Discrete Molecular Recognition Induced Higher-Order Structures

Letter pubs.acs.org/OrgLett

Discrete Molecular Recognition Induced Higher-Order Structures: Fibrous Formation Triggered by Melamine Recognition with a Cationic Ethynylpyridine Macrocyclic Host Daiki Suzuki, Hajime Abe,* and Masahiko Inouye* Graduate School of Pharmaceutical Sciences, University of Toyama, Toyama 930-0194, Japan S Supporting Information *

ABSTRACT: A tricationic shape-persistent macrocycle was obtained by methylation on the nitrogen atoms of the three 3,5-pyridylene groups of an alternating 2,6-/3,5-substituted ethynylpyridine macrocycle. The tricationic macrocycle recognized melamine in polar solvents such as DMSO and water, and the host−guest association in water induced a higher-order aggregate confirmed by UV−vis titration and dynamic light scattering experiments. Scanning electron microscopy, transmission electron microscopy, and atomic force microscopy indicated that fibrous network structures resulted from the stacking of the macrocycle and melamine complex. Scheme 1. Preparation of Macrocycle 23+·3TfO−

S

hape-persistent macrocycles (SPMs) based on a planar framework with a broad π-surface1,2 have attracted much attention in the fields of supramolecular chemistry and materials science including liquid crystals, 3 nanoporous materials,4 and photoconductive materials.5 To date, SPMs of various frameworks have been developed, and some SPMs selfaggregate by noncovalent interactions such as π-stacking and solvophobic effects.6 For example, Moore and co-workers reported that a cyclic meta-phenylene ethynylene hexamer selfassembled to form a nanofiber.6a The size and morphology of the self-assembled structure could be controlled by changing temperature and solvents. Höger et al. investigated phenylene ethynylene macrocycles with flexible long alkyl side chains, and the SPMs showed liquid-crystalline behavior.3c,d On the other hand, several kinds of SPMs have been known to accommodate a guest molecule in their cavities.6f,7 Miljanic and co-workers reported crystal structures of the host−guest complex of phenylene ethynylene macrocycles and fluorinated benzene associating through C−H···F−C interaction.7c However, there are few examples of self-aggregation systems triggered by discrete molecular recognition between SPMs and organic guests in their cavity. Yuan et al. investigated an oligoamidetype SPM that recognized an alkylammonium ion in its cavity. The initially resulting complex subsequently self-assembled to form a higher-order structure.7d Nevertheless, this type of aggregation is still rare and remains to be widely explored. Our group also has developed SPMs8 such as 1, in which three 2,6-pyridylene and three 3,5-pyridylene units were alternatingly linked by acetylene bonds9 with D3h symmetry (Scheme 1).8a In solid states, the rigid planar structure of 1 took advantage of π-stacking enhanced by the intermolecular dipole−dipole interaction to form self-assemblies. Apart from the self-assembling property, macrocycle 1 has multipoint hydrogen-bonding sites inside and showed an affinity with octyl © XXXX American Chemical Society

glucoside in apolar solvent. To give water solubility in the framework, we derivatized 1 by alkylating at the outer nitrogen atoms to produce pyridinium derivatives (Scheme 1). Herein, we report that the water-soluble tripyridinium cation 23+ recognized melamine, and that the discrete host−guest association in water induced self-assembly to form fibrous aggregates observable on the basis of dynamic light scattering (DLS) and various microscopies (Figure 1). Precursor 1 was treated with an excess amount of methyl triflate (MeOTf) in CH2Cl2 at room temperature (Scheme 1). The reaction gave one major product as a pale yellow solid, which was assigned to trication 23+·3TfO− based on 1H NMR and ESI-TOF-MS. The 1H NMR spectrum was very simple and showed three kinds of aromatic signals (δ 9.55, 9.32, 6.86 ppm in DMSO-d6) as well as the case for 1 (δ 8.75, 8.27, 7.14 ppm in CDCl3). In comparison, though the solvents used were different,10 the two signals on the “outward” pyridine of 1 moved remarkably downfield after the reaction. Thus, in the product, the three nitrogen atoms in the 3,5-pyridylene units Received: December 9, 2015

A

DOI: 10.1021/acs.orglett.5b03502 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

atoms of −NH2 groups in melamine (Hb in Figure 2c) and triple bonds of 23+. In addition, the C−H···N interaction was observed for nitrogen atoms in the triazine ring of melamine (Nb in Figure 2c) with 4-hydrogen atoms in the 3,5-pyridylene units of 23+ (Hc in Figure 2c). Overall, the host−guest complex model was stabilized with nine intermolecular hydrogen bonds. The planarity and the D3h symmetry were enforced after the association, thus self-association seems to work well by using the enlarged π-plane in this host−guest complex. The affinity of host 23+ with melamine in DMSO was studied by UV−vis titration (Figure 3). When melamine was added to a Figure 1. Schematic representation of the aggregation process triggered by molecular recognition of SPM.

were methylated. On the other hand, the “inward” three nitrogen atoms in the 2,6-pyridylene units remained intact. The decreased reactivity was presumably caused by the decreased nucleophilicity, which should be suppressed by the substituents adjacent to the inward nitrogen atoms. The signal of the tertbutyl groups of 1 disappeared after the reaction, while two singlets appeared at δ 4.41 and 4.21 ppm, each of which represents nine protons based on the integration. The three tert-butyl groups at the 2,6-pyridylene units were replaced with methyl groups by electrophilic substitution on the oxygen atoms. Taking those into account, we concluded that the product of the reaction of 1 with MeOTf must be trication 23+. The counteranion in 23+ proved to be TfO− as ESI-TOF-MS spectra showed signals of various molecular ions involving 23+ and TfO− such as [23+ + 2TfO−]+ and [23+ + 3TfO− + H]+. This salt 23+·3TfO− was soluble in various polar solvents such as H2O, DMSO, and MeOH. SPMs consisting of arylene and ethynylene units have been developed as host molecules incorporating guests within their cavities.6f,7,9 Macrocycle 23+ has three nitrogen atoms facing the cavity, and these were expected to work as convergent hydrogen-bonding acceptors with D3h symmetry. We examined melamine as a guest molecule because melamine appears to just fit in the cavity and has divergent hydrogen-bonding donors in the same planar and D3h symmetric manner as in 23+.11 High symmetry and rigidity of both the host and the guest would reduce entropic cost that inevitably arises from the host−guest complexation. Geometry optimization of the melamineincorporated 23+ was carried out using DFT (Figure 2a,b). We predicted the formation of intermolecular hydrogen bonds of the −NH2 groups in melamine (Ha in Figure 2c) with the inward nitrogen atoms of 23+ (Na in Figure 2c). Unexpectedly, the N−H···π interaction also was indicated between hydrogen

Figure 3. (a) Change of UV−vis spectrum and (b) change of absorbance at 300 nm of 23+·3TfO− in DMSO according to the addition of melamine. Association constant Ka was obtained as (1.8 ± 0.3) × 103 M−1 by a curve-fitting analysis, and the fitted curve is shown as a solid line. Conditions: [23+·3TfO−] = 2.0 × 10−5 M, [melamine] = 0 to 1.0 × 10−2 M, DMSO, 25 °C, path length = 10 mm.

DMSO solution of 23+·3TfO− (2.0 × 10−5 M), the absorption band around 300 nm was decreased with a blue shift and that around 450 nm was increased (Figure 3a). An isosbestic point at 344 nm shows the presence of only two kinds of absorptive species, probably 23+ and the complex 23+·melamine. The absorption band around 450 nm might result from the intramolecular charge transfer between electronically localized 2,6-pyridylene donors and 3,5-pyridylene acceptors in 23+, which should be enhanced by the formation of the rigid framework after the complexation. Indeed, the HOMO in 23+ mainly lies on the 2,6-pyridylene and the LUMO on the 3,5pyridylene (Figure S1). The titration curve was drawn for the absorbance at 300 nm, and the association constant Ka was obtained as (1.8 ± 0.3) × 103 M−1 by the curve-fitting analysis based on the assumption of 1:1 association (Figure 3b). Although DMSO was known to strongly interfere with specific hydrogen bonds, 23+ fairly recognized melamine by the multipoint hydrogen bonds even in such highly competitive conditions. The recognition of melamine by 23+ in H2O was also studied by UV−vis titration (Figure 4).12 When melamine was added to a H2O solution of 23+·3TfO− (2.0 × 10−5 M), the absorption

Figure 4. (a) Change of UV−vis spectrum and (b) change of absorbance at 295 nm of 23+·3TfO− in H2O according to the addition of melamine. Conditions: [23+·3TfO−] = 2.0 × 10−5 M, [melamine] = 0 to 2.5 × 10−3 M, H2O, 25 °C, path length = 10 mm.

Figure 2. (a) Top and (b) side views of 23+·melamine optimized by DFT calculation. Conditions: B3LYP/6-31G. (c) Supposed hydrogen bonds in 23+·melamine. B

DOI: 10.1021/acs.orglett.5b03502 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters band around 295 nm was weakened and a slope appeared around 400−700 nm, accompanying the color change from pale yellow to brown. Melamine is most likely incorporated into the cavity of 23+ because the spectral change resembles that in DMSO. Figure 4b shows the absorbance at 295 nm against the molar concentration of melamine. The shape of the plot, however, was different from that in DMSO and could not fit with any theoretical curve, assuming simple binding models such as 1:1 and 1:2. It was suggested that the combination of 23+ and melamine in H2O forms some kinds of higher-order aggregates through the 23+·melamine host−guest complexation. To confirm the aggregation, Tyndall scattering was tested using a green-emitting laser pointer (λem = 532 nm). After a H2O solution of 23+·3TfO− (2.0 × 10−5 M) was mixed with melamine (1.0 × 10−4 M) in a glass tube, the resulting sample mixture exhibited a clear Tyndall scattering (Figure 5a). On the

Figure 6. (a) Precipitate appears in a mixture of 23+·3TfO− and melamine in H2O after annealing treatment: mixture was heated at 45 °C for 5 min and then cooled at 25 °C for 15 min to form the precipitate shown. Conditions: [23+·3TfO−] = 1.0 × 10−4 M, [melamine] = 5.0 × 10−3 M. (b) SEM, (c) TEM, and (d) AFM images of self-aggregated fibril structure of the precipitate. (e) Line scan profile as marked in the AFM image. Figure 5. (a) Tyndall scattering of a mixture in H2O. Conditions: [23+· 3TfO−] = 2.0 × 10−5 M, [melamine] = 1.0 × 10−4 M, 25 °C. (b) Hydrodynamic diameter distribution for a sample mixture of 23+· 3TfO− and melamine in H2O measured by DLS. The sample mixture was left at 25 °C for 12 h before DLS measurement. Conditions: [23+· 3TfO−] = 2.0 × 10−5 M, [melamine] = 1.0 × 10−3 M, 25 °C.

Interestingly, the precipitate once yielded was found to never dissolve even in DMSO. This means that the discrete host− guest complex in DMSO would be a kinetic product that fails to self-assemble. Similar types of SPM-based fibril structures have been reported by Moore, Höger, and other groups.6 We also reported that precursor 1 self-assembled into stacked structures only in solid states.8a Thus, the nanofibers observed in the present study were presumably composed of the columns made of stacked 23+·melamine. On the other hand, aqueous solutions of 23+ without melamine showed no self-assembling as mentioned above. Coulomb repulsion would inhibit the selfassembly of 23+ with a large hole inside as an “apo” state of the enzyme. When melamine was incorporated within the “apohost”, the π-surface of the 23+·melamine complex became larger than that of 23+. In addition, the rigidity of the framework of 23+ would be enhanced by the formation of the complex. The enlarged and rigidified π-surface would enforce the hydrophobic π-interaction, which should more than make up for the Coulomb repulsion and the entropic loss during the self-assembly (Figure 1). In summary, we developed a water-soluble pyridine− pyridinium alternating tricationic macrocycle, which recognized melamine in polar solvents such as DMSO and H2O. The discrete molecular recognition in water induced the stacking of the host−guest complex to give columnar and eventually mesoscopic fibrous structures. This approach could be applied to various discrete host−guest combinations. The resulting higher-order structures are expected to be functional materials of interest, and such projects are now underway.

other hand, H2O solutions of 23+·3TfO− and melamine alone showed no meaningful Tyndall scattering as well as a DMSO solution of the mixture. The aggregation was quantified by DLS experiments. A sample mixture was prepared from 23+·3TfO− (2.0 × 10−5 M) and melamine (1.0 × 10−3 M) in H2O and left at 25 °C for 12 h. DLS of the resulting mixture was measured at that temperature, and the mean hydrodynamic diameter of the particles was estimated to be 2.4 μm (Figure 5b). Of course, when DLS was examined for each the aqueous solution of 23+· 3TfO− and melamine, no meaningful scattering light could be observed (Table S1). Annealing experiments for the mixture of 23+·3TfO− and melamine in H2O gave rise to insoluble materials, which were used to investigate the mesoscopic structure of the aggregates. When the mixture was heated at 45 °C for 5 min and then cooled at 25 °C for 15 min, a fluffy brown precipitate appeared (Figure 6a). This precipitate was analyzed by using various microscopes. As shown in Figure 5, a number of fibril structures were observed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The width of the fiber was estimated as more than 10 nm from all the microscopic images, and this value was much larger than that of one molecule of 23+ predicted by the DFT calculation (Figure 2). Therefore, these observed nanofibers would be some kind of bundles from the several columns that consist of the 23+·melamine complex. C

DOI: 10.1021/acs.orglett.5b03502 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters

■

2012, 1, 1335−1338. (b) Datar, A.; Gross, D. E.; Balakrishnan, K.; Yang, X.; Moore, J. S.; Zang, L. Chem. Commun. 2012, 48, 8904−8906. (c) Li, J.; Zhou, W.; Yang, J.; Lang, X.; Huang, P. J. Colloid Interface Sci. 2013, 395, 99−103. (d) Balakrishnan, K.; Datar, A.; Zhang, W.; Yang, X.; Naddo, T.; Huang, J.; Zuo, J.; Yen, M.; Moore, J. S.; Zang, L. J. Am. Chem. Soc. 2006, 128, 6576−6577. (e) Rosselli, S.; Ramminger, A.-D.; Wagner, T.; Lieser, G.; Höger, S. Chem. - Eur. J. 2003, 9, 3481−3491. (f) Xiao, D.; Zhang, D.; Chen, B.; Xie, D.; Xiang, Y.; Li, X.; Jin, W. Langmuir 2015, 31, 10649−10655. (g) Lee, S.; Hirsch, B. E.; Liu, Y.; Dobscha, J. R.; Burke, D. W.; Tait, S. L.; Flood, A. H. Chem. - Eur. J. 2016, 22, 560−569. (h) Tahara, K.; Lei, S.; Mamdouh, W.; Yamaguchi, Y.; Ichikawa, T.; Uji-i, H.; Sonoda, M.; Hirose, K.; De Schryver, F. C.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2008, 130, 6666−6667. (i) Tobe, Y.; Utsumi, N.; Kawabata, K.; Nagano, A.; Adachi, K.; Araki, S.; Sonoda, M.; Hirose, K.; Naemura, K. J. Am. Chem. Soc. 2002, 124, 5350−5364. (7) For molecular recognition by host molecule based on SPMs, see: (a) Bahr, A.; Droz, A. S.; Puntener, M.; Neidlein, U.; Anderson, S.; Seiler, P.; Diederich, F. Helv. Chim. Acta 1998, 81, 1931−1963. (b) Anderson, S.; Neidlein, U.; Gramlich, V.; Diederich, F. Angew. Chem., Int. Ed. Engl. 1995, 34, 1596−1600. (c) Popov, I.; Chen, T.-H.; Belyakov, S.; Daugulis, O.; Wheeler, S. E.; Miljanic, O. Š. Chem. - Eur. J. 2015, 21, 2750. (d) He, Y.; Xu, M.; Gao, R.; Li, X.; Li, F.; Wu, X.; Xu, D.; Zeng, H.; Yuan, L. Angew. Chem., Int. Ed. 2014, 53, 11834− 11839. (8) (a) Abe, H.; Ohtani, K.; Suzuki, D.; Chida, Y.; Shimada, Y.; Matsumoto, S.; Inouye, M. Org. Lett. 2014, 16, 828−831. (b) Abe, H.; Kurokawa, H.; Chida, Y.; Inouye, M. J. Org. Chem. 2011, 76, 309−311. (c) Abe, H.; Chida, Y.; Kurokawa, H.; Inouye, M. J. Org. Chem. 2011, 76, 3366−3371. (9) Abe, H.; Suzuki, D.; Shimizu, A.; Inouye, M. Heterocycles 2014, 88, 547−557. (10) Unfortunately, 1H NMR measurements of the 23+ salt and 1 could not be carried out in solvent of the same kind. Precursor 1 was practically insoluble in H2O, DMSO, and MeOH. (11) For examples of supramolecular structures based on melamine, see: (a) Bairi, P.; Roy, B.; Chakraborty, P.; Nandi, A. K. ACS Appl. Mater. Interfaces 2013, 5, 5478−5485. (b) Bairi, P.; Roy, B.; Nandi, A. K. Chem. Commun. 2012, 48, 10850−10852. (c) Bairi, P.; Roy, B.; Nandi, A. K. J. Phys. Chem. B 2010, 114, 11454−11461. (d) Saha, A.; Roy, B.; Garai, A.; Nandi, A. K. Langmuir 2009, 25, 8457−8461. (e) Makowski, S. J.; Lacher, M.; Lermer, C.; Schnick, W. J. Mol. Struct. 2012, 1013, 19−25. (f) Lazar, A. N.; Danylyuk, O.; Suwinska, K.; Coleman, A. W. New J. Chem. 2006, 30, 59−64. (12) We have also studied the additive effect of melamine on the NMR spectrum of a D2O solution of 23+·3TfO− (5.0 × 10−5 M). Unfortunately, the addition of melamine (10 and 20 equiv) made the sample solution turbid, and the NMR spectrum was significantly broadened, as shown in Figure S2 in the Supporting Information. The chemical shift of pyridinic protons seemed to move upfield, possibly because of host−guest association and/or π-stacking aggregation.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.5b03502. Figures S1 and S2, Table S1, experimental details for the preparation of 23+·3TfO−, details of theoretical analyses for 23+·melamine, and 1H and 13C NMR spectra for 23+· 3TfO− (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Prof. Dr. Hiromi Kitano and Dr. Makoto GemmeiIde (Graduate School of Science and Engineering, University of Toyama) for advice on the analyses of self-aggregations. DLS measurements were supported by Prof. Dr. Minoru Nakano and Dr. Keisuke Ikeda (Graduate School of Pharmaceutical Sciences, University of Toyama). NMR, IR, SEM, TEM, and AFM measurements were performed at Life Science Research Center, University of Toyama.

■

REFERENCES

(1) Reviews for shape-persistent macrocycles: (a) Höger, S. Chem. Eur. J. 2004, 10, 1320−1329. (b) Zhang, W.; Moore, J. S. Angew. Chem., Int. Ed. 2006, 45, 4416−4439. (c) Smith, M. K.; Miljanic, O. Š. Org. Biomol. Chem. 2015, 13, 7841−7845. (d) Höger, S. Angew. Chem., Int. Ed. 2005, 44, 3806−3808. (e) Iyoda, M.; Yamakawa, J.; Rahman, M. J. Angew. Chem., Int. Ed. 2011, 50, 10522−10553. (2) For examples of SPMs, see: (a) Tobe, Y.; Nagano, A.; Kawabata, K.; Sonoda, M.; Naemura, K. Org. Lett. 2000, 2, 3265−3268. (b) Sonoda, M.; Yamaguchi, Y.; Tahara, K.; Hirose, K.; Tobe, Y. Tetrahedron 2008, 64, 11490−11494. (c) Henze, O.; Lentz, D.; Schluter, A. D. Chem. - Eur. J. 2000, 6, 2362−2367. (d) Henze, O.; Lentz, D.; Schafer, A.; Franke, P.; Schluter, A. D. Chem. - Eur. J. 2002, 8, 357−365. (e) Sun, S. S.; Lees, A. J. Organometallics 2001, 20, 2353− 2358. (f) Campbell, K.; Kuehl, C. J.; Ferguson, M. J.; Stang, P. J.; Tykwinski, R. R. J. Am. Chem. Soc. 2002, 124, 7266−7267. (g) Campbell, K.; McDonald, R.; Tykwinski, R. R. J. Org. Chem. 2002, 67, 1133−1140. (h) Yamaguchi, Y.; Kobayashi, S.; Miyamura, S.; Okamoto, Y.; Wakamiya, T.; Matsubara, Y.; Yoshida, Z. Angew. Chem., Int. Ed. 2004, 43, 366−369. (i) Spitler, E. L.; McClintock, S. P.; Haley, M. M. J. Org. Chem. 2007, 72, 6692−6699. (j) Miki, K.; Fujita, M.; Inoue, Y.; Senda, Y.; Kowada, T.; Ohe, K. J. Org. Chem. 2010, 75, 3537−3540. (k) Yamasaki, R.; Shigeto, A.; Saito, S. J. Org. Chem. 2011, 76, 10299−10305. (3) (a) Zhang, J.; Moore, J. S. J. Am. Chem. Soc. 1994, 116, 2655− 2656. (b) Mindyuk, O. Y.; Stetzer, M. R.; Heiney, P. A.; Nelson, J. C.; Moore, J. S. Adv. Mater. 1998, 10, 1363−1366. (c) Höger, S.; Enkelmann, V.; Bonrad, K.; Tschierske, C. Angew. Chem., Int. Ed. 2000, 39, 2268−2270. (d) Fischer, M.; Lieser, G.; Rapp, A.; Schnell, I.; Mamdouh, W.; De Feyter, S.; De Schryver, F. C.; Höger, S. J. Am. Chem. Soc. 2004, 126, 214−222. (4) Chen, T.-H.; Popov, I.; Chuang, Y.-C.; Chen, Y.-S.; Miljanic, O. Š. Chem. Commun. 2015, 51, 6340−6342. (5) Luo, J.; Yan, Q.; Zhou, Y.; Li, T.; Zhu, N.; Bai, C.; Cao, Y.; Wang, J.; Pei, J.; Zhao, D. Chem. Commun. 2010, 46, 5725−5727. (6) For examples of self-assembly of SPMs see: (a) Li, L. G.; Che, Y. K.; Gross, D. E.; Huang, H. L.; Moore, J. S.; Zang, L. ACS Macro Lett. D

DOI: 10.1021/acs.orglett.5b03502 Org. Lett. XXXX, XXX, XXX−XXX