Designing Self-recognising Molecules using Halogen Bonding

Recent work from long-running collaboration between the research groups of Prof. Lee Brammer and Prof. Robin Perutz (University of York) reports the design of self-complementary molecules able to form halogen bonds to themselves by providing an outward-facing halogen-bond donor group and a complementary halogen-bond acceptor group (see box for a brief introduction to halogen bonds). The study was published in Chemical Science, the flagship journal of the Royal Society of Chemistry.

A series of nickel coordination complexes were designed to enable a systematic study of the geometry of halogen bonds in inorganic compounds, formed between a halogen-bond donor C–I group and a halogen-bond acceptor Ni–X group (where halogen X = F, Cl, Br or I). The self-complementary nature of these molecules promoted their formation as crystalline materials in which the molecules are organised as linear assembles due to formation of intermolecular C–I···X–Ni halogen bonds.

The geometries of the halogen bonds could then be studied by X-ray crystallography and, remarkably, the series of molecules that were prepared enabled the investigation of the effects on the halogen bonds of changing the metal halide, the ancillary ligands, the regiochemistry, and the effect of temperature and the influence of the crystalline environment. The study also enabled the use of solid-state 19F NMR spectroscopy as a probe for the effects of halogen bonding on fluoride ligands in C–I···F–Ni halogen bonds. This method provides a direct comparison between halogen bonds in the solid state, where metrical information can be obtained by crystallography and the solution phase, where 19F NMR spectroscopy can be used to determine energetics of the interactions.

A cartoon description of nickel coordination complexes organised through formation of halogen bonds

What is a halogen bond? A halogen bond is a directional intermolecular interaction between a covalently bound halogen, most often iodine, and an electron-rich site (e.g. a lone pair of electrons) in a neighbouring molecule. Thus, the halogen serves as a Lewis acid in forming such an interaction. Halogen bonds typically adopt a linear geometry. They have a number of similarities to hydrogen bonds, and can be similar strength. Halogen bonds have attracted much interest in recent years with a wide range of applications areas including supramolecular chemistry, soft matter and materials chemistry, catalysis, and medicinal chemistry involving proteins, DNA and pharmaceuticals.