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

Abstract

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).

Pre-inverse-crowns: synthetic, structural and reactivity studies of alkali metal magnesiates primed for inverse crown formation

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Compared to the organolithium reagents commonly employed in these reactions, magnesiates (as well as other types of ate, most importantly zincates 1,21-45 ) can show advantages of superior functional group tolerance and application at ambient temperature and in ethereal solvents. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Compared to the organolithium reagents commonly employed in these reactions, magnesiates (as well as other types of ate, most importantly zincates 1,21-45 ) can show advantages of superior functional group tolerance and application at ambient temperature and in ethereal solvents.…”

Solid state and solution studies of lithium tris(n-butyl)magnesiates stabilised by Lewis donors

“…A search in the Cambridge Structural Database 28 reveals that only four lithium complexes stabilised by TMPDA have been crystallographically characterised, bearing anionic groups such as phenylacetylene, 29 isocyanate, 30 boratabenzene 31 or benzyl. 24 Complex 2 resembles the dimer obtained by Weiss and co-workers when they employed the alkynyl ligand with TMEDA, although the structure found by them is Scheme 2 Synthesis of homoleptic n-butyllithium magnesiates. Diamines have proven being a key additive in the field of alkali metal-mediated magnesiation, dramatically modifying both structural pattern and reactivity, 33 so different nitrogen-donors were employed in this work (Scheme 2).…”

“…The Li-C distances range from 2.348(4) to 2.303(4) Å and the Mg-C distances from 2.205(6) to 2.298(2)Å are as expected for such species. 24 n-Butyl's less sterically demanding nature in comparison to methyl(trimethylsilyl) presumably results in two key differences between the structures. 32 Li-N distances of 1, [2.085(4) to 2.092(4) Å], are similar to those found for other lithium compounds stabilised by chelating TMPDA.…”

“…3). 24,42 When one equivalent is added to the unsolvated LiMg( n Bu) 3 , it was possible to crystallise polymeric 6, which crystallises in space group C2/c (Fig. Several alkali-metal magnesium amide complex stabilised by chiral donors 33c,38 [namely (−)-sparteine and (R,R)-TMCDA] have been characterised, but to the best of our knowledge, this is the first example of a crystallographically elucidated chiral tris-(alkyl)lithium magnesiate.…”

Structure–Reactivity Relationship in Organolithium Compounds

“…This is indicated by the Si-C α distance of 180 pm, which is the shortest Si-C α distances that is discussed here. Interestingly, the Mg-C distance (219 pm) of the bridging alkyl group is shorter than the Li-C distance (233 pm) and the lithium cation completes the coordination by bonding to PMDETA similar to the already [50]. Simultaneously, the longest Si-C trans bond (189.7 pm) is found in [(pmdeta)LiCH 2 SiMe 3 ] [31].…”