Structural chemistry of monodentate donor-solvated mixed lithium–magnesium secondary amide complexes

Abstract

The monodentate donor-solvated intermetallic lithium–magnesium amide complexes [Mg(HMDS)₃Li·(THF)] 1 [HMDS = N(SiMe₃)₂], [Mg(HMDS)₃Li·(Pyr)] 2 and [Mg{N(Cy)₂}₃Li·(THF)] 3 [N(Cy)₂ = dicyclohexylamide] have been prepared and characterised by NMR spectroscopy and X-ray crystallography. Synthesis was achieved by the reaction of equimolar amounts of n-BuLi and n,sec-Bu₂Mg with three equivalents of the appropriate amine in hexane/donor solution. The molecular structures of 1, 2 and 3 are essentially isostructural containing a central, planar LiNMgN four-membered ring: two amide units bridge to the metal centres whilst the third binds exclusively to magnesium in the terminal position to complete a three-coordinate distorted trigonal planar geometry. The lithium achieves a similar geometry with solvation from a single monodentate donor molecule. Three co-crystalline by-products were also isolated from solution and are included for completeness: [Mg(HMDS)₂(Bu)Li·Pyr] 4, [(LiHMDS·Pyr)₂] 5 and [Mg(HMDS)₂·(Pyr)₂] 6. Complex 4 exhibits a similar structure to 1, 2 and 3 with an alkyl group (consisting of disordered n- and sec-butyl groups) replacing the terminal amido functionality. Complex 4 is produced by a similar method to 2via incomplete amination in the presence of two equivalents of hexamethyldisilazane (HMDS(H)). In contrast, 5 and 6 are simple homometallic amides formed when an excess of pyridine is introduced into the reaction system. To conclude the study a series of reactions were undertaken in which the stoichiometry of both amine and donor was altered systematically. The results from this study imply that intermetallic aggregation is hindered by the presence of excess donor solvent.