Preparation and Reactivity of the Versatile Uranium(IV) Imido Complexes U(NAr)Cl₂(R₂bpy)₂ (R = Me, ^tBu) and U(NAr)Cl₂(tppo)₃

Uranium tetrachloride undergoes facile reactions with 4,4′-dialkyl-2,2′-bipyridine, resulting in the generation of UCl₄(R₂bpy)₂, R = Me, ^tBu. These precursors, as well as the known UCl₄(tppo)₂ (tppo = triphenylphosphine oxide), react with 2 equiv of lithium 2,6-di-isopropylphenylamide to provide the versatile uranium­(IV) imido complexes, U­(NDipp)­Cl₂(L)_n (L = R₂bpy, n = 2; L = tppo, n = 3). Interestingly, U­(NDipp)­Cl₂(R₂bpy)₂ can be used to generate the uranium­(V) and uranium­(VI) bisimido compounds, U­(NDipp)₂­X­(R₂bpy)₂, X = Cl, Br, I, and U­(NDipp)_2­I₂(^tBu₂bpy), which establishes these uranium­(IV) precursors as potential intermediates in the syntheses of high-valent bis­(imido) complexes from UCl₄. The monoimido species also react with 4-methylmorpholine-N-oxide to yield uranium­(VI) oxo-imido products, U­(NDipp)­(O)­Cl₂(L)_n (L = ^tBu₂bpy, n = 1; L = tppo, n = 2). The aforementioned molecules have been characterized by a combination of NMR spectroscopy, X-ray crystallography, and elemental analysis. The chemical reactivity studies presented herein demonstrate that Lewis base adducts of uranium tetrachloride function as excellent sources of U­(IV), U­(V), and U­(VI) imido species.