Complexation of U(VI) with Dipicolinic Acid: Thermodynamics and Coordination Modes

Abstract

Complexation of UO₂²⁺ with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO₄. The stability constants (log β₁ and log β₂) for two successive complexes, UO₂L and UO₂L₂^2– where L^2– stands for the deprotonated dipicolinate anion, were determined to be 10.7 ± 0.1 and 16.3 ± 0.1 by spectrophotometry. The enthalpies of complexation (Δ<i>H</i>₁ and Δ<i>H</i>₂) were measured to be −(6.9 ± 0.2) and −(28.9 ± 0.5) kJ·mol^–1 by microcalorimetry. The entropies of complexation (Δ<i>S</i>₁ and Δ<i>S</i>₂) were calculated accordingly to be (181 ± 3) and (215 ± 4) J·K^–1·mol^–1. The strong complexation of UO₂²⁺ with DPA is driven by positive entropies as well as exothermic enthalpies. The crystal structure of Na₂UO₂L₂(H₂O)₈(s) shows that, in the 1:2 UO₂²⁺/DPA complex, the U atom sits at a center of inversion and the two DPA ligands symmetrically coordinate to UO₂²⁺ via its equatorial plane in a tridentate mode. The structural information suggests that, due to the conjugated planar structure of DPA with the donor atoms (the pyridine nitrogen and two carboxylate oxygen atoms) arranged at optimal positions to coordinate with UO₂²⁺, little energy is required for the preorganization of the ligand, resulting in strong UO₂²⁺/DPA complexation