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 reveals 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 unprecedented. Ab initio and semiempirical methods (MNDO and PM3) show tetrameric aggregates, (LiCl)(4), to be favored over trimers and dimers. Among the tetramers, a distorted cube constitutes the 1 most stable arrangement. MNDO calculations find the potential energy surface for the different TMEDA-solvated tetrameric aggregates to be extremely shallow. In contrast, with the recently developed Li/PM3 parametrization, the solvated tetramers are distinct energetically. The Li NMR shifts computed by the IGLO method for several LiCl oligomers differ only slightly. Hence, Li-7 NMR spectroscopy could not detect an equilibrium between different species, if they were present in solution.