a, Initial UV-Vis spectrum of UO 2 (NO 3 ) 2 (THF) 2 (0.042 g, 0.078 mmol, 0.026 M, 1.25 equiv.) in 3 ml of 0.1 M sodium acetate buffer at pH 5.4 (blue). UV-Vis spectrum taken after mixing the aqueous layer with the DCE layer of electrochemically reduced 1 (to generate 2b) in a 0.1 M [Bu 4 N][PF 6 ] DCE solution for 2 h, indicating a residual concentration of 0.0073 M, consistent with a total quantity of captured UO 2 2+ to the DCE layer of 0.056 mmol (red). b, UV-Vis spectrum of 0.1 M aqueous sodium acetate buffer layer at pH 5.4 after mixing for 12 h with electrochemically oxidized 3N/4N in DCE. The concentration of UO 2 2+ was calculated to be 0.010 M, consistent with a total quantity of 0.031 mmol of released UO 2 2+ from the DCE layer to the aqueous phase. c, Control for UO 2 2+ migration from water to DCE in the absence of carborane (1 or 2a/b). Initial UV-Vis spectrum of UO 2 2+ in 0.1 M sodium acetate-buffered solution at pH 5.4 (blue). UV-Vis spectrum of aqueous layer after mixing for 4 h with DCE solution containing [Bu 4 N][PF 6 ] (0.1 M; red). d, Corresponding UV-Vis spectrum of DCE layer after mixing for 4 h with the aqueous layer containing UO 2 2+ shown in c. e, Control for UO 2 2+ migration from water to DCE in the presence of neutral carborane (1). Initial UV-Vis spectrum of UO 2 2+ (1.0 equiv.) in 0.1 M sodium acetate-buffered solution at pH 5.4 (blue). UV-Vis spectrum of aqueous layer after mixing for 3 h with DCE solution containing [Bu 4 N][PF 6 ] (0.1 M) and 1 (1.0 equiv.; red). f, Corresponding UV-Vis spectrum of DCE layer containing 1 after mixing for 3 h with the aqueous layer containing UO 2 2+ shown in e. The UO 2 2+ extinction coefficient was experimentally determined to be 7.715 L mol−1 cm−1 (460 nm) at pH 5.4. See Methods for experimental details.