Ligands for the alkali metals. Part 6. Some bis(crown) Schiff bases which form pocket complexes with alkali-metal cations of appropriate size

Abstract

Three homologous series of bis(crowns) have been produced by condensation of the appropriate formylbenzocrown with long-chain α,ω-diaminoalkanes. The ligands may be viewed as pocket or crevice sites for alkali-metal cations, lined with donor groups to provide ion–dipole interaction and terminated by a hydrophobic methylene chain. The interaction of these ligands with alkali-metal cations has been investigated by spectroscopic techniques and solvent extraction. Pocket complexes of these bis(crowns) with alkali-metal cations were apparently formed in ‘best-fit’ situations: thus bis(crown-5) compounds formed pocket complexes with cations larger than sodium, while bis(crown-6) ligands formed pocket complexes with cations larger than potassium. The bis(crown-4) ligands did not appear to complex strongly with any cation. The rates of trapping and release of alkali-metal cations from some of these ligands in methanol solution, investigated by temperature-jump techniques, indicate behaviour distinct from that of any of the crown and cryptand types described in the literature.