Probing the Influence of Acidity and Temperature to Th(IV) on Hydrolysis, Nucleation, and Structural Topology

Systematic control of the molar ratio between thorium hydroxides and selenic acid and their reaction temperature under hydrothermal conditions results in four novel thorium-based selenate complexes, namely, [Th₈O₄(OH)₈(SeO₄)₆(H₂O)₁₆]·(SeO₄)₂·13H₂O (Th-1), [Th₈O₄(OH)₈(SeO₄)₈(H₂O)₁₃]·7H₂O (Th-2), Th(OH)₂(SeO₄)H₂O (Th-3), and Th₃(SeO₄)₆(H₂O)₆·2.5H₂O (Th-4), as well as the thorium mixed selenite selenate compound Th(SeO₃)(SeO₄) (Th-5). Smaller [H₂SeO₄]/[Th(IV)] ratio or lower temperature give rise to the formation of octameric [Th₈(μ₃-O)₄(μ₂-OH)₈]¹⁶⁺ cores in Th-1/Th-2 and infinite [Th(μ₂-OH)₂H₂O]²⁺ chains in Th-3, respectively. Increasing the [H₂SeO₄]/[Th(IV)] ratio or elevating the temperature generates a microporous (11.3 Å voids) open-framework Th-4, a monomeric thorium species without oxo/hydroxyl ligands, and a three-dimensional thorium structure Th-5. Formation of these compounds suggests that variables including acidity and temperature play a critical role in the hydrolysis and oligomerization of Th^IV ions. Increasing acidity limits the deprotonation of water molecules and formation of nucleophilic hydroxo/oxo-aquo Th species, and high temperature appears to suppress the olation/oxolation hydrolysis reactions, which in both ways limit the formation of the thorium oligomers.