Chemical Speciation of the Uranyl Ion under Highly Alkaline Conditions. Synthesis, Structures, and Oxo Ligand Exchange Dynamics

Abstract

The tetramethylammonium counterion was used to suppress formation of insoluble uranate salts, M2U2O7, and allow for a detailed structural and spectroscopic characterization of UO2(OH)n2-n (n = 4, 5) under highly alkaline aqueous solution conditions. Single crystals of [Co(NH3)6]2[UO2(OH)4]3·H2O were obtained by cooling a dilute solution of Co(NH3)6Cl3 and UO2(NO3)2·6H2O in 3.5 M (Me4N)OH to 5 °C. The asymmetric unit contains three distinct UO2(OH)42- ions, each displaying a pseudo-octahedral coordination geometry with trans oxo ligands. The three independent UO2(OH)42- ions in the unit cell give average UO and U−OH distances of 1.82(1) and 2.26(2) Å, respectively. EXAFS data on solid [Co(NH3)6]2[UO2(OH)4]3·H2O and aqueous UO22+ in 3.5 M (Me4N)OH solution were collected at the U LIII edge, and the resulting radial distribution function shows a single asymmetric peak. For the solid and solution, curve fitting reveals two near neighbors. For the crystalline solid, the first shell was fit with two O atoms at a distance of 1.81(1) Å, and the second shell was fit with 3.9(5) O atoms at a distance of 2.21(1) Å. For the solution sample, the first shell contains two O atoms with a UO distance of 1.79(1) Å, and the second O shell was fit with 5.2(5) O atoms at a U−O distance of 2.22(1) Å. The bond distances for both the solution and solid state samples correspond relatively well with the single-crystal diffraction data; however, the second-shell coordination number is larger in solution than in the solid state, indicating a greater number of OH ligands in solution than in the solid state. Both EXAFS and X-ray diffraction analyses reveal relatively long axial UO and short equatorial U−OH bonds. Raman spectra of single crystals of [Co(NH3)6]2[UO2(OH)4]3·H2O reveal a symmetrical OUO stretch at 796 cm-1, 74 cm-1 lower than that for the uranyl aquo ion. In solution, the symmetrical OUO stretch is at 786 cm-1, 10 cm-1 lower than observed in the solid state. 18O enrichment produces a shift to 752 cm-1 confirming the assignment in solution. Luminescence spectroscopy recorded as a function of hydroxide ion concentration reveals that an equilibrium exists between two species, assigned to UO2(OH)42- and UO2(OH)53-. The vibronic structure of the luminescence bands was used to determine a vibrational energy of 790 cm-1 for UO2(OH)53- to confirm its assignment. 17O NMR and 16O/18O Raman spectroscopies also reveal an unprecedented facile ligand exchange between UO and bulk solvent oxygen atoms. Line-broadening analysis of the 17O NMR data provide activation parameters of ΔH⧧ = 9.8 ± 0.4 kcal/mol, ΔS⧧ = −18 ± 6 cal/mol·K, and kex298K = 45 ± 15 s-1. Crystal data for [Co(NH3)6]2[UO2(OH)4]3·H2O: monoclinic space group C2/c, a = 17.4130(4) Å, b = 12.1794(3) Å, c = 15.3721(4) Å, β = 120.384(1)°, Z = 4, R1 = 0.0313, wR2 = 0.0734.