Nature | http://www.nature.com | 1Article
Redox-switchable carboranes for uranium
capture and release
Megan Keener^1 , Camden Hunt^1 , Timothy G. Carroll^1 , Vladimir Kampel^2 , Roman Dobrovetsky^2 ,
Trevor W. Hayton^1 & Gabriel Ménard^1 *The uranyl ion (UO 2 2+; 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 UO 2 2+ using chelating polymers^1 ,^2 , porous inorganic^3 –^5
or carbon-based^6 ,^7 materials, as well as homogeneous^8 compounds. By contrast, the
controlled release of uranyl after capture is less established and can be difficult,
expensive or destructive to the initial material^2 ,^9. Here we show how harnessing the
redox-switchable chelating and donating properties of an ortho-substituted closo-
carborane (1,2-(Ph 2 PO) 2 -1,2-C 2 B 10 H 10 ) cluster molecule can lead to the controlled
chemical or electrochemical capture and release of UO 2 2+ 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.Known for over 50 years, carboranes have been extensively studied in
coordination chemistry (including with U), catalysis, luminescence,
and energy storage applications^10 –^15. Studies have shown that reduction
of substituted closo-carboranes to the nido-carboranes results in rup-
ture of the C–C bond and cage opening, with a simultaneous increase
in ligand bite angle, θ (Fig. 1a; closo and nido refer to 2n + 2 and 2n + 4
framework bonding electrons, respectively, where n is the number of
vertices)^11 ,^14 ,^16 –^18. We rationalized that by incorporating donating groups
to ortho-carborane, we could tune the chelating properties of the clus-
ter switching from opened to closed conformations by redox control of
the reduced and oxidized states, respectively, and enable the chemical
or electrochemical capture and release of uranyl in solution (Fig. 1a).
The closo-carborane 1,2-(Ph 2 PO) 2 -1,2-C 2 B 10 H 10 ( 1 ) was synthesized
and fully characterized, including by X-ray diffraction (XRD)
studies (Fig. 1a, Extended Data Fig. 1a)^19. The cage C–C bond length
(1.688(4) Å) and interatomic P···P distance (3.537 Å) are consistent
with previous relevant reports^19 ,^20 (all uncertainties are estimated
standard deviations or standard uncertainties). These metrics
will be used throughout to correlate coordinated and uncoordinated
carboranes, both in lieu of—yet proportional to—the traditional
bite angle θ (Fig. 1a). The cyclic voltammogram of 1 in tetrahydrofuran
(THF) revealed two quasi-reversible cathodic waves at −0.93 V
and −1.11 V relative to the ferrocene/ferrocenium (Fc/Fc+) redox couple
(Extended Data Fig. 2a). Reduction of 1 using 2.0 equiv. decamethyl-
cobaltocene (CoCp 2 ⁎) afforded the direduced nido-carborane,
[CoCp] 2 ⁎ 22 (nido−1,2−(PhPO)− 22 1,2−CB 10 H) 10 (Fig. 1a). The
solid-state structure revealed an open-cage nido-carborane with a
cleaved C–C bond (2.860 Å) and an elongated P···P distance (5.036 Å)
relative to 1 (Extended Data Figs. 1b, c). An analogous salt, [Bu 4 N] 2 [(nido-
1,2-(Ph 2 PO) 2 -1,2-C 2 B 10 H 10 )] (2b) (Fig. 1a), relevant to the electrochemi-
cal experiments, was also synthesized by reduction of 1 with KC 8 ,
followed by salt metathesis with [Bu 4 N][Cl] (see Methods).
We next investigated the coordination chemistry of 1 and 2a. Addi-
tion of 4 equiv. 1 to dimeric [UO 2 Cl 2 (THF) 2 ] 2 in deuterated dichlorometh-
ane (DCM-d 2 ) resulted in a light-yellow solution from which two new
equivalent-intensity^31 P resonances appeared at 38.8 and 38.4 ppm in
the nuclear magnetic resonance (NMR) spectrum, shifted downfield
from 1 (22.8 ppm). The inequivalent P environments suggest either an
octahedral geometry at U with two monodentate 1 ligands or a pen-
tagonal bipyramidal geometry at U with two bidentate 1 and a chloride
in the fifth equatorial site. Although attempts to obtain single crystals
for XRD studies failed, the NMR data suggest that a 2:1 adduct is formed
with a presumed formulation of UO 2 Cl 2 ( 1 ) 2. In contrast to 1 , treatment
of 2 equiv. 2a to [UO 2 Cl 2 (THF) 2 ] 2 led to clean formation of a single new
resonance at 51.1 ppm in the^31 P NMR spectrum, consistent with a biden-
tate coordination mode. XRD studies confirmed the composition as
the uranyl salt [CoCp]⁎ 222 [UOCl( 22 nido−1,2−(PhPO)− 22 1,2−CB 10 H) 10 ]
( 3 ) (Fig. 1a, c). A disubstituted uranyl salt was also obtained by
addition of 4 equiv. 2a to [UO 2 Cl 2 (THF) 2 ] 2. Monitoring the reaction by(^31) P NMR spectroscopy revealed clean conversion to a new product
with a single peak at 52.0 ppm. XRD studies on single crystals
confirmed the composition as the disubstituted complex
[CoCp]⁎ 222 [UO(nido−1,2−(Ph 22 PO)−1,2−CB 2101 H) 02 ] ( 4 ) (Fig. 1a, b).
The bond metrics for 3 and 4 are similar (Extended Data Fig. 1c).
https://doi.org/10.1038/s41586-019-1926-4
Received: 26 March 2019
Accepted: 30 October 2019
Published online: 22 January 2020
(^1) Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA, USA. (^2) School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University,
Tel Aviv, Israel. *e-mail: [email protected]