Multinuclear uranium-nitrogen cluster formation from exploration of organouranium-ammonolysis

Uranium-nitrogen compounds (U_xN_y) pose interesting structural and electronic questions. Understanding the formation and speciation of U_xN_y may provide insights in nitrogen fixation as well as the preparation of potential uranium nitride nuclear fuels, a promising Gen IV nuclear reactor fuel. Small molecular clusters offer the opportunity to study U_xN_y using solution and dynamic spectroscopies. Herein, we have explored a new method for  the synthesis of multinuclear uranium-nitrogen clusters containing uranium-nitride, -imide, and -amide moieties in excellent yield. Ammonolysis of (C₅Me₅)UBn₃ gives a mixture of the tetra- and hexa-nuclear clusters (C₅Me₅)₄U₄(µ₄-N)(µ-NH)(μ-NH₂)₇, 1, (C₅Me₅)₆U₆(µ₆-N)(µ₃-NH₂)(µ₃-NH)₇, 2, and (C₅Me₅)₆U₆(µ₃-N)₂(µ-NH₂)₁₂, 3. Compound 1 was readily separated from 2 and 3, but compounds 2 and 3 display similar solubilities. Various reaction conditions, including changing the stoichiometry, reaction time, solvent free conditions, or the use of an ammonia/dioxane solution, did not alter the identity of what clusters were produced. Only small variations in the amount of each cluster produced were observed. Once formed, the clusters do not interconvert. The clusters were characterized by single crystal X-ray diffraction as well as ¹H NMR, UV-vis-NIR, and IR spectroscopies.