Low-temperature sol-gel transformation of methyl silicon precursors to silica-based hybrid materials

Six new methyl silicon (IV) precursors of the type [MeSi{ON=C(R)Ar}3] [when R = Me, Ar = 2-C5H4N (1), 2-C4H3O (2) or 2-C4H3S (3); and when R = H, Ar = 2-C5H4N (4), 2-C4H3O (5) or 2-C4H3S (6)] were prepared and structurally characterized by various spectroscopic techniques. Molecular weight measurements and FAB (Fast Atomic Bombardment) mass spectral studies indicated their monomeric nature.1H and13C{1H} NMR spectral studies suggested the oximate ligands to be monodentate in solution, which was confirmed by29Si{1H} NMR signals in the region expected for tetra-coordinated methylsilicon (IV) derivatives. Thermogravimetric analysis of 1 revealed the complex to be thermally labile, decomposing to a hybrid material of definite composition. Two representative compounds (2 and 4) were studied as single source molecular precursor for low-temperature transformation to silica-based hybrid materials using sol-gel technique. Formation of homogenous methyl-bonded silica materials (MeSiO3/2) at low sintering temperature was observed. The thermogravimetric analysis of the methylsilica material indicated that silicon-methyl bond is thermally stable up to a temperature of 400 °C. Reaction of 2 and Al(OPri)3in equimolar ratio in anhydrous toluene yielded a brown-colored viscous liquid of the composition [MeSi{ON=C(CH3)C4H3O}3.Al(OPri)3]. Spectroscopic techniques1H,13C{1H},27Al{1H} and29Si{1H} NMR spectra of the viscous product indicated the presence of tetracoordination around both silicon and aluminum atoms. On hydrolysis it yielded methylated aluminosilicate material with high specific surface area (464 m2/g). Scanning electron micrography confirmed a regular porous structure with porosity in the nanometric range.