Supramolecular Assembly of Geometrically Unstable Hybrid Organic-Inorganic Uranyl Peroxide Cage Clusters and Their Transformations

Article abstract of DOI:10.1021/jacs.9b05599

An aromatic ligand was introduced into the synthesis of a uranyl peroxide polyoxometalate formulated as K₃₂(UO₂)₁₉(O₂)₂₆(OH)₂(C₆H₄P₂O₆)₄

Full text of DOI:10.1021/jacs.9b05599

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Article
Supramolecular Assembly of Geometrically Unstable Hybrid Organic-
Inorganic Uranyl Peroxide Cage Clusters and their Transformations
Mengyu Xu, Hrafn Traustason, Fabrice Dal Bo, Sarah M. Hickam,
Saehwa Chong, Lei Zhang, Allen G. Oliver, and Peter C Burns
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b05599 • Publication Date (Web): 24 Jul 2019
Downloaded from pubs.acs.org on July 25, 2019
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Supramolecular Assembly of Geometrically Unstable Hybrid Organic-Inorganic Uranyl Peroxide
Cage Clusters and their Transformations
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Mengyu Xu,a Hrafn Traustason,b Fabrice Dal Bo,a Sarah Hickam,a Saehwa Chong,a Lei Zhang,a
Allen G. Oliver,b and Peter C. Burns* a,b
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a. Department of Civil and Environmental Engineering and Earth Sciences, University of
Notre Dame, Notre Dame, Indiana, 46556, USA.
b. Department of Chemistry and Biochemistry, University of Notre Dame, Indiana, 46556,
USA
ABSTRACT: An aromatic ligand was introduced into the synthesis of a uranyl peroxide
polyoxometalate (POM) formulated as K32(UO2)19(O2)26(OH)2(C6H4P2O6)4⋅65 H2O that consists
of a unique “open oyster” shaped structure (U19) with intramolecular H-bonds. In the solid state,
K-π and π-π interactions, as well as K-O bonds enable the formation of a supramolecular
network between U19 clusters. U19 adopts an incomplete fullerene topology and was utilized as a
precursor from which the geometrically favored U24 structure was produced. A potassium-
encapsulated U24 structure was obtained upon heating the solution containing U19.
INTRODUCTION
Polyoxometalates (POMs) are mostly anionic metal-oxo clusters of high-valence group V/VI
cations.1-4 POMs range in size from sub‐nanoscale to protein scale and have structures and
properties with applications in analytical and clinical chemistry, catalysis, medicine, and solid-
state devices.1, 5-7 POMs exhibit substantial structural diversity owing to a variety of building
blocks8 and many thousands of combinatorial possibilities.4 Design and control of the structure
of POMs requires either bottom-up synthetic approaches or structural transformations from POM
precursors. The synthetic variables include: 1) the concentration/type of metal oxide anion and
heteroatom, 2) the presence of additional ligands, 3) the pH and ionic strength of the reaction
mixture, 4) the presence of a reducing agent, and 5) the temperature of the reaction and
processing.4 Generally, structural transformations of POM precursors are triggered by: 1) a
change of pH caused by addition of acid/base,9 2) control of the pH-dependent equilibrium
between POM monomers and dimers,10-11 3) modifying the structural “vacancy” sites of lacunary
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POMs.12-16
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The highly controllable structures and properties of POMs facilitate their use as building
blocks for functional nanosystems and nanomachines at a larger scale (>10 nm).4 Controlling
interactions between POM clusters during their assembly into extended structures is necessary
and difficult when POMs are bridged by covalent bonds.5 To control supramolecular self-
assembly, non-covalent forces such as Van der Waals interactions, hydrophobic binding, H-
bonds, and/or metal-ligand interactions may be used.17 Functionalizing POMs by addition of
organic ligands allows the use of H-bonds,18 cation-π interactions,19 and π-π interactions20
between clusters to guide the assembly of POMs into extended structures.
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High-valent actinides (An: actinide) often form linear (AnO2)+,2+ actinyl ions with multiply-
bound oxo ligands and uranyl- and neptunyl-based peroxo clusters have emerged as an extensive
family of POMs.21-23 The majority of uranyl peroxide clusters are cages in which uranyl ions are
bridged by peroxide or hydroxide ligands, with the uranyl ions arranged in topological squares,
pentagons and hexagons.22 For example, U20 and U28 both have fullerene topologies, with the
topology of U28 consisting of six hexagons in addition to 12 pentagons (Figure 1a and b, Un
represents a cluster containing n uranyl polyhedra).24 U24 adopts the sodalite topology consisting
of topological squares and hexagons (Figure 1c).25-28 U32 has a topology that contains squares,
pentagons and hexagons (Figure 1d).25
Figure 1. Graphical representation of the topologies of (a) U20, (b) U28, (c) U24 and (d) U32. Linkages between uranyl
ions are represented as yellow lines.
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2+
Uranyl peroxide clusters commonly form in alkaline aqueous solutions containing UO2 ,
H2O2 and alkali cations under ambient conditions.29 Varying the choice of alkali cation,
2+
UO2 /alkali cation ratio, pH, reaction time, and adding ligands during the synthetic process
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yields widely varied topologies and sizes of uranyl peroxide clusters.30-32 The topologies of
2+
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uranyl peroxide cages synthesized using LiOH are controlled by reaction time and UO2 /Li
ratio, forming U28 or U24.30 Controlling the UO2 /Na ratio promotes multiple cluster topologies
2+
in the NaOH system, yielding U20, U24, U28, and U32.32 U28 is a favored capsule topology when K
is the only available counter cation, perhaps because K+ has an affinity with topological
pentagons and hexagons formed by uranyl ions at room temperature during the self-assembly
process.33-34 A structural transformation methodology from a uranyl peroxide cluster precursor
attracts our attention as a possible approach to achieve structural variety in the KOH system.
Here we report synthesis of (UO2)19(O2)26(OH)2(C6H4P2O6)432- (U19) and demonstrate a direct
heating method that induces the structural transformation from U19 into the first K+-encapsulated
U24 structure. U19 is an organic-functionalized lacunary cluster that has an incomplete U20
fullerene topology with intramolecular H-bonds and a unique “open oyster” shape. The presence
of the aromatic ligand 1,2-bis(dimethoxyphosphoryl)benzene in U19 provides for non-covalent
cation-π and π-π interactions between uranyl peroxide clusters, forming a higher-dimensional
network of clusters.
EXPERIMENTAL SECTION
Caution! Experiments described herein were conducted using radioactive isotopically depleted
uranium (238U, α = 4.467 MeV) by appropriately trained personnel in laboratories equipped and
licensed to work with radioactive materials.
General Considerations. Uranyl nitrate hexahydrate (UO2(NO3)2·6H2O, International Bio-
Analytical Industries, Inc.), hydrogen peroxide (30% aqueous solution, EMD Millipore),
potassium hydroxide (KOH, Sigma- Aldrich), 1,2-bis(dimethoxyphosphoryl)benzene
(C10H16O6P2, Alpha Aesar), and deuterium oxide (D2O, Cambridge Isotope Laboratories, Inc.)
were obtained from commercial suppliers and used as received. Distilled and Millipore filtered
water with a resistance of 18.2 MΩ⋅cm was used in all reactions. Isotopic labeling of the uranyl
ion (U16O22+ → U18O2 ) was conducted by a UV light-induced oxygen exchange reaction using
2+
18
97% H2 O (Cambridge Isotope Laboratories, Inc.) and was confirmed via solution-mode Raman
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spectroscopy (Supporting Information).35
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C6H4PO3H2 (BzDPA). Ten g of 1,2-bis(dimethoxyphosphoryl)benzene was dissolved in 20
mL of 12 M HCl and refluxed for 8 h. The product was washed with cold 2 M HCl, filtered, and
dried at 60 °C for 12 hours.36 White crystals of benzene-1,2-diphosphonic acid (BzDPA) were
formed in high yield (90%). 1H NMR (400 MHz, D2O): δ = 7.85, 2H, m; 7.52, 2H, m. 13C NMR
(100 MHz, D2O): δ = 131.40, 2 CH, vt, Japp = 5; 132.86, 2 CH, vt, Japp = 12; 133.43, 2 CP, dd,
1JCP = 175, 2JCP = 10. 31P NMR (162 MHz, D2O): δ = 14.32, 2 P, s.
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K32(UO2)19(O2)26(OH)2(C6H4P2O6)4⋅65H2O (1). 0.1 mL of an aqueous 0.5 M UO2(NO3)2
solution and 0.1 mL of 30% H2O2 solution were combined in a 5 mL glass vial, resulting in
precipitation of studtite, [UO2(O2)(H2O)2](H2O)2. Subsequently, 0.2 mL of 4.0 M aqueous KOH
solution was added to dissolve the precipitate. Then 0.15 mL 0.2 M BzDPA solution was added
and 4.0 M KOH solution was titrated to obtain pH 9.5. The resulting reaction solution was left
open to the air and orange block-shaped crystals of 1 formed after four hours. Yield of 1: 30% on
the basis of uranium.
K32(U18O2)19(O2)26(OH)2(C6H4P2O6)4⋅65H2O (1*). 18O isotopically enriched 1* was obtained
following the same method as for 1, except using 0.5 M U18O2(NO3)2.
K24(UO2)24(O2)24(OH)24⋅nH2O (2). Twenty mg of 1 was dissolved in 0.5 mL 18MΩ H2O and
the resulting solution was heated in a 23 mL capacity Teflon-lined stainless-steel Parr reaction
vessel at 80°C for eight hours before cooling to room temperature. Yellow diamond-shaped
crystals of 2 formed from the resulting solution after two days. Yield of 2: 10%.
Elemental Analysis. Ten mg of 1 or 2 were dissolved in 0.5 mL 18MΩ H2O and the resulting
solution was diluted in a 5% HNO3 matrix with yttrium added as an internal standard to monitor
for instrument drift. Measurements were done using a Perkin Elmer Optima 8000 inductively
coupled plasma optical emission spectrometer (ICP-OES) and the concentration of each element
was calculated from calibration curves of standards with element concentrations in the range of
0.1 to 50 ppm. The reported value with uncertainty for each element is derived from the mean
value and standard deviation of more than nine replicate measurements. The U: P: K ratios of 1
are 19: 7.90
0.41: 32.11
0.56, and the U: K ratios of 2 are 24: 20.38
0.59.
±
±
±
Crystallographic Studies. Crystals of 1 or 2 were isolated from their mother solutions and
placed in oil to prevent their dehydration. A single crystal of each was selected using a polarized
light microscope and was placed on a cryoloop for single crystal X-ray diffraction studies.
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Diffraction data were measured using a Bruker APEX II Quazar diffractometer equipped with a
micro source sealed tube with multilayer optics that produced a monochromated MoKα X-ray
beam. Data were collected using frame widths of 0.5° in ω and an exposure time of 10 s for each
frame. Data integration and corrections for Lorentz, polarization and background effects were
performed using the Bruker APEX3 software package37 and corrections for absorption were
applied using SADABS.38 The initial structure solutions were obtained by direct methods and
least-squares refinements were performed by SHELXTL.39
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The structure of 1 contains 19 symmetrically unique U sites, 18 of which are fully occupied by
U. The site occupancy of U19 initially refined to 0.683(7), and U19 was subsequently replaced
by two closely-spaced cation sites occupied by U (U19) and K (K19A). Refinement of the site
occupancies of U19 and K19A with their total occupancy constrained to 1.0 yielded 0.608(9) U
and 0.392(9) K and a U19-K19A separation of 0.33 Å. The interstitial regions of the structure of
1 contain somewhat disordered K and H2O that were tentatively assigned as partially occupied
sites.
The phenyl rings of 1 composed of C1-C24 were constrained to the vertices of rigid hexagons
with C-C distances held at 1.39 Å. Hydrogen atoms were included on these phenyl rings as
riding atoms with the constraint that C-H be 0.93 Å and Uiso(H) is 1.2 times larger than Ueq(C).
Selected crystallographic information of 1 and 2 are listed in Table S2 and S6, respectively.
Atomic coordinates and additional crystallographic information are provided in the Supporting
Information (CIF). Bond valences incident upon cation and anion sites were calculated using
literature parameters.40-41
Raman Spectroscopy. Raman spectra were measured using a Bruker Sentinel system with
fiber optics and a video-assisted Raman probe equipped with a 785 nm, 148 mW laser and a
high-sensitivity, TE-cooled CCD detector. Spectra were collected over the range 80-3200 cm-1.
Crystalline U19 was placed under a Nikon optical microscope attached to the Raman probe for
data collection. Spectral deconvolution of the 650-900 cm-1 region of 1 was done by fitting seven
sets of Gaussian functions. Spectra were also collected using this instrument for solutions
contained within glass vials. In cases where solutions initially contained a precipitate, they were
first centrifuged before collecting Raman spectra.
Heat Treatment of Aqueous U19. Eight solutions were prepared for study that each had 20
mg of 1 dissolved in 0.5 mL 18MΩ H2O. One of these solutions was left sitting at room
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temperature for 8 hours as a control, and the others were heated at different temperatures ranging
from 40°C to 160°C for 8 hours with ramping and cooling rates of 1°C/min. Yellow precipitates
formed in the solutions that were heated above 80°C and were analyzed using powder X-ray
diffraction. Precipitates were recovered by centrifugation and the resulting clear solutions were
analyzed by ICP-OES to determine the concentration of uranium. 31P nuclear magnetic
resonance (NMR) spectra, electrospray ionization mass spectra (ESI-MS) and Raman spectra
were collected for each solution following heating. The reaction solutions were then left open to
the air under ambient conditions to allow slow evaporation of the solvent. Crystals of 2 were
recovered from the solutions heated at 80°C and 100°C after they were left standing in air for
two days.
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31P Nuclear Magnetic Resonance Spectroscopy (31P NMR). 31P NMR measurements were
performed for aqueous solutions using a 600 MHz Varian INOVA spectrometer (11.74 T) with a
pulse length of 14.8 ms, 128 scans, and a relaxation time (d1) of 10 s. Diffusion-ordered
spectroscopy (DOSY) data were collected using the modified bipolar pulse stimulated echo
(BPPSTE) sequence at 22°C, which were implemented in the Varian VnmrJ® version 2.2
revision C software package (AgilentTechnologies, Santa Clara, CA, USA).42 The gradients
were increased from 2.3 to 50.7 G cm-1 in 10 steps using 256 scans. The gradient pulse (δ) was
∙
set to 3.0 ms and the diffusion time (Δ) was set to 0.1 s. Diffusion coefficients for resolved 31P
signals were extracted from decay curves using the ‘peak height fit’ in the DOSY Transform
module of MestReNova 11.0.4. The reported diffusion coefficients are the mean values with
standard deviations of five repetitions. All samples were measured with a coaxial insert
containing D2O and 0.1 M H3PO4. 31P chemical shifts were referenced to H3PO4 at 0 ppm.
Electrospray Ionization Mass Spectrometry (ESI-MS). ESI-MS spectra were acquired
using a Bruker qTOF time-of-flight spectrometer. An aqueous solution of U19 was prepared at a
10-20 μM concentration and was directly infused into the ionization source using a syringe pump
at a flow rate of 4 μL/min. TOF data were acquired in negative-ion mode at a capillary voltage of
3600 V, end plate offset of -500 V, 0.8 bar dry gas, 1.2 L/min desolvation gas, and 180°C dry
gas temperature.
Additional Characterization Data. Details of thermogravimetric analysis (TGA), infrared
spectroscopy (IR), and powder X-ray diffraction (P XRD) methods and results are available in
Supporting Information.
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RESULTS AND DISCUSSION
Cluster synthesis and cluster salt composition of compound 1. Combining an aqueous
solution of uranyl nitrate, hydrogen peroxide, BzDPA and potassium hydroxide at pH ~9.5 under
ambient conditions resulted in the crystallization of 1 within four hours. Crystallization of 1
occurs more quickly than for most salts of uranyl peroxide clusters, which often only crystallize
after significant solvent evaporation and several days. X-ray diffraction data provided the crystal
structure of 1 that contains well-ordered positions of U, O, P and C atoms arranged in cage
clusters containing 19 UO22+ uranyl ions (Figure 2). According to the crystal structure
determination, the anionic cage of 1 has composition [(UO2)19(O2)26(O)2(C6H4P2O6)4]34-, where
O2 corresponds to peroxide. The structure determination also revealed locations of 27 K sites
distributed inside and outside of the cluster cage. Of these, 10 have refined occupancies less than
unity, and may contain a mixture of K and H2O. Despite disorder of the H2O positions, 59 H2O
sites were located, with 49 of these having occupancies modelled as less than unity.
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The elemental analysis of 1 yielded a U: P: K ratio of 19: 7.90
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0.56,
±
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indicating ~32 K cations per formula unit. Thermogravimetric analysis (TGA) yielded ~65 H2O
per formula unit in 1 (Supporting Information). Thus, the formula of 1 is assigned as
K32(UO2)19(O2)26(OH)2(C6H4P2O6)4(H2O)65, with the occurrence of hydroxyl discussed below.
Structure of U19. The cage cluster in 1 consists of 19 uranyl hexagonal bipyramids and four
BzDPA ligands (Figure 2a and b, designated U19). In the discussion that follows the cluster is
described for the case where the U19 polyhedron is occupied by U, although the structure
determination indicates that in about 40% of the clusters of the studied crystal this polyhedron is
occupied by K. Each U(VI) cation is strongly bonded to two oxygen atoms in a trans
arrangement, forming the usual UO22+ uranyl ion with an average uranium to yl-oxygen (U-Oyl)
bond length of 1.80 Å. Each uranyl ion is coordinated to six oxygen-containing ligands arranged
at equatorial positions of hexagonal bipyramids, and the average uranium to equatorial-oxygen
(U-Oeq) bond length is 2.36 Å. Nine uranyl polyhedra contain two peroxo ligands that are
bidentate to the uranyl ion in a cis arrangement and that correspond to two of the equatorial
edges of hexagonal bipyramids. Eight of these uranyl ions are also coordinated by two oxygen
atoms of diphosphonic groups, and the atom U19 is coordinated by two O2- ligands that are
likely protonated (see below). The other uranyl ions have three bidentate peroxo ligands that are
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mostly shared with adjacent uranyl polyhedra. However, in the cases of the U11, U15, U16 and
U17 polyhedra, one of the peroxo ligands is non-bridging. Eight phosphorus cations are
tetrahedrally coordinated by oxygen atoms and carbon atoms that are part of four different
benzene cycles.
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U19 is the first uranyl cluster containing 19 uranyl cations and has two distinct building units
that are linked by four BzDPA groups. One of these has eight uranyl polyhedra assembled into
two five-membered rings, and one has 11 uranyl polyhedra assembled into three five-membered
rings (Figure 2c and d). The linkages between uranyl polyhedra and BzDPA groups form
distorted five-membered rings, giving a novel overall 19:57 topology (Figure 2c, d and e).
Figure 2. Polyhedral representations of the U19 cage cluster in 1 from (a) front and (b) back. (c) Graphical
representation of the topology of U19, of which the building blocks are five-membered rings. Combined polyhedral
ball-and-stick representations of (d) the five-membered ring of uranyl ions bridged by peroxide ligands and (e) the
five-membered ring of uranyl ions bridged by BzDPA and peroxide ligands. In (a) and (b), uranyl polyhedra are yellow,
phosphorus polyhedra are magenta, and carbon-carbon bonds are black. In (b), the yellow lines correspond to linkages
between uranyl ions through peroxide ligands, the magenta lines designate linkages between uranyl ions through
BzDPA ligands. In (d) and (e), uranium and oxygen atoms are shown as yellow and red spheres, respectively.
Intramolecular H-bonds. Four inorganic lacunary uranyl peroxide clusters have been
reported to date: the open-bowl-shaped U1643 and crown-shaped U20R,43 U24R,43 and U32R.44 U19 is
an organic-functionalized lacunary cluster that has an incomplete U20 fullerene topology with
two open sides (Figure 3a). Compared to U20, omission of one uranyl ion results in an open side
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on U19 where it is terminated by the triperoxide uranyl hexagonal bipyramids U11, U15, and
U17, each of which includes a non-bridging peroxide ligand. These non-bridging peroxide
ligands are bonded to potassium cations and also likely accept H-bonds donated by interstitial
water to satisfy their bond-valence deficiency of ~0.65 valence units (vu) calculated in the
absence of H bonds. Previously reported crown-shaped uranyl peroxide clusters also feature non-
bridging peroxide groups that bond to interstitial cations and accept H bonds from interstitial
water.43-44 On the opposite side of U19 (Figure 3c), the U16 polyhedron is terminated by peroxide
oxygen atoms O97 and O85 and this peroxide is not shared by neighboring U19, whereas its
equivalent is in the case in the U20 cluster structure, leaving a gap between these two uranyl
polyhedra and producing an ‘open-oyster’ shape. The distance between U16 and U19 is ~1.5 Å
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longer than the typical U-U distance of uranyl ions bridged by peroxide ligands.
Figure 3. (a) U19 has an incomplete U20 fullerene topology. Details of the two open sides of U19: (b) Three non-bridging
peroxide ligands truncate the cluster and likely accept H bonds from interstitial H2O. (c) U19 contains two non-bridging
oxygen atoms O110 and O117 that donate H bonds to the accepters O97 and O85. Legend as in Figure 2. Potassium
atoms are shown as purple spheres.
O110 and O17, which are not peroxide oxygen atoms, terminate the U19 diperoxide
hexagonal bipyramid, and each is also bonded to one potassium cation. The calculated bond-
valence sums at these O sites from the bonds to the uranium and potassium cations are 0.52 vu
for O110 and 0.63 vu for O17. This situation is consistent with O110 and O17 being occupied by
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either H2O or OH-, as they may also accept one or two H bonds that would be important if they
are occupied by OH-. The O110-O97 and O17-O85 distances across the opening in U19 (Figure
3c) are 2.71 Å and 2.61 Å, consistent with intramolecular H bonds, and the bond-valence
strengths of the U-O bonds of O97 and O85 are 0.62 vu and 0.61 vu, consistent with them acting
as H bond acceptors. Based on the elemental analysis and the requirement of electroneutrality, a
proton is assigned to each of O110 and O17.
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Intermolecular Interaction Between U19 Clusters. In compound 1, there are T-shaped π-π
stacking interactions, which are also identified as C-H···π interactions, between BzDPA ligands
of adjacent U19 clusters that are characterized by centroid-centroid separations of ~5.11 Å and
~97° between the normals to the ring plane (Figure 4a).45 A computational study suggested that
T-shaped and slipped-parallel aromatic dimers are both enthalpically favored, and the T-shaped
dimers have the largest interaction energy when the intermolecular distance is 5.0 Å.46 The
cluster dimers are connected to other clusters with different orientations through K-O bonds and
a K-π interactions (Figure 4a). The C-H···π interactions facilitate the formation of chains of cage
clusters in the (010) plane and linked by K-O bonds (Figure 4b). K-π and K-O interactions link
the chains in the (10 ) plane and form the overall supramolecular network of 1 (Figure 4b). We
1
propose that the supramolecular interactions between clusters described here facilitate the rapid
crystallization of 1, which at four hours is much faster than is typical for salts of uranyl peroxide
cage clusters.
Raman Spectroscopy. The Raman spectra of 1 and its O18-enriched variety and their peak
assignments are shown in Figure 5. Peroxide ligands bonded to uranyl produce Raman bands in
the range of 840-870 cm-1,47 and the stretch of η2-peroxide appears at 861 cm-1 in the Raman
spectrum of studtite, [UO2(O2)(H2O)2](H2O)2.48 In Raman spectra of salts of clusters Li-U24 and
Li/K-U60, which contain uranyl diperoxide hexagonal bipyramids only, peaks near 850 cm-1 were
assigned to the O-O symmetric stretch (νs, O-O),49 although a different study assigned this
vibrational mode to uranyl-bridging hydroxyl groups.50 Here, we assigned the mode at 861 cm-1
to the νs, O-O from the eight diperoxide uranyl units since there are no uranyl-bridging hydroxyl
groups in this structure. The mode at 825 cm-1 is assigned to the νs, O-O of the ten triperoxide
uranyl units, consistent with the reported value of νs, O-O in uranyl peroxide clusters composed
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of uranyl triperoxide hexagonal bipyramids that occur in the range of 800-850 cm-1.30, 51 The
mode at 801 cm-1 is assigned to the symmetric stretch of uranyl ions (νs, O-U-O), in line with the
reported Raman spectra of uranyl peroxide clusters.30, 49 The mode at 757 cm-1 is tentatively
assigned to the symmetric vibration of uranyl (νs, O-U-O) from uranyl ions coordinated to non-
bridging peroxide groups, U11, U15 and U17, based on the corresponding values in the spectra
of uranyl triperoxide monomers that occur in the range of 680 to 740 cm-1. The mode at 680 cm-1
is attributed to the symmetric stretching vibration of PO3 (νs, P-O-P).
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Figure 4. (a) C-H···π (black dashed lines) and K-π (purple dashed lines) interactions between adjacent U19 clusters;
space-filling diagram of these interactions. (b) Graphic representation of the network composed of cluster chains.
The U19 clusters are represented as yellow spheres and are connected through K-O bonds (grey lines).
To support the Raman shift assignments, the spectrum of O18-enriched crystalline salt of 1
(1*) was collected. Note that the O18 label was placed on the uranyl ion, and no exchange of this
label into peroxide is expected. The lack of response to isotopic substitution of bands at 825 and
861 cm-1 is consistent with their assignment to peroxide. The growth of the mode at 757 cm-1 and
the decrease of that at 801 cm-1 upon isotopic substitution indicates that these are from an
isotopic pair of uranyl ions, represented as (O-U-O)A here. Two modes appear in the region of
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780-820 cm-1 and are fitted at 786 and 801 cm-1 after the shifting of the (O-U-O)A peak. They are
assigned to the vibration of O-O because they did not respond to the isotopic substitution at the
uranyl oxygen sites. These two modes have a lower wavenumber as compared to the other
peroxide peaks but are close to the values reported for the uranyl triperoxide monomers at
around 810 cm-1;51 as such, these peaks may be due to non-bridging peroxide groups in 1. The
mode at 718 cm-1 in the spectrum of 1* corresponds to the mode at 757 cm-1 in the spectrum of 1.
This peak shift is in good agreement with our assignment above and this peak is designated as
the (O-U-O)B peak.
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The empirical formulation RUO = 106.5ν1-2/3 + 0.575 developed by Bartlett52 correlates the
experimentally observed wavenumber of νs, O-U-O (ν1) with the crystallographic U-Oyl bond
lengths. The predicted value in this case is 801 cm-1, which is consistent with the experimental
value of the (O-U-O)A peak. Polyhedron U19 has a long U-Oyl bond length of 1.86 Å and the
empirically predicted peak position is 755 cm-1, which is in accord with the experimental value
of the (O-U-O)B peak. The substitution of equatorially-bound water molecules by hydroxyl
groups is expected to lead to weakening of uranyl bonds and is reflected in the redshift of Raman
peaks.53
Figure 5. Raman spectra of isotopically neutral (black) and 18O-enriched (blue) crystals of 1.
Transformation from U19 to [(UO2)24(O2)24(OH)24]24- (U24). Inclusion of KOH in the
aqueous uranyl peroxide system commonly yields U28 (K-U28), which earlier was attributed to a
cation templating effect with the uranyl five-membered ring of fullerene-topology U28 favoring
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coordination to K+.21, 33 Synthesis of uranyl peroxide clusters via structural transformation may
provide opportunities to circumvent templating effects of counter cations during the formation of
clusters. Addition of acid/base to trigger structural transformations is common in cluster
chemistry9 but is not entirely suitable for uranyl peroxide clusters because these clusters often
decompose upon significant change of pH. Recently it was observed that U60 in aqueous solution
converts into U24 upon heating,54 prompting us to heat solutions of U19 in an attempt to trigger a
transformation. Efforts to prepare other K-containing cluster structures from K-U28 through
structural transformations triggered by addition of acid or base or heating thus far have failed and
K-U28 persists in the solution. U19 is a potential POM precursor because: 1) it contains both
uranyl diperoxide units and triperoxide units that are basic building units for most uranyl
peroxide clusters,22 and 2) the defected U20 fullerene structure of U19 may be susceptible to
structural transform into a geometrically more stable structure.
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Crystals of 2, which contain the well-known U24 cluster and K counter cations, were isolated
from aqueous solutions of U19 after heat treatment from 60-100°C. U24 was first synthesized with
Li+ as the counter cation and has since been reported containing Na+, Bi3+, Pb2+ and alkaline
earth elements (Ca, Sr, Ba),26-28 as well as combinations of Li/Na and Li/K.55 U24 consists of 24
uranyl hexagonal bipyramids, each of which contains two peroxide ligands and two hydroxyl
groups (Figure 6a).21 Single-crystal X-ray diffraction data indicate U24 in 2 has composition
[(UO2)24(O2)24(OH)24]24-, where O2 corresponds to peroxide. Twenty-four K sites were located in
the crystal structure that balance the charge of the cage cluster, although six symmetrically
equivalent K sites are each split into four sites that are disordered with water oxygen atoms.
Eight K cations are inside the U24 cage, where they are positioned inside the topological
hexagons that are each defined by six uranyl ions, and these are arranged at the vertices of a cube
(Figure 6b). Each of these encapsulated K+ cations are bonded to uranyl oxygen atoms and water
(Figure 6c). A similar arrangement of the encapsulated counter ions was reported earlier for
Pb8O64+ polyoxocations in U24, where Pb2+ is also located under the hexagonal windows,27 as
well as the K cations in a salt of U24 that contains both Li and K.55
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Figure 6. (a) Polyhedral representation of U24 contained in 2. (b) K+ cations in a cubic arrangement encapsulated in
the cage of U24. (c) Each K+ is 8-coordinated by six uranyl oxygen atoms and two water oxygen atoms. Legend as in
Figure 1 except water oxygen atoms are shown in cyan.
31P NMR spectra were collected for room temperature solutions of U19 that had earlier been
heated in sealed reactor vessels to soak temperatures from 40 to 160 °C for 8 hours, as well as a
control aged at room temperature for 8 hours (Figure 7a). The disassembly of U19 started in
solutions heated to 40°C, as indicated by the decrease of the peak intensity of BzDPA ligands
contained in U19 (signal A, 17.92 ppm) and increase of the peak intensity of disassociated
BzDPA ligands (signal B, 12.20 ppm) in the solution. A phosphorus species is evident in the
spectrum from the solution heated to 80°C as peak C (20.56 ppm) and corresponds to ~14% of
the total P in solution. Signal A is absent in spectra collected for solutions heated above 100°C,
indicating full disassembly of U19. The highest yield of crystals of 2 was obtained from the
solution heated at 80°C (~10%).
Time-resolved 31P NMR spectra were collected from a solution of U19 heated at 80°C for
eight hours in the NMR spectrometer (Figure 7b). Signal C appeared within one hour of the
onset of heating and intensified along with signal B over time, accompanied with a decrease
and broadening of signal A.
Diffusion-ordered spectroscopy (DOSY) measurements were done for the room-temperature
solution that had been heated in a sealed vessel at 80°C for eight hours. From DOSY spectra,
the measured diffusion coefficient (D) for signal A is (1.44
0.46)
10−10 m2 s−1, and that
× ∙
±
for signal C is (1.11
0.17)
10−10 m2 s−1 (Figure S3). The similarity of these diffusion
× ∙
±
coefficients indicates that the species that produces signal C is similar to U19 (which produces
signal A) in size.
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Figure 7. (a)31P NMR spectra of aqueous solutions that initially contained U19 collected at room temperature after
solution aging at room temperature or after heating of solutions in a sealed vessel for 8 hours at 40 to 160 °C and
cooling to room temperature. (b) Time-resolved 31P NMR spectra collected for a solution that initially contained U19
that was heated at 80°C for 8 hours.
Crystals of K-U24 were isolated in highest yield from a solution that initially contained U19
after heating at 80°C for 8 hours (hereafter designated the reaction solution). Crystals of U19 and
U24 were dissolved in water and ESI-MS spectra were collected for comparison with that of the
reaction solution (Figure 8a). Aqueous solutions prepared by dissolving the salts of U19 and U24
each contain signals corresponding to species with charges of -6, -5 and -4. The spectrum
collected for the reaction solution contains signals corresponding to charges of -7, -6 and -5.
Peaks at m/z values 1336.3, 1641.3 and 2095.1 in the spectrum of solution into which U24 was
dissolved are assigned to [K9H9(UO2)24(O2)24(OH)24]6-, [K14H5(UO2)24(O2)24(OH)24]5- and
[K18H2(UO2)24(O2)24(OH)24(H2O)]4-, respectively. In the spectrum of U19, each peak represents
the cluster ion (UO2)19(O2)26(OH)2(C6H4P2O6)432- with varying combinations of K+ and H+
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counterions and water. For example, prominent peaks observed at m/z values 1261.2, 1529.0 and
2015.9 correspond to [K16H10(UO2)19(O2)26(OH)2(C6H4P2O6)4]6-,
[K18H9(UO2)19(O2)26(OH)2(C6H4P2O6)4]5- and [K28(UO2)19(O2)26(OH)2(C6H4P2O6)4(H2O)2]4-,
respectively. The spectrum of the solution prepared by dissolution of U19 salt followed by
heating to 80°C (the reaction solution) indicates the presence of multiple species and includes
weak signals that are attributable to U24. However, peaks observed at m/z values 1248.0, 1462.6,
1516.4 and 1781.9 are inconsistent with either U24 or U19, and likely correspond to uranyl
peroxide clusters with or without organic ligands, of which the molecular mass ranges from 8700
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to 9100 (see Supporting information, Table S1 for the assignment of all peaks in Figure 8a).
Figure 8. (a) ESI-MS spectra of the solution into which crystals of U19 or U24 was dissolved and the reaction
solution from which crystals of U24 were isolated. (b) ESI-MS spectra of aqueous solutions that initially contained
U19 collected at room temperature after solution aging at room temperature or after heating of solutions in sealed
vessels for 8 hours at 40 to 160 °C and cooling to room temperature.
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As for the 31P NMR experiment, ESI-MS spectra were also collected for room temperature
solutions of U19 that had earlier been heated in sealed vessels from 40 to 160 °C for 8 hours, as
well as a solution aged at room temperature for 8 hours (Figure 8b). The spectra indicate that
peaks attributed to U19 persist in solutions heated up to 60 C, although peaks associated with
unknown clusters are dominant for solutions heated to 40 C or higher.
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Raman spectra were collected for the various heated solutions that were initially prepared by
dissolving the salt of U19 (Figure 9). The salt of U19 (1) produces a Raman mode at 825 cm-1
assigned to the νs, O-O of the uranyl triperoxide units, and this is also present in the spectrum of
the solution produced by dissolving the salt of U19. The Raman spectra of an aqueous solution of
U24 and the reaction solution (heated to 80 C) contain a mode at 848 cm-1 that is assign to the
stretch of peroxide in the uranyl diperoxide units. The aqueous solution of U24 gives a uranyl
vibration mode at 812 cm-1, whereas the reaction solution and a solution into which the salt of
U19 was dissolved in water produce uranyl vibration modes at 804 cm-1. The signals at 1040 cm-1
are assigned to the asymmetric stretching of PO3 (νas, P-O-P). The mode at 1067 cm-1 for the
reaction solution corresponds to the asymmetric stretch of PO3 (νas, P-O-P) of dissociated
BzDPA ligands. In the room temperature spectrum of the solution heated to 60°C for 8 hours, the
signal at 825 cm-1 is reduced, while the signals at 848 cm-1 and 1067 cm-1 are strengthened
relative to the spectrum of an unheated solution, indicating conversion from uranyl triperoxide
species to uranyl diperoxide species and the dissociation of BzDPA ligands. Increasing
temperature fosters formation of uranyl diperoxide species until the temperature is above 100°C.
The modes attributable to uranyl peroxide cluster species decrease above 80°C, and are no longer
detectable for solutions heated at 140°C for 8 hours.
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Figure 9. (a) Raman spectra of solutions into which crystals of U19 or U24 were dissolved and the reaction solution
from which crystals of U24 were isolated. (b) Raman spectra of aqueous solutions that initially contained U19
collected at room temperature after solution aging at room temperature or after heating of solutions in sealed vessels
for 8 hours at temperatures from 40 to 160 °C and cooling to room temperature.
U19 is composed of topological pentagons of uranyl di or tri-peroxide units, whereas U24
contains topological squares and hexagons of uranyl diperoxide dihydroxide units. Previous
studies suggested assembly of uranyl triperoxide monomers into uranyl peroxide capsules
through tetramer, pentamer, and hexamer intermediates.50, 56 The significant topological
differences between U19 and U24 require nearly complete disassembly of U19 into uranyl peroxide
monomers or dimers followed by a reassembly into U24. When the aqueous solution of U19 is
heated between 60°C and 100°C, a disassembly of U19 is induced. Raman spectra suggest that
uranyl diperoxide species dominate in the solution and no significant Raman signal
corresponding to uranyl triperoxide species was observed. Conversion of uranyl triperoxide
monomers to diperoxide monomers can occur through the disproportionation of peroxide ligands
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to form two bridging hydroxyl ligands and free molecular oxygen.31 Hence, a disproportionation
4-
4-
reaction may occur during heating: 2 UO2(O2)3 + 2 H2O → 2 UO2(O2)2(OH)2 + O2↑.31 31
P
NMR spectra suggest the presence of a phosphorous-containing cluster in the reaction solution
where the highest yield of U24 crystals was obtained. ESI-MS indicate that a cluster of mass 8.7 –
9.1 KDa is present in the reaction solution. This unidentified cluster likely forms due to assembly
of uranyl diperoxide units and BzDPA ligands according to the 31P NMR and ESI-MS results.
A precipitate forms when an aqueous solution that initially contained U19 is heated above
100°C (see Supporting Information, Figure S4 for the ICP-OES results of the concentration of
uranium in solutions after heat treatment). Powder X-ray diffraction patterns collected for
precipitates formed in solutions heated from 140 to 200°C indicated the major phase is
metaschoepite ((UO2)4O(OH)6)(H2O)5, whereas precipitates formed at lower temperatures are
poorly crystalline (see Supporting Information, Figure S5 for the P XRD results).
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CONCLUSIONS
The self-assembly of uranyl peroxide polyhedra with benzene-1,2-diphosphonic acid formed
U19 that features a defected fullerene topology with 19 uranyl ions. This is the first example of a
uranyl peroxide cluster that contains terminal hydroxyl ligands and intramolecular H-bonds.
With the presence of benzene-containing ligands, K-π and π-π interactions along with K-O bonds
connect U19 clusters in the solid state to form a POM-based supramolecular network. These
interactions appear to accelerate the rate of cluster crystallization relative to most uranyl
peroxide cage clusters. The functionalizing of uranyl peroxide clusters using organic ligands as
shown here may provide a route to uranyl peroxide cluster-based metal organic frameworks
(MOFs).
U19 was selected to be a POM precursor to obtain other POMs via structural transformation.
Direct heating of an aqueous solution was applied to trigger the structural transformation of U19
through a disassembly and reassembly process. In response to the increasing temperature, U19
that was dissolved in water fragmented and the uranyl polyhedra reassembled into U24 above
60°C. Crystallization of U24 from the heated solution provided the first example of encapsulation
of only potassium and water in U24. Above 100°C, the uranyl peroxide cluster decomposes and
metashoepite precipitates. U28, on the other hand, is inert when exposed to the same reaction
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conditions, which indicates the reactivity of U19 may result from its unstable geometry.
Deconstruction of a geometrically unstable POM may generate building blocks that are not
commonly formed from normal synthesis approaches; in this case, uranyl diperoxide units.
Demonstrated by the successful synthesis of U24 from U19, we propose a workflow here to obtain
kinetically or thermodynamically unfavorable POM structures, which may be expanded to the
transition metal POMs synthesis. Step 1: introduce additional organic or inorganic ligands into
the POM structure to create geometrically unstable structures. Step 2: Induce fragmentation of
the unstable structure into basic building blocks that assemble into a new structure.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:.
Crystallographic information file for 1 and 2 (CIF)
Thermogravimetric analysis (TGA), infrared spectroscopy (IR), Diffusion-ordered Spectroscopy
(DOSY), Electrospray Ionization Mass Spectrometry (ESI-MS), Concentration of U in the
resulting solution from the heating experiments, and additional crystallographic information
(PDF)
AUTHOR INFORMATION
Corresponding Author
Peter C. Burns, pburns@nd.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDMENTS
This work was funded by the Chemical Sciences, Geosciences and Biosciences Division, Office
of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant No. DE-FG02–
07ER15880. We thank Dr. Jaroslav Zajicek and Dr. Evgenii Kovrigin for assisting with NMR
measurements, and Dr. William Boggess for helping with ESI-MS analyses. We also thank Dr.
Seth N. Brown for his advice on NMR spectra analyses. The Magnetic Resonance Research
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Center, Center for Environmental Science and Technology, Materials Characterization Facility,
and Mass Spectrometry and Proteomics Facility at the University of Notre Dame provided
instrumentation used in this work.
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REFERENCES
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(1) Pope, M. T.; Müller, A., Polyoxometalate chemistry: an old field with new dimensions in
several disciplines. Angew. Chem., Int. Ed. 1991, 30 (1), 34-48.
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