Strong CH•••Halide Hydrogen Bonds from 1,2,3-Triazoles Quantified Using Pre-Organized and Shape-Persistent Triazolophanes

The structures associated with halide (F-, Cl-, Br-) complexation inside CH hydrogen-bonding macrocyclic receptors, called triazolophanes, are characterized using density functional theory (DFT). The associated binding energies in the gas and solution phases are evaluated. The ruffles in the empty triazolophane become smoothed-out upon Cl- and Br--ion binding directly into the middle of the cavity. The largely pre-organized cavity morphs into an elliptical shape to facilitate shorter hydrogen bonds in the north and south regions and longer ones west and east. The smaller F- ion sits in, and flattens-out, only the north (or south) region. The 1,2,3-triazoles show shorter CH center dot center dot center dot Cl- contacts than for the phenylenes. Both Cl- and Br- show the same binding geometries but Cl- has a larger binding energy consistent with its stronger Lewis basicity. Model triads were used to decompose the overall binding energy into those of its components. In the course of this triad analysis, anion polarization was identified and its contribution to the triad center dot center dot center dot Cl- binding energy estimated. Consequently, the binding energies for the individual aryl units within the comparatively non-polarized triazolophanes were estimated. The 1,2,3-triazoles are twice as strong as the phenylenes thus contributing most of the interaction energy to Cl--ion binding. Therefore, the 1,2,3-triazoles appear to approach the hydrogen bond strengths of the NH donors of pyrrole units.