Space-confined click reactions in hierarchically organized silica monoliths

Abstract

Hybrid hierarchically organized, monolithic silica gels comprising periodically arranged mesopores and a cellular macroscopic network have been prepared via a co-condensation approach of tetrakis(2-hydroxyethyl)orthosilicate and chloromethyltrimethoxysilane or 3-(chloropropyl)-triethoxysilane, respectively. Subsequent conversion of the chloro into azido groupsvia nucleophilic substitution with NaN₃, followed by a Cu(I)-catalyzed 1,3-dipolar cycloaddition with several alkynes was successfully conducted within the monolithic gel structure. The click reaction resulted in complete conversion of the azido groups for the propylene-spacered samples with a surface coverage of about 1 group nm⁻², while for azidomethyl groups the azide functionality could still be detected viaIR spectroscopy. The monolithic structure and the periodic ordering of the mesopores was preserved during all reaction steps with an improvement of the periodicity of the mesopores, which was confirmed by SAXS, N₂-sorption analysis, SEM, and TEM.