Monomeric and Trimeric Thorium Chlorides Isolated from Acidic Aqueous Solution

Two thorium(IV) compounds, [Th(H₂O)₄Cl₄]·2 (HPy·Cl) (1) and (HPy)₃[Th₃(H₂O)₂Cl₁₀(OH)₅]·4(HPy·Cl) (2) (HPy = pyridinium), were isolated from acidic aqueous solution. The compounds were synthesized at room temperature and subsequently characterized using single crystal X-ray diffraction along with Raman and IR spectroscopies. Whereas compound 1 is built from discrete mononuclear Th(H₂O)₄Cl₄ units, compound 2 consists of a novel hydroxo-bridged trimeric [Th₃(OH)₅]⁷⁺ core. Such species are largely absent from discussions of Th solution and solid-state chemistry and their isolation may be attributed to outer coordination sphere interactions that help stabilize the structural units; extensive hydrogen bonding and $\text{π}\text{–}\text{π}$ stacking interactions are present in 1 and 2. Density functional theory calculations were performed to predict the respective vibrational frequencies of the structural units, and their relative stability was predicted at the correlated molecular theory level. Small-angle X-ray scattering analysis of [Th₃(OH)₅]⁷⁺ in water indicates that the trimeric structural unit remains intact and that it is indeed an important species that necessitates consideration in geochemical models and for design of Th materials from water.