Synthesis, Characterization, and Phosphoesterase Activity of a Series of 4f- And 4d-Sandwich-Type Germanotungstates [(n-C4H9)4N]l/ mH2[(M(H2O)3)(γ-GeW10O35)2] (M = CeIII, NdIII, GdIII, ErIII, l = 7; ZrIV</s

Article abstract of DOI:10.1021/acs.inorgchem.0c01852

We report on a family of five new 4f- and 4d-doped sandwich-type germanotungstates with the general formula [(n-C4H9)4N]l/mH2[(M(H2O)3)(γ-GeW10O35)2]·3(CH3)2CO [M(H2O)3(GeW10)2] (M = CeIII, NdIII, GdIII, ErIII, l = 7; ZrIV, m = 6), which have been synthes

Full text of DOI:10.1021/acs.inorgchem.0c01852

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Synthesis, Characterization, and Phosphoesterase Activity of a
Series of 4f- and 4d-Sandwich-Type Germanotungstates
[(n‑C4H9)4N]l/mH2[(M(H2O)3)(γ-GeW10O35)2] (M = CeIII, NdIII, GdIII, ErIII, l
= 7; ZrIV, m = 6)
Elias Tanuhadi, Emir Al-Sayed, Alexander Roller, Hana Cipcic-Paljetak, Donatella Verbanac,
̌
̌
́
and Annette Rompel*
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ABSTRACT: We report on a family of five new 4f- and 4d-doped sandwich-type germanotungstates
with the general formula [(n-C4H9)4N]l/mH2[(M(H2O)3)(γ-GeW10O35)2]·3(CH3)2CO [M-
(H2O)3(GeW10)2] (M = CeIII, NdIII, GdIII, ErIII, l = 7; ZrIV, m = 6), which have been synthesized at
room temperature in an acetone−water mixture. Among the compound series, [Zr-
(H2O)3(GeW10)2]8−, which has been obtained in the presence of 30% H2O2, represents the first
example of a 4d-substituted germanotungstate incorporating the intact dilacunary [γ-GeIVW10O36]8−
building block. All compounds were characterized thoroughly in the solid state by single-crystal and
powder X-ray diffraction (XRD), IR spectroscopy, thermogravimetric analysis (TGA), and elemental
analysis and in solution by NMR and UV−vis spectroscopy. The phosphoesterase activity of
[Ce(H2O)3(GeW10)2]9− and [Zr(H2O)3(GeW10)2]8− toward the model substrates 4-nitrophenyl
1
phosphate (NPP) and O,O-dimethyl O-(4-nitrophenyl) phosphate (DMNP) was monitored with H-
and 31P-NMR spectroscopy revealing an acceleration of the hydrolytic reaction by an order of
magnitude (kcorr = 3.44 ( 0.30) × 10−4 min−1 for [Ce(H2O)3(GeW10)2]9− and kcorr = 5.36 ( 0.05) ×
10−4 min−1 for [Zr(H2O)3(GeW10)2]8−) as compared to the uncatalyzed reaction (kuncat = 2.60
( 0.10) × 10−5 min−1). [Ce(H2O)3(GeW10)2]9− demonstrated improved antibacterial activity toward Moraxella catarrhalis (MIC
32 μg/mL), compared to the unsubstituted [GeW10O36]8− POM (MIC 64 μg/mL).
C4H9)4N]4H4[γ-SiIVW10O36] lacunary POM is the most
INTRODUCTION
■
commonly used precursor.13,14 The germanium analogue [(n-
Polyoxometalates (POMs)1 represent a broad class of anionic
C4H9)4N]4[γ-GeIVW10O34(H2O)2],15 however, has not been
inorganic clusters with versatile structural topologies resulting
reported in combination with electrophiles, e.g., lanthanides, in
in a variety of chemical and physical properties which can be
organic media yet. Also, the number of 4f-metal substituted
modulated by molecular design. These features make them
germanotungstates which have been prepared remains scarce
attractive materials in a wide range of fields like catalysis,2,3
(Table S2). Most sandwich-type compounds have been
electrochemistry,4 magneto chemistry,5 and biological chem-
studied toward their catalytic properties, such as water
istry6,7 including protein crystallography.8−10 In contrast to
oxidation16 and Mannich-type reactions.17 Owing to their
plenary POMs, lacunary POMs have well-defined vacant sites
properties as Lewis acids, 4f- and 4d-doped sandwich-POMs
and higher negative charges, which make them interesting as
are interesting candidates for hydrolysis reactions.18
multidentate inorganic ligands toward heteroatoms.11 The
The high negative charge, water solubility, and solution
combination of Keggin-type lacunary building blocks with
stability under physiological conditions have made POMs
heteroatoms has resulted in a broad variety of POM subclasses
attractive candidates for interaction studies with biological
including triangle-shaped, tetrameric and dimeric sandwich-
systems.6 Phosphodiester bonds are characterized by a high
type POMs, with the latter structural-type being the largest
stability with a half-life of 130 000 years toward hydrolysis
subfamily of heteroatom substituted POMs.12
Out of the vast variety of reaction systems using lacunary
POMs as precursors, only 18 examples applying Keggin-type
Received: June 22, 2020
lacunary precursors in organic media have been reported to
date (Table S1), although undesired isomerization and
condensation of POMs commonly encountered in aqueous
solution can be circumvented in organic solvents. Among the
lacunary building blocks used in organic solvent, the [(n-
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under physiological conditions, protecting the genetic materi-
al.19 As a matter of fact, attention has been paid to the
controlled cleavage of this relatively inert bond as a vital step in
biology.20 Being inspired by nature, the majority of researchers
uses Lewis acid containing catalysts as artificial phosphoes-
terases.21−23 Based on previous work, lanthanide(III)- and
zirconium(IV) ions are highly suitable for designing artificial
phosphoesterases, as they exhibit high charge density and
coordination number, as well as fast ligand exchange rates.24
Chemical warfare agents (CWAs), which represent a
significant threat to both military and civilian populations,
are interesting targets for artificial phosphoesterases as
commonly used CWAs such as nerve agents are organo-
phosphorus (OP) esters containing P−X bonds (X = CN, SR).
OP nerve agents are highly toxic by inactivating acetylcholi-
nesterase leading to even death in high doses. The primary and
effective way for the environmentally friendly decontamination
of nerve agents is hydrolysis. The Lewis acidic hydrolysis
catalyzed by activation of the phosphorus oxygen bond upon
coordination to the Lewis acidic sites has attracted attention as
well.25−27 A series of zirconium(IV) based hydrolytic catalysts
including metal oxides, metal hydroxides, and metal−organic
frameworks (MOFs) have been reported,25 but due to their
heterogeneous nature, it is difficult to determine the exact
structure of active sites. Hence, the development of molecular
catalysts with Lewis acidic centers for the homogeneous
degradation of nerve agents contributes to understanding the
hydrolytic mechanism and crucial factors during the
decontamination process. Hill and co-workers thoroughly
investigated the Zr-substituted POM, {[α-PW11O39Zr(μ−
OH)(H2O)]2}8−, as a homogeneous catalyst for the hydrolysis
of a nerve agent and its simulants in a buffered solution.
However, the number of studies on POM compounds as
homogeneous Lewis acid catalysts for the decontamination of
nerve agents remains scarce. Moreover, the applied Zr-POM
dimer is unstable and dissociates into its monomeric form
under turnover conditions.28,29 Attributed to their high
negative charge, strong acidity, and geometry, POMs have
been subjected to antibacterial studies exhibiting synergy with
some conventional antibiotics30 or direct antibacterial
activity31 against both Gram-negative and Gram-positive
bacteria. In general, high-nuclear, highly negatively charged
POMs exhibit a high activity.32 Moraxella catarrhalis is a Gram-
negative human mucosal pathogen, which causes middle ear
infections in infants and children and lower respiratory tract
infections in adults with chronic pulmonary disease.32,33
Among the different archetypes tested toward their anti-
bacterial properties, sandwich-type POMs are the most
promising representatives.6
[Ce(H2O)3(GeW10)2]9−, a comprehensive study on the
hydrolytic activity of [Zr(H2O)3(GeW10)2]8− toward the
nerve agent simulant O,O-dimethyl O-(4-nitrophenyl) phos-
phate (DMNP) under ambient reaction conditions (25 °C, pD
= 7.0) was carried out, and its stability under turnover
conditions was confirmed by recyclability experiments and
postcatalytic IR spectroscopy. The antibacterial activity of the
isostructural polyanions [Ce(H2O)3(GeW10)2]9− and [Zr-
(H2O)3(GeW10)2]8− was tested against Moraxella catarrhalis
thereby revealing an enhanced inhibitory effect of the
[Ce(H2O)3(GeW10)2]9− polyanion as compared to the
unsubstituted [GeW10O36]8− lacunary anion and no inhibitory
effect for the pure Ce(III) salt, which highlights the
importance of POM lacunary ligands for the enhancement of
the antibacterial properties of Lewis acidic metal centers.
RESULTS AND DISCUSSION
■
Synthesis. The first step in the synthesis of [M-
(H2O)3(GeW10)2] was the preparation of the lacunary
literature known precursor [(n-C4H9)4N]4[γ-Ge-
W10O34(H2O)2]15 ((TBA)[GeW10]) (TBA = tetrabutylammo-
nium). Upon addition of 0.5 equiv of Ln(acac)3 (acac =
acetylacetonate; Ln = CeIII, NdIII, GdIII, ErIII) and stoichio-
metric amounts of water to a white suspension of (TBA)-
[GeW10] in acetone (Scheme 1), a clear reaction mixture was
Scheme 1. Schematic Representation Showing the Synthesis
of [M(H2O)3(GeW10)2]n− Starting from the Dilacunary
a
((TBA)[GeW10] TBA = Tetrabutylammonium) Precursor
a
In contrast to the LnIII system, the addition of H2O2 to the reaction
mixture is crucial for the successful incorporation of ZrIV into the
POM architecture (M = CeIII, NdIII, GdIII, ErIII, n = 9; ZrIV, n = 8).
Blue and red spheres represent the M- and oxygen ions, respectively.
Grey for Ge and yellow polyhedra for {WO6}.
obtained. It is worth noting that elevated H2O contents, e.g., to
enable the use of pure inorganic Ln(NO3)3 salts as 4f-metal
sources instead of Ln(acac)3, resulted in the partial formation
of the Keggin ion [α-GeW12O40]4− as confirmed by SXRD
measurements (CCDC 1915317, Tables S7, S14, and S15).
The reaction was carried out under mild conditions in acetone
at room temperature (RT). Considering the low stability and
high reactivity of [γ-XW10] units (X = SiIV, GeIV, PV),34 higher
reaction temperatures than RT would have resulted in
undesired isomerization and/or partial decomposition of the
lacunary units, respectively. After 90 min of stirring and
filtration of the reaction mixture at RT, block shaped crystals of
[M(H2O)3(GeW10)2] were obtained in yields of 79% [Ce-
(H2O)3(GeW10)2]9− (CCDC 1915355), 76% [Nd-
(H2O)3(GeW10)2]9− (CCDC 1915316), 54% [Gd-
(H2O)3(GeW10)2]9−, and 40% [Er(H2O)3(GeW10)2]9−
(CCDC 1915318) based on Ln(acac)3 from a mixture of
Herein, we report on the facile synthesis of five new
monosubstituted sandwich-type germanotungstates with the
general sum formula [(n-C4H9)4N]l/mH2[(M(H2O)3)(γ-
GeW10O35)2]·3(CH3)2CO in the following termed as [M-
(H2O)3(GeW10)2] (M = CeIII, NdIII, GdIII, ErIII; l = 7 and ZrIV;
m = 6), which, to the best of our knowledge, represent the first
examples of monosubstituted 4f-germanotungstates incorpo-
rating the intact dilacunary γ-[GeW10O36]8− building block.
[Ce(H2O)3(GeW10)2]9− and [Zr(H2O)3(GeW10)2]8−, exhib-
iting the highest water solubility (c ∼ 5.3 mM) (Table S19)
among the investigated [M(H2O)3(GeW10)2] series, were
tested toward their phosphoesterase activity with the model
compound 4-nitrophenyl phosphate (NPP) (60 °C, pD = 7.0).
Characterized by a higher Lewis activity compared to
B
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acetone and diethyl ether (∼5:1, (v/v)) after approximately 1
week.
first examples of a family with distinct γ-GeW10 lacunary units
incorporating a 4f- or 4d-metal, respectively (Table S2), which
could only be achieved under the ambient reaction conditions
preventing the in situ isomerization of the lacunary precursor
to the monolacunary [β2-GeW11O39]8− building block or the
plenary [α-A-GeW12O40]4− polyanion.34 Importantly, the
successful incorporation of the heteroatoms could exclusively
be achieved in organic solution by preventing formation of
metal hydroxides commonly encountered in aqueous solution.
Despite all efforts, single-crystals of [Gd(H2O)3(GeW10)2]9−
with sufficient quality for SXRD measurements could not be
obtained. However, elemental analysis and IR spectroscopic
investigations (Figure S1) clearly indicate the successful
synthesis of pure [(n-C4H9)4N]7H2[(Gd(H2O)3)(γ-
GeW10O35)2]. Powder XRD measurements were performed
on compounds [M(H2O)3(GeW10)2] (M = CeIII, NdIII, ErIII,
and ZrIV) and compared to the corresponding simulated
spectra, thereby showing the homogeneity of all bulk
compounds (Figure S9).
Besides XRD, all five compounds were characterized by
ATR-IR spectroscopy (Figure S1) showing the terminal W
O and bridging W−O−W vibrations typical for the Keggin-
type polyoxotungstate framework. The bands at 1630 and 2960
cm−1 are attributed to vibration and deformation bands of
tetrabutylammonium methylene groups.
The number of water molecules and crystal solvents in the
compounds TBA7H2[Ce(H2O)3(GeW10O35)2]·3(CH3)2CO
(Figure S5), TBA7H2[Nd(H2O)3(GeW10O35)2]·3(CH3)2CO
(Figure S6), TBA7H2[Er(H2O)3(GeW10O35)2]·3(CH3)2CO
(Figure S7), and TBA6H2[Zr(H2O)3(GeW10O35)2]·3-
(CH3)2CO (Figure S8) was determined using thermogravi-
metric analysis (TGA). All compounds exhibit, in general,
three weight-loss regions (Tables S3−S6) that are attributed to
losses of three water ligands, acetone and tetrabutylammonium
(TBA) molecules, respectively.
The first attempts to adapt the synthetic conditions used for
the lanthanide incorporating counterparts for the synthesis of
4d sandwiches using zirconium(IV) resulted in the formation
of precipitates which despite all our efforts could not be further
characterized. It has recently been shown that the reactive
peroxide ligand can disassemble Zr4 square tetramers and cubic
hexamers that dominate the solution phase chemistry of
zirconium(IV).35 Considering this regulatory effect of peroxo-
ligands, 10 equiv of H2O2 were added to a solution of 2 equiv
of Zr(acac)4 in acetone followed by the addition of
(TBA)[GeW10] (Scheme 1). Note that acetone/H2O2 mixtures
should be handled with care due to possible formation of acetone
peroxides. Upon addition of H2O2, an immediate color change
from colorless to yellow was observed indicating coordination
of the peroxo-ligands to the ZrIV ions. Filtration of the reaction
mixture after 90 min and addition of diethyl ether (acetone/
diethyl ether, 5:1) resulted in the formation of colorless block
shaped crystals of [Zr(H2O)3(GeW10)2]8− (CCDC 2010882)
in a yield of 65% based on W after approximately 2 weeks.
Structure. Single-crystal X-ray diffraction (SXRD) meas-
urements were performed on [Ce(H2O)3(GeW10)2]9− (Tables
S8, S9), [Nd(H2O)3(GeW10)2]9− (Tables S10, S11), [Er-
(H2O)3(GeW10)2]9− (Tables S12, S13), and [Zr-
(H2O)3(GeW10)2]8− (Tables S16, S17). All polyanions
crystallize in a monoclinic space group (C2/m for [Ce-
(H2O)3(GeW10)2]9−, [Nd(H2O)3(GeW10)2]9−, and [Zr-
(H2O)3(GeW10)2]8− and P21/c for [Er(H2O)3(GeW10)2]9−).
The polyanions are isostructural with idealized C2v point group
symmetry. The architecture of [M(H2O)3(GeW10)2] repre-
sents a polyanion composed of two [γ-GeW10O35]6− lacunary
units linked by two oxygen bridging ligands resulting in a
“pacmanlike” monolacunary dimer. Occupation of the vacant
site with the corresponding heterometal results in the
monosubstituted sandwich-type germanotungstate architec-
ture. In all compounds, the metal center exhibits a distorted
square antiprism coordination geometry with three H2O
ligands and M−O bond lengths ranging from 2.195(2) to
2.308(2) Å. The three water ligands can be easily exchanged
and are a prerequisite for hydrolytic activity (Figure 1). To the
best of our knowledge, [M(H2O)3(GeW10)2] represent the
The UV/vis spectra of all five polyanions are characterized
by an absorption maximum at 275 nm attributed to the pπ(Ob)
→ dπ*(W) ligand-to-metal charge-transfer typical for the
Keggin-type framework (Figures S10, S11).36
Hydrolytic Studies. When compared to other lanthanide
substituted POMs, the cerium containing representatives are
known to be the most active phosphoesterases due to their
higher Lewis acidity and sterically less hindered Lewis acid
sites.37 To test [Ce(H2O)3(GeW10)2]9− and [Zr-
(H2O)3(GeW10)2]8− toward their potential phosphoesterase
activity, NPP as a model substrate was applied. The hydrolytic
activity of [Ce(H2 O)3 (GeW1 0 )2 ]9 − and [Zr-
1
(H2O)3(GeW10)2]8− toward NPP was monitored using H
NMR measurements on reaction mixtures of the correspond-
ing POM (0.5 mM) and NPP (1 mM) in D2O at pD = 7.0 and
60 °C (Figures 2, S12) considering the high stability of the
phosphoester bond present in NPP (t1/2 = 135 days at 50 °C)
and to create reaction conditions which are comparable to
other literature reported reaction systems.39 The NMR spectra
taken after 45, 1200, 1800, 2680, 3790, 5460, and 5580 min of
incubation reveal a gradual disappearance of the doublets
corresponding to NPP, while two new doublets assigned to the
hydrolysis product nitrophenol (NP) are observed (Figure 2,
Scheme S1). The upfield shift of the NP peaks can be
explained by the hydroxyl group in NP (Figures 2, S12). Based
Figure 1. Crystal structure of [M(H2O)3(GeW10)2]n− (n = 9 for M =
CeIII, NdIII, ErIII; n = 8 for M = ZrIV) showing the polyhedral frontal
view of the anion with the three aqua ligands bound to the
corresponding metal. Blue and red spheres represent the M - and
oxygen ions, respectively. Orange for GeIV and yellow polyhedra for
{WO6}.
1
on the H NMR integration values, the percentage of NPP
after reaction with [Ce(H2O)3(GeW10)2]9− or [Zr-
(H2O)3(GeW10)2]8− was calculated. The natural logarithm of
C
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of [Zr(H2O)3(GeW10)2]8−, exhibiting both the highest water
solubility and phosphoesterase activity expressed by 24 times
the accelerated hydrolysis of the relatively inert phosphoester
bond present in NPP, were performed at room temperature
(25 °C) and physiological pH (125 mM Tris-HCl, pH 7)
(Scheme S2). A buffered reaction system was chosen
considering the in situ formation of the acidic product
dimethyl phosphate (DMP). The stepwise hydrolysis of
DMNP to nitrophenol was monitored with 31P NMR
spectroscopy by taking aliquots after 180, 1400, 1524 1614,
and 2940 min of reaction (Figure 3). A stepwise disappearance
1
Figure 2. H NMR spectra of the hydrolysis of 4-nitrophenylphos-
phate (NPP) [1 mM] to nitrophenol (NP) with [Zr-
(H2O)3(GeW10O35)2]8− [0.5 mM] in D2O at pD 7.0 and 60°.
the NPP concentration obtained from the integration values as
a function of time fitted to a first-order decay function gave a
rate constant of kobs = 3.70 ( 0.20) × 10−4 min−1 for
[Ce(H2O)3(GeW10)2]9− (Figure S13). To correct for the
autohydrolysis of NPP, reaction mixtures lacking the POM
were prepared and measured under the same reaction
conditions. The rate constant determined from these experi-
ments (kuncat = 2.60 ( 0.10) × 10−5 min−1) was subtracted
from the rate constant (kobs = 3.70 ( 0.20) × 10−4 min−1)
obtained from the experiments investigating [Ce-
(H2O)3(GeW10)2]9− to give the corrected constant (kcorr
)
with a value of 3.44 ( 0.30) × 10−4 min−1, which is almost an
order of magnitude acceleration as compared to the rate of
spontaneous hydrolysis of NPP and more than five times
higher than other monosubstituted Ln(III) Keggin-type POTs
like [Me2NH2]11[CeIII(PW11O39)2] (k = 6.72 × 10−5 min−1)
and K4[EuPW11O39] (k = 6.42 × 10−5 min−1), whereas a
c o m p a r a b l e v a l u e c a n b e o b s e r v e d f o r
[Me2NH2]10[CeIV(PW11O39)2] (k = 3.19 × 10−4 min−1)
under similar reaction conditions.19 The comparatively high
Lewis activity of [Ce(H2O)3(GeW10)2]9− can be explained by
the thermodynamically unfavored structural conversion of the
dimeric [CeIII/IV(PW11O39)2]10−/9− POM to the monomeric
[CeIII/IVPW11(H2O)2]4−/3− species, which is a crucial step in
order to be hydrolytically active18 (Scheme S3), whereas the
architecture of [Ce(H2O)3(GeW10)2]9− exhibits a freely
accessible Lewis acid metal center without structural
conversion (Figure 1). As for [Zr(H2O)3(GeW10)2]8−, a
corrected rate constant of kcorr = 5.36 ( 0.05) × 10−4 min−1 is
observed (Figure S15), which is comparable to other
monosubstituted ZrIV compounds such as K15H[Zr(α2-
P2W17O61)2] (kcorr = 7.56 × 10−4 min−1) under similar
conditions.38 This value is 1.5 times higher than the value
observed for [Ce(H2O)3(GeW10)2]9− and 3.4 times higher
than the corrected rate constant observed for the
[GeW10O36]8− precursor (kcorr = 1.59 × 10−4 min−1) (Figure
S16), which can be attributed to the higher Lewis acid activity
of ZrIV. The pD value of the reaction mixtures after reaction
was measured thereby remaining almost unchanged (pD 7.0,
start vs pD 6.8, end of experiment).
Figure 3. 31P NMR spectra of the hydrolysis of DMNP [4.2 mM] to
nitrophenol (NP) and dimethyl phosphate (DMP) with [Zr-
(H2O)3(GeW10O35)2]8− [2.5 mM] in Tris-HCl [125 mM]/D2O
(50%) at pD 7.0 and room temperature (25 °C).
of the singlet corresponding to the substrate DMNP and
gradual appearance of a singlet attributed to the hydrolysis side
product dimethyl phosphate (DMP) can be observed (Figure
3).28 The corrected rate constant after subtracting the rate
constant obtained from reaction mixtures lacking the POM
catalyst (kuncat = 4.01 ( 0.10) × 10−5 min−1) - under elsewise
identical reaction conditions considering autohydrolysis of the
substrate - gives a value of kcorr = 2.42 ( 0.10) × 10−4 min−1
(Figure S17) which is about seven times higher than the values
obtained for the uncatalyzed reaction (kuncat = 4.01 ( 0.10) ×
10−5 min−1) and control experiments containing the
[GeW10O36]8− precursor (kcorr = 3.37 ( 0.12) × 10−5
min−1) (Figure S18), respectively. Concentration dependent
experiments on the phosphoesterase activity of [Ce-
(H2O)3(GeW10)2]9− and [Zr(H2O)3(GeW10)2]8− toward
NPP and DMNP were carried out, and the substrate
conversion for a specific reaction time (45.6 h, 49 h, and 25
h) under varying catalyst concentrations (0.5 mM, 1.0 mM,
1.25 mM, 2.5 mM) was recorded and compared (Table S20).
Solution stability studies on [Ce(H2O)3(GeW10)2]9− and
[Zr(H2O)3(GeW10)2]8− after reaction with NPP and DMNP
were intended to be performed; however, due to the low
solubility and sensitivity of the 183W nucleus (14.3% natural
abundance), the 183W NMR measurements at conditions
pertinent to the catalytic reactions were not very informative,
which is a common problem encountered in POM chemistry.39
Considering the low solubility of the polyanion, the POM
concentrations used for the hydrolysis experiments were too
The removal of toxic organophosphorus (OP) nerve agents
and pesticides remains a significant and general goal which is
mainly achieved by hydrolytically transforming the toxic OP
compounds into nontoxic forms. Considering the structural
similarity between NPP and O,O-dimethyl O-(4-nitrophenyl)
phosphate (DMNP), which is an established OP ester nerve
simulant,25 hydrolytic studies on the degradation performance
D
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low to obtain reasonable amounts for consecutive postcatalytic
PXRD measurements. Hence, to prove stability of [Ce-
(H2O)3(GeW10)2]9− and [Zr(H2O)3(GeW10)2]8− indirectly,
the IR spectra of the polyanions were recorded after
precipitation with tetrabutylammonium bromide, thereby
clearly showing the characteristic W−O−W bridging and
terminal WO vibrations in the tungsten fingerprint area
from 300−1000 cm−1, which proves the solution stability of
the polyanions after reaction with NPP (Figures S2, S3) and
DMNP, respectively (Figure S4), and represents an established
method frequently used in POM chemistry.39 Subsequent
control experiments with CeCl3 and ZrOCl2 as nonshielded
free Lewis metal centers resulted in the formation of
precipitates or insoluble Zr(IV) hydroxyl polymeric gels,
(H2O)3(GeW10)2]9− and [Zr(H2O)3(GeW10)2]8− revealed
that [Zr(H2O)3(GeW10)2]8− is a highly active recyclable
catalyst for the Lewis acidic degradation of the nerve agent
simulant DMNP. Antibacterial studies toward M. catarrhalis
revealed that the highly negatively charged polyanion [Ce-
(H2O)3(GeW10)2]9− exhibits inhibitory properties (MIC = 32
μg/mL) in contrast to Ce(NO3)3 (MIC > 256 μg/mL). These
results exemplify the potential of the underinvestigated POM
reaction systems in organic media to apply lacunary POM
ligands for the successful encapsulation of Lewis acid metal
centers further enhancing their catalytic and antibacterial
activity which opens perspectives for the tailored design of
model compounds with biotechnologically relevant applica-
tions.
1
respectively, shown by the disappearance of the H NMR
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01852.
peaks even at low concentrations of the corresponding metal
center, which additionally proves the solution stability of the
POM compounds under reaction conditions (Figure
S14).40−42 The recyclability of [Zr(H2O)3(GeW10)2]8− as a
catalyst for the decontamination of DMNP was tested in a
consecutive experiment by reloading the reaction mixture with
DMNP substrate after 31P NMR measurements confirmed that
97% of DMNP was converted to the hydrolysis product DMP
(Figure S19). A direct comparison of the turnover frequency
(TOF) values obtained for [Zr(H2O)3(GeW10)2]8− in the first
and the second reaction cycle indicates no change in the
catalytic performance of the polyanion after reaction (TOF =
0.02 h−1) (Table S18).
Antibacterial Studies. The antibacterial activity of the
water-soluble polyanions [Ce(H2O)3(GeW10)2]9− and [Zr-
(H2O)3(GeW10)2]8− of the compound series was tested
against M. catarrhalis at physiological pH (Tables S21, S22).
The minimum inhibitory concentration (MIC) of [Ce-
(H2O)3(GeW10)2]9− is compared to that of Ce(NO3)3 and
the lacunary GeW10 unit. [Ce(H2O)3(GeW10)2]9− exhibits
better antibacterial activity (MIC value of 32 μg/mL)
compared to the lacunary building block GeW10 (MIC = 64
μg/mL), whereas Ce(NO3)3 shows no inhibition activity
(MIC > 256 μg/mL) (Table S21). As for [Zr-
(H2O)3(GeW10)2]8−, no difference in the antibacterial activity
between [Zr(H2O)3(GeW10)2]8− and ZrO(NO3)2 is observed
as both the [Zr(H2O)3(GeW10)2]8− polyanion and ZrO-
(NO3)2 exhibit an inhibitory effect (Table S22). It can be
concluded that the encapsulation of the Lewis acidic metal
centers using POM lacunary ligands thereby forming the
negatively charged sandwich-POM species led to enhanced
antibacterial properties of the otherwise inactive Ce3+ metal
center, whereas the activity observed for ZrO(NO3)2 can be
most likely explained by the strong oxidizing properties of the
salt itself.
■
sı
Details on synthesis, IR, TGA, XRD, UV−vis, and NMR
measurements (PDF)
Accession Codes
CCDC 1915316−1915318, 1915355, and 2010882 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing 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
■
̈
̈
̈
Annette Rompel − Fakultat fur Chemie, Institut fur
̈
Biophysikalische Chemie, Universitat Wien, 1090 Wien,
Austria; orcid.org/0000-0002-5919-0553;
Email: annette.rompel@univie.ac.at; www.bpc.univie.ac.at
Authors
Elias Tanuhadi − Fakultat fur Chemie, Institut fur
̈
̈
̈
̈
Biophysikalische Chemie, Universitat Wien, 1090 Wien, Austria
̈
̈
̈
Emir Al-Sayed − Fakultat fur Chemie, Institut fur
Biophysikalische Chemie, Universitat Wien, 1090 Wien, Austria
Alexander Roller − Fakultat fur Chemie, Zentrum fur
Rontgenstrukturanalyse, Universitat Wien, 1090 Wien, Austria
Hana Cipcic-Paljetak − Center for Translational and Clinical
Research, Croatian Center of Excellence for Reproductive and
Regenerative Medicine, School of Medicine, University of Zagreb,
10000 Zagreb, Croatia
̈
̈
̈
̈
̈
̈
̌
̌
́
Donatella Verbanac − Faculty of Pharmacy and Biochemistry,
University of Zagreb, 10000 Zagreb, Croatia
CONCLUSIONS
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01852
■
In conclusion, the combination of the (TBA)[GeW10]
precursor with lanthanides in organic medium afforded a
versatile synthetic route for the preparation of monosub-
stituted 4f-doped germanotungstates exemplified by the
successful incorporation of early (CeIII, NdIII), mid (GdIII),
and late (ErIII) lanthanides at room temperature. Addition of
hydrogen peroxide to the reaction system expands the
synthetic route to 4d-substituted germanotungstates shown
by the crystallization and characterization of the zirconium(IV)
containing counterpart. Systematic studies on the phosphoes-
terase activity of the water-soluble polyanions [Ce-
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Nuclear magnetic resonance (NMR) measurements were
performed by Ricarda Bugl and Sabine Schneider at the
NMR center, Faculty of Chemistry, University of Vienna. The
authors wish to thank Dr. Ioannis Kampatsikas, Nadiia
Gumerova, Ph.D., Dipl.-Ing. Matthias Pretzler, and Dr. Joscha
E
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
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Breibeck for valuable discussions concerning this work and
Ass.-Prof. Dr. Peter Unfried for TGA measurements. We
gratefully acknowledge the Austrian Science Fund FWF
(P27534 and P33089) as well as the University of Vienna
for financial support. E.T. and A.R. acknowledge the University
of Vienna for awarding an Uni:docs fellowship to E.T. Bilateral
financial funding is provided by the Centre for International
Cooperation & Mobility (ICM) of the Austrian Agency for
International Cooperation in Education and Research (OeAD-
GmbH) Project No. HR 06/2020 to A.R. and the Project
“Biological profiling of polyoxometalates” to H.C.P.
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