Molecular structures of some (dimethylamino)halogenosilanes in the gas phase by electron diffraction and the crystal and molecular structures of mono- and di-chloro(dimethylamino)silane by X-ray diffraction at low temperatures

Abstract

The (dimethylamino) halogenosilanes SiH₂X(NMe₂)(X = Cl, Br, or I) have monomeric structures in the gas phase, with the three bonds at nitrogen close to coplanarity, but not exactly so; SiHCl₂(NMe₂) is also monomeric, with the nitrogen apparently planar. The conformations about the Si–N bonds suggest that repulsions between the nitrogen lone pair and the halogen atom(s) on Si are important. Dichloro(dimethylamino)silane retains its monomeric structure in the crystal at 94 K, and the molecular parameters are close to those found for the gas phase; molecules are associated only very loosely through bridging Cl atoms. In strong contrast, the crystal structure of monochloro(dimethylamino)silane at 116 K shows it to consist of dimers in the solid state, in which two strongly-distorted monomer units are linked through their nitrogen and silicon atoms in a four-membered ring. The two bonds to nitrogen formed by each silicon are not equivalent [bond lengths 181.3(13) and 205.4(13) pm; cf. 168.7(2) pm in the gas-phase monomer], but the silicon atoms are clearly five-co-ordinate, with the five bonded groups defining a trigonal bipyramid; Cl and one of the two nitrogen atoms occupy apical positions, The nitrogen atoms are four-co-ordinate, with roughly tetrahedral bond angles. The structure is compared with that of dimethylaminosilane itself, which forms a cyclic pentamer in the crystal.