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.