halogen···halogen interactions in hexah 5.pdf · 130 chapter 5 halogen···halogen interactions

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5.1 Introduction

Kitaigorodskii suggested in 1955 in The theory of close-packing of molecules [5.1],

that organic molecules in crystals tend to close pack to fill spaces as tightly as possible.

Accordingly, for a given molecule, the actual crystal structure is the one that corresponds

to the most densely packed arrangement of all possible structures. The close-packing

principle considers all organic molecules as spheres with characteristic atomic radii

(geometrical fit). This assumption leads to a simple model applicable to a wide range of

molecular crystals. In course of time, many exceptions to the model were found and the

importance of intermolecular interactions in further understanding of the packing of

molecular crystals was realized (chemical factors) [1.2]. In other words, the recognition

between molecules during crystallization is governed by geometrical or chemical factors,

that is because of shape complementarity and size compatibility (short range repulsion)

[5.1, 5.2], or specific anisotropic interactions of electrostatic or polarization origin (long

range attraction) [5.3, 5.4]. In the process of minimization of total free energy, a balance

between these geometrical and chemical factors needs to be reached, i.e. the final

structure is the result of the minimization of the internal energy, the entropy term and

electrostatic, polarization and van der Waals interactions. But the nature of this balance

between geometrical and chemical factors is poorly understood. Halogenhalogen

intermolecular interactions appear to be particularly problematic [1.2, 1.14a].

5.2 HalogenHalogen Interactions

The weak intermolecular interactions between halogen atoms have been a

subject of interest and debate for many years because of their complexity in geometrical

and chemical terms [5.5, 1.2]. Two hypotheses have been proposed to explain the

Chapter 5 130

halogenhalogen interactions (Scheme 1). One explanation is that the shape of the

atomic charge densities of halogen atoms is spherical and specifically attractive which

produces short contacts in certain directions [5.6]. This view has been strengthened by

Desiraju [5.5g] and Williams [5.5f] and variously referred to as a donor-acceptor, or

charge-transfer interaction, or electrophilic and nucleophilic attack. The alternative

model is the anisotropic repulsion hypothesis [5.5h, d]. According to this, shorter XX

contacts are observed in certain directions due to the nonspherical shape of the atomic

charge densities which produce a decreased repulsion between the atoms. Based on

theoretical investigations, Price et al. [5.5d] argued that the ClCl interactions are best

explained by the latter hypothesis. They predicted that the most important contributions

to the XX intermolecular interactions are the repulsion, electrostatic and dispersion

terms and that the contributions from charge-transfer type interactions is negligible.

Although an increase in the intermolecular attraction or a reduction in the repulsion can

both account for short van der Waals contacts in certain directions, the two are not

equivalent and have different implications.

Scheme 1

According to the increased attraction force explanation, the CXXC

interactions (X = Cl, Br, I) are classified into two types based on the values of the two

CXX angles, 1 and 2 [5.5i, 5.5e]. The type-I interactions (1 2) represent close

packing of X-atoms in a geometrical model because identical portions of the halogen

atoms make the nearest approach. The type-II interactions (1 180, 2 90) are

understood based on the X-atom polarization, X(+)X(), and represent a chemical

model with each halogen atom polarized positively in the polar region and negatively in

the equatorial region (Scheme 2). Type-II interactions are included in a larger category

HalogenHalogen Interactions in....


of X(+)Y() halogen bonds in which electrophilic halogen is involved [5.7].

Scheme 2

One of the reasons for the difficulty in understanding the nature of XX

interactions arise from the fact that the halogens are of low to high electronegativity (I to

F) and polarizability (F to I). They act, depending on the circumstances, as either

electropositive or electronegative entities in an intermolecular interaction, or in some

cases with no particular electrostatic character [5.8]. Iodine is somewhat easier to

understand, in the context of halogen bonding, when compared to the other halogens

because it is more readily polarized as I(+). Accordingly, contacts such as ICl and IBr

may be represented as I(+)Cl() and I(+)Br() and they generally have the type-II

geometry. Fluorine is very hard and non-polarisable, and it is still not really possible to

deduce the nature of its interactions with other halogens [5.9]. Desiraju and co-workers

have stated in their study that the FF interaction is not really viable [5.10]. Perhaps the

IF interaction is polarization induced. Chlorine and bromine belong to an intermediate

region and various workers analyzed them using one of the two models given above, and

have attributed the observed geometries of XX contacts to either a van der Waals (non-

Chapter 5 132

spherical atoms) or polarization (+) character of the interaction [5.5]. What is

possible is that the type-I contacts formed by Cl and Br are of the van der Waals variety

while the type-II contacts are polarization induced. If all contacts in a crystal were of the

van der Waals type, one would expect a greater degree of isotropy in the packinga

Kitaigorodskii solid [5.1].

There are several reports on halogen interactions in the literature, and in some of

the cases the discussion has been raised in the context of halogenhalogen interactions.

Trihalomesitylene structures (135X246M, X = Cl, Br, I) [2.16] are isostructural with

space group P 1 and Z = 2. In each structure, the molecules are arranged as layers of two-dimensional sheets which are formed via attractive trigonal X3-synthons. Each

halogen atom is part of the triangular X3-synthons as shown in Figure 1.













135C246M 135B246M 135I246M

(a) (b)

Figure 1. (a) Plot showing electrostatic potential in iodobenzene with maxima of negative and positive electrostatic potential indicated. This figure is reproduced from reference [2.16]. (b) Layer constituted of Br3-synthons in 135B246M. Bosch et al. [2.16] analyzed the X3-supramolecular synthons in their

investigation on 135B246M, 135I246M and some other reported structures. Their

HalogenHalogen Interactions in....


theoretical calculations on iodobenzene revealed the polarization of the iodo group in the

molecule that supports the attractive donor-acceptor model (Figure 1). The nonspherical

atomic charge distribution on the halogens results in the formation of donor-acceptor

interactions in which each halogen atom is simultaneously a donor and an acceptor. In

their investigation they strongly felt that the XX interactions must be considered as

being of the attractive donor-acceptor type as suggested by Desiraju and Williams










X-POT (X = Cl, Br, I)





Figure 2. (a) Supramolecular X3 synthon in the inclusion complexes of X-POT. (b) Hexagonal host framework formed through Br3-synthons in the crystal structure of Br-POTcollidine. Disordered collidine molecules are shown in space filling model. Nangia and co-workers [5.11] employed the halogen interactions for

constructing hexagonal host frameworks in C3 symmetric molecules, 2,4,6-tris(4-

halophenoxy)-1,3,5-triazines (X-POT, X = Cl, Br, I)). The halogen groups on trigonal X-

POT molecule, reproducibly self-assemble via the XX trimer (X3) synthon to form

hexagonal cavities that include various aromatic guest molecules (Figure 2). The

architectural isomerism from channel to cage framework and the persistent

crystallization of trigonal X-POT molecule in high-symmetry host networks is

exemplified in these host-guest complexes.

Chapter 5 134

As the intermolecular contacts acquire some distinctiveness, anisotropy enters

the crystal with a change in properties [1.2]. In a one-dimensional structure involving

halogen atoms, molecules are held relatively strongly in this direction [5.12]. In a layered

or two-dimensional crystal structure, the interactions within a layer (intralayer) are

stronger and more directional than the interactions between layers (interlayer) [2.18].

Whether the layered structures arise on account of type-II halogen interactions or the

ubiquitous stacking interactions [5.13] is hard to say from the distance-angle

criteria. However, there is a fundamental distinction