Homoleptic Uranium(IV) Alkyl Complexes: Synthesis and Characterization

The addition of 4.5 equiv of LiCH2SiMe3 to [Li(THF)](2)[U((OBu)-Bu-t)(6)], in the presence of LiCI, results in the formation of the homoleptic uranium(IV) alkyl complex [Li-14((OBu)-Bu-t)(12)CI][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 H-1 and Li-7{H-1} NMR spectroscopy, owing to the lability of the [Li-14((OBu)-Bu-t)(12)CI](+) cation. To avoid the formation of the (Li-14((OBu)-Bu-t)(12)CI](+) counterion, alkylation of UCI4 was investigated. Treatment of UCI4 with 5 equiv of LiCH2SiMe3 or (LiCH2Bu)-Bu-t at -25 degrees C in THF/Et2O affords [Li(DME)(3)][U(CH2SiMe3)(5)] (2) and[Li(THF)(4)][U((CH2Bu)-Bu-t)(5)] (3), respectively, in good yields. Similarly, treatment of UCI4 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 degrees C. The electronic and magnetic properties of 2, 3, and 5 were also investigated by NIR spectroscopy and SQUID magnetometry.