Sol–gel derived Si/C/O/N‐materials: molecular model compounds, xerogels and porous ceramics

Polymeric Si/C/O/N xerogels, with the idealized polymer network structure comprising [SiOSi(NCN)3]n moieties, were prepared by reactions of hexachlorodisiloxane (Cl3SiOSiCl3) with bis(trimethylsilyl)carbodiimide (Me3SiNCNSiMe3, BTSC). NMR and FTIR spectra indicate the existence of -NCN- andSiOSi- units in the xerogels and also in the ceramic materials obtained upon pyrolysis. The feasibility of this reaction protocol was confirmed on the molecular level by the deliberate synthesis of the macrocyclic compound [SiPh2OSiPh2(NCN)]2, the crystal structure and spectroscopic data of which are reported. The influence of pyridine as a catalyst for the cross-linking reaction was studied. The degree of cross-linking increased within the polymers with the addition of pyridine. It was shown by the reaction of hexachlorodisiloxane with excess pyridine that the latter appears to activate only one out of the two -SiCl3 moieties under formation of hexacoordinated silicon compounds. The crystal structure of Cl3SiOSiCl3(pyridine)2 is presented. Quantum chemical calculations are in support of this adduct being a potential intermediate in the pyridine catalyzed sol–gel process. The ceramic yield after pyrolysis of the Si/C/O/N-xerogels at 1000 °C, which reaches values up to 50%, was found to depend on the aging protocol (time, temperature), whereas no correlation was found with the amount of pyridine added for xerogel synthesis. The Si/C/N/O-ceramics obtained after pyrolysis at 1000 °C under NH3 are completely amorphous. Chemically they have to be considered as hybrids between an ideal [SiOSi(NCN)3]n network and glass-like Si2N2O. The products are mesoporous with closed pores and a broad pore size distribution. Copyright © 2011 John Wiley & Sons, Ltd.