X-ray Structures and Electronic Spectra of the Halogen Complexes between Halogen Donors and Acceptors with-Receptors - Halogen Bonding: Fundamentals and Applications - page 147

dibromine and ethylene leads to T-shaped structures with C 2 v symmetry,
with a separation of about 3.0 A between the bromine atom and center of
the double C - C bond [13, 14]. However, the low-temperature interaction
of bromine with highly sterically encumbered adamantylideneadamantane
in dichloromethane results in the (reversible) formation of the crystalline
adamantylideneadamantane bromonium adduct shown in Fig. 7 (as the salt
with Br 3 - [78] or Br 5 - , Rosokha et al. unpublished results) with elongated
Br - C bonds of about 2.1-2.2 A .

3.2
Complexes of Polyhalogenated Methanes with π-Donors

The Cambridge Structural Database [79] contains numerous examples of
close contacts of at least 0.2 A shorter than the sum of the van der Waals
radii that exist between aromatic or olefinic donors and the halogen atoms
of the halocarbon derivatives - especially in dichloromethane and chlo-
roform solvates. However, most of the structures involve metal-ion com-
plexes or charged species, in which the intrinsic features of the halogen
π

-bonding might be obscured by electrostatics, hydrogen bonding, crystal
packing and other extraneous factors. The intermolecular complexes between
a halocarbon and neutral aromatic donors are rather rare; and the structural
overview of some complexes of tetrabromomethane with various aromatic
donors [53, 80-82] are summarized in Table 2.
Tetrabromomethane shows two types of -bonding with aromatic donors

that are contrasted in Fig. 8a and b, showing over-the-rim coordination to the
aromatic C - C bond and over-the-center coordination to the benzene ring.
The over-the-rim coordination is generally similar to that observed in the
dibromine complexes but the C - Br distance in the former is longer, in agree-
ment with weaker acceptor abilities of tetrabromomethane. Note that picryl
bromide shows similar bromine coordination to the outer pyrene C - Cbond
with Br - C distances of 3.35 and 3.39A [83]. A second type of coordination
was reported earlier in the p -xylene complex [80] and recently in the associate
with dimethylnaphtalene [53].
Notably, tetrabromomethane prefers coordination to n-type donors, if
available, over the aromatic ring (and such a preference is also observed
in complexes of bromoform with p -dimethoxybenzene [53], and in di-
iodopolyfluorocarbons with TMPD or DAM donors [84]) - with the only ex-
ception being its complex with triphenylamine. In addition, the bromine sep-
arations with n-type oxygen and nitrogen centers are much shorter than the
bromine-carbon separation in

π

-bonded complexes, which indicates (even
with a correction for the slightly smaller van der Waals radii of nitrogen and
oxygen) stronger bonding of bromine to the former.
Spectroscopic and phase-diagram studies suggest complex formation be-
tween tetrachloromethane and chloroform with alkylbenzene donors, and

π