Journal of the American Chemical Society, Vol.117, No.46, 11507-11511, 1995
A New Highly Selective Macrocycle for K+ and Ba2+ - Effect of Formation of a Pseudo 2nd Macroring Through Complexation
A new macrocyclic compound capable of forming a pseudo second macroring upon complexation has been characterized. The compound, 5-chloro-8-hydroxyquinoline (CHQ)-substituted diaza-18-crown-6 (1), exhibits unique complexing properties. log K values for the formation of K+ and Ba2+ complexes with 1 in methanol are larger than those for tie K+ and Ba2+ complexes with all other lariat ethers. The log K value for the 1-Ba2+ complex is the same magnitude as that of the cryptand [2.2.2]-Ba2+ complex. Selectivity factors for Ba2+ over other alkalineearth cations and for K+ over Na+ in methanol are > 10(7) and similar to 10(3), respectively, which are the highest factors ever reported for lariat ethers. Moreover, the selectivity of 1 for Ba2+ over other alkaline-earth cations is larger than that of any other cryptand studied to date. UV-visible, H-1 NMR spectral, and X-ray crystallographic studies indicate that a pseudo second macroring is formed through pi-pi interaction between the two CHQ rings upon complexation with K+ and Ba2+. This effect results in a cryptate-like structure and therefore highly stable complexes with high selectivity for these cations.