Hydrothermal Syntheses, Structures, and Fluorescence Spectroscopy of New One-Dimensional Uranium Oxyfluorides Built from Edge-Sharing [UO2F5] Pentagonal Bipyramids

Abstract

Three new one-dimensional uranium oxyfluorides, (C5H14N2) U2O4F6 (AU1-3), (C5H6N)UO2F3 (AU1-4), and (C3H5N2) UO2F3 (AU1-5), have been prepared from the reactions of UO3 with HF and homopiperazine (C5H12N2), pyridine, or pyrazole at 180°C in aqueous media. In all cases, the compounds were isolated in high yield in the form of large single crystals. Single crystal X-ray diffraction, elemental analysis, and fluorescence spectroscopy have been used to characterize these compounds. Each structure contains linear one-dimensional chains formed from edge-sharing [UO2F5] pentagonal bipyramids with substantial hydrogen-bonding occurring between the protonated organic cations and the anionic chains. AU1-4 and AU1-5 luminesce brightly when irradiated with 365 nm UV light, while emission is considerably weaker from AU1-3. An energy transfer mechanism between the pyridinium and pyrazolium cations and the UO2+2 groups in AU1-4 and AU1-5, respectively, is proposed to account for some of these differences. Crystallographic data: AU1-3, triclinic, space group P1, a=7.328(2) Å, b=9.488(3) Å, c=11.106(4) Å, α=94.65(3)°, β=106.47(3)°, γ=107.79(2)°, Z=2, MoKα, λ=0.71073, R(F)=5.02% for 173 parameters with 1977 reflections with I>2σ(I); AU1-4, orthorhombic, space group Imma, a=6.793(2) Å, b=7.232(2) Å, c=17.094(7) Å, Z=4, MoKα, λ=0.71073, R(F)=3.14% for 44 parameters with 412 reflections with I>2σ(I); AU1-5, monoclinic C2/m, a=17.011(7) Å, b=6.708(3) Å, c=7.213(2) Å, β=113.31(3)°, Z=4, MoKα, λ=0.71073, R(F)=3.76% for 65 parameters with 664 reflections with I>2σ(I).