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

68 A. Nangia · G.R. Desiraju
Fig. 1 b. (RR,SS)-complex 26. (Figures adapted from Hanessian et al. [34]). Note the slight
differences in the two figures
26. Interestingly, the chirality of the diol and diamine components controls the
tertiary structure of the complex, in other words the sense of chirality with
which the cyclohexane rings wrap around the central core in the triple-stranded
helicate. Thus, whereas the (S,S)-diamine 22 gives a left-handed helicate 25 with
the (S,S)-diol 23, the (R,R)-diamine 24 gives a right-handed helicate 26 with the
same diol.
If the above example highlights the delicate way in which the handedness of
the crystal structure is related to the chirality of the molecular components, and
how relationships between molecular and crystal structure can be traced, the
structures of the three aminophenols present a case in total contrast. The crystal
structure of 4-aminophenol 27, described by Ermer and Eling [33], shows the
expected tetrahedral hydrogen bonded network of O-H◊◊◊N and N-H◊◊◊O hydrogen
bonds. However, in the X-ray structures of isomeric 2- and 3-aminophenols
28 and 29 (Scheme 5),the strong hydrogen bonded tetrahedral network is absent
and instead, an unusual N-H◊◊◊p hydrogen bond is observed. The crystal structures
of 28 and 29 were determined using low temperature neutron diffraction
data by Desiraju and co-workers [35], and this confirmed the presence of
N-H◊◊◊p hydrogen bond in both structures (Fig. 2). Surprisingly, the “normal”
hydroxy-amino recognition seen in 27 is not found in 28 and 29. In 2-aminophenol
28, each -OH group donates an O-H◊◊◊N hydrogen bond and accepts
N-H◊◊◊O and C-H◊◊◊O hydrogen bonds with three different molecules of 28.Each