Complexation process between La3+ cation and 4′-nitrobenzo-15-crown-5 in some binary mixed non-aqueous solvent

The complexation reaction between 4′-nitrobenzo-15-crown-5 (4′-NB15C5) with La3+ cation was studied in ethanol-acetonitrile (EtOH-AN), ethanol-methylacetate (EtOH-MeOAc), ethanol-1,2 dichloroethane (EtOH-DCE), and ethanol-nitrobenzene (EtOH-NB) binary mixed solvent solutions at different temperatures using the conductometric method. The results show that in most cases, the stoichiometry of the complex formed between 4′-NB15C5 and La3+ cation is 1: 1 [ML]. The stability constant of (4′-nitrobenzo-15C5La)3+ complex changes by the nature and composition of the solvents systems. The stability order of (4′-nitrobenzo-15C5La)3+ complex in the binary mixed solvents with 10 mol % of ethanol at 25°C was found to be: EtOH-MeOAc > EtOH-NB > EtOH-AN ~ EtOH-DCE. A non-linear behavior was observed for variations of logKf of the complex in terms of the solvent composition. The standard values of the thermodynamic parameters (ΔHc° ΔSc°) for complexation reaction were obtained from temperature dependence of the stability constant of the complex and the results show that, in most solvent systems, the complexation reaction between La3+ and 4′-NB15C5 is entropy stabilized, but from enthalpy view point, except in a few cases, it is athermic.