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assembled thanks to hydrogen bonding or other interactions [198-202], the
empty space potentially present in the overall crystal packing is filled by sol-
vent molecules or through interpenetration [189, 195, 196]. The XB role and
potential with respect to these phenomena of the overall crystal packing still
have to be established.
4
Conclusions
It has been shown how the XB is a specific, directional, and strong inter-
action that can be successfully employed as a general protocol to drive the
self-assembly of a wide diversity of molecular modules.
Paraphrasing Corey's historic definition of synthon [203], Desiraju defined
a supramolecular synthon as a structural unit within a supermolecule that
can be formed or assembled by known or conceivable synthetic operations
involving intermolecular interactions [204]. The robustness of the XB has
allowed several supramolecular synthons based on this interaction to be iden-
tified and some examples have been presented in this chapter.
According to Wuest's definition [205, 206], a tecton is a molecule that in-
teracts with its neighbours in strong and well-defined ways, as it inherently
possesses the molecular structure and intermolecular recognition features
to predictably self-assemble into crystalline networks. Iodine atoms, and to
a lesser extent bromine atoms, can be used to build up reliable tectons.
In fact, whenever the electropositive crown present in the polar region of
these halogens is incremented by electron-withdrawing neighbouring groups,
these halogens effectively work as “sticky sites” that direct molecular asso-
ciation. Several cases of such XB-based tectons have been discussed above.
Thanks to this potential in identifying and designing supramolecular syn-
thons and tectons, the XB can be considered as a new paradigm in supra-
molecular chemistry. Halocarbons work as effective XB-based tectons for
the construction of a wide and predictable diversity of architectures. Using
fancier words, it can be stated that halocarbons are the blocks for the con-
struction of an XB-based Legoland. While XB-based crystal engineering is
still in its infancy, the growing interest in the field promises remarkable future
advancements.
The ability of XB to control recognition, self-organization, and self-
assembly processes in the different phases of matter is clearly emerging in the
literature. This chapter focusses on self-assembly in the solid phase, while the
chapters of B. Duncan and A. Legon (in this volume) deal with the liquid crys-
talline phase and gas phase, respectively. Relatively few papers are reported
in the literature on self-assembly processes in solution [66-68, 207, 208]. Sev-
eral analytical techniques have been used to detect XB formation, to define its
nature, to establish its energetic and geometric characteristics, and to reveal
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