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

Armstrong, D.R. and Herd, E. and Graham, D.V. and Hevia, E. and Kennedy, A.R. and Clegg, W. and Russo, L. (2008) Synthesis and structural elucidation of solvent-free and solvated lithium dimethyl (HMDS) zincates. Dalton Transactions (10). pp. 1323-1330. ISSN 1472-7773

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Abstract

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(µ-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. (Abstract from: http://www.swetswise.com/eAccess/viewAbstract.do?articleID=35973847)