Synthesis and characterization of three homoleptic alkoxides of uranium: [Li(THF)]2[UIV(OtBu)6], [Li(Et2O)][UV(OtBu)6], and UVI(OtBu)6

Addition of 6 equiv of LiOtBu to a THF/Et2O solution of UCl4 at -25 degrees C generates Li(THF)]2U(OtBu)6] (1) in 61% yield. 1 is soluble in polar organic solvents and is stable for several days in THF. However, 1 slowly decomposes in benzene or hexanes, forming the dinuclear uranium(IV) species Li(THF)]U2(OtBu)9] (2) as one of the decomposition products. Alternatively, 2 can be directly prepared in moderate yield by the addition of 4.5 equiv LiOtBu to UCl4 in hexanes/THF at room temperature. The decomposition of 1 has been studied by 1H and 7Li{1H} NMR spectroscopies to elucidate the nature of this transformation. Oxidation of 1 occurs readily in the presence of 0.5 or 1 equiv of I2 to give Li(Et2O)]U(OtBu)6] (3) and U(OtBu)6 (4), respectively, in good yields. Alternately, 3 can be generated by comproportionation of 1 and 4. 1-4 have been fully characterized, including analysis by X-ray crystallography. In the solid-state these complexes possess large U-O-Cq bond angles, suggestive of a significant U-O pi interaction. In addition, we have studied the redox properties of 4 by cyclic voltammetry.