Complexation of N′-[1-(3-aminophenyl)ethylidine]isonicotinohydrazide with La^(3+), Pr^(3+), Nd^(3+), Sm^(3+), Eu^(3+) and Gd^(3+) ions and associated thermodynamics

A novel Schiff base N＇-[1-（3-aminophenyl）ethylidine]isonicotinohydrazide was prepared and its complexation behavior towards some selected lanthanides had been studied employing pH-metric and calorimetric titration and spectral techniques, pH-metric studies were carried out for the trivalent La, Pr, Nd, Sm, Eu, and Gd complexes in 30% aqueous-dioxane medium at constant ionic strength of 0.05 mol/L NaClO4 and at different temperatures of 293, 303 and 313 K. The proton-ligand formation constants of the ligand indicated the presence of only one dissociable proton while the metal-ligand formation constants were compatible with the formation of 1：1 Ln（III） complexes. The stability of the complexes followed the order： La3＋〈pr3＋〈Nd3＋〈Sm3＋〈Eu3-〉Gd3＋, showing a break at gadolinium. The thermodynamic parameters, AG, AH and AS associated with protonation and complexation reactions were negative which suggested that all reactions were exothermic and enthalpy-driven. Isothermal calorimetric studies of Gd3＋-aeINH systems at 303 K also showed exothermic nature of the complexation reaction and formation of 1：1 complex in agreement with the pH-metric data. Formation of 1：1 complexes was confirmed by the characterization of Nd（III） complex. A seven coordinated geometry was assigned for the complex based on its elemental and spectral data.