Symmetric Halogen Bonding is Preferred in Solution
Artikel i vetenskaplig tidskrift, 2012
Halogen bonding is a recently rediscovered secondary
interaction that shows potential to become a complementary molecular tool
to hydrogen bonding in rational drug design and in material sciences. Whereas
hydrogen bond symmetry has been the subject of systematic studies for
decades, the understanding of the analogous three-center halogen bonds is yet
in its infancy. The isotopic perturbation of equilibrium (IPE) technique with
13C NMR detection was applied to regioselectively deuterated pyridine
complexes to investigate the symmetry of [N−I−N]+ and [N−Br−N]+
halogen bonding in solution. Preference for a symmetric arrangement was observed for both a freely adjustable and for a
conformationally restricted [N−X−N]+ model system, as also confirmed by computation on the DFT level. A closely attached
counterion is shown to be compatible with the preferred symmetric arrangement. The experimental observations and
computational predictions reveal a high energetic gain upon formation of symmetric, three-center four-electron halogen bonding.
Whereas hydrogen bonds are generally asymmetric in solution and symmetric in the crystalline state, the analogous bromine and
iodine centered halogen bonds prefer symmetric arrangement in solution.