Chemistry Reference
In-Depth Information
a
b
c
Fig. 11 (a) Solvent switchable rotaxane. (b) Stabilization of an N-oxide fulleropyrrolidine by
encapsulation in the rotaxane. (c) Electrochemically driven molecular shuttle
The first example of a supramolecular H-bonded architecture involving fullerene
was a dimer with a pseudorotaxane-type structure (28, Fig.
10
) reported by
Diederich et al. [
121
]. For this dimer a
K
a
value of ~970 M
1
was obtained. More
robust supramolecular dimers of fullerene were obtained by employing a quadruple
array of H-bonds based on 2-ureido-4-pyrimidone (UP) (29 and 30)[
122
,
123
].
In these cases the DDAA H-bonds disposition gives rise to complementary pairs
with affinities in the range of
K
a
~10
6
M
1
.
Leigh-type clipping reactions have allowed the preparation of several fullerene
containing rotaxanes. The first bistable rotaxane containing a fullerene is depicted
in Fig.
11
(31)[
124
]. Depending on the polarity of the solvent, the macrocycle
resides preferentially over the glycylglicyne unit or over the alkyl chain. A slight
modification of the structure of the thread allows the macrocycle to shuttle in the
opposite direction, from the amide to the fullerene moiety [
125
].
This displacement of the macrocycle has been used to increase the stability of a
fulleropyrrolidine N-oxide (32, Fig.
11
)[
126
]. The formation of hydrogen bonds
between the macrocycle and the N-oxide inhibits the deoxygenation reaction, thus
enhancing the stability of the N-oxide derivative.
Remarkably, the shuttling of the macrocycle can also be stimulated electrochem-
ically by the formation of the fullerene trianion [
127
]. This electrochemically
driven molecular shuttling has recently been achieved at very low reduction
potential by introducing positive charges in the macrocycle (33, Fig.
11
). In the
latter case, only one electron is needed to induce the operation of the shuttle [
128
].
The first reported donor-acceptor supramolecular dyad based on C
60
is a
pseudorotaxane formed between the dibenzylammonium salt of a Bingel-type
fullerene and the crown ether of a zinc phthalocyanine (34)[
129
]. This complex
has an association constant of ~ 1.4
10
4
M
1
and, what is more important, an
