research papers
came to be regarded as a robust supramolecular synthon for
crystal engineering (Allen et al., 1994; Thalladi et al., 1996;
Masciocchi et al., 1998; George et al., 2004). However, these
reports each refer to a single geometrical isomer and these
structures all happen to involve molecular components with
the substituents at the distal ends, namely the 1:1 adduct of
1,4-diiodobenzene and 1,4-dinitrobenzene (Allen et al., 1994),
4-iodonitrobenzene (Thalladi et al., 1996), 4-iodo-40-nitrobi-
phenyl (Masciocchi et al., 1998) and N-4-iodophenyl-N0-40-
nitrophenylurea (George et al., 2004). Likewise, we have
observed this interaction in N-(4-iodophenylsulfonyl)-4-
nitroaniline and in N-(40-nitrodophenylsulfonyl)-4-iodoani-
line, but not in any of the other isomers in these series; indeed,
in the series (A)–(G) noted above (see x1), wherever one or
other of the iodo or nitro substituents is in the 2- or 3-posi-
tions, three-centre iodoꢀ ꢀ ꢀnitro interactions are absent.
Consistent with these observations, in 2,4,6-trinitroiodo-
benzene (picryl iodide), a three-centre interaction is formed
by the 4-nitro substituent but not by the other two nitro
groups (Weiss et al., 1999), while in 1,2-diiodo-4-nitro-5-
(butylamino)benzene, the 1-iodosubstituent participates in a
three-centre iodoꢀ ꢀ ꢀnitro interaction, while the 2-iodo substi-
tuent is involved only in a two-centre iodoꢀ ꢀ ꢀnitro interaction
(Senskey et al., 1995). Hence, it may be that the initial
acceptance of the three-centre iodoꢀ ꢀ ꢀnitro synthon owes less
to its intrinsic utility than to the chance selection of the
isomeric forms of the compounds used in the initial studies.
On the other hand, there is a three-centre iodoꢀ ꢀ ꢀnitro inter-
action in N-(4-iodophenyl)-3-nitrophthalimide, but there are
no iodoꢀ ꢀ ꢀnitro interactions at all in N-(4-iodophenyl)-2-
nitrophthalimide (Glidewell, Low, Skakle, Wardell & Wardell,
2005). Thus, in substituted aryl systems this synthon appears to
behave predictably only for specific isomeric forms, but
normally to be absent for the remaining isomeric forms.
Reported examples of structures that are described as having
been specifically and deliberately engineered by the applica-
tion of particular supramolecular synthons are often restricted
to specific isomer forms of their molecular components,
sometimes to a single isomer; in some of these cases there
must arise at least a suspicion of an element of post hoc
rationalization as opposed to reliable supramolecular design.
The difficulty of structure prediction appears to be entirely
characteristic of the crystal structures of molecular
compounds where all of the intermolecular forces are
comparatively weak, but of comparable magnitudes to the
rotational energy barriers associated with single bonds, so that
the molecular conformations are a direct reflection of the
intermolecular interactions. For this reason alone, molecular
conformations computed for isolated molecules are unlikely
ever to reproduce the conformations observed experimentally
in the crystalline state. More disturbing is the fact that, to date,
attempts to make computed predictions of the crystal and
molecular structures of even rather simple compounds have
met with only limited success (Lommerse et al., 2000;
Motherwell et al., 2002; Day et al., 2005). Compounds whose
molecules contain internal degrees of freedom, such as rota-
tions about single bonds, particularly where aryl rings are
connected to a semi-rigid unit, as in the examples discussed
here, seem to pose particular difficulty, possibly associated
with the delicate interplay of intramolecular and inter-
molecular forces.
X-ray data were collected at the EPSRC X-ray Crystal-
lographic Service, University of Southampton, UK, and at the
Daresbury SRS Station 9.8, Warrington, UK; the authors
thank the staff of these facilities for all their help and advice.
JLW thanks CNPq and FAPERJ for financial support.
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