Thorough electron density analysis of halogen bonding interactions : Crystal engineering applications. – HalX-Bond

Directional intermolecular interactions of diverse strengths are favorite tools of crystal engineering, since the orientation of molecules in the solid state can be predicted with a reasonable degree of accuracy. Three such interactions stand out: (i) hydrogen bonding, (ii) weaker C−H•••X hydrogen bonding and (iii) halogen bonding. The marked directionality of the hydrogen bond (HB) is well established and is the basis for efficient and reliable topologies of intermolecular motifs. The HB strength can be estimated from high resolution X-ray diffraction as proposed by the Nancy group and this method based on the topological analysis of the electron density is now widely used. On the other hand, the halogen bonding interaction has been much less investigated. It occurs in the systems C−Hal•••X, where an organic halogen atom approaches either a halogen atom (X = Hal) or a Lewis base (X = B). It finds its origin in the anisotropy of the electron density around the halogen nucleus, leading to a smaller effective atomic radius along the extended C−Hal bond axis than in the direction perpendicular to this axis, a feature called polar flattening and recently confirmed for the deformation density of chlorines in the crystal structure of Chloranil, also experimentally investigated by us in Nancy. Halogen bonding is characterized by: 1) a Hal•••X distance shorter than the sum of the van der Waals radii, 2) a strong linearity of the C−Hal•••B bond, 3) stronger halogen bonding in the order I > Br > Cl. These characteristics demonstrate that halogen bonding can be as effective as hydrogen bonding for driving highly specific crystal packing motifs, as demonstrated in the elaboration of extended networks based on halogenated molecules activated by fluorine atoms. Also, in the field of conducting molecular materials, oxidation of halogenated tetrathiafulvalenes to the cation radical state activates indeed the halogen atom for entering into halogen bond interaction with counter ions of Lewis base character (Br−, Ag(CN)2−). The Rennes group has been actively involved in this field and this topic has been recently reviewed by one of us. Additionally, Cl•••Cl or Br•••Br interactions have been shown by the Nancy group to play an important role in the mechanism of the ionic to neutral transition of the TTF–Chloranil (Bromanil) family. Here, it should be pointed out that (i) the quantum modeling team of Nancy has developed a new hybrid functional that avoids charge transfer overestimation in these complexes, a problem usually found with common functionals and (ii) the 4-circles diffractometer of the CRISTAL beam line run by one of us at SOLEIL has been specially designed for charge density studies on heavy elements such as bromine or iodine. This beam line will be used in this project to extend the results of the TTF–Chloranil family to other halogen bonding interactions. On the other hand, to analyze Hal•••Hal interactions, a new collaboration has been established with Pr. G. Desiraju (Hyderabad). Actually, our knowledge on halogen bonding is essentially based on a combination of: 1) statistical analysis of structural characteristics, 2) gas phase spectroscopic data, and 3) theoretical calculations on model systems. There is therefore a crucial need to experimentally determine the exact nature of the halogen bond in the solid state, and high resolution X-ray crystallography is the most appropriate tool for that. Indeed, it provides an accurate model of the electron density in the molecular and intermolecular regions. From high resolution X-ray diffraction data at very low temperatures, one can accurately model the valence electron density distribution by a multipolar expansion that can be used to determine the interactions of the halogen atom with the environment and to accurately calculate the electrostatic properties needed to build an experimental halogen bond model. On the other hand, theoretical quantum calculations carried out on periodic systems permit to retrieve crystalline electron density distributions, offering an alternative modeling to the experimental one. These calculations will be undertaken in this project to complement the experimental analysis, to support interpretations and, eventually, to bring answers in the cases where experimental conditions do not permit to measure high resolution data. In summary, this know-how of the Nancy group is combined here with (i) the experience of the Rennes group on the electrocrystallisation of conducting materials derived from halogenated TTFs, (ii) the knowledge on structural halogenated motifs and on crystal engineering brought by the Hyderabad group and (iii) the facilities at the CRISTAL beam line in SOLEIL to measure highly absorbent molecular crystals, to present an ambitious project leading to the understanding of halogen bonding interactions.