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

Hydrogen-Bonded Ribbons, Tapes and Sheets as Motifs for Crystal Engineering 123
The formation of C-H◊◊◊O hydrogen bonds between the dimethylamino and
nitro groups has been elegantly expressed by using N,N’-dimethylnitramine
(42) as a prototype molecule. The 1:1 complexes of 4-nitrobenzoic acid with
4-(N,N¢-dimethylamino)benzoic acid (43) [73] and 4-nitrobenzoic acid with
4-(N,N¢-dimethylamino) cinnamic acid (44) [74] show a balance of strong and
weak hydrogen bonding. In 43 and 44 the two acid molecules are linked by
O-H◊◊◊O hydrogen bonds to form dimers, as shown in Fig. 29. In addition, the
dimers organize themselves to provide the right orientation to form C-H◊◊◊O
bonds between the NO 2 and the N(CH 3) 2 groups.
Fig. 29. C-H◊◊◊O hydrogen bonding is evident in the tapes formed by 42, 43 and 44
42
43
44
The observation of C-H◊◊◊N hydrogen bonds between the CN groups and
the aromatic H atoms in crystalline cyanocinnamic acids has led to the identification
of 1,3,5-tricyanobenzene (45) as a candidate for the formation of sheets
stabilized by C-H◊◊◊NC bonds. Indeed, 45 forms a hexagonal network when
crystallized in a 1:1 complex with hexamethylbenzene (Fig. 30a) [75]. The C◊◊◊N
distances in the sheets are 3.471(4) and 3.516(6) Å and C-H◊◊◊N angles of 172(2)
and 180° . In the same way, Desiraju et al. identified the presence of C-H◊◊◊O
hydrogen bond donors and acceptors in the same geometrical orientation in
trimethylisocyanurate 46. Not surprisingly, hexagonal networks form in its 1:1
complex with 1,3,5-trinitrobenzene (Fig. 30b) [76].
Weak polarization induced interactions of the CN◊◊◊Cl type have also been
used in crystal structure design. Cambridge Structural Database surveys show
that this interaction has definite length (N◊◊◊Cl 3.00–3.50 Å) and angle
(CN◊◊◊Cl 100–180°) attributes and therefore a certain structure-defining
ability which can be used in crystal engineering. The structure of 2,3-dicyano-
5,6-dichlorobenzenes (47) has been studied for the formation of molecular