Double insertion of carbon dioxide into the Si−N bonds of diaminosilanes of the type Me 2 Si(NRR′) 2 gives di(carbamoyloxy)silanes Me 2 Si[(OCO)NRR′] 2 . The reactions proceed exothermically and quantitatively in most cases. A comprehensive analysis of the CO 2insertion products including single-crystal X-ray structure analyses was carried out. Quantum chemical calculations indicate an activation energy of about 124 kJ/mol for both the first and the second insertion and support the exothermal nature of the reaction. Investigation of the thermal decomposition of the di(carbamoyloxy)silanes Me 2 Si[(OCO)NRR′] 2 reveals the formation of oligo-and polysiloxanes.Depending on the thermolysis parameters, isocyanates, amines, and/or ureas are formed in addition to the siloxanes. Various methods were applied to study the decomposition process and to identify and quantify the products, including thermal analyses, mass spectrometry, and FTIR and NMR (solution and solid-state) spectroscopy. The overall reaction scheme provides a novel route to polysiloxanes which uses carbon dioxide as an oxygen source.
Large amounts of chlorosilanes, especially SiCl4 and CH3SiCl3, are produced as side-products of the industrial fabrication of solar or electronic grade silicon and the Müller-Rochow process. It was a goal of the present study to transform these compounds into useful chlorine-free precursors for Si/(C)/N ceramics via a sol-gel analogous liquid processing route. Chlorine substitution of the chlorosilanes (mixtures) with diethylamine did not yield chlorine-free products, complete reactions are only possible with lithium diethylamide. However, aminolyses with n-propylamine were successful. Transamination with ammonia was not possible with diethylaminosilanes but was with n-propylaminosilanes in various solvents. This result was attributed to steric reasons and polar interactions of the N-H groups. Colourless solid or liquid polysilazanes were obtained, depending on the silane (mixture) and the solvent. Transamination reactions of CH3Si(NH-n-Pr)3 in chloroform reproducibly yielded a cage-like oligosilazane of the composition (CH3)9Si9(NH)12N. Single crystal X-ray structure analysis revealed a seven-cyclic cluster containing four six- and three ten-membered silazane rings. This unique silazane cage as well as the other aminosilanes and the silazanes were comprehensively characterised using multi-nuclear solid state and solution NMR, elemental analyses and thermal gravimetry (TGA).
Diaminosilanes (CH 3 ) 2 Si(NHR) 2 1a, b are obtained from dichlorodimethylsilane and primary amines (R = n-hexyl, iso-propyl) in good yields. Bis-insertion of CO 2 into the Si-N bonds of the aminosilanes quantitatively gives the silylcarbamates (CH 3 ) 2 Si(OCONHR) 2 , 2a, b. Oligo-and polydimethylsiloxanes 3a, b as well as N,N -substituted ureas CO(NHR) 2 4a, b are formed upon heating the silylcarbamates 2a, b to ∼ 150 • C. The results of comprehensive NMR analyses of the aminosilanes, the two novel bis-silylcarbamates, the siloxanes and the ureas, and of single-crystal structure analyses of 2a and 4b are presented. In the crystal the n-hexyl silylcarbamate 2a shows a similar molecular packing arrangement as the iso-propyl urea derivative 4b.
Keywords: Silicon / Chelates / Density-functional calculations / HypercoordinationInsertion of phenyl isocyanate into the Si-N bond of N-(trimethylsilyl)diethylamine yields the NЈ-silylated N,N-diethyl-NЈ-phenylurea 1, which undergoes transsilylation with SiCl 4 to yield the C 3 -symmetric cationic hexacoordinate silicon complex 3s + [tris-κ-O,NЈ-(N,N-diethyl-NЈ-phenylureato)siliconium] as chloride salt, which was characterized crystallographically. The cationic complex 3s + exhibits a distorted trigonal antiprismatic coordination sphere about the silicon atom with fac arrangement of the three N-atoms (and the three O-atoms) relative to one another. This C 3 -symmetric
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