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Abstract

This work dealt with the development of a mild chemical route for making silicon nitride and silicon diimide based materials, i.e. Si₃N₄ and Si(NH)₂. The strategy we used was to synthesise partially ammonolysed silicon amides, i.e. amino silylamines such as (Me₂N)₃SiNH₂ and (Morpholine)₃SiNH₂, in order to achieve a degree of control, on their further ammonolysis. In particular we found that while (Me₂N)₄Si and (Me₂N)₃SiNH₂ are inert towards ammonia, they react in the presence of acid catalysts affording Si(NH)₂ powders. However when the ammonolysis reaction is carried out on the cyclic trisilazane [(Me₂N)₂SiNH]₃, obtained by selective self-condensation of the silylamine (Me₂N)₃SiNH₂ with triflic acid as catalyst, a Si(NH)₂ gel is obtained. In analogy with the parent SiO₂ gels, the Si(NH)₂ gels have surface area up to 1000 m²/g and a pore size distribution comparable to that of mesoporous silica. This result represents the first example of high surface area non-oxide gel.
The special reactivity of the silylamine (Me₂N)₃SiNH₂ has been used to synthesise a new class of multielement single source precursors. By reaction with LiBu the silylamine has been deprotonated affording the corresponding lithium salt (Me₂N)₃SiNH₂ (structurally characterised), that by methatetic reaction with AlCl₃ afforded LiAl[NHSi(NMe₂)₃]₄ (structurally characterised). By reaction with metal amides, using the SiNH₂ functionality, a series of mixed amides has been synthesised: Ti[NHSi(NMe₂)₃]₄, [((Me₂N)₃SiNH)₂Ti(NSi(NMe₂)₃)₂]₂, (Me₂N)B[NHSi(NMe₂)₃]₂. By reaction with organometallic compounds like AlEt₃ and (MeAlO)_n, the corresponding substituted product Al[NHSi(NMe₂)₃]₃ and [(Me₂N)₃SiNHAlO]_n has been obtained.
Some of these single source precursors, i.e. Al[NHSi(NMe₂)₃]₃ and (Me₂N)B[NHSi(NMe₂)₃]₂, have been ammonolysed with acid catalyst to give mixed silicon diimide based high surface area gels, while the polyalumionosilazane (Me₂N)₃SiNHAlO]_n has been converted in a microcrystalline Sialon ceramic.