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

Abstract

Several Lewis base adducts of the synthetically important lithium tris(n-butyl)magnesiate LiMg(ⁿBu)₃ have been prepared and structurally characterised. The complexes were prepared by a co-complexation approach i.e., by combining the monometallic ⁿBuLi and ⁿBu₂Mg reagents in hydrocarbon solution before adding a molar equivalent of a donor molecule (a bidentate amine, tridentate amine or cyclic ether). The lithium magnesiates all adopt variants of the “Weiss motif” structure, i.e., contacted ion pair dimers with a linear arrangement and metals connected by butyl anions, where tetrahedral magnesium ions are in the central positions and the lithiums occupy the outer region, solvated by a neutral Lewis donor [(donor)Li(μ-ⁿBu)₂Mg(μ-ⁿBu)₂Mg(μ-ⁿBu)₂Li(donor)]. When TMPDA, PMDETA or (R,R)-TMCDA [TMPDA = N,N,N′N′-tetramethylpropanediamine; PMDETA = N,N,N′,N′′,N′′-pentamethyldiethylenetriamine; and (R,R)-TMCDA = (R,R)-N,N,N′,N′-tetramethylcyclohexane-1,2-diamine], are employed, dimeric tetranuclear lithium magnesiates are produced. Due to the tridentate nature of the ligand, the PMDETA-containing structure (2) has an unusual ‘open’-motif. When TMEDA (TMEDA = N,N,N′,N′-tetramethylethylenediamine) is employed, a n-butoxide-containing complex [(TMEDA)Li(μ-ⁿBu)(μ-OⁿBu)Mg₂(ⁿBu)₂(μ-ⁿBu)(μ-OⁿBu)Li(donor)] has been serendipitously prepared and adopts a ladder conformation which is commonly observed in lithium amide chemistry. This complex has also been prepared using a rational methodology. When 1,4-dioxane is employed, the donor stitches together a polymeric array of tetranuclear dimeric units (6). The hydrocarbon solution structures of the compounds have been characterised by ¹H, ⁷Li, ¹³C NMR spectroscopy; 2 has been studied by variable temperature and DOSY NMR.