Elucidating trends in synthesis and structural periodicity in a series of tetravalent actinide–oxo hexamers†

Metal ion hydrolysis and condensation reactions are critical to describing the chemical behavior of the tetravalent actinides (An) due to their high charge density. 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)₄O₄]¹²⁺ core, representing the most pervasive cluster. Hexamers decorated by a range of ligands, including carboxylates, sulfates, and chlorides, have been described. Previous reports have demonstrated the formation of hexamers for Th, U, Np, and Pu both in solution and the solid state, yet little work has focused on the synthesis and properties of structurally analogous clusters across the early An series using the same complexing ligand. Here, 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 is reported. 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 underpin their formation provides insight into the ways differences in metal ion charge density manifest across the early tetravalent actinides, both in their synthetic and structural chemistry.