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

We present the syntheses and crystal structures of several adventitious hydrolysis products of UCl₄, UBr₄, UBr₅, and UO₂Cl₂. We obtained compounds containing dinuclear μ-O-bridged uranium(IV) cations, mixed-valent uranyl(V)-uranyl(VI) tetra- and octanuclear complex cations, and also a compound where the isoelectronic species UO₂ ²⁺ and UN₂ formed a complex.

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 UO₂ ²⁺ 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.

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