Thermodynamics, Optical Properties, and Coordination Modes of Np(V) with Dipicolinic Acid

Complexation of NpO₂⁺ with dipicolinic acid (DPA) has been investigated in 1 M NaClO₄ at 25 °C. Two complexes, NpO₂(DPA)⁻ and NpO₂(DPA)₂³⁻, were identified, and the stability constants (log β₁ and log β₂) were determined to be 8.68 and 12.31, respectively, by spectrophotometry. The enthalpies of the complexation (Δ<i>H</i>₁ and Δ<i>H</i>₂) were measured to be −25.2 and −45.9 kJ/mol by microcalorimetry. The entropies (Δ<i>S</i>₁ and Δ<i>S</i>₂) were calculated to be 81.6 and 81.8 J/(K mol) accordingly. The strong complexation of NpO₂⁺ with DPA is driven by both positive entropies and highly exothermic enthalpies. The crystal structure of Na₃NpO₂(DPA)₂(H₂O)₆(s) shows that, in the NpO₂(DPA)₂³⁻ complex, the Np atom sits at a center of inversion and the two DPA ligands symmetrically coordinate to Np in a tridentate mode. Due to the centrosymmetric structure of the NpO₂(DPA)₂³⁻ complex, the f → f transitions of Np(V) are forbidden, and the sharp bands originating from the f → f transitions either disappear or become very weak in the optical absorption or diffuse reflectance spectra of the NpO₂(DPA)₂³⁻ complex.