The status of research on halogen bonds and other sigma-hole interactions (June issue of Acta Crystallographica Section C: Structural Chemistry)

The cover of the June issue of Acta Crystallographica Section C: Structural Chemistry features a topical review by Lee Brammer, Anssi Peuronena, and Thomas Roseveare (University of Sheffield, UK, and University of Turku, Finland) on the status of research on halogen bonds and other sigma-hole interactions involving p-block elements in Lewis acidic roles, such as chalcogen bonds, pnictogen bonds and tetrel bonds.

Halogen bonding and related intermolecular (and sometimes intramolecular) interactions in which p-block elements in groups other than group 17 serve in a Lewis acidic role have been and continue to be extensively investigated. Their applicability in supramolecular assembly is similarly well studied, with numerous reports of applications in crystal engineering, molecular recognition, catalysis, polymers and soft matter, materials chemistry, structural biology and medicinal chemistry. Many of these areas are the subjects of reviews.

It has been noted that examples of this family of interactions, particularly halogen bonds, are implicit in reports of compounds and observed behaviour as far back as the early 19th century. Extensive historical perspectives can be found in several substantial review articles. Definition, identification and understanding of such interactions evolved slowly until the mid-20th century when work by Mulliken and others classified interactions of I2 with Lewis basic solvents as electron donor-acceptor complexes based on UV-Vis spectroscopic studies. Complementary crystallographic studies of dihalogen interactions with such solvents by Hassel and co-workers then revealed the now well-known linear geometry and short interaction distances. This class of interactions became well established in the 1960s and 1970s, as reviews by Bent on donor-acceptor interactions and by Alcock on secondary bonding attest. The prevailing description at that time was of an electron donor-acceptor interaction, with the Lewis acidic p-block atom involved in the electron-acceptor component. This bonding description is still common in current studies, but is dominant for the strongest interactions, whereas an electrostatic model for the interactions first advanced 15 years ago, and usually referred to as the sigma-hole model, is generally thought to provide the best description of weak to moderate strength interactions. Indeed the electrostatic description has become sufficiently prevalent that this class of interactions are often referred to as sigma-hole interactions, as we have done in the title of this article, although the term perhaps lacks universality since it implies a universal (electrostatic) bonding model. Alcock's earlier term `secondary bonding' is not prescriptive, but refers to interactions other than primary (covalent) bonds. Subclasses of interactions involving elements from a particular p-block group in the Lewis acidic role are most commonly named after the class of elements that comprise the group, the most common being halogen bonds (group 17), chalcogen bonds (group 16), pnictogen bonds (group 15) and tetrel bonds (group 14). In a tutorial review entitled `Hypervalency, secondary bonding and hydrogen bonding: siblings under the skin,' which also benefits from some historical perspective, Crabtree provides an important reminder that there is a continuum of behaviour, both in geometry and bonding description from weak secondary bonding through to the strongest interactions that evolve into hypervalency of the main group elements. This continuum description applies equally to hydrogen bonding.

After a period of relative dormancy, the field of halogen bonding has grown enormously in the past 25 years, accelerated initially by studies in the late 1990s/early 2000s that made clear the similarities between halogen bonding and the more widely studied hydrogen bonding, notably the matrix-isolation and gas-phase rotational spectroscopy studies of Legon and co-workers and the introduction by Resnati, Metrangolo and colleagues of perfluoroalkyl and -aryl halides to enable strong directional halogen bonding in the condensed phases. The evolution of halogen bonding is such that not only are there many reviews on this topic, but many reviews even focus on specific applications of halogen bonding. Chalcogen bonds are the next most widely studied class of secondary bonding interactions, their understanding and application having advanced considerably in the past decade. Reviews that focus specifically on these or the other classes of interactions are now also plentiful. Thus, we have endeavoured to bring together a compilation of the many topics that have been reviewed in the sections below along with tabulated and categorised lists of reviews (including survey articles in which the Cambridge Structural Database has been used as the primary source of experimental data to identify interactions, and reviews that include new computational studies of the compounds being surveyed). The aim has been to provide an ease of departure into the extensive literature on the classes of interactions that are brought together in this special issue of Acta Crystallographica Section C: Structural Chemistry entitled `Halogen, chalcogen, pnictogen and tetrel bonds: structural chemistry and beyond'.