Anion Receptors Based on Halogen Bonding with Halo-1,2,3-triazoliums

A systematic series of anion receptors
based on bidentate halogen
bonding by halo-triazoles and -triazoliums is presented. The influence
of the halogen bond donor atom, the electron-withdrawing group, and
the linker group that bridges the two donor moieties is investigated.
Additionally, a comparison with hydrogen bond-based analogues is provided.
A new, efficient synthetic approach to introduce different halogens
into the heterocycles is established using silver­(I)-triazolylidenes,
which are converted to the corresponding halo-1,2,3-triazoliums with
different halogens. Comprehensive nuclear magnetic resonance binding
studies supported by isothermal titration calorimetry studies were
performed with different halides and oxo-anions to evaluate the influence
of key parameters of the halogen bond donor, namely, polarization
of the halogen and the bond angle to the anion. The results show a
larger anion affinity in the case of more charge-dense halides as
well as a general preference of the receptors to bind oxo-anions,
in particular sulfate, over halides.