Synthesis, StructuralElucidation, and Diffusion-Ordered NMR Studies of Homoleptic AlkyllithiumMagnesiates: Donor-Controlled Structural Variations in Mixed-MetalChemistry.

This paper presents the synthesis and characterizationof new homoleptic lithium magnesiate reagents incorporating the silyl-substitutedalkyl ligand CH2SiMe3in the presence of a varietyof 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 beenassessed in both the solid state and solution using a combinationof X-ray crystallographic studies and multinuclear NMR spectroscopy,including 1H diffusion-ordered (1H-DOSY) NMRexperiments. These studies highlight the major role played by thedonor molecule in controlling the structure of the complexes as wellas the wide structural diversity available for these mixed-metal speciesranging from discrete molecules, as found for [(PMDETA)LiMg(CH2SiMe3)3] (6), to more complexsupramolecular 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 studieshave also revealed that in some cases the donor molecule can promotea 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-richtetraorganomagnesiate [(TMEDA)Li2Mg(CH2SiMe3)4] (5) along with Mg(CH2SiMe3)2. The formation of the unprecedentedcationic lithium magnesiate [{(PMDETA)2Li2Mg(CH2SiMe3)3}+{Mg3(CH2SiMe3)6(OCH2SiMe3)}−] (7) is also described, by thecontrolled exposure to oxygen of the monomeric compound [(PMDETA)LiMg(CH2SiMe3)3] (6). [ABSTRACT FROM AUTHOR]