The ability of compounds 1-3 to complex anions was evaluated by 1 H NMR titrations with Cl -and H2PO4 -tetrabutylammonium salts in d6-DMSO/H2O (0.5%). The binding affinities were derived from fitting a 1:1 binding curve to the changes observed in the chemical shifts of 1-3 over the course of the titration (Figures ). An association constant Ka of 90±15 M -1 was determined for the interaction between receptor 1 and Cl -, which is higher than the values obtained for reference compounds 2 and 3 (Table ). When titrating with H2PO4 -, significant broadening and shifts of the 1 H NMR signals of 1 and the appearance of new signals were observed (Figure ), indicative of a loss of symmetry and intermediate exchange on the chemical shift NMR time scale. UV-Vis titrations were performed, from which a Ka of 2.6(±0.4)×10 4 M -1 was estimated (Figure ). The Ka of the bisurea 2 for H2PO4 -was 20-fold lower than that of 1 and no interaction was observed with calix [4]pyrrole 3.