Solvent influence upon complex formation between dibenzo 24-crown-8 and Mg2+, Ca2+, Sr2+ and Ba2+ cations in acetonitrile-dimethylformamide binary mixtures using the conductometric method

The complexation reactions between Mg2+, Ca2+, Sr2+ and Ba2+ cations with macrocyclic ligand, dibenzo-24-crown-8 (DB24C8) were studied in acetonitrile-dimethylformamide (AN-DMF) binary mixtures at different temperatures using the conductometry method. In all cases, DB24C8 forms 1:1 complexes with these cations. The values of stability constants of complexes which were obtained from conductometric data, show that the stability of complexes is affected by the nature and composition of the solvent. In many cases, the stability constants of complexes in these solvent systems increase with increasing the concentration of acetonitrile which has a lower donor number than dimethylformamide. However, in the case of complex formation between Mg2+ and Sr2+ cations with DB24C8, an anomalous behaviour was observed for the stability constants (LogK(f)) with increasing the mole fraction of acetonitrile in AN-DMF binary mixtures at all temperatures. A non-linear behaviour was observed for variation of LogK(f) of the metal ion complexes vs the composition of the mixed solvents. This behaviour may be related to heteroselective solvation of cations and also solvent-solvent interactions in these binary systems. The obtained results show that the order of selectivity of DB24C8 ligand for the metal cations in different concentrations of acetonitrile in AN-DMF binary systems is: Ba2+ > Sr2+ > Ca2+ > Mg2+, but a reversal in selectivity is observed in a certain composition of the mixed solvents. The values of thermodynamic parameters (DeltaHdegrees(c), and DeltaSdegrees(c)) for formation of complexes were obtained from temperature dependence of stability constants of complexes using the van't Hoff plots. The results show that in most cases, the complexation reactions are enthalpy destabilized but entropy stabilized, and the values of these thermodynamic parameters are influenced by the nature and composition of the mixed solvents.