Study of complex formation between C18H36N2O6 and UO22+ cation in some binary mixed non-aqueous solutions

The complexation reaction between UO22+ cation and the macrobicyclic ligand C18H36N2O6 was studied in acetonitrile-dimethylformamide (AN-DMF), acetonitrile-tetrahydrofuran (AN-THF), acetonitrile-dichloromethane (AN-DCM) binary solvent solutions at different temperatures using the coductometric method. In most cases, C18H36N2O6 forms a 1:1 [M:L] complex with the UO22+ cation. But in some of the studied solvent systems, in addition to formation of a 1:1 complex, a 1:2 [M:L2] complex is formed in solution. A non-linear behavior was observed for changes of log Kf of the (C18H36N2O6·UO2)2+ complex versus the composition of the binary mixed solvents. The sequence of the stability of the (C18H36N2O6·UO2)2+ complex in pure solvent systems at 25 °C decreases in the order: AN > THF > DMF. In the case of binary solvent solutions, the stability constant of the complex at 25 °C was found to be: AN-DCM > AN-THF > AN-DMF. The values of thermodynamic quantities (ΔSc°,ΔHc°), for the formation of the complex were obtained from temperature dependence of the stability constant of the complex using the van't Hoff plots. The results show that in all cases, the complex is both entropy and enthalpy stabilized and both of these parameters are affected by the nature and composition of the mixed solvent systems.