The Molecular Structures of Tetra(amino)silanes

In a search for crystalline homoleptic tetra(amin)silanes with (mirror)‐symmetrical NR 2 substituents, tetrakis( 1‐pyrrolidinyl)silane (1) was prepared by an improved synthetic route, and tetrakis(morpholino)silane (2) was synthesized from SiBr 4 and morpholine/Li‐morpholide via the intermediate BrSi[N(CH 2 CH 2 ) 2 O] 3 (4) . With SiCl 4 only the monochloro stage, 3 , is reached. Tetrakis(4‐methyl‐1‐piperazinyl)silane (5) was prepared similarly, using 4‐methylpiperazine. With (CH 2 ) 6 NH or (CH 2 ) 6 NLi and SiBr 4 only [(CH 2 ) 6 N] 3 SiBr (6) was obtained. The structures of 1 , 2 and 5 were determined by single‐crystal X‐ray diffraction. While compound 1 has an irregular molecular geometry following no symmetrical pattern, molecules of 2 and 5 have similar structures with almost superimposable Si(NC 2 ) 4 cores of approximate D 2d symmetry. The overall molecular symmetry of 2 is reduced only by incongruous folding of the heterocycles, which are all in a chair conformation. The core structures of 2 and 5 are in excellent agreement with the results of density function calculations for Si(NH 2 ) 4 . Si(NMe 2 ) 4 (the crystal structure of which could not be determined) was converted into (Me 2 N) 3 SiO‐SO 2 CF 3 (7) by treatment with two equivalents of triflic acid. Reaction of 7 with 1‐naphthylamine/triethylamine affords (Me 2 N) 3 SiNHC 9 H 7 (8) . With 2,4‐difluoroaniline/NEt 3 the analogous derivative (Me 2 N) 3 SiNHC 6 H 3 F 2 (9) is obtained, but with 4‐bromoaniline the product is a silane with the ligands redistributed to give (Me 2 N 2 ),Si(NHC 6 H 4 Br), (10) , the structure of which was also determined. It has a non‐standard geometry with very flat nitrogen pyramids at all four substituents.