X-ray Structures and Electronic Spectra of the Halogen Complexes between Halogen Donors and Acceptors with-Receptors - Halogen Bonding: Fundamentals and Applications - page 145

3
Structural Features of Donor/Acceptor Complexes with Halogen Derivatives

3.1
Complexes of Dihalogen Acceptors with Aromatic and Olefinic π -Donors

In spite of the numerous spectral observations of complex formation between
aromatic and olefinic donors with the dihalogens, the preparations of the
corresponding crystalline complexes have been hindered by their enhanced
reactivity (as well as the relatively weak bonding). As such, only few examples
of the X-ray structural characterization of the corresponding intermolecular
associates are reported, the most notable exception being the dibromine com-
plex with benzene.
According to the earlier X-ray studies of Hassel and Strømme [66, 67], the
structure of the isomorphous complexes of dichlorine and dibromine with
benzene (measured at 183 Kand 230 K, respectively) are characterized by

the symmetrical location of the halogen molecules along the sixfold axis of
the aromatic ring to form infinite

∼

···
chains. However, recent X-ray measurements [68] of the Br 2 /benzene system
at lower temperatures (123 K) reveals the less symmetric arrangement of the
dihalogen, and a phase transition at 203 K that led to the diffraction pattern
originally reported by Hassel and Strømme. In the precise low-temperature
structure, the bromine atoms are positioned over the rim of the benzene
ring (Fig. 6) and oriented nearly perpendicular to the aromatic planes (with
the slight deviation

··· Ar ··· Br - Br ··· Ar ··· Br - Br ··· Ar

of typically less than 8 ◦ ). Furthermore, a pair of di-
bromines is coordinated to each benzene ring from opposite sides in the
meta -positions, which are known to be relatively more electron-rich in arenes
with acceptor substituents.
The X-ray structure of the dibromine complex with toluene (measured at
123 K) is more complicated, and shows multiple crystallographically inde-
pendent donor/acceptor moieties [68]. Most important, however, is the fact
that in all cases the acceptor shows an over-the-rim location that is simi-
lar to that in the benzene complex. In both systems, the acceptor is shifted
by

α

1.4 A from the main symmetry axis, the shortest Br ∼ ··· Cdistancesof

3.1A being significantly less than the sum of the van der Waals radii of
3.55A [20]. Furthermore, the calculated hapticity in the benzene/Br 2 com-
plex (

∼

η = 1.52) is midway between the “over-atom” ( η = 1.0) and “over-bond”

( =1.70

to 1.86 (in four non-equivalent coordination modes) and thus lies closer to
the “over-bond” coordination model. Moreover, the “over-bond” bromine is
remarkably shifted toward the ortho -and para -carbons that correspond to
the positions of highest electron density (and lead to the transition states
for electrophilic aromatic bromination [12]). Such an experimental location
of bromine is in good agreement with the results of high level theoretical

η = 2.0) coordination. In the toluene complex, the latter varies from η