The Key Role of U₂₈ in the Aqueous Self‐Assembly of Uranyl Peroxide Nanocages

Abstract

For 11 years now, the structural diversity and aesthetic beauty of uranyl-peroxide capsules have fascinated researchers from the diverse fields of mineralogy, polyoxometalate chemistry, and nuclear fuel technologies. There is still much to be learned about the mechanisms of the self-assembly process, and the role of solution parameters including pH, alkali template, temperature, time, and others. Here we have exploited the high solubility of the UO2 (2+) /H2 O2 /LiOH aqueous system to address the effect of the hydroxide concentration. Important techniques of this study are single-crystal X-ray diffraction, small-angle X-ray scattering, and Raman spectroscopy. Three key phases dominate the solution speciation as a function of time and the LiOH/UO2 (2+) ratio: the uranyl-triperoxide monomer [UO2 (O2 )3 ](4-) and the two capsules [(UO2 )(O2 )(OH)]24 (24-) (U24 ) and [(UO2 )(O2 )1.5 ]28 (28-) (U28 ). When the LiOH/U ratio is around three, U28 forms rapidly and this cluster can be isolated in high yield and purity. This result was most surprising and challenges the hypothesis that alkali templating is the most important determinant in the cluster geometry. Moreover, analogous experiments with KOH, NH4 OH, and TEAOH (TEA=tetraethylammonium) also rapidly yield U28 , which suggests that U28 is the kinetically favored species. Complete mapping of the pH-time phase space reveals only a narrow window of the U28 dominance, which is why it was previously overlooked as an important kinetic species in this chemical system, as well as others with different counterions.

Benchmarking Uranyl Peroxide Capsule Chemistry in Organic Media

“…Therefore, we suspect that there are encapsulated uranium ions. This has been observed in several single‐crystal structural studies , , …”

“…It has also been shown that, under more acidic conditions, these capsules form with oxalate or pyrophosphate bridges . Mechanisms of cluster formation are not yet fully understood, but both “bottom‐up” cluster assembly (monomeric uranyl to cluster),, and top‐down assembly (studtite mineral to cluster) have been amply demonstrated, and even an evolutionary approach (formation of a kinetically favored cluster topology followed by slow conversion into a different topology) has been reported , . Here we also investigated cluster self‐assembly in tandem with oxidation of U IV to U VI .…”

“…Extensive studies of the aqueous behavior of uranyl capsules have included formation kinetics and mechanisms,, , templating by alkali cations and oxoanions,, supramolecular assembly, and exchange of alkali cations between the capsules and the solution. Meanwhile, studies outside aqueous media have not yet been reported.…”

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

“…Speciation assignment of these bands was achieved using both isotopic labeling and DFT calculations. These techniques were highlighted in the case of uranyl triperoxide (UO 2 (O 2 ) 3 ) 4− complexes where the ν 1 mode was observed between 738 and 677 cm −1 [140]. Empirical band predictions based upon bond distances do not accurately predict vibrational frequency, indicating a need for additional computational approaches.…”

Crystallizing Elusive Chromium Polycations

“…In summary, while X-ray scattering suggested hydrolysis and condensation reactions occurred in the reaction solutions, full cluster formation was never observed prior to or in tandem with crystallization, which was unexpected based on our experience with other metal-oxo cluster systems. (Falaise and Nyman, 2016; Goberna-Ferrón et al, 2016; Sadeghi et al, 2015) For an additional control experiment, we prepared a solution of the Al13 Keggin ion, per the well-known synthesis. (Johansson, 1960) In this synthesis, 2.6 equivalents of NaOH are added dropwise to a heated aqueous solution of 0.1 molar aluminum nitrate.…”