Lithium Coordination in Chelating Silazanes of the General Formula [X−Me<sub>2</sub>Si−N−SiMe<sub>2</sub>−X]<sub>2</sub>Li<sub>2</sub><sup>†</sup> VeithMichael KobanAstrid FriesKira SpaniolPatrick ElsässerRalf RammoAndreas HuchVolker KleinsteuberUlrich 1998 New derivatives of hexamethyldisilazanelithium of the general formula [X−Me<sub>2</sub>Si−N−SiMe<sub>2</sub>−X]<sub>2</sub>Li<sub>2</sub> (X = Ph (<b>2</b>), C<sub>4</sub>H<sub>3</sub>S (<b>3</b>), NMe<sub>2</sub> (<b>4</b>), NEt<sub>2</sub> (<b>5</b>), N(H)<sup>i</sup>Pr (<b>6</b>), OPh (<b>7</b>), OSiMe<sub>3</sub> (<b>8</b>), C<sub>4</sub>H<sub>3</sub>O (<b>9</b>)) have been synthesized and characterized by spectroscopic means. All compounds except <b>3</b> have been subjected to X-ray structure determinations which reveal a common polycyclic arrangement with a central Li<sub>2</sub>N<sub>2</sub> 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 (<b>2</b>, <b>3</b>), nitrogen (<b>4</b>−<b>6</b>) or oxygen (<b>7</b>−<b>9</b>)). The common four-membered polycyclic skeleton Li<sub>2</sub>N<sub>2</sub>Si<sub>4</sub>Y<sub>4</sub> has a point symmetry of approximately <i>D</i><sub>2</sub> (222) of which only <i>C</i><sub>2</sub> (2) symmetry is retained in the crystals of <b>4</b>, <b>5</b>, <b>6</b>, and <b>9</b>, whereas all other derivatives have point symmetry <i>C</i><sub>1</sub> (1). One of the compounds crystallizes in one enantiomeric form (<b>4</b>) in an acentric structure. All other compounds crystallize in centrosymmetric structures with the two enantiomers present in the crystal. The lithium atoms in <b>2</b>−<b>9</b> 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 <b>8</b> at higher temperatures (<sup>13</sup>C SPE/MAS NMR evidence). Li−N distances in the central Li<sub>2</sub>N<sub>2</sub> 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 <b>2</b>), whereas longer Li−N bonds (2.07−2.085 Å) are encountered for the nitrogen donors with short Li−N “donor” bonds (2.157 (<b>4</b>), 2.163 (<b>5</b>), 2.121 Å (<b>6</b>)). If the donor atom (Y) is oxygen, the Li−O bonds can be either shorter than the Li−N bonds (<b>7</b>, Li−N 2.073, Li−O 1.978 Å; <b>9</b>, Li−N 2.076, Li−O 1.977 Å) or slightly longer (<b>8</b>, Li−N 2.021, Li−O 2.077 Å). It is remarkable that in the trimethylsilyloxy case <b>8</b> 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 Li<sub>2</sub>N<sub>2</sub> ring are much shorter than the remaining two (average 2.08 Å).