THE MOLECULAR-STRUCTURES OF TRIAMINOSILANES

The structural chemistry of a series of triaminosilanes XSi(NR2)(3) ha s been investigated in order to determine the ground-state configurati on and conformation of these molecules. Theoretical studies and analog y with the isoelectronic XP(NR2)(3) (X = CH2, O, Se, NSiMe3 etc.) and P(NR2)(3) molecules suggest structures of C-s or C-3 symmetry, with sm all energy differences between them. The molecular structure of HSi(NM e2)(3) (1) has been determined by a single-crystal X-ray diffraction s tudy at low temperature. Surprisingly, the molecules have a non-standa rd conformation with an irregular orientation of the three NMe2 groups . The hydride-, chloro-, methyl-, and vinyltris(morpholino)silanes XSi [N(CH2CH2)(2)O](3) (X = H, Cl, Me, Vi; 2-5) were prepared from the cor responding chlorosilanes and morpholine (2, 3) or lithium morpholide ( 4, 5). Their variable-temperature NMR spectra were studied and the mol ecular structures determined by single crystal X-ray diffraction. Comp ound 2 has a molecular geometry approaching mirror symmetry with one m orpholino group significantly different from the other two. This struc ture resembles that of isoelectronic tris(amino)phosphines. Compounds 3-5 have structures with a propeller-like arrangement of the three mor pholino rings approaching C-3 symmetry for this part of the molecules. The chore compound 3 shows the shortest Si-N bond lengths (average 1. 690 Angstrom), followed by 2 (1.708 Angstrom), 5 (1.709 Angstrom) and 4 (1.712 Angstrom). The configuration of all nitrogen atoms is almost planar, with sums of the angles close to 360 degrees, and the morpholi no rings adopt a chair conformation. According to the solution NMR spe ctra the ring inversion is slow at low temperature, but the rings rota te freely about the Si-N bonds on the NMR time scale. The molecular st ructures are discussed in terms of steric and electronic effects of th e substituents. Other model compounds, including (Me2N)(3)SiC6H4-4-Br (6), MeSi[N(CH2CH2)(2)NMe](3) (7) and PhSi[N(CH2)(4)](3) (8), could no t be crystallized, but were characterized by their analytical and spec troscopic data.