Synthesis and structural elucidation of solvent-free and solvated lithium dimethyl (HMDS) zincates

Using a co-complexation methodology the unsolvated lithium zincate LiZn(HMDS)Me2] ( 4, HMDS = 1,1,1,3,3,3-hexamethyldisilazide) was prepared by reaction of an equimolar amount of LiHMDS with Me2Zn in a non-polar toluene-hexane solvent mixture. X-Ray crystallographic studies reveal that the asymmetric unit of 4 has a dinuclear arrangement, based on a planar LiNZnC four-membered ring. As a result of intermolecular interactions between the lithium centre of one asymmetric unit and a terminal methyl group of another, 4 presents a polymeric chain array in the solid state. DFT calculations revealed that the formation of the polymer is the driving force for the success of co-complexation of LiHMDS and Me2Zn to yield the unsolvated zincate 4. The reaction of 4 with PMDETA (N,N,N,N,N-pentamethyldiethylenetriamine) afforded the new solvated zincate (PMDETA)Li(mu-Me)Zn(HMDS)Me] ( 5). X-Ray crystallographic studies show that the asymmetric unit of 5 consists of an open, dinuclear LiCZnC arrangement rather than a closed cyclic one, in which the HMDS ligand unusually occupies a terminal position on Zn. DFT computational studies showed that the structure found for 5 was energetically preferred to the expected HMDS-bridging isomer due to the steric hindrance imposed by the tridentate PMDETA ligand. The reaction of 4 with the neutral nitrogen donors 4-tert-butylpyridine and tert-butylcyanide afforded the homometallic compounds (tBu-pyr)Li(HMDS)] ( 6) and (tBuCN)Li(HMDS)] ( 7) respectively as a result of disproportionation reactions. Compounds 6 and 7 were characterized by NMR (1H, 13C and 7Li) spectroscopy.