Pillar[5]arene derivatives containing two dinitrophenyl rings: syntheses, conformations and the tubular self assembly in the solid state

Abstract

Pillar[5]arene derivatives with two dinitrophenyl rings were synthesized in 90% and 99% yields. The di(2,4-dinitro-5-fluoro-phenyl)-modified pillar[5]arene self assembled into columns in the solid state.

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In supramolecular chemistry, synthesis and functionalization of macrocyclic hosts¹ is a fundamental and challenging topic. Pillararenes,² as a new class of macrocycles, whose hydroquinone units are connected by methylene bridges at the para-position, have received much attention in recent years. Pillar[5]arenes are the greatest member of this macrocyclic family because of their easy syntheses and potential applications in the fabrication of mechanical interlocked molecules,³ supramolecular polymers,⁴ vesicles,⁵ transmembrane proton channels,⁶ metal nanoparticles,⁷ and other functional materials.⁸ Great efforts have been devoted to decorating pillar[5]arenes with different functional groups.⁹ To the best of our knowledge, most modifications of pillar[5]arenes so far endow their derivatives with flexible alkane chain linkers. Pillar[5]arenes with rigid groups, however, have rarely been explored. One example is the synthesis of a phenylethynyl group modified pillar[5]arene reported by Ogoshi et al.¹⁰ And another rigid pillar[5]arene derivative was investigated by Stoddart et al. based on p-benzoic acid modified pillar[5]arene in order to construct metal–organic framework (MOF) materials.¹¹ Such conformational rigidity often shows geometrical orientation and is useful to fabricate nanomaterials such as nanotubes and nanoparticles.¹²

We are trying to construct a rigid pillar[5]arene dimer, which is linked by two 2,4-dinitrophenyl groups. In this design, there are three holes in a molecule and two pillar cavities are parallel to each other. It can provide good opportunity to synthesize [3]rotaxane with interesting conformation and functionalization. Before getting this target compound, two intermediate pillar[5]arene derivatives 1 and 2 containing two dinitrophenyl rings were synthesized (Scheme 1). Herein, we reported the easy syntheses of compounds 1 and 2 in very high yields and their tubular self-assembly in the solid state. The rigid pillar[5]arene dimer is now ongoing process.

Scheme 1 Synthetic route of compounds 1 and 2.

Huang et al. reported the synthesis of 1,4-dimethoxypillar[4]arene[1]quinone (DMP[4]A[1]Q) and 1,4-dimethoxypillar[4]arene[1]hydroquinone (DMP[4]A[1]HQ).¹³ Then the yield of getting DMP[4]A[1]Q was improved by Wen et al.¹⁴ Based on these work, we reacted DMP[4]A[1]HQ with 1,5-difluoro-2,4-dinitrobenzene 3 through the nucleophilic aromatic substitution (S_NAr) reaction to obtain compound 1. The conversion was accomplished by mixing DMP[4]A[1]Q and 3 in THF solution at room temperature for 12 h under N₂ protection (Scheme 1). It can be seen from Table 1 that by changing the molar ratio of 3 and the catalyst NEt₃ to the reagent DMP[4]A[1]Q to 3 : 3 : 1, the yield of 1 increased to 90%. Then for further modification, 1 was dissolved in DMF under the condition of K₂CO₃ and reacted for 3 days. Compound 2 was synthesized in 99% yield.

Table 1 Yields of 1 for different molar ratio of 3 and NEt₃ to DMP[4]A[1]HQ

DMP[4]A[1]HQ (eq.)	3 (eq.)	NEt3 (eq.)	Yield of 1
1	1	1	19%
1	2	1	40%
1	2	2.5	68%
1	3	2.5	70%
1	3	3	90%

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Single crystals of 1 and 2 were obtained by slow diffusion of hexane into a dichloromethane solution of 1 and heptane into a chloroform solution of 2, respectively (Fig. 1). It was found that the pillar structure still remained in both 1 and 2 after derivatizing pillar[5]arene with two dinitrophenyl groups. We compared the distances between two adjacent bridging carbon atoms and the angles between the two bridging carbon–carbon bonds in single crystals of 1, 2, DMP[5] and DMP[4]A[1]Q (Fig. S4 and Table S1 in ESI†), and found that they have similar sizes. It shows that little geometrical effect was led to for the pillar cavity by the derivative groups. Meanwhile, the planes of two dinitrobenzene rings were almost parallel to each other and located on one side of the pillar cavity (not upside and downside of the cavity), which resulted in the side position of the fluorine atoms in 1 and the hydroxyl groups in 2. Such conformation provides the preorganization for the further synthesis of the rigid pillar[5]arene dimer and gives a new chance to functionalize pillar[5]arene on the side orientation.

Fig. 1 Crystal structures. (a) Top view and (b) side view of 1; (c) top view and (d) side view of 2. Solvent molecules and hydrogen atoms are omitted for clarity. C, black; O, red; N, blue; F, yellow.

Furthermore, we studied the supramolecular interactions and the self assembly of molecules 1 and 2 in the solid state. For compound 1, when symmetric expansion happened along the crystallographic a axis, the interesting infinite pentagonal channel existed (Fig. 2a). The main noncovalent interactions that resulted in this channel formed among the 2,4-dinitro-5-fluro-phenyl groups attached to the pillar[5]arene frameworks (Fig. 2b). Those interactions contain two C–H⋯O hydrogen bonds (d_C⋯O = 3.38 Å, θ_CH⋯O = 141.8°) between the benzene group of one molecule and the nitro oxygen atom of the adjacent molecular host, two C–H⋯O hydrogen bonds (d_C⋯O = 3.50 Å, θ_CH⋯O = 137.2°) between the methoxyl group of one molecule and the nitro oxygen atom of the adjacent molecular host, and four C–H⋯Cl hydrogen bonds (d_C⋯Cl = 3.67 Å, θ_CH⋯Cl = 146.4°) and one C–H⋯O hydrogen bond (d_C⋯O = 3.32 Å, θ_C–H⋯O = 137.7°) between dichloromethane and host molecules. These interactions expanded the molecules along a axis and formed the tubular packing mode. Seen from Fig. 2b and compared with the crystal packing of DMP[5]^2a and DMP[4]A[1]Q,¹² we can conclude that two 2,4-dinitro-5-fluoro-phenyl groups play an important role in the formation of tubular assemblies.

Fig. 2 (a) Stick representation of a 1-based column, (b) noncovalent interactions among molecules when self-assembled into a tube in the solid state.

Packing picture along the crystallographic a axis shows that the tubes arranged compactly and the whole surface was honeycombed (Fig. 3). The interactions between one tube and another include offset face-to-face π⋯π stacking among the phenyl rings in 1 and C–H⋯π hydrogen bonds. The role of solvents in the formation of this tubular structure may probably be important. Dichloromethane molecules were located outside of the channels and filled among host molecules.

Fig. 3 Sixteen neighboring tubes are shown in stick representation along the crystallographic a axis. Hydrogen atoms are omitted for clarity.

Self-assembly of macrocycle 2 in the solid state was also studied. From the crystal packing of 2, no tubular assembly formed in the solid state (Fig. S5†). Although the chemical constituents of 2 changed slightly compared to 1 (just replacing two F atoms with two OH groups), they showed obviously different assembling properties in the solid state. It can be seen from Fig. S6† that F atoms in 1 contributed to the C–H⋯F hydrogen bonds and OH groups in 2 participated in C–H⋯O hydrogen bonds in the solid state, respectively, which resulted in different effect for self-assembly of host molecules.

In crystal structure of 2, it is found that macrocycle 2 could encapsulate one n-heptane molecule into its cavity through multiple C–H⋯π hydrogen-bonding interactions (Fig. 4). It is similar with the case of 1,4-dibutoxypillar[5]arene reported by Huang et al.,¹⁵ which can recognize a n-hexane molecule inside the host cavity. It is suggested that the host–guest property of pillar[5]arene framework is not changed a lot after modifying two dinitrophenyl groups.

Fig. 4 (a) Top view and (b) side view of the crystal structure of 2·n-heptane. The red stick mode represents host 2, and the blue spacefill mode represents a n-heptane molecule. Hydrogen atoms are omitted for clarity.

In summary, two dinitrophenyl modified pillar[5]arene derivatives 1 and 2 were synthesized in 90% and 99% yields, respectively. X-ray crystal structure analysis showed that the conformation of 1 provides preorganization for the synthesis of the rigid pillar[5]arene dimer. In the solid state, compound 1 self assembled into an organic tube by multiple noncovalent interactions. Moreover, the nanotubes could further assemble into a 3D honeycombed architecture. This work gives a new chance to functionalize pillar[5]arene on the side orientation. Further functionalizations of such compounds are now in progress in our laboratory.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant 21544009, 21274132, 91227105) and Zhejiang Natural Science Foundation (Grant LY15B020007).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, X-ray crystal data and other characterizations. CCDC 1444303 and 1444304. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra28117b

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