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Rannabauer, Stefan (2004): Vom Silazan zum Nanokomposit. Dissertation, LMU München: Faculty of Chemistry and Pharmacy
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Abstract

Silazanes: Using HMDS and SiCl4 it was possible to synthesize and characterize several new silazanes. This could be done by direct reaction or with further reactive compounds. For the cyclic disilazane 2,2,4,4-tetrachloro-1,3-bis(trimethylsilyl)-[1,3,2,4]-diazadisiletidine it has been possible to proof the centric symmetry of the molecule in the solid state. Since the asymmetric unit was built up by one complete molecule and the tms-groups showed strong rotational disorder, this information was not accessible by X-ray diffraction analysis. A combination of calculations and experiments allowed to take a closer look at the conformation of the latter bis(trimethylsilylamino)­dichlorosilane. Although the molecule possesses only two NH-groups, three signals of different intensities pertaining to that bond were found in the IR-spectra. DFT calculations showed that these signals have to be related to different conformations of the molecule, which were unequivocally present in the solution. The condensation reaction of bis(trimethylsilylamino)dichlorosilane leads to poly-silicondiimide (sol-gel process) and, finally, to Si3N4. X-ray powder diffraction yields the following results: the tetrahedra Si(NH)4 or SiN4, appearing in amorphous poly-silicondiimide and the derived amorphous Si3N4, respectively, turned out to be partially edge-sharing. Si3N4 obtained from bis(trimethylsilylamino)dichlorosilane did not crystallize before reaching its decomposition temperature. Using bis(trimethylsilylamino)dichlorosilane and the corresponding secondary amine it has been possible to synthesize the substitution variants bis(trimethylsilylamino)-dialkylaminochlorosilane (alkyl = Me, Et, iPr) and bis(trimethylsilylamino)-bis(dialkylamino)silane (alkyl = Et). By using the dialkyltrimethylsilylamines, the appearance of precipitates could be suppressed. Further substitution variants of bis(trimethylsilylamino)­dichlorosilane could be obtained by further reaction with HMDS. The separation of the resulting silanes tris(trimethylsilylamino)chlorosilane and tetrakis(trimethylsilylamino)silane was found to be difficult due to the combination of thermal sensitiveness with a high boiling point. Separation from polymeric by-products was only possible using high vacuum. Reactions of metal containing silazanes: While handling the metal chlorides a crystalline oxonium salt could be isolated and its X-ray structure could be determined. In the crystal [Ti2Cl9]- showed up as a weakly coordinating anion. Therefor, the cation could be observed almost undisturbed. It consists of two molecules Et2O solvating a proton between them. The existence of the proton could be proven in solution by means of 1H NMR spectroscopy. The spectra of the 47Ti and 49Ti nuclei showed the anion to be persistent in solution. Reactions of the silazanes with TiCl4 in the presence of Et2NH did not yield the wanted titana­silazanes but ended up in the reduction of Ti(IV) and the formation of the salt [Et2NH2]+[(Et2NH)2TiCl4]-, which could be characterized using X-ray diffraction. However, the reaction of bis(trimethylsilylamino)­dichlorosilane with TiCl4 in non-polar or only weakly polar aprotic solvents quantitatively led to the crystalline titanosilazane [µ-ClTiCl2N(SiMe3)SiCl2NH2]2. This compound exhibits a planar Ti-N-Si-N ring as characteristic entity as well as Si, which is surrounded by two N and to Cl. The latter renders the compound a particularly suitable candidate for transformation into ternary silicon nitrides. In addition, while investigating the formation of [µ-ClTiCl2N(SiMe3)SiCl2NH2]2, an intermediate could be observed using NMR spectroscopy. After modification of the disilazane, and reaction of bis(trimethylsilylamino)­chlordiethylaminosilane with TiCl4, the salt [(Me3SiNH)2SiClNHEt2]+ [Et2NClSi(NSiMe3)2TiCl-µ-Cl3TiCl3] – could be isolated almost quantitatively. Because of the additional amino-group of the silazane, a reaction with TiCl4 could take place and the protons liberated during the reaction could be partially neutralized. The substance turned out to be unstable, and decomposed within several days. The products of that decomposition reaction could not be identified as yet. The change of the amino group resulted in a reaction of bis(trimethylsilylamino)­chlordimethylaminosilane with TiCl4. The main product is obtained as a yellow to orange powder and its structure could not be solved yet. However, a by-product of the reaction, the salt [Me2NH2]+ [TiCl6] –, could be characterized by X-ray diffraction. This suggests that the reaction of bis(trimethylsilylamino)­chlor­di­methyl­amino­silane with TiCl4 proceeds similar to the reaction of bis(trimethylsilylamino)­dichlorosilane with TiCl4. The salt [Me2NH2]+ [TiCl6] – was obtained also directly from Me2NH2Cl and TiCl4 as a powder. By elaborating this direct access, it could be seen that it may be possible to obtain further, yet unknown, crystalline phases by the reaction of Me2NH2Cl with TiCl4. Furthermore, reactions of the titanosilazane [µ-ClTiCl2N(SiMe3)SiCl2NH2]2 were a subject of interest. It could be shown that ammonolysis is a simple way to substitute the chlorine atoms bonded to Si. The use of secondary amines as reactants led to amorphous products, considered to be paramagnetic. The use of dialkyamino-trimethylsilylamines led to interesting reactions, and single crystalline products could be obtained. Pyrolysis: It was proven that the titanosilazane [µ-ClTiCl2N(SiMe3)SiCl2NH2]2 could be used to synthesize a nanocomposite consisting of nanocrystalline TiN and amorphous Si3N4. The elemental composition of the products showed a strong dependence on the reaction conditions of the pyrolysis. If using [µ-ClTiCl2N(SiMe3)SiCl2NH2]2, the products have been homogenous down to a few nanometers. A detailed investigation of the pyrolysis using temperature dependent X-ray powder diffraction gave no evidence for further crystalline phases. TG and MS investigations were assessed as a strong indication for a rearrangement of the titanosilazane molecule at about 120 °C. Performing the pyrolysis with ammonolized [µ-ClTiCl2N(SiMe3)SiCl2NH2]2, a less homo­genous distribution of the elements in the product resulted. The data from temperature dependent X-ray powder diffraction studies showed the existence of an additional crystalline phase besides NH4Cl and (NH4)2TiCl6 at about 250 °C.