Chemistry Reference
In-Depth Information
1.2
Significance of Halogen Bonding with Dihalogens
Much of the current research into halogen bonding involves organohalogen
or inorganic halide acceptors. However, the dihalogens (X
2
)andinterhalo-
gens (XY) continue to attract attention. This review will focus specifically on
structural and theoretical studies of such systems over the past decade. While
polyhalide anions are closely related to the neutral donor-acceptor concepts
that are the focus here, they will be discussed only when necessary to elabo-
rate on the central theme of the review.
Spectroscopic studies of the X
2
and XY charge-transfer systems have pro-
vided fundamental insights into the nature of the interaction. The charge-
transfer band of organoiodine halogen bonded complexes typically lies in the
UV region and is obscured by other chromophores in the system [9]. These
are readily observed, however, in complexes of the dihalogens. The C-X and
halogen bond vibrations are also difficult to identify unambiguously in the
organoiodines, and vibrational spectroscopy has not been commonly used in
the study of such complexes [10]. This is not the case with X
2
and XY, where
valuable structural information is readily obtained [11, 12]. Microwave stud-
ies of organohalogen complexes are also rare. Conversely, such studies are
a powerful tool for examining gas phase bond energies and geometries of
X
2
and XY complexes [13-15], and a chapter from A.C. Legon in this topic
specifically deals with this topic [16].
Halogen bonding is increasingly being discovered to play an important
role in biochemical systems, particularly in thyroid biochemistry. While halo-
gen bonding involving the halogenated thyroid enzymes is often the focus of
studies in this area, the role of I
2
is also of interest [17, 18]. Closely related
to this are investigations into the interaction of various antithyroid drugs
with I
2
[19]. Recently, it has also been found that certain psychotropic drugs
also affect thyroid function, with halogen bonding potentially playing a major
role [20]. Interestingly, halogen bonding has also been used as a spectroscopic
tool for the determination of drug concentrations in tablets and formula-
tions [21-23].
Other applications involving X
2
and XY halogen bonding include dye-
sensitized solar cells [24-26], batteries [27-29], membrane electrodes [30],
and polymorph control [31]. Iodine has been used to evaluate the active elec-
tron donor sites in zeolites by monitoring changes in the visible absorption
band [32]. Halogen bonding was even found to play an important role in the
rheology and carbonization of coal tar pitch [33] and in the recovery of metals
such as gold [34] and mercury [35]. In some cases, it is the complex itself that
is of interest, while in others, the complex is an initial intermediate that insti-
gates a chemical reaction. No matter what the process, a better understanding
of halogen bond strength and structure is desirable.
Search WWH ::
Custom Search