Structure, equilibrium and ligand exchange dynamics in the binary and ternary dioxouranium(vi)–glyphosate–fluoride system. A multinuclear NMR study

Abstract

Complex formation in the binary and ternary uranium(VI)–glyphosate–fluoride systems was investigated with the aid of multinuclear NMR spectroscopy. The stoichiometry and the equilibrium constants of the different complexes in both systems are based on the integral values of the coordinated and free ligands in the ¹H-, ¹⁹F-, ³¹P- and ¹⁷O-NMR spectra. These were measured at different uranium(VI) concentrations, varying the total ligand concentrations (glyphosate and/or fluoride) in the pH range of 7–10 using a NaClO₄ medium at constant sodium concentration, [Na⁺] = 1.00 M. Tridentate and monodentate coordination has been found for the glyphosate ligand. The proposed structures are based on other spectral parameters (chemical shifts, homo- and heteronuclear couplings) and confirmed by two-dimensional homo- and heteronuclear correlation spectra. The spectra indicate the formation of several isomers for complexes 2 and 4, which differ from one another in the position of the non-chelated glyphosate. The numerical value of the stepwise stability constants for the non-chelated glyphosates in the binary complexes 4 (log K=12) and 5 (log K=11) falls between the formation constants for U(VI) with PO₄³⁻ and HPO₄²⁻, that is an independent confirmation of the magnitude of the latter, but also a strong indication that complex formation with phosphate/phosphonate through a single oxygen bond is very strong. The line widths of the fluoride signals in the ternary complexes are independent of the free ligand concentrations, and from these similar external fluoride exchange rate, k_obs1=10 ± 2 s⁻¹ can be calculated as observed for other U(VI) ternary complexes. The exchange between the coordinated and the free glyphosate was studied by 1D ¹H magnetization transfer experiments. From these an inter-molecular ligand exchange rate, k_obs2= 0.69 ± 0.03 s⁻¹, and a faster intra-molecular exchange rate for the methylene protons can also be calculated, k_obs3 = 2.00 ± 0.21 s⁻¹. The latter is probably a result of consecutive ring openings/chelate formation prior to the dissociation of the ligand.