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

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(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.

Mixed Lithium Complexes: Structure and Applicationin Synthesis

“…These rings are linked via the zinc atoms by a non-chelating TMEDA ligand ( Figure 5.15) [58]. When PMDETA is employed in the reaction of LiHMDS and Me 2 Zn, a simple monomeric dinuclear zincate [(PMDETA) • LiZn(μ-Me)(HMDS)(Me)] 62 is isolated [61]. Owing to the tridentate nature of PMDETA, there is only one bridging ligand in the complex; however, perhaps counter-intuitively from an electronic perspective it is the alkyl group rather than the amido group that fulfills this role.…”

π-Spacer-coupled Diimidosulfinates

“…Up to now monomeric diimidosulfinates have only been known with a chelating donor base like TMEDA [31]. Otherwise only triimidosulfinates or the S(N t Bu) 4 2− anion have been known to crystallize as monomers when THF is the only donor base present [32,33].…”

“…In spite of these purification problems the metal exchange from lithium to aluminum (vide infra) worked and proved a posteriori that the reaction of the lithiated benzene with the sulfurdiimide was successful. 4 (Fig. 2) with half of the molecule in the asymmetric unit.…”

“…2 (color online). Crystal structure of [(THF) 4 Li 2 {(NSiMe 3 ) 2 S} 2 biphenyl] (1). Anisotropic displacement parameters are depicted at the 50 % probability level, and all hydrogen atoms have been omitted for clarity.…”

Lithium and Aluminum Anthracenyldiimidosulfinates

“…Since [(THF) 4 Li 2 {(NSiMe 3 ) 2 S} 2 biphenyl] also crystallizes as a monomer [20], it is anticipated that one reason for this uncommon structural motif is the aryl group connected to the sulfur atom. The biphenyl and the anthracene substituent both can form C-H··· π hydrogen bonds.…”