Thermodynamic Study of Complex Formation between Kryptofix-5 and Sn2+ in Several Individual and Binary Non-Aqueous Solvents Using a Conductometric Method

The complex formation between 1,13-bis(8-quinolyl)-1,4,7,10,13-pentaoxatridecane (Kryptofix-5) and Sn2+ ions was studied in pure acetonitrile (AN), dimethylformamide (DMF), 1,4-dioxane (DOX), and methanol (MeOH) and in acetonitrile-1,4-dioxane (AN-DOX), acetonitrile-dichloromethane (ANDCM), acetonitrile-methanol (AN-MeOH), and acetonitrile-dimethylformamide (AN-DMF) binary mixed solvent solutions at different temperatures using conductometric method. 1 : 1 [ML] complex is formed between the metal cation and ligand in most solvent systems but in the cases of AN-MeOH (MeOH = 90 mol %) binary mixture and in pure MeOH a 2 : 1 [M2L] complex was observed, that is the stoichiometry of complexes may be changed by the nature of the medium. The stability order of the (Kryptofix-5.Sn)(2+) complex in the studied binary mixed solvent solutions at 25 degrees C was found to be AN-DOX > AN-DCM > AN-MeOH > AN-DMF and in the case of pure solvents at 25 degrees C the sequence was the following: AN > DMF > DOX. A non-linear behavior was observed for changes of logK(f) of (Kryptofix-5.Sn)(2+) complex versus the composition of the binary mixed solvents, which was explained in terms of solvent-solvent intractions and also by the preferential solvation of the f species involved in the complexation reaction. The values of standard enthalpy changes (DHc degrees) for complexation reactions were obtained from the slope of the Van't Hoff plots and the changes in standard entropy (Delta S-c degrees) were calculated from the relationship DG(c,) (298.15)degrees = DHc degrees - 298.15 Delta S-c degrees The results show that in most cases, the (Kryptofix-5.Sn) (2+) complex is both enthalpy and entropy stabilized.