PNNL

Abstract

Metal ion hydrolysis and condensation reactions are critical to describing the chemical behavior of the tetravalent actinides due to their high charge density and Brønsted acidity. This recognition has fueled synthetic efforts targeting polynuclear actinide-oxo clusters. Oligomers ranging from trimers to octatriacontamers have been reported, with the hexameric unit, which typically exhibits a [An6(OH)6O6]12+ core, representing the most pervasive cluster. Hexamers decorated by a range of ligands, including carboxylates, sulfates, and chlorides, have been described. While previous reports have demonstrated the formation of hexamers for Th, U, Np, and Pu both in solution and the solid-state, there remains a lack of literature precedence for the formation of structurally analogous clusters across the early An series using the same complexing ligand. Herein, we report a series of benzoate (Bz) decorated actinide–oxo/hydroxo hexamers of the same general formula [An6O4(OH)4(Bz)12(H2O)n], where An= Th, U, Np, Pu and n = 6 for Th and 4 for U-Pu. The title compounds were characterized by X-ray diffraction, UV-vis-NIR absorbance, Raman, and infrared spectroscopy. Notably, isolation of these phases and elucidation of the parameters that control their formation provides insight into key differences in metal ion charge density across the early tetravalent actinides, both in their synthetic and structural chemistry.