Cl2MeSi–NH–BCl2 and ClMe2Si–NH–BCl2: novel processable single source precursors of amorphous Si/C/B/N ceramics

Abstract

The synthesis and polymerization of Cl2MeSi–NH–BCl2 (MADB) and ClMe2Si–NH–BCl2 (DADB) is reported. MADB and DADB are obtained in two-step reaction sequences from MeSiCl3, Me3Si–NH–SiMe3 (HMDS), and BCl3 (→ MADB) or Me2SiCl2, HMDS and BCl3 (→ DADB). The monomeric single source precursors are structurally characterized by means of NMR and IR spectroscopy and mass spectrometry. The molecular structure of DADB is additionally investigated by single crystal X-ray diffraction. It crystallizes orthorhombically in space groupPbca with a = 1334.4 pm, b = 1375.4 pm, c = 1607.8 pm, and Z = 8. In the solid state two molecules are associated via intermolecular B–N coordination, forming planar B2N2 rings with almost identical B–N bond lengths. Polymerization of MADB and DADB is performed by ammonolysis and aminolysis (MeNH2) releasing MADB–NH, DADB–NH, MADB–NMe, and DADB–NMe as colorless solids (MADB–NH) or colorless viscous liquids. The viscosity increases with increasing cross-linking density from DADB–NMe to MADB–NMe to DADB–NH. Accordingly, ceramic yields decrease from 77% (MADB–NH) to 45% (DADB–NH) to below 30% DADB–NMe). As-obtained ceramics are fully amorphous. Their near-range order is investigated using solid state 11B, 13C, and 29Si NMR spectroscopy. All materials display BN3 coordination at boron whereas the bonding situation at silicon is different. Silicon atoms in DADB–NH have ‘mixed’ SiNxCy (x + y = 4) environments, whereas the 29Si NMR spectra of MADB–NMe and DADB–NMe suggest SiN4 coordination. High temperature TGA performed in a He atmosphere up to 2000 °C indicates that DADB–NH ceramics are most stable; the onset of decomposition is above 1900 °C. SiC and Si3N4 reflections in the XRD patterns of the heated samples indicate that partial crystallization occurs during the heat treatment at elevated temperature.