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In-Depth Information
are macrocycles formed via the mechanical bond. The Sauvage group has recently
published a beautiful crystal structure of a cyclic [4]rotaxane [
153
]. The appealing
structure in Fig.
22f
by Sauvage is a doubly threaded catenane [
148
], with one ring
passing through two covalently linked “handcuff” macrocycles.
Fig.
23
is a collection of crystal structures of some exceptionally novel
architectures, all of them uniquely interlocked. A suitane (Fig.
23a
) is so-named
because a molecular “suit” mechanically envelops a multilimbed body (a suit[
n
]ane
has
n
“limbs”) [
33
,
159
]. Bohmer's multicatenane [
154
] in Fig.
23b
consists of
interwoven annulated rings with a remarkable eight interlockings among only
two components. The Leigh group used novel building materials (inorganic
macrocycles) to construct [
88
] their architecturally new [4]rotaxane in which two
rings each encircle two threads (Fig.
23c
). The cyclic, double-threaded [2]rotaxane
dimer (“daisy chain”) in Fig.
23d
is yet another mechanically bonded macrocycle
[
155
,
160
,
161
]. Bistable daisy chains [
162
-
164
] are interesting for their capacity to
dramatically change length in response to external stimuli. Trimeric daisy chains
have also been reported [
165
]. Kimoon Kim's group appropriately calls the large
macrocycle with four curcurbituril “beads” (a [5]catenane) in Fig.
23e
a molecular
necklace [
116
]. The molecular Borromean Rings (Fig.
23f
) are a tour de force in
self-assembly and chemical topology [
156
]. Their synthesis is discussed in
Sect.
4.3
. The Puddephat group obtained a crystal structure [
157
] for the doubly
interlocked [2]catenane known as a Solomon Knot (Fig.
23g
), a topology pioneered
[
166
] by Sauvage in 1994. Finally, the multiply interlocked bicyclic coordination
cages of Fujita [
158
] are presented in Fig.
23h
.
For all of the pleasure, i.e., beauty - derived from creating new interlocked
architectures, we have only scratched at the surface of what is possible. Many
of the molecules shown in this section were already framed in the context of
beauty (Borromean Rings and Solomon Knots), and we expect that aesthetic
considerations will continue to motivate new architectural developments.
4.3 Simplicity and Elegance
Simplicity is probably the least controversial trait that a molecule needs to be
beautified in the minds of chemists, who have always been drawn to Platonic
notions of beauty. And who could blame chemists for this line of thinking?
Simplicity is tied to the pleasing virtues of balance, symmetry, wholeness, and
harmony. It is also tied to coherence and comprehension, which is tied to truth,
which is tied to beauty. When a chemist examines a simple molecule, a network of
these related concepts inevitably leaves him or her with a sense of pleasure and
satisfaction.
There is no lack of beautifully simple MIMs. The quintessential example is
Schill's all-hydrocarbon [2]catenane [
167
], the simplest non-trivial topology com-
posed of the simplest atoms and bonds (Fig.
24
). Other MIMs are simplified by
highly symmetrical structures that make them “easy on the brain”, such as the
