Ligand Metalation in the Reactivity of a Tetravalent Uranium Amides

The reactions of organolithium reagents with the tetravalent UCl2[N(SiMe3)2]2 and its DME solvate have been examined. Treatment of both compounds with methyl-lithium in diethyl ether resulted in one electron reduction of the metal center and γ-deprotonation of one of the ligands. The dimeric {U[(μ-CH2−SiMe2)N(SiMe3)]2[μ-Li(DME)]}2 (1) was isolated from the reaction mixture regardless of the amount of MeLi employed. The employment of LiCH2SiMe3 in DME led instead to multiple γ-deprotonation events at the same carbon atom with formation of the trimetallic {U[(μ-CH2−SiMe2)N(SiMe3)][N(SiMe3)2]}2{U[(μ3-C-SiMe2)N(SiMe3)][N(SiMe3)2]}{μ-OMe} (2) cluster centered on a fully deprotonated carbon atom. Crystallographic analysis revealed the presence of μ-OCH3 units in the cluster as generated by DME solvent cleavage. A similar reaction carried out in the absence of DME led to the isolation of a closely related trimetallic {U[μ-(CH2−SiMe2)N(SiMe3)][N(SiMe3)2]}2{U[(μ3-C-SiMe2)N(SiMe3)][(μ-CH2−SiMe2)N(SiMe3)]} (3). One additional γ-deprotonated fragment replacing the bridging methoxy group of 2 was present in this case. The presence of a fully deprotonated carbon atom bridging three metal centers and of one silicon atom was confirmed by both X-ray structures and NMR data. An attempt to reduce UCl2[N(SiMe3)2]2 with KC8 in a coordinating solvent resulted in ligand scrambling with the formation of two products. The first is a trimeric U(III) cluster formulated as {U-μ-Cl[N(SiMe3)2][DME]}2{U-μ-Cl[N(SiMe3)2]2}{μ3-Cl}2 (4). The second was U[N(SiMe3)2]3. A similar reduction reaction carried out in noncoordinating toluene resulted instead in an attack on the ligand affording the dimeric {U-μ-Cl[N(SiMe3)2]2[N(SiMe3)]}2 (5). Alkylation of UCl2[N(SiMe3)2]2 with n-butyl-lithium in hexane surprisingly yielded the pentavalent U[(μ-CH2−SiMe2)N(SiMe3)]2[N(SiMe3)2] (6). The acquisition of one additional ligand during the reaction hinted at the presence of other products in the reaction mixture.