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12+
12NO
3
-
H
2
N
NH
2
4+
4PF
6
-
Pd
N
N
N
Pd
N
H
2
N
N
N
X
N
N
Pd
N
H
2
NH
2
X
O
N
O
X
X
N
O
Pd
N
N
H
2
N
N
N
N
N
O
X
X
X
N
X
O
NN
O
O
N
N
H
N
H
2
N
O
X
N
N
Pd
X
X
N
N
Pd
Pd
N
N
NN
NH
2
H
2
N
X
N
57
: X = None
58
: X = -C
6
H
4
-
59
: X = -(C
6
H
4
)
2
-
N
N
56
Pd
H
2
N H
2
Variations of the guest showed that bulky hydrophobic moieties
such as 1-phenylethyl or adamantyl groups favor the formation
of cage structure, whereas less hydrophobic substrates such as
dicarboxylates or acetates showed hardly any effect. Similar,
nanoscopic three-dimensional molecules,
57
−
59
, with the shape of
truncated tetrahedra were reported by Fujita and coworkers [105].
These molecules were soluble in water and capable of encapsulating
up to four guests, such as adamantane carboxylates. The assembly
formed the host
guest complex with four molecules of adamantyl
carboxylate and no intermediate inclusion complexes with one,
two, or three guests were observed. This was confirmed by X-ray
crystallographic structure of the cage
−
57
with adamantyl carboxylate
1
H NMR studies in aqueous solution. This is likely due to
the allosteric effect: the hydrophobicity of the cavity increases with
the inclusion of guest molecules in the cavity thereby increasing the
number of guests inside the cavity. The homologous Pt(II) complex
was isolated in the presence of template guest, such as adamantyl
carboxylate molecules under harsh conditions, while only traces
of the platinum cage was formed in the absence of such guest. The
cage showed remarkable kinetic stability because of the relatively
inert Pt
as well as by
pyridine bond. In contrast to its palladium counterpart, the
platinum cage is stable under highly acidic and basic conditions and
is not destroyed by strong nucleophiles. As a result the compound
was used to design a pH-responsive host
−
−
guest system. Under
basic conditions,
-dimethylaniline was effectively bound in the
hydrophobic cavity in a 1:4 (host:guest) ratio, whereas under acidic
N
,
N
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