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.