Sol–gel analogous aminolysis–ammonolysis of chlorosilanes to chlorine-free Si/(C)/N–materials

Abstract

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).