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(H 2 O) 10 ]Br 4 ( 1 ), has been previously reported. The other phase, [Th(H 2 O) 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 p K a of the organic cation in determining the distribution of the Br – ligands about the Th metal center.