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

Supramolecular Synthons and Pattern Recognition 73
Scheme 7. Outlier refcodes in the CSD study of C-H◊◊◊O hydrogen bonding in CHCl 3 and
CH 2Cl 2 solvates
frequent occurrence of the urea motif is a result of this marginally higher stability.
A more likely reason is that the synthon 35 is not common in imides such as
31 because it can occur only when both the carbonyl groups are syn to each
other. Examination of the crystal structures of the retrieved imides shows that a
large number of these compounds (such as, say, 33) exist in the conformation
wherein the two carbonyl groups are anti to one another. Thus, in ureas the
stablest molecular conformation is in consonance with the requirements for the
target tape motif, whereas in imides the conformation of the molecule is in conflict
with that necessary for the extended, linear hydrogen bonded tape.Another
possible reason for the formation of the infinite tape 34 in the ureas and the nonformation
of the infinite tape 35 for the imides is the match of donor (2 H atoms)
and acceptor (1 C=O group with 2 acceptor sites) groups in the former and
mismatch (1 donor, 4 acceptor sites) in the latter.
The examination of outlier structures in a CSD analysis is often instructive. It
is well-known that C-H◊◊◊O bonds formed by CHCl 3 are in general shorter than
those formed by CH 2Cl 2 because of carbon acidity effects [38]. However, histograms
of C-H◊◊◊O distances of these two populations show some degree of overlap
so that some of the shorter contacts in the CH 2Cl 2 sample are shorter than
the longer contacts in the CHCl 3 sample. This corresponds to the outlier region.
For example, a long and bent contact is formed by CHCl 3 in the crystal structure
of BOLVAZ (Scheme 7). Here, the acceptor oxygen is electron-deficient because it
is bonded to a Cu(II) atom and this reduces its propensity to form a strong
C-H◊◊◊O hydrogen bond (d=2.88 Å, q =125.3). Conversely, a short and linear
contact is formed by CH 2Cl 2 in the structure of PIDDOV. In this phosphonium
salt, the acceptor phenoxide O – -atom is unusually basic and forms a strong
hydrogen bond (d=1.96 Å, q =162.4). In both instances, the outlier status of the
points is confirmed by examination of the individual structures and this provides
additional insight about the chemical nature of the intermolecular contact.
6
Multipoint Recognition Patterns
The presence of two or more complementary hydrogen bond donor and acceptor
sites in a recognition event, that is multipoint recognition, confers stability