Aqueous speciation studies of europium(III) phosphotungstate

Article abstract of DOI:10.1021/ic049655h

The incorporation of lanthanide ions into polyoxometalates may be a unique approach to generate new luminescent, magnetic, and catalytic functional materials. To realize these new applications of lanthanide polyoxometalates, it is imperative to understand the solution speciation chemistry and its impact on solid-state materials. In this study we find that the aqueous speciation of europium(III) and the trivacant polyoxometalate, PW₉O ₃₄^9-, is a function of pH, countercation, and stoichiometry. For example, at low pH, the lacunary (PW₁₁O ₃₉)^7- predominates and the 1:1 Eu(PW₁₁O ₃₉)^4-, 2, forms. As the pH is increased, the 1:2 complex, Eu(PW₁₁O₃₉)₂^11- species, 3, and (NH₄)₂₂{(Eu₂PW₁₀O₃₈) ₄(W₃O₈(H₂O)₂(OH) ₄}·44H₂O, a Eu₈ hydroxo/oxo cluster, 1, form. Countercations modulate this effect; large countercations, such as K ⁺ and Cs⁺, promote the formation of species 3 and 1. Addition of Al(III) as a counterion results in low pH and formation of {Eu(H₂O)₃(α-2-P₂W₁₇O ₆₁)}₂, 4, with Al(III) counterions bound to terminal W-O bonds. The four species observed in these speciation studies have been isolated, crystallized, and characterized by X-ray crystallography, solution multinuclear NMR spectroscopy, and other appropriate techniques. These species are 1, (NH₄)₂₂{(Eu₂PW₁₀O₃₈) ₄(W₃O₈(H₂O)₂(OH) ₄}·44H₂O (P1; a = 20.2000(0), b = 22.6951-(6), c = 25.3200(7) A; α = 65.6760(10), β = 88.5240(10), γ = 86.0369(10)°; V = 10550.0(5) A³; Z = 2), 2, Al(H ₃O){Eu(H₂O)₂PW₁₁O ₃₄}·20H₂O (P1, a = 11.4280(23), b = 11.5930(23), c = 19.754(4) A; α = 103.66(3), β = 95.29(3), γ = 102.31(3)°; V = 2456.4(9) A³; Z = 2), 3, Cs ₁₁Eu(PW₁₁O₃₄)₂·28H ₂O (P1; a = 12.8663(14), b = 19.8235(22), c = 21.7060(23) A; α = 114.57(0), β = 91.86(0), γ = 102.91(0)°; V = 4858.3(9) A³; Z = 2), 4, Al₂(H₃O) ₈{Eu(H₂O)₃(α-2-P₂W ₁₇O₆₁)}₂·29H₂O (P1; a = 12.649(6), b = 16.230(8), c = 21.518(9) A; α = 11.1.223(16), β = 94.182(18), γ = 107.581(17)°; V = 3842(3) A;³; Z = 1).

Full text of DOI:10.1021/ic049655h

Inorg. Chem. 2004, 43, 7691−7701
Aqueous Speciation Studies of Europium(III) Phosphotungstate
Cheng Zhang, Robertha C. Howell, Kymora B. Scotland, Frances G. Perez, Louis Todaro, and
Lynn C. Francesconi*
Department of Chemistry, Hunter College and Graduate School of the City UniVersity of New York,
New York, New York 10021
Received March 15, 2004
The incorporation of lanthanide ions into polyoxometalates may be a unique approach to generate new luminescent,
magnetic, and catalytic functional materials. To realize these new applications of lanthanide polyoxometalates, it is
imperative to understand the solution speciation chemistry and its impact on solid-state materials. In this study we
find that the aqueous speciation of europium(III) and the trivacant polyoxometalate, PW
9
-
O
349-, is a function of
O
39)7 predominates and the
pH, countercation, and stoichiometry. For example, at low pH, the lacunary (PW11
39)4 , 2, forms. As the pH is increased, the 1:2 complex, Eu(PW11
-
O
)
11-
species, 3, and
1
:1 Eu(PW11
O
39
2
(
NH (Eu PW10
4
)
22
{
2
O
38
)
4
(W
3
O
8
(H
2
O)
2
(OH)
4
}‚44H
2
O, a Eu
8
hydroxo/oxo cluster, 1, form. Countercations modulate
+
+
this effect; large countercations, such as K and Cs , promote the formation of species 3 and 1. Addition of Al(III)
as a counterion results in low pH and formation of
to terminal W O bonds. The four species observed in these speciation studies have been isolated, crystallized,
and characterized by X-ray crystallography, solution multinuclear NMR spectroscopy, and other appropriate tech-
2 3 2 17 61 2
{Eu(H O) (R-2-P W O )} , 4, with Al(III) counterions bound
−
niques. These species are 1, (NH
6), c 25.3200(7) Å; R ) 65.6760(10), â ) 88.5240(10), γ ) 86.0369(10)
Al(H O)
95.29(3), γ ) 102.31(3)
4
)
22
{
(Eu
2
PW10
O
38
)
4
(W
3
O
8
(H
2
O)
2
(OH)
4
}‚44H
2
O (P1
h
; a
)
)
20.2000(0), b
)
22.6951-
2), 2,
3
(
)
{
°
; V
10550.0(5) Å ; Z
)
3
2 2
Eu(H O) PW11O
34}‚20H
; V
2
O (P1
)
h
, a
)
11.4280(23), b
)
11.5930(23), c
)
19.754(4) Å; R ) 103.66(3),
â
)
3
)
°
2456.4(9) Å ; Z
)
2), 3, Cs11Eu(PW11
O
34
)
2
‚
28H
2
O (P1
; V
16.230(8), c
h
; a
)
12.8663(14), b
3
1
4
1
9.8235(22), c
)
21.7060(23) Å; R ) 114.57(0), â ) 91.86(0), γ ) 102.91(0)
°
)
4858.3(9) Å ; Z
)
2),
, Al
2
(H
3
O)
8
{
Eu(H
2
O)
3
(
R
-2-P
2
W
17
O
61
)
}
2
‚
29H
2
O (P1
h
; a
)
12.649(6), b
)
)
21.518(9) Å; R )
3
11.223(16), â ) 94.182(18), γ ) 107.581(17)
°
; V
)
3842(3) Å ; Z
)
1).
Introduction
into solid-state oligomers3,4 and large wheel structures.5-8
POMs can serve as connectors and transfer agents for
different monolacunary POMs.
Polyoxometalates (POMs) containing Keggin and Wells-
Dawson moieties are chemically robust, easily modified
with respect to incorporation of transition metal ions, charge,
size, and potential, and can be rendered soluble in water or
organic solution. Due to these features, POMs have been
developed as catalysts for oxidation and acid-dependent
reactions.
Lanthanide (Ln) ions can offer unique functionality when
combined with polyoxometalates. We envision that incor-
poration of Ln ions into POMs offers unique functionality,
9
While solid-state crystal structures show the numerous
possibilities for use of lanthanide ions in forming new
polyoxometalate families, understanding the solution spe-
ciation chemistry and its impact on the solid-state chemistry
is critical to define new applications. Such an understanding
of the complex solution speciation and dynamics for POMs,
in general, and Ln POMs, specifically, is lacking. Polyoxo-
metalate composition in aqueous solution is dynamic; mul-
tiequilibria exist depending on pH, countercation, concen-
tration, and aging of the solution. Lanthanide ions also show
complex dynamic behavior in aqueous solution.
1
for example, in the creation of luminescent, magnetic, and
Lewis acid catalytic centers. In the area of developing novel
functional materials, lanthanide ions, by means of their
multiple coordination numbers, can link polyoxometalates
2
We and others have studied the aqueous chemistry of lan-
thanide complexes of the monovacant lacunary polyoxo-
10-
10-
metalates, specifically, (R
2
-P
2
W
17 61
O )
1 2 17 61
and (R -P W O )
*
Author to whom correspondence should be addressed. E-mail:
lfrances@hunter.cuny.edu.
isomers, where the lanthanide ion is incorporated into the
1
0.1021/ic049655h CCC: $27.50
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 24, 2004 7691
Published on Web 10/28/2004

Zhang et al.
“
cap” and “belt” regions of the POM, respectively.3,4,10-16
X ) As(V), n ) 9), and this anion has six oxygen atoms
available for bonding (in the AR form) and seven (in the
A number of species exist in solution and in the solid state,
and focused solution speciation studies coupled with crystal-
lography provide insight into the chemistry and stabilities
8
BR form). We isolated a unique Ln cluster tied together by
9
-
17
9
PW O34 under neutral to basic conditions. This species
of the solution species.
The trivacant polyoxotungstate, XW
tential to support lanthanide clusters. XW
from the Keggin structure (X ) P, n ) 9; X ) Si, n ) 10;
is very stable in water at pH 6.5-9. Examination of the
solution chemistry of Eu(III) and PW O34 revealed inter-
9
esting and complex behavior of lanthanide phosphotungstates
that is reported herein.
n-
9-
9
O
34 , has the po-
n-
9
O34 is derived
The objective of this work is to examine the speciation of
9
-
(
1) Key references to lanthanide luminescence: (a) Bunzli, J.-C. G.
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9
O34 as a function of solution
9
-
and PW
9
O
34
with respect to pH, countercation, and
2
357-2368. (c) Bruce, J. I.; Dickins, R. S.; Govenlock, L. J.;
stoichiometry, parameters that are well-known to influence
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studies because the shift properties allow convenient moni-
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3
1
toring by P NMR.
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12705. (e) Parker, D.; Dickins, R. S.; Puschmann, H.; Crossland, C.;
To unambiguously assign the species, we optimized
Howard, J. A. K. Chem. ReV. 2002, 102, 1977-2010. Key reviews
on new functional materials, particularly electrochromic, electrolu-
minescent, photochromic, and photoluminescent materials: (f) Kat-
soulis, D. E. Chem. ReV. 1998, 98, 359-387. (g) Yamase, T. Chem.
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AdV. Mater. 2002, 14, 225-228. Photoluminescent films: (i) Mo, Y.-
G.; Dillon, R. O.; Snyder, P. G.; Tiwald, T. E. Thin Solid Films 1999,
9-
9
reaction conditions of Eu(III) and PW O34 to isolate the
four compounds that are observed in the speciation studies.
These compounds were characterized by appropriate solution
and solid-state techniques, including multinuclear NMR and
X-ray crystallography. The solid-state crystal structures
not only are consistent with the solution species but also
reveal the unique abilities of the countercations to influence
speciation and structure.
3
55-356, 1-5. (j) Xu, L.; Zhang, H.; Wang, E.; Kurth, D. G.; Li, Z.
J. Mater. Chem. 2002, 12, 654-657. (k) Xu, L.; Zhang, H.; Wang,
E.; Wu, A.; Li, Z. Mater. Chem. Phys. 2002, 77, 484-488. (l) Wang,
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(
3
m) Wang, J.; Liu, F.; Fu, L.; Zhang, H. Mater. Lett. 2002, 56, 300-
Experimental Section
04. (n) Wang, Y.; Wang, X.; Hu, C. J. Colloid Interface Sci. 2002,
2
49, 307-315. (o) Wang, J.; Wang, H. S.; Fu, L. S.; Liu, F. Y.; Zhang,
General Methods. All reagents were commercially available and
used without further purification. Nanopure water was obtained from
a Millipore Reverse Osmosis Direct-Q System. Elemental analyses
were carried out by inductive coupled plasma atomic emission
spectrometry (ICP-AES, SECTROFLAME M120E) as described
below. IR spectra were recorded on a Perkin-Elmer 1625 FT-IR at
room temperature from KBr pellets. Sodium 9-tungstophosphate
H. J. Thin Solid Films 2002, 414, 256-261.
(
2) Key references to lanthanide Lewis acid catalysis: (a) Aspinall, H.
C. Chem. ReV. 2002, 102, 1807-1850. (b) Molander, G. A.
Chemtracts: Org. Chem. 1998, 11, 237-263. (c) Molander, G. Chem.
ReV. 1992, 92, 29-68. (d) Shibasaki, M.; Yoshikawa, N. Chem. ReV.
2
002, 102, 2187-2209. (e) Kobayashi, S.; Kawamura, M. J. Am.
Chem. Soc. 1998, 120, 5840-5841. (f) Kobayashi, S. Pure Appl.
Chem. 1998, 70, 1019-1026. (g) Aspinall, H. C.; Dwyer, J. L. M.;
Greeves, N.; McIver, E. G.; Woolley, J. C. Organometallics 1998,
(Na
9
A-PW
9
O
2
34‚16H O) was prepared according to a published
1
7, 1884-1888. (h) Xie, W.-H.; Yu, L.; Chen, D.; Li, J.; Ramirez, J.;
18
method and identified by infrared spectroscopy.
Miranda, N. F.; Wang, P. G. In EnVironmentally Benign Chemistry:
Green Chemistry; Anastas, P. T., Williamson, T. C., Eds.; Oxford
University Press: Oxford, U.K., 1998; pp 129-149.
Reaction Chemistry. Reaction of the Polyoxometalate A-r-
9-
PW
Buffer solutions of LiOAc (0.5M), NaOAc (0.5 M), KOAc (0.5
M), and CsOAc (0.5 M) were each prepared at pH 4.75 (30% D O).
(sodium salt) (0.1 g) was added with vigorous
9
O34 as a Function of Countercation and pH. Method i.
(
3) Sadakane, M.; Dickman, M. H.; Pope, M. T. Angew. Chem., Int. Ed.
2
000, 39, 2914-2916.
2
(
4) Mialane, P.; Lisnard, L.; Mallard, A.; Marrot, J.; Antic-Fidancev, E.;
9
-
9 9 34
Na A-PW O
Aschehoug, P.; Vivien, D.; Secheresse, F. Inorg. Chem. 2003, 42,
2
102-2108.
stirring into four vials each containing 3 mL of buffer.
(
(
(
5) Muller, A.; Peters, F.; Pope, M. T.; Gatteschi, D. Chem. ReV. 1998,
9-
Method ii. A 3 mL volume of A-PW
g) aqueous solution (30% D
9
O34 (sodium salt) (0.1
O) was prepared at pH 1, 3, 5, 7.3, 8,
98, 239-271.
6) Muller, A.; Krickemeyer, E.; Bogge, H.; Schmidtmann, M.; Peters,
F. Angew. Chem., Int. Ed. 1998, 37, 3360-3365.
2
and 10.35 by using HCl or NaOH to adjust the pH. The solutions
from both methods i and ii were heated to 90 °C for 2 min. The
solutions were cooled to room temperature and then placed into
10 mm NMR tubes. The 31P NMR spectra were recorded, and the
data are listed in Table S1 (Supporting Information).
7) Muller, A.; Sarkar, S.; Shah, S. Q. N.; Bogge, H.; Schmidtmann, M.;
Sarker, S.; Kogerler, P.; Hauptfleish, B.; Trautwein, A. X.; Schun-
emann, V. Angew. Chem., Int. Ed. 1999, 38, 3238-3241.
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Ed. Engl. 1997, 36, 1445-1448.
(
(
9) Belai, N.; Sadakane, M.; Pope, M. T. J. Am. Chem. Soc. 2001, 123,
9
-
3+
Reaction of A-PW
9
O34 with Eu (1:1 Stoichiometry) as a
2087-2088.
31
(
10) Bartis, J.; Dankova, M.; Blumenstein, M.; Francesconi, L. C. J. Alloys
Compd. 1997, 249, 56-68.
Function of pH Monitored by P NMR. A 0.056 mmol amount
of Eu (50 µL of 1.12 M) was added into four vials each containing
mL of H O (30% D O), respectively; A-PW (0.1527 g, 0.056
3+
(
11) Bartis, J.; Sukal, S.; Dankova, M.; Kraft, E.; Kronzon, R.; Blumenstein,
M.; Francesconi, L. C. J. Chem. Soc., Dalton Trans. 1997, 1937-
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2
2
9
1944.
(
12) Bartis, J.; Dankova, M.; Lessmann, J. J.; Luo, Q.-H.; Horrocks, W.
D., Jr.; Francesconi, L. C. Inorg. Chem. 1999, 38, 1042-1053.
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Horrocks, W. D., Jr.; Young, J., V. G.; Rheingold, A. L.; Francesconi,
L. C.; Antonio, M. R. Inorg. Chem. 2001, 40, 1894-1901.
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2715-2719.
(
(16) Sadakane, M.; Ostuni, A.; Pope, M. T. J. Chem. Soc., Dalton Trans.
2002, 63-67.
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A. L.; Francesconi, L. C. Angew. Chem., Int. Ed. 2001, 40, 4301-
4304.
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6117.
(18) Domaille, P. J. Inorg. Synth. 1990, 27, 100-101.
7692 Inorganic Chemistry, Vol. 43, No. 24, 2004

Aqueous Speciation of Eu(III) Phosphotungstate
Figure 1. P NMR spectra for reactions of PW9O34 + Eu3+ (1:1 stoichiometry) as a function of pH. The numbers represent the species, see text, that
3
1
9-
3
1
give rise to the designated P NMR resonances.
mmol) was added slowly to the above solutions with vigorous
stirring, and the pH of resulting solutions were adjusted to 1, 3, 7,
and 9 by adding diluted HCl or NaOH, followed by heating to 90
°
C for 2 min. The solutions were cooled to room temperature and
31
then placed into 10 mm NMR tubes. The P NMR spectrum was
recorded and is shown in Figure 1. The pH did not change
significantly during the NMR measurement.
9-
3+
9
A-PW O34 with Eu (1:1 Stoichiometry) as a Function of
Countercation and pH. Buffer solutions of LiOAc (0.5 M),
NaOAc (0.5 M), and KOAc (0.5 M) were prepared at three pH
values (4.5, 5.5, 6.5) (30% D
2
O). Preparation of solutions: 0.056
mmol Eu3 (50 µL of 1.12 M) was added into nine vials each
+
9-
9
containing 3 mL of buffer. A-PW O34 (0.1527 g, 0.056 mmol)
Figure 2. 1P NMR spectra of reactions of PW9O349- + Eu3+ (1:1
stoichiometry) with different countercations (see text for concentrations of
countercations, pH 7, except for Al(III), where the pH 2-3). The numbers
3
was added slowly with vigorous stirring to obtain slightly cloudy
solutions that were heated at 90 °C for 2 min to form clear solutions.
The solutions were cooled to room temperature and then placed
into 10 mm NMR tubes. The 31P NMR spectra were recorded and
are shown in Figures S1 and S2 (Supporting Information).
31
represent the species, see text, that give rise to the designated P NMR
resonances.
9-
3+
Expanded Study of A-PW
etry) as a Function of Countercation, including Cs and
Al(III). A 0.056 mmol amount of Eu3 (50 µL of 1.12 M) was
9
O
34 with Eu (1:1 Stoichiom-
same product.) To EuCl
O (25 mL) was added Na
3
‚6H
2
O (0.32 g, 0.87 mmol) dissolved in
PW 34‚15H O (2.44 g, 0.87 mmol)
+
H
2
9
9
O
2
+
to form a cloudy solution. After heating at 90 °C for 10 min, cooling
and filtering off a small amount of insoluble material, tetrabuty-
lammonium bromide (2.78 g, 8.7 mmol) was added to form a white
precipitate. This crude precipitate can be collected by filtration.
Further purification can be achieved by extracting three times with
added into four vials containing 3 mL of H
2
O (30% D
2
O).
9-
A-PW O34 (0.1527 g, 0.056 mmol) was added slowly to the above
9
solutions with vigorous stirring; the resulting solutions were heated
to 90 °C for 2 min. NaCl (0.990 g, 0.56 M), KCl (0.125 g, 0.56
M), CsCl (0.113 g, 0.22 M), and AlCl
3
(0.135 g, 0.19 M) were
2 2
CH Cl (50 mL). The organic layer was collected and the solvent
added to each of the vials during the heating stage. The vials were
cooled to room temperature, and the pH of the solutions was
measured and found in all cases except Al(III) to be ca. 7; for
Al(III), the pH was 2-3. The solutions were placed into 10 mm
NMR tubes. The 31P NMR spectra were recorded, and the spectra
are shown in Figure 2. The pH after the experiment did not change
significantly.
evaporated and dried under vacuum.
9
Reaction of A-PW O
34 with Eu3+ as a Function of Stoichi-
9
-
9
-
3+
ometry (Eu:A-PW
9
O
34 ) 0.5:1, 1:1, 2:1). The reaction of Eu
9-
and A-PW O34 in 2:1, 1:1, and 0.5:1 ratios was carried out in
9
buffer solution NaOAc (0.5 M, pH 6.5). A 50 µL (0.056 mmol)
volume of 1.12 M Eu3 each was added into three vials containing
+
3 mL of NaOAc (0.5 M, pH 6.5) (30% D
g (0.112 mmol), 0.1527 g (0.056 mmol), and 0.07635 g (0.028
mmol) of A-PW were added slowly to the above solutions with
2
O). Amounts of 0.3054
9
Reaction of A-PW O
34 with Eu3+ as a Function of Organic
9
-
Countercations, Tetrabutylammonium (TBA) Bromide and
9
Tetraethylammonium (TEA) Bromide. (The following reaction
vigorous stirring. The resulting solutions were heated to 90 °C for
2 min, followed by cooling to room temperature. The P NMR
9-
31
performed in 1:1 or 2:1 Eu:A-PW
9
O34 stoichiometry yields the
Inorganic Chemistry, Vol. 43, No. 24, 2004 7693

Zhang et al.
‚28H O,
O (2.45 g, 0.9 mmol) was added slowly
O (0.33 g, 0.9 mmol) in 7.5 mL of H
Preparation and Crystallization of Cs11Eu(PW11
. Solid Na A-PW 34‚16H
to a solution of EuCl ‚6H
O )
34 2
2
3
9
9
O
2
3
2
2
O
to obtain a slightly cloudy solution. Heating to 80 °C resulted in a
clear solution within a few minutes. Solid CsCl (2.27 g, 13.5
mmol) was added to the hot solution resulting immediately in the
formation of a white precipitate. Heating was continued for an
additional 5 min, and then the resulting slurry was cooled in an ice
bath. The solid was collected by filtration. Yield: 1.86 g, 82%.
X-ray-quality crystals were obtained by recrystallizing 0.5 g of the
white solid from 6 mL of warm water (50 °C). Anal. Calcd for
Cs11Eu(PW11
O
34
)
2
2
‚28H O: W, 53.02; Eu, 1.99; P, 0.81. Found:
-
1
W, 53.64; Eu, 2.13; P, 0.81. IR (KBr, cm ) (metal-oxygen
stretching region): 1084 (m), 1056 (m), 1023 (m), 943 (s),
776 (s).
Figure 3. 31P NMR spectra of reactions of PW9O349- + Eu3+ as a function
of stoichiometry. The boxed ratios represent the stoichiometric ratio of Eu:
9
-
-
POM; therefore, the bottom spectrum represents the 1:2 Eu:PW9O34
stoichiometry, the middle spectrum represents the 1:1 Eu:PW9O34
Preparation and Crystallization of Al
2-P ‚29H O, 4. Solid Na A-PW
mmol) was added slowly to a solution of EuCl
mmol) in 7.5 mL of H O to form a cloudy solution. Heating to 80
C resulted in a clear solution within seconds. Solid AlCl (4.34 g,
2
(H
34‚16H
‚6H
3
O)
8
{Eu(H
O (2.45 g, 0.9
2
O (0.33 g, 0.9
2 3
O) (r-
9
stoichiometry, and the top spectrum represents 2:1 Eu:PW9O349- stoichi-
2
W
17
O
61)}
2
2
9
9
O
2
ometry. The solution species are indicated by the numbers above the
resonances; see text.
3
2
°
3
spectrum is shown in Figure 3. When the experiment was carried
out in water, not NaOAc buffer, the same results are obtained with
18 mmol) was added, and the resulting clear solution was heated
for an additional 3 min. Traces of a precipitate were removed by
filtration. The solution was stored in a vial at room temperature.
After 1 month, small thick rectangular-like crystals were grown,
and X-ray-quality crystals were selected from the bulk crystals.
9-
a few more unidentified peaks for the 1:2 Eu:PW
not shown).
Preparation and Crystallization of the Individual Species
That Are Observed in Speciation Experiments. The syntheses
9
O34 combination
(
Yield: 0.7 g, 53%. Anal. Calcd for Al
(R-2-P ‚29H O: W, 65.84; Eu, 3.20; P, 1.31; Al, 0.57.
Found: W, 64.71; Eu, 3.21; P, 1.10; Al, 0.56. IR (KBr, cm
2 3 8 2 3
(H O) {Eu(H O) -
9-
of complexes 1-4 from Eu(III) and A-PW
9
O
34 were optimized,
2
W
17
O61)}
2
2
-
1
and the complexes were isolated and characterized by X-ray
crystallography, elemental analysis, infrared spectroscopy, and
multinuclear NMR.
)
(metal-oxygen stretching region): 1100 (m), 1046 (m), 954 (vs),
893 (m), 820 (s), 773 (vs), 722 (s).
Preparation and Crystallization of (NH
(H O) (OH) }‚44H O, 1. 1 was prepared by a modification
2
of the procedure reported previously. Solid Na O
4
)
22{(Eu
2
PW10
O
38
)
4
-
Analytical Techniques. Elemental Analysis by ICP. (i) Stan-
dard Solution Preparation. The standard solution of P (0.2, 0.4,
0.6, 0.8, 1.2 ppm), Eu (1, 2, 4, 6 ppm), W (20, 40, 60, 80, 120
ppm), Al (0.5, 1, 1.5, 2.5 ppm), Na (1, 3, 5, 7 ppm), and K (1, 3,
5, 7 ppm) was prepared by diluting 1000 ppm ICP standard solution
(GFS Chemicals, Inc.) with distilled water.
(ii) Sample Preparation. Crystals of complex 1-4 were
collected by filtration, air-dried, and then further dried in a
4
desiccator over CaSO , under vacuum for 1.5 h. The samples were
left in the closed desiccator overnight. Afterward, 0.0508 g of 1,
0.0470 g of 2, 0.0526 g of 3, and 0.0510 g of 4 were each dissolved
in 50 mL of distilled water. The 1 and 2 mL solutions from each
stock solution were diluted to 25 mL with water and used for ICP
measurements.
(W O
3 8
2
2
4
2
17
9
A-PW
4.90 g, 1.8 mmol) was added slowly to a solution of EuCl
0.66 g, 1.8 mmol) in 15 mL of H O. The resulting cloudy solution
9
O34‚16H
(
(
3 2
‚6H O
2
was heated to about 80 °C, and within a few seconds, a clear
solution formed. Solid NH Cl (5.16 g, 54 mmol) was added to the
4
hot solution, a white precipitate was formed immediately, and the
solution was heated for an additional 5 min. The resulting solu-
tion turned clear and then was cooled in an ice bath. The crystal-
lized solid was collected by filtration and recrystallized from
hot water. Yield: 2.78 g, 76%. X-ray-quality crystals were ob-
tained at 4 °C by recrystallizing 1 g of the white crystalline solid
4 2 38 4
from 8 mL of hot water. Anal. Calcd for (NH )22{Eu PW10O ) -
(
W
3
O
8
(H O) (OH) }‚44H O: W, 60.11; Eu, 9.25; P, 0.94. Found:
2
2
4
2
(iii) Measurement Method. The maximum wavelength for
different element was selected (P, 213.618 nm; Eu, 381.970 nm;
W, 239.709 nm; Al, 308.215 nm; Na, 589.592 nm; K, 766.496 nm).
A calibration curve for each element was constructed. After the
calibration curve of each element was completed, the concentrations
(in ppm) were determined for the two solutions of each sample
(40.64, 81.28 µg/mL of 1; 37.60, 75.20 µg/mL of 2; 42.08, 84.16
µg/mL of 3; 40.80, 81.60 µg/mL of 4). The concentrations, in ppm,
were converted to weight percent of each element.
-
1
W, 60.10; Eu, 9.30; P, 0.88. IR (KBr, cm ) (metal-oxygen
stretching region): 1092 (m), 1055 (m), 1025 (m), 951 (s), 935
(s), 820 (vs), 790 (s).
Preparation and Crystallization of Al(H
3
O){Eu(H
2
O)
2
-
PW11
mmol) in 15 mL of H
0
O
34}‚20H
2
O, 2. To a solution of EuCl
O was added Na A-PW
3
‚6H
2
O (0.66 g, 1.8
2
9
9 2
O34‚16H O (2.45 g,
.9 mmol) slowly with vigorous stirring to form a slightly cloudy
solution. Heating at 80 °C and addition of AlCl (1.74 g, 7.2 mmol)
3
resulted in a clear solution. Stirring at room temperature was
continued for 20 min. Traces of a precipitate were removed by
filtration. The solution was stored in a beaker and allowed to slowly
evaporate. After 1 week, needlelike crystals were obtained. Yield:
Collection of NMR Data. All NMR spectra were recorded on
a JEOL GX-400 spectrometer with 5 or 10 mm tubes. Resonance
frequencies are 161.8 MHz for 31P and 16.7 for W. Chemical
183
3
1
shifts are given with respect to external 85% H
3
PO
4
for P and
for 1 W. Typical acquisition parameters for
83
31
P
1
.2 g, 45%. X-ray-quality crystals were selected from the bulk
2.0 M Na
2
WO
4
3
crystals and cut into 0.1 × 0.12 × 0.25 mm blocks. Anal. Calcd
for Al(H O){Eu(H O) 34}‚20H O: W, 61.51; Eu, 4.62; P,
.94; Al, 0.82. Found: W, 61.59; Eu, 4.67; P, 0.95; Al, 0.88. IR
spectra included the following: spectral width, 10 000 Hz; acquisi-
tion time, 0.8 s; pulse delay, 1 s; pulse width, 15 µs (50° tip angle).
From 200 to 1000 scans were required. Generally, the 1 s pulse
3
2
2
PW11
O
2
0
-
1
31
(KBr, cm ) (metal-oxygen stretching region): 1093 (m), 1051
delay was sufficient for accurate integration of the P peaks and,
(m), 957(s), 833 (s).
thus, qualitative assessment of concentrations of species could be
7694 Inorganic Chemistry, Vol. 43, No. 24, 2004

Aqueous Speciation of Eu(III) Phosphotungstate
Table 1. Crystal and Structure Refinement Data for Species 1-4
1
2
3
4
empirical formula
fw
Eu8O228P4W43
12 893.11
triclinic
P 1h (No. 2)
109(2)
AlEuO41PW11
2888.26
triclinic
P 1h (No. 2)
100(2)
ClCs11EuKO93P2W22
7283.16
triclinic
P 1h (No. 2)
100(2)
K2Al4Eu2O168P4W34
9552.82
triclinic
P 1h (No. 2)
100(2)
cryst syst
space group
temp, K
wavelength, Å
a, Å
0.710 73
0.710 73
0.710 73
0.710 73
20.2000(0)
22.6951(6)
25.3200(7)
65.6760(10)
88.5240(10)
86.0360(10)
10 550.0 (5)
2
11.4280(23)
11.5930(23)
19.754 (4)
103.66(3)
95.29(3)
102.31(3)
2456.4(9)
2
3.905
27.029
12.8663(14)
19.8235(22)
21.7060(23)
114.57(0)
91.86(0)
102.91(0)
4858.3(9)
2
4.979
30.832
12.649(6)
16.230(8)
21.518(9)
111.223(16)
94.182(18)
107.581(17)
3842(3)
b, Å
c, Å
R, deg
â, deg
γ, deg
3
V, Å
Z
1
3
calcd density, g/cm
4.058
25.816
11 140
4.129
26.373
4136
-
1
abs coeff, mm
F(000)
2466
6212
θ range, deg
1.77-27.50
1.89-27.45
-14 e h e 14
-14 e k e 15
-25 e l e 25
21 306/11 183
[R(int) ) 0.0286]
full-matrix least
squares on F2
11 183/0/491
1.096
1.04-28.40
-16 e h e 16
-26 e k e 22
0 e l e 28
18 777/18 777
[R(int) ) 0.0000]
full-matrix least
squares on F2
18 777/0/705
0.986
1.04-28.74
-14 e h e 16
-21 e k e 13
-27 e l e 28
27 206/16 035
[R(int) ) 0.0500]
full-matrix least
squares on F
16 035/0/519
1.055
limiting indices
-26 e h e 26
-
29 e k e 29
32 e l e 32
-
reflcns collcd/unique
refinement meth
141 901/48 396
[
R(int) ) 0.0386]
full-matrix least
2
2
squares on F
48 396/0/1408
1.071
data/restraints/params
2
GOF on F
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak
R1 ) 0.0534,
wR2 ) 0.1292
R1 ) 0.0624,
wR2 ) 0.1347
5.373 and -4.87
R1 ) 0.0572,
wR2 ) 0.1624
R1 ) 0.0687,
wR2 ) 0.1770
11.94 and -3.807
R1 ) 0.0818,
wR2 ) 0.2135
R1 ) 0.1170,
wR2 ) 0.2391
8.071 and -7.938
R1 ) 0.0778,
wR2 ) 0.2158
R1 ) 0.0946,
wR2 ) 0.2293
9.386 and -8.749
-
3
and hole, e Å
made. For 183W spectra, typical conditions included the following:
spectral width, 10 000 Hz; acquisition time, 1.6 s; pulse delay, 0.5
s; pulse width, 50 µs (45° tip angle). From 1000 to 30 000 scans
were acquired. For all spectra, the temperature was controlled to
that, at equilibrium conditions, predominantly the monola-
7-
cunary R-PW11O39 species formed in the presence of two
22
buffers (sulfite and tris). Also, unidentified phosphorus-
containing products were formed that were buffer dependent.
In this study, we examined the solution behavior of the
31
183
(
0.2 deg. For both P and W chemical shifts, the convention
used is that the more negative chemical shifts denote more upfield
9-
A-PW
countercation (Li , Na , K , Cs , Al ) conditions by
NMR (Supporting Information Table S1). In acidic solution
9
O34 , at equilibrium, under different pH (1-10) and
resonances.
+
+
+
+
3+
31
P
Single-Crystal X-ray Structure Determination. Crystals of
1-4 were examined under a thin layer of mineral oil using a
polarizing microscope. Selected crystals were mounted on a glass
fiber and quickly placed in a stream cold nitrogen on a Bruker
SMART CCD diffractometer equipped with a sealed tube Mo anode
the major species formed are H
3
PO
4
, the Keggin anion
3-
7-
12
40
11 39
PW O , and the monolacunary Keggin anion, PW O .
In buffer (0.5 M, LiOAc, NaOAc, KOAc) at pH 4.75, only
(KR radiation, λ ) 0.710 73 Å) and graphite monochromator or
7-
two species H
Under neutral conditions, PW11
3
PO
4
and PW11
O
O
39
exist in the solution.
Nonius Kappa CCD diffractometer. The data were collected at
around 100 K. Data collection, indexing, and initial cell refinements
were all handled using SHELXTL software. The SHELX package
of software was used to solve and refine the restructures.19 The
7-
3-
39 along with PO
4
and
unidentified species were present, similar to the previous
2
2
study. In basic solutions, the major species formed are
3
-
2-
183
PO
4
and WO
4
(according to W NMR data), consistent
heaviest atoms were located by direct methods, and the remaining
atoms were found in subsequent Fourier difference syntheses. For
the ammonium salt 1, the data did not support discrimination
between oxygen and nitrogen atoms. All refinements were full-
9
-
with decomposition of the A-PW
9
O34 . The addition of the
3
+
9-
9
Al to the solution of A-PW O34 results in a lowering of
the pH to about 2-3 and the observation of two equal-
intensity peaks at -10.7 and -11.45 ppm that may indicate
2
squares on F . Crystal data and structure refinement parameters
for 1-4 are listed in Table 1. Selected bond distances for 1-4 are
given in Table 2. Final atomic coordinates and displacement
parameters of 1-4 are given in the Supporting Information.
a Wells-Dawson anion, for example, the â-Wells-Dawson
6-
anion or an Al(III) adduct of the R- P
incorporation of the Al(III) into R2-P
peaks were in small concentration (13%); the majority of
2
W
18
O
62 or possibly
1
0-
2
W
17
O61 . These two
Results
9
-
Solution Chemistry. Solution Speciation of PW
9
O
34
20,21
.
(
19) Sheldrick, G. M. SHELXTL; Brucker AXS, Inc., Bruker Advanced
9-
9 2 4 3 4
A-PW O34 is formed from Na WO and H PO at pH 9.
X-ray Solutions: Madison, WI, 1999.
Hill and co-workers examined the fundamental stability of
(20) Contant, R. Inorg. Synth. 1990, 27, 71.
(
21) Contant, R.; Herve, G. ReV. Inorg. Chem. 2002, 22, 63-111.
22) Hill, C. L.; Weeks, M. S.; Schinazi, R. F. J. Med. Chem. 1990, 33,
2767-2772.
9-
A- and B-PW
9
O34 under buffered neutral (physiological
(
31
pH 7.4) aqueous media by P NMR spectroscopy and found
Inorganic Chemistry, Vol. 43, No. 24, 2004 7695

Zhang et al.
Table 2. Selected Bond Lengths (Å) for Species 1-4
Compound 1
Eu(3)-O(1)
Eu(1)-O(1)
2.436(10)
2.346(11)
2.391(13)
2.383(11)
2.557(11)
5.907(13)
6.146(10)
4.426(13)
2.396(13)
2.479(11)
2.318(11)
2.539(11)
2.369(10)
5.827(12)
6.146(13)
6.557(12)
Eu(3)-O(16B)
Eu(3)-O(19B)
Eu(4)-O(18)
Eu(4)-O(12B)
2.377(13)
2.451(11)
2.314(13)
2.374(13)
Eu(1)-O(13)
Eu(2)-O(13)
Eu(2)-O(19B)
Eu(2)-O(16B)
Eu(1)-Eu(3)
Eu(2)-Eu(4)
Eu(1)-Eu(8)
Eu(3)-O(15)
Eu(3)-O(16)
Eu(3)-O(13)
Eu(3)-O(14B)
Eu(4)-Eu(3)
Eu(1)-Eu(4)
Eu(4)-Eu(6)
Eu(1)-Eu(6)
Eu(4)-Eu(4)
Eu(2)-Eu(3)
3.910(13)
6.536(12)
4.392(10)
Compound 2
Eu(1)-O(1)
Eu(1)-O(6)
Eu(1)-O(4A)
Eu(1)-Eu(1A)
2.431(39)
2.369(23)
2.362(23)
6.288(88)
Eu(1)-O(5A)
Eu(1)-O(3A)
Eu(1)-O(40)
2.392(20)
2.362(21)
2.407(21)
Eu(1)-O(2A)
Eu(1)-O(34)
Al(1)-O(27)
2.456(57)
2.499(21)
2.871(30)
Compound 3
Eu(1)-O(1)
Eu(1)-O(6)
Eu(1)-O(4)
2.423(1)
2.346(2)
2.373(1)
Eu(1)-O(5)
Eu(1)-O(3)
Eu(1)-O(8)
2.362(5)
2.428(0)
2.380(1)
Eu(1)-O(2)
Eu(1)-O(7)
2.350(2)
2.421(3)
Compound 4
Eu(1)-O(1)
Eu(1)-O(6)
Eu(1)-O(4)
2.460(11)
2.437(13)
2.324(11)
Eu(1)-O(5)
Eu(1)-O(3)
Eu(1)-O(51A)
2.419(10)
2.479(11)
2.380(11)
Eu(1)-O(2)
Eu(1)-O(7)
Eu(1)-Eu(1A)
2.571(13)
2.447(11)
5.366(12)
9
-
3+
the species observed in the case of Al(III) addition are
9
Reaction of PW O34 with Eu (1:1 Stoichiometry)
7-
3-
31
PW11
O
39 and PW12
O
40
.
at Different pH Values. The P NMR spectra (Figure 1)
for the reaction carried out at low pH (1-3) shows two peaks
at 5.23 ppm and -11.93 ppm that correspond to species 2,
Eu(PW O ) , and PW O , respectively. Decomposition
11 39 11 39
to PW12O40 is also observed. Three species are observed
at pH 7; these are 1, (Eu
(δ, ppm: 28.55, 22.07), 2, Eu(PW O ) (δ, ppm: 5.25
9-
with Eu3+ After
Solution Speciation of PW
9
O
34
Mixing and Before Heating. The speciation reactions of
4-
7-
9
-
3+
PW
9
O
34 with Eu involve adding the polyoxometalate to
3
-
a solution containing Eu(III) at the appropriate pH and
countercation content. The solutions are heated for 2 min at
22-
2 38 4 3 8 2 2 4
PW10O ) (W O (H O) (OH) )
4
-
3
1
9
0 °C; after this heating step, the P NMR does not change
11 39
1
1-
3
1
ppm), and 3, Eu(PW11O39)2
(δ, ppm: 0.5 ppm). The
over a period of days. Before the heating step, P NMR
analysis shows that there are unidentified, broad 31P peaks
that are likely due to chemical exchange processes of
different species. For example, Figure S3 shows the P NMR
spectra for a typical reaction of PW
spectrum at pH 9 is dominated by species 1 and 3.
9
O
34 with Eu3+ under varying pH
9
-
The reaction of PW
values (4.5, 5.5, and 6.5) was examined in the presence of
31
9-
+
+
+
9
O
34 and Eu(III), 1:1
different cations (Li , Na , K ) (Supporting Information,
Figures S1 and S2). This experiment provides the opportunity
to evaluate both the pH and counterion influence on
speciation. With Li acetate or Na acetate buffer (0.5M) at
stoichiometry, pH ca. 6-7, before heating and after heating
at 90 °C. Before heating, the spectrum is broad and the peak
positions do not correspond to any of the identified species.
However, it is apparent that the broad peaks represent
dynamic behavior of species that are in chemical exchange.
For example, the peak at 25.89 ppm is clearly the largest
peak and likely represents an average of the peaks of species
3
1
9-
3+
pH 4.5 (Figure S1), the P NMR of PW
9
O34 and Eu in
a 1:1 mole ratio showed that 2 was the major solution species
with the PW11O39 ligand as a minor species. At pH 5.5,
only species 2 was observed, while, at pH 6.5, two additional
species 1 and 3 were present in the solution in low
concentration.
Different behavior is observed with K+ counterions
(KOAc, 0.5 M). For example, at pH 4.5, the major species
observed in solution are 2, Eu(PW O ) , and free
PW O , with 3, Eu(PW O ) , in significant concen-
tration (Figure S2). Increasing the pH results in no observable
and a significant increase in the concentration
of species 3. In contrast to the Li and Na case, species 3
grows in significantly at the lower pH values with K as
the buffer, until it is the dominant species at pH 7.6. Species
1 begins to appear at a higher pH of 7.6 compared to 6.5
found for the Li and Na buffers.
Expanded Study of PW O
34 with Eu3+ (1:1 Stoichi-
9
7
-
2
2-
1
, {(Eu
2
PW10
O
38
)
4
(W
3
O
8
)(H
2
O)
2
(OH)
4
}
(28.97, 22.55
17
ppm), where the two chemically inequivalent (Eu
2 38
PW10O )
lobes of the molecule are engaged in a dynamic process.
The other peaks at 13.82 and 2.80 ppm are also broad and
may represent chemical exchange as well. The peak at 2.80
ppm appears to be an average between species 2 and 3
4
-
1
1
39
7-
11-
1
1
39
11 39 2
4-
11-
39
Eu(PW11O )
(5.28 ppm) and Eu(PW11
O )
39 2
(0.57 ppm)
7-
reflecting an exchange process involving these species. The
peak at 13.82 ppm possibly represents exchange between
all three species. Heating this solution results in clear
conversion to species 1 (28.97, 22.55 ppm) and species 2
free PW O
39
1
1
+
+
+
4-
11-
and 3, (Eu(PW11
0.57 ppm), respectively.
Conversion of the broad peaks (Figure S3) to species 1-3
O
39
)
(5.28 ppm) and Eu(PW11
O
39
)
2
+
+
(
9
-
also occurs if the solution is allowed to age for 12 h at room
temperature. Aging experiments of the heated solutions show
that the spectra do not change during a period of days,
suggesting that the system has reached equilibrium.
ometry) as a Function of Countercation. To further test
the effect of countercations on speciation, a study of the
9
-
3+
speciation of PW
9
O34 + Eu (1:1 stoichiometry, pH 7)
with an expanded series of countercations was carried out
7696 Inorganic Chemistry, Vol. 43, No. 24, 2004

Aqueous Speciation of Eu(III) Phosphotungstate
+
+
+
(
Figure 2). With Na (not shown), K , or NH
4
(not shown),
Scheme 1. Synthetic Strategy for Isolation of Species 1-4 from
Eu(III) and Isolation of PW9O349
-
three species corresponding to 1 (28.8, 22.2 ppm), 2 (5.2
ppm), and 3 (0.5 ppm) can be identified, similar to studies
+
reported above. Addition of Cs into the aqueous solution
9
-
3+
of PW
9
O34 + Eu (1:1 stoichiometry, pH 7) results in
11-
exclusively one species, 3, Eu(PW11
O
39
)
2
.
Solutions containing the countercation Al(III) show the
presence of species 4 that has been identified as Eu(R-2-
7
-
31
14
P
2
W
17
O
61
)
by P NMR and crystallography; vide infra.
3+
This phenomenon was only seen in the presence of Al and
is possibly due to the acidity that Al3 confers upon the
+
3
solution; generally, the pH after treatment with AlCl was
ca. 2-3. Also, the ability of Al(III) to bind to oxygen atoms
of polyoxometalates may influence the formation of this
4
-
species. Species 2 and 3, 1:1 Eu(PW11
Eu(PW11O )
39 2
O
39
)
and 1:2
11-
, respectively, are also found as the majority
species in this solution (Figure 2).
Table 3. Multinuclear NMR Data for 1-4
9
-
compd
31P NMR data (δ, ppm)
The reaction of Eu(III) and A-PW
9
O
34 in H
2
O, in both
9
-
1
:1 or 2:1 Eu:A-PW
9
O
34 stoichiometry, with the addition
Eu2PW10O38)4(W3O14)]30-, 1a
7-
[
31.53, 23.22
5.25
0.34
8.72, -12.23
-12.04
-12.28
+
+
of TBA or TEA , resulted in isolation, by extraction into
[Eu(H2O)xPW11O39] , 2
[Eu(PW11O39)2]1 , 3
1-
4-
organic solution, of species 2, Eu(PW11
The 31P NMR of the TBA salt (δ, ppm, 5.3, H
.6, CH CN) was identical with that for a genuine sample
of TBA H{EuPW11 39} that was prepared directly by two
methods (metathesis of the potassium salt of Eu(PW11
O39) , exclusively.
[
Eu(H2O)x(R-2-P2W17O61)]14-, 4
2
O; δ, ppm,
[Y(H O) PW O ]7-
2 x
11 39
1
1-
[
Y(PW11O39)2]
9
3
3
O
183W NMR data
4
-
O
39) ,
compd
[δ, ppm (integration)]
7
-
prepared by direct reaction of Eu(III) and PW11
O
in CH
39 or by
Y2PW10O38)4(W3O14)]30- b
[
-30.8 (2), -99.8 (2), -103.4 (2),
-110.8 (2), -124.2 (2), -129.8 (2),
23
reaction of Eu(ClO
4
)
3
and TBA
H
4 3
(PW11O
39
)
3
CN).
-
-
-
-
131.1 (2), -133.8 (2), -134.6 (2),
135.8 (2), -139.2 (2), -141.3 (1),
145.9 (2), -149.7 (2), -157.9 (2),
159.8 (2), -174.7 (2), -187.8 (2),
3
The samples of TBA H{EuPW11O39} prepared directly have
been analyzed by elemental analysis, thus far.
9
-
3+
Reaction of PW
Stoichiometries. The reaction of Eu and A-PW
.5:1, 1:1, and 2:1 Eu: POM stoichiometric ratios were
studied in NaOAc buffered solution (0.5 M, pH 6.5), and
9
O
34
with Eu under Different
3+
9-
-225.7 (2)
9
O34 in
[
[
[
Y(H2O)xPW11O39]4
- c
-107.36 (2), -116.79 (1), -126.47 (2),
-144.94 (2), -145.47 (2), -155.00 (2)
-123.93 (1), -131.45 (2), -138.44 (2),
0
Eu(H2O)x(PW11O39)]4
- c
-187.69 (2)
3
1
31
the P NMR data are shown in Figure 3. P NMR spectra
Y(PW11O39)2]11- c
-130.00 (1), -132.02 (1), -143.93 (1),
-149.42(1), -150.64 (1), -155.53 (1),
-166.64 (1), -172.38 (1), 173.23 (1),
9-
11-
show that at 0.5:1 Eu:PW
dominates while 2, Eu(PW11
at 1.99 ppm, possibly due to decomposition of PW
9
O
34 species 3, Eu(PW11
O )
39 2
,
4-
O
39
)
and an unknown species
-174.76 (1), -206.99 (1)
9
-
9
in
solution,2 are present in smaller quantities. At 1:1
4
a Chemical shifts for crystalline sample of 1 in water. The chemical shifts
for samples with high concentrations of counterions and other species are
found slightly upfield at 28.55 and 22.07 ppm. b Howell, R. C.; Perez, F.
9-
Eu:PW
9
O34 stoichiometry, three distinct species coexist in
31
solution; according to P NMR, these species are 1,
G.; Jain, S.; Horrocks, W. D.; Rheingold, A. L.; Francesconi, L. C. Angew.
2
2-
Chew., Int. Ed. 2001, 40 (21), 4031-4034. c The samples were prepared
{
(Eu
2
PW10
O
38
-
)
4
(W
3
O
8
(H
2
O)
2
(OH)
4
}
, 2, and 3. At 2:1
directly from PW11O397 for W NMR. The P NMR for these samples
-
183
31
9
Eu:PW
9
O34 , one species, 2, is observed in the aqueous
was identical with samples prepared via PW9O349
-
.
solution. The same speciation behavior is observed when the
experiment is run in water, except that there are a few more
of Crystal Structures. From careful analysis of the reactions
reported above, we chose appropriate conditions to optimize
the syntheses and isolate the four species 1-4 from reactions
9
-
small unidentified peaks in the 1:2 Eu:PW
9
O
34
sample,
9-
probably due to decomposition of PW
Addition of AlCl to solutions of 1:1 Eu:PW
chiometry resulted in the small amount Eu(R-2-P
9
in solution.
9
-
3+
9
-
of PW O
34
with Eu . This is important to confidently
O
34 stoi-
9
3
9
1
4-
identify the species and understand their formation. The
2
17 61 2
W O )
procedures are summarized in Scheme 1.
(
Figure 2). In contrast, upon addition of Al to 2:1
+
9-
4-
Species 1. The addition of NH at pH 7-9 to a solution
Eu:PW
9
O
34 stoichiometry, the Eu(PW11
39
O )
species was
4
9-
3+
of PW
colorless rectangular blocks. The P NMR chemical shifts
Table 3) for isolated crystalline samples of 1 are observed
9
O34 and Eu results in the isolation of 1 as chunky
isolated as crystals in 45% yield; vide infra.
Isolation, Characterization of the Four Complexes
Observed in Solution Speciation Studies, and Description
3
1
(
at 31.53 and 23.22 ppm and are dependent on solution
conditions; in solution with different countercations present,
the resonances are shifted upfield. We have reported the
(
23) Ho, R. K. C.; Klemperer, W. G. J. Am. Chem. Soc. 1978, 100, 6772-
774.
24) The peak at 1.99 ppm along with other small peaks were observed
6
(
+
9
-
identical structure of the K salts of Y(III) and Eu(III)
upon dissolving PW9O34 alone in NaAc (0.5 M, pH 6.5, 30%
D2O).
1
7
analogues of 1, and therefore, complete structural details
Inorganic Chemistry, Vol. 43, No. 24, 2004 7697

Zhang et al.
-
Figure 5. Ball and stick structure of Al{Eu(H2O)2(PW11O39)} , 2.
Uncoordinated H2O molecules have been removed for clarity. Legend: W,
blue; O, red; Eu, green; P, yellow; Al, blue.
Figure 4. Structure of the central W8Eu8(H2O)2(OH)4O40 unit in
2
2-
{
Eu2PW10O38)4(W3O8(H2O)2(OH)4)} , 1. Uncoordinated H2O molecules
9
-
have been removed for clarity (W, blue; O, red; Eu, green). Hydroxide and
water oxygen atoms are labeled; see text.
species is the monovacant PW11
O39 at lower pH (Table
S1), it is very reasonable that addition of Eu(III) will form
Eu(H O) PW11O39 especially in this case, with an excess
2 4
of Eu(III). P NMR and elemental analysis are consistent
with this formulation.
As shown in Figure 5, in the solid-state, 2 is an infinite
4
-
+
of the NH
4
salt are in the Supporting Information. Y(III)
3
1
and Eu(III) also behave similarly with respect to solution
9-
9
speciation of PW O34 , and thus, the Y(III) analogues are
183
useful for solution characterization using W NMR because
resonances corresponding to all W atoms can be observed.
The resonances corresponding to W atoms close to the site
of substitution are often not observed in the Eu(III) ana-
logues.
5-
one-dimensional polymer, similar to the Eu(H
2
O)
2 39
(SiW11O )
+
4
(K
salt) species published by Mialane recently. The
5-
structure is similar also to Ce(H
2
O)
3
3
(R-SiW11
O
39
)
that also
forms a one-dimensional polymer. Due to the smaller size
3
+
3+
of Eu than Ce , it is reasonable that only two water
9-
9
Species 1 forms easily with PW O34 at pH 6.5-9 in 76%
3
+
molecules are coordinated to the Eu rather than three water
yield and is uniquely stable in basic solution. From our
previous work, multinuclear NMR and luminescence excita-
tion spectra provide evidence that the cluster remains intact
3
+
3+
molecules that are found coordinated to Ce . The Eu is
coordinated to four oxygen atoms of the defect site of
R-PW11
R-PW11
O
O
39, to two water molecules, and to two neighboring
39 units through terminal oxygen atoms. As ob-
served for most lanthanide complexes of monovacant poly-
oxometalates, the Eu(III) ion is in a distorted monocapped
square antiprism environment. The Al(III) ion sits in the
space between the R-PW11O39 units and shows a weak
connection (average Al-O: 2.87(3) Å) to a terminal oxygen
atom of an adjacent polyoxometalate. The structure contains
5.3% solvent accessible space. Therefore, it is likely that
water is located in this interstitial space as well.
The Eu(III)-O bond lengths to the four oxygen atoms in
the Keggin defect are similar (average: 2.371 Å), and the
average Eu(III)-oxygen bond lengths to the two neighboring
39 moieties is longer at 2.453 Å. The Eu-O(H
distance is 2.455 Å within the range of Eu-O(H O) bond
distances. The interatomic distances between the europium
centers are 6.288 Å. The bond lengths for the atoms in the
tungsten-oxygen framework of Eu(H
pare favorably with other Keggin structures. Although 2
exists as an oligomer in the solid state, it is soluble in aqueous
31
in aqueous solution, and the P NMR data, provided in this
study, show that the cluster remains intact under basic
conditions and different countercation content. Increasing the
acid content sets up an equilibrium with complex 2,
4-
31
Eu(PW11
O39) , that can be monitored by P NMR, Figure
S4. The Y(III) analogue shows the same equilibrium, and
1
83
9-
W NMR of the Y(III) + PW
O
9 34
solution at pH 4.6
shows 6 peaks, of appropriate integration, that correspond
2
7
3
4
-
to the 1:1 Y(PW11
ment of Eu(PW11
can be viewed as the confluence of four PW10O37 units
O
O
39
)
species, consistent with our assign-
4-
39) .
9-
1
each incorporating 2 Eu(III) ions to create four Keggin-like
anions that are further tied together by three additional
tungstate units, Figure S5.17 The central core consists then
PW11O
2
O)
2
of the eight Eu(III) ions coordinated to tungstate units, shown
4,14
in Figure 4. Bond valence sums2
5,26
(Table S3) performed
on all of the oxygen atoms in this core suggest that four
oxygen atoms, O13, O14, O16, and O18, each bridging two
Eu(III) ions, are hydroxides and two oxygen atoms, O15 and
O17, are water molecules bound to Eu(III) centers. This
hydroxo/oxo core is expected given the neutral to basic pH
required for the formation of this complex and the general
Lewis acidity of the lanthanides.
4-
2
O)
2 39
(PW11O )
com-
3
1
solution and the P NMR is identical with a sample of
4-
Eu(PW11
the monomeric form.
9
Species 3. Addition of Cs to PW O
34 and Eu3+ (1:1
O39) , suggesting that the polymer dissociates into
+
9-
3+
Species 2. The addition of Al into the solution of
9
-
3+
stoichiometry) at pH 7 results in 3 as small colorless thick
PW
9
O
34
with Eu (2:1 Eu:POM) results in a drop in
rectangular crystals. The crystal structure, Figure S6, shows
pH from 7 to 2.8. As the predominant polyoxotungstate
7
-
that this species is a 1:2 Eu:PW11O39 complex, originally
(
25) Thorp, H. H. Inorg. Chem. 1992, 31, 1585-1588.
(26) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244-247.
(27) Spek, A. L. Acta Crystallogr. 1990, A 46, C34.
7698 Inorganic Chemistry, Vol. 43, No. 24, 2004

Aqueous Speciation of Eu(III) Phosphotungstate
Figure 6. Left: Ball-and-stick structure of {Eu(H2O)3(R-2-P2W17O61)}214-, 4. Right: Packing diagram of 4, viewed along the a axis. Uncoordinated H2O
molecules have been removed for clarity. Legend: W, blue; O, red; Eu, green; P, yellow; Al, pink.
minal oxygen of an adjacent polyoxoanion and to the surface
3
+
31
183
bonding of the Al cations. Both P and W NMR show
good evidence to support the notion that the dimeric complex
1
4-
{
Eu(H
2
O)
3
(R-2-P
2
W
17
O61)}
2
in the solid state dissociates
in aqueous solution to form the monomeric form Eu(R-2-
7
-
P
2
W
17 61
O )
(Table 3).
Discussion
Three parameters, pH, countercation, and stoichiometry,
have an important impact on the aqueous speciation of
9
-
3+
PW
9
O
34
and Eu . The pH influences the speciation of
9
-
the polyoxometalate PW
9
O34 , alone, to form primarily
Figure 7. Packing diagram of 4, viewed along the c axis. Uncoordinated
H2O molecules have been removed for clarity. Legend: W, blue; O, red;
Eu, green; P, yellow; Al, pink.
7
-
22
PW11
O39 ligand and decomposition products (Table S1)
to which the Eu(III) can bind. The countercations modulate
the reactivity and may be instrumental in anchoring poly-
oxometalate units together for assembling larger structures,
isolated by Peacock and Weakley.28 The 31P NMR shows
one peak at 0.34 ppm that is identical with the 1:2
11-
such as the “sandwich” complex Eu(PW O ) , 3, and
1
1
39 2
7
-
183
30
Eu:PW11
of 1:2 analogue Y(PW11
), consistent with the C
O
39 complex, prepared directly. The W NMR
the hydroxo/oxo cluster, 1.
11-
11,29
O
39
)
2
shows 11 peaks
structure revealed by X-ray
crystallography. All 11 Cs ions were located and surround
(Table
Under low pH conditions, the high acid content stabilizes
3
2
+
7-
4-
the PW11
O39 ligand as well as species 2, Eu(PW11O39) ,
the 1:1 species. As the base is increased, and proton
11-
+
the Eu(PW11
O
39
)
2
species (Figure S7), with 1 Cs ion (C7)
7-
4-
39
competition with the PW11O39 ligand and Eu(PW11O )
located in an interactive distance of 3.2 Å to both PW11
O
39
(
2) decreases, the concentration of the 1:2 complex,
lobes.
11-
Eu(PW11
where PW O
O
39
)
2
(3), is increased. Eventually at high pH,
3+
9-
34
9-
7-
Species 4. Addition of Al into the solution of PW
9
O
decomposes to PW O , lower nuclearity
9
34
11 39
3
+
2-
+
Eu (1:1 stoichiometry) results in colorless chunky prisms
phosphotungstates, as well as WO4 species 3 and 1, become
of 4 in 53% yield. X-ray crystallography reveals the dimeric
major components of the solution, as seen in Figures 1, S1,
14-
11-
structure {Eu(H
2
O)
3 2
(R-2-P W
17
O61)}
2
(Figure 6) that we
and S2. Species 1 is comprised of the PW O
fragment
1
0
38
have observed before from the reaction of excess Eu(III) with
and monotungstate units bound to Eu(III).
1
0-
14
R-2-P
the Al(III) serves as a counterion and forms weak connections
with the terminal W-O bonds of adjacent {Eu(H O) (R-2-
2
W
17
O
61
in a KCl medium. However, in this case,
Cations have a significant effect on the speciation of
9-
9
PW O34 and Eu(III). The hypothesis that large, less ex-
2
3
tensively hydrated alkali metal cations bind to the surfaces
of the polyoxometalates and “anchor” the POM units in place
14-
P
2
W
17
O61)}
2
units, seen in Figures 6 and 7.
The extended structure is composed of the anions of 4
for assembly into larger structures, such as the 1:2 sandwich
linked together by surface bound Al3 cations. The Al ions,
in a distorted trigonal prismatic coordination environment,
bind to terminal oxygen atoms of two dimers (average Al-O
distances: 2.77(5) Å) giving rise to chains along the
crystallographic a axis (Figure 6), while, down the c axis,
+
3+
31,32
structures, has been put forward.
Ion pairing of large
+
cations (Rb ) to POM surfaces was proposed by Kirby and
Baker in a study to understand the solution “syn” structure
1
6-31
+
of Th(R-2-P W O ) .
2
17 61 2
The formation of a 1:2 K :
+
SiW O species in a Cs environment has been reported
recently by Laronze. In this latter study, similar to ours,
1
1
39
3+
14-
32
Al links three {Eu(H
2
O)
3
(R-2-P
2
W
17 61
O }
2
units giving
1
0-
+
rise to the formation of discrete channels forming a porous
treatment of the trivacant A-R-SiW O
with K and
9
34
3
D structure (pore size: 17 Å × 6 Å; Figure 7). This com-
pound is less soluble in water than anions of similar size,
(30) We have also observed Eu(III) and Eu(III)-OH-Eu(III) dimeric units
serving as countercations. These moieties bind to terminal WdO bonds
of polyoxometalates: Zhang, C.; Howell, R. C.; Francesconi, L. C.
Manuscript in preparation, 2004.
probably due to the covalent bonding of the Eu3+ to a ter-
(
(
28) Peacock, R. D.; Weakley, T. J. R. J. Chem. Soc. 1971, A, 1836-39.
29) Fedotov, M. A.; Pertsikov, B. Z.; Danovich, D. K. Polyhedron 1990,
(31) Kirby, J. F.; Baker, L., C. W. Inorg. Chem. 1998, 37, 5537-5545.
(32) Laronze, N.; Marrot, J.; Herve, G. Inorg. Chem. 2003, 42, 5857-
5862.
9, 1249-1256.
Inorganic Chemistry, Vol. 43, No. 24, 2004 7699

Zhang et al.
+
excess Cs results in isolation of Cs15K(SiW11
O
39
)
2
, where
are binding to surface sites of POMs and thus stabilize the
1:2 structures in solution.
+
K is bound in the cavity. This molecule is similar to 3,
with cubic coordination about the K rather than square
antiprismatic coordination geometry that is prevalent with
the lanthanides. They propose, according to the solid-state
crystal structure, that the Cs anchors the SiW11
+
In contrast, the addition of large organic counterions such
+
as tetrabutylammonium (TBA ) chloride or tetraethylam-
+
9-
monium (TEA ) chloride into the solution of PW
9
O
34
+
+
8-
3+
9-
34
O
39 species
Eu (2:1 and 1:1 Eu:PW
formation of exclusively species 2, Eu(PW11O39) . It has
9
O
stoichiometry) results in
+
4-
in place for the formation of the 1:2 K :POM complex. To
substantiate this proposal, the solid-state structure of
been demonstrated recently, in ion-pairing experiments, that
large organic countercations do not bind to the surface of
+
Cs15K(SiW11
terminal oxygen atoms of each SiW11
O
39
)
2
shows that three Cs ions are bound to
10-
34
O
39
unit (with Cs-O
POMs to form ion pairs. The stabilization of the 1:1
15-
4-
11-
distances of ca. 3.2 Å) perhaps stabilizing the K(SiW11
O )
39 2
39
Eu(PW11O )
over the 1:2 Eu(PW11
39
O )
2
species with
solid-state structure. The molecule dissociates in aqueous
TBA and TEA, then, is consistent with their lack of binding
to the POM surfaces.
solution.
+
The effect of Al(III) as a counterion has also been probed.
Similarly, we have observed that in aqueous solution K
14-
+
11-
To understand the formation of the Eu(R-2-P
2 17 61 2
W O )
and Cs promote the formation of the 1:2 Eu(PW11
species. The formation of species 3, 1:2 Eu(PW11
O
39
)
2
11-,
species, 4, with the addition of Al(III), we analyzed solutions
O
39
)
2
9
-
3+
9
of the PW O34 ligand in the presence of Al . Treatment
in significant concentration is observed at low pH (4.5) for
9
-
+
of PW
9
O
34 with Al(III) results in a lowering of the pH to
solutions containing K buffer (Figure S2). In contrast 3
+
+
2
-3, and it is possible that condensation of two R-A-
appears only at higher pH (6.5) with Li - and Na -containing
buffers and in very low quantities (Figures 2 and S1). When
9
-
31
9
PW O34 moieties can occur. The P NMR reveals small,
+
equal-intensity peaks that likely correspond to a Wells-
Dawson molecule, possibly a â-P 62 molecule, an
R-P 62 with Al(III) bonded unsymmetrically to its faces,
or the monolacunary R2-P with Al(III) occupying
Cs is employed as a counterion, 3 is the only species formed
2
18
W O
(
Figure 2). These observations are consistent with ion pairing
+
+
2 18
W O
of K and Cs to the PW11
O39 lobes, thus anchoring the
10-
W O
2 17 61
PW11O39 lobes in position to form the 1:2 complex.
the vacancy or near the vacancy (Table S1). It is likely that
Eu(III) binds into the vacancy to form the small amount of
complex 4 observed in the solution (Figure 2).
Considering the solid-state structure of Cs11Eu(PW11
O
39
)
2
,
+
one Cs (Cs7) is in close contact with the terminal W-O
and bridging W-O oxygen atoms (3.2 Å) from both lobes
and may be a prototype as to the role large cations have in
9
-
The stoichiometry of Eu and PW
9
O34 play an important
+
role in the speciation of Eu polyoxometalates, as shown in
stabilizing this structure. The remaining 10 Cs ions surround
9
-
+
Figure 3. In an excess of PW
9
O
34 at pH 7, decomposition
39 with some unidentified P-containing
products and phosphate as well as tungstate (Table S1) results
the sandwich complex. The Cs ions may insulate and
7-
to primarily PW11
O
stabilize the sandwich Eu(PW11
Similar interaction of Cs ions with the metal oxygen
surfaces may stabilize this structure in solution as well.
O
39
)
2
ions in the crystal.
+
1
1-
4-
in formation of both Eu(PW11
O
39
)
2
, 3, and Eu(PW11O39) ,
2
. With more Eu(III) present, as in 1:1 stoichiometry, under
Ion pairing of alkali metal cations to polyoxometalates has
been quantitated in recent studies. The larger and less
extensively solvated alkali-metal cations form smaller (more
+
+
+
7-
neutral conditions and with Na , K , and NH
and lower nuclearity phosphotungstates (such as PW10
can be trapped by the excess Eu(III) ions and monotungstate
groups to form Eu hydroxo/oxo cluster 1. The 2:1 Eu:
34 stoichiometry provides an excess of Eu(III) ions
4 39
, PW11O
O
3811-)
n+
(9-n)-
intimate) association complexes with the {X VW11
40
O }
3
3,34
8
(
X ) P(V), Si(IV), Al(III)) family of polyoxometalates.
9
-
PW
9
O
The smaller association complexes impact physical properties
of the polyoxometalates, for example the diffusion coef-
ficients and electron-transfer rates of organic oxidation reac-
tions. The countercation effect can be further substantiated
by our observations on the solution speciation of the 1:2 Ln-
4
-
to promote formation of Eu(PW11
O
39) , 2.
The crystal structures of species 2 and 4, shown in Figures
-7, are instructive to examine the potential interaction of
5
oxophilic Al(III) counterions with polyoxometalate surfaces.
Species 2 and 4 show weak interactions of the Al(III) ions
with terminal oxygen atoms of the polyoxometalate. The
interaction, although weak, allows connection of the poly-
oxometalates and buildup of a porous material, as seen in
species 4.
We have investigated only the effects of pH and coun-
tercation in this study. However, other ions also can serve
as templates or structure-directing groups. This was seen
1
4-
35
(
R-1-P
W O )
2 17 61 2
species. We find that countercations are
14-
critical to the formation of 1:2 Ln(R-1-P
in aqueous solution. In these studies, P NMR shows clearly
W O )
2 17 61 2
species
31
+
that in aqueous solution the 1:2 species is maintained in Cs
buffer, whereas an equilibrium between the 1:1 and 1:2
+
+
is set up with K . Li buffer results in complete dissocia-
1
4-
tion of the 1:2 Ln(R-1-P
2
W
17 61
O )
2
species into the 1:1
7-
10-
Ln(R-1-P
2
W
17
O
61
)
species and (R-1-P
We suspect that the Cs ions and, to an extent, the K ions
2
W
O
17 61
)
ligand.
clearly in a recent report by Hill wherein the addition of
+
+
III
carbonate to an aqueous solution of Y Cl
3
followed by
addition of Na
new complex that consists of a Y
8
HA-R-PW
9
O
34 and workup forms 33% of a
(PW sandwich
anchors the Y(III) ions and the two
POM units. We find that species 1 forms without the
(
(
(
33) Grigoriev, V. A.; Hill, C. L.; Weinstock, I. A. J. Am. Chem. Soc.
3
9 34 2
O )
2000, 122, 3544-3545.
2
-
34) Grigoriev, V. A.; Cheng, D.; Hill, C. L.; Weinstock, I. A. J. Am. Chem.
Soc. 2001, 123, 5292-5307.
complex wherein CO
3
3
6
35) Cheng, Z.; Howell, R. C.; Luo, Q.; Fieselmann, H. L.; Todaro, L.;
Francesconi, L. C. 2004, manuscript in preparation.
2-
3
addition of CO . In this case, carbonate appears to template
7700 Inorganic Chemistry, Vol. 43, No. 24, 2004

Aqueous Speciation of Eu(III) Phosphotungstate
the formation of this sandwich complex. The structure is
in the solid state and in solution. The pH has a profound
37
7-
reminiscent of the Cu (PW O )
3 9 34 2
templated by nitrate anion.
effect on the speciation; at low pH, (PW11
O
39
)
predomi-
4
-
We employ multinuclear NMR coupled with X-ray
crystallography in these studies; these techniques allow us
to understand the speciation unambiguously. We and others
have also used luminescence of Eu(III) for the study of
nates and the 1:1 Eu(PW11
increased, the 1:2 Weakley complex, Eu(PW11
cies, 3, and a Eu hydroxo/oxo cluster, 1, forms. The
countercations modulate this effect. Large countercations,
O39) , 2, forms. As the pH is
11-
O
39
)
2
spe-
8
4,12,14,17
+
+
speciation of lanthanide polyoxometalates.
Eu(III)
such as K and Cs , with low hydrodynamic radii (compared
+
+
4-
luminescence is very informative to identify the number of
species in solution, the coordination environment, and the
number of bound water molecules. Luminescence studies
39
to Li and Na ) promote the formation of 1:2 Eu(PW11O )
species, 3, and 1, at low pH values. We postulate that these
large countercations can interact with the polyoxometalate
4
-
(
excitation, emission spectra, and luminescence lifetimes) of
surface and position one Eu(PW11
O
39
)
unit for binding by
3
O) -
7
-
7-
the 1:1 and 1:2 Eu:(PW11
ported.3
O
39
)
species have been re-
39
a second (PW11O )
moiety. The formation of {Eu(H
2
8-41
These studies show that the 1:1 species has four
(R-2-P W O61)} , 4, with Al(III) ions bound to terminal
2 17 2
bound water molecules, while the 1:2 species is not bound
to water molecules. These are consistent with our structural
and solution data, reported herein.
W-O bonds is observed when Al(III) is employed as a
countercation.
Acknowledgment. We acknowledge the following sources
Conclusion
of support for this research: NIH MARC/MBRS program
(
F.G.P., K.B.S.); Gertrude Elion Fellowship and Rose Kfar
We have focused this study on understanding speciation
chemistry of lanthanide complexes of the trivacant polyoxo-
Rose Dissertation Award (C.Z.); Faculty Research Award
Program of the City University of New York, National
Science Foundation Grant No. CHE-0414218, Grant NIH-
S06 GM60654 (SCORE) (L.C.F.); NSF Grant MRI0116244
for the purchase of an X-ray Diffractometer. Research
infrastructure at Hunter College is partially supported by
NIH-Research Centers in Minority Institutions Grant
RR03037-08.
9-
9-
metalate PW
9
O
34 in aqueous solution. The A-R-PW O
9 34
has 6 basic oxygen atoms that are available for bonding. The
reaction products are highly dependent on the pH, counter-
9-
9
cation, and stoichiometry between Eu(III) and PW O34 . The
four species that have been observed in solution speciation
studies have been synthesized and thoroughly characterized
(
36) Fang, X.; Anderson, T. M.; Neiwert, W. A.; Hill, C. L. Inorg. Chem.
003, 42, 8600-8602.
37) Knoth, W. H.; Domaille, P. J.; Harlow, R. L. Inorg. Chem. 1986, 25,
Supporting Information Available: Tables S1 and S2; 31P
NMR chemical shifts for speciation of PW O34 as a function of
9
2
9-
(
1
577-1584.
pH, buffer, and countercation and bond valence sum calculations;
Figures S1-S7; and CIF files for the X-ray crystal structures of
species 1-4. This material is available free of charge via the Internet
at http://pubs.acs.org.
(
(
(
38) Lis, S.; But, S. J. Alloys Compd. 2000, 300-301, 370-376.
39) Yusov, A. B.; Fedoseev, A. M. Radiokhimiya 1992, 34, 61-70.
40) Blasse, G.; Dirksen, G. J.; Zonnevijlle, F. J. Inorg. Nucl. Chem. 1981,
4
3, 2847-2853.
41) Van Pelt, C. E.; Crooks, W. J., III.; Choppin, G. R. Inorg. Chim. Acta
003, in press.
(
2
IC049655H
Inorganic Chemistry, Vol. 43, No. 24, 2004 7701