Uranium(IV) alkyl complexes of a rigid dianionic non-donor ligand: Synthesis and quantitative alkyl exchange reactions with alkyllithium reagents

Article abstract of DOI:10.1021/om301136f

Reaction of [(XA₂)UCl₃{K(dme)₃}] (XA ₂ = 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9- dimethylxanthene) with 2 equiv of ((trimethylsilyl)methyl)lithium or neopentyllithium afforded red-orange [(XA₂)U(CH₂SiMe ₃)₂] (1) and dark red [(XA₂)U(CH ₂CMe₃)₂] (2), respectively. Reaction of 1 with an additional 1 equiv of LiCH₂SiMe₃ in THF yielded yellow [Li(THF)_x][(XA₂)U(CH₂SiMe₃) ₃] (3), and reaction of [(XA₂)UCl₃{K(dme) ₃}] with 3 equiv of methyllithium in dme afforded yellow [Li(dme)₃][(XA₂)UMe₃] (4). Reaction of 1 with 2.1 equiv of LiCH₂CMe₃ in benzene resulted in rapid conversion to 2, with release of 2 equiv of LiCH₂SiMe₃. Similarly, reaction of 1 with 3.3 equiv of MeLi in THF provided 4 as the [Li(THF)_x]⁺ salt, accompanied by 2 equiv of LiCH ₂SiMe₃. These unusual alkyl exchange reactions resemble salt metathesis reactions, but with elimination of an alkyllithium instead of a lithium halide. Addition of a large excess of LiCH₂SiMe₃ to 2 or 4 did not generate detectable amounts of 1 by NMR spectroscopy, suggesting that the equilibrium in these reactions lies far to the side of complexes 2 and 4. In contrast, the reaction of [(XA₂)Th(CH ₂SiMe₃)₂] (1-Th) with 2.2 equiv of LiCH ₂CMe₃ yielded an approximate 1:1:3:1 mixture of [(XA ₂)Th(CH₂CMe₃)₂] (2-Th), [(XA ₂)Th(CH₂SiMe₃)(CH₂CMe₃)] (5-Th), LiCH₂SiMe₃, and LiCH₂CMe₃.