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)xCly]4–y (x = 4–7 and y = 2–4) or AnCl62– 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 PuCl62–; U crystallized as both U–aquo–chloro and UCl62–. 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, AnCl62– does not exist to an appreciable extent in the reaction solution. Desp...