Synthesis and pH-variable ultracentrifugation molecular weight
measurements of the dimeric, Ti–O–Ti bridged anhydride form
of a novel di-Ti
IV
-1,2-substituted -Keggin polyoxotungstate.
Molecular structure of the [(-1,2-PW
10
Ti
2
O
39
)
2
]
10
polyoxoanion †
Kenji Nomiya,*
a
Mizuto Takahashi,
a
Jason A. Widegren,
b
Takao Aizawa,
a
Yoshitaka Sakai
a
and Noriko C. Kasuga
a
a
Department of Materials Science, Faculty of Science, Kanagawa University, Hiratsuka,
Kanagawa 259-1293, Japan. E-mail: nomiya@chem.kanagawa-u.ac.jp
b
Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA
Received 16th May 2002, Accepted 31st July 2002
First published as an Advance Article on the web 10th September 2002
The preparation and characterization of a Keggin-type, novel di-Ti
IV
-1,2-substituted polyoxotungstate are described.
The dimeric, Ti–O–Ti bridged anhydride form of the di-Ti
IV
-1,2-substituted α-Keggin polyoxotungstate, K
10
[α,α-
P
2
W
20
Ti
4
O
78
]12H
2
O 1, was unexpectedly found in the varied molar-ratio reactions of tri-lacunary precursor Na
9
-
[A-PW
9
O
34
]19H
2
O with Ti(SO
4
)
2
in aqueous solution. Although this compound was first found as a minor product
in the preparation of the dimeric, tri-Ti
IV
-1,2,3-substituted species, K
10
H
2
[α,α-P
2
W
18
Ti
6
O
77
]17H
2
O 3, it was
successfully prepared as a main product in this work and structurally characterized. Compound 1, as analytically
pure, homogeneous colorless needle crystals, was obtained as a major product in 29.2% yield (2.7 g scale) from
recrystallization under acidic conditions (at pH 2.2) of the 1 : 2 molar-ratio reaction product. X-Ray structure
analysis revealed that the molecular structure of 1 consisted of a dimeric anhydride formed by two Ti–O–Ti
bonds linking two [α-1,2-PW
10
Ti
2
O
40
]
7-
Keggin units. Interestingly, ultracentrifugation molecular weight (MW)
measurements of this compound in aqueous solution showed the pH-dependent interconversion between
monomer [α-1,2-PW
10
Ti
2
O
40
]
7-
2 and dimer [α,α-P
2
W
20
Ti
4
O
78
]
10-
1; this compound was present as the
monomer under less acidic conditions (pH 7.8), while it was in dimeric form under more acidic
conditions (pH 1.0 and 2.2). Characterization of 1 was also accomplished by complete
elemental analyses, TG/DTA, FTIR and solution (
31
P and
183
W) NMR spectroscopy.
Introduction
Polyoxoanions are molecular metal-oxide clusters which are
of current interest as soluble analogs of heterogeneous metal
oxides
1a–c
and for their application to catalysis, medicine
and material sciences.
1d–i
In particular, the polyoxotungstates
substituted with the early transition metal (d
0
) ions such as V
V
and Nb
V
have been intensively studied,
2,3
because they allow
covalent bonding of organometallic fragments to specific
binding sites on the polyoxoanion surface.
3,4
Substitution of W
VI
in polyoxoanions with Ti
IV
is particu-
larly interesting, because of the expected, much higher basicity
of the resulting polyoxoanion. The ionic radius of Ti
IV
(0.75 Å)
is close to that of W
VI
(0.74 Å), a fact which suggests that
Ti
IV
should fit nicely into the polyoxotungstate framework.
However, there is a significant issue of Ti–O–Ti anhydride
formation.
5c,d,g
Indeed, the tri-Ti
IV
-1,2,3-substituted Keggin
polyoxotungstates heretofore prepared are the dimeric, Ti–O–
Ti bridged anhydride forms, e.g. [β,β-Si
2
W
18
Ti
6
O
77
]
14-
4,
5c
which
is formed from two [A-β-SiW
9
Ti
3
O
40
]
10-
Keggin units, and
[α,α-Ge
2
W
18
Ti
6
O
77
]
14-
5,
5d
which is formed from two [A-α-
GeW
9
Ti
3
O
40
]
10-
Keggin fragments, and the very recently found
[α,α-P
2
W
18
Ti
6
O
77
]
12-
3 and [α,α-Si
2
W
18
Ti
6
O
77
]
14-
,
5j
which are
formed from two [A-α-PW
9
Ti
3
O
40
]
9-
Keggin units and two
[A-α-SiW
9
Ti
3
O
40
]
10-
Keggin units, respectively. Also, the
recently found tri-Ti
IV
-substituted Dawson polyoxotungstates
† Electronic supplementary information (ESI) available: further bond
lengths and angles for compound 1. See http://www.rsc.org/suppdata/
dt/b2/b204775f/
have been elucidated to be a tetrameric, Ti–O–Ti anhydride
form, i.e. [(P
2
W
15
Ti
3
O
60.5
)
4
]
36-
6,
5g
which is composed of four
[α-1,2,3-P
2
W
15
Ti
3
O
62
]
12-
Dawson fragments to form the T
d
tetramer. On the other hand, the monomeric, mono- and
di-Ti
IV
-substituted Keggin polyoxotungstates have been realized
in [α-1,5-PW
10
Ti
2
O
40
]
7- 5b,f
and [α-PW
11
TiO
40
]
5-
.
5a,e
In our recent preparation of [α,α-P
2
W
18
Ti
6
O
77
]
12-
3,
5j
we
found that a di-Ti
IV
-substituted species, which showed different
31
P and
183
W NMR spectra from those of 3, was formed as a
minor product. Thus, we carefully examined the reactions of
tri-lacunary precursor [A-PW
9
O
34
]
9-
with Ti(SO
4
)
2
in aqueous
solution by changing their respective molar ratios and found
conditions in which the novel di-Ti
IV
-1,2-substituted α-Keggin
species is the main product. Herein, we report full details of
the synthesis and structural characterization of [α,α-P
2
W
20
-
Ti
4
O
78
]
10-
1.
Results and discussion
Synthesis, isolation and compositional characterization
The water-soluble potassium salt of 1, K
10
[α,α-P
2
W
20
Ti
4
O
78
]
12H
2
O, was obtained in 29.2% (2.7 g scale) yield by recrystalliz-
ing, under acidic conditions (unbuffered pH 2.2 solution), the
white powder obtained from 1 : 2 molar-ratio reaction of
Na
9
[A-PW
9
O
34
]19H
2
O with Ti(SO
4
)
2
4H
2
O in aqueous solu-
tion. Compound 1 was first found as a minor product in our
recent preparation of 3.
5j
We examined by
31
P NMR the form-
ation of 1 and 3 by changing the molar ratio of [A-PW
9
O
34
]
9-
and Ti(SO
4
)
2
in aqueous solution, and found the optimal molar
DALTON
FULL PAPER
DOI: 10.1039/b204775f J. Chem. Soc., Dalton Trans., 2002, 3679–3685 3679
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