Nucleophilic substitution on silica surfaces: Comparison of the reactivity of α- versus γ-chlorosubstituted silanes in the reaction with sodium azide

Abstract

Azido-group functionalized, hierarchically organized meso-/macroporous silica gels have been prepared through co-condensation of tetrakis(2-hydroxyethyl)orthosilicate with chloromethyltrimethoxysilane or 3-(chloropropyl)-triethoxysilane and subsequent conversion of the chloro groups. Azido functionalities have been obtained by nucleophilic substitution of the surface-bound chloro moieties with NaN₃ in N,N-dimethylformamide. A strong dependence of the later azide density (N₃ groups nm⁻²) in the final material on the reaction conditions, such as temperature and time, but also on the spacer length (methylene versus propyl) of the organofunctional silane has been observed. In principle, the nucleophilic substitution benefits from higher reaction temperatures and times. However, while γ-azido groups seem to be stable over a wide range of reaction temperatures and longer reaction times, α-azido moieties tend to decompose at temperatures above 60°C. The structural features of the monolithic gels and the azide functionalities on the surface were determined by IR-ATR-spectroscopy, elemental analyses, N₂-sorption analyses, small angle X-ray scattering and transmission electron microscopy.