Stepwise encapsulation of sulfate ions by ferrocenyl-functionalized tripodal hexaurea receptors

Three ferrocenyl-functionalized tripodal hexaurea anion receptors with ortho- (L²), meta- (L³), and para-phenylene (L ⁴) bridges, which showed strong binding affinities toward sulfate ions, have been designed and synthesized. In particular, meta-phenylene-bridged ligand L³, owing to its trigonal bipyramidal structure, can encapsulate two SO₄^2- ions in its "inner" and "outer" tripodal clefts, respectively, as supported by their clearly distinct NMR resonances and by molecular modeling. The sulfate complex of ortho-ligand L², (TBA)₂[SO₄⊠L²]×2 H₂O (1), displays a caged tetrahedral structure with an encapsulated sulfate ion that is hydrogen bonded by the six urea groups of ligand L². CV studies showed two types of electrochemical response of the ferrocene/ferrocenium redox couple upon anion binding, that is, a shift of the wave and the appearance of a new peak. Quantitative binding data were obtained from the NMR and CV titrations. The time capsule: Three ferrocenyl-functionalized tripodal hexaurea anion receptors were designed for sulfate recognition. Owing to its trigonal-bipyramidal shape, a meta-phenylene-bridged ligand can encapsulate two SO₄^2- ions in two distinct steps. CV studies showed two types of electrochemical response of the Fc/Fc⁺ redox couple upon anion binding, that is, a shift of the wave and the appearance of a new peak.