Evidence for Halogen Bond Covalency in Acyclic and Interlocked Halogen-Bonding Receptor Anion Recognition

The synthesis and anion binding properties
of novel halogen-bonding (XB) bis-iodotriazole-pyridinium-containing
acyclic and [2]­catenane anion host systems are described. The XB acyclic
receptor displays selectivity for acetate over halides with enhanced
anion recognition properties compared to the analogous hydrogen-bonding
(HB) acyclic receptor. A reversal in halide selectivity is observed
in the XB [2]­catenane, in comparison to the acyclic XB receptor, due
to the interlocked host’s unique three-dimensional binding
cavity, and no binding is observed for oxoanions. Notable halide anion
association constant values determined for the [2]­catenane in competitive
organic–aqueous solvent mixtures demonstrate considerable enhancement
of anion recognition as compared to the HB catenane analogue. X-ray
crystallographic analysis of a series of halide catenane complexes
reveal strong XB interactions in the solid state. These interactions
were studied using Cl and Br K-edge X-ray Absorption Spectroscopy
(XAS) indicating intense pre-edge features characteristic of charge
transfer from the halide to its bonding partner (σAX←X–* ← X1s), and providing a direct measure
of the degree of covalency in the halogen bond(s). The data reveal
that the degree of covalency is similar to that which is observed
in transition metal coordinate covalent bonds. These results are supported
by DFT results, which correlate well with the experimental data.