A new trend in metal-alkoxide chemistry: the elaboration of monophasic organic-inorganic hybrid materials

The elaboration of monophasic organic inorganic hybrid materials is an important extension of metal alkoxide chemistry. These solids are obtained by hydrolytic polycondensation from a molecular precursor containing -Si(OR)(3) groups (at least two) covalently bound to an organic moiety. Using solid state C-13 NMR it is possible to demonstrate that, in all cases studied until now, the organic units are bound to the oxide matrix after hydrolysis. Si-29 permits the evaluation of the degree of polycondensation. All the solids obtained are kinetically controlled: their texture is highly dependent on all the parameters governing the kinetics of polycondensation (catalyst, temperature, solvent, concentrations, etc.). It is also possible to demonstrate that the geometry of the organic molecule is an important factor for both the texture and the short range organization of the solid. Weak interactions (charge transfer complexes) have been evidenced as possible controlling parameters for the solid formation. Short range organization in the hybrid material can be demonstrated using the easy polymerisation of bisacetylenic units which has been observed by heating the solid formed by hydrolytic polycondensation of [(MeO)(3)Si](2)C-4. The relationship between the porosity formed in the solid after elimination of the organic unit and the size of this unit was considered. A mild selective cleavage for Si-C bonds has been found and used for eliminating the organic moiety. After cleaving and recovering the organic molecule a monodispersed porosity is observed. However, there is no relation between the size of the pores and the size of the molecule. This fact is certainly due to a reorganization of the solid occurring during the Si-C bond cleavage. (C) 1998 Elsevier Science Ltd. All rights reserved.