DOI: 10.1002/chem.201204010 Wide-Ranging Host Capability of a Pd II -Linked M 2 L 4 Molecular Capsule with an Anthracene Shell Norifumi Kishi, [a] Zhiou Li, [a] Yoshihisa Sei, [a] Munetaka Akita, [a] Kenji Yoza, [b] Jay S. Siegel, [c] and Michito Yoshizawa* [a] Introduction Coordination-driven assembly of organic ligands and metal ions plays a key role in the construction of nanoscale molec- ular architectures with well-defined cavities. [1, 2] Cage assem- blies and capsules, [3] comprising many components, often have large cavities and act as molecular hosts for highly spe- cific molecular recognition, unusual chemical reactions, sta- bilization of reactive species, and generation of unique phys- ical phenomena in the inner space. [1, 2, 4] In contrast to such large multicomponent assemblies, coordination hosts formed from a few components ( 6 components) tend to have rela- tively small cavities and manifest fewer complex phenom- ena; [5, 6] however, such few-component assemblies have an entropic advantage in favor of their formation, which makes overcoming the cavity-size issue a worthwhile challenge to- wards the development of functional molecular flasks. [4] Molecular cages, M 2 L 4 , with square-planar metals bridged by rigid bidentate “keystone” ligands show great promise for the development of assemblies with useful host capabili- ty; [7] they have a simple composition and highly symmetric framework. There have been many reports on the prepara- tion of cage-shaped M 2 L 4 structures providing wire-like frameworks with large openings; [6] however, molecular rec- ognition of organic guest molecules in these cages has been rarely achieved and limited only to molecules with binding sites, such as -CN or -SO 3  groups, to the metal centers. [8] In- corporation of large aromatic panels into molecular cages composed of wire-like frameworks greatly alters the host ca- pability of the assembly, opening the possibility to explore the host–guest chemistry in assemblies consisting of a small number of components. In contrast to previous systems, the design of molecular capsule 1 anticipates the formation of a large cavity shielded by large aromatic panels from a few components (Figure 1). Three features of the organic ligand 2 are seen as key: 1) a bent bridge with two anthracene panels to act as the “key- stone”, [9] 2) 3-pyridyl terminals to direct metal coordination, and 3) polar side chains to increase the solubility in protic solvents (Figure 1 c). Ligand 2 adopts an orthogonal confor- mation about the phenyl-anthryl and anthryl-pyridyl bonds, manifesting a rigid bent “keystone” suitable to support a closed-shell hollow M 2 L 4 molecular capsule 1, which quanti- tatively forms the complexation of 2 with square-planar Pd II ions. [10, 11] Abstract: This article reports that an M 2 L 4 molecular capsule is capable of encapsulating various neutral mole- cules in quantitative yields. The capsule was obtained as a single product by mixing a small number of components; two Pd II ions and four bent bispyridine ligands containing two anthracene panels. Detailed studies of the host ca- pability of the Pd II -linked capsule re- vealed that spherical (e.g., paracyclo- phane, adamantanes, and fullerene C 60 ), planar (e.g., pyrenes and triphen- ACHTUNGTRENNUNGylACHTUNGTRENNUNGene), and bowl-shaped molecules (e.g., corannulene) were encapsulated in the large spherical cavity, giving rise to 1:1 and 1:2 host–guest complexes, re- spectively. The volume of the encapsu- lated guest molecules ranged from 190 to 490  3 . Within the capsule, the planar guests adopt a stacked-dimer structure and the bowl-shaped guests formed an unprecedented concave-to- concave capsular structure, which are fully shielded by the anthracene shell. Competitive binding experiments of the capsule with a set of the planar guests established a preferential bind- ing series for pyrenes  phenanthrene > triphenACHTUNGTRENNUNGylACHTUNGTRENNUNGene. Furthermore, the capsule showed the selective formation of an unusual ternary complex in the case of triphenACHTUNGTRENNUNGylACHTUNGTRENNUNGene and corannulene. Keywords: anthracene · encapsula- tion · host-guest systems · molecular recognition · self-assembly [a] N. Kishi, Dr. Z. Li, Dr. Y. Sei, Prof.Dr. M. Akita, Dr. M. Yoshizawa Chemical Resources Laboratory Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku Yokohama 226-8503 (Japan) Fax: (+ 81) 45-924-5230 E-mail : yoshizawa.m.ac@m.titech.ac.jp [b] Dr. K. Yoza Bruker AXS, 3-9 Moriya-cho Kanagawa-ku Yokohama 221-0022 (Japan) [c] Prof. Dr. J. S. Siegel Organic Chemistry Institute University of Zürich Winterthurerstrasse 190 Zürich 8057 (Switzerland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201204010. Chem. Eur. J. 2013, 00,0–0  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! ÞÞ &1& FULL PAPER