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atom. Simple metal oxide precursors such as molybdenum(VI) oxide, MoO
3
or sodium
molybdate, Na
2
MoO
4
are dissolved in aqueous media. Acidification of the solution leads
to an expansion of the coordination shell from 4 to 6, from a tetrahedral to an
octahedral oxygen coordination environment. This in turn enables protonation of the
terminal oxygens and facilitates condensation reactions between the particular [MO
6
]
fragments. As a result, larger structures are self-assembled in solution.
As precursor for the ionic liquids based on Keggin-type clusters the K
8
[
D
-
SiW
11
O
39
]
ڄ
13H
2
O was used. It is derived from the Keggin anion [SiW
12
O
40
]
4-
(see Figure
13 left part), which structurally belongs to a group of clusters that are made up by
W
n
O
3n
units encapsulating one or more formally anionic subunits. The main network of
this cluster is built of a W
12
O
36
cage and therefore of four independent [W
3
O
9
] subunits
plus [SiO
4
]
4-
encapsulated as tetrahedral oxoanion. This leads effectively to a truncated
tetrahedron of T
d
-symmetry (see Figure 13 right part with the representation boiled
down to the central tetrahedron drawn into the dodecanuclear framework).
Figure 13
Different representations of the Keggin structure. left: polyhedral representation of the four
[W
3
O
9
] triads (yellow, red, blue, green) enclosing the tetrahedral template [SiO
4
]
4-
(magenta). middle:
View alongside one of the four Si-O bonds falling together with one of the eight C
3
-axes. Top [W
3
O
9
]-triad
(green) transparently colored showing the connectivity in ball and stick representation with the addenda
atoms (blue), oxygen (red) and template (pink). right: Representation boiled down to the T
d
-symmetry of
the Keggin-cluster with the three atoms of each of the four triads interconnected and the templates Si-O
bonds pointing towards the face of each triangle.
The monolacunary [
D
Ǧ
SiW
11
O
39
]
8-
is built exactly the same way only with one W-O unit
missing, creating a vacancy of one polyhedron at its place (see Figure 14 below). It was
chosen as the main precursor of this work mainly for the following three reasons: First
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