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

Flash reduction of Cp'U-3 (Cp' = C5H4SiMe3) in a column of potassium graphite in the presence of 2.2.2-cryptand generates crystalline [K(2.2.2-cryptand)][Cp'U-3], 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 crystallo-graphically similar, the hydride complex was synthesized by addition of KH to Cp'U-3 and by reduction of H-2 by the U2+ complex and was confirmed to be a different compound. Density functional theory calculations indicate a 5f(3)6d(1) quintet ground state for the [Cp'U-3](-) anion and match the observed strong transitions in its optical spectrum.