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

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 NaMgMgNa 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.