The "picrate effect" on extraction selectivities of aromatic group-containing crown ethers for alkali metal cations

As evaluated with 15 crown ethers which contain varying numbers of benzo su bstituents, the magnitude of the extraction selectivities, (and in one case , the selectivity order) of aromatic group-containing ionophores for alkali metal picrates may vary significantly from those for alkali metal salts wi th inorganic anions as a result of pi-pi interactions between picrate ion a nd an aromatic unit of the ionophore. The importance of the "picrate effect " increases as the number of benzo groups in the crown ether is enhanced an d varies with their location in the macrocycle. To verify the involvement o f picrate-crown ether pi-stacking in complexation, crown ether-alkali metal picrate complexes were examined in solution by H-1 NMR spectroscopy and so lid-state structures for nine complexes were determined by X-ray diffractio n. Dependence of the chemical shift for the picrate proton singlet in the N MR spectrum on the metal cation and/or macrocycle identity in the metal pic rate-crown ether complex was found to be a convenient tool for studying ani on-ligand pi-pi interactions in solution.