Complexes featuring a linear [N≡U≡N] core isoelectronic to the uranyl cation

The aqueous chemistry of uranium is dominated by the linear uranyl cation [UO₂]²⁺, yet the isoelectronic nitrogen-based analogue of this ubiquitous cation, molecular [UN₂], has so far only been observed in an argon matrix. Here, we present three different complexes of [UN₂] obtained by the reaction of the uranium pentahalides UCl₅ or UBr₅ with anhydrous liquid ammonia. The [UN₂] moieties are linear, with the U atoms coordinated by five additional ligands (ammonia, chloride or bromide), resulting in a pentagonal bipyramidal coordination sphere that is also commonly adopted by the uranyl cation [UO₂(L)₅]²⁺ (L, ligand). In all three cases, the nitrido ligands are further coordinated through their lone pairs by the Lewis-acidic ligands [U(NH₃)₈]⁴⁺ to form almost linear, trinuclear complex cations. Those were characterized by single-crystal X-ray diffraction, Raman and infrared spectroscopy, ¹⁴N/¹⁵N isotope studies and quantum chemical calculations, which support the presence of two U≡N triple bonds within the [UN₂] moieties.