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.