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 NaN 3 in N,N-dimethylformamide. A strong dependence of the later azide density (N 3 groups nm ¹2) 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 £-azido groups seem to be stable over a wide range of reaction temperatures and longer reaction times, ¡-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, N 2 -sorption analyses, small angle X-ray scattering and transmission electron microscopy.

Sol–gel synthesis of monolithic materials with hierarchical porosity

“…However, one has to keep in mind that changes in polarity in the sol might result in completely different phase separation behaviors, thus resulting in different porous network structures. A very detailed investigation on the formation of hybrid chloroalkyl-modified, meso-/macroporous silica monoliths and the structural changes observed due to the presence of the organofunctional silane and its organic chain, as well as due to post-synthesis processes, such as azide-alkyne Click reactions, has been presented by Keppeler et al [70][71][72] In addition to co-condensation reactions, pure silsesquioxanebased hybrids are accessible via condensation reactions of the sole organotrialkoxysilane or bis(trialkoxysilyl) precursors, such as 1,4-bis[tris(2-hydroxyethoxy)silyl]benzene or the corresponding alkoxy-derivatives. 37,73 Even dendrimeric silanes as well as cyclic preceramic precursors, such as a glycol-modified 1,3,5-trisilacyclohexane carbosilane, have been used in the formation of meso-/macroporous hybrid monoliths.…”

Theoretical Investigation of the Hydrolytic Mechanism of α-Functionalized Alkoxysilanes as Effective Crosslinkers and the Difficulty of Deep Vulcanization in RTV Silicone Rubber

“…Carbofunctionalized trialkoxysilanes (X–(CH 2 ) n –Si(OR) 3 , where X is functional amines, chlorine, carboxyl etc., n = 1, 2, 3 is called α-, β-, γ-functionalized alkoxysilane, respectively) play a great role in organosilicon chemistry. They are widely used as coupling agents to connect inorganic and organic materials possessing various structures and properties [ 1 , 2 ]. And they are also used as crosslinkers for room temperature vulcanized (RTV) silicone rubber.…”