Expanding Sapphyrin: Towards Selective Phosphate Binding

The anion‐templated syntheses and binding properties of novel macrocyclic oligopyrrole receptors in which pyrrole rings are linked through amide or imine bonds are described. The efficient synthesis was accomplished by anion‐templated [1+1] Schiff‐base condensation and acylation macrocyclization reactions. Free receptors and their host–guest complexes with hydrochloric acid, acetic acid, tetrabutylammonium chloride, and hydrogen sulfate were analyzed by single‐crystal X‐ray diffraction analysis. Stability constants with different tetrabutylammonium salts of inorganic acids were determined by standard 1H NMR and UV/Vis titration techniques in [D6]DMSO/0.5 % water solution. According to the titration data, receptors containing three pyrrole rings (10 and 12) exhibit high affinity (log Ka=5–7) for bifluoride, acetate, and dihydrogen phosphate, and interact weakly with chloride and hydrogen sulfate. The amido‐bipyrrole receptors 11 and 13 with four pyrrole rings exhibit 104‐ and 102‐fold selectivity for dihydrogen phosphate, respectively, as inferred from competitive titrations in the presence of tetrabutylammonium acetate. Expanding expanded porphyrins for selective anion binding: Eight NH donor sites arranged in a macrocyclic receptor display an unprecedented affinity and 104‐fold selectivity for dihydrogen phosphate anions versus other inorganic oxoanions in a DMSO/0.5 % water solvent mixture (see picture).