Fluorine prefers hydrogen bonds over halogen bonds! Insights from crystal structures of some halofluorobenzenes

Abstract

Abstract Crystal structures of a series of isomers of chlorofluorobenzene, bromofluorobenzene and iodofluorobenzene, all of which are liquids under ambient conditions, are determined by a technique of in situ cryocrystallography. These simple dihalo substituted benzenes provide clear insights into subtle interplay of packing interactions preferred by fluorine and heavier halogens for example, C–H···X hydrogen bonds vs. X···X halogen bonds (X=F, Cl, Br, I). The interaction patterns noted here are purely characteristic of halogens, having not been influenced by other stronger interactions. Variability of principal supramolecular synthons among the isomers highlights the importance of molecular shape and relative position of interacting atoms while preserving the basic intermolecular bonds. Mutually exclusive occurrence of homo (I···I) and hetero (I···F) halogen bonds in polymorphs of 4-iodofluorobenzene questions the robustness and reliability of these interactions.