Thermodynamic study of Eu(III) complexation by pyridine monocarboxylates

The thermodynamic parameters (log K, ΔG, ΔH and ΔS) of complexation of Eu(III), a chemical analogue of trivalent actinides, with pyridine monocarboxylates, namely, picolinic acid (pyridine-2-carboxylic acid), nicotinic acid (pyridine-3-carboxylic acid), isonicotinic acid (pyridine-4-carboxylic acid) have been studied at 1.0 M ionic strength adjusted by NaClO₄ and 298 K by potentiometry, fluorescence spectroscopy and calorimetry. The potentiometric results revealed formation of four complexes, ML_i (i = 1–4) in case of picolinate whereas only ML complexes in case of nicotinate and isonicotinate. The log K_ML for Eu(III) picolinate complex is higher than that for complexes of Eu(III) with the other two acids. The complexation reaction between Eu(III) and picolinate was found to be exothermic due to chelate formation via pyridyl nitrogen. In case of complexation of Eu(III) with nicotinate and isonicotinate, the enthalpy changes are similar as in the case of simple mono carboxylates and are positive. Life time measurements by time resolved fluorescence spectroscopy, for the decay of ⁵D₀ state of Eu(III) also indicated the formation of ML₄ with picolinate and formation of ML only with the other two acids. The experimental observations on the stability and binding mode of the complexes are corroborated by theoretical calculations using the TURBOMOLE software. The detail analysis of calculated charge values of the free ligands and the complexes indicates that charge polarization is more in the isonicotinate than in nicotinate upon complexation.