Published: August 08, 2011 r2011 American Chemical Society 13930 dx.doi.org/10.1021/ja206457b | J. Am. Chem. Soc. 2011, 133, 13930–13933 COMMUNICATION pubs.acs.org/JACS Five-Fold-Symmetric Macrocyclic Aromatic Pentamers: High-Affinity Cation Recognition, Ion-Pair-Induced Columnar Stacking, and Nanofibrillation Changliang Ren, † Victor Maurizot, ‡ Huaiqing Zhao, † Jie Shen, † Feng Zhou, § Wei Qiang Ong, † Zhiyun Du, || Kun Zhang, || Haibin Su, § and Huaqiang Zeng* ,† † Department of Chemistry and NUS MedChem Program of the Office of Life Sciences, 3 Science Drive 3, National University of Singapore, Singapore 117543 ‡ CNAB—UMR5084, Universit  e de Bordeaux, UMR CNRS 5248, Institute Europ een de Chimie et Biologie, 2 rue Robert Escarpit, 33607 Pessac Cedex, France § Division of Materials Science, 50 Nanyang Avenue, Nanyang Technological University, Singapore 639798 ) Faculty of Light Industry and Chemical Engineering, Guang Dong University of Technology, Guang Dong 510006, China b S Supporting Information ABSTRACT: Described in this study is a conceptually new class of five-fold-symmetric cavity-containing planar penta- meric macrocycles with their interior decorated by five convergently aligned, properly spaced carbonyl oxygen atoms. These cation-binding oxygens enclose a hydrophilic lumen of 2.85 Å in radius and thus display high-affinity binding toward alkali metal cations, and possibly many other cations, too. Arising from their high-affinity recognition of metal ions, these planar macrocycles form cation- or ion- pair-induced one-dimensional columnar aggregates, and subsequently fascinating fibrillation results. O ne-dimensional (1D) fibers or nanotubes assembled from organic molecules are promising materials 1 for applications as diverse as nanodevices, 1b,c,g,h sensors, 2aÀc and water trans- porters. 2d These organic columnar aggregates are predominantly stabilized by hydrogen-bonding or πÀπ stacking forces, or a combination of both. 1aÀh,2eÀ2m Although metal ions can offer unique properties and geometries for functional diversification, metalÀligand coordination bonds have received comparatively much less attention in the construction of fibrillar materials 3a derived from acyclic or macrocyclic organic molecules. 1iÀk On the other hand, macrocyclic molecules suitable for constructing metal-containing 1D materials are particularly limited and mostly rely on a G-quartet and its derivatives, 1i,3bÀ3d Schiff base macrocycles, 3eÀh and porphyrin and its analogues. 3hÀm More- over, these fibrillar ensembles are largely cation-mediated, where cations either are sandwiched between macrocycles 1i,3b,3dÀ3f or form additional coordination bonds directly and vertically with the neighboring macrocyclic ligands, 3hÀm while ion-pair-induced 1D columnar aggregation 3g is very scarce. We report here a new class of planar five-fold-symmetric macrocyclic pentamers whose appropriately sized interiors are decorated by five carbonyl oxygens that display high-affinity binding toward alkali metal ions through metalÀoxygen interatomic interactions. Cation- or scarcely reported ion-pair-induced fibrillation thus results from these high-affinity bindings and planar geometries. As illustrated in Figure 1a, pentameric molecules 1 and 2 are made of five alkylated 4(1H)-pyridone motifs meta-linked by secondary amide groups. Both carbonyl oxygens and amide protons point inward to form a continuous intramolecularly H-bonded network in a way such that the H-bonding rigidified backbone becomes increasingly curved and eventually cyclizable to arrive at a pentagon shape. 4 Ab initio calculation at the B3LYP/ 6-31G* level (Figure 1b) on pentamers 1 and 2 shows that such a pentagon shape encloses a hydrophilic oxygen-containing cavity of 2.85 Å, nearly identical to the average coordination bond distance between K + ions and covalently bound oxygen atoms. 5a These convergently aligned, properly spaced oxygens should therefore suggest high-affinity cooperative recognition of the alkali metal ions by pentameric molecules 1 and 2. Additionally, the planar geometry in 1 and 2 should promote the formation of cation- or ion- pair-mediated 1D stacked structures under suitable conditions. Pentamers 1 and 2 were both made by a stepwise construction strategy using HBTU-mediated amide coupling requiring 15À16 steps with an overall yield of 1À2%, starting from the commer- cially available diethyl 3-oxopentanedioate (Schemes S1 and S2 5b ). Their identity was unambiguously confirmed by excellent Figure 1. (a) Chemical structures of pentamers 1 and 2. (b) Top and side views of computationally optimized structures for 1 and 2, with the exterior side chains replaced by methyl groups at the B3LYP/6-31G* level, illustrating five-fold-symmetric planarity in 1 and 2. (c) Chemical and crystal structures of dimer 1q. Received: July 12, 2011