triazine with Anion-Ï Interactions

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Conformational Control of Oxacalix[3]arene[3]triazine with Anion-# Interactions Xu-Dong Wang, Qi-Qiang Wang, Yu-Fei Ao, and De-Xian Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00332 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Crystal Growth & Design

Conformational Control of Oxacalix[3]arene[3]triazine with Anion-π Interactions Xu-Dong Wang,† Qi-Qiang Wang,†, ‡ Yu-Fei Ao,†,* and De-Xian Wang†,‡, * †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China [email protected]; [email protected]

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

Recent years have witnessed the remarkable progress in the study of anion-π interactions, a new noncovalent interaction that describe the energetic favorable interaction between anions and electrondeficient aromatics.1-8 Since the pioneering theoretical predications,9-11 a growing number of experimental evidences

12-18

and theoretical calculations19-22 substantiate the importance of anion-π

interactions as driving force in supramolecular chemistry.23-26 In stark contrast to cation-π interaction that has been exemplified ubiquitous in living system, the biological function and application of anion-π interactions remains largely unexplored.27-35 For example, the Howell and Stojanorić groups independently analyzed the influence of anion-π interaction on stabilizing protein structures through the analysis of anion-π pairs, and they particularly highlighted the contribution of anion-Phe interactions.29,30 Chakravarty and coworkers examined high-resolution structures of proteins and nucleic 1 ACS Paragon Plus Environment

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acids and found that η6-type anion-π pairs plays important role in macromolecular folding and function as they often occurred in protein/nucleir acid loops and involved in conserved/coevolving sites in proteins.31 Very recently, Price and coworkers made a systematically deep study to reveal the function of anion-π interactions to stabilize the β-sheet of protein WW.33 Structural and statistical analyses of Protein Data Bank (PDB) suggested the important roles that anion-π interactions play in structural stability of proteins, however, the often observed interactions between anions and electro-rich protein side chains such as Phe, Trp, or Tyr 29-31, leave this subject a debate. To meet the confused situation of anion-π interactions in biological system, a more comprehensive picture of such weak non-covalent interactions are highly demanding. Macrocyclic molecules are of fundamental importance to supramolecular chemistry. The sophisticated design and construction of ingenious macrocyclic molecules has ever been one of the driving forces to promote the major advances of supramolecular science. Importantly, macrocyclic molecules provide ideal models to probe the nature of non-covalent interactions.36 Taking advantage of the electrondeficient feature and shape-persistent 1,3-alternate conformation of oxacalix[2]arene[2]triazines, for instance, we have demonstrated the experimental evidence, generality and structure of anion-π interactions between anions and charge neutral π receptors.12,13 One of the salient structural features of heteraaromatics is the versatile conformation contributed by the nature and selective conjugation of bridging heteroatoms. Whereas aza- or oxacalix[4](hetero)arenes dominantly adopt shape-persistent 1,3alternate conformation, larger heteraaromatics generally exhibit flexible conformations.36,37 The unique conformational structure of heteraaromatics, and our continuous interest in probing the nature of anion-π interactions encouraged us to carry out this study. Reported herein is the design, structure and anion binding properties of oxacalix[3]arene[3]triazine with halides. Conformational control of anion-π interaction on the new electron-deficient macrocyclic host molecule is demonstrated. Based on our experiences on the study of anion-π interactions, we designed the molecular model for this study as oxacalix[3]arene[3]triazine 1 (Figure 1). We envisioned the three electron-deficient ACS Paragon Plus Environment

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trazines would serve as the binding sites for anions, the composition of six aromatic building units render the macrocycle flexibility which give space to regulate its conformation after anion complexation. The target molecule was isolated from a one-pot reaction between resorcinol and cynuric chloride.38 Crystal structure of 1 is illustrated as Figure 2. As anticipated, all the bridging oxygen atoms tend to conjugate with electron-deficient triazine rather than benzene as judged from the bond lengths of 1.34 (dO-Ctriaizne) vs 1.41 Å (dO-Cbenzene), respectively. The macrocycle adopts a tighten-waist 1,3,5-alternate conformation (Figure 2A, top view), with the whole molecular backbone therefore giving a chair-like shape as observed from the side view (Figure 2B). Whereas selective conjugations of bridging oxygens, and flexibility arising from the six contained aromatic units probably lead to the resulting conformation, the intermolecular interaction might also contribute. For example, the triazine and benzene ring locating on the “chair head” and “chair end” interact with another molecule through π-π stacking and hydrogen bonding, respectively (Figure S1, B). Moreover, one host molecule intrudes into the cavity of another molecule, the intruded triazine forming hydrogen bonding with the aryl hydrogens that belong to two adjacent benzene rings, respectively (Figure S1, C). Such intermolecular interactions zoom in the lowerrim of the two benzenes and consequently lead to a tighten-waist cavity. As only one set of distinct proton and carbon signals was recorded in the NMR spectra, which indicate a fast conformation interconversion exists in solution within the NMR time scale (Figure S4 and S5). The NMR observations of 1 also meet our anticipation that flexible structure should be produced with this macrocycle.

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Figure 1. Molecular structure of oxacalix[3]arene[3]triazine 1.

Figure 2. Crystal structure of 1, top view (A) and side views (B and C). Hydrogen atoms and solvent were omitted for clarity. The probability is 25 percent. Having the molecular model oxacalix[3]arene[3]triazine in hand, we examined its binding towards halides in solid state, with the purpose to probe the effect of anion complexation on the conformation. Through slow diffusion of diethyl ether to a solution of mixture of 1 and chloride and bromide (as tetraethylammonium salts), respectively, single crystals of the complexes of [1•3Cl-] and [1•2Br-] were obtained. The structural details are illustrated in Figure 3 and 4, and Figure S2 and S3. Interestingly, the interaction of 1 and chloride gave a 1:3 complex. As shown in Figure 3, each chloride seats over the plane of one triazine ring. The distances from chlorides to the planes are 3.143 ACS Paragon Plus Environment

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(dCl4-plane), 3.228 (dCl5-plane) and 3.159 Å (dCl6-plane), respectively (Figure 3A). Such short distances (shorter than the sum of Van der Waals radius) confirm therefore the formation of anion-π interactions between chlorides and triazines. Concurrently, the chlorides show short contact with two aryl hydrogen atoms that belong to two adjacent benzenes, respectively. The distances from chloride (Cl4-Cl6) to the aryl hydrogens are in the range of 2.627-2.931 Å, indicating the existence of weak hydrogen bonding. The aforementioned cooperative multiple weak interactions consequently lead to a dramatic change of the conformation of 1. For example, in stark contrast with the initial chair-like conformation, one of the benzene ring of 1 in the complex reversely folds and intrudes within the cavity formed with other five aromatics, giving a pyramidal structure. Different to the significant intermolecular interactions in the bare host, only weak halogen bonding between the chlorine atoms on the triazine rings links two host molecules in the complex (Figure S2, B). These observations further support that cooperative anion-π interaction and hydrogen bonding most probably contribute as the dominant driving forces to change the conformation of 1.

Figure 3. Crystal structure of the [1⋅3Cl-] complex, top view (A) and side view (B). Cations and solvents were omitted for clarity. ACS Paragon Plus Environment

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Different but interesting conformational change in the [1•2Br-] complex is worth addressing. As shown in Figure 4, the host 1 binds two bromides with two of its triazines through anion-π interactions (dBr1-plane = 3.274 Å, dBr2-plane = 3.447 Å). Similar with that observed in [1•3Cl-] complex, bromides form cooperative weak hydrogen bonding with the adjacent aryl hyrogens (dBr-H = 3.003~3.214 Å). As a result of the non-covalent interactions, the macrocyclic backbone changes its shape from a chair-like to a calathiform one. The distances between the upper-rim carbon atoms of triazine rings are around 11.1 Å, indicating the resulting C3-symmetric 1,3,5-alternate conformation after the bromide complexation. In the packing mode, two complexes face to face arrayed solvent-mediated (methanol) non-covalent interactions (Figure S3, B). Bearing three triazines as the anion-binding sites, 1 in principle can maximally accommodate three halides. As the counter cation (tetraethylammonium) is unique for two complexes, the different stoichiometries obtained for chloride and bromide can be attributed to the nature of anions. Generally smaller but showing stronger binding ability to the electron-deficient aromatics,14 chloride is more possible to access the 1:3 binding than bromide. Owing to the different binding modes in [1⋅3Cl-] and [1•2Br-] complexes, the different conformation controls by anion-π interactions are consequently reasonable.


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Figure 4. Crystal structures of the [1⋅2Br-] complex, top view (A) and side view (B). Cations and solvents were omitted for clarity.

In

summary,

bearing

three

electron-deficient

triazine

rings

in

the molecular skeleton,

oxacalix[3]arene[3]triazine 1 showed powerful ability to bind anions. The intrinsic flexible conformation of 1 endowed it a good macrocyclic model to study the conformational changes after anion complexation. The outcome of X-ray crystallographic study demonstrated the unprecedented examples that cooperative anion-π interaction and hydrogen bonding could control and change dramatically the conformation of macrocycle. This study therefore provides important information for studying the function of anion-π interactions in living system.

ASSOCIATED CONTENT Supporting information Experimental details, cif documents for the crystal structures, and copies of 1H and

13

C NMR spectra.

This material is available free of charge via Internet at http://pubs.acs.org. Accession Codes CCDC 1827237-1827239 contain the supplementary crystallographic data for this paper. These data can be

obtained

free

of

charge

via

www.ccdc.cam.ac.uk/data_request/cif,

or

by

emailing

[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION Corresponding Author Prof. De-Xian Wang, Tel: +86 10 62565610. Fax: +86 10 62564723. E-mail: [email protected] ACS Paragon Plus Environment

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Dr. Yu-Fei Ao, Tel: +86 10 62564723. E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

We thank NSFC (91427301, 21502202, 21521002), MOST (2014CB643601), and Chinese Academy of Sciences (QYZDJ-SSW-SLH023) for financial support. References (1) Hay, B. P.; Bryantsev, V. S. Anion-Arene Adducts: C-H Hydrogen bonding, anion-π interaction, and carbon bonding motifs. Chem. Commun. 2008, 2417-2428. (2) Berryman, O. B.; Johnson, D. W. Experimental evidence for interactions between anions and electron-deficient aromatic rings. Chem. Commun. 2009, 3143-3153. (3) Wang, D.-X.; Wang, M.-X. Anion recognition by charge neutral electron-deficient arene receptors. Chimia 2011, 65, 939-943. (4) Frontera, A.; Gamez, P.; Mascal, M.; Mooibroek, T. J.; Reedijk, J. Putting anion-π interactions into perspective. Angew. Chem. Int. Ed. 2011, 50, 9564-9583. (5) Chifotides, H. T.; Dunbar, K. R. Anion-π interactions in supramolecular architectures. Acc. Chem. Res. 2013, 46, 894-906.


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Table of contents

Conformational Control of Oxacalix[3]arene[3]triazine with Anion-π π Interactions

Xu-Dong Wang,† Qi-Qiang Wang,†, ‡ Yu-Fei Ao,†,* and De-Xian Wang†,‡, *


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Unprecedented conformational control of macrocycle with cooperative anion-π interaction and hydrogen bonding.


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triazine with Anion-Ï Interactions

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