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3. Results and Discussion
3.1. Ionic liquids based on Keggin clusters
In this work a systematic investigation of how the alkyl chain length of the cations
effects the resulting compounds properties was conducted. As model system worked
the archetypal Keggin-cluster as monolacunary anion with Cu
II
and Fe
III
embedded as
additional transition metal in the lacuna. This cluster family is an ideal prototype as it
offers wide coordination flexibility,
[59]
good solution stability
[56]
and facile synthetic
access.
[56]
The cluster was charge balanced with tetraalkylammonium cations Q
x
(x
denotes alkyl chain length, from
n-
(C
5
H
11
)
4
N
+
(= Q
5
) to
n-
(C
8
H
19
)
4
N
+
(= Q
8
)
(
1
,
2
,
3
,
4
,
6
,
8
,
13
,
14
).
As the cluster charge determines the number of counter cations does it have important
influence on many physical parameters (viscosity, melting point, etc.). A convenient way
of influencing the charge of the cluster anion without applying completely different
cluster types is to vary the templating heteroelement or the transition metal
incorporated in the lacunary site, both of which are also of interest with respect to the
chemical reactivity. In this work six transition metal (TM)-substituted Keggin anions of
the type [SiW
11
O
39
TM(H
2
O)]
n
-
(TM =Cr
III
, Fe
III
, Co
II
, Ni
II
, Cu
II
, Mn
II
) were synthesized and
investigated for this purpose (
6
-
11
). For heteroelement variation Keggin anions of the
type [XW
11
O
39
TM(H
2
O)]
n
-
(X = B
3+
, Si
4+
, P
+
) were applied, substituted with Cu
II
(
22,
6
,
24
)
and Fe
III
(
23
,
8
,
25
) respectively. Tetraheptylammonium ions (= Q
7
)
were applied as
counterions for this investigation.
3.1.1 Structure and synthesis of the Si-templated Keggin clusters
Early transition metal polyoxoanions have traditionally posed serious challenges to the
synthetic chemist because of their stoichiometric and structural complexity. However
over the past decades an increasingly systematic approach to their synthesis was
developed mainly based on empiric observations.
[56]
A compound class of clusters that is well investigated with well established synthetic
routs is those of Keggin clusters based on tungsten or molybdenum as the addenda
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