Inclusion [2]complexes based on a pillar[5]arene with mono(ethylene oxide) substituents and vinylogous viologens

Abstract

Host–guest complexation between a pillar[5]arene with mono(ethylene oxide) substituents and vinylogous viologens was studied. It was found that the pillar[5]arene formed [2]pseudorotaxanes with these vinylogous viologens in acetonitrile with association constants up to (4.7 ± 0.3) × 10⁴ M⁻¹.

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Threaded structures,¹ such as pseudorotaxanes and rotaxanes, have been widely applied in the construction of molecular machines,² mechanically bonded macromolecules³ and functional supramolecular polymers.⁴ Pseudorotaxanes, which were the fundamental building blocks for the construction of sophisticated mechanically interlocked supramolecular assemblies with intriguing properties, have attracted much attention and been widely studied.⁵ However, the currently available host–guest recognition motifs which are good for the preparation of inclusion structures are still limited.⁶ This has impeded the development of research on threaded structures. Therefore, the discovery of new host–guest recognition motifs, especially the ones which have high binding constants will undoubtedly push forward the development of supramolecular chemistry.

Pillararenes,^7,8 as a new class of supramolecular hosts which have gained a wide range of interests since their first synthesis in 2008, are exceptional hosts for organic salts because of their rigid symmetrical pillar structures. Paraquat derivatives (N,N′-dialkyl-4,4′-bipyridinium salts) have been commonly used in the preparation of functional supramolecular systems with many kinds of hosts, including crown ethers,⁹ cyclodextrins,^5a,10 calixarenes¹¹ and cucurbiturils.^4c A series of threaded structures, including pseudorotaxanes and supramolecular polymers, have been prepared based on the pillararene/paraquat recognition motif.^7a,12 However, as π-extended viologens, vinylogous viologens, which expand the properties and functions of paraquat derivatives, such as photo-responsive isomerization, have been rarely studied as building blocks to construct threaded structures.^6b,13 Considering that the design and investigation of new recognition motifs based on pillararenes and vinylogous viologens will undoubtedly promote not only the development of pillararene host–guest chemistry but also the research on threaded structures, we are interested in the study of the host–guest complexation between pillararenes and vinylogous viologens. Herein, we report the formation of [2]pseudorotaxanes between a pillar[5]arene with mono(ethylene oxide) substituents (1 in Fig. 1)¹⁴ and vinylogous viologens (2 and 3 in Fig. 1). This is the first investigation of the host–guest chemistry between pillararenes and vinylogous viologens. The crystal structure of the complex between 1 and 2 was also reported. Furthermore, the guest-substituent effect on the host–guest complexation was addressed using similar salt guests 4 and 5 (Fig. 1).

Fig. 1 Chemical structures of host 1 and organic salt guests 2–5.

When the pillar[5]arene host 1 was mixed with an equivalent of guest 2 in acetonitrile, a yellow color was observed immediately due to charge transfer between the electron-rich aromatic rings of 1 and the electron-poor pyridinium rings of 2, providing a direct evidence for complexation. The recognition of 1 to vinylogous viologen 2 was first investigated by ¹H NMR spectroscopy. Partial proton NMR spectra of 1, 2, and an equimolar solution of 1 and 2 are shown in Fig. 2. Only one set of peaks was found for the solution of 1 and 2, indicating fast-exchange complexation on the proton NMR time scale. After complexation between 1 and 2, peaks corresponding to H_2b of guest 2 and H₃ and H₄ of host 1 shifted downfield by 0.02 ppm, 0.01 ppm and 0.10 ppm, respectively, while H₁ and H₂ of host 1 and H_2a of guest 2 shifted upfield by 0.12 ppm, 0.17 ppm and 0.01 ppm, respectively (Fig. 2). Additionally, the signals of H_2c and H_2d disappeared after complexation (Fig. 2, spectra b and c), suggesting that these protons were located in the shielding region of the cyclic pillar structure. All these chemical shift changes indicated that the complexation of 1 with 2 took place in solution.

Fig. 2 Partial ¹H NMR spectra (400 MHz, CD₃CN, 22 °C) of: (a) 1.00 mM 1; (b) 1.00 mM 1 and vinylogous viologen 2; (c) 1.00 mM vinylogous viologen 2.

To establish the binding mode between 1 and 2, a 2D NOESY experiment was carried out. The NOESY spectrum (Fig. 3) of an equimolar solution of 1 and 2 shows cross peaks A, B, C and D representing the correlations between the signal of protons H_2b of 2 and those of protons H₁, H₂, H₃, and H₄ of 1. These correlations indicated the existence of the host–guest complexation between pillararene 1 and methyl vinylogous viologen 2.

Fig. 3 2D NOESY NMR spectrum (600 MHz, CD₃CN, 22 °C, mixing time = 800 ms) of 4.00 mM 1 and 2.

Additionally, the electrospray ionization mass spectrum of an equimolar solution of 1 and 2 exhibited a peak at m/z = 1545.9 (Fig. S5, ESI†), corresponding to [1⊃2 – PF₆]⁺, which also confirmed the complexation between 1 and 2. Furthermore, the complex-related fragment peaks were also found for other host–guest complexes of pillararene 1 with other guests 3–5 (Fig. S6–S8, ESI†).

Further evidence of the formation of a 1 : 1 inclusion complex between 1 and 2 was from X-ray analysis of a single crystal grown by slow diffusion of isopropyl ether into an equimolar acetonitrile solution of 1 and 2. The crystals of 1⊃2 have a red color due to the charge transfer between the electron-rich aromatic rings of 1 and the electron-poor pyridinium rings of 2. The crystal structure of 1⊃2 shows that vinylogous viologen 2 threads through the cavity of host 1 to form an inclusion complex in the solid state, which is stabilized by C–H⋯O hydrogen bonding interactions and C–H⋯π interactions (Fig. 4). The inclusion of guest 2 into the cavity of host 1 in the solid state is in accordance with the above-mentioned disappearance of NMR signals of protons of H_2c and H_2d of 2 after the complexation between 1 and 2 in solution. Ten C–H⋯O hydrogen bonds (A–J) are formed between eight hydrogen atoms of guest 2 and eight oxygen atoms on mono(ethylene oxide) substituents of pillar[5]arene 1 (Fig. 4). Not only six pyridinium hydrogen atoms but also two N-methylene hydrogen atoms are involved in these hydrogen bonding interactions. The C–H⋯π-plane distances of two hydrogen atoms of guest 2 (K and L) are 2.96 Å and 2.91 Å, respectively, shorter than 3.05 Å, implying the existence of C–H⋯π interactions.^12a What is more interesting is that guest 2 is threaded symmetrically into the cavity of host 1. The H⋯O distances of F and D are the same and the H⋯O distances of H and C are almost the same (Fig. 4).

Fig. 4 Ball-stick views of the crystal structure of 1⊃2. Host 1 is red, guest 2 is blue, hydrogens are sky blue, oxygens are green, and nitrogens are black. PF₆⁻ counterions, solvent molecules and hydrogens except the ones involved in hydrogen bonding between 1 and 2 were omitted for clarity. The red dashed lines indicate hydrogen bonds (A–J) and the green dashed lines indicate C–H⋯π interactions (K and L). Hydrogen bond parameters: H⋯O distance (Å), C⋯O distance (Å), C–H⋯O angel (deg): A, 2.69, 3.46, 135.7; B, 2.46, 2.94, 111.7; C, 2.52, 3.33, 143.9; D, 2.58, 3.38, 141.7; E, 2.41, 3.24, 146.2; F, 2.58, 3.35, 138.8; G, 2.52, 3.31, 141.2; H, 2.51, 3.33, 145.2; I, 2.52, 3.05, 115.9; J, 2.65, 3.46, 140.9. C–H⋯π interaction parameters: C–H⋯π distance (Å), C–H⋯π angel (deg): K, 2.96, 150.2; L, 2.91, 151.2.

We further explored the difference of the host–guest binding properties of pillar[5]arene 1 with salt guests 2–5. This was carried out by isothermal titration calorimetry (ITC), which is a powerful tool for studying the host–guest complexes because it not only gives the complexation association constants (K_a) but also yields their thermodynamic parameters (enthalpy and entropy changes). The stoichiometries of the complexations between 1 and 2–5 were all determined to be 1 : 1 in solution (Fig. S1–S4, ESI†). The association constants (K_a) were determined in acetonitrile to be (4.7 ± 0.3) × 10⁴ M⁻¹ for 1⊃2, (1.2 ± 0.2) × 10⁴ M⁻¹ for 1⊃3, (2.0 ± 0.5) × 10³ M⁻¹ for 1⊃4, and (1.5 ± 0.5) × 10³ M⁻¹ for 1⊃5 (Fig. S1–S4, ESI†). It is found that the ethylene oxide substituted groups play a crucial role in the present strong host–guest binding. The difference in K_a values indicates that the substituent groups on guests have the different influence on the host–guest complexation in solution. The association constants of 1⊃2 and 1⊃3 were much higher than those of 1⊃4 and 1⊃5. This could be attributed to the rigid double bond, which can help the guest to stay in the cavity of the pillar[5]arene not only by the preorganization of the guest for the host–guest complexation but also by additional C–H⋯π interactions. Besides, the association constant of 1⊃2 (or 1⊃4) was higher than that of 1⊃3 (or 1⊃5). This demonstrated that the end phenyl groups also influenced the host–guest complexation. Because of the existence of these phenyl groups, the guest threads more difficultly through the cavity of the pillar[5]arene.

In summary, we studied the host–guest complexation between pillar[5]arene 1 with mono(ethylene oxide) substituents and vinylogous viologens 2 and 3. ¹H NMR, 2D NOESY and single crystal X-ray analysis indicated that pillar[5]arene 1 forms a stable inclusion complex with vinylogous viologen 2 both in solution and in the solid state. The good binding ability of pillar[5]arene 1 to vinylogous viologens, which largely resulted from the introduction of the ethylene oxide substituents to provide multiple hydrogen bonds between the host and guest, makes this newly established host–guest recognition motif applicable in the design and preparation of new functional supramolecular systems.

This work was supported by the National Natural Science Foundation of China (31002701).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, characterizations. See DOI: 10.1039/c3ra45169k

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