Metal-Controlled Assembly of Uranyl Diphosphonates toward the Design of Functional Uranyl Nanotubules Pius O. Adelani and Thomas E. Albrecht-Schmitt* Department of Civil Engineering and Geological Sciences and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States *S Supporting Information ABSTRACT: 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 A 2 {(UO 2 ) 2 F(PO 3 HC 6 H 4 C 6 H 4 PO 3 H)(PO 3 HC 6 H 4 C 6 H 4 PO 3 )}·2H 2 O (A = Cs + , Rb + ), revealed that A + cations can be exchanged for Ag + ions. The uranyl phenyldiphosphonate nanotubules, Cs 3.62 H 0.38 [(UO 2 ) 4 {C 6 H 4 (PO 2 OH) 2 } 3 {C 6 H 4 (PO 3 ) 2 }F 2 ]·nH 2 O, 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 3.62 H 0.38 [(UO 2 ) 4 {C 6 H 4 (PO 2 OH) 2 } 3 {C 6 H 4 (PO 3 ) 2 }F 2 ]·nH 2 O, as might be expected for exchange through a diffusion mechanism. ■ INTRODUCTION Early studies on the structural chemistry of U(VI) demonstrated the dominance of layered structures, especially among minerals, with diverse coordination environments for uranium that favor tetragonal, pentagonal, and hexagonal bipyramidal geometries in almost boundless combinations. 1 U(VI) coordination chemistry is exceptionally rich, and most compounds contain uranyl cations, UO 2 2+ , and because of the generally inert nature of the two oxo atoms, it was assumed that three-dimensional network structures would be infrequent and that curvature (nanotubules and nanospheres) would be unusual. 2,3 However, recent advances in the synthetic and structural chemistry of U(VI) has yielded unprecedented shapes, topologies, and properties with potential applications in materials chemistry and waste management. 4 This field has indeed undergone a revolution with increasingly fascinating structural architectures such as nanotubules 2 and nanospheres, 3 which are assembled using organic linkers and U(VI) cations. Uranyl oxoanion materials have displayed important properties that include selective ion- exchange, ionic conductivity, intercalation chemistry, photo- chemistry, nonlinear optics, and catalysis. 5−8 We are particularly interested in developing the solid-state chemistry of uranyl arylphosphonate compounds. 2a,9 Until recently, our understanding of the coordination chemistry of actinide arylphosphonates has been limited mostly to those of uranyl phenylphosphonate reported by Clearfield et al. and a handful of other compounds. 2g−k,10 The coordination chemistry of uranyl phenyldiphosphonate is very rich, displaying the first member in the family of uranyl nanotubules that was isolated via the transformation of one-dimensional α- and β-uranyl phenylphosphonates upon exposure to Na + or Ca 2+ cations in aqueous solution. 2i,j We have de- monstrated in our previous reports that rigid phenyldiphosphonates can be used to construct pillared uranyl structures in the presence of organic amines as structure directing agents. 9b,c When we replaced the aromatic amines with the Cs + cation to template the structure of a uranyl phenyldiphosphonate, a remarkable structure of Cs 3.62 H 0.38 - {(UO 2 ) 4 [C 6 H 4 (PO 2 OH) 2 ] 3 [C 6 H 4 (PO 3 ) 2 ]F 2 }·nH 2 O resulted. This compound possesses an elliptical uranyl nanotubular structure, and atypical ion-exchange properties where the cations on the interior of the tubes exchange much more rapidly than those on the exterior. 2a The application of nanotubular structures in the uranyl system has been highlighted previously. 3c Applications of nanotubular structures in ion-exchange studies have been partially reported by our group. 3b The aim of this paper is to communicate further advances we have made to uncover new nanotubular topologies. As an extension of the previous work on phenyldiphosphonate, we have further expanded on this system by using 4,4′-biphenylenebisphosphonate as a linker between U(VI) centers, and present here in detail the syntheses, structural characterization, and ion-exchange properties of these compounds, A 2 {(UO 2 ) 2 F(PO 3 HC 6 H 4 C 6 H 4 PO 3 H)- (PO 3 HC 6 H 4 C 6 H 4 PO 3 )}·2H 2 O (A = Cs + , Rb + )(CsUbpbp-1 and RbUbpbp-1), and Cs 3.62 H 0.738 {(UO 2 ) 4 [C 6 H 4 (PO 2 OH) 2 ] 3 [C 6 H 4 - (PO 3 ) 2 ]F 2 }·nH 2 O(CsUbbp-1). ■ EXPERIMENTAL SECTION Synthesis. UO 2 (NO 3 ) 2 ·6H 2 O (98%, International Bio-Analytical Industries), HF (48 wt %, Aldrich), 4,4′-biphenylenebisphosphonic acid (98%, Epsilon Chimie), 1,4-benzenebisphosphonic acid (95%, Epsilon Chimie), sodium chloride, potassium chloride, rubidium chloride, cesium chloride, thallium nitrate, silver nitrate, and strontium chloride hexahydrate (Alfa Aesar) were used as received. Reactions were run in PTFE-lined Parr 4749 autoclaves with a 23 mL internal Received: September 5, 2011 Published: November 3, 2011 Article pubs.acs.org/IC © 2011 American Chemical Society 12184 dx.doi.org/10.1021/ic201945p | Inorg. Chem. 2011, 50, 12184−12191