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 (Cb). The series features new cage-substituted variants of Cb, namely, 9-I-^POCb (CbI), 9,12-I₂-^POCb (CbI), 9,12-Me₂-^POCb (CbMe), 9,12-Et₂-^POCb (CbEt), and 4,5,7,8,9,10,11,12-Me₈-^POCb (CbMe). Aryl-substituted variants include 1,2-((4-MeO-Ph)₂PO)₂-Cb (Cb) and 1,2-((4-F-Ph)₂PO)₂-Cb (Cb). 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 , 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 and, surprisingly, more closely related to their hydrophobic rather than their Lewis basic properties.