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 C-3 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. (C) 2002 Wiley Periodicals, Inc.