Anion-templated self-assembly of tetrahedral cage complexes of cobalt(II) with bridging ligands containing two bidentate pyrazolyl-pyridine binding sites

摘要

The bridging ligands L1 and L2 contain twoN,N-bidentate pyrazolyl-pyridine unitslinked to a central aromatic spacer unit (1,2-phenyl or 2,3-naphthyl,respectively). Reaction with Ni(II) salts and treatment with the anionstetrafluoroborate or perchlorate result in formation ofdinuclear complexes having a 2:3 metal:ligand ratio, with one bridgingand two terminal tetradentate ligands. In contrast, reaction ofL1 and L2 with Co(II) salts, followed bytreatment with tetrafluoroborate or perchlorate, results in assembly ofcage complexes having a 4:6 metal:ligand ratio; these complexes have ametal ion at each corner of an approximate tetrahedron, and abis-bidentate bridging ligand spanning each edge. The central cavity isoccupied by a tetrahedral counterion that forms multiplehydrogen-bonding interactions with the methylene protons of thebridging ligands. The anionic guest fits tightly into the centralcavity of the cage to which it is ideally complementary in terms ofshape, size, and charge. Solution NMR experiments show that the centralanion acts as a template for cage formation, with a mixture of Co(II)and the appropriate bridging ligand alone giving no assembly into acage until the tetrahedral anion is added, at which point cage assemblyis fast and quantitative. The difference between the structures of thecomplexes with Ni(II) and Co(II) illustrate how the uncoordinatedanions can exert a profound influence on the course of the assemblyprocess.