Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Three tetravalent actinide (An(IV)) hexanuclear clusters with the octahedral core [An6(OH)4O4]12+ and (An(IV) = U(IV), Np(IV), Pu(IV)) were structurally characterized in solid state and in aqueous solution using single crystal X-ray diffraction, X-ray absorption, IR, Raman and UV-Visible spectroscopy. The observed structure, [An6(OH)4O4(H2O)8(HDOTA)4].HNO3.nH2O (An = U (I), Np (II), Pu (III)), consists of a An(IV) hexanuclear pseudo- octahedral cluster stabilized by (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) (DOTA) ligands. The six actinide atoms are connected through alternative µ3-O2- and µ3-OH- groups. EXAFS investigations combined with UV-vis spectroscopy evidence the same local structure in moderate acidic and neutral aqueous solutions. The synthesis mechanism was partially elucidated and the main physical chemical properties (pH range stability, solubility and protonation constant) of the cluster were determined. The results underline the importance (i) to consider such polynuclear species in thermodynamic models and (ii) importance of competing reactions between hydrolysis and complexation. It is interesting to note that the same synthesis route with thorium(IV) leads to the formation of a dimer, Th2(H2O)10(H2DOTA)2.4NO3.xH2O (IV), which in contrast to the structure of the other An(IV) hexamers.