Complexation and thermodynamic studies of oxathiadibenzocrown ethers with Cu2+, Pb2+, Zn2+ and Cd2+ ions

A thermodynamic study of the complexation of Cu2+, Pb2+, Zn2+ and Cd2+ ions with 1 and 2 in acetonitrile has been carried out. The study was conducted in the temperature range 283-308 K using a conductometric technique. The observed molar conductivity, I >, was found to decrease significantly for mole ratios [L](t)/[M](t) less than unity in all cases. A model involving 1:1 stoichiometry has been used to analyze the conductivity data. The stability constant, K, for each 1:1 complex was determined from the conductivity data by using a nonlinear least-squares curve fitting procedure. The results show that compound 1 has no peak selectivity for any of the metal cations, while compound 2 selectively associates with Cu2+ and Pb2+. Complexes of 1 have the following stability order Pb2+ > Cu2+ > Zn2+ > Cd2+ and Pb2+ > Cu2+ for the complexes of 2. The a dagger HA degrees and a dagger SA degrees values for the complexation process were obtained from the slope and intercept of the Van't Hoff plots respectively. All a dagger GA degrees values were negative and were determined from the Gibbs-Helmholtz equation and the significance of these values is discussed.