Synthesis and structural characterisation of mixed alkali metal–magnesium mixed ligand alkyl-amido ate complexes

Using two different reaction methodologies, two alkali metal–magnesium alkyl bis(amide) complexes were synthesised. First the lithium magnesiate LiMg{μ-N(SiMe3)2}2(tBu) (1) was prepared by combining equimolar quantities of tBuLi and Mg{N(SiMe3)2}2 in hydrocarbon solvent. An X-ray crystallographic study revealed that the asymmetric unit of 1 has a dinuclear arrangement, based on a planar Li–N–Mg–N four-membered ring. As a result of the presence of intermolecular agostic interactions between the Li centre of one asymmetric unit and a methyl group which is resident on the terminal tert-butyl group of another, 1 is polymeric in the solid-state. Second the sodium magnesiate NaMg{μ-N(SiMe3)2}2(tBu) · (OEt2) (2) was prepared by reacting two molar equivalents of Na{N(SiMe3)2} with one molar equivalent of tBuMgCl in hydrocarbon/diethyl ether solution. X-ray crystallographic analysis revealed that the asymmetric unit of 2 consisted of a dinuclear molecular arrangement. As expected it is not polymeric due to the coordination of the Lewis basic ether. Stabilizing intramolecular agostic Na⋯C bonds are observed (where C is a methyl group resident on a Si atom).

Graphical abstractThe crystal and solution structures of two mixed alkali metal–magnesium, mixed ligand alkyl-amido ate complexes are reported. Both complexes contain stabilizing alkali metal – methyl agostic interactions – intermolecular and intramolecular for Li and Na, respectively. From a supramolecular viewpoint, the lithium magnesiate complex is polymeric due to this additional agostic interaction. The sodium magnesiate complex is also stabilised by coordination to a molecule of diethyl ether, which obviates the need for additional intermolecular agostic contacts.Figure optionsDownload full-size imageDownload as PowerPoint slide