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 center dot center dot center dot I halogen bonds. The reluctance to the formation of a supramolecular assembly having a third N center dot center dot center dot 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 center dot center dot center dot F contacts of 2.96 and 3.05 angstrom. 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 center dot center dot center dot 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 center dot center dot center dot I halogen bonds.