Nanoscale Assemblies DOI: 10.1002/ange.200903910 A Spherical 24 Butyrate Aggregate with a Hydrophobic Cavity in a Capsule with Flexible Pores: Confinement Effects and Uptake–Release Equilibria at Elevated Temperatures** Christian Schäffer, Hartmut Bögge, Alice Merca, IraA. Weinstock, Dieter Rehder, Erhard T. K. Haupt,* and Achim Müller* Dedicated to Professor Martin Jansen on the occasion of his 65th birthday Compounds like zeolites exhibiting nanoscale holes and channels can serve as filters and traps or hosts for molecular guests. They play an important role in many areas of chemistry and materials science as they can be used for different tasks, for example, for separation and storage purposes. [1, 2] Our related interest concerns molecular (i.e., discrete) porous metal oxide based capsule analogues, which specifically allow encapsulation of a large number of different guests; for general remarks regarding inorganic host–guest chemistry, see Ref. [3]. It was our aim to integrate within a capsules cavity an unprecedented structurally well-defined, mostly hydrophobic aggregate. This goal was achieved with a robust, spherical, porous capsule of the type [{pentagon} 12 - {linker} 30 ]  [{(Mo)Mo 5 O 21 (H 2 O) 6 } 12 {Mo 2 O 4 (ligand)} 30 ] n[4–7] allowing, generally speaking, a wide range of applica- tions. [4e,f, 8] The capsules interiors can be modified by intro- ducing a variety of species coordinating weakly to the 30 dinuclear {Mo 2 } linkers. This approach has now allowed, as intended, the encapsulation of the quasi-spherical, partly compact 24 butyrate aggregate, whose constituents show under the confined conditions interesting interactions (an attractive, up-to-date research field) detectable by ROESY NMR spectroscopy in agreement with the related distances. Remarkably, the present scenario automatically generates an unprecedented hydrophobic cavity and shell at the center of the capsule that is spanned by 72 H atoms originating from 24 butyrate CH 3 groups. Furthermore, the resulting capsule skeleton is stable even at rather high temperatures, which allows the observation of a strong uptake–release exchange of the butyrates with the future option to introduce into the capsule system different species that can react with one another under confined conditions. The pictorial title “flex- ible” refers to one of the important properties of the capsules, that is, the option of reversible pore widening, which will be discussed in comparison with formally related scenarios of spherical viruses. Compound 1 containing the capsule 1a loaded with the 24 butyrate aggregate (Figure 1) is obtained by the reaction of an aqueous solution of heptamolybdate with hydrazinium sulfate (as reducing agent) and butyric acid and a subsequent recrystallization process. (The products recrystallization was primarily performed to obtain higher-quality crystals.) This synthetic procedure is similar to that which led to the first published related capsule-type compound with 30 acetate ligands obtained in a facile high-yield synthesis, [7] while the Figure 1. Structure of the capsule 1a showing details of the butyrate ligands coordinated to the {Mo 2 }-type linkers (see Figure 2 highlighting the packing of the encapsulates). For clarity, one of the 12 pentagonal {(Mo)Mo 5 O 21 (H 2 O) 6 } 6 units and five of the {Mo 2 O 4 (butyrate)} + link- ers (polyhedral representation in blue) are omitted; O red, C black, H gray. The H atoms were generated here and in the subsequent figures. [*] Prof. Dr. D. Rehder, Dr. E. T. K. Haupt Department Chemie, Institut für Anorganische und Angewandte Chemie, Universität Hamburg 20146 Hamburg (Germany) E-mail: erhard.haupt@uni-hamburg.de C. Schäffer, Dr. H. Bögge, Dr. A. Merca, Prof.Dr. A. Müller Fakultät für Chemie, Universität Bielefeld Postfach 100131, 33501 Bielefeld (Germany) Fax: (+ 49) 521-106-6003 E-mail: a.mueller@uni-bielefeld.de Homepage: http://www.uni-bielefeld.de/chemie/ac1/ Prof. Dr. I. A. Weinstock Department of Chemistry, Ben Gurion University of the Negev Beer Sheva, 84105 (Israel) [**] We thank Dr. RenØ Thouvenot and Prof. Pierre Gouzerh (Paris) for discussions. A.M. thanks the Deutsche Forschungsgemeinschaft (E.T.K.H. in context with HA 2822/1-1) and the Fonds der Chemischen Industrie for continuous financial support. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200903910. Angewandte Chemie 8195 Angew. Chem. 2009, 121, 8195 –8200  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim