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coordination numbers as well as easy exchange of water ligands at the addenda atoms,
the moderate strength of Mo-O-Mo type bonds allowing "split and link" type processes,
the easy exchange of electron densities without a strong tendency to form metal-metal
bonds and the presence of the terminal M=O groups preventing in principle unlimited
growth to extended structures.
[42]
1.3.3 Template assisted cluster formation
One very well reviewed structure exemplifying a self-assembly condensation process is
that of the Keggin polyoxometalate type [XM
12
O
40
]
n-
(M = Mo, W; X = B
3+
, Si
4+
, P
5+
). This
cluster-type will be used in the following to more precisely illustrate the key principles
of a templated self-assembly process of POM-frameworks.
In case of the Keggin structure, oxoanions such as borate, silicate or phosphate can act
as templates and allow the controlled Keggin anion synthesis. The typical Keggin
structure with T
d
symmetry is then formed of four aggregating triads templated by a
tetrahedral anion XO
4
n-
. This self-assembly process is exemplified by the formation of
the
D
-Keggin-type [SiW
12
O
40
]
8-
anion as shown in Figure 7 in step (2), where a
tetrahedral PO
4
3-
anion works as the template.
Figure 7
Self-assembly of the Keggin anion [
D
-SiW
12
O
40
]
4-
. (1) Oxometalate building blocks (e.g. {WO
6
},
{W
3
O
13
} are spontaneously formed in solution by acidification. (2) In the presence of tetrahedral
templates (e.g. SiO
4
4-
), the building blocks assemble to give the Keggin anion [
D
-SiW
12
O
40
]
4-
. (3) Removal
of one W center leads to the formation of a lacunary species, [SiW
11
O
39
]
8-
(the structural vacancy is
marked with an orange arrow). (4) Transition metal (M) incorporation into the structural vacancy leads to
the formation of a TM-functionalized Keggin anion [SiW
11
O
39
M(H
2
O)]
n-
. Color scheme: W (black), O (red),
Si (purple).
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