Unexpected Roles of Alkali-Metal Cations in the Assembly of Low-Valent Uranium Sulfate Molecular Complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b03182

The directing effect of coordinating ligands in the formation of uranium molecular complexes has been well established, but the role of counterions in metal-ligand interactions remains ambiguous and requires further investigation. In this work, we describe the targeted isolation, through the choice of alkali-metal ions, of a family of tetravalent uranium sulfates, showing the influence of the overall topology and, unexpectedly, the U^IV nuclearity upon the inclusion of such countercations. Analyses of the structures of uranium(IV) oxo/hydroxosulfate oligomeric species isolated from consistent synthetic conditions reveal that the incorporation of Na⁺ and Rb⁺ promotes the crystallization of 0D discrete clusters with a hexanuclear [U₆O₄(OH)₄(H₂O)₄]¹²⁺ core, whereas the larger Cs⁺ ion allows for the isolation of a 2D-layered oligomer with a less condensed trinuclear [U₃(O)]¹⁰⁺ center. This finding expands the prevalent view that counterions play an innocent role in molecular complex synthesis, affecting only the overall packing but not the local oligomerization. Interestingly, trends in nuclearity appear to correlate with the hydration enthalpies of alkali-metal cations, such that the alkali-metal cations with larger hydration enthalpies correspond to more hydrated complexes and cluster cores. These findings afford new insights into the mechanism of nucleation of U^IV, and they also open a new path for the rational design and synthesis of targeted molecular complexes.

Full text of DOI:10.1021/acs.inorgchem.9b03182

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Article
Unexpected Roles of Alkali-Metal Cations in the Assembly of Low-
Valent Uranium Sulfate Molecular Complexes
Zenghui Yue, Xiaofeng Guo, Mei-Ling Feng, Yue-Jian Lin, Yu Ju, Xiao Lin, Zhi-Hui Zhang, Xiaojing Guo,
Jian Lin,* Yu-Ying Huang, and Jian-Qiang Wang
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ABSTRACT: The directing effect of coordinating ligands in the formation
of uranium molecular complexes has been well established, but the role of
counterions in metal−ligand interactions remains ambiguous and requires
further investigation. In this work, we describe the targeted isolation,
through the choice of alkali-metal ions, of a family of tetravalent uranium
sulfates, showing the influence of the overall topology and, unexpectedly,
the UIV nuclearity upon the inclusion of such countercations. Analyses of
the structures of uranium(IV) oxo/hydroxosulfate oligomeric species isolated from consistent synthetic conditions reveal that the
incorporation of Na+ and Rb+ promotes the crystallization of 0D discrete clusters with a hexanuclear [U6O4(OH)4(H2O)4]12+ core,
whereas the larger Cs+ ion allows for the isolation of a 2D-layered oligomer with a less condensed trinuclear [U3(O)]10+ center. This
finding expands the prevalent view that counterions play an innocent role in molecular complex synthesis, affecting only the overall
packing but not the local oligomerization. Interestingly, trends in nuclearity appear to correlate with the hydration enthalpies of
alkali-metal cations, such that the alkali-metal cations with larger hydration enthalpies correspond to more hydrated complexes and
cluster cores. These findings afford new insights into the mechanism of nucleation of UIV, and they also open a new path for the
rational design and synthesis of targeted molecular complexes.
charge-balancing species, the counterions also influence the
thermodynamics and electrochemical properties, that even-
tually affect the compositions of the ML molecular complexes.
As one of the most commonly used families of counterions,
alkali-metal ions are inherent to the advanced nuclear fuel
cycles.14,15 Through the heating of fuels with salts of alkali
metals, uranium and plutonium are converted to solid-state
ternary oxides, uranates and plutonates, respectively, while
volatile fission products can be removed during the reactions.16
These uranates are one of the main constituents of yellowcake
and essential to the reprocessing of spent nuclear fuel.17
Therefore, besides the influences of pH value, temperature,
and relative concentration, exploring the effect of alkali metal
on the formation of uranium molecular complexes is of great
necessity.
INTRODUCTION
■
The mechanisms of the formation of metal−ligand (ML)
molecular complexes have been extensively studied, which,
however, were mostly limited to the interactions between the
metal ions and coordinating ligands.1 The contributions of
other species between ML complexes, such as counterions and
solutes, have been practically overlooked owing to their
relatively little influence on the local coordination of molecular
ions and thermodynamics that drives the reactions.2 Counter-
ions have been generally used as charge-balancing species to
control the ionic strength,3 metal-complex solubility,4 and
overall packing of molecular ions.5−10 Nevertheless, our recent
study demonstrates the correlation between the hydration
enthalpies of countercations (A+ = Li+, Na+, K+, Rb+, Cs+,
+
+
NH4 , or NR4 ) and the formation of thorium nitrato
molecular complexes from aqueous solution, wherein a
transition from the more hydrated [Th(NO3)5(H2O)2]− to
less hydrated [Th(NO3)6]2− ML complex occurs with
decreasing hydration enthalpies of counterions.11 Moreover,
the electrochemical properties and chemical reactivity within a
series of alkali-metal cerium(III) 1,1′-binolate complexes can
be fine-tuned by the choice of alkali metals.12 Jin and co-
workers suggest that hydrophobic tetra-n-alkylammonium
(TAA+) cations tend to stabilize higher oxidation states of
neptunium dioxo cations in aqueous solution, while the
hydrated Li+ cation does not have such an effect.13 All of these
results suggest that, in addition to their function as innocent
The importance of the coordination chemistry of uranium
sulfate is highlighted by the widespread nature of uranium
sulfate minerals and the extensive use of sulfuric acid in nuclear
fuel cycles.18,19 Sulfate can adopt rich binding modes including
monodentate and bidentate, as well as a variety of bridging
modes in both solution and solid states, endowing a rich but
Received: October 30, 2019
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unpredictable effect on the structure and speciation.3,20−23
Despite the fact that numerous uranium sulfate complexes have
been reported, the majority are based on the uranyl ions and
only a few of them incorporate the lower-oxidation-state
uranium ions (III and IV).24−29 Furthermore, UIV ions are
inclined to hydrolyze and condense to form polynuclear
species owing to the high Lewis acidity of UIV, which makes
predicting their behavior difficult in both solid and solution
states.30−36 The formation of these polyoxo/hydroxo com-
plexes and the resulting discrepancies of their solubility from
that of monomeric ions underpin several critical processes
including the spent fuel cycle and the fate of uranium in the
environment.33,37 The chemistry of low-valent uranium
clusters still remains underdeveloped. Hence, expansion of
the alkali-metal uranium molecular complexes and an in-depth
understanding of the formation mechanisms are highly
desirable.
Herein we report a series of alkali-metal-containing
uranium(IV) sulfates that are hydrothermally synthesized by
utilizing zinc amalgam as in situ reductants. We also explore
how variation of the alkali metals and systematic changes in the
acidity affect the formation of these uranium molecular
complexes by combined techniques, including X-ray crystallog-
raphy, X-ray absorption, electronic, and vibrational spectros-
copies. We provide evidence that counterions such as alkali-
metal ions can assume a vital role in mediating the nuclearity of
the uranium(IV) polyoxo/hydroxo clusters and controlling the
overall solid-state packing molecular complexes.
atoms were refined with anisotropic displacement parameters.
Selected crystallographic information is listed in Table S2. Atomic
coordinates and additional structural information are provided in the
CIFs. Numerous crystals of RbUIVSO4-1 were examined and are all
severely twinned, and the final refinement of the structural model is
less than satisfactory. As a result, only the unit cell parameters are
provided and some general bonding aspects are discussed. RbUIVSO4-
2 contains one SO42− tetrahedron disordered over two sites, as shown
in Figure S1.
Powder X-ray Diffraction (PXRD). PXRD patterns were
collected from 5 to 50°, with a step of 0.02°, using a Bruker D8
Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å)
equipped with a Lynxeye 1D detector, and selected PXRD results are
provided in Figure S2.
Extended X-ray Absorption Fine Structure (EXAFS) Spec-
troscopy. EXAFS analysis of samples that were prepared by adding
ground powder of NaUIVSO4, KUIVSO4-2, and RbUIVSO4-1 was
conducted. U L3-edge X-ray absorption spectra were collected at the
beamline 14W1 at Shanghai Synchrotron Radiation Facility.40 The
electron beam energy of the storage ring was 3.5 GeV, and the
maximum stored current was approximately 210 mA. Data were
recorded with a Si(111) double-crystal monochromator in fluo-
rescence mode. The U L3-edge EXAFS data were analyzed in terms of
the standard procedures in Demeter.41 Normalized absorption
coefficients and EXAFS theoretical fittings were obtained using
Athena and Artemis software, respectively. Theoretical phase and
amplitude functions were calculated from the program FEFF 9.0.42
A
fitting procedure was performed on the k3-weighted Fourier transform
(FT)-EXAFS from 3 to 11.5 Å−1, and an R window of 1.4−5 Å was
used for the fitting in tetravalent uranium sulfate compounds.
UV−Vis−Near-IR Spectroscopy. UV−vis−NIR data were
acquired from single crystals using a Craic Technologies micro-
spectrophotometer. Crystals were placed on quartz slides under
Krytox oil, and the data were collected from 200 to 800 nm. The
exposure time was autooptimized by the Craic software.
EXPERIMENTAL SECTION
■
Synthesis. Caution! The uranium precursor containing depleted
uranium was used in this synthesis, and 238U is an α-emitting radioisotope.
All handling of radioactive materials requires proper procedures. All
operations should be performed in a laboratory specially designed for
actinide element studies.
Fourier Transform Infrared (FTIR) Spectroscopy. The IR
spectra in the range of 400−4000 cm−1 were recorded on ground
powder using a Thermo Nicolet 6700 FTIR spectrometer equipped
with a diamond attenuated-total-reflectance accessory. The presence
of coordinating or hydrating H2O molecules as well as OH groups in
NaUIVSO4, KUIVSO4-1, KUIVSO4-2, RbUIVSO4-2, CsUIVSO4-1, and
CsUIVSO4-2 can be confirmed by the O−H stretching and the H−
O−H bending bands at 3600−2800 and ∼1600 cm−1, respectively
(Figure S3).43 RbUIVSO4-1, however, does not display any vibration
bands from H2O molecules or bridging −OH groups. Free tetrahedral
Materials. UO2(CH3COO)2·2H2O (99.9%, Changchun Institute
of Applied Chemistry, Chinese Academy of Sciences), H2SO4 (98%,
Alfa-Aesar), Na2CO3 (99.99%, Aladdin), K2CO3 (99.99%, Aladdin),
Rb2CO3 (99.99%, Aladdin), Cs2CO3 (99.99%, Aladdin), Zn (99.99%,
Aladdin), and Hg (99.9%, Aladdin) were used as received. Millipore
filtered water with a resistance of 18.2 MΩ·cm was used, and all of the
reactions were performed in poly(tetrafluoroethylene)-lined Parr
4749 autoclaves with a 23 mL internal volume.
Mixtures of UO2(CH3COO)2·2H2O (0.2 mmol, 0.0848 g), A2CO3
(A = Na, K, Rb, or Cs; 0.6−1.2 mmol), and 98% H2SO4 (2−6.44
mmol, 0.11−0.35 mL) with molar ratios of [U]/[A]/[H2SO4]
ranging from 1:6:10 to 1:3:35 were loaded into a Teflon liner
(Table S1). Additional water (1.89−1.65 mL) was added to the
mixture to keep the total volume of solutions equal to 2 mL (Table
S1). Zn amalgam was prepared according to the method by Cross et
al., and the amalgam was placed at the bottom of the liner.25 The
autoclave was sealed, placed in a box furnace, heated to 150 °C for 2
h, and then slowly cooled to room temperature at a cooling rate of 5
°C/h. The reaction products were rinsed with deionized water,
followed by rinsing with ethanol. Green crystals of seven A+−UIV−
sulfates and gold crystals of four ternary A+−UIII−sulfates were
isolated as pure or mixed phases, as shown in Figure 1.
Crystallographic Studies. Single-crystal X-ray diffraction
(SCXRD) data for all UIV molecular complexes were collected on a
Bruker D8-Venture single-crystal X-ray diffractometer equipped with
a Turbo X-ray source (Mo Kα radiation, λ = 0.71073 Å), adopting the
direct-drive rotating-anode technique and a CMOS detector at 173 or
296 K. The data frames were collected using the program APEX2 and
processed using the program SAINT routine in APEX2.38 The
structures were solved by direct methods and refined by full-matrix
least squares on F2 using the SHELXTL-2014 program.39 All non-H
2−
SO4 (Td symmetry) have four typical vibration modes, A1 (ν1
symmetric stretch at 983 cm−1), doubly degenerate E (ν2 bending at
450 cm−1), and two triply degenerate T2 modes (ν3 asymmetric
stretching at 1105 cm−1 and ν4 bending at 611 cm−1).43 Owing to the
crystal-field effects and correlation field splittings of sulfates within
solid-state compounds, splitting and shifts of those bands occur,
which result in multiple stretching (ν1 and ν3) and bending (ν2 and
ν4) bands at the ranges of 800−1300 and 400−700 cm−1
,
respectively.43
Inductively Coupled Plasma Optical Emission Spectrome-
try/Mass Spectrometry (ICP-OES/MS) Analysis. To determine
the formulas of NaUIVSO4 and RbUIVSO4-2, the molar ratios of Na/
U in NaUIVSO4 and Rb/U in RbUIVSO4-2 were measured by
dissolving 1 mg of samples in 1 mL of 20% HNO3. The
concentrations of Na/U and Rb/U in aqueous solutions were
determined by ICP-OES and ICP-MS, respectively, showing that
NaUIVSO4 has a Na/U molar ratio of 2.08 (calcd 2) and RbUIVSO4-2
has a Rb/U molar ratio of 1.86(4) (calcd 1.83).
RESULTS AND DISCUSSION
■
Synthesis. The assembly of uranium molecular complexes
is systematically controlled by introducing alkali-metal cations
with a periodic trend of the ionic radius (Na+ < K+ < Rb+ <
Cs+) and by fine-tuning of the [CO32−]/[H2SO4] ratio.
B
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Hydrothermal reactions between UO2(CH3COO)2, A2CO3,
and H2SO4 with the presence of zinc amalgam result in the
crystallization of seven alkali-metal-containing uranium(IV)
sulfates and four previously reported uranium(III) sulfates as
pure or mixed phases (Figure 1a and Table S1).25 The
[CO32−]/[H2SO4] < 0.27 for Rb, and 0.09 < [CO32−]/
[H2SO4] < 0.12 for Cs, respectively. As [CO32−]/[H2SO4] is
increased, transitions from the first phase of uranium(IV)
sulfate to the second UIV phase, KUIVSO4-2, Rb11[U6O4-
(OH)4(H2O)4](SO4)12·(H3O) (RbUIVSO4-2), and Cs4[U3O-
(SO4)7]·2.2H2O (CsUIVSO4-2) occur at 0.3 < [CO32−]/
[H2SO4] < 0.4 for K, 0.4 < [CO32−]/[H2SO4] < 0.45 for Rb,
and 0.12 < [CO32−]/[H2SO4] < 0.16 for Cs, respectively.
Furthermore, the products continue to evolve with increasing
[CO32−]/[H2SO4] to progress forward to a pure phase of
KUIVSO4-2, RbUIVSO4-2, or CsUIVSO4-2. An excess of
Na2CO3, Rb2CO3, or Cs2CO3 leads to the formation of a
black UO2 powder in each system.
Structures and Topological Descriptions. Single-crystal
X-ray analysis revealed that NaUIVSO4 and RbUIVSO4-2
crystallize in monoclinic space group C2/m and tetragonal
space group P4/mnc, respectively (Table S2). Both of their
structures are composed of discrete molecular clusters built
from the assembly of [U6O4(OH)4(H2O)4]12+ hexanuclear
cores and sulfates, charge-balanced by Na+/Rb+ cations, and
solvent species (Figure 2a,b). The [U6O4(OH)4(H2O)4]12+
octahedral core consists of six UIV cations bridged by four μ3-O
and four μ3-OH groups with U···U distances of 3.8277(4)−
3.9120(5) Å, as well as four capping H2O molecules located on
the vertexes of the equatorial plane (Figure 2c and Table S3).
While [U6O4(OH)4(H2O)6]12+ cores with one capping H2O
molecule on each vertex are well documented for a wide
variety of tetravalent metal oxohydroxo clusters, [U6O4(OH)4-
(H2O)4]12+ hexamers with two capping H2O molecules
missing are unprecedented.31,35,44−46 It is noteworthy that a
thorium hydroxoselenate, Th3O2(OH)2(SeO4)3, contains a
similar hexanuclear core, [Th6O4(OH)4]12+, without deco-
ration of the H2O molecules.47 The O or OH groups appear to
be distributed over the six μ3 nodes of the octahedron, as
confirmed by the bond valence sum of O atoms of the nodes
(Table S3). Furthermore, the average U−O bond distances
range from 2.286(10) to 2.454(5) Å, corresponding to values
between U−OH (2.40−2.50 Å) and U−O (2.20−2.27 Å)
bonds based on the published UIV hexanuclear clusters with
lower symmetry (Table S3).37 Two crystallographically
independent UIV centers can be identified in the hexanuclear
core, the polar U(1) and equatorial U(2), with square-
antiprismatic and monocapped square-antiprismatic geome-
tries, respectively (Figure 2d). The U−O bond distances range
from 2.286(10) to 2.367(6) Å for U−O/OH, from 2.389(5) to
2.454(5) Å for U−O(SO42−), and 2.649(5) Å for U−O(H2)
(Table S3). The [U6O4(OH)4(H2O)4]12+ unit is further
Figure 1. (a) Crystal images of trivalent and tetravalent uranium
sulfates. (b) Products that form as a function of [CO32−]/[H2SO4] in
the A−U−SO4 system (A = Na, K, Rb, and Cs). The acidity is
controlled by varying the molar ratio of A2CO3 and H2SO4.
resulting products were characterized for phase purity using
PXRD analysis (Figure S2), and the evolutions of the
respective phases are illustrated in a composition diagram as
a function of [CO32−]/[H2SO4] (Figures 1b and S4). For the
2−
decorated by 12 bridging μ2-SO4 (Figure 2e) rather than
2−
2−
Na+−U−SO4 system, olive prisms of NaU(SO4)2(H2O)
μ3-SO4 anions found in the 3D framework U6O4(OH)4-
(NaUIIISO4) can be obtained at low [CO32−]/[H2SO4]
(0.09−0.30), while green prisms of Na12[U6O4(OH)4(H2O)4]-
(SO4)12·(H2O)2 (NaUIVSO4) are the dominant phase when
0.30 ≤ [CO32−]/[H2SO4] ≤ 0.60 (Figure 1b). Coprecipitation
of NaUIIISO4 and NaUIVSO4 occurs in the intermediate 0.22 <
(SO4)6 and other thorium oxohydroxosulfate open frame-
works, e.g., Th3[Th6(OH)4O4(H2O)6](SO4)12(H2O)13.36,46 It
is noteworthy that μ2 ligands are inclined to terminate the
clusters and result in the formation of discrete molecular
moieties, while μ3 or μ4 ligands are capable of providing
additional bridging bonds to form 3D frameworks.48−50
KUIVSO4-1 crystallizes in an infinite 3D framework with a
chiral space group C2, and it is isotypic to the previously
reported cerium analogue K2Ce(SO4)3(H2O).51 As shown in
Figure 3a, KUIVSO4-1 is composed of two crystallographically
independent UIV centers, five K+ cations, six SO42− anions, and
two coordinating H2O molecules without inclusion of any
bridging oxo and hydroxo groups. As a result, monomeric UIV
units are the exclusive phase of uranium in KUIVSO4-1. Both
[CO32−]/[H2SO4] < 0.30 range. For the K/Rb/Cs+−U−SO4
2−
reactions, one trivalent uranium sulfate and two distinct
tetravalent uranium phases have been isolated in each system.
Mixtures of gold crystals of trivalent uranium(III) sulfate
[K5U2(SO4)6(H2O) (KUIIISO4), RbU(SO4)2 (RbUIIISO4), or
CsU(SO4)2 (CsUIIISO4)] and dark-green crystals of uranium-
(IV) sulfate [K4U2(SO4)6(H2O)2 (KUIVSO4-1), RbUIVSO4-1,
or Cs4U3(SO4)8(H2O)3·2H2O (CsUIVSO4-1)] cocrystallize
when 0.09 < [CO32−]/[H2SO4] < 0.3 for K, 0.09 <
C
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Figure 2. (a) Polyhedral representations of (a) NaUIVSO4 and (b) RbUIVSO4-2. (c) Ball-and-stick representation of the [U6O4(OH)4(H2O)4]12+
2−
hexanuclear core. (d) Coordination geometries of U(1) and U(2) in NaUIVSO4 and RbUIVSO4-2. (e) Representation of the μ2-SO4 anions
bridging the UIV ions in the hexamer. UIV atoms are shown in green, Na in light blue, Rb in orange, S in gray, O in red, and H2O molecules in dark
blue.
resulting in the formation of a UIV dimer via the bridging of
2−
four SO4 anions. This contrasts sharply with thorium(IV)
and plutonium(IV) hydroxonitrato species isolated from the
nitric acid solutions, which contain the hydroxo-bridged
[Th2(μ2-OH)2]6+ and [Pu2(μ2-OH)2]6+ dimeric moieties,
respectively.52,53 Compared to the terminal bidentate mode
−
2−
of NO3 , monodentate bridging ligation of SO4 allows
additional bridging between the metal centers and ligands,
promoting oligomerization and formation of condensed
IV
molecular complexes. Sulfate groups also join the U dimers
and monomers, and their assembly forms the fundamental
building unit of the [U3(SO4)8(H2O)3]4− layers, as shown in
Figure 4c. Two different coordination environments of UIV are
observed in CsUIVSO4-1. The coordination geometry of U(2)
in the dimer is a capped square antiprism with six O atoms
Figure 3. (a) Polyhedral representation of KUIVSO4-1. (b) Selected
coordination environments of U(1) and U(2). UIV atoms are shown
in green, K in purple, S in gray, and O in red.
2−
donated from SO4 anions and three O atoms from H2O
molecules, while that of the U(1) monomer is an irregular
muffin with nine O atoms provided by SO42− anions. The U−
O(SO42−) bond lengths range from 2.297(5) to 2.541(5) Å,
and the U−O(H2) bond distances vary from 2.431(8) and
2.483(7) Å (Table S5).
U(1) and U(2) adopt a nine-coordinate distorted monocapped
square-antiprismatic geometry, within which eight O atoms are
2−
Because CsUIVSO4-2 has been reported in our previous
study, a brief description of its structure is given for structural
donated from six SO4 and one O atom is from H2O
molecules (Figure 3b). The average U−O(SO42−) and U−
O(H2) bond distances are 2.416 and 2.461 Å, respectively
(Table S4). Interestingly, sulfates in KUIVSO4-1 exhibit
extraordinarily rich binding modes, including μ3-bridging
(monodentate, ×1; bidentate, ×1) for S(1), S(3), and S(5),
μ3-bridging (monodentate, ×3) for S(4), μ2-bridging for S(6),
and η2-terminal bidentate mode for S(2).
comparisons.35 CsUIVSO4-2 crystallizes in the space group P3
̅
,
and it features infinite sheets with open channels extending
along the c axis (Figure 4d). The anionic [U3O(SO4)7]4−
honeycomb lattice consists of [UIV3(μ3-O)]10+ trinuclear oxo-
centered UIV cores chelated by 13 SO42− anions, including one
2−
S(1)O42−, six S(2)O42−, and six S(3)O4 anions (Figure 4e).
CsUIVSO4-1 adopts a 2D anionic lamellar architecture
(Figure 4a) with orthorhombic space group Pnma. The
s t r u c t u r e o f C s U I V S O 4 - 1 i s c o m p o s e d o f
[U3(SO4)8(H2O)3]4− layers extending along the b and c axes,
with Cs+ and hydrating H2O molecules residing within the
interlamellar spacing (Figure 4a). The [U3(SO4)8(H2O)3]4−
layer consists of two crystallographically unique UIV centers,
2−
Both the μ3-O atom and S(1)O4 coordinate with three UIV
̅
ions generated from the 3 symmetry operation, and those μ3-
bridging ligands assist the assembly of the trimeric species.
S(2)O42− and S(3)O42− function as intertrimer linkers through
μ3-bridging between different trimers. The UIV center
coordinates with one μ3-O and eight O atoms donated from
2−
2−
SO4 anions, forming a 9-fold coordination geometry with a
four SO4 anions, and three coordinating H2O molecules
monocapped square antiprism around the UIV atoms (Figure
4f).
(Figure 4b). The U(1) polyhedron edge-shares with another
U(1) center generated from the 1 symmetry operation,
̅
D
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Figure 4. (a) Polyhedral representation of CsUIVSO4-1. (b) Monomeric and dimeric units in CsUIVSO4-1. (c) Selected coordination environments
of U(1) and U(2) in CsUIVSO4-1. (d) Polyhedral representations of Cs4[U3O(SO4)7]·2.2H2O (CsUIVSO4-2). (e) Polyhedral representation of the
[UIV3(μ3-O)]10+ trinuclear core decorated by sulfate groups. (f) Selected coordination environments of U(1) in CsUIVSO4-1. The UIV atoms are
shown in green, Cs in pink, K in purple, S in gray, O in red, and H2O molecules in dark blue.
During the structural refinement of RbUIVSO4-1, all
examined crystals of RbUIVSO4-1 were severely twinned and
final refinement was less than satisfactory. As a result, only the
unit cell parameters are provided, and some general bonding
aspects are discussed (Table S2). Two crystallographically
independent U atoms, four Rb atoms, and six unique S atoms
can be localized unambiguously in the difference Fourier map
(Figure S5a). The U(1)−U(2) interatomic distance is
5.370(6) Å and is in good agreement with those refined in
KUIVSO4-1, suggesting the dominance of UIV monomers in
RbUIVSO4-1 (Figure S5b). KUIVSO4-2 crystallizes as clusters
of microcrystals, making SCXRD data difficult to obtain. As a
result, EXAFS spectroscopy was used to determine the local
coordination environment of UIV in KUIVSO4-2. As shown in
Figure 5a,b, the U L3-edge spectra of KUIVSO4-2 and
NaUIVSO4 are quite similar, showing U−O/OH, U−S, and
U−U paths at the first, second, and third coordination shells,
respectively. Peaks at 1.8 Å in the FTIR spectra for all
complexes can be attributed to the backscattering of O atoms
(O/OH) in the coordination spheres. The coordination
number and U−U interatomic distance of KUIVSO4-2 from
theoretical fitting are highly consistent with those of
NaUIVSO4-2, suggesting the presence of hexanuclear UIV
moieties in KUIVSO4-2 (Table S6). Furthermore, the EXAFS
Figure 5. (a) U L3-edge k3-weighted EXAFS spectra and (b)
spectrum of RbUIVSO4-1 was collected, and its FT data show
corresponding FTs of NaUIVSO4, KUIVSO4-2, and RbUIVSO4-1.
two main peaks located at 1.8 and 2.7 Å, contrasting sharply
Solid lines: experimental data. Dotted lines: theoretical fitting. Phase
with those of hexameric NaUIVSO4 and KUIVSO4-2.
shifts (Δ) are not corrected on the FTs.
Theoretical fittings show that the first and second shells can
be attributed to the U−O(SO4) and U−S(bidentate) paths,
respectively, both of which are in good agreement with the
bonding distances of monomeric KUIVSO4-1 from the SCXRD
result (Table S4).
Structure-Directing Effect of Alkali-Metal Cations.
The series of alkali-metal uranium(IV) sulfates presented
herein affords the opportunity to investigate the effect of alkali-
metal cations on the formation of ML molecular complexes.
Structural comparisons between the alkali-metal-containing
uranium(IV) sulfates with other structurally characterized
uranium(IV) oxohydroxosulfates suggest that alkali metal plays
a significant role in determining the overall packing of the ML
molecular complexes.26,36,54 At low acidic (high [CO32−]/
[H2SO4] ratio) conditions, the presence of Na+ and Rb+ in
NaUIVSO4 and RbUIVSO4-2 promotes crystallization of 0D
12−
discrete [U6O4(OH)4(H2O)4](SO4)12
molecular moieties.
In addition, larger Cs+ in CsUIVSO4-2 promotes crystallization
of a 2D-layered [U3O(SO4)7]4− structure with an interlayer
separation of 9.859(1) Å. These results indicate that the
incorporation of counterions into ML molecular complexes
appears to result in a dimensional reduction of its overall
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topology. This idea was first proposed by Tulsky and Long,
who demonstrated that the incorporation of an ionic agent into
the covalent framework results in decreasing dimensionality of
the covalent structural unit.55 Furthermore, Krivovichev et al.
suggest that even more innocent H2O molecules can act as
dimensionality reduction agents as well.56 At high acidic (low
[CO32−]/[H2SO4] ratio) conditions, the larger ionic radii of
Cs+ enables higher coordination number with O atoms and
longer Cs−O bond distances, which promotes crystallization of
RbUIVSO4-2, whereas all coordinated O atoms of Cs+ cations
are donated from sulfate groups for the Cs compound. This
correlation between the number of coordinating H2O
molecules and the ionic sizes of the counterions can be
attributed to variation in their affinity to H2O, termed the
hydration energy (ΔHhydr) in the following order: Na+ (−415
kJ/mol) > K+ (−330 kJ/mol) > Rb+ (−305 kJ/mol) > Cs+
(−280 kJ/mol).60 The incorporation of more hydrophilic
counterions leads to the crystallization of molecular clusters
with more hydrated hexameric [U6O4(OH)4(H2O)4]12+ cores.
Particularly, O(W1) in RbUIVSO4-2 serves as a bridging unit
for the [U6O4(OH)4(H2O)4]12+ hexamer and Rb(2)+ cation,
suggesting the dominance of ion-hydration-association inter-
actions between these species.61,62 In contrast, the more
hydrophobic Cs+ cation appears to dictate different reaction
outcomes, where ion-association interactions are favored and a
less hydrated trinuclear [U3(O)]10+ core is isolated in the
resulting coordination polymer.
4−
a 2D U3(SO4)8(H2O)3 lamellar structure with an interlayer
separation of 9.486(1) Å. In contrast, the structure of
KUIVSO4-1, being isolated from a similar condition with the
presence of smaller K+ cations, features a higher 3D dimension
than that of CsUIVSO4-1.
More strikingly, a closer structural examination suggests that
counterions have a structure-directing effect on the nuclearity
of UIV ions. The formation of [U6(OH)4O4]12+ hexanuclear
clusters with the presence of Na, Rb, and K counterions is not
surprising because of the prevalence of such oligomers in both
solid and solution states for a variety of tetravalent metal ions,
including Zr, Ce, Th, U, Np, and Pu.31,45,48,49,57 Numerous
reactions with various pH conditions and different temper-
atures were attempted to prepare a hexanuclear species
templated by Cs+ cations but were unsuccessful. CsUIVSO4-
2, a much less common uranium(IV) hydro/oxo species with
trinuclear [U3(O)]10+ cores embedded, dominates at the high
[CO32−]/[H2SO4] range. This type of trinuclear moiety with a
μ3-oxo bridge has only been observed in two thorium(IV)/
uranium(IV) trimesate coordination polymers, where the
trimeric organization is stabilized by a ligand with C3
symmetry.58,59 Unlike thorium(IV)/uranium(IV) trimesate,
trinuclear formation in CsUIVSO4-2 appears to be promoted
by the Cs+ cation. How does the counterion influence the
nuclearity of UIV ions from aqueous solution? To uncover the
mechanism, the coordination environments of the alkali metals
in the oligomers have been examined and are shown in Figure
6. H2O molecules can be identified at the first coordination
spheres of Na(1)+, Rb(1)+, and Rb(2)+ for the NaUIVSO4 and
Effect of the pH on Oligomerization. The pH of the
solution was fine-tuned by changing the [CO32−]/[H2SO4]
ratio, and introduction of such a variation provides
opportunities to delineate the roles of the pH on the formation
of ML molecular complexes. Nucleation reductions with
increasing acidity of the solution can be observed in this
2−
A+−UIV−SO4
system. Specifically, a higher [CO32−]/
[H2SO4] (higher pH) ratio gives rise to the formation of a
hexanuclear [U6O4(OH)4(H2O)4]12+ core in the Na, K, and
Rb systems or a trinuclear [U3O]10+ core in the Cs system,
both of which contain oxo/hydro bridges. At the region of
lower [CO32−]/[H2SO4] ratio, molecular complexes
(KUIVSO4-1, RbUIVSO4-1, and (CsUIVSO4-1) with low
nuclearity are formed, and they are composed of purely
isolated mononuclear UO8 or UO9 centers. A U dimeric unit
can be identified in CsUIVSO4-1 as well, but the U ions in this
2−
unit are exclusively bridged by SO4 anions rather than OH
groups found in thorium and plutonium hydroxonitrato
species.52,53 These results indicate that increasing the pH can
promote hydrolysis owing to increasing hydroxyl concen-
tration, species essential to the olation/oxolation mechanisms
for aggregation of the tetravalent actinide ions. A similar
nucleation reduction with increasing acidity has been observed
in a thorium sulfate system, showing that the elevation of the
pH to 2.4 promotes the formation of Th hexamers in
Th3[Th6(OH)4O4(H2O)6](SO4)12(H2O)13, whereas lower
pH leads to the crystallization of monomeric thorium sulfate
hydrate Th(SO4)2(H2O)7·(H2O)2.46
Effects of the Nuclearity on Electronic Transitions.
The solid-state UV−vis spectra of uranium(IV) sulfate
complexes acquired from single crystals are shown in Figure
7. The tetravalent nature of U ions in uranium(IV) sulfate
complexes can be verified by the typical f−f transition bands of
UIV (5f2) at wavelengths longer than 400 nm, which can be
assigned to the transitions from ground state (3H4) to excited
63
1
3
3
1
1
states (3P2, I6, P1, P0, G4, and D2). Differences in the
nuclearity give rise to considerable variances in the spectra of
those complexes. First, the molecular complexes with low
nuclearity (monomer or dimer in KUIVSO4-1, RbUIVSO4-1,
and CsUIVSO4-1) exhibit negligible ligand-to-metal charge-
transfer (LMCT) absorbance ranging from 300 to 400 nm,
while NaUIVSO4, KUIVSO4-2, RbUIVSO4-2, and CsUIVSO4-2
featuring hexanuclear or trinuclear cores show strong
absorbance in this region. A similar evolution of the LMCT
bands from monomeric to hexameric species in the solution-
Figure 6. Selected coordination environments of alkali-metal cations:
(a) Na(1) in the structure of NaUIVSO4; (b) Rb(1) in the structure of
RbUIVSO4-2; (c) Rb(2) in the structure of RbUIVSO4-2; (d) Cs(1) in
the structure of CsUIVSO4-2. UIV atoms are shown in green, Na in
light blue, Rb in orange, Cs in pink, S in gray, O in red, and H2O
molecules in dark blue.
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Figure 8. Photographs of alkali-metal-containing uranium(IV)
sulfates showing the Alexandrite effect.
complexes. The trivalent uranium sulfates have been reported,
but the novelty of the tetravalent uranium sulfates in terms of
their composition and structure allows a systematic inves-
tigation to reveal the underlying role of alkali-metal cations in
the formation of ML molecular complexes.25 Compared to
U6O4(OH)4(SO4)6, a 3D uranium oxo/hydroxosulfato com-
plex composed of octahedral U6 oligomers, the hexameric
cores are well retained in the Na+- and Rb+-incorporated
molecular complexes (NaUIVSO4 and RbUIVSO4-2), but
dimensional reductions of the overall topologies from a
dense 3D framework to 0D discrete clusters have been
observed.36 The incorporation of Cs+ ions into the framework
under similar synthesis conditions allows for a dimensional
reduction from a 3D framework to a 2D-layered structure of
CsUIVSO4-2. More significantly, a trinuclear [UIV3(μ3-O)]10+
core rather than the commonly observed [U6O4(OH)4]12+
hexamer is embedded in CsUIVSO4-2. These results indicate
that the incorporation of counterions into ML molecular
complexes not only influences the overall solid-state packing of
uranium molecular complexes but also affects the oligomeriza-
tion of UIV ions. To the best of our knowledge, the reactions of
hydrolysis and nucleation are governed by various conditions
including the pH, temperature, relative concentration, and
ligand species, but the effect of counterions has barely been
investigated. These results expand the general perspective of
the pH and ligand as the dominant influence of the nuclearity
and shed further light on a new strategy for the design and
synthesis of targeted oligomers.
Moreover, nucleation reduction from oxo/hydroxo-bridged
hexameric or trimeric UIV cores found in KUIVSO4-2,
RbUIVSO4-2, and CsUIVSO4-2 to monomeric and dimeric
UIV centers in KUIVSO4-1, RbUIVSO4-1, and CsUIVSO4-1 with
increasing acidity represents a nice illustration that hydrolysis
and nucleation occur for UIV ions at low-acidity conditions in
an aqueous medium. Such nucleation reduction can be
associated with variation of the LMCT bands and f−f
electronic transitions, as illustrated by the UV−vis spectra of
those complexes. This correlation offers an alternative way to
determine the relative size and nuclearity of UIV oligomeric
species and design new materials with some type of optical
eccentricity.
Figure 7. UV−vis absorption spectra of uranium(IV) sulfates with
lower nuclearity (KUIVSO4-1, RbUIVSO4-1, and CsUIVSO4-1) and
higher nuclearity (NaUIVSO4, KUIVSO4-2, RbUIVSO4-2, and
CsUIVSO4-2). The arrow symbols highlight the blue shifts of the
3
3
1I6, P1, and P0 peaks of RbUIVSO4-1.
state uranium(IV) glycine system has been reported.64 Second,
an emerging peak at 673 nm for NaUIVSO4, KUIVSO4-2,
RbUIVSO4-2, and CsUIVSO4-2 reveals that the electronic
transitions of UIV in the oligomers with high nuclearity are red-
shifted to the longer wavelength. Such variations of the LMCT
and f−f transition bands are indicative and thus excellent
probes to determine not only the valence but also the
nuclearity of UIV in both solid and solution states.31,50,64,65 In
addition, noticeable blue shifts of the 1I6, 3P1, and 3P0 peaks as
3
1
well as merging of the P2 and I6 bands in the absorption
spectrum of RbUIVSO4-1 can be observed, which accounts for
the atypical optical properties of RbUIVSO4-1 compared to
other uranium(IV) sulfates. Specifically, NaUIVSO4, KUIVSO4-
1, KUIVSO4-2, RbUIVSO4-2, CsUIVSO4-1, and CsUIVSO4-2
exhibit brown color under standard fluorescence lighting but
are green when illuminated by natural or light-emitting-diode
lighting (termed the Alexandrite effect), as shown in Figure 8,
while RbUIVSO4-1 does not feature this type of dichroism and
its color remains unchanged under variable lighting conditions.
CONCLUSIONS
■
Seven tetravalent uranium sulfates and four trivalent uranium
sulfates templated by alkali-metal ions (Na+, K+, Rb+, and Cs+)
have been synthesized and characterized. The synthesis
performed highlighted the success of using Zn amalgam as
an in situ reductant for isolating low-valent uranium molecular
G
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https://pubs.acs.org/10.1021/acs.inorgchem.9b03182
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03182.
■
sı
Notes
The authors declare no competing financial interest.
PXRD patterns, possible structure of RbUSO4-1,
ACKNOWLEDGMENTS
■
structure of U6O4(OH)4(SO4)6, coordination geometry
This work was supported by grants from the National Natural
Science Foundation of China (Grants 21701184, 21876182,
and 21676291) and the Shanghai Pujiang Program (Grant
17PJ1410600)
2−
of S(2)O4 in RbUIVSO4-2, optical micrographs, FTIR
spectra, SEM images, EDS spectra, synthesis details,
crystallographic data, EXAFS analysis, and selected bond
distances (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
Jian Lin − Key Laboratory of Interfacial Physics and Technology,
Shanghai Institute of Applied Physics, Chinese Academy of
Sciences, Shanghai 201800, China; orcid.org/0000-0002-
3536-220X; Email: linjian@sinap.ac.cn
Authors
Zenghui Yue − Key Laboratory of Interfacial Physics and
Technology, Shanghai Institute of Applied Physics, Chinese
Academy of Sciences, Shanghai 201800, China; University of
Chinese Academy of Sciences, Beijing 100049, China; Shanghai
Advanced Research Institute, Chinese Academy of Sciences,
Shanghai 201210, China
Xiaofeng Guo − Department of Chemistry, Washington State
University, Pullman, Washington 99164-4630, United States;
orcid.org/0000-0003-3129-493X
Mei-Ling Feng − State Key Laboratory of Structural Chemistry,
Fujian Institute of Research on the Structure of Matter, Chinese
Academy of Sciences, Fuzhou, Fujian 350002, China;
orcid.org/0000-0003-2524-0994
Yue-Jian Lin − Department of Chemistry, Fudan University,
Shanghai 200433, China
Yu Ju − Jiangsu Key Laboratory of Advanced Catalytic Materials
and Technology, Changzhou University, Changzhou 213164,
China
Xiao Lin − Key Laboratory of Interfacial Physics and
Technology, Shanghai Institute of Applied Physics, Chinese
Academy of Sciences, Shanghai 201800, China
Zhi-Hui Zhang − Jiangsu Key Laboratory of Advanced Catalytic
Materials and Technology, Changzhou University, Changzhou
213164, China; orcid.org/0000-0002-8744-7897
Xiaojing Guo − The Education Ministry Key Laboratory of
Resource Chemistry and Shanghai Key Laboratory of Rare
Earth Functional Materials, Department of Chemistry and
Chemical Engineering, Shanghai Normal University, Shanghai
200234, China; orcid.org/0000-0001-6489-0717
Yu-Ying Huang − Shanghai Advanced Research Institute,
Chinese Academy of Sciences, Shanghai 201210, China
Jian-Qiang Wang − Key Laboratory of Interfacial Physics and
Technology, Shanghai Institute of Applied Physics, Chinese
Academy of Sciences, Shanghai 201800, China; orcid.org/
0000-0003-4123-7592
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