The effect of iron binding on uranyl(V) stability

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 trensal3− 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 UO22+ 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 trensal3− 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).