| Abstract
| - The complex formation and equilibrium dynamics have been studied by potentiometry and multinuclear NMR spectroscopy for the binary and ternary uranium(VI)−α-hydroxycarboxylate−fluoride and uranium(VI)−glycine−fluoride systems. The experiments confirmed the existence of complexes formed by deprotonation and coordination of the alcoholic OH group of glycolate and α-hydroxyisobutyrate in neutral or slightly alkaline mediums. The 19F NMR spectrum shows bidentate coordination of glycine in the ternary system.
- Equilibria, structures, and ligand-exchange dynamics in binary and ternary U(VI)−L−F- systems, where L isglycolate, α-hydroxyisobutyrate, or glycine, have been investigated in 1.0 M NaClO4 by potentiometry and 1H,17O, and 19F NMR spectroscopy. L may be bonded in two ways: either through the carboxylate end or by theformation of a chelate. In the glycolate system, the chelate is formed by proton dissociation from the α-hydroxygroup at around pH 3, indicating a dramatic increase, a factor of at least 1013, of its dissociation constant oncoordination to uranium(VI). The L exchange in carboxylate-coordinated UO2LF32- follows an Eigen−Wilkinsmechanism, as previously found for acetate. The water exchange rate, kaq = 4.2 × 105 s-1, is in excellent agreementwith the value determined earlier for UO22+(aq). The ligand-exchange dynamics of UO2(O−CH2−COO)2F3-and the activation parameters for the fluoride exchange in D2O (kobs = 12 s-1, ΔH⧧ = 45.8 ± 2.2 kJ mol-1, andΔS⧧ = −55.8 ± 3.6 J K-1 mol-1) are very similar to those in the corresponding oxalate complex, with twoparallel pathways, one for fluoride and one for the α-oxocarboxylate. The same is true for the L exchange inUO2(O−CH2−COO)22- and UO2(oxalate)22-. The exchange of α-oxocarboxylate takes place by a proton-assistedchelate ring opening followed by dissociation. Because we cannot decide if there is also a parallel H+-independentpathway, only an upper limit for the rate constant, k1< 1.2 s-1, can be given. This value is smaller than thosein previously studied ternary systems. Equilibria and dynamics in the ternary uranium(VI)−glycine−fluoridesystem, investigated by 19F NMR spectroscopy, indicate the formation of one major ternary complex, UO2LF32-,and one binary complex, UO2L2 (L = H2N−CH2COO-), with chelate-bonded glycine; log β(9) = 13.80 ± 0.05for the equilibrium UO22+ + H2N−CH2COO- + 3F- = UO2(H2N−CH2COO)F32- and log β(11) = 13.0 ± 0.05for the reaction UO22+ + 2H2N−CH2COO- = UO2(H2N−CH2COO)2. The glycinate exchange consists of a ringopening followed by proton-assisted steps. The rate of ring opening, 139 ± 9 s-1, is independent of both theconcentration of H+ and the solvent, H2O or D2O.
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