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

The synthesis and structural chemistry of two tetravalent thorium compounds precipitated from acidic bromide solutions are described. One of the phases, [Th(H2O)(10])Br-4 (1), has been previously reported. The other phase, [Th(H2O)(4)Br-4](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.