Unravelling the nature of halogen bonding: experimental and theoretical studies

Halogen bonding, namely any noncovalent interaction involving halogens as electrophilic sites, is a relatively new item in the supramolecular toolbox and shares numerous properties with the better known hydrogen bonding. The topological analysis of the X-ray multipole refined charge density proved to be an effective tool to elucidate the nature of such interaction [1]. In this Communication, the results recently obtained [2] on the Br...N interaction in two different complexes of 1,4-dibromotetrafluorobenzene with dipyridyl derivatives, that is, 1,2-bis(4-pyridyl)ethylene (bpe) and 4,4'-dipyridyl, will be presented, together with preliminary results on the I...I- interaction in the complex of 1,4-diiodotetrafluorobenzene with the cryptate K.2.2.2.?KI. The experimental charge densities have been derived from X-ray data collected at 90 K, using the aspherical atom formalism of Stewart [3] as implemented in VALTOPO [4], as well as by accurate molecular modeling calculations. Information such as: (i), topological features of the interactions, as derived from topological analysis of electron density and its Laplacian; (ii), charge transfer within the complexes; and (iii), interaction energies associated to halogen bonding, can be extracted from these studies. The potentialities of halogen bonding in the rational design of new materials for electrooptics will be also illustrated. In particular, it will be shown how the halogen bond interaction can strongly influence the second-order NLO response. 1. (a) R. Bianchi, A. Forni, T. Pilati, Chem. Eur. J., 2003, 9, 1631; (b) R. Bianchi, A. Forni, T. Pilati, Acta Crystallogr. Sect. B, 2004, 60, 559. 2. A. Forni, J. Phys. Chem. A, 2009, 113, 3403. 3. R. F. Stewart, Acta Cryst. Sect. A, 1976, 32, 565. 4. R. Bianchi, A. Forni, J. Appl. Cryst., 2005, 38, 232