LITHIUM COORDINATION IN CHELATING SILAZANES OF THE GENERAL FORMULA [X-ME2SI-N-SIME2-X](2)LI-2

New derivatives of hexamethyldisilazanelithium of the general formula [X-Me2Si-N-SiMe2-X](2)Li-2 (X = Ph (2), C4H3S (3), NMe2 (4), NEt2 (5), N(H)Pr-i (6), OPh (7), OSiMe3 (8), C4H3O (9)) have been synthesized a nd characterized by spectroscopic means. All compounds except 3 have b een subjected to X-ray structure determinations which reveal a common polycyclic arrangement with a central Li2N2 four-membered ring to whic h four similar LiNSiY rings are annealed along a common Li-N edge (Y c an either be a carbon atom of a pi-system (2, 3), nitrogen (4-6) or ox ygen (7-9)). The common four-membered polycyclic skeleton Li2N2Si4Y4 h as a point symmetry of approximately D-2 (222) of which only C-2(2) sy mmetry is retained in the crystals of 4, 5, 6, and 9, whereas all othe r derivatives have point symmetry C-1 (1). One of the compounds crysta llizes in one enantiomeric form (4) in an acentric structure. All othe r compounds crystallize in centrosymmetric structures with the two ena ntiomers present in the crystal. The lithium atoms in 2-9 are present- in a distorted tetrahedral environment constituted by two nitrogen and two Y atoms. From molecular mass determinations, the compounds seem t o retain their dimeric nature in benzene, the NMR patterns being never theless more simple than expected from the crystal structures and indi cate a dynamic behavior in solution. None of these compounds, so far, shows lithium motion in the solid state up to room temperature, althou gh phase transitions seem to occur in compound 8 at higher temperature s (C-13 SPE/MAS NMR evidence). Li-N distances in the central Li2N2 rin g depend on the nature of donor groups Y: short Li-N bonds (2.024 Angs trom) are found for the lithium atoms coordinated by organic ct-system s together with relatively long Li-C bonds (2.53 Angstrom in 2), where as longer Li-N bonds (2.07-2.085 Angstrom) are encountered for the nit rogen donors with short Li-N ''donor'' bonds (2.157 (4), 2.163 (5), 2. 121 Angstrom (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 Angs trom; 9, Li-N 2.076, Li-O 1.977 Angstrom) or slightly longer (8, Li-N 2.021, Li-O 2.077 Angstrom). It is remarkable that in the trimethylsil yloxy case 8 the Li-N distances are not equal within their standard de viations as observed in the other cases: two distances (average 1.96 A ngstrom) on opposite sides of the Li2N2 ring are much shorter than the remaining two (average 2.08 Angstrom).