Coordination polymers of tetravalent neptunium with aromatic polycarboxylate ligands

Coordination polymers are organic-inorganic complexes built up from the association of metallic centers with O- or N-donor ligands. In the particular case of actinides, previous literatures mainly have reported the synthesis of solid networks bearing U(VI) or Th(IV). 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 actinides (An), the tetravalent state has been investigated most actively and large polynuclear oxo-clusters have been isolated for U1,2 or Pu3. In contrast, there is very few data concerning Np(IV) compounds. In 2012, Takao et al.4 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 organic pollutants (e.g. phthalates). In the present work, we studied the crystallization of Np(IV) with various aromatic polycarboxylate ligands in different solvents and analyzed their crystal structures. In water, an infinite chain of Np2O2(H2O)2(1,2-bdc)2 were isolated in the presence of phthalate. This compound crystallizes as aggregates of orange plates, whereas the analogue compound with uranium is obtained as green crystals. 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 the other neptunium(V) compounds5.
The use of other solvents allowed the crystallization of large polynuclear discrete Np(IV) clusters. For example, using DMF, the hexanuclear unit 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 structures revealed 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.