Homoleptic Uranium(IV) Alkyl Complexes: Synthesis and Characterization

The addition of 4.5 equiv of LiCH2SiMe3 to [Li(THF)]2[U(OtBu)6], in the presence of LiCl, results in the formation of the homoleptic uranium(IV) alkyl complex [Li14(OtBu)12Cl][U(CH2SiMe3)5] (1) in low yield. Complex 1 has been characterized by X-ray crystallography. As a solid, 1 is thermally stable for several days at room temperature. However, 1 rapidly decomposes in C6D6, as indicated by 1H and 7Li{1H} NMR spectroscopy, owing to the lability of the [Li14(OtBu)12Cl]+ cation. To avoid the formation of the [Li14(OtBu)12Cl]+ counterion, alkylation of UCl4 was investigated. Treatment of UCl4 with 5 equiv of LiCH2SiMe3 or LiCH2tBu at −25 °C in THF/Et2O affords [Li(DME)3][U(CH2SiMe3)5] (2) and [Li(THF)4][U(CH2tBu)5] (3), respectively, in good yields. Similarly, treatment of UCl4 with 6 equiv of MeLi or KCH2C6H5 generates the U(IV) hexa(alkyl) complexes [Li(TMEDA)]2[UMe6] (4) and {[K(THF)]3[K(THF)2][U(CH2C6H5)6]2}x (5) in 38% and 70% yields, respectively. The structures of 3−5 have been confirmed by X-ray crystallography. Complexes 2, 3, and 5 are thermally stable solids which can be stored at room temperature for several days, whereas 4 decomposes upon warming above −25 °C. The electronic and magnetic properties of 2, 3, and 5 were also investigated by NIR spectroscopy and SQUID magnetometry.