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Lithium Coordination in Chelating Silazanes of the General Formula [X−Me2Si−N−SiMe2−X]2Li2

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journal contribution
posted on 19.05.1998, 00:00 by Michael Veith, Astrid Koban, Kira Fries, Patrick Spaniol, Ralf Elsässer, Andreas Rammo, Volker Huch, Ulrich Kleinsteuber
New derivatives of hexamethyldisilazanelithium of the general formula [X−Me2Si−N−SiMe2−X]2Li2 (X = Ph (2), C4H3S (3), NMe2 (4), NEt2 (5), N(H)iPr (6), OPh (7), OSiMe3 (8), C4H3O (9)) have been synthesized and characterized by spectroscopic means. All compounds except 3 have been subjected to X-ray structure determinations which reveal a common polycyclic arrangement with a central Li2N2 four-membered ring to which four similar LiNSiY rings are annealed along a common Li−N edge (Y can either be a carbon atom of a π-system (2, 3), nitrogen (46) or oxygen (79)). The common four-membered polycyclic skeleton Li2N2Si4Y4 has a point symmetry of approximately D2 (222) of which only C2 (2) symmetry is retained in the crystals of 4, 5, 6, and 9, whereas all other derivatives have point symmetry C1 (1). One of the compounds crystallizes in one enantiomeric form (4) in an acentric structure. All other compounds crystallize in centrosymmetric structures with the two enantiomers present in the crystal. The lithium atoms in 29 are present in a distorted tetrahedral environment constituted by two nitrogen and two Y atoms. From molecular mass determinations, the compounds seem to retain their dimeric nature in benzene, the NMR patterns being nevertheless more simple than expected from the crystal structures and indicate a dynamic behavior in solution. None of these compounds, so far, shows lithium motion in the solid state up to room temperature, although phase transitions seem to occur in compound 8 at higher temperatures (13C SPE/MAS NMR evidence). Li−N distances in the central Li2N2 ring depend on the nature of donor groups Y:  short Li−N bonds (2.024 Å) are found for the lithium atoms coordinated by organic π-systems together with relatively long Li−C bonds (2.53 Å in 2), whereas longer Li−N bonds (2.07−2.085 Å) are encountered for the nitrogen donors with short Li−N “donor” bonds (2.157 (4), 2.163 (5), 2.121 Å (6)). If the donor atom (Y) is oxygen, the Li−O bonds can be either shorter than the Li−N bonds (7, Li−N 2.073, Li−O 1.978 Å; 9, Li−N 2.076, Li−O 1.977 Å) or slightly longer (8, Li−N 2.021, Li−O 2.077 Å). It is remarkable that in the trimethylsilyloxy case 8 the Li−N distances are not equal within their standard deviations as observed in the other cases:  two distances (average 1.96 Å) on opposite sides of the Li2N2 ring are much shorter than the remaining two (average 2.08 Å).