Silica-attached molecular receptor complexes for benzoates and naphthoates

A series of cyclen (1,4,7,10-tetraazacyclododecane) derived molecular receptors for aromatic oxoanions, that are activated by complexation with Cd(II), have been covalently linked to 3-(glycidoxy)propyl-functionalised silica gel (70-230 mesh). These immobilised receptor complexes are highly effective for the sequestration of o-hydroxybenzoates and 2-naphthoate from aqueous solution, achieving a >80% saturation level by stirring the material in the aqueous solution for 1 h at pH 7.00 and 298 K. Examination of the uptake levels of a variety of different benzoates and naphthoates suggests that the retention mechanism involves a combination of classical hydrogen bonding and non-classical, water mediated, O-H···π hydrogen bonding. Contrary to expectations, attachment of hydroxy terminated polyether chains to the periphery of the receptor complex diminished the level of uptake.