Multiple Lanthanide Helicate Clusters and the Effects of Anions on Their Configuration

摘要

Helices are the central structural motif in biological molecules. Except for some familiar helices (single, double, and triple), the plasmid DNA has a cyclic structure, and the biologically significant multi-stranded nucleic acid configuration is quadruplex. Helices and circular helices have been fundamental topological targets for chemical synthesis for the past two decades, and much effort has been made in the design and synthesis of artificial helical molecules. Helical-metal coordination compounds (helicates), have been extensively investigated; however, the formation of the cyclic helicate, a higher nuclearity variant of the helicate, is conversely less well understood. Controlling the structure of assemblies of helicates is one of the leading challenges for supramolecular chemists. The ingenuity of Lehn and coworkers occupies a prominent position, and they have reported a series of helicates/circular helicates, leading to further improvements in this field. Although this challenging task has been quite successfully accomplished with transition metals, the polynuclear helicates with lanthanides have not been developed to such an extent because of the fact that trivalent lanthanide ions display high kinetic lability and a lack of stereochemical preference. Despite some evolution, the ability to predict and control supramolecular assembly is seriously restricted by the bewildering structure-directing factors; in particular, the anion-controlled self-assembly of lanthanide helicates is rare.