Complexation of thorium with pyridine monocarboxylates: A thermodynamic study by experiment and theory

Complexation of thorium with pyridine monocarboxylates namely picolinic acid (pyridine-2-carboxylic acid), nicotinic acid (pyridine-3-carboxylic acid) and isonicotinic acid (pyridine-4-carboxylic acid) has been studied by potentiometry and calorimetry to determine the thermodynamic parameters (log K, ΔG, ΔH and ΔS) of complexation. All the studies were carried out at 1.0 M ionic strength adjusted by NaClO4 and at a temperature of 298 K. The detailed analysis of potentiometric data by Hyperquad confirmed the formation of four complexes, MLi (i = 1–4) in case of picolinate but only one complex (ML) in case of nicotinate and isonicotinate. The stepwise formation constant for ML complex (log KML) of thorium-picolinate is higher than those of thorium-nicotinate and thorium-isonicotinate complexes. Further the changes in enthalpy during formation of thorium-picolinate complexes are negative whereas the same for the complexes of thorium with the other two isomers was positive. This difference in the complexation process is attributed to chelate formation in case of thorium-picolinate complexes in which the thorium ion is bound to the picolinate through both the nitrogen in the pyridyl ring and one of the carboxylate oxygen atoms. The complexation process of thorium-nicotinate and thorium-isonicotinate are found to be endothermic in nature and are entropy driven confirming the similar binding nature as in simple carboxylate complexes of thorium. The complexation energies, bond lengths and charges on each atom in the complexes of various possible geometries were calculated theoretically using TURBOMOLE and the most stable configurations were optimized. The detail analysis of calculated complexation energy values in solvent phase and higher value of stability constant indicates that isonicotinate forms stronger complex with thorium than nicotinate. The results are corroborating the experimental observations.

► The thermodynamic parameters have been determined for the first time.
► The Th-picolinate complexation was exothermic in nature.
► The complexation of Th(IV) with the other two isomers was endothermic process.
► Isonicotinate forms stronger complexes than nicotinate with Th(IV).
► The theoretically calculated values are in line with the experimental results.