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, UO 2 HL + , 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, UO 2 L and UO 2 L 2 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 UO 2 (HL) 2 (H 2 O) 4 with two monoprotonated HL - bonding UO 2 2+ in a tridentate mode was successfully grown from aqueous solutions, the corresponding complex species UO 2 (HL) 2 (aq) could not yet be clearly identified and characterized.