Chapter 3. Design and Structural Chemistry of Halogen-bonded Co-crystals

Abstract

Having emerged in the last few decades, one of the main directions in halogen bonding research has been that of halogen-bonded co-crystals. Analogously to hydrogen bonding, halogen bonding requires a donor and an acceptor, with the donor an electrophilic halogen species and the acceptor a nucleophilic species. The presence of an electrophilic region on the halogen atom, a σ-hole, is a sine qua non for halogen bonding, and its magnitude can be tuned by rational choice of both the halogen atom and the activating chemical moiety attached to it. On the other hand, a variety of Lewis bases and nucleophilic species have been successfully employed as halogen bond acceptors. Here, the design and structural chemistry of co-crystals based on halogen bonding is explored through a number of examples, which are organized based on the chemical nature of the donor activating groups. The relationship between hydrogen and halogen bonding is also explored, as well as the hierarchy of halogen bonds and a unique property of halogen bonding—structural equivalence of donors based on different halogen atoms and different acceptors.