Selective recognition and extraction of the uranyl ion

Article abstract of DOI:10.1021/ja1035607

A tripodal receptor capable of extracting uranyl ion from aqueous solutions has been developed. At a uranyl concentration of 400 ppm, the developed ligand extracts ～59% of the uranyl ion into the organic phase. The new receptor features three carboxylates

Full text of DOI:10.1021/ja1035607

Published on Web 09/14/2010
Selective Recognition and Extraction of the Uranyl Ion
Aaron C. Sather, Orion B. Berryman, and Julius Rebek, Jr.*
The Skaggs Institute for Chemical Biology and Department of Chemistry, The Scripps Research Institute,
10550 North Torrey Pines Road, La Jolla, California 92037
Received April 26, 2010; E-mail: jrebek@scripps.edu
The new receptor is built from a rigid skeleton featuring three
Abstract: A tripodal receptor capable of extracting uranyl ion from
aqueous solutions has been developed. At a uranyl concentration
carboxylates that converge on the uranyl ion through bidentate
interactions.13 We combined the triethylbenzene core with Kemp’s
of 400 ppm, the developed ligand extracts ∼59% of the uranyl
ion into the organic phase. The new receptor features three
carboxylates that converge on the uranyl ion through bidentate
interactions. Solution studies reveal slow exchange of the car-
boxylates on the NMR time scale. The crystal structure of the
complex shows that the carboxylates coordinate to uranyl ion
while the amides hydrogen bond to one of the uranyl oxo-oxygen
atoms. The hydrophobic coating of the ligand and its rigidity
contribute to its ability to selectively extract uranyl ion from dilute
aqueous solutions.
triacid14,15 to minimize the internal rotations of the new ligand.
The appropriate size, shape, and chemical complementarity was
found in the use of hydrazine as the linker. Condensation of the
known trihydrazide 316 with the Kemp anhydride acid chloride gave
ligand 2 in good yield (Scheme 1). The planes of the imide and
amide functions of 2 prefer to be perpendicular,17,18 and to avoid
unfavorable steric and electronic clashes, the acid and the amide
N-H are fixed near each other. The only remaining (freely rotating)
single bonds in 2 are those indicated in the scheme. In one
conformation, ligand 2 presents the three carboxyl groups well-
positioned to satisfy the equatorial coordination geometry of the
uranyl ion and simultaneously directs the three amide hydrogens
toward a uranyl oxygen atom.
Recognition of the uranyl ion (UO22+) is a long-standing goal
for purposes of environmental remediation, metallurgical extraction,
and water purification. Uranium is found in a variety of sources,
both terrestrial and aqueous. Typical uranium ores contain deposits
at ∼1000 ppm. A more subtle source of uranium is coal ash, where
it is found at 300 ppm.1 The uranium present in aqueous
environments exists naturally or as a contaminant. For instance,
some Bavarian drinking water is polluted with uranium at 10 ppb,
and contamination of the Dnieper River in Kiev has reached 11.5
ppm.2 The Earth’s oceans are a natural source where more than
99% of all global uranium resides. However, the low concentration
of uranyl ion there (3 ppb)3 and the abundance of other cations
call for chelating agents with high affinity and selectivity. We
describe here a tripodal synthetic receptor that through its preor-
ganization and hydrophobicity features, these attributes for the
selective extraction of uranyl ion from water.
Scheme 1. Synthesis of Uranyl Ligand 2
1
The H NMR spectrum of ligand 2 in MeOD (shown in Figure
1) indicates a time-averaged C3V symmetry, the conformation
expected as a result of the directing effects of the 1,3,5-triethyl-
benzene core.19 Treatment of 2 with uranyl nitrate or uranyl acetate
and triethylamine (TEA) in MeOH at room temperature results in
the quantitative formation of uranyl complex 1. Upon complexation
of uranyl ion, many of the signals in the NMR spectrum of 2 exhibit
substantial shifts (Figure 1a,b). Most affected are the methylene
protons, which are shifted ∼0.5 ppm upfield from their positions
for the deprotonated ligand.
When uranyl ion is added to a solution of 2 with no base present,
the resulting proton NMR spectrum is broad and unresolved. Upon
addition of base (TEA), the signals become sharp and resolved,
matching those in the spectrum in Figure 1b. The sharp signals
suggest that ligand 2 is in slow exchange with the complex on the
NMR time scale, a feature that is unusual for carboxylate ligands
bound to uranium.20 In fact, when less than 1 equiv of uranyl ion
is added to a basic solution of 2, separate signals appear for
uncomplexed deprotonated ligand (Figure 1a) and complex (Figure
1b) in solution [see the Supporting Information (SI)].
The linear geometry and characteristically short uranium-oxygen
bonds of the uranyl ion limit the approach of complementary anions
to the equator of the positively charged uranium center. In natural
aqueous environments, the oxophilic uranyl ion is coordinated by
three carbonate ligands.4 The small bite angle of carbonate/
carboxylate allows for three ligands to be positioned around the
metal, fully satisfying its coordination geometry. Complements to
the uranyl ion’s oxygen atoms can approach from many directions,
as is apparent from the structures of uranyl-protein5 and
uranyl-ligand6,7 complexes. Since the pioneering work of Ray-
mond,8 few examples of uranyl ligands containing only carboxylates
have been reported,9-11 even though it is the coordination motif
preferred in nature. Here we present the rigid and preorganized
tripodal carboxylate ligand 2. The crystal structure of complex 1
(see Figure 2), the first structure involving binding of uranium by
a tripodal ligand,12 confirms the 1:1 stoichiometry of the complex
and also indicates hydrogen bonding with the uranyl oxygens. This
ligand extracts uranium from aqueous solutions into organic
solutions with striking selectivity and produces a complex that
shows unusual kinetic stability on the NMR time scale.
A strong base such as TEA is not needed in order to drive
complex formation. For example, sodium acetate or pyridine are
9
13572 J. AM. CHEM. SOC. 2010, 132, 13572–13574
10.1021/ja1035607  2010 American Chemical Society

C O M M U N I C A T I O N S
solution of uranyl nitrate (1.6 equiv) in acetate buffer (pH 5) was
stirred with a chloroform solution of 2 (1 equiv), and the
concentrations of uranium in each phase were determined before
and after extraction with inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) (see the SI). The extraction
experiments were run at a series of uranium concentrations: 400,
40, and 4 ppm. At 400 ppm, 59 ( 9% of 2 successfully extracted
aqueous uranyl ion into the organic phase, and at 40 ppm, 29 (
9% of 2 removed uranium from the aqueous phase.26 At 4 ppm,
no uranium was extracted. The uranium can be recovered from the
ligand by adding 0.5 M HNO3 (see the SI). The reported tripodal
carboxylate ligands of Raymond did not extract uranyl ion at high
concentrations of NaCl.8,27 In contrast, ligand 2 is highly selective
for uranyl. The extraction of uranyl ion at 400 ppm was carried
out in the presence of the six ions that dominate the chemistry of
seawater: Cl-, Na+, Mg2+, Ca2+, K+, and SO42-.3 With these ions
present at seawater concentrations, 2 showed no diminished
function; again, ∼59% of 2 extracted uranyl ion into the organic
phase.
1
Figure 1. (a) H NMR spectrum of uranyl ligand 2 with 3 equiv of TEA
added. (b) 1H NMR spectrum of uranyl ligand 2 with 3 equiv of TEA and
1 equiv of uranyl nitrate added. The asterisks label the methylene protons
adjacent to the amide carbonyl.
sufficiently basic to produce complex 1 quantitatively. Slow
diffusion of pentane into a pyridine solution of 2 and uranyl nitrate
yielded pale-yellow single crystals suitable for X-ray diffraction.21
Ligand 2 and uranyl ion crystallize as the 1:1 complex 1 (Figure
2) with a nearby pyridinium ion to balance the charge.22,23
As anticipated, the crystal structure reveals that the three alkyl
carboxylates converge onto the uranium center, fully satisfying the
hexacoordinate geometry of the uranyl ion. In addition, the three
amide hydrogens near the inner uranyl oxo-oxygen atom produce
secondary stabilizing interactions. These amide hydrogens are able
to interact with the uranyl oxygen through three long hydrogen
bonds with an average N · · · O distance of 3.5 Å. The solid-state
structure of 1 confirms a two-component recognition motif by the
new ligand.
In summary, the new chelating ligand 2 has been synthesized to
complex uranyl ion. The crystal structure of the complex shows
that all three of the carboxylates coordinate to the uranyl ion while
the hydrogens of the amides hydrogen bond to the inner uranyl
oxo-oxygen atom. The hydrogen-bonding interaction was cor-
roborated with IR spectroscopy. In solution, the complex is in slow
1
exchange with the ligand, as shown by the sharp and resolved H
NMR signals. The hydrophobic coating of the ligand and its rigidity
all contribute to its ability to extract uranyl ion out of dilute aqueous
solutions without interference by other ions in seawater.
Acknowledgment. We are grateful to the Skaggs Institute for
support. A.C.S. and O.B.B. are Skaggs Predoctoral and Postdoctoral
Fellows, respectively. We are indebted to the M. G. Finn laboratory
for the generous use of their ICP-AES spectrometer.
Supporting Information Available: 1H NMR, 13C NMR, and mass
spectra of all compounds; experimental details of ICP-AES analysis;
single-crystal X-ray diffraction experimental procedures and data for
complex 1; and complete ref 2. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) The Economist. NoVel Sources of Uranium: Rising from the Ashes, April
8, 2010, http://www.economist.com/node/15865280?story_id)15865280,
accessed September 10, 2010.
(2) Balonov, M.; et al. Radiological Conditions in the Dnieper RiVer Basin;
Radiological Assessment Reports Series; International Atomic Energy
Agency: Vienna, 2006.
(3) Schwochau, K. Top. Curr. Chem. 1984, 124, 91.
(4) Cinneide, S. O.; Scanlan, J. P.; Hynes, M. J. Inorg. Nucl. Chem. 1975, 37,
1013.
(5) Van Horn, J.; Huang, H. Coord. Chem. ReV. 2006, 250, 765.
(6) Arnold, P. L.; Patel, D.; Wilson, C.; Love, J. B. Nature 2008, 451, 315.
(7) Sarsfield, M. J.; Helliwell, M. J. Am. Chem. Soc. 2004, 126, 1036.
(8) Franczyk, T. S.; Czerwinski, K. R.; Raymond, K. N. J. Am. Chem. Soc.
1992, 114, 8138.
(9) Stary, J.; Balek, V. Collect. Czech. Chem. Commun. 1962, 27, 809.
(10) Ramamoorthy, S.; Santappa, M. Bull. Chem. Soc. Jpn. 1969, 42, 411.
(11) Keramidas, A. D.; Rikkou, M. P.; Drouza, C.; Raptopolou, C. P.; Terzis,
A.; Pashalidis, I. Radiochim. Acta 2002, 90, 549.
Figure 2. Views of the X-ray structure of uranyl complex 1. The pyridinium
countercation and other solvent molecules have been removed for clarity.
Carbon is colored gray, nitrogen blue, oxygen red, uranium green, and
hydrogen white. (a) Top view of 1 showing the threefold symmetry. (b)
Side view of 1 showing the amide H atoms directed toward the uranyl
oxygen.
The ligand’s interaction with the uranyl oxo-oxygen atoms was
also corroborated using IR spectroscopy. The strong ν3 antisym-
24
metric stretch of the uranyl ion usually occurs at ∼920 cm-1
,
while the ν1 symmetric stretch is IR-inactive. However, hydrogen
bonding to one of the oxo-oxygens of the uranyl ion removes the
center of symmetry, so the ν1 stretch becomes IR-active.8 The IR
spectrum of complex 1 shows the strong ν3 antisymmetric stretch
of the uranyl ion at 902 cm-1 (see the SI). This stretch is revealing
because it is inversely proportional to the donor strength of the
equatorial ligands.25 The medium band that appears at 842 cm-1
is the ν1 stretch, which confirms the hydrogen-bonding interactions
between the amide hydrogens and one of the oxo-oxygens.
The application of 2 as an agent for uranyl ion sequestration
was investigated through liquid-liquid extractions. An aqueous
(12) A thorough search of the Cambridge Crystal Structure Database version
5.31 showed 1394 structures containing the uranyl ion, none of which
contained a chelate involving carboxylate ligands.
(13) For a related dicarboxylate, see: Marshall, L. R.; Parris, K.; Rebek, J., Jr.;
Luis, S. V.; Burguete, M. I. J. Am. Chem. Soc. 1988, 110, 5192.
(14) Kemp, D. S.; Petrakis, K. S. J. Org. Chem. 1981, 46, 5140.
(15) Rebek, J., Jr.; Askew, B.; Killoran, M.; Nemeth, D.; Lin, F.-T. J. Am. Chem.
Soc. 1987, 109, 2426.
(16) Restorp, P.; Berryman, O. B.; Sather, A. C.; Ajami, D.; Rebek, J., Jr. Chem.
Commun. 2009, 5692.
(17) Felten, A.; Dautrey, S.; Bodiguel, J.; Vanderesse, R.; Didierjean, C.; Arrault,
A.; Jamart-Gre´goire, B. Tetrahedron 2008, 64, 10741.
(18) Apreda, M. C.; Foces-Foces, C.; Cano, F. H.; Garcia-Blanco, S. Acta
Crystallogr. 1978, B34, 3477.
9
J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010 13573

C O M M U N I C A T I O N S
(19) For a review of benzene-based tripodal receptors, see: Hennrich, G.; Anslyn,
E. V. Chem.sEur. J. 2002, 8, 2218.
(23) Unfortunately, the crystal quality was insufficient for modeling of the highly
disordered solvent molecules, and all but two pyridine molecules were
removed with SQUEEZE during the structure refinement (see the SI).
(24) Cattalini, L.; Croatto, U.; Degetto, S.; Tondello, E. Inorg. Chim. Acta, ReV.
1971, 5, 19.
(20) (a) Zoltan, S.; Ingmar, G. Inorg. Chem. 2000, 39, 5036. (b) Beer, S.;
Berryman, O. B.; Ajami, D.; Rebek, J., Jr. Chem. Sci. 2010, 1, 43.
(21) Crystal
data
for
[C10H12N2]+[UO2(C59H75N7O15)]- · 6C5H5N:
C99H112N15O17U; M ) 1945.99; monoclinic; C2/c; a ) 40.971(19) Å, b )
21.705(8) Å, c ) 19.145(7) Å, R ) 90.00°, ꢀ ) 112.144(9)°, γ ) 90.00°;
V ) 15770(11) Å3; Z ) 8; Fcalcd ) 1.633 g mL-1; µ(Mo KR) ) 2.144
mm-1; 2θmax ) 46.78°; T ) 100(2) K; 51633 reflns collected; R1 ) 0.0910
for 7130 reflns; Rint ) 0.1315 (749 parameters) with I > 2σ(I); R1 ) 0.2345,
wR2 ) 0.2629; GOF ) 1.058 for all 11401 data; CCDC 792943.
(25) Bart, S. C.; Meyer, K. Struct. Bonding 2008, 127, 119.
(26) In these extraction experiments, complex 1 was extracted into the organic
phase with sodium as the countercation.
(27) Raymond reported a Kex value of ∼1011, which was extrapolated from
measurements at low pH.8 A direct comparison to this Kex was further
complicated by the charge differences present in the two systems.
(22) At least three different pyridine molecules are present in the unit cell.
Determining which pyridine is protonated is complicated by the presence
of uranium in the structure and the lack of high-angle data.
JA1035607
9
13574 J. AM. CHEM. SOC. VOL. 132, NO. 39, 2010