Synthesis and Structural Elucidation of Alkyl, Amido, and Mixed Alkyl−Amido “Highly-Coordinated” Zincates

The solution and solid-state characterization of the tetraorganozincates
[(TMEDA)2Li2ZnMe(NMe2)3] (2) and [(TMEDA)2Li2ZnMe4] (4) and the all-amido triorganozincate [{(TMEDA)LiZn(NMe2)3}2] (3) are reported.
X-ray crystallographic studies reveal that 2 and 4 exhibit similar structural motifs, namely trinuclear Li···Zn···Li
chain arrangements held together by a methyl and three amido ligands
in the former and exclusively by methyl groups in the latter, whereas 3 adopts a longer tetranuclear Li···Zn···Zn···Li
chain arrangement with all NMe2 ligands. The zinc coordination
in all these structures is distorted tetrahedral. DFT calculations
revealed that the formation of tetraorganozincate 4 by
cocomplexation of 2 equiv of MeLi and Me2Zn is thermodynamically
preferred over the parent triorganozincate [(TMEDA)LiZnMe3]. Preliminary studies show that the tris(amido)alkyl zincate 2 is a poor Brønsted base for metalation (zincation)
of functionalized aromatic substrates such as anisole, N,N-diisopropylbenzamide, and benzonitrile.