Alkali-metal cation binding by self-assembled cryptand-type supermolecules

Reaction of three equivalents of alkyl-bridged biscatechol ligands (1-3)-H-4 with two equivalents of [(acac)(2)TiO] in the presence of two equivalents of alkalimetal carbonate (M = Na, K) affords helicate- or meso-helicate-type supramolecular coordination compounds by spontaneous self-assembly processes. The cryptand-type structure of the binuclear tetraanions [(1)(3)Ti-2](4-) or [(2)(3)Ti-2](4-) permits inclusion of sodium cations; this was observed by Na-23 NMR spectroscopy in solution. Competition experiments with potassium or lithium cations revealed that potassium bur not lithium easily displaces the sodium ions from the cavity. Titration experiments indicated that the binding constant for potassium is significantly higher than for sodium. On the other hand, for the large tetraanion [(3)(3)Ti-2](4-) with hexamethylene spacers, no binding of alkali-metal ions could be observed by NMR spectroscopy in solution. In the solid state, however, two potassium ions could be incorporated in the interior of the cryptand-type helicate.