Surprises in the Solvent-Induced Self-Ionization in the Uranium Tetrahalide UX₄ (X = Cl, Br, I)/Ethyl Acetate System

The reaction of the uranium(IV) halides UCl₄, UBr₄, or UI₄ with ethyl acetate (EtOAc) leads to the formation of the complexes [UX₃(EtOAc)₄][UX₅(EtOAc)] (X = Cl, Br) or [UI₄(EtOAc)₃]. Thus, both UCl₄ and UBr₄ show self-ionization in ethyl acetate to a distorted pentagonal bipyramidal [UX₃(EtOAc)₄]⁺ cation and a distorted octahedral [UX₅(EtOAc)]^- anion. Surprisingly, the chloride and bromide compounds are not isotypic. While [UCl₃(EtOAc)₄][UCl₅(EtOAc)] crystallizes in the orthorhombic crystal system, space group P2₁2₁2₁ at 250 K, the bromide compound crystallizes in the monoclinic crystal system, P12₁/n1 at 100 K. Unexpectedly, UI₄ does not show self-ionization but forms [UI₄(EtOAc)₃] molecules, which crystallize in the monoclinic crystal system, P2₁/c, 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.