SIO2 AS A STARTING MATERIAL FOR THE SYNTHESIS OF PENTACOORDINATE SILICON COMPLEXES .1.

SiO2 as silica gel, fused silica, or quartzite will react readily in ethylene glycol (EG) with 1 equiv of alkali (M) hydroxide (except Na) to produce, in essentially quantitative yield, monomeric pentacoordinate glycolato silicates [M(OCH2CH2O)2SiOCH2CH2OH] containing two bidentate glycol ligands and one monodentate ligand. On heating, one EG is lost per two monomer units forming dimeric species, [M2Si2(OCH2CH2O)5], or polymers, [MSi(OCH2CH2O)2.5]x. The Na derivative precipitates out of solution as the dimer. In experiments run with fused silica, the dissolution process exhibits a first-order dependence on base concentration and silica surface area. The E(a) for silica dissolution is 14 +/- 2 kcal/mol with DELTAH(double dagger) almost-equal-to 11 kcal/mol and DELTAS(double dagger) almost-equal-to - 44 cal/mol K. In the absence of base, the silica surface reacts with EG to form an alkoxy-modified surface as determined by diffuse reflectance FTIR spectroscopy (DRIFTS). In the presence of base, only hydroxyl groups are seen on the silica surface. A mechanism for dissolution is proposed based on these observations. The dissolution process appears to be relevant to the nonaqueous synthesis of zeolites, especially silica-sodalite (ZSM-5), which are prepared under very similar conditions. The monomeric, anionic glycolato silicates exhibit trigonal-bipyramidal geometry, with no apparent contact interactions between the anionic framework and the alkali metal counterions. The monomers, dimers and polymers are characterized by chemical analysis, X-ray powder diffractometry, FTIR, solution and solid-state MAS Si-29 NMR, and thermal gravimetric analysis. The monomers dissolve readily in methanol but appear to do so via a ligand-exchange process wherein some of the glycolato ligands are displaced by methoxy ligands. On heating, the dimers/polymers decompose to phase-pure alkali silicates.