Hydrogen bond-directed assembly of silsesquioxanes cubes: synthesis of carboxylic acid POSS derivatives and the solid state structure of octa[2-(p-carboxyphenyl)ethyl] silsesquioxane

Abstract

The octa[2-(p-carboxyphenyl)ethyl] and octa[2-(4-carboxy-1,1′-biphenyl)ethyl] silsesquioxane were synthesised under mild reaction conditions upon hydrogenolysis of the corresponding benzyl-ester derivatives. The octa[2-(p-carboxyphenyl)ethyl] silsesquioxane was shown to self-assemble upon formation of carboxylic acid dimers. The presence of eight carboxylic acid functional groups allowed to obtain an ordered silsesquioxane hybrid network. The X-ray crystal structures of the octa-ester and of the octa-acid were determined. The three-dimensional conformation of the ester provided an interpenetrated compact packing of the molecular building blocks without any specific supramolecular interaction. The two-dimensional character of the acid and the directionality of the hydrogen bond pattern of a dimer of acid led to the formation of hydrogen-bonded ribbons. It showed that functionalized silsesquioxane precursors capable of hydrogen bonding are of interest to generate nanostructured materials through self-organization processes. Moreover, the use of carboxylic acid groups is interesting not only because of their ability to form carboxyl dimer structures by hydrogen bonding but also because of their ability to form metal carboxylate derivatives that may lead to new organised hybrid metal organic silica frameworks.