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 UO2(2+) in UO2(H2O)(H2L)(EDP-U1) 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 UO2(2+), resulting in a layered structure. Compounds (NEt4)2(UO2)3(HL)2(H2L)·4H2O (EDP-U2) and (H2bipy)UO2L (EDP-U3) 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 EDP-U1, the UO2(2+) exists in the form of a UO6 tetragonal bipyramid and is surrounded by four different EDP ligands in EDP-U2 and EDP-U3. (Hphen)2(UO2)2(H2L)3 (EDP-U4) features a three-dimensional framework structure with large elliptical channels along the c axis (1.3 × 1.1 nm(2)). 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.

Structural chemistry of uranium phosphonates

“…Differing from the typical hydrothermal synthesis of uranyl phosphonates, [Bmim][(UO 2 )(HPDP)] was obtained under ionothermal conditions using the ionic liquid 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) as the solvent. This structure is 2D and similar to Ba-1 [30] and EDP-U2 [37] in that uncommon uranyl square bipyramids are bridged by phosphonate groups to form chains, but the bridging linker is a propylene group in [Bmim][(UO 2 )(HPDP)] ( Fig. 10c) instead of a methylene group as in Ba-1 [30] or an ethylene group as in EDP-U2.…”

“…Similar layered structures formed by uranyl square bipyramids and increasingly long diphosphonate ligands in Ba-1 (a)[30], EDP-U2 (b)[37], and [Bmim][UO2(HPDP)] (c)[39] as well as the identical graphical representation (d).…”

“…By increasing the length of the alkyl chain, flexible 1,2-ethylenediphosphonic acid (EDP) is produced, which serves as the linker for the construction of five uranyl phosphonates including three sheets and two 3D frameworks [37,38]. In (UO 2 )(H 2 O)(H 2 EDP) (EDP-U1), the EDP ligand is doubly protonated and chelates three uranyl pentagonal bipyramids, forming a layered structure.…”

“…11. 3D framework structure with channels in EDP-U4 [37]. moieties functionalized with phosphonate groups.…”

“…In Zn 2 (phen) 2 (UO 2 ) 2 (H 2 O) 3 (hedp) 2 ·3H 2 O (ZnUP-1) and Zn 2 (bipy) 2 There are in total eight zinc uranyl phosphonates with increasing alkyl chain lengths in the phosphonate ligand that have been reported. Zn(bipy)(UO 2 )(EDP) (ZnUEDP-bipy) [62] contains a uranyl phosphonate layer isotypic to EDP-U2 [37]. The layers are modified by Zn(bipy) moieties through Zn O P and Zn-O = U connections producing the overall layered arrangement.…”