Sandwich-Type Uranyl Phosphate-Polyoxometalate Cluster Exhibiting Strong Luminescence

Article abstract of DOI:10.1021/acs.inorgchem.1c00622

A pure inorganic uranyl phosphate-polyoxometalate of Na17{Na@[(SbW9O33)2(UO2)6(PO3OH)6]}·xH2O (abbreviated as Na@U6P6, with x ≈ 46) featuring a sandwich-type structure was prepared using Keggin-type trilacunary [α-B-SbW9O33]9- units as building blocks, which were formed in situ by SbCl3 and Na2WO4·2H2O. Crystal structural analysis showed that six UO22+ cations and six PO3OH2- anions generated a wheel-like cluster unit with a Na+ center ([Na@(UO2)6(PO3OH)6]+) that is stabilized by two [α-B-SbW9O33]9- units. Na@U6P6 displayed a solid-state photoluminescence quantum yield of 33% at 300 K. The temperature-dependent fluorescence emission spectra showed that Na@U6P6 has temperature-sensitive fluorescence in which its emission intensity decreased by 77% as the temperature increased from 200 to 300 K. These results suggest that such uranyl phosphate-polyoxometalate clusters could serve as potential temperature-sensitive molecular materials.

Full text of DOI:10.1021/acs.inorgchem.1c00622

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Article
Sandwich-Type Uranyl Phosphate−Polyoxometalate Cluster
Exhibiting Strong Luminescence
Hai-Ying Wang, Xiu-Ying Zheng,* La-Sheng Long, Xiang-Jian Kong,* and Lan-Sun Zheng
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ABSTRACT: A pure inorganic uranyl phosphate−polyoxometalate of
Na17{Na@[(SbW9O33)2(UO2)6(PO3OH)6]}·xH2O (abbreviated as Na@U6P6, with x ≈
46) featuring a sandwich-type structure was prepared using Keggin-type trilacunary [α-B-
SbW9O33]9− units as building blocks, which were formed in situ by SbCl3 and Na2WO4·
2H2O. Crystal structural analysis showed that six UO2 cations and six PO3OH2− anions
2+
generated a wheel-like cluster unit with a Na+ center ([Na@(UO2)6(PO3OH)6]+) that is
stabilized by two [α-B-SbW9O33]9− units. Na@U6P6 displayed a solid-state photo-
luminescence quantum yield of 33% at 300 K. The temperature-dependent fluorescence
emission spectra showed that Na@U6P6 has temperature-sensitive fluorescence in which its
emission intensity decreased by 77% as the temperature increased from 200 to 300 K.
These results suggest that such uranyl phosphate−polyoxometalate clusters could serve as
potential temperature-sensitive molecular materials.
properties;12 its emission is derived from the highest occupied
molecular orbital (HOMO) → lowest unoccupied molecular
orbital (LUMO) (O2p → U5f) electron transition,13 which
exhibits no transition resistance. Therefore, uranyl ions possess
strong, intrinsic luminescence. Unfortunately, in-depth studies
on the luminescence properties of most reported uranyl
polyoxometalate clusters have not been conducted; such
studies could further reveal the versatility of these complexes.
Herein, a uranyl phosphate−polyoxometalate cluster with
the formula Na17{Na@[(SbW9O33)2(UO2)6(PO3OH)6]}·
xH2O (abbreviated as Na@U6P6, with x ≈ 46) was obtained
using in situ formed Keggin-type trilacunary POMs of [α-B-
SbW9O33]9− as inorganic ligands. Structural analysis showed
that Na@U6P6 is a pure sandwich-type inorganic metal cluster
with a wheel-like [Na@(UO2)6(PO3OH)6]+ unit as the
sandwich linker. Fluorescence measurements revealed that
Na@U6P6 has a solid-state photoluminescence quantum yield
(PLQY) of 33%. In addition, the temperature-dependent
fluorescence emission spectra showed that the emission
intensity of Na@U6P6 decreased by 77% as the temperature
increased from 200 to 300 K. This indicates that Na@U6P6 has
temperature-sensitive fluorescence properties over the meas-
ured experimental conditions and that these uranyl polyox-
1. INTRODUCTION
The study of actinide complexes has received extensive
attention in the past decade because of their potential
applications in the nuclear fuel cycle.1 Among the actinides,
the contribution of U and its waste to environmental pollution
2
2+
are of great concern to scientists. UO2 is the most stable
form of U and can coordinate with a variety of O/N/S-
containing ligands to form complexes with different properties;
this propensity can be used in the storage and separation of
radioactive waste. Polyoxometalates (POMs), defined as early-
transition-metal clusters (e.g., M = V, Nb, Ta, Mo, W) with
their highest oxidation states, are a class of oxygen-rich
multidentate inorganic ligands that have been widely used in
various research fields.3 Previous investigations have found that
2+
linear UO2 combines easily with lacunary POMs containing
polyoxo groups to form uranyl polyoxometalate clusters.4
2+
Trilacunary POMs are particularly suitable for chelating UO2
ions5 because the linear uranyl structure (OUO2+) can
easily coordinate with them by occupying their vacancy sites,
2+
thus making this an effective strategy for coordinating UO2
ions.
Compared with the frequent reports on trilacunary POM-
based 3d,6 3d−4f,7 or 4f clusters,8 the synthesis of POM-based
actinide clusters still remains a challenge,9,10 and the
understanding of their structure−function relationship is
limited. Since Pope and co-workers reported the first
sandwich-type uranyl polyoxometalate {(UO2)2(PW9O34)2}
cluster in 1999,5a only a few sandwich-type uranyl polyox-
ometalate clusters have been studied,5,11 and exploration of the
potential applications of such complexes is especially scant.
The uranyl ion has very good stability and attractive optical
Received: March 1, 2021
Published: April 22, 2021
https://doi.org/10.1021/acs.inorgchem.1c00622
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 6790−6795
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Article
ometalate clusters could be used as a potential temperature-
sensitive material.
ions5 because the linear uranyl structure (OUO2+) can
easily coordinate with them by occupying their vacancy sites,
2+
thus making this an effective strategy for coordinating UO2
2. EXPERIMENTAL SECTION
ions. Compound Na@U6P6 was synthesized from the reaction
of SbCl3, NaOAc·3H2O, Na2WO4·2H2O, H3PO4, and
UO2(NO3)2·6H2O in aqueous solution. During the reaction,
the trilacunary [α-B-SbW9O33]9− was formed in situ by SbCl3
and Na2WO4·2H2O. H3PO4 was involved in the coordination
of compound Na@U6P6 in the form of monoprotonation in
this acidic reaction system.
Single-crystal analysis showed that Na@U6P6 crystallizes in
̅
the triclinic P1 space group and displays a sandwich structure,
with the largest diameter being 1.9 nm (Figure 1). The cluster
Materials and Physical Measurements. All materials, reagents,
and solvents were of commercial origin and were used as received.
Caution!Although isotopically depleted uranium was used for all
experiments described here, appropriate precautions are essential for
handling all radioactive materials. IR spectra were recorded on a
Nicolet iS50 spectrophotometer with pressed KBr pellets in the range
of 4000−400 cm−1. Powder X-ray diffraction data were recorded on a
X’pert PRO powder X-ray diffractometer (Cu Kα, λ = 1.54184 Å) at
room temperature. Thermogravimetric analysis (TGA) was per-
formed on a SDT-Q600 thermal analyzer with a heating rate of 10 °C·
min−1. Inductively coupled plasma emission spectroscopy (ICP-OES)
data were obtained with a Thermo Fisher iCAP7000 spectrometer.
Emission spectra were acquired at room temperature by a steady-state
spectrometer (FLS-980, Edinburgh) with a 450 W xenon lamp. Time-
resolved photoluminescence decay curves were obtained on the same
spectrometer but with a μF2 xenon lamp. The luminescence overall
absolute quantum yield for a sample powder was collected by the
same spectrometer using a 450 W xenon lamp and an integrating
sphere (coating with a polytetrafluoroethylene-like material with a
reflectance of >99% over the spectral range of 400−1500 nm). The
temperature-dependent emission spectra were acquired by a steady-
state spectrometer (FLS-980, Edinburgh).
Preparation of the Mixed Aqueous Solution of NaOAc·3H2O
(0.2 mol·L−1) and Na2WO4·2H2O (1.0 mol·L−1). NaOAc·3H2O
(0.02 mol, 2.72 g) and Na2WO4·2H2O (0.1 mol, 32.98 g) were
dissolved in 100 mL of deionized water. The mixed aqueous solution
of NaOAc·3H2O (0.2 mol·L−1) and Na2WO4·2H2O (1.0 mol·L−1)
was obtained.
Preparation of an Aqueous Solution of SbCl3 (2.0 mol·L−1).
The aqueous solution of SbCl3 (2.0 mol·L−1) was prepared by
dissolving Sb2O3 (0.01 mol, 2.92 g) in 10 mL of HCl (37% aqueous
solution).
Figure 1. Polyhedral (a) and ball-and-stick (b) views of the uranyl
p h o s p h a t e − p o l y o x o m e t a l a t e c l u s t e r { N a @ [ ( S b -
W9O33)2(UO2)6(PO3OH)6]}17−. Colo code: Sb, gray; W, green; P,
pink; U, yellow; O, red; Na, blue.
Synthesis of Na17{Na@[(SbW9O33)2(UO2)6(PO3OH)6]}·xH2O
(Na@U6P6, with x ≈ 46). A total of 200 μL of SbCl3 (1.0 mol·
L−1) was added to the mixed aqueous solution (4.0 mL) of NaOAc·
3H2O (0.2 mol·L−1) and Na2WO4·2H2O (1.0 mol·L−1). The reaction
solution was stirred for 1−2 min until clarified. A total of 200 μL of
H3PO4 (2.0 mol·L−1) was added and stirred for 1−2 min. Then, the
aqueous solution of 200 μL of UO2(NO3)2·6H2O (2.0 mol·L−1) was
added to the above mixed solution and stirred for 10 min. The mixed
solution was further heated in a water bath at 95 °C for 5 h. The
resulting solution was filtered and left to slowly evaporate at room
temperature, and then yellow-green crystals were obtained after 1 day.
Yield: 29%, based on UO2(NO3)2·6H2O. Anal. Calcd: Na, 5.14; Sb,
3.03; P, 2.31; W, 41.12; U, 17.75. Found: Na, 5.35; Sb, 2.99; P, 2.48;
W, 41.50; U, 17.73. IR (KBr, cm−1): 3436 (s), 2924 (s), 2854 (m),
1630 (s), 1466 (w), 1457 (w), 1383 (m), 1121 (w), 1051 (s), 1009
(m), 973 (m), 938 (m), 890 (s), 842 (s), 795 (w), 761 (w), 686 (w),
607 (w), 558 (w), 507 (m), 485 (w).
of {Na@[(SbW9O33)2(UO2)6(PO3OH)6]}17− can be viewed
as a wheel-like uranyl phosphate cluster unit of
[Na@(UO2)6(PO3OH)6]+, which is sandwiched between
two trilacunary [α-B-SbW9O33]9− units (Figure 2). Six UO2
2+
cations and six PO3OH2− anions are alternately arranged
a ro u nd
a
N a+ i o n , f o r m in g t h e w h ee l - l i k e
[Na@(UO2)6(PO3OH)6]+ cluster unit (Figure 2b). The two
trilacunary [α-B-SbW9O33]9− units cover the upper and lower
faces of the wheel-like cluster unit in a centrally symmetric
manner (Figure 2a,c) to form the sandwich-type anionic core
X-ray Crystallography. Data for compound Na@U6P6 were
collected on an Agilent Technologies SuperNova Microfocus single
diffractometer using Mo Kα radiation (λ = 0.71073 Å). The structure
was solved by direct methods, and non-H atoms were refined
anisotropically by a least-squares method on F2 using the OLEX2
program.14 Crystal data as well as details of the data collection and
refinement for Na@U6P6 are summarized in Table S1. According to
the charge balance, ICP-OES, and TGA, there were about 46 guest
water molecules in compound Na@U6P6. Because of disorder, these
guest water molecules were removed using SQUEEZE.15 CCDC
2059822 contains the supplementary crystallographic data for Na@
U6P6 for this paper.
Figure 2. (a and c) Two centrally symmetric trilacunary [α-B-
9 −
SbW9 O3 3
]
units. (b) Wheel-like cluster unit of
[Na@(UO2)6(PO3OH)6]+. (d) Sandwich-type {Na@[(Sb-
W9O33)2(UO2)6(PO3OH)6]}17−. (e) Na+ ion in the same plane as
the six P atoms. (f) Alternating arrangement of six U and six P atoms.
Colo code: Sb, gray; W, green; P, pink; U, yellow; O, red; Na, blue.
3. RESULTS AND DISCUSSION
Synthesis Strategy and Crystal Structure Analysis.
2+
Trilacunary POMs are particularly suitable for chelating UO2
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of {Na@[(SbW9O33)2(UO2)6(PO3OH)6]}17− (Figure 2d).
The trilacunary [α-B-SbW9O33]9− unit is a common inorganic
building block that can coordinate with metal ions, such as
UO22+ (Figure S1), through three lacunary sites to form POM-
based metal complexes.16 Alternatively, the sandwich-type
anionic core of {Na@[(SbW9O33)2(UO2)6(PO3OH)6]}17− can
be viewed as two [(α-B-SbW9O33)(UO2)3]3− units (Figure
S1c) that are connected by six PO3OH2− units and a central
Na+ ion. Interestingly, the templated Na+ ion is located in the
center of the sandwich-type Na@U6P6 complex because it
Luminescence Properties. The results of the solid-state
luminescence measurements show that Na@U6P6 exhibits
intense photoluminescence emissions with 441 nm excitation
at 300 K. The luminescence spectrum (Figure 4a) is
2+
coordinates with the acyl O atom of UO2 in the axial
direction, which is in the same plane as the six P atoms (Figure
2f). The resulting uranyl phosphate polyoxometalate sandwich
structure is significantly different from previously reported
sandwich-type uranyl polyoxometalate clusters that comprise
2+
two lacunary POM units directly connected through UO2
linkers.5 In addition, the anionic cluster core of {Na@[(Sb-
W9O33)2(UO2)6(PO3OH)6]}17− as a node can be extended
into a purely inorganic three-dimensional metal frame if the
peripheral Na+ ions act as linkers (Figure 3). All of the U
Figure 4. (a) Solid-state fluorescence emission spectra of Na@U6P6
and UO2(NO3)2·6H2O under excitation at 441 nm. (b) Photo-
luminescence decay curve of Na@U6P6. (c) Emission map of spectra
recorded from 100 to 300 K. (d) Variation trend of each emission
peak of Na@U6P6.
characterized by five broad emission peaks from 450 to 650
nm with a maximum emission peak at 517 nm, which is
attributable to uranyl ion emission from the triplet to singlet
state.17 Additionally, four shoulders were noted at 496, 540,
565, and 590 nm. The spectrum shows a typical vibronic
structure, resulting from the symmetrical vibration of the O
UO bonds, corresponding to the electronic transitions of S11
→ S00 and S10 → S0ν (ν = 0−3).18 In comparison with the
emission peaks in the spectrum of UO2(NO3)2·6H2O, the five
emission peaks of Na@U6P6 were red-shifted by varying
degrees, which is probably because of the changes in the
number of donor atoms in the equatorial plane of the U
coordination sphere.19,20 Specifically, U6+ exhibits a hexagonal-
bipyramidal structure in UO2(NO3)2·6H2O but displays a
pentagonal-bipyramidal geometry in the Na@U6P6 cluster.
The luminescence decay time of Na@U6P6 was measured at
room temperature with excitation and emission bands at 441
and 517 nm, respectively. As shown in Figure 4b, the
photoluminescence lifetime (τ) of Na@U6P6 was determined
to be 121.90 μs using the exponential decay functional
equation I = A exp(−t/τ), with a preexponential factor (A) of
4898. The emission colors of Na@U6P6 and UO2(NO3)2·
6H2O are both green, like that of UO22+, under 365 nm UV
irradiation (Figure S6a) and correspond to the CIEx,y
coordinate values of (0.20, 0.66) and (0.20, 0.60), respectively.
Compared with UO2(NO3)2·6H2O, the values of Na@U6P6
significantly shifted toward the y coordinate (Figure S6b).
Furthermore, the PLQY of solid-state Na@U6P6 was collected
at room temperature with an excitation band at 441 nm. The
PLQY of Na@U6P6 was 33%, which is higher than that of
UO2(NO3)2·6H2O (27%). The luminescence of the uranyl ion
is related to the type of complex that it is a part of, the
presence of water molecules around the U6+ atom, counter-
balance ions, and the solvent type because all of these affect the
Figure 3. Pure inorganic three-dimensional metal frame of compound
Na@U6P6.
atoms in this complex show a distorted pentagonal-bipyramidal
configuration. Except for the two acyl O atoms in the axial
direction, the other five-coordinated O atoms are in the same
plane. Two are connected to [α-B-SbW9O33]9− units by W−
O−U bridges, and the other three are connected to P atoms by
U−O−P bridges (Figure S2b). Notably, [α-B-SbW9O33]9− was
generated in situ by SbCl3 and Na2WO4·2H2O. The [α-B-
SbW9O33]9− unit was confirmed to have been formed in situ
instead of [PW9O34]9− based on the following points: (a) the
coordination mode of {SbO3} (Figure S3a), and not that of
{PO4} (Figure S3b), was observed; (b) the bond lengths of
1.978(8)−2.002(8) and 1.494(8)−1.566(9) Å for Sb···O and
P···O, respectively, are representative of those of [α-B-
SbW9O33]9− (Table S2); (c) the characteristic peaks of [α-B-
SbW9O33]9− appeared in the IR spectra of Na@U6P6 (Figure
S4).
The results of bond-valence-sum (BVS) calculations for
Na@U6P6 show that all of the W, Sb, P, and U atoms are in the
6+, 3+, 5+, and 6+ oxidation states, respectively (Table S3).
Terminal atoms O54, O49, and O58 on the P1, P2, and P3
atoms, respectively, demonstrated BVS results of 1.38, 1.41,
and 1.34, respectively, suggesting that they are all monoproto-
nated.
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vibration of the OUO double bonds.21 The greater the
vibration of the OUO bonds, the greater the energy loss
during the transition process, which negatively impacts the
luminescence. In Na@U6P6, the central Na+ ion is coordinated
of Structure and Functional Regulation of Hybrid Materials
of Ministry of Education, Institutes of Physical Science and
Information Technology, Anhui University, Hefei 230601,
China; orcid.org/0000-0002-4016-9519;
2+
with six acyl O atoms of six UO2 ions in the axial direction,
Email: xyzheng@ahu.edu.cn
further stabilizing the OUO double bonds and enhancing
Xiang-Jian Kong − Collaborative Innovation Center of
Chemistry for Energy Materials, State Key Laboratory of
Physical Chemistry of Solid Surface and Department of
Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China; orcid.org/
0000-0003-0676-6923; Email: xjkong@xmu.edu.cn
the luminescence effects of Na@U6P6.
To evaluate the potential of Na@U6P6 in temperature
sensing, the temperature-dependent emission spectra of Na@
U6P6 were obtained (Figure 4c,d) between 100 and 300 K.
The temperature-dependent luminescent behavior of Na@
U6P6 is significantly different from that of UO2(NO3)2·6H2O
(Figure S7). As presented in Figure 4c, the emission intensity
of Na@U6P6 shows a small change over the temperature range
100−200 K, suggesting that Na@U6P6 has a weak temperature
sensitivity in this temperature range. As the temperature rose
to 300 K, the emission intensity of Na@U6P6 dropped sharply
by 77% and showed a thermal sensitivity of 0.79%·K−1. This
indicated that Na@U6P6 has a strong temperature sensitivity in
the range 200−300 K. Normally, the fluorescence intensity
decreases with an increase in the temperature because more
nonradiative transitions occur. For Na@U6P6, the rapid
decrease in the fluorescence intensity from 200 to 300 K
may be attributed to the quenching effect of heavy metals on
uranyl fluorescence.
Authors
Hai-Ying Wang − Collaborative Innovation Center of
Chemistry for Energy Materials, State Key Laboratory of
Physical Chemistry of Solid Surface and Department of
Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China
La-Sheng Long − Collaborative Innovation Center of
Chemistry for Energy Materials, State Key Laboratory of
Physical Chemistry of Solid Surface and Department of
Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China; orcid.org/
0000-0002-0398-4709
Lan-Sun Zheng − Collaborative Innovation Center of
Chemistry for Energy Materials, State Key Laboratory of
Physical Chemistry of Solid Surface and Department of
Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China
4. CONCLUSION
In summary, a uranyl phosphate polyoxometalate cluster, Na@
U6P6, was obtained, stabilized by two Keggin-type trilacunary
[α-B-SbW9O33]9− units formed in situ. Structural analysis
showed that Na@U6P6 displayed a sandwich-type structure, in
which six UO22+ ions and six PO3OH2− ions are templated by a
Na+ ion to form a wheel-like cluster unit that is further
sandwiched by two [α-B-SbW9O33]9− units. Interestingly,
Na@U6P6 exhibited a PLQY of 33% at room temperature,
and its emission intensity decreased significantly when the
temperature was increased from 200 to 300 K, indicating that
Na@U6P6 has temperature-sensitive characteristics over the
measured experimental conditions. This work suggests that
uranyl phosphate polyoxometalate complexes could serve as
luminescent materials.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00622
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grants 21871224, 21673184, 21721001,
and 21901002) and the Open Project of the State Key
Laboratory of Physical Chemistry of the Solid Surface of
Xiamen University (Grant 201822).
REFERENCES
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00622.
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AUTHOR INFORMATION
Corresponding Authors
Xiu-Ying Zheng − Collaborative Innovation Center of
Chemistry for Energy Materials, State Key Laboratory of
Physical Chemistry of Solid Surface and Department of
Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China; Key Laboratory
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