198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

Hydrogen-Bonded Ribbons, Tapes and Sheets as Motifs for Crystal Engineering 101
describe particular types of packing. For example, MacDonald and Whitesides
have distinguished tapes from ribbons formed by amides in the following
manner: “a tape motif is generated when each molecule is hydrogen bonded to
two neighboring molecules, and when the hydrogen bonds between any two
molecules form an eight membered ring; a ribbon motif is generated when each
molecule is hydrogen bonded to three or more molecules, regardless of the
connectivity of hydrogen bonds between the molecules” [20].
Since this article is a compilation of structures that possess different functionalities
and geometries, we will distinguish between tapes and ribbons in the
following manner. A tape will form when the functional groups involved in
intermolecular interactions on a molecule are antiparallel or at 120 o to each
other – thus the molecule can interact with only two adjacent molecules in a
linear or zigzag fashion. In a ribbon pattern, the functional groups may be oriented
in different directions, giving some flexibility in the recognition to adjacent
molecules. A sheet will involve the formation of two-dimensional arrays
stabilized by intermolecular interactions. Most of the examples presented here
will involve compounds with functional groups built around a cyclic template.A
schematic representation of tapes, ribbons, cyclic and sheet aggregates is shown
in Fig. 2. The triangle represents a template which may be the same or a different
d
a
b
c
e
Fig. 2 a – e. Schematic diagrams of: a, b tapes; c ribbons; d cyclic aggregates; e sheet aggregates