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crystallographic investigations on MIMs and their supramolecular precursors, his
extraordinary attention to detail in the presentation of ball-and-stick representations
of MIMs is a heroic legacy in itself. He was always at pains to ensure that few if any
atoms were obscured behind others, that bonds crossed over each other as clearly as
possible close to their mid-points, and that the elevation chosen illustrated the most
salient aspects of the MIM's geometry. His immense talent in the design arena was
matched by his complete command of the graphics programs required to give
expression to his artistic genius. Williams' handiwork is featured throughout this
chapter, for example in Figs.
12a,b
,
13
,
14b
, c, and
23a
,d.
It is important now to discuss a different kind of mechanically interlocked solid-
state structure made possible through the development of metal-organic
frameworks (MOFs) [
90
-
93
]. MOFs are infinite, robust, crystalline networks com-
posed of organic struts coordinated to metals or metal clusters as secondary
building units and have very high porosities on account of their rigid reticular
architectures. The massive amount of void space in some kinds of MOFs sometimes
coaxes them into forming two or more interpenetrated networks (Fig.
15a
) in the
same crystal, i.e., catenated frameworks [
94
-
96
]. Although researchers interested
in high surface area and porosity want to avoid catenated MOFs, they are marvelous
structures from the perspective of topology: these infinite catenated networks are
the molecular cousins of the sculpture in Fig.
6e
. Recently, MIMs have also been
introduced into MOFs [
97
-
100
]. The example in Fig.
15
from the Yaghi and
Stoddart groups incorporates a molecular catenane into a catenated MOF [
99
].
The beauty of this framework lies in the unprecedented complexity, linker size, and
Fig. 15 Catenated MOF with catenated linkers [
99
]. (a) Ball-and-stick representation of the X-ray
crystal structure of the MOF with the pendent catenanes omitted in order to permit the visualiza-
tion of the interpenetrated networks, colored in
green
and
gray
.(b) The same MOF with pendant
crown-10 macrocycles included in
red
; only CBPQT
4+
is omitted. (c) Complete view of the MOF
structure, in which the CBPQT
4+
cyclophanes are assigned space-filling representations in
blue
.
(d) Simplified illustration of the catenated MOF for conceptual purposes
