A chiral spherical molecular assembly held together by 60 hydrogen bonds

Spontaneous self-assembly processes that lead to discretespherical molecular structures are common in nature. Sphericalviruses 1 (such ashepatitis B) and fullerenes 2 are well-known examples inwhich non-covalent and covalent forces,respectively, direct the assembly of smaller subunits intolarger superstructures. A common feature of theseshell-like architectures is their ability to encapsulateneutral and/or charged guests whose size, shape and chemicalexteriors complement those of the host's innersurface 3 , 4 . Their interiors can often beregarded as a new phase of matter 5 , capable of controlling the flowof reactants, transients and products, and of catalysingreactions of both chemical and biological relevance. Suchproperties have inspired the recent emergence ofmonomolecular 5 , 6 , 7 and supramolecular dimeric molecularcapsules 8 , 9 , many of which have been basedon the head-to-head alignment of bowl-shapedpolyaromatic macrocycles such as calix[4]arenes 5 , 7 , 9 . But true structural mimicry offrameworks akin to viruses and fullerenes, which are based onthe self-assembly of n > 3 subunits,and where surface curvature is supplied by edge sharing of regularpolygons, has remained elusive. Here we present anexample of such a system: a chiral spherical molecular assemblyheld together by 60 hydrogen bonds (1) ( Fig. 1 ). We demonstrate the ability of 1, which consists of six calix[4]resorcinarenes 2 and eight water molecules, to self-assemble and maintain its structure in apolar media and to encapsulate guest species within a well-defined cavity that possesses an internal volume of about 1,375 Å 3 . Single crystal X-ray analysis shows that its topology resembles that of a spherical virus 1 and conforms to the structure of a snub cube, one of the 13 Archimedean solids 10 . Figure 1 The structure of 1a : a , cross-sectional view (inset, structural formula for 2a ); b–d , space-filling views along the crystallographic four-fold rotation axis ( b ), three-fold rotation axis ( c ), and two-fold rotation axis ( d ); e , cut-away view along the three-fold rotation axis; f , solid-state packing, where the shaded grey spheres also represent 1a (solvent molecules have been omitted for clarity). The spheroid is held together by 60 hydrogen bonds (hydrogen bond donor/hydrogen bond acceptor/hydrogen bond type/number of hydrogen bonds): calixarene/calixarene/intramolecular/24; calixarene/calixarene/intermolecular/12; calixarene/water/intermolecular/12; water/calixarene/i