Halogen Bond - Applications - Crystal Engineering

Crystal Engineering

Crystal engineering is a growing research area that bridges solid-state and supramolecular chemistry. This unique field is interdisciplinary and merges traditional disciplines such as crystallography, organic chemistry, and inorganic chemistry. In 1971, Schmidt first established the field with a publication on photodimerization in the solid-state. The more recent definition identifies crystal engineering as the utilization of the intermolecular interactions for crystallization and for the development of new substances with different desired physicochemical properties. Before the discovery of halogen bonding, the approach for crystal engineering involved using hydrogen bonding, coordination chemistry and inter-ion interactions for the development of liquid-crystalline and solid-crystalline materials. Furthermore, halogen bonding is employed for the organization of radical cationic salts, fabrication of molecular conductors, and creation of liquid crystal constructs. Since the discovery of halogen bonding, new molecular assemblies exist. Due to the unique chemical nature of halogen bonding, this intermolecular interaction serves as an additional tool for the development of crystal engineering.

The first reported use of halogen bonding in liquid crystal formation was by H. Loc Nguyen. In an effort to form liquid crystals, alkoxystilbazoles and pentafluoroiodobenzene were used. Previous studies by Metrangolo and Resnati demonstrated the utility of pentafluoroiodobenzene for solid-state structures. Various alkoxystilbazoles have been utilized for nonlinear optics and metallomesogens. Using another finding of Resnati (e.g. N−I complexes form strongly), the group engineered halogen-bonded complexes with iodopentafluorobenzene and 4-alkoxystilbazoles. X-ray crystallography revealed a N−I distance of 2.811(4) Å and the bonding angle to be 168.4°. Similar N−I distances were measured in solid powders. The N−I distance discovered is shorter than the sum of the Van Der Waals radii for nitrogen and iodine (3.53 Å). The single crystal structure of the molecules indicated that no quadrupolar interactions were present. Interestingly, the complexes in Figure 4 were found to be liquid-crystalline.

To test the notion of polarizability involvement in the strength of halogen bonding, bromopentafluorbenzene was used as a Lewis base. Consequently, verification of halogen bond complex formation wasn’t obtained. This finding provides more support for the dependence of halogen bonding on atomic polarizability. Utilizing similar donor-acceptor frameworks, the authors demonstrated that halogen bonding strength in the liquid crystalline state is comparable to the hydrogen-bonded mesogens.