Donor-Dictated Interlocking Co-Complexation Reactions of LiNHDipp with Dimethylzinc: Synthesis and Structures of New Methyl(amido)zincates

A systematic study of the interlocking co-complexation reactions between the primary lithium amide LiNHDipp (Dipp = 2,6-diisopropylphenyl) and dimethylzinc in the presence of different donor ligands is presented which concludes that the final outcome of these reactions is largely dictated by the type of structure that is formed when the donor is coordinated to the lithium amide. When chelating diamine TMEDA (N,N,N',N'-tetramethylethylenediamine) is employed [{Li-2(NHDipp)(2)(TMEDA)}(infinity)] (1) is obtained, where Li2N2 rings are connected by TMEDA bridges generating a polymeric chain arrangement which does not form a co-complex with Me2Zn even in the presence of an excess of TMEDA. The tridentate ligand PMDETA (N,N,N',N '',N ''-pentamethyldiethylenetriamine) when reacted with LiNHDipp forms monomeric [(PMDETA)Li(NHDipp)](4) which successfully forms a mixed-metal co-complex with Me2Zn affording dialkyl(amido)zincate [(PMDETA)LiZn(NHDipp)(Me)(2)] (2). When the co-complexation reaction is carried out in the presence of monodentate tetrahydrofuran (THF), zincate [(THF)(3)LiZn(NHDipp)(Me)(2)] (3) is obtained which was found to partially decompose in hexane solution after long periods of time at room temperature (2 weeks) to afford the unprecedented zinc-rich zincate [(THF)(3)LiZn2(Me)(3)(NHDipp)(2)](5). This compound presents a unique structure in the solid state previously unknown in organozincate chemistry with a trinuclear Li center dot center dot center dot Zn center dot center dot center dot Zn chain arrangement where the metals are connected by only two amido bridges and therefore both zinc centers exhibit trigonal planar geometries. 5 can be prepared in good yields by the rational reaction of LiNHDipp with a 2:1:3 mixture of Me2Zn, NH(2)Dipp and THF. The different solid-state structural motifs of compounds 1, 2, 4, and 5 have been revealed by X-ray crystallographic studies. Multinuclear NMR (1(H), C-13 and Li-7) spectroscopic data recorded in C6D6 solution are also reported for compounds 1-6. Mixed-metal compounds 2 and 5 constitute the first examples of crystallographically characterized alkyl(amido)zincates containing a primary amide.