The molecular structures of triaminosilanes

The structural chemistry of a series of triaminosilanes XSi(NR2)(3) has been investigated in order to determine the ground-state configuration and conformation of these molecules. Theoretical studies and analogy 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 small energy differences between them. The molecular structure of HSi(NMe2)(3) (1) has been determined by a single-crystal X-ray diffraction study at low temperature. Surprisingly, the molecules have a non-standard 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 corresponding chlorosilanes and morpholine (2, 3) or lithium morpholide (4, 5). Their variable-temperature NMR spectra were studied and the molecular structures determined by single crystal X-ray diffraction. Compound 2 has a molecular geometry approaching mirror symmetry with one morpholino group significantly different from the other two. This structure resembles that of isoelectronic tris(amino)phosphines. Compounds 3-5 have structures with a propeller-like arrangement of the three morpholino 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 morpholino rings adopt a chair conformation. According to the solution NMR spectra the ring inversion is slow at low temperature, but the rings rotate freely about the Si-N bonds on the NMR time scale. The molecular structures are discussed in terms of steric and electronic effects of the 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 not be crystallized, but were characterized by their analytical and spectroscopic data.