Coordination polymers of tetravalent uranium and neptunium with aromatic polycarboxylate ligands

Coordination polymers are organic-inorganic complexes built up from the association of metallic centers with organic (e.g. O- or N-donor) ligands. In the particular case of actinides (An), precedent studies have reported mainly the synthesis of solid networks bearing U(VI) or Th(IV), while trans-uranium elements have been much less studied due to their high radiotoxicity and limited amount of the material source. Among the possible oxidation states of An, the tetravalent state has been investigated most actively and large polyoxo clusters have been isolated for U or Pu. In contrast, there are very few data concerning Np(IV) compounds. In 2012, Takao et al. reported the presence of a hexanuclear cluster of Np(IV) in an aqueous solution, which is the only polyoxo cluster reported for Np(IV) thus far. The knowledge of the formation of such polynuclear An(IV) species could be of significant importance for the fate of An in contaminated soils containing O-donor ligands, such as humic acids, or other organic pollutants (e.g. phthalates).
In the present work, we studied the crystallization of U(IV) and Np(IV) with various aromatic polycarboxylate ligands in different solvents and analysed their crystal structures. In an aqueous medium, an infinite chain of An2O2(H2O)2(1,2-bdc)2 (An = U, Np) were isolated in the presence of phthalate. This compound crystallizes as aggregates of green or orange plates for U and Np, respectively. With mellitic acid the oxidation of Np(IV) to Np(V) was observed and led to large green plates. Single-crystal XRD analysis revealed layers of {NpO7H2O0-2} units linked to each other via trans-dioxo neptunyl bonds. Similar coordination environments have been observed in other neptunium(V) compounds. To the contrary, the same synthesis procedure with U(IV) led to an U(IV)-based compound: U2(OH)2(H2O)2(mel), in which two uranium atoms are linked by hydroxo groups; mellitate ligands stabilize and connect these dinuclear units.
The use of other solvents allowed the crystallization of large polynuclear discrete Np(IV) clusters. For example, using DMF, the hexanuclear moiety of [Np6O4(OH)4] has been obtained with different dicarboxylic ligands and is the basic building unit to form an open-framework structure. The corresponding structure reveals for the first time the isolation of the hexanuclear cluster An6O8 with Np(IV). These clusters are linked by the ligand creating tetrahedral and octahedral voids in the structure.