Tuning of the Anion Affinity and Selectivity of Tripyrromethane-Based Receptors by Simple Structural Modification

Tripyrromethane-based tripodal compounds 1–3 were synthesized as anion receptors. It was revealed by ¹H NMR spectroscopic analyses carried out in CDCl₃ that tripyrromethane 1 with an ester group was capable of recognizing F⁻, Cl⁻, Br⁻, and H₂PO₄⁻ with relatively high selectivity for F⁻. By contrast, receptor 2 having an additional hydrogen bond donor of an amide group binds F⁻ and H₂PO₄⁻ with much improved affinity relative to receptor 1. On the other hand, receptor 3 bearing formyl groups on its two pyrrole subunits showed markedly enhanced selectivity for F⁻ presumably due to the increased acidity of the pyrrolic NHs and the resulting strengthened intermolecular hydrogen bonds. In addition, tripodal receptors 1–3 were found to bind H₂PO₄⁻, a tetrahedral oxyanion, more efficiently than 2-dimensional macrocyclic calix[4]pyrrole 4.