Vom Silazan zum Nanokomposit

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.