X-ray Structures and Electronic Spectra of the Halogen Complexes between Halogen Donors and Acceptors with-Receptors - Halogen Bonding: Fundamentals and Applications - page 156

to 1.9, but X-ray measurements of the benzene/Br 2 associate at 230 Kare
consistent with the symmetrical arrangement of dibromine over the ring
center. -Bonded complexes of tetrabromomethane also show over-the-rim

and over-the-center arrangement of the coordinated bromine atom. Fur-
thermore, the halide associates with tetracyanoarenes are characterized by
significant scattering of the anion positions over the aromatic ring. In the
complexes with tetracyanoethylene, the bromide anion is located over the
double bond and also shifted toward the cyano group. Such structural data
suggests that various modes of coordination are possible. Such a structural
variability on the halogen

π

π -bonding is reminiscent of that observed in the

π -complexes of aromatic ion-radicals with their diamagnetic parents [93].

The latter suggests that in long-distance -bonding, the subtle balance be-

tween the attractive interaction of partially occupied frontier orbitals vis á vis
the repulsion of filled atomic orbitals led to several shallow, close in energy,
local minima involving various mutual donor/acceptor arrangements. As
such, we posit that the interactions between halogen acceptors and

π

π -donors

(similar to ion-radical -bonding) can be readily modulated by tempera-

ture, electrostatics, crystal packing, solvation, etc., to produce a variety of
polymolecular associates within a relatively narrow range of intermolecular
separations.
Spectral studies of the intermolecular interaction of dihalogens, halo-
carbons and halide anions with various organic

π

-receptors (including the
unified Mulliken dependence of their absorption bands) show the direct rela-
tionship of the spectral characteristics and formation thermodynamics of the
corresponding associates with those of traditional organic donor/acceptors
complexes. This indicates the common (charge-transfer) origin of the long-
distance bonding of halogen centers. Such a conclusion is of particular in-
terest for the

π

-interactions of halides, since the formation constants of the
halide complexes with neutral

π

-acceptors, together with their intense ab-
sorptions and compression of the intermolecular separations found by X-ray
structural analysis, indicate the existence of substantial anion-

π

interactions.
As such, we believe that the relatively strong complex formation together with
the distinctive colorations of various anion-

π

interactions encourage their
use in the design of anion-sensing receptors, provided systems with multicen-
tered binding sites are offered for optimal recognition.

π

Acknowledgements We thank the R.A. Welch Foundation for financial support of this
study.

References

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