Biological encapsulants such as ferritin, [1] lumazine synthase, [2] and viral capsids [3] achieve their selective separation and sequestration of substrates by providing: 1) a guest microenvironment isolated from the surroundings, 2) favorable interactions complementing a size and shape match with the encapsulated guests, and 3) sufficient flexibility to allow guests to be incorporated and released.[4] These biological hosts self-assemble from multiple copies of identical protein subunits, the symmetries and connection properties [5] of which dictate the hollow polyhedral structures of the encapsulant. In order to create abiological molecular systems that are capable of expressing functions of similar complexity to biological systems and to explore new applications of synthetic hosts, [6] there is a need to create synthetic capsules capable of tightly and selectively binding large substrates.Taking inspiration from natural systems [1][2][3] and from other previously reported metal-organic capsules, [7] we report the design and synthesis of a series of metallo-supramolecular cage molecules capable of selectively encapsulating large aromatic guests. The necessary features to achieve this function are: 1) small pore sizes to isolate guests from the environment, [8] 2) large cavity sizes to ensure sufficient volume for the guests of interest, 3) enough flexibility and lability to allow guests to enter and exit the host, and 4) regions of the cage walls rich in p-electron density to provide favorable interactions with targeted guests.[9] The selective encapsulation of large aromatic molecules is an attractive goal since their physicochemical properties are similar, which can render their separation difficult. The higher fullerenes represent particularly attractive targets because their potential applications [10] remain difficult to explore because of the challenges associated with their separation, despite recent advances. [11] Employing principles of geometric analysis, [5] we determined that combination of the C 4 -symmetric tetrakis-bidentate ligand shown in Figure 1 with the C 3 -symmetric iron(II) tris(pyridylimine) center would result in the formation of an O-symmetric cubic structure of general formula M 8 L 6 , in which the corners of the cube are defined by the metal centers and the faces by the ligands (Figure 1). This cage represents the first example of a new class of closed-face metallosupramolecular cubic hosts to be synthesized. In order to provide favorable binding sites for our target guests we incorporated porphyrin moieties, which have previously been demonstrated to interact with large aromatic molecules, [11a-c, 12] into our design. This design also provides for small pore sizes and the potential to create new chemical functionality through the introduction of different metal ions into the centers of the N 4 macrocycle and by substituting these metals axial ligands. We chose to employ labile iron(II) centers with pyridylimine ligands as chelating agents to allow for the formation of the liga...

Biochemical systems are adaptable, capable of reconstitution at all levels to achieve the functions associated with life. Synthetic chemical systems are more limited in their ability to reorganize to achieve new functions; they can reconfigure to bind an added substrate (template effect) or one binding event may modulate a receptor's affinity for a second substrate (allosteric effect). Here we describe a synthetic chemical system that is capable of structural reconstitution on receipt of one anionic signal (perchlorate) to create a tight binding pocket for another anion (chloride). The complex, barrel-like structure of the chloride receptor is templated by five perchlorate anions. This second-order templation phenomenon allows chemical networks to be envisaged that express more complex responses to chemical signals than is currently feasible.