Encapsulation of divalent tetrahedral oxyanions of sulfur within the rigidified dimeric capsular assembly of a tripodal receptor: first crystallographic evidence of thiosulfate encapsulation within neutral receptor capsule.

A simple tris(2-aminoethyl)amine based meta-chloro substituted tripodal thiourea receptor L has been extensively studied with two divalent oxyanions of sulfur, such as sulfate and thiosulfate, with identical dimensionality. The solid state crystal structure of the anion complexes with L reveal that the anions are encapsulated within the dimeric rigid capsular assembly of the receptor via N-H···anion interactions. To the best of our knowledge this is the first report on the encapsulation of thiosulfate within dimeric capsular assembly of a neutral receptor. The tight capsular sizes for both anion complexes are quite comparable, whereas the coordination mode of the anions and the hydrogen bonding parameters are significantly varied. The three dimensional solid state structural orientations of the capsular complexes are mainly governed by the Cl···Cl (for thiosulfate complex) and Cl···S (for sulfate complex) halogen bonding interactions. The solution-state binding and encapsulation of oxyanions by N-H···anion hydrogen bonding has also been confirmed by quantitative (1)H NMR titration and 2D NOESY NMR experiments. Both the experiments confirm that in contradiction of 2:1 solid state binding, in solution the studied anions are bound within the pseudocavity of the receptor with 1:1 binding stoichiometry. Moreover, the change in chemical shifts of thiourea -NH protons and the binding constant values suggest the receptor-sulfate interaction is more energetically favorable compared to the receptor thiosulfate interaction.