Effect of an Intramolecular Hydrogen Bond Belt and Complexation with

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Effect of an Intramolecular Hydrogen Bond Belt and Complexation with the Guest on the Rotation Behavior of Phenolic Units in Pillar[5]arenes Tomoki Ogoshi,* Keisuke Kitajima, Takamichi Aoki, Tada-aki Yamagishi, and Yoshiaki Nakamoto* Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan

ABSTRACT The rotation behavior of the phenolic units in pillar[5]arenes has been studied by means of the dynamic 1H NMR measurements. In acetone-d6, the individual conformers of pillar[5]arene were observed under -60 °C, indicating that the rotation movement was slow on the NMR time scale under -60 °C. In permethylated pillar[5]arene in acetone-d6, the peaks were not split even at -90 °C, strongly suggesting that the rotation movement was fast on the NMR time scale at -90 °C. This is due to the lack of the intramolecular hydrogen bond belt in permethylated pillar[5]arene. In the mixture of pillar[5]arene and viologen guest in acetone-d6, the split peaks were found at -30 °C and did not change under -60 °C. By formation of the host-guest complex between pillar[5]arene and viologen guest, the rotation movement was slow on the NMR time scale under -30 °C and almost stopped under -60 °C. SECTION Macromolecules, Soft Matter

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tudy of the conformational characteristics of the macrocyclic hosts is extremely important because their conformational flexibility is intimately related to their host-guest properties. Cyclodextrins (CDs) are cyclic oligosaccharide composed of the glucopyranose residues. Since the O(2)H group of the glucopyranose unit can form a hydrogen bond with the O(3)H group of the adjacent glucopyranose unit, CDs form stable rigid structures, and their ring flipping movement of the glucopyranose constituent unit is suppressed.1-7 On the other hand, permethylated CDs are flexible structures, and thus, the flipping of the glucopyranose unit easily takes place due to the lack of the intramolecular hydrogen bond.8-12 The intramolecular hydrogen bond belt has an effect on the conformational flexibility of CDs. For the evaluation of the conformational characteristics of the macrocyclic hosts, dynamic 1H NMR, calorimetry measurements, and molecular simulations have been carried out.1,13,14 We have first synthesized an interesting pillar-shaped novel host and named it “pillar[5]arene” (Figure 1a).15-17 The composition of pillar[5]arene is a cyclic pentamer composed of phenolic units, which is almost analogous to typical calixarenes.18-23 However, because its repeating units are connected by methylene bridges at the para position, pillar[5]arene forms a unique symmetrical pillar architecture, which is quite different from the basket-shaped structure of the meta-bridged calixarenes. Since pillar[5]arene is a novel host and has a different architecture compared to CDs and calixarenes, pillar[5]arene should show new molecular recognition and self-assembly properties. To reveal these properties, investigation of the conformational characteristics of

r 2010 American Chemical Society

Figure 1. (a) Pillar[5]arene, (b) DMpillar[5]arene, and (c) C8BpyC8.

pillar[5]arene is especially important. Pillar[5]arene has OH groups at both ends, which is the same as CDs. As with CDs, the OH groups of pillar[5]arene should form the intramolecular hydrogen bond belt. Therefore, in this Letter, by the variable-temperature 1H NMR measurements, we examined the intramolecular hydrogen bond belt of pillar[5]arene and the effect of the intramolecular hydrogen bond on the conformational characteristics of pillar[5]arene. The conformational flexibility of permethylated pillar[5]arene (DMpillar[5]arene, Received Date: December 21, 2009 Accepted Date: February 3, 2010 Published on Web Date: February 05, 2010

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DOI: 10.1021/jz900437r |J. Phys. Chem. Lett. 2010, 1, 817–821

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Figure 2. Variable-temperature 1H NMR spectra of pillar[5]arene (10 mM) in (a) acetone-d6 and (b) methanol-d4.

Figure 1b) was also investigated. Furthermore, change in the conformational characteristics of pillar[5]arene by complexation of the viologen guest molecule (C8BpyC8, Figure 1c) was reported. The dynamic 1H NMR technique was applied to examine the intramolecular hydrogen bond belt of pillar[5]arene (Figure 2). In acetone-d6 at 30 °C (Figure 2a), the singlet peaks at 7.93, 6.67, and 3.59 ppm were ascribable to the protons from the OH, phenyl, and methylene groups, respectively. At -50 °C, the OH proton peak shifted to a lower magnetic field (8.51 ppm). The chemical shift of the OH protons reflects the degree of the intramolecular hydrogen bond interactions. As the intramolecular hydrogen bond interaction was stronger, the chemical shift of the OH protons shifted to a lower magnetic field.24 Therefore, by cooling, the intramolecular hydrogen bond belt of pillar[5]arene in acetone-d6 became stabilized. Under -60 °C in acetone-d6 (Figure 2a), the proton signals from pillar[5]arene were split, indicating that the conformational interconversion of pillar[5]arene under -60 °C was slow on the NMR time scale. The conformational interconversion should derive from the rotation of the phenolic units (Figure 3). The phenolic units rotated


around the methylene bridges as the axis. Since the rotation movement was slow on the NMR time scale under -60 °C, these split peaks were observed. The complicated split peaks should derive from individual conformers. The lower row of Figure 3 shows the possible conformers by rotation of the phenolic units. To investigate the solvent effect on the rotation behavior, the variable-temperature 1H NMR measurements of pillar[5]arene in methanol-d4 were carried out (Figure 2b). The split peaks were not found at -60 °C in methanol-d4, while pillar[5]arene in acetone-d6 at the same temperature showed the split peaks (Figure 2a). In methanol-d4, the split of these peaks was found under -70 °C. At temperatures under -80 °C, the signals in acetone-d6 were clearly split, whereas the split signals in methanol-d4 were broadened. These differences in the variable-temperature 1H NMR spectra between the solvents indicated that the rotation movement in methanold4 was faster than that in acetone-d6. Since the intramolecular hydrogen bond belt should be strong in an aprotic solvent of acetone-d6, the rotation movement in acetone-d6 was slow on the NMR time scale under -60 °C (Figure 4b). In contrast, in a polar-protic solvent of methanol-d4, the intramolecular

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the split peaks were observed at -30 °C (Figure 5). The foundation indicated that the rotation movement was slow on the NMR time scale under -30 °C, which was higher than that in the absence of C8BpyC8 (Figure 2a). Under -60 °C, the split peaks were not changed, indicating that the rotation movement almost stopped under -60 °C (Figure 4c). Without

hydrogen bond belt should weaken, and/or the rotational behavior of phenoxide as generated by proton exchange with methanol may be different from that of the phenolic component. Therefore, the rotation movement in methanol-d4 freely occurred compared to that in acetone-d6. The variable-temperature 1H NMR measurements of DMpillar[5]arene in acetone-d6 were carried out (Supporting Information). At 30 °C, as with pillar[5]arene, proton signals from the aromatic, methoxy groups, and methylene bridge were observed as singlets. However, unlike pillar[5]arene, these peaks were not split at -90 °C. In case of pillar[5]arene in acetone-d6, the rotation movement was slow on the NMR time scale under -60 °C (Figure 2a). Due to the lack of the intramolecular hydrogen bond belt in DMpillar[5]arene, the rotation movement of the phenolic units was fast on the NMR time scale even at -90 °C (Figure 4a). Pillar[5]arene is composed of the electron donor of hydroquinone; thus, pillar[5]arene captures electron-acceptor guests such as viologen and pyridinium derivatives.15 Therefore, in the presence of dioctylviologen salt (C8BpyC8, Figure 1c), we investigated the rotation behavior of pillar[5]arene. In the mixture of C8BpyC8 (2.5 mM) and pillar[5]arene (2.5 mM),

Figure 5. Variable-temperature 1H NMR spectra of the mixture of pillar[5]arene (2.5 mM) and C8BpyC8 (2.5 mM) in acetone-d6. The peaks from C8BpyC8 showed open circles.

Figure 3. Conformations of pillar[5]arene.

Figure 4. Schematic representation of the rotation behavior of phenolic units in (a) DMpillar[5]arene, (b) pillar[5]arene, and (c) the hostguest complex between pillar[5]arene and C8BpyC8.


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C8BpyC8, the peaks of pillar[5]arene were gradually split by cooling to -90 °C (Figure 2a). Thus, in the absence of C8BpyC8, the rotation movement of pillar[5]arene at -60 °C was slow on the NMR time scale but did not stop. From these results, C8BpyC8 slowed/stopped the rotation movement of the phenolic units in pillar[5]arene. In conclusion, by using the dynamic 1H NMR measurements, we successfully clarified the rotation behavior of novel host compounds of pillar[5]arenes. The rotation behavior of pillar[5]arene largely depended on measurement temperature, kinds of solvents, and the addition of the guest. Formation of the intramolecular hydrogen bond network stabilized the conformation of pillar[5]arene and slowed the rotation movement. In acetone-d6, the rotation movement of pillar[5]arene was slow on the NMR time scale under -60 °C. Since permethylated DMpillar[5]arene did not form the intramolecular hydrogen bond belt, the rotation movement in DMpillar[5]arene was fast on the NMR time scale even at -90 °C. By formation of the host-guest complex between pillar[5]arene and C8BpyC8, the rotation movement of the phenolic units was slow on the NMR time scale under -30 °C and almost stopped under -60 °C. In other words, C8BpyC8 played a role as a “braking agent” for the rotation movement of pillar[5]arene. The information on the conformational characteristics of pillar[5]arene initially clarified in this study is important to reveal its host-guest and self-assembly properties and accelerate the research of pillar[5]arene. We are going to investigate the effect of the kinds of guests, concentration, and feed ratio of the viologen guest molecules on the rotation behavior in detail and try to strictly control the rotation movement (stop/go or slow/accelerate) by stimuli such as photo, oxidation/reduction, temperature, and chemicals.

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SUPPORTING INFORMATION AVAILABLE Experimental

section and the variable-temperature 1H NMR spectra of DMpillar[5]arene in acetone-d6. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Address: Department of Chemistry and Chemical Engineering, Graduate School of Natural Science and Technology, Kanazawa University, Kakumamachi, Kanazawa, 920-1192, Japan. Tel: þ81-76-234-4775. Fax: þ81-76-234-4800. E-mail: [email protected].

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ACKNOWLEDGMENT This work was supported by a Grant-in-Aid

(KIBAN C-20550120 and WAKATE B-21750140) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Effect of an Intramolecular Hydrogen Bond Belt and Complexation with

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