[UO₂(NH₃)₅]Br₂ ∙ NH₃: synthesis, crystal structure, and speciation in liquid ammonia solution by first-principles molecular dynamics simulations

Pentaammine dioxido uranium(VI) dibromide ammonia (1/1), [UO₂(NH₃)₅]Br₂·NH₃, was synthesized in the form of yellow crystals by the reaction of uranyl bromide, UO₂Br₂, with dry liquid ammonia. The compound crystallizes orthorhombic in space group Cmcm and is isotypic to [UO₂(NH₃)₅]Cl₂·NH₃ with a = 13.2499(2), b = 10.5536(1), c = 8.9126(1) Å, V = 1246.29(3) ų and Z = 4 at 123 K. The UO₂²⁺ cation is coordinated by five ammine ligands and the coordination polyhedron can be best described as pentagonal bipyramid. Car–Parrinello molecular dynamics simulations are reported for [UO₂(NH₃)₅]²⁺ in the gas phase and in liquid NH₃ solution (using the BLYP density functional). According to free-energy simulations, solvation by ammonia has only a small effect on the uranyl–NH₃ bond strength.