X-ray Structures and Electronic Spectra of the Halogen Complexes between Halogen Donors and Acceptors with-Receptors - Halogen Bonding: Fundamentals and Applications - page 149

Fig. 8 Two modes: over-the-rim ( a ) and over-the-center ( b ) coordination of tetrabro-
momethane to aromatic donors (data from [53, 80])

zene is observed in the complexes of chloroform with calixarene [85]. In the
CHCl 3 complex with 2,2 -dimethoxy-9,9 -biacridine, one chlorine atom is lo-
cated over the aromatic bond (with C - Cl separations of 3.25 and 3.33A)
and another chlorine is coordinated to the carbon (with a 3.33 A separa-
tion) [86] On the other hand, coordination close to the aromatic ring center is
observed in the chloroform complex with cobalt-coordinated diphenylphos-
phinomethane (with Cl - C separations varying from 3.27 to 3.58 A )orinthe
tetrachloromethane complex with dicorannulenobarrelene (with Cl - Csepa-
rations varying from 3.27 to 3.46A) [87, 88].
Finally, a weak -type halogen bonding involving a cyclopentadienyl ring

and the iodine atom of an iodofluorocarbon [89] is a rare example of a

π

-
bonded iodocarbon derivative, in contrast to numerous examples of halogen
bonding of the latter with n-type electron donors [2, 20].

π

3.3
Complexes of Halide Anions with Aromatic and Olefinic π -Receptors

X-ray structural information on halide binding to neutral organic -receptors

is limited to a few recent reports [23, 24, 62, 89-91] 2 . In fact, the slow diffusion
of hexane into a dichloromethane solutions of tetracyanopyrazine contain-
ing the alkylammonium salts of either chloride, bromide or iodide affords
yellow to red crystals with UV-Vis absorptions closely resembling the elec-

π

2 Anion-

interaction is also recognized in halide associates with aromatic rings when the lat-
ter represents a part of the (positively charged) metal-ion complex and/or when

π

π -bonding is

supported by hydrogen bonding.