Identification of the +2 Oxidation State for Uranium in a Crystalline Molecular Complex, [K(2.2.2-Cryptand)][(C₅H₄SiMe₃)₃U]

Flash reduction of Cp′3U (Cp′ = C5H4SiMe3) in a column of potassium graphite in the presence of 2.2.2-cryptand generates crystalline [K­(2.2.2-cryptand)]­[Cp′3U], the first isolable molecular U2+ complex. To ensure that this was not the U3+ hydride, [K­(2.2.2-cryptand)]­[Cp′3UH], which could be crystallographically similar, the hydride complex was synthesized by addition of KH to Cp′3U and by reduction of H2 by the U2+ complex and was confirmed to be a different compound. Density functional theory calculations indicate a 5f36d1 quintet ground state for the [Cp′3U]− anion and match the observed strong transitions in its optical spectrum.