Complexity in seemingly simple sodium magnesiate systems

Abstract

A systematic study both in the solid- and solution-state, was carried out for a series of sodium magnesiates containing the utility amide ligand 1,1,1,3,3,3-hexamethyldisilazide (HMDS). The first complex considered is the donor-free bisamido monoalkyl polymeric complex [Na(μ-HMDS)₂Mg(ⁿBu)]_∞1. The reactivity of 1 with common tertiary bidentate donors including N,N,N′,N′-tetramethylethylenediamine (TMEDA) or its chiral relative (1R,2R)-tetramethylcyclohexyldiamine [(R,R)-TMCDA] is detailed. Surprisingly, the products of these reactions are not simple diamine adducts but are solvent separated sodium magnesiate systems [(TMEDA)₂·Na]⁺[Mg(HMDS)₃]⁻2 and [{(R,R)-TMCDA}₂·Na]⁺[Mg(HMDS)₃]⁻3. By concentrating on the likely equilibria which may give rise to formation of 2, a potential intermediate complexed ion pair [{(TMEDA)₂·Na}(μ-ⁿBu)Mg(HMDS)₂] 4 was isolated. Additionally, the novel “inverse magnesiates” [{Na(μ-HMDS)}₂Mg(μ-ⁿBu)₂·(TMEDA)]_∞5 and [{Na(μ-HMDS)}₂Mg(μ-ⁿBu)₂·{(R,R)-TMCDA}]_∞6, were obtained by reacting solutions of composition “NaMg(HMDS)(ⁿBu)₂” (a likely by-product in the formation of 2 from 1), with TMEDA or (R,R)-TMCDA. The structure and nature of these bimetallic complexes have been determined using a combination of X-ray crystallographic studies and multinuclear NMR spectroscopy.