Syntheses and Structures of Uranyl Ethylenediphosphonates: From Layers to Elliptical Nanochannels

Abstract

A family of uranium diphosphonates have been hydrothermally synthesized through the reaction of ethylenediphosphonic acid (EDP, H₄L) and uranyl nitrate/zinc uranyl acetate in the presence of organic templates, such as tetraethyl ammonium (NEt₄⁺), 4,4′-bipyridine (bipy), and 1,10-phenanthroline (phen). The UO₂²⁺ in UO₂(H₂O)­(H₂L)­(<b>EDP-U1</b>) is equatorially five-coordinated by four phosphonate groups and one aqua ligand, forming a pentagonal bipyramid. Each EDP ligand is doubly protonated and chelates three UO₂²⁺, resulting in a layered structure. Compounds (NEt₄)₂­(UO₂)₃­(HL)₂­(H₂L)·​4H₂O (<b>EDP-U2</b>) and (H₂bipy)­UO₂L (<b>EDP-U3</b>) have the same layered structure in which NEt₄⁺ and protonated bipy fill in the uranyl–phosphonate interlayers, respectively, and play a role to balance the negative charges. Different from that in <b>EDP-U1</b>, the UO₂²⁺ exists in the form of a UO₆ tetragonal bipyramid and is surrounded by four different EDP ligands in <b>EDP-U2</b> and <b>EDP-U3</b>. (Hphen)₂­(UO₂)₂­(H₂L)₃ (<b>EDP-U4</b>) features a three-dimensional framework structure with large elliptical channels along the <i>c</i> axis (1.3 × 1.1 nm²). Monoprotonated phen molecules fill in these channels and hold together through strong π···π interactions. All of the four compounds have been characterized by IR and photoluminescent spectroscopy. Their characteristic emissions have been attributed as transition properties of uranyl cations. The ion-exchange study indicates that [Co­(en)₃]³⁺ could partially replace the protonated phen molecules