Hybrid uranyl-carboxyphosphonate cage clusters

Article abstract of DOI:10.1021/ic4008262

Two new hybrid uranyl-carboxyphosphonate cage clusters built from uranyl peroxide units were crystallized from aqueous solution under ambient conditions in approximately two months. The clusters are built from uranyl hexagonal bipyramids and are connected by employing a secondary metal linker, the 2-carboxyphenylphosphonate ligand. The structure of cluster A is composed of a ten-membered uranyl polyhedral belt that is capped on either end of an elongated cage by five-membered rings of uranyl polyhedra. The structure of cluster B consists of 24 uranyl cations that are arranged into 6 four-membered rings of uranyl polyhedra. Four of the corresponding topological squares are fused together to form a sixteen-membered double uranyl pseudobelt that is capped on either end by 2 topological squares. Cluster A crystallizes over a wide pH range of 4.6-6.8, while cluster B was isolated under narrower pH range of 6.9-7.8. Studies of their fate in aqueous solution upon dissolution of crystals by electrospray ionization mass spectrometry (ESI-MS) and small-angle X-ray scattering (SAXS) provide evidence for their persistence in solution. The well-established characteristic fingerprint from the absorption spectra of the uranium(VI) cations disappears and becomes a nearly featureless peak; nonetheless, the two compounds fluoresce at room temperature.

Full text of DOI:10.1021/ic4008262

Article
pubs.acs.org/IC
Hybrid Uranyl-Carboxyphosphonate Cage Clusters
Pius O. Adelani,† Michael Ozga,† Christine M. Wallace,† Jie Qiu,† Jennifer E. S. Szymanowski,†
Ginger E. Sigmon,† and Peter C. Burns*,†,‡
‡
†Department of Civil and Environmental Engineering and Earth Sciences and Department of Chemistry and Biochemistry,
University of Notre Dame, Notre Dame, Indiana 46556, United States
S
* Supporting Information
ABSTRACT: Two new hybrid uranyl-carboxyphosphonate
cage clusters built from uranyl peroxide units were crystallized
from aqueous solution under ambient conditions in approx-
imately two months. The clusters are built from uranyl
hexagonal bipyramids and are connected by employing a
secondary metal linker, the 2-carboxyphenylphosphonate
ligand. The structure of cluster A is composed of a ten-
membered uranyl polyhedral belt that is capped on either end
of an elongated cage by five-membered rings of uranyl
polyhedra. The structure of cluster B consists of 24 uranyl
cations that are arranged into 6 four-membered rings of uranyl polyhedra. Four of the corresponding topological squares are
fused together to form a sixteen-membered double uranyl pseudobelt that is capped on either end by 2 topological squares.
Cluster A crystallizes over a wide pH range of 4.6−6.8, while cluster B was isolated under narrower pH range of 6.9−7.8. Studies
of their fate in aqueous solution upon dissolution of crystals by electrospray ionization mass spectrometry (ESI-MS) and small-
angle X-ray scattering (SAXS) provide evidence for their persistence in solution. The well-established characteristic fingerprint
from the absorption spectra of the uranium(VI) cations disappears and becomes a nearly featureless peak; nonetheless, the two
compounds fluoresce at room temperature.
choice to introduce curvature in structural units containing
uranyl polyhedra.34,35 The different binding preferences of the
bifunctional 2-carboxyphenylphosphonate ligand allow for the
construction of heterobimetallic U(VI)-3d complexes.36 When
nickel metal ions are incorporated in the structure of uranyl
carboxyphosphonate, a remarkable heteropolyoxometalate
structure results: [H3O]4[Ni(H2O)3]4{Ni[(UO2)-
(PO3C6H4CO2)]3(PO4H)}4·2.72H2O.37
Our current research efforts are focused on incorporating
organic moieties as linkers in the design of uranyl peroxide cage
clusters to vastly expand their structural topologies, properties,
and functions. Herein, we report the synthesis, structural
characterization, and spectroscopic properties of two hybrid
uranyl-carboxyphosphonate cage clusters, (A) and (B).
INTRODUCTION
■
The development of new synthetic strategies to assemble
uranyl peroxide clusters with unique topologies and tunable
properties is presently a key goal in the field of nanostructure
actinide chemistry. To date, over 35 notable actinide peroxide
clusters have been synthesized in aqueous solution under
ambient conditions.1−20 Burns and his co-workers have
demonstrated that uranyl peroxide clusters can be extended
significantly through the incorporation of different anions such
as nitrate, oxalate, phosphate, and phosphonates.8−10,13,15,17
Most of the earlier investigations in this area of study were
centered on the synthesis and structures of actinide clusters,
and future research efforts on their properties and applications
hold considerable promise. However, because of the radio-
activity of actinides, their applications are likely to differ from
those of transition metal polyoxometalates. Notwithstanding,
actinide peroxide clusters are inspiring the development of new
techniques in advanced nuclear energy systems such as mass-
based separations of used nuclear fuel and materials fabrication,
as well as models for actinide transportation in the environ-
ment.2,12,19,20
EXPERIMENTAL SECTION
■
Synthesis. Caution! While the uranium used in these studies was
isotopically depleted, precautions are needed for handling radioactive
materials, and all studies should be conducted in a laboratory dedicated to
studies of toxic and radioactive materials.
Distilled and Millipore filtered water with a resistance of 18.2 MΩ
cm was used in all reactions.
2-Carboxyphenylphosphonic acid (2-CPPA). Diethyl (2-
ethoxycarbonylphenyl)phosphonate (96%, Epsilon Chimie) was
dissolved in 20 mL of concentrated hydrochloric acid and allowed
In the past two decades, several research groups have shown
that actinide atoms can be incorporated in heteropolytungstates
for use in sequestration and storage of actinide waste.21−30
Further studies have examined the incorporation of uranyl
peroxo in polyoxotungstates, thus inspiring their use as
potential candidates in the design of catalysts.31−33 Phospho-
nate derivatives are pliable and have proven to be a ligand of
Received: April 4, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic4008262 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
to reflux for 24 h. The white crystals of 2-carboxyphenylphosphonic
acid (2-CPPA) formed during cooling to room temperature. The
crystals were filtered and dried in an oven at 60 °C.
Table 1. Crystallographic Data for A and B
cluster A
cluster B
9582.60
[K18Li12][(UO2)20(HO2CC6H4PO3)10(O2)20(OH)10](H2O)n (A) and
[K3Li21[(UO2)24(HO2C-C6H4PO3)8(O2)24(OH)8](H2O)n (B). Crystals of
A were synthesized by loading a mixture of solutions containing
UO2(NO3)2·6H2O (0.5 M, 0.1 mL), H2O2 (30%, 0.1 mL), LiOH (2.4
M, 0.1 mL), KCl (0.5M, 0.1 mL), and 2-CPPA (0.25 M, 0.2 mL) in a 4
mL scintillation vial with initial pH ranging over 4.8−6.8. The resulting
solution was left standing opened to air to permit slow evaporation,
and yellow tablets of A appeared after approximately two months
(yield: 44% on the basis of uranium). Yellow crystals of B were
isolated, along with fine-grained yellow precipitates that we have been
unable to identify, by slightly modifying a mixture of the solution
above. We increased the pH slightly with LiOH (2.4 M, 0.12 mL) and
reduced the concentration of KCl (0.5 M, 0.05 mL) to minimize the
precipitates formed. The initial pH values ranged from 6.9−7.8 (yield:
∼10% on the basis of uranium).
Crystallographic Studies. A single crystal of each of the
compounds was mounted on a cryoloop and optically aligned on a
Bruker APEX II Quazar CCD X-ray diffractometer using a digital
camera. Initial intensity measurements were performed using a IμS X-
ray source and a 30 W microfocused sealed tube (Mo Kα, λ = 0.71073
Å) with a monocapillary collimator. Semiempirical corrections for
absorption were applied to the full sphere of data collected in each
case using the program SADABS. Data were integrated using the
Bruker APEX II software, and the SHELXTL system of programs was
used for the solution and refinement of each structure.38 Refinement
of crystal structures such as these that contain cage clusters built from
uranyl polyhedra is particularly challenging because of the X-ray
scattering dominance of uranium as compared to the lighter atoms
(i.e., H, C, and O), the presence of relatively large voids with little
electron density, and disorder of counterions and water molecules.
These factors result in relatively weak and poor diffraction at higher
angles, and limit the resolution of the crystal structure. Soft constraints
were added to the refinements in this study to improve the geometries
of the disordered counterions and lighter elements. Despite the
shortcomings of X-ray diffraction for studying such structures, the
method provides definitive details of many aspects of the structures.
Lithium cations are especially difficult to locate and may be mostly
disordered. Chemical analyses and charge-balancing considerations
provide further details of the formula for the crystals studied. Selected
crystallographic data are presented in Table 1. Atomic coordinates,
bond distances, and additional structural information are provided in
the crystallographic data (CIF).
Elemental Analysis. To confirm the presence of U, P, K, and Li
cations, crystals of each of the compounds were washed lightly with
deionized water under vacuum and were subsequently dissolved for
analysis using a PerkinElmer ICP-OES. Energy dispersive spectra
(EDS/EDX) were collected for single crystals corresponding to each
cluster using a LEO EVO-50XVP variable-pressure/high-humidity
scanning electron microscope. Spectra collected for each compound
confirmed the presence of U, P, and K for A and B, respectively,
consistent with the results from ICP-OES which also confirmed the
presence of Li cations.
Small-Angle X-ray Scattering. Small-angle X-ray scattering
(SAXS) data were collected using a Bruker Nanostar equipped with
a Cu microfocus source, Montel multilayer optics, and a HiSTAR
multiwire detector. Data were collected with the sample chamber
under vacuum and a sample-to-detector distance of 26.3 cm. Crystals
were first isolated from their mother solution by vacuum filtration.
They were rinsed gently using water and were then harvested from the
filter membrane prior to dissolution in ultrapure water. The resulting
solutions were drawn into 0.5 mm diameter glass capillaries, and the
ends of each capillary were sealed using wax. Water in an identical
capillary was used for background measurement.
formula mass
color and habit
space group
a (Å)
9354.88
yellow, tablet
P21/c (No. 14)
17.975(3)
34.567(5)
21.848(3)
90
yellow, tablet
C2/c (No. 15)
35.433(3)
24.290(2)
32.991(1)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å)
102.498(2)
90
104.549(1)
90
13253(4)
2
27484(4)
4
Z
T (K)
100
100
λ (Å)
0.71073
2.344
0.71073
2.316
ρcalcd (g cm−3
)
μ (Mo Kα) (mm−1
)
12.605
14.263
a
2
2
R(F) for Fo > 2σ (Fo
)
0.065
0.074
b
2
Rw(Fo )
0.208
0.247
a
b
2
2
R(F) = ∑||Fo| − |Fc||/∑|Fo|. R(Fo ) = [∑w(Fo − Fc2)2/
4
∑w(Fo )]1/2
.
180 °C dry gas temperature). The samples were introduced by direct
infusion at 7 μL/min and scanned over the range m/z 500−5000 with
data averaged over 5 min. The data were deconvoluted using MaxEnt
software.39
Spectroscopic Properties. Absorption and fluorescence data
were acquired from a single crystal of each compound using a Craic
Technologies UV−vis−near-IR (NIR) microspectrophotometer with a
fluorescence attachment. The absorption data were collected in the
range of 250−1200 nm at room temperature. Excitation was achieved
using 365 nm light from a mercury lamp for the fluorescence
spectroscopy. The IR spectra were collected from single crystals of A
and B using a SensIR Technology IlluminatIR FT-IR micro-
spectrometer. A single crystal of each compound was placed on a
glass slide and the spectrum was collected with a diamond ATR
objective.
RESULTS
■
Structure of [K18Li12][(UO2)20(HO2CC6H4PO3)10(O2)20-
(OH)10](H2O)n (A). The X-ray crystal structure analysis
provided the formula {[K18Li4][(UO2)20(HO2CC6H4PO3)10-
(O2)20(OH)10](H2O)n}8−, and the remaining charge is
presumably balanced by K+ and Li+ cations that are located
within or between the cage clusters. The structure contains
considerable electron density that we attribute to unassigned
disordered Li+ and H2O molecules. SQUEEZE was applied to
the data to confirm that a 3,588 Å3 void contains about 2,380
electrons. ICP-OES analyses indicate 18 K+ and 12 Li+ cations
per formula unit; 8 Li+ cations were not assigned in the crystal
structure presumably because of their weak X-ray scattering.
The overall structure of A consists of 20 uranyl hexagonal
bipyramids and 10 carboxyphosphonate groups, forming
nanoscopic cage clusters approximately 13.7 × 18.1 Å in
diameter, as measured from the outer oxygen atoms of the
uranyl cations (see Figure 1a, b). The basic uranyl diperoxide
hexagonal bipyramidal units are arranged in two patterns: 2
five-membered uranyl rings are positioned at either end of the
cage as shown in Figure 2a, and the remaining uranyl cations
are assembled into a rare ten-membered uranyl belt (see
Figures 2b and 3a). These two distinct uranyl arrangements are
loosely linked through the carboxyphosphonate moiety.
Topological pentagons are common graphical representations
Electrospray Ionization Mass Spectrometry. ESI-MS spectra
were collected in negative-ion mode using a Bruker micrOTOF-Q II
high-resolution quadrupole time-of-flight (Q-TOF) spectrometer
(3600 V capillary voltage, 0.8 bar nebulizer gas, 4 L/min dry gas,
B
dx.doi.org/10.1021/ic4008262 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Polyhedral representations and ellipsoid diagrams of A (a, b)
and B (c, d). UO8 units = yellow, phosphorus = magenta, oxygen =
red, carbon = black, hydrogen = white. Ellipsoids are shown at the 50%
probability level.
Figure 3. Connectivity diagrams in A (a) and B (b), the yellow nodes
represent uranyl atoms and links indicate connections between them.
all other patterns that have been reported in crystal
structures.13,15,18 In uranyl peroxide pyrophosphate/phosphate
cage clusters, uranyl belts are arranged in a zigzag pattern to
accommodate the coordinated pyrophosphate/phosphate moi-
eties.13,15 The fragments of uranyl belts from the recently
reported chiral uranyl peroxo cage clusters are built from only
four uranyl cations.18
All the uranium centers in A are coordinated by two nearly
linear oxo atoms, forming a classical uranyl ion, UO22+ unit, and
the O−U−O bond angles range from 177.8(5) ° to 179.6(5) °
with normal U−O bond distances that range from 1.765(11) Å
to 1.819(10) Å. Six oxygen atoms are coordinated to the uranyl
cations at the equatorial vertices of hexagonal bipyramids with
U−O bond distances that range from 2.288(12) to 2.522(11)
Å; the longer U−O bond lengths reveal the presence of
hydroxyl groups. The equatorial ligands for the uranyl cations
consist of two bidentate peroxide groups, with the remaining
two O2 atoms from phosphonate-carboxylate groups and
phosphonate-hydroxyl groups for the five and ten-membered
rings, respectively. The calculated bond-valence sums for the
uranyl cations and the absorption spectrum (see Figure 5) are
consistent with the formal valence of U(VI).40,41 The P−O
bond lengths from the phosphonate groups range from
1.507(11) Å to 1.562(12) Å. The phenyl ring bearing the
P(1) atom is disordered; therefore, the bond lengths from the
P(1) atom are outside the normal range. The C−O bonds
within the carboxylate moiety range between 1.209(19) and
1.420(20) Å. Atom C(7) is also disordered, and the C−O bond
lengths are not included within the above bond distance values.
The protonation of the C−O group is indicative of the terminal
and slightly elongated C−O bonds.
Figure 2. Comparison of structural fragments from A (a,b) and B
(c,d) clusters. These fragments correspond to topological pentagons
(a) and the 10-membered uranyl belt in A, squares and fused squares
in B (c,d). Legend as in Figure 1.
in uranyl peroxide clusters. Uranyl belts are rare, and the side-
by-side arrangement pattern we reported herein is distinct from
C
dx.doi.org/10.1021/ic4008262 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Structure of [K3Li21][(UO2)24(HO2CC6H4PO3)8(O2)24-
(OH)8](H2O)n (B). The X-ray crystal structure analysis supports
the formula [K3][(UO2)24(HO2CC6H4PO3)8(O2)24(OH)8]-
(H2O)n}21−, with disordered electron density within and
between the cage clusters left unassigned. SQUEEZE was
applied to the data to confirm that a 10,717 Å3 void contains
about 5,857 electrons. We attribute the unassigned electron
density to disordered Li+ and H2O molecules. Atomic
percentages for U, P, and K from EDX analysis are consistent
with the result from the X-ray diffraction. The X-ray scattering
is ineffective for locating Li atoms in the crystal structure;
therefore ICP-OES analyses are necessary. The ratios of the U/
P/K/Li atoms varied slightly as a result of difficulties we
encountered in separating the crystals from the unidentified
fine-grained yellow precipitates formed. This prompted us to
assume that the remaining 21 electrons are balanced with Li+
cations, since all the K+ ions have been accounted for through
the single crystal X-ray solution.
Figure 4. ESI-MS data for samples made by dissolving single crystals
of A and B in ultrapure water.
The cluster B is topologically divergent from A in that there
are 8-membered uranyl double rings positioned at the middle
of the elongated cage which is capped at either end by 2 four-
membered uranyl rings (see Figure 1c and d). The cage is
approximately 13.4 × 17.9 Å in diameter, as measured from the
outer oxygen atoms of the uranyl cations. Only one unique
coordination arrangement is formed around the uranyl cations:
2 four-membered uranyl rings (see Figures 2c and 3b), and the
remaining sixteen uranyl cations are assembled into 4 four-
membered uranyl rings that are fused together through the
carboxylate groups into uranyl pseudobelts as shown in Figure
2d.
after day 1 indicated the presence of an additional species in
solution. Therefore, we investigated further by slowly
evaporating the solution of A. Crystals of uranyl peroxide
clusters containing the common U24 cluster were isolated. The
deconvolution of the ESI-MS data collected from the solution
of B in ultrapure water gave an average mass of 9701 Da, and its
assigned charge states from the isotopic separation reveals −7
(m/z 1347), −6 (m/z 1578), −5 (m/z 1902), and −4 (m/z
2404).
Spectroscopic Properties. The UV−vis−NIR absorption
spectra for uranyl(VI) complexes [i.e., the benchmark
compound, UO2(NO3)2·6H2O] contain the characteristic
equatorial U−O charge transfer bands of uranyl centered at
325 nm, as shown in the Figure 5. In addition, the axial U−O
charge transfer bands are observed around 421 nm with a
characteristic vibronic fine-structure. The absorption spectrum
from a single crystal of A is similar to that of B and their spectra
are comparable to those of reported uranyl peroxide
clusters.16,18 The characteristic fingerprint of uranyl nitrate
hexahydrate is not evident, instead there is a relatively broad
spectrum that is almost a featureless peak from 300 to 550 nm.
The spectra reveal a shoulder around 354 nm and a distinct
peak around 416 nm. The loss of the fine structure in the
absorption spectra of uranyl peroxide clusters and the
significantly higher molar absorptivity are presumably due to
the intense ligand-to-metal charge transfer between coordinated
peroxide and the uranyl(VI) moiety and the resulting bent
configuration of the U-(O2)-U moiety (asymmetric mole-
cules).43,44 It has also been well established that most uranyl-
containing compounds emit green light centered near 520 nm.
The charge-transfer-based emission is in fact vibronically
coupled to both bending and stretching modes of the uranyl
cation, yielding a well resolved five-peak pattern at room
temperature.45 We and others have shown that luminescent
properties can be enhanced by stronger metal-to-aromatic
ligand interactions from carboxyphosphonates and pyridinedi-
carboxylates.46,47 Despite the loss of vibronic fine-structure in
the absorption spectra of uranyl peroxide clusters, intense
emission with vibronic coupling are clearly observed in the
fluorescence spectra of compounds A and B (see Figure 6).
The two uranyl-carboxyphosphonate cage clusters reveal four
peaks that are clearly resolved at 488, 543, 585, and 611 nm.
These clusters are red-shifted by approximately 34 nm relative
The 24 uranium cations are each part of a typical UO22+ unit,
with an average O−U−O bond angle of 177(8)° and U−O
bond length of 1.783(15) Å. The equatorial ligands consist of
two bidentate peroxide groups, one oxygen from the
phosphonate and one hydroxyl group for each uranyl ion
within the four-membered uranyl ring. The sixteen uranyl
cations of the fused four-membered ring are coordinated to two
O2 atoms from phosphonate and carboxylate moieties, along
with the four oxygen atoms of the two bidentate peroxide
groups. The calculated bond-valence sums at the uranium
centers and the absorption spectrum are in agreement with
U(VI) (see Figure 5).40,41 The P−O and C−O bond lengths
from the phosphonate and carboxylate moieties are within the
ranges observed in A.
SAXS and ESI-MS Studies of Dissolved A and B.
Characterization of the clusters in solution was performed using
SAXS and ESI-MS to probe their behavior in ultrapure water.
The radius of gyration, Rg, of the macroions in solution was
determined using Guinier analysis of the very low angle
scattering data.42 The value of Rg derived from the data (see
SAXS Supporting Information, Figure S1) was found to be 7.7
and 7.4 Å for A and B, respectively; these are comparable to the
estimated experimental values derived from the crystallographic
data, 7.9 and 7.8 Å, respectively, for A and B. The ESI-MS data
are shown in Figure 4 and are highly reproducible. Data for A
and B were deconvoluted using the MaxEnt software. Data
from a dissolved single crystal of compound A was collected on
the same day that the solution was prepared, and although the
cluster was still present, other species present indicated partial
disintegration, either in solution or in the spectrometer. The
assignment of charge state from the isotopic separation reveals
−6 (m/z 1290), −5 (m/z 1552), and −4 (m/z 1945), yielding
an average formula weight of 8004 D. The ESI-MS data of A
D
dx.doi.org/10.1021/ic4008262 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 5. Absorption spectra of the two uranyl peroxide clusters (A and B), and uranyl nitrate hexahydrate.
Figure 6. Luminescent spectra (365 nm excitation) of the two uranyl peroxide clusters (A and B), and the uranyl nitrate hexahydrate.
to the six-band emission pattern in the spectrum of the
benchmark compound, UO2(NO3)2·6H2O (487, 509, 532, 558,
586, and 612).
and the broad bands around 3139−3500 cm−1 are associated
with free water molecules.36,37
The infrared bands from 700 to 775 cm−1, as shown in
Supporting Information, Figure S2, are dominated by the O−
P−O bending, phenyl ring, and P−C stretching vibrations. The
asymmetric and symmetric stretching modes of the uranyl
cation, UO22+, range from 879 to 977 cm−1.13,17,18,34 The group
of peaks around 1040−1140 cm−1 is attributed to the P−O and
P−O symmetric and asymmetric stretching modes of the
phosphonates, while the bands around 1312−1445 cm−1 are
assigned to phenyl ring stretching vibrations. The ν(C−O) of
the carboxylate groups is located between 1590 and 1614 cm−1
DISCUSSION
■
The use of counterions and pH is very significant when
constructing different topologies in uranyl peroxide clusters.
Since similar counterions (Li+ and K+) were employed in the
synthesis of the two cage clusters reported herein, the dominant
role of either of the species is difficult to explain in the presence
of the varied pH conditions and steric influence of the phenyl
rings. Miro’s studies based on density functional theory (DFT)
calculations of counter cations indicate a strong propensity for
square and pentagon topologies when Li and K cations are
used, respectively.49,50 However, we can presume that the low
E
dx.doi.org/10.1021/ic4008262 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
ratio of K+ to Li+ used in the synthesis of B relative to A might
have some effects in the isolation of clusters with topological
squares. Only two of the oxygen atoms of the P−O group are
employed in joining the pentagons and the 10-membered
uranyl belt together in compound A. One of the oxygen atoms
of the carboxylate group coordinates to each of the five uranyl
cations of the pentagons. For compound B, all the oxygen
atoms of the phosphonate along with one of the carboxylate
groups, are involved in linking the uranyl cations. This is
presumably responsible for the higher stability of B in solution.
Attempts were made to incorporate transition metals in the 2-
carboxyphenylphosphonate ligand, without success. Because of
the high pH and the presence of H2O2, the expected differential
binding preferences of the phosphonate and carboxylate groups
around the uranium and the transition metals are not strictly
based on Pearson’s principle of hard/soft acid/base.36,37,48
Moreover, the H2O2 group solubilizes uranium oxides, and its
propensity for uranium is greater than transition metals.
Whereas the four and five-membered rings of uranyl
hexagonal bipyramids found in A and B are common features
of uranyl peroxide cage clusters, both clusters also contain
novel units built from uranyl polyhedra. In cluster A, the belts
consisting of 10 uranyl hexagonal bipyramids are reminiscent of
the chains found in studtite, [(UO2)O2(H2O)2]·2H2O.12 In
both cases adjacent uranyl ions are bridged by bidentate peroxo
groups that are in trans arrangements in their respective
polyhedra, as also recently reported in four-membered belts in
two clusters.18 In contrast, most clusters are built from uranyl
hexagonal bipyramids containing two peroxo ligands in a cis
arrangement, including the belt-like unit consisting of 10
polyhedra in a cluster consisting of 20 uranyl polyhedra and 10
pyrophosphate units.9 Cluster B is built from the well-known
four-membered ring of uranyl hexagonal bipyramids, with each
polyhedron containing two peroxo groups in a cis arrangement.
The linkage of four such rings into a larger ring structure,
consisting of 16 polyhedra, is novel. Note that adjacent four-
membered rings in which the uranyl ions are all bridged by
peroxo groups are linked through vertex-sharing only, which is
another unusual feature of this cluster. Another fascinating
feature that distinguishes these two structures from hybrid
uranyl-organic cage clusters recently published by Burns and
co-workers is the steric influence from the phenyl rings.9,51 The
phenyl rings are arranged on the outer periphery of the
spherical core thus influencing the chelating and packing
patterns.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported as part of the
Materials Science of Actinides, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under Award DE-
SC0001089.
REFERENCES
■
(1) Burns, P. C.; Kubatko, K.; Sigmon, G.; Fryer, B. J.; Gagnon, J. E.;
Antonio, M. R.; Soderholm, L. Angew. Chem., Int. Ed. 2005, 44, 2135−
2139.
(2) Soderholm, L.; Almond, P. M.; Skanthakumar, S.; Wilson, R. E.;
Burns, P. C. Angew. Chem., Int. Ed. 2008, 47, 298−302.
(3) Forbes, T. Z.; McAlpin, J. G.; Murphy, R.; Burns, P. C. Angew.
Chem., Int. Ed. 2008, 47, 2824−2827.
(4) Sigmon, G. E.; Unruh, D. K.; Ling, J.; Weaver, B.; Ward, M.;
Pressprich, L.; Simonetti, A.; Burns, P. C. Angew. Chem., Int. Ed. 2009,
48, 2737−2740.
(5) Sigmon, G. E.; Weaver, B.; Kubatko, K.; Burns, P. C. Inorg. Chem.
2009, 48, 10907−10909.
(6) Sigmon, G. E.; Ling, J.; Unruh, D. K.; Moore-Shay, L.; Ward, M.;
Weaver, B.; Burns, P. C. J. Am. Chem. Soc. 2009, 131, 16648−16649.
(7) Unruh, D. K.; Burtner, A.; Pressprich, L.; Sigmon, G. E.; Burns, P.
C. Dalton Trans. 2010, 39, 5807−5813.
(8) Ling, J.; Wallace, C. M.; Szymanowski, J. E. S.; Burns, P. C.
Angew. Chem., Int. Ed. 2010, 49, 7271−7273.
(9) Ling, J.; Qiu, J.; Sigmon, G. E.; Ward, M.; Szymanowski, J. E. S.;
Burns, P. C. J. Am. Chem. Soc. 2010, 132, 13395−13402.
(10) Ling, J.; Qiu, J.; Szymanowski, J. E. S.; Burns, P. C. Chem.Eur.
J. 2011, 17, 2571−2574.
(11) Nyman, M.; Rodriguez, M. A.; Alam, T. M. Eur. J. Inorg. Chem.
2011, 2197−2205.
(12) Burns, P. C. Mineral. Mag. 2011, 75, 1−25.
(13) Unruh, D. K.; Ling, J.; Qiu, J.; Pressprich, L.; Baranay, M.; Ward,
M.; Burns, P. C. Inorg. Chem. 2011, 50, 5509−5516.
(14) Sigmon, G. E.; Burns, P. C. J. Am. Chem. Soc. 2011, 133, 9137−
9139.
(15) Ling, J.; Ozga, M.; Stoffer, M.; Burns, P. C. Dalton Trans. 2012,
41, 7278−7284.
(16) Qiu, J.; Ling, J.; Sui, A.; Szymanowski, J. E. S.; Simonetti, A.;
Burns, P. C. J. Am. Chem. Soc. 2012, 134, 1810−1816.
(17) Ling, J.; Qiu, J.; Burns, P. C. Inorg. Chem. 2012, 51, 2403−2408.
(18) Qiu, J.; Nguyen, K.; Jouffret, L.; Szymanowski, J. E. S.; Burns, P.
C. Inorg. Chem. 2013, 52, 337−345.
(19) Armstrong, C. R.; Nyman, M.; Shvareva, T.; Sigmon, G. E.;
Burns, P. C.; Navrotsky, A. Proc. Natl. Acad. Sci. U. S. A. 2012, 109,
1874−1877.
In conclusion, the synthesis of the two hybrid uranyl cage
clusters was achieved by connecting the uranyl moieties
together using 2-carboxyphenylphosphonate ligands as secon-
dary metal linkers. Nevertheless, the central question that will
be addressed by the ongoing studies is whether we can
incorporate 3d metal ions other than tungstometalate frag-
ments to probe further the structural variation and electronic
properties of such clusters.
(20) Burns, P. C.; Ewing, R. C.; Navrotsky, A. Science 2012, 335,
1184−1188.
(21) Kim, K.; Pope, M. T. J. Am. Chem. Soc. 1999, 121, 8512−8517.
(22) Kim, K.; Pope, M. T. J. Chem. Soc., Dalton Trans. 2001, 986−
990.
(23) Kim, K.; Gaunt, A.; Pope, M. T. J. Cluster Sci. 2002, 13, 423−
436.
(24) Gaunt, A. J.; May, I.; Helliwell, M.; Richardson, S. J. Am. Chem.
Soc. 2002, 124, 13350−13351.
ASSOCIATED CONTENT
* Supporting Information
SAXS and IR spectra; and crystallographic data (CIF) of the
two new clusters. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
(25) Choppin, G. R.; Wall, D. E. J. Radioanal. Nucl. Chem. 2003, 255,
47−52.
S
(26) Gaunt, A. J.; May, I.; Copping, R.; Bhatt, A. I.; Collison, D.;
Danny, F., O.; Travis, H., K.; Pope, M. T. Dalton Trans. 2003, 3009−
3014.
(27) Gaunt, A. J.; May, I.; Collison, D.; Holman, K. T.; Pope, M. T. J.
Mol. Struct. 2003, 656, 101−106.
(28) Craciun, C.; Rusu, D.; Pop-Fanea, L.; Hossu, M.; Rusu, M.;
David, L. J. Radioanal. Nucl. Chem. 2005, 264, 589−594.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pburns@nd.edu.
■
F
dx.doi.org/10.1021/ic4008262 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(29) Khoshnavazi, R.; Eshtiagh-hossieni, H.; Alizadeh, M. H.; Pope,
M. T. Polyhedron 2006, 25, 1921−1926.
(30) Antonio, M. R.; Chiang, M. Inorg. Chem. 2008, 47, 8278−8285.
(31) Mal, S. S.; Dickman, M. H.; Kortz, U. Chem.Eur. J. 2008, 14,
9851−9855.
́
(32) Miro, P.; Ling, J.; Qiu, J.; Burns, P. C.; Gagliardi, L.; Cramer, C.
J. Inorg. Chem. 2012, 51, 8784−8790.
(33) Nyman, M.; Burns, P. C. Chem. Soc. Rev. 2012, 41, 7354−7367.
(34) Adelani, P. O.; Albrecht-Schmitt, T. Inorg. Chem. 2011, 50,
12184−12191.
(35) Adelani, P. O.; Albrecht-Schmitt, T. E. Angew. Chem., Int. Ed.
2010, 49, 8909−8911.
(36) Adelani, P. O.; Albrecht-Schmitt, T. Cryst. Growth Des. 2011, 11,
4676−4683.
(37) Adelani, P. O.; Oliver, A. G.; Albrecht-Schmitt, T. Inorg. Chem.
2012, 51, 4885−4887.
(38) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 211.
(39) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse,
C. M. Science 1989, 246, 64−71.
(40) Burns, P. C.; Ewing, R. C.; Hawthorne, F. C. Can. Mineral.
1997, 35, 1551.
(41) Brese, N. E.; O’Keeffe, M. Acta Crystallogr. 1991, B47, 192.
(42) Feigin, L. A.; Svergun, D. I. Structure Analysis by Small-Angle X-
Ray and Neutron Scattering; Plenum Press: New York, 1987.
(43) Goff, G. S.; Brodnax, L. F.; Cisneros, M. R.; Peper, S. M.; Field,
S. E.; Scott, B. L.; Runde, W. H. Inorg. Chem. 2008, 47, 1984−1990.
(44) Zanonato, P. L.; Di Bernardo, P.; Grenthe, I. Dalton Trans.
2012, 41, 3380−3386.
(45) Liu, G.; Beitz, J. V. In The Chemistry of the Actinide and
Transactinides Elements; Morss, L. R., Edelstein, N. M., Fuger, J., Eds.;
Springer: Heidelberg, Germany, 2006; p 2088.
(46) Adelani, P. O.; Albrecht-Schmitt, T. Inorg. Chem. 2010, 49,
5701−5705.
(47) Frisch, M.; Cahill, C. L. Dalton Trans. 2006, 4679−4690.
(48) Adelani, P. O.; Oliver, A. G.; Albrecht-Schmitt, T. E. Cryst.
Growth Des. 2011, 11, 3072−3080.
(49) Miro, P.; Pierrefixe, S.; Gicquel, M.; Gil, A.; Bo, C. J. Am. Chem.
Soc. 2010, 132, 17787−17794.
(50) Miro, P.; Bo, C. Inorg. Chem. 2012, 51, 3840−3845.
(51) Liao, Z.; Ling, J.; Reinke, L. R.; Szymanowski, J. E. S.; Sigmon,
G. E.; Burns, P. C. Dalton Trans. 2013, 42, 6793−6802.
G
dx.doi.org/10.1021/ic4008262 | Inorg. Chem. XXXX, XXX, XXX−XXX