Hybrid Silica Gels Containing 1,3-Butadiyne Bridging Units. Thermal and Chemical Reactivity of the Organic Fragment

Abstract

Silica gel containing diyne units have been obtained from molecular organosilicon precursors and the properties associated to the very reactive unsaturated organic moieties have been explored. The sol−gel polymerization of 1,4-bis(trimethoxysilyl)-1,3-butadiyne ((MeO)3SiC⋮CC⋮CSi(OMe)3) quantitatively led to a silsesquioxane network, [O1.5SiC⋮CC⋮CSiO1.5]n, consisting of siloxanes chains with bridging diyne units. The derived xerogels were characterized by IR and 13C and 29Si CP MAS NMR spectroscopies. The major environment of the Si atom corresponded to a T2 CSi(OR)(OSi)2 substructure, and only minor Si−C bond cleavage occurred during the sol−gel condensation. The chemical reactivity of the hybrid organic−inorganic gel was studied and used as a tool for the study of the organization of the solid induced by the organic moieties. Upon heating, in the solid state, the diyne fragments undergo a polyaddition to give an ene−yne structure. The polymerization, observed in the solid state, suggests favorable arrangements of the organic fragments within the amorphous solid. The resulting composite material consists in a network made of interpenetrating ene−yne and siloxane polymers. On the other hand, the organic diyne fragments in the hybrid gel have been removed, leaving silica behind, in two ways: (i) The thermal oxidation in air led to microporous silicas with N2 BET surface areas in the range 300−350 m2 g-1. (ii) Interestingly, the smooth Si−C bond cleavage by MeOH catalyzed by NH4F gave highly porous silica with N2 BET surface areas up to 950 m2 g-1. The latter elimination of the organic moiety under mild reaction conditions is of particular interest since it gives rise to silica with a surface area significantly higher than that produced upon thermal oxidation and higher than that of the originating hybrid precursor.