Tuning Phosphine Oxide-Substituted ortho-Carboranes for Improved Biphasic Electrochemical UO22+ Capture and Release.

We report the synthesis and characterization of a series of new, tunable 1,2-bis(diarylphosphine oxide)-ortho-carboranes, derivatives of our previously reported uranyl (UO22+) capture agent 1,2-(Ph2PO)2-1,2-C2B10H10 (POCb). The series features new cage-substituted variants of POCb, namely, 9-I-POCb (POCbI), 9,12-I2-POCb (POCbI2), 9,12-Me2-POCb (POCbMe2), 9,12-Et2-POCb (POCbEt2), and 4,5,7,8,9,10,11,12-Me8-POCb (POCbMe8). Aryl-substituted variants include 1,2-((4-MeO-Ph)2PO)2-Cb ((OMe)POCb) and 1,2-((4-F-Ph)2PO)2-Cb ((F)POCb). The effects of electron-withdrawing (EWG) and electron-donating (EDG) groups on resulting carborane redox potentials were assessed using electrochemical means, and the resulting Lewis basicities were quantified using empirical and competition-based NMR experiments. In organic solution, carboranes substituted with EWGs exhibited weaker coordination to UO22+, whereas those with EDGs exhibited stronger coordination. Similar to the previously reported unsubstituted POCb, the tunable new series of carboranes were electrochemically reduced and used for the biphasic capture of UO22+ from an aqueous to an organic phase and back again (release) through electrochemical oxidation. Extraction and back-extraction efficiencies were determined by analyses of the aqueous phases by ICP-OES. While all reduced nido-carboranes efficiently extracted UO22+ in high yields (78-88%)─with seemingly no correlation to the aforementioned measured Lewis basicities─we found the back-extraction of UO22+ to be significantly improved from POCb and, surprisingly, more closely related to their hydrophobic rather than their Lewis basic properties.

Selective heterogeneous capture and release of actinides using carborane-functionalized electrodes.

We report the heterogenization of molecular, electrochemically switchable ortho-substituted carboranes (POCb, POCb-Pyr) for selective metal capture. Films of POCb and POCb-Pyr on glassy carbon and carbon fiber (CF) electrodes demonstrated heterogeneous electrochemical behaviour that was enhanced by the inclusion of single-walled carbon nanotubes (CNTs). Galvanostatically charged CF|CNT|POCb and CF|CNT|POCb-Pyr electrodes selectively captured and released actinides (Th4+, UO22+) from mixed solutions containing alkali (Cs+), lanthanide (Nd3+, Sm3+) and actinide (Th4+, UO22+) metal ions.

Selective electrochemical capture and release of uranyl from aqueous alkali, lanthanide, and actinide mixtures using redox-switchable carboranes.

We report the selective electrochemical biphasic capture of the uranyl cation (UO2 2+) from mixed-metal alkali (Cs+), lanthanide (Nd3+, Sm3+), and actinide (Th4+, UO2 2+) aqueous solutions to an organic, 1,2-dichloroethane (DCE), phase using the ortho-substituted nido-carborane anion, [1,2-(Ph2PO)2-1,2-C2B10H10]2- (POCb2-). The reduced POCb2- is generated by electrochemical reduction of the closo-carborane, POCb, prior to mixing with the aqueous mixed-metal solution. Subsequent UO2 2+ release from the captured product, [UO2(POCb)2]2-, was performed by galvanostatic bulk electrolysis of the DCE phase and back-extraction of UO2 2+ to a fresh aqueous phase. The selective capture and release of UO2 2+ was confirmed by combined ICP-OES and NMR spectral analyses of the aqueous and organic phases, respectively, against the newly synthesized nido-carborane complexes, [[CoCp*2][Cs(POCb)]]2, [CoCp*2]3[Nd(POCb)3], [CoCp*2]3[Sm(POCb)3], and [CoCp*2]2[Th(POCb)3].