Aqueous macrocyclic N₄O₄ chelates of tetravalent uranium

The coordination chemistry of lanthanides and early actinides complexes in aqueous solutions is of great interest both for understanding basic chemistry as well for applications in environmental and/or medical hazards-decontamination.

We have recently reported of our study regarding the mono-capped square antiprismatic [U^IVDOTA(L_ax)] complex and the effect of the axial ligand (L_ax) substitution on the complex electronic configuration and redox properties in aqueous solutions¹. The U^IV cation coordinative sphere in this complex is saturated by the DOTA^4- chelate, through its four N-donor and four carboxylate groups (N₄O₄), and an axially bonded small ligand such as H₂O, OH^- and F^-. The rigid macrocyclic coordination of the U^IV cation inhibits its thermodynamically-favored oxidation by dissolved dioxygen in aqueous solutions. We look to further investigate the use of likewise N₄O₄-framework macrocyclic ligands as potential chelates for the U^IV cation. Such compounds are mostly discussed today with regard to their metaloorganic characteristics in non-aqueous solutions and were shown before to introduce some exceptional properties to the chelated U^IV and U^V cations². Elaborate study of similar complexes in aqueous solutions may hold great prospect for development of novel nuclear fuel cycle related processes such as nuclear waste treatment techniques.

We herein report of both the deciphered crystal structure of the [U^IVDOTA(F)]^- complex and the synthesis and characterization (UV-VIS, ¹H-NMR, electrochemistry) of the [U^IVDOTAM]⁴⁺ complex, another N₄O₄-framwork macrocyclic complex. The substitution of the U^IV coordination by carboxylate groups of the DOTA^4- chelate in favor of amide O-donor groups of the DOTAM chelate has great effect on the cation stability in aqueous solutions.

1. Dovrat et al., Chemistry an European J. submitted
2. Hümmer et al., Chem. 2017, 56, 6, 3201-3206