From Thorium to Plutonium: Trends in Actinide(IV) Chloride Structural Chemistry

A series of eighteen tetravalent actinide (An = Th, U, Pu) compounds were synthesized from acidic aqueous solutions containing thorium, uranium, or plutonium and a series of protonated nitrogen heterocycles. The compounds were characterized using Raman, IR, and optical absorption spectroscopies. The structures were determined using single-crystal X-ray diffraction and found to consist of [An(H₂O)Cl]^4- (x = 4-7 and y = 2-4) or AnCl₆^2- molecular units. Breaks in the structural chemistry of the early actinides were observed, with Th adopting exclusively Th-aquo-chloro species and Pu forming only PuCl₆^2-; U crystallized as both U-aquo-chloro and UCl₆^2-. The relationship between the solid-state structural units and the solution species was interrogated using UV-vis-near-IR absorption spectroscopy. A comparison of the solution and solid-state spectra suggested that, although prevalent in the solid state, particularly for U and Pu, AnCl₆^2- does not exist to an appreciable extent in the reaction solution. Despite the identification of U-aquo-chloro species in solution, there are limited reports of these complexes in the solid state. Isolation of these unique actinide(IV) chlorides as reported in this work may point to the importance of nonbonding interactions in the stabilization and precipitation of An^IV structural units.