Lithium-7 and sodium-23 NMR studies of the complexation of Li+ and Na+ ions with 1,13-dibenzo-24-crown-8 in binary nitromethane-acetonitrile mixtures

Li-7, Na-23 and C-13 NMR measurements were used to study the stoichiometry and stability of Li+ and Na+ complexes with dibenzo-24-crown-8 in binary ni tromethane-acetonitrile mixtures. The resulting chemical shift-mel ratio da ta clearly reveal the formation of both 1:1 and 2:1 (metal/ligand) complexe s in solution. Formation of the two adducts in nitromethane and acetonitril e solutions was further supported by monitoring the molar conductance of Li + and Na+ solutions as a function of macrocycle/metal ion mel ratio. Stepwi se formation constants of the 1:1 and 2:1 complexes were evaluated from com puter fitting of the NMR mel ratio data to equations which relate observed metal ion chemical shifts to formation constants. In all solvent systems, s odium forms more stable complexes with the crown ether than lithium. There is an inverse linear relationship between the logarithms of the stability c onstants and the mol fraction of acetonitrile in the solvent mixtures.