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Reduction of uranyl(VI) to UV and to UIV is important in uranium environmental migration and remediation processes. The anaerobic reduction of a uranyl UVI complex supported by a picolinate ligand in both organic and aqueous media is presented. The [UVI O2 (dpaea)] complex is readily converted into the cis-boroxide UIV species via diborane-mediated reductive functionalization in organic media. Remarkably, in aqueous media the uranyl(VI) complex is rapidly converted, by Na2 S2 O4 , a reductant relevant for chemical remediation processes, into the stable uranyl(V) analogue, which is then slowly reduced to yield a water-insoluble trinuclear UIV oxo-hydroxo cluster. This report provides the first example of direct conversion of a uranyl(VI) compound into a well-defined molecular UIV species in aqueous conditions.

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).

Herein we report the assembly of large uranium(IV) clusters with novel nuclearities and/or shapes from the controlled hydrolysis of UCl4 in organic solution and in the presence of the benzoate ligands. {U6 }, {U13 }, {U16 }, {U24 }, {U38 } oxo and oxo/hydroxo clusters were isolated and crystallographically characterized. These structural snapshots indicate that larger clusters are slowly built from the condensation of octahedral {U6 } building blocks. The uranium/benzoate ligand ratio, the reaction temperature and the presence of base play an important role in determining the structure of the final assembly. Moreover, the isolation of different size cluster {U6 } (few hours), {U16 } (3 days), {U24 } (21 days) from the same solution in a chosen set of conditions shows that the assembly of uranium oxo clusters in hydrolytic conditions is time dependent.

We have identified a polydentate aminocarboxylate ligand that stabilizes uranyl(V) in water. The mononuclear [UO2(dpaea)]X, (dpaeaH2 = Bis(pyridyl-6-methyl-2-carboxylate)-ethylamine; X = CoCp2*+ or X = K(2.2.2.cryptand) complexes have been isolated from anaerobic organic solution, crystallographically and spectroscopically characterized both in water and organic solution. These complexes disproportionate at pH ≤ 6, but are stable in anaerobic water at pH 7–10 for several days.

The importance of uranyl(V) (UO2+ ) species associated with environmental and geologic applications is becoming increasingly evident, but the tendency of the uranyl(V) cation to disproportionate in water has prevented the isolation of stable complexes. Here we demonstrate that in the presence of the tridentate complexing dipicolinate (dpa2- ), a ligand highly abundant in soil, the uranyl(V) species can be stabilized and isolated in anoxic basic water. Stable uranyl(V) dipicolinate complexes are readily formed from the reduction of the uranyl(VI) analogue both in organic solution and in basic water, and their solution and solid-state structure were determined. A bis-dpa UV O2+ complex was obtained from water at pH 10, while at higher pH values, a trinuclear mono-dpa cation-cation complex was isolated. These results present the second ever isolated water stable uranyl(V) complex. Moreover, we demonstrate that dipicolinate complexes of UVI O22+ , UV O2+ and UIV are strongly luminescent with a signature characteristic of each oxidation state. This provides unique examples of luminescent UV and UIV compounds.