Sequential Barium‐Catalysed N−H/H−Si Dehydrogenative Cross‐Couplings: Cyclodisilazanes versus Linear Oligosilazanes

Starting from Ph₃SiH, the barium precatalyst Ba[CH(SiMe₃)₂]₂?(THF)₃ was used to produce the disilazane Ph₃SiN(Bn)SiPh₂NHBn ( 4 ) by sequential N?H/H?Si dehydrogenative couplings with BnNH₂ and Ph₂SiH₂. Substrate scope was extended to other amines and hydrosilanes. This smooth protocol gives quantitative yields and full chemoselectivity. Compound 4 and the intermediates Ph₃SiNHBn and Ph₃SiN(Bn)SiHPh₂ were structurally characterised. Further attempts at chain extension by dehydrocoupling of Ph₂SiH₂ with 4 instead resulted in cyclisation of this compound, forming the cyclodisilazane c‐(Ph₂Si‐NBn)₂ ( 5 ) which was crystallographically authenticated. The ring‐closure mechanism leading to 5 upon release of C₆H₆ was determined by complementary experimental and theoretical (DFT) investigations. Ba[CH(SiMe₃)₂]₂?(THF)₃ and 4 react to afford the reactive Ba{N(Bn)SiPh₂N(Bn)SiPh₃}₂, which was characterised in situ by NMR spectroscopy. Next, in a stepwise process, intramolecular nucleophilic attack of the metal‐bound amide on the terminal silicon atom generates a five‐coordinate silicate. It is followed by turnover‐limiting β‐C₆H₅ transfer to barium; this releases 5 and forms a transient [Ba]?Ph species, which undergoes aminolysis to regenerate [Ba]?N(Bn)SiPh₂N(Bn)SiPh₃. DFT computations reveal that the irreversible production of 5 through such a stepwise ring‐closure mechanism is much more kinetically facile (ΔG^≠=26.2 kcal mol^?1) than an alternative σ‐metathesis pathway (ΔG^≠=48.2 kcal mol^?1).     

Aminoderivate hydrierter Oligosilane: Darstellung,Charakterisierung und Eigenschaften

Summary Amino derivatives of linear and branched tri- und tetrasilanes R₂N-H₂Si(SiH₂)SiH₂-NR₂ H₃SiSiHNR₂SiHNR₂SiH₃, R₂N-H₂SiSiH₂SiH₂SiH₂-NR₂ und (R₂N-H₂Si)₂SiHSiH(SiH₂-NR₂)₂ with R = Et, SiMe₃ are formed by the reaction of the corresponding bromooligosilanes with suitable amines or alkali metal amides. Product distribution and yields are strongly influenced by the nucleophilicity of the amino reagent and by the structure of the SiSi-backbone. The structures proposed for the aminopolysilanes thus prepared are proved by²⁹Si-,¹H-NMR-and MS-investigations.

     

Vacuum‐UV Irradiation‐Based Formation of Methyl‐Si‐O‐Si Networks from Poly(1,1‐Dimethylsilazane‐co‐1‐methylsilazane)

The vacuum‐UV (VUV)‐induced conversion of commercially available poly(1,1‐dimethylsilazane‐co‐1‐methylsilazane) into methyl‐Si‐O‐Si networks was studied using UV sources at wavelengths around 172, 185, and 222 nm, respectively. Time‐of‐flight secondary ion mass spectroscopy (TOF‐SIMS), X‐ray photo electron spectroscopy (XPS), and Fourier transform infrared (FTIR) measurements, as well as kinetic investigations, were carried out to elucidate the degradation process. First‐order kinetics were found for the photolytically induced decomposition of the Si? NH‐Si network, the subsequent formation of the methyl‐Si‐O‐Si network and the concomitant degradation of the Si? CH₃ bond, which were additionally independent of the photon energy above a threshold of about 5.5 eV (225 nm). The kinetics of these processes were, however, dependent on the dose actually absorbed by the layer and, in the case of Si‐O‐Si formation, additionally on the oxygen concentration. The release of ammonia and methane accompanied the conversion process. Quantum‐chemical calculations on methyl substituted cyclotetrasilazanes as model compounds substantiate the suggested reaction scheme. Layers <100 nm in thickness based on mixtures of poly(1,1‐dimethylsilazane‐co‐1‐methylsilazane) and perhydropolysilazane (PHPS) were coated onto polyethylene terephthalate (PET) foils by a continuous roll to roll process and cured by VUV irradiation by using wavelengths <200 nm and investigated for their O₂ and water vapor‐barrier properties. It was found that the resulting layers displayed oxygen and water vapor transmission rates (OTR and WVTR, respectively) of <1 cm³ m^?2 d^?1 bar^?1 and <4 g m^?2 d^?1, respectively.     

Synthesis and Characterization of Silazane-Based Polymers as Precursors for Ceramic Matrix Composites

The goal of this investigation was to optimize the synthesis of silazane-based polymers for processing fibre-reinforced ceramic matrix composites (CMCs). Liquid oligomeric silazanes were synthesized by ammonolysis of chlorosilanes and characterized spectroscopi- cally (FTIR, NMR) as well as by elemental analysis. The silazanes were obtained in high yield and purity. Different functional groups (system S1: Si—H, Si—CH₃, Si—CH=CH₂) and different degrees of branching in the Si—N backbone [system S2; Si(NH)₃, Si(NH)₂] were realized in order to study the properties of the silazanes that are dependent on the molecular structure. For processing ceramics via pyrolysis of pre-ceramic oligomers, molecular weight, rheological behaviour, thermosetting and ceramic yield were investigated systematically and correlated with the molecular structure of the silazanes. Low molecular weights (500–1000 g mol⁻¹) as well as low viscosity values (0.1–20 Pa s) enable processing of the silazanes in the liquid phase without any solvent. Due to the latent reactivity of the functional groups, curing of the polymers via hydrosilylation is achieved. Structural changes and weight loss during polymer curing as well as the organic/inorganic transition were monitored by FTIR spectroscopy and differential thermogravimetric analysis. With increasing temperature (room temperature to 800 °C) the hydrogen content decreases from 7 to < 0.5 wt% due to the formation of gaseous molecules (NH₃, CH₄, H₂). High ceramic yields up to 80% were reached by branching the oligomers, thus reducing the amount of volatile precursor fragments. Up to 1300 °C, ceramic materials remained amorphous to X-rays. At higher temperatures (1400–1800 °C) either SiC or SiC/Si₃N₄ composites were selectively crystallized, depending on the pyrolysis conditions. The utility of the optimized precursors for CMCs has been demonstrated by infiltration of fibre preforms and subsequent pyrolysis. © 1997 by John Wiley & Sons, Ltd.