Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries^**

Graphical Abstract

Isolation of the first crystal structures of Pu(C₂O₄)₂ ⋅ 6 H₂O and Th(C₂O₄)₂ ⋅ 6 H₂O has resulted in the identification of a previously unreported change in oxalate coordination and the presence of two coordinated water molecules. This structure was determined to persist across the actinide(IV) series: Th, U, Np, and Pu.

Abstract

Plutonium(IV) oxalate hexahydrate (Pu(C₂O₄)₂ ⋅ 6 H₂O; PuOx) is an important intermediate in the recovery of plutonium from used nuclear fuel. Its formation by precipitation is well studied, yet its crystal structure remains unknown. Instead, the crystal structure of PuOx is assumed to be isostructural with neptunium(IV) oxalate hexahydrate (Np(C₂O₄)₂ ⋅ 6 H₂O; NpOx) and uranium(IV) oxalate hexahydrate (U(C₂O₄)₂ ⋅ 6 H₂O; UOx) despite the high degree of unresolved disorder that exists when determining water positions in the crystal structures of the latter two compounds. Such assumptions regarding the isostructural behavior of the actinide elements have been used to predict the structure of PuOx for use in a wide range of studies. Herein, we report the first crystal structures for PuOx and Th(C₂O₄)₂ ⋅ 6 H₂O (ThOx). These data, along with new characterization of UOx and NpOx, have resulted in the full determination of the structures and resolution of the disorder around the water molecules. Specifically, we have identified the coordination of two water molecules with each metal center, which necessitates a change in oxalate coordination mode from axial to equatorial that has not been reported in the literature. The results of this work exemplify the need to revisit previous assumptions regarding fundamental actinide chemistry, which are heavily relied upon within the current nuclear field.