Peroxo-Cerium(IV)-Containing Polyoxometalates: [CeIV6(O2)9(GeW10O37)3]24-, a Recyclable Homogeneous Oxidation Catalyst

Article abstract of DOI:10.1021/acs.inorgchem.9b01164

The class of peroxo-cerium-containing polyoxometalates has been discovered via the synthesis of the 9-peroxo-6-cerium(IV)-containing 30-tungsto-3-germanate, [Ce^IV₆(O₂)₉(GeW₁₀O₃₇)₃]^24- (1). Polyanion 1 consists of a cyclic [Ce₆(O₂)₉]⁶⁺ assembly that is stabilized by three dilacunary [GeW₁₀O₃₇]^10- Keggin fragments. The title polyanion 1 is solution-stable, on the basis of ¹⁸³W nuclear magnetic resonance, and was shown to act as a recyclable homogeneous catalyst for the selective, microwave-activated sulfoxidation of the model substrate methionine to the sulfoxide in the absence and to the sulfone in the presence of hydrogen peroxide. Solution and solid-state Raman as well as solid-state infrared studies of 1 demonstrated the complete loss (and regain) of the nine peroxo groups in situ during the catalytic cycle, suggesting that the peroxo-free {Ce₆(GeW₁₀)₃} skeleton remains most likely intact during the catalytic cycle. Solid-state X-ray photoelectron spectroscopy measurements showed that peroxo loss is accompanied by reduction of the cerium ions from +4 to +3, which is fully reversible. Density functional theory calculations are in complete agreement with all of these observations and furthermore suggest that the reduction of the six cerium(IV) ions is accompanied by the formation of molecular dioxygen.

Full text of DOI:10.1021/acs.inorgchem.9b01164

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Peroxo-Cerium(IV)-Containing Polyoxometalates:
IV
24−
[
Ce (O ) (GeW O ) ] , a Recyclable Homogeneous Oxidation
6 2 9 10 37 3
Catalyst
†
†
§
†
†
§
Hafiz M. Qasim, Wassim W. Ayass, Patrice Donfack, Ali S. Mougharbel, Saurav Bhattacharya,
§
§
∥
†
†
Talha Nisar, Torsten Balster, Albert Sole-Daura, Isabella Romer, Joydeb Goura, Arnulf Materny,
́
̈
Veit Wagner,§ Josep M. Poblet, Bassem S. Bassil,* and Ulrich Kortz*
∥
,†,‡
,†
†
Jacobs University, Department of Life Sciences and Chemistry, Campus Ring 1, 28759 Bremen, Germany
‡
University of Balamand, Faculty of Arts and Sciences, P.O. Box 100, 1300 Tripoli, Lebanon
§
Jacobs University, Department of Physics and Earth Sciences, Campus Ring 1, 28759 Bremen, Germany
∥
̀
Universitat Rovira i Virgili, Departament de Química Física i Inorganica, c/Marcel·lí Domingo 1, 43007 Tarragona, Spain
*
S Supporting Information
ABSTRACT: The class of peroxo-cerium-containing polyoxometa-
lates has been discovered via the synthesis of the 9-peroxo-6-
c e r i u m ( I V ) - c o n t a i n i n g 3 0 - t u n g s t o - 3 - g e r m a n a t e ,
IV
24−
[
[
[
Ce (O ) (GeW O ) ] (1). Polyanion 1 consists of a cyclic
6 2 9 10 37 3
6
+
Ce (O ) ] assembly that is stabilized by three dilacunary
6
2 9
1
0−
GeW O ]
Keggin fragments. The title polyanion 1 is
1
0
37
183
solution-stable, on the basis of W nuclear magnetic resonance,
and was shown to act as a recyclable homogeneous catalyst for the
selective, microwave-activated sulfoxidation of the model substrate
methionine to the sulfoxide in the absence and to the sulfone in the
presence of hydrogen peroxide. Solution and solid-state Raman as
well as solid-state infrared studies of 1 demonstrated the complete
loss (and regain) of the nine peroxo groups in situ during the catalytic cycle, suggesting that the peroxo-free {Ce (GeW ) }
6
10 3
skeleton remains most likely intact during the catalytic cycle. Solid-state X-ray photoelectron spectroscopy measurements
showed that peroxo loss is accompanied by reduction of the cerium ions from +4 to +3, which is fully reversible. Density
functional theory calculations are in complete agreement with all of these observations and furthermore suggest that the
reduction of the six cerium(IV) ions is accompanied by the formation of molecular dioxygen.
9
INTRODUCTION
sulfone. Herein, we report on the discovery and character-
■
ization of the first peroxo-cerium(IV)-containing POM.
One of the main challenges in homogeneous oxidation
catalysis is the ability to synthesize and test “recyclable”
catalysts, with reproductive efficiency and quantitative
recuperation after the catalytic cycle. There has been much
interest in transition metal complexes for the catalytic
EXPERIMENTAL SECTION
■
General Information. All reagents and chemicals were of high
purity and were used as purchased without further purification. The
FT-IR spectra (KBr pellet) were recorded on a Nicolet-Avatar 370
spectrometer. Thermogravimetric analysis (TGA) was carried out
with a TA Instruments Q 600 device at a heating rate of 5 °C/min
under a nitrogen atmosphere. The W and H NMR spectra were
recorded on a JEOL ECS400 instrument. Elemental analysis was
performed at CREALINS (Villeurbanne, France).
Synthesis of Na-1. CeCl ·7H O (0.066 g, 0.177 mmol) was
dissolved in 20 mL of a 2 M NaCl aqueous solution at pH 5.0.
Subsequently, Na [A-α-GeW O ]·18H O (0.500 g, 0.177 mmol)
synthesized according to the published procedure was added. The
reaction mixture was stirred for 30 min at 50 °C, and then 1 mL of
30% hydrogen peroxide was added to the solution, which was kept at
50 °C for an additional 1 h, at which point a color change to yellow
1
2
oxidation of organic substrates. In this regard, polyoxometa-
lates (POMs), as discrete polynuclear metal-oxo complexes
with a large structural and compositional diversity, are also
attractive candidates, as well as in other areas such as
magnetism, material science, and biomedicine. Peroxo-
containing POMs are known, and some are highly active in
1
83
1
3
3
2
4
1
0
9
34
2
oxidation catalysis, including the Venturello ion
10
3−
5
[
-
PW O (O ) ] . Peroxo-containing polyoxomolybdates and
4 8 2 8
niobates as well as lanthanum-containing peroxopolytung-
states have also been synthesized.
6
−8
Our group has reported
dimeric and trimeric peroxo derivatives of zirconium- and
hafnium-containing POMs, which were active in the
stoichiometric oxidation of methionine to the sulfoxide or
Received: April 23, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
was observed. The reaction mixture was cooled to room temperature
and then filtered. The solution was left for 1 day in a closed vial. On
the following day, 0.5 mL of 30% H O was added and the mixture
cooled and ultraviolet-enhanced back-illuminated CCD detector
having a chip size of 2048 × 1024 pixels (CCD 3500, Jobin Yvon).
As the single-crystal XRD data suggested not significantly different
bond strengths for the peroxo groups in the POM, high-spectral
resolution optics (grating and detector) were needed for the Raman
experiments. With these settings, the most characteristic Raman
spectral region of the POM (240−1226 cm ) required twice the
CCD spectral window and was recorded with an exposure time of
≤120 s and over three averaged signal accumulations per spectral
window. Toluene was used for both wavenumber position and
intensity calibration of the Raman setup. The measurement control
software NGSLabSpec (Jobin Yvon) was also used for Raman data
analysis. The raw spectra were noise-filtered by being processed five
times through a 4-degree and 7-size Savitsky−Golay least-squares
smoothing algorithm, and then the baseline caused by a fluorescence
background in each measured spectrum was approximated by a line fit
(with a set of manually fixed fitting nodes for all spectra) and
subtracted.
2
2
heated to 50 °C for 15 min. The reaction mixture was then filtered at
room temperature. After the solution had been left for 1 week in a
closed container, a dark red crystalline product was obtained, which
was collected and air-dried. Yield: 0.030 g (10%). IR (Figure S1).
−
1
IV
Elemental Anal. Calcd for Na [Ce (O ) (GeW O ) ]·100H O
2
4
6
2
9
10 37
3
2
(
Na-1): Na, 5.03; Ce, 7.65; Ge, 1.96; W, 50.0. Found: Na, 5.02; Ce,
7.76; Ge, 1.97; W, 50.1. The number of crystal waters was confirmed
by TGA (Figure S2).
We have also made the silicon analogue of 1,
IV
24−
[
Ce (O ) (SiW O ) ] (2), on the basis of single-crystal XRD
6 2 9 10 37 3
and TGA. Na [Ce (O ) (SiW O ) ]·∼100H O (Na-2) is iso-
24
6
2
9
10 37
3
2
morphous with Na-1: triclinic, P1
̅
, a = 22.2638(7) Å, b = 23.5142(7)
Å, c = 23.5205(6) Å, α = 119.8558(13)°, β = 102.2457(17)°, γ =
3
9
9.0349(18)°, V = 9922.5(5) Å , R = 0.0546. See also Table S1.
Single-Crystal X-ray Diffraction. A single crystal of Na-1 was
mounted on a Hampton cryoloop in light oil for data collection at 100
K. Indexing and data collection were performed on a Bruker D8
SMART APEX II CCD diffractometer with κ geometry and Mo Kα
radiation (graphite monochromator; λ = 0.71073 Å). Data integration
XPS Measurements. To measure the photoelectron spectra of
differently treated layers of the Ce-containing polyanion salt Na-1, a
50 nm thick Au film was deposited by means of an electron beam
evaporator on a piece of a silicon wafer. The as-prepared, the
depleted, and the regenerated POM were dispersed in acetone and
deposited by drop casting. After deposition, the samples were
introduced into the XPS vacuum vessel, which was equipped with a
photoelectron spectrometer consisting of a hemispherical analyzer
(Specs Phoebos 100) and a Mg/Al X-ray gun (Specs XR-50) with an
angle of 45°. For all measurements, Mg Kα1,2 radiation (E = 1253.6
eV) was used as source of excitation, whereas the analyzer was
operated in fixed analyzer transmission mode with a pass energy of 50
eV. The energetic shift in binding energy positions due to charging of
the samples was referenced to the C 1s peak. Overview (not shown
here) and Ce 3d spectra were measured for the differently treated
POM samples. The measured data were analyzed using the CASAXPS
software. The background was subtracted using Shirley’s method. The
different Ce 3d doublets were fitted using two simplified Voigt
functions with the same full width at half-maximum and an intensity
11
was performed using SAINT. Routine Lorentz and polarization
corrections were applied. A multiscan absorption correction was
12
performed using SADABS. Direct methods (SHELXS97) success-
fully located the tungsten atoms, and successive Fourier syntheses
13
(
SHELXL14) revealed the remaining atoms. Refinements were full
matrix least-squares against |F2| using all data. In the final refinement,
all nondisordered heavy atoms (W, Ge, Ce, and Na) were refined
anisotropically; oxygen atoms and disordered countercations were
refined isotropically. No hydrogen atoms were included in the models.
For the sake of overall consistency, we show in the CIF file the same
formula unit as in the text, with the exact number of countercations
and crystal waters on the basis of elemental analysis and TGA, as this
reflects the true bulk composition of the compound. Single crystals of
Na-1recharged were obtained after the catalytic reaction and recharging
of peroxo-depleted 1 with fresh H O , and the XRD measurements
2
2
ratio of 0.6 between the 3d3/2 and 3d5/2 peaks. Spectra of
demonstrated that 1 and 1recharged are identical. See Tables S1 and S2
for crystallographic details.
SO Formation. The homogeneous oxidation of DL-methionine (S)
by 1 was performed by dissolving S (10 mg, 0.067 mmol) in 2.5 mL of
water, and then Na-1 (100 mg, 0.009 mmol) was added before the
solution was heated by microwave irradiation (power of 30 W) at 50
IV
(
NH ) Ce(NO ) and CeCl were taken as references for the Ce
4 2 3 6 3
III
and Ce oxidation states, respectively (see the Supporting
Information).
Computational Details. DFT calculations were carried out at the
1
4
15
B3LYP level using the Gaussian 09 package. The MWB28 basis
16
17
set was used to describe Ce, while the LANL2DZ basis set was
used for the remaining atoms, with the exception of oxygen atoms
directly bonded to Ce, which were described by a Pople-type double-ζ
°
C for 50 min.
SO Formation. The homogeneous oxidation of DL-methionine
2
(
S) by 1 and hydrogen peroxide was performed by dissolving S (13
18
basis set supplemented with polarization functions. Solvent effects
mg, 0.087 mmol), Na-1 (92 mg, 0.0084 mmol), and 0.05 mL of 30%
of water were included in the calculations by means of the IEF-PCM
H O (0.75 mmol) in 2.5 mL of water. The solutions were then
2
2
19
implicit solvation model as implemented in Gaussian09. Geometry
optimizations were performed without any symmetry restriction. Due
to the complexity of the system, we set the optimization convergence
criteria to a maximum step size of 0.01 au and a root-mean-square
force of 0.0017 au. However, it must be pointed out that the
heated by microwave irradiation at 50 °C (power of 30 W) for 35
min.
Catalyst Recovery. The depleted POM catalyst was recharged by
addition of 0.05 mL of 30% H O to the solution. The formation of
2
2
charged 1 can be seen by a change in the color of the solution from
−
4
maximum force at the last step of geometry optimization is <3 × 10
light yellow (SO formation pathway) or light red (SO formation
2
au in all cases, indicating that the structures are rather relaxed. As
frequency calculations of such large molecules are highly expensive,
we used a model system to compute the Raman spectrum. In the
model, the three tungsten-oxo Keggin units were omitted, keeping the
oxygen atoms directly bonded to Ce, which were capped by hydrogen
atoms. Prior to the frequency calculation, the Ce-peroxo core was fully
optimized using the default convergence criteria within a cluster
model approach, i.e., freezing the position of the capping oxygens as
in the optimized structure of the real system to reproduce the rigidity
induced by the POM fragments.
pathway) to dark red (Figure S5). Polyanion 1 can be easily
recrystallized after several catalytic cycles by addition of 2 mL of a 2
M NaCl solution to the reaction mixture. The turbid solution turns
transparent upon being heated at 50 °C for 5 min. Single-crystal XRD
of these crystals confirmed the presence of polyanion 1.
Solution Raman Studies. The 514.5 nm line of an argon ion
laser source (Inova 308, Coherent Inc.) was focused using a
microscope setup equipped with a long-working distance 50×
objective (MPlan, Olympus) to excite the sample contained in a
quartz glass cell with a maximum laser power of ∼10 mW at the
sample cell. Then the scattered light collected in a 180° back-
scattering geometry through the same objective was filtered by a
Notch filter for Rayleigh scattering rejection, dispersed in a single
monochromator (Triax 550, Jobin Yvon) using an entrance slit width
of 100 μm and a high-resolution diffraction grating with 2400
grooves/mm, and detected on a high-pixel density liquid nitrogen-
RESULTS AND DISCUSSION
■
Synthesis and Structure. We have synthesized the 6-
cerium(IV)-9-peroxo-containing 30-tungsto-3-germanate
IV
24−
[Ce (O ) (α-GeW O ) ]
(1) in a simple one-pot
6
2
9
10 37 3
B
DOI: 10.1021/acs.inorgchem.9b01164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
reaction of Ce3+ ions with the trilacunary Keggin-based POM
of nine peroxo groups, as compared to six peroxo groups in
10−
18−
precursor [α-GeW O ]
at pH 5 in a sodium chloride
[M (O ) (OH) (γ-SiW O ) ] . In 1, each cerium(IV) ion
9
34
6
2 6
6
10 36 3
solution. Polyanion 1 was isolated as a hydrated sodium salt,
is coordinated to three side-on peroxo bridges and three oxo
ligands from the lacunary POM unit, resulting in a
coordination number of 9. On the other hand, the
zirconium/hafnium centers in [M (O ) (OH) (γ-
IV
Na [Ce (O ) (α-GeW O ) ]·100H O (Na-1), which crys-
2
4
6
2 9
10 37 3
2
tallizes in triclinic space group P1
ray diffraction analysis of Na-1 revealed a cyclic [Ce (O ) ]
core that is surrounded by three [α-GeW O ] dilacunary
̅
(Table S1). Single-crystal X-
6
+
6
2 9
6
2
6
6
10−
18−
SiW O ) ]
are each coordinated to two side-on peroxo
10
37
10 36 3
Keggin units (Figure 1). The central Ce-peroxo wheel
bridges, two hydroxo bridges, and two oxo ligands from the
lacunary POM unit, resulting in a coordination number of 8. It
should also be noted that polyanion 1 contains {GeW }
1
0
Keggin units, although the synthesis was performed using
GeW }, indicating partial decomposition of the latter, which
{
9
is expected at the synthetic pH of 5. We studied the pH
window in which polyanion 1 can be formed and discovered
that the desired product could be obtained only at pH 5.
Slightly higher or lower pH values did not result in the
formation of polyanion 1. We also added extra sodium
tungstate during the reaction and used mono- and dilacunary
POM precursors but did not see an improvement in the rather
low yield
183W Solution Nuclear Magnetic Resonance (NMR).
NMR spectroscopy was performed to study the solution
stability of polyanion 1. The room-temperature W NMR
spectrum of Na-1 dissolved in water shows six peaks (Figure 2)
1
83
Figure 1. Combined polyhedral/ball-and-stick representation of
2
4−
6+
[
(
Ce (O ) (GeW O ) ] (1), top. View of the [Ce (O ) ] core
6 2 9 10 37 3 6 2 9
6
+
bottom left) of 1 and the [M (O ) (OH) ] (M = Zr or Hf) core
6 2 6 6
(
bottom right) of the structurally related [M (O ) (OH) (γ-
6 2 6 6
18−
SiW O ) ]
(see ref 9a). Color code: green Ce, light-gray Zr/
10
36 3
Hf, red peroxo, pink hydroxo, blue octahedra WO , and yellow
6
tetrahedra GeO4.
Figure 2. 183W NMR spectrum of Na-1 dissolved in H O/D O. The
2
2
comprises six equivalent CeIV centers, as confirmed by bond
valence sum (BVS) calculations. Two Ce ions occupy the
inset shows the structure of polyanion 1 highlighting the magnetically
inequivalent tungsten centers for one of the three structurally
equivalent Keggin units.
20
IV
lacunary site of each {α-GeW } unit and are connected by a
1
0
2
IV
2
side-on η -peroxo bridge. In turn, the three {Ce (O )GeW }
2
10
2
2
fragments are connected to each other via two side-on η ,η -
peroxo bridges, one above and one below the overall plane of
the polyanion, resulting in an assembly with C3v point group
symmetry. The Ce−O bond lengths range from 2.237(15) to
.446(16) Å, and the Ce−peroxo bond lengths range from
.307(15) to 2.446(14) Å. The O−O bond lengths are
between 1.489(19) and 1.52(2) Å, which are in good
agreement with the value of 1.49 Å reported for the crystal
structure of hydrogen peroxide. Such a linkage mode
between Ce centers has been reported before in coordination
chemistry.
The structure of the [Ce (O ) ] core (Figure 1, bottom
left) in 1 closely resembles that of [M (O ) (OH) ] (M =
at −99.4, −109.9, −113.0, −130.6, −207.9, and −296.2 ppm,
with a 2:2:2:2:1:1 intensity ratio, which is in agreement with
the solid-state structure of 1. Polyanion 1 has C3v symmetry,
resulting in six types of non-equivalent W atoms in the
mentioned intensity ratio (Figure 2, inset). The four downfield
peaks with equal intensity ratios (between −99 and −131
ppm) are assigned to the four W pairs shown in red, pink,
turquoise, and maroon, respectively. The peak at −207.9 ppm
is tentatively assigned to the unique W (blue) of the “outer
triad”, and the most upfield signal at −296.2 ppm is tentatively
assigned to the unique W (green) that is connected to the two
Ce centers.
2
2
2
1
IV
2
2
6+
6
2 9
6
+
Catalytic Oxidation of Methionine. As polyanion 1 is
stable in solution, we decided to test it as a homogeneous
catalyst for the sulfoxidation of the model substrate DL-
methionine (S). In previous work, we have shown that peroxo-
6
2 6
6
IV
IV
Zr
or Hf ) in our reported [M (O ) (OH) (γ-
6 2 6 6
9a
1
8−
SiW O ) ] (Figure 1, bottom right). However, there is
1
0
36 3
a small but important difference; polyanion 1 contains a total
C
DOI: 10.1021/acs.inorgchem.9b01164
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Inorganic Chemistry
Article
ZrIV and -HfIV POMs convert S to the sulfoxide SO, and in
some cases, oxidation of SO to the sulfone SO was also
2
9
IV
observed in the presence of hydrogen peroxide. As the Ce -
peroxo assembly in 1 possesses nine peroxo units per
polyanion, we first tested 1 for oxygen transfer to methionine
(
S). Mixing 1 with methionine in an aqueous solution at a
POM:S ratio of 1:7.4 resulted in quantitative conversion of S
to SO after reaction for 50 min under mild (50 °C) microwave
heating. It should be noted that a stoichiometric POM:S ratio
of 1:9 led to 90% conversion under the same conditions. If
after quantitative conversion excess methionine was added, it
remained in the S form without any oxidative conversion,
suggesting complete discharge of 1 to a peroxo-free polyanion
{
Ce (GeW ) } (vide infra). In the case of excess hydrogen
6 10 3
peroxide with no POM present at all, S is converted to SO,
without further oxidation to SO . Both conversions were
2
1
followed in solution by H NMR (Figure S3). Next, we studied
the oxidation of S in the presence of both H O and 1. Mixing
2
2
Figure 3. Hydrogen peroxide-based transformation of SO to SO2
using 1 as the catalyst (see the text for details).
1
with methionine in an aqueous solution, with a POM:S ratio
of 1:10 and in the presence of excess H O , led to quantitative
2
2
conversion of S to SO within 35 min during 50 °C microwave
2
heating (Scheme 1). If more S is added to the reaction mixture,
the structure of 1 remained intact after addition of H O
2
2
(
Figure 1 and Table S2).
Because the POM solution changed color during the
Scheme 1. Reaction Pathways from Methionine (S) to the
Sulfoxide (SO) and/or Sulfone (SO ) Product(s) in the
2
oxidation process, we monitored this by ultraviolet−visible
(
(
Presence of Hydrogen Peroxide and/or Polyanion 1
UV−vis) spectroscopy as a function of methionine added
Figure S6). The obtained spectra showed a decrease in
absorbance after the addition of each aliquot of methionine,
followed by a complete return to the original spectrum after
the addition of hydrogen peroxide, indicating regeneration of
polyanion 1. We decided to further investigate this
phenomenon of 1 during the oxidation process by Raman
spectroscopy.
Solution Raman Spectroscopy. To better understand
the solution behavior of 1 toward methionine oxidation,
Raman spectroscopy measurements in solution were per-
formed. As one can see by the star-marked peaks at 838 and
then it is fully converted to SO , until all hydrogen peroxide is
2
−
1
exhausted. If fresh H O is added, then the catalytic process
852 cm in Figure 4a, the Raman spectrum of a solution of
polyanion 1 indicates the vibrational fingerprints correspond-
ing to the two distinct types of peroxo groups [double bridges
vs single bridges (see Figure 1)] embedded in the polyanion.
This assignment was further supported by theoretical
calculations. A density functional theory (DFT)-simulated
spectrum for a model system of polyanion 1 (Figure S7) also
associates these peaks with the bridging peroxo groups. The
2
2
starts again and was performed 10 times with the same
catalytic turnover. Overall, the observed reactivity suggests that
oxidation of S to SO is faster than oxidation of SO to SO2.
Accordingly, H O can oxidize S to SO, but it appears not to
2
2
be electrophilic enough to oxidize SO to SO . On the other
2
hand, the peroxo groups in the structure of 1 are more reactive
IV
because of Ce , which is a Lewis acid center. This fact renders
−
1
1
capable of oxidizing S as well as SO.
The progress of the oxidation reaction in terms of product
WO signature of 1 appears at around 955 cm , and the
characteristic O−O peak for free H O is expected at around
2
2
1
−1
selectivity is shown in Figure 3 and was monitored by H
875 cm . Figure 4a shows the spectrum of a solution of 1 after
conversion of S to SO, revealing the disappearance of the
peroxo signals after depletion of 1 (red spectrum). Addition of
H O forms the regenerated polyanion, which exhibits similar
NMR. Within 1 min, all of S is transformed to SO, which is
then further converted quantitatively to SO in 35 min (Figure
2
3
). Upon performance of the reaction with 1 and H O at
2 2
2
2
room temperature, the SO:SO product ratio was 80:20 after
Raman signals (blue spectrum) as originally measured for 1.
Such a regenerated polyanion could be crystallized, and XRD
data showed the presence of 1, indicating complete
reversibility (vide supra).
These three Raman measurements indicate (i) complete
transfer of the peroxo groups from 1 to S in the oxidative
formation of SO and (ii) completely reversible uptake of
peroxo groups between two solution-stable forms of 1, the
depleted and regenerated forms.
2
2
5 min. Therefore, mild microwave heating at 50 °C is needed
to ensure complete oxidation (Figure 3). In the presence of
H O and in the absence of 1, no formation of SO is seen by
2
2
2
1
H NMR (Figure S3), which confirms the catalytic role of 1 in
the conversion of SO to SO2.
For both oxidation reactions (S to SO and SO to SO ), we
2
were able to fully recover the pristine polyanion 1 by simple
addition of H O2 to a solution containing the depleted
2
(
peroxo-free) polyanion catalyst {Ce (GeW ) } (Figure S5).
Raman spectroscopy was also used to monitor the solution
6
10 3
In fact, we could even crystallize such regenerated polyanion 1,
and X-ray diffraction (XRD) measurements demonstrated that
behavior of 1 during the catalytic oxidation of S to SO . As one
2
can see in Figure 4b, addition of H O to 1 alters slightly its
2
2
D
DOI: 10.1021/acs.inorgchem.9b01164
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Inorganic Chemistry
Article
−
1
Figure 5. FT-IR spectra (fingerprint region from 400 to 1800 cm )
for the sodium salt of polyanion 1 at three different stages: original
sample (black), depleted sample (green), and regenerated sample
(blue). The dotted line indicates the peak for the peroxo groups.
the recrystallized salt with the absence of the peak for the
peroxo groups. This suggests the structural integrity of the
trimeric POM skeleton throughout the catalytic peroxo loss/
uptake cycles.
To better account for the presence or absence of the peroxo
groups between the different forms, we measured the solid-
state Raman spectra for the three samples. As one can see in
Figure S8, the spectrum for the salt of the depleted polyanion
demonstrated the absence of any peroxo signal, in analogy with
the solution Raman studies.
We also performed X-ray photoelectron spectroscopy (XPS)
measurements on the three POM salts to confirm the oxidation
state of the cerium centers upon loss of the peroxo groups. In
Figure 6, the photoelectron spectra of the Ce 3d level for the
salts of the pure, depleted, and regenerated polyanion 1 are
shown. The spectra show a structure that is more complex than
Figure 4. Raman spectral fingerprint evolution of the polyanion 1
catalyst alone and during the sulfoxidation reaction at different
reaction times (trxn): (a) SO formation pathway and (b) SO2
formation pathway.
Raman peroxo signals (green vs blue spectrum), and the
−
1
hydrogen peroxide peak appears as expected at 875 cm .
Midway during the catalytic process, the Raman spectrum of
the solution (pink spectrum) reveals a decrease in the intensity
of the peroxide signal due to its consumption during the
oxidation cycle, as well as a “readjustment” of the peroxo
signals for 1. At the end of the cycle (turquoise spectrum), the
peroxide signal has disappeared, and the POM spectrum
displays peroxo signals identical to those of the original form
(
bottom green spectrum). When a new cycle is restarted by
addition of fresh H O (top blue spectrum), the Raman
2
2
spectrum observed is identical to that of 1 upon the first
addition of hydrogen peroxide (bottom blue spectrum). We
speculate that when hydrogen peroxide is added to the
solution, it interacts with the peroxo groups in 1, leading to
some sort of “activation” of the POM structure. Such a feature
4b
of POMs has been discussed previously, with more studies
being needed to fully understand the phenomenon.
Solid-State Characterization. To fully characterize the
depleted” form of polyanion 1, we tried to crystallize it from
“
solution after loss of the peroxo groups. Despite using a
multitude of crystallization methods, no crystalline salt was
obtained, leading always to amorphous precipitates. The
Fourier transform infrared (FT-IR) spectrum for the
amorphous sodium salt of 1 after depletion (Figure 5) is
virtually identical to that of the original Na-1 salt and that of
Figure 6. Photoelectron spectra and fits for the Ce 3d3/2 and 3d5/2
doublet structures of an as-prepared layer containing polyanion 1
(
bottom spectrum), depleted 1 (middle spectrum), and regenerated 1
(top spectrum). The Ce oxidation state is easily reflected by the
presence or absence of the peak around 917 eV.
E
DOI: 10.1021/acs.inorgchem.9b01164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a simple chemically shifted 3d doublet peak as expected from
consistent with the experimental observation that polyanion 1
loses peroxo groups upon being heated in solution. Such
elimination may occur either before or after the transfer of the
electrophilic oxygen to the substrate. Figure 8 shows the
3
the spin−orbit split core level with j quantum numbers of /
2
5
and / . In the structure of 1 , cerium is coordinated by
2
oxygens. This chemical bonding leads to a hybridization of the
23−25
4
f state with the 2p orbitals of oxygen.
Such hybridization
results in an additional splitting of the final electron state of the
IV
emission process with respect to the Ce oxidation state,
which is the origin of the additional peak visible at a binding
energy of 916.7 eV. This peak is also seen in the spectrum of
the cerium(IV) reference compound (NH ) Ce(NO ) and is
4
2
3 6
missing in the cerium(III) reference compound CeCl (Figure
3
S9). This range of interest is colored yellow in Figure 5. The
other two peak structures originate from electronic states of
IV
III
both Ce and Ce compounds. Both the original 1 and the
IV
regenerated 1 exhibit the peak characteristic for the Ce
oxidation state. After depletion of 1, only CeIII can be
detected, indicating complete reduction of cerium from the +4
to +3 oxidation state. During the regeneration step, the +4
oxidation state is fully recovered.
The findings described above lead to the conclusion that
polyanion 1 can lose the peroxo groups while retaining its
overall structure, resulting in the reduction of its cerium
centers from CeIV to Ce . This feature is however fully
reversible, and polyanion 1 can be regenerated to its original
form after depletion by addition of hydrogen peroxide. To
better understand the experimental findings, we decided to
perform computational studies on the structure of 1 and
possibly develop a mechanistic pathway for the peroxo loss.
Computational Studies. To shed more light on the
structural and electronic properties of polyanion 1, we have
optimized its structure using DFT methods. Despite the size
and complexity of the anion, we have reproduced rather well
the observed structure (Table S3). The MO orbital analysis
III
Figure 8. Transformation of all-CeIV polyanion 1 to the all-CeIII
derivative. The spin density and schematic molecular orbital diagram
III/IV
are shown for the Ce
mixed valence intermediate.
electronic change when one peroxo unit is detached from 1,
IV
transferring two electrons to two neighboring Ce ions,
III/IV
resulting in a Ce
mixed valence polyanion species and 1
(Figure 7) confirms that all six Ce ions have a formal oxidation
equiv of molecular dioxygen. There are two types of peroxo
2
groups in 1, namely, the Ce−η (OO)−Ce single side-on
2
2
bridge within a Keggin unit of 1 and the Ce−η ,η (OO) −Ce
2
double side-on bridge linking two neighboring Keggin units.
Figure 8 highlights that the first two electrons probably
originate from a π* orbital located in one of the three unique
2
η -OO groups and that this process will be equivalent for the
other two unique peroxo groups. However, we cannot discard
the possibility that the other six peroxo groups are somehow
involved in this redox process, as well. In such a
III
VI
{
Ce 6(GeW ) } compound, the occupied f orbitals are
10 3
26
energetically above the oxo orbitals.
III
Finally, a preliminary all-Ce polyanion structure was
computed assuming the overall elimination of three O2
molecules and the transfer of six electrophilic oxygen atoms
to substrate (S or SO) molecules. The formation of μ-oxo
bridges in the depleted POM after the oxidation of the
substrate is consistent with the proposed mechanism of
thioether oxidation by Ti-substituted POMs. The position
of the bridging oxo groups linking the three Keggin units can
be seen in the two views shown in Figure S10. This figure also
shows the localized nature of the six unpaired electrons on the
Figure 7. Frontier molecular orbital diagram for polyanion
2
4−
[
Ce (O ) (GeW O ) ] (1).
6 2 9 10 37 3
27
state of +4. As expected in compounds containing peroxo
groups, the highest occupied molecular orbitals (HOMOs)
correspond to the π*O−O antibonding orbitals. This fact
combined with the 4f nature of the LUMO is strong evidence
of the fully oxidized nature of the Ce ions. It is worth
mentioning that the energy gap between the occupied and
unoccupied frontier orbitals is rather large, suggesting stability
for 1.
III
respective Ce ions. We are performing additional exper-
imental and theoretical studies to determine the kinetics of
oxygen transfer reactions that will be reported elsewhere.
III
CONCLUSIONS
The formation of an all-Ce derivative of 1, as suggested by
■
XPS (vide supra), requires an internal redox reaction, and most
likely upon heating, three peroxo groups of 1 act as a reductant
and are transformed to three dioxygen molecules. This is
We have discovered the class of peroxo-cerium-containing
POMs, and the 9-peroxo-6-cerium(IV)-containing 30-tungsto-
IV
24−
3-germanate, [Ce (O ) (GeW O ) ] (1), represents its
6
2 9
10 37 3
F
DOI: 10.1021/acs.inorgchem.9b01164
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
first member. The title polyanion was characterized by a
multitude of solid-state and solution techniques, including IR
spectroscopy, single-crystal XRD, XPS, 183W NMR, Raman
spectroscopy, and DFT calculations. We believe that there is
an entire family of peroxo-cerium POMs of various shapes and
sizes, and the future will show how large this field really is. The
highlight of 1 is undoubtedly the fact that it contains two
different redox-active sites, namely, the peroxo groups and the
cerium(IV) ions. Interestingly, both are directly connected to
each other. Polyanion 1 is an attractive oxidation catalyst, as we
have shown using the model substrate DL-methionine (S) that
can be cleanly and quantitatively oxidized to the sulfoxide
Josep M. Poblet: 0000-0002-4533-0623
Ulrich Kortz: 0000-0002-5472-3058
Funding
U.K. thanks the German Research Council (DFG, KO-2288/
6-1), Jacobs University, and CMST COST Action CM1203
PoCheMoN) for support. J.M.P. thanks the Spanish Ministry
2
(
of Science (CTQ2017-87269-P) and the Generalitat de
Catalunya (2017SGR629) for support. J.M.P. also thanks the
ICREA foundation for an ICREA ACADEMIA award.
Notes
The authors declare no competing financial interest.
(
SO) by the POM alone and to the sulfone (SO ) in the
2
additional presence of H O . Our studies show that the
trimeric POM skeleton {Ce (GeW ) } is robust and can most
likely survive the loss of all nine peroxo groups, which is
accompanied by reduction of all six Ce ions to Ce with
ACKNOWLEDGMENTS
2
2
■
6
10 3
Figure 1 was generated with Diamond, version 3.2 (copyright
Crystal Impact GbR).
IV
III
concomitant O formation. If we assume that three peroxo
2
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Corresponding Authors
Jacobs University, Department of Life Sciences and
■
*
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