Experimental and Theoretical Study of Halogen-Bonded Complexes of DMAP with Di- and Triiodofluorobenzenes. A Complex with a Very Short N•••I Halogen Bond

X-ray single crystal structures are reported for 2:1 halogen-bonded complexes of 4-(N,N-dimethylamino)pyridine (DMAP) with 1,4- and 1,3-diiodotetrafluorobenzene and 1,3-diiodo-4,5,6-trifluorobenzene and for a 3:1 complex of DMAP with 1,3,5-triiodotrifluorobenzene. The complex between DMAP and 1,4-diiodotetrafluorobenzene shows the shortest halogen bond recorded for fluorinated iodoarenes. Model systems based on complexes between the same iodofluorobenzenes and ammonia are investigated by Hartree-Fock and DFT calculations to gain insights into the features of halogen bonding in di- and triiodo systems. The calculations reveal a weak charge-transfer component to the halogen bond and account for the lengthening of the C-I bond on complexation in terms of the C-I antibonding character observed within the localized molecular orbital describing the N center dot center dot center dot I bond.