Co-Crystallization of Sym-Triiodo-Trifluorobenzene with Bipyridyl Donors: Consistent Formation of Two Instead of Anticipated Three N•••I Halogen Bonds

The potential triple-halogen-bond acceptor, sym-triiodo-trifluorobenzene IFB (1), has been co-crystallized with a series of bipyridyl derivatives (2−4) to gain insight to the factors controlling formation of multiple halogen bonds with a single aromatic system. Co-crystals 5−7 were obtained that consistently contained two N···I halogen bonds. The reluctance to the formation of a supramolecular assembly having a third N···I halogen bond does not depend on the size of the bispyridine donor systems (2−4). Apparently, there are limitations to the number of halogen bonds that can be formed with a single aromatic halogen donor. The solid-state structure of co-crystal (5) contains short I···F contacts of 2.96 and 3.05 Å. DFT calculations were performed at the PBE0/(apc1-aSDBDZ)//PBE0/(pc1-SDBDZ) level of theory to investigate the nature of the interaction between the pyridine nitrogen and IFB (1). These calculations reveal a weakening of N···I interactions as more pyridine moieties coordinate to the IFB (1), which might be a contributing factor to the consistent formation of two rather than three N···I halogen bonds.