X-ray Structures and Electronic Spectra of the Halogen Complexes between Halogen Donors and Acceptors with-Receptors - Halogen Bonding: Fundamentals and Applications - page 137

Keywords Anion- π interaction · Charge transfer · Electronic spectra ·

Halogen π -bonding · X-ray crystallography

1
Introduction

The direct intermolecular (non-covalent) interaction involving a halogen cen-
ter represents an important example of long-distance bonding [1-4] (see also
the chapter by Legon in this volume). Indeed, in the solid-state the interac-
tions of halogen-based (electron-density) acceptors with various organic and
inorganic donors are recognized as a powerful motif for the supramolecu-
lar design of non-linear optical materials and even pharmaceutical drugs -
and also for the control of solid-state reactivity, chiral resolution, etc. [5-11].
On the other hand, solution studies have shown that such interactions are
critical for the reactivity of halogenated derivatives [1, 12-14] and in mo-
lecular recognition [15, 16]. Halogen interactions also play important roles
in biochemical systems [17]. As such, the quantitative characterization of
halogen bonding (this generic term is usually reserved for systems involving
halogen-based electron acceptors [18, 19]) 1 ; and in general, the non-covalent
interactions involving halogen centers are of prime importance for chemical
fundamentals and for a variety of applications in crystal engineering, in bio-
chemistry, and in organic synthesis.
Most of the recent studies of halogen bonding have concentrated on
the structural features of solid-state complexes with n-type (electron)
donors [2, 20-22]. By comparison, the nature and properties of the -mode

for halogen-centered interactions is much less explored, in spite of their cru-
cial role in the molecular reactivity of aromatic and olefinic compounds, as
well as in their synthetic prospects for the development of

π

π

-receptors and
other applications [1, 12-14, 23-25] (see also the chapter by Legon in this
volume). Accordingly, this chapter is focused on the long-distance interac-
tions of halogen centers with organic (olefinic and aromatic) -receptors.

In particular, two basic types of complexes are considered: (i) associates be-
tween dihalogens as well as halocarbon acceptors with organic

π

π -donors, and

(ii) associates of halide anions with organic -acceptors.

It is important to recognize that the intermolecular long-distance bond-
ing with the participation of halogen derivatives represents a specific ex-
ample of the broad general area of donor/acceptor interactions. Moreover,
the complexes of molecular iodine, bromine and chlorine with aromatic
donors represent classic examples of charge-transfer compounds [26-28] that
are vital for the development of Mulliken theory of intermolecular associ-
ation [29-31]. The latter thus provides the convenient framework for the

π

1 For a discussion on usage of the term halogen bonding , see [18, 19].