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