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

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 UO2L22– where L2– 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 (ΔH1 and ΔH2) were measured to be −(6.9 ± 0.2) and −(28.9 ± 0.5) kJ·mol–1 by microcalorimetry. The entropies of complexation (ΔS1 and ΔS2) 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.