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

Supramolecular Synthons and Pattern Recognition 77
bonding -OH group in picric acid 45 is capable of disrupting the recognition
motif 49 of dibenzylidene ketones with trinitrobenzenes. There is thus a limit to
synthon robustness and an appreciation of these limits is advantageous in the
design of newer crystal structures within the same family.
Two recent examples illustrate the consequences of N-H◊◊◊O and C-H◊◊◊O
hydrogen bonds co-existing in a crystal structure. MacGillivray and Atwood
observed that in doubly protonated 50, the in-in conformation is more stable
than the out-out form by 21 kcal mol –1 stabilised, in part, by the two intraionic
N + -H◊◊◊O hydrogen bonds [50]. In the 1:2 complex 52 of [2.2.2]cryptand 50 and
2,6-dihydroxybenzoic acid (51) (Scheme 11a), there are two interionic N + -H◊◊◊O
hydrogen bonds (N◊◊◊O 2.68, 2.68 Å; N-H◊◊◊O 177, 166°) on the outer surface
between the cryptand and the acid and four intraionic C-H◊◊◊O hydrogen bonds
(C◊◊◊O 3.01, 2.91, 3.04, 2.96 Å; C-H◊◊◊O 111, 110, 117, 117°) inside the cavity. Since
the in-in, in-out and out-out conformations of the cryptand are in rapid equilibrium
in solution, the stabilisation of the out-out conformation is attributed to
the four weak hydrogen bonds. This example underscores the idea that if the
strength of the strong hydrogen bonds in two distinct orientations are comparable,
then the weak interactions play a dominant structure-determining role in
the crystal.
In an example that illustrates “supramolecular weaving”, Stoddart and coworkers
found that in the X-ray structure of the 1:2 complex 55 between
BPP34C10 53 and the ammonium salt 54, the two cations are threaded co-directionally
through the centre of the macrocycle (Scheme 11b) [51]. This arrangement
is stabilised by a total of six N + -H◊◊◊O hydrogen bonds between the two
NH 2 + centres and three oxygen atoms from each polyether macrocycle (N◊◊◊O
b
Scheme 11 53
a
50
52:50.51
55:53.54
51
54