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

66 A. Nangia · G.R. Desiraju
structure?” Because there is a one-to-many correspondence between functional
groups and supramolecular synthons, this question may be answered in several
ways: (1) not possible to predict; (2) difficult to answer; (3) has multiple answers.
The first response is perhaps the technically most correct given the state-ofthe-art
today, but along with its certitude is associated a rather static and pessimistic
frame of mind that is not conducive to the improvement of our undertanding
of these matters. We choose therefore to explore the second and third
options as answers.
The earliest attempt to correlate molecular structures with crystal structures
was by Robertson who discussed the packing of planar fused-ring aromatic
hydrocarbons in the 1950s [31]. He suggested that aromatic disk-like molecules
with an area large in comparison with their thickness (coronene, ovalene) tend
to stack in columns while those with a smaller area (naphthalene, anthracene)
tend to be steeply inclined. This issue was re-examined by Desiraju and Gavezzotti
in 1989 given the enhanced perception of intermolecular forces and a
knowledge of the ubiquitous herringbone and stacking motifs in crystal structures
of aromatic compounds [32]. These authors divided polynuclear aromatic
hydrocarbons into four categories. Small arenes (benzene, naphthalene, anthracene)
have herringbone or inclined “T”-geometry structures, while larger arenes
pack as sandwich herringbone (pyrene, perylene), g-structures (Robertson’s
coronene group renamed) and b-structures (tribenzopyrene).
Based on the above classification along with the carbon to hydrogen atom
ratio and the surface area of the molecule, a prediction of the aromatic hydrocarbon
crystal structures was shown to be possible. However, one must be aware
that such predictions are accurate because the forces in hydrocarbon crystals are
only of the isotropic variety. Nonetheless, different packing modes do exist even
for aromatic hydrocarbons, and polymorphs of molecules as distinct as toluene,
perylene, quaterphenyl and dibenzanthracene have been observed.
Most organic compounds contain heteroatoms and the intermolecular interactions
in their crystal are a complex mosaic of forces of varying strengths,
directionalities and distance dependence characteristics [2, 5, 6]. In such situations,
direct and simple extrapolations from molecular to crystal structure are
tricky and difficult in the best cases and impossible in the worst.We present here
two examples of molecule Æ crystal extrapolation to convey the mixed feelings
associated with such correlations. Both examples are based on compounds
containing hydroxy and amino groups. Ermer and Eling [33] and Hanessian
et al. [34] pointed out independently that molecular recognition between these
groups is robust and predictable because of the complementary 1 : 2 and 2 : 1
hydrogen bond donor:acceptor ratios present. The coordination environment
about these N- and O-atoms is accordingly expected to be tetrahedral.
Hanessian and co-workers reported the self-assembly of supramolecular
hydrogen bonded structures of complexes of C 2 symmetrical chiral 1,2-diols
and 1,2-diamines [34]. In the 1 : 1 complex 25 of (1S,2S)-1,2-diaminocyclohexane
22 and (1S,2S)-1,2-cyclohexanediol 23 (Scheme 4), the cyclohexane rings align
into four vertical columns and the polar hydrogen bonding groups face inward.
The structure is a pleated sheet-like array of eight-membered, square planar,
hydrogen bonded units in which the oxygen and nitrogen atoms are tetracoor-