X-ray Structures and Electronic Spectra of the Halogen Complexes between Halogen Donors and Acceptors with-Receptors - Halogen Bonding: Fundamentals and Applications - page 146

Fig. 6 Molecular structure of the Br 2 complex with benzene (from [68])

calculations, which consistently favor both over-atom and over-bond (i.e.,
η

2 -) coordinations without a significant energy barrier between
them [69-74].
TheonlyreportedX-raystructureofa

1 -and

η

-bonded diiodine exists in the
I 2 /coronene associate [75], which shows the I 2 to be located symmetrically
between the aromatic planes and to form infinite donor/acceptor chains.
η 2 -Coordination of diiodine over the outer ring in this associate is similar to
that observed in the bromine/arene complexes (vide supra), and the I - Csep-
aration of 3.20A is also significantly contracted relative to the sum of their
van der Waals radii [75]. For the highly reactive dichlorine, only X-ray struc-
tures of its associates are observed with the n-type coordination to oxygen of
1,4-dioxane [76], and to the chlorinated fullerene [77].
It is should be noted that high reactivity precludes the X-ray structural
characterization of the

π

-complexes between dihalogens and olefinic accep-
tors. Indeed, quantum mechanical calculation of the interaction between

π

Fig. 7 Molecular structure of adamantylideneadamantane bromonium, as its salt with

Br 5 - counterion (Rosokha et al. unpublished results)