Complexation of 4'-nitrobenzo-15-crown-5 with Mg2+, Ca2+, Sr2+ and Ba2+ metal cations in acetonitrile-methanol binary solutions

The complex formation reactions between Mg2+, Ca2+, Sr2+ and Ba2+ metal cations with macrocyclic ligand, 4'-nitrobenzo-15C5, were studied in acetonitrile (AN)-methanol (MeOH) binary mixtures at different temperatures using conductometric method. The results show that 4'-nitrobenzo-15C5 forms 1:1 [ML] complexes with Mg2+, Ca2+ and Sr2+ metal cations in solutions. But in the case of Ba2+ cation a 1:2 [ML2] complex is formed in these solvent systems. The stability of the complexes is sensitive to the solvent composition and a non-linear behavior was observed for variation of logK (f) of the complexes versus the composition of the binary mixed solvents. The stability constants of complexes decrease suddenly with increasing the concentration of methanol in this binary system. The values of thermodynamic parameters (Delta H (c) (A degrees) and Delta S (c) (A degrees) ) for formation of (4'-nitrobenzo-15C5.Mg)(2+), (4'-nitrobenzo-15C5.Ca)(2+) and (4'-nitrobenzo-15C5.Sr)(2+) complexes were obtained from temperature dependence of the stability constants and the results show that these parameters are affected by the nature and composition of the mixed solvents. A non-linear behavior is observed between the Delta S (c) (A degrees) and the composition of the mixed solvents.