Chemical speciation of the uranyl ion under highly alkaline conditions. Synthesis, structures, and oxo ligand exchange dynamics

The tetramethylammonium counterion was used to suppress formation of insolu ble uranate salts, M2U2O7, and allow for a detailed structural and spectros copic characterization of UO2(OH)(n)(2-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)(6)Cl-3 and U O2(NO3)(2). 6H(2)O in 3.5 M (Me4N)OH to 5 degrees C. The asymmetric unit co ntains three distinct UO2(OH)(4)(2-) ions, each displaying a pseudo-octahed ral coordination geometry with trans oxo ligands. The three independent UO2 (OH)(4)(2-) ions in the unit cell give average U=O and U-OH distances of 1. 82(1) and 2.26(2) Angstrom, 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 co llected at the U L-III 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) Angstrom, and the second shel l was fit with 3.9(5) O atoms at a distance of 2.21(1) Angstrom. For the so lution sample, the first shell contains two O atoms with a U=O distance of 1.79(1) Angstrom, and the second O shell was fit with 5.2(5) O atoms at a U -O distance of 2.22(1) Angstrom. The bond distances for both the solution a nd solid state samples correspond relatively well with the single-crystal d iffraction data; however, the second-shell coordination number is larger in solution than in the solid state, indicating a greater number of OH ligand s in solution than in the solid state. Both EXAFS and X-ray diffraction ana lyses reveal relatively long axial U=O and short equatorial U-OH bonds. Ram an spectra of single crystals of [Co(NH3)(6)](2)[UO2(OH)(4)](3). H2O reveal a symmetrical O=U=O stretch at 796 cm(-1), 74 cm(-1) lower than that for t he uranyl aquo ion. In solution, the symmetrical O=U=O stretch is at 786 cm (-1), 10 cm(-1) lower than observed in the solid state. O-18 enrichment pro duces a shift to 752 cm(-1) confirming the assignment in solution. Luminesc ence spectroscopy recorded as a function of hydroxide ion concentration rev eals that an equilibrium exists between two species, assigned to UO2(OH)(4) (2-) and UO2(OH)(5)(3-). The vibronic structure of the luminescence bands w as used to determine a vibrational energy of 790 cm(-1) for UO2(OH)(5)(3-) to confirm its assignment. O-17 NMR and O-16/O-18 Raman spectroscopies also reveal an unprecedented facile ligand exchange between U=O and bulk solven t oxygen atoms. Line-broadening analysis of the O-17 NMR data provide activ ation parameters of Delta H double dagger = 9.8 +/- 0.4 kcal/mol, Delta S d ouble dagger = -18 +/- 6 cal/mol.K, and k(ex)(298K) = 45 +/- 15 s(-1). Crys tal data for [Co(NH3)(6)](2)[UO2(OH)(4)](3). H2O: monoclinic space group C2 /c, a = 17.4130(4) Angstrom, b = 12.1794(3) Angstrom, c = 15. 3721(4) Angstrom, beta = 120.384(1)degrees, Z = 4, R1 = 0.0313, wR2 = 0.073 4.