Chiral molecular fluoridosilicates and their twin polymerization for the preparation of fluorine-doped mesoporous silica and microporous carbon

Abstract

The syntheses of chiral pentacoordinated fluoridosilicates and their conversion into fluorine-doped silica and mesoporous carbon are reported. Starting from 4H,4′H-2,2′-spirobi[benzo[d][1,3,2]dioxasiline] (1) and 8,8′-di-tert-butyl-4H,4′H-2,2′-spirobi[benzo[d][1,3,2]dioxasiline] (2), the corresponding fluorido complexes 3 and 4 were obtained by the addition of potassium fluoride to the spirocyclic silicon salicylic alcoholates 1 and 2 in the presence of 18-crown-6. The pentacoordinated geometry around the silicon atom of the SiO₄F type in 3 and 4 was proved using single-crystal X-ray diffraction analysis. DSC measurements were used to study the thermal behavior of the compounds with regard to their usability in the twin polymerization process. Hence, polymerization of 3 and 4 and the simultaneous twin polymerization of the fluoridosilicate 3 with spirocyclic 1 gave fluorine-doped phenolic resin/silica composites as confirmed by solid state NMR spectroscopic studies (¹H, ¹³C{¹H}, ¹⁹F, and ²⁹Si{¹H}). Carbonization of the as-obtained hybrid materials and subsequent treatment with hydrofluoric acid resulted in microporous carbon with surface areas up to 1060 m² g⁻¹. Calcination of the hybrid materials produced fluorine-doped silica with BET surface areas up to 239 m² g⁻¹. Additionally, the synthesis of hybrid materials using the additive assisted twin polymerization of 1 with either tetra-n-butylammonium fluoride or KF gave either microporous carbon (1219 m² g⁻¹) or mesoporous silica (687 m² g⁻¹).