Complex Formation of Lanthanide Ions with Sulfonated Crown Ethers in Aqueous Solution

Abstract A new type of water-soluble crown ether (3′-sulfobenzo-12-crown-4 (SB12C4), 3′-sulfobenzo-15-crown-5 (SB15C5), 3′-sulfobenzo-18-crown-6 (SB18C6), di(3′-sulfo)dibenzo-18-crown-6 (DSDB18C6), di(3′-sulfo)dibenzo-21-crown-7 (DSDB21C7), and di(3′-sulfo)dibenzo-24-crown-8 (DSDB24C8)) has been prepared. The complex formation constants (β) of lanthanide ions with sulfonated crown ethers in aqueous solution were determined via the solvent-extraction method. The stability of the resulting complexes increases with the number of sulfonic acid groups, 18C6 < SB18C6 < DSDB18C6. For mono and disulfonated crown ether complexes, the stability varies as SB18C6 < SB15C5 < SB12C4 and DSDB18C6 < DSDB21C7 < DSDB24C8. The β values of all the complexes of lanthanide ions decrease with the atomic number, which is a characteristic of macrocyclic ligands, and is quite opposite to the conventional complexing tendency. The extractive separability of lanthanide ions was found to improve by adding sulfonated crown ethers into the aqueous phase as ion-size selective masking reagents. The number of water molecules in the first coordination sphere of the central Eu3+ in the complex was determined by a laser-induced luminescence study. In addition, the fluorescence spectra were measured in order to evaluate the structure of the complexes. The stabilization of the complexes formed is discussed while taking into account an outer-sphere electrostatic attraction between the sulfonic acid group and the metal ion.