A new structural family of heteropolytungstate lacunary complexes
with the uranyl, UO
2
2
, cation
Andrew J. Gaunt,*
a
Iain May,
a
Roy Copping,
a
Anand I. Bhatt,
a
David Collison,
b
O. Danny Fox,
c
K. Travis Holman
d
and Michael T. Pope*
d
a
Centre for Radiochemistry Research, Department of Chemistry,
The University of Manchester, Manchester, UK M13 9PL.
E-mail: gaunt@lanl.gov; Fax: 44 (0)161 275 4616; Tel: 44 (0)161 275 4657
b
Department of Chemistry, The University of Manchester, Manchester, UK M13 9PL
c
Research and Technology, BNFL, Sellafield, Seascale, Cumbria, UK CA20 1PG
d
Department of Chemistry, Georgetown University, Box 571227, Washington, DC 20057-1227,
USA. E-mail: popem@georgetown.edu
Received 14th March 2003, Accepted 9th June 2003
First published as an Advance Article on the web 27th June 2003
The reaction of UO
2
2+
with the trivacant lacunary polyoxometalate anions, [SbW
9
O
33
]
9-
and [TeW
9
O
33
]
8-
, yields the
novel isostructural complexes [(UO
2
)
2
(H
2
O)
2
(SbW
9
O
33
)
2
]
14-
(1) and [(UO
2
)
2
(H
2
O)
2
(TeW
9
O
33
)
2
]
12-
(2), respectively.
The complex anions contain two [XW
9
O
33
]
n-
(X = Sb
III
or Te
IV
) anions linked by two UO
2
2+
cations. Each uranyl
moiety bonds to two unsaturated oxygen atoms of each lacunary anion in the complex. Each [XW
9
O
33
]
n-
anion has
six unsaturated oxygen atoms meaning that in 1 and 2 each [XW
9
O
33
]
n-
anion has two unsaturated oxygen atoms
which remain uncoordinated to uranium with the result being the formation of an ‘open’ sandwich structure. The
fact that a third UO
2
2+
cation is not coordinated to form a ‘closed’ sandwich structure (as is observed for first row
d-block transition metals) is attributed to the steric hindrance of the axial ‘yl’ oxygen atoms of the uranyl group.
The products, prepared as NH
4
+
salts, have been characterised by single crystal X-ray diffraction, elemental analysis,
TGA analysis, IR, Raman and UV/vis spectroscopy, which indicate that the O donor atoms of the lacunary
heteropolytungstate anions are strongly coordinating to U() in the equatorial plane, weakening the uranyl U–O
axial bonds.
Introduction
The synthesis and characterisation of heteropoly compounds
containing uranium is of current interest because of their
potential use for nuclear waste sequestration and storage.
1
Until
recently the chemistry of the uranyl cation, UO
2
2+
, with poly-
oxometalates has been largely neglected. Actinyl species are
thought to be weakly coordinating to monovacant lacunary
anions, such as [PW
11
O
39
]
7-
, because of the steric constraints of
the linear dioxo group.
2
However, Pope and co-workers have
reported several structural characterisations of complexes
of UO
2
2+
with various di- and trivacant lacunary hetero-
polyanions. The first structurally characterised uranyl–poly-
oxometalate complex was from the reaction with [PW
9
O
34
]
9-
yielding the sandwich complex [Na
2
(UO
2
)
2
(PW
9
O
34
)
2
]
12-
in
which two UO
2
2+
ions are sandwiched between two [PW
9
O
34
]
9-
ions and are also linked to each other through Na
+
ions.
3
More recently the crystal structure and characterisation of the
NpO
2
+
analogue has been reported.
4
[γ-SiW
10
O
36
]
8-
reacts with
UO
2
2+
to form the tetrameric complex [{Na(OH
2
)}
4
-
(UO
2
)
4
(SiW
10
O
36
)
4
]
22-
, which contains a U–O–Na array that
links the four [γ-SiW
10
O
36
]
8-
anions and four UO
2
2+
cations.
5
Of most interest to this present study was that of the
interaction between UO
2
2+
and [NaAs
4
W
40
O
140
]
27-
(a cyclic
assembly of four [AsW
9
O
33
]
9-
anions linked by WO
6
octa-
hedra), in which both plenary, [(UO
2
)
3
(H
2
O)
6
As
3
W
30
O
105
]
15-
,
and lacunary, [(UO
2
)
3
(H
2
O)
5
As
3
W
29
O
104
]
19-
, complexes are
formed.
6
The plenary anion consists of three B-[AsW
9
O
33
]
9-
anions linked by three uranyl cations. The [AsW
9
O
33
]
9-
anions
are also linked through three corner shared WO
6
octahedra.
First row transition metal ions react with lone pair contain-
ing [XW
9
O
33
]
x-
anions to yield closed dimeric structures in
which three metal cations are sandwiched between two lacunary
polyoxometalate anions with general formula [(XW
9
O
33
)
2
-
M
3
(H
2
O)
3
]
n-
(n = 12, X = As
III
, Sb
III
, M = Cu
2+
, Zn
2+
; n = 10,
X = Se
IV
, Te
IV
, M = Cu
2+
).
7
Trivacant polyoxoanions also form
novel structural types with 4f-elements such as [Eu
3
(H
2
O)
3
-
(SbW
9
O
33
)(W
5
O
18
)
3
]
18-
,
8
and very recently the crown shaped
ring complexes [K{Eu(H
2
O)
2
(AsW
9
O
33
)}
6
]
35-
and [Cs{Eu-
(H
2
O)
2
(AsW
9
O
33
)}
4
]
23-
in which Eu
3+
ions link a cyclic array of
lacunary anions with encapsulation of the alkali metal ion.
9
We report here the interaction of UO
2
2+
with [SbW
9
O
33
]
9-
and [TeW
9
O
33
]
8-
resulting, not in complexes analogous to those
with [AsW
9
O
33
]
9-
, but unexpectedly dimeric lacunary com-
plexes with formulae [(UO
2
)
2
(H
2
O)
2
(SbW
9
O
33
)
2
]
14-
and [(UO
2
)
2
-
(H
2
O)
2
(TeW
9
O
33
)
2
]
12-
.
Experimental
General
Chemicals were obtained from commercial suppliers and used
without further purification. Water was purified by distillation.
Syntheses
Na
9
[SbW
9
O
33
]19.5H
2
O was prepared according to a published
literature method.
10
Na
8
[TeW
9
O
33
]19.5H
2
O. was prepared using the same
method as for Na
9
[SbW
9
O
33
]19.5H
2
O, but replacing Sb
2
O
3
with TeO
2
as follows: Na
2
WO
4
2H
2
O (40 g, 121 mmol) was
dissolved in boiling H
2
O (80 cm
3
). TeO
2
(2.14 g, 13.44 mmol)
was dissolved in concentrated HCl (10 cm
3
) and the resulting
solution added dropwise to the tungsten containing solution.
The mixture was refluxed for 1 h then allowed to cool to room
temperature. Slow evaporation in air over several days yielded
colourless crystals of the target compound (25.39 g, 66% yield),
which were washed with ice-cold H
2
O (5 cm
3
) and dried in air.
(NH
4
)
14
[(UO
2
)
2
(H
2
O)
2
(SbW
9
O
33
)
2
]24H
2
O. Na
9
[SbW
9
O
33
]
19.5H
2
O (1.14 g, 0.40 mmol) was suspended in H
2
O (100 cm
3
)
DOI: 10.1039/ b302955g
3009
This journal is © The Royal Society of Chemistry 2003 Dalton Trans. , 2003, 3009–3014