Probing the Influence of Acidity and Temperature to Th(IV) on Hydrolysis, Nucleation, and Structural Topology

Article abstract of DOI:10.1021/acs.inorgchem.7b02335

Systematic control of the molar ratio between thorium hydroxides and selenic acid and their reaction temperature under hydrothermal conditions results in four novel thorium-based selenate complexes, namely, [Th₈O₄(OH)₈(SeO₄)₆(H₂O)₁₆]·(SeO₄)₂·13H₂O (Th-1), [Th₈O₄(OH)₈(SeO₄)₈(H₂O)₁₃]·7H₂O (Th-2), Th(OH)₂(SeO₄)H₂O (Th-3), and Th₃(SeO₄)₆(H₂O)₆·2.5H₂O (Th-4), as well as the thorium mixed selenite selenate compound Th(SeO₃)(SeO₄) (Th-5). Smaller [H₂SeO₄]/[Th(IV)] ratio or lower temperature give rise to the formation of octameric [Th₈(μ₃-O)₄(μ₂-OH)₈]¹⁶⁺ cores in Th-1/Th-2 and infinite [Th(μ₂-OH)₂H₂O]²⁺ chains in Th-3, respectively. Increasing the [H₂SeO₄]/[Th(IV)] ratio or elevating the temperature generates a microporous (11.3 ? voids) open-framework Th-4, a monomeric thorium species without oxo/hydroxyl ligands, and a three-dimensional thorium structure Th-5. Formation of these compounds suggests that variables including acidity and temperature play a critical role in the hydrolysis and oligomerization of Th^IV ions. Increasing acidity limits the deprotonation of water molecules and formation of nucleophilic hydroxo/oxo-aquo Th species, and high temperature appears to suppress the olation/oxolation hydrolysis reactions, which in both ways limit the formation of the thorium oligomers.

Full text of DOI:10.1021/acs.inorgchem.7b02335

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Probing the Influence of Acidity and Temperature to Th(IV) on
Hydrolysis, Nucleation, and Structural Topology
Jian Lin,† Meiying Qie,† Linjuan Zhang,† Xiaomei Wang,† Yuejian Lin,‡ Wei Liu,§ Hongliang Bao,†
and Jianqiang Wang*,†
†Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 2019 Jia Luo Road, Shanghai 201800, China
‡Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China
§School for Radiological and Interdisciplinary Sciences and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher
Education Institutions, Soochow University, 199 Renai Road, Suzhou 215123, China
S
* Supporting Information
ABSTRACT: Systematic control of the molar ratio between thorium hydroxides and
selenic acid and their reaction temperature under hydrothermal conditions results in
four novel thorium-based selenate complexes, namely, [Th8O4(OH)8-
(SeO4)6(H2O)16]·(SeO4)2·13H2O (Th-1), [Th8O4(OH)8(SeO4)8(H2O)13]·7H2O
(Th-2), Th(OH)2(SeO4)H2O (Th-3), and Th3(SeO4)6(H2O)6·2.5H2O (Th-4), as
well as the thorium mixed selenite selenate compound Th(SeO3)(SeO4) (Th-5).
Smaller [H2SeO4]/[Th(IV)] ratio or lower temperature give rise to the formation of
octameric [Th8(μ3-O)4(μ2-OH)8]16+ cores in Th-1/Th-2 and infinite [Th(μ2-
OH)2H2O]2+ chains in Th-3, respectively. Increasing the [H2SeO4]/[Th(IV)] ratio
or elevating the temperature generates a microporous (11.3 Å voids) open-framework
Th-4, a monomeric thorium species without oxo/hydroxyl ligands, and a three-
dimensional thorium structure Th-5. Formation of these compounds suggests that
variables including acidity and temperature play a critical role in the hydrolysis and
oligomerization of ThIV ions. Increasing acidity limits the deprotonation of water
molecules and formation of nucleophilic hydroxo/oxo-aquo Th species, and high temperature appears to suppress the olation/
oxolation hydrolysis reactions, which in both ways limit the formation of the thorium oligomers.
transuranium elements both in solution and in the solid state.
For example, reaction between ThIV and 1,4,7,10-tetraazacy-
clododecane-1,4,7,10-tetraacetic acid (DOTA) leads to the
formation of Th2(H2O)10(H2DOTA)2·4NO3·xH2O, which
contrasts sharply to [An6(OH)4O4(H2O)8(HDOTA)4]·HCl/
HNO3·nH2O (An = U, Np, Pu) isolated from the same
synthesis route.21 Third, although most of the current research
is focused on the chemical properties of thorium, perhaps its
most revolutionary potential use involves thorium molten-salt
reactor (TMSR). The thorium blueprints, which gathered dust
in the archives for decades, have retrieved great attentions
because of its advanced safety features and its abundance in the
earth’s crust compared to uranium.22,23
INTRODUCTION
■
Thorium, as the most abundant radioactive element in nature,
has attracted intensive interests for the last decades owing to
the following reasons. First, as the softest of the tetravalent ions
with high charge density and large ionic radius, hydrolysis and
condensation reactions are prevalent in aqueous chemistry of
Th(IV), leading to the formation of oxo-hydroxo clusters. A
variety of polynuclear Th(IV) complexes, including dimers
[Th2(μ2-OH)2]6+,1 trimers [Th3(μ3-O)(OH)(H2O)2]9+,2 tet-
ramers [Th4Cl8(μ4-O)(EO4)3] (EO4 = tetraethylene glycol),3
4−6
hexamers [Th6(μ3-OH)4(μ3-O)4(H2O)6]12+
,
octamers
7
[Th8(μ3-O)4(μ2-OH)8]16+
,
and decamers [Th10F16-
(O)8(NH3)32]8+,8 have been isolated from the solution via
hydrolysis, nucleation, and condensation. The consequence of
the formation of these oligomeric species is the modification of
the mobility of the thorium, which largely affects the fate and
migration of it in the environment.9−11 Second, thorium has
long been used as a surrogate or benchmark for studying the
tetravalent transuranic ions, especially Pu(IV) due to their
similarities in coordination.12,13 Both Th(IV) and Pu(IV) are
typically bonded to eight or nine oxygen atoms, yielding
coordination geometries based on square antiprism, tricapped
trigonal prism, monocapped square antiprism, etc.14−20 In
many cases, however, thorium is not a proper mimic of
79Se is one of the prominent long-lived products created
during the fission of actinides in nuclear reactors.24 The solid-
state chemistry of selenium oxo-anion compounds is intricate
owning to the diverse oxidation and coordination that selenium
can adopt, either selenite SeIVO322−− or selenate SeVIO4
.
25−28 In
2−
the former, selenite anion SeO3 possesses a stereochemically
active lone pair of electrons on the Se center, which is typically
three coordinated with O, forming trigonal pyramidal
Received: September 11, 2017
© XXXX American Chemical Society
A
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Table 1. A Summary of the Reactant Compositions and Reaction Temperatures for the Formation of Th-1 to Th-5
temperature
[H2SeO4]/[Th(IV)]
1 M H2SeO4 (mL)
ThIV (mmol)
H2O (mL)
pH
120 °C
150 °C
180 °C
220 °C
1:1
0.3
0.3
0.3
0.3
0.3
0.3
1.7
2.49
2.12
1.46
1.10
0.97
RXN-1
RXN-2
RXN-3
RXN-4
RXN-5
RXN-6
RXN-7
RXN-8
RXN-9
RXN-10
RXN-11
RXN-12
RXN-13
RXN-14
RXN-15
RXN-16
RXN-17
RXN-18
RXN-19
RXN-20
1.5:1
2:1
0.45
0.6
1.55
1.4
2.5:1
3:1
0.75
0.9
1.25
1.1
Table 2. Crystallographic Data for [Th8O4(OH)8(SeO4)6(H2O)16]·(SeO4)2·13H2O (Th-1), [Th8O4(OH)8(SeO4)8(H2O)13]·
7H2O (Th-2), Th(OH)2(SeO4)H2O (Th-3), Th3(SeO4)6(H2O)6·2.5H2O (Th-4), and Th(SeO3)(SeO4) (Th-5)
compound
formula mass
color
Th-1
Th-2
Th-3
Th-4
Th-5
3656.00
colorless
block
3512.00
colorless
plate
427.03
1689.88
colorless
prism
501.96
colorless
prism
colorless
plate
habit
space group
a (Å)
C2/c
P1
P21/n
P42/nmc
26.5725(11)
26.5725(11)
9.4065(5)
90
P21/c
̅
21.613(4)
15.178(3)
20.848(4)
90
12.3121(5)
13.2312(5)
18.9373(8)
94.0342(17)
90.6768(16)
110.9379(15)
2871.9(2)
2
7.0650(5)
7.0890(5)
11.0337(7)
90
7.0410(7)
9.5539(8)
9.0213(8)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
113.328(3)
90
97.038(2)
90
90
103.141(3)
90
90
6280(2)
4
548.45(6)
4
6641.9(7)
8
590.96(9)
4
Z
T (K)
173
296
296
296
173
λ (Å)
0.710 73
28.000
3.867
0.710 73
0.710 73
27.576
0.710 73
27.499
0.710 73
27.525
maximum 2θ (deg)
27.574
ρcalcd (g cm−3
)
4.061
5.172
3.380
5.642
μ (Mo Kα) (mm−1
)
23.647
1.022
25.834
33.797
20.081
37.530
GOF on F2
1.097
1.053
1.082
1.100
a
b
R1, wR2 [I > 2σ(I)]
0.0354,0.0767
0.0610, 0.0875
0.0316, 0.0763
0.0507, 0.0899
0.0203, 0.0419
0.0254, 0.0440
1.57, −1.24
0.0406, 0.0962
0.0726, 0.1191
2.37, −1.76
0.0339, 0.1024
0.0360, 0.1036
3.82, −3.34
a
b
R1, wR2 (all data)
(Δρ)max, (Δρ)min/e Å−3
2.70, −3.47
2
4.79, −2.48
a
b
R(F) = ∑||Fo| − |Fo||/∑|F0|. Rw (Fo ) = [∑[w(Fo − Fc2)2]/∑wFo ]1/2
2
4
geometry.29 Examples of thorium-bearing selenite compounds
for which detailed structural information is available include α-
Th(SeO3 )2 , β-Th(SeO3 )2, Th(Se2O5)2 , and Th-
(IO3)2(SeO3).16,30 As the more soluble species of selenium,
especially for the polynuclear species, remains relatively
underexplored. Variables including temperature, relative
concentration, and pH value can play a critical role in
influencing the formation of oligomers.2,6,34,35 In an effort to
explore how the systematic changes in acidity and temperature
affect the mechanisms of cluster formation and the nature of
resulting species, herein we report the reactions between
thorium hydroxides and selenic acid under hydrothermal
conditions (120−220 °C). Four new thorium selenate hydrates
[Th8O4(OH)8(SeO4)6(H2O)16]·(SeO4)2·13H2O (Th-1),
[Th8O4(OH)8(SeO4)8(H2O)13]·7H2O (Th-2), Th(OH)2-
(SeO4)H2O (Th-3), and Th3(SeO4)6(H2O)6·2.5H2O (Th-4),
as well as the thorium selenite selenate mixed-anion complex
Th(SeO3)(SeO4) (Th-5), were isolated at different reaction
conditions. The pure obtained phases (Th-1, Th-3, Th-4, and
Th-5) were further characterized by thermogravimetric analysis
(TGA) and infrared (IR) spectroscopy.
2−
selenate SeO4 is analogous to sulfate, and it adopts a
2−
tetrahedral geometry. While it might be expected that SeO4
compounds would be much simpler, in fact, SeO42− can exhibit
different binding modes, monodentate or multidentate due to
the additional Se−O bond selenate possesses, resulting in a rich
but unpredictable effect on the speciation and structure.7,28 For
instance, a hexanuclear Th(IV) core containing compound
Th3(μ3-O)2(μ3-OH)2(SeO4)3 and four octanuclear Th(IV)
hydroxide/oxide selenates, including [Th8(μ3-O)4(μ2-
OH)8 (H2 O)1 5 (SeO4 )8 ·7.5H2 O], [Th8 (μ3 -O)4 (μ2 -
OH)8 (H2 O)1 7 (SeO4 )8 ·nH2 O], [Th9 (μ3 -O)4 (μ2 -
O H ) 8 ( H 2 O ) 2 1 ( S e O 4 ) 1 0 ] , a n d T h 9 ( μ 3 -O ) 4 ( μ 2 -
OH)8(H2O)21(SeO4)10·nH2O, were synthesized via the hydro-
thermal condition and evaporation, respectively.7,30 A family of
h i g h l y p o r o u s u r a n y l s e l e n a t e n a n o t u b u l e s ,
EXPERIMENTAL SECTION
31
■
K5[(UO2)3(SeO4)5](NO3)(H2O)3.5
,
(H3O)2K[(H3O)
Synthesis. Caution! 232Th is an α-emitting radioisotope, and standard
precautions for handling radioactive materials should be followed when
working with the quantities used in the syntheses that follow.
@([18]crown-6)][(UO2)3(SeO4)5](H2O)4,32 and (C4H12N)14-
[(UO2)10(SeO4)17(H2O)],33 were isolated from the solutions
by Krivovichev et al.
Materials. Th(NO3)4·5H2O (Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences), NH3·H2O (28 wt % in
H2O, 28.0−30.0% NH3 basis, Aladdin), and H2SeO4 (40 wt %, 99.95%
trace metals basis, Aladdin) were all used as received. Distilled and
Despite a handful of thorium selenate compounds that have
been synthesized, an in-depth understanding of formation
processes including hydrolysis, nucleation, and condensation,
B
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Article
Millipore filtered water with resistance of 18.2 MΩ·cm was used in all
reactions.
centrosymmetric space groups C2/c and P1, respectively
(Figure 1a,b). The building blocks of Th-1 and Th-2 are
̅
Amorphous Th hydroxide was precipitated from 0.3 mL of solution
of 1 M Th(NO3)4 in H2O with 0.3 mL of 28 wt % NH3·H2O. The
resulting white precipitate was washed several times with distilled
water, until the pH of the supernatant was near neutral. Then, 0.3,
0.45, 0.6, 0.75, or 0.9 mL of 1 M H2SeO4 solution was added to the
washed precipitate with the [H2SeO4]/[Th(IV)] ratio of 1:1, 1.5:1,
2:1, 2.5:1, or 3:1, respectively, as shown in Table 1. Additional water
(1.7, 1.55, 1.4, 1.25, or 1.1 mL) was added to the mixture to keep the
total volume of all reactions consistent. The pH values of the mixtures
were 2.49, 2.12, 1.46, 1.10, and 0.97, respectively. Four sets of the five
resulting mixtures were transferred to 23 mL Teflon-lined autoclaves
and heated to 120, 150, 180, or 220 °C for 2 d followed by cooling to
room temperature at a rate of 5 °C/h. The products were washed with
deionized (DI) water to remove soluble solids, followed by rinsing
with ethanol. Single crystals of Th-1, Th-2, Th-3, Th-4, or Th-5 were
isolated as pure phases or mixtures from the aforementioned reactions.
Crystallographic Studies. Single-crystal X-ray diffraction data for
Th-1, Th-2, Th-3, Th-4, and Th-5 were collected on a Bruker D8-
Venture single-crystal X-ray diffractometer equipped with a Turbo X-
ray Source (Mo Kα radiation, λ = 0.710 73 Å) adopting the direct-
drive rotating-anode technique and a CMOS detector at 173 or 298 K.
The data frames were collected using the program APEX2 and
processed using the program SAINT routine in APEX2.36 The
structures were solved by direct methods and refined by the full-matrix
least-squares on F2 using the SHELXTL-2014 program.37 All non-H
atoms were refined with anisotropic displacement parameters. During
the refinement of Th-1, large areas of residual electron density were
located in the difference Fourier map with a short distance to four O
atoms coordinating with Se(4), suggesting disorder of these O atoms
(Figure S1). Large displacement parameters compared to other O
atoms further indicate disorder of these O atoms. Modeling these
disorder atoms, O(13), O(14), O(15), and O(16), over two sites with
0.545/0.455 occupancy gave reasonable displacement parameters and
Se−O bond distances (Table S1). Selected crystallographic
information is listed in Table 2. Atomic coordinates and additional
structural information are provided in the CIFs (Supporting
Information).
Powder X-ray Diffraction (PXRD). PXRD patterns were collected
from 10° to 60°, with a step of 0.02°, using a Bruker D8 Advance X-ray
diffractometer with Cu Kα radiation (λ = 1.540 56 Å) equipped with a
Lynxeye one-dimensional (1D) detector.
Fourier Transform Infrared (FTIR) Spectrum. The IR spectra
were recorded on ground powder of Th-1, Th-3, Th-4, and Th-5
using an FTIR spectrometer instrument in the range of 400−4000
cm−1 (Thermo Nicolet 6700 spectrometer), equipped with a diamond
attenuated total reflectance (ATR) accessory. The IR spectrum of Th-
2 was not included in this study due to the unsuccess of obtaining pure
phase of Th-2.
Thermal Stability Analysis. TGA of Th-1, Th-3, Th-4, and Th-5
was performed on a NETZSCH STA 449F3 instrument in the range
of 30−900 °C under a nitrogen flow at a heating rate of 10 °C min−1
(Supporting Information, S1 and Figure S2).
Scanning Electron Microscopy and Energy-Dispersive X-ray
Spectrometry (SEM-EDS) Analysis. SEM images and EDS data
were collected on a Phenom ProX SEM with the energy of the
electron beam being 15 kV. Crystals of Th-1, Th-2, Th-3, Th-4, and
Th-5 were mounted directly on the carbon conductive tape. The
spectra acquisition time was 60 s, and all EDS results are provided in
the Supporting Information (Figure S3).
Figure 1. (a) Polyhedral representation of Th-1 viewed down the c
axis. (b) Polyhedral representation of Th-2 viewed down the a axis. (c)
Ball-and-stick representation of the [Th8(O)4(OH)8]16+ core. The Th
2−
centers are shown in gray, bridging SeO4 anions are in light blue,
dissociated SeO42− anions in Th-2 are in dark blue, μ3-O atoms are in
red, and μ2-OH are in purple.
similar to the thorium(IV)−selenate clusters reported by
Knope, et.al., all of which contain an octanuclear core
[Th8(μ3-O)4(μ2-OH)8]16+ as shown in Figure 1c.7 This
contrasts sharply with the most commonly observed
hexanuclear oligomers [M6(μ3-O)4(μ2-OH)4]12+, which are
prevalent in tetravalent metal ions (Zr,38 Ce,39,40 Th,4,5
U,41,42 Np,43 Pu44) interlinked by carboxylate or sulfate groups.
Although the H atoms are not explicitly located in the structural
refinement of Th-1 and Th-2, charge balance requirements
suggest the octanuclear core is constructed from eight ThIV
cations bridged by four μ3-O and eight μ2-OH groups.
Calculations have demonstrated that the highest symmetry
structure, with a μ2-OH/μ3-O/μ2-OH stacking geometry
(Figure 1c), a positioning that maximizes OH group distances,
provides the most energetically favorable positions for protons.7
The thorium atoms in Th-1 occupy five crystallographic
centers, while Th-2 contains eight crystallographically inde-
pendent ThIV centers. All of the ThIV sites are nine-coordinate,
and the coordination environments of them can best be
described as a distorted tricapped trigonal prism. The Th−Th
RESULTS
■
Structure Description. Five thorium selenate compounds
were obtained and identified by single-crystal X-ray diffraction
analysis, of which Th-1, Th-2, Th-3, and Th-4 are new, but Th-
5 has been previously reported.
Th-1 and Th-2. Both Th-1 and Th-2 adopt three-
dimensional (3D) framework, and they crystallize in
C
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bonding distances range from 3.8562 (9) to 4.1933 (7) Å,
which agree well with the literature (Tables S1 and S2).7 Each
[Th8(μ3-O)4(μ2-OH)8]16+ unit is further decorated by 16 or 13
water molecules and 10 or 13 SeO4 ions in Th-1 and Th-2,
respectively.
1.636(4) Å, both are in agreement with the expected bond
lengths (Table S3).
Th-4. Single-crystal X-ray diffraction studies reveal that Th-4
is isotypic to Th3(SO4)6(H2O)6·H2O and crystallizes in the
tetragonal space group, P42/nmc (No. 137).45 The unit-cell
parameters of Th-4 (a = b = 26.573(1) Å, c = 9.4065(5) Å, V =
6641.9(7) Å3) are larger than those of Th3(SO4)6(H2O)6·H2O
(a = b = 25.890(4) Å, c = 9.080(2)Å, V = 6086.3(17)Å3) due to
2−
The ligation modes between ThIV and bridging ligands,
particularly selenates, are expected to play a central role in
directing the structure and building of the framework in Th-1
and Th-2. All of the Th-OSeO3 linkages are monodentate,
consistent with most other structures.7,30 Compared to the
bidentate mode, monodentate ligation prevails in Th-selenate
oligomers, because it allows additional bridging between ThIV
and promotes oligomerization. Within the four unique
selenates in Th-1, two of them are μ4-SeO42−, one is μ3-
2−
the steric effect created by the larger bridging units SeO4 in
Th-4. The key feature of Th-4 is that it adopts a 3D structure
with a microporous open framework (Figure 3a). The ThIV
2−
SeO42−, and the fourth SeO4 is dissociated and not involved
in coordinating with ThIV (Figure 1a). The presence of
dissociated selenate anions in Th-1 suggests the cationic nature
of its framework [Th8O4(OH)8(SeO4)6(H2O)16]4+. Within the
2−
eight crystallographically independent SeO4 anions in Th-2,
five of them are μ4-SeO42−, two are μ3-SeO42−, and one is
2−
monodentate nonbridging η1-SeO4
.
Th-3. Th-3 crystallizes in the monoclinic space group of P21/
n, featuring a 3D framework as shown in Figure 2a. Th-3 is
Figure 3. (a) View of Th-4 showing the open framework extending
down the c axis and the open channels with a dimension of 11.3199(5)
Å. (b) Polyhedral representation of the channel showing Th-4 is
2−
composed of discrete ThIV monomers and SeO4 anions. Th
2−
polyhedra are shown in gray, and SeO4 polyhedra are in light
blue. Hydration water and hydrogen atoms were omitted for clarity.
Figure 2. (a) Polyhedral representation of Th-3 viewed down along
the a axis. (b) Depiction of the Th-3 showing the linking of ThIV
bridged by μ2-OH. The Th centers are shown in gray, SeO4 anions
2−
cations and SeO4 units do not fill all of the space in the
2−
crystals; consequently, large free voids in the structure are
observed. The result of such a combination is a 3D framework
with a system of square channels extending down the c axis.
These channels have a dimension of 11.3199(5) Å, and the
solvent-accessible volume calculated by PLATON is 1520.3 Å3,
corresponding to 22.9% of the unit-cell volume of Th-4.46 In
comparison, the structure of Th3(SO4)6(H2O)6·H2O contains
11.5 Å square channels, and the void volume (1566.7 Å3)
corresponds to 25.79% of the unit-cell volume (6086.3 Å3).
Water molecules were identified in the square channels of Th-
4, while no specific species were identified in those of
Th3(SO4)6(H2O)6·H2O.
are in light blue, O atoms are in red, μ2-OH are in purple, water
molecules are in dark blue, and hydrogen atoms are in pink.
composed of 1D [Th(OH)2H2O]2+ chains bridged by μ3-
2−
SeO4 ions into a 3D architecture (Figure 2b). There are one
symmetrically independent ThIV, one μ4-SeO42−, two μ2-OH,
and one water molecules in the structure. ThIV center adopts a
capped square antiprism coordination geometry, as it is
coordinated to four oxygen atoms from OH−, four oxygen
atoms from SeO42−, and one oxygen atom from water molecule.
Th−O(SeO3) and Th−O(H) bond distances range from
2.495(4) to 2.517(4) Å and from 2.376(5) to 2.402(5) Å,
respectively. The Th−OH2 bond distance is 2.657(5) Å, which
is longer than Th−O(SeO3) and Th−O(H) bonds. The Th−
Th distances within the hydroxo bridged units are 4.0184 (5)
and 4.0708 (5) Å, and the average Se−O bond distance is
Another key feature of Th-4 is that the structure of Th-4 is
based on discrete ThIV−SeO42− monomers without μ3-O or μ2-
OH linkages (Figure 3b). There are two crystallographically
2−
independent Th centers, two μ3-SeO42−, and two μ4-SeO4
groups in Th-4. Both ThIV centers are coordinated by nine O
D
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atoms (two from water and seven from SeO42−) in a
monocapped square antiprism. The coordinated water
molecules have Th(1)−O(w) bond distances ranging from
2.493(9) and 2.605(11) Å. The remaining Th−OSeO3 bond
distances range from 2.344(9) to 2.513(8) Å, in a good
agreement with Th3(SO4)6(H2O)6·H2O (Table S4).45
Th-5. Th-5 adopts a 3D structure formed from ThO9
tricapped trigonal prisms, trigonal pyramidal selenite
SeIVO32−, and tetrahedral selenate SeVIO42−, as shown in Figure
4. Th-5 was obtained as a result of the partial reduction of
Figure 5. IR spectra of compounds Th-1, Th-3, Th-4, and Th-5.
These modes could split and shift due to the correlation field
splittings and crystal-field effects within solid-state compounds.
In these particular cases, peaks ranging from 800 to 1000 cm−1
could be assigned to ν3 stretch of the SeO42− ions. Peaks in the
lower frequency range (450 to 500 cm−1) are associated with
2−
the ν4 bending mode. The ideal SeO3 trigonal pyramid has a
C3v symmetry consisting of four vibrational frequencies: two
nondegenerate ν1 (symmetric stretching) and ν2 (symmetric
bending mode) modes and two other doubly degenerate ν3
(asymmetric stretching) and ν4 modes (asymmetric bending
mode).55 The most striking differences between the spectrum
of Th-5 with the spectra of other complexes are the broad
bands located at ∼670 and 520 cm−1, which could be assigned
to the ν3 asymmetric bending and ν4 asymmetric stretching
Figure 4. Polyhedral representation of Th-5 projected along the a axis.
ThIV polyhedra are shown in gray, SeO42− polyhedra are in light blue,
2−
and SeO3 polyhedra are in purple.
selenate to selenite in acidic aqueous solutions. Selenic acid is
strong acid, and it decomposes above 200 °C, liberating oxygen
gas and being reduced to selenous acid.47 As a result, mixed-
valent selenite selenate compounds are common and represent
a growing family of transition-metal and f-block oxoanion
solids. Examples of these materials include Ag4(Mo2O5)-
(SeO4)2(SeO3)2,48 Au2(SeO3)2(SeO4),49 Nd2(SeO4)-
(SeO3)2(H2O)2,50 Er(SeO3)(SeO4)0.5H2O,51 [C5H14N]-
[ U O 2 ( S e O 4 ) ( S e O 2 O H ) ] , 5 2 [ C 5 H 1 4 N ] 4 -
[(UO2)3(SeO4)4(HSeO3)(H2O)](H2SeO3)(HSeO4),53 etc.
The ThO9 polyhedra edge-share to form 1D chains through
2−
SeO3 bands, respectively.
DISCUSSION
■
Systematic control of the reaction temperature and ratio of
reactants involving thorium hydroxide with selenic acid led to
the isolation of five distinct phases, Th-1, Th-2, Th-3, Th-4,
and Th-5. Powder X-ray diffraction (PXRD) analysis was
conducted to confirm the phase purity and composition of the
products (RXN-1 to RXN-20; Figure S4). The domain of
formation of these phases is represented in a composition
diagram as shown in Figure 6. Rectangular block crystals of Th-
1 and trace amount of plates of Th-2 can be obtained in RXN-1
(120 °C, [H2SeO4]/[Th(IV)] = 1). Further increase of the
reaction temperature to 150 °C (RXN-6) results in the
cocrystallization of Th-2 and Th-3. Pure phase of colorless
prisms of Th-3 can be synthesized in RXN-2, RXN-7, RXN-11,
RNX-12, and RXN-16, which locate at the relatively small
[H2SeO4]/[Th(IV)] region across a wide temperature range.
Th-4 becomes the dominant phase when [H2SeO4]/[Th(IV)]
equals 2.0 and 2.5 with the temperature below 220 °C (RXN-4,
RXN-9, and RXN-13). Square plates of Th-5 dominates at the
high temperature and large [H2SeO4]/[Th(IV)] regions
(RXN-5, RXN-10, RXN-15, RXN-18, RXN-19, and RXN-20).
The isolation of 3D octameric [Th8(μ3-O)4(μ2-OH)8]16+
cores in Th-1/Th-2 and 1D [Th(OH)2H2O]2+ chains in Th-
3 suggests that olation/oxolation hydrolysis reactions take place
to form Th−O/OH−Th bonding at areas with small
[H2SeO4]/[Th(IV)] or with low temperature. Monomeric
thorium selenate products, Th-4, without oxo/hydroxyl ligands
the bridging of SeO3 along the a axis. The ThIV−SeO3
2−
2−
2−
chains are further connected by SeO4 anion to form the 3D
framework.
Spectroscopic Properties. The IR spectra were collected
for Th-1, Th-3, Th-4, and Th-5 synthesized as pure phase as
shown in Figure 5. The presence of water and hydroxyl group
in Th-1, Th-3, and Th-4 can be easily confirmed by the broad
stretching bands of O−H ranging from 2500 to 3700 cm−1 and
the H−O−H bending mode approximately at 1600 cm−1
(1624, 1629, 1623 cm−1).54 No water bands can be observed
in the spectrum of Th-5. The OH stretching band for Th-3 is
much sharper than those for Th-1 and Th-4, possibly because
Th-3 only contains one coordinating H2O and two μ2-OH
groups, while both Th-1 and Th-4 have multiple crystallo-
2−
graphically independent H2O molecules. The free SeO4 ion
under perfect Td symmetry exhibits the following internal
modes: the symmetric stretching mode ν1 (833 cm−1), the
symmetric bending mode ν2 (335 cm−1), the asymmetric
stretching ν3 (875 cm−1), and bending modes ν4 (432 cm−1),
respectively, within which ν3 and ν4 bands are IR-active.54
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
because temperature removes H2O from the structure. The
prevailing of the fifth phase Th-5 can be further associated with
the thermal instability of selenic acid at high temperature, being
reduced to selenous acid and promoting the formation of Th-
5.47 Loiseau et. al’s study of solvothermal synthesis of ThIV/UIV
terephthalates (100−150 °C) also shows that oligomers can be
synthesized at a much broader acidity range at low temperature
than that at high temperature.6,35
CONCLUSION
■
This study demonstrates the richness of thorium selenate
hydrate chemistry in terms of the structural diversity under
different hydrothermal conditions. By systematic control of the
[H2SeO4]/[Th(IV)] (from 1:1 to 3:1) and the temperature
(120−220 °C), reactions between thorium hydroxide and
selenic acid gave rise to four new thorium-based selenate phases
and a thorium selenite selenate complex. At low temperature
(120 and 150 °C) and [H2SeO4]/[Th(IV)] = 1, two thorium
oxo-hydroxo-selenate oligomers Th-1 and Th-2, built from
[Th8(μ3-O)4(μ2-OH)8]16+ cores, were synthesized. The ele-
vation of the temperature of synthesis or the increase of the
[H2SeO4]/[Th(IV)] ratio (up to 2) resulted in the formation
of a thorium hydroxide selenate Th-3, which is composed of
infinite [Th(OH)2H2O]2+ chains. A microporous open-frame-
work structure Th-4 constructed from ThO9 monomeric and
2−
Figure 6. Composition diagram of the ThIV−SeO4 system as
functions of temperature and [H2SeO4]/[Th(IV)].
prevail at large [H2SeO4]/[Th(IV)] or high-temperature
regions. By changing the [H2SeO4]/[Th(IV)] ratio, one of
the most significant impacts on the generation of polynuclear
motifs is the change of hydroxyl concentration, species involved
in olation/oxolation mechanisms for the condensation of ThIV
ions. For the titled reactions, the concentration of ThIV
remained almost constant, and increasing [H2SeO4]/[Th(IV)]
would increase the acidity of the system (from pH 2.49 to
0.97), which limits the deprotonation of water molecules and
formation of nucleophilic hydroxo/oxo-aquo Th species.
Consequently, the higher is the acidity of the system, the
lower will be the concentration of such active species and
subsequently the less will be the nuclearity of the condensed
thorium clusters. Such a dimensional reduction from acidity
variation has also been explored with the sulfuric acid in our
previous study, within which a transition from octahedral
hexamers Th3[Th6(OH)4O4(H2O)6](SO4)12(H2O)13 to mono-
meric thorium sulfate hydrate Th(SO4)2(H2O)7·(H2O)2) was
observed.4 The same trend was also illustrated by a large family
of ThIV/UIV-based dicarboxylate complexes under solvothermal
conditions.2,6,35 Higher acidity can further induce the reduction
of SeO42− to less soluble SeO32−, resulting the crystallization of
Th-5 at large [H2SeO4]/[Th(IV)] areas.56 In the meanwhile,
there is a competition between SeO42−, oxo/hydroxo, and H2O
for coordination with ThIV. Larger [H2SeO4]/[Th(IV)]
2−
SeO4 anions was isolated when [H2SeO4]/[Th(IV)] ranges
from 2 to 2.5 and the temperature is below 220 °C. Th-5
became the dominant phase either at 220 °C or [H2SeO4]/
[Th(IV)] = 3.
This dimensional reduction from octameric cores and infinite
ThIV-hydroxo chains to monomeric ThIV monomeric species
highlights the important effects of the acidity and temperature
on the hydrolysis and condensation of ThIV. The lower is the
acidity of the system or the lower is the reaction temperature,
the higher will be the nuclearity of the condensed thorium
clusters. These findings give predictive insights into how acidity
and temperature impact the composition and structure of the
solid precipitates, which would have implications on both the
environmental fate and separation processes in nuclear fuel
cycle of thorium.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02335.
Thermogravimetric analysis and curves, ball-and-stick
representation of the coordination of Se(4), SEM images
and EDS spectra, powder X-ray diffraction (PXRD)
patterns, selected bond distances, EXAFS spectroscopy
(PDF)
2−
introduces more SeO4 anions, competing for Th ligation
with oxo/hydroxo groups. This can be confirmed by the local
environment of Th under different [H2SeO4]/[Th(IV)] ratio in
solution environments (Supporting Information, S2, Figure S5,
Table S6). The competition between oxo/hydroxo groups and
oxo-anions for metal ligation has been demonstrated for the
harder ZrIV and HfIV ions.34,57,58
There is also a trend of dimensional reduction from
octameric [Th8(μ3-O)4(μ2-OH)8]16+ cores to [Th-
(OH)2H2O]2+ chains and subsequently to monomeric ThIV
units along with the increasing temperatures, which suggests
oligomeric hydroxide species could be thermodynamically more
stable at low temperature. Indeed, all of the other four Th(IV)
hydroxo/oxo octamers were isolated from evaporation at room
temperature with/without pretreatment of heat.59 High
temperature favors formation of low-hydrated structures,
Accession Codes
CCDC 1571722−1571725 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wangjianqiang@sinap.ac.cn.
■
F
DOI: 10.1021/acs.inorgchem.7b02335
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
ORCID
Article
the Fate of Radionuclides 79Se and 129I in the Environment. Inorg.
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Jian Lin: 0000-0002-3536-220X
Jianqiang Wang: 0000-0003-4123-7592
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Albrecht-Schmitt, T. E. Unusual Coordination for Plutonium(IV),
Cerium(IV), and Zirconium(IV) in the Cationic Layered Materials
[M2Te4O11]X2 (M = Pu, Ce, Zr; X = Cl, Br). Inorg. Chem. 2012, 51,
11949−11954.
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.inorgchem.7b02335
Inorg. Chem. XXXX, XXX, XXX−XXX