Uranyl ion coordination with rigid aromatic carboxylates and structural characterization of their complexes

Article abstract of DOI:10.1039/c3cc43358g

Uranyl complexes of rigid aromatic carboxylates were synthesized and their solid-state structures characterized by X-ray crystallography. The new ligands create cavities lined with endohedral functions to encapsulate the uranyl ion.

Full text of DOI:10.1039/c3cc43358g

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Uranyl ion coordination with rigid aromatic
carboxylates and structural characterization of their
complexes†
Aaron C. Sather,a Orion B. Berryman,b Curtis E. Moorec and Julius Rebek Jr.*a
Cite this: Chem. Commun., 2013,
49, 6379
Received 6th May 2013,
Accepted 30th May 2013
DOI: 10.1039/c3cc43358g
www.rsc.org/chemcomm
Uranyl complexes of rigid aromatic carboxylates were synthesized
and their solid-state structures characterized by X-ray crystallography.
The new ligands create cavities lined with endohedral functions to
encapsulate the uranyl ion.
The principal form of uranium on Earth is the uranyl dication (UO22+).
Recovery of this ion is a longstanding goal for the purposes of
environmental remediation, nuclear waste processing, and
harvesting for energy production.1 The distinctive shape of the uranyl
ion not only provides a basis for ligand design,2 but its unique
coordination environment has also given rise to the controlled self-
assembly of uranyl polyhedra,3 and remarkable reactivity at the
otherwise inert uranyl oxygen atoms.4 The linear UO22+ prefers to
interact with hard oxygen donors and its characteristically short
uranium–oxygen bonds limit coordination to five or six donor atoms
about its equator. This is evidenced by the predicted form of uranium
in sea water, uranyl tricarbonate UO2(CO3)34À.5 Due to the small bite
angle of the planar carbonate ligand, three fit around the metal center
with optimal placement of the oxygen atoms. Chelation of this type
Scheme 1 Synthesis of 3,5-diphenyl-1H-pyrazole-4-carboxylic acid 4 and original
2,6-terphenyl carboxylic acid ligand 5. DCM = dichloromethane; TFA = trifluoroacetic
acid.
has been effectively extended to synthetic ligands using carboxylates panels to surround the uranyl ion, and we provide solid-state
as the coordinating unit.2,6 Specifically, three 2,6-terphenyl carboxylic
structures of their respective complexes. Notably, the crystal
structures of UO2(9) and UO2(10) (Fig. 3) are only the second and
third examples of coordination between a tripodal carboxylate ligand
and the uranyl ion.9
acid ligands have been shown to assemble around the uranyl ion,
encapsulating it within the cavity created by the flanking aromatic
panels of the ligand.7 The new apolar environment surrounding the
metal excludes polar solvent molecules, enhancing the attraction
Our first ligand preserves the aromatic panels of the 2,6-terphenyl
between the carboxylates and the uranyl ion—akin to effects observed system but replaces the central ring with a 1H-pyrazole (Scheme 1).
within enzyme interiors.8 We report here the synthesis of two new The 3,5-diphenyl-1H-pyrazole-4-carboxylic acid 4 was obtained in
ligand systems that expand this notion and incorporate aromatic three steps from b-keto ester 1.10 Acylation of 1 with benzoyl chloride
at room temperature with N,N-dimethylaniline as the solvent yields
the substituted 1,3-diketone 2. Subsequent condensation with
hydrazine and acetic acid afforded pyrazole 3 in excellent yield.
Finally, removal of the tert-butyl ester with trifluoroacetic acid (TFA)
in dichloromethane gave pyrazole ligand 4 as the TFA salt. The
biphenyl pyrazole ligand class was initially chosen as an alternative
to 5 because it maintains the flanking phenyl groups while allowing
a rapid, modular synthesis: desymmetrizing the backbone can be
achieved by altering the acid chloride component in the first step
and further modifications can be made through the use of sub-
stituted hydrazine derivatives in the condensation step.
a The Skaggs Institute for Chemical Biology and Department of Chemistry,
The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla CA,
92037, USA. E-mail: jrebek@scripps.edu
b The Chemistry and Biochemistry Department, University of Montana,
32 Campus Drive, Missoula MT, 59812, USA
c The Department of Chemistry and Biochemistry, The University of California,
San Diego, 9500 Gilman Drive, M/C 0358, La Jolla CA, 92093, USA
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of ligands and complexes. CCDC 930587, 930588 and 940964. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc43358g
c
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pyrazole ligands, and forms bifurcated hydrogen bonds with
the carboxyl groups of the adjacent ligands.
In both uranyl complexes with 4, it is clear that the open geometry
characteristic of the pyrazole ring impacts the coordination behavior
when compared with 5. The wider angle between the phenyl rings
precludes effective encapsulation of the targeted ion and solvent
molecules are free to interact with the complex. Perhaps tethering
three molecules of 4 to a single scaffold or careful modification of the
flanking phenyl rings by the addition of appropriate bulk could
provide the desired encapsulation behavior, but we have yet to observe
this with pyrazole ligand class 4.
Fig. 1 X-ray crystal structure of UO2(4)2(pyr)2 highlighting the incorporation of
pyridine ligands and the p–p stacking interactions.
Uranyl complexes with the new pyrazole-based ligand were
prepared by combining methanolic solutions of uranyl nitrate hexa-
hydrate and 4 in the presence of excess triethylamine (TEA). The
resulting bright yellow solution was evaporated and crystallized by
diffusing pentane onto pyridine solutions of the complex yielding pale
yellow prisms suitable for X-ray diffraction‡ (Fig. 1). In contrast to our
expectations, only two 3,5-diphenyl pyrazole ligands coordinate the
uranyl ion in a trans orientation. In addition, two pyridine molecules
serve as ancillary ligands, and are sandwiched between the phenyl
rings of 4 at p–p stacking distances.11 The closest contact of the
inclined aromatics has a CÁÁÁC distance of 3.80 Å and the stacking is
easily seen in the space filling view of the structure (Fig. 1). An
additional molecule of pyridine also forms a hydrogen bond with the
N–H of the pyrazole ring at a NÁÁÁN distance of 2.82 Å (see ESI†).
As pyridine clearly influences the coordination behavior of 4
with the uranyl ion, it was omitted from further crystallization
attempts. By diffusing diethyl ether onto a methanolic solution
of 4, TEA, and uranyl nitrate, single crystals of a new complex
were obtained12 (Fig. 2) (see ESI†). As was observed previously
with the terphenyl carboxylate ligand 5,7 three pyrazole ligands
surround the uranyl ion in the new complex. However, the
structures of the two systems differ by the inclusion of solvent
molecules. The splayed geometry of the flanking phenyl rings
of 4 ((B1531) compared to 5 (B1231), Scheme 1) creates an
aperture that allows access of solvent molecules to the interior
of the complex. In stark contrast with the original system based
on 5, the solution behavior of both UO2(4)2(pyr)2 and UO2(4)3 is
dynamic and the signals in the 1H NMR spectra from 4 are
broad and unresolved (see ESI†). As highlighted in Fig. 2, the
uranyl complex co-crystallized with a molecule of diethyl ether,
which fills the open space directly above a uranyl oxygen atom.
Additionally, the counter (triethylammonium) cation for the
negatively charged complex is positioned between two of the
The original terphenyl motif 5 was revisited to improve encapsu-
lation of the uranyl ion by fusing the three aromatic ligands onto a
single core. We chose 1,3,5-triethylbenzene as the appropriate
scaffold,13 as its alternating geometry preorganizes and directs
ligands on the 2, 4 and 6 positions. Lithium–halogen exchange of
6
14 with nBuLi at À78 1C followed by addition of trialdehyde 715 and
warming to room temperature gave benzylic alcohols 8 as a mixture
of diastereomers. The phenyl groups flanking the methyl ester of 6
behave as protecting groups, shielding the ester from the lithium
species formed under the reaction conditions. Triols 8 were con-
verted into two ligands: oxidation with pyridinium chlorochromate
(PCC) followed by acidic hydrolysis of the esters gave triketone
ligand 9; reduction with triethylsilane and borontrifluoride diethyl
etherate then ester hydrolysis yielded 10.
The solution behavior of rigid tripodal ligands 9 and 10 with the
uranyl ion were examined by 1H NMR. The respective complexes were
prepared as described above by treating dilute methanolic solutions
of either ligand with TEA followed by addition of uranyl nitrate.
While the free methylene ligand 10 showed the expected, time-
averaged C3v symmetry in solution (provided by the steric gearing of
the 1,3,5-triethylbenzene core), ligand 9 showed multiple resonances
for the ethyl feet, suggesting a less symmetric geometry (Cs) in
solution—with two terphenyl arms of 9 on one side of the benzene
core and the third arm on the other (see ESI†). Despite the solution
behavior of 9 and 10, 1:1 complexes with UO2 were formed with each
1
ligand. H NMR spectra show the UO2 complex with the compara-
tively more rigid 9 has some broad resonances, while the spectrum of
the complex with 10 is sharp and well resolved (see ESI†).
Yellow, single crystals of UO2(9) suitable for X-ray diffraction were
obtained by layering cyclopentane on dilute dichloromethane solu-
tions of the complex‡ (Fig. 3, left). All three carboxylates of 9 are
directed inward, surrounding the uranyl ion. The upper phenyl rings
of the ligand close around the top of the uranyl ion and shield it from
solvent molecules. The negatively charged complex is accompanied by
a triethylammonium counter cation that is sandwiched between the
phenyl groups of adjacent complexes. Single crystals of UO2(10) were
isolated by diffusing methanol into DMF solutions of the complex
and its solid state structure closely resembles that of UO2(9)‡ (Fig. 3,
right). While these crystal structures reveal promising ligand leads, the
inherent solubilities of 9 and 10 and the solution behavior of 9 are
current limitations. Optimization of the linker may restore the desired
time-averaged C3v symmetry of 9 and the addition of solubilizing
groups at the periphery of the ligands could increase their potential
for use as effective extractants for the uranyl ion.16 Additionally, crystal
structures UO2(9) and UO2(10) show that incorporation of a hydroxyl
group on carbons 1 or 2 (Scheme 2), would position a hydrogen bond
Fig. 2 X-ray crystal structure of the UO2(4)3 complex highlighting the open
spaces for the accessibility of solvent to the uranyl ion.
c
6380 Chem. Commun., 2013, 49, 6379--6381
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rcalcd = 1.617 g mLÀ1, m(Cu-Ka) = 10.49 mmÀ1, ymax = 57.71, T =
100(2) K, 17 762 reflections collected, R1 = 0.0218 for 2709 reflections,
Rint = 0.039, (304 parameters) with I > 2s(I), and R1 = 0.0242, wR2À
=
:
0.0648, GooF = 1.09 for all 3067 data, CCDC #930587. Et3NH+Á9UO2
C72H51O11U, 1.5(C5H10), 2.5(CH2Cl2), C6H16N, M = 1749.86, Monoclinic,
P21/n, a = 19.6677(9) Å, b = 18.8428(9) Å, c = 22.6324(10) Å, a = 90.001, b =
109.593(2)1, g = 90.001, V = 7901.8(6) Å3, Z = 4, rcalcd = 1.471 g mLÀ1
,
m(Mo-Ka) = 2.286 mmÀ1, ymax = 25.2421, T = 100(2) K, 64 835 reflections
collected, R1 = 0.0472 for 10 896 reflections, Rint = 0.0846, (949 para-
meters) with I > 2s(I), and R1 = 0.0915, wR2 = 0.1028, GooF = 1.010 for all
16 177 data, CCDC #930588. Et3NH+Á10UO2À: C72H57O8U, C6H16N,
4(CH4O), M = 1518.57, Monoclinic, P21/n, a = 14.2622(12) Å, b =
18.6958(15) Å, c = 26.384(2) Å, a = 90.001, b = 93.484(3)1, g = 90.001,
V = 7022.2(10) Å3, Z = 4, rcalcd = 1.436 g mLÀ1, m(Cu-Ka) = 7.02 mmÀ1
,
ymax = 68.001, T = 100(2) K, 56 112 reflections collected, R1 = 0.0228 for
11 660 reflections, Rint = 0.030 (900 parameters) with I > 2s(I), and R1 =
0.0246, wR2 = 0.0568, GooF = 1.062 for all 12 372 data, CCDC #940964.
Fig. 3 X-ray crystal structures of the UO2(9) and UO2(10) complexes. The
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Scheme 2 (a) PCC, DCM, 2 h, r.t.; (b) CHCl3, Et3SiH, BF3ÁEt2O, 0 1C, 45 min; (c)
AcOH, 48% HBr in H2O, 130 1C.
donor in close proximity to the uranyl oxygen atom, which could
improve affinity.
In summary, we have synthesized and structurally characterized
the uranyl complexes of rigid aromatic carboxylate ligands 4, 9, and
10. With the pyrazole core, the adjacent phenyl groups are open wide,
allowing solvent molecules to interact freely with the interior of
the complex. The terphenyl-based ligands 9 and 10 more-or-less
encapsulate the uranyl ion with aromatic panels. Their poor solubility
and the unfavourable solution geometry of 9 will be addressed in our
future pursuit of the encapsulation of the uranyl ion.
14 E. Reisner and S. J. Lippard, Eur. J. Org. Chem., 2008, 156–163.
´
15 A. Ana Arda, C. Venturi, C. Nativi, O. Francesconi, G. Gabrielli,
Notes and references
˜
´
F. J. Canada, J. Jimenez-Barbero and S. Roelens, Chem.–Eur. J., 2010,
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16 For instance, 9 and 10 are able to extract B20% of the uranyl ion
‡ Crystal data: (pyr)242UO2: C42H32N6O6U, 2(C5H5N), M = 1112.96,
Monoclinic, P21/c, a = 16.8356(11) Å, b = 9.0754(6) Å, c = 14.9921(10) Å,
a = 90.001, b = 93.824(2)1, g = 90.001, V = 2285.5(3) Å3, Z = 2,
from a 400 ppm aqueous solution into the organic phase (see ESI†).
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 6379--6381 6381