High-yield molecular borazine precursors for Si-B-N-C ceramics

The synthesis of Si-B-N-C ceramic materials can be accomplished via two different routes using B-tris(trichlorosilylvinyl)borazine (2) as starting material. This silyl-functionalized ethinylborazine is obtained by Pt-catalyzed hydrosilylation of B, B', B"-tri-ethynylborazine (1) with HSiCl3 in quantitative yield with a selectivity of 80% beta-substituted product. Amminolysis of 2 with methylamine leads to the soluble silazane polymer P3 which contains intact borazine rings connected by -CH=CH-Si-NMe linkages. In a second approach, the trichlorosilyl. groups of 2 are hydrogenated to yield the B-tris(hydrosilylvinyl)borazine (4). With polymer P3 or 4, a highly durable Si-B-N-C ceramic is obtained after pyrolysis under inert atmosphere. The composition SiBN1+xC2 of the ceramic material corresponds exactly to the backbone of the precursor molecules 2 or 4, and very compact materials are obtained in each case. The ceramic yield of approximately 94% starting from the silane precursor 4 sets a new standard for this type of ceramics using the pyrolysis of a single site molecular precursor. Conductivity measurements show a semiconductor behavior of the ceramic at about 10(2) (Qm)(-1) at room temperature. The composition of the ceramic was characterized by laser-ablation ICP-MS, which was used for that purpose for the first time. The very satisfying results demonstrate the high potential of this direct solid sampling technique.