Most of the receptors for anions operate through electrostatic interactions, which include hydrogen bonding. Such interactions are not esp. strong, which prevents anion recognition to occur in highly solvating media, like water. We considered that metal-ligand interactions are strong enough to more than compensate the endothermic anion dehydration process and we designed a variety of receptors contg. transition metal centers. As an example, polyamine cages contg. a pair of coordinatively unsatd. CuII ions selectively include ambidentate anions, e.g. dicarboxylates, in aq. soln. Fluorescent sensing is achieved by taking profit from the "chemosensing ensemble" paradigm. In particular, a neg. charged fluorescent indicator, included within the dimetallic cage and quenched through the interaction with the metals, recovers its full fluorescence when displaced by an ambidendate anion displaying higher affinity towards the host. Discrimination is attained when the assocn. const. decreases along the series: envisaged anion > indicator > interfering anion.
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