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 Bronsted basic mixed-metal tris-amide compounds of empirical formula [MMg((NPr2)-Pr-i)(3)] [where M = Li (1), Na (2)] towards phenylacetylene (HC CPh) 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((NPr2)-Pr-i)(3)] produce heteroanionic bis(amido)-mono( acetylido) [LiMg((NPr2)-Pr-i)(2)(C CPh)](2) (3) and mono(amido)-bis(acetylido) [(TMEDA)center dot Na(C CPh)(2)Mg( (NPr2)-Pr-i)](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 center dot center dot center dot Mg center dot center dot center dot Mg center dot center dot center dot 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. H-1 and C-13 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.