The effect of iron binding on uranyl(v) stability

Abstract

Here we report the effect of UO₂ ⁺···Fe²⁺ cation-cation interactions on the redox properties of uranyl(v) complexes and on their stability with respect to proton induced disproportionation. The tripodal heptadentate Schiff base trensal^3- ligand allowed the synthesis and characterization of the uranyl(vi) complexes [UO₂(trensal)K], 1 and [UO₂(Htrensal)], 2 and of uranyl(v) complexes presenting UO₂ ⁺···K⁺ or UO₂ ⁺···Fe²⁺ cation-cation interactions ([UO₂(trensal)K]K, 3, [UO₂(trensal)] [K(2.2.2crypt)][K(2.2.2crypt)], 4, [UO₂(trensal)Fe(py)₃], 6). The uranyl(v) complexes show similar stability in pyridine solution, but the presence of Fe²⁺ bound to the uranyl(v) oxygen leads to increased stability with respect to proton induced disproportionation through the formation of a stable Fe²⁺-UO₂ ⁺-U⁴⁺ intermediate ([UO₂(trensal)Fe(py)₃U(trensal)]I, 7) upon addition of 2 eq. of PyHCl to 6. The addition of 2 eq. of PyHCl to 3 results in the immediate formation of U(iv) and UO₂ ²⁺ compounds. The presence of an additional UO₂ ⁺ bound Fe²⁺ in [(UO₂(trensal)Fe(py)₃)₂Fe(py)₃]I₂, 8, does not lead to increased stability. Redox reactivity and cyclic voltammetry studies also show an increased range of stability of the uranyl(v) species in the presence of Fe²⁺ with respect both to oxidation and reduction reactions, while the presence of a proton in complex 2 results in a smaller stability range for the uranyl(v) species. Cyclic voltammetry studies also show that the presence of a Fe²⁺ cation bound through one trensal^3- arm in the trinuclear complex [{UO₂(trensal)}₂Fe], 5 does not lead to increased redox stability of the uranyl(v) showing the important role of UO₂ ⁺···Fe²⁺ cation-cation interactions in increasing the stability of uranyl(v). These results provide an important insight into the role that iron binding may play in stabilizing uranyl(v) compounds in the environmental mineral-mediated reduction of uranium(vi).