In a search for crystalline homoleptic tetra(amin0)silanes with (mirror)-symmetrical NR2 substituents, tetrakis( 1 -pyrro1idinyl)silane (1) was prepared by an improved synthetic route, and tetrakis(morpho1ino)silane (2) was synthesized from SiBr4 and morpholine/Li-morpholide via the intermediate BrSi [N(CH,CH,),O], (4). With SiC14 only the monochloro stage, 3, is reached. Tetrakis(4-methyl-1-piperaziny1)silane ( 5 ) was prepared similarly, using 4-methylpiperazine. With (CH&NH or (CH,),NLi and SiBr4 only [(CI12)6N]3SiBr (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 tollowing no symmetrical pattern, molecules of 2 and 5 have similar structures with almost superimposable Si(NC& cores of approximate Dad 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(NMea)4 (the crystal structure of which could not be determined) was converted into (Me,N),SiO-S02CF3 (7) by treatment with two equivalents of triflic acid. Reaction of 7 with 1-naphthylamine/triethylamine affords (Me2N)3SiNHC9H7 ( 8 ) . With 2,4-difluoroaniline/NEt, the analogous derivative (Me2N)3SiNHC6H3F2 (9) is obtained, but with 4-bromoaniline the product is a silane with the ligands redistributed to give (MezN),Si(NHC6H4Br), (lo), the structure of which was also determined. It has a non-standard geometry with very flat nitrogen pyramids at all four substituents. TntroductionSilicon nitride, Si3N4, plays an important role in contemporary technology both as a refractory material and as a component of electronic devices"]. In all modifications of this binary compound the silicon atoms are tctracoordinatcd by four nitrogen atoms, and the nitrogen atoms are tricoordinated by silicon atoms[2d~h]. Such molecular compounds, in which silicon is in a homoleptic bonding situation with nitrogen ligands, are rare and their structural chemistry is poorly understood. A survey of a modern data base for species with an SiNS unit shows only a limited number of compounds that have actually been structurally characteri~ed[~-~'I. In many of these structures the SiN, units are part of cyclic or polycyclic systems and represent a very special bonding situation[''-"J. Detailed PEIIla] and TR[2'b,c] spectral data are available for several of the simple prototypes, but the results of these studies are not always conclusive regarding the structure of the compounds.During the course of our current investigations of small hydrogen-rich silicon compounds with siliconlcarbon LZ2] , silicon/nitrogen Fz3I, silicon/oxygen or mixed frameworks we have recently carried out a systematic study of the structural chemistry of tsi(amino)silane~ ['~]. This work has now been extended to tetra(amino)silanes, and a parallel study of tetra(amino)methanes is underway["] As a first part of...
Nitrogen planarity / SilanesThe structural chemistry of a series of triaminosilanes XSi(NR& has been investigated in order to determine the ground-state configuration and conformation of these molecules. Theoretical studies and analogy with the isoelectronic XP(NR& (X = CH2, 0, Se, NSiMe3 etc.) and P(NR2)3 molecules suggest structures of C, or C3 symmetry, with small energy differences between them. The molecular structure of HSi(NMe& (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 hydrido-, chloro-, methyl-, and vinyltris(morpholino)silanes XSi[N(CH,CH,),O], (X = H, C1, 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 , symmetry for this part of the molecules. The chloro compound 3 shows the shortest Si-N bond lengths (average 1.690 A), followedby2 (1.?08A),5 (1.709A) and4 (l.?12A).Theconfiguration of all nitrogen atoms is almost planar, with sums of the angles close to 360", and the rnorpholino 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)3SiC6114-4-Br (61, MeSi[N(CH2CH2)2NMe]3 (7) and PhSi[N(CH,),], ( 8 ) , could not be crystallized, but were characterized by their analytical and spectroscopic data.
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