Chemistry Reference
In-Depth Information
Fe
+
Fe
+
N
H
H
N
O
O
C O
CO
P
O
O
H
H
N
N
+
O
H
H
N
C
160
140
120
100
80
60
40
20
0
0
+
N
H
N
C
N
N
N
O
H
H
O
O
O
O
P
P
N
N
O
O
O
O
(b)
H
H
O
H
N
N
N
N
C
N
+
N
H
C
H
1
O
+
0.2
0.4 0.6
Eqv. anion per FC
0.8
N
H
N
H
O
O
O
O
C
P
O
C
O
N
H
N
H
(a)
+
+
FIGURE 8.8
(a) Schematic representation of the double hydrogen-bonding interaction
between two ferrocenium-urea branches of dendrimer
. (b) Potential shift of the ferrocene
redox couple versus equivalents of H
2
PO
4
added. Concentration of
14
10
5
M.
14
2.5
Even in the presence of Cl
, HSO
4
, and H
2
PO
4
(4 equiv. of each anion) the
cathodic shift of the observed half-wave potential is within a few mVof the potential
shift observed with only 4 equiv. of H
2
PO
4
(in the absence of Cl
and HSO
4
). We
also explored the dependence of the half-wave potential of each dendrimer on the
concentration of H
2
PO
4
. Interestingly, the apparent half-wave potential value levels
off at 0.5 equiv. of anion, which strongly suggests that each H
2
PO
4
anion is bound by
two ferrocenyl-urea branches (Figure 8.8), in agreement with the fact that the two
N-H groups can bind with two adjacent oxygen atoms in an oxyanion to yield an
eight-membered chelate ring and the C
O group from the adjacent dendritic arm can
bind with the OH groups in H
2
PO
4
. These macromolecules also exhibit a dendritic
effect in going from the first to the second and third generations, but the degree of
steric congestion in the fourth generation seems to deteriorate the binding ability
toward hydrogenphosphate.
A simple and efficient synthetic route has been used to incorporate poly(propy-
leneimine) dendrimers containing 4, 8, and 64 amidoferrocenyl moieties into
MCM-41, a mesoporous silicatewith a highly regular structure composed of channels
in a hexagonal arrangement, thus giving rise to a novel type of redox-active materials
that constituted the first examples of mesoporous silica hosts containing dendritic
guest molecules within their channels (Figure 8.9) [69]. One significant feature of
these composite materials is that the guest dendrimers, containing a controlled
number of ferrocenyl units, are easily accessible to electrochemical oxidation.
Platinum disk electrodes coated with layers of dend-n-MCM-41/graphite powder/
polystyrene composites have been studied by cyclic voltammetry and differential
pulse voltammetry showing a well-defined reversible redox process. These modified
electrodes are very stable and multiple successive cyclic voltammetric scans can be
carried out with no loss of electroactivity. Likewise, after standing in air for several
days, the redox response was practically unchanged. These mixed materials can be
ΒΌ
