Identifying the Missing Protonated Complex Species and Revisiting the Overestimated Stability Constants of Two Known Complexes of Uranyl(VI) with Dipicolinic Acid

The complexation of uranium­(VI) [U­(VI)] with dipicolinic acid was revisited with respect to the missing protonated complex species existing in acidic solutions. For the first time, the presence of the protonated complex, UO2HL+, in aqueous solutions was confirmed and the stability constant was determined by fluorescence spectroscopy. Considering the protonated species, which was missing in previous investigations, the overestimated stability constants of the two known complexes, UO2L and UO2L2 2–, were carefully reevaluated with potentiometry using N-(2-hydroxyethyl)­ethylenediamine-N,N′,N′-triacetic acid as a competing ligand. While a single crystal of the solid compound UO2(HL)2(H2O)4 with two monoprotonated HL– bonding UO2 2+ in a tridentate mode was successfully grown from aqueous solutions, the corresponding complex species UO2(HL)2(aq) could not yet be clearly identified and characterized.