Uranyl sequestration: Synthesis and structural characterization of uranyl complexes with a tetradentate methylterephthalamide ligand

Article abstract of DOI:10.1039/c1cc11329a

Uranyl complexes of a bis(methylterephthalamide) ligand (LH₄) have been synthesized and characterized by X-ray crystallography. The structure is an unexpected [Me₄N]₈[L(UO₂)]₄ tetramer, formed via coordination of the two MeTAM units of L to two uranyl moieties. Addition of KOH to the tetramer gave the corresponding monomeric uranyl methoxide species [Me₄N]K₂[LUO₂(OMe)].

Full text of DOI:10.1039/c1cc11329a

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COMMUNICATION
Uranyl sequestration: synthesis and structural characterization of uranyl
complexes with a tetradentate methylterephthalamide ligandwz
Chengbao Ni,a David K. Shuha and Kenneth N. Raymond*ab
Received 7th March 2011, Accepted 21st March 2011
DOI: 10.1039/c1cc11329a
Uranyl complexes of a bis(methylterephthalamide) ligand (LH4)
have been synthesized and characterized by X-ray crystallo-
graphy. The structure is an unexpected [Me4N]8[L(UO2)]4
tetramer, formed via coordination of the two MeTAM units of
L to two uranyl moieties. Addition of KOH to the tetramer
gave the corresponding monomeric uranyl methoxide species
[Me4N]K2[LUO2(OMe)].
(such as ethylene and thiophene) linked ligands14–16 exhibit the
most planar coordination mode about UO22+ and that the 4Li
ligand binds most strongly to UO22+ in all bis-Me-3,2-HOPO
ligands.17 Pellet-Rostaing and coworkers have reported uranyl
sequestration studies of a series of water soluble five-carbon
linked bis-CAM(S) ligands9 as well as calixarene ligands
incorporating two CAM(S) or 1,2-HOPO units.10 They
showed that the efficiency of the bis-CAM(S) ligands depends
on the rigidity and steric hindrance of the spacers and that the
combination of calixarene and CAM(S) or 1,2-HOPO features
different uranyl affinities at different pH.9,10
The uranyl ion (UO22+) is the most common species for
uranium under oxidizing conditions and in vivo.1–3 During
the past few decades, various approaches have been used to
the design of uranyl-selective chelators: several multidentate
ligands based on phosphonic ligands,4–7 siderophore-based
units (Scheme 1),8 or a combination of both4–10 have been
identified as effective. Studies by Durbin, Raymond, and
coworkers have evaluated the efficacy of multidentate ligands
containing catechol derivatives, 3-hydroxy-N-methyl-2-(1H)-
pyridinone (Me-3,2-HOPO), and 1-hydroxypyridin-2-one
(1,2-HOPO) binding units. Several of these ligands are orally
Despite the fact that the MeTAM unit binds more strongly
than CAM(S) or HOPO,8 uranyl sequestration
2+
to UO2
studies with MeTAM ligands have attracted less attention,
due primarily to the very severe toxicity associated with the
earlier 3Li-(MeTAM) and 4Li-(MeTAM) ligands.13 In addition,
the diprotic nature of MeTAM gives the corresponding uranyl
complex ꢀ2 (or more) charge, which greatly complicates the
crystallization process for structural analysis. Given these
issues, the strong binding affinity of MeTAM makes this class
of ligands attractive for nuclear waste remediation. Due to the
low-toxicity and improved solubility of the 5LiO backbone as
shown by previous studies with the 5LiO-(Me-3,2-HOPO)
ligand,13 we prepared the MeTAM analog 5LiO-(MeTAM)
(LH4, Scheme 2) to study the structures of its uranyl complexes.
Herein, we describe the synthesis and characterization of the
uranyl complexes with L. X-Ray crystallography has revealed
that [LUO2]2ꢀ forms an unexpected tetramer [L(UO2)]48ꢀ, in
2+
active decorporation agents and effective in reducing UO2
in both kidneys and skeleton (removal efficiency in the sequence
MeTAM 4 CAM(S) 4 CAM(C) 4 Me-3,2-HOPO 4
1,2-HOPO).8,11–13 Of the ligands evaluated, the 5LiO-
(Me-3,2-HOPO) and 5Li-CAM(S) ligands were identified as
low or non-toxic ligands with high efficiency.13 Subsequently,
tetradentate bis-Me-3,2-HOPO ligands for uranyl sequestration
have been intensely studied.14–17 It has been shown that 2Li
2+
which the two MeTAM units of each L bind to two UO2
ions. The addition of KOH changes the preferred coordination
nature of the tetramer to give the corresponding uranyl
methoxide species [LUO2(OMe)]3ꢀ with the two MeTAM
units of L binding to the same uranyl moiety.
The uranyl complex [Et3NH]2[LUO2]ꢁ2H2O was prepared
by reaction of UO2(NO3)2ꢁ6H2O with LH4 in the presence of a
slight excess of Et3N and isolated as a brown solid.z
Scheme 1 Siderophore-based binding units.
a Chemical Sciences Division, Lawrence Berkeley National
Laboratory, Berkeley, CA 94720, USA
b Department of Chemistry, University of California, Berkeley,
CA 94720, USA. E-mail: raymond@socrates.berkeley.edu;
Fax: +1 510-486-5283; Tel: +1 510-642-7219
w Part 63 in the series ‘‘Specific Sequestering Agents for the Actinides.’’
For Part 62, see ref. 17.
z Electronic supplementary information (ESI) available: Experimental
details. CCDC_816794 and 816795. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1cc11329a
Scheme 2 The 5LiO-(MeTAM) ligand (LH4).
c
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different (2.345(6) A and 2.380(5) A) with the shorter
U–Ophenolate distance corresponding to the smaller torsion
angle (12.9(2)1) between the MeTAM plane and the O–U–O
plane. Thus, the bending of the MeTAM unit may lengthen
the U–Ophenolate distance slightly. The U–Oamide distance of
8ꢀ
2.365(5) A in [L(UO2)]4
is similar to the U–Ophenolate
distances, indicating strong interactions between uranium
and the amide oxygen. Such unusually strong interactions
are precedented by those between the uranyl ion and a
DMF or DMSO molecule occupying the fifth position of
Me-3,2-HOPO complexes.14 The average MeTAM bite angle
is 66.4(2)1, close to those in Na4[U(catechol)]20 and Me-3,2-
HOPO uranyl complexes.14–16,19
The sum of the five equatorial O–U–O angles in
8ꢀ
[L(UO2)]4 is 360.5(2)1 and the mean deviation of the six
equatorial atoms (five oxygens and one uranium) from the
equatorial plane is 0.078(4) A, suggesting a good planar
geometry around UO22+. Unlike the monomeric bis-Me-3,2-
HOPO uranyl complexes, in which the Ophenolate–U–Ophenolate
angle can be viewed as a ‘‘ligand bite angle’’ and depends
strongly on the linker length (65.2(2)1 to 94.1(1)1),15 there is no
such angle in [L(UO2)]48ꢀ. Thus, a comparison of this angle is
not applicable. Instead, comparison of the remaining three
angles to the average value of 75.71, obtained by assuming
each ligand (MeTAM or HOPO) bite angle is approximately
66.51, can be employed.z In a highly relaxed uranyl coordination
environment, such as with untethered binding units, the
remaining angles should be close to 75.71, as is exactly the
case in the untethered uranyl complex (Pr-Me-3,2-HOPO)2-
UO2(DMF) with angles of 74.2(2), 76.8(2), and 76.1(2)1,
respectively.14 In [L(UO2)]48ꢀ, the corresponding angles of
76.0(2), 77.1(2), and 74.6(2)1 are within 1.51 of the average
value, suggesting an equally highly relaxed uranyl coordination
environment. Thus, the bridging coordination of L and the
8ꢀ
Fig. 1 Crystal structure of [LUO2]4 (30% probability) without
hydrogen atoms. The four ligands are presented in four different
ellipsoid formats. Carbon atoms are grey, nitrogen atoms blue, oxygen
atoms red, and uranium atoms green.
[Et3NH]2[LUO2]ꢁ2H2O is highly soluble in MeOH or H2O.
Attempts to crystallize the complex as the [Et3NH]+ salt from
various solvents were unsuccessful. By replacing [Et3NH]+
with Me4N+, red crystals readily formed with slow diffusion
of Et2O into a MeOH/DMF (50 : 1) solution of the uranyl
complex.
The solid state structure of [LUO2]2ꢀ is a tetramer (Fig. 1);
[Me4N]8[L(UO2)]4 crystallizes in space group I41/acd with a
large unit cell (ca. 42 300 A3). There is one [L(UO2)]2ꢀ moiety
in the asymmetric unit and there are eight tetramers in the unit
tetramer formation are consequences of achieving a highly
2+
relaxed and planar geometry around UO2
.
8ꢀ
cell.z As shown in Fig. 1, [L(UO2)]4 has a helical structure,
Addition of a slight excess of KOH into a MeOH solution of
[Et3NH]2[LUO2]ꢁ2H2O causes an immediate color change
from brown to bright red. The 1H NMR spectrum showed
the appearance of a methoxide ligand at 5.54 ppm. In
addition, the amide proton signals suggested the presence of
multiple species.z These observations indicate that the
dominant species is the uranyl methoxide and that the
methoxide ligand forms an equilibrium with other molecules,
such as MeOH or DMSO, in solution.
in which the ligands adopt a bridging coordination mode with
two TAM units in each L coordinated to two uranyl ions.
Uranyl complexes in which the bis-CAM or bis-Me-3,2-HOPO
ligands show a bridging coordination mode have been
reported; however, this has only been observed in ligands
with either very rigid backbones16,18 or very long linker
lengths.15 Considering the moderate length and high flexibility
of the 5LiO backbone and the fact that the closely related
5Li-(Me-3,2-HOPO) ligand forms
a
monomeric uranyl
By using a combination of K+ and Me4N+ ions, large dark
complex,14 the bridging coordination of L in [L(UO2)]4 is
surprising. The fifth position of the uranyl ions in the tetramer
is occupied by an amide oxygen atom, due probably to ligand
distortion which brings the amide oxygen atom in the vicinity
of the uranyl ion.
red crystals of [Me4N]K2[LUO2(OMe)] were obtained.
ꢀ
8ꢀ
[Me4N]K2[LUO2(OMe)] crystallizes in space group P1 with
two [LUO2(OMe)]3ꢀ moieties in the asymmetric unit. The
OamideꢁꢁꢁK+ꢁꢁꢁOoxo interactions link [LUO2(OMe)]3ꢀ moieties
to form a three dimensional network in the solid state.z
Similar K+ꢁ ꢁ ꢁOoxo interactions have been observed in
the solid state structures of other uranyl complexes.2,21–25
Interestingly, one oxo group in [LUO2(OMe)]3ꢀ shows
interactions with two K+ ions (2.825(3) and 3.164(2) A), while
the other oxo group shows no such interaction. Aside from the
association of K+ ions, the most striking differences between
In [L(UO2)]48ꢀ, the slightly lengthened U–O(oxo) distances
(1.785(5) and 1.794(6) A) than those in bis-Me-3,2HOPO or
1,2-HOPO complexes15,16,19 are probably due to the presence
of more negatively charged ligands. The U–Ophenolate distances
are in the range 2.342(6) to 2.397(5) A, comparable to the
average U–O distance of 2.371(4) A in Na4[U(catechol)].20
A
8ꢀ
close examination of the structure revealed that the average
U–Ophenolate distances to the two MeTAM units are slightly
[LUO2(OMe)]3ꢀ and [L(UO2)]4 are the coordination mode
of L and the fifth ligand of UO22+. In [LUO2(OMe)]3ꢀ, the
c
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Chem. Commun., 2011, 47, 6392–6394 6393

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In summary, we have synthesized and structurally charac-
terized the uranyl complexes of a bis-MeTAM ligand for the
8ꢀ
first time. Structural analysis of [L(UO2)]4 shows that the
tetramer formation is governed by the highly relaxed and
planar coordination geometry around UO22+. The corres-
ponding uranyl methoxide complex forms a monomeric salt
structure with inclusion of a methoxide ligand. The results
presented highlight the effect of uranyl coordination geometry
in the molecular structures and will provide information for
the design of uranyl sequestering agents based on the
TAM unit.
We thank Tiffany A. Pham and Drs. Geza Szigethy, Ga-lai
´
Law, and Christopher M. Andolina for help and discussions.
This research is supported by the Director, Office of Science,
Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences of the U.S. Department of Energy
at LBNL under Contract No. DE-AC02-05CH11231.
Fig.
without hydrogen atoms. Up: structure of [K2LUO2(OMe)]ꢀ; down:
side view of [LUO2(OMe)]3ꢀ
2
Crystal structure of [LUO2(OMe)]3ꢀ (30% probability)
.
Notes and references
2+
two MeTAM units of L are coordinated to one UO2
ion
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(Fig. 2) with a methoxide ligand occupying the fifth position.
The dramatically different coordination mode of L is most
likely due to the combined effects of the methoxide ligand and
K+ ions. The methoxide ligand is a much stronger donor
(U–O(3) = 2.288(3) A)26 than the amide oxygen and its
coordination makes the second MeTAM unit free of distortion.
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the second MeTAM unit within the vicinity of uranyl ion, to
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As shown in Fig. 2, [LUO2(OMe)]3ꢀ shows a rather ruffled
structure with large torsion angles (25.4(2) to 33.7(2)1)
between the MeTAM planes and the corresponding O–U–O
planes. The bent binding of the MeTAM units is partially due
to the length of the 5LiO backbone, since a similarly ruffled
structure with bent HOPO binding was observed in the
5Li-(Me-3,2-HOPO) uranyl complex.14 The K+ꢁ ꢁ ꢁOphenolate
interactions further contribute to the bending because the
torsion angles mentioned above are always larger for the
MeTAM unit with K+ꢁ ꢁ ꢁOphenolate interactions than those
without in [LUO2(OMe)]3ꢀ. The sums of the equatorial angles
(360.4(2) and 360.3(2)1) are close to 3601 and the mean
deviation of the equatorial atoms from the equatorial planes
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is ca. 0.065 A, suggesting a similarly good planar geometry
2+
around UO2
as the tetramer. Due to the presence of
K+ꢁ ꢁ ꢁO(1) interaction, the U–O(1) distance is ca. 0.01 A
longer than the corresponding U–O(2) distance. The
U–Ophenolate distances (2.350(2) to 2.412(2) A) and the
21 C. J. Burns, D. L. Clark, R. J. Donohoe, P. B. Duval, B. L. Scott
and C. D. Tait, Inorg. Chem., 2000, 39, 5464.
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MeTAM bite angles (65.81(8) to 66.44(8)1) are similar to those
8ꢀ
in [LUO2]4
(see above). The remaining three equatorial
angles in [LUO2(OMe)]3ꢀ are in the narrow range of
65.81(8) to 78.02(9)1, which are considerably closer to 75.71
than those in the 5Li or m-xyl-(Me-3,2-HOPO) uranyl
complexes,14,15 indicating a much more relaxed coordination
environment around UO22+ in [LUO2(OMe)]3ꢀ than those in
the Me-3,2-HOPO complexes.z
c
6394 Chem. Commun., 2011, 47, 6392–6394
This journal is The Royal Society of Chemistry 2011