Letter pubs.acs.org/OrgLett
Solvent-Dependent Self-Assembly and Crystal Structures of a SalenBased Macrocycle Masahide Tominaga,*,† Eri Takahashi,† Hiroyuki Ukai,† Kazuaki Ohara,† Tsutomu Itoh,‡ and Kentaro Yamaguchi*,† †
Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan Center for Analytical Instrumentation, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
‡
S Supporting Information *
ABSTRACT: A salen-based macrocycle possessing adamantane units was constructed from two ethylenediamine and two disubstituted adamantanebearing salicylaldehyde components. The addition of methanol to a tetrahydrofuran solution of the macrocycle afforded the crystals via hollow spherical and fused aggregates, and X-ray crystallographic analysis showed that the macrocycle had a rhomboidal structure. The use of chloroform instead of methanol gave the crystals of a square-shaped macrocycle without the formation of self-assembled materials.
M
components were induced to form nanospheres in aqueous solution; these nanospheres displayed sensitive and selective detection of aromatic-deficient organic molecules.19 However, the solvent-dependent molecular assembly of salen-derived macrocycles has not been investigated. We report herein the solvent-dependent self-assembly and crystal structures of a salen-based macrocycle consisting of two salen and two diethynyladamantane portions (1). After addition of methanol to a tetrahydrofuran solution of 1, the macrocycles were induced to form hollow spherical and fused aggregates and finally transformed into the crystals 1a. X-ray crystallographic analysis revealed that the macrocycle had a rhomboidal structure. When chloroform was employed instead of methanol, the macrocycles formed different crystals 1b without the generation of well-defined aggregates. In contrast, the macrocycle had a square framework. Previously, we demonstrated that adamantane-derived macrocycles form hollow spherical and networked aggregates, and eventually the crystals, in aqueous solution.20 The adamantane moieties in these molecules play a central role in the formation of these nanostructures and the crystallization because of their aliphatic properties and symmetrical shape. Thus, we designed a salen-based macrocycle containing adamantyl groups (1) for assembly into supramolecular materials. The macrocycle consists of two salen and two 1,3diethynyladamantane portions and therefore contains both polar and apolar moieties within the macrocyclic skeleton. Furthermore, the macrocycle is composed of rigid adamantane and aromatic segments and relatively flexible diimine containing ethylene components. We synthesized the macro-
acrocyclic compounds containing a cavity are versatile building blocks for the fabrication of self-assembled nanostructures such as vesicles, tubes, and helices1−3 and the construction of porous organic crystalline materials.4,5 The size, rigidity, and presence of functional groups on the macrocyclic framework influence the conformation of the single molecules and their arrangement in supramolecular materials. Further, the solvents used in the molecular assembly of the macrocycles play an essential role in determining the morphologies of the selfassembled nanostructures and the structures of the crystalline materials. This is because the solvents are involved in cooperative intermolecular interactions, which include hydrogen bonding, π-stacking, van der Waals contacts, and hydrophobic interactions.6,7 Therefore, the understanding of solvent-dependent self-assembly or crystallization behavior is vitally important for precise structural predictions and preparation of solvato-controlled supramolecular materials. Salen and related compounds are fascinating organic molecules and are often used as ligands in metal complexes; such complexes have been well investigated as catalysts.8 Salen derivatives contain two imine nitrogen atoms and two hydroxyl groups, which interact with hydrogen-bonding organic compounds, ionic salts, and metal ions and give the molecules attractive host−guest properties.9 Thus, salen units have been incorporated into macrocycles, cages, tweezers, and helices as molecular building blocks.10−13 Diverse salen-containing functional molecules and their applications have been reported by Kleij, MacLachlan, Nabeshima, and other groups.14−17 Further, salen-containing nanoarchitectures are being developed as a class of higher order assemblies. MacLachlan and co-workers have demonstrated that macrocycles possessing salen derivatives are directed into one-dimensional aggregates through intermolecular interactions with alkali metal and ammonium salts.18 Tetraphenylethylene macrocycles containing salen © XXXX American Chemical Society
Received: January 24, 2017
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DOI: 10.1021/acs.orglett.7b00264 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters cycle according to Scheme 1. We chose 1,3-diethynyladamantane as the disubstituted adamantane derivative; this can be Scheme 1. Synthesis of a Salen-Based Macrocycle with Adamantyl Groups
Figure 1. (a) Top and (b) side views of macrocycle 1 in crystal 1a showing the thermal ellipsoids.
CH···O and CH···π interactions between the phenol, ethylene, and adamantane moieties (Figure 2).
Figure 2. Packing diagram of macrocycle 1 in crystal 1a: (a) side and (b) top views of the network structure.
Pale yellow single crystals of 1b were obtained from a solution of 1 in chloroform and tetrahydrofuran. The macrocycle has an almost square structure, unlike the rhomboidal framework observed in crystal 1a (Figure 3). The
prepared in two steps from adamantane according to the report by Malik and co-workers.21 A Sonogashira coupling reaction between 1,3-diethynyladamantane and 2-hydroxy-4-iodobenzaldehyde gave 2 in 53% yield. The reaction of a 1:1 ratio of 2 and ethylenediamine in a mixture of acetonitrile and chloroform afforded macrocycle 1 in 72% yield. The 1H NMR spectrum of the macrocycle in THF-d8 contained one singlet for the imine protons at 8.34 ppm and one singlet for the hydroxyl protons at 13.03 ppm, indicating that the product had a symmetric cyclic structure. High-resolution mass spectrometry showed the molecular ion at m/z = 897.4374 for [M + H]+, which is in agreement with the molecular weight of [2 + 2] condensation product 1. The molecular structure of the macrocycle was clearly determined by X-ray crystallographic analysis. Pale yellow single crystals of 1a were obtained from a solution of 1 in methanol and tetrahydrofuran. Macrocycle 1 has a rhomboidal architecture with a cavity (Figure 1). The centroid−centroid distances are 12.35 and 14.03 Å for the phenyl rings and 14.38 Å for the adamantyl groups, and the distances between the nitrogen atoms in the imine components are 17.71 and 18.15 Å. The four phenyl rings are located almost perpendicular to the plane of the macrocyclic skeleton. Two of the four hydroxyl groups are positioned on the upper side, and the others are on the lower side. The macrocycles are aligned in columnar structures along the c axis through CH···O interactions between the phenol groups. These structures form networks through
Figure 3. (a) Top and (b) side views of macrocycle 1 in crystal 1b showing the thermal ellipsoids. Chloroform and tetrahydrofuran molecules have been omitted for clarity.
centroid−centroid distances are 13.58 and 13.62 Å for the phenyl rings and 16.19 Å for the adamantyl groups, and the distances between the nitrogen atoms in the imine units are 16.73 and 16.74 Å. The arrangement of the four phenyl rings and the orientation of the four hydroxyl groups are analogy to those in crystal 1a. The macrocycles are assembled into layers through CH···O interactions between the hydrogen atoms of the imine and the hydroxyl groups (Figure S1, Supporting Information). Chloroform and tetrahydrofuran molecules are accommodated within the internal spaces of the macrocycles. B
DOI: 10.1021/acs.orglett.7b00264 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters The chloroform molecules interact with the phenol groups in the macrocycles through CH···π interactions. The aggregation behavior of the macrocycle was next examined in the crystallization solvents. Macrocycle 1 is soluble in tetrahydrofuran but only slightly soluble in methanol and chloroform. Macrocycle 1 was dissolved in tetrahydrofuran (0.25 mM) at 40 °C and allowed to stand for 5 days at 25 °C. A field-emission scanning electron microscopy (FE-SEM) study showed no evidence of formation of well-defined supramolecular nanostructures. When methanol was added to a tetrahydrofuran solution of 1 (methanol/tetrahydrofuran = 3:2, v/v, 0.25 mM) and the solution was allowed to stand at 25 °C, the crystals were generated after 11 h. To examine self-assembly behavior up to crystal formation, we monitored the changes in the sizes and morphologies of the aggregates as a function of time by FE-SEM. After 0.25 h, the presence of spherical objects with a diameter of approximately 280 nm was confirmed (Figure 4a). After 0.5 h, burst spherical aggregates were partly
Figure 5. Schematic representation of solvent-dependent self-assembly and crystal structures of macrocycle 1.
ple hydroxyl groups positioned on the surface of the spheres. After 11 h, in the early stages after crystal formation of 1a, randomly fused aggregates of spheres were appeared in addition to microsized solids, demonstrating that the phase transition to the crystals occurred by way of the hollow spheres and their fused aggregates. These findings provide clear evidence for hollow spherical aggregation, fusion events, and phase transition. In a more polar mixed solvent of methanol and tetrahydrofuran (7:3, v/v), the sizes of spherical aggregates and self-assembly behaviors were similar to that in a methanol/ tetrahydrofuran (3:2, v/v) mixture, which were evidenced by SEM experiments (Figure 4d and Figure S3, Supporting Information). The crystals were generated after 2 h under this condition. When water as a poor solvent was used in place of methanol, the yellow precipitates were immediately produced in a H2O/tetrahydrofuran (3:2, v/v) mixture. The SEM micrographs exhibited microsized plate and rod-shaped materials and hollow spherical aggregates, where burst spheres were also monitored (Figure S4, Supporting Information). In a mixture of H2O and tetrahydrofuran (3:5, v/v) of 1 (0.25 mM), sphere formation, fusion events, and subsequent transformation into the crystals was also appeared (Figure S5, Supporting Information). These results indicate that the polarity in a mixed solvent mainly influences crystallization properties rather than the size, stability, and morphology of the self-assembled nanostructures. The self-assembly behavior and crystal structure in a mixture of chloroform and tetrahydrofuran were different from those in a mixture of methanol and tetrahydrofuran. Chloroform was added to a tetrahydrofuran solution of 1 (chloroform/ tetrahydrofuran = 3:2, v/v, 0.25 mM), and the solution was allowed to stand at 25 °C. The crystals appeared after 28 h. After 0.25 and 0.5 h, SEM images indicated no formation of spherical aggregates and only the presence of unordered aggregates. At the time of crystal formation, the SEM images showed the crystals and unordered aggregates. Similar results were obtained in a chloroform/tetrahydrofuran (1:1, v/v) and (3:7, v/v) mixture of 1 (0.25 mM). In crystal 1a, formed in the polar solvent, the macrocycles had a rhomboidal structure, which was densely aligned into molecular networks without channels. In contrast, the macrocycle in crystal 1b, formed in the relatively apolar solvent, was a square structure, which was arranged into a layer structure with channels that accom-
Figure 4. SEM images after (a) 0.25 h and (c) 3 h and (b) TEM image after 0.25 h obtained from a methanol/tetrahydrofuran (3:2, v/v) solution of 1 (0.25 mM). (d) SEM image obtained from a methanol/ tetrahydrofuran (7:3, v/v) solution of 1 (0.25 mM) after 0.25 h.
observed, indicating that the spheres were hollow (Figure S2, Supporting Information). The creation of hollow spheres was also supported by transmission electron microscopy (TEM) measurements, which showed the presence of spherical aggregates with a diameter of approximately 240 nm and obvious contrast between the outer and inner sides of the circles (Figure 4b). The thickness of the membranes in the hollow spheres was estimated to be ca. 35 nm, confirming their multilayer structures. On the basis these outcomes, 1D columnar structures composed of the macrocycles observed in the X-ray analysis presumably align to form the membrane of the hollow spheres in solution (Figure 5). The polar hydroxyl and imine moieties on the macrocycles are located on the outside of the hollow spheres. In tetrahydrofuran, hollow spherical aggregates and the crystals were not observed even after 1 month. Therefore, the driving force for the hollow sphere formation is ascribed to the effect of the polar solvent. Over time, irregularly fused aggregates made up of several spheres were observed, and their number gradually increased as the number of spherical aggregates decreased. Morphological changes thus occurred through neck formation between the spheres (Figure 4c). Fusion events between the spheres mostly arose from hydrogen-bonding interactions between the multiC
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Organic Letters
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modated chloroform and tetrahydrofuran molecules. The choice of solvent crucially influenced the morphology of the nanostructures in solution and the molecular structures and packing of the macrocycles in the solid state. In conclusion, we have demonstrated solvent-dependent selfassembly and crystal structures of salen-based macrocycles containing adamantyl groups. The macrocycle afforded the crystals via hollow spheres in a polar solvent, whereas the crystals were obtained without the formation of supramolecular materials in relatively apolar solvents. Remarkably, the difference in the shape and conformation of the macrocycles and their packing in the solid state were determined by the organic solvent used. These findings contribute to the understanding of the molecular self-assembly of macrocycles and the preparation of channel-containing crystalline materials with potential porosity. Studies of the coordination of these salen-containing macrocycles to transition metals and their selfassembly behavior and crystal structure are currently underway.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00264. Synthetic procedures, characterization data, NMR spectra, SEM images, and crystal data (PDF) X-ray crystallographic file of 1a (CIF) X-ray crystallographic file of 1b (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Masahide Tominaga: 0000-0003-3199-1882 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant No. JP16K05801. REFERENCES
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DOI: 10.1021/acs.orglett.7b00264 Org. Lett. XXXX, XXX, XXX−XXX
