Abstract
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).