Shape-Selective Supramolecular Capsules for Actinide Precipitation and Separation

Abstract

Improving actinide separations is key to reducing barriers to medical and industrial actinide isotope production and to addressing the challenges associated with the reprocessing of spent nuclear fuel. Here, we report the first example of a supramolecular anion recognition process that can achieve this goal. We have designed a preorganized triamidoarene receptor that induces quantitative precipitation of the early actinides Th(IV), Np(IV), and Pu(IV) from industrially relevant conditions through the formation of self-assembled hydrogen-bonded capsules. Selectivity over the later An(III) elements is shown through modulation of the nitric acid concentration, and no precipitation of actinyl or transition-metal ions occurs. The Np, Pu, and Am precipitates were characterized structurally by single-crystal X-ray diffraction and reveal shape specificity of the internal hydrogen-bonding array for the encapsulated hexanitratometalates. This work complements ion-exchange resins for 5f-element separations and illustrates the significant potential of supramolecular separation methods that target anionic actinide species.

Figure 1

(Left) Schematic of the precipitation of An(IV) nitratometalates by L from biphasic nitric acid/toluene mixtures and the release of the loaded metal using water. (Right) ²³⁸Pu-containing precipitate on an organic/aqueous interface.

Figure 2

(A) Capsule geometry obtained from the X-ray crystal structure of 1-Pu (side-on view). For clarity, all hydrogen atoms except those involved in hydrogen bonding and a disorder component of the amide arm are omitted (where shown, thermal displacement ellipsoids are drawn at 50% probability). N–H and O–H hydrogen atoms were located in the difference Fourier map and (O1) is 1/3% occupied on a crystallographic special position. Atom colors: Pu = green; oxygen = red; nitrogen = blue; carbon = silver; hydrogen = white. (B) ChemDraw representation of the capsular hexanitratometalate complex [{An(IV)(κ²-NO₃)₆}⊂(L₂)(H₃O)₂]_n (C) Top-down view of a two-capsule segment of the extended structure highlighting the bridging H₃O⁺ units.

Figure 3

Precipitation arising from single-metal solutions of ²³⁷Np (474 ppm) and ²³⁸Pu (486 ppm) and mixed-metal solutions of U and Th (595 and 580 ppm, respectively) in 0.8–8.0 M HNO₃/toluene equal-volume biphasic mixtures after the addition of L (10-fold excess L relative to metal) at 298 K. Lines have been drawn for ease of comprehension.

Figure 4

Precipitation arising from a mixed rare-earth solution (∼500 ppm), single-metal solutions of ²⁴³Am and ^245/248Cm (230 and 265 ppm respectively), and mixed-metal solutions of Bk/Cf (373 and 245 ppm, respectively) in 6.0 to 8.0 M HNO₃/toluene equal-volume biphasic mixture after the addition of L (5-fold excess L relative to metal) at 298 K. Rare-earth precipitations are plotted using data that were previously used to generate a figure found in ref (17).

Figure 5

(Left) X-ray crystal structure of 1-Am (side-on view). For clarity, all hydrogen atoms except those involved in hydrogen bonding and a disorder component of the amide arm are omitted (where shown, thermal displacement ellipsoids are drawn at 50% probability). N–H and O–H hydrogen atoms were located in the difference Fourier map, and (O1)H is 1/2% occupied on a crystallographic special position. Atom colors: Am = pink; oxygen = red; nitrogen = blue; carbon = gray; hydrogen = white. (Right) Selected bond lengths from the X-ray crystal structures of 1-Am and 1-Eu.