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

Complexation of NpO(2)(+) with dipicolinic acid (DPA) has been investigated in 1 M NaClO(4) at 25 degrees C. Two complexes, NpO(2)(DPA)(-) and NpO(2)(DPA)(2)(3-), were identified, and the stability constants (log beta(1) and log beta(2)) were determined to be 8.68 and 12.31, respectively, by spectrophotometry. The enthalpies of the complexation (DeltaH(1) and DeltaH(2)) were measured to be -25.2 and -45.9 kJ/mol by microcalorimetry. The entropies (DeltaS(1) and DeltaS(2)) were calculated to be 81.6 and 81.8 J/(K mol) accordingly. The strong complexation of NpO(2)(+) with DPA is driven by both positive entropies and highly exothermic enthalpies. The crystal structure of Na(3)NpO(2)(DPA)(2)(H(2)O)(6)(s) shows that, in the NpO(2)(DPA)(2)(3-) 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 NpO(2)(DPA)(2)(3-) 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 NpO(2)(DPA)(2)(3-) complex.