Reactions of Actinide Ions with 2,6-Pyridinedicarboxylic Acid in Aqueous Solutions and the Structure of Crystalline Compounds

The results of studying the interaction of An(III–VI) with 2,6- pyridinedicarboxylic acid (2,6-PDCA) are summarized. The stability constants are estimated for complexes An^IV(PDC)_n^4–2n (n = 1–3). The total stability constant exceeds 10²⁵ mol^–3 L^–3 and increases in the series from Th(IV) to Pu(IV) by 8 orders of magnitude. The difference in the stability of the An(III) and An(IV) complexes leads to rapid oxidation of Pu(III) to Pu(IV) with oxygen in the presence of 2,6-PDCA. An(V) and An(VI) ions form 1 : 1 and 1 : 2 complexes with PDC. The formation of An(IV,V,VI) complexes leads to a shift in the redox potential of the (V)/(IV) and (VI)/(V) couples. An(V) disproportionate in the presence of 2,6-PDCA anions. For An(VI) complexes of 1 : 2 composition, the f–f transition bands are weakened, which is associated with the centrosymmetrical structure of the coordination polyhedron of the An atom. The stability of the An(VI) complexes was evaluated. An(III) ions form compounds with PDC^2– in which the metal : ligand ratio is 1 : 3. Compounds An^IV(PDC)₂ and those with an anionic moiety in the form of An^IV(PDC)₃^2– were obtained. The cations were imidazole, Cs⁺, Ln²⁺, and Ln³⁺. An(V) forms compounds with one and two PDC^2– ions. Compounds of UO₂²⁺ with PDC^2–, as well as UO₂(PDC)₂^2– compounds with organic and inorganic outer-sphere cations have been studied. According to X-ray diffraction analysis, the AnO₂(PDC)₂^2– have the center of symmetry. In the isostructural series [Li₂AnO₂(PDC)₂]·2H₂O, actinide contraction is expressed in a decrease in the An=O distances in the “yl” groups from U to Pu. In all cases, PDC^2– is coordinated in the tridentate fashion through two oxygen atoms of carboxyl groups and a nitrogen atom.