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 (ΔH1 and ΔH2) were measured to be −25.2 and −45.9 kJ/mol by microcalorimetry. The entropies (ΔS1 and ΔS2) 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.

在25°C的1 M NaClO4溶液中，对锕系元素NpO2+与二水杨酸（DPA）的络合作用进行了研究。鉴定出两种络合物，即NpO2(DPA)−和NpO2(DPA)23−，并分别通过光谱光度法确定了其稳定性常数（log β1和log β2），分别为8.68和12.31。络合反应的焓变（ΔH1和ΔH2）通过微量热法测定为−25.2和−45.9 kJ/mol，相应的熵变（ΔS1和ΔS2）计算为81.6和81.8 J/(K mol)。NpO2+与DPA的强络合作用既受正熵效应驱动，也受高度放热的焓变驱动。Na3NpO2(DPA)2(H2O)6(s)的晶体结构表明，在NpO2(DPA)23−络合物中，Np原子位于中心反转点，两个DPA配体以三齿模式对称地与Np配位。由于NpO2(DPA)23−络合物的中心对称结构，Np(V)的f → f跃迁被禁阻，源于f → f跃迁的尖锐吸收带在NpO2(DPA)23−络合物的光学吸收或漫反射光谱中消失或变得非常微弱。