Proton-Induced Assembly–Disassembly Modulation of Spiroborate

Article pubs.acs.org/Macromolecules

Proton-Induced Assembly−Disassembly Modulation of Spiroborate Twin-Bowl Polymers Bearing Pyridyl Groups Hiroshi Danjo,*,† Masahiro Hamaguchi,† Kohei Asai,‡ Mizuki Nakatani,‡ Hiroko Kawanishi,‡ Masatoshi Kawahata,§ and Kentaro Yamaguchi§ †

Department of Chemistry and ‡Graduate School of Natural Science, Konan University, 8-9-1 Okamoto, Higashinada, Kobe 658-8501, Japan § Kagawa School of Pharmaceutical Sciences, Tokushima Bunri University, 1314-1 Shido, Sanuki, Kagawa 769-2193, Japan S Supporting Information *

ABSTRACT: Twin-bowl-shaped tris-(spiroborate) cyclophanes bearing pyridyl groups have been prepared for the construction of proton-responsive supramolecular polymers. Preparation of the pyridyl twin bowls was carried out by the reaction of 6,6′-(3-pyridyl)-2,2′,3,3′-tetrahydroxy-1,1′-binaphthyls and an equimolar amount of boric acid in N,Ndimethylformamide in self-organization manner, as previously reported. The reversible acid/base response of the pyridyl twin bowls was evaluated by the addition of hydrochloric acid and aqueous sodium hydroxide. The assembly disassembly modulation of the supramolecular polymers composed of pyridyl twin bowls and the tricationic iridium-(III) complex was also examined. Dissociation of the supramolecular polymers occurred by the addition of hydrochloric acid, and its reconstruction was realized by the addition of aqueous sodium hydroxide. The opposite behavior was observed when the dianionic palladium-(II) complex was employed as a guest. The addition of acid led to the formation of the aggregate that was dissociated by the addition of base. Scheme 1. Preparation of Pyridyl Twin-Bowl rac-2·TMA3

1. INTRODUCTION Supramolecular polymers are one of the most promising candidates for the development of functional soft materials because of their dynamics in the polymerization process, and a number of supramolecular polymer systems have been developed so far.1−5 Such secondary interactions as hydrogen bonding,6−16 hydrophobic interaction,17−25 π−π or CH−π interaction,26−29 electrostatic interaction,30−35 and donor− acceptor interaction36−40 have been employed for the polymerization, and the reversibility of these interactions has allowed researchers to design dynamic polymer materials possessing fascinating functions, such as self-healing41−45 or stimulusresponsive46−56 functions. Previously, we have reported the preparation of twin-bowlshaped tris-(spiroborate) cyclophanes as molecular connecting modules that glue cationic guests to each other to form supramolecular polymers.57−60 In this system, the host and the guest are bound mainly via electrostatic interaction, and this makes it possible to modulate the supramolecular polymerization by adjusting the countercharge with metal cations, such as potassium and barium cations. In this case, however, the added metal ion could not be removed, so the change of the polymer structure was irreversible. To realize the reversible regulation of our supramolecular polymer system, we next planned to apply proton as the simplest cationic chemical stimulus: protonation and deprotonation of the spiroborate twin bowl would be easily carried out by the addition of © XXXX American Chemical Society

appropriate acid and base.56,61,62 For this purpose, we designed spiroborate twin bowls 2·TMA3 bearing pyridyl groups as the proton-accepting functionality (Scheme 1). We present herein the preparation and structure elucidation of the pyridyl twin bowls and discuss their proton-responsive supramolecular polymerization behavior. Received: September 6, 2017

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DOI: 10.1021/acs.macromol.7b01924 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

2. RESULTS AND DISCUSSION 2.1. Preparation of Pyridyl Twin Bowls rac-2·TMA3. The preparation of the pyridyl twin bowl was carried out according to a previously reported procedure (Scheme 1).57 A N,N-dimethylformamide (DMF) solution of rac-6,6′-di-(3pyridyl)-2,2′,2;,3,3′-tetrahydroxy-1,1′-binaphthyl, generated by the deprotection of rac-1a with BBr3, and an equimolar amount of boric acid were heated at 150 °C, and the tris-(spiroborate) cyclophane was exclusively formed after 96 h. It was then treated with tetramethylammonium hydroxide to exchange the countercation to give rac-2a·TMA3 (TMA: tetramethylammonium). Pyridyl twin-bowl rac-2b·TMA3 bearing n-octyl groups was prepared in a similar manner. The precise structure of the pyridyl twin bowl was unambiguously determined by single-crystal X-ray diffraction analysis of rac-2a·TMA3. In the crystal, it was confirmed that six 3-pyridyl groups were located on the rim of the two bowl-shaped cavities (Figure 1).63 Both of these cavities were occupied

Figure 2. Partial 1H NMR spectra (500 MHz, 25 °C in DMSO-d6) of (a) rac-2a·TMA3 (2 mM), (b) (a) + 2 M HCl-(aq) (6 equiv to 2a3−), (c) (b) + 2 M NaOH-(aq) (6 equiv to 2a3−), and (d) (c) + 2 M NaOH-(aq) (3 equiv to 2a3−).

deprotonation of pyridinium moiety to regenerate 2a3− with releasing OH−. This acid/base cycle was repeated three times without any decomposition of components (Figure S1). 2.3. Evaluation of the Supramolecular Polymerization of Pyridyl Twin Bowls. The molecular recognition behavior of the pyridyl twin bowl was evaluated by the use of [Ir-(tpy)2](PF6)3 (3·(PF6)3, tpy = 2,2′:6″,2″-terpyridine) as the guest. In the presence of 2a3−, the proton signals of 33+ were significantly broadened and shifted upfield, indicating the association of 33+ with 2a3− (Figure 3b). After the addition of 6 equiv of HCl, all the signals of 2a3− exhibited downfield shifts. In addition, the signals of 33+ were sharpened and shifted downfield (Figure 3c). This change would indicate that the dissociation of the host−guest complex took place by the protonation of 2a3−, in which electrostatic repulsion occurred between [2a·H6]3+ and 33+. The signals of 33+ were again broadened and shifted upfield when 9 equiv of NaOH was added to the solution; in this case, deprotonation took place to regenerate 2a3−, which again associated with 33+ (Figure 3e). From these results, we could show schematically the protoninduced assembly disassembly modulation of the supramolecular polymer as shown in Figure 3f. The polymerization behavior of pyridyl twin-bowl 2a3− could be reversibly controlled by a Brønsted acid and a Brønsted base. The existence of H+ led to the disassembly of supramolecular polymer [2a·3], and its removal resulted in the reconstruction of the polymer structure. The same assembly disassembly modulation was also observed with rac-2b·TMA3 (Figure S2). This phenomenon was also evaluated by the dynamic light scattering (DLS) experiment (Figure 4a). In the mixed solvent of DMF and chloroform (1:3), the averaged hydrodynamic diameter (DH) of rac-2b·TMA3 was 1.8 nm, and the mixture of rac-2b·TMA3 and 3·(PF6)3 gave a peak at DH = 11.2 nm, which corresponds to ca. 36 nm of polymer length by treatment with the Stokes−Einstein equation for cylindrical model.67−69 When 6 equiv of HCl was added, DH of the mixture was diminished to 1.8 nm, but it was again increased to 7.1 nm, corresponding to ca. 18 nm of polymer length, after the addition of 9 equiv of NaOH. Although an incomplete recovery of DH of the supramolecular polymer was observed after the addition of NaOH, which might be caused by the intervention of water or sodium ions, these results also indicated the proton-induced assembly disassembly modulation of the supramolecular polymer.

Figure 1. Crystal structure of rac-2a·TMA3. Front (left) and top (right) views. One of the TMA ions and solvent molecules (DMF) are omitted for clarity.

by tetramethylammonium cations that showed van der Waals contact with each other with penetrating the central crownether-like cavity of 2a3−. This implied that pyridyl twin bowl 23− acted as a molecular connecting module that glued cationic guests, in a manner similar to the previously reported spiroborate twin bowls. 2.2. Evaluation of Acid/Base Response of Pyridyl Twin Bowls. To observe the protonation/deprotonation of 23−, a 1 H NMR experiment was performed in DMSO-d6 with the use of 2 M aqueous solutions of HCl and NaOH. All the aromatic proton signals of 2a3− were shifted downfield by the addition of HCl-(aq) (Figure 2). Particularly, remarkable downfield shifts were observed for the signals assigned to pyridyl protons, implying protonation of the pyridine nitrogens. These downfield shifts continued to increase until the addition of 6 equiv of HCl, where the six pyridyl groups would be fully protonated to form [2a·H6]3+. These spectral changes were reversibly canceled by the addition of NaOH-(aq). Incomplete upfield shifts were observed by the addition of 6 equiv of NaOH, and further addition of 3 equiv of NaOH gave almost the same 1 H NMR spectrum as that before the addition of HCl. This inconsistency in acid/base amount might be caused by the intervention of spiroborate moieties (Scheme 2).64−66 In 2a·H3, tetracoordinated spiroborate linkage was destabilized, and tricoordinated borate would be predominant due to the strong electron-withdrawing nature of the pyridinium moiety. The addition of OH− reacted with more acidic tricoordinated borate center rather than less acidic pyridinium proton to form [2a·H3·(OH)3]3−. Further addition of OH− would lead to the B

DOI: 10.1021/acs.macromol.7b01924 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 2. A Proposed Deprotonation Pathway of 2a·H3

The protonated pyridyl twin bowl was expected to act as a cationic host molecule and associate with anionic guests. To investigate the molecular recognition behavior of the cationic twin bowl, we employed the Pd-(II)−dithiolene complex ([Pd-(dmit)2]2− (dmit: 1,3-dithiole-2-thione-4,5-dithiolate), 42−) as the dianionic guest. The mixture of rac-2b·TMA3 and 4·TBA2 (TBA: tetra-n-butylammonium) in DMF/chloroform (1:3) gave an averaged DH of 5.0 nm, indicating the weak association, probably due to the presence of TBA cations (Figure 4b). After the addition of 5 equiv of HCl to this mixture, a remarkable increase of DH (1351 nm) was observed, indicating that cationic twin-bowl [2b·H5]2+ would interact with 42− to give some aggregation product. Similar 1H NMR spectra were observed for [2b·H5]2+ in the presence or absence of 42−, implying that [2b· H5]2+ associated with 42− to form a supramolecular polymer rather than the random aggregate (Figure S4). The increase of DH was canceled by the addition of 8 equiv of NaOH (5.4 nm) and again increased by the addition of 8 equiv of HCl (1505 nm). Figure 3. Partial 1H NMR spectra (500 MHz, 25 °C in DMSO-d6) of (a) 3·(PF6)3 (2 mM), (b) 3·(PF6)3 and rac-2a·TMA3 (2 mM, 1:1, after removal of TMAPF6), (c) (b) + 2 M HCl-(aq) (6 equiv to 2a3−), (d) (c) + 2 M NaOH-(aq) (6 equiv to 2a3−), and (e) (d) + 2 M NaOH-(aq) (3 equiv to 2a3−). (f) Schematic representation of the proton-induced assembly disassembly modulation of the supramolecular polymer.

3. CONCLUSION Tris-(spiroborate) cyclophanes bearing six 3-pyridyl groups at the rim of the bowl-shaped cavities were prepared, and their precise structures were determined by X-ray crystallographic analysis. The proton-induced assembly disassembly modulation of the supramolecular polymer composed of the pyridyl twin bowl and cationic iridium-(III) complex was confirmed by 1H NMR and DLS measurements. The supramolecular polymer was dissociated by the addition of HCl-(aq) and again associated by the addition of NaOH-(aq). The molecular recognition behavior of the protonated, hence cationic, pyridyl twin bowl was also evaluated by DLS measurement. Remarkable association was observed between the pyridyl twin bowl and dianionic Pd-(II) guest after the protonation by HCl, and the deprotonation by NaOH again gave the isolated monomeric components.

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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.macromol.7b01924. Detailed experimental procedures for synthesis and characterization of all new compounds and 1H NMR data of the acid−base experiments of rac-2a·TMA3 and 2a·TMA3 in the presence of guests (PDF) X-ray diffraction data of rac-2a·TMA3 (CIF)

Figure 4. Size characterization of supramolecular associates using dynamic light scattering (DLS, 1.0 mg/mL in DMF/chloroform (1:3), 20 °C). (a) Size distribution of (i) rac-2b·TMA3, (ii) rac-2b·TMA3 + 3·(PF6)3, (iii) (ii) + 2 M HCl-(aq) (6 equiv to 2b3−), and (iv) (iii) + 2 M NaOH-(aq) (9 equiv to 2b3−). (b) Size distribution of (i) rac-2b·TMA3, (ii) rac-2b·TMA3 + 4·TBA2, (iii) (ii) + 2 M HCl-(aq) (5 equiv to 2b3−), (iv) (iii) + 2 M NaOH-(aq) (8 equiv to 2b3−), and (v) (iv) + 2 M HCl-(aq) (8 equiv to 2b3−).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.D.). C

DOI: 10.1021/acs.macromol.7b01924 Macromolecules XXXX, XXX, XXX−XXX

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Hiroshi Danjo: 0000-0001-9850-8301 Masatoshi Kawahata: 0000-0003-2865-4113 Funding

This work was supported by JSPS KAKENHI Grant No. 26410103. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Drs. N. Mizuno and S. Baba (Japan Synchrotron Radiation Research Institute (JASRI)) for invaluable help in data collection in the X-ray analysis of rac-2a·TMA3. The synchrotron radiation experiment was performed at the BL38B1 stations of SPring-8 with the approval of JASRI (Proposal No. 2016A1104). We also thank Prof. Dr. S. Iwatsuki (Konan University) for inspiring discussion in the acid−base reaction of the spiroborate structure.

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DOI: 10.1021/acs.macromol.7b01924 Macromolecules XXXX, XXX, XXX−XXX