Th(IV) Bromide Complexes: A Homoleptic Aqua Ion and a Novel Th(H₂O)₄Br₄ Structural Unit

Abstract

The synthesis and structural chemistry of two tetravalent thorium compounds precipitated from acidic bromide solutions are described. One of the phases, [Th­(H2O)10]­Br4 (1), has been previously reported. The other phase, [Th­(H2O)4Br4]­(HPy·Br)2 (2), is a novel compound and exhibits Th–Br coordination. While complexes with such Th–Br bonding have been observed in the solid state, most have been isolated from nonaqueous solutions. Notably, the two compounds crystallize from HBr(aq) solutions containing pyridinium (HPy+) counterions; in the absence of HPy+, only 1 is observed. The reaction energies of stepwise bromide addition were predicted using electronic structure theory. These calculations highlighted an overall thermodynamic driving force for Th–Br complexation up to the addition of four bromide ions. Electrostatic potential (ESP) surfaces were calculated to better understand the role of HPy+ in the isolation of 2. Consistent with our previous work, the ESP surfaces point to the importance of the pK a of the organic cation in determining the distribution of the Br– ligands about the Th metal center.

Elucidating Actinide–Pertechnetate and Actinide–Perrhenate Bonding via a Family of Th–TcO4 and Th–ReO4 Frameworks and Solutions

“…The last 2 decades has yielded structural insights into Th­(IV) coordination and speciation . Specifically relevant to the current study includes inorganic Th-oxoanion complexes with halogens − borate, sulfate, ,− molybdate, , chromate, nitrate, selenate, , and arsenate; assembled as oxoclusters, chains, sheets, and frameworks. , In addition to the specific knowledge of aqueous thorium complexation, structure, and reactivity; studies are carried out to expand our knowledge of trends across the f-block; and for example comparing structure and bonding of Ce­(IV), Th­(IV), U­(IV), Np­(IV), and Pu­(IV) …”

Structural chemistry of penta- and hexanitrato thorium(iv) complexes isolated using N–H donors

“…Building on the approaches used to study Th-Cl and Th-Br systems, 27,28 electronic structure calculations at the density functional theory and correlated molecular orbital theory levels were used to predict the relative energetics between a range of Th 4+ -aquo-nitrate complexes. The lowest energy structures for different species were predicted as a function of the number of ligands in the first coordination shell (Table 3).…”

“…27 In an effort to further understand the influence of counterions on actinide speciation, we have looked to other monoanionic ligand systems that similarly form relatively weak Th complexes. 28 Thorium remains our metal of choice for this work as it is stable under ambient conditions, is redox-inactive, is the least susceptible to hydrolysis and condensation, and can adopt a range of accessible coordination numbers. As a hard Lewis acid, thorium 4+ forms stronger complexes with harder ions; this is reflected in the formation constants for ThF 3+ (8.87) and ThCl 3+ (1.70).…”

“…27 In an effort to further understand the influence of counterions on actinide speciation, we have looked to other monoanionic ligand systems that similarly form relatively weak Th complexes. 28…”

“…25,26 With this in mind, our group has been examining the effects of outer sphere interactions on actinide solid-state structural chemistry. [27][28][29][30] We previously reported a series of Th-aquo-chloro complexes, where the identity of the solid-state structural unit and the chloride distribution about the metal center, in particular, had some dependence on the pK a of the N-heterocycle employed. 27 In an effort to further understand the influence of counterions on actinide speciation, we have looked to other monoanionic ligand systems that similarly form relatively weak Th complexes.…”