A new chiral uranyl phosphonate framework consisting of achiral building units generated from ionothermal reaction: structure and spectroscopy characterizations

Abstract

The ionothermal reactions of uranyl nitrate and 1,3-pbpH₄ (1,3-pbpH₄ = 1,3-phenylenebis(phosphonic acid) ligand in ionic liquids of [C₄mim][Dbp], [C₄mpyr][Br], and [Etpy][Br], respectively, afforded three new uranyl phosphonates, namely [C₄mim][(UO₂)₂(1,3-pbpH)(1,3-pbpH)·Hmim] (1), [UO₂(1,3-pbpH₂)H₂O·mpr] (2), and [Etpy][UO₂(1,3-pbpH₂)F] (3). Compound 1 exhibits a rare example of a chiral uranyl phosphonate 3D framework structure built from achiral building units of tetragonal bipyramidal uranium polyhedra and 1,3-pbp ligands. The structure adopts a network with channels extending along the b axis, which are filled with C₄mim⁺ and protonated 1-methylimidazole. In sharp contrast, compounds 2 & 3 both show pillared topology composed of uranyl pentagonal bipyramid polyhedra and phosphonate ligands. The layers are neutral in compound 2 with N-methylpyrrole molecules in the interlayer space, while compound 3 adopts anionic layer, and the charge is compensated with N-ethyl-pyridinium cations between the layers. Although compounds 1, 2, and 3 were synthesized under identical conditions with sole variation of the ionic liquid species, the resulting structures show a rich diversity in the local coordination environment of uranyl ions, the protonation of the phosphonate ligand, the conformation of ionic liquid ions, and the overall arrangement of the structure. All compounds were characterized by absorption, temperature dependent fluorescence, as well as infrared and Raman spectroscopies.