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Redox-switchable carboranes for uranium capture and release

Nature volume 577, pages 652–655 (2020)Cite this article

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Abstract

The uranyl ion (UO22+; U(vi) oxidation state) is the most common form of uranium found in terrestrial and aquatic environments and is a central component in nuclear fuel processing and waste remediation efforts. Uranyl capture from either seawater or nuclear waste has been well studied and typically relies on extremely strong chelating/binding affinities to UO22+ using chelating polymers1,2, porous inorganic3,4,5 or carbon-based6,7 materials, as well as homogeneous8 compounds. By contrast, the controlled release of uranyl after capture is less established and can be difficult, expensive or destructive to the initial material2,9. Here we show how harnessing the redox-switchable chelating and donating properties of an ortho-substituted closo-carborane (1,2-(Ph2PO)2-1,2-C2B10H10) cluster molecule can lead to the controlled chemical or electrochemical capture and release of UO22+ in monophasic (organic) or biphasic (organic/aqueous) model solvent systems. This is achieved by taking advantage of the increase in the ligand bite angle when the closo-carborane is reduced to the nido-carborane, resulting in C–C bond rupture and cage opening. The use of electrochemical methods for uranyl capture and release may complement existing sorbent and processing systems.

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Fig. 1: Chemical or electrochemical capture and release of UO22+ with resulting complexes shown.
Fig. 2: Electrochemical setup and quantification data for the capture (blue) and release (red) of UO22+ in solution.
Fig. 3: Simplified depiction of half H-cell and spectroscopic measurements for the biphasic electrochemical capture/release of dissolved UO22+ (yellow sphere) from/to buffered aqueous solutions.

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Data availability

X-ray data are available free of charge from the Cambridge Crystallographic Data Centre (htpps://www.ccdc.cam.ac.uk/data_request/cif) under reference numbers CCDC-1903723 (1), CCDC-1903724 (2a), CCDC-1903725 (3) and CCDC-1903726 (4). All other data generated or analysed during this study are included in the published article.

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Acknowledgements

We thank H. Zhou and G. Wu for assistance with NMR and XRD experiments, respectively. J. Kaare-Rasmussen is thanked for experimental support. The US-Israel Binational Science Foundation (grant 2016241), the ACS Petroleum Research Fund (58693-DNI3), the US Department of Energy, Office of Basic Energy Sciences (contract DE-SC-0001861), the University of California, Santa Barbara, and Tel Aviv University are thanked for financial support.

Author information

Authors and Affiliations

  1. Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, USA

    Megan Keener, Camden Hunt, Timothy G. Carroll, Trevor W. Hayton & Gabriel Ménard

  2. School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv, Israel

    Vladimir Kampel & Roman Dobrovetsky

Authors
  1. Megan Keener
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  2. Camden Hunt
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Contributions

M.K. carried out the synthetic work and analytical characterization, performed the electrochemical–chemical monophasic reactions and acquired most of the NMR and XRD data. C.H. devised the electrochemical setup and the mono- and biphasic experiments. T.G.C. and M.K. devised all the biphasic capture and release experiments. T.G.C. and V.K. synthesized precursor 1. R.D. performed all DFT studies. M.K., C.H. and G.M. wrote the manuscript with input from all authors. R.D., T.W.H. and G.M. assisted with data analysis. G.M. directed the research.

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Correspondence to Gabriel Ménard.

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The authors declare no competing interests.

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Peer review information Nature thanks Vincent Lavallo, Zuowei Xie and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Solid-state molecular structures and electron density surfaces.

a, b, Solid-state molecular structures of complexes 1 (a) and 2a (b). H atoms, \({[{{\rm{CoCp}}}_{2}^{\ast }]}^{+}\)counter cations (2a) and all co-crystallized solvent molecules are omitted for clarity. c, Selected interatomic distances and angles for complexes 1, 2a, 3 and 4. d, Electron density surfaces with colour-coded electrostatic potentials obtained from DFT calculations using optimized structures of 1 and the anion of 2a, labelled 2 (negative values (red) are indicative of higher electron density).

Extended Data Fig. 2 Cyclic voltammetry data for 1 and 4.

a, Cyclic voltammogram of 1 (0.5 mM) in a 0.1 M [Bu4N][PF6] THF solution, measured using a 3-mm-diameter glassy carbon working electrode and a platinum-wire counter electrode, referenced to the Fc+/Fc redox couple (scan rate, 100 mV s−1). The quasi-reversible redox event exhibits two cathodic waves at −0.93 V and −1.11 V. b, Cyclic voltammogram of 4 (1.0 mM) in a 0.1 M [Bu4N][PF6] PC solution, obtained using a 3-mm-diameter glassy carbon working electrode and a platinum-wire counter electrode, referenced to the Fc+/Fc couple (scan rate, 100 mV s−1). The quasi-reversible redox event exhibits one anodic wave at −0.42 V.

Extended Data Fig. 3 31P{1H} NMR spectra from competition experiments of 1, 2a and TPO.

a, 31P{1H} NMR spectrum of 2.0 equiv. 1 with 1.0 equiv. UO2Cl2(TPO)2 in DCM-d2. Relative integrations are shown in red. b, 31P{1H} NMR spectra of in situ reactions of 1.0 equiv. 2a with 1.0 equiv. UO2Cl2(TPO)2 in DCM-d2. Rapid precipitation of a yellow solid was observed. Top, 31P{1H} NMR spectrum of the DCM-d2 supernatant. An unknown byproduct at 47 ppm is observed. Bottom, 31P{1H} NMR spectrum of the filtrate dissolved in PC with a DCM-d2 capillary tube.

Extended Data Fig. 4 31P{1H} NMR spectra from competition experiments of PC with UO2Cl2(TPO)2.

a, Initial 31P{1H} NMR spectrum of UO2Cl2(TPO)2 in DCM-d2, displaying the proposed cis:trans isomers. b, 31P{1H} NMR spectrum after the addition of 20.0 equiv. PC to the solution in a. c, 31P{1H} NMR spectrum after the addition of 40.0 equiv. PC to the solution in a.

Extended Data Fig. 5 31P{1H} NMR spectra for chemical capture and release of UO22+.

Reactions were carried out in a PC:benzene (3:1) solvent mixture with a MeCN-d3 capillary tube insert using Mes3P as the analytical standard. A relaxation delay of 30 s was used to obtain accurate integrations of all species (see Methods). a, 31P{1H} NMR spectrum of 4.0 equiv. TPO and 2.0 equiv. 1. b, 31P{1H} NMR spectrum of 4.0 equiv. TPO and 2.0 equiv. 1 in the presence of 0.5 equiv. [UO2Cl2(THF)2]2. c, 31P{1H} NMR spectrum of 4.0 equiv. \({{\rm{CoCp}}}_{2}^{\ast }\) added to the reaction in b. d, 31P{1H} NMR spectrum after addition of 4.0 equiv. [Fc][PF6] to the reaction in c. The reaction conditions are detailed in Methods.

Extended Data Fig. 6 Monophasic electrochemical capture and release of UO22+ by GBE.

Reactions were carried out in a PC:benzene (3:1) solvent mixture with a MeCN-d3 capillary tube insert with [Ph3PNPPh3][PF6] as the analytical standard. A relaxation delay of 40 s was used to obtain accurate integrations of all species (see Methods). a, A: 31P{1H} NMR spectrum of 6.0 equiv. TPO and 5.0 equiv. 1, with 1.0 equiv. [Ph3PNPPh3][PF6] as the analytical standard. B: 31P{1H} NMR spectrum of 6.0 equiv. TPO and 5.0 equiv. 1 in the presence of 0.5 equiv. [UO2Cl2(THF)2]2. Cycles 1–6: 31P{1H} NMR spectra of charged (blue) and discharged (red) solutions. An unknown species begins to appear at 45 ppm after multiple cycles. Detailed experimental conditions are given in Methods. b, Plot of integrated values for all 31P-containing species, obtained from the charged spectra versus the charge cycle number. The repeated cycling resulted in loss of electrochemically generated 3N (average loss of 15.6% per cycle) due to presumed chloride migration over the anion-exchange membrane. There was little change in the yield of TPO (average loss of 0.3% per cycle) with larger losses in 1 (average loss of 7.2% per cycle) and 4N (average loss of 3.4% per cycle). The values of per cent loss per cycle were estimated from the calculated trendlines by taking the ratio of the slope versus the y-intercept values.

Extended Data Fig. 7 Stepwise procedure for the biphasic electrochemical capture and release of UO22+ (yellow U).

For simplicity, only half of the H-cell is displayed here. For the full cell design, see Extended Data Fig. 8c, d. a, 31P{1H} NMR spectrum of DCE layer containing only 1 and [Bu4N][PF6] ([PF6]− resonance not shown) before charging. b, Top inset, UV-Vis spectrum of aqueous phase containing 1.25 equiv. UO22+ (from UO2(NO3)2(THF)2) before mixing with the DCE phase. Bottom inset, 31P{1H} NMR spectrum of DCE layer containing 2b (major) and 1 (minor) after charging 1 (from a) galvanostatically. c, Mixing of the phases in b for 2 h. d, Top inset, UV-Vis spectrum of aqueous phase after mixing with the DCE phase, revealing approximately 0.35 equiv. UO22+ remaining. Bottom inset, 31P{1H} NMR spectrum of the DCE layer after mixing with aqueous phase, showing captured products 3N/4N (major) and 1 (minor). e, 31P{1H} NMR spectrum of the DCE layer following phase separation and galvanostatic discharge. A broad peak is observed at 38 ppm, which we attribute to an adduct of UO22+ with 1. This, together with the broadened peak of 1, accounts for ~75% of products. The remaining unknown byproducts are marked with # or *. f, Top inset, UV-Vis spectrum of aqueous phase after addition of fresh buffer to the discharged DCE solution (in e) and mixing for 12 h. The spectrum reveals the release of approximately 0.50 equiv. UO22+. Bottom inset, 31P{1H} NMR spectrum of the DCE layer after mixing with fresh buffer, showing the free carborane 1 (major), as well as unknown byproducts at 44 ppm and 20 ppm marked by * (~20% of total).

Extended Data Fig. 8 Diagrams for mono- and biphasic electrochemical cells.

a, Schematic of the divided H-cell used for the monophasic galvanostatic bulk electrolysis cycling experiments with UO22+. b, Photograph of the divided H-cell used for the monophasic galvanostatic bulk electrolysis cycling experiments with UO22+. c, Schematic of the two-compartment H-cell used for the biphasic electrochemical capture and release of UO22+. d, Photograph of the two-compartment H-cell used for the biphasic electrochemical capture and release of UO22+.

Extended Data Fig. 9 31P{1H} NMR spectra for biphasic electrochemical capture and release of UO22+.

a, 31P{1H} NMR spectrum of electrochemically reduced 1 in DCE to produce 2b. b, 31P{1H} NMR spectrum of DCE layer following UO22+ capture from the aqueous layer containing UO2(NO3)2(THF)2 in 0.1 M sodium acetate buffer. c, 31P{1H} NMR spectrum of an electrochemically oxidized DCE layer containing 3N/4N following extraction of UO22+ into 0.1 M sodium acetate buffer. Minor unknown byproducts (marked by asterisks) are also observed. Detailed experimental conditions are found in Methods.

Extended Data Fig. 10 UV-Vis spectra for the biphasic electrochemical capture and release of UO22+ and controls.

a, Initial UV-Vis spectrum of UO2(NO3)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 [Bu4N][PF6] DCE solution for 2 h, indicating a residual concentration of 0.0073 M, consistent with a total quantity of captured UO22+ 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 UO22+ was calculated to be 0.010 M, consistent with a total quantity of 0.031 mmol of released UO22+ from the DCE layer to the aqueous phase. c, Control for UO22+ migration from water to DCE in the absence of carborane (1 or 2a/b). Initial UV-Vis spectrum of UO22+ 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 [Bu4N][PF6] (0.1 M; red). d, Corresponding UV-Vis spectrum of DCE layer after mixing for 4 h with the aqueous layer containing UO22+ shown in c. e, Control for UO22+ migration from water to DCE in the presence of neutral carborane (1). Initial UV-Vis spectrum of UO22+ (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 [Bu4N][PF6] (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 UO22+ shown in e. The UO22+ 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.

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Keener, M., Hunt, C., Carroll, T.G. et al. Redox-switchable carboranes for uranium capture and release. Nature 577, 652–655 (2020). https://doi.org/10.1038/s41586-019-1926-4

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