THE BICYCLIC STRUCTURE OF A NOVEL TMEDA-SOLVATED LITHIUM-CHLORIDE TETRAMER [(LICL)(4).3.5TMEDA](2) - X-RAY STRUCTURAL-ANALYSIS AND MO INVESTIGATIONS

Crystals of [(LiCl)(4).3.5TMEDA](2), 1, were obtained as an unexpected byproduct in two different lithiation reactions, X-ray analysis revea ls that 1 forms a pair of TMEDA-bridged tetrameric units (TMEDA N,N,N' ,N'-tetramethylethylenediamine) each with fused six- and four-membered rings and tetracoordinated lithiums. The tetrameric unit in 1 is unpr ecedented. Ab initio and semiempirical methods (MNDO and PM3) show tet rameric aggregates, (LiCl)(4), to be favored over trimers and dimers. Among the tetramers, a distorted cube constitutes the 1 most stable ar rangement. MNDO calculations find the potential energy surface for the different TMEDA-solvated tetrameric aggregates to be extremely shallo w. In contrast, with the recently developed Li/PM3 parametrization, th e solvated tetramers are distinct energetically. The Li NMR shifts com puted by the IGLO method for several LiCl oligomers differ only slight ly. Hence, Li-7 NMR spectroscopy could not detect an equilibrium betwe en different species, if they were present in solution.