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

Hydrogen-Bonded Ribbons, Tapes and Sheets as Motifs for Crystal Engineering 105
3.1
Carboxylic Acids
Carboxylic acids are commonly used as pattern controlling functional groups
for the purpose of crystal engineering. The most prevalent hydrogen bonding
patterns formed by carboxylic acids are dimers and catemers. The preference of
pattern formation is based on the size of the R group in RCOOH. Acids containing
small substituent groups (formic acid, acetic acid) form the catemer
synthon, while most others (especially aromatic carboxylic acids) form dimers,
although not exclusively [16]. Figure 8 shows the hydrogen bonding pattern of
benzoic acid (6) and 3-(4-chlorophenyl)prop-2-ynoic acid (7) [15].
6 7
Fig. 8. Carboxylic acid dimer formed by 6 and polymeric hydrogen bonds formed by 7
The dimer synthon formed by the carboxyl group has been used to assemble
a variety of supermolecules due to its bidentate character which increases the
strength of the interaction. For example, terephthalic acid (8) [39] and isophthalic
acid (9) [40] form one dimensional tapes. Trimesic acid (2) with its
threefold molecular symmetry forms a two dimensional hydrogen bonded sheet
[41] and adamantane-1,3,5,7-tetracarboxylic acid (10) with its tetrahedrally
disposed carboxy functionality forms a diamondoid network (Fig. 9) [42].
Our group has induced the formation of a hexameric cyclic array by appropriately
modifying isophthalic acid. The reasoning behind this approach is that
a bulky substituent at C-5 on isophthalic acid might disrupt the tape packing
motif in the solid state. The structure of 5-decyloxyisophthalic acid (11) was
successfully characterized by X-ray crystallography yielding the anticipated
hydrogen bonding pattern (Fig. 10) [43]. Vapor pressure osmometry experiments
indicated the presence of the cyclic hexamer in solution as well as in the
solid state. The crystal structure of 5-octyloxyisophthalic acid (12) has also been
determined and is observed to form the hydrogen bonding pattern in the same
space group (R3 – ) [44].
The crystal structures of several 5-alkoxyisophthalic acids form tapes and
ribbons when cocrystallized with pyrazine, pyrimidine and ethanol [45]. The
carboxylic acid units of 13 engage in hydrogen bonding to other acids as well as
to the bases and alcohol (Fig. 11a,b). In Fig. 11c the pyrimidine acts as a spacer
between 14, extending the tape formation. However, when acid 14 is recrystallized
in its pure form, the formation of the carboxylic acid dimer motif occurs