NUCLEAR-MAGNETIC-RESONANCE STUDIES OF SODIUM-ION COMPLEXES WITH SEVERAL CROWN-ETHERS IN BINARY ACETONITRILE DIMETHYLSULFOXIDE MIXTURES

Na-23 NMR measurements were used to determine the stoichiometry and st ability of Na+ complexes with 12-crown-4 (12C4), 15-crown-5 (15C5), be nzo-15-crown-5 (B15C5), 18-crown-6 (18C6), dicyclohexyl-18-crown-6 (DC 18C6) and dibenzo-18-crown-6 (DB18C6) in binary acetonitrile-dimethyls ulfoxide mixtures. With the exception of 12C4, in all other cases, the variation of the Na-23 chemical shift with the crown/Na+ mole ratio i ndicated the formation of 1:1 complexes. However, in the case of 12C4, evidence for the formation of both 1:1 and 2:1 (ligand to metal) comp lexes was observed. The formation constants of the resulting complexes were evaluated from the computer fitting of the mole ratio data to th e equations which relate the observed chemical shifts to the formation constants. It was found that, in pure acetonitrile, the stabilities o f the resulting 1 : 1 complexes vary in the order 15C5 > DC18C6 > B15C 5 > 18C6 > DB18C > 12C4, while in pure dimethylsulfoxide the stability order is DC18C6 > 18C6 > 15C5 > B-15C5 > DB18C6 > 12C4. The observed changes in the stability order could be related to the specific intera ctions between some crown ethers and acetonitrile. It was found that, in all complexes studied, an increase in the percentage of dimethylsul foxide in the acetonitrile-dimethylsulfoxide solvent mixtures would dr astically decrease the stability of the resulting complexes.