Solvent Influence upon Complex Formation between Dibenzo-18-crown-6 with the Y3+ Metal Cation in Pure and Binary Mixed Organic Solvents

Abstract

The complexation reaction between the Y3+ cation with the macrocyclic ligand dibenzo-18-crown-6 (DB18C6) was studied in acetonitrile–dimethylformamide (AN-DMF), acetonitrile–ethanol (AN-EtOH), acetonitrile–propanol (AN-PrOH), and dimethylformamide–methanol (DMF-MeOH) binary mixtures at different temperatures using a conductometric method. In all cases, DB18C6 forms a 1:1 complex with the Y3+ cation. The stability order of (DB18C6.Y)3+ complex changes with the composition of the mixed solvents. For example, the stability constant of the complex in the binary mixed solutions with 75 % AN at 25 °C varies in order: AN-EtOH > AN-PrOH ∼ AN-DMF, but in the case of 50 % AN was found to be: AN-DMF > AN-PrOH ∼ AN-EtOH. A nonlinear behavior was observed between the log Kf of the complex versus the composition of the mixed solvents, which may be related to solvent–solvent interactions which result in changing the solvation of the metal cation, ligand, and even the resulting complex in the binary mixed solvents. The values of the standard thermodynamic parameters (ΔS°c, ΔH°c) for formation of the (DB18C6.Y)3+ complex were obtained from the temperature dependence of the formation constant of the complex using van't Hoff plots. The obtained results show that the complexation reaction between DB18C6 with this metal cation in most cases is entropy-destabilized, but from the enthalpy viewpoint, depending on the nature of the medium, the complexation process is enthalpy-stabilized or -destabilized.