Metal-Controlled Assembly of Uranyl Diphosphonates toward the Design of Functional Uranyl Nanotubules

Two uranyl nanotubules with elliptical cross sections were synthesized in high yield from complex and large oxoanions using hydrothermal reactions of uranyl salts with 1,4-benzenebisphosphonic acid or 4,4′-biphenylenbisphosphonic acid and Cs⁺ or Rb⁺ cations in the presence of hydrofluoric acid. Disordered Cs⁺/Rb⁺ cations and solvent molecules are present within and/or between the nanotubules. Ion-exchange experiments with A₂{(UO₂)₂F­(PO₃HC₆H₄C₆H₄PO₃H)­(PO₃HC₆H₄C₆H₄PO₃)}·2H₂O (A = Cs⁺, Rb⁺), revealed that A⁺ cations can be exchanged for Ag⁺ ions. The uranyl phenyldiphosphonate nanotubules, Cs_3.62H_0.38[(UO₂)₄{C₆H₄(PO₂OH)₂}₃{C₆H₄(PO₃)₂}­F₂]·nH₂O, show high stability and exceptional ion-exchange properties toward monovalent cations, as demonstrated by ion-exchange studies with selected cations, Na⁺, K⁺, Tl⁺, and Ag⁺. Studies on ion-exchanged single crystal using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS) provide evidence for chemical zonation in Cs_3.62H_0.38[(UO₂)₄{C₆H₄(PO₂OH)₂}₃{C₆H₄(PO₃)₂}­F₂]·nH₂O, as might be expected for exchange through a diffusion mechanism.