Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by the [α-SiW9O34]10- Polyoxometalate

Article abstract of DOI:10.1021/acs.inorgchem.5b00786

The reaction of Na₁₀[α-SiW₉O₃₄] with tetravalent metallic cations such as 4f ((NH₄)₂Ce(NO₃)₆), 5d (HfCl₄), or 5f (UCl₄ and Th(NO₃)₄) i

Full text of DOI:10.1021/acs.inorgchem.5b00786

Article
pubs.acs.org/IC
Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by
the [α-SiW9O34]10− Polyoxometalate
Sylvain Duval,*,† Sebastien Beghin,† Clement Falaise,† Xavier Trivelli,‡ Pierre Rabu,§,∇
́
́
́
and Thierry Loiseau†
†Unite
90108, 59652 Villeneuve d’Ascq, France
‡Unite
́
de Glycobiologie Structurale et Fonctionnelle (UGSF)−UMR CNRS 8576, Universite de Lille Nord de France, Bat C9, BP
́ ́
de Catalyse et Chimie du Solide (UCCS)−UMR CNRS 8181, Universite de Lille Nord de France, USTL-ENSCL, Bat C7, BP
́
90108, 59652 Villeneuve d’Ascq, France
§Institut de Physique et Chimie des Mater
France
́
iaux de Strasbourg, UMR 7504 CNRS-University of Strasbourg, 67034 Strasbourg cedex 2,
Gaspard Monge, F-67000, Strasbourg, France
∇International Center for Frontier Research in Chemistry (icFRC), 8, Allee
́
S
* Supporting Information
ABSTRACT: The reaction of Na10[α-SiW9O34] with tetrava-
lent metallic cations such as 4f ((NH4)2Ce(NO3)6), 5d
(HfCl4), or 5f (UCl4 and Th(NO3)4) in a pH 4.7 sodium
acetate buffer solution leads to the formation of four sandwich-
3
t y p e
p o l y o x o m e t a l a t e s
[ C e ( μ
-
4
O ) 2 ( S i W 9 O 3 4 ) 2 (CH 3 C O O ) 2 ] 1 0 − ( 1 ) , [ U4 ( μ3 -
O)2(SiW9O34)2(CH3COO)2]10− (2), [Th3(μ3-O)(μ2-
OH)3(SiW9O34)2]13− (3), and [Hf3(μ2-OH)3(SiW9O34)2]11−
(4). All four compounds consist of a polynuclear cluster
fragment stabilized by two [α-SiW9O34]10− polyanions.
Compounds 1 and 2 are isostructural with a tetranuclear core (Ce4, U4), while compound 3 presents a trinuclear Th3 core
bearing a μ3-O-centered bridge. It is an unprecedented configuration in the case of the thorium(IV) cluster. Compound 4 also
possesses a trinuclear Hf3 core but with the absence of the μ3-O bridge. The molecules have been characterized by single-crystal
X-ray diffraction, 183W and 29Si nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, thermogravimetric
analysis (TGA), and scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis.
compounds have a tendency to grow in nuclearity. From this
point of view, we are interested in the use of such inorganic
moieties to coordinate clusters of tetravalent 4f (cerium), 5d
(hafnium), and 5f (uranium, thorium) cations. To our
knowledge, only few examples of polyoxometalates coordinated
to tetravalent 4f, 5d, or 5f metallic cations have been described
in the literature. They are built from the association of vacant
Keggin- or Dawson-type polyoxotungstates associated with
tetravalent metallic cations (zirconium, hafnium, cerium,
uranium, or thorium), sometimes mixed with 3d metals, giving
rise to 1:2 sandwich-type compounds, such as
[MIV(XW11O39)2]12−, [MIV(XW17O61)2]16−, or more-complex
sandwiched systems [{MIV(H2O)}2(μ-OH)2(γ-SiW10O36)2]10−
and [{MIV(H2O)}4(μ4-O)(μ-OH)6(γ-SiW10O36)2]8− (with M =
Zr, Hf, Ce, U, Th and X = P, Si, Ge).14−28 Some more-complex
molecules with three or six condensed metallic zirconium and
hafnium cations were also obtained by various groups.29,30
Recently, some of these complexes containing tetravalent
uranium were characterized by small-angle X-ray scattering.31
1. INTRODUCTION
Heteropolyoxometalates (POMs) are discrete molecular
assemblies based on the condensation of transition metals.
They are known for their large composition and structural
diversity, making them attractive for their properties in
catalysis, magnetism, materials science, or luminescence.1−8
POMs are usually constructed from polycondensation
processes, in aqueous acidic solution, of metal-oxo species.
Incorporation of various templating agents (using tetrahedral
XO4 units, where X = PV, SiIV, GeIV, etc., or trigonal XO3 units,
where X = AsIII, SbIII, BiIII, etc.) or heterometals increase the
structural diversity of this family of molecules and helps to
modulate the properties of the related materials. This also can
be obtained via controlled hydrolysis with vacant polyoxome-
talates species. For instance, trivacant Keggin polyoxometalates
are particularly known in POM chemistry to act as a
polydentate ligand, thus allowing the coordination of different
metallic species. Most examples found in the literature include
sandwich-like compounds with incorporation of polymetallic 3d
and 4d transition-metal−oxo clusters (for example, see the
nonexhaustive series of compounds containing Fe, Co, Zr, Mo,
etc.).9−13 Over the years, the heterometallic core in these
Received: April 8, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b00786
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Table 1. Crystal Data and Structure Refinement for Compounds 1−4
1
2
3
4
formula
C4H6Ce4Cs1.5Na8.5O97.5Si2W18
C4H6U4Cs3Na7O97.7Si2W18
6494.55
Na12RbO98Si2Th3W18
5990.95
Na3Rb8.5O95.25Cl0.5Si2Hf3W18
formula weight
temperature
crystal color
crystal size
5934.83
100 K
yellow
6238.14
100 K
100 K
100 K
dark brown
colorless
colorless
0.20 mm × 0.15 mm × 0.15 mm 0.45 mm × 0.15 mm × 0.10 mm 0.30 mm × 0.15 mm × 0.15
0.40 mm × 0.40 mm × 0.20
mm
mm
crystal system
monoclinic
C2/c
monoclinic
C2/c
trigonal
triclinic
space group
P63/m
P1
̅
a
25.7962(8) Å
19.0538(5) Å
18.0991(5) Å
90°
100.5227(15)°
90°
8746.4(4) Å3
25.8519(12) Å
19.0881(9) Å
18.1113(9) Å
90°
100.472(3)°
90°
8788.4(7) Å3
12.1735(3) Å
12.1735(3) Å
33.1941(8) Å
90°
90°
120°
4260.13(18) Å3
12.6921(5) Å
16.9827(7) Å
21.3707(10) Å
94.684(2)°
97.557(2)°
101.571(2)°
4445.8(3) Å3
2
b
c
α
β
γ
volume
Z
4
4
2
density, ρcalculated
μ
4.6153 g cm−3
26.411 mm−1
1.34°−26.58°
−32 ≤ h ≤ 32
−23 ≤ k ≤ 23
−22 ≤ l ≤ 22
60648
4.9481 g cm−3
32.206 mm−1
1.33°−26.57°
−31 ≤ h ≤ 32
−23 ≤ k ≤ 23
−19 ≤ l ≤ 22
45044
4.6701 g cm−3
30.191 mm−1
1.23°−33.17°
−18 ≤ h ≤ 18
−18 ≤ k ≤ 18
−51 ≤ l ≤ 25
46721
4.66 g cm−3
31.466 mm−1
1.48°−36.40°
−15 ≤ h ≤ 15
−21 ≤ k ≤ 21
−26 ≤ l ≤ 26
110396
θ range
limiting indices
collected reflections
unique reflections
9055
9060
5525
18167
[R(int) = 0.0376]
614
[R(int) = 0.0693]
370
[R(int) = 0.0579]
218
[R(int) = 0.0506]
758
parameters
goodness-of-fit on F2
1.093
1.034
1.291
1.081
final R indices [I > 2σ(I)] R1 = 0.0271
wR2 = 0.0611
R1 = 0.0512
wR2 = 0.1311
R1 = 0.0697
wR2 = 0.1447
4.19 and −6.19 e Å−3
R1 = 0.0331
wR2 = 0.0694
R1 = 0.0359
wR2 = 0.0702
2.25 and −2.53 e Å−3
R1 = 0.0381
wR2 = 0.904
R1 = 0.0416
wR2 = 0.922
4.29 and −4.85 e Å−3
R indices (all data)
R1 = 0.0318
wR2 = 0.633
largest diff. peak and hole 1.97 and −1.77 e Å−3
precautions with suitable care and protection for handling such substances
have been followed.
Generally, these types of tetravalent clusters are not easily
stabilized or crystallized from solution. They generally require
the presence of several organic acids or amino acids to prevent
the infinite condensation process of the metallic cations and
stop at a nanoparticle size in discrete coordination compounds.
Some nice recent examples included an octanuclear thorium
cluster organized in a boat shape and stabilized by coordination
of several selenite oxides,32 polynuclear cerium clusters up to 22
metal centers,33 and an U38 nano poly oxo cluster composed of
tetravalent uranium and containing distorted UO2 fluorite core,
which is stabilized by THF solvent molecules and benzoate
ligands.34 The recent reports of Knope and Soderholm35 and
Nyman and Burns36 gave a good survey of the poly oxo clusters
occurrence for actinides.
Herein, we report on the synthesis, characterization, and
structural description of four new sandwich-type polyoxome-
talates containing 4f, 5d, and 5f polyoxometallic clusters,
Cs2Na8[Ce4(μ3-O)2(SiW9O34)2(CH3COO)2]·40H2O (1),
Cs3Na7[U4(μ3-O)2(SiW9O34)2(CH3COO)2]·24H2O (2),
Na12Rb[Th3(μ3-O)(μ2-OH)3(SiW9O34)2] (3), and
Rb8.5Na3[Hf3(μ2-OH)3(SiW9O34)2]Cl0.5·24H2O (4). These
molecules are built from the association of tetravalent cerium,
hafnium, uranium, and thorium cations with the trivacant
[SiW9O34]10− polyoxometalate.
The compounds have been synthesized by using the following
chemical reactants: ammonium cerium nitrate ((NH4)2CeIV(NO3)6,
Acros Organics, 99%), uranyl chloride (UCl4, obtained from the
protocol using the reaction of hexachloropropene with uranium
oxide), thorium nitrate (Th(NO3)4·4H2O, Prolabo, 99.99%), Cs-
(NO3) (Aldrich, 99%), CsCl, Rb(NO3) (Alfa Aesar, 99.8%), RbCl
(Acros Organics, 99.8%), and acetate buffer (solution made by using
glacial acetic acid (Carlo Erba reagent, 99.9%) and sodium acetate
(Merck, 99%)). The starting chemical reactants (except UCl4) are
commercially available and have been used without any further
purification. Na10[SiW9O34]·12H2O was synthesized according to a
literature procedure.37
Compound 1. (NH4)2CeIV(NO3)6 (328 mg, 0.60 mmol) was
dissolved in 5 mL of a 1 M sodium acetate buffer (pH 4.7).
Na10[SiW9O34]·12H2O solid (500 mg, 0.20 mmol) was added to the
cerium solution. The resulting mixture was heated at 50 °C for 30 min
until the yellowish solid dissolved. Cs(NO3) (13 mg, 0.067 mmol) was
added to the solution, which was heated for an additional 15 min at 50
°C. The solution was cooled to room temperature and left to
crystallize by slow evaporation. After 1 day, yellow crystals suitable for
X-ray diffraction (XRD) analyses started to appear. After 1 week, the
crystals were filtered off, washed with a small amount of water, and
dried at room temperature (mass (m) = 112 mg, yield = 18%).
IR (cm−1): 996(w), 956(m), 872(s), 852(m), 793(s), 757(s),
717(s), 673(s), 643(s), 516(s).
Compound 2. Na10[SiW9O34]·12H2O (250 mg, 0.10 mmol) was
dissolved in 10 mL of 1 M acetate buffer (pH 4.7). The solution was
added to solid UCl4 (108 mg, 0.30 mmol). The brown solution was
stirred for 10 min and CsCl (24 mg, 0.14 mmol) was added to the
2. EXPERIMENTAL SECTION
Synthesis. Caution! Uranyl chloride (UCl4) and thorium nitrate
(Th(NO3)4·4H2O) are radioactive and chemically toxic reactants, so
B
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solution. After stirring for an additional 10 min, the solution was left to
crystallize at room temperature. After one night, brown crystals
suitable for XRD analyses started to appear. The crystals were quickly
collected (m = 45 mg, yield = 14%) before crystallization of
Na[UVIO2(OAc)3] (OAc = acetate) as yellow crystals,38 indicating
the oxidation of the uranium.
the molecular composition, and can be found in the Supporting
Information (Figure S7).
Thermogravimetric Analysis. The thermogravimetric experi-
ments have been carried out on a thermoanalyzer (SETARAM, Model
TGA 92) under an air atmosphere with a heating rate of 1 °C min−1
from room temperature up to 600 °C for 1 and 800 °C for 2 and 4.
Compound 3 was not pure, so no TGA was performed on it.
Infrared Spectroscopy. Infrared spectra of compounds 1−4 were
measured on Perkin−Elmer Spectrum Two spectrometer, between
4000 cm−1 and 400 cm−1, that was equipped with a diamond
attenuated total reflectance (ATR) accessory.
Magnetic Measurements. The magnetic properties of compound
2 have been investigated by using a Quantum Design Squid vibrating
sample magnetometer (VSM). The temperature and field variation of
the moment was measured on powder samples immobilized in a
gelatin capsule. The magnetic susceptibility was measured under a field
of 5000 Oe between 1.8 K and 300 K. The magnetization versus field
variation was recorded between −70 000 Oe and +70 000 Oe. The
measured magnetic moment was corrected for sample size effect,
sample diamagnetism (based on Pascal’s constant corrections42,43),
and diamagnetism of the sample holder.
IR (cm−1): 996(w), 952(m), 875(s), 851(m), 789(s), 759(s),
724(s), 674(s), 647(s), 514(s).
Compound 3. Na10[SiW9O34]·12H2O (310 mg, 0.124 mmol) was
dissolved in 3 mL of 1 M acetate buffer (pH 4.7). The solution was
added to solid Th(NO3)·4H2O (210 mg, 0.40 mmol). The colorless
solution was stirred for 10 min and Rb(NO3) (14 mg, 0.095 mmol)
was added to the solution. After stirring another 10 min, the solution
was left to crystallize at room temperature. After 3 weeks, a mixture of
a white microcrystalline powder and crystals suitable for XRD analyses
was obtained.
IR (cm−1): polyanion part: 983(w), 936(m), 863(m), 808(m),
769(m). Thorium acetate part: 1056(w), 1047(w), 1026(m), 956(m),
674(s), 639(s), 609(m), 498(s), 474(m).
Compound 4. HfCl4 (32 mg, 0.10 mmol) was dissolved in 7 mL of
a 0.5 M acetate buffer (pH 4.7). Na10[SiW9O34]·12H2O (250 mg, 0.10
mmol) was added to the metal solution, followed by the addition of 20
drops of 1 M RbCl solution until precipitation occurred. The solution
then was heated to 60 °C for 15 min and cooled to room temperature
and allowed to crystallize. After 2 days, suitable colorless crystals for
XRD studies were obtained. The solution was left alone for 14 days
and the crystals were collected (m = 90 mg, yield = 42.5%).
IR (cm−1): polyanion part: 1014(w), 994(w), 938(m), 883(m),
750(s), 666(s), 527(s), 464(s).
Single-Crystal X-ray Diffraction. Crystals of 1−4 were selected
under observation using a polarizing optical microscope and glued on
a glass fiber or a mitegen loop for single-crystal XRD experiments.
Because of the instability of the crystalline network, which is due to the
loss of crystallization water, all measurements were performed at 100 K
under a nitrogen flow. X-ray intensity data were collected on a Bruker
Model X8-APEX2 CCD area-detector diffractometer using Mo Kα
radiation (λ = 0.71073 Å) with an optical fiber as the collimator.
Several sets of narrow data frames (10 s per frame) were collected at
different values of θ for two initial values of ϕ and ω, respectively,
using 0.3° increments of ϕ or ω. Data reduction was accomplished
using SAINT V7.53a.39 The substantial redundancy in data allowed a
semiempirical absorption correction (SADABS V2.10)40 to be applied,
on the basis of multiple measurements of equivalent reflections. The
structure was solved by direct methods, developed by successive
difference Fourier syntheses, and refined by full-matrix least-squares on
all F2 data using the SHELX program suite.41 Hydrogen atoms of the
acetates ligands for compounds 1 and 2 were included in calculated
positions and allowed to ride on their parent atoms. The final
refinements include anisotropic thermal parameters of all non-
hydrogen atoms, except the oxygen atoms of the water molecules
and the atoms being NPD if refined anisotropically. The crystal data
are given in Table 1. Supporting Information is available in CIF
format.
3. RESULTS
Synthesis. The four compounds have been synthesized in
an acetate buffer solution at pH 4.7. SEM images of the crystals
obtained after crystallization are presented in Figure S1 in the
Supporting Information. The use of buffered solvent ensures
that the solution will remain at a stable pH during the synthesis
and crystallization processes. The pH of the solutions of the
synthesis during the crystallization of the four compounds
remains up to 4.2 with the cationic stoichiometry used in the
synthetic procedure. Furthermore, as already seen in the
literature,44 the acetate ligands may be helpful regarding the
control of the polycondensation processes of tetravalent species
by playing the role of a ligand giving support to the
coordination of the cluster. The tetravalent uranium cation is
known to be unstable and, when subjected to air and water
oxidation, form the uranyl {UO2}2+ species.45 It is interesting to
note that no precautions have been made regarding these
oxidizing factors to obtain compound 2. The stabilization of the
4+ oxidation state of uranium in this molecule can be explained
using several parameters. The first one is the high charge
density of the uranium atoms, causing a preferential electro-
static interaction with the oxygen atoms of the polyanion in the
relatively acidic medium (buffer solution at pH ∼4.7). The
second parameter is of geometric consideration. The U4+ atoms
are located in the center of an “Archimedean” antiprism. The
last factor is the unique electron transfer phenomenon
occurring between the U4+ cations and the W6+ centers of
the polyanion through the U−O−W bond observed in the
sandwich complex [U(P2W17O61)]16‑.46 This type of geo-
metrical environment, which is present in the compounds
U(SO4)2·4H2O, U(CH3COO)4 and U(C2O4)·6H2O, has been
also shown to strongly stabilize the U4+ cation,47−49 thus
preventing the crystallized reduced form to be oxidized for
several days even if the solid is left in the mother liquor.
Structure Description. The structures of compounds 1−4
consist of a sandwich-type polyoxometalate (Figure 1). In these
molecules, the central heterometallic core corresponding to a
tetranuclear group {M4(μ3-O)2} (M = Ce, U) for 1 and 2,
respectively, and a trinuclear group {Th3(μ3-O)(μ2-OH)3} for
3 or {Hf3(μ2-OH)3} for 4, are coordinated by the oxygen atoms
of the vacancies of two [SiW9O34]10‑ subunits (Figure 1).
Compounds 1 and 2 are isostructural and each Ce4+ or U4+
center has a coordination number of eight with an
NMR Spectroscopy. The 29Si NMR and 183W NMR measure-
ments were carried out on 5 mm tubes. The crystals of compounds 1
and 4 were dissolved in a 90:10 H2O/D2O mixture (0.75 mL) to get a
concentration of ∼25 mM. The spectra were recorded on a Bruker 400
MHz spectrometer. Chemical shift were referenced to the 183W
resonance of an external 2 M Na2WO4 solution in alkaline D2O and
29Si chemical shifts were referenced to TMS. The pH of the solutions
is ∼5.2 for compound 1 and ∼5.4 for compound 4.
Powder Diffraction. Microcrystalline powder patterns were
recorded on a Bruker D8 Avance diffractometer (Cu Kα radiation)
equipped with a lynx eye fast detector. Each pattern was recorded in
the 2θ range of 5°−50° with a 0.5 s/step scan.
SEM/EDX Analysis. Photographs of the crystals were performed
on a Hitachi Model S3400N system equipped with a tungsten
filament. EDX measurements were performed on compounds 2 and 3
to estimate the Na/Cs and Na/Rb ratios, with respect to the rest of
C
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cation (2.237(5)−2.406(5) Å for 1, compared with 2.254(12)−
2.398(11) Å for 2). The same observation is made with the
external atoms (average 2.485 Å for 1 and 2.484 Å for 2), but
their M−(μ3-O) distances are longer for 2 (2.144(11) Å) than
for 1 (2.128(5) Å). This structural observation shows that the
sandwich complex with cerium is more compact than that with
uranium, causing the external cation to be slightly off-center
from the compound. It is worth noting that a mirror plane is
present in the molecular unit. This plane goes through the
inversion center and the silicon atoms of the POMs.
Furthermore, a C2 axis going through the two inner cerium
atoms is also present. Thus, the molecular unit in this system
possesses a C2h symmetry. In these compounds, it is interesting
to notice the occurrence of Archimedean square antiprismatic
units MO8 involved in a tetranuclear subcluster, which differs
from that existing in a fluorite-type structure, with a regular
cubic environment for the metallic element. The electro-
neutrality of both compounds is ensured by the presence of
mixture of alkali metals. In both structures, three Cs+ and seven
Na+ cations surrounded by water molecules or terminal oxygen
atoms of the [SiW9O34] unit were crystallographically located.
In compound 3, the three thorium cations have a
coordination number of seven. This coordination sphere was
confirmed by BVS calculation performed by considering a
coordination number of seven or nine for the cation (Table
S2).50,51 From these values, a coordination number of seven
fitted best with the tetravalent nature of the thorium.
Compound 3 crystallizes in a P63/m hexagonal space group.
The asymmetric unit is composed of one thorium atom, one
{W3O13} group belonging to the SiW9O34 unit, and an oxygen
atom corresponding to a μ2-hydroxo bridge. Another oxygen
atom type adopts a μ3-oxo bridging mode, with a 1/6
multiplicity located on a C3-axis. The silicon atom is also
located on this axis. Application of the C3 symmetry axis going
through the silicon atom and the oxygen atom of the μ3-oxo
bridge and application of a mirror plane going through the
{Th3O4} central core give the complete sandwich-type
molecule. The mirror planes parallel and perpendicular to the
principal axis in the molecular structures define an average D3h
symmetry for the compound (Figure 3b). In the molecular
structure, each of the three cations share four oxygen atoms
with the two [SiW9O34] units with distances ranging from
2.400(4) Å to 2.406(4) Å. The distances observed for the μ2-
hydroxo ranges from 2.422(9) Å to 2.425(9) Å and those of the
μ3-oxo bridges are of 2.267(3) Å. Electroneutrality of
compound 3 is ensured by a mixture of 10 Na+ and 3 Rb+
cations located in the lattice of the molecule. All cations are
located around the polyanion architecture, interacting with
some terminal oxygen atoms of the [SiW9O34] subunits and
surrounded by several water molecules. This type of trinuclear
arrangement possessing a μ3-oxo bridge is unique in the
chemistry of thorium. A similar arrangement with a μ3-oxo
bridge has already been observed for the U4+ cation in a
coordination polymer based on U4+−trimesate coordination.53
As far as we know, only one example of trinuclear thorium
arrangement has been reported in the literature.54 The
structure of this thorium cluster differs largely from our
compound because the three thorium cations are bridged by six
μ2-hydroxo atoms to form a trinuclear [Th3(OH)6]6+ unit;
however, no μ3-oxo bridge is present. In this literature
compound, each thorium atom is thus coordinated to four
μ2-hydroxo groups. Its coordination sphere is completed by
Figure 1. Polyhedral representations of the anionic part of (a)
compounds 1 and 2, (b) compound 3, and (c) compound 4. Side view
on the left and top view of the central metallic cluster on the right.
“Archimedean” square antiprism geometry. Their tetravalent
oxidation state was confirmed by BVS calculation (Table S2 in
the Supporting Information).50,51 Both systems crystallized in
the monoclinic space group C2/c. The asymmetric unit is
constituted of one unique α-SiW9O34 unit, two crystallo-
graphically different cations (cerium for 1 and uranium for 2)
and one acetate ligand. The complete molecule is obtained by
the application of an inversion center located between the two
μ3-oxo bridges. The external cation (Ce or U) is coordinated by
two [SiW9O34]10− units via five oxygen atoms of the vacancies
of the POMs. Its coordination sphere is completed by one μ3-
oxo bridge and one chelating acetate ligand positioned in an
almost-equatorial fashion, compared to the axis of the two
[SiW9O34] units. In contrast, the internal metallic cation (Ce or
U) is close to the inversion center of the molecular species and
is coordinated by the two μ3-oxo bridges and the two
[SiW9O34] units via six oxygen atoms (Figure 2a). Careful
Figure 2. Ball and stick representation of the tetranuclear subunit-
based compounds 1 and 2: (a) top and side views of the tetranuclear
cluster and (b) view of the sandwich architecture.
examination of the distances in both tetranuclear clusters
(Table 2) reveals a small increase of several distances for the
compound containing the uranium. These modifications are
due to the slight increase in ionic radius when going from
cerium(IV) (1.11 Å) to uranium(IV) (1.14 Å).52 The
equatorial distances between the oxygen atoms of the
polyanion and the cation are roughly equal for the internal
D
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Table 2. Comparison of Bond Distances for Compounds 1 and 2
Compound 1
Compound 2
Ceint
Ceext
Uint
Uext
M−(OPOMax) (Å)
M−(OPOMeq) (Å)
M−(μ3-O) (Å)
M−(OAc) (Å)
2.548(5)−2.565(5)
2.237(5)−2.406(5)
2.220(5)−2.231(5)
2.269(6)−2.400(6)
2.482(5)−2.488(5)
2.128(5)
2.574(11)−2.606(11)
2.254(12)−2.398(11)
2.248(11)−2.252(11)
2.312(12)−2.408(12)
2.479(11)−2.490(11)
2.144(11)
2.461(5)−2.463(5)
2.482(12)−2.497(12)
of the smaller ionic radius of the hafnium cation (0.97 Å), its
coordination number is generally limited to six, similar to that
observed for most of the 3d or 4d transition metals (although
some rare examples present higher coordination numbers).55
This preferential coordination number limits the nuclearity of
the cluster to a simple trinuclear cluster fragment, in agreement
with the C3 symmetry of the polyanion, without any μ3-O
bridge. This arrangement with three μ2-OH bridges has already
been observed with the zirconium cation (ionic radius = 0.98
Å).29 In our species, the distances between the metallic cation
and the oxygen atoms belonging to the two silicotungstic units
and the oxygen atoms of the hydroxo bridges are in the same
range (M−-OPOM = 2.04(2)−2.16(2) Å from ref 29 and
2.076(8)−2.118(8) Å for (4)) and M−OH-M = 2.06(2)−
2.13(2) Å from ref 29 and 2.041(8)−2.112(8) Å for (4)), in
agreement with the close ionic radii of both cations.
Figure 3. Ball-and-stick representation of the trimeric compound 3:
(a) top and side views of the trimetallic cluster and (b) view of the
sandwich architecture.
Thermal Behavior. Thermogravimetric (TG) analyses
were performed on the compounds to determine the water
content of the systems (Figure S4 in the Supporting
Information). Calculations were carried out based on the
observation of the first loss of weight (i.e., 200 °C for
compound 1, 260 °C for compounds 2 and 4) and compared
with the calculated values obtained from the RX studies. They
indicated that the three compounds contain 40 (obs., 11.5%;
calc., 8.4%), 24 (obs., 6.8%; calc., 6.6%), and 24 (obs., 6.9%;
calc., 6.6%) hydration water molecules for the cerium, uranium,
and hafnium clusters, respectively. From the structural point of
view, all the water molecules could not be found from the XRD
studies, because of the important disorder of the water
molecules, which is a classical issue in polyoxometalate systems
provoking differences between calculated and observed
percentages. For the first two systems, a second weight
decrease can be observed until 400 °C for 1 and 500 °C for
2. This step could correspond to the loss of the organic part
(acetate ligand) coordinated to the cerium and the uranium.
From the calculations, 3 acetate molecules (obs., 3.2%; calc.,
1.97%) and 1.5 acetate molecules (obs., 1.24%; calc., 1.88%) are
lost for compounds 1 and 2, respectively (two molecules being
coordinated to the cluster). At higher temperature, a small
increase in weight is observed for the uranium compound. It
could correspond to the formation of uranium oxide (U3O8).
Since compound 3 was not obtained pure, no TG measurement
was performed.
Infrared Spectroscopy and Powder Diffraction. The
infrared (IR) spectra of four compounds was collected and
compared to the one of the SiW9O34 precursors (Figure 4).
Informations regarding the POM moiety can be obtained in the
range of 1100−400 cm−1. This domain corresponds to W−O
and Si−O vibrations. The latter ones are generally overlapped
with the W−O vibrations in the cases of silicotungstate
compounds. From the spectra, it appears that the polyanion
backbone is strongly modified, which is consistent with the
coordination of heterometals on the POM vacancies. We
mainly observed some variations at ∼15−25 cm−1 from the
four nitrogen atoms belonging to a porphyrin molecule to give
all three thorium atoms a square antiprism geometry.
In compound 4, the three hafnium centers possess a
coordination number of six with a trigonal prismatic geometry.
BVS calculations confirmed the +4 character of each hafnium
cation in this geometry. Each hafnium atom is coordinated to
four oxygen atoms belonging to the two polyoxometalates
(from 2.076(8) Å to 2.118(8) Å) and two atoms belonging to
the μ2-OH bridges (from 2.041(8) Å to 2.112(8) Å). The
compound crystallized in the triclinic P1 space group. The
̅
inversion center is located outside the molecular unit and, the
anionic part of the compound contains a chloride atom located
on this inversion center with a multiplicity of 1/2. It is probably
coming from the chloride hafnium salt used in the synthesis.
The molecular unit possesses a C3-axis going through the two
Si atoms of the POMs. Three mirrors parallel and on mirror
perpendicular to the main C3-axis give the molecule a D3h
symmetry. Electroneutrality is ensured by three sodium and 8.5
Rb+ cations surrounded by water molecules and oxygen atoms
from the [SiW9O34] unit.
The major difference between the two isostructural
compounds 1 and 2 and compound 3 lies in the nuclearity of
the polyoxo subcluster stabilized by the two SiW9O34 moieties.
From the four tetravalent cations used in these studies, the
compound bearing thorium possesses the largest ionic radius
(1.19 Å),52 giving it a soft acid character, greatly favoring
olation condensation processes, compared to U4+ and Ce4+,
which are harder acids and are more easily condensed by
oxolation processes. From a general point of view, increasing
the nuclearity of a cluster is easier when it is monitored by
oxolation condensation reaction rather than olation reaction.
This phenomenon could explain why, under the same synthetic
conditions, the nuclearity of the thorium cluster is limited to
three. This cluster has three hydroxo bridges and only one oxo
bridge, whereas with uranium and cerium, the clusters are
tetrameric and constructed around two oxo bridges. The
molecular arrangement observed for compound 4 slightly
differs from the other trinuclear cluster with thorium. Because
E
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characteristic transitions in the visible region, but very few data
exist regarding its association with polyoxometalates.17,46 The
spectrum of the {UIV4(SiW9O34)2} system was recorded
between 400 and 900 nm both in solution and in solid state
(Figure S3 in the Supporting Information). The spectra present
very broad absorbance but some transitions are still observable.
They concern two U4+−W6+ transitions: 3H4 to 3P2 at 431 nm,
3H4 to 3P1 at 552 nm, and two f-f transitions of U4+: 3H4 to 3P0
3
1
at 644 nm and H4 to D2 at 665 nm. All these transitions are
characteristic of a U4+-containing species. In the solid state, the
transitions are much broader and almost disappear, probably
because of the charge-transfer interactions occurring between
the U4+ and the W6+ of the polyanion.17
183W and 29Si NMR Studies. Solution stabilities studies of
compounds 1 and 4 were performed by 29Si and 183W NMR
experiments. The 29Si spectra were recorded using a solution
that contained a concentration of compound of ∼25 mmol L−1.
They both show only one single moderately sharp signal, in
agreement with 2 equiv of [SiW9O34] unit in each compound,
respectively located at −84 ppm and −85 ppm for molecule 1
and 4 with a line width (Δν1/2) of 10 Hz (see Figure S5 in the
Supporting Information). 183W NMR spectra of the two
systems and the chemical shift with the related intensities are
respectively shown in Figure 6 and Table 3. The spectrum of
{Ce4(μ3-O)2(SiW9O34)2(CH3COO)2} exhibits a five-line setup
with resonances going from −147.9 ppm to −180.2 ppm, with
a relative intensity of 1:2:2:2:2. The chemical shifts are located
in the usual range for tungsten atoms in an octahedral oxygen
environment. The molecular compound possess a plane of
symmetry going through the two external cerium atoms and a
C2 axis going through the two inner cerium atoms giving a
global C2h symmetry for the compound. The combination of
29Si and 183W NMR signals is consistent with this C2h
symmetry. The resonances located at −147.9 ppm and
−174.4 ppm appear to be slightly broadened (Δν1/2 = 7.9−
9.4 Hz) and could correspond to the three tungsten atoms of
Figure 4. IR spectra in the polyoxometalate region of {SiW9O34}
(spectrum a), {Ce4(μ3-O)2(SiW9O34)2(OAc)2} (spectrum b), {U4(μ3-
O)2(SiW9O34)2(OAc)2} (spectrum c), {Hf3(μ2-OH)3(SiW9O34)2}
(spectrum d), and {Th3(μ3-O)(μ2-O)3(SiW9O34)2} (spectrum e).
W−Ob−W, W−Oc−W, and W−Od vibrations (see Table S1
in the Supporting Information). Spectra of compounds 1 and 2
are almost similar, and this confirms the same mode of
coordination of the cerium and the uranium clusters to the
SiW9O34 polyanion. The spectrum of compound 4 shows
slightly modified W−Ob−W, W−Oc−W, and W−Od
vibrations, in comparison with compounds 1 and 2. This
difference must be assigned to the different coordination
geometries (prismatic) of the hafnium atoms constituting the
trinuclear cluster. The spectrum of the thorium-containing
compound 3 appears different than the other ones, and
important vibrations that did not belong to the sandwich
compound are observed. They are, in a large part, due to
impurities that are present in the crystallized preparation. This
impurity was clearly identified by powder XRD analysis (see the
powder patterns in Figure S2) and IR spectroscopy. It consists
of a polymeric chain based on the coordination of acetates
around the thorium cation. For instance, the comparison of the
calculated XRD pattern of the UIV(CH3COO)4 chain with our
impurities shows close diffraction Bragg peaks with a slight shift
in the lower θ region, in agreement with the greater ionic radius
of the thorium over the uranium.56 The presence of these
acetate ligands is also visible in the IR spectrum in the region of
1400−1600 cm−1 (not shown) and in the POM region below
1100 cm−1, where several vibrations belonging to the organic
component are visible (Figure 4). Finally, comparison of
simulated powder patterns obtained from the structural
resolution with the experimental ones show that compounds
1, 2, and 4 are obtained as a pure crystalline phase (Figure 5).
Ultraviolet−Visible (UV-vis) Spectroscopy. It is well-
known from the literature that U4+-containing systems exhibit
the [SiW9O34] cap, the three others sharper signals (Δν1/2
=
5.3−7.5 Hz) being related to the six tungsten atoms of the
polyanion belt. Compound {Hf3(μ2-OH)3(SiW9O34)2}
presents a two-line spectrum with a relative intensity of 2:1,
respectively located at −145 and −175 ppm, in the usual range
for polyoxometalates. This observation combined with the 29Si
spectrum is reliable, with an average D3h symmetry of the
molecular unit of the compound in solution. The sharp signal
(Δν1/2 = 5.3 Hz) at −175 ppm is related to the three tungsten
atoms of the cap and the very broad signal centered at −145
ppm is attributed to the six atoms of the belt. This signal
appears to be affected by moderately slow dynamic proton
hopping occurring between protons of solvent (H2O−D2O)
and oxygen atoms belonging to the μ2-OH bridges of the
{Hf3O3} moieties. BVS calculation of O71 (1.43) shows that it
is slightly different than O69 (1.24) and O70 (1.21).
Furthermore, the distances are shorter for Hf−O71 bonds
(average 2.045 Å) than Hf−O69 bonds (2.10 Å) or Hf−O70
bonds (2.109 Å). We stated in the crystallographic study that
the three bridges of the trinuclear hafnium belt are protonated;
however, based on the distances and BVS, it remains possible
that one of the three oxygen bridges (O71) could be partially
deprotonated, allowing this proton hopping and causing the
broadening of the signal centered at −145 ppm. Nevertheless,
these multiplicities observations confirm the stability of both
compounds in solution, at least over the measurement period
(4 days) needed to obtain the spectra.
F
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Inorganic Chemistry
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Figure 5. Calculated (black line) and experimental (red line) powder X-ray diffraction (XRD) patterns of [CeIV4(μ3-O)2(SiW9O34)2(CH3COO)2]·
40H2O (spectrum a), [UIV4(μ3-O)2(SiW9O34)2(CH3COO)2]·24H2O (spectrum b), [ThIV3(μ3-O)(μ3-O)3(SiW9O34)2] (spectrum c), and [Hf3(μ2-
OH)3(SiW9O34)2]·24H2O (spectrum d).
Magnetism. To ascertain the formal valence of the uranium
ions, we investigated the magnetic properties of compound 2 as
a function of temperature. The temperature variation of the
magnetic susceptibility and the magnetization curve at 1.8 K is
shown in Figure S6 in the Supporting Information. The
magnetic susceptibility (χ(T)) increases continuously between
room temperature and 1.8 K, with a slight upturn below 15 K.
Consistently, the χT product decreases from 3.74 K emu mol−1
at 300 K (0.94 K emu/U mol) to 0.15 K emu mol−1 at 1.8 K
(0.04 K emu/U mol). The constant decrease of the χT vs T
curve indicates the predominance of temperature-dependent
effects. On this basis, we have tentatively fit the temperature
variation of the magnetic susceptibility by using the relation χ =
C/(T + A) + B. The first term holds for localized electron
systems within the frame of crystal field approach, which is
usually the starting point for analyzing the properties of actinide
systems.57 The parameter B is often introduced to take into
account possible electron delocalization (and Pauli para-
magnetism), yet it can phenomenologically be used to account
for other temperature-independent effects, including van Vleck
paramagnetism.57,58 The value of the Curie constant (C) is
2
dependent on the effective moment: μeff = 8C.
As shown in Figure S6, a nice fit of our experimental data was
obtained above 15 K, with refined values of C = 2.50(1) K emu
mol−1, A = 71.7(3) K, and B = 0.00595(3) emu mol−1. The
parameter B is quite large. In the present case, we do not expect
much delocalization, since the shortest inter-ion distance (Uint−
Uint = 3.68 Å) is higher than the critical distance reported for
the occurrence of inter-ion interaction (3.4−3.6 Å).57 Hence, B
most likely reflects the occurrence of coupling between the
ground state and excited states leading to van Vleck
paramagnetism. It is worth noting that any error in the
diamagnetic correction can also contribute to the value of B.
From the C value for tetrameric units, we can deduce the
G
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Article
Figure 6. 183W NMR spectra of {Ce4(μ3-O)2(SiW9O34)2(CH3COO)2} (spectrum a) and {Hf3(μ2-OH)3(SiW9O34)2} (spectrum b).
Table 3. 183W Chemical Shifts for Compounds 1 and 4
δ (ppm)/Relative Intensity/Δν1/2 (Hz)
compound
{Ce4(μ3-O)2(SiW9O34)2(CH3COO)2}, 1
belt
−171/2/6.5
−176.7/2/7.5
cap
−147.9/1/7.9
−174.4/2/9.4
−180.2/2/5.3
−145/2/not measured
{Hf3(μ2-OH)3(SiW9O34)2}, 4
−175/1/5.3
effective moment μeff = 3.5 μB per UIV atoms. The μeff value at
room temperature is consistent with that found in the literature
for U4+ ions (2.5−3.5 μB /U),46,59−62 especially in poly-
oxometalates.46,63,64 Below 15 K, a steeper increase of the
magnetic susceptibility is observed. Such changes in the
magnetic behavior can stem either from ion−ion interactions
or changing populations of low-lying crystal field states. The
magnetization versus field curve at low temperature, M(H),
indicates a small magnetic moment for the {UIV4(SiW9O34)2}
compound (2). This paramagnetic behavior agrees with a
coupling of the nonmagnetic (3H4, mJ = 0) ground state with
low-lying excited states.59−61,65 The overall magnetic behavior
is consistent with what was reported in the literature for U4+
compounds within the localized electron model.34,61,62,65−67
topologies of the cluster is mainly due to the ionic radius of
the cation, the biggest one (i.e., the thorium) being the most
difficult to grown in nuclearity. As far as we know, these
compounds are the first examples of polynuclear tetravalent (4f
or 5f) clusters stabilized by polyoxometalate ligands that have
been unambiguously characterized by X-ray diffraction. In
addition, the solution studies carried out by 1D 29Si and 183W
NMR measurements attested of the stability in solution of the
tetrameric cerium and trimeric hafnium clusters associations.
The magnetic behavior of the {UIV4(SiW9O34)2} compound (2)
is fully consistent with the presence of UIV ions. These results
open the pathway for further works regarding the use of POMs
to bind and stabilize tetravalent cations clusters. We are
currently working on pH and stoichiometric modulation to see
their impact on the sizes and topologies of the obtained
clusters.
4. CONCLUSION
A simple synthetic strategy was adopted and efficiently applied
to obtain four new compounds based on the trivacant
[SiW9O34]10− building block. Association of this polyanion
with Ce4+ or U4+ give us the isostructural moieties [MIV4(μ3-
O)2(SiW9O34)2(CH3COO)2]10− (1 and 2) containing a
tetranuclear core. Surprisingly, the use of Th4+ leads only to
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b00786.
■
S
the formation of
a
molecule [Th3 (μ3 -O)(μ2 -
Synthesis details for UCl4; comparison of {SiW9O34}
main vibrations of compounds 1−4 (Table S1); BVS
data for compounds 1−4 (Table S2); SEM images of
compounds 1−4 (Figure S1); calculated XRD spectra for
UIV(CH3COO)4 and experimental spectra for ThIV3(μ3-
O)(μ3-O)3(SiW9O34)2 (Figure S2); UV-vis spectra of
OH)3(SiW9O34)2]13− (3) constructed around the trinuclear
cationic core. Compound 4, which involved the hafnium cation
([Hf3(μ2-OH)3(SiW9O34)2]11−), is also arranged in a trinuclear
core but with the absence of the μ3-O bridge, relevant to the
geometry of this type of cation. In the four compounds, the
inorganic moieties play an interesting role in stabilizing these
polymetallic tetravalent species to a discrete nanometric
molecular level. The main structural difference on the
[UIV4(μ3-O)2(SiW9O34)2(CH3COO)2]·24H2O (Figure
3
S 3 ) ; T G A c u r v e s o f [ C e ( μ
-
4
O)2(SiW9O34)2(CH3COO)2]·40H2O, [U4(μ3-
H
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Inorganic Chemistry
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O)2(SiW9O34)2(CH3COO)2]·24H2O, and [Hf3(μ2-
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OH)3(SiW9O34)2]·24H2O (Figure S4); 29Si NMR
spectra of Ce4(μ3-O)2(SiW9O34)2(CH3COO)2 and
Hf3(μ2-OH)3(SiW9O34)2 (Figure S5); magnetization
and magnetic susceptibility data for compound 2 (Figure
S6); and EDX measurements for compounds 2 and 3
(Figure S7) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: (33) 3 20 434 973. Fax: (33) 3 20 43 48 95. E-mail:
sylvain.duval@univ-lille1.fr.
(25) Kikukawa, Y.; Yamaguchi, S.; Tsuchida, K.; Nakagawa, Y.;
Uehara, K.; Yamaguchi, K.; Mizuno, N. J. Am. Chem. Soc. 2008, 130,
5472−5478.
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(29) Finke, R. G.; Rapko, B.; Weakley, T. J. R. Inorg. Chem. 1989, 28,
1573−1579.
The authors would like to thank Mrs. Nora Djelal, Laurence
Burylo, and Mr. Philippe Devaux for their technical assistances
with the SEM images, TG measurements, and powder XRD
(30) Al-Kadamany, G.; Mal, S. S.; Milev, B.; Donoeva, B. G.;
Maksimovskaya, R. I.; Kholdeeva, O. A.; Kortz, U. Chem.Eur. J.
2010, 16, 11797−11800.
(UCCS). The “Fonds Europee
(FEDER)”, “CNRS”, “Region Nord Pas-de-Calais”, and
“Ministere de l’Education Nationale de l’Enseignement Super
́ ́ ́
n de Developpement Regional
́
(31) Shiryaev, A. A.; Shilov, V. P.; Fedoseev, A. M.; Volkov, V. V.
Radiochemistry 2014, 56, 489−492.
̀
́
-
ieur et de la Recherche” are acknowledged for funding of X-ray
diffractometers. The present work is part of the research activity
supported by the Labex NIE (Nanostructires in Interaction
with their Environment, http://www.labex-nie.com/).
(32) Knope, K. E.; Vasiliu, M.; Dixon, D. A.; Soderholm, L. Inorg.
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