Azomethine derivatives. Part 20. Crystal and molecular structures of the lithioketimine [{Li(NCBut2)}6] and lithioguanidine [{Li[NC(NMe2)2]}6]; electron-deficient bridging of Li3 triangles by methyleneamino-nitrogen atoms

The lithioketimine Li(NCBut2)(1) and lithioguanidine Li[NC(NMe2)2](2) have remarkably similar hexameric structures [{Li(NCR2)}6](R = But or NMe2) in the crystal phase, based on slightly folded chair-shaped Li6 rings held together by triply-bridging methyleneamino-groups, NCR2, thus providing examples of electron-deficient bridging by the nitrogen atoms of organonitrogen ligands. The mean distance between adjacent metal atoms in the Li6 rings is 2.35(2)A in (1), and 2.445(2)A in (2), and the mean dihedral angles between Li6 chair seats and backs are 85 and 78° respectively. The nitrogen atoms of the bridging methyleneamino-groups are approximately equidistant from the three bridged metal atoms, the mean Li–N distance being 2.06(1)A in (1) and 2.00(1)A in (2). The NC distances of 1.30(1) and 1.244(3)A respectively lie within the range expected for carbon–nitrogen double bonds. Features of these structures are compared with those of related compounds, and some bonding implications are discussed.