On the Aqueous Chemistry of the U^IV–DOTA Complex

Abstract

The 1,4,7,10‐tetrazacyclodecane‐1,4,7,10‐tetraacetic acid (DOTA) aqueous complex of UIV with H2O, OH−, and F− as axial ligands was studied by using UV/Vis spectrophotometry, ESI‐MS, NMR spectroscopy, X‐ray crystallography, and electrochemistry. The UIV–DOTA complex with either water or fluoride as axial ligands was found to be inert to oxidation by molecular oxygen, whereas the complex with hydroxide as an axial ligand slowly hydrolyzed and was oxidized by dioxygen to a diuranate precipitate. The combined data set acquired shows that, although axial substitution of fluoride and hydroxide ligands instead of water does not seem to significantly change the aqueous DOTA complex structure, it has an important effect on the electronic configuration of the complex. The UIV/UIII redox couple was found to be quasi‐reversible for the complex with both axially bonded H2O and hydroxide, but irreversible for the complex with axially bonded fluoride. Intriguingly, binding of the axial fluoride renders the irreversible one‐electron UV/UIV oxidation of the [UIV(DOTA)(H2O)] complex quasi‐reversible, which suggests the formation of the short‐lived pentavalent form of the complex, an aqueous non‐uranyl chelated UV cation.

Oligomers Intermediates in Between Two New Distinct Homonuclear Uranium(IV) DOTP Complexes**

“…Finally, the coordination number of the chelated “in‐cage” U 4+ cation in 2 is an opened question. We did not find any similar spectral change (UV‐Vis, AT‐IR 1 H‐NMR, 31 P‐NMR, not shown) for the complex in the solution in the presence of excess F − analogous to that recorded for the U IV DOTA complex [1a] . Together with the lack of significant pH‐dependent spectral shift, it is tempting to suggest that the 9 th coordination position is left empty as was reported for the other smaller size Ln 3+ cations complexes [1f,7–9] .…”

“…Due to its similar characteristics to those reported for the U IV DOTA complex, [1a] as well its irreversibility and stability, it is plausible to argue that 2 is a 1 : 1 N 4 O 4 chelated U IV cation inside the DOTP “cage”. The pH‐driven, putative formation of the thermodynamically stable N 4 O 4 complex at pH>8.0 can be related to similar reports regarding the “out‐of‐cage”→“in‐cage” of Ln 3+ DOTA complexes.…”

Controlling the Reduction of Chelated Uranyl to Stable Tetravalent Uranium Coordination Complexes in Aqueous Solution

“…Experimental confirmation of inert U IV complexes toward oxidation or hydrolysis in aqueous solutions is unusual, with only a few known examples. 27,30,39,40 Extended aging of U IV in aqueous media can also lead to changes in metal-ion speciation; 35,40 however, this was not observed here via UV−vis or investigated via other techniques, which suggests the U IV -343-HOPO complex is very stable, a likely result of the thermodynamic benefits of the chelate effect and the known selectivity of 343-HOPO for tetravalent metal cations. 41 To improve our understanding of U IV and U VI coordination by the 343-ligands, we turned to X-ray absorption spectroscopy.…”

“…The chemically-mediated reduction of U, from +VI to +IV, is known in synthetic schemes, 25−27 biological systems, 28 and the environment, 29 yet the reduction is complicated by the known instability of U IV in the presence of molecular oxygen. 27,30 This complication makes the U VI /U IV redox couple hard to evaluate and has thus far limited the applicability of chelation-driven reduction as a means for U decorporation, although recent efforts have elucidated some details about U two-electron processes in both aqueous and nonaqueous systems. 27,31−33 Here we began our investigation into uranium chelation by looking at fundamental binding interactions of U IV and U VI complexes featuring 343-HOPO as well as three synthetic analogs via UV−vis spectrophotometry.…”

“…These results are particularly notable as tetravalent actinides are prone to hydrolysis, even under very acidic conditions, 35 thus electrochemical characterization of aqueous U IV complexes with organic ligands is scarce, with only a few known examples studied in aqueous systems. 30,36 ■ RESULTS AND DISCUSSION Optical Absorption Spectroscopy. The UV−visible (UV−vis) absorbance spectra of 343-HOPO under acidic and neutral pH conditions upon addition of U IV or U VI are shown in Figure 1.…”