Surprises in the Solvent-Induced Self-Ionization in the Uranium Tetrahalide UX (X = Cl, Br, I)/Ethyl Acetate System.

The reaction of the uranium(IV) halides UCl, UBr, or UI with ethyl acetate (EtOAc) leads to the formation of the complexes [UX(EtOAc)][UX(EtOAc)] (X = Cl, Br) or [UI(EtOAc)]. Thus, both UCl and UBr show self-ionization in ethyl acetate to a distorted pentagonal bipyramidal [UX(EtOAc)] cation and a distorted octahedral [UX(EtOAc)] anion. Surprisingly, the chloride and bromide compounds are not isotypic. While [UCl(EtOAc)][UCl(EtOAc)] crystallizes in the orthorhombic crystal system, space group 222 at 250 K, the bromide compound crystallizes in the monoclinic crystal system, 12/1 at 100 K. Unexpectedly, UI does not show self-ionization but forms [UI(EtOAc)] molecules, which crystallize in the monoclinic crystal system, 2/, at 100 K. The compounds were characterized by single-crystal X-ray diffraction, IR, Raman, and NMR spectroscopy, as well as molecular quantum chemical calculations using solvent models.