Ion-pairs of structurally related polyoxotantalate clusters and divalent metal cations

We report cation-binding behaviors of two {P4Ta6}-type polyoxometalate (POM) clusters withcis-andtrans-subunit geometry in aqueous solution. Two types of divalent cations, Sr(2+)and Pb2+, possess different binding affinities and sites when interacting with {P4Ta6}. Sr(2+)ions only form solvent-share ion-pairs with {P4Ta6}, whereas Pb(2+)ions form contact-ion pairs, selectively interacting with the phosphate groups at different locations and subsequently induce the aggregation of {P4Ta6}, confirmed by isothermal titration calorimetry (ITC), NMR, and small-angle X-ray scattering (SAXS) techniques. Despite Sr(2+)and Pb(2+)possessing similar ionic sizes and hydration energies, Pb(2+)ions with stronger coordination capability result in a closer association with the clusters, eventually to selective interaction that depends on the chelating effect, charge density, and the acidity of phosphate groups at specific sites.