Uranium Chemistry in liquid Ammonia: Compounds obtained by adventitious Presence of Moisture or Air

Abstract

UCl4, UBr4, UBr5, or UO2Cl2 reacted with excess liquid ammonia—in adventitious presence of moisture and/or air—and formed some peculiar uranium compounds of which we present the crystal structures. [(NH3)7(N3)U(µ‐O)U(NH3)8]Cl5∙7NH3 contains a dinuclear µ‐O‐bridged uranium(IV) cation, [{(NH3)4UO2}2(µ‐O)]Cl2∙4NH3 features a dinuclear µ‐O‐bridged uranyl(VI) cation, while the compounds [(U(VI)O2)2(U(V)O2)2(µ3‐O)2(NH3)12]Br2∙6NH3 and [(U(VI)O2)4(U(V)O2)4(µ3‐O)4(NH3)22]Br4∙16NH3 are mixed‐valent containing uranyl(V)‐uranyl(VI) units. For these tetra‐ and octanuclear complex cations we observed that the O atoms of the uranyl(V) units can be µ2‐ and even µ3‐bridging to uranyl(VI) units, while the O atoms of the latter are acting as terminal ligands only. [(NH3)8U(µ‐N)U(NH3)5(µ‐N)UO2(NH3)4]Br6∙18NH3 presents the first example of a compound where the isoelectronic species UO22+ and UN2 formed a complex with the NUN unit bridging to the U atom of the uranyl(VI) cation. As it is can be difficult to distinguish between N and O atoms with X‐ray diffraction, quantum‐chemical calculations at the DFT‐PBE0/TZVP level of theory were carried out which unequivocally confirmed the atom assignments in the crystal structures. The chemical bonding in the complex cations was studied using intrinsic bonding orbitals and allowed for an additional discrimination of the U(V) and U(VI) atoms in the mixed‐valent compounds.