Uranium(IV) Alkyl Complexes of a Rigid Dianionic NON-Donor Ligand: Synthesis and Quantitative Alkyl Exchange Reactions with Alkyllithium Reagents

Reaction of [(XA₂)­UCl₃{K­(dme)₃}] (XA₂ = 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(CH₂SiMe₃)₂] (1) and dark red [(XA₂)­U­(CH₂CMe₃)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 LiCH₂SiMe₃. 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(CH₂SiMe₃)₂] (1-Th) with 2.2 equiv of LiCH₂CMe₃ yielded an approximate 1:1:3:1 mixture of [(XA₂)­Th­(CH₂CMe₃)₂] (2-Th), [(XA₂)­Th­(CH₂SiMe₃)­(CH₂CMe₃)] (5-Th), LiCH₂SiMe₃, and LiCH₂CMe₃.