Structural Relationships, Interconversion, and Optical Properties of the Uranyl Iodates, UO₂(IO₃)₂and UO₂(IO₃)₂(H₂O): A Comparison of Reactions under Mild and Supercritical Conditions

The uranyl iodates, UO 2 (IO 3 ) 2 ( AU1 − 8 ) and UO 2 (IO 3 ) 2 (H 2 O) ( AU2 − 8 ) have been prepared from the reaction of UO 3 with I 2 O 5 under hydrothermal conditions. Equilibrium between AU1 − 8 and AU2 − 8 is established after 3 days at 400 °C, resulting in the isolation of an equimolar ratio of these compounds. Lowering of the reaction temperature to 180 °C or shortening of the reaction duration to 1 day allows for the isolation of AU2 − 8 in pure form. At 327 °C, AU2 − 8 undergoes dehydration and structural rearrangement to AU1 − 8, which is a reversible process. Single-crystal X-ray diffraction, bond valence sum calculations, EDX, DSC, TGA, and vibrational and fluorescence spectroscopy have been used to characterize these compounds. Crystallographic data are as follows: AU1 − 8, monoclinic, space group P 2 1 / n, a = 4.2454(8) Å, b = 16.636(5) Å, c = 5.284(1) Å, β = 107.57(2)°, and Z = 2; AU2 − 8, orthorhombic, space group Pbcn, a = 8.452(2) Å, b = 7.707(2) Å, c = 12.271(3), and Z = 4. The structure of AU1 − 8 is pseudo-three-dimensional with one-dimensional uranyl iodate chains forming close I···O interchain contacts. Each one-dimensional chain consists of edge-sharing UO 8 hexagonal bipyramids and chelating iodate anions that also bridge the UO 2 2+ units through μ 3 -O atoms. AU2 − 8 has a corrugated layered structure containing pentagonal bipyramidal UO 7 units that are formed from the binding of four oxygen atoms from four bridging iodates and the coordination of a water molecule to the UO 2 2+ moiety. Both of these compounds fluoresce when irradiated with UV light (365 nm), yielding emission spectra characteristic of the uranyl dication.