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((n)Bu)3 have been prepared and structurally characterised. The complexes were prepared by a co-complexation approach i.e., by combining the monometallic (n)BuLi and (n)Bu2Mg 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-(n)Bu)2Mg(μ-(n)Bu)2Mg(μ-(n)Bu)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-(n)Bu)(μ-O(n)Bu)Mg2((n)Bu)2(μ-(n)Bu)(μ-O(n)Bu)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 (1)H, (7)Li, (13)C NMR spectroscopy; 2 has been studied by variable temperature and DOSY NMR.

Alkali-Metal-Mediated Synergistic Effects in Polar Main Group Organometallic Chemistry

“…In general, these reactions produce predictable structures with Weiss motifs as observed in the (anion) homoleptic [{(R,R)-TMCDA}Li( n Bu) 2 Mg( n Bu)] 2 (63), but in the presence of an alcohol a heteroleptic variant with an alternative ladder motif can be accessed in [(TMEDA)Li( n Bu)(O n Bu)Mg( n Bu)] 2 , (64, Scheme 40). 76 When mixed together in hexane, n-butyllithium and di-nbutylmagnesium react with the aminopyridine 2-Me 3 SiNH-6-MeC 5 H 3 N (L 1 H) to produce the mixed lithium−magnesium polymer [(L 1 )Li(μ 2 -n Bu)Mg(L 1 )] ∞ (65, Scheme 41). The polymer propagates through an unusual Mg-n Bu-Li bridge.…”

Structural and metal–halogen exchange reactivity studies of sodium magnesiate biphenolate complexes

“…2,3 The synergic behaviour arising from the combination of two metal centres within a complex gives these reagents unique and often enhanced reactivity, allowing them to perform transformations that would otherwise be unachievable using their monometallic parent complexes. 1,4,5 Not only do these bimetallic reagents allow for reactions to be carried out under milder conditions 6 (i.e., carried out at temperatures at or close to ambient temperature), they also exhibit a much greater functional group tolerance compared to monometallic bases. 7 Consequently, bimetallic magnesiates have attracted much attention for use in metal-halogen exchange reactions.…”