Preparation and solid state structures of tetra(amino)silane organolithium compounds containing a Li8 core forming a...

Tris(dimethylamino)silyl substituted aniline HNPh{Si(NMe2)3} (1) was prepared in high yield by metathesis reaction of lithium anilide with tris(dimethylamino)chlorosilane. 1 reacts with 1 equiv of n-butyl lithium at elevated temperatures to yield the corresponding lithium salt [LiNPh{Si(NMe2)3}] (2). Treatment of 1 with 2 equiv of n-butyl lithium leads to dilithiated compounds [Li2N(C6H4){Si(NMe2)3}]4 (3a) or [Li2(THF)2N(C6H4){Si(NMe2)3}]2 (3b) depending on the solvent used for crystallization, the former being crystallized in heptane, the latter in tetrahydrofuran (THF). Single crystal analyses of 3a and 3b confirm the occurrence of two lithium atoms per ligand, where the anilino nitrogen atom and one ortho carbon atom of the phenyl group are deprotonated. In both cases organolithium compounds with polynuclear lithium cores are formed. The molecular structure of 3a represents the first example of an organolithium compound consisting of a Li8 core that forms a polyhedron in which two trigonal prisms share one square face and are twisted by 90 degrees. Such a polyhedron is known in mathematics as the Johnson solid J26 or as Gyrobifastigium and is rare in chemistry. The coordinating carbon atoms are situated above the other square faces with three close contacts and one slightly longer contact to four lithium atoms. Furthermore, the central four lithium atoms form an Li4N4 square with almost linear N-Li-N bonds, a rare structural situation. The molecular structure of 3b shows each lithium atom to be additionally coordinated by a molecule of THF compared to that of 3a. Thus, a dimeric structure is found with a Li4 core that forms a planar parallelogram. The carbon atoms are found in close contact to a triangular Li3 face on each side of the Li4 plane.