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) hydrolysable Si(OR)3 groups have been synthesised. Their hydrolysis and polycondensation gave rise to new hybrid organic–inorganic materials incorporating dibenzo-18-crown-6 ether moieties covalently linked to silica by two or four Si–C bonds. The complexation of alkali metal cations (Na+ and/or K+) by these materials was investigated. A survey of the uptake of cations showed that four types of chelating sites exist within these materials, the ratio of which depends on the nature of the precursor (flexibility of the spacers between the benzene rings and the silicon centres and number of hydrolysable Si(OR)3 groups) and also on the degree of condensation σ of the polysiloxane 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 routes of incorporation of salts within these hybrid materials are not equivalent.