Thermodynamic, Structural, and Computational Investigation on the Complexation between UO₂²⁺ and Amine-Functionalized Diacetamide Ligands in Aqueous Solution

Abstract

The stability constants (log β), enthalpies of complexation (ΔH), and entropies of complexation (ΔS) for the complexes of uranium(VI) with a series of amine-functionalized diaetamide ligands, 2,2'-benzylazanediylbis(N,N'-dimethylacetamide) (BnABDMA), 2,2'-azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2'-methylazanediylbis(N,N'-dimethylacetamide) (MABDMA), in aqueous solution were determined by potentiometry and calorimetry. Electronspray ionization mass spectrometry was used to verify the presence of uranium(VI) complexes in solution. The thermodynamic data indicate that the binding strengths of the three ligands with UO₂²⁺ follow the order BnABDMA < ABDMA < MABDMA, parallel to the order of the protonation constants as well as the order of the stability of the Nd³⁺ complexes, suggesting that the complexation of UO₂²⁺ with the ligands consist predominantly of electrostatic interactions. Denisty functional theory calculations were conducted to reveal the structures, electronic charge distribution, and energetics of the uranium(VI) complexes, providing insight into the thermodynamic trends of the complexation. Extended X-ray absorption fine structure spectroscopy was used to identify the structures of the uranium(VI) complexes in aqueous solution.