Synthesis, Structural Elucidation, and Diffusion-Ordered NMR Studies of Homoleptic Alkyllithium Magnesiates: Donor-Controlled Structural Variations in Mixed-Metal Chemistry

This paper presents the synthesis and characterization of new homoleptic lithium magnesiate reagents incorporating the silyl-substituted alkyl ligand CH2SiMe3 in the presence of a variety of Lewis base donors, namely tetrahydrofuran (THF), 1,4-dioxane, N,N,N′,N′-tetramethylethylenediamine (TMEDA), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA). The constitution of these bimetallic compounds has been assessed in both the solid state and solution using a combination of X-ray crystallographic studies and multinuclear NMR spectroscopy, including 1H diffusion-ordered (1H-DOSY) NMR experiments. These studies highlight the major role played by the donor molecule in controlling the structure of the complexes as well as the wide structural diversity available for these mixed-metal species ranging from discrete molecules, as found for [(PMDETA)­LiMg­(CH2SiMe3)3] (6), to more complex supramolecular arrangements, as in the 1D-polymeric chain [{(THF)­LiMg­(CH2SiMe3)3}∞] (2) or in the stoichiometrically distinct dioxane solvates [{(dioxane)2LiMgR3}∞] (3) and [{(dioxane)­Li2Mg2R6}∞] (4). Furthermore, these studies have also revealed that in some cases the donor molecule can promote a redistribution process, as shown for the reaction of triorganomagnesiate [LiMg­(CH2SiMe3)3] (1) with 1 molar equiv of TMEDA, which led to the formation of lithium-rich tetraorganomagnesiate [(TMEDA)­Li2Mg­(CH2SiMe3)4] (5) along with Mg­(CH2SiMe3)2. The formation of the unprecedented cationic lithium magnesiate [{(PMDETA)2Li2Mg­(CH2SiMe3)3}+{Mg3(CH2SiMe3)6(OCH2SiMe3)}−] (7) is also described, by the controlled exposure to oxygen of the monomeric compound [(PMDETA)­LiMg­(CH2SiMe3)3] (6).