The molecular structures of tetra(amino)silanes

In a search for crystalline homoleptic tetra(amino)silanes with (mirror)-symmetrical NR2 substituents, tetrakis(1-pyrrolidinyl)silane (1) was prepared by an improved- synthetic route, and tetrakis(morpholino)silane (2) was synthesized from SiBr4 and morpholine/Li-morpholide via the intermediate BrSi[N(CH2CH2)(2)O](3) (4). With SiCl4 only the monochloro stage, 3, is reached. Tetrakis(4-methyl-1-piperazinyl)silane (5) was prepared similarly, using 4-methylpiperazine. With (CH2)(6)NH or (CH2)(6)NLi and SiBr4 only [(CH2)(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(NC2)(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(NH2)(4). Si(NMe2)(4) (the crystal structure of which could not be determined) was converted into (Me2N)(3)SiOSO2CF3 (7) by treatment with two equivalents of triflic acid. Reaction of 7 with 1-naphthylamine/triethylamine affords (Me2N)(3)SiNHC9H7 (8). With 2,4-difluroaniline/NEt3 the analogous derivative (Me2N)(3)SiNHC6H3F2 (9) is obtained, but with Li-bromoaniline the product is a silane with the ligands redistributed to give (Me2N)(2)Si(NHC6H4Br)(2) (10), the structure of which was also determined. It has a non-standard geometry with very flat nitrogen pyramids at all four substituents.