22-Isopolytungstate fragment [H2W22O 74]14 coordinated to lanthanide ions

Article abstract of DOI:10.1021/ic801946m

The novel polyanions [Ln₂(H₂O)₁₀W ₁₁O₇₁(OH)₂]^8- (Ln = La (1), Ce (2), Tb (3), Dy (4), Ho (5), Er (6), Tm (7), Yb (8), Lu (9), Y (10)) have been synthesized by reaction of WO

Full text of DOI:10.1021/ic801946m

Inorg. Chem. 2009, 48, 1559-1565
22-Isopolytungstate Fragment [H2W22O74]14-Coordinated to Lanthanide
Ions
Amal H. Ismail, Michael H. Dickman, and Ulrich Kortz*
School of Engineering and Science, Jacobs UniVersity, P.O. Box 750 561, 28725 Bremen, Germany
Received October 14, 2008
The novel polyanions [Ln2(H2O)10W22O71(OH)2]8- (Ln ) La (1), Ce (2), Tb (3), Dy (4), Ho (5), Er (6), Tm (7), Yb
(8), Lu (9), Y (10)) have been synthesized by reaction of WO42- and lanthanide ions in acidic aqueous medium.
The low symmetry (Ci) polyanion family [Ln2(H2O)10W22O71(OH)2]8- consists of the isopolyanion [H2W22O74]14- and
two {Ln(H2O)5}3+ supporting units. The [H2W22O74]14- cluster, which consists of two undecatungstate {W11} fragments,
acts as a tridentate ligand to two Ln3+ ions. Polyanions 1-10 are isostructural, and the coordination number of the
lanthanide ions correlates with their sizes. All compounds have been fully characterized in the solid state by Fourier
transform infrared spectroscopy, single-crystal X-ray diffraction, thermogravimetric analysis, and elemental analysis.
¯
Single-crystal X-ray diffraction analyses show that 1-10 crystallize as sodium salts in the triclinic space group P1.
Introduction
Although the first POM was synthesized already in 1826
by Berzelius,1a the mechanism of POM formation is still not
completely understood and is often described as a self-
assembly. POMs can usually be isolated from aqueous
solution as solid salts with appropriate counter cations which
may be alkali metal cations (e.g., Li+, Na+, K+, Rb+, Cs+)
or organic cations (e.g., guanidinium, alkylamines). Two
main types of POMs are known, based on their chemical
composition: isopoly and heteropoly anions. Isopolyanions
are represented by the general formula [MmOy]n-, where M
is the addendum atom, usually molybdenum or tungsten and
less frequently vanadium, niobium, or tantalum in their higher
oxidation states. Heteropolyanions are represented with the
general formula [XxMmOy]q- (x e m) where X, the hetero-
atom, can be one of many elements of the periodic table,
most commonly P5+, As5+, Si4+, Ge4+, and B3+.1
Polyoxometalates (POMs) represent a large class of
nanosized metal-oxo anions. POMs are remarkable not only
in terms of molecular and electronic structural versatility but
also because of their reactivity and relevance in fields such
as photochemistry, analytical chemistry, clinical chemistry,
magnetism, catalysis, biology, medicine, and materials sci-
ence.1 Polyoxoanions which result from the condensation
of metalate anions in acidified solutions (usually aqueous)
can efficiently absorb light in the near UV-vis region and
can therefore be used as photocatalysts.2 The intrinsic
chemical properties of POMs as electron acceptors or as very
strong Brønsted acids (in their protonated form) make them
useful as catalysts in organic transformations, and their
abilities to form peroxo complexes or to serve as supports
for catalytically active metal ions in high oxidation states
are particularly useful in oxidation reactions.1b,3
Lanthanide-containing POMs have been investigated less
than their 3d-transition-metal-substituted analogues. Because
of the larger size of the lanthanide ions compared to 3d
metals, they are not fully incorporated into the lacunary
site(s) of vacant POM precursors. Because of their higher
* To whom correspondence should be addressed. E-mail: u.kortz@
jacobs-university.de. Fax: +49-(0)421-200-3229.
(1) (a) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag:
Berlin, 1983. (b) Pope, M. T.; Mu¨ller, A. Angew. Chem., Int. Ed. Engl.
1991, 30, 34. (c) Polyoxometalates: From Platonic Solids to Anti-
RetroViral ActiVity; Pope, M. T., Mu¨ller, A., Eds.; Kluwer: Dordrecht,
The Netherlands, 1994. (d) Mu¨ller, A.; Reuter, H.; Dillinger, S. Angew.
Chem., Int. Ed. Engl. 1995, 34, 2328. (e) Hill, C. Polyoxometalates;
Chem. ReV. 1998 (special thematic issue on polyoxometalates). (f)
Polyoxometalate Chemistry: From Topology Via Self-Assembly to
Applications; Pope, M. T.; Mu¨ller, A., Eds.; Kluwer: Dordrecht, The
Netherlands, 2001. (g) Polyoxometalate Chemistry for Nano-Composite
Design; Yamase, T., Pope, M. T., Eds.; Kluwer: Dordrecht, The
Netherlands, 2002. (h) Choppin, G. R. J. Nucl. Radiochem. Sci. 2005,
6, 1.
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10.1021/ic801946m CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 4, 2009 1559
Published on Web 12/30/2008

Ismail et al.
coordination numbers, each lanthanide ion can be used as a
linker to one or more other lacunary POM units. This
approach has resulted in polymeric or unusually large
molecular POM assemblies.4 Also, lanthanide-containing
POMs can be of interest because of photoluminescence, as
well as catalytic, electrochemical, and magnetic properties.5
Our group has also been working on lanthanide-containing
heteropolytungstates. We have reported the ytterbium-
containing tungstoarsenate [YbAs2W20O68(H2O)3]7- resulting
from the interaction of the monolacunary [As2W20O68-
(H2O)]10- with Yb3+ ions in acidic aqueous medium. The
polyanion consists of two (R-AsIIIW9O33) fragments con-
nected by a V-shaped (H2O)Yb(OW(H2O))2 fragment.4j The
monolanthanide-containing polyanion family [Ln(ꢀ2-
SiW11O39)2]13- (Ln ) La, Ce, Sm, Eu, Gd, Tb, Yb, Lu) has
also been synthesized and structurally characterized. These
polyanions are composed of two chiral (ꢀ2-SiW11O39) units
sandwiching the Ln3+ ion.4s Recently, we reported the 20
[Ln(W5O18)2]n-.4a,6 This polyanion is now known for Ln3+
) La, Ce, Pr, Nd, Sm, Ho, Yb, and Y, and also for Ce4+, all
synthesized by reaction of the lanthanide ions with Na2WO4
in hot aqueous (pH 6.5-7.5) solution. The structure of the
D4d [Ln(W5O18)2]n- polyanions consist of two monolacunary
Lindqvist based fragments [W5O18]6- encapsulating a central
metal ion exhibiting a square-antiprismatic coordination.
Yamase et al. reported the crystal structures of the Pr, Nd,
Dy, Sm, Eu, Gd, and Tb derivatives with different types of
alkali counter cations.7 The lanthanum analogue
[La(W5O18)2]9- was reported in 2005,8 and also the actinide
analogues [Th(W5O18)2]8- and [U(W5O18)2]8- have been
studied crystallographically.5b,9 Very recently our group
reported on the synthesis and solid state structure of the
yttrium-derivative [YW10O36]9-, as well as its solution
properties by 183W and 89Y NMR.10 Very recently, Cao and
co-workers reported on a pentadecatungstate ring capped by
two Ce3+ ions.11
cerium containing 100-tungsto-10-germanate [Ce20Ge10W100
-
The isopolyanion family [LnIIIW10O36]9- (Ln ) Pr, Nd,
Sm, Eu, Tb, Dy) has shown high luminescence quantum
efficiency.12 The decatungstoterbate [Tb(W5O18)2]9- showed
green-emissive luminescence.7h Furthermore, Griffith
and co-workers studied the lanthanoisopolytungstates
[LnIIIW10O36]9- (Ln ) Y, La, Ce, Pr, Sm, Eu, Gd, Dy, Er,
Lu) as calalytic oxidants with H2O2 for alcohol oxidations
and alkene epoxidations.5b This provides an impetus to
prepare other, novel isopolyanion structures coordinated to
lanthanide ions.
O376(OH)4(H2O)30]56- which is the third largest molecular
polytungstate known to date.4t We synthesized this polyanion
by reaction of the trilacunary POM precursor
[R-GeW9O34]10- with Ce3+ ions in acidic aqueous medium.
Until very recently, only one family of lanthanide-
containing isopolyanions had been reported. Peacock and
Weakley were the first to describe the sandwich-type
decatungstate
family
of
the
general
formula
Herein we report on the synthesis of a novel class of 22-
isopolytungstates {W22} coordinated to two external lan-
thanide ions, [Ln2(H2O)10W22O71(OH)2]8- (Ln3+ ) La (1),
Ce (2), Tb (3), Dy (4), Ho (5), Er (6), Tm (7), Yb (8), Lu
(9)) and yttrium, [Y2(H2O)10W22O71(OH)2]8- (10).
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Dickman, M. H.; Gama, G. J.; Kim, K-C.; Pope, M. T. J. Cluster Sci.
1996, 7, 567. (c) Wassermann, K.; Dickman, M. H.; Pope, M. T.
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W. P.; Morley-Smith, N.; Nogueira, H. I. S.; Shoair, A. G. F.;
Suriaatmaja, M.; White, A. J. P.; Williams, D. J. J. Organomet. Chem.
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Chen, W.; Li, Y.; Wang, Y.; Wang, E.; Su, Z. J. Chem. Soc., Dalton
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A.; van Slageren, J.; La¨ge, M.; Krebs, B. J. Cluster Sci. 2007, 16,
711.
Experimental Section
Synthesis. We used all reagent-grade chemicals as purchased
without further purification.
Na2La2[La2(H2O)10W22O72(OH)2]·44H2O (NaLa-1). A sample
of 10.00 g of Na2WO4 ·2H2O (30.3 mmol) was dissolved in 20 mL
H2O, followed by dropwise addition of a solution of 1.02 g of
LaCl3 ·7H2O (2.8 mmol) in 5 mL of H2O. Then 2.5 mL of 12 M
(6) Iball, J.; Low, J. N.; Weakley, T. J. R. J. Chem. Soc., Dalton Trans.
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1994, C50, 1849. (h) Ozeki, T.; Takahashi, M.; Yamase, T. Acta
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J.; Nogueira, H. I. S. Acta Crystallogr. 2005, E61, 1370.
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1560 Inorganic Chemistry, Vol. 48, No. 4, 2009

[H2W22O74]14- Coordinated to Lanthanide Ions
HCl was added dropwise to the resulting solution (local formation
and redissolution of hydrated tungsten oxide and lanthanum
hydroxide was observed), and then the pH was adjusted to 2.2 using
4 M aqueous HCl. The mixture was heated to 90 °C for 1 h with
vigorous stirring. A white precipitate, which had formed, was
filtered off and discarded. After cooling to room temperature, the
filtrate was kept in an open vial for crystallization. Colorless crystals
were obtained after several days, which were filtered off and air-
dried. Although additional product continued to form subsequently,
it contained NaCl impurities. Yield: 0.87 g (9.2%). The compound
NaLa-1 could also be synthesized in higher yield under analogous
reaction conditions by heating to 45 °C instead of 90 °C. Yield:
1.51 g (16%). IR for NaLa-1 in cm-1 (only between 400-1000
cm-1): 943(m), 868(sh), 820(s), 711(s), 513(w), 419(w). Anal. Calcd
(%) for NaLa-1: Na, 0.7; W, 59.1; La, 8.1. Found (%): Na, 1.1;
W, 59.7; La, 7.7.
and a heating rate of 5 °/min. Elemental analyses were carried out
by Eurofins Umwelt West GmbH, Wesseling, Germany.
X-ray Crystallography. All crystals were mounted in Hampton
cryoloops using light oil for data collection at low temperature.
Indexing and data collection were performed using a Bruker X8
APEX II CCD diffractometer with kappa geometry and Mo KR
radiation (λ ) 0.71073 Å). Data integration and routine processing
were performed using the SAINT software suite. Further data
processing, including multiscans absorption corrections, was per-
formed using SADABS.13 Direct method (SHELXS97) solutions
successfully located the W atoms, and successive Fourier syntheses
(SHELXL97) revealed the remaining atoms.14 Refinements were
full-matrix least-squares against F2 using all data. Some lanthanides,
cations, and waters of hydration were modeled with varying degrees
of occupancy, a common situation for polyoxotungstate structures.
For NaTb-3, NaHo-5, and NaLu-9, disordered lanthanide counter
cations were found in addition to the nondisordered Ln3+ ions
bridging the {W22} polyanion. In the final refinements, all non-
disordered heavy atoms (W, Ln) were refined anisotropically, while
the O and Na atoms and some disordered lanthanides were refined
isotropically. No H atoms were included in the models. The
crystallographic data are provided in Table 1.
Na2Ce2[Ce2(H2O)10W22O72(OH)2]·44H2O (NaCe-2). A sample
of 10.00 g of Na2WO4 ·2H2O (30.3 mmol) was dissolved in 20 mL
of H2O followed by dropwise addition of 2.5 mL of 12 M HCl
(local formation and redissolution of hydrated tungsten oxide was
observed). Then a solution of 1.03 g of CeCl3 ·7H2O (2.8 mmol)
in 5 mL of H2O was added immediately and dropwise. Finally, the
pH was adjusted to 2.2 using 4 M aqueous HCl. The mixture was
vigorously stirred at room temperature for 1 h. A yellow precipitate,
which had formed, was filtered off and discarded. The filtrate was
kept in an open vial for crystallization at room temperature. After
a few hours, a yet unidentified polycrystalline material started to
form and was filtered off successively until yellow crystals of
NaCe-2 started to appear. These crystals were in turn filtered off
and air-dried. Yield: 0.63 g (3.8%). Although additional product
continued to form subsequently, it contained NaCl impurities. IR
Results and Discussion
The ten novel lanthanide containing isopolytungstates
[Ln2(H2O)10W22O71(OH)2]8- (Ln ) La (1), Ce (2), Tb (3),
Dy (4), Ho (5), Er (6), Tm (7), Yb (8), Lu (9), Y (10)) have
been prepared by reaction of WO42- and lanthanide ions in
a ratio of 11:1 in aqueous medium at pH 2.2. We discovered
that three parameters are crucial for the successful formation
of pure salts of 1-10: reaction temperature, concentration
of reagents, and pH of the solution. For example, the
lanthanum analogue 1 forms only if the solution is heated.
If the synthesis is performed at room temperature, a different
product is formed as based on IR which has not been fully
characterized yet but appears to be {W56} based on prelimi-
nary crystal structures (vide infra). On the other hand, the
cerium analogue 2 forms only at room temperature and is
isolated as a second batch using fractional crystallization.
The first crop of crystals appears to be identical with the
room temperature product of the La synthesis described
above, as based on IR. If the synthesis solution of 2 is heated,
the well-known metatungstate ion [H2W12O40]6- is formed.
Polyanions 3-10 also form upon heating, but the yield is
much higher at room temperature. The yield is also affected
by the concentration of the starting material. A tungstate
concentration as high as 1.5 M is needed to obtain a pure
crystalline product. Lower concentrations result in lower
yields and non-crystalline white powder as the main product.
It is interesting that formation of a considerable amount of
unidentified precipitate was also detected during the optimal
synthetic procedure, which partially accounts for the low
yields observed. This precipitate had an IR signature different
from 1-10. Finally, we discovered that the pH of the reaction
solution affects the formation and yields of compounds 1-10.
A pH window of 2.0-3.5 can be considered for a successful
for NaCe-2: 944(m), 870(sh), 820(s), 721(s), 514(w), 419(w) cm-1
.
Anal. Calcd (%) for NaCe-2: Na, 0.7; W, 59.1; Ce, 8.2. Found
(%): Na, 0.9; W, 58.6; Ce, 8.0.
Na5Tb[Tb2(H2O)10W22O72(OH)2]·41H2O (NaTb-3). A sample
of 10.00 g of Na2WO4 ·2H2O (30.3 mmol) was dissolved in 20 mL
of H2O, and then 2.5 mL of 12 M HCl was added dropwise. A
sample of 0.93 g of Tb(CH3COO)3 ·H2O (2.8 mmol) in 5 mL of
H2O was added dropwise to the acidified tungstate solution, and
then the pH was adjusted to 2.2 using 4 M aqueous HCl. The
mixture was vigorously stirred at room temperature for 1 h, and
then filtered and allowed to crystallize at room temperature.
Colorless crystals were obtained after several days, which were
filtered off and air-dried. Yield: 0.65 g (7.0%). Although additional
product continued to form subsequently, it contained NaCl impuri-
ties. IR for NaTb-3: 944(m), 870(sh), 820(s), 721(s), 514(w),
419(w) cm-1. Anal. Calcd (%) for NaTb-2: Na, 1.7; W, 60.0; Tb,
7.1. Found (%): Na, 1.8; W, 58.9; Tb, 6.6.
Na8[Dy2(H2O)10W22O72(OH)2]·49H2O (Na-4), Na5Ho[Ho2(H2O)10-
W22O72(OH)2]·45H2O (NaHo-5), Na8[Er2(H2O)10W22O72(OH)2]·
44H2O (Na-6), Na8[Tm2(H2O)10W22O72(OH)2]·41H2O (Na-7),
Na8[Yb2(H2O)10W22O72(OH)2]·46H2O (Na-8), Na5Lu[Lu2(H2O)10-
W22O72(OH)2]·44H2O (NaLu-9), Na8[Y2(H2O)10W22O72(OH)2]·
46H2O (Na-10). Compounds Na-4 through Na-10 were synthesized
using similar procedures (see Supporting Information).
In this paper, “Ln” will be used as an abbreviation for the
lanthanide ions La3+, Ce3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, Yb3+
,
Lu3+, as well as for the yttrium ion Y3+, since the ionic radius of
the latter falls in the range of lanthanides.
Instrumentation. Infrared (IR) spectra were recorded on KBr
pellets using a Nicolet Avatar 370 spectrometer. Thermogravimetric
analyses (TGA) were performed using a TA Instruments SDT Q600
instrument between 20-900 °C with a 100 mL/min flow of nitrogen
(13) Sheldrick, G. M. SADABS; University of Go¨ttingen: Go¨ttingen,
Germany, 1996.
(14) Sheldrick, G. M. SHELXS-97, Program for Solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
Inorganic Chemistry, Vol. 48, No. 4, 2009 1561

Ismail et al.
Figure 1. Formation scheme of two different isopoly dodecatungstates
(left), and two lanthanide containing isopolytungstates (right) as a function
of pH. The color code is as follows: W (black); O (red); Ln (blue); H2O
(pink).
synthetic procedure, with a pH of 2.2 being optimal. A pH
lower than 2.0 did not result in any product, and a pH higher
than 3.5 produced paradodecatungstate ([H2W12O42]10-) as
the main product.
The synthetic procedures of 2-10 are very similar to
Peacock and Weakley′s [Ln(W5O18)2]n- and only differ in
the pH of the reaction.4a These authors worked at pH ∼ 7,
whereas we used pH ∼ 2. It is well-known that pH is a
crucial parameter in POM synthesis in general. For tung-
states, it is also known that paratungstate, [H2W12O42]10-
,
dominates in weakly acid solution, and metatungstate,
[H2W12O40]6-, in strongly acidic solution.1a Continuous
acidification leads eventually to the tungsten oxide hydrate,
WO3 · 2H2O.15
Figure 1 shows the formation of different polytungstate
structures in the absence and presence of lanthanide ions.
Peacock and Weakley were able to isolate the sandwich-
type, lanthanide-containing decatungstate [Ln(W5O18)2]n-
{LnW10} in a pH range where paratungstate is the major
product if no lanthanide ions are present. The pH range of
formation for 1-10 falls in the same range of formation for
metatungstate, if no lanthanide ions are present. Therefore,
the presence of lanthanide ions in solution is essential for
the isolation of {LnW10} at pH 5-7 and 1-10 at pH ∼2.
We discovered experimentally that the presence of the
lanthanide ions is needed at a very early stage in the synthesis
of 2-10. If the preacidified tungstate solution is left alone
for too long before addition of the lanthanide solution, no
lanthanide containing polyanion is formed but rather the
metatungstate ion [H2W12O40]6-. For the synthesis of 1 the
situation is even more critical, as the presence of La3+ ions
(15) Fuchs, J.; Hartl, H.; Schiller, W. Angew. Chem., Int. Ed. Engl. 1973,
12, 420.
1562 Inorganic Chemistry, Vol. 48, No. 4, 2009

[H2W22O74]14- Coordinated to Lanthanide Ions
ing coordination sphere is filled by five aqua ligands, giving
a total coordination number of eight for each metal ion
resulting in idealized square-antiprismatic geometry (see
Figure 2). For the lanthanum and cerium derivatives, 1 and
2, an extra coordination site was found due to the larger size
of the early lanthanides, hence leading to a coordination
number of nine and idealized monocapped antiprismatic
geometry (see Figure 2). This extra coordination site has
important consequences with respect to the solid state
structures of 1 and 2. We observed an extra µ2-oxo bridge
between the lanthanum/cerium ions of one polyanion to a
tungsten center of an adjacent polyanion, resulting in two
intermolecular bridges and an overall 1D chain arrangement
of the polyanions in the solid state (see Figure 3). Average
Ln-O bond lengths were calculated for 1-10 and are shown
in Table 2. As expected, a difference of about 0.2 Å was
observed between the Ln-O bonds of the early (La and Ce)
and late (Tb-Lu) lanthanides. For yttrium, the average Y-O
bond lengths fall in the range of the late lanthanides.
The undecatungstate subunit {W11}, two of which com-
prise the {W22} fragment in 1-10, can be considered an
assembly of four distinct “building blocks”, see Figure 4:
- One tetratungstate {W4} fragment (light blue in Figure
4) formed from four edge-shared WO6 octahedra connected
via a µ4-oxo bridge. This tetratungstate fragment can be
viewed as a dilacunary form of the Lindqvist structure.1
- One tritungstate {W3} fragment (green in Figure 4)
formed from three edge-shared WO6 octahedra connected
via a µ3-hydroxo bridge (vide infra). This fragment can be
viewed as a single “triad” of the well-known Keggin or
Wells-Dawson ions.1
Figure 2. Polyhedral representation of [Ln2(H2O)10W22O72(OH)2]8- (1-2)
(left) and [Ln2(H2O)10W22O72(OH)2]8- (3-10) (right). The structures of both
polyanions are virtually identical, except the coordination number (8 vs 9)
of the lanthanide ions (see text for details). The color code is as follows:
WO6 octahedra (light blue, green, red); Ln (blue); H2O (pink), O (red).
The WO6 octahedra were colored differently for clarity. See also text and
Figure 4.
was essential even before the first acidification step. An
additional practical complication of this situation is that
because of the formation of hydroxides the solution turns
rather cloudy making it difficult to detect the redissolution
of tungsten oxide which is formed while adding HClaq. The
synthesis procedure was more straightforward for the late
lanthanides Tb-Lu (3-9), and for the early 4d metal Y (10).
When applying such synthesis procedure to the early
lanthanides, then different products are obtained. Preliminary
X-ray diffraction (XRD) analysis revealed the novel, lan-
thanide-containing 56-tungsten isopolyanion {W56} family
{[Ln2(H2O)11W28O93(OH)2]14-}2, Ln ) La-Gd. So far, we
have obtained single-crystal XRD analyses for the lanthanum
and cerium derivatives.16 After successive filtrations, another
crystalline product is formed which turned out to be
{Ln2W22} based on IR and single crystal XRD. Polyanions
1-10 are isostructural and crystallize as hydrated sodium
- Two ditungstate {W2} fragments (red in Figure 4), each
formed from two edge-shared WO6 octahedra, which are
corner-sharing on one side.
This undecatungstate fragment {W11} was first reported
by Fuchs in 1988.17 Here we report on the lanthanide capped
22-isopolytungstate {W22}, which represents two {W11}
fragments fused via two W-O-W′ bridges (see Figure 4).
Recently Cronin et al. reported on {W36} which is a cyclic
trimer of {W11} linked via three extra WO6 bridges and
stabilized by a central potassium ion.18 Figure 5 summarizes
the three isopolytungstate structures {W11}, {W36}, and
{W22} and the respective formation conditions. The {W22}
isopolytungstate as such offers two tridentate ligand sites,
which are coordinated to two Ln3+ centers in 1-10. To the
best of our knowledge, polyanions 1-10 represent the largest
lanthanide-containing isopolytungstates known to date.
We also tried to synthesize the lanthanide-free isopolya-
nion [H2W22O74]14- by following the same synthetic proce-
dure as 1-10 but in the absence of lanthanides. We were
able to isolate a solid compound with an IR spectrum very
similar to that of 1-10 (Supporting Information, Figure S2)
and different from paratungstate and metatungstate (Sup-
porting Information, Figure S3).
j
salts in the common triclinic space group P1. This is reflected
in their virtually identical IR spectra (see Supporting
Information, Figure S1). Single-crystal XRD on the salts of
polyanions La (1), Ce (2), Tb (3), Ho (5), and Lu (9) showed
extra lanthanides as counter cations. These results were also
confirmed by elemental analysis. The structure of polyanions
1-10 comprises the [H2W22O74]14- isopolyanion coordinated
to two {Ln(H2O)5}3+ supporting ions. This 22-isopolytung-
state consists of two undecatungstate units {W11} fused by
two corner sharing W-O-W bridges (see Figure 2). Hence
this structure can be described as a dimeric molecular entity
composed of two {LnW11} half-units related by an inversion
center, with point group symmetry Ci. Each Ln3+ is linked
to the {W22} unit through three µ2-oxo bridges, two bridges
to one {W11} half-unit and one bridge to the other half-unit.
For the smaller lanthanides and yttrium (3-10), the remain-
(16) The cerium derivative {[Ce2(H2O)11W28O93(OH)2]14-}2 crystallizes in
j
the triclinic system, space group P1, with a ) 21.629(2) Å, b )
26.869(3) Å, c ) 31.611(3) Å, R ) 80.463(3)°, ꢀ ) 72.936(4)°, γ )
75.999(4)°, V ) 16951.2 Å3, and Z ) 2. The full characterization
will be published soon.
(17) Lehmann, T.; Fuchs, J. Z. Naturforsch. B 1988, 43, 89.
(18) Long, D-L.; Abbas, H.; Ko¨gerler, P.; Cronin, L. J. Am. Chem. Soc.
2004, 126, 13880.
Inorganic Chemistry, Vol. 48, No. 4, 2009 1563

Ismail et al.
Figure 3. Polyhedral representation of polyanions 1 and 2 forming 1D chains in the solid state. The color code is as follows: WO6 octahedra (red); La or
Ce (blue); H2O (pink), O (red).
Table 2. Average Ln-O Bond Lengths (Å) [Ln2(H2O)10W22O72(OH)2]8-
(La (1), Ce (2), Tb (3) Dy (4), Ho (5), Er (6), Tm (7), Yb (8), Lu (9), Y
(10))
Ln
average Ln-O(W)
average Ln-OH2
La (1)
Ce (2)
Tb (3)
Dy (4)
Ho (5)
Er (6)
Tm (7)
Yb (8)
Lu (9)
Y (10)
2.551(15)
2.523(11)
2.324(13)
2.382(10)
2.339(11)
2.36(3)
2.310(11)
2.302(8)
2.301(11)
2.339(7)
2.562(18)
2.533(12)
2.337(15)
2.388(12)
2.388(12)
2.33(4)
2.350(11)
2.344(8)
2.328(12)
2.361(7)
Bond valence sum (BVS) calculations for 1-10 confirmed
that all tungsten atoms are in the +6 oxidation state and that
all lanthanide ions and yttrium are in the +3 oxidation state.19
The main purpose of our BVS studies on 1-10 was indeed
to check all polyanion oxygens for possible protonation. As
stated above, one monoprotonated oxygen was identified in
the asymmetric half-unit, namely the µ3-oxo bridge of the
{W3} fragment connecting the three tungsten atoms W5, W6,
and W7 (see Supporting Information, Figure S4). This results
in two OH groups for the dimeric form and a total charge of
-8 for all polyanions 1-10. As usual, terminal W)O
oxygen BVS values are low (∼1.5) but this is typical for
the distorted W)O geometry and does not indicate additional
protonation.
Figure 4. Polyhedral representation of the {W22} unit of 1-10 which is
composed of two {W11} half-units. The latter are composed of di-, tri- and
tetrameric tungsten-oxo building blocks shown in different color: {W4} (light
blue), {W3} (green), {W2} (red).
Polyanions 1-10 represent the first examples of lan-
thanide-containing isopolyanions with “exposed” lanthanide
centers, bearing aqua ligands, which are expected to be labile.
These water molecules represent good candidates for ligand
substitution by other mono- or polydentate ligands, including
chiral ones. The resulting chiral polyanions could be interest-
ing for catalytic applications. Also, lipophilic alkyl am-
monium salts of 1-10 could be useful for homogeneous
catalysis applications in organic media. We plan to perform
such studies in the near future.
hydration and the thermal stability. The TGA graphs of all
ten compounds showed a region of weight loss due to
dehydration (see Supporting Information, Figures S5-S14).
This region corresponds to a two-step weight loss in the range
of ∼25-200 °C and ∼200-400 °C for the salts of
derivatives 3-10. The first step is attributed to the release
of all lattice water molecules, and the second step to the
release of the coordinated water molecules, corresponding
approximately to the calculated 10 waters per molecular
formula. For the La and Ce derivatives 1 and 2, this region
exhibits a one-step only weight loss in the range ∼25-400
°C making the steps for the loss of crystal and coordinated
water molecules indistinguishable. This reflects nicely the
Thermogravimetric analyses (TGA) were performed on
the salts of 1-10 to determine the respective degree of
(19) Altermatt, D.; Brown, I. D. Acta Crystallogr. 1985, B41, 244.
1564 Inorganic Chemistry, Vol. 48, No. 4, 2009

[H2W22O74]14- Coordinated to Lanthanide Ions
We speculate that Li+ ion exchange may have stripped the
Lu3+ and Y3+ from the polyanion framework, thus initiating
a decomposition pathway resulting in various products. We
also performed 183W NMR studies on Fuchs′ original {W11},
but we observed too many signals which we attributed to a
mixture of [H2W12O42]10- and {W11}.
Conclusions
We have successfully synthesized and structurally char-
acterized by IR spectroscopy, TGA, and single crystal X-ray
diffraction 10 novel isopolyoxotungstate anions containing
lanthanides: [Ln2(H2O)10W22O72(OH)2]8- (Ln3+ ) La (1), Ce
(2), Tb (3), Dy (4), Ho (5), Er (6), Tm (7), Yb (8), Lu (9),
Y (10)). These polyanions were synthesized in aqueous acidic
medium starting from sodium tungstate and Ln3+ salts.
Polyanions 1-10 were isolated as sodium or mixed sodium/
lanthanide salts, and they all crystallized in the triclinic space
j
group P1. Single crystal X-ray structural analysis showed
that all polyanions are isostructural and are composed of a
22-tungsten isopolyanion unit {W22} coordinated to two
lanthanide ions bearing terminal aqua ligands. The 22-
tungstate unit {W22} is assembled from two {W11} subunits
fused via two corner- sharing W-O-W bridges. The
undecatungstate subunit has been reported before in a
monomeric and trimeric form. Possible applications of
polyanions 1-10 in the field of homogeneous catalysis may
prove possible if pure lipophilic organic salts can be prepared.
Chiral derivatives of 1-10 can be envisioned by replacing
the terminal water ligands on the lanthanide centers by chiral
organic ligands. This will be the scope of further studies.
Figure 5. Polyhedral/ball-and-stick representation of isopolytungstates
containing the {W11} unit. The color code is the same as in Figure 4:
tungsten (black); potassium (gray), oxygen (red).
different solid state packing of 1 and 2, compared to 3-10
(vide supra). The degree of hydration was calculated for all
compounds and gave a range of 41-49 water molecules per
formula unit. These results were also supported by elemental
analysis. Furthermore, the absence of any additional weight
loss indicated that all 10 compounds, after loss of the water
molecules, were thermally stable up to 800-900 °C (see the
Supporting Information).
Acknowledgment. U.K. thanks Jacobs University and the
Fonds der Chemischen Industrie for research support. A.I.
appreciates the grant from the Ministry of Higher Education
of the Arab Republic of Egypt, allowing her to pursue her
Ph.D. studies in the laboratory of U.K. in Bremen. A.I. deeply
thanks her colleagues Dr. Bassem Bassil and Dr. Sib Sankar
Mal for their help with writing this manuscript and perform-
ing NMR measurements, respectively. Figures 1-5 were
generated by Diamond Version 3.1f (copyright Crystal
We tried very hard to obtain solution 183W NMR spectra
of the diamagnetic derivatives 9 and 10 (Lu and Y analogues)
but without success. The same also applies for 89Y NMR.
Both compounds were not very soluble. Although heating
to ∼80 °C eventually led to partial dissolution, 183W NMR
of these solutions resulted in a single peak for metatungstate
([H2W12O40]6-) indicating decomposition of 9 and 10 at this
temperature. The 89Y spectrum of polyanion 10, when
dissolved upon heating, showed a signal typical for free Y3+
ions. We also tried cation exchange using a Li+ loaded resin,
which led eventually to complete dissolution of the polyanion
salts. However, the 183W NMR spectra for both 9 and 10
were not conclusive and showed a large number (>11) of
peaks probably arising from a mixture of species in solution.
Impact GbR). Meanwhile, the Cronin group (Glasgow) has
20
isolated a {W22} ion with the formula [H4W22O74]12-
.
Note Added after ASAP Publication. After ASAP
publication, ref 20 and related text in the Acknowledgment
section was added. This article was published ASAP on
December 30, 2008. The corrected article was published
ASAP on January 12, 2009.
Supporting Information Available: Further details are given
in Figures S1-S14. This material is available free of charge via
the Internet at http://pubs.acs.org.
(20) Miras, H. N.; Yan, J.; Long, D.-L.; Cronin, L. Angew. Chem., Int.
Ed. 2008, 47, 8420.
IC801946M
Inorganic Chemistry, Vol. 48, No. 4, 2009 1565