Dissecting the complex recognition interfaces of potent tetrazole- and pyrrole-based anion binders.

Tetrazoles are potent anion binders. We report here a new family of tetrazole-pyrrole-amide hosts that arise when a tetrazole is incorporated as a new binding element alongside the well-known amidopyrrole anion-binding scaffold. In addition to reporting three new, modular synthetic routes that can be used to access these structures, we also report that the new hosts are highly potent binders of chloride. Along the way, we carried out studies of a pyrrole ester control compound that, surprisingly, bound anions almost as strongly as did the amide analogues. This led us to investigate further the relative importance of the amide NH in halide binding. We report that, despite the regular appearance of this close amide NH---Cl contact in calculated and experimental X-ray structures, the amide NH in this family of anion hosts does not hydrogen bond strongly to chloride in solution.