Organic-inorganic hybrid materials. Preparation and properties of dibenzo-18-crown-6 ether-bridged polysilsesquioxanes

Dibenzo-18-crown-6 ether derivatives bearing two (1, 2) or four (3) hydroly sable Si(OR)(3) groups have been synthesised. Their hydrolysis and polycond ensation gave rise to new hybrid organic-inorganic materials incorporating dibenzo-18-crown-6 ether moieties covalently linked to silica by two or fou r Si-C bonds. The complexation of alkali metal cations (Na+ and/or K+) by t hese materials was investigated. A survey of the uptake of cations showed t hat four types of chelating sites exist within these materials, the ratio o f which depends on the nature of the precursor (flexibility of the spacers between the benzene rings and the silicon centres and number of hydrolysabl e Si(OR)(3) groups) and also on the degree of condensation sigma of the pol ysiloxane network. Furthermore, the complexation of K+ by the precursors 1, 2 and 3 and of Na+ by 1 were performed quantitatively. During the sol-gel polymerisation of these complexes it was shown that about 95% of the alkali cations were retained within the xerogel. This study proves that the two r outes of incorporation of salts within these hybrid materials are not equiv alent.