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).