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’ SiN_xC_y (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.