Synthesis and characterisation of new bimetallic alkali metal–magnesium mixed diisopropylamide-acetylides: structural variations in bimetallic lithium- and sodium-heteroleptic magnesiates

Abstract

The reactivity of the Brønsted basic mixed-metal tris-amide compounds of empirical formula [MMg(NⁱPr₂)₃] [where M = Li (1), Na (2)] towards phenylacetylene (HCCPh) has been investigated and has led to the synthesis of a series of mixed-metal acetylido-amido-magnesiates. Thus, 1 and 2 molar equivalents of the alkyne with [MMg(NⁱPr₂)₃] produce heteroanionic bis(amido)-mono(acetylido) [LiMg(NⁱPr₂)₂(CCPh)]₂ (3) and mono(amido)-bis(acetylido) [(TMEDA)·Na(CCPh)₂Mg(NⁱPr₂)]₂ (4) (TMEDA = N,N,N′,N′-tetramethylethylenediamine) respectively. X-Ray crystallographic studies reveal that the new compounds adopt a different structural motif. Complex 3 can be defined as an inverse crown structure, having a cationic eight-atom [(NaNMgN)₂]²⁺ ring which hosts in its core two acetylido ligands. On the other hand, 4 adopts a tetranuclear Na⋯Mg⋯Mg⋯Na near-linear chain arrangement, held together by acetylido and amido bridges. The metal coordination geometries in both structures are distorted tetrahedral, and the sodium cations at the end of the mixed-metal chain carry terminal chelating TMEDA ligands. ¹H and ¹³C NMR spectral data recorded in C₆D₆ solutions are also reported for 3 and 4, and are consistent with the solid-state structures being retained in solution.