Study of Complex Formation between Kryptofix 21 and UO22+ Cation in Some Binary Mixed Non-Aqueous Solutions

The complexation reaction between UO22+ cation and the macrocyclic ligand, Kryptofix 21, was studied in acetonitrile-methanol (AN-MeOH) and acetonitrile-ethylacetate (AN-EtOAc) binary solvent solutions at different temperatures using the conductometric method. In most cases, Kryptofix 21 forms a 1 : 1 [M : L] complex with the UO22+ cation. But in some of the studied solvent systems, 1 : 2 [M : L-2] and also 1 : 3 [M : L-3] complexes are formed in solutions. The results obtained in this study show that the mechanism of the complexation process between the uranyl cation and Kryptofix 21 changes with the nature and composition of the solvent system. In the case of the binary solvent solutions (mol % AN = 50 and 60), the order of stability constant of the complex at all studying temperatures was found to be: AN-EtOAc > AN-MeOH. The values of thermodynamic quantities (Delta S-c degrees, Delta H-c degrees) for the formation of (Kryptofix 21-UO2)(2+) complex were obtained from temperature dependence of the stability constant of the complex using the van't Hoff plots. The results show that the values of these parameters are influenced by the nature and composition of the mixed solvents and is most solvent systems, the 1 : 1 complexation reaction UO22+ between and the macro-cyclic ligand is athermic.