Coordination chemistry 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 (An), previous literature mainly has 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 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 are very few data concerning Np(IV) compounds. In 2012, Takao et al.4 reported the formation of Np(IV) hexanuclear cluster in an aqueous solution. 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.
In this 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.5 This compound crystallizes as orange aggregates, 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, involving layers of {NpO7H2O0-2} units linked to each other via trans-dioxo neptunyl bonds.
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 (Figure 1, left). The corresponding structures revealed the isolation of the hexanuclear cluster An6O8 with Np(IV), the first example of hexanuclear Np(IV) complex characterized in the solid state. These clusters are linked by the ligand creating tetrahedral and octahedral voids in the structure. The formation of larger neptunium-based polyoxo clusters will be also presented.