Halogen Bond Asymmetry in Solution

Halogen bonding is the noncovalent interaction of halogen atoms in which they act as electron acceptors. Whereas three-center hydrogen bond complexes, [D center dot center dot center dot H center dot center dot center dot D](+) where D is an electron donor, exist in solution as rapidly equilibrating asymmetric species, the analogous halogen bonds, [D center dot center dot center dot X center dot center dot center dot D](+), have been observed so far only to adopt static and symmetric geometries. Herein, we investigate whether halogen bond asymmetry, i.e., a [D-X center dot center dot center dot D](+) bond geometry, in which one of the D-X bonds is shorter and stronger, could be induced by modulation of electronic or steric factors. We have also attempted to convert a static three-center halogen bond complex into a mixture of rapidly exchanging asymmetric isomers, [D center dot center dot center dot X-D](+) (sic) [D-X center dot center dot center dot D](+), corresponding to the preferred form of the analogous hydrogen bonded complexes. Using N-15 NMR, IPE NMR, and DFT, we prove that a static, asymmetric geometry, [D-X center dot center dot center dot D](+), is obtained upon desymmetrization of the electron density of a complex. We demonstrate computationally that conversion into a dynamic mixture of asymmetric geometries, [D center dot center dot center dot X-D](+) (sic) [D-X center dot center dot center dot D](+), is achievable upon increasing the donor-donor distance. However, due to the high energetic gain upon formation of the three-center-four electron halogen bond, the assessed complex strongly prefers to form a dimer with two static and symmetric three-center halogen bonds over a dynamic and asymmetric halogen bonded form. Our observations indicate a vastly different preference in the secondary bonding of H+ and X+. Understanding the consequences of electronic and steric influences on the strength and geometry of the three-center halogen bond provides useful knowledge on chemical bonding and for the development of improved halonium transfer agents