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

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.