Cerium(IV), neptunium(IV), and plutonium(IV) 1,2-phenylenediphosphonates: Correlations and differences between early transuranium elements and their proposed surrogates

摘要

The in situ hydrothermal reduction of Np(VI) to Np(IV) and Pu(VI) to Pu(IV) in the presence of 1,2-phenylenediphosphonic acid (PhP2) results in the crystallization of Np[C_6H_4(PO_3H) _2]_2?2H_2O (NpPhP2) and Pu[C _6H_4(PO_3H)(PO_3H_2)][C _6H_4(PO_3H)(PO_3)]?2H _2O (PuPhP2), respectively. Similar reactions have been explored with Ce(IV) resulting in the isolation of the Ce(IV) phenylenediphosphonate Ce[C _6H_4(PO_3H)(PO_3H_2)][C _6H_4(PO_3H)(PO_3)]?2H _2O (CePhP2). Single crystal diffraction studies reveal that although all these three compounds all crystallize in the triclinic space group P1?, only PuPhP2 and CePhP2 are isotypic, whereas NpPhP2 adopts a distinct structure. In the cerium and plutonium compounds edge-sharing dimers of MO _8 polyhedra are bridged by the diphosphonate ligand to create one-dimensional chains. NpPhP2 also forms chains. However, the NpO_8 units are monomeric. The protonation of the ligands is also different in the two structure types. Furthermore, the NpO_8 polyhedra are best described as square antiprisms (D_(4d)), whereas the CeO_8 and PuO _8 units are trigonal dodecahedra (D_(2d)). Bond-valence parameters of R_o = 1.972 and b = 0.538 have been derived for Np ~(4+) using a combination of the data reported in this work with that available in crystallographic databases. The UV-vis-NIR absorption spectra of NpPhP2 and PuPhP2 are also reported and used to confirm the tetravalent oxidation states.