Experimental and database studies of three-centered halogen bonds with bifurcated acceptors present in molecular crystals, cocrystals and salts

We present a database and experimental study of three-centered halogen bonds involving bifurcated halogen bond acceptors. Our study encompasses three cocrystals with nitrogen, oxygen and sulfur atoms as acceptors and iodine atoms as donors, as well as an overview of three-centered halogen-bonded systems contained within the Cambridge Structural Database based on organic iodine as the donor atom and nitrogen, oxygen, fluorine, sulfur, chlorine, bromine or iodine atoms as acceptors. The results demonstrate that three-centered halogen bonding arrangements are a possible outcome of cocrystallisation experiments and that they exhibit well-defined geometries. The occurrence of such three-centered interactions is considerably rarer for halogen bonds than for hydrogen bonds and, at the current level of understanding, their formation cannot be readily predicted. The strong preference for binary two-center interactions makes a clear difference between halogen and hydrogen bonds. However, as potential guidelines towards developing halogen-bonded architectures based on bifurcated acceptors, the database search indicates particular preference for bifurcation of neutral sulfur or anionic chloride and bromide acceptors.