THE MOLECULAR-STRUCTURES OF TETRA(AMINO)SILANES

In a search for crystalline homoleptic tetra(amino)silanes with (mirro r)-symmetrical NR2 substituents, tetrakis(1-pyrrolidinyl)silane (1) wa s prepared by an improved- synthetic route, and tetrakis(morpholino)si lane (2) was synthesized from SiBr4 and morpholine/Li-morpholide via t he intermediate BrSi[N(CH2CH2)(2)O](3) (4). With SiCl4 only the monoch loro 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 str uctures of 1, 2 and 5 were determined by single-crystal X-ray diffract ion. While compound 1 has; an irregular molecular geometry following n o symmetrical pattern, molecules of 2 and 5 have similar structures wi th almost superimposable Si(NC2)(4) cores of approximate D-2d symmetry . The overall molecular symmetry of 2 is reduced only by incongruous f olding of the heterocycles, which are all in a chair conformation. The core structures of 2 and 5 are in excellent agreement with the result s of density function calculations for Si(NH2)(4). Si(NMe2)(4) (the cr ystal structure of which could not be determined) was converted into ( Me2N)(3)SiOSO2CF3 (7) by treatment with two equivalents of triflic aci d. Reaction of 7 with 1-naphthylamine/triethylamine affords (Me2N)(3)S iNHC9H7 (8). With 2,4-difluroaniline/NEt3 the analogous derivative (Me 2N)(3)SiNHC6H3F2 (9) is obtained, but with Li-bromoaniline the product is a silane with the ligands redistributed to give (Me2N)(2)Si(NHC6H4 Br)(2) (10), the structure of which was also determined. It has a non- standard geometry with very flat nitrogen pyramids at all four substit uents.