Crystal engineering through halogen bonding: Complexes of nitrogen heterocycles with organic iodides

X-ray analysis has revealed that 4,4'-bipyridine, 1,2-bis(4-pyridyl)ethylene, and hexamethylenetetramine form donor-acceptor complexes with 1,4-diiodobenzene, 1,4-diiodotetrafluorobenzene, and tetraiodoethylene in which the N...I distance is longer than that for the corresponding I-2 complexes. As opposed to the corresponding I-2 complexes that are molecular adducts, these complexes have extended structures and longer N...I distances. Steric and electronic effects influence the strength of the N...I interaction and the crystal packing. An additional complex of 1,2-bis(4-pyridyl)ethane and 1,4-dibromotetrafluorobenzene has also been investigated for comparison of N...I and N...Br interactions.