Self-assembly of anion-binding supramolecular cage complexes

Three tetradentate ligands, in which two bidentate pyrazolyl–pyridine binding sites are connected by an aromatic spacer unit, have been used to prepare adamantoid tetrahedral cages of the form [Co4L6(X)][X]7 (where X is a uninegative, noncoordinating counterion such as perchlorate, tetrafluoroborate, or hexafluorophosphate). In these complexes an approximately tetrahedral array of metal ions occurs, with a bridging ligand spanning each of the six edges of this tetrahedron; each metal ion is accordingly six coordinate and the cages can have either T or C3 symmetry, depending on the ligand. The central cavity of each cage is occupied by an anion. In the cases where the anion is a good fit for the central cavity, it is tightly bound (no exchange in solution with external anions) and acts as a template for assembly of the cage, with a mixture of Co(II) and the bridging ligand in the correct proportions not assembling into the Co4L6 cage until the templating anion is added. With a longer bridging ligand, the central cavity is too large to encapsulate the anion completely, and accordingly the encapsulated anion can exchange freely with external anions; this behavior can be “frozen out” in the NMR spectra at low temperatures. The host–guest chemistry of the cage complexes is therefore strongly dependent on the size of the central cavity. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:567–573, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10101