The effect of iron binding on uranyl(v) stability

Here we report the effect of UO(2)(+)···Fe(2+) 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 [UO(2)(trensal)K], 1 and [UO(2)(Htrensal)], 2 and of uranyl(v) complexes presenting UO(2)(+)···K(+) or UO(2)(+)···Fe(2+) cation–cation interactions ([UO(2)(trensal)K]K, 3, [UO(2)(trensal)] [K(2.2.2crypt)][K(2.2.2crypt)], 4, [UO(2)(trensal)Fe(py)(3)], 6). The uranyl(v) complexes show similar stability in pyridine solution, but the presence of Fe(2+) bound to the uranyl(v) oxygen leads to increased stability with respect to proton induced disproportionation through the formation of a stable Fe(2+)–UO(2)(+)–U(4+) intermediate ([UO(2)(trensal)Fe(py)(3)U(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 UO(2)(2+) compounds. The presence of an additional UO(2)(+) bound Fe(2+) in [(UO(2)(trensal)Fe(py)(3))(2)Fe(py)(3)]I(2), 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 Fe(2+) 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 Fe(2+) cation bound through one trensal(3–) arm in the trinuclear complex [{UO(2)(trensal)}(2)Fe], 5 does not lead to increased redox stability of the uranyl(v) showing the important role of UO(2)(+)···Fe(2+) 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).