Heterobimetallic Uranium(V)-Alkali Metal Alkoxides: Expanding the Chemistry of f-Block Elements

Heterobimetallic uranium(V) alkoxides incorporating monovalent alkali metal counterions display remarkable structural versatility, dictated by the steric demands of the alkoxide ligands and the ionic radius of the alkali metal. Compounds of the general formula [UM(O<i>^t</i>Bu)₆] (<b>UM-O<i>^t</i>Bu</b>-type: M = Na, K, Rb, Cs) were obtained by: (i) reacting [U(O<i>^t</i>Bu)₅(py)] with equimolar amounts of alkali metal silylamides in <i>tert</i>-butyl alcohol, and (ii) oxidative transformation of [UM₂(O<i>^t</i>Bu)₆] (M = Na, K, Rb, Cs) upon reaction with iodine. <i>Trans</i>-alcoholysis of uranium heterobimetallic <i>tert</i>-butoxides with sterically less demanding <i>iso</i>-propyl alcohol yields oligomeric or polymeric <i>iso</i>-propoxide derivatives of the general formula [UM(O<i>ⁱ</i>Pr)₆]<i>_n</i>, where the nuclearity depends on the alkali metal (<i>n</i> = 2 for M = Li; <i>n</i> = ∞ for M = Na, K, Rb). The capacity of alkali metal ions to adopt flexible coordination geometries results in different structural types ranging from finite clusters to infinite chains, with [ULi(O<i>ⁱ</i>Pr)₆]₂ (<b>ULi-O<i>ⁱ</i>Pr-1</b>) found to be dimeric, whereas [UM(O<i>ⁱ</i>Pr)₆]_∞ (<b>UM-O<i>ⁱ</i>Pr-2</b>-type, M = Na, K) and [URb(O<i>ⁱ</i>Pr)₆]_∞ (<b>URb-O<i>ⁱ</i>Pr-3</b>) exhibit a polymeric architecture. These findings provide fresh insights into the structure-directing influence of alkali metals on actinide coordination chemistry and broaden the chemistry of actinide alkoxides. All compounds were unambiguously characterized in both solution and solid-state through NMR and IR spectroscopic studies, as well as single crystal X-ray diffraction analysis.