PNNL

Abstract

A series of eighteen actinide (IV) compounds were synthesized from acidic aqueous media containing thorium, uranium, or plutonium. The metal cation adopts a variety of different structural units with the general formula [An(H2O)xCly]4-y (x=4-7, y=2-4) or AnCl62- charge balanced by protonated nitrogen-containing heterocycles. Compound identities were classified by single crystal X-ray diffraction and subsequently characterized by Raman, IR, and optical absorption spectroscopies. Discrepancies across the early actinides were observed in this family of An(IV)-chlorides, with Th adopting exclusively Th-aquo-chloro species, Pu forming only PuCl62-, and U residing somewhere in between, crystallizing both as U-aquo-chloro and UCl62- monomeric units. Reports of An-H2O-Cl species are largely absent from tetravalent An solid state discussions despite their identification in chloride solutions. The isolation of unique An(IV)-chlorides may be attributed to nonbonding interactions provided by outer coordination sphere counter-ions that could play a role in stabilizing these structural units. Differences in the structural chemistry of Th, U, and Pu also demonstrate a break in the series despite their common oxidation state and nearly identical synthetic procedures.