.
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15. Crystal data for Q[8]·C₃PA²⁺: (C₄₈H₄₈N₃₂O₁₆)·(C₂₁H₄₄N₂)²⁺·(H₂O)_18.5·2Br⁻, M_r = 4293.72, monoclinic, space group P21/c, a = 25.547(10) Å, b = 24.005(10) Å, c = 15.446 Å, β = 92.070(6)°, V = 9466(6) ų, Z = 2, D_c = 1.506 g cm⁻³, F(000) = 4508, GOF = 1.027, R₁ = 0.1270 (I > 2σ(I)), wR₂ = 0.3676 (all data). For complex Q[8]·C₃PA²⁺, higher symmetry was excluded with the program PLATON, but it is possible that this structure suffers from partial merohedral twinning. The bromide anions in the lattice are ill-defined, but the main part of the molecule is well-defined.

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Footnote

† Electronic supplementary information (ESI) available: Details of experimental procedures for both guests and Cucurbit[8]uril and single-crystal X-ray crystallographic data. CCDC 1018028 and 1018029. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra08636h

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